High Lithium Ionic Conductivity in the Li 1+x Al x Ge y Ti 2-x-y (PO 4 ) 3 NASICON Series Pilar Maldonado-Manso, † Enrique R. Losilla, † Marı ´a Martı ´nez-Lara, † Miguel A. G. Aranda, † Sebastia ´ n Bruque,* ,† Fatima E. Mouahid, ‡ and Mohammed Zahir ‡ Departamento de Quı ´mica Inorga ´ nica, Cristalografı ´a y Mineralogı ´a, Universidad de Ma ´ laga, 29071-Ma ´ laga, Spain, and L.P.C.M. De ´ partement de Chimie, Faculte ´ des Sciences, Universite ´ Chouaib Doukkali, El Jadida 24000, Morocco Received November 15, 2002. Revised Manuscript Received February 24, 2003 Two Li 1+x Al x Ge y Ti 2-x-y (PO 4 ) 3 (0.2 e x e 0.8; y ) 0.8, 1.0) solid solutions have been prepared as polycrystalline powders. These compounds crystallize in the NASICON-type structure, R3 h c space group, and the crystal structures have been characterized by the Rietveld method with laboratory X-ray powder diffraction data. The cell parameters evolution along the two series agrees with the substitution of larger Ti 4+ by smaller Ge 4+ and Al 3+ cations. The electrical properties have been characterized by an impedance study. Bulk conductivity values at room temperature are close to 10 -3 S‚cm -1 with low activation energies (≈0.35 eV). The trajectories of the Li + cations have been simulated from the bond valence sum calculation. Structural keys, which justify the high ionic conductivity and the low activation energy, are discussed. Introduction All-solid-state lithium batteries have a very important market due to their unmatchable properties of high potential (E red ° )-3.024 V), very light weight, and hence, very high energy-density storage properties. Thus, there is an intense research effort in three- dimensional Li-based solid electrolytes for all solid lithium batteries. Li analogues of NASICON-type ma- terials are promising candidates as electrolytes in these cells if the conductivity at room temperature is en- hanced. There are many reports dealing with NASI- CON-type Li + ion conductor materials. 1-3 The original NASICONs are solid solutions derived from NaZr 2 P 3 O 12 by partial replacement of P by Si, with Na excess to balance the negatively charged framework, to yield the general formula Na 1+x Zr 2 P 3-x Si x O 12 (0 e x e 3). 4,5 The NASICON structure has a negatively charged 3D framework, of general formula M 2 X 3 O 12 , within which the Na + cations reside in fully or partially occupied sites. The framework is built of XO 4 tetrahedra linked by corners to MO 6 octahedra. Each XO 4 tetrahedron shares each corner with one MO 6 octahedron and, conversely, each MO 6 octahedron shares each corner with a differ- ent XO 4 group. The interstitial voids generated within the network are of two types known as M1 and M2 sites (1:3 multiplicity). The M1 site is coordinated by a trigonal antiprism of oxygens and the M2 site has a distorted 8-fold coordination. The large charge-compen- sating Na + cations are located in these two positions. In general, the properties of the NASICON-type Li + ion conductor compounds strongly depend on the chemical stoichiometry and the thermal treatment. LiM 2 IV (PO 4 ) 3 (M IV ) Ti, Zr, Hf, Ge, Sn) systems have been widely studied. 1-3,6-11 For M ) Zr, Hf, Sn composi- tions, a triclinic phase has been reported at low temperature 12-14 and for M ) Ti, Ge, the compounds are rhombohedral at all reported temperatures. 15,16 The M ) Ti system is probably the most studied system because the smaller size of the Ti 4+ cations makes the size of the sites in the channels more appropriate for lithium cations. 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