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Solid State Ionics 225 (2012) 626630

Contents lists available at SciVerse ScienceDirect

Solid Stat

.eformance and reliability.To date, the sulde glass electrolytes in the systems the Li2SP2S5

[35] have been extensively investigated since it is possible to preparethe Li2SP2S5 glass electrolytes with high lithium content by simple andquick mechanical milling due to good glass forming ability of P2S5. In ad-dition, after the glass electrolytes are heat-treated at the crystallizationtemperature, the highly lithium ion conducting crystalline phases suchas thio-LISICON analogs are precipitated in the Li2SP2S5 system andthose phases show higher conductivities exceeding over 104 S cm1

than mother glasses [6,7].

and a relevant mechanism is suggested to explain the enhancement ofconductivity. In addition, basic electrochemical properties of the pre-pared crystalline sample with the highest ionic conductivity as solidelectrolytes are reported.

2. Experimental

The amorphous and crystalline Li2SP2S5P2Se5 electrolytes wereprepared by high-energy mechanical milling and post heat-treatment.Reagent-grade chemicals of crystalline Li2S (Alfa, 99.9%), P2S5 (Aldrich,Numerous attempts have been made in theconductivity of sulde electrolytes by increasining of ortho-oxosalts [8]. In particular, the add

Corresponding author at: Division of Materials ScUniversity, 17 Haengdang-dong, Seongdong-gu, SeoulTel.: +82 2 2220 0503; fax: +82 2 2220 4011.

E-mail address: dwshin@hanyang.ac.kr (D. Shin).

0167-2738/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.ssi.2012.05.013tivity, low electronic con-er awide potential rangebatteries with high per-

um into thio-LISICON lattice via the addition of P2Se5 to Li2SP2S5 sys-tem with the purpose of enhancing the ionic conductivity. The effectsof P2Se5 addition on the conductivity and structure were investigatedductivity and good electrochemical stability ovfor constructing the all-solid-state lithium ion1. Introduction

Demands on power sourceswith higperformance are ever increasing and it sinvestigate and develop the all-solid-son the inorganic solid electrolytes [1,solid-state lithium ion batteries is consties of the solid electrolytes. Thus, it is esolid electrolyteswith high lithium iony density and high safetymotivates researchers toium ion batteries basedperformance of the all-y affected by the proper-l to develop the inorganic

former such as SiS2 and GeS2 to the glass has reported to effectively en-hance the ionic conductivities, so called the mixed former effect [9,10].Incorporation of elements with larger ionic size or higher polarizabilityof their electronic shells than sulfur can expand the crystal lattice andlower the electrostatic binding energy required to separate the Li ionfrom its charge compensating anion site, which offer better Li+ transportpathways.

Motivated bymixed anion effect, we tried to incorporate the seleni-SuldeMechanical millingCharacterization of amorphous and crystaall-solid-state lithium ion batteries

Junghoon Kim a, Yongsub Yoon a, Minyong Eom b, Doa Division of Materials Science & Engineering, Hanyang University, 17 Haengdang-dong, Seb Department of fuel cells and hydrogen technology, Hanyang University, 17 Haengdang-do

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 September 2011Received in revised form 3 May 2012Accepted 9 May 2012Available online 31 May 2012

Keywords:Solid electrolyteAll-solid-stateLithium ion battery

The amorphous and crystalchanical milling method andporated into the amorphousof the Se incorporated netwLISICON III phase was preciand crystalline electrolytesthe crystalline electrolyte wroom temperature.

j ourna l homepage: wwwpast to enhance the ionicg the Li2S content or dop-ition of a second network

ience & Engineering, Hanyang, 133791, Republic of Korea.

rights reserved.ine Li2SP2S5P2Se5 solid electrolytes for

wook Shin a,b,dong-gu, Seoul, 133791, Republic of KoreaSeongdong-gu, Seoul, 133791, Republic of Korea

75Li2S(25-x)P2S5xP2Se5 solid electrolytes were prepared by simple me-at-treatment. The structural analysis shows that selenium is effectively incor-atrix and crystal lattice by mechanical milling, which results in the formationk units. In addition, the high lithium ion conducting phase similar to thio-ted at whole composition range after heat-treatment. For both amorphouse ionic conductivities were enhanced with an increase of Se contents, and2 mol% of P2Se5 showed the highest conductivity of 6104 S cm1 at

2012 Elsevier B.V. All rights reserved.

e Ionics

l sev ie r .com/ locate /ss i99%) and amorphous P2Se5 were used as starting materials. Here, theamorphous P2Se5 was prepared by heating the elements mixed to stoi-chiometric ratio in an evacuated and sealed quartz ampoule. Themeltingwas performed at 450 C for 12 h and then cooled in the air to room tem-perature [11].

Since the most remarkable effect of P2Se5 addition was observedin the composition system with 75 mol% of Li2S in preliminary experi-ments, the composition systemof 75Li2S(25-x)P2S5xP2Se5was selected

system by mechanical milling and subsequent heat-treatment.However, as shown in Fig. 3, noticeable changes in XRD patterns

are observed when adding the P2Se5. As the P2Se5 increases, the in-tensity ratios of other peaks relative to the peak at 30 drastically de-crease. This result indicates that the crystalline phase preferentiallygrows into plate-like structure in the crystal plane corresponding tothe peak at 30 and the crystal growth in other planes is suppressed.

Fig. 2. DTA curves of the 75Li2S(25-x)P2S5xP2Se5 (0x3) glass samples prepared

627J. Kim et al. / Solid State Ionics 225 (2012) 626630to investigate the sole effect of the Se substitution without the change ofLi ion content. Furthermore, since the selenide has a higher electronicconductivity than the sulde, the content of P2Se5 was limited below3mol% to inhibit the decrease of Li ion transport number. The startingmaterialswere thoroughlymixed in appropriatemolar ratios and theme-chanical milling was carried out for 20 h using a high energy planetaryball mill (Fritsch Pulverisette 7) by repeating milling for 40 min and rest-ing for 20 min. The rotation speed was xed at 450 rpm. To prepare thecrystalline samples, the obtained glassy powders were heated to 230 Cwhich were slightly above the rst crystallization temperature. All theprocesses were carried out in a dry Ar-lled glove box.

Amorphous and crystalline phases were examined with a X-ray dif-fractometer (XRD; Rigaku Ultima IV) at room temperature. To preventair exposure, the samples were sealed in an air tight container coveredwith polyimide thin lm and mounted on the X-ray diffractometer.

Raman spectrumwas recorded using a JY LabRamHR equippedwitha 514.5 nm line of an Ar-ion laser and a liquid-nitrogen cooled CCDdetector. The mechanically milled samples were pelletized by the colduniaxial press under 4 metric tons and sealed in a small quartz box,and spectra were taken by focusing the microscope on the surface ofthe sample through the quartz.

Ionic conductivitiesweremeasured using AC impedance spectroscopy(Solartron 1260 impedance analyzer). For AC impedance measurements,gold was coated on both faces of the pellets using a Cressington 108 autosputter coater. The pellets were then placed in a two-electrode samplecell. The impedance of the selected cells was measured from 5MHz to1 Hz and the conductivity was determined using complex impedanceplot.

DC polarization was also measured to determine lithium ion trans-port number of the samples. Lithium foils and SUS (Stainless Use Steel)plates were attached onto both faces of the pelletized sample as non-blocking electrodes and blocking electrodes, respectively.

Electrochemical stability of the asymmetric Li/75Li2S23P2S52P2Se5glass-ceramic/SUS cell was evaluated by using cyclic voltammetry. Thecyclic voltammogram was obtained using a Solartron 1287 electro-chemical interface at the scan rate of 5 mV S1 in the scan range be-tween0.3 and 5 V.

3. Results and discussion

The XRD patterns of the 75Li2S(25-x)P2S5xP2Se5 (0x3) sam-ples obtained by mechanical milling are shown in Fig. 1. The XRD pat-terns of the pure Se and as-prepared P2Se5 are also shown in Fig. 1 asreferences. From XRD patterns, it can be seen that selenium sourceused to prepare the P2Se5 is crystalline powder with trigonal selenium(t-Se) phase (JCPDS #6-362) and the as-prepared P2Se5 clearly exhibitsamorphous phase without any distinct crystalline phases after melting-quenching of stoichiometric mixture of P and Se powders. After me-chanical milling of the 75Li2S(25-x)P2S5xP2Se5 (0x3) samples,no XRD peaks related to the P2Se5 and notable XRD peaks excepthalo patterns are observed in all samples over the whole compositionrange of x=0 to x=3, indicating that these samples become amor-phous state after mechanical milling of 20 h.

Fig. 2 shows DTA curves of the 75Li2S(25-x)P2S5xP2Se5 (0x3)glass samples prepared by mechanical milling. Although the phenome-non of glass transition is not clearly observed due to the lowprecision ofthe instrument, the distinct exothermic peaks due to crystallization areobserved in all glass samples. An exothermic peak at 204 C assigned tocrystallization is observed for 75Li2S25P2S5 glass (x=0). The crystalli-zation temperature of the glass shifts to the higher temperature byaddition of 2 mol% P2Se5, and then shifts to the lower temperature byfurther addition of P2Se5.

However, the DTA curve of as-prepared P2Se5 (x=100) has an en-dothermic peak at 105 C despite the fact that the XRD pattern of thatshows the amorphous phase, which is probably caused by insufcient

cooling rate during synthesis process of P2Se5. From the fact that theendothermic peak temperature of 105 C is coincident with the melt-ing point of PxSe1x (x=0.18) [12], it is speculated that the nano-crystalline particles with the P-poor composition were partially formedin glassymatrix because the cooling rate ofmelted P2Se5 in air was slight-ly slow to prohibit partial crystallization completely. Although the pre-pared P2Se5 shows the endothermic reaction, any reactions related tothe prepared P2Se5 are not observed for the 75Li2S(25-x)P2S5xP2Se5(1x3) glasses, indicating that the all added P2Se5 would be incorpo-rated into the glass structure.

Fig. 3 showsXRDpatterns of the 75Li2S(25-x)P2S5xP2Se5 (0x3)samples after heat-treatment of mechanically milled powders at abovetheir rst crystallization temperatures for 3 h. Crystalline phases whichis similar to, but not completely coinciding with, the Li3.2Ge0.2P0.8S4thio-LISICON III are precipitated in the amorphous 75Li2S(25-x)P2S5xP2Se5 samples at all composition range (0x3). It hasbeen reported that the Li4xGe1xPxS4 thio-LISICON III crystals showhigh ionic conductivity of 6.4104 S cm1 [13], and the metastablethio-LISICON III analogs are precipitated in the Li2SP2S5 system whichcould not be synthesized by the conventional solid-state reaction [14].From the XRD results, it is conrmed that the crystalline phase similarto thio-LISICON III was also precipitated in the P2Se5 added Li2SP2S5

Fig. 1. XRD patterns of the pure Se, as-prepared P2Se5, 75Li2S(25-x)P2S5xP2Se5(0x3) samples mechanically milled for 20 h.by mechanical milling and the as-prepared P2Se5.

Fig. 4. Raman spectra of the (a) pure Se, as-prepared P Se , amorphous and (b) crystalline

628 J. Kim et al. / Solid State Ionics 225 (2012) 626630We speculate that the Se ions incorporated into the crystal structuremainly substitute the S sites doubly bonded with P (P_S). Sincethis doubly bonded Se normally tends to attract alkali ions such asLi+, the Li+ ions clustered in a specic crystallographic plane willplay as an obstacle impeding the crystal growth during heat treat-ment. For this reason, the main peak at 30 maintains its intensity dueto the crystal growth along a specic directionwith plate-like crystalliteshape, while the other XRD peaks are decreased and broadened withthe increase of P2Se5. In addition, the XRDpeaks are shifted to lower an-gles, which indicates that the crystal lattice is expanded due To A LargerIonic Radius Of Se2 ion (1.98 ) than S2ion (1.84 ). From the aboveresults, it is concluded that the selenium substitutes the sulfur and is in-corporated into the lattice of thio-LISICON III analogs.

In Fig. 4, Raman spectra of the amorphous and crystalline75Li2S(25-x)P2S5xP2Se5 (0x3) samples were measured toobserve the change of the local structure. The Raman spectra ofthe pure t-Se and the prepared P2Se5 are also shown in Fig. 4(a) asreferences.

Typically, the Raman peak of trigonal Se (t-Se) is known to be lo-1

Fig. 3. XRD patterns of the 75Li2S(25-x)P2S5xP2Se5 (0x3) samples prepared byheat-treatment of mechanically milled powders at 230 C for 3 h in the range from16 to 32.cated at ~237 cm [15,16]. In the Raman spectrum of the pure Seused for preparing the P2Se5, a Raman peak at 239 cm1 assignedto the A1 symmetric stretching mode of Sen chains is observed, whichagree well with the reported papers. On the other hand, it is knownthat the PxSe1x glasses (x=0.150.40) show the strong Raman peakobserved near 237 and 250 cm1 contributed to the A1 and E vibrationmodes of Se\Se bond in Sen chains and Se8 rings, respectively, and per-sists upon P alloying. In addition, the local units consisting of P and Sesuch as pyramidal P(Se1/2)3 or quasi tetrahedral Se_P(Se1/2)3 unit con-nected each other by Se atoms or Se\Se chains also occur in the Se-richPxSe1x glasses [12,17]. Comparing the pure Se and as-prepared P2Se5,one can see that for the as-prepared P2Se5 the peak at 258 cm1

(A1 and E modes of Se8 rings) is prominent, which implies that the as-prepared P2Se5 have a large amount of Se8 rings and some of P-relatedstructural unit after melting-quenching. However, in this work, theRaman peaks attributed to the P-bonded structural unit are not ob-served due to dominant Se\Se bonds and very low concentration ofP-bonded units. In particular, the decreased scattering intensity and in-creased noise ratio due to a decreased laser power and accumulationtimes to avoid localized heating of samples was expected to be one ofreasons which made it difcult to observe the P-bonded units.

For the amorphous 75Li2S25P2S5 (x=0) sample, two peaks at-tributed to the PS43units are located at 421 and 267 cm1, which isassigned to A1 symmetric stretching and F2 symmetric bending mode,respectively [18,19]. However, when the P2Se5 is added to the75Li2S25P2S5 (x=0) glass, noticeable changes of the Raman spec-tra are identied.

Although it is expected from XRD and DTA data that all added P2Se5are incorporated into the glass structure, the Raman peak at 258 cm1

originated from the as-prepared P2Se5 is still observed in the mechani-cally milled 75Li2S(25-x)P2S5xP2Se5 (1x3). It looks like that theP2Se5 is simply mixed and remains in 75Li2S25P2S5 glass matrix. How-ever, considering several results in Raman data, one can speculate thatthe Raman peak at 258 cm1 of the P2Se5 and that of mechanicallymilled 75Li2S(25-x)P2S5xP2Se5 (1x3) don't have the completelysame origin: (1) the strong Raman peak at 421 cm1 attributed tothe A1 symmetric stretchingmode of PS43units is drastically decreasedeven if a very small amount of P2Se5 (3 mol%) is added to the75Li2S25P2S5 glass; (2) if the Raman peak at 258 cm1 is only attribut-ed to the Se-Se bond in Se8 rings, the relatively high intensity ofthe peak compared with a small amount of added P2Se5 is not wellexplained; (3) the Raman peak at 258 cm1 has very low intensity atthe simply mixed state before mechanical milling and is overlappedwith that at 267 cm1 of PS43units assigned to F2 symmetric bending

2 5

75Li2S(25-x)P2S5xP2Se5 (0x3) samples.

amorphous 75Li2S23P2S52P2Se5 electrolyte by adding P2Se5 mightbe due to more open structure facilitating better Li ions transportcompared to pure Li2SP2S5 system (x=0). In addition, the highestlithium ion conductivity of the crystalline 75Li2S23P2S52P2Se5 elec-trolyte may be mainly resulted from not only the formation of highLi+ conducting phase similar to the thio-LISICON III and the disap-pearance of the P2S64unit with low ionic conductivity by the heat-treatment but also the incorporation of selenium into the lattice ofthio-LISICON III by substitution of sulfur sites attributed to the de-crease of the electrostatic binding energy required to separate theLi ion.

On the other hand, the decreased ionic conductivity of the amor-phous sample (x=3) is due to the P2S64 unit with low conductivity.For the crystalline sample (x=3), the decrease of ionic conductivitiesis resulted from the suppression of crystal growth by incorporation ofexcessive amount of selenium.

Fig. 6 shows the DC polarization curves of the non-blocking andblocking symmetric cells at room temperature. The current throughthe cell with non-blocking electrodes shows the almost constant valuewith time due to the Li ion drift and leakage current by electrons/holes. Themeasured current value is 2102 A at 1400 s after applyinga constant DC voltage of 1 V to the crystalline 75Li2S23P2S52P2Se5sample. The dc conductivity dcLi calculated from the current and theapplied voltage is 5.8104 S cm1 at 25 C, which is almost samewith the value obtained from the Arrhenius plot previously shown inFig. 5(a).

On the other hand, the current through the cell with blocking elec-trodes exhibits the typical space charge polarization relaxation andthen the leakage current due to the conduction by electrons/holes.The residual leakage current is about 6.6107 A after 1400 s, which

(b) Lithium ion conductivity at 25 C and activation energy as a function of P2Se5 content.Error bars for these measurements are shown on the represented data points.

629J. Kim et al. / Solid State Ionics 225 (2012) 626630mode. And then, the intensity of Raman peak at 258 cm1 graduallyincreases during mechanical milling; and (4) when the P2Se5 is added,new peaks attributed to the P2S64units appear at 384 cm1.

From above results, it can be speculated that the 75Li2S25P2S5 andadded P2Se5 is not simply mechanically mixed in this glass system, butrather they have reacted with each other to form a new structural unitsand network.

We attribute the decrease of PS43 units and the increase of P2S64

units by the addition of P2Se5 to break-up of the partial P\Se bondingin P2Se5 bymechanical milling, which results in the separation of P ionsfrom the chain structure. Here, the separated P ions react with PS43

ions to form the P2S64 units with P\P homo bond by mechanical mill-ing; 3(PS4)3+P5+2(P2S6)4. In addition, the PS4xSex3 unitswith various S\P\Se bonding are partially formed by substitutionof the S in PS43 unit with Se by mechanical milling; PS43+xSe2PS4xSex3+xS2.

Typically, the vibration frequencies are inuenced by the size andmass of atoms. The substitution of S atoms by large and heavier Seatoms induces modications in binding lengths, binding energiesand consequently vibration frequencies, which leads to a shift tolower wave numbers. However, in this work, the Raman peak shiftedto a lower wave number attributed to the symmetric stretching modeof PS4xSex3 units is not observed while the Raman peak shiftedfrom 267 cm1 to 258 cm1 attributed to the symmetric bendingmode of PS4xSex3 units is observed. We suggest that this is resultedfrom the lowered symmetry of PS4xSex3 units. The PS4xSex3

units show unbalanced symmetry by substitution of S atoms by largeand heavier Se atoms, which makes structural units vibrate by mainlybending mode than stretching mode.

For this reason, as the content of P2Se5 increases in the amorphous75Li2S(25-x)P2S5xP2Se5 (1x3) even with the small amounts ofP2Se5 added to suldematrix, large amounts of PS43units are convertedinto the PS4xSex3 units and the P2S64 units.

However, in present, it is difcult to thoroughly analyze the structuralchange and clearly distinguish the peaks attributed to the PS4xSex3

units and Se8 rings due to the similar peak position at 258 cm1. Furtherinvestigation will be required to clearly explain the structural change byaddition of the P2Se5.

After heat-treatment of the amorphous 75Li2S(25-x)P2S5xP2Se5(1x3) samples, the peaks at around 384 cm1 assigned to the P2S64units disappear, suggesting that the PS43 units are formed by the reac-tion between the P2S64 unit and sulfur ions produced by mechanicalmilling; P2S64+2 S22(PS4)3.

The temperature dependence of the lithium ion conductivities ofthe amorphous and crystalline 75Li2S(25-x)P2S5xP2Se5 (0x3)samples is shown in Fig. 5(a). The lithium ion conductivities are linearand obey the Arrhenius law, indicating that no structure or phasechanges occur in the observed temperature range. The activation energyfor Li+ conduction is determined from the slope of linear Arrhenius plot.

Fig. 5(b) shows the lithium ion conductivity at 25 C and the activa-tion energy of the amorphous and crystalline 75Li2S(25-x)P2S5xP2Se5(0x3) samples as a function of P2Se5 content, which is determinedfrom Fig. 5(a). Comparedwith the amorphous samples, the higher ionicconductivities and the lower activation energies are obtained in thecrystalline samples at the whole composition range of x=0 to x=3,which is attributed to the formation of fast lithium ion conductingphase similar to the thio-LISICON III as shown in Fig. 3. The lithiumion conductivities of both amorphous and crystalline samples increasewith x, reaching a maximum value of 6104 S cm1 for the crystal-line sample with x=2, while further addition of P2Se5 reduces the lith-ium ion conductivities of both the amorphous and crystalline samples.The activation energy also reached a minimum value of 27.2 kJ mol1

at x=2. The small amount of P2Se5 added to the Li2SP2S5 system hasa signicant effect on the conductivity.

As conrmed by the results of XRD and Raman spectroscopy, one

of the reasons for the slightly increased ionic conductivity of theFig. 5. (a) Temperature dependence of lithium ion conductivities of the amorphous (lledsymbols) and crystalline (empty symbols) 75Li2S(25-x)P2S5xP2Se5 (0x3) samples.is about 4 orders of magnitude smaller than that of the cell with non-

Fig. 7. Cyclic voltammogram of the asymmetric Li/75Li2S23P2S52P2Se5 sample/SUS

630 J. Kim et al. / Solid State Ionics 225 (2012) 626630blocking electrodes. Although the P2Se5, known as electronic conductorwidely studied for semiconductors and photoconductors, is added tothe sulde electrolyte, electronic conduction through the cell is almostnegligible. The dc conductivity dc-electron obtained using blocking elec-trode is 1.9108 S cm1 at 25 C. The lithium ion transport numberis calculated to be >0.99997 indicating that the 75Li2S23P2S52P2Se5sample is pure lithium ion conductor.

Fig. 7 shows the cyclic voltammogram of the crystalline 75Li2S23P2S52P2Se5 sample. A cathodic current peak due to the lithiumdeposition reaction is observed on a cathodic sweep from 0.3 to0 V, and then an anodic current peak due to the dissolution reactionof metallic lithium (LiLi++e) is observed at around +0.2 V onan anodic weep. There are no signicant current peaks due to the elec-trolyte decomposition or phase change over thewhole range from0.3to+5.0 V, suggesting the sample has the high electrochemical stability.

4. Conclusions

The ionic conductivity of the 75Li2S25P2S5 solid electrolyte isenhanced by adding up to 2 mol% of P2Se5. The crystalline 75Li2S23P2S52P2Se5 solid electrolyte shows the highest conductivity of6104 S cm1 at room temperature, which is 2.6 times larger valuethan the pristine amorphous electrolyte. Structural analyses show thatthe addition of P2Se5 results in the preferential crystal growth andexpansion of lattice structure, indicating the incorporation of seleni-um. Furthermore, the lowered electrostatic binding energy requiredfor Li ion conduction due to the incorporation of selenium followed bythe formation of the Se substituted thio-LISICON III structure leadsto the higher ionic conductivity. The crystalline 75Li2S23P2S52P2Se5

Fig. 6. The DC polarization curves of the crystalline 75Li2S23P2S52P2Se5 sample afterapplying a constant DC voltage of 1 V.solid electrolyte exhibits a negligible electronic conductivity andhigh electrochemical stability. On the basis of high conductivity andstability, we expect that the various selenide materials can be usedfor the enhancement of electrolyte properties.Acknowledgments

This research was supported by a grant from the FundamentalR&D Program for Core Technology of Materials funded by the Minis-try of Knowledge Economy, Republic of Korea.

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Characterization of amorphous and crystalline Li2SP2S5P2Se5 solid electrolytes for all-solid-state lithium ion batteries1. Introduction2. Experimental3. Results and discussion4. ConclusionsAcknowledgmentsReferences