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Electrochimica Acta 55 (2010) 2958–2963

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

ole of amorphous boundary layer in enhancing ionic conductivity ofithium–lanthanum–titanate electrolyte

o Meia, Xiao-Liang Wangb, Jin-Le Lana, Yu-Chuan Fenga,ong-Xia Genga, Yuan-Hua Lina, Ce-Wen Nana,∗

Department of Materials Science and Engineering, and State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, ChinaCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA

r t i c l e i n f o

rticle history:eceived 1 October 2009eceived in revised form 9 January 2010ccepted 11 January 2010

a b s t r a c t

The low ionic conductivity is a bottleneck of the inorganic solid state electrolyte used for lithium ionbattery. In ceramic electrolytes, grain boundary usually dominates the total conductivity. In order toimprove the grain boundary effect, an amorphous silica layer is introduced into grain boundary of ceramicelectrolytes based on lithium–lanthanum–titanate, as evidenced by electron microscopy. The results

vailable online 18 January 2010

eywords:olid state electrolyteithium lanthanum titanateithium ionic batteryomposites

showed that the total ionic conductivity could be to be enhanced over 1 × 10−4 S/cm at room temperature.The reasons can be attributed to removing the anisotropy of outer-shell of grains, supplement of lithiumions in various sites in grain boundary and close bindings among grains by the amorphous boundarylayer among grains.

© 2010 Elsevier Ltd. All rights reserved.

morphous layer

. Introduction

Safety and volume issue are the most concerning ones whenhe lithium ion batteries are used as power sources in electricalehicles. Using an inorganic solid state electrolyte as a substituteor organic liquid electrolytes is considered to be an efficient wayo avoid the fire and explosion accident in lithium ion batteries.owever, low ionic conductivity of inorganic solid state electrolytes

imits their application. In polycrystalline ceramic electrolytes, it ishe migration pathway and content of lithium ions in grain bulk,ores and potential barrier in grain boundary that dominates theigration of lithium ions [1–6].Many studies have been carried out on solid state electrolytes

o enhance their ionic conductivity. Among them, modifying therystalline structure by doping various ions is a usual way tomprove the conductivity in grain bulk [7]. However, the enhance-

ent is limited because it is the grain boundary not the grain bulkhat determines the total conductivity. Thus increasing the grain-

oundary conductivity is critical and effective to enhance the totalonductivity. Much effort has been devoted to enhance the grain-oundary conductivity, and among them adding second phases hasttracted much attention. It is believed that the second phases

∗ Corresponding author. Fax: +86 10 6277 3587.E-mail address: cwnan@tsinghua.edu.cn (C.-W. Nan).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.01.036

would introduce some space charge layer, adjust compositionsand modify the grain boundary, thus affect the ionic conductivity[8–14].

Lithium lanthanum titanium oxide (LLTO) is an attractive solidstate electrolyte for its high bulk conductivity of about 10−3 S/cmat room temperature, and lithium ions transfer in two-dimensionalway in its special superstructure lattice [7,15–17]. However, itsgrain-boundary conductivity is quite low, e.g., less than 10−5 S/cm[7,15,16,18–23]. Few studies have been devoted to analyzing thereasons of low grain-boundary conductivity and making attemptto improve it. Our previous studies showed that silica seems tobe beneficial to the enhancement of the grain-boundary conduc-tivity in LLTO ceramics. By introducing silica, the grain-boundaryconductivity increases obviously and the total conductivity of theLLTO ceramics could attain 8.9 × 10−5 S/cm at room temperature.The results demonstrated that the enhancement is attributed tothe modification of microstructure and abatement of detour effect[24–26].

In this paper, in order to further improve the conductivity ofLLTO ceramics, the processing is modified to try to obtain a moreuniform silicon-rich layer in grain boundary [24]. Moreover, the

effect of silicon-rich layer is further analyzed. The results show thatan amorphous layer in the grain boundary covers the LLTO grainsand affects the anisotropy of outer-shell of LLTO grains, leading toa decrease in the potential barrier for migration of lithium ionsamong grains and thus an increase in the total conductivity.

A. Mei et al. / Electrochimica Acta 55 (2010) 2958–2963 2959

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Table 1Fitting parameters of impedance plots for the LLTO and LLTO/Si ceramics.

Sample T/◦C Rb/� Rgb/� CPEgb

Cp/�F n

LLTO 30 115 5004 0.224 0.93950 68 1996 0.255 0.92970 37 913 0.281 0.92590 22 507 0.386 0.896110 14 335 0.670 0.842

LLTO/Si 30 213 1492 0.210 0.86850 118 694 0.247 0.86270 69 322 0.306 0.85490 46 168 0.458 0.830110 32 93 0.862 0.789

distribution of silica and enhances the effect of silica to grain bound-ary, in spite of that the effect of silica is still not so clear. In orderto closely analyze and explain the effect of silica in the LLTO/Siceramics, phases and microstructure were further characterized.

ig. 1. Electrochemical impedance spectra of the LLTO and LLTO/Si ceramics at 30 ◦C,here the insert picture is the equivalent circuits.

. Experimental

Li0.5La0.5TiO3 (LLTO) powder was prepared by wet chemicalethod as described before [24]. The processing for introduction of

ilica into LLTO was modified to get uniform distribution of silica.he LLTO powder was dispersed in ethanol and stirred by ultrasonicispersion then followed by adding a certain amount of deionizedater and ammonia. Afterwards, the tetraethoxysilane (TEOS) was

dded into the suspension. Based on the pervious results that intro-uction of about 5 vol.% SiO2 led to a maximum increase in the totalonductivity of the LLTO ceramics [24], only 5 vol.% of SiO2 mixtureas stirred and heated at 150 ◦C to obtain silica doped LLTO pow-er (denoted as LLTO/Si). The composite powder was pressed intoellets of 12 mm in diameter, followed by sintering at 1350 ◦C forh.

The phases in the ceramic samples were identified by X-rayiffraction (XRD, 2500, Rigaku). Microstructure of the pellets wasbserved by scanning electron microscopy (SEM, JSM-6460LV,EOL). High resolution transmission electron microscopy (HRTEM,010F, JEOL) was used to observe the grain boundary. Com-osition analysis of the samples by energy dispersive of X-raypectroscopy (EDX, Oxford Instrument) and Raman SpectrometerRaman, RM2000, Renishaw) was also performed. For electrical

easurement, Au was sputtered on the sample surfaces as elec-rodes. The electrical conductivity of the samples was measuredsing electrochemical impedance spectroscopy with ChenhuaHI760B Electrochemical Workstation over the frequency range

rom 0.1 Hz to 0.1 MHz. The testing temperature was controlledy Cincinnati Sub-Zero MCB-1.2-AC Environmental Chambers from0 to 110 ◦C. The density of samples was measured by Archimedesrinciple.

. Results and discussion

Lithium ionic conductivity of the LLTO based ceramics waseasured by electrochemical impedance and Fig. 1 shows the

ypical plots for the LLTO polycrystalline solid electrolyte with ion-locking electrodes at 30 ◦C, including one arc and a straight line24,25]. The arc in the high frequency range is due to the ioniconductivity of the ceramic samples. The long straight line in theow frequency range corresponds to the blocking effects of Au elec-

rodes. As shown, the arc of the LLTO/Si ceramics is much smallerhan that of the LLTO, indicating the resistance of the LLTO ceram-cs decreases obviously after introducing silica. Considering such anrreversible cells of Au||polycrystalline LLTO||Au, a simplified equiv-lent circuit as shown in the inset of Fig. 1 can be used to calculate

Notes: The impedance spectroscopes are fitted by ZView program. Rb and Rgb are theresistances of grain bulk and grain boundary, respectively. Constant phase element(CPE) is a substitute for conventional capacitor C in order to reflect the fact that thesemicircle is suppressed. The Cp is the pseudo-capacitance and n is the CPE exponent.

the resistances, where the grain bulk component is described bya resistance (Rb) with capacitance ignored, and the grain bound-ary component is described by a resistance (Rgb) in parallel witha constant phase element (CPEgb). The electrode polarization ismodeled by a constant phase element (CPEe). Such an equivalentcircuit can be used to well fit the curves to refine the contributionsfrom grain and grain boundary [24]. The fitting calculations werecarried out by ZView program to obtain the resistances of grainand grain boundary. The fitting parameters for the two samples atdifferent temperatures are listed in Table 1. The errors of all resul-tant parameters are below 5%. As seen, although the resistance ofthe grain bulk Rb increases after introducing silica, the resistanceRgb of grain boundary decreases obviously after adding silica. Theconductivity values are finally obtained by using these resistancevalues, the sample thickness and electrode area. The total and grainbulk conductivity of the LLTO/Si ceramics were 1.11 × 10−4 and0.77 × 10−3 S/cm at 30 ◦C, while the total and grain bulk conduc-tivity of the LLTO ceramics were 3.3 × 10−5 and 1.1 × 10−3 S/cm at30 ◦C. Compared with the conductivity of the LLTO ceramics, thetotal conductivity of the LLTO/Si ceramics is enhanced obviouslyup to over 10−4 S/cm at 30 ◦C which is the highest value achievedin the LLTO based ceramics so far [24,25]. It seems that the introduc-ing ultrasonic dispersion in the experiments promotes the uniform

Fig. 2. XRD patterns of the LLTO and LLTO/Si ceramics, where the inset plots are forpeaks at (2 0 0), (2 2 0) and (4 2 0).

2960 A. Mei et al. / Electrochimica Acta 55 (2010) 2958–2963

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ig. 3. Comparison between Raman spectra of the LLTO and LLTO/Si ceramics.

Fig. 2 shows the XRD patterns of the LLTO and LLTO/Si ceramics.ll the diffraction peaks in two patterns can be indexed based on

CPDS card 46-0466 and are identified to LLTO without additionalhases. No diffraction peaks for silica or related silicate appear inRD patterns. The silica introduced might be in the form of amor-hous state.

ig. 4. SEM micrographs of the cross-sections of (a) LLTO and (b) LLTO/Si ceramics.

Fig. 5. (a) SEM micrograph of grain boundary–grain junctions in the LLTO/Si ceram-ics; and EDX for (b) point A and (c) point B.

A close check of the XRD patterns (e.g., in the ranges of 46–49◦,68–71◦ and 124–132◦) as shown in the inset of Fig. 2 shows that,except for Cu K�2 diffraction peaks, no peak split is observed for thepeaks of (2 0 0), (2 2 0) and (4 2 0), demonstrates that the LLTO unitcell has tetragonal symmetry (P4/mmm) [7,18]. Furthermore, inboth patterns, the superstructure lines appear as marked by arrows,indicating that the LLTO lattice presents ordered occupation of La3+,Li+ and vacancy in the a–b plane. The ordered occupation means aLa3+-rich plane and a La3+-deficit plane laminating along c-axis inthe LLTO lattice. The laminated structure builds up the superstruc-ture based on cubic unit cell and thus forms the tetragonal unit cell.Meanwhile, the crystalline structure affects the migration of Li ionsin the lattice. In the superstructure lattice Li ions migrate more eas-ily along the La3+-deficit plane, i.e., along the a–b plane of the LLTOlattice [15,17].

The Raman spectra of the LLTO and LLTO/Si ceramics are shownin Fig. 3. These newly obtained spectra are similar to the spectrawe presented previously [24]. In the measured frequency range,the spectra consist basically of four bands at about 140, 237, 311and 455 cm−1, and a broad high-frequency band extending from

500 to 600 cm−1 which is made up of two components at about526 and 575 cm−1. The number of the modes is in agreement withthe prediction for tetragonal (P4/mmm) double-perovskite cell inconsistent with the XRD results. The six bands correspond to six

A. Mei et al. / Electrochimica Acta 55 (2010) 2958–2963 2961

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ig. 6. (a) HRTEM micrograph for grain and grain boundary-grain junction in the LLFT image, (c) selected-area electron diffraction (SAED) of the inert grain interior ahe FFT image of the grain outer-shell.

aman-active modes (i.e., 2A1g, B1g and 3Eg respectively). All theseaman-active modes involve vibration of Ti and O atoms. Bands And C are assigned to Ti vibration in-plane and along c-axis. Bandsand E are assigned to O vibration in-plane, while O atoms vibra-

ion along c-axis are responded to bands D and F assigned [27–29].he bands arising from lithium vibration might be overdamped.nother possibility is that the lithium vibration is coupled to oxy-en and titanium vibration [27–30].

As seen from Fig. 3, the bands in the spectrum of the LLTO/Sieramics are strengthened and sharper compared with bands in

he spectrum of the LLTO. A similar tendency was also observedn lithium content dependent Raman spectra of LLTO [27,28,30], in

hich the intensity of bands increased with decreasing the contentf lithium ions in grains. This might suggest that the lithium con-ent in the grain bulk of the LLTO/Si ceramics decreases. Thereby

ceramics, (b) lattice image of the grain interior (region A) with the inset showing itsHRTEM image of the grain outer-shell and grain boundary where the inset shows

a possible deduction is that the lithium in the grain bulk mightreact with silica to form some amorphous lithium silicate basedsubstance with a decrease in the lithium content in the LLTO grainbulk. Thus it was considered that the introduction of silica wouldaccelerate grain growth of LLTO during high temperature sintering.But actually, the comparison between the grain bulk resistances Rb(see Table 1) of the LLTO and LLTO/Si samples demonstrates thatthe grain size does not increase after the introduction of silica asalso shown in the SEM observation (Fig. 4). As known, Rb ∝ �b/dand Rgb ∝ �gb(t/d), where d and t are respectively the grain size

and thickness of the boundary layer. An increase in d leads to adecrease in Rb. However, the Rb values of the LLTO/Si sample arenot lower than those for the LLTO sample (Table 1). Thus similargrain sizes in both samples imply a bit lower grain bulk conductiv-ity in the LLTO/Si than in LLTO sample, which is due to the decrease

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962 A. Mei et al. / Electrochim

n the lithium content in the grain bulk of the LLTO/Si ceramicss discussed above. Similarly, the comparison between the grainoundary resistances Rgb (see Table 1) of the LLTO and LLTO/Si sam-les illustrates that t cannot be a main reason for their difference.urthermore, the pseudo-capacitance Cp (Table 1) of grain bound-ry does not change obviously after adding silica. All these implyhat the geometric changes at grain boundary level are not so sig-ificant to contribute to the conductivity enhancement observed

n the LLTO/Si ceramics.Fig. 4(a) and (b) shows typical SEM photos of cross-sections of

he LLTO and LLTO/Si ceramics. It can be seen that two ceramicamples show similar micrographs with the average grain sizes ofround 10 �m. The relative density of the LLTO/Si ceramic is upo about 96.5% which is a little bit higher than that of the LLTOeramic (95.3%). Thus, the introduction of appropriate amount ofilica does not significantly affect both the grain size as concludedrom the fitting parameters above, and the relative density [24].

However, further examination of the LLTO/Si ceramics revealsilicon-rich boundaries. Fig. 5(a) shows a typical SEM micrographf a triangle grain boundary region in the LLTO/Si ceramics, andrain boundary regions exhibit different composition from grainulk as evidenced by EDX. By comparing two EDX plots, shown inig. 5(b) and (c), for points A and B, it can be seen that the silicononcentrates in the grain boundary regions. Besides, the morphol-gy of the grain boundary appears smooth and grains are closelyompacted together.

In order to closely check the grain boundary HRTEM was used.ig. 6 shows TEM micrographs of the grain boundary in the LLTO/Sieramics. Two typical regions (Fig. 6(a)), the interior region in therain bulk (region A) and the grain boundary region (region B), arexamined by further HRTEM. As shown in Fig. 6(b), an irregular con-rast can be seen in the interior region of the grain bulk. The brightnd the dark spots locate alternatively along the c-axis, indicatinghat the modulated structure or superstructure as detected by XRDxists within the grain. Meanwhile, the corresponding selected-rea electron diffraction pattern (shown in Fig. 6(c)) and Fourierransformed (FFT) image (inset in Fig. 6(b)) present the extra super-tructure reflections (h/2 0 0)p, (0 0 l/2)p and (h/2 0 l/2)p (here theubscript p being used to refer to the ideal cubic perovskite cell).t implies that the LLTO phase has a tetragonal symmetry withuperstructure [31–33]. Fig. 6(b) also shows the spacing of 0.36 nma-axis) and 0.72 nm (c-axis) for the tetragonal lattice [7,32–35].

Fig. 6(d) shows the HRTEM micrograph of the region B inig. 6(a), which clearly demonstrates that the grain boundary inhe right part of the micrograph is amorphous. The EDX focused onhe grain boundary region shows possible existence of lanthanumnd titanium besides silicon. Thus the amorphous phase in the grainoundary is some silicates probably with lanthanum and titanium.onsidering the conclusion from the Raman spectra above, the sil-

cates maybe contain lithium as well. The comparison of Fig. 6(b)nd (d), illustrates that the lattice near the grain boundary is nothe same as that in the interior region of grains. In Fig. 6(b), theiffraction spots present uniform intensity, but the superstructureeflections disappear in the FFT image shown in the inset of Fig. 6(d).hus the superstructure disappears in the outer-shell of grain nearhe grain boundary, and the outer-shell near the grain boundaryppears to be cubic symmetry with the spacing intervals of about.39 nm [7].

Based on these analyses, we could give an explanation on theffect of silica, as schematically shown in Fig. 7. As shown in Fig. 7(a),n the LLTO grain bulk, the superstructure causes the lithium ions

o migrate two-dimensionally in the La3+-deficit plane and therebyithium ions migrate from grains into grains only along the con-uction plane. However, in normal polycrystalline LLTO, the grainsre randomly orientated, and thus the conduction planes betweeneighboring grains separated by boundary are mismatching to each

Fig. 7. Schematic of migration of Li+ in normal LLTO (a) and LLTO/Si ceramics (b).

other, which causes high potential barrier for lithium ions to acrossthe grain boundary. Such low grain-boundary conduction pathsbecome the bottleneck of migration of lithium ions in ceramics.

After introducing silica, some amorphous silicate layer (Fig. 6)forms in grain boundary. As schematically shown in Fig. 7(b), theamorphous layer induces transfer of the symmetry of grain outer-shell from tetragonal to the cubic (see Fig. 6) and thus removesthe anisotropy of the grain outer-shell. The migration of lithiumions in cubic system is in three-dimensional ways. The migrationpath in the outer-shell with cubic symmetry results in increasingthe conduction plane in the outer-shell [36], which implies thelithium ions can easily access the grains via grain boundary fromnearby grains. Moreover, the amorphous layer is not an inert phase,but contains lithium ions, as indicated by Raman spectroscopy, soas to provide lithium ions in various sites and be convenient fordelivery of lithium ions to the conduction planes where it can beinserted. Thus the potential barrier decreases and grain-boundaryconductivity increases obviously, as observed in experiments.

4. Conclusions

The conductivity of the LLTO based ceramics has been increasedobviously up to over 1 × 10−4 S/cm. The conductivity enhancementroots in the amorphous silicate layer introduced in grain bound-

removes the anisotropy of grain outer-shell and supplies lithiumions in various sites in grain boundary. Therefore, the LLTO grainswith core–shell structure and amorphous layer in grain boundarydecrease the potential barrier and increase the conductivity obvi-ously.

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A. Mei et al. / Electrochim

cknowledgements

The authors would like to thank Drs. S. Yokoishi and Y. Minamidaor helpful discussions.

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