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A high-entropy metal oxide as chemical anchor of polysulfide for lithium-sulfur batteries
Energy Storage Materials 23 (2019) 678–683
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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm
A high-entropy metal oxide as chemical anchor of polysulfide for lithium-sulfur batteries Yuenan Zheng a, 1, Yikun Yi b, 1, Meihong Fan a, Hanyu Liu c, Xue Li c, Rui Zhang a, Mingtao Li b, **, Zhen-An Qiao a, * a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, Changchun, 130012, China b School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, 710049, China c State Key Laboratory of Superhard Materials & Innovation Center for Computational Physics Methods and Software, College of Physics, Jilin University, Changchun, 130012, China
A R T I C L E I N F O
A B S T R A C T
Keywords: High-entropy metal oxide Immobilizing lithium polysulfides Synergistic contribution
Lithium-sulfur (Li-S) battery is anticipated as one of the most promising candidates for the next-generation rechargeable cell. In order to conquer the shuttle effect of dissolution lithium polysulfides (LIPSs), the chemical interactions between sulfur host and LIPSs on the performance of Li-S batteries has recently been highlighted. Herein, a facile strategy is proposed, which produces a high-entropy metal oxide (HEMO-1) as an anchor to restrain LIPSs in Li-S batteries via chemical confinement as demonstrated by strong bonding interaction between HEMO-1 and Li2S6. The as-prepared HEMO-1 incorporates five metal components, Ni, Mg, Cu, Zn and Co, into a metal oxide crystalline structure. The homogeneously dispersive multiple metal active species in HEMO-1 favor the restriction of LIPSs and facilitate the redox reaction in the cathode of Li-S batteries. Especially, the synergistic contribution of Li-O and S-Ni bonds from the interaction between HEMO-1 and LIPSs effectively alleviate the shuttle of LIPSs between the cathode and anode. As the promoter catalyst for LIPSs, HEMO-1 shows a competitive reversible capacity, outstanding cycling stability, and a low capacity decay of 0.077% per cycle after 600 cycles. This study not only presents a high-entropy oxide promoter for polysulfide immobilizing in Li-S batteries, but also provides an avenue of high-entropy metal oxides to a variety of energy conversion and storage fields.
1. Introduction As the demands for environmentally friendly electric vehicles and portable electronics devices ever-increasing, rational design and fabrication of advanced battery system has been gradually realized as indispensable and urgently needed [1]. Li-S batteries have been intensely studied over the past decades [2–4]. Taking consideration of plentiful advantages of Li-S batteries, such as high theoretical capacity and energy density of 1,675 mA h g1 and 2,500 kW kg1, respectively, the low cost and abundant sulfur resources as well as fewer safety worries, Li-S battery has been regarded as one of the most promising candidates to satisfy the needs of energy storage facilities [5–11]. Nevertheless, the complex multielectron conversion of sulfur species in a working cell, the insulating feature of sulfur and LIPSs and the shuttle effect of dissolution
LIPSs constitute the choke point of Li-S batteries development [12–15]. Furthermore, these issues result in lessening of cycle capacity and lower coulombic efficiency, and eventually impede practical implements of Li-S batteries [16–18]. In order to conquer these problems, an enormous endeavor has been contributed to introducing functional catalysts into cathode to anchor LIPSs and enhance the performance of Li-S batteries [19–29]. Originally, porous carbons were popular host materials in cathode to encapsulate sulfur in their porous networks, such as mesoporous carbons [30–34] and carbon nanotubes [35–37]. However, they can only partially confine polysulfide due to the physical interaction with LIPSs and volume expansion during discharge [38,39]. Recently polar multicomponent metal oxides are proposed to act as additives to restrain polysulfide by chemical immobilization instead of physical interaction [40–44]. For
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Li), [email protected] (Z.-A. Qiao). 1 Yuenan Zheng and Yikun Yi contributed equally to this work. https://doi.org/10.1016/j.ensm.2019.02.030 Received 4 January 2019; Received in revised form 24 February 2019; Accepted 27 February 2019 Available online 28 February 2019 2405-8297/© 2019 Elsevier B.V. All rights reserved.
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structure of HEMO-1 is shown in Fig. 2a (the oxygen, nickel, magnesium, zinc, copper and cobalt atoms are marked with red, white, orange, green, blue and purple, respectively) [52]. The calcination temperatures of HEMO-1 were investigated at 750 and 1000 C to obtain the entropy-stabilized oxides. X-ray diffraction (XRD) patterns depict the phase evolution in Fig. 2b. The as-milled mixed oxides were completely converted to a single phase when the calcination temperature was 1000 C. The observed peaks could be well attributed to the (111), (200), (220), (311) and (222) planes of HEMO-1, exhibiting a cubic rock salt structure. In addition, the entropy-driven transitions exhibit reversibility in a certain temperature range. When the single phase HEMO-1 obtained at 1000 C was thermal treated again at 750 C, several stray peaks appeared accompany with peaks attributed to HEMO-1, however, single phase HEMO-1 without impurities can get gain when it was thermally treated to 1000 C again. Scanning electron microscopy (SEM) images in Fig. 2c showed the morphology of HEMO-1. The particles have irregular shape and an average size of 300 nm. Clear lattice fringes could be observed in the high-resolution transmission electron microscopy (HRTEM) image (inset in Fig. 2c), indicating the high crystallinity of the material. The average distance spacing between the adjacent lattice plane is 0.24 nm, corresponding to the (111) plane of HEMO-1. N2 adsorption/desorption analysis (Fig. 2d) manifests Brunauer-Emmett-Teller (BET) surface area of HEMO-1 to be 24.7 m2 g1, which is crucial to exert LiPS absorptivity and enhance the interaction between HEMO-1 and LiPSs [53,54].
example, Xiong S. reported spinel ZnCo2O4 porous particles@N-doped reduced graphene oxide (ZnCo2O4@N-RGO) was prepared to mitigate the shuttle effect of LIPSs by chemical confinement measure. Zhang Q. et al. synthesized Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite nanoparticles as a promoter to anchor LiPSs and regulate Li2S deposition, where the metal-sulfur and Li-O interactions were suggested [45]. These metal oxides or mixed metal oxides show good electronic conductivity and chemical stability. What's more, the multicomponent metal oxides exhibit preferable properties to the single-metal ones, on account of the synergistic effect between metal ions [46–50]. In view of these merits, it is desirable to design efficient and characteristic metal oxides catalyst contributing to Li-S batteries. As a new class of crystalline high-entropy ceramics, high-entropy metal oxides are single phase and entropy-stabilized crystalline solid solutions, which present distinct structure and compelling electrochemical properties [51]. The as-prepared HEMO-1 incorporates five metal components into a single lattice with elements uniform distribution of Ni, Mg, Cu, Zn and Co [52]. High dispersity of metal species in HEMO-1 is conducive to exposure of active sites, which are undoubtedly advantageous to confine LIPSs in the process of charging and discharging. Furthermore, HEMO-1 possesses thermodynamic and chemical stability, as well as good uniformity without any agglomerated metal species, even after thermal treatment at 1000 C. Based on the above comprehension, we propose to use HEMO-1 as an immobilizing mediator to LiPSs in the sulfur cathode (Fig. 1). The reasons that HEMO-1 was chosen as an additive to restrain polysulfide and sulfur species in cathode are as follow: Firstly, intrinsic thermodynamic stability and metallic conductivity of HEMO-1 contribute to the cycling stability and electron transport in the working cell. Secondly, HEMO-1 exposes affluent active sites with random occupancies of five metal components to anchor LiPSs. Thirdly, the synergistic contribution of multicomponent in HEMO-1 is beneficial to absorptivity of Li2S6 and electrochemical conversion. This work not only puts forward a feasible and valid approach of using HEMO-1 as a promoter to build better Li-S batteries but also extends the development of high-entropy materials. 2. Results and discussion 2.1. Preparation and characterization HEMO-1 was prepared via a modified method [52], employing a mechanical ball milling measure, without redundant tableting dispose in order to expediently conduct subsequent experiment. The as-milled composite was further thermally treated in air atmosphere at 1000 C for 2 h with 10 C/min, and the brownish-black HEMO-1 powder was obtained without any post process. The as-prepared HEMO-1 incorporates five metal components into a single lattice with elements uniform distribution of Ni, Mg, Cu, Zn and Co. The simulative crystal
Fig. 2. a) The simulative crystal structure of HEMO-1, b) XRD patterns c) SEM image (inset: a corresponding lattice pattern HRTEM image) of HEMO-1, and d) N2 adsorption/desorption isotherm curve (inset: pore size distribution graph).
Fig. 1. Schematic illustration of cell configuration using HEMO-1 as chemical anchor of polysulfide in cathode for enhancing performance of Li-S batteries. 679
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To further understand the structure of HEMO-1, scanning transmission electron microscopic (STEM) characterization were performed, confirming the bulk morphology with rough surface (Fig. 3a). Furthermore, the energy dispersive X-ray (EDX) mapping of HEMO-1 clearly indicated the presence of cobalt, nickel, cuprum, zinc, magnesium and oxygen elements throughout the whole structure (Fig. 3c–h). A homogeneous dispersion of these metal elements could be found in each HEMO-1 particle. The specific EDX analysis of every element content and the X-ray photoelectron spectroscopy (XPS) survey of HEMO-1 can be found in Figs. S1 and S2 Supporting Information. The XPS measurement was performed to study the surface chemistry of HEMO-1 before and after absorptivity of Li2S6. The property of the interaction of HEMO-1 with Li2S6 was mainly detected from Ni 2p and O1s spectra mainly. As showed in Fig. 4a, the O1s spectra of HEMO-1 consist of three components of oxygen species. The peak located at about 529.3 eV is attributed to lattice oxygen (O2), while that at 530.34 and 531.5 eV can correspond to adsorbed oxygen species (O2 2 ) and hydroxyls species (OH), respectively [55]. Upon contacting with Li2S6, the lattice oxygen component shifted 0.6 eV to higher binding energy resulting from the interaction of Li and O presumably [45,56,57]. Previous research reported that the activity and mobility of oxygen species would be enhanced with binding energy increasing of O1s [58], which could facilitate O species in HEMO-1 bonding to Li ones in LiPSs. Furthermore, the intensity of adsorbed oxygen species decreased after absorptivity of Li2S6, and we speculate that it is induced by the increasing affinity between HEMO-1 and Li2S6. Fig. 4b shows the deconvoluted high resolution spectrum of Ni, and a doublet 2p3/2 and 2p1/2 is observed. Peaks located at 855.68 and 861.81 eV in HEMO-1 are indexed as the Ni 2p3/2, while 873.17 and 879.97 eV are attributed to Ni 2p1/2, in accord with previous research [59,60]. Comparing to the XPS peaks of HEMO-1, the doublet of Ni 2p spectrum in HEMO-1@Li2S6 integrally shifted 0.5 eV towards higher binding energy, suggesting that the HEMO-1 have a stronger redox ability of the Ni species and strong chemical interaction with LIPSs [61, 62], which may play an important role in enhancing the absorptivity performance of Li2S6. Furthermore, above the XPS analysis is in accordance with the followed simulative calculation in Fig. 5. In addition, the spectrums of Zn 2p and Mg 1s of HEMO-1@Li2S6 both shifted to higher binding energy after absorptivity of Li2S6 compared to pristine HEMO-1 (Fig. 4c and d). These important shifts are attributed to chemical anchor effect and imply the mutual transfer of electrons between HEMO-1 and Li2S6, which is beneficial to enhance electrochemical property. To sum up, the XPS analysis examines the interaction between HEMO-1 and LiPSs, which indicates: (1) the bonding effect of Li-O and Ni-S improve immobilizing behavior for Li2S6 remarkably; (2) the synergistic effect of multielement in HEMO-1 contributes to absorptivity of
Fig. 4. High resolution XPS spectra of O 1s a), Ni 2p b), Zn 2p c) and Mg 1s d) of HEMO-1 before and after adsorption of Li2S6.
Li2S6 and electrochemical conversion [47]. To further investigate the interfacial binding interactions between HEMO-1 and polysulfide species, the First-principle calculations based on density functional theory (DFT) were implemented to obtain the binding energies and bond distances in HEMO-1@Li2S6 model (Fig. 5a and b). Impressively, the (100) plane of HEMO-1 renders a higher binding energy of 6.916 eV to Li2S6 than a limited binding energy of 0.742 eV provided by Ketjen Black (KB) carbon with Li2S6 in Fig. 5c. The calculated binding energies of Li2S6 to (100) plane of HEMO-1 and amorphous KB basic plane present evident distinction, which mainly is attributed to much stronger polar-polar interaction [7] between polysulfide and HEMO-1 than the case of Li2S6 on KB. The simulated bond distance of Li-O in HEMO-1@Li2S6 model is merely 1.842 Å, even lesser than that in bulk Li2O [45], showing the formation of strong interaction between Li and O species (Fig. 5a; Table S1, Supporting Information). Meanwhile, the binding distance of S-Ni is very approximate to the corresponding bond length in NiS (2.263 Å) crystal [63]. Above the results clearly suggest that the synergic contribution of Li-O and S-Ni binding interactions enables HEMO-1 possess remarkable confinement to polysulfide. In order to further discuss the advantages of HEMO-1 than single metal oxide, we predicted the interaction between NiO and Li2S6 by DFT.
Fig. 3. Elemental analysis of HEMO-1. a) STEM image, b) merge elements image of HEMO-1, c-h) EDX elemental mappings of c) O, d) Co, e) Ni, f) Cu, g) Mg and h) Zn. 680
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different content of KB carbon. The sharp and enhanced cathodic peak of the HEMO-1 cell in the low-voltage range is owing to the anchoring effect, promoting the conversion of LIPSs in the subsequent reductions [64–66]. Then the capacity performances of HEMO-1 were tested by galvanostatic charge and discharge at a voltage range of 1.7–2.6 V with different current densities. Progressively increasing current densities from 0.1 to 0.2, 0.5 and 1C with 10 cycles retaining at each rate, the HEMO-1/KB/S cell delivers discharge specific capacities of 1148, 849, 762 and 634 mAh g1, respectively (Fig. 6b). When abruptly turning current rate back to 0.1C again, a reversible capacity of 835 mAh g1 is retained. Additionally, the cycling performance is likewise important for Li-S batteries. During the cycling process, the ability to retard LIPSs shuttle is the key for a stable capacity. The dual bonding interaction and synergistic effect of Li-O and S-Ni from HEMO-1@Li2S6 are proved in the XPS analysis and DFT calculation, which provide the theoretical proofs to boost the cycle performance of Li-S batteries. Galvanostatic charge-discharge profiles of the HEMO-1/KB/S and KB/S cathodes for the initial and after 100 cycles are shown in Fig. 6c. The two separate discharge plateaus and continuous charge plateaus are consistent with the two steps of complete reduction reactions and the oxidation reactions of long-chain Li2Sn (4 n 8) mentioned in CV results, respectively. Both the two plateaus of HEMO-1 cell are lengthened compared with the KB/S cell, indicating the promotion of the LIPSs conversion. The energy efficiency of HEMO-1 we have calculated is 90.45% from the charge/discharge profiles, which is of practical interest [67]. The catalytic effect of HEMO-1 on the LIPSs conversion was demonstrated by CV in symmetric cells with HEMO-1@KB carbon as working and counter electrode in a 0.2 M Li2S6 electrolyte. For comparison, KB carbon as control was conducted in a 0.2 M Li2S6 electrolyte. During cycling, the LiPSs species in electrolyte solution reversibly convert on the interface of electrodes, with oxidation on working electrode and reduction on counter electrode or vice versa, thus resulting in the current curves and peaks. The reactions and results at negative potentials are symmetrical with that at positive potentials because of the identical working and counter electrode. HEMO-1 displays larger current density than bare KB carbon in Fig. 6d, which indicates HEMO-1 is beneficial to promote the conversion of LIPSs and accelerate their redox reactions [45,68]. The advantage of multicomponent HEMO-1 is again revealed by CV in symmetric cells with comparison between HEMO-1@KB carbon and NiO@KB carbon electrode. As shown in Fig. S4, HEMO-1 and NiO cells both display larger current density than bare KB carbon cell. Notably, HEMO-1 exhibits more distinct improvement than NiO, verifying a better electrocatalytic activity and catalytic effect of HEMO-1. As shown in Fig. 6e, with an initial capacity of 1191 mAh g1, the cell with HEMO-1/KB/S cathode exhibits a discharge capacity of 768 mAh g1 after 100 cycles at 0.1C, and 664 mAh g1 after 200 cycles. In contrast, the routine KB/S cell only maintains a discharge capacity of 597 mAh g1 after 100 cycles, despite the similar initial capacity of 1131 mAh g1, displaying a dramatic capacity decay compared with the HEMO-1 cell. Meanwhile, to validate the synergistic affinity of multielement in HEMO-1 to LiPSs, NiO/KB/S cathodes were prepared as control, which only retained the capacity of 667 mAh g1 after 100 cycles (Fig. S5, Supporting Information). Further increasing the chargedischarge rate to 0.5C (Fig. 6f), the cell with KB/S cathode demonstrates a dramatic decay of the capacity during 120 cycles. With a distinct enhancement, HEMO-1 cell delivers an initial discharge capacity of 890 mAh g1 and still retained 479 mAh g1 after 600 cycles, displaying a capacity decay of only 0.077% per cycle. As shown in electrochemical impedance spectra (EIS) of the two cells (Fig. S6, Supporting Information), the intercepts in the high-frequency regions which represent the bulk electrolyte resistance are comparable. While the semicircles in the high-medium-frequency regions, reflecting the electrode/electrolyte interfacial resistance, display a great difference, and the favorable lessening of interfacial resistance could effectively enhance the ability of charge transfer at a large current density. The morphological change of the HEMO-1/KB/S composite electrodes before and after 100 cycles is
Fig. 5. a) Geometry configuration of Li2S6 binding to HEMO-1. (the oxygen, nickel, magnesium, zinc, copper, cobalt, lithium and sulfur atoms are marked with red, white, orange, green, blue, purple, luminous yellow and light green, respectively. 2.276 Å and 1.842 Å are simulative bond distances of S-Ni and LiO, respectively.) b) Geometry configuration of Li2S6 binding to bare KB carbon. (the carbon, lithium and sulfur atoms are marked with brown, luminous yellow and light green, respectively.) c) Comparison of calculated binding energies of Li2S6 with HEMO-1 and KB carbon. d) Ultraviolet/visible absorption spectra of a Li2S6 solution prior to and after the additives of HEMO-1 and KB carbon. Inset image shows a photograph of a Li2S6 visual adsorption result by using HEMO-1 and bare KB carbon. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As shown in Figs. S3a–3c, the (100) plane of NiO renders a lower binding energy of 2.529 eV to Li2S6 than HEMO-1 (6.916 eV). The simulated bond distance of Li-O (1.906 Å) and Ni-S (2.475 Å) in NiO@Li2S6 model were larger than Li-O and Ni-S in HEMO-1@Li2S6 model, respectively. Therefore, the DFT results show that the HEMO-1 containing multiple metal species possess the stronger interaction and better affinity to polysulfide than monometallic NiO. It is well known that effective chemical adsorption towards polysulfide species could alleviate the shuttling effect of LIPSs. Herein, the chemical adsorption experiments of HEMO-1 and KB carbon were carried out by utilizing Li2S6 solution to investigate the interaction between HEMO-1 and LIPSs. As the resulting photographs show (inset in Fig. 5d), the solution containing HEMO-1 changed to transparent visibly, whereas it still held yellow for the soluble polysulfide with alone KB carbon. It also can be clearly seen that the absorption peak of Li2S6 in the visible light range apparently disappeared after adding HEMO-1, but remained after adding KB carbon (Fig. 5d). This difference suggests strong adsorption of Li2S6 molecules to HEMO-1, owing to bonding effects of Ni-S and Li-O. The above polysulfide adsorption results suggest that HEMO-1 can forcefully trap soluble LIPSs, further confirming that the stronger affinity of HEMO-1 towards Li2S6 than KB carbon.
2.2. Electrochemical property To directly verify the role of HEMO-1 promoter in Li-S batteries, the electrochemical properties of the HEMO-1/KB/S cathode were evaluated in coin cells. Cyclic voltammetry (CV) curves of the HEMO-1/KB/S and KB/S electrodes (vs Liþ/Li) are presented in Fig. 6a. Both the cathodes exhibit two cathodic peaks locating at 2.2–2.4 V and 1.9–2.1 V, which are associated with the formation of long-chain Li2Sn (4 n 8) and the further conversion to insoluble Li2S, respectively [7]. Besides, the prominent oxidation peak located at 2.3–2.5 V corresponds to the reverse reactions. In addition, Fig. 6a shows a slightly shift of the cathodic peak in the high-voltage range in the HEMO-1 cell, which is attributed to the 681
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Fig. 6. a) CV curves of the HEMO-1 and KB cathodes at a scan rate of 0.1 mV s1. b) Rate capability of HEMO-1. c) Discharge-charge profiles of HEMO-1 and KB at 0.1C. d) CV of symmetric cells of HEMO-1@KB carbon and KB carbon electrodes with and without Li2S6. e) Cycling performance and CE of HEMO-1 and KB at 0.1C. f) Long-term cycling performance and CE of HEMO-1 and KB at 0.5C.
3. Conclusion
shown in Fig. S7, Supporting Information. Distinct morphological evolution is displayed, the surface of electrode material after 100 cycles became smooth and porous structure reduced, which may be due to the extensive contact area of the electrolyte/cathode, and this is conducive to lessen the transport distances of LIPSs and electrons and promote to enhance electrochemical property [69,70]. These analyses all rationally suggest the HEMO-1 promoter shows higher capacity performance, consisting with the above electrochemical results. The long cycling performance of different materials for Li-S batteries were summarized in Table S2, the longer cycles and comparable decay of HEMO-1 indicate its feature to function as a LIPSs anchor in Li-S batteries.
In summary, an efficient sulfur host material composed of HEMO-1 was designed for immobilizing LiPSs in Li-S batteries cathode, which was afforded via a facilely mechanochemical-assisted method. The resultant HEMO-1 incorporates five metal components, Ni, Mg, Cu, Zn and Co, into a metal oxide crystalline structure with a homogeneous distribution of multicomponent. In addition, The HEMO-1 functions as a cathode and exhibits a competitive reversible capacity, outstanding cycling stability and a low capacity decay after 600 cycles. Specifically, the homogeneous distribution of multiple metal active sites in HEMO-1
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accelerates the redox reaction kinetics of LIPSs. Due to the synergistic contribution of Li-O and S-Ni bonds from the interaction between HEMO1 and LIPSs, the interfacial affinity of LiPSs as well as suppression of the shuttle effect are effectively enhanced. This work not only presents a high-entropy oxide promoter for polysulfide immobilizing in Li-S batteries, but also is anticipated to extend the viability of high-entropy metal oxides to comprehensive varieties of energy conversion and storage fields.
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Conflicts of interest There are no conflicts to declare. Data availability The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgment This work was supported by the Young Thousand Talented Program and the National Natural Science Foundation of China (21671073 and 21621001); the “111” Project of the Ministry of Education of China (B17020); the Program for JLU Science and Technology Innovative Research Team; the National Key Research and Development Program of China (No. 2017YFA0204800). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.02.030. References [1] M. Armand, J.M. Tarascon, Nature 451 (2008) 652–657. [2] A. Manthiram, Y.Z. Fu, S.H. Chung, C.X. Zu, Y.S. Su, Chem. Rev. 114 (2014) 11751–11787. [3] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19–29. [4] D. Larcher, J.M. Tarascon, Nat. Chem. 7 (2015) 19–29. [5] X.L. Ji, L.F. Nazar, J. Mater. Chem. 20 (2010) 9821–9826. [6] A. Eftekhari, D.W. Kim, J. Mater. Chem. A 5 (2017) 17734–17776. [7] Z. Sun, J.Q. Zhang, L.C. Yin, G.J. Hu, R.P. Fang, H.-M. Cheng, F. Li, Nat. Commun. 8 (2017) 14627–14634. [8] M.A. Pope, I.A. Aksay, Adv. Energy Mater. 5 (2015), 1500124. [9] J.C. Guo, Y.H. Xu, C.S. Wang, Nano Lett. 11 (2011) 4288–4294. [10] J.S. Kim, T.H. Hwang, B.G. Kim, J. Min, J.W. Choi, Adv. Funct. Mater. 24 (2014) 5359–5367. [11] Y. Yang, G.Y. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018–3032. [12] D. Zheng, G.W. Wang, D. Liu, J.Y. Si, T.Y. Ding, D.Y. Qu, X.Q. Yang, D.Y. Qu, Adv. Mater. Technol. 3 (2018), 1700233. [13] P.G. Bruce, L.J. Hardwick, K.M. Abraham, MRS Bull. 36 (2011) 506–512. [14] J.Q. Huang, Q. Zhang, F. Wei, Energy Storage Mater. 1 (2015) 127–145. [15] M.R. Busche, P. Adelhelm, H. Sommer, H. Schneider, K. Leitner, J. Janek, J. Power Sources 259 (2014) 289–299. [16] D.H. Liu, C. Zhang, G.M. Zhou, W. Lv, G.W. Ling, L.J. Zhi, Q.-H. Yang, Adv. Sci. 5 (2018), 1700270. [17] Y. V Mikhaylik, J.R. Akridge, J. Electrochem. Soc. 151 (2004) A1969–A1976. [18] M. Hagen, D. Hanselmann, K. Ahlbrecht, R. Maça, D. Gerber, J. Tübke, Adv. Energy Mater. 5 (2015), 1401986. [19] H.Q. Wang, W.C. Zhang, J.Z. Xu, Z.P. Guo, Adv. Funct. Mater. 28 (2018), 1707520. [20] Z.-W. Zhang, H.-J. Peng, M. Zhao, J.-Q. Huang, Adv. Funct. Mater. 28 (2018), 1707536. [21] R. Ponraj, A.G. Kannan, J.H. Ahn, D.-W. Kim, ACS Appl. Mater. Interfaces 8 (2016) 4000–4006. [22] Q. Pang, D.P. Kundu, M. Cuisinier, L.F. Nazar, Nat. Commun. 5 (2014) 4759–4766. [23] S.A. Ahad, P. Ragupathy, S. Ryu, H.-W. Lee, D.K. Kim, Chem. Commun. 53 (2017) 8782. [24] X. Liang, A. Garsuch, L.F. Nazar, Angew. Chem. 127 (2015) 3979–3983. [25] H.-S. Kang, Y.-K. Sun, Adv. Funct. Mater. 26 (2016) 1225–1232.
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A high-entropy metal oxide as chemical anchor of polysulfide for lithium-sulfur batteries Yuenan Zheng a, 1, Yikun Yi b, 1, Meihong Fan a, Hanyu Liu c, Xue Li c, Rui Zhang a, Mingtao Li b, **, Zhen-An Qiao a, * a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, Changchun, 130012, China b School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, 710049, China c State Key Laboratory of Superhard Materials & Innovation Center for Computational Physics Methods and Software, College of Physics, Jilin University, Changchun, 130012, China
A R T I C L E I N F O
A B S T R A C T
Keywords: High-entropy metal oxide Immobilizing lithium polysulfides Synergistic contribution
Lithium-sulfur (Li-S) battery is anticipated as one of the most promising candidates for the next-generation rechargeable cell. In order to conquer the shuttle effect of dissolution lithium polysulfides (LIPSs), the chemical interactions between sulfur host and LIPSs on the performance of Li-S batteries has recently been highlighted. Herein, a facile strategy is proposed, which produces a high-entropy metal oxide (HEMO-1) as an anchor to restrain LIPSs in Li-S batteries via chemical confinement as demonstrated by strong bonding interaction between HEMO-1 and Li2S6. The as-prepared HEMO-1 incorporates five metal components, Ni, Mg, Cu, Zn and Co, into a metal oxide crystalline structure. The homogeneously dispersive multiple metal active species in HEMO-1 favor the restriction of LIPSs and facilitate the redox reaction in the cathode of Li-S batteries. Especially, the synergistic contribution of Li-O and S-Ni bonds from the interaction between HEMO-1 and LIPSs effectively alleviate the shuttle of LIPSs between the cathode and anode. As the promoter catalyst for LIPSs, HEMO-1 shows a competitive reversible capacity, outstanding cycling stability, and a low capacity decay of 0.077% per cycle after 600 cycles. This study not only presents a high-entropy oxide promoter for polysulfide immobilizing in Li-S batteries, but also provides an avenue of high-entropy metal oxides to a variety of energy conversion and storage fields.
1. Introduction As the demands for environmentally friendly electric vehicles and portable electronics devices ever-increasing, rational design and fabrication of advanced battery system has been gradually realized as indispensable and urgently needed [1]. Li-S batteries have been intensely studied over the past decades [2–4]. Taking consideration of plentiful advantages of Li-S batteries, such as high theoretical capacity and energy density of 1,675 mA h g1 and 2,500 kW kg1, respectively, the low cost and abundant sulfur resources as well as fewer safety worries, Li-S battery has been regarded as one of the most promising candidates to satisfy the needs of energy storage facilities [5–11]. Nevertheless, the complex multielectron conversion of sulfur species in a working cell, the insulating feature of sulfur and LIPSs and the shuttle effect of dissolution
LIPSs constitute the choke point of Li-S batteries development [12–15]. Furthermore, these issues result in lessening of cycle capacity and lower coulombic efficiency, and eventually impede practical implements of Li-S batteries [16–18]. In order to conquer these problems, an enormous endeavor has been contributed to introducing functional catalysts into cathode to anchor LIPSs and enhance the performance of Li-S batteries [19–29]. Originally, porous carbons were popular host materials in cathode to encapsulate sulfur in their porous networks, such as mesoporous carbons [30–34] and carbon nanotubes [35–37]. However, they can only partially confine polysulfide due to the physical interaction with LIPSs and volume expansion during discharge [38,39]. Recently polar multicomponent metal oxides are proposed to act as additives to restrain polysulfide by chemical immobilization instead of physical interaction [40–44]. For
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Li), [email protected] (Z.-A. Qiao). 1 Yuenan Zheng and Yikun Yi contributed equally to this work. https://doi.org/10.1016/j.ensm.2019.02.030 Received 4 January 2019; Received in revised form 24 February 2019; Accepted 27 February 2019 Available online 28 February 2019 2405-8297/© 2019 Elsevier B.V. All rights reserved.
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structure of HEMO-1 is shown in Fig. 2a (the oxygen, nickel, magnesium, zinc, copper and cobalt atoms are marked with red, white, orange, green, blue and purple, respectively) [52]. The calcination temperatures of HEMO-1 were investigated at 750 and 1000 C to obtain the entropy-stabilized oxides. X-ray diffraction (XRD) patterns depict the phase evolution in Fig. 2b. The as-milled mixed oxides were completely converted to a single phase when the calcination temperature was 1000 C. The observed peaks could be well attributed to the (111), (200), (220), (311) and (222) planes of HEMO-1, exhibiting a cubic rock salt structure. In addition, the entropy-driven transitions exhibit reversibility in a certain temperature range. When the single phase HEMO-1 obtained at 1000 C was thermal treated again at 750 C, several stray peaks appeared accompany with peaks attributed to HEMO-1, however, single phase HEMO-1 without impurities can get gain when it was thermally treated to 1000 C again. Scanning electron microscopy (SEM) images in Fig. 2c showed the morphology of HEMO-1. The particles have irregular shape and an average size of 300 nm. Clear lattice fringes could be observed in the high-resolution transmission electron microscopy (HRTEM) image (inset in Fig. 2c), indicating the high crystallinity of the material. The average distance spacing between the adjacent lattice plane is 0.24 nm, corresponding to the (111) plane of HEMO-1. N2 adsorption/desorption analysis (Fig. 2d) manifests Brunauer-Emmett-Teller (BET) surface area of HEMO-1 to be 24.7 m2 g1, which is crucial to exert LiPS absorptivity and enhance the interaction between HEMO-1 and LiPSs [53,54].
example, Xiong S. reported spinel ZnCo2O4 porous particles@N-doped reduced graphene oxide (ZnCo2O4@N-RGO) was prepared to mitigate the shuttle effect of LIPSs by chemical confinement measure. Zhang Q. et al. synthesized Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite nanoparticles as a promoter to anchor LiPSs and regulate Li2S deposition, where the metal-sulfur and Li-O interactions were suggested [45]. These metal oxides or mixed metal oxides show good electronic conductivity and chemical stability. What's more, the multicomponent metal oxides exhibit preferable properties to the single-metal ones, on account of the synergistic effect between metal ions [46–50]. In view of these merits, it is desirable to design efficient and characteristic metal oxides catalyst contributing to Li-S batteries. As a new class of crystalline high-entropy ceramics, high-entropy metal oxides are single phase and entropy-stabilized crystalline solid solutions, which present distinct structure and compelling electrochemical properties [51]. The as-prepared HEMO-1 incorporates five metal components into a single lattice with elements uniform distribution of Ni, Mg, Cu, Zn and Co [52]. High dispersity of metal species in HEMO-1 is conducive to exposure of active sites, which are undoubtedly advantageous to confine LIPSs in the process of charging and discharging. Furthermore, HEMO-1 possesses thermodynamic and chemical stability, as well as good uniformity without any agglomerated metal species, even after thermal treatment at 1000 C. Based on the above comprehension, we propose to use HEMO-1 as an immobilizing mediator to LiPSs in the sulfur cathode (Fig. 1). The reasons that HEMO-1 was chosen as an additive to restrain polysulfide and sulfur species in cathode are as follow: Firstly, intrinsic thermodynamic stability and metallic conductivity of HEMO-1 contribute to the cycling stability and electron transport in the working cell. Secondly, HEMO-1 exposes affluent active sites with random occupancies of five metal components to anchor LiPSs. Thirdly, the synergistic contribution of multicomponent in HEMO-1 is beneficial to absorptivity of Li2S6 and electrochemical conversion. This work not only puts forward a feasible and valid approach of using HEMO-1 as a promoter to build better Li-S batteries but also extends the development of high-entropy materials. 2. Results and discussion 2.1. Preparation and characterization HEMO-1 was prepared via a modified method [52], employing a mechanical ball milling measure, without redundant tableting dispose in order to expediently conduct subsequent experiment. The as-milled composite was further thermally treated in air atmosphere at 1000 C for 2 h with 10 C/min, and the brownish-black HEMO-1 powder was obtained without any post process. The as-prepared HEMO-1 incorporates five metal components into a single lattice with elements uniform distribution of Ni, Mg, Cu, Zn and Co. The simulative crystal
Fig. 2. a) The simulative crystal structure of HEMO-1, b) XRD patterns c) SEM image (inset: a corresponding lattice pattern HRTEM image) of HEMO-1, and d) N2 adsorption/desorption isotherm curve (inset: pore size distribution graph).
Fig. 1. Schematic illustration of cell configuration using HEMO-1 as chemical anchor of polysulfide in cathode for enhancing performance of Li-S batteries. 679
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To further understand the structure of HEMO-1, scanning transmission electron microscopic (STEM) characterization were performed, confirming the bulk morphology with rough surface (Fig. 3a). Furthermore, the energy dispersive X-ray (EDX) mapping of HEMO-1 clearly indicated the presence of cobalt, nickel, cuprum, zinc, magnesium and oxygen elements throughout the whole structure (Fig. 3c–h). A homogeneous dispersion of these metal elements could be found in each HEMO-1 particle. The specific EDX analysis of every element content and the X-ray photoelectron spectroscopy (XPS) survey of HEMO-1 can be found in Figs. S1 and S2 Supporting Information. The XPS measurement was performed to study the surface chemistry of HEMO-1 before and after absorptivity of Li2S6. The property of the interaction of HEMO-1 with Li2S6 was mainly detected from Ni 2p and O1s spectra mainly. As showed in Fig. 4a, the O1s spectra of HEMO-1 consist of three components of oxygen species. The peak located at about 529.3 eV is attributed to lattice oxygen (O2), while that at 530.34 and 531.5 eV can correspond to adsorbed oxygen species (O2 2 ) and hydroxyls species (OH), respectively [55]. Upon contacting with Li2S6, the lattice oxygen component shifted 0.6 eV to higher binding energy resulting from the interaction of Li and O presumably [45,56,57]. Previous research reported that the activity and mobility of oxygen species would be enhanced with binding energy increasing of O1s [58], which could facilitate O species in HEMO-1 bonding to Li ones in LiPSs. Furthermore, the intensity of adsorbed oxygen species decreased after absorptivity of Li2S6, and we speculate that it is induced by the increasing affinity between HEMO-1 and Li2S6. Fig. 4b shows the deconvoluted high resolution spectrum of Ni, and a doublet 2p3/2 and 2p1/2 is observed. Peaks located at 855.68 and 861.81 eV in HEMO-1 are indexed as the Ni 2p3/2, while 873.17 and 879.97 eV are attributed to Ni 2p1/2, in accord with previous research [59,60]. Comparing to the XPS peaks of HEMO-1, the doublet of Ni 2p spectrum in HEMO-1@Li2S6 integrally shifted 0.5 eV towards higher binding energy, suggesting that the HEMO-1 have a stronger redox ability of the Ni species and strong chemical interaction with LIPSs [61, 62], which may play an important role in enhancing the absorptivity performance of Li2S6. Furthermore, above the XPS analysis is in accordance with the followed simulative calculation in Fig. 5. In addition, the spectrums of Zn 2p and Mg 1s of HEMO-1@Li2S6 both shifted to higher binding energy after absorptivity of Li2S6 compared to pristine HEMO-1 (Fig. 4c and d). These important shifts are attributed to chemical anchor effect and imply the mutual transfer of electrons between HEMO-1 and Li2S6, which is beneficial to enhance electrochemical property. To sum up, the XPS analysis examines the interaction between HEMO-1 and LiPSs, which indicates: (1) the bonding effect of Li-O and Ni-S improve immobilizing behavior for Li2S6 remarkably; (2) the synergistic effect of multielement in HEMO-1 contributes to absorptivity of
Fig. 4. High resolution XPS spectra of O 1s a), Ni 2p b), Zn 2p c) and Mg 1s d) of HEMO-1 before and after adsorption of Li2S6.
Li2S6 and electrochemical conversion [47]. To further investigate the interfacial binding interactions between HEMO-1 and polysulfide species, the First-principle calculations based on density functional theory (DFT) were implemented to obtain the binding energies and bond distances in HEMO-1@Li2S6 model (Fig. 5a and b). Impressively, the (100) plane of HEMO-1 renders a higher binding energy of 6.916 eV to Li2S6 than a limited binding energy of 0.742 eV provided by Ketjen Black (KB) carbon with Li2S6 in Fig. 5c. The calculated binding energies of Li2S6 to (100) plane of HEMO-1 and amorphous KB basic plane present evident distinction, which mainly is attributed to much stronger polar-polar interaction [7] between polysulfide and HEMO-1 than the case of Li2S6 on KB. The simulated bond distance of Li-O in HEMO-1@Li2S6 model is merely 1.842 Å, even lesser than that in bulk Li2O [45], showing the formation of strong interaction between Li and O species (Fig. 5a; Table S1, Supporting Information). Meanwhile, the binding distance of S-Ni is very approximate to the corresponding bond length in NiS (2.263 Å) crystal [63]. Above the results clearly suggest that the synergic contribution of Li-O and S-Ni binding interactions enables HEMO-1 possess remarkable confinement to polysulfide. In order to further discuss the advantages of HEMO-1 than single metal oxide, we predicted the interaction between NiO and Li2S6 by DFT.
Fig. 3. Elemental analysis of HEMO-1. a) STEM image, b) merge elements image of HEMO-1, c-h) EDX elemental mappings of c) O, d) Co, e) Ni, f) Cu, g) Mg and h) Zn. 680
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different content of KB carbon. The sharp and enhanced cathodic peak of the HEMO-1 cell in the low-voltage range is owing to the anchoring effect, promoting the conversion of LIPSs in the subsequent reductions [64–66]. Then the capacity performances of HEMO-1 were tested by galvanostatic charge and discharge at a voltage range of 1.7–2.6 V with different current densities. Progressively increasing current densities from 0.1 to 0.2, 0.5 and 1C with 10 cycles retaining at each rate, the HEMO-1/KB/S cell delivers discharge specific capacities of 1148, 849, 762 and 634 mAh g1, respectively (Fig. 6b). When abruptly turning current rate back to 0.1C again, a reversible capacity of 835 mAh g1 is retained. Additionally, the cycling performance is likewise important for Li-S batteries. During the cycling process, the ability to retard LIPSs shuttle is the key for a stable capacity. The dual bonding interaction and synergistic effect of Li-O and S-Ni from HEMO-1@Li2S6 are proved in the XPS analysis and DFT calculation, which provide the theoretical proofs to boost the cycle performance of Li-S batteries. Galvanostatic charge-discharge profiles of the HEMO-1/KB/S and KB/S cathodes for the initial and after 100 cycles are shown in Fig. 6c. The two separate discharge plateaus and continuous charge plateaus are consistent with the two steps of complete reduction reactions and the oxidation reactions of long-chain Li2Sn (4 n 8) mentioned in CV results, respectively. Both the two plateaus of HEMO-1 cell are lengthened compared with the KB/S cell, indicating the promotion of the LIPSs conversion. The energy efficiency of HEMO-1 we have calculated is 90.45% from the charge/discharge profiles, which is of practical interest [67]. The catalytic effect of HEMO-1 on the LIPSs conversion was demonstrated by CV in symmetric cells with HEMO-1@KB carbon as working and counter electrode in a 0.2 M Li2S6 electrolyte. For comparison, KB carbon as control was conducted in a 0.2 M Li2S6 electrolyte. During cycling, the LiPSs species in electrolyte solution reversibly convert on the interface of electrodes, with oxidation on working electrode and reduction on counter electrode or vice versa, thus resulting in the current curves and peaks. The reactions and results at negative potentials are symmetrical with that at positive potentials because of the identical working and counter electrode. HEMO-1 displays larger current density than bare KB carbon in Fig. 6d, which indicates HEMO-1 is beneficial to promote the conversion of LIPSs and accelerate their redox reactions [45,68]. The advantage of multicomponent HEMO-1 is again revealed by CV in symmetric cells with comparison between HEMO-1@KB carbon and NiO@KB carbon electrode. As shown in Fig. S4, HEMO-1 and NiO cells both display larger current density than bare KB carbon cell. Notably, HEMO-1 exhibits more distinct improvement than NiO, verifying a better electrocatalytic activity and catalytic effect of HEMO-1. As shown in Fig. 6e, with an initial capacity of 1191 mAh g1, the cell with HEMO-1/KB/S cathode exhibits a discharge capacity of 768 mAh g1 after 100 cycles at 0.1C, and 664 mAh g1 after 200 cycles. In contrast, the routine KB/S cell only maintains a discharge capacity of 597 mAh g1 after 100 cycles, despite the similar initial capacity of 1131 mAh g1, displaying a dramatic capacity decay compared with the HEMO-1 cell. Meanwhile, to validate the synergistic affinity of multielement in HEMO-1 to LiPSs, NiO/KB/S cathodes were prepared as control, which only retained the capacity of 667 mAh g1 after 100 cycles (Fig. S5, Supporting Information). Further increasing the chargedischarge rate to 0.5C (Fig. 6f), the cell with KB/S cathode demonstrates a dramatic decay of the capacity during 120 cycles. With a distinct enhancement, HEMO-1 cell delivers an initial discharge capacity of 890 mAh g1 and still retained 479 mAh g1 after 600 cycles, displaying a capacity decay of only 0.077% per cycle. As shown in electrochemical impedance spectra (EIS) of the two cells (Fig. S6, Supporting Information), the intercepts in the high-frequency regions which represent the bulk electrolyte resistance are comparable. While the semicircles in the high-medium-frequency regions, reflecting the electrode/electrolyte interfacial resistance, display a great difference, and the favorable lessening of interfacial resistance could effectively enhance the ability of charge transfer at a large current density. The morphological change of the HEMO-1/KB/S composite electrodes before and after 100 cycles is
Fig. 5. a) Geometry configuration of Li2S6 binding to HEMO-1. (the oxygen, nickel, magnesium, zinc, copper, cobalt, lithium and sulfur atoms are marked with red, white, orange, green, blue, purple, luminous yellow and light green, respectively. 2.276 Å and 1.842 Å are simulative bond distances of S-Ni and LiO, respectively.) b) Geometry configuration of Li2S6 binding to bare KB carbon. (the carbon, lithium and sulfur atoms are marked with brown, luminous yellow and light green, respectively.) c) Comparison of calculated binding energies of Li2S6 with HEMO-1 and KB carbon. d) Ultraviolet/visible absorption spectra of a Li2S6 solution prior to and after the additives of HEMO-1 and KB carbon. Inset image shows a photograph of a Li2S6 visual adsorption result by using HEMO-1 and bare KB carbon. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As shown in Figs. S3a–3c, the (100) plane of NiO renders a lower binding energy of 2.529 eV to Li2S6 than HEMO-1 (6.916 eV). The simulated bond distance of Li-O (1.906 Å) and Ni-S (2.475 Å) in NiO@Li2S6 model were larger than Li-O and Ni-S in HEMO-1@Li2S6 model, respectively. Therefore, the DFT results show that the HEMO-1 containing multiple metal species possess the stronger interaction and better affinity to polysulfide than monometallic NiO. It is well known that effective chemical adsorption towards polysulfide species could alleviate the shuttling effect of LIPSs. Herein, the chemical adsorption experiments of HEMO-1 and KB carbon were carried out by utilizing Li2S6 solution to investigate the interaction between HEMO-1 and LIPSs. As the resulting photographs show (inset in Fig. 5d), the solution containing HEMO-1 changed to transparent visibly, whereas it still held yellow for the soluble polysulfide with alone KB carbon. It also can be clearly seen that the absorption peak of Li2S6 in the visible light range apparently disappeared after adding HEMO-1, but remained after adding KB carbon (Fig. 5d). This difference suggests strong adsorption of Li2S6 molecules to HEMO-1, owing to bonding effects of Ni-S and Li-O. The above polysulfide adsorption results suggest that HEMO-1 can forcefully trap soluble LIPSs, further confirming that the stronger affinity of HEMO-1 towards Li2S6 than KB carbon.
2.2. Electrochemical property To directly verify the role of HEMO-1 promoter in Li-S batteries, the electrochemical properties of the HEMO-1/KB/S cathode were evaluated in coin cells. Cyclic voltammetry (CV) curves of the HEMO-1/KB/S and KB/S electrodes (vs Liþ/Li) are presented in Fig. 6a. Both the cathodes exhibit two cathodic peaks locating at 2.2–2.4 V and 1.9–2.1 V, which are associated with the formation of long-chain Li2Sn (4 n 8) and the further conversion to insoluble Li2S, respectively [7]. Besides, the prominent oxidation peak located at 2.3–2.5 V corresponds to the reverse reactions. In addition, Fig. 6a shows a slightly shift of the cathodic peak in the high-voltage range in the HEMO-1 cell, which is attributed to the 681
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Fig. 6. a) CV curves of the HEMO-1 and KB cathodes at a scan rate of 0.1 mV s1. b) Rate capability of HEMO-1. c) Discharge-charge profiles of HEMO-1 and KB at 0.1C. d) CV of symmetric cells of HEMO-1@KB carbon and KB carbon electrodes with and without Li2S6. e) Cycling performance and CE of HEMO-1 and KB at 0.1C. f) Long-term cycling performance and CE of HEMO-1 and KB at 0.5C.
3. Conclusion
shown in Fig. S7, Supporting Information. Distinct morphological evolution is displayed, the surface of electrode material after 100 cycles became smooth and porous structure reduced, which may be due to the extensive contact area of the electrolyte/cathode, and this is conducive to lessen the transport distances of LIPSs and electrons and promote to enhance electrochemical property [69,70]. These analyses all rationally suggest the HEMO-1 promoter shows higher capacity performance, consisting with the above electrochemical results. The long cycling performance of different materials for Li-S batteries were summarized in Table S2, the longer cycles and comparable decay of HEMO-1 indicate its feature to function as a LIPSs anchor in Li-S batteries.
In summary, an efficient sulfur host material composed of HEMO-1 was designed for immobilizing LiPSs in Li-S batteries cathode, which was afforded via a facilely mechanochemical-assisted method. The resultant HEMO-1 incorporates five metal components, Ni, Mg, Cu, Zn and Co, into a metal oxide crystalline structure with a homogeneous distribution of multicomponent. In addition, The HEMO-1 functions as a cathode and exhibits a competitive reversible capacity, outstanding cycling stability and a low capacity decay after 600 cycles. Specifically, the homogeneous distribution of multiple metal active sites in HEMO-1
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accelerates the redox reaction kinetics of LIPSs. Due to the synergistic contribution of Li-O and S-Ni bonds from the interaction between HEMO1 and LIPSs, the interfacial affinity of LiPSs as well as suppression of the shuttle effect are effectively enhanced. This work not only presents a high-entropy oxide promoter for polysulfide immobilizing in Li-S batteries, but also is anticipated to extend the viability of high-entropy metal oxides to comprehensive varieties of energy conversion and storage fields.
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Conflicts of interest There are no conflicts to declare. Data availability The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgment This work was supported by the Young Thousand Talented Program and the National Natural Science Foundation of China (21671073 and 21621001); the “111” Project of the Ministry of Education of China (B17020); the Program for JLU Science and Technology Innovative Research Team; the National Key Research and Development Program of China (No. 2017YFA0204800). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.02.030. References [1] M. Armand, J.M. Tarascon, Nature 451 (2008) 652–657. [2] A. Manthiram, Y.Z. Fu, S.H. Chung, C.X. Zu, Y.S. Su, Chem. Rev. 114 (2014) 11751–11787. [3] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19–29. [4] D. Larcher, J.M. Tarascon, Nat. Chem. 7 (2015) 19–29. [5] X.L. Ji, L.F. Nazar, J. Mater. Chem. 20 (2010) 9821–9826. [6] A. Eftekhari, D.W. Kim, J. Mater. Chem. A 5 (2017) 17734–17776. [7] Z. Sun, J.Q. Zhang, L.C. Yin, G.J. Hu, R.P. Fang, H.-M. Cheng, F. Li, Nat. Commun. 8 (2017) 14627–14634. [8] M.A. Pope, I.A. Aksay, Adv. Energy Mater. 5 (2015), 1500124. [9] J.C. Guo, Y.H. Xu, C.S. Wang, Nano Lett. 11 (2011) 4288–4294. [10] J.S. Kim, T.H. Hwang, B.G. Kim, J. Min, J.W. Choi, Adv. Funct. Mater. 24 (2014) 5359–5367. [11] Y. Yang, G.Y. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018–3032. [12] D. Zheng, G.W. Wang, D. Liu, J.Y. Si, T.Y. Ding, D.Y. Qu, X.Q. Yang, D.Y. Qu, Adv. Mater. Technol. 3 (2018), 1700233. [13] P.G. Bruce, L.J. Hardwick, K.M. Abraham, MRS Bull. 36 (2011) 506–512. [14] J.Q. Huang, Q. Zhang, F. Wei, Energy Storage Mater. 1 (2015) 127–145. [15] M.R. Busche, P. Adelhelm, H. Sommer, H. Schneider, K. Leitner, J. Janek, J. Power Sources 259 (2014) 289–299. [16] D.H. Liu, C. Zhang, G.M. Zhou, W. Lv, G.W. Ling, L.J. Zhi, Q.-H. Yang, Adv. Sci. 5 (2018), 1700270. [17] Y. V Mikhaylik, J.R. Akridge, J. Electrochem. Soc. 151 (2004) A1969–A1976. [18] M. Hagen, D. Hanselmann, K. Ahlbrecht, R. Maça, D. Gerber, J. Tübke, Adv. Energy Mater. 5 (2015), 1401986. [19] H.Q. Wang, W.C. Zhang, J.Z. Xu, Z.P. Guo, Adv. Funct. Mater. 28 (2018), 1707520. [20] Z.-W. Zhang, H.-J. Peng, M. Zhao, J.-Q. Huang, Adv. Funct. Mater. 28 (2018), 1707536. [21] R. Ponraj, A.G. Kannan, J.H. Ahn, D.-W. Kim, ACS Appl. Mater. Interfaces 8 (2016) 4000–4006. [22] Q. Pang, D.P. Kundu, M. Cuisinier, L.F. Nazar, Nat. Commun. 5 (2014) 4759–4766. [23] S.A. Ahad, P. Ragupathy, S. Ryu, H.-W. Lee, D.K. Kim, Chem. Commun. 53 (2017) 8782. [24] X. Liang, A. Garsuch, L.F. Nazar, Angew. Chem. 127 (2015) 3979–3983. [25] H.-S. Kang, Y.-K. Sun, Adv. Funct. Mater. 26 (2016) 1225–1232.
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