Research Status and Development Trend of Aluminum-Air CR2016 battery

The theoretical specific energy of aluminum-air fuel cells can reach 8100Wh/kg, with the advantages of low cost, high specific energy density and specific power density. As a special fuel cell, aluminum-air CR2016 battery have great commercial potential in special, civilian, underwater power systems, backup power sources for telecommunications systems, and portable power sources.

Overview of Metal-Air CR2016 battery Lithium-ion CR2016 battery have high specific energy and are currently mature and commercially available secondary CR2016 battery. However, in recent years, facing the tremendous development of mobile electronic devices and electric vehicles, lithium-ion CR2016 battery have been difficult to meet their large capacity needs, especially for power battery systems that are highly dependent on energy. Therefore, metal-air CR2016 battery with several times larger specific capacity than lithium-ion CR2016 battery have emerged, such as zinc-air CR2016 battery, aluminum-air CR2016 battery, magnesium-air CR2016 battery, lithium-air CR2016 battery, etc.

Since the positive active material of this type of battery mainly comes from oxygen in the air, the theoretical amount of positive active material is infinite, so the theoretical capacity of the battery mainly depends on the amount of negative metal, and this type of battery has a larger specific capacity.

Among them, the theoretical specific energy of aluminum-air fuel cells can reach 8100Wh/kg, with the advantages of low cost, high specific energy density and specific power density. As a special fuel cell, aluminum-air CR2016 battery have great commercial potential in special, civilian, underwater power systems, telecommunications system backup power sources and portable power supplies.

Structure and principle of aluminum-air CR2016 battery Based on existing research results and battery characteristics, aluminum-air CR2016 battery have the following characteristics:

(1) High specific energy. Aluminum-air CR2016 battery are a new type of high-specific energy battery with a theoretical specific energy of 8100Wh/kg. The products currently under development can reach 300-400Wh/kg, which is much higher than the specific energy of various CR2016 battery today.

(2) Moderate specific power. Since the working potential of the air electrode is far away from its thermodynamic equilibrium potential, its exchange current density is very small, and the battery is highly polarized during discharge, resulting in the battery's specific power of only 50-200W/kg.

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

(4) Non-toxic, no harmful gas is generated. The battery electrochemical reaction consumes aluminum, oxygen and water to generate Al2O3˙nH2O, which can be used for drying adsorbents and catalyst carriers, grinding and polishing abrasives, ceramics and excellent precipitants for sewage treatment.

(5) Strong adaptability. The battery structure and the raw materials used can be changed according to the practical environment and requirements, and have strong adaptability.

(6) Aluminum, the raw material for the battery negative electrode, is cheap and easy to obtain. Compared with other metals, the price of metallic aluminum is relatively low, and the manufacturing process of the metal anode is relatively simple. Aluminum anode (negative electrode) Aluminum (Al) is an ideal electrode material. The theoretical energy density of metallic aluminum is 8.2W˙h/g, which is second only to lithium's 13.3W˙h/g among common metals. The electrode potential is relatively negative, and it is the light metal battery material with the highest mass specific energy except for metallic lithium. The mass specific energy of aluminum-air CR2016 battery can actually reach 450Wh/kg, and the specific power can reach 50~200W/kg. It has the advantages of high theoretical capacity, low consumption rate, light weight, negative potential, abundant resources and easy processing, and has been widely studied. However, since aluminum is a very active amphoteric metal, the current development of aluminum anode is still 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 easy to corrode by hydrogen evolution in a strong alkaline environment, affecting the electrode potential. The product floats in the electrolyte and affects the progress of the entire electrochemical reaction.

(3) The unique semi-open system of air CR2016 battery makes the air electrode easily affected by external humidity, resulting in "flooding" or "drying up" of the aluminum anode, or even "alkali climbing" or "leakage", which damages the structure of the entire air battery. In order to solve the above problems, domestic and foreign scholars have conducted research from 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, which will aggravate the hydrogen evolution corrosion of aluminum at the phase interface, especially iron will form a local galvanic cell with aluminum, resulting in an exponential increase in electrochemical corrosion. Alloy components that can improve both chemical activity and corrosion resistance can be added to aluminum. The conditions that need to be met by the elements added to aluminum alloying are:

① The melting point of the alloying element must be lower than that of metal Al;

② The solid saturation in Al is higher;

③ The electrochemical activity is higher than Al;

④ The solubility in the electrolyte is higher;

⑤ It has a higher hydrogen evolution overpotential.

In addition, processing the anode metal into ultrafine-grained materials can further improve the anode efficiency.

2. Adding slow-release agents to the electrolyte Since anode alloying has certain cost issues, people often choose to add some slow-release agents to the electrolyte to ensure the performance of aluminum-air CR2016 battery. Some carboxylic acid, amine, amino acid slow-release agents and their inhibitory efficiency on aluminum corrosion are shown in Table 1: Researchers use natural substances as inhibitors of metal aluminum corrosion. Experiments have shown that organic amines, pyrrole, etc. have a significant inhibitory effect on aluminum corrosion. By adding organic matter and water-soluble compounds to strong alkaline electrolytes, the electrochemical behavior of aluminum metal anodes is studied to reduce the corrosion rate of aluminum, thereby improving the performance of aluminum-air CR2016 battery.

3. Heat treatment process Heat treatment affects the properties of alloys by changing the distribution of trace elements in aluminum alloys and the microstructure of the alloy surface, and belongs to the research scope of process science. The best heat treatment process can be found through appropriate orthogonal experiments. Electrolyte The electrolyte of aluminum-air CR2016 battery is mostly neutral salt solution or strong alkaline solution. When using a neutral electrolyte, the anode self-corrosion is small, but the aluminum anode surface is severely passivated, which reduces the working voltage, makes it difficult to increase the power and current of the battery, and also causes voltage hysteresis. The product aluminum hydroxide colloid will also settle and block the electrolyte. Therefore, this type of battery can only be used as a low-power power output device. When using a strong alkaline electrolyte, the passivation of aluminum is reduced, and the alkaline solution can absorb a certain amount of the reaction product aluminum hydroxide, so the battery performance is relatively good. However, aluminum is an amphoteric metal, and in a strong alkaline environment, it will undergo strong hydrogen evolution corrosion, releasing a large amount of hydrogen, reducing the output power of the battery and the utilization rate of the anode, which is more serious at high current density. If you simply want to solve the above problems, you can choose to regularly replace the electrolyte and add additives to the electrolyte that can activate the aluminum anode surface and inhibit aluminum hydrogen evolution corrosion to solve the above problems. The air electrode (positive electrode) cathode is the reaction site of O2, which is breathable, conductive, waterproof, corrosion-resistant and catalytic, and is often called the air electrode. The air electrode generally consists of a porous catalyst layer, a conductive current collector and a waterproof and breathable layer. The porous catalyst layer is the main place where oxygen is reduced. Here, the diffused oxygen, oxygen reduction catalyst and thin layer electrolyte form a three-phase interface electrochemical active site. The conductive current collector mainly plays the role of conductivity and mechanical support. The waterproof and breathable layer has a loose porous and hydrophobic structure, which not only provides the catalyst layer with the gas required for the reaction, but also prevents the electrolyte from flooding the gas diffusion channel. The catalyst layer is the most critical part of the air electrode and plays a decisive role in its electrochemical performance. The performance of aluminum-air CR2016 battery depends largely on the selected cathode catalyst. The performance of the air electrode can directly affect the electrode reaction balance. Therefore, improving its performance can improve the utilization rate of the aluminum-air battery anode to a certain extent and inhibit the self-corrosion of the anode aluminum. Commonly used catalysts for aluminum-air CR2016 battery are as follows:

(1) Precious metal catalysts. Platinum and silver are commonly used. Their catalytic activity is high and stable, but due to their high price and shortage of resources, their adoption rate is not high.

(2) Metal macrocyclic compound catalysts. Organometallic macrocyclic compounds have good catalytic activity for oxygen reduction, especially when they are adsorbed on carbon with a large surface area. Their activity and stability can be significantly improved by heat treatment. Therefore, they are expected to replace precious metal oxygen reduction catalysts. Common methods for synthesizing metal macrocyclic compounds include thermal decomposition and precursor preparation. However, since the heat treatment process of the thermal decomposition method will cause the metal macrocyclic compound to react with the carbon matrix, the catalyst prepared by the precursor method has poor activity, so there are certain problems in its application.

(3) Perovskite oxide catalysts. Perovskite oxides have high catalytic activity for oxygen reduction and precipitation, and are inexpensive, so they have broad application prospects in aluminum-air CR2016 battery and fuel cells. Current research on perovskite oxygen electrode catalysts mainly focuses on improving preparation methods and finding new substitution elements to improve catalytic performance. The amorphous precursor method, especially the malic acid precursor method, can prepare perovskite oxides with fine grains and large specific surface area, thereby greatly improving their catalytic activity. It is currently the best method for preparing perovskite oxides.

(4) Cheap catalysts. The most important representative is manganese dioxide catalyst. Its biggest advantage is that the raw materials are abundant and the cost is low. It can be widely used in CR2016 battery with aqueous or non-aqueous electrolytes. However, the electrocatalytic activity of single manganese dioxide has certain limitations, so people have never stopped researching in this area.

(5) AB2O4 spinel oxide catalyst. The lattice of spinel is face-centered cubic. There are 32 densely packed O2- ions in the unit cell, and 64 tetrahedral voids and 32 octahedral voids are occupied by metal ions. The dehydration activity of spinel is related to the fraction of B ions located in the tetrahedral voids. The larger the fraction, the higher the acidity of the catalyst surface and the greater the dehydration activity. Generally, aluminum-air CR2016 battery do not use this catalyst.

(6) Other metal and alloy catalysts. Nickel is relatively cheap and has high corrosion resistance under anode polarization conditions in alkaline electrolytes. At the same time, nickel has the highest oxygen evolution efficiency among metal elements, so nickel is traditionally used as the anode material for alkaline water electrolysis. Nickel-iron, nickel-cobalt and other alloy catalysts are also often used. They have good catalytic activity and corrosion resistance, and are also a catalyst direction that can be considered for aluminum-air CR2016 battery.

(7) Composite catalyst. Two or more catalysts are combined together to better improve the catalytic activity of the air electrode of aluminum-air CR2016 battery. Application prospects of aluminum-air CR2016 battery At present, aluminum-air CR2016 battery have not been widely promoted and applied in industrial and civilian fields, mainly because the material preparation technology needs to be improved and the understanding of the concept of secondary charging and discharging needs to be improved. At the technical level: the measured specific energy and discharge efficiency of aluminum-air CR2016 battery are quite different from the theoretical values. The main technical problems include:

(1) The self-corrosion and hydrogen evolution of the aluminum anode largely restrict its discharge efficiency, and the surface passivation of the aluminum anode affects its discharge response time;

(2) The matching of the electrolyte and the anode, which can not only form a fast anodic oxidation reaction response mechanism with the aluminum electrode, but also maintain the high efficiency and stability of ion transfer, the recyclability of the oxidation product, etc.;

(3) The structure of the air electrode, the self-consumption of the conductive collector current, the oxygen reduction ability of the air electrode catalyst, etc., all need to be further optimized and improved. The concept of aluminum-air battery as a secondary battery: aluminum-air battery is a metal fuel cell, and it is generally considered to be a primary battery. This is a misunderstanding of the charge and discharge cycle unit. The lithium-ion battery we commonly use now is a classic secondary battery, which can achieve instant charge and discharge conversion. If the aluminum-air battery can realize the industrialized charge and discharge process, it can also be regarded as a secondary battery from the perspective of this large cycle system, and this is one of the key technologies to solve its promotion and application.

Pure aluminum or alloy generates Al2O3˙nH2O through anodization reaction (discharge), and Al2O3˙nH2O generates Al2O3 through roasting. It can be further regenerated into aluminum through electrolytic reduction (charging); Al2O3 and Al2O3˙nH2O can also be used as raw materials for the preparation of chemical aluminum oxide. The high specific energy of aluminum-air battery and its safety and environmentally friendly characteristics determine that it will have a broad development prospect, and it may be first used in mobile devices such as power vehicles and mining in the future. Aluminum-air CR2016 battery can be designed as integrated battery packs and stored at gas stations or dedicated charging stations like gasoline and other fuels. When the anode is consumed during use, the battery pack can be directly replaced. The discharged battery pack is handed over to a professional technology company for separation and recycling of Al2O3˙nH2O and secondary assembly of the battery pack.

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