Structure and electrochemistry of NaFePO4 and Na2FePO4F cathode materials prepared via mechanochemical route

Nanostructured NaFePO4 (space group Pmnb) and Na2FePO4F (space group Pbcn) were prepared by a quick and facile mechanochemically assisted solid state synthesis. Low-crystalline Na2FePO4F was formed as a result of direct mechanochemical reaction of NaFePO4 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, Mössbauer, and NMR spectroscopy. Electrochemical properties were studied by galvanostatic cycling in lithium cells and GITT. Although NaFePO4 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 Na2FePO4F and accomplished with the NaLiFePO4F formation. Li+ diffusion coefficient in NaLiFePO4F at different delithiated/lithiated states was determined from GITT. Carbon-coated Na2FePO4F 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.