Meaning
Increase in internal force exerted by gases or solids against the walls of a sealed container. In lithium cells, pressure buildup occurs when the electrolyte decomposes into gaseous products during overcharge or high temperature exposure. Rigid housings must withstand these forces without leaking or bursting.
Gas Generation
Chemical reactions at the electrode surface release carbon dioxide and other vapors. While some of this gas is absorbed into the solid electrolyte interphase, excessive pressure buildup indicates a failure of the safety window. Vents are designed to open at a specific set point to prevent a catastrophic explosion.
Monitoring the rate of gas release helps in diagnosing the health of the battery.
Mechanical Strain
Physical expansion of the anode during lithiation also contributes to the total load. This pressure buildup compresses the separator and can reduce the ion flow between the electrodes. If the pressure becomes too high, the separator pores may close and stop the reaction entirely.
Proper spacing in the module design allows for some of this natural growth.
Containment Boundary
Enclosures are tested to a safety factor that is several times the expected operating force. A breach in the containment boundary leads to the ingress of moisture and oxygen. These contaminants react violently with the lithium and lead to fire.
Engineers use thickness and material strength to manage the pressure buildup safely. The end of the battery life is often defined by the point where the housing can no longer safely contain the internal stresses.
