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Batteries 2018, 4, 8 7 of 10 The high coulombic efficiency registered for the SC-NMO again suggests a highly reversible insertion of Na+ ions. Indeed, as reported in Figure 5d, the amount of Na+ equivalents in SC-NMO is between 0.63 eq. Na+ and 0.26 eq. Na+, i.e., well within the range where high reversibility is observed [13]. As a matter of fact, the potential profiles of the first and second charge plotted vs. Na+ equivalents (Figure 5d) almost overlap. 3. Materials and Methods 3.1. Solution Combustion Synthesis of Na0.44MnO2 The powder sample of nominal composition Na0.44MnO2 (solution combustion synthesis Na0.44MnO2, SC-NMO) was prepared starting from appropriate amounts of NaNO3, Mn(NO3)2.4H2O and urea, CO(NH2)2 (Sigma-Aldrich S.r.l, Milano, Italy). The chemicals were dissolved in deionized water in the appropriate stoichiometric ratio and heated at 80 ◦C under continuous stirring for 30 min. The temperature was then increased to 100 ◦C to achieve complete evaporation of the solvent until a viscous light pink resin was obtained, which spontaneously ignited to produce a black precursor. This latter was subsequently treated in a furnace at a temperature between 500 ◦C and 700 ◦C for maximum 2 h. The Na/Mn ratio was modified between 0.47–0.51 to identify the Na excess suitable to compensate for the loss of alkali metal during the high temperature synthesis steps. For comparison, a sample of the same nominal composition (solid-state synthesis Na0.44MnO2, SS-NMO) was prepared according to the solid-state synthesis procedure reported by Sauvage et al. [13], which foresees a 10 wt.% excess of Na precursor, corresponding to a starting stoichiometry of Na0.49MnO2, to compensate for the Na ion volatility at high temperature. The characterization of this sample was already described in our previous work [29]. 3.2. Morphological and Structural Characterization of SC-NMO SC-NMO powder was sputter-coated with Au and analyzed through Scanning Electron Microscopy (SEM, Mira 3, Tescan, Brno, Czech Republic), using a beam acceleration voltage of 20 kV. The average dimensions of the slabs were calculated by measuring the width and length of at least 75 single slabs through the ImageJ® free software. X-ray Diffraction Patterns (XRDP, BRUKER diffractometer D8 Advance, Cu-Kα: λ = 0.154 nm, Bruker Italia S.r.l., Milano, Italy) were acquired on the SC-NMO powder using an Al sample holder. The data were acquired in the 2θ range from 10◦ to 45◦ with scan step of 0.02 and a fixed counting time per step of 8 s. The powder underwent a Thermo-Gravimetric Analysis (TGA, Q5000, TA Instruments, New Castle, DE, USA) under static air, with a heating ramp of 10 ◦C/min from room temperature up to 800 ◦C. 3.3. Casting of the Electrodes The NMO (both SC or SS) powders were mechanically sieved (mesh size ≤ 45 μm), after hand-grinding to separate possible agglomerates of particles. The active material, the conductive carbon (SuperC65®, IMERYS, Switzerland) and binder polyvinylidene fluoride (PVdF, Solef 6020, Solvay Polymer specialties, Bruxelles, Belgium) were mixed via stirring overnight in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich) in the weight ratio 85:10:5, respectively. In detail, 1 mL of NMP was used for 0.588 g of solid (solid content of 34.8%). Slurries with a wet thickness of 150 μm were cast on aluminum foil (Hydro, 0.020 mm thick). The slurries were dried overnight in an oven at 60 ◦C, and then round disc electrodes with a diameter of 12 mm-diameter were cut. The electrodes were dried under vacuum in a glass oven (Büchi) for 3 h at room temperature, 3 h at 60 ◦C and finally for 12 h at 100 ◦C. Subsequently, electrodes were pressed (8 ton for 10 s), weighed and dried again under vacuum for 3 h at room temperature and 8 h at 100 ◦C. The final electrodes had an average active material mass loading of 2.7 to 3.2 mg cm−2.PDF Image | Sodium-Ion Batteries Obtained through Urea Based
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