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Freestanding Mn3O4@CNF/S Paper Cathodes with High Sulfur Loading for Lithium-Sulfur batteries Xin Chen, Lixia Yuan, Zhangxiang Hao, Xiaoxiao Liu, Jingwei Xiang, Zhuoran Zhang, Yunhui Huang, and Jia Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18154 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

Freestanding Mn3O4@CNF/S Paper Cathodes with High Sulfur Loading for Lithium-Sulfur batteries Xin Chen, †,‡ Lixia Yuan,*, ‡ Zhangxiang Hao, ‡ Xiaoxiao Liu, ‡ Jingwei Xiang, ‡ Zhuoran Zhang, ‡ Yunhui Huang‡ and Jia Xie*,† †

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and

Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science

and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China *

Correspondence to J. Xie (email: [email protected]) or to L. Yuan (email: [email protected]).

ABSTRACT: Freestanding paper cathodes with layer-by-layer structure are synthesized for high-loading lithium-sulfur (Li-S) battery. Sulfur is loaded in a three dimensional (3D) interconnected nitrogen-doped carbonfiber (CNF) framework impregnated with Mn3O4 nanoparticles. The 3D-interconnected CNF framework creates an architecture with outstanding mechanical properties. Synergetic effects generated from physical and chemical entrapment could effectively suppress the dissolution and diffusion of the polysulfides. Electrochemical measurements suggest that the rationally designed structure endows the electrode with high utilization of sulfur and good cycle performance. Specifically, the cathode with high areal sulfur loading of 11 mg cm-2 exhibits a reversible areal capacity over 8 mAh cm-2. The fabrication procedure is low cost and readily scalable. We believe that this work will provide a promising choice for potential practical applications. KEYWORDS: lithium-sulfur batteries; free-standing paper cathode; high sulfur loading; nitrogen-doped carbonfiber; Mn3O4 nanoparticle

to meet these two requirements because their theoretical gravimetric energy density is as high as 2600 Wh kg-1 and sulfur as the cathode material is cheap and abundant.2-4 However, severe challenges remain to be overcome before using rechargeable Li-S batteries commercially. Firstly, sulfur cannot be used directly due to its intrinsic insulating property which limits the sulfur loading in the cathode. Secondly, the shuttling of the intermediate lithium polysulfides (LiPSs) in the

INTRODUCTION Since invention, rechargeable lithium ion batteries have been widely applied to power electronic devices, from cars to microchips.1 Recently, the fast growing markets of electric vehicle and grid-scale energy storage not only extend the application of lithium-ion batteries but also bring further requirements especially in reducing cost and increasing energy density. Li-S batteries have the potential 1

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electrolyte leads to the loss of active material and the structural instability of the cathode. The soluble LiPSs can further spread to the anode and react with the lithium anode. Thirdly, during the electrochemical cycle, the sulfur cathode faces large volume change when sulfur is reduced to Li2S, which further destabilize the cathode.5 All of above cathode related problems limit the electrochemical performance of Li-S batteries. To improve sulfur cathode related issues, various strategies have been adopted to fabricate carbon-sulfur cathode composites which can increase the conductivity of the sulfur electrode and contain LiPSs intermediates physically. It is shown that physical confinement by using carbon materials alone, such as meso/microporous carbon6-7, carbon nanotubes8, graphene7, hollow carbon nanofibers/nanosphere9 cannot hold LiPSs well enough. Although these carbon-sulfur materials deliver high specific capacity in the first place, the capacity usually decays quickly after limited cycles because most of the carbon-based materials are not polar and cannot provide enough interaction force in restricting polar LiPSs. Chemical interaction between polar hosts and LiPSs has a significant effect on keeping LiPSs within the cathode. Chemical absorption along with physical containment shows better results in materials, which were based on nitrogen-doped graphene,10 metal-organic frameworks (MOFs),11 TiO212, Ti4O713, Nb2O514, MnO215-19, La2O320 and metal sulfide (TiS2,21 FeS2, CoS2)22. As expected, Li-S batteries based on these composite cathodes exhibit better cycling

performance. However, the high specific capacity in previous reports are often obtained with low sulfur-loading in the electrode (< 2 mg cm-2), which limits their applications in the industry. Thus, it is essential to construct unique structures that exhibits stable cycling performance without reducing the areal energy density. Some unique structured materials with high sulfur-loading have been developed,23 such as sandwich-structured electrode materials,24 gel based sulfur cathodes25 and graphene foam-based electrode,26 and others. Although above works have improved the areal sulfur capacity and gravimetric energy density of Li-S batteries, the researches of low-cost and long-life Li-S batteries with high sulfur mass loading are still challenging at present stage. Herein, inspired by the flexible layer-by-layer structure,27-29 we have designed and prepared a freestanding woven paper electrode. Sulfur is restrained in the interconnected porous Mn3O4@Carbon Nanofibers (Mn3O4@ CNF). In regard to the absorbing materials, most metal oxides (TiO, Ti4O7, Co3O4, etc.) are usually synthesized via complex processing routes. Compared to the previous studies, our method is very simple and economic since only one-step pyrolysis process is needed for the Mn3O4 preparation. In addition to its effective chemical absorption of polysulfides and better stability than MnO2 in the electrolyte after binding with 30 polysulfides, Mn3O4 also has many advantages such as its resource abundance, environmental friendliness and low cost. In addition, Mn3O4 are coated on the 2


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acetate and ethylsilicate （TEOS）in N, N-dimethylformamide (DMF) was assembled into woven nanofibers by electrospinning. After carbonization at 700 ºC in nitrogen atmosphere, manganese acetate was transformed into Mn3O4 nanoparticles and TEOS became SiO2 within the carbonized PAN (cPAN). The Mn3O4/CNF was acquired by removing the SiO2 in NaOH aqueous solution and porous nanofibers were generated. After that, the Mn3O4/CNF paper was cut into small circular pieces to dip into the sulfur-containing slurry and transferred to a vacuum oven at 80 °C overnight. Sulfur was introduced into the nanofibers by heating in a sealed reactor at 155 ºC for 12 h. The binder-free Mn3O4@CNF/S cathodes (sulfur content in electrode: ~50 %) were directly used as electrodes for test. Since the obtained free-standing and woven paper electrode can be used directly without traditional binders, carbon black and current collector, it is possible to achieve high sulfur areal mass loading and areal capacity without any addition. As shown in Figure 1b, the layer-by-layer structured electrodes are designed by stacking layers of Mn3O4@CNF/S to boost the sulfur areal mass loading and areal capacity. By stacking three layers of Mn3O4@CNF/S electrode, the areal sulfur mass loading of 6 mg cm-2 and areal sulfur capacity of 6 mAh cm-2 can be achieved respectively.

carbon nanofibers and the resulted hybrid can guarantee the electronic conductivity to facilitate the charge transfer within the electrode. Besides, the electrospinning technology is a good candidate to produce fibers.31-32 The Mn3O4/CNF hybrid can be easily prepared by electrospinning. We boost the sulfur mass loading to 11 mg cm-2 by simply stacking several layers of Mn3O4@CNF/S, which in turn affords a reversible areal capacity up over 8 mAh cm-2 along with stable cycling performance. The good electrochemical performance can be ascribed to three factors. Firstly, the porous CNF can host sulfides inside the carbon nanofiber and provide physical containment effect as well as void space for nucleation and deposition during lithiation. Secondly, the Mn3O4 nanoparticles distributed inside the nanofiber and on the surface of the nanofiber provide chemical absorption effect to further prevent the dissolution and diffusion of LiPSs. The last but not the least the three-dimensional (3D) interconnected carbon nanofibers offer good ionic and electron transport for the sulfur cathode which allow much higher area sulfur loading.

RESULTS AND DISCUSSION The procedure of fabricating the free-standing paper cathode was schematically illustrated in Figure 1a. Typically, a mixing precursor composed with polyacrylonitrile (PAN)，manganese

3



Figure 1. Schematic diagram of the preparation process for the Mn3O4@CNF/S electrode. (a) Preparation of the Mn3O4@CNF/S composite. (b) Enhancing the thickness of the cathode by stacking layers to boost the sulfur areal mass loading in the coin cells.

Figure 2c-d shows the field-effect scanning electron microscopy (FE-SEM) pictures of the Mn3O4@CNF, which are similar to the precursor nanofibers prepared by electrospinning (Figure 2a-b). The images reveal a smooth carbon surface with uniform fibrous morphology. The fibers with ~500 nm diameter are tangled and interweaved, forming a 3D network configuration. Abundant porous spaces exist in fiber matrix (Figure S1), favoring the active material storage and ionic transport through electrode-electrolyte interfaces. The TEM and high-resolution TEM (HR-TEM) images show a similar thickness of the carbon frame and nanoparticles (Figure 2f-h). The image in Figure 2g reveals that the Mn3O4 nanoparticles have a representative polycrystalline structure with 0.51 nm lattice fringes corresponding to the d-spacings of (101) planes. From the TEM micrographs of the Mn3O4@CNF composite, the size of Mn3O4 nanoparticles is ~ 15 nm. Therefore, the polymer-carbonized process serves not only to carbonize the PAN but also to generate nanoscale

particles in the fibrous carbon. Figure 2e shows the XRD pattern of the nanocomposite. The Mn3O4@CNF diffraction peak of the carbon element is observed at 25.7º. The diffraction peaks of Mn3O4 in Mn3O4@CNF nanocomposites match well with the reported tetragonal structure of Mn3O4 nanoparticles (JCPDS card no. 24-0734), which belongs to the tetragonal hausmannite Mn3O4 structure30. The peak at 2θ values of 18.14, 29.04, 32.53, 36.29, 38.15, 44.59, 50.89, 58.69, 60.06 and 64.78, corresponding to the (101), (112), (103), (211), (004), (220), (105), (321), (224) and (400) planes of body-centered tetragonal hausmannite Mn3O4 33 respectively. Thermogravimetric analysis (TGA) reveals that the content of Mn3O4 is about 20 wt% in the Mn3O4@CNF composite (Figure S2). Nitrogen sorption measurements (Figure S3) reveals that the Mn3O4@CNF hybrid has a specific surface area of about 40 m2 g-1. The shape of the adsorption-desorption isotherms implies the Mn3O4@CNF hybrid possesses mainly macropores. TEM elemental mappings 4


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(Figure 2i-m) in a single nanofiber of Mn3O4@CNF/S further show that most sulfur is uniformly distributed in the nanofibers. The XRD patterns (Figure 2e) of Mn3O4@CNF/S composite suggest that sulfur exists as the conventional orthorhombic structure. Compared to the pure sulfur sample, the intensity of the S peak is reduced obviously in the XRD

pattern of Mn3O4@CNF/S. It suggests that the uniform impregnation and distribution of sulfur within the Mn3O4@CNF/S electrode. The sulfur introduced into the nanofibers is difficult to run away from the Mn3O4 nanofibers due to the physical barrier of the nanofibers and the chemical attraction of nanoparticles.

Figure 2. FE-SEM photographs of (a-b) precursor nanofibers and (c-d) Mn3O4@CNF. (e) X-ray diffraction patterns of Mn3O4@CNF, Mn3O4@CNF/S and pure sulfur. (f-h) TEM photographs of Mn3O4@CNF and high-resolution TEM image of Mn3O4 nanocrystallites with lattice planes. (i-m) Element mappings of Mn3O4@CNF/S.

Cyclic voltammetry (CV) tests are aimed to study the electrochemical behavior of the Mn3O4@CNF/S electrodes (Figure S4). Two pairs of peaks are exhibited, demonstrating typical sulfur

cathode redox reactions. During the discharge process, the peaks centered at 2.25V and 1.9V indicated the formation of the LiPSs intermediate (Li2Sn, 4≤n≤8) accompanied with the phase transfer from 5



solid to liquid, and their further reduction to short-chain LiPSs (Li2S2/Li2S). The two overlapped anodic peaks at around 2.4V and 2.5V belong to the oxidation of Li2S2 and Li2S to the high-order LiPSs respectively and eventually transformed to elemental sulfur. Moreover, it is worth mentioning that the broadened peaks of both the cathodic and anodic peaks suggest slower electrochemical kinetics resulted from the high loading of sulfur (6 mg cm−2) and the increased thickness of the electrode. Figure3a shows the discharge-charge curves of the battery with an areal sulfur loading of 6 mg cm-2 during cycles at 0.2 C. Two discharging platforms located at 2.3 and 2.0 V, and two charging platforms at 2.3 and 2.4 V are shown in the voltage-capacity diagrams. Such results are in accordance with the CV analysis. The cycling performances of the electrodes are shown in Figure 3b. The electrodes are discharged at 0.05 C for 2 cycles to activate the electrode and facilitate the wetting of the electrolyte. 24 The initial discharge capacities of the electrodes are 1130 mAh g-1 (red profile) and 1180 mAh g-1 (black profile). Even after 100 charge/discharge cycles, the electrodes still maintain a high discharge capacity of 780 mAh g-1 at 0.1 C (1 mA cm-2) and 700 mAh g-1 at 0.2 C (2 mA cm-2), with a capacity retention of approximately 70%. At a current density of 0.1 C, the cell with the sulfur loading of 11 mg cm−2 delivers an initial discharge capacity of 11.6 mAh cm-2 (1054.8 mAh g-1) and retains 8.2 mAh cm-2 (744.4 mAh g-1) after 70 cycles (blue profile). At

present stage, it is very difficult to completely avoid the shuttling phenomenon and the loss of active cathode material in lithium sulfur battery, which in turn leads to the decreased coulombic efficiency and decreased specific capacity. The situation will be more severe for high areal sulfur loading electrodes. However, the Mn3O4@CNF/S electrode with high sulfur loadings still exhibit good cycling performance at 0.1 C for 2000 h (100 cycles), which indicates a good improvement in preventing the loss of active sulfur materials in the electrodes and mitigating the shutting phenomenon. The electrode with areal sulfur loading of 6 mg cm-2 is further charged/discharged from 0.05 to 0.8 C rate (1C = 10 mA cm-2) for 5 cycles at each current rate (Figure 3c). Specific capacities as high as 1150, 880, 780, 720 and 570 mAh g-1 are achieved at 0.05, 0.1, 0.3, 0.5 and 0.8 C respectively. Furthermore, the discharge capacity can recover as the current density switches back to the original state, demonstrating the good structure stability of Mn3O4@CNF/S electrode. The results of voltage-capacity diagrams in different rates are shown in Figure 3d. Higher charging/discharging rates lead to lower charge/discharge voltage platforms, but the two discharge plateaus are shown in all tests. For a control experiment purpose, we perform electrochemical tests using Mn3O4@CNF without sulfur as the cathode to determine the capacity contribution of the Mn3O4. The result shows that the capacity contribution of the Mn3O4 to can be basically ignored (Figure S5). 6


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Figure 3. Electrochemical characterization of the Mn3O4@CNF/S electrode. (a) Galvanostatic charge-discharge profiles of the electrode with areal sulfur loading of 6 mg cm−2 at different cycles at 0.2 C. (b) Cycling performances of the electrodes at 0.1 C and 0.2 C. (c) rate capability and d) voltage profiles at various rate capabilities from 0.05 to 0.8 C.

To further study the electrochemical performance of the Mn3O4@CNF/S electrode with the layer-by-layer structure, the Mn3O4@CNF/S cathodes with higher sulfur-loading have also been fabricated. By stacking layers of Mn3O4@CNF/S electrodes together, the areal sulfur loading can be easily increased from 4 (two layers) to 11 mg cm-2 (five layers). At

a current density of 0.05 C, the cells with different sulfur loading (five, four, three, two and one layer) deliver the initial discharge capacity of 11.6 (1055), 10.4 (1100), 9 (1120), 6.9 (1145), 4.8 mAh cm-2 (1190 mAh g-1), respectively, presenting not only good areal capacity but also good specific capacities (Figure 4a). 7



Figure 4. (a) Voltage profiles of layer-by-layer structured Mn3O4@CNF/S electrodes during cycling at 0.05 C. (b) Comparison with recent publications and our reports on Li–S batteries. (c) Areal capacities and columbic efficiency of electrodes with different sulfur areal mass loading at 0.2 C and (d) at 0.1 C.

The cycling performance of the Mn3O4@CNF/S electrodes are shown in Figure 4c and 4d. At a current density of 0.1 C, the cells deliver an initial discharge capacity of 12.3 (1116), 6.78 (1130), 4.8 mAh cm-2 (1200 mAh g-1) and retains 6.3 (570), 4.7 (780), 3 mAh cm-2 (770 mAh g-1) after 100 cycles, corresponding to the area mass loadings of 11, 6, 4 mg cm-2, respectively (Figure 4d, Figure S6b). With cathodes with 9.5 and 8 mgsulfur cm-2 area mass loading, the batteries exhibit the

initial discharge capacity of 10.45 (1100), 9 mAh cm-2 (1120 mAh g-1) and retain 6.7 (710), 5.8 mAh cm-2 (720 mAh g-1) after 100 cycles respectively (Figure 4c, Figure S6a). The Mn3O4@CNF/S electrodes with a slightly lowered sulfur-loading deliver 728 mAh g-1 at 0.5 C after activating for 3 cycles and show good capacity retention of 77% over 200 cycles. (Figure S7). Compared with the reported cathodes which apply well-designed electrode structures to 8


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obtain high-loading sulfur cathodes (Table S1), our work still realizes a very high sulfur loading and areal specific capacity (Figure 4b). In addition, during the charge–discharge cycles, the Mn3O4@CNF/S batteries show high coulombic efficiency over 95%. Above results suggest that the 3D structure of the Mn3O4@CNF cathode and the introduction of Mn3O4 nanoparticles with chemical absorption of LiPSs could suppress the dissolution and diffusion of LiPSs effectively, thus, the cathodes based on Mn3O4@CNF/S composites with high areal sulfur loading demonstrate good electrochemical performance. The interaction between Mn3O4@CNF and LiPSs is investigated by X-ray photoelectron spectroscopy (XPS) analysis. The LiPSs (mainly Li2S6) are dissolved in DOL/DME solvents, forming a lucid yellow solution (1mg mL-1). The Mn3O4@CNFs (20 mg) is added in the above solution (10 mL). The Li2S6 solution fades with the addition of Mn3O4@CNF (Figure S8), indicating strong adsorption capability of Mn3O4@CNF. In contrast, with the addition of CNF, the color of Li2S6 solution remains lucid yellow, similar to the original solution. The S2P spectrum of Li2S6 (Figure 5a) displays two 2p3/2 contributions34 at 164.3/163.1 and 162.9/161.7 eV, which correspond to the bridged S–S (S0) bond and terminal Li–S (S-1) bond35, respectively. Besides, the two contributions exhibit ~2:1 ratio, which is approximately equal to the 2:4 ratio of terminal/bridging sulfur in LiPSs. Figure 5b shows very different S 2p peaks for the Mn3O4@CNF/Li2S6 sample: the peaks of

the bridging and terminal sulfur atoms shift to 165.57/164.39 and 164.38/163.88 eV, with splitting energy of 1.18eV. The peak shifts suggest the existence of S in higher valence states. In addition to the shift of the peak location, the ratio of bridging to terminal sulfur atoms decreases to 1:2. Obviously, the terminal sulfur is affected by Mn3O4 more seriously due to its higher electron density. Furthermore, for the Mn3O4@CNF/Li2S6 sample, the two additional S 2p peaks at 168.09 and 169.16 eV should be ascribed to the SOx species. It could be assumed that Li2S6 is partially oxidized by the Mn3O4 nanoparticles. Figure 5e displays the wide spectrum of the XPS spectra for Mn3O4@CNF composites. The Mn 2p XPS spectra of Mn3O4@CNF in Figure 5c shows the strong peaks at 653.4eV and 641.5 eV, corresponding to the binding energy of Mn 2p1/2 and Mn 2p3/2, respectively, and the energy separation between the Mn 2p1/2 and Mn 2p3/2 peaks is 11.9 eV. The strong peak located at 640.87 eV and 642.9eV (Figure 5d) indicate that the contribution from the Mn4+ ions for the Mn3O4@CNF/Li2S6 sample significantly decreases and the Mn3+ ions increases comparing to the Mn3O4@CNF. These results suggest that Mn3O4 is partially reduced during the electrode process. Similar thiosulfate mechanism has also been reported by Nazar’s group.15 The spectrum of two S 2p sharp peaks located at about 168.09 and 169.16 eV displays the polythionate in the Mn3O4-Li2S6 material, which is considered to be poorly soluble to alleviate the shuttle of LiPSs. The transformation from Mn4+ to Mn3+ also 9



corresponds to the change of S 2p spectrum, resulting in the increasing of the peak of Mn3+ at 642.9 eV in the Mn 2p3/2 XPS spectrum. The dissolution of long-chain LiPSs can be suppressed due to the lower LiPSs transformed by the poorly soluble S2O32- complex. As schematically illustrated in Figure 5f, at the beginning of discharge, LiPSs (Li2Sn, 4≤n≤8) can be transformed to S2O32- firstly due to the reaction with Mn3O4 at the surface. Secondly, the intermediate S2O32- tends to

form short-chain LiPSs (Li2Sn, n

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