Ageing mechanisms in lithium-ion batteries are of great importance to increase their lifetime and user acceptance. One degradation mechanism is the fracture of micrometre-sized cathode particles. To fully understand this mechanical degradation, a material characterization on the particle level is required. These experimental investigations help to identify aspects for material improvements and thus help to produce better batteries. In this study, polycrystalline NMC cathodes are investigated in order to characterize their microstructure, defects in the material and the mechanical properties of grain boundaries.

The material description starts with a microstructure characterization of the polycrystalline NMC particles. EBSD measurements are conducted to derive a grain size distribution and to examine the texture of the material. In addition, mechanical defects such as micropores and chemical defects such as inhomogeneities are investigated using FIB-SEM and EDS analyses. Their influence on the stress distribution within the cathode particles during the electrochemical cycling is analysed in particle-level finite element models.

Since cracking of the NMC polycrystals is typically observed along the grain boundaries, the mechanical properties and in particular the strength of the grain boundaries are measured. Due to the small size of the particles, this can only be done through in situ micro tensile tests. The resulting force-displacement curves are used for the description of cohesive zones representing the grain boundaries in simulation models. The effect of different grain boundary strengths on the fracture behaviour of the polycrystals is numerically investigated. A focus is put on the number of fractured grain boundaries and crack patterns.

The exact knowledge of the experimentally determined material properties on the particle level and the numerically conducted sensitivity analysis allow for an improved understanding of ageing mechanisms and material improvements resulting in greater battery lifetimes.