The critical role of point defects in improving the specific capacitance of δ-MnO2 nanosheets

3D porous nanostructures built from 2D delta-MnO2 nanosheets are an environmentally friendly and industrially scalable class of supercapacitor electrode material. While both the electro-chemistry and defects of this material have been studied, the role of defects in improving the energy storage density of these materials has not been addressed. In this work, delta-MnO2 nanosheet assemblies with 150m(2) g(-1) specific surface area are prepared by exfoliation of crystalline KxMnO2 and subsequent reassembly. Equilibration at different pH introduces intentional Mn vacancies into the nanosheets, increasing pseudocapacitance to over 300 Fg(-1), reducing charge transfer resistance as low as 3 Omega, and providing a 50% improvement in cycling stability. X-ray absorption spectroscopy and high-energy X-ray scattering demonstrate a correlation between the defect content and the improved electro-chemical performance. The results show that Mn vacancies provide ion intercalation sites which concurrently improve specific capacitance, charge transfer resistance and cycling stability.