Modeling non-equilibrium microstructure in electrochemistry

Daniel Cogswell
Samsung Advanced Inst. of Technology

Reaction-driven phase transformations that involve charge transfer accompanied by ion intercalation or deposition are common in electrochemistry. Examples include Li-ion, metal-air and lead-acid batteries, as well as metal electrodeposition/dissolution. Despite complex thermodynamics, the standard kinetic model is the Butler-Volmer equation with a dilute solution approximation. In contrast, we have developed a thermodynamically consistent phase-field model that allows for rigorous treatment of phase boundary morphology and concentrated solutions thermodynamics. Application of the model to two-phase nucleation and coexistence in LiFePO4, phase-separation in porous electrodes, and dendritic growth during electrodeposition will be presented. The phase-field model reveals several important observations about the cathode material LiFePO4. First, phase-separation is a dynamic process that can be suppressed at high discharge rates. Second, elastic coherency strain leads to the formation of low-energy phase boundaries along (101) planes, and elastic relaxation near surfaces leads to the formation of a striped morphology with a characteristic length scale that can be used to estimate of the interfacial energy. Third, the role of nucleation, which is difficult to observe experimentally and beyond the reach of ab initio calculations, can be determined. Dendrite growth during electrodeposition is another challenging problem with important technological relevance for advanced battery technologies. Although the phase-field method has succeeded at quantitatively modeling dendritic solidification morphology, there has been only limited application to electrochemical deposition. We have developed an electrochemical phase-field model for electrodeposition in a binary electrolyte, and find that it is consistent with fractal geometries have been observed experimentally. Daniel A. Cogswell (1,2), Todd R. Ferguson(2), Peng Bai (2), and Martin Z. Bazant (2) - (1) Samsung Advanced Institute of Technology America; (2) Department of Chemical Engineering, MIT

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