Lithium-ion battery charge and discharge ratio

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

The multiplier index is particularly important when batteries are used as energy carriers for electric vehicles. Imagine that if you drive an electric car to do some business, and you find that the power is running out halfway, and you find a charging station to charge the car, and it is not full after an hour of charging, it may delay your work. Or maybe your electric car is climbing a steep hill and, no matter how hard you hit the gas, it's slow as a tortoise and you can't get up and want to push it.

Obviously, we do not want to see these scenes above, but it is the current situation of lithium ion batteries, charging time is long, discharge can not be too strong, otherwise the battery will soon aging, and even may occur safety problems. But in many applications, we need batteries with a high rate of charge and discharge performance, so we are stuck in the "battery" here again. In order for lithium-ion batteries to develop better, it is important to understand what limits their power performance.

The charge and discharge rate performance of lithium ion battery is directly related to the migration ability of lithium ion in the positive and negative electrode, electrolyte and the interface between them. All factors affecting the migration rate of lithium ion (these factors can also be equivalent to the internal resistance of the battery) will affect the charge and discharge rate performance of lithium ion battery. In addition, the heat dissipation rate inside the battery is also an important factor affecting the performance of the multiplier. If the heat dissipation rate is slow, the heat accumulated during charging and discharging at a high rate cannot be transferred out, which will seriously affect the safety and service life of the lithium ion battery. Therefore, the study and improvement of charge and discharge rate performance of lithium ion battery mainly focus on improving the migration speed of lithium ion and the heat dissipation rate inside the battery.

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

The rate at which lithium ions detach and embed in the positive/negative active material, that is, the rate at which lithium ions escape from the positive/negative active material or find a place in the active material from the surface of the positive/negative electrode, is an important factor affecting the charging and discharging rate.

Worldwide every year, for example, there are many marathon, though all basically the same time, the road width is limited, however, are too many people have participated (and sometimes up to tens of thousands of people), cause mutual crowded, plus participants physical quality is uneven, the team finally become a super long front. Some arrive at the finish line quickly, some arrive a few hours late, some run into a faint and stop halfway.

The diffusion and movement of lithium ions in the positive/negative electrode is basically the same as that of marathon, with some running slowly and some running fast. In addition, the different length of the road they choose seriously restricts the time of the end of the race (everyone finishes the race). Therefore, we do not want to run a marathon. It is better for everyone to run 100 meters. The distance is short enough so that everyone can reach the finishing line quickly. In this way, the referee a ring, thousands of troops and horses together to the end of the race, the end of the race quickly, with excellent performance.

In the positive material, we hope that the electrode sheet should be thin enough, that is, the thickness of the active material should be small, which is equivalent to shorten the race distance, so we hope to increase the compaction density of the positive material as much as possible. In the active material, there should be enough pore space to leave a channel for lithium ions to compete. At the same time, these "runways" should be evenly distributed, not in some places and not in some places. This requires optimizing the structure of the anode material, changing the distance and structure between particles, and achieving uniform distribution. The above two points are actually contradictory. To improve the compaction density, 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 making the material thinner. Therefore, it is necessary to find a balance point to achieve the optimal migration rate of lithium ions.

In addition, the diffusion coefficient of lithium ions is significantly affected by the anode materials. Therefore, it is an important direction to select anode materials with high lithium ion diffusion coefficient to improve the multiplier performance.

The treatment idea of negative electrode materials is similar to that of positive electrode materials. It is mainly based on the structure, size and thickness of materials to reduce the concentration difference of lithium ions in negative electrode materials and improve the diffusion ability of lithium ions in negative electrode materials. 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 electrolytes

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

In swimming, how to reduce the resistance of water (electrolyte) becomes the key to speed. In recent years, swimmers generally wear shark suits, which can greatly reduce the resistance of water on the human body surface, thus improving the performance of athletes and becoming a very controversial topic. Lithium ions have to shuttle back and forth between the positive and negative poles, just like swimming in the "swimming pool" made up of electrolyte and battery shell. The ionic conductivity of electrolyte, like the resistance of water, has a great impact on the speed of lithium ions swimming. At present, the organic electrolyte used in lithium ion battery, whether it is liquid electrolyte or solid electrolyte, its ionic conductivity is not very high. The resistance of electrolyte becomes an important part of the resistance of the whole battery.

In addition to improving the ionic conductivity of electrolytes, the chemical and thermal stability of electrolytes should also be emphasized. When charging and discharging at a high rate, the range of the electrochemical window of the battery is very wide. If the chemical stability of the electrolyte is not good, it is easy to be oxidized and decomposed on the surface of the anode material, affecting the ionic conductivity of the electrolyte. The thermal stability of electrolyte has a great impact on the safety and cycle life of li-ion battery, because the electrolyte will generate a lot of gas when it is decomposed by heat. On the one hand, it poses a hidden danger to battery safety; on the other hand, some gas will destroy SEI film on the negative electrode surface, affecting its cycle performance.

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

3. Reduce the internal resistance of the battery

There are several different kinds of materials and their interfaces, which form resistance values, but all have an effect on ion/electron conduction.

In general, conductive agents will be added inside the positive active material, so as to reduce the contact resistance between the active material and the positive matrix/collector fluid, improve the conductivity of the positive material (ion and electron conductivity), and improve the multiplier performance. Different materials and different shapes of conductive agents will affect the internal resistance of the battery, and thus affect its multiplier performance.

The positive and negative collector (pole ear) is the carrier of energy transfer between lithium ion battery and the outside world. Therefore, the multiplier performance and cycle life of lithium ion battery can be improved by changing the material, size, extraction method and connection process of the collector fluid.

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

During the first cycle of a lithium ion battery, a layer of solid electrolyte (SEI) film will be formed in the negative electrode as lithium ions are embedded in the negative electrode. Although the SEI film has good ionic conductivity, it still has some hindering effect on the diffusion of lithium ions, especially in the case of high rate charge and discharge. With the increase of cycle times, SEI film will fall off, peel off and deposit on the surface of the negative electrode, leading to the increase of the internal resistance of the negative electrode, which becomes the factor affecting the cycle ratio performance. Therefore, controlling the change of SEI film can also improve the multiplier performance in the long-term cycle of li-ion battery.

In addition, the absorbance and porosity of the membrane also have a great influence on the permeability of lithium ions, and also affect the multiplier performance of lithium ion battery to a certain extent (relatively small).

The page contains the contents of the machine translation.