Investigation On Electrochemical Performance Of CoB-type Alloys In A Wide Temperature Range

Some metal borides, especially cobalt boride related alloys, have been explored in alkaline secondary batteries. Due to the high discharge capacity, it is regarded as one of the most promising candidates for anode materials. In this thesis, the Co-B and Co-Ni-B alloys were prepared by a chemical reduction method and the electrochemical performance was investigated. In the case of amorphous CoB alloy, the La-Mg-Ni-based alloy as an additive was introduced by mechanical milling, and the effect of the rare earth base alloy on the electrochemical properties in a wide temperature range was systematically investigated.The Co-B binary alloys were prepared via the reduction of cobalt chloride hexahydrate, cobalt sulfate heptahydrate or cobalt nitrate hexahydrate with sodium borohydride solution. The experimental results show that, as anode materials in rechargeable batteries, Co-B alloy prepared from cobalt chloride hexahydrate has a maximum specific capacity of 844.6 mAh/g, and it remains to be around 450 mAh/g after initial several cycles and keeps unchangeable in 40 cycles at a discharge current of 100mA/g. By contrast, the maximum specific capacity of Co-B alloy prepared from cobalt sulfate heptahydrate and cobalt nitrate hexahydrate is 686.3 and 359.0 mAh/g, respectively.The CoB-x La_0.7Mg_0.3Ni_3.5（x= 0, 5, 10, 20 and 30 wt.%） composites exhibit a superior electrochemical property. The high-rate dischargeability（HRD） is significantly increased to82.5 %（x = 30） from 48.6（x = 0） at a discharge density of 900 mA/g. The capacity retention rate C100/Cmax increases firstly from 37.1 %（x = 0） to 45.6 %（x = 5）, then decreases to 42.4 %（x = 30）. According to our experimental results, the addition of 5 wt.%La_0.7Mg_0.3Ni_3.5 alloy is the optimal content to be introduced, with an improved electrochemical kinetics and a satisfactory electrochemical property at a low temperature of-30 ℃.Used as a negative electrode in the alkaline rechargeable batteries, ternary Co-Ni-B amorphous alloy exhibits an excellent electrochemical performance in the temperature range of-30 to 50 ℃. Specifically, the discharge capacity after 100 cycles still keeps up at about 400 mAh/g at a temperature range of-15 to 25 ℃, respectively. Even at a much lower temperature of-30 ℃, the maximum discharge capacity remains to be 408.3 mAh/g and slowly decays to 317.1 mAh/g after 100 cycles. Moreover, the high-rate dischargeability at900 mA/g is 29.6 % at-30 ℃. These properties are much better than those of conventional hydrogen storage alloy electrodes at the corresponding temperatures.