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Factors affecting fast charging of lithium batteries(I)
- Jul 25, 2018 -

The charged ions of the lithium battery 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 at the same time, excess electrons are generated to pass 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.


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Fast charging requirements for each part of the battery

For the battery, if the power performance is to be improved, it is necessary to work hard in all parts of the battery, mainly including positive pole, negative electrode, electrolyte, diaphragm and structure design.

1. Positive electrode

In fact, a variety of positive materials can be used to make fast charge batteries, mainly to ensure that the properties include conductance (reduction of resistance), diffusion (reaction kinetics), life, safety, proper processing performance.

Of course, there may be a difference in the problems to be solved for each specific material, but the common positive materials can be met by a series of optimizations, but different materials are also different:

1) Lithium ferric phosphate may be more focused on solving the problems of conductivity and low temperature. Carbon coating, moderate nanoscale, it is the most typical strategy to form ion conductors on the surface of a particle.

2) The ternary material itself has a good conductance, but its reactivity is too high, so the ternary material has little work of nanocrystallization,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. Put forward higher requirements.

3) Lithium manganate is more important for life, there are a lot of fast-charge batteries of lithium manganate on the market.

2. 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. The kinetic requirements, but also to solve the safety problems caused by the increased tendency of lithium dendrite formation, so the main technical difficulty of the fast charging core is the insertion of lithium ions in the negative electrode.

1) The current dominant negative material in the market is still graphite (about 90% of the market share), because the price of the graphite is low, and the processing performance and energy density of the graphite are excellent, and the disadvantages are relatively few. Graphite negative electrode is of course also a problem, its surface is more sensitive to the electrolyte and the imbedding reaction of lithium has strong directionality. Therefore, it is the direction to make the graphite surface treatment, improve its structural stability and promote the diffusion of lithium ion on the base.

2) Hard carbon and soft carbon materials have also developed a lot in recent years: hard carbon materials have high lithium potential, and there are micropores in the material, so the reaction kinetics is good, and the compatibility of the soft carbon materials with the electrolyte is good, and the MCMB material is very representative, but the efficiency of hard carbon materials is low and the cost is high, so the amount of the material is far from now. And graphite, more used in some special batteries.

3. How about lithium titanate? The advantages of lithium titanate are high power density, safer, and obvious disadvantages. The energy density is very low, and the calculation cost is high according to Wh. Therefore, lithium titanate battery is a technology that has advantages in certain occasions, but it is not suitable for many occasions where the cost and cruising range are high.

4. Silicon anode materials are an important development direction, but 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.