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The above considerations have a profound impact on the atomistic methodology for studying interfacial processes. Any attempts to set up the parameters of the interface (treated atomistically) according to equilibrium thermodynamics will encounter problems. Due to the finite size of the system, idealized conditions, etc., thermodynamic characteristics may not be realizable at all. As a consequence, there will be a problem with comparisons of the computationally estimated parameters with experimentally measured ones. In particular, many discrepancies between theory and experiment in electrochemical deposition of divalent metals are rooted here. Extremely high estimates of activation energies for the first electron transfer hindered by the strongly bound solvation sphere of the divalent ions lead to gigantic overestimates of deposition overpotentials,50 which can result from ignorance of the interfacial inhomogeneity. negative electrode (a) -3.5 VRPZC PZC (b) 0 VRPZC positive electrode (c) +2.5 VRPZC NEXT GENERATION ELECTRICAL ENERGY STORAGE Figure 3.2.6. The interfacial structure of electrolyte (salt of LiPF6 and solvent of ethylene carbonate mixed with dimethyl carbonate) at the negative electrode, point of zero charge (PZC), and positive electrode, calculated from molecular dynamics simulations. Voltages are relative to potential of zero charge (RPZC). From Ref. 45. Electrochemistry, on the other hand, involves intrinsically non-equilibrium electron distributions, which conflict with the ground-state Kohn-Sham DFT formulation. Constant-voltage DFT calculations are, therefore, demanding. They require referencing the Fermi level to some external potential. For the hydrogen fuel cell and electrochemical capacitor applications, this has been achieved by using specialized boundary conditions that combine DFT and implicit solvent-like methods such as the effective medium approach and joint DFT.51-56 As an example, the effective medium method references the metallic electrode Fermi level to the standard hydrogen electrode and is held constant in molecular dynamics simulations with an extended Lagrangian, enabling DFT/ molecular dynamics simulations of potential-dependent interface/interphase electrochemical reactions. Currently adapted potentiostat schemes either introduce the grand-canonical approach at the expense of eliminating the interface per se57 or introduce artificial boundary conditions53,55 that ignore, to some extent, the response of the electrolyte and thus simulate non-equilibrium situations. Mechanistic Understanding of Interfaces and Interphases: The structure and function of the SEI in Li-ion battery anodes have been under investigation for over 30 years, and a general understanding of the primary components of the SEI has emerged from characterization experiments.58-66 During the initial lithiation cycles, an SEI forms on the anode surface due to the electrochemical instability of electrolyte components, including additives and salt. In favorable situations, the reactions that form the SEI are self-limiting, preventing continuous loss of electrolyte components that form the SEI. An ideal SEI allows interfacial Li-ion transport at rates comparable to what can be achieved in the bulk. It must be mechanically stable and electrically insulating, inhibiting further reduction of the electrolyte. Formation of the SEI is now understood to be one of the most important and fundamental reactions in Li-ion batteries and is critical to attaining reversible cycling performance. Electron microscopy, spectromicroscopy, scanning probe techniques, etc.—often implemented in parallel on the same electrodes—have been instrumental for understanding the SEI structure and its time-dependent structural evolution in an electrochemical cell.1,67 There are, nonetheless, persistent gaps of knowledge regarding interface/interphase structure-property relationships, which inhibit our ability to effectively design interfaces with explicit, desirable function. PANEL 2 REPORT 99PDF Image | Next Generation Electrical Energy Storage
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