The Ionic Conduction Properties of Na3OCl with O2-/Cl- Non-Stoichiometry
Authors: Ishigami Kana , Miyazaki Reona , Hihara Takehiko 
Published: December 2024

Background
Na-rich antiperovskites (APs), such as Na3OBH4 and Na3OBr have focused much attention as the candidate for the solid-state Na+ conductors because of their high ionic conductivities, facile composition tuning and electrochemical stability [1]. However, the experimental studies on Na+ conduction in the Na-rich APs are rather scarce compared to a number of theoretical studies such as first principle simulation and molecular dynamics simulation [2]. The preparation of pure Na2O which is the raw material for Na-rich APs is the one of the bottlenecks for the synthesis [3]. The facile purification method for high purity Na2O has been reported by R. Miyazaki et al. [4]. We reported that the ionic conductivity of Br-rich sample (Na2.95O0.95Br1.05) was higher than the stoichiometric Na3OBr [4]. The Na+ vacancies are introduced as the charge compensation by the replacement of O2- with Br-, which can be the origin for the increase in the ionic conductivity [4]. In this study, Na3OCl was prepared using pure Na2O. The relationship between the introduction of excess Cl- ions and the ionic conductivity was investigated.
Experimental
All experiments were conducted in the Ar-filled glove box in which the O2 concentration was maintained at approximately 2 ppm. For preparation Na3OCl, pure-Na2O prepared by following the previous method [4] and NaCl (99.95%, Wako) were mixed in a molar ratio of 1:1 and ball-milled using ZrO2 jar with 10 pieces of balls (φ10 mm) at 600 rpm for 24 h. The ball-milled powder was pelletized to ca. 1 mm thickness using a die with 10mm in diameter at approximately 270 MPa for 30 s. The pellet was sintered at 200-400 ℃ in the electric furnace set in the glove box. Additionally, Cl--rich Na3OCl (Na3-x
O1-x
Cl1+x
: x=0.5, 1) was prepared using excess NaCl by similar procedures above. In this study, Na2.95O0.95Cl1.05 and Na2.9O0.9Cl1.1 are abbreviated as Cl 5 mol% and Cl 10 mol%, respectively. XRD measurement was conducted for all prepared samples to confirm the crystal structure and lattice parameter. Ionic conductivity was measured for the pellets prepared by the uniaxial pressing and the sintering. Stainless-steels were set on both sides of the pellet. The impedance was measured in the frequency range between 1 MHz and 1 Hz. The measurement temperature was between 130 ℃ and 250 ℃ at the interval of 10 ℃. The cross-section of the pellets was observed by FIB-SEM. The atomic distribution of the pellet cross section was analyzed by EDS.
Results and discussions
Figure (a) shows the XRD patterns of stoichiometric Na3OCl, Cl 5 mol% and Cl 10 mol%. All diffraction peaks were indexed by the AP structure with space group of Pm-3m. It is shown that Na-rich AP with excess Cl- was successfully synthesized by ball-milling. Figure (b) shows the relationship between the lattice parameters of ball-milled samples and excess Cl- concentration in Na3OCl. The lattice expansion was observed for Cl 5 mol%. On the other hand, further expansion was not observed for Cl 10 mol%, indicating the solubility limit of Cl. It is considered that the lattice expansion was caused by the replacement of O2- with Cl- because the ionic size of Cl- (1.81 Å) is larger than that of O2- (1.40 Å) [5]. Figure (c) shows the ionic conductivities of the sintered pellets of stoichiometric Na3OCl, Cl 5 mol% and Cl 10 mol%. The ionic conductivity of Cl 5 mol% was the highest in the three samples (2.99×10-6 Scm-1 at 240℃). It is considered that the highest value was obtained due to the introduction Na+ vacancies by the replacement of O2- ions with Cl- ions. On the other hand, the ionic conductivity of Cl 10 mol% was 4.98×10-7 Scm-1 at 240℃. From the XRD pattern, the unreacted NaCl marked as black circles in the figure was observed for Cl 10 mol%. This residual NaCl can partially explain the decrease in the conductivity of Cl 10 mol%.
References
[1] J. A. Dawson, et al., J. Mater. Chem. A, 2021, 9, 18746
[2] B. A. Goldman, et al., J. Mater.
Chem. A, 2022, 10, 2249-2255
[3] L. Gao, et al., J. Mater. Chem. A, 2020, 8, 21265-21272
[4] R. Miyazaki, et al, J. Mater. Chem. A, 2023, 11, 25734-25742
[5] R. D. Shannon, Acta Cryst. 1976, A32, 751-767
Acknowledgement
This work was supported by JSPS KAKENHI Grant Numbers 23K23447 and the NAGAI Foundation for Science and Technology.

Figure 1