Novel layered double hydroxide precursor derived high-Co9S8-content composite as anode for lithium-ion batteries

Journal of Alloys and Compounds 768 (2018) 485e494

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Novel layered double hydroxide precursor derived high-Co9S8-content composite as anode for lithium-ion batteries Lan Yang, Hui Li, Shilin Zhang, Feng Yao, Jinmeng Lv, Sailong Xu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2018 Received in revised form 23 July 2018 Accepted 25 July 2018 Available online 26 July 2018

Layered double hydroxides (LDHs), also known as brucite (Mg(OH)2)-like anionic clay compounds, are very convenient precursors with a unique flexibility of tuning component type and molar ratio toward composite nanomaterials in energy storage, such as lithium-ion batteries (LIBs). Conventional binary LDH precursors are typically converted to active/non-active transition-metal oxide composites as anode nanomaterials for LIBs, but either with the aid of additionally introducing highly conductive carbonaceous matrix, or possessing relatively high-content non-active components that greatly lower the reversible specific capacity. Herein, we demonstrate a rational design of a novel single-source precursor of dodecyl sulfonate-intercalated Co2þCo3þAl3þ-layered double hydroxide (Co2þCo3þAl3þ-LDH) and its conversion to high-Co9S8-content composite (Co9S8/S-doped carbon/Al2O3) as high-efficiency anode nanomaterials for LIBs. In-situ X-ray diffraction (XRD) reveals the controllable topotactic transformation via tuning calcination temperature and time. Electrochemical test shows that the composite electrode delivers a reversible capacity of 970 mA h g1 after 200 cycles at 100 mA g1, and in particular, a longterm cycling stability of 780 mA h g1 after 500 cycles at 1 A g1, manifesting highly enhanced electrochemical performances compared with the counterpart derived from a conventional binary LDH precursor. Monitoring the discharged/charged states by in-situ XRD and ex-situ Raman spectra provides a direct support to the enhancement. Our results show that the LDH precursor-based approach provides an alternative to prepare diverse transition metal sulfides for energy storage. © 2018 Elsevier B.V. All rights reserved.

Keywords: Layered double hydroxides Cobalt sulfide In-situ analysis Anode nanomaterials Lithium-ion batteries

1. Introduction Layered double hydroxides (LDHs), also well-known as huge family of anionic clay compounds possessing structures based on brucite (Mg(OH)2)-like layers, have stirred extensive interest in energy storage and conversion [1e6], especially including rechargeable secondary batteries, supercapacitors, and full cells. LDHs possess the general chemical formula of [M2þ13þ xþ n 2þ (such as Mg2þ, Zn2þ, Co2þ xM x(OH)2] (A )x/n$yH2O, where M and Ni2þ) and M3þ (such as Al3þ and Fe3þ) present tunable cations occupied well-orderedly within a LDH layer, Ane means an anion  (such as CO2 3 , NO3 , and surfactant anion) intercalated between LDH interlayer galleries, and x is M3þ/(M2þ þ M3þ) molar ratio [7]. By virtue of the unique flexibility of easily tailoring types of cations and intercalated anions, as well as cationic molar ratios over a wide range, LDHs have been investigated intensively as very tunable

* Corresponding author. E-mail address: [email protected] (S. Xu). https://doi.org/10.1016/j.jallcom.2018.07.292 0925-8388/© 2018 Elsevier B.V. All rights reserved.

precursors to prepare anode nanomaterials for lithium-ion batteries (LIBs). Various LDH precursors were subjected directly to thermal decomposition to prepare transition-metal oxides used as anode nanomaterials for LIBs, in the main form of conventional active/ nonactive or bi-component-active binary LDH precursors [8e12]. The LDH precursors involved ZnAl-LDH nanoarrays used to synthesize ZnO/ZnAl2O4 [9], nanoparticle-like ZnFe-LDH to yield ZnO/ ZnFe2O4 [10], and CoFe-LDH precursor to form CoO/CoFe2O4 nanoparticles [11], as well as MgZnFe-LDH to generate ZnFe2O4/ MgFe2O4/MgO [12]. To alleviate their problems of rapid capacity fading and poor cycling stability, introducing carbonaceous support (such as carbon nanotube (CNT) and graphene (G)) is one common improvement strategy [13e16]. The LDH/carbon composites mainly included Co2þFe2þFe3þ-LDH/CNT to form CoFe2O4/CNTs [14], Co2þFe3þ-LDH/G to prepare CoO/CoFe2O4/G [15], and NiAl-LDH/ glucose mixture to synthesize Ni@NiO/Al2O3/C [16]. Recently, via hydrothermal synthesis-assisted sulfuration, three-dimensional graphene aerogel supported NiCo-LDH precursor was derived to

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NiCo2S4/Ni0.96S composite as anode nanomaterials for LIBs [17]. All the LDH-derived transition-metal oxides and sulfides indeed exhibited their greatly enhanced electrochemical performances. However, most of the enhancements were achieved with the aid of additionally introducing highly conductive carbonaceous matrix. Alternatively, introduction of non-active components (such as Al2O3) into composites is one effective approach to boost cycling stability and rate capability. The non-active component has been demonstrated to play an important role in effectively buffering volume changes of the electrochemically actives and preventing aggregation of the active nanoparticles during the cycles [18]. Typically, the strategy were employed to cathode or anode nanomaterials for LIBs, either mainly by coating several-nanometerthick ultrathin Al2O3 using an advanced atomic layer deposition (ALD) technique [19], by elaborately controlled wet-chemical method [20,21], or by incorporating Al2O3 and the active via a high-energy mechanical milling [22]. Similarly, for the Alcontaining LDH precursors derived multiple-component electrode composites [16,23,24], the formation of Al2O3 indeed greatly enhanced cycling stability and rate capability of the resulting composites. However, the enhancement compromised at the cost of remarkably lowering the reversible capacities. Therefore, it is a challenge to directly convert conventional Al-containing LDH precursors to composites possessing high-content active component and low-content non-active Al2O3 when combining non-active Al2O3 and carbonaceous matrix for synergistic lithium storage, because of the intrinsic feature of possessing a relatively highcontent non-active components that greatly lower the specific capacity. Herein, we report a rational design of a novel single-source precursor of dodecyl sulfonate (DSe)-intercalated Co2þCo3þAl3þlayered double hydroxide (Co2þCo3þAl3þ-DSe-LDH) and its conversion to high-Co9S8-content Co9S8/S-doped carbon/Al2O3 composite (Co9S8/S-C/Al2O3) used as anode nanomaterials for LIBs (Fig. 1). In distinct contrast to a conventional binary Co2þAl3þ-LDH with a relatively low Co2þ/Al3þ molar ratio typically ranging between 2: 1 and 4: 1, the unique Co2þCo3þAl3þ-DSe-LDH precursor is rationally designed to contain a remarkably high molar-ratio Co/Al (8.6: 1), and well-intercalated DSe anions confined between the LDH interlayer galleries. The novel ternary LDH precursor is converted to the Co9S8/S-C/Al2O3 composite with the following advantages of improving the electrochemical performances: (i) the high-content Co component capable of forming high-content electrochemically active Co9S8 that can contribute to specific capacity; (ii) the intercalated DSe anion as dual molecular S and C sources transformed to the conductive S-doped carbon coating; and (iii) low-content Al component to form non-active Al2O3 matrix that facilitates cycling stability. In-situ XRD characterization reveals the topotactic conversion of the LDH precursor. The electrochemical tests show that the Co9S8/S-C/Al2O3 electrode delivers the excellent electrochemical performances for lithium-ion battery,

which were supported by monitoring the discharge/charge process using in-situ XRD and ex-situ Raman spectroscopy. 2. Experimental section 2.1. Preparation of Co2þCo3þAl3þ-DSe-LDH precursor Nanosized Co2þCo3þAl3þ-DSe-LDH precursor was prepared via a modified conventional precipitation method. Briefly, a mixture of Co(NO3)2$6H2O (18 mmol), Al(NO3)3$9H2O (2 mmol), H2O2 (1 mL) and SDS (10 mmol) was dissolved in 100 mL of deionized and decarbonated water, then titrated by adding a basic solution of NaOH (100 mmol, 100 mL) with strongly stirring under N2 atmosphere for 40 min to form a deep green solution at room temperature until pH ¼ 10.5. The slurry was then transferred into a Teflonlined stainless steel autoclave with a capacity of 200 mL. The autoclave was sealed and maintained at 120  C for 24 h. The Co2þCo3þAl3þ-DSe-LDH precursor was obtained by resining with distilled water and ethanol several times, and during at 80  C for 24 h in a vacuum oven. 2.2. Preparation of Co9S8/S-C/Al2O3 The Co2þCo3þAl3þ-DSe-LDH precursor was heated at 700  C with a ramping rate of 5  C min1 and kept for 2 h under an Ar atmosphere. The above-obtained product (0.3 g) and thioacetamide (TAA, 4 mmol) were used as starting material, and added into 50 ml ethanol under strongly stirring. The mixture was then transferred into a Teflon-lined stainless steel autoclave, and maintained at 160  C for 24 h. The Co9S8/S-C/Al2O3 composite was obtained by resining with distilled water and ethanol several times, and during in a vacuum oven at 60  C for 12 h. To highlight the contribution of the novel high-Co-content Co2þCo3þAl3þ-DSe-LDH precursor, a conventional binary Co2þAl3þ-DSe-LDH precursor was prepared to yield Co9S8/Co/S-C/ Al2O3 with a low-content Co9S8. The Co/Co9S8/S-C/Al2O3 was prepared from Co2þAl3þ-DSe-LDH under the same experimental conditions, except for a different Co2þ/Al3þ molar ratio of 3: 1, without the oxidation of H2O2, and no further sulfurization of TAA. 2.3. Material characterization Powder XRD measurements were carried out by using a Rigaku XRD-6000 diffractometer (Cu Ka, 40 kV, 30 mA, l ¼ 0.1542 nm) over the 2q range between 3 and 70 . The morphology of the nanocomposites was visualized by using a scanning electron microscope (SEM, ZEISS Supra 55) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) characterization was conducted through a JEOL JEM-2100F electron microscope with an accelerating voltage of 200 kV. A confocal Raman spectrometer (Renishaw RM2000) was used to record Raman spectroscopy,

Fig. 1. Schematic illustration of preparing Co9S8/S-C/Al2O3 composite derived from a Co2þCo3þAl3þ-DSe-LDH single-source precursor.

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which is equipped with a 532 nm excitation laser (laser spot size of 0.5 mm) and operated at a low power level (ca. 2 mW). A X-ray photoelectron spectroscopy (XPS) (Thermo VG Scientific) was operated using Al Ka 1486.6 eV radiation at 400 W (15 kV) under high vacuum, to measure the samples dried onto silicon wafers. Elemental analysis was employed the metal ions by using a Shimadzu inductively coupled plasma emission spectrometer (ICP-ES). The contents of carbon, sulfur, oxygen, and hydrogen of the calcined products were analyzed using the vario EL cube (Elementar, Hanau, Germany). 2.4. Electrochemical measurement All the electrochemical measurements were carried out within Swagelok-type cells that were pre-assembled in an Ar-filled glove box. A mixture of active material, Poly (vinyl difluoride) (PVDF), and Ketjen black was prepared with a 70/10/20 weight ratio, and then pasted on a Cu foil to get the working electrodes. The loading mass of the composite was about 1.0 mg cm2, and the thickness of the composite electrode is approximately 10.62 mm by measuring the cross section of the cast film. A lithium foil was used as the counter, and a glass fibber (GF/D, purchased from Whatman) as a separator. The electrolyte used was composed of a solution of 1M LiPF6 salt in dimethyl carbonate (DMC)/ethylene carbonate (EC) (1/1 in wt. %) (from Tianjing Jinniu Power Sources Material Co. Ltd.). Cyclic voltammetry curves were recorded by using an Autolab Electrochemical workstation (PGSTAT302N, Metrohm Autolab, Switzerland). Galvanostatic cycling and the charge-discharge profiles were conducted on the assembled cells using a battery tester (LAND CT2001A) in the voltage range of 0.01e3.0 V (vs. Liþ/Li). Insitu XRD measurement was measured on a Bruker D8A25 Advanced diffractometer, using a specifically developed electrochemical cell that was assembled with a beryllium window used as a current collector. The electrochemical measurements were controlled by a Mac pile system. Subsequently, the cell was galvanostatically cycled at 100 mA g1 in the potential window of 0.01e3.0 V, for in-situ XRD tests, each scan was collected between 10 and 80 at a scanning speed of 7 per second. EIS measurement was performed over the frequency range between 100 kHz and 0.1 Hz at open circuit potential. 3. Results and discussion 3.1. Preparation of precursor Various previous studies have demonstrated that multicomponent anode nanomaterials for LIBs are able to synergistically enhance the electrochemical performances of the integrated composites [16,23e25]. From a viewpoint of LDH precursors, a traditional binary LDH, such as a DSe-intercalated Co2þAl3þ-DSeLDH, possesses a Co2þ/Al3þ molar ratio range typically between 2: 1 and 4: 1. A challenge thus occurs, i.e., if a high Co2þ/Al3þ molar ratio of 4: 1 were chosen, non-active Al2O3 component can be formed after the completion of calcination, but with a high molar fraction of 1/5. The high-content Al2O3 formed thus significantly lowers the specific capacity of the composite [16]. From a viewpoint of electrodes, high-efficiency lithium-ion battery necessitates the resulting composite to possess high-content Co9S8, low-content Al2O3 and S-doped carbon, the latter two of which have been well recognized to greatly boost the electrochemical performances [26]. To achieve the enhancement, a rational chemical balance is considered between the LDH precursor and the resulting Co9S8/S-C/ Al2O3 composite. Therefore, based on the unique flexibility of easily tuning cationic type and ratio of LDHs [7], novel ternary Co2þCo3þAl3þ-DSe-LDH is designed as a precursor, with high-

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content Co, low-content Al, as well as the intercalated surfactant anions used as the molecular carbon and sulfur sources. Each componential counterpart can be converted to high-content Co9S8, low-content Al2O3, and S-doped carbon during the pyrolysis, respectively. The Co2þCo3þAl3þ-DSe-LDH precursor was prepared by using a conventional co-precipitation method [27], but with a necessary addition of the exact amount H2O2 to in-situ oxidize the starting Co2þ to Co3þ during the co-precipitation process. Fig. 2a shows a characteristic XRD pattern of a hydrotalcite-like compound, reflecting the formation of the LDH precursor. One can clearly see that the (003) basal reflection peak of the precursor shifts remarkably to a lower angle 2q < 10 (Fig. 2a) than that of the a 2þ 3þ  conventional CO2e 3 - intercalated Co Al -LDH (2q ¼ 11.6 , JCPDS card No. PDF #51-0045). The pronounced shift strongly suggests the successful intercalation of the DSe anions between the LDH interlayer galleries, as reported previously for various surfactantintercalated LDH precursors, such as a methyl methacrylate/DSe co-intercalated CoAl-LDH [28] and a sodium dodecylbenzene sulfonate (SDBS)-intercalated NiAl-LDH [29]. Note that both DSe-intercalated Co2þAl3þ-LDH and DSe-intercalated a-Co(OH)2 could also be formed, both with the similar hydrotalcite-like XRD characteristics. To rule out both possibilities, we further carried out the element analysis by using the ICP-ES and XPS techniques. The ICP-ES result shows that the Co/Al molar ratio is 8.6, i.e., a [Co2þ þ Co3þ]/Al3þ molar ratio of 8.6: 1, closely approximate to that of the starting materials (18: 2). The XPS result yields a Co2þ/Co3þ molar ratio of 3.3 (Fig. S1). As a result, the Co2þ/ Co3þ/Al3þ molar ratio is determined to be 6.6: 2: 1 for the designed Co2þCo3þAl3þ-LDH precursor, giving rise to a Co2þ/[Co3þ þ Al3þ] molar ratio of 2.2. Indeed, the value of Co2þ/[Co3þ þ Al3þ] falls into the very range of M2þ/M3þ molar ratio typically between 2 and 4 for most traditional LDHs. The above-combined XRD, ICP, and XPS results indeed manifest the successful formation of the distinct DSe-intercalated Co2þCo3þAl3þ-LDH precursor, instead of DSeintercalated a-Co(OH)2 or DSe-intercalated Co2þAl3þ-LDH. Furthermore, considering the theoretical length of DSe anion (1.85 nm) [30] and the thickness of LDH layer as 0.48 nm [31], as well as the LDH basal spacing calculated to be 3.02 nm, the intercalated DSe anions are arranged in a bilayer arrangement tilted at an angle of 60 , i.e., an energetically favorable arrangement with two SO2 3 groups linked to the adjacent LDH layers and tailgroupto-tailgroup dislocation of hydrocarbon chains (as shown in inset of Fig. 2a). Therefore, the unique Co2þCo3þAl3þ-DSe-LDH precursor is obtained, with the expected high Co/Al molar ratio of 8.6: 1 and the well-intercalated DSe surfactant anions. 3.2. Characterization of composite The Co2þCo3þAl3þ-DSe-LDH precursor was subjected to the thermal decomposition in Ar, and in-situ XRD characterization was carried out to monitor the topotactic transformation of the Co2þCo3þAl3þ-DSe-LDH precursor. Fig. 2b shows that after the calcination at 200  C and 300  C, the basal (00l) reflection peaks (such as (003) and (006)) of the LDH precursor decrease, whereas the non-basal (012) and (110) peaks are still resolved, thereby demonstrating the collapse of the layered structures resulting from the dehydroxylation of the LDH precursor [32]. With the calcination temperature increasing, the phase of Co9S8, accompanied with the metallic Co phase, is found to appear from 400  C, and increase in intensity remarkably afterwards. After the completion of the calcination at 700  C for 2 h, the product exhibits the XRD reflection peaks at 15.5, 29.8, 31.2, 36.2, 39.5, 47.6, 52.1, 61.2, and 62 , which can be indexed to (111), (311), (222), (400), (331), (511), (440), (533), and (622) of a cubic Co9S8 phase with space group Fm3m (JCPDS 73-

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Fig. 2. (a) XRD patterns of the Co2þCo3þAl3þ-DSe-LDH precursor, with a standard XRD pattern of Co2þAl3þ-CO2e 3 -LDH (JCPDS card No. PDF #51-0045) for comparison, in-situ XRD patterns with 2q ranging from 3 to 70 for the topotactic transformation from the Co2þCo3þAl3þ-DSe-LDH precursor to the intermediate Co/Co9S8/S-C/Al2O3 obtained via the calcination from 200  C to 700  C; and (c) XRD patterns of the resulting Co9S8/S-C/Al2O3 composite obtained via sulfuration of the intermediate Co/Co9S8/S-C/Al2O3. Inset of Fig. 2a shows a possible arrangement of the intercalated DSe anions.

1442), respectively. Note that the XRD reflection peaks of metallic Co are still clearly visible between 400  C and 700  C, which is ascribed to the carbothermal reduction in the presence of the intercalated DSe surfactant anions. Similar observations were reported previously [29,33], such as for the metallic phases obtained by calcining the DSe intercalated NiAl-LDH [33] or SDBS intercalated NiAl-LDH [29]. However, in previous studies [33,34], the metallic phases were either recognized as conductive but nonactive component [33], or removed by acid leaching [34]. By contrast, in our case, a further sulfuration was subsequently applied at 160  C for 24 h to make the utmost of the metallic Co component. As a result, no XRD reflection peaks of metallic Co are observed, only the Co9S8 phase is clearly visible (Fig. 2c). The disappearance of metallic Co strongly suggests that the metallic cobalt reacts with the sulfur to produce Co9S8 during the sulfuration process. In addition, no characteristic XRD reflection peak is visible for the amorphous Al2O3 phase, consistent with the previous studies concerning the Al2O3 phase derived from various Al-containing LDH [35]. Furthermore, a broad hump peak ranging between 20 and 30 is found, attributable to the amorphous sulfur-doped carbon derived from the SDS surfactant during the calcination process [36,37]. Therefore, a topotactic transformation is tentatively proposed as “LDH precursor-Co/Co9S8/S-C/Al2O3 intermediate-Co9S8/ S-C/Al2O3”, i.e., the initial collapse of the layered structures of Co2þCo3þAl3þ-DSe-LDH precursor formed between 200 and 300  C, the subsequent formation of metallic Co and nucleation of Co9S8 at 300  C, and the eventual growth of both Co9S8 and Co between 500 and 700  C, as well as the conversion of the metallic Co to Co9S8 via the sulfuration. The understanding of the topotactic transformation could facilitate avoiding either the effect of nonactive metallic Co on lowering the specific capacity of the composite, or the necessary utilization of acid leaching to remove the inert Co component. The morphology of the Co9S8/S-C/Al2O3 composite was observed by using SEM and TEM. SEM image shows a nanoparticle-

like morphology of the Co9S8/S-C/Al2O3 composite with a relatively homogenous size distribution (Fig. 3a). TEM image confirms the nanoparticle-like feature (Fig. 3b and c). By manually measuring the dimension sizes of over 100 nanoparticles from Fig. 3c, the resulting histogram profile shows that a mean particle size distribution is determined to be 57.2 ± 1.6 nm via Gaussian fitting the numbers obtained (inset of Fig. 3c). The relatively homogenous size distribution is ascribed to the confinement effect from both Al2O3 and carbon formed during the calcination process, well consistent with our previous study showing the narrow size distribution of Co2SnO4/Co3O4 nanoparticles co-defined by carbon and Al2O3 that were derived from a ternary Co2þAl3þSn4þ-LDH precursor [25]. HRTEM image further reveals that the lattice fringes are clearly identified with a mean spacing of 0.245 nm (Fig. 3d), corresponding to the (400) plane of Co9S8. In addition, curved graphene-like sheets are clearly observed at the edge of the crystalline nanoparticles, attributable to the amorphous carbon that are converted from the intercalated surfactant anions during the carbothermal process [29]. The STEM/EDX images reveal a homogeneous distribution of the coexisting Co, S, C and Al of the composite (Fig. 3e and f). To quantitatively determine the contents of the Co9S8/S-C/Al2O3 composite, TG analysis was carried out in air from room temperature to 700  C at a rate of 5  C min1. Two stages of weight loss are visible for the Co9S8/S-C/Al2O3 composite (Fig. S2). The first weight loss before 500  C originates from the removal of oxygencontaining groups and the oxidation of Co9S8, and the second is related to a large continuous weight loss in the range from 510 to 600  C [38,39]. The latter behavior could be caused by the combustion of carbon in air. Therefore, the content of the S-doped carbon is measured to be 12%, and the content of Co9S8/Al2O3 is 88%. By referring to the Co/Al molar ratio determined by ICP-ES, the content of Co9S8 is 78.8%. Raman spectrum was utilized to examine the carbon characteristic of the composite. The peak is centered at 1335 cm1

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Fig. 3. (a) SEM image, (b, c) TEM images, (d) HRTEM observations of Co9S8/S-C/Al2O3, as well as (e) STEM image and (f) corresponding element mapping of Co, Al, S and C.

(Fig. S3), which corresponds to the D band associated with the defects and disorder in the carbon layer. The peak at 1587 cm1 corresponds to the G band, well related to the co-planar vibration of sp2-bonded carbon atoms. The intensity ratio (ID/IG) is thus determined to be 1.16, strongly suggestive of the correlation between a large number of disordered carbon edge sites and the lowgraphitization carbon of the Co9S8/S-C/Al2O3 composite. The low graphitization can be caused by the sulfur doping to the carbon structure and the low-temperature calcination [40,41]. XPS technique was employed to characterize the chemical states of the composite. Fig. 4a shows a survey spectrum with the main peak values centered at 74.6, 161.9, 284.5, 530.0, and 781 eV, which are assigned to the binding energies of Al 2p, S 2p, C 1s, O 1s, and Co 2p, respectively. The O 1s spectrum is attributed to the Al2O3 or surface adsorption of O2. In the case of Co 2p, the bands centered at 778.2 and 794.4 eV correspond to the spin-orbit split peaks of Co 2p3/2 and Co 2p1/2, respectively (Fig. 4b). The weak binding energies centered at around 783.8 and 800.1 eV could be identified as the shake-up peaks of Co2þ [42,43]. In the case of C 1s spectrum (Fig. 4c), the spectrum can be de-convoluted into three types of the C-C (284.6 eV), C-S (285.2 eV) and C-O (288.1 eV) groups [44]. In the case of S 2p spectrum (Fig. 4d), the S 2p band is attributed to three different types of S species: the bands at 161.9, 163.1, and 168.6 eV that are ascribed to CoeS, CeSeC, and sulfone, respectively [34]. The existence of sulfone could be originated from the starting material of the intercalated SDS anions, and the C-S-C moiety from the doped sulfur atom through carbothermal process in the carbon matrix [29], reflecting the formation of the S-doped carbon during the carbothermal process. In the case of Al 2p spectrum, a peak is observed at 74.5 eV (Fig. S4a), which corresponds to the Al3þ species bonded to oxygen, indeed confirming the presence of the amorphous Al2O3 phase. In addition, the XPS spectra of O 1s were fitted by two different types of O species (Fig. S4b): the band centered at 531.0 eV attributed to the lattice oxygen bound to metal

cations in Al2O3, and the ones at 532.2 eV corresponding to the surface oxygen [45]. By perusal of the above XRD and XPS results, as well as SEM/ TEM visualizations of Co9S8 nanoparticles, we can conclude that the Co9S8/S-C/Al2O3 composite is obtained by combination of the initial calcination of the rationally designed the DSe-intercalated Co2þCo3þAl3þ-LDH as a precursor, and the subsequent sulfuration of the metallic Co phase. The resulting Co9S8/S-C/Al2O3 composite consists of high-content active Co9S8 nanoparticles, low-content non-active Al2O3 matrix, and conductive S-doped carbon coating. These componential and nanostructural features are expected to synergistically boost the electrochemical performances of the of the Co9S8/S-C/Al2O3 composite [46]. 3.3. Electrochemical testing We initially carried out the CV measurements on the Co9S8/S-C/ Al2O3 electrode between 0.01 and 3 V at 0.1 mV s1. In the first reduction sweep (cathodic scanning curve), a weak and broad peak at 0.68 V is observed (Fig. S5), corresponding to the decomposition of the electrolyte and the formation of solid electrolyte interface (SEI). A strong reduction peak is visible at 1.12 V, reflecting the reduction of Co9S8 to metallic Co via the following conversion reaction [40]. Co9S8 þ 16Li 4 8Li2S þ 9Co The sharp peak also appears in the subsequent two discharge curves, accompanied with an apparent shift to a high potential voltage of 1.31 V and was significantly decreased in intensity. These imply the microstructure rearrangement of electrode materials and the irreversible transformation in the first cycle. In the first oxidation sweep (anodic scanning curve), an oxidation peak at 2.03 V is clearly visible, corresponding to the reverse reaction

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Fig. 4. XPS spectra of the Co9S8/S-C/Al2O3 composite for (a) survey, (b) Co 2p, (c) C 1s and (d) S 2p.

producing the broad peak during the sweep process, i.e., the sulfuration of metallic Co and oxidation of the SEI [47]. To support the aspect, we carried out the in-situ XRD measurements on the Co9S8/S-C/Al2O3 electrode coated on the Be foil at the selected discharged/charged states between 0.01 and 3.0 V at a current density of 100 mA g1. Fig. 5a shows that during the first lithiation process (corresponding to discharge curve), the reflection peaks of (311) and (440) of the crystalline Co9S8 phase are dramatically decreased in intensity. Note that no new phase is observed during the discharge process, which is ascribed to the formation of amorphous cobalt owing to the very small dimensional size. The results are in good agreement with the previous study showing a transformation from crystalline NiO to amorphous

Ni during the discharge process, which was inspected by an ex-situ XRD technique [48]. Importantly, during the following first delithiation curve (corresponding to charge curve), the reflection peaks of (311) and (440) of the Co9S8 phase are clearly visible to increase in intensity, indicative of the gradual conversion from the amorphous Co to the crystalline Co9S8. To further support the above conversion reaction, ex-situ Raman spectrum was combined to characterize the Co9S8/S-C/Al2O3 electrode after discharged and charged between 0.01 and 3.0 V. For the fresh electrode, the Co9S8 phase is clearly visible (Fig. 5b, curve I). During the discharge cycle, the peaks collected at 3.0 V are observed at 190, 460, 500 and 650 cm1 (Fig. 5b, curve II), which can be well related to the Co9S8 phase [49,50]. During the discharge cycle, the peak collected at

Fig. 5. (a) The in-situ XRD patterns collected at the selected discharged/charged states between 0.01 and 3.0 V at 100 mA g1. (b) Raman spectra of (I) the fresh Co9S8/S-C/Al2O3 electrode, (II) the electrode discharged at 3.0 V, (III) the electrode discharged at 0.01 V, and (IV) the electrode charged at 3.0 V.

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0.01 V is observed at 370 cm1 can be ascribed to the formation of the Li2S (Fig. 5b, curve III) [51], and the spectrum recorded at 3.0 V during the charge cycle corresponds to Co9S8 (Fig. 5b, curve IV). The combination of the above in-situ XRD patterns and ex-situ Raman spectra clearly confirms that the electrochemical active Co9S8 undergoes a reversible conversion change upon lithiation and subsequent delithiation, thereby underpinning the good reversibility of the composite electrode. The “Co9S8-Co/Li2S-Co9S8” phase transition behavior was also reported by an elaborately designed in-situ TEM study of Co9S8/Co-filled CNT electrode [52], which has clarified that single crystalline Co9S8 nanowire converts to Co and Li2S matrix during the lithiation process. In this regard, the combination of in-situ XRD and ex-situ Raman techniques might provide a simple alternative approach to easily reveal the conversion mechanism. The discharge/charge profiles of the Co9S8/S-C/Al2O3 electrode was measured between 0.01 and 3.0 V at 100 mA g1. Fig. 6a shows that the initial discharge/charge specific capacities are 2180 and 1100 mA h g1, respectively, thus yielding a Coulombic efficiency of 50.4%. Such a Coulombic efficiency reveals a large irreversible capacity loss in the first cycle, which is typically attributed to the formation of both the SEI film and the electrolyte decomposition [17]. After the second cycle, the discharge and charge capacities

491

decrease to 1137 and 1083 mA h g1, respectively. A good overlap is also observed between the 2nd and 3rd charge/discharge profiles, suggestive of a possibility of possessing good cycling stability of the composite electrode. To highlight the contribution of high-content-Co9S8 derived from the novel high-Co-content Co2þCo3þAl3þ-DSe-LDH precursor, low-content Co9S8 containing Co9S8/Co/S-C/Al2O3 composite was prepared for comparison from a traditional binary Co2þAl3þ-DSeLDH precursor by using the same LDH precursor-derived synthesis route. The Co9S8/Co/S-C/Al2O3 composite was obtained, with the XRD patterns similar to those of the above Co9S8/S-C/Al2O3 composite, except for the metallic Co phase formed during the pyrolysis of the Co2þAl3þ-DSe-LDH precursor (Fig. S6a). The ICP data show that the Co/Al molar ratio of the as-obtained Co9S8/Co/S-C/Al2O3 composite is 2.7: 1, which is close to that of the starting materials (3: 1) but quite smaller than that (8.6: 1) of the Co9S8/S-C/Al2O3 composite. SEM image shows a particle-like morphology and a good particle size distribution of the Co9S8/Co/S-C/Al2O3 composite (Fig. S6b). TEM image depicts the co-existence of nanoparticles and matrix, a mean particle size is determined to be 61.9 ± 4.9 nm via Gaussian fitting (Fig. S6c). In addition, the weight fraction of the carbon is quantitatively determined to be 12.8% by using TG

Fig. 6. (a) The first three charge-discharge profiles of Co9S8/S-C/Al2O3 at a current density of 100 mA g1, (b) cycling behavior at a 100 mA g1, (c) rate capacity from 100, through 200, 400, 800, 1000, 2000, to 100 mA g1, (d) long-cycling performance at a current density of 1000 mA g1, and (e) Nyquist plots of the Co9S8/S-C/Al2O3 electrodes before and after rate test.

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technique (Fig. S7a). By combining the ICP-ES result of Co/Al molar ratio (2.7: 1) and the XPS result of Co 2p spectrum (Fig. S7b), the mass content of Co9S8 in the Co9S8/Co/S-C/Al2O3 composite is thus 29.7%, quite lower than that of the Co9S8/S-C/Al2O3 composite (78.8%). The difference reflects the great flexibility of LDH precursor-based synthesis route. For both electrodes, we then evaluated the cycling stability at 100 mA g1. Fig. 6b displays that over those 200 cycles, the Co9S8/SC/Al2O3 electrode is able to retain a reversible capacity of 970 mA h g1, whereas the Co9S8/Co/S-C/Al2O3 electrode exhibits only a low reversible capacity of 575 mA h g1 (Fig. S6d). Apparently, the remarkable difference is ascribed to the remarkable difference in content of the electrochemically active Co9S8 between those two electrodes. In addition, the distinct contrast strongly suggests the reproducibility of the LDH precursor-based synthesis route, and in particular, the rational designing of the novel Co2þAl3þ-DSe-LDH precursor with high-content active and lowcontent non-active components. Note that over those 200 cycles, the reversible capacities and Coulombic efficiencies remain very stable. We attribute the cycling stability to the contributions from non-active Al2O3 and amorphous carbon as a buffering matrix, and also from the amorphous carbon as conductive support. The contributions are indeed consistent with the previous studies [25,53] showing the benefit of both Al2O3 and carbon in enhancing the cycling stabilities of CoO/Co3O4/N-C/Al2O3 and Co2SnO4/Co3O4/ Al2O3/C derived from LDH precursors. In particular, after the 200 cycles, the Co9S8/S-C/Al2O3 electrode is able to retain a high reversible capacity of 970 mA h g1. We also examined the rate capabilities of the Co9S8/S-C/Al2O3 electrode. Fig. 6c shows that the electrode exhibits the rate capabilities of 986, 890, 845, 740, 623, and 498 mA h g1 at the current densities of 100, 200, 400, 600, 1000, and 2000 mA g1. When the current density is reset to 100 mA g1, the reversible specific capacity is capable of returning to 963 mA h g1, this suggests that the Co9S8/S-C/Al2O3 electrode remains an actually stable behavior during cycling. We further estimated the long-term cycling performance of the Co9S8/S-C/ Al2O3 electrode at a large current density of 1000 mA g1. Fig. 6d shows that the charge capacity of Co9S8/S-C/Al2O3 electrode is 1002 mA h g1 in the first cycle, then decreases gradually until the 76th cycling, and then increases until the 158th cycle, and eventually hovers at 780 mA h g1 after the 500th cycle at the large current density. The electrochemical behaviors have been recognized to correlate well to the following possibilities: (i) the initial formation of stable SEI film, (ii) the subsequent pulverization of active components, and (iii) eventually the good preservation of the overall morphology of active components [54e56]. In our case, the decreases before the 76th cycling could be typically attributed to the formation of both the SEI film and the electrolyte decomposition, starting from the 76th cycle, the increased capacity could be ascribed to the improved Li-diffusion kinetics by activation effect of electrodes during long term charging-discharging process and the reversible reaction between metal particles and electrolytes [57]. The similar behavior of capacity increase during cycling was also typically observed for metal oxide-based composites [58,59]. Note that the Coulombic efficiency in the preliminary 120 cycles appeared to be lower than that of the electrode in the subsequent cycles. We hypothesize that the relatively large irreversible capacity might be due to the increasing layer of SEI formation and decreasing dimensional size of active nanoparticles during the initial 120 cycles, as reported previously by previous studies such as 3D mesoporous Co3O4 network [58] and graphene-supported Mn0.25Co0.75O solid solution [59]. Further comparison of reversible specific capacity was summarized between the Co9S8/S-C/Al2O3 and the Co9S8-containing electrodes reported previously (Fig. S8). We find that the reversible

charge capacity of our composite electrode could be comparable to those of the other electrodes. The comparable electrochemical performances are ascribed to the following features of the Co9S8/SC/Al2O3 composite. (i) The well-dispersed high-content Co9S8 nanoparticles can guarantee the high reversible capacity. (ii) The low-content S-doped carbon can facilitate increasing the electronic conductivity, delivering a low charge-transfer resistance, and thus enhancing the cycling stability and rate capability [60]. EIS measurement was performed to the Co9S8/S-C/Al2O3 electrodes before and after the rate test, and the post-first-cycled Co9S8/S-C/Al2O3 electrode (Fig. 6e). A slight difference in semicircle diameter is observed, which corroborates the preservation of relatively good electron transport between the active nanoparticles and the carbon support. (iii) Low-content non-active Al2O3 and S-doped carbon can be utilized as buffering matrix to confine the remarkable volumetric expansion upon lithiation/delithiation and also to prevent the growth and agglomeration of active material during the cycle process, thus facilitating the cycling stability [61]. To support the point, we further carried out TEM imaging on the Co9S8/S-C/ Al2O3 electrode after the long-term cycling. The SEM visualization shows the nanoparticle-like morphology of the post-cycled electrode (Fig. S9a), which is further supported by the TEM image (Fig. S9b). Quantitative measurement of the dimension sizes shows that a mean particle size distribution is 47.5 ± 3.4 nm (inset of Fig. S9b), indeed smaller than that (57.2 ± 1.6 nm) of the nanoparticle before the long-term cycling. The remarkable decrease strongly confirms the decrease-increase-stable trend of reversible capacity resulting from the change of dimensional size owing to the pulverization of active components during lithiation/delithiation process. By the virtue of the unique flexibility of chemically balancing types and molecule ratios of active/non-active cations, our method can be extended to design and prepare diverse transition metal sulfides for high-efficiency lithium storage. 4. Conclusion We designed and prepared the single-source precursor of novel Co2þCo3þAl3þ-DSe-LDH precursor, and the derived Co9S8/S-C/Al2O3 composite containing high-content active and low-content nonactive components. In-situ XRD patterns revealed the topotactic transformation of the distinct ternary LDH precursor to the resulting Co9S8/S-C/Al2O3 composite. We developed the composite as a synergistic anode nanomaterial for LIBs. Electrochemical tests show that the Co9S8/S-C/Al2O3 electrode indeed delivers a superior electrochemical performance, i.e., a reversible capacity of 970 mA h g1 after 200 cycles at 100 mA g1 and an exceptionally high reversible capability (780 mA h g1) after 500 cycles at 1 A g1. The in-situ XRD and ex-situ Raman results reveal the direct and complementary support to the conversion mechanism of the Co9S8-based anode for LIBs, and the EIS result manifests the improved conductivity, all underpin the enhancements. Our results demonstrate that the LDH precursor-derived strategy could provide an alternative approach to prepare diverse transition metal sulfides, based on the great versatility in varying metal cations and surfactant interlayer anions of LDH precursors. Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program, 2014CB932102), and the National Natural Science Foundation of China. Appendix A. Supplementary data Supplementary data related to this article can be found at

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https://doi.org/10.1016/j.jallcom.2018.07.292. References [1] J. Yu, Q. Wang, D. O'Hare, L. Sun, Preparation of two-dimensional layered double hydroxide nanosheets and their applications, Chem. Soc. Rev. 46 (2017) 5950e5974. [2] Y. Jia, L. Zhang, G. Gao, H. Chen, B. Wang, J. Zhou, M.T. Soo, M. Hong, X. Yan, G. Qian, J. Zou, A. Du, X. Yao, A heterostructure coupling of exfoliated NieFe hydroxide nanosheet and defective graphene as a bifunctional electrocatalyst for overall water splitting, Adv. Mater. 29 (2017) 1700017. [3] X. Wang, Q. Wang, X. Hou, Y. Liu, P. Zheng, J. Huo, L. Yin, S. Guo, Facile fabrication of two-dimensional reduced graphene oxide/CoAl-layered double hydroxides nanocomposites for lithium-oxygen battery with improved electrochemical performance, J. Alloys Compd. 744 (2018) 196e203. [4] D. Wang, J. Wu, D. Bai, R. Wang, F. Yao, S. Xu, Mesoporous spinel ferrite composite derived from a ternary MgZnFe-layered double hydroxide precursor for lithium storage, J. Alloys Compd. 726 (2017) 306e314. [5] L. Zhang, H. Yao, Z. Li, P. Sun, F. Liu, C. Dong, J. Wang, Z. Li, M. Wu, C. Zhang, B. Zhao, Synthesis of delaminated layered double hydroxides and their assembly with graphene oxide for supercapacitor application, J. Alloys Compd. 711 (2017) 31e41. [6] G. Fan, F. Li, D.G. Evans, X. Duan, Catalytic applications of layered double hydroxides: recent advances and perspectives, Chem. Soc. Rev. 43 (2014) 7040e7066. [7] P.J. Sideris, U.G. Nielsen, Z. Gan, C.P. Grey, Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy, Science 321 (2008) 113e117. [8] Z. Quan, E. Ni, Y. Ogasawara, N. Sonoyama, Nano-size multiple metal oxide anode electrodes synthesized from layered double hydroxides electrochemical reaction mechanism and surface morphology change during reaction with lithium ion, Solid State Ionics 268 (2014) 268e272. [9] J. Liu, Y. Li, X. Huang, G. Li, Z. Li, Layered double hydroxide nano-and microstructures grown directly on metal substrates and their calcined products for application as Li-ion battery electrodes, Adv. Funct. Mater. 18 (2008) 1448e1458. [10] M.A. Woo, T.W. Kim, I.Y. Kim, S.-J. Hwang, Synthesis and lithium electrode application of ZnO-ZnFe2O4 nanocomposites and porously assembled ZnFe2O4 nanoparticles, Solid State Ionics 182 (2011) 91e97. [11] M. Li, Y.-X. Yin, C. Li, F. Zhang, L.-J. Wan, S. Xu, D.G. Evans, Well-dispersed bicomponent-active CoO/CoFe2O4 nanocomposites with tunable performances as anode materials for lithium-ion batteries, Chem. Commun. 48 (2012) 410e412. [12] D. Wang, J. Wu, D. Bai, R. Wang, F. Yao, S. Xu, Mesoporous spinel ferrite composite derived from a ternary MgZnFe-layered double hydroxide precursor for lithium storage, J. Alloys Compd. 726 (2017) 306e314. [13] J.M. Lee, T.H. Gu, N.H. Kwon, S.M. Oh, S.-J. Hwang, Rapid synthetic route to nanocrystalline carbon-mixed metal oxide nanocomposites with enhanced electrode functionality, J. Phys. Chem. C 120 (2016) 8451e8460. [14] H. Fu, Z.-j. Du, W. Zou, H.-Q. Li, C. Zhang, Simple fabrication of strongly coupled cobalt ferrite/carbon nanotube composite based on deoxygenation for improving lithium storage, Carbon 65 (2013) 112e123. [15] D. Du, W. Yue, Y. Ren, X. Yang, Fabrication of graphene-encapsulated CoO/ CoFe2O4 composites derived from layered double hydroxides and their application as anode materials for lithium-ion batteries, J. Mater. Sci. 49 (2014) 8031e8039. [16] Z. Liang, R. Huo, Y.X. Yin, F. Zhang, S. Xu, Y.G. Guo, Carbon-supported Ni@NiO/ Al2O3 integrated nanocomposite derived from layered double hydroxide precursor as cycling-stable anode materials for lithium-ion batteries, Electrochim. Acta 108 (2013) 429e434. [17] D. Bai, F. Wang, J. Lv, F. Zhang, S. Xu, Triple-confined well-dispersed biactive NiCo2S4/Ni0.96S on graphene aerogel for high-efficiency lithium storage, ACS Appl. Mater. Interfaces 8 (2016) 32853e32861. [18] S. Kalluri, M. Yoon, M. Jo, H.K. Liu, S.X. Dou, J. Cho, Z.P. Guo, Feasibility of cathode surface coating technology for high-energy lithium-ion and beyondlithium-ion batteries, Adv. Mater. 29 (2017) 1605807. [19] P. Yan, J. Zheng, X. Zhang, R. Xu, K. Amine, J. Xiao, J.-G. Zhang, C.-M. Wang, Atomic to nanoscale investigation of functionalities of an Al2O3 coating layer on a cathode for enhanced battery performance, Chem. Mater. 28 (2016) 857e863. [20] W. Zhang, Z.-X. Chi, W.-X. Mao, R.-W. Lv, A.-M. Cao, L.-J. Wan, One-nanometer-precision control of Al2O3 nanoshells through a solution-based synthesis route, Angew. Chem. Int. Ed. 53 (2014) 12776e12780. [21] B. Han, T. Paulauskas, B. Key, C. Peebles, J.S. Park, R.F. Klie, J.T. Vaughey, F. Dogan, Understanding the role of temperature and cathode composition on interface and bulk: optimizing aluminum oxide coatings for Li-ion cathodes, ACS Appl. Mater. Interfaces 9 (2017) 14769e14778. [22] D. Applestone, S. Yoon, A. Manthiram, Cu2SbeAl2O3eC nanocomposite alloy anodes with exceptional cycle life for lithium ion batteries, J. Mater. Chem. 22 (2012) 3242e3248. [23] Y. Feng, H. Zhang, L. Fang, W. Li, Y. Wang, Novel three-dimensional flower-like porous Al2O3 nanosheets anchoring hollow NiO nanoparticles for highefficiency lithium ion batteries, J. Mater. Chem. A 4 (2016) 11507e11515. [24] Q. Li, Z. Yi, Y. Cheng, X. Wang, D. Yin, L. Wang, Microwave-assisted synthesis

[25]

[26]

[27]

[28] [29]

[30] [31] [32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

493

of the sandwich-like porous Al2O3/RGO nanosheets anchoring NiO nanocomposite as anode materials for lithium-ion batteries, Appl. Surf. Sci. 427 (2018) 354e362. B. Wu, S. Zhang, F. Yao, F. Zhang, S. Xu, Synergistic lithium storage of a multicomponent Co2SnO4/Co3O4/Al2O3/C composite from a single-source precursor, RSC Adv. 5 (2015) 69932e69938. I.D. Scott, Y.S. Jung, A.S. Cavanagh, Y. Yan, A.C. Dillon, S.M. George, S.H. Lee, Ultrathin coatings on nano-LiCoO2 for Li-ion vehicular applications, Nano Lett. 11 (2011) 414e418. W. Peng, H. Li, S. Song, Synthesis of fluorinated graphene/CoAl-layered double hydroxide composites as electrode materials for supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 5204e5212. J. Sun, H. Liu, X. Chen, D.G. Evans, W. Yang, X. Duan, Carbon nanorings and their enhanced lithium storage properties, Adv. Mater. 25 (2013) 1125e1130. S. Zhang, F. Yao, L. Yang, F. Zhang, S. Xu, Sulfur-doped mesoporous carbon from surfactant-intercalated layered double hydroxide precursor as highperformance anode nanomaterials for both Li-ion and Na-ion batteries, Carbon 93 (2015) 143e150. M.A. Drezdzon, Synthesis of isopolymetalate-pillared hydrotalcite via organicanion-pillared precursors, Inorg. Chem. 27 (1988) 4628e4632. M. Meyn, K. Beneke, G. Lagaly, Anion-exchange reactions of layered double hydroxides, Inorg. Chem. 29 (1990) 5201e5207. X. Zhao, F. Zhang, S. Xu, D.G. Evans, X. Duan, From layered double hydroxides to ZnO-based mixed metal oxides by thermal decomposition: transformation mechanism and UV-blocking properties of the product, Chem. Mater. 22 (2010) 3933e3942. Z. Liang, R. Huo, S. Yin, F. Zhang, S. Xu, Eco-efficient synthesis route of carbonencapsulated transition metal phosphide with improved cycle stability for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 921e925. J. Wang, L. Li, X. Chen, Y. Lu, W. Yang, Monodisperse cobalt sulfides embedded within nitrogen-doped carbon nanoflakes: an efficient and stable electrocatalyst for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 11342e11350. X. Zhao, L. Wang, X. Xu, X. Lei, S. Xu, F. Zhang, Fabrication and photocatalytic properties of novel ZnO/ZnAl2O4 nanocomposite with ZnAl2O4 dispersed inside ZnO network, AIChE J. 58 (2012) 573e582. J.M. Calderon, M. Yoshimura, Hydrothermal processing of high-quality multiwall nanotubes from amorphous carbon, J. Am. Chem. Soc. 123 (2001) 741e742. J. Zhang, C.P. Yang, Y.X. Yin, L.J. Wan, Y.G. Guo, Sulfur encapsulated in graphitic carbon nanocages for high-rate and long-cycle lithiumesulfur batteries, Adv. Mater. 28 (2016) 9539e9544. M. Li, H. Zhou, W. Yang, L. Chen, Z. Huang, N. Zhang, C. Fu, Y. Kuang, Co9S8 nanoparticles embedded in a N, S co-doped graphene-unzipped carbon nanotube composite as a high performance electrocatalyst for the hydrogen evolution reaction, J. Mater. Chem. A 5 (2017) 1014e1021. R. Tian, Y. Zhou, H. Duan, Y. Guo, H. Li, K. Chen, D. Xue, H. Liu, MOF-derived hollow Co3S4 quasi-polyhedron/MWCNT nanocomposites as electrodes for advanced lithium ion batteries and supercapacitors, ACS Appl. Energy Mater. 1 (2018) 402e410. W. Shi, J. Zhu, X. Rui, X. Cao, C. Chen, H. Zhang, H.H. Hng, Q. Yan, Controlled synthesis of carbon-coated cobalt sulfide nanostructures in oil phase with enhanced Li storage performances, ACS Appl. Mater. Interfaces 4 (2012) 2999e3006. Y. Zhou, D. Yan, H. Xu, S. Liu, J. Yang, Y. Qian, Multiwalled carbon nanotube @a-C@Co9S8 nanocomposites: a high-capacity and long-life anode material for advanced lithium ion batteries, Nanoscale 7 (2015) 3520e3525. H. Li, Y. Gao, Y. Shao, Y. Su, X. Wang, Vapor-phase atomic layer deposition of Co9S8 and its application for supercapacitors, Nano Lett. 15 (2015) 6689e6695. D. Du, W. Yue, Y. Ren, X. Yang, Fabrication of graphene-encapsulated CoO/ CoFe2O4 composites derived from layered double hydroxides and their application as anode materials for lithium-ion batteries, J. Mater. Sci. 49 (2014) 8031e8039.  n, M.J. Biggs, W. Skinner, T.J. Bandosz, Effect M. Seredych, E. Rodríguez-Castello of visible light and electrode wetting on the capacitive performance of S- and N-doped nanoporous carbons: importance of surface chemistry, Carbon 78 (2014) 540e558. W. Li, S. Zhang, Q. Fan, F. Zhang, S. Xu, Hierarchically scaffolded CoP/CoP2 nanoparticles: controllable synthesis and their application as a well-matched bifunctional electrocatalyst for overall water splitting, Nanoscale 9 (2017) 5677e5685. J. Chen, X. Wang, J. Wang, P.S. Lee, Sulfidation of NiMn-layered double hydroxides/graphene oxide composites toward supercapacitor electrodes with enhanced performance, Adv. Energy Mater. 6 (2016) 1501745. Y. Zhou, D. Yan, H. Xu, J. Feng, X. Jiang, J. Yue, J. Yang, Y. Qian, Hollow nanospheres of mesoporous Co9S8 as a high-capacity and long-life anode for advanced lithium ion batteries, Nano Energy 12 (2015) 528e537. J.H. Jang, B.M. Chae, H.J. Oh, Y.K. Lee, Understanding conversion mechanism of NiO anodic materials for Li-ion battery using in situ X-ray absorption near edge structure spectroscopy, J. Power Sources 304 (2016) 189e195. Z. Li, W. Li, H. Xue, W. Kang, X. Yang, M. Sun, Y. Tang, C.S. Lee, Facile fabrication and electrochemical properties of high-quality reduced graphene oxide/cobalt sulfide composite as anode material for lithium-ion batteries, RSC Adv. 4 (2014) 37180e37186.

494

L. Yang et al. / Journal of Alloys and Compounds 768 (2018) 485e494

[50] J. Mujtaba, H. Sun, G. Huang, Y. Zhao, H. Arandiyan, G. Sun, S. Xu, J. Zhu, Co9S8 nanoparticles encapsulated in nitrogen-doped mesoporous carbon networks with improved lithium storage properties, RSC Adv. 6 (2016) 31775e31781. [51] J.J. Chen, R.M. Yuan, J.M. Feng, Q. Zhang, J.X. Huang, G. Fu, M.S. Zheng, B. Ren, Q.F. Dong, Conductive lewis base matrix to recover the missing link of Li2S8 during the sulfur redox cycle in LieS battery, Chem. Mater. 27 (2015) 2048e2055. [52] Q. Su, G. Du, J. Zhang, Y. Zhong, B. Xu, Y. Yang, S. Neupane, W. Li, In situ transmission electron microscopy observation of electrochemical sodiation of individual Co9S8-filled carbon nanotubes, ACS Nano 8 (2014) 3620e3627. [53] B. Wu, S. Zhang, F. Yao, R. Huo, F. Zhang, S. Xu, Nitrogen-doped carbon and high-content alumina containing bi-active cobalt oxides for efficient storage of lithium, J. Colloid Interface Sci. 462 (2016) 183e190. [54] J. Luo, J. Liu, Z. Zeng, C.F. Ng, L. Ma, H. Zhang, J. Lin, Z. Shen, H.J. Fan, Threedimensional graphene foam supported Fe3O4 lithium battery anodes with long cycle life and high rate capability, Nano Lett. 13 (2013) 6136e6143. [55] M. Chen, J. Liu, D. Chao, J. Wang, J. Yin, J. Lin, H. Fan, Z. Shen, Porous a-Fe2O3 nanorods supported on carbon nanotubes-graphene foam as superior anode for lithium ion batteries, Nano Energy 9 (2014) 364e372. [56] X. Fan, J. Shao, X. Xiao, L. Chen, X. Wang, S. Li, H. Ge, Carbon encapsulated 3D

[57]

[58]

[59]

[60]

[61]

hierarchical Fe3O4 spheres as advanced anode materials with long cycle lifetimes for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 14641e14648. R. Wu, D.P. Wang, X. Rui, B. Liu, K. Zhou, A.W.K. Law, Q. Yan, J. Wei, Z. Chen, Insitu formation of hollow hybrids composed of cobalt sulfides embedded within porous carbon polyhedra/carbon nanotubes for high-performance lithium-ion batteries, Adv. Mater. 27 (2015) 3038e3044. S. Zhu, J. Li, X. Deng, C. He, E. Liu, F. He, C. Shi, N. Zhao, Ultrathin-nanosheetinduced synthesis of 3D transition metal oxides networks for lithium ion battery anodes, Adv. Funct. Mater. 27 (2017) 1605017. F. Wang, S. Zhang, D. Bai, F. Zhang, S. Xu, Graphene-supported binary active Mn0.25Co0.75O solid solution derived from a CoMn-layered double hydroxide precursor for highly improved lithium storage, RSC Adv. 6 (2016) 19716e19722. J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen: an overview of advanced heteroatom doped carbons with boron, sulphur and phosphorus for energy applications, Energy Environ. Sci. 6 (2013) 2839e2855. Y. Xu, G. Jian, Y. Liu, Y. Zhu, M.R. Zachariah, C.S. Wang, Superior electrochemical performance and structure evolution of mesoporous Fe2O3 anodes for lithium-ion batteries, Nano Energy 3 (2014) 26e35.