Fracture testing of lithium‐ion battery cathode secondary particles in‐situ inside the scanning electron microscope

Fracture of cathode secondary particles is a critical degradation mechanism in lithium-ion batteries. The microindentation strength of LiNi0.8Mn0.1Co0.1O2 secondary particles is measured in situ in the scanning electron microscope (SEM), enabling dynamical imaging of fracture. Crack propagation is intergranular between primary particles when induced by compressing between flat platens (analogous to calendaring), and with a cono-spherical indenter (representing particle-particle contact). Fracture occurs directly beneath the cono-spherical tip and at the centre of secondary particles when compressed between flat platens. Finite element modelling of stress states provides insight into the dependence of fracture load upon cohesive strength and particle toughness. Secondary particle indentation strength decreases with increasing secondary particle size, with cycling, and with increasing state of charge. The indentation strength decrease is greatest in earlier stages of delithiation. The novel microindentation technique allows assessment of strength and toughness of different cathode morphologies, aiding prediction of optimal particle structure and processing conditions

Mechanical properties of cathode materials for lithium-ion batteries

Mechanochemical degradation processes such as the fracture of cathode particles play a major role in limiting the service life of advanced lithium-ion batteries (LIBs). In order to help alleviate the degradation of battery performance, it is necessary to measure the relationship between the degradation of the mechanical properties of cathodes and their concomitant degradation of electrochemical performance. In this review, measurements of the mechanical properties of LIB cathode materials are summarized from the literature, along with the range of experimental methods used in their determination. Dimensional changes that accompany charge and discharge are compared for active materials of olivine, spinel, and layered atomic structures. The sensitivity of indentation hardness, Young's modulus and fracture strength to grain size, porosity, state of charge and charge/discharge history are critically reviewed and discussed with reference to the behavior of conventional, electrically inactive solids. This approach allows for the identification of microstructural properties that dictate the mechanical properties of LIB cathode materials