Crystal structures and ionic conductivity in Li2OHX (X = Cl, Br) antiperovskites☆
Graphical abstract
Ionic conductivity for Li2OHCl and Li2OHBr. The cubic phases show dominant high Li+-ion conductivity, while the orthorhombic phase of Li2OHCl exhibited reduced ionic conductivity.
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], [2], [3]]. The exploration of novel solid electrolyte materials with high Li-ion conductivity has thus been accelerated recently. Li3−nOHnX (n = 0–1, X = 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], [5], [6], [7], [8], [9], [10], [11]].
Li-containing antiperovskite-structure n = 0 compounds, Li3OCl and Li3O(Cl0.5Br0.5), were first reported to show high conductivity (>10−3 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 (10−8–10−7 S/cm at room temperature) with increased activation energies were then confirmed in Li3OCl (n = 0) [13,14].
On the other hand, Li2OHCl and Li2OHBr (n = 1) have been reported to be rather stable antiperovskite-structure compounds that show high ionic conductivity. Li+-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], [16], [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.
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 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.
Section snippets
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 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
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 Li+ sites in the octahedra and the Cl− and Br− ions in the cavity of corner-sharing Li-octahedra. Li2OHCl exhibited the structural transition from a cubic () to an orthorhombic (Pmc21) phase between 27 and 37 °C. The transition
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
References (21)
- et al.
Physica B
(1997) - et al.
Solid State Ionics
(2016) - et al.
Nat. Nanotechnol.
(2017) - et al.
Nat. Mater.
(2015) - et al.
Adv. Mater.
(2017) - et al.
Z. Kristallogr.
(2003) - et al.
Chem. Mater.
(2013) - et al.
Phys. Chem. Chem. Phys.
(2015) - et al.
J. Mater. Chem. A
(2016) - et al.
Adv. Sci.
(2016)
Cited by (28)
Elucidating the effects of −OH content on phase transition and Li-ion transport of anti-perovskite solid electrolytes
2024, Electrochemistry CommunicationsAdjustable antiperovskite ZnCCe<inf>3−x</inf>Ni<inf>x</inf>/CNFs as an efficient bifunctional Zn-air batteries electrocatalyst
2024, Journal of Alloys and CompoundsLithium mobility along conduction channels of ceramic LiTa<inf>2</inf>PO<inf>8</inf>
2023, Journal of the European Ceramic SocietySolid-state electrolytes for lithium-ion batteries
2023, Comprehensive Inorganic Chemistry III, Third Edition
- ☆
Dedicated to the occasion of the 70th birthday of Prof. Kenneth Poeppelmeier