Hard carbon anodes for advanced sodium ion batteries: A review on sodium storage mechanism and strategies to improve the initial Coulombic efficiency

• Sodium storage mechanisms of HC anodes are reviewed and discussed. • The reasons causing the unsatisfied ICE by using HC anodes are summarized. • Strategies on improving ICEs of HC anodes are reviewed and analyzed. • Challenges for the development of HC based anodes are discussed and outlooked. Benefitting from the merits of affordable cost, inexhaustible sodium resources and excellent safety properties, sodium ion batteries (SIBs) have attracted extraordinary attentions and shown great potential in grid-scale energy storage system in recent decades. The electrochemical performance of advanced electrode materials affects the commercialization of SIBs. Among all kinds of electrode materials, hard carbon (HC) is regarded as a most attractive anode material and has received tremendous attention owing to its low-price, wide availability, excellent reversible capacity, and low working potential. Nevertheless, the major issue is the low initial Coulombic efficiency (ICE) of HC with low energy density, and thus severely restricting the commercial applications in SIBs. Therefore, in this review, the sodium storage mechanism and the reasons causing the unsatisfied ICE of HC anodes are discussed and summarized in detail. Meanwhile, the strategies on improving the ICEs for HCs are summarized, including structure and morphology design, defect engineering, surface engineering, heteroatom doping, optimization of electrolyte, selection of electrode configurations and pre-sodiation according to the intrinsic property of HC and external factor to introduce. Additionally, the prospects and perspectives on development of HC anodes with the desirable ICEs are also briefly outlined, contributing to the commercialization of SIBs. It is anticipated that this review can provide guidance on developing and designing the suitable HC electrodes for high-performance SIBs, which will be conducive to realizing the sustainable and renewable society.