Classification, fabrication, and modification of carbon spheres for sodium-ion batteries

Carbonaceous materials for sodium-ion batteries (SIBs) have garnered considerable attention due to their abundant availability, renewable nature, and cost-effectiveness. Compared with other forms of carbonaceous materials, carbon spheres (including hard carbon and soft carbon) are promising anode candidates due to their potential high tap density, and excellent structural stability, which make them suitable for commercialization applications. Recently, great efforts have been devoted to developing high-performance carbon spheres and certain progress has been achieved. However, to our best knowledge, there is a scarcity of specialized reviews focusing on carbon spheres for SIBs. A timely review could significantly accelerate their commercial application. In this paper, we first introduce the classification of carbon spheres that are employed as anodes in SIBs. Then, the precursors, preparation methods, and performance optimization of carbon spheres are summarized and analyzed. Finally, some prospectives of carbon spheres have been provided to give a reference for the design of commercializable carbon spheres for advanced SIBs.