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

Abstract

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, 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 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 (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+xM_2-xAl_xP₃O₁₂ (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.