For the ideal lithium-ion battery, the capacity balance will not change in its cycle cycle, and the initial capacity in each cycle should be a certain value. Actually, the situation is much more complex. Any side reaction that can initiate or consume lithium ions may lead to the change of battery capacity balance. This will have a serious impact on the cycle performance of the battery.
There are many factors affecting the cycle life of lithium-ion batteries, but the fundamental reason is that the number of lithium ions involved in energy transfer is decreasing. It should be noted that the total amount of lithium in the battery has not been reduced, but there are less “activated” lithium ions. They are confined in other places or the active channel is blocked and cannot freely participate in the process of cyclic charge and discharge.
Then, as long as we find out where these lithium ions that should have participated in the redox reaction have gone, we can find out the mechanism of capacity decline, and take targeted measures to delay the capacity decline trend of lithium-ion batteries and improve the cycle life of lithium-ion batteries.
1. Deposition of metal lithium
Through the previous decomposition, we know that lithium should not exist in the metal form of lithium in lithium-ion batteries. Lithium elements either exist in the form of metal oxides, carbon lithium compounds or ions.
On the surface of the negative electrode, metal lithium is easy to deposit. For certain reasons, when lithium ions migrate to the negative electrode surface, some lithium ions do not enter the negative electrode active material to form stable compounds, but deposit on the negative electrode surface to become metal lithium after obtaining electrons, and no longer participate in the subsequent cycle process, resulting in a decrease in capacity.
This situation is generally caused by several reasons: charging exceeds the cut-off voltage; High rate charging; Insufficient negative material. When overcharged or the cathode material is insufficient, the cathode cannot accommodate the lithium ions migrated from the cathode, resulting in the deposition of metal lithium. During high rate charging, the amount of lithium ions reaching the negative electrode in a short time is too large, resulting in blockage and deposition.
The deposition of metal lithium will not only reduce the cycle life, but also lead to positive and negative short circuit, resulting in serious safety problems.
To deal with this problem, we should reasonably mix the positive and negative materials, and strictly limit the service conditions of lithium-ion batteries to prevent exceeding the service limit. Of course, starting from the magnification performance, the cycle life can also be improved locally.
2. Analysis of cathode materials
Although lithium containing metal oxides as cathode materials have sufficient stability, they will continue to be analyzed in the long-term use process, and some electrochemical inert substances (such as Co3O4, Mn2O3, etc.) and some combustible gases will sprout, which will destroy the capacity balance between electrodes and cause irreversible loss of capacity.
This situation is particularly obvious in the case of overcharge, and sometimes even violent analysis and gas release will occur, which will not only affect the battery capacity, but also cause serious safety risks.
In addition to strictly limiting the charging cut-off voltage of the battery, improving the chemical and thermal stability of the cathode material is also a feasible way to reduce the decline rate of cycle life.
3. SEI film on electrode surface
As mentioned earlier, for lithium-ion batteries with carbon material as negative electrode, during the initial cycle, the electrolyte will form a layer of solid electrolyte (SEI) film on the electrode surface. Different negative electrode materials will have certain differences, but the components of SEI film are mainly composed of lithium carbonate, lithium alkyl ester, lithium hydroxide, etc. of course, there are also salt analysis products, as well as some polymers.
The formation process of SEI membrane will consume lithium ions in the battery, and the SEI membrane is not stable. It will continue to break during the cycle, expose new carbon surfaces, and then react with electrolytes to form a new SEI membrane, which will continue to cause the continuous loss of lithium ions and electrolytes, resulting in the decline of battery capacity. SEI film has a certain thickness. Although lithium ions can penetrate, SEI film will block some diffusion channels on the negative electrode surface, which is not conducive to the diffusion of lithium ions in the negative electrode material, which will also reduce the battery capacity.
4. Influence of electrolyte
In the process of continuous circulation, the electrolyte will be continuously analyzed and volatilized due to the limitations of chemical stability and thermal stability, which will accumulate for a long time, resulting in the reduction of the total amount of electrolyte, insufficient infiltration of positive and negative materials, incomplete charge and discharge reaction, resulting in the decline of the actual use capacity.
Because the oxidation potential of impurities is generally lower than the positive potential of lithium-ion battery, it is easy to oxidize on the positive surface, and the oxide is reduced on the negative electrode, continuously consuming positive and negative active substances, causing self discharge, that is, changing the battery discharge under abnormal use.
The electrolyte also contains a certain amount of water, which will react with lifp6 in the electrolyte to process LIF and HF. HF will then destroy SEI membrane and generate more LIF, resulting in LiF deposition, continuous consumption of active lithium ions and decline of battery cycle life.
From the above decomposition, it can be seen that electrolyte has a very important impact on the cycle life of lithium-ion battery. Selecting appropriate electrolyte will clearly improve the cycle life of battery.
5. Falling off of positive and negative materials
The active substances of the positive and negative electrodes are fixed on the substrate through the adhesive. In the long-term use process, due to the failure of the adhesive and the mechanical vibration of the battery, the active substances of the positive and negative electrodes continue to fall off and enter the electrolyte solution, which leads to the continuous reduction of the active substances that can participate in the electrochemical reaction and the continuous decline of the cycle life of the battery.
6. External use factors
Lithium ion batteries have reasonable service conditions and ranges, such as charge discharge cut-off voltage, charge discharge ratio, working temperature range, storage temperature range, etc. However, in actual use, abuse beyond the permitted scope is very common. Long-term unreasonable use will lead to irreversible chemical reaction inside the battery, damage the battery mechanism, accelerate the aging of the battery, and cause a rapid decline in cycle life. In serious condition, it will also cause safety accidents.
7. Safety of lithium ion battery
The internal reason for the safety problem of lithium-ion battery is that the heat inside the battery is out of control and the heat is continuously accumulated, resulting in the continuous rise of the internal temperature of the battery, and its external performance is the violent energy release phenomena such as combustion and explosion.
Battery is a high-density carrier of energy. In essence, there are unsafe factors. The higher the energy density, the greater the impact of violent energy release, and the more prominent the safety problem. Gasoline, natural gas, acetylene and other high-energy carriers also have the same problems. There are countless safety accidents every year.
Different electrochemical systems, different capacities, process parameters, use environment and use degree have a great impact on the safety of lithium-ion batteries.
Because the battery stores energy, in the process of energy release, when the heat initiation and accumulation speed of the battery is greater than the heat dissipation speed, the internal temperature of the battery will continue to rise. Lithium ion batteries are composed of highly active cathode materials and organic electrolyte. Under heating conditions, they are very prone to violent chemical side reactions. This reaction will generate a lot of heat and even lead to “heat out of control“, which is an important reason for dangerous accidents of batteries.