Mg2+ hybrid ion storage

Electrochimica Acta xxx (xxxx) xxx

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Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage Pengcheng Jing, Huimin Lu*, Wenwen Yang, Yuan Cao School of Materials Science and Engineering, Beihang University, Xue Yuan Road No.37, Hai Dian District, Beijing, 100191, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 June 2019 Received in revised form 10 October 2019 Accepted 9 November 2019 Available online xxx

Magnesium ion batteries (MIBs) and magnesium/lithium hybrid ion batteries (MLIBs) are promising candidates for next-generation energy storage, due to the large capacity, abundant magnesium reserves, and safe operation. Nevertheless, the use of MIBs and MLIBs is limited by sluggish Mg2þ kinetics and Liþdependent storage in cathode materials, respectively. Herein, an efficient interlayer expansion strategy is employed to deal with the problems in both MIBs and MLIBs based on VS2. The interlayer spacing of VS2 is enlarged to 1.0 nm by octylamine intercalations via one-pot solvothermal reaction, generating dramatically enhanced electrochemical performance in MIBs and MLIBs. Moreover, various ex situ methods are used to confirm a Liþ/Mg2þ co-intercalation in the expanded VS2, with most of the capacity controlled by pseudo-capacitance. This work expands the application of interlayer expansion strategy to enhance magnesium ion and lithium/magnesium hybrid ion storage. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Magnesium ion batteries Interlayer expansion Intercalations VS2 Solvothermal reaction

1. Introduction Rechargeable metal ion batteries have become a promising renewable energy storage system to power electric devices and vehicles, as is witnessed by the successful development of lithium ion batteries (LIBs) in the past decades. Although LIBs possess high energy density and power density, they may not be the most suitable batteries for next-generation large-scale energy storage devices [1]. The two main disadvantages of LIBs lie in potential safety problems caused by dendrite growth and subsequent shortcircuiting, and increasing lithium cost because of low lithium mineral reserves [2]. Recently, multivalent-metal ion batteries, as potential alternatives to LIBs, are drawing increasing attention. This new type of battery system uses low-cost and abundant metal as anodes, such as magnesium, aluminum, zinc, and calcium [3e6]. Among all those anodes, magnesium stands out because of its dendrite-free plating, relatively low density and polarization [7]. Magnesium ion batteries (MIBs), having been investigated intensively, show some superior advantages compared with LIBs. First, attributed to its bivalent nature, magnesium offers a specific capacity of 2205 mAh$g1, which is comparable with that of lithium

* Corresponding author. School of Materials Science and Engineering, Beihang University, Xue Yuan No.37, Hai Dian District, Beijing, 100191, China. E-mail address: [email protected] (H. Lu).

metal and graphite (3861 mAh$g3, 372 mAh$g3, respectively) [8]. Second, magnesium content is 1450 times higher than lithium in the earth’s crust (2.9% for magnesium vs. 0.002% for lithium) [9], and the extraction of magnesium from minerals is much simpler and safer than lithium. Moreover, differing from LIBs using lithium as anodes, magnesium anodes are not prone to form dendrites during operation in most non-aqueous electrolytes like all-phenylcomplex (APC), which is an important characteristic of MIBs [10]. Although MIBs have many intriguing advantages, they still suffer from some systematic problems related to anode, electrolyte, cathode, and current collector compatibility. Generally, these problems can be divided into two parts, namely, electrolytecentered and cathode-centered problems. With respect to electrolyte-centered problems, traditional and industrialized lithium-analogous magnesium electrolytes, such as Mg(PF6)2, offer wide electrolyte voltage windows and are compatible with nonnoble current collectors like aluminum [11]. However, they are incompatible with magnesium anodes due to the formation of a passive MgF2 layer on the surface, which hinders the electrochemical reaction [12]. With the use of halide and chlorinecontaining electrolytes, typically APC, the electrolyte voltage window can only exceed 3.0 V with noble current collectors like platinum and molybdenum [13]. More importantly, those electrolytes are incompatible with inexpensive aluminum due to serious corrosion [14,15]. Therefore, it is urgent to develop magnesium electrolytes with wide electrochemical windows, high

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Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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compatibility with magnesium anodes, and non-noble current collectors. The cathode-centered challenge of MIBs is to develop highcapacity cathode materials with superior reversibility. Due to strong polarizing nature of Mg2þ, it has been difficult to find suitable cathode materials for MIBs [9]. The first breakthrough of MIBs came in 2000, when Aurbach et al. reported a Chevrel phase Mo6S8 with excellent Mg2þ reversibility and intercalation kinetics [16]. Subsequently, an increasing number of cathode materials have been identified for MIBs, including chalcogenides (MoS2 [17], CoS [18], WSe2 [19], etc.), oxides (MnO2 [20], V2O5 [21], etc.), silicates, sulfur, and Prussian blues, of which the most promising type may be chalcogenides, especially two-dimension transition metal dichalcogenides (TMDs), due to a relatively soft electrostatic interaction between Mg2þ and the host. However, even nanosized TMDs with normal spacing, such as MoS2, cannot reversibly accommodate a reasonable amount (at least 150 mAh$g1) of Mg2þ [22], which may be related to the narrow Mg2þ transport channel. To solve this problem, many strategies, including exfoliation and interlayer expansion [23], have been employed to enlarge the Mg2þ transport channel. Indeed, interlayer expansion strategy has been widely used to efficiently enhance the performance of different types of rechargeable batteries [24e26]. The art of interlayer expansion lies in the use of suitable pillars inserted between the two-dimension interlayers of TMDs with a purpose of widening the Mg2þ transport channel. As a result, greater amounts of Mg2þ can be accommodated in the host. For instance, a significant improvement in capacity (from 55 mAh$g1 of normal MoS2 to 105 mAh$g1 of expanded MoS2) for MIBs can be seen when carbon or polyethylene oxide pillared MoS2 with larger interlayer spacings are used as cathode materials [17]. Furthermore, to expand the options of cathode materials and to achieve clear steering of the sluggish Mg2þ diffusion in the host, Mg2þ/Liþ dual ion batteries (MLIBs) have been proposed in recent years [27]. This system allows the intercalation of Liþ or both Liþ and Mg2þ into cathode materials, and the plating of Mg2þ on the magnesium anode. In this way, most LIBs cathode materials can be employed to MLIBs on the condition that the electrochemical windows of cathode materials and electrolytes are compatible. Other materials with modified structures may also store Mg2þ besides Liþ, including interlayer-expanded MoS2 [17], nanosized Li4Ti5O12 [28], and electrospun V2MoO8 [9]. This type of cathode materials is expected to show better performance than those that can only store Liþ because it requires less electrolyte and depends less on the concentration of Li salt. VS2, similar to MoS2, is one of the typical members of TMDs with hexagonal structure. Each layer of VS2 consists of a sandwich SeVeS framework where each vanadium layer is bonded with two sulfide layers, and all layers stack together by weak Van der Waals force to constitute a layered structure with a normal interlayer spacing of 0.576 nm [29]. This VS2 with normal spacing (N-VS2) is able to support fast Liþ diffusion in LIBs and MLIBs [30,31], yet unable to reversibly store Mg2þ [32], which is also similar to normal spacing MoS2 [22]. Drawing an analogy between VS2 and MoS2, it is expected that interlayer-expanded VS2 (E-VS2) may show similar Mg2þ and Liþ/Mg2þ storage mechanism to interlayer-expanded MoS2 in the equal expanding standard. Therefore, employing interlayer expansion strategy, we choose N-VS2 as a basic model to synthesize E-VS2 and examine its electrochemical performance as cathode material for MIBs and MLIBs. In this work, we report an interlayer-expanded, binder-free, and flower-like VS2 nanosheets assemblies on carbon paper (E-VS2@C) via a facile solvothermal method for both MIBs and MLIBs. The intercalations of octylamine and/or NH4þ into VS2 produces a largely expanded interlayer spacing, which widens the ion diffusion

channel, alleviates the electrostatic interaction between ions and SeVeS monolayers, and mitigates the volume changes during cycling. Additionally, the hierarchical VS2 nanostructure shortens the ion transportation path. As a result, the presented MIBs display excellent electrochemical performance that a normal-spacing VS2 could never achieve; and MLIBs with dramatically improved synthetical performance. More importantly, ex situ XRD, TEM, XPS, and ICP-OES were employed to unveil the storage mechanism of MLIBs. 2. Experimental 2.1. Synthesis of expanded VS2 on carbon paper Typically, 2 mmol NH4VO3 and 10 mmol CH3CSNH2 (TAA) were added to 20 ml octylamine in a 50 ml Teflon-lined autoclave. The mixture was stirred for 2 h at room temperature (RT). Afterwards, carbon paper (16 mm in radius) pre-annealed at 700  C for 10 min was placed in the above mixture. The autoclave was sealed and kept at 160  C/24 h, and cooled to RT. The carbon paper and the product in the bottom were washed with distilled water and ethanol several times, and then dried at 60  C/15 h/vacuum. The resulting carbon paper and dried powder are denoted as E-VS2@C and E-VS2, respectively. 2.2. Synthesis of normal VS2 First, 2 mmol NH4VO3 was added to the mixture of 30 mL distilled water and 2 mL ammonium hydroxide. Then 20 mmol TAA was added to the above mixture and stirred for 2 h at RT. The autoclave was sealed and kept at 180  C/20 h, and cooled to RT. The product was washed with distilled water and ethanol for several times, and then dried at 60  C/15 h/vacuum. The dried product is denoted as N-VS2. 2.3. Materials characterization XRD (Rigaku, D/MAX 2200pc) with Cu Ka radiation was employed to record XRD patterns. Micro-Raman spectroscopy (Renishaw InVia) was employed to record Raman spectra (532 nm). Field-emission scanning electron microscope (FESEM, Zeiss, Merlin Compact) and TEM (JEOL JEM-2100) with an energy dispersive spectrometer (EDS) element analysis system were employed to study the morphology and composition. X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi) measurements with mono Al Ka radiation were performed to record XPS spectra. Avantage software was used to study the elemental constituent, and C 1s binding energy (284.6 eV) was used to calibrate all the measured elements. Inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 7500ce) was used to calculate the Li/Mg ratio at discharged and charged states. Thermal analyzer (NETZSCH TGA/STA 449 F3) was used in the Ar atmosphere at a heating rate of 10  C$min1 to obtain the thermogravimetric analysis (TGA) data. The surface area was determined by nitrogen adsorptionedesorption isotherms using the BrunaueremmetteTeller (BET) method on Micromeritics ASAP 2460. 2.4. Electrochemical measurements The electrochemical performance of E-VS2@C and N-VS2 in MIBs and MLIBs was studied in CR2505 coin cells with magnesium foils as counter electrodes and Whatman glass fiber as separators. EVS2@C, with an average loading of 1.0 mg cm2, was directly used as positive electrode, while N-VS2 electrodes were made by blending active materials, acetylene black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 6:3:1 (a mass loading of

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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1.5 mg cm2). 0.4 M of APC (2PhMgCl-AlCl3) and 1.0 M of LiCl-APC electrolytes were prepared based on literature [32]. The galvanostatic cycling was conducted on LAND CT2001A between 0.25 and 2.0 V for MIBs and MLIBs. Cyclic voltammetry (CV, 0.2e0.7 mV s1) and electrochemical impedance spectroscopy (EIS) measurements (0.01e10000 Hz) were conducted using an electrochemistry workstation (Gamry 2800). CVs of MIBs, MLIBs, and LIBs were measured at a sweeping rate of 0.2 mV s1. The electrochemical performance in LIBs was investigated using 1.0 M LiPF6 in EC:DMC ¼ 1:1 wt % between 1.1 and 2.8 V. The specific capacity was calculated based on the mass loading of VS2. 3. Results and discussion Binder-free E-VS2 nanosheets assemblies on carbon paper were synthesized through a one-step solvothermal reaction between ammonium vanadate and thioacetamide in octylamine solvent. The digital photograph in Fig. S1 shows a significant difference between the annealed carbon paper and the one after solvothermal reaction, indicating the deposition of E-VS2 on carbon paper. The XRD

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patterns of E-VS2@C, E-VS2, and N-VS2 are illustrated in Fig. 1a. The pattern of N-VS2 prepared via hydrothermal reaction can be directly indexed as the hexagonal VS2 (JCPDS 89e1640). However, after solvothermal reaction, the diffraction peaks of E-VS2 are completely distinct from those of VS2. Two newly formed sharp peaks at 8.7 and 17.6 can be indexed to enlarged (001) and (002) planes of VS2, the d-spacings of which are 1.0 nm and 0.5 nm, respectively. The shift of (001) and (002) planes to lower degrees indicates a layer expanding along c-axis, which can be attributed to octylamine and/or NH4þ intercalated between VS2 monolayers [33]. Similar lattice expansion and diffraction peaks shift can also be observed in amorphous carbon pillared MoS2 [17]. With respect to E-VS2@C, the diffraction peaks of E-VS2 besides graphitic carbon paper confirms the presence of E-VS2 on carbon paper during solvothermal reaction. Fig. 1b demonstrates the Raman spectra of EVS2 and N-VS2, both of which show similar signals. The characteristic signals at around 282 cm1 and 409 cm1 are attributed to the VeS in-plane Eg vibration and the out-of-plane Ag vibration, respectively [34,35]. The amount of octylamine in E-VS2 is further calculated by comparing the weight loss percentages of fresh E-VS2

Fig. 1. Structure and composition characterizations of expanded VS2 and N-VS2 (a) XRD patterns of E-VS2@C, E-VS2, and N-VS2; (b) Raman spectra of expanded VS2 and N-VS2; (c)e(e) V 2p, S 2p, C 1s, and N 1s XPS spectra of expanded VS2.

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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and annealed E-VS2. According to TGA curves in Fig. S2, the weight proportion of intercalated octylamine is 16 ± 1% [33,36]. N2 adsorptionedesorption isotherm curve of E-VS2 is presented in Fig. S3, and the corresponding BET surface area is determined and compared with some works in related fields (Table S1). The BET surface area of E-VS2 is 8.5 m2 g1, which is relatively low and may be resulted from the stack of E-VS2 nanosheets assemblies. XPS measurement was further employed to study the elemental constituent and chemical states of the as-prepared E-VS2 (Fig. 1cef). The high-resolution spectra of V 2p, as shown in Fig. 1c, can be separated into three sets of doublet peaks. The higher doublet peaks at 524.1 and 516.6 eV, corresponding to V2p1/2 and V2p3/2, are ascribed to V4þ species [37]. The medium doublet peaks at 527.8 and 515.5 eV are assigned to V3þ species, and the low doublet peaks at 521.7 and 513.9 eV are related to V2þ species [38,39]. The presence of low valent V3þ and V2þ may be attributed to partial reduction of V4þ by octylamine and H2S during solvothermal reaction [40]. The spectra of S 2p in Fig. 1d are divided into two sets of doublet peaks, of which the major doublet peaks at 162.4 and 161.2 eV are ascribed to S 2p1/2 and S 2p3/2 of S2- state, the minor doublet peaks at 163.9 and 162.4 are attributed to S with low coordination state (S0 and S) [41,42]. The peak at 168.8 eV, corresponding to the characteristic signal of sulfate, is attributed to the

slight oxidation of S2- in the air [33]. The divided spectrum of C 1s in Fig. 1e shows three peaks at 284.6, 285.4, and 286.3 eV, each of them can be assigned to CeC, CeN, and CeO bonds, respectively [43]. The fitted spectrum of N 1s in Fig. 1f presents two peaks located at 402.0 and 400.0 eV, indicating the existence of NeH bond from octylamine and/or NH4þ from the decomposition of thioacetamide, and NeC bond from N-doped amorphous carbon due to the carbonization of octylamine during solvothermal reaction [33]. The morphology of E-VS2@C, E-VS2, and N-VS2 is uncovered by SEM and TEM. Fig. 2a shows the low-magnification SEM image of EVS2@C, on which VS2 nanosheets assembled in flower-like structure are deposited on the carbon fibers and in the intervals. Upon magnification, as shown in Fig. 2b, thin VS2 nanosheets with an average thickness of ~20 nm are randomly distributed on the carbon fibers and are tightly connected via the wrinkled nanosheet edges. On the contrary, E-VS2 collected in the bottom of the autoclave shows a seriously aggregated bulk structure with no nanosheets, indicating an inferior morphology and highlighting the role of carbon paper to avoid VS2 aggregation of nanosheets (Fig. S4). NVS2 produced from hydrothermal reaction in Fig. 2ced shows a microscale and flower-like structure, where thick VS2 sheets with an average thickness of ~200 nm are bonded tightly and show a less open morphology than E-VS2@C. The TEM image in Fig. 2e further

Fig. 2. Morphology characterizations of E-VS2@C and N-VS2. (a)e(b) SEM images of E-VS2@C; (c)e(d) SEM images of N-VS2; (e) TEM image of expanded VS2 nanosheets assemblies exfoliated from E-VS2@C; (f) HRTEM image of expanded VS2 nanosheets assemblies.

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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confirms the superior morphology of the open and hierarchical expanded VS2 nanosheets assemblies derived from carbon paper. As shown in Fig. 2f, the high-resolution TEM (HRTEM) reveals a largely expanded interlayer spacing of 1.00 nm, corresponding to the expanded (100) plane of N-VS2, which demonstrates a narrow interlayer spacing of 0.57 nm [31]. The expanded spacing of (100) plane in E-VS2@C is in good consistence with the XRD pattern. The EDS mapping of expanded VS2 nanosheets assemblies is shown in Fig. S5, indicating a uniform distribution of element V, S, C, and N. We first measured the electrochemical performance of E-VS2@C in MIBs given the potential of the expanded interlayers of VS2 to store Mg2þ, as shown in Fig. 3. CVs of E-VS2@C and N-VS2 were tested after 118 times of activation cycles at 100 mAh$g1. As shown in Fig. 3a, two cathodic peaks at 1.27 and 0.76 V are observed, which are attributed to the intercalation of Mg2þ [44]. Only one broad anodic peak at 1.28 V is observed, which may be caused by a onestep de-intercalation of Mg2þ [7]. For N-VS2 with a normal interlayer spacing, the CV curve with no redox peaks demonstrates extremely low current of 0.014 mA, indicating negligible Mg2þ storage. Fig. 3b illustrates the discharge and charge curves of EVS2@C. The system takes around 118 times of activating discharge and charge progress to reach a high capacity of 234 mAh$g1 from

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near zero, with a high coulombic efficiency of 100.4%. The activated curves show two discharge slopes and one quasi-plateau, presenting good consistence with the CV peaks. As a comparison, NVS2 has little capacity distribution (Fig. S6), highlighting the effect of interlayer expansion on the electrochemical performance of MIBs. The detailed discharge-charge curves during activation are shown in Fig. S7. The cycling performance was measured to further investigate the performance of E-VS2@C. As shown in Fig. 3c, EVS2@C experiences an obvious activation during the first 118 cycles to reach a good capacity of 234 (±4) mAh$g1 under the current density of 100 mA g1, and maintains 222 (±4) mAh$g1 after 200 cycles (94.9% of the 118th cycle). The activation cycling, of which the first 80 cycles from near zero capacity are shown in Fig. S8, may be attributed to large polarization caused by the intercalated octylamine and/or NH4þ. Specifically, in those intercalated octylamine molecules, N atoms with non-bonding pairs tend to interact with positively charged V4þ in SeVeS sandwich-like monolayers, leaving alkyl chains vertical to SeVeS monolayers. Those nonconductive alkyl chains are likely to generate large polarization and intensively resist divalent Mg2þ intercalation during the activation cycles. It is noted that the activation cycles may vary from 50 to 120 cycles due to some minor differences between cells, but the

Fig. 3. Electrochemical performance of E-VS2@C and N-VS2 in MIBs. (a) CV curves between 0.25 V and 2.0 V at a sweeping rate of 0.2 mV s1; (b) Dischargeecharge curves (EVS2@C), and (c) Cycling performance at a current density of 100 mA g1; (d) Rate capability at different current densities; (e) Long-term cycling performance at 1000 mA g1 for 1000 cycles.

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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discharge-charge curves and maximal capacities remained similar. Although expanding the discharge cut-off voltage may fit the polarization and shorten the activation cycles, the cycling performance can degrade fast (Fig. S9). To identify the structures of EVS2@C after activation, the samples at the activated discharged and charged states were measured by ex situ XRD method (Fig. S10). There is no obvious peaks’ shift in the position of Mg2þ intercalating (001) Van der Waals plates, but there is a large decrease in the intensity of the peaks, indicating a decline of crystallinity. The negligible intensity of (002) planes is likely to be assigned to the overall amorphization of the structure as seen on activated discharge, coupled with the anticipated lower intensity of this peak. As for N-VS2, the host has been storing little Mg2þ throughout the cycling, at around 20 mAh$g1 corresponding to only 0.043 Mg2þ for each VS2. The capability of E-VS2@C against different current densities after activation is shown in Figs. 3d and S11. With the increasing current densities from 100 to 2000 mA g1, capacities of 237, 222, 208, 189, 179, and 147 (±3) mAh$g1 are obtained, indicating a reasonable rate capability as is compared in Fig. S12 with other materials. The cycling performance of E-VS2@C under

high current density of 1000 mA g1 was further tested. As shown in Fig. 3e, E-VS2@C exhibits an initial activation with the highest capacity of 172 (±2) mAh$g1. After 1000 cycles, a high capacity of 147 (±2) mAh$g1 can be attained, corresponding to 86 (±1) % retention of the highest capacity. Additionally, a performance comparison of this material with some reported materials is shown in Table S2. All of those results suggest that E-VS2@C is a promising cathode material for MIBs. The electrochemical performance of E-VS2@C in MLIBs was further investigated. Fig. 4a shows the CV curves of E-VS2@C and NVS2. For N-VS2, four redox peaks at around 1.74/1.61, 1.66/1.53, 1.60/ 1.50, and 1.49/1.38 V are observed, which are all attributed to Liþ intercalation/deintercalation rather than Mg2þ [32]. However, after interlayer expansion, E-VS2@C shows quite different CV profile. First, E-VS2@C displays lower cathodic peaks at 1.62, 1.34, 1.28, and 0.97 V, which may be due to larger polarization for Liþ intercalation in the VS2 host after insertion of octylamine or/and NH4þ [23,45,46]. Despite the relatively lower operating voltage, the current density of E-VS2@C is enhanced due to expanded interlayer induced pseudo-capacitance effect [17,24,47]. Second, the distinct

Fig. 4. Electrochemical performance of E-VS2@C and N-VS2 in MLIBs. (a) CV curves between 0.25 V and 2.0 V at a sweeping rate of 0.2 mV s1; (b) Dischargeecharge curves (EVS2@C), and (c) Cycling performance at a current density of 100 mA g1; (d) Rate capability at different current densities; (e) Long-term cycling performance at 2000 mA g1 for 2000 cycles.

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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cathodic peak at 0.68 V suggests the co-intercalation of Mg2þ besides Liþ, the lower voltage of Mg2þ intercalation in MLIBs than in MIBs may also result from the polarization mentioned above and the already intercalated Liþ at higher voltage. The co-intercalation of both Mg2þ and Liþ into E-VS2@C is also confirmed by comparing with electrochemical behavior of LIBs (Fig. S13). The galvanostatic discharge and charge curves of E-VS2@ in MLIBs and LIBs, and N-VS2 in MLIBs are in good consistency with the CV analysis (Figs. 4b and S13). The low plateau voltages in the three systems in the first discharge may be attributed to the large polarization in the initial intercalation. Fig. 4c shows the cycling performance of E-VS2@C and N-VS2 at 100 mA g1. It is obvious that E-VS2@C exhibits higher performance than N-VS2. A higher first-cycle coulombic efficiency of 87.6 (±0.1)% for E-VS2@C is obtained, with the initial discharge and charge capacity of 309 and 270 (±5) mAh$g1, respectively. The cycling of EVS2@C is stable after a few cycles’ activation, and maintains a capacity of 257 (±5) mAh$g1 after 100 cycles, which is 95 (±0.2)% retention of the second cycle. The activation of E-VS2@C in MLIBs takes less cycles than that in MIBs, which may be due to smaller intrinsic polarization of Liþ than Mg2þ. Our results are compared with several recently published cathode materials for MLIBs in Table S3, suggesting that E-VS2@C is a promising 300-Wh$kg1class cathode material. The rate capability of E-VS2@C and N-VS2 at various current densities from 100 to 5000 mA g1 was measured, as shown in Figs. 4d and S14. E-VS2@C delivers robust discharge capacities of 270, 229, 206, and 138 (±3) mAh$g1 at the current densities of 100, 1000, 2000, and 5000 mA g1, respectively. In contrast, N-VS2 shows relatively low capacity along the whole rate cycling. Long-term cycling stability of E-VS2@C and N-VS2 under a high current density of 2000 mA g1 is further compared in Fig. 4e. It is observed that E-VS2@C displays superior stability and delivers a

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capacity of 190 (±3) mAh$g1 after 2000 cycles, 88. (±0.6) % retention of the first discharge, while N-VS2 shows inferior cycling stability and lower capacity (116 mAh$g1 and 77% retention after 2000 cycles). The synthetical improvement in the electrochemical performance of VS2 is attributed to the enlarged interlayer spacing which widens the ion diffusion channel and reduces the intensive interaction between Liþ/Mg2þ and SeVeS sandwich monolayers. To further study the superior Liþ/Mg2þ storage mechanism of EVS2@C in MLIBs, ex situ XRD, HRTEM, XPS, and ICP-OES were conducted at the initial cycle. Fig. 5a shows the XRD patterns of the asprepared, discharged, and charged samples. It is shown that the position of the expanded Mg2þ/Liþ intercalating (001) Van der Waals plates demonstrate no obvious shifts at discharged and charged states, which indicates a good structure stability during cycling. The negligibility of (002) peaks, which is similar to the XRD observations in MIBs, may also result from an overall amorphization of the structure, coupled with low intensity of this peak. These air-aged samples in Fig. S15 show shifts of the two main peaks, while these instantly measured samples show the existence of main peak (001). This means that instantly detected samples rule out of air contamination. On the basis of the XRD analysis, the TEM measurement of the discharged and charged samples should avoid air exposure as possible. Therefore, two boxes were used to place each sample and two vacuum bags to contain the two boxes separately, taking as short time to transfer the samples to the TEM chamber as possible. The HRTEM images are illustrated in Fig. 5b. The discharged and charged samples show an interlayer spacing of 1.02 nm and 1.00 nm, respectively, both corresponding to the (001) planes of expanded VS2. These results, in good conformity with the XRD analysis, indicate decreased volume changes during Liþ/Mg2þ intercalation/deintercalation. Furthermore, the Liþ/Mg2þ cointercalation mechanism is unveiled by XPS and ICP-OES, as

Fig. 5. (a) Ex situ XRD patterns of E-VS2@C in MLIBs; (b) HRTEM images of the discharged and charged samples; (c) Li 1s and Mg 2p XPS spectra of the discharged and charged samples.

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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Fig. 6. EIS of (a) fresh E-VS2@C and N-VS2 in MLIBs, (b) the 5th and 10th cycled E-VS2@C. (c) CVs of E-VS2@C at different sweeping rates. (d) linear fitting of the Log(i) vs. Log(v) plots.EIS of (a) fresh E-VS2@C and N-VS2 in MLIBs, (b) the 5th and 10th cycled E-VS2@C. (c) CVs of E-VS2@C at different sweeping rates. (d) linear fitting of the Log(i) vs. Log(v) plots.

shown in Fig. 5c and Table S4. In the Li 1s and Mg 2p contained XPS spectra, both Li and Mg are observed in the discharged sample. After charge, the Li 1s signal almost disappears and Mg 2p signal shows a dramatical decrease, confirming the storage of both Liþ/ Mg2þ in E-VS2@C. The quantitative variation of Liþ/Mg2þ in the discharged and charged samples is also presented in Table S4. All of the above results and analyses confirm that the energy storage mechanism of E-VS2@C in MLIBs is mainly resulted from Liþ/Mg2þ co-intercalation. The electrochemical impedance spectra (EIS) of E-VS2@C and NVS2 were measured. As shown in Fig. 6a, the semicircle radius in the high-frequency limitation of E-VS2@C is remarkably smaller than that of N-VS2, indicating a smaller charge transfer resistance for EVS2@C. Meanwhile, the steeper slope in the low-frequency limitation of E-VS2@C demonstrates a faster ion diffusion in the expanded VS2. The EIS plots of E-VS2@C after 5 and 10 cycles were also measured, as displayed in Fig. 6b. The semicircle radii in the highfrequency limitation decrease along with the steeper slopes as the cycle number increases, which is attributed to enhanced reaction activeness and diffusion kinetics during activation. To better understand the electrochemical kinetics of E-VS2@C, CVs at different sweeping rates from 0.2 to 0.7 mV s1 were conducted. The general method to analyze the electrochemical kinetics is presented as the following equation [48].

i ¼ avb or LogðiÞ ¼ LogðaÞ þ bLogðvÞ

(1)

where i is the current (mA), v is the sweeping rate (mV$s1), and a and b are adjustable parameters, of which b usually ranges from 0.5 to 1.0. The b value of 0.5 means a diffusion-controlled process, while 1.0 represents a capacitive-controlled process. The b value of the selected reduction and oxidation peaks are 0.790 and 0.883, respectively, which indicates that most capacity in E-VS2@C is capacitive-controlled (Fig. 6ced).

4. Conclusions In summary, we report interlayer-expanded, binder-free VS2 nanosheets assemblies on carbon paper via a facile solvothermal method for both MIBs and MLIBs. The intercalation of octylamine and/or NH4þ in VS2 produces largely expanded interlayer spacing of 1.0 nm, which widens the ion diffusion channel and mitigates the volume changes during cycling. Meanwhile, the open and hierarchical VS2 nanostructure shortens the ion transportation path. As a result, the presented MIBs displays excellent electrochemical performance that a normal-spacing VS2 can never achieve; and MLIBs with dramatically enhanced synthetical performance. A Liþ/Mg2þ co-intercalation mechanism is confirmed in MLIBs, with most of the capacity contribution supported by pseudo-capacitance. This work expands the application of interlayer expansion strategy to enhance magnesium and magnesium/lithium hybrid storage. Acknowledgements This work was supported by grant from Ministry of Science and Technology of the People’s Republic of China (863 project 2012AA062302). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135263. References [1] S.W. Kim, D.H. Seo, X.H. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithiumion batteries, Adv. Energy Mater. 2 (2012) 710e721. [2] Z.X. Xu, J.L. Wang, J. Yang, X.W. Miao, R.J. Chen, J. Qian, R.R. Miao, Enhanced performance of a lithium-sulfur battery using a carbonate-based electrolyte,

Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263

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Please cite this article as: P. Jing et al., Interlayer-expanded and binder-free VS2 nanosheets assemblies for enhanced Mg2þ and Liþ/Mg2þ hybrid ion storage, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135263