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.