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Crystal structures and ionic conductivity in Li2OHX (X = Cl, Br) antiperovskites
Journal of Solid State Chemistry 286 (2020) 121263
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Crystal structures and ionic conductivity in Li2OHX (X ¼ Cl, Br) antiperovskites☆ Anucha Koedtruad, Midori Amano Patino, Noriya Ichikawa, Daisuke Kan, Yuichi Shimakawa * Institute for Chemical Research, Kyoto University, Kyoto, 611-0011, Japan
A B S T R A C T
Crystal structures and ionic conductivity of antiperovskites Li2OHX (X ¼ Cl, Br), which are potential solid electrolyte materials for all-solid-state Li-ion batteries, were investigated. Li2OHCl showed a Pmc21 orthorhombic structure at room temperature and changed to a Pm3m cubic one between 27 and 37 C. Li2OHBr, on the other hand, crystallized in a Pm3m cubic structure and showed no phase transition in the measured temperature range. The cubic phases of both compounds showed dominant high Liþ-ion conductivity, while the orthorhombic phase of Li2OHCl exhibited reduced ionic conductivity. The Liþ-ion order in the orthorhombic Li2OHCl makes the crystal structure highly anisotropic and two dimensional, which disturbs the hopping of the Liþ ions.
1. Introduction All-solid-state Li-ion batteries are expected to be the next generation batteries that have high energy densities and also highly stable and safe properties [1–3]. The exploration of novel solid electrolyte materials with high Li-ion conductivity has thus been accelerated recently. Li3nOHnX (n ¼ 0–1, X ¼ Cl, Br) have great potential to meet such expectations and have been studied intensively. These compounds crystallize in an antiperovskite structure, which is the same as that of the perovskite structure but with the anion and cation positions switched [4–11]. Li-containing antiperovskite-structure n ¼ 0 compounds, Li3OCl and Li3O(Cl0.5Br0.5), were first reported to show high conductivity (>103 S/ cm at room temperature) with low activation energies (0.18–0.26 eV) [12]. However, subsequent studies revealed that the observed high conductivity was due not to ionic but to electronic conduction of LiCl‧ xH2O, which was easily formed when the samples were exposed to moisture. The reduced ionic conductivities (108–107 S/cm at room temperature) with increased activation energies were then confirmed in Li3OCl (n ¼ 0) [13,14]. On the other hand, Li2OHCl and Li2OHBr (n ¼ 1) have been reported to be rather stable antiperovskite-structure compounds that show high ionic conductivity. Liþ-ion vacancies in the antiperovskite structure play an essential role in the high ionic conductivity [15]. An interesting difference between the compounds is that Li2OHCl had an orthorhombic structure at room temperature, while Li2OHBr had a cubic structure. The room-temperature orthorhombic phase in Li2OHCl changed to a
high-temperature cubic phase, and consequently a significant increase in conductivity was observed [15–17]. It is important to clarify the differences in crystal structures and conducting properties between the antiperovskites Li2OHCl and Li2OHBr, and the phase transition in Li2OHCl makes the compound especially interesting in terms of structure-property relationships. In this work, we have made nearly single-phase samples of Li2OHCl and Li2OHBr and analyzed their crystal structures in detail with synchrotron X-ray diffraction (SXRD) data. We also analyzed the temperature dependence of the crystal structures and the conduction properties measured by AC impedance and DC polarization techniques. The relation of crystal structures and conducting properties will be discussed. 1.1. Experiments Li2OHCl and Li2OHBr were prepared by solid-state reaction. The mixture of LiOH and LiCl, or LiBr, was ground in a glovebox under an Ar atmosphere and placed in a vacuum-sealed glass tube. The tubes were heated in a furnace at 265 C for 1 day and then quickly cooled to room temperature. The phases of the synthesized samples were identified by a conventional X-ray diffraction (XRD) method with Cu-Kα radiation. Crystal structures were analyzed by using SXRD data. The SXRD measurements were performed at the BL02B2 beamline in SPring-8 and the TPS09A beamline in Taiwan Photon Source with a wavelength of 0.59979 Å and 0.82656 Å, respectively. The powder sample was packed into a sealed glass capillary tube and was rotated during the measurement to minimize
☆ Dedicated to the occasion of the 70th birthday of Prof. Kenneth Poeppelmeier * Corresponding author. . E-mail address: [email protected] (Y. Shimakawa).
https://doi.org/10.1016/j.jssc.2020.121263 Received 30 December 2019; Received in revised form 5 February 2020; Accepted 14 February 2020 Available online 22 February 2020 0022-4596/© 2020 Elsevier Inc. All rights reserved.
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Journal of Solid State Chemistry 286 (2020) 121263
Fig. 2. SXRD patterns and the results of structure analysis for (a) cubic Li2OHCl (67 C), (b) orthorhombic Li2OHCl (27 C), and cubic Li2OHBr (67 C). The red marks and green solid line represent observed and calculated patterns, respectively. The green vertical marks indicate the peak positions of Li2OHX (X ¼ Cl, Br). The blue line below is the difference between the observed and calculated intensities. A very small amount of LiCl phase is included in the refinement for the orthorhombic Li2OHCl. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Refined structure parameters obtained from SXRD data for Li2OHCl and Li2OHBr. Atom
Site
x
y
z
g
B (Å2)
0 0 0.5
0 0 0.5
0.667 1.0 1.0
5.4(2) 1.27(4) 1.74(2)
0.039(5) 0.026(11) 0.011(2) 0.499(1)
0.25(1) 0.0 0.0 0.25(2)
1.0 1.0 1.0 1.0
3.0(3) 7.3(4) 1.10(6) 1.59(3)
0 0 0.5
0 0 0.5
0.667 1.0 1.0
3.1(4) 2.7(1) 1.71(3)
a
(a) Cubic Li2OHCl Li 3d 0.5 O 1a 0 Cl 1b 0.5 (b) Orthorhombic Li2OHClb Li1 2a 0.0 Li2 2b 0.5 O 2a 0.0 Cl 2b 0.5 (c) Cubic Li2OHBrc Li 3d 0.5 O 1a 0 Br 1b 0.5
Fig. 1. SXRD patterns at temperatures from 7 to 67 C for (a) Li2OHCl and (b) Li2OHBr.
absorption. Diffraction data were collected at temperatures from 7 to 67 C. The obtained data were analyzed with the Rietveld method using the program RIETAN-VENUS [18]. The conductivity of the samples was measured by AC impedance spectroscopy using the Solatron1260 impedance analyzer and by the DC polarization method using the Keithley 2450 source-measure unit. The samples were pelletized into discs 10 mm in diameter and 3 mm thick, and for the conductivity measurements they were sandwiched by graphite electrodes. The AC impedance spectra at temperatures from room temperature to 80 C were collected from 10 MHz to 1 kHz with an applied voltage of 0.1 V under N2 flow. The DC polarization at room temperature was measured by applying a voltage of 0.1 V to complete polarization. The current as a function of time was collected.
a
Cubic (Pm3m), a ¼ 3.91424(3) Å and Rwp ¼ 8.89%. Orthorhombic (Pmc21), a ¼ 3.87243(6) Å, b ¼ 3.82595(6) Å, c ¼ 7.99957(13) Å, and Rwp ¼ 10.03%. c Cubic (Pm3m), a ¼ 4.05268(4) Å and Rwp ¼ 6.73%. b
phase, LiCl (less than 1.42 wt%), was detected in the Li2OHCl sample. Fig. 1 shows temperature-dependent SXRD patterns of the synthesized samples. For Li2OHCl it is clear that a structural phase transition occurs between 27 and 37 C, and the diffraction peaks observed at temperatures from 7 to 27 C are indexed with an orthorhombic unit cell, while those at temperatures from 37 to 67 C are indexed with a cubic unit cell. For Li2OHBr, on the other hand, no evidence of such a structural phase
2. Results and discussion Nearly single-phase samples of both Li2OHCl and Li2OHBr were obtained by the present synthesis. A very small amount of a secondary 2
A. Koedtruad et al.
Journal of Solid State Chemistry 286 (2020) 121263
Fig. 3. Crystal structures of (a) cubic Li2OHX (X ¼ Cl, Br) and (b) orthorhombic Li2OHCl.
Fig. 5. DC polarization currents of (a) cubic Li2OHCl at 60 C, (b) orthorhombic Li2OHCl at 25 C, and (c) cubic Li2OHBr at 30 C.
and the Cl and Br ions are located at the 12-fold coordinated cavity of the octahedral network. Note that one third of the Li sites in the octahedra are vacant. The refined cubic lattice parameters for Li2OHCl and Li2OHBr are respectively 3.91424(3) and 4.05268(4) Å, which agree with those reported previously [15,19]. The lattice parameter for the cubic Li2OHCl is slightly smaller than that for Li2OHBr because the ionic radius of Cl is smaller than that of Br. The room-temperature (27 C) crystal structure for Li2OHCl was refined with an orthorhombic cell with the space group Pmc21. The SXRD pattern with the refinement result are presented in Fig. 2b. The refined crystal structure is a distorted antiperovskite with the lattice parameters a ¼ 3.87243(6) Å, b ¼ 3.82595(6) Å, and c ¼ 7.99957(13) Å. The Li-ion vacancies are ordered, and the OH ions are coordinated square-planarly
Fig. 4. Typical Nyquist plots of (a) cubic Li2OHCl at 60 C, (b) orthorhombic Li2OHCl at 25 C, and (c) cubic Li2OHBr at 30 C.
transition is seen in the diffraction patterns in the measured temperature range, and the observed diffraction peaks are well indexed with a cubic unit cell. The crystal structures of both cubic phases were then analyzed by the Rietveld structure refinement method with the SXRD data. Fig. 2a and c respectively show the SXRD patterns for Li2OHCl and Li2OHBr collected at 67 C with their refinement results, and the refined structure parameters are listed in Table 1. Both patterns are well reproduced with cubic antiperovskite crystal structure models with the space group Pm3m (Fig. 3a). The OH ions are coordinated by Liþ ions, forming octahedra, 3
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Journal of Solid State Chemistry 286 (2020) 121263
the most likely carrier, as suggested in a previous paper [19]. They would travel through the vacancies of the octahedra. Thus, the high symmetry of the three-dimensional cubic phase should give rise to the high Liþ-ion conduction with low activation energy, whereas Liþ-ion order in the orthorhombic Li2OHCl makes the crystal structure highly anisotropic and two-dimensional, leading to the low conductivity. As discussed with the data from NMR spectroscopy and molecular dynamics simulation, the OH rotation also influences the Liþ-ion hopping [15,17,19]. In the two-dimensional orthorhombic structure with the Liþ vacancy ordering, such rotation is limited by the electrostatic repulsion between Hþ and Liþ ions. This would also disturb the hopping of the Liþ ions, leading to the reduction in conductivity for the orthorhombic Li2OHCl. Fig. 6. Temperature and Li2OHBr.
dependence
of
ionic
conductivity
for
Li2OHCl
3. Conclusions We synthesized Li2OHCl and Li2OHBr by solid-state reactions and investigated their crystal structures by temperature-dependent SXRD. The structure analysis revealed that both compounds crystalized in an antiperovskite structure with vacancies in one third of the Liþ sites in the octahedra and the Cl and Br ions in the cavity of corner-sharing Lioctahedra. Li2OHCl exhibited the structural transition from a cubic (Pm3m) to an orthorhombic (Pmc21) phase between 27 and 37 C. The transition was caused by the ordering of the Liþ-ion vacancies, and the vacancy-ordered structure was highly anisotropic and two-dimensional. On the other hand, Li2OHBr had a cubic structure with space group Pm3m, and no such structural transition was observed in the measured temperature range. The conducting properties of Li2OHCl and Li2OHBr were found to be dominantly ionic, and the electronic contributions were very low. The cubic phases of both compounds showed high Liþ-ion conductivity of 107 - 106 S/cm with activation energies of about 0.55–0.57 eV. The orthorhombic Li2OHCl phase, on the other hand, showed low Liþ-ionic conductivity because of its two-dimensional distorted crystal structure, which disturbs the Liþ-ion hopping.
by Liþ ions, as reported previously [20,21]. Consequently, the structure is highly anisotropic and two-dimensional, as shown in Fig. 3b. It is interesting to focus on the difference in structural transition behaviors between the compounds. Although the present compounds have Li-ion vacancies and contain hydroxide ions in the structures, the tolerance factor, t ¼ (rCl/Br þ rLi)/√2(rO þ rLi), for the parent (n ¼ 0) antiperovskite compounds give useful information on the structural stability. With ionic radii of Cl (0.181 nm), Br (0.196 nm), O2 (0.140 nm), and Liþ (0.068 nm), the calculated t for Li3OCl and Li3OBr are respectively 0.85 and 0.90. These results clearly indicate that the structural mismatch for the Cl antiperovskite is more significant than that for the Br compound. The Cl ion is too small to occupy the cavity of corner-sharing octahedra. To reduce this structural instability, the ordering of the Liþion vacancies and the distortion of the structure are induced by the structural transition. We then discuss the conducting properties of the synthesized compounds. The Nyquist plots of the impedance spectra for both cubic Li2OHCl (60 C) and Li2OHBr (30 C) are respectively shown in Fig. 4a and c. The impedance, Z ¼ Z0 iZ00 , shows a typical semi-circle behavior, and the total conductivity (σ) is calculated from intersections at the realpart axes. The obtained conductivities for the Cl and Br compounds are respectively 1.01 106 and 1.33 107 S/cm. The values agree well with those reported in previous papers [15,16]. The Nyquist plot for the orthorhombic Li2OHCl at room temperature is also shown in Fig. 4b, giving σ ¼ 6.83 109 S/cm. DC polarization measurements were performed to evaluate the ionic contribution to the total conductivity, and the results are shown in Fig. 5. By applying the voltage, the currents of the samples become constant due to depleting the mobile ions, and the values reached to 2.31 1011, 1.46 1011, 1.16 1011 A respectively for the cubic Li2OHCl, the orthorhombic Li2OHCl and the cubic Li2OHBr. The electronic conductivities estimated from the currents are found to be of the order of 1011 S/cm, which are very low compared to the total conductivity. When the applied voltage was set to zero after 30 min, the inverse current was observed, also confirming the ionic conduction in each sample. All the results clearly show that the electronic contribution is very low and that the ionic conduction is dominant in the present compounds. Temperature dependence of the ion conductivity was then measured, and the Arrhenius plot results are shown in Fig. 6. Both cubic Li2OHCl and Li2OHBr shows high ion conductivity at high temperatures. The conductivity decreases with decreasing temperature to about 40 C, and the linear relations in the Arrhenius plots give the activation energies Ea by using the equation σ ¼ σ0 exp(Ea/kT) [12,13]. The Ea for the cubic Li2OHCl and Li2OHBr are respectively 0.57 and 0.55 eV, which are consistent with the reported values [13,16]. Note that the conductivity of Li2OHCl significantly decreases at the structural phase transition temperature. The result strongly suggests that the ionic conductivity for the orthorhombic phase is significantly lower than that for the cubic phase. Although the present conduction measurement results cannot identify the mobile ionic species—either Liþ, Hþ, or OH—the Liþ ions would be
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Masato Goto, Zhenhong Tan, Satoshi Sugano, and Yoshihisa Kosugi in Kyoto University, Syogo Kawaguchi in SPring-8, Chun Chuang and Hwo-Shuenn Sheu in NSRRC, and Wei-Tin Chen in National Taiwan University for help in the SXRD experiments. The SXRD experiments were conducted at the Japan Synchrotron Radiation Research Institute, Japan (proposal Nos: 2018B1710 and 2019B1757) and the National Synchrotron Radiation Research Center, Taiwan (proposal Nos: Nos: 2017-1-125 and 2019-1-198). This work was partly supported by Grantsin-Aid for Scientific Research (Nos: 16H02266, 17K19177, and 19H05823) and by a grant for the Integrated Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also supported by Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks. References [1] D. Lin, Y. Liu, Y. Cui, Nat. Nanotechnol. 12 (2017) 194–206. [2] Y. Wang, W.D. Richards, S.P. Ong, L.J. Miara, J.C. Kim, Y. Mo, G. Ceder, Nat. Mater. 14 (2015) 1026–1031. [3] Y. Guo, H. Li, T. Zhai, Adv. Mater. 29 (2017) 1700007. [4] C. Eilbracht, W. Kockelmann, D. Hohlwein, H. Jacobs, Physica B 234 – 236 (1997) 48–50. [5] K. Friese, A. Hӧnnerscheid, M. Jansen, Z. Kristallogr. 218 (2003) 536–541. 4
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Journal of Solid State Chemistry 286 (2020) 121263 [14] X. Lü, G. Wu, J.W. Howard, A. Chen, Y. Zhao, L.L. Daemen, Q. Jia, Chem. Commun. 50 (2014) 11520–11522. [15] A.-Y. Song, Y. Xiao, K. Turcheniuk, P. Upadhya, A. Ramanujapuram, J. Benson, A. Magasinski, M. Olguin, L. Meda, O. Borodin, G. Yushin, Adv. Energy Mater. 8 (2018) 1700971. [16] Z.D. Hood, H. Wang, A.S. Pandian, J.K. Keum, C. Liang, J. Am. Chem. Soc. 138 (2016) 1768–1771. [17] G. Schwering, A. Honnerscheid, L.V. Wullen, M. Jansen, ChemPhysChem 4 (2003) 343–348. [18] F. Izumi, K. Momma, Solid State Phenom. 130 (2007) 15–20. [19] J.A. Dawson, T.S. Attari, H. Chen, S.P. Emge, K.E. Johnston, M.S. Islam, Energy Environ. Sci. 11 (2018) 2993–3002. [20] J. Howard, Z.D. Hood, N.A.W. Holzwarth, Phys. Rev. Mater. 1 (2017), 075406. [21] J. Howard, N.A.W. Holzwarth, Phys. Rev. B 99 (2019), 014109.
[6] A. Emly, E. Kioupakis, A.V.D. Ven, Chem. Mater. 25 (2013) 4663–4670. [7] Z. Lu, C. Chen, Z.M. Baiyee, X. Chen, C. Niu, F. Ciucci, Phys. Chem. Chem. Phys. 17 (2015) 32547–32555. [8] R. Mouta, E.M. Diniz, C.W.A. Paschoal, J. Mater. Chem. A 4 (2016) 1586–1590. [9] X. Lü, J.W. Howard, A. Chen, J. Zhu, S. Li, G. Wu, P. Dowden, H. Xu, Y. Zhao, Q. Jia, Adv. Sci. 3 (2016) 1500359. [10] Y. Li, W. Zhou, S. Xin, S. Li, J. Zhu, X. Lu, Z. Cui, Q. Jia, J. Zhou, Y. Zhao, J.B. Goodenough, Angew. Chem. Int. Ed. 55 (2016) 9965–9968. [11] S. Li, J. Zhu, Y. Wang, J.W. Howard, X. Lü, Y. Li, R.S. Kumar, W. Wang, L.L. Daemen, Y. Zhao, Solid State Ionics 284 (2016) 14–19. [12] Y. Zhao, L.L. Daemen, J. Am. Chem. Soc. 134 (2012) 15042–15047. [13] I. Hanghofer, G.J. Redhammer, S. Rohde, I. Hanzu, A. Senyshyn, H.M.R. Wilkening, D. Rettenwander, Chem. Mater. 30 (2018) 8134–8144.
5
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Crystal structures and ionic conductivity in Li2OHX (X ¼ Cl, Br) antiperovskites☆ Anucha Koedtruad, Midori Amano Patino, Noriya Ichikawa, Daisuke Kan, Yuichi Shimakawa * Institute for Chemical Research, Kyoto University, Kyoto, 611-0011, Japan
A B S T R A C T
Crystal structures and ionic conductivity of antiperovskites Li2OHX (X ¼ Cl, Br), which are potential solid electrolyte materials for all-solid-state Li-ion batteries, were investigated. Li2OHCl showed a Pmc21 orthorhombic structure at room temperature and changed to a Pm3m cubic one between 27 and 37 C. Li2OHBr, on the other hand, crystallized in a Pm3m cubic structure and showed no phase transition in the measured temperature range. The cubic phases of both compounds showed dominant high Liþ-ion conductivity, while the orthorhombic phase of Li2OHCl exhibited reduced ionic conductivity. The Liþ-ion order in the orthorhombic Li2OHCl makes the crystal structure highly anisotropic and two dimensional, which disturbs the hopping of the Liþ ions.
1. Introduction All-solid-state Li-ion batteries are expected to be the next generation batteries that have high energy densities and also highly stable and safe properties [1–3]. The exploration of novel solid electrolyte materials with high Li-ion conductivity has thus been accelerated recently. Li3nOHnX (n ¼ 0–1, X ¼ Cl, Br) have great potential to meet such expectations and have been studied intensively. These compounds crystallize in an antiperovskite structure, which is the same as that of the perovskite structure but with the anion and cation positions switched [4–11]. Li-containing antiperovskite-structure n ¼ 0 compounds, Li3OCl and Li3O(Cl0.5Br0.5), were first reported to show high conductivity (>103 S/ cm at room temperature) with low activation energies (0.18–0.26 eV) [12]. However, subsequent studies revealed that the observed high conductivity was due not to ionic but to electronic conduction of LiCl‧ xH2O, which was easily formed when the samples were exposed to moisture. The reduced ionic conductivities (108–107 S/cm at room temperature) with increased activation energies were then confirmed in Li3OCl (n ¼ 0) [13,14]. On the other hand, Li2OHCl and Li2OHBr (n ¼ 1) have been reported to be rather stable antiperovskite-structure compounds that show high ionic conductivity. Liþ-ion vacancies in the antiperovskite structure play an essential role in the high ionic conductivity [15]. An interesting difference between the compounds is that Li2OHCl had an orthorhombic structure at room temperature, while Li2OHBr had a cubic structure. The room-temperature orthorhombic phase in Li2OHCl changed to a
high-temperature cubic phase, and consequently a significant increase in conductivity was observed [15–17]. It is important to clarify the differences in crystal structures and conducting properties between the antiperovskites Li2OHCl and Li2OHBr, and the phase transition in Li2OHCl makes the compound especially interesting in terms of structure-property relationships. In this work, we have made nearly single-phase samples of Li2OHCl and Li2OHBr and analyzed their crystal structures in detail with synchrotron X-ray diffraction (SXRD) data. We also analyzed the temperature dependence of the crystal structures and the conduction properties measured by AC impedance and DC polarization techniques. The relation of crystal structures and conducting properties will be discussed. 1.1. Experiments Li2OHCl and Li2OHBr were prepared by solid-state reaction. The mixture of LiOH and LiCl, or LiBr, was ground in a glovebox under an Ar atmosphere and placed in a vacuum-sealed glass tube. The tubes were heated in a furnace at 265 C for 1 day and then quickly cooled to room temperature. The phases of the synthesized samples were identified by a conventional X-ray diffraction (XRD) method with Cu-Kα radiation. Crystal structures were analyzed by using SXRD data. The SXRD measurements were performed at the BL02B2 beamline in SPring-8 and the TPS09A beamline in Taiwan Photon Source with a wavelength of 0.59979 Å and 0.82656 Å, respectively. The powder sample was packed into a sealed glass capillary tube and was rotated during the measurement to minimize
☆ Dedicated to the occasion of the 70th birthday of Prof. Kenneth Poeppelmeier * Corresponding author. . E-mail address: [email protected] (Y. Shimakawa).
https://doi.org/10.1016/j.jssc.2020.121263 Received 30 December 2019; Received in revised form 5 February 2020; Accepted 14 February 2020 Available online 22 February 2020 0022-4596/© 2020 Elsevier Inc. All rights reserved.
A. Koedtruad et al.
Journal of Solid State Chemistry 286 (2020) 121263
Fig. 2. SXRD patterns and the results of structure analysis for (a) cubic Li2OHCl (67 C), (b) orthorhombic Li2OHCl (27 C), and cubic Li2OHBr (67 C). The red marks and green solid line represent observed and calculated patterns, respectively. The green vertical marks indicate the peak positions of Li2OHX (X ¼ Cl, Br). The blue line below is the difference between the observed and calculated intensities. A very small amount of LiCl phase is included in the refinement for the orthorhombic Li2OHCl. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Refined structure parameters obtained from SXRD data for Li2OHCl and Li2OHBr. Atom
Site
x
y
z
g
B (Å2)
0 0 0.5
0 0 0.5
0.667 1.0 1.0
5.4(2) 1.27(4) 1.74(2)
0.039(5) 0.026(11) 0.011(2) 0.499(1)
0.25(1) 0.0 0.0 0.25(2)
1.0 1.0 1.0 1.0
3.0(3) 7.3(4) 1.10(6) 1.59(3)
0 0 0.5
0 0 0.5
0.667 1.0 1.0
3.1(4) 2.7(1) 1.71(3)
a
(a) Cubic Li2OHCl Li 3d 0.5 O 1a 0 Cl 1b 0.5 (b) Orthorhombic Li2OHClb Li1 2a 0.0 Li2 2b 0.5 O 2a 0.0 Cl 2b 0.5 (c) Cubic Li2OHBrc Li 3d 0.5 O 1a 0 Br 1b 0.5
Fig. 1. SXRD patterns at temperatures from 7 to 67 C for (a) Li2OHCl and (b) Li2OHBr.
absorption. Diffraction data were collected at temperatures from 7 to 67 C. The obtained data were analyzed with the Rietveld method using the program RIETAN-VENUS [18]. The conductivity of the samples was measured by AC impedance spectroscopy using the Solatron1260 impedance analyzer and by the DC polarization method using the Keithley 2450 source-measure unit. The samples were pelletized into discs 10 mm in diameter and 3 mm thick, and for the conductivity measurements they were sandwiched by graphite electrodes. The AC impedance spectra at temperatures from room temperature to 80 C were collected from 10 MHz to 1 kHz with an applied voltage of 0.1 V under N2 flow. The DC polarization at room temperature was measured by applying a voltage of 0.1 V to complete polarization. The current as a function of time was collected.
a
Cubic (Pm3m), a ¼ 3.91424(3) Å and Rwp ¼ 8.89%. Orthorhombic (Pmc21), a ¼ 3.87243(6) Å, b ¼ 3.82595(6) Å, c ¼ 7.99957(13) Å, and Rwp ¼ 10.03%. c Cubic (Pm3m), a ¼ 4.05268(4) Å and Rwp ¼ 6.73%. b
phase, LiCl (less than 1.42 wt%), was detected in the Li2OHCl sample. Fig. 1 shows temperature-dependent SXRD patterns of the synthesized samples. For Li2OHCl it is clear that a structural phase transition occurs between 27 and 37 C, and the diffraction peaks observed at temperatures from 7 to 27 C are indexed with an orthorhombic unit cell, while those at temperatures from 37 to 67 C are indexed with a cubic unit cell. For Li2OHBr, on the other hand, no evidence of such a structural phase
2. Results and discussion Nearly single-phase samples of both Li2OHCl and Li2OHBr were obtained by the present synthesis. A very small amount of a secondary 2
A. Koedtruad et al.
Journal of Solid State Chemistry 286 (2020) 121263
Fig. 3. Crystal structures of (a) cubic Li2OHX (X ¼ Cl, Br) and (b) orthorhombic Li2OHCl.
Fig. 5. DC polarization currents of (a) cubic Li2OHCl at 60 C, (b) orthorhombic Li2OHCl at 25 C, and (c) cubic Li2OHBr at 30 C.
and the Cl and Br ions are located at the 12-fold coordinated cavity of the octahedral network. Note that one third of the Li sites in the octahedra are vacant. The refined cubic lattice parameters for Li2OHCl and Li2OHBr are respectively 3.91424(3) and 4.05268(4) Å, which agree with those reported previously [15,19]. The lattice parameter for the cubic Li2OHCl is slightly smaller than that for Li2OHBr because the ionic radius of Cl is smaller than that of Br. The room-temperature (27 C) crystal structure for Li2OHCl was refined with an orthorhombic cell with the space group Pmc21. The SXRD pattern with the refinement result are presented in Fig. 2b. The refined crystal structure is a distorted antiperovskite with the lattice parameters a ¼ 3.87243(6) Å, b ¼ 3.82595(6) Å, and c ¼ 7.99957(13) Å. The Li-ion vacancies are ordered, and the OH ions are coordinated square-planarly
Fig. 4. Typical Nyquist plots of (a) cubic Li2OHCl at 60 C, (b) orthorhombic Li2OHCl at 25 C, and (c) cubic Li2OHBr at 30 C.
transition is seen in the diffraction patterns in the measured temperature range, and the observed diffraction peaks are well indexed with a cubic unit cell. The crystal structures of both cubic phases were then analyzed by the Rietveld structure refinement method with the SXRD data. Fig. 2a and c respectively show the SXRD patterns for Li2OHCl and Li2OHBr collected at 67 C with their refinement results, and the refined structure parameters are listed in Table 1. Both patterns are well reproduced with cubic antiperovskite crystal structure models with the space group Pm3m (Fig. 3a). The OH ions are coordinated by Liþ ions, forming octahedra, 3
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Journal of Solid State Chemistry 286 (2020) 121263
the most likely carrier, as suggested in a previous paper [19]. They would travel through the vacancies of the octahedra. Thus, the high symmetry of the three-dimensional cubic phase should give rise to the high Liþ-ion conduction with low activation energy, whereas Liþ-ion order in the orthorhombic Li2OHCl makes the crystal structure highly anisotropic and two-dimensional, leading to the low conductivity. As discussed with the data from NMR spectroscopy and molecular dynamics simulation, the OH rotation also influences the Liþ-ion hopping [15,17,19]. In the two-dimensional orthorhombic structure with the Liþ vacancy ordering, such rotation is limited by the electrostatic repulsion between Hþ and Liþ ions. This would also disturb the hopping of the Liþ ions, leading to the reduction in conductivity for the orthorhombic Li2OHCl. Fig. 6. Temperature and Li2OHBr.
dependence
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3. Conclusions We synthesized Li2OHCl and Li2OHBr by solid-state reactions and investigated their crystal structures by temperature-dependent SXRD. The structure analysis revealed that both compounds crystalized in an antiperovskite structure with vacancies in one third of the Liþ sites in the octahedra and the Cl and Br ions in the cavity of corner-sharing Lioctahedra. Li2OHCl exhibited the structural transition from a cubic (Pm3m) to an orthorhombic (Pmc21) phase between 27 and 37 C. The transition was caused by the ordering of the Liþ-ion vacancies, and the vacancy-ordered structure was highly anisotropic and two-dimensional. On the other hand, Li2OHBr had a cubic structure with space group Pm3m, and no such structural transition was observed in the measured temperature range. The conducting properties of Li2OHCl and Li2OHBr were found to be dominantly ionic, and the electronic contributions were very low. The cubic phases of both compounds showed high Liþ-ion conductivity of 107 - 106 S/cm with activation energies of about 0.55–0.57 eV. The orthorhombic Li2OHCl phase, on the other hand, showed low Liþ-ionic conductivity because of its two-dimensional distorted crystal structure, which disturbs the Liþ-ion hopping.
by Liþ ions, as reported previously [20,21]. Consequently, the structure is highly anisotropic and two-dimensional, as shown in Fig. 3b. It is interesting to focus on the difference in structural transition behaviors between the compounds. Although the present compounds have Li-ion vacancies and contain hydroxide ions in the structures, the tolerance factor, t ¼ (rCl/Br þ rLi)/√2(rO þ rLi), for the parent (n ¼ 0) antiperovskite compounds give useful information on the structural stability. With ionic radii of Cl (0.181 nm), Br (0.196 nm), O2 (0.140 nm), and Liþ (0.068 nm), the calculated t for Li3OCl and Li3OBr are respectively 0.85 and 0.90. These results clearly indicate that the structural mismatch for the Cl antiperovskite is more significant than that for the Br compound. The Cl ion is too small to occupy the cavity of corner-sharing octahedra. To reduce this structural instability, the ordering of the Liþion vacancies and the distortion of the structure are induced by the structural transition. We then discuss the conducting properties of the synthesized compounds. The Nyquist plots of the impedance spectra for both cubic Li2OHCl (60 C) and Li2OHBr (30 C) are respectively shown in Fig. 4a and c. The impedance, Z ¼ Z0 iZ00 , shows a typical semi-circle behavior, and the total conductivity (σ) is calculated from intersections at the realpart axes. The obtained conductivities for the Cl and Br compounds are respectively 1.01 106 and 1.33 107 S/cm. The values agree well with those reported in previous papers [15,16]. The Nyquist plot for the orthorhombic Li2OHCl at room temperature is also shown in Fig. 4b, giving σ ¼ 6.83 109 S/cm. DC polarization measurements were performed to evaluate the ionic contribution to the total conductivity, and the results are shown in Fig. 5. By applying the voltage, the currents of the samples become constant due to depleting the mobile ions, and the values reached to 2.31 1011, 1.46 1011, 1.16 1011 A respectively for the cubic Li2OHCl, the orthorhombic Li2OHCl and the cubic Li2OHBr. The electronic conductivities estimated from the currents are found to be of the order of 1011 S/cm, which are very low compared to the total conductivity. When the applied voltage was set to zero after 30 min, the inverse current was observed, also confirming the ionic conduction in each sample. All the results clearly show that the electronic contribution is very low and that the ionic conduction is dominant in the present compounds. Temperature dependence of the ion conductivity was then measured, and the Arrhenius plot results are shown in Fig. 6. Both cubic Li2OHCl and Li2OHBr shows high ion conductivity at high temperatures. The conductivity decreases with decreasing temperature to about 40 C, and the linear relations in the Arrhenius plots give the activation energies Ea by using the equation σ ¼ σ0 exp(Ea/kT) [12,13]. The Ea for the cubic Li2OHCl and Li2OHBr are respectively 0.57 and 0.55 eV, which are consistent with the reported values [13,16]. Note that the conductivity of Li2OHCl significantly decreases at the structural phase transition temperature. The result strongly suggests that the ionic conductivity for the orthorhombic phase is significantly lower than that for the cubic phase. Although the present conduction measurement results cannot identify the mobile ionic species—either Liþ, Hþ, or OH—the Liþ ions would be
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Masato Goto, Zhenhong Tan, Satoshi Sugano, and Yoshihisa Kosugi in Kyoto University, Syogo Kawaguchi in SPring-8, Chun Chuang and Hwo-Shuenn Sheu in NSRRC, and Wei-Tin Chen in National Taiwan University for help in the SXRD experiments. The SXRD experiments were conducted at the Japan Synchrotron Radiation Research Institute, Japan (proposal Nos: 2018B1710 and 2019B1757) and the National Synchrotron Radiation Research Center, Taiwan (proposal Nos: Nos: 2017-1-125 and 2019-1-198). This work was partly supported by Grantsin-Aid for Scientific Research (Nos: 16H02266, 17K19177, and 19H05823) and by a grant for the Integrated Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also supported by Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks. References [1] D. Lin, Y. Liu, Y. Cui, Nat. Nanotechnol. 12 (2017) 194–206. [2] Y. Wang, W.D. Richards, S.P. Ong, L.J. Miara, J.C. Kim, Y. Mo, G. Ceder, Nat. Mater. 14 (2015) 1026–1031. [3] Y. Guo, H. Li, T. Zhai, Adv. Mater. 29 (2017) 1700007. [4] C. Eilbracht, W. Kockelmann, D. Hohlwein, H. Jacobs, Physica B 234 – 236 (1997) 48–50. [5] K. Friese, A. Hӧnnerscheid, M. Jansen, Z. Kristallogr. 218 (2003) 536–541. 4
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