Stability of lithium ion conductor NASICON structure glass ceramic in acid and alkaline solution

Stability of lithium ion conductor NASICON structure glass ceramic in acid and alkaline aqueous solutionKun He ⁎,Chengkui Zu,Yanhang Wang,Bin Han,Xianyin Yin,Huifeng Zhao,Yonghua Liu,Jiang ChenInstitute of Quartz and Special Glasses,China Building Materials Academy,Beijing 100024,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 25August 2013Received in revised form 5November 2013Accepted 6November 2013Available online xxxx Keywords:Glass ceramic Solid electrolyte Lithium/air battery Alkaline solutionThe stability of lithium ion conducting glass ceramic Li 1.5Al 0.5Ge 1.5(PO 4)3in different aqueous solutions was analyzed with X-ray diffraction (XRD),scanning electron microscopy (SEM)and impedance analysis methods.The XRD results showed that LAGP glass ceramic decomposed into impurity phase Li 3PO 4after being immersed in LiOH alkaline aqueous solution for 6months.The results of impedance analysis indicated that the conductivity of glass ceramic immersed in 0.1M LiOH alkaline aqueous solution at room temperature for 6months obviously decreased.However,no conductivity change was observed for samples that were immersed in 1.0M LiOH solution and saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2O solutions.This result clearly indicates that Li 1.5Al 0.5Ge 1.5(PO 4)3glass ceramic has excellent stability in saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2O solutions,suggesting that it is a promising solid electrolyte and electrode protective layer for practical application in lithium/water battery.©2013Elsevier B.V.All rights reserved.1.IntroductionDuring the past few years,much attention has been paid on lithium/water and lithium/air batteries,because they have stood a good chance in achieving better discharge capacities than conventional batteries owing to the fact that they use lithium metal as the anode [1,2].Lithium/water battery has great potential use especially in high-discharge-capacity power supplies for the long term operation of deep sea applications.The theoretical capacity of lithium/air battery is about 5.8–11.4kWh/kg.The solid electrolyte is one of the most important components that control the properties of lithium/water secondary battery.In the past decades,lots of investigations have been carried out on Li-analogs of NASICON structured lithium ion conductor with the general formula of LiGe 2(PO 4)3,because it is particularly stable against Li metal and can be used as the solid electrolyte and the protective layer for the lithium metal electrode [3–12].Though the stability of Li 1+x +y Ti 2−x Al x P 3−y Si y O 12in acid solu-tions had been reported [13],the stability of Li 1.5Al 0.5Ge 1.5(PO 4)3(denoted as LAGP)glass ceramic specimens soaked in aqueous solution of Li salt and LiOH alkaline aqueous solutions is scarcely understood up to now.Therefore,in the present work,we investigated the stability of the LAGP glass ceramic specimens that were soaked in different types of aqueous solutions (such as saturated LiCl-HAc-H 2O solutions,saturated LiCl-LiOH-H 2O solutions and various concentrations of LiOH solution)with XRD,SEM and AC impedance measurement.2.Experiment proceduresStoichiometric amounts of Li 2CO 3,Al 2O 3,GeO 2and NH 4H 2PO 4were used as the starting materials to prepare Li 1.5Al 0.5Ge 1.5(PO 4)3glass ceramic by a conventional solid solution method.10%excess Li 2CO 3was employed to compensate the loss of lithium during the meltingSolid State Ionics 254(2014)78–81⁎Corresponding author.Tel./fax:+860165723301.E-mail address:cbmahekun@ (K.He).Fig.1.XRD pro files of LAGP specimens immersed in various aqueous solutions at room temperature for 6months:(a)the pristine plate,(b)the specimen immersed in saturated LiCl-HAc-H 2O,(c)the specimen immersed in saturated LiCl-LiOH-H 2O,(d)the specimen immersed in 1.0M LiOH,and (e)the LAGP plate immersed in 0.1M LiOH.0167-2738/$–see front matter ©2013Elsevier B.V.All rights reserved./10.1016/j.ssi.2013.11.011Contents lists available at ScienceDirectSolid State Ionicsj o ur n a l h o m e p a g e :w ww.e l s e v i e r.c om /l o c a t e /s s iprocess.The powders were weighed,mixed and milled in a high energy milling machine for 60min.Then they were transferred to an electric furnace and heated to 700°C in an alumina crucible for 2h in order to decompose ammonia,carbon dioxide and water vapor out from the starting materials.Finally,the powders were melted at 1350°C for 2h and poured onto preheated stainless steel plates.The cast glass sheets were annealed at 470°C for 2h to release the thermal stresses and then cooled down to room temperature.To obtain the glass ceramic specimens,the annealed glass sheets were crystallized at 825°C for 6h with a heating rate of 5°C/min.Crystalline phases were identi fied by X-ray diffraction (XRD)analy-sis at room temperature using Cu-K αradiation.The microstructures of the crystalline phases were observed by means of field emission scan-ning electron microscopy (SEM,JSM-5510LV,Japan)on fractured surfaces.The ionic conductivities were determined using impedance spectroscopy (Solartron2016impedance analyzer)in the frequencyrange of 0.1–106Hz,with a voltage amplitude of 5mV.After polishing,platinum was sputtered onto the two parallel faces of the specimens in order to ensure the necessary electrical contacts.The platinum coated specimens were assembled into a cell using stainless steel blocking electrodes in a cell fixture.3.Results and discussion 3.1.Phase analysis of glass ceramicThe XRD patterns for the LAGP glass ceramic specimens immersed in various aqueous solutions for 6months at room temperature are shown in Fig.1.The NASICON structure phase of LiGe 2(PO 4)3(JCPDS card 80-1923)was found to be the major crystalline phase in all LAGP glass ceramic specimens.As shown in this figure,no change in the XRD patterns were observed in saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2OFig.2.SEM photographs of LAGP specimens immersed in various aqueous solutions at room temperature for 6months:(a)the pristine plate,(b)the specimen immersed in saturated LiCl-HAc-H 2O,(c)the specimen immersed in saturated LiCl-LiOH-H 2O,(d)the specimen immersed in 1.0M LiOH,and (e)the specimen immersed in 0.1M LiOH.79K.He et al./Solid State Ionics 254(2014)78–81solutions,compared to the pristine LAGP plate.Interestingly,the Li 3PO 4phase (JCPDS card 87-0039)was observed for LAGP specimens im-mersed in 0.1M and 1.0M LiOH aqueous solutions at room temper-ature for 6months (cf.,Fig.1(d)and (e)),indicating that LAGP glass ceramics were unstable in LiOH alkaline aqueous solutions.Besides,the diffraction intensity of Li 3PO 4phase from specimen immersed in 0.1M LiOH aqueous solution was much higher than that immersed in 1.0M LiOH aqueous solution.This is relevant information from a prac-tical standpoint for the glass ceramics in lithium/water batteries,be-cause strong alkaline solutions cannot be used as electrolytes at the water electrode.The stability of the LAGP glass ceramic plates immersed in various aqueous solutions was also con firmed from the SEM observations.Fig.2shows the SEM images of the LAGP specimens immersed in saturated LiCl-HAc-H 2O,saturated LiCl-LiOH-H 2O,and 1.0M LiOH solutions at room temperature for 6months.No signi ficant surface morphology change can be observed in LAGP specimens after being immersed in saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2O solutions,compared to the surface of pristine LAGP plate.However,obvious surface morphology change is observed in the specimen immersed in 0.1M LiOH aqueous solution at room temperature for 6months (cf.,Fig.2(d)).The surface of the sample is corroded seriously and rhombohedral crystal grains are observed clearly.The morphology change may be due to the dissolution of the decomposition reaction of crystalline phases and residual glass phase of LAGP specimens immersed in alkaline solutions,which is supported by XRD investi-gations (cf.,Fig.1).It is known that the glass phase of glass ceramic is more easily corroded than the crystal phases when the samples are immersed in strong alkaline solution.The room temperature complex impedance plots for LAGP speci-mens immersed in various aqueous solutions for different times at room temperature are shown in Fig.3.As can be seen,there is only one depressed semicircle and oblique line observed for all specimens.The depressed semicircle represents the grain boundary resistance while the intercept of the semicircle on the real axis at high frequency (low x value of abscissa axis)represents the bulk resistance.In addition,the appearance of a low frequency oblique line in the case of ionic blocking electrodes is an indication of the ionic nature of the NASICON structure material [14].Accordingly,the total resistance of the specimen is obtained from the right intercept of the semicircle with the real axis in the plots,while the total conductivity (σt )of LAGP glass ceramic is calculated by σt =H /(S·R t ),where H is the sample's thickness and S is the sample's surface area [15].In Fig.3(a)and (b),the complex impedance plots of the pristine LAGP plates are compared with the LAGP specimens immersed in the saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2O solutions at room tempera-ture for 7days,60days and 180days,respectively.The impedance pro-files of LAGP specimens immersed in the saturated LiCl-HAc-H 2O and LiCl-LiOH-H 2O solutions almost show no change except that the grain boundary resistances slightly fluctuate.In the same way,Fig.3(c)and (d)show the LAGP specimens immersed in 1.0and 0.1M LiOH alkaline aqueous solutions for different times.The grain boundary resistance of LAGP plates slightly increases after being immersed in 1.0M LiOH alka-line aqueous solution for 6months.Especially,it should be noted that the grain boundary resistances of LAGP plates immersed in 0.1M LiOH alkaline aqueous solution sharply increase with immerse times (cf.,Fig.3(d)),attributing to the resistance of decomposition product such as impurity phase Li 3(PO 4)3segregatedplex impedance plots at room temperature of the LAGP specimens immersed in (a)saturated LiCl-HAc-H 2O,(b)saturated LiCl-LiOH-H 2O,(c)1.0M LiOH,and (d)0.1M LiOH aqueous solutions for different times.80K.He et al./Solid State Ionics 254(2014)78–81from LAGP glass ceramic.Moreover,these resistance changes may be due to the significant loss of lithium ions in the major conducting phase LiGe2(PO4)3.The results indicate that the stability of LAGP plates not at all depends on the pH value of alkaline solution,which is a little different from the result reported by Shimonishi[13,16].From the complex impedance plots of Fig.3,the room temperature conductivities of LAGP plates immersed in various aqueous solutions for different times are summarized in Table1.The conductivity of LAGP plates shows no significant change after being immersed in these aqueous solutions for different times,except that in0.1M LiOH alkaline aqueous solution for several months.Though both of the samples in0.1M and1.0M LiOH alkaline aqueous solutions contain Li3PO4impure phase according to XRD patterns(cf.,Fig.1),the conduc-tivity of sample immersed in1.0M LiOH aqueous solution is much higher than that immersed in0.1M LiOH aqueous solution.This is prob-ably because there are more Li+in1.0M LiOH aqueous solution,which are greatly beneficial to the conduction of lithium ions.In other worlds, the decreasing effect of conductivity from Li3PO4impure phase plays a less important role than the increasing effect of conductivity from Li+ that dissociated from1.0M LiOH alkaline aqueous solution.Hence,it can be concluded that the LAGP glass ceramics could be used not only as the solid electrolyte but also as the protective layer of lithium metal electrode for lithium/water batteries with saturated LiCl-HAc-H2O and LiCl-LiOH-H2O aqueous solutions.However,the activation energy of conduction Ea,the bulk conductivity and the grain boundary conductiv-ity of LAGP specimens are not analyzed in the present study,which needs more work in the next phase of our work.4.ConclusionsThe lithium ion conducting glass ceramic Li1.5Al0.5Ge1.5(PO4)3,which was immersed in LiOH alkaline aqueous solution for6months,partly decomposed into impurity phase Li3PO4.However,the glass ceramics were excellently stable in saturated LiCl-HAc-H2O and LiCl-LiOH-H2O aqueous solutions.On the basis of the above results,it can be concluded that Li1.5Al0.5Ge1.5(PO4)3glass ceramics are promising solid electrolytes and electrode protective layers for practical application in lithium/water batteries.References[1]S.J.Visco,E.Nimon,L.C.De Jonghe,Encyclopedia of electrochemical power sources,Secondary Batteries—Metal–Air Systems|Lithium–Air2009.376.[2]K.M.Abraham,Z.Jiang,J.Electrochem.Soc.143(1996)1.[3]H.Aono,N.Imanaka,G.-Y.Adachi,Acc.Chem.Res.27(1994)265–270.[4]V.Thangadurai,A.K.Shukla,J.Gopalakrishnan,J.Mater.Chem.9(1999)739–741.[5] 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times.Immersed aqueous solutions Conductivities at room temperature(×10−4S cm−1)Just after For7days For60days For180daysSaturated LiCl+LiOH solution 2.592 2.514 2.601 2.496 Saturated LiAC+HAc solution 2.621 2.515 2.423 2.6201.0M LiOH alkaline solution2.736 2.760 2.838 2.4630.1M LiOH alkaline solution 3.432 3.3820.6070.55181K.He et al./Solid State Ionics254(2014)78–81。