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Nanomaterials 2022, 12, 984 2 of 11 components, the SIB cathode materials that use layered transition metal oxides have been identified as potential candidates. [23–27]. Layered-type sodium transition metal oxides (NaTMO2, where TM = Mn, Fe, Ni, V, Ti, etc.) can provide higher capacities, faster Na+-ion diffusion, and high energy densities. In accordance with the notation from Delmas, NaTMO2 is categorized into two types, O3 and P2, according to different oxygen stacking orderings, where Na+ is located in a unit cell between layers in the octahedral (O) and prismatic (P) sites [28–31]. Compared with P2-type transition metal oxides, O3-type materials are promising candidates as a cathode for an SIB full cell. In particular, O3-type layered cathode materials facilitate Na+ insertion into the material, enabling the fabrication of practical sodium-ion full cells with hard carbon as an anode material. Different cathodes of O3-type layered materials have been intensively studied. The oxygen stacking is different in the O3, P2, and P3-type structures. For instance, the oxygen stacking of O3, P2, and P3-type structures is ABCABC, ABBA, and ABBCCA, respectively [32–34]; however, because of the Na+ ion greater radius, many structural trans- formations are inevitable in the host structure during charging/discharging, leading to poor cyclic performance and low energy efficiency [35–37]. Layered NaMnO2 provide significant energy density, and Fe is electrochemically active in the Na-ion cathode. Researchers have attempted Fe doping in NaMnO2 to improve the electrochemical performance through a synergistic effect [38,39]. Partial Fe doping in NaMnO2 will provide higher initial capacities; however, the material has limitations, such as low rate performance, and huge capacity fading during cycling. To address these issues, several scientific techniques have been used on the material. [40–44]. In this study, we developed an O3-type layered NaFe0.5Mn0.5O2 (NFM) nanocomposite material through a facile solution combustion technique followed by calcination. The as- synthesized material was applied as a cathode for the SIB half-cell. The material exhibits higher specific charge/discharge capacity in initial cycles at a lower current rate; however, it suffers from poor rate capability and drastic capacity fading when cycled at higher current rates, which limits its potential applications. To overcome these limitations, we applied carbon into the host NFM material to form a composite, which was then used as the cathode in the sodium-ion energy storage device. To the best of our knowledge, this is the first time that the physical, chemical and electrochemical properties of as-synthesized NFM/carbon composite material have been investigated and reported. Preparation of NFM active material using solution combustion method will also be newly discussed. 2. Materials and Methods 2.1. Chemicals The chemicals used are as follows: sodium nitrate (NaNO3) and 2-methylimidazole (CH3C3H2N2H) from Sigma Aldrich, Seoul, Korea; manganese (II) nitrate hexahydrate (Mn(NO3)2·6H2O), and iron (III) nitrate enneahydrate (Fe(NO3)3·9H2O) from Junsei Chem- ical, Tokyo, Japan; and glycine fuel (C2H5NO2) from Dae-Jung Chemicals, Siheung-si, Korea. All reagents were analytical and used directly in the reactions without any further treatment. 2.2. Preparation of Sodium Iron Manganese Oxide/Carbon Composite NaFe0.5Mn0.5O2/C The NaFe0.5Mn0.5O2/carbon composite materials were prepared by a facile solution combustion synthesis technique, continued via dry solid-state technique. Figure 1 repre- sents the schematic illustration of NaFe0.5Mn0.5O2/carbon nanocomposite materials.PDF Image | NaFe0 Nanocomposite as a Cathode for Sodium-Ion Batteries
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