Determining the Fundamental Failure Modes in Ni-rich Lithium Ion Battery Cathodes

Challenges associated with in-service mechanical degradation of Li-ion battery cathodes has prompted a transition from polycrystalline to single crystal cathode materials. Whilst for single crystal materials, dislocation-assisted crack formation is assumed to be the dominating failure mechanism throughout battery life, there is little direct information about their mechanical behaviour, and mechanistic understanding remains elusive. Here, we demonstrated, using in situ micromechanical testing, direct measurement of local mechanical properties within LiNi0.8Mn0.1Co0.1O2 single crystalline domains. We elucidated the dislocation slip systems, their critical stresses, and how slip facilitate cracking. We then compared single crystal and polycrystal deformation behaviour. Our findings answer two fundamental questions critical to understanding cathode degradation: What dislocation slip systems operate in Ni-rich cathode materials? And how does slip cause fracture? This knowledge unlocks our ability to develop tools for lifetime prediction and failure risk assessment, as well as in designing novel cathode materials with increased toughness in-service

Electrochemical-mechanical corrosion phenomena

The substantial number of parameters and their interdependence makes the modelling and prediction of erosion-corrosion rates a challenging task, yet owing to increasing computational capacity one that is potentially solvable. To accurately estimate the rate of material loss, universally accepted explanations of the mechanisms of combined erosion and corrosion must be developed. The absence of reliable data can undermine the safety and predictability of industrial processes, such as oil and gas transportation. The aim of this work is to demonstrate the application of an electrode scratching technique coupled with microstructural characterisation for improved understanding of synergy in erosion-corrosion. In this PhD thesis, a rotating disc electrode scratching setup, with well-defined and controlled flow conditions, was developed to study the factors affecting erosion-corrosion. Linear potentiodynamic and potentiostatic polarization techniques were used to reproducibly monitor the kinetics of dissolution and repassivation of API X65 carbon steel and AISI 316L stainless steel electrodes upon scratching. Samples characterised using high-resolution scanning electron microscopy (SEM) and white-light interferometry (WLI), confirm the synergy; the losses due to erosion-corrosion are larger than that of the summation of the separate contributions of erosion and corrosion. Focused Ion Beam (FIB) milling was implemented for in-situ lift-out of lamellae from scratched samples for Transmission Electron Microscopy (TEM) characterisation. Distinct microstructural changes in the vicinity of scratches were confirmed with nanoscale grain refinement and orientation changes observed. These results, coupled with electrochemical data and micro-hardness measurements, suggest time-dependent surface-hardening processes affect material loss rates during mechanical-electrochemical coupled corrosion. Samples subject to jet impingement erosion-corrosion confirm similar microstructural changes take place, thus making an electrode scratching setup a versatile tool for studying erosion-corrosion, as well as a quick assay tool for development and testing of corrosion inhibitor formulations for industrial applications as demonstrated in this thesis.Open Acces