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A Flexible Free-standing MoS2 /Carbon Nanofibers Composite Cathode for Rechargeable Aluminum-Ion Batteries Wenwen Yang, Huimin Lu, Yuan Cao, Binbin Xu, Yan Deng, and Wei Cai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05292 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Sustainable Chemistry & Engineering

A Flexible Free-standing MoS2/Carbon Nanofibers Composite Cathode for Rechargeable Aluminum-Ion Batteries Wenwen Yang, Huimin Lu,∗ Yuan Cao, Binbin Xu, Yan Deng, and Wei Cai School of Materials Science and Engineering, Xue Yuan No.37, HaiDian District, Road, Beijing 100191, China E-mail: [email protected]

Abstract Rechargeable aluminum-ion batteries are considered promising candidates for the new generation of energy storage systems owing to their high capacity, low cost and high security. The most urgent challenge to be addressed for the practical application of aluminum-ion batteries is exploring cathode materials with simple fabrication processes and preeminent electrochemical performance. Herein, a flexible free-standing MoS2 /carbon nanofibers composite has been successfully synthesized by electrospinning and annealing treatment and investigated as a cathode material for rechargeable aluminum-ion batteries, delivering an initial discharge capacity of 293.2 mAh g −1 at a current density of 100 mA g −1 and maintaining 126.6 mAh g −1 after 200 cycles. The novel free-standing MoS2 /carbon nanofibers composite can provide new ideas for the use of transition-metal sulfide as cathode materials for aluminum storage and facilitate the commercial adoption of aluminum-ion batteries.

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Keywords Aluminum-ion battery, Free-standing, MoS2 , Carbon nanofibers, Aluminum intercalation

Introduction Rechargeable lithium-ion batteries (LIBs) have been commercialized and achieved great success since the 1990s. 1 However, LIBs suffer from high-cost and safety concerns such as combustion and even explosion. 2 Other rechargeable metal-ion batteries, such as sodium, 3 potassium, 4 calcium, 5 magnesium, 6 and aluminum 7 batteries are considered substitutes for LIBs due to their abundant reserves and low costs. 8 Among these non-lithium battery systems, aluminum-ion batteries (AIBs) have received extensive attention due to the advantages of high volumetric capacity (8040 mAh cm−3 ), high gravimetric capacity (2980 mAh g −1 ), low cost, abundant reserve, high security, and incombustibility. 9 However, although researchers have made great efforts, AIBs have been far from practical applications over the past 30 years due to problems such as cathode material disintegration, low cell discharge voltage, capacitive behavior without discharge voltage plateaus and insufficient cycle life with rapid capacity decay. 10–13 In 2015, Lin et al. made a breakthrough in AIBs, developing an ultrafast rechargeable aluminum-ion battery with a graphite foam cathode and an ionic liquid electrolyte. Since then, researchers have made enormous efforts to investigate novel cathode materials to promote the capability for fast AlCl4 – and Al3+ ion intercalation, including graphitic materials (graphitic foam, 14 graphene, 15–18 and graphite 19 ), transition metal chalcogenides (CuS, 20 Mo6 S8 , 21 SnS2 , 22 Ni3 S2 , 23 Co9 S8 , 24 MoS2 , 25 NiS, 26 VS2 , 27 and VS4 , 28 ), transitional metal oxides (V2 O5 , 29 TiO2 , 30 CuO, 31 and MoO3 32 ), and nonstoichiometric compounds (Cu2 – x Se 33 and WO3 – x 34 ). For carbon materials and nonstoichiometric compounds, the energy storage mechanism of AIBs is considered as the intercalation and de-intercalation of AlCl4 – into electrode materials which does not take advantage of the trivalent characteristics of aluminum, leading to insufficient discharge capacity of these ma2


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terials. Furthermore, the large size of chloroaluminum anion may leads to a large volume expansion which results in structure disintegration and electrochemical performance decay. For some metal oxides and metal sulfides, researchers suggested that it is Al3+ intercalate and de-intercalate into the cathode materials during the electrochemical reaction, resulting in three-electron electrochemical reactions with high capacities compared with those of carbon materials. 35 The exploration among transition metal sulfides of cathode materials for Al-ion batteries has attracted great interest. Molybdenum disulfide (MoS2 ) has been widely used in the energy storage field. 25,36–40 As a typical layered transition metal sulfide, MoS2 has a structure analogous to that of graphene, but with a different stacking sequence in which S – Mo – S units are sandwich structures held together by covalent bonding and van der Waals forces. 40 MoS2 has excellent electrochemical storage performance for lithium and sodium with an attractive host for electrochemical intercalation and de-intercalation. MoS2 has seldom been used to improve the electrochemical performance of aluminum-ion batteries until the recent study of the MoS2 microsphere cathode for AIBs, where the AIBs exhibited an attractive electrochemical performance with a discharge specific capacity of 253.6 mAh g 1 at a current density of 20 mA g 1 and a discharge capacity of 66.7 mAh g 1 at a current density of 40 mA g 1 after 100 cycles. 25 Although a high specific capacity could be maintained at a low current density, the rate capacity was deficient and the current density was too small for practical application due to the poor electron conductivity and structural instability of MoS2 . Therefore, it is desirable to explore new structured electrode materials with high structural stability and an enhanced rate capability for rechargeable AIBs. One solution is to prepare metal sulfide and carbon (e.g., carbon nanotubes, graphene, carbon nanofibers, mesoporous carbon, and carbon microsphere) hybrid materials. In metal sulfide/carbon composites, carbon can not only provide physical support for the metal sulfides which can retard the volume convergent-divergence of metal sufides during the charging-discharging cycles due to the good stability of the carbon framework, but also greatly enhance the electron and ion transport. 41 Thus, the cycle stability and rate performance of metal sulfides

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can be further improved. 37 In this study, we successfully synthesized a flexible, free-standing MoS2 /carbon nanofibers (MoS2 /CNFs) composite via a simple electrospinning and subsequent annealing procedure that incorporated the poorly conductive MoS2 into carbon nanofibers as a new type of binderfree cathode material for aluminum-ion batteries. The as-prepared MoS2 /CNFs exhibited excellent cycle stability and remarkable rate capacity, delivering an initial discharge capacity of 293.2 mAh g −1 at 100mA g −1 and the discharge capacity remained steady at approximately 130mAh g −1 with a coulombic efficiency above 95% after 200 cycles. During the rate performance test, the capacity decreased only slightly as the current density increased and was then restored to its previous state, and it remained stable in subsequent cycles after the current density was restored. Furthermore, the aluminum storage mechanism of the MoS2 /CNFs is confirmed to be that Al3+ intercalates/de-intercalates into the interlayer between individual MoS2 layers corresponding to (002) plane during the charging-discharging process, which is confirmed by the ex-situ X-ray photoelectron spectroscopy(XPS) and X-ray diffraction(XRD) results.

Experimental Section The flexible, free-standing MoS2 /CNFs composite was synthesized via electrospinning and subsequent annealing, and a schematic of the preparation procedure is illustrated in Figure 1a. (NH4 )2 MoS4 and PAN were added to N, N-dimethyformamide(DMF) and stirred to obtain the precursor for electrospinning. Then the as-prepared precursor was electrospun into fiber networks and thermal treated to obtain the final MoS2 /carbon nanofiber networks composite. The as-prepared MoS2 /CNFs composite is a flexible, nonwoven textile, as shown in Figure 1b. The integrality and flexibility can be maintained after long-term electrochemical cycling, as shown in Figure 1c. The as-prepared flexible free-standing MoS2 /CNFs composite was directly used as the cathode for AIBs and subjected to the characterizations and

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electrochemical tests as below. The detailed fabrication and cell fabrication processes are provided in the Supporting Information.

Figure 1: a) Illustrative diagram of the preparation process of the flexible free-standing MoS2/CNFs composite. b, c) Photographs of the MoS2/CNFs cathode before and after cycling.

Result and Discussion The X-ray diffraction(XRD) pattern of the as-prepared MoS2 /CNFs structure (Figure 2) shows three diffraction peaks at 2θ = 13.8◦ , 32.2◦ and 58◦ , which are indexed to the (002), (100) and (110) planes of MoS2 (JCPDS No. 37-1492). The broad diffraction peak at 2θ = 23.8◦ is attributed to the amorphous carbon nanofiber. The carbon content of the asprepared MoS2 /CNFs composite is approximately 38.5 wt%, and the contents of Mo and S are 32.7 wt% and 23.47 wt%, respectively, as measured by inductively coupled plasma optical emission spectroscopy(ICP-OES). The atomic ratio of S to Mo is approximately equal to 2, further confirming that the as-prepared composite is MoS2 /CNFs. To investigate the electrochemical reaction mechanism during the charging-discharging process, the ex-situ XRD was conducted at the fully discharged (0.2V) and charged (1.8V) states, as shown in Figure 2. After fully discharged, the diffraction peak at 13.8◦ which is indexed to (002) 5



disappeared and it reappeared after fully charged. The ex-situ XRD results indicate that during the discharge process Al3+ intercalated into the interlayers of MoS2 corresponding to (002) plane and after fully charged the Al3+ de-intercalated from the MoS2 structure. Note that the recovered (002) diffraction peak is broader than its initial state, which might be attributed to the trapped Al3+ .

Figure 2: Ex-situ XRD patterns of the MoS2/CNFs electrode. The SEM images of the MoS2 /CNFs are shown in Figure 3a,b. The hybrid fibers are uniform, continuous, randomly oriented and interconnected nonwoven nanofibers that form a porous 3D network. The carbon nanofibers show a smooth surface without any structure can be found on the surface of the carbon nanofibers, as the MoS2 nanostructure was too small to observe at the current magnification. Transmission electron microscopy(TEM) was performed to further investigate the nanostructure of the MoS2 /CNFs. As shown in Figure 3c,d, MoS2 is uniformly embedded in CNFs without aggregation, which can be further confirmed by element mapping. The high-resolution transmission electron microscopy(HRTEM) results of MoS2 /CNFs are shown in Figure 3e. The lattice fringe spacing is 0.62 nm for MoS2 , which corresponds to the hexagonal (002) plane of the MoS2 crystal. Figure 3f-h 6


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show the element mapping of the hybrid nanofibers of the selected section in Figure 3c, confirming that C, Mo and S are uniformly distributed along the nanofiber.

Figure 3: a, b) SEM. c, d) TEM. e) HR-TEM. (f-h) Element mapping images of C, S and Mo. X-ray photoelectron spectroscopy(XPS) experiments of the MoS2 /CNFs cathode in its pristine, fully charging(1.8V) and fully discharging(0.2V) states were conducted to further explore the aluminum storage mechanism of the MoS2 /CNFs during the processes of charging and discharging. Before XPS test, the samples were adequately washed with anhydrous methanol to eliminate the effect of the pollution and residual electrolyte on the surface of the tested MoS2 /CNFs cathode, and the XPS test was carried out with 60 s etching to further eliminate the interference factors. The narrow spectrum scan results are shown in Figure 4. In the pristine state, there are no Al 2p signal, while in fully discharging(0.2 V) state, Al 2p peaks appeared, and their content decreased obviously in the fully charging (1.8 V) state. The remaining Al 2p signal is caused by the trapped Al3+ species in MoS2 structure which is corresponding to the broader (002) diffraction peak after fully charging in ex-situ XRD result. This phenomenon can also partly account for the low coulombic efficiency and irreversible capacity at the beginning of the cycles. However, the trapped species perhaps also good for further smoother intercalation/de-intercalation of Al3+ as it might weaken the van der Waals force between the MoS2 layers correspond to (002) planes thus providing more open spaces in the subsequent long-term cycles. 42 The result of Cl 2p is in contrast to Al 2p, as shown in Figure 4b. For AlCl4 – intercalated-type active materials, the intensity 7



changes of Al 2p and Cl 2p XPS signals are similar, which significantly increase during charge process as AlCl4 – intercalates into cathodes and decrease during discharge process due to the de-intercalation of AlCl4 – . 14 While for Al3+ intercalated-type cathodes, Al 2p peak is significantly more intense in the fully discharged state versus that of the fully charged state as Al3+ intercalates into cathodes during discharge process and de-intercalates during charge process. 43 In our result, the Al 2p signal is attributed to the incorporation of Al3+ into MoS2 and Cl 2p signal is determined by the AlCl4 – intercalation into carbon nanofibers. Note that the contribution of the carbon nanofibers to the capacity is negligible compared to the MoS2 as confirmed by the galvanostatic chargedischarge cycles of the Al/CNFs batteries(Figure S1), so the Al signal depends on the aluminum storage mechanism of MoS2 rather than carbon nanofibers. For Mo 3d, the peaks at 232.2 eV and 228.9 eV correspond to Mo 3d 3/2 and Mo 3d 5/2 in MoS2 in its initial state. After discharging, the Mo 3d spectra shift toward low binding energies of 231.4 eV and 228 eV , which are attributed to Mo 3d 3/2 and Mo 3d 5/2 in the low-valence state, indicating the reduction of the valence state of Mo during discharging due to the intercalation of Al3+ . After charging to 1.8V , the Mo 3d peaks at low binding energy return to their initial states, confirming the occurrence of oxidation during charging due to the de-intercalation of Al3+ . The variation of the Mo valence is consistent with the pervious reported MoS2 electrode in AIBs. 25 The changes of the Al 2p and Mo 3d peaks illustrate that during discharging process Al3+ intercalates into the MoS2 structure which results in the reduction of Mo, and during charging process Al3+ de-intercalates from MoS2 structure which induces the oxidation of Mo. S 2p spectra are consistent at different charge and discharge states, demonstrating that the valence of S doesn’t change during the electrochemical redox process, as shown in Figure 4d, hence the charges are equilibrated by Mo with different valence during the electrochemical reaction process. The same phenomenon occurs in AIBs with some other metal chalcogenides as cathodes in which the valence of S and Se are stale during charge-discharge process. 25,33 Figure 4e revealed that after charging, the 284.9 eV C 1s peak developed a shoulder at higher

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energy(285.9 eV ), indicating the electrochemical oxidation of the carbon nanofibers. And the 285.9 eV peak disappeared after discharging which illustrates the reduction of the carbon nanofibers during discharging process. However, the contribution of the CNFs to the capacity is negligible and the main function of the CNFs is to enhance the transport of electrons and ions during the oxidation-reduction reactions and provide mechanical supporting and protection for the MoS2 nanostructure during the repetitive intercalation/de-intercalation of Al3+ thus alleviate the expansion and structure degradation of the MoS2 nanostructure. In the AlCl3 /1-ethy-3-methylimidazolium chloride ([EMIm]Cl) electrolyte, the predominant anions in basic melts (AlCl3 /[EMIm]Cl mole ratio < 1) are Cl – and AlCl4 – , while in acidic melts (AlCl3 /[EMIm]Cl mole ratio > 1) chloroaluminate anions Al2 Cl7 – is formed. The intercalation/de-intercalation of Al can only occur in acidic melts with the AlCl3 /[EMIm]Cl mole ratio between 1.1 ∼ 1.3. Al3+ intercalation involves that aluminum in Al2 Cl7 – firstly lose its bond to Cl – and then to diffuse as Al3+ in cathode active materials. 44 Based on the above results, we propose that the electrochemical reactions in Al/(MoS2 /CNFs) cell can be simplified as: In the discharging process:

Cathode : Anode :

MoS2 + 3xe− + xAl3+ → AlxMoS2

(1)

Al + 7AlCl4 − → 4Al2 Cl7 − + 3e−

(2)

AlxMoS2 → MoS2 + 3xe− + xAl3+

(3)

4Al2 Cl7 − + 3e− → Al + 7AlCl4 −

(4)

In the charging process:

Cathode : Anode :

The circuit potential of the Al/(MoS2 /CNFs) battery is above 1.5 V , as shown in Figure S2. The series connection of two Al/(MoS2 /CNFs) batteries can light up an LED lamp, as shown in Figure S3. The galvanostatic charge-discharge cycles of the Al/(MoS2 /CNFs) 9



Figure 4: a) XPS data of the Al 2p peak of the MoS2/CNFs cathode: pristine, fully charged (1.8 V) and fully discharged (0.2 V). b) Cl 2p. c) Mo 3d. d) S 2p. e) C 1s. battery at a current density of 100 mA g −1 in a voltage range of 0.1 ∼ 2.0 V vs. Al3+ /Al over 200 cycles is shown in Figure 5a. The discharge specific capacities are 293.2 mAh g −1 , 251.4 mAh g −1 and 232 mAh g −1 , and the corresponding charge specific capacities are 339.6 mAh g −1 , 283.7 mAh g −1 and 251.5 mAh g −1 in the first three cycles. After 15 cycles, the discharge specific capacity is stabilized and the coulombic efficiency is above 95%. The discharge capacity is at a steady state at approximately 130 mAh g −1 after 200 cycles. After long-term cycling, the pouch cells were opened and the MoS2 /CNFs electrode mat still remains a whole body without any obvious cracking or broken pieces observed. The nanostructure maintained a good fiber morphology with continuously interconnected networks, as 10


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shown in Figure S4. The repaid capacity decay in the first few cycles is a common problem in most AIBs systems, the specific reason is still unclear, the most accepted explanation is that the obvious capacity loss in the first few cycles maybe due to the formation of solid electrolyte interphase(SEI) layer which is also observed in our SEM patterns after cycling(Figure S4) and the decomposition of electrolyte or possible side reactions. 20,23,25,27,28 Further research is needed to understand the problem thoroughly. Figure 5b shows the chargedischarge curves at the 1st, 2nd and 3rd cycles, which exhibit two pairs of obvious and stable charging and discharging plateaus. The charge plateaus are around 1 V and 1.15 V , and the discharging plateaus appear at 0.55 V and 0.8 V , which are in good agreement with the cyclic voltammogram (CV) curves. CV was conducted to analysis the aluminum storage behavior in the MoS2 /CNFs electrode at a scan rate of 0.5 mV /s, as shown in Figure 5c. The cathodic peak at 0.55 V and 0.8 V are associated with the intercalation of Al3+ into MoS2 structure which is consistent with the results reported in the literature. 25 It is noted that the electrochemical tests for AlCl4 – intercalation-type active materials are starting by an oxidation process in the cathodes, while for cation intercalation-type it is starting by a reduction process in the cathodes. In this study, all the electrochemical tests were carried out from a reduction process in the cathode and, consequently, the aluminum storage mechanism in MoS2 is Al3+ intercalation, confirmed both by electrochemical tests and characterization results. To evaluate the rate performance of the MoS2 /CNFs, charge-discharge cycles at different current densities from 100 mA g −1 to 250 mA g −1 were performed, as shown in Figure 5d. The specific capacity decreased gradually with increasing current density. The discharge capacity remained at 147.2 mAh g −1 after 20 cycles at a current density of 100 mA g −1 and subsequently decreased to 129.7, 118.3, 111.9 mAh g −1 after every 10 charge-discharge cycles at a current density of 150, 200 and 250 mA g −1 , respectively. As the current density returned to 100 mA g −1 , the discharge capacity was restored to 148.6 mA g −1 and remained stable in subsequent cycles, exhibiting excellent rate performance and high cycling stability. The excellent rate performance, high capacity and high cycling stability for AIBs

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with a MoS2 /CNFs cathode can be attributed to the unique structure of the as-prepared MoS2 /CNFs: 1) The advantage of binder-free materials can avoid active material exfoliation from the current collector due to the failure of the binder during long-term cycling and avoid side reactions between the binder and ionic liquid electrolyte. 2) The carbon nanofibers are highly conductive, enhancing the transport of electrons during the oxidation-reduction reactions. 3) The carbon nanofibers can provide mechanical support and protection for the MoS2 nanostructure during the cycling process of Al3+ intercalation/de-intercalation, alleviating the expansion and structure degradation of the MoS2 nanostructure. 4) Finally, the one dimensional structure of carbon nanofibers can increase the contact area with electrolyte and decrease ion diffusion length, which enhance ion access and charge transfer. 33,37

Figure 5: a). Cycling performance and corresponding coulombic efficiency at a current density of 100 mA g −1 . b). the 1st , 2nd and 3rd charge/discharge curves at a current density of 100 mA g −1 . c) CV curves. d) Rate performance at different current densities.

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Conclusion In summary, we prepared a flexible free-standing MoS2 /CNFs composite by electrospinning and subsequent annealing which exhibited excellent electrochemical performance in AIBs and the aluminum storage mechanism was confirmed by ex situ XRD and XPS, indicating that Al3+ intercalates/de-intercalates into the interlayer between individual MoS2 layers corresponding to (002) plane during the charging-discharging process. The flexible freestanding MoS2 /CNFs composite can provide new ideas for the use of transition-metal sulfides as cathode materials for aluminum storage and facilitate the commercial application of AIBs.

Associated Content Supporting Information Material synthesis, material characterization, cells fabrication and electrochemical tests, charge-discharge curves of the CNFs, Open circuit test of the Al/(MoS2 /CNFs) battery, The Al/(MoS2 /CNFs) battery lights up LED, SEM of the MoS2 /CNFs cathode after 200 cycles, element content of the as-prepared MoS2 /CNFs composite analyzed by ICP-OES.

Author Information Corresponding Authors *E-mail: [email protected]

ORCID Wenwen Yang: 0000-0002-7016-5666

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Acknowledgments This work was supported by grant from the Science and Technology Ministry of China (863 project 2012AA062302).

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Synopsis Burgeoning aluminum-ion batteries based on flexible free-standing MoS2 /carbon nanofibers composite cathode to develop high efficient and sustainable energy resources.

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Graphical TOC Entry

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Carbon Nanofibers Composite

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