With the development of social science and technology, people's basic necessities are inseparable from the support of energy. From small objects such as mobile phones and laptops, to the means of transport, have become an indispensable part of people's lives. Crude oil and other traditional energy although there is a high energy output capacity, but its environmental unfriendly (such as a large number of carbon dioxide emissions) and a series of shortcomings have become today's society can not be ignored. And new energy sources such as solar energy, wind energy, etc., although there is a huge potential, but because of its use of the environment at that time a strong dependence, so to do some of the need for continuous function of electronic equipment (such as cars, etc.) is still a challenge. So to achieve the support of these forces will depend on the improvement of electronic energy storage equipment, which is applied to different areas, with different sizes of the battery.
In recent years, scientists have done a lot of research on batteries, such as alkaline batteries (such as Fe / Ni batteries and Zn / Mn batteries), the traditional lead-acid batteries, lithium-sulfur batteries, and much attention lithium-ion battery. Compared to other batteries, lithium-ion battery has been favored by people because of its high energy than the energy, high operating voltage, long cycle life, low self-discharge, no memory and green, etc., widely used in mobile phones and Notebook computers, etc., is also the next generation of rechargeable hybrid and electric car ideal choice.
Lithium-ion battery using a similar rocking chair-like work principle, charge and discharge process Li + between the positive and negative pole back and forth between the shuttle, from one side to the other side, reciprocating cycle, to achieve the battery charge and discharge process. As a variety of electrochemical storage in the material system and design is different, technical indicators are also different. Japan's new energy industry technology development agency (NEDO) Li-EAD plan to set the 2030 battery to 700Wh / kg high performance indicators. At present, lithium-ion battery can not achieve this goal, and lithium - air battery theoretical calculation of energy density can reach 12000Wh / Kg, exceeding the NEDO proposed indicators. Before we elaborate on the lithium air battery, we briefly introduce the lithium-ion battery.
The principle of lithium-ion battery
Lithium-ion battery consists of three parts: the cathode (usually the layered structure of lithium cobalt oxide and cobalt nickel manganese lithium compounds, spinel structure of lithium manganate, olivine structure of lithium iron phosphate), negative (usually graphite Layer) and electrolyte; in which the redox reaction occurs in the positive and negative, the electrolyte as a transport ion transport medium. Specifically, in the discharge process, the lithium ion in the internal electric field through the electrolytic solution to the positive pole, while the negative electrons run away through the external circuit flow to the positive (specific reaction is intercalation, interested students can google [ 3]). In the electronic from the negative to the process of migration will work, this work is used to support the work of electronic equipment, the energy. As the name implies, the charging process is the reverse process of the discharge process.
Lithium ion battery is the main limitation
There are many factors that affect lithium ion, such as temperature, rapid charge and discharge, the theoretical capacity of the material and energy density. Where energy density and theoretical capacity are two major aspects of limiting lithium-ion batteries. Here we first talk about an important concept - energy density (Energy Density). Energy density, also known as energy, written interpretation refers to the storage of energy in a certain space or mass material size, popular point refers to the unit volume or unit mass contains the energy. In the battery industry, it is often used to compare the amount of battery power stored per unit weight of the battery. For example, the energy density of the existing commercial battery is relatively high lithium ion battery, the energy density of about 500Wh / Kg, as mentioned earlier, this energy density is not enough to replace gasoline (energy density of about 13 000Wh / Kg) used to achieve the car Pure electricization.
There is no damage to the traditional lithium-ion battery energy density (0.05-0.1 kwh / Kg), only the vehicle's energy density (13 kwh / Kg, Chevrolet Vaughan) is less than 1% Relative to the traditional energy such as inefficient lithium-ion battery why so favored by people and recognized it?
This is from the lithium-ion battery structure. Careful readers have noticed that in Figure 1, the positive and negative style of the wind seems not the same. Such as the negative pole is some of the frame and the positive pole is some flake blue brick, but these are not the focus, this is only the author wants to represent the positive and negative materials are different. But their common ground is the mung bean-like lithium ions are ordered in orderly line. This is because lithium ions through the electrolyte migration to the negative process, with some of the previously arrived lithium ion confluence. If there is no lamellar structure, these first to the lithium ions will form a crystal structure, academic called dendrites. These crystals will grow rapidly and connect the positive and negative, so that the entire battery from the internal short circuit. Popular as we all have to stop, at the entrance do not let each other cause the traffic jam long team. While the positive and negative layers of the structure is able to orderly storage of these different times the arrival of the lithium ion function, such as parking spaces. So the positive and negative poles with an ordered layered structure are indispensable for rechargeable batteries (Figure 2). But the positive and negative materials, and the electrolyte does not give energy in the discharge process. Thus dragging down the overall energy density of the battery.
The layered structure can store lithium ions in order to prevent the formation of dendrites.
Another major factor affecting the lithium-ion battery is the capacity of the electrode material itself. It is worth noting that the positive electrode is an important part of the lithium-ion battery, and its performance largely determines the final performance of the battery. Many lithium-ion batteries Of the major technological progress and the cathode material technology is closely related to enhance. The known positive electrode materials which can be put into practical use include lithium cobalt oxide and cobalt-nickel-manganese lithium compounds having a layered structure, lithium manganate of the spinel structure, lithium iron phosphate of olivine structure.
However, with the growing demand for the global electric vehicle market, the development of lithium-ion battery has been seriously hindered, the development of the bottleneck is mainly to enhance the charge and discharge capacity of the cathode material to meet the high specific energy, high charge and discharge power Claim. The actual specific capacity of the positive electrode material used is between 120 and 250 mAh / g, which is still very low compared to today's commercially available carbon anode material (actual specific capacity of 330-360 mAh / g) The relatively low specific capacity / energy density is the current research situation of cathode materials, it is restricting the development of lithium-ion battery is the primary factor. Second, the cost of cathode materials is also affecting the development of high-capacity lithium-ion battery is one of the important factors. In general, the preparation of the cathode material requires the use of rare transition metal elements (such as cobalt, nickel, etc.) in large quantities. On the one hand, cobalt, nickel and other metal resources in the Earth's reserves are limited, not suitable for large-scale mining and excessive consumption (contrary to the national sustainable development strategy); the other hand, the use of rare metal elements will lift the battery manufacturing costs , Is not conducive to the future high-performance lithium-ion battery large-scale popularization (for example, in the field of energy storage and other practical applications). In addition, the heavy use of cobalt, nickel, manganese and other heavy metals on the soil, water and other environments have a greater harm, and human and animal and plant life poses a serious threat.
However, the method is always more than the problem, in order to further improve the energy density of rechargeable batteries, reduce the weight of the battery has become a breakthrough. Although it is not possible to find a material with a higher energy-to-mass ratio than metal lithium, we can give the battery a weight-reducing and improve the overall energy density of the battery. One of the most representative is the lithium - air battery. Lithium-air battery theoretical calculation of energy density can reach 12000Wh / Kg, which is comparable to the ultra-high energy density of gasoline, it is expected to completely replace the gasoline, the real long journey of pure electric vehicles (Figure 3).
What is lithium - air battery?
Simply put, with the traditional lithium-ion battery to transition metal oxide as a cathode material is different from the lithium-air battery is a lithium metal as a negative electrode to the oxygen in the air as a positive electrode reactant battery. One advantage of metal lithium instead of graphite as the negative electrode is that lithium (3860 mAh · g-1) has a specific capacity of nearly 10 times the specific capacity of graphite (372 mAh · g-1). Like all batteries, lithium-air batteries are composed of three basic components: positive, negative, electrolyte, external circuit connected by the conductive wire conduction, the internal circuit connected by the electrolyte transfer ions. Its working principle shown in Figure 4:
Figure 4: Lithium - air battery working principle diagram.
Lithium-air batteries are made of lithium metal as the anode, and the porous electrode composed of carbon-based materials is positive. During the discharge process, the metal lithium in the negative loss of electrons into lithium ions, electrons through the external circuit to reach the porous cathode, the oxygen in the air reduction, and lithium ions through the electrolyte to reach the porous cathode, and oxygen and electrons to form lithium peroxide (Li2O2) (Main product). This reaction continues and the battery can supply energy to the load. The charging process is just the opposite, under the action of the charging voltage, the discharge product produced in the discharge process is first oxidized in the porous positive electrode, re-release of oxygen, lithium ions in the negative electrode was reduced to metal lithium.
Since the anode material is a very light porous carbon material and oxygen is taken from the environment, the weight of the lithium-air battery depends primarily on the cathode material and the electrolyte. Reduced the attached lithium-air battery has a higher energy density compared to lithium-ion batteries.
Lithium-air battery anode material is metal lithium, the cathode is able to pass through the O2 porous carbon-based materials, we usually according to the electrolyte will be different lithium battery into four categories: aprotic lithium - air battery, water system lithium - air Batteries, mixed lithium-air batteries and solid-state lithium-ion batteries.
Figure 5: four types of lithium - air battery structure diagram.
Aprotic lithium - air battery:
A typical aprotic lithium-air battery consists of a metal lithium anode, a cathode of a porous carbon-based material with catalyst particles added, and an aprotic solvent electrolyte that dissolves lithium salts. Commonly used aprotic electrolytes include organic carbonates, ethers, esters, lithium salt solvents and the like. Aprotic electrolyte is the most widely used electrolyte, the advantages of high oxygen solubility, little corrosion of lithium, the battery structure is simple, operable, the disadvantage is that the discharge product is solid, easy to block the air cathode, and lithium oxide only Li2O2 Can be broken in the charging process, the battery cycle performance is poor.
Water system lithium - air battery:
Water system Lithium-air battery consists of lithium metal anode, water electrolyte and porous carbon cathode. The water and electrolyte combine lithium salts dissolved in water. It avoids the problem of cathode clogging because the reaction product is water-soluble. Compared with aprotic solvents, water design has a high practical discharge potential. However, lithium metal reacts violently with water, so the water design requires a solid electrolyte interface between lithium and the electrolyte.
Mixed system lithium - air battery:
Water system - aprotic lithium-air battery or a mixed-system lithium-air battery, its design attempts to combine the advantages of non-proton and water system battery design. The common feature of the hybrid design is a two part connected by a lithium conductive film (part of which is water and a part is aprotic). When the cathode is in contact with the water surface, the anode is adjacent to the aprotic end. Lithium conductive ceramics are commonly used as films for connecting two electrolytes.
Solid lithium - air battery:
Current solid lithium-air batteries use lithium as a negative electrode, ceramic, glass or glass ceramic as an electrolyte, and porous carbon as a positive electrode. The anode and cathode are usually separated from the polymer-ceramic composite, reinforcing the charge transfer on the anode, and combining the cathode with the electrolyte. The polymer ceramic composite reduces the overall impedance. Solid-state battery design enhances safety and eliminates the possibility of ignition rupture, but the disadvantage is that most glass-ceramic electrolytes have low conductivity.
Lithium-air battery advantages and defects?
Lithium - air battery applied to the field of automotive concept, as early as 1970 was put forward, but by the time of material technology development limit, has not been in-depth study, has not yet achieved commercial applications. With the development of electric vehicle industry and the upgrading of materials science and technology, lithium - air battery has begun to attract attention, one of the reasons is its high theoretical energy. Lithium and oxygen (air) in the ratio, theoretically can make the electrochemical battery has the highest energy. In fact, the theoretical energy of nonaqueous lithium-ion batteries is about 12kWh / Kg, which is equivalent to the theoretical energy of gasoline (13kWh / Kg), much higher than zinc air batteries, lithium-ion batteries, lithium-sulfur batteries (Figure 3 ). In practice, each lithium-air battery has a specific energy of 1.7 kWh / kg, which is five times larger than a commercial lithium-ion battery, enough to run a 2-ton all-electric vehicle (FEV), just use 60 kg battery can travel 500 km.
Another important advantage of lithium-air batteries is that the active material of the positive electrode is directly from the surrounding air and is therefore inexhaustible for inexhaustible use and does not need to be stored inside the battery, thus reducing costs and reducing The weight of the battery, the battery's energy density depends entirely on the metal lithium side. And in the battery charge and discharge the whole process, will not produce environmentally harmful substances, is completely zero pollution of the green process.
However, careful readers should note that in the so-called "(metal) lithium-air (oxygen) battery" working environment, the actual function of the role of oxygen in the air. Therefore, not as good as the name, lithium - air battery on the working environment or have some requirements. Therefore, there are many problems with lithium-air batteries that have not been solved: the effects of H2O and CO2 in the atmosphere on the side effects, the release of the discharge product leads to clogging of the air circuit, the large charge-discharge overvoltage-induced catalyst problem, and the air electrode Carbon-based fluid corrosion. More research shows that the atmosphere of nitrogen is also unwilling to participate in this reaction.
At the same time, the inhibition of Li2O2 precipitation reaction is directly related to the discharge capacity of the battery. Another problem with the precipitation of Li2O2 is that the overvoltage is large at the time of charging, which not only affects the conversion efficiency of energy, but also causes the oxidation of Li2O2 problem.
Lithium ions and oxygen coexistence conditions, the carbon material potential increases, the formation of lithium carbonate, too high voltage may lead to decomposition of the electrolyte, so the air electrode has a variety of discussions. It is generally believed that the structure, composition and catalytic activity of the cathode of the lithium-air battery have an important effect on the specific capacity and cycle performance of the battery. For example, Bruce et al. Reported that α-MnO2 nanowires were compounded with carbon, Reversible.
With the increasing shortage of energy such as oil and coal and the increasing environmental pollution, it is imperative to develop efficient clean energy, and the superior theoretical performance of lithium - air battery will undoubtedly make it the focus of scientific research and commercial application. At present, various types of lithium-air batteries have their own advantages and disadvantages, whether it is due to liquid-phase electrolyte evaporation or porous carbon electrode material conduction catalytic performance and affect the battery performance, lithium-air batteries want to achieve commercial applications, Competitive market positioning, must solve the cycle life, energy efficiency, air filtration membrane, metal lithium protection and other key issues. Related areas of scientific research workers are also constantly strive to jointly promote the lithium - air battery to achieve practical application. Compared with the traditional metal air battery, lithium - air battery has a smaller size, lighter weight, higher operating voltage, higher specific energy characteristics, and thus in the military, field, electric vehicles, water and other fields Has a broad application prospects.