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

Highlights: • Na₂FePO₄F is prepared by mechanochemically assisted solid state synthesis. • The crystal and local structure are studied by XRPD, FTIR, Mössbauer, and NMR. • Na⁺/Li⁺ ion exchange is completed with the formation of NaLiFePO₄F. • The average D_Li (10⁻¹⁵ cm² s⁻¹) is determined from GITT measurements. - Abstract: 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