Quantifying Helium Leak Rates for Commercial Pouch Cell Quality Control
Helium leak quantification below 1e-6 mbar L/s requires background suppression, flow conversion modeling, and strict pouch heat seal quality control.
Electrolyte Solvent Outgassing
Meaning
Chemical decomposition reactions generate gaseous byproducts within a sealed lithium-ion cell when high temperatures or voltages cause the breakdown of carbonate components. This phenomenon occurs when the liquid electrolyte becomes unstable and releases molecules like carbon dioxide or ethylene into the internal volume of the cell. While some electrolyte solvent outgassing is expected during the initial formation cycle, excessive gas production during normal use indicates a failure or a safety risk.
The accumulation of these gases increases the internal pressure and can lead to the swelling or venting of the battery pouch. This process stops when the reactive species are consumed or the cell is disconnected from the circuit.
Internal Pressure
Buildup of gas within the hermetic enclosure puts mechanical stress on the seals and the outer casing of the battery. As electrolyte solvent outgassing continues, the pouch may expand like a balloon, which can damage the surrounding module structure. This expansion also creates gaps between the electrodes, increasing the internal resistance and reducing the power output of the cell.
In extreme cases, the pressure may exceed the burst strength of the material, leading to a controlled or uncontrolled release of the contents. Monitoring the thickness of a cell is a common way to detect this issue.
Degradation Pathway
Side reactions at the surface of the cathode or anode are usually the source of the gaseous emissions. When a cell is overcharged, the high potential strips electrons from the solvent molecules, causing them to fragment. This electrolyte solvent outgassing is often accelerated by the presence of moisture or other contaminants that catalyze the breakdown.
The specific composition of the gas can tell engineers which part of the chemistry is failing. Analysis of the gas through chromatography reveals the health of the internal components and the quality of the manufacturing environment.
Cell Deformation
Physical changes to the battery geometry can lead to mechanical interference with cooling plates or electrical connectors. When electrolyte solvent outgassing causes a pouch to swell, it may press against neighboring cells, creating hot spots or short circuits. This deformation is often irreversible even if the gas is eventually reabsorbed or vented.
Designers must include enough space in the battery pack to accommodate some expansion without compromising the system. Preventing this gas production through the use of stable additives and careful voltage limits is a priority for battery researchers. Reliable operation depends on keeping these chemical reactions under control.
Standardization of High Sensitivity Helium Mass Spectrometry Leak Thresholds for Commercial Pouch Cell Procurement
Commercial pouch cell procurement contracts require specifying helium mass spec leak thresholds below 2.69 × 10⁻⁶ mbar·L/s to prevent moisture ingress and HF acid formation.