XPS and ionic conductivity studies on Li2O-Al2O3(TiO2 or GeO2)-P2O5 glass-ceramics

Lithium ion conducting glass-ceramics composed of the crystalline conductin g phase LiM2(PO4)(3) (M = Ti and Ge) in which the M4+ ions are partially su bstituted by Al3+ ions (Li-analogue of NASICON) have been synthesized by he at-treatment of Li2O-Al2O3-MO2-P2O5 (M = Ti and Ce) glasses. The as-prepare d and annealed glasses and glass-ceramics have been characterized by XRD, D SC, XPS and conductivity techniques. The glass transition temperature, Tg O f the annealed glasses was found to be 601 degreesC and 474 degreesC for th e M = Ti and Ge glass systems whereas the crystallization temperature, T-c of the annealed glasses are 644 degreesC and 578 degreesC respectively. XPS studies show that the O Is spectra for all the glasses and glass-ceramics could be deconvoluted into two peaks corresponding to the non-bridging (NBO ) and bridging oxygen (BO) atoms. The binding energies (BE) of Li, Ti, Ge, Al and P have also been listed. In the Li2O-Al2O3-TiO2-P2O5 system, the dec onvoluted Ti 2p XPS spectra indicate the existence of two oxidation states of titanium, 3 + and 4 +. The Ti 2p and Ge 3d core levels are characterized by high BEs, suggesting that Ti4+-O and Ge4+-O bonds are highly ionic in c haracter. The glass-ceramics show fast ion conduction (sigma (303 K) = 6.53 x 10(-4) (M = Ti) and 3.99 x 10(-4) S cm(-1) (M=Ge)) and low E-a value (0. 31 eV). These sigma values are slightly higher than the corresponding cryst alline Li1+xM2-xAlxP3O12 (M = Ti and Ge) phases and four to five orders of magnitude higher than the respective glassy phases. Plausible explanation f or the enhancement of sigma in glass-ceramics is given. It is suggested tha t the M = Ge glass-ceramics can be used as a solid electrolyte in the all-s olid stare Li-ion rechargeable battery. (C) 2000 Elsevier Science B.V. All rights reserved.