A Novel Janus-Structured Composite Polymer Electrolyte Constructs a Stable Electrode/Electrolyte Interface for a Solid Lithium Metal Battery

Composite polymer electrolytes (CPEs) have emerged as promising candidates for solid-state batteries because of their flexibility and safety. However, practical applications remain limited by low lithium-ion conductivity, uneven lithium dendrite growth, and high interfacial impedance, leading to poor Coulombic efficiency and rapid capacity decay. A bilayer CPE with a Janus structure (J-CPE) is proposed for enhancing interfacial compatibility, with the LLZTO-PEO layer (L-CPE) toward the cathode and the Si 3 N 4 –PEO layer (S-CPE) toward the Li–metal anode. The amorphous Si 3 N 4 suppresses side reactions between the CPE and Li metal, facilitating a stable and ionically conductive solid electrolyte interface layer film that is rich in Li 3 N and LiSi x N y . The reduced interfacial resistance promotes uniform Li + transport and inhibits lithium dendrite growth. Consequently, the Li symmetric cell demonstrates stable cycling at 0.1 mA cm –2 for 1800 h, and the Li|J-CPE|LFP full cell delivers 600 cycles at 1C with an 86.0% capacity retention. This J-CPE strategy offers new insights for improving interfacial stability in solid-state batteries.