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Materials Letters 62 (20

A novel sulfide-based composite electrolyte Li4SiS4−La2S3by spark plasma sintering

Zhanqiang Liu, Fuqiang Huang ⁎, Zhenzhu Cao, Jianhua Yang, Minling Liu, Yaoming Wang

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,Chinese Academy of Sciences, Shanghai 200050, PR China

Received 22 May 2007; accepted 21 August 2007Available online 27 August 2007

Abstract

A novel composite electrolyte of Li4SiS4−La2S3, prepared from quenching method, was sintered by spark plasma sintering (SPS) technique.The composition of the Li4SiS4−La2S3 composite was found to keep unchanged before and after the SPS process. The lithium ion conductivitieswere measured in the temperature range from 30 °C to 210 °C. The SPS process improved the lithium ion conductivity at 30 °C by nearly oneorder of magnitude and decreased the activation energy correspondingly, compared with the cool-pressed sample. The total lithium ionconductivity at 30 °C was 1.20×10−4 S/cm, which is much higher than the value of Li4SiS4 (∼10−8 S/cm) in the literature.© 2007 Elsevier B.V. All rights reserved.

Keywords: Composite materials; Sintering; Lithium ion conductivity; Sulfide

1. Introduction

The rapid development and extensive application of electronicproducts, such as laptop computer, cellular phone and electricvehicle, demand higher-energy-capacity and safer lithium ionsecondary batteries [1,2]. The traditional liquid electrolytes havemany disadvantages, such as leakage, inflammability and narrowrange of operating temperature. Many efforts have been dedicatedto developing all-solid-state lithium ion secondary batteries thatconsist of solid lithium ion electrolytes [2,3]. The solid electrolytesshould have the similar lithium ion conduction property as thetraditional liquid electrolytes. Inorganic materials and organicpolymers are the two candidates as solid electrolytes. For inorganicelectrolytes, sulfide-based electrolytes generally have higherlithium ion conductivity by several orders of magnitude thanoxide-based electrolytes, owing to the higher polarizability of thelarge-size sulfur atom than that of oxygen. The more electron-negative oxygen anion has a larger attraction to the lithium ion.Therefore, the average free pathmigration of lithium ions decreasesin these oxides, as compared with the sulfide-based electrolytes.

⁎ Corresponding author. Tel.: +86 21 52411620; fax: +86 21 52413903.E-mail address: huangfq@mail.sic.ac.cn (F. Huang).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.08.055

For a specific lithium ion conductor, the ionic conductivity maybe improved with lithium ion densification to reduce the resistancefrom the porous boundary area. Recently, spark plasma sintering(SPS) method has been used to improve the electrical orelectrochemical properties of some ionic conductors and cathodematerials [4–7]. SPS is a sintering technique for materials to berapidly consolidated in a short time by using of microscopic elec-trical discharge between particles under a high pressure [8]. To thebest of our knowledge, SPS has not been applied to prepare sulfide-based lithium ion solid electrolyte.

Lithium ion conductivity enhancement in some composite elec-trolytes has been observed by dispersing a second phase, insulatingand inert material into an ion conducting glass system [9]. In thisletter, a new composite electrolyte of Li4SiS4−La2S3 was proposed.La2S3 is expected to be well-crystallized, and the low temperaturemelting Li4SiS4 [10,11] is to form amorphous or poor-crystallizedLi4SiS4.Here, the sampleswere prepared by quenchingmethod andsintered by the SPS process. The lithium ion conductivities of thesamples were measured in the temperature range from 30 °C to 210 °C.

2. Experimental

Lanthanum sulfide (La2S3), silicon sulfide (SiS2) and lithiumsulfide (Li2S) were used as the starting materials for preparing

Fig. 1. XRD patterns of the samples before and after the SPS process.

1367Z. Liu et al. / Materials Letters 62 (2008) 1366–1368

the aimed product, Li4SiS4−La2S3. All these raw materials werestoichiometrically synthesized in this lab, and the purities wereconfirmed by X-ray diffraction analysis. Mixture of Li2S, SiS2and La2S3 with the molar ratio of 6:3:0.5 was loaded into anevacuated and sealed silica tube. The tube was placed into thefurnace, heated at 750 °C for 10 h and then quenched in water.The product was collected and ground into fine powder.

Some powderwas cold-pressed into aϕ10×1mmpellet underthe pressure of 80 MPa, and indium plates were attached to bothsides of the pellet as the current collectors. Some powder wasloaded into a spark plasma sintering (SPS) furnace (SPS-2040,Sumitomo Coal Mining Co., Tokyo, Japan). Heat treatment wasset at 400 °C for 5 min under an argon atmosphere. The heatingrate was controlled at 100 °C/min, and the employing pressurewas 50MPa. Indium plates were also attached to both sides of theas-obtained ϕ10 mm pellet as the current collectors.

Ionic conductivity was measured in a dry argon flow bycomplex impedance on an impedance analyzer (Chenhua 660B) inthe frequency range of 0.1 Hz and 0.1 MHz over the temperaturerange from 30 °C to 210 °C.

Fig. 2. Complex impedance plots for the cold-pressed sample.

3. Results and discussion

The samples before and after SPS treatment were examined by X-raypowder diffraction patterns, as shown in Fig. 1. The strong peaks wereindexed to be La2S3, and the weak peaks to be Li4SiS4. Li2S and SiS2stoichiometrically reacted to form Li4SiS4. There was no reaction foundbetween La2S3 and Li2S or SiS2. Therefore, the samples can both berecognized as a composite of two materials, Li4SiS4−La2S3. The peaksof Li4SiS4 were very weak, although the amount of Li4SiS4 was muchhigher than that of La2S3. The line broadening of the Li4SiS4 peaksindicates that the average particle of Li4SiS4 is in the small size (about35 nm). Furthermore, the cold water quenching process restrained thecrystallization of Li4SiS4 in a certain extent. The reported melting pointof Li4SiS4 is around 710 °C [10], which is a little lower than ourexperiment temperature, 750 °C. Therefore, Li4SiS4 may be the mixtureof amorphous and poor-crystallized (nanostructured) forms. The XRDpatterns also show that the peaks of La2S3 do not change significantlybefore and after the SPS process. This implies that the compositions ofthe composite kept same during the SPS process. So, the as-preparedsamples are consistent with the expected composite, the combination ofcrystallites (well-crystallized La2S3) and the amorphous form (poor-crystallized Li4SiS4).

The conductivities of the cold-pressed pellet were examined from theAC impedance plots, as shown in Fig. 2. For a typical ionic conductor, asemicircle spectrum should be presented in the high-frequency range and aspike in the low-frequency range. However, for the cold-pressed sample,the semicircle is incomplete even at 30 °C and only spikes left withincreasing the measuring temperature. The inset figure is the AC imped-ance plots in the high-frequency region for the measuring temperaturesfrom 120 °C to 210 °C. The total resistances at each temperature wereobtained from the intersections of the semicircles with the real axis at thelower frequency side. The total lithium ion conductivities at 30 °C (σ30)and 210 °C (σ210) were calculated to be 3.10×10−5 S/cm and4.88×10−3 S/cm, respectively. These values are much higher than thoseof Li4SiS4, and the room temperature conductivity of the Li4SiS4 wasreported to be 5.0×10−8 S/cm [10–12].

The impedance spectrums of the SPS pellet were shown in Fig. 3,which are similar to those in Fig. 2. This may suggest that the SPSprocess did not remarkably affect the lithium ion transfer character ofcomposite electrolyte, Li4SiS4−La2S3. The big difference is that thereis no typical semicircle as observed in Fig. 2. It can be inferred that thegrain-boundary resistance was decreased after the SPS process. Thetotal lithium ion conductivity at 30 °C was calculated to be

Fig. 3. Complex impedance plots for the sample after SPS process.

Fig. 4. Temperature dependence of the electrical conductivities for the cold-pressed and SPS pellets, respectively.

1368 Z. Liu et al. / Materials Letters 62 (2008) 1366–1368

1.20×10−4 S/cm, nearly one order of magnitude higher than that of thecold-pressed pellet. Therefore the SPS process successfully increasedthe lithium ion conductivity of the composite electrolyte by reducingthe grain-boundary resistance.

Fig. 4 shows the temperature dependences of conductivities forcold-pressed pellet and SPS pellet, respectively. The plots of log(σT)against 103/T indicate that the conductivities of these two samples inthe temperature range from 30 °C to 210 °C follow the ArrheniusEquation: σT=σ0exp(−Ea/RT), where σ0 is the pre-exponential factor,Ea the activation energy for conduction, R the gas constant and T theabsolute temperature. So it can be concluded that both of the obtainedsamples are of good lithium ion conductors. The activation energies forconduction of the cold-pressed pellet and SPS pellet were calculated tobe 36.5 kJ mol−1 and 31.0 kJ mol−1, respectively. The SPS processobviously decreased the activation energy of the Li4SiS4−La2S3composite. And the activation energies are in consistent with their totalconductivity values.

The improved performance results from the enhanced densificationof lithium ion caused by the SPS process. It was measured that thedensities of the cold-pressed pellet and the SPS pellet are about 1.27and 2.03 g/cm3, respectively. In the denser SPS pellet, the Li4SiS4−La2S3composite may obtain more and tight contacts in the interface (boundaryarea) of Li4SiS4 and La2S3. This kind of contacts provides more paths forlithium ions to migrate more easily from grain to gain. Thus, the grain-boundary resistancewas correspondingly decreased. For the cold-pressed

process, there is no enough driven force to build up somany such contactsbetween gains.

4. Conclusion

A novel composite electrolyte of Li4SiS4−La2S3 powder wassuccessfully synthesized by quenching method, and the pelletswere prepared by the cold-pressed and SPS processes, respectively.This composite system has much higher lithium ion conductivitythan that of pure Li4SiS4. After the SPS process, the XRD patternsverified that the composition of the Li4SiS4−La2S3 composite wasfound to keep unchanged. The composite achieved nearly an orderof magnitude improvement in lithium ion conductivity. Thecomposite may obtain more and tight contacts in the interface(boundary area) of Li4SiS4 and La2S3. This kind of contactsprovides more paths for lithium ions to migrate more easily. Sparkplasma sintering can be an excellent technique for improving theconduction properties of sulfide-based lithium ion conductorsystems.

Acknowledgement

This research was supported by the Science and TechnologyCommission of Shanghai Municipality (Grant No. 05JC14080).

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