Studies On Improving The Safety Of Sealed Ni-Cd And Ni-MH Batteries

Ni-Cd and Ni-MH batteries are playing important roles in rechargeable battery market and exhibiting more extensive applications for its high rate charging capability, long cycle lifetime, broad operating temperature rang and the low cost of usage. However, the currently commercialized Ni-Cd and Ni-MH batteries cannot satisfactorily meet the performance requirements of some important applications, especially in the safety performance of both batteries when they are charged/discharge at high rate current. Thus the tasks of this thesis are to deal with the basic and applied problems associated with improving the safety of Ni-Cd and Ni-MH batteries. The main results and new findings are summarized as follows:1. The effects of MH material as negative electrode additive on gas consumption reactions in Ni-Cd were investigate during charge and overcharge. It has found that the internal gas pressure within conventional Ni-Cd cell will be kept higher level for long time if the battery is charged or overcharged at high rate charging current. However, the internal gas pressure of sealed Ni-Cd cell system can be effectively eliminated by the addition of MH material in the negative electrode. The experimental results show that the gas consumption can achieve by both chemical and electrochemical reactions. In addition, the addition of MH material can slow capacity fade rate and reduce overpotential during charge/discharge stage.2. The effects of pulse charging technique on inner pressure and cycling characteristics of sealed Ni-MH batteries were investigated by comparison with the conventional direct current (dc) charging. The electrochemical impedance spectroscopy of cycled Ni-MH batteries have been measured. The micrographs of individual electrodes in the batteries have also been examined by SEM. Experimental results show that pulse charging is an effective approach to lower the internal pressure of battery during charge and overcharge, moreover the appropriate frequency is tp=5s and tn=ls in our experiments. The battery charged by pulse charge exhibits slower capacity fade rate and smaller resistance at the same charge-discharge rate. In addition, the anode charged by pulse charging exhibits less serious pulverization than that charged by DC charging. The possible explanation is that the short relaxation periods interspersed during the chargingprocess can effectively eliminate the concentration polarization and increase the power transfer rate, thus accelerating the charging process, reducing the gas evolution reaction, slowing down the pulverization rate of electroactive materials and keeping the water absorption capability of separator constant,...