Origin of High Li+ Conduction in Doped Li7La3Zr2O12 Garnets

Gallium-Doped Li7La3Zr2O12 Garnet-Type Electrolytes with High Lithium-Ion Conductivity. Jian-Fang Wu , En-Yi Chen , Yao Yu , Lin Liu , Yue Wu , Wei Ko...
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Origin of High Li+ Conduction in Doped Li7La3Zr2O12 Garnets Yan Chen,† Ezhiylmurugan Rangasamy,‡ Chengdu Liang,‡ and Ke An*,† †

Chemical and Engineering Materials Division and ‡Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

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octahedral sites, distinguished from the influenced sites near the dopants, is the key to the ionic conductivity, which is determined by the valence and the content of aliovalent doping. The LLZO, LLZO-Al24, and LLZO-Zn60 samples were synthesized at 1000 °C with trivial impurities (Figure 1a).

ubstituting a native ion in the crystals with a foreign ion that has a difference in valence, termed as aliovalent doping, has been widely attempted to upgrade solid-state ionic conductors for various charge carriers including O2−, H+, Li+, Na+, and so forth.1−4 The doping aids to promote the high-conductive framework and dredge the tunnel for fast ion transport. The garnet-type Li7La3Zr2O12 (LLZO) is a fast Li+ solid conductor, which received vast attention as an electrolyte candidate for allsolid-state lithium ion batteries, showing great potential to offer high energy density and minimize battery safety concerns to meet extensive applications in large energy storage systems such as electric vehicles and aerospace.5−8 In the Li-stuffed garnet framework of LLZO, the 3D pathway formed through the incompletely occupied tetrahedral sites bridged by a single octahedron enables the superior Li+ conductivity.9,10 For the purpose of optimal performance, many efforts of aliovalent doping have been made throughout metal elements (Al3+, Ta5+) and metalloid elements (Ga3+, Te6+) in the periodic table with various valences11−14 to stabilize the high-conductive phase and increase the Li vacancy concentration.7,10,15 However, the governing mechanism of the high conductivity through aliovalent doping is still not fully understood. Doping does not result in a much different garnet framework of highconductive cubic phase from that of the low-conductive tetragonal phase.16,17 The aliovalent doping does not tremendously change the Li vacancy concentration, either, because of the “sufficient” vacancies (∼16 vacancies distributed in the 3Dconnecting 24 tetrahedral sites plus 48 octahedral sites) preexisting in both cubic and tetragonal phases although with different arrangements.16,17 The slight structure tuning above is not fully responsible, however, for ionic conductivity varying by orders of magnitude16 among those “similar” garnets. It is noted that the vacancies in tetragonal LLZO reside in the tetrahedral sites but hardly help the conduction. Therefore, rather than the global concentration, the vacancy density in the right site, the bridging octahedral site, is thought to be critical for fast Li+ transport.16,18 The doping may alter the vacancy distribution in the octahedral site to impact the conductivity. With this hypothesis, a question is then raised: is there a rule to control the vacancy distribution by aliovalent doping to trigger the fast Li+ conduction? In this work, we chose two Li-site-doping garnets by two distinct dopants (Al, Zn): Li6.28Al0.24La3Zr2O12 (abbr. LLZOAl24) and Li5.8Zn0.6La3Zr2O12 (abbr. LLZO-Zn60), along with LLZO as reference. The two dopants belong to different families and groups, have different contents and valences, and bring different amounts of vacancies. We then discerned a common rule that the dopants obey in redistributing the vacancies in the framework of the garnets. The Li vacancy density in the active © XXXX American Chemical Society

Figure 1. (a) Neutron diffraction and Rietveld refinement of synthesized LLZO-Al24 garnet at 1000 °C. The possible Li+ hopping paths (sticks) are composed by tetrahedral and octahedral Li+ sites (spheres). (b) Lithium occupancies in the tetrahedral (tet.) and octahedral (oct.) sites as a function of temperature.

Neutron diffractions performed at Vulcan instrument (SNS, ORNL)19 confirm the dopants residing at tetrahedral Li-site (see Supporting Information for details), which agrees with the conclusions by density functional theory (DFT), nuclear magnetic resonance, and other ex situ neutron diffraction studies.20−22 With the dopants participating, the global Li-site occupancies as a function of temperature are revealed in Figure 1b. The distributions in the three samples are consistent at 1000 °C. In response to cooling, the phase transition takes place in LLZO at 630 °C, leading to an ordered distribution of Li+. The cubic phase is stabilized in LLZO-Al24 and LLZO-Zn60; however Al3+ and Zn2+ seem to alter the distribution differently: Li+ migrate from octahedral sites to tetrahedral sites in LLZOReceived: July 1, 2015 Revised: August 6, 2015

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DOI: 10.1021/acs.chemmater.5b02521 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 2. (a) Ideal model of 3D Li+ fast pathway in disordered cubic LLZO. Both the tetrahedral (tet.) sites and octahedral (oct.) sites are partially occupied. (b) In LLZO-Al24, the pathway is mostly maintained with a few blocked by Al dopant. Despite of the vacancies trapped beside Al3+, there are sufficient vacancies distributed in the active oct. sites that contribute to conduction. (c) In LLZO-Zn60, the pathway is highly suppressed by the Zn2+. There lack vacancies at the active oct. sites to conduct Li+ transport along the pathway because all oct. vacancies are trapped at the influenced sites beside Zn2+.

locally they do not reside at the neighboring sites in order to lower the electrostatic energy. By differentiating effects of the driving forces, we unraveled how the dopant impacts the Li+ and Li vacancy distribution at RT, which is found consistent in both doped garnets. At the Li+ favorite tetrahedral sites, the vacancy quantity is altered with the dopant amount. It is found that each dopant brings about 3.22 and 3.17 vacancies at the tetrahedral sites per unit cell in LLZOAl24 and LLZO-Zn60, respectively (Table S10). Interestingly, this ratio ( f) is close to 3.2, regardless of the dopant type and its valence. In the octahedral sites, the vacancies tend to stay in the influenced sites first and then, if more vacancies exist, the active sites due to the Li−dopant repulsion. At this point, it leads to a fatal difference of active vacancy density in LLZO-Al24 and LLZO-Zn60. The former one has sufficient vacancy to deplete all the influence octahedral sites, and there remain 5.96 vacancies per unit cell in the active octahedral sites; the latter one has insufficient vacancies, and all of them are bound at the influenced sites (Table S11). As demonstrated below, this difference is the cause of the different electrical performances of the two materials. The active vacancy density at the octahedral sites, denoted by v, determines the Li+ conductivity. Employing supervalent dopant may increase Li vacancies in the octahedral sites, but the dopant itself also plays a role to squeeze the Li+ pathway in comparison to the ideal 3D pathway (Figure 2). First, the electrostatic repulsion of dopants lowers the probability of Li+ occupation at the influenced octahedral sites. Second, the dopant in tetrahedral site physically blocks the pathway passing its site. It directly reduces the active vacancy density since the immobile vacancies at the influenced octahedral sites do not effectively contribute to the Li+ conduction. Consider a particular amount of Li ions are substituted by M dopants (+n valence, at tetrahedral Li-site) in Li7‑nxMxLa3Zr2O12 (where x should be small), in one unit cell (8 times formula), knowing the total vacancy amount 16 + 8(n − 1)x, vacancy number in tetrahedral sites 8f x (f ≈ 3.2 for both Al and Zn experimentally), and the number of influenced octahedral sites 32x, we then estimate v using a simple relation of the dopant’s concentration and the valence: v = 16 − (40 − 8n + 8f)x. When v > 0, such as in LLZOAl24 (Figure 2b), a number of vacancies reside in the active octahedral sites that enable a fast Li+ pathway bridging the two neighboring tetrahedral sites. The fast pathways throughout the garnet 3D framework result in the high conductivity in LLZO-

Al24, while it is reversed in LLZO-Zn60. The paradoxical trends are caused by the joint effect from two types of driving forces: entropy and electrostatic repulsions. The entropy dominates the distribution at the high temperature (i.e., 1000 °C), and it promotes the tendency toward a particular disordered structure at high temperatures: Li+ fully occupy the octahedral sites and randomly scatter in 1/3 of the tetrahedral sites.17 The refined results in LLZO are consistent with such entropy controlling. The Al3+ and Zn2+ dopants unlikely ruin this configuration although the vacancy concentration is slightly increased after doping. The effect of electrostatic repulsion is less shown on altering the vacancy distribution, despite of the reduced probability of Li+ presence at the supervalent dopant’s neighboring octahedral sites. As a result, the change of octahedral site vacancy at 1000 °C is consistent with the dopant content. The rest of the vacancies that compensate the charge difference are in the tetrahedral sites, where the vacancy is favored. As the temperature is reduced, the effect of entropy is weakened and not able to maintain the distribution at the high temperature. The effects of electrostatic repulsion, including both Li+−Li+ and Li+−dopant pairs, gradually dominate and redistribute Li+ between tetrahedral and octahedral sites. The Li+−Li+ repulsion drives Li+ to reside at a tetrahedral site for increasing their mutual distances,23 and the Li+−dopant repulsion influences the dopant’s neighboring Li-sites. Around the supervalent dopant ion, the four nearest octahedral sites are most influenced. The strong repulsion makes them unfavorable for Li+ occupying, and thus, the vacancies can be trapped and deactivated beside the dopants. The inaccessible Li vacancies were demonstrated in Ga-doped LLZO as a defect cluster.24 Although the octahedral vacancy is thought to help the conduction,16,18 the contribution is limited with a dopant neighbor. Here we denote those octahedral sites beside a dopant as “influenced sites”, and the other octahedral sites that are “far” away from the influence of dopants as “active sites”. The multiple states of vacancy may bring various activation energies (slope in the Arrhenius plots) of Li+ conductivity in doped garnets.25 The dominant repulsion also has impacts on the distribution in tetrahedral sites. However, the dopant at a tetrahedral site unlikely trap the neighboring tetrahedral vacancies due to the larger distance of two tetrahedral sites and screening of the octahedron between them. Here we assume that the dopant ions are homogeneously distributed in the tetrahedral sites and that B

DOI: 10.1021/acs.chemmater.5b02521 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Al24 (>10−4 S cm−1). When v ≤ 0, such as in LLZO-Zn60 (Figure 2c), there is a lack of vacancies in the active octahedral sites because all octahedral vacancies are trapped and deactivated at the influenced sites that are not able to contribute to Li+ conduction. The fully occupied octahedral sites provide less chance for Li+ hopping through and thus lead to a slow pathway. It is noted that the immobile dopant ions do not block all of the possible pathways; however, it is the overwhelming slow pathways that limit the Li+ conduction in LLZO-Zn60 (∼10−7 S cm−1, details to be published in a separate paper). The tetrahedral vacancy quantity may not be the bottleneck of Li conduction. Although abundant vacancies reside in the tetrahedral site, the Li+ mobility here strongly depends on the accessibility of the octahedral bridges that converge on that site. As a large atomic displacement parameter (Uiso) usually suggests a mobile ion, the refined Uiso values of Li in tetrahedral sites (see Supporting Information for details) reflect the fast pathways in LLZO-Al24 (100Uiso = 6.34) and the slow pathways in LLZOZn60 (100Uiso = 0.65) and LLZO (100Uiso = 0.71) at RT. The v-dependence of the Li+ conductivity (σLi) is shown in Figure 3a. Poor σLi is received for v ≤ 0, while the positive v

low σLi. Therefore, Figure 3b includes the phase boundary (dashdot line) between cubic and tetragonal phase, which is according to the estimation by DFT.17 The map in Figure 3b is hereby divided into three portions by the phase boundary and the contour of v = 0. The red-colored portion stands for the cubic garnet phase with improved σLi. Also, there is an underlying upper limit of the valence because the Li site has difficulty adapting to high-valence dopant. They may substitute for other ions (La3+ or Zr4+) or have very low solubility in the garnets. The feasibility of the Li-site dopants with large n thus needs to be taken into account. For the doping optimization of the garnet electrolytic performance, some key guidance can be provided with the relationship of ionic conductivity and aliovalent dopants (Figure 3b). (1) It is impossible for +2 dopants from IIA, IB, or IIB groups in the periodic table, such as Zn2+, to generate a positive v in a stabilized cubic garnet. Therefore, seldom results have been reported on this type of material design for a fast Li+ conductor. (2) For the purpose of high Li+ conduction, metalloid dopants with high valence in IVA and VA groups (Si, Ge, Sn, As, and Sb) will potentially impart a large v and then high σLi if the compounds are able to synthesize with dopants substituting in the right site in LLZO. The transition metals at VIB−VIIIB groups, which have high and usually variant valences, are additional options for the dopant as well. As an example, Fe was reported to replace Li and stabilize the cubic LLZO.30 (3) In a particular doped garnet system, the largest v is reached at the phase boundary theoretically. However, it is not a distinct boundary. The compositions near the phase boundary probably cause the presence of the low Li-conducting tetragonal phase, due to either the thermodynamic equilibrium of two-phase coexistence or the local inhomogeneous distribution of dopants. Therefore, while approaching the boundary with increasing v, a compromise needs to be made by lowering the dopant concentration. (4) The dopants that have the same valence (in the same group, for example), such as Al3+ and Ga3+,12,15 receive similar σLi. At this point, besides feasibility and solubility, the extrinsic factors thus become more important in aliovalent dopant selection, including the improvement of sintering density, formation of secondary phases with low σLi, and so forth. (5) The concept may be further generalized to garnet systems with aliovalent doping in other sites. If there is a dopant ion at La-site or Zr-site, the most influenced Li+ or vacancies are also at the nearest neighboring octahedral sites, like our cases. It probably draws a similar map as Figure 3b for the active vacancy density and ionic conductivity, which has been evidenced by the fact that the compositions near the cubic and tetragonal phase boundary usually achieve the highest ionic conductivities in many garnet systems,31 with doping in various sites. In summary, engineering of vacancy can be manipulated through aliovalent doping to tune vacancy quantity, control vacancy distribution, and alter the charge carrier pathway in solid electrolytes. The active vacancy density, controlled by the valence and the concentration of aliovalent doping, determines the ionic conductivity. The doping−performance relationship provides the direction of searching high ionic conductive solid electrolytes and accelerates the process of material findings.

Figure 3. (a) Relationship between Li+ conductivity (σ) and the active octahedral vacancy density (v) in a unit cell. Our hypothesis agrees with the measured data in the literature. (b) Contour map indicates v as a function of the valence n and doping level x of M in the Li7‑nxMxLa3Zr2O12 garnet (assuming f = 3.2). The dash-dot line is a reference phase boundary between the cubic phase and the tetragonal phase (t-phase).

boosts σLi with a strong correlation. Our material systems15,16 LLZO (v = 0), LLZO-Al24 (v = 5.96), and LLZO-Zn60 (v = −13.60) agree with this dependence. Moreover, a number of reported similar material systems12,21,22,26−29 with various Al and Ga dopant contents are also covered by the trend we hypothesize. Recalling that the same rule of Li+ and Li vacancy distribution is induced from LLZO-Al24 and LLZO-Zn60, which contain different divalent and trivalent dopants with different doping levels, it is possible to generalize this simple rule in other tetrahedral Li-site doped LLZO garnets to calculate the value of v and thus to estimate the improvement of σLi. A map is constructed in Figure 3b to indicate v as a function of n and x. Generalizing from Al and Zn, here we assume f ≈ 3.2 for possible dopants universally. Although f is estimated experimentally so far, slight adjustment of f does not significantly change the nature of the map. As a direct indication in the simple model (Figure 3), a tendency of large v and thus high σLi is predicted from low content of dopant with high valence. However, there has to be a lower limit of concentration because the low concentration of dopant may lead to the loss of stability of the disordered cubic phase. The phase transition to an ordered tetragonal phase will rearrange the Li+ and fulfill all octahedral sites, resulting in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02521. C

DOI: 10.1021/acs.chemmater.5b02521 Chem. Mater. XXXX, XXX, XXX−XXX

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(15) Rangasamy, E.; Wolfenstine, J.; Sakamoto, J. The role of Al and Li concentration on the formation of cubic garnet solid electrolyte of nominal composition Li7La3Zr2O12. Solid State Ionics 2012, 206, 28−32. (16) Awaka, J.; Kijima, N.; Hayakawa, H.; Akimoto, J. Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure. J. Solid State Chem. 2009, 182, 2046−2052. (17) Bernstein, N.; Johannes, M. D. Origin of the structural phase transition in Li7La3Zr2O12. Phys. Rev. Lett. 2012, 109, 205702. (18) Xu, M.; Park, M. S.; Lee, J. M.; Kim, T. Y.; Park, Y. S.; Ma, E. Mechanisms of Li+ transport in garnet-type cubic Li3+xLa3M2O12 (M = Te, Nb, Zr). Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 052301. (19) An, K.; Skorpenske, H.; Stoica, A.; Ma, D.; Wang, X.; Cakmak, E. First in situ lattice strains measurements under load at VULCAN. Metall. Mater. Trans. A 2011, 42, 95−99. (20) Rettenwander, D.; Blaha, P.; Laskowski, R.; Schwarz, K.; Bottke, P.; Wilkening, M.; Geiger, C. A.; Amthauer, G. DFT study of the role of Al3+ in the fast ion-conductor Li7−3xAl3+xLa3Zr2O12 garnet. Chem. Mater. 2014, 26, 2617−2623. (21) Lee, J. M.; Kim, T.; Baek, S. W.; Aihara, Y.; Park, Y.; Kim, Y. I.; Doo, S. G. High lithium ion conductivity of Li7La3Zr2O12 synthesized by solid state reaction. Solid State Ionics 2014, 258, 13−17. (22) Buschmann, H.; Dölle, J.; Berendts, S.; Kuhn, A.; Bottke, P.; Wilkening, M.; Heitjans, P.; Senyshyn, A.; Ehrenberg, H.; Lotnyk, A.; Duppel, V.; Kienle, L.; Janek, J. Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12. Phys. Chem. Chem. Phys. 2011, 13, 19378−19392. (23) Jalem, R.; Yamamoto, Y.; Shiiba, H.; Nakayama, M.; Munakata, H.; Kasuga, T.; Kanamura, K. Concerted migration mechanism in the Li ion dynamics of garnet-type Li7La3Zr2O12. Chem. Mater. 2013, 25, 425− 430. (24) Jalem, R.; Rushton, M. J. D.; Manalastas, W., Jr.; Nakayama, M.; Kasuga, T.; Kilner, J. A.; Grimes, R. W. Effects of gallium doping in garnet-type Li7La3Zr2O12 solid electrolytes. Chem. Mater. 2015, 27, 2821−2831. (25) Thangadurai, V.; Narayanan, S.; Pinzaru, D. Garnet-type solidstate fast Li ion conductors for Li batteries: critical review. Chem. Soc. Rev. 2015, 43, 4714−4727. (26) Li, Y.; Han, J. T.; Wang, C. A.; Vogel, S. C.; Xie, H.; Xu, M.; Goodenough, J. B. Ionic distribution and conductivity in lithium garnet Li7La3Zr2O12. J. Power Sources 2012, 209, 278−281. (27) Shimonishi, Y.; Toda, A.; Zhang, T.; Hirano, A.; Imanishi, N.; Yamamoto, O.; Takeda, Y. Synthesis of garnet-type Li7−xLa3Zr2O12−1/2x and its stability in aqueous solutions. Solid State Ionics 2011, 183, 48−53. (28) Chen, R. J.; Huang, M.; Huang, W. Z.; Shen, Y.; Lin, Y. H.; Nan, C. W. Effect of calcining and Al doping on structure and conductivity of Li7La3Zr2O12. Solid State Ionics 2014, 265, 7−12. (29) Tadanaga, K.; Takano, R.; Ichinose, T.; Mori, S.; Hayashi, A.; Tatsumisago, M. Low temperature synthesis of highly ion conductive Li7La3Zr2O12−Li3BO3 composites. Electrochem. Commun. 2013, 33, 51− 54. (30) Rettenwander, D.; Geiger, C. A.; Amthauer, G. Synthesis and crystal chemistry of the fast Li-ion conductor Li7La3Zr2O12 doped with Fe. Inorg. Chem. 2013, 52, 8005−8009. (31) Thompson, T.; Wolfenstine, J.; Allen, J. L.; Johannes, M.; Huq, A.; David, I. N.; Sakamoto, J. Tetragonal vs. cubic phase stability in Al-free Ta doped Li7La3Zr2O12 (LLZO). J. Mater. Chem. A 2014, 2, 13431− 13436.

Details of experimental and neutron diffractions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE). Neutron work at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, BES, DOE. The authors thank Mrs. R. Mills, Mr. M. Frost, and Mr. H. Skorpenske from SNS for their technical support. The authors thank Mrs. G. Zhu for the technical support.

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DOI: 10.1021/acs.chemmater.5b02521 Chem. Mater. XXXX, XXX, XXX−XXX