Direct 4D Observations of Electrochemically Induced Intergranular Cracking in NMC811 Particles

Establishing the nature of crack formation is paramount to understanding the degradation modes that govern decline in battery performance. Cracking has several possible origins; however, it can be classified in two general cases: mechanically induced, during manufacturing, or electrochemically induced, during operation. Accurate and repeatable tracking of operational cracking without the convolution of pre-existing cracks is highly challenging; observing these features requires the highest resolutions possible for 3D imaging techniques, necessitating very small sample geometry, while also achieving realistic electrochemical performance. Here, we present a technique in which particle cracking can be completely attributed to electrochemical stimulation via a pseudo in-situ laboratory X-ray nano computed tomography (CT) technique. Observed upon electrochemical delithiation of NMC811, was significant cracking within the core of secondary particles which propagated radially. We also noted that contrary to crystallographic data, which suggests the unit cell volume reduces upon charging, significant volume expansion of the secondary particle occurred.