MAPPING OF TRANSITION-METAL REDOX ENERGIES IN PHOSPHATES WITH NASICONSTRUCTURE BY LITHIUM INTERCALATION

The position of several transition metal redox energies with respect t o the Fermi energy of lithium in phosphates with sodium super ionic co nductor (NASICON) framework were determined electrochemically upon lit hium intercalation. The materials studied were Li2NaV2(PO4)(3), LiNa2F eV(PO4)(3), TiNb(PO4)(3), LiFeNb(PO4)(3), and Li2FeTi(PO4)(3) all havi ng NASICON framework. The positions of the redox couples were found ce ntered at the following energies: V4+/V3+ at 3.8 eV, Fe3+/Fe2+ at 2.8 eV, Ti4+/Ti3+ at 2.5 eV, Nb5+/Nb4+ at 2.2 eV, Nb4+/Nb3+ at 1.8 eV, and V3+/V2+ at 1.7 eV below the Fermi energy of lithium. Several of these redox couples were found to overlap each other. Some of these materia ls look promising as positive electrode materials for Li+-ion batterie s.