Synthesis and Conductivity Characterization of Anti-Perovskite Na3OX Solid Electrolytes for All Solid Na-Ion Batteries ()
1. Introduction
There have been considerable attentions on secondary ion batteries due to the industry developments on computers, smartphones, and electric vehicles (EV). Lithium-ion batteries (LIB) are now the main product using in wide area [1] [2] . However, with the cost increase of LIB as well as energy resource risk and environmental problems, sodium-ion batteries (SIB) have been attracting much attention as an alternative energy storage system in recent years [1] [2] [3] . In the research on battery with higher voltage, higher capacity and stable cycle characteristics, synthesizing various materials such as electrodes and electrolytes were important topics [4] [5] [6] [7] [8] . Since most of the commercial batteries, for both LIB and SIB, use a liquid electrolyte (LE), a solid electrolyte (SE) is necessary for achieving all solid state batteries [9] [10] [11] [12] . Comparing to LE, SE is electrically and chemically stable and has the advantage of being extremely safe because it can be charged quickly and is nonflammable. Comparing with lithium (Li), sodium (Na) has its price advantage in high volume demand for high-density and large-capacity batteries with the widespread use of EV [13] [14] . Therefore, it is urgent to improve the performance of Na-SE material (application requirement: σ > 10−3 S/cm at room temperature) [15] [16] , which is relatively low at present. Thus, this work is aimed on changing the composition of halogen elements to introduce additional lattice distortion and lower the ionic conduction barriers.
Compared with sorts of SE materials, Na-rich anti-perovskite SE has its unique advantages of loosening the bottleneck and increasing the ion conductivity by increasing a lattice distortion [17] [18] [19] [20] . In a Na-rich anti-perovskite structure, theoretical calculations indicated that Na3OX (X = Cl, Br, I) materials give a relatively lower activation energy and lead to a higher conductivity, as shown in Figure 1 [21] [22] [23] .
This study aims to clarify the relationship between the structure of SE Na3OX (X = Br, I), where changing halogen elements and their ratios, and then to improve their conductivity based on results of a structural investigation. We first synthesized the anti-perovskite type Na3OX (X = Br, I) electrolytes. The ionic conductivity measurements indicated that the conductivity of SE Na3OX has been improved by introducing lattice distortion. A relatively higher ionic conductivity of σ = 1.55 × 10−7 S/cm at 60˚C has been obtained for Na3OBr0.6I0.4, which is about three orders higher than that of Na3OBr in literature [24] .
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Figure 1. Crystal structure of an anti-perovskite Na3OX (X = Cl, Br, I) lattice.
2. Experimental Methods
We added different ratio of X (X = Br, I) ions into sodium oxide to make high-purity synthetic Na3OX crystal, then using X-ray diffraction (XRD) measurements to identify their crystal structure and material purity. And we used the Williamson-Hall analysis to evaluate the lattice distortion of the fabricated samples. Impedance measurements were carried out to analyze the ion conductivity activation energy. Scan electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to measure the surface morphology and ion distribution of the sample.
(1)
The Na3OX (X = Br, I) SE materials were synthesized by above mentioned reactions. First, NaX (X = Br, I) materials were crushed and mixed in a glove box filled with N2. Then quickly grind sodium oxide (0.8Na2O + 0.2Na2O2) was added with the above halogen mixture. After mixing, it was filled in an alumina crucible and heated for 6 hours at 330˚C. Then, to promote the reaction, the obtained sintered body was re-crushed and reheated to obtain a pure reacting material.
XRD measurements were performed at room temperature using a system Altima IV manufactured by Rigaku, in the range of about 2θ = 30˚ to 110˚ in steps of 0.01˚. The ionic conductivity measurements were carried out with the equipment provided by JFE Techno Research. The crushed sample powder was placed in a measurement cell (inner diameter 10 mm) and pressed in an Ar glove box at a pressure of 2.0 tons (250 MPa) for 15 minutes to form pellets with a thickness of about 0.5 mm. The AC impedance characteristic was carried out by the sandwiched two-terminal method using a blocking electrode and evaluated with a device Bio Logic VSP. The measuring temperatures were changed from 20˚C to 60˚C in a running range of 1 to 50 MHz. The whole measurements were carried out in the same glove box to avoid the deoxidization of the samples. The Na3OX (X = Br0.8I0.2, Br0.7I0.3, and Br0.6I0.4) samples were measured. The Na3OBr sample was highly insulated and could not give a good result.
3. Results and Discussion
3.1. Results
Figure 2 shows the XRD measurement results of the Na3OX samples (5 compositions in total, Na3OBr, Na3OBr0.9I0.1, Na3OBr0.8I0.2, Na3OBr0.7I0.3, and Na3OBr0.6I0.4). It was confirmed that all compositions up to Br1−xIx (x = 0 to 0.4) had the anti-perovskite structure with Pm-3m cubic symmetry. It is also demonstrated that the impurity peaks could hardly be confirmed, and the weight of the precursor substance with respect to Na3OX (X = Br, I) was estimated by using the crystal structure analysis software PDXL2 based on the crystallographic information framework (CIF) and data obtained from the results of the Rietveld analysis [25] , which will be described later. These results are different from the XRD pattern reported by Wang et al. [13] , in which a remarkable peak split was observed when compared with the measurement results of our Br1−xIx (x = 0 to 0.4) eutectics. Such a peak split was attributed to be a result of vigorous segregation of two types of halogen ions because the precursor samples were not sufficiently mixed. In our sample, only when x = 0.5 was exceeded, peak split and impurity precipitation became not negligible. It is considered that segregation occurred in this case because I (iodine) exceeded the solid solution limit, which might be in the range of x = 0.4 to 0.5.
Rietveld analysis was performed for each composition using the structural analysis software EXPO2014 [26] . Currently, the only CIF data presented in major databases are X = Cl, Br, I, and Cl0.5Br0.5, all of which are cubic perovskites belonging to the symmetry Pm-3m. In addition, from the waveform pattern in Figure 2, it can be inferred that the symmetry of the X = Br1−xIx eutectics, which is currently unknown, belongs to same cubic symmetry up to x = 0.4. Therefore, for the synthesized Na3OBr1−xIx eutectics, Rietveld analysis was performed from x = 0 to 0.4, but all fittings were performed assuming cubic symmetry Pm-3m. As a result, the parameters of each composition were refined, and a CIF file containing the information was obtained. According to the fitting results of Na3OBr and Na3OBr1−xIx (x = 0 to 0.5), it was confirmed for the first time that all compositions of Br1−xIx (x = 0 to 0.4) had the anti-perovskite structure with Pm-3m cubic symmetry.
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Figure 2. X-ray diffraction patterns of Na3OBr1−xIx (x = 0 to 0.4).
From the above-mentioned analysis, the crystal structure for each composition could be identified, so the counter value width was extracted for each XRD peak and Williamson-Hall (WH) plots were obtained, as shown in Figure 3. The half width component has been corrected by the correction formula obtained from the standard sample. The slope of the WH plots exhibits a lattice distortion
, which represents a relative change of the lattice constant (∆d/d). It is clear from Figure 3 that the introduction of significant strain was successfully achieved in the Br1−xIx eutectics, which gave rise to a maximum distortion at near x = 0.2. The relative distortions comparing with Na3OBr were
= 2.3, 5.5, 4.2, and 3.4 × 10−3, for Na3OBr1−xIx (x = 0.1, 0.2, 0.3, and 0.4) eutectics, respectively. These distortions might lower the activation energy (ΔEa) by loosening the bottleneck and increase the ion conductivity in the synthesized Na3OBr1−xIx eutectics. The Na3OBr0.5I0.5 sample was not the anti-perovskite crystal structure.
Figure 4 shows the impedance results, the Cole-plots, for three Na3OBr1−xIx (x = 0.2, 0.3, and 0.4) samples measured at 60˚C. A relatively high ionic conductivity of σ = 1.55 × 10−7 S/cm was obtained from Na3OBr0.6I0.4, as shown in Figure 4. The ionic conductivities of all samples were obtained by fitting the Cole-plots of the impedance measurements and shown in Figure 5. The measurement temperature was changed from 20˚C to 60˚C. Since the conductive type of SE follows the Arrhenius type, the slope of the straight line corresponds to the conductive activation energy. It is clear that the ionic conductivity of our anti-perovskite SE Na3OBr0.6I0.4 is three orders higher than that of Na3OBr in literature [24] . This high conductivity is considered to come from the lattice distortion caused by doping I (iodine) ions. In the impedance measurements, the resistance values of the grain boundaries and bulk had been separated using the equivalent circuit fitting software pyZwx developed by NIMS [27] . So the fitting results shown in Figure 4 are relatively credible.
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Figure 3. Williamson-Hall plots of Na3OBr1−xIx (x = 0 - 0.4).
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Figure 4. Cole-plots of Na3OBr1−xIx (x = 0.2 - 0.3) at 60˚C.
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Figure 5. Arrhenius plot of ionic conductivity (σ) versus 1000/T of Na3OBr1−xIx (x = 0 - 0.4).
Furthermore, the activity energy of ionic conductivity, ΔEa, was determined by fitting the Arrhenius plots in Figure 5. As claimed in literature [24] , ΔEa for Na3OBr was 1.14 eV, which decreased to 0.72, 0.79, and 0.64 eV for our Na3OBr1−xIx (x = 0.2, 0.3, and 0.4), respectively. Although the activation energy did not completely decrease with the distortion increase, the bromine-iodine (B-I) mixture gave rise to the high conductivity performance by lowering the activity energy, which will be discussed below.
Since the B-I mixture played an important role in increasing ionic conductivity, the ratio of B/I and their distribution should be taken into account in the synthesis process. Figure 6 shows the surface morphology and ion distribution of Na3OX crystals measured by SEM-EDS (Ultim Max, Oxford Instruments). Compared with pure Na3OBr, the Br-I mixture Na3OX got a broader eutectic. However, there was still obviously visible bromide crystal in the Br-I mixture Na3OX surface, which might due to a lack of enough reaction time.
3.2. Discussions
A significant improvement in conductivity was observed with the Na3OBr1−xIx eutectic compared to Na3OBr. However, the conductivity of the Na3OBr0.6I0.4 eutectic was significantly superior to that of the Na3OBr0.8I0.2 eutectic, even which had the largest strain. Assuming strain introduction optimizing the bottleneck as intended and destabilizing the stronger Na-O bond, a reduction in ion transfer energy and defect formation energy should be confirmed. Since the conduction activation energy is the sum of the ion transfer energy and the defect formation energy, it is considered that the conductivity reduction can be evaluated by comparing with the strain and the conductivity activation energy. It had been demonstrated that the Br-I mixture introduced additional lattice distortion in the eutectic, which led to a large reduction of the conductive activation energy from 1.14 to 0.64 eV. But the reduction of the activation energy was not completely following with the distortion increase. The reason of a rough relationship between the activation energy and the distortion was thought to be an effect of the polarizability of the introduced X ions (X: Br = 4.16, I = 6.43 [10−24 cm3]) [28] [29] . When the ligand ions constituting the bottleneck are highly polar in the case of Na ion diffusing, they are flexibly deformed even if the bottleneck radius is narrow, and the Na ions could push away the ligands so that they could
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Figure 6. SEM images and EDS pictures of Na3OBr1−xIx (x = 0, 0.2, and 0.4) samples.
diffuse with low energy. From these facts, it is considered that finding a compositional balance that maximizes the contribution of both strain and polarity is an effective way to optimize the effect of improving the conductivity in the Na3OX eutectic. It is also considered that the impedance measurement result itself changes significantly due to the influence of the surrounding environment and the pellet preparation method, which will be published next.
4. Conclusion
This research succeeded in synthesizing Na3OBr1−xIx (x = 0 to 0.4) with high purity, and confirmed that they have Pm-3m cubic symmetry. In addition, it was confirmed that a large crystal strain was introduced by B-I mixture eutecticization, which reduced the ion conduction activation energy, leading to the improvement of the ionic conductivity. The conductivity was maximum at σ = 1.55 × 10−7 S/cm (60˚C) for Na3OBr0.6I0.4. A significant improvement up to several orders was achieved compared with Na3OBr. The process optimization and high temperature measurements are in progress, which will be published later.
Acknowledgements
We would like to show our gratitude to the Dr. Guan Sujun for sharing his pearls of wisdom on the research direction, and his kindly comments and help during lab experiment that greatly improved the manuscript.