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Bi0.94Sb1.06S3 Nanorod Cluster Anodes for Sodium-ion Batteries: Enhanced Reversibility by the Synergistic Effect of the Bi2S3–Sb2S3 Solid Solution Yubao Zhao, and Arumugam Manthiram Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02833 • Publication Date (Web): 13 Aug 2015 Downloaded from http://pubs.acs.org on August 17, 2015

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Chemistry of Materials

Bi0.94Sb1.06S3 Nanorod Cluster Anodes for Sodium-ion Batteries: Enhanced Reversibility by the Synergistic Effect of the Bi2S3–Sb2S3 Solid Solution Yubao Zhao and Arumugam Manthiram* Materials Science and Engineering Program and Texas Materials Institute University of Texas at Austin, Austin, Texas, 78712, United States.

ABSTRACT: Bi0.94Sb1.06S3 solid solution anode with a nanorod cluster morphology has been synthesized by a hydrothermal reaction and investigated in sodium-ion batteries in comparison to the binary counterparts Bi2S3 and Sb2S3. The Bi0.94Sb1.06S3-graphite composite obtained by a mechanical milling of 80 wt. % Bi0.94Sb1.06S3 and 20 wt. % graphite exhibits much improved cycle stability with 79% capacity retention after 200 cycles in the full voltage window of 0.01 – 2.8 V compared to its Bi2S3-graphite (58 % retention) and Sb2S3-graphite (10 % retention) counterparts, demonstrating a synergistic effect. Cyclic voltammetry scans indicate that the polarization overpotential associated with the conversion reaction in Bi0.94Sb1.06S3 is lower than those in Sb2S3 and Bi2S3. Cycling under controlled voltage windows of 2.8 – 0.85 V corresponding to a conversion reaction and 0.85 – 0.01 V corresponding to an alloying reaction reveals both highly reversible conversion and alloying reactions in the Bi0.94Sb1.06S3 solid solution anode compared to that in the binary counterpart anodes.



INTRODUCTION

The demand for efficient, low-cost energy storage technologies is increasing dramatically in recent years to reduce environmental pollution and to utilize renewable sources. Lithium ion batteries (LIBs) are now utilized as an effective energy storage approach in the consumer electronics and electric vehicles (EV).[1-8] However, due to the limited abundance and specific localized locations of lithium deposits, the interest in sodium-ion batteries (NIBs) is growing exponentially as sodium is abundant and inexpensive, especially for the large-scale energy storage systems.[9-14] The pursuit of low-cost, highperformance NIBs is driving the exploration of new cathode and anode materials. The exploration of cathode materials for NIBs has shown fruitful achievements in recent years, such as some layered transition-metal oxides and polyanionic compounds.[14,15] However, the larger radius of Na+ (102 pm) compared to that of Li+ (76 pm) causes a series of problems for the anodes of NIBs, such as large volume change and severe pulverization of the active materials during sodiation/desodiation, slow ionic diffusion rate, and instability of the solid-electrolyte interface (SEI) layer. These problems cause poor reversibility, low rate-capacity, making the development of high-performance anode materials for NIBs challenging.[14-18]

Among the various anode materials for NIBs, the metal sulfides have drawn much research interest due to their high theoretical capacities, e.g., SnS2 (1231 mAh g–1), Sb2S3 (947 mAh g–1), FeS2 (894 mAh g–1), MoS2 (669 mAh g–1), and Bi2S3 (625 mAh g–1).[19-24] During the sodiation process, the metal sulfide (MSx) first converts to metal (M) and Na2S (Equation 1, conversion reaction). The produced metal (M) then reacts with Na+ to form an M-Na alloy, providing the metal is electrochemically active for the alloying reaction (Equation 2). There could also be an intercalation reaction before the conversion reaction if the metal sulfide has an appropriate structure, such as MoS2 and Sb2S3.[22,23] However, the sluggish kinetics of the conversion reaction is one of the key reasons leading to poor reversibility of the discharge/charge cycles and thus severe capacity fade.[20,22] Improving the reversibility of the conversion reaction in the electrode is of great significance to develop a metal sulfide anode material with both high capacity and stable cycle performance for NIBs. MSx + 2x Na+ + 2x e- ⇌ x Na2 S + M +

-

M + y Na + y e ⇌ Na y M

(1) (2)

Intensive research efforts have been devoted to prolong the cycle life and improve the reversible capacity of metal sulfide anodes, such as limited voltage window during cycling and dispersion of the nano-sized metal sulfide particles in a carbon matrix. With the strategy of avoiding

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the poorly reversible conversion reaction by limiting the voltage window, Hu et al reported that MoS2 nano-flowers show a stable cycle performance with a discharge capacity of 350 mA h g–1 at a current density of 0.05 A g–1 in a limited voltage window of 0.4 – 3.0 V (vs. Na+/Na).[22] In this strategy, although the cycle life was prolonged, the capacity was low due to the limited voltage window. Embedding the nano-sized metal sulfide particles in a conductive matrix could alleviate the capacity fade caused by active materials pulverization. Nanosize Sb2S3 particles wrapped by reduced graphene oxide were reported to exhibit a charge capacity of 730 mAh g–1 at a current density of 50 mA g–1 for 50 cycles.[23] Nano-sized SnS2 loaded onto reduced graphene oxide was reported to exhibit a reversible capacity of 630 mA h g–1 at a current density of 0.2 A g–1 with stable cycle performance.[19] Development of new approaches to improve the cycle stability of the metal sulfides anode is important for practical application. In the various reports, synergistic effect was observed with binary and ternary alloy systems as anodes for LIBs and NIBs, delivering much improved cycle stability compared to their pure elemental counterparts, e.g., Bi-Sb,[18] Ge-Si,[25] Ge-Sn-Sb,[26] and Sb-Sn.[27] In the supercapacitor application, NiCo2S4 has also been reported to have better electrochemical performance than its single-metal sulfide counterparts.[28,29] Herein, we present our strategy of designing ternary metals sulfides as a stable NIB anode with enhanced reversibility of the conversion reaction. Sb2S3 and Bi2S3 are electrochemically active for the sodiation/desodiation reaction. The combination of conversion reaction (Equations 3 and 5) and the alloying reaction (Equations 4 and 6) makes Sb2S3 and Bi2S3 deliver high theoretical capacities of, respectively, 947 and 625 mAh g–1. Sb2 S3 + 6 Na+ + 6 e- ⇌ 2 Sb + 3 Na2 S +

(3)

2 Sb + 6 Na + 6 e- ⇌ 2 Na3 Sb

(4)

Bi2 S3 + 6 Na+ + 6 e- ⇌ 2 Bi + 3 Na2 S

(5)

+

2 Bi + 6 Na + 6 e ⇋ 2 Na3 Bi -

(6)

Detailed CV scan studies and the cycling tests in controlled voltage ranges indicate a synergistic effect between Sb and Bi in the Bi0.94Sb1.06S3 solid solution during charge/discharge, exhibiting improved reversibility of the conversion reaction and thus enhanced cycle performance compared to its counterparts, Bi2S3 and Sb2S3. Furthermore, the Bi0.94Sb1.06S3-graphite composite anode exhibits high-rate capacity: the reversible charge (desodiation) capacities at the current densities of 0.1 and 10 A g–1 are, respectively, 564 and 256 mAh g–1. More importantly, the Bi0.94Sb1.06S3-graphite anode delivers a stable cycle performance: the capacity retention is 79% after 200 full discharge/charge cycles in the voltage range of 0.01 – 2.80 V (vs. Na+/Na) at the current density of 1 A g–1. 

EXPERIMENTAL SECTION

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In a typical synthesis of Sb2S3, 0.65 g of SbCl3 was dissolved in 1.1 mL of concentrated HCl (37 wt. %), followed by the addition of 3.25 g citric acid into the beaker. Under magnetic stirring, 40 mL of H2O (18 MΩ·cm at 25 oC) was slowly poured into the beaker, producing a transparent colorless solution. Then, 0.83 g of thioacetamine was dissolved in the solution. The solution was megnetically stirred for 30 min and then transferred into a 100 mL Teflon-lined autoclave for a hydrothermal reaction at 200 oC for 24 h in an oven. For the synthesis of Bi2S3 nano-rods, the experimental procedure was the same as the above-mentioned steps except that the metal precursor was 0.68 g of Bi(CH3COO)3, and the amount of the thioacetamine was 0.82 g. For the synthesis of Bi0.94Sb1.06S3, 0.30 g of SbCl3 and 0.45 g of Bi(CH3COO)3 were dissolved in 1.1 mL HCl (37 wt. %). The amount of citric acid and thioacetamine were, respectively, 3.25 g and 1.03 g. Other procedures were the same as that for the synthesis of Sb2S3. The products from the autoclaves were cleaned by repeated washing with water and ethanol, followed by drying in a vaccum oven. The Sb2S3-graphite, Bi2S3-graphite, and Bi0.94Sb1.06S3-graphite samples were prepared by mixing the as-prepared metal sulfides with graphite (20 wt. %) via a mechical milling method in a Fritsch Pulverisette 6 planetary mill. These metal sulfidegraphite composite samples were used as active materials for electrochemcial tests in sodium-ion batteries. Electrochemical tests: The electrode was prepared by a doctor-blade method, coating the slurry onto a copper foil. The slurry for electrode casting was prepared by mixing the metal sulfide-graphite composite sample, Super P conductive carbon black, and sodium carboxymethyl cellulose (Mw ~ 70,000) in the weight ratio of 7 : 2 : 1 in a glass vial followed by adjusting the viscocity of the slurry by water and megnetic stirring overnignt. The electrochemical performance of the electrodes was assessed within CR2032 coin cells with glass fiber as separator and sodium metal as the counter/reference electrode. The mass loading of the active material (Bi0.94Sb1.06S3-graphite) was ca. 1.3 mg per electrode (1.13 cm2). The coin cells were assembled in an argon-filled glovebox with O2 and H2O level below 0.1 ppm. The electrolyte was composed of 1 M NaClO4 in ethylene carbonate (EC)/propylene carbonate (PC) (1 : 1 v/v) with 5 wt. % fluoroethylene carbonate (FEC) as an additive. Charge/discharge performance was assessed with an Arbin battery cycler under galvanostatic condition. Characterization: Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images were obtained, respectively, on a Hitachi S5500 STEM and a JEOL-2010F TEM. Cyclic voltammetry (CV) was carried out with a VoltaLab PGZ402 potentiostat at a scan rate of 0.05 mV s–1 at 0.01 – 2.8 V (vs. Na+/Na). X-ray diffraction (XRD) data were collected with a Rigaku MiniFlex 600 X-ray diffractometer with Cu Kα radiation.

Materials synthesis: Sb2S3, Bi2S3, and the Bi0.94Sb1.06S3 solid solution were prepared by hydrothermal reactions.


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RESULTS AND DISCUSSION

Figure 1. Electron microscopy and spectrometry characterization of Bi0.94Sb1.06S3 nanorod clusters. (a, b), Scanning electron microscopy (SEM) images; (c, d), high resolution transmission electron microscopy (HRTEM) images; (e), bright filed scanning transmission electron microscopy (BF-STEM) image; (f) Energy-dispersive X-ray spectroscopy (EDS) line scan of the elements S, Bi, and Sb along the dashed arrow marked in (e); (g) SEM image of the line-scan area; (h, i, and j) EDS elemental mapping of S, Bi and Sb of the area marked in (e). As shown in the scanning electron microscopy (SEM) images, Bi0.94Sb1.06S3 clusters are composed of nanorods with a width of ca. 50 nm and length of several micrometers (Figures 1(a) and 1(b)). The crystal phase of Bi0.94Sb1.06S3 clusters was confirmed by X-ray diffraction (XRD). As shown in Figure 2, the diffraction peaks of Bi0.94Sb1.06S3 clusters match perfectly with the standard diffraction patterns of orthorhombic Bi0.94Sb1.06S3.[30] Figures 1(c) and (d) show the high-resolution transmission electron microscopy (HRTEM) images of the Bi0.94Sb1.06S3 nanorods. The spacings of 0.28 nm and 0.37 nm correspond, respectively, to the lattice distances of (400) and (011) planes of Bi0.94Sb1.06S3 crystals (Pnma space group) with a = 1.132 nm, b = 0.395 nm, and c = 1.116 nm.[30] Figure 1e shows the bright filed scanning transmission electron microscopy (BF-STEM) image of the Bi0.94Sb1.06S3 nanorods. The Energy-dispersive X-ray spectroscopy (EDS) line scan was conducted along the dashed arrow to investigate the elemental distribution in the Bi0.94Sb1.06S3 nano-rods. As shown in Figures 1(f) and 1(g), S Kα, Bi Lα, and Sb Lα signal intensities raise and drop simultaneously along the scanning track. Figures 1(h), 1(i), and 1(j) show the ele-

mental mappings of S, Bi, and Sb, respectively, indicating uniform distribution of the elements. All these characterizations confirm the pure crystal phase of the orthorhombic Bi0.94Sb1.06S3.

Intensity (a.u.)

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10

20

30

40 o 2 ( )

50

60

70

Figure 2. X-Ray Diffraction (XRD) pattern of Bi0.94Sb1.06S3 nanorod clusters.

Figure S1 in the supporting information shows the SEM and TEM images of the hydrothermally synthesized Bi2S3 nanorods. The nanorods have a width of ca. 50 nm



0.70 0.811.25

-0.4

0.99 0.75 0.55 0.41 0.24

1st 2nd 3rd 4th 5th

0.0 0.5 1.0 1.5 2.0 2.5 + Voltage (V vs. Na /Na)

1.5 1.0 0.5 0.0

0

150 300 450 600 Specific Capacity (mAh/g)

Figure 3. (a) Cyclic voltammetry (CV) scans of Bi0.94Sb1.06S3graphite electrode with sodium as the counter electrode/reference electrode at a scan rate of 0.05 mV s–1. (b) Galvanostatic intermittent titration technique (GITT) profiles of Bi0.94Sb1.06S3-graphite electrode; the current pulse of C/30 lasted for 1 h and the relaxation time was 1 h.

Specific Capacity (mAh/g)

Bi0.94Sb1.06S3-Graphite Bi2S3-Graphite

600 400 200 0

0

50

100 Cycle Number

150

200

120th Cycle 200th Cycle

80 60 40 20 0

Sb2S3

Bi0.94Sb1.06S3

Bi2S3

-Graphite

-Graphite

-Graphite

Figure 4. (a) Cycle performance of Bi0.94Sb1.06S3-graphite, Sb2S3-graphite, and Bi2S3-graphite electrodes at galvanostatic condition with a current density of 1 A/g and in the voltage range of 0.01 – 2.80 V (vs. Na+/Na) in sodium-ion batteries. (b) Capacity retention of the electrodes in sodium-ion batteries after 120 and 200 cycles. (a) 0.1 0.3 0.5 1

600 400

0.1 3

60

5 7

40

10

200

20

Current Density Unit: A/g 0

0

10

20 30 Cycle Number

80

40

50

0

Coulombic Efficiency (%)

100

800

(b)

+

2.0

Sb2S3-Graphite

800

(b) 100

Voltage (V vs. Na /Na)

0.0

-0.2

Discharge Charge

2.5

+

0.2

(b) 3.0 Voltage (V vs. Na /Na)

(a) 0.4

(a)

Capacity Retention (%)

Sb2S3-graphite, Bi2S3-graphite, and Bi0.94Sb1.06S3graphite composite samples were prepared by mechanical milling of the metal sulfides with natural graphite flakes for electrochemical performance tests. Figure S5 in the supporting information shows the XRD patterns of the composite samples. The sodiation/desodiation process of Bi0.94Sb1.06S3 was studied with cyclic voltammetry (CV) scans. As shown in Figure 3(a), in the first cathodic scan, there are 3 cathodic peaks located at 1.15, 0.75, and 0.24 V (vs. Na+/Na), which, respectively, conrespond to electrolyte decomposition (solid-electrolyte interface (SEI) layer formation), conversion reaction (Equation 1), and alloying reaction (Equation 2). The anodic scans give three peaks located at 0.70, 0.81, and 1.25 V (vs. Na+/Na). The peaks at 0.70 and 0.81 V (vs. Na+/Na) derive from the alloying reaction (reverse reaction of Equation 2), and the broad peak at 1.25 V (vs. Na+/Na) derives from the conversion reaction (reverse reaction of Equation 1). From the 3rd scan, the stable current peaks of the conversion reaction are located at 0.99 V (vs. Na+/Na) and the alloying reactions are located at 0.55 and 0.41 V (vs. Na+/Na). A polarization overpotention of 0.26 V for the conversion reaction is observed in the Bi0.94Sb1.06S3graphite electrode.

Figure S6(b) in the supporting information shows the CV scan profiles of Sb2S3-graphite electrode. The cathodic peak at 0.91 V (vs. Na+/Na) and anodic peak at 1.25 V (vs. Na+/Na) derive from the conversion reaction (Equation 3). The alloying reaction (Equation 4) gives the cathodic and anodic peaks, respectively, at 0.47 and 0.74 V (vs. Na+/Na).

Charge Capacity (mAh/g)

and a length of less than 1 m. In the HRTEM image, the spacing of 0.504 nm corresponds to the distance between the (120) planes of Bi2S3 crystals (Pnma space group) with a = 1.115 nm, b = 0.398 nm, and c = 1.130 nm. Figure S2 in the supporting information shows the XRD profile of Bi2S3 nanorods, and all the peaks match well with the standard diffraction patterns of the orthorhombic Bi2S3 (PDF# 170320). The SEM image in Figure S3(a) in the supporting information shows the morphology of Sb2S3 wires. In the HRTEM image, the lattice distance of 0.32 nm corresponds to the spacing between the (210) planes of orthorhombic Sb2S3 crystals (Pnma space group) with a = 1.096 nm, b = 0.381 nm, and c = 1.098 nm (Figure S3c). As shown in Figure S4 in the supporting information, the XRD profile of Sb2S3 sample matches well with the standard diffraction pattern (PDF# 42–1393).

Current (mA)

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Current Density Unit: A/g 0.5 0.3 10 7 5 3 1 0.1

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

100 200 300 400 500 600 700 Specific Capacity (mAh/g)

Figure 5. Rate capabilities and potential profiles of Bi0.94Sb1.06S3-graphite electrode at various current densities in sodium-ion battery.


0

0.01 - 2.80 V (vs. Na /Na) 0 50 100 150 Cycle Number Bi0.94Sb1.06S3-Graphite

400 0.85 - 2.80 V (vs. Na /Na) +

200

200 100 0

(g) 400

50

100 150 Cycle Number

Bi0.94Sb1.06S3-Graphite +

300 0.01 - 0.85 V (vs. Na /Na)

200

100

50


200 Sb S - Graphite 2 3 +

0

0.01 - 2.80 V (vs. Na /Na) 0

50


200

400 300 200 100 Sb2S3- Graphite +

0

0.85 - 2.80 V (vs. Na /Na) 0 50 100 150 Cycle Number

200

200

0.5 A/g 1 A/g

300 200 100 Sb S - Graphite 2 3 +

0 0

0.01 - 0.85 V (vs. Na /Na) 50


200

Bi2S3- Graphite +

600 0.01 - 2.80 V (vs. Na /Na)

0.5 A/g 1 A/g

400 200 0

(f) 300 0.5 A/g 1 A/g 5 A/g

(h) 400 0.5 A/g 1 A/g

200

0 0

400

(e) 500 0.5 A/g 1 A/g 5 A/g

300

0

600


+

(c) 800 0.5 A/g 1 A/g


200 Bi Sb S -Graphite 0.94 1.06 3


400

(b) 800


600

(d) 500 Charge Capacity (mAh/g)

0.5 A/g 1 A/g 2 A/g 5 A/g



(a) 800


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250

0

50


Bi2S3- Graphite +

0.85 - 2.80 V (vs. Na /Na)

200

200

0.5 A/g 1 A/g 5 A/g

150 100 50 0

0

(i) 400 Charge Capacity (mAh/g)

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50


Bi2S3- Graphite +

300 0.01 - 0.85 V (vs. Na /Na)

200

0.5 A/g 1 A/g

200 100 0 0

50


200

Figure 6. Cycle performance of (a, d, and g) Bi0.94Sb1.06S3-graphite, (b, e, and f) Sb2S3-graphite, and (c, f, and i) Bi2S3-graphite electrodes in sodium-ion batteries in the charge-discharge voltage ranges of (a, b, and c) 0.01 – 2.80 V (vs. Na+/Na), (d, e, and f) 0.85 – 2.80 V (vs. Na+/Na), and (g, h, and i) 0.01 – 0.85 V (vs. Na+/Na).

CV scan profiles of the Bi2S3-graphite electrode are shown in Figure S6(d) in the supporting information. The alloying reaction gives the cathodic current peaks at 0.57 and 0.36 V (vs. Na+/Na); the anodic peaks of the alloy reactions are located at 0.67 and 0.81 V (vs. Na+/Na). For the conversion reaction, the cathodic and anodic current peaks are, respectively, at 1.06 and 1.78 V (vs. Na+/Na). It is worth noting that the overpotentials of the converion reactions in the Bi2S3-graphite electrode and Sb2S3graphite electrode are, respectively, 0.72 and 0.34 V (vs. Na+/Na), while that in the Bi0.94Sb1.06S3-graphite electrode is as low as 0.26 V, indicating an improved diffusion rate and/or lowered intrinsic activation barrier. Figure 3(b) shows the galvanostatic intermittent titration technique (GITT) curves of the Bi0.94Sb1.06S3-graphite electrode, presenting the discharge/charge profiles close to the thermodynamic equilibrium potentials.[31-34] In the charge curve, there are two plateaus with potentials of 0.75 and 1.25 V (vs. Na+/Na). Such an operating voltage of Bi0.94Sb1.06S3-graphite electrode eliminates the sodiumdentrite hazard and makes it a safe anode for sodium-ion batteries. The cycle stability of the electrodes was compared by full discharge/charge cycling under galvanostatic condition at current density of 1 A g–1 in the voltage range of 0.01 – 2.80 V (vs. Na+/Na) (Figure 4a). The Sb2S3-graphite

electrode cycles stably for 50 cycles, but severe capacity decay is observed after that. For the Bi2S3-graphite electrode, the capacity decays from the start of the cycling. It is noteworthy that the Bi0.94Sb1.06S3-graphite anode cycles stably for more than 100 cycles, and the capacity decay is also mild after 120 full charge-discharge cycles. Figure 4(b) compares the capacity retention of the electrodes after 120 and 200 cycles (normallized by the charge capacity of the 1st cycle under a current density of 1 A g–1 ). The 120th cycle capacity retentions of Sb2S3-graphite and Bi2S3graphite electrodes are, respectively, 78 % and 70%, while that for the Bi0.94Sb1.06S3-graphite electrode is as high as 99%. After 200 full discharge/charge cycles, the Bi0.94Sb1.06S3-graphite electrode gives a remarkable charge capacity retention of 79 %, while those of the Sb2S3graphite and Bi2S3-graphite electrodes drop to, respectively, 10% and 58%. The graphite makes negligible contribution to the capacity of the compositions in NIBs (Figure S7). The rate capability of Bi0.94Sb1.06S3-graphite electrode was tested at galvanostatic condition with a series of current densities (Figure 5). The theoretical capacity of Bi0.94Sb1.06S3-graphite is 637 mAh g–1. At a current density of 0.1 A g–1 (0.16 C), the reversible charge capacity is 564 mA h g–1, and it also shows a remarkably high initial Coulombic efficiency (ICE) of 78 %. When the current density



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increases to 0.3 (0.47), 0.5 (0.78C), and 1 A g–1 (1.6 C), the charge capacities moderately decrease to, respectively, 556, 551, and 534 mA h g–1. At very high current densities of 3 (4.7 C), 5 (7.9 C), 7 (11.0 C), and 10 A g–1 (15.7 C), the Bi0.94Sb1.06S3-graphite electrode delivers, respectively, remarkable charge capacities of 487, 426, 347, and 256 mA h g–1, which are 86 %, 76 %, 62 % and 45 % of the charge capacity at the current density of 0.1 A g–1. After the highcurrent-rate cycling, the Bi0.94Sb1.06S3-graphite electrode still delivers a charge capacity of 617 mA h g–1 at a current density of 0.1 A g–1. It is obvious that the Bi0.94Sb1.06S3graphite exhibits excellent rate performance. Figure 5b shows the potential profiles of the Bi0.94Sb1.06S3-graphite electrode at galvanostatic condition with different current densities. The continuous discharge/charge potential profiles are similar in shape to the GITT curves, indicating that the electrode is very close to the equibrilium during continuous discharge/charge.[31-34] This also demonstrates good sodium-ion diffusion rate and high electrical conductivity of the Bi0.94Sb1.06S3-graphite electrode. The pristine and cycled Bi0.94Sb1.06S3-graphite electrodes were also characterized by SEM and XRD, showing uncracked surface and amorphous active material on the cycled electrode (Figure S8 and S9). In order to have a deep understanding of the improved electrochemical performance of the ternary metals sulfide versus the binary metal sulfides in NIBs, the cycle performances of the electrodes with controlled reaction type were assessed. Within a voltage window of 0.85 – 2.80 V (vs. Na+/Na), only the conversion reaction occurs; when the discharge/charge voltage window is controlled at 0.01 – 0.85 V (vs. Na+/Na), only the alloying reaction occurs in the electrodes. Figures 6(a), 6(b), and 6(c) show, respectively, the cycle performances of Bi0.94Sb1.06S3graphite, Sb2S3-graphite, and Bi2S3-graphite electrodes under the full discharge/charge voltage range of 0.01 – 2.80 V (vs. Na+/Na). The improved cycle stability of Bi0.94Sb1.06S3-graphite electrode compared to the other electrodes is obviously observed under a series of current densities. The reversibility of the conversion reaction in each electrode was studied by cycling the electrodes at the voltage range of 0.85 - 2.80 V (vs. Na+/Na) (Figures 6(d), 6(e) and 6(f)). In the Bi2S3-graphite electrode, the capacity decays from the start of the cycling, and with the increase in current density, the capacity fade becomes more severe. For the Sb2S3-graphite electrode, the capacity fades fast after 50 cycles. The capacity fade indicates the poor reversibility of the conversion reaction in the Bi2S3-graphite and Sb2S3-graphite electrodes. However, in the Bi0.94Sb1.06S3-graphite electrode, an obviously improved reversibility of the conversion reaction is demonstrated by the stable cycle performance for 200 cycles without any capacity decay at various current densities. This is consistent with the CV scan results of lower polarization potential of the ternary metals sulfides electrode. With the voltage window of 0.01 – 0.85 V (vs. Na+/Na), the electrodes cycled with the alloying reactions only (Figures 6(g), 6(h) and 6(i)). For the Bi2S3-graphite electrode, the capacity of the alloying reaction decreases gradually after

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40 stable cycles, while for the Bi0.94Sb1.06S3-graphite and Sb2S3-graphite electrodes, the alloying reaction exhibits good reversibility and there is no capacity fade at different current densities for 200 cycles. These comparisons clearly indicate that the ternary metal sulfide possesses good reversibility of the conversion reaction, thus exhibiting stable cycle performance in the full discharge/charge cycles.



CONCLUSIONS

The ternary Bi0.94Sb1.06S3 solid solution sample with a nanorod cluster morphology has been hydrothermally synthesized and electrochemically tested in NIBs. At a current density of 0.1 A g–1, the Bi0.94Sb1.06S3-graphite anode displays a reversible charge capacity of 564 mA h g–1 with an initial Coulombic efficiency of 78 %. At very high current densities of 3, 5, 7, and 10 A g–1, the Bi0.94Sb1.06S3graphite anode delivers, respectively, remarkable charge capacities of 487, 426, 347, and 256 mA h g–1, which are 86 %, 76 %, 62 % and 45 % of the charge capacity at the current density of 0.1 A g–1. Most importantly, the Bi0.94Sb1.06S3-graphite anode shows much improved reversibility compared to its binary counterparts, Bi2S3-graphite and Sb2S3-graphite anodes. The Bi0.94Sb1.06S3-graphite anode delivers a charge capacity retention of 79 % after 200 cycles under a current density of 1 A g–1 (1.6 C), while its binary counterparts Bi2S3-graphite and Sb2S3-graphite exhibit capacity retentions of, respectively, 58 % and 10 %. CV scans studies show that the conversion reaction of the Bi0.94Sb1.06S3-graphite electrode has lowered polarization overpotential. The discharge/charge cycling of Bi0.94Sb1.06S3-graphite electrode in the voltage range of 0.85 – 2.80 V (vs. Na+/Na) delivers unvarying charge capacity for 200 cycles, indicating a highly reversible conversion reaction in the ternary sulfide. The design of ternary metals sulfide solid solutions could be considered as a potential approach to the development of novel anode materials with high rate capability and long cycle life for NIBs.

ASSOCIATED CONTENT Supporting Information. SEM and TEM images; XRD patterns; NIB cycle performance of graphite anode. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under award number DE-SC0005397.

REFERENCES


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