Atypical Hydrogen Bond Interaction Enables Anion‐Rich Solvation Structure in Polymer Electrolytes for High‐Voltage Flexible Lithium Metal Batteries

Flexible lithium metal batteries (LMBs) using polymer-based solid-state electrolytes (PSSEs) are highly desirable for wearable applications because of the potential advantages in energy density and safety. Recently, ether-based polyelectrolytes have received extensive attention because of their good stability, high ionic conductivity, and Li metal anode compatibility. However, it typically forms organic-rich cathode electrolyte interphase (CEI) at the cathode, which is still a pain point that impedes the high-voltage performance. To address this challenge, herein a fluorinated plasticizer, bis(2-fluoroethyl) ether (BFE) is reported, which can be easily blended into ether-based PSSE and enables high-voltage-stable flexible LMBs. The BFE and PSSE molecules form an atypical hydrogen bond interaction, which weakens the interaction between PSSE and lithium ions. This leads to the formation of an anion-rich solvation structure that generates inorganic-rich and high-voltage-stable CEI. The oxidation stability of PSSE is improved from 4.4 V to over 4.7 V after introducing the BFE molecules. LMBs using BFE-blended PSSE can couple with high-voltage cathode and retain 80% capacity after 480 cycles at 1C. Full cells show high energy density (752.2 Wh L^-1) outstanding capacity retention per cycle (99.88%), and high flexibility with almost identic charge/discharge characteristics after 4000 bending cycles.