Meaning
Dimensional differences exist between the crystalline unit cell volumes of coexisting phases or during the charging cycle of insertion electrodes. This lattice volume mismatch generates internal mechanical stress that drives particle fracture and capacity degradation in secondary batteries. It occurs in both anode and cathode chemistries when guest ions are inserted or extracted.
The degree of mismatch determines the mechanical stability of the electrode.
Physical Mechanism
Phase-separating materials like lithium iron phosphate or sodium transition metal oxides transition between distinct crystalline phases with different lattice parameters. This transition generates a high-strain interface where the lattice volume mismatch is localized, creating shear stresses at the phase boundary. The elastic energy stored in this interface scales with the size of the phase mismatch.
If the mismatch exceeds the critical strain limit, the crystal lattice relaxes through the formation of defects and dislocations.
Degradation Consequence
Repeated mechanical strain during cycling leads to the disintegration of active material particles and the disruption of the conductive network. Because of lattice volume mismatch, the secondary particle agglomerates suffer from cracking, exposing fresh surface area to side reactions with the electrolyte. This process accelerates the consumption of active lithium and increases cell impedance.
Material Selection
Designers select dopants to adjust the lattice parameters and reduce the volumetric change during cycling. Mitigating lattice volume mismatch improves the long-term cycle life of the battery. This optimization decreases the warranty risk of the system.
