Competitive network modification in non-oxide chalcogenide glasses structural and motional properties of glasses in the system Li2S-P2S5-B2S3 studied by multinuclear NMR techniques

Abstract

Non-oxide chalcogenide glasses based on more than one network former species have certain advantages for applications as solid electrolytes in batteries. To elucidate the influence of competitive glass-formation on the structural and motional properties of ionically conductive chalcogenide glasses, the system (Li2S)0.67[(B2S3)1−y (P2S5) y ]0.33 has been characterized comprehensively by DSC, electrical conductivity and6Li,7Li,31P, and11B solid state NMR techniques. The data obtained provide the first systematic characterization of the coformer effect in a chalcogenide glass system. Homogeneous glassy samples are formed fory=0.3 and 0.9≦y≦1.0, and microphase separated glasses for 0≦y≦0.2. The presence of a coformer leads to an increase in the electrical conductivity and a decrease of the activation energy, as compared to either binary system (Li2S-B2S3 or Li2S-P2S5), but only if homogeneous glasses are formed. The NMR data, in conjunction with systematic DSC and NMR studies of the binary systems (Li2S)x(B2S3)1−x (0.50≦x≦0.75) and (Li2S) x (P2S5)1−x (0.50≦x≦0.70), lead to the following conclusions: 1)11B MAS-NMR is well-suited to quantitate the amounts of three- and four coordinated boron atoms; in the binary glasses, the fraction of four-fold boron (N4) increases with decreasing Li2S/B2S3 ratio; in the ternary glasses N4 increases with increasingy. 2)31P MAS-NMR spectra of the binary glasses discriminate between three different phosphorus microenvironments, assigned to sulfide-analogs of metaphosphate, pyrophosphate, and orthophosphate species, respectively. These results suggest the applicability of network modification models originally developed for oxide glasses. For the ternary glasses, the DSC and NMR data of glasses with low phosphorus contents are consistent with a phase separation model involving a Li-rich thioborate glass that contains all of the phosphorus component and a glass phase less rich in lithium containing the four-fold boron atoms. In addition to the structural studies, the7Li spin-spin relaxation times (T 2) are used to characterize the mobility of the Li atoms. The activation energy of Li motion, determined from temperature dependentT 2 measurements and analyzed by using BPP theory differs from that determined from conductivity measurements by a factor of 2–3, possibly reflecting the inapplicability of this theory to the lithium diffusion process.