Editor's note
Although it is theoretically possible to increase the cruising range by stacking the battery load, it also increases the cost and weight, which is not worth the cost in terms of economy and safety.
Improving the battery specific energy has become the core technical direction to improve the cruising range of electric vehicles.
Although it is theoretically possible to increase the cruising range by stacking the battery load, it also increases the cost and weight, which is not worth the cost in terms of economy and safety.
Ai Xinping, a professor at Wuhan University, said that with the existing technical conditions of the whole vehicle, the most reasonable design should be 300w/kg for the battery life of 300Km; 400wh/kg for the battery life of 400km; and if the monomer is 500wh/kg, The battery life will reach 500km.
In fact, it is generally believed in the industry that the short-term goal of lithium battery technology is to achieve 300 wh/kg through high-nickel ternary positive electrode and silicon-carbon negative electrode; the medium-term (2025) target is based on lithium-rich manganese-based/high-capacity Si-C negative electrode. Monomer 400wh / kg; in the long term is the development of lithium sulfur, lithium battery, to achieve monomer specific energy of 500wh / kg.
In this regard, Ai Xinping said that by 2020, it will reach 300 wh/kg, and there is no technical risk except for the safety. As for the medium-term target, according to the calculation results, 400 wh/kg requires a positive electrode capacity of 250 mAh/g and a negative electrode capacity of 800 mAh/g. This requirement is also feasible with the current material system.
In the long-term goal, the theoretical value of lithium sulfur and lithium space far exceeds 500 wh/kg (2600 wh/kg of lithium-sulfur and 11,000 wh/kg of lithium gas), but its feasibility remains to be considered.
Among them, lithium space is a battery system using metal lithium as a negative electrode and oxygen in air as a positive electrode. Of course, the oxygen electrode requires porous carbon as a reaction carrier. Although much progress has been made in catalyst selection, mechanism research, electrolyte selection, and chargeability over the years, as a product, lithium-ion batteries have four fatal defects:
The first is the problem of moisture control. The lithium-ion battery is an open system, which is different from the lithium-ion battery. The lithium space uses oxygen in the air, and the air contains water. The lithium reacts with water. It is both difficult to solve with oxygen and water.
Second, it is the catalytic reduction of oxygen. The reaction rate of oxygen is very slow. To increase the reactivity of oxygen, it is necessary to use a highly efficient catalyst. The current catalysts are all precious metals. Therefore, it is necessary to develop efficient and inexpensive catalysts, which has been a shortcoming that restricts the development of fuel cells.
Third, it is the chargeability of the metal lithium negative electrode. It is also the problem of lithium dendrite that has been studied in the industry. In the past 60 years, countless scientific researchers have gone on and on, and there is still no slight progress.
Fourth, it is the re-decomposition of the discharge product. The discharge product of a lithium-ion battery is a lithium oxide, and it is difficult to re-catalyze the solid lithium oxide into oxygen and lithium.
The feasibility of a lithium-ion battery that has gathered so many years of problems can be said to be very embarrassing. Looking at the lithium-sulfur battery, the negative electrode uses metal lithium and the positive electrode uses sulfur. The sulfur capacity is very high, reaching 1600 mAh/g, which is why everyone studies it.
However, lithium-sulfur batteries also have a lot of pain points, and the first one is the poor performance of the electrode cycle. When the sulfur electrode is discharged, it does not directly generate lithium sulfide, but is gradually reduced, accompanied by the formation of lithium polysulfide intermediate product; lithium polysulfide is dissolved in the electrolyte, and dissolution loss occurs. On the one hand, the dissolved polysulfide will diffuse to the negative electrode and then oxidize in the positive electrode, resulting in a shuttle effect, resulting in low coulombic efficiency and high self-discharge; on the other hand, dissolved lithium polysulfide will also be preferentially on the positive surface during charging. Deposition causes the electrode to be deactivated due to clogging of the surface pores, and therefore, the electrode cycle performance is poor.
At present, the method of scientific research is to use porous carbon materials to block and desorb polysulfide ions and reduce its dissolution loss. This strategy seems to be very effective academically, but the actual effect is very limited. The main difference between the two is that the laboratory research work is based on a small button cell, the electrode is very thin, the sulfur load is not high, the total sulfur is about a few milligrams; and the actual battery has a large sulfur content. (Grade), and the electrode is very thick and the unit sulfur load is very high.
For example, in the lithium sulphur battery 863 project involving Professor Ai Xinping, the laboratory can recycle 1000 times of sulfur/carbon composite materials, which can only be circulated several times in actual batteries, and sometimes even one time can not be discharged. This reason.
The second problem with lithium-sulfur batteries is the chargeability of the lithium negative electrode, which is also a problem that is difficult to solve in a short time. The electrochemical reaction must contain several processes in series. The first process is the transfer of reactants from the bulk solution to the electrode surface, called liquid phase mass transfer; the second process is the reactants get or lose electrons on the electrode surface. The process of forming the product is referred to as the electrochemical reaction step. Which speed is slow, which step is controlled by the electrode reaction.
For lithium electrodes, the electron exchange process is very fast, so liquid phase transport is the reaction control step, that is, the step of transferring lithium ions from the solution body to the electrode surface is relatively slow. This brings up some problems. The liquid phase transfer is actually affected by convection. As long as there is gravity, there will be convection, and the convection speed of each point on the electrode surface is not the same. Therefore, the reaction speed of each point is different. . Where the length is long, the transmission distance of lithium ions is shorter, and the deposition speed of lithium is getting faster and faster, which is the reason for the growth of lithium dendrites.
Of course, the distance between the positive and negative electrodes is different, and the current distribution is different, which is also an important cause of lithium dendrite growth. Obviously, these factors are difficult to avoid in actual batteries. Therefore, the problem of lithium chargeability caused by dendrite growth cannot be said to be impossible, but it is still difficult to find an effective solution.
The third problem is that the lithium-sulfur battery has a relatively low volumetric energy density and may only be equivalent to a lithium iron phosphate battery. Because sulfur is an insulator, letting it conduct electricity, let it react, and disperse it, it is necessary to use a large amount of carbon with a high specific surface, resulting in a very small density of sulfur/carbon composite; in addition, the reaction of sulfur is dissolved and redeposited first, so There must be a large number of liquid phase transport channels on the electrodes.
However, most of the lithium-sulfur battery sulfur electrode pole pieces are incapable of being pressed, what kind of coating is what, and the porosity is particularly high, so the volumetric energy density is very low. For cars, especially for passenger cars, when the energy density reaches a certain value, the volumetric energy density is even more important, because passenger cars do not have as many places to install batteries. So in this sense, at least in the field of automotive power, lithium-sulfur batteries are hopeless.
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