Why Is the Open-Circuit Voltage of a Hand-Assembled Coin Cell Always Low? Six Root Causes

1. Why Coin Cell OCV Consistency Matters for Material Evaluation

Lithium-ion coin cells are commonly used for the preliminary electrochemical evaluation of new materials and new processes. By measuring specific capacity, median voltage, cycle life, and related indicators, researchers can quickly assess a material’s potential. However, small deviations introduced during assembly can cause the open-circuit voltage (OCV) to drop abnormally, which directly affects the resulting test conclusions. From the electrode (pole piece) to the separator, and from the electrolyte to the sealing step, an oversight at any stage can lead to an abnormal OCV. The following six categories of factors are worth checking in any laboratory.

Figure 1. Overview of six coin cell assembly-related factors — electrode defects, separator porosity, assembly concentricity, electrolyte filling, sealing process, and lithium foil/auxiliary materials — that can cause open-circuit voltage (OCV) to drop in a lithium coin cell.

2. Six Key Factors Behind Abnormal OCV Drop

2.1 Factor 1: Electrode Defects

Two distinct problems fall under this category. Visible surface defects — flaking, cracking, or uneven coating — create an uneven distribution of active material, which disrupts electrode integrity and hinders lithium-ion intercalation and deintercalation, lowering OCV. Separately, a dull die-cutting blade or poorly controlled cutting process can leave burrs on the electrode edge. During assembly or during the resting period that follows, these burrs can pierce the separator and create a micro-short circuit between the positive and negative electrodes, disrupting the cell’s internal potential balance.

2.2 Factor 2: Separator Porosity

Separator pore size affects OCV in two opposing ways. When porosity is too small, lithium-ion transport is hindered and internal resistance increases; polarization that develops during the resting period gradually lowers OCV, and electrolyte wetting is also impaired. When porosity is too large, the separator’s mechanical strength is insufficient, making a short circuit more likely, which likewise affects OCV.

2.3 Factor 3: Assembly Concentricity and Alignment

When the cathode and anode electrodes are not well centered relative to the outer casing, the electrodes experience uneven mechanical stress and the inter-electrode spacing becomes inconsistent, producing an abnormal local electric field and an uneven lithium-ion distribution. Edge contact between electrodes arising from poor concentricity can also trigger micro-short circuits, causing part of the current to be lost internally and resulting in a lower OCV.

2.4 Factor 4: Electrolyte Filling Volume and Degradation

Electrolyte quantity must fall within a defined range. An insufficient filling volume leaves the electrode and separator inadequately wetted, so the electrode reaction is incomplete. An excessive filling volume raises internal pressure during sealing and makes leakage more likely. Electrolyte degradation is a separate but related concern: decomposition of the lithium salt reduces conductivity, and moisture reacting with the lithium salt generates hydrofluoric acid (HF), which corrodes the electrodes — both of which cause an abnormal OCV.

2.5 Factor 5: Sealing Process

Sealing quality affects OCV through two failure modes. Insufficient sealing tightness allows the electrolyte to leak and external air to enter, which oxidizes the electrolyte or the lithium metal anode and shifts the electrode potential. On the pressure side, excessive sealing pressure can damage the electrodes and separator, triggering micro-short circuits, while insufficient pressure results in inadequate sealing and poor internal contact — both of which lower OCV.

2.6 Factor 6: Lithium Foil and Auxiliary Materials

Surface scratches, pits, an oxide layer, or uneven lithium deposition on the lithium foil counter electrode increase internal resistance, reduce the effective reaction area, and allow a passivation layer to affect the electrode potential. Poor-quality auxiliary materials — gaskets, foam nickel, and similar components — can cause poor contact and increased internal resistance, both of which lower OCV.

In summary, An abnormal drop in open-circuit voltage during the resting period after coin cell assembly is generally the result of several factors acting together, including the surface appearance and quality of internal materials, assembly concentricity, and sealing pressure. Reducing OCV drop during coin cell fabrication therefore requires optimization across material selection, electrode preparation, and assembly process, with strict control at every step to ensure the resulting cell’s quality and performance.

3. Addressing Assembly Variability: The IEST CAAS1200 Automated Coin Cell Assembly System

The IEST CAAS1200 replaces several of the manual assembly steps described above with robotic and vision-guided control, targeting the concentricity, sealing, and electrolyte-dosing factors identified in Sections 2.3–2.5.

• Controlled-atmosphere environment: water and oxygen content inside the glovebox is kept below 0.1 ppm, reducing environmental interference during assembly.
• High-Precision Assembly: Robotic arm ensures a concentricity of ±0.2 mm, with real-time visual inspection for immediate error correction.
• Advanced Sealing Technology: Dual-control mechanism for both crimping pressure and thickness, ensuring superior consistency.
• Curled Electrode Solution: Custom-designed suction pads securely handle curled electrodes — solving a long-standing industry challenge.
• Precise Electrolyte Dispensing: Achieves ±2 µL metering accuracy, eliminating manual uncertainties.

Additional system specifications and customization are available on request from IEST Instrument’s customer service team.

Figure 2. The IEST Automatic Coin Cell Assembly System (CAAS1200), integrating a controlled-atmosphere glovebox, robotic assembly arms, and vision-guided electrode alignment.

4. Validation: OCV Consistency Across 200 LFP Coin Half-Cells

LiFePO4 (LFP) coin half-cells assembled using the CAAS1200 automated coin cell assembly system were tested across 5 groups of 40 cells each (200 cells total). The resulting OCV values fell between 3.24 V and 3.28 V across all five groups, indicating that the open-circuit voltage results were consistent and stable across this sample set.

Figure 3. Open-circuit voltage (OCV) of LiFePO4 (LFP) coin half-cells assembled using the IEST CAAS1200 automated coin cell assembly machine, across 5 test groups of 40 cells each (200 cells total), with OCV values falling between 3.24 V and 3.28 V.

5. Summary

IEST Automatic Coin Cell Assembly Machine(CAAS1200) uses a high-precision robotic arm, a vision-based inspection system, and an automated sealing unit to perform automated, high-precision coin cell assembly with stable sealing pressure. The system addresses the industry-wide difficulty of handling curled electrodes, and provides real-time process monitoring along with material and process traceability. It is applicable to the systematic electrochemical evaluation of cathode and anode materials for the lithium (and sodium) battery industry.

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