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Thermal and electrical properties of rapidly quenched Li2S-SiS2-Li2O-P2O5 oxysulfide glasses
Solid State Ionics 113–115 (1998) 733–738
Thermal and electrical properties of rapidly quenched Li 2 S-SiS 2 Li 2 O-P2 O 5 oxysulfide glasses Akitoshi Hayashi*, Ryuhei Araki, Ryoichi Komiya, Kiyoharu Tadanaga, Masahiro Tatsumisago, Tsutomu Minami Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 599 -8531, Japan
Abstract We have systematically investigated the thermal and electrical properties of (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses in the wide range of lithium ion concentration. These oxysulfide glasses with y 5 0.05 exhibited high Tc–Tg values, which is a measure of the glass stability against crystallization, and high ion conductivities at room temperature in the middle range of lithium ion concentration around x 5 55 mol%. In the same composition range, large amounts of silicon atoms coordinated with both sulfur and oxygen atoms, i.e. SiO n S 42n (n 5 1, 2, 3) tetrahedral units, were observed by 29 Si MAS-NMR measurements. The presence of such unique structural units brought about the high glass stability and high ion conductivity of the glasses with y 5 0.05. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Lithium ion conductivity; Thermal property; Oxysulfide glass; Solid-state NMR; Rapid quenching; Glass structure
1. Introduction During the last decade the properties of sulfidebased lithium ion conducting glasses have been studied extensively because of their much higher performance as solid electrolytes compared to that of oxide-based glasses [1–4]. In particular, the Li 2 SSiS 2 glasses have several excellent properties such as high lithium ion conductivities of 10 24 –10 23 S cm 21 at room temperature, high glass transition temperature, and easy preparation without vacuum sealing [5,6]. We have previously prepared (100 2 z)(0.6Li 2 S ? 0.4SiS 2 ) ? zLi x MO y oxysulfide glasses (Li x MO y : ortho-oxosalt, M 5 P, Si, Ge) by using a twin-roller *Corresponding author.
quenching technique and investigated their properties and glass structure [7–10]. The investigation of thermal and electrical properties of these glasses revealed that the addition of small amounts of Li x MO y improved the glass stability against crystallization and kept high conductivity of about 10 23 S cm 21 at room temperature. We have also clarified by using solid-state NMR and X-ray photoelectron spectroscopy that the silicon-centered structural units, where silicon atoms were coordinated with both sulfur and oxygen atoms, were mainly present in the 60Li 2 S ? 40SiS 2 glass to which small amounts of Li x MO y were added [11,12]. The relationship between the thermal and electrical properties and the glass structure of these oxysulfide glasses was pointed out in a previous paper [13]. The fact that such unique structural units are
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00398-1
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A. Hayashi et al. / Solid State Ionics 113 – 115 (1998) 733 – 738
present in glass is of great interest from the viewpoint of glass structure. To understand the glass structure of the Li 2 S-SiS 2 -based oxysulfide glasses, we have systematically done the structural analyses for the glasses with a wide range of lithium ion concentration including the compositions with 60 mol% Li 2 S and reported the structure of the glasses in the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide system by using 29 Si MAS-NMR [14]. In this oxysulfide system, x indicates the total lithium ion concentration, Li 2 Ch (Ch 5 S 1 O), and y indicates the amount of Li 2 O-P2 O 5 glasses added to the Li 2 SSiS 2 glasses. The nominal mole ratio of Li 2 S to SiS 2 is kept equal to the ratio of Li 2 O to P2 O 5 in these oxysulfide glasses. In the present paper, we report the glass stability and lithium ion conductivity for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses (45 # x # 67; y 5 0, 0.05, 0.20) in the wide range of lithium ion concentration. The composition dependence of the thermal and electrical properties of these glasses is discussed on the basis of the glass structure reported in our previous paper [14].
surements were performed for the flakelike glasses in dry Ar atmosphere by using a Solartron 1260 impedance analyzer in the temperature range 25– 2008C and the frequency range 100 Hz–15 MHz. 29 Si MAS-NMR spectra were obtained by using a Varian UNITY INOVA 300 NMR spectrometer. Detailed conditions for NMR measurements were described in the previous paper [14].
2. Experimental
Table 1 Glass transition temperatures (Tg), crystallization temperatures (Tc), and Tc–Tg for (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses
Li 2 O-P2 O 5 glasses were prepared from reagentgrade chemicals Li 2 CO 3 and NH 4 H 2 PO 4 . The mixture of these chemicals was put into a platinum crucible and kept at 8008C to 9008C for 1 h. The molten sample was pressed between two stainless steel plates to obtain plate-like Li 2 O-P2 O 5 glasses. The ground Li 2 O-P2 O 5 glasses and regent-grade chemicals of Li 2 S and SiS 2 were used as starting materials to prepare oxysulfide glasses in the systems (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20). The mixture of these materials was heated in a carbon crucible at 10008C for 2 h in an electric furnace located in a dry N 2 -filled glove box. The molten samples were rapidly cooled by using a twin-roller quenching technique to prepare flakelike glasses of 20 mm in thickness. Differential thermal analysis (DTA) was carried out for the powdered samples sealed in an Al-pan by using a Rigaku thermal analyzer. Conductivity mea-
3. Results and discussion The glass transition temperatures (Tg) and the crystallization temperatures (Tc) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses were determined by using DTA in the wide range of lithium ion concentration. The compositions and the values of Tg, Tc, and Tc–Tg for these glasses are listed in Table 1. Fig. 1 shows the composition dependence of the Tc–Tg values for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) glasses prepared in the present study; the value
x 45 50 55 60 63 65 67 45 48 50 53 55 58 60 63 65 67 55 60 65
y
0.00
0.05
0.20
Tg / 8C
Tc / 8C
Tc–Tg / 8C
319 336 341 346 305 274 307 343 357 344 391 363 331 313 318 318 274 322 326 310
420 441 449 425 367 352 349 451 479 473 533 494 470 461 412 380 320 432 467 390
101 105 108 79 62 78 42 108 122 129 142 131 139 148 94 62 46 110 141 80
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735
systems. It is interesting to note that the enhancement of Tc–Tg with the addition of Li 2 O-P2 O 5 is markedly observed in the middle range of lithium ion concentration around x 5 55 and no enhancement is observed in both ends of x, namely around the compositions of x 5 45 and 67. The 0.80(60Li 2 S ? 40SiS 2 ) ? 0.20(60Li 2 O ? 40P2 O 5 ) oxysulfide glass, which corresponds to the composition with x 5 60 and y 5 0.20, also exhibits large Tc–Tg value of 1408C. Fig. 2 shows the composition dependence of the electrical conductivities at 258C (s25 ) for the glasses with y 5 0, 0.05, and 0.20. All the marks used in this figure are the same ones used in Fig. 1. The s25 values for the glasses with y 5 0 reported by Kennedy [5] are also plotted for comparison (the open rhombuses). We newly measured the conductivities of the twin-roller quenched glasses with y 5 0 (the
Fig. 1. Composition dependence of the Tc–Tg values for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) glasses. The numbers of the abscissa indicate the total lithium ion concentration x, in mol% of Li 2 Ch (Ch 5 S 1 O). The open circles, closed circles, and closed triangles denote the values for y 5 0, y 5 0.05, and y 5 0.20, respectively.
of Tc–Tg is used as one of the measures of the glass stability against crystallization. The abscissa indicates the total lithium ion concentration x, in mol% of Li 2 Ch (Ch 5 S 1 O). The open circles, closed circles, and closed triangles denote the values for the glasses with y 5 0, 0.05, and 0.20, respectively. In this figure, the expressions of xLi 2 S ? (100 2 x)SiS 2 and xLi 2 O ? (100 2 x)P2 O 5 are simplified as those of Li 2 S-SiS 2 and Li 2 O-P2 O 5 , respectively. The Tc–Tg values for the glasses with y 5 0.05, ranging 50–1508C, are larger than those for the glasses with y 5 0 in the almost entire range of lithium ion concentration; in particular, very large Tc–Tg values of about 1508C are obtained in the middle of the x range around x 5 55. These results indicate that the addition of small amounts of Li 2 OP2 O 5 to Li 2 S-SiS 2 glasses brings about the improvement of the glass stability against crystallization in the wide range of lithium ion concentration of the
Fig. 2. Composition dependence of the electrical conductivities at 258C (s25 ) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] glasses. All the marks used in this figure are the same ones used in Fig. 1. The s25 values for the Li 2 S-SiS 2 glasses reported by Kennedy [5] are also plotted for comparison (the open rhombuses).
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open circles) for which the preparation was carried out in a highly dehydrated, newly equipped glove box keeping H 2 O levels lower than 1 ppm. The values of s25 in the present study are a little higher than those reported by Kennedy in the whole lithium ion concentration range. However, the composition dependences of them are very similar; both conductivities show a maximum at around x 5 60. We can compare the conductivities for the (1 2 y)(Li 2 S-SiS 2 ) ? y(Li 2 O-P2 O 5 ) (y 5 0.05, 0.20) oxysulfide glasses with those for the sulfide glasses with y 5 0 prepared in our present study. High conductivities of about 10 23 S cm 21 are obtained in the wide range of lithium ion concentration around x 5 55 for the glasses with y 5 0.05. The composition dependence of s25 is very similar to that for the glasses with y 5 0. It is noteworthy that both the glasses with y 5 0 and 0.05 have almost the same values of s25 of about 10 23 S cm 21 . However, the values of s25 for the glasses with y 5 0.20 are about 24 21 10 S cm , which are lower than those for the glasses with y 5 0 and 0.05. Fig. 3 shows the composition dependence of the activation energy for conduction (Ea) for the glasses with y 5 0, 0.05, and 0.20. All the marks used in this figure are the same ones used in Fig. 2. In the glasses with y 5 0.05, the Ea values are minimized at x 5 60, the composition of which is 0.95(60Li 2 S ? 40SiS 2 ) ? 0.05(60Li 2 O ? 40P2 O 5 ); the value of activa21 tion energy reaches 27 kJ mol . Such a composition dependence of Ea for the glasses with y 5 0.05 is similar to that for the glasses with y 5 0. The Ea values for the glasses with y 5 0.20 are found to be larger than those of the other glasses. Figs. 2 and 3 suggest that the composition with the maximum of the lithium ion conductivity basically corresponds to that with the minimum of the activation energy for the glasses with y 5 0 and 0.05. The composition dependence of ionic concuctivity and activation enegy is very similar in both glasses in the wide range of lithium ion concentration. We previously carried out 29 Si MAS-NMR measurement for the glasses with y 5 0.05 in the wide range of lithium ion concentration and reported the local structure around silicon atoms in these glasses [14]. Fig. 4 shows the composition dependence of the fraction of silicon structural units in the glasses with y 5 0.05 calculated from the peak area of the
Fig. 3. Composition dependence of the activation energy for conduction (Ea) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] glasses. All the marks used in this figure are the same ones used in Fig. 2.
29
Si NMR spectra reported in the reference [14]. The open circles and closed circles denote the fraction of SiS 4 tetrahedral units in which a silicon atom is coordinated with only sulfur atoms and SiO n S 42n (n 5 1, 2, 3) tetrahedral units in which a silicon atom is coordinated with both sulfur and oxygen atoms, respectively. Large amounts of SiO n S 42n tetrahedral units are present in the middle range of lithium ion concentration around x 5 55 while they are small in both ends of x. On the other hand, the composition dependence of the amounts of SiS 4 tetrahedral units is opposite to that of the amounts of SiO n S 42n tetrahedral units. The amounts of SiO n S 42n tetrahedral units show a maximum and those of SiS 4 tetrahedral units are at minimum around x 5 55. High conductivities of about 10 23 S cm 21 have been obtained in the middle composition range around x 5 55 for the glasses with y 5 0.05 as shown in Fig. 2. The presence of large amounts of the silicon-centered structural units of SiO n S 42n proba-
A. Hayashi et al. / Solid State Ionics 113 – 115 (1998) 733 – 738
737
atoms have never been observed as crystal phases in the precipitates from these oxysulfide glasses. The oxysulfide glasses containing large amounts of such structural units are not easily crystallized because the crystallization requires large rearrangement of chemical bonds in these glasses. Therefore, the glass stability against crystallization is improved in the middle range of lithium ion concentration in the oxysulfide glasses with y 5 0.05.
4. Conclusion
Fig. 4. Composition dependence of the fraction of silicon structural units for the 0.95[xLi 2 S ? (100 2 x)SiS 2 ] ? 0.05[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses. The open circles and closed circles denote the fraction of SiS 4 and SiO n S 42n (n 5 1, 2, 3) tetrahedral units, respectively.
bly brought about the high conductivity at the composition as we already pointed out in the (100 2 z)(0.6Li 2 S ? 0.4SiS 2 ) ? zLi x MO y oxysulfide glasses (Li x MO y : ortho-oxosalt, M 5 P, Si, Ge) [9,13]. From our preliminary experiment, two kinds of crystal phases, Li 2 SiS 3 (lithium meta-thiosilicate) and Li 4 SiS 4 (lithium ortho-thiosilicate), were found to be mainly precipitated in the heating of these oxysulfide glasses in the present composition range; both crystal phases consist of only SiS 4 tetrahedral units in which silicon atoms are coordinated with only sulfur atoms. The glasses with y 5 0.05 in both ends of x were found to be easily crystallized into the crystal phases containing only SiS 4 tetrahedral units because large amounts of SiS 4 units were present in such range of x (see Fig. 4). The amounts of SiS 4 tetrahedral units decreased while those of SiO n S 42n (n 5 1, 2, 3) tetrahedral units increased in the middle range of lithium ion concentration around x 5 55. Such unique structural units in which silicon atoms are coordinated with both sulfur and oxygen
We have systematically investigated the thermal and electrical properties of (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses in the wide range of lithium ion concentration. The Tc–Tg values for the glasses with y 5 0.05 were larger than those for the glasses with y 5 0 in the wide range of lithium ion concentration. High conductivities of about 10 23 S cm 21 were obtained for the glasses with y 5 0 and 0.05. Both glasses exhibited the similar composition dependence in conductivities and activation energies. However, the oxysulfide glasses with y 5 0.20 exhibited lower conductivities and higher activation energies than the glasses with y 5 0 and 0.05. In the middle range of lithium ion concentration around x 5 55, a large number of structural units in which a silicon atom is coordinated with both sulfur and oxygen atoms, i.e. SiO n S 42n (n 5 1, 2, 3) tetrahedral units, were observed in the glasses with y 5 0.05 by 29 Si MAS-NMR measurements. The presence of large amounts of these structural units brought about the high glass stability and high conductivity of these oxysulfide glasses.
Acknowledgements This work was supported by the ‘‘Research for the Future’’ Program from the Japan Society for the Promotion of Science. It was also partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.
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References [1] H. Wada, M. Menetrier, A. Levasseur, P. Hagenmuller, Mat. Res. Bull. 18 (1983) 189. [2] A. Pradel, T. Pagnier, M. Ribes, Solid State Ionics 17 (1985) 147. [3] Z. Zhang, J.H. Kennedy, Solid State Ionics 38 (1990) 217. [4] A. Pradel, M. Ribes, Mat. Chem. Phys. 23 (1989) 121. [5] J.H. Kennedy, Mat. Chem. Phys. 23 (1989) 29. [6] J.H. Kennedy, Z. Zhang, H. Eckert, J. Non-Cryst. Solids 123 (1990) 328. [7] M. Tatsumisago, K. Hirai, T. Minami, K. Takada, S. Kondo, J. Ceram. Soc. Japan 101 (1993) 1315. [8] K. Hirai, M. Tatsumisago, T. Minami, Solid State Ionics 78 (1995) 269.
[9] M. Tatsumisago, K. Hirai, T. Hirata, M. Takahashi, T. Minami, Solid State Ionics 86–88 (1996) 487. [10] M. Tatsumisago, K. Hirai, M. Takahashi, T. Minami, Phys. Chem. Glasses 38 (1997) 63. [11] K. Hirai, M. Tatsumisago, M. Takahashi, T. Minami, J. Am. Ceram. Soc. 79 (1996) 349. [12] A. Hayashi, M. Tatsumisago, T. Minami, Y. Miura, J. Am. Ceram. Soc. 81 (1998) 1305. [13] A. Hayashi, M. Tatsumisago, T. Minami, Y. Miura, Phys. Chem. Glasses 39 (1998) 145. [14] A. Hayashi, R. Araki, K. Tadanaga, M. Tatsumisago, T. Minami, Phys. Chem. Glasses, accepted for publication.
Thermal and electrical properties of rapidly quenched Li 2 S-SiS 2 Li 2 O-P2 O 5 oxysulfide glasses Akitoshi Hayashi*, Ryuhei Araki, Ryoichi Komiya, Kiyoharu Tadanaga, Masahiro Tatsumisago, Tsutomu Minami Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 599 -8531, Japan
Abstract We have systematically investigated the thermal and electrical properties of (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses in the wide range of lithium ion concentration. These oxysulfide glasses with y 5 0.05 exhibited high Tc–Tg values, which is a measure of the glass stability against crystallization, and high ion conductivities at room temperature in the middle range of lithium ion concentration around x 5 55 mol%. In the same composition range, large amounts of silicon atoms coordinated with both sulfur and oxygen atoms, i.e. SiO n S 42n (n 5 1, 2, 3) tetrahedral units, were observed by 29 Si MAS-NMR measurements. The presence of such unique structural units brought about the high glass stability and high ion conductivity of the glasses with y 5 0.05. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Lithium ion conductivity; Thermal property; Oxysulfide glass; Solid-state NMR; Rapid quenching; Glass structure
1. Introduction During the last decade the properties of sulfidebased lithium ion conducting glasses have been studied extensively because of their much higher performance as solid electrolytes compared to that of oxide-based glasses [1–4]. In particular, the Li 2 SSiS 2 glasses have several excellent properties such as high lithium ion conductivities of 10 24 –10 23 S cm 21 at room temperature, high glass transition temperature, and easy preparation without vacuum sealing [5,6]. We have previously prepared (100 2 z)(0.6Li 2 S ? 0.4SiS 2 ) ? zLi x MO y oxysulfide glasses (Li x MO y : ortho-oxosalt, M 5 P, Si, Ge) by using a twin-roller *Corresponding author.
quenching technique and investigated their properties and glass structure [7–10]. The investigation of thermal and electrical properties of these glasses revealed that the addition of small amounts of Li x MO y improved the glass stability against crystallization and kept high conductivity of about 10 23 S cm 21 at room temperature. We have also clarified by using solid-state NMR and X-ray photoelectron spectroscopy that the silicon-centered structural units, where silicon atoms were coordinated with both sulfur and oxygen atoms, were mainly present in the 60Li 2 S ? 40SiS 2 glass to which small amounts of Li x MO y were added [11,12]. The relationship between the thermal and electrical properties and the glass structure of these oxysulfide glasses was pointed out in a previous paper [13]. The fact that such unique structural units are
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00398-1
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A. Hayashi et al. / Solid State Ionics 113 – 115 (1998) 733 – 738
present in glass is of great interest from the viewpoint of glass structure. To understand the glass structure of the Li 2 S-SiS 2 -based oxysulfide glasses, we have systematically done the structural analyses for the glasses with a wide range of lithium ion concentration including the compositions with 60 mol% Li 2 S and reported the structure of the glasses in the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide system by using 29 Si MAS-NMR [14]. In this oxysulfide system, x indicates the total lithium ion concentration, Li 2 Ch (Ch 5 S 1 O), and y indicates the amount of Li 2 O-P2 O 5 glasses added to the Li 2 SSiS 2 glasses. The nominal mole ratio of Li 2 S to SiS 2 is kept equal to the ratio of Li 2 O to P2 O 5 in these oxysulfide glasses. In the present paper, we report the glass stability and lithium ion conductivity for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses (45 # x # 67; y 5 0, 0.05, 0.20) in the wide range of lithium ion concentration. The composition dependence of the thermal and electrical properties of these glasses is discussed on the basis of the glass structure reported in our previous paper [14].
surements were performed for the flakelike glasses in dry Ar atmosphere by using a Solartron 1260 impedance analyzer in the temperature range 25– 2008C and the frequency range 100 Hz–15 MHz. 29 Si MAS-NMR spectra were obtained by using a Varian UNITY INOVA 300 NMR spectrometer. Detailed conditions for NMR measurements were described in the previous paper [14].
2. Experimental
Table 1 Glass transition temperatures (Tg), crystallization temperatures (Tc), and Tc–Tg for (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses
Li 2 O-P2 O 5 glasses were prepared from reagentgrade chemicals Li 2 CO 3 and NH 4 H 2 PO 4 . The mixture of these chemicals was put into a platinum crucible and kept at 8008C to 9008C for 1 h. The molten sample was pressed between two stainless steel plates to obtain plate-like Li 2 O-P2 O 5 glasses. The ground Li 2 O-P2 O 5 glasses and regent-grade chemicals of Li 2 S and SiS 2 were used as starting materials to prepare oxysulfide glasses in the systems (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20). The mixture of these materials was heated in a carbon crucible at 10008C for 2 h in an electric furnace located in a dry N 2 -filled glove box. The molten samples were rapidly cooled by using a twin-roller quenching technique to prepare flakelike glasses of 20 mm in thickness. Differential thermal analysis (DTA) was carried out for the powdered samples sealed in an Al-pan by using a Rigaku thermal analyzer. Conductivity mea-
3. Results and discussion The glass transition temperatures (Tg) and the crystallization temperatures (Tc) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses were determined by using DTA in the wide range of lithium ion concentration. The compositions and the values of Tg, Tc, and Tc–Tg for these glasses are listed in Table 1. Fig. 1 shows the composition dependence of the Tc–Tg values for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) glasses prepared in the present study; the value
x 45 50 55 60 63 65 67 45 48 50 53 55 58 60 63 65 67 55 60 65
y
0.00
0.05
0.20
Tg / 8C
Tc / 8C
Tc–Tg / 8C
319 336 341 346 305 274 307 343 357 344 391 363 331 313 318 318 274 322 326 310
420 441 449 425 367 352 349 451 479 473 533 494 470 461 412 380 320 432 467 390
101 105 108 79 62 78 42 108 122 129 142 131 139 148 94 62 46 110 141 80
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735
systems. It is interesting to note that the enhancement of Tc–Tg with the addition of Li 2 O-P2 O 5 is markedly observed in the middle range of lithium ion concentration around x 5 55 and no enhancement is observed in both ends of x, namely around the compositions of x 5 45 and 67. The 0.80(60Li 2 S ? 40SiS 2 ) ? 0.20(60Li 2 O ? 40P2 O 5 ) oxysulfide glass, which corresponds to the composition with x 5 60 and y 5 0.20, also exhibits large Tc–Tg value of 1408C. Fig. 2 shows the composition dependence of the electrical conductivities at 258C (s25 ) for the glasses with y 5 0, 0.05, and 0.20. All the marks used in this figure are the same ones used in Fig. 1. The s25 values for the glasses with y 5 0 reported by Kennedy [5] are also plotted for comparison (the open rhombuses). We newly measured the conductivities of the twin-roller quenched glasses with y 5 0 (the
Fig. 1. Composition dependence of the Tc–Tg values for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) glasses. The numbers of the abscissa indicate the total lithium ion concentration x, in mol% of Li 2 Ch (Ch 5 S 1 O). The open circles, closed circles, and closed triangles denote the values for y 5 0, y 5 0.05, and y 5 0.20, respectively.
of Tc–Tg is used as one of the measures of the glass stability against crystallization. The abscissa indicates the total lithium ion concentration x, in mol% of Li 2 Ch (Ch 5 S 1 O). The open circles, closed circles, and closed triangles denote the values for the glasses with y 5 0, 0.05, and 0.20, respectively. In this figure, the expressions of xLi 2 S ? (100 2 x)SiS 2 and xLi 2 O ? (100 2 x)P2 O 5 are simplified as those of Li 2 S-SiS 2 and Li 2 O-P2 O 5 , respectively. The Tc–Tg values for the glasses with y 5 0.05, ranging 50–1508C, are larger than those for the glasses with y 5 0 in the almost entire range of lithium ion concentration; in particular, very large Tc–Tg values of about 1508C are obtained in the middle of the x range around x 5 55. These results indicate that the addition of small amounts of Li 2 OP2 O 5 to Li 2 S-SiS 2 glasses brings about the improvement of the glass stability against crystallization in the wide range of lithium ion concentration of the
Fig. 2. Composition dependence of the electrical conductivities at 258C (s25 ) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] glasses. All the marks used in this figure are the same ones used in Fig. 1. The s25 values for the Li 2 S-SiS 2 glasses reported by Kennedy [5] are also plotted for comparison (the open rhombuses).
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open circles) for which the preparation was carried out in a highly dehydrated, newly equipped glove box keeping H 2 O levels lower than 1 ppm. The values of s25 in the present study are a little higher than those reported by Kennedy in the whole lithium ion concentration range. However, the composition dependences of them are very similar; both conductivities show a maximum at around x 5 60. We can compare the conductivities for the (1 2 y)(Li 2 S-SiS 2 ) ? y(Li 2 O-P2 O 5 ) (y 5 0.05, 0.20) oxysulfide glasses with those for the sulfide glasses with y 5 0 prepared in our present study. High conductivities of about 10 23 S cm 21 are obtained in the wide range of lithium ion concentration around x 5 55 for the glasses with y 5 0.05. The composition dependence of s25 is very similar to that for the glasses with y 5 0. It is noteworthy that both the glasses with y 5 0 and 0.05 have almost the same values of s25 of about 10 23 S cm 21 . However, the values of s25 for the glasses with y 5 0.20 are about 24 21 10 S cm , which are lower than those for the glasses with y 5 0 and 0.05. Fig. 3 shows the composition dependence of the activation energy for conduction (Ea) for the glasses with y 5 0, 0.05, and 0.20. All the marks used in this figure are the same ones used in Fig. 2. In the glasses with y 5 0.05, the Ea values are minimized at x 5 60, the composition of which is 0.95(60Li 2 S ? 40SiS 2 ) ? 0.05(60Li 2 O ? 40P2 O 5 ); the value of activa21 tion energy reaches 27 kJ mol . Such a composition dependence of Ea for the glasses with y 5 0.05 is similar to that for the glasses with y 5 0. The Ea values for the glasses with y 5 0.20 are found to be larger than those of the other glasses. Figs. 2 and 3 suggest that the composition with the maximum of the lithium ion conductivity basically corresponds to that with the minimum of the activation energy for the glasses with y 5 0 and 0.05. The composition dependence of ionic concuctivity and activation enegy is very similar in both glasses in the wide range of lithium ion concentration. We previously carried out 29 Si MAS-NMR measurement for the glasses with y 5 0.05 in the wide range of lithium ion concentration and reported the local structure around silicon atoms in these glasses [14]. Fig. 4 shows the composition dependence of the fraction of silicon structural units in the glasses with y 5 0.05 calculated from the peak area of the
Fig. 3. Composition dependence of the activation energy for conduction (Ea) for the (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] glasses. All the marks used in this figure are the same ones used in Fig. 2.
29
Si NMR spectra reported in the reference [14]. The open circles and closed circles denote the fraction of SiS 4 tetrahedral units in which a silicon atom is coordinated with only sulfur atoms and SiO n S 42n (n 5 1, 2, 3) tetrahedral units in which a silicon atom is coordinated with both sulfur and oxygen atoms, respectively. Large amounts of SiO n S 42n tetrahedral units are present in the middle range of lithium ion concentration around x 5 55 while they are small in both ends of x. On the other hand, the composition dependence of the amounts of SiS 4 tetrahedral units is opposite to that of the amounts of SiO n S 42n tetrahedral units. The amounts of SiO n S 42n tetrahedral units show a maximum and those of SiS 4 tetrahedral units are at minimum around x 5 55. High conductivities of about 10 23 S cm 21 have been obtained in the middle composition range around x 5 55 for the glasses with y 5 0.05 as shown in Fig. 2. The presence of large amounts of the silicon-centered structural units of SiO n S 42n proba-
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atoms have never been observed as crystal phases in the precipitates from these oxysulfide glasses. The oxysulfide glasses containing large amounts of such structural units are not easily crystallized because the crystallization requires large rearrangement of chemical bonds in these glasses. Therefore, the glass stability against crystallization is improved in the middle range of lithium ion concentration in the oxysulfide glasses with y 5 0.05.
4. Conclusion
Fig. 4. Composition dependence of the fraction of silicon structural units for the 0.95[xLi 2 S ? (100 2 x)SiS 2 ] ? 0.05[xLi 2 O ? (100 2 x)P2 O 5 ] oxysulfide glasses. The open circles and closed circles denote the fraction of SiS 4 and SiO n S 42n (n 5 1, 2, 3) tetrahedral units, respectively.
bly brought about the high conductivity at the composition as we already pointed out in the (100 2 z)(0.6Li 2 S ? 0.4SiS 2 ) ? zLi x MO y oxysulfide glasses (Li x MO y : ortho-oxosalt, M 5 P, Si, Ge) [9,13]. From our preliminary experiment, two kinds of crystal phases, Li 2 SiS 3 (lithium meta-thiosilicate) and Li 4 SiS 4 (lithium ortho-thiosilicate), were found to be mainly precipitated in the heating of these oxysulfide glasses in the present composition range; both crystal phases consist of only SiS 4 tetrahedral units in which silicon atoms are coordinated with only sulfur atoms. The glasses with y 5 0.05 in both ends of x were found to be easily crystallized into the crystal phases containing only SiS 4 tetrahedral units because large amounts of SiS 4 units were present in such range of x (see Fig. 4). The amounts of SiS 4 tetrahedral units decreased while those of SiO n S 42n (n 5 1, 2, 3) tetrahedral units increased in the middle range of lithium ion concentration around x 5 55. Such unique structural units in which silicon atoms are coordinated with both sulfur and oxygen
We have systematically investigated the thermal and electrical properties of (1 2 y)[xLi 2 S ? (100 2 x)SiS 2 ] ? y[xLi 2 O ? (100 2 x)P2 O 5 ] (45 # x # 67; y 5 0, 0.05, 0.20) oxysulfide glasses in the wide range of lithium ion concentration. The Tc–Tg values for the glasses with y 5 0.05 were larger than those for the glasses with y 5 0 in the wide range of lithium ion concentration. High conductivities of about 10 23 S cm 21 were obtained for the glasses with y 5 0 and 0.05. Both glasses exhibited the similar composition dependence in conductivities and activation energies. However, the oxysulfide glasses with y 5 0.20 exhibited lower conductivities and higher activation energies than the glasses with y 5 0 and 0.05. In the middle range of lithium ion concentration around x 5 55, a large number of structural units in which a silicon atom is coordinated with both sulfur and oxygen atoms, i.e. SiO n S 42n (n 5 1, 2, 3) tetrahedral units, were observed in the glasses with y 5 0.05 by 29 Si MAS-NMR measurements. The presence of large amounts of these structural units brought about the high glass stability and high conductivity of these oxysulfide glasses.
Acknowledgements This work was supported by the ‘‘Research for the Future’’ Program from the Japan Society for the Promotion of Science. It was also partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.
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