Fiber-Type Ni(OH)2 Electrode with α/γ Phase Transformation: High-Capacity and High-Voltage Performances of Nickel-Metal Hydride Battery

Fiber-type electrodes with 20 mol%Al-added nickel hydroxide were prepared by electrodeposition on nickel-plated carbon fibers. Discharge curves of the nickel-metal hydride (Ni-MH) test cells containing the fiber-type electrodes exhibited a wide plateau region at a 1.3 V discharge voltage and a high 305 mAh/g discharge capacity at the 1 C rate. Even after a 200 C charging, a 220 mAh/g discharge capacity was maintained. Based on the cycle-life performance measurement, the cell exhibited a good voltage retention after 700 cycles. Synchrotron XRD patterns showed that the as-prepared and discharged fiber-type electrodes consisted of α-Ni(OH)2 phases. An approximate 80% α-Ni(OH)2 phase was transformed into a γ-NiOOH phase after charging to 318 mAh/g. The active material layer contracted during the charge process, but returned to its original volume after discharging. The active-material layer did not expand more than its original volume, thus maintaining contact between the active material and fiber substrate which leads to a good voltage retention during cycling.