Ti-Ni Alloys in Flexible Secondary Battery

Superelastic Ti-Ni alloys have been known to be available for fabricating highly flexible secondary battery with long cycle life which is required for mobile electrical appliances such as wearable computer.  In this presentation, we will introduce various electrodes system using Ti-Ni alloys, such as LiFePO₄/Graphite/TiNi, sulfide/Ti-Ni, LiNiTiO₂/Ti-Ni and Si/Ti-Ni electrodes.  In the LiFePO₄/Graphite/Ti-Ni system in which Ti-Ni alloys are used as current collector shows excellent flexibility and fatigue life, which is ascribed to the superelasticity.  Ti-Ni alloys are also proved to be electrochemically stable for Li ion, which is necessary condition for current collector material in Li ion secondary battery.  In the sulfide/Ti-Ni electrode, sulfides such as Ti sulfide and Ni sulfide act as a cathode and Ti-Ni substrates act as a current collector.  The sulfides are made on the surface of Ti-Ni current collector by annealing at sulfur atmosphere.  In the LiNiTiO₂/Ti-Ni electrode, LiNiTiO₂ act as a cathode and Ti-Ni substrates act as a current collector. LiNiTiO₂ is formed on the surface of Ti-Ni substrate by annealing under oxygen atmosphere after deposition of Li thin film.  These methods are cost effective comparing with conventional method including multi-stage processes such as mixing, pasting and drying.  In the silicon/Ti-Ni electrode, Si act as an anode and Ti-Ni substrates act as a current collector.  The stress-induced martensitic transformation in Ti-Ni substrate accommodate the stress in Si developed by large volume change accompanied by lithiation and delithiation, thus improve cycle life of Si electrode.