Structure and function of hard carbon negative electrodes for sodium-ion batteries

Practical utilisation of renewable energy from intermittent sustainable sources such as solar and wind relies on safe, reliable, cost-effective, and high-capacity energy storage systems to be incorporated into the grid. Among the most promising technologies aimed towards this application are sodium-ion batteries(SIBs). Currently, hard carbon is the leading negative electrode material for SIBs given its relatively good electrochemical performance and low cost. Furthermore, hard carbon can be produced from a diverse range of readily available waste and renewable biomass sources making this an ideal material for the circular economy. In facilitating future developments on the use of hard carbon-based electrode materials for SIBs, this review curates several analytical techniques that have been useful in providing structure-property insight and stresses the need for overall assessment to be based on a combination of complementary techniques. It also emphasises several key challenges in the characterisation of hard carbons and how various in situ and operando techniques can help unravel those challenges by providing us with a better understanding of these systems during operation thereby allowing us to design high-performance hard carbon materials for next-generation batteries.