| Supercapacitors (SCs) are regarded as a new promising type of device for energy storage and conversion, due to its high power density, rapid charging and discharging, and long cycle life. But, some bottlenecking problems such as low energy density and large leakage current limit SCs’development. Developing new type of electrode materials is the key to resolve these problems. Transition metal oxides (e.g. MnO2) and sulfides are regarded as the most promising supercapacitor materials due to their high theoretical capacitances, excellent cycling performance, abundance, low cost and environment friendliness. But weak conductivity and low specific surface area of the materials make their capacities far less than the theoretical values. Usually designing a unique morphology is used to increase the materials’specific surface area, and inducing other highly conductive materials is used to enhance the conductivity. However, most of these reports only focus on one of the two factors. Research on enhancing materials’properties by designing combination of different supercapacitive materials on the micron or nano scale is too limited, and therefore the effects of different components’sizes and their synergy on the performance are still unclear so far. It is a great challenge to realize low-cost and large-scale fabrication of flexible SCs with high performance (high energy density and cycle stability).To address the problems above, our work successfully designed and prepared hierarchically porous MnO2 microspheres doped with Fe3O4 nanoparticles for SCs. Since MnO2 and Fe3O4 are both pseudocapacitive, the composite materials combine both the advantages of micron-nano scales and a synergistic effect between different components, and the hierarchical architecture facilitates the ion infiltration and transfer. On this basis, we further improved the capacitive performance by coating their surface by highly conductive SnO2 nanoparticles, and achieved MnO2-Fe3O4-SnO2 ternary composite electrode materials. In addition, we extended this design concept to the bimetallic sulfides’system due to that metal sulfides have lower electronegatity and higher electrochemical activities than oxides. We investigated the preparation of composite electrode materials with mechanically stable nanotube-built multi-tripod structures on a flexible fiber cloth, and systematically discussed the charging and discharging behaviors of the SCs with such electrodes. The main research contents are as follows:(1) Novel MnO2 based micron-nano composite electrode materials were prepared, and the structure, size and composition are designed for the capacitance improvement. Hierarchically porous yet densely packed MnO2 microspheres doped with Fe3O4 nanoparticles are synthesized via a one-step and low-cost ultrasound assisted method. The scalable synthesis is based on Fe2+ and ultrasound assisted nucleation and growth at a constant temperature in a range of 25-70 ℃. The results show that single-crystalline Fe3O4 particles of 3-5 nm in diameter are homogeneously distributed throughout the spheres and none are on the surface. A systematic optimization of reaction parameters results in isolated, porous, and uniform Fe3O4-MnO2 composite spheres. The spheres’average diameter is dependent on the temperature, and thus is controllable in a range of 0.7-1.28μm. The involved growth mechanism is discussed. The specific capacitance is optimized at an Fe/Mn atomic ratio of r=0.075 to be 448 F g-1 at a scan rate of 5 mV s-1, which is nearly 1.5 times that of the extremely high reported value for MnO2 nanostructures. Especially, such a structure allows significantly improved stability at high charging rates. The composite has a capacitance of 367.4 F g-1 at a high scan rate of 100 mV s-1, which is 82% of that at 5 mV s-1. Also, the electrode has an excellent cycling performance with capacitance retention of 76% after 5000 charge/discharge cycles at 5 A g-1. The excellent capacitor performance and cycle stability can be attributed to the synergistic effect between different components, micro-nano size effect, and the hierarchically porous architecture.(2) Using SnO2 improves the conductivity of the Fe3O4-MnO2 composite. Hierarchical MnO2-based ternary composite nanostructures were prepared by coating SnO2 nanoparticles on the surface of porous Fe3O4-MnO2 spheres under hydrothermal conditions. SnO2 nanoparticles were uniformly coated on the composite spheres’ surface, and the content of SnO2 wre controlled by controlling the concentration of Sn4+. Optimization composition results in a Fe3O4-MnO2-SnO2 composite electrode material with 5.9 wt.% Fe3O4 and 5.3 wt.% SnO2, leading to a high specific areal capacitance of 1.12 F cm-2 at a scan rate of 5 mV s-1. This value is 2-3 times the values for MnO2-based binary nanostructures at the same scan rate. In addition, the optimal ternary composite has a good rate capability and an excellent cycling performance with capacitance retention of ～90% after 5000 charge/discharge cycles at 7.5 mA cm-2. An integrated device made by connecting two identical SCs in series can power a light-emitting diode indicator for more than 10 min. The enhancement is attributed to the improvement of the conductivity with remaining the high porosity.(3) Silver sputtering was used to improve non-conductive textile cloth. It not only facilitates nucleation and induces generation of novelnanostructures on ligaments of a textile, but also improves their mechanical stability and conductivity. Using hydrothermal synthesis, the precursors with the unique network-like structure was obtained by adjusting the reaction parameters, and then the composite sulfides with nanotube-built multi-tripod structures were obtained by the subsequent sulfidation. Benefiting from intriguing compositional and structural features, the silver-sputtered cloth supported FeCo2S4-NiCo2S4 composites, using as an additive-free electrode, shows a specific capacitance of 1518.5 F g-1 at 5mA cm-2. All-solid-state symmetric SCs employing such electrodes deliver enhanced energy density of 45.77 Wh kg-1 (at 1070 W kg-1) as well as achieve outstanding cycling stability (capacitance retention of 92% after 3000 cycles at 8.5 mA cm-2) and superior flexibility and reliability (no degradation under large twisting), which is closely related to the unique multi-tripod structure with high mechanical stability. A mobile phone can be recharged with a wrist band made from connecting five SCs. This work provides a general, low-cost route to wearable power sources. |
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