Non-linear cracking response to voltage revealed by operando X-ray tomography in polycrystalline NMC811

Abstract

To understand fracture behaviour in battery materials, X-ray computed tomography (X-ray CT) has become the primary technique for non-destructive particle and crack analysis. Cracking causes performance decline in polycrystalline NMC811 by exposing new surfaces for parasitic electrolyte reactions and disconnecting active material from the electrode matrix. First cycle crack formation has been documented, but definitive electrochemically induced particle fracture is challenging to assess due to complex sample preparation and high-resolution X-ray imaging requirements. Presented here is an operando X-ray CT technique that enables accurate observation of fracture behaviour during de-/lithiation. A non-linear relationship between fracture behaviour and cell voltage was uncovered, and evidence of particle reformation during re-lithiation. Using a grey level analysis algorithm for fracture detection, we expedite damage evaluation in several thousands of particles throughout the electrochemical process, understanding crack initiation, propagation, and closure on a large, statistical scale and give the ability to track any one of the thousands of particles through its individual electrochemical history. Additionally, we explore the effects of continued volumetric hysteresis on particle damage. For the first time, we demonstrate the complex plurality of fracture behaviour in commercial lithium-ion battery materials, aiding in designing mitigation strategies against particle fracture.