Influencing Factors on Li-ion Conductivity and Interfacial Stability of Solid Polymer Electrolytes, Exampled by Polycarbonates, Polyoxalates and Polymalonates

The application of solid polymer electrolytes (SPEs) in all-solid-state(ASS) batteries is hindered by lower Li+-conductivity and narrower electrochemical window. Here, three families of ester-based F-modified SPEs of poly-carbonate (PCE), poly-oxalate (POE) and poly-malonate (PME) were investigated. The Li+-conductivity of these SPEs prepared from pentanediol are all higher than the counterparts made of butanediol, owing to the enhanced asymmetry and flexibility. Because of stronger chelating coordination with Li+, the Li+-conductivity of PME and POE is around 10 and 5 times of PCE. The trifluoroacetyl-units are observed more effective than -O-CH2-CF2-CF2-CH2-O- during the in situ passivation of Li-metal. Using trifluoroacetyl terminated POE and PCE as SPE, the interfaces with Li-metal and high-voltage-cathode are stabilized simultaneously, endowing stable cycling of ASS Li/LiNi0.6Co0.2Mn0.2O2 (NCM622) cells. Owing to an enol isomerization of malonate, the cycling stability of Li/PME/NCM622 is deteriorated, which is recovered with the introduce of dimethyl-group in malonate and the suppression of enol isomerization. The coordinating capability with Li+, molecular asymmetry and existing modes of elemental F, are all critical for the molecular design of SPEs.

Automated summary

This study investigates the application of ester-modified solid polymer electrolytes (SPEs), including poly-carbonate (PCE), poly-oxalate (POE), and poly-malonate (PME), in all-solid-state (ASS) batteries. The Li+-conductivity of these SPEs, prepared from pentanediol, is higher than those made of butanediol due to enhanced asymmetry and flexibility. PME and POE exhibit Li+-conductivity around 10 and 5 times that of PCE, respectively, thanks to their stronger chelating coordination with Li+. The use of trifluoroacetyl terminated POE and PCE as SPEs stabilizes the interfaces with Li-metal and high-voltage-cathode simultaneously, enabling stable cycling of ASS Li/LiNi0.6Co0.2Mn0.2O2 (NCM622) cells. The cycling stability of Li/PME/NCM622 is deteriorated due to enol isomerization of malonate, which is recovered by introducing a dimethyl-group in malonate and suppressing enol isomerization. The coordinating capability with Li+, molecular asymmetry, and existing modes of elemental F are critical for the molecular design of SPEs.