Changing the pore structure and surface chemistry of hard carbon by coating it with a soft carbon to boost high-rate sodium storage

Changes to the microstructure of a hard carbon (HC) and its solid electrolyte interface (SEI) can be effective in improving the electrode kinetics. However, achieving fast charging using a simple and inexpensive strategy without sacrificing its initial Coulombic efficiency remains a challenge in sodium ion batteries. A simple liquid-phase coating approach has been used to generate a pitch-derived soft carbon layer on the HC surface, and its effect on the porosity of HC and SEI chemistry has been studied. A variety of structural characterizations show a soft carbon coating can increase the defect and ultra-micropore contents. The increase in ultra-micropore comes from both the soft carbon coatings and the larger pores within the HC that are partially filled by pitch, which provides more Na⁺ storage sites. In-situ FTIR/EIS and ex-situ XPS showed that the soft carbon coating induced the formation of thinner SEI that is richer in NaF from the electrolyte, which stabilized the interface and promoted the charge transfer process. As a result, the anode produced fastcharging (329.8 mAh g^–1 at 30 mA g^–1 and 198.6 mAh g^–1 at 300 mA g^–1) and had a better cycling performance (a high capacity retention of 81.4% after 100 cycles at 150 mA g^–1). This work reveals the critical role of coating layer in changing the pore structure, SEI chemistry and diffusion kinetics of hard carbon, which enables rational design of sodium-ion battery anode with enhanced fast charging capability.

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