Meaning
Thermodynamic properties indicate the tendency of liquid electrolyte components to evaporate into a gaseous state at a specific temperature within a closed system. This organic solvent vapor pressure is a primary factor in determining the internal pressure of a battery cell during its operation and storage. If the pressure becomes too high, it can stress the mechanical seals or trigger the safety vent, leading to a loss of electrolyte and a reduction in cell performance.
The solvents used in lithium ion batteries, such as ethylene carbonate and dimethyl carbonate, each have a unique pressure profile that changes with the ambient heat. Understanding these values is essential for designing cell housings that can withstand the internal forces generated during high temperature use.
Boiling Point
Temperature at which the vapor pressure of the liquid equals the external atmospheric pressure determines how quickly a solvent will evaporate. In a battery, the organic solvent vapor pressure increases as the cell gets hotter, even if the liquid does not reach its actual boiling point. This relationship is often described by the Clausius Clapeyron equation, which shows an exponential rise in pressure with increasing temperature.
If the cell is exposed to extreme heat, the solvents will turn into gas rapidly, creating a dangerous buildup of pressure. This is why the thermal limits of a battery are so strictly controlled by the battery management system.
Internal Pressure
Force exerted by the gas molecules against the walls of the cell container must be balanced by the strength of the materials used in the assembly. A high organic solvent vapor pressure can cause the casing of a pouch cell to swell or the cap of a cylindrical cell to bulge. This physical deformation can damage the internal structure of the cell, leading to short circuits or a loss of contact between the layers.
Manufacturers must choose solvent mixtures that provide a good balance between ionic conductivity and low vapor pressure. The use of additives can also help to stabilize the electrolyte and reduce the amount of gas produced during normal operation.
Safety Margin
Design limits for the cell housing and the safety vent are set based on the maximum expected pressure at the top of the operating temperature range. The organic solvent vapor pressure provides the data needed to calculate these limits and ensure that the cell will not burst under normal conditions. If the pressure exceeds the safety margin, the vent is designed to open and release the gas in a controlled manner.
This prevents a more violent explosion of the cell casing and protects the surrounding battery pack. Every cell design undergoes rigorous pressure testing to verify that these safety features work as intended.
