Hard carbon for sodium storage: Mechanism and performance optimization

Due to the shortage of lithium resource reserves and the pressure of rising prices, sodium-ion batteries have regained the attention of the public, and shown great potential for application in the fields of grid energy storage and low-speed vehicles to achieve the purpose of complementing lithium-ion batteries, so it is imperative to promote the commercial application of sodium-ion batteries. For sodium-ion battery anode materials, hard carbon is the material most likely to be used commercially. However, there is still much work to be done before its commercialization. This review provides a comprehensive overview of the current research status from the following three aspects. First, the microstructure and sodium storage active sites of hard carbon are described. Then, the mechanism of sodium storage in hard carbon is investigated, which can be broadly categorized into four model, "insertion–filling", "adsorption–insertion", "adsorption–filling", and "multistage". Finally, from the perspective of improving the electrochemical performance of hard carbon, the performance improvement strategies proposed in recent years are summarized. Combined with the performance of hard carbon commercial products of some enterprises, the future development goal of hard carbon is prospected, hoping that all sectors of society can work hard for this common goal.