Salt Template Tuning Microstructure of Hard Carbon Anodes with Extended Plateau Capacity

Abstract

The microstructure (pore structure and microcrystal structure) plays a crucial role in determining the capacity, especially the plateau capacity of hard carbon (HC) anodes for sodium-ion batteries (SIBs). However, it is difficult to modulate the pore structure and the microcrystal structure effectively and simultaneously. Here, we synthesized HCs with tailored pore structures and microcrystal structures from sodium carboxymethyl cellulose (CMC-Na) by adding the inorganic salt KCl as a microstructure modulator. Due to the hindrance of KCl to the over-graphitization of pseudo-graphite domains, the optimized HC (KHCM600) with more abundant C═O groups, closed pores, and expanded interlayer spacing (>0.37 nm) was fabricated, which provides sufficient active sites not only for Na⁺ adsorption but also for Na⁺ intercalation and pore filling. Furthermore, the minimized lateral width of pseudo-graphitic domains in KHCM600 is achieved concurrently to improve the accessibility of Na⁺ to the filling sites and intercalation sites. The KHCM600 exhibits a high reversible sodium storage capacity of 364.3 mAh g^–1 at 30 mA g^–1 and a high plateau capacity of 281.9 mAh g^–1, with a superior initial Coulombic efficiency of 88.2%. It also demonstrated an excellent rate performance (196.6 mAh g^–1 at 2 A g^–1). This study provides an innovative approach to simultaneously modulate the pore structure and microcrystal structure of HC, thereby improving sodium storage performance.