Meaning
Mechanical stress from repeated lattice expansion and contraction leads to the separation of primary particles within a cathode sphere. This phenomenon, known as intergranular boundary micro-cracking, increases the internal resistance of the cell by breaking the electrical pathways between particles. It reduces the capacity and cycle life of the battery over time.
Structural Failure
Volume changes during charge and discharge cycles are anisotropic, creating severe shear stresses between the boundaries. These shear stresses generate microscopic fractures that allow the liquid electrolyte to penetrate deep into the secondary particle. The electrolyte then reacts with the newly exposed active material surfaces, which accelerates transition metal dissolution.
This reaction further degrades the cathode structure.
Sourcing Impact
Battery pack buyers use the presence of this degradation mode to explain why some cells exhibit rapid capacity fade. When evaluating high-nickel cathode materials, sourcing engineers request scanning electron microscopy data from cycled cells to check for intergranular boundary micro-cracking. Suppliers with advanced coating technologies or doped chemistries that resist cracking are preferred.
This enables the purchase of cells that hold their value and capacity over long warranties.
Mitigation Strategy
Single-crystal cathode materials are increasingly sourced to eliminate the grain boundaries where cracking begins. Because single-crystal particles consist of a single grain, they do not suffer from the same localized stress concentration that occurs in polycrystalline structures. This significantly extends the high-voltage cycle life of the cells.
Sourcing agreements should specify the ratio of single-crystal to polycrystalline material to balance cost and longevity.
