Incoming Batch Inspection and Degradation Testing Procedures for Sodium Ion Shipments
Immediate AC-IR screening and differential capacity testing reveal hidden transit degradation and cell capacity variance in sodium ion shipments.
Gassing Mechanics
Meaning
The chemical and physical processes that govern the generation, accumulation, and release of gas within an electrochemical cell during its manufacture, formatting, and subsequent operating life. This phenomenon occurs when the electrolyte decomposes under high voltage, elevated temperature, or during the initial formation cycle. It is measured by monitoring cell volume changes, internal pressure, or through gas chromatography analysis of the released compounds.
The boundary of these processes is reached when the gas volume exceeds the mechanical tolerance of the cell casing, leading to venting or mechanical failure of the container.
Chemical Origin
The primary generation of gas occurs during the first charging cycle when the electrolyte undergoes reduction at the anode to form the protective solid electrolyte interphase. This reaction produces gases like ethylene, carbon monoxide, and hydrogen. While this initial production is expected and often managed by a vacuum-sealing step after formation, subsequent gassing indicates degradation.
High-voltage operation can oxidize the solvent at the cathode, generating carbon dioxide. Moisture contamination within the cell also reacts with the salt to produce corrosive hydrofluoric acid and hydrogen gas.
Physical Consequence
The accumulation of these gases increases the internal pressure of the cell, leading to swelling or pouch distortion. This deformation reduces the mechanical stack pressure within the cell, which increases the distance between the electrodes and raises the internal resistance. If the gas remains trapped between the electrode layers, it blocks the path of ions, leading to uneven current distribution and localized plating.
In severe cases, the pressure causes the cell to vent, releasing flammable gases and electrolyte into the surrounding pack environment.
Prevention Method
Minimizing these degradation reactions involves the use of high-purity raw materials and the inclusion of sacrificial electrolyte additives. These additives polymerize or decompose before the primary solvent, forming a stable passivation layer that prevents further electrolyte reduction. Additionally, maintaining tight control over the upper cutoff voltage prevents cathode degradation and subsequent gas generation.
Packaging designs often include a designated degassing pouch or a pressure-relief vent to manage the physical consequences of gas generation safely.