1 Introduction

The organic Li-ion salt electrolytes are widely used in lithium-ion batteries at present, but their flammability may cause serious safety issues [1,2,3,4,5,6,7], and the stability problem of the aqueous electrolytes with lithium metal has seriously limited the voltage range [2]. Therefore, inorganic solid-state electrolytes are now attracting much attention. Garnet-type Li7La3Zr2O12 (LLZO) as the inorganic solid electrolyte has excellent thermal performance, high chemical compatibility against molten lithium, high conductivity, wide electrochemical window and has been given much attention [8,9,10,11,12,13,14,15]. LLZO has cubic and tetragonal crystalline phases. The lithium-ion conductivity of cubic phase LLZO is about 2 or 3 orders of magnitude higher than that of tetragonal phase LLZO [16]. Hence, formation and stabilization of cubic phase is an effective approach for increasing the total conductivity of LLZO. Ga [17], Al [18,19,20], W [21], Ta [1, 3, 22], Nb [23], Sc [24], Hf [24, 25], Ti [26], Te [27], etc., were doped in LLZO, respectively, to stabilize the cubic structure of LLZO by previous researchers. Our research found that the room-temperature ionic conductivities of LLZO could be improved significantly by the co-doping of Ba, Y and Al in LLZO [28]. In addition, minimizing the grain boundary resistance is also an effective approach for enhancing the total conductivity in this solid electrolyte [24].

In this paper, the Ba, Y and Al co-doped LLZO was prepared by the solid-state reaction method. Effect of sintering on the crystallographic structure, morphology, total conductivity, relative density and contractibility rate of the prepared solid electrolyte was studied, respectively. The sintered samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) techniques, respectively.

2 Experimental

2.1 Preparation of solid electrolyte sample

Samples of Ba, Y and Al co-doped LLZO (composition: Li6.52La2.98Ba0.02Zr1.9Y0.1Al0.2O12) were prepared by solid-state reaction method in the literature [28]. Stoichiometric amounts of Li2CO3 (analytical reagent (AR) grade), La2O3 (99.999%), BaCO3 (AR), ZrO2 (AR), Y2O3 (99.999%) and Al2O3 (99.999%) were selected as original materials. The starting materials were mixed and ball-milled for 12 h with isopropyl alcohol as the dispersing reagent and then heat-treated at 900 °C for 6 h and then ball-milled again for 12 h. Then, the obtained powder was putted into ϕ = 13 mm model of stainless steel and pressed into pellets with 20 MPa pressure. The pellets were covered with mother powder and sintered at 1160–1200 °C for 12 h and at 1190 °C for 3–18 h, respectively, and the solid electrolyte samples were obtained.

2.2 Characterization techniques

An X-ray diffractometer (XRD, Bruker AXS D8 Advance) with Cu Kα radiation at 40 kV and 30 mA in the 2θ ranges from 10° to 60° at scanning rate of 2 (°)·min−1 was used to identify the phase composition of the sintered samples. The cross-sectional morphologies of the sintered samples were measured using a scanning electron microscope (SEM, HITACHI S3400N). A solartron impedance analyzer (Solartron 1260 &1287) with 100 Hz–10 MHz and 10 mV was used to identify EIS measurements of the sintered samples to measure the ionic conductivity. The relative densities of the sintered samples were measured by the Archimedes' method. The contractibility rates were measured by micrometer. The contractibility rate of the sintered sample was the ratio between contractibility value of the diameter of the sample after sintering and the diameter before sintering. Elemental analysis of sintered sample (sintering at 1190 °C for 12 h) was conducted with ICP-AES (VISTA-MPX ICP-OES) with ICP as light source.

3 Results and discussion

3.1 XRD studies

XRD patterns of Ba, Y and Al co-doped LLZO samples at different sintering temperatures are shown in Fig. 1a. The peaks are labeled with the standard pattern known as cubic garnet phase Li5La3Nb2O12 (PDF No.45–0109, the vertical lines in the bottom). From Fig. 1a, it can be seen that with temperature at 1160–1190 °C, the diffraction peaks are well matched with the standard peaks of the cubic garnet phase Li5La3Nb2O12, indicating that the solid electrolytes samples are cubic garnet phase. At 1200 °C, the sample is primarily a cubic garnet phases with small amounts of impurities such as La2Zr2O7.

Fig. 1
figure 1

XRD patterns of Ba, Y and Al co-doped LLZO samples at a different temperatures and b different times

Full size image

XRD patterns of Ba, Y and Al co-doped LLZO samples at different sintering times are shown in Fig. 1b. From Fig. 1b, it can be seen that as time for 3 h, along with the peaks of the cubic garnet phase sample, diffraction peak at 2θ = 30.5° splits into two peaks, and there are four diffraction peaks in 50° ≤ 2θ 55°, indicating the existence of the tetragonal garnet phase in the sample [29, 30]. And the sample is a mixture of tetragonal garnet phase and cubic garnet phases. With time for 6–15 h, the samples are cubic garnet phase. With t = 18 h, the sample is primarily a cubic garnet phases with small amounts of impurities such as La2Zr2O7.

3.2 SEM image

SEM surface morphologies of the cross section of Ba, Y and Al co-doped LLZO samples at different temperatures are shown in Fig. 2. From Fig. 2, at 1160–1170 °C, the grain sizes in the samples become bigger with temperature increasing, and the grains are not in good contact with each other, implying a low density. With temperature at 1180–1190 °C, those images of the samples exhibit the distinctively flattened morphology without any noticeable grain boundaries, and the pores are observed, indicating that controlling temperature helps to eliminate the samples’ grain boundary resistances and improve the densifications. Therefore, the higher densities and conductivities in the samples could be obtained by controlling temperature. The appearance of the pores could be due to the evaporation of lithium compounds. The evaporation of lithium compounds can be compensated with mother powder covering the surface during the sintering process, but the inner part cannot get completely compensation. With temperature at 1190–1200 °C, the grain sizes of the sample become smaller with temperature increasing.

Fig. 2
figure 2

SEM images of Ba, Y and Al co-doped LLZO samples at different temperatures: a 1160 °C, b 1170 °C, c 1180 °C, d 1190 °C, and e 1200 °C

Full size image

SEM surface morphologies of the cross section of the Ba, Y and Al co-doped LLZO samples for different time are shown in Fig. 3. From Fig. 3, it can be seen that with time for 3–9 h, the grain sizes in the samples become bigger with time increasing. With time for 12–18 h, those images of the samples exhibit the distinctively flattened morphology without any noticeable grain boundaries, and the pores become more and bigger with time increasing. Thus, the higher densities and conductivities in the samples could be obtained by controlling time.

Fig. 3
figure 3

SEM images of Ba, Y and Al co-doped LLZO samples at different times: a 3 h, b 6 h, c 9 h, d 12 h, e 15 h, and f 18 h

Full size image

3.3 Ionic conductivity

The impedance spectra of Ba, Y and Al co-doped LLZO samples at different temperatures measured at 25 °C in air are shown in Fig. 4a. From Fig. 4a, all the plots of the samples show only a single semicircle at high-frequency region and a remarkable diffusion tail at the low-frequency region. The semicircle represents the grain boundary impedance. The straight line corresponds to Warburg-type impedance. The bulk resistance, usually appearing at high frequency, cannot be detected because it is out of the frequency range of the analyzer [25]. The impedance spectra were fitted with an equivalent circuit model ( ), where R and CPE1 represent the resistance and the capacitance associated with the sintered sample, respectively. CPE2 reflects the capacitance from the Ag paint current collector. The ionic conductivity (σ) was calculated using the formula: σ = (1/R)·(L/S), where σ, R, L and S represent the total ionic conductivity, total resistance obtained in EIS measurement, thickness and the effective area of the pellet, respectively. The total ionic conductivities were obtained by analyzing the impedance data with ZsimpWin software for simulation, and the values are shown in Table 1. From Table 1, σ increases significantly from 1.23 × 10−5 to 2.96 × 10−4 S·cm−1 with T from 1160 to 1190 °C, and due to that with temperature increasing, the grains contact better with each other. The result is supported by SEM analysis result in Fig. 2a–d. It is beneficial for improving the ionic conductivity. With temperature changing from 1190 to 1200 °C, σ decreases from 2.96 × 10−4 to 1.92 × 10−4 S·cm−1; due to that as temperature further increases, small amounts of impurities such as La2Zr2O7 are produced. The result is supported by XRD analysis result (Fig. 1a). And the results indicate that higher conductivities in the samples could be obtained by controlling temperature.

Fig. 4
figure 4

Impedance spectra of Ba, Y and Al co-doped LLZO samples at a different temperatures and b different times measured at 25 °C (Z′- the real part of impedance, Z′′- the imaginary part of impedance.)

Full size image
Table 1 Conductivities of samples at different temperatures (S·cm−1)
Full size table

The impedance spectra of the Ba, Y and Al co-doped LLZO samples at different time measured at 25 °C in air are shown in Fig. 4b. From Fig. 4b, the plots of the samples show only a single semicircle at high frequency region and a remarkable diffusion tail at the low-frequency region. The total conductivity values of the samples are shown in Table 2. From Table 2, σ increases significantly from 6.29 × 10−5 to 2.96 × 10−4 S·cm−1 with time increasing from 3 to 12 h; due to that as time increases, the grains contact better with each other. The result is supported by SEM analysis result in Fig. 3a–d. As time increases from 12 to 18 h, σ decreases from 2.96 × 10−4 to 1.60 × 10−4 S·cm−1; due to that as time further increases, the pores become more and bigger in the sample. The result is supported by SEM analysis result in Fig. 3d–f. And the results indicate that higher conductivities in the samples could be obtained by controlling time.

Table 2 Conductivities of samples at different time (S·cm−1)
Full size table

3.4 Relative density and contractibility rate

The relative density and contractibility rate of the Ba, Y and Al co-doped LLZO samples at different temperatures are shown in Fig. 5a. As can be seen, the relative density and contractibility rate increase from 92.47% and 10.15% to 94.19% and 18.61%, respectively, as temperature increases from 1160 to 1190 °C. As temperature increases from 1190 to 1200 °C, the relative density and contractibility rate decrease from 94.19% and 18.61% to 93.10% and 17.86%, respectively. The result is consistent with SEM and ionic conductivity analysis result.

Fig. 5
figure 5

Relative density and contractibility rate of Ba, Y and Al co-doped LLZO samples at a different temperatures and b different time

Full size image

The relative density and contractibility rate of the Ba, Y and Al co-doped LLZO samples at different time are shown in Fig. 5b. As can be seen, the relative density and contractibility rate increase significantly from 86.32% and 6.58% to 94.19% and 18.61%, respectively, with time changing from 3 to 12 h. As time changes from 12 to 18 h, the relative density and contractibility rate decrease from 94.19% and 18.61% to 91.91% and 18.37%, respectively. The result is consistent with SEM and ionic conductivity analysis result.

3.5 Composition

The composition of Ba, Y and Al co-doped LLZO sample (sintering at 1190 °C for 12 h) is shown in Table 3. As shown, Li:La:Ba:Zr:Y:O = 6.52:2.98:0.02:1.9:0.1:0.2:12 (atom fraction ratio), the composition of the sample is Li6.52La2.98Ba0.02Zr1.9Y0.1Al0.2 O12. Al was substituted in Li site in the formation process of the LLZO [18,19,20]. Alkali earth cations have been long considered to be confined exclusively to dodecahedral 8-coordinated sites (La sites) [3]. The ionic radius of Y3+, Zr4+, La3+ and Ba2+ are 0.088, 0.072, 0.103 and 0.135 nm, respectively. Because the ionic radius of Y3+ is the closest to that of Zr4+ among the ions, doping Y on Zr site is easier to occur. As shown in Table 3, the La:Ba molar ratio and Zr:Y molar ratio are 2.98:0.02 and 1.8:0.1, respectively, which well accords with the nominal stoichiometry and confirms the substitution of La and Zr sites with Ba2+ and Y3+.

Table 3 Composition of sintered sample (1190 °C, 12 h)
Full size table

4 Conclusion

The cubic garnet phase Ba, Y and Al co-doped LLZO is obtained, and the room-temperature total conductivity of the Ba, Y and Al co-doped LLZO solid electrolyte is improved significantly by eliminating the grain boundary resistances and improving the densifications with controlling temperature and time, respectively. Sintering at 1160–1190 °C for 12 h and for 6–15 h at 1190 °C, respectively, the Ba, Y and Al co-doped LLZO solid electrolytes are cubic garnet phase. Sintering at 1180–1190 °C for 12 h and for 12–18 h at 1190 °C, respectively, SEM images of the cross section of the Ba, Y and Al co-doped LLZO solid electrolytes exhibit the distinctively flattened morphology without any noticeable grain boundaries. The total conductivity, relative density and contractibility rate of Li6.52La2.98Ba0.02Zr1.9Y0.1Al0.2O12 solid electrolyte are 2.96 × 10−4 S·cm−1, 94.19% and 18.61%, respectively.