Crystal Structure and Electrochemistry of Na2-XLixFePO4f (0≤x≤1) New Cathode Materials for Li- and Na-Ion Batteries

Sodium-based cathode materials are interested both for sodium-ion and lithium-ion batteries application. In 2007, Ellis et al. [1] synthesized a new sodium-based flourophosphate Na₂FePO₄F. This material operates at 3.3 V and has two-dimensional (2D) pathways for intercalation/deintercalation of alkaline ions and a single-phase (solid solution) mechanism of Li (de)intercalation.

                In the present study, Na_2-xLi_xFePO₄F with 0≤x≤1 were prepared by mechanochemically assisted solid state synthesis. Na₂FePO₄F was synthesized via two-step approach including a covalent precursor formation (NaFePO₄) and incorporation of ionic NaF into its framework. Both steps consist of preliminary mechanical activation (MA) using AGO-2 planetary mill and subsequent heat treatment in Ar flow. Na_2-xLi_xFePO₄F compositions were obtained from NaFePO₄, LiFePO₄, NaF and LiF with the correspondent ratio. Crystal and local structure was studied by X-ray, neutron diffraction (IBR-2 reactor of JINR, Dubna) using Rietveld refinement (Fig. 1); FTIR and Mössbauer spectroscopy. Phase transformations under heating and cooling of the NaFePO₄+NaF activated mixture were investigated by in situ XRD, using a HTK 1200N temperature-controlled X-ray chamber. Particle size and morphology were studied by SEM and TEM. The samples were cycled in Swagelok-type half-cells with Li anode and LiPF₆-based electrolyte at 2.0-4.2 V range and 0.1C-10C cycling rates. GITT method was applied for the evaluation of D_Li.

                It has been shown that poor-crystalline Na₂FePO₄F was formed directly at the stage of MA. The completion of the synthesis and crystallization of the final product with Pbcn space group and average particle size of about 100 nm occur upon the subsequent heating of the activated mixture to 600°C. Refined cell parameters, presented in Table 1, well correlate with the literature data [1]. On contrast, the structure of as prepared Na_2-xLi_xFePO₄F (0.5≤x≤1.0) is well described in Pnma space group. Mössbauer spectra are fitted by one doublet indicating that all Fe ions are in 2+ state and octahedral coordination.

On galvanostatic charge-discharge curves, a solid solution-like sloping voltage profile was observed for all samples. When cycled with Li anode, Na/Li exchange in Na­₂FePO₄F cathode with the formation of NaLiFePO₄F was completed after the 4^th cycle. The structure of the product was maintained (S.g. Pbcn), while its lattice parameters decrease. Discharge capacity was 115, 110 and 55 mAh×g^-1 for Na₂FePO₄F, Na_1.5Li_0.5FePO₄F and NaLiFePO₄F, respectively (Fig. 2). The determined Li⁺ diffusion coefficient D_Li in NaLiFePO₄F, prepared by electrochemical Na/Li exchange from Na­₂FePO₄F, was 10^-15 cm²×s^-1 (Fig. 3), while it was only 10^-17 cm²×s^-1 for NaLiFePO₄F and Na_1.5Li_0.5-xFePO₄F (S.g. Pnma) prepared by solid state synthesis.

[1] B. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar // Nat. Mater. 6 (2007) 749–753.