Molten salt Mg2+/Mg bifunctional system for enhancing graphitization of low-temperature biomass-derived carbon

Abstract

The development of low-temperature graphitization methods for biomass-derived carbon is critical for sustainable lithium-ion battery anodes. Herein, we propose a MgCl₂–NaCl–KCl molten salt system exploiting the bifunctional role of Mg²⁺/Mg to achieve graphitization at 750 °C. The strong oxygen affinity of Mg²⁺ synergizes with electrochemical polarization to rapidly deoxygenate carbon surfaces, while Mg deposition catalyzes the conversion of SiO₂ impurities to conductive SiC (via SiO₂ + 2 Mg → Si + 2MgO; Si + C → SiC) and promotes carbon rearrangement. The resulting graphite/SiC composite exhibits enhanced crystallinity (27% graphitization degree) and electrochemical performance, delivering a capacity of 198 mAh g⁻¹ at 0.1 A g⁻¹ and retaining 70.2% capacity after 6000 cycles. In summary, this study fully exploited the strong bonding capability of Mg²⁺ with oxygen and the reduction and catalytic properties of deposited Mg. This dual-functional role not only significantly reduced the required temperature for the reaction process (750 °C) but also maintained the graphitization process of biomass-derived carbon at high temperatures (> 750 °C). This method provides a low-cost, low-energy-consumption pathway for the high-value-added application of biomass-derived hard carbon materials in lithium-ion battery graphite anodes.