XPS and ionic conductivity studies on Li 2O–Al 2O 3–(TiO 2 or GeO 2)–P 2O 5 glass–ceramics

Abstract

Lithium ion conducting glass–ceramics composed of the crystalline conducting phase LiM 2(PO 4) 3 (M=Ti and Ge) in which the M 4+ ions are partially substituted by Al 3+ ions (Li-analogue of NASICON) have been synthesized by heat-treatment of Li 2O–Al 2O 3–MO 2–P 2O 5 (M=Ti and Ge) glasses. The as-prepared and annealed glasses and glass–ceramics have been characterized by XRD, DSC, XPS and conductivity techniques. The glass transition temperature, T g of the annealed glasses was found to be 601°C and 474°C for the M=Ti and Ge glass systems whereas the crystallization temperature, T c of the annealed glasses are 644°C and 578°C respectively. XPS studies show that the O 1s 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 Li 2O–Al 2O 3–TiO 2–P 2O 5 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 Ti 4+–O and Ge 4+–O bonds are highly ionic in character. The glass–ceramics show fast ion conduction ( σ 303 K=6.53×10 −4 (M=Ti) and 3.99×10 −4 S cm −1 (M=Ge)) and low E a value (0.31 eV). These σ values are slightly higher than the corresponding crystalline Li 1+ x M 2− x Al x P 3O 12 (M=Ti and Ge) phases and four to five orders of magnitude higher than the respective glassy phases. Plausible explanation for the enhancement of σ 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 state Li-ion rechargeable battery.