Covalently Anchored Polymer Electrolytes With Built‐In Ion Regulation for Stable Lithium Metal Batteries

Uncontrolled anion migration in conventional liquid electrolytes (LE) induces severe concentration polarization and heterogeneous lithium deposition, challenges that become even more pronounced at low temperatures because of sluggish ion transport and interfacial kinetics. Here, we report a covalently anchored polymer electrolyte with built‐in ion regulation for stable lithium metal batteries (LMBs). The semi‐solid electrolyte (PAB‐GPE) is constructed via in situ copolymerization of potassium allyltrifluoroborate and pentaerythritol tetraacrylate, forming a covalently cross‐linked 3D polymer network. Within this architecture, tethered BF 3 − moieties suppress anion migration and interfacial accumulation through electrostatic repulsion toward mobile TFSI − anions, while immobilized K + sites act as built‐in competitors to regulate the Li + flux coordination environment and homogenize Li + flux. This synergistic ion regulation enables selective lithium‐ion transport, alleviates concentration polarization, and promotes uniform lithium deposition, especially under subzero conditions. As a result, PAB‐GPE delivers a high Li + transference number of 0.65 at −20°C and markedly reduced kinetic hysteresis. Li||Li symmetric cells exhibit stable cycling for over 2000 h at −20°C, while Li||NCM811 full cells achieve improved cycling stability and capacity retention at both ambient and low temperatures. This work provides a materials‐design strategy for polymer electrolytes with built‐in ion regulation toward stable and low‐temperature LMBs.