Nickel-hydrogen Battery Cathode Materials For The ¦Â-type Nickel Hydroxide Modified Surface Preparation Technology And High-temperature And High-rate Electrochemical Performance

Nickel metal-hydride(Ni-MH) batteries for EV/HEV were used successfully several years ago,power Ni-MH battery requires that positive materials are characterized with high specific energy density,and long cycle life,thereby the improvement of the above-mentioned properties is important to the positive material.The poor high-temperature performance and high rate charge/discharge of spherical nickel hydroxide becomes the factor for limitation of use as the positive electrode active material in MH-Ni battery.The thesis deals with preparation and modification of spherical nickel hydroxide used in the power Ni-MH batteries.The main works and results are summarized as follows.The spherical nickel hydroxide with high tap density was prepared by means of controlled crystallization,the relation between the preparation condition and electrochemical activity,crystal structure of the product were investigated.The SEM images showed that the microstructures of nickel hydroxide were diverse obviously through changing the pH value at late anaphase of the synthesis of the nickel hydroxide.The reason was the formation of nanocrystallites of nickel hydroxide.It was found by X-Ray Diffraction that the full width of half maximum intensity of XRD patterns of the samples in(001) and(101) lines increase.The spherical nickel hydroxide coated nanocrystallites maintains higher specific capacity than the commercial products,the discharge capacity at 50℃,0.2C rate up to 85%of its nominal capacity.The spherical cobalt-coated nickel hydroxide,calcium-coated nickel hydroxide,yttrium-coated,ytterbium-coated nickel hydroxide were synthesized by controlling the precipitations accurately.We systemically studied the effects on the structure,morphology and electrochemical performance ofβ-Ni(OH)₂。The results indicated that the electrochemical conductivity was improved when theβ-Ni(OH)₂ was coated Co(OH)₂;the discharge capacity of the nickel hydroxide electrodes containing the hydroxides of various rare elements was improved at high temperature. Surface microcapulation of Ni(OH)₂ with Co-Y composite hydroxides can greatly increase the electrochemical performance ofβ-Ni(OH)₂.Its charge/discharge efficiency are more than 90%when it is charged at 1C rate and discharged at 3C.More additives of Ca(OH)₂ and Y(OH)₃ which would come into self-existent phase lead to the deterioration of conductivity and electrochemical performance of Ni(OH)₂.Constant current charge-discharge test,cyclic voltammetric and AC impedance experiments showed that the electrochemical behaviors of different doping elements are not same during the course of charged/discharged at high temperature;The diffusion coefficient of proton was increased by the coating content of Co(OH)₂ at 25℃,then it increased at first and afterwards decreased following the coating content of Co-Yb composite hydroxides at 60℃.The polarization processes of the spherical Ni(OH)₂ electrodes were analyzed and modeled with R(R₁(R₂C)) equivalent circuit by EIS measurements.Proton diffusion impedance drastically increases when more content of rare earth hydroxide.The mechanisms was studied that the effect of Yb(OH)₃ on structure and electrochemical behaviors of spherical nickel hydroxide coated at high temperature;The structure changes of spherical nickel hydroxide during charge/discharge process were studied by XRD,CV,EIS measurements were used to study electrochemical behaviors of nickel electrode at high temperature.The results indicated that the peaks of(001) and(101) ofβ-Ni(OH)₂ in full charge state were broader strongly than the discharge state when the ytterbium hydroxide was coated.The discharge capacity of the ytterbium-coated nickel hydroxide was higher than that of the conventional nickel hydroxide at 60℃,ytterbium was found to increase obviously oxygen evolution over potential of nickel electrode. The variety of transfer resistances of ytterbium-coated nickel hydroxide relative to the conventional nickel hydroxide was found in disparate SOC at 60℃.Meeting the requirement of high-power type Ni-MH battery,the new type nickel hydroxide was prepared which characterized with high tap density and high conductivity.The specific capacity of the nickel hydroxide coated nano grain was up to 254mAh/g.its capacity deceased to 91%of its nominal capacity after 500 cycles.The tap density was high as 2.28/cm³.