Fracture Testing of Lithium‐Ion Battery Cathode Secondary Particles in‐situ inside the Scanning Electron Microscope

Cathode particle fracture affects battery performance. Here, the strength of NMC811 secondary particles is measured and fracture mechanisms imaged in situ within a scanning electron microscope. Flat compression platens are used to mimic the loads in calendaring, and cono-spherical indentation to explore contact between particles. Effects of cycling and delithiation upon particle indentation strength are investigated.

Abstract

Fracture of cathode secondary particles is a critical degradation mechanism in lithium-ion batteries. The microindentation strength of LiNi_0.8Mn_0.1Co_0.1O₂ 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.

Zum Volltext