What factors will limit the charge and discharge rate performance of lithium-ion batteries?

The charge-discharge rate of a lithium-ion battery determines how fast we can store a certain amount of energy into the battery, or how fast the energy inside the battery is released. Of course, this storage and release process is controllable and safe, without significantly affecting battery life and other performance metrics.

Magnification indicators are particularly important when batteries are used as power tools, especially for energy vehicles. Imagine if you drive an electric car to do things, halfway to find that there is no electricity, find a charging station to charge, charging for an hour is not full, it is estimated that everything to be done is delayed. Or your electric car is climbing a steep slope. No matter how you step on the throttle (electrical door), the car is slow like a tortoise, so you can't wait for it.

Obviously, these scenes are things we don't want to see, but they are the current status of lithium-ion batteries. It takes a long time to charge and the discharge should not be too strong. Otherwise, the battery will soon age and may even have safety problems. But in many applications, we all need the battery to have a large rate of charge and discharge performance, so we are stuck in the "battery" again. In order to achieve better development of lithium-ion batteries, it is necessary to figure out which factors are limiting the rate performance of the battery.

The charge-discharge rate performance of lithium-ion batteries is directly related to the migration ability of lithium ions between the positive and negative electrodes, the electrolyte, and the interface between them. All the factors affecting the migration speed of lithium ions (these influence factors can also be equivalent to batteries). The internal resistance) will affect the charge and discharge rate performance of the lithium ion battery. In addition, the internal heat dissipation rate of the battery is also an important factor affecting the rate performance. If the heat dissipation rate is slow, the accumulated heat during charging and discharging at a large rate cannot be transmitted, which will seriously affect the safety and life of the lithium ion battery. Therefore, research and improvement of the charge-discharge rate performance of lithium-ion batteries are mainly from the aspects of improving the migration speed of lithium ions 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 embedded inside the positive/negative active material, that is, the speed at which lithium ions run out of the positive/negative active material, or from the positive/negative surface to the inside of the active material to find a position to “settle” How fast is the speed, which is an important factor affecting the charge and discharge rate.

For example, there are many marathons in the world every year. Although everyone starts at the same time, the road width is limited, but there are many people involved (sometimes as many as tens of thousands), causing mutual crowding and the body of the participants. The quality is uneven, and the team will eventually become an extra long battle. Someone quickly reached the end, some people were late for a few hours, some people ran to fainting, and they stopped eating halfway.

The diffusion and movement of lithium ions in the positive/negative poles is basically the same as that of the marathon. They run slower and run faster, and the length of the roads they choose varies, which seriously restricts the end of the game (everyone is run it). So, we don't want to run a marathon but is better that everyone runs 100 meters. The distance is short enough. Everyone can reach the end quickly. In addition, the runway should be wide enough, don't crowd each other, and the roads should not be twisted and twisted. The straight line is the best is to reduce the difficulty of the game. In this way, the referee rang aloud, and the thousands of horses and horses rushed to the end. The game ended quickly and the rate performance was excellent.

At the positive electrode material, we want the pole piece to be thin enough, that is, the thickness of the active material is small, which is equivalent to shortening the running distance, so it is desirable to increase the compaction density of the positive electrode material as much as possible. Inside the active material, there should be enough holes clearance to leave the passage for the lithium ions. At the same time, the distribution of these "runway" should be uniform. There should be some places, and some places are not. This is to optimize the structure of the positive electrode material. Change the distance and structure between the particles to achieve a uniform distribution. The above two points are actually contradictory, increasing the compaction density. Although the thickness is thinner, the particle gap will become smaller, and the runway will appear crowded. Conversely, maintaining a certain particle gap is not conducive to making the material thin. So you need to find a balance point to achieve the best lithium ion migration rate.

In addition, the positive electrode materials of different materials have a significant influence on the diffusion coefficient of lithium ions. Therefore, selecting a positive electrode material with a relatively high lithium ion diffusion coefficient is also an important direction for improving the rate performance.

The treatment idea of the anode material is similar to that of the cathode material, and it mainly starts from the structure, size and thickness of the material, reduces the concentration difference of lithium ions in the anode material, and improves the diffusion ability of lithium ions in the anode material. Taking carbon-based anode materials as an example, in recent years, researches on nano-carbon materials (nanotubes, nanowires, nanospheres, etc.) can significantly improve the specific surface area, internal structure and diffusion channel of the anode materials by replacing the traditional layered structure of the anode, thus greatly improving the multiplier performance of the anode materials.

2. Improve the ionic conductivity of the electrolyte

Lithium ions play in the positive/negative materials, and the game in the electrolyte is swimming.

In swimming competitions, how to reduce the resistance of water (electrolyte) is the key to speed improvement. In recent years, swimmers have generally wore shark suits, which can greatly reduce the resistance of water formation on the surface of the human body, thereby improving athletes' performance and becoming a very controversial topic.

Lithium ions need to shuttle back and forth between the positive and negative electrodes. Just like swimming in the "pool" composed of electrolyte and battery case, the ionic conductivity of the electrolyte is the same as the resistance of water. The speed of swimming for lithium ions is very large influences. At present, the organic electrolyte used in lithium ion batteries, whether it is a liquid electrolyte or a solid electrolyte, has a low ionic conductivity. The resistance of the electrolyte becomes an important part of the overall battery resistance, and the influence on the high rate performance of the lithium ion battery cannot be ignored.

In addition to increasing the ionic conductivity of the electrolyte, it is also important to focus on the chemical and thermal stability of the electrolyte. When charging and discharging at a large rate, 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 influence on the safety and cycle life of the lithium ion battery, because the electrolyte will generate a lot of gas when it is decomposed by heat, which poses a hidden danger to the safety of the battery on the one hand, and some gas on the surface of the negative electrode on the other hand. The SEI membrane produces a destructive effect that affects its cycle performance.

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

3. Reduce the internal resistance of the battery

This involves the interface between several different substances and substances, the resistance values they form, but all have an effect on the ion/electron conduction.

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

The current collector (polar ear) of the positive and negative electrodes is a carrier for transferring electric energy between the lithium ion battery and the outside, and the resistance value of the current collector 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 of the electrolyte with the positive and negative materials affects 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 resistance between the electrolyte and the electrode, which is an important research direction to improve the rate performance.

During the first cycle of lithium-ion battery, as lithium ions are embedded in the negative electrode, a solid electrolyte (SEI) film is formed on the negative electrode. Although the SEI film has good ionic conductivity, it still diffuses lithium ions. There is a certain hindrance, especially when charging and discharging at a large rate. As the number of cycle increases, the SEI film will continuously fall off, peel off, and deposit on the surface of the negative electrode, resulting in an 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 during long-term cycling of the lithium ion battery.

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 the lithium ion battery to some extent.

The page contains the contents of the machine translation.