A novel sol–gel synthesis route to NaVPO4F as cathode material for hybrid lithium ion batteries

Sodium vanadium fluorophosphate, NaVPO4F, a cathode material for hybrid lithium ion batteries has been synthesized via a modified sol–gel method followed by heat treatment. The vanadium (Ш) gel precursor as the reaction intermediate phase can be facilely prepared in ethanol under ambient conditions, and this synthesis considerably simplifies the conventional high-temperature fabrication of VPO4. X-ray diffraction (XRD) results indicate a phase transition of NaVPO4F from the monoclinic crystal to the tetragonal symmetry structure. Meanwhile, the scanning electron microscope (SEM) images show the obvious spatial rearrangements on the morphology of samples. The hybrid lithium ion batteries based on the tetragonal NaVPO4F exhibit an even discharge plateau at 3.6 V vs. Li in the limited voltage range of 3.0–4.2 V, and the discharge capacity retention is up to 98.7% after 100 cycles at C/4 rate. With voltage excursion to 3.0–4.5 V, the initial charge and discharge deliver the reversible storage capacity of 117.3 and 106.8 mAh g−1, respectively. Furthermore, the prepared NaVPO4F has a capacity retention of 83% after 100th cycle at 2 C rate. The electrochemical properties reveal the reversible mixed alkali ion (Li+, Na+) insertion reactions for this fluorophosphate material.