The charge-discharge rate performance of a lithium-ion battery is directly related to the mobility of lithium ions at the positive and negative electrodes, the electrolyte, and the interface between them. The internal resistance) will affect the charge-discharge rate performance of lithium-ion batteries. In addition, the heat dissipation rate inside the battery is also an important factor affecting the rate performance. If the heat dissipation rate is slow, the heat accumulated during high rate charge and discharge cannot be transferred out, which will seriously affect the safety and life of the lithium-ion battery. Therefore, the research and improvement of the charge-discharge rate performance of lithium-ion batteries mainly starts from two aspects: improving the lithium-ion migration speed and the heat dissipation rate inside the battery.

1. Improve the lithium ion diffusion capacity of the positive and negative electrodes

The rate at which lithium ions are deintercalated and intercalated inside the positive/negative active material, that is, the speed at which lithium ions run out of the positive/negative active material, or enter the active material from the positive/negative surface to find a place to "home" How fast is the speed, which is an important factor affecting the charging and discharging rate.


For example, there are many marathons around the world every year. Although everyone starts at the same time, the width of the road is limited, and many people (sometimes as many as tens of thousands of people) participate, causing mutual crowding and the physical health of the participants. The quality is uneven, and the team of the competition will eventually become a super long front. Some people arrived at the finish line quickly, some were a few hours late, and some ran into a coma and stopped halfway.

The diffusion and movement of lithium ions in the positive and negative poles is basically similar to that of a marathon. There are slow runners and fast runners. In addition, the length of the road chosen by each person is different, which seriously restricts the time for the end of the race (everyone finished running). So, we don't want to run a marathon. It's better for everyone to run 100 meters. The distance is short enough so that everyone can reach the finish line quickly. In addition, the track should be wide enough, not crowded with each other, and the road should not be twisty and meandering. The straight line is Best of all, to lower the difficulty of the game. As a result, the referee gave an order, thousands of troops rushed to the finish line together, the game ended quickly, and the multiplier performance was excellent.

At the cathode material, we hope that the pole piece should be thin enough, that is, the thickness of the active material should be small, which is equivalent to shortening the distance of the race, so we hope to increase the compaction density of the cathode material as much as possible. Inside the active material, there should be enough pore gaps to leave a channel for lithium ions to compete. At the same time, these "tracks" should be evenly distributed, not in some places, but not in some places. This requires optimizing the structure of the positive electrode material. Change the distance and structure between particles to achieve an even distribution. The above two points are actually contradictory. If the compaction density is increased, although the thickness becomes thinner, the particle gap will become smaller, and the runway will appear crowded. On the contrary, maintaining a certain particle gap is not conducive to thinning the material. Therefore, it is necessary to find a balance point to achieve the best lithium ion migration rate.


In addition, the cathode materials of different materials have a significant effect on the diffusion coefficient of lithium ions. Therefore, choosing a cathode material with a relatively high lithium ion diffusion coefficient is also an important direction to improve rate performance.

The processing idea of negative electrode material is similar to that of positive electrode material. It mainly starts from the structure, size and thickness of the material to reduce the concentration difference of lithium ions in the negative electrode material and improve the diffusion ability of lithium ions in the negative electrode material. Taking carbon-based anode materials as an example, in recent years, research on nano-carbon materials (nanotubes, nanowires, nanospheres, etc.) can significantly improve the specific surface area, internal structure and Diffusion channel, thereby greatly improving the rate performance of the negative electrode material.

2. Improve the ionic conductivity of the electrolyte

Lithium ions play a race in the positive/negative electrode material, but swimming in the electrolyte.

In swimming competitions, how to reduce the resistance of water (electrolyte) has become the key to speed improvement. In recent years, swimmers generally wear shark suits, which can greatly reduce the resistance formed by water on the surface of the human body, thereby improving the performance of athletes, and it has become a very controversial topic.

Lithium ions have to shuttle back and forth between the positive and negative electrodes, just like swimming in the "swimming pool" formed by the electrolyte and the battery case. influences. The organic electrolytes currently used in lithium-ion batteries, whether liquid electrolytes or solid electrolytes, have low ionic conductivity. The resistance of the electrolyte becomes an important part of the entire battery resistance, and its impact on the high-rate performance of lithium-ion batteries cannot be ignored.

In addition to improving the ionic conductivity of the electrolyte, the chemical and thermal stability of the electrolyte also needs to be focused on. During high-rate charge and discharge, the electrochemical window of the battery varies widely. If the chemical stability of the electrolyte is not good, it is easy to oxidize and decompose on the surface of the positive electrode material, which affects the ionic conductivity of the electrolyte. The thermal stability of the electrolyte has a great impact on the safety and cycle life of the lithium-ion battery, because a lot of gas will be generated when the electrolyte is thermally decomposed, which on the one hand poses a hidden danger to the safety of the battery, and on the other hand, some gases are harmful to the surface of the negative electrode. The SEI film has a destructive effect, which affects its cycle performance.

Therefore, choosing an electrolyte with high lithium ion conductivity, good chemical and thermal stability, and matching electrode materials is an important direction to improve the rate performance of lithium ion batteries.

3. Reduce the internal resistance of the battery

There are several different substances and interfaces between substances involved here, and the resistance values they form, but all have an effect on the conduction of ions/electrons.

Generally, a conductive agent is added inside the positive electrode active material, thereby reducing the contact resistance between the active materials, between the active material and the positive electrode matrix/current collector, improving the electrical conductivity (ionic and electronic conductivity) of the positive electrode material, and improving the rate performance. Conductive agents of different materials and shapes will affect the internal resistance of the battery, thereby affecting its rate performance.

The current collectors (pole tabs) of the positive and negative electrodes are the carriers of electrical energy transfer between the lithium ion battery and the outside world. The resistance value of the current collectors also has a great influence on the rate performance of the battery. Therefore, by changing the material, size, extraction method, connection process, etc. of the current collector, the rate performance and cycle life of the lithium-ion battery can be improved.

The degree of infiltration between the electrolyte and the positive and negative materials will affect the contact resistance at the interface between the electrolyte and the electrode, thereby affecting the rate performance of the battery. The total amount of electrolyte, viscosity, impurity content, pores of positive and negative materials, etc., will change the contact impedance between the electrolyte and the electrode, which is an important research direction to improve the rate performance.

During the first cycle of a lithium-ion battery, as lithium ions are inserted into the negative electrode, a solid-state electrolyte (SEI) film will be formed on the negative electrode. Although the SEI film has good ionic conductivity, it still affects the diffusion of lithium ions. It has a certain hindering effect, especially when charging and discharging at a high rate. With the increase of the number of cycles, the SEI film will continue to fall off, peel off, and deposit on the surface of the negative electrode, resulting in a gradual increase in the internal resistance of the negative electrode, which becomes a factor affecting the cycle rate performance. Therefore, controlling the variation of the SEI film can also improve the rate performance of Li-ion batteries during long-term cycling.

In addition, the liquid absorption rate and porosity of the separator also have a great influence on the passage of lithium ions, and also affect the rate performance (relatively small) of lithium ion batteries to a certain extent.