Dual-functional chemical pre-sodiation of carbon-coated hard carbon anodes with initial Coulombic efficiency up to 99.5% for sodium-ion batteries

Hard carbon (HC) has been regarded as a promising anode material for sodium-ion batteries (SIBs), yet its practical commercial application is hindered by the low initial Coulombic efficiency (ICE) and unstable solid electrolyte interphase (SEI). Herein, we propose a dual-functional strategy that synergizes surface engineering and solution chemical pre-sodiation in SIBs. Along with the graphitic carbon coating on HC acting as a conductive buffer network and shield the surface defects, we also facilitated a potential difference to drive sodium ions into the material by leveraging the low potential advantage of sodium biphenyl (Na-Bp), thereby inducing the formation of a pre-SEI layer and subsequent formation of a thin, dense and NaF rich inorganic components of mature SEI layer during cycling. This innovative approach effectively compensated for the irreversible loss of sodium ions during the initial cycle and significantly enhanced long-term cycling stability. Thanks to the seamless integration of these two strategies, the pre-sodiated electrode (pCH₄-HC) exhibited a desirable ICE of 99.5% and a high reversible capacity of 321.7 mA h g⁻¹. Last but not the least, when pCH₄-HC was coupled with NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode, the full-cell pCH₄-HC∥NFM demonstrated excellent cycling stability and rate performance, underscoring the substantial benefits of pre-sodiation technology for rapid full-cell matching. It validated the robust adaptability of our dual-strategy approach, broadening the scope of pre-sodiation technology and accelerating the development of high performance in SIBs.