Solid State Ionics 179 (2008) 2379–2382

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Solid State Ionics

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Nickel sulfide cathode in combination with an ionic liquid-based electrolyte forrechargeable lithium batteries

Jia-Zhao Wang a,d,⁎, Shu-Lei Chou a,d, Sau-Yen Chew a,d, Jia-Zeng Sun b,d, Maria Forsyth b,d,Douglas R. MacFarlane c,d, Hua-Kun Liu a,d

a Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australiab Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australiac School of Chemistry, Monash University, Clayton, Victoria 3800, Australiad ARC Center of Excellence for Electromaterials Science, University of Wollongong, NSW 2522, Australia

⁎ Corresponding author. Institute for SuperconductUniversity of Wollongong, NSW 2522, Australia. Fax: +6

E-mail address: jiazhao@uow.edu.au (J.-Z. Wang).

0167-2738/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.ssi.2008.09.007

a b s t r a c t

a r t i c l e i n f o

Article history:

Nickel sulfides, pure Ni3S2 a Received 8 April 2008Received in revised form 2 September 2008Accepted 4 September 2008

Keywords:Nickel sulfidesSolvothermal synthesisIonic liquid-based electrolyteLithium rechargeable batteries

nd a mixture of Ni7S6–NiS, were synthesized through a solvothermal process. Thenickel sulfide powders were characterized by X-ray diffraction, scanning electron microscopy, andelectrochemical testing. The results showed that the capacity of NiS–Ni7S6 is much higher than that ofNi3S2, when used as the cathode in a lithium cell with an organic solvent-based electrolyte, l M lithium bis(trifluoromethanesulfonyl)amide (LiNTf2) in poly(ethylene glycol) dimethyl ether 500. The NiS–Ni7S6electrodes were also tested with an ionic liquid electrolyte consisting of 1 M LiNTf2 in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C3mpyr][NTf2]) to compare with organic-solvent basedelectrolytes. The results revealed that the ionic liquid is a useful solvent for use with this cathode material.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Transport is one of the largest sources of greenhouse gas emissionsand fossil-fuel consumption. Thishas led to agrowingdemand forelectricvehicles/hybrid electric vehicles (EV/HEVs) to reduce global warming, airpollution, and the consumption of fossil fuels. For EV/HEV applications, arechargeable lithium battery is required, with high power, high energydensity, and high rate capability, together with a high level of safety. Oneproblem that is retarding the development of large lithium batteries forelectric vehicles is that the flammable organic-solvent based electrolytescurrently used in commercial lithium batteries still raise safety concerns.Recently, room temperature ionic liquids (RTILs) have attracted attentionas potentially safe electrolytes for large-size lithium secondary batteries,owing to their unique properties, such as non-flammability and non-volatility [1]. This non-flammability is effective in preventing batteriesfrom catching fire, and a non-volatile electrolyte prevents batteries frombursting. It has also been shown that some ionic liquids provide a highlystable solid electrolyte interphase (SEI) layer on Li metal, such that stablecycling of Li becomes possible without dendrite growth [2,3]. Variouscathodematerials, such as LiCoO2, LiFePO4, and sulfur, havebeen tested inionic liquid-based electrolyte. The results showed that the cathodematerials were stable in the ionic liquid-based electrolyte, and thecapacity retentions were very good during cycling [4–7]. However, at

ing and Electronic Materials,1 2 4221 5731.

l rights reserved.

present, there have still been no reports on the use of RTILs with metalsulfide cathode for lithium metal batteries.

Metal sulfides are known to be promising materials for secondarylithium metal batteries because of their high theoretical capacity. Avariety of metal sulfides have been considered for cathodematerials inlithium batteries [8–13], and nickel sulfides, in particular, have beenattracting attention because of their high theoretical capacity(590 mAh/g for NiS and 462 mAh/g for Ni3S2) compared with theconventional LiNO2 (275 mAh g−1).

Traditionally, nickel sulfides were synthesized by solid-state reac-tions at high temperature and by a ball-milling method [14–16].Recently, the solvothermal method has been proved to be an effectivesynthesis technique for preparing nickel sulfides [17,18] with differentmorphologies. In this paper, nickel sulfideswith apetal-likemorphologywere synthesized solvothermally with varying S/Ni molar ratios. PureNi3S2 and a mixture of NiS–Ni7S6 were obtained and tested as cathodematerials with an organic solvent-based electrolyte consisting of l Mlithium bis(trifluoromethanesulfonyl)amide (LiNTf2) in poly(ethyleneglycol) dimethyl ether 500. A ionic liquid-based electrolyte, l M LiNTf2 inN-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)amide([C3mpyr][NTf2]),wasalso tested in theNiS–Ni7S6 cells. Itwas found thatthe NiS–Ni7S6 cathode material had good cycling stability in this ionicliquid-based electrolyte.

2. Experimental

Nickel sulfide powders were synthesized via a simple solvothermalsynthesis method [17]. NiCl2 H2O and Na2S2O3 were first added to

Fig. 1. X-ray diffraction patterns of the products prepared in hydrazine hydrate:(a) combination of NiS (o) and Ni7S6 (⁎), with a molar ratio of 1:1 for NiCl2 H2O toNa2S2O3; (b) Ni3S2, with a molar ratio of 2:1 for NiCl2 H2O to Na2S2O3.

Fig. 2. SEM images of nickel sulfide powders: (a) NiS–Ni7S6, (b) Ni3S2.

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35 ml hydrazine hydrate. The molar ratio of NiCl2 H2O to Na2S2O3 was1:1 and 2:1, respectively, for the two solutions (amount of Na2S2O3:0.001 mol). Sealed autoclaves containing these precursors weremaintained at 170 °C for 24 h and then cooled to room temperature.After cooling, the mixture was centrifuged, and the separated blackprecipitate was washed thoroughly. Finally, the black mass was driedat 100 °C for 4 hrs under vacuum.

Phase analysis was performed by powder X-ray diffraction (XRD)using a Phillips 1730 X-ray generator and diffractometer with Cu Kαradiation. The morphology and structure of the nickel sulfide powderswere investigated by scanning electronmicroscopy (SEM) using a JEOLJEM-3000 30 kV instrument.

To fabricate the nickel sulfide electrodes, 70 wt.% nickel sulfideswere mixed with 20 wt.% carbon black and 10 wt.% polyvinylidenefluoride (PVDF) binder, using N-methyl-2-pyrrolidinone (NMP) as adispersant to form a slurry. The slurry was spread onto aluminum foilsubstrates. After drying under vacuum, the electrodes were cut to a1×1 cm2 size. CR2032 coin-type cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany), using lithiummetal foil asthe counter electrode. Three electrolytes were used, including aconventional commercial organic solvent-based electrolyte of 1 MLiFP6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate

(DMC), a home-made organic solvent-based electrolyte of l M lithiumbis(trifluoromethanesulfonyl)amide (LiNTf2) in poly(ethylene glycol)dimethyl ether 500 (PEGDME 500), and an ionic liquid-basedelectrolyte comprised of 1 M LiNTf2 in N-methyl-N-propyl pyrrolidi-nium bis(trifluoromethanesulfonyl)amide ([C3mpyr][NTf2]), whichwas prepared as previously reported [19]. The cells were galvanosta-tically charged and discharged in the range of 1.0–3.0 V at a currentdensity of 50 mA g−1. Cyclic voltammetry (CV) measurements wereperformed using a CHI 660 Electrochemical Workstation at a scanningrate of 0.1 mV s−1.

3. Results and discussion

The X-ray diffraction patterns of the nickel sulfide powders areshown in Fig. 1. When the reaction was carried out in hydrazinehydrate with a 1:1 molar ratio of NiCl2 H2O to Na2S2O3, a mixture withtwo phases, NiS (JCPDS Card No. 12-0041) and Ni7S6 (JCPDS Card No.14-0364) was produced (Fig. 1(a)). When the molar ratio was changedto 2:1 (NiCl2 H2O to Na2S2O3), pure Ni3S2 (JCPDS Card No. 30-0863)compound was obtained (Fig. 1(b)). The X-ray diffraction peaks for theprepared nickel sulfide powders are broad, indicating their nanocrys-talline nature.

SEM images of the samples are shown in Fig. 2. It can be seen thatthe nickel sulfide powders in both the NiS–Ni7S6 mixture and the pureNi3S2, which were prepared in hydrazine hydrate with NiCl2 H2O andNa2S2O3 using an autoclave, have a petal-likemorphology. The particlesize of the powders was 2–10 μm.

Fig. 4.Discharge capacities vs. cycle number for the NiS–Ni7S6 and Ni3S2 electrodes with

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The electrochemical properties of the electrode materials weredetermined using cyclic voltammetry (CV) and charge/dischargetesting. Fig. 3 shows the CV curves for the nickel sulfide samples.The cells were assembled with 1 M LiNTf2 in PEGDME 500 as theelectrolyte and cycled between 1.0–3.0 V vs. Li/Li+, with a potentialsweep rate of 0.10 mV s−1. For the cell containing NiS–Ni7S6, two peaksare present at around 1.80 V and 1.30 V during discharging. Also, wecan find a large charging current peak at 1.90 V and a small peak atabout 2.60 V versus Li/Li+ (Fig. 3(a)). The CV curve of NiS–Ni7S6 issimilar to that of the single phase NiS which was prepared in polyol bya reflux method in our previous work [8]. It is suggested that thecharge–discharge reaction mechanism of NiS–Ni7S6 is similar to thatof the NiS electrode in lithium cells. The charge–discharge reactionmechanism of NiS has been recently determined by Lee et al., using anex-situ X-ray diffraction method [20]. They reported that NiSdischarges in a two-step process. The first step is the transformationof NiS to Ni3S2 at higher potential, and the second step is theconversion of Ni3S2 to Ni at lower potential. The final dischargeproducts were Li2S and nickel metal. Nickel sulfide is regenerated inthe fully charged state. The CV curve of Ni3S2 shows just one reductionpeak at lower potential during discharge (Fig. 3(b)), which is

Fig. 3. Cyclic voltammograms of the nickel sulfide electrodes in lithium cells with 1 MLiNTf2-PEGDME 500 electrolyte: (a) NiS–Ni7S6, (b) Ni3S2.

1 M LiNTf2-PEGDME 500 electrolyte.

attributed to the Ni3S2 reacting with lithium to form Ni metal andLi2S. The charging–discharging reaction of Ni3S2 can be expressed as:

Ni3S2 þ 4Li↔3Ni þ 2Li2S

According to the CV performance, all the redox processes tookplace in the voltage range between 1.0 and 3.0 V (vs. Li/Li+). Therefore,a potential range of 1.0–3.0 V (vs. Li/Li+) was selected for thecontinuous charge–discharge cycling process. Fig. 4 shows dischargecapacities versus cycle number for cells made with the Ni3S2 and NiS–Ni7S6 electrodes and home-made organic solvent-based electrolyte,consisting of 1 M LiNTf2 in PEGDME 500. It can be seen that thedischarge capacity of NiS–Ni7S6 is much higher than that of Ni3S2,which is associated with the different mechanisms behind thedischarge reactions, as demonstrated by the CV tests for the twoelectrode materials in lithium cells, i.e. the NiS discharges in a two-step process, while the Ni3S2 just discharges in a one-step process.Therefore, the theoretical capacities are different for the cathodematerials containing NiS (590 mAh/g) [20] and Ni3S2 (462 mAh/g) [9].

Fig. 5. Discharge capacities vs. cycle number for the NiS–Ni7S6 electrodes using threedifferent electrolytes: 1 M LiPF6-EC+DMC, 1 M LiNTf2-PEGDME 500, and l M LiNTf2 in[C3mpyr][NTf2].

Fig. 6. The charge/discharge profiles for the NiS–Ni7S6 electrodes using two differentelectrolytes: 1 M LiNTf2-PEGDME 500 and l M LiNTf2-[C3mpyr][NTf2].

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TheNiS–Ni7S6,which had a higher capacity, was selected for testingin an ionic liquid-based electrolyte of l M LiNTf2 in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C3mpyr][NTf2]).The compatibility of this cathode material with the ionic liquid-basedelectrolyte was examined, with an emphasis on stable cycling inlithium cells. The samples were also tested with a conventionalcommercial organic solvent-based electrolyte of 1M LiPF6 in amixtureof ethylene carbonate (EC) and dimethyl carbonate (DMC) forcomparison. The results showed that the capacity of the NiS–Ni7S6using the commercial organic solvent-based electrolyte was muchlower than that of the samples with either the home-made organicsolvent-based electrolyte or the ionic liquid-based electrolyte (Fig. 5),This may be because the dissolution of the active material in both thehome-made organic solvent-based electrolyte and the ionic liquid-based electrolyte is reduced compared with the conventional organicsolvent-based electrolyte. Therefore, the active materials utilization ofthe NiS–Ni7S6 cathode with the home-made organic solvent-basedelectrolyte orwith the ionic liquid-based electrolyte is higher than thatin the conventional organic electrolyte, and consequently, thedischarge capacities are higher [21].

The NiS–Ni7S6 electrodes showed capacity retention of ~350 mAh g−1

after 20 cycles in both electrolytes, the 1M LiNTf2-PEGDME, and the l MLiNTf2-([C3mpyr][NTf2]).

The discharge curves of the cells with ionic liquid electrolyte andhome-made organic solvent-based electrolyte are shown in Fig. 6. Thedischarge profiles of the NiS–Ni7S6 electrodes show two plateaus,which could be assigned to the two-step reactions of NiS–Ni7S6 withlithium during the discharge process in both the ionic liquid-basedand the home-made organic-solvent based electrolytes. However, thedischarge plateaus of the NiS–Ni7S6 electrodes in the two electrolyteswere slightly different. The potential of the transformation of NiS–Ni7S6 to Ni3S2 in the ionic liquid-based electrolyte was slightly lowerthan for the electrode in the organic electrolyte. This phenomenon hasbeen observed in recent research work on ionic liquid-based electro-lyte for lithium batteries [7,22].

The battery testing results indicated that the ionic liquid-basedelectrolyte consisting of l M LiNTf2-([C3mpyr][NTf2]) is a suitable

candidate for use with NiS–Ni7S6 cathode in lithium cells. Batterieswith ionic liquid-based electrolytes are even more favorable forelectric vehicle applications in the near future, due to their non-flammability, non-volatility, high thermal stability, and good chemicalstability properties. Therefore, ionic liquids have become the mostpromising solvents for lithium battery electrolytes and are expected toimprove cell safety when they replace the flammable electrolytescurrently used in lithium batteries.

4. Conclusions

Nickel sulfides, Ni3S2 and Ni7S6–NiS, were synthesized via asolvothermal method using Teflon lined stainless steel autoclaves.The nickel sulfide particles, consisting of Ni3S2 and NiS–Ni7S6prepared in hydrazine hydrate, have a petal-like morphology. Thecapacity of NiS–Ni7S6 was much higher than that of Ni3S2. The NiS–Ni7S6 cells showed good performance with both a novel ionic liquid-based electrolyte, 1M LiNTf2-[C3mpyr][NTf2], and an organic solvent-based electrolyte, 1 M LiNTf2-PEGDME 500. Nickel sulfide cathode incombination with a non-flammable ionic liquid-based electrolyte is avery promising system for large batteries for electric vehicles in thefuture. The rechargeable NiS/Li batteries have a potential to becommercialized when the techniques are matured.

Acknowledgments

This research was supported by the ARC Center of Excellencefunding under grant number CE0561616, administered through theUniversity of Wollongong. We thank Dr. T. Silver for critical reading ofthe manuscript.

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