A New Ceramic Sr 2 Fe 8 O 18 : Crystal Structure and Analysis of Application on Solid Electrolytes

: All-solid-state batteries have been expected to overcome the safety problem of present lithium-ion batteries including organic liquid electrolytes. The materials with high ionic conductivity are urgently needed. In this paper, we reported a new ionic crystal Sr 2 Fe 8 O 18 which can be applicated on solid electrolyte. Sr 2 Fe 8 O 18 is a typical p-type semiconductor and shows a layered monoclinic crystal structure. The resistivities of Sr 2 Fe 8 O 18 in the temperature range of 20~145°C were above 10 7 Ω•cm. The microstructure of Sr 2 Fe 8 O 18 was flaky, and the size of flaks were 1μm ~ 5μm. The E - P curve suggested that it was a ferroelectric semiconductor and had small ferroelectric effect. The dielectric response study (Cole-Cole plot) showed that Sr 2 Fe 8 O 18 had two separated relaxation time, each of which contained a group of relaxation. The ionic conductivity σ of the sample was calculated to be 0.2196×10 -4 S/cm. The conductive mechanism which confirmed by the results of First principle calculation at 300K is mainly sublattice vacancy cation diffusion with self-diffusion coefficient D of 1.794×10 -5 cm 2 /s. Fe ion has two dimensional diffusion path (x and y axial), and Sr ion has on dimensional diffusion path (x axial). The crystal structure of Sr 2 Fe 8 O 18 shows tremendous potential application on the solid electrolyte preparation.


Introduction
Current commercial lithium-ion batteries use combustible organic liquid electrolytes and thus suffer from fire risks during overcharge or abused operations, especially in large-scale applications [1][2][3]. Getting solid is the most important study point in the secondary battery research area. However, the development of solid-state batteries was has largely been hindered by the availability of solid electrolytes with fast ion conductivity. Therefore, the materials with high ionic conductivity are urgently needed for the development of solid-state secondary batteries. Up to now, solid electrolytes containing Li ion has attracted extensive research [4][5][6][7][8][9][10][11][12][13][14][15][16], while the reports about other solid electrolytes were rare. The Brownmillerite Sr2Fe2O5, which belongs to the oxygen-deficient perovskite family, has earned much attention because it has many interesting physical properties, such as being ion conducting, ferroelectric, ferromagnetic etc. [17][18][19]. Due to its special crystal structure, there were abundant research about structural characterization of oxygen-deficient perovskites [20][21][22][23][24]. The latest report about Ca2Fe0.5Ga1.5O5 by Hona et al. [25] pointed out that in oxygen deficient perovskites, the B-site cations usually form BO6, BO5, or BO4 polyhedral depending on the structure, while the A-site cations reside in spaces between the polyhedral. Changes in the A or B-site cations in oxygen-deficient perovskites can change crystal structures and lead to significant differences in electrical properties. This characteristic of oxygen-deficient perovskites indicates enormous potential in applications of sensing, solid-oxide fuel cells, and electrocatalysts.
Among all research of oxygen-deficient perovskites, materials with 1:1 atom proportion of Sr to Fe (or A-site to B-site) significantly outnumbered those with other atom proportions of Sr to Fe, such as that in Sr4Fe6O13. We are interested in finding new material based on Brownmillerite and exploring better properties. Therefore, in this paper, we reported a new compound Sr2Fe8O18 with high ionic conductivity which have an atom proportion of Sr: Fe= 1:4. The crystal material, electronic structure, and the basic electrical properties (ferroelectricity, dielectric response, and ionic conductivity) were studied. The analysis of conductive mechanism of Sr2Fe8O18 indicated that the crystal structure of Sr2Fe8O18 shows tremendous potential application on the solid electrolyte preparation.

Experimental section
The Sr2Fe8O18 ceramic was synthesized directly by pure SrCO3 (> 99.9%) and Fe2O3 (> 99.9%) powders via solid-state method. The mixed powders were pressed into pellets after 24h ball milling, and sintered at 1220°C for 3h. The phase structure were examined by X-ray diffraction (XRD) in the 2θ range of 20 -50° with a step size of 0.02° using the Bruker D8 advance at 40kV/40mA with Cu Kα radiation (λ=0.154nm). The polished samples after thermal etching were prepared and observed using a Scanning Electron Microscope (JSM-6510-LV, JEOL, Japan). The ceramic pellets for electrical characterization were polished and coated with silver electrodes. The resistivities were measured using an Keithley digital multimeter (DMM7510 7 1/2, United States) from room temperature to 435°C. The polarization-electric field hysteresis loops were measured with a Precision LC ferroelectric test system (Radiant Technologies, Northford, United States) at room temperature. EIS (Electrochemical Impedance Spectroscopy) was carried out using electrochemical workstation measurement system in the frequency range of 0.1Hz to 100kHz.

Results and Discussions
The XRD patterns of the Sr2Fe8O18 sample is shown in Fig.1 (a). The sample shows a monoclinic structure (space group: P1 21 1) with a=8.4009Å, b=8.4623 Å, c=6.4648 Å, β=111.676, and Vcell=427.09 Å 3 . The crystal structure was calculated and refined [26] with R factor of Rp=5.81, Rwp=7.39, Re=5.42, Chi 2 =1.86, and the parameter detail of crystal structure was shown in Table 1. Fig. 1 (b) ~ (e) shows the graphical results of refinement and the detail of crystal structure of Sr2Fe8O18. Fig. 1 (b) shows the graphical results of refinement. Fig. 2 (c)~(e) show the detail of crystal structure of Sr2Fe8O18. There were two molecules in one unit cell, in other words, Z=2. The whole structure composed by layered FeO6 and the FeO6 was a distorted oxygen octahedrons. Every four FeO6 was a repetitive unit, where three of them were connected by edge and one by angle. Sr2Fe8O18 has excess oxygen ions, therefore, it has many cation vacancy, shows character of p-type semiconductor. Those cation vacancy provides the possibility for sublattice vacancy cation diffusion which shows ionic conduction macroscopically. Furthermore, the geometry of crystal structure had been optimization by First principle calculation. Fig. 2 shows the relative results of First principle calculation [27][28][29][30] and the resistivities of Sr2Fe8O18 in the temperature range of room temperature to 435°C. Fig. 2 (a) shows the energy optimizing results during the last time of geometry optimization. The geometry optimization was taken twice. The plane wave basis set cut-off of the first time was 340eV with the function of GGA by Perdew, Burke and Ernzerhof [31], and the energy decreased from about -16380eV to -16440eV . The plane wave basis set cut-off of the second time was 340eV with the function of GGA by Perdew-Wang 1991 (PW91) [32], and the energy decreased from about -16440eV to -16470eV. Fig. 2 (b) ~ (e) shows the final crystal structure after geometry optimization. The whole structure is composed by FeO4 which is connected by angle and SrO6 which is connected by edge. Every three FeO4 that are connected by O1 is a repeat unit, and every repeat unit is connected by O5. Fig. 2 (f) shows the resistivity of Sr2Fe8O18 in the temperature range of room temperature to 435°C. It was the typical resistivity characteristic of semiconductor, and the relative relationship between resistivity and temperature was: where ρ0 is the resistivity in room temperature; ρ is the resistivity; T is the temperature; α is the temperature coefficient of resistivity, the α could be calculated. In this study, the α of Sr2Fe8O18 was -0.02334. The insulating property is important to solid electrolyte. Therefore, the resistivities in the temperature range of 20~145°C are shown in the inset of Fig. 2 (f). The resistivities of Sr2Fe8O18 in the temperature range of 20~145°C were from 1.5556 × 10 8 to 1.1227× 10 7 Ω•cm, respectively. Fig. 3 shows the SEM photograph of Sr2Fe8O18 sample. In the first sight, the Sr2Fe8O18 ceramic composed by granule microstructure. But take a good look at the photograph, every granule was actually layered.
Therefore, it also can be seen form the zoom-in picture on the right side, the microstructure of Sr2Fe8O18 was flaky, and the size of flaks were 1μm ~ 5μm. In view of this feature, Sr2Fe8O18 might have the potential applications in the area of energy storage and conversion [33][34][35][36]. where c is the capacitance; V is the bias voltage, QF which is here can be expressed as polarization P (stands for the polarization intensity), is caused by ferroelectric effect, and is the polarized charge quantity nonlinear to V; R is the resistance of the conductive layer, ΔV/Δt is the scan velocity of V, A is the scan speed of voltage, and QT is the total charge quantity. If QF is negligible, in other words, the material don't have obvious ferroelectric effect, the QT -E curve will be parabolic, like the Fig. 1 (c ) in the ref. 37. Therefore, the E-P curve of Sr2Fe8O18 suggested that it was a ferroelectric semiconductor which do showed the ferroelectric effect but was smaller than the traditional one. The remanent polarization Pr was around 0.1737μC/cm 2 , and the coercive field Ec was around 1.9309 kV/cm. The dielectric spectroscopy shows the dynamical dielectric, which is important for discussing the physical properties of ionic crystal material at the molecular level. Fig. 4 (b) and (c) show the dielectric response of frequency and the Cole-Cole plot [38] of Sr2Fe8O18, respectively. Fig. 46 (a) shows the dielectric response of frequency (form 40Hz to 110MHz) of Sr2Fe8O18. The pink area is the relaxation rang of Sr2Fe8O18. The ε ′ (the real part of the complex dielectric constant) decrease with increase of frequency, and the ε ″ (the imaginary part of the complex dielectric constant) first increase and then decrease with increase of frequency. Thus, the ε ″ has the maximum value, and at this point of frequency, there is a following relationship: where ω is the frequency; τ is the relaxation time; εs is the shunt dielectric constant; ε∞ is the dielectric constant at infinity frequency. Therefore, the relaxation time could be calculated via this relationship, and the value of relaxation time τ was 0.6007×10 -7 . Fig. 4 (b) shows the Cole-Cole plot from the data of Fig.   4 (a). It can be seen that Sr2Fe8O18 had two separated relaxation time, and each one contained a group of relaxation. As previous report, the first group relaxation times were from grain, and the second group were from grain boundary.  Fig. 5 (a) shows the Nyquist plot and the fitting curve with the equivalent circuit shown in Fig. 5 (b). According to the test method described at experiment, the equivalent circuit could be shown as Fig. 5(b). Rb is the resistance of solid electrolyte; Voigt component Where v(0) and v(t) are the initial particle velocity and at time t, respectively. 〈 ( ) • (0)〉 is the value of VACF. Therefore, the cation self-diffusion coefficient of Sr2Fe8O18 can be calculated as 1.794×10 -5 cm 2 /s. Fig. 6

Conclusions
In summary, a new oxygen-deficient perovskite compound Sr2Fe8O18 which can be a promising solid electrolyte, its microstructure and its physical properties were reported for the first time.  Fig. 1 (a) The XRD pattern of the Sr2Fe8O18 sample. Fig. 1