Covalently Grafted Poly(vinylidene fluoride) Copolymer as a Cathode Binder and Its Ionic Conductivity Assessment toward Filler-Free Gel Polymer Electrolyte

Abstract

Polymer materials with covalently grafted pendant functional groups rather than those in a composite state are highly attractive for next-generation rechargeable batteries owing to their ability to provide both mechanical robustness and efficient ion transport. In this work, a multifunctional polymer system poly(vinylidene fluoride)-g-poly[(methacrylic acid)-random-(poly(ethylene glycol) methyl ether methacrylate)] (PVMAPE) was synthesized via atom transfer radical polymerization (ATRP) of tert-butyl methacrylate (t-BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMEM) after being initiated from a PVDF macroinitiator. Subsequently, the graft copolymer underwent selective hydrolysis of the t-BMA ester moieties to get the desired graft copolymer. X-ray photoelectron spectroscopy confirmed “halogen exchange” on the PVDF backbone prior to grafting, with a substitution level of approximately 8%, generating ATRP initiation sites that enabled homogeneous graft-chain distribution throughout the matrix. The synthesized polymer was first evaluated as a sodium-ion battery cathode binder, delivering an initial discharge capacity of 121.81 mA h g–1 and retaining 75% of its capacity after 100 cycles (90% with respect to the second cycle). Subsequently, it was investigated as a gel polymer electrolyte (GPE) for both lithium- and sodium-ion conduction. Electrochemical impedance spectroscopy revealed maximum ionic conductivities of 1.03 × 10–4 and 4.6 × 10–5 S cm–1 at 343 K, with activation energies of 0.153 and 0.161 eV, respectively. Both the lithium- and sodium-based GPEs exhibited superior ion-transport characteristics, highlighting the versatility of the developed polymer as both an electrochemically active binder and an ion-conducting electrolyte.