Thermal and diffusion induced stresses in a structural battery under galvanostatic cycling

When charging or discharging a structural battery composite heat will be generated and the active electrode materials will expand or shrink, inducing internal stresses within the material. These stresses may cause mechanical and/or electrical failure. It is therefore crucial to be able to predict the stress state when evaluating the performance of the material. In this paper, a semi-analytical framework to predict the thermal and diffusion induced stresses in a structural battery under galvanostatic cycling is presented. The proposed model is a concentric cylinder (CC) model coupled with an axisymmetric diffusion model and a one-dimensional heat generation model. The present study shows that the heat generated during electrochemical cycling must be accounted for when evaluating the internal stress state in structural battery composites. Furthermore, the results show that the charge/discharge current, lamina dimensions and residual stresses have significant effect on the internal stress state and effective properties of the composite lamina

Initial study of the microstructure of carbon fibres acting as negative electrodes in structural battery composites

Structural composite batteries are a novel type of multifunctional devices, which have a great potential to remarkably reduce the mass of electric vehicles, and thus increase their energy efficiency. In these batteries, carbon fibres (Carbon fibres) play dual roles: reinforcements (as in CARBON FIBRE composites) and negative electrodes (as in batteries). However, the relationship between the microstructure and the electrochemical property of the Carbon fibres is not well understood. In this study, the microstructure of two Carbon fibres, M60J and IMS65, were studied by using scanning electron microscopy and transmission electron microscopy. Detailed microstructural features were revealed, and correlated to the electrochemical properties of the Carbon fibres. The more disordered microstructure, and rather large pores are the reasons for the better electrochemical properties of IMS65 compared to M60J

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 \ua0and \ua0and the in-plane shear modulus \ua0are strongly affected by the SOC while the longitudinal stiffness \ua0is 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 \ua0≥ 0.4 and Young’s modulus of the coating of 1\ua0GPa or higher, the transverse properties \ua0and \ua0change 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 \ua0is 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

Specimen preparation for transverse modulus measurement of carbon fibres using focused ion beam

Transverse Young’s modulus of carbon fibres is an important material property for micromechanical modeling and design of carbon fibre reinforced composites. To accurately measure their transverse Young’s modulus is of special importance for applications in novel multifunctional devices, such as structural composite batteries. However, experimental measurement of their transverse Young’s modulus is still largely lacking due to experimental challenges. In this study, we successfully prepared high quality longitudinal cross sections from a commercial carbon fibre using precision ion milling in a combined focused ion beam and scanning electron microscope (FIB/SEM) instrument. These cross sections were then directly used in an atomic force microscope (AFM) and a nanoindentation equipment to measure the transverse Young’s modulus. Here, the entire procedure is described in detail. In particular, the most critical aspects for specimen preparation are identified and discussed

Orthotropic criteria for transverse failure of non-crimp fabric-reinforced composites

In this paper, a set of failure criteria for transverse failure in non-crimp fabric-reinforced composites is presented. The proposed failure criteria are physically based and take into account the orthotropic character of non-crimp fabric composites addressing the observed lack of transverse isotropy. Experimental data for transverse loading out-of-plane in combination with in-plane loads are scarce. Therefore, to validate the developed criteria, experimental data are complemented with numerical data from a representative volume element model using a meso-micromechanical approach. The representative volume element model also provides a deeper understanding of how failure occurs in non-crimp fabric composites. Strength predictions from the developed set of failure criteria show good agreement with the experimental and numerical data

On the coupled thermo–electro–chemo–mechanical performance of structural batteries with emphasis on thermal effects

Carbon fibre (CF) based structural batteries is a type of battery designed to sustain mechanical loads. In this paper, a fully coupled thermo–electro–chemo–mechanical computational modelling framework for CF based structural batteries is presented. We consider the combined effects of lithium insertion in the carbon fibres leading to insertion strains, and thermal expansion/shrinkage of the constituents leading to thermal (free) strains, while assuming transverse isotropy. The numerical studies show that the developed framework is able to capture the coupled thermo–electro–chemo–mechanical behaviour. Moreover, it is found that the dominating source for heat generation during galvanostatic cycling is associated with discontinuities in the electrical and chemical potentials at the fibre/electrolyte interface. Further, a limited parameter study shows that the temperature change during electrochemical cycling is significantly influenced by the applied current, thermal properties of the constituents and heat exchange with the surroundings. Finally, for large temperature variations, e.g. as identified during relevant (dis)charge conditions, the magnitude of the thermal strains in the structural battery electrolyte (SBE) are found to be similar to the insertion induced strains

Effect of specimen width on strength in off-axis compression tests

Compression tests have been performed according to ASTM D6641 to check whether 12 mm is a sufficient width for off-axis tests of a unidirectional Non Crimp Fabric (NCF) reinforced carbon-fibre composite. Various off-axis angles are tested in a larger context and it is important to establish a representative material volume. The test matrix consists of two different widths for two off-axis cases, 15° and 20° with a total sample size of 24. A two-sample T-test is performed for each off-axis angle to check if there is a statistically significant difference of the compressive strength between specimens with different widths. The null hypothesis, that there is no difference between the mean values is tested with a double-tailed test on a 5 % significance level. Neither of the cases may be rejected, i.e. there is no statistically significant difference on the 5 % level. The 15° off-axis case returns a p-value of 7.4 % and the 20° off-axis case gives a p-value of 21.3 %. It can be concluded that the effect is small and not statistically significant. It means that remaining off-axis testing in the larger context can proceed with the nominal width of 12 mm

X-ray tomography based numerical analysis of stress concentrations in non-crimp fabric reinforced composites - assessment of segmentation methods

In this study two automated segmentation methodologies of an X-ray computer tomography based numerical analysis are compared. These are then assessed based on their influence on the stress distribution results of finite element models of glass fibre reinforced composites made out of non-crimp fabrics. Non-crimp fabrics reinforced composites are commonly used for wind turbine blades due to their high stiffness to weight ratio for the dominating bending load. Finite element modelling based on X-ray computer tomography allows the reduction of the cost and can accelerate the development process of the key material parameters of wind turbine blades. Recent research progress in the last years has laid the basis for such a procedure. Those processes must be easy applicable, fast and accurate. The main challenge in current methodologies is the segmentation part. The segmentation methods applied for this study have overcome this issue by being automated. This allows for a comparatively fast transfer from X-ray computer tomographic data to finite element results

The transition from out-of-plane to in-plane kinking due to off-axis loading

A comprehensive test campaign has been performed on coupon level to gain fundamental understanding of compressive failure in unidirectional NCF composites for aerospace applications. A subset of this study is focusing on the effect of off-axis loading, where a number of laminates have been tested with fibres oriented in off-axis angles in the interval 0-20\ub0 in steps of 5\ub0. Our hypothesis is that 0\ub0 laminates fail by kinking out-of-plane and as the off-axis angle is increased, there is a shift to in-plane kinking as the in-plane shear component increases. The contribution from this shear component on kinking will have little effect on the compressive strength until in-plane kinking becomes "dominant" over out-of-plane kinking. Preliminary results indicate a transition from out-of-plane to in-plane governed kinking to occur at an off-axis angle between 10\ub0 and 15\ub0