Pseudocapacitive materials have been extensively investigated as a new type of fast charging energy storage material. For example, Nb2O5 is a promising pseudocapacitive 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 has a low bulk electrical conductivity of ~3.4 x 10-5 S cm-1 at 300 K, which 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 on an Nb4N5/Nb2O5-reduced graphene oxide (rGO) composite structure with improved kinetics prepared via partial nitridation of Nb2O5. Nb2O5/Nb4N5 has a core/shell structure confirmed by TEM and XPS, and the nitride shell thickness can be controlled by varying the nitridation time. The excellent electrical conductivity of Nb4N5 (~2000 S cm-1) and the direct transformation from Nb2O5 to Nb4N5 with almost no structural change facilitate the interparticle electron conduction pathway and improve the interface contact between the electrochemically active material and the conductive coating. Thin film studies were conducted to understand the fundamental contribution of the conductive nitride shell. Very high capacity retention at high charging/discharging speed was achieved in these materials, though the high power density comes at the cost of some capacity because the nitride is electrochemically inactive. The optimal nitride content was found to maximize electric conductivity without blocking Li+ pathway into Nb2O5. Some slurry electrodes were also tested to explore the potential for scale-up. The results indicate that metal oxide/metal nitride composites could provide a new design for high power electrode materials.

Nb4N5/Nb2O5-rGO composites as pseudocapacitive anode materials for high power lithium ion batteries

253rd National Meeting (2017)

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