Functional Separator with Poly(Acrylic Acid)‐Enabled Li 2 CO 3 ‐Free Garnet Coating for Long‐Cycling Lithium Metal Batteries

Lithium metal batteries (LMBs) possess a theoretical energy density far surpassing that of commercial lithium-ion batteries (LIBs), positioning them as one of the most promising next-generation energy storage systems. Modifying separators with composite coatings comprising oxide solid-state electrolyte (SSE) particles and polymers can improve the cycling stability and safety of LMBs. However, exposure to air forms Li₂CO₃ on oxide SSE particles, diminishing their ion flux regulation at the electrode/electrolyte interface. Utilizing the reaction of Li₂CO₃ with polyacrylic acid (PAA) to form lithium polyacrylate (LiPAA), an ultra-thin composite coating on polyethylene (PE) separator with Li₂CO₃-free Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) particles and LiPAA binder is fabricated in one step. The exposed Li₂CO₃-free LLZTO surface increases the ionic conductivity and lithium ion (Li⁺) transference number of the functional separator, resulting in small resistance and uniform Li deposition of the Li metal anode. Consequently, the Li//LiCoO₂ cell with the functional separator exhibits a significantly improved life of 980 cycles with 80.9% capacity retention under lean-electrolyte conditions. Both the Li//LiCoO₂ coin cell and pouch cell using thin Li foil anode demonstrate good cycling stability and high mechanical robustness. This study provides a green and scalable approach for fabricating advanced separators for LMBs.