Unveiling the SEI Formation in a Potassium Ion Battery Using Distribution of Relaxation Time

Abstract Understanding of solid electrolyte interphase (SEI) formation process in novel battery systems is of primary importance. Alongside increasing powerful in-situ techniques, searching for readily-accessible, non-invasive, and low-cost tools to probe battery chemistry is highly demanded. Here, we applied distribution of relaxation time (DRT) analysis to interpret in-situ electrochemical impedance spectroscopy results during cycling, which is able to distinguish various electrochemical processes based on their time constants. By building direct link between SEI layer and the cell performances, it allows us track the formation and evolution process of SEI layer, diagnose the failure of cell, and unveil the reaction mechanism. For instance, in a K-ion cell using SnS2/N-doped reduced graphene oxide (N-rGO) composite electrode, we found that the ion-transport in the electrolyte phase is the main reason of cell deterioration. In the electrolyte with potassium bis(fluorosulfonyl)imide (KFSI), the porous structure of the composite electrode was reinforced by rapid formation of a robust SEI layer at SnS2/electrolyte interface and thus the KFSI-based cell delivers a high capacity and good cycleability. This method lowers the barrier of in-situ EIS analysis, and helps public researchers to explore high-performance electrode materials.