Overview of coals as carbon anode materials for sodium-ion batteries

Energy storage is an important technology in achieving carbon neutrality goals. Compared with lithium-ion batteries, the raw materials of sodium-ion batteries are abundant, low-cost and highly safe. Furthermore, their costs are expected to be further reduced as large-scale applications take off, making them viable for energy storage applications. The primary anode material for sodium-ion batteries is hard carbon, which has a high sodium-ion storage capacity but is relatively expensive, limiting its applications in energy storage. In order to widen the applications of sodium-ion batteries in energy storage and other fields, it is particularly important to develop anode materials that have both high performance and low cost. Coals, with abundant reserves and worldwide availability, can serve as low-cost carbon sources for anode materials. Additionally, coals of different grades of metamorphism have different structural characteristics that can be tailored for the structural characteristics of coal-based anode materials for sodium-ion batteries. Recent researches on tailoring coals as the anode materials for sodium-ion batteries is summarized and the recent progress made towards mitigating the existing issues is analysed in this review. Specifically, the impacts of different grades of metamorphism on the sodium-ion storage performance of coal-based anode materials prepared with direct carbonization are discussed in detail. Studies on improving the electrochemical performances of coal-based anode materials through pore and microcrystalline structure controls, and surface as well as interface modifications are presented. Finally, the advantages and disadvantages of different preparation methods are identified. To make the industrial applications of coal-based anode materials for sodium-ion batteries more viable, the importance of the de-ashing process is introduced.