The Nanoscale Additives For Ni(OH)₂ Electrode And Their Effects On The High-rate Performance Of The MH/Ni Batteries

As one of the most widely used rechargeable batteries, nickel-metal hydride (MH/Ni) batteries are required to improve the high power characteristics considering application for electric vehicles. The high power performance of MH/Ni battery is affected strongly by the positive electrode because of the poor conductivity of the active material β-Ni(OH)₂. In this work, nanosized additives such as nanoscale CoO, multiwalled carbon nanotubes (CNTs), nanoscale α-Ni(OH)₂ and surface-modified amorphous nanosized carbon were synthesized as additives for the Ni(OH)₂ electrodes. The morphologies and structures of the nanoscale additives were charactered by XRD, SEM and TEM. The influences of these nanosized additives on the overall performance of Ni/MH batteries, especially the high-rate capability, were evaluated by the means of electrochemical measurements, including electrochemical impedance spectrum, cyclic voltammetry and charge-discharge cycling.CoCO₃ nanorods as precursor were synthesized by precipitation method and short rod-like nanoscale CoO was prepared by decomposing CoCO₃ in vacuum. Compared with the electrodes with usual sub-micron CoO, the electrochemical measurements indicate that the electrodes with nanoscale CoO exhibit lower Ohm impedance and the lower electrochemical reaction impedance, narrower redox potential space and high reaction potential. The specific capacity of the electrodes with nanoscale CoO is up to 283 mAh/g, which is close to the theoretical specific capacity of β-Ni(OH)₂. The sealed MH/Ni batteries with nanoscale CoO present better high-rate performance. At 10 C discharge rate the capacity of the batteries with 8 wt.% CoO in the positive electrodes still remains 90% of original capacity, and the lifespan is 165 cycles, which is 43% longer than the usual batteries with 115 cycles.Well crystallized multi-walled carbon nanotubes (CNTs) with diameter about 10 nm were synthesized by chemical vapor decomposition (CVD) method. After purification and ball-milling, the as-prepared CNTs were added to the positive electrodes of MH/Ni batteries as additives. During the process of transformation from CoO to CoOOH, a complex conductive network was created with CNTs as the frame. Because of the high conductivity and intension characteristics of CNTs. thecharge-transfer capability was improved and the integrality of the complex condutive network was enhanced. The electrochemical measurements show that the impedance of the electrodes was lessened and the exchange current density was increased by addition of CNTs. Further researches on sealed batteries show that the batteries with CNTs in the positive electrodes exhibit improved capacity, modified discharge stability, restrained internal resistance and prolonged lifespan. The advantages of CNTs are more obvious when discharged at high current rates. 0.5 wt.% was proved a desired amount for CNTs and the capacity of the batteries with 0.5 wt.% CNTs maintained 85% after discharging at 10 C rate even for 120 cycles.Surface modified β-Ni(OH)₂ by amorphous nanoscale carbon was prepared by the decomposition of glucose in hydrothermal condition. Electrochemical measurements show that though the carbon modification will enhance the discharging stability, the utility of β-Ni(OH)₂ dimished to 87%, which is 10% lower than the electrode without carbon modification at 0.2 C and 1 C. However, at a high discharge rate of 3 C, the carbon-modified β-Ni(OH)₂ electrodes can be discharged stably for 30 cycles without any specific capacity loss and the discharge voltage is 30 mV higher than the electrode without carbon modification. In addition, a proper mount of 6 wt.% CoO was nesessary to the carbon modified β-Ni(OH)₂ electrodes for the high-rate performance.Well-crystallized nanoscale α-Ni(OH)₂ with diameter of 20-30 nm and the tap density of 1.7 g/ml was synthesized through co-precipitation in alcohol-water system. Influnce of the complexing agent and the ageing procedure on the microscopic morphologies of α-Ni(OH)₂ were studied. The tap density of β/α-Ni(OH)₂ biphase powder with 10 wt.% nanoscale α-Ni(OH)₂ could get to 2.41 g/ml. Electrochemical investigation shows that the nanoscale α-Ni(OH)₂ exhibits better electrochemical activities than β-Ni(OH)₂. The discharge capacity increases and the cyclic stability is enhanced for the biphase electrodes with proper mount of α-Ni(OH)₂. The biphase electrode with 10 wt.% nanoscale α-Ni(OH)₂ has a best overall electrochemical performances. Excessive addition of nanoscale α-Ni(OH)₂ does no good to improve the performances of the biphase electrode material.