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

Lithium ion conducting glass-ceramics composed of the crystalline conducting phase LiM2(PO4)(3) (M = Ti and Ge) in which the M4+ ions are partially substituted by Al3+ ions (Li-analogue of NASICON) have been synthesized by heat-treatment of Li2O-Al2O3-MO2-P2O5 (M = Ti and Ce) glasses. The as-prepared and annealed glasses and glass-ceramics have been characterized by XRD, DSC, XPS and conductivity techniques. The glass transition temperature, Tg Of the annealed glasses was found to be 601 degreesC and 474 degreesC for the 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 deconvoluted 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 character. 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 crystalline Li1+xM2-xAlxP3O12 (M = Ti and Ge) phases and four to five orders of magnitude higher than the respective glassy phases. Plausible explanation for the enhancement of sigma in glass-ceramics is given. It is suggested that the M = Ge glass-ceramics can be used as a solid electrolyte in the all-solid stare Li-ion rechargeable battery. (C) 2000 Elsevier Science B.V. All rights reserved.