Structural batteries are hybrid and multifunctional composite materials able to carry load and store electrical energy as a lithium ion battery. In such a device, carbon fibres are used as the primary load carrying material, due to their excellent strength and stiffness properties, but also as the active negative electrode providing the battery function. Structural batteries can be made utilising a traditional battery structure similar to a composite laminate in which also the positive electrode is reinforced with carbon fibres coated with lithium oxides particles.

The research comprises pursuing several research tracks involving studying electrode materials (one being carbon fibres), matrix materials which provide stiffness for the composite while still being ionically conductive (structural battery electrolytes), device development, etc. Possible ground-breaking applications for such innovative material concepts are chassis for mobile phones, laptops or car- and airplane structures that simultaneously carry load and store electrical energy, allowing substantial weight savings.

Supercapacitors provide exceptional power density, high dimensional stability, and very long cycle life, making them excellent candidates for new structural energy storage systems. They typically use high surface area carbon electrodes laminated with a separator (Shirshova et al., 2013 and Carlson and Asp, 2013). This architecture maps onto conventional composite laminate designs, if suitable constituents are developed. However, there are several challenges. One is to provide a ‘structural electrode’, a conductive carbon-rich electrode with high electrochemical surface area and maximum mechanical performance, in particular stiffness and strength. Here, we use both intrinsically high strength nanoscale carbons (nanotubes), and combine high strength carbon fibres with nanostructured carbon matrices.

For example, structural supercapacitor composites can combine carbon fibre fabrics with thin electric double-layer capacitor based on carbon nanotube (CNT) fibre veils as interleaves, (Senokos et al., 2018) as shown in Figure 1. The first step is to produce a thin (100 micron-thick) electrochemical double-layer capacitor (EDLC) by laminating two CNT fibre electrodes with a polymer electrolyte. This material can then be integrated between structural woven CF lamina by a conventional vacuum resin infusion manufacturing process. This strategy produces structural composites with electrochemical properties that are very resilient to mechanical deformations. In-situ galvanostatic charge discharge measurements during bending show almost no changes in capacitance even at very large deformations (Figure 2 a-d). This resilience is due to the high toughness of the CNT fibre fabrics.

Another strategy uses high surface area carbon aerogels as a monolithic porous ‘matrix’ for conventional structural carbon fibres. The aerogel provides both electrochemical activity and mechanical rigidity (Qian et al., 2013 and Nguyen et al., 2019) schematically shown in Figure 2.

The second challenge is the development of a ‘structural electrolyte’ that can carry mechanical shear loads whilst providing a high ionic conductivity. Here, we use nanostructured polymer systems that organise to form bicontinuous structural and ionic-conducting phases, though thermally-induced phase segregation (Kerbs et al., 2013)) or self-assembly of block copolymers (Leijonmarck et al., 2013). For example, a fine-tuned mixture of a structural matrix with a lithium salt, segregates upon curing into a bi-continuous microstructure of stiff matrix with channels for ion transport (Shirshova et al., 2013). A more top-down approach, can exploit CNT fibre electrodes: by making macroscopic patterns in the electric double-layer capacitor part of the structural supercapacitor to promote the interconnection of CF plies by epoxy (Figure 3 e-f). This method is not only simple, but also enables the design and fabrication of composites with different balance of mechanical and electrochemical properties.

Realizing high energy/power densities and excellent mechanical properties simultaneously presents numerous challenges in terms of materials properties, fabrication routes, testing, evaluation, etc. Further details are discussed in specific reviews (Asp and Greenhalgh, 2014; González et al., 2017) on the topic.