A “grafting technique” to tailor the interfacial behavior of hard carbon anodes for stable sodium-ion batteries†

Abstract

Hard carbon (HC), a commercially available anode for sodium-ion batteries (SIBs), suffers from an unstable solid electrolyte interphase (SEI) and massive irreversible sodium loss caused by oxygen-containing functional groups, resulting in reduced initial coulombic efficiency (ICE) and shortened calendar life. Conventional ways to minimize irreversible sodium loss by removing functional groups inevitably reduce the sodium storage capacity in the slope region. Herein, we report a novel strategy for grafting a highly fluorinated molecule, 4-(2,2,2-trifluoroacetyl)-benzoic acid (FB), on the HC surface (FHC), which functionally enhances the reversible sodium storage behavior in the slope region and contributes to the architecture of a robust NaF-rich SEI. The FB molecule is capable of reversibly storing sodium ions by internal structural switching and supplying extra F atoms to enable the formation of a ∼5 nm thin, NaF-rich, robust SEI on the FHC surface. The as-optimized FHC with 90.0% ICE exhibited a reversible sodium storage capacity of ∼359.0 mA h g⁻¹ and is capable of servicing >5000 cycles at a high current density of 2.0 A g⁻¹, whereas the commercial HC is capable of servicing only 340 cycles. Thus, our work provides a novel perspective on HC interface design.