The low capacity of the battery cell is a visual judgment made based on the comparison between the lower cabinet capacity after capacity division and the design value. If the battery cell data shows that the capacity of the lower cabinet is lower than the design value, the first reaction is to confirm whether the capacity division process is set incorrectly (such as whether there are abnormalities in discharge current, charging time, cut-off voltage, and capacity division temperature); ① If there is no problem with the capacity division step setting, it is necessary to replace the test points and re divide the battery cells to see if there is a problem with the capacity division equipment and channels If there are no abnormalities in the capacity data after replacing the equipment, then there is a problem with the equipment If the low capacitance still exists after retesting, it can be confirmed that the low capacitance problem really exists.
After confirming the existence of low capacity, it is necessary to further confirm the frequency and severity of low capacity occurrence, and grasp the actual situation of low capacity as a whole. We're about to start analyzing. We need a more systematic approach. Before analyzing the system, the low capacity battery cells that were previously retested and fully charged can be disassembled to check the interface. If there are no issues, it is likely that the positive electrode coating is too light or the design margin is insufficient; If there is a problem with the interface, it may be due to other aspects of the process or design.
Next, we will investigate the reasons for low capacity from both the design and process aspects.
1 Design end
1. Is the combination of material systems reasonable
The matching between the negative electrode and electrolyte has a particularly significant impact on the capacity of the battery cell. For newly introduced negative electrodes or new electrolytes, if repeated tests show that the battery cells will precipitate lithium and have low capacity each time, there is a high possibility of material mismatch. The reason for the mismatch may be:
① The SEI film formed during formation is not dense enough, too thick, or unstable
② It is also possible that the PC in the electrolyte caused the graphite layer to peel off
③ It cannot be ruled out that the excessive compaction of the designed surface density may cause the battery cell to be unable to adapt to high rate charging and discharging.
2. Is the capacity design margin sufficient
① Starting from the gram capacity of the positive electrode material: Due to errors in positive and negative electrode coating, capacity distribution cabinets, and the influence of adhesive on capacity, a certain capacity margin must be left in the design. For new materials, accurate evaluation of the gram capacity of the positive electrode in this system is crucial. Not the same positive electrode has the same performance when paired with any negative electrode or electrolyte. The capacity ratio, charging cut-off current, charging and discharging rate, and type of electrolyte all affect the performance of the positive electrode. So if the designed capacity is too large, it will cause a situation where the designed capacity is "virtual high", resulting in the actual capacity of the divided cells being "low capacity".
② From the perspective of excessive negative electrode and high CB value, excessive negative electrode can also affect the positive electrode performance to a certain extent, thereby affecting the capacity of the battery cell. Excess negative electrode is not enough as long as lithium does not precipitate. Continuing to increase the negative electrode excess at the lower limit without lithium deposition will result in a 1% to 2% increase in positive electrode performance. When the negative electrode excess is too high, the positive electrode performance will decrease again, because more irreversible lithium is needed to form a film during formation, resulting in a decrease in the initial discharge capacity.
3. Is the injection volume insufficient and is the retention coefficient too low
When the injection volume is low, the corresponding retention capacity will also decrease. When the retention capacity of the battery cell is too low, the insertion and extraction of lithium ions in the positive and negative electrodes will be affected, leading to low capacity. When the liquid holding capacity of the battery cell is insufficient, the positive and negative electrode plates will be relatively dry, and a thin layer of lithium will be formed on the negative electrode, which can be a factor in the low capacity caused by poor liquid holding capacity.
2 Process end
1. Is the density of the coated surface strictly in accordance with the process parameters
Lightly coating the positive or negative electrode can directly cause low capacity of the battery cell. When the positive electrode coating is too light, there will be no abnormalities at the interface of the fully charged core. At this time, the problem needs to be determined by baking and measuring the weight of the positive electrode sheet. If the weighing weight is smaller than the design value, then there is a situation where the coating surface density is less than the set value. In addition, coating the positive or negative electrode on the positive and negative sides can also cause low capacity, with the negative electrode being mainly coated on a lighter side. Even if the positive electrode is coated heavily, although its performance may decrease, the total capacity may not decrease but may even increase.
2. Check if there is any overpressure in the roller press
Overpressure from rolling can damage the structure of the material, thereby affecting the capacity utilization. The most direct phenomenon of roller overpressure is the shining of the polarizer. The positive electrode experiences overvoltage, causing the active material particles with damaged structures to be unable to properly intercalate lithium, resulting in capacity decay. If the negative electrode experiences overvoltage, it cannot accept the lithium ions released from the positive electrode, which can slightly lead to surface lithium deposition and capacity decay.
3. Whether various tolerance requirements are met during the assembly process
During the battery assembly process, issues such as poor alignment of electrode plates and wrinkled separators can cause internal short circuits or increased local resistance in the battery, affecting battery charging and discharging and leading to a decrease in capacity. The battery cell diaphragm produces wrinkles inside the battery cell, and the areas where the wrinkles occur have insufficient lithium insertion in the negative electrode, which does not turn golden and affects the battery cell capacity.
4. Whether the water content exceeds the standard abnormally
Excessive water content can also cause low capacity. When the water content in the water electrode of the battery cell exceeds the standard before injection, the dew point of the injection glove box is not qualified, the electrolyte water content exceeds the standard, and moisture is introduced into the degassing seal, the battery cell may have low capacity.
5. Is the humidity or capacitance temperature normal during the production of battery cells
Improper control of environmental humidity and temperature can have an impact on batteries. High humidity may cause water splitting reactions inside the battery, damaging electrode materials and electrolyte; Low temperature, slow chemical reaction rate inside the battery, and difficulty in lithium ion diffusion can all lead to a decrease in battery capacity.
6. Other factors
① The battery itself has certain problems: some metal foreign objects or magnetic substances during the battery production process may potentially increase the self discharge rate of the battery cells. This type of battery cell may experience low capacity when divided.
② Storage time and conditions: If the storage time of the battery before capacity division is too long or the storage conditions are improper, such as high temperature and high humidity environment, it will cause chemical reactions inside the battery, leading to electrode material aging and electrolyte deterioration, thereby reducing the battery capacity.
