*Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

it suitable for energy storage applications, the SPE is formed by blending a durable, cross-linked epoxy-based matrix with a fast-diffusing lithium salt/ionic liquid electrolyte. This blending process is carried out using a convenient one-pot curing method. In a successful endeavor, Kwon et al. [100] managed to create epoxy-based cross-linked SPEs by incorporating plasticized lithium salt, ionic liquid, and an inorganic slurry containing Al2O3 nanowires, all prepared in a single-pot process (illustrated in **Figure 7a**). According to their findings, when these SPEs were utilized alongside activated carbon electrodes, they exhibited excellent supercapacitor performance, demonstrating notably high-power density and energy density values. Specifically, the results showed a power density of 9.3 kW at 44 Wh kg−1 and an energy density of 75 Wh kg−1 at 382 W kg−1.

Recently, Wang et al. [101] successfully fabricated liquid crystalline epoxy network (LCEN) composites with aligned CNT sheets, demonstrating electrically controlled actuation behavior (**Figure 7b**). The incorporation of CNTs into the vitrimer network directly promoted the photothermal conversion effect, resulting in higher thermal energy that activated transesterifications. This process facilitated Joule heating and played a crucial role in introducing structural anisotropy to the composite film. As a result, the composite film exhibited spontaneous bending behavior when powered by electricity, eliminating the need to induce macroscopic liquid crystalline orientation. Furthermore, the film displayed self-healing properties, which allowed it to recover not only through exposure to light but also by using electricity after being broken under a voltage of 1.18 V mm−1. Additionally, the authors showcased the potential application of the conductive LCEN composite as self-healing supercapacitors (**Figure 7c**).

#### **Figure 7.**

*(a) Chemical structures of epoxy-based solid polymer electrolytes containing an ionic liquid, a lithium salt, and inorganic Al2O3 nanowires. Reprinted with permission from [100]. Copyright 2018 American Chemical Society. (b) Fabrication of aligned CNT sheet-reinforced LCEN composite. (c) Flexible LCEN composite supercapacitor with self-healing behavior. Reprinted with permission from [101]. Copyright 2020 American Chemical Society.*

Additionally, there is a growing interest in epoxy composite-based rechargeable batteries with self-healing capabilities. Sun et al. have achieved significant breakthroughs in the field of highly conductive self-healing SPEs [102]. Their innovative approach involved incorporating disulfide dynamic covalent bonds into an epoxy matrix to create a unique and recyclable electrolyte named RFSPE-3. To fabricate this self-healing and recyclable electrolyte, the researchers strategically combined diglycidyl ether of bisphenol A (DGBE) and poly(ethylene glycol) diglycidyl ether (PEGDGE) as the matrix components. The crosslinker used was 2-aminophenyl disulfide (2-AFD), which provided the disulfide bonds crucial for the exceptional self-healing ability and recyclability of the electrolyte. The resulting RFSPE-3 exhibited impressive characteristics. The inclusion of epoxy resin endowed the electrolyte with superior mechanical strength, boasting a tensile strength exceeding 20 MPa. Meanwhile, the dynamic exchange of disulfide bonds in 2-AFD contributed to enhanced self-healing capabilities, achieving a remarkable healing efficiency of over 95% within just 2 h at room temperature. Moreover, RFSPE-3 demonstrated exceptional ion conductivity, reaching 10−3 S cm−1, and even after multiple healing processes, there were no changes observed in its ionic conductivity. Furthermore, this electrolyte exhibited an inhibitory effect on the growth of lithium dendrites, ensuring excellent cycling stability for up to 1800 h at 1 mAh cm−2. These promising results pave the way for the development of advanced and long-term rechargeable batteries, with potential applications in various fields requiring durable and efficient energy storage solutions. The combination of high mechanical strength, excellent self-healing properties, and outstanding ion conductivity positions RFSPE-3 as a promising candidate for next-generation energy storage devices.

Sun and Wu [103] adopted a similar strategy by incorporating disulfide dynamic networks into an epoxy matrix. Their approach involved introducing the disulfide bond through a disulfide-containing aliphatic polyamine as the epoxy-curing agent. The resulting SPEs exhibited remarkable properties, being optically transparent and capable of self-healing above the *T*g. An intriguing observation was the substantial decrease in curing time, from 102 to just 1.4 mins, achieved by raising the temperature from 40 to 100°C. Among various formulations tested, the SPE with 15% LiTFSI content demonstrated the highest ion conductivity. Specifically, it exhibited values of 3.35 × 10−6 Scm−1 at 80°C and 8.31 × 10−6 Scm−1 at 100°C. Although this conductivity was slightly lower compared to other self-healing SPEs, it still marked a significant advancement in the field. By successfully incorporating disulfide dynamic networks into the epoxy matrix, Sun and Wu's work represents a notable step forward in the development of solid polymer electrolytes with enhanced properties, showing potential for various applications in the realm of materials science and beyond.

#### *3.6.2 Energy-harvesting devices*

Conductive self-healing ECs find another crucial application in the realm of energy harvesting devices. The advancement of energy harvesting technologies represents a significant stride towards achieving an environmentally friendly society. Among these technologies, triboelectric nanogenerators (TENGs) have emerged as a highly regarded and eco-conscious power source. They offer numerous benefits, including cost-effectiveness, simple production processes, abundant material choices, impressive conversion efficiency, and the ability to design them in various versatile configurations [104–108]. These advantages have drawn considerable attention to TENGs as a sustainable and green solution. These TENGs capitalize on the

#### *Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

phenomena of triboelectrification and electrostatic induction, making them a highly promising option for efficiently converting ambient mechanical energies into electricity, particularly in situations where low frequencies are involved [109]. To enhance the output power of conventional TENGs, researchers have employed patterned triboelectric layers with micro-nanostructures or self-assembled monolayers (SAMs) with different functional groups, effectively increasing the contact surface area [110, 111]. Nevertheless, enduring challenges persist, such as the decline in overall performance and the restricted lifespan of TENG devices, primarily attributable to interface friction between dissimilar materials and frequent mechanical deformation, including twisting, stretching, bending, and compressing. A viable approach to address these challenges involves incorporating a "self-healing" capability into the TENGs. This self-healing feature enables the repair of broken micro-nanostructures in the triboelectric layer, thereby restoring the overall performance of the nanogenerators [105, 112].

Ye et al. [113] developed a self-healable vitrimer-based triboelectric nanogenerator (VITENG) by employing a rapid thiol-Michael reaction with diacrylate poly(dimethylsiloxane) (AA-PDMS) elastomer. They introduced 2,3-dihydroxypropyl methacrylate (DHPMA) to generate hydroxy end dangling side chains within the matrix, which enhanced the flexibility and transesterification efficiency of the PDMS network. This innovation resulted in an impressive healing efficiency of 84% within a mere 15 mins for the developed vitrimer elastomer. The research findings revealed that the dangling hydroxy end chains not only enhanced the material's flexibility and stretchability but also significantly improved its self-healing ability. The low *T*<sup>g</sup> system facilitated efficient collisions during the dynamic bond exchange process, contributing to this enhancement. An important outcome of their work was that VITENG exhibited a remarkable healing effectiveness of 100% and demonstrated a stretchability of 125%. Moreover, the VITENG showcased excellent output performance, producing 135 V of energy at low frequencies and applied forces. This remarkable performance makes the developed VITENG a highly efficient energy harvester with great potential for powering a wide array of soft devices.
