Research progress on hard carbon materials in advanced sodium-ion batteries

Sodium-ion batteries have recently emerged as a promising alternative energy storage technology to lithium-ion batteries due to similar mechanisms and potentially low cost. Hard carbon is widely recognized as a potential anode candidate for sodium-ion batteries due to its high specific surface area, high electrical conductivity, abundance of resources, and low cost. Despite recent progress in the study of hard carbon materials, the understanding of the intrinsic charge storage mechanism behind their charging/discharging behavior remains controversial. In order to gain a comprehensive understanding of the sodium storage mechanism of hard carbon as well as to rationally design high-performance hard carbon anode materials, this review discusses four typical hard carbon storage models and its mechanism for storing sodium ions. It comprehensively elucidates the key bottleneck issues of the hard carbon anode structure and electrolyte in sodium-ion batteries and proposes several solutions to enhance the performance of hard carbon materials through structural design and electrolyte optimization. In addition, the design and manufacturing progress of coal-based hard carbon materials are discussed in detail. Finally, several possible strategies for the development of high-performance hard carbon negative electrode sodium-ion batteries in the future are proposed.