Study On High Temperature Performance Of Spherical Nickel Hydroxide

Nickel-metal hydride (Ni-MH) batteries will be one of the best choices tobe used as the power sources for electric vehicle (EV) and hybrid electricvehicle (HEV) in the near tens of years, because they have the characteristicsas high reversible storage capacity, excellent long-term cycle life and goodsafety performance. However, in working process, the temperature incrementof the Ni-MH batteries can not be avoided, and the performance of the Ni-MHbatteries will drop rapidly when working at high temperature because of thecharging efficiency descent of the nickel hydroxide which is the cathodematerials in the Ni-MH battery. So, to improve the high-temperatureperformance of the nickel hydroxide became an urgent problem need to solve.Thermodynamic theory shows that the heterogeneous nucleation takespriority to homogeneous nucleation. Based on this theory, the film, which iscomposed of ytterbium hydroxide insoluble calcium salt (phosphate calciumand fluoride calcium) and cobalt hydroxide, was coated on the sphericalnickel hydroxide by layered-precipitation and co-precipitation process. Andthe electrochemical analyses showed that the surface coating could improvethe high temperature performance of the nickel hydroxide.After comparing the results between the phosphate calcium-coated nickelhydroxide and fluoride calcium-coated nickel hydroxide which containedsame amount of the calcium and cobalt (2%) by layered-precipitation, theconclusion was made that the former one has better high temperatureperformance than the latter one. And the optimal coating amounts of the twocalcium salts are the same as 2% (atom ratio to nickel). The samples coatedwith this amount showed the best high temperature performance at 60℃. Thedischarge capacity of the calcium phosphate-coated samples reached to199mAh/g and the capacity conservation rate reached to 81%, which is muchhigher than that of the uncoated sample (121mAh/g, 49%).At the same time, different amounts of ytterbium hydroxide and sameamount of cobalt hydroxide (2%) were both coated on the surface of the nickelhydroxide by two processes, the layered-precipitation process andco-precipitation process. The results showed that co-precipitation process wasmore effective than the layered-precipitation process, meanwhile both surfacemodification process could maintain the nickel hydroxide's capacityreservation rate to be higher than 90% at high temperature. The optimalYb-coating amount for layered-precipitation process was 2%, the dischargecapacity of the nickel hydroxide which was modified by this process couldreach to 210mAh/g at 60℃. And the optimal Yb-coating amount forco-precipitation was 0.75%;the discharge capacity of the sample which wasmodified by this process could reach 231mAh/g at 60℃.The cyclic voltammetry test showed, at room temperature and hightemperature, the oxidation potential, the oxygen evolution potential and thedifference between these two potentials of the nickel hydroxide increasedafter surface coating. It will increase the charging efficiency of the nickelhydroxide and improve the electrochemical performance at high temperature.