For a good lithium-ion battery, the capacity balance will not change in its cycle, and the initial capacity of each cycle must be a certain value. In fact, the situation is more complicated. Any side reactions that can initiate or consume lithium ions can cause the battery’s capacity balance to change. This will have a strong impact on battery cycle performance.
Many factors affect the life of a lithium-ion battery, but the main reason is that the amount of lithium ion involved in power delivery decreases. It should be noted that the amount of lithium in the battery has not decreased, but there are fewer “active” lithium ions. They are blocked in other areas where the active channel is blocked and cannot participate in the cyclic charging and discharging process.
Then, as long as we find out where the lithium ions that should participate in the redox reaction went, we can identify the capacity reduction process and take targeted measures to delay the capacity failure of the lithium-ion battery and improve the life of lithium-ion batteries.
- Support lithium metal
Because of past failures, we know that lithium does not have to be in the form of lithium metal found in lithium-ion batteries. Lithium elements are either in the form of metal oxides, lithium-carbon compounds or ions.
On a negative electrode, lithium falls easily. For some reason, when lithium ions move to the surface of the negative electrode, some lithium ions do not enter the active substance of the negative electrode to form a stable compound, and are placed on the electrode not good enough to turn into lithium after they gain electrons. , and no longer participate in the next cycle, resulting in reduced capacity.
This situation is usually caused by several reasons: the load exceeds the cut-off voltage; Fast charging; Bad things are not enough. When it is overcharged or the cathode material is insufficient, the cathode cannot accept the lithium ions that migrate from the cathode, resulting in the deposition of lithium failure. During a high charge, the number of lithium ions reaching the negative electrode in a short time is large, resulting in a breakdown and deposit. The supply of lithium metal will cause the life to be shortened, but also lead to short circuits and poor quality, causing serious safety problems.
To solve this problem, the positive and negative elements should be mixed well, and the working condition of the lithium-ion battery should be good enough to avoid exceeding the working limit. Of course, with increased efficiency, the working life in the area can also be improved.
- Analysis of the cathode material
Although lithium has metal oxides as a cathode material with sufficient stability, they will continue to be examined in the process of long-term use, and some weak electrochemical materials (such as Co3O4, Mn2O3, etc.) and some flammable gases will be emitted. Which destroys the stability between the electrodes and causes irreversible power loss.
This situation is especially evident when you transfer it, and sometimes even violent tests and gassing will occur, which will not only affect the capacity of the battery, but also cause great danger to safe life. In addition to effectively limiting battery discharge voltage, improving the chemistry and stability of the cathode material is also a possible way to reduce the rate of degradation and service life.
- SEI film on the electrode surface
As mentioned earlier, for lithium-ion batteries with carbon material as the negative electrode, during the first cycle, the electrolyte will form a solid film of electrolyte (SEI) on the electrode surface. Different negative electrode materials will have some differences, but the part of the SEI film is mainly composed of lithium carbonate, lithium alkyl ester, lithium hydroxide, etc. Of course, there are also salt test products, as well as some polymers.
The process of forming the SEI membrane will consume the lithium ions in the battery, and the SEI membrane is unstable. It will continue to break down during the cycle, exposing new carbon elements, and then reacting with electrolytes to form a new SEI membrane, which will continue to cause the loss of lithium ions and electrolytes in -continue, resulting in the failure of the battery. SEI film has some thickness. Although lithium ions can penetrate, the SEI film will block some diffusion channels on the surface of the negative electrode, which does not facilitate the diffusion of lithium ions into the material of the negative electrode, which will also reduce battery capacity.
- Effects of electrolytes
In the continuous diffusion process, the electrolyte will be continuously analyzed and changed due to the limits of chemical stability and thermal stability, which will accumulate to a long time. Time, causing a decrease in electrolyte concentration, insufficient seepage and poor quality, insufficient charge and discharge reaction, resulting in failure in the ability to use. Because the oxidation potential of impurity is usually lower than the positive potential of lithium-ion batteries, it is easy to oxidize on the positive surface, and the oxide is reduced to the negative electrode, and continues to consume the good and the bad things work, do it yourself. -wind, that is to change the discharge of the battery if it is not used.
- Sounds good and bad
The active elements of the positive and negative electrodes are fixed in the fruit thanks to the adhesive. In the process of long-term use, due to the failure of the adhesive and the mechanical vibration of the battery, the active elements of the positive and negative electrodes continue to fall and enter the solution electrolyte, which leads to continuous reduction. Active substances that can participate in electrochemical reactions and continuous reduction in the battery life cycle.
- External use
Lithium-ion batteries have reasonable operating conditions and limits, such as charge discharge voltage, charge discharge rate, operating temperature limit, storage temperature limit, etc. However, in actual use, abuse beyond acceptable limits is common. Unreasonable use for a long time will cause an irreversible chemical reaction in the battery, damage the battery system, accelerate the battery’s aging, and lead to a rapid reduction in service life. In the worst case, it will also cause a security emergency.
- Lithium-ion battery storage
The internal problem of lithium-ion battery storage is that the temperature inside the battery is not controlled and the temperature continues to accumulate, which causes the temperature inside the battery to rise and go to face energy release factors such as combustion and explosion.
A battery is a high energy carrier. Of course, there are things that are dangerous. At the height of power, the greater impact of violent power releases is of increasing security concern. Gasoline, gas, acetylene and other high energy carriers have the same problem. There are many security incidents every year. Different electrochemical processes, different capacities, process methods, environmental conditions and the degree of use have a great impact on the safety of lithium-ion batteries.
Since the battery stores energy, in the energy release process, when the initial battery temperature and the total temperature are higher than the battery temperature, the temperature inside the battery will continue to increase. Lithium-ion batteries are composed of a highly active cathode material and an organic electrolyte. Under heating conditions, they often undergo aggressive chemical reactions. This reaction will produce excessive heat and even lead to “uncontrolled fire”, which is the main cause of dangerous battery accidents.
