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New lithium ion conducting glass-ceramics prepared from mechanochemical Li2S–P2S5 glasses
Solid State Ionics 154 – 155 (2002) 635 – 640 www.elsevier.com/locate/ssi
New lithium ion conducting glass-ceramics prepared from mechanochemical Li2S–P2S5 glasses M. Tatsumisago *, S. Hama, A. Hayashi, H. Morimoto, T. Minami Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakeuncho, Sakai, Osaka 599-8531, Japan Accepted 18 March 2002
Abstract Amorphous solid electrolytes in the system Li2S – P2S5 were prepared from a mixture of crystalline Li2S and P2S5 using a mechanical milling technique at room temperature. In the composition range xV87.5 of xLi2S(100 x)P2S5, conductivities of the glassy powders mechanically milled for 20 h were as high as 10 4 S cm 1 at room temperature. The heat treatment of the 80Li2S20P2S5 glassy powders at around 220 jC produced dense glass-ceramics with high conductivity around 10 3 S cm 1 at room temperature. The crystallization of conductive phases of Li7PS6, Li3PS4 and unknown crystals from the glass matrix and the decrease of grain boundaries by the softening of the glassy powders are simultaneously achieved at relatively low temperatures, around 220 jC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical milling; Solid electrolyte; Lithium ion conduction; Glass-ceramics; Amorphous powder
1. Introduction Glassy solid electrolytes with high lithium-ion conductivity have attracted strong interest because of their potential application to solid-state lithium secondary batteries with high safety and high energy densities [1,2]. The mechanochemical synthesis using a high-energy ball mill is one of the most attractive techniques to prepare glassy solid electrolyte powders at room temperature [3]. We have succeeded in the preparation of glassy powders in the systems Li2S – SiS2 [3], Li2S –SiS2 –Li4SiO4 [4] and Li2S –SiS2 –Li3PO4 [5] by mechanical milling *
Corresponding author. Tel.: +81-722-549331; fax: +81-722549913. E-mail address: [email protected] (M. Tatsumisago).
and found that they have high lithium-ion conductivity, more than 10 4 S cm 1 at room temperature. These powders can be directly used for electrolytes in the solid-state batteries as in the prepared form, although the bulk glasses prepared by melt quenching have to be pulverized into fine powders. Such mechanochemically prepared fine powders are expected to bring about the good interfacial contacts between the solid electrolyte and the electrode materials. In fact, we have found that the solid-state electrochemical cells, using the mechanochemically prepared glassy electrolytes of Li2S – SiS2 – Li4SiO4, were charged and discharged with excellent cycling performance [6]. Very recently, we have discovered that a new type of glass-ceramics [7] with high lithium-ion conductivity can be obtained by heating the mechanochemi-
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 0 9 - X
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cally prepared glassy powders in the system Li2S – P2S5 [8], up to above their crystallization temperatures. By using the softening of the original glassy powders during the glass-ceramics formation, the good interfacial contacts between the electrolytes and the electrode materials are expected to be much improved in constructing the solid-state lithium secondary batteries. The present paper reports the preparation and characterization of the highly conductive Li2S –P2S5 glass-ceramics, and the formation process of the glass-ceramics is discussed on the basis of the diffraction measurements and microstructure observation.
2. Experimental procedure Reagent-grade Li2S (Furuuchi Chemicals, 99.9%) and P2S5 (Aldrich Chemicals, 99%) crystalline powders were used as starting materials for sample preparation. The mechanical milling treatment was carried out for the batches (1 g) of the mixed materials at the composition xLi2S(100 x)P2S5 (x=60, 70, 75, 80, and 87.5) in an alumina pot (volume=45 ml) with alumina balls (10 balls with a diameter 10 mm) using a high-energy planetary ball mill (Fritsch Pulverisette 7). The rotation speed was fixed at 370 rpm and all the procedures were carried out at room temperature in a dry N2-filled glove box. Electrical conductivities were measured for the pelletized samples obtained by the cold press (3700 kg/cm2) of the powdered samples; the diameter and thickness of the pellets were 10 mm and about 1 mm, respectively. Carbon paste was painted as the electrodes on both faces of the pelletized sample. Stainless steel plates were used as the current collector, which supported the pelletized sample. The measurements were carried out in dry Ar atmosphere using Solartron 1260 impedance analyzer in a frequency range from 100 Hz to 15 MHz. X-ray diffraction (XRD) measurements (CuKa) were performed using a Shimadzu XRD-6000 diffractometer. Differential thermal analysis (DTA) was carried out using Rigaku thermal analyzer, Thermo-plus 8110, for the above powders sealed in an Al pan in a dry N2 atmosphere. Microstructure observation for the mechanically milled powders was performed by a JEOL JSM-5300 scanning electron microscope (SEM).
3. Results and discussion 3.1. Mechanochemical preparation and characterization of Li2S –P2S5 glass powders Fig. 1 shows the XRD patterns of the xLi2S(100 x)P2S5 powders that have been subjected to mechanical milling for 20 h. Halo patterns and no XRD peaks are observed for xV75, which indicates that these samples become amorphous after milling for 20 h. Halo patterns are basically observed also in the samples with xz80, whereas the XRD peaks due to Li2S are still present after milling for 20 h. After mechanical milling for 40 h, the x=80 samples became almost amorphous with a trace of Li2S crystals. The x=87.5 sample could not be amorphous within a milling period of V65 h. Zang and Kennedy [9] have reported that glass formation in the Li2S – P2S5 system that was obtained using melt quenching was limited to compositions with xV70. The amorphous-forming region obtained via mechanical milling in the present study extends up to the x=80 composition. It is concluded that amorphous materials with extremely high Li2S concentration (x=80), which could not be prepared via
Fig. 1. X-ray diffraction patterns of the xLi2S(100 x)P2S5 powders mechanically milled for 20 h.
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
conventional melt quenching, can be prepared using mechanical milling. Thermal analyses were carried out for all the amorphous powders with XRD halo patterns obtained using mechanical milling. Fig. 2 shows the DTA curve for the amorphous 80Li2S20P2S5 powder mechanically milled for 40 h as an example. A very sharp exothermic peak, which is due to crystallization, is observed at 210 jC. In addition, the phenomenon of glass transition is observed at around 190 jC. The temperatures of glass transition and crystallization were observed in all the amorphous powders prepared by mechanical milling, indicating that these powders are in glassy state. Fig. 3 shows the temperature dependence of electrical conductivity for the pelletized samples of the xLi2S(100-x)P2S5 powders that have been mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5). The data were shown in the temperature ranges up to around the glass transition temperatures. The conductivities of all the mechanically milled glasses follow the Arrhenius equation. Fig. 4 shows the composition dependence of the room temperature conductivity r25 and the activation energy for conduction Ea, as determined from Fig. 3. The value of r25 increases as x increases and attains a maximum value for a composition of x=75 – 80. Further increase in x results in a decrease in r25. The composition dependence of Ea corresponds to that of r25: the Ea value decreases up to x=75, and then increases as x increases further. The increase of
Fig. 2. DTA curve for the 80Li2S20P2S5 powders mechanically milled for 40 h.
637
Fig. 3. Temperature dependence of the electrical conductivity of the xLi2S(100 x)P2S5 samples mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5).
r25 and the decrease of Ea, with increasing x up to 75 mol%, are due to the increase of carrier concentration and/or the mobility of Li+ ions. The conductivity decrease with further addition of Li2S is mainly due to the presence of low-conducting Li2S crystals in the samples.
Fig. 4. Composition dependence of the conductivity at 25 jC (r25) and activation energy (Ea) for conduction of the xLi2S(100 x)P2S5 samples mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5).
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3.2. Preparation and characterization of Li2S –P2S5 glass-ceramics Fig. 5 shows the temperature dependence of electrical conductivity for the 80Li2S20P2S5 pelletized sample of the amorphous powder mechanically milled for 40 h. Open and closed circles denote heating and cooling processes, respectively. The amorphous sample before heating exhibits high conductivity of 110 4 S cm 1 at room temperature. This value is as high as that of the Li2S –SiS2 amorphous materials prepared by mechanical milling [3]. The conductivities of the amorphous sample follow the Arrhenius equation up to the glass transition temperature in the heating process. After the sample was heated up to 215 jC, which is beyond the crystallization temperature, the conductivity changes in a different manner in the cooling run. The conductivity of the heated sample is 610 4 S cm 1 at room temperature and this value is extremely high as a pelletized sample which is prepared by cold press of solid electrolyte powders. The activation energy of 37 kJ mol 1 for conduction in the heating run decreased to 28 kJ mol 1 in the cooling run. In our previous letter, the conductivity of the resultant glass-ceramics obtained from the milled glass of the same composition was reported to be a little higher than 910 4 S cm 1 at room temperature [7]. This discrepancy is probably caused by the difference in the starting materials of
Fig. 5. Temperature dependence of electrical conductivity of the 80Li2S20P2S5 powders mechanically milled for 40 h. Open and closed circles denote heating and cooling runs, respectively.
Fig. 6. X-ray diffraction patterns of the 80Li2S20P2S5 powders mechanically milled for 40 h before and after the conductivity measurements shown in Fig. 5.
P2S5; we previously used 98% pure P2S5 powder from Koujyundo Chemicals [7], while we used 99% pure P2S5 powder from Aldrich Chemicals in the present study. Fig. 6 shows the XRD patterns of the 80Li2S20 P2S5 samples before and after the conductivity measurements shown in Fig. 5. The Li3PS4 and unknown crystals are precipitated in addition to the Li7PS6 crystal in the sample after the conductivity measurements. In our previous work [7], only the Li7PS6 crystal was precipitated after heating up to 250 jC of the mechanically milled glass; this discrepancy is also due to the difference in the starting materials of P2S5. Some impurities in the P2S5 from Koujyundo Chemicals probably prevented the precipitation of the Li3PS4 crystals in the previous work. These crystals of Li3PS4 and Li7PS6 are expected to exhibit higher conductivities than the original glasses because the higher conductivity was observed in the glassceramics. However, the sintered Li3PS4 crystal has already been reported to exhibit low conductivity, in the order of 10 7 S cm 1 at room temperature [10]. We measured the conductivity of the Li7PS6 crystal, which has never been reported so far, and found that the value of the pelletized Li7PS6 crystal powder was 810 5 S cm 1 at room temperature. Since these two crystals of Li3PS4 and Li7PS6 exhibit a sign of phase
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
transformation in the temperature range 150 – 200 jC on the basis of the thermal analyses [10], high conductive phases of these crystals may be stabilized in the present case of glass-ceramics. The existence of the high temperature phase of these crystals and the characterization of the unknown crystals are now under study. Fig. 7 shows the SEM micrographs of the fracture surfaces of the 80Li2S20P2S5 pelletized samples before and after the conductivity measurements shown in Fig. 5. The contact between grains is so close that the grain boundary cannot be distinguished clearly after the conductivity measurements. The amorphous powders must be softened by heating over the glass transition temperature. In general, the large impedance due to grain boundaries prevents highly ionic conduction in the pelletized and sintered materials [11]. Both the complex impedance plots for the glass-ceramics and the unheated glassy powders showed only one semicircle, so that we could not distinguish the impedance of grain and grain boundary. However, since the pelletized sample obtained in this study has very dense microstructure with few grain boundaries, the decrease of the impedance due to good contact among grain boundaries improves the conductivity of the 80Li2S20P2S5 pelletized materials. It was revealed that the conductivity improvement with forming the 80Li2S20P2S5 glass-ceramics was brought about by the following two reasons: one is the softening of amorphous powders to decrease grain boundary impedance, and the other is the crystallization with precipitating highly conductive phases of Li3PS4, Li7PS6 and unknown crystals. This glassceramics preparation procedure using the amorphous fine powders with high lithium-ion concentration obtained by mechanical milling is a very simple method because all the synthesis processes are carried out at low temperatures, around 215 jC. The lithium secondary batteries coming on to the market at present usually include LiCoO2 as cathode and carbon as anode materials. These electrode materials are solid powders. The construction of the close contact between solid electrolyte and electrode powders is indispensable to realize the superior solid-state lithium secondary batteries. The softening of the Li2S–P2S5 amorphous powder obtained in the present study must form the close solid/solid contact between
639
Fig. 7. Scanning electron micrographs of the fracture surfaces of the 80Li2S20P2S5 pelletized samples (a) before and (b) after the conductivity measurements shown in Fig. 5.
electrolyte and electrode powders without any chemical reactions because of low heat-treatment temperatures. The development of many kinds of glass-
640
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
ceramics solid electrolytes with high conductivity will be expected using the new process with softening and crystallization of mechanically milled amorphous fine powders.
(B) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References 4. Conclusion The heat treatment of the mechanochemically prepared Li2S – P2S5 glassy powders at around their crystallization temperatures produced highly conductive lithium-ion glass-ceramics. The crystallization of conductive phases of Li7PS6, Li3PS4 and unknown crystals from the glass matrix and the decrease of grain boundaries by the softening of the glassy powders are simultaneously achieved during the heat treatment.
Acknowledgements This work was supported by the Grant-in-Aid for Scientific Research on Priority Areas (B) and Section
[1] J.R. Akridge, H. Vourlis, Solid State Ion. 28 – 29 (1988) 841. [2] K. Iwamoto, N. Aotani, K. Takada, S. Kondo, Solid State Ion. 79 (1995) 288. [3] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 82 (1999) 1352. [4] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Ceram. Soc. Jpn., Int. Ed. 108 (2000) 128. [5] M. Tatsumisago, H. Yamashita, A. Hayashi, H. Morimoto, T. Minami, J. Non-Cryst. Solids 274 (2000) 30. [6] R. Komiya, A. Hayashi, H. Morimoto, M. Tatsumisago, T. Minami, Solid State Ion. 140 (2001) 83. [7] A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, T. Minami, Chem. Lett. (2001) 872. [8] A. Hayashi, S. Hama, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 84 (2001) 477. [9] Z. Zhang, J.H. Kennedy, Solid State Ion. 38 (1990) 217. [10] M. Tachez, J.P. Malugani, R. Mercier, G. Robert, Solid State Ion. 14 (1984) 181. [11] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, Solid State Ion. 47 (1991) 257.
New lithium ion conducting glass-ceramics prepared from mechanochemical Li2S–P2S5 glasses M. Tatsumisago *, S. Hama, A. Hayashi, H. Morimoto, T. Minami Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakeuncho, Sakai, Osaka 599-8531, Japan Accepted 18 March 2002
Abstract Amorphous solid electrolytes in the system Li2S – P2S5 were prepared from a mixture of crystalline Li2S and P2S5 using a mechanical milling technique at room temperature. In the composition range xV87.5 of xLi2S(100 x)P2S5, conductivities of the glassy powders mechanically milled for 20 h were as high as 10 4 S cm 1 at room temperature. The heat treatment of the 80Li2S20P2S5 glassy powders at around 220 jC produced dense glass-ceramics with high conductivity around 10 3 S cm 1 at room temperature. The crystallization of conductive phases of Li7PS6, Li3PS4 and unknown crystals from the glass matrix and the decrease of grain boundaries by the softening of the glassy powders are simultaneously achieved at relatively low temperatures, around 220 jC. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical milling; Solid electrolyte; Lithium ion conduction; Glass-ceramics; Amorphous powder
1. Introduction Glassy solid electrolytes with high lithium-ion conductivity have attracted strong interest because of their potential application to solid-state lithium secondary batteries with high safety and high energy densities [1,2]. The mechanochemical synthesis using a high-energy ball mill is one of the most attractive techniques to prepare glassy solid electrolyte powders at room temperature [3]. We have succeeded in the preparation of glassy powders in the systems Li2S – SiS2 [3], Li2S –SiS2 –Li4SiO4 [4] and Li2S –SiS2 –Li3PO4 [5] by mechanical milling *
Corresponding author. Tel.: +81-722-549331; fax: +81-722549913. E-mail address: [email protected] (M. Tatsumisago).
and found that they have high lithium-ion conductivity, more than 10 4 S cm 1 at room temperature. These powders can be directly used for electrolytes in the solid-state batteries as in the prepared form, although the bulk glasses prepared by melt quenching have to be pulverized into fine powders. Such mechanochemically prepared fine powders are expected to bring about the good interfacial contacts between the solid electrolyte and the electrode materials. In fact, we have found that the solid-state electrochemical cells, using the mechanochemically prepared glassy electrolytes of Li2S – SiS2 – Li4SiO4, were charged and discharged with excellent cycling performance [6]. Very recently, we have discovered that a new type of glass-ceramics [7] with high lithium-ion conductivity can be obtained by heating the mechanochemi-
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 0 9 - X
636
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
cally prepared glassy powders in the system Li2S – P2S5 [8], up to above their crystallization temperatures. By using the softening of the original glassy powders during the glass-ceramics formation, the good interfacial contacts between the electrolytes and the electrode materials are expected to be much improved in constructing the solid-state lithium secondary batteries. The present paper reports the preparation and characterization of the highly conductive Li2S –P2S5 glass-ceramics, and the formation process of the glass-ceramics is discussed on the basis of the diffraction measurements and microstructure observation.
2. Experimental procedure Reagent-grade Li2S (Furuuchi Chemicals, 99.9%) and P2S5 (Aldrich Chemicals, 99%) crystalline powders were used as starting materials for sample preparation. The mechanical milling treatment was carried out for the batches (1 g) of the mixed materials at the composition xLi2S(100 x)P2S5 (x=60, 70, 75, 80, and 87.5) in an alumina pot (volume=45 ml) with alumina balls (10 balls with a diameter 10 mm) using a high-energy planetary ball mill (Fritsch Pulverisette 7). The rotation speed was fixed at 370 rpm and all the procedures were carried out at room temperature in a dry N2-filled glove box. Electrical conductivities were measured for the pelletized samples obtained by the cold press (3700 kg/cm2) of the powdered samples; the diameter and thickness of the pellets were 10 mm and about 1 mm, respectively. Carbon paste was painted as the electrodes on both faces of the pelletized sample. Stainless steel plates were used as the current collector, which supported the pelletized sample. The measurements were carried out in dry Ar atmosphere using Solartron 1260 impedance analyzer in a frequency range from 100 Hz to 15 MHz. X-ray diffraction (XRD) measurements (CuKa) were performed using a Shimadzu XRD-6000 diffractometer. Differential thermal analysis (DTA) was carried out using Rigaku thermal analyzer, Thermo-plus 8110, for the above powders sealed in an Al pan in a dry N2 atmosphere. Microstructure observation for the mechanically milled powders was performed by a JEOL JSM-5300 scanning electron microscope (SEM).
3. Results and discussion 3.1. Mechanochemical preparation and characterization of Li2S –P2S5 glass powders Fig. 1 shows the XRD patterns of the xLi2S(100 x)P2S5 powders that have been subjected to mechanical milling for 20 h. Halo patterns and no XRD peaks are observed for xV75, which indicates that these samples become amorphous after milling for 20 h. Halo patterns are basically observed also in the samples with xz80, whereas the XRD peaks due to Li2S are still present after milling for 20 h. After mechanical milling for 40 h, the x=80 samples became almost amorphous with a trace of Li2S crystals. The x=87.5 sample could not be amorphous within a milling period of V65 h. Zang and Kennedy [9] have reported that glass formation in the Li2S – P2S5 system that was obtained using melt quenching was limited to compositions with xV70. The amorphous-forming region obtained via mechanical milling in the present study extends up to the x=80 composition. It is concluded that amorphous materials with extremely high Li2S concentration (x=80), which could not be prepared via
Fig. 1. X-ray diffraction patterns of the xLi2S(100 x)P2S5 powders mechanically milled for 20 h.
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
conventional melt quenching, can be prepared using mechanical milling. Thermal analyses were carried out for all the amorphous powders with XRD halo patterns obtained using mechanical milling. Fig. 2 shows the DTA curve for the amorphous 80Li2S20P2S5 powder mechanically milled for 40 h as an example. A very sharp exothermic peak, which is due to crystallization, is observed at 210 jC. In addition, the phenomenon of glass transition is observed at around 190 jC. The temperatures of glass transition and crystallization were observed in all the amorphous powders prepared by mechanical milling, indicating that these powders are in glassy state. Fig. 3 shows the temperature dependence of electrical conductivity for the pelletized samples of the xLi2S(100-x)P2S5 powders that have been mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5). The data were shown in the temperature ranges up to around the glass transition temperatures. The conductivities of all the mechanically milled glasses follow the Arrhenius equation. Fig. 4 shows the composition dependence of the room temperature conductivity r25 and the activation energy for conduction Ea, as determined from Fig. 3. The value of r25 increases as x increases and attains a maximum value for a composition of x=75 – 80. Further increase in x results in a decrease in r25. The composition dependence of Ea corresponds to that of r25: the Ea value decreases up to x=75, and then increases as x increases further. The increase of
Fig. 2. DTA curve for the 80Li2S20P2S5 powders mechanically milled for 40 h.
637
Fig. 3. Temperature dependence of the electrical conductivity of the xLi2S(100 x)P2S5 samples mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5).
r25 and the decrease of Ea, with increasing x up to 75 mol%, are due to the increase of carrier concentration and/or the mobility of Li+ ions. The conductivity decrease with further addition of Li2S is mainly due to the presence of low-conducting Li2S crystals in the samples.
Fig. 4. Composition dependence of the conductivity at 25 jC (r25) and activation energy (Ea) for conduction of the xLi2S(100 x)P2S5 samples mechanically milled for 20 h (x=60, 70, and 75) and 40 h (x=80 and 87.5).
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3.2. Preparation and characterization of Li2S –P2S5 glass-ceramics Fig. 5 shows the temperature dependence of electrical conductivity for the 80Li2S20P2S5 pelletized sample of the amorphous powder mechanically milled for 40 h. Open and closed circles denote heating and cooling processes, respectively. The amorphous sample before heating exhibits high conductivity of 110 4 S cm 1 at room temperature. This value is as high as that of the Li2S –SiS2 amorphous materials prepared by mechanical milling [3]. The conductivities of the amorphous sample follow the Arrhenius equation up to the glass transition temperature in the heating process. After the sample was heated up to 215 jC, which is beyond the crystallization temperature, the conductivity changes in a different manner in the cooling run. The conductivity of the heated sample is 610 4 S cm 1 at room temperature and this value is extremely high as a pelletized sample which is prepared by cold press of solid electrolyte powders. The activation energy of 37 kJ mol 1 for conduction in the heating run decreased to 28 kJ mol 1 in the cooling run. In our previous letter, the conductivity of the resultant glass-ceramics obtained from the milled glass of the same composition was reported to be a little higher than 910 4 S cm 1 at room temperature [7]. This discrepancy is probably caused by the difference in the starting materials of
Fig. 5. Temperature dependence of electrical conductivity of the 80Li2S20P2S5 powders mechanically milled for 40 h. Open and closed circles denote heating and cooling runs, respectively.
Fig. 6. X-ray diffraction patterns of the 80Li2S20P2S5 powders mechanically milled for 40 h before and after the conductivity measurements shown in Fig. 5.
P2S5; we previously used 98% pure P2S5 powder from Koujyundo Chemicals [7], while we used 99% pure P2S5 powder from Aldrich Chemicals in the present study. Fig. 6 shows the XRD patterns of the 80Li2S20 P2S5 samples before and after the conductivity measurements shown in Fig. 5. The Li3PS4 and unknown crystals are precipitated in addition to the Li7PS6 crystal in the sample after the conductivity measurements. In our previous work [7], only the Li7PS6 crystal was precipitated after heating up to 250 jC of the mechanically milled glass; this discrepancy is also due to the difference in the starting materials of P2S5. Some impurities in the P2S5 from Koujyundo Chemicals probably prevented the precipitation of the Li3PS4 crystals in the previous work. These crystals of Li3PS4 and Li7PS6 are expected to exhibit higher conductivities than the original glasses because the higher conductivity was observed in the glassceramics. However, the sintered Li3PS4 crystal has already been reported to exhibit low conductivity, in the order of 10 7 S cm 1 at room temperature [10]. We measured the conductivity of the Li7PS6 crystal, which has never been reported so far, and found that the value of the pelletized Li7PS6 crystal powder was 810 5 S cm 1 at room temperature. Since these two crystals of Li3PS4 and Li7PS6 exhibit a sign of phase
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
transformation in the temperature range 150 – 200 jC on the basis of the thermal analyses [10], high conductive phases of these crystals may be stabilized in the present case of glass-ceramics. The existence of the high temperature phase of these crystals and the characterization of the unknown crystals are now under study. Fig. 7 shows the SEM micrographs of the fracture surfaces of the 80Li2S20P2S5 pelletized samples before and after the conductivity measurements shown in Fig. 5. The contact between grains is so close that the grain boundary cannot be distinguished clearly after the conductivity measurements. The amorphous powders must be softened by heating over the glass transition temperature. In general, the large impedance due to grain boundaries prevents highly ionic conduction in the pelletized and sintered materials [11]. Both the complex impedance plots for the glass-ceramics and the unheated glassy powders showed only one semicircle, so that we could not distinguish the impedance of grain and grain boundary. However, since the pelletized sample obtained in this study has very dense microstructure with few grain boundaries, the decrease of the impedance due to good contact among grain boundaries improves the conductivity of the 80Li2S20P2S5 pelletized materials. It was revealed that the conductivity improvement with forming the 80Li2S20P2S5 glass-ceramics was brought about by the following two reasons: one is the softening of amorphous powders to decrease grain boundary impedance, and the other is the crystallization with precipitating highly conductive phases of Li3PS4, Li7PS6 and unknown crystals. This glassceramics preparation procedure using the amorphous fine powders with high lithium-ion concentration obtained by mechanical milling is a very simple method because all the synthesis processes are carried out at low temperatures, around 215 jC. The lithium secondary batteries coming on to the market at present usually include LiCoO2 as cathode and carbon as anode materials. These electrode materials are solid powders. The construction of the close contact between solid electrolyte and electrode powders is indispensable to realize the superior solid-state lithium secondary batteries. The softening of the Li2S–P2S5 amorphous powder obtained in the present study must form the close solid/solid contact between
639
Fig. 7. Scanning electron micrographs of the fracture surfaces of the 80Li2S20P2S5 pelletized samples (a) before and (b) after the conductivity measurements shown in Fig. 5.
electrolyte and electrode powders without any chemical reactions because of low heat-treatment temperatures. The development of many kinds of glass-
640
M. Tatsumisago et al. / Solid State Ionics 154 – 155 (2002) 635–640
ceramics solid electrolytes with high conductivity will be expected using the new process with softening and crystallization of mechanically milled amorphous fine powders.
(B) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References 4. Conclusion The heat treatment of the mechanochemically prepared Li2S – P2S5 glassy powders at around their crystallization temperatures produced highly conductive lithium-ion glass-ceramics. The crystallization of conductive phases of Li7PS6, Li3PS4 and unknown crystals from the glass matrix and the decrease of grain boundaries by the softening of the glassy powders are simultaneously achieved during the heat treatment.
Acknowledgements This work was supported by the Grant-in-Aid for Scientific Research on Priority Areas (B) and Section
[1] J.R. Akridge, H. Vourlis, Solid State Ion. 28 – 29 (1988) 841. [2] K. Iwamoto, N. Aotani, K. Takada, S. Kondo, Solid State Ion. 79 (1995) 288. [3] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 82 (1999) 1352. [4] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Ceram. Soc. Jpn., Int. Ed. 108 (2000) 128. [5] M. Tatsumisago, H. Yamashita, A. Hayashi, H. Morimoto, T. Minami, J. Non-Cryst. Solids 274 (2000) 30. [6] R. Komiya, A. Hayashi, H. Morimoto, M. Tatsumisago, T. Minami, Solid State Ion. 140 (2001) 83. [7] A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, T. Minami, Chem. Lett. (2001) 872. [8] A. Hayashi, S. Hama, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 84 (2001) 477. [9] Z. Zhang, J.H. Kennedy, Solid State Ion. 38 (1990) 217. [10] M. Tachez, J.P. Malugani, R. Mercier, G. Robert, Solid State Ion. 14 (1984) 181. [11] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, Solid State Ion. 47 (1991) 257.