Sol–gel synthesis and electrochemical properties of fluorophosphates Na2Fe1−xMnxPO4F/C (x = 0, 0.1, 0.3, 0.7, 1) composite as cathode materials for lithium ion battery

Abstract

Fluorophosphates Na₂Fe_1−xMn_xPO₄F/C (x = 0, 0.1, 0.3, 0.7, 1) composite were successfully synthesized via a sol–gel method. The structure, morphology and electrochemical performance of the as prepared materials were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge/discharge measurements. XRD results show that, consistent with Na₂FePO₄F, Na₂Fe_0.9Mn_0.1PO₄F (x = 0.1) crystallize in a two-dimensional (2D) layered structure with space groupPbcn. However, increasing the content of Mn to x ≥ 0.3 results in a structure transition of Na₂Fe_1−xMn_xPO₄F from the 2D layered structure of Na₂FePO₄F to the three-dimensional (3D) tunnel structure of Na₂MnPO₄F. SEM and TEM analysis indicates nanostructured primary particles (about tens of nanometres in diameter) are obtained for all samples due to uniform carbon distribution and low calcining temperature used. Na₂FePO₄F is able to deliver a reversible capacity of up to 182 mA h g⁻¹ (about 1.46 electrons exchanged per unit formula) with good cycling stability. Compared with Na₂FePO₄F, partial replacement of Fe by Mn in Na₂Fe_1−xMn_xPO₄F increases the discharge voltage plateau. Similar to Na₂FePO₄F, iron-manganese mixed solid solution Na₂Fe_1−xMn_xPO₄F (x = 0.1, 0.3, 0.7) also show good cycling performance. Furthermore, Na₂MnPO₄F with high electrochemical activity was successfully prepared for the first time, which is able to deliver a discharge capacity of 98 mA h g⁻¹. The good electrochemical performance of Na₂Fe_1−xMn_xPO₄F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.