(Invited) Sodium Vanadium Fluorophosphates for Na-Ion Batteries

Abstract

Among polyanionic-based electrode materials developed for Na-ion batteries, one of the most promising families turns out to be Na3V2(PO4)2F3-yOy (0 ≤ y ≤ 2).[1] We will first give a summary of all the studies performed for few years in our groups in order to reach an in-depth understanding of the crystallochemistry of this compound, i.e. the fine control of the triptych Synthesis conditions, Composition and Structure. Oxygen partial substitution for fluorine was shown to significantly impact the structural and transport properties of Na3V2(PO4)2F3-yOy, as well as its electrochemical performance.[2-3] The richer the content in oxygen in Na3V2(PO4)2F3-yOy, the higher the electronic conductivity of the pristine material is, in relation with the mixed valence state V3+/V4+ within the bi-octahedral units V2O8F3-yOy. Nuclear magnetic resonance spectroscopy was shown to be the best probe for determining the oxygen defects’ concentration. Interestingly, we have also shown that Na3V2(PO4)2F3-y O y compounds are stable in aqueous media, with no water intercalation in the channels of the structure and no Na+/H+ or F-/OH- exchange despite of long immersion times.[4] These materials can thus be used for electrode formulation in aqueous media.[5] The electrochemical behavior and performance of Na3V3+ 2(PO4)2F3, Na3V3+V4+(PO4)2F2O and Na3V4+ 2(PO4)2FO2 will be compared for different cycling potential windows, with careful attention to their chemical stability and to the structural and redox processes involved.[4, 6-7] The study of Na3V2(PO4)2F3 samples prepared according to different synthesis routes and with or without carbon coating has demonstrated that despite subtle differences between them (morphology and nature of the carbon coating, amount of oxygen defects) the phase diagram observed during Na+ extraction is only slightly altered at moderate cycling rates.[8] The optimized carbon-coated one was shown to deliver promising electrochemical performance, at room temperature and at 0°C, at very high rates (up to 25C), with no major loss of reversible capacity and no significant modification of the phase diagram observed within the composition range Na3V2(PO4)2F3 - Na1V2(PO4)2F3. 18650 prototypes of 75 Wh/kg made of this optimized first generation of Na3V2(PO4)2F3 have demonstrated exceptional rate and electrochemical cycling capabilities with more than 4000 charge and discharge cycles performed at 1C rate.[8] New progresses are rapidly expected, for instance with the optimization of the electrolyte and electrode formulation, considering especially that already appealing results were obtained in non-optimized cells prepared in our research laboratories. The authors acknowledge the RS2E French Network and the Alistore European Research Institute for funding, as well as the French National Research Agency (STORE-EX Labex project ANR-10-LABX-76-01 and SODIUM Descartes project ANR-13-RESC-0001-02) and the European Union’s Horizon 2020 research and innovation program under grant agreement No 646433-NAIADES.