In this study, we present coil-structured, pseudocapacitive yarns as promising electrodes for achieving stretchable and highly performing supercapacitors. The bare CNT coiled yarns are fabricated by inserting a giant twist (50 000 twists m− 1) into CNT spun yarns, which are drawn from a multiwalled carbon nanotube (MWNT) forest.[25] Although the bare CNT coiled yarns can be used as stretchable electrodes for EDLC-based supercapacitor without further processing,[24] we deposited MnO 2, which is a promising pseudocapacitive material because of its high theoretical capacitance, low cost, and environmental friendliness [26] to enhance dramatically its energy storage capacity. A scanning electron microscope (SEM) image of our novel core (CNT)–shell (MnO 2)-structured hybrid coiled yarn is shown in Figure 1A. The diameter of the single hybrid coiled electrode is≈ 80 µm and about 200 uniform microloops are contained in a centimeter of coiled electrode. The resulting aspect ratio (ratio of coiled electrode’s length to diameter) is high (about 210) and the electrode volume is impressively small (≈ 6.5× 10− 5 cm 3) that the volume of presented coiled CNT electrode is one-twentieth the size of previously reported coiled CNT/nylon fiber.[21]

A high-resolution SEM surface image of the bare CNT coiled yarn (Figure 1 B) shows closely packed and uniaxially aligned CNT bundles, which implies an effective electron pathway for longitudinal current collecting. After the MnO 2 deposition (Figure 1 C), a mesoporous film, which is morphologically characteristic of MnO 2,[27] is uniformly formed on the surface of the coiled electrode. It is experimentally confirmed that strong adhesion of the MnO 2 thin shell to the porous CNT surface is achieved so that high electrochemical stability could be demonstrated against repeated mechanical deformation by loop opening or electrochemical reaction, which will be discussed later. In this study, the mass of MnO 2 was measured using an electrochemical quartz-crystal microbalance (EQCM). From the slope of the charge–mass graph measured by EQCM, the mass of MnO 2 per unit charge transferred during deposition is determined to be 5.4× 10− 4 g C− 1, as shown in Figure S1 in the Supporting Information. To measure the thickness of the MnO 2 shell layer, cross-sectional analysis was performed by cutting the hybrid coiled electrode along its diameter using a Ga ion beam, as shown in Figure 1 D. Figure 1 E shows a magnified SEM image of the denoted rectangle at the edge part of the hybrid coiled yarn cross-section shown in Figure 1 D. The nanoscopic MnO 2 shell with thickness≈ 1 µm is distinguished from the CNT core part. Because only nanoscopic surfaces of MnO 2