Designing of 3D LSG/CoNi2S4 nanosheets micro-electrodes for flexible highenergy integrated micro-supercapacitors

Over the past years, self-powered micro-systems such as wireless sensor networks, wearable and biomedical implants, micro-robots, active tags and remote sensors have been developed dramatically [1-2]. Accordingly, extensive efforts have been devoted to design of in plane energy storage systems with high energy and power, small volume, light weight and low cost that can integrated on flexible chips [3-4]. Presently, although commercially thin-film micro-batteries are the major power sources for these devices but micro-supercapacitors (MSCs) are the most promising candidates because of their higher power density, much longer operating lifetime, excellent safety and better rate capability [5-6]. In order to introduce high performance MSCs, various kinds of carbon have been utilized. But, owning to EDLC mechanism, carbon based MSCs have a low energy density. In addition, conventional lithographic techniques are not cost-effective [7-8]. Thus, a facile and low cost preparation process resulting flexible high energy MSC devices is still challenging. Herein, we report LSG/CoNi2S4//LSG interdigitated microelectrodes for the first time prepared by a facile, scalable and low cost process as an all-solid-state, flexible integrated hybrid asymmetric micro-supercapacitor. The LSG/CoNi2S4//LSG MSCs were prepared by two steps of LSG patterning and selective electrodeposition of CoNi2S4 nanosheets (Scheme 1). Firstly, a volume of 16 ml of 2.7 mg ml-1 GO aqueous dispersion was drop-cast on polyethylene terephthalate (PET) sheet glued to the surface of a LightScribe DVD disc. After then, the interdigitated micro-patterns were directly LightScribed onto the GO film by a consumer-grade LightScribe DVD burner. In the second step, CoNi2S4 nanosheets were selectively electrodeposited on one set of the LSG microelectrodes by cyclic voltammetry at a scan rate of 5 mV s-1 for 3-15 cycles within a voltage range of -1.2 to 0.2 V. Deposition bath contained 5 mM CoCl2·6H2O, 7.5 mM NiCl2·6H2O and 0.75 M thiourea with the pH value of ∼6. 178 Scheme 1 Schematic illustrating the fabrication processes of an asymmetric LSG/CoNi2S4//LSG micro-supercapacitor. Scheme 1 Schematic illustrating the fabrication processes of an asymmetric LSG/CoNi2S4//LSG microsupercapacitor. Scheme 1 Schematic illustrating the fabrication processes of an asymmetric LSG/CoNi2S4//LSG micro-supercapacitor. The composition and morphology of the samples was characterized using XRD, XPS, EDAX, SEM and TEM. All these techniques verified successfully synthesis of 3D LSG/CoNi2S4 nanosheets micro-electrodes. All-solid-state flexible MSCs were prepared by coating PVA-KOH gelled electrolyte onto them. The stack capacitance of the devices can be arrived to an ultra-high volumetric capacitance of 122.4 F cm-3 with 15 deposition cycles. The excellent cycling stability of the device was also confirmed by retaining 93.9% of the initial capacitance after 10,000 cycles. These MSCs can achieve energy densities of up to 49 Wh l-1 which is comparable to that of lead acid batteries. We believe that this work not only introduces a new flexible integrated asymmetric MSCs with ultra-high energy/power densities, but also constitutes a step forward towards novel, low-cost and flexible in-plane interdigitated microelectrodes that can be widely applied in various applications, such as Li-ion batteries, biosensors, gas sensors and other electronic devices.