Degradation mechanism of room temperature Na/Ni3S2 cells using Ni3S2 electrodes prepared by mechanical alloying

Abstract

Ni3S2 powder has been fabricated by mechanical alloying Ni and S powders and the electrochemical properties of Na/Ni3S2 cells fabricated from Na anodes and Ni3S2 cathodes and 1 M sodium trifluoromethanesulfonate in tetraethyleneglycol dimethylether liquid electrolyte have been investigated. Upon discharging, the Ni3S2 cathode transformed to amorphous Na2S, Ni and residual unconverted Ni3S2. On charging, the pristine structure of Ni3S2 is fully recovered. The reversibility of this anode/cathode pair has been demonstrated and the discharge capacity loss of Na/Ni3S2 cells has been investigated over 100 cycles. From TEM, EDS, XRD, and EIS results, the degradation mechanism appears to be the formation of nano-particles of Ni3S2 and Na2S which become detached from the bulk cathode material causing the continuous increase in the interfacial resistance. Furthermore, many small cracks in Ni3S2 cathode material are caused during repeated cycling and this is an additional phenomenon that leads to further degradation of the discharge capacity during cycling. (C) 2013 Elsevier B.V. All rights reserved.