Iron-Doped Sodium-Vanadium Fluorophosphates: Na3V2-yO2-yFey(PO4)2F1+y (y < 0.3).

The sodium-vanadium fluorophosphate family has been actively investigated recently, but few examples tackle chemical doping or the substitution of vanadium. This work presents a series of iron-doped compounds Na3V2-yO2-yFey(PO4)2F1+y (y ≤ 0.3) prepared by hydrothermal synthesis with low iron content. The amount of iron in the structure is confirmed by X-ray and neutron powder diffraction, electronic paramagnetic resonance, magnetic susceptibility measurements, and solid-state nuclear magnetic resonance (ssNMR). The degree of vanadium substitution, together with the solubility limit for iron in sodium-vanadium fluorophosphates, has been calculated by ssNMR and magnetic susceptibility measurements to be y = 0.3 based on the synthetic route used here. The introduction of small amounts of Fe3+ to the structure leads to the reduction of a fraction of V4+ to V3+, and the voltage profiles do not change with the introduction of iron to the structure. In situ synchrotron X-ray diffraction demonstrates that the electrochemical-structural changes during charge and discharge are very similar to those observed in the V3+/V4+ mixed-valent Na3V2O1.6(PO4)2F1.4, which could be related to the existence of both iron dopant and V3+ in the phase.

Polymeric Redox-Active Electrodes for Sodium-Ion Batteries.

Polymer binding agents are critical for the good performance of the electrodes of Na- and Li-ion batteries during cycling as they hold the electroactive material together to form a cohesive assembly because of their mechanical and chemical stability as well as adhesion to the current collector. New redox-active polymer binders that insert Na+ ions and show adhesion properties were synthesized by adding polyether amine blocks (Jeffamine) based on mixed propylene oxide and ethylene oxide blocks to p-phenylenediamine and terephthalaldehyde units to form electroactive Schiff-base groups along the macromolecule. The synthetic parameters and the electrochemical properties of these terpolymers as Na-ion negative electrodes in half cells were studied. Reversible capacities of 300 mAh g-1 (50 wt % conducting carbon) and 200 mAh g-1 (20 wt % conducting carbon) were achieved in powder and Cu-supported electrodes, respectively, for a polySchiff-polyether terpolymer synthesized by using a poly(ethylene oxide) block of 600 g mol-1 in place of one third of the aniline units. The new redox-active polymers were also used as a binding agent of another anode material (hard carbon), which led to an increase of the total capacity of the electrode compared to that prepared with other standard fluorinated polymer binders such as poly(vinylidene) fluoride.