Computational modelling of structural battery composites

Doctoral thesis, 2022

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.