Quantifying Micro Leak Thresholds for Lithium Ion Envelope Integrity Assurance
Micro leak rates above 1.0e-6 mbar L/s drive moisture ingress and hydrofluoric acid formation, degrading lithium ion cell capacity and triggering transport failure.
SEI Breakdown
Meaning
Decomposition of the solid electrolyte interphase layer on a lithium-ion cell’s anode due to high temperatures or electrochemical abuse. This deterioration exposes the active lithium-carbon structure directly to the liquid electrolyte. Preventing sei breakdown is a critical design requirement for avoiding thermal runaway in high-power lithium batteries.
Thermal Trigger
Heat is the primary cause of this protective layer’s instability. When the cell’s internal temperature exceeds eighty degrees Celsius, the organic components of the passivation layer begin to decompose. This initiatory sei breakdown is exothermic, generating additional heat that further raises the cell temperature.
The reaction represents the first phase of thermal runaway, leading to the subsequent degradation of the cathode and separator.
Chemical Release
The decomposition process releases volatile gases like ethylene and carbon dioxide, which increase the internal pressure of the cell pouch. As the passivation layer degrades, fresh electrolyte reacts with the exposed active material, generating more heat and consuming active lithium ions. Sourcing materials that form a thermally resilient interphase raises the temperature at which sei breakdown occurs, improving safety.
Lifetime Impact
Repeated cycling at high rates or low temperatures causes micro-cracking of the protective layer, leading to localized breakdown. The continuous repair of this layer consumes active lithium and liquid solvent, resulting in capacity fade and internal resistance growth. Monitoring the rate of sei breakdown through electrochemical impedance spectroscopy allows engineers to predict the remaining useful life of the cells in demanding applications.