Basic knowledge of air battery
Air battery refers to a secondary battery in which oxygen is used as the active material of the positive electrode, metals are commonly used as the active material of the negative electrode, and the electrolyte is alkaline (KOH).
Because there are many types of metal materials for the negative electrode, so there are many types of air batteries. Generally, the metal selected as the negative electrode material is named the first word of the battery name, and the air battery is the battery name, such as zinc. Air batteries with negative electrodes are called zinc-air batteries, and aluminum-air batteries with aluminum as the negative electrode material are called aluminum-air batteries. Because zinc materials are easily available and the price is not high, there are more zinc-air battery products.
(1) Structure of zinc-air battery
The structure of the zinc-air battery is shown in Figure 1. The air plate is composed of four layers: one is the isolation layer; the other is the catalyst layer, which is surface-modified activated carbon or carbon black material; the third is the collector layer, which is made of plastic and conductive metal mesh; the four layers are air The diffusion layer uses cellulose as the diffusion paper, and there is a water-containing film made of polytetrafluoroethylene resin between the catalyst layer and the air diffusion layer. The finished zinc-air battery is made up of a set of single cells in series. The on-board zinc-air power battery pack also includes an air circulation guarantee system and a battery pack thermal management system to ensure long-term stable operation of the power battery pack. The air circulation guarantee system regulates the amount of air entering the anode of the zinc-air battery, and can automatically cut off the air when the battery is not in use. The thermal management system is mainly used to ensure the reliable operation of the zinc-air battery pack.
The charging concept of the zinc-air battery is different from that of other batteries. The “charging” here is to remove the zinc particles that have been chemically reacted to form zinc oxide and refill the zinc particles. The method is simple and does not take long. The vehicle-mounted zinc-air power battery pack usually adopts a mechanical charging mode, that is, “charging” by changing the zinc plate or zinc particle electrolyte. The replaced zinc oxide is recovered and processed in a special factory to realize the recycling of zinc. This special “charging” generally only takes a few minutes to change once, for example, it takes 100s to change a 20kg battery pack meter. Therefore, if you want to “charge” the vehicle-mounted zinc-air battery, you only need to set up a mechanical overall replacement station for zinc plates or zinc pellet boxes and electrolyte container boxes along the road. The effect is like the current gas station of fuel vehicles, and it can be the owner of the vehicle. Provide great convenience.
(2) Working principle of zinc-air battery
As shown in Figure 2, the zinc-air battery uses zinc (Zn) as the negative electrode, the air electrode as the positive electrode (the active material is oxygen), and the KOH solution as the electrolyte. The chemical reaction of a zinc-air battery is similar to that of an ordinary alkaline battery. Under the action of a special catalyst, when the battery is discharged, the zinc ingests and transports the oxygen absorbed from the air in the carbon block, and a chemical reaction between zinc and oxygen produces zinc oxide (ZnO). ). Zinc-air battery is actually a half battery and half fuel cell. First, the negative electrode active material is encapsulated inside the battery like zinc-manganese batteries and lead batteries, which has the characteristics of a battery; secondly, the positive electrode active material comes from the oxygen contained in the air outside the battery, and theoretically has unlimited capacity, which is the fuel cell’s Typical characteristics.
The chemical reaction between the positive and negative electrodes of zinc-air batteries is irreversible. Unlike other forms of alkaline batteries, there is no charging process and the reacted substances have to be removed, so the consumption of zinc metal is relatively large. The operating voltage of a single zinc-air battery is 1.1~14V.
(3) Classification of zinc-air batteries
Zinc-air batteries can be divided into three categories according to their charging methods and the characteristics of their applications in electric vehicles and other fields: direct rechargeable zinc-air batteries, mechanically rechargeable zinc-air batteries, and injected zinc-air batteries.
①Direct rechargeable zinc-air battery. The direct rechargeable zinc-air battery directly charges the zinc electrode of the zinc-air battery. During this process, the electrochemical activity of zinc in alkaline solution is very large, and the thermodynamic properties are unstable. The charging product zincate is in strong alkali. The solubility in the solution is high, and the electrode deformation, dendritic growth, self-corrosion and passivation are prone to phenomenon, which leads to the gradual failure of the electrode. In addition, the air electrode has poor reversibility, the electrolyte is easily carbonated in the atmosphere, and the electrolyte is greatly affected by the humidity of the air. When the relative humidity of the air is low, the battery will lose moisture, leading to insufficient electrolyte or battery failure; when the relative humidity of the air is high, the electrolyte becomes thinner and the conductivity decreases, and it may flood the catalyst layer of the gas electrode and reduce the electrode. Active, leading to battery failure. Therefore, the application of direct rechargeable zinc-air batteries is subject to certain restrictions. But the newly developed rechargeable zinc-air battery has reached a cycle life of more than 100 times.
②Mechanical rechargeable zinc-air battery. In view of the problems of direct rechargeable zinc-air batteries, mechanical charging can be used according to the discharge characteristics of zinc-air batteries and their own characteristics. Mechanical charging means that after the battery is fully discharged, the used zinc electrode in the battery is taken out, replaced with a new zinc electrode, or the entire battery pack is completely replaced, and the entire process is controlled in a relatively short time (3~5min ). This method is very beneficial for popularizing zinc-air battery electric vehicles. The used zinc electrode or zinc-air battery can be recycled and reprocessed in a special zinc recycling plant to realize environmentally friendly and pollution-free production.
③Injected zinc-air battery. The basic principle of the injected zinc-air battery is similar to that of the mechanically rechargeable zinc-air battery, in essence it is to replace the zinc electrode active material. This kind of battery continuously sends the prepared zinc paste into the battery by squeezing and conveying, and at the same time extracts the reacted mixture out of the battery, so that when it is used in electric vehicles, the battery system only needs to carry the zinc paste. If the fuel tank is filled with enough zinc paste fuel, the continuous driving of the vehicle can be realized.
4) Features and applications of zinc-air batteries
a. Greater than energy. The theoretical specific energy of the zinc-air battery is 1350Whkg, the actual specific energy is 180~230whkg; the energy density is 230W-h/L.
b. Short charging time. With mechanical charging mode, the charging time is only a few minutes. Battery capacity is not affected by discharge intensity and temperature
c. Stable performance. The single battery has good consistency, can be deeply discharged, the battery capacity is not affected by the discharge intensity and temperature, and can work under the environmental conditions of -20~80℃. When discharging, there is no pressure and no gas generation, which can be sealed and maintenance-free, which is convenient for the energy management of the battery pack.
d. Good security. Even if there is an open flame, short circuit, puncture and impact externally, it will not burn or explode.
e. Environmental protection. The positive electrode of the battery adopts activated carbon and copper mesh, and the negative electrode adopts metallic zinc, and no toxic substances are used.
f. Recyclable. After the zinc electrode is used up, it can be reused through regeneration reduction.
g. Easy to charge. Since the charging of the zinc-air battery is mainly to replace the electrode plate, the regeneration of the electrode plate can be carried out intensively. The distribution of the plates can be arranged like a store, without the need to build a dedicated charging station. This not only saves a lot of upfront investment, but also brings a lot of convenience to users.
② Disadvantages. Zinc-air batteries are very sensitive to moisture and carbon dioxide. If the relative humidity changes, the characteristics of the battery will change accordingly. The critical relative humidity of zinc-air battery is about 60%, if the deviation is too large, it will seriously affect the battery’s use effect. After research, if the humidity is less than 60%. The battery will lose moisture, and when the humidity is greater than 60%, the moisture will be too much, and the battery may have quasi-leakage. The carbon dioxide that enters with the air will chemically react with the electrolyte (KOH) to acidify the electrolyte to form carbonate (or carbonate), crystallize on the electrode, and the cathode will be damaged and the air passage will be blocked. Danger.
③ Application. Zinc-air batteries are mostly used in pure electric commercial vehicles. For example, the zinc-air battery postal vehicle developed in Germany uses the zinc-air battery developed by Israel Electric Fuel Co., Ltd. The zinc-air battery developed by Dreisback Electromotive of the United States has been used in buses and trucks with a total mass of 9t. The MB410 electric van of Germany Benz Motors has a standard total mass of 4000kg and uses a 150kWh zinc-air battery. Electric trucks, electric buses and electric service vehicles in Stockholm, Sweden, use zinc-air batteries with a specific energy of 180W·h/kg, an energy density of 100Wh/L, and a driving range of 350~425km. China’s electric buses and sanitation vehicles have also been installed and put into test operation of the city’s public transportation and sanitation systems.