Hard Carbons for Sodium-Ion Battery Anodes: Synthetic Strategies, Material Properties, and Storage Mechanisms

Sodium-ion batteries are attracting much interest due to their potential as viable future alternatives for lithium-ion batteries, in view of the much higher earth abundance of sodium over that of lithium. Although both battery systems have basically similar chemistries, the key celebrated negative electrode in lithium battery, namely, graphite, is unavailable for the sodium-ion battery due to the larger size of the sodium ion. This need is satisfied by "hard carbon", which can internalize the larger sodium ion and has desirable electrochemical properties. Unlike graphite, with its specific layered structure, however, hard carbon occurs in diverse microstructural states. Herein, the relationships between precursor choices, synthetic protocols, microstructural states, and performance features of hard carbon forms in the context of sodium-ion battery applications are elucidated. Derived from the pertinent literature employing classical and modern structural characterization techniques, various issues related to microstructure, morphology, defects, and heteroatom doping are discussed. Finally, an outlook is presented to suggest emerging research directions.