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Enhanced ionic conductivity of sulfide-based solid electrolyte by incorporating lanthanum sulfide
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 15497–15501 www.elsevier.com/locate/ceramint
Enhanced ionic conductivity of sulfide-based solid electrolyte by incorporating lanthanum sulfide Zhanqiang Liu, Yufeng Tang, Xujie Lü, Guohao Renn, Fuqiang Huangnn Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 16 May 2014; received in revised form 2 July 2014; accepted 2 July 2014 Available online 10 July 2014
Abstract Solid electrolyte is the key point for developing all-solid-state lithium-ion battery with good recyclability and no safety problem. Here, a crystalline phase of lanthanum sulfide (La2S3) was incorporated into glassy sulfide-based solid electrolytes (Li2S–SiS2 or Li2S–P2S5) to form two composite systems Li2S–SiS2–xLa2S3 and Li2S–P2S5–yLa2S3. The results demonstrated that La2S3 has the ability to suppress the crystallization of Li2S–SiS2 or Li2S–P2S5 during the synthetic process. The lithium-ion conductivities of the designed composites can be improved by more than one order of magnitude in comparison with the pristine samples. And both of these two composites have a wide electrochemical window of more than 8 V vs. Li þ /Li. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Powders: solid state reaction; B. Composites; C. Ionic conductivity; E. Batteries; Lanthanum sulfide
1. Introduction Traditional liquid electrolytes used in lithium-ion battery contain toxic and flammable solvents, which bring serious problems such as leakage, inflammability and narrow operating temperature range. For safety concerns and particular applications, the next-generation lithium-ion batteries have been heading for replacing the traditional liquid electrolytes with solid electrolytes [1]. However, the solid electrolytes have not been widely used because their ionic conductivities are still much lower than those of the liquid electrolytes. To meet the requirements of high performance solid-state batteries, advanced solid electrolytes having similar ionic conductivity with the liquid electrolyte are highly required. Many inorganic solid electrolytes have been investigated among which sulfidebased electrolytes show great promising prospects. n
Corresponding author. Tel.: þ86 21 69987741; fax: þ 86 21 5241 3903. Corresponding author. Tel.: þ 86 21 5241 1620; fax: þ86 21 5241 6360. E-mail addresses: [email protected] (G. Ren), [email protected] (F. Huang). nn
http://dx.doi.org/10.1016/j.ceramint.2014.07.011 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Sulfide-based electrolytes have much higher lithium-ion conductivity than oxide-based electrolytes owing to their higher polarization ability of S2 than O2 . Li2S P2S5 [2–5], Li2S SiS2 [6–8] and Li2S GeS2 [9–11] are currently the most studied sulfide-based electrolytes. In these systems, P2S5, SiS2 or GeS2 serve as network former and Li2S is network modifier. It is well known that the lithium-ion conductivities of these sulfide electrolytes can be improved by reducing the crystallinity to form poorly-crystallized or amorphous phases. Unfortunately, these systems usually have low-melting points and are easy to form good crystalline phase. So it is very important to develop strategies to stabilize the glassy phase of sulfide-based electrolytes. Although remarkable progress has been made in the preparation of the glassy systems by doping oxides, sulfides, lithium halides or other chemical materials to improve ionic conductivities, an effective strategy still represents a big challenge [6,12–19]. In this paper, we propose a facile route to improve the ionic conductivities of Li2S SiS2 and Li2S P2S5 systems by incorporating lanthanum sulfide (La2S3). The La2S3 in the composites can suppress the crystallization of Li2S SiS2 or Li2S P2S5, and the ionic conductivities of these composites can be significantly enhanced.
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2. Experimental details La2S3, SiS2, P2S5 and Li2S were used as the starting materials. All the raw materials were stoichiometrically synthesized in our lab, and the purities were confirmed by X-ray diffraction. For the incorporation of La2S3 into Li2S SiS2, the molar ratio of Li2S, SiS2 and La2S3 was controlled as 6: 3: x (x=0, 0.1, 0.3, 0.5, 0.75 and 1.0). For the addition of La2S3 into Li2S P2S5, the molar ratio of Li2S, P2S5 and La2S3 was controlled as 4: 1: y (y=0, 0.1, 0.2, 0.3 and 0.5). The fully mixed precursors were loaded into evacuated and sealed silica tubes. The tubes were placed into the furnace, heated at 750 1C for 10 h and then quenched in water. The final products were collected and ground into fine powders. X-ray diffraction (XRD) analysis was carried out with an X-ray diffractometer (Rigaku D/Max 2550V, 40 kV 40 mA) with CuKα radiation in the 2θ range from 201 to 601. For ionic conductivity measurement, powder sample was cold-pressed into a ϕ10 1 mm pellet and both sides of the pellet were attached with indium plates as current collectors. Then the measurement was conducted in a dry argon flow by complex impedance on an impedance analyzer (Chenhua 660B) in the frequency range of 0.1 Hz and 0.1 MHz over the temperature range from 30 to 210 1C. DC polarization measurement was carried out to determine the electronic conductivity. Indium foil was used as blocking electrode and lithium plate as unblocking electrode. Time dependence of the electrical current was measured under a constant voltage 1 V for 1600 s. Cyclic voltammogram (CV) of the asymmetric Li/sample/Pt cell was performed on Chenhua 660B to evaluate the electrochemical stability of the electrolytes at the scan rate of 10 mV s 1. The Lithium plate and Pt plate in the cell are serve as the reference/counter and working electrodes, respectively.
Fig. 1. X-ray diffraction patterns for the samples 3Li4SiS4 xLa2S3.
3. Results and discussion XRD patterns of 6Li2S 3SiS2 xLa2S3 with various La2S3 contents are shown in Fig. 1. It can be seen that Li4SiS4 was obtained at x ¼ 0 [8]. With the increasing of La2S3, the peaks indexed to La2S3 appear and become stronger gradually. The unchanged phase of La2S3 before and after the synthetic process indicates that there is no reaction between La2S3 and Li2S or SiS2. Since the melting temperature of Li4SiS4 is around 750 1C, the quenching process should result in the poor crystallinity of Li4SiS4. And it can be observed that the peaks indexed to Li4SiS4 become relatively weaker with the increasing of x and almost disappeared at x ¼ 1.0. The poor-crystal Li4SiS4 transforms into amorphous state step by step, implying that the added La2S3 can efficiently suppress the crystallization of Li4SiS4 during the preparation process. Usually, the coordination number of La3 þ is six to nine, while Si4 þ is 4-coordinated in Li4SiS4. Considering other conditions, such as the reaction temperature, reaction time and especially the big difference of atomic radius between La3 þ and Si4 þ , the possibility of substitution of Si4 þ by La3 þ is very low. So it can be concluded that the final samples can be
Fig. 2. Complex impedance plots for 3Li4SiS4 xLa2S3 with x¼0.3. (a) Complex impedance plots. (b) Enlarged plots from 0 to 500 Ω.
recognized as a kind of composite, poor-crystallized Li4SiS4/ well-crystallized La2S3 (3Li4SiS4 xLa2S3). Ionic conductivities of the obtained samples were examined through the AC impedance spectroscopy. Fig. 2 shows the impedance plots of 3Li4SiS4 xLa2S3 with x ¼ 0.3 at various temperatures. A semicircle in the high-frequency range with a spike in the low frequency range was observed at 30 1C. As typical ionic conductor, the semicircle is interpreted as a
Z. Liu et al. / Ceramics International 40 (2014) 15497–15501
parallel combination of a resistance and a capacitance, which are attributed to the bulk and grain boundary contribution, respectively. The spike is caused by the electrode contribution, where Warburg impedance exists. By increasing the measurement temperatures over 60 1C, no semicircle can be observed but only spikes left. The total resistance at each temperature can be calculated from the intersection of the semicircle with the real axis at the lower frequency side. Then the total lithium-ion conductivity at each temperature can be derived. Fig. 3 shows the temperature dependences of conductivities for the series of 3Li4SiS4 xLa2S3. The plots of log(σT) vs. 103/T indicate that the conductivities of these composites in the temperature range from 30 to 210 1C follow the Arrhenius equation: σT ¼ σ0exp( Ea/RT), where σ0 is the pre-exponential factor, Ea the activation energy for conduction, R the gas constant and T the absolute temperature. So it can be inferred that the system of 3Li4SiS4 xLa2S3 is a good lithium-ion conductor. Then the corresponding activation energies were calculated based on the Arrhenius equation. To further demonstrate the effect of the amount of La2S3 on the Li4SiS4 system. The composition dependences of the conductivity and the activation energy for are shown in Fig. 3 (the inset figure). The ionic conductivity and activation energy of the obtained Li4SiS4 are calculated to be 3.3 10 6 S cm 1 and 0.43 eV, respectively. As reported, however, the ionic conductivity and activation energy of the fine crystalline Li4SiS4 are 5.0 10 8 S cm 1 and 0.60 eV, respectively [20]. The reason for the big differences is the poor crystallinity of the obtained Li4SiS4, which is caused by the quenching process. So the above result demonstrates that poor crystallinity is benefit for the lithium ion diffusion in Li4SiS4. The XRD analysis has disclosed that the poor crystallinity will turn into amorphous gradually with the increasing of La2S3, suggesting the ionic conductivity might be further enhanced. As shown in Fig. 3, the conductivities of Li4SiS4 xLa2S3 increase initially and then decrease as the La2S3 content increasing from x¼ 0 to 1.0. The corresponding activation energy almost follows the reverse trend. Apparently, the content of La2S3 was optimized to be x¼ 0.3. And the corresponding conductivity and activation energy were calculated to be
Fig. 3. Temperature dependences of the electrical conductivities for the samples of 3Li4SiS4 xLa2S3. The inset depicts the composition dependence of the conductivities and the activation energies for the samples 3Li4SiS4 xLa2S3.
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3.4 10 5 S cm 1 and 0.38 eV, respectively. The optimized conductivity is ten times higher than that of the pristine Li4SiS4 after La2S3 added. Since La2S3 is ionic-insulating, heavy addition will block the lithium ions and subsequently reduce the ionic conductivity of the composite. Fig. 4 shows the time dependence of the DC conductivity obtained from current after applying a constant 1 V DC voltage on sample 3Li4SiS4 0.3La2S3 at room temperature. When using lithium as nonblocking electrode, the DC conductivity is almost constant with time and the value, around 2.8 10 5 S cm 1, is in good agreement with the number obtained from AC impedance shown above. When using indium as blocking electrode, a large decrease of DC conductivity, resulted from the accumulation of lithium atoms on the electrode, can be observed initially and then the conductivity become almost constant. The constant conductivity of 1.80 10 8 S cm 1 is about three orders of magnitude lower than that obtained using the nonblocking electrode. Therefore, it can be concluded that the electronic conduction in the total conductivity is almost negligible. Electrochemical stabilities of the obtained samples were also studied by cyclic voltammetry with a potential range from 0.50 to þ 10.0 V vs. Li þ /Li. Fig. 5 shows the cyclic voltammogram of the sample 3Li4SiS4 3La2S3. A cathodic current due to lithium deposition (Li þ þ e -Li) is observed at around 0 V on the cathodic sweep up to 0.50 V, and then an anodic current due to lithium dissolution (Li-Li þ þ e ) is observed at around 0.2 V on the an anodic sweep. There is no significant anodic current in the potential up to 10 V, which means the sample of 3Li4SiS4 0.3La2S3 has a wide electrochemical window up to 10 V vs. Li þ /Li. In order to verify our statement on the effects of La2S3 addition, La2S3 was also introduced into the Li2S P2S5 system. As shown in Fig. 6, the phases of the series of 4Li2S P2S5 yLa2S3 were examined by X-ray powder diffraction. At the point of y ¼ 0, the main peaks are indexed to Li3 þ 0.55P1 0.11S4 [21]. Similar to Li4SiS4, the peaks of Li3 þ 0.55P1 0.11S4 also become weaker and weaker gradually with La2S3 adding, which implies the crystallinity of Li3 þ 0.55P1 0.11S4 is becoming weaker. Interestingly, with
Fig. 4. Time dependence of DC conductivity for the sample 3Li4SiS4 0.3La2S3 after applying a constant DC voltage of 1 V at room temperature.
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Fig. 5. Cyclic voltammogram of the sample 3Li4SiS4 0.3La2S3 at room temperature with scanning rate of 10 mV s 1.
Fig. 7. Temperature dependences of the electrical conductivities for the samples 2.25Li3 þ 0.55P1 0.11S4 2yLaS2.
Fig. 8. Cyclic voltammogram of the sample 2.25Li3 þ 0.55P1 0.11S4 0.2LaS2 at room temperature with scanning rate of 10 mV s 1. Fig. 6. X-ray diffraction patterns for the samples 2.25Li3 þ 0.55P1 0.11S4 yLa2S3.
increasing y up to 0.5, new peaks of LaS2 come up gradually rather than the originally added La2S3. The conversion of La2S3 to LaS2 should be resulted from the high vapor pressure of P2S5 [1,22]. Consequently, the final products should be a composite in the form of poor-crystallized Li3 þ 0.55P1 0.11S4 and well-crystallized LaS2 (2.25Li3 þ 0.55P1 0.11S4 2yLaS2). The conductivities and the corresponding activity energies of 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 were also measured by AC impedance spectroscopy. The temperature dependences of conductivities are shown in Fig. 7. Just as the former studied Li4SiS4 xLa2S3 system, the plots of log(σT) against 103/T indicate that the conductivities of these composite samples in the temperature range from 30 to 210 1C also follow the Arrhenius equation. The reported data for Li3 þ 0.55P1 0.11S4 is also displayed in Fig. 7 as reference. It can be seen that for the sample with y¼ 0 at 30 1C, both of the conductivity, 1.82 10 5 S cm 1 and the activity energy, 0.43 eV are similar with the reported data. With LaS2 existed, the conductivity of 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 reached a maximum value of 7.41 10 5 S cm 1 at y ¼ 0.1. Then the conductivity decreased gradually with the increase of LaS2 due to its insulating nature.
The DC conductivity was studied on 2.25Li3 þ 0.55 P1 0.11S4 0.2LaS2. As shown in Fig. 4, the DC conductivities with nonblocking or blocking electrodes were measured to be 6.07 10 5 or 3.40 10 8 S cm 1, respectively. Therefore, the electronic conduction in the total conductivity is almost negligible. And the obtained value of lithium-ion conductivity is comparative to that measured by AC impedance spectroscopy. As shown in Fig. 8, cyclic voltammogram test for 2.25Li3 þ 0.55 P1 0.11S4 0.2LaS2 shows that the cathodic current and anodic current due to lithium deposition and dissolution can be observed, respectively. And it can be seen that 2.25Li3 þ 0.55P1 0.11S4 0.2LaS2 also has a wide electrochemical window of more than 8 V vs. Li þ /Li. Finally, the lithium-ion conductivities of both Li2S SiS2 and Li2S P2S5 systems can be improved by the incorporation of lanthanum sulfide. The formation of composite structure should be the main reason for the enhancement. To explore the enhancement mechanism of ionic conductivity in multiphase composite electrolyte systems, Agrawal [23] proposed several models, all of which emphasize the existence of a space-charge region at the interface between the host and the dispersoid. The host is assigned as the ionic conducting solid and the
Z. Liu et al. / Ceramics International 40 (2014) 15497–15501
dispersoid as the insulating materials. In our study, Li4SiS4 or Li3 þ 0.55P1 0.11S4 in poor crystallinity (partially in amorphous state after lanthanum sulfides addition) are the host and lanthanum sulfide is the dispersoid, respectively. The amorphous character is beneficial for host covering on dispersoid with the largest interface area. Along the interface between the poor crystalline Li4SiS4 or Li3 þ 0.55P1 0.11S4 and the good crystalline La2S3 or LaS2, lithium-ion transfers much freely and speedily. Another reason is the large amount of defects existing in the poor crystalline Li4SiS4 or Li3 þ 0.55P1 0.11S4 will decrease the activation energy of the lithium ion. In addition, the existing La ion in the interface will coordinate with sulfur anion and then decrease the attraction between sulfur and lithium. 4. Conclusions In this paper, a composite structure of sulfide-based solid electrolyte with poor-crystallized host and well-crystallized dispersoid was designed to increase the ionic conductivity by adding lanthanum sulfide into the Li2S SiS2 or Li2S P2S5 systems. The lanthanum sulfide in the as-prepared 3Li4SiS4 xLa2S3 and 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 composite electrolytes can suppress the crystallization of Li4SiS4 or Li3 þ 0.55P1 0.11S4. The electrochemical impedance measurements demonstrate that the ion conductivities of the designed solid electrolytes were enhanced by one magnitude in comparison with the pristine lithium-ion electrolytes (from 3.3 10 6 to 3.4 10 5 S cm 1 for 3Li4SiS4 xLa2S3, and from 1.82 10 to 7.41 10 5 S cm 1 for 2.25Li3 þ 0.55P1 0.11 S4 2yLaS2). Both of these composites have a very wide electrochemical window of more than 8 V vs. Li þ /Li, which is important for practical applications. The mechanism for the enhanced ionic conductivity was demonstrated to be related to the formation of multiphase composites, poor crystallinity of the lithium-ion conducting phase and the existing of La ion in the interface. Acknowledgments This work was financially supported by National Science Foundation of China (Grant nos. 51125006, 21203234) and Science and Technology Commission of Shanghai Grant 12XD1406800. References [1] K. Takada, Progress and prospective of solid-state lithium batteries, Acta Mater. 61 (2013) 759–770. [2] A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, T. Minami, Preparation of Li2S-P2S5 amorphous solid electrolytes by mechanical milling, J. Am. Ceram. Soc. 84 (2001) 477–479.
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[3] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, New, highly ionconductive crystals precipitated from Li2S-P2S5 glasses, Adv. Mater. 17 (2005) 918–921. [4] J.E. Treveya, Y.S. Jungb, S.H. Lee, High lithium ion conducting Li2S– GeS2–P2S5 glass–ceramic solid electrolyte with sulfur additive for all solid-state lithium secondary batteries, Electrochim. Acta 56 (2011) 4243–4247. [5] T. Ohtomo, A. Hayashi, M. Tatsumisago, Y. Tsuchida, S. Hama, K. Kawamoto, All-solid-state lithium secondary batteries using the 75Li2S 25P2S5 glass and the 70Li2S 30P2S5 glass–ceramic as solid electrolytes, J. Power Sour. 233 (2013) 231–235. [6] S. Sahami, S.W. Shea, J.H. Kennedy, Preparation and conductivity measurements of SiS2–Li2S–LiBr li-ion conductive glasses, J. Electrochem. Soc. 132 (1985) 985–986. [7] H Takahara, M. Tabuchi, T. Takeuchi, H. Kageyama, J. Ide, K. Handa, Y. Kobayashi, Y. Kurisu, S. Kondo, R. Kanno, Application of lithium metal electrodes to all-Solid-State lithium secondary batteries using Li3PO4–Li2S–SiS2 glass, J. Electrochem. Soc. 151 (2004) A1309–A1313. [8] M. Murayama, R. Kanno, M. Irie, S. Ito, T. Hata, N. Sonoyama, Y. Kawamoto, Synthesis of new li-ionic conductor thio-LISICON lithium silicon sulfides system, J. Solid State Chem. 168 (2002) 140–148. [9] K. Itoh, M. Sonobe, K. Mori, M. Sugiyama, T. Fukunaga, Structural observation of Li2S–GeS2 superionic glasses, Physica B 385–386 (2006) 520–522. [10] J. Saienga, S.W. Martin, The comparative structure, properties, and ionic conductivity of LiIþ Li2S þGeS2 glasses doped with Ga2S3and La2S, J. Non-Cryst. Solids 354 (2008) 1475–1486. [11] Y. Ito, A. Sakuda, T. Ohtomo, A. Hayashi, M. Tatsumisago, Preparation of Li2S–GeS2 solid electrolyte thin films using pulsed laser deposition, Solid State Ionics 236 (2013) 1–4. [12] Y. Ooura, N. Machida, M. Naito, T. Shigematsu, Electrochemical properties of the amorphous solid electrolytes in the system Li2S– Al2S3–P2S5, Solid State Ionics 225 (2012) 350–353. [13] Z. Nagamedianova, A. Hernandez, E. Sanchez, Conductivity studies on LiX–Li2S–Sb2S3 P2S5, Ionics 12 (2006) 315–322. [14] J.E. Trevey, J.R. Gilsdorf, S.W. Miller, S.H. Lee, Li2S–Li2O–P2S5 solid electrolyte for all-solid-state lithium batteries, Solid State Ionics 214 (2012) 25–30. [15] A. Hayashi, T. Fukuda, H. Morimoto, M. Tatsumisago, Amorphous solid electrolytes in the system Li2S–Al2S3–SiS2 prepared by mechanical milling, J. Mater. Sci. 39 (2004) 5125–5127. [16] Z. Nagamedianova, E. Sanchez, Preparation and characterization of novel Li2S–Sb2S3 and Li2S–P2S5Sb2S3 glassy systems, J. Non-Cryst. Solids 329 (2003) 13–16. [17] Z. Nagamedianova, E. Sanchez, Synthesis and characterization of new glasses based on Li2S–P2S5–Sb2S3 system, J. Mater. Sci-Mate. El 18 (2007) 547–552. [18] M. Yamashita, H. Yamanaka, Formation and ionic conductivity of Li2S– GeS2–Ga2S3 glasses and thin films, Solid State Ionics 158 (2003) 151–156. [19] J. Saienga, Y. Kim, B. Campbell, S.W. Martin, Preparation and characterization of glasses in the LiIþ Li2Sþ GeS2 þ Ga2S3 system, Solid State Ionics 176 (2005) 1229–1236. [20] B.T. Ahn, R.A. Huggins, Synthesis and lithium conductivities of Li2SiS3 and Li4SiS4, Mater. Res. Bull. 24 (1989) 889–897. [21] M. Murayama, N. Sonoyama, A. Yamada, R. Kanno, Material design of new Li-ionic conductor, thio-LISICON, in the Li2S–P2S5 system, Solid State Ionics 170 (2004) 173–180. [22] I.G. Vasilyeva, R.E. Nikolaev, The La2S3–LaS2 system: Thermodynamic and kinetic study, J. Solid State Chem. 183 (2010) 1747–1751. [23] R.C. Agrawal, R.K. Gupta, Superionic solid: composite electrolyte phase–an overview, J. Mater. Sci. 34 (1999) 1131–1162.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 15497–15501 www.elsevier.com/locate/ceramint
Enhanced ionic conductivity of sulfide-based solid electrolyte by incorporating lanthanum sulfide Zhanqiang Liu, Yufeng Tang, Xujie Lü, Guohao Renn, Fuqiang Huangnn Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 16 May 2014; received in revised form 2 July 2014; accepted 2 July 2014 Available online 10 July 2014
Abstract Solid electrolyte is the key point for developing all-solid-state lithium-ion battery with good recyclability and no safety problem. Here, a crystalline phase of lanthanum sulfide (La2S3) was incorporated into glassy sulfide-based solid electrolytes (Li2S–SiS2 or Li2S–P2S5) to form two composite systems Li2S–SiS2–xLa2S3 and Li2S–P2S5–yLa2S3. The results demonstrated that La2S3 has the ability to suppress the crystallization of Li2S–SiS2 or Li2S–P2S5 during the synthetic process. The lithium-ion conductivities of the designed composites can be improved by more than one order of magnitude in comparison with the pristine samples. And both of these two composites have a wide electrochemical window of more than 8 V vs. Li þ /Li. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Powders: solid state reaction; B. Composites; C. Ionic conductivity; E. Batteries; Lanthanum sulfide
1. Introduction Traditional liquid electrolytes used in lithium-ion battery contain toxic and flammable solvents, which bring serious problems such as leakage, inflammability and narrow operating temperature range. For safety concerns and particular applications, the next-generation lithium-ion batteries have been heading for replacing the traditional liquid electrolytes with solid electrolytes [1]. However, the solid electrolytes have not been widely used because their ionic conductivities are still much lower than those of the liquid electrolytes. To meet the requirements of high performance solid-state batteries, advanced solid electrolytes having similar ionic conductivity with the liquid electrolyte are highly required. Many inorganic solid electrolytes have been investigated among which sulfidebased electrolytes show great promising prospects. n
Corresponding author. Tel.: þ86 21 69987741; fax: þ 86 21 5241 3903. Corresponding author. Tel.: þ 86 21 5241 1620; fax: þ86 21 5241 6360. E-mail addresses: [email protected] (G. Ren), [email protected] (F. Huang). nn
http://dx.doi.org/10.1016/j.ceramint.2014.07.011 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Sulfide-based electrolytes have much higher lithium-ion conductivity than oxide-based electrolytes owing to their higher polarization ability of S2 than O2 . Li2S P2S5 [2–5], Li2S SiS2 [6–8] and Li2S GeS2 [9–11] are currently the most studied sulfide-based electrolytes. In these systems, P2S5, SiS2 or GeS2 serve as network former and Li2S is network modifier. It is well known that the lithium-ion conductivities of these sulfide electrolytes can be improved by reducing the crystallinity to form poorly-crystallized or amorphous phases. Unfortunately, these systems usually have low-melting points and are easy to form good crystalline phase. So it is very important to develop strategies to stabilize the glassy phase of sulfide-based electrolytes. Although remarkable progress has been made in the preparation of the glassy systems by doping oxides, sulfides, lithium halides or other chemical materials to improve ionic conductivities, an effective strategy still represents a big challenge [6,12–19]. In this paper, we propose a facile route to improve the ionic conductivities of Li2S SiS2 and Li2S P2S5 systems by incorporating lanthanum sulfide (La2S3). The La2S3 in the composites can suppress the crystallization of Li2S SiS2 or Li2S P2S5, and the ionic conductivities of these composites can be significantly enhanced.
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2. Experimental details La2S3, SiS2, P2S5 and Li2S were used as the starting materials. All the raw materials were stoichiometrically synthesized in our lab, and the purities were confirmed by X-ray diffraction. For the incorporation of La2S3 into Li2S SiS2, the molar ratio of Li2S, SiS2 and La2S3 was controlled as 6: 3: x (x=0, 0.1, 0.3, 0.5, 0.75 and 1.0). For the addition of La2S3 into Li2S P2S5, the molar ratio of Li2S, P2S5 and La2S3 was controlled as 4: 1: y (y=0, 0.1, 0.2, 0.3 and 0.5). The fully mixed precursors were loaded into evacuated and sealed silica tubes. The tubes were placed into the furnace, heated at 750 1C for 10 h and then quenched in water. The final products were collected and ground into fine powders. X-ray diffraction (XRD) analysis was carried out with an X-ray diffractometer (Rigaku D/Max 2550V, 40 kV 40 mA) with CuKα radiation in the 2θ range from 201 to 601. For ionic conductivity measurement, powder sample was cold-pressed into a ϕ10 1 mm pellet and both sides of the pellet were attached with indium plates as current collectors. Then the measurement was conducted in a dry argon flow by complex impedance on an impedance analyzer (Chenhua 660B) in the frequency range of 0.1 Hz and 0.1 MHz over the temperature range from 30 to 210 1C. DC polarization measurement was carried out to determine the electronic conductivity. Indium foil was used as blocking electrode and lithium plate as unblocking electrode. Time dependence of the electrical current was measured under a constant voltage 1 V for 1600 s. Cyclic voltammogram (CV) of the asymmetric Li/sample/Pt cell was performed on Chenhua 660B to evaluate the electrochemical stability of the electrolytes at the scan rate of 10 mV s 1. The Lithium plate and Pt plate in the cell are serve as the reference/counter and working electrodes, respectively.
Fig. 1. X-ray diffraction patterns for the samples 3Li4SiS4 xLa2S3.
3. Results and discussion XRD patterns of 6Li2S 3SiS2 xLa2S3 with various La2S3 contents are shown in Fig. 1. It can be seen that Li4SiS4 was obtained at x ¼ 0 [8]. With the increasing of La2S3, the peaks indexed to La2S3 appear and become stronger gradually. The unchanged phase of La2S3 before and after the synthetic process indicates that there is no reaction between La2S3 and Li2S or SiS2. Since the melting temperature of Li4SiS4 is around 750 1C, the quenching process should result in the poor crystallinity of Li4SiS4. And it can be observed that the peaks indexed to Li4SiS4 become relatively weaker with the increasing of x and almost disappeared at x ¼ 1.0. The poor-crystal Li4SiS4 transforms into amorphous state step by step, implying that the added La2S3 can efficiently suppress the crystallization of Li4SiS4 during the preparation process. Usually, the coordination number of La3 þ is six to nine, while Si4 þ is 4-coordinated in Li4SiS4. Considering other conditions, such as the reaction temperature, reaction time and especially the big difference of atomic radius between La3 þ and Si4 þ , the possibility of substitution of Si4 þ by La3 þ is very low. So it can be concluded that the final samples can be
Fig. 2. Complex impedance plots for 3Li4SiS4 xLa2S3 with x¼0.3. (a) Complex impedance plots. (b) Enlarged plots from 0 to 500 Ω.
recognized as a kind of composite, poor-crystallized Li4SiS4/ well-crystallized La2S3 (3Li4SiS4 xLa2S3). Ionic conductivities of the obtained samples were examined through the AC impedance spectroscopy. Fig. 2 shows the impedance plots of 3Li4SiS4 xLa2S3 with x ¼ 0.3 at various temperatures. A semicircle in the high-frequency range with a spike in the low frequency range was observed at 30 1C. As typical ionic conductor, the semicircle is interpreted as a
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parallel combination of a resistance and a capacitance, which are attributed to the bulk and grain boundary contribution, respectively. The spike is caused by the electrode contribution, where Warburg impedance exists. By increasing the measurement temperatures over 60 1C, no semicircle can be observed but only spikes left. The total resistance at each temperature can be calculated from the intersection of the semicircle with the real axis at the lower frequency side. Then the total lithium-ion conductivity at each temperature can be derived. Fig. 3 shows the temperature dependences of conductivities for the series of 3Li4SiS4 xLa2S3. The plots of log(σT) vs. 103/T indicate that the conductivities of these composites in the temperature range from 30 to 210 1C follow the Arrhenius equation: σT ¼ σ0exp( Ea/RT), where σ0 is the pre-exponential factor, Ea the activation energy for conduction, R the gas constant and T the absolute temperature. So it can be inferred that the system of 3Li4SiS4 xLa2S3 is a good lithium-ion conductor. Then the corresponding activation energies were calculated based on the Arrhenius equation. To further demonstrate the effect of the amount of La2S3 on the Li4SiS4 system. The composition dependences of the conductivity and the activation energy for are shown in Fig. 3 (the inset figure). The ionic conductivity and activation energy of the obtained Li4SiS4 are calculated to be 3.3 10 6 S cm 1 and 0.43 eV, respectively. As reported, however, the ionic conductivity and activation energy of the fine crystalline Li4SiS4 are 5.0 10 8 S cm 1 and 0.60 eV, respectively [20]. The reason for the big differences is the poor crystallinity of the obtained Li4SiS4, which is caused by the quenching process. So the above result demonstrates that poor crystallinity is benefit for the lithium ion diffusion in Li4SiS4. The XRD analysis has disclosed that the poor crystallinity will turn into amorphous gradually with the increasing of La2S3, suggesting the ionic conductivity might be further enhanced. As shown in Fig. 3, the conductivities of Li4SiS4 xLa2S3 increase initially and then decrease as the La2S3 content increasing from x¼ 0 to 1.0. The corresponding activation energy almost follows the reverse trend. Apparently, the content of La2S3 was optimized to be x¼ 0.3. And the corresponding conductivity and activation energy were calculated to be
Fig. 3. Temperature dependences of the electrical conductivities for the samples of 3Li4SiS4 xLa2S3. The inset depicts the composition dependence of the conductivities and the activation energies for the samples 3Li4SiS4 xLa2S3.
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3.4 10 5 S cm 1 and 0.38 eV, respectively. The optimized conductivity is ten times higher than that of the pristine Li4SiS4 after La2S3 added. Since La2S3 is ionic-insulating, heavy addition will block the lithium ions and subsequently reduce the ionic conductivity of the composite. Fig. 4 shows the time dependence of the DC conductivity obtained from current after applying a constant 1 V DC voltage on sample 3Li4SiS4 0.3La2S3 at room temperature. When using lithium as nonblocking electrode, the DC conductivity is almost constant with time and the value, around 2.8 10 5 S cm 1, is in good agreement with the number obtained from AC impedance shown above. When using indium as blocking electrode, a large decrease of DC conductivity, resulted from the accumulation of lithium atoms on the electrode, can be observed initially and then the conductivity become almost constant. The constant conductivity of 1.80 10 8 S cm 1 is about three orders of magnitude lower than that obtained using the nonblocking electrode. Therefore, it can be concluded that the electronic conduction in the total conductivity is almost negligible. Electrochemical stabilities of the obtained samples were also studied by cyclic voltammetry with a potential range from 0.50 to þ 10.0 V vs. Li þ /Li. Fig. 5 shows the cyclic voltammogram of the sample 3Li4SiS4 3La2S3. A cathodic current due to lithium deposition (Li þ þ e -Li) is observed at around 0 V on the cathodic sweep up to 0.50 V, and then an anodic current due to lithium dissolution (Li-Li þ þ e ) is observed at around 0.2 V on the an anodic sweep. There is no significant anodic current in the potential up to 10 V, which means the sample of 3Li4SiS4 0.3La2S3 has a wide electrochemical window up to 10 V vs. Li þ /Li. In order to verify our statement on the effects of La2S3 addition, La2S3 was also introduced into the Li2S P2S5 system. As shown in Fig. 6, the phases of the series of 4Li2S P2S5 yLa2S3 were examined by X-ray powder diffraction. At the point of y ¼ 0, the main peaks are indexed to Li3 þ 0.55P1 0.11S4 [21]. Similar to Li4SiS4, the peaks of Li3 þ 0.55P1 0.11S4 also become weaker and weaker gradually with La2S3 adding, which implies the crystallinity of Li3 þ 0.55P1 0.11S4 is becoming weaker. Interestingly, with
Fig. 4. Time dependence of DC conductivity for the sample 3Li4SiS4 0.3La2S3 after applying a constant DC voltage of 1 V at room temperature.
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Fig. 5. Cyclic voltammogram of the sample 3Li4SiS4 0.3La2S3 at room temperature with scanning rate of 10 mV s 1.
Fig. 7. Temperature dependences of the electrical conductivities for the samples 2.25Li3 þ 0.55P1 0.11S4 2yLaS2.
Fig. 8. Cyclic voltammogram of the sample 2.25Li3 þ 0.55P1 0.11S4 0.2LaS2 at room temperature with scanning rate of 10 mV s 1. Fig. 6. X-ray diffraction patterns for the samples 2.25Li3 þ 0.55P1 0.11S4 yLa2S3.
increasing y up to 0.5, new peaks of LaS2 come up gradually rather than the originally added La2S3. The conversion of La2S3 to LaS2 should be resulted from the high vapor pressure of P2S5 [1,22]. Consequently, the final products should be a composite in the form of poor-crystallized Li3 þ 0.55P1 0.11S4 and well-crystallized LaS2 (2.25Li3 þ 0.55P1 0.11S4 2yLaS2). The conductivities and the corresponding activity energies of 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 were also measured by AC impedance spectroscopy. The temperature dependences of conductivities are shown in Fig. 7. Just as the former studied Li4SiS4 xLa2S3 system, the plots of log(σT) against 103/T indicate that the conductivities of these composite samples in the temperature range from 30 to 210 1C also follow the Arrhenius equation. The reported data for Li3 þ 0.55P1 0.11S4 is also displayed in Fig. 7 as reference. It can be seen that for the sample with y¼ 0 at 30 1C, both of the conductivity, 1.82 10 5 S cm 1 and the activity energy, 0.43 eV are similar with the reported data. With LaS2 existed, the conductivity of 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 reached a maximum value of 7.41 10 5 S cm 1 at y ¼ 0.1. Then the conductivity decreased gradually with the increase of LaS2 due to its insulating nature.
The DC conductivity was studied on 2.25Li3 þ 0.55 P1 0.11S4 0.2LaS2. As shown in Fig. 4, the DC conductivities with nonblocking or blocking electrodes were measured to be 6.07 10 5 or 3.40 10 8 S cm 1, respectively. Therefore, the electronic conduction in the total conductivity is almost negligible. And the obtained value of lithium-ion conductivity is comparative to that measured by AC impedance spectroscopy. As shown in Fig. 8, cyclic voltammogram test for 2.25Li3 þ 0.55 P1 0.11S4 0.2LaS2 shows that the cathodic current and anodic current due to lithium deposition and dissolution can be observed, respectively. And it can be seen that 2.25Li3 þ 0.55P1 0.11S4 0.2LaS2 also has a wide electrochemical window of more than 8 V vs. Li þ /Li. Finally, the lithium-ion conductivities of both Li2S SiS2 and Li2S P2S5 systems can be improved by the incorporation of lanthanum sulfide. The formation of composite structure should be the main reason for the enhancement. To explore the enhancement mechanism of ionic conductivity in multiphase composite electrolyte systems, Agrawal [23] proposed several models, all of which emphasize the existence of a space-charge region at the interface between the host and the dispersoid. The host is assigned as the ionic conducting solid and the
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dispersoid as the insulating materials. In our study, Li4SiS4 or Li3 þ 0.55P1 0.11S4 in poor crystallinity (partially in amorphous state after lanthanum sulfides addition) are the host and lanthanum sulfide is the dispersoid, respectively. The amorphous character is beneficial for host covering on dispersoid with the largest interface area. Along the interface between the poor crystalline Li4SiS4 or Li3 þ 0.55P1 0.11S4 and the good crystalline La2S3 or LaS2, lithium-ion transfers much freely and speedily. Another reason is the large amount of defects existing in the poor crystalline Li4SiS4 or Li3 þ 0.55P1 0.11S4 will decrease the activation energy of the lithium ion. In addition, the existing La ion in the interface will coordinate with sulfur anion and then decrease the attraction between sulfur and lithium. 4. Conclusions In this paper, a composite structure of sulfide-based solid electrolyte with poor-crystallized host and well-crystallized dispersoid was designed to increase the ionic conductivity by adding lanthanum sulfide into the Li2S SiS2 or Li2S P2S5 systems. The lanthanum sulfide in the as-prepared 3Li4SiS4 xLa2S3 and 2.25Li3 þ 0.55P1 0.11S4 2yLaS2 composite electrolytes can suppress the crystallization of Li4SiS4 or Li3 þ 0.55P1 0.11S4. The electrochemical impedance measurements demonstrate that the ion conductivities of the designed solid electrolytes were enhanced by one magnitude in comparison with the pristine lithium-ion electrolytes (from 3.3 10 6 to 3.4 10 5 S cm 1 for 3Li4SiS4 xLa2S3, and from 1.82 10 to 7.41 10 5 S cm 1 for 2.25Li3 þ 0.55P1 0.11 S4 2yLaS2). Both of these composites have a very wide electrochemical window of more than 8 V vs. Li þ /Li, which is important for practical applications. The mechanism for the enhanced ionic conductivity was demonstrated to be related to the formation of multiphase composites, poor crystallinity of the lithium-ion conducting phase and the existing of La ion in the interface. Acknowledgments This work was financially supported by National Science Foundation of China (Grant nos. 51125006, 21203234) and Science and Technology Commission of Shanghai Grant 12XD1406800. References [1] K. Takada, Progress and prospective of solid-state lithium batteries, Acta Mater. 61 (2013) 759–770. [2] A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, T. Minami, Preparation of Li2S-P2S5 amorphous solid electrolytes by mechanical milling, J. Am. Ceram. Soc. 84 (2001) 477–479.
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