1. The working principle of Ni-MH battery
The Ni-MH battery is an alkaline storage battery with metal hydride as the negative electrode, NiOOH as the positive electrode, and KOH aqueous solution as the electrolyte. The electrochemical formula is:
(-)M/MH| KOH(6mol/L)|Ni(OH)2/NiOOH(+) hydrogen storage alloy has the function of absorbing and releasing hydrogen when the potential changes. The battery uses this function to realize charging and discharging.
The electrode reaction of Ni-MH battery during charge and discharge is as follows:
(1) Negative reaction
The normal charge and discharge reaction is: M+xH2O+xe–(charge)⇌(discharge)MHx+xOH–
In the formula, M stands for hydrogen storage material, MHx stands for metal hydride, and the standard electrode potential is E0=-0.93V (VS, Hg/HgO)
Overcharge reactions are:
MHx+2yH2O + 2yе–→MHx+2y + 2yOH–
1/2yO2 + MHx+2y→MHx+yH2O
1/2 pO2 + M→MHx-2rOp+rH2
Over-discharge reactions include:
MHp+pOH–→M+ pH2O+ pe–
(2) Positive electrode reaction
The normal charge and discharge reaction is: Ni(OH)2 +OH–(charge)⇌(discharge)NiOOH + H2O+e–
The standard electrode potential is E0=0.39v (VS, Hg/HgO).
The overcharge response is
The overdischarge reaction is
At this time, hydrogen diffuses through the diaphragm and reacts with M on the negative electrode as follows:
(3) Battery reaction
During normal charging and discharging, the general reaction formula of the battery can be expressed as
x Ni（OH）2+ M(charge)⇌(discharge)x NiOOH +MHx
The standard battery electromotive force is E0 = 1.32V. During overcharge, the gas inside the battery undergoes a recombination reaction to maintain the pressure balance inside the battery and the battery voltage is stable. Oxygen is precipitated on the positive electrode, and after reaching the surface of the negative electrode through the porous diaphragm, it recombines with the adsorbed hydrogen atoms to form water. Due to the excess design capacity of the negative electrode, when overcharged, water molecules are reduced to form adsorbed hydrogen atoms, rather than hydrogen atoms recombine to form hydrogen molecules and precipitate. In the above process, nickel is a good catalyst. After the Ni-MH battery is activated, the concentration of metal nickel in the surface layer of the hydrogen storage alloy is greatly increased. When the battery is overcharged, the hydrogen atoms adsorbed on the surface of the metal nickel increase, which is conducive to the progress of the recombination reaction. Therefore, the oxygen evolution will not affect the pressure inside the battery. During the overcharging process, the substance inside the battery does not change due to the evolution of oxygen, but the material balance is achieved through the precipitation and reaction consumption of oxygen.
According to the above reaction, it can be seen that the reactions that occur on the positive and negative electrodes of Ni-MH batteries belong to the solid phase transition mechanism and do not involve the generation of any soluble metal ion intermediate products. Therefore, the positive and negative electrodes of the battery have high structural stability. sex. In addition, Ni-MH batteries generally adopt the configuration of excess capacity of the negative electrode. When the battery is overcharged, the oxygen precipitated on the positive electrode can be reduced to water on the surface of the metal hydride electrode; It can be absorbed by the metal hydride electrode, so that the battery has good resistance to overcharge and overdischarge. At the same time, the battery does not generate and consume additional electrolyte components (including KOH and H2O) during the working process, so that the Ni-MH battery can be fully sealed and maintenance-free.
2. Charging and discharging characteristics of Ni-MH battery
Charge a single battery of 100Ah at a rate of 1C. The charging characteristic curve is shown in Figure 3. In the initial stage of charging, the battery terminal voltage rises rapidly. After the battery is fully charged, continuing to charge will cause the voltage to drop because of overcharge. Charging will cause gassing, and there may even be a danger of explosion. Therefore, overcharging of Ni-MH batteries should be avoided.
The discharge test is carried out at a rate current of 1C. The discharge characteristic curve is shown in Figure 4. The terminal voltage of a fully charged Ni-MH battery decreases slowly at the initial stage of discharge, and when the battery capacity is close to exhaustion, the battery The terminal voltage has only begun to drop significantly. Under the same conditions, Ni-MH batteries can release more energy than lead-acid batteries.