Bridging Polymer Electrolyte Design and Electrode Architecture for Stable Operation of Polymer‐Based Solid‐State Lithium Metal Batteries

Deep eutectic solvent‐based solid polymer electrolytes (DES‐SPE) offer a compelling platform for lithium metal batteries (LMB), uniting the mechanical robustness of polymers with DES‐enabled ionic transport. However, their practical deployment remains limited by interfacial incompatibility with cathodes and an incomplete understanding of solvation dynamics. Herein, a DES‐SPE is developed, and its solvation environment is elucidated using nuclear magnetic resonance spectroscopy and molecular dynamics simulations. The results reveal a bicontinuous nanostructure in which DES domains are preserved, maintaining DES‐like conduction pathways. This phase separation prevents direct contact between DES and lithium metal, effectively suppressing parasitic reactions and improving interfacial stability. To address cathode integration, a porous‐structured electrode (PSE) is introduced to promote spontaneous electrolyte infiltration and uniform interfacial contact with high‐loading cathodes. The optimized DES‐SPE (LDP373) achieves a high ionic conductivity of 0.719 mS cm −1 at 30°C and a transference number of 0.626. When implemented in a bi‐stack pouch cell with a 2 mAh cm − 2 PSE‐LiNi 0.83 Mn 0.06 Co 0.11 O 2 cathode and a 40 µm lithium metal anode, the system delivers over 80% capacity retention after 120 cycles at 0.1 C and 25°C. These findings underscore a design strategy integrating solvation structure control and electrode engineering for high‐performance solid‐state LMBs.