KOH and NaOH aqueous solutions are alkaline, and batteries using them as electrolytes are called alkaline batteries, including iron-nickel, nickel-cadmium, nickel-hydrogen, nickel-hydride, and zinc-silver batteries. At present, nickel-cadmium batteries are commonly used in alkaline batteries. The basic structure is shown in Figure 1. It is mainly composed of nickel hydroxide positive plate, cadmium negative plate and diaphragm. The electrolyte is potassium hydroxide aqueous solution, and an appropriate amount of lithium hydroxide is added. The active material on the positive plate of the battery is composed of nickelous hydroxide and graphite powder. Graphite does not participate in the chemical reaction, and its main function is to enhance conductivity. The active material on the negative plate is composed of cadmium oxide powder and iron oxide powder. The function of iron oxide powder is to make the cadmium oxide powder have higher diffusivity, prevent agglomeration, and increase the capacity of the electrode plate. The upper cover of the alkaline battery case must be tightly closed, otherwise the electrolyte will produce carbonate harmful substances. In order to inject or pour the electrolyte into the casing, there are cover holes for installing automatic valves on the battery casing cover. It can prevent the electrolyte from splashing, and can automatically discharge the gas when the air pressure in the casing exceeds the external atmospheric pressure, and the external air is not easy to contact with the electrolyte. Alkaline batteries have high energy density and small size, but are expensive and have a memory effect when discharged.
The working principle of the alkaline battery is the same as that of the acid battery, the main difference is that the chemical reaction caused by the use of different electrolytes is different. For example, when a nickel-metal hydride battery is discharged, the active material of the positive electrode is NiOOH, and the active material of the negative electrode is H2; when charging, the active material of the positive electrode is Ni(OH)2, and the active material of the negative electrode is H2O, and the working electrolyte is 30% hydroxide. Potassium solution, the electrochemical reaction occurs when charging is as follows:
H2O+e (charging) → (1/2) H2+ОH–
It can be seen from the reaction that when the battery is charged, the hydrogen evolved from the negative electrode is stored in the container, and the positive electrode changes from nickel hydroxide to nickelous hydroxide (NiOOH) and H2O. When discharging, the reaction that occurs is the reverse reaction of the above reaction, hydrogen is consumed at the negative electrode, and the positive electrode changes from nickelous hydroxide to nickel hydroxide. When an alkaline battery is overcharged, oxygen is released from the positive plate and hydrogen is released from the negative plate. Due to the large area of the hydrogen electrode with catalyst, and the oxygen can diffuse to the surface of the hydrogen electrode at any time, hydrogen and oxygen can be easily recombined to form water inside the battery, so that the gas pressure in the container remains unchanged.
Nickel-metal hydride batteries have good low-temperature discharge characteristics. Even at an ambient temperature of 120°C, the discharged electricity can reach more than 85% of the nominal capacity with a large current (at a 1C discharge rate), and a small current discharge rate is used. When the nickel-metal hydride battery discharges power, it can reach more than 90% of the nominal power. However, when the nickel-metal hydride battery is at a high temperature (above +40°C), the storage capacity will drop by 5% to 10%. This capacity loss due to self-discharge (the higher the temperature, the greater the self-discharge rate) is reversible, and the maximum capacity can be restored after a few charge-discharge cycles. The self-discharge rate of NiMH batteries is very small. At room temperature, after the nickel-metal hydride battery is fully charged for 28 days, the battery power can still maintain between 75% and 85% of the nominal power.
Compared with the nickel-cadmium battery of the same volume, the power of the nickel-metal hydride battery can be doubled, and after the number of charge and discharge cycles reaches 500, the power of the nickel-metal hydride battery does not decrease significantly. The number of charge and discharge cycles can reach 1000 times. In nickel-metal hydride batteries, the expensive toxic substance – metal cadmium is not used. Therefore, nickel-metal hydride batteries will not pollute the environment after production, use and disposal, and are called green batteries. Ni-MH battery has no memory effect, can be charged at any time, and does not need to be discharged before charging, so it is very convenient to use. This battery is the preferred battery type in photovoltaic power generation systems.