High lithium ion conductivity in mechanically milled Nb-doped m-Li^sub 3^Fe^sub 2^(PO^sub 4^)^sub 3

Monoclinic lithium iron phosphate (m-Li3Fe2(PO4)3) doped with 0-15 mol.% Nb2O5 is mechanically milled from a glassy nanopowder. The microstructures are observed using field emission scanning electron microscopy. X-ray diffraction and X-ray photoelectron spectroscopy are used to determine the m-Li3Fe2(PO4)3 crystallinity. AC impedance and DC polarization are used to investigate the m-Li3Fe2(PO4)3 electrical properties. The mechanically milled as-prepared monoclinic powders are amorphous and are sintered at 800 °C for 12 h to produce NASICON-type crystals whose lattice parameters and sinterability change with increasing Nb concentration. The 5 mol.% Nb2O5-doped m-Li3Fe2(PO4)3 shows the maximum ionic conductivity of 7.40 × 10-6 S cm-1 (Ea = 0.47 eV) and an electronic conductivity of ~10-9 S cm-1. However, m-Li3Fe2(PO4)3 doped with >10 mol.% Nb2O5 shows even lower ionic conductivity and sinterability, and NbOPO4 is detected as the impurity phase. Consequently, Li ion migration may be enhanced by the addition of the appropriate amount of Nb2O5. Therefore, Nb-doped m-Li3Fe2(PO4)3 is a promising solid electrolyte for application in all-solid-state Li ion batteries.