A Novel Fast Lithium-Ionic Conductor
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A NOVEL FAST LITHIUM-IONIC CONDUCTOR. L. MORENO-REAL*, T.RAMIREZ-CARDENAS*, S.BRUOUE-, M.MARTINEZ-LARA- AND J.R.RAMOSBARRADO** *Departamento d8 Quimica Inorg~n1ca. Universidad de M61laga. "*Departamento de Fisica Aplicade. Universided de M6laga. Apartado 59, 29071 MALAGA. SPAIN. ABSTRACT The lithium derivatives from anhydrous niobium (Y) phosphate were made through NbOPO4.H 20 and LiCl by solid state reaction at 200 °C and subsequent annealed at 500°C. The solid with the highest lithium content, exhibits the maximum conductivity of all materials prepared. The electrical conductivity of this solid ranges from 10- 10 - cm-
at 373K to 3.62.10-3n
1
cmI at 683K.
INTRODUCTION Anhydrous niobium (V) phosphate is a promising thermal and chemically stable inorganic host whose properties have not been studied sufficiently. The literature contains information on the synthesis of niobium phosphates [1], spectroscopic and structural properties [2.3] and on some of .their intercalative reactions into the hydrate phases [4]. However, no evidence has been found for the use of the niobium phosphate as adequate host matrix in fast alkali ion conduction. Structurally, 0NbOPO 4 is a 3D tetragonal solid, consisting of [NbO 6 ] octahedra surrounded by four [P0 4 ] tetrahedra in the ab plane. The individual layers are held together by shared trans oxigen atoms of the [Nb0 6 ]. These structural features are shown in Fig. 1 [5].
b
a
FIGURE 1.Structure of s- NbOPO 4 This 3D skeleton structure of the a-NbOPO4 provides the presence of empty rigid tunnels along the c axis (Fig. la) and smaller interconnected cavities in the a or b direction (Fig. lb). The size or diameter of these micropores is sufficiently large to accommodate small cations as Li+, Na+, Cu+, Ag+, etc,, and it may permit rapid three or preferable mono- dimensional diffusion (linear passageway in c
direction) of these cations within the Intracrystalline space.
Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
688
The hydrates of niobyl phosphate NbOPO4 .nH2 0 (n13) are lamellar solids and have a tetragonal symmetry with the same a lattice constant than the corresponding for the anhydrous a-NbOPO4 defined before. The c parameter dependes on the water molecule number, n- I per formula, located in their interlayer space; the other molecule is coordinated to the niobium atom. This water molecule is acid [2,31 and can be more reactive in acid-base and ion-exchange reactions. At 200° C is stable the monohydrated phase NbOPO 4.H2 0 and above this temperature the layer structure evolves irreversibly to the NbOPO 4 framework by the collapse of the layer structure. The aim of this work is the preparation of a ionic conductor by lithium ion insertion into the channels of the NbOPO 4 framework. The chemical strategy is to begin from open layer lattice (niobyl phosphate hydrate) energizing the water molecules by partial dehydration, then to exchange the proton from the residual acid water by lithium, and finally to complete the dehydration and so to provoke th
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