A668 Journal of The Electrochemical Society 161 (5) A668-A674 (2014)0013-46512014161(5)A6687$3100 copy The Electrochemical Society

Ta-Doped Li7La3Zr2O12 for Water-Stable Lithium Electrode ofLithium-Air BatteriesK Ishiguroa H Nemoria S Sunahiroa Y Nakataa R Sudoa M Matsuiablowast Y TakedaalowastO Yamamotoalowastlowastz and N Imanishialowast

aGraduate School of Engineering Mie University Tsu Mie 514-8507 JapanbJST PRESTO 4-1-8 Honcho Kawaguchi Saitama 332-0012 Japan

Garnet-type high lithium ion conductivity solid electrolytes with nominal compositions of Li7-xLa3Zr2-xTaxO12 (x = 0ndash07) wereprepared by using a sol-gel precursor and are proposed as the protective layer for a water-stable lithium electrode for lithium-airrechargeable batteries The Li675La3Zr175Ta025O12 (LLZ-025Ta) composition sintered at 1180C for 36 h showed the highestrelative density of 967 and resistance to water permeation The LLZ-025Ta was stable in contacting with lithium metal and alsoin a saturated LiOH10 M LiCl aqueous solution The total and bulk conductivities of the sintered LLZ-025Ta pellet were 520times 10minus4 and 655 times 10minus4 S cmminus1 at 25C respectively The LiLLZ-025TaLi cell showed a stable cell resistance of 310 cm2 at25C for 4 months However polarization experiments on the LiLLZ-025TaLi cell suggest a lithium dendrite short-circuit after100 s at 05 mA cmminus2 and after 2000 s of polarization at 01 mA cmminus2 at room temperaturecopy 2014 The Electrochemical Society [DOI 1011492013405jes] All rights reserved

Manuscript submitted January 29 2014 revised manuscript received February 26 2014 Published March 8 2014

Rechargeable lithium-air batteries are attracting increased atten-tion as potential power sources for electric vehicles and grid energystorage because they have far higher energy density and a lower mate-rials cost than lithium ion batteries1ndash5 The calculated specific energydensities are 3460 Wh kgminus1 for a non-aqueous system with the cellreaction of 2Li + O2 = Li2O2 and an open-circuit voltage (OCV) of296 V6 and 1910 Wh kgminus1 for a aqueous system with the cell reactionof 4Li + 6H2O + O2 = 4(LiOH middot H2O) and an OCV of 30 V7 Thesecalculated energy densities are several times higher than those of con-ventional lithium-ion batteries However many problems should besolved to realize rechargeable lithium-air batteries with the high spe-cific energy and power densities that have been projected Previousstudies on the non-aqueous system have used pure oxygen because thelithium metal electrode requires protection from the water and carbondioxide in air8 One approach to solve this problem is to use a water-stable lithium electrode as proposed by Visco et al in 20049 Theconcept of this electrode is to protect lithium metal with a water-stablelithium conducting solid electrolyte of Li1+xTi2-xAlx(PO4)3 (LTAP)The water-stable lithium electrode is a key component for the aque-ous system The aqueous rechargeable lithium-air system cannot berealized without the water-stable lithium electrode LTAP has beenextensively used as the protective layer for the lithium metal elec-trode in previous studies on the aqueous lithium-air system10ndash12 Thelithium ion conductivity of LTAP was reported to be as high as 7times 10minus4 at 25C13 However LTAP is unstable in contacting withlithium metal14 Therefore another protective layer must be used be-tween lithium metal and LTAP Visco et al9 and Imanishi et al10 haveused Li3N and a lithium conducting polymer electrolyte respectivelywhich are stable in contacting with lithium metal but unstable in wa-ter The Li3N thin layer was prepared by a sputtering method whichis expensive and not a convenient method for the fabrication of largesize systems The polymer electrolyte protective layer is preferablefor large size systems but the room temperature conductivity is lowand the system must be operated at above 60C to obtain sufficientconductivity

In 2007 Weppner and colleagues15 reported a lithium conductingsolid electrolyte with a nominal composition of Li7La3Zr2O12 (LLZ)This compound has a garnet-type structure and the conductivity wasreported to be 244 times 10minus4 S cmminus1 at 25C Furthermore LLZ isstable in molten lithium metal Shimonishi et al16 reported that LLZwas stable in a saturated LiOH10 M LiCl aqueous solution ThusLLZ is an attractive candidate for the protective layer of a water-stable lithium electrode for the aqueous lithium-air system and is

lowastElectrochemical Society Active MemberlowastlowastElectrochemical Society Fellow

zE-mail yamamotochemmie-uacjp

also useful to protect reaction of the lithium anode with non-aqueouselectrolyte such as dimethyl sulfide which is not stable in contactwith a bare lithium anode17 Recently there have been many reportson the doping of LLZ to enhance the lithium ion conductivity Ohtaet al reported Nb-doped LLZ with a high conductivity of 8 times 10minus4 Scmminus1 at 25C18 which was sintered at 1200C for 36 h in an aluminacrucible However the Nb-doped LLZ was unstable in contactingwith lithium metal19ndash21 Allen et al22 reported that Al-free Ta-dopedLLZ Li675La3Zr175Ta025O12 which was prepared by hot pressing at1050C had a high lithium ion conductivity of 87 times 10minus4 S cmminus1 at25C Wang and Lai23 synthesized Li7-xLa3Zr2-xTaxO12 (x = 0ndash2) at1130C for 36 h in an alumina crucible using 05 mol NaHCO3 andα-Al2O3 as a sintering agent and reported the highest total lithiumion conductivity of 69 times 10minus4 S cmminus1 and a bulk conductivity of 96times 10minus4 S cmminus1 at 25C for the x = 03 composition In addition Liet al24 reported the lithium ion conductivity of Li7-xLa3Zr2-xTaxO12

(x = 0ndash10) that was prepared by conventional solid-state reaction at1120ndash1230C in an alumina crucible the highest total lithium ionconductivity was 1 times 10minus3 S cmminus1 at 25C which is comparable tothe bulk conductivity was found for Li64La3Zr14Ta06O12 sintered at1140C for 16 h More recently Inada et al25 reported a total electricalconductivity of 41 times 10minus4 S cmminus1 at 27C for Li675La3Zr175Ta025O12

and 61 times 10minus4 S cmminus1 for Li65La3Zr15Ta05O12 which was sinteredat 1100C for 15 h using a Pt-Au crucible The Ta content in LLZthat exhibited the highest conductivity was different in these reportsAllen et al22 reported a reduction in the lithium conductivity from 87times 10minus4 to 37 times 10minus4 S cmminus1 at 25C by the addition of 02 molesAl into Li675La3Zr175Ta025O12 The difference in the conductivityfrom report to report may be due to the difference in the Li and Alcontents of the samples as a result of the different preparation methodsThese previous papers have reported that Ta-doped LLZ has a highlithium ion conductivity that is comparable or higher than that of LTAPHowever the stability in contact with lithium metal and in aqueoussolution as well as the electrochemical performance at the interfacebetween the lithium metal electrode and Ta-doped LLZ have not yetbeen studied in detail In this work we have investigated Ta-dopedLLZ with high lithium ion conductivity as a potential candidate forthe protective layer for water-stable lithium electrodes in lithium-airbatteries

Experimental

Ta-doped LLZ was prepared by using a sol-gel precursor Theprecursors for the nominal composition of Li7-xLa3Zr2-xTaxO12

(x = 0ndash07 denoted as LLZ-xTa) were prepared by using a previ-ously reported sol-gel method19 Stoichiometric amounts of chemicalgrade LiNO3 La(NO3)3 middot 6H2O ZrO(NO3)2 middot 2H2O and TaCl5 weredissolved in water to which citric acid and ethylene glycol (11 molar

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Journal of The Electrochemical Society 161 (5) A668-A674 (2014) A669

ratio) were added Excess lithium (10 at) was added to compensatefor the expected lithium loss during high temperature sintering Thesolution was stirred for several hours on a hot plate The obtained solidwas heated at 400C for 10 h in open air and then ground to a pow-der The powder was ball-milled with ZrO2 balls in a ZrO2 vessel for2 h at 200 rpm by using high energy mechanical milling (HEMMFritsch planetary micro mill) and then calcined at 900C for 12 hin an alumina crucible The calcined pellets were reground and thenball-milled with ZrO2 balls in a ZrO2 vessel for 2 h at 200 rpm byusing HEMM The obtained powders were isostatically pressed intopellets and sintered at 1180C for 36 h in a covered alumina crucibleThe pellets were covered with the mother powder (prepared at 900Cfor 12 h in an alumina crucible) The sintering temperature used wasslightly higher than those reported by Wang et al23 (1130C) and Liet al21 (1140C) because the aim was to prepare a dense pellet thatwas resistant to water penetration

The crystal structures of the sintered samples were analyzed byX-ray diffraction analysis (XRD Rigaku RINT 2500) with Cu Kαradiation in the 2θ range from 10 to 90 at a scanning step rate of002 sminus1 The microstructure and morphology of the sintered pelletswere observed using scanning electron microscopy (SEM Hitachi S-4000) and electron probe micro analysis (EPMA JEOL JXA-8530F)The particle size distribution of the powder was estimated by using alaser diffraction particle size analyzer (Nikkiso Microtrac MT 3300EXII) Elemental analysis for Li La Zr Al and Ta in the sinteredsamples was conducted by using inductively coupled plasma spec-troscopy (ICP Agilent Technology 700 ICP-OES) The samples werecompletely dissolved into a mixed solution of hydrochloric acid andnitric acid (31 vv) for ICP analysis The relative density of the sin-tered pellets was estimated from the ratio of the density calculatedfrom the lattice constant and that calculated from the weight andvolume

The electrical conductivity of the sintered samples (ca 10 cm di-ameter and ca 01 cm thick) with gold-sputtered electrodes was mea-sured by using an impedancegain phase analyzer (Solartron 1260)in the frequency range from 01 Hz to 1 M Hz with the bias voltageset at 10 mV Bulk and grain boundary resistances were estimatedfrom complex impedance plots by using Zview 226 Direct currentmeasurements of the LiLLZ-xTaLi cell were performed by using theimpedancegain phase analyzer (Solartron 1260) with an electrochem-ical interface (Solartron 1287) LiLLZ-xTaLi cells were prepared bysandwiching lithium metal foils (Honjyo Metal 200 μm thick) witha Cu thin foil lead and the LLZ-xTa pellet in a plastic envelope Theenvelope was then evacuated heat-sealed and subjected to an isostaticpressure of 150 MPa to ensure good contact between the Li and LLZ-xTa The stability of the sintered LLZ-xTa pellets was investigatedby immersion into distilled water saturated LiOH and a mixture ofsaturated LiOH10 M LiCl aqueous solutions at 25C for one weekThe pellets immersed in these solutions were then carefully washedwith distilled water and dried at 120C for overnight before measuringelectrical conductivity and XRD patterns The change in conductivitywith the period of immersion in the aqueous solutions was also mea-sured by using a H-type cell separated by the sintered LLZ-025Tapellet where an aqueous solution of the saturated LiOH10 M LIClwas used at both sides and platinum plates were used as the elec-trode The H-type cell was also used to measure the water permeationthrough the sintered pellets where the saturated LiCl aqueous solu-tion was on one side of the cell and distilled water on the other sideThe chloride ion content in the distilled water side was measured withrespect to time using a chloride ion meter (Kasahara CL-5Z) Theion exchange of Li+H+ for LLZ-025Ta in water was examined byanalysis of the lithium ions in solution A fine powdered sample ofLLZ-025Ta (003 g) was immersed in 100 mL distilled water andthe Li+ content in the water was analyzed by ICP spectroscopy as afunction of the immersion period

Results and Discussion

Figure 1 shows room temperature powder XRD patterns of thenominal Li7-xLa3Zr2-xTaxO12 compositions from sol-gel precursors

Figure 1 XRD patterns of Li7-xLa3Zr2-xTaxO12 sintered at 1180C for 36 has a function of x The reference pattern of the cubic Li7La3Zr2O12 wassimulated based on the structure from Ref 26

sintered in an aluminum crucible at 1180C for 36 h Almostdiffraction peaks are assigned to the garnet-related structure reportedby Awaka et al27 Trace impurity phases of LiTaO3 La2Zr3O7 andLi5La3Ta2O12 were observed for LLZ-01Ta LLZ-05Ta and LLZ-07Ta respectively The lattice parameter of Li7-xLa3Zr2-xTaxO12 de-creases with increasing x because the six coordination ionic radius ofTa5+ (0064 nm) is less than that of Zr4+ (0072 nm) The lattice param-eter of the nominal composition Li675La3Zr175Ta025O12 was 12959nm which is lower than that observed by Wang et al23 (129801nm) and comparable to that reported by Li et al24 (129597 nm)for Li675La3Zr175Ta025O12 The lattice parameters of LLZ are af-fected somewhat by the Li+ and Al3+ contents in the lattice

To obtain water permeation-free dense pellets the optimum par-ticle size of the precursor powders of LLZ-025Ta calcined at 900Cwas evaluated A mixture of powders with average particle sizes of28 and 12 μm gave the highest relative density and conductivitythese were produced by high speed mechanical milling at 200 rpmfor 2 h The pellets sintered at 1180C for 36 h showed a relativedensity of 967 for LLZ-025Ta 962 for LLZ-03Ta and 960for LLZ-07Ta which are higher than that of Li675La3Zr175Ta025O12

reported by Wang and Lai23 (937) and Inada et al25 (92) How-ever a dense pellet of LLZ-01Ta could not be obtained The densityof a sintered pellet of LLZ-025Ta produced by using an average par-ticle size of 108 μm was 916 Water permeation tests confirmedthat a pellet of LLZ-025Ta with a relative density of 967 preventedwater permeation as shown in Fig 2 The dense LLZ-025Ta pelletshows no detectable chloride ions in the distilled water side of the

Figure 2 Water permeation test results () Li675La3Zr175Ta025O12 sinteredat 1180C for 36 h in an alumina crucible to obtain a relative density of 967and () La7La3Zr2O12 sintered at 1180C for 36 h in an alumina crucible toobtain a relative density of 90

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A670 Journal of The Electrochemical Society 161 (5) A668-A674 (2014)

Table I Bulk conductivity and molar ratio of element for the cubic Li7-xLa3Zr2-xMx(M = Nb Ta)O12

Sintering BulkNominal temperature Mole ratio Lattice conductivity

composition (C) Li La Zr Ta or Nb Al parameter at 25C Reference

Li7La3Zr2O12 1100 676 35 20 0 026 12975 29Li7La3Zr2O12+05 wt Al2O3 1180 684 ndash 20 0 025 12968 548 times 10minus4 30Li64La3Zr14Ta06O12 1140 632 291 ndash 06 008 12954 10 times 10minus3 24Li675La3Zr175Nb025O12 1150 646 338 175 023 012 12952 613 times 10minus4 30Li65La3Zr15Ta05O12 1100 639 30 148 050 0 ndash 61 times 10minus4 25Li675La3Zr175Ta025O12 1180 640 311 175 025 015 12959 655 times 10minus4 This studyLa65La3Zr15Ta05O12 1180 623 315 15 050 0054 12933 45 times 10minus4 This studyLa63La3Zr13Ta07O12 1180 617 318 13 071 0016 12917 32 times 10minus4 This study

Total conductivity

H-type cell after 4 weeks but chloride ions were detected for the lessdense LLZ (90) after a few days The prevention of water perme-ation is an important requirement for LLZ as a protective layer for thewater-stable lithium electrode in aqueous lithium-air batteries

The stoichiometric composition of Li7La3Zr2O12 with the hightemperature cubic phase and high lithium ion conductivity has notbeen reported The composition of Li7La3Zr2O12 is stable in thetetragonal phase at room temperature27 and transitions to the hightemperature cubic phase28 Recently Matsui et al29 confirmed thephase transition of Li7La3Zr2O12 from the tetragonal to cubic phase at650C by high temperature XRD analysis Doping with Al is consid-ered to stabilize the high temperature cubic phase with high lithiumion conductivity at ambient temperature2830 Rangasamy et al28 re-ported that at least 0204 moles of Al in Li7La3Zr2O12 is required tostabilized the cubic phase at room temperature Stabilization of thehigh temperature cubic phase of Li7La3Zr2O12 was also achieved bythe substitution of Ta for Zr without Al25 Therefore the mole ra-tio of the constituent elements in sintered LLZ-xTa is an importantfactor to discuss with respect to the electrical conductivity and elec-trochemical properties of doped LLZ Table I summarizes the atomicratios of LLZ-xTa and the high temperature cubic phase of doped LLZreported previously along with the bulk electrical conductivity andlattice parameters where the ratio was normalized according to Zr+ Ta (or Nb) at 2 The Al may come from the aluminum crucible usedfor the high sintering temperature and the content may depend on thesintering temperature and the amount of doped Ta (or Nb) The atomicratio of LiLaZrTaAl for LLZ-025 Ta was 640311175025015Geiger et al30 observed an atomic ratio of LiLaZrAl in LLZ to be6763520026 and Li et al24 reported the atomic ratio of LiLaAlin LLZ-06Ta to be 632291008 If all Al3+ substitutes for Li+ inthe lattice sites then the stoichiometric amount of Li+ is 630 for thepresent result and 622 and 616 for that reported by Geiger et al andLi et al respectively The observed Li+ contents were higher than theestimated contents Some Al3+ may be substituted for Li+ in the latticesites and the other be in the grain boundary Variation of the elemental

ratios in the sintered pellets was observed from sample to sampledue to the difficulty in controlling Li loss during the high temperaturesintering and Al diffusion from the alumina crucible via the motherpowder on the alumina crucible However it could be concluded fromthe results shown in Table I that the Al content in LLZ-xTa decreaseswith increasing Ta content

Figure 3 shows impedance profiles for the AuLLZ-xTaAu cellsat 25C as a function of x where the LLZ-xTa samples were sinteredat 1180C for 36 h The impedance profiles of LLZ-025Ta stored ina dry box with silica gel showed no aging effect after 4 months whileLi675La3Zr175Nb025O12 (LLZ-025Nb) showed a significant aging ef-fect of the grain boundary resistance19 These impedance profiles showa small semicircle in the frequency range of 106 to 105 Hz which is at-tributed to the grain boundary resistance of the pellet (Rgb) The grainboundary specific conductivity for LLZ-025Ta of 251 times 10minus3 S cmminus1

is comparable to that for LLZ-03Ta of 245 times 10minus3 S cmminus1 reportedby Wang and Lai23 and higher than that for 05 wt Al2O3 doped LLZof 167 times 10minus3 S cmminus131 The lower grain boundary sepcific conduc-tivity for LLZ-05Ta and LLZ-07Ta compared with LLZ-025Ta maybe due to the high grain boundary resistance of the impurity phasesThe intercepts of the small semicircles on the real axis at high fre-quency represent the bulk resistance (Rb) No semicircle due to thebulk was observed because this was out of the frequency range of theimpedance analyzer32 The highest bulk conductivity of 655 times 10minus4

S cmminus1 at 25C observed for LLZ-025Ta as shown in Fig 3b Wangand Lai23 and Li et al24 reported the highest bulk conductivities of 96times 10minus4 S cmminus1 for x = 03 and 11 times 10minus3 S cmminus1 for x = 06 inLLZ-xTa respectively The bulk conductivity may depend on the con-tents of Li+ and Al3+ ions in the lattice sites which depend on x andthe sintering conditions The low electrical conductivity of our samplewith x = 025 in LiyLa3Zr2-xTaxO12 compared with Wang and Lai23

and Li et al24 may be due to the high Al content introduced by hightemperature sintering with an alumina crucible Allen et al22 observedthat the conductivity of the nominal compositions Li675La3Zr175Ta025

O12 and Li615La3Zr175Ta025Al02O12 at 25C were 87 times 10minus4 and

Figure 3 (a) impedance profiles of the AuLi7-xLa3Zr2-xTaxO12Au cell and (b) bulk and total conduc-tivities of Li7-xLa3Zr2-xTaxO12 as a function of x at25C

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Journal of The Electrochemical Society 161 (5) A668-A674 (2014) A671

Figure 4 (a) bulk conductivity vs y in Li7-x-3yAlyLa3Zr2-x(Ta or Nb)xO12and (b) bulk conductivity vs x in LixLa3(Zr Ta Nb)2O12 curves at 25C

37 times 10minus4 S cmminus1 respectively where these samples were sinteredat 1050C and 40 MPa Inada et al25 also reported a total conductivityof 61 times 10minus4 S cmminus1 for LLZ-05Ta without Al and with a Li contentof 639 which was sintered at 1000C using a Pt-Au alloy crucibleThe bulk conductivity was not reported but the impedance profilesuggested a high bulk conductivity of ca 1 times 10minus3 S cmminus1 LLZ-06Ta with an Al atomic ratio of 008 reported by Li et al24 showeda high conductivity of 10 times 10minus3 S cmminus1 Wang and Lai23 and Ran-gasamy et al28 did not report the atomic content ratio in LLZ-xTaThe Al content in the lithium lattice sites could not be determinedby ICP analysis however the Al content in the lattice could be esti-mated from the Li content by assuming charge compensation by Al3

to achieve neutrality As such the lattice Al contents in LLZ-025TaLLZ-05Ta and LLZ-07Ta prepared at 1180C and LLZ-06Ta pre-pared at 1140C were estimated to be 0117 009 0043 and 0027moles respectively from the mole ratio of elements shown in Table IThe bulk conductivities of these samples decreased with the decreasein the lattice Al content down to 0043 and then increased as shownin Fig 4a Therefore the conductivity behavior of LLZ-xTa could notbe explained by only the lattice Al content but the content of Li inLLZ-xTa may play an important role Figure 4b shows the bulk elec-trical conductivity of LLZ-xTa (or Nb) at 25C as a function of thelithium contents presented in Table I The maximum bulk conductivitywas obtained at x = 068 for Li632La3Zr14Ta06O12 the Al content ofwhich was 008 The cubic garnet-type LLZ with a low Al content andan optimum Li content (ca 632) exhibits the highest conductivity

Stability of the high lithium ion conductivity solid electrolyte insaturated LiOH aqueous solution is an important requirement for theprotective layer of a water-stable lithium electrode for the aqueouslithium-air batteries Shimonishi et al16 investigated the stability ofLLZ in aqueous solutions and reported that the electrical conductivityof LLZ decreased significantly by immersion in distilled water 1 M

Figure 5 SEM images for (a) pristine Li675La3Zr175Ta025O12 immersed in(b) distilled water (c) saturated LiOH aqueous solution and (d) an aqueoussolution of saturated LiOH and 10 M LiCl at room temperature for one week

LiOH and 01 M HCl while LLZ immersed in a saturated LiOHLiClaqueous solution for one week exhibited a similar impedance profileto that for pristine LLZ The stability of high lithium ion conductiv-ity LLZ-025Ta in aqueous solution was examined Figure 5 showsSEM images of the sintered pellet surfaces before and after immersionin distilled water saturated LiOH aqueous solution and an aqueoussolution of saturated LiOH10 M LiCl for one week at room tem-perature Significant changes in the surface morphology are evidentfor the pellet immersed in distilled water while no change was ob-served for the pellets immersed in saturated LiOH aqueous solutionand the saturated LiOH10 M LiCl aqueous solution However thepellet immersed in the saturated LiOH aqueous solution was bro-ken XRD patterns of the pellets before and after immersion in theseaqueous solutions were the same as that of the pristine LLZ-025TaGalven et al33 reported that in ambient air a spontaneous Li+H+ ex-change in the garnet-type tetragonal Li7La3Sn2O12 occurs which leadto the formation of Li7-xHxLa3Sn2O12 and this sensitivity to moistureshould be generalized to most of the lithium garnet We have also con-firmed the Li+H+ exchange for LLZ-025Ta powder in distilled waterby analysis of the Li content in the distilled water after immersionFigure 6 shows the change in the concentrations of Li+ Zr4+ andTa5+ in distilled water with immersion time The lithium ion concen-tration increases with time while the other ions were not detected

Figure 6 Temporal change in Li+ Zr4+ and Ta5+ ion concentrations ofdistilled water (100 mL) containing Li675La3Zr175Ta025O12 powder (003 g)at room temperature

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A672 Journal of The Electrochemical Society 161 (5) A668-A674 (2014)

Figure 7 Impedance profiles of the Ptsaturated LiOH10 M LiCl aque-ous solutionLi675La3Zr175Ta025O12saturated LiOH10 M LiCl aqueoussolutionPt cell at room temperature

Approximately 20 Li+ in Li675La3Zr175Ta025O12 were exchanged byH+ after 40 days where 003 g LLZ-025Ta powder was immersedin 100 mL water The Li+H+ exchange rate may be dependent onthe Li+ ion concentration in the distilled water in the first few hoursthe Li+ ion concentration in the distilled water increased significantlyThe high grain boundary resistance reported by Shimonishi et al16

for LLZ immersed in distilled water could be explained by the for-mation of a high resistance phase of Li7-xHxLa3Zr2O12 The surfaceof sintered LLZ-025Ta stored for several weeks was examined usingEPMA which revealed carbon on the surface The carbon could beconsidered as a reaction product of the lithium released and atmo-spheric CO2 The stability of LLZ-025Ta in the saturated LiOH10 MLiCl aqueous solution was confirmed by the change in resistance of thePtsaturated LiOH10 M LiCl aqueous solutionLLZ-025TasaturatedLiOH10 M LiCl aqueous solutionPt cell with storage time at roomtemperature The cell resistance was slightly decreased with the stor-age time and then became constant for one month as shown in Fig 7The total conductivity of LLZ-025Ta is comparable to that measuredusing the AuLLZ-025TaAu cell The high grain boundary resistancemay be due to reaction with water and impurity phases in the grainsandor on the surface The excellent stability of LLZ-025Ta in thesaturated LiOH10 M LiCl aqueous solution confirms that this highlithium ion conductivity solid electrolyte could be used as the pro-tective layer of the water-stable lithium metal electrode because thereaction product of the aqueous lithium-air battery is LiOH and theaqueous solution is saturated at only around 5 discharge depth

The stability of LLZ-xTa in contacting with lithium metal is theother important requirement for the electrolyte in solid state batteriesand as a protective layer for the water-stable lithium electrode Nb-doped LLZ exhibits a high electrical conductivity of 8 times 10minus4 Scmminus1 at 25C18 However the impedance profiles of the LiLLZ-

025NbLi cell showed a significant increase of the interface resistance(Ri) with lithium metal over the storage period19 The increase ofRi could be explained by the reduction of Nb+5 in contacting withlithium The impedance of the LiLLZ-025TaLi cell was examinedwith respect to the storage time The cell was stored in a dry box atroom temperature and the cell impedance was measured at 25 and 0Cover a long period The contribution of Rgb and the interface resistancebetween lithium and LLZ-025Ta were not clearly evident from theroom temperature impedance measurement but were observed at 0CFigure 8 shows the impedance profiles of the LiLLZ-025TaLi cell at25 and 0C as a function of the storage time at room temperature Theimpedance profiles measured at 0C show two semicircles in the highand low frequency ranges The high frequency range semicircle from106 to 105 Hz may be due to the contribution of Rgb in LLZ-025Tabecause the AuLLZ-025TaAu cell showed a similar semicircle inthe same frequency range The low frequency range semicircle from105 to 102 Hz may be due to the contribution of Ri which consistsof the resistance of the interlayer produced between lithium metaland LLZ-025Ta and a charge transfer resistance34 Rb Rgb and Ri

were estimated by using the equivalent circuit shown in Fig 8 Thegrain boundary resistances showed no change with aging in contrastto that for Nb-doped LLZ19 The interface resistance was decreasedslightly for two weeks and then remained constant for 4 months Thesteady interface resistance was as low as 180 cm2 at 0C and around20 cm2 at 25C Figure 9 shows impedance profiles for LiLLZ-xTa(x = 025 05 and 07)Li measured at 25 and 0C after overnightstorage The interface resistance increased with increasing x The totalconductivity of LLZ-07Ta is approximately half that for LLZ-025Tabut the interface resistance for LLZ-07Ta is significantly higher thanthat for LLZ-025Ta It is not clear why the interface resistance forLLZ-07Ta is so high compared to that for LLZ-025Ta but LLZ-07Ta had a low Al content and the grain boundary resistance wasconsiderably higher than that for LLZ-025Ta as shown in Fig 3 Thegrain boundary phase at the surface may affect the interface resistanceandor LLZ with a high Ta content is unstable in contact with lithiummetal as observed for LLZ-Nb The activation energy for the interfaceresistance estimated from the temperature dependence of the inverseinterface resistance is 38 kJ molminus1 for LLZ-025Ta and 51 kJ molminus1

for LLZ-07Ta in the temperature range from 25 to 0C which arecompared with those for the bulk conductivity of LLZ-025 Ta at26 kJ molminus1 and grain boundary conductivity at 43 kJ molminus1 Thehigh interface resistance and activation energy for LLZ-07Ta suggestthat the interlayer formed between lithium metal and LLZ-07Ta hasa high barrier for lithium transport The interlayer may be affected bythe impurity phase in LLZ-07Ta

LLZ-025Ta exhibits high electrical conductivity and low interfaceresistance and stability between lithium metal and the electrolyte Theother requirement of the solid electrolyte of a water-stable lithiumelectrode is reversible lithium stripping and deposition at high currentdensity There have been only a few reports on the electrochemicalbehavior of the interface between lithium metal and LLZ Kotobukiet al35 reported an abnormal increase in cell voltage for LiLLZLi that

Figure 8 Impedance profiles for LiLi675La3Zr175Ta025O12Li measured at 25 and 0C as a functionof the room temperature storage time

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Journal of The Electrochemical Society 161 (5) A668-A674 (2014) A673

Figure 9 Impedance profiles for LiLLZ-xTaLimeasured at 25 and 0C as a function of x

was as low as 50 μA cmminus2 after polarization for 70 s where the LLZpellet was sintered at 1230C for 36 h and the electrical conductivitywas 18 times 10minus4 S cmminus1 at room temperature However the reasonfor the abnormal behavior was not explained We have previouslyreported19 that the LiLLZ-025NbLi cell exhibited a short-circuit af-ter 200 s polarization at 05 mA cmminus2 Figure 10 shows the changein cell voltage with the polarization period at 05 mA cmminus2 for theLiLLZ-xTaLi cell at 25C Steady cell voltages are obtained for shortperiods and the cell resistance calculated from the cell voltage justafter polarization is comparable to that estimated from the impedanceprofile of the cell at 25C Abrupt drops in cell voltage after polar-ization for a short period were observed for all LiLLZ-xTaLi cellsThis may be due to short-circuit by the formation of lithium den-drites as observed for the LiAl2O3-doped LLZLi cell31 A seriousproblem for the lithium metal electrode is lithium dendrite formationby lithium deposition which results in short-circuiting of the cell Thelithium metal anode has not been used for conventional rechargeablelithium batteries with liquid electrolytes because of dendrite forma-tion on the lithium surface during the charge process Thus most ofthe attempts to stabilize the lithium metal electrode have focused onthe solid electrolyte36 Monroe and Newman37 predicted that if a ho-mogenous solid electrolyte with a modulus of 6 GPa were obtainedthen the lithium dendrite problem would be solved The modulusof ceramics materials is generally higher than 6 GPa therefore weshould expect no lithium dendrite formation between LLZ and lithiummetal However Ishiguro et al19 reported that the LiLLZ-025NbLicell showed a short-circuit after 230 s polarization at 05 mA cmminus2

and no short-circuit after 40 h at 01 mA cmminus2 where the LLZ wassintered at 1150C for 36 h and the relative density was 928 Morerecently Sudo et al31 reported that the Li05 wt doped LLZLicell showed a cell voltage fluctuation after 250 s polarization and a

Figure 10 Cell voltage vs polarization period curves for LiLLZ-xTaLi at05 mA cmminus2 and 25C

short-circuit after 1000 s of polarization at 05 mA cmminus2 where theLLZ was sintered at 1180C for 36 h and the relative density was937 The LiLLZ-025TaLi and LiLLZ-07TaLi cells were shortndashcircuited after 100 and 240 s polarization at 05 mA cmminus2 respectivelywhich are comparable or shorter times than those for LLZ-025Nb and05 wt Al2O3 doped LLZ Short-circuiting was observed after 33min of polarization at 01 mA cmminus2 The mechanism for dendriteformation between lithium metal and solid electrolyte has yet tobe clarified Sudo et al31 found some black spots on the 05 wtAl2O3 doped LLZ part of a Liethylene carbonate-diethyl carbonate-LiClO405 wt Al2O3 doped LLZLi cell after 280 s polarization at05 mA cmminus2 The black spots were considered to be lithium dendritegrowth through the grain boundaries and voids in LLZ There was noclear dependence of the period to short-circuit on the relative densityof LLZ but the lithium ion diffusion kinetics at grain boundaries maybe a key factor because LLZ-07Ta with the high grain boundary re-sistance showed longer short-circuit period than LLZ-025Ta with thelow grain boundary resistance and a slightly lower relative densityHowever the short-circuit period for LLZ-025Ta at 05 and 01 mAcmminus2 is too short for practical lithium-air batteries Therefore thegrain boundaries of the water permeation-free Ta doped LLZ withhigh conductivity should be improved to protect the lithium dendriteformation

Conclusions

Water penetration-free and lithium-stable high lithium ion conduc-tivity solid electrolyte of the cubic garnet-type with the nominal com-position Li7-xLa3Zr2-xTaxO12 was prepared by using a sol-gel precursorsintered at 1180C for 36 h The highest conductivity of 520 times 10minus4

S cmminus1 at 25C was obtained for the nominal Li675La3Zr175Ta025O12

composition Elemental analysis of Li675La3Zr175Ta025O12 by ICPshowed the atomic ratios of Al and Li were 015 and 640 respec-tively The dependence of the bulk electrical conductivity on the Liand Al contents in LLZ was discussed and compared with previouslyreported results It was concluded that the highest bulk conductivitycould be found at ca y = 632 in LiyLa3Zr2-xTaxO12 and with low Alcontent

Polarization studies on the LiLLZ-xTaLi cell revealed short-circuiting by the formation of lithium dendrites The short-circuitperiod depended on the polarization current density the grain bound-ary resistance and the interface resistance between lithium metal andLLZ-xTa The LiLLZ-025TaLi cell was short-circuited after 100spolarization at 05 mA cmminus2 and after 2000 s polarization at 01 mAcmminus2 Therefore future work should be focused on optimization ofthe lithium ion conductivity of the grain boundary and the suppressionof lithium dendrite formation at high current density

Acknowledgment

This study was supported by Japan Science and TechnologyAgency (JST) under the ldquoAdvanced Low Carbon Technology Re-search and Development Programrdquo

) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 163118172206Downloaded on 2014-08-23 to IP

A674 Journal of The Electrochemical Society 161 (5) A668-A674 (2014)

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