Ni sulfide/Ti50Ni50 electrode with the superelasticity

Ni sulfides layers were formed on the surface of a Ti50Ni50 alloy by reacting sulfur with a Ni film deposited on the alloy, and then microstructures, transformation behavior, shape memory characteristics, superelasticity and electrochemical properties were investigated. When a Ni film deposited on a Ti50Ni50 alloy was annealed under the sulfur pressure of 100 kPa at 623 K, sulfides layers consisted of NiS and NiS2 were formed. When annealing was made at 648 K with annealing time less than 0.9 ks, sulfides layers with a mixture of NiS and NiS2 were formed, while only NiS2 was formed when it was made for 1.8 ks. When annealing was made at 673 K with annealing time longer than 0.9 ks, only NiS2 was formed. With raising annealing temperature and prolong annealing time, NiS changed into NiS2 accompanied with a morphological change from a particulate-like to a dense film-like. A Ti50Ni50 alloy with surface NiS2 layer showed the two-stage B2–R–B19′ transformation behavior, the shape memory effect and a partial superelasticity with a superelastic recovery ratio of 78%. NiS2 cathode showed a clear discharge behavior with multivoltage plateaus induced by intermediate reaction products; NiS and Ni3S2. The initial discharge capacity was 743 mA h g−1 corresponding to 85% of theoretical capacity and 65% of capacity duration is obtained at 20th discharge.