Novel BCC Anode Materials for High-Power Alkaline MH-Air Batteries

Monday, 25 May 2015: 09:40
Continental Room A (Hilton Chicago)
H. Tan (Liox), N. Weadock, B. Fultz (California Institute of Technology), and R. V. Bugga (NASA Jet Propulsion Laboratory)
Aqueous metal hydride air batteries (MH-Air) offer an inexpensive and safe alternative to the Li-ion batteries for electric vehicles.  To be competitive with Li-ion batteries, the energy density of a metal hydride battery system can be improved by introducing a low-mass bifunctional air cathode and developing a higher capacity anode material. V-based BCC alloys are promising high-capacity anode materials, and discharge capacities of greater than 450 mAh/g have been reported for TiV2.1-xCrxNi0.3 alloys.1 V-based BCC alloys containing Ti and Ni have been shown to separate chemically into a two-phase mixture with a minority Ni-Ti phase within a V-rich matrix,2 and we report the effects of this unmixing on electrochemical properties. 

The Ti-V-Cr-Ni BCC electrode materials were prepared by arc melting and rapid cooling with an Edmund Buhler compact MAM-1 with an attached suction casting crucible. The control samples were made by conventional arc melting under Ar. As-cast ingots were pulverized by hydriding in a Sievert’s apparatus. The samples were analyzed by x-ray diffractometry (XRD), scanning and transmission electron microscopy (SEM, TEM) and energy-dispersive x-ray spectrometry (EDS). The anode was prepared by pressing a mixture of alloy and Ni powders onto Ni mesh. Electrochemical cycling experiments were conducted in a half cell containing  30% aqueous KOH electrolyte, a Ni(OH)2 counter electrode and an Hg/HgO reference electrode. The electrode was charged at 100mA/g and discharged three times at 167mA/g, 50mA/g, and 10mA/g; each to a cut-off voltage of -0.7V vs. Hg/HgO.

EDS mapping showed, as expected,2 a Ti-Ni rich secondary phase in the arc melted sample that permeates the micron-scale primary V phase. (Fig. 1, top, Ni phase in green.) No phase segregation is resolved by EDS for the suction cast alloy. (Fig. 1, bottom). For the first 100 charge-discharge cycles, both materials gave similar capacities. However, at each cycle, the suction cast alloy consistently discharges with 20%-40% more capacity during the high-rate step (167mA/g, ~C/3, 1st step in Fig. 2).

It has been argued that the Ni-Ti phase is important for the electrocatalytic dissociation of H2 molecules during charge, and the matrix V-rich phase is the storage region for hydrogen.2 Reducing the length scale of the phase segregation of the Ni-Ti regions by rapid cooling may therefore improve the kinetics of the material by reducing the hydrogen diffusion length and by increasing surface area for electrocatalysis.

  1. Inoue, et al. Electrochimica Acta, (2012), 59, 23-31.
  2. Tsukahara, et al., J. Alloys Comp. (1997), 253-4, 583-6.