A research group led by Prof. WU Zhongshuai and Prof. BAO Xinhe from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences assembled planar sodium ion micro-capacitors with high volumetric performance, remarkable mechanical flexibility and high-temperature stability.Their findings were published in Advanced Science.
Microscale electrochemical energy storage devices have been extensively acknowledged as key power sources for miniaturized smart and integrated electronics, such as remote sensors, microrobots and self-powered microsystems.
Hybrid ion micro-capacitors (HIMCs), consisting of a battery-type electrode and a supercapacitor-type electrode, can synchronously combine the merits of high energy density of micro-batteries and high power density of micro-sueprcapacitors, They are regarded as a highly competitive class of next-generation microscale energy storage devices.
Lithium ion micro-capacitors have many merits on performance, but their massive applications would be likely prevented by the restrained source and rising cost of lithium. Alternatively, much attention have been devoted to sodium ion energy storage devices, thanks to the abundant sodium resource, low cost, and comparable electrochemical properties of sodium to lithium. However, sodium ion micro-capacitors (NIMCs) are rarely reported due to the lack of cost-effetive fabrication method and reasonable design of micro-electrodes.
Schematic diagram of planar sodium ion micro-capacitors (Image by ZHENG Shuanghao)
To overcome this issue, the researchers reported a prototype assembling of planar NIMCs based on the interdigital microelectrodes of urchin-like sodium titanate as Faradaic anode and nanoporous activated graphene as non-Faradaic cathode along with high-voltage ionogel electrolyte on a single flexible substrate.
Through effectively coupling with battery-type anode and capacitor-type cathode, the resultant all-solid-state NIMCs working at 3.5 V exhibited high volumetric energy density of 37.1 mWh cm-3 and ultralow self-discharge rate of 44 h from Vmax to 0.6 Vmax, both surpassing the most reported hybrid micro-supercapacitors.
The researchers adjusted graphene layer covered on the top surface of interdigital microelectrodes, and the NIMCs unveil remarkably enhanced power density owing to the establishment of favorable multi-directional fast ion diffusion pathways that significantly reduce the charge transfer resistance.
Meanwhile, the as-fabricated NIMCs present excellent mechanical flexibility without capacitance fade under repeated deformation, and electrochemical stability at high temperature of 80 ℃ because of using non-flammable ionogel electrolyte and in-plane geometry. Therefore, these flexible planar NIMCs with multi-directional ion diffusion pathways hold tremendous potential for microelectronics.
This work was supported by National Natural Science Foundation of China, National Key R&D Program of China, etc. (Text by ZHENG Shuanghao and HOU Xiaocheng)