Compositional engineering of composite polymer electrolytes for all solid-state batteries to simultaneously improve reaction kinetics and long-term stability

All-solid-state batteries (ASSB) with Li anodes are urgently required to increase both energy density and safety. However, interfacial instabilities between the solid electrolyte and Li anode (SEI), and between the solid electrolyte and cathode (CEI) limit stable operation at elevated temperatures. In this study, a composite polymer electrolyte (CPE) comprising succinonitrile (SN) and lithium difluorophosphate (LiDFP) is used to significantly increase the cycle life and Li⁺ transport kinetics. SN successfully enhances the electrochemical performance of CPE by improving the compatibility and interface contact with the Li anode and the Li⁺ transport kinetics. LiDFP improves the oxidation stability of CPE, suppresses the increase in resistance, and promotes Li ⁺ conductivity by forming a favorable SEI and CEI consisting of LiF and Li₃PO₄. Most of all, the combination of SN and LiDFP result in synergistic effects. Therefore, CPE with SN and LiDFP enables stable cyclability of Li-iron phosphate/Li cell over 1000 cycles with capacity retention of 76 % and high average Coulombic efficiency of 99.5 % at 3C and 60 °C. Understanding novel composite solid electrolytes aids in the design of advanced CPE and even ceramic solid electrolytes for high-performance ASSBs.