21700 battery

Briefly describe the impact of the formation process of 21700 battery on the performance of the battery

The impact of the formation process of 21700 battery on the performance of the battery. For the formation of lithium iron phosphate battery, due to the low intrinsic conductivity of lithium iron phosphate, activation is relatively difficult. Therefore, it should be considered that before the formation, the electrolyte fully infiltrates the electrode, and the electrode is left at room temperature for more than 7 hours, and aged at high temperature (50-60) for more than 2 hours. It is best to consider low current and high voltage formation during the formation. The impact of the formation process of 21700 battery on the performance of the battery Formation is an important process in the production process of 21700 battery. During the formation, a passivation layer, namely the solid electrolyte interface film, is formed on the surface of the negative electrode. The quality of the SEI film directly affects the electrochemical properties of the battery, such as cycle life, stability, self-discharge, and safety, and meets the requirements of "maintenance-free" sealing of secondary batteries. Different formation processes form different SEI films, and the impact on the performance of lithium batteries is also very different. The traditional small current pre-charging method is conducive to the formation of a stable SEI film, but long-term small current charging will increase the impedance of the formed SEI film, thereby affecting the rate discharge performance of lithium-ion batteries, and the long process time affects production efficiency. The formation process of lithium iron phosphate system usually selects the following charging current 0.05C~0.2C, cut-off voltage 3.6~3.7V, charging cut-off current 0.025C~0.05C, after standing for a period of time, 0.1~0.2C discharge to 2.5V, and stand for a period of time. Under different charging and discharging mechanisms, the difference in charging current affects the formation and quality of SEI, and the standing time and charging cut-off current affect the formation process time of the battery. The battery formation process of lithium iron phosphate system needs to select a suitable cut-off voltage. From the perspective of material crystal structure, when the charging voltage is greater than 3.7V, the lattice structure of lithium iron phosphate may be damaged, thereby affecting the cycle performance of the battery. The results of some internal resistance experiments and electrode SEM observations also prove the correctness of the following conclusions: 1. Appropriately reducing the formation voltage and shortening the formation time can effectively reduce the generation of lithium precipitation on the negative electrode surface, thereby obtaining a negative electrode with a smoother surface. This is because when the formation voltage is high, the gas production rate inside the battery is fast, so that the gas inside the battery cannot be discharged in time, and deposits on the surface of the diaphragm, affecting the contact balance between the diaphragm and the negative electrode. In the process of lithium ion deintercalation, the lithium ions are over-embedded in some areas due to the imbalance of contact between the two, causing the surface of the negative electrode to be rough, and finally affecting the battery performance. 2. After the internal resistance test of the iron-lithium battery after formation, it was found that the internal resistance of the battery can be reduced by appropriately reducing the formation voltage and shortening the formation time. The high internal resistance caused by the high formation voltage is also related to the rough surface of the negative electrode and the formation of white spots, because the white spots are lithium compounds, which have poor conductivity and therefore the internal resistance of the battery is large. 3. Properly reducing the formation voltage in the formation process design can increase the initial charge and discharge capacity of the lithium battery and improve the cycle performance of the battery. Excessive formation voltage can easily cause lithium and its compounds to deposit on the surface of the negative electrode, increase the irreversible capacity of the lithium battery, and inevitably affect the capacity of the battery. Due to the presence of lithium and its compounds, the capacity of the battery decays faster and faster during the charge and discharge cycle, affecting the battery cycle life. Lithium iron phosphate battery formation recommendations 1) Discharge gram capacity of the positive electrode 2) The battery's first charge and discharge efficiency 3) Battery voltage platform 4) Constant current and constant voltage charging ratio of lithium batteries 5) The difference in charge and discharge voltage platforms of lithium batteries 6) The relationship between the capacities of 0.2c and 0.5c Steps in the lithium battery formation process The formation process of the SEI film during the lithium battery formation process specifically includes the following four steps: Step ①: Electrons are transferred from the inside of the current collector-conductive agent-graphite particles to point A where the SEI film is to be formed; Step ②: The solvated lithium ions diffuse from the positive electrode to point B on the surface of the SEI film being generated under the wrapping of the solvent; Step ③: The electrons at point A diffuse to point B through the electron tunneling effect; Step ④: The electrons that jump to point B react with lithium salts, solvated lithium ions, film-forming agents, etc., and continue to generate SEI film on the surface of the original SEI film, thereby increasing the thickness of the SEI film on the surface of the graphite particles and eventually forming a complete SEI film. It can be seen that the overall reaction process of SEI formation can be specifically decomposed into the above four step-by-step reactions. The four step-by-step reaction processes determine the formation process of the entire SEI film. The formation of the 21700 battery is the first charge of the battery, which activates the active substances in the battery and generates a dense film on the surface of the anode to protect the entire chemical interface. Formation is also called activation. After the lithium battery is manufactured, the internal positive and negative electrode materials are activated through a certain charging and discharging method to improve the overall performance of the battery. Formation is a very complex process and is also a very important process that affects the performance of lithium batteries.

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