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Batteries 2018, 4, 8 3 of 10 2. Results and Discussion 2.1. Structural and Morphological Characterization of the as Prepared SC-Powder Prior to the electrochemical characterization, all materials underwent a structural investigation to identify the appropriate synthesis conditions. Figure 2a shows the X ray diffraction patterns, XRDP, of the samples prepared with different Na/Mn ratios in the range 0.47–0.51, treated at 800 ◦C for 2 h. Results show that the sample prepared with a nominal ratio of 0.50 is single-phase, with the desired structure of Na0.44MnO2, i.e., an orthorhombic Pbam space space group isostructural to Na4Mn4Ti5O18 (JCPDS: 01-076-0785) [28]. Samples with a lower Na/Mn ratio (0.47 and 0.49) contain Mn2O3 impurities (JCPDS: 01-71-0635), confirming a partial volatility of the alkali metal ions during the thermal treatment. Instead, the sample prepared with a higher Na content (Na/Mn ratio = 0.51) shows impurity peaks associated to the presence of α-NaMnO2 (JCPDS: 01-25-0845). Accordingly, the study on the required minimum temperature and time of the thermal treatment for obtaining single-phase Na0.44MnO2 was performed with a Na/Mn ratio of 0.50. Figure 2b reports the evolution of the diffraction patterns at different annealing temperatures. The XRDP show that at least 700 ◦C is required to achieve the preparation of phase-pure Na0.44MnO2 with the desired structure. Indeed, the mixture treated at 500 ◦C shows peaks related to the presence of crystalline Mn2O3 (JCPDS: 01-71-0635), weak peaks due to β-Na0.70MnO2 (JCPDS: 01-27-0752) and additional broader peaks of unidentified phases. After the thermal treatments at 600 ◦C, the dominant phase present is β-Na0.70MnO2, still partially detectable after the thermal treatment of 1 hour at 700 ◦C. Only after being treated at 700 ◦C for 2 h is the sample single-phase and able to be indexed according to the reflections of Na0.44MnO2, isostructural to Na4Mn4Ti5O18 (JCPDS: 01-76-0193). Figure 2. (a) X-Ray Diffraction Pattern, XRPD, of the samples prepared using different Na/Mn ratios in the precursor solution. Pink circles indicate the peaks of the Mn2O3 impurity while blue triangles those of α-NaMnO2; (b) XRDP of the Na0.44MnO2 powders treated at increasing temperature. Pink circles indicate the peaks of the Mn2O3 impurity, green triangles those of β-Na0.7MnO2 and yellow squares are related to the Na0.44MnO2 phase.PDF Image | Sodium-Ion Batteries Obtained through Urea Based
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