Lithium-ion 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.

During 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 pass through the positive 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.

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

The requirements of fast charging for all parts of the battery

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

positive electrode

In fact, almost all kinds of cathode materials can be used to make fast-charging batteries. The important properties to be guaranteed include conductivity (reduce internal resistance), diffusion (ensure reaction kinetics), life (don’t explain), and safety (don’t explain) , 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 There are sometimes many anti-uses in China), and more attention is paid to safety and suppression of side reactions (with electrolyte). After all, the current life of ternary materials lies in safety, and recent battery safety accidents have also occurred frequently. Put forward higher requirements.

C. Lithium manganate is more important in terms of service 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 important 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 the graphite negative electrode. The surface is relatively sensitive to the electrolyte, and the lithium intercalation reaction has a strong directionality. Therefore, it is important to work hard to improve the structural stability of the graphite surface and promote the diffusion of lithium ions on the substrate. 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 imagine that graphite is the same cheap, I am afraid that it is not hopeful from an industrial point of view), so the current consumption 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 an important development direction, and Panasonic’s new 18650 battery has begun the commercial process of such materials. However, how to achieve a balance between the pursuit of nanometer performance and the general micron-level requirements of battery industry-related materials is still a more challenging task.

Diaphragm

Regarding power-type batteries, high-current operation imposes higher requirements on their safety and lifespan. Diaphragm coating technology cannot be circumvented. Ceramic coated diaphragms are rapidly being pushed out because of their high safety and the ability to consume impurities in the electrolyte. In particular, the effect of improving the safety of ternary batteries is particularly significant.

The most important system currently used for 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 mechanical support effect of fiber-related diaphragms is better. Excellent, and it has a lower tendency to block the diaphragm pores during service.

After 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. Jiangsu Qingtao Energy Co., Ltd. supported by the technical support of the Nan Cewen research group of the School of Materials and Materials of Tsinghua University has some representative in this regard. Working, the diaphragm is shown in the figure below.

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, efforts can be made 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 within the battery and the rate of insertion of electrode holes will affect the rapid charging ability of lithium-ion batteries.

Overview of fast charging technology routes for mainstream manufacturers

Ningde era

Regarding the positive electrode, CATL developed the “super electronic network” technology, which makes lithium iron phosphate have excellent electronic conductivity; on the negative electrode graphite surface, the “fast ion ring” technology is used to modify the graphite, and the modified graphite takes into account both super fast charging and high With the characteristics of energy density, the negative electrode no longer has excessive by-products during fast charging, so that it has 4-5C fast charging capacity, realizing 10-15 minutes fast charging and charging, and can ensure the energy density of the system level above 70wh/kg, achieving 10,000 Cycle life.

In terms of thermal management, its thermal management system fully recognizes the “healthy charging interval” of the fixed chemical system at different temperatures and SOCs, which greatly broadens the operating temperature of lithium-ion batteries.

Waterma

Waterma is not so good lately, let’s just talk about technology. Waterma uses lithium iron phosphate with a smaller particle size. At present, the common lithium iron phosphate on the market has a particle size between 300 and 600 nm, while Waterma only uses 100 to 300 nm lithium iron phosphate, so lithium ions will have The faster the migration speed, the larger the current can be charged and discharged. For systems other than batteries, strengthen the design of thermal management systems and system safety.

Micro Power

In the early days, Weihong Power chose lithium titanate + porous composite carbon with spinel structure that can withstand fast charging and high current as the negative electrode material; in order to prevent the threat of high power current to battery safety during fast charging, Weihong Power Combining non-burning electrolyte, high-porosity and high-permeability diaphragm technology and STL intelligent thermal control fluid technology, it can ensure the safety of the battery when the battery is quickly charged.

In 2017, it announced a new generation of high-energy density batteries, using high-capacity and high-power lithium manganate cathode materials, with a single energy density of 170wh/kg, and achieving 15-minute fast charging. The goal is to take into account life and safety issues.

Zhuhai Yinlong

Lithium titanate anode is known for its wide operating temperature range and large charge-discharge rate. There is no clear data on the specific technical methods. Talking to the staff at the exhibition, it is said that its fast charge can achieve 10C and the life span is 20,000 times.

The future of fast charging technology

Whether the fast charging technology of electric vehicles is a historical direction or a short-lived phenomenon, in fact, there are different opinions now, and there is no conclusion. As an alternative method to solve mileage anxiety, it is considered on the same platform with battery energy density and overall vehicle cost.

Energy density and fast charge 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 prevent the so-called “range anxiety”, the demand for battery rate charging performance will be reduced; at the same time, if the battery power is large, if the battery cost per kilowatt-hour is not low enough, then is it necessary? 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 promoted on a large scale, the energy density and fast charging technology which develops fast, and the two technologies that cut costs down, may play a decisive role in its future.