Sol-gel synthesis and electrochemical properties of fluorophosphates Na(2)Fe(1-x)Mn(x)PO(4)F/C (x=0, 0.1, 0.3, 0.7, 1) composite as cathode materials for lithium ion battery

摘要

Fluorophosphates Na(2)Fe(1-x)Mn(x)PO(4)F/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 Na(2)FePO(4)F, Na(2)Fe(0.9)Mn(0.1)PO(4)F (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 Na(2)Fe(1-x)Mn(x)PO(4)F from the 2D layered structure of Na(2)FePO(4)F to the three-dimensional (3D) tunnel structure of Na(2)MnPO(4)F. 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. Na(2)FePO(4)F 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 Na(2)FePO(4)F, partial replacement of Fe by Mn in Na(2)Fe(1-x)Mn(x)PO(4)F increases the discharge voltage plateau. Similar to Na(2)FePO(4)F, iron-manganese mixed solid solution Na(2)Fe(1-x)Mn(x)PO(4)F (x 0.1, 0.3, 0.7) also show good cycling performance. Furthermore, Na(2)MnPO(4)F 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 Na(2)Fe(1-x)Mn(x)PO(4)F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.