Improved Cyclability of LithiumâOxygen Batteries by Synergistic

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Improved Cyclability of Lithium-Oxygen Batteries by Synergistic Catalytic Effects of Two-Dimensional MoS2 Nanosheets Anchored on Hollow Carbon Spheres Anjun Hu, Chaozhu Shu, Xuemei Qiu, Minglu Li, Ruixin Zheng, and Jianping Long ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06496 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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ACS Sustainable Chemistry & Engineering

Improved Cyclability of Lithium−Oxygen Batteries by Synergistic Catalytic Effects of Two-Dimensional MoS2 Nanosheets Anchored on Hollow Carbon Spheres Anjun Hu,† Chaozhu Shu,*,†,‡ Xuemei Qiu,† Minglu Li,† Ruixin Zheng,† Jianping Long,*,† †College

of Materials and Chemistry & Chemical, Engineering, Chengdu University

of Technology, 1#, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, P. R. China ‡Institute

for Superconducting and Electronic Materials, University of Wollongong,

Squires Way, North Wollongong, NSW 2500, Australia *Corresponding author: Email: [email protected] (Chaozhu Shu); [email protected] (Jianping Long)

ABSTRACT: The design and development of high-efficient electrocatalysts plays a decisive role in improving the stability of lithium−oxygen (Li−O2) batteries. Here, two-dimensional (2D) MoS2 nanosheets anchored on hollow carbon spheres (MoS2/HCS) composites is designed and reported as promising cathode catalysts for Li−O2 batteries. The MoS2/HCS based Li−O2 battery shows superior electrochemical performance in terms of high capacity (4010 mA h g−1) and enhanced cycling performance (104 cycles). X-ray photoelectron spectroscopy (XPS) results reveal that the formation of Li2CO3 and other side products can be effectively alleviated when MoS2/HCS electrode is used as cathode. On the basis of

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experimental studies, it is found that the synergistic effects, originated from superior catalytic property of MoS2 nanosheets and good electrical conductivity of HCS with high surface area, is the main reason for performance improvement. The synergistic effects induced by the dispersed MoS2 nanosheets anchored on nanostructured HCS cathodes provide a promising strategy for developing catalysts of O2 electrode for Li−O2 batteries with excellent performance. KEYWORDS: Li−O2 batteries, composite catalysts, MoS2, Hollow carbon spheres, Synergistic effects INTRODUCTION With the remarkable theoretical energy density up to 3600 Wh kg−1, rivaling that of all state-of-the-art secondary batteries, Li−O2 batteries have lead a new wide wave of research attentions in pursuit of technological breakthrough toward practical applications of this energy storage technology.1,2 The electrochemical energy storage in aprotic Li−O2 batteries is based on a reversible redox chemistry: 2(Li+ + e−) + O2 ↔ Li2O2 (E0 = 2.96 V vs Li/Li+), which involves both the oxygen reduction reaction (ORR) for Li2O2 formation during discharging and oxygen evolution reaction (OER) from Li2O2 decomposition during charging on cathodes.3 Since mass transfer pathway and products storage space is indispensable inside the oxygen electrode to complete the electrochemical reaction, porous materials with superior conductivity and


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large surface area are usually utilized.4−6 Carbonaceous materials have been extensively studied as catalysts since they possess excellent electrical conductivity, low cost, light weight, as well as easy fabrication of porous structure.7−9 Among carbonaceous materials, hollow carbon spheres (HCS) have shown growing research interests owing to their tunable porous spherical structure, high porosity, large interior void, and superior conductivity. These merits greatly enrich the already broad application of carbonaceous materials in catalysis and energy storage.10,11 However, these carbonaceous materials are extremely unstable at high charge potentials and easily react with aggressive oxygen species and ether-based electrolytes to generate Li2CO3 and other side products, thereby promoting degradation of the O2 electrode and eventually leading to the premature failure of batteries.12,13 Designing composite cathodes by introducing non-carbon catalysts on the surface of carbonaceous materials is considered to be an effective approach to relieve carbon related parasitic reaction by lowering the charge overpotential.14−16 Two-dimensional

(2D)

nanostructured

transition

metal

dichalcogenides (TMDs) have shown numerous promising applications in energy storage and conversion field.17,18 As a typical member, molybdenum disulfide (MoS2) arouses wide research effort in the exploration of its application in electrochemical energy storage and conversion systems owing to its fascinating catalytic properties.19,20



Among various MoS2-based nanostructures, such as nanosheets, nanospheres, nanotubes, and mesoporous/microporous structures, 2D layered nanosheets stand out due to their highly exposed active edge sites and fast ion diffusion paths.21−23 However, the inherently low electrical conductivity and easy aggregation of MoS2 nanosheets severely hamper their further development.24 Various approaches have been reported in recent years to solve the above issues, in which the composite strategies between the layered MoS2 nanosheet and conductive carbonaceous materials are considered to be a promising approach.25 Herein, porous 2D MoS2 nanosheets grown on the HCS (MoS2/HCS) is fabricated and studied as effective O2 electrode catalyst for Li−O2 battery. The uniform HCS templated by sacrificial SiO2 spheres serve as substrates for the growth of MoS2 nanosheets. By designing such a unique porous architecture, the synergistic effects between MoS2 nanosheets with high catalytic activity and HCS with high electronic conductivity and large surface area in the MoS2/HCS composites can remarkably improve performance of Li−O2 batteries. Moreover, the instability issue of HCS caused by their reactivity toward Li2O2 and electrolytes can be alleviated since the effective cover of MoS2 nanosheets on HCS surface, leading to improved cyclability of Li−O2 batteries. Compared with Li−O2 batteries based on single component (MoS2-only and HCS-only), the MoS2/HCS based Li−O2 battery exhibits


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enhanced cycling stability (up to 104 cycles), ultimately implying that MoS2/HCS composite serves as effective electrocatalysts for Li−O2 batteries in terms of improved energy efficiency as well as cyclability.

EXPERIMENTAL SECTION Materials synthesis Synthesis of Hollow Carbon Spheres (HCS). First, 3.46 mL of Tetrapropylorthosilicate (TPOS) was dissolved into a dispersion including 70 mL of ethanol, 10 mL of deionized (DI) water, and 3 mL of NH3·H2O (25 wt%) and then kept stirring for 15 min. After that, 0.56 mL of formaldehyde (37 wt%) and 0.4 g of resorcinol were sequentially dissolved into the above dispersion and kept stirring for 24 h. After that, the precipitates were collected with the aid of centrifugation, and dried at 60 °C for 12 h. Finally, the sample was carbonized at 700 °C for 5 h under N2 atmosphere and SiO2 templates were removed by hydrofluoric acid (HF) (10 wt%) to obtain HCS for further use. Synthesis of MoS2 Nanosheets Anchored on HCS (MoS2/HCS). The obtained 100 mg of HCS was dissolved into 60 mL of DI water containing (NH4)6Mo7O24·4H2O (0.1 mmol) and thiourea (2.8 mmol) by stirring. Then the above solution was transferred into an autoclave with capacity of 100 mL and the reaction was conducted at 200 °C for 24 h. Finally, the final sample was rinsed with DI water several times followed by drying at 60 °C for 12 h.



Synthesis of Pristine MoS2 Nanosheets. For comparison, the pristine MoS2 nanosheets were also prepared via the same method as those for MoS2/HCS in the absence of HCS.

Materials characterization The morphological evolutions of catalysts were observed using field-emission SEM (FEI Inspect F50) and TEM (JEOL 2100F). Crystallographic structures were obtained by using XRD (Bruker D8 ADVANCE). Raman spectra were analyzed on Renishaw 200 Raman microscope. The valence state of surface elements was analyzed by using XPS (ESCALAB 250Xi). Thermogravimetric analysis (TGA) was carried out on TA Q500 thermal analyzer system. The N2 adsorption/desorption isotherms were tested on a specific surface and pore analyzer (Micrometrics ASAP 2460).

Cathode preparation The inks were obtained by mixing 80 wt% samples (HCS, MoS2, MoS2/HCS) and 20 wt% poly(vinylidene fluoride) (PVDF) in N-methyl-2-pyrrolidone (NMP). After that, the inks were pasted on carbon papers. For comparison, the MoS2@HCS electrode was fabricated by mechanical mixture of 80 wt% MoS2 and HCS (7:3 by weight based on TGA result) and 20 wt% PVDF in NMP. All the above cathodes were dried at 100 °C prior to use.

Assembly and Measurements of Li−O2 batteries


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The electronic conductivity of MoS2/HCS, MoS2@HCS, MoS2, and HCS catalysts

were

obtained

on

a

thermoelectric

tester

(Joule

Yacht/NAMICRO-3L), as shown in Table S1. The electrochemical measurements were conducted by using CR2032-typed cell with air windows on the side of cathodes. A Li−O2 battery was fabricated in a glove box filled with Ar using a Li anode, a glass fiber separator, 1 M bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) dissolved in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte and an as-fabricated cathode. The cyclic voltammetry (CV) analysis were carried out at scan rate of 0.05 mV s−1 within a voltage range of 2.0−4.5 V on an electrochemical workstation (Autolab/PGSTAT302N). The galvanostatic discharge-charge curves were obtained on a battery tester (LAND CT 2001A) within a voltage range of 2.0−4.5 V. The specific capacities and current densities were normalized on the basis of the mass of catalyst and binder.

RESULTS AND DISCUSSION Structure and composition of catalysts The

fabrication

strategy

of

MoS2/HCS

composites

was

schematically illustrated in Figure 1. First, SiO2 sphere templates were prepared using sol-gel method by a modified Stöber reaction26 followed by

encapsulated

resorcinol-formaldehyde

with (RF)

polymer by

a

layer

derived

from

condensation-polymerization



procedure, forming the SiO2@SiO2/RF spheres with a diameter of ∼150 nm (Figure S1a). After that, the SiO2@SiO2/RF spheres were converted into SiO2@SiO2/C through carbonization under N2 environment (Figure S1b). Then, the silica template was etched from SiO2@SiO2/C by hydrofluoric acid (HF) treatment, forming the HCS. Finally, MoS2 nanosheets were successfully anchored onto HCS surface (MoS2/HCS) by a facile hydrothermal method.

Figure 1. Schematic illustration of the fabrication of MoS2/HCS composites. The morphologies of HCS, pristine MoS2, and MoS2/HCS were characterized by FE-SEM and TEM. The hollow structural HCS with ordered pores on the carbon shells are obtained with uniform size distribution (Figure 2a-c). For each hollow sphere, the thickness of the ACS Paragon Plus Environment

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shell is ~30 nm and envelops a large hollow space with diameter of ~150 nm. The MoS2 nanosheets synthesized via the same method except for the absence of HCS are severely stacked together forming a dense nanosheet-assembled structure (Figure S2 and S3). During the hydrothermal process, the HCS were served as the substrate on which the MoS2 nanosheets grew to form the MoS2/HCS while the porous carbon shell keeps structure integrity. The huge surface area induced by the HCS enables numerous exposed sites for supporting MoS2 nanosheets and relieves its agglomeration. Interestingly, after the formation of MoS2 layer, the MoS2/HCS with coarse surface can be observed compared with the smooth surface of primary HCS, indicating the formation of dense MoS2 nanosheets coating (Figures 2d-e). As shown by the corresponding FESEM images (Figures S4 and S5), the HCS particles and MoS2/HCS have

good

dispersibility

and

uniform

size

distribution.

From

high-resolution TEM result of MoS2/HCS (Figure 2f), the layer number of MoS2 nanosheets anchored onto the HCS is in the range of four and nine. The measured lattice spacing of MoS2 in MoS2/HCS is approximately 0.65 nm, which corresponds to the (002) plane of hexagonal-phase (2H) MoS2 (Figure 2g). The selected-area electron diffraction (SAED) result of MoS2/HCS (Figure 2i) reveals the distinct diffraction ring patterns of (110), (103), (101), and (002), indicating the polycrystal nature of as-synthesized MoS2/HCS composites. As shown in



Figure 2i, the EDX mappings of MoS2/HCS show the homogenous distribution of Mo, S, and C elements on the hollow shell, further indicating the successful loading of MoS2 nanosheets over HCS surface.

Figure 2. TEM images of (a-c) HCS and (d-f) MoS2/HCS, (g) HRTEM image, (h) SAED pattern and (i) EDX mappings of C, Mo, and S of MoS2/HCS. The crystal structures and phase purities of pristine MoS2 and MoS2/HCS were analyzed by XRD. The XRD patterns of pristine MoS2 and MoS2/HCS shown in Figure 3a are clearly indexed to the 2H-MoS2 ACS Paragon Plus Environment

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(JCPDS Card No. 37-1492). Note that, the (002) peak of MoS2/HCS is presented at 14.1°, which is lower compared with the (002) peak of MoS2 (14.4°) (insert of Figure 3a). This blue shift indicates an increased lattice spacing of the (002) plane in MoS2/HCS, which can also be verified from Figures 2g and S3b, where the lattice spacing increased from 0.62 to 0.65 nm. The presence of 2H-MoS2 in MoS2/HCS are further confirmed by Raman spectra, as displayed in Figure 3b. Two peaks of pristine MoS2 centered at 384.3 and 411.6 cm−1 are in good agreement with E1 2g and A1g phonon modes of 2H-MoS2, respectively, while the above two peaks are moved to 380.9 and 405.5 cm−1 in MoS2/HCS. It has been proved that as the layer number of MoS2 reduces, the frequency difference between the E1 2g and A1g will gradually decrease.27 Thus, the decrease of frequency difference for MoS2/HCS (25.4 cm−1) as compared to pristine MoS2 (27.3 cm−1) suggested that the layer number of MoS2 nanosheets in MoS2/HCS is lower than that in pristine MoS2 (insert of Figure 3b), which is also consistent with the results shown in Figures 2f and S3b. Additionally, the characteristic peaks centered at 1341 and 1593 cm−1 in MoS2/HCS can be attributed to the D and G bands of carbon materials, respectively. The surface chemical states of MoS2/HCS were studied by XPS. As shown in Figure S6, the XPS survey spectra corroborated the existence of C, Mo, S, and O elements. The atomic ratio of Mo to S is approximately 1:2, which correspond to the EDX result



(Figure S7), matching the nominal ratio of MoS2. As shown in Figure 3c, the strong peaks in C 1s spectrum centered at 287.4, 286.2, and 284.8 eV are attributed to C=O, C−O, and C−C, respectively. For Mo 3d spectrum, two peaks at 233.1 and 229.5 eV can be assigned to the Mo 3d3/2 and Mo 3d5/2, respectively, suggesting the presence of Mo4+ (Figure 3d).28 Notably, the single peak of 236.5 eV can be ascribed to the oxidized Mo in air.29 Two peaks at 162.4 and 163.8 eV in S 2p spectrum correspond to the S 2p3/2 and S 2p1/2, respectively (Figure 3e). The loading mass of MoS2 in the MoS2/HCS was quantitatively determined to be approximately 70.16% by thermogravimetric analysis (TGA) (Figure S8). The N2 adsorption-desorption isotherm curves were conducted to obtained surface areas and pore structures of three samples. The MoS2/HCS with a IV-shaped isotherm characterized by a long and narrow hysteresis loop at relative pressure (P/P0) from 0.4 to 1.0 (Figure 3f) indicates its mesoporous feature.30 Benefiting from the introduction of HCS with well-defined hollow structure (Figure S9), the surface area and pore volume of MoS2/HCS are 244.98 m2 g−1 and 0.41 cm³ g−1, respectively, which are larger than those of bare MoS2 (6.37 m2 g−1 and 0.02 cm³ g−1), as shown in Figure S10. The pore-size distribution curves of MoS2/HCS were calculated on the basis of the adsorption curve by using the Barrett-Joyner-Halenda (BJH) model (inset of Figure 3f). The pore size of MoS2/HCS exhibited bimodal distributions with larger pore


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(7.66 nm) contributed to HCS (inset of Figure S9) and smaller pore (3.88 nm) associated with MoS2 nanosheets (inset of Figure S10). The high surface area and large pore volume of MoS2/HCS electrode result in copious exposed sites for reactions and enough space to accommodate products, which is expected to facilitate performance improvement (vide infra).



Figure 3. (a) XRD patterns and (b) Raman spectra of pristine MoS2 and MoS2/HCS composites (the inset is an enlarged view for pristine MoS2 and MoS2/HCS composites). High-resolution XPS spectra of (c) C 1s, (d) Mo 3d, and (e) S 2p of MoS2/HCS composites. (f) N2 adsorption-desorption isotherm and pore-size distribution (inset) of the MoS2/HCS.

Electrochemical Performance of Li−O2 batteries To better verify the synergetic catalytic effects of MoS2/HCS on improving battery performance, control electrodes, including mechanical mixture of MoS2 and HCS (MoS2@HCS), bare MoS2, and bare HCS electrodes, were also studied (cathode fabrication is shown in Experimental Section). For better understanding of the catalytic property, cyclic voltammetry (CV) curve of Li−O2 batteries with MoS2/HCS, MoS2@HCS, MoS2, and HCS cathodes were conducted under O2, as shown in Figure S11. Compared with other three cathodes, the MoS2/HCS cathode exhibited significantly lower OER and higher ORR onset potentials as well as obviously larger anodic and cathodic peak currents, indicating the best catalytic activity of MoS2/HCS towards ORR and OER among the studied electrodes. In general, the broad ORR peak at approximately 2.35 V could be assigned to the formation of Li2O2, while the OER peaks around 3.3~3.6 V could be attributed to the oxidation of Li2O2.31,32 The full discharge and charge curves of Li−O2


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batteries based on the MoS2/HCS, MoS2@HCS, bare MoS2, bare HCS cathodes were shown in Figures 4a-c and S12-14. It can be seen from Figures 4a and b that the MoS2/HCS based Li−O2 battery can achieve a capacity up to 4010 mA h g−1 due to the improved ORR activity via synergistic catalytic effects, while capacities of MoS2@HCS, MoS2, and HCS electrodes were 3741 mA h g−1, 3107 mA h g−1 and 3505 mA h g−1, respectively. Additionally, the MoS2/HCS based Li−O2 battery showed a negligible discharge capacity (62 mA h g−1) under Ar (Figure S15), suggesting that the excellent capacity of MoS2/HCS cathode was indeed derived from the ORR rather than lithium intercalation reaction. For the charging process, the highest Coulombic efficiency of approximately 96.4% was obtained from MoS2/HCS cathode while other cathodes showed relatively poor Coulombic efficiencies of approximately 93.5% (MoS2@HCS), 92.6% (MoS2), and 85.8% (HCS), respectively. Notably, Li−O2 batteries based on MoS2@HCS, bare MoS2, and HCS cathodes exhibited rapid capacity decay after 10 cycles. In contrast, the MoS2/HCS cathode exhibited a more stable cyclability and higher capacity of 1500 mA h g−1 with a superior Coulombic efficiency of 93.12% after the 10th cycle. The greatly improved electrochemical behavior is primarily derived from the synergistic effects of two components in MoS2/HCS. The in-situ chemical growth of MoS2 nanosheets on HCS enables intimate contact between the two components. In addition, the MoS2



nanosheets on HCS surface can effectively protect HCS from the direct reaction with Li2O2 without weakening the conductivity of HCS. However, the mechanically mixed MoS2@HCS electrode exhibits lower capacity and unsatisfying cycling life because of poor contact between two components, which not only limits the catalytic activity of MoS2 on the HCS, but also leads to an unprotected HCS surface. In the case of bare MoS2 electrode, although the cycle performance keeps relatively stable, the poor capacity of bare MoS2 based Li−O2 battery is observed because of its inherent low conductivity. For bare HCS electrode, the absence of both non-carbon catalysts and effective protection of carbon leads to extremely fast capacity decay. The rate capability was evaluated at various current densities (Figure 4d). The discharge voltage plateau of the MoS2/HCS based Li−O2 battery was obviously higher than those of other three cathodes at each different rate. The achieved excellent rate capability of the MoS2/HCS cathode originates from the following synergetic merits: (i) the superior catalytic activity of MoS2 nanosheets and high electronic conductivity of HCS can trigger a “highway” for fast charge transport; (ii) the ordered porous structure of oxygen electrode induced by HCS can avoid clogging of the channels by the discharge product and ensure the fast diffusion of Li+ and O2 during cycling.


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Figure 4. (a) Full discharge and charge curves and (b) corresponding cycling capacities and Coulombic efficiency of MoS2/HCS based Li−O2 battery during 10 cycles. (c) Comparison of capacity retentions in selected cycles of MoS2/HCS, MoS2@HCS, MoS2, and HCS electrodes. (d) Rate capability of these four electrodes. The long-term cyclability is another prerequisite for the practical applications of Li-O2 batteries. When tested in a fully discharged/charge cycling mode, O2 is reduced to form O2−* intermediate (这里需要说明* 代表什么) on the cathode surface. O2−* is proved to be highly aggressive towards cell components (such as electrolyte and carbon base electrode) and causes decomposition of them to generate side products on the



cathode surface.33 These side products can not be effectively decomposed during charging, resulting in poor energy efficiency and the battery life. Therefore, the capacity-fixed cycling scheme is widely employed to investigate the cycling stability. To this end, the cyclability of MoS2/HCS, MoS2 and HCS cathodes were investigated at rate of 200 mA g−1 with limited capacity of 1000 mA h g−1. It is worth noting that the MoS2@HCS electrode is not performed under this protocol due to its poorer cycling performance than that of the MoS2/HCS electrode in full discharge and charge tests. Figure 5 shows the cycling performance of MoS2/HCS, MoS2 and HCS based Li−O2 batteries. The MoS2/HCS cathode can keep stable for more than 104 cycles when the voltage fall below 2.0 V. In contrast, the MoS2 and HCS electrodes can only operate for 82 and 61 cycles, respectively, under the same condition. Figure 6 displays the comparison of the discharge and charge voltage curves for three cathodes over the selected 61 cycles. As shown in Figures 6a and b, the MoS2/HCS based Li−O2 battery remains stable over 61 cycles with no obvious change for the discharge/charge voltage platforms, leading to a relatively low overpotential of 1.44 V. However, the terminal voltage of the MoS2/HCS cathode dropped sharply after 61 cycles, which can be ascribed to the formation of unwanted side products from binder degradation due to the extreme nucleophilicity of the superoxide radical intermediates formed during discharge/charge34,35 and the pore clogging


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caused by the accumulation of insoluble film formed on the cathode surface (see ex-situ SEM results below) 36. In contrast, the MoS2 and HCS based batteries exhibit more dramatically increase of overpotentials upon cycling (Figures 6c and d), resulting in the fast decay of terminal voltage. Undoubtedly, the above results clearly show the difference in cycling performance of these electrodes, the MoS2/HCS cathode exhibited a lower charge overpotential than other two cathodes, indicating the synergistic effects of the MoS2/HCS cathode endows the reaction with faster OER/ORR kinetics. The extremely low overpotential endowed the MoS2/HCS cathode with a longer cycle life. In sharp contrast, the high overpotential of MoS2 and HCS cathodes will further accelerate the electrolyte decomposition, thus forming numerous side products. Therefore, the copious accumulation of residual Li2O2 and side products during cycling leads to a larger overpotential, resulting in rapid degradation of battery performance. The above superior electrochemical behavior of the MoS2/HCS cathode originates from the highly catalytic MoS2 nanosheets modified conductive HCS. Moreover, the synergistic effects of HCS substrate and MoS2 can indeed enable the Li−O2 batteries with enhanced cycle stability. For comparison, the MoS2/HCS electrode reported herein demonstrated a very competitive performance in terms of cyclability, compared with reported Li−O2 batteries based on other carbon electrodes and the results are given in Table S2.



Figure 5. Cycling performance and the variation of the terminal discharge voltages for the Li−O2 batteries based on MoS2/HCS, MoS2, and HCS electrodes at rate of 200 mA g−1 with limited capacity of 1000 mA h g−1.

Figure 6. (a) Discharge and charge curves (times vs voltage) of the Li−O2 batteries with the MoS2/HCS, MoS2, and HCS cathodes during 61 cycles


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and corresponding voltage curves at rate of 200 mA g−1 with limited capacity of 1000 mA h g−1.

Discharge/charge products on the cathode surface To investigate the improved cyclability of the MoS2/HCS cathode, the ex-situ SEM, XRD, and XPS characterizations were carried out to analyze the morphological and structural changes of three electrodes during different cycles. As shown in Figures 7a-i, the typical film-shaped products completely covered on the surface of all three electrodes after discharging and then obviously disappeared after charging. The corresponding XRD results (Figure S16) suggested that the discharge products of all three cathodes were predominantly Li2O2 at initial discharge stage, and then they are completely decomposed after the first cycle. Therefore, at least during the initial cycle, all three electrodes were capable of reversibly forming and decomposing Li2O2 products.



Figure 7. SEM images of (a-c) MoS2/HCS, (d-f) MoS2, and (g-i) HCS electrodes (a, d, g) in pristine states, (b, e, h) after the 1st discharge to 2.0 V and (c, f, i) after the 1st charge to 4.3V at 200 mA g−1. The MoS2/HCS cathode after various cycles and the different cathodes after 61 cycles were then investigated by XPS characterizations. The Li 1s spectrums were shown in Figure 8. After the first charge, Li2O2 (54.3 eV) 37 could barely be observed (Figures 8a and S17), indicating its fully decomposition by the MoS2/HCS, MoS2, and HCS electrodes in agreement with the results of XRD and SEM in Figures 7a-i and S16. ACS Paragon Plus Environment

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However, as the cycle number of the MoS2/HCS electrode increases to 61 and 104 cycles, both Li2CO3 (55.5 eV)37 and other side products (56.3 eV)38 appeared to remain and were not completely decomposed, which may cause irreversible chemical reactions in TEGDME electrolyte (Figures 8a).39 Therefore, even if HCS is effectively covered by MoS2 nanosheets in MoS2/HCS, certain extent of fading in cycle life seems to be inevitable. However, compared with the other two electrodes, the lowest peak areas of Li2CO3 and side products were observed after 61 cycles by using MoS2/HCS electrode (Figures 8b). Moreover, SEM results of the three electrodes after 61 cycles showed that MoS2 and HCS electrodes were unable to recover to the initial state (Figures S18-19), while MoS2/HCS electrode can maintain the pristine structure, although some discharge products and side products left on its surface (Figure S20). The above results indicate that the synergistic catalytic activity of MoS2 nanosheets and HCS can effectively control and retard the formation of side products on MoS2/HCS electrode. On the one hand, the porous structure of HCS provides large porosity and high surface area, which leads to effective mass transport pathway and provides enough deposition space for Li2O2 formation. On the other hand, the introduction of MoS2 nanosheets significantly enhances the catalytic activity for Li2O2 decomposition, while the MoS2 nanosheets grown on HCS maintain structural integrity during cycling.



Figure 8. XPS spectra of (a) MoS2/HCS cathode after various9 cycles and (b) various electrodes (MoS2/HCS, MoS2, and HCS) after 61th cycle. The greatly improved electrocatalytic properties of MoS2/HCS cathode can benefit from the following synergistic effects of HCS matrix and MoS2 nanosheets, as illustrated in Figure 9a. Firstly, the in-situ chemical growth of MoS2 nanosheets on HCS enables intimate contact between two components. The MoS2 nanosheets anchored onto HCS surface with the large specific area can be uniformly dispersed and thus expose more active sites to accelerate the reaction kinetics, while the nanosheets can effectively protect HCS from the direct reaction with Li2O2 without reducing the conductivity of HCS, leading to reduced undesired side products. In addition, the superior electronic conductivity of HCS and good catalytic property of MoS2 nanosheets can provide a fast pathway for charge transfer and mass transport; Last but not least, the ordered porous structure of oxygen electrode induced by HCS can avoid clogging of the channels by the discharge product and ensure effective diffusion of O2 and Li+ during cycling, while the MoS2 nanosheets grown on HCS ACS Paragon Plus Environment

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maintain structural integrity during cycling. These above desirable credentials together indicate that the synergistic effects between MoS2 nanosheets and HCS is critical for the improvement of catalytic properties. Figure 9b schematically illustrates the reaction process on MoS2/HCS during cycling. The MoS2/HCS cathode is capable of promoting the reversible formation and decomposition of Li2O2 effectively due to the synergistic effects. In discharging step, conductive HCS with high surface area and MoS2 nanosheets with excellent catalytic activity can contribute to the formation of Li2O2 in complementary manner. Li2O2 first easily grows on the surface of MoS2/HCS electrode under the surface mediated mechanism,40,41 and then grows into thin film that is connected tightly with the surface of oxygen electrode. For decomposition of Li2O2, the porous structure induced by the HCS facilitates the Li+ and O2 diffusion, whereas electron transfer for Li2O2 conversion reactions can be accelerated along conductive HCS. Furthermore, the good contact between film-like products and catalysts facilitates the decomposition of Li2O2 because O2 and Li+ can be released from the film surface,42,43 ultimately leading to enhanced electrochemical reversibility of MoS2/HCS based Li−O2 battery.



Figure 9. Schematic of the (a) synergistic effects and (b) electrochemical reactions of MoS2/HCS cathode.

CONCLUSION In this study, MoS2/HCS composite catalyst with HCS as a hollow shell on which MoS2 nanosheets were anchored was developed and studied as catalysts for Li−O2 batteries. The results show that MoS2/HCS electrode exhibited promising performance in terms of a high capacity, reduced overpotential, and enhanced cyclability. The improved performance of this tunable porous spherical structure of MoS2/HCS can be attributed to the synergistic effects of MoS2 nanosheets with good catalytic activities and HCS shell with high electronic conductivity, facilitating the ORR and OER during cycling. The formation of Li2CO3 and other side products can be effectively reduced by using this composite, ultimately enabling improved cycling stability of MoS2/HCS based Li−O2 battery. Therefore, strategy of anchoring highly active MoS2 nanosheets on uniformly dispersed nanostructural HCS is expected to provide a new way to design other highly efficient O2 electrode catalysts for Li−O2 batteries.

ASSOCIATED CONTENT ACS Paragon Plus Environment

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SEM, TEM and HRTEM images; XPS survey spectrum; EDS result; TGA curve; N2 absorption-desorption isotherm and pore-size distribution results; CV curves; Full discharge and charge profiles; Comparison of cycling capacities and Coulombic efficiencies; ex-situ XRD, XPS, and SEM results; Performance comparison with other carbon-based cathodes.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C. Z. Shu) *E-mail: [email protected] (J. P. Long) ORCID A. J. Hu: 0000-0003-4025-0330 J. P. Long: 0000-0001-7245-8991 C. Z. Shu: 0000-0003-4025-0330

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was funded by the Cultivating Program of Middle-Aged Key Teachers of CDUT (KYGG201709). The author greatly appreciate CDUT for the financial support to UOW. REFERENCES



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TOC/Abstract Graphic

Two-dimensional MoS2 nanosheets anchored on hollow carbon spheres composites (MoS2/HCS) is designed and reported as promising O2 electrode catalysts for Li−O2 batteries.


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