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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Rechargeable Aluminum-Ion Battery Based on MoS2 Microsphere Cathode Zhanyu Li,† Bangbang Niu,† Jian Liu,† Jianling Li,*,† and Feiyu Kang‡ †
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, No. 30 College Road, Beijing 100083, China ‡ Shenzhen Key Laboratory for Graphene-based Materials and Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China S Supporting Information *
ABSTRACT: In recent years, a rechargeable aluminum-ion battery based on ionic liquid electrolyte is being extensively explored due to three-electron electrochemical reactions, rich resources, and safety. Herein, a rechargeable Al-ion battery composed of MoS2 microsphere cathode, aluminum anode, and ionic liquid electrolyte has been fabricated for the first time. It can be found that Al3+ intercalates into the MoS2 during the electrochemical reaction, whereas the storage mechanisms of the electrode material interface and internal are quite different. This result is confirmed by ex situ X-ray photoelectron spectroscopy and X-ray diffraction etching techniques. Meanwhile, this aluminum-ion battery also shows excellent electrochemical performance, such as a discharge specific capacity of 253.6 mA h g−1 at a current density of 20 mA g−1 and a discharge capacity of 66.7 mA h g−1 at a current density of 40 mA g−1 after 100 cycles. This will lay a solid foundation for the commercialization of aluminum-ion batteries. KEYWORDS: molybdenum disulfide, microsphere, cathode material, Al-ion battery, aluminum-storage mechanism a discharge capacity of 60 mA h g−1 at a high current density of 12 000 mA g−1 and stably cycled over 4000 cycles. To further improve the discharge capacity, Lu et al.9 prepared graphene nanoribbons on highly porous 3D graphene foam as a cathode for Al-ion battery. This battery shows exceptionally high electrochemical performance due to extra spaces and short diffusion paths for the AlCl4− anions intercalated/deintercalated into the electrode material. From the above results, it can be seen that the discharge specific capacity of the graphite-like electrode material is low because the aluminum-storage mechanism is AlCl4−, which does not exhibit the characteristics of trivalent aluminum. Therefore, metal oxides or metal sulfides10−16 have recently been widely studied due to their three electron electrochemical reactions, which improves the electrochemical performance of aluminum-ion batteries to some extent. However, two-dimensional (2D) transitionmetal dichalcogenides with weak interlayer interactions have not been extensively investigated as an electrode material for AIB. Molybdenum disulfide (MoS2), a layered transition-metal chalcogenide, has a sandwich-like structure, in which each MoS2 layer consisted of S−Mo−S trilayers through strong ionic/ covalent bonds. Individual MoS2 layers with an interlayer spacing of 0.62 nm is separated by van der Waals gap. MoS2 can
1. INTRODUCTION The rechargeable aluminum-ion battery (AIB) is receiving burgeoning attention since the drawbacks of Li-ion battery (LIB) have become more and more prominent, such as the increasing cost, safety issue, the dendrite formation of metallic lithium, the relatively lower capacity, and the scarcity of the Li element in the crust.1−3 Because aluminum reserves are abundant in nature and can be used directly as the electrode anode, the price of AIB is lower than that of LIB.4 Besides, aluminum-ion batteries are safer because aluminum anodes do not form dendrites on the surface of the electrode material.5 More importantly, Al3+ transfers three electrons during electrochemical reactions due to its trivalent nature. This leads to excellent gravimetric capacity of 2980 mA h g−1 and a volumetric capacity of 8046 mA h cm−3, which are higher than those of the graphite-based LIB (≈372 mA h g−1 and ≈800 mA h cm−3, respectively) and magnesium-ion battery (MIB) (2205 mA h g−1 and 3833 mA h cm−3, respectively).6,7 In 2015, Dai et al.5 developed an ultrafast rechargeable aluminum-ion battery, which exhibits excellent discharge voltage platform near 2.0 V and a discharge specific capacity of about 70 mA h g−1, with a Coulombic efficiency of approximately 98%. This battery shows such excellent electrochemical performance mainly due to the use of graphitic foam cathode. Therefore, a series of graphite-like cathode materials have been explored recently. In 2016, Dai et al.8 reported a monolithic three-dimensional graphitic foam (3DGF) as cathode for rechargeable aluminum-ion battery, which shows © 2018 American Chemical Society
Received: January 3, 2018 Accepted: February 22, 2018 Published: February 22, 2018 9451
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Research Article
ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Hydrothermal Process for the Formation of MoS2 Microspheres
Figure 1. (a, b) SEM images of MoS2 microspheres before cycling. (c) Energy spectrum of the as-prepared MoS2 microspheres. (d) SEM images of the MoS2 microspheres after 100 cycles.
2. RESULTS AND DISCUSSION 2.1. Structural Characterization and Aluminum-Storage Mechanism. The synthesis mechanism of MoS2 microspheres is shown in Scheme 1. First, it can be seen that thiourea is adsorbed around the ammonium heptamolybdate as the nuclei by hydrothermal treatment, which is the synthesis of the MoS2 microsphere precursor. Then, the Mo source is grown from the inside to the outside and the S source is grown from the outside to the inside by heat treatment in a nitrogen atmosphere to obtain the MoS2 microspheres. Nitrogen protection is to avoid the formation of molybdenum oxides, which will affect the experimental results. The obtained MoS2 microstructure is basically similar to the microstructure of graphene. Figure 1 shows the field emission scanning electron microscopy images of MoS2 microspheres before and after cycling. It can be clearly seen from the figure that the prepared MoS2 is a microsphere consisted of well-dispersed layered nanosheets. And the thickness of nanosheets is about a dozen nanometers. To confirm the composition and content of the as-
display two most common polymorphs, trigonal prismatic coordination and octahedral coordination, belonging to 2H semiconducting phase and 1T metallic phase, respectively.17−19 MoS2 has been studied in single-electron and multielectron reversible batteries, such as LIB and MIB.20,21 Nevertheless, its application has not been reported yet in AIB. In this study, we successfully prepared MoS2 microspheres by a simple hydrothermal method. The battery system consists of MoS2 microspheres cathode, metal aluminum anode, and room-temperature ionic liquid electrolyte and shows excellent electrochemical performance. It is noteworthy that Al3+ can be inserted into the electrode material accompanied with phase transformation at the electrode material interface and internal, which is confirmed by ex situ X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) etching techniques. The elements species and valence changes of original MoS2 and MoS2 during electrochemical reactions are also characterized by XPS, and the structural changes are observed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). 9452
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) SEM image of as-prepared MoS2 microspheres. TEM images of MoS2 microspheres before (b, c) and after (d−f) test. EDS images at different depths after cycling: 1 → (g), 2 → (h), and 3 → (i).
discharging. The formation of the SEI film can be attributed mainly to the decomposition of ionic liquid electrolytes, adsorption, and other side effects. For a more clear observation of MoS2 microspheres morphology characteristics on the electrode material surface before and after charging and discharging, the transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) images are shown in Figure 2. It can be found from Figure 2a,b that MoS2 microspheres are composed of a well-dispersed nanosheet, which is the same as the above results. Figure 2c shows the high-resolution transmission electron microscopy (HRTEM) image of the as-prepared MoS2 microspheres. Its interplanar spacing is 0.62 nm, which is consistent with the (002) lattice spacing of the MoS2 crystal
prepared MoS2, the EDS image is shown in Figures 1c and S1. It can be seen that its composition mainly includes Mo and S elements and their ratio is about 2:1. The results suggest that the synthetic material is pure-phase MoS2 and no other impurity elements are present. To compare the microstructure of the surface of the electrode material before and after the electrochemical cycle, the scanning electron microscopy (SEM) image of the MoS2 microspheres after 100 cycles is shown in Figure 1d. In addition, the SEM image of MoS2 after 10 cycles is shown in Figure S2. It is clear that the surface of the electrode material after cycling has a blurred film, which is mainly attributed to solid electrolyte interface (SEI) film. The microstructure of MoS2 is different before and after its cycling due to the formation of SEI film during charging and 9453
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Research Article
ACS Applied Materials & Interfaces
Figure 3. Ex situ XPS images of MoS2 microspheres: (a) spectra of Mo 3d; (b) spectra of S 2p; (c) spectra of Al 2p of various discharge and charge states before etching; (d, g) spectra of Mo 3d; (e, h) spectra of S 2p; and (f, i) spectra of Al 2p after etching for various times (0, 60, and 120 s).
structure (PDF # 77-1716). Figure 2d,e shows the TEM images of the MoS2 microspheres after discharging at the first cycle. It is clear that its lattice spacing increased to 0.73 nm and some of the lattice stripes appear distorted (as indicated by the red circle). This phenomenon is mainly due to Al3+ inserted into the lattice spacing of MoS2. After the subsequent charging process, the HRTEM images show an interplanar spacing of 0.63 nm, as shown in Figure 2f, which reveals that the
electrochemical reaction of Al3+ inserted into individual MoS2 layers separated by van der Waals gap is reversible. It is also found that there is a layer of SEI film (2−3 nm) on the surface of the electrode material, which is consistent with the observed results of SEM. EDS characterization was carried out for more intuitive observation of the composition and ingredient of the electrode materials interface at different depths after cycling. The results are shown in Figure 2g−i. Obviously, its main 9454
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Research Article
ACS Applied Materials & Interfaces
mechanism of inserted Al3+ is different at the electrode material interface and internal. As shown in Figure 3f, Al content gradually decreases as the etching time increases. This indicates that electrolyte adsorption is present in addition to irreversible inserting reaction of Al3+ on the surface of the electrode material. But only the former is included in the electrode material internal. It can be found from Figure 3g that two pairs of Mo 3d binding energy peaks were observed due to phase transition on the surface of the electrode material, when discharged to 0.8 V. There is only a pair of binding energy peaks observed after Ar+ etching for 60 or 120 s. This interesting phenomenon reveals that the phase transition occurs only on the surface of the cathode material during electrochemical reaction. As shown in Figure 3h,i, the content of Al 2p increases and the content of S 2p decreases with the increase of etching depth, which suggests that Al3+ is preferentially inserted in the cathode material internal during the charging−discharging process. The ex situ XPS images of other charge−discharge states after Ar+ sputtering for 60 and 120 s are shown in Figure S6. It can be found from Figure S6a,d that Mo 3d maintains a pair of binding energy peaks during the entire charging and discharging process after etching. In Figure S6c,f, the content of A is the highest, when the discharge is complete. These phenomena are similar to those discussed above. As discussed above, in this battery system, the dissolution/ deposition of Al occurs at the anode side and the corresponding Al3+ intercalation/deintercalation occurs at the cathode side. Accordingly, the electrochemical reactions of MoS2 microspheres as aluminum-ion battery can be described by the following equations In the discharge process
components include Mo, S, Al, and C elements, in which the C element is mainly derived from the binder poly(tetrafluoroethylene) (PTFE). It is found that the content of Al gradually decreased along with the increase of the electrode surface depth. This phenomenon implies that the content of Al on the surface of the electrode material is derived from not only the ionic liquid electrolyte decomposition/adsorption but also the electrode material, in which the Al can only be inserted but not be deinserted. However, the Al in the electrode material internal mainly comes from the latter, which also results in irreversible capacity and low Coulombic efficiency during the charging−discharging process. To probe the valence changes and the aluminum-storage mechanism of MoS2 microspheres during the charging− discharging process, X-ray photoelectron spectroscopy (XPS) experiments of the cathode material at various charge and discharge states and different etching levels were carried out. Before etching and after etching for 60 and 120 s, a wide-scan ex situ XPS survey was performed to observe the presence of various elements in the electrode material, as shown in Figures S3−S5. The three spectra reveal the presence of Mo, S, C, Al, and Cl elements, in which the presence of Cu can be attributed to the Cu substrate. The presence of the C peak is mainly derived from air-contaminated carbon and carbon in PTFE. The detailed narrow spectrum scan results are shown in Figure 3. It can been seen from Figure 3a that the binding energies of Mo 3d include a pair of peaks at 232.3 and 229.1 eV before test, which are attributed to Mo 3d3/2 and Mo 3d5/2 in the MoS2, respectively.22,23 After discharging to 0.8 V, the binding energy at 232.3 eV separates into two peaks at 233.1 and 231.6 eV, and the binding energy at 229.1 eV also splits into two peaks at 229.8 and 228.3 eV.24,25 The binding energies at 233.1 and 229.8 eV are attributed to Mo 3d3/2 and Mo 3d5/2 in the high valence state, respectively, and the binding energies at 231.6 and 228.3 eV belong to Mo 3d3/2 and Mo 3d5/2 in the low valence state, respectively. This phenomenon implies that Al3+ may be inserted into different locations on the surface of the electrode material and causes a phase structure transition during the discharging process. When the discharge is completed, the two peaks of the high valence state are completely eliminated and only the two peaks of the low valence state are left. The charging process and the discharging process are just the opposite, which is consistent with the result in Figure 3a. As shown in Figure 3b, the binding energy of S 2p contains two peaks at 163.1 and 161.8 eV during the entire charging−discharging process, which are attributed to S 2p1/2 and S 2p3/2, respectively.24 This result reveals that the valence of S does not change during the electrochemical reaction process. It can be found from Figure 3c that the content of Al gradually increases during the discharging process and its content gradually decreases during the charging process, which confirms the inserting−deinserting process of Al3+ in the cathode material. After charging, residual Al can be attributed mainly to the formation of SEI film on the electrode surface, which induces the consumption of a small amount of Al3+ or AlxCly− and other side effects. This also gives a reasonable explanation for the following low Coulombic efficiency during the electrochemical process. Figure 3d shows the binding energies of Mo 3d at different depths after the first cycle. Obviously, the binding energy of Mo 3d contains only a pair of peaks. However, the position of the peak gradually shifts to lower binding energy as the etching depth increases. This is mainly due to the fact that the
Cathode: MoS2 + x Al3 + + 3x e− → Alx MoS2
Anode: x Al + 7x AlCl−4 → 4x Al 2Cl −7 + 3x e−
In the charge process Cathode: Al xMoS2 − 3e− → MoS2 + x Al3 +
Anode: 4x Al 2Cl −7 + 3x e− → x Al + 7x AlCl−4
To further confirm its electrochemical reaction mechanism, the ex situ XRD pattern of MoS2 electrode was detected as shown in Figure S7. It can be clearly seen that the diffraction peak corresponding to (002) plane disappears completely when discharged to 0.5 V and the intensities of the other diffraction peaks also decrease. This phenomenon indicates that Al3+ is intercalated into MoS2 in the discharge process and induces the phase transition of its structure. When charged to 2.0 V, the diffraction peak (002) appeared again and became broader. This implies that the electrochemical performance of MoS2 microspheres is affected to a certain degree due to the formation of SEI film and phase transition in the electrochemical cycle. To further analyze the intercalation and deintercalation positions of Al3+ at the interface and internal of the electrode material, the crystal structure of MoS2 is shown in Scheme 2 9455
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
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ACS Applied Materials & Interfaces
Scheme 2. (a) Crystal Structure of MoS2; Al3+ Intercalated and Deintercalated into MoS2 Microspheres at the (b) Electrode Material Interface and (c) Internal
Figure 4. Electrochemical performance of MoS2 microspheres: (a) discharge−charge curves at the current density of 20 mA g−1 from the first to the third cycle, (b) the first discharge−charge curves at different current densities, and (c) the cycling performance at the current density of 40 mA g−1 for 100 cycles.
A1 site, which will cause loss of capacity and phase transition. However, the A2 site constructed by the van der Waals force has a small electrostatic effect so that the intercalation and deintercalation of Al3+ is reversible at the A2 site. According to the above discussion, the process of intercalation and deintercalation of Al3+ on the surface of the electrode material is shown in Scheme 2b. It can be found that Al3+ is first
during the charging−discharging process. As shown in Scheme 2a, the packing of MoS2 units has two different positions for Al3+ intercalated and deintercalated, where the smaller site A1 is made up of S−Mo−S ion bonds and the larger site A2 is constructed by individual MoS2 layers separated by van der Waals gap. It is necessary to overcome the strong electrostatic effect for the high energy density of Al3+ intercalated into the 9456
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
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ACS Applied Materials & Interfaces
Figure 5. CV graphs of MoS2 microspheres: (a) CV graph under the voltage range of 0.5−2.0 V at a scan rate of 2 mV s−1; (b) CV graph at different scan rates (1, 2, and 5 mV s−1). Electrochemical impedance spectroscopy (EIS) measurements of MoS2 microspheres: (c) Nyquist plots of MoS2 microspheres and (d) equivalent circuit model of this system.
attenuating, its Coulombic efficiency increases. Our work shows a certain degree of advantages compared to the cathode materials reported in the previous papers, as shown in Table S1. In addition, it can be seen that the difference between charge and discharge curves is relatively large regardless of the current density, which is mainly due to the electrochemical polarization caused by large charge density of Al3+ under the effect of electrostatic effects during the discharging−charging process. To study the cycle performances and Coulombic efficiencies of MoS2 microspheres, they are subjected to 100 charging− discharging cycles with a cutoff voltage between 2.0 and 0.5 V at a current density of 40 mA g−1, as shown in Figure 4c. And the first three cycles are primarily to activate the electrode material at a current density of 20 mA g−1. It can be found that the reversible specific capacity decreases from 77.7 to 66.7 mA h g−1 and Coulombic efficiency increases from 83.6 to 91.5%. The rapid capacity decay in the first three cycles could be attributed to the formation of the SEI layer and the decomposition of the liquid electrolyte or possible side reactions. The Al-ion storage behavior and kinetics analysis of the MoS2 electrode were investigated by the cyclic voltammetry (CV) curves shown in Figure 5a,b. It can be found from Figure 5a that there are two cathodic peaks in the MoS2 electrode material. The broad cathodic peak at 0.5−0.9 V is associated with the intercalation of Al ion and induces irreversible phase transition. This implies that its kinetics analysis is difficult due to the phase transition occurred in the electrode. As shown in Figure 5b, the redox peak is not particularly notable regardless of the scan rate. Considering the insignificant redox peaks, the energy-storage process for the MoS2 electrode in the AIB is
intercalated in the A1 site and then intercalated in the A2 site from the A1 site during the discharging process and that Al3+ is first deintercalated from the A2 site to the A1 site and finally deintercalated the electrode material from the A1 site during the charging process at the cathode material interface. However, as shown in Scheme 2c, Al3+ is intercalated/ deintercalated directly at the A2 site during the charging− discharging process at the cathode material internal. Therefore, the phase structure transition is only generated on the surface of the electrode material, which is consistent with the results discussed above. 2.2. Electrochemical Performance. To study the electrochemical performance of MoS2 microspheres as aluminum-ion batteries, as shown in Figure 4, the galvanostatic discharge− charge measurements were executed. It can be clearly seen from Figure 4a that the discharge specific capacities of MoS2 microspheres are 253.6, 224.4, and 217.7 mA h g−1 and the corresponding charge specific capacities are 128.1, 121.6, and 119.4 mA h g−1 at the current density of 20 mA g−1 in the first, second, and third cycles, respectively. Obviously, MoS 2 microspheres exhibit low Coulombic efficiency as cathode materials for aluminum-ion batteries. The reason for this can be attributed to the MoS2 phase transition, the formation of the SEI film as well as other side effects on the electrode material surface, which is consistent with the results obtained from the above ex situ XPS and HRTEM images. Figure 4b shows the initial discharge−charge curves of MoS2 microspheres at different current densities (20, 30, and 40 mA g−1). The MoS2 microspheres deliver specific discharge capacities of 138.5 and 77.7 mA h g−1 at current densities of 30 and 40 mA g−1, respectively. Although its reversible capacity is rapidly 9457
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
ACS Applied Materials & Interfaces mainly not a diffusion-dominated battery behavior but more of capacitive effects (both surface pseudocapacitance and doublelayer capacitance), which means that charge is stored between the electrode and electrolyte interface forming an electric double layer. This result is basically consistent with the results reported in the literature.21,26,27 Electrochemical impedance spectroscopy (EIS) measurements were carried out to further characterize the kinetic behavior of MoS2 electrodes. Figure 5c shows the Nyquist plots of MoS2 electrodes in the AIB. The plot shows a single semicircle from the high- to medium-frequency region, which is associated with the charge-transfer kinetics between the electrode interface and electrolyte, and a slope in the lowfrequency region indicating aluminum-ion diffusion or Warburg diffusion process into the bulk of the electrodes.28 The experimental results can be fitted using the equivalent circuit model, as shown in Figure 5d. In the equivalent circuit, Rs, Rsf, and Rct are the resistances of the electrolyte, electrode material surface, and charge transfer, respectively. CPE1 and CPE2 are constant phase elements of SEI film and Al-ion interfacial transfer, respectively. Zw is associated with the Warburg impedance of solid-phase diffusion. We can find by fitting that the solution impedance of MoS2 electrodes is 44.47 Ω and the impedance caused by charge transfer is 98.8 Ω. It does not show the SEI film impedance due to the low thickness of the SEI film, as observed in the HRTEM image.
ACKNOWLEDGMENTS
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00100. Experimental details; the component of MoS2 microspheres; ex situ XRD of MoS2; ex situ XPS of MoS2 before and after etching (PDF)
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This work was financially supported by the National Natural Science Foundation of China (nos. 51772025 and 51572024).
3. CONCLUSIONS In summary, a rechargeable aluminum-ion battery composed of MoS2 microsphere cathode, aluminum anode, and ionic liquid electrolyte has been fabricated for the first time. The phasetransition mechanism is confirmed by ex situ XPS and XRD with etching techniques during the charging−discharging process on the surface of the electrode material. The irreversible electrochemical properties and low Coulombic efficiency caused by phase transition are also confirmed. However, the electrochemical polarization of MoS2 electrode materials is relatively large due to the large energy density of Al3+ intercalated and deintercalated in the cathode material. To overcome its defects, future research on eliminating the electrostatic effect between Al3+ and electrode material by extending the MoS2 layer spacing is needed. In this paper, the study of phase-transition mechanism has played a guiding role in the study of two-dimensional transition-metal sulfide as cathode material for aluminum-ion battery.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jianling Li: 0000-0002-3915-9540 Notes
The authors declare no competing financial interest. 9458
DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
Research Article
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DOI: 10.1021/acsami.8b00100 ACS Appl. Mater. Interfaces 2018, 10, 9451−9459
