Performance of Lead Acid Battery Using Graphite Based Composite As Cathode Current Collector

Abstract

Introduction Lead acid batteries have been widely used for more than 100 years. They are used for vehicles and backup power supplies. However, when overdischarged, lead acid battery cannot be recharged and its performance is greatly declined. Recently, Iwai et al. had revealed that the cause of not being able to be recharged is the nucleation of α-PbO2 on cathode material, β-PbO2, and that, when gold or platinum is used as the current collector, formation of α-PbO2 is prevented due to local cell reaction, then that lead acid battery can be recharged even after overdischarge [1,2]. In this study, we investigated the possibility of realizing the lead acid battery with high resistance to overdischarge using graphite materials, such as natural expanded graphite sheet as a current collector. We think that graphite materials will be advantageous for cost reduction, weight reduction and improvement of processability for lead acid batteries. Experiment We developed graphite based composite material as a current collector which was prepared by the following procedure, in order to improve water repellency. Resin materials such as polyethylene, polypropylene (PP), ethylene propylene copolymer and fluororesin were dispersed with graphite sheet, preheated in air at 100 degrees for 10 minutes, and then heated at 160 degrees to 200 degrees for 10 minutes. Then, the graphite-based composite material was immersed in sulfuric acid (H2SO4) and the weight variation was measured. Also, the surface electric resistance of the graphite composite material was measured by four-terminal method. We decided to use PP as resin for graphite based composite materials, because graphite based composite material using PP have a low electric resistance and PP is easily dispersed in graphite sheet. Then, we chose PP contents of 5% from the viewpoint of water resistance. We used two-electrode glass cell for charge-discharge experiments. The cathode paste was prepared by mixing purchased β-PbO2 powder as an active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 80:15:5 in weight. The cathode was fabricated by pressing the paste on a current collector. In this study, we tested three kinds of current collectors, Pb plate, graphite sheet and graphite-based composite material. Each current collectors were used in 35 wt% H2SO4. Lead plate and 35 wt% H2SO4 were employed as anode and electrolyte, respectively. Firstly, we discharged the cell at 9 mAg-1 for 30 min and charged at 180 mAg-1 for 20 min. We repeated this cycle 20 times in order to stabilize the charge-discharge reaction. Secondly, we deeply discharged the cell down to 0 V at 9 mAg-1 and opened the circuit for 48 h. Finally, we once again charged the cell at 180 mAg-1 for 60 min, then we discharged the cell at 9 mAg-1 for 30 min and charged at 180 mAg-1 for 20 min and repeated the cycle to study the cell performance. Results and discussion Figure 1 (A) and (B) show the charge-discharge curves for the lead acid battery using Pb plate and graphite sheet as a current collector, respectively. In the case of Pb plate, the lead acid battery cannot be recharged after the open circuit. On the other hand, in the case of graphite sheet, it can be recharged even after the open circuit. Electrode potential does not come out for carbon, so it is thought that carbon has a role of stopping the local cell reaction. From this reason, it can be explained that the formation of α-PbO2 is prevented on cathode β-PbO2 then that the similar performance was shown when using gold or platinum as a current collector. In figure 2, performance of lead acid battery using graphite based composite as cathode current collector is shown. We discharged the cell at 9 mAg-1 to 0 volt and charged at 180 mAg-1 for 60 min and repeated the cycle between 0 volt and 2 volt. Between 1.9 V and 1.3 V, potential flat area is clearly observed. Just only inserting a thin graphite based composite, lead acid battery repeating charge / discharge cycles between 0 volt and 2 volt so many times without decay was realized. This result may be caused by improvement of water repellency of current collector by impregnate with PP into graphite sheet.