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Phase transition and conductivity improvement of tetragonal fast lithium ionic electrolyte Li7La3Zr2O12
Solid State Ionics 253 (2013) 137–142
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Phase transition and conductivity improvement of tetragonal fast lithium ionic electrolyte Li7La3Zr2O12 X.P. Wang ⁎, Y. Xia, J. Hu, Y.P. Xia, Z. Zhuang, L.J. Guo, H. Lu, T. Zhang, Q.F. Fang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China
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Article history: Received 25 June 2013 Received in revised form 19 September 2013 Accepted 20 September 2013 Available online 8 October 2013 Keywords: Lithium ionic conductor Li7La3Zr2O12 Phase transition Conductivity
a b s t r a c t The phase transition processes, lithium ionic diffusion and conductivity of solid lithium ionic electrolyte Li7La3Zr2O12 with tetragonal symmetry have been carefully investigated by variable-temperature X-ray diffraction (XRD), internal friction (IF) and alternate current (AC) impedance spectroscopy, respectively. Multiple phase transition processes were observed in temperature range from 25 °C to 1000 °C, which correspond to the tetragonal– cubic phase transition around 100 °C–150 °C and the cubic–tetragonal phase transition around 800 °C–900 °C, respectively. During the IF measurement, a weak IF peak (labeled as PT) and a strong IF peak (labeled as PC) were observed in the tetragonal and cubic Li7La3Zr2O12 compounds, and the mechanism related with the dramatic difference of the relaxational strength was ascribed to the different distribution of lithium ions: order in tetragonal phase and disorder in cubic phase. The measurement of conduction property revealed an abrupt increase of conductivity in temperature range 150 °C–200 °C, which also originated from the tetragonal–cubic phase transformation. Based on the irreversible characteristic of tetragonal–cubic phase transition around 100 °C–150 °C, a novel synthesized method of cubic Li7La3Zr2O12 was proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Crystalline Li-rich oxides that have high ionic conductivity but insulate to electrons are of great importance in current electrochemical applications such as all-solid-state rechargeable lithium ion batteries, powerful supercapacitors and electrochromic displays. Earlier reaches for solid lithium electrolytes were generally focused on the following candidates [1]: Li2 + 2xZn1 − xGeO4 (LISICON), Li3N, Li-β-alumina, NASICON-type Li1.3Ti1.7Al0.3(PO4)3, and perovskite-type (La, Li) TiO3. However, none of them was completely suitable for solid-state electrolytes owing to either low lithium ionic conductivity or poor chemical stability. In the last few years, a series of garnet-like structural oxides have been developed as a novel family of solid state lithium electrolytes by Weppner [2–5]. Among them, Li7La3Zr2O12 has been recently paid much attention as a promising solid electrolyte for all-solid-state rechargeable batteries owing to its good ionic conductivity and high chemical stability against metal Li [5]. For pure Li7La3Zr2O12 compound, it generally crystallizes in a tetragonal structure with space group I41/acd, the low crystalline symmetry and order distribution of lithium ions make the tetragonal phase exhibit an extremely poor ionic conductivity (~10−6 S/cm at 300 K)[6]. Although the ionic conductivity of cubic Li7La3Zr2O12 (space group Ia-3d) can reach up to 3.1 × 10−4 S/cm at room temperature [7], the high conductive phase is relatively difficult to be obtained, which needs partial cationic substitution, such as Li by ⁎ Corresponding author. Tel.: +86 551 65591125; fax: +86 551 65591434. E-mail address: [email protected] (X.P. Wang). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.09.029
Al and Zr by Ta, Nb, Si, Ga or Y, to stabilize the cubic phase to room temperature [8–12]. It was worth noting that the cubic Li7La3Zr2O12 without any cationic substitution had been also successfully synthesized at 700 °C by a modified sol–gel process [13], but the preparation route was relatively complex compared with traditional solid state reaction method. Moreover because of the large grain boundary resistance, the total ionic conductivity in the sol–gel case was two orders of magnitude lower than that of solid state sintered cubic samples. Investigation of structural stability of Li7La3Zr2O12 revealed that the tetragonal phase was actually unstable, in which a phase transition was evidenced from tetragonal symmetry to cubic symmetry in the temperature range 100 °C–150 °C by tracing the merging process of the splitting (121) and (112) Bragg peaks [14]. Toda and his co-workers also found a reversible tetragonal–cubic phase transition through annealing the tetragonal Li7La3Zr2O12 at a higher temperature of 450 °C for 20 h in air, and the reason related with the phase transition was explained by CO2 absorption because the cubic phase was not obtained by annealing at 450 °C in pure oxygen and argon atmospheres [15]. Moreover, Kokal firstly revealed that the crystal structure of Li7La3Zr2O12 was closely related with the synthesis temperature during the sol–gel synthesis routes [16], in which the cubic Li7La3Zr2O12 could be formed at 978 K, but if further increasing the sintered temperature up to 1073 K, this oxide crystallized into tetragonal garnet-related phase. Thus according to the reported experimental results, it is obvious that the phase structure of the tetragonal Li7La3Zr2O12 is actually very complex, and to truly understand structural properties of the tetragonal Li7La3Zr2O12 as well as its phase transition processes, it still needs systematic investigation. In this paper, the phase complexity of the tetragonal Li7La3Zr2O12 was
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carefully investigated by variable-temperature X-ray diffraction (XRD) and internal friction (IF) methods and multiple tetragonal–cubic phase transitions are found. Moreover, it is particularly important that the cubic Li7La3Zr2O12 induced by the tetragonal–cubic phase transition can be stable to room temperature, which results in a great improvement of lithium ionic conductivity. 2. Experimental The polycrystalline ceramics samples of Li7La3Zr2O12 in our investigation were prepared by conventional solid-state reaction method from a stoichiometric mixture of high purity La(OH)3 (99.99%), Li2CO3 (99.9%) and ZrO2 (99.9%). To compensate the loss of Li induced by the high temperature sintering, 10% wt.% excess of Li2CO3 was added. After having been well mixed in an agate mortar, the initial powders were heated at 1000 °C for 5 h, and a pure tetragonal Li7La3Zr2O12 with garnet-type structure would be obtained. During our sample preparation, the initial reaction powder Li7La3Zr2O12 was shaped into a pellet with a press of 100 MPa at first, and then put on an aluminum crucible covered with the same Li7La3Zr2O12 reactant powder to avoid aluminum contamination at high temperature. The synthesized powder sample was uniaxially shaped in a round mold at first, and then pressed in an isostatic press with a press of about 200 MPa. To get high-density ceramic sample, the discs were sintered at 1050 °C for 10 h, and then cooled down to room temperature with furnace. The dimension of the finally sintered samples was about 28 mm in diameter and 1.5 mm in thickness, and the density of the sample was about 93% of their theoretical density calculated from X-ray diffraction (XRD) data. The room temperature XRD was recorded on a θ/θ Bragg–Brentano X'Pert MPD PRO diffractometer (Cu Kα1 + 2 radiations). These patterns were collected in a scanning range from 10° to 70° with a step of 0.033°. The thermo-diffractograms were recorded in the same X'Pert MPD PRO apparatus equipped with a higher temperature heating chamber. The variable-temperature patterns were collected between 25 °C and 950 °C with the same counting step of 0.033°. Whole pattern matching refinement of the powder XRD patterns was performed using Fullprof program, in which the pseudo-Voigt profile function was used as the profile function in the refinements. Internal friction (IF) is an energy dissipation of mechanical energy, which is manifested by stress-stained hysteresis in the case of cyclic loading. Generally, IF (Q−1) can be written as [17]:
Q
−1
¼
ΔW 2πW
ð1Þ
where W is the maximum elastic stored energy during one cycle, ΔW is the corresponding energy absorption during that cycle. In this investigation, the low frequency IF measurements were carried out on a computer-controlled torsion pendulum under the forced vibration mode. The bar samples for the IF measurement were sawed from the sintered dense disc with a dimension of about 27 × 1 × 1.5 mm3. The maximum torsion strain amplitudes were kept to be 2.5 × 10−5 with a heating/cooling rate of 2 K/min in all measurements. The conduction properties were evaluated by alternate current (AC) impedance spectroscopy, in which the conductivity measurement of the Li7La3Zr2O12 compound was isothermally collected in air on a computer-controlled Hioki 3531 Z-Hitester frequency response analyzer in the frequency range 42 Hz–5 MHz. The square pellet for the electrical measurement was also cut from the sintered Li7La3Zr2O12 bulk, and the typical size of the square samples was about 10 × 10 × 1.5 mm3. Prior to each impedance measurement, the sample was stabilized at a constant temperature for 30 min to ensure thermal equilibrium. By analyzing the impedance spectra at different temperature, the resistances of the Li7La3Zr2O12 compound will be obtained. Thus based on Ohm's law, the ionic conductivity of the samples can be deduced.
3. Experimental results and discussion 3.1. Structural characterization Fig. 1 shows the room temperature powder XRD pattern of the initial Li7La3Zr2O12 compound in the scanning range from 15° to 75°, in which the obvious splitting of the diffraction lines demonstrates a typical tetragonal garnet-type phase. For the obtained tetragonal phase, the refinement of the XRD profile can be well indexed using space group I41/acd (no. 142), as shown in Fig. 1. Beside the symbols, the calculated, Bragg positions and defference pattern of the Rietveld refinement from the XRD data are also presented in Fig. 1, respectively. The lattice constants determined from the refinement are a = 13.126(2)Å and c = 12.675(8) Å with a cell ratio of c/a = 0.9657, which are well consistent with the reported values of a = 13.134(4)Å and c = 12.663(8) Å [6].
3.2. Internal friction analysis Fig. 2 illustrates the IF spectra of the tetragonal Li7La3Zr2O12 sample at a measuring frequency of 1 Hz in air under different thermal runs. During our experiments, the measuring temperature range actually covered from −50 °C to 300 °C, but only the IF spectra from −50 °C to 210 °C are presented, as shown in Fig. 2, because the profiles at higher temperature only exhibited a flat background and no IF peak was detected. As can be seen from this figure, a small IF peak, labeled as PT, is observed in the first heating run with a heating rate of 2 K/min, and the peak position locates around 48 °C. After subtracted the IF background, the peak height of the PT peak is only about 0.01. But in the following cooling run, the peak height of the IF peak dramatically increases up to 0.09, accompanying the peak position shifting towards about 16 °C. To clearly describe the evolution of the IF spectra, the strong IF peak is denoted as PC peak here. In the next heating/cooling run, the strong PC peak is found to appear stably, and the relaxational strength hardly changes, as shown in Fig. 2, where the IF spectra under the second heating/cooling run are present as an example. On the issue of obvious shift of peak position between the heating run and cooling run, it is due to the regular thermal hysteresis. Further investigations related with the IF peak are carried out in different environmental condition including vacuum, oxygen and argon, and similar evolution of the IF spectra is also observed in the different heating/cooling runs. These experimental results demonstrate that the evolution of the IF peak is independent on the external atmosphere condition. Noting that Li7La3Zr2O12 could exhibit two different phase structures: cubic and tetragonal, depending on the sintering temperature, as
Fig. 1. Observed, calculated, Bragg positions and defference of the Rietveld refinement from the XRD patterns of tetragonal Li7La3Zr2O12. The space groups used in the refinement is I41/acd (no.142) for the tetragonal phase. The inset gives the enlarged local fitting result.
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in cubic phase, the lithium ions have good mobility, and therefore lead to high ionic conductivity and strong relaxational IF peak. More direct evidence about the tetragonal–cubic transformation will be presented in the following variable-temperature XRD patterns.
3.3. Thermo-XRD analysis
Fig. 2. The temperature dependence of internal friction for the tetragonal La7La3Zr2O12 under different thermal runs.
reported by Kokal [16]. Moreover compared with the ordering distribution of lithium ions for the tetragonal garnet Li7La3Zr2O12 [6], the cubic phase has much higher ionic conductivity because of the disordering distribution of lithium ions [18]. So according to the reported results, the mechanism related with the dramatic difference of the peak height could be understood from the variation of the crystal structure of Li7La3Zr2O12 as well as lithium ion distribution with temperature. With regard to crystal structure of garnet-like Li7La3Zr2O12, La3+ ions occupy the eight-coordinate sites, and Zr4+ ions stay on the sixcoordinate octahedral sites, and there are two possible sites to accommodate Li ions: 24d and 48g. Since in Li7La3Zr2O12, the tetrahedral 24d sites can't enough to accommodate all Li ions, so the extra Li+ must enter the octahedral 48g sites that are empty for the standard garnet structure (If the occupied positions deviate from the octahedral center, it is called 96h). Further analyzing by the neutron diffraction patterns, the accommodation of lithium ions in both 24d sites and 48g sites are partially occupied, the occupation for the tetrahedral sites (24d) and octahedral sites (48g) is about 36.0% and 41.8%, respectively [7]. Because in garnet-like Li7La3Zr2O12, the interstitial tetrahedral sites (24d) are surrounded by face-sharing octahedral sites (48g), so the diffusion routes for lithium lions only have two possibilities: 48g↔24d and 48g ↔48g. Dynamic simulation based on bond-valence analysis also revealed that in garnet-type lithium electrolytes, the disordering distribution of lithium ions provided a 3D diffusion channels for carries in terms of the possible routes: 48g → 24d → 48g [19]. According to relaxational theory of internal friction, the relaxational strength of IF peak is proportional to the concentration and mobility of diffusion carriers. While in Li7La3Zr2O12, the diffusion ions are just the lithium ions that occupied in tetrahedral sites (24d) and octahedral sites (48g). So combined with the relaxational theory of internal friction and lithium occupation in Li7La3Zr2O12, the mechanism of the IF peak is suggested to originate from the following migration routes: 48g↔24d and 48g↔48g. Actually in our previous IF investigation on cubic garnet-type Li5La3M2O12(M = Ta, Nb, Bi) compounds [20–22], similar results were also obtained, in which a strong IF peak was observed at around room temperature, and both the relaxational strength and peak position were very close to that of PC peak in Li7La3Zr2O12, and the mechanism of the IF peak had been similarly described with the following two diffusion process ways: 48g↔24d and 48g↔48g. As for the obvious difference of relaxational strength between in the tetragonal phase and cubic phase, that is owing to the difference of lithium ion distribution between the tetragonal phase and the cubic phase. Since the order distributed lithium ions in the tetragonal phase only exhibit weak mobility, it results in poor conductivity as well as low IF intensity. On the contrary benefitting from disorder distribution
Fig. 3 gives the room temperature XRD patterns of the initial and thermal cycling Li7La3Zr2O12 compounds, in which the thermal powder is ground from the corresponding IF samples. As noted from this figure the initial tetragonal phase is found to transform into cubic structure after the first heating/cooling run in the temperature range 25 °C– 300 °C, implying that the tetragonal phase is actually metastable and tends to transform into high symmetric cubic phase in the investigation temperature range. It is worth pointing out that the thermal samples from the IF experiments are slowly cooled to room temperature (2 K/min) and not quenched, the maintenance of high temperature cubic phase evidences that the tetragonal–cubic phase transition is irreversible, as proved by IF spectra, in which the strong PC peak is found to stably appear after the first heating/cooling run. From another aspect, this XRD result also gives a direct evidence that the dramatic variation of IF spectra is really induced by the tetragonal–cubic phase transition during the heating measurement. Fig. 4 shows the variable-temperature XRD patterns of tetragonal La7La3Zr2O12 in the temperature range 25 °C–400 °C with an interval of 25 °C. As can be seen from this figure the splitting tetragonal diffraction lines are gradually merged into the simple cubic diffraction lines with increasing temperature, which further confirms the occurrence of tetragonal–cubic phase transformation. To clearly understand the detailed phase transition process of Li7La3Zr2O12 compound, the evolutions of the (213) and (426) diffraction lines with temperature are used as the signs to trace the phase transition process, as shown in Fig. 5(a) and (b). It is found that the two splitting diffraction lines associated with the tetragonal phase are gradually decreased, and finally merged into the cubic diffraction lines. In Fig. 5(c), the variations of diffraction intensity of the (213) and (426) diffraction lines with temperature are especially illustrated, and the intensity of the two diffraction lines are noted to dramatically decrease in the temperature range 100 °C–150 °C. This result clearly gives the temperature region of the tetragonal–cubic phase transformation, which is also in good agreement with the reported temperature region by Geiger [14]. To further explore the phase stability of Li7La3Zr2O12 compound at higher temperature, the evolution of in-situ XRD patterns of Li7La3Zr2O12 with temperature in the temperature from 400 °C to 1000 °C is also studied, as shown in Fig. 6. It is found that the transformed cubic phase
Fig. 3. The room temperature XRD patterns of the initial and heated Li7La3Zr2O12 samples, the tetragonal phase was found to transform into cubic structure after heating to 300 °C.
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Fig. 4. Powder XRD patterns of tetragonal La7La3Zr2O12 as a function of temperature. A phase transition from tetragonal to cubic symmetry was observed between 100 °C–150 °C, and the splitting diffraction lines in tetragonal phase disappeared after the tetragonal phase transform into cubic structure.
in the temperature range 100 °C–150 °C can be maintained to 800 °C, but further increasing temperature, the cubic phase gradually transformed into tetragonal phase again above 900 °C. Interestingly, the transformed tetragonal Li7La3Zr2O12 compound can be maintained to room temperature with furnace cooling. In addition, it is noted that although the phase above 900 °C exhibits typical tetragonal structure, it is slightly different from the initial tetragonal phase before heating (see Fig. 3). The main difference between the initial phase and high temperature XRD patterns lies in the relative intensity of the diffraction lines. The reason possibly originates from the preferred orientation of the surface of the heated sample induced by the high temperature sintering process. According the XRD results above, it clearly indicates that the undoped tetragonal Li7La3Zr2O12 compound is actually unstable, which exhibits complex and multiphase tetragonal–cubic–tetragonal transformation process. Especially in terms of the forgoing IF results that the evolution of IF spectra isn't independent on the variation of the external measurement environment, the tetragonal–cubic phase transformation at lower temperature should be intrinsic, obviously different from the reversible tetragonal–cubic phase transition induced by CO2 absorption at around 400 °C. On the other hand, the temperature regime of the phase transition at around 100 °C–150 °C is relatively low, it is therefore excluded the potential influence of lithium volatilization. In this condition, this lower temperature tetragonal–cubic phase transition should be related with the variation of a temperature-dependent redistribution of Li ions, from order distribution in the tetragonal phase to disorder distribution in cubic phase. As for the cubic–tetragonal phase transition around 800 °C–900 °C, it can be similarly ascribed to the disorder–order transition of lithium distribution. During the investigation of Li7La3Zr2O12 structure, Janani and the coworkers had successfully synthesized a pure cubic Li7La3Zr2O12 in the temperature range 700 °C–800 °C by a modified sol–gel method without any cationic substitution [13], which indicated that the cationic substitution was not essential to form cubic Li7La3Zr2O12 phase. Kokal further expanded the heated temperature range [16], and revealed that the crystal structure of Li7La3Zr2O12 was closely dependent on sintering temperature, once the heated temperature increased up to 800 °C–900 °C, it was not formed cubic phase again but tetragonal
Fig. 5. Variation with temperature of the typical splitting diffraction lines associated with the tetragonal–cubic phase transformation in La7La3Zr2O12 compound: (a) Evolution of (213) diffraction line; (b) Evolution of (426) diffraction line; (c) Evolution of the intensity for the (213) and (426) diffraction lines, respectively.
phase. This result is in agreement with the observed cubic–tetragonal phase transition process at 900 °C in our investigation. 3.4. Conductivity measurement The variation of lithium ionic conductivity of tetragonal Li7La3Zr2O12 with temperature is determined by AC impedance spectroscopy. The typical complex spectra at four difference temperatures of 150 °C, 250 °C, 350 °C and 450 °C are shown in Fig. 7. As can be seen that in each Nyquist spectrum, one or two pressed semicircles are observed at the high frequency regime, such compressed semicircles are contributed to the resistance of both bulk and grain boundaries. At low frequency region, an obvious polarized tail is observed, which indicates
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Fig. 6. Evolution of in-situ XRD patterns of Li7La3Zr2O12 with temperature in the temperature from 400 °C to 1000 °C. Fig. 8. Temperature dependence of total ionic conductivity for tetragonal Li7La3Zr2O12.
that the conductivity of the Li7La3Zr2O12 compound is actually ionic in nature, as reported in Ref. [7,23]. Since the bulk resistance and grainboundary resistance are not obviously separated from each other from the impedance plots, therefore the lithium ionic conduction properties of the sample are evaluated by using total conductivity. For the initial tetragonal Li7La3Zr2O12, the total lithium ionic conductivity determined from the AC impedance spectra was only about 1 × 10−7 S/cm at room temperature. Compared with the reported value of 3.1 × 10−4 S/cm in Al stabilized cubic phase [7], this conductivity is actually too poor to be applied for practical application. Fig. 8 presents the lithium ionic conductivity of the temperature dependence of total ionic conductivity for the tetragonal Li7La3Zr2O12 during the initial heating process, and an obvious enhancement of conductivity is observed in the temperature range 150 °C–200 °C, in which the conductivity was found to dramatically increase from 2.9 × 10−4 S/cm at 150 °C to 2.0 × 10−3S/cm at 251 °C. This abrupt variation of conductivity is also ascribed to the tetragonal–cubic phase transition, as verified by the thermal XRD and IF measurements. Benefitting from the tetragonal–cubic phase transformation, the order distribution of lithium ions in the tetragonal
phase is induced into disorder distribution, and thus greatly increases its conductivity. As have been pointed out by XRD and IF measurements, the obtained cubic phase after the phase transition can be maintained to room temperature, the irreversible characteristic of phase transition reveals a novel method to obtain undoped cubic Li7La3Zr2O12 with high ionic conductivity. 4. Conclusion The phase transition processes of garnet-related type Li7La3Zr2O12 with tetragonal symmetry were carefully investigated by IF and variable-XRD methods, and multiple irreversible phase transition processes are found, which are the tetragonal–cubic phase transition located around 100 °C–150 °C and cubic–tetragonal phase transition around 800 °C–900 °C, respectively. Compared with the weak IF peak in the tetragonal Li7La3Zr2O12, the IF peak in the cubic phase is much stronger, and the mechanism related with variation of relaxational strength of
Fig. 7. Impedance spectra of tetragonal Li7La3Zr2O12 at four different temperatures: (a) t = 150 °C; (b) t = 250 °C; (c) t = 350 °C; (a) t = 450 °C.
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the IF peak was suggested to originate from the order–disorder transition of lithium distribution induced by the tetragonal–cubic phase transformation. During the investigation of ionic conductivity, an abrupt variation of conductivity was found in the temperature range 150 °C– 200 °C, which also corresponds to tetragonal–cubic phase transformation. In terms of the irreversible characteristic of tetragonal–cubic phase transition, a novel method was proposed to obtain the undoped cubic Li7La3Zr2O12 with high ionic conductivity. Acknowledgments This work was subsidized by the National Natural Science Foundation of China (Grant Nos: 11374299, 11274305). References [1] [2] [3] [4] [5] [6]
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Phase transition and conductivity improvement of tetragonal fast lithium ionic electrolyte Li7La3Zr2O12 X.P. Wang ⁎, Y. Xia, J. Hu, Y.P. Xia, Z. Zhuang, L.J. Guo, H. Lu, T. Zhang, Q.F. Fang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 19 September 2013 Accepted 20 September 2013 Available online 8 October 2013 Keywords: Lithium ionic conductor Li7La3Zr2O12 Phase transition Conductivity
a b s t r a c t The phase transition processes, lithium ionic diffusion and conductivity of solid lithium ionic electrolyte Li7La3Zr2O12 with tetragonal symmetry have been carefully investigated by variable-temperature X-ray diffraction (XRD), internal friction (IF) and alternate current (AC) impedance spectroscopy, respectively. Multiple phase transition processes were observed in temperature range from 25 °C to 1000 °C, which correspond to the tetragonal– cubic phase transition around 100 °C–150 °C and the cubic–tetragonal phase transition around 800 °C–900 °C, respectively. During the IF measurement, a weak IF peak (labeled as PT) and a strong IF peak (labeled as PC) were observed in the tetragonal and cubic Li7La3Zr2O12 compounds, and the mechanism related with the dramatic difference of the relaxational strength was ascribed to the different distribution of lithium ions: order in tetragonal phase and disorder in cubic phase. The measurement of conduction property revealed an abrupt increase of conductivity in temperature range 150 °C–200 °C, which also originated from the tetragonal–cubic phase transformation. Based on the irreversible characteristic of tetragonal–cubic phase transition around 100 °C–150 °C, a novel synthesized method of cubic Li7La3Zr2O12 was proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Crystalline Li-rich oxides that have high ionic conductivity but insulate to electrons are of great importance in current electrochemical applications such as all-solid-state rechargeable lithium ion batteries, powerful supercapacitors and electrochromic displays. Earlier reaches for solid lithium electrolytes were generally focused on the following candidates [1]: Li2 + 2xZn1 − xGeO4 (LISICON), Li3N, Li-β-alumina, NASICON-type Li1.3Ti1.7Al0.3(PO4)3, and perovskite-type (La, Li) TiO3. However, none of them was completely suitable for solid-state electrolytes owing to either low lithium ionic conductivity or poor chemical stability. In the last few years, a series of garnet-like structural oxides have been developed as a novel family of solid state lithium electrolytes by Weppner [2–5]. Among them, Li7La3Zr2O12 has been recently paid much attention as a promising solid electrolyte for all-solid-state rechargeable batteries owing to its good ionic conductivity and high chemical stability against metal Li [5]. For pure Li7La3Zr2O12 compound, it generally crystallizes in a tetragonal structure with space group I41/acd, the low crystalline symmetry and order distribution of lithium ions make the tetragonal phase exhibit an extremely poor ionic conductivity (~10−6 S/cm at 300 K)[6]. Although the ionic conductivity of cubic Li7La3Zr2O12 (space group Ia-3d) can reach up to 3.1 × 10−4 S/cm at room temperature [7], the high conductive phase is relatively difficult to be obtained, which needs partial cationic substitution, such as Li by ⁎ Corresponding author. Tel.: +86 551 65591125; fax: +86 551 65591434. E-mail address: [email protected] (X.P. Wang). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.09.029
Al and Zr by Ta, Nb, Si, Ga or Y, to stabilize the cubic phase to room temperature [8–12]. It was worth noting that the cubic Li7La3Zr2O12 without any cationic substitution had been also successfully synthesized at 700 °C by a modified sol–gel process [13], but the preparation route was relatively complex compared with traditional solid state reaction method. Moreover because of the large grain boundary resistance, the total ionic conductivity in the sol–gel case was two orders of magnitude lower than that of solid state sintered cubic samples. Investigation of structural stability of Li7La3Zr2O12 revealed that the tetragonal phase was actually unstable, in which a phase transition was evidenced from tetragonal symmetry to cubic symmetry in the temperature range 100 °C–150 °C by tracing the merging process of the splitting (121) and (112) Bragg peaks [14]. Toda and his co-workers also found a reversible tetragonal–cubic phase transition through annealing the tetragonal Li7La3Zr2O12 at a higher temperature of 450 °C for 20 h in air, and the reason related with the phase transition was explained by CO2 absorption because the cubic phase was not obtained by annealing at 450 °C in pure oxygen and argon atmospheres [15]. Moreover, Kokal firstly revealed that the crystal structure of Li7La3Zr2O12 was closely related with the synthesis temperature during the sol–gel synthesis routes [16], in which the cubic Li7La3Zr2O12 could be formed at 978 K, but if further increasing the sintered temperature up to 1073 K, this oxide crystallized into tetragonal garnet-related phase. Thus according to the reported experimental results, it is obvious that the phase structure of the tetragonal Li7La3Zr2O12 is actually very complex, and to truly understand structural properties of the tetragonal Li7La3Zr2O12 as well as its phase transition processes, it still needs systematic investigation. In this paper, the phase complexity of the tetragonal Li7La3Zr2O12 was
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carefully investigated by variable-temperature X-ray diffraction (XRD) and internal friction (IF) methods and multiple tetragonal–cubic phase transitions are found. Moreover, it is particularly important that the cubic Li7La3Zr2O12 induced by the tetragonal–cubic phase transition can be stable to room temperature, which results in a great improvement of lithium ionic conductivity. 2. Experimental The polycrystalline ceramics samples of Li7La3Zr2O12 in our investigation were prepared by conventional solid-state reaction method from a stoichiometric mixture of high purity La(OH)3 (99.99%), Li2CO3 (99.9%) and ZrO2 (99.9%). To compensate the loss of Li induced by the high temperature sintering, 10% wt.% excess of Li2CO3 was added. After having been well mixed in an agate mortar, the initial powders were heated at 1000 °C for 5 h, and a pure tetragonal Li7La3Zr2O12 with garnet-type structure would be obtained. During our sample preparation, the initial reaction powder Li7La3Zr2O12 was shaped into a pellet with a press of 100 MPa at first, and then put on an aluminum crucible covered with the same Li7La3Zr2O12 reactant powder to avoid aluminum contamination at high temperature. The synthesized powder sample was uniaxially shaped in a round mold at first, and then pressed in an isostatic press with a press of about 200 MPa. To get high-density ceramic sample, the discs were sintered at 1050 °C for 10 h, and then cooled down to room temperature with furnace. The dimension of the finally sintered samples was about 28 mm in diameter and 1.5 mm in thickness, and the density of the sample was about 93% of their theoretical density calculated from X-ray diffraction (XRD) data. The room temperature XRD was recorded on a θ/θ Bragg–Brentano X'Pert MPD PRO diffractometer (Cu Kα1 + 2 radiations). These patterns were collected in a scanning range from 10° to 70° with a step of 0.033°. The thermo-diffractograms were recorded in the same X'Pert MPD PRO apparatus equipped with a higher temperature heating chamber. The variable-temperature patterns were collected between 25 °C and 950 °C with the same counting step of 0.033°. Whole pattern matching refinement of the powder XRD patterns was performed using Fullprof program, in which the pseudo-Voigt profile function was used as the profile function in the refinements. Internal friction (IF) is an energy dissipation of mechanical energy, which is manifested by stress-stained hysteresis in the case of cyclic loading. Generally, IF (Q−1) can be written as [17]:
Q
−1
¼
ΔW 2πW
ð1Þ
where W is the maximum elastic stored energy during one cycle, ΔW is the corresponding energy absorption during that cycle. In this investigation, the low frequency IF measurements were carried out on a computer-controlled torsion pendulum under the forced vibration mode. The bar samples for the IF measurement were sawed from the sintered dense disc with a dimension of about 27 × 1 × 1.5 mm3. The maximum torsion strain amplitudes were kept to be 2.5 × 10−5 with a heating/cooling rate of 2 K/min in all measurements. The conduction properties were evaluated by alternate current (AC) impedance spectroscopy, in which the conductivity measurement of the Li7La3Zr2O12 compound was isothermally collected in air on a computer-controlled Hioki 3531 Z-Hitester frequency response analyzer in the frequency range 42 Hz–5 MHz. The square pellet for the electrical measurement was also cut from the sintered Li7La3Zr2O12 bulk, and the typical size of the square samples was about 10 × 10 × 1.5 mm3. Prior to each impedance measurement, the sample was stabilized at a constant temperature for 30 min to ensure thermal equilibrium. By analyzing the impedance spectra at different temperature, the resistances of the Li7La3Zr2O12 compound will be obtained. Thus based on Ohm's law, the ionic conductivity of the samples can be deduced.
3. Experimental results and discussion 3.1. Structural characterization Fig. 1 shows the room temperature powder XRD pattern of the initial Li7La3Zr2O12 compound in the scanning range from 15° to 75°, in which the obvious splitting of the diffraction lines demonstrates a typical tetragonal garnet-type phase. For the obtained tetragonal phase, the refinement of the XRD profile can be well indexed using space group I41/acd (no. 142), as shown in Fig. 1. Beside the symbols, the calculated, Bragg positions and defference pattern of the Rietveld refinement from the XRD data are also presented in Fig. 1, respectively. The lattice constants determined from the refinement are a = 13.126(2)Å and c = 12.675(8) Å with a cell ratio of c/a = 0.9657, which are well consistent with the reported values of a = 13.134(4)Å and c = 12.663(8) Å [6].
3.2. Internal friction analysis Fig. 2 illustrates the IF spectra of the tetragonal Li7La3Zr2O12 sample at a measuring frequency of 1 Hz in air under different thermal runs. During our experiments, the measuring temperature range actually covered from −50 °C to 300 °C, but only the IF spectra from −50 °C to 210 °C are presented, as shown in Fig. 2, because the profiles at higher temperature only exhibited a flat background and no IF peak was detected. As can be seen from this figure, a small IF peak, labeled as PT, is observed in the first heating run with a heating rate of 2 K/min, and the peak position locates around 48 °C. After subtracted the IF background, the peak height of the PT peak is only about 0.01. But in the following cooling run, the peak height of the IF peak dramatically increases up to 0.09, accompanying the peak position shifting towards about 16 °C. To clearly describe the evolution of the IF spectra, the strong IF peak is denoted as PC peak here. In the next heating/cooling run, the strong PC peak is found to appear stably, and the relaxational strength hardly changes, as shown in Fig. 2, where the IF spectra under the second heating/cooling run are present as an example. On the issue of obvious shift of peak position between the heating run and cooling run, it is due to the regular thermal hysteresis. Further investigations related with the IF peak are carried out in different environmental condition including vacuum, oxygen and argon, and similar evolution of the IF spectra is also observed in the different heating/cooling runs. These experimental results demonstrate that the evolution of the IF peak is independent on the external atmosphere condition. Noting that Li7La3Zr2O12 could exhibit two different phase structures: cubic and tetragonal, depending on the sintering temperature, as
Fig. 1. Observed, calculated, Bragg positions and defference of the Rietveld refinement from the XRD patterns of tetragonal Li7La3Zr2O12. The space groups used in the refinement is I41/acd (no.142) for the tetragonal phase. The inset gives the enlarged local fitting result.
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in cubic phase, the lithium ions have good mobility, and therefore lead to high ionic conductivity and strong relaxational IF peak. More direct evidence about the tetragonal–cubic transformation will be presented in the following variable-temperature XRD patterns.
3.3. Thermo-XRD analysis
Fig. 2. The temperature dependence of internal friction for the tetragonal La7La3Zr2O12 under different thermal runs.
reported by Kokal [16]. Moreover compared with the ordering distribution of lithium ions for the tetragonal garnet Li7La3Zr2O12 [6], the cubic phase has much higher ionic conductivity because of the disordering distribution of lithium ions [18]. So according to the reported results, the mechanism related with the dramatic difference of the peak height could be understood from the variation of the crystal structure of Li7La3Zr2O12 as well as lithium ion distribution with temperature. With regard to crystal structure of garnet-like Li7La3Zr2O12, La3+ ions occupy the eight-coordinate sites, and Zr4+ ions stay on the sixcoordinate octahedral sites, and there are two possible sites to accommodate Li ions: 24d and 48g. Since in Li7La3Zr2O12, the tetrahedral 24d sites can't enough to accommodate all Li ions, so the extra Li+ must enter the octahedral 48g sites that are empty for the standard garnet structure (If the occupied positions deviate from the octahedral center, it is called 96h). Further analyzing by the neutron diffraction patterns, the accommodation of lithium ions in both 24d sites and 48g sites are partially occupied, the occupation for the tetrahedral sites (24d) and octahedral sites (48g) is about 36.0% and 41.8%, respectively [7]. Because in garnet-like Li7La3Zr2O12, the interstitial tetrahedral sites (24d) are surrounded by face-sharing octahedral sites (48g), so the diffusion routes for lithium lions only have two possibilities: 48g↔24d and 48g ↔48g. Dynamic simulation based on bond-valence analysis also revealed that in garnet-type lithium electrolytes, the disordering distribution of lithium ions provided a 3D diffusion channels for carries in terms of the possible routes: 48g → 24d → 48g [19]. According to relaxational theory of internal friction, the relaxational strength of IF peak is proportional to the concentration and mobility of diffusion carriers. While in Li7La3Zr2O12, the diffusion ions are just the lithium ions that occupied in tetrahedral sites (24d) and octahedral sites (48g). So combined with the relaxational theory of internal friction and lithium occupation in Li7La3Zr2O12, the mechanism of the IF peak is suggested to originate from the following migration routes: 48g↔24d and 48g↔48g. Actually in our previous IF investigation on cubic garnet-type Li5La3M2O12(M = Ta, Nb, Bi) compounds [20–22], similar results were also obtained, in which a strong IF peak was observed at around room temperature, and both the relaxational strength and peak position were very close to that of PC peak in Li7La3Zr2O12, and the mechanism of the IF peak had been similarly described with the following two diffusion process ways: 48g↔24d and 48g↔48g. As for the obvious difference of relaxational strength between in the tetragonal phase and cubic phase, that is owing to the difference of lithium ion distribution between the tetragonal phase and the cubic phase. Since the order distributed lithium ions in the tetragonal phase only exhibit weak mobility, it results in poor conductivity as well as low IF intensity. On the contrary benefitting from disorder distribution
Fig. 3 gives the room temperature XRD patterns of the initial and thermal cycling Li7La3Zr2O12 compounds, in which the thermal powder is ground from the corresponding IF samples. As noted from this figure the initial tetragonal phase is found to transform into cubic structure after the first heating/cooling run in the temperature range 25 °C– 300 °C, implying that the tetragonal phase is actually metastable and tends to transform into high symmetric cubic phase in the investigation temperature range. It is worth pointing out that the thermal samples from the IF experiments are slowly cooled to room temperature (2 K/min) and not quenched, the maintenance of high temperature cubic phase evidences that the tetragonal–cubic phase transition is irreversible, as proved by IF spectra, in which the strong PC peak is found to stably appear after the first heating/cooling run. From another aspect, this XRD result also gives a direct evidence that the dramatic variation of IF spectra is really induced by the tetragonal–cubic phase transition during the heating measurement. Fig. 4 shows the variable-temperature XRD patterns of tetragonal La7La3Zr2O12 in the temperature range 25 °C–400 °C with an interval of 25 °C. As can be seen from this figure the splitting tetragonal diffraction lines are gradually merged into the simple cubic diffraction lines with increasing temperature, which further confirms the occurrence of tetragonal–cubic phase transformation. To clearly understand the detailed phase transition process of Li7La3Zr2O12 compound, the evolutions of the (213) and (426) diffraction lines with temperature are used as the signs to trace the phase transition process, as shown in Fig. 5(a) and (b). It is found that the two splitting diffraction lines associated with the tetragonal phase are gradually decreased, and finally merged into the cubic diffraction lines. In Fig. 5(c), the variations of diffraction intensity of the (213) and (426) diffraction lines with temperature are especially illustrated, and the intensity of the two diffraction lines are noted to dramatically decrease in the temperature range 100 °C–150 °C. This result clearly gives the temperature region of the tetragonal–cubic phase transformation, which is also in good agreement with the reported temperature region by Geiger [14]. To further explore the phase stability of Li7La3Zr2O12 compound at higher temperature, the evolution of in-situ XRD patterns of Li7La3Zr2O12 with temperature in the temperature from 400 °C to 1000 °C is also studied, as shown in Fig. 6. It is found that the transformed cubic phase
Fig. 3. The room temperature XRD patterns of the initial and heated Li7La3Zr2O12 samples, the tetragonal phase was found to transform into cubic structure after heating to 300 °C.
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Fig. 4. Powder XRD patterns of tetragonal La7La3Zr2O12 as a function of temperature. A phase transition from tetragonal to cubic symmetry was observed between 100 °C–150 °C, and the splitting diffraction lines in tetragonal phase disappeared after the tetragonal phase transform into cubic structure.
in the temperature range 100 °C–150 °C can be maintained to 800 °C, but further increasing temperature, the cubic phase gradually transformed into tetragonal phase again above 900 °C. Interestingly, the transformed tetragonal Li7La3Zr2O12 compound can be maintained to room temperature with furnace cooling. In addition, it is noted that although the phase above 900 °C exhibits typical tetragonal structure, it is slightly different from the initial tetragonal phase before heating (see Fig. 3). The main difference between the initial phase and high temperature XRD patterns lies in the relative intensity of the diffraction lines. The reason possibly originates from the preferred orientation of the surface of the heated sample induced by the high temperature sintering process. According the XRD results above, it clearly indicates that the undoped tetragonal Li7La3Zr2O12 compound is actually unstable, which exhibits complex and multiphase tetragonal–cubic–tetragonal transformation process. Especially in terms of the forgoing IF results that the evolution of IF spectra isn't independent on the variation of the external measurement environment, the tetragonal–cubic phase transformation at lower temperature should be intrinsic, obviously different from the reversible tetragonal–cubic phase transition induced by CO2 absorption at around 400 °C. On the other hand, the temperature regime of the phase transition at around 100 °C–150 °C is relatively low, it is therefore excluded the potential influence of lithium volatilization. In this condition, this lower temperature tetragonal–cubic phase transition should be related with the variation of a temperature-dependent redistribution of Li ions, from order distribution in the tetragonal phase to disorder distribution in cubic phase. As for the cubic–tetragonal phase transition around 800 °C–900 °C, it can be similarly ascribed to the disorder–order transition of lithium distribution. During the investigation of Li7La3Zr2O12 structure, Janani and the coworkers had successfully synthesized a pure cubic Li7La3Zr2O12 in the temperature range 700 °C–800 °C by a modified sol–gel method without any cationic substitution [13], which indicated that the cationic substitution was not essential to form cubic Li7La3Zr2O12 phase. Kokal further expanded the heated temperature range [16], and revealed that the crystal structure of Li7La3Zr2O12 was closely dependent on sintering temperature, once the heated temperature increased up to 800 °C–900 °C, it was not formed cubic phase again but tetragonal
Fig. 5. Variation with temperature of the typical splitting diffraction lines associated with the tetragonal–cubic phase transformation in La7La3Zr2O12 compound: (a) Evolution of (213) diffraction line; (b) Evolution of (426) diffraction line; (c) Evolution of the intensity for the (213) and (426) diffraction lines, respectively.
phase. This result is in agreement with the observed cubic–tetragonal phase transition process at 900 °C in our investigation. 3.4. Conductivity measurement The variation of lithium ionic conductivity of tetragonal Li7La3Zr2O12 with temperature is determined by AC impedance spectroscopy. The typical complex spectra at four difference temperatures of 150 °C, 250 °C, 350 °C and 450 °C are shown in Fig. 7. As can be seen that in each Nyquist spectrum, one or two pressed semicircles are observed at the high frequency regime, such compressed semicircles are contributed to the resistance of both bulk and grain boundaries. At low frequency region, an obvious polarized tail is observed, which indicates
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Fig. 6. Evolution of in-situ XRD patterns of Li7La3Zr2O12 with temperature in the temperature from 400 °C to 1000 °C. Fig. 8. Temperature dependence of total ionic conductivity for tetragonal Li7La3Zr2O12.
that the conductivity of the Li7La3Zr2O12 compound is actually ionic in nature, as reported in Ref. [7,23]. Since the bulk resistance and grainboundary resistance are not obviously separated from each other from the impedance plots, therefore the lithium ionic conduction properties of the sample are evaluated by using total conductivity. For the initial tetragonal Li7La3Zr2O12, the total lithium ionic conductivity determined from the AC impedance spectra was only about 1 × 10−7 S/cm at room temperature. Compared with the reported value of 3.1 × 10−4 S/cm in Al stabilized cubic phase [7], this conductivity is actually too poor to be applied for practical application. Fig. 8 presents the lithium ionic conductivity of the temperature dependence of total ionic conductivity for the tetragonal Li7La3Zr2O12 during the initial heating process, and an obvious enhancement of conductivity is observed in the temperature range 150 °C–200 °C, in which the conductivity was found to dramatically increase from 2.9 × 10−4 S/cm at 150 °C to 2.0 × 10−3S/cm at 251 °C. This abrupt variation of conductivity is also ascribed to the tetragonal–cubic phase transition, as verified by the thermal XRD and IF measurements. Benefitting from the tetragonal–cubic phase transformation, the order distribution of lithium ions in the tetragonal
phase is induced into disorder distribution, and thus greatly increases its conductivity. As have been pointed out by XRD and IF measurements, the obtained cubic phase after the phase transition can be maintained to room temperature, the irreversible characteristic of phase transition reveals a novel method to obtain undoped cubic Li7La3Zr2O12 with high ionic conductivity. 4. Conclusion The phase transition processes of garnet-related type Li7La3Zr2O12 with tetragonal symmetry were carefully investigated by IF and variable-XRD methods, and multiple irreversible phase transition processes are found, which are the tetragonal–cubic phase transition located around 100 °C–150 °C and cubic–tetragonal phase transition around 800 °C–900 °C, respectively. Compared with the weak IF peak in the tetragonal Li7La3Zr2O12, the IF peak in the cubic phase is much stronger, and the mechanism related with variation of relaxational strength of
Fig. 7. Impedance spectra of tetragonal Li7La3Zr2O12 at four different temperatures: (a) t = 150 °C; (b) t = 250 °C; (c) t = 350 °C; (a) t = 450 °C.
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the IF peak was suggested to originate from the order–disorder transition of lithium distribution induced by the tetragonal–cubic phase transformation. During the investigation of ionic conductivity, an abrupt variation of conductivity was found in the temperature range 150 °C– 200 °C, which also corresponds to tetragonal–cubic phase transformation. In terms of the irreversible characteristic of tetragonal–cubic phase transition, a novel method was proposed to obtain the undoped cubic Li7La3Zr2O12 with high ionic conductivity. Acknowledgments This work was subsidized by the National Natural Science Foundation of China (Grant Nos: 11374299, 11274305). References [1] [2] [3] [4] [5] [6]
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