Meaning
Localized mechanical tension arises within the grains and at the grain boundaries of polycrystalline materials during thermal or chemical transitions. This microstructural stress occurs when individual crystal grains expand or contract at different rates or in different directions. In battery cathodes, the anisotropic change in lattice parameters during delithiation is the primary driver of this phenomenon.
Internal forces can exceed the fracture toughness of the material even if the bulk stress remains low.
Lattice Interaction
Atoms moving through the crystal lattice create pressure gradients that vary by crystallographic orientation. Such microstructural stress leads to the formation of microcracks that sever the electrical pathways between grains. The loss of connectivity reduces the capacity of the cell because certain areas become inactive.
Grain size influences the magnitude of these forces.
Cycle Degradation
Repeated charging and discharging cycles exacerbate the separation of particles. High microstructural stress eventually causes the secondary particles to crumble into smaller fragments. Electrolyte then penetrates these new surfaces and creates resistive films.
This process is a major contributor to the rise in internal resistance over time.
Structural Modification
Doping or coating the grains can stabilize the architecture. These interventions minimize the mismatch between adjacent crystals.
