Charge–discharge performance of Cr-substituted V-based hydrogen storage alloy negative electrodes for use in nickel-metal hydride batteries

To improve the charge–discharge cycle durability of a TiV2.1Ni0.3 alloy negative electrode with a discharge capacity of ∼470 mAh g−1, the vanadium constituent was partially substituted with chromium. The TiV2.1−xCrxNi0.3 (x = 0.1–0.4) alloys, which were prepared by arc-melting, were composed of two phases, similar to the TiV2.1Ni0.3 alloy. Each constituent was distributed in both phases, and the V and Cr content in the primary phase was higher than that in the secondary phase, although the Ti and Ni content was higher in the secondary phase. The maximum discharge capacity for the TiV2.1−xCrxNi0.3 (x = 0.1–0.4) negative electrodes showed a slight decrease as the x value increased, and their cycle durability was significantly improved due to the effective suppression of the dissolution of V. In particular, the loss of discharge capacity per cycle for the TiV1.7Cr0.4Ni0.3 negative electrode was about one-tenth that for the TiV2.1Ni0.3 negative electrode. The high-rate dischargeability (HRD) was also greatly improved by increasing the Cr content. At 200 mA g−1 the variations of the HRD and the charge transfer resistance (Rct) with the Cr content were similar, while at 400 mA g−1 the change in the HRD at a lower Cr content was markedly different from the change in Rct. Moreover, at a lower Cr content the potential at a 50% degree of discharge stagnated at specific discharge currents over 200 mA g−1. These results strongly suggest that hydrogen diffusion in the primary phase served as the main hydrogen reservoir.