THE INFLUENCE OF ANION INCORPORATION ON SOME PHOSPHATE-BASED ION CONDUCTING GLASSES

Abstract

Fast ion conducting glasses have been widely investigated in view of their potential application as solid electrolyte materials in high energy density batteries. Li2O:P2O5 and Ag2O:P2O5 are some of the few oxide glasses which have received much attention because of their high ionic conductivity near room temperature, ease of preparation, low melting points and wide glass-forming region. In particular, phosphate glasses are structurally interesting because they accept a wide range of anion substitution. The substitution of different anions in oxide glasses has a strong influence on the physicochemical properties such as conductivity, hygroscopic nature, chemical stability and glass structure, and hence it is of great interest to study the effect of different anion substitution/incorporation on the physical and structural characteristics of phosphate-based ion conducting glasses. In addition, the objective of the investigation on different cations such as Li+ and Ag+ ion is to see the characteristics of different cation interactions with the phosphate network, which lead to the differences in conduction processes. In the present investigation, the synthesis and characterization of three types of glass systems, xS:(1-x)AgPO3, xAg2S:(1-x)AgPO3 (I); xF2:( 1-x)Ag PO3, xAgF:( 1-x)AgPO3 (III) and xF2: (1-x)LiPO3, xLiF:(1-x)LiPO3 (Ill) are reported. The glass forming regions in three types of glass systems are found to be comparable. In type I and II glass systems, glass transition temperature (Tg) increases with the increase of x, while it decreases gradually in type Ill glass. Tg for type Ill is higher than that of type I and II. These variations could be related to the changes in the glass structure and different effects of anion substitution and cation interaction with the phosphate network. The P 2p binding energy is found to increase with the increase of sulfur content in S:AgPO3. While it appears to decrease in Ag2S:AgPO3, F2:AgPO3 and AgF:AgPO3 systems, no appreciable decrease is observed in F2:LiPO3 and LiF:LiPO3 systems. This behaviour may be attributed to the depolymerization of the phosphate chain, different anion substitution in different oxygen positions as well as the different cation-anion interaction in the phosphate network. The increase in conductivity (?) and the decrease in activation energy with increasing x in the present systems except the F2:LiPO3 system are attributed to the polarizability of sulfur and the structural modification by the different anion substitution/incorporation together with the increase of charge carrier concentration. Among the glass compositions studied, the 0.33 Ag2S: 0.67AgPO3 glass has the highest a at room temperature, its value being 6.11 x10-5 (?-1 ,cm-1 ). For glasses of analogous compositions in the three types of systems, Ag2S:AgPO3 glasses have the highest a, followed by the AgF:AgPO3, S:AgPO3 , F2:AgPO3 , LiF:LiPO3 and F2:LiPO3 glasses. Fourier-transform infrared absorption (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) show that sulfur and fluorine participate in the phosphate network. Various structural species such as P(O,S)4, P(O,F)4, P2(O,F)7, P2O74-, PO3, P-F, P-S are presented. The absence of Ag-F bonds in AgF:AgPO3, and the presence of Li-F bonds in LiF:LiPO3 glasses suggest that different cations interact differently with the phosphate network. Structural modifications in the phosphate network through sulfur and fluorine incorporation, accompanied by the increase of charge carrier concentration are considered to be responsible for enhancement in conductivity.