Carbon Nanomaterials for Flexible Energy Storage Devices

This chapter discusses the necessity to develop flexible energy storage devices. The main efforts are first paid to describe the history and recent advancements in flexible supercapacitors/batteries. Detailed discussion is focused on electrode materials, device configuration/structure (1-dimensional (ID) fiber and 2D planar shapes), and their mechanical/electrochemical performances (flexibility, stability, and capacity). Special attention is paid to carbon-based materials (e.g., carbon nanotubes (CNTs), graphene, and carbon composites) for supercapacitors, conductive carbon cloth-, paper-, and insulating textile-based electrodes for batteries, etc. Some recent applications of flexible supercapacitors/batteries are further described. Finally, emphasis is given to integrated energy storage devices, and the remaining difficulties are summarized.

Overview of Flexible Energy Storage Devices

Nowadays, the advent of flexible electronic devices such as smart watches, bendable screens, and wearable sensors has attracted great interest. Compared with conventional electronic facilities, flexible electronics possess many advantages of being flexible, lightweight, wearable, and even implantable. To satisfy the requirements of high-performance flexible electronics, considerable efforts have been devoted to seeking compatible power systems such as flexible supercapacitors and lithium-ion batteries (LIBs).

Flexible Supercapacitors

Traditional supercapacitors in rigid planar structures cannot meet the rapid development of portable and wearable electronic devices which demand the power sources of small size, light weight, and high flexibility. Hence, it is urgent to develop flexible supercapacitors.

Mechanism and Advancement of Supercapacitors

According to the energy storage mechanism, supercapacitors can be divided into two categories, namely electrochemical double-layer capacitors (EDLCs) and pseudocapacitors [1,2]. As illustrated in Figure 2.1a, the capacitance in EDLCs mainly relies on the accumulation/desorption of ions at the electrode/electrolyte interface. Differentially, the capacitance in pseudocapacitors stems from fast and reversible redox reactions occurring on the surface of electroactive materials (such as transition metal oxides and conducting polymers) (Figure 2.1b), ensuring good rate capability and high power density.

The pseudocapacitors can be further classified into three faradaic mechanisms based on the various charge transfer processes, including underpotential deposition, redox pseudocapacitance, and intercalation pseudocapacitance as illustrated in Figure 2.2. The underpotential deposition happens when the surface of metallic materials is covered with a monolayer of metal ions or protons whose potential is beyond the redox potential of the materials. Redox pseudocapacitance indicates that active materials (transition metal oxides or conducting polymers) are electrochemically

FIGURE 2.1 Schematics of (a) carbon-based EDLC and (b) Mn0₂-based pseudocapacitor. (Reprinted with permission from Ref. [3]. Copyright 2014, Royal Society of Chemistry.)

FIGURE 2.2 Schematic of three types of charge storage mechanisms for pseudocapacitors: (a) underpotential deposition, (b) redox pseudocapacitance, and (c) intercalation pseudocapacitance. (Reprinted with permission from Ref. [4]. Copyright 2014, Royal Society of Chemistry.)

absorbed by ions on or near the surface, accompanied by faradaic charge transfer process. In the case of intercalation pseudocapacitance, cations are intercalated into the lattice framework of active materials involving a faradaic charge transfer without structural distortion.

To assess the electrochemical performance of supercapacitors, cyclic voltammetry (CV) and galvanostatic charge and discharge tests are usually conducted. The specific capacitance of the supercapacitor can be obtained based on the charge- discharge curve: C = 2i/(mAV/At), where m is the mass of the active material in a single electrode, i is the discharge current, and AVI At corresponds to the slope of the discharge curve. Specific volumetric or areal capacitance of the supercapacitor can also be obtained. Energy density and power density of supercapacitor can be calculated according to the equations E = 1/2CV², P = W4/?,, where R_s (Q) is the equivalent series resistance of the system. According to the above analysis, the energy density can be enhanced by increasing the specific capacitances and/or widening the potential window.