RETRACTED ARTICLE: Structural, optical and electrochemical properties of a new phosphate-based compounds Na₂Mn_2−xNi_xFe(PO₄)₃ as negative electrode for sodium-ion batteries

The development of attractive negative electrode materials with high energy density, excellent structural stability and safety is crucial to advance the practical applications of sodium-ion batteries in the market. Hence, the combination of abundant and environmentally benign elements (such as sodium, iron and phosphorus) with the use of polyanionic frameworks holds significant promise. Herein, we report the synthesis, crystal structure, and the electrochemical properties of a series of new compounds belonging to the Alluaudite family Na₂Mn_2−xNi_xFe(PO₄)₃ (where x = 0.6, 1.6). These materials were successfully prepared by co-precipitation method, and thoroughly characterized using X-Ray Diffraction analysis, Raman Spectroscopy, Scanning Electron Microscopy, UV vis spectroscopy and impedance spectroscopy technique. Their electrochemical behavior as anode materials for Sodium ion batteries was investigated by the galvanostatic charge–discharge cycling at C/5 current rate and over 0.1–3.6 V voltage range. Initial discharge capacities of 340 mAh g⁻¹ and 370 mAh g⁻¹ were respectively achieved during the first cycle by Na₂Mn_1.4Ni_0.6Fe(PO₄)₃ and Na₂Mn_0.4Ni_1.6Fe(PO₄)₃, corresponding to an irreversible reaction where about seven sodium ions per formula unit were stored. In fact, fewer Na ions were involved in the electrochemical reaction during the subsequent charge and discharge cycles, indicating that this material undergoes an irreversible conversion-type reaction.