Abstract:

Modified phosphate glasses can be used in all-solid-state batteries as solid electrolytes and cathodes due to their high ionic conductivity. The properties of fast ionic conducting glasses are strongly related to the structure of the glass networks. However, most previous works have focused on improving the ionic conductivity of such glasses by composition adjustments, while structural studies are scant. Structural investigations are essential to understand the composition dependence of the glass structure, which is valuable for improving the ionic conductivity and developing new ionic conducting glasses. In this work, phosphate ionic conducting glasses with compositions of 100LiO_1/2-(100-x)PO_5/2-xTeO₂ (x = 0, 10, 20, 25, 30) were synthesized, and their structures were investigated using Raman and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. When x = 0, Raman and ³¹P magic angle spinning (MAS) NMR spectra showed that most of the phosphorus species were Q_0Te⁽²⁾ species, while the concentration of Q_0Te⁽¹⁾ species was negligible. Q_mTe⁽ⁿ⁾ represents the phosphorus species with n bridging oxygen atoms (the oxygen atoms in P—O—P and P—O—Te linkages are both considered to be bridging oxygen atoms), and m Te atoms are connected to this [PO₄] tetrahedron. When PO_5/2 is substituted with TeO₂, long P—O—P chains are broken into short chains, and Q_0Te⁽²⁾ species gradually transform into Q_1Te⁽²⁾ and Q_0Te⁽¹⁾ species. Two-dimensional (2D) refocused incredible natural abundance double quantum transfer experiment (INADEQUATE) spectra proved that no isolated phosphorus species existed in the glasses; Q_0Te⁽²⁾, Q_1Te⁽²⁾, and Q_0Te⁽¹⁾ species were connected with each other through P—O—P linkages. Three- and four-coordinated Te were observed in the static ¹²⁵Te wideband uniform-rate smooth truncation quadrupolar Carr-Purcell-Meiboom-Gill (WURST-QCPMG) spectra. When the concentration of TeO₂ was low, four-coordinated Te was dominant. However, with the increase in TeO₂, the proportion of three-coordinated Te gradually increased, while that of four-coordinated Te decreased. The experimental contents of P—O—P, P—O—Te, and Te—O—Te linkages in these glasses were calculated from the deconvolutions of ³¹P and ¹²⁵Te NMR spectra. Then, the experimental contents were compared with the theoretical contents calculated according to a random distribution model. It was found that the experimental contents of homonuclear P—O—P and Te—O—Te linkages were slightly higher than their corresponding theoretical values, while the experimental content of heteronuclear P—O—Te was slightly lower than the theoretical value. These results indicated a weak priority for homonuclear connectivities. In this glass system, Li⁺ ions preferred to stay around [PO₄] tetrahedrons rather than tellurium oxygen polyhedrons. However, a small number of Li⁺ ions still interacted with tellurium oxygen polyhedrons to form [TeO₃] units. During the substitution of PO_5/2 by TeO₂, the fractions of bridging oxygen atoms in these glasses were almost unchanged, resulting in a slight change in the glass transition temperature. This work provides a comprehensive description of glass networks, depending on their compositions, which could be valuable for improving the ionic conductivity and for designing new fast ionic conducting glasses through structural modifications.

Key words: Phosphotellurite glass, Solid-state NMR, Fast ionic conductor, Glass structure, Raman spectrum