Overcoming copper-induced conversion reactions in nickel disulphide anodes for sodium-ion batteries

Employing copper (Cu) as an anode current collector for metal sulphides is perceived as a general strategy to achieve stable cycle performance in sodium-ion batteries, despite the compatibility of the aluminium current collector with sodium at low voltages. The capacity retention is attributed to the formation of copper sulphide with the slow corrosion of the current collector during cycling which is not ideal. Conventional reports on metal sulphides demonstrate excellent electrochemical performances using excessive carbon coatings/additives, reducing the overall energy density of the cells and making it difficult to understand the underlying side reaction with Cu. In this report, the negative influence of the Cu current collector is demonstrated with in-house synthesised, scalable NiS2 nanoparticles without any carbon coating as opposed to previous works on NiS2 anodes. Ex situ TEM and XPS experiments revealed the formation of Cu2S, further to which various current collectors were employed for NiS2 anode to rule out the parasitic reaction and to understand the true performance of the material. Overall, this study proposes the utilisation of carbon-coated aluminium foil (C/Al) as a suitable current collector for high active material content NiS2 anodes and metal sulphides in general with minimal carbon contents as it remains completely inert during the cycling process. Using a C/Al current collector, the NiS2 anode exhibits stable cycling performance for 5000 cycles at 50 A g−1, maintaining a capacity of 238 mA h g−1 with a capacity decay rate of 8.47 × 10−3% per cycle.