Characterisation of a structural battery composite and its constituents

Doctoral thesis, 2022

The structural battery composite is a recently successfully developed multifunctional lithium-ion battery. It is safer and capable to carry mechanical load compared to commercially available liquid electrolyte batteries. This makes it possible to apply the structural batteries to replace parts of the structural components in a system and thus reduce the weight of the whole system. The structural battery composite uses carbon fibre, an excellent lightweight material, as the anode material and uses a semi-solid structural battery electrolyte (SBE) material. The entire battery behaves as a solid material. The overall mechanical properties of the structural battery composite material are excellent due to the reinforcement of the carbon fibres and the mechanically robust SBE matrix.

In this thesis, first of all, a multifunctional structural battery composite is manufactured. The structural battery composite uses the lithium storage capacity of carbon fibre for the first time and therefore, has an energy density of 24 Wh/kg and an elastic modulus of 25 GPa. Secondly, characterisation methods were developed for a number of important components in the structural battery composite. This includes precise measurements of transverse and shear moduli on micron-scale carbon fibres, the effect of lithiation on the carbon fibre anode mechanical properties, and 3D reconstruction and simulation of the SBE. For the pristine carbon fibres, focused ion beam combined with scanning electron microscopy (FIB/SEM) was used to accurately mill flat surfaces in different orientations on the carbon fibres, followed by indentation test using atomic force microscopy, and nanoindentation. The elastic hysteresis of the carbon fibres was observed in the experiments. For the first time, the moduli in the transverse and shear directions were derived in conjunction with an accurate orthotropic mechanical model. For the study of lithiation effects on the carbon fibre anode, the focus is on volume expansion and modulus changes. The volume expansion was obtained by analysis of SEM and optical micrographs. By using the protection of hydrophobic ionic liquids, the samples were successfully transferred into a vacuum environment in the SEM and subjected to transverse compression experiments. The transverse modulus of the carbon fibres is found to be doubled after lithiation. Finally, the microstructure of the SBE was reconstructed in 3D. The geodesic tortuosity of the SBE was found to be approximately 1.8. Meanwhile, the elastic modulus and ionic conductivity of the SBE were experimentally measured and simulated. In terms of elastic modulus, the results were consistent, and in terms of ionic conductivity, the simulated result overestimated the measured result.