Utilization of heavy alkali dopants as a beacon to study the cathode electrolyte decomposition layer in lithium-ion batteries

Abstract

We introduce low levels of CsClO 4 and RbClO 4 into the electrolyte of LiCoO 2 electrochemical half-cells to probe the composition of the passivation film on the surface of the cathode, the electrolyte decomposition layer (EDL). The advantages of these heavy alkali dopants lie in their large ionic radii, which limit intercalation, yet their strong light scattering cross-section creates a beacon that highlights the formation of products near the cathode surface. Detailed surface analysis and depth profiling with X-ray photoelectron spectroscopy, and bulk analysis utilizing X-ray absorption spectroscopy, show evidence for the formation of Cs/Rb compounds, such as carbonates, halides, and perchlorates, similar to those formed by lithium in previous studies, but also reveal the significantly reduced mobility of the Cs/Rb relative to Li in the non-uniform EDL. This unique approach could open several presently untapped techniques to gather new information on the EDL in Li-ion batteries.