High Energy Density Aqueous Metal Hydride-Air Batteries

Li-ion batteries exhibit poor abuse tolerance which necessitate complex charge management electronics, elaborate thermal management and adequate mechanical reinforcements, which result in reduced system-level energy densities and higher battery costs.  These constraints make the aqueous systems more appealing, especially if their energy densities can be further improved.  In this context, we are developing low cost and safe (aqueous) rechargeable metal-hydride / air batteries with high energy densities (200 Wh/kg and 400 Wh/l at cell level) and long cycle life for electric vehicle applications under an ARPA-E sponsored program.  Our effort is focused on developing: i) new metal hydride alloys with higher hydrogen absorption capacities, ii) stable electro-catalysts for bifunctional oxygen electrode or suitable dual-electrodes for oxygen reduction/oxidation, iii) Stable hydroxyl membranes for improved cell durability and easier water management, and iv) New cathode designs with dispersed catalysts over nanostructured frameworks.

For the metal hydride anodes, the desired performance parameters include: i) High hydrogen absorption of > 1.5 w% in the gas phase with low absorption pressures (below 2 bar) and desorption temperatures (ambient), ii) High electrochemical capacity of 400 mAh/g, iii) Low corrosion rate and long cycle life (> 1000 cycles) and iv) fast electrode kinetics to operate at  > C/3 rate. We initially focused on AB₅ alloys, supplied by BASF (e.g., La_10.5Ce_4.3Pr_0.5Nd_1.4Ni_64.3Co_5.0Al_6.0Mn_4.6Cu_3.4)¹ to establish the baseline for MH-air cells.    In parallel, we have started developing metal hydride alloys of V-based BCC Alloys, specifically quaternary alloys of Ti-V-Ni-Cr, which reportedly have a maximum discharge capacity of 450 mAh/g with 90% capacity retention over 30 cycles.² For the metal hydride air cells, we have been focusing on both the options of bifunctional air cathode as well as dual-electrode configuration, i.e., with an auxiliary electrode to support the oxygen evolution reaction (OER).   Our initial studies for the bifunctional air cathode are based on thin-film noble metal catalysts, dispersed on nanostructures. For the dual electrode configuration, we are examining low-cost commercial electrodes based on manganese oxide catalysts for oxygen reduction, and Ni-based electrodes for OER. In either case, we have been able to demonstrate good cycling characteristics in laboratory MH-air test cells.  In addition, we performed fundamental studies on the MH anodes as well as air cathodes, using the Differential Electrochemical Mass Spectroscopy (DEMS) technique (developed at Liox with swage-lock test cells coupled with mass spectrometer), to understand the gaseous environments within a MH-air cell.

1. Young et al., J. of Alloys and Compounds 509 (2011) 4896.
2. Inoue et al., Electrochimica Acta (2012), 59, 23-31