Nickel disulfide nanosheet as promising cathode electrocatalyst for long-life lithium–oxygen batteries

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Nickel disulfide nanosheet as promising cathode electrocatalyst for long-life lithium–oxygen batteries Bobae Ju 1, Hee Jo Song 1, Gwang-Hee Lee, Myeong-Chang Sung, Dong-Wan Kim * School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 02841, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Nickel disulfide Lithium-oxygen battery Electrocatalyst Carbon-free Long-term stability

Lithium–oxygen batteries (LOBs) are considered as next-generation energy storage systems owing to their high energy densities. In order to achieve high-performance LOBs, it is necessary to develop efficient electrocatalysts that exhibit reversible formation and decomposition of discharge products on the oxygen-electrode side. In this study, single-crystalline NiS2 nanosheets (NiS2-NSs) are fabricated as an efficient electrocatalyst in an oxygenelectrode for high-performance LOBs. Ni(OH)2-NSs are prepared through a hydrothermal reaction and subsequently reacted with sulfur by a solid/gas phase reaction process to form NiS2-NSs. As an electrocatalyst in an oxygen-electrode, the single-crystalline NiS2-NSs can reversibly form and decompose the discharge products during the discharging and charging processes, respectively. In particular, the NiS2-NSs more effectively decompose the discharge products compared to the Ni(OH)2-NSs owing to its high affinity to oxygenated species. In addition, the NiS2-NSs exhibit a long-term cyclability over 300 cycles at a current density of 1000 mA g
1 with a cut-off capacity of 1000 mA h g
1. Moreover, NiS2-NSs without conducting agent exhibit an electrocatalytic activity and its LOB performance can be further maximized through addition of a redox mediator.

1. Introduction Rechargeable lithium–oxygen batteries (LOBs) attract significant attention for application in next-generation automobile technologies owing to their high theoretical specific energy density of 3505 W h kg
1, which is approximately 5–10 times higher than those of conventional lithium-ion batteries (LIBs) [1–3]. In an LOB system using an aprotic electrolyte, the electrochemical reaction between Li-ion and oxygen can be expressed as: 2Liþ þ 2e
þ O2 ↔ Li2O2 (E0 ¼ 2.96 V vs. Li/Liþ), during which an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) occur upon the discharging and charging processes, respectively [1,2]. However, some critical issues such as the low energy efficiency and the short cycle life hinder their applications [4]. In particular, the sluggish kinetics of the ORR and OER lead to a high potential gap between the ORR and OER. In order to overcome this obstacle, it is necessary to use an effective electrocatalyst in an oxygen-electrode (or cathode) [5,6]. Extensive studies have been carried out to develop suitable electrocatalysts for LOBs, such as noble metals (Au, Pt, Ru) and transition-metal oxides, carbides, and nitrides [7–12]. However, considering the significant drawbacks, including the high cost and the low catalytic activity and stability, it is required to develop novel

electrocatalysts for LOBs. Transition-metal sulfides (TMSs) attract interest as energy conversion and storage materials owing to their low costs, earth-abundance, and higher electron conductivities and electrochemical activities compared to those of their oxide counterparts [13–16]. Recently, they have been reported as LOB electrodes. For example, Sennu et al. synthesized Co3S4 with a high reversibility of 95.72% during the first discharging and charging process [17]. Dou et al. and Lin et al. reported an excellent intrinsic oxygen affinity of Co9S8; nanocage- and sisal-shaped Co9S8 structures exhibited discharge capacities of 7000 and 6875 mA h g
1, respectively, at a current density of 50 mA g
1 [13,18]. Additionally, Ma et al. reported a flower-like NiS, which exhibited a full-discharge capacity of 6733 mA h g
1 at a current density of 75 mA g
1 [19]. However, although NiS2 has been reported in diverse applications such as water splitting electrocatalysts and supercapacitors [20,21], it has not been reported as an electrocatalyst for LOBs. To the best of our knowledge, for the first time, we utilized a twodimensional (2D) single-crystalline NiS2 nanosheets (NiS2-NSs) as an effective ORR/OER electrocatalyst for LOBs. As the number of crystal boundaries in a single-crystalline structure is smaller than that of a polycrystalline structure, the single-crystal structure promotes electron

* Corresponding author. E-mail address: [email protected] (D.-W. Kim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ensm.2019.06.017 Received 20 April 2019; Received in revised form 7 June 2019; Accepted 18 June 2019 Available online xxxx 2405-8297/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: B. Ju et al., Nickel disulfide nanosheet as promising cathode electrocatalyst for long-life lithium–oxygen batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.06.017

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Fig. 1. (a) XRD patterns of the Ni(OH)2-NSs and NiS2-NSs. FESEM images of the (b) Ni(OH)2-NSs and (c) NiS2-NSs. (d) Survey, (e) Ni 2p, and (f) S 2p XPS of the Ni(OH)2-NSs and NiS2-NSs.

analysis was performed by X-ray photoelectron spectroscopy (XPS, Kalpha þ, Thermo Fisher Scientific) and Raman spectroscopy (LabRam ARAMIS IR2, HORIBA JOBIN YVON).

movement and exhibits an excellent catalytic activity [22,23]. In addition, the 2D morphology is suitable for large numbers of Li-ion and O2 species [24,25]. Single-crystalline NiS2-NSs were obtained through a hydrothermal reaction and a subsequent solid/gas phase reaction. The synthesized NiS2-NSs electrode exhibited effective formation and decomposition of the discharge products and excellent cycle stability. In addition, a NiS2-NSs electrode without conducting agent exhibited a good electrocatalytic performance.

2.4. Electrochemical measurements The electrochemical performances of the electrocatalysts were evaluated using Swagelok-type cells with gas holes. The oxygen-electrodes were prepared by mixing the as-prepared electrocatalysts (45 wt%), Super P carbon black (45 wt%, MMM Carbon), and polytetrafluoroethylene (PTFE, 10 wt%, Aldrich), followed by coating on a Ni foam. The loading weight on the Ni foam was adjusted to 0.5 mg. The cells consisted of a lithium foil as an anode, glass fiber (Whatman) as a separator, 1 M of LiNO3 in N,N-dimethylacetamide (DMAc, Aldrich) as an electrolyte, oxygen-electrode, and carbon cloth as a gas diffusion layer. 0.1 M of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, Aldrich) was added as a redox mediator. All of the cells were assembled in an Ar-filled glove box. All of the measurements were carried out under a 1.5-bar dry oxygen atmosphere (>99.999%) to avoid negative effects of humidity. Cyclic voltammetry (CV) and galvanostatic discharge–charge profiles were recorded using an automatic battery cycler (WBCS 3000, WonaTech). An electrochemical impedance spectroscopy (EIS) analysis was performed with an electrochemical workstation (Ivium-n-Stat electrochemical analyzer, Ivium Technologies B⋅V.). The impedance response was collected by applying AC voltages of 10 mV while maintaining a constant DC voltage in the frequency range of 0.01 Hz–100 kHz. All of the above measurements were conducted at room temperature.

2. Experimental section 2.1. Synthesis of a Ni(OH)2-NSs precursor The Ni(OH)2-NSs precursor was synthesized by a simple hydrothermal method. First, Ni(OCOCH3)2⋅4H2O (8 mmol, 98%, Sigma-Aldrich) was completely dissolved in deionized water (100 mL) under magnetic stirring. Second, the pH of the solution was adjusted to 9.1–9.2 using an ammonia solution (28–30%). Third, the solution was transferred to a 200-mL Teflon-lined autoclave and heated at 170  C for 12 h. After the hydrothermal reaction, the product was washed several times with deionized water and ethanol, and then dried to obtain a Ni(OH)2-NSs precursor. 2.2. Synthesis of NiS2-NSs The NiS2-NSs were synthesized through a solid/gas phase reaction. Ni(OH)2-NSs (46 mg) and sulfur powder (322 mg) were placed in the center and upstream zones of a quartz tube, respectively, in a single-zone horizontal tube furnace. After flushing with Ar gas, the tube furnace was heated to 400  C at a ramp rate of 5  C min
1 and kept at this temperature for 1 h under an Ar gas flow (20 sccm).

3. Results and discussion Fig. 1a shows the XRD patterns of the Ni(OH)2-NSs and NiS2-NSs. After the hydrothermal reaction, a crystalline Ni(OH)2 phase (hexagonal polymorph, JCPDS No. 14–0117) was obtained; no secondary peaks were observed (lower panel in Fig. 1a). After the thermal reaction of the Ni(OH)2-NSs precursor with the sulfur powder, the Ni(OH)2 phase was completely transformed to the NiS2 phase (cubic polymorph, JCPDS No. 11–0099); no secondary peaks were observed (upper panel in Fig. 1a). However, there were some differences in XRD peak ratios between the

2.3. Characterization The crystal structures were analyzed using X-ray diffraction (XRD, Smartlab, Rigaku). The morphologies were identified by field-emission scanning electron microscopy (FESEM, SU-70, Hitachi) and transmission electron microscopy (TEM, JEM-2100F, JEOL). The chemical 2

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Fig. 2. (a,d) TEM images, (b,e) HRTEM images, and (c,f) corresponding FFT patterns of the Ni(OH)2-NSs and NiS2-NSs, respectively. (g) Schematic of the phase transformation from the Ni(OH)2-NSs to the NiS2-NSs.

partial surface oxidation of the NiS2-NSs owing to the air exposure [30]. In the S 2p spectrum of the NiS2-NSs, two peaks at 162.5 and 163.7 eV corresponding to the Ni–S bonding are observed, which are not observed in that of the Ni(OH)2-NSs (Fig. 1f) [30]. This demonstrates the successful phase transformation from the Ni(OH)2-NSs to the NiS2-NSs. The morphologies and microstructural properties of the Ni(OH)2-NSs and NiS2-NSs were characterized in detail by TEM. Fig. 2a–c show TEM images of the Ni(OH)2-NSs. Ni(OH)2-NSs exhibited a sheet morphology with sheet sizes of 100–200 nm (Fig. 2a), which is consistent with the SEM image in Fig. 1b. In the high-resolution TEM (HRTEM) image, the Ni(OH)2-NSs exhibited continuous lattice fringes (Fig. 2b), indicating a good crystallinity of Ni(OH)2. In addition, three sets of lattice fringes had the same interplanar spacing of 0.27 nm corresponding to the (100) planes of the hexagonal Ni(OH)2 structure. Furthermore, the angles between adjacent lattice fringes were 120 . The fast Fourier transform (FFT) pattern of the Ni(OH)2-NSs for the [001] zone axis was also indexed to the (100) plane (Fig. 2c), revealing the single-crystal form of the Ni(OH)2-NSs with a preferential {001} orientation, as expected from the XRD data. Fig. 2d–f show TEM images of the NiS2-NSs. The NiS2-NSs also exhibited a sheet morphology (Fig. 2d), confirming the small change in the morphology between the Ni(OH)2-NSs and NiS2-NSs. In the HRTEM image, the NiS2-NSs also exhibited a high crystallinity and clear lattice fringes in the whole region (Fig. 2e). Two sets of lattice fringes had the same interplanar spacing of 0.28 nm under an angle of 90 , corresponding to the {200} planes of the cubic NiS2 structure. The FFT pattern of the NiS2-NSs for the [001] zone axis was also indexed to the (200) planes (Fig. 2f), revealing the single-crystal form of the NiS2-NSs with a

measured and reference data. The measured peak intensity ratio of the (001) plane to the (100) plane (I(001)/I(100)) for the Ni(OH)2-NSs was 3.2, while that calculated from reference data was 2.2. Similarly, the measured and calculated peak intensity ratios of the (200) and (111) planes (I(200)/I(111)) for the NiS2-NSs were 5.5 and 5.0, respectively, implying possible crystallographic orientations of the Ni(OH)2-NSs and NiS2-NSs. Fig. 1b and c show SEM images of the Ni(OH)2-NSs and NiS2-NSs, respectively. The Ni(OH)2 precursor exhibited a uniform thin-nanosheet morphology (Fig. 1b). Although the NiS2-NSs had slightly rough surfaces owing to the gas release and dehydration of Ni(OH)2-NSs during the annealing [26], a negligible change in sheet-shaped morphology of the NiS2-NSs was observed, which indicates that the NiS2-NSs maintained their initial morphology after the thermal reaction at 400  C (Fig. 1c). The chemical states and molecular environments of the Ni(OH)2-NSs and NiS2-NSs were characterized by XPS (Fig. 1d–f). As shown in Fig. 1d, both survey spectra show Ni 2p and O 1s bands corresponding to the Ni(OH)2-NSs and NiS2-NSs, whereas S 2s and 2p bands were detected only for NiS2-NSs. High-resolution XPS of Ni 2p and S 2p were measured near 890–850 and 170–160 eV, respectively. Both Ni 2p XPS can be deconvoluted into spin–orbit doublets and shake-up satellites (Fig. 1e). For the Ni(OH)2-NSs, the Ni 2p spectrum showed only one pair of spin–orbit doublet peaks at 855.4 and 872.9 eV in the 2p3/2 and 2p1/2 regions, respectively, corresponding to the Ni–O bonding [27]. However, two pairs of spin–orbit doublet peaks were observed in the XPS of the NiS2-NSs. The intense peaks at 854.2 and 871.3 eV represent the Ni–S bonding, which are consistent with a previously reported NiS2 structure [28,29], while the two peaks at 855.8 and 873.7 eV are attributed to the

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Fig. 3. (a) First and second galvanostatic discharge–charge curves at a current density of 500 mA g
1. (b) CV curves of the Ni(OH)2-NSs and NiS2-NSs at a scan rate of 0.1 mV s
1.

Fig. 4. (a–c) Ex-situ SEM images and (d–f) XPS of the NiS2-NSs before and after the first discharge–charge cycle: (a,d) fresh, (b,e) after the first discharged state, and (c,f) after the first charged state. The cells were measured at a current density of 1000 mA g
1 and cut-off capacity of 1000 mA h g
1. (g) Schematic of the formation and decomposition of the discharge products during the cycling.

calculated based on the weights of the electrocatalysts because Super P electrode exhibited inferior electrochemical ORR/OER activities and poor cycling performance (Fig. S1). Fig. 3a shows galvanostatic discharge–charge profiles of the Ni(OH)2-NSs and NiS2-NSs electrodes at a current density of 500 mA g
1. The discharge step was analyzed at a cutoff potential of 2.0 V, while the charge step was carried out to the same discharge capacity. As shown in Fig. 3a, the NiS2-NSs electrode exhibited a first discharge capacity of 22500 mA h g
1, which is much higher than that of the Ni(OH)2-NSs electrode (18900 mA h g
1). In the subsequent

preferential {200} orientation. When the crystal structure was changed, the preferred crystallographic orientation changed from the (001) plane in the hexagonal structure to the (200) plane in the cubic structure (Fig. 2g). The electrochemical properties of the Ni(OH)2-NSs and NiS2-NSs for LOBs were evaluated. As-synthesized Ni(OH)2-NSs and NiS2-NSs were employed in oxygen-electrodes, which consisted of Super P carbon black as a conductive agent and Ni(OH)2-NSs or NiS2-NSs as electrocatalysts at a ratio of 1:1. All of the electrochemical measurement results were 4

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Fig. 5. Galvanostatic discharge–charge curves of the (a) Ni(OH)2-NSs and (b) NiS2-NS electrodes at a current density of 500 mA g
1 and those of the (c) NiS2-NSs electrode at a current density of 1000 mA g
1. (d) Cycling performances of the Ni(OH)2-NSs and NiS2-NSs electrodes at a current density of 1000 mA g
1. (e) Comparison of the cut-off capacity and cycle performance with those of previously reported TMS-based electrocatalysts.

clearly observed (Fig. 4c). The formation and decomposition of the discharge products were further analyzed by XPS (Fig. 4d–f). For the fresh electrode (Fig. 4d), no Li 1s peak was observed in the XPS. However, the Li 1s spectrum was observed after the first discharge step (Fig. 4e). The well-defined peaks at 54.5 and 55.4 eV correspond to Li2O2 and Li2CO3, respectively [22,32]. After the charge step, the NiS2-NSs electrode exhibited no trace of Li2O2, indicating the reversible decomposition of the discharge product on the NiS2-NSs electrode. Therefore, the NiS2-NSs can act as an ORR/OER electrocatalyst in an LOB (Fig. 4f). The film-like morphology of the discharge products, which uniformly covered the electrode surface, can promote a larger charge transfer than that in the bulk Li2O2 and enable a rapid decomposition of the discharge products during the charging [33]. Li2CO3 was formed by the oxidation of the Super P in the electrode or by the reaction with the electrolyte [33–35]. The small amount of Li2CO3 remained after the recharging may have a small effect on the cycling performance. Fig. 4g illustrates the formation and decomposition of the discharge products during the discharge–charge cycling process. In addition, we further investigated the reversible formation and decomposition of the discharge products by Raman spectroscopy (Fig. S4). The NiS2-NSs electrode after discharging showed the shoulder-like peak at around 790 cm
1 corresponding to the formation of Li2O2, whereas this peak did not appear in the fresh and charged electrode. Meanwhile, the extra peaks at 480 cm
1 were attributed to the vibration of A1g mode of NiS2 [36]. Fig. 5a and b show galvanostatic discharge–charge profiles of the Ni(OH)2-NSs and NiS2-NSs electrodes in a potential range of 2.0–4.8 V at a current density of 500 mA g
1 and cut-off capacity of 1000 mA h g
1. Both Ni(OH)2-NSs and NiS2-NSs electrodes exhibited similar first discharge profiles with a potential plateau around 2.7 V. In addition, they exhibited the same ORR activity up to 100 cycles. However, the NiS2-NSs electrode exhibited a lower potential than that of the Ni(OH)2-NSs electrode during the first charging process, indicating an efficient

charge step, the NiS2-NSs exhibited a longer plateau around 3.7 V than that of the Ni(OH)2-NSs. In the second cycle, the NiS2-NSs also exhibited a higher discharge capacity of 14000 mA h g
1 than that of the Ni(OH)2NSs (10000 mA h g
1). Furthermore, in the second charge step, the NiS2NSs exhibited a plateau similar to that of the first step at 3.7 V, whereas the Ni(OH)2-NSs exhibited a short plateau and their overpotential rapidly increased. Fig. 3b shows the cyclic voltammetry (CV) results at a potential range of 2.3–4.3 V obtained at a scan rate of 0.1 mV s
1. Compared to that of the Ni(OH)2-NSs electrode, the CV curve of the NiS2NSs electrode showed higher ORR and OER current densities during the cathodic and anodic scans. Furthermore, the NiS2-NSs had smaller specific surface area than that of the Ni(OH)2-NSs (Fig. S2 and Table S1). These suggest that the NiS2-NSs had a catalytic role in the discharge and charge process and higher electrocatalytic activity than that of the Ni(OH)2-NSs. As the sulfur atoms in the NiS2-NSs had high affinities to oxygenated species, such as O2* and LiO2*, the NiS2-NSs exhibited a higher electrocatalytic activity than that of the Ni(OH)2-NSs [31]. Meanwhile, both the Ni(OH)2-NSs and NiS2-NSs electrodes showed low electrochemical reactions when measured in Ar-saturated electrolyte (Fig. S3). So, most of the electrochemical reactions corresponded to the formation/decomposition of the discharge products by catalytic activities of the NiS2-NSs (or Ni(OH)2-NSs), not the electrochemical reaction between Li-ion and NiS2-NSs (or Ni(OH)2-NSs). In order to demonstrate the reversible formation and decomposition of the discharge product in the NiS2-NSs electrode, the morphologies and chemical states of the electrode surface before and after the cycling were investigated. Fig. 4a–c show ex-situ SEM images of the NiS2-NSs electrode before and after the first discharging and charging at a current density of 1000 mA g
1 and cut-off capacity of 1000 mA h g
1. The NiS2NSs were clearly observed on the surface of the fresh electrode (Fig. 4a). After the discharging, discharge products with a film-like morphology were formed on the electrode surface (Fig. 4b). After the charging, most of the discharge products were decomposed and the NiS2-NSs could be

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Fig. 6. Galvanostatic discharge–charge curves at the (a) 1st, (b) 50th, and (c) 100th cycles of the 50-wt% NiS2-NSs, 100-wt% NiS2-NSs, and 100-wt% NiS2-NSs with TEMPO at a current density of 500 mA g
1. (d) Cycle stability of the 100-wt% NiS2-NS electrode.

performance evaluation (Fig. 5d). Compared to the Ni(OH)2-NSs electrode, the NiS2-NSs electrode exhibited a lower discharge–charge potential gap and decomposed discharge products at a lower potential in all of the cycles, yielding a good cycling stability. Its high performance is superior to those of previously reported TMS electrocatalysts such as MoS2 NSs with Au nanoparticles (50 cycles, 200 mA g
1), nanorod Bi2S3 on nickel foam (146 cycles, 500 mA g
1), flower-like MoS2 with carbon nanotubes (CNTs) (141 cycles, 100 mA g
1), and MoS2 with CNTs (132 cycles, 200 mA g
1) (Fig. 5e and Table S2) [37–40]. Further, the electrochemical properties of NiS2-NSs electrode without conducting agent were evaluated. Fig. 6a–c show galvanostatic discharge–charge profiles based on 50 wt% of NiS2-NSs (including 50 wt% Super P), 100 wt% of NiS2-NSs, and 100 wt% of NiS2-NSs with TEMPO at a current density of 500 mA g
1 and cut-off capacity of 1000 mA h g
1. Although the carbon-free NiS2-NSs electrode exhibited comparable ORR/ OER properties during the first cycle and stable discharge profiles up to 100 cycles compared to those of the 50 wt% NiS2-NSs electrode (Fig. 6a), a higher potential was required to decompose the discharge products, which was attributed to the absence of the conducting agent (Fig. 6b). However, this electrode exhibited a stable cycling performance up to 100 cycles (Fig. 6b and c). The carbon-free NiS2-NSs electrode exhibited 235 cycles in the galvanostatic discharge–charge cycling performance evaluation (Fig. 6d). It is known that an electrode containing carbonaceous materials promotes the formation of insulating byproducts from the decomposition of the electrolyte, leading to degradation in LOB performance [35,41]. As the NiS2-NSs without carbon can act as an electrocatalyst, they can be utilized as a carbon-free electrocatalyst. In order to optimize the LOB system, we also measured the electrochemical performance of the carbon-free electrode upon addition of the well-known redox mediator, TEMPO, which easily decomposes Li2O2 to O2 and Li-ion in the electrolyte [42,43]. As expected, the carbon-free NiS2-NSs

decomposition of the discharge products. The charge potential of the NiS2-NSs electrode increased during the initial cycles; subsequently, stable charge profiles were observed after the 20th cycle. Indeed, the Ni 2p peaks at 854.2 and 871.3 eV corresponding to the Ni–S binding were still observed in the Ni 2p XPS after the 20th discharging and charging process, which demonstrates that the NiS2-NSs electrode acted as an electrocatalyst after the 20th cycle (Fig. S5). In addition, the NiS2-NSs electrode exhibited a lower potential in all charge profiles up to 100 cycles. After 100 cycles, the potential gaps of the Ni(OH)2-NSs and NiS2NSs electrodes between the discharge and charge potentials at half capacity were 1.58 and 1.42 V, respectively, indicating a higher electrocatalytic OER activity of the NiS2-NSs electrode than that of the Ni(OH)2NSs electrode. An EIS analysis was carried out to further analyze the kinetics of the Ni(OH)2-NSs and NiS2-NSs electrodes. In the Nyquist plots, the NiS2-NSs electrode exhibited a significantly smaller semicircle (2483, 929, 634 Ω mg
1 at OCV, after 5th and 10th charged state, respectively) than that of the Ni(OH)2-NSs electrode (4617,1581, 1410 Ω mg
1), demonstrating a faster charge transfer of NiS2 than that of Ni(OH)2, which led to the higher ORR and OER activities (Fig. S6). Fig. 5c shows galvanostatic discharge–charge profiles of the NiS2-NSs electrode measured at a higher current density of 1000 mA g
1. The NiS2NSs electrode exhibited a similar tendency in discharge profiles with a potential plateau around 2.7 V. The potential gap of the NiS2-NSs electrode increased owing to the increase in the rate. In addition, stable charge profiles were observed even after the continuous cycling. After 150 cycles, the Ni(OH)2-NSs electrode exhibited unstable discharge–charge profiles. The terminal potential gap, particularly at the discharge potential, rapidly increased, while the NiS2-NSs electrode maintained a constant potential gap over a longer cycling (Fig. 5c and Fig. S7). The NiS2-NSs and Ni(OH)2-NSs electrodes exhibited 314 and 177 cycles, respectively, in the galvanostatic discharge–charge cycling 6

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electrode with TEMPO exhibited a stable potential plateau during the first charging process (Fig. 6a). Furthermore, it exhibited a superior electrochemical performance with a potential gap of only 1.07 V between the discharge–charge curves even at the 100th cycle (Fig. 6b and c).

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4. Conclusion In summary, we developed single-crystalline NiS2-NSs as an electrocatalyst in an oxygen-electrode for rechargeable LOBs. The singlecrystalline Ni(OH)2-NSs were synthesized by the hydrothermal synthesis and then reacted with evaporated sulfur through the gas/solid phase reaction to form the single-crystalline NiS2-NSs. As an electrocatalyst in the oxygen-electrode, the NiS2-NSs exhibited excellent ORR/OER performances with a low potential gap of 1.42 V after 100 cycles. Particularly, they exhibited a stable charge profile over a long-term cycling at a fixed capacity of 1000 mA h g
1 and high current rate of 1000 mA g
1. In addition, the carbon-free NiS2-NSs electrode exhibited effective electrocatalytic properties. TEMPO could maximize the electrochemical performance of the carbon-free NiS2-NSs electrode. Therefore, the developed NiS2-NSs are very promising for application as an LOB electrocatalyst. Conflicts of interest There are no conflicts to declare. Acknowledgements This work is supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT, South Korea (2019R1A2B5B02070203) and by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT, South Korea (2018M3D1A1058744). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.06.017. References [1] L. Ma, T. Yu, E. Tzoganakis, K. Amine, T. Wu, Z. Chen, J. Lu, Fundamental understanding and material challenges in rechargeable nonaqueous Li-O2 batteries: recent progress and perspective, Adv. Energy Mater. 8 (2018) 1800348. [2] P. Zhang, Y. Zhao, X. Zhang, Functional and stability orientation synthesis of materials and structures in aprotic Li-O2 batteries, Chem. Soc. Rev. 47 (2018) 2921–3004. [3] J.-C. Kim, G.-H. Lee, S. Lee, S.-I. Oh, Y. Kang, D.-W. Kim, Tailored porous ZnCo2O4 nanofibrous electrocatalysts for lithium-oxygen batteries, Adv. Mater. Interfaces 5 (2018) 1701234. [4] Z. Chang, J. Xu, X. Zhang, Recent progress in electrocatalyst for Li-O2 batteries, Adv. Energy Mater. 7 (2017) 1700875. [5] Y.J. Lee, S.H. Park, S.H. Kim, Y. Ko, K. Kang, Y.J. Lee, High-rate and high-arealcapacity air cathodes with enhanced cycle life based on RuO2/MnO2 bifunctional electrocatalysts supported on CNT for pragmatic Li–O2 batteries, ACS Catal. 8 (2018) 2923–2934. [6] G.-H. Lee, S. Lee, J.-C. Kim, D.W. Kim, Y. Kang, D.-W. Kim, MnMoO4 electrocatalysts for superior long-life and high-rate lithium-oxygen batteries, Adv. Energy Mater. 7 (2017) 1601741. [7] R.A. Wong, C. Yang, A. Dutta, M. O, M. Hong, M.L. Thomas, K. Yamanaka, T. Ohta, K. Waki, H.R. Byon, Critically examining the role of nanocatalysts in Li–O2 batteries: viability toward suppression of recharge overpotential, rechargeability, and cyclability, ACS Energy Lett 3 (2018) 592–597. [8] J. Kim, H. Jo, M. Wu, D.H. Yoon, Y. Kang, H.K. Jung, Mesoporous amorphous binary Ru-Ti oxides as bifunctional catalysts for non-aqueous Li-O2 batteries, Nanotechnology 28 (2017) 145401. [9] G.-H. Lee, M.-C. Sung, J.-C. Kim, H.J. Song, D.-W. Kim, Synergistic effect of CuGeO3/graphene composites for efficient oxygen-electrode electrocatalysts in LiO2 batteries, Adv. Energy Mater. 8 (2018) 1801930. [10] W.-J. Kwak, K.C. Lau, C.-D. Shin, K. Amine, L.A. Curtiss, Y.-K. Sun, A Mo2C/carbon nanotube composite cathode for lithium-oxygen batteries with high energy efficiency and long cycle life, ACS Nano 9 (2015) 4129–4137.

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