Symposium EN08—Progress in Understanding Charge Transfer at Electrochemical Interfaces in Batteries

The design and achievement of superior battery systems (i.e., those that are more efficient, higher in energy density, safer, more environmentally friendly, etc.) requires knowledge of fundamental chemical and physical properties of the system and its components while under true operating conditions. Such advanced knowledge can be obtained through both modeling and experimental efforts. In the case of battery systems designed to replace the well-known lithium ion battery systems, new chemistries in particular are being explored, classified, and improved using state-of-the-art characterization and computational tools. New analytical tools have been developed for the study of these advanced battery systems in destructive, post-mortem modes as well as while the battery is under operating conditions. Further, certain aspects of the battery chemistry and operation can also be modeled and reproduced with varying degrees of fidelity through computational studies that cross scales from the atomic and molecular to the sizes of pores and beyond. The combination of these experimental techniques and computational tools will eventually lead to predictive understanding of battery components and their operation in the complete battery system. This presentation will focus on progress in merging both post-mortem and operando studies that use advanced spectroscopies and computational methods to understand components of complex battery systems. In this work, we focus on enabling the prediction and creation of bespoke interfaces and transformative interphases for energy storage through atomistic- and molecular-level understanding of the role of interface structure and composition on interphase properties, as well as the design and discovery of novel interface constructs for facile charge transport.

Identification of oxygen as a high energy density storage material marked a paradigm shift from solid-state intercalation chemistry form of energy storage such as Li-ion batteries. Nearly three decades ago, Abraham and co-workers demonstrated a rechargeable metal-oxygen (Li-O₂) battery. Li-O₂ batteries display a theoretical energy density of over 3505 Wh. Kg^-1, much higher than Li-ion batteries. However, Li-O₂ electrochemistry is non-trivial. The electrolyte, electrocatalysts, and solid-porous O₂-scaffold influences the multistep O₂ Û Li_xO_y reaction. The reversibility of the transformation of O₂ Û Li_xO_y is crucial for stable battery performance, and this is directly associated with the kinetics of oxygen evolution reaction (OER) and the formation of insoluble oxides (Li_xO_y) during oxygen reduction reaction (ORR). There has been intense research on the Li-O₂ battery system's different components over the last two decades. Our strategy here is to develop efficient soluble redox mediators/electrocatalysts that reside at porous O₂-electrode|electrolyte interface. This is in contrary to the conventional approach of immobilization of electrocatalyst(s) on the porous cathode (O₂)-scaffold. The efficiency of the redox-mediator is finely determined by the redox reversibility of the transition metal site and oxygen binding capability of the complex. While this strategy may lead to efficient metal-oxygen batteries, it will also pose new and non-trivial challenges on interfacial charge transfer and transport. This presentation will discuss porphyrin (bare hemin and heme proteins, Rudra N. Samajdar et al. J. Phys Chem. C 2019) and phthalocyanines (Pc) (Subhankar Mandal et al. submitted, 2021) from groups 6-12, e.g., MnPc, FePc, CoPc, NiPc, CuPc, ZnPc, and MgPc (d⁰ system, group 2) as the redox mediators in Li-O₂ batteries. Due to the redox mediator's residence at the electrode | electrolyte interface, the insulating Li-oxides do not form on the air-cathode scaffold and block the pores. In the context of the porphyrin, due to their superior ability to combine with O₂, the protein performs better than the bare heme. In the case of the Pcs, the electrochemical processes are observed to be highly dependent on the metal centre. The experimental findings in case of the Pcs are additionally supported by theoretical studies based on DFT.

Deposition of thin protective films on the cathode surface is an efficient approach to overcome interfacial degradation processes and control ionic mobility across the interface. An essential requirement of a good coating layer is that in addition to surface passivation, it will enable unimpeded ion transport between the electrode and electrolyte. To date, the rational design of such protective layers is limited, mostly due to the lack of sensitive characterization tools that can provide atomic-molecular level insight into both the structure and function of thin amorphous layers.
Here I will present a new approach to examine thin interphases and gain atomic level insight into their composition, 3D structure and lithium ion transport properties by using solid state nuclear magnetic resonance (ssNMR) spectroscopy. The approach is based on (i) 10-10⁴ fold increase in ssNMR sensitivity provided by Dynamic nuclear polarization (DNP), a process in which the high electron spin polarization is transferred to surface nuclei in the sample, enabling the detection of otherwise invisible nanometer-thick layers, and (ii) tracking ⁶Li-⁷Li isotope exchange processes across the electrode-electrolyte interface.
I will describe the application of this approach to a novel surface treatment for high energy cathodes, lithium rich LiNi_xMn_yCo_zO₂ (NMC), which leads to substantial improvements in rate performance and capacity retention. Specifically, I will show how the combination of DNP and ssNMR provides a detailed chemical map of the surface composition and structure of this lithium-silicate protection layer. The permeability of the coating and the role of lithiated interphases was assessed by ^6,7Li exchange experiments on coated and uncoated NMC and further compared to Electrochemical Impedance Spectroscopy (EIS) results.
The combination of structural insight from high sensitivity ssNMR and lithium exchange brings us closer to understanding the functionality of electrode surface layers.

Nuclear magnetic resonance (NMR), a quantitative and non-destructive atomic-isotope specific tool, is idea for studying a system without compromising control of both pressure and temperature. We will show how NMR and associated capabilities (1D and 2D solid/liquid state NMR spectroscopy, pulsed-field-gradient-NMR, and DFT computational modeling of NMR parameters) combined with molecular dynamics simulations can be utilized to study the solvation structures and the molecular-level dynamics of electrolytes in solution and at the solid-electrolyte interface. The following examples will be highlighted to demonstrate the power of our combinations of approaches: (i) Mechanisms of desolvation associated with Mg(TFSI)₂ + G₂ on surfaces of nano-sized MgO solids; (ii) Role of solvent rearrangement on Mg²⁺ solvation structures in dimethoxyethane solutions; and (iii) Origin of unusual acidity and Li⁺ diffusivity in a series of water-in-salt electrolytes.

References
Hu, J. Z.; Rajput, N. N.; Wan, C.; Shao, Y. Y.; Deng, X. C.; Jaegers, N. R.; Hu, M.; Chen, Y. W.; Shin, Y.; Monk, J.; Chen, Z.; Qin, Z. H.; Mueller, K. T.; Liu, J.; Persson, K. A., Mg-25 NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes. Nano Energy 2018, 46, 436-446. https://doi.org/10.1016/j.nanoen.2018.01.051
Hu, J.Z.; Jaegers N. R.; Y. Chen, Y.; Han, K.; Wang, H.; Murugesan, V.; and Mueller, K.T. Adsorption and thermal decomposition of electrolytes on nanometer magnesium oxide: An in situ ¹³C MAS NMR study. ACS Applied Materials & Interfaces 2019, 11, 42:38689-38696. DOI:10.1021/acsami.9b11888
Chen, Y.; Jaegers, N. R.; Wang, H.; Han, K.; Hu, J. Z.; Mueller, K. T.; and Murugesan, V. Role of Solvent Rearrangement on Mg²⁺ Solvation Structures in Dimethoxyethane Solutions Using Multimodal NMR Analysis. J. Phys. Chem. Lett. 2020, 11, 15, 6443–6449. https://doi.org/10.1021/acs.jpclett.0c01447.
Han, K.; Zhou Yu, Wang, H.; Redfern, P. C.; Ma, L.; Cheng, L.; Chen, Y.; Hu, J. Z.; Curtiss, L. A.; Xu, K.; Murugesan, V.; and Mueller, K. T. Origin of Unusual Acidity and Li⁺ Diffusivity in a Series of Water-in-Salt Electrolytes. J. Phys. Chem. B 2020, 124, 5284−529. https://pubs.acs.org/doi/10.1021/acs.jpcb.0c02483

While several multivalent battery working ions such as Mg²⁺ or Ca²⁺ present the theoretical promise of high pack-level energy density, numerous challenges stand in the way of their successful development to commercialization. One of the key challenges is maintaining electrolyte stability during the working ion (de)solvation and charge transfer events at the potential extremes of the anode and cathode electrified interfaces. Stability under these conditions is heavily influenced by the interactions of the strongly polarizing multivalent cation of interest with the supporting salt or solvent species. In this work, we evaluate the efficacy of using selective coordination strategies to tune interfacial stability at magnesium and calcium battery electrodes. A combination of electrochemical, solvation, and surface analyses will be presented to provide a holistic view of the extent to which targeted salt or solvent additives mitigate electrolyte decomposition. These findings help guide electrolyte design efforts aimed at achieving high voltage multivalent batteries.

Electrochemical stability windows of electrolytes largely determine the limitations of operating regimes of lithium-ion batteries, but the degradation mechanisms are difficult to characterize and poorly understood. Using computational quantum chemistry to investigate the oxidative decomposition mechanisms that govern voltage stability of multi-component organic electrolytes, we find that electrolyte decomposition is a process involving the solvent and the salt anion and requires explicit treatment of their coupling. We find that the ionization potential of the solvent-anion system is often lower than that of the isolated solvent or the anion. This mutual weakening effect is explained by the formation of the anion-solvent charge-transfer complex, which we study for 16 anion-solvent combinations relevant for Li-ion battery electrolytes. This understanding of the oxidation mechanism allows to formulate a simple predictive model that explains experimentally observed trends in the onset voltages of degradation of electrolytes near the cathode. This model opens opportunities for rapid rational design of stable electrolytes for high-energy batteries. [1]
Our charge complex model suggests an explanation of why ionic liquids are typically found to be more electrochemically stable at high voltage than conventional organic solvent-based electrolytes. At the same time, ion complexes in ionic liquids affect ionic transport due to correlation effects. Using molecular dynamics simulations of transport properties, we find that strong ionic interactions result in significant deviations from ideal solution behavior. By adopting rigorous concentrated multicomponent solution theory, we show that anomalously low and even negative Li transference numbers emerge in vast range of different ionic liquid chemistries, suggesting a universal behavior of this class of electrolytes. Finally, we propose chemical strategies for mitigating the detrimental effects of clustering and for increasing transference number and conductivity. [2-4]

[1] E. Fadel et al, “Role of solvent-anion charge transfer in oxidative degradation of battery electrolytes”, Nature Comm., 10, 3360 (2019)
[2] N. Molinari, J. P. Mailoa, B. Kozinsky, “General Trend of Negative Li Effective Charge in Ionic Liquid Electrolytes”, J. Phys. Chem. Lett. 10, 2313 (2019)
[3] N. Molinari, J. P. Mailoa, N. Craig, J. Christensen, B. Kozinsky, “Transport Anomalies Emerging from Strong Correlation in Ionic Liquid Electrolytes”, J. Power Sources, 428, 27 (2019)
[4] N. Molinari and B. Kozinsky, “Chelation-Induced Reversal of Negative Cation Transference Number in Ionic Liquid Electrolytes”, J. Phys. Chem. B 124, 13, 2676–2684 (2020).

Layered oxide cathode materials are widely used in modern Li-ion batteries. However, during device operation they suffer from surface degradation, transition metal oxide dissolution and destructive phase transitions at the cathode-electrolyte interface. One way to suppress the chemical instability and reactivity of the cathode with the electrolyte is to coat the cathode surface with inert oxides.

We investigate niobium oxide cathode coatings with an intention of improving performance at ultra-high C rates and long-term cyclability of Li-ion batteries. The effect of the coating was evaluated in a thin-film LiCoO₂ model system. The niobum oxide was deposited by atomic layer deposition (ALD), which enables a conformal coating over the cathode material. The niobium oxide film was lithiated by annealing it together with the cathode. The electrochemical properties could be significantly improved compared to uncoated samples by systematically investigating the battery performance as a function of coating thickness. Thereby, 47 % remaining initial capacity was observed at 100 C for 30 nm niobum oxide-coated cathode films. For the same coating thickness, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode. Impedance investigation indicated a strong retardation of the interfacial resistance growth during cycling. Elemental analysis revealed a bulk and surface contribution of the niobium oxide coating. These results suggest that ALD as coating method can also have a strong impact for high rate discharging of bulk cathode powders.

Redox-active polymers (RAPs) are a promising material for energy storage in flow batteries due their large size preventing detrimental redox material crossover and adjustable molecular chemistry and architecture for optimized performance. There has been a recent effort to understand the physics governing charge diffusion in RAPs, both in the bulk and near surfaces. We use simulations and theory to show how a variety of molecular charge transport mechanisms affect diffusive motion in RAP solutions. To simulate these RAP solutions, we employ a hybrid Brownian dynamics and kinetic Monte Carlo model that accounts for both the conformational degrees of freedom and charge hopping dynamics. This model uses a coarse-grained representation that can be altered to reflect a variety of physicochemical properties of RAPs, in particular using the molecular charge hopping kinetics as either a tunable variable or an input parameter. We perform these simulations for both isolated, single-chain and interacting, multi-chain systems to demonstrate the interplay between a variety of transport mechanisms (e.g. intra-polymer self-exchange, polymer segmental motion, inter-polymer collisions). We develop theoretical scaling arguments to describe the diffusive motion of charge via these mechanisms, showing excellent agreement with simulations. Our predictions suggest the existence of three charge transport regimes, which distinguish between inter- and intra-molecular processes and dilute and semi-dilute solutions. We extend our understanding of these solution dynamics to model of the kinetics of surface adsorption, providing a molecular connection between surface interactions and charge transport.

One of the biggest challenges facing the development of multivalent batteries is the fact that the majority of promising electrolytes demonstrated to date are incompatible with either metallic anodes or high voltage cathodes, both of which are required for multivalent systems to compete with existing Li-ion technologies. For example, many electrolytes shown to exhibit reversible plating/stripping of Mg metal rely on the presence of significant concentrations of Cl^-, precluding their use with oxide cathode materials due to parasitic reactions that take place at high voltage. In light of this limitation, it is imperative that a more complete understanding be developed of the specific role of Cl^- in promoting reversibility in multivalent systems in order to develop design rules for synthesizing new electrolytes that can avoid the use of Cl^- altogether. Through systematic rotating disk and ring-disk electrode (RDE and RRDE) investigations of metal plating/stripping in multivalent electrolytes utilizing the bis-(trifluoromethane sulfonyl) imide (TFSI^-) anion with varying concentrations of Cl^-, a cooperative effect is observed between both anions that yields the overall more reversible behavior of mixed TFSI^-/Cl^- electrolytes relative to electrolytes containing only TFSI^-. This effect is shown to be general across multiple multivalent systems (i.e., Mg, Zn and Cu), and mechanistic understanding of the role of Cl^- in improving reversibility of TFSI-based electrolytes is provided through the combination of R(R)DE experimental results with computational evaluation of the activity and stability of various TFSI- and Cl-based solution complexes. The cooperative anion effect can be further generalized to other electrolyte systems, suggesting a set of general design principles for developing new multivalent electrolytes.

Rechargeable Mg batteries hold a great potential for a new low-cost, high energy density energy storage solution due to the high volumetric capacity (3,833 mAh cm^-3) and natural abundance (2.33% in earth crust) of Mg. Despite its promising electrochemical metrics, the absence of proper electrolytes that enable reversible Mg plating-stripping and compatible pairing with cathode materials hindered the practical use of Mg metal anodes. More importantly, there is very limitedfundamental understanding of the critical factors to drive the reversible Mg anodes at the Mg/electrolyte interface. Here, we investigated the interfacial chemistry between Mg and modified Mg cationic clusters in Mg(TFSI)₂ with Mg(BH₄)₂ cosalt through operando electrochemical analyses to rationalize the improved reversibility of Mg plating-stripping. Meanwhile, the electrochemically evolved artificial film was developed to stabilize the fresh Mg deposits, which showed ultrafast charge transfer and low film resistance in the conventional electrolyte of Mg(TFSI)₂-MgCl₂.

In bulk battery materials, solid-state diffusion of ions frequently limits rate capabilities, and phase transitions or phase separation processes often limit stability. Many of these limitations can be overcome by moving to the nanoscale, as diffusion distances are shortened, and phase transitions or phase separation can either be suppressed, or occur on much shorter length-scales. Nanoporous materials, in particular, are very promising, as they offer reduced diffusion lengths without significantly reducing electrical conductivity, as often occurs in nanocrystal based materials. While the advantages of nanostructured electrode materials can be significant, they have the marked disadvantage of increased interfacial area that can lead to increased SEI formation and reduced coulomb efficiency. In this talk, we explore a number of systems where we seek to optimize the trade-off between bulk kinetic limitations and interfacial or charge transfer limitations, with a specific focus on how nanoscale architecture can be tuned to produce an ideal balance between these limitations. Systems that will be addressed include high rate nanoporous cathodes based on both conventional oxides, using LiNi_0.80Co_0.15Al_0.05O₂ (NCA) as a prime example, and on metal phosphates, using LiVPO₄F (LVPF) as an example. In both cases, nanoporous materials are produced by combining polymer templating with sol-gel methods. For both systems, in addition to optimizing nanoscale size, we find that the key to high rate behavior is incorporation of a conductive artificial SEI layer. In the oxide cathodes, in particular, conjugated polymers can be highly effective at increasing stability and rate capabilities, a process that should derive from a combination of enhanced electrical conductivity and reduced SEI formation. A variety of new conjugated polymer binders will be discussed. Finally, we will consider nanostructured alloy anodes produced by selective etching of metals. In these systems, dramatic structural changes upon lithiation require the use of nanophase materials to achieve stable cycling. Here, the interplay between nanoscale architecture, material composition, and surface layers will be explored using operando transmission X-ray microscopy (TXM), which allows us to directly image the evolution of the nanoscale pore structures during battery operation. Taken together these results demonstrate the rich design space that is available when nanoscale architecture and interfacial engineering are combined to optimize the performance of battery materials.

Electrode deformations during charging and discharge process are directly related to the power performance and cyclability of the energy storage devices. Understanding the electrode deformation and changes in mechanical responses during electrochemical events and their impact on devices’ performance is crucial for achieving high power and high energy storage devices. In-situ atomic force microscopy (AFM) is well suited to tackle this task as it allows tracking local sub-nanometer volume changes and other mechanical responses. With a spatial resolution of tens of nanometer, this approach is capable of mapping the electro-chemo-mechanical behavior under the conditions close to the device operation and access information about local redox mechanisms and investigate its heterogeneity.

In this work, the local electro-chemo-mechanical coupling behaviors of proton insertion into WO₃ electrodes is studied. The concept of mechanical cyclic voltammetry (mCV) curves is introduced, and the relationship between electrochemical current and strain are discussed in the context of simplified models. The results show that different mechanical responses are involved during ion insertion and extraction. The mechanical CV mapping highlighted the local heterogeneity and showed that the charging mechanisms varied across the electrode. These local variations could be further corelated to local morphology, crystal orientations or chemical compositions. Besides providing information of redox heterogeneity, the mCV methodology allows us to circumvent the common issues that are often encountered in the global electrochemical characterizations, for example cell resistance and unwanted parasitic reaction. We further demonstrate that the mCV approach is applicable to a variety of energy storage materials with increasing complexity of current-deformation relationships.

The work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

Rechargeable batteries with high energy density are of significant importance to the energy storage market, especially for portable electronics and electric vehicles. Although the use of lithium (Li) metal anode and high capacity cathode at high operation voltage can achieve the high energy density of batteries, the development of Li metal batteries is greatly limited by the inferior performances because of the issues related to the Li metal anode (dendrite growth and low Coulombic efficiency) and the intercalation cathode under high voltages and challenging temperatures. The main reason is the electrode/electrolyte interphases on both Li metal anode and the high voltage cathode are not stable and effective enough to protect the two electrodes from the continuous side reactions with the electrolyte. In this presentation, we report to design and in situ generate highly stable electrode/electrolyte interphases on both Li anode and LiCoO₂ cathode with an advanced electrolyte, with which greatly improved cell performances of Li||LiCoO₂ batteries are achieved under a high charge voltage of 4.5 V and a wide temperature range from -30 to 55 °C. More details will be discussed at the presentation.

As the energy density of lithium ion batteries (LIBs) increases, safety of the battery is raising intensive concern. Flammability of the electrolyte is a critical link of the overall safety property of LIBs. The conventional approach to reduce the flammability of liquid electrolytes is to introduce a certain amount of a miscible organic phosphate solvent as flame retardant (FR) additive or co-solvent into the LiPF₆-organocarbonates electrolytes.[1] However, due to the intercalation of the phosphate molecules into the graphite electrode, the introduction of such FR usually induces incompatibility between the FR and the graphite electrode and deterioration in battery cycling performance. Moreover, as the FR is gradually consumed by the parasitic reactions at the electrode/electrolyte interfaces, the flame retarding effectiveness of the FR decays.[2] In response to these challenges, the concept of localized high-concentration electrolyte (LHCE) with an organic phosphate solvent FR is utilized to adjust the electrode/electrolyte interphases on both graphite anode and high voltage cathode. By adopting the unique solvation structure of LHCE and the incorporation of a small amount of electrolyte additive, the electrode/electrolyte interphases can be designed and tuned. Consequently, the graphite||LiNi_0.8Mn_0.1Co_0.1O₂ cells using such phosphate-based LHCEs can achieve the capacity retention of 85.5% after 500 charge/discharge cycles, being significantly higher than that of conventional LiPF₆-organocarbonate electrolyte (75.8%). Meanwhile, ignition test shows that these LHCEs are much less flammable than the conventional LiPF₆-organocarbonate electrolytes. More details will be reported during the presentation.

References
[1] X. Yao, S. Xie, C. Chen, Q. Wang, J. Sun, Y. Li, S. Lu, Journal of power sources 2005, 144, 170.
[2] T. Dagger, C. Lürenbaum, F. M. Schappacher, M. Winter, Journal of Power Sources 2017, 342, 266.

Non-aqueous Rechargeable Mg batteries (RMBs) represent a safer, cheaper and more powerful alternative to lithium ion battery technology.^{1, 2} However, the high charge density of Mg²⁺ ions triggers the formation of ion impermeable deposits with solvents at electrode/electrolyte interfaces called Solid-Electrolyte Interfaces (SEIs). To overcome the stability and reversibility issues caused by SEIs, the electrolytes used must be stable against reductive reactions at the anode/electrolyte interface during charge/discharge cycles in RMBs.^{3, 4} Magnesium halide salts are an exciting prospect as stable and high-performance electrolytes for RMBs. By nature of their complex equilibria, these salts exist in solution as a variety of electroactive species (EAS) in equilibrium with counter ions such as AlCl₄^-. Here we investigated ion agglomeration and transport of several such EAS in MgCl₂ salts dissolved in ethereal solvents under both equilibrium and operating conditions using large scale atomistic simulations.⁵ We find that the solute morphology is strongly characterized by the presence of clusters and is governed by the solvation structures of EAS. Specifically, the isotropic solvation of Mg²⁺ results in slow formation of bulky cluster, compared with chain-like analogues observed in the Cl-containing EAS such as Mg₂Cl₃⁺, MgCl⁺ and Mg₂Cl₂²⁺. We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl₂ based electrolyte by at least an order of magnitude. We highlight the role of the solvent in determining cluster morphology and ion mobility by controlling ion diffusion pathways and ion coverage. Our findings of the cluster formation, morphology and kinetics can provide useful insight into the electrochemical reactions at the anode/electrolyte interface in RMBs.
References

1. C. B. Bucur, T. Gregory, A. G. Oliver and J. Muldoon, The Journal of Physical Chemistry Letters, 2015, 6, 3578-3591.
2. M. John, B. C. B. and G. Thomas, Angewandte Chemie International Edition, 2017, 56, 12064-12084.
3. P. Baofei, H. Jinhua, H. Meinan, B. S. M., V. J. T., Z. Lu, B. A. K., Z. Zhengcheng and L. Chen, ChemSusChem, 2016, 9, 595-599.
4. K. A. See, K. W. Chapman, L. Zhu, K. M. Wiaderek, O. J. Borkiewicz, C. J. Barile, P. J. Chupas and A. A. Gewirth, Journal of the American Chemical Society, 2016, 138, 328-337.
5. V. Vasudevan, M. Wang, J. A. Yuwono, J. Jasieniak, N. Birbilis and N. V. Medhekar, The Journal of Physical Chemistry Letters, 2019, 10, 7856-7862.

The development of new energy storage systems has attracted substantial attention in the scientific community. The overall performance of energy storage in batteries, including rates of charge/discharge and system reliability, largely depends on the resiliency of the electrode/electrolyte interface. Despite recent progress, major challenges lie developing new electrolytes and electrodes for redox-flow batteries to enhance charge transfer rates and to achieve extended operation lifetimes. In recent years, bulk-scale methods have been used to characterize and understand redox-active electrolyte behavior at interfaces, however, bulk measurements tend to 'average out' and obscure charge transport events at the molecular level. In this work, we use single molecule methods to directly characterize charge transport at the electrode interface. In particular, we use a custom electrochemical scanning tunneling microscope-break junction (ECSTM-BJ) instrument to understand interfacial charge transport and redoxmer degradation at the molecular level. ECSTM-BJ enables the direct measurement of molecular charge transport and conductance as a function of applied bias, external gate voltage, and the chemical identity of the redox-active species or supporting electrolytes. Materials with different chemical structures and charge transport channels are identified as different molecular subgroups, thereby revealing otherwise hidden molecular sub-populations that underlie bulk-scale behavior. Using this approach, we investigate two different redox-active systems: viologen-based molecules and mesolytic cleavage of redoxmers. Electrostatic gating is used to tune charge transport in viologen-based molecules by adjusting the potential drop across the double layer and the associated redox states in the electrochemical environment. In contrast to non-redox-active counterparts, our results show that the conductance of viologen-containing molecules is a strong function of the gate voltage as the viologen redox center is reduced. Moreover, the reduced state of the molecules weakens the interaction between the molecules and the electrode, which is indicated by changes in junction length. In a related study, we show that intermolecular charge transport is effectively regulated by the formation of a pH-responsive supramolecular host-guest complex. In this way, we present intriguing new avenues for intermolecular complexation for controlling the charge transport process. Finally, we discuss the direct molecular characterization of self-immolating redoxmers that degrade upon being damaged via mesolytic cleavage. Interestingly, our results show that strong electric fields near electrode interfaces induce mesolytic cleavage, followed by the dimerization of the reaction products, which is detected by single molecule conductance. Overall, our results highlight the use of single molecule techniques to characterize charge transport at the electrode/electrolyte interface.

The essential electrochemical processes at electrode interfaces are closely related to the changes of coordination environments and solvation structures of cations. The thermodynamics of these processes including reactivity and stability of interfaces are largely determined by the spatial speciation profile in the interfacial regions whereas kinetics of the charge transfer is governed by the stiffness of solvation spheres of cations and their coordination states. However, the atomistic details and thermodynamic characteristics of ion (de-)solvation process in the confined environment (e.g., near an electrode) remain largely unknown. Specifically, for multivalent cations that are prone to the formation of ion-pairs and neutral aggregates in low dielectric solvents and show a non-trivial concentration dependence of population of various ionic species these characteristics are very difficult to probe both experimentally and computationally. The key questions we want to address are the following: 1) how different the population of ionic/neutral species at the interface is as compared to the bulk electrolyte at various electrode potentials; 2) in which form the active matter (metal cations) is delivered to the interface; 3) what is the dynamics of the cation solvation and coordination environment along the most thermodynamically favorable pathways of a cation that approaches the interface from a bulk electrolyte; and finally 4) what is the locus and a particular cation solvation/coordination state at which the charge transfer occurs.
Using a multiscale methodology based on classical and ab initio free energy sampling techniques we analyze a solution of MgTFSI₂ in THF next to graphene and show that the anion-coordinated species are the vehicle that delivers the Mg-cation to the interfaces at shorter distances and in lower coordination states. This allows us to formulate the key and general interfacial descriptors that can be used to control the balance between the chemical stability of an interface and the overpotential associated with the charge transfer kinetics. We identified the optimal free energy pathways and a generalized coordinate that describe the (de-)solvation dynamics of the Mg-cation. Finally, we show how the interfacial solvation dynamics can be manipulated using the ion solvation multiplicity.

[1] A. Baskin and D. Prendergast, J. Phys. Chem. Lett. 10, 4920−4928 (2019)
[2] A. Baskin and D. Prendergast, J. Phys. Chem. Lett. 11, XXX, 9336–9343 (2020

Interfacial charge transfer processes strongly influence the performance of elecrochemical enenergy storage devices in many different ways. For example, ion deposition from electrolytes onto electrodes influences the efficiency, morphology, and stability of anodes based on high-capacity metals. Electronic transfer between electrodes and solid eletrolytes can impart undesirable electrical conductivtity and precipitate harmful chemical reactions. Ion transport in solid electrolytes can be highly sensitvie to internal interfaces such as grain or particle boundaries. This presentation will summarize recent computational studies of the atomic-scale phenomena that underlie charge transfer processes on electrode surfaces, between elecrodes and electrolytes, and at internal interfaces in solid electrolytes. Multiple computational techniques, ranging from large scale classical MD to many-body perturbation theory, are used to tackle the different length scales spanned by these systems.

Multivalent batteries, particularly Mg-metal batteries, are promising technology for next-generation energy storage applications due to their theoretically high volumetric energy density. One of the critical challenges in multivalent batteries is to create a stable and functional electrochemical interphase (solid electrolyte interphase) that facilitates reversible plating and stripping of charge carrying ions and impedes electronic transport to limit electrolyte degradation. This requires fundamental understanding of the reactivity of electrolyte (solvent and salt) species with Mg-metal anode. To address this, we employ in-operando X-ray photoelectron spectroscopy (XPS) experiments in combination with density functional theory (DFT) calculations to elucidate the decomposition mechanisms of two commonly used ethereal solvents, diglyme (G2) and tetrahydrofuran (THF) on the Mg-metal anode surface. The solvent molecules are exposed to sputter cleaned Mg-metal surface and high-resolution XPS spectra are collected at different temperatures to analyze the reaction products. DFT calculations are performed to model the adsorption and to compute the reaction energy profiles to elucidate decomposition reaction pathways of G2 and THF on pristine Mg (0001) surface. The decomposition pathways are modeled as a series of C-O and C-H bond cleavage reactions. The climbing image nudge elastic band (CI-NEB) calculations are carried out to compute the transition states and energy barriers for the various C-O and C-H bond cleavage reactions. Further, Bader charge analysis is performed to estimate the effect of charge transfer on the stability of intermediate species formed during the surface decomposition reactions. The computed reaction energy profiles indicate higher reactivity of diglyme molecule compared to a THF molecule at Mg (0001) surface, in agreement with experimental observations. Our approach enables a detailed atomic-level understanding of the reaction mechanisms and explain the nature of key reaction products of solvent decomposition on Mg-metal anode surface.

The use of lithium metal anodes is expected and desired in the new generation of high energy density batteries. However, the high reactivity of Li metal electrodes creates a series of issues that keep them away from practical applications. For example, electrolyte degradation reactions due to electron transfer from the surface induce the formation of a solid-electrolyte interphase (SEI) film, that starts at open circuit conditions. This multicomponent SEI film has different passivation properties and morphologies depending on the electrolyte. However, during battery charge, another phenomenon takes place simultaneously with SEI growth, and this is cation electrodeposition and metal nucleation or “plating”. As expected, at fixed current rate, phenomena such as cation diffusion and desolvation depend strongly on the type of electrolyte and the stage of formation of the SEI. Here we analyze cation electrodeposition from first principles simulations, including ab initio constrained molecular dynamics coupled to thermodynamic integration in the Blue Moon ensemble. We characterize the possible free energy pathways of the cation in various environments and the influence of the electrolyte composition and its decomposition, and the surface structure.

A major breakthrough in battery materials is required to meet the ever-increasing proliferation of portable electronic devices, electric vehicles and their variants, as well as the need for incorporating renewable energy resources into the main energy supply [1]. In this context, lithium-sulfur (Li-S) batteries are promising candidates due to their very high energy density (2,600 Wh kg^-1) and specific capacity (1,675 mAh g^-1) and significantly lower weight and cost, compared to lithium-ion batteries (LIBs) [2]. Fully packaged, it is expected that future Li-S batteries can operate at close to 500 Wh kg^-1, which is more than twice the energy density of LIBs (200 Wh kg^-1) [3]. However, several issues including the dissolution of Li-Polysulfide (PS) species into the electrolyte have prevented this technology from being broadly commercialized [4]. In this talk, I will present our computational approach of attaching functional groups to the cathode surface to create thermodynamic barriers to the dissolution of PS in an electrolyte. Beginning with theoretical analysis of the structure and interactions of generic LiPS moieties and a large set of functional groups via high-throughput density functional theory (DFT) calculations, I show that it is possible to identify functional groups that anchor to the cathode and exert high enough binding affinity to LiPS formed near electrode/electrolyte interface to prevent their loss to the solution. The selected candidates are used in detailed molecular dynamics (MD) studies to develop a fundamental understanding of the effect of various electrolyte variables (components, PS chain length, salt concentration) on structural and dynamical properties at the functionalized interface [5, 6]. In addition, I show how we are applying such a multi-scale modeling approach using our group's newly developed computational infrastructure that combines DFT calculations with MD simulations, resulting in a database of well-characterized materials to be used for building machine-learning models as well as for testing computationally identified structures in experiments. This work provides crucial information to mitigate the dissolution of PS species during cycling, which is the main reason for rapid capacity decay in Li-S batteries.
References
1. Larcher, D. and J.-M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage.Nature chemistry, 2015. 7(1): p. 19.
2. Manthiram, A., X. Yu, and S. Wang, Lithium battery chemistries enabled by solid-state electrolytes. Nature Reviews Materials, 2017. 2(4): p. 1-16.
3. Fang, R., et al., More reliable lithium-sulfur batteries: status, solutions and prospects. Advanced materials, 2017. 29(48): p. 1606823.
4. Manthiram, A., Y. Fu, and Y.-S. Su, Challenges and prospects of lithium–sulfur batteries. Accounts of chemical research, 2013. 46(5): p. 1125-1134.
5. Rajput, N.N., et al., Elucidating the solvation structure and dynamics of lithium polysulfides resulting from competitive salt and solvent interactions. Chemistry of Materials, 2017. 29(8): p. 3375-3379.
6. Andersen, A., et al., Structure and dynamics of polysulfide clusters in a nonaqueous solvent mixture of 1, 3-dioxolane and 1, 2-dimethoxyethane. Chemistry of Materials, 2019. 31(7): p. 2308-2319.

Rechargeable all solid-state lithium-sulfur batteries (ASLSBs) hold tremendous promise for use in electric vehicles due to their high theoretical capacity (~1675 Ah/kg; ~5-6 times higher than state-of-the-art Li-ion batteries), high promised energy density (400 Wh/kg) and cycle life, and low costs owing to natural abundance of sulfur. Lithium argyrodites (e.g., Li₇PS₆, and its halogen-doped derivatives) have emerged as a lucrative class of solid-state electrolytes (SSEs) for ASLSBs; owing to their high Li-ion conductivity (~10^-3 S/cm), good elastic stiffness (~30 GPa), and low flammability. Despite their promise, ASLSBs (even using argyrodite SSEs) remain far from commercialization due to paucity of fundamental understanding of physical factors underlying key electrochemical phenomenon including ion-conduction, charge transport, structural evolution, and interfacial reactions (e.g., dendrite growth, electrolyte decomposition etc.). Specifically, such knowledge gap has thwarted development of high-performance SSEs that provide rare combination of superionic Li conduction and interfacial stability. Here, we integrate ab initio molecular dynamics (AIMD) simulations, density functional theory (DFT) calculations, liquid-phase synthesis and characterization methods to advance the understanding of lithium-ion conduction and interfacial reactions in fluorine (F) containing lithium argyrodites. The choice of F-containing argyrodites was motivated by recent reports indicating that F (or LiF) could suppress formation of deleterious dendrites at the Li-anode. Our AIMD simulations showed that lithium argyrodite doped with F alone, Li₆PS₅F possesses much lower Li-ion conductivity (~0.32 mS/cm) as compared to state-of-the-art Li₆PS₅Cl electrolytes (~0.9 mS/cm). Interestingly, we found that simultaneous doping of argyrodites with two dissimilar halogens (e.g., F and Cl) results in much higher Li⁺ ion conductivity as compared to the counterparts doped with one halogen. For instance, we found that the Li⁺ ion conductivity of Li₆PS₅F_0.5Cl_0.5 electrolyte (2 mS/cm) is ~2.2 times higher than that of Li₆PS₅Cl (0.9 mS/cm) and ~6 times that of Li₆PS₅F (0.32 mS/cm); consistent with our experimental measurements. Careful analysis of our AIMD trajectories, and DFT calculations indicate that this enhanced Li-ion diffusion can be attributed to the unique defect structure stabilized by F ions, and halogen-mediated hopping of Li ions across the SSE. More importantly, we found that such mixed halogen doping enhances stability of SSE against Li-anode, as evidenced by our galvanostatic Li stripping/plating experiments that show flat voltage profile and low polarization voltage for Li₆PS₅F_0.5Cl_0.5 even at current density of 0.2 mA/cm²; in contrast, the cells with Li₆PS₅F and Li₆PS₅Cl electrolytes die at 0.05 mA/cm² and 0.1 mA/cm² respectively. AIMD simulations show that this enhanced interfacial stability in Li₆PS₅F_0.5Cl_0.5 SSE is enabled by formation of strong Li-F bonds at the anode/electrolyte interface, which reduce the dissociation of PS₄^3- tetrahedra. These results will be discussed in the context of developing novel solid-state electrolytes for high-performance all solid-state lithium-sulfur batteries.

Interfacial charge-transfer processes in batteries govern their performance and stability, where both electronics and ionics play crucial roles. However, the electronic and atomistic understanding of such interface processes is still difficult in the conventional computational as well as experimental approaches. From theoretical side, we have addressed this issue based on the first-principles density functional theory (DFT) calculations of the explicit solid-solid interfaces. Firstly, we explored techniques to obtain the energetically probable disordered solid-solid interfaces. If soft materials consist of the interface, one can use the DFT molecular dynamics (MD) sampling. We applied this procedure for the Li-intercalated graphite LiC_x anode / Li₂CO₃ inorganic solid electrolyte interphase (SEI) [1]. For the solid electrolyte (SE) - cathode interfaces in all-solid-state batteries (ASSBs), we developed, “Heterogeneous Interface (HI)-CALYPSO method”, allowing systematic sampling of heterogeneous solid-solid interface structures, based on the particle swarm optimization (PSO) [2].

With the obtained probable interfaces, we calculated the free/potential energy profiles of the Li-ion transfer across the interfaces. For the LiC_x anode / Li₂CO₃ SEI interface, we demonstrated that the Li-ion energy (a sort of electrochemical potential) in the anode is much higher than in the SEI [1]. The energy difference is attributed to the negative-charge environment surrounding the Li-ions. The obtained energy profiles correspond to the discharge process where Li-ions flow into the electrolyte side. To explain the charging process, we proposed that the energy profiles are modulated by the electric field by the excess electrons. These results also suggest a continuous energy profile model for Li-ion in the whole battery. We have also examined the energy profiles across the β-Li₃PS₄ (LPS) SE / LiCoO₂ (LCO) cathode interfaces in ASSB, which supports the current profile model. Though several hypotheses still exist, the present results will give a novel and useful insight into the Li-ion transport in batteries.

The work was partly supported by MEXT as “Program for Promoting Researches on the Supercomputer Fugaku” (Fugaku Battery & Fuel Cell Project) and JSPS KAKENHI “Interface IONICS” Grant Number JP19H05815.

References: [1] T. Baba, Y. Tateyama et al., Phys. Chem. Chem. Phys. 22, 10764 (2020). [2] B. Gao, R. Jalem, Y. Ma, Y. Tateyama, Chem. Mater. 32, 85 (2020).

The inherent reactivity of magnesium metal toward electrolytes promotes the formation of insulating surface films which inhibit ionic charge transfer at the anode-electrolyte interface of magnesium metal batteries. It remains a significant challenge to realize reversibility with high energy efficiency due to the exceedingly high overpotential to plate and strip the anode at high current densities. Here, we will discuss recent efforts in our group to control interphase generation at the magnesium anode-electrolyte interface with de novo designed materials. We find that tailoring the solvation environment of magnesium in the interphase is a critical factor and explains both successes and failures in rechargeable cells. We further find that both structure and dynamics play important roles, which we elucidate by comparing molecular dynamics simulations with Raman, IR, and X-ray spectroscopic studies as well as X-ray scattering. Together, these colective insights feed back into the design of artificial interphases, which have decidedly different design rules than those that are used in alkali metal batteries (i.e., Li-metal, Na-metal, or K-metal). We will also highlight how computational materials design and high-throughput screens becomes indispensible tools to quickly navigate important structure–transport relationships in artificial interphses for magnesium metal batteries.

Sodium-nickel chloride batteries are based on abundant, non-critical raw materials (namely rock salt, nickel, and alumina). These batteries have a proven track record for backup power applications, but also show great potential for large-scale stationary electricity storage. To enable increased deployment of this battery technology, charge and discharge rate capabilities need to be improved (>1C), and cell production cost needs to be reduced.
While lithium-ion battery cells are currently developed towards generation 3b, the current sodium-nickel chloride cell design stems from the early 1990s. In this study, we assess the rate limiting processes in state-of-the-art sodium-nickel chloride cells and propose measures to improve their performance. We deconvolute the battery's performance into anode and cathode processes in order to develop next-generation sodium-nickel chloride batteries with enhanced cell design and chemistry. By these measures, we target to improve nominal discharge current rates from currently C/3 to 1C (200 mA/cm²), and maximum pulse discharge rates from currently 1.5C to 3C (600 mA/cm²) [1].
On the anode side, we investigate plating and stripping of liquid sodium metal from a ceramic Na-β"-alumina electrolyte at 250 °C, applying a specially designed high-temperature electrochemical cell. Employment of a porous carbon electrode coating allows us to (1) prevent dewetting of plated liquid sodium from, and (2) to supply liquid sodium upon stripping to, the sodiophobic Na-β"-alumina surface. By this measure we are able to eliminate the mass transport limitations at the sodium/ Na-β"-alumina interface and demonstrate current densities above 1000 mA/cm² at cumulative plated capacity of 10 Ah/cm² without dendrite formation [2]. These values are two orders of magnitude higher than the corresponding values measured at room temperature [3] and exceed practical continuous current densities by a factor of 5.
On the cathode side, we characterize the chlorination and de-chlorination reaction of nickel by cyclic voltammetry, galvanostaic techniques, and electrochemical impedance spectroscopy. We deconvolute the cathode behavior into single processes by dedicated experiments. We identify the rate-limiting processes and link the overall electrochemical performace of the battery to the local degree of chlorination. By enhancing the general understanding of the working principles in sodium-nickel chloride cells, we strive to enhance cycle-life and increase charge and discharge rates for next-generation sodium-nickel chloride batteries.


[1] M. V.F. Heinz, G. Graeber, D. Landmann, C. Battaglia, J. Power Sources 2020, 465, 228268
[2] D. Landmann, G. Graeber, M. V. F. Heinz, C. Battaglia, Materials Today Energy 2020, 18, 100515
[3] M.-C. Bay, M. Wang, R. Grissa, M. V. F. Heinz, J. Sakamoto, C. Battaglia, Adv. Energy Mater. 2019, 201902889

Until now, a key barrier toward realizing a high energy density Mg²⁺ battery has been the limited understanding of mechanisms governing multivalent metal electrodeposition. This, compounded with recent observations of Mg dendrites, highlights the need for better fundamental insight into multivalent systems. We present a comprehensive study of electrodeposition in practical coin cell configurations to evaluate the mechanisms of growth from common Mg(TFSI)₂-based electrolytes. Our findings indicate a transition from charge transfer-limited to diffusion-limited electrodeposition processes that govern the morphological evolution of Mg deposits. We observe the signature of cell shorting under a wide range of current densities which we attribute to 3-D hemispherical growth of Mg deposits that form under mixed diffusion and kinetic control and are distinguished from traditional fractal dendrites. Our results highlight synergy with classical electrochemical theories for growth and lay groundwork for future approaches to achieve stable electroplated multivalent metal electrodes.

High-energy density batteries are the critical component which could enable electric aircraft and more powerful portable devices. The Li metal anode offers the highest theoretical specific capacity among the available anode materials. The most significant barrier to the development of secondary Li-metal batteries is the dendritic electrodeposition of Li during the battery charging process. In this presentation, we will characterize the onset time of dendritic growth that occurs when charging Li-metal anodes. Using a combination of electrochemical techniques and optical microscopy, we observe that Li dendrites initiate at a time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. The dendrite onset time (τ_onset) is shown to increase with increasing temperature and decrease with increasing current density and initial solid electrolyte interphase (SEI) thickness. These observations form the basis of an analytical transport model wherein the Li⁺ concentration available for Li plating decreases gradually within the SEI as the layer becomes thicker during electrodeposition. At t = τ_onset, the mobile Li⁺ concentration at the Li–SEI interface approaches zero. Due to this Li⁺ concentration depletion, the roughness of the Li electrode amplifies, eventually producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for Li plating. Model predictions of how τ_onset varies with current density, initial SEI thickness, and temperature are found to be in qualitative agreement with experimental observations. The analytical model explains mechanistically how transport limitations within the SEI control the onset time of dendrites during Li electrodeposition. Our results highlight the need for stable SEI materials that suppress or eliminate solid-state Li⁺ concentration depletion, which is shown herein to be the key process controlling the formation of dendrites during Li-metal battery charging.

Lithium-based energy storage system has faced a critical limit of gravimetric/volumetric capacity in commercial graphite anode (340 mAh g^-1). Lithium metal is a holy grail for the next generation anode with high specific capacity (3850 mAh g^-1), but still has big challenges caused by lithium dendrites and large volume changes particularly at high current density and capacity. Three-dimensional (3D) lithium host materials may compensate for the volume change with pores in the host material but limited lithium-ion diffusion through porous media has resulted in clogging at the pore inlet and thereby initiated lithium dendrite growth on the surface. While artificial solid-electrolyte interphase (SEI) coated lithium metal anode could avoid lithium dendrite growth on the surface but repeated large volume change may cause the detachment of the SEI layer. In this study, a two-dimensional metal-organic framework (MOF) has been attached to carboxylated CNTs via strong coordination bonding. We observed that MOF-layered 3D CNT scaffolds stored a large amount of lithium metal without dendrite growth and volume expansion until a high capacity of 24 mAh cm^-2 at a high current density of 8mA cm^-2. With the utilization of inner void space to store lithium, high volumetric capacity (~1000 mAh cm^-3) and gravimetric capacity (~1690 mAh g^-1) was achieved, which are much higher than those of commercial graphite anode (570 mAh cm^-3 and ~340 mAh g^-1). This study discloses a promising approach for a high-energy-density anode with lithium metal, which has been arduous in utilizing lithium metal for Li batteries in practice.

Managing electrochemical interfaces in molten sodium batteries is key to achieving high battery performance. Traditional molten sodium batteries operate a high temperature (~300 °C) which facilitates charge transfer at the interfaces between molten sodium and solid electrolyte, solid electrolyte and catholyte, and catholyte and current collector. Dramatically lowering the operating temperature to near the melting temperature of sodium (97.8 °C) introduces substantial charge transfer resistances at these key interfaces. To enable high battery efficiency, longevity, and resiliency, the molten sodium battery must be re-engineered from the bottom up to improve charge transfer and lower interfacial resistances. Here, we describe a low-temperature molten sodium battery capable of high performance, long-term operation at the low temperature of 110 °C. This is achieved by the modification of battery interfaces and materials design of the metal-halide sodium iodide molten salt catholyte. The use of engineered Sn-based coatings on the solid electrolyte surface at the interface with molten sodium to increase sodium wetting on NaSICON is shown to dramatically lower interfacial resistance, allowing battery operation at higher current densities. The activation of the cathode current collector to enhance charge transfer at the catholyte-current collector interface further reduces resistance at this interface, enabling high cyclability of the battery. Materials design of the metal-halide sodium iodide molten salt catholyte results in further improvements in interfacial resistance, resulting in a high-performance, low-temperature molten sodium battery of great promise for large-scale electrochemical energy storage. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & 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.

Development of a new class of redox active, fully inorganic, low temperature (c.a. 100 °C) molten salt electrolytes is necessary for the continued development of safe, low cost sodium battery technologies. The discussed, redox active, NaI-AlBr₃ system demonstrates many physical properties that are desirable for the use in a battery system. These properties have been investigated and experimentally evaluated with focus to energy storage considerations, including the conductivity and melting point of various compositions at different temperatures allowing the construction of a simple phase diagram. The electrochemical properties of the various molten salt composition showed lower than expected iodide oxidation current density along with variable diffusion coefficients of reactant. Choice of electrode material was found to significantly affect the reaction kinetics, where carbon is found to hinder reaction rate as compared to tungsten, potentially dictating battery cell engineering and cost.


This work was supported by the U.S. Department of Energy’s Office of Electricity through the Energy Storage Research Program, managed by Dr. Imre Gyuk. 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. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

During the last decade, the (dis)charge behaviour of LiFePO₄ (LFP) particles has been widely studied. While LFP typically charges in a phase separation reaction forming LiFePO₄ and FePO₄-rich domains, these domains vanish into a single Li_xFePO₄ solid solution at faster rates or smaller particle size.¹ Using phase-field modelling, an involvement of the interface has been proposed where autocatalytic reactions and resistances may affect the LFP charging dynamics and hence the rate-limiting steps.^2,3 However, due to the lack of understanding of the LiFePO₄ interfaces, existing experiments do not suffice to validate this model.

Here, we rationalize the interface dependent delithiation mechanism in LFP particles by introducing coatings that affect ion dynamics and hence the resistances.⁴ With a combination of electronic structure calculations and ab-intio molecular dynamics modelling, we show that insulating oxide coatings such as ZnO hamper lithium ion interface diffusion, whereas carbon coatings lead to faster Li ion transport. We validate our models by synthesizing coated LiFePO₄ nanoparticles and measuring their electronic structure using ultraviolet absorption and x-ray photoemission spectroscopy. Measuring lithium ion interface diffusivity on the coated LFP nanoparticles via muon spin spectroscopy confirms our calculations.⁵ Finally, using operando x-ray diffraction, we can track interface dependent changes in the delithiation behaviour. Based on our findings, we offer a rational design strategy for interfaces in lithium ion batteries that homogenize the current density and reduce the strain throughout the battery materials providing longer battery lifetime at increased rate capabilities.

References
[1] Y. Li, Solid State Ionics 323, 142 (2018).
[2] N. Nadkarni et al., Phys. Rev. Mater. 2, 8, 85406 (2018).
[3] M. Z. Bazant, Faraday Discuss. 199, 423-463 (2017).
[4] P. Benedek et al., In Preparation.
[5] P. Benedek et al., ACS Appl. Mater. Interfaces 12, 14, 16243 (2020).

Further development in the field of modern battery materials requires advanced characterization techniques that can yield insights into the atomic and chemical processes occurring during charging and discharging. Therefore, over the last years, a variety of in situ/operando methods were developed, probing the transfer of ions and electrons mainly at the electrode-electrolyte interface [1,2]. To achieve a complete picture of the processes in the electrodes, additional bulk sensitive methods, probing the whole volume of the material, would be beneficial. By developing an operando SQUID magnetometry technique, our group has established such a method, which enables a continuous monitoring of the oxidation states of the transition metal ions and therefore an identification of the redox active ions [3]. In this talk, first the electrochemical cell that enables operando measurements in a SQUID magnetometer will be presented and then an overview about recent results obtained with this new characterization technique will be given.

Measurements of the variation of the magnetic susceptibility of the technologically important class of NMC cathode materials have shown that different charge transfer processes occur for the two compositions LiNi_0.33Mn_0.33Co_0.33O₂ and LiNi_0.6Mn_0.6Co_0.2O₂. For Li_xNi_0.33Mn_0.33Co_0.33O₂ first Ni acts as redox active ion and changes its oxidation state stepwise from Ni²⁺ to Ni³⁺ and then from Ni³⁺ to Ni⁴⁺. Upon extracting more than 66% of Li, a simultaneous oxidation of Co³⁺ and O^2- ions was observed that is associated with irreversible capacity losses [4]. The charge compensation process for Li_xNi_0.6Mn_0.6Co_0.2O₂ could also be divided into three steps. First a simultaneous oxidation of Ni²⁺ and Co²⁺ (1 ≥ x > 0.53), followed by a sole Ni³⁺ oxidation (0.53 > x > 0.21), and finally by a contribution of Ni³⁺ and Co³⁺ or by Co³⁺ and O^2- (x < 0.21) [5].

The potential of the developed operando technique will also be demonstrated for post Li-ion batteries. Studies on NaV₂(PO₄)₃ cathodes revealed that vanadium is the only ion undergoing oxidation and reduction upon battery operation and that parasitic side reactions take place in the first charging cycle [6].

All in all, we could proof that operando magnetic susceptibility measurements can serve as a sensitive fingerprint for the charge compensation processes taking place in Li-ion and Na-ion battery cathodes. Therefore, this technique is highly suitably as complementary method for the characterization of the electron transfer reactions at electrode interfaces.

[1] M. G. Boebinger et al., ACS Energy Lett. 5 (2020) 335
[2] D. Liu et al., Adv. Mater. 31 (2019) 1806620
[3] S. Topolovec et al., J. Solid State Electrochem. 20 (2016) 1491
[4] G. Klinser et al., Appl. Phys. Lett. 109 (2016) 213901
[5] G. Klinser et al., J. Power Sources 396 (2018) 791
[6] G. Klinser et al., Phys. Chem. Chem. Phys. 21 (2019) 20151

The solid electrolyte interphase (SEI) is known to have a direct impact on sodium ion battery performance. The composition of the SEI has been shown to have a compositional dependence upon electrolyte, cycling time, anode material, and sodium salt. For hard carbon anodes with organic solvent and NaPF₆ there are a number of common features. These involve the fluoridation of the hard carbon surface suggested to result from the salt dissociation, in addition the breakdown of the electrolyte molecules can result in carboxyl, carbonyl and carbonate motifs being observed, together with both inorganic and organic SEI constituents.^1,2 This make the atomic scale understanding of the anode electrolyte interphase, and its influence on both charge transfer and electrochemical performance challenging.
In this study, we combine atomic scale computational modelling with experimental characterisation of anode electrolyte interphases to investigate the structure, charge transfer, and electronic processes at the hard carbon anode electrolyte interphase in Na-ion batteries.^3,4 Hard carbons are complex disordered carbon structures with randomly oriented graphene sheets, closed and open pore systems, and turbostratically stacked graphitic stacks. Adding to their complexity, these anode materials are also prone to defects and oxygen functionalities. This complex anode material structure leads a plethora of different anode electrolyte interphase structures, which all can have different influence on the electrochemical performance.^4–7 Here, we construct, based on experimental guidance, a number of different interphase models to capture the effect of anode surface termination, roughness, defects, functional groups, pore size, and combinations of these on the electrochemical performance and charge transfer across the electrolyte anode interphase. Through a combination of ab initio molecular dynamics, and density functional theory simulations, the electronic structure, ionic diffusion, and electrolyte dissociation products can be investigated. These simulations showed, using an organic solvent based electrolyte that the local structure of the anode surface directly influences the solvent molecule breakdown and interphase immobile solvent molecule layer. Introducing carbon vacancy defects, and oxygen functionalities, together with different pore systems were further shown to affect the organic solvent molecule breakdown and lead to irreversible capacity loss. Similarly, the sodium ion transfer from the electrolyte bulk to the anode (both through intercalation and surface adsorption) was probed at different interphases to explore the effect of morphology, the liquid phase composition, and electronic structure on charge transfer.

Acknowledgment
The financial support from EPSRC (Engineering and Physical Sciences Council) under the grant number EP/M027066/1, and EP/R021554/2, is acknowledged.

References
¹ G.G. Eshetu, T. Diemant, M. Hekmatfar, S. Grugeon, R.J. Behm, S. Laruelle, M. Armand, and S. Passerini, Nano Energy 55, 327 (2019).
² C. Bommier and X. Ji, Small 14, 1703576 (2018).
³ H. Au, H. Alptekin, A.C.S. Jensen, E. Olsson, C.A. O’keefe, T. Smith, M. Crespo-Ribadeneyra, T.F. Headen, C.P. Grey, Q. Cai, A.J. Drew, and M.M.-M. Titirici, Energy Environ. Sci. (2020).
⁴ H. Alptekin, H. Au, A.C. Jensen, E. Olsson, M. Goktas, T.F. Headen, P. Adelhelm, Q. Cai, A.J. Drew, M.-M. Titirici, and A.C. Jensen, ACS Appl. Energy Mater. acsaem.0c01614 (2020).
⁵ E. Olsson, J. Cottom, H. Au, Z. Guo, A.C.S. Jensen, H. Alptekin, A.J. Drew, M.-M. Titirici, and Q. Cai, Adv. Funct. Mater. 30, 1908209 (2020).
⁶ E. Olsson, G. Chai, M. Dove, and Q. Cai, Nanoscale 11, 5274 (2019).
⁷ A.C.S. Jensen, E. Olsson, H. Au, H. Alptekin, Z. Yang, S. Cottrell, K. Yokoyama, Q. Cai, M.-M. Titirici, and A.J. Drew, J. Mater. Chem. A 8, 743 (2020).

Transportation accounts for 23% of energy-related carbon dioxide emissions and electrification is a pathway toward ameliorating these growing challenges. All solid state batteries could potentially address the safety and driving range requirements necessary for widespread adoption of electric vehicles. However, the power densities of all-solid state batteries are limited because of ineffective ion transport at solid|solid interfaces. New insight into the governing physics that occur at intrinsic and extrinsic interfaces are critical for developing engineering strategies for the next generation of energy dense batteries. However, buried solid|solid interfaces are notoriously difficult to observe with traditional bench-top and lab-scale experiments. This talk will discuss opportunities for tracking phenomena and mechanisms in all solid state batteries in-situ using advanced synchrotron techniques. Synchrotron techniques that combine reciprocal and real space techniques are capable of tracking multi-scale structural phenomena from the nano- to meso-scale. This talk will discuss the role microstructure plays on transport and interfacial properties that govern adhesion. Quantification of salient descriptors of structure in solid state batteries is critical for understanding the mechanochemical nature of all solid state batteries.

Rechargeable multivalent batteries have attracted extensive research due to their low cost and higher energy density compared to Li-ion batteries. However, most cathode materials have several major challenges such as slow charge/discharge cycle and irreversible phase transitions that hinder their practical use as multivalent cathodes. Therefore, identifying and synthesizing the ideal electrode material is therefore essential in advancing multivalent battery technology. Furthermore, fundamental nature of the diffusion mechanism and its effects on the host structure is necessary to improve the performance of these systems.
Manganese-based spinel oxides are a particular class of promising materials for multivalent electrodes due to their increased energy density and higher specific capacity. However, their performance is highly dependent on their synthesis conditions and phase purity due to the existence of a multiple Manganese oxidation states (Mn²⁺, Mn³⁺, Mn⁴⁺). In this presentation, we highlight the use of epitaxial thin films, grown using Molecular Beam Epitaxy (MBE), a technique that provides precise control over interface structure and thickness, to study host structures for multivalent cathodes. We begin with epitaxial Fe₃O₄ thin film as a model system to highlight the utility of well-defined interfaces and smooth surfaces to perform fundamental studies in cation diffusion. In addition, we investigate the electronic and structural properties of Mn₃O₄ spinel epitaxial films. In particular, we highlight the phase selective growth of phase pure Mn₃O₄ is highly dependent on the growth parameters such as flux rate and oxygen partial pressure. Furthermore, to understand the evolution of Mn₃O₄, thermal annealing of the material is performed to promote cation diffusion and structural transitions. These changes are monitored using high-resolution in-situ and ex-situ characterization techniques such as x-ray photoelectron spectroscopy, x-ray absorption near edge structure and x-ray diffraction.

Increasing the energy storage capacity of portable, rechargeable Li-ion batteries would find the most gains by implementing a Li-metal anode. Li-metal anodes at high-charging rates are needed for high power applications, such as for electric vehicles. However, Li-metal anodes experience significant capacity losses and limited lifetimes as a result of the uncontrolled Li morphology during plating and stripping, especially at high-charge rate. The resultant morphology is commonly seen as high aspect ratio grains, which have a large surface area that can react with the electrolyte and form the solid electrolyte interphase (SEI) layer. The SEI layer is a determinant factor of the performance of the Li anode, as cyclic Li plating and stripping steps produce significant surface volume changes that present fresh surfaces for further SEI formation. This causes the SEI to continuously grow over the lifetime of the battery. This is a barrier for implementation of Li-metal anodes into commercial batteries, as the continuous formation of SEI consumes the Li inventory (resulting in capacity loss) and electrolyte (reducing pathways for Li-ion transport through the cell).
The visualization of the Li/SEI/electrolyte interfaces allows for understanding the co-dependence of the different stacked materials within a battery and the failure mechanisms that result from effects of stack pressure, electrolyte, separator, and cycling rate. Cryogenic electron microscopy techniques allow for the intact characterization of solid-liquid interfaces from atomic-scale resolution [1], up to complete cross-sectional visualization of frozen coin-cell batteries. We present analysis of SEI/Li metal interfaces using cryogenic focused ion beam (FIB) cross-sectional imaging in a scanning electron microscope, and the subsequent lamella preparation for cryo-scanning/transmission electron microscopy analysis [2]. Additionally, using a cryo stage on a laser plasma-FIB [3], we visualize a complete coin cell in cross section at representative areas of a millimeter in width. Both structural mapping methods are combined with elemental mapping using x-ray spectroscopy for identification of the electrode/electrolyte interfaces. We propose a mechanism for Li plating through a separator based on our analysis, as it is known that Li metal itself does not have the mechanical integrity to puncture the polymeric material unassisted [4,5].
References:
[1] Y Li et al., Science 358 (2017), p. 506.
[2] MJ Zachman et al., Nature 560 (2018), p. 345.
[3] MP Echlin et al., Materials Characterization 100 (2015), p. 1.
[4] CD Fincher et al., Acta Materialia 186 (2020), p. 215.
[5] This work was funded by Sandia National Laboratories’ Laboratory Directed Research and Development program. It was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. 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-NA-0003525.

While there is considerable interest in multivalent batteries, calcium has not attracted much attention compared to magnesium or aluminum until a recent work showing that calcium can be plated and stripped at room temperature with good capacities and stabilities. Additionally, because of the higher mobility of Ca²⁺ compared to that of Mg²⁺ as a result of the lower charge density of Ca²⁺, the interest in calcium batteries has been growing rapidly. The problem now inhibiting the development of calcium batteries is the understanding of the solvation and charge transfer mechanism at the electrode/electrolyte interface and how different types of anions will affect the calcium solvation at this interface while under in-situ/operando electrochemical conditions. It has been reported that when Ca(BH₄)₂ is substituted by the more weakly coordinated calcium hexafluoroisopropoxyborate (Ca(BHFIP)₂), ionic conductivity increases by an order of magnitude, which demonstrates the strong coordination of BH₄^- compared to Ca²⁺ in THF. To probe the calcium coordination chemistry difference at the electrode/electrolyte interface as a function of the type of anions, total electron yield (TEY) mode soft X-ray absorption spectroscopy (XAS) was employed under operando electrochemical conditions. It was observed that the Ca(BHFIP)₂ in THF shows a small shoulder on the right side of the L₂-edge and a pre-edge peak on the left side of the L₂-edge peak. A less prominent pre-edge peak is also observed on the left side of the L₃-edge peak. These signatures become more prominent when the potential is swept negatively and are not reversible when the potential is swept positively in a cyclic voltammetry cycle. On the other hand, the Ca(BH₄)₂ in THF is more reversible, and no shoulder peak on the right side of the L₂-edge peak was observed. The detailed solvation structure of the calcium still needs to be resolved with the DFT calculations but such TEY spectra indicate their differences in the calcium coordination chemistry at the electrode/electrolyte interface as a result of the anions and it will guide the future development of electrolytes for calcium batteries.

The pursuit of eco-friendly battery technologies that meet the industrial requirements for portable, stationary and automotive applications in terms of safety, cost, energy density, performance reliability and sustainability has propelled research efforts toward battery chemistries beyond Li-ion. Thus far, none of the battery chemistries that promise high energy densities such as those based on Li metal (Li-S, Li-air) and multivalent batteries such as Mg have demonstrated their potential owing to intrinsic material challenges. For example, Li-S and Li-air suffer from complex cathode conversion reactions and dendrite prone Li metal. On the other hand, issues in multivalent batteries like Mg stem from the high polarizability of Mg²⁺ that results in strong interaction with solvents and host solid state structures. ^[1,2,3,4] This unfortunate phenomena has often resulted in sluggish diffusion kinetics and high overpotentials, which undermine the practicality of Mg batteries. Herein, we will discuss how fundamental studies we conducted inspired a potential path forward towards overcoming these challenges and allowed us to discover new possibilities towards enabling Mg rechargeable batteries.
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References
[1] Choi J. W. Aurbach D. Nature Reviews Materials 1, 2016, 16013, 1.
[2] Mohtadi R., Orimo S., Nature Reviews Materials, 2016, 2,16091, 1311.
[3] Mohtadi R., Mizuno F. Beilstein J. Nanotechnol. 2014, 5, 1291.
[4] Kar M., Tutusaus O., MacFarlane D.R., Mohtadi R., Energy & Environmental Science, 2019, 12, 566.

Rechargeable batteries based on divalent Ca anodes have attracted considerable research attention due to their high theoretical energy densities, reduction potentials approaching lithium’s, high crustal abundance, and low propensity for dendrite formation [1,2]. The highly reductive nature of calcium means that a solid-electrolyte interphase (SEI) layer spontaneously forms to protect the growing deposit from parasitic reactions. Many SEI components formed from typical calcium electrolytes (e.g. CaF₂) are poor ionic conductors [3], leading to failure upon cycling [4], but a select few (e.g. borate species, CaH₂) support limited calcium transport, enabling more extensive cycling [3,5]. To date, the formation mechanisms and transport properties of interphases in Ca electrodeposits have been sparsely studied.

In this contribution, we present multimodal characterization of electrodeposited and cycled Ca metal from Ca(BH₄)₂-THF and related electrolytes, with emphasis on the properties of native interphases. We identify primary and secondary phases in electrodeposits using techniques including XRD, SEM, and ToF-SIMS. We discuss the transport properties of key interphase components as determined through electrochemical methods. We then show results of characterization of thin calcium lamellae including TEM and STEM imaging, selected-area diffraction, and EDS and EELS mapping, revealing the chemical identity, nano-scale structure, and spatial distribution of interphases, while drawing connections to bulk microstructure when possible. We also discuss the use of cryogenic techniques to freeze the native electrolyte in place, allowing for insight into an evolving calcium deposit under realistic battery operating conditions.

Our results are an important step towards the rational design and engineering of favorable protective interphases required for high efficiency plating and stripping of calcium metal anodes [6].

References:
[1] M. E. Arroyo-de Dompablo et al. Chem. Rev. 2020, 120(14), 6331-6357.
[2] A. M. Melemed et al. Batteries and Supercaps 2020, 3(7), 570-580.
[3] J. Forero-Saboya et al. Energy Env. Sci. 2020, 13(10), 3423-3431.
[4] A. Shyamsunder et al. ACS Energy Lett. 2019, 4(9), 2271-2276.
[5] D. Wang et al. Nat. Mater. 2018, 17(1), 16-20.
[6] This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

The objective in electrocatalysis and energy storage is to correlate electrochemical activity with nanostructured electrochemical interfaces (electrodes) [1]. However, it is challenging to quantify the heterogeneity of electrode structures or study the local structure-activity relationship for these interfaces using conventional macroscopic electrochemical techniques. Scanning electrochemical cell microscopy (SECCM) is a new nanoelectrochemical scanning probe technique designed to investigate the local electrochemical properties of electrode surfaces. In this study, the electrochemically reversible [Ru (NH₃)₆]^3+/2+ electron transfer process at a highly ordered pyrolytic graphite (HOPG) surface was recorded with SECCM technique using Park NX12 AFM system. A single barrel glass nanopipette with a Ag/AgCl quasi-reference counter electrode(QRCE) is utilized [2]. Using previous successful experience in commercializing pipette-based electrochemical microscopy [3], Park Systems’s hardware and software enable localized nanoscopic cyclic voltammetry each time the meniscus contacts the surface. The redox process at the HOPG surface is detected with high reproducibility and robustness, with a current limit as minute as a few pA. Position dependence electrochemical current mapping is demonstrated. These results suggest that the effectiveness of Park Systems’s commercial SECCM option for quantitative electroanalysis at the nanoscale. This capability could also facilitate the rational design of functional electromaterials with potential applications in energy storage (battery) studies and corrosion research.

[1] Guell, A. G., Ebejer, N., Snowden, M. E., Macpherson, J. V., & Unwin, P. R. (2012). Structural correlations in heterogeneous electron transfer at monolayer and multilayer graphene electrodes. Journal of the American Chemical Society, 134(17), 7258-7261.
[2]Gao, R., Edwards, M. A., Qiu, Y., Barman, K., & White, H. S. (2020). Visualization of Hydrogen Evolution at Individual Platinum Nanoparticles at a Buried Interface. Journal of the American Chemical Society, 142(19), 8890-8896.
[3] Shi, W., Goo, D., Jung, G., Pascual, G., Kim, B., & Lee, K. Simultaneous Topographical and Electrochemical Mapping using Scanning Ion Conductance Microscopy–Scanning Electrochemical Microscopy (SICM-SECM).

Carbon-fiber electrodes exhibit many desirable material properties including conductivity, porosity, and chemical stability, and, accordingly, are employed in a range of electrochemical technologies.¹ These electrodes are particularly important in redox flow battery (RFB) systems, as they support multiple critical functions including providing surface area for electrochemical reactions, distributing liquid electrolytes, and conducting electrons and heat. While commercial materials possess suitable permeability and conductivity, the performance of pristine substrates is limited and oxidative treatment prior to use is common practice.^2,3 Partial oxidation of the electrode surface has been shown to introduce a range of oxygen functional groups, which improve hydrophilicity and, in some cases, catalyze redox reactions, as well as to increase surface area available for the electrochemical reactions.^4,5 Both effects are posited to improve electrode performance but their relative importance and effectiveness remains unclear.⁶

Here, we critically assess the nature of the surface area generated by thermal pretreatment in air and its role on RFB electrode performance. Using Freundenberg H23 as a model substrate, we systematically vary pretreatment time and temperature to create different surface morphologies and develop structure-function relations. First, we use Brauner Emmanuel Teller gas adsorption (e.g., N₂, Kr, Ar/CO₂) and mercury porosimetry to estimate physical surface area and pore size distribution. In parallel, we use voltammetry and impedance spectroscopy to determine electrochemically active surface area with different electrolyte compositions. In general, we find that only a fraction of the physical surface area is accessible for electrochemical redox reactions due, in large part, to the nanoscopic dimensions of the added surface area. Second, we develop a simple mathematical model to assess the effectiveness of the available surface area of carbon fibers for Faradaic reactions. We find that, under typical RFB operating conditions, external mass transfer limitations govern performance, limiting the utilization of recessed areas generated during pretreatment. Ultimately, these results highlight the potential limits of performance improvement strategies based on increasing surface area of fibrous substrates and motivate new approaches for electrode development.

Acknowledgements
The authors acknowledge the financial support of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the United States Department of Energy. The author also acknowledges the Center for Nanoscale Systems and the NSF’s National Nanotechnology Infrastructure Network (NNIN) for the use of Nanoscale Analysis facility for electrode property characterization.

References
1. M. H. Chakrabarti et al., Journal of Power Sources, 253, 150–166 (2014).
2. T. J. Rabbow, M. Trampert, P. Pokorny, P. Binder, and A. H. Whitehead, Electrochimica Acta, 173, 17–23 (2015).
3. N. Pour et al., The Journal of Physical Chemistry C, 119, 5311–5318 (2015).
4. B. Sun and M. Skyllas-Kazacos, Electrochim. Acta, 37, 8 (1992).
5. T. J. Rabbow and A. H. Whitehead, Carbon, 111, 782–788 (2017).
6. K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Applied Materials & Interfaces, 10, 44430–44442 (2018).

Deep eutectic solvents (DESs) are emerging as promising electrolytes for electrochemical energy storage applications. Electroactive nitroxide-radical-containing organics can be dissolved in DESs to facilitate redox reactions; however, mechanistic insight of their charge transfer kinetics at the electrode surface is limited. Here, we investigate the mechanism underlying the electrochemical oxidation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO). Using polarization measurements on a platinum rotating disk electrode (RDE) and micro-electrode, we show that the anodic charge transfer coefficient (α) for one-electron transfer oxidations of TEMPO and 4-hydroxy-TEMPO approaches 0.9 in DES as well as in aqueous electrolytes, i.e., a significant deviation from α ≈ 0.5 expected for symmetric redox behavior. To explain this observation, a two-step oxidation mechanism is proposed wherein the nitroxide-containing species undergo fast charge transfer at an electrode surface followed by slow rate-limiting desorption of the adsorbed oxidized species. Numerical simulations are reported to characterize how the proposed two-step mechanism manifests in transient electrochemical behavior of the 4-hydroxy-TEMPO oxidation reaction, and good agreement with experiments is noted. Further evidence, based on voltammetry coupled with gravimetry (e-QCM), will be presented in support of the surface adsorption-desorption phenomena. Ramifications of the mechanistic understanding gained to the design of future high-performance DES electrolytes for energy storage will be discussed.

Long-lasting, high performance redox flow batteries (RFBs) will require robust electrode interfaces capable of enabling thousands of cycles while sustaining high rates of charge transfer. In this context, understanding the mechanisms of charge transfer as affected by processes such as precipitation/adsorption, molecular or electrode degradation, and the distinct reactivity of heterogeneous surface features, seems of great relevance to develop electrodes that ensure sustained RFB performance. We posit that to achieve this understanding, a versatile approach that achieves a quantitative evaluation of electron transfer rates and relates them to these chemical processes and structural attributes is necessary.

In this presentation, we will discuss how we have combined the redox imaging capabilities of scanning electrochemical microscopy (SECM) with other in-situ and ex-situ characterization techniques such as atomic force microscopy (AFM) and secondary-ion mass spectrometry (SIMS), as well as with versatile multi-layer graphene (MLG) electrodes to elucidate reaction attributes of model redox-active dialkoxybenzene derivatives, 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (C1) and 2,3-dimethyl-1,4-dialkoxybenzene (C7).[1] Specifically, during cycling of the redox mediators, we observed the formation of interfacial films with distinct redox and mechanical properties compared to those of cleaved graphitic surfaces, and exclusively during reduction of electrogenerated radical cations. These films modified the rate and distribution of the electron transfer rate constants causing a significant deviation from Butler-Volmer kinetics. However, their impact was also found to be highly dependent on the surface features investigated, which were conveniently probed using the spatially-resolved capabilities of SECM. Bolstered by our findings with C1 and C7, we have also investigated other molecules used as catholytes in RFBs as well as the impact of redox-active concentration on the observed rate constants. The diversity of effects observed through our electrochemical imaging methodology highlights the importance of understanding how heterogeneous reactivity might impact RFB performance.

[1] Watkins, T.; Sarbapalli, D.; Counihan, M.J.; Danis, A.S.; Zhang, J.; Zhang, L.; Zavadil, K.R.; Rodríguez-López, J. A Combined SECM and Electrochemical AFM Approach to Probe Interfacial Processes Affecting Molecular Reactivity at Redox Flow Battery Electrodes. J. Mater. Chem. A. 2020, 8, 15734-15745.

For charge transfer to occur in a redox flow battery the molecules must come in proximity to the electrode surface and because of this the binding energy of the constituent molecules is a crucial aspect of the net charge transfer of the system. In this work the process of computing charged binding energies is discussed along with theoretical ambiguities. The methods of using either cluster or periodic models are compared. The results overall suggest that charged redox flow molecules can bind strongly to a carbon surface. Results also show that binding energies are strongly affected by an applied potential. For cases where spontaneous charge transfer occurs within standard density functional theory simulations, constrained density functional theory is used to get a more physical binding energy. Finally some results of the electronic coupling for diabatic states prepared with constrained density functional theory are presented in the context of Marcus theory.