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H2V3O8 as a High Energy Cathode Material for Nonaqueous Magnesium-Ion Batteries Mohadese Rastgoo-Deylami, Munseok S Chae, and Seung-Tae Hong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01381 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Chemistry of Materials

H2V3O8 as a High Energy Cathode Material for Nonaqueous Magnesium-Ion Batteries Mohadese Rastgoo-Deylami, Munseok S. Chae, and Seung-Tae Hong* Department of Energy Science and Engineering, DGIST (Daegu Gyeongbuk Institute of Science and Technology), Daegu 42988, South Korea

ABSTRACT: Magnesium-ion batteries (MIBs) suffer from a low energy density of cathode materials in a conventional nonaqueous electrolyte, contrary to the expectation due to the divalent Mg ion. Here, we report H2V3O8, or V3O7∙H2O, as a high-energy cathode material for MIBs. It exhibits reversible magnesiation-demagnesiation behavior with an initial discharge capacity of 231 mAh g−1 at 60 °C, and an average discharge voltage of ~1.9 V vs. Mg/Mg2+ in an electrolyte of 0.5 M Mg(ClO4)2 in acetonitrile, resulting in a high energy density of 440 Wh kg−1. The structural water remains stable during cycling. The crystal structure for Mg0.97H2V3O8 is determined for the first time. Bond valence sum difference mapping shows facile conduction pathways for Mg ions in the structure. The high performance of this material with its distinct crystal structure employing water–metal bonding and hydrogen bonding provides insights to search for new oxide-based stable and high-energy materials for MIBs.

INTRODUCTION A development of rechargeable batteries with low-cost, high safety and high energy density is necessary for portable and large-scale applications such as electric vehicles, plug-in hybrid electric vehicles, and smart grid. During last decades, lithium-ion batteries (LIBs) have been the most prevalent energy storage devices for mobile applications due to their high energy and power densities. However, the safety concerns, high cost, and low abundance of lithium lead to increasing research activities on new battery chemistry based on magnesium,1 calcium,2 or zinc3 as the carrier ions. Magnesium-ion batteries (MIBs) or magnesium batteries (MBs) have received attention because of the potential benefits of using Mg element, such as its natural abundance in the earth’s crust, low redox potential (– 2.37 V vs. SHE) that enables a high cell-voltage when appropriate anode and cathode materials are combined.1,4 Furthermore, due to the divalency nature of Mg ion, the capacity could be twice the capacity of the best hosts available for the monovalent ion when Mg2+ intercalates into the same number of vacant sites of the host structure and the host can afford the redox electrons. In addition, magnesium has higher melting point than lithium and dendrite-free deposition/stripping properties, making the operation of MIBs or MBs safer than LIBs.5 Discovery of high-energy host materials exhibiting reversible magnesiation and demagnesiation is one of the greatest challenges for the development of practical MIBs. To date, a few materials, such as α-V2O5,6-7 Prussian blue analogs,8 Chevrel phase Mo6T8 (T= S or Se),4,9 MnO2,10 NaV3O8(H2O)1.5,11 Na3V2(PO4)3,12 CoS,13 TiS2,14-15 , TiSe2,16 and MgFeSiO4,17 have been reported as cathode

candidates for nonaqueous MIBs. However, most of these materials suffer from low electrochemical performance as electrode materials. For example, sulfides have low discharge voltages, and the oxides and phosphates have low Mg-diffusion rates, capacities, or cycling stabilities. Layered vanadium oxides have been explored for monovalent ion batteries. Among different types of layered vanadium oxides, hydrated vanadium oxide, H2V3O8 (or V3O7∙H2O), showed its potential as a cathode material for lithium, sodium, and hybrid magnesiumlithium ion batteries.18-20 It was first synthesized by Theobald and Cabala in 1970,21 with its crystal structure determined by Oka et al. in 1990,22 and improved further by Mettan et al. in 2015.23 Figure 1a shows its crystal structure that can be described as a stacking of V3O8 layers along the a-axis, providing interlayer vacant sites. Each V3O8 layer consists of corner- or edge-shared VO6 octahedra and VO5 square pyramids, forming a twodimensional slab in the bc-plane of the structure. Structural water molecules are positioned at both sides of the V3O8 slab, where the water is directly bonded to the vanadium atom of a VO5 polyhedron (green color in Figure 1a) and hydrogen bonds connect two neighboring V3O8 layers. Because of hydrogen bond vibrations, elastic buffer spaces are created between V3O8 layers.18 As a result, the distortion of the unit cell during electrochemical intercalation and de-intercalation of guest ions takes place relatively easily without destroying the crystal structure. In addition, H2V3O8 with a reduced oxidation state (+4.67) shows high electronic conductivity with the vanadium atoms being electrochemical redox centers. With all these properties, H2V3O8 is considered a promising electrode material for rechargeable batteries.

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In this study, H2V3O8 was explored as a cathode material for MIBs using a nonaqueous electrolyte. We presented high magnesium-ion storage performance and evidence for the electrochemical intercalation of Mg ions into H2V3O8 using cyclic voltammetry, X-ray diffraction, elemental analyses, transmission electron microscopy, Xray photoelectron spectroscopy, and thermogravimetry. Fourier electron density analysis was used to determine the positions of the inserted Mg in the crystal structure. Bond valence sum mapping calculations were used to provide additional support for the facile magnesium-ion diffusion pathways in the structure. a

Figure 1. (a) The crystal structure of H2V3O8, with V(1)O6 octahedra (yellow), V(2)O5 (green), and V(3)O5(blue) square pyramids. Hydrogen atoms are bonded to the oxygen atom (purple), which are not shown. The vanadium labels refer to those in Table S1. (b) Powder X-ray Rietveld refinement profile at 25 °C, (Red points: experimental data, Green line: calculated data, Pink line: difference, Black bars: Bragg positions), (c and d) TEM images, and (e) HR-TEM image of H2V3O8 nanowire particles.

EXPERIMENTAL SECTION Synthesis of H2V3O8 nanowires. H2V3O8 nanowires were synthesized via one-step hydrothermal method. First, vanadium oxide (0.237 g, V2O5, ≥ 99.0%, SigmaAldrich) was added to 50 mL deionized water with stirring for 30 min, followed by the addition of polyethylene glycol (0.040 g, M.W. 4000, Alfa Aesar) with stirring for another 30 min. Then, hydrogen peroxide (10 ml, 30%, extra pure grade, Duksan Pure Chemical, Korea) was slowly added to the solution with stirring for 24 h at room temperature. The final solution was transferred to a Teflon-lined stainless-steel autoclave (100 ml) and heated at 180 °C for 60 h. Finally, the H2V3O8 crystal precipitates were collected by filtration, washed with deionized water and absolute ethanol several times, and dried at 80 °C for 4 h. Materials Characterization. To analyze the morphology and elemental compositions of the samples,

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field-emission transmission electron microscopy (FETEM, Hitachi HF-3300) with an energy dispersive X-ray spectrometry (EDX) attachment, high-resolution fieldemission scanning electron microscopy (HR FE-SEM, Hitachi SU-8020), and inductively coupled plasma analysis (ICP, Varian 700-ES) were used. The amounts of structural water in the samples after the discharge and charge processes were analyzed by thermogravimetric analysis (TGA, Rigaku TG 8120). X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) analysis was performed for the initial, charged, and discharged electrodes. Electrochemical Characterization. For the electrochemical measurements, the working electrodes consisted of H2V3O8 powder, conductive carbon (Super P carbon black, Timcal Graphite & Carbon), and a poly(vinylidene fluoride) binder (W#1300, Kureha Co.) with a 8:1:1 weight ratio, which were mixed and dispersed in dried N-methyl-2-pyrrolidone (NMP), coated on Al foil (~20 μm, as the current collector) and dried at 80 °C in an oven for 10 h to remove NMP. Then, to create a good contact between the active materials and Al foil, the electrodes were pressed by an electrode rolling press (Wellcos Co., Korea). The loading of the active material and the porosity of the electrodes were around 1.6 mg and 62%, respectively, with an electrode area of 1.53 cm2. A glass fiber (GF/A, Whatman) and 0.5 M magnesium perchlorate (Mg(ClO4)2, ACS reagent grade, Alfa Aesar) in acetonitrile (AN, 99.8%, Samchun, Korea) were used as a separator and electrolyte, respectively. A thick activated-carbon pellet (1 g, SX-plus) and a Ag/Ag+ electrode was used as the counter and reference electrodes, respectively. The water content of the electrolyte was 48 ppm according to the water analysis (Metrohm, 831 KF coulometer). Cyclic voltammetry (CV) and galvanostatic discharge– charge measurements were performed using a homemade cell (Figure S1) and EC-Lab software on a Biologic VMP3 multichannel potentiostat (Biologic Science Instruments SAS). Structural Analysis. Powder X-ray diffraction (XRD) data were collected at 25 °C using an X-ray diffractometer (Rigaku Miniflex 600) with a Cu X-ray tube (λ = 1.5418 Å), a secondary graphite (002) monochromator, and an angular range of 5° ≤ 2θ ≤ 80°. The crystal structures for H2V3O8 were refined using the powder profile refinement program GSAS,24 where the initial structural models for H2V3O8 were adopted from previous reports.22 For higher-resolution data to determine and refine the inserted magnesium positions of Mg0.97H2V3O8, a PANalytical Empyrean X-ray diffractometer was used with a Cu Kα1 X-ray (λ = 1.5406 Å) with a primary Ge (111) monochromator, a positionsensitive PIXcel3D 2×2 detector, an angular range of 5° ≤ 2θ ≤ 150°, a step of 0.013000°, and a total measurement time of 12 h at room temperature. The crystal structure determination of the magnesium-


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Figure 2. (a) Initial galvanostatic discharge–charge profiles for H2V3O8 in 0.5 M Mg(ClO4)2 in acetonitrile at 25 °C and 60 °C at a current density of 10 mA g−1, (b) initial discharge–charge profiles at various current densities, (c) galvanostatic cycling performance at various current densities, and (d) galvanostatic cycling performance at 40 mA g−1 for 100 cycles at 60 °C (blue: charge capacity; black: discharge capacity; Coulombic efficiency is defined as Qch/Qdisch).

inserted-phase Mg0.97H2V3O8 was performed using a combination of the powder profile refinement program GSAS and the single-crystal structure refinement program CRYSTALS25 in a similar way to that described in our previous works.26-27 For a 3D view of the Fourier density maps, MCE was used.28 Le Bail fitting was carried out for the Mg0.97H2V3O8 phase with a H2V3O8-like structural model, where the cell parameter was only slightly different from that of the H2V3O8. The structure factors were extracted and used as input data for CRYSTALS. The position of magnesium was then confirmed from the electron-density maps (Fourier and difference Fourier maps), completing the structural model for Mg0.97H2V3O8. Finally, the Rietveld refinement was applied. Bond Valence Sum Maps. The 3D bond valence sum difference maps (BVS-DMs)29 calculation was performed with the code 3DBVSMAPPER,30 which was written in the Perl script language, in Materials Studio.31 The absolute values of the difference |Δv| between the calculated valence of the magnesium ion at each point on a 3D grid within the unit cell and the ideal valence of 2 are plotted as isosurfaces, such that the plausible diffusion pathways can be graphically visualized, where points less than 1.6 Å away from the oxygen atoms were excluded for the calculation. RESULTS AND DISCUSSION Characterization of the synthesized H2V3O8. Powder X-ray Rietveld refinement was performed to confirm the

crystal structure of the synthesized H2V3O8 (orthorhombic, space group Pnma, a = 16.844 (1) Å, b = 3.625 (1) Å, c = 9.309 (1) Å), as shown in Figure 1b. The refined atomic parameters and interatomic distances are presented in Tables S1 and S2, respectively. There are no noticeable impurities in the XRD pattern with sharp peaks indicating high crystallinity. TEM images of H2V3O8 particles (Figures 1c-d and S2) show a nanowire morphology with a diameter of 100–200 nm. The nano-size of the particles can be beneficial to the diffusion of magnesium ions in the structure of H2V3O8 during the discharge–charge process. The interplanar distance of 0.34 nm in Figure 1e corresponds to d(011) spacing of H2V3O8, confirming the structure. The EDX elemental mappings also verified a homogenous distribution of vanadium and oxygen in the H2V3O8 particle (Figure S3). The hydrogen or water content of H2V3O8 was determined by TGA measurement in the range of 25–400 °C with 5 °C min−1 rate under N2 flowing atmosphere. The observed weight loss (6.37%) corresponds to one equivalent of molecular water per formula unit (Figure S4), which is consistent with the chemical formula V3O7∙H2O. Electrochemical Magnesium Storage Performance. The discharge–charge performance for H2V3O8 was investigated at two different temperatures: ambient (25 °C) and elevated (60 °C). Figure 2a shows that the specific capacity of H2V3O8 is 80 mAh g–1 at 25 °C with a current density of 10 mA g−1, and it increases dramatically to 231 mAh g−1 at 60 °C. Such an increased capacity at elevated



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Figure 3. (a) Cyclic voltammograms of H2V3O8 at various scan rates for H2V3O8 in 0.5 M Mg(ClO4)2 in acetonitrile at 60 °C. (b) Log-log plot of cathodic peak current dependence on the scan rate. (c) Plot of cathodic peak current (i/v½) dependence on the square root of scan rate (mV s−1)½. (d and e) Contribution ratio of the diffusion-controlled intercalation and surfacelimited capacitive reaction as a function of the cyclic voltammogram scan rate for reduction peak 1 (d), and oxidation peak 2 (e).

temperature (e.g., 60 °C) was also observed in other magnesium host materials, like cubic and layered TiS2,14-15 where the capacities at ambient temperature were negligible. Considering that 1.0 e− charge transfer corresponds to 94.75 mAh g−1 per formula unit of H2V3O8, the capacity of 231 mAh g–1 corresponds to 2.44 e– charge, and the chemical formula for the discharged electrode at 60 °C is Mg1.22H2V3O8. The capacity is reversible, as it is almost maintained for the second cycle onward at a current density of 10 mA g–1 (Figure 2c). That reversible capacity is relatively high among the cathode materials for Mg batteries: ~160 mAh g–1 at 60 °C for both cubic and layered TiS2,14-15 and 80 mAh g–1 at 25 °C for Chevrel phase.4 However, it is lower than the value (304.2 mAh g−1) recently reported for the same material, H2V3O8.20 We have verified that the reason for the difference in capacity comes from the amount of water content in the organic electrolyte. The capacity of 80 mAh g–1 at 25 °C in the relatively dry electrolyte increased to ~260 mAh g–1 in a wet electrolyte (Figure S5), where the water content was 48 ppm and 5,790 ppm, respectively. Such a significant increase in capacity in a wet organic electrolyte was also observed in α-V2O5 due to co-intercalation of Mg ions and protons.7 The sloping curve between 1.0 and −1.4 V (vs. AC) indicates a single-phase-like Mg insertion mechanism (Figure 2a), which was also confirmed by structural analysis (vide infra). The average discharge voltage is ~1.9 V vs. Mg/Mg2+ (−0.7 V vs. AC). The AC reference electrode voltage was estimated to be 2.6 V vs. Mg/Mg2+, by comparing the CV curves using the AC reference electrode with that using the Ag/Ag+ reference electrode; the Ag/Ag+ electrode voltage was calibrated with the ferrocene/

ferrocenium redox couple (Figure S6a, b, and c). The specific energy density of the cathode material is 440 Wh kg−1 in a nonaqueous electrolyte: twice that of thiospinel,14 and four times that of Chevrel phase.4 The rate capability of H2V3O8 at different current densities is presented in Figures 2b and 2c, indicating reduced capacities with increasing current density. The discharge capacities of H2V3O8 are 231, 201, 170, and 97 mAh g−1 at 10, 20, 40, and 80 mA g−1, respectively. In addition, the discharge capacity remained 220 mAh g−1 at 10 mA g−1 over 30 cycles at various current densities. Figure 2d presents the cycling performance of H2V3O8 for 100 cycles at 40 mA g−1. The capacity slowly decreases to 132 mAh g−1 (77% of capacity retention) with the coulombic efficiency of each cycle above 98%. The XRD pattern of the electrode after 100 cycles showed that the intensity of the whole pattern was slightly reduced, but the crystallinity of the original structure was almost maintained, demonstrating high stability of the H2V3O8 structure (Figure S7) for reversible magnesiation at 60 °C. One of the possible mechanisms for the capacity reduction could be dissolution of the host material during cycles, as evidenced by the observed vanadium atoms (4.2 ppm) in the ICP elemental analysis of electrolyte solution even after 10 cycles. A comprehensive analysis should be performed to understand the capacity fading mechanism as a further study. Magnesium Storage Mechanism. Cyclic voltammograms of H2V3O8 were recorded at 60 °C with various scan rates from 0.1 to 0.3 mV s−1 to determine the magnesium storage mechanism of the electrochemical reaction in the voltage range between −1.5 and 1 V vs. AC (Figure 3a). There is one pair of reduction and oxidation peaks. The oxidation peak is at 0.49 V vs. AC at 0.1 mV s−1,


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Figure 4. (a) (Mg/V) atomic ratios for MgxH2V3O8 electrode samples during the discharge–charge cycle, where the x values were estimated from ICP, FE-SEM EDX and HR-TEM EDX elemental analyses (see details in Tables S3–S5). Fe-TEM EDX elemental mapping of (b) initial (x = 0), (c) discharged (x = 0.97), and (d) charged (x = 0) electrodes.

but the reduction peak is too broad to define a precise potential, suggesting that the magnesiation reaction is sluggish, whereas the demagnesiation reaction is relatively fast (Figure S8). Two typical mechanisms may explain the magnesium storage reaction in this work: (i) bulk diffusion-controlled intercalation and (ii) surface-controlled capacitive reactions. The intercalation reaction generally involves structural changes that can be clearly observed. Due to the nanosize, i.e., large surface area, of H2V3O8 nanowires, the contribution from the capacitive reaction might not be negligible. According to the power-law relationship between the current and scan rate, expressed as i = ɑνb , (1) where i is the measured current (A), v is scan rate (mV s−1), a and b are adjustable parameters—b = 0.5 represents an intercalation reaction, and b = 1 indicates surface-redox (pseudocapacitive) or surface-adsorption (electrostatic double-layer capacitive) reactions. Using log(v) vs. log(i) plots, the b-values were calculated as 0.60 and 0.61 for the reduction and oxidation peaks, respectively (Figure 3b), suggesting a mechanism combining both intercalation and surface reactions. The contribution ratios between intercalation and surface reactions were calculated by the equation32 i/v1/2 = a1v1/2 + a2, (2) where a1v and a2v1/2 correspond to the surface and intercalation reactions, respectively, and a1 and a2 can be is observed in the particles of the discharged electrode, which is absent in the initial or charged electrodes (Figures 4b–d). Whether the structural water of the host material (V3O7∙H2O) is maintained during cycles is a great concern because it could affect the stability of the material, and eventually the cyclability performance. To analyze the water content in H2V3O8, TGA analysis was performed for

determined from i/v1/2 vs. v1/2 plots (Figure 3c). The proportions of the intercalation reaction at various rates are presented for the reduction peaks (Figure 3d) and the oxidation peaks (Figure 3e). Around 80–83% of the magnesium ions are intercalated at a scan rate of 0.10 mV s−1, and the rest (17%) are surface ions that are mostly pseudocapacitive because electrostatic double-layer capacitance is insignificant due to low specific area of the material. When increasing the scan rate to 0.30 mV s−1, the surface capacitance slowly increases. These results clearly show that the diffusion-controlled intercalation reaction is the predominant Mg-storage mechanism for H2V3O8. Elemental Analyses as Evidence of Mg Intercalation. EDX FE-SEM, EDX HR-TEM, and ICP analyses were conducted to quantify the magnesium content in H2V3O8 particles during the electrochemical magnesiation– demagnesiation process (Figure 4 and Tables S3–S5). The analyses for the discharged electrode after thorough washing showed 0.97 mole of magnesium ions per formula unit of H2V3O8, suggesting the chemical formula as Mg0.97H2V3O8 (Figure 4a). The content of magnesium is about 80% of that calculated from the discharge capacity (Figure 4a, 231 mAh g−1 at 10 mA g−1, or Mg1.22H2V3O8). The value of ~80% matches well with the portion (80–83%, Figure 3d) of the bulk-intercalated ions in the total magnesium ions stored, suggesting that only the bulkintercalated magnesium ions remained while the surface ions were washed out during the sample preparation process for the elemental analyses. A homogenous distribution of magnesium ions three samples: the pristine electrode, the discharged electrode to −1.5 V (vs. AC), and the recharged electrode to 1 V at 60 °C. The weight losses observed in TGA plots (Figure S9) were almost the same, confirming that the extracted water was negligible during the electrochemical cycle. X-ray Photoelectron Spectroscopic Analysis. The existence of magnesium and the oxidation state of



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a c Figure 6. (a) (010) view of the observed Fourier map for Mg0.97H2V3O8. The map width and thickness are 7 Å and 2 Å, respectively with the center of map at (0.124, 0.250, 0.322). (b) The powder X-ray Rietveld refinement profile for Mg0.97H2V3O8, recorded at 25 °C. Red points: experimental data; green line: calculated data; pink line: difference; bars: Bragg positions for Mg0.97H2V3O8 (black) and Al (red). (c) The crystal structure of Mg0.97H2V3O8, with V(1)O6 octahedra (yellow), V(2)O5 (green), and V(3)O5(blue) square pyramids. Mg–oxygen bonds are drawn with Mg represented by the blue spheres. Hydrogen atoms are bonded to the oxygen atom (O6, purple), but are not shown. (d) Local environment around the inserted Mg site. The atom labels refer to those in Table S6.

vanadium during the electrochemical cycle were determined by XPS analysis. The Mg 1s peak appears at 1304.5 eV for the discharged samples (Figure 5a), where the peak intensity increases in proportion to the depth of

discharge, and almost disappears for the recharged sample. These results also confirmed reversible magnesiation and demagnesiation during the electrochemical cycle, conforming to the previous elemental analyses. The insignificant but observable weak


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Figure 7. Ex-situ XRD patterns of the MgxH2V3O8 (0 ≤ x ≤ 0.97) electrodes that were electrochemically prepared at the AC/0.5M Mg(ClO4)2 in AN/H2V3O8 cell during the first cycle in a voltage range between −1.5 to 1 V vs. AC at 60 °C. Pink: discharge (magnesiation), green: charge (demagnesiation).

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Mg0.97H2V3O8

Figure 8. (a) Evolution of unit cell parameters (a, b, and c), and volume, and (b) the local structure around vanadium atoms and average (V–O) interatomic distances in MgxH2V3O8 (0 ≤ x ≤ 0.97) upon magnesiation. peak in the recharged sample at point 11 indicates that a small amount of magnesium was not completely extracted from the sample charged to 1 V vs. AC. The XPS spectra exhibit V 2p spin-orbit spectra, with binding energies of V5+ 2p1/2, V4+ 2p1/2, V5+ 2p3/2, and V4+ 2p3/2 at 524.6, 523.4, 517.5, and 516.3 eV, respectively (Figure 5b). The V5+ 2p3/2 intensity decreases during the first discharge and increases in the subsequent charge, whereas that of V4+ 2p3/2 exhibits the opposite behavior, in good agreement with the electrochemical results. Structure Determination of the Magnesiated Phase. The crystal structure of Mg0.97H2V3O8 was determined and refined for the first time in this study, where the intercalated Mg position was identified by Fourier electron-density map calculation, as shown in Figure 6a. The Rietveld refinement profile is presented in Figure 6b (see Figure S10 for a full 2θ range). The crystal structure for the final structural solution is presented in Figure 6c. The details of the refined parameters and interatomic distances are summarized in Tables S6 and S7,

respectively. It is clearly shown that the inserted Mg ions are located in the cavity sites of the interlayer between V3O8 layers. Figure 6d shows the local environment of the intercalated Mg atom that is bonded to five oxygen atoms (O3, O5, O8, and two O6 atoms) with interatomic Mg–O distances of 2.125, 2.321, 1.941, and 2.228 Å, respectively. It forms a square pyramidal geometry of MgO5 with a Mg atom at the center. Evolution of Crystal Structure During Cycle. The series of ex-situ powder XRD patterns were recorded for the MgxH2V3O8 (0 ≤ x ≤ 0.97) electrode samples taken at various points during the galvanostatic discharge–charge cycle at 60 °C (Figure 7). The patterns clearly change during the magnesiation/demagnesiation process and show a reversibility of the structure for a complete cycle. The (200) peak shifts to higher angles with magnesiation indicating a reduction of the interlayer distance due to an attractive interaction between the positively charged magnesium ions and the surrounding negatively charged oxygen atoms. The (301), and (502) peaks also shift



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slightly to the right in the first discharge (Figures 7a, b, and c, respectively), and all return back—but not completely— to the initial angles. The incomplete reversibility is consistent with the XPS results where a small amount of magnesium is observed in the charged sample (Figure 5a). It is also noted that the XRD pattern for the fully discharged electrode at 25 °C (Figure S11) is similar to the pattern for the partially discharged electrode at 60 °C (point 2 in Figure 7), suggesting that the fully discharged phase at 25 °C is equivalent to a partially discharged phase at 60 °C. The variations in unit cell parameters, volume, and interatomic distances of (V–O) bond were determined using Rietveld refinement (x = 0 and 0.97) or Le Bail fit (the other x values) of the powder XRD data, as presented in Figure 8. With magnesiation during discharge, the unit cell parameters change anisotropically (Figure 8a). The aaxis decreases by 3.85% (16.845 to 16.196 Å), indicating the decrease in the interlayer distance, whereas the b-axis increases by 4.22 % (3.625 to 3.778 Å) and the c-axis increases by 0.17% (9.309 to 9.325 Å). Overall, the unitcell volume increases slightly by 0.35% (568.5 to 570.5 Å3). It is also noted that the reduction in vanadium oxidation state leads to a relatively large increase in the average V–O interatomic distances (Figure 8b): V1–O (from 1.93 to 2.01 Å), V2–O (1.89 to 1.94 Å) and V3–O (1.82 to 1.95 Å). Bond Valence Sum Difference Maps (BVS-DMs) Calculation. The BVS-DMs are generally used to find plausible locations for intercalated ions and to probe ion conduction pathways in inorganic materials. Thus, a threedimensional (3D) BVS-DM calculation was performed using the crystallographic information derived from the XRD Rietveld refinement (Table S6). Figure 9 shows BVS-DM isosurfaces at |Δv| = 0.2 valence units (v.u.) for Mg ions. Possible magnesium-ion-diffusion pathways are noted within the interlayer, providing additional support for the electrochemical magnesium intercalation into H2V3O8. Remarks on a cell test using Mg metal anode. Because Mg metal is not reversibly deposited/stripped in AN-based electrolytes, an ethereal electrolyte, magnesium bis(trifluoromethylsulfonyl)imide in triglyme (1:5 molar ratio), and Mg metal anode were used to test an electrochemical performance of H2V3O8. The electrolyte is known to be stable up to 3.2 V vs Mg metal.17 Unfortunately, the cell does not show a reversible behavior (Figure S12). A further systematic investigation should be made to develop an appropriate electrolyte that are compatible to both Mg metal and H2V3O8.

crystal structure is highly stable during electrochemical cycling at 60 °C, maintaining the initial water content. CVs, galvanostatic cycles, XPS, powder XRD, elemental analyses, and BVS-DM calculations unequivocally show evidence of reversible magnesiation of H2V3O8 and a facile conduction pathway network for magnesium ions. H2V3O8 is among the high-energy MIB cathode materials using a nonaqueous electrolyte. The high cycling stability of the material appears to come from its unique crystal structure with direct bonding between structural water to transition metal and hydrogen bonding connecting neighboring layers. It is distinct from crystal-watercontaining structures lacking in such a direct water-metal bonding. The importance and role of water-metal and hydrogen bonding should be investigated further. In addition, to make H2V3O8 useful as a cathode material for MIBs, anode materials that behave reversibly with the electrolyte used in this work and/or other electrolytes that are compatible with both H2V3O8 and Mg anodes should be developed as a further study. These results provide general insights into some of the important factors governing the high energy capacity and stability of a cathode material for MIBs and open up possibilities to discover new oxide materials with high performance.

CONCLUSION H2V3O8 (= V3O7∙H2O) has been demonstrated as a highenergy cathode material for magnesium batteries, exhibiting an initial capacity of 231 mAh g−1 at a current density of 10 mA g–1 with an average discharge voltage of 1.90 V vs. Mg/Mg2+, and the energy density of 440 Wh Kg−1 in an electrolyte of 0.5 M Mg(ClO4)2 in AN at 60 °C. The

Figure 9. 3D bond valence difference map iso-surfaces (light blue) for H2V3O8 with the iso-surfaces of |0.2| valence units (Oxygen atoms: red balls; V(1)O6 octahedra: yellow; V(2)O5 square pyramids: green; V(3)O5 square pyramids: blue).

a c

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Scheme of the cell, additional SEM images, TEM-EDX mapping, TGA results, galvanostatic profile with a wet organic electrolyte, reference electrode calibration CVs, XRD patterns after 100 cycles, ICP, TGA and EDX results, Rietveld refinement results for H2V3O8 and Mg0.97H2V3O8, cell test result using Mg metal anode.

Corresponding Author *E-mail: [email protected]. Fax: +82 53 785 6409.

ORCID Seung-Tae Hong: 0000-0002-5768-121X Mohadese Rastgoo-Deylami: 0000-0001-6711-5249 Munseok S. Chae: 0000-0002-4450-0846

ACKNOWLEDGMENT This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D1A1069707). We also thank the support from LG Chem.

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Page 11 of 11 SYNOPSIS TOC

H2V3O8

V(2)O5

0.5 M Mg(ClO4)2 in acetonitrile

1.0

3.6

0.5

3.1

0.0

2.6

−0.5

2.1

−1.0

1.6

−1.5

1.1 0

50

100

150

Capacity (mAh

200

g−1)

250


Voltage (V) vs. AC

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


Water

V(1)O6

V(3)O5 Mg

a c

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H2V3O8 as a High Energy Cathode Material for ... - ACS Publications

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