A flexible solid-state electrolyte based on comb-like PEG-functionalized covalent organic frameworks for lithium metal batteries

Abstract

Solid-state batteries have great potential for increased safety and energy density, but most solid-state electrolytes show relatively low ionic conductivity and can't fully meet the requirements of solid-state batteries. Herein, vinyl-functionalized covalent organic frameworks (Bt-COFs) based on a CN bond, poly(ethylene glycol) methyl ether methacrylate (PEGMA), and LiTFSI were used to prepare a composite solid polymer electrolyte via a simple in situ polymerization technique. Bt-COF can improve the ionic conductivity of polyethylene glycol (PEG)-based electrolytes by creating a fast highway for Li⁺ conduction along one-dimensional nanochannels through the robust network of covalent organic backbones. At the same time, the Bt-COF-modified composite solid polymer electrolyte with a variety of “lithium-friendly” elements results in the construction of a LiF-rich SEI to regulate Li plating/stripping, which can effectively reduce the growth of dendrites. It has also been shown that the introduction of Bt-COF can effectively reduce the crystallinity of PEGMA and improve the ionic conductivity of composite solid electrolytes. The Bt-COF_2%-SPE with 2 wt% Bt-COF shows a high ionic conductivity of 5.3 × 10⁻⁵ S cm⁻¹ and an excellent lithium ion transference number (t_Li⁺) of 0.47 at room temperature. Simultaneously, the Li∥Li symmetric cell with Bt-COF_2%-SPE as the electrolyte exhibits remarkable cycling stability over 450 h. Moreover, the Li∥Bt-COF_2%-SPE∥LiFePO₄ cell shows a high discharge specific capacity of 146.2 mA h g⁻¹, and a capacity of 126.7 mA h g⁻¹ is also delivered with a coulombic efficiency of 86.6% after 200 cycles at 60 °C. Thus, solid-state electrolytes based on Bt-COF show broad prospects for practical application in lithium metal batteries.