Effect of Chromium on electrochemical and mechanical properties of beta-Al2O3 solid electrolyte

In order to improve the performance of the beta-Al2O3 solid electrolyte, the influence of Cr doping during the preparation of Na-beta-Al2O3 by solid state method was studied, the α-Al2O3 and Na2C2O4 was used as raw materials, and Li+ was added to stabilize β″ phase. The results showed that without Cr2O3 doping, the β″ phase content of the electrolyte was 92.28%, and the relative density was 99.07% of the theoretical density. The appropriate amount of Cr element could stabilize the β″ phase and refine the grain, when the doping amount of Cr2O3 was 0.15 wt%, the content of β″ phase could reach 97.93%, the relative density was 99.07%, the ionic conductivity was 0.082 S · cm−1 at 350 °C, and the bending strength was 253 Mpa. In addition, the excessive Cr element tends to gather in the grain boundary, resulting in the melting state and non-uniform size of the grains, and it will also hinder the formation of the β″ phase.


Introduction
Beta alumina ceramics are well-known fast-ionic conductor of Na + and consist of two similar crystalline phases of β and β″-Al 2 O 3 [1][2][3]. Sodium-sulfur battery has strict requirements for the composition, density and ionic conductivity of electrolyte due to its high operating temperature of 350°C [4][5][6][7][8]. More and more researchers begin to focus on the preparation process of beta-Al 2 O 3 ceramics. Usually, the performances of electrolyte can be improved by studying the sintering process, raw materials, and element doping [9,10]. For example, the timetemperature curve must be strictly controlled during the sintering process to avoid excessive growth of grain, and intermittent batch sintering is currently the most common sintering process [11]. α-Al 2 O 3 and other hydrated alumina groups are commonly used as aluminum based raw materials at present, and the selection of raw materials is an important factor to determine the production process, cost and quality of BASE [12]. In addition, the structure and properties of BASE can be ameliorated by adding specific elements. For example, the Li + or Mg 2+ was usually added in BASE to stabilize β″ phase and increase its content [13]. Zhao et al [14] studied the co-doping of ZrO 2 and TiO 2 into Mg 2+ stabilized Na-β″-Al 2 O 3 , and the results indicated that the density and bending strength of Na-β″-Al 2 O 3 could be significantly improved.
In many reports, the addition of Cr in the ceramics material could optimize the electrochemical properties and improve the compactness and morphology. For example, Ma et al [15]  Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In this paper, Li + stabilized Na-beta-Al 2 O 3 with Cr doped was obtained by solid state method. The structures of beta-Al 2 O 3 was characterized and analyzed by XRD and SEM. The density, ionic conductivity and bending strength of obtained ceramics had been ameliorated by the doping.

Experimental
2.1. Synthesis α-Al 2 O 3 (model VK-L30) and Na 2 C 2 O 4 (AR) were mixed according to the molar ratio of Al 2 O 3 :Na 2 O=5:1. Li + was used to stabilize β″ phase, and Li 2 CO 3 was added according to the mass fraction of Na 2 O:10.76 wt% and Li 2 O:0.7 wt%, and then a quantitative amount of Cr were added (0-1.5 wt%, table 1). The mixed powders were ball-milled for 10 h, and calcined at 1250°C for 2 h after dried to obtained precursor powder of beta-Al 2 O 3 . And then 5 wt% polyvinyl alcohol aqueous solution (concentration of 5%) was mixed with precursor powders after 10 h ball-milled. The mixed powders were pressed with the mold of Φ16 mm×2 mm (D) and 3 mm (D)×4 mm (W)×40 mm (L). The sample pieces were sintered at 1600°C for 10 min, then cooled to 1450°C and held for 2 h to obtain more β″ phase.

Characterization
The thermal decomposition process was studied by differential scanning calorimeter (204/1/F) and thermogravimetric (STA449C) in the air. The analysis process was heating to 1400°C at a rate of 10°C min −1 .
Where I β was the the peak intensities at 44.50°and I β″ was that at 45.90°. The three-point bending testing with electro-mechanical universal testing machine (CMT5254) measurements of the sinters were carried out to evaluated the bending strength (3×4×40 mm), and the loading rate of indenter was 0.05 mm min −1 . And the density (D b ) of electrolytes was determined by Archimedean principle.
The ionic conductivitiy of the electrolytes were tested by frequency response analyzer (Solartron 1260) and electrochemical interface (SI 1287) from 50°C to 400°C with 50°C increments. The impedance spectra were recorded at the frequency range of 0.1 Hz-10 MHz with signal amplitude of 20 mV.

Results and discussion
3.1. Thermogravimetric analysis Figure 1 was the TG/DSC curves of the mixed powders with 0 wt% Cr 2 O 3 and 0.15 wt% Cr 2 O 3 . In figure 1(a), there was an endothermic peak near 120°C and the mass loss was 1.3 wt%, which was the evaporation of the alcohol in the mixture. The mass loss was about 5.10 wt% and there was an obvious exothermic peak during 200°C and 530°C. This was the decomposition of Na 2 C 2 O 4 (Na 2 C 2 O 4 →Na 2 CO 3 +CO). The theoretical mass loss of Na 2 C 2 O 4 was 4.88 wt%. There was an obvious endothermic peak between 530°C and 830°C, it was the decomposition process of Na 2 CO 3 (Na 2 CO 3 →Na 2 O+CO 2 ) and Li 2 CO 3 (Li 2 CO 3 →Li 2 O+CO 2 ). The theoretical mass loss was about 7.45 wt%, while the actual mass loss was 7.22 wt%. After 800°C, the quality change was unconspicuous, but the DSC curve presented an exothermic trend. Accroding to the Na 2 O-Al 2 O 3 binary phase diagrams [20], it was caused by the formation of NaAlO 2 (Al 2 O 3 +Na 2 O→NaAlO 2 ), and with the increase of temperature, there would be the process of the formation of β phase and the transformation of β″ phase. The TG/DSC curve in figure 1(b) was sinmilar to that in figure 1(a), which indicated that Cr 2 O 3 doping doesn't have a significant effect on the reaction of the raw material and the phase transition. Referring to the literature [9, 10], 1250°C was set as the calcination temperature of precursor powders.    morphology of the sinters changed to the molten state, which inferred that the molten state impeded the formation of the β″-Al 2 O 3 phases. Figure 5 was the cross section microstructure of the electrolytes with different doping amounts of Cr 2 O 3 . There were many pores in the structure of the undoped sample and the particles were nonuniform. The particles became denser and more uniform by doping appropriate content of Cr 2 O 3 . When the content of Cr 2 O 3 was 0.01-0.15 wt%, the microstructure was mainly composed of lath-like grains, and the size of particles and pores decreased with the increase of Cr 2 O 3 . When the content of Cr 2 O 3 was 0.25 wt%, the particle size increased slightly. When the content of Cr 2 O 3 was 0.5 wt%, the microstructures of the sample were mainly composed of cone-like and rod-like grains, and the amount of the pores was also increased. At the same time, the surface of   the cone-like grains appeared the molten state. When the content of Cr 2 O 3 was 1-1.5 wt%, all the particles of the sample presented the molten state, the microstructure was composed of long rod-like grains, and the distributions of the grains appeared disorderly and the particle sizes were not uniform, and the sizes of the pores became larger and the quantities increased. When the Cr 2 O 3 content was no more than 0.15 wt%, the nucleation of the crystal became easier with the increase of Cr 2 O 3 content. When the Cr 2 O 3 content was more than 0.15 wt%, with the increase of Cr 2 O 3 content, the Cr 3+ tended to gather in the grain boundary or the vicinity of the grain boundary to lead the size of the grain to become uneven and promote the grains to become molten state [15,17]. It inhibited the growth of grains and reduced the densities of the sintering samples. Figure 6 were the density and the relative density of electrolytes with different doping amounts of Cr 2 O 3 . The relative density of the undoped sample was 96% (the measured density was 3.1129 g cm −3 ). The relative density of the sinter reached the maximum of 99.07% (3.2101 g cm −3 ) when the Cr 2 O 3 content was 0.15 wt%. Considering the microstructure of the beta-Al 2 O 3 solid electrolytes, it was found that the homogenization of grains could make the microstructure compact and increase the bulk density. Therefore, an appropriate content of Cr 2 O 3 could improve the β″-Al 2 O 3 phase contents and increase the bulk densities of the sinters. Because the size of the particles and pores could increase by the excessive Cr 2 O 3 doping, the bulk densities of the samples decreased.  Figure 8 was the AC impedance spectra of electrolytes at 350°C with different doping contents of Cr 2 O 3 . Figure 9 was the equivalent circuit showed the beta-Al 2 O 3 solid electrolytes contacting with the silver electrode. R g was the resistance of grains, R gb was the resistance of grain boundary, CPE gb was the capacitance of grain boundary and CPE ct was the capacitance of interface.   Cr 2 O 3 doping would make the grains grow disorderly and the size of particles and pores become larger to reduce the conductivity of the samples. Figure 10 was the Arrhenius plot of electrolytes with different doping amounts of Cr 2 O 3 , the relationships between conductivity and temperature were well fitted by Arrhenius equation.

Ionic conductivity analysis
In the above equation (2), A was Arrhenius constant, Ea was the activation energy, R was the gas constant, T was the thermodynamic temperature.
The quantitative relationship between the activation energy and the conductivity was defined by Arrhenius equation. The activation energy of the sinters was determined by the slope of the Arrhenius plot and listed at table 2. The low value of Ea indicated that Na + was easier to migrate. When the Cr 2 O 3 content was 0.15 wt%, the activition energy of the sinter reach the minmum of 0.148 eV.

Conclusions
In this paper, the influence of Cr doping with different content during the preparation of Na-beta-Al 2 O 3 by solid state method was studied. The results showed that a small amount Cr could optimize the electrochemical and mechanical properties of the ceramics material. However, the excessive Cr would lead the grains to molten state  and make the pores become larger to deteriorate the ionic conductivity. When the content of Cr 2 O 3 doping of the beta-Al 2 O 3 solid electrolyte was 0.15 wt%, the β″-Al 2 O 3 phase content was 97.93%, the microstructure was uniform and compact, the relative density was 99.07% of the theoretical density, the bending strength was up to 253 MPa and the ionic conductivity could reach 0.082 S cm −1 at 350°C.