Assembly of Aqueous Rechargeable Magnesium Ions Battery Capacitor

Research Article pubs.acs.org/journal/ascecg

Assembly of Aqueous Rechargeable Magnesium Ions Battery Capacitor: The Nanowire Mg-OMS-2/Graphene as Cathode and Activated Carbon as Anode Hongyu Zhang, Ke Ye,* Kai Zhu, Ruibai Cang, Xin Wang, Guiling Wang, and Dianxue Cao* Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China S Supporting Information *

ABSTRACT: The aqueous magnesium ion battery (AMIB) system is an attractive candidate in the aqueous batteries due to its high safety properties, similar electrochemical characteristics to lithium and low cost in energy storage applications. The magnesium octahedral molecular sieves of Mg-OMS-2 and the Mg-OMS-2/Graphene composite, depending on their unique 2 × 2 tunnel structure that can provide enough large pore size for magnesium ions insertion/deinsertion into/from the lattice of host materials, are utilized as the cathode materials in the AMIB system and exhibit good battery performances in the three different magnesium salt electrolytes. The Mg-OMS-2/Graphene, not only maintaining the tunnel structure but also possessing the excellent electrochemical property, obtains better rate ability and cycle performance than that of Mg-OMS-2. Furthermore, the Mg-OMS-2/Graphene//AC system is first assembled as aqueous rechargeable magnesium ion battery capacitor. The discharge capacity of this system remains to be 44.1 mAh g−1 at the current density of 100 mA g−1 after 500 cycles and the capacity retention rate is 95.8%. KEYWORDS: Green energy storage system, Magnesium ion, Aqueous battery capacitor, Nanowire Mg-OMS-2/Graphene

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interests. Aurbach’s group16−18 and Nazar’s group19−21 have many excellent works, which accumulated a wealth of experience for researchers on magnesium ion batteries. Therefore, the aqueous magnesium ion batteries (AMIB)22−24 emerged. The radius of magnesium ion is smaller than that of lithium ion and the electrochemical property of magnesium is similar to that of lithium. And not only that, the Mg metal anode can exhibit a higher volumetric energy density with 3833 mAh cm−3 than that of Li metal with 2046 mAh cm−3. Moreover, it can also utilize the potential secure advantages, namely, without the formation of dendritic structure.25 The AMIB system is promising to bring new break in the energy storage systems. Manganese oxide octahedral molecular sieves (OMS),26,27 one type of MnO2, have been extensively used in the

INTRODUCTION Aqueous rechargeable batteries, one of novel green energy storage systems, owning the properties of high ionic conductivity, low cost and inherent safety by using the aqueous electrolytes, have aroused much attention.1−5 This aqueous system can not only solve the environmental problem by cutting down the fossil fuel consumption but also reduce the production cost and expand production scale, which promises to be the most extensively applied prospect in energy storage systems. In the past several years, many aqueous battery systems have been invented. The aqueous lithium ion batteries (ALIB)6−8 first came out, such as VO2/LiMn2O4,9 LiV3O8/ LiNi0.81Co0.19O2,10 LiTi2(PO4)3/LiMn0.05Ni0.05Fe0.9PO411 and AC/LiMn2O4.12 However, the expensive lithium resource is the main constraint for the large-scale production. In addition, aqueous sodium ion batteries (ASIB)13−15 also did not show good rate performance due to the large radius of sodium ion, which is difficult to insert/deinsert from the lattice of host materials. The magnesium ion battery has attracted a lot of © 2017 American Chemical Society

Received: March 31, 2017 Revised: June 4, 2017 Published: July 11, 2017 6727

DOI: 10.1021/acssuschemeng.7b00982 ACS Sustainable Chem. Eng. 2017, 5, 6727−6735

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ACS Sustainable Chemistry & Engineering catalysis,28,29 lithium battery,30,31 supercapacitor32,33 fields due to its unique tunnel structure. One of these OMS, cryptomelane OMS-2,34−36 constituted by 2 × 2 and 1 × 1 tunnels, has been a powerful candidate as cathode material in lithium batteries. However, the inferior conductivity37 is an inherent defect for manganese oxide materials, which affects the large-scale application due to the poor rate ability and cycle performance. To overcome this defect, graphene is a superb choice to enhance the electrical conductivity of manganese oxide octahedral molecular sieves. Because graphene38−41 is only an atomically thin corresponding to two-dimensional (2D) form film of carbon allotrope with the properties of ultra high mechanical and heat conduction performance, highly electrical conductivity and large specific surface area, it has been attracted a great deal of attention and already applied in many fields such as battery42−44 and supercapacitor.45−47 It is noteworthy that graphene has been always employed to form composites in the battery fields. In this work, we not only prepared the Mg-OMS-2 but also synthesized the composites of Mg-OMS-2/Graphene, which were used as the cathodes in aqueous rechargeable magnesium ion battery capacitor for the first time. Comparing with the two obtained cathode materials, Mg-OMS-2/Graphene exhibited the better rate ability and cycle performance in MgCl2 and Mg(NO3)2 electrolytes. More importantly, we first achieved the assembly of Mg-OMS-2/Graphene//Activated Carbon (AC) aqueous rechargeable magnesium ion battery capacitor, which utilizes the Mg-OMS-2/Graphene as cathode and AC as anode in 0.5 mol dm−3 Mg(NO3)2 electrolyte and shows a high energy density and good cycling stability. Therefore, AMIB capacitor can be the most promising candidate for the second generation of energy storage system.

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Electrochemical Measurement of Mg-OMS-2 and Mg-OMS2/Graphene Electrodes. The prepared cathode slurry was made up of 10% acetylene black, 10% polyvinylidene difluoride (PVDF) binder and 80% Mg-OMS-2 or Mg-OMS-2/Graphene dispersed in 1-methyl2-pyrrolidone (NMP). The obtained electrodes were prepared by coating the slurry onto carbon fiber cloths (CFC, 1.0 cm × 1.0 cm) with the mass loadings of active material about 5 mg cm−2, and then the electrodes were dried in vacuum at 80 °C for 24 h. LAND Battery Testing system (model CT2001A, Land, China) was used to measure the galvanostatic charge and discharge tests. The electrochemical workstation (VMP3/Z, Biologic, France) was used to test cyclic voltammetry (CV). The electrochemical tests of Mg-OMS-2 or MgOMS-2/Graphene electrode were performed by utilizing a conventional standard three electrode electrochemical cell including a working electrode (Mg-OMS-2 or Mg-OMS-2/Graphene), a counter electrode (graphite rod) and a reference electrode (saturated calomel electrode, SCE). The analytical grade chemical reagents and Milli-Q water were utilized to prepare the MgCl2, Mg(NO3)2 and MgSO4 electrolytes with different concentrations. Electrochemical Measurement of the Mg-OMS-2/Graphene//AC Battery Capacitor. The battery capacitor system of Mg-OMS-2/Graphene//AC is composed by Mg-OMS-2/Graphene cathode and AC anode, which is also prepared by mixing the slurry of 10% acetylene black, 10% polyvinylidene difluoride (PVDF) binder and 80% Mg-OMS-2/Graphene or AC dispersed in 1-methyl-2pyrrolidone (NMP). And the electrodes were also prepared by coating the slurry onto carbon fiber cloths. This two electrodes system was fabricated by holding Mg-OMS-2/Graphene as cathode, a filter paper as the traditional battery membrane and AC as anode, which just likes the sandwich together using the two designed titanium frames on each side. The two electrodes sandwich is immersed in 0.5 mol dm−3 Mg(NO3)2 electrolyte, which is bubbled by nitrogen for 1 h to exclude oxygen before measures.51

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RESULTS AND DISCUSSION Characterization of the Mg-OMS-2 and Mg-OMS-2/ Graphene. The X-ray diffraction patterns of Mg-OMS-2 and Mg-OMS-2/Graphene are shown in Figure 1. The two samples,

EXPERIMENTAL SECTION

Preparation and Characterization of Mg-OMS-2 and MgOMS-2/Graphene. The Mg-OMS-2 material was prepared by onestep hydrothermal route described previously.48 Primarily, 0.0095 mol MnSO4·H2O and 0.0024 mol MgSO4·7H2O were dissolved in 0.2 mol dm−3 H2SO4 for 1 h. And then, 0.0075 mol KMnO4 was added quickly in the mix solution for stirring 2 h. At last, the obtained above solution was transferred into a Teflon-lined autoclave at 120 °C for 24 h. The resulting solids (Mg-OMS-2) were washed with deionized water (DDW) before drying at 60 °C in the air. The Mg-OMS-2/Graphene material was prepared by the hydrothermal method described previously.49 First, the layered birnessite as the precursor was obtained by conventional stoichiometry.50 Second, 0.8 g obtained layered birnessite was added into 0.5 mol dm−3 (NH4)2S2O8 solution at 80 °C for stirring 24 h. And then, 40 mL tetrabutylammonium hydroxide solution was immediately added in the above suspension for stirring 12 h. After that, 50 mg graphene (commercialization, the optimum amount showed in Figure S1) was added in this mix solution for stirring another 12 h. Subsequently, 2.5 mol dm−3 MgCl2 solution was mixed with the obtained solution for stirring 2 h. Finally, the above precipitates were washed with centrifugation until neutral and transferred into Teflon-lined autoclave at 120 °C for 24 h. The resulting solids (Mg-OMS-2/Graphene) were washed with deionized water (DDW) before drying at 60 °C in the air. The prepared samples were identified by using the X-ray diffractometer (XRD, Rigaku TTR III) with Cu Ka radiation (λ = 0.151 417 8 nm). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) was characterized with a FEI Teccai G2 S-Twin microscope combining with energy dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy with Al Kα radiation (XPS, ThermoESCALAB 250) was used to investigate oxidation and reduction states of Mn.

Figure 1. XRD patterns of the Mg-OMS-2 and Mg-OMS-2/Graphene.

of which crystal structure belongs to the tetragonal I4/m space group (a = 9.8 Å, b = 9.8 Å, c = 2.8 Å; α, c and β close to 90°), maintain the crystal type of cryptomelane and match very well with the standard crystallographic tables (JCPDS card 291020). Meanwhile, there are no peak for other type of manganese dioxide in the XRD patterns of two samples, which also demonstrated that the prepared Mg-OMS-2 and Mg-OMS2/Graphene owned a high purity and crystallinity (the MgOMS-2 can be abbreviated to Mg0.12MnO2 by ICP analysis). The peak of graphene is also observed at 2θ = ∼26° in Figure 1. Comparing with the conventional cryptomelane (OMS-2), the 6728


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timate contact between Mg-OMS-2 nanowire and graphene sheets. Figure 2d shows the HRTEM image of Mg-OMS-2/ Graphene. It can be seen that the d spacing of lattice fringes is 0.69 nm, which corresponds to the width of 2 × 2 tunnels. This result is in good line with the (110) plane of Mg-OMS-2/ Graphene from the peak at 2θ = 12.7° in XRD pattern, which also strongly certifies the existence of 2 × 2 tunnels and indicates that the Mg-OMS-2/Graphene well keeps the cryptomelane structure. The tunnel is always orderly grown along only one direction (c-axis), which forms the nanowire structure. The elemental mapping images of Figure 2e reveal the existence and homogeneous dispersion of Mg through the single nanowire, which confirms that magnesium ions doped into the host material and then forms the Mg-OMS-2. Meanwhile, uniformly dispersed Mn and O are also detected in Figure 2e. Therefore, it is reasonable to postulate that the Mg-OMS-2/Graphene composite not only maintains the 2 × 2 tunnels but also owns excellent properties such as a large specific surface area and highly electrical conductivity resulting from graphene. These materials are very promising to apply as the cathode material for aqueous rechargeable magnesium ion battery system. Electrochemical Performance of Mg-OMS-2 and MgOMS-2/Graphene in Aqueous Magnesium Salt Solutions. The electrochemical behavior of Mg2+ insertion/ deinsertion in Mg-OMS-2 and Mg-OMS-2/Graphene is primarily investigated by cyclic voltammetry (CV), which displays the kinetic characteristic intuitively. Figure 3 exhibits the CV curves of Mg-OMS-2 and Mg-OMS-2/Graphene electrodes in 0.5 mol dm−3 MgCl2, Mg(NO3)2 and MgSO4 electrolytes at scan rates of 1, 2, 5, 10, 20, 30 mV s−1 in the potential range from −0.65 to +0.75 V. This potential range ensures that the magnesium salts aqueous solutions are not decomposed to hydrogen and oxygen gas during the charging and discharging progress, which guarantees that the total capacity is from the host material. It is very obvious that not only Mg-OMS-2 but also Mg-OMS-2/Graphene shows two couples of oxidation/reduction peaks at 0.5 V/0 and 0.3 V/− 0.2 V in Figure S2, which corresponded to the insertion/ deinsertion process of magnesium ions into the lattice of host materials.30 Moreover, the current densities of Mg-OMS-2/ Graphene in the three magnesium salts electrolytes (Figure 3d−f) are higher than that of Mg-OMS-2 in these electrolytes (Figure 3a−c). The reason is that the Mg-OMS-2/Graphene composite may own a higher electrical conductivity than the pure Mg-OMS-2 and so exhibits better electrochemical performance than Mg-OMS-2. To further study the insertion/deinsertion behavior of Mg2+ ions, galvanostatic charge and discharge measurement is utilized to evaluate in the potential range from −0.65 to +0.75 V at different current densities. Figure 4a−c reveals the charge and discharge curves of Mg-OMS-2 electrodes in 0.5 mol dm−3 electrolytes (MgCl2, MgSO4 and Mg(NO3)2) and that of MgOMS-2/Graphene electrodes in these three magnesium salts electrolytes are also exhibited in Figure 4d−f. It is obvious from Figure 4 that the Mg-OMS-2/Graphene electrode shows better rate performance than that of Mg-OMS-2 in the three electrolytes. The initial discharge capacities of Mg-OMS-2 electrodes at the current density of 20 mA g−1 in MgCl2, Mg(NO3)2 and MgSO4 electrolytes are 155.2, 171.8, 112.5 mAh g−1, but that of the Mg-OMS-2/Graphene electrodes in the three electrolytes reached 171.8, 232.4, 161.4 mAh g−1, respectively. Furthermore, all the discharge curves in Figure 4

prepared samples are doped with magnesium ions, which may substitute with the sites of Mn2+ ions52 and utilize to improve the property of reversible multielectron redox transformations.53 The prepared Mg-OMS-2 or Mg-OMS-2/Graphene, which belongs to one of the octahedral molecular sieves family, keeps the tunnel structure of conventional cryptomelane (OMS-2) and contains 2 × 2 tunnels with a pore size of 4.6 Å or even larger that based on the doped metal, so it can improve enough space for Mg2+ ion (the radius is 0.72 Å) to insert/deinsert into/from the lattice. Moreover, the Mg-OMS2/Graphene composite, which inherited the high electrical conductive performance of graphene, is promising to display a better electrochemical property in AMIB system. The detailed morphological and structural analysis of MgOMS-2 and Mg-OMS-2/Graphene are conducted by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX), as shown in Figure 2.

Figure 2. TEM images with different magnifications of Mg-OMS-2 (a and b); TEM image (c) and HRTEM image (d) of Mg-OMS-2/ Graphene; TEM image of single nanowire of Mg-OMS-2/Graphene (e) and the corresponding distribution of the Mg, Mn, O elemental mapping images.

Figure 2a,b shows the TEM images of Mg-OMS-2 with different magnifications. It is effortless to discover that the MgOMS-2 exhibits a nanowire structure, which is uniformly distributed. The nanowires of Mg-OMS-2 are generally 0.2−2 μm in length and 10−15 nm in diameter. The TEM image of Mg-OMS-2/Graphene is also shown in Figure 2c. The MgOMS-2/Graphene also maintains the nanowire structure and the graphene sheets can also be clearly observed in Figure 2c. Meanwhile, it is clear that Mg-OMS-2 is successfully grown on the graphene surfaces in the yellow square of Figure 2c and the Mg-OMS-2/Graphene composite demonstrate a highly in6729


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Figure 3. Cyclic voltammograms (CVs) of the Mg-OMS-2 electrodes in 0.5 mol dm−3 MgCl2 (a), Mg(NO3)2 (b), MgSO4 (c) and Mg-OMS-2/ Graphene electrodes for 0.5 mol dm−3 MgCl2 (d), Mg(NO3)2 (e), MgSO4 (f) at the different scan rates.

Figure 4. Galvanostatic charge and discharge curves of Mg-OMS-2 in 0.5 mol dm−3 MgCl2 (a), Mg(NO3)2 (b), MgSO4 (c) and Mg-OMS-2/ Graphene in 0.5 mol dm−3 MgCl2 (d), Mg(NO3)2 (e), MgSO4 (f).

for Mg-OMS-2 and Mg-OMS-2/Graphene demonstrate a large flat discharge plateau at around 0 V. These results are in good accordance with the cathodic peak of CVs (Figure 3), which is associated with the insertion of magnesium ions into lattice of Mg-OMS-2 or Mg-OMS-2/Graphene and similar to the insertion of lithium ions into magnesium octahedral molecular sieves.30

Meanwhile, it is also distinctly observed in Figure 4 that not only Mg-OMS-2 but also Mg-OMS-2/Graphene shows better rate performance in the MgCl2 and Mg(NO3)2 electrolytes than that in the MgSO4 electrolyte. For electrolytes, the viscosity as one of the transport properties is an important parameter to examine the battery performance, which is also important for aqueous strong electrolytes, namely, the electrostatic interactions (Coulombic forces) between ions.54 The Jones−Dole 6730


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Figure 5. Cycle performance of Mg-OMS-2 (a) and Mg-OMS-2/Graphene (b) electrodes at 100 mA g−1 in 0.5 mol dm−3 MgCl2, Mg(NO3)2 and MgSO4.

Figure 6. Potential-time curves of Mg-OMS-2/Graphene and AC electrodes vs SCE reference electrode and the voltage−time curve of Mg-OMS-2/ Graphene//AC battery capacitor at 100 mA g−1 (a); charge−discharge curves of Mg-OMS-2/Graphene//AC at different current densities (b); cycle performance of Mg-OMS-2/Graphene//AC battery capacitor at a current density of 100 mA g−1 (c); Ragone plots of Mg-OMS-2/Graphene//AC battery capacitor in 0.5 mol dm−3 Mg(NO3)2 electrolyte (d).

viscosity B-coefficient, which represents the solvational properties between the water and ions, is an important understanding and expression for the structure and destruction of ionic processes. The reason is that the hydrated Mg2+ ions in MgSO4 electrolyte occur dehydration more difficultly on the interface and the bare Mg2+ ions insert into the lattice of Mg-OMS-2 or Mg-OMS-2/Graphene more difficultly than those in MgCl2 and Mg(NO3)2 electrolytes, which is proved by the positive Jones− Dole coefficient (B)55 of SO42− (+0.206 at room temperature),56 meaning to make the structure of the water more

closely (the structure-making). However, the Jones−Dole coefficient (B) of Cl− and NO3− are −0.005 and −0.04556 at room temperature, respectively, and vice versa. Figure 5 reveals the cycle performance of Mg-OMS-2 and Mg-OMS-2/Graphene electrodes in the three magnesium salt electrolytes at a current density of 100 mA g−1 in the potential range from −0.65 to +0.75 V. Both of the two electrodes have been experienced the activation in the first 10 cycles, which are not the battery performance, as shown in Figure S3. The cycle performance can also testify that the Mg-OMS-2 and Mg-OMS6731


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Figure 7. XRD patterns of the Mg-OMS-2/Graphene fresh and reduced electrodes at 20 mA g−1 with −0.65 V (a), a zoomed view of 200 plane (b) and 310 plane (c) in XRD patterns.

the current density of 100 mA g−1 between 0 and 2.0 V, as shown in Figure 6c. And after 500 cycles, the discharge capacity remains to be 44.1 mAh g−1 and the capacity retention rate is 95.8%, which shows a better cycling stability. However, the columbic efficiency is 92.2%, which is due to the irreversible capacity and IR drop. Ragone plot is displayed in Figure 6d, illustrating the relationship of energy density and power density. This Mg-OMS-2/Graphene//AC system can reach a energy density of 46.9 Wh kg−1, which is higher than that of the reported system such as 38.0 Wh kg−1 of LiMn2O4//AC,60 19.5 Wh kg−1 of NaMnO2//AC59 and 8.5 Wh kg−1 AC//AC61 in 0.5 mol dm−3 Mg(NO3)2 electrolyte. This good performance is likely caused by the smaller ion radius of Mg2+ (0.072 nm) compared with that of Na+ (0.117 nm), the higher ionic conductivity of Mg2+ (43.8) than that of Li+ (38.6) and the MgOMS-2/Graphene composite owning the excellent electrochemical performance that successfully achieved by matching with AC. For the first time, the Mg-OMS-2/Graphene//AC system achieves the assembly of aqueous rechargeable magnesium ions battery capacitor, which supplies a new thought for the future aqueous rechargeable battery system. Discussion about the Mechanism of Mg2+ Insertion/ Deinsertion into/from the Mg-OMS-2/Graphene Electrode. To study the reaction mechanism and check the change of their structures and phases, the compared XRD patterns between Mg-OMS-2/Graphene fresh and reduced electrodes at 20 mA g−1 with −0.65 V are shown in Figure 7a. It is obvious that both the fresh and reduced Mg-OMS-2/Graphene electrodes maintain the tunnel structure; however, in Figure 7b,c, the diffraction peaks corresponding to the (200) and (310) planes of the Mg-OMS-2/Graphene reduced electrode shifted toward the lower angles by comparison with that of the fresh electrode, which is due to the Mg2+ ions insertion into the lattice of host material. Moreover, it can be seen that a part of the tetragonal phase for the fresh electrode transforms to the new cubic phase and hexagonal phase after the reduced process. Based on the comparison of XRD patterns of the Mg-OMS-2/ Graphene fresh and reduced electrodes, the possible reaction mechanism is as follows, which is similar to the reaction mechanism of Li+ ion in lithium battery that reported,30 namely, the insertion/deinsertion process of Mg2+ ions into/ from the lattice of host material is as below: primarily, the

2/Graphene electrodes show better electrochemical performance in the MgCl2 and Mg(NO3)2 electrolytes than that in the MgSO4. Clearly, the Mg-OMS-2/Graphene electrodes show better cycle performance than Mg-OMS-2 in all the three magnesium salt electrolytes, which is due to their large specific surface areas and highly electrical conductivity combining with graphene. In Figure 5b, the Mg-OMS-2/Graphene electrodes show the initial discharge capacity of 117.0, 115.0 and 81.6 mAh g−1 and the capacity retentions are 75.7%, 93.0% and 57.5% after 300 cycles at 100 mA g−1 in 0.5 mol dm−3 MgCl2, Mg(NO3)2 and MgSO4 electrolytes, respectively. However, the Mg-OMS-2 electrodes show the initial discharge capacity of 112.2, 110.2 and 74.5 mAh g−1 and the capacity retentions are 46.2%, 59.4% and 36.9% after 300 cycles at 100 mA g−1 in the three magnesium salt electrolytes in Figure 5a. Electrochemical Performance of the Battery Capacitor Mg-OMS-2/Graphene//AC. To further explore the aqueous rechargeable magnesium battery system, the battery capacitor is assembled by Mg-OMS-2/Graphene as cathode and AC as anode in 0.5 mol dm−3 Mg(NO3)2 electrolyte. Figure 6a exhibits potential-time curves of Mg-OMS-2/ Graphene and AC electrodes vs SCE reference electrode and the voltage−time curve of Mg-OMS-2/Graphene//AC (weight ratio of Mg-OMS-2/Graphene to AC is ∼1:2, which can achieve the maximum matching degree as shown in Figure 6a) battery capacitor at a current density of 100 mA g−1. The MgOMS-2/Graphene cathode presents plateaus, indicating the insertion/deinsertion of magnesium ions into/from the lattice of the host material, which shows the battery performance. And the AC anode presents the characteristic of electric double layer capacitance, which shows the typical linear relationship with time. Not only that, this Mg-OMS-2/Graphene//AC aqueous rechargeable magnesium ion battery capacitor can achieve maximum operational voltage of 2 V, which is higher than other aqueous battery capacitor reported such as 1.7 V for Na4Mn9O18//AC,57 1.8 V for LiMn2O4//AC58 and 1.9 V for NaMnO2//AC.59 Figure 6b shows the charge/discharge curves of Mg-OMS-2/Graphene//AC at different current densities, which can obtain the discharge time of 1700 s at the density of 100 mA g−1. It also shows the capacitor performance more obviously as the current density increases. The Mg-OMS-2/ Graphene//AC system exhibits a good cycle performance at 6732


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Figure 8. XPS patterns of the Mg-OMS-2/Graphene fresh and reduced electrodes (a) and Mg 1s core level spectra (b), Mn 2p core level spectra of fresh (c) and reduced (d) electrode.

hydrated Mg2+ ions diffuse to the surface of Mg-OMS-2/ Graphene electrode; and then, the dehydration happens on the interface; finally, the bare Mg2+ ions inserted into the lattice of host material.24 The above two couples of redox peaks shown in Figure 3 could be tentatively ascribed to Mg2+ ions insert/ deinsert into the 2 × 2 tunnels. In the two-electrode system of Mg-OMS-2/Graphene//AC, the AC is utilized as anode to adsorb/desorb Mg2+ ions,61,62 which can achieve the transmission of Mg2+ ions by matching with the Mg-OMS-2/ Graphene cathode. For the aqueous electrolyte, the secondary reaction (e.g., hydrogen and oxygen evolution) should be taken into account during the charge and discharge process. To eliminate the occurrence of secondary reaction or the decomposition of aqueous electrolytes, the potential of −0.65 ∼ +0.75 V are selected to be the most appropriate potential window. To effectively check the changes in the oxidation/reduction state of manganese element, XPS measurement is employed to further explore the reaction mechanism and examine if the magnesium ions insert into the lattice of Mg-OMS-2/Graphene during the discharge process or not. Figure 8a reveals the survey spectra of Mg-OMS-2/Graphene for the fresh and reduced electrodes. It is obvious that Mg 1s, Mg 2s and Mg 2p are found in both the fresh and reduced electrodes and the peak intensity of Mg 1s at 1302.1 eV for the reduced electrode becomes larger than that of the fresh electrode, as shown in Figure 8a,b. This result is well consistent with the main diffraction peaks of (110), (200) and (310) planes of the reduced electrode (Figure 7) shifted toward the lower angles, proving that much more Mg2+ ions insert into the lattice of MgOMS-2/Graphene electrode after the discharge process.

Moreover, the F 1s peak at 686.8 eV of PVDF and C 1s peak at 283.8 eV of acetylene black are also observed in Figure 8a. In the Mn 2p XPS spectrum of fresh electrode (Figure 8c), the two main peaks with binding energy of 653.6 and 641.8 eV are evidently observed, which are assigned to Mn 2p1/2 and Mn 2p3/2. This result is in good line with the binding energy values and associated with the coexistence of Mn3+ and Mn4+ species that reported in the literatures.63,64 The percentage of Mn3+ and Mn4+ are calculated to be 54.6% and 45.4% by fitting the fresh electrode in Figure 8c. In the Supporting Information, the XPS data of oxidized electrode is also fitted to obtain the 48.1% Mn3+ and 51.9% Mn4+ in Figure S4, which means that a part of Mn3+ oxidized to Mn4+ during the charging process. In Figure 8d, two main peaks at 653.5 and 641.5 eV are detected for the reduced electrode and well associated with the coexistence of Mn2+, Mn3+ and Mn4+ species, which is similar to the reports.65 And the percentage of 34.2% Mn2+, 28.6% Mn3+ and 37.2% Mn4+ for the reduced sample are also obtained by fitting the XPS data, which means that a part of Mn4+ transforms to Mn3+ and Mn3+ transforms to Mn2+ during the discharged process. This result well explains the insertion/deinsertion of Mg2+ ions by the changed valence of manganese during the charged/ discharged process, which is in good accordance with the two pairs of redox peaks showed in CVs (Figure 3).

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CONCLUSIONS In summary, we successfully synthesized the Mg-OMS-2 nanowire and Mg-OMS-2/Graphene composite by the hydrothermal method. The Mg-OMS-2/Graphene shows better rate ability and cycle performance than Mg-OMS-2. The Mg-OMS6733


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ACS Sustainable Chemistry & Engineering 2/Graphene composite not only maintains the 2 × 2 tunnels but also owns the excellent electrochemical performance with a unique nanowire structure anchoring on the graphene sheets. The initial discharge capacities of Mg-OMS-2/Graphene electrodes in 0.5 mol dm−3 MgCl2, Mg(NO3)2 and MgSO4 electrolytes achieved 171.8, 232.4 and 161.4 mAh g−1 at 20 mA g−1 and the capacity retentions are 75.7%, 93.0% and 57.5% after 300 cycles at 100 mA g−1 in the three electrolytes, respectively. Moreover, the Mg-OMS-2/Graphene//AC system, which reached a high energy density of 46.9 Wh kg−1 and a capacity retention rate of 95.8% after 500 cycles, is first assembled as the aqueous rechargeable magnesium ions battery capacitor, indicating that the AMIB system is very promising for the application in the energy storage systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00982. The cycle performance of different amount of graphene in composites, cyclic voltammograms, cycle performance of Mg-OMS-2 and Mg-OMS-2/Graphene, XPS patterns of Mn 2p core level spectra of oxidized electrode (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected] (K. Ye). *E-mail address: [email protected] (D. Cao). ORCID

Dianxue Cao: 0000-0001-9138-7295 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51672056), Natural Science Foundation of Heilongjiang Province of China (LC2015004), Heilongjiang Postdoctoral Scientific Research Developmental Fund (LBHQ16044) and Fundamental Research Funds for the Central Universities (HEUCF171003).

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Assembly of Aqueous Rechargeable Magnesium Ions Battery Capacitor

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