Continuum-Scale 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 due to their potential use as next-generation high-voltage batteries with great intrinsic safety. Among the different types of solid electrolytes, polymer electrolytes could provide a pathway to solid-state Li-metal batteries by solving current problems such as dendrite growth [1] and flammability [2]. Especially their mechanical properties, stability and tunability make them promising candidates for the use in ASSBs, both as solid polymer electrolytes (SPE) as well as composite electrolytes in combination with other types of solid electrolytes [3].

In this contribution, we derive a continuum model for transport of charge and mass in polymer electrolytes, and compare the results with the widely-used and well parametrized concentrated solution theory (CST) and experiment.

Our thermodynamically consistent approach is based on modeling the free energy [4,5], in which we include contributions from mechanical deformation, configurational entropy and electric fields to reproduce the behaviour of SPEs. 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. Furthermore, we account for convective effects and use the velocity of the polymer matrix 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 [6]. This allows to parametrize our continuum model with data from atomic scale simulations.

We validate our transport model by comparing it to experimental and CST results from Steinrück et al. [7] for the case of the canonic polymer electrolyte polyethylene glycole (PEO) with LiTFSI in a symmetric Li/electrolyte/Li cell. 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 compare our numerical results for this electrolyte, which was recently developed by Nguyen et al. [2], with cell experiments. Altogether, we find that our transport model serves as a good first step towards the theory-based spatially and time-resolved description of processes in ASSBs with polymer electrolytes.

We gratefully acknowledge funding and support by the BMBF (Federal Ministry of Education and Research) within the FestBatt project. This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe).

[1] R. Khurana et al., Journal of the American Chemical Society, 2014, 136, 7395-7402 [2] H.-D. Nguyen et al., Energy Environ. Sci., 2018, 11, 3298. [3] S. A. Aziz et al., Journal of Science: Advanced Materials and Devices 3, 2018, 1-17 [4] M. Schammer et al., J. Electrochem. Soc., 2021, 168 026511 [5] L. von Kolzenberg et al., Batteries & Supercaps, 2022, 5, e20210021 [6] D. Diddens et al., Macromolecules, 2010, 43, 2028–2036 [7] H.-G. Steinrück et al., Energy Environ. Sci., 2020, 13,4312