Figure 1: Schematic diagram of the working principle of a lithium 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), traditional lead-acid batteries, lithium-sulfur batteries, and lithium-ion batteries that have attracted much attention. Compared with other batteries, lithium-ion batteries are favored by people because of their high specific energy, high operating voltage, long cycle life, low self-discharge, no memory and environmental protection. They are widely used in mobile phones and Laptops, etc., are also ideal for next-generation plug-in hybrids and electric vehicles .
Lithium-ion battery adopts a rocker-like working principle. During charging and discharging, Li+ shuttles back and forth between the positive and negative electrodes, swinging from side to side, and reciprocating to realize the charging and discharging process of the battery. Since various electrochemical energy storage materials are different in material system and design, technical indicators are also different. Japan's New Energy Industry Technology Development Agency (NEDO)'s Li-EAD program sets a high-performance indicator that the battery will reach 700Wh/kg by 2030. At present, lithium-ion batteries can not achieve this goal, and the theoretical calculation energy density of lithium-air batteries can reach 12000Wh/Kg, exceeding the index proposed by NEDO. Before we elaborate on the lithium-air battery, let's briefly introduce the lithium-ion battery.
Principle of lithium ion battery
A lithium-ion battery consists of three parts: a positive electrode (usually a layered structure of lithium cobaltate and a cobalt nickel manganese lithium compound, a spinel-structured lithium manganate, an olivine-structured lithium iron phosphate), and a negative electrode (usually graphite). Layer) and electrolyte; wherein the redox reaction occurs in the positive and negative electrodes, and the electrolyte acts as a medium for transporting ions. Specifically, during the discharge process, lithium ions run through the electrolyte to the positive electrode under the action of the internal electric field, while the electrons that run away at the negative electrode flow to the positive electrode through the external circuit (the specific reaction is intercalation, and interested students can google [ 3]). The migration of electrons from the negative electrode to the positive electrode will do work, which is the energy used to support the operation of the electronic device. As the name suggests, the process of charging is the reverse of the discharge process.
Main limitations of lithium-ion batteries
There are many factors that affect lithium ions, such as temperature, rapid charge and discharge, theoretical capacity of materials, and energy density. Among them, energy density and theoretical capacity are two main aspects limiting lithium-ion batteries. Here we first talk about an important concept - Energy Density. Energy density, also called specific energy, written interpretation refers to the amount of energy stored in a certain space or mass material. Popular point is the energy contained in a unit volume or unit mass. In the battery industry, it is often used to compare the amount of electricity stored in a unit of weight. For example, lithium ion batteries with high energy density in existing commercial batteries have an energy density of about 500 Wh/Kg. As described above, such energy density is not enough to replace gasoline (energy density is about 13 000 Wh/Kg) for realizing automobiles. Purely electric.
There is no harm without comparison. The energy density of traditional lithium-ion batteries (0.05-0.1 kwh/Kg) is less than 1% of the energy density of motor gasoline (13 kwh/Kg, Chevrolet Volanda)! Why are lithium-ion batteries so inefficient compared to traditional energy sources so popular and recognized?
This starts with the structure of a lithium-ion battery. Careful readers have noticed that in Figure 1, the styles of the positive and negative electrodes seem to be different. For example, the negative electrode is some frame and the positive electrode is some piece of blue brick, but these are not the key points, but the authors want to indicate that the materials forming the positive and negative electrodes are different. But what they have in common is the orderly queuing of mung bean-like lithium ions. This is because during the migration of lithium ions through the electrolyte to the negative electrode, it will merge with some of the lithium ions that have arrived earlier. If there is no layered structure, these first-time lithium ions will form a crystal structure, which is academically called dendrites. These crystals will grow faster than the normal and negative poles, making the entire battery short-circuited from the inside. In general, it is like everyone has to go to the parking, and at the entrance, they are not allowed to make a long queue of traffic jams. The layered structure of the positive and negative electrodes plays the role of orderly storing the lithium ions coming at different times, like a parking space. Therefore, positive and negative electrodes with an ordered layered structure are indispensable for rechargeable batteries (Fig. 2). However, the positive and negative materials, as well as the electrolyte, do not give energy during discharge discharge. This drags down the overall energy density of the battery.
Figure 2: Working principle of lithium battery: a. lithium metal battery; b. lithium ion battery. The layered structure can store lithium ions in an orderly manner to prevent the formation of dendrites.
Another major factor affecting lithium-ion batteries 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 Significant technological advances are closely related to the technological advancement of cathode materials. 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 having a spinel structure, and lithium iron phosphate having an olivine structure.
However, with the increasing demand of the global electric vehicle market, the development of lithium-ion batteries has been seriously hindered. The development bottleneck is mainly how to improve the charge-discharge ratio capacity of the cathode material to meet high specific energy and high charge-discharge power. Claim. Compared with today's commercial carbon anode materials (actual specific capacity of 330-360 mAh/g), the actual specific capacity value of the cathode material that has been put into use is only between 120 and 250 mAh/g, which is still far lower. The capacity of the anode material; its relatively low specific capacity/energy density is the current research status of cathode materials, which is the primary factor that restricts the development of lithium-ion batteries. Second, the cost of the cathode material is also one of the important factors affecting the development of high-capacity lithium-ion batteries. In general, the preparation of a positive electrode material requires the use of a rare transition metal element (e.g., cobalt, nickel, etc.) in large quantities. On the one hand, metal resources such as cobalt and nickel have limited reserves on the earth, which are not suitable for large-scale mining and excessive consumption (which runs counter to national sustainable development strategies); on the other hand, the use of rare metal elements will raise the manufacturing cost of batteries. It is not conducive to the large-scale popularization of high-performance lithium-ion batteries in the future (for example, practical applications in fields such as energy storage power stations). In addition, the heavy use of heavy metals such as cobalt, nickel and manganese will cause greater harm to the environment such as soil and water, and pose a serious threat to the lives of humans, animals and plants.
solution
However, the method is always more problematic. In order to further increase the energy density of the rechargeable battery, reducing the weight of the battery has become a breakthrough. Although it is temporarily impossible to find a material with a higher energy-to-mass ratio than lithium metal, we can reduce the weight of the battery as a whole, thereby increasing the overall energy density of the battery. The most representative one is the lithium-air battery. The theoretical energy density of lithium-air batteries can reach 12000Wh/Kg, which is comparable to the ultra-high energy density of gasoline, making it possible to completely replace gasoline, and truly realize the ultra-long journey of pure electric vehicles (Figure 3).
Figure 3: Comparison between various types of batteries.
What is a lithium-air battery?
To put it simply, unlike a conventional lithium ion battery using a transition metal oxide as a positive electrode material, a lithium-air battery is a battery in which lithium metal is used as a negative electrode and oxygen in the air is used as a positive electrode reactant. One advantage of lithium metal instead of graphite as the negative electrode is that lithium metal (3860 mAh·g-1) has a specific capacity of nearly 10 times that of graphite (372 mAh·g-1). Like all batteries, lithium-air batteries are also composed of three basic parts: positive electrode, negative electrode, and electrolyte. The external circuit is connected by wires to conduct electrons, and the internal circuit is connected by electrolyte to transfer ions. Its working principle is shown in Figure 4:
Figure 4: Schematic diagram of the working principle of lithium-air battery.
The lithium-air battery has lithium metal as a negative electrode, and a porous electrode composed of a carbon-based material is a positive electrode. During discharge, metallic lithium loses electrons in the negative electrode to become lithium ions. Electrons reach the porous positive electrode through an external circuit, reducing oxygen in the air, and lithium ions pass through the electrolyte to reach the porous positive electrode, forming lithium peroxide with oxygen and electrons (Li 2 O 2 ) (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 generated during the discharge is first oxidized in the porous positive electrode, and the oxygen is released again, and the lithium ion is reduced to metallic lithium at the negative electrode.
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 is mainly determined by the cathode material and the electrolyte. The lithium-air battery in the middle is reduced, so it has a higher energy density than the lithium ion battery.
Lithium-air battery classification
The anode material of lithium-air battery is lithium metal, and the positive electrode is porous carbon-based material that can pass O2. We usually classify lithium-air batteries into four categories according to different electrolytes: aprotic lithium-air battery, water system lithium-air Battery, hybrid lithium-air battery and solid state lithium-air battery.
Figure 5: Schematic diagram of the structure of four types of lithium-air batteries.
Aprotic lithium-air battery:
A typical aprotic lithium-air battery design consists of a metallic lithium anode, a porous carbon-based material cathode to which catalyst particles are added, and an aprotic solvent electrolyte that dissolves the lithium salt. Commonly used aprotic electrolytes include organic carbonates, ethers, esters, lithium salt solvents, and the like. The aprotic electrolyte is the most widely used electrolyte at present. The advantages are high oxygen solubility, low corrosiveness to lithium, simple battery structure and good operability. The disadvantage is that the discharge product is solid and easily blocks the air positive electrode, and only Li in the lithium oxide 2 O 2 can be decomposed during the charging process, and the battery cycle performance is poor.
Water system lithium-air battery:
The water system lithium-air battery consists of a lithium metal anode, a water electrolyte, and a porous carbon cathode. The water electrolyte incorporates a lithium salt dissolved in water. It avoids the problem of cathodic clogging because the reaction product is water soluble. The water design has a higher actual discharge potential than an aprotic solvent. However, lithium metal reacts violently with water, so the design of water requires a solid electrolyte interface between lithium and electrolyte.
Hybrid Lithium-Air Battery:
Water System - An aprotic lithium-air battery, or a hybrid system lithium-air battery, is designed to combine the advantages of aprotic and water system battery designs. A common feature of hybrid design is a two-part (a part of which is water and a part of aprotic) connected by a lithium conductive film. When the cathode is in contact with the water surface, the anode is adjacent to the aprotic end. Lithium conductive ceramics are generally used as a film connecting two electrolytes.
Solid state lithium-air battery:
Current solid-state 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 typically separated by a polymer-ceramic composite that enhances charge transfer on the anode and bonds the cathode to the electrolyte. Polymer ceramic composites reduce the overall impedance. Solid-state battery design enhances safety and eliminates the possibility of ignition cracking, but has the disadvantage that most glass ceramic electrolytes have low electrical conductivity.
Advantages and disadvantages of lithium-air batteries?
The idea of ​​applying lithium-air batteries to the automotive field was proposed as early as 1970. However, due to the limitations of material technology development at that time, it has not been studied in depth and has not yet been commercialized. With the development of the electric vehicle industry and the improvement of materials science and technology, lithium-air batteries have also begun to receive much attention. One of the reasons is that their theoretical specific energy is high. The ratio of lithium to oxygen (in air) can theoretically maximize the energy of an electrochemical cell. In fact, the theoretical energy of a non-aqueous lithium-ion battery is about 12kWh/Kg, which is equivalent to the theoretical energy of gasoline (13kWh/Kg), far higher than zinc-air batteries, lithium-ion batteries , lithium-sulfur batteries, etc. (Figure 3 Shown). In practice, the specific energy of each lithium-air battery is also 1.7kWh/kg, which is five times larger than a commercial lithium-ion battery, enough to run a 2 ton all-electric vehicle (FEV), just use The 60 kg battery can travel for 500 km.
Another major advantage of lithium-air batteries is that the active material oxygen of the positive electrode is directly derived from the surrounding air, so it is inexhaustible and does not need to be stored inside the battery, which reduces the cost and reduces the cost. The weight of the battery, the energy density of the battery is completely dependent on the metal lithium side. In the whole process of charging and discharging the battery, there is no environmentally harmful substance, and it is completely a green process with zero pollution.
However, careful readers should note that in the so-called "(metal) lithium-air (oxygen) battery" working environment, the actual functioning is oxygen in the air. Therefore, it is not as good as the name. Lithium-air batteries still have certain requirements for the working environment. Therefore, there are still many problems in the lithium-air battery that have not been solved: the side reaction caused by the influence of H 2 O and CO 2 in the atmosphere, the clogging of the air circuit caused by the discharge of the discharge product, and the catalyst problem caused by the large charge and discharge overvoltage. And corrosion of the air electrode char collector. More research has shown that nitrogen in the atmosphere is not involved in this reaction.
At the same time, the inhibition of the precipitation reaction of Li 2 O 2 is directly related to the discharge capacity of the battery. Another problem with the precipitation of Li 2 O 2 is that the overvoltage at the time of charging is large, which is not only related to the energy conversion efficiency but also causes Li 2 . O 2 precipitates a new problem such as oxidation of the carrier carbon.
Under the condition that lithium ions and oxygen coexist, the potential of the carbon material rises to generate lithium carbonate, and excessively high voltage may cause decomposition of the electrolyte, so there are various discussions on the air electrode. It is generally believed that the structure and composition of the positive electrode of lithium-air battery and the catalytic activity of air catalyst have an important influence on the specific capacity and cycle performance of the battery. For example, Bruce et al. reported that the nanowire of α-MnO 2 is compounded with carbon and has a high Reversibility.
future
With the depletion of energy sources such as oil and coal and the increasing environmental pollution, it is imperative to develop efficient and clean energy, and the superior theoretical performance of lithium-air batteries will undoubtedly become the focus of research and business applications. At present, various types of lithium-air batteries have their own advantages and disadvantages. Whether it is due to liquid electrolyte evaporation or porous carbon electrode material conduction catalytic performance affecting battery performance, lithium-air batteries want to achieve commercial applications, find Competitive market positioning must address key issues such as cycle life, energy efficiency, air filtration membranes, and metal lithium protection. Researchers in related fields are also working hard to promote the practical application of lithium-air batteries. Compared with traditional metal air batteries, lithium-air batteries have smaller volume, lighter weight, higher operating voltage and higher specific energy characteristics, so they are used in military, field, electric vehicles, water and other fields. Has broad application prospects.
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