Iron and Titanium-Based Electrode Materials for Sodium-Ion Batteries
Recent studies in NIBs focused on the development of electrode materials. A number of promising electrode materials, including cathodes such as α-NaFeO2-based NaMO2 (M = Fe, Co, Mn, Ni, Ti, etc.) [1, 2], polyanion-based Na3V2(PO4)3 , Na3MPO4CO3 , Na4Fe3(PO4)2(P2O7) , and Prussian blue , and anodes such as hard carbon , Na2Ti3O7 , P2-type Na2/3Co1/3Ti2/3O2 , and carboxylate-based materials  have already been explored for NIBs. The development of feasible electrode materials by an optimized composition with high capacity, long cycle life, and low cost still remains a challenging issue that warrants further investigation. Looking back to the history, iron (Fe) always has the priority being used when it comes to transition metal oxides because the elemental abundance in the Earth’s crust is of primary importance for large-scale batteries [1, 11]. In this pursuit, herein, O3-type Na[Fe1/3Ni1/3Ti1/3]O2 cathode material and orthorhombic Nax[FeTi]O4 (x = 1 and 4/3) anode materials are synthesized by a solid-state reaction method, and studied as a positive electrode and negative electrodes for NIBs, all tested in a half cell and at 20 ºC, using 1 M NaClO4 in 1:1 weight ratio of propylene carbonate (PC) and dimethyl carbonate (DEC) as electrolyte, and a glass fiber filter (GF/D, Whatman) as separator. The obtained Na[Fe1/3Ni1/3Ti1/3]O2 cathode material has an initial discharge capacity of 115 mAh g-1 under a current density of 10 mA g-1 (0.1 C) in the voltage range of 1.5-4.0 V, and shows reversible discharge capacity values of 91.3, 89.7, 79.2, 69.7, 46.2, and 30.9 mAh g-1, respectively at 0.2, 0.5, 1, 2, 5, and 10 C. The potentials reported in this work refer to the Na+/Na couple. The obtained Na4/3[FeTi]O4 anode material exhibits a first charge capacity of 119.6 mAh g-1 under a current density of 17.7 mA g-1 (0.1 C) in the voltage range of 0.01-2.5 V, and delivers reversible charge capacity values of 84.5, 73.1, 61.4, 47.5, and 36.7 mAh g-1, respectively at 0.5, 1, 2, 5, and 10 C. The even more promising thing is as-prepared orthorhombic Na[FeTi]O4 exhibits an initial charge capacity of about 181 mAh g-1 at 0.1 C, and good cycling performance as well.
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