Meaning
Mechanical stress concentrations initiate localized grain boundary separations or crystal defects within active material particles during electrode cycling. This process of microcrack nucleation marks the onset of structural degradation that eventually leads to macroscopical particle pulverization. It occurs primarily at high states of charge where lattice strain is maximized.
The phenomenon reduces the long-term structural integrity of the electrode.
Initiation Stage
Localized shear stresses build up at the boundaries of anisotropic grains as they expand and contract during lithium-ion transport. In the context of microcrack nucleation, these stresses exceed the cohesive energy of the grain boundaries at specific critical voltages or states of charge. This mechanical failure begins at defect sites or voids within the polycrystalline structure.
Once a nucleus is formed, it acts as a stress concentrator that accelerates crack propagation.
Degradation Impact
The formation of tiny fissures allows the liquid electrolyte to penetrate the interior of the active particles. This penetration during microcrack nucleation triggers the formation of secondary solid electrolyte interphase layers on the newly exposed internal surfaces. This side reaction consumes active lithium and increases the overall resistance of the cell.
Design Prevention
Engineers specify single-crystal morphologies or apply surface coatings to prevent stress concentrations. Minimizing microcrack nucleation prolongs the electrochemical life of the cell. This improvement reduces the frequency of battery replacements.
