Superelastic electrodes using Ti–Ni shape memory alloys

Ni and Ti sulfides are formed on the surface of a Ti50Ni50 alloy by annealing the alloy at 873 K for 0.24–72 ks under the sulfur pressure of 160 kPa, and then microstructures, martensitic transformation behavior, shape memory characteristics, superelasticity and electrochemical properties are investigated by means of scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, thermal cycling tests under constant load and tensile tests. NiS2 particles are formed first on the surface of the alloy, and then are grown and coalesced with increasing annealing time. When annealing time is longer than 1.2 ks, in addition to NiS2, Ti8.2S11 sulfide is formed, and therefore the surface sulfide layers is consisted of NiS2 and Ti8.2S11. A Ti50Ni50 alloy with the surface sulfide layers shows the shape memory effect and superelasticity clearly. A Ti50Ni50 alloy with the surface sulfide layers shows clear discharge behavior with an increase of annealing time. Multi-voltage plateaus of 1.89, 1.70 and 1.42 V are observed at a cell with electrode annealed for 1.2 ks and an additional plateau at 2.0 V appeared at cells of 3.6 and 10.8 ks. NiS2 is not transformed into pure Ni and Li2S during discharging process directly but is transformed by way of intermediate phases such as NiS and Ni3S2.