Multifunctionalization of fiber-reinforced composites, especially by adding energy storage capabilities, is a promising approach to realize lightweight structural energy storages for future transport vehicles. Compared to conventional energy storage systems, energy density can be increased by reducing parasitic masses of non-energy-storing components and by benefitting from the composite meso-and microarchitectures. In this paper, the most relevant existing approaches towards multifunctional energy storages are reviewed and subdivided into five groups by distinguishing their degree of integration and their scale of multifunctionalization. By introducing a modified range equation for battery-powered electric aircrafts, possible range extensions enabled by multifunctionalization are estimated. Furthermore, general and aerospace specific potentials of multifunctional energy storages are discussed. Representing an intermediate degree of structural integration, experimental results for a multifunctional energy-storing glass fiber-reinforced composite based on the ceramic electrolyte Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 are presented. Cyclic voltammetry tests are used to characterize the double-layer behavior combined with galvanostatic charge-discharge measurements for capacitance calculation. The capacitance is observed to be unchanged after 1500 charge-discharge cycles revealing a promising potential for future applications. Furthermore, the mechanical properties are assessed by means of four-point bending and tensile tests. Additionally, the influence of mechanical loads on the electrical properties is also investigated, demonstrating the storage stability of the composites.

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Structural supercapacitors are very interesting multifunctional devices combining the properties of an electrical energy storage device and a structural component simultaneously. These types of supercapacitors are mostly equipped with solid state electrolytes, instead of traditional liquid electrolytes, avoiding leakage and safety problems and supporting the mechanical performance of the composite materials. In the present study, the Lithium-ion based solid ceramic electrolyte Li1.4Al0.4Ti1.6(PO4)3 was successfully synthesized by sol-gel method. Its electrical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Results show that Li1.4Al0.4Ti1.6(PO4)3 possesses a conductivity of 2.94×10−4 S/cm at room temperature and a specific capacity of 55.57 μF/g. The as-prepared samples were embedded into fiber composite material using the aviation approved resin RTM6 with an injection process making the composite structure flexible. Subsequently, the specific capacity and conductivity were tested getting values of 53.44μF/g and 2.00×10−4 S/cm respectively. The reason for electrical properties loss was investigated by computerized tomography (CT) and EIS tests and the results provide reference for the future research.

In the present work, we report on structural supercapacitors which are based on NASICON-type solid electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP). The nanostructured electrodes incorporate single-wall carbon nanotubes (SWCNTs) mixed with the LATP electrolyte. The complete energy storage devices are manufactured in a sandwich structure consisting of two nanostructured electrode layers which are separated by a pure LATP layer. The as-prepared specimens are embedded in composite materials with Airstone 880/886H epoxy resin as matrix. Their electrical properties are characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). At ambient temperature, the addition of 6.5 wt. % SWCNTs results in a distinct improvement by reducing the total resistance of the embedded devices and enhances the capacitance from 0.025 mF g−1 to 3.160 mF g−1 at a scan rate of 5 mV s−1. Electrical measurements of two types of specimens are then applied under different temperatures from ambient temperature to 80 °C. It is observed that the equivalent series resistance (ESR) of device with SWCNTs decreases greatly and capacitance increases comparing with the device without SWCNTs. As a conclusion, the structural supercapacitors acquire excellent performance through high efficient double layer effects realized by nanostructured electrode/electrolyte interphase (large surface electrode areas).