Thermodynamics of the 23A battery system

At 20℃, the charge and discharge curves and corresponding temperature changes of the two batteries are basically similar, indicating that there is no significant difference in the discharge capacity, heat generation and heat dissipation of the two batteries at room temperature. At 60℃, the discharge capacity of the negative copper-plated 23A battery is much higher than that of the blank 23A battery. Comparing the charging curves of the two, it can be seen that the charging time of the negative copper-plated 23A battery is nearly 100mAh longer than that of the blank 23A battery. The temperature of the blank 23A battery during the charging process is always high, and the temperature at the end of the charging is 5℃ higher than that of the negative copper-plated 23A battery. The difference in temperature between the two batteries during charging may be the main reason for the low charging efficiency of the blank 23A battery at high temperature. The thermal effect of MH/Ni 23A battery includes reversible thermal effect, Joule thermal effect and reaction heat when the oxygen precipitated from the positive electrode is compounded on the negative electrode at the end of charging. From the perspective of reversible heat, MH/Ni 23A battery releases heat during charging. Joule thermal effect is the heat generated by conductive components, diaphragms, electrolyte ohmic resistance, etc. and the heat generated by electrochemical and concentration reaction impedance. It is related to the internal resistance of the 23A battery and the current passing through. These two thermal effects are almost the same in the two batteries studied in this paper. Oxygen recombination in MH/Ni batteries is an important source of heat, because all the energy of this reaction is converted into heat, so it is a large heat source. In summary, MH/Ni batteries release heat from the beginning of charging. As charging proceeds, electrochemical and concentration polarization increase. Oxygen precipitation and its recombination at the end of charging generate a large amount of heat, which aggravates the rise in 23A battery temperature. The reversible thermal effect is determined by the thermodynamic properties of the MH/Ni 23A battery system. For high-power MH/Ni batteries such as power tools and HEVs, from the perspective of 23A battery design, polarization heat has been greatly reduced. Therefore, appropriately reducing the oxygen recombination performance of the negative electrode and reducing the heat generated by oxygen recombination is an effective way to improve the high-temperature performance of high-power MH/Ni batteries. SaiKai believes that chemical copper plating of nitrogen storage alloy can quickly transfer electrons to the alloy surface, which is conducive to the smooth progress of electrochemical reactions on the electrode surface. At the same time, it inhibits hydrogen desorption, helps nitrogen atoms diffuse into the bulk phase, and reduces the internal pressure of the 23A battery. In the internal pressure curve, the highest internal pressure of the blank 23A battery is slightly lower than that of the Cu-coated 23A battery. The internal pressure rise rate during charging is not much different, but the internal pressure of the Cu-coated 23A battery drops slowly during discharge, because the copper plating on the negative electrode surface does not have a strong catalytic effect on the recombination of oxygen on the negative electrode surface as the hydrogen storage alloy, which leads to a decrease in the oxygen recombination rate and a slow decrease in internal pressure. T. Saikai et al.'s research shows that after chemical copper plating, the composition of the gas in the 23A battery changes from the original H2 volume accounting for more than 75% to O2 accounting for the main component, and H2 volume accounting for only less than 15%. These data fully illustrate that when the internal pressure of the 23A battery is not much different, the increase in O2 content means that the O2 used for negative electrode recombination is reduced, and the blank 23A battery generates more heat in a sister time due to the large amount of oxygen recombination, the 23A battery temperature rises rapidly, negative voltage appears, and charging is terminated.

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