Ni Composites as High-Performance

Dec 17, 2013 - Figure 1a shows the XRD pattern of the deposit prepared in the typical solution (0.2 M TU) at 1 A dm–2 for 10 min. .... Such good rat...
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Electrodeposition of Nis3S2/Ni Composites as High-Performance Cathodes for Lithium Batteries Chang-Wei Su,*,†,‡ Jun-Min Li,†,‡ Wei Yang,†,‡ and Jun-Ming Guo†,‡ †

Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission & Ministry of Education, Yunnan University of Nationalities, D-306, Guang-Jing Compound, Kunming 650500, P. R. China ‡ Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan University of Nationalities, Kunming 650500, P. R. China ABSTRACT: An alternative electrodeposition technology, which provides an efficient way to complete a better interface contact between electroactive materials and metal foil substrates, was used to prepare Ni3S2/Ni composites in aqueous solutions contained thiourea (TU). The deposition parameters, such as time and TU concentration, were optimized. Surface morphology, chemical composition, and crystal structure of Ni3S2/Ni composites were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD), respectively. A Ni3S2/Ni composite, which consisted of submicrometer grains with size in the range of 0.2−2 μm, was obtained. Galavostatic battery testing shows that the submicrometer Ni3S2/Ni4 composites as cathodes for lithium batteries exhibit a high specific capacity of 338 mAh g−1 at a current density of 0.17 A/g (∼0.6 C), extraordinary capacity retention as well as 95.3% after 100 cycles, and outstanding rate performance, for example, still delivering 180 mAh g−1 at 1.7 A g−1 (∼6 C). The performances may benefit from their miscellaneous submicrometer structures and subsequently formed tremella-like structures during discharge/charge cycles.

1. INTRODUCTION Nickel sulfides as a cathode material for rechargeable lithium batteries were first studied in 1969 by Jasinski and Burrows, but the highest utilization was about 60%.1 Recently, they are attracted tremendous attention in the field of lithium batteries due to their low cost and high theoretical capacities, assuming complete discharge to Ni and Li2S.2−18 Among them, NiS2−8 and Ni3S213−18 were widely studied. Han et al.3 found that NiS first transformed to Ni3S2 after discharging at a high potential of 1.75 V vs Li+/Li by the reaction 3NiS + 2Li+ + 2e− → Ni3S2 + Li 2S (1.75 V)

surface sulfuration of nickel possess good electrical contact and, consequently, can be directly utilized without polymer binder and graphite.14,16−18 Exhilaratingly, the Ni3S2/Ni electrodes also exhibit good electrochemical performances, for example, a reversible capacity of 421 mAh g−1 after 60 cycles at a current density of 50 mA g−1.18 Electrodeposition is also a great approach to anchor directly electroactive materials on current collectors without using any polymer binder, conducting agent, and high pressure.19 In this paper, an alternative electrodeposition technology was used to prepare Ni3S2 active materials, forming the excellent electrical contact between the active materials and Cu foil substrates. The resultant submicrometer Ni3S2/Ni4 composites as cathodes of lithium batteries exhibited extraordinary capacity retention and high-rate performance, benefiting from their miscellaneous structures, in which Ni is similar to graphite mixed in cathode materials and can play a role of transportation of electrons.

(1)

Hence, the theoretical capacity of NiS could be mainly contributed to the discharge of Ni3S27 by Ni3S2 + 4Li+ + 4e− → 3Ni + 2Li 2S (1.34 V)

(2)

In addition, the potential difference between aforementioned two discharge steps is up to 0.41 V. So, the practical capacity of NiS might be only utilized that of Ni3S2. The formation of Ni3S2 for lithium batteries or supercapacitors can be realized by mechanical alloying with metallic nickel and sulfur powder as precursors,13 sintering of nickel and sulfur at 700 °C,15 electrodepositing from an aqueous solution composed of 50 mM NiCl2 and 1 M thiourea,19 and surface sulfuration of nickel substrates by L-cysteine14,18 or thiourea17 in a sealed Teflon-lined autoclave or by sulfur in a open reactor filled with an aqueous solution including hydrazine and NaOH.16 The self-supported Ni3S2/Ni electrodes obtained by © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Electrodeposition of Ni3S2/Ni Composites. The typical solution for Ni3S2/Ni composites electrodeposition consisted of 0.25 M NiSO4·6H2O, 0.20 M thiourea (TU), and 0.03 M citric acid as the sources of nickel and sulfur and pH buffer reagent, respectively. However, Han et al.20 pointed out Received: July 19, 2013 Revised: November 28, 2013 Published: December 17, 2013 767

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2.2. Characterizations. The morphologies and energydispersive X-ray spectroscopy (EDX) of the synthesized Ni3S2/ Ni composites on Cu foils were investigated by scanning electron microscopy (SEM, QUANTA 200, America FEI). Chemical composition of the composites was semiquantitatively analyzed using EDX data. The collection time for EDX data was 50 s. The X-ray diffraction (XRD) patterns of Ni3S2/ Ni composites scraped off from Cu foils were recorded using a D/max-TTRIII diffractometer with Cu Kα radiation over 2θ range of 10°−90°. 2.3. Electrochemical Measurements. The electrochemical performance was evaluated using CR2025 coin cells which are assembled in a high-purity argon-filled glovebox (Mikrouna, Super 1220/750). The prepared Ni3S2/Ni/Cu foils were cut into disks with a diameter of 1.6 cm (∼2.0 cm2) and regarded as working electrode without any polymer binder. Lithium foil was used as both counter and reference electrodes, and Celgard 2320 was used as separator membrane. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). The weight specific capacity was calculated using the weight difference between raw and Ni3S2/ Ni-carried Cu foil disks by a semimicro balance (readability: 0.01 mg, Mettler Toledo, MS105DU). The cyclic voltammogram (CV) was obtained on an electrochemical workstation (CHI 604D, Shanghai Chenhua) at a scanning rate of 0.5 mV/s. The galvanostatic discharge/ charge cycles were carried out using the Land battery system (CT2001) with a potential window of 3.00−0.01 V vs Li+/Li.

that TU concentration was a key parameter influencing sulfur content in Ni−S deposit. Herein, other solutions containing different TU concentrations from 0.1 to 0.5 M were also studied. The pH of the solution was adjusted to 3.0 with NaOH and H2SO4. To avoid significant changes in the solution composition over the course of several experiments, an electrolyte would not used any more if electrolyzed over 0.5 Ah/L. All chemicals were of analytical reagent grade and deionized water was used. Ni3S2/Ni composites were galvanostatically electrodeposited on Cu foils (4 × 5 cm2) at room temperature in a singlecompartment glass cell with two-electrode configuration using a dc power supply. An IrO2/Ti plate was employed as inert anode. The current density was keep a constant of 1 A dm−2, and the deposition time was between 5 and 50 min. Prior to electrodeposition, Cu foils were dipped into 10% H2SO4 to remove surface oxides and then rinsed thoroughly with deionized water. After electrodeposition, the resultant Ni3S2/ Ni/Cu foils were rinsed immediately using deionized water and then dried under vacuum at 80 °C for 12 h. The Ni3S2/Ni composite, which electrodeposited in the typical solution at 1 A dm−2 for 10 min, would be studied in detail, and its thickness and weight are about 6 μm and 1 mg cm−2, respectively.

3. RESULTS AND DISCUSSION 3.1. Electrodeposition. Figure 1a shows the XRD pattern of the deposit prepared in the typical solution (0.2 M TU) at

Figure 2. Changes of chemical compositions of the deposits with (a) TU concentration at 1 A dm−2 for 10 min and (b) electrodeposition time at 1 A dm−2 for the 0.2 M TU solution.

Figure 1. (a) XRD pattern and (b) EDX spectrum of the Ni3S2−Ni4 composite. 768

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Figure 3. SEM images of Ni3S2−Ni composites electrodeposited for (a) 5, (b) 10, (c) 30, and (d) 50 min.

1 A dm−2 for 10 min. The diffraction peaks located at 2θ = 44.5°, 51.8°, and 76.4° can be assigned to these of Ni (JCPDS No. 040850), while other peaks can be indexed to the phase of Ni3S2 (JCPDS No. 44-1418). The result indicates that the deposit is composed of two phases of Ni and Ni3S2, which is a Ni3S2/Ni composite. The EDX of the Ni3S2/Ni composite is shown in Figure 1b. It can be seen that the atomic percentage of Ni and S is 55.22:16.13. According to the value, the mole ratio of Ni3S2 and Ni of the deposit is calculated to be nearly 1:4. The corresponding sample is referred as Ni3S2/Ni4. The signal of Cu is from the Cu foil substrate, but O is possible from hydroxide precipitates which were embedded into the Ni3S2/Ni composite during elctrodeposition because of high pH value nearby the hydrogen evolution material of Ni3S2. The pure Ni3S2 and other metal sulfides could be synthesized by potentiodynamic electrodeposition19,21 or periodic potential reversal electrodeposition.22−24 But, it is not necessary to remove Ni from the deposits during electrodeposition because Ni in the Ni3S2/Ni composites takes as a conductor of electrons similarly to graphite mixed into electrode materials of LIBs. Furthermore, the current efficiency will become lower because of the existence of reversal potential steps to dissolve Ni deposite. The Ni3S2/Ni composites can be also electrodeposited from other baths,19,20 for examples a bath containing NiCl2 and TU.19 A possible electrodeposition mechanism from TUcontaining baths has been proposed.19

Ni 2 + + 2e− → Ni

(4)

From Figure 2a, it can be observed that the content of S increases slowly then rapidly with increasing TU concentration. The inflection appears at about 0.3 M TU, which is corresponding to the concentration of Ni2+ ions. According to the following reaction 5, dissociative TU molecules will increase rapidly when the TU total concentration exceed that of Ni2+ ions. The dissociative TU molecules could be decomposed electrochemically to S2− ions (the reaction 619,24) which then form Ni3S2 with Ni by the reaction 7.17,18 Thereby, S in the deposits increases rapidly. TU + Ni 2 + → [NiTU]2 +

(5)

TU + 2e− → S2 − + CN− + NH4 +

(6)

2S2 − + O2 + 2H 2O + 3Ni → Ni3S2 + 4OH−

(7)

The increase of O in the deposits may ascribe to the increase of hydroxide precipitates in deposits due to the capacity improvement of hydrogen evolution for Ni−S deposits. The content of S decreases to a limit value of ∼10% with electrodepostion time from 5 to 50 min. The result indicates that the deposition of S is a diffusion-control process. The highest O content at 5 min possibly results from oxidation of the Cu foil substrate, which can be detected until 50 min. Subsequently, the content of O increases from a lowest point of 3.96% at 10 min to 8.74% at 50 min due to pH increasing nearby the electrode with electrodeposition time. 3.2. Morphology. Figure 3 shows SEM images of Ni3S2/Ni composites electrodeposited in the typical solution for different

3[NiTU]2 + + 6e− → Ni3S2 + CN− + 2NH4 + + TU (3) 769

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Figure 4. SEM images of Ni3S2−Ni composites electrodeposited from electrolytes containing (a) 0.1, (b) 0.3, and (c) 0.5 M.

indicate the structural change of the material, which is testified by aftermentioned XRD data after discharge−charge cycles. Another remarkable characteristic of the discharging curves is a higher initial capacity. This irreversible phenomenon has been also observed in the Ni3S2 materials prepared by other methods,14,16−18 which could be associated with the formation of a solid electrolyte interface (SEI) layer on the surface of the electrode. A low sloping curve (between 1.50 and 1.90 V) and a plateau at 1.95 V can be observed in the first three charges, implying that a two-step charge occurs. The low sloping curve may be attributed to the part decomposition of SEI, so it disappears after 10 cycles when the stable SEI has been formed. A pair of discharge (1.45 V) and charge (1.95 V) plateaus become broad with cycling in the first 10 cycles, implying an activation process for the Ni3S2/Ni4 composite during discharge/charge cycles. Figure 5b presents the cyclic behavior of Ni3S2/Ni4 composite with Coulombic efficiency, from which it is clear to see that the composite possesses excellent cyclic capacity with Coulombic efficiency nearly 100%. However, the first Coulombic efficient is below 80%, indicating irreversible lithium ions consumptions due to the formation of SEI. The discharge capacities are 338 and 322 mAh g−1 in the 2nd and 100th, respectively, showing an extraordinary capacity retention of 95.3%. According to reaction 2, it is calculated to be a theoretical capacity of 445 mAh g−1 for Ni3S2 active materials. It can be deduced that the content of Ni3S2 in the Ni3S2/Ni4 composite exceeds 50 wt % and reaches minimally 71 wt %, which may be ascribed to the following two reasons: (i) there is a negative deviation from the EDX data of S content, and

durations ranging from 5 to 50 min. There are many tiny grains of from 80 to 300 nm in size after 5 min (see Figure 3a), implying the growth of Ni3S2/Ni composite grains follows a progressive nucleation. Herein, it can be seen from Figure 3b,c that the numbers as well as the size of the grains grow as the electrodeposition time prolongs. As shown in Figure 3b, the Ni3S2/Ni4 composite electrodeposited for 10 min consists of submicrometer grains with size in the range of 0.2−2 μm. However, diastemata between grains can be filled when the electrodeposition time prolongs to 50 min, as shown in Figure 3d. Such a filled structure is not beneficial to the transportation of Li+ ions. So it is suggested that electrodeposition duration should not exceed 30 min. Figure 4 shows the change in the surface morphology of the samples electrodeposited for 10 min with TU concentration. At 0.1 M TU, a great deal of uniform and fine grains decorated by flocky flowers can be found in Figure 4a. Increasing TU concentration, the grains become big and the distribution of grain size is not uniform as shown in Figures 4b and 3b. A rimous structure can be observed in Figure 4c when TU increases to 0.5 M, similarly to amorphous Ni−P25 deposits. Amorphous Ni−S deposits have been obtained from solutions containing high TU concentration.20 3.3. Electrochemical Performances. Figure 5a shows the discharge−charge voltage profiles of the Ni3S2/Ni4 composite at 0.17 A/g (∼0.6 C) in the range of 0.01−3.00 V. A voltage plateau at 1.18 V followed by a sloping curve is exhibited in the first discharge, while it shifts to 1.45 V in subsequent discharge processes. The increase of the voltage plateau would 770

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Figure 6. CV curves of the Ni3S2−Ni4 composite (a) before and (b) after 50 discharge/charge cycles at a scan rate of 0.05 mV s−1.

between the first and subsequent three cycles is found. In the first cathodic scan, a strong reduction peaks occur at very negative potential of 0.13 V, which is attributed to the formation of SEI and the formation of Ni. In subsequent three cathodic scans, a strong reduction peak (C1) and a weak shoulder (C2) are present at 1.34 and 0.60 V, respectively. The corresponding anodic peaks of A1 and A2 are observed at 2.00 and 1.68 V, respectively. The couple redox peaks of C1 and A1 can be assigned to the formation and decomposition of SEI, whose peak currents decrease along with cycling in the first four cycles and disappear after 50 discharge/charge cycles (see Figure 6b) due to forming a stable SEI. Another couple redox peaks of C2 and A2 result from the insertion and extraction of Li+ ions (Ni3S2 + 4Li+ + 4e− ↔ 3Ni + 2Li2S), whose peak currents increase along with cycling in the first four cycles and limited to −0.41 and 0.53 mA, respectively, after 50 discharge/charge cycles (see Figure 6b), resulting in high capacity retention (see Figure 5b). The morphology and structure of the Ni3S2/Ni4 composite electrodes in charge state were characterized after 50 discharge/ charge cycles. The electrodes were washed gradually with 1-methyl-2-pyrrolidone (C5H9NO) acetone and deionized water. As shown in Figure 7a, the two phases of Ni and Ni3S2 are detected, indicating that Ni3S2 is a stable sulfide in a Ni-rich circumstance. The strongest peak of Ni at 2θ = 44.5° becomes narrower than that of as-deposited Ni3S2/Ni composite (Figure 1a) due to the Ni recrystallization during discharge/charge processes. Comparing the morphologies of Ni3S2/Ni4 composite as-deposited (Figure 3b) with discharged/ charged (Figure 7b), it can be deduced that in the discharge/ charge processes the submicrometer particles transform to

Figure 5. (a) Discharge/charge curves and (b) cyclic behavior and Coulombic efficiency of the Ni3S2−Ni4 composite at 0.17 A/g (∼0.6C) in the range of 0.01−3.00 V. (c) Rate performance of the Ni3S2−Ni4 composite.

(ii) hydroxides or corresponding oxides generated during drying them under vacuum at 80 °C for 12 h contribute to their capacities in the low potential range below 1.0 V due to their higher capacity (∼1000 mAh g−1). Figure 5c shows the rate performance of the Ni3S2/Ni4 composite. It can be seen that the decrease of capacity along with increasing current density. The capacity is about 300 and 180 mAh g−1 at 0.17 and 1.7 A g−1 (∼6 C), respectively. In addition, after 40 cycling at different current densities, the discharge and charge capacities recover their original values at 0.17 A g−1. Such good rate performance and high capacity retention suggest that the Ni3S2/Ni4 composite can be a cathode candidate for lithium batteries. Cyclic voltammetry curves (CVs) of the Ni 3 S 2 /Ni 4 composite before and after 50 discharge/charge cycles are shown in Figure 6. As shown in Figure 6a, the profiles of CVs of the 2nd to 4th are similar, whereas an obvious difference 771

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51062018, 51262031), the Natural Science Foundation of Yunnan (2010FXW004), Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province (2010UY08, 2011UY09), Yunnan Provincial Innovation Team (2011HC008), and the Education Department foundation of Yunnan Province (2013J006).

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(1) Jasinski, R.; Burrows, B. Cathodic Discharge of Nickel Sulfide in a Propylene Carbonate-LiClO4 Electrolyte. J. Electrochem. Soc. 1969, 116, 422−424. (2) Han, S. C.; Kim, H. S.; Song, M. S.; Kim, J. H.; Ahn, H. J.; Lee, J. Y. Nickel Sulfide Synthesized by Ball Milling as an Attractive Cathode Material for Rechargeable Lithium Batteries. J. Alloys Compd. 2003, 351, 273−278. (3) Han, S. C.; Kim, K. W.; Ahn, H. J.; Ahn, J. H.; Lee, J. Y. Charge− Discharge Mechanism of Mechanically Alloyed NiS Used as a Cathode in Rechargeable Lithium Batteries. J. Alloys Compd. 2003, 361, 247− 251. (4) Wang, J.; Chew, S. Y.; Wexler, D.; Wang, G. X.; Ng, S. H.; Zhong, S.; Liu, H. K. Nanostructured Nickel Sulfide Synthesized via a Polyol Route as a Cathode Material for the Rechargeable Lithium Battery. Electrochem. Commun. 2007, 9, 1877−1880. (5) Takeuchi, T.; Sakaebe, H.; Kageyama, H.; Handa, K.; Sakai, T.; Tatsumia, K. Modification of Nickel Sulfide by Surface Coating with TiO2 and ZrO2 for Improvement of Cycle Capability. J. Electrochem. Soc. 2009, 156, A958−A966. (6) Ni, S. B.; Yang, X. L.; Li, T. Fabrication of a Porous NiS/Ni Nanostructured Electrode via a Dry Thermal Sulfuration Method and Its Application in a Lithium Ion Battery. J. Mater. Chem. 2012, 22, 2395−2397. (7) Idris, N. H.; Rahman, M. M.; Chou, S. L.; Wang, J. Z.; Wexler, D.; Liu, H. K. Rapid Synthesis of Binary α-NiS-β-NiS by Microwave Autoclave for Rechargeable Lithium Batteries. Electrochim. Acta 2011, 58, 456−462. (8) Aso, K.; Hayashi, A.; Tatsumisago, M. Synthesis of NiS−Carbon Fiber Composites in High-Boiling Solvent to Improve Electrochemical Performance in All-Solid-State Lithium Secondary Batteries. Electrochim. Acta 2012, 83, 448−453. (9) Apostolova, R. D.; Tysyachnyi, V. P.; Shembel, E. M. Electrolytic Binary Co and Ni Sulfides in Electrodes of Lithium and Lithium-Ion Low-Temperature Batteries. Russ. J. Electrochem. 2010, 46, 100−106. (10) Aso, K.; Kitaura, H.; Hayashi, H.; Tatsumisago, M. Synthesis of Nanosized Nickel Sulfide in High-Boiling Solvent for All-Solid-State Lithium Secondary Batteries. J. Mater. Chem. 2011, 21, 2987−2990. (11) Liu, X. J.; Xu, Z. Z.; Ahn, H. J.; Lyu, S. K.; Ahn, I. S. Electrochemical Characteristics of Cathode Materials NiS2 and FeDoped NiS2 Synthesized by Mechanical Alloying for Lithium-Ion Batteries. Powder Technol. 2012, 229, 24−29. (12) Mahmood, N.; Zhang, C. Z.; Hou, Y. L. Nickel Sulfide/ Nitrogen-Doped Graphene Composites: Phase-Controlled Synthesis and High Performance Anode Materials for Lithium Ion Batteries. Small 2013, 9, 1321−1328. (13) Zhu, X. J.; Wen, Z. Y.; Gu, Z. H.; Huang, S. H. RoomTemperature Mechanosynthesis of Ni3S2 as Cathode Material for Rechargeable Lithium Polymer Batteries. J. Electrochem. Soc. 2006, 153, A504−A507. (14) Wang, Q.; Gao, R.; Li, J. H. Porous, Self-Supported Ni3S2/Ni Nanoarchitectured Electrode Operating Through Efficient LithiumDriven Conversion Reactions. Appl. Phys. Lett. 2007, 90, 143107−1−3.

Figure 7. (a) XRD pattern and (b) SEM image of the Ni3S2−Ni4 composite after 50 discharge/charge cycles.

tremella-like structures, which may be responsible for long cycle life and good high-rate performance of the electrodeposited Ni3S2/ Ni4 composite. The morphology variation, which is tending toward one with improved electrochemical performances during cycling, has been also observed in other redox-type electrodes, for example, the CuxO/Cu electrode ascribing to an electrochemical activation and reconstruction effect.26

4. CONCLUSION Ni3S2/Ni composites are successfully electrodeposited on a Cu foil substrate in aqueous solutions contained TU at a constant current density of 1 A dm−2. The content of S (or Ni3S2) in the deposites can be adjusted by changing the concentration of TU and a submicrometer Ni3S2/Ni4 (1:4 mole ratio between Ni3S2 and Ni) composite is prepared in 0.2 M TU. The submicrometer Ni3S2/Ni4 composite exhibits good cycle stability and rate capability when used as cathodes in lithium batteries. It can deliver a high capacity of 180 mAh g−1 even at a high rate of 1.7 A g−1 (∼6 C). Hence, the Ni3S2/Ni4 is expected to be a promising candidate as cathode material in lithium batteries.

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