Research Status and Development Trend of Aluminum Air Battery

The theoretical specific energy of aluminum air fuel cells can reach 8100Wh/kg, which has the advantages of low cost, high specific energy density and high specific power density. As a special fuel cell, aluminum-air batteries have great commercial potential in applications such as military, civilian, and underwater power systems, telecom system backup power sources, and portable power supplies.

Metal Air Battery Overview

Lithium-ion batteries have higher specific energy and are currently researching mature and large-scale commercial secondary batteries. However, in recent years, lithium-ion batteries have been difficult to meet in the face of tremendous developments in mobile electronic devices and electric vehicles. Its large-capacity demand, especially for the power-dependent battery system is very strong. Therefore, metal-air batteries with a specific capacity several times larger than lithium-ion batteries have emerged, such as zinc-air batteries, aluminum air batteries, magnesium air batteries, and lithium-air batteries.

Since the positive active material of such batteries is mainly derived from oxygen in the air, theoretically, the amount of active material of the positive electrode is infinite, so the theoretical capacity of the battery depends mainly on the amount of negative electrode metal. Such batteries have a larger specific capacity.

Among them, the theoretical specific energy of aluminum air fuel cell can reach 8100Wh/kg, which has the advantages of low cost, high specific energy density and high specific power density. As a special fuel cell, aluminum-air batteries have great commercial potential in applications such as military, civilian, and underwater power systems, telecom system backup power sources, and portable power supplies.

Aluminum air battery structure and principle

Analyze from the existing research results and battery characteristics, aluminum air battery has the following characteristics:

(1) High specific energy. Aluminum air battery is a new type of high specific energy battery, the theoretical specific energy can reach 8100Wh/kg The current research and development of the product has reached 300-400Wh/kg, much higher than the specific energy of all types of batteries.

(2) Medium specific power. Since the working potential of the air electrode is far from its thermodynamic equilibrium potential, its exchange current density is very small, and the polarization of the battery is large, resulting in the battery's specific power can only reach 50-200W/kg.

(3) Long service life. The aluminum electrode can be continuously replaced, so the life of the aluminum air battery depends on the working life of the air electrode.

(4) Non-toxic and no harmful gas. The electrochemical reaction of the battery consumes aluminum, oxygen, and water to produce Al2O3?nH2O, which can be used for drying adsorbents and catalyst supports, abrasives for polishing, ceramics, and excellent precipitants for sewage treatment.

(5) strong adaptability. The structure of the battery and the raw materials used can vary according to the practical environment and requirements, and have a strong adaptability.

(6) The battery anode material aluminum is cheap and easy to get. Compared with other metals, the price of metal aluminum is relatively low, and the manufacturing process of metal anodes is relatively simple.

Aluminum anode (negative)

Aluminum (Al) is an ideal electrode material, the theoretical energy density of metal aluminum is 8.2W?h / g, in common metals, second only to lithium 13.3W?h / g, the electrode potential is more negative, in addition to Light metal battery materials with the highest specific mass energy other than lithium metal. The mass ratio of the aluminum air battery can actually reach 450Wh/kg, and the specific power can reach 50~200W/kg. With the advantages of high theoretical capacity, low consumption rate, light weight, negative potential, abundant resources and easy processing, it has been widely studied.

However, since aluminum is a very active amphoteric metal, the development of aluminum anodes is currently affected by the following problems.

(1) There is a passivation film on the surface of aluminum, which affects the electrochemical activity of aluminum.

(2) Aluminum is an amphoteric metal element, which determines that it is prone to hydrogen evolution in a strongly alkaline environment, affecting the electrode potential, and the product floating in the electrolyte affects the entire electrochemical reaction.

(3) The unique semi-open system of the air battery makes the air electrode vulnerable to the influence of the external humidity, and the aluminum anode is “submerged” or “dried”, and even “climbing alkali” or “leakage”, and thus the whole air The structure of the battery causes damage. In order to solve the above problems, domestic and foreign scholars have studied the following three aspects:

1. Aluminum anode alloying

Industrial-grade aluminum (99.0%) contains more impurities, such as iron (0.5%), silicon, copper, manganese, magnesium, and zinc, etc., which will increase the hydrogen evolution corrosion of the aluminum at the phase interface, especially, the iron will form local part with aluminum. The primary battery causes multiplication of electrochemical corrosion. Aluminum can be added to the alloy which can increase the chemical activity and increase the corrosion resistance.

The alloys to be alloyed need to meet the following conditions: 1 the melting point of the alloying element is lower than that of the metal Al; 2 the solid saturation is high in Al; 3 the electrochemical activity is higher than Al; 4 the solubility in the electrolyte is relatively High; 5 has a high hydrogen evolution overpotential. In addition, processing the anode metal into an ultrafine grained material can further increase anode efficiency.

2, adding slow release agent in the electrolyte

Since the anode alloying has a certain cost problem, people often choose to add some release agents to the electrolyte to ensure the performance of the aluminum air battery. Some carboxylic acids, amines, amino acid slow release agents and their inhibitory effects on aluminum corrosion are shown in Table 1:

Researchers have used natural materials as corrosion inhibitors for metal aluminum. Experiments have shown that organic amines, pyrrole, etc. have a significant inhibitory effect on aluminum corrosion. By adding organic substances and water-soluble compounds to the strong alkaline electrolyte to study the electrochemical behavior of the aluminum metal anode, the corrosion rate of aluminum is reduced, and the performance of the aluminum air battery is further improved.

3, heat treatment process

Heat treatment affects the performance of the alloy by changing the distribution of trace elements in the aluminum alloy and the microstructure of the alloy surface, which is a research area of ​​technology. The best heat treatment process can be found by suitable orthogonal experiments.

Electrolyte

Aluminum air battery electrolyte is mostly neutral salt solution or strong alkaline solution. When a neutral electrolyte is used, the self-corrosion of the anode is small, but the surface of the aluminum anode is seriously inactivated, so that the operating voltage is reduced, the power and current of the battery are difficult to increase, and the voltage lags, and the product aluminum hydroxide colloid also settles and blocks. Electrolyte, so this type of battery can only be used as a low-power power output device.

When a strong alkaline electrolyte is used, the passivation of aluminum is reduced, and the alkaline solution can absorb a certain amount of reaction product aluminum hydroxide. The performance of the battery is relatively good, but aluminum is an amphoteric metal and occurs in a strong alkaline environment. Strong hydrogen evolution corrosion, release a large amount of hydrogen, reduce the output power of the battery and the anode utilization, is more serious at high current densities. If it is simply to solve the above problems, you can choose to replace the electrolyte regularly and add additives that can activate the aluminum anode surface and inhibit aluminum corrosion and hydrogen evolution corrosion to solve the appeal problem.