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Is the important process in the process of lithium-ion battery production, into in the cathode surface to form a layer of passivation layer, namely the film of solid electrolyte interface (SEI), the stand or fall of SEI film since the influence to the battery cycle life, electrochemical properties such as stability, self-discharge, safety, satisfy the secondary battery seal "maintenance", while different into a process of formation of the SEI film is different, there is also a very big difference influence on the performance of the battery.
Traditional way of small current prefi lled help stabilize the SEI film formation, but small current charging for a long time can lead to formation of the SEI film impedance increases, which affects the lithium ion battery discharge performance, process time influence the production efficiency.
Different lithium battery systems have different chemical processes. This paper analyzes the lithium iron phosphate battery system.
The formation process of lithium iron phosphate system is usually selected as follows:
Charging current 0.05 C ~ 0.2c, the cut-off voltage of 3.6 ~ 3.7 V, charging cut-off current 0.025c ~ 0.05 C, static set a period of time (10-20min), 0.1 ~ 0.2c discharge to 2.5v, and static for a period of time (20-60min).
Under different charging and discharging mechanisms, the difference of charging current affects the formation and quality of the SEI, the static time and the charging current affect the process time of the battery.
Battery of lithium iron phosphate system into process need to choose the appropriate cut-off voltage, from material on the crystal structure, when the charging voltage is greater than 3.7 V, may lead to damage the lattice structure of lithium iron phosphate, thus influence on the performance of the cell cycle.
Partial internal resistance experiment and poly-slice SEM observations also prove the following conclusions:
1. Reducing the reduced to the voltage and reducing the time can effectively reduce the production of the negative electrode surface, so as to get a relatively smooth negative electrode plate.
This is because when turned to high voltage, battery internal gas rate faster, so that the gas inside the battery does not discharge in time, deposition in the surface of the diaphragm, diaphragm contact with negative balance.
In the process of lithium ion removal, the influence of the contact imbalance between the two is caused by overembedding of lithium ions in some areas, causing the negative surface to be not smooth, and finally affecting the battery performance.
2. After being turned into the battery internal resistance test, the internal resistance of the battery can be reduced by reducing the reduced to the voltage and reducing the time.
The high internal resistance caused by high voltage is also not smooth with the negative surface, which is related to the formation of white spots, because the white spot is a lithium compound, and the internal resistance of the battery is too large.
3. In the process design, it can improve the first charge and discharge capacity of the battery and improve the circulation performance of the battery.
Too much into the voltage from the cathode surface deposition of lithium and its compounds, improves the irreversible capacity of the battery, will inevitably affect the capacity of the battery, as a result of the existence of lithium and its compounds, result in the battery charge and discharge cycle capacity attenuation faster and faster, affect the battery cycle life.