A Highly Compatible Deep Eutectic Solvent‐Based Poly(ethylene) Oxide Polymer Electrolyte to Enable the Stable Operation of 4.5 V Lithium Metal Batteries

Insufficient ionic conductivity, limited Li⁺ transfer number (t_Li+), and narrow electrochemical windows have heavily restricted the actual application of PEO （poly(ethylene) oxide）-based polymer electrolytes (PEs). Herein, a novel deep eutectic solvent (DES)-based PEO PE for stabilizing high voltage lithium metal battery (LMB) is designed. The DES reduces the crystallinity of PEO while promoting the dissociation of LiTFSI to release more free Li⁺, thus facilitating the transport of Li⁺ in the PEO matrix. In addition, the interaction between DES and the PEO, thereby improving the stability of the PEO-based PE under high voltage. Consequently, the PEO-DES-FEC (for short PDF) PE possesses satisfactory ionic conductivity, good mechanical properties, and high electrochemical stability. Meanwhile, PDF PE can build a robust/uniform LiF-rich solid electrolyte interface (SEI) to ensure electrode/electrolyte interface stability. As a concept proof, the Li symmetrical battery and Li||LiFePO₄ LMBs of PDF PE exhibit good cycle stability. Applied to the high voltage Li||NCM811 LMBs, the PDF PE-based cell has excellent cycling performance at 4.3 and 4.5 V. This tactic is one of the successful demonstrations of PEO-based electrolytes under 4.5 V high voltage conditions, which breaks through the voltage constraint of conventional PEO-based PEs.