Versatile Solvent‐Free Synthesis of Composite Polymer Electrolytes for Thin High‐Performance Solid‐State Lithium Metal Batteries

The development of high‐performance solid‐state lithium metal batteries (SSB s ) relies on the invention of efficient composite polymer electrolytes (CPE s ) that offer both high ionic conductivity and mechanical stability. However, mixing polymers and inorganic particles often leads to inhomogeneous distributions, inhibiting ion movement. This work introduces a novel solvent‐free synthesis for thin CPE films, enabling scalable and straightforward electrolyte fabrication. The proposed hybrid electrolyte system consists of a self‐crosslinking polyether matrix incorporating lithium‐ion‐conducting ceramic particles. The synthesis method facilitates homogeneous dispersion of Li 6.45 Al 0.05 La 3 Zr 1.6 Ta 0.4 O 12 (LLZO), thus preventing agglomeration and affording consistent electrochemical performance with film thicknesses of ≈30 µm. The ability to mix polymers and incorporate additives further boosts the electrolyte's tunability, providing a versatile approach. Electrochemical characterization reveals that the fabricated hybrid CPEs exhibit superior ionic conductivity (0.27 mS cm −1 at 60 °C) and compatibility with lithium metal, while their implementation in high‐mass‐loading lithium iron phosphate (LFP, 7 mg cm 2 ) cathodes demonstrates exceptional cycling performance of over 200 cycles at 80% state of health (SOH) at 0.25 C. The CPEs are characterized by small amplitude oscillatory shear (SAOS) in the linear regime, Young module, tensile strength, scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDX), and electrochemically in Li||Li and NMC 622 (LiNi 0.6 Mn 0.2 Co 0.2 O 2 ) /LFP||Li cells.