Many closed-cell foams exhibit an elongated cell shape in the foam rise direction, resulting in anisotropic compressive properties, e.g. modulus and strength. Nevertheless, the underlying deformation mechanisms and how cell shape anisotropy induces this mechanical anisotropy are not yet fully understood, in particular for the foams with a high cell face fraction and low relative density. Moreover, the impacts of mesostructural stochastics are often overlooked. This contribution conducts a systematic numerical study on the anisotropic compressive behaviour of low-density closed-cell foams (with a relative density <0.15), which accounts for cell shape anisotropy, cell structure and different mesostructural stochastics. Representative volume elements (RVE) of foam mesostructures are modelled, with cell walls described as Reissner–Mindlin shells in a finite rotation setting. A mixed stress–strain driven homogenization scheme is introduced, which allows for enforcing an overall uniaxial stress state. Uniaxial compressive loadings in different global directions are applied. Quantitative analysis of the cell wall deformation behaviour confirms the dominant role of membrane deformation in the initial elastic region, while the bending contribution gets important only after buckling, followed by membrane yielding. Based on the identified deformation mechanisms, analytical models are developed that relate mechanical anisotropy to cell shape anisotropy. It is found that cell shape anisotropy translates into the anisotropy of compressive properties through three pathways, cell load-bearing area fraction, cell wall buckling strength and cell wall inclination angle. Besides, the resulting mechanical anisotropy is strongly affected by the cell shape anisotropy stochastics while almost insensitive to the cell size and cell wall thickness stochastics. The present findings provide deeper insights into the relationships between the anisotropic compressive properties and mesostructures of low-density closed-cell foams.

Structural power composites, multifunctional materials that can withstand mechanical loads while storing/delivering electrical energy, are gaining significant interest. However, a consequence of melding disparate structural and electrochemical technologies is that there are no common characterization and reporting protocols, undermining the advancement of this emerging field. This Perspective paper sets out the challenges and resulting issues in the literature and recommends best practices and requirements for future protocols for reporting multifunctional performance. A key recommendation is that a “universal coupon” should be developed to be used for both mechanical and electrochemical characterization of cells, and hence credibly declare multifunctional performance. Ultimately, such a universal coupon can simultaneously characterize both functions, so as to glean electrochemical–mechanical coupling phenomena. This article recommends reporting guidelines so as to avoid the current ambiguities associated with normalization and permit robust comparison across the literature. The aspiration is that the guidelines and framework outlined in this paper lay the groundwork for formal standard methods to be developed and agreed upon. Establishing robust characterization and clearer reporting permits researchers and industry to take an informed view of the literature and provides a better grounding for the adoption of this technology, underpinning future industrialization of these emerging materials.

Laminated structural batteries present a transformative solution to reducing weight constraints in electric vehicles. These structural batteries are based on a multifunctional material that incorporates an energy storage function within a carbon fiber-reinforced polymer. Despite the potential of this technology, the intricate morphology of fiber-matrix or electrode-electrolyte interfaces and the impact of long-term cycling at low current rates (C-rates) on these interfaces remain insufficiently understood. This study addresses these critical knowledge gaps by examining the influence of matrix composition on the long-term electrochemical performance of structural battery electrodes and exploring advanced techniques to investigate carbon fiber-matrix interfaces. Localized imaging and X-ray scattering techniques were used to characterize morphological changes at the electrode-electrolyte interfaces by analyzing negative structural electrodes. The findings revealed that the matrix composition influences long-term electrochemical behavior and fiber-matrix interface formation. While the intrinsic properties of carbon fibers largely remain unaffected by long-term cycling, cycling promotes debonding at fiber-matrix interfaces. Nonetheless, residual regions of adhesion persist, underscoring the potential for preserving multifunctionality even under prolonged cycling conditions. These insights advance the understanding of interface dynamics, which is critical for optimizing structural battery technologies.

Structural multifunctional materials have the potential to transform current technologies by implementing several functions to one material. In a multifunctional structural battery, mass saving and energy efficiency are created by the synergy between the mechanical and electrochemical properties of the material's constituents. Consequently, structural batteries could e.g. mitigate electric vehicle overweight or enable thinner portable electronics. This requires combining the best composite and battery manufacturing practices. In the present work this is achieved through the infusion of a stack of carbon fibre-based electrodes with a hybrid polymer-liquid electrolyte. The realised full cell structural battery is based on carbon fibre electrodes with a lithium iron phosphate (LiFePO4) coating on the positive side. This battery laminate shows a very good balance between energy density, stiffness and strength of 33.4 Wh/kg, 38 GPa and 234 MPa, respectively. To push these performances further, different improvement strategies are discussed, and the results are compared with previously published target performances. Ultimately, we demonstrate the feasibility of designing and manufacturing all-fibre solid-state structural batteries as a material solution for future lightweight electric commodities.

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.

Bicontinuous solid-liquid electrolytes can combine high ionic conduction with high mechanical performance and provide an opportunity to realize laminated structural batteries. Polymerization-induced phase separation is a facile one pot reaction to make these electrolytes. It is a versatile method but requires control over the complex interaction of various parameters to tune the morphologies and properties of biphasic electrolytes as it is highly system dependent. This study examines the effects of thiol-ene chemistry and parameters such as porogen type and content, thiol content, and salt concentration in the liquid electrolyte, linking these factors to their curing behavior, morphology, and multifunctional properties. We present a toolbox showing how different morphologies and properties can be reached by changing these parameters. The porogen type and a 10% increase in the porogen content affected ionic conductivity by an order of magnitude. Thiol-ene chemistry accelerates the curing process but reduces mechanical properties while slightly increasing the ionic conductivities for small amounts of thiol. The best negative structural electrode, containing carbon fibers as negative electrode, showed increased rate capability compared to previous work and a discharge capacity of 219 mA h g-1 at a current density of 18 mA g-1 (∼0.08C). The results also indicate the potential of applying the concept of highly concentrated electrolytes in structural electrodes to improve safety and capacity retention while maintaining high specific capacities and good rate capability. Interestingly, the increased ionic conductivity of the electrolyte does not always imply an improved electrochemical performance of the structural electrode. 

This study centers on investigating the influence of conductive additives, carbon black (Super P) and graphene, within the context of LiFePO4 (LFP)-impregnated carbon fibers (CFs) produced using the powder impregnation method. The performance of these additives was subject to an electrochemical evaluation. The findings reveal that there are no substantial disparities between the two additives at lower cycling rates, highlighting their adaptability in conventional energy storage scenarios. However, as cycling rates increase, graphene emerges as the better performer. At a rate of 1.5C in a half-cell versus lithium, electrodes containing graphene exhibited a discharge capacity of 83 mAhgLFP−1; those with Super P and without any additional conductive additive showed a capacity of 65 mAhgLFP−1 and 48 mAhgLFP−1, respectively. This distinction is attributed to the structural and conductivity advantages inherent to graphene, showing its potential to enhance the electrochemical performance of structural batteries. Furthermore, LFP-impregnated CFs were evaluated in full cells versus pristine CFs, yielding relatively similar results, though with a slightly improved outcome observed with the graphene additive. These results provide valuable insights into the role of conductive additives in structural batteries and their responsiveness to varying operational conditions, underlining the potential for versatile energy storage solutions.

Tow-based discontinuous composites are an attractive alternative material to conventional continuous composites as they offer in-plane isotropy, enhanced manufacturability allowing to achieve complex 3D shapes with high curvatures and local reinforcement in critical areas, while also maintaining high strength and stiffness, therefore expanding the design space significantly. In addition, the use of ultra-thin tapes and optimised manufacturing methods can increase the mechanical properties even further and change the damage mechanisms. Fatigue, however, could be a limiting design factor, as the fatigue behaviour of these materials has not been fully characterised. This work presents a complete study on the fatigue response of ultra-thin tow-based discontinuous composites: fatigue S–N curves are measured, and the damage and failure mechanisms are characterised utilising optical and scanning electron microscopy. Finally, a critical interpretation of the results is also presented by comparing the performance of ultra-thin tow-based discontinuous composites against other similar fibre reinforced composites and metals. It is shown that the optimised manufacturing methods combined with low tape thickness leads to enhanced quasi-isotropic fatigue performance. In addition, the fatigue limit was raised significantly compared to other discontinuous composites, and the tow-based discontinuous composites outperformed their metal counterparts when the results were normalised with density.

This study presents the fabrication of LiFePO4 (LFP)-coated carbon fibers (CFs) as a positive electrode component for structural batteries, utilizing a spray coating technique. The successful coating of CFs through this method demonstrated their usefulness as efficient current collectors. The electrodes obtained using this method underwent electrochemical evaluations. Throughout the extended cycling tests at C/7, the maximum specific discharge capacity reached 146 mAh/g, maintaining a 77% capacity retention after 100 cycles. In rate performance assessments at the faster C-rate of 1.5C, the capacity measured 123 mAh/g, with a retention of 96%. The application of spray coating emerges as a promising technique for electrode production in structural batteries, showcasing its potential for optimizing performance in multifunctional energy storage systems.

Most products today have several functions, but these are achieved by integrating different monofunctional devices and/or materials in a system. Having several functions simultaneously in one single material has many potential advantages, such as a structural material that can also store energy, have self-sensing or self-healing capability or any other physical function. This would lead mass and resource savings, being more energy efficient and thus more sustainable. This paper presents a mini review on how carbon fibres can be used for integrating several functions simultaneously in a high-performance load carrying structural material using the electrical and electrochemical properties of carbon fibres. Through this carbon fibre composites can also store energy like a lithium-ion battery, be used as a strain sensor, have electrically controlled actuation and shape-morphing, and be used as an energy harvester.