Wire-Shaped 3D-Hybrid Supercapacitors as Substitutes for Batteries

Highlights The flexible 3D porous structure with a large surface area provides pathways for rapid ion/electron transport and ion diffusion as well as numerous electroactive sites. The wire-shaped supercapacitor exhibits a high energy density of 153.3 Wh kg−1 and a power density of 8810 W kg−1. The hybrid device demonstrates excellent durability under various mechanical deformations. Electronic supplementary material The online version of this article (10.1007/s40820-019-0356-z) contains supplementary material, which is available to authorized users.


S1.1 Equations
The optimum mass ratio of positive electrode to negative electrode was calculated by Eq. S1: where m is the mass of electroactive materials, V is the potential window and C represents the specific capacitance, respectively.
The gravimetric, volumetric, areal and length capacitance of NiCo LDH/3D-Ni nanostructures electrode materials were estimated from the cyclic voltammetry and galvanostatic charge/discharge profiles using Eqs. S2-S7: Cyclic Voltammetry: Length capacitance: Nano-Micro Letters S2/S18 Where C is the capacitance (F), Cg is the gravimetric capacitance (F g -1 ), Cv is the volumetric capacitance (F cm -3 ), Ca is the areal capacitance (F cm -2 ), Cl is the length capacitance (F cm -1 ), ν is the sweep rate (mV s -1 ), △V is the potential window (V), I is the discharge current (A), △t is the discharge time (s), v is the volume of the electrode material (cm 3 ), m is the mass of the active material (g), A is the area of the electroactive material (cm 2 ), l is the length of the electrode material and ∫idV is the integral area of the CV curve (A).

S1.3 Calculating the Energy Density and Power Density of the Hybrid Supercapacitors
where Ct is the specific capacitance of the supercapacitor based on the total mass of the two electrodes (F g -1 ), V is the potential window in the discharge process, and t is the discharge time (s).
The calculated capacitance of the hybrid supercapacitor was, The XRD pattern of NiCo LDH is shown in Fig. S4a. All the diffraction peaks are in good agreement with the standard spectrum 1) (JCPDS No. 00-038-0715) of Ni(OH)2, 2) (JCPDS No. 01-073-6993) of Co(OH)2, and 3) NiCO LDH (No. 01-033-0429). The typical Raman spectrum of NiCo LDH/3D-Ni is shown in Fig. S4b. Remarkable peaks in the Raman spectrum of the NiCo LDH/3D-Ni were shown at 191, 305, 467, and 528 cm -1 . The peaks at 191, 467, and 528 cm -1 are related to the Co(OH)2 phase, and the peaks at 305, 467, and 528 cm -1 are related to the Ni(OH)2 phase. The Ni-OH/Co-OH symmetric (A1g(T)) mode and NiO-/Co-O symmetric stretching (Ag) mode were detected at 467 and 528 cm -1 . The Eg(T) mode and Eg symmetry mode for the Ni(OH)2 and Co(OH)2 were detected at 191 and 305 cm -1 , respectively. With these measurements, the NiCo S8/S18 LDH active material was successfully synthesized. The elemental composition and the oxidation state of NiCo LDH/3D-Ni were evaluated by XPS, as shown in Fig. S4c-f. A typical XPS survey spectrum of NiCo LDH/3D-Ni is displayed in Fig. S4c, where nickel (Ni 2p), cobalt (Co 2p), oxygen (O 1s), and carbon (C 1s) elements were presented. The high-resolution Ni 2p spectrum in Fig. S4d exhibited two major peaks with the binding energy at 873.3 eV (Ni 2p1/2) and 855.7 eV (Ni 2p3/2) accompanied with two shakeup satellite peaks, which indicated the existence of the Ni 2+ state. As shown in Fig. S4e, the high resolution Co 2p spectrum displayed two main peaks located at 797.2 eV (Co 2p1/2) and 781.3 eV (Co 2p3/2) accompanied by two shakeup satellites, which confirmed the cobalt is in an ionic state (Co 2+ ). Finally, the high resolution O1 peak in Fig. S4f shows a major peak at 531.3 eV, attributed to hydroxyl ions revealing the formation of metallic hydroxides [S2-S4]  The nitrogen adsorption/desorption isotherms and the pore size distribution results of the two samples are shown in Fig. S8. Fig. S8a-b shows that the 3D-S10/S18 Ni/Ni has a BET surface area of 0.9746 m 2 g -1 with meso/macro porosity, whereas the NiCo LDH/3D-Ni electrode has abundant pore distribution ranging from micropores to macropores with a 3.5215 m 2 g -1 BET surface area as shown in Fig. S8c-d.

Fig. S10
Nyquist plots of NiCo LDH/3D-Ni nanostructures electrode before and after 10,000 cycles The Nyquist plots were fitted well with the equivalent circuit model of the inset in Fig. S10. The equivalent circuit model contains several elements such as Rs, CPEDL, RCT, WO, CPEL, and RL. RS, the surface resistance, is the equivalent series resistance (ESR), which generally describes the resistance of the electrolyte combined with the internal resistance of the electrode. The transfer resistance (RCT) demonstrates the rate of redox reactions at the electrodeelectrolyte interface. The slight increase of charge transfer resistance after 10,000 cycles might be due to the loss of adhesion of some active materials from the current collector due to the continuous adsorption and desorption of OHions. However, as shown in Fig. 1b-c and the Fig. S10, the overall structural stability of the active materials could be confirmed by no obvious mechanical deformation at the surface of the electrode. The Nyquist plot of 3D-NiCo LDH/Ni after 10,000 cycles exhibited a semi-circle arc at the high frequency region followed by a straight line at the low frequency region, which validates stable capacitive behavior. CPEDL is the constant phase element (CPE) representing double layer capacitance, which occurs at interfaces between solids and ionic solutions due to the separation of ionic and/or electronic charges. WO is the Warburg element, which represents the diffusion of ions into the porous electrode in the intermediate frequency region and is a result of the frequency dependence of this diffusion. RL is the leakage resistance which is placed in parallel with CPEL which denotes faradaic capacitance. S11/S18   (d) series at a scan rate of 20 mV s -1 and a current density of 20 A g -1 S15/S18 Figure S15a-b show the CV and GCD curves of single and two wire supercapacitors connected in parallel. Compared with a single device (1.8 V), the output current and the discharge time of the two devices connected in parallel are increased by a factor of two compared with a single device at the same constant current density of 20 A g -1 . Fig. S15c-d show the CV and GCD curves of single and two wire supercapacitors connected in series. Compared with a single device (1.8 V), the output of the two devices connected in series exhibited a larger potential window of 3.6 V.

Fig. S22
Galvanostatic charging profile of the NiCo-LDH/3D-Ni//Mn3O4/3D-Ni hybrid supercapacitor before the LED test at a current density of 20 A g -1