Analysis Approach for Low Capacity Issue of Battery Cells

The determination of low battery capacity (low capacity) for battery cells is based on a straightforward comparison between the post-formation (post-charging/discharging cycle) capacity and the designed capacity value. If the capacity measured after the formation process is lower than the designed value, the first response should be to confirm whether there are errors in the formation process settings (such as discharge current, charging time, cut-off voltage, and formation temperature). ①If the formation step settings are correct, it is necessary to change the testing point and re-perform the formation process on the battery cell to check if there are issues with the formation equipment or channels. ②Assuming no abnormalities are found in the formation data after changing the equipment, then the original equipment is likely problematic. ③If the re-test still shows low capacity, it can be confirmed that the low-capacity issue truly exists.

After confirming the existence of low capacity, it is necessary to further determine the frequency and severity of the low-capacity occurrences to grasp the actual situation of low capacity from an overall perspective. This requires a more systematic approach. Before conducting a systematic analysis, it is advisable to first disassemble the re-charged low-capacity battery cells to inspect the interface. If no issues are found, it is likely due to insufficient positive electrode coating weight or inadequate design margin. If there are interface problems, it may be due to other issues in the manufacturing process or design. Next, we will investigate the causes of low capacity from the design end and the process manufacturing end.

I. Design End

1. Material system compatibility: In particular, the compatibility between the negative electrode and electrolyte has a significant impact on battery cell capacity. For newly introduced negative electrodes or electrolytes, if repeated tests show that each battery cell experiences lithium plating and low capacity, there is a high likelihood of material mismatch. The reasons for mismatch may include: ①Inadequate density, thickness, or instability of the SEI (Solid Electrolyte Interphase) film formed during formation; ②Possible delamination of the graphite layer caused by PC (propylene carbonate) in the electrolyte; ③Excessively high designed areal density or compaction density, making the battery cell unable to adapt to high-rate charging and discharging.
2. Adequacy of capacity design margin: ①Starting from the gravimetric capacity of the positive electrode material: Due to errors in positive/negative electrode coating, formation cabinet accuracy, and adhesive effects on capacity, a certain capacity margin must be reserved during design. For new materials, accurate assessment of the gravimetric capacity of the positive electrode in the specific system is crucial. The same positive electrode material may not exhibit the same gravimetric capacity when paired with different negative electrodes or electrolytes. Factors such as formation rate, charge cut-off current, charge/discharge rate, and electrolyte type can all affect the performance of the positive electrode. Therefore, if the designed gravimetric capacity is too high, it can lead to an "overestimated" design capacity, resulting in "low capacity" in the actual formation process. ②From the perspective of negative electrode excess and high CB (capacity balance) value: Negative electrode excess can also affect the gravimetric capacity of the positive electrode to some extent, thereby impacting battery cell capacity. Negative electrode excess is not just about "no lithium plating." Increasing negative electrode excess beyond the lower limit of no lithium plating can improve the gravimetric capacity of the positive electrode by approximately 1% to 2%. However, when negative electrode excess is too high, the gravimetric capacity of the positive electrode will decrease due to the need for more irreversible lithium to form the SEI film during formation, resulting in reduced first-cycle discharge capacity.
3. Insufficient electrolyte injection and low electrolyte retention coefficient: When the electrolyte injection amount is low, the corresponding electrolyte retention amount will also decrease. When the electrolyte retention amount of the battery cell is insufficient, the lithium ion insertion/extraction effect at the positive and negative electrodes will be affected, leading to low capacity. When the electrolyte retention amount of the battery cell is insufficient, the positive and negative electrode sheets will be relatively dry, and a thin layer of lithium plating will form on the negative electrode, which can be considered a factor causing low capacity due to poor electrolyte retention.

II. Process Manufacturing End

1. Compliance with coating areal density process parameters: Whether the positive or negative electrode coating is too light can directly cause low battery cell capacity. When the positive electrode coating is too light, there will be no abnormalities in the fully charged cell interface. At this point, it is necessary to determine the problem by baking and measuring the weight of the positive electrode sheet. If the measured weight is less than the designed value, it indicates that the coating areal density is less than the set value. In addition, uneven coating on the positive or negative electrode (especially on the negative electrode) can also cause low capacity. Although heavier positive electrode coating may reduce the gravimetric capacity, the total capacity may not decrease and may even increase slightly.
2. Investigation of over-pressing during rolling: Over-pressing during rolling can damage the material structure, thereby affecting capacity performance. The most direct manifestation of over-pressing is a shiny appearance of the electrode sheet. Over-pressing of the positive electrode can cause the active material particles with damaged structures to be unable to properly insert/extract lithium ions, resulting in capacity decay. Over-pressing of the negative electrode can prevent it from accepting lithium ions from the positive electrode, leading to surface lithium plating and capacity decay.
3. Compliance with various tolerance requirements during assembly: Issues such as poor electrode alignment, separator wrinkles, etc., in the battery assembly process can cause internal short circuits or increased local resistance, affecting battery charging/discharging and resulting in reduced capacity. Wrinkles in the cell separator of the cell can also cause insufficient lithium insertion/extraction on the negative electrode, affecting cell capacity.
4. Abnormal water content: Excessive water content can also cause low capacity. When the water content of the electrode sheet exceeds the standard before electrolyte injection, the dew point of the glove box is not qualified, the water content of the electrolyte exceeds the standard, or when water is introduced during degassing or secondary sealing, the battery cell may experience low capacity.
5. Normalcy of humidity and temperature during cell production: Improper control of environmental humidity and temperature is crucial for battery performance. High humidity can cause water decomposition reactions inside the battery, damaging electrode materials and electrolytes; low temperature can slow down lithium ion diffusion, leading to reduced battery capacity.
6. Other factors:
   ①Battery-related issues: Metallic foreign matter or magnetic substances in the battery production process can potentially increase the self-discharge rate. Such battery cells may exhibit low capacity after formation.
   ②Storage time and conditions: Long storage time or improper storage conditions, such as high temperature and humidity, can cause electrode material aging and electrolyte degradation, leading to reduced battery capacity.

above steps, when step by step, can generally help to identify the cause of low capacity.