Meaning
Spherical clusters of individual nanometer-sized transition metal oxide crystals are assembled during precursor synthesis to optimize the tapped density and processability of active materials. In conventional cathode chemistries, secondary particle agglomerates provide a high surface area that facilitates rapid lithium-ion exchange with the electrolyte. This morphology allows for faster charging rates than larger, single-crystal equivalents.
The structural integrity of these clusters prevents early electrode degradation.
Particle Morphology
Precipitation of metal hydroxides in a stirred tank reactor determines the size and shape of these multicrystalline clusters. In NMC and LCO chemistries, secondary particle agglomerates usually range from five to fifteen micrometers in diameter. These structures hold together through grain-boundary adhesion, which is established during high-temperature sintering.
Although this clustered arrangement shortens the lithium-ion diffusion path through the solid state, it creates numerous internal grain boundaries that are vulnerable to chemical attacks.
Mechanical Degradation
Volume changes during charge-discharge cycles generate stress that breaks the clusters apart. As lithium ions are repeatedly extracted and reinserted, secondary particle agglomerates experience anisotropic lattice contraction, causing them to fracture along grain boundaries. This mechanical breakdown, known as particle pulverization, isolates inner grains from the conductive network.
Liquid electrolyte penetrates the newly formed cracks, leading to parasitics and accelerated capacity loss.
Processing Constraints
High-pressure calendering during electrode fabrication must be carefully controlled to prevent premature cracking of the clusters.
