Development of sodium ion conducting mixed anion glassy solid-state electrolytes

Abstract

The short-range order (SRO) structures of glasses in the Na4P2S 7-xOx, 0 ≤ x ≤ 7, series were investigated on samples prepared by planetary ball milling (PBM). Like other glasses prepared by PBM, the glassy nature of the prepared materials was confirmed by x-ray diffraction (XRD) showing that they were structurally amorphous and by differential scanning calorimetry (DSC) showing that each composition exhibited a reproducible glass transition temperature (Tg). The short-range order (SRO) structure of these glasses were determined using Raman, Fourier Transform Infrared (FT-IR), and 31 P Magic Angle Spinning NMR (MAS NMR) spectroscopies to investigate how oxygen and sulfur are bonded in the various SRO structural units that were found in these glasses. The three spectroscopic techniques gave further evidence that the resulting glasses were completely reacted from their polycrystalline starting materials Na2S, P2S5 and P2O5. It was found that significant disproportionation reactions occur in these compositions in which the 28 original as batched and expected pyro-phosphate SRO units, P 1 = 2 Na/P, for x = 0 undergo a transformation to form ortho-phosphate (P 0 , 3 Na/P) and meta-phosphate (P 2 , 1 Na/P) SRO structures with increasing x such that there is a conservation of Na + charge, on average 2Na/P. In this nomenclature, the superscript describes the number of bridging sulfurs (BSs) and the number of Na per P plus one (arising from the P +5 valency) describing the number of non-bridging sulfurs (NBSs) on each of the SRO units. As seen in earlier work on other mixed oxy-sulfide (MOS) glass forming systems, the stronger Lewis base O = , compared to S = , is found to preferentially form bridging oxygen (BOs) in the form of P 2 SRO units by bonding to the stronger Lewis acid P +5 , compared to Na + . This leaves the weaker Lewis base S = to bond to the weaker Lewis acid, Na + in the form of non-bridging sulfurs (NBSs) on the charge compensation required P 0 groups. Using both charge balance and quantitative 31 P MAS NMR measurements, a complete composition map of all of the SRO structures present in these glasses has been created.