Physical Science Research Frontiers 2016 60 Structure of Li 2 S-P 2S 5 sulfide glasses Sulfide glasses are materials of interest for solid electrolytes in lithium ion batteries (LIBs) [1], because the realization of an all-solid-state battery will enable the miniaturization of battery packages and reduce safety issues compared with those of LIBs with an organic electrolyte. Significant progress has been made so far with the discovery of numerous sulfide compounds with high ionic conductivities such as Li 7 P 3 S11 [2], Li 10 GeP 2 S 12 [3], Li 10 SnP 2 S12 [4], and Li 7 P 2 S8 I [5]. All these materials are derived from Li 2 S- P 2 S 5 sulfide glasses. Therefore, the nature of Li 2 S- P 2 S 5 glasses must be clarified in detail to continue the development of advanced Li ionic conductors targeted at the realization of all-solid-state batteries. To clarify the origin of the ionic conduction, we present a comparative fundamental study of the structures of the 67Li 2 S-33P 2 S 5 (67Li 2 S), 70Li 2 S-30P 2 S 5 (70Li 2 S), and 75Li 2 S-25P 2 S 5 (75Li 2 S) glasses. The lithium ionic conductivities of 67Li 2 S, 70Li 2 S, and 75Li 2 S were found to be 5.6×10 –5 S/cm, 1.4 × 10 –4 S/cm, and 3.0 × 10 –4 S/cm, respectively. We performed high energy X-ray diffraction measurements at SPring-8 BL04B2 , and analyzed the environment of the Li + ions by structural analyses combining X-ray and neutron diffraction measurements with the aid of density functional theory (DFT), reverse Monte Carlo (RMC) simulation, and Raman spectroscopy to reveal the relationship between structural properties and Li ionic conduction. Binary Li 2 S-P 2 S 5 glasses, which consist of PS x polyhedral anions, are well-known superionic conductors. To quantitatively evaluate the fraction of PS x polyhedral anions, the Raman spectra of the 67Li 2 S, 70Li 2 S, and 75Li 2 S glasses were obtained, as shown in Fig. 1(a). It is known that bands in the frequency range of 330–480 cm –1 are sensitive to the S-P-S bond angle. We assigned the three bands at approximately 425 cm –1 , 410 cm –1 , and 390 cm -1 to the stretching vibration of the P-S bonds in the PS 4 3– ( ortho - thiophosphate) ion, P 2 S 7 4– ( pyro -thiophosphate) ion, and P 2 S 6 4– (an ethanelike structure with a P-P bond) ion, respectively. The ratios of the PS 4 3– , P 2 S 7 4– , and P 2 S 6 4– ions were estimated using a Lorentzian function, shown as dotted lines in Fig. 1(b), and are summarized as open circles, open triangles, and open squares in Fig. 1(c), respectively. It is clear that the ratio of PS 4 3– ions increases with the Li 2 S content, while the ratios of P 2 S 7 4– and P 2 S 6 4– ions decrease. Intriguingly, it was found that P 2 S 6 4– ions exist in these glasses with ratios of approximately 33.0%, 18.3%, and 4.4% in 67Li 2 S, 70Li 2 S, and 75Li 2 S, respectively, whereas they should not be contained in the stoichiometric compositions (0PS 4 :100P 2 S 7 in 67Li 2 S, 50PS 4 :50P 2 S 7 in 70Li 2 S, and 100PS 4 :0P 2 S 7 in 75Li 2 S). This means that there is a sulfur deficiency in these glasses. Figure 2 shows experimental X-ray total structure factors, S X ( Q ), for the 67Li 2 S, 70Li 2 S, and 75Li 2 S glasses. Oscillations in S X ( Q ) remain up to the high Q region, which is evidence for well-defined short-range order in the formation of P-S bonds. The difference between the three compositions is not significant in both sets of diffraction data. To uncover the relationship between the glassy structure and the high ionic conductivity for these glasses, we modeled the atomic structure of the Li 2 S-P 2 S5 glasses by DFT/RMC simulation using X-ray and neutron diffraction data, Fig. 1. (a) Raman spectra in the range of 330–480 cm –1 for Li 2 S-P 2 S5 glasses. Black, blue, and green lines represent 75Li 2 S, 70Li 2 S, and 67Li 2 S glasses, respectively. (b) Spectral decomposition of Raman spectrum for 70Li 2 S glass. Blue line, experimental data; dotted lines, fitting result for all PS polyhedral anions (light blue), PS 4 (black), P 2S 7 (red), and P 2 S 6 (blue). (c) PS x polyhedral fractions for Li 2 S-P 2S 5 glasses derived from Raman spectra (open marks) and DFT/RMC model (filled marks). 2000 1500 1000 500 67Li 2 S 70Li 2 S 70Li 2 S 75Li 2 S 1600 1200 800 400 Intensity (arb. units) 450 400 350 Wavenumber (cm –1 ) EXP. FIT PS 4 PS 4 P 2S 7 P 2S 7 P 2S 6 P 2S 6 (a) (c) (b) 100 80 60 40 20 0 Fraction (%) 80 75 70 65 60 Li 2 S Content (mol%) Raman PS 4 PS 4 P 2 S 7 P 2 S 7 P 2 S 6 P 2 S 6 DFT/RMC Research Frontiers 2016 61 fixing the ratios of the PS 4 3– , P 2 S 7 4– , and P 2 S 6 4– ions on the basis of Raman spectroscopy measurements to reproduce the plausible glassy structures. The total structure factors S X ( Q ) of the Li 2 S-P 2 S 5 glasses derived from the DFT/RMC model are shown in Fig. 2 as lines. The DFT/RMC model is consistent with the experimental data. The DFT/RMC structure is consistent with both the diffraction data and the Raman data, and we compared the electronic structure in terms of each PS x polyhedral anion for the 70Li 2 S glass. Figures 3(a) and 3(b) show the partial density of states (p-DOS) of the 70Li 2 S glass for the S 3 p -orbital and P 3 p -orbital, respectively. It is apparent that the orbitals form a hybrid orbital between the phosphorus and sulfur; the highest occupied molecular orbital (HOMO) is located at −4.0 to −0.5 eV and the lowest unoccupied molecular orbital (LUMO) is located at 1.5 to 5.0 eV. The positive charge of the P ion is large owing to the hybrid orbital. However, the p-DOS plots of the P ion reveal that the P 2 S7 anion only differs from the PS 4 and P 2 S 6 anions as follows ( Fig. 3(b)). A shallow level appears near the bottom of the LUMO at approximately 2.0 eV in the P 2 S 7 anion, which is related to a covalent bond between the P ion and the bridging sulfur (BS) ion in the P 2 S7 anion. This electron transfer is expected to weaken the positive charge of the P ions, which attract Li + ions to the P 2 S 7 anions more strongly than the other PS x polyhedral anions. Furthermore, it is easy for the attracted Li + ions to remain around the P 2 S 7 anions, which may suppress the lithium ionic conduction in solid electrolytes. In this study, we found that P 2 S6 4– ions as well as PS 4 3– and P 2 S 7 4– ions are present in 67Li 2 S, 70Li 2 S, and 75Li 2 S glasses on the basis of Raman spectroscopy measurement. Density functional theory and reverse Monte Carlo simulations (DFT/RMC) quantitatively reproduced the results of high-energy X-ray diffraction, neutron diffraction, and Raman spectroscopy, fixing the ratios of PS 4 3– , P 2 S 7 4– , and P 2 S 6 4– ions. The electronic structure of the DFT/RMC model suggests that the existence of the P 2 S 7 anion may suppress lithium ionic conduction. Thus, the control of the edge sharing between PS x anions and Li ions without electron transfer between the P ion and the BS ion is expected to facilitate lithium ionic conduction in a solid electrolyte, which should contribute to the development of all-solid batteries. References [1] Y. Kato et al .: Nature Energy 1 (2016) 16030. [2] Y. Seino et al .: Energ. Environ. Sci. 7 (2014) 627. [3] N. Kamaya et al .: Nat. Mater. 10 (2011) 682. [4] P. Bron et al .: J. Am. Chem. Soc. 135 (2013) 15694. [5] E. Rangasamy et al .: J. Am. Chem. Soc. 137 (2015) 1384. [6] K. Ohara, A. Mitsui, M. Mori, Y. Onodera, S. Shiotani, Y. Koyama, Y. Orikasa, M. Murakami, K. Shimoda, K. Mori, T. Fukunaga, H. Arai, Y. Uchimoto, and Z. Ogumi: Sci. Rep. 6 (2016) 21302 . K. Ohara a,b, *, A. Mitsui a,c , Y. Uchimoto a and Z. Ogumi a a Kyoto University b Japan Synchrotron Radiation Research Institute (JASRI) c Toyota Motor Corporation *Email: ohara@spring8.or.jp Fig. 2. Total structure factors S ( Q ) at room temperature for Li 2 S-P 2 S 5 glasses derived from X-ray diffraction. Circles, experimental data; lines, DFT/RMC model. Fig. 3. Partial DOS for (a) S 3 p -orbital and (b) P 3 p -orbital of 70Li 2S glass. 4 2 0 –2 S X ( Q ) 20 15 10 5 0 X-ray 67Li 2 S 70Li2S 75Li2S Q (Å –1 ) 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Density of States (e V –1 atom –1 ) –15 –5 0 5 –10 Energy (eV) –15 –5 0 5 –10 Energy (eV) (a) (b) PS 4 PS 4 P 2 S 7 (1) P 2 S 7 P 2 S 7 (2) P 2 S 6 P 2 S 6
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