Recently, nickel hydrogen battery is taking a great attention as the most promising candidate for power sources of hybrid vehicles. Therefore, it is important to develop a property of nickel hydrogen battery. When nickel hydrogen battery is cycled at a shallow depth of discharge, the cell capacity degrades in comparison with that after normal discharge-charge cycles[1]. Sato et al. reported that the formation of γ-NiOOH (oxidant of β-NiOOH) is the major factor of such capacity degradation, and that γ-NiOOH formation doesn't occur from the interface between electrolyte and β-NiOOH but from the interface between current collector (CC) and cathode active material (β-NiOOH)[2]. However, the precise mechanism of γ-NiOOH formation from CC side is not clarified. We focused attention to the local cell reaction (LCR) taking place at the interface of β-NiOOH and CC in the open circuit[3]. In the previous study, by employing various metals as CC, we investigated about the difference of LCR and the effect on the battery performance. We revealed that the degradation of cell capacity occurs only when nickel mesh CC is used and no degradation occurs when gold or platinum mesh is used. In addition, we also found that the degraded sample shows a XRD peak between 10° and 15° in 2θ[4,5]. In this study, we investigated the oxidation state of nickel in NiOOH by XPS measurement to verify that the product of LCR is the reductant of β-NiOOH.
Experiment
We fabricated the cathode by mixing powder of the synthesized β-NiOOH, acetylene black and PTFE at the ratio of 80:15:5 by weight. Platinum plate was used as counter electrode and Ag/AgCl electrode was used as reference. The electrolyte was an 8M potassium hydroxide. We used nickel, platinum and gold mesh as CC to investigate the difference of changes in the oxidation state of nickel in NiOOH due to LCR.
We discharged the cell to the cutoff voltage of 0.19 V (vs. SHE) and then charged for 6h. We repeated this process for 3 times to stabilize the cell reaction. After the stabilization, we discharged the cell to SOC 40% or 20%. Then, we conducted the charge-discharge cycles with rest time period at the every end of charge to cause LCR during rest. For the charge-discharge cycles with rest, the cell was charged for 50 minutes, take rest for 2 h, discharged for 40 minutes and this cycle was repeated, i.e., we conducted the cycles with rest between SOC 40-60% and that between SOC 20-40%. All of the charge-discharge cycles were conducted at 30mA/g. We also conducted XRD measurement of the surface of cathode using RINT-TTR (Rigaku co., CuKα, 200 mA, 30 kV) to investigate the change of crystal structure and XPS measurement of cathode using ESCA-3400 (Shimadzu co., MgKα, 1253.6 eV) to check the oxidation state of nickel in NiOOH.
Results and Discussion
Fig.1 shows the discharge curves in charge-discharge cycles with rest at SOC 40-60% region. Fig.1 (a), (b) and (c) represent the results for nickel, platinum, and gold mesh CCs, respectively. Fig.2 also represents the corresponding X-ray diffraction results. When we use platinum or gold mesh with higher potential than β-NiOOH, the reduction of β-NiOOH doesn’t occur and the capacity loss is not observed after 50th cycle. On the other hand, when we use nickel mesh with lower potential than β-NiOOH, β-NiOOH acts as cathode and CC acts as anode in LCR, and consequently, the reduction of β-NiOOH occurs. We named the reductant of β-NiOOH as γ´-NiOOH which is different from γ-NiOOH. Due to the formation of γ´-NiOOH, the capacity loss and the XRD peak are observed between 10° and 15° in 2θ after 35th cycle as depicted by arrow in Fig.2 (a).
Fig.3 shows the results for XPS measurement and (a), (b) and (c) represent the results for nickel, platinum, and gold mesh, respectively. Only when we use nickel mesh, the chemical shift of Ni 2p XP spectrum indicating the reduction of nickel in β-NiOOH is observed. Considering the results for Fig.1 and Fig.2, it is verified that the γ´-NiOOH is the reductant of β-NiOOH. It is concluded that our local cell reaction model for the capacity loss due to the γ´-NiOOH formation is consistent with our present experimental results.
References
[1] F. Fourgeot, S. Deabate, F. Henn and M. Costa, Ionics6 (2000) 364-368.
[2]Y. Sato, S. Takeuchi and K. Kobayakawa, J. Power Sources, 93 (2001) 20-24.
[3] T. Yao, T. Iwai and H. Tagashira, PCT patent (2013), PCT /JP2013/ 74165.
[4] T. Iwai and T. Yao, 225th ECS Abst. (2014), No.1639.
[5] T.iwai, S.Takai, and T.Yao, 226th ECS Abst.(2014), No.2250.