Sodium-ion battery is attracting much attention as large-scale energy storage devices, because sodium is an earth-abundant element and it is of low cost. Hard carbon is one of the most promising anode material for the sodium-ion battery owing to its large reversible capacity of ca. 300 mAhg^–1[1]. However, the reaction mechanism has not been fully elucidated.

Both lithium and sodium can intercalate into hard carbon. Their charge–discharge curves show similar profiles, having a slope and a flat region, therefore the reaction mechanism has been considered to be similar. However, our ab initio calculations and experimental results revealed the clear differences. We built several models of hard carbon with two graphene sheets that have typical point defects (MV, DV, SW). Initially, sodium adsorption onto the point defects occurs well above the sodium plating potential, and induces expansion of interlayer distance of the two graphenes. Then, sodium intercalation into the interlayer space becomes possible[2]. On the other hand, lithium ions intercalate into the interlayer at the slope region followed by the aggregation into nanopores[3].

[1] D. A. Stevens and J. R. Dahn, J. Electrochem. Soc., 147 (4), 1271 (2000)

[2] P. Tsai et al., J. Mater. Chem. A, 3, 9763-9768 (2015)

[3] Park C. W. et al., Carbon, 38, 995-1001(2000)