Study On The Preparation And Electrochemical Analysis Of Ni-MH Batteries For EVs And HEVs

Nickel-metal hydride batteries are nowadays the most promising system for electric vehicle (EV) and hybrid electric vehicle (HEV). In this thesis, doped spherical Ni(OH)2 was prepared by ammonia complexing liquid phase co-deposition method, using graphite powder and acetylene-black as dopants. The XRD and SEM results indicated the interrelation of carbon dopants in the spherical Ni(OH)2. With electronic conductor increasing, the discharge capacity is decreased and the plateau voltage and the discharge ability at high rate are increased.Influence of nickel electrode thickness is studied and analyzed by kinetics of discharge process and structure of porous nickel electrode as well as the effect of electrode packing density on electrochemical performances is also studied. Decreasing electrode thickness can decrease charge voltage and concentration polarization ,improve charge efficiency, increase absorb ability of capillary, shorten average diffuse distance, improve utilization of active material, increase electrode area, decrease current density, decrease electrochemistry polarization, improve reaction rate of discharge process, and increase discharge voltage and capacity. The result also shows that decreasing electrode packing density can decrease charge voltage and concentration polarization, improve charge efficiency and improve discharge capacity. The result shows that ,for SC size MH-Ni batteries , when nickel electrode thickness is 0.61～0.62mm and packing density is 3.2g/cm3, the electrochemical performances are the best.We analyzed the effect of electrolyte composing and content on electrochemical performance of MH/Ni batteries, and confirmed that the optimum electrolyte composing was 4 mol/L KOH +3 mol/L NaOH +15g/L LiOH and the optimum electrolyte content was 5.0g-5.2g. Pretreatment before the activation affects electrochemical performance of MH-Ni battery. That can debase internal pressure, reduce leak electrolyte and shorten activation periods. Adopting activated means with variational current and increasing current step by step can improve capacity, heighten discharge plateau voltage, reduce internal resistance and reduce self-discharge rate. For SC size MH-Ni battery, this paper confirmed the best activation institution which adapted to practical produce. That is charged for 16h at 0.1Crate, then discharged at 0.2C rate down to 0.9V; charged for 8h at 0.2C rate, then discharged at 0.5C rate down to 0.9V; charged for 2.5h at 0.5C rate, then discharged at 0.5C rate down to 1.0V.The XRD and SEM results showed that the pulverization and oxidation of active materials on the positive and negative electrodes and expansion of electrodes are the main reasons for electrochemical performance deterioration of nickel-metal hydride batteries.We also study and explore the technological process of dipolar battery preparation.