Charge and discharge ratio of lithium ion battery

The charge and discharge ratio of lithium-ion batteries determines how fast we can store a certain amount of energy in the battery, or at how fast we can release the energy in the battery. Of course, this process of storage and release is controllable, safe, and does not significantly affect the battery life and other performance indicators.

The ratio index is particularly important when the battery is used as an electric tool, especially the energy carrier of electric vehicles. Imagine that if you drive an electric car to do business, you find that there is almost no electricity in the middle, find a charging station to charge it, and fill it up for an hour. It is estimated that everything that needs to be done has been delayed. Or maybe your electric car is climbing a steep slope, no matter how much you press the throttle(electric door), but the car is slow like a turtle, can not make it, I would like to come down and push.

Obviously, these scenes are not what we want to see, but it is the current situation of lithium-ion batteries. It takes a long time to charge, and the discharge can not be too violent. Otherwise, the battery will quickly age and may even have safety problems. But in many applications, we need the battery to have a large charge and discharge performance, so we are stuck here again. In order to achieve better development of lithium-ion batteries, it is necessary to understand what factors are limiting the doubling performance of batteries.

The recharge and discharge ratio performance of lithium-ion batteries is directly related to the ability of lithium ions to migrate at the positive and negative poles, electrolytes, and interfaces between them. All factors that affect the rate of lithium ions migration(these influence factors can also be equivalent to batteries. Internal resistance), Will affect the charge and discharge ratio of lithium ion batteries. In addition, the heat dissipation rate inside the battery is also an important factor that affects the performance of the ratio. If the rate of heat dissipation is slow and the heat accumulated when the rate of mass is charged and discharged can not be transmitted, it will seriously affect the safety and life of the lithium ion battery. Therefore, to study and improve the recharge and discharge performance of lithium-ion batteries, mainly from two aspects of improving the rate of lithium-ion transport and heat dissipation inside the battery.

1. Improve the Lithium-ion diffusion capability of positive and negative poles

The rate of deembedding and embedding of lithium ions inside positive/negative active substances, that is, the speed at which lithium ions run out of positive/negative active substances, Or how fast it is to find a position inside the active material from the positive-negative surface, 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 basically the same time, the width of the road is limited and the number of people involved is large(sometimes as many as tens of thousands), resulting in mutual congestion. The physical quality of the participants is uneven. The team in the game will eventually become an extremely long front. Someone arrived at the finish line quickly. Someone arrived a few hours late. Someone ran into a faint and stopped eating.

The diffusion and movement of lithium ions at the positive/negative poles is basically the same as that of the marathon. Slow running and fast running, together with the different lengths of their respective choices, severely restrict the time for the end of the game(all people finish). So, we don't want to run marathons, it's best if we all run 100 meters, the distance is short enough for everyone to get to the finish line quickly, and the runway should be wide enough not to crowd each other, the roads should not meander, straight lines are the best. To reduce the difficulty of the game. As a result, the referee rang and rushed to the end of the game. The game quickly ended and the performance was excellent.

In the positive material, we hope that the electrode is thin enough, that is, the thickness of the active material is small, which is equivalent to shortening the distance of the race, so we hope to increase the compaction density of the positive material as much as possible. Inside the active material, there must be enough holes to allow the lithium ions to play, and at the same time these "runways" must be evenly distributed, not in some places, and not in some places, which requires the optimization of the structure of the cathode material. Change the distance and structure between particles to achieve uniform distribution. The above two points are actually contradictory and increase the compaction density. Although the thickness becomes thinner, the particle gap will become smaller and the runway will appear crowded. Conversely, maintaining a certain amount of particle gap is not conducive to making the material thin. Therefore, it is necessary to find a balance point to achieve the best rate of lithium ion migration.

In addition, the positive materials of different materials have a significant impact on the diffusion coefficient of lithium ions. Therefore, choosing a positive material with a high diffusion coefficient of lithium ions is also an important direction to improve the performance of the ratio.

The treatment of negative materials is similar to that of positive materials. It also focuses on the structure, size, and thickness of materials, reducing the concentration difference of lithium ions in negative materials, and improving the diffusion of lithium ions in negative materials. ability. Taking carbon-based negative polar materials as an example, in recent years, research on nanocarbon materials(nanotubes, nanowires, nanowires, etc.) has replaced traditional negative layered structures, which can significantly improve the specific surface area of negative polar materials. Internal structure and diffusion channels, Therefore, the doubling performance of the negative electrode material is greatly improved.

2. Improve Ionic conductivity of electrolytes

Lithium ions play in the positive-negative material is a race, in the electrolyte in the competition is swimming.

Swimming competition, how to reduce the resistance of water(electrolyte), has become the key to speed increase. In recent years, swimmers have generally worn shark suits. This kind of bathing suit can greatly reduce the resistance formed by water on the surface of the human body, thereby improving the athlete's performance and becoming a very controversial topic.

Lithium ions travel back and forth between positive and negative poles, just as they swim in a "swimming pool" composed of electrolytes and battery cases. The Ionic conductivity of electrolytes is the same as the resistance of water, which has a very large effect on the speed of lithium-ion swimming.. At present, the organic electrolytes used in lithium ion batteries, whether they are liquid electrolytes or solid electrolytes, have not had high Ionic conductivity. The resistance of electrolytes becomes an important part of the battery resistance, and the influence on the high-magnification performance of lithium-ion batteries can not be ignored.

In addition to improving the Ionic conductivity of electrolytes, we also need to pay attention to the chemical stability and thermal stability of electrolytes. When the high magnification is charged and discharged, the electrochemical window of the battery changes in a very wide range. If the chemical stability of the electrolyte is not good, it is easy to oxidize and decompose on the surface of the cathode material, affecting the Ionic conductivity of the electrolyte. The thermal stability of the electrolyte has a great influence on the safety and recycling life of the lithium ion battery, because the electrolyte generates a lot of gas when it is thermally decomposed. On the one hand, it poses a hidden danger to the battery safety, and on the other hand, some gases are on the negative surface. The SEI membrane has a destructive effect. It affects its cycling performance.

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

3. Reduce the internal resistance of the battery

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

Generally, conductive agents are added inside positive polar active substances, thereby reducing the contact resistance between active substances, active substances and positive basic/collector fluids, improving the conductivity(ion and electron conductivity) of positive polar materials, and improving The rate performance. Different materials of different shapes of conductive agents, will have an impact on the internal resistance of the battery, and then affect its ploidy performance.

The positive and negative collector fluid(polar ear) is the carrier of electrical energy transfer between the lithium ion battery and the outside world. The resistance value of the collector fluid also has a great influence on the multiplier performance of the battery. Therefore, by changing the material, size, extraction method, and connection process of the set fluid, the doubling performance and cycle life of the lithium ion battery can be improved.

The degree of infiltration of electrolytes and negative electrode materials will affect the contact resistance at the interface between electrolytes and electrodes, thus affecting the performance of the battery. The total amount of electrolytes, viscosity, impurity content, porosity of positive and negative electrode materials, etc., will change the contact impedance of electrolytes and electrodes, which is an important research direction for improving the performance of magnification.

During the first cycle of lithium ion batteries, as lithium ions are embedded in the negative electrode, a layer of solid electrolyte(SEI) membrane is formed at the negative electrode. Although the SEI membrane has good Ionic conductivity, it still has a certain effect on the diffusion of lithium ions. Obstructive effect, especially when the high rate is charged and discharged. With the increase of the number of cycles, the SEI membrane will continue to fall off, peel off, and deposit on the negative surface, resulting in a gradual increase in the internal resistance of the negative electrode, which has become a factor that affects the performance of the cycle doubling rate. Therefore, controlling the change of SEI membrane can also improve the doubling performance of lithium ion batteries during long-term cycle.

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.