A metal hydride (MH)/air secondary battery is one of the next generation energy storage devices, since a high energy density and a high power density are expected from its unique property that the active mass of the positive electrode is oxygen in air. We have developed this type of battery and demonstrated that a non-packaged cell fabricated using PTFE container has about 900 Wh/L [1] and a laminate cell using polyethylene film about 772 Wh/L [2], which are beyond the energy density of lithium ion secondary batteries. However, the laminate cell also indicated some issues on the structure and components of the cell, causing the electrolyte’s leakage and a poor cycleability. This work was done to improve the charge-discharge performance of the MH/air laminate cell, especially the cycling performance, and evaluated the charge-discharge behaviors of the cell obtained with some modification in the process and materials.

The laminate cell consisted of an MH electrode (23 mm x 23 mm), an air electrode (23 x 23 mm), and a separator with 6 mol/L KOH solution between the electrodes, which were thermally sealed and packed in polyethylene (PE) film having an open area (13 x 13 mm) on the air electrode. All the components were the same as those used in our previous work [1]. We examined the design, size, and thickness of PE film to improve the adhesion to the electrode and between PE films and to inhibit the leakage of the alkaline solution. A water repellent film attached to the air electrode to prevent the flooding of the solution was also modified to suppress the increase in internal pressure by oxygen generated during charge. The cell was operated with constant current (0.05 C) at room temperature in a desiccator, and the cell voltages during charge and discharge were measured, in which charge was limited for 2 hours and the cut-off voltage of discharge was 0.5 V.

Some modifications in the structure and components of the laminate cell, e.g., the change in the electrode’s tab from Ni ribbon to Ni mesh and the optimization on the thickness of PE film and the thermal bonding conditions such as heating temperature and time, resulted in no leakage of the electrolyte during charge-discharge cycles and no expansion of the laminate package by oxygen, and the cycling performance was significantly enhanced compared to our previous cell [2]. The cell presented very stable charge and discharge voltages for 20 cycles and the current efficiency was more than 90 %. More detailed results will be shown in this paper.

This work was financially supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors acknowledge FDK Corporation for supplying the MH negative electrode.

Reference

[1] T. Gejo, K. Kawaguchi, M. Morimitsu, 2017 TMS Annual Meeting & Exhibition, Pan American Materials Congress, San Diego, USA (2017).

[2] S. Kino, M. Morimitsu, The 232th ECS Meeting, Abs#104870, Washington, D. C., USA (2015).