Meaning
This physical phenomenon occurs when mechanical pressure applied to a battery cell forces the liquid electrolyte out of the porous electrodes and separator. When the internal or external compressive forces exceed the capillary forces holding the liquid, the electrolyte is expelled. Electrolyte squeeze out depletes the active material pores of ionically conductive liquid, increasing the transport resistance within the cell.
The process is a major concern in pouch cells housed in rigid modules that do not allow for natural expansion during cycling. It stops occurring when the mechanical pressure is released or when the capillary pressure balances the external load.
Expulsion Dynamics
As a lithium-ion cell cycles, the intercalation of lithium ions causes the anode and cathode materials to swell and contract. If the cell is held in a fixed-volume enclosure, this swelling creates a high mechanical pressure against the rigid walls. This compressive stress squeezes the liquid out of the center of the electrode stack and toward the margins of the cell.
The center of the electrodes becomes dry, which restricts lithium ion transport and causes uneven current density distribution. This localized dry-out accelerates lithium plating and can lead to rapid, localized capacity loss and premature cell failure.
Purchasing Considerations
Sourcing and design teams use squeeze out data to specify the optimal pre-charge compression for battery pack assemblies. The purchase specifications for foam pads and spring-loaded retention plates are directly calculated from the cells pressure-displacement curves. Sourcing agreements require manufacturers to define the maximum continuous mechanical pressure the cells can endure before ionic conductivity is degraded.
Procuring cells with optimized electrode porosity and high-surface-area separators minimizes the risk of dry-out under extreme mechanical constraints. This data ensures that the battery pack operates reliably under the various swell profiles encountered over its lifetime.
Technical Limits
The rate and extent of liquid displacement depend on the viscosity of the electrolyte and the pore size distribution of the separator. Lower-viscosity solvents are more easily expelled, whereas gel polymer electrolytes or highly tortuous separators resist this mechanical squeezing. The process cannot be easily measured in real-time, requiring indirect methods such as electrochemical impedance spectroscopy or post-mortem analysis.
At low pressures, the liquid remains locked within the pores by surface tension, meaning that a minimum threshold pressure must be exceeded. This threshold varies with the surface energy of the materials and the wetting characteristics of the electrolyte.
