A Rechargeable Aluminum-Ion Battery Based on MoS2 Microsphere

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A Rechargeable Aluminum-Ion Battery Based on MoS2 Microsphere Cathode Zhanyu Li, Bangbang Niu, Jian Liu, Jianling Li, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00100 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

A Rechargeable Aluminum-Ion Battery Based on MoS2 Microsphere Cathode Zhanyu Lia, Bangbang Niua, Jian Liua, Jianling Lia*, Feiyu Kangb a

School of Metallurgical and Ecological Engineering, University of Science and

Technology Beijing, No. 30 College Road, Beijing 100083, China. b

Shenzhen Key Laboratory for Graphene-based Materials and Engineering

Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China * Corresponding author e-mail: [email protected] (J.Li)

Mo S

A2 Al

M S 3+ Al

ACS Paragon Plus Environment

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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 comprised 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, while the storage mechanism of the electrode material interface and internal are quite different. This result is confirmed by ex-situ XPS and XRD 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 the 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

1. Introduction The rechargeable aluminum-ion battery (AIB) is receiving burgeoning attention since the drawbacks of Li-ion battery (LIB) become more and more prominent, such as the increasingly 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 cheaper than that of LIB 4. Besides, aluminum ion batteries are safer because aluminum anode 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. This leads to excellent gravimetric capacity of 2980 mA h g−1 and a volumetric capacities of 8046 mA h cm−3, which are higher than that 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, ACS Paragon Plus Environment

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respectively) 6, 7. In 2015, Dai et al 5 developed an ultrafast rechargeable aluminium-ion battery, which exhibits excellent discharge voltage platform near 2.0 V and the discharge specific capacity of about 70mAh g-1 with a coulombic efficiency of approximately 98 %. This battery shows such excellent electrochemical performance, which is mainly due to the use of graphitic foam cathode. Therefore, a series of graphite-like cathode materials has been explored recently. In 2016, Dai et al

8

reported a monolithic 3D graphitic

foam (3DGF) as cathode for rechargeable aluminium-ion battery, which shows a discharge capacity of 60 mA h g−1 at a high current density of 12000 mA g-1 and stably cycled over 4000 cycles. In order to further improve the discharge capacity, Lu et al 9 prepared graphene nanoribbons on highly porous 3D-graphene (GNHPG) foam as a cathode for Al-ion battery. This battery shows an 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, the metal oxides or metal sulfides

10-16

have recently been widely studied due to their three charge energy

storage, which improve the electrochemical performance of aluminum ion batteries to some extent. However, 2D transition metal dichalcogenides (TMDCs) with weak interlayer interactions has not been extensively investigated as an electrode material for AIB. Molybdenum disulfide (MoS2), layered transition metal chalcogenide, has a sandwich-like structure, in which each MoS2 layer is 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 display two most common polymorphs being trigonal prismatic coordination and octahedral coordination belongs to 2H semiconducting phase and 1T metallic phase, respectively 17-19

. MoS2 has been studied in a single electron and multi-electron reversible battery,

such as LIB and MIB

20, 21

. Nevertheless, its application has not been reported yet in ACS Paragon Plus Environment


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AIB. In this study, we successfully prepared the 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 XPS and XRD etching techniques. The elements species and valence changes of original MoS2 and MoS2 during electrochemical are also characterized by XPS, and the structural changes are observed by transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS).

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 been 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 obtained 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. Scheme 1 Schematic illustration of the hydrothermal process for the formation of MoS2 microspheres.

(NH4)2CS 180 ºC Hydrothermal

N2 450 ºC MoS2

(NH4)6Mo7O24·4H2O

Mo S

Figure 1 shows the field-emission scanning electron microscopy (FESEM) images of


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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. In order to confirm the composition and content of as-prepared MoS2, the EDS energy spectrum is shown in Figure 1(c) and Figure S1. It can be seen that its composition mainly includes Mo and S elements and the ratio of both is about 2: 1. The results suggest that the synthetic material is pure phase MoS2 and no other impurity elements are present. In order to compare the microstructure of the surface of the electrode material before and after the electrochemical cycle, the SEM image of the MoS2 microspheres after 100 cycles is shown in Figure 1(d). In addition, the SEM image of MoS2 after 10 cycles in the 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 discharging. The formation of the SEI film can be attributed mainly to the decomposition of ionic liquid electrolytes, adsorption and other side effects.

(a)

(b)

(c)

Mo

Counts/cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d) S

CO 0

1

2

3

4

5

keV

Figure 1(a) and (b) The SEM images of MoS2 microspheres before cycling. (c) The energy



spectrum of as-prepared MoS2 microspheres. (d) The SEM images of MoS2 microspheres after 100 cycles.

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 the Figure 2(a) and (b) that MoS2 microspheres are composed of a well-dispersed nanosheet, which is the same as the above results. Figure 2(c) shows the HRTEM image of as-prepared MoS2 microspheres. Its interplanar spacing is 0.62 nm, which is consistent with the (002) lattice spacing of the MoS2 crystal structure (PDF#77-1716). Figure 2(d-e) shows the TEM image of 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 shows an interplanar spacing of 0.63 nm as shown in the Figure 2(f), 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. The 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 2(g-i). Obviously, its main components include Mo, S, Al and C elements, in which the C element is mainly derived from the binder 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 not only from 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. ACS Paragon Plus Environment

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(a)

(b)

(c)

(d)

(e)

(f)

1 2 3

(h)

1 After 1st cycle

C

S

(i) Mo

2 After 1st cycle

Mo 3 After 1st cycle

C Al

Al

S

Counts/cps

Mo

S

Counts/cps

(g) Counts/cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C

Mo

Al

Mo

Mo 4

8

12

Energy/kev

16

20

4

8

12

16

20

Energy/kev

4

8

12

16

20

Energy/kev

Figure 2(a) The SEM image of as-prepared MoS2 microspheres. The TEM images of MoS2 microspheres before (b-c) test and after (d-f) test. The EDS energy spectrum at different depths after cycling: 1→(g), 2→(h), 3→(i).

In order to probe the valence changes and aluminum storage mechanism of MoS2 microspheres during charging-discharging process, the X-ray photoelectron spectroscopy (XPS) of the cathode material at various charge and discharge states and different etching levels were carried out. Before etching and after etching for 60s and 120s, a wide scan ex-situ XPS survey were performed to observe the presence of various elements in the electrode material as shown in the Figure S3, Figure S4 and Figure S5, respectively. The three spectrum reveals 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 ACS Paragon Plus Environment


PTFE. The detailed narrow spectrum scan results are shown in Figure 3. It can been seen from the Figure 3(a) that the binding energies of Mo 3d include a pair of peaks at 232.3 eV and 229.1 eV before test, which are attributed to Mo 3d3/2 and Mo 3d5/2 in the MoS2, respectively 22, 23. After dicharging to 0.8 V, the binding energy at 232.3 eV separates into two peaks at 233.1 eV and 231.6 eV, respectively, and the binding energy at 229.1 eV also splits into two peaks at 229.8 eV and 228.3 eV, respectively 24, 25

. The binding energies at 233.1 eV 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 eV and 228.3 eV are belong to Mo 3d3/2 and Mo 3d5/2 in the low valence state, respectively. This phenomenon implies that Al3+ may be inserted into the different locations in the surface of the electrode material and causes a phase structure transition during discharging process. When the discharge is completed, the two peaks of the high valence state are completely eliminated and only two peaks of the low valence state are left. The charging process and the discharging process is just the opposite, which is consistent with the result in Figure 3(a). As shown in the Figure 3(b), the binding energy of S 2p contains two peaks at 163.1 eV 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 the Figure 3(c) that the content of Al gradually increases during the discharging process and its content gradually decreases during 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 amounts 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 3(d) shows the binding energy of Mo 3d at different depths after 1st cycle. Obviously, the binding energy of Mo 3d contains only a pair of peaks. However, the position of the peak gradually shift to lower binding energy as the etching depth ACS Paragon Plus Environment

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increases. This is mainly due to the fact that the inserted mechanism of Al3+ is different at the electrode material interface and internal. As shown in the Figure 3(f), 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 the Figure 3(g) 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 presented after Ar+ etching for 60s or 120s. This interesting phenomenon reveals that the phase transition occurs only on the surface of the cathode material during electrochemical reaction. As shown in the Figure 3(h-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 charging-discharging prcoess. The ex-situ XPS spectra of other charge-discharge states after Ar+ sputtering for 60s and 120s are shown in Figure S6. It can be found from the Figure S6(a) and (d) that Mo 3d maintains a pair of binding energy peaks during the entire charging and discharging process after etching. In the Figure S6(c) and (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 equation: In the discharge process: Cathode: MoS2 +xAl3+ +3xe- →AlxMoS2 -

-

Anode: xAl+7xAlCl4 →4xAl2 Cl7 +3xeIn the charge process: Cathode: Alx MoS2 -3e- →MoS2 +xAl3+



-

-

Anode: 4xAl2 Cl7 + 3xe- →xAl+7xAlCl4

Intensity (a.u.)

233.1 232.3 231.6

charge 2.0V

(b)

229.8 229.1 228.3

charge 1.85V discharge 0.5V discharge 0.8V

S 2p 0s

charge 2.0V

Al 2p 0s

(c) charge 2.0V charge 1.85V

75.5

163.1

charge 1.85V

161.8

discharge 0.5V discharge 0.8V

before test

Intensity (a.u.)

Mo 3d 0s

Intensity (a.u.)

(a)

discharge 0.3V

discharge 0.8V

before test before test

238 236 234 232 230 228 226 224 222

166 165 164 163 162 161 160 159 158

Binding energy (eV)

(d) Mo 3d

After 1st cycle

80

Binding energy (eV)

(e) S 2p

After 1st cycle

(f)

78

76

74

After 1st cycle

Al 2p

120 s 163.1

161.8

60 s

(h) S 2p

229.8

0s 238 236 234 232 230 228 226 224 222

Binding energy (eV)

120 s

(i)

76

74

72

70

Al 2p

discharge 0.8V

75.5

120 s

60 s

discharge 0.8V

78

Binding energy (eV)

163.1

120 s

161.8

60 s

Intensity (a.u.)

228.3

80

Binding energy (eV)

Intensity (a.u.)

233.1

0s

166 165 164 163 162 161 160 159 158

Binding energy (eV) discharge 0.8V

60 s

0s

0s 238 236 234 232 230 228 226 224 222

75.5

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

60 s

231.6

70

120 s 120 s

(g)Mo 3d

72

Binding energy (eV)

229.1 232.3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 s

0s 166 165 164 163 162 161 160 159 158

Binding energy (eV)

0s 80

78

76

74

72

70

Binding energy (eV)

Figure 3 The Ex-situ XPS spectra 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, (f, i) spectra of Al 2p after etching for various times (0s, 60s and 120s).

In order to further confirm its electrochemical reaction mechanism, the ex-situ XRD pattern of MoS2 electrode was detected as shown in the Figure S7. It can be clearly seen that the diffraction peak corresponding to (002) plane disappears


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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. In order 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 the Scheme 2 during the charging-discharging process. As shown in the Scheme 2(a), the packing of MoS2 units has two different positions for Al3+ intercalated and deintercalated, where smaller site A1 is made up of S-Mo-S ion bonds, 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 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 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 2(b). It can be found that Al3+ is first intercalated in the A1 site and then intercalated in the A2 site from the A1 site during the discharging process and 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 the Scheme 2(c), Al3+ is intercalated/deintercalated directly at the A2 site during charging/discharging process at the cathode material internal. Therfore, the phase structure transition is only generated on the surface of the electrode material, which is consistent with the results discussed above. Scheme 2 (a) crystal structure of MoS2; Al3+ intercalated and deintercalated into MoS2 microspheres at the electrode material interface (b) and internal (c).



(a)

Al

A

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2

Mo S Al3+

(b) Discharge 0.8V

Discharge

Charge

Charge

0.5V

1.85V

2.0V

(c) Discharge

Discharge

Charge

Charge

0.8V

0.5V

1.85V

2.0V

3.2 Electrochemical performance In order to study electrochemical performance of MoS2 microspheres as aluminum-ion batteries, as shown in the Figure 4, the galvanostatic discharge/charge measurements were executed. It can be clearly seen from the Figure 4(a) that the discharge specific capacities of MoS2 microspheres are 253.6 mA h g-1, 224.4 mA h g-1 and 217.7 mA h g-1 and corresponding charge specific capacities are 128.1 mA h g-1, 121.6 mA h g-1 and 119.4 mA h g-1 at the current density of 20 mA g-1 from the first to the third cycle, respectively. Obviously, MoS2 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. Figure 4(b) shows the initial discharge-charge curves of MoS2 microspheres at the different current densities (20 mA g-1, 30 mA g-1 and 40 mA g-1). The MoS2 microspheres deliver specific discharge capacity of 138.5 mA h g-1 and 77.7 mA h g-1 at the current density of 30 mA g-1 and 40 mA g-1, respectively. Although its reversible capacity is rapidly attenuating, its coulombic efficiency increases. Our work shows a certain degree of advantages compared with the cathode materials reported in the previous papers as


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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 discharging- charging process. In order to study the cycle performances and coulombic efficiency of MoS2 microspheres, it is subjected to 100 charging-discharging cycles with a cut-off voltage between 2.0 V and 0.5 V at a current density of 40 mA g-1 as shown in the Figure 4(c). 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 mA h g-1 to 66.7 mA h g-1 and coulombic efficiency increases from 83.6 % to 91.5 %. The rapid capacity decay in the first 3 cycles, which could be attributed to the formation of the SEI layer and the decomposition of the liquid electrolyte or possible side reactions. (a)

(b) 2.0

2.0 1st 2nd 3rd

1.6 1.2

1.2

20 mA g

0.8

-1

0.8 0.4

Specific Capacity (mAh g-1)

0

50

100

150

200

0

250

Specific Capacity (mAh g-1)

50

100

150

200

Specific Capacity (mAh g-1) 100

250

80 200 150

40 mA g

Discharge Charge

-1

60 40

100 50 0

20 mA g 0

20

20

-1 40

60

80

100

0

250

Coulombic Efficiency (%)

0.4

(c)

20 mA g-1 30 mA g-1 40 mA g-1

1.6

Potential (V)

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120

Cycle Number

Figure 4 The 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 the different current densities, (c) The cycling performance at the current density of 40 mA g-1 for 100 cycles.



The Al-ion storage behavior and kinetics analysis of the MoS2 electrode were investigated by the cyclic voltammetric curves (CVs), as shown in the Figure 5(a-b). It can be found from the Figure 5(a) that there are two cathodic peaks in the MoS2 electrode material. The broad cathodic peak at 0.5 V-0.9 V is associated with the intercalation of Al ion and induced irreversible phase transition. This implies that its kinetically is difficult due to the the phase transition occurred in the electrode. As shown in the Figure 5(b), the redox peak is not particularly noticeable regardless of the scan rate. Considering the insignificant redox peaks, the energy storage process for the MoS2 electrode in the AIB is mainly not diffusion-dominated battery behavior but a more capacitive effects (both surface pseudocapacitance and double layer capacitance), which means charge is stored between the electrode and electrolyte interface forming an electric double layer. This result is basically consistent with the reported results in the literature 21, 26, 27. The electrochemical impedance spectroscopy (EIS) measurements were carried out to further characterize the kinetic behavior of MoS2 electrodes. Figure 5(c) 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 low frequency 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 the Figure 5(d). In the equivalent circuit, Rs, Rsf and Rct are the resistance of the electrolyte, the resistance of electrode material surface and the resistance of charge-transfer, respectively. The CPE1 and CPE2 are constant phase element 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 thin thickness of the SEI film, as observed in the HRTEM.


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(b) 0.08 -2

Current Density (mA cm )

-2

Current Density (mA cm )

(a) 0.06

1st 2nd 3rd

0.04 0.02 0.00

-0.02

1 mV s-1 2 mV s-1 5 mV s-1

0.04 0.00

-0.04

-0.04 -0.06

2 mV s-1

-0.08

-0.08

-0.12

-0.10 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Potential vs Al (V)

(c) 800

(d)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

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Z'/Ω Figure 5 The CV graph of MoS2 microspheres: (a) the CV graph under the voltage range of 0.5 V-2.0 V at a scan rate of 2 mV s-1; (b) the CV graph at the different scan rate (1 mV s-1, 2 mV s-1 and 5 mV s-1). The electrochemical impedance spectroscopy (EIS) measurements of MoS2 microspheres: (c) Nyquist plots of MoS2 microspheres (d) Equivalent circuit model of this system.

3. Conclusions In summary, a rechargeable aluminum-ion battery comprised of MoS2 microspheres cathode, aluminum anode and ionic liquid electrolyte has been fabricated for the first time. The phase transition 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. In order 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 2D transition metal sulfide as cathode material for aluminum ion battery.



Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no.51772025, no 51572024).

Supporting Information Supporting Information Available: Experimental details, the component of MoS2 microspheres, ex-situ XRD of MoS2, ex-situ XPS of MoS2 before and after etching. (PDF)

Author Information Corresponding Author E-mail: [email protected] (J.L.) ORCID Jianling Li: 0000-0002-3915-9540 Notes The authors declare no competing financial interest.

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A Rechargeable Aluminum-Ion Battery Based on MoS2 Microsphere

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