Quasi‐Ionic Liquid Enabling Single‐Phase Poly(vinylidene fluoride)‐Based Polymer Electrolytes for Solid‐State LiNi_0.6Co_0.2Mn_0.2O₂||Li Batteries with Rigid‐Flexible Coupling Interphase

Abstract Poly(vinylidene fluoride)‐based polymer electrolytes are being intensely investigated for solid‐state lithium metal batteries. However, phase separation and porous structures are still pronounced issues in traditional preparing procedure. Herein, a bottom‐to‐up strategy is employed to design single‐phase and densified polymer electrolytes via incorporating quasi‐ionic liquid with poly(vinylidene fluoride‐co‐hexafluoropropylene). Due to strong ion/dipole–dipole interaction, the optimized polymer electrolyte delivers high room‐temperature ionic conductivity of 1.55 × 10⁻³ S cm⁻¹, superior thermal and oxidation stability of 4.97 V, excellent stretchability of over 1500% and toughness of 43 MJ cm⁻³ as well as desirable self‐extinguishing ability. Furthermore, the superb compatibility toward Li anode enables over 3000 h cycling of Li plating/stripping and ≈98% Coulombic efficiency in Li||Cu test at 0.1 mA cm⁻². In particular, lithium metal battery Li||LiNi_0.6Co_0.2Mn_0.2O₂ exhibits a room‐temperature discharge retention rate of 96% after 500 cycles under a rate of 0.1 C, which is associated with the rigid‐flexible coupling electrodes/electrolytes interphase. This investigation demonstrates the potential application of quasi‐ionic liquid/polymer electrolytes in safe lithium metal batteries.