What factors affect the fast charge capacity of lithium batteries?

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

Microscopic process of charging

Lithium batteries are called "rocking chair type" batteries, and charged ions move between the positive and negative electrodes to achieve charge transfer, powering external circuits or charging from an external power source. During the specific charging process, the external voltage is applied to the two poles of the battery, lithium ions are deintercalated from the positive electrode material, enter the electrolyte, and excess electrons are generated through the positive current collector, and move to the negative electrode through an external circuit; lithium ions are in the electrolyte. The positive electrode moves toward the negative electrode and passes through the separator to reach the negative electrode; the SEI film passing through the surface of the negative electrode is embedded in the negative graphite layered structure and bonded to the electron.

During the entire ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will have an impact on fast charge performance.

Fast charge, requirements for all parts of the battery

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

Positive electrode

In fact, almost all kinds of positive electrode materials can be used to make fast-filled batteries. The main performances required to be guaranteed include conductance (reduced internal resistance), diffusion (guaranteed reaction kinetics), longevity (no need to explain), and safety (not required). Explain), proper processing performance (the specific surface area cannot be too large, reduce side reactions, for safety services).

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

A. lithium iron phosphate may be more focused on solving problems of conductance and low temperature. Carbon coating, moderate nanocrystallization (note that it is moderate, definitely not as fine as the simple logic), the formation of ionic conductors on the surface of the particles is the most typical strategy.

B. The conductivity of the ternary material itself is relatively good, but its reactivity is too high, so the ternary material has little work of nanocrystallization (nanocrystallization is not an antidote to the performance improvement of the metallurgical material, especially in the field of batteries. There are sometimes many reactions in the system. More attention is paid to safety and inhibition (and electrolyte) side effects. After all, the main goal of ternary materials is safety. The recent battery safety accidents are also frequent. Higher requirements were raised.

C. Lithium manganate is more important for life, there are many fast-charge batteries of lithium manganate on the market.

Negative electrode

When the lithium ion battery is charged, lithium migrates to the negative electrode. The excessively high potential caused by the fast charge and high current will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode rapidly accepting lithium will become larger, and the tendency to generate lithium dendrites will become larger. Therefore, the negative electrode must not only satisfy the lithium diffusion during fast charging, but also to solve the safety problems caused by the increased tendency of lithium dendrite formation, so the main technical difficulty of the fast charge core is the insertion of lithium ions in the negative electrode.

A. At present, the dominant anode material in the market is still graphite (accounting for about 90% of the market share), the root cause is none--cheap, and the comprehensive processing performance and energy density of graphite are excellent, and the disadvantages are relatively few. . Of course, graphite anodes also have problems. The surface is sensitive to electrolytes, and the lithium intercalation reaction has strong directionality. Therefore, it is mainly necessary to work hard to carry out graphite surface treatment, improve its structural stability, and promote the diffusion of lithium ions on the substrate direction.

B. Hard carbon and soft carbon materials have also developed in recent years: hard carbon materials have high lithium insertion potential, micropores in the materials, and good reaction kinetics; and soft carbon materials have good compatibility with electrolytes, MCMB The materials are also very representative, but the hard and soft carbon materials are generally low in efficiency and high in cost (and Imagine that graphite is as cheap as I hope from an industrial point of view), so the current usage is far less than graphite, and more used in some special On the battery .

C. How about lithium titanate? To put it simply: lithium titanate has the advantages of high power density, safer, and obvious disadvantages. The energy density is very low, and the calculation cost is high according to Wh. Therefore, the viewpoint of lithium titanate battery is a useful technology that is advantageous in specific situations, but it is not suitable for many occasions where the cost and cruising range are high.

D. Silicon anode material is an important development direction. Panasonic's new 18650 battery has begun commercial process for such materials. But how to achieve a balance between the pursuit of performance in nanotechnology and the battery industry's general micron-scale requirements for materials is still a challenging task.

Diaphragm

For power batteries, high current operation provides higher requirements for safety and longevity. Diaphragm coating technology is inseparable. Ceramic coated membranes are rapidly being pushed away due to their high safety and the ability to consume impurities in the electrolyte. Especially for the safety of ternary batteries, the safety effect is particularly remarkable.

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

After the coating, the separator has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform, resulting in short circuit. Jiangsu Qingtao Energy Co., Ltd., technical support of the Academic Researcher of Tsinghua University School of Materials, has some representative aspects in this respect.

Electrolyte

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

If these requirements are to be met, the key is to use additives and functional electrolytes. For example, the safety of ternary fast-charged batteries is greatly affected by it. It is necessary to add various anti-high temperature flame-retardant and anti-overcharged additives to protect them to a certain extent. The problem of the old lithium titanate battery, high temperature flatulence, but also rely on high-temperature functional electrolyte to improve.

Battery structure design

A typical optimization strategy is the stacked VS winding type. The electrodes of the laminated 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 work hard on the number of poles to solve internal resistance and heat dissipation problems. In addition, the use of high-conductivity electrode materials, the use of more conductive agents, and the coating of thinner electrodes are also strategies that can be considered.

In short, factors affecting the internal charge movement of the battery and the rate of embedding the electrode cavity will affect the rapid charging capability of the lithium battery.

The future of fast charging technology

The fast charging technology of electric vehicles is the direction of history or the glimpse of the past. In fact, there are many different opinions and there is no conclusion. As an alternative to solving mileage anxiety, it is considered on a platform with battery energy density and overall vehicle cost.

Energy density and fast charge performance, in the same battery, can be said to be incompatible in both directions, not both. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough, the load of one car is large enough to avoid the so-called "mileage anxiety", the demand for battery rate charging performance will be reduced; at the same time, the power is large, if the cost of battery power is not low enough, then whether it is necessary Ding Kezhen’s purchase of electricity that is “not anxious” requires consumers to make choices. If you think about it, fast charge will have value. Another angle is the cost of fast charging facilities, which is of course part of the cost of electrifying the entire society.

Whether the rapid charging technology can be widely promoted, who develops faster in energy density and rapid charging technology, and who reduces the cost of the two technologies so much, may play a decisive role in their future.

The page contains the contents of the machine translation.