Charge and discharge rate of lithium ion battery

Lithium ion batteries charge and discharge rate determine the speed of how fast we can storage certain amount of energy into battery inside, or how fast release the inside energy of the battery. Of course, the storage and release process is controllable and safe, which will not significantly affect the service life of battery and other performance indicators.

Ratio index, the battery as a power tool, especially when electric vehicle energy carrier, is particularly important. Imagine, if you drove a electric cars to handle affairs, halfway found almost no electricity, find a charging stations and didn't full charge for an hour, estimates that want to do things delayed. Or your electric cars in climbing a steep hill, no matter how trample accelerator (switch), the car is slow as a turtle, don't want to, your right down the cart.

Obviously, the above is we don't want to see the scene, but it is the current status of lithium ion batteries, the charging takes a long time, discharge nor too fierce, otherwise the battery will be aging soon, perhaps even a security issue. But in many applications, we all need a battery with large ratio of charge and discharge performance, so we stuck in the "battery" again here. For lithium ion battery for better development, it is necessary for us to clear up, is what factors in limiting the rate of battery performance.

Lithium ion battery charging and discharging rate performance, and lithium ion in anode, electrolyte, are directly related to the ability to migrate and the interface between them, all the influencing factors of lithium ion migration velocity (these factors can be equivalent to battery internal resistance), will influence on the performance of the lithium ion battery charging and discharging rate. In addition, the battery internal cooling rate, is also an important factor to influence the ratio performance, if the cooling rate is slow, big ratio when the charge and discharge accumulated heat transfer out, will seriously affect the safety of the lithium ion battery and life. Therefore, the research and improvement of the lithium ion battery charging and discharging rate performance, mainly from the lithium ion migration velocity and improved battery internal cooling rate two aspects.

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

Lithium ion embedded on the inside of the positive/negative active material take off and embedding rate, namely lithium ion ran out from inside the positive/negative active material, the speed of the positive/negative or from inside the surface into the active substances, to find a "home" how fast, this is an important factor affect the charging and discharging rate.

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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 soon reach the destination, some late for a few hours, someone ran to the blackout, quit halfway.

Lithium ion in the movement and diffusion of positive/negative, basic about the same, and marathon run slow, fast, and path length is differ, their choice of the end of the game in the time all of (run).So, we don't want to run a marathon, everyone ran best hundred meters, the distance is short enough, everyone can quickly reach the finish line, in addition, the runway to wide enough, don't crowded each other, also do not want a winding road, straight line is the best, to reduce the game more difficult. In this way, the referee a voice to ring, hammering together towards the end of the game quickly over, ratio performance.

In the positive CaiLiao Chu, we hope the pole piece should be enough thin, namely, the smaller the thickness of the active material that is equal to shorten the distance of the race, so I hope as soon as possible to improve the anode material compaction density. Within the active material, want to have enough hole clearance, lithium ion to set aside the game channel, at the same time, these "runway" to uniformly distributed, not in some places there are, in some places, it will optimize the structure of the anode materials, change the distance between the particles and the structure, uniform distribution. The above two points, it is contradictory, and improve the compaction density, while the thickness thinning, but the gap will be smaller particles, the runway is crowded, on the other hand, maintain a certain particle clearance, is not conducive to the thin material. So I need to find a balance, in order to achieve the best lithium ion migration rate.

In addition, the anode material of different materials and lithium ion diffusion coefficient has a significant impact. Therefore, choose the positive material of the lithium ion diffusion coefficient is higher, and is also the important direction of improve the rate performance.

Anode materials processing train of thought, similar to the anode material, is mainly from the aspects of the structure of the material, size, thickness, reduce the concentration difference in lithium ion in anode materials improve lithium ion diffusion capacity in anode materials.Carbon negative electrode material, for example, in recent years the study of nanostructured carbon materials (nanotubes and nanowires, nano ball, etc.), to replace the traditional anode layer structure, can significantly improve the specific surface area, the internal structure of anode materials and diffusion channels, thus greatly enhance the cathode material ratio performance.

2. Improve the ionic conductivity of electrolyte

Lithium ion in anode materials are/is playing racing, sports is swimming in the electrolyte.

Swimming competition, how to reduce the resistance of water (the electrolyte), has become the key to speed up. In recent years, a swimmer in the shark costume generally, the swimsuit can greatly reduce the resistance of water in the human body surface, so as to improve the performance of athletes, and become a very controversial topic.

Lithium ion in shuttling back and forth between positive and negative, as in electrolyte and a battery shell of inside swimming pool to swim, the ionic conductivity of electrolyte like water resistance, the lithium ion swimming speed has great impact. Organic electrolytes used in lithium ion batteries, whether liquid electrolyte, or solid electrolyte, the ionic conductivity is not very high. The resistance of the electrolyte become an important part of the whole battery resistance, effect on the performance of lithium ion battery high-magnification nots allow to ignore.

In addition to improve the ionic conductivity of electrolyte, also need to focus on chemical stability and thermal stability of electrolyte. In the big ratio to charge and discharge, battery range is very wide electrochemical window, if the chemical stability of the electrolyte is not good, easy in the anode material surface oxidation decomposition, influence the ionic conductivity of the electrolyte. Thermal stability of electrolyte is security and cycle life of lithium ion battery has very big effect, because the thermal decomposition of electrolyte will produce a lot of gas, on the one hand, the battery safety hidden trouble, on the other hand some of the gas on the SEI film on the surface of the cathode damage, affecting the cycle performance.

Therefore, choose to have higher lithium ionic conduction ability, good chemical stability and thermal stability, and matching the electrolyte and electrode material is to improve the performance of lithium ion battery ratio is an important direction.

3. Reduce the internal resistance of the battery

Here involves several different material and the interface between the material, they formed by the resistance value, but will have an effect on ion/electrical conduction.

General within the positive active material can add conductive agent, thereby reducing active substances, with the positive matrix/active materials between the contact resistance, to improve the anode materials of electrical conductivity (ion and electron conductivity), increase rate performance. Different materials in the shape of a conductive agent, will have an effect on the battery internal resistance, thus influence its performance ratio.

Set fluid (ears) extremely negative is lithium ion battery for electrical power transmission carrier with the outside world, collection of fluid resistance ratio of battery performance also has a great influence. Therefore, by changing the material, size, set fluid extract method, connection technology and so on, can improve the performance and cycle life of lithium ion batteries.

Electrolyte and anode materials are infiltrating extent, will affect the interface between electrolyte and electrode contact resistance, thus affect the ratio of the battery performance. The amount of electrolyte, viscosity, impurity content, is the porosity of the anode materials, etc., will change the resistance of the electrolyte and electrode contact, is to improve the rate performance of the important research direction.

Lithium ion battery in the process of the first cycle, as lithium ion anode, embedded in the cathode will form a solid electrolyte layer (SEI) film, although SEI film has good ionic conductivity, but will still be on lithium ion diffusion has a certain effect, especially when large rate charge and discharge. With the increase of cycling times, SEI will continue to fall off, stripping, deposition in the cathode surface, leading to negative resistance increases gradually, become the influence factors of circulation ratio. Therefore, control the change of the SEI film, also can improve lithium ion battery recycling ratio in the process of performance for a long time.

In addition, the isolation membrane fluid absorption rate and porosity of lithium ion also affected by sex, also can to some extent influence on the performance of the lithium ion battery ratio (relatively small).

The page contains the contents of the machine translation.