Overview of hard carbon anode for sodium-ion batteries: Influencing factors and strategies to extend slope and plateau regions

Sodium-ion batteries (SIBs) are regarded as one of the most promising choices for future large-scale energy storage systems due to their abundant sodium source and similar advantages to lithium-ion batteries (LIBs). However, the successful commercialization of SIBs predominantly relies on the progress in advanced anode material development. Hard carbon has received much attention as a promising anode material for energy storage systems because of its low cost, abundant source and high capacity. Based on the investigations regarding Na+ storage, the charge-discharge curve of hard carbon can be usually divided into two parts: the slope region at high voltage (>0.1 V vs. Na+/Na) and the plateau region (<0.1 V vs. Na+/Na) at low voltage. Both regions are closely related to the overall electrochemical performance of SIBs. Herein, we thoroughly explore the contributing factors that influence the slope and plateau regions of hard carbon and provide a comprehensive summary of the strategies used to extend the slope/plateau region, including heteroatom doping, pore structure design, interlayer space as well as electrolyte system. This review offers deep insights into the strategies of achieving high-performance hard carbon electrodes, which present promising opportunities for the development of next-generation sustainable batteries beyond LIBs.