Electrochemical Insight into the Sodium-Ion Storage Mechanism on a Hard Carbon Anode

Hard carbon (HC) has been actively investigated as a high-capacity and low-cost anode material for sodium-ion batteries (SIBs); however, its sodium-storage mechanism has remained controversial, which imposes great difficulties in the design and construction of better microstructured HC materials. To obtain a deeper understanding of the Na-storage mechanism, we comparatively investigated electrochemical behaviors of HC and graphite for Na- and Li-storage reactions. The experimental results reveal that the Na-storage reaction on HC at a low-potential plateau proceeds in a manner similar to the Li+-insertion reaction on graphite but very differently from the Li+-storage process on HC, suggesting that the Na-storage mechanism of HC at a low-voltage plateau operates through the Na+ intercalation into the graphitic layers for the formation of sodium–graphite intercalation compounds (Na–GICs) and is consistent with the “adsorption–intercalation” mechanism. Our work might provide new insight for designing better HC materials of high-energy density SIBs.