Computational modelling of structural battery composites

Batteries and surrounding structures (e.g. battery modules and packs) in electrical vehicles and devices are often designed in a way that prevents the electro-chemically active part of the battery cells from being exposed to mechanical loads during operation/service. This means that the energy storage capability is added as a monofunctional addition to the system (i.e. it only provides one functionality, storing energy). Hence, one of the main drawbacks of the existing technology is its energy storage to weight ratio, in terms of the complete system. A viable route to improve this ratio is to develop energy storage solutions with the ability to sustain mechanical loads. Indeed, by adding this additional functionality, such solutions offer significant system mass and volume savings and allow for innovative future design of electric vehicles and devices.The structural battery composite material is made from carbon fibre reinforced structural battery electrolyte (SBE), and exploits the multifunctional capability of the material constituents to facilitate electrical energy storage in structural components. Due to its inherent multifunctionality, the physical phenomena occurring within the material during operation will interact. Further, due to the fact that the studied material is intended to perform multiple functions some of the couplings between the physical processes are expected to be more pronounced, and critical to design, as compared to conventional batteries. Hence, to accurately predict and evaluate the combined performance of structural batteries, coupled multiphysics models are needed.In this thesis, a computational modelling framework to predict the coupled thermo-electro-chemo-mechanical performance of structural batteries is developed. The framework is utilized to study the essential couplings between the physical processes and numerical predictions are compared favourably with experimental data. It is shown that two-way coupling between the electro-chemical and mechanical processes is important to account for when evaluating the combined electro-chemo-mechanical performance of structural batteries. Further, it is shown that the convective contribution to the mass flux of ions in the SBE, as well as the thermal effects during operations are crucial to consider when evaluating the combined performance. Moreover, the framework is extended to study an electro-chemically driven actuator and sensor utilizing carbon fibre-SBE electrodes. Finally, in addition to the modelling work a laminated structural battery with unprecedented multifunctional (i.e. combined mechanical and electro-chemical) performance is manufactured and characterized, featuring an energy density of 24 Wh/kg and an elastic modulus of 25 GPa and tensile strength exceeding 300 MPa

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

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

On the multifunctional performance of structural batteries

The structural battery composite is a composite material that can store electrical energy (i.e. work as a battery) while simultaneously provide mechanical integrity in a structural system. Due to its inherent ability to store energy this material offers significant weight savings on a system level. For this reason, this type of material has the potential to revolutionize future design of electric vehicles and devices.Because of the multifunctional nature of this material novel design frameworks and multiphysics models are needed to predict and evaluate its multifunctional performance. Furthermore, charging and discharging the structural battery will generate heat and alter the volume and elastic properties of the constituents during operation. This will affect the effective properties of the material and generate internal stresses which can cause mechanical and/or electrical failure. For these reasons, it is crucial to be able to predict the multifunctional performance of the material and how it varies during operation.In this thesis, modelling frameworks to predict and analyse the multifunctional performance of structural batteries is developed. The frameworks are used to estimate the multifunctional performance of different material designs and to study the mechanical consequences from electrochemical cycling. We demonstrate how the material design can be altered to enhance different performances. Furthermore, significant changes in effective elastic properties for the structural battery composite with change in state of charge (SOC) are found. This illustrates the need to consider changes in the elastic properties with SOC when designing structural battery components. Finally, it is shown that the properties of the constituents, charge/discharge current, lamina dimensions and residual stresses have significant effect on the internal stress state and the elastic properties of the composite lamina. The results also show that the heat generated during electrochemical cycling must be accounted for when evaluating the internal stress state in structural batteries

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

Multifunctional Carbon Fiber Composites: A Structural, Energy Harvesting, Strain-Sensing Material

Multifunctional structural materials are capable of reducing system level mass and increasing efficiency in load carrying structures. Materials that are capable of harvesting energy from the surrounding environment are advantageous for autonomous electrically powered systems. However, most energy harvesting materials are non-structural and add parasitic mass, reducing structural efficiency. Here, we show a structural energy harvesting composite material consisting of two carbon fiber (CF) layers embedded in a structural battery electrolyte (SBE) with a longitudinal modulus of 100 GPa-almost on par with commercial CF pre-pregs. Energy is harvested through mechanical deformations using the piezo-electrochemical transducer (PECT) effect in lithiated CFs. The PECT effect creates a voltage difference between the two CF layers, driving a current when deformed. A specific power output of 18 nW/g is achieved. The PECT effect in the lithiated CFs is observed in tension and compression and can be used for strain sensing, enabling structural health monitoring with low added mass. The same material has previously been shown capable of shape morphing. The two additional functionalities presented here result in a material capable of four functions, further demonstrating the diverse possibilities for CF/SBE composites in multifunctional applications in the future

Conceptual design framework for laminated structural battery composites

The structural battery composite is a class of composite materials with ability to provide mechanical integrity in a structural system while simultaneously store electrical energy (i.e. work as a battery). In this paper a framework to estimate the mechanical and electrical performance of laminated structural battery composites is proposed. The mechanical performance of the battery composite laminate is assessed by estimating the in-plane elastic properties of the laminate using Classical Laminate Theory. The electrical performance is assessed estimating the specific capacity and energy density of the component. The developed framework is applied on an A4 sized structural battery composite demonstrator, as part of the Clean Sky 2 project SORCERER [1] to demonstrate the capabilities of the framework. The design process for the demonstrator is presented and mechanical and electrical performance metrics are estimated for three laminate configurations, one promoting structural performance, one promoting electrical performance and one intermediate. As the material provides both load carrying and electrical energy storage capabilities, the laminate configuration can be alternated to provide suitable performance based on the purpose of the component

Electro-chemo-mechanically coupled computational modelling of structural batteries

Structural batteries are multifunctional composites that combine load-bearing capacity with electro-chemical energy storage capability. The laminated architecture is considered in this paper, whereby restriction is made to a so called half-cell in order to focus on the main characteristics and provide a computational tool for future parameter studies. A thermodynamically consistent modelling approach is exploited for the relevant electro-chemo-mechanical system. We consider effects of lithium insertion in the carbon fibres, leading to insertion strains, while assuming transverse isotropy. Further, stress-assisted ionic transport is accounted for in addition to standard diffusion and migration. The relevant space-variational problems that result from time discretisation are established and evaluated in some detail. The proposed model framework is applied to a generic/idealized material representation to demonstrate its functionality and the importance of accounting for the electro-chemo-mechanical coupling effects. As a proof of concept, the numerical studies reveal that it is vital to account for two-way coupling in order to predict the multifunctional (i.e. combined electro-chemo-mechanical) performance of structural batteries

Unit cells for multiphysics modelling of structural battery composites

To predict the multifunctional performance of structural battery composites, multiple physical phenomena need to be studied simultaneously. Hence, multiphysics models are needed to evaluate the complete performance of this composite material. In this study the coupled analysis for multiphysics modelling of structural battery composites is presented and modelling strategies and unit cell designs are discussed with respect to the different physical models. Furthermore, FE-models are setup in the commercial Finite Element (FE) software COMSOL to study if existing physics-based modelling techniques and homogenization schemes for conventional lithium ion batteries can be used to describe the electrochemical behaviour of structural battery composites. To predict the microscopic behaviour, the local variation of the mass and charge concentrations need to be accounted for. Hence, refined models with appropriate boundary conditions are needed to capture the microscopic conditions inside the material. The numerical results demonstrate that conventional physics-based 1D battery models and homogenization schemes based on porous media theory can be used to predict the macroscopic electrical behaviour of the fibrous structural battery. For future work electrochemical experiments on battery cell level are planned to validate the numerical results

Variationally consistent modeling of a sensor-actuator based on shape-morphing from electro-chemical–mechanical interactions

This paper concerns the computational modeling of a class of carbon fiber composites, known as shape-morphing and strain-sensing composites. The actuating and sensing performance of such (smart) materials is achieved by the interplay between electrochemistry and mechanics, in particular the ability of carbon fibers to (de)intercalate Li-ions repeatedly. We focus on the actuation and sensing properties of a beam in conjunction with the appropriate “through-the-thickness” properties. Thus, the electro-chemo-mechanical analysis is essentially two-dimensional, and it is possible to rely heavily on the results in Carlstedt et al. (2020). More specifically, the cross-sectional design is composed of two electrodes, consisting of (partly) lithiated carbon fibers embedded in structural battery electrolyte (SBE), on either side of a separator. As a result, the modeling is hierarchical in the sense that (macroscale) beam action is combined with electro-chemo-mechanical interaction along the beam. The setup is able to work as sensor or actuator depending on the choice of control (and response) variables. Although quite idealized, this design allows for a qualitative investigation. In this paper we demonstrate the capability of the developed framework to simulate both the actuator and sensor modes. As proof of concept, we show that both modes of functionality can be captured using the developed framework. For the actuator mode, the predicted deformation is found to be in close agreement with experimental data. Further, the sensor-mode is found to agree with experimental data available in the literature