Intragranular Fracture Mechanism of Highly Crystalline Lithium Manganese Oxide during Lithium Insertion/Extraction Reactions

Lithium-ion batteries require long cycle lives for efficient automobile and stationary applications. Therefore, the lithium-insertion materials used as positive and negative electrodes in batteries need to be highly durable. Lithium-insertion materials normally change their lattice dimensions during charge and discharge; therefore, particle fracture leading to the deterioration of the materials is one of the greatest hindrances to extending the cycle life. Despite its importance, no one has experimentally elucidated the fracture mechanism in detail. To avoid or suppress particle fracture, it is required to understand the fracture mechanisms, with an emphasis on the relationship between the change in lattice dimensions and the generation/propagation of cracks on the particles. In this study, we examined particle fracture phenomena using single-crystal LiMn2O4 (LMO) having an octahedral shape with a crystal size of ∼5 μm. Accelerated cycle tests and overcharge tests revealed that during the lithium extraction reaction of LMO in the 4 V region, cracks are induced along the {111} fracture planes by stress corrosion cracking, wherein the acid generated by the decomposition of the electrolyte accelerates the crack propagation. During the lithium-insertion reaction of LMO in the 3 V region, wherein the structural transformation between the cubic and tetragonal lattices takes place, heavy cracks are caused by strong internal stresses at grain boundaries, {100} fracture planes at the cubic/tetragonal grain boundary, and {110} fracture planes at the tetragonal/tetragonal grain boundary. The results obtained herein suggest that crack patterns are closely related to the change in lattice dimensions for lithium-insertion materials. These findings provide useful insights into the development of lithium-insertion materials having excellent cyclability by designing particle morphology and size.