The Development of High Capacity V-Based BCC Electrodes for Aqueous Metal Hydride Air Systems

Abstract

The energy density of traditional Ni-MH batteries could be dramatically increased by developing a higher capacity metal hydride alloy. An even larger increase in energy density for MH-type electrochemical cells would be seen by pairing the high capacity metal hydride with an air cathode to create an MH-air battery. Vanadium based BCC alloys are a promising choice for high capacity anode materials, and discharge capacities of greater than 450 mAh/g have been reported in the literature for TiV2.1-xCrxNi0.3 alloys.1 The kinetics and power density of these alloys are further improved by reducing the microstructure via rapid solidification.2 Pure vanadium metal forms a dihydride with a total hydrogen content of 4 wt.% or a theoretical electrochemical capacity of 1068 mAh/g. Practical cycling capacity is limited by the fact that the VH and VH2 phase transitions differ by several orders of magnitude in pressure.3 The plateau pressures of these phase transitions are strongly affected by the composition and lattice parameter of host metal alloy, and the kinetics depend on microstructure and electrode design. We initially identified a ternary TiVNi alloy4 that delivers close to the theoretical capacity of about 1100 mAh/g by accessing both the monohydride and dihydride phase transitions. The transitions manifest as two voltage plateaus in the charge/discharge profile of the metal hydride electrode, as seen in the accompanying figure. The difference in voltage plateaus is directly related to the difference in gas phase plateau pressures via the Nernst equation. From the figure we see a difference of ~120mV, corresponding to a 4 – 5 order of magnitude difference in plateau pressure. This alloy has poor cycle life, so Cr is added to prevent corrosion resistance. We will present results on the effect of composition on the capacity and cycle life of the alloy, and the role that microstructural reduction through rapid solidification and ball milling play on improving the kinetics. 1. H. Inoue, S. Koyama, and E. Higuchi, Electrochimica Acta, 59, 23–31 (2012). 2. H. Tan, N. Weadock, B. Fultz, and R. V. Bugga, Meet. Abstr., MA2015-01, 19–19 (2015). 3. M. Tsukahara et al., J. Alloys Compd., 224, 162–167 (1995). 4. C. Iwakura, W.-K. Choi, R. Miyauchi, and H. Inoue, J. Electrochem. Soc., 147, 2503–2506 (2000). Figure 1