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

Fluorophosphates Na2Fe1-xMnxPO4F/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, Na2Fe0.9Mn0.1PO4F (x = 0.1) crystallize in a two-dimensional (2D) layered structure with space group Pbcn. However, increasing the content of Mn to x >= 0.3 results in a structure transition of Na2Fe1-xMnxPO4F 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 Na2Fe1-xMnxPO4F increases the discharge voltage plateau. Similar to Na2FePO4F, iron-manganese mixed solid solution Na2Fe1-xMnxPO4F (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 Na2Fe1-xMnxPO4F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.