Article Electrochemistry, 82(10), 880–883 (2014)

Preparation of Novel Electrode Materials Based on Lithium Niobium SulfidesAtsushi SAKUDA,a,* Tomonari TAKEUCHI,a Hironori KOBAYASHI,a

Hikari SAKAEBE,a Kuniaki TATSUMI,a and Zempachi OGUMIb

a Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Scienceand Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

b Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

*Corresponding author: a.sakuda@aist.go.jp

ABSTRACTNovel lithium niobium sulfide electrode materials, xLi2S·NbS2·(2−x)S (x = 1/2–2), were mechanochemicallysynthesized using Li2S, NbS2, and S8 as starting materials, and their electrochemical performance was investigated.The electrodes with x = 1/2 were in amorphous phase, while those with x = 1 and 3/2 are assigned as cubic phase.Cells prepared with these electrodes charge and discharge reversibly, with a capacity of ca. 400mAhg−1. Within thewide operational voltage range 1.0–3.0V, Li3NbS4 charges and discharges with ca. 4.5-electron processes betweenLi0.4NbS4 and Li4.9NbS4. Li3NbS4 discharges even at a high current density of 2000mAg−1 at 30°C.

© The Electrochemical Society of Japan, All rights reserved.

Keywords : Lithium Secondary Battery, Niobium Sulfide, Mechanical Milling, Electrode Material

1. Introduction

Rechargeable batteries with high energy densities are requiredfor developing high-performance electric vehicles (EVs). Commer-cially produced lithium-ion batteries usually use lithium transitionmetal oxides or phosphates such as LiCoO2, Li(Ni,Co,Mn)O2,LiMn2O4, and LiFePO4 as positive electrode materials. Theseelectrodes exhibit well-balanced electrode performance, a veryimportant requirement for commercial batteries. However, theircapacities are insufficient for increasing the energy density ofbatteries required for long-distance EV drives. Thus, the develop-ment of novel electrode materials is one of the most importantresearch subjects for material scientists in the field of rechargeablebatteries.

Metal sulfides are potential candidates for positive electrodematerials in rechargeable batteries with high energy densities.1–10

However, because they do not contain lithium, their application tothe positive electrode in cells with lithium-free negative electrodematerials such as carbon or silicon-based alloys is limited.

Several studies have been conducted on lithium-containing metalsulfides as electrode active materials.4,5,7,8 Most of these electrodescharge and discharge at relatively low potentials, and thus aregenerally studied as negative electrodes. As a positive electrodematerial, composite materials comprising Li2S and transition metals(Fe, Cu, and Co) have been reported.11–13 However, there are fewstudies on lithium-containing metal sulfides as a positive electrodematerial with a high capacity.

We recently reported novel lithium-containing metal sulfides,Li2TiS3 and Li3NbS4, mechanochemically synthesized using ballmilling techniques.14 They are considered to be rock-salt-typestructures; lithium and titanium or niobium ions are expected tooccupy the same crystal site. When charged and discharged in thevoltage range 1.5–3.0V vs. Li counter electrodes, the Li2TiS3and Li3NbS4 electrodes showed relatively high capacities of 425and 386mAhg¹1, corresponding to 2.5- and 3.5-electron processes,respectively.14 Details regarding the electrode performance ofLi3NbS4 were not reported in the previous study. For example,charge-discharge curves have been reported only in the voltagerange 1.5–3.0V.

In this paper, we report on the preparation of some novel lithiumniobium sulfides, xLi2S·NbS2·(2¹x)S (Li2xNbS4; x = 1/2, 1, 3/2,and 2), where the x = 3/2 sample was previously reported asLi3NbS4, and their electrochemical performance. We also report thecharge-discharge characteristics of the cells using the lithiumniobium sulfide electrode in the wide cutoff voltages and underhigh current densities.

2. Experimental

Lithium niobium sulfides, Li2S-NbS2-S8, were mechanochemi-cally synthesized at room temperature using a planetary ball millapparatus (P-7, Fritsch GmbH). In an argon-filled glove box, themixture of lithium sulfide (Li2S, 99.9%, Mitsuwa Pure Chemicals),niobium disulfide (NbS2, 99%, High Purity Chemicals), and sulfur(S8, 99.9%, Wako Pure Chemical Industries) was placed into azirconia pot (45mL) along with zirconia balls (4mm in diameter,90 g). The total weight of the mixture was fixed to 1 g. A stainlesssteel over pot was used to avoid exposure to air. The rotation speedand time of ball milling were fixed at 510 rpm and 60 h (60min ©60), respectively. Powder XRD measurements were performed atroom temperature over the 2ª angle range 25° ¯ 2ª ¯ 80°, witha step size of 0.1° using a NEW D8 ADVANCE (Bruker AXS)diffractometer operating with CuKA radiation. The calculatedpowder X-ray data of rock-salt-type Li3NbS4 were obtained usingthe software program Powder Cell.15

The electrochemical cells were also constructed in an argon-filledglove box. The working electrodes were prepared from Li2S-NbS2-S(10mg), acetylene black (AB, 1 or 2mg), and polytetrafluoro-ethylene (PTFE) powder (0.7mg). A mixture of Li2S-NbS2-S, AB,and PTFE was ground using agate mortar and attached to an Almesh current corrector. A 1-M solution of LiPF6 in a 50:50 (byvolume) mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) (Tomiyama Pure Chemical Industries Ltd.) was used as theelectrolyte, and a lithium foil disk was used as the counter electrode.Electrochemical measurements were performed at 30°C using acharge-discharge unit (TOSCAT-3100, Toyo System) at variouscurrent densities between 5 and 2000mAg¹1. The cell was cycledbetween 1.5 and 3.0V, or between 1.0 and 3.0V.

Electrochemistry Received: April 15, 2014Accepted: July 15, 2014Published: October 5, 2014

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.82.880JOI:DN/JST.JSTAGE/electrochemistry/82.880

880

3. Results and Discussion

Table 1 summarizes the composition and molar ratios of theinitial powder (Li2S, NbS2, and S8) for the present samples (x =1/2–2). The S/Nb ratio was fixed to 4, and the lithium compositionwas varied from 1 to 4. Figure 1 shows the XRD patterns of theprepared Li2S-NbS2-S. The x = 3/2 samples (Li3NbS4) wereassigned as the rock-salt-type cubic phase (Fm�3m).14 The simulatedXRD pattern of rock-salt-type Li3NbS4 is also shown in Fig. 1,assuming that the unit cell contains four formula units of [Li3/4/Nb1/4]4a[S]4b with the cell parameter of 5.13¡. The simulated dataare listed in Table 2. The pattern of the x = 1 sample was similarto that of the Li3NbS4 (x = 3/2 sample). The XRD patterns ofthe x = 1/2 sample showed no obvious peaks, indicating that theprepared sample was amorphous. The x = 2 sample mainlyconsisted of Li2S with minor impurities and an unknown phase.Small impurity peaks were observed, such as those of zirconia-basedmaterials, which were contaminated by the zirconia pot and ballsduring the ball milling process. The amount of contaminationseemed to depend on the hardness of both the starting and obtainedmaterials. The contamination tended to increase with increasing Li2Scomposition in the starting materials.

Figure 2 shows an SEM image of the x = 3/2 sample (Li3NbS4).Although the particle size was not uniform, 0.1–10µm particleswere observed in the SEM image. There were some large particleswith size more than 10µm.

Figures 3(a)–3(d) shows the charge-discharge curves of thecells using the prepared samples. The initial charging capacities ofthe samples of x = 1/2, 1, 3/2, and 2 were 88, 187, 289, and343mAhg¹1, respectively. The initial discharging capacities were446, 413, 386, and 357, respectively, for the same samples. Theinitial charging capacity increased with increasing initial lithiumcontent, and values were close to those calculated assuming that allof the contained lithium ions can be extracted during charge. Incontrast, discharging capacity decreased with increasing initiallithium content; the discharge capacities of around 400mAhg¹1

were larger than the initial charging capacity. The cells can bereversibly charged and discharged with the capacity correspondingto the initial discharging capacity after the 2nd cycle. Figures 3(e)–3(h) show charge-discharge characteristics as a function of lithiumcontent. The prepared samples of x = 1/2–3/2 can be charged anddischarged approximately between Li0.4NbS4 and Li4NbS4. Thedischarge voltages were almost identical, although the chargingvoltage of the x = 3/2 sample was higher than those of x = 1/2 and1. The charge-discharge profile of the x = 2 sample differed slightlyfrom those of the others, and the x = 2 sample showed a lowercapacity than the other samples. This may be related to the residualLi2S, which is an electronic insulator and is reported to be elec-trochemically inactive in carbonate-based electrolytes.11,16,17 Theelectrode performance of Li3NbS4 of lower discharge cutoff wasalso investigated. Figure 4(a) shows the charge-discharge curves ofLi3NbS4 cycled between 1.0 and 3.0V. The initial dischargingcapacity largely increased from 386 to 486mAhg¹1 when the cut-off voltage was lowered from 1.5 to 1.0V. Figure 4(b) also shows

Table 1. Composition of prepared lithium niobium sulfides, xLi2S·NbS2·(2¹x)S (Li2xNbS4 composition), and molar ratio of startingmaterials.

x Li2S NbS2 S8 Composition

1/2 0.5 1 1.5 LiNbS41 1 1 1 Li2NbS43/2 1.5 1 0.5 Li3NbS42 2 1 0 Li4NbS4

30 40 50 60 70 802θ /° (CuKα)

Li2S

ZrO2 Contamination

Inte

nsity

(ar

b. u

nit)

Li3NbS4 (simulated)

NbS2

S8

(a) x = 1/2

(b) x = 1

(c) x = 3/2(Li3NbS4)

(d) x = 2

Figure 1. XRD patterns of xLi2S·NbS2·(2¹x)S; (a) x = 1/2, (b)x = 1, (c) x = 3/2 (Li3NbS4), and (d) x = 2. The XRD patterns ofLi3NbS4 simulated with a rock-salt structure, Li2S, NbS2, S8, andZrO2 (contamination), are shown for comparison.

Table 2. Calculated powder X-ray data for Li3NbS4 for a =5.13¡. (The data were obtained by using CuKA radiation.)

h k l 2ª/deg d/¡ Intensity

1 1 1 30.15 2.962 2

2 0 0 35.95 2.565 100

2 2 0 50.26 1.814 66

3 1 1 59.74 1.547 0.2

2 2 2 62.69 1.481 19

4 0 0 73.83 1.283 8

3 3 1 81.77 1.177 0.1

4 2 0 84.37 1.147 22

4 2 2 94.72 1.047 16

5 μm

Figure 2. SEM image of Li3NbS4 particles.

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charge-discharge characteristics as a function of lithium content. Thecapacity of 486mAhg¹1 corresponds to ca. 4.5-electron processes,and the reaction was reversible during repeated cycling. Li3NbS4could be charged and discharged such that it ranged from Li0.4NbS4to Li4.9NbS4 over the voltages 1.0–3.0V. The estimated energydensity of Li/Li3NbS4 was 923Whkg¹1. This value is higher thanthose of commercially available lithium-ion batteries such as Li/Li1¹xCoO2 and Li/Li1¹x(Ni,Mn,Co)O2 (600–800Whkg¹1). Thus, itcan be concluded that Li3NbS4 shows a relatively high energy

density. The charge-discharge mechanisms of the electrode materialsreported in this study are unclear at the present stage. A betterunderstanding of the redox mechanism is needed for the improve-ment of electrode performance and further development of relatedelectrode materials. We will continue to investigate this charge-discharge mechanism in the future.

The discharge performance of Li3NbS4 at high current densitieswas also investigated. Figure 5 shows charge-discharge curves ofLi3NbS4 under discharge current densities varying from 5 to 2000

0

1

2

3

0 100 200 300 400 500

Cel

l vol

tage

/ V

Capacity / mAh g-1

(a) x = 1/2

0

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2

3

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l vol

tage

/ V

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0

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0 1 2 3 4 5

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4

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l vol

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/ V

x in LixNbS

4

10 mA g-1, 30°C

5th-1st

1st5th-2nd

10 mA g-1, 30°C

3rd-1st

1st 3rd-2nd

(b) x = 1

(c) x = 3/2 (Li3NbS4)

(d) x = 2

(e) x = 1/2

(f) x = 1

(g) x = 3/2 (Li3NbS4)

(h) x = 2

0

1

2

3

0 1 2 3 4 5

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l vol

tage

/ V

x in LixNbS

4

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x in LixNbS

4

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2nd,1st

1st 3rd, 2nd

0

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l vol

tage

/ V

Capacity / mAh g-1

1st

1st3rd,2nd

3rd,2nd

20 mA g-1, 30°C

20 mA g-1, 30°C

Figure 3. Charge-discharge curves of samples of (a) x = 1/2, (b)x = 1, (c) x = 3/2 (Li3NbS4), and (d) x = 2, and charge-dischargecharacteristics described as a function of lithium content: (e) x =1/2, (f ) x = 1, (g) x = 3/2 (Li3NbS4), and (h) x = 2.

0

1

2

3

0 100 200 300 400 500

Cel

l vol

tage

/ V

Capacity / mAh g-1

1st 5th-2nd

5th-1st

0

1

2

3

0 1 2 3 4 5

Cel

l vol

tage

/ V

x in LixNbS

4

Li3NbS4

(a)

(b)

Figure 4. (a) Charge-discharge curves of the cell using Li3NbS4cycled between 1.0 and 3.0V vs. the Li counter electrode and (b) thecharge-discharge characteristics described as a function of lithiumcontent.

Cel

l vol

tage

/ V

0

1

2

3

0 100 200 300 400Capacity / mAh g-1

Charging (100 mA g-1, CC-CV)

40 mA g-1

80 mA g-1

120 mA g-1

160 mA g-1

200 mA g-1

5 mA g-1400800120016002000

Figure 5. (Color online) Charge-discharge curves of the cell usingLi3NbS4 at various discharge current densities from 5 to 2000mAg¹1.

Electrochemistry, 82(10), 880–883 (2014)

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mAg¹1. The cells were charged at a current density of 100mAg¹1

with the CC-CV mode. Even at high current densities of more than1000mAg¹1, the cell can be sufficiently discharged, indicating thatLi3NbS4 essentially exhibits a superior rate capability. The powder-compressed pellet of the prepared Li3NbS4 showed a high electronicconductivity of 2.0 © 10¹3 S cm¹1. This would relate to a favorablerate capability of Li3NbS4.

4. Conclusion

Novel lithium niobium sulfides xLi2S·NbS2·(2¹x)S (x = 1/2, 1,3/2, and 2) were mechanochemically prepared as positive electrodematerials for lithium/metal sulfide secondary batteries. All cellsusing prepared lithium niobium sulfides reversibly charged anddischarged. The initial charging capacity increased with increasinglithium content (x), while the discharge capacities were approx-imately similar, irrespective of the x value. The cell with the x = 3/2sample (Li3NbS4) can be reversibly charged and discharged with ahigh capacity of 486mAhg¹1, which corresponds to 4.5-electronprocesses when the cell was cycled between 1.0 and 3.0V. Li3NbS4can be discharged even at the current density 2000mAg¹1 at 30°C.It shows a large capacity and superior rate capability; it is a suitablecandidate for positive electrode materials for lithium/metal sulfidesecondary batteries. We need to further investigate the fundamentalredox mechanism for further developing high-performance novelelectrode materials.

Acknowledgments

This work was supported by the “Research and DevelopmentInitiative for Scientific Innovation of New Generation Battery(RISING Project)” of the New Energy and Industrial Technology

Development Organization (NEDO), Japan. We thank Mr. K.Okamura, Dr. B. Y. Hamdi, and Dr. M. Shikano of AIST for theirsupport with data analysis.

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