Publications

Carbon fibre based electrodes offer the potential to significantly improve the combined electrochemical and mechanical performance of structural batteries in future electrified transport.

This review compares carbon fibre based electrodes to existing structural battery electrodes and identifies how both the electrochemical and mechanical performance can be improved. In terms of electrochemical performance achieved to date, carbon fibre based anodes outperform structural anode materials, whilst carbon fibre based cathodes offer similar performance to structural cathode materials. In addition, while the application of coating materials to carbon fibre based electrodes can lead to improved tensile strength compared to that of uncoated carbon fibres, the available mechanical property data are limited; a key future research avenue is to understand the influence of interfaces in carbon fibre based electrodes, which are critical to overall mechanical integrity.

This review of carbon fibre based electrode materials, and their assembly strategies, highlights that research should focus on sustainable electrode materials and scalable assembly strategies.

Structural batteries utilise the bifunctionality of carbon fibres to act as a load-bearing structure, but also as a conductive current collector for a battery electrode. Lithium-ion transport during the cycling of structural battery cathodes coated with different morphologies is investigated using Iron X-Ray Absorption Near Edge Spectroscopy (Fe XANES) and correlated to electrochemical performance.

Two contrasting morphologies were produced using slurry coating and electrophoretic deposition (EPD) of lithium-iron phosphate (LFP) onto continuous carbon fibres. The ability to study the different structural battery cathode morphologies operando allows for a comparative analysis of their impact on cycling performance. The EPD-coated fibres exhibited a more homogeneous, thinner coating around the fibre compared to the thick, one-sided coating produced using slurry coating. Despite a lower initial capacity and 30 % lithium re-intercalation loss in the first cycle, EPD-coated fibres exhibited more stable capacity retention over time compared to slurry-coated counterparts. Electrochemical Impedance Spectroscopy (EIS) revealed initially high ionic resistance for the EPD-coated fibres, but a larger increase in resistance in the slurry coated electrodes over multiple cycles. 

This study demonstrated an innovative and novel method of analysing in greater detail, the cycling ability of the coated cathode material on carbon fibres using synchrotron radiation.

This study presents the first demonstration of the use of X-ray diffraction (XRD) to quantify the radial or transverse deformation in Hexcel IM7 PolyAcryloNitrile (PAN)-based carbon fibres at temperatures as low as 200 K (-70 °C).

The Coefficient of Thermal Expansion (CTE) is a critical design parameter that needs to be precisely quantified for the next generation of carbon fibre-based Liquid Hydrogen (LH₂) storage tanks for net-zero aviation. This variable quantitatively describes the thermal mismatch between the fibre and the resin that is the driver for microcracking and tank leakage. However, quantification of the CTE of the fibres is experimentally challenging. The results provide unique insights, indicating that the microscopic transverse CTE of the fibre (α₂₂) is equal to 26.2 × 10^-6 K^-1 and is governed by van der Waals forces, similar to those in the basal c-axis (out-of-plane) direction of graphite and the radial direction of multi-wall carbon nanotubes.

Taking into account the microcrack-induced relaxation effect reported in polycrystalline graphite, the macroscopic fibre transverse CTE was determined to be 7.86 × 10^-6 K^-1. XRD data were also collected on Hexcel IM7/8552 Uni-directional (UD) and Quasi-isotropic (QI) composite laminates to investigate the influence of the interaction of the resin matrix with the fibre lattice and the stacking sequence on the development of thermal fibre lattice strain.

In the UD laminate, the presence of resin induces an additional transverse strain in the fibres as a result of resin contraction during cooling, leading to the development of a compressive strain in the fibre direction. This behaviour was found to be in good agreement with numerical simulations, with a 13 % error at the lowest measured temperature.

In contrast, the fibres in the QI configuration were reinforced in the transverse direction, effectively mitigating the influence of resin contraction. These CTE values, insights, and resulting models are essential for multi-scale modelling, design and certification of carbon fibre composite LH₂ tanks that are required to achieve net-zero aviation.

This paper demonstrates a new approach that exploits both lattice strain mapping via Wide Angle X-ray Scattering (WAXS) and Digital Volume Correlation (DVC) of Computed Tomography (CT) to understand the material response at different length scales in Carbon Fibre Reinforced Polymers (CFRPs) under in-situ loading, a phenomenon of substantial importance for the modelling, design, and certification of composite structures. WAXS gives insight into fibre lattice strain, while DVC provides sub-laminate response in the CFRP. A detailed numerical simulation was also developed to compare with these novel experimental methods.

This approach is the first demonstration that the strain within the crystalline regions of the fibre is distinct from the sub-laminate behaviour, with up to 80 % and 36 % differences in the longitudinal and transverse directions, respectively, as a result of the complex microstructure of the fibres.

An improved understanding of composite behaviour is fundamental to understanding how strain accommodation leads to structural failure, providing routes to refine part rejection criteria and reduce the environmental impact of this increasingly widespread material class.

Accurate state-of-charge estimation is critical to the safe and reliable operation of Li-ion batteries. The accuracy of established state-of-charge estimation methods is limited due to measurement drift over many cycles or models which are challenging to reliably parametrise. Ultrasound sensing techniques have the potential to overcome these issues by providing a direct link between the state-of-charge and physical properties of the cell. Ultrasonic time-of-flight is the predominant measurement used to predict state-of-charge, but this measurement is complicated by changes in cell thickness during cycling, the effect of which has gone underappreciated until now.

Unlike time-of-flight, ultrasonic velocity is unaffected by cell thickness changes, and this paper demonstrates for the first time the advantages of using velocity to predict state-of-charge. Velocity is found to be more consistent than time-of-flight at a range of different C-rates, temperatures, and across multiple cycles. This culminates in a large improvement in the prediction of state-of-charge during a US06 drive cycle, where the mean difference between predicted and actual state-of-charge is improved from 12.3 % to 3.8 % when velocity is used instead of time-of-flight. These results are a significant step towards the realisation of state-of-charge prediction using ultrasound and hence safer and more reliable Li-ion battery operation.