Journal of Alloys and Compounds 590 (2014) 147–152

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Solid state lithium ionic conducting thin film Li1.4Al0.4Ge1.6(PO4)3

prepared by tape casting

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.12.100

⇑ Corresponding author. Tel./fax: +86 27 87558142.E-mail address: chibo@hust.edu.cn (B. Chi).

Ming Zhang a, Zheng Huang a, Junfang Cheng a, Osamu Yamamoto b, Nobuyuki Imanishi b, Bo Chi a,⇑,Jian Pu a, Jian Li a

a Center for Fuel Cell Innovation, State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University ofScience and Technology, Wuhan 430074, Chinab Department of Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514-8507, Japan

a r t i c l e i n f o

Article history:Received 18 October 2013Received in revised form 11 December 2013Accepted 11 December 2013Available online 18 December 2013

Keywords:Solid electrolyteLAGPLithium air batteriesTape castingWater stable lithium electrolyte

a b s t r a c t

Solid-state lithium ionic conducting ceramic thin film Li1.4Al0.4Ge1.6(PO4)3 is prepared by tape castingmethod. The thermal decomposition of the green tape is investigated by TG/DTA analysis. And the sinter-ing parameters are optimized in air. The resultant sample shows a total lithium ionic conductivity of3.38 � 10�4 S cm�1 at 25 �C and activation energy of 30.57 kJ mol�1. The thickness of the tape sheetcan be controlled to be about 75 lm. The ionic conductivity of the prepared sample reported in this studyis comparable with those reported for commercial LATP plate, showing the potential application for lith-ium-air batteries.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recently rechargeable lithium air batteries have attracted awide focus all over the world due to the requirement for develop-ing electrochemical energy storage devices with high energy den-sities that can promote the mass production of hybrid electricalvehicles and electrical vehicles [1–3]. Two types of lithium air bat-teries have been developed recently [4], aqueous and non-aqueouselectrolyte systems lithium air batteries. The non-aqueous typelithium-air battery shows a high weight-specific energy density[5], but it has some several problems that must be overcome, suchas lithium corrosion by water and CO2 ingression when operated inair, precipitation of high resistance reaction products on the porousair cathode electrode, and high polarization during the charge anddischarge processes. However, these problems observed for thenon-aqueous electrolyte lithium air system could be removed inthe aqueous electrolyte lithium air battery. While it is impossibleto ignore that lithium metal can react vigorously with water toproduce LiOH and hydrogen gas in aqueous electrolyte lithiumair battery. Therefore, Visco et al. [6] and Imanishi et al. [7] haveproposed the water stable lithium electrode, which consists of alithium metal anode, a lithium conducting polymer electrolyteconsisting of polyethylene oxide based lithium ion polymer

electrolyte, and a water stable lithium ion conducting solidelectrolyte Li1+xAlxTi1�x(PO4)3 (LATP). The polymer electrolytewas used as an interlayer to prevent the direct contact of lithiummetal with solid state lithium ion conducting electrolyte LATP,because LATP is unstable in contact with lithium metal.

The key material of the water stable lithium electrode is thewater stable lithium ion conducting solid state electrolyte. Re-searches on solid state lithium ion conducting electrolytes havebeen attracting more and more attention in the past several yearsdue to the wide range of potential important technological applica-tions in the high energy density rechargeable lithium air batteriesand other lithium metal batteries. But most lithium batteries arecoupling a polymer membranes (such as PP, PE) with a plasticizingorganic solvent (such as EC, DMC, EMC) which may bring the usualdrawbacks related to the presence of safety problems, for instance,liquid electrolyte leakage, lithium anode corrosion and limitationsfor miniaturization [8–10]. Some inorganic solid state lithium ionconductor can overcome those problems, such as LIPON [11,12],LISICON [13], NASICON glass ceramic [4,14,15], perovskiteLi0.5�3xLa0.5+xTiO3 [16–18]. However, the conductivity of LIPONwas reported to be 3 � 10�6 S cm�1 at room temperature [12].Thio-LISICON Li2S–P2S5 glass has a high conductivity at roomtemperature while it is difficult to fabricate and shows poor chem-ical durability [19]. The ionic conductivity of Li0.5�3xLa0.5+xTiO3 wasreported only to be 4 � 10�7 S cm�1 by film deposited usingradio-frequency magnetron sputtering [20] and it was observed

148 M. Zhang et al. / Journal of Alloys and Compounds 590 (2014) 147–152

that the LLTO-10 wt% Al2O3 composite electrolyte exhibited a highbulk conductivity as high as 9.33 � 10�4 S cm�1, but the grainboundary conductivity was only to be 2.38 � 10�5 S cm�1 by anceramic processing route including tape casting and relativelylow-temperature calcination [16]. Recently, NASICON glass cera-mic LATP (OHARA Inc., Japan) was reported to have a conductivity3.3 � 10�4 S cm�1 at room temperature. While it is a little moreexpensive to use on a large scale because of the high preparationcost, and it is very difficult to prepare large size LATP sheet becauseof the preparation process [13,21]. We have reported NASICONlithium ion conductor Li1.4Al0.4Ge1.6(PO4)3 (LAGP) with a maximumtotal conductivity of 1.22 � 10�3 S cm�1 at room temperatureusing a citric acid as precursor prepared by the sol–gel method,and the all-solid-state Li7Ti5O12/Li1.4Al0.4Ge1.6(PO4)3/LiCoO2 cancharge and discharge normally with a current 1 lA cm�2 at 60 �C,while the discharge capacity of the battery was only 5 lAh, whichwas little lower than the charge capacity. And the thickness of thisLAGP was about 2 mm, which was too thick for application.

Thus although LAGP pellets made by dried-pressing methodhave a high conductivity, it is very difficult to prepare for a largescale and the decrease of the thickness is difficult to carry out.Alternative route for large-scale preparation and the attempt to de-crease the thickness of the electrolyte are desired. In this research,we have attempted to prepare for a thin LAGP film (micron-grade)by using tape casting method to scale up the productivity and re-duce the preparation cost. The preparation and the performanceof the products will be discussed in detail.

2. Experimental

Li1.4Al0.4Ge1.6(PO4)3 powders were prepared using a precursor from the sol–gelmethod with citric acid, as reported previously [4]. Stoichiometric amounts ofGe(OC2H5)4 (Aldrich), LiNO3, Al(NO3)3�9H2O, and NH4H2PO4 were dissolved into a0.2 M aqueous solution of citric acid and stirred continuously with a magnetic stir-rer to obtain a homogeneous solution. A certain volume of ethylene glycol (citricacid: ethylene glycol molar ratio = 1:1, and (citric acid + ethylene glycol):(Li+ + -Al3+ + Ge4+) molar ratio = 4:1) was added to the mixed solution in order to preventthe formation of hard agglomerates and promote the poly-esterification and poly-condensation. The mixed solution was kept at 80 �C for several hours during thesol–gel preparation process. After the formation of the homogeneous solution, thegel was kept at 170 �C for 24 h to allow the evaporation of water and promotethe esterification and polymerization between ethylene glycol and citric acid, andthen heated at 500 �C for 4 h to obtain an organic metal salt powders. The obtainedpowders were ground with an agate mortar and pestle before sintering at 800 �C for5 h to prepare LAGP powders.

Then the as-prepared LAGP powders were ball-milled with ZrO2 balls for 6 husing high energy mechanical milling (HEMM) (Fritsch Planetary Micro Mill) withethanol (95%) as solvent, dried in air, and then dispersed in ethanol (95%) and tol-uene mixed solution (3:7 volume ratio). Menhaden fish oil (2 wt% to LAGP) wasadded into the slurry as a dispersant. Then the mixed slurry was ball-milled againfor 24 h using HEMM. Then a certain amount of poly (vinyl butyral) (Aldrich, ButvarB-98, 7 wt% to LAGP) as a binder and butyl benzyl phthalate (7 wt% to LAGP) as theplasticizer were added into the mixed slurry, and the slurry was ball-milled usingHEMM for another 24 h. Before tape-casting, the slurry was de-foamed for 3 minto remove the air bubbles by using a planetary vacuum mixer (Thinky, Japan).Tape-casting was performed on a silicon coated polyethylene substrate foil withdouble blades of gap heights of 700 lm and 400 lm. The casting speed was con-stant at 60 cm min�1. After tape-casting, the green sheets were left to dry at roomtemperature for 24 h in air. Then the green sheets were cut into small pieces of 1.2–1.5 cm � 1.2–1.5 cm. Then two or three pieces of the green sheets were hot pressedtogether at 90 �C for 10 min and sintered at various temperatures under air. Theflowchart for LAGP tape-sheet preparation is shown Fig. 1.

Scanning electron microscope (SEM, Hitachi SEM S-4000) is used to analyze themicro-morphology of the as-prepared LAGP powders and LAGP sheets. Differentialthermal analysis (TG-DTA) was carried out using a Rigaku Thermo-Plus TG8120 toanalyze the sintering temperature. The crystal structure of the samples was ana-lyzed using X-ray diffraction (XRD; Rigaku RINT 2500) with Cu Ka radiation inthe 2h range from 10� to 90� at a scanning step rate of 0.02 deg s�1. Gold filmwas sputtered on both sides of the sintered pellets as the blocking electrodes in or-der to evaluate the electrochemical properties of the LAGP sheet. The gold coatedLAGP specimen was assembled into a sealed sandwich cell using a stainless steelas a current collector in a dry chamber with silica-gel which was slightly lower thanthat under air to test the electrochemical impedance spectra (EIS). EIS measure-ments of the LAGP were carried out using an impedance analyzer (Solartron 1260

and 1287) in the temperature range from 20 �C to 80 �C and the frequency rangeof 0.01 Hz to 1 MHz. At each different temperature, the specimen was equilibratedfor 20 min before the impedance measurement. A nonlinear instant fit program inthe Z-View software was used to analyze the impedance profiles.

3. Results and discussion

Fig. 2 shows SEM images of the powders. It confirms that finepowders without agglomeration are obtained after HEMM. TheBET surface area of the powders increases from 1.46 m2 g�1 to6.67 m2 g�1 after ball-milling.

To exhibit the decomposition profiles of the organics in theLAGP casting tape, the TG and DTA results are presented in Fig. 3.The first weight loss occurs at around 200 oC, due to the oxidationloss of organic addition. This process lasts to about 500 oC until allthe organics are completely eliminated. The weight loss and exo-thermic peaks could be attributed to the oxidization reaction com-bined with the evaporation of volatile products such as H2O andCO2. When the temperature exceeds 600 oC, no further weight losscan be observed since all the organic additives have been con-sumed. When the temperature is above 1050 oC, an apparent endo-thermic peak appears because of the evaporation of Li at the hightemperature along with the side reactions.

Fig. 4 shows the XRD patterns of the LAGP green sheet sinteredat 900 �C (a), 950 �C (b), and 1000 �C (c) for 5 h. Sample sintered at900 �C for 5 h shows a dominating LAGP phase and a small butunobvious impurity GeO2 phase as in Fig. 4a. The impurity phasesAlPO4 can be found at 27�, 28�, 36� and GeO2 phase at 2h of 21� asshown in Fig. 4b and c, which matches well with the reported re-sult for LAGP sintered above 950 oC [4]. The AlPO4 phase is an elec-trically insulating phase and its existence can decrease theelectrical conductivity of the specimen. Thus, the sintering temper-ature should be controlled to be below 950 oC in order to get a rel-ative pure LAGP phase with a high conductivity.

XRD patterns of the LAGP green sheet sintered at 900 oC for 8 h,12 h, and 15 h are shown in Fig. 5. All the main diffraction peaksmatch well with the NASICON-type LiGe2(PO4)3 structure(PDF#80-1924). The crystal lattice parameters are calculated byusing the Jade cell refinement program. The a-lattice constant de-creases from 0.8280 nm to 0.8259 nm and the c-lattice constant in-creases from 2.0474 nm to 2.0625 nm for the samples treated at900 oC for 8 h and 12 h respectively. While the c-lattice constantdecreases to 2.0450 nm and a-lattice constant increases to0.8270 nm for the sample LAGP treated at 900 oC for 15 h. Wenet al. [22] reported that c-lattice of Li1.5Al0.5Ge1.5(PO4)3�xLi2O,(x = 0.0–0.20) decreased with the lower Li2O content. The decreaseof the c-lattice parameter for LAGP sintered for 15 h may be due tothe evaporation of Li+.

Fig. 6 shows the SEM images of the samples sintered at 900 �Cfor different time. The grain sizes increase with the sintering time,along with the grain boundary sizes decrease. LAGP sintered for 8 hhas the smallest grain size and the largest grain boundary asshown in Fig. 6a. With the increase of the sintering time, the grainsize increases and the grain boundary decreases as shown inFig. 6b, together with the cracks, as shown in Fig. 6c.

Typical EIS profiles measured at room temperature are shownin Fig. 7 for LAGP tape sheets sintered at 900 �C for various time.The impedance profiles consist of a semicircle in the high fre-quency and a linear region at the low frequency, and the equivalentcircuit is inserted into Fig. 7. The intercept of the semicircle in thehigh frequency represents the bulk resistance Rb, and the diameterof the semicircle is attributed to the grain boundary resistance Rgb.Rb values are 3045.81 X cm, 2746.05 X cm, 2981.87 X cm, and Rgb

values are 318.88 X cm, 213.46 X cm, and 284.1 X cm for LAGPsample sintered at 900 �C for 8 h, 12 h and 15 h, respectively. Allthe resistance values are obtained from the simulated results by

Fig. 1. Flowchart for LAGP tape-sheet preparation process.

Fig. 2. SEM pictures for LAGP powder before (a) and after (b) using the HEMM.

Fig. 3. TG–DTA curves for LAGP green tape-sheet.

Fig. 4. XRD patterns of LAGP tape-sheet sintered at (a) 900 �C, (b) 950 �C, (c)1000 �C for 5 h.

M. Zhang et al. / Journal of Alloys and Compounds 590 (2014) 147–152 149

using the equivalent circuit with a nonlinear instant fit program inthe z-view software. The result confirms that Rb and Rgb are depen-dent on the sintering time. Variation of initial resistance at highfrequency region depends on the density of the prepared castingsheets. With a long heat treating time, Rb and Rgb decrease along

with the increasing density of the sheets. The calculated electricalconductivity shows an obvious temperature and time dependence.The relative densities are measured to be 83%, 88%, and 87% for thesamples sintered at 900 �C for 8 h, 12 h, and 15 h, respectively. The

Fig. 5. XRD patterns and lattice parameters of LAGP tape-sheets sintered at 900 �Cfor (a) 8 h, (b) 12 h and (c) 15 h.

Fig. 6. SEM pictures for LAGP tape sheets sintered at 900 �C for (a) 8 h, (b) 12 h and(c) 15 h.

Fig. 7. Impedance profiles of LAGP tape-sheets sintered at (j) 900 �C for 8 h, (d)12 h and (.) 15 h at 25 �C.

Fig. 8. Arrhenius plots for LAGP tape-sheets sintered at (j) 900 �C for 8 h, (d) 12 hand (.) 15 h between 20 �C to 80 �C.

150 M. Zhang et al. / Journal of Alloys and Compounds 590 (2014) 147–152

highest conductivity is observed for LAGP sintered at 900 oC for12 h along with the highest relative density. The bulk and grain

boundary conductivity measured at 25 �C of LAGP sintered at900 �C for 12 h are calculated to be 3.64 � 10�4 S cm�1 and4.68 � 10�3 S cm�1, respectively, and the total conductivity rt is3.38 � 10�4 S cm�1 at 25 �C, this value is comparable to the com-mercial solid state lithium ion conductor LATP [23] (Ohara Inc., Ja-pan) with the electrical conductivity of 3.3 � 10�4 S cm�1. The totalconductivities of LAGP sintered at 900 �C for 8 h and 15 h are2.97 � 10�4 S cm�1 and 3.06 � 10�4 S cm�1, respectively. The dis-crepancy could be explained as a result of different density derivedfrom the sintering time.

The activation energy (Ea) and conductivity (r) for differentsamples are found to fit the Arrhenius equation.

r ¼ Ae�EaRT ð1Þ

logr ¼ log A� Ea

2:3RT¼ log Aþ B

Tð2Þ

where Ea is the activation energy; A is the pre-exponential factor; Tis the absolute temperature and R is the gas constant. The parame-ters, as the slope B and log A, are determined in Fig. 8. Ea can be cal-culated from the slope of the conductivity dates measured in thetemperature range from 20 �C to 80 �C. The corresponding Ea for

Fig. 9. Relationship for the sintering time with (j) conductivity rt, (d) the relativedensity and (.) the activation energy Ea.

Fig. 10. Temperature dependence impedance plots for LAGP annealed at 900 �C for12 h.

M. Zhang et al. / Journal of Alloys and Compounds 590 (2014) 147–152 151

the samples sintered at 900 �C for 8 h, 12 h, and 15 h is31.76 kJ mol�1, 30.57 kJ mol�1, and 30.96 kJ mol�1, respectively.

The high conductivity of the sample sintered at 900 �C for 12 hmay be due to the increasing of the relative density by the properlyheat-treating process, which mainly contributes to decrease theresistance of grain bulk and grain boundary impedance. And theconductivity decrease of the sample sintered at 900 �C for 15 hmay be due to the formation of the cracks in the grain boundaryand the evaporation of Li+ at a high temperature for a long sinter-ing time (as shown in Fig. 6c), which can also cause the increase ofEa of the sample from 30.57 kJ mol�1 to 30.96 kJ mol�1 comparedto that for sample sintered for 12 h. The relationship of sinteringtime, relative density, conductivity and the activation energy is

Fig. 11. Photographs of different sizes LAGP tape-sheet after sinter

summarized in Fig. 9. It can be found that LAGP sheet sintered at900 �C for 12 h exhibits the highest conductivity and relative den-sity, together with the lowest activation energy among all thesamples.

Temperature dependence impedance plots for LAGP (withthickness of 228 lm) sintered at 900 �C for 12 h are shown inFig. 10. The resistance decreases with the testing temperature from20 �C to 80 �C, and no charge transfer effect is found because of thetest environment in a dry chamber with silica-gel. At 60 �C, the to-tal resistance is only 18.31 X cm2, and rt at 60 �C is calculated tobe 1.25 � 10�3 S cm�1, which is comparable to that of LAGP(1.22 � 10�3 S cm�1, 25 �C) pellet prepared by dried-pressing andsintered at 900 �C for 11 h. Imanishi et al. [7,23] reported aqueouslithium air batteries by using a water stable lithium anode con-sisted of lithium metal, LiPON or polymer interlayer, and LATP so-lid state membrane. LAGP pellet has been proven to be stable insaturated LiOH and LiCl aqueous solution in our previous report[4]. Thus the prepared casting sheet can be used as a solid statemembrane for aqueous lithium air batteries to protect lithium an-ode from reacting with aqueous electrolyte solution. Meanwhile, itis necessary to reduce the thickness of the ceramic electrolyte so asto reduce the weight of the cell to meet the requirement of appli-cation. Philippe et al. [24] reported thin membranes with thicknessabout 50 lm were preferred with a significant reduction in areaspecific resistance and cell weight. We have tried to make a thinLAGP sheet about 75 lm in thickness (shown in Fig. 11). The diffi-culty is to keep both a good mechanical stability and high lithiumion conductivity for the membrane. The average three-point bend-ing strength of LAGP films was measured to be 48.02 N mm�2,which is lower than that of commercial LATP as 154 N mm�2. Fur-ther optimization in preparation process will be going onto modifythe densification of electrolyte, ionic conductivity and hencemechanical properties of LAGP sheet. Several approaches couldbe adopted to enhance the ambience temperature conductivity aswell as to improve the mechanical stability, such as adding someepoxy resin to fill the micro-porous of the film [25], or Nano cera-mic fillers [26] to decrease the sintering temperature and improvethe density of the film.

4. Conclusions

A low cost solid-state lithium ion conductor LAGP sheet hasbeen prepared by tape casting method. LAGP sintered at 900 �Cfor 12 h shows a total lithium ion conductivity of 3.38 � 10�4 -S cm�1 at 25 �C and activation energy of 30.57 kJ mol�1, with rela-tive density of 88%. The ionic conductivity is similar with thatreported for commercial LATP plate (3.3 � 10�4 S cm�1) of OharaInc., Japan. The thickness of the tape sheet can be controlled toabout 75 lm. The method adopted in this research can be scaledup easily, which makes it possible for LAGP as potential solid-statelithium ion conductor in lithium-air batteries.

ing at 900 �C for 12 h, and a thin one with a thickness 75 lm.

152 M. Zhang et al. / Journal of Alloys and Compounds 590 (2014) 147–152

Acknowledgements

The authors would like to thank Program for Changjiang Schol-ars and Innovative Research Team in University (PCSIRT No.IRT1014) for financial support and Materials Characterization Cen-ter of Huazhong University of Science and Technology for samplescharacterization assistance.

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