Research project: Hector: Charge Storage with Metal Nitrides

Molybdenum nitride has been prepared by solution phase ammonolysis of MoCl₅ or Mo(NMe₂)₄ followed by heating at various temperatures. The cubic (γ-Mo₂N) and hexagonal (δ₁-MoN) phases were obtained under various heating conditions, with very different phase behaviours from the two precursor systems and rod/tube morphologies seen after lower temperature treatments. The chloride-derived materials behaved largely as double layer capacitors and showed their best performance (275 F g^-1) when produced at low temperatures, whereas the amide-derived materials (Figure) showed much stronger redox features and had reasonable capacities (up to 161 F g^-1) despite relatively low surface areas. This work continues with a recently published study of Mn₃N₂ and current work in titanium and vanadium nitrides.

Research in sodium batteries has recently regained significant momentum due to the importance of lithium batteries and a recognition that lithium is a finite resource. Negative electrode materials are less well developed than for lithium and we have shown that a number of metal nitrides work well in this application, including the first report of nitrides in sodium batteries with Ni₃N. The Figure shows the performance of Cu₃N, which has a moderate capacity and good cycling performance (89 mA h g^-1 on the 50th cycle at 0.1× theoretical capacity per hour). Good performance has also been achieved vs lithium in a number of cases, and often it has been possible to apply ex situ diffraction experiments to understand how the reactions proceed. A key result of these investigations is that most of these materials react mainly at their surfaces, so optimisation of the synthesis to produce even higher surface areas could improve their capacities.