One work area of the Degradation project is building towards a comprehensive model of lithium transport and diffusion behaviour and of mechanical properties such as shear yield and fracture, as a step towards extending battery lifetime. The team is building a continuum model to understand the origins and long-term effects of mechanical strain and deformation in single cathode particles under a range of practical usage scenarios.

Novel optical microscopy methods, developed by researchers at Cambridge, have been applied to follow intraparticle lithium transport in real-time and with nanoscale resolution. In parallel, ⁷Li magic-angle spinning nuclear magnetic resonance (MASNMR) methods have obtained ‘hopping frequency’ values as a function of state-of-charge. The frequency at which Li ‘hops’ between lattice sites can be related to Li diffusion and was therefore used to parameterise finite-element method modelling. Researchers concluded Li diffusion is heavily dependent on state-of-charge and follows non-Fickian behaviour and have reported accurate diffusion coefficients that are essential for modelling but previously unknown.

The project has also collaborated with FutureCat researchers to develop new methods for measuring mechanical properties of individual cathode particles. Namely, a simple Vickers indentation method has been presented to measure critical shear yield strength parameters (τc). The results are the first of their kind and researchers have found τc is heavily dependent on state-of-charge. These values are essential input parameters for constructing the more comprehensive models that are under development and that are the long-term objective of this project. Revised understanding of the chemomechanical behaviour of single crystals during cycling is also expected to guide protocols to increase cycle lifetime.

Image: Comparison of simulation and experimental imaging results, both conducted at a delithiation rate of C/3. The predicted degree of delithiation (1-θ) on the basal plane of the particle at various times during the charge.

Case study published December 2022.