Review—Hard Carbon Negative Electrode Materials for Sodium-Ion Batteries

A first review of hard carbon materials as negative electrodes for sodium ion batteries is presented, covering not only the electrochemical performance but also the synthetic methods and microstructures. The relation between the reversible and irreversible capacities achieved and microstructural features is described and illustrated with specific experiments while discussing also the effect of the electrolyte. A summary of the current knowledge is given while emphasizing the possibility of further performance improvements by thoroughly mastering structure-property relationships and also discussing the main current bottleneck to maximize energy density in real applications: the first cycle irreversible capacity. Finally, a short conclusion and perspectives session is provided highlighting necessary developments in the field to turn the present optimistic research prospects into tangible practical products.