All solid state lithium ion rechargeablebatteries using NASICON structured electrolyte

J. K. Feng1,2, B. G. Yan1,3, J. C. Liu3, M. O. Lai1 and L. Li*1

NASICON (Sodium super ionic conductor) structured Li1?5Al0?5Ge1?5(PO4)3 (LAGP) solid

electrolyte is synthesized through a solid state reaction. The total conductivity of the LAGP

electrolyte is 761025 S cm21 with a potential window larger than 6 V. All solid state lithiumbatteries are fabricated using LiMn2O4 as a cathode, LAGP as an electrolyte and lithium metal as

an anode. The LiMn2O4/LAGP/Li cell can deliver a capacity of about 80 mAh g21 in the first

discharge cycle and increases gradually with charge/discharge cycles, indicating that LAGP can

be used as a promising electrolyte for lithium rechargeable batteries.

Keywords: Lithium ion batteries, Thin film batteries, Solid electrolyte, Sputtering deposition

This paper is part of a special issue on Functional Materials for Device and Energy Applications

IntroductionLi ion batteries have been widely used in almost all typesof electronic products and more recently in electronicvehicles. The current state-of-the-art Li-ion batteriesconsist of cathode and anode electrode separated by athin membrane with organic liquid electrolyte. Manytypes of structures of cathode materials are used, such asLiCoO2, LiMnxNiyCo(1-x-y)O2,

1 LiMn2O4,2 LiFePO4,

35

but only graphite is commercially used as the anodematerial although a lot of other types of anode are beingstudied. One of most noticeable anodes is lithiumtitanate.6,7 Although LiFePO4 is commonly recognizedto be the safest cathode, possible growth dendrite andother defects from copper and aluminium substratesmay cause short circuits. Therefore the only way todesign hazard free Li ion rechargeable batteries iscompletely replace currently used polymer separatorand organic electrolyte by inflammable ionic conductor.

All solid state Li ion rechargeable batteries withinorganic ceramics as the solid electrolytes have attract-ed growing interests in recent years since the solidelectrolytes possess many advantages including highsafety, high reliability, and wide potential windows. Tofabricate all solid state batteries, solid electrolytes withhigh Li ionic conductivities, wide potential window andhigh stability with both the cathode and anode materialsare essential. Several kinds of solid electrolytes withhigh ionic conductivity and good electrochemical abilityhave been studied, such as LiPON, sulphide glass,Li1zxMxTi2-x(PO4)3, Li-V-Si-O and Li-La-Ti-O et al.

814

Some of these electrolytes show low stability when

contacted with cathodes or anodes, and some of themare moisture sensitive.15

NASICON structured Li1?5Al0?5Ge1?5(PO4)3 (LAGP)has been reported to have a conductivity as high as1023y1025 S cm21 with an electrochemical potentialwindow of about 6 V. It is also known that LAGP iselectrochemically stable when contacted with Li metal.Therefore LAGP is a promising solid state electrolytefor all solid state Li batteries.1622 However, only littleresearch has been reported using LAGP as the electro-lyte for the Li metal batteries.

In the present study, all solid state Li batteries usingLiMn2O4 as a cathode, LAGP as an electrolyte and Limetal as an anode are fabricated. Electrochemicalproperties of the all solid state batteries are investigated.The results show that LAGP can be a good electrolytefor the all solid state Li batteries.

ExperimentalLAGP was synthesised through a typical solid statemethod as reported before.16,20,21 In a typical process,Li2CO3 (99%), GeO2 (99%), Al2O3 (99%) and NH4H2PO4(98%) of analytical grade (all from Aldrich, USA) in astoichiometric ratio were thoroughly mixed by ballmilling for 48 h and then heated in a platinum crucibleat 700uC for 2 h to release the volatile compounds. Thepowder was reground and ball milled again for another24 h, after which the powder was heated to 1350uC with aheating rate of 1uC min21 and homogenised at 1350uCfor 1 h. The melted LAGP was quickly cooled to 500uCand held for 2 h at 500uC, followed by cooling down toroom temperature. The as synthesised LAGP was thencrystallized at 850uC for 10 h. Finally the sintered LAGPwas shaped to pellets of 8 mm diameter and 1 mm inthickness.

The structures of the as synthesised LAGP pelletswere measured using Shimadzu XRD-6000 X-ray dif-fraction with Cu Ka radiation (l51?5406 A), at a scanrate of 2 deg/min. Surface morphologies of the LAGP

1Materials Science Group, Department of Mechanical Engineering,National University of Singapore, Singapore 1175762School of Materials Science and Engineering, Shandong University,China3Institute of Advanced Shaping and Surface Engineering, HunanUniversity, China

*Corresponding author, email luli@nus.edu.sg

2013 W. S. Maney & Son Ltd.Received 15 April 2013; accepted 25 April 2013DOI 10.1179/1753555713Y.0000000085 Materials Technology 2013 VOL 28 NO 5276

pellets before and after coating LiMn2O4 were char-acterized with a Hitachi S-4100 field emission scanning(FE-SEM) electron microscopy.

Ionic conductivity was measured using AC impedance(Solartron1287 electrochemical interface combined witha Solartron1260 frequency response analyser) in thefrequency range between 1021 and 106 Hz based onChowdari et al.11 To study the ionic conductivity of theLAGP electrolyte, the LAGP powder was pressed into apellet of 1mm thickness and 50 mm2 area at 10 MPa,and the pellets were then sintered at 850uC for 10 h. Ptwas coated on both sides of the LAGP electrolyte pellet,followed by heating the Pt coated specimens to 400uCfor half an hour. The Pt coated pellets in the form as Pt/LAGP/Pt were assembled into a cell using stainless steel(SS) as a current collector to test the AC impedance.

All solid state battery was prepared by deposition of a300 nm thick LiMn2O4 layer on the LAGP pellet byradio frequency (rf) magnetron sputtering at 600uC. Thesputtering was carried out in an Ar/O2 (9 : 1) atmosphereat a working pressure of 1?4 Pa and a sputtering powerof 100 W. A Pt film was sputtered on the LiMn2O4surface as a positive current collector.

A Li metal foil was directly pressed onto the LAGPpellet, forming a LiMn2O4/LAGP/Li all solid state cellin the glove box. The battery performance of the Li/LAGP/LiMn2O4 cells was characterised by galvano-static cycling between 2 and 4?5 V and cyclic voltam-metry (CV) at a scanning rate of 0?1 mV s21 with aSolartron 1287 and 1256 two-terminal cell test systemwith a Lab-made cell.

Results and discussionFigure 1 shows the XRD pattern of as prepared LAGP.Although 25% percentage of Ge4z ion is replaced byAl3z ion, the XRD diffraction can be indexed as pureNASICON structure LiGe2(PO4)3 (JCPDS41-0034)since ionic radii of Ge4z (0?053 nm) and Al3z

(0?050 nm) are similar, which is similar with thosereported before.1621 Sharp diffraction peaks of theNASICON structure indicate a good crystallinity aftercrystallization at 850uC. The scanning electron micro-graphs of the as synthesised pellet are shown in Fig. 2. Itcan be seen that the as synthesised LAGP was well

compacted together without appearance of porosity andcracks and no clear crystalline grain can be observed.This type of morphology was observed before.16

To measure the conductivity of the as-preparedLAGP, AC impedance measurement was performedusing a Pt/LAGP/Pt electrode. Typical impedancespectrum of the LAGP electrolyte pellet is shown inFig. 3. The spectrum is composed of only one semicirclein the high frequency range and a straight line in the lowfrequency range, which is well known due to the bulkresistance and the Warburg resistance. No secondsemicircle due to the grain boundary resistance isobserved, indicating extremely low grain boundaryresistance of the LAGP electrolyte. It is known thatthe LAGP will form a glass-ceramic structure duringfast cooling.16,20 The lower grain boundary resistancewill greatly facilitate the Li ions transportation duringthe charge and discharge processes. The total ionicconductivity can be calculated from

s~L

RtotS

where Rtot is the total resistance which is equal to sum ofbulk resistance, Rb and grain boundary resistance, Rgbof the samples, L is the thickness of the electrolyte pellet,S is the cross sectional area of the electrolyte. The Rtotcan be obtained from the right intercept of the semicirclewith the real axis ray. If Rtot is taken to be 2700 V, S is50 mm2, and L is 1 mm, the total conductivity, s is761025 S cm21 which is a little lower than the best

1 XRD spectrum of as prepared LAGP

2 Surface morphology of as prepared LAGP

3 AC impedance of LAGP pellet

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Materials Technology 2013 VOL 28 NO 5 277

reported data16,20 but higher than that of the widely usedLiPON.8

Figure 4 shows the cyclic voltammetry (CV) spectrumof the solid electrolyte (Pt/LAGP/Li) measured at roomtemperature with a scan rate of 0?1 mV s21 in thevoltage range from 20?4 to 6 V. Figure 4 reveals thatthe Li metal can reversibly be deposited and dissolvedbetween 20?4 and 0?6 V. No other reactions areobserved even at 6 V, indicating that LAGP can workat least up to 6 V potential while for most of cathodematerials the highest potential is only up to 5 V. Theresult is also consistent with previous reports.16,17 Thehigh ionic conductivity and high potential window makethe LAGP be a promising electrolyte for the all solidstate Li batteries.

Figure 5 shows the surface morphology of theLiMn2O4 layer deposited on the LAGP pellet. Thecathode film appears smooth, dense, and free of cracksand pin holes. The grain diameter is about 40 nm. Theinsert in Fig. 5 shows the LAGP pellets before (white)and after (black) deposition of the cathode, showing thatthe LAGP electrolyte is fully covered by black LiMn2O4.

As shown in Fig. 6, a pure LAGP phase (Fig. 6a) canbe seen before LiMn2O4 deposition. After deposition anew diffraction peak at 2h 518?6uC appears and thediffraction can be indexed to be the spinel LiMn2O4(111) diffraction.23 The week LiMn2O4 diffraction peak

is due to thin LiMn2O4 film character. There is nodistinguished change in the diffraction from the LAGPpellet, implying that the LAGP structure is stable and noreaction with LiMn2O4 at the present depositionconditions.

To test the electrochemical performance of theLiMn2O4/LAGP/Li cell, CV measurement was carriedout between 2 and 4?5 V. Figure 7 shows a typical CVcurve of the LiMn2O4/LAGP/Li cell at a scan rate of0?1 mV s21. From Fig. 7, two separated pairs of peakscan clearly be seen, which are resulted from the typicallithium intercalation and de-intercalation feature ofLiMn2O4. The slightly large separated distance betweenanodic and catholic peaks is mainly due to largepolarization caused by the relatively lower conductivityof the LAGP compared with the liquid electrolytes aswell as low conductivity of the cathode material sincethere is no conductive additive is used as the traditionalbulk Li ion rechargeable batteries. It is also noted thatthe pairs of anodic and cathodic peaks are week andbroad which are resulted from large polarization.Figure 8 gives the 150 typical charge/discharge curvesof the LiMn2O4/LAGP/Li cell at a constant current of

4 Potential window of LAGP pellet

5 SEM of LiMn2O4 deposited on LAGP pellet with insert

showing overview of LAGP pellets before (white) and

after (black) LiMn2O4 deposition

6 XRD spectra of LAGP pellets a before and b after

LiMn2O4 deposition

7 CV curve of LiMn2O4/LAGP/Li cell at scan rate of

0?1 mV s21

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278 Materials Technology 2013 VOL 28 NO 5

25 mA g21 in a potential range from 2 to 4?5 V due tothe fact that polarization was very high resulting in largedrop in discharge voltage and increase in charge voltage.The voltage profiles show an expected potential plateaufor LiMn2O4 with the first charge capacity of 80 mAh g

21

which is due to the well known lithium intercalation andde-intercalation reaction of LiMn2O4.

2325 The subse-quent cycles show a similar shape with an increasedcapacity as well as coulombic efficiency. We ascribe theincreased capacity to be caused by activation andreduction in impedance between the electrode layers andelectrolyte. When a Li metal foil is pressed directly ontothe electrolyte, interface impedance is high due to theformation of solid interface film (such as Li2O).

25,26 Inaddition, the Li metal foil is also unable to fully contactwith the electrolyte. With cycling, the impedance layermay be broken and the contact area between the Li metaland the electrolyte may also increase. After 150 charge/discharge cycles, the capacity increases from 80 to115 mAh g21, which is also reported in other papers.24

The charge/discharge spectra of the LiMn2O4/LAGP/Lifull cell are almost similar during cycling demonstratingthat the LiMn2O4 /LAGP/Li cell has a good cyclability.The reason for the high capacity retention in the currentstudy may be contributed reduced Mn2z disslovationwhen the solid electrolyte is used.27,28

ConclusionsNASICON structured LAGP was prepared and testedas the electrolyte for all solid state lithium batteries. Theresults show that LAGP has a 6 V potential window andan ionic conductivity of 7?561025 S cm21. For the firsttime, a LiMn2O4/LAGP/Li cell was constructed and

tested. The cell showed an 80 mAh g21 first chargecapacity and 115 mAh g21 after 150 cycles, suggestingthat LAGP can be used as a promising electrolyte forhigh performance lithium metal rechargeable batteriesand an electrolyte for test high potential cathodematerials.

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8 First 150 charge/discharge curves of LiMn2O4/LAGP/Li

cell at current density of 25 mA g21

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