batteries aaa.Effects of stepped formation process and constant current formation process on lithium battery performance

　　After the liquid injection of the lithium battery is completed, during the first charge and discharge process, the electrode material and the electrolyte will undergo an electrochemical reaction at the solid-liquid interface to form a solid electrolyte interface film (SEI film) covering the surface of the electrode material. SEI The quality of the membrane directly determines the cycle performance of the battery. SEI is composed of Li2O, LiF, LiCl, Li2CO3, LiCO2-R, alkoxide and non-conductive polymer. It has a multi-layer structure. The side close to the electrolyte is porous and the side close to the electrode is dense. On the one hand, the formation of the SEI film consumes part of the lithium ions, which increases the irreversible capacity of the first charge and discharge and reduces the charge and discharge efficiency of the electrode material; on the other hand, the SEI film is insoluble in organic solvents and can exist stably in organic electrolyte solutions. Can improve the cycle life of lithium batteries.

　　The formation process has an important impact on the formation of the SEI film of lithium batteries and directly affects the performance of the battery. Usually the formation potential of the SEI film is in the range of 0.6V-0.8V, so the current in the initial stage of formation is often kept at a very small state to ensure that the SEI film is formed more densely and is conducive to improving the cycle life.

　　Although the traditional low-current precharging method helps to form a stable SEI film, long-term low-current charging will increase the resistance of the formed SEI film, thus affecting the battery cycle, rate performance, etc.; the length of the formation time will also Affects the formation of the SEI film of the battery, because the formation of the lithium-ion battery is a first activation process. As charging proceeds, the internal voltage of the battery increases and is accompanied by the generation of gas. Once the gas production rate is higher than the exhaust of the liquid injection hole rate, the gas will accumulate between the separators inside the battery, which will affect the formation of the SEI film on the surface of the negative electrode. Therefore, it is necessary to select the appropriate current and formation time.

　　At present, the formation process is mainly divided into two categories, the multi-step stepped formation process and the constant flow formation process. So which formation process is more suitable?

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

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　　Group A multi-step formation process:

　　Charging (0.05CC/4h→0.1CC/2h→0.2CC/1h→0.4CCCV/4.2V→0.02Ccut)→Station 0.5h→Discharge (0.5C to cutoff voltage)→Station 0.5h, cycle three times and then 0.2C /2h, charge to 4.0V.

　　Group B adopts constant current formation process:

　　Charging (0.2C/2.5h)→Standing for 12h→Discharging (0.2C cut-off voltage)→Standing for 0.5h→Charging (0.2C/4.2V, 0.02Ccut-off)→Standing for 0.5h→Discharging (0.2C cut-off voltage)→ Rest for 0.5h → charge (0.2C/4.0V).

　　As you can see, Group A uses slow charging with a small current, gradually increasing the current, and reducing the charging time at the same time. Group B uses a 0.2C charge/discharge current, and there are not many parameters that change. So what are the differences between these two formation processes on the SEI and electrochemical performance of the battery?

　　1. SEI film

　　Through SEM analysis of the negative electrode surfaces of both groups of batteries, it can be found that the electrode surface is covered by the SEI film, but it is impossible to see any difference in thickness or coverage area between the two.

　　2. Electrochemical performance

　　By analyzing the basic electrochemical properties of the two sets of batteries, it can be concluded that the capacity of the lithium battery cathode material using the stepped formation process is 3mAh/g higher than that of the constant current formation process, and the charge and discharge efficiency of the entire battery is higher. After 50 cycles, the specific capacity decay rate of the constant flow formation process is slower than that of the step formation process.

　　In terms of first effect, the constant current type is lower than the step type, but the second cycle is higher than the step type. This also explains that the degree of reversible reaction of the battery formed by the constant current type is higher than that of the step type formation process, that is, Less irreversible capacity loss.

　　3. SEI component analysis

　　By analyzing the SEI of the two groups of batteries, the following conclusions can be drawn:

　　1. The lithium ion content on the CMS surface of the negative electrode of the lithium-ion battery after step-type formation is higher than that of constant-current formation. This is because a variety of lithium-containing compounds are formed under different current densities, resulting in excessive lithium content.

　　2. XPS results show that Li2CO3 or LiCO2-R exists in the SEI films of both groups of batteries.

　　3. The SEI thickness formed by two chemical formation processes is greater than 3nm.

　　Through the analysis of the above two sets of batteries with different formation processes, it can be concluded that different current sizes and times have different effects on battery performance. The composition and properties of the SEI film are also different, which will inevitably affect the performance of the battery.

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