Basic knowledge of Ni-Cd battery and Ni-MH battery

Alkaline batteries, also known as alkaline dry batteries, alkaline zinc-manganese batteries, and alkaline manganese batteries, are one of the best-performing varieties in the zinc-manganese battery series. The power alkaline battery suitable for electric vehicles is a rechargeable battery, which is a general term for a secondary battery with potassium hydroxide (KOH) aqueous solution as the electrolyte.

According to the different active material materials of the plates, alkaline batteries can be divided into zinc-silver storage batteries, iron-nickel storage batteries, nickel-cadmium storage batteries and nickel-hydrogen storage batteries. Under normal circumstances, the KOH in the electrolyte does not directly participate in the electrode reaction, which is a major feature of alkaline batteries different from lead-acid batteries. Compared with lead-acid batteries, alkaline batteries have the characteristics of high energy density, high mechanical strength, stable working voltage, and high power density. They are the main body of power batteries for electric vehicles produced in industrial production, and they are also used in mass-produced electric vehicles so far. One of the largest battery types.

1). Nickel-cadmium (Ni-Cd) battery
The nickel-cadmium battery is a storage battery with nickel oxyhydroxide as the positive electrode, metal cadmium as the negative electrode, and water-soluble potassium hydroxide solution as the electrolyte.
(1) Working principle
During the chemical reaction of charging and discharging nickel-cadmium batteries, the electrolyte is basically not consumed. In order to increase life and improve high-temperature performance, lithium oxide is usually added to the electrolyte. The chemical reaction equation of nickel-cadmium battery:

Cd＋NiO₂＋2H₂O [Charge]↔[Discharge] Cd(OH)₂＋Ni(OH)₂
(2) Structure
Each single cell of a nickel-cadmium battery is composed of a positive plate, a negative plate and a separator installed between the positive plate and the negative plate, as shown in Figure 1. The single batteries are installed in different plastic shells in different combinations to obtain the required nickel saw battery assemblies (battery packs) with different voltages and different capacities. There are many different specifications and models of nickel-cadmium battery assemblies on the market. Available for selection. After the electrolyte is filled and charged, current can be drawn from the battery terminal. Figure 2 shows the appearance of the nickel saw battery pack.

(3) Features and applications
① Features. The working voltage of nickel-cadmium batteries is relatively low. The nominal voltage of the single battery is 1.2V, the specific energy is 55W·h/kg, the specific power can exceed 225W/kg, and the cycle life is more than 2000 times; it can be quickly charged and recharged. It can recover 50% of the power in 15 minutes, and 100% of the power can be recovered after 1 hour of charging, but in general, it takes 6 hours to fully charge; the deep discharge reaches 100%, and the self-discharge rate is less than 0.5%/day. It can work normally under ambient temperature conditions of -40 ~ 80℃.
The cost of nickel-cadmium batteries is 4 to 5 times that of lead-acid batteries, and the initial purchase cost is relatively high. However, the specific energy and cycle life of nickel-cadmium batteries are much higher than those of lead-acid batteries. Therefore, when electric vehicles are actually used, the total The cost will not exceed the cost of lead-acid batteries.

Nickel-cadmium batteries have a memory effect, that is, if the battery is not fully charged and discharged for a long time, it is easy to leave traces in the battery and reduce the battery capacity. For example, a nickel-cadmium battery can only be charged after it has discharged 80% of its power for a long time, and it can only discharge 80% of its power after a period of time when the battery is fully charged.
Cadmium (Cd) used in nickel-cadmium batteries is a harmful heavy metal and must be effectively recycled after the batteries are scrapped.

② Application. Because nickel-cadmium batteries have better performance than lead-acid batteries, they are widely used in hybrid vehicles. Nickel-cadmium batteries are used in Chrysler’s TE vans, Peugeot 106 hybrid vehicles, Citroen AX-EV, and hybrid vehicles produced by Honda Motor Co. and Nissan Motor Co., Ltd. of Japan.

2). Ni-MH battery
A battery with a nickel compound (usually nickel hydroxide) as the active material of the positive plate, a hydrogen storage alloy as the negative plate material (the active material is hydrogen), and the electrolyte is a mixture of water-soluble potassium hydroxide and lithium hydroxide is called nickel hydrogen Battery.

(1) Structure
The positive electrode of the nickel-metal hydride battery is a paste made of spherical nickel hydroxide [Ni(OH)₂] powder and additives such as cobalt (Co), metal, resin, and adhesives. It is coated on the positive plate with an automatic sound applicator, and then After drying treatment, it becomes a foamed nickel hydroxide positive electrode plate. Adding Ca, Co, Zn or rare earth elements to the cathode material Ni(OH)₂ can significantly improve the performance of the stable electrode. When polymer materials are used as adhesives or foamed nickel electrodes made by extrusion and rolling, and nickel powder, graphite, etc. are used as conductive agents, the discharge performance at high currents can be improved.

The key technology of the negative electrode of the nickel-hydrogen battery is the hydrogen storage alloy, which requires the hydrogen storage alloy to be able to withstand repeated cycles of gas storage and degassing stably. Hydrogen storage alloy is a multi-metal alloy crystal lattice base block that allows hydrogen atoms to enter or separate. It uses five basic elements of titanium-vanadium-zirconium-nickel-chromium (Ti-V-Zr-Ni-Cr) and is combined with A sintered alloy of metal elements such as cobalt (Co) and manganese (Mn) is hydrogenated, crushed, shaped and sintered to form a negative electrode plate. The type and performance of hydrogen storage alloys have a direct impact on the performance of nickel-metal hydride batteries. The negative electrode neither dissolves nor crystallizes during charging or discharging, and the electrode will not have structural changes. While maintaining its own chemical function, it also guarantees its own mechanical robustness. Hydrogen storage alloys generally require heat treatment and surface treatment to increase the corrosion resistance of the hydrogen storage alloys, which is conducive to improving the specific energy, specific power and service life of the nickel-hydrogen battery.

The basic unit of the nickel-hydrogen battery is a single battery. Each single battery is composed of a positive plate, a negative plate and a separator installed between the positive plate and the negative plate. The shape of the battery is round and square, as shown in the figure. 3 shown. The rated voltage of each battery is 13.2V (the maximum voltage when charging is 16.0V), and then the batteries are combined into a nickel-hydrogen battery assembly (battery pack or battery box) of different voltages and different capacities according to the requirements of use, as shown in Figure 4 Shown. Ni-MH battery has a specific energy of 70W·h/kg, an energy profile of 165W·h/L, a specific power of 220W/kg at a depth of 50% of discharge, and 200W/kg at a depth of 80% of discharge. Ni-MH batteries can greatly improve the power performance of electric vehicles.

(2) Working principle
As shown in Figure 5, when the battery is charged, water is decomposed into hydrogen ions (H+) and hydroxide ions (OH-) in the electrolyte solution, H+ is absorbed by the negative electrode, and the negative electrode is converted from metal to metal hydride (adsorbed hydrogen); OH- is absorbed by the positive electrode, and nickel hydroxide is converted into nickel hydroxide. In the discharge process, H+ leaves the negative electrode, OH- leaves the positive electrode, H+ and OH- combine to form water in the electrolyte potassium hydroxide and release electrical energy.

(3) Features and applications
① Features. Compared with lead-acid batteries, nickel-metal hydride batteries have the following characteristics.
a. High specific power. The specific power of Ni-MH batteries can reach 200W/kg, which is twice that of lead-acid batteries, which can improve the starting performance and acceleration performance of the vehicle. At present, the specific power of commercial Ni-MH power batteries has reached 1350W/kg.
b. Higher than energy. The nominal voltage of the nickel-hydrogen battery is 1.2V, and the specific energy can reach 70~80W·h/kg, which is conducive to extending the driving range of electric vehicles.
c. Long life. At 80% depth of discharge, the cycle life of Ni-MH batteries can reach 1,000 times (or 10 years), which is 3 times that of lead-acid batteries. The cycle life of 100% discharge depth is more than 500 times, and it can be used for more than 5 years in hybrid vehicles.
d. No heavy metal pollution. There are no heavy metal elements such as Pb and Cd in the nickel-hydrogen battery, which will not cause pollution to the environment.
e. Resistance to overcharge and overdischarge. Ni-MH batteries have high-rate discharge characteristics, can be discharged at 3C in a short time, and the instantaneous pulse discharge rate is very high. Ni-MH batteries have good overcharge and overdischarge performance.
f. It can be charged quickly. The Ni-MH battery can be charged 60% of the power within 15 minutes, and can be fully charged within 1 hour, and the time for emergency supplementary charging is short.
g. No memory effect. So it can be recharged and released.
h. Wide operating temperature range. The normal use temperature range is -30 ~ 55℃; the storage temperature range is -40 ~ 70℃.
i. Safe and reliable. Carry out short circuit, squeeze, acupuncture, safety valve working ability, drop, heating, vibration resistance and other safety and reliability tests on Ni-MH batteries, without explosion or combustion. Using a fully enclosed shell, it can work normally in a vacuum environment.

② Application. Automobile power battery packs are often charged and discharged, and the charging and discharging are carried out irregularly, which has a serious impact on the life of the battery. Matsushita Battery Company conducted a simulation test on nickel-metal hydride batteries by simulating the driving conditions of hybrid electric vehicles, and proved that the characteristics of nickel-metal hydride batteries hardly change, and that nickel-hydrogen batteries are more suitable for hybrid vehicles.

At present, Japan, the United States, Germany, France, etc. all have examples of applying nickel-metal hydride batteries to electric vehicles.
Japan’s Matsushita Battery Company has successfully developed high-energy nickel-hydrogen battery packs suitable for pure electric vehicles and high-power battery packs suitable for hybrid vehicles, which are used in Toyota Prius, LS600h hybrid vehicles and FCHV-adv fuel cell vehicles.

At present, nickel-metal hydride power batteries are mainly used in hybrid vehicles, and they are basically not used in pure electric vehicles. The reasons are as follows.
a. The voltage platform is low.
b. Low energy density.
If nickel-metal hydride batteries are used as power battery packs on electric vehicles, the cost will be greatly increased, and the volume of the battery pack will increase by about 3 times.