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Improved Li+ diffusion enabled by SEI film in a high-energy-density hybrid magnesium-ion battery
Journal of Power Sources 441 (2019) 227190
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Improved Liþ diffusion enabled by SEI film in a high-energy-density hybrid magnesium-ion battery Xusheng Wang a, Jingyi Ding b, Jitao Chen b, Mianqi Xue a, c, * a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China c Beijing National Laboratory for Molecular Sciences, Beijing, 100190, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� SEI film enables the improved Liþ diffusion in a-MoS5.7 for a Li–Mg hybrid system. � A high energy density of 761 Wh kg 1 is realized. � A considerable capacity retention of 90% is achieved over 500 cycles.
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid battery Cathode SEI film Ion diffusion High energy density
We report a high-energy-density Li–Mg hybrid battery with a sulfur-rich amorphous molybdenum polysulfide (aMoS5.7) as the cathode. The solid electrolyte interphase (SEI) film, which forms during the cycling process, enables the improved Liþ diffusion and the successful capacity delivery, and thus a high energy density of 761 Wh kg 1. The stable SEI film also facilitates a considerable capacity retention of 90% over 500 cycles.
1. Introduction The appealing natures of magnesium (Mg) as a rechargeable anode for Mg-ion batteries, have been widely characterized in the early elec trochemical studies [1,2]. Building feasible rechargeable Mg-ion batte ries depends on the stable operation of electrolyte and cathode [3,4]. Classical all-phenyl complex (APC) electrolyte and some newly-developed non-nucleophilic Mg-ion electrolytes have realized their successful preparations and applications [3,5]. Compared to the flourishing development of electrolytes, high-performance cathode
materials are still lacking [6]. Insertion-type cathodes (such as TiS2 and Mo6S8) face the issue of low capacity [7], and conversion-type cathodes (such as sulfur, Br2, and CuS) face the issues of slow kinetics (high temperature is required) and poor cycling life [8]. By using the hybrid ion electrolytes, researchers have realized the higher-reactivity Liþ instead of lower-reactivity Mg2þ to participate in the cathode reaction [9], which is conductive to improve the capacity delivery, kinetics, and cycling life. Wu et al. have used the synergistic insertion of Liþ and Mg2þ to enhance the reversible capacity and kinetics property of Li4Ti5O12 cathode [10]. Gao et al. have studied the
* Corresponding author. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (M. Xue). https://doi.org/10.1016/j.jpowsour.2019.227190 Received 4 August 2019; Received in revised form 17 September 2019; Accepted 19 September 2019 Available online 17 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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electrochemical behavior of TiS2 cathode in a long-life Li–Mg hybrid battery [4]. Sun et al. have reported a high-rate and long-life VS2 cathode in a Li–Mg hybrid battery [11,12]. Sun et al. have used Prussian blue analogues as the cathode of Li–Mg hybrid battery with a high discharge potential of over 2 V [12]. Tian et al. have fabricated a high-capacity Mg-organic battery using a Li–Mg hybrid electrolyte [13]. Despite these great achievements, the energy density of such hybrid system is barely satisfactory. Considering that the discharge potential is limited by the relatively narrow electrochemical window of ether-based hybrid electrolyte [14], the capacity delivery becomes the key factor to improve the energy density of Li–Mg hybrid battery. In this point, the Liþ diffusion in the cathode material is essential to improving the charge storage (i.e. capacity delivery) [15,16]. The ion diffusion inside the bulk can be improved by using the amorphous materials as the cathode, in which their one-dimensional disordered structure allows rich active reaction sites and fast iondiffusion kinetics [17]. And the improved ion diffusion across the electrolyte-electrode interface relys on the construction of stable SEI film [18]. Considering both the formation potential of SEI film and the discharge potential as high as possible, the disulfide bond with a suitable reduction/oxidation potential (1.5–2.5 V vs Li/Liþ) is favorable to improve the energy density [19]. Therefore, the transition metal poly sulfides assembled with disulfide bonds should be the right choice for the cathode material of hybrid battery. Here, we develop a sulfur-rich amorphous MoSx (x ¼ 5.7, a-MoS5.7) polysulfide as a high-energy-density and long-life cathode in a Li–Mg hybrid battery. One of the highest energy density of 761 Wh kg 1 is realized among the reported (hybrid) Mg-ion batteries, and one of the highest capacity retention of 90% is achieved (tested at 300 mA g 1 over 500 cycles) compared to the reported Mg–S batteries. The stable SEI film with both valence-divided Mo and S elements is formed on the surface of a-MoS5.7, and it contributes to the improved Liþ diffusion and a long cycling life.
atomic emission spectroscopy (ICP-AES, PROFILE SPEC, Leeman). The morphological and microstructural characterizations of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 materials were assessed by scanning electron microscopy (SEM, S4800, Hitachi), transmission electron mi croscopy (TEM, JEM-2100 F, JEOL), and energy-dispersive X-ray spec troscopy (EDX, S4800, Hitachi). The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd.). The type of X-ray source is monochromatic Al target. The binding energy scale calibration was performed using the standard XPS peaks of Au, Ag, Cu metals within the range of 1–1400 eV to realize the linear results. With regard to the correction for sample charging, the binding energy of C 1s peak was fitted to be 284.8 eV using the software of CasaXPS to realize the correction. 2.4. Preparation of the a-MoS5.7 electrodes The MoS5.7 powders (75 wt%), carbon black powders (15 wt%), and sodium carboxymethylcellulose (10 wt%, CMC) were mixed into a slurry, which was spread onto a copper foil and then dried at 90 � C in an oven. Some disks (11 mm in diameter) were tailored from the dried copper foil and then rolled as the a-MoS5.7 electrodes. After that, the aMoS5.7 electrodes were dried in an oven at 80 � C overnight under vac uum. The average loading of a-MoS5.7 is 2.0 mg cm 2. 2.5. Electrochemical performance of the a-MoS5.7 electrodes The cell performance of a-MoS5.7 electrodes were characterized with polished Mg metal foils (11 mm in diameter) as the counter electrodes, glass fibers as the separators, and 0.25 mol L 1 Mg(AlCl2EtBu)2 in tetrahydrofuran (APC) and 0.25 mol L 1 Mg(AlCl2EtBu)2 þ 0.5 mol L 1 LiCl in tetrahydrofuran (APC þ LiCl) as the electrolytes, and then assembled into LIR2032-type coin cells in an argon-filled glove box in which the moisture and oxygen contents were below 1.0 ppm. The electrolyte volume is 150 μL per cell. The cycling tests of assembled cells were performed on a Land CT2001A battery testing system within a voltage range of 0.2–1.8 V versus Mg2þ/Mg. The specific capacities and energies were referenced to the mass of the active a-MoS5.7 material. The electrochemical impedance spectroscopy (EIS) patterns were measured on an Autolab electrochemical workstation. The cycle number of battery performance starts after an activation process.
2. Experimental section 2.1. Synthesis of the (NH4)2Mo2S12⋅2H2O material (NH4l)6Mo7O24⋅4H2O (4.0 g, Energy Chemical, 98%) and NH2OH⋅HCl (3.0 g, J&K Scientific, 98.5%) were added into deionized water (60 mL) under continuous stirring, and then the (NH4)2Sx solution (60 mL, 20 wt% in water) was mixed with the above solution. The asprepared solution was put into an oven for 1 h (50 � C), then cooled to room temperature and filtered. The filtrate was stood in an oven for 4 h (90 � C) and filtered after being cooled to room temperature. Then the (NH4)2Sx solution (20 mL) was added to the filtrate (placed in a jar) under continuous stirring. After a degassing process for 3 min under Ar gas, the jar was sealed and placed in the dark environment for 12 h. Then the (NH4)2Mo2S12⋅2H2O material was obtained by filtration and fol lowed by further washing with ice-cold water (30 mL, three times), isopropanol (20 mL, two times), carbon disulphide (20 mL, two times), and diethyl ether (20 mL, two times). After that, the purified (NH4)2Mo2S12⋅2H2O material was dried in an oven under vacuum for 12 h at room temperature, and then stored in a glove box.
3. Results and discussion The a-MoS5.7 is synthesized via the low-temperature thermal treat ment of (NH4)2Mo2S12⋅2H2O, whose X-ray diffraction (XRD) pattern is shown in Fig. 1a. Fig. 1b displays the structural schematic diagram of a single molecule of (NH4)2Mo2S12⋅2H2O [20]. As exhibited in Fig. S1 (Supporting Information), only Mo and S elements can be found in the energy-dispersive X-ray spectroscopy (EDX) image of a-MoS5.7, so the N and O elements are successfully removed. Then the molar ratio of Mo to S elements is determined to be 5.7. The XRD pattern of a-MoS5.7 (Fig. 1a) indicates that it has an amorphous structure. The surface chemical states of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figs. S2a and c (Sup porting Information), the survey XPS spectra of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 also exhibit the main difference of N element. As displayed in Fig. 1c and d, the fitting curves of Mo element in the (NH4)2Mo2S12⋅2H2O reveals a valence state of Mo4þ with double-peak positions of 229.7 eV (3d5/2) and 232.8 eV (3d3/2), respectively, and the valence state of Mo element does not change in the a-MoS5.7, which can be confirmed by its double-peak positions of 229.5 and 232.7 eV. The insets in Fig. 1c and d shows the scanning electron microscopy (SEM) images of (NH4)2Mo2S12⋅2H2O and a-MoS5.7, respectively. We can see that the rod-like morphology does not change significantly. The transmission electron microscopy (TEM) image (Fig. S3a, Supporting Information) of a-MoS5.7 rod further determines the thin-lamella stacked
2.2. Synthesis of the a-MoS5.7 material The a-MoS5.7 material was synthesized by removing the nitrogen, hydrogen, and oxygen elements in the (NH4)2Mo2S12⋅2H2O material at 220 � C for 1 h under Ar gas. 2.3. Material characterizations The crystallographic structures of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 materials were characterized by X-ray diffraction (XRD, PANalytical diffractometer) with Cu Kα radiation (λ ¼ 1.5416 Å). The value of 5.7 in the a-MoS5.7 material was determined by inductively coupled plasma2
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Journal of Power Sources 441 (2019) 227190
Fig. 1. a) XRD patterns of the (NH4)2Mo2S12⋅2H2O and a-MoS5.7. B) Schematic of a single molecule (NH4)2Mo2S12⋅2H2O. c) Mo 3 d region in the XPS spectrum of (NH4)2Mo2S12⋅2H2O. The inset displays the SEM image of (NH4)2Mo2S12⋅2H2O, and the scale bar is 0.5 μm. d) Mo 3 d region in the XPS spectrum of a-MoS5.7. The inset displays the SEM image of a-MoS5.7, and the scale bar is 1.0 μm. e,f) Possible structural transformation from the [Mo2S12]2- to the a-MoS5.7.
configuration. And in Fig. S3b (Supporting Information), its high-resolution TEM image reveals no crystal lattice, and the corre sponding selected area electron diffraction (SAED) pattern demonstrates no clear diffraction spot. Both results affirm the amorphous nature of a-MoS5.7. The fitting pattern of S 2p region in (NH4)2Mo2S12⋅2H2O (Fig. S2b, Supporting Information) reveals two chemical states of S element, corresponding to the terminal sulfur (I S 2p) and bridging sulfur (II S 2p) ligands (S2), respectively, the double-peak (2p3/2/2p1/2) positions of which are separately 162.1/163.2 and 163.4/164.6 eV. And as far as the a-MoS5.7 is concerned (Fig. S2d, Supporting Information), the double-peak positions of the two S2 ligands are 162.2/163.4 and 163.5/164.7 eV, respectively, which are almost identical to the fitting results of (NH4)2Mo2S12⋅2H2O, yet with the changed relative contents of the two S2 ligands, suggesting that a small amount of terminal S2 ligands is lost on the surface. Therefore, the possible structural transformations from [Mo2S12]2- to a-MoS5.7 are shown in Fig. 1e and f. The main dif ference is the disappearance of crystal unit cell and part of the terminal S2 ligands. The battery performance was tested by assembling the a-MoS5.7 cathode and Mg anode into coin cells. The electrolytes are based on a phenylmagnesium chloride-AlCl3 system with or without the addition of
LiCl. The galvanostatic discharge-charge curves of a-MoS5.7 in the electrolyte containing LiCl is shown in Fig. 2a. The discharge capacity is 812 mAh g 1 at a current density of 50 mA g 1 with an average voltage plateau of 1.02 V, and the discharge energy density is as high as 761 Wh kg 1. The charge voltage is 1.33 V, thus allowing an energy efficiency of 71%. Considering that the lithiated products of metal sulfides always include Li2S, we propose the discharge mechanism of a-MoS5.7 þ xLiþ þ e 1 → MoS5.7-0.5x þ 0.5 xLi2S with a theoretical specific capacity of 1096 mAh g 1. Therefore, the utilization ratio of S in a-MoS5.7 for the reaction with Liþ should be no more than 74.1% ((812/1096)%). In view of the amount of Liþ and electron is equal, the value of x should be no more than 8.4. So in this case, the final discharge products should be MoS1.5 and Li2S. The cycling stability of a-MoS5.7 was assessed at a current density of 50 mA g 1. As displayed in Fig. 2b, the capacity delivery is still as high as 769 mAh g 1 after 50 cycles, and the capacity retention reaches up to 95%. In the electrolyte system without LiCl, the capacity delivery of a-MoS5.7 is negligible (less than 1 mAh g 1), and no improvement is observed after 1000 cycles (Fig. S4, Supporting Infor mation), indicating that Mg2þ does not react directly with a-MoS5.7. The rate capability of a-MoS5.7 is demonstrated in Fig. 2c, in which high capacities of 411, 317, and 199 mAh g 1 can still be realized at the 3
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Journal of Power Sources 441 (2019) 227190
Fig. 2. a) Galvanostatic discharge-charge curves of the a-MoS5.7 tested at 50 mA g 1. B) Cycling perfor mance of the a-MoS5.7 tested at 50 mA g 1. c) Rate capability of the a-MoS5.7 (1C corresponds to 1096 mAh g 1). d) Cycling performance of the a-MoS5.7 tested at 300 mA g 1. e) Comparison in the capacity and energy density between the a-MoS5.7 and some previously reported works of (hybrid) Mg-ion bat tery. The red star represents this work. f) Compari son in the cycle number and capacity retention between the a-MoS5.7 and some previously reported works of Mg–S battery. The two hollow red stars represent this work.
current densities of 500, 1000, and 2000 mA g 1, respectively. When the current density recovers to 100 mA g 1, the capacity delivery returns to 627 mAh g 1, suggesting an excellent rate capability of a-MoS5.7. As shown in Fig. 2d, the a-MoS5.7 still maintains a high capacity of 539 mAh g 1 after 500 cycles tested at a current density of 300 mA g 1, with a high capacity retention of 90%, manifesting a long-life cycling stability of a-MoS5.7. Fig. 2e exhibits the comparison of capacity delivery and energy density between a-MoS5.7 and some published works of (hybrid) Mg-ion batteries [2,4,11–13,21–29]. It can be seen that the a-MoS5.7 presents significant advantages in both capacity delivery and energy density. In Fig. 2f, the comparison of cycling life and capacity retention between a-MoS5.7 and some published works of Mg–S batteries dem onstrates that the cycling stability of a-MoS5.7 is superior to that of the reported Mg–S batteries [30–39]. Therefore, the a-MoS5.7 combines the strengths of high capacity, high energy density, and long cycling life compared with both Mg-ion and Mg–S batteries. Fig. 3a displays the electrochemical impedance spectroscopy (EIS) patterns of the coin cells before and after test (charged state) in both the pure Mg-ion and the LiCl-containing electrolytes. It can be seen that the charge-transfer resistance (Rct, the semicircle diameter enclosed by the high-frequency region) is reduced after test in the pure Mg-ion electro lyte, but still greater than 104 Ω, which reveals that the charge-transfer process is very difficult in a-MoS5.7, that is, Mg2þ cannot diffuse in aMoS5.7, thus leading to almost no capacity delivery in the pure Mg-ion electrolyte. In the LiCl-containing electrolyte, the Rct values before and after test are both much smaller than the values in the pure Mg-ion electrolyte. The involvement of Liþ greatly improves the charge-transfer process in a-MoS5.7. Therefore, the working principle of this Li–Mg hybrid battery is based on the Liþ-involved cathode reaction and Mg2þinvolved anode reaction (Fig. 3b). As exhibited in Fig. 3c, in the LiCl-containing electrolyte, the Rct value is significantly reduced after test, which suggests that the
electrochemical discharge/charge behavior enables a battery activation process, thereby lowering the kinetic hindrance of charge transfer [11]. The EIS patterns before/after test in the pure Mg-ion electrolyte and the EIS pattern before test in the LiCl-containing electrolyte have a common feature: there is no obvious SEI resistance-based semicircle. Fig. 3d shows the full view of EIS pattern after test in the LiCl-containing electrolyte. It can be seen that there is another small semicircle before the Rct-related semicircle in higher frequency, which proves that a SEI film is formed. The SEM image of a-MoS5.7 rod tested in the LiCl-containing electrolyte is demonstrated in Fig. 3e. Compared with the initial state of a-MoS5.7, the rod-like morphology does not change significantly after test, but a film is formed on the surface, which should be assigned to a SEI film, and this is consistent with the conclusion of EIS pattern. Fig. 3f exhibits the linear relationship between Zre and ω 1/2 (ω represents angular frequency; ω ¼ 2πf) (after test in the pure Mg-ion electrolyte, before test in the LiCl-containing electrolyte, and after test in the LiCl-containing electrolyte). The fitted slopes correspond to the Warburg factor σ, whose square (σ2) is inversely proportional to the ion-diffusion coefficient D, with numbers separately being 3040, 8948, and 102. Therefore, a much faster ion-diffusion rate is realized for the tested a-MoS5.7 electrode in the LiCl-containing electrolyte. The involvement of Liþ generates the stable SEI film on the surface of a-MoS5.7, and considering that the SEI film is an electrical insulator and yet an ionic conductor, the improved Liþ diffusion across the solid-liquid interface is enabled by the SEI film, which is a key point for the suc cessful capacity delivery of a-MoS5.7. The Mo 3 d XPS region of long-life cycled a-MoS5.7 electrode (charged state) is shown in Figure 4a. It can be seen that the peak deconvolution produces two kinds of Mo species with poor Mo4þ (I Mo 3 d) and rich Mo5þ (II Mo 3 d), which are located at 230.6/233.7 and 232.0/235.1 eV, respectively. As exhibited in Figure 4b, the S 2p XPS region of long-life cycled a-MoS5.7 electrode reveals two kinds of S 4
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Fig. 3. a) EIS patterns of the coin cells before and after test (charged state) in both the pure Mg-ion and the LiCl-containing electrolytes. B) Schematic working principle of the a-MoS5.7-based Li–Mg hybrid battery. c) EIS patterns of the coin cells before and after test (charged state) in the LiCl-containing electrolyte system. d) Magnified EIS pattern of the coin cell after test in the LiCl-containing electrolyte system. e) SEM image of the a-MoS5.7 rod tested in the LiCl-containing electrolyte. The scale bar is 500 nm. f) Ion-diffusion analysis of the different coin cells. The legends in panels (c,d,f) are consistent with panel (a) in colour and shape. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
species of S22 (I S 2p) and S6þ (II S 2p), which are separately located at 161.6/162.8 and 168.0/169.1 eV. Due to the presence of SEI film on the surface of a-MoS5.7, these information about Mo and S elements ob tained from XPS deconvolution correspond to the analyses of SEI film in elemental composition and valence. The C 1s XPS spectra of a-MoS5.7 before and after testing were also collected to characterize the compo sition of SEI film. As displayed in Fig. S5a (Supporting Information), the C 1s spectrum of a-MoS5.7 is simulated into two peaks, which can be separately attributed to C–C bond (284.8 eV) and C–O bond (286.7 eV). These carbon-based signals come from the absorbed carbon materials in the air, which are used to correct the sample charging. As exhibited in Fig. S5b (Supporting Information), the C 1s spectrum of long-life cycled a-MoS5.7 (no air contact) demonstrates three peaks with an additional – O bond compared to that of initial a-MoS5.7, indicating the peak of C– carbonate or organic carbonyl in the SEI film. Therefore, the SEI film formed on the surface of cycled a-MoS5.7 has a significant change compared to the surface chemical state of initial a-MoS5.7. The XPS spectra of Mo and S elements for the discharged a-MoS5.7 electrode were also collected. As shown in Fig. S6a (Supporting Information), the Mo5þ
(I Mo 3 d) and Mo6þ (II Mo 3 d) species are separately located at 232.0/ 235.1 and 233.2/236.3 eV. And as exhibited in Fig. S6b (Supporting Information), the S22 (I S 2p) and S6þ (II S 2p) species are located at 163.4/164.6 and 169.2/170.4 eV, respectively. Therefore, the discharge process induces the valence increase of both the Mo and S elements. The XPS spectra of Li 1s and Mg 2p for the discharged a-MoS5.7 cathode and the charged Mg anode were separately collected (the electrodes were washed with tetrahydrofuran and then dried). As shown in Figure 4c, only a characteristic peak of Li 1s (55.3 eV) appears for the discharged a-MoS5.7 cathode. This result indicates that Mg2þ does not react with a-MoS5.7 or deposits on the electrode surface. As for the charged Mg anode, the only peak located at 49.4 eV should be assigned to the Mg 2p, suggesting a pure Mg2þ-based deposition without the interference of Liþ. All these above results are consistent with the schematic in Fig. 3b. As can be seen in Fig. 4d, the surface of fresh Mg anode is smooth with no impurity. In comparison, the surface of charged Mg anode (Fig. 4e) is covered with nanoparticles, and their average particle size is below 100 nm (the inset of Fig. 4e). The corresponding EDX elemental mapping image of charged Mg anode is exhibited in 5
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Fig. 4. a) Mo 3 d region in the XPS spectrum of the long-life cycled a-MoS5.7 cathode. B) S 2p region in the XPS spectrum of the long-life cycled a-MoS5.7 cathode. c) Adjacent Li 1s and Mg 2p regions in the XPS spectra of the discharged a-MoS5.7 cathode and the charged Mg anode. d,e) SEM images of the fresh Mg anode and the charged Mg anode, respectively. The inset in panel (e) is the enlarged SEM image of charged Mg anode. The scale bars are 5, 5, and 0.3 μm in panels (d), (e), and the inset of (e). f) EDX elemental mapping image of the charged Mg anode.
Fig. 4f, in which only Mg element is detected with a uniform distribu tion. Therefore, the nanoparticles can be assigned to the deposition of Mg2þ onto Mg anode. And this result is in accordance with the conclu sion in Fig. 4c.
manuscript. Acknowledgements This work was supported by the National Key Research and Devel opment Program of China (2016YFB0700604), the National Natural Science Foundation of China (21622407, 21673008, 21875266), the Beijing National Laboratory for Molecular Sciences (BNLMS201909), and the “transformational Technologies for Clean Energy and Demon stration”, Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21010214).
4. Conclusions In summary, we build a high-performance Li–Mg hybrid battery using a-MoS5.7 as the cathode. Characterizations of a-MoS5.7 reveal that it has a rod-like morphology and an amorphous sulfur-rich structure bridged by Mo centres and two kinds of S2 ligands. This hybrid battery demonstrates both high energy density and long cycling life. The greatly improved kinetics is realized in the Li–Mg hybrid electrolyte with a much smaller charge-transfer resistance and a faster ion-diffusion rate compared to those in the pure Mg-based electrolyte. A stable SEI film is formed on the surface of a-MoS5.7 rod, which is crucial to the improved Liþ diffusion and then successful capacity delivery. Characterizations of the SEI film show that both Mo and S elements have two valence states. And characterizations of the Mg anode prove the dendrite-free Mg2þ deposition/stripping. The Liþ-storage (or Naþ-, Kþ-storage) cathode materials [40–43] should be featured by high capacity and suitable re action potential to build better hybrid Mg-ion battery.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227190. References [1] D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, Prototype systems for rechargeable magnesium batteries, Nature 407 (2000) 724–727. [2] Y. Zhang, J. Xie, Y. Han, C. Li, Dual-Salt Mg-based batteries with conversion cathodes, Adv. Funct. Mater. 25 (2015) 7300–7308. [3] J. Muldoon, C.B. Bucur, T. Gregory, Quest for nonaqueous multivalent secondary batteries: magnesium and beyond, Chem. Rev. 114 (2014) 11683–11720. [4] T. Gao, F. Han, Y. Zhu, L. Suo, C. Luo, K. Xu, C. Wang, Hybrid Mg2þ/Liþ battery with long cycle life and high rate capability, Adv. Energy. Mater. 5 (2015) 1401507.
Author contributions All authors have given approval to the final version of the 6
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Improved Liþ diffusion enabled by SEI film in a high-energy-density hybrid magnesium-ion battery Xusheng Wang a, Jingyi Ding b, Jitao Chen b, Mianqi Xue a, c, * a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China c Beijing National Laboratory for Molecular Sciences, Beijing, 100190, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� SEI film enables the improved Liþ diffusion in a-MoS5.7 for a Li–Mg hybrid system. � A high energy density of 761 Wh kg 1 is realized. � A considerable capacity retention of 90% is achieved over 500 cycles.
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid battery Cathode SEI film Ion diffusion High energy density
We report a high-energy-density Li–Mg hybrid battery with a sulfur-rich amorphous molybdenum polysulfide (aMoS5.7) as the cathode. The solid electrolyte interphase (SEI) film, which forms during the cycling process, enables the improved Liþ diffusion and the successful capacity delivery, and thus a high energy density of 761 Wh kg 1. The stable SEI film also facilitates a considerable capacity retention of 90% over 500 cycles.
1. Introduction The appealing natures of magnesium (Mg) as a rechargeable anode for Mg-ion batteries, have been widely characterized in the early elec trochemical studies [1,2]. Building feasible rechargeable Mg-ion batte ries depends on the stable operation of electrolyte and cathode [3,4]. Classical all-phenyl complex (APC) electrolyte and some newly-developed non-nucleophilic Mg-ion electrolytes have realized their successful preparations and applications [3,5]. Compared to the flourishing development of electrolytes, high-performance cathode
materials are still lacking [6]. Insertion-type cathodes (such as TiS2 and Mo6S8) face the issue of low capacity [7], and conversion-type cathodes (such as sulfur, Br2, and CuS) face the issues of slow kinetics (high temperature is required) and poor cycling life [8]. By using the hybrid ion electrolytes, researchers have realized the higher-reactivity Liþ instead of lower-reactivity Mg2þ to participate in the cathode reaction [9], which is conductive to improve the capacity delivery, kinetics, and cycling life. Wu et al. have used the synergistic insertion of Liþ and Mg2þ to enhance the reversible capacity and kinetics property of Li4Ti5O12 cathode [10]. Gao et al. have studied the
* Corresponding author. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (M. Xue). https://doi.org/10.1016/j.jpowsour.2019.227190 Received 4 August 2019; Received in revised form 17 September 2019; Accepted 19 September 2019 Available online 17 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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electrochemical behavior of TiS2 cathode in a long-life Li–Mg hybrid battery [4]. Sun et al. have reported a high-rate and long-life VS2 cathode in a Li–Mg hybrid battery [11,12]. Sun et al. have used Prussian blue analogues as the cathode of Li–Mg hybrid battery with a high discharge potential of over 2 V [12]. Tian et al. have fabricated a high-capacity Mg-organic battery using a Li–Mg hybrid electrolyte [13]. Despite these great achievements, the energy density of such hybrid system is barely satisfactory. Considering that the discharge potential is limited by the relatively narrow electrochemical window of ether-based hybrid electrolyte [14], the capacity delivery becomes the key factor to improve the energy density of Li–Mg hybrid battery. In this point, the Liþ diffusion in the cathode material is essential to improving the charge storage (i.e. capacity delivery) [15,16]. The ion diffusion inside the bulk can be improved by using the amorphous materials as the cathode, in which their one-dimensional disordered structure allows rich active reaction sites and fast iondiffusion kinetics [17]. And the improved ion diffusion across the electrolyte-electrode interface relys on the construction of stable SEI film [18]. Considering both the formation potential of SEI film and the discharge potential as high as possible, the disulfide bond with a suitable reduction/oxidation potential (1.5–2.5 V vs Li/Liþ) is favorable to improve the energy density [19]. Therefore, the transition metal poly sulfides assembled with disulfide bonds should be the right choice for the cathode material of hybrid battery. Here, we develop a sulfur-rich amorphous MoSx (x ¼ 5.7, a-MoS5.7) polysulfide as a high-energy-density and long-life cathode in a Li–Mg hybrid battery. One of the highest energy density of 761 Wh kg 1 is realized among the reported (hybrid) Mg-ion batteries, and one of the highest capacity retention of 90% is achieved (tested at 300 mA g 1 over 500 cycles) compared to the reported Mg–S batteries. The stable SEI film with both valence-divided Mo and S elements is formed on the surface of a-MoS5.7, and it contributes to the improved Liþ diffusion and a long cycling life.
atomic emission spectroscopy (ICP-AES, PROFILE SPEC, Leeman). The morphological and microstructural characterizations of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 materials were assessed by scanning electron microscopy (SEM, S4800, Hitachi), transmission electron mi croscopy (TEM, JEM-2100 F, JEOL), and energy-dispersive X-ray spec troscopy (EDX, S4800, Hitachi). The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd.). The type of X-ray source is monochromatic Al target. The binding energy scale calibration was performed using the standard XPS peaks of Au, Ag, Cu metals within the range of 1–1400 eV to realize the linear results. With regard to the correction for sample charging, the binding energy of C 1s peak was fitted to be 284.8 eV using the software of CasaXPS to realize the correction. 2.4. Preparation of the a-MoS5.7 electrodes The MoS5.7 powders (75 wt%), carbon black powders (15 wt%), and sodium carboxymethylcellulose (10 wt%, CMC) were mixed into a slurry, which was spread onto a copper foil and then dried at 90 � C in an oven. Some disks (11 mm in diameter) were tailored from the dried copper foil and then rolled as the a-MoS5.7 electrodes. After that, the aMoS5.7 electrodes were dried in an oven at 80 � C overnight under vac uum. The average loading of a-MoS5.7 is 2.0 mg cm 2. 2.5. Electrochemical performance of the a-MoS5.7 electrodes The cell performance of a-MoS5.7 electrodes were characterized with polished Mg metal foils (11 mm in diameter) as the counter electrodes, glass fibers as the separators, and 0.25 mol L 1 Mg(AlCl2EtBu)2 in tetrahydrofuran (APC) and 0.25 mol L 1 Mg(AlCl2EtBu)2 þ 0.5 mol L 1 LiCl in tetrahydrofuran (APC þ LiCl) as the electrolytes, and then assembled into LIR2032-type coin cells in an argon-filled glove box in which the moisture and oxygen contents were below 1.0 ppm. The electrolyte volume is 150 μL per cell. The cycling tests of assembled cells were performed on a Land CT2001A battery testing system within a voltage range of 0.2–1.8 V versus Mg2þ/Mg. The specific capacities and energies were referenced to the mass of the active a-MoS5.7 material. The electrochemical impedance spectroscopy (EIS) patterns were measured on an Autolab electrochemical workstation. The cycle number of battery performance starts after an activation process.
2. Experimental section 2.1. Synthesis of the (NH4)2Mo2S12⋅2H2O material (NH4l)6Mo7O24⋅4H2O (4.0 g, Energy Chemical, 98%) and NH2OH⋅HCl (3.0 g, J&K Scientific, 98.5%) were added into deionized water (60 mL) under continuous stirring, and then the (NH4)2Sx solution (60 mL, 20 wt% in water) was mixed with the above solution. The asprepared solution was put into an oven for 1 h (50 � C), then cooled to room temperature and filtered. The filtrate was stood in an oven for 4 h (90 � C) and filtered after being cooled to room temperature. Then the (NH4)2Sx solution (20 mL) was added to the filtrate (placed in a jar) under continuous stirring. After a degassing process for 3 min under Ar gas, the jar was sealed and placed in the dark environment for 12 h. Then the (NH4)2Mo2S12⋅2H2O material was obtained by filtration and fol lowed by further washing with ice-cold water (30 mL, three times), isopropanol (20 mL, two times), carbon disulphide (20 mL, two times), and diethyl ether (20 mL, two times). After that, the purified (NH4)2Mo2S12⋅2H2O material was dried in an oven under vacuum for 12 h at room temperature, and then stored in a glove box.
3. Results and discussion The a-MoS5.7 is synthesized via the low-temperature thermal treat ment of (NH4)2Mo2S12⋅2H2O, whose X-ray diffraction (XRD) pattern is shown in Fig. 1a. Fig. 1b displays the structural schematic diagram of a single molecule of (NH4)2Mo2S12⋅2H2O [20]. As exhibited in Fig. S1 (Supporting Information), only Mo and S elements can be found in the energy-dispersive X-ray spectroscopy (EDX) image of a-MoS5.7, so the N and O elements are successfully removed. Then the molar ratio of Mo to S elements is determined to be 5.7. The XRD pattern of a-MoS5.7 (Fig. 1a) indicates that it has an amorphous structure. The surface chemical states of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figs. S2a and c (Sup porting Information), the survey XPS spectra of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 also exhibit the main difference of N element. As displayed in Fig. 1c and d, the fitting curves of Mo element in the (NH4)2Mo2S12⋅2H2O reveals a valence state of Mo4þ with double-peak positions of 229.7 eV (3d5/2) and 232.8 eV (3d3/2), respectively, and the valence state of Mo element does not change in the a-MoS5.7, which can be confirmed by its double-peak positions of 229.5 and 232.7 eV. The insets in Fig. 1c and d shows the scanning electron microscopy (SEM) images of (NH4)2Mo2S12⋅2H2O and a-MoS5.7, respectively. We can see that the rod-like morphology does not change significantly. The transmission electron microscopy (TEM) image (Fig. S3a, Supporting Information) of a-MoS5.7 rod further determines the thin-lamella stacked
2.2. Synthesis of the a-MoS5.7 material The a-MoS5.7 material was synthesized by removing the nitrogen, hydrogen, and oxygen elements in the (NH4)2Mo2S12⋅2H2O material at 220 � C for 1 h under Ar gas. 2.3. Material characterizations The crystallographic structures of (NH4)2Mo2S12⋅2H2O and a-MoS5.7 materials were characterized by X-ray diffraction (XRD, PANalytical diffractometer) with Cu Kα radiation (λ ¼ 1.5416 Å). The value of 5.7 in the a-MoS5.7 material was determined by inductively coupled plasma2
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Fig. 1. a) XRD patterns of the (NH4)2Mo2S12⋅2H2O and a-MoS5.7. B) Schematic of a single molecule (NH4)2Mo2S12⋅2H2O. c) Mo 3 d region in the XPS spectrum of (NH4)2Mo2S12⋅2H2O. The inset displays the SEM image of (NH4)2Mo2S12⋅2H2O, and the scale bar is 0.5 μm. d) Mo 3 d region in the XPS spectrum of a-MoS5.7. The inset displays the SEM image of a-MoS5.7, and the scale bar is 1.0 μm. e,f) Possible structural transformation from the [Mo2S12]2- to the a-MoS5.7.
configuration. And in Fig. S3b (Supporting Information), its high-resolution TEM image reveals no crystal lattice, and the corre sponding selected area electron diffraction (SAED) pattern demonstrates no clear diffraction spot. Both results affirm the amorphous nature of a-MoS5.7. The fitting pattern of S 2p region in (NH4)2Mo2S12⋅2H2O (Fig. S2b, Supporting Information) reveals two chemical states of S element, corresponding to the terminal sulfur (I S 2p) and bridging sulfur (II S 2p) ligands (S2), respectively, the double-peak (2p3/2/2p1/2) positions of which are separately 162.1/163.2 and 163.4/164.6 eV. And as far as the a-MoS5.7 is concerned (Fig. S2d, Supporting Information), the double-peak positions of the two S2 ligands are 162.2/163.4 and 163.5/164.7 eV, respectively, which are almost identical to the fitting results of (NH4)2Mo2S12⋅2H2O, yet with the changed relative contents of the two S2 ligands, suggesting that a small amount of terminal S2 ligands is lost on the surface. Therefore, the possible structural transformations from [Mo2S12]2- to a-MoS5.7 are shown in Fig. 1e and f. The main dif ference is the disappearance of crystal unit cell and part of the terminal S2 ligands. The battery performance was tested by assembling the a-MoS5.7 cathode and Mg anode into coin cells. The electrolytes are based on a phenylmagnesium chloride-AlCl3 system with or without the addition of
LiCl. The galvanostatic discharge-charge curves of a-MoS5.7 in the electrolyte containing LiCl is shown in Fig. 2a. The discharge capacity is 812 mAh g 1 at a current density of 50 mA g 1 with an average voltage plateau of 1.02 V, and the discharge energy density is as high as 761 Wh kg 1. The charge voltage is 1.33 V, thus allowing an energy efficiency of 71%. Considering that the lithiated products of metal sulfides always include Li2S, we propose the discharge mechanism of a-MoS5.7 þ xLiþ þ e 1 → MoS5.7-0.5x þ 0.5 xLi2S with a theoretical specific capacity of 1096 mAh g 1. Therefore, the utilization ratio of S in a-MoS5.7 for the reaction with Liþ should be no more than 74.1% ((812/1096)%). In view of the amount of Liþ and electron is equal, the value of x should be no more than 8.4. So in this case, the final discharge products should be MoS1.5 and Li2S. The cycling stability of a-MoS5.7 was assessed at a current density of 50 mA g 1. As displayed in Fig. 2b, the capacity delivery is still as high as 769 mAh g 1 after 50 cycles, and the capacity retention reaches up to 95%. In the electrolyte system without LiCl, the capacity delivery of a-MoS5.7 is negligible (less than 1 mAh g 1), and no improvement is observed after 1000 cycles (Fig. S4, Supporting Infor mation), indicating that Mg2þ does not react directly with a-MoS5.7. The rate capability of a-MoS5.7 is demonstrated in Fig. 2c, in which high capacities of 411, 317, and 199 mAh g 1 can still be realized at the 3
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Fig. 2. a) Galvanostatic discharge-charge curves of the a-MoS5.7 tested at 50 mA g 1. B) Cycling perfor mance of the a-MoS5.7 tested at 50 mA g 1. c) Rate capability of the a-MoS5.7 (1C corresponds to 1096 mAh g 1). d) Cycling performance of the a-MoS5.7 tested at 300 mA g 1. e) Comparison in the capacity and energy density between the a-MoS5.7 and some previously reported works of (hybrid) Mg-ion bat tery. The red star represents this work. f) Compari son in the cycle number and capacity retention between the a-MoS5.7 and some previously reported works of Mg–S battery. The two hollow red stars represent this work.
current densities of 500, 1000, and 2000 mA g 1, respectively. When the current density recovers to 100 mA g 1, the capacity delivery returns to 627 mAh g 1, suggesting an excellent rate capability of a-MoS5.7. As shown in Fig. 2d, the a-MoS5.7 still maintains a high capacity of 539 mAh g 1 after 500 cycles tested at a current density of 300 mA g 1, with a high capacity retention of 90%, manifesting a long-life cycling stability of a-MoS5.7. Fig. 2e exhibits the comparison of capacity delivery and energy density between a-MoS5.7 and some published works of (hybrid) Mg-ion batteries [2,4,11–13,21–29]. It can be seen that the a-MoS5.7 presents significant advantages in both capacity delivery and energy density. In Fig. 2f, the comparison of cycling life and capacity retention between a-MoS5.7 and some published works of Mg–S batteries dem onstrates that the cycling stability of a-MoS5.7 is superior to that of the reported Mg–S batteries [30–39]. Therefore, the a-MoS5.7 combines the strengths of high capacity, high energy density, and long cycling life compared with both Mg-ion and Mg–S batteries. Fig. 3a displays the electrochemical impedance spectroscopy (EIS) patterns of the coin cells before and after test (charged state) in both the pure Mg-ion and the LiCl-containing electrolytes. It can be seen that the charge-transfer resistance (Rct, the semicircle diameter enclosed by the high-frequency region) is reduced after test in the pure Mg-ion electro lyte, but still greater than 104 Ω, which reveals that the charge-transfer process is very difficult in a-MoS5.7, that is, Mg2þ cannot diffuse in aMoS5.7, thus leading to almost no capacity delivery in the pure Mg-ion electrolyte. In the LiCl-containing electrolyte, the Rct values before and after test are both much smaller than the values in the pure Mg-ion electrolyte. The involvement of Liþ greatly improves the charge-transfer process in a-MoS5.7. Therefore, the working principle of this Li–Mg hybrid battery is based on the Liþ-involved cathode reaction and Mg2þinvolved anode reaction (Fig. 3b). As exhibited in Fig. 3c, in the LiCl-containing electrolyte, the Rct value is significantly reduced after test, which suggests that the
electrochemical discharge/charge behavior enables a battery activation process, thereby lowering the kinetic hindrance of charge transfer [11]. The EIS patterns before/after test in the pure Mg-ion electrolyte and the EIS pattern before test in the LiCl-containing electrolyte have a common feature: there is no obvious SEI resistance-based semicircle. Fig. 3d shows the full view of EIS pattern after test in the LiCl-containing electrolyte. It can be seen that there is another small semicircle before the Rct-related semicircle in higher frequency, which proves that a SEI film is formed. The SEM image of a-MoS5.7 rod tested in the LiCl-containing electrolyte is demonstrated in Fig. 3e. Compared with the initial state of a-MoS5.7, the rod-like morphology does not change significantly after test, but a film is formed on the surface, which should be assigned to a SEI film, and this is consistent with the conclusion of EIS pattern. Fig. 3f exhibits the linear relationship between Zre and ω 1/2 (ω represents angular frequency; ω ¼ 2πf) (after test in the pure Mg-ion electrolyte, before test in the LiCl-containing electrolyte, and after test in the LiCl-containing electrolyte). The fitted slopes correspond to the Warburg factor σ, whose square (σ2) is inversely proportional to the ion-diffusion coefficient D, with numbers separately being 3040, 8948, and 102. Therefore, a much faster ion-diffusion rate is realized for the tested a-MoS5.7 electrode in the LiCl-containing electrolyte. The involvement of Liþ generates the stable SEI film on the surface of a-MoS5.7, and considering that the SEI film is an electrical insulator and yet an ionic conductor, the improved Liþ diffusion across the solid-liquid interface is enabled by the SEI film, which is a key point for the suc cessful capacity delivery of a-MoS5.7. The Mo 3 d XPS region of long-life cycled a-MoS5.7 electrode (charged state) is shown in Figure 4a. It can be seen that the peak deconvolution produces two kinds of Mo species with poor Mo4þ (I Mo 3 d) and rich Mo5þ (II Mo 3 d), which are located at 230.6/233.7 and 232.0/235.1 eV, respectively. As exhibited in Figure 4b, the S 2p XPS region of long-life cycled a-MoS5.7 electrode reveals two kinds of S 4
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Fig. 3. a) EIS patterns of the coin cells before and after test (charged state) in both the pure Mg-ion and the LiCl-containing electrolytes. B) Schematic working principle of the a-MoS5.7-based Li–Mg hybrid battery. c) EIS patterns of the coin cells before and after test (charged state) in the LiCl-containing electrolyte system. d) Magnified EIS pattern of the coin cell after test in the LiCl-containing electrolyte system. e) SEM image of the a-MoS5.7 rod tested in the LiCl-containing electrolyte. The scale bar is 500 nm. f) Ion-diffusion analysis of the different coin cells. The legends in panels (c,d,f) are consistent with panel (a) in colour and shape. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
species of S22 (I S 2p) and S6þ (II S 2p), which are separately located at 161.6/162.8 and 168.0/169.1 eV. Due to the presence of SEI film on the surface of a-MoS5.7, these information about Mo and S elements ob tained from XPS deconvolution correspond to the analyses of SEI film in elemental composition and valence. The C 1s XPS spectra of a-MoS5.7 before and after testing were also collected to characterize the compo sition of SEI film. As displayed in Fig. S5a (Supporting Information), the C 1s spectrum of a-MoS5.7 is simulated into two peaks, which can be separately attributed to C–C bond (284.8 eV) and C–O bond (286.7 eV). These carbon-based signals come from the absorbed carbon materials in the air, which are used to correct the sample charging. As exhibited in Fig. S5b (Supporting Information), the C 1s spectrum of long-life cycled a-MoS5.7 (no air contact) demonstrates three peaks with an additional – O bond compared to that of initial a-MoS5.7, indicating the peak of C– carbonate or organic carbonyl in the SEI film. Therefore, the SEI film formed on the surface of cycled a-MoS5.7 has a significant change compared to the surface chemical state of initial a-MoS5.7. The XPS spectra of Mo and S elements for the discharged a-MoS5.7 electrode were also collected. As shown in Fig. S6a (Supporting Information), the Mo5þ
(I Mo 3 d) and Mo6þ (II Mo 3 d) species are separately located at 232.0/ 235.1 and 233.2/236.3 eV. And as exhibited in Fig. S6b (Supporting Information), the S22 (I S 2p) and S6þ (II S 2p) species are located at 163.4/164.6 and 169.2/170.4 eV, respectively. Therefore, the discharge process induces the valence increase of both the Mo and S elements. The XPS spectra of Li 1s and Mg 2p for the discharged a-MoS5.7 cathode and the charged Mg anode were separately collected (the electrodes were washed with tetrahydrofuran and then dried). As shown in Figure 4c, only a characteristic peak of Li 1s (55.3 eV) appears for the discharged a-MoS5.7 cathode. This result indicates that Mg2þ does not react with a-MoS5.7 or deposits on the electrode surface. As for the charged Mg anode, the only peak located at 49.4 eV should be assigned to the Mg 2p, suggesting a pure Mg2þ-based deposition without the interference of Liþ. All these above results are consistent with the schematic in Fig. 3b. As can be seen in Fig. 4d, the surface of fresh Mg anode is smooth with no impurity. In comparison, the surface of charged Mg anode (Fig. 4e) is covered with nanoparticles, and their average particle size is below 100 nm (the inset of Fig. 4e). The corresponding EDX elemental mapping image of charged Mg anode is exhibited in 5
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Fig. 4. a) Mo 3 d region in the XPS spectrum of the long-life cycled a-MoS5.7 cathode. B) S 2p region in the XPS spectrum of the long-life cycled a-MoS5.7 cathode. c) Adjacent Li 1s and Mg 2p regions in the XPS spectra of the discharged a-MoS5.7 cathode and the charged Mg anode. d,e) SEM images of the fresh Mg anode and the charged Mg anode, respectively. The inset in panel (e) is the enlarged SEM image of charged Mg anode. The scale bars are 5, 5, and 0.3 μm in panels (d), (e), and the inset of (e). f) EDX elemental mapping image of the charged Mg anode.
Fig. 4f, in which only Mg element is detected with a uniform distribu tion. Therefore, the nanoparticles can be assigned to the deposition of Mg2þ onto Mg anode. And this result is in accordance with the conclu sion in Fig. 4c.
manuscript. Acknowledgements This work was supported by the National Key Research and Devel opment Program of China (2016YFB0700604), the National Natural Science Foundation of China (21622407, 21673008, 21875266), the Beijing National Laboratory for Molecular Sciences (BNLMS201909), and the “transformational Technologies for Clean Energy and Demon stration”, Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21010214).
4. Conclusions In summary, we build a high-performance Li–Mg hybrid battery using a-MoS5.7 as the cathode. Characterizations of a-MoS5.7 reveal that it has a rod-like morphology and an amorphous sulfur-rich structure bridged by Mo centres and two kinds of S2 ligands. This hybrid battery demonstrates both high energy density and long cycling life. The greatly improved kinetics is realized in the Li–Mg hybrid electrolyte with a much smaller charge-transfer resistance and a faster ion-diffusion rate compared to those in the pure Mg-based electrolyte. A stable SEI film is formed on the surface of a-MoS5.7 rod, which is crucial to the improved Liþ diffusion and then successful capacity delivery. Characterizations of the SEI film show that both Mo and S elements have two valence states. And characterizations of the Mg anode prove the dendrite-free Mg2þ deposition/stripping. The Liþ-storage (or Naþ-, Kþ-storage) cathode materials [40–43] should be featured by high capacity and suitable re action potential to build better hybrid Mg-ion battery.
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Author contributions All authors have given approval to the final version of the 6
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