Molecular-Level CuS@S Hybrid Nanosheets Constructed by Mineral

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Energy, Environmental, and Catalysis Applications

Molecular-Level CuS@S Hybrid Nanosheets Constructed by Mineral Chemistry for Energy Storage Systems Sijie Li, Peng Ge, Feng Jiang, Christopher W. Foster, Craig E Banks, Wei Xu, Yu Zhang, Wanwan Hong, Chenyang Zhang, Wei Sun, Jiugang Hu, Hongshuai Hou, Yuehua Hu, and Xiaobo Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16428 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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ACS Applied Materials & Interfaces

Molecular-Level CuS@S Hybrid Nanosheets Constructed by Mineral Chemistry for Energy Storage Systems Sijie Li1, Peng Ge1, Feng Jiang2, Christopher W. Foster3, Craig E. Banks3, Wei Xu1, Yu Zhang1, Wanwan Hong1, Chenyang Zhang1, Wei Sun2*, Jiugang Hu1, Hongshuai Hou1, Yuehua Hu2* and Xiaobo Ji1* 1State

Key Laboratory of Powder Metallurgy, College of Chemistry and Chemical

Engineering, Central South University, Changsha, 410083, China.

2School

of Minerals Processing and Bioengineering, Central South University,

Changsha, 410083, China.

3Faculty

of Science and Engineering, Manchester Metropolitan University,

Manchester, M1 5GD, UK

ABSTRACT: The transition-metal sulfide, CuS, is deemed a promising material for energy storage, mainly derived from its good chemisorption and conductivity, whilst serious capacity fading limits its advancement within reversible lithium storage. Learning from the gold extraction method utilizing a lime-sulfur-synthetic-solution, CuS@S hybrid utilizing CaSx as both the sulfur resource and reductant-oxidant is prepared, which is an efficient approach to apply the metallurgy for the preparation of electrode materials. Regulating the amount of CuCl2, the CuS@S is induced to reach a 1

ACS Paragon Plus Environment

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molecular-level hybrid. When utilized as anode within lithium-ion battery, it presents specific capacity of 514.4 mAh g−1 at 0.1 A g-1 over 200 cycles. Supported by the analyses of pseudo-capacitive behaviors, it is confirmed that the CuS matrix with the suitable content of auxiliary sulfur could improve the durability of the CuS-based anode. Expanding the wider application within lithium-sulfur battery, the synchronous growth of CuS@S exhibits stronger chemisorption with polysulfides than the mechanical mixture of CuS and S. A suite of in-situ electrochemical impedance spectroscopy further investigates the stable resistances of the CuS@S within the charge/discharge process, corresponding to the reversible structure evolution. This systematic work may provide a practical fabricating route of metal sulfides for scalable energy storage applications.

KEYWORDS: CuS@S hybrid, molecular-level dispersion, mineral chemistry, energy storage, electrochemistry

1. INTRODUCTION Over the recent decade, advanced lithium-storage technologies have significantly reduced the amount of pollution created from fossil fuels and energy crisis.1,2 As the critical members of energy-storage systems, the traditional electrode materials are regularly designed, accompanying with the high-energy consumption and contamination. To pursue an improved greener environment, the applicable materials and fabricated manners should be effectively explored. Recently, the typical transitionmetal chalcogenides (TMDs), containing CoS, NiS, MoS2, have attracted numerous 2


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attention owing to their promising energy storage prospects.3-10 Among them, copper sulfide (CuS) possesses a fascinating electrochemical performances with a high theoretical capacity (560 mAh g-1) and good electrical conductivity (10-3 S cm-1).11-14 Nonetheless, serious capacity fading upon the multiple cycling of the CuS-based materials, mainly resulted from the destruction of structure and the “shuttling effect” of reactive polysulfide intermediates,15 inevitably restricts its’ extensive application within energy storage systems. As shown in previous reports16-19, the morphology-controlled engineering and building of a multi-functional composite of the targeted samples have been recognized as effective strategies to solve the aforementioned constraints of copper sulfide. The CuS nanorods with a grain size of sub-10 nm with more active reaction sites gave rise to a shortened lithium-ions pathway of and effective charge transport, which has been verified to greatly improve the capacity retention and rate capability.12 Moreover, a number of novel nanostructures have been also constructed, such as nanoflakes20, nanosheets17, nanowires15 and sphere-like hierarchical structures

18,

typically an

improvement within the lithium storage behavior is mainly attributed to an increased electrolyte wetting and shortened Li-ion diffusion distances. Remarkably, hierarchical nanostructures have earned enormous attention within energy storage devices, as their specific structural traits suppress self-aggregation and maintain structure stability.11,18 Thusly, the fabrication of a great hierarchical nanostructured CuS-based material is therefore worthy to design for the enhanced lithium storage performances. So far almost all of these CuS-based materials are prepared through the prevalent synthesis routes 3



reported within previous literatures, such as hydrothermal/solvothermal synthesis20,21, calcination22, microwave growth17, chemical vapor deposition23 and template-directed methods24. Nevertheless, the abovementioned manners are generally facing with a variety of undesirable issues such as low yield, high cost and energy-extensive consumption processes under high temperature and pressure conditions3,25. Moreover, in most cases, two or more assorted methods are involved within these synthesis routes, which are quite complex for the control of morphologies. To realize the future promise of green chemistry, it is particularly important and challengeable to develop a fast and reliable approach without utilizing external assistance to obtain novel CuS-based hierarchical nanostructures with a controllable morphology. It should be noted that sulfur has been reported as the promised energy storage materials for lithium storage, due to its high theoretical Li-storage capacity of 1675 mAh g-1.1,26-28 However, the intermediate polysulfides are gradually generated with varying chain lengths within the charge-discharge process, which are soluble and further react with the lithium anode to form Li2S or Li2S2 at the surface of electrode, resulting in the self-discharge and capacity degradation upon cycling.29,30 At this point, the relevant research efforts have been put into practice for pursuing the durability of electrodes by filling sulfur into conductive additives or carbon skeletons31-34, inserting polysulfide-blocking interlayers35, modifying the separator and optimizing electrode architecture36,37. In particular, the in-situ impregnation of sulfur into the multifunctional matrix of the metal sulfide with electroactive and conductive properties could improve the corresponding electrochemical properties, which has been 4


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recognized as an effective strategy within the construction of the metal sulfide and sulfur16,38. However, the complex energy-consuming processes above could hardly suit the scaled-up practical productions. Targeting the aforementioned issues, the hierarchical three-dimensional CuS@S hybrid is designed through a one-step self-assembly, under ambient temperature and pressure for the first time. The growth of CuS together with in-situ impregnating sulfur are simultaneously manipulated by employing calcium polysulfides (CaSx) as the sulfur resource and reductant-oxidant, which expectedly brings about the intimate, homogenous and molecular-level dispersed hybrid for promoted electrochemical performances. The effects of tuned CuCl2 dosage upon the structure and morphology are investigated, and the formation mechanism of CuS@S hierarchical threedimensional nanostructure is proposed from lime-sulfur-synthetic-solution (LSSS) method. The lithium-ion storage performances of CuS@S with the different content of CuS are investigated systematically, in which the CuS@S sample exhibited a remarkable capability and high-rate properties within LIBs. At the same time, the insitu formation of the CuS@S further displays the stronger chemical adsorptions with polysulfides than the mixture of CuS and S within the Li-S battery. It is believed that this elaborate work would be highly informative towards the engineering morphology and nanostructure of the copper sulfide and sulfur to facilitate Li-storage performances, and offer a pragmatic preparation methodology of electrode materials for energy storage systems.

5



2. EXPERIMENTAL SECTION 2.1 Materials preparation Briefly, 3 g CuCl2 (Adamas Reagent Co., Ltd) was first dissolved within 50 mL deionized water. Then, the above-obtained solution was added dropwise into 50 mL CaSx (Lianyungang Lanxing Industrial Technology Co., Ltd) with magnetic stirring. After complete mixing, the obtained solution was further stirred at ambient temperature for 2h. Finally, the resulted products were collected by filtration and washed with deionized water and ethanol, and finally dried at 60 oC overnight. The as-prepared sample was named CuS@S-3. Utilizing the same conditions, the weight of CuCl2 changed to 1g, 2g and 4g, and the products were named CuS@S-1, CuS@S-2 and CuS@S-4. For comparison, 1g CuS@S-1/2/3/4 was dissolved in 200 mL carbon disulfide with magnetic stirring for 30 min. Then, the remnant production was filtered and washed by deionized water and dried at 60 oC overnight (donated as CuS -1/2/3/4). 2.2 Materials Characterization The crystalline structures were explored by X-ray powder diffraction (XRD, monochromatic Cu Kα radiation, the scan rate of 10 oC min-1) and Raman spectroscopy (Renishaw InVia, UK). The surface valence state of the target sample was probed by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063). Field-emission scanning electron microscopy (FESEM, using JSM-7600F, JEOL) together with energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and highresolution transmission electron microscope (HRTEM) equipped with selected area 6


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electron diffraction (SAED, using JEM-2100F at 200 kV, JEOL) were utilized in the characterization of the as-prepared samples’ nanostructure and morphology. Brunauere-Emmette-Teller (BET) surface area and pore size distribution were investigated

from

N2

adsorption-desorption

isotherms

(ASAP2460).

Thermogravimetric analyses (TGA, NETZSCH STA449F3) were operated on a Diamond TG thermos-analyzer in Ar atmosphere, determining the content of sulfur in CuS@S. High-performance liquid chromatography (HPLC) analyses were obtained by an Agilent 1260 HPLC system with a diode array detector at 254 nm; separation was conducted on a Waters Symmetry C18 column (150 mm × 3.9 mm i.d., 5 lm, Waters, MA, USA); Isocratic mobile phase was consisted of methanol and water (97:3, v/v) with a flow rate of 1.0 mL/min. 2.3 Electrochemical measurements The CR2016-type cells were assembled in a MBraun glovebox fulfilled by highly pure Ar atomsphere (O2 and H2O levels＜0.5 ppm). Metallic lithium was used as the counter/reference electrode and glass fiber utilized as separator. The working electrodes were prepared by mixing 70 wt% as-prepared active materials, 15 wt% poly (vinylidene fluoride) (PVDF) as binder and 15 wt% conductive super P with N-methylpyrrolidone (NMP) as the solvent. The slurry was then uniformly painted on a copper foil and subsequently dried for 10 h at 80 oC in a vacuum oven. The mass loading of as-prepared active material about 1.0-1.5 mg cm-1. The electrolytes were used: (1) 1 mol L-1 LiClO4 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v-v); (2) 1 mol L-1 lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in a mixture 7



of 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1:1, v-v) with 5 wt% of LiNO3 (Guotai-Huarong New Chemical Materials Co., Ltd.). Galvanostatic cycling tests of the cells were performed using Land battery (CT2011) and Arbin battery cycler (BT2000) in the voltage range of 0.01-3.0 and 1.5-3.0 V (vs. Li+/Li). Utilized MULTI AUTOLAB M204 instruments for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies were tested at various conditions. 3. RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns; (b) TGA curves; (c) Raman spectra; XPS spectra: (d) Cu 2p and (e) S 2p of CuS@S; (f) HPLC curves of CS2, CS2-S and CS2-CuS@S. Learning from the gold extraction with lime-sulfur-synthetic-solution (LSSS) method, 8


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multi-functional hierarchical CuS@S nanosheets are successfully prepared from a red brown solution of CaSx and CuCl2 particles. Note that the formation mechanism is the combination of the disproportionation reaction of Sx2- and the catalysis of Cu2+ in an open architecture environment. As shown in chemical equations and a general reaction (1-4), in the process of stirring, the O2 and CO2 can be dissolved in the CaSx liquid to increase the acidity of the system, thus strengthening the oxidation and disproportionation of Sx2-. At the same time, Cu2+ further catalyzes the formation of S2and in-situ produces CuS. Such low-cost raw materials and high-yield CuS@S products are shown in an optical image (Figure S1). Sx2- → (x-1) S0 + S2-

(1)

Sx2- + 2 H+ → (x-1) S0 + H2S

(2)

Cu2+ + S2- → CuS

(3)

Cu2+ + Sx2-+ 2 H+ → CuS + (x-2) S0 + H2S

(General reaction)

(4)

Figure 1a gives the XRD patterns of the as-prepared CuS@S-1/2/3/4. Diffraction peaks of four samples are well indexed to the standard card of sulfur (JCPDS NO. 080247), and the peak at ~ 48.1o is assigned to (110) diffraction of CuS (JCPDS NO. 011281)39. Furthermore, the existence and composition of sulfur component are confirmed by HPLC analyses. As illustrated in Figure 1f, the retention time of CS2 extract of CuS@S is 11.7 min, which corresponds to the characteristic peak of sulfur. The additional XRD patterns of CuS-1/2/3/4 products with the washing of CS2 are shown in Figure S2. As a reference, the retention time of S (soluble in CS2) is also 11.7 min and retains the same peak height, confirming that the S component of CuS@S 9



exists in the form of elemental sulfur. Additionally, there is also a small bulge at 11.7 min of the pure CS2 sample, since CS2 itself contained trace amounts of sulfur. The content of sulfur is evaluated by TGA in Figure 1b. There are 86.9, 83.3, 70.8 and 62.7 wt% weight loss of CuS@S-1/2/3/4 respectively, which is closely related to the oxidation and sublimation of sulfur. The final oxidation product at 800 oC in Ar atmosphere is Cu1.81S, corresponding with the XRD pattern in Figure S3. Through the equation: sulfur% = 1- 1.81x ×MCuS / MCu1.81S, where x is the weight loss of CuS@S1/2/3/4, the sulfur content of CuS@S-1/2/3/4 sample are about 84.6, 80.4, 65.7 and 56.2 wt%, respectively. This resultant reduction in sulfur content further verifies the reaction mechanism mentioned above, that is, the increase of Cu2+ will catalyze the generation of CuS. The structural features of CuS@S are investigated by Raman spectra. As shown in Figure 1c, the strong Raman peaks at 151, 217 and 470 cm-1 are observed for the CuS@S-1 sample, which is identified as the sulfur.40 From the curves of CuS@S-2/3/4, the CuS phase is distinguished by the band at 467 cm-141, and the relative peaks of CuS@S-1 are not found, mainly ascribed to that the characteristics of CuS Raman peak at 467 cm-1 is overlapped with the sharp sulfur peak at 470 cm-1. Moreover, the intension and width of peaks are gradually getting stronger and wider from CuS@S-2, CuS@S-3 to CuS@S-4, which is well consistent with the analyses of TGA, revealing that the increased CuS content e is produced upon the increase of the CuCl2 raw material. The disappearance of sulfur peak will be explained in detail by the following SEM images. XPS measurements is employed to obtain the information on the surface 10


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elements and valence states of the as-prepared CuS@S. As shown in Figure S4, the signals of Cu, S and O elements are displayed in survey spectrum.11,14,42 The existence of O element may derive from the air, which is adsorbed on the surface of sample. For the Cu 2p peaks (Figure 1d), the high-resolution XPS spectrum of the Cu 2p3/2 peaks are centered at 932.7 and 933.8 eV through a Gaussian fitting method, and the peaks at 952.5 and 953.9 eV are assigned to Cu 2p1/2. The S 2p spectra (Figure 1e) depicts that the peaks at 162.5 and 163.6 eV are associated with S2p3/2, the peaks at 163.1 and 164.3 eV are related to S2p1/2, and the peaks at 164.5 and 165.7 eV are fitted into elemental sulfur (S0), which is in good accordance with the different state of sulfur in as-prepared CuS@S.

‘ Figure 2. SEM images, energy dispersive spectrometer (EDS) and N2 adsorption and 11



desorption isotherm and pore size distribution curves of (A1-A4) CuS@S-1, (B1-B4) CuS@S-2, (C1-C4) CuS@S-3 and (D1-D4) CuS@S-4. The effects for tuned CuCl2 dosage on the structure and morphology of the asprepared CuS@S samples are systematically investigated, which is revealed by SEM in Figure 2. The images of CuS@S-1 with different magnification are shown in Figure 2(A1-A2), which are uniform and monodispersed bulks with the diameters in the range of 300-500 nm. With the minimum amount of reactant CuCl2, the main composite of products are sulfur particles and a few CuS nanosheets encapsulated in sulfur bulks. By gradually increasing the amount of CuCl2 salt (Figure 2(B1-B2)), it is found that some CuS as the bridge are well connected with sulfur particles to form micron size of plates for CuS@S-2, and much more CuS are generated at the surface to construct hierarchical three-dimensional nanosheets. For the CuS@S-3 exhibited in Figure 2(C1-C2), such homogenous and molecular-level dispersed hybrid is formed by fully saturating sulfur through the CuS frame. Note that the agglomerate status of nanosheets start to appear in Figure 2(D1-D2) for CuS@S-4 sample when the quantity of CuCl2 rises to the supersaturated state. These thought-provoking morphological evolutions demonstrate that the hierarchical three-dimensional CuS@S structures are molded through controlling the CuCl2 dosage, perhaps leading to the disappearance of sulfur peak in Raman analyses above. As shown in Figure 2(A3-D3), the At % ratio of Cu is altered in the order of CuS@S-1 < CuS@S-2 < CuS@S-3 < CuS@S-4, confirming the increasing growth of CuS with the augmentation of CuCl2 salt. The transmutation of the pore distribution and specific surface area are investigated in Figure 2(A4-D4). It is 12


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clear that CuS@S-1/2/3/4 are characteristically mesoporous materials with a typical diameter of 10-20 nm. Moreover, the pore volume is gradually increased from CuS@S1 to CuS@S-4 as in Figure S5a. Similarly, the specific surface area displays 0.32 m2 g-1 for CuS@S-1, 12.2 m2 g-1 for CuS@S-2, 25.7 m2 g-1 for CuS@S-3 and 68.0 m2 g-1 for CuS@S-4, which are reasonable for the SEM analyses of structural evolution from 1D sulfur bulks (CuS@S-1) to 3D homogenous CuS and S hybrid nanosheets (CuS@S4). The increased surface area and pore volume could improve electrolyte wetting together with buffered spaces during the lithium-ions insertion/extraction processes. Notably, CuS@S-3 is the most uniform of the hierarchical 3D nanostructures with large pore distribution as well as a higher specific surface area when compared to the other three samples. Through the above analyses, it can be seen that the appropriate catalysis of Cu2+ has an important influence upon the morphology of the as-formed product.

13



Figure 3. SEM images, N2 adsorption and desorption isotherm and pore size distribution curves of the (A1-A3) CuS-1, (B1-B3) CuS-2, (C1-C3) CuS-3 and (D1D3) CuS-4; (E) energy dispersive spectrometer (EDS) of CuS. Stimulated by this fascinating advancement of the as-prepared CuS@S architectures, the SEM images of the CuS-1/2/3/4 samples and their corresponding products of CuS@S-1/2/3/4 washed with CS2, are further investigated in Figure 3. Moreover, 14


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Figure 3E shows that the At % ratio of Cu : S is about 1 : 1, further confirming the successful dissolution and removal of S. It is obvious that the morphology of CuS-1 is a chaotic stack without the formation of an orderly 2D lamellar structure, due to the presence of a few shapeless CuS sheets that are generated by a very small amount of the CuCl2 reactant, therefore it is difficult to effectively self-assemble into micro-plates. From CuS-2 to CuS-4, they all keep an intact plate shape composed of hierarchical nanosheets, with the thickness of the cross section increasing in the following sequence： CuS-2 (0.5 μm) < CuS-3 (0.9 μm) < CuS-4 (1.1 μm), which is well consistent with the analyses of TGA and EDS, derived from the increase of the CuS products. With the analysis of BET, it is found that the pore volume and specific surface area of CuS samples also gradually increase from samples CuS-2 to CuS-4, which fits well with the dilatant plates. This regular phenomenon reveals that the adjusting of the CuCl2 reactant dosage could control the content of CuS, and also manipulate the dimension of the single CuS@S nanosheet to form more fluffy 3D structure, which could provide a greater pore volume and higher specific surface area. Interestingly, all of the pore volume and specific surface area within the CuS samples are larger than the corresponding CuS@S products (Figure S5), with the pore size distribution of CuS2/3/4 samples exhibiting an extra sharp peak of 3-4 nm (Figure 3(B3-D3)). Convincingly demonstrating that the dissolution and removal of sulfur within the mesopores of the CuS, when the pore size is of 3-4 nm for CuS-2/3/4 nanosheets. Based on the comprehensive exploration of SEM and BET for CuS@S and CuS samples, the CuS@S-3 possesses the most uniform structural dispersion of hierarchical nanosheets, 15



and the hybrid of CuS and S with in-situ molecular-level dispersion within 5 nm are accompanied by the appropriate size and specific surface area.

Figure 4. (A1-A2) SEM images of the CuS@S-3 and (A3) the corresponding elemental mapping; TEM, HRTEM images and the selected area diffraction pattern (SAED) of the (B1- B3, D) CuS@S-3, (C1-C3, E) CuS-3. It is clear that the features of the CuS@S-3 with homogenous and molecular-level dispersity are stable as exhibited in Figure 4. The interconnected leaf-like nanosheet16


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assembled hierarchical 3D plates are investigated by SEM through different angles in Figure 4A. From the element mapping images of CuS@S-3 (Figure 4A3), the uniform distributions of two Cu and S elements strongly supports the molecular-level hybrid between CuS and S within the CuS@S-3 sample. TEM images of the CuS@S-3 further show that the nanosheets are interlocked with horizontal and vertical orientation to create the plate object without any excess sulfur. In the HRTEM (Figure 4B-2), the vertical CuS@S-3 nanoplates with thickness of 6.5-7.0 nm are observed. The SAED pattern of CuS@S-3 reveals six concentric rings corresponding to (101), (103), (006), (110), (114) and (116) diffraction planes, which coincides well with the XRD pattern. The lattice-fringe distances of 0.33 and 0.19 nm are related to the (101) and (110) plane respectively as shown in Figure 4D. Compared with the number of surficial defects for CuS@S-3 sample (Figure 4D), it is expected that the increasing surficial defects of CuS-3 (Figure 4E) are caused by the elimination of sulfur. Furthermore, it should be noted, that the morphology of CuS-3 (Figure 4C) is almost consistent with that of CuS@S-3, which further testifies that the sulfur is immersed within the CuS skeleton at a molecular level in the CuS@S-3 sample. Table 1. The comparison of CuS@S-3 and other copper sulfides as LIBs anodes. Materials

morphology

Voltage (V)

Current density (mA g-1)

Cycles (n)

Capacity (mAh g-1)

Ref

CuS

Hybrid network

1.0-3.0

112

100

468.3

22

CuS

nanorods

0.01-3.0

112

250

390

12

CuS-CNTs

nanosheets

0.01-3.0

200

180

477

43

CuS/graphene

hierarchical microparticles

0-3.0

50

100

568

11

17



CuS@S-3

hierarchical 3D nanosheets

0.01-3.0

100

200

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514.4

This work

Figure 5. (a) Cycling performance of the CuS@S-3 and CuS-3 electrodes at a current density of 0.1 A g-1 for LIBs; (b) Galvanostatic discharge-charge profiles of CuS@S-3 at 0.1 A g-1; (c) Rate performance of the CuS@S-3 and CuS-3 electrodes at various current densities; (d) Long-term cycling performance and coulombic efficiency of the CuS@S-3 and CuS-3 electrodes for 1 000 cycles at 1.0 A g-1. Generated by the growth of the morphological features, their effects on the electrochemical performances as LIBs anodes are investigated and presented within 18


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Figure S6. At a current density of 0.5 A g-1, CuS@S-1/2/3/4 deliver the initial charge capacities of 1142.8, 974.3, 1292.9 and 826.1 mAh g-1, with a coulombic efficiency (CE) of 63.6%, 71.7%, 76.9% and 72.6%, respectively. However, the capacity of CuS@S-1 is significantly fades to 91.7 mAh g-1 over 10 cycles, originating from that the lowest content of CuS, which is not stabilized within the initial high capacity of sulfur within the CuS@S-1 electrode. With the increase of CuS, the reversible capacities of CuS@S-2/3/4 still retain 220.3, 465.1 and 344.2 mAh g-1 after 100 cycles. It is worth noting that the capacity retention of CuS@S is effectively improved due to the stable CuS matrix, allowing access to the high theoretical capacity (1675 mAh g-1) of the sulfur. While excessive CuS would reduce the content of available sulfur, resulting in a low capacity contribution of the sulfur. As shown in SEM and TEM images, and after precisely calculated by TG analyses, the CuS@S-3 electrode possesses an optimal ratio of CuS and S with a homogenous and molecular-level nanosheet-assembled hierarchical 3D structure, which may be responsible for its superior electrochemical capability. To further explore the role of sulfur in the CuS@S3 electrode and its effect on electrochemical performance, Figure 5a exhibits the cycling and rate stability of CuS@S-3 compared with CuS-3. After the removal of S, the CuS3 electrode shows an initial capacity of 513.2 mAh g-1 at 0.1 A g-1 and a serious capacity decay after 150 cycles. While CuS@S-3 electrode delivers a higher initial capacity of 684.7 mAh g-1, presenting remarkable repeatability with a reversible capacity of 514.4 mAh g-1 over 200 cycles, which is far superior than previously reported copper sulfides (as LIBs anodes) presented in Table 1. Correspondingly, the total energy density 19



delivered by CuS@S-3 electrode after 200 loops is 617.3 Wh Kg-1. The corresponding galvanostatic discharge and charge curves of CuS@S-3 electrode between 0.01 and 3.0 V are displayed in Figure 5b, which is quite consistent with the potential position of the CV redox peaks (Figure S7). Two sloping plateaus at 2.05-1.93 V and 1.69-1.54 V are observed in the discharge process. The plateau at 2.05-1.93 V can be attributed to the formation of LixCuS by the intercalation of lithium ions into the CuS lattice and the transformation of sulfur to soluble high-order lithium polysufides (Li2Sx, 4 ≤ x < 8). The plateau at 1.69-1.54 V corresponds to the generation of Cu and Li2S from the LixCuS and the conversion of soluble polysufides to insoluble low-order Li2S2 or Li2S.11 In the charge process, there are two plateaus at about 1.91 and 2.35 V during the first charge process, which coincides well with previous reported studies.15 The effects of sulfur upon the rate performances over a rate range of 0.1-2.0 A g-1 can be observed for CuS@S-3 and CuS-3 electrodes, in Figure 5c. Compared with the CuS3 electrode, the rate performance of the CuS@S-3 electrode delivers charge capacities of 862.5, 751.5, 663.9, 548.2 and 429.2 mAh g-1 at 0.1, 0.2, 0.3, 0.5 and 1.0 A g-1, respectively. Even at a higher current density of 2.0 A g-1, the discharge capacities of 327.6 mAh g-1 are obtained. When the current density returns back to 0.1 A g-1 after 55 cycles, it is expected that the capacity is recovered back to 629.6 mAh g-1, exhibiting a remarkable rate performance. However, at the very high current density of 2.0 A g-1, there is almost no capacity left in CuS-3 electrode. Therefore, long-term cycle stabilities of CuS@S-3 and CuS-3 electrodes at a high current density of 1.0 A g-1 is confirmed in Figure 5d. The CuS@S-3 electrode still exhibits a reversible capacity about 334.8 20


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mAh g-1 over 1,000 charge/discharge cycles with average coulombic efficiency of 100%, implying a highly reversible process of the lithium ions insertion/extraction. Considering the lack of the contribution of sulfur, the specific capacity of CuS-3 is significantly lower than CuS@S-3 electrode. As a result, the great rate performance and extraordinary cycling stability of the CuS@S-3 may be derived from its suitable content of sulfur and uniform nano-construction.

Figure 6. CV curves of the (a) CuS@S-3, (b) CuS-3 electrode at various scan rates (0.3, 21



0.5, 0.7 and 0.9 mV s−1); (c) Capacitive contribution (pink for the CuS@S-3 and yellow for the CuS-3) and diffusion contribution (green) at a scan rate of 0.5 mV s-1; (d) Normalized contribution ratio of capacitive capacities at different scan rates; Nyquist plots at charged condition of the (e) CuS@S-3, (f) CuS-3 for 10th, 30th, 50th and 80th. Cyclic voltammetry (CV) analysis as an authoritative technique is carried out to understand the in-depth kinetics. Figure 6a-b displays a typical scan-rate-dependent CV curves for the CuS@S-3 and CuS-3 electrodes with increasing scan rates from 0.3 to 0.9 mV s-1. The stronger intensity of cathodic peak can be obviously noticed for CuS@S-3 than CuS-3 electrode, indicating a more stable redox reaction for CuS@S-3 electrode. The fundamental kinetics of electrodes, are further explored via the Trasatti analysis44-46 to quantify the pseudo-capacitive contributions, which is determined through the utilization of Dunn’s equation i (V) = k1 v + k2 v1/2, where i (V) is the current response i at the corresponding fixed potential V, and v is the scan rate. As a consequence, 60.7% of total capacity is recognized as the capacitive contribution for CuS@S-3 electrode at 0.5 mV s-1, which is much higher than the 45.7% presented by the CuS-3 sample (Figure 6c). The improved capacitive contribution performance of the CuS@S-3 over the CuS-3 electrode may well be due to the in-situ hybrid of elemental sulfur, which leads to the superior rate performances. Upon increasing the scan rate, the ionic-diffusionally controlled capacity is further decreased, while the capacitive capacity is increases gradually as expected (Figure 6d). The resistances of both anodes are also investigated in detail via EIS over the frequency range of 0.01-105 Hz. For the CuS@S-3 and CuS-3 electrodes, the Nyquist plots upon the charge cycles 22


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10th, 30th, 50th and 80th are shown in Figure 6e-f. In the Nyquist plots, the curves are composed of two parts: (1) the semicircle in the high frequency region stands for the resistance (R) of electrolyte, SEI film and charge-transfer; (2) the straight line in the low frequency region is related to the Warburg impedance (Ws), reflecting the diffusional process of lithium ions in the anodes.47-49 Upon the 80th discharge/charge cycle, the resistances of the CuS@S-3 and CuS-3 are , with the 276 Ω and 316 Ω respectively at . The lower resistance of the CuS@S-3 anode demonstrates that more sulfur is able to provide a matrix with the high lithium-ion conductivity, thus reducing the internal resistance.5,50,51 In addition, the resistance change of the CuS-3 electrode is unsteady, corresponding to its unsharp redox peaks in CV analysis above, which should be responsible for its inferior electrochemical performances.

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Figure 7. (a) Cycling performance of the CuS@S-3 and CuS+S electrodes at the current density of 1C in Li-S battery; (b) Galvanostatic discharge-charge profiles of the CuS@S-3 at 1C (c) Rate performance of the CuS@S-3 and CuS+S electrodes at various rates; (d) Long-term cycling performance and coulombic efficiency of the CuS@S-3 and CuS+S electrodes for 200 cycles at 2C. Considering that the Li-S battery will be a promising next-generation energy storage system owing to the abundance and cheap sulfur1, the as-prepared materials are applied as Li-S battery cathodes and their electrochemical performances are investigated at a current density of 1 C (1 C = 1675 mA g-1) between 1.5 and 3.0V in Figure S8. It is 24


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obvious that the CuS@S-3 electrode delivers the highest reversible capacity of 581.3 mAh g-1 over 100 cycles. By contrast, the specific capacities of CuS@S-1/2/4 are 82.7, 105.6 and 308.7 mAh g-1, respectively, demonstrating that the suitable content of the CuS additive and effective structure is of importance for the electrochemical properties. To further distinguish the advantages of the CuS@S-3 structure, the CuS+S electrode prepared by mixing CuS and S powder (35: 65) directly (as typically prepared within the literature) with similar stoichiometric proportions of CuS@S-3 is designed as a comparable sample. As shown in Figure 7a, the CuS+S electrode displays an initial capacity of 379.1 mAh g-1 at 1 C and only retains 54.9 mAh g-1 over 200 cycles. However, the CuS@S-3 electrode possesses reversible capacity of 543.1 mAh g-1 with almost no obvious capacity fading from the 100th to 200th cycle. From the comparison, it is obvious that the CuS@S-3 hybrid nanocomposite with the in-situ homogenous and molecular-level dispersed growth of CuS and S, presents a highly superior sulfur utilization and cycling stability than the literature standard CuS+S mixture. The corresponding charge and discharge profiles of the CuS@S-3 electrode at 1 C are depicted in Figure 7b. There are two discharge voltage plateaus at ~ 2.1 and 1.66 V and two charge voltages at ~ 2.3 and 1.9 V, which are different from the typical Li-S battery. As the cycle proceeds, the discharge platform at 2.1 V and charge platform at 2.3 V fade away gradually. After 30 discharge/charge cycles, only a very flat discharge and charge plateau occur at 1.66 V and 1.9 V respectively. These similar charge and discharge profiles also could be observed in previous work13 and will be further analysed in-detail in the following CV discussion. The rate performances utilizing 25



different current densities and the long-term cycling performance of the CuS@S-3 and CuS+S electrodes are tested and are depicted in Figure 7c-d respectively. Expectedly, the CuS@S-3 electrode demonstrates outright advantages within the cycling stability and rate performance. Even at a high current density of 2 C, the reversible capacity of 402.1 mAh g-1 are obtained after 200 cycles, which further indicates a better electrochemical performance by in-situ growth CuS additive in the as-prepared CuS@S-3 cathode.

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Figure 8. The CV curves of the (A1) CuS@S-3 and (B1) CuS+S electrodes at various scan rates from 0.1 to 0.9 mV s−1 in voltage range of 1.5-3.0 V; The CV curves of the (A2) CuS@S-3 and (B2) CuS+S electrodes at a scan rate of 1.0 mV s-1 in voltage range of 1.5-3.0 V; The CV curves of the CuS@S-3 electrode at a scan rate of 0.5 mV s-1 in voltage range of (C1) 1.5-2.2 V, (C2) 1.5-2.6 V. To further explore the changing charge/discharge properties during the electrochemical analysis, the CV analyses are conducted at various scan rates from 0.1 to 0.9 mV s−1 over a voltage range of 1.5-3.0 V (Figure 8(A1-B1)). During the charge 27



process, both CuS@S-3 and CuS+S electrodes display two peaks at ~ 2.3 and 1.92 V, matching well with the charge plateaus (Figure 7b). As is readily understood, the charge voltage at ~ 2.3 V is usually exhibited for typical Li-S battery. It will be reasonable to propose that the cathodic peak at 1.92 V is corresponding to the reaction of CuS. The CV curves of CuS+S electrode presents the suppression of the sharp peaks compared with CuS@S-3. Thus, the higher peak intensities of the CuS@S-3 electrode demonstrate an enhanced cycling stability than that of the CuS+S electrode. In addition, it should be noted that the cathodic peak at 2.3 V declines sharply upon increasing of the scan rate from 0.1 to 0.9 mV s-1, while the intensity of the cathodic peaks at 1.9 V increases simultaneously. The differentiation and evolution processes of the oxidation peaks are further clearly demonstrated in Figure 8(A2-B2), and the intensity of anodic peak at ~ 2.0 V decreases gradually along with the exhaustion of the high potential cathodic peak at 2.42 V. After 30 discharge/charge cycles, the intensity of the two redox peaks mentioned above could hardly be discovered. Such interesting redox behavior attracted our attention for further in-depth exploration. The CV analysis of the CuS@S3 electrode is firstly conducted within the voltage window of 1.5-2.2 V (Figure 8C1), the anodic peak at a high potential of about 2.0 V has not been detected. Subsequently, the CV measurement of the same cell is tested over the voltage window 1.5-2.6 V (Figure 8C2). The discharge peak at 2.0 V appears together with the charge peak at 2.38 V, revealing that the high potential redox peak is related to the migration of the sectional and external sulfur, while the low potential redox peak is ascribed to the stable phase reaction of elemental sulfur existing in the CuS matrix. More importantly, the stronger 28


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intensities of these redox peaks in the CuS@S-3 electrode reveal that it must possess an improved chemical adsorption with polysulfides than the CuS+S electrode, which may arise from the uniform molecular-level hybrid of elemental sulfur and the CuS matrix.

Figure 9. Nyquist plots at discharged condition of the 29


(a) CuS@S-3 and (b) CuS+S


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electrodes for 3rd, 10th, 20th and 50th; The liner relation of ω1/2 vs. -Z’’ of the (c) CuS@S3 and (d) CuS+S electrodes; The EIS analysis of the CuS@S-3: (e) various phase evolutions at different potentials and (f, g) the variation of resistance of the target sample. ω = 2πf

Eqs 1

Zre = R + σω-1/2

Eqs 2

D = 0.5 R2T2/A2n4F4C2σ2

Eqs 3

In order to evaluate the resistances of the CuS@S-3 and CuS+S electrodes, EIS spectrum at discharged condition for 3rd, 10th, 20th and 50th are illustrated in Figure 9ab. With the cycles, both the resistance of the CuS@S-3 and CuS+S electrodes show a tendency to decrease, which are in favour for the improvement of the electron conductivity of the reaction between Li and S, providing a matrix of high ion conductivity.52 For the CuS@S-3 electrode, the resistance values are decreased continuously from 69.1 Ω at 3rd cycle to 27.3 Ω at 50th cycle. While the resistances of the CuS+S electrode is decreased only from 94.8 Ω at 3rd cycle to 44.3 Ω at 50th cycle. The EIS spectrum of CuS@S-3 electrode demonstrates that the uniform nanostructure material presents a lower internal resistance, promoting highly reversible charge/discharge processes. By utilizing Eqs 1-353,54, the diffusion coefficient (D) of Li-ions is calculated, where R is the gas constant (8.314 J mol-1 K-1), T is the absolute temperature (293.15 K), A is the contact area of electrodes (1.15 cm2), n is the transfer electrons, F is the Faraday constant (96485 C mol-1), C is the molar concentration of Li-ions and σ is the Warburg coefficient that can be obtained from the fitting line of ZRe 30


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and ω-1/2. The D value of the CuS@S-3 increases from 1.10×10-11 cm2 s-1 (3rd cycle) to 2.47×10-11 cm2 s-1 (50th cycle), whereas the D value of CuS+S increases from 8.15×10-12 cm2 s-1 (3rd cycle) to 1.53×10-11 cm2 s-1 (50th cycle). Moreover, at each cycle, the D values of CuS@S-3 are larger than these of the CuS+S electrodes as summarized in Figure 9c-d and Table S1, which indicating the lower resistance and faster transferring Li-ions contribute to the improvement of electrochemical properties in the CuS@S-3 electrode. Figure 9e-f demonstrates the in situ EIS spectra of the CuS@S-3 electrode at various charge and discharge depths of the Li-S battery, during the 10th delithiation and lithiation processes at 1C, and a series of potential points are selected, containing 1.5, 2.0, 2.5 and 3.0 V at charge, 2.3, 1.8 and 1.5 V at discharge. From the fitting data of Nyquist plots, R values of CuS@S-3 at different voltages are summarized in Table S2. In the delithiation step, the R value is decreased continuously from 95.84 Ω at the beginning of charge state at 1.5 V to 33.95 Ω at fully charged state at 3.0 V, corresponding to the formation of CuS upon the extraction of Li-ions. In the subsequent lithiation process, the Li-ions insertion to form a Li2S matrix causes a gradual volume expansion. Thusly the R values increases from 33.95 Ω (fully charged state) to 96.60 Ω (discharge state) at 1.5 V. Notably, the R value at the initial charge state (95.84 Ω at C1.5 V) is similar to the final discharge state (96.60 Ω at D1.5 V), suggesting that the great reversibility of the structural evolution, over the charge-discharge processes, could be ascribed to the stable internal resistances. 4. CONCLUSIONS In summary, multi-functional hierarchical CuS@S nanosheets are successfully 31



prepared through the LSSS methodology, utilizing CaSx and CuCl2, for reversible lithium storage, in which the CaSx is employed as both the sulfur resource and reductant-oxidant. Through tuning the dosage of the CuCl2 reactant, the CuS@S is constructed with molecular-level dispersion and stable hierarchical architecture. Among the optimized CuS@S sample, the obtained CuS displayed an excellent plate matrix and the sulfur is significantly impregnated within the meso-pores. Surprisingly, the CuS@S-3 electrode as LIBs anode exhibits a considerable capacity of 334.8 mAh g−1 over 1000 cycles at a high current density of 1.0 A g-1. Compared to other samples, it is concluded that the CuS matrix along with an appropriate content of sulfur provides a highly reversible process of the lithium ions insertion/extraction. Supported by the kinetic origin analyses, the affecting of pseudo-capacitive behaviors is quantitatively determined, which is in favor of a great rate performance. When applied in a Li-S battery, CuS is effectively employed as synchronous growth additive for the target CuS@S sample due to its strong chemical adsorptions with polysulfides over that of the mechanical mixture of CuS+S electrode. Benefitting from the analysis of in-situ EIS, the stable internal resistances of CuS@S are distinguished by its reversible structure evolution. Thus, it is of great significance to open up a new road about multifunctional electrode materials with homogeneous nanostructure for multi-faceted and large-scale applications. AUTHOR INFORMATION Corresponding Author * Email address: [email protected]; Tel: +86 731-88879616; Fax: +86 731- 88879616. 32


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[email protected]; [email protected] ACKNOWLEDGMENT This work was financially supported by National Key Research and Development Program of China (2017YFB0102000), National Natural Science Foundation of China (51622406, 21673298 and 21473258), National Postdoctoral Program for Innovative Talents (BX00192), China Postdoctoral Science Foundation (2017M6203552), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), Innovation Mover Program of Central South University (2017CX004, 2018CX005), Hunan Provincial Science and Technology Plan (2017TP1001), Provincial Natural Science Foundation of Hunan (2016TP1009), Hunan Provincial Natural Science Foundation of China (2018JJ3633), Postgraduate Electronic Design Competition of China (502241802) and the Fundamental Research Funds for the Central Universities of Central South University (2018zzts013 and 2018zzts369). National Mittal Student Innovation Program (201810533258).

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Molecular-Level CuS@S Hybrid Nanosheets Constructed by Mineral

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