Lithium-ion batteries have been extensively investigated for a range of portable electronic devices, but a major downside to current technology is the slow charging speeds. A potential solution is to utilize pseudocapacitve materials, in which the diffusion length for lithium ions is short. For example, Nb₂O₅ is a promising anode material with almost no kinetics limitations for solid-state Li⁺ diffusion due to the facile two-dimensional Li⁺ transport pathways within the structure. However, it is also a large bandgap semiconductor, with a low bulk electrical conducitivity of ~3.4 x 10^-5 S cm^-1at 300 K. The low electronic conductivity hinders its high-rate performance, especially for thick electrodes with high mass loadings.

Numerous composite structures have been explored to improve the sluggish mobility of electrons in Nb₂O₅, such as blending with carbon nanotube, graphene composites, and carbon coating. Here we report an Nb₄N₅/Nb₂O₅/reduced graphene oxide (rGO) composite structure with improved kinetics prepared via partial nitridation of Nb₂O₅. Nb₄N₅ shows excellent metallic conductivity of 2000 S cm^-1, which facilitates the interparticle electron conduction pathway in combination with the conductive graphene scaffold. Very high capacity retention at high charging/discharging speed is achieved in these materials, though the high power density comes at some cost of the capacity because the nitride is electrochemically inactive. One important advantage of the niobium oxide/nitride system, compared to pure Nb₂O₅/carbon composites is improved interface between the electrochemically active material and the conductive coating, since Nb₂O₅ can transform to Nb₄N₅ with almost no structural change. The composite also has a high volumetric capacity, since carbon additive is not needed in slurry electrodes. We believe metal oxide/metal nitride composites could provide a new design for high power electrode materials.