Rhombohedral form of Li3V2(PO4)3 as a cathode in Li-ion batteries

Ion exchange from the monoclinic NASICON phase of Na3V2(PO4)(3) results in the new lithiated form of Li3V2(PO4)(3), with unit cell parameters a = 8.316(1) Angstrom and c = 22.484-(1) Angstrom in the space group R (3) over bar. It exhibits a reversible V3+/V4+ redox couple at an attractive average potential of similar to3.8V vs Li corresponding to the extraction of 2 Li in the voltage window 3.0-4.1 V (125 mA.h/g). Oxidation is characterized by a two-phase process with an equilibrium potential of 3.77 V, which results in an 8% contraction of the c axis of the original rhombohedral unit cell as a result of partial population of the Li sites along the c axis. This is supported by solid-state Li-7 NMR data, which shows that the transformation is accompanied by conversion of the site in the R (3) over bar cell at 85 ppm to one Li site at 62 ppm in LiV2(PO4)(3). At least 1.3Li can be reversibly reinserted into the framework on discharge.