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

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/LiNi_0.6 Co_0.2 Mn_0.2 O₂ (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.