MH-Ni battery additive on the electrochemical properties of high temperature
Abstract: constant current charge-discharge and cyclic voltammetry of a mixture containing Ce and Y additives on the nickel electrode and the MH-Ni battery at high temperature electrochemical properties, cycle of Ni (OH) 2 powder by XRD test for B type. constant current charge and discharge results show that at high temperatures contain additives MH-Ni battery coulombic efficiency was higher than that without additives MH-Ni battery. Ni electrode cyclic voltammetry experiments showed that under high temperature, the anode area of nickel electrode without additives Ni (OH) 2 oxidation peak overlap and peak oxygen evolution, which contain additives, nickel electrode Ni (OH) 2 oxidation peak and peak to peak voltage difference of oxygen evolution increased significantly, nickel electrode and the MH-Ni battery charge and discharge at high temperature performance has improved significantly.
Key words: additive; high temperature; MH-Ni battery
CLC number: TM912 2 Document code: A Article ID :0367-6234 (2006) 02-0247-03
In recent years, emerging countries in the world of electric vehicles has greatly promoted the high specific energy, high specific power, high temperature, long life green battery technology. MH-Ni battery as a green pollution-free, maintenance-free, safe, high specific energy and specific power, high-rate charge and discharge performance, long cycle life characteristics of a power source for electric vehicle ideal for [1 3], but the battery at a relatively high temperature conditions often need to work in the development time the need to further improve the MH-Ni battery high temperature performance, in which the key is to improve the cathode of the high temperature performance, because the battery capacity is restricted by the cathode, while the positive charge late in the existence of OH-oxidation reaction of O2, while the oxygen precipitation in the negative on the compound and released a lot of heat, to promote the battery temperature, reducing the cathode active material utilization. solutions are: 1) the battery externally forced cooling; 2) to study the chemical composition of active materials, grain degree of structural defects and surface activity; 3) Select positive additives, such as adding a certain amount of rare earth oxides, Ca2 +, or TiO2, etc. [4 ~ 6].
This choice of transition element yttrium (Y) and rare earth element cerium (Ce) after deposition by chemical additives, added to the nickel electrode active material, and the use of constant current charge-discharge and cyclic voltammetry study of their electrochemical properties of high temperature on the battery effects.
1.1 Ni electrode and the preparation of MH-Ni battery
The measurement of good Y and Ce nitrate solution mixed according to the molar ratio of 1:1 by drops added to the KOH solution, obtained with additive mixture of Y and Ce. Then prepared additives (3%) plus zinc and Cover Nickel Cobalt Hydroxide (97%) and moderate mixed adhesive to the nickel foam after filling the matrix, obtained after roll of nickel electrode. to the positive electrode and hydrogen storage alloy negative electrode and the cell membrane involved in the C-type steel , into the electrolyte, sealing, obtained by room temperature into MH-Ni battery, the nickel electrode containing additives and MH-Ni battery electrodes were marked as B and B cells, the nickel electrodes without additive and MH-Ni battery electrodes were marked as A and A battery for comparison experiments.
1.2 nickel electrode electrochemical and MH-Ni battery charge and discharge test
Nickel electrode as working electrode, Hg / HgO electrode as reference electrode, volume large enough for secondary storage alloy negative electrode, 6mol/dm3KOH solution as electrolyte, electrochemical test instrument in solartron1280B on 25 C and 55 C nickel electrode under 0 25mV / s scan speed cyclic voltammetry. on the A, B cell as room temperature and high temperature 1C charge-discharge (100% DOD) experiments, analysis of additives on the nickel electrode and the MH- Ni battery at high temperature electrochemical properties.
2 Results and discussion
2.1 Ni (OH) 2 structure analysis
Figure 1 (a), (b) respectively after 50 cycles is given after the non-Y and Ce additives Ni (OH) 2 and containing additives, Ni (OH) 2 in the XRD diagram, can be seen from the Figure 1 Both Ni (OH) 2 structure are identical, all B-type, indicating the additive did not change after the Ni (OH) 2 in the crystal.
2.2 1C temperature properties of two cell
MH-Ni battery in the process, the nickel electrode electrochemical reaction between normal
Ni (OH) 2 + OH--> NiOOH + H2O + e-. (1)
When the MH-Ni battery charging late and over-charging, the nickel electrode for the electrochemical reaction occurs
4OH--> 2H2O + O2 |^ +4 e-. (2)
Reaction (1) is expected, while reaction (2) is needed to be suppressed. A, B cell at room temperature (25 C) and high temperature (55 C) next to 1C charge-discharge curve shown in Figure 2,3 shows, the discharge performance parameters listed in Table 1,2. from Figure 2,3 and Table 1,2 shows that when the charge and discharge at 25 C, A, B cells are in charge of a voltage when the late jump, a second voltage level that oxygen evolution platform, two battery discharge capacity and coulomb efficiency is very similar to the Coulomb efficiency of the battery MH-Ni battery is defined as the actual discharge capacity and charge capacity ratio, that is, Coulomb efficiency = discharge capacity / charge capacity x 100%, which shows that charge at room temperature, the nickel electrode oxidation reaction and oxygen evolution on a larger potential difference, can there be until the latter part of the oxygen precipitates, hence the higher charge efficiency, this time to improve battery efficiency of additives on the role of little, and, as the additive reduces the conductivity of the cathode active material, resulting in B cell discharge voltage slightly lower than the A battery. but when at 55 C, B cells in the discharge capacity and coulomb efficiency was significantly higher than A batteries, and Figure 2 shows the charging curve, the end of charging, the battery A no apparent oxygen evolution platform, battery B is a more significant platform for analysis of oxygen because oxygen evolution of nickel cathode overpotential and the temperature in the high temperature cathode overpotential of oxygen evolution decreased, making oxygen evolution potential and Ni (OH) 2 oxidation reaction of potential overlap, thus the late charge is always accompanied by oxygen precipitation, the longer the charge, the more intense oxygen evolution reaction, And when the temperature is higher than 40 C, the nickel electrode surface state has changed, making the activation of oxygen evolution point of Ni (IV) distribution has been significantly improved, favorable oxygen evolution reaction , while mixed positive additives at high temperature increased oxidation of nickel electrodes and oxygen evolution potential difference between the improved efficiency of the battery charging temperature.
2.3 Ni electrode cyclic voltammetry experiments
Cyclic voltammetry to study the use of additives on the nickel electrode mechanism of high temperature electrochemical performance, Figure 4 (a), (b), respectively A, B electrodes at 25 C and 55 C, the cyclic voltammetry. By the Figure 4 (a) Obviously, the two electrodes significantly at 25 C, the oxidation peak and reduction peak, and the two anode oxidation peak in the region and oxygen evolution peaks are clearly separated, indicating that at 25 C, whether or not containing additives, nickel electrode Ni (OH) 2 oxidation reaction with oxygen were precipitates can be separated. by Figure 4 (b) can see, 55 C under, A, B electrode although there is an evident reduction potential, but in the anode A regional analysis of electrode, the oxidation peak and peak oxygen overlap, while the B electrode Ni (OH) 2 oxidation peak and peak to peak potential of oxygen evolution clearly separated, indicating that 55 C, non-additive nickel electrode Ni (OH) 2, oxidation potential and oxygen evolution potential overlap, that is, on the nickel electrode at a high temperature oxygen evolution overpotential decreased, while mixing additives with the nickel electrode potential drop, an increase of oxidation potential of oxygen evolution potential and the difference to increase the charge efficiency of nickel electrode under high temperature, which is the battery at 25 C and 55 C, 1C charge-discharge performance test results are consistent. In addition, the added Y and Ce compounds at high temperature to improve battery performance The mechanism of cell cycle in the process of change of the valence of further research.
At high temperatures, Y and Ce mixture of additives significantly increased the oxidation potential of nickel electrode and the oxygen evolution potential difference, thereby significantly improve the high temperature charge efficiency of nickel electrode, can play a role in improving battery performance.
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