Li+ concentration waves created in the electrolyte during operation of Li-ion batteries with porous graphite-based electrodes

Summary:

In this presentation, Li+ concentration waves in the electrolyte of a C/Li cell during the galvanostatic operation are investigated with the help of the P2D model and experimental measurements with a four-electrode setup. The simulations reveal that Li+ concentration waves in the electrolyte are generated by the waves of reaction distribution inside the porous electrode. Two factors are essential for the generation of reaction distribution waves. The reaction heterogeneity (kinetic factor) results in a non-uniform electrode utilization. The non-uniform electrode utilization tends to be mitigated when going through the voltage transitions in the OCV curve (thermodynamic factor) by forming reaction distribution waves. In the experiments, a four-electrode device (WE/RE1/RE2/CE) is used to validate the electrolyte concentration waves inside the separator region. The potential differences between CE and RE2, RE1 and RE2, show apparent fluctuations during operation, demonstrating waves in the electrolyte concentration. The alignments of the transition in battery output voltage and the fluctuations in the potential differences illustrate electrolyte concentration waves’ dependency on thermodynamics and the porous electrode’s reaction kinetics.
Such phenomena are not specific to graphite-based porous electrodes. It is expectable that the porous electrodes with flat and sloping parts in OCV curves (Figure S15), i.e. spinel LiMn2O4, LiV2O5, NaCoO2, will also generate reaction distribution waves and electrolyte concentration waves at low-current applications. The results in the present paper reveal that one porous electrode inside a battery will influence the other (porous) electrode through the electrolyte. These results also provide an important viewpoint for understanding the reaction nature and explaining voltage artifacts in porous electrodes. In addition, the finding of the dependency of the reaction distribution on both thermodynamics and kinetics is highly relevant for the scientific community in the field of rechargeable batteries and applications. The presented results will also broaden the design prospective of porous electrodes and enhance the performance of future Li-ion batteries.