Lithium insertion into NASICON frameworks

Intercalation of lithium into the vacant sites in the host framework structure of sodium silicon conductor (NASICON) with various transition metal phosphates can be achieved electrochemically at room temperature. The framework compounds M-2(PO4)(3) with M = (Ti, Nb), (Mn, Nb), and M'Nb-0.5(1.5)(PO4)(3) with M' = (Mn, Fe, Cr), were electrochemically characterized in a cylindrically shaped cell made in our laboratory. To understand the role of trivalent cation (Mn, Fe, Cr) on the position of the octahedral Nb5+/Nb4+, Nb4+/Nb3+ (niobium) redox couple in the framework compounds having the same polyanion, (PO4)(3-), we have investigated various transition metal phosphates. The three trivalent cation substitutions vary in electrochemical behavior in the manner in which the (PO4) tetrahedron stabilizes the redox couples in the octahedral sites. The position of the Nb5+/Nb4+ redox couple for M' = (Mn, Fe, Cr) lies at 1.9, 2.1, and 2.3 V and the position of the Nb4+/ Nb3+ redox couple for all trivalent cations lies at 1.6 V. For the parent compound, TiNb(PO4)(3), the first two equivalents of Li appear to be inserted at an average voltage of around 2.2 V. The reason for this difference in the position of the redox energies is related to changes in the stability of the octahedral sites in the structure. (C) 2003 The Electrochemical Society.