Recently, an important research development has greatly stimulated the field of electrochemical energy storage. One from the University of Illinois at Chicago (UIC), Argonne National Laboratory and California State University North A joint scientific research team at California State University (Northridge) published an article in Nature magazine--
A lithium-air battery that can be circulated over more than 700 times in an air-like atmosphere has been successfully fabricated. This breaks the previous limitation that lithium-air batteries can only use pure oxygen, and has a short cycle life. This allows people to see that this has a very high theoretical energy. Density battery replaces existing lithium ion, breaking the potential of electric vehicle mileage bottleneck.
What is a lithium-air battery? What is the difference between a lithium-air battery and a lithium-ion battery? Why is this breakthrough in lithium-air batteries important? This is the first reason why lithium-ion batteries have low energy density.
Lithium-ion batteries are by far the most successful rechargeable batteries. They are called 'lithium-ion batteries' because in batteries, whether they are charged or discharged, they are all lithium ions (Li +) Shuttle back and forth between the two electrodes to form the current. Lithium ions need to 'embed' on their surface when they reach the electrode, and 'de-intercalation' when they leave. In order to ensure a good 'embedded-uninserted' reaction, The anode of a lithium-ion battery is usually graphite, and the cathode is usually a compound of lithium. For example, in the cathode of the most popular 'ternary lithium' battery, in addition to lithium, nickel, cobalt, and manganese are also required. Elements that make up together a compound of nickel-cobalt-manganese-manganate (LiNi 0.3Co 0.3Mn 0.3O2), and nickel, cobalt and manganese are much heavier than lithium.
Therefore, in a lithium-ion battery, although only one lithium ion with a relative atomic mass of 3 (one relative of one atom of the mass of one carbon atom) is required to carry one unit of charge, its cathode is There is also a need for nickel, cobalt, manganese, iron, phosphorus, carbon and other atom-constituting compounds that are much heavier than lithium to 'receive' this lithium ion. This leads to a positive charge of only 1 at the cathode. A 'mall' with a relative molecular mass approaching 100. Together with the weight of the anode and other materials and structures, the energy density of a lithium-ion battery can't keep up. That's why one car carries half a ton of lithium-ion batteries. In electric vehicles, the cruising range is far less than the average car with just a few tens of liters of gasoline.
In a Li-ion battery, in order to stably 'store' lithium ions carrying charges (gray spheres in the figure), a large number of other structures are required, such as lithium compounds (blue, red steric structures) and graphite ( Red layered structure), the relative atomic mass of these elements are much larger than that of lithium, which leads to the limited energy density of lithium-ion batteries. In an ideal lithium-air battery, these elements are not needed anymore. Lithium metal and oxygen in the air can!
Lithium-air batteries are different. Unlike lithium-ion batteries, which require lithium compounds and graphite electrodes, lithium-air batteries can directly use lithium metal (Li) and oxygen in the air (O 2) As an electrode. In the most ideal case, when the battery is discharged, lithium peroxide generates lithium peroxide from elemental oxygen (Li). 2O2), In the external circuit to generate current; Lithium peroxide and lithium in the decomposition of lithium oxide during charging. The whole process without the participation of other elements of greater quality, and the cathode can even directly use the weight and cost can be negligible air!
Therefore, lithium-air batteries can achieve much higher energy densities than lithium-ion batteries. In fact, because lithium is the lightest metal element in the periodic table, oxygen is from the air, and lithium-air batteries have electricity. The highest theoretical energy density in chemical batteries—in other words, the mass of lithium-air batteries can store and release more energy than all other electrochemical energy storage media.
The theoretical energy density of non-liquid lithium-air batteries can reach 12kWh/kg, which is 5 to 10 times that of existing lithium-ion batteries and almost equal to about 13kWh/kg of gasoline. If lithium-air batteries can eventually reach the market, Electric vehicles will also have the same cruising range as petrol vehicles, which will completely break the bottleneck of cruising range caused by the low energy density of lithium-ion batteries, which is of great significance for the future development of clean energy.
However, these are all theoretical analyses. It is not an easy task to achieve such an ideal situation.
Until now, lithium-air batteries, which can be said to use air as the cathode, have all depended on pure oxygen. This is because, in addition to oxygen, nitrogen in the air, carbon dioxide, and water vapor all participate in the reaction, making this process extremely incomparable. Complex. The oxidation of the anodic lithium, and the reaction of the cathode lithium ions with the carbon dioxide and water vapor in the air generate undesirable by-products.
Because of the electrode, other chemical reactions on the electrolyte, and the chemical properties of metal lithium and oxygen are more active, the cycle life of lithium-air batteries has also been very short. In addition, the pure oxygen environment requires that the lithium air must be equipped with oxygen storage devices when used. For example, a large oxygen cylinder, which allows the high energy density of lithium-air batteries to be directly amortized by large and heavy oxygen storage tanks, and the capacity of the battery also depends on the capacity of the oxygen cylinder. What's more, if you want to be electric Lithium-air batteries are used in cars. Oxygen bottles, in addition to a significant increase in weight, increase safety risks.
In fact, if it is not because of the above defects, lithium-ion batteries will not be used to remotely use complex electrodes. Because lithium-air batteries that directly use lithium metal as electrodes cannot directly obtain the required oxygen in the air, some scientists even simply The lithium air battery is called 'lithium-oxygen battery'.
After many years of development, these problems have always been clouded by lithium-air batteries, not to mention competition with lithium-ion batteries. Until this time, the University of Illinois at Chicago, Argonne National Laboratory and California State University North The breakthrough of the Ling School has brought hopeful brightness to this excellent performance that exists only in theory.
If you want to solve the fatal defects of lithium-air batteries, you must find ways to prevent the various chemicals contained in the air - nitrogen, carbon dioxide, water vapor and other components involved in side reactions. These side effects will be electrodes, lithium ions and electrolytes Influencing, producing unwanted by-products. Researchers have conducted in-depth research using computer simulations (density functional analysis) and experimental research methods on this issue. Finally, they found an answer: On lithium metal electrodes. Add a protective layer.
The core of the technology is that at the anode, they add a layer of Lithium Carbonate/Carbon (LiCO 3/C) Composition of a dense protective coating.
The coating process is unusually simple: Lithium metal and carbon dioxide pass through 10 cycles of charge and discharge, and a chemical reaction on the surface of the electrode can be completed. Lithium carbonate will prevent the entry of compounds other than lithium ions, thereby protecting the anode. Destruction of components other than oxygen in the air. In the atmosphere, lithium carbonate does not react spontaneously with water vapor in the air, so this protective layer does not participate in the chemical reaction of the battery, nor does it Destroyed. Under the protection of the coating, the lithium retention rate of a single cycle is as high as 99.97%, which is much better than the lithium air battery without coating.
Figure 丨 dense anode protection layer (scale bar: the green line in the figure is 1 μm in length)
Figure is passing through Li
2CO
3 Coated oxygen molecules
To test the performance of this battery, the researchers used molybdenum disulfide (MoS) that had previously been reported by other studies. 2The nanosheets were used as cathodes and used 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) and Dimethyl sulfoxide (DMSO). The composition of the mixture as an electrolyte. With the anode, cathode, and electrolyte working together, the lithium-air battery was placed in a simulated air environment - 79% nitrogen, 21% oxygen, 500ppm carbon dioxide, and 45% relative. Humidity, temperature 25 C.
After testing, after 700 cycles of charging and discharging, the lithium-air battery did not show any failure. This is beyond the expectations of many people and has even reached the cycle life of some mature commercial batteries (such as lead-acid batteries).
The research team therefore came to the conclusion that 'protected lithium anodes, electrolyte mixtures, and high-performance air cathodes, working together under simulated air conditions, can effectively increase the number of cycles in lithium-air batteries.'
At the same time, Argonne National Laboratories continues to perform computer simulations of this battery reaction, in order to further understand the reaction mechanism, so as to enhance the performance of the battery in the future, and provide theoretical support for possible commercialization in the future.
It should be pointed out that although this study is far from commercial applications and its energy density is not far from optimum, it is undoubtedly a major advance in the development of lithium-air batteries.
The results of this study demonstrate that lithium-air batteries can indeed shield other gases from interference, directly acquire oxygen from air-like atmospheres, get rid of dependence on oxygen storage devices, and have a long cycle life. This is undoubtedly greatly enhanced. Researchers and industry confidence in the future development of this revolutionary battery technology:
Since the most important problems have clear solutions, the rest may not be fatal at all! It may not be long before researchers can create energy densities much higher than existing lithium-ion battery technologies. New batteries, and this will undoubtedly completely change the existing energy landscape.
