Analysis of Causes for Low Cells Capacity
Low cell capacity is an intuitive judgment based on comparing the post-formation discharge capacity with the design value. If the measured capacity falls below the design specification, the first step is to verify whether the formation process parameters are correctly set (e.g., discharge current, charge duration, cutoff voltage, and formation temperature).
① If the formation steps are confirmed correct, retest the cell using alternative equipment or channels to rule out potential issues with the formation system.
② If the capacity remains normal after equipment replacement, the original formation equipment is faulty.
③ If the low-capacity issue persists after retesting, the cell is confirmed to exhibit genuine low capacity.
After confirming low capacity, further analysis is required to determine its frequency and severity. Before systematic root-cause analysis, disassemble and inspect the fully charged low-capacity cells. If no abnormalities are found, potential causes may include insufficient positive electrode coating weight or inadequate design margin. If defects are detected, design or manufacturing issues should be considered.
Root Cause Analysis: Design and Manufacturing Perspectives
I. Design-Related Factors
1. Material Compatibility
The compatibility between the negative electrode and electrolyte critically impacts capacity. For newly introduced anode materials or electrolytes, repeated lithium plating observed during testing strongly indicates material mismatch. Potential mismatch mechanisms include:
① Poorly formed, overly thick, or unstable SEI layer during formation.
② PC (propylene carbonate) in the electrolyte causing graphite exfoliation.
③ Excessive designed electrode areal density/compaction density hindering high-rate charge/discharge capability.
2. Capacity Design Margin
Positive Electrode Specific Capacity: Design must account for coating tolerances, formation equipment errors, and capacity loss from tab adhesion. For new materials, accurately evaluate the achievable specific capacity under the given system (anode/electrolyte pairing). Note that specific capacity varies with formation rate, charge cutoff current, cycling rates, and electrolyte formulation. Overestimating positive electrode capacity leads to inflated design values and actual low-capacity cells.
Negative Electrode Excess and CB Value: Excessive negative electrode loading initially enhances positive electrode utilization by 1–2% but beyond optimal levels, excessive irreversible lithium consumption during SEI formation reduces first-cycle discharge capacity.
3. Electrolyte Filling and Retention
Insufficient electrolyte filling reduces lithium-ion intercalation/deintercalation efficiency. Cells with inadequate electrolyte retention exhibit dry electrodes and thin lithium plating on the anode surface, directly contributing to capacity loss.
II. Manufacturing-Related Factors
1. Coating Areal Density Deviation
Underweight coating of positive/negative electrodes directly causes low capacity. For positive electrodes, confirm coating weight via post-drying gravimetric analysis. Non-uniform coating thickness ("yin-yang coating"), particularly negative electrode undercoating, is another contributor. Overcoating of positive electrodes may lower specific capacity but often increases total capacity.
2. Over-Compaction During Calendering
Over-compaction damages active material structure, evidenced by shiny electrode surfaces. In cathodes, this disrupts lithium deintercalation; in anodes, it induces surface lithium plating and capacity fade.
3. Assembly Tolerances
Poor electrode alignment, separator wrinkles, or internal micro-shorts increase local impedance and degrade capacity. Wrinkled separators cause incomplete lithium intercalation (non-golden anode appearance) at affected regions.
4. Moisture Content Control
Elevated moisture levels (from electrodes, electrolyte, improper glovebox dew point, or degassing processes) trigger side reactions and capacity loss.
5. Environmental Controls
High humidity accelerates hydrolysis reactions, while low temperatures impede lithium-ion diffusion, both reducing capacity. Formation temperature deviations also affect capacity measurement accuracy.
6. Other Factors
Foreign Contamination: Metallic/magnetic impurities increase self-discharge, leading to apparent low capacity post-formation.
Pre-formation Storage: Prolonged storage under high temperature/humidity degrades electrodes and electrolytes, causing capacity loss.
Ⅲ. Conclusion
By systematically investigating these factors—from material compatibility and design margins to process controls and environmental conditions—the root cause of low capacity can be effectively identified and addressed.
TOB NEW ENERGY provides advanced battery tester empower engineers and researchers to unlock comprehensive insights into battery performance through precise measurements and controlled experiments. Quantify energy storage capabilities with laboratory-grade accuracy using our battery capacity analyzers.