Insights into the diverse precursor-based micro-spherical hard carbons as anode materials for sodium–ion and potassium–ion batteries

In the past decade, the growth of the renewable energy sector worldwide and the depleting resources for Li-ion battery (LIB) technology have pushed the case for complementary storage technologies, especially for stationary energy storage. Sodium-ion batteries (SIB) are on the verge of large-scale commercialization, and potassium-ion battery (PIB) technology is a relative newcomer in the field of future storage technologies. Suitable, low-cost, and environmentally benign commercial electrode materials are the most researched battery systems in this context. Like LIBs, carbonaceous materials can potentially be the first commercial anode material for sodium-ion and potassium-ion batteries. Micro-spherical hard carbons (MSHCs) have the added advantage of higher packing density and low surface area to volume ratio with good structural stability. The metal-ion storage capacity is critically dependent on the morphology, surface area, surface defects, degree of graphitization, and porosity. The review addresses the influence of diverse precursors and synthesis conditions on the electrochemical storage of micro-spherical hard carbons, focusing on the storage-capacity and storage-mechanism correlation and the precursors' structural influence in designing anodes for sustainable, green, and safe SIBs and PIBs.