Measuring Lithium Concentration Gradients during Fast Charging in an Optically Accessible Li-Ion Cell and Comparing to a 1D Numerical Model

Abstract

Lithium-ion transport limitations hinder the fast charging capabilities of Li-ion batteries. At fast charging rates, insufficient transport can result in lithium concentration polarization, cell damage, and capacity loss. The magnitude of the lithium concentration polarization has been estimated with pseudo-2D (P2D) models, but has yet to be validated under fast charge conditions. In this research, we designed and built an optically accessible Li/Li symmetric cell and measured lithium concentration polarization in situ using FTIR. This novel cell design enabled the rapid collection of IR spectral images between electrodes at small timescales. From these in situ measurements, we can measure dynamic lithium concentration polarization at timescales relevant to fast charging. The IR imaging method was demonstrated in a Li/Li symmetric cell with an electrolyte of 1M LiPF6 in 50/50 ethylene carbonate/ethyl methyl carbonate (vol./vol) revealing lithium concentration gradients of over 2mM/mm. Results were compared with 1D numerical models utilizing concentration-dependent lithium transport properties. The comparison between our experimental and computational results gives insight into the limitations of models to adequately simulate transport in highly polarized cells. This test also showed promise for accurately measuring lithium transport properties in future experiments.