High Lithium-Ion-Conducting NASICON-Type Li1+xAlxGeyTi2−x−y(PO4)3 Solid Electrolyte

A water-stable solid electrolyte is a key material without which aqueous lithium-air batteries could not be operated. In this study, we have examined the electrical conductivity and mechanical properties of a water-stable lithium ion conducting solid electrolyte, Li1+xAlxGeyTi2-x-y(PO4)3 with the NASICON-type structure, as a function of the Al and Ge content. Li1+xAlxGeyTi2-x-y(PO4)3 was synthesized by the conventional solid-state reaction method. The highest lithium-ion conductivity of 1.0×10-3 S cm-1 at 25 °C and the highest three-point bending strength of 90 N mm-2 at room temperature were observed for a pellet of Li1.45Al0.45Ge0.2Ti1.35(PO4)3 sintered at 900 °C.

Since the report by Aono et al., the NASICON-type lithium-ion-conducting solid electrolytes have been extensively examined. The highest electrical conductivity of 4.62 × 10 −3 S cm −1 at 27°C was reported for the Li1.5Al0.5Ge1.5(PO4)3 glass-ceramic by Thokchom and Kumar (2010). However, Fu (1997) studied the Li1−xAlxGe2−x(PO4)3 glass-ceramics and found the electrical conductivity of Li1.5Al0.5Ge1.5(PO4)3 was 4.0 × 10 −4 S cm −1 at 25°C, and also Xu et al. (2007) found an electrical conductivity of 7.25 × 10 −4 S cm −1 at room temperature for the Li1.5Al0.5Ge1.5(PO4)3-0.05 Li2O glass-ceramic. The preparation of glass-ceramics is somewhat complex, and the effect of aging on the electrical conductivity is questionable. The Li1+x+yAlx(Ge, Ti)2−xSiyP4−yO12 glass-ceramics have been commercialized by Ohara, Ltd., Japan. The glass-ceramic is water-permeation free, and the electrical conductivity is 10 −4 S cm −1 at room temperature. Recently, Zhang et al. (2013) reported that the electrical conductivity of Li1.4Al0.4Ti1.6(PO4)3 was enhanced by a partial substitution of Ge for Ti. The highest electrical conductivity of 1.3 × 10 −3 S cm −1 at 25°C was observed in Li1.4Al0.4Ge0.2Ti1.4(PO4)3, where the content of Al was fixed to 0.4, and the Li1.4Al0.4GexTi1.6−x(PO4)3 powders were prepared by a sol-gel method using expensive Ti and Ge alkoxides. In this study, we have examined the electrical conductivity and mechanical properties of the Li1+xAlxGeyTi2−x−y(PO4)3 system in the range of x = 0.30-0.55 and y = 0.1-0.3 using less expensive starting materials. Water-stable high lithium-ionconducting solid electrolytes have potential applications for aqueous lithium-air batteries (Zhang et al., 2010;Bruce et al., 2012) and aqueous lithium batteries with aqueous cathodes (Lu et al., 2011;Zhao et al., 2013). These electrolytes have been used as a protective layer for the lithium metal electrode to avoid direct contact with the aqueous solution, because lithium metal reacts vigorously with water.

MaTerials anD MeThODs
The NASICON-type Li1+xAlxGeyTi2−x−y(PO4)3 lithium-ionconducting solid electrolytes were prepared by conventional solid-state reaction. Corresponding amounts of chemical reagent grade Li2CO3, TiO2, GeO2, Al2O3, and NH4H2PO4 were ball milled with zirconia balls in a zirconia vessel for 2 h at 400 rpm using high energy mechanical milling (HEMM) with a planetary micro mill (Fritsch Pulverisette 7), and the mixed powders were then pressed into pellets at 150 MPa and calcined at 600°C for 4 h. The calcined pellets were reground and again ball milled using HEMM. The obtained powders were isostatically pressed into pellets at 150 MPa and sintered at various temperatures (850-1,000°C) for 7 h. Tape-cast Li1.45Al0.45Ge0.2Ti1.35(PO4)3 films were prepared using a previously reported method (Zhang et al., 2015). Briefly, fine Li1.45Al0.45Ge0.2Ti1.35(PO4)3 powders prepared by the solid-state reaction were dispersed in a mixed solution of ethanol and toluene (1:1 v/v) using menhaden fish oil [2 wt% to Li1.45Al0.45Ge0.2Ti1.35(PO4)3] as a dispersant. The mixed slurry was ball milled for 10 h using HEMM. Polyvinyl alcohol [8 wt% to Li1.45Al0.45Ge0.2Ti1.35(PO4)3] was then added to the mixed slurry as a plasticizer and ball milled for another 12 h. After tape casting, the green sheets were kept in a sealed box with a small amount of ethanol in a refrigerator to slow the drying process at 5°C for 24 h. Several green sheets were hot pressed at 90°C for 10 min and then sintered at 900°C for 7 h.
The crystal structures of sintered samples were analyzed by X-ray diffraction (XRD) analysis using a Rigaku RINT 2500 diffractometer with Cu-Kα radiation in the 2θ range from 10° to 90° at a scanning step rate of 0.02°s −1 . The relative density of the sintered samples was estimated from the ratio of the density calculated from the lattice constants and that calculated from the volume and weight of the sintered body. The electrical conductivity of the sintered pellets (ca. 12 mm diameter and 1 mm thick) with gold sputtered electrodes were measured using an impedance phase analyzer (Solartron 1260) in the frequency range of 0.1 Hz-1 MHz with the bias voltage at 10 mV. Bulk and grain boundary conductivities of the sintered samples were estimated from complex impedance plots using Zview 2. Threepoint bending strength of the sintered pellets (ca. 0.24 mm thick and ca. 15 mm wide) was measured at room temperature using a materials tester (Shimadzu EZ-SX 500N).
resUlTs anD DiscUssiOn Figure 1 shows XRD patterns of the Li1.5Al0.5Ge0.2Ti1.3(PO4)3 samples sintered at various temperatures for 7 h with a silicon internal standard to measure the lattice constant. An impurity phase of AlPO4 was observed for the sample sintered at 850°C. At sintering temperatures as low as 850°C, the reaction was not completed. All diffraction lines of the samples sintered at 900, 950, and 1,000°C could be indexed as the NASICON-type structure (Perez-Estebanez et al., 2014). Figure 2 shows the relative density of Li1.5Al0.5Ge0.2Ti1.3 (PO4)3 pellets sintered at various temperatures. The sample with the impurity phase that was sintered at 850°C showed a low relative density of 87%. The highest relative density of 95.5% was observed for the sample sintered at 900°C, and the relative density decreased with further increase of the sintering temperature. The decreasing of the relative density may be due to the evaporation of lithium compounds at these higher temperatures. Figure 3 shows impedance profiles of Li1.5Al0.5 Ge0.2Ti1.3(PO4)3 samples (ca. 1 mm thick) sintered at various temperatures and measured at 25°C. The impedance profiles showed a large semicircle followed by a straight line. The semicircle may be attributed to the grain boundary resistance (Bruce and West, 1983). The intercept of the semicircle on the real axis at high frequency represents the bulk resistance, and the diameter of the semicircle indicates the grain boundary resistance. The semicircle due to the bulk resistance was out of the frequency range for the impedance analyzer used. The samples sintered at 900, 950, and 1,000°C showed almost the same bulk conductivity of ca. 2 × 10 −3 S cm −1 , while the sample sintered at 850°C showed a low bulk conductivity of ca. 10 −3 S cm −1 . The low bulk conductivity may be due to the non-equilibrium phase of Li1.5Al0.5Ge0.2Ti1.3 (PO4)3 prepared at the lower sintering temperature. The grain boundary resistance was dependent on the sintering temperature, and the sample sintered at 900°C with the highest relative density exhibited the lowest grain boundary resistance.
The electrical conductivity, relative density, and three-point bending strength for the Li1+xAlxGe0.2Ti1.8−x(PO4)3 system sintered at 900°C for 7 h were examined as a function of x. Figure 4 shows the XRD patterns of Li1+xAlxGe0.2Ti1.8−x(PO4)3. Almost all the diffraction lines for Li1+xAlxGe0.2Ti1.8−x(PO4)3 were indexed with the NASICON-type structure. However, Li1+xAlxGe0.2Ti1.8−x(PO4)3 with x = 0.45, 0.5, and 0.55 also showed diffraction lines due to AlPO4. The changes in the lattice parameter with x in Li1+xAlxGe0.2Ti1.8−x(PO4)3 are shown in Figure 5. The a lattice parameter of 0.812 at x = 0.30 increased to 0.882 nm at x = 0.40, and the c lattice parameter of 2.171 nm decreased to 2.043 nm at x = 0.4. Cretin and Fabry (1999) reported that the a parameter decreases and the c parameter increases with increasing x in Li1+xAlxTi2−x(PO4)3, while Aono et al. (1990) found both the a and c parameters decreased with increasing x. The decrease of the c parameter can be attributed to the substitution of Al 3+ with a small ionic radius (0.53 nm) for Ti 4+ with a large ionic radius (0.605 nm) in the octahedral sites. Several factors play a determinant role in the inference of the cation substitution on the structure (Delmas et al., 1981). Additional Li + ions are located in the unoccupied Li sites by the substitution of Al 3+ for Ti 4+ and Ge 4+ sites, as observed in Na1+xAlxTi2−x(PO4)3 (Maldonado-Manso et al., 2005), to maintain charge neutrality. The additional lithium ions in these sites lead to repulsion along the a axis. The reason for the smaller c and larger a parameters for Li1.55Al0.55Ge0.2Ti1.25(PO4)3 compared to those for Li1.5Al0.5Ge0.2Ti1.3(PO4)3 is not clear but may be due to the formation of AlPO4 impurity phases. These results suggest that the solubility limit of Al in Li1+xAlxGe0.2Ti1.8−x(PO4)3 is x = 0.4, as observed by Aono et al. (1990) for Li1+xAlxTi2−x(PO4)3. Figure 6 shows impedance profiles for Li1+xAlxGe0.2Ti1.8−x(PO4)3 measured at 25°C as a function of x. The lowest grain boundary resistance was observed for Li1.45Al0.45Ge0.2Ti1.35(PO4)3. The equivalent circuit in Figure 6 assumes a general model comprising grains and uniform grain boundaries that are parallel or perpendicular to the current flow. This results in the one with two parallel resistancecapacitance elements, one for the perpendicular grain boundary (Rp1 and CPE1) and one from the parallel grain boundary (Rp2 and CPE2) connected in parallel. In microcrystalline ceramics, where the effective grain boundary width is negligible compared to the grain size, the contribution of the parallel grain boundary can be neglected. However, parallel grain boundary contribution must be taken into account if the parallel grain boundary conductivity becomes significantly larger than that of the grain and/or if the effective grain boundary width is no longer negligible with respect to the grain size (Bouchet et al., 2003). The change in the grain boundary resistance with x could be explained by the change of the relative density as shown in Figure 7. Li1.55Al0.55Ge0.2Ti1.25(PO4)3 with the AlPO4 impurity phase had similar impedance profiles to those of Li1.50Al0.5Ge0.2Ti1.3(PO4)3 with the AlPO4 impurity phase sintered at 850°C, which revealed a high grain boundary resistance and low bulk conductivity. Figure 7 shows the electrical conductivities of total, grain bulk, grain boundary, and the relative density of Li1+xAlxGe0.2Ti1.8−x(PO4)3 measured at 25°C that are plotted as a function of x. The highest total conductivity of     1.0 × 10 −3 S cm −1 and the highest relative density of 95.8% were observed for Li1.45Al0.45Ge0.2Ti1.35 (PO4)3 at x = 0.45. The grain bulk conductivities at x = 0.40 and 0.50 are higher than that at x = 0.45.
The reason for this tendency is not yet clarified. Aluminum composition in the grain bulk may slightly deviate from the nominal one by its accumulation at the grain boundary region. As another thought, the estimation of the bulk conductivity could be influenced by a parallel grain boundary conduction path as illustrated in the equivalent circuit in Figure 6, although the grain boundary conductivities of Li1+xAlxGe0.2Ti1.8−x(PO4)3 at x = 0.40 and 0.50 were lower than that for Li1.45Al0.45Ge0.2Ti1.35(PO4)3. As a rough tendency, it is possible to state that conductivity becomes maximum around x = 0.45, and as leaving from the composition, the bulk and the grain boundary conductivity decreases. Figure 8 shows the temperature dependence of the total electrical conductivity for Li1.45Al0.45Ge0.2Ti1.35(PO4)3. The activation energy for the electrical conduction was calculated to be 31 kJ mole −1 , which is comparable to that for Li1.4Al0.4Ge0.2 Ti1.4(PO4)3, as reported previously (Zhang et al., 2013). Figure 9 shows the dependence of the three-point bending strength on the Al content for Li1+xAlxGe0.2Ti1.8−x (PO4)  Li1.4Al0.4Ge0.2Ti1.4(PO4)3 prepared by tape casting using powder prepared by the sol-gel method (Zhang et al., 2015) and lower than that of 140 N mm −2 for a polished Ohara plate of Li1+x+yAlx(Ti,Ge) SiyP3−yO12 glass-ceramics, the conductivity of which is as low as 1 × 10 −4 S cm −1 at room temperature. The high bending strength of Li1.45Al0.45Ge0.2Ti1.35(PO4)3 prepared by the conventional solidstate reaction is quite attractive for applications, such as the water-stable protective layer in aqueous lithium batteries. Zhang et al. (2013) examined the electrical conductivity dependence on the Ge content in Li1.4Al0.4GexTi1.6−x(PO4)3, and the highest conductivity was observed for Li1.4Al0.4Ge0.2Ti1.4(PO4)3 prepared by the sol-gel method. We also examined the effect of the Ge content in Li1+xAlxGeyTi2−x−y(PO4)3. Figure 10 shows impedance profiles measured at 25°C of Li1+xAlxGe0.3Ti1.7−x(PO4)3 as a function of x. The highest bulk conductivity of 1.39 × 10 −3 S cm −1 and total conductivity of 8.95 × 10 −4 S cm −2 were observed for using expensive Ti and Ge alkoxides. Here, we prepared Li1.45Al0.45Ge0.2Ti1.35(PO4)3 films by the tape casting method using powders prepared by the conventional solid-state reaction using TiO2 and GeO2. The impedance profile of the film measured at 25°C is shown in Figure 12. Zhang et al. (2015) reported an aging effect on the electrical conductivity of a tape-cast film stored in an air atmosphere. Therefore, the impedance was measured for a film stored for 1 week in an Ar glove box. The total and bulk conductivities at 25°C were estimated to be 1.01 × 10 −3 and 2.21 × 10 −3 S cm −1 , respectively, which are comparable to those of a sintered plate prepared from a pressed green body.

cOnclUsiOn
The NASICON-type water-stable lithium-ion-conducting solid electrolyte of Li1+xAlxGeyTi2−x−y(PO4)3 was prepared using conventional solid-state reaction at 900°C for 7 h. The highest lithium-ion conductivity of 10 −3 S cm −1 at 25°C was found for the Li1+xAlxGeyTi2−x−y(PO4)3 composition with x = 0.45 and y = 0.2. The relative density of the sintered pellets was as high as 95.8%, and the three-point bending strength was 90 N mm −2 . Tape cast films of Li1.45Al0.45Ge0.2Ti1.35(PO4)3 were prepared using powder prepared by solid-state reaction. The total and bulk electrical conductivities of the film were comparable with those of a sintered plate prepared from a pressed green body. This water-stable high lithium-ion-conducting solid electrolyte has potential application as the protective layer of lithium metal electrodes in aqueous lithium-air batteries and lithium batteries with aqueous liquid cathodes.
aUThOr cOnTriBUTiOns SX, HN, and PX did preparation of NASICON-type oxide, characterization, and conductivity measurements. SX also did the tape casting of the ceramics. SM, MM, YT, and OY supported and gave an advice on each experimental work. NI managed the research direction and provided necessary instructions and opportunities for discussions among all authors.