Lithium-ion Diffusion in Solid Electrolyte Interface (SEI) Predicted by Phase Field Model
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Lithium-ion Diffusion in Solid Electrolyte Interface (SEI) Predicted by Phase Field Model Pengjian Guan and Lin Liu Department of Mechanical Engineering, The University of Kansas, Lawrence, KS 66045, U.S.A. ABSTRACT Solid electrolyte interface (SEI) layer plays a key role in lithium-ion batteries’ degradation research. However, SEI layer microstructure prediction still needs further investigation, especially the lithium-ion diffusion in SEI layer considering its morphology evolution during the growth of SEI. Due to the unique advantage of avoiding explicitly tracking the interfaces with sharp composition gradients, a phase field model is developed to simulate the SEI formation and its morphology evolution that is regarded as a solidification process. Fick’s law and mass balance are applied to investigate lithium-ion concentration distribution and diffusion coefficients of different SEI layers (i.e., compact and porous SEI layers) predicted by the developed phase field model. The simulation results show lithium-ion diffusion coefficients between 298K and 318K are 1.34-1.87(10-16) m2/s and 1.73-2.18(10-12) m2/s for compact SEI and porous SEI layer, respectively. The developed model has great potential to be extended to three dimensional spaces for SEI layer spatial growth investigation and other interfaces with complex morphology evolution. INTRODUCTION Lithium-ion battery (LIB) suffers severe electrochemical degradations (1-8), which limit maximizing its performance. Although the main degradation mechanisms may change with different active materials in LIB (2), it is well believed that carbonaceous lithium-ion intercalation electrode, contacting the electrolyte solution, is covered by a passivation layer named solid electrolyte interface (SEI) layer. SEI can prevent the exfoliation of the graphite materials and further electrolyte decomposition (9). However, SEI layer growth can cause battery capacity fade and cell internal resistance increase (10-15). Many pioneering researchers focus on the research of SEI layer formation and compositions, SEI layer thickness growth prediction and measurement (11, 16-19). However, SEI layer microstructure prediction still needs further investigation, especially lithium-ion diffusion inside SEI layer considering its morphology evolution during the growth of SEI. Based on our previous work (20), the formation and morphology evolution of SEI layer are simplified as a solidification process. Reactions happen with reacting electrons from electrode to form SEI species (solid phase) in electrolyte solution (liquid phase) that may undergo decomposition (21). Instead of directly tracking the interface between two phases, a diffusive interface between electrolyte solution and SEI species is assumed to be governed by a dimensionless phase field variable . The SEI layer formation and growth investigated by Deng et al. via phase field simulation provides the physical justification of assuming the phase transformation governed by gradient
free energy (22). SEI species accumulate and form a lay
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