Non-Linear Damage Response to Voltage Revealed by Operando X-ray Tomography in Polycrystalline NMC811

Abstract

To understand the fracture behaviour in battery materials, X-ray computed tomography (X-ray CT) has been employed widely and has become the primary technique for non-destructive particle analysis. Cracking is known to cause decline in cell performance of polycrystalline NMC811 as it exposes new surfaces for parasitic electrolyte reactions and disconnects active material from the electrode matrix. First cycle crack formation has been documented previously; however, definitive electrochemically induced particle fracture can be difficult to assess due to complicated sample preparation methods and the availability of high-resolution X-ray imaging. Here we present an operando X-ray CT technique that enables accurate observation of fracture behaviour as the particles are de-/lithiated. We uncover a non-linear relationship between fracture behaviour and the cell voltage, and evidence of particle reformation as re-lithiation occurs, we also see the effect multiple cycles have on the severity of cracks. Using a grey level analysis algorithm for fracture detection, we have expedited the damage evaluation of thousands of particles throughout the electrochemical process to successfully understand crack initiation, propagation, and eventual partial particle restoration on a large statistical scale. In addition we explored the effects of continued volumetric hysteresis on particles’ damage fingerprints. For the first time, we demonstrate the complex plurality of fracture behaviour in commercial lithium ion battery materials, that will help to design mitigation strategies to combat the effects of particle fracture.