Covalent organic framework-regulated ionic transportation for high-performance lithium-ion batteries

Abstract

Separators of current lithium ion batteries (LIBs) based on transition metal oxide cathodes need to be highly conductive for lithium ions but not for transition metal ions (TMIs). Herein, a novel polymer separator that can regulate the transportation of lithium ions and TMIs is reported, and it is fabricated by coating a custom-designed covalent organic framework (COF) from 1,3,5-tris(4-aminophenyl)benzene and 2,5-dimethoxybenzene-1,4-dialdehyde on a commercial polymer separator (PS). Electrochemical measurements combining physical characterization demonstrate that the lithium ion transference number (t_Li⁺) of the as-fabricated separator (PS@COF) is two times that of the uncoated PS, while the dissolved TMIs from cathodes are effectively isolated from anodes, leading to significantly enhanced cycling stability and rate capability of Li[Li_0.2Mn_0.55Ni_0.15Co_0.1]O₂ and LiNi_0.8Co_0.1Mn_0.1O₂-based LIBs. Theoretical calculations suggest that the COF provides the localized negatively charged groups (–OCH₃) for transporting lithium ions and the polar groups (–CN–) with lone pair electrons for chelating TMIs. Our strategy shares a perspective for performance improvement of LIBs and holds great promise for the application of covalent organic frameworks (COFs) in energy storage and conversion.