Effect of α-PbO2 formation on Cycle Performance in Lead Acid Battery

Introduction

 Lead acid battery has been widely used for more than 100 years and still now, it is very important for vehicles. In recent years, lead acid battery has been attracted a large interest for the load leveling because of its low cost. Despite of its long history of study, some chemical reactions in a cell have not been clarified yet. PbO₂ of cathode material has two types of crystal structures, i.e. α-PbO₂ and β-PbO₂. It is reported that PbO₂ exists as β-PbO₂ in an acidic region and as α-PbO₂ in an alkaline region^[1]. However, α-PbO₂ also coexists with β-PbO₂in the cathode of lead acid battery, the reason of which is not yet revealed well.

In the previous study^[2], we reported that α-PbO₂ formed due to the local cell reaction at the interface between β-PbO₂ and current collector (CC) in the open circuit^[3]. Formation of α-PbO₂ was restricted by employing the nobler CC against β-PbO₂. In this study, we investigated the effect of local cell reaction on the cycle performance of the battery.

Experiment

β-PbO₂ (A Johnson Matthey Company) was mechanically crushed in the mortar for 1day. The cathode was fabricated by mixing powder of the crushed PbO₂ as the active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 80:15:5 percent by weight. Lead plate and 30wt% sulfuric acid were used as counter electrode and the electrolyte, respectively. Lead plate or gold mesh was employed as CC for the cathode to see the difference of local cell reaction between cathode active material and CC. At first, we discharged the cell for 30min at 9mAg^-1 and charged for 20 min at 180mAg^-1 for 20 cycles to make the cell reaction stable. Then, we discharged the cell to the cutoff voltage of 0V at 9mA/g and opened the circuit. After the rest for 2 d, the cell was charged for 1h at 180mA/g. Then, the cell was discharged for 30min at 9mA/g and charged for 20 min at 180mAg^-1for 20 cycles to examine the cell performance after the rest. X-ray diffraction of the cathode was measured using RINT-TTR (Rigaku co., CuKα, 200mA, 30kV) for each cathode.

Results and Discussion

Fig.1 shows XRD patterns of cathode after the charge-discharge cycles. (A) and (B) correspond to the results of using lead plate and gold mesh CCs, respectively. The peak of α-PbO₂ was detected around 28°in 2θ as depicted by arrow for the cathode using lead CC as shown in (A), while it was not observed for Au mesh CC in (B). Fig.2 shows Charge-discharge curves of the cell. (A) and (B) also correspond to the cathode using lead plate and gold mesh as CC, respectively. As for (A), Charge-discharge after the rest was not possible at all, while it was possible as for Au CC as shown in (B).

It is known that β-PbO₂ has the higher potential than α-PbO₂^[1]. Therefore, when lead plate was used as CC, the reduction of β-PbO₂ occured by local cell reaction at the interface of β-PbO₂ and lead plate, and consequently, α-PbO₂ formed. It is considered that the formation of α-PbO₂caused by local cell reaction degrades the cycle performance of the battery.

References

[1] Y. Matsuda, Z. Takehara (eds.), Denchi Binran, third edition, Maruzen, Tokyo (2001) 168-172.

[2] T. Iwai, Daishi Kitajima, Shigeomi Takai and T. Yao, 226^th ECS Abst(2014), No.30.

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