Effect of Imidazolium Concentration in Densely Functional Polymer Binder on Robust Electrochemical Kinetics and Cycling Performance of Lithium Iron Phosphate Cathode

Abstract

The inherently poor conductivity of the olivine structure in LiFePO₄ results in limited electrochemical performance, thus restricting its applications in Li-ion batteries. To address this issue, this manuscript explores the use of an ion-conducting binder based on a high-density functional poly(ionic liquid) (HFPIL) and investigates the impact of enhanced ion conductivity on electrochemical performance. High-density water-soluble polymethylene-based functional binders, such as poly(hydroxycarbonylmethylene) (PFA), poly(hydroxycarbonylmethylene-co-oxycarbonylmethylene 1-allyl-3-methylimidazolium) (PMIF), and poly(oxycarbonylmethylene 1-allyl-3-methylimidazolium) (PMAI), are synthesized and characterized using nuclear magnetic resonance and Fourier-transform infrared spectroscopy. Water-soluble binders show better long cycling and rate studies compared to N-methyl pyrrolidone-soluble poly(vinylidene fluoride) binders. The PMAI binder shows excellent cycle stability, retaining 103% of initial capacity at 1C after 200 cycles and 94% at 5C after 290 cycles. The densely imidazolium-functionalized poly(ionic liquid) reduces charge transfer resistance, lowers Li-ion desolvation activation energy, and increases Li⁺ diffusion coefficient. The improved performance of the cathodic half-cell containing the PMAI binder (PMAI/LFP) is attributed to the ion conduction properties of the imidazolium-functionalized polymer, which participates in cathode-electrolyte interphase (CEI) formation as confirmed by the X-ray photoelectron spectroscopy and mitigates thick CEI formation. The HFPIL also shows better peeling strength and crack-free cycled electrode. These findings provide valuable insights into designing better binders for active materials suffering from poor ionic conductivity.