Regulating the Li-O coordination in polymer electrolytes via semi-ionic C-F bonds for high-voltage solid lithium metal batteries

Abstract

Poly(ethylene oxide) (PEO) based electrolytes are of special potential for solving the safety problem of lithium metal batteries due to their high processability and outstanding solvation ability of lithium salts. Unfortunately, their insufficient ionic conductivity and limited electrochemical stability greatly hinder their applications. Herein, we propose a strategy based on regulating the Li-O coordination for high-voltage solid lithium-metal batteries, by generating a fluorinated PEO with C-F bonds in a semi-ionic state (SIF-PEO) as the electrolyte. The Li+ solvation structure in SIF-PEO/LiTFSI electrolyte can be modulated by SIF bonds, and the Li-O coordination is weakened which effectively boosts the Li+ ion transport ability. The density functional theory (DFT) calculation reveals that the introduction of SIF bonds into PEO depresses the highest occupied molecular orbital (HOMO) energy level which contributes to a higher oxidative decomposition potential of the electrolyte. As a consequence, the ionic conductivity and cathodic oxidation potential of SIF-PEO are significantly elevated as compared to pristine PEO. The SIF-PEO electrolyte shows highly stable cycling performance of Li symmetric cells at a current density of 0.1 mA cm−2 (up to 550 h) with a low overpotential of 136 mV. Remarkably, the assembled lithium metal batteries with high-voltage cathode deliver outstanding capacity retention and cycling stability (Coulombic efficiency, 99.79 %, 500 cycles at 0.5C with LiNi0.5Co0.2Mn0.3O2) at 40 °C. This work provides a powerful engineering avenue to regulate polymer electrolytes for high-voltage all-solid-state lithium metal batteries.