What are the factors that restrict the rapid charging capacity of lithium batteries?

The microscopic process of charging

Lithium batteries are called "rocker type" batteries, with electric ions moving between positive and negative poles, achieving charge transfer, supplying external circuits or charging from external power sources. In the specific charging process, the external voltage is loaded at the two poles of the battery, and the lithium ion is de-embedded from the positive electrode material and enters the electrolyte. At the same time, excess electrons are generated through the positive electrode set fluid and move to the negative electrode via the external circuit; Lithium-ion moves from positive to negative in the electrolyte and passes through the diaphragm to reach the negative electrode; The SEI membrane that passes through the negative surface is embedded in the negative graphite layered structure and binds to electrons.

During the entire operation of ions and electrons, cell structures that affect charge transfer, whether electrochemical or physical, will have an impact on rapid charging performance.

Quick charge, requirements for all parts of the battery

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

positive electrode

In fact, all kinds of positive materials can be used to make fast charging batteries, The main properties to be guaranteed include conductivity(reduced internal resistance), diffusion(guaranteed reaction Kinetics), lifetime(no explanation required), safety(no explanation required), proper processing performance(not too large for surface area, reduced side effects, For security). Of course, there may be differences in the problems to be solved for each specific material, but our common positive materials can meet these requirements through a series of optimizations, but different materials also differ:

A, lithium iron phosphate may be more focused on solving electrical conductivity and low temperature problems. Carbon coating, moderate nanotechnology(note, moderation, absolutely not the finer and better simple logic), the treatment of Ionic conductors on the surface of 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 nano-crystallization (nano-crystallization 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 also many lithium manganate fast rechargeable batteries on the market.

negative electrode

When the lithium ion battery is charged, lithium migrates to the negative electrode. The excessively high potential caused by fast charging of large currents will lead to a more negative potential. At present, the dominant anode material in the market is still graphite (about 90% of the market share), the root cause is no him - cheap (you are too expensive every day, exclamation mark!), and the comprehensive processing performance and energy of graphite The density is relatively good and the disadvantages are relatively small. 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.

A,The negative material that currently dominates the market is still graphite(accounting for about 90 % of the market share). The basic reason is nothing-cheap(you are always afraid of batteries, exclamation points! ), and graphite comprehensive processing performance, energy density are relatively good, relatively few disadvantages. Of course, there are also problems with graphite negative poles. The surface is more sensitive to electrolytes. The embedding reaction of lithium has strong directionality. Therefore, graphite surface treatment, improving its structural stability, and promoting the diffusion of lithium ions on the base are the main needs. The direction of effort.

B, hard carbon and soft carbon materials have also developed a lot in recent years: hard carbon materials have high lithium potential, and there are micro-pores in the material, so the reaction Kinetics is good; The compatibility of soft carbon materials with electrolytes is good, and MCMB materials are also very representative, but the general efficiency of hard and soft carbon materials is low and the cost is high(and it is as cheap as graphite, I'm afraid it is not promising from an industrial point of view). Therefore, the current amount is far less than graphite. More for some special batteries.

C, Some people will ask me how lithium titanate is. Simply put: Lithium titanate has the advantage of high power density, safety, obvious disadvantages, low energy density, and high cost in Wh. Therefore, the author's view on lithium titanate batteries has always been that it is a useful technology that has advantages in specific situations, but it is not suitable for many occasions where the cost and mileage requirements are high.

D, Silicon negative electrode material is an important development direction, Panasonic's new 18650 battery has begun the commercial process of such materials. But how to achieve a balance between the performance of nano-crystals and the general micron level requirements of the battery industry is still a challenging task.

Diaphragm

For power type batteries, high current operation provides higher requirements for their safety and life. The diaphragm coating technology is unwound, and the ceramic coating diaphragm is rapidly being pushed away because of its high safety and can consume impurities in the electrolyte, especially for the improvement of the safety of the ternary battery. The current system used in ceramic diaphragm is to coat aluminum oxide particles on the surface of the traditional diaphragm. The more novel approach is to apply solid electrolyte fibers to the diaphragm. The internal resistance of such a diaphragm is lower, and the fiber is used for the diaphragm. The mechanical support effect is better. In addition, it has a lower tendency to block the diaphragm hole during service.

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 the fast charging large current, the electrolyte must meet the following characteristics: A) can not be decomposed, B) high conductivity, C) is inert to the positive and negative electrode material, can not 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 rechargeable batteries is greatly affected by it. It is necessary to add a variety of anti-high temperature, flame retardant, and anti-charging additives to enhance its safety to a certain extent. The old and difficult problem of lithium titanate batteries, high temperature gas, must also rely on high-temperature functional electrolytes to improve.

Battery structure design

A typical optimization strategy is the laminated VS winding type. The electrodes of the laminated battery are quite parallel, and the winding type is quite connected in series. Therefore, the former has a much smaller internal resistance and is more suitable for power-type situations. In addition, you can also work on the number of polar ears to solve the problem of internal resistance and heat dissipation. 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 that affect the internal charge movement of the battery and the hole rate of the embedded electrode will affect the rapid charging ability of the lithium battery.

The page contains the contents of the machine translation.