Screening additive for stable solid electrolyte interphase in polymer lithium battery by coulometric titration time...

The stability of solid electrolyte interphase (SEI) is critical to the performance of solid-state polymer lithium metal batteries. While electrolyte additives can markedly improve SEI quality, characterizing its temporal evolution remains challenging. This study employs the coulometric titration time analysis (CTTA), a highly sensitive electrochemical method, to evaluate the effects of various additives on the long-term stability of SEI in the poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE). Using this method, a range of additives, including ion-conductive fillers, inert fillers, plasticizers, and lithium salts, are systematically screened. We demonstrate that lithium difluoro(oxalato)borate (LiDFOB) acts as a highly effective functional additive, preferentially decomposes at the lithium anode to form a thin, inorganic-rich passivation SEI layer, which mitigates the degradation of both PEO and Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI). With the incorporation of LiDFOB, the time required to consume the same amount of deposited lithium extends by approximately 14-fold compared to the additive-free SPE, indicating significantly suppressed interfacial side reactions. As a result, a Li | PEO-LiTFSI-5LiDFOB | LiFePO₄ cell exhibits outstanding cycling stability, retaining 93.5% of its capacity after 200 cycles at 0.5C. These findings underscore the utility of CTTA as a powerful diagnostic tool for screening interfacial-stabilizing additives and provide clear guidelines for designing long-life polymer-based solid-state batteries.