Rhombohedral Form of Li₃V₂(PO₄)₃ as a Cathode in Li-Ion Batteries

Abstract

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) Å and c = 22.484(1) Å in the space group R3̄. It exhibits a reversible V3+/V4+ redox couple at an attractive average potential of ∼3.8V 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 7Li NMR data, which shows that the transformation is accompanied by conversion of the site in the R3̄ 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.