CuFeS2 Quantum Dots Anchored in Carbon Frame: Superior Lithium

Aug 28, 2017 - State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, The Key Laboratory of L...
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CuFeS2 Quantum Dots Anchored in Carbon Frame: Superior Lithium Storage Performance and the Study of Electrochemical Mechanism Peisheng Guo, Huawei Song, Yuyi Liu, and Chengxin Wang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Herein, we report a simple and quick synthetic route to prepare the pure CuFeS2 quantum dots (QDs) @C composites with the unique structure of CuFeS2 QDs encapsulated in the carbon frame. When tested as anode materials for the lithium ion battery, the CuFeS2 QDs @C composites based electrodes exhibit excellent electrochemical performances. When charge−discharge occurred with a current density of 0.5 A g−1, the electrodes exhibit a high reversible capacity (760 mA h g−1) for as long as 700 cycles, which indicates the superior cycling life. Detailed investigations of the morphological and structural changes of CuFeS2 QDs by ex situ XRD, ex situ Raman, and ex situ TEM reveal an interesting electrochemical reaction mechanism, a hybrid of a lithium−copper iron sulfide battery and lithium−sulfur battery. The direct observation of orthorhombic FeS2 by HRTEM and the existence of Li2FeS2 detected by Raman support our assertion. We believe such an electrochemical mechanism would attract more attention to the CuFeS2 nanomaterials as lithium ion battery anode materials. The excellent electrochemical properties would be derived from the unique structure, which include CuFeS2 QDs encapsulated in the carbon frame. KEYWORDS: quantum dots, long cycling life, high capacity, hybrid of lithium−copper iron sulfide battery and lithium−sulfur battery, transition metal sulfides

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INTRODUCTION Rapid developments in science and technology have resulted in the ever-increasing demand for energy resources. However, with the unceasing consumption of nonrenewable fossil fuel, the reserves of fossil energy cannot maintain the sustainable development of human society and result in a series of environmental problems, such as air and water pollution.1−3 Therefore, it is extremely urgent to put emphasis on exploiting renewable energy sources and energy storage devices. As a state-of-the-art energy storage device, the lithium ion battery has been utilized in many applications, such as portable electronics, electric vehicles (EVs), and hybrid electric vehicles (HEVs).4−6 Nevertheless, with ever-growing markets of portable electronics and electric vehicles, the demand for a lithium ion battery that exhibits high capacity, long cycle stability, and safety becomes increasingly imperative.7,8 With anode materials taken into consideration, the search for electrode materials with superior electrochemical performance © 2017 American Chemical Society

that can transcend state-of-the-art graphite materials still remains a great challenge. Among the large number of anode materials, transition metal sulfides, such as Co9S8,9−11 FeS2,12 CoS2,13−15 NiS,16,17 ZnS,18−20 and SnS2,21,22 have attracted great interest due to their high theoretical capacities, availability, low cost, safety, and environmental friendliness. Moreover, owing to the intrinsic properties of high electrical conductivity, outstanding mechanical and thermal stability, and superior redox reversibility, transition metal sulfides are considered alternative anode materials for the lithium ion battery. Specifically, binary metal sulfides, due to the introduction of another metal ion that enriched the connections between atoms and resulted in the diversification of crystal structures, possess unique properties, for example, Received: May 12, 2017 Accepted: August 28, 2017 Published: August 28, 2017 31752

DOI: 10.1021/acsami.7b06685 ACS Appl. Mater. Interfaces 2017, 9, 31752−31762

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cycle life of CuFeS2-based electrodes have been improved, which is far more than in previous cases. Moreover, by confining the sulfur by carbon frame and trapping the polysulfide anions by Lewis acid−base interactions of the substoichiometric metal sulfides, it can help sulfur species form from the electroactive species. Detailed investigations of the morphological and structural changes of CuFeS2 QDs by ex situ XRD, ex situ Raman, and ex situ TEM reveal an interesting electrochemical reaction mechanism, a hybrid of a lithium− copper iron sulfide battery and lithium−sulfur battery. The direct observation of orthorhombic FeS2 by HRTEM and the existence of Li 2FeS 2 detected by Raman support our speculation. We believe such an electrochemical mechanism would attract more attention to CuFeS2 nanomaterials as lithium ion battery anode materials.

higher electrical conductivity, electrochemical activity, and capacity compared to the monometal sulfides, forecasting them as promising anode materials for outstanding performance in lithium ion batteries. As a degenerate semiconductor, CuFeS2 presents a low optical band gap of 0.53 eV, 23 antiferromagnetism,24 rectification,25 and large thermoelectric power.24 CuFeS2, a natural mineral with a special golden luster, exhibits a tetragonal structure, in which Fe and Cu ions are tetrahedrally coordinating with sulfur in the lattice. Due to the distinctive structure, CuFeS2 exhibits high conductivity and superior electrochemical performance. However, even if it is stable in the terrestrial circumstance and made up of earth abundant elements of Cu and Fe, comprehensive research is rare regarding finding a stable synthetic route for CuFeS2 quantum dots and exploring their electrochemical performance as anode materials for lithium ion batteries. In order to hinder the voltage depression, Eda et al.26 employed CuFeSx (1.6 < x ≤ 2.0) as a kind of additive. Linna Hu et al.27 explored its electrochemical performance as lithium ion battery cathode materials. The initial capacity was 1100 mA h g−1 at 14 mA g−1, and its specific discharging capacity was 500 mA h g−1 at 350 mA g−1. However, the cycle life was not presented. Gufeng He et al.28 reported the CuFeS2 electrochemical properties as anode materials by controlling the shape with PVP. The reversible capacity was 700 mA h g−1 after 60 cycles at 100 mA g−1, and the retention was 77.7%, compared with the second cycle. Jinbao Zhao et al.29 improved the cycling stability of CuFeS2 spike-liked nanorods as lithium ion battery anode materials by use of an ether-based electrolyte. The initial capacity was 632.6 mA h g−1 without an ether-based electrolyte and 674.6 mA h g−1 with one, respectively. And the retention was 10.1% without an ether-based electrolyte and 63.0% with an ether-based electrolyte after 50 cycles, respectively. Nevertheless, all these studies faced a large capacity fade and poor cycle life, which hindered the application of CuFeS2 as an alternative anode material for lithium ion batteries. The poor cycling life would originate in the pulverization resulted from large volume change, agglomeration of nanoparticles, and poor electrochemical kinetics. Therefore, we cannot sufficiently emphasize how important it is to improve the cycle stability of CuFeS2-based electrodes. In this study, we report a simple synthetic route to synthesize CuFeS2 quantum dots (QDs) anchored in a carbon frame at large scale and free from contaminant phases. To the best of our knowledge, there are rare reports on CuFeS2 quantum dots with good phase purity and outstanding stability. When used as anode materials for lithium ion batteries, the CuFeS2 QDs @C composites exhibit superior electrochemical properties. The CuFeS2 QDs @C composites present a high specific capacity of 760 mA h g−1 for 700 cycles with a current density of 0.5 A g−1. Such superior electrochemical performance should result from the special synthetic route. First, in this in situ carbon coated method, the carbon frame would hinder the growth of the CuFeS2 crystal, just like the nanoconfined effect of MOF, so we can prepare the CuFeS2 QDs with sizes of 5−10 nm, which can enhance the electrochemical kinetics by reducing the sizes of CuFeS2 QDs and shortening the diffusion distance of the lithium ion. Second, the carbon frame can effectively relax the large volume change and prevent aggregation of the pulverized nanoparticles during the lithiation−delithiation process. Last, the carbon frame is fluffy, which is beneficial to the infiltration of the electrolyte. In our work, by in situ carbon coating, the

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EXPERIMENTAL SECTION

Synthesis. In a representative synthesis, 4 mmol of Li2S and 200 mg of glucose dissolve in 20 mL of ethylene glycol by stirring for 30 min. Then, 2 mmol of C4H6FeO4 and 2 mmol of C4H6CuO4 H2O were added into the solution followed by magnetic stirring for 60 min. Afterward, a 10 mL solution was transported into the oven that was preheated to 300 °C and maintained for 5 min. Then, the dried and black powders were transported into the tube furnace and calcined in vacuum at 300 °C for 30 min with a heating rate of 10 °C min−1. Finally, the black powders were collected by centrifugation, ultrasonically washed with deionized water and alcohol several times, and dried in air at 80 °C for 24 h. Characterization. The powders were placed on the glass substrate to obtain the XRD patterns. The XRD was performed on a Rigaku SmartLab, in which the voltage and current of X-ray source operation are 40 kV and 30 mA. Phase authentication was executed with the assistance of Jade software. The images of morphologies were obtained by field-emission scanning electron microscopy (FESEM, Carl Zeiss, Geminia). Transmission electron microscopy (TEM) was performed on an FEI Tecnai G2 F30 at an accelerating voltage of 300 kV. The energy dispersive X-ray spectrum was collected by INCA300, which was installed on the TEM. X-ray photoelectron spectroscopy (XPS) was acquired by ESCALab250. Fourier Transform Infrared Spectroscopy (FT-IR) was carried out using EQUINOX 55. Thermogravimetry (TG) was operated on a TG209F1 libra. Elemental analysis (EA) was acquired by Vario EL. Raman spectra were measured by inVia Reflex with a 785 nm laser source. Electrochemical Test. The working electrode was composed of the active material (CuFeS2 QDs @C), conductive agent (acetylene black), and binding agent (PVDF) with the ratio 7:2:1. First, N-methyl pyrrolidone was used as the solvent to dissolve the PVDF, and then the active material and conductive agent were added into the solution to form a slurry by stirring for several hours. Afterward, the slurry was coated on the copper foil and transported into the oven at 90 °C in air. Pure lithium foil was employed as the counter electrode and reference electrode. All the electrochemical tests were executed in the button cell, which was assembled in an argon-filled glovebox with the concentrations of oxygen and moisture were maintained below 1 ppm. When the polypropylene micromembrane was applied as a separator, the electrolyte is the solution that includes 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC/DEC in 1:1 weight ratio). The charge/discharge performances were recorded using the NEWARE battery test system at various current rates in the voltage range between 0.005 and 3.0 V. Cyclic voltammetry (CV) was recorded by the CHI 660 electrochemical workstation (0.2 mV s−1, 0.0050−3.0000 V).

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RESULTS AND DISCUSSION Structure and Morphologies. The phase was identified by means of X-ray Powder Diffraction (XRD). As shown in Figure 1f, all the diffraction peaks can be ascribed to the standard card 31753

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Figure 1. Structural characterizations of as-synthesized CuFeS2 QDs @C composites. (a and b) Representative SEM images at different magnifications. (c, d, and e) Typical TEM image of CuFeS2 QDs @C composites at different magnification; the insets of (d) are the details of a particle and the corresponding FFT image, and the inset of (e) shows the measurement of the crystal interfaces. (f) XRD pattern of as-synthesized CuFeS2 QDs @C composites; as reference given in the bottom: JCPDS card no. 83-0983. (g) Representative EDS spectrum. (h) Scanning TEM (STEM) and element mapping images.

Figure S1d. Moreover, the architecture of the CuFeS2 QDs @C composites is fluffy, just like the foams, as shown in Figure 1a and b. More morphologies and crystal information can be obtained from the TEM and HRTEM images. As shown in Figure S2a and b, we can observe that the sizes of CuFeS2 QDs are 5−10 nm. Figure 1g shows the EDX spectrum, which shows that the CuFeS2 QDs @C composites are composed of the elements Cu, Fe, S, and C. Element mapping characterizations were implemented to further explore the structure of the asprepared products. As shown in Figure 1h, the Cu, Fe, S, and C elements exhibit identical distribution. More crystal structure information can be gained from HRTEM, as shown in Figure 1d and e. The HRTEM image of single CuFeS2 QDs in Figure 1e exhibits a basal space of 0.3002 nm, which is well matched with the (112) lattice fringe of tetragonal CuFeS2. Figure 1d shows the crystallization of a single QD; as shown in the

(JCPDS No. 83-0983), which indicates the as-synthesized products were pure CuFeS2 with a tetragonal crystal structure. As for the wide peak between 20° and 28°, it can be assigned to amorphous carbon that generated from carbonization of glucose and ethylene glycol. Moreover, no other peaks can be found from the XRD pattern, making clear that the assynthesized products had good purity and the impurity of the CuS phase was nonexistent. The morphologies of CuFeS2 QDs @C composites are shown using SEM and TEM. As shown in Figure 1a, b and Figure S1a, b, and c, the CuFeS2 QDs are arranged in the carbon frame, and all the CuFeS2 QDs are coated with carbon, which can be proved by element mapping, as shown in Figure S1e. The distributions of C, Cu, Fe, and S elements are consistent. Meanwhile, by observing the HRTEM image, the CuFeS2 QDs are well coated by the carbon frame, as shown in 31754

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Figure 2. X-ray photoelectron spectroscopy, FT-IR spectrum, and TG curve of CuFeS2 QDs @C composites: (a) Cu 2p, (b) Fe 2p, (c) S 2p, and (d) C 1s; (e) FT-IR spectrum; (f) TG curve and DTA curve.

oxidation of the copper ion on the surface. Moreover, the satellite peak of Cu 2p3/2, which resulted from the Cu2+ and is located at 942.0 eV, was not clearly observed.31,32 Therefore, we can determine that the oxidation state of Cu is +1. As shown in Figure 2b, the Fe 2p spectrum shows two contributions, 2p3/2 and 2p1/2, at 710.8 and 724.8 eV, which can be ascribed to the Fe3+.33,34 The peak located at 161.4 eV (Figure 2c) can be assigned to the S 2p3/2 of CuFeS2, and the binding energy value of 162.4 eV (Figure 2c) is consistent with the value of S 2p1/2 in CuFeS2.35 All the binding energy values of S 2p3/2 and S 2p1/2 are characteristic of CuFeS2. The binding energy value of 163.5 eV (Figure 2c) of S 2p can be contributed to the typical metal−

detailed inset image, we can observe the ordered arrangement of the atoms, and the corresponding FFT indicates the single QD is a single crystal. Combined with the lattice spacing and the angle between each other, we can obtain the specific crystal constant, as shown with red arrows. The XPS spectra were applied to distinguish the elements oxidation state, as shown in Figure 2. In Figure 2a, the Cu 2p spectrum displays two peaks. The peak located at 931.8 eV can be assigned to the Cu 2p3/2 of Cu+, and the peaks at 952.0 eV can be ascribed to the Cu 2p1/2 of Cu+, which is consistent with previous work.30 The peak located at 933.0 eV can be assigned to the existence of Cu2O, which have resulted from the 31755

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Figure 3. CV curves and charge/discharge profiles of the CuFeS2 QDs @C composites based electrode: (a) the first five CV curves recorded with a sweeping rate of 0.2 mV s−1 between 0.0050 to 3.0 V; (b) the first five charge/discharge profiles of CuFeS2 QDs @C composites at the current density of 50 mA g−1 between 0.005 to 3.0 V.

s−1 in the potential window of 0.0050 to 3.0 V. In the first anodic sweep, the CV displays four peaks at 2.10 V (a), 1.45 V (b), 1.08 V (g), and 0.55 V (h) while the first cathodic sweep exhibits three peaks at 1.56 V (d), 1.95 V (e), and 2.40 V (f). After the first cycle, by careful observation, we can see that the products exhibit consistent CV curves, indicating the uniform electrochemical reaction and outstanding cycle stability. The following four cycles show the same peaks at 2.10 V (a), 1.56 V (i), and 0.80 V (c). In the first cycle, peak (a) can be assigned to the lithium ion that reacts with CuFeS2 to form the LiCuFeS2 phase, which coexists with CuFeS2. In the following cycles, peak (a) would be assigned to the reduction of sulfur species.41 Peak (b) maybe resulted from the conversion of Cu+ to Cu0, which is similar to the study of Cu2S.42−44 Peak (g), which only occurred in the first cycle, can be attributed to the formation of SEI film, which is similar to the research of Co2SiO445 and CuCo2S4.46 Peak (h) can be ascribed to the reduction Fe3+ to Fe2+/Fe0, which is in good agreement with the iron oxides.47−49 In the cathodic sweep, the CV curves display the same peaks at 1.60 V (d), 1.96 V (e), and 2.40 V (f). Peak (d) may be caused by the reoxidation of the transition metal, at this point iron. Peak (e) can be assigned to the copper metal oxidized to Cu+. The peak (f) can be designated to the transformation between LiCuFeS2 and Li1−xCuFeS2 in the first cycle and the transformation between Li2FeS2 and FeSy in the following cycles, which is common to FeS2 study.12 In the following four anodic sweeps, peak (i) would be attributed to reduction of Cu+ and the formation of Li2FeS2. Peak (c) can be assigned to the reduction between Fe3+/Fe2+ to Fe2+/Fe0. According to the CV curves, with the increase of cycling times, the current intensity of peaks (a) and (f) become much weaker, indicating the intercalation process would fade away slowly. Figure 3b shows the discharge−charge profiles of the CuFeS2 QDs @C composites based electrode at the current density of 50 mA g−1. In the first discharge profile, we can observe four plateaus, which is well matched with the CV curves. The capacity of first discharge process is 1540 mA h g−1. In the first charge process, three plateaus can be seen and the capacity is 1079 mA h g−1. The initial Coulombic efficiency is 70.0%, and the capacity fade is caused by the irreversible process of initial intercalation and the formation of SEI film that is ahead of the

sulfur (M−S) bond of CuFeS2, which is familiar in the NiCo2S4.36 Meanwhile, combined with FT-IR spectrum analysis, the component peak would also result from the carbon−sulfur (C−S) bond. Also, the low intensity peak located at 168.8 eV (Figure 2c) would be assigned to the shakeup satellite of the sulfur ion, which is consistent with the nickel cobalt sulfur ternary compounds.36−40 The binding energy of C 1s located at 284.8 eV (Figure 2d) is well matched with the value of the CC/C−C bond. The peak at 286.0 eV would be attributed to the C−S or O−C−O bond. The peak of 289.0 eV would result from the bond of O−CO. All the values are in good agreement with the values in the NIST XPS database, which confirmed that the as-prepared samples are CuFeS2. FT-IR analysis was used to clarify the bond. Figure 2e represents the characteristic peaks of CuFeS2 QDs @C composites. The peaks at 462.8 and 595.9 cm−1 result from the stretching vibration of C−S. The peak at 1126.2 cm−1 can be assigned to the stretching vibration of C−O, and the stretching vibration of CC appears at 1616.0 cm−1. The weak peak at 3403.0 cm−1 can be attributed to the crystal water. Furthermore, TG and EA were carried out to confirm the carbon content. As shown in Figure 2f, the TG curve exhibits an obvious platform of 119.5% between 400 to 550 °C. In area I, the DTA curve presents the endothermic peak, indicating that in area I the mass change would result from the evaporation of adsorbed water. In area II, the DTA curve exhibits the exothermic peak, indicating that the mass change would be due to the burning of carbon and oxidization of CuFeS2. In area III, the DTA curve shows neither an endothermic nor exothermic peak and the TG curve demonstrates an obvious platform, which indicates the complete burning of carbon and oxidization of CuFeS2. According to the XRD pattern (Figure S3), when CuFeS2 QDs @C composites were calcined at 550 °C in the air, the products are CuSO4, 0.5Fe2O3, and SO2. By calculation, the carbon content is likely 4.48%, which is close to 3.55% that was measured by EA. Electrochemical Reaction Mechanism and Performance. Figure 3a exhibits the CV curves of the CuFeS2 QDs @C composites based electrode required at a scan rate of 0.2 mV 31756

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ACS Applied Materials & Interfaces conversion process. The discharge−charge profiles of the second and fifth cycle indicate identical features to the CV curves, with the reversible capacity of about 1050 mA h g−1. Such high reversible capacity indicates fewer side reactions and superior cycle stability. In order to confirm the electrochemical reaction mechanism of CuFeS2 QDs @C composites as anode materials for lithium ion batteries, ex situ XRD and ex situ TEM were applied to characterize the samples after a galvanostatic charge−discharge process. Upon lithiation to a potential of 1.90 V (peak (a)), the products are CuFeLi0.9S2, which can be proved by the ex situ XRD (Figure 4). Moreover, from the TEM images, as shown in

CuFeS2 + x Li → LixCuFeS2

(1)

LixCuFeS2 + (4 − x)Li → Cu + Fe + 2Li 2S

(2)

Based on the above reactions, if Cu and Fe are theoretically capable of accepting one and three electrons, the theoretical capacity of CuFeS2 is 584 mA h g−1. However, in this study, the reversible capacity can be 1050 mA h g−1, which is much higher than the theoretical capacity. Therefore, the electrochemical reaction mechanism is not just the conversion of the Cu and Fe ion; there are more reactions in the following cycling process. In the Li-FeS2 battery system, according to J. R. Dahn and Fong’s study,50 the products of the delithiation process are FeSy and element sulfur. While the products are so complicated and give rise to much controversy, the reaction processes have been deduced according to the following reactions:41,50−53 2Li 2S + Fe0 ↔ Li 2FeS2 + 2Li+ + 2e−

(3)

Li 2FeS2 ↔ Li 2 − xFeS2 + x Li+ + xe− (0.5 < x < 0.8) (4)

Li 2 − xFeS2 ↔ FeSy + (2 − y)S + (2 − x)Li+ + (2 − x)e−

(5)

Such reactions not only occur at the Li-FeS2 battery system but also exist in the Li-FeS battery system.54 Therefore, we can speculate that these reactions would take place at the oxidation of the Fe0 and Li2S system. Hence, these reactions would also exist in the Li-CuFeS2 battery system. In order to affirm our conjecture, the ex situ XRD, ex situ Raman, and ex situ TEM were employed to characterize the products in the first delithiation process. In the first delithiation process, peak (d) would be assigned to reactions 3 and 4. Upon delithiation to a potential of 1.75 V (peak (d)), the XRD pattern indicates that the products are composed of FeS and Cu. Furthermore, HRTEM images reveal the existence of cubic FeS2 and Cu particles, as shown in Figure 6a and b. Figure 6a exhibits the single copper particles, and the FFT image indicates the good crystal structure for copper, in which the indexes of the crystal face marked with red arrows (inset of Figure 6a). Figure 6b displays an HRTEM image of cubic FeS2. The interlayer distances are 0.2285, 0.1652, and 0.2415 nm, which can be ascribed to the (−2−1−1), (−3−11), and (−102) plane of cubic FeS2, as marked with red arrows in the FFT image (inset of Figure 6b). The FFT image proved the particle is FeS2 with a cubic structure. Unfortunately, we cannot directly observe the occurrence of the Li2FeS2 to prove our deduction. In consequence, peak (d) would be assigned to the oxidation of Fe0 to Fe2+/Fe3+ and the oxidation of S2−/S22−,55 and the Cu particles are not oxidized. Upon delithiation to a potential of 2.2 V (peak (e)), the peaks that displayed in the XRD pattern can be indexed to the hexagonal Cu1.92S and the peaks caused by the Cu and FeS become weaker, indicating the conversation of Cu to Cu+, which is familiar to the study of Cu2S.42 Additionally, the HRTEM image and FFT image demonstrate the formation of Cu1.92S, as shown in Figure 6c and d. Due to full charge to a potential of 3.00 V, the XRD pattern only displays the peaks that indexed to hexagonal Cu1.92S and FeS2. Hence, peak (f), which grew weaker with the increase in cycle number, can be ascribed to reaction 5. Furthermore, during the first charge process, HRTEM images can support our assertions by directly observation of hexagonal Cu1.92S (right of Figure 6e) and orthorhombic FeS2 particles

Figure 4. Ex situ XRD analysis of CuFeS2 QDs @C composites based electrode: waterfall diagram of recorded XRD patterns at different potentials, D for discharge process and C for charge process of first cycle.

Figure 5a and b, the particles coated with a carbon layer can be observed clearly. Furthermore, from the HRTEM image and corresponding FFT image (Figure 5c), the interlayer distances of 0.3145 and 0.3221 nm can be assigned to the (002) and (100) plane of hexagonal CuFeLi0.9S2, indicating that the products after lithiation are hexagonal CuFeLi0.9S2. Upon lithiation to a potential of 1.2 V (peak (b)), the peaks at 43.5° and 53.6° can be seen from the XRD pattern distinctly, indicating the existence of copper. Moreover, the TEM image exhibits typical nanoparticles anchored in the carbon frame (Figure 5d) and the HRTEM image displays a clear basal space of 0.2043 nm, which is in good agreement with the (111) lattice fringe of the crystalline copper. The FFT image (Figure 5f) of area 1 exhibits two basal spaces of 0.2054 and 0.1746 nm, which well match the (111) and (200) lattice fringes of copper, respectively. The FFT image (Figure 5f) of area 2 indicates the existence of hexagonal FeS. When the products underwent full lithiation to a potential of 0.005 V, the XRD pattern indicates the coexistence of hexagonal FeS, Fe, and Cu. As shown in Figure 5h, the distance of SEI film is just 2−3 nm. The HRTEM image and corresponding FFT image (Figure 5i) demonstrate the good crystallization of hexagonal FeS. The interlayer distances of 0.1907, 0.1829, and 0.1866 nm can be attributed to the (2−31), (−2−31), and (−400) plane of hexagonal FeS, and the indexes of crystal face are marked by the red arrows in the FFT image, demonstrating the conversion between Fe3+ and Fe2+/Fe0. Therefore, the electrochemical reaction mechanism can be shown as follows, which is consistent with previous research.27 31757

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Figure 5. Ex situ TEM images of CuFeS2 QDs @C composites based electrodes. (a and b) TEM images of the samples discharged to 1.90 V; (c) HRTEM image and corresponding FFT image. (d and e) TEM images of the samples discharged to 1.20 V; (f) corresponding FFT images in different areas. (g and h) TEM images of the samples discharged to 0.0050 V; (i) HRTEM image and corresponding FFT image of (h).

By analysis of the first lithiation−delithiation process of the Li-CuFeS2 battery system and using the electrochemical reaction mechanism of FeS and FeS2 for reference, we can speculate the electrochemical reaction mechanism. In the first discharge process, the reaction mechanism follows reactions 1 and 2. However, after the first delithiation process, the reaction mechanism is composed of reactions 3, 4, 5, and 6.

(left of Figure 6e). The phases of Cu 1.92 S and FeS 2 nanoparticles were confirmed by the corresponding FFT images. In order to confirm the existence of Li2FeS2 and the product of FeSy, Raman spectra were applied to analyze the composites during the first delithation process, as shown in Figure S5. Upon delithiation to 1.75 and 2.2 V, the Raman spectra exhibit uniform five peaks. The peaks located at 154 and 601 cm−1 can be ascribed to the existence of Fe2O3,56 which may result from the oxidation of Fe ion on the surface. The peak located at 295 cm−1 can be attributed to the FeS,57 and the peak at 396 cm−1 can be assigned to the existence of Li2FeS2, which well match the infrared study of Li2FeS2.58 The peaks located at 193, 1092, and 1598 cm−1 are due to the vibration of the carbon bond. Upon delithiation to 3.0 V, the peak located at 396 cm−1 disappeared and the peak located at 227 cm−1 can be assigned to Fe3S4,59 indicating the transformation between Li2FeS2 and Fe3S4. This can demonstrate the mechanism that we presented and confirm the product is Fe3S4.

2Cu + Li 2S ↔ Cu 2S + 2Li+ + 2e−

(6)

As the electrochemical reaction mechanism proposes above, the Li-CuFeS2 system turned into a hybrid of a Li−S battery and lithium−copper iron sulfide battery after the first delithiation process. Such a model can explain the high capacity of CuFeS2, which far exceeds the theoretical capacity that was calculated based on the conversion reaction of the transition metal. In spite of the achievement of higher capacity, the hybrid system of a Li−S battery and lithium ion battery faces several issues. First, the lithium polysulfides dissolution and reaction issues have to be overcome, which is similar to the case of the Li−S 31758

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Figure 6. Ex situ TEM images of CuFeS2 QDs @C composites based electrodes. (a and b) Typical TEM images of CuFeS2 QDs @C composites based electrodes when charged to 1.75 V; (a) representative TEM image of Cu particles, and insets in (a) are the detail of Cu particle and corresponding FFT image. (b) TEM image of cubic FeS2 particle, and insets in (b) are detail of cubic FeS2 particle and corresponding FFT image. (c and d) Representative TEM images when charged to 2.2 V; (d) HRTEM image of Cu1.95S particle, and insets are detail and corresponding FFT image. (e) TEM image when charged to 3.0 V; left inset is a detailed and corresponding FFT image of FeS2 particle, and the right inset is a detailed and corresponding FFT image of Cu2S particle.

battery. Then, the large volume change and Fe0 particles aggregation during the transformation of Fe0 ↔ FeSy have to be reduced. Last, the sluggish reaction kinetics that hindered the application of such a battery system should be improved. All these issues will result in a large capacity fade, so the cycling stability has to be improved. To further explore the electrochemical performance, the cycling stability was tested by the galvanostatic charge− discharge process. Upon charge−discharge with a current density of 0.2 A g−1, the initial discharge capacity is 1174 mA h g−1, and the charge capacity of the first charge process is 900 mA h g−1. Therefore, the initial Coulombic efficiency is 76.7%.

After cycling for 250 cycles, the discharge and charge capacity were 825.6 and 825.5 mA h g−1, respectively. And the capacity retention can be 91.7%, indicating almost no capacity decay. Moreover, except for the first cycle, the Coulombic efficiency is close to 100% in the following 249 cycles, as shown in Figure 7a. Figure 7c exhibits the stable cycling performance of the CuFeS2 QDs @C composites based electrode at 0.5 A g−1. The sample displays remarkable cycling stability that 92.5% of the capacity can be retained after 700 cycles, compared with the second cycle. Rate is an important factor to evaluate the electrochemical performance. The CuFeS2 QDs @C composites based electrode was tested utilizing varying current 31759

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Figure 7. Electrochemical performances of the CuFeS2 QDs @C composites based electrodes: (a and c) cycling performance; (a) cycling performance and Coulombic efficiency of CuFeS2 QDs @C composites based electrodes at 0.2 A g−1 between 0.005 to 3.0 V; (c) long cycling performance at 0.5 A g−1 between 0.005 to 3.0 V; (b) rate performance of CuFeS2 QDs @C composites based electrodes.

reduce the size of the material, which can provide more reaction sites and shorten the diffusion distance of the lithium ion. In this study, the sizes of CuFeS2 QDs are only 5−10 nm. Such a small size can enhance the reaction kinetics, and the carbon frame is beneficial to the transfer of electrons.

density, as shown in Figure 7b. The reversible capacities for different rates were 1150 mA h g−1 (0.05 A g−1), 1060 mA h g−1 (0.1 A g−1), 960 mA h g−1 (0.2 A g−1), 850 mA h g−1 (0.4 A g−1), 700 mA h g−1 (0.8 A g−1), 450 mA h g−1 (1.6 A g−1), 250 mA h g−1 (3.2 A g−1), and 110 mA h g−1 (6.4 A g−1). When the current reversed to 0.1 A g−1, the reversible capacity can recover to 900 mA h g−1, which is almost 90% of the reversible capacity at 0.1A g−1. Even with discharge−charge at 1.6 A g−1, the reversible capacity of 450 mA h g−1 can be achieved, which still transcends the theoretical capacity of graphite. Thus, the CuFeS2 QDs @C composites based electrodes display a long cycling life at varying current density and superior rate performance. Such outstanding electrochemical performance can be attributed to the unique structure of CuFeS2 QDs anchored in the carbon frame. By taking the issues discussed above into consideration, this unique structure can mitigate the capacity fade. First, as for the lithium polysulfides dissolution and reaction issues, an effective approch includes coating the sulfur with carbon or others, which is familiar to the study of Li−S batteries. On the one hand, in this structure, the sulfur generated by the electrochemical reaction is still encapsulated in the carbon frame, which can prevent the polysulfide dissolution in the electrolyte. On the other hand, the substoichiometric metal sulfides, which exhibit Lewis acid characteristics, can trap polysulfide anions.60 For the large volume change and Fe0 nanoparticles agglomeration, the carbon coated layer can be used as a buffer layer and hinder the movement of Fe0 nanoparticles, which can slow down the pulverization caused by large volume change and reduce the agglomeration of Fe0 nanoparticles. The homogeneous distributions of C, S, Cu, and Fe elements confirmed our analysis, as shown in Figure S4. Finally, with respect to the sluggish reaction kinetics, the state-of-the-art method is to

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CONCLUSION In summary, we have introduced a simple and rapid preparation method to synthesize pure CuFeS2 QDs @C composites. The as-prepared samples display a unique structure of CuFeS2 QDs encapsulated in the carbon frame, and the sizes of CuFeS2 QDs are only 5−10 nm. When used as lithium ion battery anode materials, the CuFeS2 QDs @C composites exhibit superior electrochemical performance in Li storage. Even if under a current density of 0.5 A g−1, the CuFeS2 QDs @C composites are able to maintain a high reversible capacity of 760 mA h g−1 for 700 cycles. When tested with varying current density, the CuFeS2 QDs @C composites can retain the reversible capacity of 450 and 250 mA h g−1 at the current density of 1.6 and 3.2 A g−1, demonstrating the outstanding rate performance. By ex situ XRD, Raman, and TEM characterization and analysis, one possible electrochemical reaction mechanism has been proposed to process the high reversible capacity that exceeds the theoretically based value in the conversion reaction of transition metals Cu and Fe. The results indicate the LiCuFeS2 system is supposed to be a hybrid of a lithium−copper iron sulfide battery and lithium−sulfur battery. The direct observation of FeS2 and Cu1.92S by HRTEM supports our deduction. Moreover, the existence of Li2FeS2 and Fe3S4 detected by Raman spectra further proved our deduction. However, evidence of sulfur reduction is not directly observed. Therefore, more characterization techniques, such as in situ XPS, in situ XRD, should be applied to confirm the evidence of sulfur reduction and productions during the first delithiation 31760

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process to clarify the electrochemical mechanism of such a hybrid of a lithium−copper iron sulfide battery and lithium− sulfur battery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06685. SEM, TEM images and elements mapping of CuFeS2 QDs @C composites, XRD pattern of CuFeS2 QDs @C composites when calcined in the air at 550 °C, elements mapping and Raman spectra during the first lithiation− delithiation process (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-20-8411-3901. E-mail: [email protected]. ORCID

Huawei Song: 0000-0001-9091-2297 Chengxin Wang: 0000-0001-8355-6431 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 (U1401241). REFERENCES

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