Meaning
Phase equilibrium properties dictate the pressure exerted by the gaseous components of a liquid mixture within a sealed battery container. This electrolyte vapor pressure is a function of temperature and the mole fraction of the various solvents such as ethylene carbonate and ethyl methyl carbonate. It determines the mechanical stress placed on the cell housing and the likelihood of solvent leakage through seals.
Thermodynamic Behavior
Raoult’s law provides a starting point for calculating the partial pressures of each component in the liquid phase. The total electrolyte vapor pressure rises exponentially as the cell heats up during fast charging or high discharge cycles. Non-ideal interactions between the lithium salts and the organic solvents lower the activity of the volatile species.
Seal Integrity
Hermetic closure must withstand the maximum expected internal pressure without allowing moisture ingress or solvent escape. If the electrolyte vapor pressure exceeds the burst strength of the pouch seal, the cell loses its electrolytic balance. This leads to increased internal resistance and eventual capacity loss.
Selection Criteria
Solvent blending allows manufacturers to adjust the boiling point and volatility of the electrolyte mixture. Choosing a formulation with a lower electrolyte vapor pressure improves the safety margin at high operating temperatures. This specification is a primary factor in the qualification of chemicals for automotive grade cells.
