Study of Memory Effect of Nickel Hydrogen Battery

Introduction

Recently, the nickel hydrogen battery is taking a great attention as the most promising candidate for the power sources of hybrid vehicles. Therefore, it is important to develop a property of nickel hydrogen battery. When a nickel hydrogen battery has been cycled at a shallow depth of discharge, the battery will produce a less capacity than that corresponding to normal discharge-charge cycles^[1].

In the open circuit, local cell reaction between electrode material and current collector (CC) will occur. Till now, the effect of local cell reaction has not been studied enough. In this study, we focused on the local cell reaction between the cathode material (β-NiOOH) and CC for nickel hydrogen battery. The relationship between local cell reaction and the memory effect was investigated.

Experiment

The cathode was fabricated by mixing powder of the synthesized β-NiOOH as the active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 80:15:5 by weight. Platinum plate was used as counter electrode and Ag/AgCl reference electrode was used as reference. The electrolyte was an 8M potassium hydroxide. Three kinds of CC, platinum mesh, gold mesh and nickel mesh were used to investigate the local cell reaction between electrode material and CC.

  We discharged the electrode material to the cutoff voltage of 0.19V(vs. SHE) at 30mA/g and charged for 6h at 30mA/g. Then, we discharged to the half of the cell capacity at 30mA/g. After that, we opened the circuit and set rest for 1d for nickel mesh and 3d for gold and platinum mesh(Ex.1).

Next, we set charge discharge cycle test having rest at the every end of charge and discharge. We charged for 1h at 30mA/g, take rest for 2h, discharged for 40 min at 30mA/g, take rest for 2h, then cycled the above process (Ex.2).

X-ray diffraction analysis of the surface of cathode was conducted using RINT-TTR (Rigaku co., CuKα, 200mA, 30kV) .

Results and Discussion

    For Ex.1, XRD peak characteristic to γ-NiOOH^[2] was observed between 10°and 15° in 2θ for the case of nickel CC.  On the other hand, no peak characteristic to γ-NiOOH  was observed for the case of gold or platinum CC. At the working electrode, β-NiOOH acts as cathode and nickel acts as anode during rest by local cell reaction. On the other hand, β-NiOOH acts as anode and gold or platinum acts as cathode during rest. It is indicated that γ-NiOOH, cause of memory effect^[1], was produced only when β-NiOOH acts as cathode^[3].

Fig.1 and Fig.2 show the experimental results of Ex.2. Fig.1 shows the discharge curves and XRD pattern when using nickel as CC. The critical reduction in cell capacity was observed and the peak characteristic to γ-NiOOH was observed between 10°and 15° in 2θ. Fig.2 shows the discharge curves and XRD pattern using platinum as CC. The critical reduction in cell capacity was not observed and the peak characteristic to γ-NiOOH was not observed.

It is concluded that when β-NiOOH acts as cathode by local cell reaction, γ-NiOOH is formed and memory effect is observed and that, when β-NiOOH acts as anode, γ-NiOOH is not formed and memory effect is not observed.

References

[1] Yuichi Sato et al., J. Power Sources 93 (2001) 20-24.

[2] J. Pan et al., Electrochemica Acta 54 (2009) 3812-3818

[3] T. Yao, T. Iwai and H. Tagashira, PCT/JP2013/ 74165 (2013)