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

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.