What are the problems facing the development of lithium-sulfur batteries

Lithium-sulfur batteries also have many pain points, the first of which is the poor circulation of the electrodes. When the sulfur electrode is discharged, lithium sulfide is not directly generated, but gradually recovered, along with the formation of lithium polysulfide intermediate products; lithium polysulfide will dissolve in the electrolyte and lose dissolution. On the one hand, the dissolved lithium polysulfide will be dispersed to the negative electrode for recovery, and then oxidized on the positive electrode, resulting in a complexation effect, resulting in low coulomb power and high self-discharge; on the other hand, the dissolved lithium polysulfide will also take precedence on the positive electrode surface during the charging process. Accumulation causes the electrode to be inactivated due to the obstruction of the outer surface hole, and therefore, the electrode circulation function is poor. At present, the method of the scientific research community is to use porous carbon materials to block and desorb polysulfide ions and reduce its dissolution loss. This strategy appears to be very effective academically, but its practical use is very limited. The important difference between the two is that the research work in the laboratory is based on a very small button battery, the electrode is very thin, the sulfur load is not high, and the total sulfur content is about a few milligrams; while the sulfur content of the practical battery is relatively high. Large (gram level), and the electrode is very thick, the unit sulfur load is very high. The second problem of lithium-sulfur batteries is the rechargeability of the lithium negative electrode, which is also difficult to deal with in a short time. Electrochemical reaction must include several processes in series. The first process is the transfer of the reactant from the bulk solution to the surface of the electrode, which is called liquid phase mass transfer; the second process is that the reactant gains or loses electrons on the surface of the electrode. The process that constitutes the product is called the electrochemical reaction process. Whichever is slower, the electrode response is controlled by which process. Regarding the lithium electrode, the electron exchange process is very fast, so the liquid phase transfer is the reaction control process, that is, the transfer of lithium ions from the solution body to the surface of the electrode is relatively slow. This brings about some problems. The liquid phase transfer is affected by convection in practice. As long as there is gravity, there will be convection. However, the convective velocity of each point on the surface of the electrode is not the same. Therefore, the reaction velocity of each point is also It's different. Whichever place grows faster, the transmission interval of lithium ions is shorter, and the accumulation speed of lithium becomes faster and faster. This is the reason for the growth of lithium dendrites. Of course, if the interval between the positive and negative electrodes is different, the current spread is also different, which is also an important reason for the growth of lithium dendrites. Obviously, these factors are difficult to guard against in the practice of batteries. Therefore, the problem of lithium rechargeability caused by dendrite growth cannot be said to be without a method, but it is currently difficult to find an effective method. The third problem is that the volumetric energy density of lithium-sulfur batteries is relatively low, which may only be comparable to lithium iron phosphate batteries. Since sulfur is an insulator, to make it conductive, to react, and to disperse, it is necessary to choose a lot of carbon with a high specific surface, resulting in a very small density of sulfur/carbon composite materials; in addition, the reaction of sulfur is to dissolve and then accumulate. Therefore, there must be many liquid phase transport channels on the electrode. At present, most of the sulfur electrode pole pieces of lithium-sulfur batteries cannot be pressed. They can be painted as they are. The porosity is extremely high, so its volumetric energy density is very low. For cars, especially passenger cars, when the energy density reaches a certain value, the volumetric energy density is even more important, because passenger cars do not have so many local batteries. So in this sense, at least in the field of vehicle power, there is no expectation for lithium-sulfur batteries. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: Analysis of the difference between batteries and fuel power batteries