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

Abstract

Introduction Lead Acid Batteries (“LAB”) have been widely used for more than 100 years. LAB cannot be recharged after over-discharged, and its performance is greatly declined. Iwai et al. have found that the above deterioration is caused by the formation of α-PbO2 on the surface of cathode active material β-PbO2 due to the reduction by local cell reaction. They also have revealed that the formation of α-PbO2 can be prevented by using gold or platinum as cathode current collector. At that case, the LAB can be recharged even after over-discharge.[1,2] We revealed, furthermore, that LAB using graphite sheet as cathode current collector also has high resistance to over-discharge.[3] This LAB can charge/discharge without performance degradation even after full discharge followed by 48 h rest period. In order to improve sulfuric acid resistance, we tried to mix four kinds of resin (polyethylene, polypropylene, ethylene propylene copolymer and fluororesin) to the graphite sheet. It is revealed that the weight of graphite sheet did not increase by immersion test into sulfuric acid for more than or equal to 5 wt% polypropylene mix, and that surface electrical resistance of the sheet did not increase for less than or equal to 5 wt% polypropylene mix.[4] We decided to use graphite sheet with 5 wt% polypropylene as cathode current collector. For this LAB, we have investigated long term charge deep-discharge cycle performance including various kinds of rest-period. We also investigated the rechargeablity after 0V discharge followed by various long rest period. Experiment We made the resin-dispersed graphite sheet in order to improve electrolyte repellency compared to normal graphite sheet. We used polypropylene for resin material and dispersed it by 5 wt% into expanded graphite, then we made it into sheet by rolling mill. The heat treatment was preheating at 100℃ for 10 min and then at 180℃ for 10 min to disperse resin evenly.We used two-electrode glass cell for charge/discharge experiments. The cathode active material was prepared by mixing β-PbO2 powder, acetylene black and PTFE at the ratio of 80:15:5 in weight. We added acetylene black as a conductive additive and PTFE as a binder. The cathode was made by pressing the paste on a current collector. We used 35% H2SO4 as electrolyte and lead plate as anode.First experiment is the long term deep-discharge durability test. The cycle was consisted of 4 times of discharge down to 0V and full charge, and then 48 h rest period. Charge/discharge rate was 180 mAg-1 for charge and 9 mAg-1 for discharge. The stabilization cycle prior to deep-discharge was consisted of 20 cycles of discharge at 9 mAg-1 for 30 min and charge at 180 mAg-1 for 20 min. We determined charge and discharge rates by weight of cathode active material.The second experiment is to evaluate durability against over-discharge followed by various rest period. After the stabilization cycle, we discharged the cell down to 0V and kept it in open-circuit for various period, and then rechargeablity was examined by charging the cell. Results and discussion Fig.1 shows the result of the first experiment. The LAB using 5 wt% polypropylene-dispersed graphite sheet current collector continued the cycle of 4 times charge/deep-discharge and 48 h rest for more than 5 months. The cause of the breakage of the cell after 5 months was due to fall of active material off the current collector. It is thought that this cell could charge/discharge for longer term by improving the filling method of active material. This result shows that this type of battery has high durability to deep-discharge for long term.Fig.2 shows the result of the second experiment. The rest period was 2 d or 1 week. It was found that the cell was rechargeable for either rest period. It was confirmed that The LAB using 5 wt% polypropylene-dispersed graphite sheet current collector has high durability to both over-discharged and long term rest.