Electrochemically Induced Structural Transformation in a γ-MnO2

May 13, 2015 - In the present study, an in-depth investigation on the structural transformation in a mesoporous γ-MnO2 cathode during electrochemical...
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Electrochemically Induced Structural Transformation in a γ‑MnO2 Cathode of a High Capacity Zinc-Ion Battery System Muhammad H. Alfaruqi,†,§ Vinod Mathew,†,§ Jihyeon Gim,† Sungjin Kim,† Jinju Song,† Joseph P. Baboo,† Sun H. Choi,‡ and Jaekook Kim*,† †

Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong dong, Buk-gu, Gwangju 500-757, South Korea ‡ Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang 790-834, South Korea S Supporting Information *

ABSTRACT: In the present study, an in-depth investigation on the structural transformation in a mesoporous γ-MnO2 cathode during electrochemical reaction in a zinc-ion battery (ZIB) has been undertaken. A combination of in situ Synchrotron XANES and XRD studies reveal that the tunneltype parent γ-MnO2 undergoes a structural transformation to spinel-type Mn(III) phase (ZnMn 2 O 4 ) and two new intermediary Mn(II) phases, namely, tunnel-type γ-ZnxMnO2 and layered-type L-ZnyMnO2, and that these phases with multioxidation states coexist after complete electrochemical Zn-insertion. On successive Zn-deinsertion/extraction, a majority of these phases with multioxidation states is observed to revert back to the parent γ-MnO2 phase. The mesoporous γMnO2 cathode, prepared by a simple ambient temperature strategy followed by low-temperature annealing at 200 °C, delivers an initial discharge capacity of 285 mAh g−1 at 0.05 mA cm−2 with a defined plateau at around 1.25 V vs Zn/Zn2+. Ex situ HR-TEM studies of the discharged electrode aided to identify the lattice fringe widths corresponding to the Mn(III) and Mn(II) phases, and the stoichiometric composition estimated by ICP analysis appears to be concordant with the in situ findings. Ex situ XRD studies also confirmed that the same electrochemical reaction occurred on repeated discharge/charge cycling. Moreover, the present synthetic strategy offers solutions for developing cost-effective and environmentally safe nanostructured porous electrodes for cheap and eco-friendly batteries.

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INTRODUCTION The continuous research to meet the rising global energy demand has highlighted the absolute necessity of developing sustainable energy storage systems and solutions. The rechargeable Li-ion battery (LIB), an energy storage/conversion device, dominates the present day battery market for portable device applications. Nevertheless, the crucial issues of safety, eco-friendliness, and cost-effectiveness, as well as the controversies associated with the available global lithium resources, have led to the intense exploration of new battery chemistries.1−4 Sodium-ion, potassium-ion, and aluminum-ion batteries are plausible alternatives since these devices are based on relatively abundant and cheap sodium, potassium, and aluminum elements, respectively. However, these battery systems, which realize energy storage/conversion similar to that technology in lithium batteries (i.e., via ion-intercalation/ deintercalation), suffer from complicated issues of safety, processing costs, and environmental concerns.5−8 Although earlier attempts to study Zn-ion intercalation met with limited success, the recent development of an aqueous zinc-ion battery (ZIB), which facilitates energy storage/conversion via Zn-ion intercalation/deintercalation, has paved the way not only to © XXXX American Chemical Society

realizing safe and environmentally benign energy devices but also to reducing the processing costs of next generation batteries.9−15 The amount of guest ion-insertion/deinsertion occurring across the electrodes is one of the crucial factors determining the energy storage capability of a rechargeable battery, and, hence, the cathode represents one of its prime components. Among the majority of transition metal oxides (TMO) used in present day battery cathodes, manganese dioxide (MnO2) has gained renewed interest, due to the abundance, low cost, and lower toxicity of the manganese element.16−21 In fact, MnO2 was used in 1865 when the primary Zn-MnO2 cell was introduced. The secondary ZnMnO2 cell, which is composed of a Zn anode, MnO2 cathode, and alkaline KOH electrolyte, stores energy via conversion reactions occurring at the electrodes. Although both the primary and secondary Zn/MnO2 cells are being widely used in portable electronic devices, the factors of capacity decline and low Coulombic efficiencies affect their performance.22−25 A Received: December 22, 2014 Revised: April 27, 2015

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Figure 1. Schematic illustration of the reaction pathway of Zn-insertion in the prepared γ-MnO2 cathode.

these polymorphs is dependent on the type of linkage between the fundamental octahedral units in the crystallographic structure. Some polymorphs possess tunnel-like structures that facilitate guest-ion insertion/deinsertion during electrochemical reactions. A very striking example is the report on divalent Mg-ion insertion in tunnel-type and layered-type MnO2 structures.27 Further, an α-MnO2 cathode possessing 2 × 2 tunnels (size ∼ 4.6 Å) demonstrated enhanced Zn-ion insertion/deinsertion and hence impressive electrochemical properties.12−14 Meanwhile, a recent study reported that larger 3 × 3 tunnels (size ∼ 7.0 Å) in a todorokite-type MnO2 led to enhanced electrode properties.15 The low temperature γ-MnO2

recent study demonstrated energy storage/conversion via Liion intercalation/deintercalation rather than the heterogeneous conversion reaction across the electrodes when the electrolyte KOH was replaced with LiOH in a secondary Zn-MnO2 cell.26 Generally, MnO2 exists in a variety of crystallographic polymorphs, namely, α, β, γ, δ, λ, and ε-type MnO2. The fundamental unit in the crystal structure of MnO2 polymorphs is composed of Mn4+ ions occupying octahedral holes formed by hexagonally close-packed (hcp) oxide ions. Precisely, every Mn4+ ion is surrounded by six oxygen neighbors to form a fundamental MnO6 octahedral unit, and these units are, in turn, linked via the edges and/or corners. The realization of each of B

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Chemistry of Materials phase comprised of randomly arranged 1 × 1 (size ∼2.3 × 2.3 Å, pyrolusite) and 1 × 2 (size ∼2.3 × 4.6 Å, ramsdellite) tunnel blocks demonstrated considerable electrochemical performances versus lithium and sodium and is therefore interesting to investigate for alternative applications.28−34 Nanostructured mesoporous materials with a porous structural framework are beneficial in various fields including chemical separation, catalysis, microelectronics, optics, and energy storage/conversion.35−38 Specific to the realm of rechargeable batteries, mesoporous electrodes offer a relatively large electrode/electrolyte contact area and, therefore, tend to enhance the diffusion kinetics by reducing the diffusion pathway for electronic and ionic transport. In addition, the high surface area of these electrodes ensures facile guest-ion insertion/deinsertion.39 The major strategies adopted to develop mesoporous electrodes include the utilization of soft/ hard templates or structure directing agents (SDA) during synthesis.40−45 The use of SDAs, however, presents the drawbacks of enhancing the production costs and making the sample impurity-prone. On the other hand, few reports have focused on direct synthesis via thermal decomposition or ureaassisted hydrothermal methods for the preparation of rechargeable battery electrodes with mesoporous properties.33,34,46,47 Therefore, the large-scale synthesis of mesoporous manganese dioxide cathodes, preferably at lower temperatures, is an attractive goal to pursue. From the perspective of developing low-cost and environmentally safe rechargeable batteries that can be easily scaled-up for large-scale production, the present study reports on a high surface area mesoporous γ-MnO2 cathode obtained by a simple template-free ambient temperature strategy and subsequent annealing at low temperatures for ZIB applications. Although the possibility of Zn-insertion was initially suggested in αMnO2 with sufficiently wide 2 × 2 tunnel-type structures, its detailed mechanism has not been examined so far.12−14 Herein, we performed a series of in situ XANES and synchrotron XRD investigations combined with ex situ XRD studies to understand the feasibility of Zn-insertion/deinsertion in the prepared orthorhombic γ-MnO2 cathode. The results indicate that, on complete electrochemical Zn-insertion, the tunnel-type γMnO2 undergoes a transformation to spinel-type ZnMn2O4 (ZMO), γ-ZnxMnO2 (tunnel-type), and L-ZnyMnO2 (layeredtype) phases which is accompanied by the reduction of manganese from Mn(IV) to the Mn(III) and Mn(II) states, respectively. Precisely, in the early stages of Zn-insertion, a portion of the γ-MnO2 undergoes a transformation into spineltype ZMO, as shown in Figure 1. As the discharge reaction proceeds further, the 1 × 2 tunnels of γ-MnO2 are occupied by zinc to form a γ-ZnxMnO2 phase with the parent orthorhombic structure, in addition to the increase in the spinel ZMO phase. During the intermediate stages of the discharge reaction, as more zinc is inserted into vacant tunnels, the Zn-containing tunnels tend to expand and open-up the structural framework to form a layered-type (L-ZnyMnO2) phase, as shown in Figure 1. On complete Zn-insertion or at the final stage (Figure 1), the three phases, viz., spinel-type ZMO, tunnel-type γ-ZnxMnO2, and layered-type L-ZnxMnO2, coexist. On subsequent charging or Zn-deintercalation, all of the structural variations revert back almost entirely to the parent orthorhombic lattice of γ-MnO2. Furthermore, the mesoporous electrode coated on A4 paper was used to fabricate a flexible zinc-ion test-cell that powered an LED with zinc foil as the anode. In contrast to the fabrication of sodium-ion, potassium-ion, or aluminum-ion batteries, ZIBs

only require an open-air environment for their fabrication, thereby markedly reducing the processing costs.

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EXPERIMENTAL SECTION

Materials. Mesoporous γ-MnO2 was synthesized by a simple redox reaction process. A total of 0.05 mol of KMnO4 and 0.15 mol of MnCl2 were dissolved separately in 100 mL of distilled water. Subsequently, KMnO4 solution was dropped slowly into MnCl2 solution under stirring at room temperature. The product was filtered, washed with distilled water, and then dried at 100 °C for several days before annealing at 200 °C for 24 h. Powder and Synchrotron X-ray Diffraction (XRD) Studies. The powder XRD patterns were measured using a Shimadzu X-ray diffractometer with Cu Kα radiation (λ = 1.54056) operating at 40 kV and 30 mA within a scanning angle, 2θ, range of 10−80° in steps of 0.01°. The in situ Synchrotron XRD measurements were performed at beamline 1D KIST-PAL, Pohang Accelerator Laboratory (PAL), using a MAR345-image plate detector operating at 2.5 GeV with a maximum storage current of 200 mA. The X-ray beam was focused by a toroidal mirror and monochromatized to 12.4016 keV (0.9997 Å) by a double bounce Si(111) monochromator. The Si(111) monochromator and a Si(111) analyzer crystal were used to provide a high-resolution configuration in reciprocal space. The patterns were recorded based on a wavelength value of 1.00076 Å. However, the XRD patterns displayed in the present study were plotted after the recalculation of the 2θ values based on conventional Cu Kα radiation (λ = 1.5414 Å). During the preparation of the in situ cell, the electrode active material mixed with 20% carbon black, and 10% TAB binder was cast on stainless steel mesh and assembled in a spectro-electrochemical cell. The cell was cycled to a fully charged/discharged state by a portable potentiostat at a constant rate of 0.1 mA cm−2. Kapton tape was applied on the apertures of the outer cases of the test cell. Electron Microscopy, Surface Area, and Ex Situ Studies. The SEM images were obtained using an S-4700 HITACHI model operating at 15 kV under a low vacuum. The field emission transmission electron microscopy (FE-TEM) pictures were recorded using an FEI Tecnai F20 at 200 kV. The mesoporous characteristics (specific surface area and pore diameter distributions) of the final sample were studied by the Brunauer Emmet and Teller (BET) method using an ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA. For ex situ studies, the test cell with γ-MnO2 electrode was discharged/charged under a current density of 0.05 mA cm−2 for specific cycles and maintained at respective cutoff potentials for 12 h. The recovered electrode was then rinsed with acetone and dried in an Ar filled glovebox at ambient temperature before TEM, ICP-AES (inductively coupled plasma atomic emission spectrometer), and XRD studies. The ex situ XRD studies were performed using a high resolution XPERT Pro PANalytical Model X-ray diffractometer, and the data were collected over the angular 2θ range of 20−60°. The cathode and anode components of the test cells were removed after the electrochemical discharge/charge reactions. On the other hand, the electrolyte recovered from the test cell was sealed in a vial in a glovebox for ICP analysis. The ICP-AES was performed using OPIMA 4300 DV from PerkinElmer. Electrochemical Insertion Studies. The electrode was fabricated by pressing a mixture of 70% active material, 20% Ketjen black, and 10% teflonated acetylene black (TAB) onto a stainless-steel mesh and dried under vacuum at 120 °C for 12 h to form the cathode. Zn metal foil (Goodfellow, thickness = 0.25 mm) was used as the anode, and the electrolyte comprised a solution containing 1 mol l−1 ZnSO4 (pH 4.0). 2032-type coin cells were assembled by sandwiching a filter paper (Whatman grade) filled with the electrolyte between the prepared cathode and the zinc foil anode. The cells were kept for 12 h before the electrochemical measurements. The discharge/charge measurements were performed at room temperature using a BTS 2004H (NAGANO KEIKI Co., LTD, Ohta-ku, Tokyo, Japan). The cyclic voltammetry (CV) test was carried out using an AUTOLAB potentiostat model PGSTAT302N. C

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Figure 2. (a) Cyclic voltammogram curves recorded at various intervals until 40 cycles for the Zn/MnO2 cell. The initial three voltage profiles of the fabricated Zn/MnO2 cell cycled within the potential window of 0.8−1.8 V at current densities of (b) 0.05 and (c) 0.5 mA cm−2. (d) The cycle performance of the test cell within 0.8−1.8 V at a current density of 0.5 mA cm−2. In Situ XANES Studies. The in situ X-ray absorption measurements were carried out on beamline 7D of the Pohang Accelerator Laboratory (PAL). The radiation was monochromatized by a Si(111) double-crystal monochromator. Higher-order harmonic contaminations were eliminated by detuning the monochromator to reduce the incident X-ray intensity by approximately 30%. The spectra for the Kedges of Mn were taken in transmittance mode at room temperature. The intensity of the incident beam (I0) was measured with a He-filled IC Spec ionization chamber, and that of the transmitted beam (IT) with an N2-filled chamber. The obtained data were analyzed with ATHENA in the IFEFFIT 1.2.11 suite of software programs.48 During the preparation of the in situ cell, the electrode active material mixed with ketjen black and TAB binder (in the ratio mentioned above) was cast on a stainless steel mesh and assembled in a spectroelectrochemical cell. The cell was fully discharged and charged by a portable potentiostat at current densities of 0.1 and 0.2 mA cm−2, respectively. Kapton tape was applied on the apertures of the outer cases of the test cell. The in situ XANES measurements were performed at various depths of discharge (DOD) and states of charge (SOC) of the spectro-electrochemical cell to understand the change in the Mn oxidation states during Zn-intercalation/deintercalation.

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When the as-prepared sample was annealed at a low temperature of 200 °C, γ-MnO2 was formed. The thermal analysis of γ-MnO2 clearly confirmed that an annealing temperature of 200 °C was sufficient to ensure the removal of water, and the XRD pattern of the resulting sample was well indexed to the orthorhombic γ-MnO2 phase.33,34 The XRD pattern of the prepared γ-MnO2 is provided in the Supporting Information (Figure S1). Electron microscopy studies of the annealed γ-MnO2 clearly revealed criss-crossed nanowire/fiber crystallites with an average diameter and length in the ranges of 2−3 and 25−40 nm, respectively, with a disordered porous network. The N2 adsorption−desorption studies suggested a type IV isotherm that is characteristic of mesoporous materials, and the pore size distribution plot also indicated the presence of mesopores (pore diameter, 3.77 nm) in the annealed γMnO2. Furthermore, BET studies performed on the mesoporous γ-MnO2 revealed the overall surface area and pore volume to be 148 m2 g−1 and 0.4283 cm3 g−1, respectively, as presented in detail in our earlier reports.33,34 Cyclic Voltammetry and Electrochemical Studies. The cyclic voltammetry (CV) curves obtained using the fabricated Zn/γ-MnO2 cell within the potential range of 1−1.8 V for 40 cycles at a sweep rate of 0.5 mV s−1 are provided in Figure 2a. The electrolyte used was 1 mol L−1 ZnSO4. The initial electrochemical cycling clearly reveals two peaks at around 1.1 and 1.6 V. On consecutive cycling, the former peak shifts toward higher potentials (∼1.2 V) while an increase in the peak currents is observed in addition to a new peak appearing at 1.3

RESULTS AND DISCUSSION

The formation of manganese dioxide by the redox reaction is written as below: 3MnCl 2 + KMnO4 + 4H 2O → 4MnO2 + K+ + 6Cl− + 4H+ D

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Figure 3. Electrochemical (a) discharge and the successive (b) charge profiles obtained for the fabricated spectro-electrochemical cell cycled within 0.8−1.8 V at a current rate of 0.05 mA cm−2. Mn K-edge XANES spectra of Zn/MnO2 taken in situ during (c) discharging, normalized spectra; (d) charging, normalized spectra; (e) discharging, derivative spectra; and (f) charging, derivative spectra.

registered initial discharge and charge capacities of 285 and 165 mAh g−1, respectively. The discharge curve reveals two gradually sloping plateaus at average potentials of 1.4 and 1.27 V and, therefore, clearly indicates the stable guest-ion insertion properties of the mesoporous γ-MnO2 cathode. In fact, the plateau voltages are clearly different from those observed in the electrochemical profiles of alkaline Zn/γ-MnO2 cells.49 However, the existence of plateau regions is similar to those observed in the voltage profiles during our earlier studies on Li/Na-ion insertion in mesoporous γ-MnO2.33,34 It appears that the nanosized mesopores in the structure tend to enhance the electrode/electrolyte contact area, thereby favoring Zn-ion insertion. Figure 2c shows the initial three discharge/charge profiles of the γ-MnO2 cathode cycled under a current density of 0.5 mA cm−2. At such high current densities, initial and second discharge capacities of 165 and 219 mAh g−1 are delivered by the γ-MnO2 cathode, respectively. The cyclability data in Figure 2d reveal that the discharge capacity is 158 mAh g−1 after 40 cycles, this value being 37% less than the highest capacity of 250 mAh g−1 registered by γ-MnO2. Reports in literature indicate that the capacity fading likely results from manganese dissolution, rather than from the structural distortion of MnO2.33,34 Although further studies are required to improve the cycling capacities of the γ-MnO2 cathode, the Coulombic efficiency data clearly indicate that there are very little or no irreversible capacity losses under long-term cycling.

V. Alternatively, in the higher potential region, the peak at 1.6 V tends to slightly shift toward lower potentials (∼1.57 V) and a shoulder peak develops at 1.63 V. Clearly, the two consistent peaks at 1.2 and 1.3 V may represent Zn-insertion into the γMnO2 host and the consequent reduction of Mn(IV) to the Mn(III)/Mn(II) states. Similarly, the appearance of a peak and a shoulder at around 1.6 and 1.65 V, respectively, probably correspond to Zn-extraction from the γ-MnO2 cathode as the Mn(III)/Mn(II) states undergo oxidation to the Mn(IV) state. However, the CV profiles for the first cycle with one redox peak appears to be different from the observed two redox peaks for the successive cycles. The reason for this may be related to the gradual activation of the electrode, as observed from the increasing peak currents on successive cycling. Further, the two clear redox peaks observed on successive cycling may correspond to the presence of two different phases arising as a result of structural transition and is discussed later. Further, the almost superimposable reduction peaks on the 30th and 40th cycles of the CV curves indicate not only that a similar reaction occurs throughout cycling but also that the structural distortion/transformation associated with the change in manganese oxidation states is reversible in the present γMnO2. The Zn/γ-MnO2 cell, which was fabricated under openair conditions, displayed an open-circuit voltage (OCV) of 1.4 V. Figure 2b shows the initial three discharge/charge profiles of γ-MnO2 in the potential range of 1.0−1.8 V vs Zn2+/Zn at a current density of 0.05 mA cm−2. The mesoporous cathode E

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Chemistry of Materials In Situ XANES Studies. The in situ Mn K-edge spectra were recorded at various depths of discharge (DOD) and states of charge (SOC) of the Zn/γ-MnO2 test cell cycled under a current density of 0.05 mA cm−2 to understand the charge compensation during the electrochemical reaction. The voltage−time profiles for the first discharge/charge cycle of the Zn/γ-MnO2 test cell used for the in situ experiments are shown in Figure 3a,b, respectively. The blue color dots in the profile indicate the various DOD/SOCs at which the XAFS scans were initiated. In total, 11 in situ scans were performed, of which the initial six steps correspond to different progressively decreasing DODs and the latter six steps represent the various incrementing SOCs. The Mn K-edge XANES spectra recorded for the selected DODs and SOCs are displayed in Figure 3c,d, respectively. As observed in Figure 3c, the intensity of the main absorption peak denoting an electronic 1s to 4p transition decreases as the discharge proceeds from 1.8 to 0.8 V. The corresponding derivative spectra plotted in Figure 3e demonstrate these variations more exquisitely. The derivative peak at 6558 eV tends to decrease remarkably as discharging proceeds from step 1 and is hardly observed in step 6. Reversely, when the cell is charged from 0.8 to 1.8 V, the XANES spectrum is shifted toward higher energies and the main absorption peak increases gradually, as shown in Figure 3d. Furthermore, the corresponding derivative spectra in Figure 3f reveal that the intensity of the peak at 6558 eV increases remarkably as the charging progresses from step 6 to step 11. In order to elucidate further on the variation in the manganese oxidation states, the three XANES spectra corresponding to the initial (step 1), fully discharged (step 6), and fully charged (step 12) states compared with those of standard manganese (II/III/IV) oxides (for standard Mn(IV)O2, the γ-MnO2 sample prepared in the present study was used) are illustrated in Figure 4a. The corresponding derivative spectra in Figure 4b clearly reveal that a weak pre-edge peak at 6539.8 eV is observed for standard MnO, Mn3O4, and αMn2O3 whereas the prepared γ-MnO2 powder exhibits two peaks at 6541.0 and 6542.6 eV. It is well-known that the preedge peak is associated with a 1s → 3d electric dipole transition that is forbidden according to the selection rules. However, the forbidden transition of a 1s electron to an unoccupied 3d orbital of Mn4+ ion is partially allowed due to the pure electric quadrupole coupling and/or the Mn 3d and O 2p orbital mixing.50−52 All of the weak pre-edge peaks are an indication of octahedral coordination around a manganese atom. The spectrum of the initial fresh electrode (step 1) and that in the completely charged state (step 11) are similar and therefore suggest that the electrochemical discharge/charge process is reversible and no byproducts are formed during the reaction. As anticipated, the main absorption edges of the spectra pertaining to steps 1 and 11 are similar to those of γ-MnO2 prepared in the present study. More explicitly, the derivative spectra of the scans at steps 1 and 11 show two pre-edge peaks (peak A) and the main absorption peak (peak C) that are slightly shifted from those observed for the γ-MnO2 powder prepared in the present study. This minute shift may arise as a result of the strong metal support interaction (SMSI) between manganese and carbon used for the electrode fabrication.53,54 Further, this interaction is known to be enhanced in nanoscale materials, and therefore, the present γ-MnO2 cathode with particle dimensions less than 50 nm may demonstrate higher Mn-C interaction and thereby lead to marginal shift from the characteristic XANES

Figure 4. Comparison of Mn K-edge XANES of initial state (step 1, 1.8 V), fully discharged state (step 6, 0.8 V), and fully charged state (step 11, 1.8 V) with standard manganese oxides; (a) normalized spectra and (b) corresponding derivative spectra.

peaks corresponding to Mn(IV) state. Therefore, it is reasonable to conclude that the manganese oxidation state after one complete discharge/charge cycle may be close to Mn(IV). On the other hand, when the cell is fully discharged (step 6), the rising part of the absorption edge appears to be situated near that of Mn3O4, which is generally explained by the existence of mixed Mn(II/III) states. Under full discharge conditions (step 6), the main absorption peak in Figure 4a is shifted to energies that are between Mn3O4 and MnO. The derivative spectra clearly show that the pre-edge peak (peak A) of step 6 appears to be related to that of Mn3O4. More importantly, the main peak (peak B) in the derivative spectra is situated near that of Mn3O4. However, peak C that appears for Mn3O4 and γ-MnO2 is not observed for the sample at step 6. Therefore, these observations suggest that the Mn K-edge absorption spectra taken under full discharge conditions (step 6) can be explained by the existence of manganese in the Mn(III) and Mn(II) states. In addition, it is highly feasible that separate phases of Mn(III) and Mn(II) as in Mn2O3 and MnO, respectively, exist unlike in the case where Mn(II)/Mn(III) states are present in a single Mn3O4 phase. Overall, the in situ XANES studies clearly indicate that the oxidation state of Mn(IV) in the fresh electrode most probably transforms into distinct phases with Mn(III) and Mn(II) states after one complete discharge reaction. Further, the mixed Mn(III)/ Mn(II) states revert back to the original Mn(IV) state after complete charge cycle. Therefore, the in stu XANES studies F

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Figure 5. (a) Electrochemical discharge profile of the fabricated spectro-electrochemical cell cycled within 1.4−0.8 V at a current rate of 0.1 mA cm−2. The horizontal bars indicate the locations at which the synchrotron XRD scans were initiated. The in situ XRD patterns obtained within selected scanning angle (2θ) domains of (b) 9.2−13.65°, (c) 18.9−24.8°, (d) 26−30°, (e) 31.5−35.6°, (f) 35.7−41.2°, and (g) 41.3−46° are presented.

corresponding to spinel-type ZnMn2O4 (ZMO) begin to appear until the characteristic (103) reflection line at 32.7° dominates the spectrum in the completely discharged state (cyan colored triangles in Figure 5d−g). Although the exact reasons for the domination of this diffraction line, which is the second highest characteristic peak of ZnMn2O4, are presently unknown, earlier investigations have also reported this trend.13,15 The observation indicates that, upon Zn-insertion, a part of the orthorhombic γ-MnO2 is transformed into the spinel phase accompanied by the anticipated reduction in the oxidation state of manganese from Mn(IV) to Mn(III). In addition, it is interesting to note that the there is little or no significant variation in the peak positions of ZMO for the entire electrochemical reduction/discharge reaction shown in Figure 5. Meanwhile, significant variations are observed in the intensities of the parent (131) and (300) reflection lines (gray colored circles in Figure 5f,g) at 37 and 42°, respectively, of γ-MnO2 during discharge cycling. Also, additional reflection lines characteristic of the γ-MnO2 phase evolve with increasing intensities along the (110), (120), (031), (130), (230), (040), (200), and (002) planes at scanning angles (2θ) of 21.3, 27.5, 34.3, 35.1, 38.4, 38.8, 40.4, and 44.3°, respectively, as observed from the XRD patterns recorded in Figure 5c−g. These diffraction lines (marked by magenta colored circles in Figure 5) probably arise from Zn-intercalation into γ-MnO2, and the intercalated phase may be represented by γ-ZnxMnO2. Precisely, upon electrochemical discharge cycling, the emergence of diffraction lines attributed to γ-ZnxMnO2 with increasing intensities results in an apparent reduction in the intensities of the reflection lines of the parent γ-MnO2 (gray colored circles in Figure 5f,g).21 However, the observation of

clearly indicate that the oxidation state of Mn(IV) in the fresh electrode transforms into distinct phases with Mn(III) and Mn(II) states after one complete discharge reaction and reverts back to the parent state after subsequent charging. Since the XANES spectra identify the variations over a short-range ordered structure, the XRD studies may shed more light on the long-range structural ordering in the cathode host. In Situ Synchrotron XRD Studies. In situ synchrotron XRD analysis of the cathode in a specially designed spectroelectrochemical cell was performed during the initial discharge/ charge cycle within the potential window of 0.8−1.8 V at a current density of 0.1 mA cm−2. The initial discharge profile is provided in Figure 5a, and the sequentially numbered horizontal bars indicate the specific depths of discharge (DODs), wherein the XRD scans were initiated. The subsequent charge profile and the corresponding in situ XRD data recorded at progressive states of charge (SOCs) are provided in the Supporting Information (Figure S2). The in situ XRD patterns at selected scanning angle domains during discharge cycling are presented in Figure 5b−g. The preliminary XRD pattern related to the OCV of the spectroelectrochemical cell reveals only two major peaks, namely, 37 and 42°, for γ-MnO2. The peak position of the latter reflection line lies in close vicinity to that of the stainless steel (SS) mesh used in the fabrication of the electrode. The synchrotron XRD patterns, in Figure 5b−g, clearly reveal not only the presence of new peaks with varying intensities but also show apparent changes in the intensities of the preliminary peaks. This suggests that new phases evolve and major structural variations occur in γ-MnO2 during Zn-insertion or the electrochemical discharge reaction. In fact, new peaks with increasing intensities G

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Chemistry of Materials

inserted phases. The longer the length of the line, the earlier the correspondig phase is formed during electrochemical discharge or Zn-intercalation. In other words, the spinel phase is initially formed followed by the tunnel-type and layered-type phases. Further, all these structural variations in γMnO2 revert almost entirely to the parent orthorhombic structure, as evidenced by the patterns recorded during the charge cycle (Supporting Information Figure S2). However, a small hump corresponding to the high intensity line of spineltype ZnMn2O4 phase (2θ ∼ 32°) appears to be retained even after complete charging. This behavior clearly suggests that the spinel phase may not completely revert back to the orthorhombic phase and most likely may contribute to some of the specific capacity loss observed during extended cycling. More importantly, the preliminary reflection lines of γ-MnO2 are restored with higher intensities, and simultaneously the diffraction lines of the γ-ZnxMnO2 phase tends to diminish and completely disappear at the end of the charge cycle. The disappearance of all the reflection lines ascribed to γ-ZnxMnO2 after complete charging further strengthens the notion that zinc deintercalation from the 1 × 2 tunnels of the γ-MnO2 structure is feasible. Although Zn-intercalation/deintercalation was initially believed to be possible only in α-MnO2 with wider 2 × 2 tunnels, the realization of Zn-insertion in the other polymorphs of MnO2, viz., γ and δ, has also been reported.12−14 However, the present study clearly confirms that the geometric dimension of the 1 × 2 tunnels is quite sufficient for Znaccommodation/transportation. Concordantly, a recent report revealed that morphologically tailored β-MnO2 pyrolusite with even 1 × 1 tunnels (∼2.3 Å) demonstrates Na-insertion/ deinsertion.55 Ex Situ TEM, ICP Studies (cathode). In order to further confirm the formation of different phases due to Zn-insertion, ex situ analyses were performed on the electrodes recovered from the test cell after discharge cycle measurements. The ex situ HR-TEM image of the electrode recovered after initial discharge, in Figure 6, clearly reveals distinguishable lattice fringes. The fringe width values estimated at a few locations, in Figure 6, well correspond to ZnMn2O4, γ-ZnxMnO2, and LZnyMnO2 phases. Although the observation appears to indicate that there are separate domains related to each of these phases formed during Zn-intercalation, further sophisticated characterization like STEM imaging is required to confirm this.

little or no significant shift in the peak positions of the characteristic γ-MnO2 reflection lines during the entire discharge cycle conforms with the flat plateau observed in the discharge profile of γ-MnO2. The major reflection lines of the parent γ-MnO2 were used to calculate the lattice parameters and unit cell volumes before and after the reduction of Mn. It was estimated that the orthorhombic unit cell expanded by 9.21% after complete discharge, which clearly indicates that it occurs most probably due to Zn-insertion. In addition, as the characteristic lines of the γ-ZnxMnO2 phase evolve with increasing intensities on electrochemical Mn reduction, it is observed that there are little or no significant shifts in their respective scanning angle (2θ) positions, further confirming that Zn-intercalation occurs via a two-phase transition process. Moreover, it is interesting to note that all of the diffraction planes attributed to the γ-ZnxMnO2 phase possibly pass through the 1 × 2 tunnels in the ramsdellite block of γMnO2. However, the results thus suggest that, in addition to the spinel-type transformation, further Zn-insertion or continuous electrochemical discharge leads to the possible expansion of the 1 × 2 tunnels and that the corresponding planes contribute to the new diffraction peaks characteristic of γ-ZnxMnO2 phase. In other words, it is most likely that the planes along which the increasing amounts of zinc ions are inserted in the 1 × 2 tunnels leads to the evolution of reflection lines related to γ-ZnxMnO2 phase with increasing intensities as the discharge reaction proceeds until 0.8 V. The occurrence of Zn-insertion into γ-MnO2 also implies that the manganese oxidation state is reduced from Mn(IV) to Mn(II), as identified by the in situ XANES studies. Thus, the present in situ investigations clearly confirm that Zn-insertion into γ-MnO2 leads to the reduction of Mn(IV) to Mn(II) although this was hypothetically suggested in earlier reports.12−14 More interestingly, from the intermediate stages of discharge, new peaks corresponding to the characteristic (001) and (002) reflection lines attributed to layered-type MnO2 at scanning angles 12.7° and 24.4°, respectively, begin to evolve until the peak intensities become noticeable at complete discharge (green colored squares in Figure 5b,c). This behavior clearly suggests that, in addition to the two structural variations, a portion of the orthorhombic γ-MnO2 undergoes a phase transition to a layered type Zn-inserted MnO2 (L-ZnyMnO2) polymorph with monoclinic symmetry at intermediate discharge. The formation of L-ZnyMnO2 may be explained as follows: As the discharge reaction proceeds toward the lower cutoff potential with apparently increasing amount of Zn-insertion, the 1 × 2 tunnels tend to expand further and ultimately open up the structural framework of γ-MnO2, leading to the formation of a layeredtype polymorph. Therefore, the in situ XRD studies clearly indicate that the parent orthorhombic γ-MnO2 phase not only undergoes phase transitions to spinel-type (ZnMn2O4) but also evolves with reflections attributed to tuunnel-type (γZnxMnO2) and layered-type (L-ZnyMnO2) polymorphs during electrochemical Zn-intercalation and that all of these Mn(III) and Mn(II) phases coexist at full discharge, as observed in the in situ patterns in Figure 5. In fact, the existence of separate Mn(III) and Mn(II) states upon the electrochemical reduction of Mn(IV) was also confirmed by the in situ XANES studies. Representative colored lines (green for layered, magenta for tunnel, and cyan for spinel phases) drawn (in Figure 5b,c,e) from the scan wherein the highly intense characteristic diffraction peaks of each phase begin to emerge until the end of discharge indicate the sequence of the formation of Zn-

Figure 6. HR-TEM image of the cathode recovered from the test cell after completion of the initial discharge cycle. H

DOI: 10.1021/cm504717p Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials Moreover, the ICP-AES analysis of the electrode recovered after initial discharge was performed and the results (provided in Supporting Information Table S1) revealed that the molar stoichiometric ratio of Zn:Mn is 1.2:1. The high zinc concentration clearly suggests that phases in addition to ZnMn2O4 are formed after complete discharge. In other words, the high zinc content may be related to the formation of the γ-ZnxMnO2 and L-ZnyMnO2 phases with Mn(II) states and is supportive of the results observed from the in situ XRD studies. Further, the formation of the Mn(II) phases at full discharge presents the possibility of Mn2+ ions dissolving into the aqueous electrolyte solution. Therefore, ICP analysis was performed on the electrolyte extracted from the test cell after initial discharge. Considering that a volume of 300 μL electrolyte (1 M ZnSO4 dissolved in DI water) was used in the test cell, the estimated manganese content (Supporting Information Table S2) in the electrolyte after first discharge is observed to be significant. On the other hand, no manganese dissolution was observed when a freshly prepared γ-MnO2 electrode was immersed in the aqueous electrolyte for 3 days. These results thus suggest that the origin of the significant manganese dissolution in the electrolyte recovered after the initial discharge of the test cell may be related to the existence of Mn(II) phases upon complete electrochemical Zn-insertion. Overall, the ex situ studies tend to support the conclusions drawn from the in situ XRD and XANES studies. The structural transformation of orthorhombic γ-MnO2 to spinel (ZnMn2O4) and layered (L-ZnyMnO2) phases as a result of electrochemical Zn-intercalation may be well understood by considering the case of Li-insertion in MnO2. In fact, the γMnO2 structure, which is composed of R-MnO2 (ramsdellite) and β-MnO2 (pyrolusite) phases, is usually represented by the distribution of manganese in a slightly distorted hexagonalclose-packed (hcp) array of oxygen anion sublattice. Obviously, the individual R-MnO2 and β-MnO2 structures share an hcp array of oxygen network.28,56−58 On the other hand, the ZnMn2O4 and L-ZnyMnO2 phases share a cubic close-packing (ccp) array of oxygen network. Numerous studies have reported that chemical and electrochemical Li-intercalation or lithiation in R-MnO2 and β-MnO2 introduces a shear motion that transforms the hcp oxygen sublattice to a ccp structure. This action leads to an anisotropic expansion of the orthorhombic unit cell and ultimately leads to the formation of a spinel-type Li-inserted phase (LiMn2O4) under continuous lithiation.28,59,60 Structural phase transitions occurring in MnO2 polymorphs due to electrochemical and chemical Li-intercalation have been reported by several workers.61−63 In the realm of rechargeable lithium and sodium batteries, different MnO2 polymorphs (α, β, γ, and λ) have been extensively investigated as intercalation cathode hosts.33,34,55,64,65 An early report identified the possibility of the formation of a γ-LixMnO2 (orthorhombic) and Li4Mn5O12 (spinel) phase upon chemical lithiation or Li-insertion in γ-MnO2.17 Similarly, a chemical sodiation study has revealed that the insertion of Na-ions in the tunnels of MnO2 polymorphs lead to the rearrangement of the structure to form different polymorphs.66 Also, an electrochemically inactive MnO2 polymorph, β-MnO2 prepared with exposure of preferred facets, demonstrated reversible Naintercalation.55 Concordantly, the insertion of even divalent ions such as Mg2+ and Zn2+ into MnO2 polymorphs with unique tunnel structures and the possible formation of structurally distorted phases on ion-intercalation have been realized.27,67 Specific to Zn-ion battery applications, an α-MnO2

cathode that exhibited reversible Zn-insertion was also reported.13 Structural studies using ex situ SAED and XRD characterizations were performed by Lee at al. to identify a reversible phase transition from the tunneled to layered polymorph in α-MnO2 due to electrochemical Zn-intercalation.68 Moreover, the trace of voltage profile pertaining to the present γ-MnO2 is almost similar to that reported for electrochemical Li/Na insertion in our earlier studies.33,34 Therefore, in the present case, it may be reasonable to argue that the hcp array of oxygen in the parent γ-MnO2 structure experiences a shear upon Zn-insertion toward cubic closepacking of the oxygen array due to the electrostatic interactions between the zinc and manganese ions. This transformation may lead to the formation of spinel-type ZnMn2O4 and layered-type (L-ZnyMnO2) phase domains that share a ccp oxygen anion sublattice. However, the formation of γ-ZnMnO2 domains with the hcp structure may also be related to the variation in the lattice volume (∼9.21%) of the orthorhombic unit cell of γMnO2 as a result of Zn-insertion. Another important factor worth noting is that the structurally distorted Zn-inserted phases may form as a result of the Jahn−Teller effect due to the electrochemical reduction of mangnese from Mn(IV) to the Mn(III) states. In fact, the Jahn−Teller effect has been known to contribute to capacity fading in electrodes based on transition metal ions.69,70 However, investigations in the recent past have indicated that the utilization of submicrometer or nanoscale electrode materials circumvents the problems of Jahn−Teller distortion in transition metal-based electrodes and thereby facilitates the realization of improved electrode capacity retentions.20,71,72 The average particle size of the γ-MnO2 electrode prepared in the present study lies well within the threshold limits (