J. Qian, S.P. Lau
The Hong Kong Polytechnic University,
Keywords: MnO2, printable, supercapacitor, Na-ion battery, energy storage devices
Summary:Printable electronics is of great interest in the areas ranging from thin film transistors (TFTs), energy storage devices, solar cells to micro electro-mechanical systems (MEMS). To date, preparations of various inks composed of semiconductors, biological materials, carbon, and conductive oxides have been reported. Specially, owing to its abundance, high theoretical capacity and environmental compatibility, manganese dioxide (MnO2) is usually regarded as an ideal candidate for the electrode materials of portable devices, water treatment, up-conversion as well as photocatalysis. The conventional MnO2 electrodes are mainly prepared by two approaches: (1) nanostructured MnO2 or MnO2-containing composite precipitates via wet chemical process; (2) direct electrodeposition or chemical deposition on various substrates (e.g. glass, quartz, copper or aluminum foil). These existing preparation methods suffer from higher cost, complicated processes and superfluous contaminations. On the other hand, during the coating process, the introduction of insulating binders would cause agglomeration in the inks, leading to the reduction of electrical conductivity. By now, it still remains a great challenge to synthesize MnO2 inks with high reliability and versatility. Hence, the development of environmental-benign aqueous MnO2 inks is desirable for high-efficient and large-scale printable processes. Nevertheless, few research works on aqueous MnO2 inks have been reported to date. In this work, aqueous inorganic ink comprised of hexagonal MnO2 nanosheets was synthesized via a chemical reduction method. The MnO2 ink exhibits long term stability. Continuous thin films can be formed on various substrates without using any binder. To investigate the electrochemical performance of the MnO2 electrode for rechargeable batteries, the additive-free MnO2 ink was spray printed on the commercial copper foil to form the robust and continuous MnO2 thin films. The MnO2 thin films was annealed at 300°C for 2 h, and then served as the anodes for Li-ion and Na-ion batteries respectively. When serving as the anode for Li-ion battery, an initial charge and discharge capacity of 1369 mAh·g-1 and 1957 mAh·g-1 was achieved at 320 mA·g-1. The discharge capacity reached 496 mAh·g-1 after 300 cycles at 1 A·g-1 and could still remain at 210 mAh·g-1 when the current density is increased to 2 A·g-1. When serving as the anode for Na-ion battery, an initial charge and discharge capacity of 555 mAh·g-1 and 1096 mAh·g-1 was achieved respectively at 320 mA·g-1. The discharge capacity could still remain 87 mAh·g-1 after 160 cycles at 320 mA·g-1. When the current density is increased to 1 A·g-1, a discharge capacity of 94 mAh·g-1 could be achieved. To obtain a flexible electrode for electrochemical capacitors, we printed the MnO2 ink on commercially available A4 paper pre-treated by multi-walled carbon nanotubes. The electrode exhibited a maximum specific capacitance of 1035 F·g-1 (91.7 mF·cm-2). Both paper-based symmetric and asymmetric capacitors were assembled. A maximum specific energy density of 25.3 Wh·kg-1 and power density of 81 kW·kg-1 were achieved. The device could maintain a 98.9% capacitance retention for 10,000 cycles at 4 A·g-1. The MnO2 ink could be a versatile candidate for large-scale production of flexible and printable electronic devices for energy storage and conversion.