Fluorinated Copolymer Functionalized with Ethylene Oxide as Novel Water-Borne Binder for a High-Power Lithium Ion Battery: Synthesis, Mechanism, and Application

A novel water-borne fluorinated binder is synthesized via copolymerizing 2-(perfluorohexyl) ethyl methacrylate (PFHEMA) and poly(ethylene glycol) methacrylate (PEGMA) to improve the performance of lithium ion battery with LiFePO₄-based cathode materials. The resulting copolymer binders can self-assemble into 150-220 nm particles stably dispersed in aqueous solution. Self-dispersed fluorinated binders (SF binders) with the PFHEMA to PEGMA ratio of 3:1 effectively reduce the overpotential during the high-discharge current density compared with the conventional PVDF cathode binder. Further increasing the PEGMA amount yet decreases the electrochemical performance of SF binders, inconsistent with the expected Li⁺ conduction of the PEO moiety. Molecular dynamics simulations show that the PEO segments reduce the Li⁺ and PF6^- interaction and increase the amount of unpaired Li⁺. In contrast, the PEO moiety wrapping around Li⁺ can decrease its mobility. These competing effects lead to the observed optimum ratio of PEO to fluorinated moieties. The novel SF binders are fully compatible with LiFePO₄-based cathode materials and feature small impedance after charging and discharging. Coin cells assembled with the SF cathode binder demonstrated excellent cyclic performance after 150 cycles with negligible decay and near-100% column efficiency. The superior performance of the novel water-borne SF binders makes them excellent candidates for the environmentally friendly production of high-power lithium ion batteries.