In-situ construction of Li₃N/Li₂CO₃ hybrid SEI to improve interface stability of quasi-solid-state lithium metal batteries

Quasi-solid-state lithium metal batteries (QSSLMBs) using gel polymer electrolytes (GPEs) promise safer high-energy storage, but their performance is limited by interfacial instability that requires both fast Li⁺ transport and durable electrode-electrolyte contact. Here we present an in-situ two-step strategy that couples electrochemical reduction with thermal curing to transform a spin-coated LiNO₃ precursor into a functionally synergistic Li₃N/Li₂CO₃ composite artificial SEI. The electrochemical step contributes to the formation of Li₃N-containing ion-conducting species, while the curing step during GPE polymerization is associated with a more carbonate-rich surface that improves interfacial compatibility. DFT calculations show stronger ethylene carbonate adsorption on Li₂CO₃ than on LiF, consistent with markedly improved wettability, as the electrolyte contact angle decreases from about 81° on cycled bare Li to about 11° on the engineered interface. The optimized anode enables stable symmetric cell cycling with 70 mV polarization for over 1200 h at 0.8 mA cm⁻², and LiFePO₄ full cells retain 95.5% capacity after 250 cycles at 0.3C and 94.4% after 300 cycles at 1C. This work shows that cooperative regulation of ion transport and contact retention within an inorganic-dominant multicomponent SEI can improve the stability of quasi-solid-state lithium metal interfaces.