Carbon Excess CN: A Potential Candidate as Li-Ion Battery Material.

Xu et al.'s recent experimental work ( Adv. Mater. 2017, 29, 1702007) suggested that CN is a potential candidate as Li-ion battery with unusual electrochemical characteristics. However, the obvious capacity loss (from 787.3 to 383.3 mA h·g) occurs after several cycles, which restricts its high performance. To understand and further solve this issue, in the present study, we have studied the intercalation processes of Li ions into CN via first-principle simulations. The results reveal that the Li-ion theoretical capacity in pure CN is only 133.94 mA h·g, the value is obviously lower than experimental one. After examining the experimental results in detail, it is found that the chemical component of the as-generated C N structure is actually CN with N excess. In this case, the calculated theoretical capacity is 837.06 mA h·g, while part of Li ions are irreversibly trapped in CN, resulting in the capacity loss. This phenomenon is consistent with the experimental results. Accordingly, we suggest that N excess CN, but not pure CN, is the proposed Li-ion battery material in Xu et al.'s experiment. To solve the capacity loss issue and maintain the excellent performance of CN-based anode material, the CN with slightly excess C (CN), which has been successfully fabricated in the experiment, is considered in view of its relatively low chemical activity as compared with N excess CN. Our results reveal that the C excess CN is a potential Li-ion battery material, which exhibits the low open circle voltage (0.12 V), high reversible capacity (840.35 mA h·g), fast charging/discharging rate, and good electronic conductivity.