An interfacial layer constructed by in situ polymerizing trimethyl phosphate and ethylene carbonate enabling durable...

Composite solid electrolytes (CSEs) based on garnet Li₇La₃Zr₂O₁₂ (LLZO) are promising for solid-state batteries due to high ionic conductivity and flexibility. However, interfacial instability with lithium metal anodes promotes dendrite growth and side reactions. To address this issue, an in situ polymerized interfacial layer (TPELL layer) was constructed by copolymerizing trimethyl phosphate (TMP) and ethylene carbonate (EC) between the Li anode and CSE. The copolymerization of EC and TMP effectively regulates the solvent structure within the interfacial layer, which in turn modulates the Li⁺ coordination environment and enhances the mechanical robustness of the interface. Furthermore, the incorporation of EC and LiPF₆ promotes the formation of a more stable lithium metal anode/electrolyte interface. Consequently, the Li||LiFePO₄ battery with CSE containing interfacial layer formed by copolymerization of TMP and EC exhibits a capacity retention of 93.83 % after 750 cycles at 1C and the Li||LiNi_0.8Co_0.1Mn_0.1O₂ battery maintains 130.1 mAh g^-1 after 350 cycles at 0.5C. Post-cycling analysis indicates that the preserved integrity of the cathode structure can be ascribed to the formation of a multifunctional cathode electrolyte interphase (CEI) derived from TPELL components, which effectively suppresses transition-metal dissolution and alleviates interfacial resistance. Moreover, this in situ polymerization strategy facilitates intimate interfacial contact at both the anode and cathode, thereby providing a promising pathway to address the persistent electrode/electrolyte interfacial challenges in solid-state lithium metal batteries.