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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP SIMULATION OF CHARGE TRANSFER REACTION AT THE COMPLEX INTERFACES IN LI-ION BATTERIES The ultimate goal of coupled simulation methods is to understand the structure-function relationship associated with charge transfer and interface evolution. This set of theory-based images shows examples of connecting length scales from Ångstrom to micron and properties including mechanical, structural, chemical, and charge and mass transfer. It highlights the importance of linking multiscale and multiphenomena modeling methods. From left to right, the panels depict electron tunneling through an atomic-layer-deposited oxide film to electrochemically reduced ethylene carbonate molecules, modeled by constrained DFT; DFT tight binding modeling of Li+ transfer from a liquid electrolyte through an inorganic (Li2CO3) interphase layer into a Li metal anode; Li+ transfer from liquid electrolyte into an organic (ethylene dicarbonate) interphase layer; and the structure of a multicomponent solid-electrolyte interphase. The complexity and heterogeneity of the models increase from left to right, along with the level of theory required. More accurate electronic structure methods are needed to deal with charge transfer and Li motion in inorganic solids (e.g., Li2CO3), while more coarse-grained methods such as force field-based molecular dynamics and phase field models are needed to depict liquid-state fluctuations and to identify hotspots at liquid-solid interfaces, mechanical deformations, and multi-component interphase morphologies. The coarse-grained methods, therefore, provide well-equilibrated starting interfacial configurations for accurate electronic structure calculations. In turn, electronic structure calculations provide parameterization for the coarse-grained simulations. The modeling results will help better interpret measurements such as electrochemical impedance spectroscopy. Images from Li et al., Computational exploration of the Li-electrode|electrolyte interface in the presence of a nanometer thick solid-electrolyte interphase layer, Acc. Chem. Res., 2016, 49 (10), 2363-2370; K. Leung et al., Using atomic layer deposition to hinder solvent decomposition in lithium ion batteries: First-principles modeling and experimental studies, J. Amer. Chem. Soc., 2011, 133 (37), 14741; O. Borodin and D. Bedrov, Interfacial structure and dynamics of the lithium alkyl dicarbonate SEI components in contact with the lithium battery electrolyte, J. Phys. Chem. C, 2014, 118, 18362-18371; J. Christensen and J. Newman, A mathematical model for the lithium-ion negative electrode solid electrolyte interphase, J. Electrochem. Soc., 2004, 151, A1977-A1988. e- transfer through SEI DFTB DFT Multi-layer Film Electrode Li+ transfer into/through SEI MD Li+ transfer (rare event) Interphase formation/evolution Phase Field & EIS ReaxFF-MD Increasing SEI complexity Active Material Solution Grain boundary • Amorphous, possibly polymer, outer later • Porous • Partially reduced • Compact, polycrystalline inner layer • Highly reduced The combination of DFT → density functional tight binding → ReaxFF naturally bridges the length scales from sub-nanometer to submicron but has not been widely used for electrochemical energy storage. With these size- bridging methods, parameters used in continuum modeling can be simulated, such as the activity coefficients in real electrolytes with large concentrations of salt. Beyond simulating the operation and degradation of materials for energy storage, these size and length-scale bridging strategies can also be used to simulate material synthesis processes. 30 PRIORITY RESEARCH DIRECTION – 2PDF Image | Next Generation Electrical Energy Storage
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