Effects of state of charge on elastic properties of 3D structural battery composites

The effects of state of charge (SOC) on the elastic properties of 3D structural battery composites are studied. An analytical model based on micromechanical models is developed to estimate the effective elastic properties of 3D structural battery composite laminae at different SOC. A parametric study is performed to evaluate how different design parameters such as volume fraction of active materials, stiffness of constituents, type of positive electrode material, etc. affect the moduli of the composite lamina for extremes in SOC. Critical parameters and configurations resulting in large variations in elastic properties due to change in SOC are identified. As the extreme cases are of primary interest in structural design, the effective elastic properties are only estimated for the electrochemical states corresponding to discharged (SOC = 0) and fully charged (SOC = 1) battery. The change in SOC is simulated by varying the volume and elastic properties of the constituents based on data from literature. Parametric finite element (FE) models for square and hexagonal fibre packing arrangements are also analysed in the commercial FE software COMSOL and used to validate the analytical model. The present study shows that the transverse elastic properties E-2 and G(23) and the in-plane shear modulus G(12) are strongly affected by the SOC while the longitudinal stiffness E-1 is not. Fibre volume fraction and the properties of the coating (such as stiffness and Poisson's ratio) are identified as critical parameters that have significant impact on the effect of SOC on the effective elastic properties of the composite lamina. For configurations with fibre volume fraction V-f >= 0.4 and Young's modulus of the coating of 1 GPa or higher, the transverse properties E-2 and G(23) change more than 30% between extremes in SOC. Furthermore, for configurations with high volume fractions of electrode materials and coating properties approaching those of rubber the predicted change in transverse stiffness E-2 is as high as +43%. This shows that it is crucial to take effects of SOC on the elastic properties into account when designing 3D structural battery composite components.