A Reversible Lithium Intercalation Process in an ReO[sub 3]-Type Structure PNb[sub 9]O[sub 25]

Abstract

Among the large family of Wadsley-Roth-type phases, PNb9O25PNb9O25 presents, as other shear structures such as H­Nb2O5,H­Nb2O5, GeNb18O47,GeNb18O47, and VNb9O25,VNb9O25, a peculiar arrangement of 3×3×∞3×3×∞ ReO3ReO3 -type blocks connected through XO4XO4 tetrahedra. The crystal structure was redetermined by combined Rietveld refinements of X-ray and neutron powder diffraction data, for the first time in the tetragonal space group I 4¯4¯ with a=15.615(1)Åa=15.615(1)Å and c=3.829(1)Å.c=3.829(1)Å. The electrochemical insertion of lithium into LixPNb9O25LixPNb9O25 reaches a value of x≃13.5,x≃13.5, between 3.0 and 1.0 V vs. Li+/LiLi+/Li at a slow cycling rate. Within this voltage window, in situ X-ray diffraction reveals that the reversible intercalation of lithium occurs through three single- and two two-phase regions. The variation of the lattice parameters as a function of xx indicates a global change of ΔV/V=+10%ΔV/V=+10% associated with the reduction of Nb5+Nb5+ to Nb4+Nb4+ and then partly to Nb3+.Nb3+. At voltages lower than 1 V vs. Li+/Li,Li+/Li, extra irreversible phenomena, such as electrolyte and/or carbon reduction were identified but, remarkably, highly crystalline particles of LixPNb9O25LixPNb9O25 remain unaltered when discharging the cell for prolonged time down to 0.02 V vs. Li+/Li.Li+/Li. The behavior of LixPNb9O25LixPNb9O25 is very similar to that of H­LixNb2O5H­LixNb2O5 (LixNbNb9O25)(LixNbNb9O25) but different to that of LixVNb9O25LixVNb9O25 where irreversible reduction of V5+V5+ to V2+V2+ (in tetrahedral coordination) takes place. © 2002 The Electrochemical Society. All rights reserved.