Mechanical response of the negative electrode in structural batteries with nonuniform microstructure

A proposed structural (negative) electrode consists of carbon fibres embedded in a Structural Battery Electrolyte (SBE). The carbon fibers swell during lithiation, causing internal stresses in the composite. Geometrical features such as e.g. interfibre distance affect the resulting stresses in the SBE. For proper functioning of the structural battery, it is important to ascertain that the compressive stresses generated in the SBE do not result in material degradation. This study investigates how the nonuniform distribution of fibres inevitably caused by the battery manufacturing process affects the local stress field variations in the SBE matrix. For this, Scanning Electron Microscopy (SEM) images of the processed battery composite are produced and are used to calculate statistics of interfibre distances. A procedure for reconstruction of statistically equivalent Representative Volume Elements (RVEs) is developed and the generated RVEs are analyzed in a boundary value problem with specified chemical expansion of fibres and a recently developed constitutive model for the SBE matrix. The calculated extremes in the local compressive stress show approximately 25% higher values than those in an idealized reference model with a uniform (hexagonal) fibre pattern. The importance of considering manufacturing induced nonuniform microstructure in structural batteries is thus demonstrated.