Polymer‐Nanodomains Regulation for Optimized Ionic Translocation Enabling High‐Voltage and Practical Utility Lithium Metal Batteries

High-voltage solid-state lithium metal batteries (HVSSLMBs) integrate high-voltage cathodes (HVCs) with lithium metal anodes, offering great promise for next-generation energy storage. However, high-voltage charging induces lattice oxygen oxidation at the cathode, generating reactive oxygen species (ROS) that degrade inorganic solid electrolytes. In addition, the low room-temperature ionic conductivity and limited electrochemical stability of polymer electrolytes hinder their application with HVCs. Here, we design a composite polymer electrolyte (CPE) with dual-domain-coupled secondary nanoconfinement via precise polymer-nanodomain engineering. This structure combines a mesoporous framework with dynamic segmental motion, forming interconnected ion channels between free and anchored domains. The anchoring domains with high negative surface potential preload lithium ions, thereby increasing the local ion density and promoting a continuous high-conductance path. Benefiting from the protection mechanism of the nanoconfined system, the chemical stability of the polymer is greatly improved, thus achieving operation at high voltage. Cells paired with 4.8 V LRMO (Li_1.2Ni_0.13Co_0.13Mn_0.54O₂) retained 80.37% capacity after 200 cycles. Notably, this study reported the first-ever assembly of a large-format pouch cell combining a CPE with the LRMO cathode, delivering an impressive energy density of 419.47 Wh kg^-1 at a capacity of 4.61 Ah. This advancement marks a significant milestone in the application of HVSSLMBs.