Andrew L. Hector
University of Southampton, Highfield, Southampton, SO17 1BJ
Over the last 25 years lithium-ion batteries have been a key component in the miniaturisation and mobilisation of electronics. The worldwide need for effective charge storage is increasing as transport and levelling of loads due to renewable power generation, and both the supply of lithium and the limited geographical spread of exploitable lithium sources are concerns. Interest in sodium cells as an alternative has ramped up rapidly in recent years, with ~1800 papers published in 2014 compared with ~100 in 2007.
Metal nitrides have been examined fairly extensively as charge storage electrodes. In aqueous supercapacitors their high electronic conductivity results in high charge/discharge rate capability while surface oxidation provides redox capacity. In lithium batteries they typically act as conversion electrodes, with formation of the metal plus lithium nitride during reduction and reformation of the metal nitride on oxidation. We published the first paper on a metal nitride in a sodium cell in 2013, where we showed that nickel nitride had a capacity and cycle life in the low potential region that was comparable with the best negative electrode materials available at the time. Since then we have shown higher capacities and good cycling stability in manganese and copper nitride. In tin nitride we have maintained the good stability but achieved a doubling of the capacity. These results will be presented in the context of other improvements in these cells in the recent literature, including work by others on the vanadium nitride system.