The dielectric layer is an electrically insulating film between conductive electrodes to store electrical energy through dielectric polarization (Figure 1A). The capacitance, C , and maximum energy density, U max , of a SDC depend on the dielectric constant, ϵ r , and breakdown strength, E b , of the dielectric layer, which are calculated as: C = ϵ 0 ϵ r A l (1) U max = 1 2 ϵ 0 ϵ r E b 2 (2) where ϵ 0 is the vacuum permittivity (8.85 × 10^–12 F m⁻¹), A is surface area of conductive electrodes, l is the distance between two electrodes. According to (Eqs. 1–2), both a high dielectric constant and breakdown strength of the dielectric layer are required to achieve a high energy density (Wu et al., 2018; Guo et al., 2020). In addition, good mechanical properties and flexibility as well as excellent interfacial adhesion with the polymer matrix are also the key requirements for the dielectric layer to ensure a simultaneous structural integrity of SDCs (Carlson and Asp, 2013; Chan et al., 2018a). Given the above multifunctional demands, conventional ceramic dielectrics were not suitable for the dielectric layers in SDCs because of their brittle nature despite their high dielectric constants (Bai et al., 2000). The majority of research works in SDCs have been focused on developing novel polymer and graphene-based dielectric layers, including polymer films (Carlson et al., 2010), glass fibre reinforced polymer (GFRP) films (O'Brien et al., 2011), and graphene oxide (GO) films (Chan et al., 2018b; Chan et al., 2018c), to achieve mechanical flexibility together with high specific capacitance. Myriad efforts have been made to improve the dielectric constant, breakdown strength, and mechanical properties of these novel dielectric layers.

Dielectric constant. A high dielectric constant allows easy polarization under external electric fields, beneficial to a high capacitance of SDCs. Polymers usually exhibit relatively low dielectric constants of 2–10, leading to low specific capacitance per unit area (0.21—1.86 μF m⁻²) and low energy density (0.034—0.089 J g⁻¹) of the resulting SDCs (Carlson et al., 2010). One effective strategy to improve the dielectric constants is the use of high-dielectric nano- and micro-fillers such as BaTiO₃ and carbon-based fillers in polymer-based films by providing abundant interfaces for interfacial polarization, forming numerous micro-capacitor networks (Mao et al., 2010; Rahman et al., 2013). In addition, GO films were also used as dielectric layers in SDCs exhibiting much higher specific capacitance of 205.0 μF m⁻² (Chan et al., 2018b; Chan et al., 2018c; Chan et al., 2021) compared to their polymer-based counterparts (0.21—1.86 μF m⁻²) (Carlson et al., 2010). This is because the high electronegativity of oxygen-containing functional groups enhanced the dipolar polarization, resulting in high dielectric constants of 900—2075 for GO films.

Breakdown strength. The energy density of SDCs is directly proportional to the square of dielectric breakdown strength according to Eq. 2. Therefore, increasing the breakdown strength of dielectric layer could be effective in improving the energy density of SDCs significantly. Although the introduction of nano- and micro-fillers enhanced the dielectric constants of nanocomposites, the breakdown strength might even decrease because of the low breakdown strength of fillers and the formation of conductive networks. To mitigate the breakdown strength reduction due to nanofillers, one possible solution is to introduce a buffer layer on the surface of conductive fillers (Yang et al., 2014). In addition to polymer composite films, the GO film exhibited an excellent dielectric breakdown strength due to the presence of oxygen-containing functional groups that hinder the charge transport in the sp² network, making GO an excellent insulator to supress the leakage current and achieve good breakdown strength (Wu et al., 2013).

Mechanical properties. The intrinsic mechanical properties of dielectric layers are important for the overall strengths and stiffnesses of SDCs. GO films are good candidates for dielectric layers because of their excellent intrinsic mechanical properties. Incorporating a GO film dielectric into CFRP electrodes to build an SDC only marginally reduced the tensile modulus and strength of the CFRP by 0.4 and 6.3%, respectively (Chan et al., 2018c). However, delamination cracks within the GO film were found under interlaminar shear loads. Further improving the interlayer strengths between GO sheets was obtained by chemically crosslinking GO using polyallylamine (PAA) (Chan et al., 2020). The PAA-modified GO imparted a 27% higher interlaminar shear strength of the SDC made therefrom against that of an unmodified sample, significantly reducing the possibility of shear-induced delamination. On the other hand, the chemical crosslinking restricted the orientation of polar functional groups, reducing the capacitance and energy density of the SDC. By contrast, a significant reduction of 35% in the tensile modulus was observed when a PET film with thickness of 125 µm was used as the dielectric layer (Carlson and Asp, 2013). This was because the PET film was much softer than the CFRP composite. Nano- and micro-fillers for improving the dielectric constant could potentially improve the mechanical properties of the resultant nanocomposites because of the strong and stiff fillers (Li et al., 2019; Ahmed et al., 2021). However, the dispersion and agglomeration issues always led to inferior properties when a large amount of fillers were used (Zare, 2016). Therefore, it is rather challenging to achieve both dielectric and mechanical properties in the same dielectric layer while maintaining good interfacial adhesion between dielectric and CFRP layers, necessitating the development of novel strategies.

Achieving simultaneous dielectric and mechanical properties. The foregoing discussion indicates that it is rather challenging to achieve both dielectric and mechanical properties in the same dielectric layer while maintaining good interfacial adhesion between dielectric and CFRP layers. To improve the overall performance of SDCs, novel strategies have been developed by tailoring the chemical compositions of dielectric layers. A recent effort was made to investigate the effects of chemical composition of GO on enhancing both mechanical and dielectric properties of the resulting film for SDC applications (Chan et al., 2021). The mechanical and dielectric properties of GO films were closely related to the carbon-to-oxygen atomic ratio. It is well known that the high dielectric constant of GO arises from the abundant oxygen-containing functional groups because of the extensive dipolar polarization. Interestingly, the partial elimination of functional groups through a mild chemical reduction created vacancy defects in GO, leading to the local redistribution of positive and negative charges and forming extra dipole moments to further increase the dielectric constant. Nevertheless, the electrical conductivity of the GO film increased significantly after excessive reduction, making them unsuitable for dielectric layers in SDCs. Furthermore, the removal of functional groups restored the sp² carbon bonds in GO, leading to improved reduced elastic modulus and hardness of the film. The work demonstrated that both mechanical and dielectric properties of GO films were enhanced simultaneously through a mild reduction whilst retaining the electrical insulating nature of GO, resulting in improved overall performance of SDCs.

In summary, polymer composite and GO films are two commonly used dielectric layers to improve both the mechanical and dielectric properties of SDCs. Generally, nano- and micro-fillers can act as reinforcement in polymer composites to enhance the structural performance when added in small amounts. The introduction of high-dielectric-constant fillers such as BaTiO₃, graphene, and MWCNTs into polymer films can induce interfacial polarization and formation of micro-capacitors, improving the capacitive performance of SDCs. Nevertheless, the high capacitance could lead to deteriorated dielectric breakdown strength. Conventional two-phrase composites might not be able to achieve both high dielectric constant and breakdown strength simultaneously. Novel three-phase composites (Shen et al., 2013; Yang et al., 2014) need to be developed as new dielectric materials to optimize the overall performance of SDCs. Alternatively, tuning the chemical composition of GO film by controlling the reduction degree led to both high dielectric and mechanical properties (Chan et al., 2021), making it an ideal dielectric layer for SDCs.

In addition to the dielectric layer, the properties of CFRP electrodes should also be optimized to improve the electrical conductivity without sacrificing the excellent mechanical properties of CFRP. A high electrical conductivity reduces the ESR, which is the major bottleneck limiting the power density and capacity of energy storage devices. Due to the presence of insulating epoxy matrix (10^–12 S m⁻¹) between fibres (Figure 1B), the electrical conductivity of CFRP (∼700 S m⁻¹) is much lower compared to metal-based electrodes (Jia et al., 2014; Chan et al., 2019). To achieve high electrical conductivity of CFRP composite, conductive polymers such as polyaniline can be used instead of epoxy (Salinas-Torres et al., 2013; Hirano et al., 2016). However, the mechanical properties of SDCs might not be as good as those using epoxy as the matrix. To simultaneously improve the electrical conductivity and mechanical properties of CFRP electrodes, one common strategy was incorporating conductive nanoparticles, such as metal and carbon nanofillers, in the epoxy matrix, so that conduction paths were created between fibres (Moisala et al., 2006; Chan et al., 2019). It should be noted that the highly conductive resins can only be used to prepare CFRP prepregs after removing the sizing on the surfaces of carbon fibres, while insulating resins have to be used to bind different capacitive layers together to prevent any electric short-circuit.

The interfacial properties between the CFRP electrodes and dielectric layer also play an important role in determining the mechanical and dielectric properties of SDCs. Delamination between CFRP electrodes and dielectric layers is the most common failure mode for SDCs under mechanical loadings, especially shear and tension (Carlson et al., 2010; Chan et al., 2018c). The interfacial interactions between dielectric layers and CFRP electrodes were enhanced by forming strong covalent bonds through creating functional groups on the surface of dielectric layers, significantly improving the interlaminar shear strength of the SDCs (Carlson et al., 2010). In terms of dielectric properties, common CFRP electrodes were made from carbon woven fabrics containing wavy carbon tows, giving rise to a larger total surface area than that of the unidirectional fibre mat having straight fibres. The presence of micro-sized pores between fibres due to the waviness improved the capacitance by forming a conformable interface with the dielectric layer (Figure 1C). In view of the above, an excellent flexibility of the dielectric layer is essential to ensure their good conformation in the interface region without increasing the separation between the two electrodes (Chung, 2018).

Among all structural energy storage composites, SDCs exhibit the best mechanical performance because of the absence of electrolyte. Although solid polymer electrolytes have been developed for SBs and SSCs, the presence of ionic liquids could affect the crystallinity of polymer matrix, reducing their mechanical properties (Demir et al., 2020). The use of ionic liquid also restricts the working voltage of SBs and SSCs to below 4 V, after which the decomposition of electrolyte might occur (Demir and Searles, 2020; Sharma and Kumar, 2020). The large-scale fabrication of SBs is also challenging because of the high reactivity of lithium, requiring a controlled environment such as glove box for the assembly of SBs (Johannisson et al., 2018). In addition, the service life of SBs is limited by the depletion of active species on the electrodes which takes much shorter time than the common service life of composite structures in aircraft, making them not economical for long-term applications. Despite the lower energy density of SDCs than SSCs and SBs, the broader working voltage allowed SDCs to be used in UAVs, more-electric-aircraft, and military aircraft, in which the most frequently applied voltages onboard ranged from 115 to 540 V (Wheeler and Bozhko, 2014; Alexander et al., 2018). In this case, there was no need to connect SDCs in series, resulting in a lower equivalent capacitance to efficiently provide sufficient voltage for onboard applications. Besides, the fast charge and discharge rates made SDCs superior in applications requiring high power density. SDCs can also be integrated with sustainable energy to supply power to UAV and aircraft avionics. For instance, a GO-based SDC was developed to store the electrostatic energy harvested from the friction between air and aircraft surfaces to power the navigation lights of the aircraft, reducing the total electrical load, as shown in Figure 2.