How do you improve the cycle life of lithium battery?

Battery becomes not so durable after using for some time, so does the capacity, which means the cycle life is constantly attenuated.

The attenuation of cycle life, in fact, is the actual available capacity of the battery at present, relative to its rated capacity when leaving the factory, a changing trend.

For an ideal lithium-ion battery, the capacity balance does not change over the course of the cycle, and the initial capacity in each cycle should be a certain value. Any side reaction that can generate or consume lithium ions may lead to the change of battery capacity balance. Once the battery capacity balance state changes, this change is irreversible and can be accumulated through multiple cycles, which has a serious impact on the battery cycle performance.

There are many factors that affect the cycle life of lithium-ion batteries, but the underlying 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 does not decrease, but there are fewer "activated" lithium ions, which are confined in other places or blocked in the active channels, unable to freely participate in the cycle of charging and discharging.

So, as long as we figure out where all the lithium ions that should be involved in the REDOX reaction go, we can figure out the mechanism of capacity decline and take targeted measures to delay the capacity decline trend of lithium battery and improve the cycle life of lithium battery.

1. Deposition of lithium metal

Based on the above analysis, we know that there should be no metal form of lithium in lithium ion batteries. Lithium elements are either in the form of metal oxides and lithium carbon compounds, or in the form of ions.

Lithium metal deposition generally occurs on the surface of the cathode. Due to certain reasons, when lithium ions migrate to the surface of the negative electrode, some lithium ions do not enter the active material of the negative electrode to form stable compounds, but deposit on the surface of the negative electrode after gaining electrons to become lithium metal, and no longer participate in the subsequent cycle process, resulting in the reduction of capacity.

In this case, there are several reasons: charging over the cut-off voltage; High power charging; Insufficient anode material. When overcharging or the cathode material is insufficient, the negative electrode cannot hold the lithium ions migrated from the positive electrode, resulting in the deposition of lithium metal. When charging at a high rate, the amount of lithium ions reaching the negative electrode in a short time is too much, resulting in blockage and deposition.

Lithium metal deposition, not only will cause a decline in cycle life, when serious will lead to positive and negative short circuit, causing serious safety problems.

To solve this problem, it is necessary to have a reasonable ratio of anode and cathode materials, and strictly limit the service conditions of lithium batteries to avoid exceeding the service limit. Of course, starting from the multiplier performance, can also locally improve the cycle life.

2. Decomposition of anode materials

Lithium metal oxides, as anode materials, have sufficient stability. However, they will continue to decompose in long-term use, producing some electrochemical inert substances (such as Co3O4, Mn2O3, etc.) and some combustible gases, which will destroy the capacity balance between electrodes and cause irreversible loss of capacity.

This situation is particularly obvious in the case of over charging, sometimes even violent decomposition and gas release, not only affect the battery capacity, but also cause serious safety risks.

In addition to strictly limiting the cut-off voltage of battery charging, improving the chemical and thermal stability of anode materials is also a feasible method to reduce the rate of cycle life decline.

SEI film on the electrode surface

Said before, in front of the carbon materials as anode of lithium ion battery, in the process of primary loop, the electrolyte will form a solid electrolyte layer on the surface of the electrode (SEI) film, different anode materials have some differences, but the SEI film composition is mainly composed of lithium carbonate, alkyl lithium, lithium hydroxide, etc, of course, there are salt decomposition product, there are some polymer, etc.

EI film is not stable and will constantly break during the cycle to expose new carbon surface and then react with electrolyte to form a new SEI film, which will continuously cause the continuous loss of lithium ions and electrolyte and lead to the reduction of battery capacity. SEI film has a certain thickness. Although lithium ions can penetrate, SEI film will cause partial diffusion channel blockage on the surface of the negative electrode, which is not conducive to the diffusion of lithium ions in the negative electrode material, which will also cause the decline of battery capacity.

4. Electrolyte effects

In the continuous cycle, due to the limitation of chemical stability and thermal stability, electrolytes will decompose and volatilize continuously and accumulate over a long period, leading to a decrease in the total amount of electrolytes, insufficient infiltration of positive and negative electrode materials, incomplete charging and discharging reactions, and a decrease in the actual use capacity.

Electrolyte containing active hydrogen material and iron, sodium, aluminum, nickel and other metal ion impurities. Because the oxidation potential of impurities is generally lower than the positive potential of lithium ion battery, easy to oxidation on the surface of the positive electrode, the oxide and reduction in the negative electrode, the constant consumption of positive and negative active substances, causing self-discharge, that is, in the case of abnormal use to change the battery discharge. Battery life is determined by the number of cycles of charge and discharge, and the electrolyte containing impurities directly affects the number of cycles.

The electrolyte also contains a certain amount of water, which will react with LiFP6 in the electrolyte to produce LiF and HF, which in turn will destroy the SEI film, generate more LiF, cause LiF deposition, constantly consume the active lithium ions, and reduce the battery cycle life.

From the above analysis, it can be seen that electrolyte has a very important impact on the cycle life of lithium ion battery. The selection of appropriate electrolyte can significantly improve the cycle life of the battery.

5. The isolation membrane is blocked or damaged

The function of the separator is to separate the positive and negative electrodes of the battery and prevent short circuit. During the cycle of li-ion battery, the drain and failure of the barrier film is one of the important reasons for the deterioration of the early performance of the battery. This is mainly due to the lack of electrochemical stability and mechanical properties of the isolation membrane itself, as well as the deterioration of the infiltration of the electrolyte to the isolation membrane in the process of repeated charging. Due to the drying of the isolation film, the ohmic resistance of the battery increases, leading to the blockage of charging and discharging channels, incomplete charging and discharging, the battery capacity cannot be restored to the initial state, greatly reducing the battery capacity and service life.

6. Positive and negative electrode materials fall off

Is the cathode active material, is through the adhesive and fixed in the base of the above, in the process of long-term use, due to the failure of adhesive and battery by mechanical vibration, is the cathode active material loss, into the electrolyte solution, it can lead to participate in the electrochemical reaction of the active material shrinking declining cycle life of the battery.

The long-term stability of the binder and the good mechanical properties of the battery will slow down the decline of the battery cycle life.

7. External use factors

Lithium ion battery has reasonable use condition and range, such as cut-off voltage of charge and discharge, charge and discharge ratio, working temperature range, storage temperature range and so on. However, in actual use, abuse beyond the allowable scope is very common. Long-term irrational use will lead to irreversible chemical reactions inside the battery, damage the battery mechanism, accelerate the aging of the battery, and cause a rapid decline in cycle life. In serious cases, it will also cause safety accidents.

The page contains the contents of the machine translation.