Symposium EN01 : Solid-Solid Interfaces in Batteries, Energy Storage and Conversion—Diagnostic and Modeling

Solid state energy storage devices are typically realized as planar structures fabricated by thin film techniques, though new options are on the horizon including 3D configurations achieved by thin film or particle composite approaches. Here we focus on the former, recognizing that thin film methods offer precision control of nanostructure synthesis, particularly using physical vapor deposition (PVD) and atomic layer deposition (ALD). When combined with microfabrication for patterning to produce 3D scaffolds, these methods enable exploration of high performance 3D architectures of dense micro- and nano- scale solid state electrochemical structures. Interdigitated electrode structures are achieved by conformal ALD deposition of both electrode and solid electrolyte materials, leading to a fully conformal 3D solid state battery that demonstrates the benefits of 3D architectures. These synthesis processes and resulting structures also serve as valuable testbeds to study interface reactions, properties, and architectures, using a variety of characterization techniques including XPS/surface analysis, FIB cross-sections, ToF-SIMS, Kelvin probe microscopy, TEM/SEM, and others.

This work has been supported by Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DESC0001160.

Lithium-ion batteries have been used in portable equipment as a key component in an information society. In addition, much larger batteries are now required for energy storage in vehicles and smart grids. We have to tackle the inherent issues, which originate from non-aqueous electrolytes, in lithium-ion batteries again for developing the large batteries. The increasing size makes the safety issues more serious due to the increasing amounts of flammable electrolytes and the lowering heat radiation. Moreover, much longer durability is required; however, the current liquid electrolytes are not stable enough to meet the requirement. Solid electrolytes are expected to provide fundamental solutions to these issues.
Solid-state batteries had been suffering from the low power density due to low ionic conductivities of solid electrolytes. However, the highest ionic conductivities have reached 10⁻² S cm⁻¹ among sulfide, which is comparable to, or even higher than that of the liquid electrolytes, when the transport number of unity is taken into account. Since ionic conductivities of solid electrolytes have become high enough, interfaces between the battery materials are now playing critical roles in battery performance, which can be recognized in a recent paper that reports higher power density in solid-state batteries than in liquid systems [1].
In the solid-state batteries reported therein, surface of the cathode material, LiCoO₂, is covered with LiNbO₃, although the LiNbO₃ is not highly-conductive. Because sulfide electrolytes exhibit very high resistance at the interface to high-voltage cathodes, even such a poor ionic conductor lowers the interfacial resistance by eliminating the origin of the huge resistance, when it is an oxide-based solid electrolyte. In addition, Li₄Ti₅O₁₂ is used as the anode in spite of the low transport properties. The reason can be understood from computation results suggesting that electronic and ionic transports are enhanced at the Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ interface.

[1] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba. R. Kanno, Nat. Energy 1, 16030 (2016).

Interfacial processes at charge-storing materials have a great impact on such critical operational characteristics as rate capabilities, cycle and calendar life, and safety. For example, solid-electrolyte interfaces that form spontaneously at Li-ion electrodes under operation serve to control surface reactivity, yet may not yield optimized electrode and battery performance. Recent advancements in nanoscale coatings protocols open new opportunities to apply deliberately designed surface coatings at practical electrode structures and materials. Initiated chemical vapor deposition (iCVD) has emerged as one such tool, providing the capability to generate ultrathin polymer coatings with thickness control at the nanoscale [1], even on complex three-dimensional (3D) substrates [2]. The siloxane-based polymers most commonly generated by iCVD contain ether-like functionalities that support the solvation and transport of Li⁺ salts, such that they are readily transformed into Li-ion conductors [3]. We are evaluating iCVD as a means to coat the surfaces of conventional powder-composite electrodes, graphite-based anodes and metal oxide/phosphate-containing cathodes, with ultrathin (5–50 nm) polysiloxanes. The resulting polymer-modified electrodes are assembled into Li-ion coin cells containing liquid electrolytes, in which we examine such properties as specific power, coulombic efficiency, cycle life, and tolerance to overcharge/overdischarge conditions. In pursuit of advanced 3D battery designs, we also continue to explore iCVD-based polymers as nanoscale solid-state Li-ion conductors.

1. Tenhaeff, W.E.; Gleason, K.K. Adv. Func. Mater. 2010, 18, 979.
2. Sassin, M.B.; Long, J.W.; Wallace, J.M.; Rolison, D.R. Mater. Horizons 2015, 2, 502.
3. B. Reeja-Jayan, N. Chen, J. Lau, J. A. Kattirtzi, P. Moni, A. Liu, I. G. Miller, R. Kayser, A. P. Williard, B. Dunn, and K. Gleason, Macromolecules 48, 5222 (2015).

The ionic and electronic charge carrier distribution is discussed for various interfaces of interest in batteries (electrode/electrolyte; electrode/passivation layer; solid electrolyte grain boundaries in multiphase electrodes). Such modified carrier concentrations are of significant influence on charge transport (conductivity) and transfer (transfer resistance) and indirectly for the storage behavior. However, space charge zones can be of great significance for the storage directly. The extreme case is met if the abrupt contact of two phases allows for storing Li or Na in a job-sharing way even if none of the two phases allow for storage themselves. This is relevant the more so since this storage can be extremely fast. Thermodynamics and kinetics of this storage mode are set out in greater detail [1-4].

[1] L. J. Fu, C. C. Chen, D. Samuelis, and J. Maier, Phys. Rev. Lett. 2014, 112, 208301(1-5).
[2] C.-C. Chen, L.-J. Fu, and J. Maier, Nature, 2016, 536, 159-164.
[3] C.-C. Chen and J. Maier, Phys. Chem. Chem. Phys., 2017, 19, 6379-6396
[4] C.-C. Chen, E. Navickas, J. Fleig, and J. Maier, Adv. Funct. Mater., 2017, submitted.

Solid-state Li batteries are promising energy-storage devices owing to their high-energy densities with improved safety. However, the large interface resistance at the interfaces of solid-electrolytes and electrodes hinders the development of solid-state Li batteries.
We fabricated thin-film Li batteries with Li₃PO₄ and LiNi_0.5Mn_1.5O₄ interface resistance below ~ 5 Ωcm²; the value is smaller than that observed in liquid-electrolyte-based Li-ion batteries. Furthermore, the activation energy of the interface resistance was found to be ~ 0.3 eV, which is comparable to that of ionic migration in Li-superionic conductors. The fabricated interface showed no degradation even after 100 cycles of charging and discharging at a rate of 3600C (current density of 14 mA/cm²), indicating the formation of a very stable interface. These studies strongly encourage solid-state Li battery research, by demonstrating the low interface resistance leading to the fast charging and discharging.

All-solid-state rechargeable lithium batteries with inorganic solid electrolytes are expected as a next generation battery with high safety and high energy density. Sulfide and oxide glass-based electrolytes with both high conductivity and good ductility have been developed. Glass electrolytes are useful as a precursor for precipitating metastable crystalline phases, which are difficult to prepare by a conventional solid phase reaction. The conductivity enhancement is achieved by crystallization of Li₇P₃S₁₁ metastable phases. These sulfide glass-based electrolytes also have favorable ductility for forming good solid-solid interfaces with electrode active materials. Oxide glass electrolytes in the system Li₃BO₃-Li₂SO₄-Li₂CO₃ also have a similar relative density to those sulfide systems. Li metal is the most attractive material as a negative electrode because of its extremely high theoretical capacity and the lowest negative electrochemical potential. Interface modification is needed for achieving stable Li dissolution/deposition cycling performance. It is noteworthy that inserting Au thin films into the Li/Li₃PS₄ interface improves Li reversibility. Good formability of active materials is important to form close solid-solid interfaces with solid electrolytes. Amorphous materials have a potential of an electrode active material because of their high capacity and good cyclability based on free volume in amorphous structure. Amorphous oxide electrodes in the system LiCoO₂-Li₂SO₄ prepared via mechanochemistry have good formability and electrical conductivity, and thus a positive electrode layer with only amorphous Li_1.2Co_0.8S_0.2O_2.4 without any conductive additives functions in a solid-state cell. Amorphous electrolyte and electrode materials with high conductivity and good ductility are useful for forming favorable solid-solid interfaces.

All-solid-state Lithium-ion batteries (ASS-LIBs) have been regarded as promising next-generation batteries. Recently, sulfide-based electrolytes have attracted considerable attention because of the high Li-ion conductivities comparable to those of the ordinary organic-liquid electrolytes. However, large interfacial resistance is usually observed if we combine sulfide electrolytes and oxide cathodes [1]. On the other hand, it has been also observed that the interfacial resistance is reduced dramatically by interposing an insulating buffer layer to the interfaces [1]. Compared to these observations, little is known about the atomistic aspects of the solid-solid interfaces in the ASS-LIBs. Thus, we have investigated those properties such as potential origins of the interfacial resistance (e.g. space-charge layer model, reaction layer model) as well as the buffer layer effect by using first-principles DFT+U calculations [2.3].
As a representative model system, LiCoO₂ (LCO), β-Li₃PS₄ (LPS), and LiNbO₃ (LNO) were selected for cathode, sulfide electrolyte, and buffer layer, respectively. We carried out first-principles calculations of the several possible interface configurations and obtained the stable structures and the electronic states. Besides, we calculated the site-dependent Li chemical potentials with respect to Li metal. The results indicate that the Li depletion can proceed at the beginning of the charge process, which may correspond to the space-charge layer scenario, and the interposition of buffer layer can suppress the depletion. [2] Furthermore, we evaluated the interfacial ion diffusion by examining possible exchange of cations between the cathode and the electrolyte. The results show that the Co and P mixing is preferred at the LCO/LPS interface, and the LNO interposition can suppress these mixings. Interestingly, the Li-depletion tendency still exists under these circumstances [3]. Therefore, the Li-depletion is likely to be a major factor of the interfacial resistance. These aspects would be useful for future improvement of the interfacial resistance of ASS-LIBs.
References [1] K. Takada, Acta Mater., 61 (2013), 759-770. [2] J. Haruyama, Y. Tateyama et al., Chem. Mater., 26 (2014) 4248-4255. [3] J. Haruyama, Y. Tateyama et al., ACS Appl. Mater. Interfaces, 9 (2017) 286-292.

The reactions occurring at interfaces have a profound impact on the performance of a rechargeable alkali-ion batteries. In this talk, we will present useful insights into materials selection strategies for enabling stable electrode/solid electrolyte interfaces in all-solid-state sodium-ion batteries using a hierarchy of first principles approximations. We will demonstrate how thermodynamic approximations based on assumptions of fast alkali diffusion and multi-species equilibrium can be used to effectively screen combinations of Na-ion electrodes, solid electrolytes and buffer oxides for electrochemical and chemical compatibility, as well as mechanical compatibility. We find that exchange reactions, especially between simple oxides and thiophosphate groups to form PO₄^3–, are the main cause for large driving forces for cathode/solid electrolyte interfacial reactions. High reactivity with large volume changes are also predicted at the Na anode/solid electrolyte interface, while the Na₂Ti₃O₇ anode is predicted to be much more stable against a broad range of solid electrolytes. We identify several promising binary oxides with similar or better chemical compatibility with most electrodes and solid electrolytes than the commonly used Al₂O₃. Finally, we show that ab initio molecular dynamics simulations of the NaCoO₂/Na₃PS₄ interface model predict that the formation of SO₄^2– -containing compounds and Na₃P are kinetically favored over the formation of PO₄^3– -containing compounds, in contrast to the predictions of the thermodynamic models.

Solidifying the components of a Li-ion battery consisting of a high energy capacity oxide cathode (e.g. NMC, NCA), a Li metal anode, and a sulfide-based glass ceramic solid electrolyte (SE), is a pathway to overcome the challenges of liquid electrolyte cells, namely dendrite growth and safety concerns without compromising high energy density. Even though the conductivity of a few sulfide solid electrolytes (SSEs) surpasses that of liquid electrolytes, multiple interfacial phenomena at both the cathode and anode interfaces play a crucial role in affecting efficient battery performance. Such phenomena include solid-electrolyte interphase (SEI) formation and mechanical deformation; the SEI forms due to poor chemical and electrochemical stability of SSEs while mechanical deformation arises from volume changes experienced by the cathode during cycling and also the rigid nature of SSEs. In the past several decades, tremendous effort has been made to study the SEI for SSEs, however, the properties of cathodic and anodic SEIs and their effects on long-term All Solid State Lithium Battery (ASSLB) cycling are still not well-understood.
In this study, we applied an argyrodite- (Li₆PS₅Cl) based Li ion conducting SSE for fabrication of an all solid-state Li-metal battery. Li₆PS₅Cl was synthesized by mechanical milling followed by heat treatment. It exhibits a room temperature ionic conductivity of 1 mS/cm. An ASSLB was fabricated with this electrolyte, an NCA cathode and a Li metal anode. The as-fabricated ASSLB with the LiNbO₃ coating on the cathode exhibits a room temperature capacity of 140 mAh/g at 0.1C with an initial Coulombic efficiency of 68%, compared with 85% for a liquid cell counterpart. There is 10% capacity loss for the initial 20 cycles followed by negligible loss after 90 cycles. Even when replacing the Li metal with a LiIn alloy (0.625 V vs Li/Li⁺), a similar capacity fade is observed which clearly indicates that the cathode is the dominant contributor to the capacity decay. Although it is not fully understood, part of the irreversible capacity loss arising at the 1^st cycle is due to electrochemical decomposition of the SSE at both the cathode and anode interfaces. The SEI was characterized via X-ray photoelectron spectroscopy (XPS) and Raman and the results match with DFT-based calculations. The electronically insulating nature of the SEI prevents further electrochemical decomposition of the SSE within the operating voltage of NCA (2.5 to 4.3 V vs. Li/Li⁺) after the 1^st cycle. Further degradation up to 20 cycles may arise due to volume change of the cathode during cycling and also the poor mechanical property of the SEI. Major cracks are visible in SEM images taken of the cathode composite after the 1^st cycle. We will discuss a few mitigation strategies for improving ASSLB.

Among the problems on the electrode/solid electrolyte interface, we focused on the lithium concentration chage in the solid electrolyte around the electrode/solid electrolyte interface. Oxide-based all-solid-state battery was introduced into the ERD chamber, and the battery charging reaction was carried out in it. The lithium concentration change in the electrode was observed by the ERD, which was consistent with electrochemical data (open circuit voltage curve). Based on these data, we next investigated the lithium concentration change around the solid electrolyte/current collector interface at a given applied voltage and detected a lithium concetration change region around the interface. Role of such lithium concentration change on the charge-discharge reaqctions will be discussed.

Diffusion is modeled in a storage particle where ions and electrons are inserted at non-coincident surfaces. Under a set of simplified but reasonable assumptions, a design rule based on transport properties is derived for the optimal geometry of both the particle and its interfaces with the ion- and electron-providing contact phases. This result can guide the rational design of storage particle dimensions and solid-solid contact geometry in a battery electrode. It can also be used to estimate to what extent the inactive solid mass loading (e.g., carbon or solid electrolyte coating) in an electrode can be reduced without slowing the discharging. However, the guideline should be applied with care, as material-specific factors can cause the discharge behavior to deviate from the model presented here.

Solid-solid interfaces involving ion conducting materials represent some of the most critical components in many electrochemical systems. The interfacial mass transport and charge transfer behavior both in a static state and upon electrochemical cycling often dictate the performance of the system. Interfaces of solid electrolytes for solid-state batteries form an important example. Unexpectedly high resistivity is often observed at electrolyte-electrolyte/electrode interfaces, and the underlying mechanism is not yet clear. Continuous ion hopping from one mobile ion site to adjacent vacancy lattice sites forms the basis for ion conduction in solids. Interfacial ion transport essentially is determined by the atomic structure, and the distribution of electrons and ions at the atomic scale. Studying these embedded interfaces with a non-centrosymmetric structure is challenging. Scanning transmission electron microscopy (STEM) represents a unique tool for such studies owning to its unprecedented spatial resolution. During the past few years, atomic-scale imaging and electron energy loss spectroscopy (EELS) in STEM have revealed critical structural and chemical features at solid electrolyte interfaces that are responsible for the degradation or enhancement in battery performance. In situ biasing platforms have been integrated to study the formation and stability of interfaces in batteries. These atomic-scale and in situ studies have provided significant insight and continue to highlight the importance of microscopy for battery research. Recent developments in fast cameras and highly stable electronics have enabled the emergence of functional imaging methods such as ptychography, differential phase contrast imaging, and vibrational spectroscopy. These techniques, once tailored for battery research, will not only allow us to probe the atomic structure and chemical species, but will also facilitate the ability to map, directly or indirectly, functionality associated with local nanofeatures in battery materials and devices. The evolving integration of new and emerging microscopy techniques into battery research, from atomic-resolution to in situ and functional imaging, will be demonstrated by several recent studies on solid-electrolyte materials.

Acknowledgement
Research sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Microscopy performed as part of a user project at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE User Facility.

The properties of lithium ion battery (LIB) cathodes strongly depend on the lithium ions diffusion across the interfaces and from the surfaces during charge/discharge process. Since this behavior determines the stability, lifetime and reliability, direct visualization of lithium site is required to understand the mechanism of the lithium ions diffusion. Aberration corrected STEM is very powerful imaging technique to directly observe the atomic columns inside a crystal. In this study, aberration corrected HAADF and ABF STEM are applied to directly observe the interface and the surface of the olivine Li_xFePO₄ and delithiated olivine (FePO₄) [1,2], and the mechanism of the lithiation/delithiation will be discussed based on the observation results.
For the cathode of LiFePO₄, previous studies showed that the lithiation/delithiation in LiFePO₄ is basically the two-phase process, that is, LiFePO₄/FePO₄ interfaces propagate through the bulk region with inserting/extracting lithium. TEM observations showed that the average particle size is of the order of µm, and most particles have core-shell structures. HAADF STEM revealed that the phase interface is parallel to the {100} plane, and the lattice variation widths are highly orientation-dependent. The a-axis lattice length (l_a) shows a narrow variation width, less than 10 nm, whereas the change of b-axis lattice length (l_b) is gradual and extends over 30 nm. The two phase interface is thus found to have very complicated feature, which is related to the lithiation/delithiation mechanism.
In order to understand the surface reconstruction during lithiation/delithiation, the (010) LiFePO₄ surfaces was directly observed by STEM [4]. Commercially available LiFePO₄ singe crystals (Oxide Co., Japan) were used for all experiments. Crystals were cut perpendicular to their (010) axis and polished, and the structures of (010) surfaces before and after chemical delithiation were characterized by STEM. It was found that P and Fe atom columns undergo comparatively large displacements near the surface, which was consistent with the results from first-principles calculations. The magnitudes of the P and Fe displacements were also found to depend on the location of the outmost Li sites.
Acknowledgements
A part of this work was supported by the Research & Development Initiative for Scientific Innovation of New Generation Batteries II (RISING II).
References [1] S. D. Findlay et al., Microscopy, 66 (2017) 3, [2] R.Huang and Y.Ikuhara, Curr. Opin.Sol.Stat. & Mat.Sci.,16 (2012)31, [3] A. Nakamura, et al., Chem. Mater., 26 (2014) 6178, [4] S. Kobayashi et al., Nano Lett. 16 (2016) 5409

In this work, we report a new approach to enhance the cycle stability of aggregated cathode particles for lithium ion battery at both room and elevated temperatures. We discover that infusion of a solid electrolyte into the grain boundaries of the cathode secondary particles can dramatically enhance the capacity retention and voltage stability of the battery. We find that the solid electrolyte infused in the boundaries not only acts as a fast channel for Li ion transport, but also most importantly prevents penetration of the liquid electrolyte into the boundaries, consequently eliminating the detrimental factors that include solid-liquid interfacial reaction, intergranular cracking, and layer to spinel phase transformation. The present work, for the first time, reveals unprecedented insight as how the cathode behaves in the case of not contacting with the liquid electrolyte, ultimately points toward a general new route, via grain boundary engineering, for designing of better batteries of both solid liquid and solid state systems.

This talk will present the results of in-situ characterization of cathode materials for Li-ion battery by using various Scanning Probe Microscopy based techniques, including DART and Band Excitation Electrochemical Strain Microscopy (DART/BE-ESM), Biased AFM and bimodal Dual AC imaging techniques. In these SPM techniques, DART/BE-ESM allows the high frequency periodic bias to be applied on the sample surface of the electrochemically active materials. The bias will induce the local periodic oscillatory displacement caused by the Li-ions redistribution within the material, and the surface deformation caused by the Li-ion re-distribution is measured and defined as electrochemical strain. Biased AFM can apply a positive or negative point DC bias in contact mode, and hence to study the influences of positive and negative biases on the surface deformation of the cathode materials. This process can be used to study the effects of charging and discharging processes to the cathode materials. Finally, bimodal Dual AC Imaging technique is used to study the composition and properties (elasticity) changes due to the Li-ions redistribution under the electrical field. With the capability of the in-situ characterization at the nano- to micro-scale of the topography, surface deformation (electrochemical strain), ionic movement as well as the corresponding changes of the composition and properties in the cathode materials, including thin film cathodes and nanoparticles for cathode materials, this work will provide the fundamental understanding of the structure-property-functionality relationships at the nano- to micro-scales for the cathode materials used for Li-ion rechargeable batteries.

Many lithium ion solid electrolytes containing sulfur are not thermodynamically stable against low-voltage anodes like lithium metal. Instead, a Li₂S-containing passivation layer forms at the interface, and the transport properties of this layer can limit the overall performance and stability of a solid state battery. Transport in Li₂S can also limit the discharge rate of sulfur-based cathodes. Despite this technological relevance, the defect chemistry of lithium sulfide has rarely been explored. Here the ionic and electronic transport properties of Li₂S are probed in bulk samples, as well as in thin films prepared by both sputtering and molecular beam epitaxy. One key finding is that the ionic conductivity is higher in thin films than in bulk samples. The origin of this enhanced transport is discussed in terms of doping and interfacial effects. These results help to understand and predict the performance of Li₂S-containing passivation layers and sulfur-containing cathodes in solid state batteries.

The pressing demand of high-capacity and high-power batteries leads to extensive efforts on various material and technical developments, such as high-voltage electrodes and anionic redox. In these systems, batteries often operate beyond the thermodynamic stability of the battery materials, which leads to unusual surface/interface behavior and bulk redox reactions. The mechanism of such complex systems is often under debate and requires incisive tools for measuring the chemical states of specific elements. Recent developments of soft X-ray spectroscopy (SXS), especially new types of soft X-ray absorption (sXAS) and resonant inelastic X-ray scattering (RIXS), have provided us an inherently elemental and chemical sensitive technique for probing both the surface/interface and the bulk (100 nm probe depth) materials of batteries.

The focus of this presentation is to provide extensive examples on probing the key electron and oxidation states of both the transition metals and oxygen for studying the battery electrodes and interfaces. In particular, recent technical developments of various sXAS detection channels extracted from high-effeciency RIXS, including high-resolution inverse partial fluorescence yield (iPFY) and super partial florescence yield (sPFY), will be introduced. We will discuss the methodology for quantitatively define the oxidation states of transition metals through sXAS and calculations. For some novel transition metal redox reactions that cannot be resolved by conventional sXAS, RIXS provides extra sensitivity to clarify the mechanism by further resolving the energies of emitted photons. For light elements involved in battery interfaces, we show that sXAS is also a sensitive probe for specific chemical bonds. We then showcase some recent examples on clarifying the intrinsic oxygen state evolution (oxygen redox) in battery materials through soft X-ray RIXS. In summary, due to the energy range of the dipole-allowed transitions in both the low-Z elements, e.g., C, N, O, and 3d Transition metals, SXS is the most direct probe of the valence states of these elements, which provides unique information on the chemical states of battery electrodes and interfaces.

Most solid materials have defects such as surfaces, interfaces, and grain boundaries that serve as pathways for transport; as a result, the properties of polycrystalline solids can be vastly different from its intrinsic properties. Diffusion in polycrystalline materials plays an important role in a wide range of electrochemical systems, from batteries to solid oxide fuel cells. Due to the computational expense in explicitly considering the grain boundary network, establishing rational design rules for nanocrystalline materials with desired transport properties remains a challenge. We apply the Smoothed Boundary Method to evaluate the effective diffusivity of polycrystalline materials with a range of morphologies. We find that the anisotropy of grain morphologies plays a critical role in the overall transport behavior, which cannot be quantified using the classical mean field theories. The results are used to obtain an expression for mixed-pathway transport that is capable of universally predicting the effective diffusivity in complex polycrystalline solids without the use of computationally intensive simulations. Applications to battery materials are highlighted.

Cerium-based oxide (ceria) is among the most investigated materials in heterogeneous chemical catalysis as an active support and electrochemical catalysis as a non-metallic oxygen-ion-conducting electrode for applications such as three-way catalysts (TWCs) and solid oxide fuel cells (SOFCs). In such devices, ceria is typically polycrystalline and is used in close contact with other components at high temperatures (> 600°C). Thus, during device operation, cation impurities from other components can be diffused into the ceria, especially through grain boundaries, which are a fast cation transport path.
In this study, we investigated metal impurities (Ni, Pt, and Al) diffusion phenomenon through the grain boundaries of acceptor-doped ceria as a function of temperature, pO₂, and the type and concentration of dopant using dense polycrystalline thin films prepared by pulsed laser deposition (PLD). The remarkably high grain boundary density of thin films with vertically-oriented, nanosized-columnar grains enables accurate analysis of the diffusion kinetics and the solubility of metal impurities by means of time-of-flight secondary ion mass spectroscopy (ToF-SIMS). It is revealed that impurities diffuse unexpectedly quickly, and a considerable amount of impurities dissolves inside ceria grain boundaries. Furthermore, we monitored how the oxygen-ion transport properties and surface oxidation reactivity of ceria change according to the presence of impurities (Co, Ni, Cu, Au, and Pt), demonstrating the importance of diffusion control of the impurities in the grain boundaries.

High ionic conductivity is desired to optimize electrolyte performance, though it is significantly degraded by grain boundaries (GBs), which act as blocking layers in polycrystalline electrolytes. Given the rich diversity in GB types, and the complex interplay between structure, composition, and chemistry at the atomic and nanoscale [1-3], there is considerable opportunity to elucidate fundamental science and performance optimization of GBs. Hence, studies should rely on GB datasets correlated across many length scales, with the aim of generalizing high spatial resolution observations to an entire GB population. This should facilitate bottom-up design of GBs with optimized properties, which remains a considerable challenge. By combining suitable modeling approaches with experimental measurements interrogating materials over different length scales, it becomes possible to estimate the electrical properties of individual GBs.

Here a novel correlated approach is employed combining precession electron nanodiffraction (PED) orientation imaging and electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning TEM to elucidate the GB transport properties in oxygen-conducting Gd_0.11Pr_0.04Ce_0.85O_2-δ [3]. Nanoscale EELS measurements of GB solute segregation are generalized to the entire boundary population via GB character determined using PED. Composition data are used to estimate carrier concentration and migration activation energy, which enables prediction and mapping of the distribution of GB ionic conductivity. The applicability of conventional GB models—used widely to predict defect distribution and transport properties—to the presented data is also evaluated.

Acknowledgements
NSF Graduate Research Fellowship (DGE-1211230), NSF grant DMR-1308085, ASU’s John M. Cowley Center for High Resolution EM.

References
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[2] W.J. Bowman, M.N. Kelly, G.S. Rohrer, C.A. Hernandez, P.A. Crozier. Nanoscale (2017).
[3] W.J. Bowman, A.D. Darbal, P.A. Crozier. (Submitted).

Solid electrolytes are generating considerable interest for all-solid-state Li-ion batteries to address safety and performance issues. Grain boundaries have a significant influence on solid electrolytes and are key hurdles that must be overcome for their successful application. However, grain boundary effects on ionic transport are not fully understood, especially at the atomic scale. The Li-rich anti-perovskite Li₃OCl is a promising solid electrolyte, although there is debate concerning the precise Li-ion migration barriers and conductivity. Using Li₃OCl as a model electrolyte, we apply large-scale molecular dynamics simulations to analyze the ionic transport at stable grain boundaries.

Our results predict high concentrations of grain boundaries and clearly show that Li-ion conductivity is severely hindered through the grain boundaries. The activation energies for Li-ion conduction traversing the grain boundaries are consistently higher that of the perfect crystal, confirming the high grain boundary resistance in this material. Using our results, we propose a polycrystalline model to quantify the impact of grain boundaries on conductivity as a function of grain size. Such insights provide valuable fundamental understanding of the role of grain boundaries and how tailoring the microstructure can lead to the optimization of new high-performance solid electrolytes.

All-solid-state Li-ion battery based on solid electrolyte materials is a promising next-generation energy storage technology, providing intrinsic safety and higher energy density. Currently, high interfacial resistance and interfacial degradation at the solid electrolyte-electrode interfaces is the key bottleneck, limiting cycling and rate performance. Fundamental understanding about the interfaces is essential, yet lacking, due to the difficulty of directly access in experiments and the complicated microstructure to construct in modeling.

In this presentation, I will show how we use first principles computation to bring new understanding about these buried interfaces. Using our developed computation approach based on large materials database, we calculated the true electrochemical stability window of solid electrolytes and predicted interphase decomposition products, which are verified by in-situ experiments. I will discuss the critical role of decomposition interphase layers and their effects on the battery performance. From these insights, we are able to classify different interface types for different solid-electrolyte and electrode pairs and estimate their impacts on battery performance. Moreover, specific interfacial engineering strategies are proposed to address potential interface issues.

In addition, I will present the predicted chemistry trend and novel strategies to enable Li metal anode. Previous research efforts to stabilize Li metal anode was greatly impeded by the lack of knowledge about Li-stable materials chemistry. With first-principles calculations based on large materials database, we found that most oxides, sulfides, and halides, which were commonly studied as protection materials, are reduced by Li metal due to the reduction of metal cations. On the contrary, nitride anion chemistry exhibits unique stability against Li metal, which is either thermodynamically intrinsic or a result of stable passivation. Many nitrides materials may be promising candidates for Li metal anode protection to achieve long-term stability. This series of computational study provides novel insights and general guidance for material design and interfacial engineering in all-solid-state Li-ion batteries.

[1] Y. Zhu, X. He, Y. Mo, Origin of Outstanding Stability in the Lithium Solid Electrolyte Materials: Insights from Thermodynamic Analyses Based on First Principles Calculations. ACS Appl. Mater. Interfaces, 7, 23685-23693 (2015);

[2] Y. Zhu, X. He, Y. Mo, First principles study on electrochemical and chemical stability of solid electrolyte-electrode interfaces in all-solid-state Li-ion batteries. Journal of Materials Chemistry A, 4, 3253-3266 (2016)

[3] F. Han^§, Y. Zhu^§, X. He, Y. Mo, C. Wang, Electrochemical Stability of Li₁₀GeP₂S₁₂ and Li₇La₃Zr₂O₁₂Solid Electrolytes. Adv. Energy Mater., 6, 1501590 (2016) (^§ co-first author)

[4] Y. Zhu, X. He, Y. Mo, Strategies Based on Nitride Materials Chemistry to Stabilize Li Metal Anode. Adv. Sci., 1600517 (2017)

All-solid-state Li-ion battery is a promising next-generation energy storage technology with intrinsic safety, high energy density, and good stability. Currently, high interfacial resistance at the solid-solid interfaces are the critical issues limiting the cycling and rate performance of all-solid-state battery. While ionic transport has been well studied in the crystalline bulk phases, the fundamental understanding about the ion transport at the interfaces is still lacking due to the difficulties of constructing atomistic models for interfaces. I will present our recent study of using large-scale molecular dynamics simulations to directly model the ionic transport at these buried solid-solid interfaces. Using molecular dynamics simulations, we reveal the mechanisms of ion transport at solid interfaces. The origin of ion transport changes at interfaces are illustrated. Moreover, we will also discuss how such interface structure and properties change impact the performance and failure of all-solid-state Li-ion batteries.

The electrochemical stability of the solid electrolyte component is one of the very important criteria for consideration when developing solid-state Li ion batteries. For example, resistive interphase materials produced over the course of cycling operation can impede Li ion transport across grain boundary regions, severely compromising the overall battery performance. In this presentation, we report our results on combined experimental and first-principles DFT investigations on the cause of capacity fading at charging step of an air-isolated Li ion battery with the following components: a garnet solid electrolyte (with a nominal formula Li_6.625La₃Zr_1.625Ta_0.375O₁₂, assigned as LLZrTaO), a LiFePO₄ + carbon (LFP + C) cathode, and a Li metal anode. Our analysis revealed that the decomposition route may involve the formation of defective garnet and formation of products composed of light elements. Surveying by DFT approach the various known garnet compounds in the general formula Li_5+xLa₃M₂O₁₂ (M for x = 0: Nb, Ta; M for x = 2: Zr, Ti, Hf), we determined that the smaller the M cation electronegativity in the host framework, the more stable is the garnet material against our predicted decomposition route.¹

Reference:
Jalem et al., J. Mater. Chem. A 2016, 4, 14371–14379.

Li₃PO₄ based materials (e.g., Li₃PO_4-xN_x) are used for solid electrolytes in thin film Li ion batteries. Recently, its application as non-volatile memory devices is also being explored: Au/Li₃PO₄/Li as well as Ni/Li₃PO₄/Li stacked structures were found to exhibit two different voltage states, viz., high and low voltage states, which can be controlled by applied voltages [1, 2]. To develop novel memory devices using this phenomenon, the understanding on Li ion distribution near the metal/Li₃PO₄ interfaces and atomic structures of the two voltage states are crucially important.

In this study, first we investigated the Li ion distributions in the γ-Li₃PO₄ connected with the metal electrodes, using the combination of the defect formation energy calculations from first principles and the continuum-model calculations [3]: A one-dimensional continuum-model was adopted, and the charge density of Li defects (vacancy and interstitial) and the electrostatic potential in the solid electrolyte were determined self-consistently. We found that Li vacancies are hardly formed near the Au(111)/γ-Li₃PO₄ and Ni(111)/γ-Li₃PO₄ interfaces. On the other hand, the accumulation of formation of Li⁺ interstitials near the interfaces is seen, and its depth reaches ca. 10 Å from the interface. When the possibility of Au-Li alloy formation in the electrode is considered, the amount of Li⁺ interstitials decreases as compared to the pure Au case. This suggests the possibility of Au-Li alloy formation using the accumulated Li⁺ interstitials at the interface [4].

Next, we investigated the relation between the atomic structure of the interface and high- and low- voltage states of the system. The voltages were estimated using the method widely used for studies in Li ion batteries [5]. For simplicity, Li adsorption on the metal electrode was considered without taking account of the Li₃PO₄ electrolyte. We found that the amount of Li atoms staying at the surface determine the two voltage states. For the Ni case, higher and lower Li coverages are likely to correspond to the low and high voltage states, respectively [2]. On the other hand, for the Au case, a higher Li ratio in the Au electrode indicates the low voltage state and a partially alloyed Au-Li surface exhibits the high voltage state. Although preliminary calculations taking account of the Li₃PO₄ electrolyte suggest that the details of atomic arrangements of the two states may change by considering the electrolyte, we conclude that the switching mechanism between the two voltage states can be attributed to the formation of Li⁺ interstitials at the interface and the variation of the Li densities at the metal electrode.

This work was supported by CREST, JST.

[1] I. Sugiyama et al., APL Mater. 5, 046105 (2017).
[2] W. Liu et al., in preparation.
[3] S. Kasamatsu et al., Solid State Ionics 183, 20 (2011).
[4] K. Shimizu et al., in preparation.
[5] M.K. Aydinol et al., Phys. Rev. B 56, 1354 (1997).

Sodium based electrochemical cells are gaining interest in industry due to the high availability and abundance of sodium. A sodium ion conducting electrolyte is required for the cell to function. Sodium zirconium gallate (Na_0.7Ga_4.7Zr_0.3O₈) belongs to the beta gallate rutile structure and was recently discovered. Preliminary measurements show it to be a one-dimensional sodium ion conductor. Synthesis of Na-β″-alumina + 3 mol.%Y₂O₃-stabilized zirconia (YSZ) by a vapor phase process led to a slightly textured microstructure with accompanying anisotropic transport. Here we report on the synthesis of Na_0.7Ga_4.7Zr_0.3O₈ + YSZ composites by a vapor phase process and observe no evidence of crystallographic texturing either in microstructure or powder diffraction. Furthermore, ionic transport is isotropic. The kinetics of vapor phase transformation in Na_0.7Ga_4.7Zr_0.3O₈ + YSZ composites are reported. The results show a similar behavior to what was observed previously for Na-β″-alumina + YSZ composites.

Both safety and high energy density are essential for the large size batteries propelling electric vehicle. All solid state batteries have received great attention as a next generation power source due to its safe characteristics with non-flammable solid electrolyte. A solid electrolyte with high ion conductivity is key to the development of all solid state battery. Solid electrolytes were mainly synthesized by conventional solid state method. Recently, liquid based new synthesis method was proposed to avoid mechanical milling process which takes long time to prepare. Moreover, new materials might be obtained via liquid based synthesis method which cannot be successfully achieved by solid state synthesis.
In this study, tetrahydrofuran (THF) and anhydrous ethanol were used as a solvent for liquid based synthesis. Li2S, P2S5 and LiBr with a stoichiometric ratio were added in solvent, and then dried at 150 C under vacuum to obtain solid electrolyte powder. Crystallinity was examined by X-ray diffraction method and ion conductivity was evaluated by AC impedance technique. All solid state batteries were fabricated using LiNi0.8Co0.1Mn0.1 (NCM811) active material and Li6PS5Br solid electrolyte obtained by liquid based synthesis process. Reaction mechanism was investigated and the detailed results will be discussed.

Polymer-based solid electrolytes recently emerge as new reliable lithium ion battery materials by not only showing better safety (e.g., flammability and toxicity) over current state-of-the-art carbonate based liquid electrolytes. Especially when it comes to lithium metal, they offer sufficient mechanical stability to suppress dendrite formation and thus prevent thermal runaway reactions.¹
Existing polymer based electrolytes suffer from low ionic conductivity – especially in the ambient temperature range - or poor redox stability against the electrode material. Self-assembled block-copolymers offer a new way to overcome these issues. Independent tuning of the unique components allows high ionic conductivity within one microdomain, while the inactive matrix ensures overall mechanical integrity.²
We prepared new high-performance solid block-copolymer/lithium salt hybrid electrolytes by molecular design of polymer structure, comprises high ionic conductivity at ambient temperature (10^-3 S/cm @ 20°C), high lithium ion transference number and good redox stability within the scope (0 – 4 V against Li/Li⁺). We interpret the good performance of our polymer electrolyte by deciphering the hybrid structure with temperature dispersive in situ X-ray scattering studies from -15 to 105 °C.


References

1 Aurbach, D., Zinigrad, E., Cohen, Y. & Teller, H. A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions. Solid State Ionics 148, 405-416 (2002).
2 Young, W. S., Kuan, W. F. & Epps, T. H. Block Copolymer Electrolytes for Rechargeable Lithium Batteries. J Polym Sci Pol Phys 52, 1-16 (2014).

Supercapacitors have attracted more and more attention because of their higher energy density than traditional capacitors, and larger power density, shorter recharging time and longer cycle lifespan than rechargeable batteries [1, 2]. But low specific surface area about conventional electrode materials limits its capacitance, novel electrode materials can be improved into mesoporous electrodes materials [3, 4]. Mesoporous materials, due to high surface areas and tuneable pore sizes, exhibit a considerable application prospects in catalysis, adsorption, sensors, lithium-ion batteries, drug delivery, and nanodevices [5]. However, it is difficult to obtain mesoporous transition metal oxides because the hydrolysis and condensation of non-silica precursors are generally hard to control, while the thermal breakdown can also destroy structural integrity [6]. In this work, we synthesized series of mesoporous transition metal oxides via soft template method and explored their pseudocapacitive behaviour. Mesoporous transition metal oxides present excellent electrochemical performance, because the mesoporous structure can increase the contact area of electrode materials and electrolyte, shorten the transportation of ions and electrons path length and quicken the rate of diffusion, which can be expected to be applied to the construction of a new type of energy storage.

Reference
[1]T.L. Wang, M.T. Liu, H.W. Ma, Nanomaterials 2017, 7, 140.
[2]Y. Fu, M.T. Liu, H.W. Ma, T.L. Wang, K.R. Hu, C. Guan, Electrochimica Acta 2016, 191,916.
[3]W. Li, J Liu, D.Y. Zhao, Nature Reviews Materials 2016, 1, 1.
[4]Y. Ren, Z. Ma, P.G. Bruce, Chemical Society Reviews 2012, 41, 4909.
[5]U. Ciesla, F. Schüth, Microporous and Mesoporous Materials 1999, 27, 131.
[6]D. Gu, F. Schüth, Chemical Society Reviews 2014, 43,313.

Perovskites belong to a family of compounds with crystal structure as ABX₃ in which A and B are cations and X is an anion. These perovskites find useful applications in the areas of fuel cells, hydrogen gas sensors and steam electrolysis. With a cage like structure perovskites are now being used in absorbing and transporting small ions and molecules like protons. Thus, their proton conduction at high temperatures is now being utilized in fabricating membranes for reactors for catalytic dehydrogenation/hydrogenation processes. In comparison to the already studied BaZrO₃, SrZrO₃ has lower proton conduction at varied temperature window.
In our study we have analysed the relative proton conduction between two perovskites, that is, BaZrO₃ and SrZrO₃. In these two materials individual dynamical and thermodynamical energy barriers for proton transfer from one oxygen atom to another were determined. Thermodynamics of proton conductivity in these systems were addressed by performing activation energy barrier calculations for proton hopping transition states using Nudged Elastic Bands (NEB) theory and ab-initio quantum chemical calculations. MD simulations were also performed to determine the activation energy from trajectories of SrZrO₃. In the end correlations and comparisons were made between the energy barriers obtained from dynamical (MD) and thermodynamical (NEB) runs that were performed on SrZrO₃. In order to study the electronic conductivity their band structures shall also be determined. Subsequently, similar operations are being performed for the sandwich of SrZrO₃ and BaZrO₃ as a model for electrode electrolyte assembly. We shall also report the interfacial properties of BaSrZrO₃.

Herein, we investigated the performance of Panasonic NCR18650B battery in regular cycling, fast charging and shelf life testing using an optimized equivalent circuit model in EIS fitting. NCR 18650B battery, already commercially used in TESLA EV and many other portable electronics, still lack of global understanding in aging and monitoring SOC/SOH in real time. We employ electrochemical impedance spectroscopy (EIS) as a primary technique combining with galvanotactic intermittent titration technique (GITT) testing and thermal evaluation. With an optimized choosing of equivalent circuit in fitting, and a creative choosing of key factor and indicator in aging, we are allowed to observe the electrochemical phenomena within the cell dynamically and more in detail.

Interfacial resistance is one of the severe problems in composite electrodes of all-solid-state batteries (ASSBs), especially oxide-type ASSBs. Conflicts between poor sinterability and possible unfavorable reaction with active materials limit applicable materials and processes. This work consists two parts: 1) distribution of ions and structural distortion around surface of solid electrolytes, 2) based on results of the first part, a novel approach is proposed to decrease grain boundary resistance among nonsintered solid electrolyte particles. The concept is successfully demonstrated, and the nonsintered grain boundary resistance of a highly conducting solid electrolyte (Li_1.3Al_0.3Ti_1.7(PO₄)₃) was suppressed by being coated with poorly conducting solid electrolyte (Li₂SiO₃). The total conductivity (mainly contributed by grain boundary resistance) was improved by one order of magnitude. Increased total conductivity and variation of apparent activation energy are well explained from the viewpoint of defect chemistry.

Solid oxide fuel cell is one of the most promising energy conversion device due to its several advantages involving the high efficiency and high power density without any harmful byproducts. However, mechanically and chemically unstable state of SOFC system in high operating temperature in the range of 700-800 °C still remain challenge to solve for universal commercialization. In order to preserve the stability for long time operation, we applied thin protection layer on the surface of the unstable electrode scaffold. A novel infiltration method using biopolymer was adopted to coat the few nanometers deep protection layer. The porous scaffold made of Sm_0.5Sr_0.5CoO_3-δ (SSC) was coated by thin film of Gd_0.2Ce_0.8O_2-δ (GDC) to be used as electrode. Using the unique property of the biopolymer, uniform and thin GDC layer in a thickness of ~3 nm was successfully covered on the SSC cathode surface. With GDC/SSC hetero-interface, the performance enhancement can be achieved because of enlarged TPB on the cathode surface. Interestingly, as well as performance enhancement, it demonstrated the excellent stability at 650 °C for 100 h compared with non-GDC coated SSC cathode and discretely GDC coated SSC cathode. With surface analysis, it can be ascribed that the enhanced durability is from the suppression of Sr segregation with thin GDC layer.

For exemplary catalytic activity for oxygen reduction reaction (ORR) platinum and its alloys have been widely used. This, however, remains challenging to develop as platinum suffers from both costs and degradative activity. Alternatives have recently attracted attention, and being more abundant at lower costs, non-precious transmission metal oxides (TMO) including MnO_x, Co₃O₄, and Fe₃O₄ provide a larger surface area and maximize catalytically active sites per volume and mass^1,2. This is achieved by dispersing TMOs on a highly conductive carbon structure, thereby creating a synergetic effect in the generation of electrons.

In this study we use a conductive 3D carbon structure like graphene oxide and TiO₂ or ZrO₂ to create a hybrid structure to increase electronic conductivity and surface area. GO, however, contains large quantities of O-containing functional groups which do not bind to nanoparticles (non-reactive) unlike the wrinkles and edges of GO. To combat the non-reactive O-containing functional groups species, functionalization of the basal plane of GO using acid treatments could be used. Specifically, the acid treatments performed using hydrobromic and/or oxalic acid to create hydroxyl and/or carboxyl groups would better react with nanoparticles (P25) or precursors (ZrOCl₂) to form hybrid structures.

Recent results of ZrO₂/GO hybrids indicated that hydroxylated GO had the best ORR performance in 0.1 M KOH as demonstrated with an increase of electron transfer number, current density and onset/half-wave potential. This was comparable to the performance of Pt/C while TiO₂ on carboxylated GO exhibited the best ORR performance compared to other TiO₂/GO hybrids. Analysis using X-ray diffraction indicated a reoccurring (002) diffraction GO peaks among most hybrids with the exception of hydroxylated ZrO2/GO and carboxylated TiO₂/GO. This affinity of metal oxides to the basal planes of graphene flakes prohibits graphene restacking for both hybrid structures. Other experimental analysis indicates the strong ORR performance and reaction route is the result of strong tethering of metal oxide particles on the basal plane of graphene and the particle-graphene interfaces (compared to graphene alone). These phenomena were also observed in density theory calculations.

This work was supported by NASA ASTAR Fellowship (NNX15AW57H). In addition, S.G. and M.H.L acknowledge the NASA MIRO Program (NNX15AQ01A).

References:

¹ G. Wu and P. Zelenay, Acc. Chem. Res. 46, 1878 (2013).

² Y. Sun, Q. Wu, and G. Shi, Energy Environ. Sci. 4, 1113 (2011).

Lithium-ion batteries are promising for large-scale energy storage applications due to their high capacity and relatively lightweight compared with conventional Lead-acid batteries. Despite their high promise, current LIBs are limited with reliability and scalability because of the lack of an accurate model to show the real state of health the batteries. Herein, we compressively investigated the internal battery dynamics of bulk, interfaces throughout the frequency range in 18650 lithium-ion batteries by modeling and driving simulation. Specifically, we simulated daily driving situation that the battery experiences in real life by using driving simulation program. We also investigated the degradation of the battery dynamics by EIS spectroscopy, which has been proved to be validated for studying the interfacial dynamics for various electrochemical systems. Finally, we developed a model for battery interface dynamics by using COMSOL modeling techniques to further investigate the interface dynamics of the batteries during the daily driving and aging situations, Butler-Volmer equation for electrochemical kinetics at interfaces, Fick’s law of diffusion and other theories were used to build the model to simulate the experimental results. As a result, accurate SoH model was determined then we also discuss the possible strategies to develop the lifetime of the batteries. Our results provide a broader understanding of the state of health of 18650 lithium-ion battery in particular and lithium-ion batteries in general.

All-solid-state lithium (ion) batteries show great advantages over traditional lithium ion batteries by replacing flammable organic electrolytes with solid electrolytes, provide enhanced safety, higher working voltage, easy packing et al., and have attracted great interests from fundamental study to commercial application. However, two core elements are required before their real application. Firstly, solid electrolyte has to satisfy (1) high ionic conductivity, (2) negligible electronic conductivity, (3) good chemical/electrochemical stability, (4) easy preparation, (5) low cost and so on. Secondly, the interfacial engineering between solid and solid is challenging because of their stiffness in nature.
Oxide-based solid electrolytes meet most of above requirements, and up to now, the highest ionic conductivity can reach 10^-3 S cm^-1 with the aid of high temperature and high pressure during preparation.[i] However, the sluggish interfacial engineering retards the development of oxide-based all-solid-state batteries. The simple press of solid electrolytes with electrode materials could not facilitate the solid-solid interface. While post high temperature treatment could improve the adhesion between solid electrolyte and electrode materials, chemical reactions always accompany at this high temperature, which are detrimental to the interfacial properties.[ii] Although sputtering technique can enable the interface in film batteries, the energy density is far away from enough. Therefore, it is urgent to develop a new technique for constructing both solid electrolyte and interfacial engineering at low temperatures, at least under the reaction temperature of solid electrolyte and electrode materials.
Cold sintering technique, enhancing the interfacial properties (grain boundaries and interfaces between solid electrolytes and electrode materials) by dissolution-precipitation of target materials with the aid of liquid solution at low temperatures, is ideal for the assembly of solid-state batteries.[iii] Herein, with the help of cold sintering technique, we successfully prepared the solid electrolytes Li_1.3Al_0.3Ti_1.7 (PO₄)₃ (LATP), Li_1.3Al_0.3Ge_1.7(PO₄)₃ (LAGP), and composite cathode LiNi_0.6Mn_0.2Co_0.2O₂/TiN/LAGP. The ionic conductivities of LATP and LAGP are close to 10^-4 S cm^-1 after heat treatment at 650 °C, which is about 200-300 °C lower than that of conventional sintering. The composite cathode LiNi_0.6Mn_0.2Co_0.2O₂/TiN/LAGP exhibits a specific capacity of 132 mAh g^-1 when test in liquid electrolyte, while further increase of ionic conductivity could enable it in all-solid-state batteries.

[i] J. C. Bachman et al., Chem. Rev. 2016, 116, 140-162.

[ii] M. Fingerle et al., Chem. Mater.2017, 29, 7675-7685.

[iii] J. Guo et al., Angew. Chem. Int. Ed. Engl. 2016, 55, 11457.

The next generation of energy storage and sensing devices may largely benefit from fast Li⁺ ceramic electrolyte conductors to allow for safe and efficient batteries and real time monitoring anthropogenic CO₂. Recently, Li solid state conductors based on Li garnet structures received attention due to their fast transfer properties and safe operation over a wide temperature range. Through this presentation basic theory and history of Li garnets will first be introduced and critically reflected towards new device opportunities. Central to our research is the fundamental investigation of the electrochemomechanic characteristics and design of electrode and Li garnet interfaces to new battery architectures and sensors. Here, we firstly present new Li garnet battery architectures for which we discuss lithium titanate and antimony electrodes in their making, electrochemistry and assembly to full battery architectures^1-4. Secondly, new insights on processing of Li garnet thin films are presented based on first developed field maps for pulsed laser deposition processing. Here, the thermodynamic stability range of maximum Li conduction, phase and nanostructure is discussed using high resolution TEM studies, near order Raman investigations on the Li-bands and electrochemical transport measurements. Finally, we present for the very first time a new type of a CO₂ sensor based on Li zirconate garnet structure electrolytes and show the full material development up to the successful engineering and control of the sensing performance. The insights provide novel aspects of material structure designs for both the Li-garnet structures (bulk to films) and their interfaces to electrodes, which we either functionalize to store energy or to track chemical species such as anthropogenic CO₂ for next generation batteries and environmental sensors.

1) Interface Engineered All Solid State Li Ion Batteries Based on Garnet Type Fast Li Conductors
J van den Broek, S Afyon, JLM Rupp
Advanced Energy Materials 6 (19), 2016

2) A shortcut to garnet-type fast Li ion conductors for all solid state batteries
S Afyon, F Krumeich, JLM Rupp
Journal of Materials Chemistry A 3 (36), 18636-18648, 2015

3) On the chemical stability of post lithiated garnet Al stabilized Li 7 La 3 Zr 2 O 12 solid state electrolyte thin films
M Rawlence, I Garbayo, S Buecheler, JLM Rupp
Nanoscale 8 (31), 14746-14753, 2016

4) Investigating the all solid state batteries based on lithium garnets and a high potential cathode LiMn1. 5Ni0. 5O4.
C Hänsel, S Afyon, JLM Rupp
Nanoscale 8, 18412-18420, 2016

Rechargeable, thin film all-solid-state Li-ion batteries (TFSSLIBs) with high specific power and energy density are highly desirable to energize an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. TFSSLIBs are also attractive for fundamental studies aimed at understanding how battery geometry, dimensions, composition and the resulting interfaces affect performance. For example, thin film fabrication methods enable precise control over electrode and electrolyte thickness, morphology, geometry and interface area (i.e. 1D, 2D or 3D type electrode). Furthermore, TFSSLIBs are vacuum compatible, meaning that techniques that generally require vacuum such as SEM, TEM, auger electron spectroscopy, secondary ion mass spectroscopy, and Kelvin probe force microscopy can be readily applied to characterize TFSSLIB, often in operando mode. In my presentation, I will discuss recent experiments and modeling efforts to understanding the factors that limit TFSSLIB performance, including the role of interfaces, and which take advantage of thin film fabrication techniques and vacuum based characterization methods.

Doped Li₇La₃Zr₂O₁₂ (LLZO) garnet has room-temperature Li-ion conductivity of 0.4 mS/cm [1] and elastic modulus sufficiently large to suppress the morphological instability of Li metal during plating [2]. Although it shows very good cycle life in low current scenarios, the Li/LLZO interface still suffers from dendrite formation and eventual failure above a critical current [3]. Such failure events may owe in part to mechanical phenomena, so understanding the electromechanical response of polarized Li/LLZO interfaces has great significance to the development of solid-state Li batteries. We have produced a model that shows the impacts of mechanical effects on the DC response and electrochemical impedance of LLZO.

Both theoretical research [4] and TEM observations [5] of metal/solid-electrolyte interfaces have probed the structure of space-charge layers (SCLs) and their importance in determining battery performance. A theoretical study by Braun et al. [4] highlights the influence of deformation stress on SCLs within solid electrolytes between biased blocking electrodes. For dynamic scenarios where Faradaic currents can flow through solid electrolytes, we are still pursuing a clear picture that can advance the understanding of dendrite nucleation.

To achieve this goal, Newman’s concentrated solution theory has been generalized in a thermodynamically consistent way [6] to form a dynamic electrochemomechanical model of solid electrolytes. Poisson’s equation releases the constraint of local electroneutrality and brings in the Lorentz force. Onsager–Stefan–Maxwell multicomponent-diffusion theory provides relationships between current flow and thermodynamic driving forces such as gradients in ion concentration, pressure, and voltage. A momentum balance couples the electrical, mechanical, and electrochemical processes, all of which contribute substantially to the structure of SCLs. This new framework has been used to model the steady-state response of galvanostatically polarized Li/LLZO/Li cells.

Interfacial impedance impacts the dynamic behaviour of solid electrolytes substantially. We will discuss how it affects the distribution of Li-ion concentration, stress, and voltage under both direct and alternating currents.

Bulk impedance can be extracted from the model by linear perturbation analysis. Impedance reveals information about structural changes in SCLs; it also exhibits several novel features associated with stress, such as the appearance of acoustical waves at high frequency. The discussion will close with an analysis of how electrochemomechanical interactions impact the impedance spectra of Li/LLZO/Li cells.

Reference:
1.R. Murugan, et al. Angew. Chem 46, 7778 (2007).
2.C. Monroe and J. Newman, J. Electrochem. Soc. 152, A396 (2005).
3.A. Shara, et al. J. Power Sources 302, 135 (2016).
4.S. Braun, et al. J. Phys. Chem. C 119, 22281 (2015).
5.K. Yamamoto, et al. Angew. Chem 49, 4414 (2010).
6.P. Goyal and C. W. Monroe, J. Electrochem. Soc. 164, E3647 (2017).

For several decades, the miniaturization of nomadic electric systems has made the world of energy storage evolve. Indeed, the designs of these devices are getting progressively smaller and smaller, yet more powerful. This of course, makes it more difficult integrating these conventional batteries into these modern devices including; integration incompatibilities due to their sizing, life-time and limited cyclability, risks of inflammation due to metallic lithium, risk of solvents leaking into liquid electrolytes. All of the above factors limit the development of these new technologies. In response to these difficulties, the all-solid-state thin film microbatteries "lithium-free" have provided answers to the needs of the industrialists. They can reach a potential of 0V [1], far below the capability of lithium and lithium-ion batteries. Furthermore, these micro batteries are thinner, more flexible, safer due to the absence of liquid electrolyte and metallic lithium electrode. These micro-devices can be integrated into diverse applications as connected watches, SmartCards, RFID Tags, or even be integrated into state of the art contact lenses to order to power the autofocus system for medical applications.

Li-Free microbatteries are composed of a platinum current collector, LiCoO₂ as positive electrode, LiPON glass as solid electrolyte and a current collector of copper. The lithium contained in insertion material of cathode is electroplated on the current collector during the initial charge, to create an anode of metallic lithium and make the battery operational. However, this technology offering a lot of advantages still remains misunderstood and consequently difficult to control.

The thin-film design makes individual separation of elements difficult after cycling. Therefore, it is complicated to characterize every layer individually. Analysis of the surface with X-ray Photoelectron Spectrometry (XPS) and Auger Electron Spectroscopy (AES) determines the chemical composition of the surface of Li-Free microbatteries after cycling. Characterizing techniques by the use of Electrochemical Impedance Spectroscopy (EIS), can provide answers to the questions on Li-Free, and finally, can control the manufacturing process and understand the functioning and ageing of these technologies. This study is focused on the non-destructive characterization of lithium-free microbatteries by means of the Electrochemical Impedance Spectroscopy coupled with X-ray Photoelectron Spectrometry and Auger Electron Spectroscopy, and shows mechanisms necessary during the first charges and discharges of the battery for good performances in cycling. With this adapted protocol to start the Li-Free [2], this battery can now exhibit a life duration and cyclability similar to lithium metallic and lithium-ion microbatteries.

References :
[1] Neudecker and al., J. Electrochem. Soc., 147 (2) 517-523 (2000)
[2] Larfaillou and Guy-Bouyssou. U.S. Patent 2015325878 (2015)

While there have been recent advances in solid ion conductors exhibiting conductivities comparable to liquid electrolytes, how to best capitalize on these materials discoveries to enable new energy storage technology is currently not known. Of particular interest is the integration solid-state electrolyte into solid-state batteries; however, numerous questions remain. What are the design rules? How are solid-solid interfaces formed? How does charge transfer occur at interfaces?
Essentially, there are two interfaces of interest in solid-state batteries, distinguished by the electrode type; the alkali metal electrode and the ceramic electrode. Several key scientific challenges related to these two interfaces must be addressed to mature solid-state batteries. This discussion is centered on the alkali metal/solid electrolyte interface. The solid electrolyte based on garnet-type oxide, of nominal composition Li₇La₃Zr₂O₁₂ (LLZO), is used as a model system that simultaneously exhibits fast-ion conductivity and stability against metallic Li.
To date, there are few examples of bulk-scale Li-ion conducting solid electrolytes that are stable at the Li/Li⁺ redox potential. This paper elucidates stability and kinetics as a function of LLZO composition and surface chemistry. EIS, TEM, and electron spin resonance analysis describes the subtle interaction at the interface between Li metal and LLZO. Simple approaches to achieving negligible Li-LLZO interface resistance (~1 Ohm.cm^-2) with no coatings is described. Based on this mechanism, a simple procedure for removing these surface layers is demonstrated, resulting in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. Combined, the demonstrated stability and low interfacial resistance suggests a pathway to achieving viable high energy density solid-state batteries enabled by the LLZO-based solid electrolyte.

Solid electrolytes are considered to be able to suppress the Li dendrite growth due to its high shear modulus. However, Li dendrites are still found to grow inside the pores and along grain boundaries, but the driving force for its growth is still unclear. In this paper, garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ (LLZO) is chosen to be studied by Density Functional Theory (DFT), because of its high conductivity and stability again Li metal. Both cubic and tetragonal LLZO structures were investigated for their interfaces with Li, to understand the nucleation of Li metal on the pores surfaces inside LLZO. The partial density of states (PDOS) and the charge transfer was analyzed when the number of electrons and Li atoms fluctuates on the LLZO surface. It was found that external electrons would be trapped on the La atoms at the (110) surfaces, which tends to pass the electrons to the adsorbed Li-ions on the surface and reduce them to Li metal. To suppress the Li dendrite growth, different La substitutions are tested, and their impact on the distribution of external electrons and implication on Li-dendrite formation inside LLZO were analyzed. Similar electronic structure analysis was performed on the crystalline Li₂PO₂N surface, to compare the Li plating resistance with LLZO.

The cubic garnet phase of Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte with a Li-ion conductivity approaching 10^-3 mS/cm at room temperature. Its interfacial stability with Li metal remains an issue, however, and its study is made challenging by rapid oxidization in air. We have developed a UHV system in which lithium can be sputtered onto LLZO and its interface immediately characterized by X-ray photoelectron spectroscopy (XPS). Here, we exploit this sytem to investigate the stability of Al, Ta, and Nb-doped LLZO against metallic lithium. We find that the redox reaction at the interface is strongly dependent on the particular dopant. For Nb-doped LLZO, Nb is reduced from 5+ to 4+ by lithium; however, for the Ta and Al-doped electrolytes, Zr is reduced from 4+ to 3+. Considering the molar ratio of Nb doping element, the total charge transfer between Li and the LLZO is approximately the same for all three samples. Despite the relatively large change in valence, we find that the change in binding energy is less than 0.5 eV (toward lower binding energies) in the La-core level spectra for all samples. Among them, the Nb doped sample has the largest energy shift. The energy shifts are likely associating with the in-built interface potential; therefore, the Nb-doped LLZO is expected to show more upward band bending at the interface. Furthermore, to investigate how sensitive this interfacial redox reaction is to the oxidation layer at LLZO surface, we compared the results with those from LLZO surfaces that had been treated differently before lithium deposition. To completely remove the Li₂CO₃ species at the surface, some samples were heated in UHV or sputtered with Ar ions. Others were lightly sanded in a glove box, with a small portion of the oxidation layer left on surface.

Lithium (Li) metal is an attractive material for the use as the negative electrode of next-generation batteries such as Li-air and Li-sulfur batteries due to its high theoretical capacity (3860 mAh g^-1) and the lowest electrochemical potential (-3.040 V vs. SHE). The use of a solid electrolyte is a potential solution to these issues inherent in Li metal. Garnet-type cubic Li₇La₃Zr₂O₁₂ (LLZ) is promising as a solid electrolyte due to various advantages, including high Li-ion conductivity, high chemical stability against Li metal and high stiffness. However, it is known that the resistance of the Li/LLZ interface (R_int) is high,which interferes with the operation of a LLZ-based solid-state battery at a practical rate. To date, several attempts have been made to reduce R_int; application of high external pressure and temperature, tuning the chemical composition of LLZ, modification of the surface morphology of LLZ by optimization of the particle and grain sizes, and the insertion of a lithiophilic layer between Li and LLZ. Although these studies have provided potential strategies to reduce R_int toward the successful operation of all-solid-state batteries, there is still limited information regarding how R_int dynamically changes during repetitive Li deposition/dissolution cycles.
To address this issue, AC impedance spectroscopy with a three-electrode setup, in which the interface between a working electrode and electrolyte can be examined independently from the other interface between a counter electrode and the electrolyte, is necessary. In the present work, we attempted to individually trace the dynamic change in R_int at a Li/LLZ interface during Li deposition and dissolution reactions through the use of the three-electrode AC impedance technique. As a result, we clarified that the trace the dynamic changes in the charge transfer resistance at the Li/LLZ interface during Li dissolution and deposition. R_int increased and decreased during Li dissolution and deposition, respectively, and the increase during dissolution was not completely offset during the subsequent deposition process. Importantly, R_int was almost constant when Li deposition proceeded without prior Li dissolution, which suggests that the formation of voids at the Li/LLZ interface during Li dissolution is a critical factor that influences R_int. Based on the results obtained through the present work, we strongly encourage the development of a strategy to prevent the formation of voids at the Li/LLZ interface, particularly during Li dissolution, toward the real application of a Li metal anode in all-solid-state secondary batteries.

The impact of surface chemistry on the interfacial resistance between the Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolyte and a metallic Li electrode is revealed. Control of surface chemistry allows the interfacial resistance to be reduced to 2 Ω cm², lower than that of liquid electrolytes, without the need for interlayer coatings. A mechanistic understanding of the origins of ultra-low resistance is provided by quantitatively evaluating the linkages between interfacial chemistry, Li wettability, and electrochemical phenomena. A combination of Li contact angle measurements, X-ray photoelectron spectroscopy (XPS), first-principles calculations, and impedance spectroscopy demonstrates that the presence of common LLZO surface contaminants, Li₂CO₃ and LiOH, result in poor wettability by Li and high interfacial resistance. On the basis of this mechanism, a simple procedure for removing these surface layers is demonstrated, which results in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. The low interfacial resistance is maintained over one-hundred cycles and suggests a straightforward pathway to achieving high energy and power density solid-state batteries.

The recent attempts to model the Li-dendrite formation using the phase-field method will be briefly reviewed. We will start with a nonlinear phase-field model for Li dendrite growth in Li metal/liquid electrolyte binary systems accounting for the Butler-Volmer electrochemical reaction kinetics. The dendritic patterns are examined as a function of applied voltage and initial electrode surface morphology. Then we further advance our phase field model by implicitly incorporating the effect of a solid electrolyte interphase (SEI) layer (or any artificial electrode protective coating layers). The impacts of key SEI properties (e.g. Li ion diffusion coefficient, interfacial energy anisotropy, charge transfer barrier, etc.) on Li plating and dendrite formation will be discussed. We will then introduce a multiphase multigrain phase-field model of Li electrodeposition at a Li metal anode in polycrystalline solid electrolyte, based on which the Li dendrite growth and nucleation mechanisms will be examined in terms of solid electrolyte electro-chemical-mechanical properties.

Electrochemical Capacitors (ECs), also known as supercapacitors [1], have now reached the technical maturity for complementing- and sometimes replacing- batteries in a broad range of applications. Conventional electrolytes based on acetonitrile or propylene carbonate solvents have been mainly used in combination with ammonium cations and fluoride anions. Designing solid electrolytes for ECs would be of great interest to solve packaging issues, corrosion, self-discharge or leaks. Moreover these solid electrolytes could be used for flexible supercapacitors applications.
Ionogels are quasi-solid electrolyte obtained from the trapping of an Ionic Liquid (IL) into a silica scaffold using a sol-gel process [2]. In a first part, we will present the electrochemical performance of ionogel electrolytes and carbon-based supercapacitor cells using ionogel electrolytes [3,4]. Cyclic voltammetries and electrochemical impedance spectroscopy plots in a large temperature range (-40°C to +60°C) of cells will be presented and discussed. We will show that porous carbon materials can achieve high capacitance (90 F.g^-1) in these solid-state like electrolytes thanks to the ionic liquid confinement in carbon nanopores (pore size < 1 nm). In a second step, we will present the results of an in-situ X-Ray scattering study combined with modelling to explain the confinement effect of ionic liquids trapped into carbon nanopores [5]. In such confined environment, ionic liquids ions can form co-ion pairs by breaking the Coulombic ordering rules, leading to an increase of capacitance thanks to the existence of a superionic state theoretically proposed by Kornyshev et al [6].

References
[1] Simon and Y. Gogotsi, Nature Materials, 7 (2008) 845-854.
[2] M. Brachet, T. Brousse, J. Le Bideau, ECS Elec. Letters, 11 (2014) A112-A115.
[3] L. Negre, B. Daffos, P.L. Taberna and P. Simon, JECS 162 (2015) A5037-A5040.
[4] L. Nègre, B. Daffos, P.L. Taberna and P. Simon, Electrochimica Acta 206, 490-495 (2016).
[5] R. Futamura, T. Iiyama, Y. Takasaki, Y. Gogotsi, M. J. Biggs, M. Salanne, J. Ségalini, P. Simon, K. Kaneko, Nature Materials (2017), DOI: 10.1038/nmat4974
[6] S. Kondrat and A. Kornyshev, J. Phys.: Condens. Matter 23, 022201- 022205 (2011).

Solid-state electrolytes have been explored increasingly for use in Li⁺ batteries on account of their ability to address the safety issues, packaging constraints, and volumetric design concerns associated with liquid electrolytes. Until recently however, their low ionic conductivity limited the achievable power density. Along with poor conductivity, high interfacial resistances stemming from the processing conditions or material incompatibility have limited their use in commercial cells. Recently, ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest because of their high ionic conductivity, stability, and solution processability. There have been few studies, however, of the electrode-ionogel interface. This presentation will review our work directed at understanding the interface for solution-processable ionogel electrolytes with various electrode materials.

Recent work has shown that processing the ionogel as a sol on certain positive electrodes led to interfacial barriers for Li⁺ intercalation/deintercalation. This detrimental effect has been attributed to several possible sources such as surface reduction by the sol’s catalyst or silica nucleated on the surface hindering electrolyte percolation. Using XPS, Raman spectroscopy, and electrochemical testing to probe the electrode-ionogel interface, the surface reactions were identified as being the source of the interfacial barriers. Our results indicate that the acidity of the sol led to breakdown of the solvent and organic acid, forming an organic surface layer which impedes Li⁺ transport. By adjusting the pH of the sol or by adding a surface coating, these interfacial reactions can be avoided, leading to stable cycling.

Interface between solid electrolyte interphase (SEI) and the anode material plays an important role for Li⁺ ion transport during the charging and discharging processes. Understanding of the structure and Li⁺ ion transport behavior at the interface is crucial for further improvement of the Lithium ion batteries (LIBs). Recent experimental studies such as XPS and TOF-SIMS have revealed the chemical components and their spatial distribution in the SEI layer. However, it is still difficult to characterize the interfacial structure experimentally. DFT-MD simulation technique is a powerful tool to investigate the structure and the dynamics in the atomic-scale. In this study, we investigate the interfacial structure and Li⁺ ion transport process at the interfaces between Li-intercalated graphite and amorphous Li₂CO₃, a model inorganic SEI, by the DFT-MD simulation. Effect of the functional groups at the graphite edges is also examined.
The Li-intercalated graphite was set to Li₁₀C₂₄₀, corresponding to the dilute stage 1. The edge carbons were terminated by the -H only (reduced surface model) or by -H:-COOH:-OH = 4:2:4 (oxidized surface model). Li⁺ and CO₃^2- were located on the graphite surface randomly. To obtain the stable interfacial structure, we performed the DFT-MD simulation for at least 5ps with NVT ensemble at 298K. In order to investigate the Li⁺ ion migration at the interface, the blue-moon ensemble technique was applied to the Li⁺ ion position. For all DFT-MD calculations, we used the PBE exchange correlation functional.
Regarding the equilibrium structures, it is demonstrated that the the oxidized surface termination is more close to the Li₂CO₃ SEI than the hydrogen-terminated reduced surface. The large stabilization of the oxidized surface is attributed to strong interaction between the surface functional groups and the carbonates.
In the investigation of Li⁺ ion transport, we applied constrained DFT-MD for the Li⁺ ion position. For the reduced surface, Li⁺ ion drags the CO₃^2- unit to around the graphite edge and then releases it to move into the anode. On the other hand, the oxidized surface shows correlation between the inserted Li⁺ ion and the nearest Oxygen of the surface functional group (COOH or OH) instead of the CO₃^2- unit. Therefore, Oxygen in the termination is likely to support the Li⁺ ion insertion. However, the free energy profiles in these two cases do not look so different. In the presentation, we will propose possible mechanisms of these observations with the detailed analysis.

Characterizing interfaces and interphases in primarily lithium-ion batteries is in one way simple but the result can be difficult to interpret. It is simple in the sense that it is easy to take a battery apart and then use different techniques to study the composition and morphology of interfaces of electrodes and separators post mortem. It is, however, difficult to study in situ how an interface forms and evolves during battery operation.
The SEI (Solid Electrolyte Interphase) on negative electrodes can be described as a mixture of inorganic and organic compounds where the inorganic compounds are formed closer to the electrode surface. The layer is a consequence of the low potential – close of that of lithium – where for lithium-ion batteries the reduction of the thermodynamically instable organic solvent (below 0.8V vs. Li⁺/Li) is taking place. There are even descriptions of the SEI consisting of an inner, more dense inorganic layer, where electrons can tunnel through until a certain thickness of the layer has been obtained where the SEI becomes electronically insulating but where ions can penetrate. How the different SEI-compounds interplay to form a well-functioning layer is not yet clear. This presentation aims at bringing some light on these complex issues based on the combination of different techniques.
On the surface of a positive electrode is also influenced by the electrolyte composition (and then primarily of the electrolyte salt) but in a different way compared to the negative electrode. In general the surface film is thinner and the reactions can more be described as corrosion reactions.
The presentation willinclude a description of how the crosstalk in the redox chemistry between the electrodes during battery operation will influence the interfacial chemistry, respectively. What is the difference to a the interface compositions in so-called half-cells compared to those for full-cell Li-ion and Na-ion batteries. Typical chemistries involve graphite and silicon as negative electrode materials and different nickel, cobalt and manganese oxide materials as positive electrode materials.

Comprehensive understanding about the solid electrolyte interphase layer (SEI) is the major knowledge gap in the development of the lithium sulfur (Li-S) batteries. Various ex situ studies, about the interfacial reaction mechanism gave overly simplistic views that lapse the transient species/structures that are critical to interfacial process. However, the underlying challenge is to detect, identify and quantify the reactions at the interface which typically span over a wide spatial and temporal region and are inaccessible by any single spectroscopic and/or classical computational methods. Hence, it is critical to develop in situ multimodal approach that can provide unprecedented chemical imaging of complex interfaces in wide lateral (ranging from subatomic to micron) and temporal scales (few ns to seconds). Herein, we report an in-situ X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) combined with ab initio molecular dynamics (AIMD) computational modelling to gain fundamental understanding about the complex interfacial interactions in Li-S batteries. A multimodal approach involving AIMD modelling and in situ XPS and NMR characterization uniquely reveals the chemical identity and distribution of active participants of interfacial reactions as well as SEI layer evolution mechanism.

Solid-solid interfaces are ubiquitous in lithium ion and other high energy density batteries. Even batteries based on organic liquid electrolytes exhibit interfacial ("solid electrolyte interphace" or "SEI") films. The atomic structure, function, and evolution of such solid-solid interfaces are challenging to elucidate using imaging and other experimental methods. Electronic structure calculation techniques provide a complementary approach. In particular, a comparative study of liquid- and solid-electrolyte batteries permits cross-polination of methods, perspectives, and paradigms. Applying solid state thermodyamic perspectives, we show that organic electrolyte molecules, and even most components of the SEI, are profoundly metastable. Under certain conditions, SEI components will evolve and decompose chemically or electrochemically. Lithium metal, silicon anode, and high voltage oxide surfaces will be used as example. Applying liquid state kinetic perspectives and LiPON as example, we show that the speciation at the interfaces in all-solid batteries are not necessarily governed by thermodynamics alone; the degradation barrier may be sufficiently high to permit the existence of metastable components. Our comparative study therefore yields an unified perspective on interfaces in solid- and liquid- based batteries.

This work was supported by Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Electronic, mechanical, transport, and electrochemical properties of composite materials are significantly influenced by the presence and characteristics of interfaces such as grain boundaries and interfaces between electrodes and electrolytes. Atomistic structures at interfaces can be characterized by electron and x-ray characterization techniques, and at the same time atomistic and first principles modeling has been used to obtain interfacial structures as well as properties. In this talk, we will discuss efforts to combine atomistic modeling (including using first principles density functional theory and interatomic potentials) and electron and x-ray measurements, under an automated framework making use of machine learning and computer vision, to produce atomistic structures at interfaces. Relevant electronic and transport properties of the interfaces will also be discussed.

Conventional rechargeable lithium-ion batteries (LIBs) utilize solid electrodes and liquid electrolytes. Combustible organic electrolytes pose potential safety risks, including volatilization, flammability and even explosion. Solid-state (SS) inorganic electrolytes have the potential to eliminate these safety concerns, while providing high-energy LIBs. Two major challenges, however, have been hindering the practical high-performance all-SS battery applications: (1) the rather low ionic conductivities of SS electrolytes compared to liquid counterparts and (2) high resistance at the electrolyte/electrode interfaces that further curtails the ion migration. Sulfide-based electrolytes are a possible solution for the former, displaying ionic conductivities comparable to the organic counterparts, with potential enhancements by dopants.1 Besides, the interfacial resistances can be lowered by a rational design of the interfaces and the use of buffer layers.2
To tackle these issues, an automated computational procedure is adopted in this work for predicting novel SS electrode/electrolyte interfaces with lower interfacial resistances and high ionic conductivities. The procedure comprises several steps, first of which involves the generation of convex hulls by pre-screening the bulk structures of sulfide-based electrolytes with diverse compositions, phases, vacancies and doping. Subsequently, the promising electrolyte materials are screened this time for their ionic conductivity using molecular dynamics addressing Li-ion conduction. Stable surfaces of the selected bulk structures are then generated through random cuts and interfaced with the known cathode surfaces (e.g. LiCoO2, Li-metal), while minimizing the lattice mismatch. Our research group’s ab initio random structure searching (PyAIRSS) code3 is used for the random search of the initial structures, and for introducing possible dopants and vacancies into the lattices. All electronic structure calculations are performed with the plane-wave density functional theory (DFT) using the CASTEP code.4 Further details of this procedure will be discussed in this contribution along with the most promising interfaces predicted by this approach.
1 C. George, et al. Chem. Mat. 28, 7304-10 (2016); M. Butala et al. Chem. Mat., 29,3070-82 (2017); K. See et al. J. Am. Chem. Soc. 136, 16368.77 (2014)
2 J. Haruyama, et al. Chem. Mater. 26, 4248-4255 (2014)
3 C. J. Pickard, R. J. Needs, PRL 97, 045504 (2006); Medeiros et al., ACS Nano, 11, 6178–6185 (2017).
4 M. D. Segall et al., J. Phys.: Condens. Matter, 14, 2717 (2002).

While advances have been made in crystal structure prediction, there have been limited attempts at predicting interfacial structure. The Minima Hopping Method (MHM) is a structure prediction method that uses short molecular dynamics escape trials to explore a potential energy landscape, and a fingerprinting approach to avoid sampling previously visited minima [1] . The MHM has been successfully employed in studying molecular clusters, bulk solids, and surfaces. In this work, we extend the MHM to search for low-energy interfacial structures at solid-solid boundaries. We use as a test system non-stoichiometric grain boundaries in strontium titanate (SrTiO₃), which have been the subject of previous structure prediction methods[2,3]. For each non-stoichiometric grain boundary considered, our MHM approach is able to predict lower energy structures than previously reported. Our method is simple to generalize to other interfacial systems and may be used with both empirical potentials as well as density functional theory to explore feature-rich and complex interfacial potential energy landscapes. We will also illustrate applications of the minima hopping method to grain boundary structures in lithium-ion cathode materials.
This work was supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of the Science, Basic Energy Science, award number DE-AC0206CH11.
[1] M. Amsler and S. Goedecker, The Journal of Chemical Physics, 133, 224104 (2010).
[2] A. L-S. Chua, N.A. Benedek, L. Chen, M.W. Finnis, and A.P. Sutton. Nature Materials, 9, 418, (2010).
[3] G. Schusteritsch and C.J. Pickard. Physical Review B. 90, 035424 (2014).

Solid state batteries (SSBs) have been developed to achieve higher energy density and safety by using inflammable solid electrolyte and lithium anode. Nevertheless, the spread of SSBs are impeded by the interface challenges of physical mismatch, chemical reaction, and space charge effect. It is therefore necessary to engineer an interface to reduce the side reactions.[1]
LATP (Li_1.3Al_0.3Ti_1.7 (PO₄)₃) solid electrolyte is widely investigated for its high ionic conductivity. However, the chemical instability of LATP against Li metal has hindered its application in solid-state batteries because of the side reaction between LATP with Li. The Ti⁴⁺ in LATP is easily reduced by Li metal into Ti³⁺, forming some interphases at the LATP/Li interface.[2] In a recent study done by Janek et al., a mixed (ionic/electronic) conducting interphase (MCI) was observed at the LAT (Ge)P/Li interface, which functioned similarly to the solid electrolyte interphase (SEI) layer formed in batteries with liquid electrolytes.[3]
To improve the stability of LATP against Li metal, intermediate layers such as polymer electrolytes can be utilized at the LATP/Li interface. The side-reactions can be partially mitigated by the chemical stability of the polymer interlayer, however, this introduces additional interfaces (LATP/polymer, Li/polymer) which may have a negative effect on cell performance.[4] Recently, Hu et al. show that introducing an ultrathin Al₂O₃ via ALD on garnet electrolyte (Li₇La₃Zr₂O₁₂) can dramatically increase the wetting and stability against Li metal after forming a Li-Al-O intermediate layer.[5] It is therefore assumed that ALD coating on LATP can be an effective method in stabilizing the LATP/Li interface. In order to understand the influence of interlayer ionic conductivity on the stability of LATP/Li interface, both Li-ion conducting Li₃PO₄ and non-conducting Al₂O₃ interlayers are studied in our design.
Herein, we applied atomic layer deposition coating on LATP surfaces to stabilize the LATP/Li interface by reducing the side-reactions based on our ALD experience [6,7]. In comparison with bare LATP, the Al₂O₃ coated LATP by atomic layer deposition exhibits a stable cycling behavior with smaller voltage hysteresis for 600 hours, as well as small resistance. More importantly, based on our advanced characterization by HRTEM-EELS, the lithium penetration into LATP bulk and Ti⁴⁺ reduction is significantly limited. The results suggest that atomic layer deposition is very effective in improving interface stability of solid-state electrolyte/ electrode.
Reference
[1] Sun, C. W, et al. Nano Energy 2017, 33, 363.
[2] Zhu, Y, et al. ACS Appl Mater Interfaces 2015, 7, 23685.
[3] Hartmann, P, et al. The Journal of Physical Chemistry C 2013, 117, 21064.
[4] Borghini, M. C, et al. Journal of Power Sources 1997, 68, 52.
[5] Han, X, et al. Nat Mater 2017, 16, 572
[6] J. Liu et al. Nanotechnology 2015,26,024001
[7] X. Meng, et al. Adv. Mater. 2012, 24,3589.

Photolithography has been extensively used to define features and create patterns for devices in the semiconductor industry. Technological advances of photolithography allow for the fabrication of more complex patterns including high aspect ratio (HAR) and tilted geometries with spatial and thickness control, potentially at submicron scales. The use of microfabrication techniques in the production of batteries has not really been considered, but it can open up new fabrication routes especially for integrated on-chip energy storage devices. One important consideration for the development of on-chip batteries is to fabricate a mechanically rigid solid electrolyte with spatial and thickness control from their photopatterning functionality.
In this study, the photopatterning of a lithium-ion conducting solid electrolyte is demonstrated. The approach is taken by modifying a negative photoresist SU-8 with LiClO₄, and the resulting lithium modified SU-8 electrolyte is evaluated as a promising gel polymer electrolyte with a room temperature ionic conductivity of 52 μS cm^-1.¹ Half-cell testing and electrochemical analysis validate its potential use in lithium-ion batteries. Electrochemical impedance spectroscopy shows an increase in the charge transfer resistance after the first cycle due to the reduction of ether linkages that can form a passivation layer at the solid electrolyte interphase (SEI).² However, the interface becomes very stable and the charge transfer resistance remains constant after 30 charge-discharge cycles. The stable cyclic voltammetry result further validates that there are no other side reactions once the initial SEI forms. The modified SU-8 electrolyte also possesses excellent mechanical integrity, is thermally stable up to 250 °C and can be photopatterned with micron-scale resolution. From this advance in materials design, there is the unique opportunity to incorporate semiconductor processing technology into battery fabrication.

References:
1. C. Choi et al., Adv. Mater., accepted (2017).
2. D. Aurbach et al., J. Electrochem. Soc., 135, 1863, (1988).

Anatase TiO₂ is a promising anode material that shows robust cyclability and high specific energy and power. TiO₂ is often studied as a model insertion electrode due to the well-characterized first-order phase transformation that occurs between the oxidized anatase TiO₂ phase and reduced Li_0.6TiO₂ phase. The relatively slow transport of Li⁺ cations through TiO₂ can impede the switching speed of these electrodes, so nanostructured electrodes have been engineered with high specific surface area and shortened diffusion path lengths. However, both the energetics and kinetics of charging can change dramatically once the electrode is structured with mesoporous nanocrystalline grains. Despite a wealth of synthetic strategies, and detailed theoretical models of lithium insertion in nanostructured electrodes, significant uncertainty remains about the microscopic behavior of lithium insertion and macroscopic ramifications of nanostructuring in real phase-transforming insertion electrodes. This talk will investigate the transformation pathways in nanocrystalline anatase films using a robust in situ optical characterization technique relying on the electrochromic visible color change of TiO₂ during lithiation.

The distribution of sizes and shapes of nanocrystal grains in a mesoporous electrode can convolute the effects of surface faceting and grain morphology on electrochemical transformations. A surfactant-mediated colloidal synthesis is used to create monodisperse ensemble electrodes of particles with controlled morphologies ranging from isotropic 10nm particles to 100nm x 15nm platelets. Potentiostatic charge titration experiments reveal that nanocrystal size and shape impact the onset potential of lithium insertion. The kinetics of this transformation are studied by observing the electrochromic color change that occurs upon lithium insertion. This electrochromic response, attributed to localized charged Ti³⁺ defects in the lattice, occurs only upon lithium insertion, and is not confounded by capacitive charge compensation. The kinetics of lithium insertion and de-insertion are observed and modeled for different initial charge states and applied overpotentials, revealing nucleation and growth kinetics that correlate with nanocrystal size and shape. A qualitative nucleation and growth model suggests that lithium insertion is geometrically constrained, consistent with current models of 1-dimensional diffusion in anatase TiO₂. These results are placed in context with existing studies of nanocrystalline insertion electrodes, and the role of grain morphology, solid-solid interfaces and ensemble behaviors are explored. A general strategy for probing electrochemical transformations through in situ spectroelectrochemical techniques is developed, and applied to nanocrystalline TiO₂ as a functional model system for solid-state ion conductors and battery electrodes.

Metal-ceramic Interfaces are common to a wide range of technological applications, including Li-ion batteries, superalloy coatings, gate-oxides in microelectronics, etc. Transport of atoms along metal-ceramic interfaces critically affects the integrity and performance in all of these applications. While grain boundaries, dislocations, and surfaces are well-studied high-diffusivity paths, little is known about diffusion along metal-ceramic interfaces. We present the results of an extensive series of first-principles calculations of point defect formation and migration energies at/near Ni/α-Al₂O₃ interfaces. The dominant point defect is shown to be Ni vacancies under most experimental conditions. The Ni vacancy formation energy is nearly a factor of two smaller than that in the bulk metal. This leads to fast vacancy diffusion along the Ni/α-Al₂O₃ interface. We generalize this result to a wide range of other metal-ceramic interfaces using data readily available for many metal-ceramic systems and predict which should lead to high-diffusivity interface paths. We then validate these predictions by performing additional first-principles calculations on the Cu/α-Al₂O₃ system. Work partly performed under the auspices of US DOE by LLNL under Contract DE-AC52-07NA27344.

Solid oxide fuel cells (SOFC) are electrochemical devices that convert chemical energy into electrical energy. SOFC consist of three main elements, i.e., the cathode, the electrolyte, and the anode. The electrolyte must be stable in reducing and oxidizing environments in addition to high ionic conductivity with low electronic conductivity. Zirconia-based ceramics as fluorite structure oxide have been the most favored electrolyte with high ion conductivity for SOFC. Even though yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) are typically used for the SOFC at high temperatures, their performance is not optimal at operating temperatures with respect to ionic conductivity and stabilization respectively. Many works on ion-diffusion have been concentrated in bulk YSZ and ScSZ systems. However, to improve the ionic conductivity and stability of electrolyte materials, it is an essential to seek more sustainable alternative systems such as heterogeneous doping and heterolayer structures. In this work, we consider three main mechanisms contributing to the enhancement of ionic diffusion at the interface: 1) the influence of cation distribution, 2) the nature of the interfaces, and 3) the influence of the crystal structure. For these considerations, different possible compositions of YSZ/ScSZ systems (within the superlattice model) have been performed using classical molecular dynamics simulations in this work. Our main research objective is to examine the oxygen ion diffusion mechanism in the vicinity of the YSZ/ScSZ interface. Additionally, several physical properties (namely, the relative energies and radial distribution function for pairs of ions) of these systems are systematically studied as a function of cation distribution and temperatures. A linear relation has been found between the mean square displacement and time, from which the ion diffusion coefficient is calculated. The respective activation energies and diffusion coefficients agree well with experimental findings.