Symposium EN21 : Next-Generation Solid-State Super Ion Conductors

Halide-substituted lithium-argyrodites, Li₆PS₅X (X=Cl, Br, I) are a promising family of lithium-ion solid electrolytes, with potential applications in all-solid-state lithium-ion batteries. Changing X from I to Cl produces a strong increase in lithium-ion conductivity, which has been attributed to increased crystallographic disorder for X and S ions across 4a and 4c sites. A previous molecular dynamics study [1] has predicted that efficient long-ranged Li-ion transport only occurs for such X/S disordered systems, with this attributed to changes in the rates of lithium-ion jumps between lattice sites. A microscopic explanation for this change in lithium-ion dynamics, however, is lacking.

To study this behaviour, we have performed a series of ab initio molecular dynamics simulations of ordered and disordered Li₆PS₅X. In contrast to previous computational studies, we have analysed the lithium-ion dynamics in terms of transitions between local potential minima (inherent structures) in lithium-ion configuration space, which allows us to resolve non-trivial lithium motion [3].

We find that in fully ordered Li₆PS₅X, the lithium ions arrange in six-coordinate octahedra around the (4a/4c) S anions. Interestingly, this is the case for X ordered over the 4a sites and ordered over the 4c sites, indicating a strong preference for separated (4a/4c) S ions at both sites to be octahedrally coordinated by Li. Lithium motion consists of concerted processes within individual octahedra; specifically, octahedral rotations and internal reorganisations via trigonal prismatic configurations; neither of which give long ranged lithium diffusion.

For anion-disordered Li₆PS₅X, this preferred octahedral coordination is disrupted. We observe a number of Li_xS coordination environments with x ≠ 6, and 6-fold coordination environments increasingly deviate from ideal octahedral symmetry. This increased Li-coordination disorder facilitates long-ranged Li-ion diffusion between LixS polyhedra. We note that for pairs of S ions in adjacent 4a / 4c sites, it is not possible for both S ions to simultaneously achieve ideal octahedral coordination. We therefore propose that the capacity for long-ranged lithium transport in anion-disordered Li₆PS₅X arises from geometric frustration of preferred octahedral lithium configurations, which produces a highly disordered network of lithium-ion polyhedra, and enables fast lithium-ion diffusion.

[1] Rao et al. Sol. Stat. Ionics 2013, 230, 72.
[2] de Klerk et al. Chem. Mater. 2016, 28, 7955.
[3] Stillinger and Weber, Science 1984, 225, 983.

Lithium super-ionic conductor materials are the key component in enabling all-solid-state Li-ion batteries. However, it has not yet been understood why only a few materials as super-ionic conductors can achieve exceptionally higher ionic conductivity than typical solids and how one can design fast ion conductors following simple principles. Using ab initio modeling, we show that fast diffusion in super-ionic conductors happens through unique concerted migration mechanism of multiple ions with low energy barrier in contrast to isolated ion hopping in typical solids. We elucidate that low energy barriers of the concerted ionic diffusion are a result of unique mobile ion configurations and strong mobile ion interactions in super-ionic conductor materials, such as Li₁₀GeP₂S₁₂, lithium garnet, NASICON, etc. Our theory provides a conceptually simple framework for guiding the design of super-ionic conductor materials. Using first principles computation, we demonstrate this strategy by designing a number of novel fast ion conducting materials, which have comparable high Li+ ionic conductivity to the known Li super-ionic conductor materials. In summary, our proposed theory and identified mechanism are universally for fast diffusion in a broad range of ionic conducting materials, and provide a general framework and a universal strategy to design solid materials with fast ionic diffusion. This study has recently been published in Nature Communications [1].

[1] Xingfeng He, Yizhou Zhu, Yifei Mo, “Origin of fast ion diffusion in super-ionic conductors”, Nat. Commun. (accepted)

Solid electrolyte materials have attracted immense interest owing to their stability and wide voltage windows needed for high energy density all-solid states batteries. From the solid electrolyte, achieving ionic conductivity higher than the liquid electrolytes is crucial for high power and high energy density of the battery. Li₇P₃S₁₁ has been considered as promising with high Li ionic conductivity comparable to that of a liquid electrolyte at room temperature. However, due to its structural complexity from low symmetry, its structural properties that enhance ionic diffusivity are still unknown.
Here, we demonstrate various new (meta-)stable Li ions interstitial sites using ab-initio molecular dynamics calculations. Moreover, from these new sites, a ground state configuration different from the previously reported ground state configuration with new Li⁺ ordering was found. To understand the Li⁺ diffusion mechanism near the anode and the cathode, defect formation energies are calculated based on the new ground structure. Diffusivities at the various defect concentrations are estimated using AIMD calculation.
These results suggest that the PS₄^3-(thiophosphate) and pyrothiophosphate(P₂S₇^4-) which construct the frame in the Li₇P₃S₁₁ form unique local environment that offer diffusion mechanism of Li ions close to the liquid-like behavior and allow ultrahigh conductivity. We suggest that our conclusion opens up the new possibility for high performance superionic conductors having the liquid-like diffusion behavior.

We compile data and physics-based machine learned models for solid Li-ion electrolyte performance to assess the state of materials discovery efforts in solid-state batteries and build new insights for future efforts. Candidate electrolyte materials must satisfy several requirements, chief among them fast ionic conductivity and robust electrochemical stability. In order to probe the interplay of these properties, we first build and validate a machine learning-based model for predicting ionic conductivity. We find this model performs 3x better than trial-and-error guessing and successfully identifies several new materials that demonstrate exceptional ionic conductivity. Then, drawing on DFT-based electrochemical stability models, we examine the predicted performance of thousands of candidate materials and quantify the likelihood of breakthrough solid electrolyte discoveries. This analysis suggests that two electrolytes are likely to be necessary in solid-state Li-ion batteries with Li metal anodes. We also find evidence to suggest the halide-based materials may be particularly promising solid electrolyte materials. Given the long time required for new materials characterization and the urgent need for improved energy storage technology, this work is an effort to extract as much information as possible from today’s limited existing data in order to provide a clear path forward for accelerating tomorrow’s efforts.

Ceramic electrolytes hold considerable promise for next-generation batteries because their elastic moduli can in principle suppress dendrite nucleation from morphological instability. Lithium-ion-conductive garnet oxides based on Li₇La₃Zr₂O₁₂ have room-temperature conductivity approaching 1 mS/cm; certain dopants extend the window of voltage stability to a range where lithium metal is stable. Through the crystal lattice, cations move with near-unit transference. Given these favorable properties, LLZO surprisingly still exhibits a ‘critical current’, above which lithium dendrites form.

Our group has put significant effort into the development of consistent transport theories to describe solid electrolytes of various types, including ionomer gels, glasses, and ceramics. We have extended multicomponent transport theory to account for excluded-volume effects, which arise from the thermodynamics of material volume, and have developed transport constitutive laws that describe space charging at the interfaces of ceramics.

This talk will summarize our recent progress toward developing a theoretical model that can be used to rationalize the critical current of LLZO in electromechanical terms. We describe a variety of new measurements that help to characterize elastic solid electrolytes, lay out the modifications of familiar transport laws that are needed to account rigorously for the energetic impact of electrolyte elasticity, and examine how electrochemical/mechanical coupling affects practical data such as impedance spectra. Interfaces are found to affect critical currents by changing the balance of bulk ohmic loss and capacitive surface charging, the latter of which leads to a buildup of stresses within the material. Our theory produces scaling laws that agree well with experiments, predicting how the critical current varies with temperature and interfacial properties.

There is great interest is developing solid state lithium electrolytes for use in an all solid-state battery to replace the flammable organic electrolyte. Previous efforts trying to understand the structure-function relationships resulting in high ionic conductivity materials have relied on ab-initio molecular dynamics (AIMD). Such simulations, however, are computationally demanding and cannot be applied to large systems containing more than a few hundred atoms in a reasonable time frame. Herein, we propose using machine learning artificial neural networks (ANN) to supply the forces and energies used during the MD simulations, and to eliminate the need of costly ab-initio force and energy evaluation methods, such as density functional theory (DFT). After carefully training a robust artificial neural network for four and five element systems, we obtain nearly identical lithium ion diffusivities for Li₁₀GeP₂S₁₂ (LGPS) when benchmarking the ANN-MD results with DFT-MD. We find that ANN-MD simulations allow the study of systems that require high number of atoms, such as finer resolution of concentrations, grain boundaries, or vacancy/dopant concentrations. To demonstrate the power of the outlined ANN-MD approach we apply it to a chlorine doped LGPS system to calculate the effect of concentrations of chlorine on the lithium diffusivity at a resolution that would be unrealistic to model with DFT-MD. These larger systems may also allow for the study of diffusivities at more moderate realistic temperatures in the future.

Metal borohydrides are a family of materials recently discovered to with high ionic conductivities, making them promising candidates as electrolytes for solid-state batteries (SSBs). However, there are no studies assessing the thermodynamic properties or discussing the suitability of metal borohydrides as electrolytes in SSBs, especially for beyond-lithium applications. We investigate the electrochemical stability, interfacial characteristics, mechanical properties, and ionic conductivities of Li, Na, Ca, and Mg borohydrides using first-principles calculations. Our results suggest that Li and Na borohydrides are unstable at high voltages. However, the corresponding decomposition products, i.e., B₁₂H₁₂^2–-containing phases, have wide electrochemical windows which protect the electrolyte, leading to large electrochemical windows as wide as 5 V. In addition, our simulations indicate that metal borohydrides are ductile, suggesting facile processing. However, their low shear moduli may result in metal dendrite formation. For Ca and Mg borohydrides, while they possess reasonably good electrochemical stability, the low cationic diffusivity may impede their practical use. Finally, the anion rotation barrier was shown to correlate with the superionic phase transition temperature, suggesting that anion mixing may be a potential approach to achieve room-temperature superionic conductivity.

Solid-state super ion conductors are key enabling elements for emerging energy storage technologies, such as sodium batteries. Here we describe the design and performance of emerging sodium battery chemistries including Na-NiCl₂, Na-I₂, and aqueous Na-based systems. Each of these technologies leverages the unique properties of the solid state electrolyte NaSICON (Na Super Ion CONductor). NaSICON provides exceptional low-to-intermediate temperature sodium ion conductivity, exhibits excellent chemical and mechanical stability, and can be produced in a range of form factors on an industrial scale. This material has facilitated the effective development of a suite of Na-based energy storage technologies that not only show promise for high capacity, high performance energy storage, but also operate at temperatures below 200 °C, utilize low cost components, and avoid the use of hazardous, explosive organic-based electrolytes. In order to expand the use of NaSICON outside nonaqueous energy storage systems, however, several “weak links” in the NaSICON phase chemistry and ceramic composition must be addressed. Here we describe our efforts to tune these characteristics during synthesis, successfully improving NaSICON’s stability in aggressive aqueous environments. Advancing the development of highly conductive, chemically stable solid state ion conductors stands to open the door to a range of innovative new sodium-based energy storage technologies needed to meet the ever-growing demand for safe, reliable, and effective electrical energy storage and delivery.

Sandia National Laboratories is a multi-mission 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.

Zn/MnO₂ alkaline batteries are currently attracting a lot of attention due to their potential as safe, low cost, high energy density rechargeable batteries for use in grid storage applications. These batteries are traditionally primary batteries at ~$18/kWh with long shelf life and have the lowest bill of materials cost, lowest manufacturing capital expenses, and an established supply chain for high volume manufacturing. However, this battery chemistry has yet to reliably realize > 5000 cycles, which equates to roughly 10-15 years of battery life which is needed for grid storage applications. One of the main failure mechanisms for these cells is the poisoning of the MnO₂ cathode material during cycling due to zinc crossover from the anode. Advanced separators that successfully stop or limit zinc crossover are crucial to increase the cycle lifetimes of these batteries. Here, a commercial ceramic sodium ion conductor which nearly eliminates zinc crossover is evaluated as a separator in these batteries. The ionic conductivity of this separator is measured to be 3.5 mS/cm while its thickness is 1.0 mm resulting in large total membrane resistance of 26 Ω. An attempt was made to reduce this resistance by decreasing the thickness of this membrane to 0.5 mm. The effect of this reduction in thickness is demonstrated in reduced polarization of the discharge curves resulting in higher discharge potentials. To evaluate the zinc blocking performance of the membrane on battery cycle life, cells cycled under limited depth of discharge (DOD) utilizing a 30 wt% NaOH electrolyte were used and compared to traditional Celgard and cellophane separators. For a 5% DOD at a C/5 rate, the cycle lifetime was increased by over 20% using the thinner ceramic separator when compared to traditional separators. SEM/EDS and XRD characterization showed limited amounts of zinc species on the cathode utilizing the ceramic separator. Further improvement in cycle lifetimes should occur with even thinner ceramic sodium ion conducting materials.

This work was supported by Sandia National Laboratories and by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability. We thank Dr. Imre Gyuk, Manager of the Energy Storage Program for continued support. Sandia National Laboratories is a multi-mission 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.

Sulfide-based sodium solid electrolytes are expected to improve next-generation energy storage technology on the basis of energy density, safety, cost, etc. Yet inherent limitations, especially the water and air stability, represent grand challenges for these materials to be processed into thin films for real batteries in electric vehicles and portable electronics [1-4]. Therefore, the exploration of new Na-ion superionics is of special importance to develop new robust Na-batteries and has attracted intense attention in recent years. Here, we show that Na₄P₂S₆ and Na₃SbS₄ hold excellent air and water tolerance and desirable ionic conductivities as solid electrolytes [5-6]. Soft-chemistry approaches were used to produce these new solid electrolytes, and their electrolyte properties were investigated using various electrochemical methods. Their lattice structures and ion mobility were studied by combining first-principles analysis with advanced electron microscopy imaging, and these properties were further linked to their macroscopic performance. We also show that both materials hold promising electrochemical properties for batteries that employ sodium metal anodes. Our results provide valuable insights into the design of superionic materials with high stability and tolerance for next-generation solid-state batteries. Their potential applications in other electrochemical devices will also be discussed.

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.

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4. H. Wang, Y. Chen, Z.D. Hood, G. Sahu, A.S. Pandian, J.K. Keum, K. An, C. Liang. Angewandte Chemie International Edition 55, 30 (2016): 8551-8555.
5. Hood et. al., In preparation.
6. Hood et. al., In preparation.

The contribution addresses advantages and disadvantages of solid electrolytes for batteries [1]. Much of the enthusiasm for solid electrolytes is due to the prejudice that they are thermodynamically more stable than liquids. With very few exceptions, however, this is not the case and passivation layers have to be invoked. In fact, in particular the superionic conductors experience an internal conflict, as the same reason that leads to the superionic property is typically detrimental to thermodynamic stability. This is discussed especially for the recently found superconductor in the Li-Sn-S system [2]. The most severe difference between solids and liquids naturally lies in the mechanical behavior. The pros (form stability and better suppression of dendrite growth) have to be contrasted with the major disadvantage, in particular the difficulty of forming well-behaved contacts.
Two points which reveal characteristic differences in the transport behavior are treated in greater detail: Single ion conductivity in superionic conductors and interfacial effects. Two alternative approaches are discussed that have the potential to combine advantages of the solid matter with advantages of the liquid state [3,4].
Finally, the applicability of solid electrolytes is considered in conjunction with ionic and electronic conductivities of the electrode phases under concern [5].

[1] B. Lotsch and J. Maier, J. Electroceramics, published online 2017, DOI 10.1007/s10832-017-0091-0.
[2] T. Holzmann, L. M. Schoop, M. N. Ali, I. Moudrakovski, G. Gregori, J. Maier, R. J. Cava, B. V. Lotsch, Energy Environ. Sci., 2016, 9, 2578-2585.
[3] C. Pfaffenhuber, M. Göbel, J. Popovic, and J. Maier, Phys. Chem. Chem. Phys. 2013, 15, 18318-18335.
[4] K.-D. Kreuer, A. Wohlfarth, C. C. de Araujo, A. Fuchs, and J. Maier, ChemPhysChem, 2011, 12 2558-2560.
[5] C. Zhu, R. Usiskin, Y. Yu, and J. Maier, Science, 2017, accepted.

Electrochemical energy storage devices are clean and efficient, but their current cost and performance limit their use in many transportation and stationary applications. Lithium-ion batteries are one of the leading candidates for these large applications, however their current use of liquid electrolytes negatively effects their lifetime and safety. Furthermore, the liquid electrolyte's potential stability window, thermal stability, and volatility are of particular concern in larger applications. Solid-state electrolytes are investigated as one of the best solutions to overcome these challenges. However, the ionic conductivity of many solid electrolytes is still much lower than that of the liquid counterparts. In this talk, a new design approach based on lattice dynamics properties of several families of Li-ion conductors is proposed, we show that a correlation exists between the enthalpy of migration and the average vibrational frequency of Lithium in these structures. Moreover, we will also show that the stability of Li-ions conductor in contact with Li metal can also be linked to this concept by showing that there is a correlation between the oxidative voltage and the average vibrational frequency of anions in these structures.

Using elemental lithium (Li) as anodes will be highly advantageous in electrochemical energy storage devices in terms of capacity and cell voltage. Li has a high theoretical capacity (3860 mAhg^-1) and the lowest potential among all elements (-3.040 V versus a standard hydrogen electrode). However, the main challenge preventing this beneficial anode selection is formation of un-wanted Li-dendrites during the charge and discharge cycles of a battery. The dendrite formation can lead to performance failures such as short circuiting. The solution to this challenge is to design a solid-state electrolyte that is mechanically robust enough to block dendrite propagation while simultaneously allowing fast conduction of ions. Using super ion conductor solid state electrolytes will enable fast charge-discharging which is a crucial goal in developing the next generation of high-performance batteries. However, high-rate cycling will be hindered if the large amount of inevitable heat generated by joule heating is not accounted for in cell design. Overheating batteries can cause catastrophic safety issues such as ignition. Therefore, information on the thermal behavior of each battery component over the operating temperature range is necessary. Herein, we will present thermal conductivity measurements of Li_1+x+yY_xZr_2−x(PO₄)₃ (x = 0.15, -0.3 ≤ y ≤ 0.4) over a wide temperature range from ~ 20 to 973 K. Stabilization of the rhombohedral phase is obtained by adding 15% Y³⁺. The charge imbalance caused by this substitution is taken into account by adding equivalent charges from excess Li⁺. The thermal diffusivity measurements are conducted on spark plasma sintered pellets with a diameter of 12.7 mm via a NETZSCH laser flash LFA-457 from room temperature to 973 K. The obtained results indicate that thermal discursivities of the samples range from 0.6 to 0.3 mm² s^-1 as the temperature changes from ~300 to 973 K. The density is measured by the Archimedes method. The heat capacity measurements are conducted on a NETZSCH DSC 404 C. The low temperature measurements of thermal conductivity (20 K- room temperature) are carried out on polished bars using a custom designed apparatus according to [Pope and Tritt et al., Cryogenics 41 (2001) 725–731].

Ionic conductivities of solid state fast ion conductors are typically reported as three values for a given temperature: the bulk conductivity, grain boundary conductivity, and the total ionic conductivity, as deduced from electrochemical impedance spectroscopy measurements and fits. In this presentation, we will discuss two factors that greatly affect total ionic conductivity, that can be difficult to discern from conventional impedance spectroscopy measurements: 1) Nano-scale grain size scaling and 2) Porosity. The systems with which these results will be discussed include thin films of Na_1+xZr₂Si_xP_3-xO₁₂ (sodium super ionic conductor, NaSICon) and LiZr₂P₃O₁₂ (lithium super ionic conductor, LiSiCon) and Li₅La₃Ta₂O₁₂ (LLTO) ceramics. We will show that when film thicknesses are sub-micron, discerning individual EIS features associated with grain and grain boundary effects is difficult, if not impossible. Further, as grain sizes scale into the nanoscale, unexpected trends in the composition dependence of ionic conductivity in NaSICon will be revealed. Typically, as the silicon content increases toward x=0.2, the ionic conductivity increases. In chemically-derived thin films, however, the competition of grain size scaling and concomitant increase in grain boundary volume and the increased ionic conductivity with silicon content results in a compositionally-dependent peak in room temperature ionic conductivity at a composition of x=0.25. This study demonstrates the significant role that grain boundaries can have on overall ionic conduction properties. Finally, we will show how phase control is vital in the processing of LLTO ceramics and that through proper batching and processing controls, ceramics with densities greater than 98% are possible without the aid of sintering pressure. The resulting phase-pure and dense ceramics possess total room temperature ionic conductivities of 2x10^-5 S/cm. Further, we will show that the high activation energy of LLTO makes it competitive with the zirconium garnet counterpart at elevated temperatures.

The management of waste products generated during the recycling of spent nuclear fuel remains an important consideration in the development of processes such as pyroprocessing. We describe here the application of ion-selective ceramics as electrochemical filters for the separation of fission product waste from molten salt electrolytes used in spent nuclear fuel recycling. As part of a larger emphasis on separation and recycling of spent nuclear fuel, pyroprocessing allows for the electrochemical separation of recyclable actinides from waste fission products (FPs). In the course of this process, high-heat generating contaminants such as Cs⁺ or Sr²⁺ remain dissolved in the eutectic chloride molten salt electrolytes used for actinide separation. Removal of such short-lived, high heat-generating FPs is an important objective in the consolidation of radioactive waste and is key to recycling the LiCl-KCl molten salts. Here, we explore an electrochemical approach designed around the use of ion-conducting ceramics to selectively separate Cs⁺ and Sr²⁺ from LiCl-KCl eutectic molten salts. We will describe a ceramic-mediated scheme for ion filtration that relies on the selective transport of ions through designed solid state ion conductors, based on ionic size and ionic charge. Having demonstrated the feasibility of this approach in high temperature molten LiCl-KCl-based salts, we discuss the stability and performance of selective garnet-based and NaSICON-based ceramic ion conductors in electrochemically isolating Cs⁺ from a mixed salt melt. In addition to potentially improving the recycling of spent nuclear fuels, this ceramic-mediated approach to chemical separations may impact additional technologies ranging from energy storage to advanced chemical purification.

Sandia National Laboratories is a multi-mission 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.

A large fraction of the natural gas reserves is considered stranded. Because transportation of natural gas is impractical and expensive, interest has increased in the field of gas-to-liquids technology. Two routes are considered in this work: methane dehydroaromatization (MDA) and steam methane reforming (SMR). Methane is converted into benzene and hydrogen in the MDA reaction (equation 1) and into CO, CO₂ and H₂ in the SMR reaction (equation 2) together with the water gas shift reaction (equation 3).
6 CH₄ ↔ C₆H₆ + 9 H₂ (1)
CH₄ + H₂O ↔ CO + 3 H₂ (2)
CO + H₂O ↔ CO₂ + H₂ (3)
The MDA conversion rate is limited to 7-8% at 700°C [1] using molybdenum supported on H-ZSM-5 zeolite. The SMR methane conversion rate increases with the amount of steam. For a steam to carbon ratio of 3, the methane conversion rate varies within 20-45% in the temperature range 500 to 700 °C with Ru or Ni based catalysts and various flow rates [2]. These conversion rates can be increased using a membrane reactor that combines chemical conversion process with a hydrogen separation membrane that removes the hydrogen from the products and shifts the equilibrium to higher yields [3-4].
High temperature proton conductors, such as yttrium doped barium zirconate (BaZr_1-xY_xO_3-d, BZY), were discovered in 1981 [5] and have been widely investigated since. A hydrogen flux of about 5 mL^.min^-1.cm^-2 can be achieved through a 25 micron thick BZY membrane at a current density of 0.7 A^.cm^-2 at 700°C. Details about the membrane preparation using the unique and cost-effective solid-state reactive sintering technique will be presented. Shortly, dense and defect free BZY membranes can be fabricated on a BZY/Ni support with only one high-temperature sintering step. Tailored electrodes were designed for the MDA and SMR applications. Very encouraging results using membrane reactors have been obtained so far: enhanced conversion rates and longer catalyst lifetime. Nevertheless some limitations, such as upscaling and decrease of the total area specific resistance, still need to be addressed for commercialized applications.

References
[1] L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett., 21 (1993) 35-41.
[2] P. Hacarlioglu, Y. Gu, S. T. Oyama, Journal of Natural Gas Chemistry 15 (2006) 73-81.
[3] S.H. Morejudo, R. Zanon, S. Escolastico, I. Yuste-Tirados, H. Malerød-Fjeld, P.K. Vestre, W.G. Coors, A. Martínez, T. Norby, J. M. Serra, C. Kjølseth, Science 353 (2016) 563-566.
[4] V. Kyriakou, I. Garagounis, A. Vourros, E. Vasileiou, A. Manerbino, W.G. Coors, M. Stoukides, Applied Catalysis B Environmental 186 (2015) DOI: 10.1016/j.apcatb.2015.12.039.
[5] H. Iwahara, T. Eska, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359-363.

Stationary energy storage systems (ESS) are considered a key component for Grid-Modernization, as they buffer the intermittent generation of renewable energy resources while improving the flexibility and reliability of the electric power grid. Rechargeable sodium (Na) based batteries provide great promise for ESS applications by utilizing earth abundant sodium resources which are much less susceptible to supply chain risks than lithium based systems. While several types of sodium batteries have been reported over recent years, sodium-metal halide (Na-MH) batteries are a compelling candidate for ESS application due to their ability to operate at lower temperatures and their inherent safety over traditional sodium-sulfur technologies. Research at PNNL has been primary focused on developing advanced Na-MH battery technologies that require operating temperatures less than 200°C (from > 300°C for traditional systems) to enable low-cost manufacturing process to be employed. Critical to this objective is the development of high performance, low-temperature cathode material. It was found that lowering operating temperature helps suppress degradation and retains higher energy density while providing opportunities to deploy conventional high temperature polymers as sealing materials. This presentation will detail the materials and cell assembly advancements developed to enable lower temperature Na-MH battery technology.

Oxygen-conducting solid oxide fuel cells (SOFCs) have attracted a lot of researchers as an alternative power source because they have high energy conversion efficiency and no use of precious metal catalysts. The high operating temperatures (800-1000°C), however, hinder durability and low fabrication costs of the SOFCs. Proton-conducting SOFCs is an alternative solution due to their low operating temperatures (400-600°C) [1].
BaCe_1-x-yZr_xY_yO₃ is a combination BaCeO₃ and BaZrO₃ with Y as a dopant. BaCeO₃ shows high conductivity but low stability, while BaZrO₃ shows good stability but low grain boundary conductivity [2]. BaCe_1-x-yZr_xY_yO₃ can overcome the weak points of the two oxides. The material, however, shows many different configurations of Ce, Zr, and Y in the B site of the ABO₃ perovskite structure. We employed a genetic algorithm and lattices dynamics to find an energetically favorable solid solution structure that can represent the BaCe_1-x-yZr_xY_yO₃ solid solution. The electronic information of the solid solution was obtained by density functional theory.

[1] W. Sun, M. Liu and W. Liu, Adv. Energy Mater., 2013, 3, 1041.
[2] Lei Yang, Shizhong Wang, Kevin Blinn, Mingfei Liu, Ze Liu, Zhe Cheng, Meilin Liu, Science, 2009, 326, 9127.

A decade ago we reported (Alicia B. Brune; Robert I. Mangham; William T. Petuskey in Journal of Materials Science 43(2), 621 (2008)) on the preparation and characterizations of two hydrogenated pyrochlores (abbreviated as HPN and HPNT) obtained from the corresponding oxides Pb_1.5Nb₂O_6.5 (PN) and Pb₂Nb_1.33Ti_0.67O_6.67 (PNT), by ion exchange in acidic aqueous media. Measured electrical conductivities were attributed to dominant proton transport. Now, complementary data are presented supporting such interpretation and providing insights on transport mechanisms. Information on hydroxyl groups, proton dynamics, and proton locations was acquired by infrared and Raman spectroscopy, solid state ¹H NMR, and neutron diffraction, respectively. Results suggest that proton transport occurs through tunnels within the bulk of the pyrochlore structure, in addition to surface transport at the lower temperatures. Tunnel dimensions are shown to formally allow the transport of larger ions, with the role of lattice dynamics being unknown. Findings are compared to literature data on pyrochlores, particularly those obtained by ion exchange. Applications of HPN and HPNT in ion transport membranes face challenges regarding densification and structural stabilization.

Lithium ion secondary batteries have been widely used with high capacity and energy density. However, safety problems such as explosion of organic electrolytes have been raised, all solid lithium secondary batteries which do not use organic electrolytes have been attracting attention as next generation batteries. Garnet-type solid electrolyte has good advantages with stable in the atmosphere and electrochemical potential window. In this study, gallium-doped LLZO material, which are cubic single phase and nano-grade size, was synthesized by Taylor reactor process. As a result, the ionic conductivity of the Ga-LLZO material was about 1.2x10^-3 S/cm at room temperature, indicating a wide electrochemical window of 0~6.0 V. Furthermore, all solid lithium batteries were fabricated using Ga-LLZO material and the electrochemical properties were investigated in more detail.

Ion conducting ceramic electrolytes continue to emerge as key components in the advance of electrical energy storage technologies, such as sodium-based batteries. These solid state electrolytes are challenged with providing not only high ionic conductivity, but also serving as a robust physical, electrical, and selective ionic barrier between anolytes and catholytes in these systems. Meeting these requirements means optimizing phase chemistry for selective, facile ion transport while maintaining critical chemical, thermal, and mechanical stability against molten, organic, and potentially aqueous media. Here, we specifically explore a family of NaSICON (Na Super Ion CONducting) separators that may exhibit ionic conductivities greater than 1 mS/cm at room temperature and show excellent chemical stability. We characterize the multi-phasic character of NaSICON, and we discuss how variations in NaSICON microstructure influence both ceramic integrity and ionic conductivity. Meanwhile, strategic changes in NaSICON composition are shown to impact not only ion transport, but also the phase distribution and chemical stability of the ceramic. Critically, these studies reveal strongly interdependent relationships between ceramic composition, structure, and ultimate performance in these complex, functional materials. Learning to manipulate such fundamental materials relationships is key to optimizing stable NaSICON application in advanced ion transport technologies.

Sandia National Laboratories is a multi-mission 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.

High temperature fuel cells (SOFC) operate at temperatures from 500°C to 1000°C. The electrolytes of these electrochemcial energy converters are thin ceramic metal oxide membranes (zirconia or cerium oxides) membranes in which oxygen ions and vacancies are the electric charge carriers. Their mobility is thermally activated which mandates the high temperature. Under humid atmosphere, numerous metal oxides show proton conductivity at a temperature considerably lower than for oxygen ion conductivity. Yet, fuel cell devleopers hope for substantial improvement of the conductivity to make such electrolyte membranes before they are fit for deployment in SOFC.
We have recently shown that the protons in such ceramic membranes couple to the phonons generated by thermal excitation in a way that they form new quasi-particles which follow characteristically the Holstein polaron concept <!--[endif]---->^{1, 2}. With a suite of analytical techniques we could identify which particular phonon modes propell the protons through the electrolyte membrane during SOFC operation. As these phonon modes scale characteristically with the lattice constant of the proton conductor, we could show that external lattice strain considerable affects the proton conductivity. We have demonstrated this expermentally for compressive strain. Extrapolation of the activation energy to tensile strain in ultrathin membranes suggests that epitxial films with the desirable orientation of lattice planes could further lower the activation energy for proton transport not too distant from the ambient temperature <!--[endif]---->^{3, 4}.<!--![endif]----><!--![endif]---->
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1. A. Braun and Q. Chen: Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors. Nature Communications 8, 15830 (2017).
2. A.L. Samgin: Lattice-assisted proton motion in perovskite oxides. Solid State Ionics 136, 291 (2000).
3. Q. Chen, T.-W. Huang, M. Baldini, A. Hushur, V. Pomjakushin, S. Clark, W.L. Mao, M.H. Manghnani, A. Braun and T. Graule: Effect of Compressive Strain on the Raman Modes of the Dry and Hydrated BaCe0.8Y0.2O3Proton Conductor. The Journal of Physical Chemistry C 115, 24021 (2011).
4. Q.L. Chen, A. Braun, A. Ovalle, C.D. Savaniu, T. Graule and N. Bagdassarov: Hydrostatic pressure decreases the proton mobility in the hydrated BaZr0.9Y0.1O3 proton conductor. Applied Physics Letters 97, 041902 (2010).

A key challenge for lithium metal batteries is the development of a solid electrolyte with a combination of high ionic conductivity and high modulus. The development of composite electrolytes, where multiple materials provide multiple functionalities, provides a promising approach to meet all needs. Here, we developed NASICON-based composite electrolytes though a cold sintering process that utilizes a small amount of solvent and uniaxial pressure to sinter ceramics at low temperatures (<150 ^oC). By tuning the composition of LATP-based electrolytes, conductivities at room temperature in excess of 10^-4 S/cm were achieved using cold sintering; these conductivities are comparable to what is obtained from samples sintered through conventional means (T > 800 ^oC). SEM images of fracture surfaces of composite electrolytes suggest strong cohesion between grains that is almost identical with that of neat LATP. Cycling of lithium half cells shows no evidence of dendrite growth, where current densities of 0.1 mA/cm² show good cycling stability over 700 hr.

Abstract
Rechargeable electrochemical storage systems are increasing in importance for many fields of daily life. High-capacity energy storage devices, such as lithium-ion (Li-ion) batteries are used in a growing number of applications, including portable electronics, medical, transportation, frequency regulation of grid-connected large energy storage and uninterruptible power supply (UPS). In each of these applications, the charge/discharge time and capacity of energy storage devices (Wh/L and Wh/kg) are key performance parameters. In addition, the size, weight, flexibility, and/or cost ($/kWh) of such energy storage devices are also key parameters. Further, safety is necessary for all applications and safety is managed both at the cell level, pack level and at the system level. One way to improve the cell safety is to incorporate high ion conducting non-flammable electrolyte such as solid electrolytes.
New inorganic and organic polymeric materials have been discovered recently showing promising ionic conductivities in the vicinity of liquid electrolytes. One of the key challenge is to integrate these new materials in a cost effective way replacing liquid/gel electrolytes. High volume manufacturing issue needs to be addressed to successful commercialization of non-flammable high ion-conducting materials. In this talk some of the manufacturing challenges will be addressed.

The garnet-type Li⁺ ion conductor Li₇La₃Zr₂O₁₂ (LLZO) is a promising candidate as a solid electrolyte for all-solid-state Li-ion batteries. Significant progress towards understanding the structure and properties of LLZO, conventionally synthesized using solid state reaction, has already been made in the last decade. In this talk, I will summarize our recent efforts on the synthesis of nanostructured LLZO using electrospinning, cellulose templating, low-temperature (< 900 ^oC) sol-gel, and molten salt-based methods as well as our current understanding of the phase evolution of LLZO nanostructures during the formation and calcination processes. The properties of nano-sized LLZO when used as ceramic fillers within solid composite polymer electrolytes and as ceramic pellets will be discussed and compared to conventional LLZO prepared by solid state reaction.

Recently, solid-state electrolytes have been a highly active area of research for future Li-ion batteries due to the potential for drastically improved energy density and safety. Among these materials, garnet structured lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) shows particular promise owing to the high conductivity (0.1-1 mS cm^-1) of its cubic polymorph, inertness, and electrochemical stability against metallic lithium. Herein we report the facile synthesis of phase-pure cubic LLZO using molten salt synthesis in a eutectic mixture of LiCl:KCl at 900°C. Fine powders of Al- and Ga-doped LLZO were obtained with primary particle sizes ranging from 0.3-3 µm. Depending on the consolidation conditions, pellets with up to 86% relative density could be obtained, with conductivity values ranging from 0.20-0.31 mS cm^-1. It is also observed that the effect of proton exchange/hydration has a profoundly deleterious effect on sintering and densification, and that this effect can be mitigated by the simple addition of LiOH before sintering to reverse protonation and aid densification. Qualitative discussion of mechanisms of formation are discussed, in addition to implications for scalable processing of LLZO electrolytes.

Lithium-conducting solid-state electrolytes (SSEs) are a promising platform for achieving the high energy density, long-lasting, and safe rechargeable batteries needed for a wide range of applications. By eliminating the need for flammable and unstable liquid electrolytes, SSEs dramatically reduce the risk of fire while enabling high voltage and energy density chemistries including Li metal. In particular, the ceramic oxide material cubic garnet Li₇La₃Zr₂O₁₂ (LLZO) is a promising option due to its stability and high ionic conductivity. Two major challenges to commercialization are manufacturing of thin layers and creating stable, low-impedance, interfaces with both anode and cathode materials. Atomic Layer Deposition (ALD) has recently been demonstrated as a powerful method for depositing both solid electrolytes and interfacial layers to improve stability and performance at electrode-electrolyte interfaces in battery systems. The self-limiting reactions afford the ability to conformally coat arbitrary geometries for 3D batteries, powders, and porous cathodes, an important advantage over the current state-of-the-art LiPON solid electrolyte and previous approaches for thin-film LLZO.
In this study¹, we present the first reported ALD of the pentenary oxide Al-doped LLZO. Constituent binary processes are successfully combined in a thermal ALD process at 225°C to deposit high purity, dense amorphous LLZO films. The cycle-by-cycle growth of the multi-component film is quantified by in situ quartz crystal microbalance (QCM) measurements. We demonstrate the ability to tune composition within the amorphous as-deposited film, anneal to achieve the desired cubic garnet phase, and characterize the annealed films via in situ synchrotron XRD during annealing. The ability to conformally coat high aspect ratio structures of arbitrary composition with ultrathin layers of dense, amorphous LLZO is shown, demonstrating the potential for integration into 3D battery architectures, including porous electrode structures. Approaches to overcome Li loss and phase segregation during annealing are demonstrated and discussed.
The film exhibits preferential orientation when annealed on single crystal substrates, and the cubic-tetragonal phase transition was observed at ~500°C, significantly lower temperature than reported for bulk synthesis methods. By tuning the composition and annealing conditions, the resulting film can be tuned from pure-phase La₂Zr₂O₇ pyrochlore, a high temperature thermal barrier material with several commercial applications, to high purity tetragonal LLZO or high phase-purity cubic LLZO, the superionic solid electrolyte material.

Recently we have made considerable advances in the direct electrodeposition at modest temperatures of high performance nearly 100% solid lithiated transition metal oxide cathodes. The capacities are near-theoretical, and the crystallinities and electrochemical potential of the oxides are comparable to powders synthesized at much higher temperatures (700 ~ 1000°C). The electrodeposited materials have good electrical conductivities, and appear to be textured in such a fashion to enable good ionic conductivity. In a more speculative direction, we are exploring the electrodeposition of solid electrolytes. An electrodeposited solid electrolyte would enable new form factors of batteries and the use of new classes of electrode materials.

For over 30,000 years, the general practice of sintering ceramics has involved a high temperature thermal treatment to drive the transport processes to densify the particles and minimize the surface energy of the material. Typical sintering temperatures consider 0.6 to 0.8 of the melting temperature (T_m) for many oxides; this means we sinter around 800 ^oC to 1200 ^oC for 2 to 10 hours. Here we introduce a broad body of systems that utilize a transient aqueous based liquid phase (1 to 10 wt%) that sinters under a uniaxial pressure, while being heated from room temperature to 250 ^OC, over a time period of 10 to 60 minutes.
Given the massive drop in sintering temperature of the ceramic, this offers many new opportunities in material design, especially in composites. We will show three different types of polymer ceramic composites with high percentages of ceramic, 100% to 60%, with the thermoplastic polymers for dielectric applications, ionic electrolytes, and semiconducting composites. We will also show data with CSP with multilayer ceramics and printable electronics.

A central goal in current battery research is to increase the safety and energy density of Li-ion batteries. Electrolytes nowadays typically consist of lithium salts dissolved in organic solvents. Solid electrolytes could facilitate safer batteries with higher capacities, as they are compatible with Li metal anodes, prevent Li dendrite formation and sulphur shuttling, and eliminate risks associated with flammable organic solvents. Less than 10 years ago, LiBH₄ was proposed as a solid state electrolyte. It showed a high ionic conductivity, but only at elevated temperatures. Since then strategies have been developed to extend the high ionic conductivity of LiBH₄ down to room temperature, and other light metal hydrides have been explored as solid electrolytes [1].
Using LiBH₄ as an example we will discuss how the properties of solid electrolytes can be modified by forming nanocomposites with metal oxides, leading to an enhancement of the room temperature ionic conductivity of more than three orders of magnitude[2]. DSC measurements combined with solid state NMR allow to identify how the nanoconfinement and presence of interfaces modify the phase stability and the Li⁺ mobility [3]. Systematic studies show how the ionic conductivity can be optimized by tuning the nanostructure and interfaces in these nanocomposites. Finally, promising results have been obtained in using these materials as solid-state electrolytes in next generation all-solid state lithium-sulphur batteries. [4]

[1] de Jongh et al. J. Appl. Phys. A (2016), 122:251.
[2] Blanchard et al., Adv. Funct. Mater. 25 (2015), 182.
[3] Verkuijlen et al., J. Phys. Chem. C 116 (2012) 22169.
[4] Blanchard et al, J. Electrochem. Soc. (2016).

The development of solid Li⁺ conducting electrolytes for all-solid-state batteries has attracted a great deal of research interest. Polymer-based electrolytes are attractive because they are not brittle and can more easily form interfaces to electrodes compared to inorganic electrolytes, but they also display poor mechanical properties and low ionic conductivities at room temperature. The use of composite polymer electrolytes (CPEs) comprising a polymer electrolyte embedded with ceramic fillers has been an attractive strategy for enhancing the mechanical stability and ionic conductivity of the polymer. The ceramic filler can increase the ionic transport of Li⁺ in the CPE by several orders of magnitude, for example by decreasing the crystallinity of the conducting polymer and creating space charge regions that can enhance the Li⁺ diffusion.

Recently, the garnet-type Li⁺ ion conductor Li₇La₃Zr₂O₁₂ (LLZO) has attracted substantial interest for its potential as ceramic filler in CPEs. Studies from our lab on the use of LLZO nanowires as ceramic fillers in polyacronitrile (PAN)-based polymer electrolytes showed that very small wt% of LLZO was needed to improve the ionic conductivity by 3 orders of magnitude to 1.31 x 10^-4 S/cm. It was demonstrated that the preferred Li diffusion pathway was through the interface between the LLZO and polymer, motivating the use of nanostructured LLZO fillers.
Here, LLZO nanoparticles were synthesized using a novel molten salt reaction. This method yields size-homogenous and non-agglomerated nanoparticles, which are ideal for use as ceramic fillers in CPEs. The synthesis and structural characterization of the nanoparticles will be presented. The effect of LLZO wt%, amount of Li salt, and CPE processing conditions on the ionic conductivity of the solid electrolyte composite films will also be discussed.

MgO stabilized zirconia has been received attention due to excellent ionic conductivity, fracture toughness, and thermal shock resistance. Zirconia (ZrO₂) has three different phase with the temperature such as monoclinic (RT to 1170 °C), tetragonal (1170 °C to 2370 °C), and cubic fluorite structure (above 2370 °C). The cubic in ZrO₂ is the most useful phase for various applications, and controlling the ZrO₂ phases is a main issue in research field. The cubic phase can be stabilized in RT by doping the lower-valence cations (Mg²⁺) than Zr⁴⁺ because of creation an oxygen vacancy. The stabilized cubic phase could be monoclinic and tetragonal by long-term use in a high temperature above 1500 °C.
In this study, the phase stabilization of MgO stabilized zirconia with Mn doping was studied in terms of the valence state of Mn and the local atomic structure of zirconia. Content of cubic phase was observed by morphology and crystallographic analysis in MgO partially stabilized zirconia (MgPSZ) with MnO₂ addition (5 and 10 mol%) and MgO fully stabilized zirconia (MSZ) with 5 mol% MnO₂ addition. Although 5 mol% Mn-doped MgPSZ exhibited 66.11% cubic phase fraction, 10 mol% Mn-doped MgPSZ and 5 mol% Mn-doped MSZ had 95.63 and 98.72 % cubic phase fraction, respectively. The conductivity of Mn-doped MgPSZ and MSZ were measured by DC 4-point probe method, and it increased with MSZ, 5 mol% Mn-doped MgPSZ, 5 mol% Mn-doped MSZ, and 10 mol% Mn-doped MgPSZ in order.
The local atomic structure of zirconia was identified through extended X-ray absorption fine structure around Zr K-edge and the coordination of Zr-O bonding was decrease by the oxygen vacancy generation with Mn doping. Valence state of doped Mn ion was reduced from Mn⁴⁺ to Mn²⁺, and it led to generation of oxygen vacancy due to charge compensation, which was identified by X-ray photoelectron spectroscopy. In addition, it indicates that oxygen vacancy formation by substitution of Mn²⁺ in the Zr⁴⁺ site in MgPSZ and MSZ increased cubic phase fraction. However, 10 mol% Mn doping caused electrical conductivity and it is the reason why 10 mol% Mn-doped MgPSZ had higher conductivity than that of 5 mol% Mn-doped MSZ in spite of cubic phase fraction. It indicated that the conductivity of 5 mol% Mn-doped MSZ increased only the ion conduction without electronic conduction.

Magnesium batteries are a class of multivalent-ion battery systems that offer the promise of high volumetric energy densities in comparison with the lithium-based counterparts. Unlike lithium metal, magnesium is relatively inexpensive and considered extremely safe because of its dendrite-free deposition. Therefore, magnesium is considered as an ideal candidate for the construction of safer next-generation battery systems. However, intercalation of magnesium-ions (Mg²⁺) into host crystal lattices has proven to be extremely challenging. Consequently, there are only a handful of cathode materials that can reversibly intercalate Mg²⁺, and their specific capacities are considerably low. Moreover, the limited availability of Mg electrolytes that have a wide voltage window, makes it difficult to screen for Mg²⁺ intercalation (cathode) materials. Unfortunately, the few electrolytes that do support high reversibility and possess anodic stability, make use of flammable organometallic reagents and solvents, thereby rendering the battery unsafe.

Polymers like PEO, PVdF have been known for a long time to be a robust, and highly conductive medium for solid-state Li-ion batteries.¹ Their conductivity could be further enhanced by using ceramic fillers such as SiO₂, Al₂O₃ etc. Following their successful integration into Li-battery systems, there have been several reports on such Mg-ion conducting solid polymer electrolytes having a high conductivity.^2–4 However, studies to determine their Mg-deposition overpotential and cycling efficiency have been scarce.

Here we propose a safe, composite polymer electrolyte that supports high reversibility with Mg-metal anode. Its Mg-ion transport was confirmed by performing DC polarization tests, and by measuring the cationic transference number. Raman spectra of the polymer composites were obtained at different stages of the synthesis to understand the Mg chemical environment. Its compatibility with Mg-metal anode and the reversibility of the Mg-metal battery were tested by cycling a symmetric Mg | Mg two-electrode cell. When the Mg-battery was cycled at moderate current densities at room temperature, a low overpotential of < 0.2 V was observed for up to 100 cycles. We believe that this composite polymer electrolyte would be a significant step forward towards the development of safe rechargeable solid-state Mg-metal batteries.

References:
1. Xue, Z., He, D. & Xie, X. Poly(ethylene oxide)-based electrolytes for lithium-ion batteries. J. Mater. Chem. A 3, 19218–19253 (2015).
2. Pandey, G. P., Agrawal, R. C. & Hashmi, S. A. Magnesium ion-conducting gel polymer electrolytes dispersed with nanosized magnesium oxide. J. Power Sources 190, 563–572 (2009).
3. Shao, Y. et al. Nanocomposite polymer electrolyte for rechargeable magnesium batteries. Nano Energy 12, 750–759 (2015).
4. Song, S. et al. Communication—A Composite Polymer Electrolyte for Safer Mg Batteries. J. Electrochem. Soc. 164, A741–A743 (2017).

Development of high performance and reliable energy storage devices that are lightweight and flexible with improved safety are imperative for next generation energy storage technologies, including lithium (Li)-ion batteries, supercapacitors, fuel cells, and many others. High-performance solid state electrolytes have the potential to address various challenges in these electrical energy storage devices, especially if they can meet requirements of (i) high ionic conductivity; (ii) sufficient mechanical strength or modulus to suppress dendrites formation; (iii) high transference number; and (iv) wide electrochemical stability window. However, dry solid-polymer electrolytes lack sufficient ion conductivity to meet cell power requirements, being typically around 10^-5-10^-9 S/cm. On the other hand, while gel polymer electrolytes can provide adequate ion conductivity, their weak mechanical properties diminish advantage over traditional liquid electrolytes, and limit their utilization. This project focuses on cultivating the fundamental understanding for the development of novel gel polymer electrolytes simultaneously providing high conductivity and tailored mechanical modulus. In the first system, mechanically robust crosslinked PEO membranes were synthesized and doped with sodium triflate and tetraglyme. The relationships between ion conductivity (reached up to 10^-4-10^-3 S/cm at r.t.) and salt/gel content, glass transition temperature (T_g) and decoupling are investigated. In the second system, mechanically tailored novel single-ion conducting polymer electrolytes (SCPEs), poly[(4-styrenesulfonyl) (trifluromethane-sulfonyl)imide] (poly(STF)) and their copolymers were successfully synthesized. The SCPEs were synthesized by covalently attaching anionic moieties to the polymer that only allows specific cations, such as lithium ions, to move freely and provide ionic conductivity, i.e. the transference number is close to 1. The novel SCPE membrane includes a membrane with PDMS as polymer backbone to afford mechanical robustness and flexibility, and poly(STF) as a side chain to provide single-ion conductivity. The effect of the monomer type, the degree of polymerization of the side chain, and different cations is also studied. The obtained polymer membrane showed 100% elongation and 10^-4 S/cm single-ion conductivity (no salt) at 30 ^oC after doped with propylene carbonate.

The sluggish electrode reaction rates, especially in the cathode part which occupies the biggest portion of polarization resistance, restricts the practical use of solid oxide fuel cells (SOFCs). Mixed ionic and electronic perovskite oxides (MIECs) with the formula of ABO_3-δ used as SOFC cathodes are gaining intensive research interest, due to their low-cost, potentially high catalytic activity, and compositional diversity. In this report, we focus on one most promising parent MIEC, i.e., BaFeO_3-δ (BFO), and systematically investigate the effect of doping on BFO as cobalt-free cathode materials. The common lanthanide ions including La³⁺, Sm³⁺ and Gd³⁺ were chosen to partially substitute the A-site of BFO to obtain Ba_0.95La_0.05FeO_3-δ (BLF), Ba_0.95Sm_0.05FeO_3-δ (BSF), and Ba_0.95Gd_0.05FeO_3-δ (BGF), while the less reducible metal ions Zr⁴⁺and Ce⁴⁺ were doped into the B-site, and BaFe_0.95Zr_0.05O_3-δ (BFZ) and BaFe_0.95Ce_0.05O_3-δ (BFC) were achieved. The materials are prepared carefully, and their structural, electronic, electrocatalytic properties are characterized and compared. XRD reveals 5 mol% single A-site or B-site dopant is sufficient to stabilize the cubic phase of BFO, as predicted by the lattice calculation. XPS and iodometric titration demonstrates that neither of the two doping sites has obvious advantage over the other towards the formation of additional oxygen vacancies. B-site doped BFO shows a lower electrical conductivity than A-site doped ones, however, they have much quicker response to electrical conductivity relaxation, likely originating from the expanded lattice size. With the largest oxygen vacancy concentrations, Ba_0.95La_0.05FeO_3-δ and BaFe_0.95Zr_0.05O_3-δ stand out from the A-site and B-site doped BFO, respectively. With a similar amount of oxygen vacancies, B-site doping is more advantageous for enhancing oxygen bulk diffusion kinetics, and thus ORR activity. Even though introducing high-valence single dopant can improve the structural stability, it reduces the oxygen vacancy concentration. As a result, doping with either isovalent or lower-valence elements is preferred. We found that 5 mol% isovalent Ca²⁺ doping in the Ba²⁺ site and 5 mol% lower-valence In³⁺ in the Fe³⁺/Fe⁴⁺ site is found to successfully achieve cubic BaFeO_3-δ. This is in contrast with the typical approach of substituting elements of higher valence. However, without resorting to co-doping strategy, the phase of BaFe_0.95In_0.05O_3-δ (BFI) is rhombohedral, while Ba_0.95Ca_0.05FeO_3-δ (BCF) is a mixture of the cubic phase together with BaFe₂O₄ impurities. Thanks to the large oxygen vacancy concentration and fast oxygen mobility, the novel Ba_0.95Ca_0.05Fe_0.95In_0.05O_3-δ exhibits a favorable ORR activity. The significantly enhanced performance, compared with BFI and BCF, is attributed to the presence of the cubic phase and the large oxygen vacancies brought by the isovalent substitution in the A-site and lower-valence doping in the B-site.

Nowadays, by no means fortuitous, pollution-free and bio-regenerative solid oxide fuel cells (SOFCs) have arisen to be a competitive candidate as next generation renewable energy, which exhibiting high energy efficiency and flexible fuel choices. However, fast oxide-ion transportation of electrolyte could only be ensured in high working temperature by conventional views, which can decrease the voltage loss and further determine the electrical performance of SOFCs. Herein we report an in-situ and non-contact method to monitor the working condition of SOFCs and it is potential to become a promising optical temperature sensor to detect the working temperature of electrolyte materials. With the combinative protocol between density functional theory calculation and upconversion (UC) luminescence, the entanglement between thermal-driven formed O-ion Frenkel pair (native solubilizer) and Bi³⁺ dopant (competitive inhibitor) in La₂Mo₂O₉ derivatives has been unraveled, especially at a lower temperature required by a future SOFCs device. It is a potential route for screening and characterizing the candidate electrolyte onsets in lower temperature without sacrificing electrical performance.

Over the past decade, significant efforts have been devoted to developing cost-effective and high-performance anode materials for solid oxide fuel cell (SOFC). Decorating the anode with highly catalytic catalyst nanoparticles and tailoring the crystal chemistry of anode materials are two main strategies to realize excellent performance of anode materials.
In this work, a series of double perovskites Sr₂FeMo_0.65M_0.35O₆ with outstanding performance are developed. Through in situ exsolution, several high performance anode materials, metallic nanoparticle catalysts decorated ceramics, were prepared from Sr₂FeMo_0.65M_0.35O₆ (SFMM, M = Co, Ni, Cu). The in situ reduction converts the pure perovskite phase into mixed phases containing Ruddlesden-Popper structure Sr₃FeMoO₇, perovskite SrFe_1-yMo_yO₃ and metallic nanoparticle catalysts. The electrochemical performance of SFMCo and SFMNi ceramic anodes is greatly enhanced by the in situ exsolved Co-Fe and Ni-Fe alloy nanoparticle catalysts that homogenously distribute on the ceramic backbone surface. Owing to the catalytically inactive feature of Cu metal, the Cu nanofiber decorated SFMCu exhibits relatively poor anode performance. The maximum power densities of La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ electrolyte supported single cells with SFMCo and SFMNi anodes reach 820, and 960 mW cm^-2 in wet H₂ at 850 ^oC, respectively. The Sr₂FeMo_0.65Ni_0.35O₆ anode also shows excellent structural stability and good coking resistance in wet CH₄. The prepared SFMCo and SFMNi materials are potential high performance anode for SOFC.
A novel double perovskite Sr₂FeMo_2/3Mg_1/3O_6-δ was designed and prepared as anode material for SOFCs. The structure study shows that the doped Mg actually takes the Fe-site, leading to the formation of a number of anti-site defects and the presence of Fe_B-O-Fe_B’ bonds. First-principles computation reveals that the presence of Fe_B-O-Fe_B’ bonds can promote the easy formation and fast migration of oxygen vacancies in the lattice, which are the key to affect the anode reaction kinetics. In an electrolyte (300 μm) supported single cell, the Sr₂FeMo_2/3Mg_1/3O_6-δ anode demonstrates excellent cell performance with maximum power density of 803, 1038 and 1316 mW cm^-2 at 800, 850 and 900 ^oC, respectively. The Sr₂FeMo_2/3Mg_1/3O_6-d shows suitable thermal expansion coefficient (16.9 × 10^-6 K^-1), and good tolerance to carbon deposition and sulfur poisoning. The designed Sr₂FeMo_2/3Mg_1/3O_6-δ is an attractive anode material for SOFCs.

The ever-increasing demands for safe, energy dense and low cost energy storage systems have been driving interests in beyond Li-ion batteries such as those based on magnesium metal and all solid state battery systems^[1,2]. In particular, rechargeable magnesium batteries have been receiving increased interests driven by the numerous recent advancements, particularly in the field of developing practical Mg electrolytes^[1,3,4]. These progresses have been made primarily in developing novel salt systems for Mg batteries, including those for conducting Mg²⁺ ions even in the solid state at elevated temperatures^[3]. However, the solvent ingredient of the electrolyte remains largely unaddressed. In fact, the incompatibility of all non-ethereal solvents with Mg metal have confined the solvent choices to those non optimal solvent choices.
Herein, we address this key challenge and offer a path towards overcoming this limitation in Mg batteries by developing novel class of Mg conducting and compatible electrolyte systems. We also demonstrate how the utility of these novel solvent systems extends to other battery beyond Mg batteries.
References
[1] Choi J. W. Aurbach D. Nature Reviews Materials 1, 2016, 16013 1-16.
[2] Manthiram A., Yu X., Wang S. Nature Reviews Materials 2, 2017 16103.
[3] Mohtadi R., Mizuno F. Beilstein J. Nanotechnol. 2014, 5, 1291–1311.
[4] Mohtadi R., Orimo S., Nature Reviews Materials, 2016, 2,16091.

Metal thiophosphate materials family offers a materials toolbox with broad functionality that includes magnetism, ferrielectricity and electron correlations. One of their signature distinctions from transition metal dichalcogenides is the high mobility of certain ions, such as Cu+1 across the van-der-Waals gap. Here we report on heterostructure engineering of layered ferrielectric CuInP₂S₆, which controllably introduces 1D and 2D chemical boundaries into the crystal on bulk scale. The methodology relies on ionic mobility within the whole cation sublattice, both within and across the layers. We used high temperature x-ray diffraction, in-situ electron microscopy and atomic crorce microscopy to show that Cu-deficient Cu_1-xIn_1+x/3P₂S₆ material forms a single phase at high temperature, and spontaneously phase separates into ferrielectric (CuInP₂S₆) and paraelectric (In_4/3P₂S₆) phases coexisting within a single crystal. The high temperature (500 K) structure is heavily disordered , indicating mobility of both Cu⁺ and In³⁺ ions within the lattice. However, the framework of P₂S₆ anions remains invariant across this transition. We propose that this transition can be understood as eutectic melting on the cation sublattice, conceptually similar to intermediate temperature behavior of halide superionic conductors. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size, forming an intricate mesh of in-layer heterostructures comprised of domains with distinct cation compositions. Heterostructures can be formed, destroyed, and reformed by thermal cycling. Using this mode of lattice manipulation, we demonstrate that the ferroelectric T_c can be both increased to a nearly record level (about 20K higher than the pure bulk CuInP₂S₆ of 305K) and completely suppressed well below room temperature, without changing the physical sample, chemical composition, or loss of reversibility. Therefore a combination of ionic conductivity and several partially incompatible lattice structures enables creating complex multifunctional materials. To this end, we will further demonstrate how this methodology applies to other combinations of thiophosphates, including the composites made of initially ferroelectric and antiferromagnetic phases. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.
[1] Susner et al., “Metal Thio- and Selenophosphates as Multi-Functional van der Waals Layered Materials”, Advanced Materials, 10.1002/adma.201602852 (2017).
[2] Susner et al., Cation-eutectic transition via sublattice melting in CuInP₂S₆/In_4/3P₂S₆ van der Waals layered crystals, ACS Nano, 11, (2017), 7060.

Abstract
The La₂Mo₂O₉ (LAMOX) is a fast oxy-ion conductor. Its conductivity is subject to a structural phase transition from room temperature non-conductive monoclinic to high temperature conductive cubic phase at about 580^oC. The origin of the conductivity in cubic LAMOX has been suggested to be due to a “disorder” in the O sublattice. The order/disorder phase transition of LAMOX is suppressed by most of the substituents which stabilizes the highly conducting cubic phase to room temperature. Various partial substituents are possible on the cationic sublattices of LAMOX: K⁺, Sr²⁺, Ba²⁺, Gd³⁺, Sm³⁺, Nd³⁺, Dy³⁺ for La³⁺ and V⁵⁺, S⁶⁺, Cr⁶⁺, W⁶⁺, Nb⁵⁺ for Mo⁶⁺ position. All these substitutions tends to suppress the resistive transition and to stabilize the cubic phase at room temperature. The phase transition is key for origin of fast ion conductivity in LAMOX based system. In spite of that structure of LAMOX is not still well understood. It may be due to (i) slight distortion from two phases, and (ii) the symmetry breaking spontaneous strains associated with the cubic-monoclinic phase transition is extremely small. To better characterize this phase transition, we have studied the structure of LAMOX-based systems by means of X-ray powder diffraction and Raman spectroscopy as a function of temperature. The evolution of structure at high temperature in pure LAMOX system and suppression of phase transition in La_2-xR_xMo_2-yP_yO_9-d (R = Gd, Sm, Dy, Nd and P = W, V, Nb) are studied; dopants content x and y are varying in between (1 to 50 at%). The compositions (x & y) are dopant sensitive for cubic-phase of LAMOX . Structural study by XRD exhibit splitting of diffraction pattern in pure LAMOX in between 24-27^o , 2theta, which are disappeared in doped system. Pure-LAMOX, XRD patterns agrees with monoclinic form (ICSD 172479) while for doped LAMOX system, XRD match with cubic structure (ICSD 420672). Further insight into the structure of pure LAMOX and doped LAMOX are provided by Raman spectroscopy. For LAMOX system, we observed three bands with maxima at around 80, 350 and 870 cm^-1; however pure-LAMOX, splitting in vibration mode around 870 cm^-1 is detected. According to literature data, modes near 870 cm^-1 are associated with symmetric and asymmetric stretching vibrations of tetrahedral MoO₄ units generate Raman bands. The structural variations in doped and pure-LAMOX system are confirmed by room temperature Raman bands. The electrical conductivity and dielectric properties are studied in the temperature range 300 to 700^oC and frequency range from 0.1 to 1 MHz. The conductivity of this compounds show Arrhenius type behaviour. The random free-energy model has been used to analyze the frequency dependence of the conductivity. The charge carrier relaxation time and activation have been determined from the conductivity spectra using this model. We have observed that the dielectric relaxation peaks arise from the diffusion of oxygen ions via vacancies.