2D mesoporous MnO2 nanosheets for high-energy asymmetric micro-supercapacitors in water-in-salt gel electrolyte

Abstract

Two-dimensional (2D) mesoporous nanosheets as electrodes present a critical materials platform for dramatically boosting the performance of planar micro-supercapacitors (MSCs) due to their unique features of interconnected porous network, enriched nanopore arrays, and high specific surface area. However, efficient strategies for constructing such complicated mesoporous architectures are very limited. Here, we developed a supramolecular bottom-up self-assembly strategy for direct synthesis of ultrathin mesoporous manganese dioxide (m-MnO2) nanosheets for high-energy all-solid-state planar asymmetric MSCs (AMSCs). The m-MnO2 nanosheets exhibited ultrathin thickness of 10 nm, uniformly interconnected network of mesopores with size of 5–15 nm, high surface area of 128 m2 g−1, and remarkably enhanced capacitance of 243 F g−1 at 1 mV s−1 in comparison with non-mesoporous MnO2 nanosheets (123 F g−1). Further, all-solid-state AMSCs were assembled based on m-MnO2 nanosheets as positive electrode, porous VN nanosheets as negative electrode, and electrochemically exfoliated graphene as conductive agent and metal-free current collector in “water-in-salt” gel electrolyte. Importantly, our all-solid-state AMSCs operated stably at 2.0 V and offered impressive energy density of 21.6 mWh cm−3, outperforming most reported MnO2 based MSCs, and two times higher than lithium thin-film batteries (≤10 mWh cm−3). Also, they presented long-term cycling stability with 90% capacity retention after 5000 cycles, outstanding flexibility without observable capacitance decay under different bending angles, and facile serial and parallel interconnection for creating high-voltage and high-capacitance integrated bipolar cells. Therefore, our proposed strategy will open many opportunites for patterning novel 2D mesoporous metal oxide nanosheets for high-performance microscale electrochemical energy storage devices.