7 Key Points to Systematically Analyze Low Capacity in Battery Cells, Especially During Mass Production
Capacity is one of the most critical performance indicators of lithium-ion batteries. Whether during sample validation or mass production, low capacity is one of the most troublesome problems. However, the causes of low capacity are often not singular—it usually results from a combination of factors involving design, material selection, process, operation, and testing.
This article introduces seven key points to help you establish a systematic approach to analyzing low capacity, particularly useful for failure analysis and improvement during the mass production stage.
1. Is the Low Capacity Really “Low”? — First Eliminate Formation Test Errors
When encountering a low-capacity issue, the first step is not to disassemble the cell, but to confirm whether the issue is real. In the formation and grading stage, parameter setting errors are the most common sources of false low capacity, such as:
Discharge current too high;
Charge cutoff current set too high;
Insufficient constant-voltage charging time;
Process file accidentally modified.
If the formation settings are correct, change the formation cabinet test point and re-test the cells. If the re-test still shows low capacity, the issue can be considered real. Meanwhile, it is recommended to keep 3 fully charged low-capacity cells for later interface analysis or comparison testing.
2. Is It an Occasional Problem or a Systematic Low Capacity? — First Determine “Frequency” and “Severity”
Once low capacity is confirmed, immediately clarify its distribution characteristics.
If the model shows long-term low capacity: the problem is mostly related to design or material selection. For example:
Cell design margin too small;
Positive and negative electrode matching not fully validated;
Long-term potential process anomalies (e.g., poor mixing uniformity, coating deviation).
If the low capacity occurs occasionally: focus first on process and operational changes. For example:
Was the negative electrode compressed too hard?
Was the aging time shortened?
Were key parameters recently modified (e.g., drying, electrolyte volume, formation current)?
Additionally, record the proportion of low-capacity cells and the degree of capacity deviation. Though this data may not directly reveal the root cause, it is very useful for evaluating whether to relax the capacity criteria or assess shortage risks.
3. What Can Disassembling 3 Cells Reveal? — Early Directional Judgment
Before conducting systematic analysis, you can first disassemble three re-tested low-capacity cells:
If interfaces appear normal → likely caused by light coating on the positive electrode or insufficient design margin;
If interfaces appear abnormal → likely related to process defects or material mismatch.
Though simple, this step often quickly identifies the analysis direction. Experienced engineers can often judge the problem’s origin from just a few electrode photos.
4. How to Conduct a Systematic Analysis? — Prepare Comparative Samples
To make the analysis more convincing, structured comparative sample design is essential. The recommendation is:
Low-capacity cells: 8 pieces (divided into Group A and Group B, 4 each);
Capacity-qualified cells: 8 pieces (also divided into Group A and Group B, 4 each).
Group A is used for positive electrode analysis, and Group B for negative electrode interface analysis. This grouping provides statistical validity and effectively avoids accidental errors.
5. What Does Positive Electrode Weighing Reveal? — Verify Whether Coating Is Too Light
Discharge the Group A cells to about 3.0V (recommended: 0.5C to 3.0V, then 0.2C to 2.5V). Then disassemble and take out the positive electrode sheets, bake them in vacuum at above 130°C for 24 hours, and weigh them.
If the positive electrode sheet weight of low-capacity cells is significantly lower than that of the qualified group or below the process control range, it can be basically determined that light coating of the positive electrode caused the low capacity. Two key points to note:
The post-baking weight error is very small (within 2%), so the weighing results are highly reliable;
This method is not applicable to negative electrodes, since after formation the negative electrode gains weight due to lithium intercalation, requiring additional experimental estimation.
Although simple, this step can quickly confirm whether the issue lies in process control.
6. What Can Negative Electrode Interface Analysis Reveal? — Especially When the Positive Electrode Is Fine
If the positive electrode is normal, focus should shift to the negative electrode interface. Disassemble the Group B cells after full charge and observe the negative electrode surface using a microscope or SEM. Common abnormalities include:
Non-uniform or excessively thick SEI film;
Local lithium plating;
Surface pores blocked, hindering lithium-ion diffusion;
Local deactivation or evident irreversible reactions.
The general trend is: the lower the capacity, the more severe the interface abnormalities.
This indicates that the root cause of low capacity may be severe side reactions at the interface and reduced usable negative electrode capacity, suggesting further review of the formation procedure, electrolyte formulation, or material surface treatment process.
7. From Phenomenon to Root Cause — Systematic Traceback and Verification
After comprehensive analysis, a judgment route can be formed as follows:
| Phenomenon | Primary Suspected Cause | Verification Method |
|---|---|---|
|
Positive electrode weight too light |
Insufficient coating thickness, abnormal material ratio |
Weight comparison, cross-sectional thickness measurement |
|
Interface abnormality, lithium plating |
Formation protocol, electrolyte additives |
SEM/EDS analysis, formation curve comparison |
|
Occasional low capacity |
Operational fluctuation, shortened drying or aging |
Batch traceability, process parameter comparison |
|
Batch low capacity |
Insufficient design margin or material system mismatch |
Design review, DOE verification |
Empirical conclusion: the root cause of low capacity is rarely a single issue—most are “edge effects from multiple factors combined.” Therefore, systematic analysis and cross-verification are extremely important; never be misled by a “single-point anomaly.”
Conclusion
Low capacity itself is not terrible—chaotic analysis is. Low-capacity issues in cells are not a disaster, but a systemic signal. What’s truly concerning is skipping verification and relying on guesswork.
Only through a scientific path and rigorous logic—progressing from test confirmation → frequency judgment → structured comparison → interface analysis → design verification—can problems truly be “seen” and ultimately solved.
Once you understand the logic behind low capacity, you’ve mastered the essence of cell analysis.
