Meaning
Dimensional reductions in the repeating structural unit of a crystal lattice during delithiation influence the mechanical integrity of active materials. This unit cell volume contraction occurs as lithium ions are removed from the cathode, causing the remaining atomic layers to shift closer together. The extent of this change varies by chemistry, with some materials shrinking by more than five percent.
Crystalline Strain
Repeated changes in size during charging and discharging create internal tension within the electrode particles. Unit cell volume contraction leads to the formation of microcracks that eventually isolate parts of the material from the electrical network. This mechanical fatigue is a primary cause of capacity loss over hundreds of cycles.
Modern electrode designs often use dopants to stabilize the lattice and reduce the severity of these changes.
Material Degradation
Physical breakdown of the host structure exposes new surfaces to the electrolyte. When unit cell volume contraction triggers cracking, it allows secondary reactions to occur deeper within the particle. These reactions consume the electrolyte and form resistive layers that slow down the movement of ions.
Volumetric Density
Pack design must account for the physical shifting of the materials at the microscopic level. While unit cell volume contraction reduces the size of the crystals, it often leads to macroscopic expansion of the electrode due to crack formation. Managing these forces is necessary for maintaining the structural safety of the battery.
