In situ formation of Li_xSi alloy protective layer for high stability quasi-solid-state batteries

Lithium metal batteries are considered promising candidates for next-generation energy storage systems owing to the high theoretical capacity and low electrochemical potential of lithium metal. However, conventional liquid electrolytes face challenges such as dendrite formation and "dead lithium" accumulation, limiting performance and safety of lithium metal batteries. In this study, a high-interface-compatible polyvinylidene fluoride PVDF-based quasi-solid-state electrolyte (PVDF@PS) was prepared using a simple direct scraping method, incorporating a high-affinity polyacrylic acid (PAA) binder. Under in-situ pressure during battery encapsulation, high-purity silicon was lithiated to form a lithium-silicon alloy, reducing the nucleation and diffusion barriers of lithium. Concurrently, a small amount of lithium reacted with the silicon surface oxides, forming a stable passivation layer of Li₄SiO₄, significantly mitigating the risk of dendrite penetration that could cause short circuits. Additionally, the PAA binder was lithiated into a highly flexible Li-PAA structure. The combination of these three components forms an alloy protective layer, enhancing the electrolyte/electrode interface stability with both flexibility and robustness. This design effectively regulates the volume changes of the lithium anode during the dynamic plating/stripping process, promotes uniform lithium deposition, and suppresses dendrite formation. Furthermore, the protection layer reduces side reactions by isolating the lithium metal from direct interaction with the electrolyte. As a result, compared to uncoated PVDF-based quasi-solid-state electrolytes, PVDF@PS demonstrates excellent stability and a notably low overpotential (20 mV) in lithium symmetric cells at various current densities. In LiFePO₄ (LFP) full cells, it retains 80.3 mAh g^-1 capacity after 1300 cycles, even under a high charge/discharge rate of 5C.