Zwitterionic polymer as binder for LiFePO4 cathodes in lithium-ion batteries

Abstract

Polymer binder plays a decisive role in the electrodes of lithium-ion batteries (LIBs) to stick the active materials and conductive additives firmly onto the current collector and to maintain the integrity of electrodes during charging and discharging. The widely used binder for the LiFePO₄ (LFP) cathodes is polyvinylidene fluoride (PVDF), which suffers poor adhesion strength and is lack of both electronic and lithium-ion conductivities. Herein, we have synthesized a series of zwitterionic polymers (ZIPs) and then are used as aqueous polymer binder for LFP cathodes. Ascribed to the zwitterionic segment in ZIPs, the ZIPs show excellent structural stability in liquid electrolyte and high adhesion. In addition, the ethylene oxide (EO) units and the zwitterionic segment in the binder effectively promote the Li⁺ transportation as indicated by the higher Li⁺ conductivity and the two times higher diffusion coefficient of lithium ions (D_Li⁺) at 3.48 × 10⁻¹² cm² s⁻¹ than those at case of PVDF. The LFP/Li half cells employing the optimized ZIP binder exhibit a high discharge specific capacity of 111.2 mAh g⁻¹ and a high capacity retention of 93.4 % at 3C current density after 500 cycles, which is much better than the cells employing the PVDF binder.