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NiS2/FeS Holey Film as Freestanding Electrode for High-Performance Lithium Battery Kun Liang, Kyle Marcus, Shoufeng Zhang, Le Zhou, Yilun Li, Samuel T. De Oliveira, Nina Orlovskaya, Yong-Ho Sohn, and Yang Yang* requirement to develop clean, sustainable, and efficient energy-storage systems with high performance.[4] With a two-electron reaction in the electrochemical process, Li-S batteries can deliver a high theoretical specific capacity (1675 mA h g−1) and energy density (2567 W h kg−1).[1,5] Therefore, Li-S batteries are one of the most promising candidates for energy-storage devices. However, significant challenges still remain, such as poor conductivity of S and corresponding products (Li2S and Li2S2), shuttle effects, volume expansion, and fast capacity decay.[6] To overcome these limitations, great progress has been achieved to enhance the electrochemical performance. Zhou et al. proposed a flexible Li-S battery electrode with high sulfur loading by adopting graphene foam-based electrode.[7] The electrode retained an extremely high capacity of 13.4 mA h cm−2 and preserved stable cycling performance with less than 0.1% capacity decay per cycle. Li et al. developed a 3D porous N-doped graphitic carbon–Co composite by using metal organic framework (MOF) polyhedron as precursor.[8] The material delivered a specific capacity of 1670 mA h g−1, which is almost the same as the theoretical specific capacity. To date, most of the efforts have been employed to develop conductive carbon-based materials as cathodes, but the nonpolar feature will reduce the interaction with polar Li2S and Li2Sn, causing poor cycling performance.[9] Very recently, inorganic host materials (TiO2, MnO2, Co9S8, CoS2, MoS2, SnS2) are introduced as cathode materials to further enhance the electrochemical performance.[10–12] Among those metal sulfide cathodes, nickel disulfide (NiS2) is a typical pyrite-type structure compound with good conductivity of 55 S cm−1.[13] Lu et al. fabricated S/NiS2–C composites, which shows good cycling performance and delivered a specific capacity of 730 mA h g−1 after 200 cycles at 0.5 C.[13] It is reported that NiS2 can be used as promising alternative electrocatalyst for polysulfide reduction.[14] As one of iron sulfides, FeS possesses specific electrontransfer ability and lower bandgap, so FeS shows excellent conductivity of 80 S cm−1.[15,16] With high sulfur content, NiS2 can provide a high theoretical specific capacity (870 mA h g−1).[17] However, sluggish diffusivity of Li ion and low surface area contribute to deterioration of specific capacity.[18] FeS possesses excellent electron-transfer ability; therefore, NiS2/FeS composite could accelerate the diffusion of Li ion and intermediate phase evolution.[19] The residual NiFe alloy substrate improves

In this work, a freestanding NiS2/FeS holey film (HF) is prepared after electrochemical anodic and chemical vapor deposition treatments. With the combination of good electrical conductivity and holey structure, the NiS2/FeS HF presents superior electrochemical performance, due to the following reasons: (i) Porous structure of HF provides a large surface area and more active sites/ channels/pathways to enhance the ion/mass diffusion. Moreover, the porous structure can reduce the damage from the volumetric expansion. (ii) The asprepared electrode combines the current collector (residual NiFe alloy) and active materials (sulfides) together, thus reducing the resistance of the electrode. Additionally, the good conductivity of HF can improve electron transport. (iii) Sulfides are more stable as active materials than sulfur, showing only a small capacity decay while retaining high cyclability performance. This work provides a promising way to develop high energy and stable electrode for Li-S battery. In recent decades, Li-ion batteries have been implemented in many kinds of electronics and play a crucial role in energystorage devices.[1,2] With the current advances in nanotechnology, specific capacity and energy density are significantly improved.[3] However, it is still difficult to satisfy the increasing demand for rechargeable energy-storage devices. There is an urgent Dr. K. Liang, K. Marcus, Prof. Y. Yang NanoScience Technology Center University of Central Florida Orlando, FL 32826, USA E-mail: [email protected] K. Marcus, Dr. L. Zhou, Prof. Y.-H. Sohn, Prof. Y. Yang Department of Materials Science and Engineering University of Central Florida Orlando, FL 32826, USA S. Zhang Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry Jilin University Changchun 130023, P. R. China Y. Li Department of Chemistry Rice University 6100 Main Street, Houston, TX 77005, USA S. T. De Oliveira, Prof. N. Orlovskaya Department of Mechanical and Aerospace Engineering University of Central Florida Orlando, FL 32826, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201701309.

DOI: 10.1002/aenm.201701309

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mostly distributed in a range of 10–50 nm, which is typically a mesoporous structure. The morphology was further examined by transmission electron microscopy (TEM). A highly porous structure with a pore size less than 10 nm can be observed, as shown in Figure 1c, confirming the mesoporous structure. The high porosity increases the surface area and provides numerous active sites for ion/mass diffusion, significantly enhancing the electrochemical performance. The selected area electron diffraction pattern, as presented in Figure S2a (Supporting Information), confirms that only the crystalline from NiFe alloy can be observed, indicating amorphous characteristics of the sulfides HF. Energy-dispersive X-ray spectrum elemental mappings were employed, as exhibited in Figures S2b and S3 (Supporting Information), suggesting uniform distributions of Ni, Fe, and S throughout the entire HF. The composition of the as-prepared sulfides HF was analyzed by X-ray diffraction Figure 1.  a) Schematic illustration outlining sulfides HF fabrication. b,c) Top-view SEM and TEM (XRD) and Raman analysis. It is noted that images of HF after sulfuration, respectively. The scale bars donate 200 and 20 nm, respectively. only strong peaks from deposited NiFe alloy can be found in Figure S4 (Supporting Information), further confirming the amorphous feature of sulfides conductivity of the holey film (HF) electrode, enhancing elecHF. Raman scattering is sensitive to the short-range ordering, tron-transport properties. The porous structure can increase which provides a useful approach to amorphous material. As the surface area, shortening the transfer path of electron and Li displayed in Figure 2a, apparent modes at 270 and 476 cm−1 ion.[20] This HF structure combines electron transport and ion transport to augment the electrochemical performance. Addiare well indexed with NiS2 Raman feature.[23] Moreover, the tionally, FeS can be employed as a counter electrode to improve modes from FeS at 214 and 282 cm−1 were identified.[24] From the performance for quantum dots-sensitized solar cells, owing the above analysis, it is known that the as-prepared HF is a to superior activity for the reduction of Sn2− to S2−, which composite of amorphous NiS2/FeS and crystalline NiFe alloy, can reduce the shuttle effect to improve the capacity retenwhich can be directly used as an electrode in lithium battery tion and cycling performance in Li-S batteries.[1,21] Therefore, without additional current collectors. The residual NiFe alloy can be employed as a current collector, which can improve the it is feasible to composite NiS2 together with FeS as cathode electrical conductivity of the electrode. for Lithium battery. Herein, a freestanding NiS2/FeS HF was The chemical composition of sulfides HF was investigated prepared by electrochemical anodic and chemical vapor deposiby X-ray photoelectron spectroscopy (XPS). In the survey scan tion (CVD) treatments. With the combination of good electric spectrum (Figure S5, Supporting Information), Ni, Fe, and S conductivity and high porous structure, the NiS2/FeS HF precan be detected, indicating the sulfide composite was prepared. sents superior electrochemical performance. The as-prepared High-resolution XPS profiles were performed to identify the electrode can deliver a high specific capacity of 580 mA h cm−3, chemical states, as presented in Figure 2b–d. In the Ni 2p3/2 and exhibits small capacity decay with excellent cycling performance. Note that the as-prepared sample is an ultrathin freespectrum, peak located at 852.8 eV can be ascribed to Ni metal, standing HF, which can be used for microsized flexible and which mainly comes from the residual NiFe alloy.[25] As shown wearable electronic devices. Therefore, volumetric capacity (Cv, in Figure 2b, the peaks at the binding energies of 854.6 and 856.0 eV are attributed to 2p3/2 of Ni2+ and Ni3+, respectively.[26] mA h cm−3) is employed to evaluate the capacity in this study.[22] The existence of Ni3+ is due to Ni3S2 or a slight oxidation of A typical process to fabricate a NiS2/FeS HF is schematically illustrated in Figure 1a (for details, see Experimental Section): NiS2 on the surface, which is also reported previously.[27] The (i) NiFe thin film with thickness of 2 µm was electrodeposited ratio of Ni2+ and Ni3+ is estimated to be 11.3:1. Therefore, in a plating solution. (ii) Freestanding film was achieved after the main phase is NiS2. In Figure 2c, the Fe metal peak can removing the substrate. (iii) HF was obtained by electrochembe observed at 707 eV, which is derived from residual NiFe ical anodization. (iv) NiS2/FeS HF was finally produced after in HF.[28] The Fe 2p3/2 peak can be identified at 712 eV, conCVD treatment. A porous structure can be observed (Figure S1, sistent with the reported result of FeS.[29] In the S 2p spectrum, Supporting Information), revealing anodic treatment is useful a shoulder peak at 161.5 eV corresponding to S 2p1/2 can be to create HF with high surface area. From the top-view scanobserved, which is a characteristic peak of FeS.[30] Two fitted ning electron microscopy (SEM) image, a rough surface with peaks located at 162.6 and 163.7 eV are ascribed to the sulfuruniform pores can be found in Figure 1b. The pore size is binding energies of S 2p3/2 and S 2p1/2 in NiS2.[13] A slight shift Adv. Energy Mater. 2017, 7, 1701309

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Figure 2.  a) Raman spectra of the sulfides HF. b–d) XPS profiles of Ni 2p3/2 peak, Fe 2p3/2 peak, and S 2p peak, respectively.

toward lower binding energy can be found in S 2p3/2, due to local band bending effects on the FeS and Ni3S2 surfaces.[31] The peak with binding energy of 168 eV is ascribed to the SO covalent bond, owing to sulfur oxidation in air.[32] The reason why SO cannot be detected in the XRD curves is probably owing to the low concentration in HF. A standard CR 2032 coin cell was assembled to evaluate the electrochemical performances of the amorphous NiS2/ FeS HF cathodes. Cyclic voltammetry (CV) with a scan rate of 0.1 mV s−1 was performed in a voltage window of 1.6–2.8 V to test electrochemical behavior. As presented in Figure 3a, a reduction peak located at 1.85 V may be owing to the transformation of sulfides to Li2NiS2 and Li2FeS. Another strong peak at 1.68 V contributes to the further transformation from Li2S to lithium polysulfides (LiPSs). One oxidation peak with voltages of 2.22 V reveals the reversed conversion.[16] While the other weak oxidation peak at 2.4 V is owing to the soluble Li2S8 to S8.[33] Moreover, the peak current weakens in the second and third cycles, due to the as-produced polysulfide reaction with electrolyte.[12] Small shifts in reduction and oxidation peaks can be observed, denoting a decrease in polarization, which is helpful in improving the cycling performance.[34] The interfacial interactions between the active materials and LiPSs were further investigated by first-principle calculations based on density functional theory (DFT). A 2 × 2 × 1 supercell of the (NiS2)0.6(FeS)0.4-(001) plane was built to simulate the adsorption of Li2S2. The optimized adsorption geometry structure is shown in Figure S6 in the Supporting Information. Theoretical calculations demonstrate that the Li2S2 should interact with the outermost sulfur atoms in prior to obtain stable phase,

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and the calculated binding energy was 1.92 eV, which is smaller than that of NiS2.[13] The smaller binding energy means that the sulfide composite presents stronger corresponded to the moderate LiPSs than the host materials.[35] Galvanostatic charge–discharge profiles with different current densities were carried out to further analyze the electrochemical performance. As shown in Figure 3b, two plateaus around 1.9 and 1.8 V can be observed in discharge curve, associated with the formation of Li2NiS2 and Li2FeS and longchain LiPSs. Moreover, the two-step discharge process implies sluggish electrochemical kinetics. In the charge curve, only a single plateau at 2.2 V is observed, which can be attributed to low Li+ diffusion and can be explained with the combination of Equations (1) and (2).[36] Since the active materials are typical conversion-type cathodes, the transfer mechanism can be descripted as follows[37] NiS2 + 4Li ↔ Ni + 2Li 2S (1) FeS + 2Li ↔ Fe + Li 2S (2) nLi 2S ↔ ( 2n − 2) Li + Li 2Sn (3) With a small current density of 10 mA cm−3, the sulfides electrode can deliver the highest specific capacity of 580 mA h cm−3. With an increase in current density, the charge plateaus maintain stable, while the discharge plateaus and capacity decrease, which is due to the higher resistance and kinetic overpotentials at higher current densities. The rate performance at current densities from 10 to 250 mA cm−3 was further investigated, as presented in Figure 3c

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Figure 3.  Electrochemical performances of sulfides HF electrode. a) Cyclic voltammograms at a scan rate of 0.1 mV s−1. b) Charge–discharge profiles with different current densities. c) Rate performance. The unit of the current density is mA cm−3. d) Long-term cyclability at a current density of 50 mA cm−3.

and Figure S7 in the Supporting Information. The specific capacity is decreased from 580 mA h cm−3 (116 µA h cm−2) to 230 mA h cm−3 (46 µA h cm−2) with increasing current density. The retention of capacity is calculated to be about 40%. However, the discharge capacity can return up to 390 mA h cm−3 (78 µA h cm−2) when the current density is reduced to 10 mA cm−3, indicating excellent rate capability. The areal capacity of sulfides HF is higher than those of the most commercial microbattery systems, but lower than those of Li-S battery systems, due to the low sulfur mass (Table S2, Supporting Information). The areal capacity can meet the requirement for microsized flexible and wearable electronic devices. To further investigate the electrochemical performance, the long-term cycling test was performed at a current density of 50 mA cm−3. As exhibited in Figure 3d, the as-prepared sulfides HF still showed excellent cycling performance at a higher current density. The electrode retained a specific capacity of 270 mA h cm−3 after 1000 cycles. The specific capacity decay rate is only 0.01% per cycle, which is better than that of previously reported.[7,8,11,38] The corresponding Coulombic efficiency of the electrode was nearly 100%, further demonstrating the excellent Li storage mechanism. The specific capacity of 580 mA h cm−3 is nearing that of most commercial cathodes (Table S1, Supporting Information). The as-prepared electrode can deliver an energy density of 1044 mW h cm−3, which is higher than that for most thin-film Li-S batteries (Table S1, Supporting Information). Due to the low sulfur mass and low contribution of NiS2/FeS active Adv. Energy Mater. 2017, 7, 1701309

mass in the electrode, the capacity and energy density values are still lower than those of S-based Li-S battery, but, it is a new way to develop thin-film electrode for Li-S battery. In order to further understand the kinetics of Li-ion insertion/extraction at the interface of electrolyte and electrode and the rate of Li diffusion in HF, CV measurements were performed at different scan rates. As shown in Figure 4a, CV curves were recorded at scan rates of 0.2, 0.5, 0.75, and 1 mV s−1 after 5th, 10th, 15th, and 20th cycles. The voltage window was set to 1.6–2.8 V. It is noted that the peak current increased with increasing the scan rates. The peak current versus square root of scan rate was fitted, as presented in Figure 4b. The slopes of the anodic and cathodic peak are 0.09 and 0.15, respectively, indicating diffusion-limited reactions. In this kind of reaction, the peak current and scan rates follow the Randles–Sevcik equation[39] I p = ( 2.69 × 10 5 ) n 3/2 AD 1/2ν 1/2 ∆C0 (4) where Ip is the peak current, n is the numbers of transfer electron, A is the surface area of electrodes, D is the diffusion coefficient, ν is the scan rate, and ΔC0 is the concentration of reaction. It is worth pointing out that the slope of cathodic peak is higher than that for anodic peak, reflecting the anodic peak covers higher Li-ion transfer coefficient. Electrochemical impedance spectroscopy (EIS) and corresponding equivalent circuit are displayed in Figure S8 in the Supporting Information. The electrode before and after 10, 100, and 1000 cycles clearly shows similar bulk solution resistance,

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Figure 4.  a) CV curves with different scan rates after 5th, 10th, 15th, and 20th cycles. b) Corresponding peak current versus square root of scan rates.

indicating just very little Li2Sx dissolved in the electrolyte. The diameter of the semicircle, meaning the charge-transfer resistance, becomes larger after further cycles, owing to the formation of LiPSs. Warburg resistance is associated to Li-ion diffusion. According to the slope of inclined line in medium-low frequency region, it is found that the Li-ion diffusion is much harder after cycling test, owing to the poor conductivity of asproduced LiPSs to render rapid electron transport in electrode. All above, the freestanding sulfides HF exhibits excellent electrochemical performance as an Li-S battery, due to the following reasons: (i) Porous structure provides a large surface area and more active sites/channels/pathways to enhance the ion/mass diffusion. Moreover, the porous structure can also reduce the damages from volumetric expansion. (ii) The HF integrates the current collector (residual NiFe alloy) and active materials (sulfides) together, reducing the resistance of the electrode. Additionally, the good conductivity of HF can improve electron transport. (iii) The sulfides are more stable as active materials than sulfur, showing only a small capacity decay while retaining high cyclability performance. In summary, a freestanding NiS2/FeS HF was prepared after electrochemical anodic and CVD treatments. With the combination of good electrical conductivity and highly porous structure, the NiS2/FeS HF presents superior electrochemical performance. This work provides a promising way to develop high energy and stable electrode for Li-S battery.

Experimental Section Fabrication of HF: The NiFe deposit was obtained through electrochemical deposition. In a typical process, 0.3 m NiSO4, 0.05 m NiCl2, 0.05 m FeSO4, 0.05 m NaCl, and 0.5 m H3BO3 were dissolved in distilled water. After that, 0.01 m saccharin was added to the electrolyte. The solution was stirred for 60 min at room temperature. A homemade two-electrode system with polished Ni foil as cathode and Pt-coated titanium mesh as anode was employed to deposit NiFe layer. The cathode current density was set at 25 mA cm−2 to perform electrochemical deposition for 10 min. The freestanding Ni layer could be obtained after removing the deposit from the substrate. Subsequent electrochemical anodic treatments were performed with a constant voltage of 20 V for 30 min in an electrolyte of 0.1 m NH4F with 1 m deionized water in ethylene glycol. The sample was washed with ethanol and deionized water three times, then dried under air flow. The sulfuration was performed in a CVD system with two heating zones. Briefly, S powder and anodized sample were placed at the

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upstream and downstream side of the tube in different heating zones, respectively. The tube was evacuated to a pressure of 50 mTorr for 10 min and purged with Ar to remove the residual air. Then, the S powder zone and HF zone were set to 150 and 250 °C, respectively. The reaction was performed for 30 min with Ar (100 sccm) as carrier gas, followed by natural cooling. Characterization: An SEM (ZEISS ULTRA 55) and a high-resolution transmission electron microscopy (FEI Tecnai F30) were used to analyze the morphology and elemental mapping of the samples. A PANalytical Empyrean diffractometer (PANalytical B.V.) configured with a Cu Kα radiation was used to record XRD curves. A Renishaw Raman RE01 scope (Renishaw, Inc.) using a 532 nm excitation argon laser was employed for Raman spectra. Chemical composition of the samples was examined by XPS (Physical Electronics). Electrochemical Measurements: Standard two-electrode 2032 coin cells were assembled with the sulfides HF as the working electrode, and Li foil as the counter electrode. 1 m bis(trifluoromethanesulfonyl)­ imide lithium in a mixture of 1,2-dimethoxyethane and 1,3-dioxolane (v/v = 1:1) was used as electrolyte, then 2 wt% of LiNO3 was added in electrolyte. The as-assembled cells were operated in a voltage window of 1.6–2.8 V. Galvanostatic charge/discharge curves were recorded by a Landt 2001A test system (Wuhan Land Electronic Co. Ltd., China). A CHI 760E electrochemical station (CH Instruments, USA) was used to test CV and EIS with frequency range from 10 mHz to 100 kHz. Computational Methods: The theoretical calculations based on DFT were performed with the CASTEP program embedded in Materials Studio package.[40] The interaction between ionic cores and the valence electrons is described by the projector-augmented wave pseudopotentials, and the exchange–correlation interactions are treated using the Pedrew–Burke–Ernzerhof functional, respectively.[41] The kinetic energy cutoff was set as 300 eV. Here, the doping ratio for NiS2 and FeS was 3:2 according to the experimental characterization. A 2 × 2 × 1 supercell of the (NiS2)0.6(FeS)0.4-(001) plane was built to simulate the adsorption of Li2S2. The 10 Å vacuum slab was added on the top to avoid the boundary effect along the direction of c axis. The binding energy can be obtained by E binding = E (NiS 2 ) 0.6 (FeS) 0.4  E [Li2S 2 ] − E (NiS 2 ) 0.6 (FeS) 0.4 − Li2S 2 



(5)

where E[(NiS2)0.6(FeS)0.4], E[Li2S2], and E[(NiS2)0.6(FeS)0.4 − Li2S2] are the (NiS2)0.6(FeS)0.4-(001) supercell energy, the single Li2S2 molecule energy, and the total energy, respectively.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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Acknowledgements K.L., K.M., and S.Z. contributed equally to this work. Y.Y. and K.L. designed the experiments. K.L. and K.M. performed the materials synthesis and characterization. S.Z. performed the computational simulation. L.Z. and Y.H.S. assisted in TEM. Y.L. assisted in XPS. S.D.O. and N.O. assisted in Raman. K.L. and K.M. wrote the manuscript. Y.Y. corrected the manuscript. All authors approved the manuscript. This work was financially supported by the University of Central Florida through a start-up grant (No. 20080741).

Conflict of Interest The authors declare no conflict of interest.

Keywords high cyclability, holey films, lithium batteries, metal sulfides, porous structures Received: May 14, 2017 Revised: June 9, 2017 Published online: July 21, 2017

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