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.
Thermal Excursions
Meaning
The temporary or permanent deviation of an electrochemical cell’s temperature beyond its specified safe operating limits during charging, discharging, or environmental exposure. This thermal deviation indicating that the heat generation rate within the cell has exceeded the heat dissipation capability of the packaging or cooling system. It is monitored using external thermocouples, internal temperature sensors, or through changes in the cell’s electrochemical response.
The boundary of this phenomenon is reached when the temperature triggers self-sustaining exothermic reactions, leading to thermal runaway and catastrophic failure.
Thermal Trigger
These temperature shifts are typically initiated by high-rate operation, internal short circuits, or high ambient temperatures. During rapid charging or high-current discharge, Joule heating from internal resistance generates significant thermal energy within the cell. If the thermal management system fails to remove this heat, the temperature rises rapidly, initiating the excursion.
An internal short circuit caused by manufacturing defects or dendrite growth can also trigger a localized temperature spike. These triggers must be identified early to prevent the heat from spreading to adjacent cells and causing a cascading failure within the battery pack.
Internal Damage
Exposure to elevated temperatures during these excursions causes severe damage to the internal chemistry of the cell. When the temperature exceeds eighty degrees Celsius, the solid electrolyte interphase on the anode begins to decompose, exposing the reactive carbon to the electrolyte and triggering further exothermic reactions. This decomposition increases the internal resistance and consumes active ions, resulting in permanent capacity loss.
If the temperature continues to rise, it can melt the polymer separator, causing a massive internal short circuit that releases the stored chemical energy in the form of heat and gas.
Safety Management
Preventing the catastrophic consequences of these temperature events requires a robust thermal management design and active safety systems. Battery management systems continuously monitor temperature inputs and will reduce the current or disconnect the pack if a thermal excursion is detected. Passive safety features, such as ceramic-coated separators and flame-retardant electrolyte additives, are also integrated to increase the thermal tolerance of the cell.
Additionally, thermal barriers between cells are used to prevent heat propagation from one failing cell to its neighbors. These protective measures ensure that localized thermal events do not compromise the safety of the entire battery system.