Water-soluble densely functionalized poly(hydroxycarbonylmethylene) binder for higher-performance hard carbon anode-based sodium-ion batteries

Abstract

Hard carbon (HC) is a promising anode material in developing sodium-ion batteries (SIBs). However, due to diminished ion diffusion kinetics, low initial coulombic efficiency (ICE) and continuous electrolyte decomposition, HC-based SIBs have led to sluggish rate capability and specific capacity. Herein, we utilised a polymer with dense functional groups containing carboxylic acid in its side chains as a binder that demonstrates ion transport, defect passivation and better mechanical stability. The consecutive polar functional groups provide ion transfer channels and enhanced adhesion to electrode components. The poly(hydroxycarbonylmethylene) (PFA) binder for the HC electrode achieved the highest ICE of 80.8% and a specific capacity of 288 mA h g⁻¹ and 254 mA h g⁻¹ at 30 mA g⁻¹ and 60 mA g⁻¹, respectively, which are superior to those for HC electrodes containing PAA and PVDF binders. Anodic half-cells containing the PFA binder showed high capacity retention of 85.4% (250 cycles) and 91.6% (200 cycles) at current densities of 60 mA g⁻¹ and 30 mA g⁻¹ respectively. In addition, the presence of dense polar groups boosted the diffusion kinetics and lowered the Na⁺ activation energy. XPS and SEM studies further verified that the dense functional groups influence the formation of a thin, stable and inorganic-rich solid–electrolyte interface and crack-free electrodes. Therefore, polymers with dense functional groups will help in the early adoption of SIBs in the market.