The impact of pretreatment process on the performance of lithium ion battery 18650 price!

　　lithium ion battery 18650 price have become the focus of many battery companies and automobile manufacturing companies due to their good safety and comprehensive electrochemical properties. If there are trace amounts of moisture and free acid in the organic electrolyte of a lithium-ion battery, it will react with the lithium salt in the electrolyte to form fluoride and deposit on the surface of the negative electrode. The existence of this deposit will play an important role in the formation of the SEI film. .

　　However, excessive water and acid content will not only cause the decomposition of the lithium salt LiPF6, but also damage the SEI film. The reaction rate between LiPF6 and water increases as the temperature increases. The reaction rate constant at 40°C is 3 to 4 times that at 20°C, and at 60°C it increases to 8 to 12 times that at 20°C. The decomposition of lithium salt and the generation of HF gas will cause the battery cycle life to decay, cause serious gas swelling, and pose certain safety issues.

　　This article focuses on the lithium iron phosphate 20Ah power battery with high moisture content and difficult to dry. It uses the SEI film-forming reaction that occurs on the electrode surface during the first charge to remove the moisture inside the battery, and studies the effect of different pretreatment processes on battery moisture removal. Investigate the effects of preformation process and high-temperature storage on battery performance (capacity, electrode status, battery thickness, and internal resistance).

　　1. Experiment

　　1 battery production

　　Use lithium iron phosphate as the positive electrode material and graphite as the negative electrode material, mix them with binders, conductive agents, and solvents in a certain proportion to form positive and negative electrode slurries, and then evenly coat them on the surfaces of aluminum foil and copper foil, dry them, and roll them. Press, cut and dry to make the pole pieces required for the experiment. The pole pieces are rolled, assembled, injected, pre-treated, sealed, etc. to produce a square lithium iron phosphate power battery with a nominal capacity of 20Ah.

　　2 Pre-formation process and formation process

　　The preformation process is shown in Table 1 and Table 2

　　The formation process is shown in Table 3 and Table 4.

　　3 test equipment

　　All electrical performance tests are conducted using the ArbinBT2000 battery testing system. High-temperature storage uses MTL-02S thermostatic box for temperature control.

　　2. Results and discussion

　　This experiment was conducted under the condition that the moisture inside the battery is relatively high. For lithium iron phosphate power batteries with large battery capacity, when the internal moisture is high, it is easy to cause metal lithium to precipitate, affecting the battery's capacity and even creating a dead zone. In addition, the moisture inside the battery will react with lithium salt (lithium hexafluorophosphate) to generate hydrogen fluoride (HF). Since HF is highly corrosive, it will have certain side effects on both the positive and negative electrodes inside the battery, which will definitely affect the cycle life of the battery. In this case, consider adopting different pretreatment processes, high-temperature storage and other methods to eliminate moisture.

　　1Influence of preprocessing process

　　Consider changing the pretreatment parameters to achieve complete reaction and eliminate excess moisture and gas. The two pretreatment processes are shown in Table 1 and Table 2. The test results of charging, discharging capacity, internal resistance and thickness of the battery after different pretreatment processes are shown in Table 5. It can be seen from Table 5 that the average discharge capacity of process 1 is 20.700Ah, which is greater than the average discharge capacity of process 2, 20.684Ah. The two processing processes have basically no impact on the internal resistance of the battery. When the formation voltage is 3.65V, the pretreatment process has no effect on the battery thickness. When the formation voltage rises to 3.9V, the thickness of the battery in process 1 is 28.34mm, which is significantly lower than the 28.51mm in process 2. This shows that increasing the pretreatment time has no advantage in eliminating gas inside the battery.

　　Taking into account the three factors of discharge capacity, internal resistance, and thickness, process 1 is better than process 2, indicating that extending the pretreatment time did not achieve the expected effect.

　　2 Impact of high temperature storage

　　Since the reaction rate between LiPF6 and water accelerates as the temperature increases, the decomposition of the lithium salt will produce HF gas, causing the battery to bulge and reduce its cycle life. After pretreatment, high-temperature storage at 45°C was added for 8 hours to speed up the reaction and remove excess moisture inside the battery. After pre-processing process 1, the test results to examine the impact of high-temperature storage on battery charging, discharging capacity, internal resistance and thickness are shown in Table 6.

　　It can be seen from Table 6 that the average discharge capacity without high-temperature treatment is 20.700Ah, which is greater than the average discharge capacity of 20.623Ah after 8 hours of high-temperature storage. The internal resistance of the battery increased slightly after being left at high temperature for 8 hours. This may be because the reaction was accelerated by standing at high temperature, resulting in a thick film on the electrode surface. The average battery thickness was 28.38mm without high-temperature standing treatment, and the battery thickness was 28.62mm after high-temperature standing treatment for 8 hours. This shows that high-temperature standing can indeed accelerate the reaction between water and lithium salts, producing more HF. It is beneficial to the consumption of water, but the gas after the reaction cannot be discharged well and remains inside the battery, causing the battery to bulge.

　　3. Influence of formation cut-off voltage

　　By adjusting the formation voltage, the dead zone of the negative electrode sheet and lithium precipitation caused by moisture can be avoided, and irreversible capacity loss can be reduced as much as possible. The morphology of the negative electrode plate after battery formation and dissection is shown in Figure 1. A summary of different experimental conditions and electrode plate states is shown in Table 7.

　　It can be seen from the experimental results that after high-temperature storage is increased, a large amount of lithium is precipitated in the pole pieces, which will cause a large loss of lithium and affect the performance of the battery. In the four sets of experiments without adding high-temperature storage processes, due to the influence of moisture inside the battery, although the lithium precipitation phenomenon can be avoided under the formation cutoff voltage of 3.65V, there is a small amount of dead zone, which affects the capacity of the battery. . Under the condition of 3.9V formation cut-off voltage, when pre-formation process 1 is used, the pole piece is in the best condition, without dead zone and lithium precipitation. However, after pre-formation process 2 is used, there is a small amount of lithium precipitation on the negative electrode piece. After comprehensive analysis, it can be seen that the pole piece is in the best condition when the preformation process 1, no high-temperature storage, and the formation cutoff voltage is 3.9V.

　　3. Conclusion

　　The presence of trace amounts of moisture in lithium-ion batteries can have a significant impact on battery performance. For situations where the battery moisture is high, the various performance of the battery and the state of the negative electrode can be improved by adjusting the pre-formation process, high-temperature storage and formation voltage. Finally, it was discovered that although standing at high temperature can speed up the reaction and force the water to react completely, it cannot escape from the battery, causing the battery to bulge and have a low capacity. After increasing the formation cut-off voltage from 3.65V to 3.9V, the thickness of the battery becomes thinner and the capacity is better. At the same time, after dissection, the negative electrode plate has no dead space and lithium precipitation.

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