Structure and electrochemistry of NaFePO 4 and Na 2 FePO 4 F cathode materials prepared via mechanochemical route

Abstract Nanostructured NaFePO 4 (space group Pmnb ) and Na 2 FePO 4 F (space group Pbcn ) were prepared by a quick and facile mechanochemically assisted solid state synthesis. Low-crystalline Na 2 FePO 4 F was formed as a result of direct mechanochemical reaction of NaFePO 4 with NaF. It crystallizes upon subsequent heating to 600 °C and decomposes at higher temperatures. Crystal and local structure were analyzed by XRD using Rietveld refinement, FTIR, Mossbauer, and NMR spectroscopy. Electrochemical properties were studied by galvanostatic cycling in lithium cells and GITT. Although NaFePO 4 showed some electrochemical activity, neither electrochemical nor chemical Na + /Li + exchange was observed by XRD. On contrary, electrochemical and chemical Na + /Li + ion exchange occurred in the case of Na 2 FePO 4 F and accomplished with the NaLiFePO 4 F formation. Li + diffusion coefficient in NaLiFePO 4 F at different delithiated/lithiated states was determined from GITT. Carbon-coated Na 2 FePO 4 F shows discharge capacity of 116 mA h g −1 at 0.1 C rate within the 2.0–4.2 V voltage range and a good cyclability.