Dynamic Mechanics of Micro-Crack Growth in Electrolyte-Swollen Polyolefin Separators
Electrolyte solvent plasticization lowers polyolefin yield stress, driving sub-critical micro-crack propagation under cyclic stack pressure.
Electrolyte Plasticization
Meaning
Softening of a semi crystalline polymer happens when small molecules penetrate the molecular chains and reduce the glass transition temperature. Electrolyte plasticization occurs when the organic solvents in a lithium ion battery interact with the separator or the binder material. This interaction increases the flexibility of the polymer but simultaneously reduces its mechanical stiffness and puncture resistance.
Chain Interaction
Solvent molecules occupy spaces between the polymer strands and increase the free volume within the material. As electrolyte plasticization progresses, the intermolecular forces that hold the polymer chains together are weakened. This allows the material to deform more easily under external pressure or internal stress.
Mechanical Alteration
Physical properties such as the Young’s modulus decrease upon exposure to the electrolyte. While some electrolyte plasticization can improve the contact between the electrode and the separator, excessive softening can lead to the collapse of the pore structure.
Operational Boundary
Chemical resistance is measured by monitoring the change in mechanical properties over time while the material is submerged in solvent. Once electrolyte plasticization reaches a critical level, the separator can no longer prevent the growth of lithium dendrites. Testing protocols often include a creep test to ensure the material does not flow under the pressure of the cell stack.
Selection of resistant polymer grades is essential for high temperature battery applications.