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

Abstract

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.

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.