Functional Separator with Poly(Acrylic Acid)‐Enabled Li2CO3‐Free Garnet Coating for Long‐Cycling Lithium Metal Batteries

Abstract 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 Li 2 CO 3 on oxide SSE particles, diminishing their ion flux regulation at the electrode/electrolyte interface. Utilizing the reaction of Li 2 CO 3 with polyacrylic acid (PAA) to form lithium polyacrylate (LiPAA), an ultra‐thin composite coating on polyethylene (PE) separator with Li 2 CO 3 ‐free Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) particles and LiPAA binder is fabricated in one step. The exposed Li 2 CO 3 ‐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//LiCoO 2 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//LiCoO 2 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.