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Investigation of the Mechanism of Mg Insertion in Birnessite in Non-aqueous and Aqueous Rechargeable Mg-ion Batteries Xiaoqi Sun, Victor Duffort, B. Layla Mehdi, Nigel D. Browning, and Linda F. Nazar Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03983 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

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Chemistry of Materials

Investigation of the Mechanism of Mg Insertion in Birnessite in Non-aqueous and Aqueous Rechargeable Mg-ion Batteries

Xiaoqi Sun,† Victor Duffort,† B. Layla Medhi,‡ Nigel D. Browning,‡ Linda F. Nazar*,† †

Department of Chemistry, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

‡

Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory,

Washington 99354, United States

ABSTRACT The magnesium battery is an energy storage system that potentially offers high energy density, but development of new high voltage cathode materials and understanding of their electrochemical mechanism is critical to realize its benefits. Herein, we synthesize the layered MnO2 polymorph (the birnessite phase), as a nanostructured phase supported on conductive carbon cloth, and compare its electrochemistry and structural changes when it is cycled as a positive electrode material in a Mg-ion battery under non-aqueous or aqueous conditions. XPS and TEM studies show that a conversion mechanism takes place during cycling in a non-aqueous electrolyte, with the formation of MnOOH, MnO and Mg(OH)2 upon discharge. In aqueous cells, on the other hand, intercalation of Mg2+ ions takes place, accompanied by expulsion of interlayer water and transformation to a spinel-like phase as evidenced by X-ray diffraction. Both systems are structurally quasi-reversible. The sharp contrast in behavior in the two electrolytes points to the important role of the desolvation energy of the Mg2+ cation in non-aqueous systems.

Keywords: Mg-battery, birnessite, conversion reaction, Mg intercalation, spinel

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INTRODUCTION Lithium-ion batteries dominate today’s rechargeable batteries in both commercial applications and academic research. However, lithium batteries – which would employ lithium metal as the negative electrode - are limited to very special cases such as polymer electrolytes at present,1,2 as the formation of dendrites during the electrodeposition process is very difficult to prevent. To overcome these limitations, current Li-ion technology uses graphite negative electrodes, reducing the theoretical capacity density from 2046 mAh L-1 to ~850 mAh L-1 and significantly increasing the overall cost. Clearly, utilization of metal negative electrodes (anodes) is a promising way of achieving batteries with higher energy density and lower cost. Amongst the various candidates, batteries using Mg metal as the negative are of special interest. 3 In addition to magnesium’s desirable electrochemical properties, including a volumetric specific capacity of 3833 mAh L-1 and the absence of dendrite growth on deposition,4 magnesium metal is already a material of broad industrial interest offering good availability and safe handling in ambient atmosphere. The first rechargeable Mg battery prototype was reported over 15 years ago by Aurbach et al.5 following two major breakthroughs: the development of electrolytes based on the association of a Grignard species R-Mg-Cl with a strong Lewis acid such as AlCl3

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and the introduction of Chevrel

phases, Mo6Ch8 with Ch = S, Se, as positive electrode materials coupled with a Mg anode.8,9 Despite a low operating voltage of about 1.1 V vs. Mg on discharge, Chevrel electrodes are difficult to fully recharge at room temperature due to reactivity with the electrolyte at potentials higher than 1.8 V.9-11 The development of magnesium batteries has since been hindered by the difficulty in finding new, chorine free electrolytes able to reversibly strip and cast Mg at low overpotential which are stable at high potential. Research into new positive electrode materials has also proven that the diffusion of Mg2+ ions in solid host structures is more hindered than that of monovalent alkali cations such as Li+ and Na+. 12 - 16 In order to design new electrode materials, fundamental studies focusing on the electrochemical mechanism rather than performance are needed at this point to understand the key parameters underpinning the development of magnesium based batteries. Promising candidates for positive electrodes with a higher voltage than Chevrel phases are

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manganese oxides, which have been extensively investigated in the Li-ion battery literature owing to their rich crystal chemistry.17-28 The edge sharing MnO6 octahedra can form ribbons of finite width that interconnect to generate tunnel structures such as hollandite (α-MnO2) or todorokite: these consist of tunnels spanned by (2x2) or (3x3) octahedra, respectively. Alternatively, bidimensional structures consisting of stacked MnO2 planes crystallize either without water in the interlayer space (i.e., O’3-NaMnO2), or insert structural water. The latter are exemplified by the minerals birnessite (Fig. 1)29 and buserite, structures that are characterized by interlayer distances of ~7 Å and ~10 Å respectively.30

Figure 1. Birnessite crystal structure showing a water monolayer between the MnO2 sheets.

Studies of these materials as cathodes for Mg-ion batteries are rather limited. It was recently shown that magnesium cells using α-MnO2 positive electrodes undergo a “conversion” mechanism, resulting in poor cyclability.31 Comprehensive analysis by the Toyota team demonstrated that the Mg2+ intercalation barrier into such structures at an early stage is comparable with Li+, but thermodynamics drive the transformation of the highly unstable product into the rock salt oxide (Mg, MnO) on the outer particle surface.31,32 This conversion mechanism is also applicable to some other MnO2 oxides in non-aqueous Mg cells, according to experimental studies that show the surface area is the largest factor influencing their electrochemistry.33 On the other hand, two very recent studies have emphasized the beneficial role of water content in the electrolyte in facilitating Mg2+ intercalation using anhydrous layered MnO234 and spinel MgMn2O435 positive electrode materials, which we became aware of while completing this work. These studies parallel our interest in exploring the behavior of birnessite in both aqueous and

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non-aqueous electrolytes in order to identify the main factors that induce intercalation or a conversion mechanism. Here, we report a one-step hydrothermal synthesis of nanometric platelets of birnessite Mg0.15MnO2.0.9H2O supported on carbon cloth. The electrochemical performance of this material was examined in aqueous media using a 0.5 M Mg(ClO4)2 electrolyte and in non-aqueous media using 0.25 M Mg(TFSI)2 in diglyme. A detailed analysis of the reaction mechanisms (by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Karl-Fischer titration and transmission electron microscopy (TEM) studies) reveals a facile intercalation process in aqueous media and a sluggish conversion mechanism in non-aqueous cells. Based on the contrast of these two systems, we propose that the principal factor limiting the performance of non-aqueous cells is the anion desolvation energy, supporting recent pioneering theoretical studies in both electrolyte and cathode development for Mg-batteries.36,37

Experimental Methods Synthesis – The birnessite Mg0.15MnO2·0.9H2O/carbon cloth composite (labelled Mg-bir/CC) was prepared by a hydrothermal method. Typically, 150 mg Mg(MnO4)2·xH2O (Sigma-Aldrich) was dissolved in 36 mL deionized water and transferred to a Teflon™ autoclave. A square piece of carbon cloth (7 × 7 cm) (Fuel Cell Earth, 99.5% carbon content) was placed in the autoclave to serve as the conductive substrate. The mixture was heated at 160 °C for 2 hrs. After cooling to room temperature, the product was washed with water and dried in air. Characterization – Powder X-ray diffraction (XRD) analyses were performed on a PANalytical Empyrean diffractometer using Cu Kα radiation. The structural water content was determined by thermogravimetric analysis (TGA, TA Instruments SDT Q600) using birnessite powder prepared by the same method (but not supported on carbon cloth) with a 5 °C min-1 heating ramp under N2 flow. Scanning electron microscope (SEM) studies were conducted using a Zeiss Ultra field emission or Zeiss Leo 1530 SEM equipped with an energy dispersive X-ray spectroscopy (EDX) detector. Typical transmission electron microscopy (TEM) images were acquired on a Zeiss Libra 200MC TEM. High

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resolution TEM (HRTEM) images and EELS spectra were obtained with FEI TITAN 80-300 eV TEM/STEM operated at 300 kV. The microscope is outfitted with a spherical aberration corrector for the probe-forming lens, enabling sub-Angstrom resolution in the TEM and STEM mode. The electron energy loss spectra were acquired with a monochromated electron gun, providing chemical and electronic structure analysis with atomic level resolution measurements. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo VG Scientific ESCALab 250 instrument. Spectra were processed using Gaussian-Lorentzian functions and a Shirley-type background with CasaXPS software, and referenced to adventitious carbon at 285.0 eV. The water content in the electrolyte was analyzed by Karl-Fischer titration (Mettler Toledo Coulometric Karl Fischer Titrator C30). The electrolyte was extracted using dry acetonitrile (Caledon, 99.9%, subsequently treated with rigorously dried molecular sieves). Two to three measurements were taken for each sample. Electrochemistry – Mg-bir/CC laminates were punched into disks and directly used as the electrodes after drying at 100 °C under vacuum overnight. The loading of birnessite on the electrode was ~ 1.5 mg/cm2. In the aqueous system, the material was examined in an electrolyte comprised of 0.5 M Mg(ClO4)2 (Sigma-Aldrich, 99.0%) in deionized water, using Pt gauze (Sigma-Aldrich, 99.9%) as the counter electrode and Ag/AgCl as the reference electrode in T-shape Swagelok® 3-electrode cells. For the non-aqueous system, 0.25 M Mg(TFSI)2 in diglyme electrolyte was prepared by dissolving 140 °C vacuum-dried Mg(TFSI)2 salt (Solvionic, 99.5%) in diglyme (Sigma-Aldrich, 99.5%), which was previously purified by distillation. The water content in electrolyte was less than 50ppm. Carbon counter electrodes were prepared by casting Black Pearls® 2000 (BP2000, Cabot, surface area ~ 1500 m2/g) and poly(vinylidene fluoride) (PVDF, Sigma-Aldrich, average Mw ~ 534,000) with a 3:2 weight ratio in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich, 99.5%) slurry onto carbon cloth. The loading of BP2000 was ~ 10 mg/cm2. The magnesium reference electrode was polished with carbide paper (Mastercraft®, 180 grit SiC) and cleaned with a Kimwipe inside an Ar-filled glovebox before use. The voltage profile for the non-aqueous system was studied in 3-electrode cells obtained from DPM Solutions Inc.38 The cell has a cylindrical geometry. A Mg ring, sandwiched in-between the positive and

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negative electrodes, is used as the reference electrode. Long term cycling for the non-aqueous system was carried out in 2325 type coin cells. Galvanostatic studies were performed on a VMP3 cycler (Biologic Instruments™, 1C = 250 mA/g as calculated from a one electron transfer per formula unit).

RESULTS AND DISCUSSION Materials Characterization The diffractogram of the pristine sample of Mg0.15MnO2·0.9H2O/carbon cloth composite (labelled Mg-bir/CC) (Fig. 2a), is composed of the amorphous carbon cloth scattering and the diffraction peaks of the birnessite phase (ICSD-068917).29 The (001) peak at 12.5° is characteristic of the ~7 Å spacing of the MnO2 sheets in birnessite due to its monolayer of structural water (Fig. 1). It allows clear differentiation from the parent buserite structure that features a double layer of structural water and a interplanar distance of ~10 Å. The large breadth of the diffraction peaks is in good agreement with the birnessite nano-morphology observed by electron microscopy (Fig. 2b). The structural water content in birnessite was determined to be 0.9 by TGA (Fig. S1), in agreement with previously reported values of 0.85 29. A homogeneous ~1 µm thick coating of birnessite Mg0.15MnO2.0.9H2O is wrapped around the ~7.5 µm diameter fibers of the carbon cloth (Fig. 2c). The mineral coating is composed of thin platelets radially arranged around the carbon fiber surface, with the platelet plane (of Mg2+ mobility) perpendicular to the fibers (Fig. 2d). This assembly offers an optimal configuration that can be used directly as a positive electrode material for magnesium cells, owing to the underlying electronically conductive substrate and the nanometric morphology of the active material that benefits the typically low mobility of Mg2+ by providing short diffusion pathways.

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Figure 2. (a) XRD comparison of Mg-bir/CC composite (blue; only the index for the peak with highest intensity of the clump is shown for the purpose of clarity) and bare carbon cloth (red); Bragg peak positions for birnessite are noted by the greens tick marks and reflections from the carbon cloth are indicated by asterisks; (b) TEM image of Mg-bir/CC (c)(d) SEM images of Mg-bir/CC. Electrochemistry Aqueous cell The good performance of birnessite cathodes in Mg aqueous cells or using an organic solvent with a significant amount of water (i.e. 5-20 % wt.) has been previously established.34,35 However, in those studies, the birnessite structure was obtained through the electrochemical conditioning of a non-hydrated manganese oxide, either MnO234 or MgMn2O4.35 Owing to the disordered nature of the birnessite framework, detailed structural information is not readily accessible using X-ray diffraction. Therefore

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substantial differences in the local arrangement of the different materials, induced by the conditioning process, could lead to significant differences in the electrochemical mechanism that are not observable by XRD. For this reason, we verified the behavior of the Mg-bir/CC composite electrodes in aqueous media. Sloping curves were observed for the discharge and charge voltage profiles (Fig. 3a), consistent with previous studies.34, 35 As expected from its low surface area, the capacitive response of carbon cloth (~0.05 mAh g-1 of active material, Fig. S2) is negligible compared to the initial capacity of 150 mAh/g, i.e. insertion of 0.3 Mg2+, as observed here and by others34, 35. The expected reversible “theoretical” capacity is 0.5 Mg2+, based on a one electron transfer (since deep reduction of Mn3+ to Mn2+ is typically not very reversible). However, different from previous reports, capacity fading is observed over the first 20 cycles (Fig. 3b). This is due to the enhancement of Mn2+ dissolution due to the nanometric size of our active material, as well as the low loading used in this study. Further evidence of this was found by post-mortem SEM imaging of the dried separator facing the anode, showing the precipitation of large

1.5 100 1.0 1st 2nd 3rd 10th

0.5 0.0

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-0.5 0

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40 60 80 100 120 140 160 Capacity (mAh/g)

Capacity (mAh/g)

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100 50 50

Charge Discharge

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0 60 80 100 120 140 160 Cycle number

Coulombic Efficiency (%)

manganese containing particles (Fig. S3). Ewe vs. Ag/AgCl (V)

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Figure 3. Electrochemistry of Mg-bir/CC in an aqueous electrolyte: voltage profile (a) and capacity and coulombic efficiency evolution (b) at 2C (a “C” rate being defined with respect to one e- transfer).

XRD analysis of the composite electrode in the charge (demagnesiated) and discharge (magnesiated) states (Fig. 4a) confirms the magnesium intercalation process in aqueous cells. However, modification of the intercalate concentration changes the electrostatic interaction responsible for the layered arrangement, resulting in a major topotactical transformation of the structure. Upon discharge, the intercalated Mg2+ allows for more efficient screening of the interslab electrostatic repulsion. This triggers a contraction of the interlayer spacing from 7 Å to 4.86 Å, as evidenced by the shift of the first 8


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diffraction peak from 12.5 degrees (2θ) to 18.6 degrees. Conversely, the removal of Mg2+ upon charge induces an expansion of the interlayer distance to the 10 Å spacing characteristic of a double layer of water between the MnO2 layers in the buserite structure. The (001) and (002) buserite reflections are clearly visible at 9.2 ° and 18.4 °. The structural evolution in the discharged material implies not only a contraction of the interlayer spacing but also a glide of the MnO2 planes. Owing to the poor crystallinity of the phases, full structure determination was not possible. However, the interlayer distance of the magnesiated phase (4.86 Å) corresponds exactly to the distance between the octahedral slabs in the MgMn2O4 spinel structure (Fig. 4b-d). Comparison of the diffractogramms of the discharged phase and MgMn2O4 clearly shows the strong relationship between the two structures. Some of the reflections of the spinel structure are extinct in the discharged phase, the most evident being the (103) reflection at 33 ° (Fig. 4d). Thus we propose that the structure of the discharged phase is based on the same layer arrangement as the spinel phase; e.g. Jahn-Teller distorted, partially occupied triangular slabs of MnO6 octahedra interconnected by

(101)

Figure 4. (a) XRD patterns of cycled birnessite electrodes in the aqueous cell: blue – pristine, green – discharged, and red – charged. The tick marks represent the birnessite29 (blue) and buserite39 (red) phases, respectively; for the purpose of clarity, only the index for the peak with highest intensity is shown. Arrows indicate the shift of the first reflection corresponding to the “interlayer” distance in the three materials; (b) Representation of the spinel MgMn2O4 showing the stacking of (c) the kagome layers40. (d) comparison of the XRD patterns of discharged Mg-bir/CC and the spinel MgMn2O4.

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tetrahedral MgO4 and octahedral MnO6 moiéties. The appearance of a new translational symmetry element - as evidenced by the disappearance of the (103) spinel reflection - suggests a slightly different stacking scheme that does not exactly reproduce the tridimensional structure of MgMn2O4 (Fig. 4c). Attempts to manually determine the precise stacking sequence using the FAULTS software41 were unsuccessful. The hypothesis of a local structure similar to the spinel phase is further supported by a previous study that clearly shows the formation of spinel in the discharged state, 35 although it was not reported as such. In the latter study, we note that the birnessite phase was obtained through a conditioning of the spinel phase where the original kagome layers are probably preserved in the birnessite phase, thus favoring the re-formation of the spinel structure upon re-magnesiation. On the other hand, in the present study, there is no reason to presume a similar arrangement of manganese vacancies in the triangular lattice of the pristine birnessite phase. Therefore we believe that creation of the manganese vacancies occurs during the contraction of the interlayer spacing: the stabilization of Mg2+ in the tetrahedral interlayer site generates a very short (1.84 Å) Mn-Mg distance, forcing Mn out of the triangular lattice and into the interlayer space. The arrangement of the created vacancies would induce a different stacking scheme than in the spinel structure. The formation of such a small interlayer spacing raises the question of the fate of the water. There is no available site to accommodate water in the interlayer space of the discharged phase. The formation of hydroxl groups, as observed in layered MnOOH, would imply the reorganization of the entire close packed oxygen framework and therefore is unlikely. Hence, water is most likely expelled from the structure. We note that conversion of the layered structure to the spinel is reminscent of the conversion of metastable layered Li0.5MnO2 to the more thermodynamically spinel LiMn2O4 on cycling in non-aqueous media,42 where similar driving forces are undoubtedly responsible.

Non-aqueous cells Traditional Mg-electrolytes based on Grignard compounds (RMgCl in association with AlCl3) are considered highly corrosive to oxide materials, and their sensitivity to moisture could interfere with the

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structural water contained in birnessite. The electrochemical behavior of Mg-bir/CC in non-aqueous solvents was therefore investigated using the electrolyte 0.25 M Mg(TFSI)2/diglyme. This electrolyte, reported for the first time by Ha et al.,43 exhibits a large overpotential (> 2 V) when significant current flows through a Mg anode in our experience. However, magnesium metal can still be used as a reference electrode owing to the extremely low current passed in this case. Therefore we first investigated the electrochemistry in a 3-electrode cell,38 with a magnesium reference electrode and a capacitive-carbon counter electrode.44 Fig. 5a and b show the representative voltage profiles for the 8th cycle at a current density of C/10. The working electrode exhibits a voltage plateau at ~1.4 V during discharge and ~1.7 V during charge. Such flat plateaus suggest a 2-phase reaction mechanism.45 The almost linear voltage evolution of the carbon electrode with respect to capacity proves its double layer capacitor behavior,46 validating the use of Mg metal as a reference electrode in this system. However, the 3-electrode cells showed limited life time. Owing to the gradual increase of the water content in the electrolyte (see later)

the surface of the reference electrode is progressively passivated by a film of magnesium hydroxide. Figure 5. Electrochemistry of Mg-bir/CC in the non-aqueous electrolyte: (a) voltage profile of the working electrode and (b) counter electrode in a 3-electrode cell with a Mg reference at C/10; (c) 11



capacity and (d) coulombic efficiency evolution at C/10 and C/5; (e) voltage profile evolution at C/10. Thus, typical 2-electrode coin cells were used to study long term cycling performance and water content evolution. At both C/5 and C/10 rates, a long conditioning process is observed before the Mg-bir/CC electrodes achieve their maximum reversible capacity (Fig. 5c). A capacity of about 135 mAh g-1 are obtained after 20 cycles at C/10 before the capacity slowly decays, whereas at a C/5 rate, about 50 cycles are necessary to achieve a capacity of 90 mAh g-1 that is maintained over 100 cycles. The higher capacity sustained at a C/10 rate compared to a C/5 rate indicates slow kinetics of the electrochemistry process, in complete contrast to the aqueous case where Mg2+ ions show good solid state diffusion at a 2C rate, and where the attendant phase transformation to the spinel is accompanied by water egress.

Mechanistic study Water evolution in the non-aqueous cell No significant changes were observed in the diffractograms of the charged and discharged samples (Fig. S4), showing that owing to the intrinsic disorder of the birnessite structure and the extremely small particle size, XRD is not an ideal probe of the changes involved upon cycling. However, we observed a progressive broadening of the (001) peak as a function of the cycle number; after 20 cycles this feature is barely visible whereas the other peaks remain (Fig. 6a). A similar evolution was observed by thermodiffraction upon dehydration of Mg0.15MnO2.0.9H2O (Fig. S5), suggesting the release of

* carbon cloth

* (001)

(-111) *

(114) * 20 cycles

1 cycle pristine

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20

30 40 50 2θ (degrees)

60

70

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structural water into the electrolyte. This was confirmed by the continuous increase in the water content

Intensity (a. u.)

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1k

100

10

b 0

10 20 30 cycle number

40

Figure 6. (a) XRD patterns of cycled electrodes (charged states) in the non-aqueous cell; (b) results of Karl-Fischer titration of the electrolyte to determine water content.

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of the electrolyte (Fig. 6b) up to the 20th cycle, as monitored by Karl-Fischer titration.

Interestingly,

the number of cycles required to release all of the structural water into the electrolyte corresponds to the number of cycles of the conditioning mechanism (Fig. 5c). Addition of a small fraction of water (0.3 % wt. (3,000 ppm)) to the electrolyte showed the same conditioning process (Fig. S6), suggesting that the increase in reversible capacity comes from the modification of the active material structure rather than the overall water concentration.

XPS studies The evolution of the chemical speciation upon cycling was studied by ex-situ XPS; the results are summarized in Fig. 7, and Fig. S7. The Mn 2p3/2 spectra were fit by sextuplets using parameters previously established for each oxidation number and chemical environment (Table S1).47-49 The O 1s spectra were fit by singlets of increasing binding energy for M-O, M-OH and H2O (Table S2). Details on the fitting procedure can be found in the corresponding section of the supplementary information. The pristine sample shows results consistent with previously reported values.47 The manganese in birnessite exhibits mixed valence states, comprised of about 77% Mn4+, 20% Mn3+ and 3% Mn2+ (Fig. 7a) in good agreement with the chemical formula Mg0.15MnO2·0.9H2O. The oxygen spectrum (Fig. 7d) was fit with a main oxide component (54%) and two hydrogenated oxygen environments; hydroxide (30%) and structural water (16%). Upon discharge, the shift to lower binding energy in the Mn 2p3/2 spectrum (Fig. 7b) is characteristic of the reduction of the manganese ions. The shoulder at 647 eV represents a highly characteristic satellite of MnO,49 clearly proving the existence of this oxide at the surface of the discharged sample. The remaining part of the Mn 2p3/2 spectrum is well fit using a component typical of MnOOH.49 Consistent with this, the O 1s spectrum of the discharged sample (Fig. 7e) shows an increase of hydroxide species (up to 62%) whereas the oxide content decreases to 21%. The large excess of hydroxide with respect to the oxide component suggests the formation of Mg(OH)2. A small fraction of the O 1s signal (3%) was assigned to water; however, the increase of the binding energy with respect to the pristine sample, suggests that the remaining water is physisorbed and not chemically bonded.50 The

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additional peak at 533 eV corresponds to the oxygen in the TFSI- anion,51 which is supported by the presence of corresponding signals in the C 1s (Fig. S7c), and N 1s, F 1s and S 2p regions (Fig. S7d). The O1s spectrum could not be fit without this contribution. Furthermore, significant signal from the TFSI was found only in the case of the discharged, i.e. magnesiated, sample despite extensive washing of the electrode. This indicates - importantly - that TFSI- anions are chemically bonded to the surface of the cathode, since the same washing procedure efficiently removed contamination from the electrolyte on the charged cathodes.

a

4+

Mn in Bir. = 77% 3+ Mn in Bir. = 20% 2+ Mn in Bir. = 3%

oxide = 54% hydroxide = 30% water = 16%

Mn 2p3/2

MnO = 53% MnOOH = 47%

b

c

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Mn in Bir. = 44% 3+ Mn in Bir. = 42% MnO = 14%

650

648

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Intensity (a. u.)

Intensity (a. u.)

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646 644 642 640 Binding Energy (eV)

638

oxide = 21% hydroxide = 62% water = 3% O in TFSI = 14%

e

oxide = 43% hydroxide = 43% water = 14%

f

536

534 532 530 Binding Energy (eV)

528

Figure 7. Mn 2p3/2 (a)-(c) and O 1s (d)-(f) XPS spectra of pristine (a)(d), discharged (b)(e) and charged (c)(f) Mg-bir/CC electrodes; fits are shown in color as labelled. In the charged sample, the Mn 2p3/2 peak shifts back to higher binding energy (Fig. 7c), implying re-oxidation of the Mn species. However the final Mn4+ concentration (44%) is lower than in the pristine 14


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sample (77%), in agreement with the irreversible capacity observed in the first discharge. The remainder of the Mn 2p3/2 spectrum is fit by 42% Mn3+, corresponding to a birnessite environment, and 14% MnO. Similarly, the “oxygen speciation” evolves from a hydroxide environment to an oxide environment close to that observed in the pristine sample.

These changes in the Mn 2p and O 1s spectra - comparing the

pristine, discharge and charged electrodes - are summarized in Fig S7a, b. The evolution of the nature of the surface species upon magnesiation, as resolved by XPS, points at a conversion mechanism involving the formation of manganese oxyhydroxide (MnOOH) and manganese oxide (MnO) as well as magnesium hydroxide (Mg(OH)2), summarized by the following equation: Mg0.15MnO2 + H2O (structural or electrolyte) + x Mg2+ + 2x e- (2x-0.71) MnO + (1.71-2x) MnOOH + (x+0.15) Mg(OH)2 The voltage plateau observed in the first section of the discharge in the three electrode cell (Fig. 5a) is consistent with this proposed multiphase process. The reversible capacity (135 mAh g-1) obtained after conditioning of the electrode at a C/10 rate, corresponds to about a 0.5 electron transfer per Mn atom. However, the change in the mean oxidation state (as measured by XPS) between the discharged and charged sample, +2.5 and +3.3 respectively (i.e., a 0.8 e- transfer), is larger than expected showing that the reaction occurs predominantly at the surface of the material. This behavior is typical of conversion reactions, where nucleation and growth of the conversion products take place at the interphase between electrode and electrolyte. Propagation of the reaction to the bulk is hindered if the conversion product does not allow for facile Mg2+ diffusion. Assuming a simplistic two phase model, we estimate that about 60 % of the active material is involved in the electrochemical process. We see that despite the nanoscopic morphology of the active material, the reaction cannot proceed to the core of the particles implying the formation of an interphase that does not conduct Mg2+ ions. The gradual decrease of the ionic conductivity of the solid interphase gives rise to the sloping voltage curve observed in the second part of the discharge.7

HRTEM and Electron Energy Loss Spectroscopy

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HRTEM analysis was carried out to further characterize the species at the discharged and charged states. The cathodes used were cycled at 90 °C at C/10 with a carbon counter electrode. We employed a higher temperature here to push the reaction further to completion, so that the products would be easier to track by TEM where only small area could be sampled each time. As shown in Fig. S8a, capacities of ~ 210 mAh/g are obtained under these conditions, which approximates to a 0.8 electron transfer. A conditioning process is also required for the cell to achieve maximum capacity (Fig. S8b), indicating the same conversion mechanism as at room temperature. However, fewer cycles are required for maximum capacity (i.e. 2 cycles at 90 °C as opposed to 20 cycles at room temperature) because of the high temperature, which allows the electrode and electrolyte to reach the equilibrium state more quickly. The electron energy loss spectra (EELS) shown in Fig. 8a and b are consistent with a conversion reaction. The small changes in the oxygen K-edge indicate new oxide species are formed on discharge, which agrees with the XPS result where the transformation from the layered oxide and structural water matrix to the hydroxide, oxyhydroxide and oxide mixture is observed. The reversible change of the Mg K-edge intensity and its energy indicates that Mg-oxide is formed at the cathode during discharge and consumed on charge, with the positive charges on Mg2+ compensated by the reduction or oxidation of Mn. The corresponding change in the Mn valence can be evaluated by the Mn-L2,3 ratio (Fig. 8a). To a first approximation - in the absence of a detailed calibration curve - we have scaled the observed Mn valence in the pristine birnessite, which is known to be +3.7 based on chemical analysis. The Mn valence decreases to +3.2 upon discharge and increases to +3.5 upon charge (Fig. 8c). The difference in these values between the Mn valence calculated from the electrochemical capacities on discharge and charge of +2.9 and +3.7 respectively (Fig. S8a), indicates an inhomogeneous reaction within the cathode matrix. We were not able to spatially resolve EELS spectra of only the surface owing to the very highly divided nature of the birnessite. We expect that the core remains unreacted birnessite, and reduction of manganese proceeds on the shell of the particles as previously confirmed for K-αMnO2.31

Thus the Mn

valence derived from EELS is the expected average. Finally, the high resolution TEM images shown in Fig. 8d-8f demonstrate that there is no major change in the overall morphology of the electrode materials during cycling.

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Figure 8. EELS spectra of the pristine, discharged and charged cathode material: (a) O-K edge and Mn L-edge; (b) Mg K-edge; (c) Bar plot showing the quantification of the Mn valence state determined from the L3/L2 ratio; High-resolution TEM images of the (d) pristine, (e) discharged and (f) charged cathode materials show their polycrystalline nature.

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CONCLUSIONS We showed that using an aqueous electrolyte, Mg2+ is reversibly inserted into layered manganese oxides. At low intercalate concentration, the interlayer electrostatic repulsion are accommodated by a double layer of water typical of the busserite structure leading to a ~10 Å interlayer distance. Upon magnesium insertion the interlayer contracts to 4.86 Å, suggesting that similarly to the spinel structure, the electrostatic repulsions are screened by tetrahedral MgO4 and octahedral MnO6. The excellent rate performance of the Mg-bir/CC and other birnessite type electrode materials

35

in aqueous electrolyte

clearly show that using interstitial structural water is a very efficient way to achieve high Mg2+ solid state mobility. However, in addition to the problem of manganese dissolution, aqueous electrolytes are not compatible with Mg anodes, limiting the interest of aqueous Mg batteries to fundamental studies. In the light of the aqueous results, the very different behavior of Mg-bir/CC electrode in non-aqueous media, which exhibits a conversion mechanism prominently at the surface of the particles despite their small size, cannot only result from the slow diffusion kinetics of Mg2+ ions. Our XPS results show clear evidence of the presence of TFSI- anions strongly bonded to the surface of the discharged sample, which suggest that in addition to the formation of thermodynamically favorable oxides32, a limiting factor is also the disruption of the ion pairing of the electrolyte salt. The high energy of ion pairing in Mg(TFSI)2 in diglyme was recently calculated and measured, showing a well-defined Mg-O(TFSI) distance of 2.08 Å, i.e. smaller than the distance observed in solids such as MgO (2.11 Å).36 Moreover, the importance of the H2O/Mg ratio in the electrolyte rather than the overall water concentration showed by this study and others,34,35 clearly links the increase of performance to the full solvation of Mg2+ by water molecules when H2O/Mg > 6. These findings show that solid state diffusion of Mg2+ might not be the most penalizing factor when moving from a singly charged ion (Li+, Na+) battery to a multivalent battery. The development of new electrode materials for Mg batteries should also focus on parameters leading to efficient disruption of the ion pairing in the electrolyte. For example, it was recently shown that the accessibility of molybdenum centers at the surface of the Chevrel phase, Mo6S8, plays a major role in the transport of Mg2+ from the

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electrolyte solution to the bulk of the electrode, because of preferential bonding with the anion (Cl-).37 ACKNOWLEDGMENTS This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences, and as part of the Chemical Imaging Initiative conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). The HRTEM work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. We would like to thank Jeff Aguiar for his help with the quantitative analysis of the energy loss spectra. PNNL, a multiprogram national laboratory, is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830. NSERC is acknowledged by LFN for a Canada Research Chair.

ASSOCIATED CONTENT Supporting Information Available: thermogravimetric analysis data of birnessite, electrochemistry data, XRD patterns, thermodiffraction of Mg-bir/CC, SEM/EDX of separator after discharge in an aqueous electrolyte, and XPS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure for Table of Contents

MnO6 octahedra

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Investigation of the Mechanism of Mg Insertion in ... - ACS Publications

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