Author’s Accepted Manuscript Efficient sulfur host based on NiCo2O4 hollow microtubes for advanced Li-S batteries Azhar Iqbal, Zahid Ali Ghazi, Abdul Muqsit Khattak, Aziz Ahmad www.elsevier.com/locate/yjssc
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S0022-4596(17)30365-1 http://dx.doi.org/10.1016/j.jssc.2017.09.009 YJSSC19939
To appear in: Journal of Solid State Chemistry Received date: 3 July 2017 Revised date: 28 August 2017 Accepted date: 11 September 2017 Cite this article as: Azhar Iqbal, Zahid Ali Ghazi, Abdul Muqsit Khattak and Aziz Ahmad, Efficient sulfur host based on NiCo2O4 hollow microtubes for advanced Li-S batteries, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.09.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Efficient sulfur host based on NiCo2O4 hollow microtubes for advanced Li-S batteries Azhar Iqbal1*, Zahid Ali Ghazi2, Abdul Muqsit Khattak2, Aziz Ahmad2
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Institute of Engineering Research, Hefei Guoxuan High-tech Power Energy Co., Ltd, Hefei,
230011, P. R. China. 2
National Center for Nanoscience and Technology, Beijing 100190, P. R. China
*Corresponding author: Azhar Iqbal Email:
[email protected] Tel: +86-551-62100921 Fax: +86-551-62100123 Address: Institute of Engineering Research, Hefei Guoxuan High-tech Power Energy Co., Ltd, Hefei, 230011, P. R. China.
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Abstract High energy density and cost effectiveness make lithium-sulfur battery a promising candidate for next-generation electrochemical energy storage technology. Here, we have synthesized a highly efficient sulfur host namely NiCo2O4 hollow microtubes/sulfur composite (NiCo2O4/S). The hollow interior cavity providing structural integrity while sufficient self-functionalized surfaces of NiCo2O4 chemically bind polysulfides to prevent their dissolution in the organic electrolyte. When used in lithium-sulfur batteries, the synthesized NiCo2O4/S cathode delivers high specific capacity (1274 mAh g-1 at 0.2 C), long cycling performance at 0.5 C, and good rate capability at high current rates.
Graphical abstract
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Key Words: Hollow microtubes; Sulfur host; Metal oxides; Li-Sulfur batteries; Polysulfide confinement; Cycling performance.
1. Introduction Lithium-sulfur batteries having six-fold specific energy compared with conventional lithium-ion batteries have attracted tremendous attention. For the emerging energy market demands, Lithium–sulfur batteries are one of the most promising candidates [1-3], as they possess a theoretical capacity and energy density of 1,675 mAh g-1 and 2,500 Wh kg-1, respectively, superior to the state-of-art lithium-ion batteries. Factors that contribute to the advancement of LiS batteries are the relatively low mass of sulfur, high abundance and its variable oxidation states. The reversible reaction taking place in Li-S batteries is; Li+ + 2e- xS
Li2Sx
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Although significant developments have been made for designing state-of-the-art Li–S batteries in the past two decades, practical applications are currently hindered by several obstacles [4]. Capacity degradation and low charging efficiency are the main challenges for the practical application of lithium-sulfur batteries [5-7]. Both of these problems are related to the “polysulfide shuttle effect”, attributed to the dissolution of lithium polysulfides (Li 2Sn, the series of sulfur reduction intermediates) into liquid electrolytes, leading to the loss of active material (i.e., sulfur) during cell operation. On charge/discharge cycling, the dissolved lithium polysulfides diffuse to the negative (lithium) electrode, where they are chemically reduced to form thick insoluble and insulating layers of Li2S2−x on the surface of the metallic lithium, resulting in high surface impedance [8]. This not only causes capacity decay, but also leads to the polysulfides shuttle effect. A large fraction of the capacity is consumed by redox reactions of polysulfides at both the electrodes surfaces [9, 10].
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In the past decades, to address these challenging issues, various strategies have been developed to improve the conductivity of the active material and trap the polysulfides within the cathode side, such as investigating new electrolytes[11-13], modifying the separator [14-17], protecting the lithium anode [18,19] and inserting polysulfides blocking interlayers [20,21]. Significant progresses on sulfur-based cathodes have been made in recent years, spanning from nanocomposites with nanoporous carbon [22, 23], surface coating [24], new binders [25, 26]. Non-polar carbon materials can physically confine polysulfides in the pores but this strategy is effective only on short- and medium-term charge/ discharge cycling. Because of the weak intermolecular interactions, the polysulfides diffuse out of the cathodes and join the flow towards the anode. Recently, cathode (sulfur host) materials that show strong chemical interactions with polysulfides have been studied and appear to be an effective approach to trap polysulfides and stabilize the capacity [27]. Beyond carbonaceous compounds, materials such as metal chalcogenides (oxides and sulfides) have been evaluated. Li et.al developed a very efficient sulfur host based on conductive polar TiO@C hollow nanospheres for lithium-sulfur batteries. The synthesized host (TiO@C hollow nanospheres) having high conductivity and strong adsorption ability for lihium polysulfides (LiPSs), thus resulted in high discharge capacity of >1100 mAh/g at 0.1 C along with stable cycling performance. Based on DFT calculation results, it is assumed that the electrochemical performance of the TiO@C-HS/S composite originates from the unique surface chemical properties of Titanium monoxide (TiO) [28]. These materials possess intrinsic polarity, where the surface metal or chalcogen ions synergistically interact with Sx2- and Li+ ions. Spectroscopic evidence of the chemical interaction between these oxides and polysulfides is clear [29]. Herein, we present hollow microtubes consisted of NiCo2O4 as the sulfur host. The concept of NiCo2O4 hollow microtubes and sulfur composite (hereafter denoted as NiCo2O4/S) has several advantages. Specifically, the hollow interior cavity provides structural stability by accommodating volume expansion during cycling while outer hollow NiCo2O4 mixed metal oxide nanosheets not only provide large amount of sulfur encapsulation but also offer a relatively large functional surface for chemically binding polysulfides and thus preventing their dissolution. Consequently, when evaluated as cathode material for lithium-sulfur batteries, the as
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prepared NiCo2O4/S composite exhibits improved electrochemical performances. The interior hollow provides structural integrity
2. Experimental 2.1 Preparation of hollow NiCo2O4 microtubes NiCo2O4 hollow microtubes were prepared using a previously reported method with some modification [30]. Typically, 3 ml of 1,3-propanediol was used to completely dissolve 0.5 mmol of Ni(Ac)2·4H2O and 1.0 mmol of Co(Ac)2·4H2O. To this solution 47 ml of isoprapanol was added and a pink solution was obtained. Then, the resulting mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 160 °C for 12 h. After cooling to room temperature naturally, the solution was centrifuged at 8000 rpm for 10 min to isolate the precipitate. The obtained precipitate was washed with ethanol for several times and vacuum dried in an oven at 80 °C overnight. Finally, the as-prepared precursor was calcined at 300 °C in air for 3 h with a ramping rate of 2 °C min-1 to obtain NiCo2O4 hollow microtubes. After thermal treatment the obtained NiCo2O4 hollow microtubes maintain uniform tubular morphology. Finally sulfur was introduced into NiCo2O4 hollow microtubes host by melt-diffusion method to form NiCo2O4/S composite.
2.2 Materials characterization Scanning electron microscopy (SEM) images were taken using Hitachi S-4800. Transmission electron microschopy (TEM) was performed at Tecnai G20 S-TWIN. The crystal phase was examined by XRD, Panalytical X’Pert-Pro MPD.vRaman spectra were recorded on Horiba (LabRAM HR Evolution). ESCALAB 250 Xi XPS system of Thermo Scientific was used for Xray photon spectroscopy (XPS) analysis with chamber 1.5 × 10-9 mbar pressure and 500 µm X5
ray spot. Thermogravimetric analysis was performed using TGA/DTA PerkinElmer Diamond instrument. The BET surface area of the materials was measured using a Micromeritics ASAP 2420 system instrument.
2.3 Electrochemical Measurements NiCo2O4 hollow microtubes powder was mixed with Super-P carbon black and PVDF binder with mass ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent to make electrode slurry. The slurry was coated onto an Al foil current collector using doctor blade. The electrode film was then dried in vacuum oven at 80 °C overnight and cut into 8 mm circular discs. Lithium metal as anode and celgard separator were used to assemble 2032-type coin cells inside Ar-filled glove box. The electrolyte used was 1.0 M lithium bis-trifluoromethanesulfonimide (LITFSI) and 2 % LiNO3 in tetraethylene glycol dimethyl ether (TEGDME) solvent. For Galvanostatic charge/discharge measurement LAND work station was employed. CHI660 electrochemical workstation was used for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements in a voltage cut off window of 1-3 V. CV measurement was performed with a scan rate of 0.2 mV/s. EIS measurement was carried out using open-circuit potential at 5 mV AC oscillation amplitude in the frequency range of 100 KHz to 0.01 Hz.
3. Results and discussion The X-ray diffraction pattern of the synthesized pristine NiCo2O4 hollow microtubes is shown in Fig. 1a. The peak positions and relative intensities of the corresponding peaks can be directly indexed to the characteristic spinel NiCo2O4, and matches well with the standard pattern (JCPDS card No. 02-1074). As the surface of the NiCo2O4 hollow microtubes is covered by thin nanosheets like structure that is why broad diffraction peaks with lower intensities are obtained [31]. From XRD pattern, it can also be seen that no additional signals corresponding to impurities like NiO or Co3O4 are detected, this point towards high phase purity of the as synthesized NiCo2O4 hollow microtubes. Fig. 2b depicts the XRD patterns of the pure sulfur and NiCo2O4/S composite. XRD confirmed both the reflection peaks of sulfur (23.18°, 25.67° and 27.7°, corresponding to the same orthorhombic structure as the elemental sulfur) and NiCo2O4 hollow microtubes.
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To further understand the structural features, Raman spectroscopy analysis of the pure and NiCo2O4/S composite was performed (Fig. 2a). The bands visible in the low wave number region (400-800 cm-1) are characteristics of the Ni-O and Co-O vibrational modes of NiCo2O4 spinel oxide. The bands at 180, 456, 504 and 644 cm-1 are assigned to the F2g, Eg, F2g and A1g active modes of NiCo2O4 hollow microtubes [31-33]. According to Ma et al. the evolution of NiCo2O4 hollow microtubes proceeds through the initial formation of tetragonal prisms that finally evolve into completely hollow microtubes consisting of nanosheets [30]. Thermogravimetric analyses were carried out under nitrogen atmosphere to determine the sulfur content in the NiCo2O4/S composite. The initial low weight loss (~3%) may be due to the adsorbed water. The weight loss of NiCo2O4/S composite clearly reveals that the sulfur content is about 27% (Fig. 2b). The morphology of NiCo2O4 hollow microtubes is investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images presented in Fig. 3a,c indicate the uniform size of NiCo2O4 hollow microtubes covered by ultrathin sheet like structures. The representative TEM image (Fig. 3b) clearly indicates the hollow structure as can be seen from the prominent contrast between the inner center and outer edges. Transmission electron microscopy (TEM) as well as the SEM images (Fig. 3a,b,c) also illustrate that the outer surface of the NiCo2O4 hollow microtubes are covered by NiCo2O4 nanosheets like structures. Furthermore, high magnification TEM images (Fig.S1, see ESI) clearly indicate the presence of nanosheets like structures. These nanosheets are actually the building blocks of the 3D microsized configuration as pointed out by Ma et al. [30]. Similar phenomenon was also reported by Dai et al. [34], who prepared uniform α-Ni(OH)2 hollow spheres constructed from ultrathin nanosheets with the thickness of ~3.5 nm as a hollow sulfur nanosphere host for Li-S batteries. These NiCo2O4 nanosheets like structures are expected to result in efficient surface bonding with sulfur species and thus will effectively alleviate the polysulfides dissolution in electrolytes during charge/discharge cycling. After mixing with sulfur, NiCo2O4/S composite retained well the size and tubular shape of the pristine NiCo2O4 hollow microtubes (Fig.3c). Energy dispersive X-ray spectroscopy (EDX) elemental mapping (Fig.3d-g) confirmed the presence of the constituent elements as well as the homogenous sulfur distribution in NiCo2O4/S composite. Nitrogen sorption measurements were carried out to further explore the microstructure of hollow microtubes (Fig. 4a, b). For pristine NiCo2O4 hollow microtubes, a high Brunauer-Emmett-Teller 7
(BET) surface area of 79.68 m2 g-1 and pore volume of 0.29 cm3 g-1 are obtained. The corresponding pore size distribution showed that NiCo2O4 hollow microtubes possess abundant mesopores with average pore width of 14.9 nm. Mesoporous texture with high surface area endows NiCo2O4 hollow microtubes sufficient electrode/electrolyte contact area for the mitigation of polysulfides dissolution due to strong interactions between polysulfides and NiCo2O4. After sulfur infiltration, the BET surface area and pore volume of the prepared NiCo2O4 hollow microtubes decrease sharply to 12.8 m2 g-1 and 0.68 cm3 g-1, respectively. Similarly, the peak intensity of the pore size distribution curve of NiCo2O4/S composite also showed a sharp decrease in the mesoporous range. All these changes indicate the complete diffusion of sulfur into the void space of NiCo2O4 hollow microtubes. Fig. 5a represents the XPS survey spectrum of the cycled NiCo2O4/S composite. The intensities of Ni and Co signals from NiCo2O4 are markedly reduced with the appearance of the sulfur signal in the spectrum after cycling, which confirms the formation of NiCo2O4/S composite. The S2p XPS spectra of NiCo2O4/S composite consist of three peaks (Fig. 5b). The dual peaks assigned to S2p3/2 and S2p1/2 located at 163.7 and 164.1 eV. The other pronounced peak located at 169.6 eV usually suggest the presence of SO42-/ S2O32- species which probably results from the strong S-O interaction between the encapsulated S and oxygenated NiCo2O4 framework [35]. More interestingly, compared to the pure elemental sulfur S2p peak (~164.08 eV), NiCo 2O4/S composite S2p is shifted to low binding energy (~163.7 eV). This shift in binding energy further indicates the existence of strong S-M (M = Ni or Co) interaction [28, 36]. Furthermore, the chemical interactions between polysulfides and Co/Ni atoms in NiCo2O4/S composite (after cycling) are also evident from the decreased binding energies of Co2p3/2 and Ni2p3/2 peaks (Fig. 5c,d) which are in good agreement with XPS spectra of Co9O8 and Ni(OH)2 reported by Chen et al. and Dai et al. respectively [34, 37]. We then evaluated the electrochemical performance by fabricating coin cells using NiCo 2O4/S composite as the cathode and lithium foil as the anode. Cyclic voltammetry profile of NiCo2O4/S cathode conducted in the potential window of 1.0 to 3.0 V at a scanning rate of 0.2 mV s-1 is presented in Fig. 6a. Two separate well-defined cathode peaks observed at 2.3 V and 2.0 V indicating that sulfur is reduced in two steps. It has been reported that the peak observed at 2.3 V corresponds to the reduction of elemental sulfur to high order polysulfides (Li2Sn, 4 < n < 8), while the peak at 2.0 V corresponds to further reduction of polysulfides to solid lithium sulfides 8
(Li2S2/Li2S) [38]. Similarly one oxidation peak at 2.5 V in the anodic scan is observed, which is associated with the final formation of the elemental sulfur from polysulfides conversion. Fig. 6b shows the galvanostatic charge/discharge curves of NiCo2O4/S composite studied at charge/discharge rate of 0.5 C within a potential window of 1.5 to 3.0 V. Charge/discharge profiles clearly show two reduction plateaus and one oxidation plateau consistent with CV result. The two plateaus observed in the discharge process correspond to the two step reduction of elemental sulfur into polysulfides. The presence of these plateaus indicates typical redox reactions of Li-S electrochemistry. To evaluate the stability of NiCo2O4/S composite, long term cycling was performed at 0.5 C (Fig. 6c). The specific capacity is calculated based on the mass of sulfur. At a high current density (0.5 C), NiCo2O4/S composite delivers initial discharge capacity of 910 mAh g-1. The rapid capacity loss in the first few cycles may be caused by the active sulfur redistribution and volumetric expansion during initial lithiation process [10]. After 200 charge/discharge cycles, NiCo2O4/S shows much better stability with reversible capacity of 601 mAh g-1. Furthermore, NiCo2O4/S cathode had much high coulombic efficiency (>99.3%), indicating that the shuttling effect of the polylsufides or the dissolution of polysulfides in the organic electrolyte is effectively suppressed during charge/discharge cycling. This also suggests that NiCo2O4 hollow microtubular host acts as a robust reservoir for trapping polysulfides both by physical and chemical confinement. Additionally, the hollow interior within the microtubulor structure is expected to improve the structural integrity for the enhanced charge/discharge cycling stability [29, 30N]. For the evaluation of current density robustness, the coin-cells with NiCo2O4/S cathode were subjected to various C-rates from 0.1 C to 1 C as shown in Fig. 7a. At 0.1 C, the material achieved an initial discharge capacity as high as 1274 mAh g-1, corresponding to 76% sulfur utilization based on the theoretical capacity (1675 mAh g-1) of sulfur cathode [39]. Further, at 0.2, 0.5, and 1 C, the discharge capacities obtained are 994, 760 and 608 mAh g-1, respectively. At high current density (1 C), the specific capacity (608 mAh g-1) is still high and the cycling performance remained stable (581 mAh g-1 for the subsequent investigated cycles). Moreover, NiCo2O4/S composite recovered most of its capacity (957 mAh g-1) when the current rate was restored to 0.1 C. This further confirms the improved electronic/ionic transport processes and fast reaction kinetics enabled by the hollow microtubes framework. Fig. 7b shows the cycling 9
performance of NiCo2O4/S composite at a high current density (1 C). After 150 charge/discharge cycles, 571 mAh g-1 capacity was maintained which is 60% of the initial discharge capacity. The improved electrochemical performance of NiCo2O4/S composite can be attributed to the efficient physical and chemical binding of polysulfides by NiCo2O4 as well as the hollow interior that provides structural integrity during charge/ discharge cycling. EIS analysis before and after 20 cycles at the rate of 1 C further confirmed the stability of NiCo2O4/S composite (Fig. 8). To further explain the obtained results, the experimentally observed impedance spectra are fitted to a proper equivalent circuit consistent with the different processes occurring in the electrodes using ZView software. At high frequency regime on real axis of Nyquist plots, the intercept indicates the resistance of the electrochemical system (R s) including electrolyte resistance, active materials intrinsic resistance as well as resistance due to current collectors [35]. A semicircle at medium-to-high frequency is assigned to the charge transfer resistance (Rct) occurring at the electrode/electrolyte interface [40]. While a straight line in the low frequency region corresponds to Warburg impedance (W) that represents the lithium ion diffusion in the bulk of the electrode. The system resistance (Rs) and the charge transfer resistance (Rct) measured from the equivalent circuit before cycling are 3.89 and 63.8 Ω, while after cycling are 5.0 and 67.79 Ω, respectively. The small changes in the values of R s and Rct before and after cycling which could be assigned to the passivation layer formation on the surface of microtubes, once again ensured the high conductivity and good electrochemical stability of NiCo2O4/S cathode.
4. Conclusion NiCo2O4 hollow microtubes and sulfur composite (NiCo2O4/S) has been synthesized and evaluated as an efficient cathode material for lithium-sulfur batteries. A high discharge capacity of 1274 mAh g-1 is obtained at 0.2 C that corresponds to 76% of the theoretical capacity (1675 mAh g-1). High rate capability and stable cycling performance at 0.5 C can be attributed to the lower charge transfer resistance (Rct), strong chemical interactions between polysulfides and Ni/Co cations, hollow interior cavity that provides structural integrity and the ultrathin nanosheets like structures on the surface of NiCo2O4 microtubes that physically and chemically
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entrap polysulfides, thus preventing their dissolution in the organic electrolyte during charge/discharge cycling.
Acknowledgement This work was financially supported by the National 863 Program of China (2015AA034601).
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Figure Captions Figure 1 (a) XRD patterns of pristine NiCo2O4 hollow microtubes and (b) NiCo2O4/S composite Figure 2 (a) Raman spectra and (b) TGA curves of pristine NiCo2O4 hollow microtubes and NiCo2O4/S composite. Figure 3 (a, b) SEM and TEM images of NiCo2O4 hollow microtubes. (c) SEM image of NiCo2O4/S composite, and EDX elemental mapping of (d) Nickel, (e) Cobalt, (f) Sulfur and (g) Oxygen. Scale bar in (a) and (c) is 2 µm. Figure 4 (a) N2 sorption isotherms and (b) the corresponding pore size distribution of the pristine and NiCo2O4/S composite. Figure 5 (a) XPS survey spectrum of NiCo2O4/S composite after cycling, and high resolution XPS spectra of (b) S2p (c) Ni 2p and (d) Co 2p of the synthesized pristine NiCo2O4/hollow microtubes and NiCo2O4/S composite. Figure 6 (a) CV profile of the pristine NiCo2O4 hollow microtubes. (b) Charge/discharge curves of NiCo2O4/S composite at a rate of 0.5 C. (c) Cycling performance of the cathode over 200 cycles at charge/discharge rate of 0.5 C. Figure 7 (a) Rate performance of the cathode at different current densities. (b) Long cycling performance at high charge/discharge rate (1 C). 15
Figure 8 EIS curves of the cathode before and after 20 cycles. Inset shows the corresponding equivalent circuit.
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Highlights: 1. NiCo2O4 hollow microtubes/sulfur composite (NiCo2O4/S) was synthesized and evaluated
as an efficient host for polysulfides confinement. 2. Strong chemical interactions, ultrathin nanosheets like structures over the surface of NiCo2O4 microtubes as well as the hollow interior cavity that provides structural integrity resulted in better cycling performance. 3. NiCo2O4/S composite delivers high specific capacity (1274 mAh g-1) at 0.2 C, long cycling performance at 0.5 C, and good rate capability.
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