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.

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.

Tow Based Discontinuous Composites (TBDCs) are a new class of composite materials which combine in-plane isotropy, high strength and stiffness and enhanced manufacturability. However, due to their complicated micro-architecture, characterising the performance of these materials and predicting their response is challenging. This work develops a complete experimental and analytical framework which identifies all the key properties in the performance of the TBDCs, characterises them experimentally and builds an analytical predictive tool for both the stiffness response and the strength of the TBDC material. Fractography is also utilised to identify the damage mechanisms and correlate them with the analytical predictions. A parametric study is developed which shows the critical effect that the tape thickness and mode II fracture toughness have on the TBDCs. Finally, the performance of the material is compared to similarly developed TBDCs from the literature and shows the significant strength and stiffness increases recorded through the combination of the thin high-modulus tapes and the increased fibre volume fractions.

This study investigates the viability of implementing multifunctional structural power composites in a four-seater air taxi, the CityAirbus. For a given specific energy of the power source, the cruise endurance can be approximately doubled by using structural power composites as opposed to conventional batteries. Replacing all the eligible composite mass and batteries with structural power composites can reduce the CityAirbus weight by 25%. To achieve the current design performance, the minimum required elastic modulus, strength, specific energy and power for the structural power composite are 54 GPa, 203 MPa, 74 Wh/kg and 376 W/kg, respectively: current state-of-the-art structural power composites are now approaching this level of performance. Hence, structural power composites are considered feasible for adoption in the urban air mobility sector and have the potential to improve endurance and facilitate commercialization. This paper also discusses several key challenges that must be addressed to realize the adoption of structural power composites in future electric air taxis. 

A structural battery is a multifunctional battery that can carry a load while storing energy. Structural batteries have been a cutting-edge research focus in the last decade and are mainly based on polyacrylonitrile (PAN)-carbon fibers (CFs). In this work, positive electrodes based on PAN-carbon fibers were manufactured with powder impregnation (siphon impregnation) technique using a water-based slurry containing lithium iron phosphate (LFP) as the active electrode material and the water-soluble binder polyethylene glycol (PEG). Different coating compositions, electrode-drying temperatures, and coating parameters were investigated to optimize the coating uniformity and the electrochemical performances. Scanning electron microscopy results showed that the electrode materials coat the CFs uniformly, conformably, and individually. Electrochemical characterization of pouch cells shows that the electrodes containing 6 wt% PEG dried at 140 degrees C have the best battery performance, delivering a first discharge capacity of 151 mAh g-1 and capacity retention higher than 80% after 100 cycles. Moreover, excellent capacity reversibility was achieved when the electrodes were cycled at multiple C-rates attesting to their stability. The results demonstrate that CFs perform excellently as current collectors in positive electrodes.

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 autono-mous 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 deforma-tions 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.