Effect of the electrode particle shape in Li-ion battery on the mechanical degradation during charge-discharge cycling

The investigation addresses the effect of shape and aspect ratio, of typical electrode particles of Li-ion cell material, on the extent of fracture surface created due to intercalation-induced strain energy. Nodular, fibrous, and flaky-shaped particles were studied approximating them to sphere, cylinder, and disc geometries, respectively. Analytical expressions for stress distribution in slab and cylindrical-shaped particles were derived using thermal stress analogy. Such results are already available for spherical particles. Finite element study was carried out using COMSOL (R) Multiphysics package to complement the analytical work as well as for verification. The spatial and temporal variations of stresses and strain energy in the electrode particles of different shapes were established. Reportedly, solid electrolyte interphase formed on the fracture surface as well as the fracture-induced isolation of electrode material are the main causes of performance degradation and in this context, the intercalation-induced strain energy density becomes important. The sphericity of a particle, that is, the ratio of the surface area of a sphere to that of the particle of equal volume, was found to fittingly describe the effect of shape. The average strain energy density stored in a particle increases with the increasing sphericity. Therefore, fibrous and flaky-shaped particles are expected to have lower tendency for mechanical degradation than the nodular ones. The analysis is restricted only to the mechanics of mechanical degradation and not to the process or the chemistry point of view.