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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP solvents at the electrified interface (see Figure 3.2.5 for example with electrolyte of LiPF6 salt and ethylene carbonate/dimethyl carbonate solvent).31-33 These techniques can potentially explore the role of grain boundaries and other defects in accelerating transport. Including atomic polarizability in the force field should significantly improve the accuracy of the simulated activation energies, allowing comparison to experiments. Molecular dynamics based on DFT, combined with rigorous statistical mechanical potential-of-mean- force techniques, is more suited for the final step involving Li-ion entry into the anode to “combine” with an electron.34 Advanced computational platforms and algorithms that offer improved processing speed will advance the level of realism that may be preserved in future large-scale simulations. DFT methods, including joint DFT, are particularly suited for elucidating the activation energies associated with SEI formation mechanisms. Proton transfer from solvent to material surfaces upon oxidation of ethylene carbonate molecules in the electrolyte has been predicted even under moderate voltage conditions.26,35 This explains previous experimental observations.36 Interfacial diffusion properties have also been successfully predicted using electronic structure methods. Joint DFT calculations have recently been used to compute activation energies for in-plane motions of Li and Na ad-atoms on minimal, halide-salt models for the SEI in acetonitrile to understand how these species influence transport in the SEI. Notably, activation barriers for Li ad-atom diffusion in SEIs composed of halide salts were found to be substantially lower than for salts, such as Li2CO3, that form spontaneously in the SEI on a lithium anode.2,37,38 Indirect confirmation of these predictions has been reported from experiments in which the stability of Li electrodeposition in SEI enriched in halides salts was compared with deposition in a spontaneously formed SEI.39,40 Electric double layers develop at electrified interfaces in mobile dielectric media,41 and their effects on transport and electrochemical reactions (such as SEI formation) are not well understood. The solvent composition and salt concentrations in the electrolyte at the Helmholtz layer are different from the bulk. Moreover, even the bulk speciation in complex organometallic electrolytes is not well known. Currently, there are only a few studies on this topic.42 Application of the large external fields in energy storage devices may also cause non-equilibrium ion43,44 and solvent distributions, producing gradations in ionic conductivity across an electrolyte. For example, enrichment of ethylene carbonate solvent at the interface and in the Li solvation shell in mixed electrolytes can explain why it is preferentially reduced to form the SEI (Figure 3.2.6). Further, the electrolyte composition has been shown to depend on the electrode potential in multicomponent liquid electrolytes. As the potential is decreased, the interfacial electrolyte layer is enriched with ethylene carbonate while the dimethyl carbonate is depleted.45 Changes in Li+ solvation structure upon application of a potential bias have also been predicted. Manipulation of the double layer structure and interfacial ion composition, through changes in the SEI-formation potential and ion concentration, presents new opportunities for tailoring electrochemical reactions and stability. These effects are not limited to lithium-ion batteries. The highly nontrivial solvation shell of magnesium salts in diglyme/oligomer electrolyte has, for example, been elucidated in combined DFT modeling and measurements.46 Additionally, recent continuum models show that they couple in non-trivial ways to the mechanics of other components in an electrochemical cell (e.g., separator and electrode) to either enhance or exacerbate the stability of metal deposition processes at electrified interfaces.2,47,48 Intriguingly, the inner Helmholtz layer dominated by bis(trifluoromethanesulfonyl)imide anions at high salt concentrations was an important factor in preventing aluminum current collector corrosion.49 Likewise, permanently anchoring a fraction of such anions in a liquid electrolyte has been reported to improve interface conductivity at high potential biases and to stabilize otherwise unstable electrodeposition of metals such as Li, Na, and Al. These considerations give us confidence that rational design of electrochemical interfaces can be used to advantage in managing chemical and physical kinetic processes at interfaces. Fundamentally, the spatial inhomogeneity of the phases (solid-liquid) and reduced dimensionality of the interfacial phase are the key factors that limit the applicability of universal thermodynamic concepts. For inhomogeneous phases, the boundary conditions that determine the initial and end points are not well defined. This, in turn, invokes the details of the process (e.g., its path), which eventually define the details of the interfacial chemistry (otherwise overlooked in bulk-phase thermodynamic studies) as a reflection of the fact that something may happen at the interface and at the interface only. The interfacial speciation and the deformation of the solvation structures of ions in the interfacial regions are manifestations of the same idea.26 98 PANEL 2 REPORTPDF Image | Next Generation Electrical Energy Storage
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