What are the influencing factors of the fast charging capacity of lithium batteries

　　Each type of lithium battery has an optimal charging current value under different state parameters and environmental parameters. Then, from the perspective of the battery structure, what are the factors that affect this optimal charging value?

　　Each type of lithium battery has an optimal charging current value under different state parameters and environmental parameters. Then, from the battery structure, what are the factors that affect this optimal charging value?

　　The micro process of charging

　　Lithium batteries are called "rocking chair type" batteries. Charged ions move between the positive and negative electrodes to realize charge transfer, and supply power to external circuits or charge from an external power source. In the specific charging process, the external voltage is applied to the two poles of the battery, and the lithium ions are extracted from the positive electrode material and enter the electrolyte. At the same time, excess electrons are generated through the positive electrode current collector and move to the negative electrode through the external circuit; the lithium ions are in the electrolyte. It moves from the positive electrode to the negative electrode, passing through the diaphragm to the negative electrode; the SEI film passing through the surface of the negative electrode is embedded in the graphite layered structure of the negative electrode and combines with electrons.

　　During the entire operation of ions and electrons, the battery structure that affects charge transfer, whether electrochemical or physical, will affect the fast charging performance.

　　Quick charge, the requirements for each part of the battery

　　For batteries, if you want to improve power performance, you need to work hard in all aspects of the battery, including the positive electrode, negative electrode, electrolyte, separator, and structural design.

　　positive

　　In fact, almost all kinds of cathode materials can be used to make fast-charging batteries. The main properties that need to be guaranteed include conductivity (reduction of internal resistance), diffusion (ensure of reaction kinetics), life (no explanation required), and safety (no need for explanation). Explanation), proper processing performance (the specific surface area should not be too large to reduce side reactions and serve safety).

　　Of course, the problems to be solved for each specific material may be different, but our common cathode materials can meet these requirements through a series of optimizations, but different materials are also different:

　　A, lithium iron phosphate may be more focused on solving the problems of conductivity and low temperature. Carrying out carbon coating, moderate nanoization (note that it is moderate, it is definitely not a simple logic that the finer the better), and the formation of ion conductors on the surface of the particles are the most typical strategies.

　　B. The ternary material itself has relatively good electrical conductivity, but its reactivity is too high, so ternary materials rarely carry out nano-scale work (nano-ization is not a panacea-like antidote to the improvement of material performance, especially in the field of batteries (Sometimes there are a lot of side effects), and more attention is paid to safety and suppression of side reactions (with electrolyte). After all, the main life of ternary materials is safety, and recent battery safety accidents have also occurred in this regard. Put forward higher requirements.

　　C and lithium manganate are more important to life. There are also many lithium manganate-based fast-charge batteries on the market.

　　negative electrode

　　When a lithium ion battery is charged, lithium migrates to the negative electrode. The excessively high potential caused by fast charging and large current will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode to quickly accept lithium will increase, and the tendency to generate lithium dendrites will increase. Therefore, the negative electrode must not only satisfy the lithium diffusion during fast charging. The kinetics requirements of the lithium ion battery must also solve the safety problem caused by the increased tendency of lithium dendrites. Therefore, the main technical difficulty of the fast charging core is the insertion of lithium ions in the negative electrode.

　　A. At present, the dominant negative electrode material in the market is still graphite (accounting for about 90% of the market share). The fundamental reason is cheap, and the comprehensive processing performance and energy density of graphite are relatively good, with relatively few shortcomings. . Of course, there are also problems with graphite negative electrodes. The surface is sensitive to the electrolyte, and the lithium intercalation reaction has strong directionality. Therefore, the graphite surface treatment to improve the structural stability and promote the diffusion of lithium ions on the substrate is mainly required to work hard. direction.

　　B. Hard carbon and soft carbon materials have also seen a lot of development in recent years: hard carbon materials have high lithium insertion potential and have micropores in the materials, so the reaction kinetics are good; and soft carbon materials have good compatibility with electrolyte, MCMB The materials are also very representative, but hard and soft carbon materials are generally low in efficiency and high in cost (and if you want to be as cheap as graphite, I am afraid that it is not hopeful from an industrial point of view), so the current dosage is far less than graphite, and more used in some specialties On the battery.

　　C, how about lithium titanate? To put it briefly: the advantages of lithium titanate are high power density, safer, and obvious disadvantages. The energy density is very low, and the cost is high when calculated by Wh. Therefore, the viewpoint of lithium titanate battery is a useful technology with advantages in specific occasions, but it is not suitable for many occasions that require high cost and cruising range.

　　D, silicon anode materials are important development directions, and Panasonic's new 18650 battery has begun the commercial process of such materials. However, how to achieve a balance between the pursuit of performance in nanotechnology and the general micron-level requirements of the battery industry is still a challenging task.

　　diaphragm

　　For power batteries, high-current operation provides higher requirements for their safety and life. Diaphragm coating technology cannot be bypassed. Ceramic coated diaphragms are rapidly being pushed out because of their high safety and the ability to consume impurities in the electrolyte, especially for the improvement of the safety of ternary batteries.

　　The main system currently used in ceramic diaphragms is to coat alumina particles on the surface of traditional diaphragms. A relatively novel method is to coat solid electrolyte fibers on the diaphragm. Such diaphragms have lower internal resistance and the fibers have a better mechanical support effect on the diaphragm. Excellent, and it has a lower tendency to block the diaphragm pores during service.

　　After the coating, the diaphragm has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform and cause a short circuit. The Jiangsu Qingtao Energy Company, supported by the technical support of the Nan Cewen research group of the School of Materials and Materials, Tsinghua University, has some representative in this regard. Work.

　　electrolyte

　　The electrolyte has a great influence on the performance of fast-charging lithium-ion batteries. To ensure the stability and safety of the battery under fast charging and high current, the electrolyte must meet the following characteristics: A) cannot be decomposed, B) high conductivity, and C) is inert to the positive and negative materials. React or dissolve.

　　If you want to meet these requirements, the key is to use additives and functional electrolytes. For example, the safety of ternary fast-charging batteries is greatly affected by it, and it is necessary to add various anti-high-temperature, flame-retardant, and anti-overcharge additives to them to improve its safety to a certain extent. The old and difficult problem of lithium titanate batteries, high-temperature flatulence, also has to be improved by high-temperature functional electrolyte.

　　Battery structure design

　　A typical optimization strategy is the stacked VS winding type. The electrodes of the stacked battery are equivalent to a parallel relationship, and the winding type is equivalent to a series connection. Therefore, the internal resistance of the former is much smaller and it is more suitable for the power type. occasion.

　　In addition, you can also work hard on the number of tabs to solve the problems of internal resistance and heat dissipation. In addition, using high-conductivity electrode materials, using more conductive agents, and coating thinner electrodes are also strategies that can be considered.

　　In short, the factors that affect the charge movement inside the battery and the rate of insertion of the electrode holes will affect the fast charging ability of the lithium battery.

　　The future of fast charging technology

　　Electric vehicle fast charging technology is a historical direction or a short-lived phenomenon. In fact, there are different opinions now, and there is no conclusion. As an alternative solution for mileage anxiety, it is considered on the same platform with battery energy density and overall vehicle cost.

　　Energy density and fast charging performance, in the same battery, can be said to be two incompatible directions, and cannot be achieved at the same time. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough and the battery capacity of a vehicle is large enough to avoid the so-called "range anxiety", the demand for battery rate charging performance will be reduced; at the same time, if the power is large, if the battery cost per kilowatt-hour is not low enough, then whether Ding Kemao's purchase of electricity that is sufficient for "not anxious" requires consumers to make a choice. If you think about it, fast charging has value. Another point of view is the cost of fast charging facilities, which of course is part of the cost of the entire society to promote electrification.

　　Whether fast charging technology can be widely promoted, the energy density and fast charging technology which develops fast, and whoever reduces the cost by the two technologies, may play a considerable decisive role in its future.