Nitrogen/Carbon Atomic Ratio-Dependent Performances of Nitrogen-Doped Carbon-Coated Metal Oxide Nanocrystals for Anodes in Lithium-Ion Batteries

We report the hydrothermal synthesis of the N-doped carbon-coated NiO nanocrystals (N–C–NiO NCs) with tunable N/C atomic ratios using the nitrogen-containing ionic liquids (ILs) as new carbon precursor, and the N-doped carbon layer composition-dependent performances of N–C–NiO NCs anode for lithium-ion batteries (LIBs). The results indicate that the N-doped carbon coating can significantly enhance the electronic conductivity, effectively avoid the problems of cracking or pulverization of the NiO, and prevent the aggregation of the active materials upon cycling. These properties make the synthesized material a promising anode material for LIBs. The N–C–NiO NCs with the N/C atomic ratio of 21.2% in the N-doped carbon layer show a high specific capacity of ∼710 mAh g^–1 at a current rate of 0.3 C (very closed to the theoretical capacity of 718 mAh g^–1 for NiO), a high rate capability (still able to deliver a discharge capacity of ∼430 mAh g^–1 at a current density of 10 C), and good capacity retention upon cycling (maintains at 710 mAh g^–1 at least up to the 50th cycle) compared with those of pristine NiO nanoparticles. Moreover, the electrochemical performances of the N–C–NiO NCs depend on the composition (N/C atomic ratios) in the N-doped carbon layer and are enhanced with increasing of the N/C ratios. Our approach offers an effective and convenient technique to improve the specific capacities and rate capabilities of highly insulating electrode materials for batteries and may also provide general and effective approach toward the synthesis of other metal oxides coated with N-doped carbon layer