Unravelling the Surface Structure of MgMn2O4 Cathode Materials for

Jul 10, 2017 - The spinel MgMn2O4, a cathode material with theoretical capacity of 272 mA h g–1, holds promise for future application in high volume...
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Unravelling the Surface Structure of MgMn2O4 Cathode Materials for Rechargeable Magnesium-Ion Battery Quang Duc Truong,*,† Murukanahally Kempaiah Devaraju,† Phong D. Tran,‡ Yoshiyuki Gambe,† Keiichiro Nayuki,§ Yoshikazu Sasaki,§ and Itaru Honma*,† †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aobaku, Sendai 980-8577, Japan Department of Advanced Materials Science and Nanotechnology, University of Science and Technology of Hanoi, 18 Hoang Quoc Viet, Cau Giay, Hanoi 100000, Vietnam § Field Solution Division, JEOL Ltd., 1156 Nakagami, Akishima, Tokyo 196-0022, Japan ‡

S Supporting Information *

ABSTRACT: The spinel MgMn2O4, a cathode material with theoretical capacity of 272 mA h g−1, holds promise for future application in high volumetric magnesiumion batteries. Atomic-resolution imaging of the structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn2O4 for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO2 was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/ Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries.

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retention of the bulk MgMn2O4 spinel phase. This new finding provides a critical step toward understanding the intrinsic structure and control of the electrochemical properties of spinel MgMn2O4 oxide in a Mg-ion battery.

INTRODUCTION The increasing demand for higher energy density devices for future applications as in hybrid electric vehicles (HEV) or electric vehicles (EV) and portable electronics (laptops and smartphones) is challenging that request to development of novel energy storage systems beyond Li-ion technology.1−3 Among them, the rechargeable magnesium battery has attracted attention owing to the high natural abundance of Mg and its safety, high specific capacity (2205 A h kg−1), and especially high volumetric energy density (3833 mA h cm−3).4−6 The MgMn2O4 spinel oxide is an attractive electrode material for Mg-ion batteries thank to its superior specific capacity (272 mA h g−1) and high energy density (1000 W h kg−1).7−9 The structural investigation by advanced electron microscopy on the surface structure of LiMn2O4 spinel oxide during the electron beam irradiation or electrochemical cycling reveals the relocation of transition metal ions as Frenkel defect, phase transition, and surface reconstruction, which may initiate the capacity fade during charge/discharge cycling.10−13 Thus, understanding the surface structure of the MgMn2O4 spinel oxide is particularly important for elucidating its charge− discharge behavior and further boosting the energy density of the Mg-ion batteries. In this work, we report on an investigation of the surface structure of magnesium manganese spinel oxide (MgMn2O4) using spherical aberration-corrected scanning transmission electron microscopy (STEM) and show the existence of a thin stable rocksalt MgMnO2 phase with © 2017 American Chemical Society

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RESULTS AND DISCUSSION The MgMn 2 O 4 nanocrystals were synthesized by the amorphous metal complex method with annealing at 650 °C for 12 h. The electrochemical properties of MgMn2O4 in a Mgion battery were performed in a coin cell on multichannel battery testers (Hokuto Denko, Japan) as described elsewhere.14 MgMn2O4 was used as cathode by mixing with acetylene black and poly(tetrafluoroethylene) (PTFE) binder. The three-electrode Mg-ion cells were used with Maxsorb MSC-30 nanoporous activated carbon as counter and quasireference electrode (QRE). The electrolyte consists the solution of 0.5 M Mg(ClO4)2 in acetonitrile, a solvent with high oxidation resistance ability. The activated carbon enables reversible charge transport via electrochemical double layer capacitance (EDLC) due to its high surface area.9,15 The cyclic voltammograms (CVs) of the cell containing MgMn2O4 electrode are shown in Figure 1a. The CV curves show a broadened oxidation peak at 3.3 V and one reduction Received: March 28, 2017 Revised: July 10, 2017 Published: July 10, 2017 6245

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cations Mg and Mn occupy tetrahedral and octahedral sites, respectively, and their oxide ions can be organized into cubic spinel struture in space group Fd3̅m with a lattice parameter of a = 8.52 Å. Figure 2a (and Figure S2) illustrates the MgMn2O4 cubic spinel with Fd3m ̅ symmetry, regarded as the phase in this study. The structure is composed of interconnected chains of edge-sharing MnO6 octahedrals, cross-linked by corner-sharing single MgO4 tetrahedrals. Annular bright-field/high-angle annular dark-field (ABF/ HAADF) imaging was performed on an aberration-corrected JEM-ARM200F, equipped with a cold field emission electron gun. The camera length was 6 cm. The convergence semiangle for the incident probe was set to 29 mrad. Most of the ADF images were collected for a half-angle range of 90−370 mrad.14,16 During the observation of the synthesized particles by STEM, we observed the cubic spinel phase. For STEM imaging, the sample was tilted to be viewed along the [110] axis of the cubic spinel structure where like-atom columns are parallel to the electron beam. Panels b and c of Figure 2 show HAADF and ABF images viewed along the [110] direction of the MgMn2O4 particle crystallizing with the space group Fd3̅m. The two-dimensional atomic arrangement of a unit cell is overlaid on the image for the comparison. The ADF image contrast roughly correlates to with atomic number, according to a Z1.7 relationship. Thus, the periodically arrayed brighter contrasts in Figure 2b exhibit the Mn columns and the dimmer contrasts are sites of Mg and O atoms. Eight Mn columns form with each other a diamond configuration in which the number of Mn atoms in Mnα columns (brighter contrast) are twice that of Mnβ columns (see quantitative analysis of the image intensity in Figure 2iii). Two Mg columns can be clearly seen at tetrahedral sites. Oxygen atoms (Z = 8) are barely visible in the HAADF image. Nevertheless, the oxygen columns can be visualized in a line profile (Figure 2i). The image intensity line profiles show the spatial distribution of Mn, O, and Mg in the structure with two Mg columns separated by ∼2.16 Å at the center. Figure 2c shows an ABF image of the same region with line profiles in Figure S3. Similarly, the Mn columns can be visualized directly as strong dark contrasts. In the ABF image, the contrast varies as the Z1/3 relationship, in comparison with the Z1.7 dependence of the ADF image. Thus, light atoms (Mg/ O) show much higher intensity and are clearly resolved from each other. The dark contrasts of periodic Mn, Mg, and O columns arrange into a pattern matching well with the corresponding structure shown schematically in the inset of Figure 2c. The EDS spectra and EDS elemental mapping in Figure 2d,e confirm that the nanocrystal is truly MgMn2O4 and how its homogeneous elemental distribution is in the particle. More information in the different-scale mapping can be found in Figure S4. Observation of Rocksalt MgMnO2. We now investigate the surface structure of the MgMn2O4 nanocrystals. Figure 3a displays the HAADF-STEM images with visualization of both the bulk and the surface. We observed a distinct area with different atomic ordering, located at the surface of the nanocrystal. The bulk of the crystal shows diamond configuration of the spinel framework (Figure 3b); however, the surface exhibits clearly a diamond configuration with visible contrast at center octahedral sites and the absence of contrast at the tetraheral sites (Figure 3c). The contrast intensity at the Mnα column is significantly reduced, even equal to that at the Mnβ column, while contrast at center octahedral sites gradually becomes brighter. At some diamond, we critically observed

Figure 1. (a) Cyclic voltammogram (CV) curves recorded on a spinel MgMn2O4 electrode in coin cells at potential scan rate of 0.1 mV s−1 using Maxsorb MSC-30 nanoporous carbon on stainless steel as the anode and 0.5 M Mg(ClO4)2 in ACN as the electrolyte. The bottom axis is the measured voltage versus carbon while the top axis is the estimated voltage quoted against Mg/Mg2+ couple. (b) Five galvanostatic charge/discharge cycles of spinel oxide at a rate of 0.0139 C. The right axis is the estimated voltage quoted against Mg/ Mg2+ couple.

peak centered at 2.8 V versus Mg/Mg2+. The CV curves were identically reproduced at subsequent cycles, suggesting the good cyclic performance of the cell. The spinel MgMn2O4 exhibited a wide voltage plateau at around 2.8 V versus Mg/ Mg2+ with an initial discharge capacity of 97 mA h g−1 at a current rate of 0.0139 C (Figure 1b). Due to the sluggish intercalation/deintercalation kinetics of Mg ions within the crystal lattice, the small C rate was used. Interestingly, the specific capacity increased continuously cycle by cycle and reached 169 mA h g−1 after 27 cycles. At the eighth and 17th cycles, the discharge capacities were determined to be 135 and 154 mA h g−1, corresponding to the insertion of 0.5 and 0.57 Mg atoms per formula unit, respectively. The extraction of bivalent Mg ions may introduce a severe structural deformation that requires many electrochemical cycles for gradual extraction/insertion of ions to accommodate the structural change. Thus, the gravimetric charge/discharge capacities appear to be not high in initial cycles but increased continuously cycle to cycle. During the electrochemical cycling, the average Coulombic efficiencies progressively increased and reached 99%. (Supporting Information Figure S1). After 30 cycles, the charge/discharge capacities become gradually stable at 171 mA h g−1. Atomic Structure of MgMn 2 O 4 . The magnesium manganese spinel oxide composes of tetragonally packed oxide Mg ions and octagonally packed oxide Mn ions. The 6246

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Figure 2. STEM characterization of MgMn2O4. (a) Schematic illustrations for the MgMn2O4 cubic structure viewed along [110] projection showing separated columns of Mg, Mn, and O atoms. (b) HAADF image of MgMn2O4 particle viewed along [110] direction. (c) ABF image of MgMn2O4 particle viewed along [110] direction. The superimposed atomic arrays on the inset indicate the locations of atom columns. Scale bar, 5 Å. (d) STEM image and elemental mapping. (e) EDS spectra showing pristine MgMn2O4. (i, ii, iii) Intensity line profiles taken along the highlighted lines in panel b.

lower Mn oxidation state than the bulk counterparts with oxygen vacancy. It was reported that Li+ can be extracted to leave Mn4+ in the spinel framework [Mn2]O4.10 Thus, it can be deduced that the stable surface layer of rocksalt is formed by the reconstruction of the spinel phase via the disproportionation reaction 2Mn3+ = Mn4+ + Mn2+. In this manner, Mg atoms were extracted from the spinel leaving Mg1−xMn2O4 with Mn4+ in the framework [Mn2]O4. Such Mg atoms were inserted into 16c vacancies of another cell at the surface to form rocksalt MgMnO2 containing Mn2+. Detailed analysis of HAADF images presented in Figure 5 reveals weak contrasts at the center of the diamond configuration of the rocksalt phase at the surface. It is noted that the atom density is the same in transition metal sites along the [110] direction of the ideal rocksalt structure (Figure 3d); the contrasts at the octahedral sites are equal to those of other transition metal columns. However, the image intensity line profiles reveal the atom density at the octahedral sites are ∼10% (Figure 5a) to 50% (Figure 5b) those at the Mnα

equal contrast at three Mn sites (Figure S5). By matching the ADF image with the schematic illustration of atomic arrays presented in Figure 3d, it clearly reveals the presence of a rocksalt structure on the surface of the single crystal of the spinel phase (Figure S6). Formation of Rocksalt. Rocksalt MgMnO2 has the cubic structure with lattice parameter a = 4.336 Å. As shown in Figure 2a and Figure 3d, spinel and rocksalt share the same space group Fd3̅m. MgMn2O4 with a spinel structure can be written as [Mg2+]8a[Mn23+]16d[□]16c[O4]32e (□ represents the vacancies) in which Mn distributes onto octahedral interstitials, 16d sites, in the Wyckoff position, while, in the rocksalt structure, cations occupy both 16c and 16d sites. Thus, the spinel can be transferred to rocksalt by insertion of Mg cations into 16c vacant sites (Figure 4),17 followed by Frenkel-defectmediated relocation of Mg from tetraheral sites to octahedral sites.12 It is known that in LiMn2O4 cathode materials, the disproportionation reaction 2Mn3+ = Mn4+ + Mn2+ occurs at its surface. As a result of charge balance, the surface contains a 6247

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Figure 3. STEM characterization of MgMn2O4 surface. (a) HAADF image of MgMn2O4 particle viewed along [110] direction with visualization of both the bulk and the surface. (b, c) Magnified images shows the spinel phase in the bulk (blue), while a new phase (red diamond) is found at the surface. (d) Schematic illustrations for the rocksalt MgMnO2 structure viewed along [110] axis. Mg/Mn occupy octahedral sites at the center of the diamond. Scale bar, 1 nm.

Figure 5. (a, b) Enlarged one-cell row HAADF STEM images and line intensity profiles taken at the center rows denoted by blue arrows. (c, d) Line intensity profiles taken at the center rows denoted by green arrows, across the cell in panels a and b. The profiles indicate the absence of contrast at the tetragonnal sites, suggesting the overlapping of spinel [Mn2]O4 phase and rocksalt phase along [110] direction (R, rocksalt; S, spinel). See Figure S6 for positions of line profiles.

Figure 4. Schematic illustration of the insertion of Mg cations into 16c vacant sites of spinel to form rocksalt.

columns. It can be deduced that the rocksalt phase is only partly present in this observed area along the [110] direction and the overlapped structure consists of the spinel [Mn2]O4 phase. This observation is consistent with above assumption on the disproportionation reaction. While most of observed area shows the stacking of rocksalt with the spinel [Mn2]O4 phase, we critically observed the stacking region with the spinel MgMn2O4 phase (Figure 6). Theoretical investigation based on molecular-dynamics simulations revealed that the pristine spinel metal manganese oxide surfaces are unstable and therefore a reconstruction is expected.18 To examine the stability of the surface against activation induced by electron beam energy during STEM observation in the current experiment, we applied an electron beam (probe current ∼ 35 pA) on narrow regions of the crystal surface (2.4 × 2.4 nm2) for a certain period of time (90.45 s). While dynamic fluctuation of in the column intensity of tetrahedral 8a sites was observed in Figure 7 and Supporting Information Movie S1, no structural transformation was detected. The stability of the spinel surface under electron

Figure 6. Line profiles indicate the stacking region of rocksalt with spinel MgMn2O4 phase. Red rectangles indicate the stacking region of rocksalt with spinel MgMn2O4 phase. (a) Visible contrast detected at octohedral sites, denoted by green asterisks, indicates the presence of rocksalt. (b) Line profiles taken horizontally at the cells marked by green arrows. Visible contrast detected at tetrahedral sites, denoted by orange asterisks, indicates the presence of spinel phase.

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It was suggested that the presence of rocksalt with different cation ordering reduces the overall electrical conductivity of the material due to electron scattering at the phase interface. Furthermore, in cubic spinel compounds, the Li or Mg ion diffuses along the (110) channel with the lowest energy barrier. The transition in the cation ordering at the surface inevitably disturbs the magnesium-ion diffusion pathway. Therefore, it appears that the presence of rocksalt at the spinel surface in cathode materials inhibits the intercalation reaction and therefore battery performance as we discussed in an early part. The synthesized spinel exhibited medium electrochemical performance with extraction/insertion of 0.63 Mg per spinel formula. To further investigate the chemical composition of surface and bulk, X-ray photoelectron spectroscopy (XPS) measurements were performed (Figure 8). For surface analysis, an XPS Figure 7. (a) HAADF image of spinel surface. Red rectangle indicates the confined region irradiated by electron beam. (b−n) Sequence of HAADF-STEM images showing dynamic fluctuation of in the column intensity of tetrahedral 8a sites, but no structural transformation is observed. Each subsequent image is acquired with a frame time of 7.5375 s with total time of 90.45 s.

beam irradiation reveals that the formation of the rocksalt phase is a native reaction occurring during the synthesis process rather than an electron-beam-induced reconstruction. This native surface layer may arise from long-time annealing at high temperature. The rocksalt surface may be formed due to the side reaction with CO2 and moisture or during the ion milling process in STEM preparation. A rocksalt LiMnO2 phase has been observed on the surface of cathode materials including spinel and layer oxides using advanced electron microscopy.10,13,19,20 When cubic spinel LiMn2O4 is damaged purposefully with a very high electron dose, the LiMn2O4 spinel structure converts into a rocksalt structure.10 By electrochemical cycling of cubic spinel LiMn2O4, migration of transition metal ions into empty octahedral sites forms a rocksalt structure. The surface of layer oxide LiMO2 R3̅m structure also undergoes reconstruction into a rocksalt structure upon electrochemical cycling and electrolyte exposure.19,20 The intercalation of Mg as crystal water into the spinel by electrochemical cycling enables the phase transition to a layered Birnessite structure.21,22 Thus, it does call into question the phase transition of spinel to rocksalt during the electrochemical charge/discharge in magnesiumbased electrolytes. As mentioned above, in the rocksalt phase, the atom densities at the cation sites 16c and 16d were found to be identical, namely, 50% Mg and 50% Mn. This evidence reveals that during the local reorganization of the spinel phase, Mg atoms migrate and occupy the Mn sites. The presence of Mn atom chains along the [110] direction in the spinel phase is energyunfavorable due to strong electrostatic repulsion between highvalence Mn3+ ions. The chains of alternating MgO6 and MnO6 are more thermodynamically stable, thus promoting the interexchange of Mg/Mn cations. This cations exchange disorder, accompanied by Frenkel-defect-mediated relocation of cations, led to formation of a new stable phase, namely, rocksalt. The experimental analysis of the chemical composition of the surface has proved difficult due to the overlap of rocksalt and the [Mn2]O4 spinel framework. Ongoing efforts are being carried out to determine the cation valence on the spinel cathode and the local chemical component.

Figure 8. Mn 2p XPS profiles measured in the bulk (top) and on the surface (bottom) of the sample.

spectrum was record with very small angle, θ = 3°. For the bulk analysis, the sample was etched using an Ar-ion gun for 10 min with the sputtering rate of 1.1 nm/min to remove the surface part. The oxidation state of Mn in the spinel is 3+, so that the Mn 2p is observed at 641.6 ± 0.2 eV in the bulk part with a satellite peak that appears for the 2p1/2 component at 663.7 ± 0.2 eV, whereas surface MgMnO2 shows a Mn 2p core peak of Mn2+ with a 2p3/2 maximum at 640.7 ± 0.2 eV and is characterized by a satellite peak located at 4.5 eV higher. In order to investigate the structural evolution and the role of the rocksalt phase during the electrochemical cycling, the cathode sample after charge/discharge cycles was collected and characterized. The sample on the current collector was disassembled in the glovebox and washed with ACN. The ordered spinel structure was maintained after electrochemical cycling as indicated in high-resolution TEM images (Figure S7). To observe the change in resistance upon cycling, we performed electrochemical impedance spectroscopy (EIS) measurements of the coin cells. Figure 9 shows typical Nyquist plots, indicating the charge-transfer resistance (Rct) of cells before and after cycling to be 894 and 1431 Ω, respectively. The increase in the resistance after cycling reveals the growth of the rocksalt phase inward in the bulk spinel. 6249

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electrode. Cyclic voltammetry measurements were performed at 1 mV s−1 using a Solartron analytical potentiostat.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01252. Electrochemical performance of the sample, more ADFSTEM images, line profiles, EDS elemental mapping, and HRTEM images of sample after electrochemical cycling (PDF) Movie comprised of sequential HAADF-STEM frames showing the dynamic fluctuation of in the column intensity of tetrahedral 8a sites (AVI; frame rates changed for compatibility with media formats and movie repeated 10 times for clarity) (AVI)

Figure 9. Nyquist plots of MgMn2O4 electrodes in cells before and after electrochemical cycling.

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CONCLUSIONS In summary, atomic-resolved imaging of the structure of MgMn2O4 spinel provides insight into the atomic structure and phase transition of a complex spinel oxide. The magnesium intercalation and local atomic rearrangement of Mg/Mn atoms through cation interexchange, as well as Frenkel-defect formation, leads to the phase transformation from spinel to rocksalt. This native surface reconstruction occurred during the synthesis process, opening the opportunity to control the surface structure in order to protect the unstable bulk spinel phase for battery application. The structural analysis in the present work unravels the complex surface reconstruction and facilitates fundamental understanding on spinel cathode oxide for magnesium-ion batteries.

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AUTHOR INFORMATION

Corresponding Authors

*(Q.D.T.) E-mail: [email protected]. *(I.H.) E-mail: [email protected]. ORCID

Quang Duc Truong: 0000-0003-1505-1989 Phong D. Tran: 0000-0002-9561-6881 Notes

The authors declare no competing financial interest.

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METHODS

ACKNOWLEDGMENTS This research work was financially supported by Japan Society for Promotion of Science (JSPS, Grant No. PU15903), Japan, the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), and the Core Technology Consortium for Advanced Energy Devices, Tohoku University, Japan. This work was partially supported by ALCA-SPRING (ALCA-Specially Promoted Research for Innovative Next Generation Batteries) from Japan Science and Technology Agency (JST).

MgMn2O4. MgMn2O4 materials were prepared by amorphous metal complex method. Typically, magnesium nitrate, Mg(NO3)2 (10 mmol, Kanto Chemicals Co., Inc.), and manganese nitrate, Mn(NO3)2 (20 mmol, Kanto), were dissolved in a solution of citric acid (30 mmol, Kanto). After complete dissolution of the starting materials, the obtained solution was heated with continuous stirring at 80 °C for 2 h. Then, the hot plate temperature was set at 120 °C. After 24 h reaction, a gel-like specimens resin was obtained. For pretreatment of samples, the gel-like matter was subjected to further heat treatment. The resulting carbonate precursor was treated at 650 °C for 12 h in air to crystallize spinel oxide. X-ray Photoelectron Spectroscopy. XPS analyses were performed using a ULVAC PHI 500 (Versa Probe II) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. The spectra were collected in the constant pass energy mode at 200.0 eV. Charge referencing was corrected using the adventitious hydrocarbon at a binding energy of 284.6 eV. Electrochemical Measurement. The electrochemical performance of MgMn2O4 was investigated using coin-type cells (CR2032). The working electrodes are composed of 80 wt % MgMn2O4, 10 wt % PTFE (poly(tetrafluoroethylene)) as a binder, and 10 wt % acetylene black. These materials were ground by conventional agar mortar to make an electrode paste. The prepared paste was spread uniformly, rolled into a sheet, and then dried in a vacuum oven for 4 h at 160 °C. The cathode sheet was punched into circular discs and cut into wafers (7 mm in diameter, 0.025 mm in thickness, and 0.5 mg). The capacitive anode was prepared by mixing 80 wt % Maxsorb MSC-30, 10 wt % PTFE (poly(tetrafluoroethylene)) as a binder, and 10 wt % acetylene black. The tested cells were assembled inside an argon-filled glovebox. The cathode and anode electrodes were separated by a microporous polypropylene film. The electrolyte consists of the solution of 0.5 M Mg(ClO4)2 in acetonitrile. The charge−discharge cycling was performed galvanostatically between −1.0 and 1.0 V versus C on multichannel battery testers (Hokuto Denko) at charge/ discharge rate of 0.0139 C. Current densities and specific capacities were calculated on the basis of the weight of MgMn2O4 cathode in the

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