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Solid State Ionics 179 (2008) 1714 – 1716 www.elsevier.com/locate/ssi
New lithium ion conductor, thio-LISICON lithium zirconium sulfide system Zhanqiang Liu, Fuqiang Huang, Jianhua Yang, Baofeng Wang, Junkang Sun ⁎ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 12 July 2007; received in revised form 23 January 2008; accepted 28 January 2008
Abstract A new lithium ion conductor, thio-LISICON (LIthium SuperIonic CONductor), Li2ZrS3 was synthesized by solid state reaction. It has the lithium ion conductivity of 7.3 × 10− 6 S cm− 1 at 30 °C. Its conductivity were successfully improved by introducing interstitial lithium ions with Zr4+ partially substituted by Zn2+ (Li2 + 2xZnxZr1 − xS3; x = 0, 0.05, 0.1, 0.2, 0.4). The optimum x value for introducing the highest content of interstitial lithium ions was found to be 0.1. And the highest ionic conductivity at 30 °C was reached to be 1.2 × 10− 4 S cm− 1. © 2008 Elsevier B.V. All rights reserved. Keyword: Lithium ion conductor; Lithium zirconium sulfide; All-solid-state lithium secondary batteries; Thio-LISICON
1. Introduction With the rapid development of the science and technology in laptop computer, cellular phone, digital camera and electric vehicle, the demands for high energy capacity of lithium ion secondary batteries are increasing [1,2]. And safety is the most important issue for energy application in this new century. The traditional liquid electrolytes have many disadvantages, such as leakage, inflammability and narrow range of operating temperature. Therefore many efforts have been dedicated to developing all-solid-state lithium ion secondary batteries that consist of solid lithium ion electrolytes [2,3]. The solid electrolytes should have the similar lithium ion conduction property as the traditional liquid electrolytes. Inorganic materials and organic polymers are the two candidates chosen as solid electrolytes. For inorganic electrolytes, sulfide-based electrolytes generally have higher lithium ion conductivity by several orders of magnitude than oxide-based electrolytes, owing to the higher polarizability of the large-size sulfur atom than that of oxygen. The stronger electronegativity oxygen anion has a larger attraction to lithium ion. So the average free
⁎ Corresponding author. Tel.: +86 21 52413214; fax: +86 21 52413903. E-mail address:
[email protected] (J. Sun). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.055
path migration of lithium ions decreases in these oxides, as compared with the sulfide-based electrolytes. Currently, the research works are being mostly focused on sulfide glass because of its actually higher lithium ion conductivity than the corresponding crystalline materials [4–7]. But the crystalline materials should have higher conductivities if their crystal structures have been well designed or modified, such as aliovalent substitution [8]. Here we successfully obtained a new lithium ion conductor, Li2ZrS3, by solid state reaction and its highest ionic conductivity at 30 °C was improved to 1.2 × 10− 4 S cm− 1 with Zr4+ partially substituted by Zn2+. 2. Experimental Series of Li2 + 2xZnxZr1 − xS3 (x = 0, 0.05, 0.1, 0.2, 0.4) samples were synthesized by solid state reaction. The starting materials, ZnS, Li2S, sulfur and zirconium powder, were well mixed in the exact molar ratio and put into sealed quartz tubes in an argon filled glove box and then heated in the furnace at the temperature of 650 °C for 10 h. The tubes were slowly cooled to room temperature and samples were finally obtained. X-ray diffraction (XRD) analysis was carried out to identify the samples, using an X-ray diffractometer (Rigaku D/Max 2550V, 40 kV 40 mA) with CuKα radiation in the 2θ range from 20° to 70°. To avoid the attack of water and oxygen in air, the
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powder samples were pasted with vegetable oil before the XRD analysis. Ionic conductivity was measured in dry Argon atmosphere by complex impedance on an impedance analyzer (Chenhua 660B) in the frequency range of 0.01 Hz and 0.1 MHz. The pelletized sample, which was measured for electrical conductivities, was obtained by cold pressing (80 MPa) the powder samples in the hard steel die and the diameter and thickness of the pellets were about 10 mm and 1 mm, respectively. Indium plates (~ 0.03 mm thick) were attached to both faces of the pelletized sample to serve as current collectors. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of power samples of Li2 + 2xZnxZr1 − xS3 (x = 0, 0.05, 0.1, 0.2, 0.4) obtained at 650 °C. The baseline due to vegetable oil is also presented. The ´ ) is a little larger than that of Zn2+ (0.74 Å´) radius of Zr4+ (0.80 Å and the difference is about 7.5%. It is generally accepted that unlimited solubility is usually observed if the difference in the atomic radii of the components is not more than 15% in solid
Fig. 1. X-ray diffraction patterns of power samples of Li2 + 2xZnxZr1 − xS3 (x = 0, 0.05, 0.1, 0.2, 0.4) and the enlarged patterns from 25° to 32°. (a) XRD patterns; (b) Enlarged patterns from 25° to 32°.
Fig. 2. Complex of impedance plots for series of Li2 + 2xZnxZr1 − xS3 (x = 0, 0.05, 0.1, 0.2, 0.4) at the temperature of 30 °C.
solutions [9]. The XRD results in this experiments show that solid solution was formed with the substitution of Zr4+ by Zn2+ when the content of Zn2+ was low (x b 0.2). But the patterns show that ZnS peaks appears with increasing the value of x up to 0.4. Zr4+ could not be fully replaced by Zn2+ and the ideal unlimited solid solutions could not be formed if the content of Zn2+ was too high. This may result from the big difference of valences between Zr4+ and Zn2+ although they have the very similar radii. The substitution of Zr4+ by Zn2+ resulted in the decrease of interplanar distance, d, and correspondingly the increase of 2θ. This explains why the right shifts of the diffraction peaks were observed with the increase of x as shown in Fig. 1. The lithium ion conductivities of the series of Li2 + 2xZnxZr1 − xS3 (x= 0, 0.05, 0.1, 0.2, 0.4) were examined from the impedance plots, which are shown in Fig. 2. The total resistances at 30 °C for each sample were obtained from the intersections of the semicircles with the real axis at the lower frequency side. From Fig. 2, the total conductivity value at 30 °C of the new thioLISICON, Li2ZrS3, was calculated to be 7.3 ×10− 6 S cm− 1. This value is relatively higher than those of other sulfide thioLISICON. Li3PS4 [10,11], Li4GeS4 [12,13], Li2SiS3 and Li4SiS4 [14,15] were reported to have the lithium ion conductivities of 10− 7–10− 9 S cm− 1. For a thio-LISICON with low conductivity, some research works showed that aliovalent substitution is a good method to improving the conductivities. Interstitial lithium ions or lithium vacancies, which are beneficial to increase the lithium ion concentration or improve the lithium ion transfer ability, can be introduced to the host structure after aliovalent substitutions. For example, the lithium ion conductivities were increased from 5.0× 10− 8 S cm− 1 to 2.3 ×10− 7 S cm− 1 and 6.4× 10− 4 S cm− 1 when the aliovalent substitutions proceeded as Si4+↔ Li+ + Al3+ and Si4+ + Li+ ↔ P5+, respectively [8]. In this paper, Zn2+ was chosen to partially replace Zr4+ in order to introduce interstitial lithium ions. With increasing the content of Zn2+, it can be observed from Fig. 2 that the highest lithium ion conductivity, 1.2 × 10− 4 S cm− 1, was obtained at the composition x = 0.1 in Li2 + 2xZnxZr1 − xS3. This value is comparable for those of the lithium-silicon system (6.4 × 10− 4 S cm− 1) [8], lithium-
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germanium system (6.5 × 10− 5 S cm − 1 ) [12] or lithiumphosphorus system (1.5 × 10− 4 S cm− 1) [16]. The introducing of interstitial lithium ions apparently improved the conductivity. But the lithium ion conductivities did not constantly increase with increasing the content of Zn2+. The conductivity subsequently decreased gradually with adding Zn2+ more than x = 0.1. So the introduced interstitial lithium ions reached the maximum value at the point of x = 0.1 and no more interstitial lithium ions could be introduced but ZnS with continuously increasing the content of Zn2+. The XRD patterns in Fig. 1 confirmed this supposition with that a little content of ZnS was detected if x was higher than 0.1. The emerging of ZnS in the system may block the transfer of lithium ions and result in the decrease of ionic conductivity. 4. Conclusions A new lithium ion conductor, Li2ZrS3 was synthesized by solid state reaction with its lithium ion conductivity at 30 °C was calculated to be 7.3 × 10− 6 S cm− 1. The highest ionic conductivity at temperature of 30 °C was improved to 1.2 × 10− 4 S cm− 1 by introducing interstitial lithium ions with Zr4+ partially substituted by Zn2+ (Li2 + 2xZnxZr1 − xS3; x = 0, 0.05, 0.1, 0.2, 0.4). This value is comparable for those of the lithium-silicon (germanium or phosphorus) systems. The optimum value of x for Zn 2+ substituting Zr4+ to introduce the highest content of interstitial lithium ions was found to be 0.1. The extra ZnS presented in the sample results in the decrease of ionic conductivity. Acknowledgement This research was supported by the Science and Technology Commission of Shanghai Municipality (Grant No. 05JC14080).
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