Crystal Structure and Electrochemistry of Na2-XLixFePO4F (0≤x≤1) New Cathode Materials for Na- and Li-Ion Batteries

A series of the mixed sodium-lithium iron fluorophosphates Na 2-x Li x FePO 4 F with 0≤x≤1 has been synthesized by mechanochemically assisted solid-state route. Their crystal and local structure, and electrochemical properties were investigated by X-ray and neutron powder diffraction, 57 Fe Mössbauer spectroscopy and electrochemical measurements with GITT. Three single-phase compounds Na 2 FePO 4 F, Na 1.5 Li 0.5 FePO 4 F and NaLiFePO 4 F were obtained. The former crystallize with space group Pbcn , while the latter - with Pnma , differing by Na + /Li + distribution. All as-prepared Na 2-x Li x FePO 4 F samples are electrochemically active in Li cell pointing to the occurrence of a gradual Na + /Li + ion exchange. Ex situ XRD and EDX study indicates that it results in the formation of lithium rich phases: NaLiFePO 4 F (S.G. Pbcn) for Na 2 FePO 4 F and Na 0.3 Li 1.7 FePO 4 F (S.G. Pnma ) for Na 1.5 Li 0.5 FePO 4 F. Charge-discharge curves exhibit sloping profiles with an average potential of 3.3-3.4 V vs. Li/Li+. The average value of D Li determined by GITT is 10 -15 and 10 -16 cm 2 s -1 for NaLiFePO 4 F and Na 0.3 Li 1.7 FePO 4 F, respectively.