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 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, Tg 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, Tc 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 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 (σ303 K=6.53×10−4 (M=Ti) and 3.99×10−4 S cm−1 (M=Ge)) and low Ea value (0.31 eV). These σ 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 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.