When compared to expensive lithium metal, the metal sodium resources on Earth are abundant and evenly distributed. Therefore, low-cost sodium-ion batteries are expected to replace lithium-ion batteries and become the most likely energy storage system for large-scale applications. Among the many anode materials for sodium-ion batteries, hard carbon has obvious advantages and great commercial potential. In this review, the adsorption behavior of sodium ions at the active sites on the surface of hard carbon, the process of entering the graphite lamellar, and their sequence in the discharge process are analyzed. The controversial storage mechanism of sodium ions is discussed, and four storage mechanisms for sodium ions are summarized. Not only is the storage mechanism of sodium ions (in hard carbon) analyzed in depth, but also the relationships between their morphology and structure regulation and between heteroatom doping and electrolyte optimization are further discussed, as well as the electrochemical performance of hard carbon anodes in sodium-ion batteries. It is expected that the sodium-ion batteries with hard carbon anodes will have excellent electrochemical performance, and lower costs will be required for large-scale energy storage systems.

Having a high theoretical capacity density of 4200 mAh g⁻¹, silicon has been highlighted as one of the most promising anode materials for lithium-ion batteries. Countless silicon-based materials have been proposed and reported in research articles, mostly synthesized using bottom-up methods. While the infamous volume expansion issue can be settled with the bottom-up processes, the complicated protocols and high cost leave a non-neglectable gap between laboratory-scale and mass production. The top-down ball-milling method is still favored by industrial suppliers because of its simplicity and cost-effectiveness, even with compromised electrochemical performances. This paper reviews the latest development of ball-milling-based silicon anode materials. Although the ball-milling process seems straightforward, the procedures and parameters influencing the product have hardly been discussed in research papers compared to the bottom-up ones. This paper reviews recent advances in ball-milling-based silicon anode materials, provides a material comparison, and discusses how ball milling can provide lithium-ion batteries with greater possibilities at a larger scale.