9840 Chem. Commun., 2012, 48, 9840–9842 This journal is c The Royal Society of Chemistry 2012

Cite this: Chem. Commun., 2012, 48, 9840–9842

Experimental visualization of lithium conduction pathways in garnet-type

Li7La3Zr2O12w

Jiantao Han,*aJinlong Zhu,

aYutao Li,

bXiaohui Yu,

aShanmin Wang,

aGang Wu,

aHui Xie,

b

Sven C. Vogel,*aFujio Izumi,

cKoichi Momma,

dYukihiko Kawamura,

eYunhui Huang,

f

John B. Goodenoughband Yusheng Zhao*

a

Received 16th July 2012, Accepted 7th August 2012

DOI: 10.1039/c2cc35089k

The evolution of the Li-ion displacements in the 3D interstitial path-

ways of the cubic garnet-type Li7La3Zr2O12, cubic Li7La3Zr2O12,

was investigated with high-temperature neutron diffraction (HTND)

from RT to 600 8C; the maximum-entropy method (MEM) was

applied to estimate the Li nuclear-density distribution. Temperature-

driven Li displacements were observed; the displacements indicate that

the conduction pathways in the garnet framework are restricted to

diffusion through the tetrahedral sites of the interstitial space.

Recently the Li-ion conductors with a garnet-type framework

have attracted attention because of their high conductivity as

solid-state electrolytes for lithium batteries. Thangadurai

et al.1 initially reported Li-ion conduction in Li5La3M2O12

(M = Nb, Ta) as a new class of Li-ion conductors, which are

different from the well known Li-ion conductors, viz. layered

Li3N and Li-b-alumina,2 Li1.3Ti1.7Al0.3(PO4)3 with a NASICON-

type framework (open channel),3 interstitial (Li14ZnGe4O16), and

defective perovskite (Li3xLa(2/3)�xTiO3) structures.4 The garnets

contain a cubic La3M2O12 framework with a 3D interstitial

pathway of 9 interconnected interstitial sites per formula unit

(f.u.), and hence isotropic Li-ion conductivity occurs in three

dimension similar to the case of Li-ion conductors with the

NASICON framework.5 The highest Li-ion conductivity has

been reported in a nominal cubic Li7La3Zr2O12 (c-LLZO) that

is stable at 1230 1C, which is needed for sintering a dense

ceramic.6 Significant advances in this field have established (1)

the existence of a second tetragonal phase (t-LLZO) with a much

lower conductivity than c-LLZO;7 (2) a small concentration of

only about 1 wt%Al appears to be sufficient to stabilize a nominal

c-LLZO phase8 and introduce more Li+ vacancies; (3) the

maximum Li concentration in the garnet framework was

predicted to be 7.5 Li per f.u. with 7.0 Li per f.u. representing

a practical upper limit;9 (4) Li6La3ZrTaO12 (sLi,bulk E 2.5 �10�4 S cm�1 and Ea E 0.4 eV) shows a little bit lower

conductivity and higher activation energy than the reported

c-LLZO, but it opens a much easier preparation route, and

c-Li6.5La3Zr1.5Ta0.5O12 has shown higher Li-ion conductivity,

sLi E 10�3 S cm�1 at room temperature.10

The ab initio calculations by Xu et al.11 have indicated two

possible pathways for Li transport in a garnet framework: one

bypasses the 24d tetrahedral sites and the other is via the 24d

sites. The pathway contains the tetrahedral 24d sites that are

occupied by Li in a Li5La3M2O12 garnet as well as octahedral

sites that bridge the tetrahedral sites with shared faces on

opposite sides of the octahedral (see Fig. S1a, ESIw). A similar

3D interstitial pathway is found in the [M2]O4 framework of

the A[M2]O4 spinels. Adams and Rao12 have argued that the

lower energy pathway is via the 24d sites. Using high-temperature

neutron diffraction (HTND), we have studied the evolution with

temperature of the Li distribution and displacements in the 3D

interstitial pathways of the c-LLZO polycrystalline samples pre-

pared by solid-state reaction as reported elsewhere,6 but with7LiCO3 in the starting mixture. The ND data were collected at

different temperatures (RT, 200, 400, 600 1C) with a high-pressure,

preferred-orientation (HIPPO) neutron time-of-flight powder

diffractometer at the Lujan Neutron Scattering Center, Los

Alamos National Laboratory. Bulk samples were placed in a

vanadium can and neutron time-of-flight data were collected

under vacuum at room temperature. Neutrons were detected with

42 panels of 3He detector tubes arranged in three rings with

nominal diffraction angles of 401, 901, and 1441. For each of four

different temperatures from RT to 600 1C, structures were refined

with the General Structural Analysis System GSAS developed by

Larson and Von Dreele.13 The refinement results have been

obtained with the crystal model reported by Cussen.14 The unit-

cell parameters, atomic positions, isotropic displacement para-

meters, and peak widths in the profile function were all refined.

Fig. 1 shows the Rietveld refinement patterns for c-LLZO

obtained by time-of-flight ND data at room temperature and

600 1C, respectively. The patterns were indexed on the cubic

a LANSCE-Lujan Neutron Scattering Center, Los Alamos NationalLaboratory, Los Alamos, New Mexico 87545, USA.E-mail: jthan0509@gmail.com, sven@lanl.gov,yusheng.zhao@unlv.edu

b Texas Materials Institute, ETC 9.102, The University of Texas atAustin, TX 78712, USA

cQuantum Beam Center, National Institute for Materials Science,Namiki, Tsukuba, Ibaraki 305-0044, Japan

dNational Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba,Ibaraki 305-0005, Japan

e Research Center for Neutron Science and Technology, Tokai-mura,Naka-gun, Ibaraki 319-1106, Japan

f School of Materials Science and Engineering, Huazhong Universityof Science and Technology, Wuhan, Hubei 430074, Chinaw Electronic supplementary information (ESI) available: Experimentaldetails and Fig. S1–S4. See DOI: 10.1039/c2cc35089k

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 9840–9842 9841

space group Ia%3d (no. 230); there was no evidence of a tetragonal

modification. The fitting was satisfactory (Rwp = 2.99%, Rp =

0.46%, and w2 = 2.34 for RT) with precisely refined atomic

positions as well as anisotropic atomic displacement parameters

for all atoms under the classical harmonic-oscillation model.

The crystal structure and possible Wyckoff positions of Li

are shown in Fig. S1 (ESIw). The Li-ion conduction pathways

in the Cussen model consist of three distinguishable positions:

the 24d tetrahedral sites of the garnet structure, the 48g center

of the octahedral sites bridging the 24d sites; and where only

one of the two bridged 24d sites is occupied by Li+, a 96h

position in the bridging site near the face shared by the empty

24d neighbor. It was postulated that Li is not stable in an

octahedral site that bridges two occupied 24d sites,9 but it is

stable in a 48g position if the octahedral site bridges two empty

24d sites.

Fig. S2 (ESIw) shows the temperature dependence of the

cubic a lattice parameter; it reveals a small deviation from

linear thermal expansion, but with no evidence of a phase

transition. Our data indicate only a minor 1.1% enlargement

of the a parameter with temperature from RT to 600 1C. On

the other hand, the rise in temperature caused a pronounced

increase in the displacement parameters of all the atoms,

particularly the Li atoms, as is shown in Fig. S3 (ESIw). Theaverage preferred directions of the Li1 (24d) displacements are

towards the two opposing tetrahedral-site edges common to

two interfaces with bridging octahedra, see Fig. 1(b) and the

green ellipsoids of Fig. 2(a). These average preferred displace-

ment directions are indicative of individual displacements

toward the center of one of the four shared faces with a

bridging octahedron. The smaller Li2 (48g) and Li3 (96h)

displacements are along the axis of a bridge between 24d sites;

there is no indication of a bypass of the 24d sites. The strong

increase with temperature of the Li+ displacements relative to

those of the framework ions signals an enhancement of Li+

transport and disorder within the interstitial pathways in a

relatively stable background of framework ions.

From Fig. S3 (ESIw), the amplitude of the Li1 (24d)

displacements appears to increase above 600 1C at the expense

of those of the Li3 (96h), but not of the Li2 (48g); the Li+ ions

are transported between the Li1 (24d) and Li3 (96h) sites across

a common face, and the data appear to be consistent with a

more rapid cooperative intersite transfer across two shared

faces that involves shifting of Li3 (96h) sites within octahedra

via a 48g position as the Li1 (24d) occupancy shifts from one

side to the other of the bridging octahedron.

To obtain experimental evidence for the Li+ pathways and

the evolution with temperature of Li+ motions, the maximum-

entropy (MEM) was applied with the Dysnomia program to

estimate the density distribution of the neutron-scattering

Fig. 1 Neutron-diffraction patterns with Rietveld refinement for the

time-of-flight neutron diffraction profiles measured for c-7Li7La3-

Zr2O12 at room temperature and 600 1C, respectively.

Fig. 2 (a) Anisotropic harmonic lithium vibrations in c-7Li7La3Zr2O12

shown as green thermal ellipsoids obtained from Rietveld analysis of

room-temperature neutron diffraction data; (b) nuclear distribution of

lithium calculated by the MEM from neutron powder diffraction data

obtained at 600 1C; three-dimensional 7Li nuclear-density data shown as

blue contours (equi-value 0.15 fm A�3 of the negative portion of the

coherent nuclear scattering density distribution). The green octahedra

represent ZrO6 and the purple dodecahedra represent LaO8 units; (c), (d),

(e) and (f) show the distribution of lithium calculated by MEM from

neutron powder diffraction data obtained at room temperature, 200, 400

and 600 1C, respectively, and three-dimensional Li nuclear-density data

shown as blue contours (equi-value 0.15 fm A�3 of the negative portion

of the coherent nuclear scattering density distribution).

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9842 Chem. Commun., 2012, 48, 9840–9842 This journal is c The Royal Society of Chemistry 2012

lengths,15 which correspond to the nuclear-density distribution.

The negative nuclear densities obtained with MEM analysis of

the HTND data are shown in Fig. 2.

Since 7Li is the only negative neutron-scattering element in

the sample, all the stripes observed in the Fourier maps of

Fig. 2 are the Li-diffusion pathways. Fig. 2(c)–(f) demonstrate

a clear enhancement of the Li displacements, hence diffusivity,

with increasing temperature from ambient to 600 1C. The

broken stripes correspond to Li displacements located at 24d,

48g, and 96h sites, and displacements between 96h sites signal

interchange of the occupied 24d site of a pair of tetrahedral

sites. As temperature increases above about 400 1C, the broken

stripes become connected to form a continuous 3D network of

moving Li+ ions in the interstitial, interlocking network of

24d–96h–48g–96h–24d segments penetrating in 3D the garnet

framework. Thus, our results indicate that the Li+ ions in

c-LLZO move only within the predicted low-energy pathway

and that as the temperature increases from room temperature to

600 1C, the displacements of all the Li+ ions remain confined to

the interstitial pathways, including the 24d sites. Fig. 3 shows

the two-dimensional contour maps sliced on (001) planes with

z = 0.4 at T = room temperature, 200, 400, and 600 1C,

respectively; they show a clear delocalization process of lithium

located at (24d) and (96h) sites. Although the thermal vibrations

of the other atoms, La, Zr and O, are also enhanced based on

the MEM analysis, they remain in their initial positions and do

not have diffusion pathways such as Li.

In summary, polycrystalline c-LLZO was prepared by conven-

tional solid-state synthesis and investigated by high-temperature

neutron diffraction techniques coupled with the MEM

analysis. Temperature-driven dynamic Li+-ion displacements

show a Li 3D diffusion pathway consisting of interlocking

Li(24d)–Li(96h)–Li(48g)–Li(96h)–Li(24d) chain segments. The

results indicate that c-LLZO possesses a stable cubic phase at

high temperature and that the Li+ diffusion pathway in the

compound contains the tetrahedral 24d sites; there is no

evidence for Li+ bypassing the 24d sites.

The authors would like to acknowledge: Office of Vehicle

Technologies of the U.S. Department of Energy under Contract

No. DE-AC02-05CH11231 and the BATT Program subcontract

no. 6805919.TA.

Notes and references

1 V. Thangadurai, H. Kaack and W. Weppner, J. Am. Ceram. Soc.,2003, 86, 437–440; V. Thangadurai and W. Weppner, Adv. Funct.Mater., 2005, 15, 107–112; V. Thangadurai and W. Weppner,J. Solid State Chem., 2006, 179, 974–984.

2 U. V. Alpen, A. Rabenau and G. H. Talat, Appl. Phys. Lett., 1977,30, 621–623; G. C. Farrington and B. S. Dunn, Solid State Ionics,1982, 7, 267–281.

3 B. S. Dunn and G. C. Farrington, Solid State Ionics, 1983, 9–10,223–225.

4 N. Hirose and J. Kuwano, J. Mater. Chem., 1994, 4, 9–12; C. Y. Sunand K. Z. Fung, Solid State Commun., 2002, 123, 431–436.

5 A. K. Padhi, K. S. Nanjundaswamy, C. Masquelier andJ. B. Goodenough, J. Electrochem. Soc., 1997, 144, 2581–2586;M. Catti, S. Stramare and R. Ibberson, Solid State Ionics, 1999,123, 173–180; H. P. Y. Hong,Mater. Res. Bull., 1978, 189, 117–124.

6 R. Murugan, V. Thangadurai and W. Weppner, Angew. Chem.,Int. Ed., 2007, 46, 7778–7781.

7 J. Awaka, N. Kijima, H. Hayakawa and J. Akimoto, J. Solid StateChem., 2009, 182, 2046–2052.

8 S. Ohta, T. Kobayashi and T. Asaoka, J. Power Sources, 2011, 196,3342–3345; C. Galven, J. L. Fourquet, M. P. Crosnier-Lopez andF. Le Berre, Chem. Mater., 2011, 23, 1892–1900; Y. T. Li,C. Wang, H. Xie, J. Cheng and J. B. Goodenough, Electrochem.Commun., 2011, 13, 1289–1292.

9 H. Xie, J. A. Alonso, Y. T. Li, M. T. Fernandez-Diaz andJ. B. Goodenough, Chem. Mater., 2011, 23, 3587–3589.

10 A. Logeat, T. Koohler, U. Eisele, B. Stiaszny, A. Harzer,H. Ehrenberg and B. Kozinsky, Solid State Ionics, 2012, 206, 33–38.

11 M. Xu, M. S. Park, J. M. Lee, T. Y. Kim, Y. S. Park and E. Ma,Phys. Rev. B, 2012, 85, 052301–052305.

12 S. Adams and R. P. Rao, J. Mater. Chem., 2012, 22, 1426–1434.13 A. C. Larson and R. B. Von Dreele, General Structure Analysis System

(GSAS), Los Alamos National Laboratories, Los Alamos, NM, 1990.14 E. J. J. Cussen, J. Mater. Chem., 2010, 20, 5167–5173.15 S. Nishimura, G. Kobayashi, K. Ohoyama, R. Kanno,

M. Yashima and A. Yamada, Nat. Mater., 2008, 7, 707–711;H. Buschann, J. Dolle, S. Berendts, A. Kuhn, P. Bottke,M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lotnyk,V. Duppel, L. Kienle and Janek, Phys. Chem. Chem. Phys., 2011, 13,19378–19392; F. Izumi and K. Momma, IOP Conf. Ser.: Mater.Sci. Eng., 2011, 18, 022001.

Fig. 3 Two-dimensional contour maps sliced on the (001) plane with

z = 0.4 at (a) room temperature, (b) 200, (c) 400 and (d) 600 1C; Li

delocalizes along the 3-dimensional chain, Li(24d)–Li(96h)–Li(48g)–

Li(96h)–Li(24d) direction whereas Zr, La and O remain near their

original positions.

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