Solid State Ionics 268 (2014) 135–139

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Solid State Ionics

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Crystal structure, migration mechanism and electrochemicalperformance of Cr-stabilized garnet

Shufeng Song, Binggong Yan, Feng Zheng, Hai Minh Duong, Li Lu ⁎Materials Science Group, Department of Mechanical Engineering, National University of Singapore, Singapore

⁎ Corresponding author.E-mail address: luli@nus.edu.sg (L. Lu).

http://dx.doi.org/10.1016/j.ssi.2014.10.0090167-2738/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 July 2014Received in revised form 9 October 2014Accepted 9 October 2014Available online xxxx

Keywords:GarnetTransportation mechanismSolid electrolyteLithium-ion conductor

Garnet-structured Li7La3Zr2O12 is stabilized through Cr3+ doping at Zr4+ sites. The Rietveld refinement of pow-der XRD data is used to analyze the crystal structure and Cr site. The migration mechanisms of lithium ions aredependent on the hopping of lithium ions at different sites. Three possible migration pathways are proposed,in which lithium ions transport via the tetrahedral sites. Small doping of Cr3+ at Zr4+ sites leads to high totalconductivity of the 0.1 mol Cr-stabilized Li7.1La3Zr1.9Cr0.1O12 to be 5.2 × 10−4 S cm−1 at 27 °C. The sampleshows a wide electrochemical stability up to 6 V.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Conventional lithium ion batteries have been facing a challenge inenergy density, safety, and durability, due to small potential windowsup to 4.5 V and flammable liquid organic electrolytes currently used.Some of cathode materials with high potential above 4.5 V are unableto be used [1]. In addition, recent, renaissance of interest in Li–S andexploitation in Li–O2, hybrid aqueous and non-aqueous electrolyte airbatteries require to develop new solid electrolytes with high ionic con-ductivity and structural stability for Li–S and Li–O2 batteries [2,3].Therefore it is timely urgent to develop nonflammable solid electrolyteswith large potential windows. The new inorganic crystallized composi-tion, Li10GeP2S12, shows the highest lithium ion conductivity ever mea-sured for a solid, about 12mS cm−1 at 300K,which even exceeds that ofsolvent based systems [4]. Overall, the work from Kamaya et al. willprobably stir significant and renewed interest in ceramic solid electro-lytes, and it may lead to breakthroughs similar to the discoveries ofbeta alumina and NASICON-type structures, which are nowadays usedin Na–S batteries [5].

Developed in Weppner's group [6], the novel class of fast Li-ionconducting inorganic oxides with the nominal chemical compositionLi5La3M2O12 (M = Nb, Ta), possessing a garnet structure, has beenproved to be a potential category and therefore has attracted a lot of at-tentions in recent years. Both the niobium and tantalum members ex-hibit the same order of magnitude of bulk conductivity (10−6 S cm−1

at 25 °C). The activation energies of ion transportation (b300 °C) are0.43 and 0.56 eV for Li5La3Nb2O12 and Li5La3Ta2O12, respectively. The

activation energy of organic liquid electrolyte LiPF6 in EC/DMC isabout 0.14 eV, which is much smaller than that of inorganic solid elec-trolytes [7]. However, the cation transference number of organic liquidelectrolyte LiPF6 in EC/DMC is just about 0.38 [8], while the transferencenumber of solid electrolytes is near 1.

Lithium-stuffed garnet Li7La3Zr2O12 (LLZO) with high conductivityof 2.4 × 10−4 S cm−1 at 25 °C was followed reported by the samegroup [9]. The garnet structure A3B3C2O12 contains a B3C2O12 frame-work structure with B cations in dodecahedral sites and C cations in oc-tahedral sites [10]. The refinement was initiated with the garnetframework having La, Zr, and O located at 24c-B sites, 16a-C sites, and96h-O sites, respectively. Fourier maps to locate the Li+ within theframework identified two positions that can be assigned to the 24dsites and the displaced 96h bridging octahedral sites [11].

It has been reported that tetrahedrally-structured LLZO has a Li-ionconductivitywhich is approximately two and three orders ofmagnitudelower than that of the cubic LLZO [12]. Thus, to obtain a stabilizedcubic-structured LLZO becomes a main challenge. One hypothesis isthat super-valent cation substitution reduces the Li content and/orincreases Li vacancy concentration, while maintaining oxygen stoi-chiometry, and hence stabilizes the cubic phase [13]. McGinn et al.[14] report that Al could be used to substitute for Li to stabilize thecubic phase. Other people report that Ga3+, Si4+, Ge4+, etc. couldstabilize the cubic structure of LLZO [15,16]. However, substitutionsfor Li+ result in the obstruction of ionic transport path, thus reducingthe conductivity.

Likewise, substitution on the 24c site (La3+) is another approach tocreate Li vacancy or increase Li content. The Ca2+, Ba2+, Mg2+, Sr2+,Ce4+, etc. are reported to stabilize the cubic structure by many authors[17–19].

136 S. Song et al. / Solid State Ionics 268 (2014) 135–139

The cubic garnet framework LLZO has La, Zr, and O located at do-decahedral (24c) sites, octahedral (16a) sites, and 96h-O sites, respec-tively, while the Li sites have not been conclusive. Adams et al. [20]used the bond valence calculations based on their Rietveld refinementsof XRD data that show that the tetrahedrally coordinated 24d site Li(1),and the Li(2) site pair (on 96h sites around an octahedrally coordinated48g site) are the only low-energy sites for Li in cubic phase and nointerstitial sites exist. As seen in Fig. 1 [20], a 3D network of pathwaysof lowest activation energy for Li+-ion migration in LLZO involvesboth site types: Li(1)–Li(2)–Li(2)–Li(1) paths are interconnected atthe Li(1) site forming the pathway network. Thereby substitution onthe 16a site (Zr4+) will not block the Li-ion transport path and thusshould lead to higher conductivity than that observed for the 24dsite (Li+) and 24c site (La3+) substitutions. Goodenough and the co-authors deduct the practical upper limit for the Li concentration withb7.5 per formula unit, and suggest that the highest conductivity willbe found at a nominal Li = 6.4 ± 0.1 [11]. Subsequently, they obtainthe highest room-temperature conductivity of 10−3 S/cmwith the sub-stitution of Ta5+ on the 16a site (Zr4+) [21].

To our best knowledge, there are few works reporting the lower-valent substitution on 16a site (Zr4+) as in LixB3C2O12, probably dueto the cubic garnet phase that is difficult to obtain for x N 7.0. Herein,we report the chromium-stabilized LLZO with pure cubic garnetphase, and moisture and air resistance. Cr3+ ions with incompletelyfilled 3d orbits would show strong preference for the octahedral lattice

Fig. 1. Projections of half the unit cell of cubic Li7La3Zr2O12 on the a–c plane: (a) Polyhedralstructure model; (b) Network of Li sites; (c) Bond valence-based model of Li+ ion trans-port pathways shown as three isosurfaces of low Li site energy superimposed on stickmodel of the LLZ structure. Li(1) on 96h sites: small red spheres (a and b); Li(2) on 24dsites: blue spheres; La: large green spheres; ZrO6: blue octahedra in (a), as blue orangestick model in (c). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.).

replacing Zr4+ ions thereby contributing positively to the structurestabilization.

2. Experimental

Chromium-stabilized Li7.1La3Zr1.9Cr0.1O12 was prepared by a con-ventional solid-state reaction using stoichiometric amounts of high pu-rity chemicals, Li2CO3 (Sigma, 99%, with 10 wt.% excess to compensatefor the loss of lithium during annealing), La2O3 (Sigma, 99.99%; driedat 900 °C for 12 h prior using), ZrO2 (Inframat Advanced Materials,99.9%), and Cr2O3 (Alfa, 99%). All the raw precursors were first ball-milled using zirconia balls for about 6 h in absolute ethanol in air, andthis procedure was repeated after calcination at 900 °C for 12 h. Subse-quently, the calcined powder was pressed into pellets and calcinedagain at 1125 °C for 12 h,while the pelletswere coveredwith rawpow-ders. The ball-milled and pelleted processes were repeated againfollowed by sintering at 1230 °C for 16 h. X-ray diffraction (ShimadzuXRD-6000 and 7000 Cu-Kα) was employed tomonitor crystallographicstructures after each step of calcination processing. Lattice constantswere obtained and occupancy of Cr sitewas analyzed by Rietveld refine-ment of the powder XRD data using GSAS software.

Electrical conductivity measurements of the pellets were performedin air using lithium-ion-blocking gold electrodes in the temperaturerange from 27 °C to 200 °C using a Solartron 1260 + 1287 System,applying 10 mV in the frequency range of 1 MHz–1 Hz, without anypressure applied. The electrochemical stability window was measuredby Linear Sweep Voltammetry run on Pt\Li7.1La3Zr1.9Cr0.1O12 pellet\Licells in a voltage range from -0.5 V to 6 V vs. Li+/Li and with a0.5 mV s−1 scan rate.

The lithium transference number was determined by Hebb–Wagnerpolarization measurement. The test was performed on sintered pelletsby applying a constant voltage of 1 V, using lithium metal as the refer-ence electrode and Au as lithium-ion-blocking electrode.

3. Results and discussion

Fig. 2 shows the XRD spectra of Li7.1La3Zr1.9Cr0.1O12 powders cal-cined at 900 °C and 1125 °C for 12 h, and the sample sintered at eachstep together with the standard diffraction spectrum of Li5La3Nb2O12

(PDF 45-0109). The measured powder XRD pattern of Li7La3Zr2O12

(LLZO) matches well with the standard one of the known garnetphase Li5La3Nb2O12 indicating that the present garnet structure is ableto accommodate cations with different valence states and differentsizes without any change in the symmetry [9]. Both Li7.1La3Zr1.9Cr0.1O12

powders calcined at 900 °C and 1125 °C for 12 h show pure tetragonalgarnet structure (I41/acd). Whereby well-crystallized cubic garnetstructure (Ia3d) was achieved after annealed at 1230 °C for 16 h. It

Fig. 2. XRD spectra of Li7.1La3Zr1.9Cr0.1O12 for each sintering step together with thestandard diffraction spectra of Li5La3Nb2O12.

Table 1Structural parameters of “Li7.1La3Zr1.9Cr0.1O12” garnet fromRietveld refinement of powderXRD data.

Atom Site x, y, z Occupancy Uiso

Li1 24d 3/8, 0, 1/4 0.564 0.00997Li2 96h 0.707595, 0.588605, 0.0816 0.451 0.0427La 24c 1/8, 0, 1/4 1.0008 0.01327Zr 16a 0, 0, 0 0.95 0.03539Cr 16a 0, 0, 0 0.05 0.08O 96h 0.28005, 0.102304, 0.194092 1.0029 0.00285Rp/Rwp 0.0797/0.1067CHI2 6.368

137S. Song et al. / Solid State Ionics 268 (2014) 135–139

should be pointed out that the suppression of lithium loss is importantduring both calcination and sintering processes; otherwise, La2Zr2O7

and other impurities like La2O3 would appear.To understand arrangement of cations, Rietveld refinement was

employed [5,11]. Fig. 3 and Table 1 show the results from the Rietveldrefinement of powder XRD data of Li7.1La3Zr1.9Cr0.1O12. All garnetframeworks were revealed in the space group Ia3d. The lattice constantobtained from Rietveld fit is 13.073 Å, slightly larger than that of bareLLZO (12.98 Å) [22]. It is supposed that the lattice constant ofLi7.1La3Zr1.9Cr0.1O12 should be smaller than that of LLZO since the ionicradius of Cr3+ (0.76 Å) is smaller than that of Zr4+ (0.84 Å). However,there is no direct relationship between ionic radii and lattice constants.The lattice constant decreased from 12.975 Å to 12.967 Å with substitu-tionY3+ for Zr4+, though the radius of Y3+ (1.02Å) ismuchhigher thanthat of Zr4+. It is noted that Cr3+ occupies the 16a site obtained fromRietveld fitting. This sub-valent substitution on the 16a site would in-crease the Li content and decrease Li vacancy concentration to stabilizethe cubic structure through

Cr2O3 ¼ 2Cr0

Zr þ 2Li•24d=96h þ 3O�o:

On the other hand, the substitution on 16a site will not block theLi-ion transport path and thus leads to higher conductivity.

Fig. 4(a) schematically shows the basic unit cell of cubicLi7.1La3Zr1.9Cr0.1O12. The Cr and Zr ions are located at Wyckoff posi-tions of 16a sites, forming Zr/CrO6 octahedra. La ions are located atWyckoff positions of 24c sites, forming LaO8 dodecahedra. Thereare two kinds of Li sites, the tetrahedral sites [Li(1) 24d sites] and theoctahedra sites [Li(2) 96h sites]. The latter Wyckoff position 96h indi-cates split-atom 96h sites from 48g sites.

From a structural view point, the Li-ion migration pathway shouldcorrespond to the Li-ion arrangement in the framework. As seen fromFig. 4(b), by paying attention only to the Li ions in the framework, a3D-loop network of the Li-ion migration pathway is formed. In thisloop structure, every tetrahedral Li(1)O4 is linked to four octahedralLi(2)O6, and each octahedral Li(2)O6 is linked to two tetrahedral Li(1)O4 by opposing faces. Therefore, simultaneous occupation of adjacenttetrahedral and octahedral sites necessitates short Li⋯Li contacts(1.98 Å) and the observed displacement of Li+ away from one of theshared faces increases this separation to 2.71 Å as compared against2.53 Å [23]. For our Cr-stabilized LLZO, the substitution of Cr for Zr re-sulted in large lattice constant (13.073 Å), and this large lattice constantwould induce large Li⋯Li distance, 2.71 Å. On the other hand, thisrelatively large Li⋯Li distance would improve the stability of crystalstructure. This displacement from the center of the octahedron away

Fig. 3.Observed, calculated, and difference patterns for the Rietveld refinement of powderXRD data of Li7.1La3Zr1.9Cr0.1O12. The short vertical lines below the profiles mark the peakpositions of all possible Bragg reflections.

from one shared face requires the movement toward the other sharedface and so the stability conferred by this mechanism is dependent onone of the two oxide tetrahedra remaining unoccupied [17].

Fig. 4(c) reveals an insight into the Li-ion migration mechanismwhere the Li ion arrangement can be simply drawn as a 2D-loop.There are three possible migration mechanisms according to which Lisites can be served as the trigger for ion mobility and reconfigurationof surrounding Li neighbors. The Li-ion migration mechanism in theloop structure looks like the Domino Effect. If the Li(1) sites serve asthe trigger, when Li(1) ions move toward their vacancy of Li(2) sitestermed as Li(2v) neighbor, that Li(2) ions will be pushed to vacancy ofLi(1) sites termed as Li(1v), then next Li(2) ions will be pushed tothe vacancy of Li(2) sites nearby. The Li-ion migration pathway is:Li(1)-Li(2v)⋯Li(2)-Li(1v)⋯Li(2)-Li(2v). If the Li(2) sites serve as thetrigger, there are two possibilities. Firstly, when Li(2) ions move to-ward their vacancy of Li(1) sites termed as Li(1v) neighbor, nextLi(2) ions will be pushed to their vacancy of Li(2) sites termed asLi(2v), then next Li(1) ions will be pushed to the vacancy ofLi(2) sites nearby. The Li-ion migration pathway is: Li(2)-Li(1v)⋯Li(2)-Li(2v)⋯Li(1)-Li(2v). Secondly, when Li(2) ions move towardtheir vacancy of Li(2) sites termed as Li(2v) neighbor, the Li(1) ionswill be pushed to their vacancy of Li(2) sites termed as Li(2v), thennext Li(2) ions will be pushed to the vacancy of Li(1) sites nearby. TheLi-ion migration pathway is: Li(2)-Li(2v)⋯Li(1)-Li(2v)⋯Li(2)-Li(1v).Clearly, the Li-ion migration pathway in the compound via Li(1) sitesdid not bypass Li(1) sites, which is in agreement with experimentaland theoretical studies by Goodenough et al. [24]. The lithiummobilityin cubic garnet is much more complex.

A typical impedance plot of Li7.1La3Zr1.9Cr0.1O12 at 27 °C is shown inFig. 5(a), which shows a depressed rather than ideal semicircle and in-clined rather than vertical capacitive tail. The appearance of the tail atlow frequencies in the case of ionically blocking electrodes is an indica-tion that the pellet is ionically conducting in nature [25]. Upon close ex-amination of the Nyquist plot, there is only one semicircle, suggestingthat the grain and grain boundary resistance cannot be separated. In arepresentative electrical equivalent circuit (Fig. 5(a)), Rs representsthe resistance of current electrodes and Rt the total resistance of grainand grain-boundary [26,27]. CPE is a non-ideal capacitance so-calledconstant phase element contributions. The total resistance and totalconductivity are determined based on the real part of impedance Zvalue at the minimum between the high frequency and the low fre-quency tail. The total conductivity of the pellet at 27 °C is calculatedby the equation.

σ t ¼1Rt

� dS

where Rt, d and S are the total resistance, the thickness of sample,the electrode area, respectively. For the sample (d = 3.26 mm,S = 54.9 mm2), the total conductivity of the pellet at 27 °C is obtainedto be 5.2 × 10−4 S cm−1. To obtain activation energy of Li conduction,the impedance of the pellets wasmeasured at 5 different temperatures.

Fig. 4. (a) Crystal structure of Li7.1La3Zr1.9Cr0.1O12, (b) 3D-loop network of Li-ion distribu-tion and migration pathway, (c) 2D schematics of the Li migration mechanism.

Fig. 5. Impedance and Arrhenius plots of Li7.1La3Zr1.9Cr0.1O12. (a) Impedance plot at 27 °C.The inset shows impedance plot from 100 kHz to 1 Hz. (b) Arrhenius plot from 27 °C to200 °C.

138 S. Song et al. / Solid State Ionics 268 (2014) 135–139

The temperature dependence of the conductivity can be expressed bythe Arrhenius equation.

σ ¼ A exp −Ea=kTð Þ

where A represents the pre-exponential factor, k is the Boltzmann con-stant, and T is the absolute temperature. The activation energy Ea for theconduction is calculated to be 0.39 eV. The conductivity and activationenergy were comparable to that of pure LLZO (2.4 × 10−4 S cm−1 and0.34 eV).

A high conductivity is not a sufficient property to make an electro-lyte to be useful in practical terms. A wide electrochemical stability isalso an essential parameter for ensuring good performance in recharge-able lithium batteries. The electrochemical stability window of the Cr-stabilized LLZO has been evaluated by running a cyclic voltammogramof cells, in which Li and Pt serve as reference/counter and workingelectrodes, respectively. Fig. 6 displays the current–potential curve ofCr-stabilized LLZO. The open voltage (OCV) of the cell was about 3.1 V.First, the potential was swept from OCV to−0.5 V (cathodic direction)and then from −0.5 V to 6 V (anodic direction). The cathodic currentcorresponds to lithium deposition, and the anodic current correspondsto lithium dissolution. No obvious current was observed except theones that are due to the lithium deposition and dissolution, indicatingthat the Cr-stabilized LLZO electrolyte has a wide electrochemical win-dow up to 6 V.

The Hebb–Wagner polarization method has been developed todetermine the minority charge carrier conductivity of predominantionic conductors. Under a dc potential of less than the voltage thatmight decompose the sample, all charge carriers migrate initially, butonly electrons and holes move under steady-state conditions [28]. Fig. 7reveals the Hebb–Wagner polarization data at 27 °C, obtained usinglithium metal as reversible electrode and Au as lithium-ion-blocking

Fig. 6. Current–potential curve of Li7.1La3Zr1.9Cr0.1O12.

139S. Song et al. / Solid State Ionics 268 (2014) 135–139

electrode by applying 1 V. This low potential should avoid the samples todecompose. The steady state current, I, of electrons and holes is deter-mined using the equation [29]

I ¼ kTqL

� σe 1−exp −Eq.

kT

� �h iþ σh exp −Eq

.kT

� �−1

h in oð1Þ

where k, T, q, L and E are the Boltzmann constant, the absolute tem-perature, the elementary charge, cell constant (electrolyte thickness/electrode area) and the applied voltage, respectively. Employing theHebb–Wagner theory, the excess electron minority charge carrierconductivity (σe) was measured to be 6.7 × 10−7 S cm−1 at 27 °C(hole conductivity (σh) is considered to be a very small quantityand is therefore neglected), which is 3 order of magnitude lower thantotal conductivity. The lithium transference number was found to beclose one (tLi+ = (σtotal − σe) / σtotal ≈ 1).

4. Conclusions

Partial substitution of Zr4+ with Cr3+ in LLZO with the expectedcomposition of Li7.1La3Zr1.9Cr0.1O12with cubic garnet structurewas pre-

Fig. 7. Polarization current–time curves, obtained at 27 °C by Hebb–Wagner measure-ments with a lithiummetal reversible electrode and Au lithium-ion-blocking electrode.

pared. The crystal structure was determined by the Rietveld refinementof powder XRD data. All the garnet frameworks were revealed in thespace group Ia3d, and the Cr3+ ion occupied the 16a sites. Themigrationmechanism is governed by the trigger Li-ion sites. Three possiblemigra-tion pathways consisted Li(1)-Li(2v)⋯Li(2)-Li(1v)⋯Li(2)-Li(2v), Li(2)-Li(1v)⋯Li(2)-Li(2v)⋯Li(1)-Li(2v), and Li(2)-Li(2v)⋯Li(1)-Li(2v)⋯Li(2)-Li(1v). It was concluded that the Li-ion migration pathway in the com-pound via Li(1) sites did not bypass Li(1) sites. The total conductivityat 27 °C and activation energy for lithium conduction were calculatedto be 5.2 × 10−4 S cm−1 and 0.39 eV, respectively. The Cr-stabilizedLi7.1La3Zr1.9Cr0.1O12 has a wide electrochemical stability window up to6 V. This work showed that partial substitution of Zr4+ with Cr3+

could stabilize the cubic garnet structure and improve the ionic conduc-tivity of LLZO.

Acknowledgments

This work was supported by SERC 2011 Public Sector ResearchFunding (PSF) Grant R-265-000-424-305. The authors also acknowl-edge Xiaoyong Fan and Siheng Li for their discussion on the EIS, andChunfu Lin and Yaqi Zhu for the Rietveld refinement.

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