Magnesium-Ion Storage Capability of MXenes - ACS Applied Energy

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Magnesium-Ion Storage Capability of MXenes Meng-Qiang Zhao, Chang E. Ren, Mohamed Alhabeb, Babak Anasori, Michel W. Barsoum, and Yury Gogotsi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02253 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Energy Materials

Magnesium-Ion Storage Capability of MXenes Meng-Qiang Zhao,

†,§

Chang E. Ren,† Mohamed Alhabeb, † Babak Anasori,† Michel

W. Barsoum,‡, * Yury Gogotsi†,*

†A.J.

Drexel Nanomaterials Institute and Department of Materials Science and

Engineering Department, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA

‡

Department of Materials Science and Engineering Department, Drexel University, 3141

Chestnut Street, Philadelphia, PA 19104, USA

§Department

of Physics and Astronomy, University of Pennsylvania, 209 South 33rd

Street, Philadelphia, PA 19104, USA

AUTHOR INFORMATION

Corresponding Author

ACS Paragon Plus Environment

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*Email: [email protected], [email protected]


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ABSTRACT: Rechargeable magnesium-ion batteries (MIBs) with Mg metal anodes have been attracting attention due to their potential safety, low cost, and high theoretical energy density. Nevertheless, developing a high-energy-density MIB with long cycling life and reasonable rate capability is still a huge challenge due to the lack of high-performance cathodes beyond the Chevrel phases. Here, we investigate the mechanism of Mg-ions uptake and storage by MXenes, that have been theoretically predicted to be promising candidates for MIB cathodes. Flexible and conductive 3D macroporous Ti3C2Tx MXene films were fabricated and tested as MIB cathodes after the incorporation of Mg ions from a Mg2+-containing electrolyte. The 3D MXene cathode exhibited promising cycling stability accompanied with good rate performance. A 3D Mg0.21Ti3C2Tx MXene electrode delivered, at 0.5 C, 1 C and 5 C, capacities of ~210 mAh g-1, ~140 and ~55 mAh g-1, respectively. A reversible intercalation charge storage mechanism was demonstrated and a possible redox reaction mechanism proposed. Higher capacities of ~240 mAh g-1 were achieved by MXenes with other compositions, such as Mo2CTx, based cathodes. Considering the large family of 2D transition metal carbides and nitrides, with over 30


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different MXenes synthesized to date, this work suggests the availability of a variety of high-rate cathode candidates for MIBs.

KEYWORDS: MXene, magnesium-ion battery, cathode, pre-intercalation, mechanism


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INTRODUCTION



Rechargeable lithium-ion batteries (LIBs) are currently the most appealing option for future electric transportation and large-scale grid storage.1-2 However, the unbalanced distribution of Li resources worldwide as well as serious safety issues concerning Li metal anodes restrict the penetration of LIBs into these large-volume markets. Besides, after continuous improvements on the energy density and cost reduction in the past decades, advancements at the materials level are approaching a fundamental limit in LIB technologies.3-5 As a result, increasing attention has been paid to the so-called “beyond Li-ion” technologies, such as metal-air6-8, metal-S9-11, Na-ion12-13, K-ion14-15, solid state1617,

and multivalent-ion batteries18-19.

Batteries based on multivalent ions, such as Mg2+, Ca2+, Zn2+, and Al3+, are particularly attractive because they promise to offer superior volumetric energy densities compared to LIBs.18-19 Among which, rechargeable magnesium-ion batteries (MIBs) - with magnesium, Mg, metal anodes - have attracted much attention since the pioneering work of Aurbach et al.20 MIBs are safer than LIBs due to the absence of dendrite growth, lower cost because of the earth-abundant Mg, and the possibility of handling the metal in


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ambient environment, and they offer high energy capacity due to the divalency of the Mg ions.1, 18, 21 Considerable progress has been achieved by developing bismuth (Bi), tin (Sn), antimony (Sb), and their alloys as anodes for MIBs.22-25 Nevertheless, developing a highenergy-density MIBs, with long cycling lifes and reasonable rate capabilities is still a considerable challenge due to the lack of suitable Mg-ion electrolytes and highperformance cathodes.26 A variety of materials have been studied as cathodes, including oxides (e.g., V2O5,27-28 MnO2,29 MoO3,30 TiO231-32), chlorides,33 chalcogenides (e.g., TiS2,34 TiSe2,35 MoS2,36 WSe237), fullerenes,38 and polyanions (e.g., silicates,39 borates,40 Prussian-blue structures41), aiming to go beyond the prototypical Chevrel phases, Mo6Ch8 where Ch = S, Se. In recent years, batteries composed of sulfur (S) cathodes and Mg anodes are attracting much attention due to their higher charge-storage capability compared to conventional MIBs, which is attributed to the redox couple of Mg and S.42 However, most of the aforementioned cathode materials exhibit sluggish kinetics and the conversion materials lack the required cycling life. Until recently, a second benchmark cathode material, after the Chevrel phases, was reported by Sun et al, through the demonstration of a high capacity and promising cycling life of a TiS2 thiospinel cathode.34


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These discoveries notwithstanding, the development of high-performance Mg cathodes represents a great challenge that needs to be solved before commercialization of MIBs is possible. With a combination of metallic conductivity, large surface area, excellent cation intercalation capacity, and low ion diffusion barriers, MXenes - a family of two-dimensional (2D) transition metal carbides and nitrides – may provide a promising solution to the challenges associated with MIB cathode development.43-45 MXenes have the formula Mn+1XnTx, where M is an early transition metal, such as Ti, Nb, V, Ta, Cr, and Mo, X is carbon and/or nitrogen, n = 1, 2 or 3, and Tx refers to surface functional groups such as OH, O, and/or F.43,

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Using density functional theory, DFT, methods, Eames et al.

predicted that the M2C MXenes containing light transition metals (e.g. Ti, V, and Cr) with nonfunctionalized or O-terminated surfaces should possess capacities of > 400 mAh g-1 for Mg-ion storage.47 Similar predictions were also made by Xie et al., who further predicted the stable multilayer adsorption of Mg ions between MXene layers, leading to a significant increase in their theoretical capacities.48


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Our group has experimentally demonstrated the superior capacity for reversible intercalation into MXenes of many organic molecules and metal cations including Li+, Na+, K+, Mg2+, Al3+, etc.49-50 So far, MXenes have been tested in a variety of energy storage devices, including supercapacitors,51-52 LIBs53-54 and beyond55-59. The storage of multivalent ions in MXenes is of particular interest here. When used as a supercapacitor electrode, Ti3C2Tx had a capacity of 400 F cm-3 and impressive cycling stability in an aqueous MgSO4 electrolyte.60 Coupled with a Mg metal anode, MXenes delivered promising electrochemical properties in terms of high capacity, superior rate performance, and stable cycling life as a cathodes in hybrid Mg2+/Li+ batteries.61 Recently, Beidaghi et al. showed the promise of MXenes cathode candidates in Al-ion batteries.62 Xu et al. demonstrated the promising Mg-ion storage capability of MXenes by intercalating cationic surfactant through the reversible intercalation of Mg ions.63 Herein, we report on a more detailed investigation of Mg-ion insertion into MXenes and their storage capability in a typical MIB system. 

RESULTS AND DISCUSSION


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A schematic of the structure of a MXene-based MIB is shown in Figure 1. Similar to other 2D materials, the stacking or aggregation of MXene nanosheets limits their electrochemical performance. In order to facilitate ion transport through the electrodes, three-dimensional (3D) macroporous MXene films need to be fabricated, coupled with Mg metal anodes. The conventional all phenyl complex (APC) is employed as the electrolyte, and a glass microfiber paper is used as the separator.

Figure 1. Schematic showing the concept of using MXenes as cathode materials in MIBs with a Mg metal anode in the all phenyl complex (APC) electrolyte. Pre-Intercalation of Mg Ions. To date, the most studied and understood MXene is Ti3C2Tx and it is for this reason it was chosen here. Starting with Ti3C2Tx, we fabricated 3D macroporous films using a sacrificial template method, the details of which are


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described elsewhere.58 These free-standing films are flexible and exhibit electrical conductivities of ~200 S cm-1 (Figure 2a). Cross-sectional scanning electron microscope (SEM) images reveal that the ~10-µm thick 3D Ti3C2Tx films are comprised of stacked hollow Ti3C2Tx spheres, with diameters of 1-2 µm. The wall thicknesses were ~10 nm and the porosity is estimated to be ~90 %. (Figure 2b-c). To serve as a MIB cathode, the 3D framework was first annealed at 120 oC under vacuum to get rid of interlayer water molecules. They were then pre-intercalated with Mg ions. Our previous work has shown that Mg2+ could spontaneously intercalate into MXene layers in aqueous electrolytes.49 Here the Mg2+ was incorporated by soaking our films into a mixed solution of H2O/acetonitrile containing 0.1 M MgCl2. Acetonitrile is added in order to facilitate the soaking of the 3D MXene films into the solution due to their hydrophobicity originated from the lotus effect.58 Figure 2c and 2d show cross-sectional transmission electron microscope (TEM) images of a few Ti3C2Tx layers before and after the incorporation of the Mg2+ ions, which resulted in an increase in interlayer spacing, d. X-ray diffraction (XRD) patterns using Cu K radiation of a typical Ti3C2Tx film before and after Mg2+ intercalation are shown in Fig. 2e. After Mg2+ intercalation (top pattern in


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Fig. 2e), the (002) peak is at 5.7o 2; before intercalation (lower pattern in Fig. 2e) it was 6.5o 2. It follows that Mg2+ intercalation increases the d spacing from 13.6 to 15.5 Å (Figure 2e). Energy-dispersive X-ray spectroscopy (EDX) in the TEM yielded a molar ratio of Mg2+ to Ti3C2Tx (or 3Ti), after intercalation, of 0.58:1.0 (Figure 2f). Henceforth, the 3D MXene films intercalated with Mg in the H2O/acetonitrile solution is denoted as Mg0.58Ti3C2Tx. The incorporation of Mg2+ was also conducted in an APC electrolyte to avoid the presence of water and other contaminants, resulting in a Mg0.21Ti3C2Tx composition, that had less Mg2+ (see Table 1).


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Figure 2. (a) Digital and (b) cross-sectional SEM images of a 3D macroporous Ti3C2Tx film; TEM image of Ti3C2Tx layers, (c) before and, (d) after the incorporation of Mg2+ ions; (e) Comparison of XRD patterns for the 3D films before, and after, the incorporation of Mg2+ ions; (f) EDX spectrum of Mg2+ incorporated in 3D film.


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Electrochemical Performance. The cyclic voltammetric (CV) curves of the Mg0.21Ti3C2Tx cathode (Figure 3a) show two sets of magnesiation/demagnesiation peaks at 0.5/2.0 V and 1.5/2.5 V vs. Mg/Mg2+, respectively. At a charging rate of 1 C, an initial capacity of 170.5 mAh g-1 is obtained, with a Coulombic efficiency of 88.1% (Figure 3b). The capacity reduces to ~140 mAh g-1 during the following several cycles, with Coulombic efficiencies close to 100%, followed by further reduction in capacity, that finally stabilized at ~50 mAh g-1 (Figure 3b). Reasons for this capacity decay are discussed below. To check whether Mg2+ pre-intercalation is necessary, we tested a 3D electrode that was not pre-intercalated (Figure S1) and observed a quite small capacity of 3.1 mAh g-1 at 0.5 C initially. This value however, increased rapidly upon cycling. After ~10 cycles, the capacity was ~70 mAh g-1, and after 35 cycles, it further increased to ~100 mAh g-1. This increase in capacity is attributable to improved electrolyte and Mg2+ ions accessibility upon cycling, which is common for MXene-based film electrodes.54, 60 Both TEM and XRD of cycled samples showed clear evidence for an increase in d spacing after cycling (see Figure S2). Consequently, Mg2+ pre-incorporation is necessary if MXene are electrodes to serve as MIB cathodes. This result is in a good agreement with a previous report.63


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Figure 3c shows the rate performance of the 3D Mg0.21Ti3C2Tx cathode. At 0.5 C, the MXene electrode delivers a capacity of ~210 mAh g-1. At 1 and 3 C, capacities of ~140 and ~70 mAh g-1 are recorded, respectively. Even at the high rate of 5 C, a capacity of ~55 mAh g-1 is obtained. Compared to other reported MIB cathodes18, it is obvious that MXene electrodes exhibit better rate performances, that we ascribe to the excellent electron and ion transport properties of Ti3C2Tx. The charge-discharge profiles at different rates are shown in Figure 3d. Two sets of charge-discharge plateaus are observed at 0.5 and 1 C, respectively, which correspond to the two sets of peaks in the CV curves shown in Figure 3a. Note that the capacity decay at higher current rates can be ascribed to changes in capacities generated during the second set of plateaus at 0.6/2.0 V, which were significantly shortened at 1 C and disappeared at 3 and 5 C. Here again the reason for this state of affairs is discussed below.


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Figure 3. Electrochemical performance of the 3D Mg0.21Ti3C2Tx film electrodes: (a) CV at 0.1 mV s-1; (b) Cycling profile at 1 C; (c) Rate performance; (d) Charge-discharge curves at different rates (1 C = 100 mA g-1).

Charge Storage Mechanism. To probe the Mg-ion storage mechanism in MXenes, samples at different charge-discharge states during cycling (labelled 1-3 in Figure 4a) were collected and characterized by EDX. As mentioned above, the molar ratio of Mg2+ to Ti3C2Tx in the original APC electrolyte-soaked sample was 0.21:1.0. In addition, a large amount of Al3+ and Cl– were also present, with n(Al3+):n(Ti3C2Tx) = 1.11 : 1 and n(Cl-


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):n(Ti3C2Tx) = 1.03:1, respectively (Table 1 and Figure S3a). This indicates that during the soaking process intercalation of entire salt molecules - (RMgCl)2AlCl3, where R = phenyl – between the MXene layers in the APC electrolyte occurred. When fully charged (sample 1 in Figure 4a), the Mg2+ ions almost completely disappeared from the electrodes, indicating a full demagnesiation. In the meantime, the amounts of both, Al3+ and Cl–, were largely reduced. When the electrode was discharged from 2.85 to 1.5 V (Sample 2 in Figure 4a), which was just past the first discharge plateau, the molar ratio of Mg2+ to Ti3C2Tx increased to 0.37:1.0. Concomitantly, the Al3+ content also decreased. When the electrode was further discharged to 0.3 V (Sample 3 in Figure 4a), no obvious changes in the amount of Mg2+, Al3+, and Cl– were observed. Considering the stoichiometric ratio in the electrolyte, 0.2 mole Al3+ could combine with 0.6 mole Cl–, and the molar ratio of Mg2+ to the rest of Cl– was close to 1:1, corresponding to RMgCl. As a result, it is reasonable to conclude that the reversible intercalation of Mg ions into MXenes occurred during the first set of discharge/charge plateaus (~2.0/2.4 V), in the form of RMgCl rather than bare Mg2+ ions. Atomistic details of what happens to the intercalated ions, such as the possibility of desolvation of Mg-ions during the intercalation, are to be examined in


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the future.64 Since no additional intercalation of Mg ions was observed during the second set of discharge/charge plateaus (~0.8/1.9 V), we assigned it to a redox reaction between Mg ions and Ti3C2Tx. This is because the O-containing functional groups render the Ti3C2Tx flakes a titanium oxide-like surface, capable of redox reaction with Mg2+ ions.65-66 Besides, the reversibility of the second set of plateaus (Figure 3c-d and 4b) minimizes the possibility of side reactions between the Mg2+ ions and water molecules. Therefore, the MXene cathode in a MIB system possibly combines intercalation- and conversion-type charge storage mechanisms. More evidence of the redox reaction between Mg ions and MXenes would require additional experiments, including in-situ characterization and detailed impedance spectroscopy studies. Table 1. EDXs data showing the element changes per MXene formula unit, (viz. normalized by 3 Ti) during the charging and discharge process shown in Figure 4a.

n(Mg)

n(Al)

n(Cl)

n(Ti3C2Tx)

Initial

0.21

1.11

1.03

1

Sample 1

0.01

0.33

0.36

1


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Sample 2

0.37

0.18

1.19

1

Sample 3

0.38

0.20

1.08

1

Figure 4b shows charge-discharge profiles of the 3D Mg0.21Ti3C2Tx MXene cathode for the 2nd, 20th, and 100th cycles. Compared to the 2nd cycle, the plateau set at 0.9/1.9 V, corresponding to the redox reaction between Mg ions and Ti3C2Tx, disappears while that related to the intercalation process (1.9/2.4 V) is well preserved at the 20th cycle. This indicates good reversibility of the intercalated Mg-ions between the MXenes layers, but a significant irreversibility for the conversion-type charge storage although it provides the larger capacity. Such irreversibility can be attributed to the large activation barrier to recharge the Mg-containing compounds like MgO, which is common for many conversiontype MIB cathodes.21 It may be formed as a result of interaction of Mg2+ with O and/or OH on the MXene surface. TEM analysis showed that MXene sheets maintained their 2D morphology and single-crystal structure after cycling (Figure S4), suggesting that no phase transformation occurred during cycling. A similar phenomenon was observed after


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the rate performance tests. As mentioned above, the charge-discharge plateaus at 0.6/2.0 V were significantly shortened at 1 C and disappeared at 3 and 5 C, which can be explained by the continuous cycling during tests and the irreversibility of the redox reaction. For the 100th cycle, no obvious charge-discharge plateaus were observed, which can be attributed to the further enlarged interlayer spacing that, in turn, allowed fast diffusion of Mg ions.67 The disappearance of charge-discharge plateaus upon cycling is frequently observed for MXene-based electrodes.54

Figure 4. (a) The second-cycle charge-discharge profile at 0.5 C. Samples at different charge-discharge states are labeled as 1, 2, and 3. Inset sketch shows the proposed RMgCl intercalated MXene structure. (b) Charge-discharge curves at the 2nd, 20th, and 100th cycles at 1 C.


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The 3D films with higher Mg loading (Mg0.58Ti3C2Tx), obtained by soaking in the H2Oacetonitrile electrolyte, were also tested as the cathodes. At 1 C, an initial charge capacity of ~ 200 mAh g-1 was recorded, which reduced to ~100 and ~50 mAh g-1 at the 2nd and 8th cycles, respectively (Figure S5a). Besides, a low Coulombic efficiency ranging from 50-70% was recorded during these 8 cycles (Figure S5b). The poor cycling stability and low Coulombic efficiency compared to the APC electrolyte-soaked 3D MXene film can be attributed to the presence of water molecules and OH groups in the electrode originating from the soaking process. Similarly, two sets of charge-discharge plateaus corresponding to the reversible intercalation and redox reaction process, respectively, were observed. 

CONCLUSIONS In summary, free-standing and flexible 3D Ti3C2Tx films were fabricated and tested as

cathodes in MIBs. Pre-incorporation of Mg ions was achieved by soaking films in a Mg2+containing electrolyte. At 0.5 C, 1 C and 5 C, the cathodes delivered capacities of ~210 mAh g-1, ~140 and ~55 mAh g-1, respectively. This performance is better, especially at high rates, than other reported MIB cathodes and bodes well for use of MXenes in MIBs.


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A reversible intercalation of Mg2+ ions between MXene flakes was demonstrated, accompanied with a possible redox reaction charge storage mechanism. The former exhibited excellent cycling stability; the latter led to significant irreversibility. Further efforts, devoted to a detailed understanding of the redox reactions occurring between the Mg ions and MXenes, are needed to improve the cycling stability. Considering the variety of MXene compositions, this work suggests that a large family of cathode materials are potentially available for MIBs.

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EXPERIMETAL SECTION Synthesis and Delamination of Ti3C2Tx. First, 1.98 g of lithium fluoride (LiF) (Alfa Aesar,

98.5%) was added to 20 mL of 9.0 M HCl (Fisher, technical grade) aqueous solution. The mixture was stirred for 5 min to dissolve the salt. Then, 2 g of Ti3AlC2 powder - the synthesis of which is described elsewhere68 - was added into the mixture over the course of 10 min to avoid overheating of the solution as a result of the reaction's exothermic nature. The reaction mixture was kept at 35 oC for 24 h while stirring, after which the solid residue was washed with distilled water, centrifuged

(3500 rpm), and decanted until the pH of the supernatant reached approximately 6. The final powder, with a small amount of water, was filtered through a polyvinylidene fluoride filter (0.45 μm pore size) and dried in air to obtain multi-layer Ti3C2Tx powder. The as-produced Ti3C2Tx powder was dispersed in deaerated water with a 1:20 weight ratio of Ti3C2Tx:H2O. The suspension


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was hand shaken for 10 min, then centrifuged at 3500 rpm for 1 h, and the supernatant fluid – comprised of a colloidal suspension of delaminated Ti3C2Tx flakes with a concentration of ≈ 2.0 mg mL-1 - was collected. Synthesis of PMMA Spheres. The monodispersed poly(methyl methacrylate) (PMMA) spheres with a diameter of 2-3 µm were synthesized via a dispersion polymerization method. In a typical process, the radical initiator (azoisobutyronitrile) (Sigma-Aldrich, USA) and poly(vinyl pyrrolidone) as a stabilizer were dissolved at 0.1 and 4 wt.%, respectively, in methanol at room temperature. The solution was purged with Ar to remove oxygen, and methyl methacrylate monomer (MMA, Sigma-Aldrich, USA) was added. The concentration of MMA was 10 wt.%. The reacting mixture was then stirred at 55 oC. After 24 h, a white product was collected by centrifugation. After washing with methanol, these PMMA spheres were dispersed in water by probe sonication for 0.5 h before use. Fabrication of 3D Macroporous MXene Films. A Ti3C2Tx colloidal suspension (Supporting Information) (2 mg mL-1) and the poly(methyl methacrylate) (PMMA) sphere dispersed in water (10 mg mL-1, 2-3 μm in size) were directly mixed together and stirred for 10 min. The Ti3C2Tx to PMMA spheres mass ratio was controlled at 1:4. The mixture was then filtered through a polypropylene membrane (3501 Coated PP, Celgard LL, Charlotte, NC) to yield a film. The latter was dried in air at room temperature for 10 min and peeled off from the polypropylene membrane, yielding a flexible, freestanding


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Ti3C2Tx/PMMA hybrid film. The latter was annealed at 450 °C for 1 h under flowing Ar to burn out the PMMA spheres, leaving a macroporous film. Incorporation of Mg Ions into the 3D Macroporous MXene Films. The MXene films were cut into 2x2 cm2 squares and soaked in a premixed solution of H2O/acetonitrile with 0.1 M MgCl2 for 10 h to incorporate Mg ions. The H2O/acetonitrile volumetric ratio was 1:1. The soaked films were then rinsed in a deionized (DI) water bath 3 times to remove any Mg salts present. The films were dried in a vacuum oven overnight at 120 oC. The incorporation of Mg ions into MXenes from the APC electrolyte was conducted in an Arfilled glove box. 3D films with similar size were soaked in the APC electrolyte for 10 h, washed using tetrahydrofuran (THF) 3 times to remove excess salts and then dried in a glove box at room temperature overnight. Characterization. The samples were characterized using a scanning electron microscope (SEM) (Zeiss Supra 50VP, Germany) and a transmission electron microscope (TEM) (JEOL JEM-2100, Japan) using an accelerating voltage of 200 kV. The TEM samples were prepared by sonicating the 3D MXene film in ethanol for 10 min. Several drops of the resulting dispersion were dropped onto a lacey carbon coated TEM


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grid and air dried. The X-ray diffraction (XRD) patterns were recorded by a powder diffractometer (Rigaku Smart Lab, USA) with Cu Kα radiation at a step scan of 0.02° and dwell time of 0.5 s. EDX analysis was conducted using an energy dispersive X-ray spectrometer (Oxford EDS, with INCA software) incorporated in the TEM on several points to average the MXene-Mg ratio. Electrochemical Measurements. The APC electrolyte was synthesized as described before.29 A Lewis base, 0.4 M phenylmagnesium chloride in tetrahydrofuran (PhMgCl in THF, Aldrich, 99%) solution was mixed with a Lewis acid, 0.2 M aluminum trichloride (AlCl3, Aldrich 99.999%) in THF solution (Aldrich, inhibitor-free, anhydrous) in an Ar glove box. The obtained solution was vigorously stirred overnight before using as the electrolyte. To test electrode storage properties, pristine and Mg2+-incorporated MXene films were cut into self-supported electrodes. The electrode assembly was sandwiched between a specialized glassy carbon cup as the cathode current collector and a stainless steel plate as the anode current collector in a customized airtight cell described elsewhere.29 The battery cells were assembled in an Ar-filled glove box using the MXene film electrode as


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the cathode and a Mg metal foil as the anode. The two were electronically separated by a glass microfiber saturated with the APC electrolyte. The loading of the MXene film electrodes was 0.5-1.0 mg cm-2. The charge-discharge measurements were performed at different current densities (1 C = 100 mA g-1) in the voltage range from 0.3 to 2.85 V vs. Mg2+/Mg using an Arbin system (Arbin BT-2143-11U, College Station, TX, USA). Cyclic voltammetry (CV) were conducted using a VMP3 potentiostat (Biologic, France) and measured between 0.3 and 2.85 V vs. Mg2+/Mg at a scan rate of 0.1 mV s-1.

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ASSOCIATED CONTENT

Supporting Information Available: Electrochemical performance of 3D Ti3C2Tx films with and without Mg2+-ion incorporation, TEM, XRD, and EDXs analysis of 3D Ti3C2Tx film electrodes before and after cycling, and electrochemical performance of the 3D Ti3C2Tx film with Mg2+ incorporated from H2O/acetonitrile electrolyte.

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AUTHOR INFORMATION


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Corresponding Author *Email: [email protected]; *Email: [email protected] ORCID Meng-Qiang Zhao: 0000-0002-0547-3144 Babak Anasori: 0000-0002-1955-253X Michel W. Barsoum: 0000-0001-7800-3517 Yury Gogotsi: 0000-0001-9423-4032 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT



This work was supported by the Toyota Research Institute of North America. M.A. was supported by the Libyan North America Scholarship Program funded by the Libyan Ministry of Higher Education and Scientific Research. The authors thank Dr. Bing Hao for preparing the Table of Content and schematic images. The authors thank Dr. Ruigang Zhang and Dr. Chen Ling the Toyota Research Institute of North America for valuable discussions. The authors thank the Core Research Facilities of Drexel University for providing access to XRD, SEM and TEM.

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Magnesium-Ion Storage Capability of MXenes - ACS Applied Energy

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