A Hierarchically Designed Janus Polymer Electrolyte for High‐Performance Lithium‐Metal Batteries

Abstract Practical implementations of solid polymer electrolytes (SPEs) in solid‐state lithium‐metal batteries (SSLMBs) are inhibited by the limited lithium‐ion (Li⁺) transport and poor‐quality interface between SPEs and both electrodes. SPEs exhibit lower ionic conductivity than other electrolytes and are oxidized and decomposed by oxide‐based cathode materials in high‐voltage windows. SSLMBs are also long‐term destabilized by parasitic side reactions at the electrode–electrolyte interfaces and Li dendrite formations. This study proposes a selectively designed Janus‐structured polymer electrolyte (JPE), which is more physically and chemically compatible with the electrodes than other SPEs. The proposed JPE includes a cathode‐facing composite polymer electrolyte (C‐CPE) containing succinonitrile and Li₇La₃Zr₂O₁₂, and an anode‐facing composite polymer electrolyte (A‐CPE) incorporating fluoroethylene carbonate (FEC). The C‐CPE layer provides additional Li⁺ paths and increases the antioxidant properties, improving the high‐voltage tolerance of the SSLMB, while the A‐CPE layer alleviates side reactions with the Li metal anode and improves the stability against protruding dendrites. Full cells of Li|JPE|Ni_0.8Co_0.15Al_0.05O₂ and Li|JPE|LiCoO₂ remain stable over 1600 cycles at 1 C, demonstrating the potential of the JPE structures for SSLMBs. Moreover, symmetric Li||Li cells assembled with the JPE cycle over 2500 h at 0.1 mA cm⁻² and 1000 h at 0.5 mA cm⁻².