Thermodynamically Consistent Transport Model for Polymer Electrolytes in All-Solid-State Batteries

Abstract

All-solid-state batteries (ASSB) are experiencing a growing scientific interest in recent years as potential next-generation high-voltage batteries with great intrinsic safety. Polymer electrolytes could provide a pathway to solid-state Li-metal batteries by solving current problems such as dendrite growth and flammability. Especially the mechanical properties and stability make polymer electrolytes promising candidates, as shown by the large number of different polymers being discussed. In this contribution, we derive a continuum transport model for charge and mass transport in polymer electrolytes and compare the results with the widely-used concentrated solution theory (CST). The methodological framework for our thermodynamically consistent multi-physics approach is based on modeling the free energy. Our model for the free energy of polymer electrolytes includes contributions from mechanical deformation, configurational entropy and electric fields. We use the Ogden model for compressible rubber-like materials to describe the mechanics of the highly elastic polymers, and model the configurational entropy via the Flory-Huggins model for polymer solutions. We account for convective effects and use the species velocity of the dominant polymer as convection velocity. Although this choice for the convection velocity is in contrast to other models, it is closely related to models on smaller length scales, e.g. molecular dynamics simulations. This allows to parametrize our continuum model with data from atomic scale simulations. In a first step, we validate our transport model by comparing it to experimental and CST results for the case of the canonic polymer electrolyte polyethylene glycole (PEO) with a Li salt in a symmetric Li/electrolyte/Li cell from Steinrück et al.. Our numerical results are in very good agreement with the experimental and CST results for the current density, species velocities and concentration distributions. Next, we focus on a novel single-ion conducting polymer electrolyte (SIC) consisting of an ether-free, nanostructured multi-block copolymer, plasticized with ethylene carbonate. We use this electrolyte, which was recently developed by Nguyen et al., and compare our numerical results with cell experiments. Altogether, we find that our transport model serves as a first step towards the theory-based spatially and time-resolved description of processes in ASSBs with polymer electrolytes.