Binder-Free Electrospun Ni-Mn-O Nanofibers Embedded in Carbon Shells with Ultrahigh Energy and Power Densities for Highly Stable Next-Generation Energy Storage Devices.

We demonstrate the fabrication of binder-free electrospun nickel-manganese oxides embedded into carbon-shell fibrous electrodes. The morphological and structural properties of the assembled electrode materials were elucidated by high-resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy, and glancing-angle X-ray diffraction. The fibrous structure of the electrodes was retained even after annealing at high temperatures. The X-ray photoelectron spectroscopy and HR-TEM analyses revealed the formation of nickel and manganese oxides in multiple oxidation states (Ni2+, Ni3+, Mn2+, Mn3+, and Mn4+) embedded in the carbon shell. The embedded nickel-manganese oxides into the carbon matrix fibrous electrodes exhibit an excellent capacitance (1082 F/g) in 1 M K2SO4 at 1 A/g and possess a high rate capability of 73% at 5 A/g. The high rate capability and capacitance can be attributed to the presence of carbon cross-linked channels, the binder-free nature of the electrodes, and various oxidation states of the Ni-Mn oxides. The asymmetric supercapacitor device constructed of the as-fabricated nanofibers and the bio-derived microporous carbon as the positive and negative electrodes, respectively, sustains up to 1.9 V with a high specific capacitance at 1.5 A/g of 108 F/g. The nanofibrous//bio-derived device exhibits an outstanding specific energy of 54.2 W h/kg with a high specific power of 1425 W/kg. Interestingly, the tested device maintains a high capacitive retention of 92% upon cycling over 10,000 charging/discharging cycles.