Holey graphene/MnO2 nanosheets with open ion channels for high-performance solid-state asymmetric supercapacitors

Holey graphene/MnO2 (HG/MnO2) composites with open ion channels are synthesized by an electrostatic self-assembly method. The HG with rich in-plane nanopores is prepared via a mild etching reaction first, followed by modified with poly-diallyldimethylammoniumchloride (PDDA) to transfer the surface charge of HG nanosheets from negative to positive, which are eventually assembled on the negatively charged MnO2 nanosheets via electrostatic attraction. As a result, the HG allows ions to pass through the graphene sheet, improving the ion transport channels. Besides, the electrostatic self-assembly between HG and MnO2 enables the composite a high conductivity, providing effective electron transport pathways. The HG and MnO2 sheets are observed to be tightly bounded by the transmission electron microscope (TEM), and the HG content in the composites is determined to be 9.6% to 20% by the thermogravimetric (TG) test. The HG/MnO2-2 electrode with the HG content of around 14.8% displays the large specific capacitance of 219.3 F g(-1) at 0.5 A g(-1) and the high rate capacity of 134.7 F g(-1) at 10 A g(-1). Furthermore, the as-prepared solid-state asymmetric supercapacitors (SSAS) achieve a wide stable operating voltage of 1.8 V, a high energy density of 16.8 Wh kg(-1) at the power density of 224.6 W kg(-1), and low capacitance degradation of only 6.3% after 5000 cycles.