Kinetics of the Thermally-Induced Structural Rearrangement of γ

Article pubs.acs.org/JPCC

Kinetics of the Thermally-Induced Structural Rearrangement of γ‑MnO2 Wesley M. Dose,† Neeraj Sharma,‡ Nathan A. S. Webster,§ Vanessa K. Peterson,∥ and Scott W. Donne*,† †

Discipline of Chemistry, University of Newcastle, Callaghan, NSW 2308, Australia School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia § CSIRO Process Science and Engineering, Box 312, Clayton South, VIC 3169, Australia ∥ Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ‡

S Supporting Information *

ABSTRACT: This work presents a temperature-dependent and time-resolved Xray and neutron diffraction study of the thermally induced structural rearrangement of γ-MnO2. Here, we study electrochemically prepared γ-MnO2, the manganese dioxide phase used in the majority of battery applications, which we find to be ∼64% ramsdellite [a = 4.4351(6) Å, b = 9.486(2) Å, c = 2.8128(7) Å, and V = 118.33(3) Å3] and ∼36% pyrolusite [a = 4.718(3) Å, c = 2.795(2) Å, and V = 62.22(8) Å3]. Taking a deeper look at the kinetics of the structural rearrangement, we find two steps: a fast transition occurring within 4−8 min with a temperaturedependent ramsdellite to pyrolusite transformation (rate constant 0.11−0.74 min−1) and a slow transition over 4 h that densifies (with changes in unit cell and volume) the ramsdellite and pyrolusite phases to give structures that appear to be temperature-independent. This effectively shows that γ/β-MnO2 prepared in the range of 200−400 °C consists of temperature-independent structures of ramsdellite, unit cell a = 4.391(1) Å, b = 9.16(5) Å, c = 2.847(1) Å, and V = 114.5(6) Å3, and pyrolusite, unit cell a = 4.410(2) Å, c = 2.869(2) Å, and V = 55.79(4) Å3, with a temperature-dependent pyrolusite fraction between 0.45 and 0.77 and increasing with temperature. Therefore, we have linked the temperature and time of heat treatment to the structural evolution of γ-MnO2, which will aid the optimization of γ/β-MnO2 as used in Li-primary batteries.

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INTRODUCTION Manganese oxides are a versatile group of compounds finding use as catalytic materials,1,2 ion-sieves,3−5 biosensors,6 magnetic materials,7 as electrodes in supercapacitors,8−12 and in aqueous and nonaqueous batteries.13−17 The reason for this lies in the structural diversity of manganese oxides, in which manganese can be found in numerous oxidation states (II, III, IV, and VII, being the more common) as well as having polymorphism.18,19 Many manganese oxide phases can be derived from other manganese oxide precursors. For example, β-MnO2, Mn2O3, Mn3O4, and MnO can be produced by thermal decomposition of γ-MnO220 (eq 1, where the consecutive steps are labeled A− D), which is itself an intergrowth of two manganese dioxide phases; namely, ramsdellite (r-MnO2, space group Pbnm21) and pyrolusite (β-MnO2, space group P42/mnm22).13,23 The structure of the ramsdellite and pyrolusite phases (showing the 2 × 1 and 1 × 1 tunnels, respectively) and the γ-MnO2 intergrowth are shown in Figure 1.

structural reflections, as described by Chabre and Pannetier.13 The γ-MnO2 structure contains a number of other structural defects, including twinning planes (Tw or Mt), substitution of Mn(IV) by Mn(III), and Mn vacancies.13,26−28 Associated with the last two defects is the so-called “structural water”, where OH− substitutes for O2− to compensate for charge.26−28 Step A in the thermal decomposition of γ-MnO2 (eq 1) is of particular commercial significance since the synthesis of suitable cathode materials for nonaqueous lithium batteries involves thermal treatment of γ-MnO2 in the range of 250−400 °C.13,29,30 The purpose of the thermal treatment is to remove the structural water,29 but this process also initiates a structural rearrangement of the ramsdellite domains to form the pyrolusite phase, thus increasing Pr.13 This gives rise to a continuum of metastable structures referred to as γ/β-MnO2 (step Ai in eq 1) that have different electrochemical properties depending on the extent of the structural conversion.31,32 While it is common to characterize the structure of the starting γ-MnO2 and the thermally derived γ/β-MnO2 using ex situ X-ray diffraction (XRD), no studies have monitored in situ

The phase fraction of pyrolusite domains in the ramsdellitebased γ-MnO2 structure, denoted Pr, can be determined from an X-ray diffraction pattern using peak-fitting of four major

Received: July 11, 2014 Revised: September 4, 2014 Published: September 17, 2014

© 2014 American Chemical Society

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Figure 1. Schematic of the component phases of γ-MnO2 and its thermally induced structural evolution. Mn atoms are shown in green, O atoms in red, and the [MnO6] octahedra in blue. The diagram shows the structure of single-phase ramsdellite (r-MnO2)24 and pyrolusite (β-MnO2),25 and unit cell parameters are determined for these phases within γ-MnO2. The two steps in the γ-MnO2 to γ/β-MnO2 structural evolution are also shown, indicating the rapid change in the pyrolusite phase fraction (Pr) with little variation in the unit cell parameters during S1, followed by slower changes to the unit cell parameters with constant Pr during S2. Underlined parameters are those determined in this work.

the progress of the thermally induced structural rearrangement or determined the kinetics of this process. Therefore, there remains considerable scope for the optimization of the time and temperature of the thermal synthesis of γ/β-MnO2 through an understanding of the kinetics and process of the structural transformation. Here we present the results of a temperaturedependent and time-resolved in situ XRD and neutron powder diffraction (NPD) study under nonisothermal and isothermal heating conditions.

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EXPERIMENTAL SECTION γ-MnO2 was prepared via anodic electrolysis and hence termed electrolytic manganese dioxide (EMD). Deposition took place onto two titanium electrodes, each 144 cm2, submerged in a deposition bath with 0.1 M MnSO4 and 0.25 M H2SO4 at 97 °C, and with an applied anodic current density of 65 A m−2. The deposited material was mechanically removed from the electrode, washed thoroughly with water, milled to a powder, and sieved to give a mean particle size of 13 μm determined by laser particle size distribution. The crystal form of this γ-MnO2 starting material was determined using ex situ XRD data collected using a Panalytical X’Pert Pro MPD, employing Cu Kα radiation (1.5418 Å) operated at 40 kV and 40 mA. In situ XRD data were collected in transmission mode on an Inel Equinox 3000 diffractometer fitted with a CPS120 position sensitive detector spanning 120° in 2θ. The instrument utilized Mo Kα radiation (0.7107 Å) and was operated at 40 kV and 40 mA. Samples were loaded into 0.7 mm quartz capillaries, which were inserted into a fitting33 shown schematically in Figure 2 (not to scale). This fitting was positioned in a goniometer head and attached to the diffractometer. A gentle flow of compressed air was passed through the capillary via a Teflon tube, and a plug of glass wool was inserted into the funneled end of the capillary in order to prevent the sample from being flushed out. The sample was heated at 200 °C min−1 to 200, 250, 300, 350,

Figure 2. Schematic of experimental setup during in situ X-ray diffraction on the Inel Equinox 3000 (not to scale).

or 400 °C using a hot air blower and held at temperature for ∼120 min. The hot air blower was controlled using a K-type thermocouple positioned directly underneath the capillary/ sample on the vacuum side of the vacuum furnace. The goniometer head and capillary were oscillated about the capillary axis in order to ensure uniformity of heating and improved particle statistics. The presence of the Teflon tube meant that the capillary was only oscillated through 180° to avoid tangling. Data were collected continuously throughout the heating ramp and isothermal treatment, with individual data sets collected for 0.5 min in the 2θ angular range 0.3−86° with a step size of 0.034°. XRD data were analyzed using peak-fitting with an asymmetric Lorentzian function for each reflection superimposed on a flat background. The pyrolusite fraction (Pr) was calculated based on fits of the γ-MnO2 (ramsdellite phase) 110, 130, 221, and 240 reflections, as described by Chabre and Pannetier.13 Examples of the peak fits and the average mean 24258

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squared error for the data sets are given in the Supporting Information. In situ NPD data were collected using WOMBAT,34 the high-intensity neutron powder diffractometer at the Open Pool Australian Light-water (OPAL) research reactor at the Australian Nuclear Science and Technology Organisation (ANSTO). WOMBAT features an area detector that continuously covers 120° in 2θ and has a relatively intense neutron beam, allowing the rapid collection of data. A neutron beam with a wavelength of 2.4166(1) Å was used, determined using the Al2O3 NIST standard reference material 676. Data were obtained with an exposure time of 1 min per pattern in the angular range 30−150° in 2θ. The temperature of the sample was controlled using a vacuum furnace with a niobium element. The sample was held open to the air using a quartz insert sample holder and heated using either an isothermal or nonisothermal protocol. Isothermal experiments were carried out at 250, 350, or 400 °C, in which the sample was heated at 20 °C min−1 to the desired temperature and then held at this temperature for at least 4 h before allowing the sample to cool. Nonisothermal experiments involved a temperature ramp at 2 °C min−1 from ambient temperature to 450 °C, after which the sample was allowed to cool. Diffraction patterns were collected every minute during heating and cooling, and also during the 4 h “soak” time at the highest temperature. Data were analyzed using Gaussian peak-fitting of the γ-MnO2 ramsdellite 021, 200, and 121 reflections using LAMP.35 Example fits and the determined parameters are given in the Supporting Information. γ/β-MnO2 samples for ex situ analysis were prepared by heating γ-MnO2 in a MTI GSL1300X tube furnace using a similar heating profile to the in situ NPD experiments. Samples were removed after 21, 30, or 240 min at 250, 350, or 400 °C, giving a total of nine γ/β-MnO2 samples, and quenched in air. High-resolution NPD data were collected using ECHIDNA, the high-resolution neutron powder diffractometer at the OPAL research reactor at ANSTO.36 The neutron beam wavelength was ∼1.62 Å and was determined accurately using the LaB6 NIST standard reference material 660b. Data were obtained in the 2θ angular range of 6−164° with a step size of 0.125°. Rietveld refinements were performed using the ex situ diffraction data with GSAS.37,38 All peak positions determined from the XRD or NPD diffraction patterns collected above room temperature were corrected for the thermal expansion effects using thermal expansion coefficients for heat treated EMD determined in our previous work.39

Figure 3. X-ray diffraction pattern of γ-MnO2 (Cu Kα radiation, 1.5418 Å) with the ramsdellite reflections labeled (top). Rietveld refinement profile using γ-MnO2 neutron powder diffraction data (1.6255 Å) (bottom). Structural models for ramsdellite24 and pyrolusite25 are used with the reflection markers shown as magenta and cyan vertical bars, respectively. The observed data is shown in black, calculated pattern in red, background in green, and the difference between the observed and calculated in blue. The observed 021, 200, and 121 reflections are labeled (black text), as are the expected positions of the 110, 130, and 221 reflections (gray text).

MnO2 (unit cell parameters of ramsdellite and pyrolusite phases in γ-MnO2 are shown in Figure 1). A number of authors have used NPD to resolve the structure of manganese dioxide phases, including ramsdellite,24 pyrolusite,25 and chemically prepared γ-MnO2.40 However, to our knowledge this is the first time that the structure of electrochemically prepared γ-MnO2 (or EMD), the form used most widely in battery applications, has been determined. Rietveld refinements revealed that the γ-MnO2 sample consisted of ∼64% ramsdellite and ∼36% pyrolusite. For this material the unit cell and profile parameters, as well as the atom position and displacement factors, were refined for both phases. The final refinement parameters for γ-MnO2 are listed in Table 1. The refined unit cell and atomic parameters are listed in Tables 2 and 3, respectively. For comparison, the unit cell parameters of the pure ramsdellite phase reported by Post and

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RESULTS AND DISCUSSION XRD and NPD patterns of the γ-MnO2 sample studied show features that have been attributed to structural disorder (Figure 3). On the basis of the model proposed by Chabre and Pannetier,13 this disorder arrises from “De Wolff” disorder: the intergrowth of ramsdellite and pyrolusite, microtwinning about the (021) and (061) planes of ramsdellite, and point defects such as the substitution of Mn(IV) by Mn(III) and Mn vacancies. Rietveld analysis of the NPD data revealed that γMnO2 can be described by the orthorhombic (Pnma) ramsdellite24 and tetragonal (P42/mnm) pyrolusite25 models. The starting atom coordinates for the refinements were taken from these references. While the structural model does not take into account the intergrowth nature of the material, it gives an indication of the structure of the individual phases within γ-

Table 1. Final Rietveld Refinement Parameters for γ-MnO2 γ-MnO2 no. of variables Rp wRp χ2 24259

29 0.0172 0.0230 2.891

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regions within the structure that do not adjust with respect to the other phase. Stresses between the phases would be expected in this latter case. The in situ XRD experiments revealed that when γ-MnO2 is heated to 200, 250, 300, 350, or 400 °C and held at this temperature, the most pronounced change in the pattern occurs in the range of 2θ = 8−17° (Figure 4). Here the γ-MnO2 ramsdellite 110 and 130 reflections shift toward each other before reaching a steady separation. Complete conversion to βMnO2 is indicated by these peaks merging to form the pyrolusite 110 reflection. The structural change is most pronounced at 400 °C, where the ramsdellite 110 and 130 reflections almost merge completely. At each temperature, the change in the ramsdellite 110 and 130 peak positions occurs almost immediately. As discussed previously, these structural changes have been attributed to the conversion of γ-MnO2 to γ/β-MnO2 and the increase of the pyrolusite phase at the expense of the ramsdellite phase.13,29 Pr shows a rapid increase in the first 8 min (Figure 5), followed by a more gradual increase and then a plateau. At t =

Table 2. Unit-Cell Parameters and Volume for Pure Ramsdellite (as Reported by Post and Heaney24), Pure Pyrolusite (as Reported by Bolzan et al.25), and γ-MnO2 (This Work) Determined Using Neutron Powder Diffraction ramsdellite pyrolusite 4* γ-MnO2

a (Å)

b (Å)

4.5219(3) 4.4041(1)

9.2734(7)

V (Å3)

c (Å)

2.8638(2) 2.8765(1) Ramsdellite 9.486(2) 2.8128(7) Pyrolusite 2.795(2)

4.4351(6) 4.718(3)

120.09(2) 55.79(3) 118.33(3) 62.22(8)

Table 3. Atomic Coordinates and Isotropic Atomic Displacement Parameters (Uiso) for γ-MnO2 Determined Using Neutron Powder Diffraction atom

x

Mn1 O1 O2

0.121(1) 0.040(2) 0.289(1)

Mn1 O1

0 0.357(3)

7* γ-MnO2

z

Uiso (Å2)

0.25 0.75 0.25

0.007(2) 0.020(4) 0.004(2)

0 0

0.48(7) 0.13(2)

y Ramsdellite −0.014(3) 0.229(1) 0.274(1) Pyrolusite 0 0.357(3)

Heaney24 and of the pure pyrolusite phase reported by Bolzan et al.25 are listed in Table 2. Compared to the single-phase structures, the unit cell of both the ramsdellite and pyrolusite components of γ-MnO2 are distorted. The pyrolusite component of γ-MnO2 is expanded along the a axis [7.13(7)%], resulting in a larger unit cell volume than pure pyrolusite [11.5(2)%]. Conversely, the ramsdellite component has a similar unit cell volume compared to its pure form [−1.47(4)% difference], although it is anisotropically distorted with changes in all cell axes, −1.92(2)% for a, −1.78(3)% for c, and 2.29(3)% for b. It is possible that the expansion/contraction behavior observed in one of the phases is closely connected to the behavior of the other phase. The extent to which the expansion/contraction in one phase influences the behavior of the other is dependent on the extent of mixing of the ramsdellite and pyrolusite phases in the γ-MnO2. Molecular level mixing will mean that there is a strong interaction between the phases; however, larger scale mixing (even on the nanometer scale) may lead to isolated

Figure 5. Time dependence of the phase fraction of pyrolusite (Pr) during isothermal heat treatment at 200 °C (deg), 250 °C (Δ), 300 °C (▽), 350 °C (+), and 400 °C (×). The solid colored lines show the fit of a first-order growth rate equation to the data. Errors are omitted for clarity.

0, γ-MnO2 has a Pr = 0.4, indicating that there is a higher fraction of the ramsdellite phase (2 × 1 tunnels) compared to the pyrolusite phase (1 × 1 tunnels). The increase in Pr indicates the rearrangement of a fraction of the ramsdellite to pyrolusite, with higher temperature giving rise to a greater conversion. For instance, after heating at 200 °C, Pr = 0.45, compared to Pr = 0.77 at 400 °C. While this temperaturedependent transition is expected and consistent with previous

Figure 4. Time dependence of a selected region of the XRD patterns collected every 30 s during isothermal treatment at 350 °C. The ramsdellite 110 and 130 reflections are identified. Dashed lines are to guide the eye only (left). Difference between the position (2θ) of the ramsdellite 130 and 110 peaks with respect to time (right). 24260

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where T is the temperature in °C and is shown by the red line in Figure 6]. However, we have previously shown that the magnitude of the pyrolusite fraction with respect to temperature is dependent on the environment in which the sample is heated.42 A flowing air atmosphere, as used in the current work, has been shown to slightly retard the structural transition, giving rise to lower Pr values than achieved in static air.42 The rate constant for the structural change is minimum at 300 °C and increases below and above this temperature. The cause of the slower kinetics at 300 °C relative to other temperatures is not understood at this time. Temperature-dependent NPD patterns of γ-MnO2 were collected during heating at 2 °C min−1 (Figure 7). The onset temperature for the γ-MnO2 to γ/β-MnO2 structural change is 110 °C. Changes occurring at temperatures higher than 400 °C are likely due to isolated domains in which MnO2 is reduced to form Mn2O3.43 Therefore, we attribute the structural changes between 110 and 400 °C to the γ-MnO2 to γ/β-MnO2 transition. From the XRD data, after heat treatment at 400 °C, Pr = 0.77, suggesting a complete phase change to β-MnO2 (Pr = 1) is not realized prior to the thermal decomposition of the material. Generally, the γ/β-MnO2 crystal structure becomes more compact with increasing temperature, as expected from the comparative unit cell size of the ramsdellite and pyrolusite phases.13 The changes in the reflections up until 400 °C are nonuniform, showing greater contraction in the ramsdellite (200) crystal plane [−1.10(1)%] compared to the ramsdellite (021) and (121) planes [−0.18(1)% and −0.14(1)%, respectively]. Furthermore, a slight expansion is observed between 220 and 300 °C in the (021) and (121) crystal planes [0.15(1)% and 0.07(1)%, respectively], although the structure subsequently recontracts in the range of 300−400 °C [−0.12(1)% and 0.07(3)%, respectively]. The full width at half-maximum (fwhm) of the 200 reflection is subtly smaller than the 021 reflection during heating, which may indicate an anisotropic crystallization process and the formation of larger crystallites in the a axis direction compared to the b/c axis direction. However, further work is required to verify this

reports,13,30 the rate at which it occurs has not previously been determined. After only 4−8 min, the extent of the γ-MnO2 to γ/β-MnO2 phase change thermodynamically possible at a given heating temperature is essentially complete, even at the lower temperatures. Given the long heating times typically used to prepare γ/β-MnO2 for nonaqueous batteries (2−48 h),29,30,41 this is a surprising result. The change in Pr with respect to time (Figure 5) was fitted with the first-order rate expression: Pr = (Pmax − P0)(1 − exp−kt ) + P0

(2)

where P0 and Pmax are the initial and maximum Pr values, respectively, t is the time (min), and k is the rate constant (min−1). The fitting parameters and residuals for these fits are included in the Supporting Information. The trends in Pmax and k with respect to temperature (Figure 6) show that Pmax

Figure 6. Temperature dependence of the rate constant (k) (gray lines to guide the eye only) and maximum pyrolusite phase fraction (Pmax) determined from the first-order growth kinetic 2. The red line is a fit of a quadratic equation [Pmax(T) = 5.5(1) × 10−6T2 − 1.7(8) × 10−3T + 0.6(1), where T is the temperature in °C] to the Pmax data (r2 = 0.995).

undergoes quadratic growth between 200 and 400 °C [Pmax(T) = 5.5(1) × 10−6T2 − 1.7(8) × 10−3T + 0.6(1),

Figure 7. (a) Selected region of the in situ neutron diffraction patterns [2.4166(1) Å] during isothermal treatment at 350 °C. Dashed lines are to guide the eye only. (b) Changes in the d spacing of the γ-MnO2 ramsdellite 021, 200, and 121 reflections obtained using LAMP as a function of temperature. Errors are smaller than the symbols. 24261

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observation. These trends represent the structural changes during the γ-MnO2 to γ/β-MnO2 transition. However, the subtle movement of the ramsdellite 021, 200, and 121 reflection positions mean these changes are not observed in the in situ XRD patterns. While the reason for the behavior is not fully understood at this time, the nonlinear change in the crystal structure with respect to temperature may partially explain electrochemical performance data reported for γ/β-MnO2. Previously we have shown that γ/β-MnO2 exhibits superior discharge capacity after thermal treatment at 250 and 350 °C but poorer performance after treatment at 200, 300, and 400 °C.30 The similar spacing of the ramsdellite (021) and (121) crystal planes at 250 and 350 °C (Figure 7) is correlated to electrochemical performance data that indicate optimal lithium ion insertion, diffusion, and a higher lithium content in γ/βMnO2 prepared at 250 and 350 °C, compared to that in γ/βMnO2 prepared at 200, 300, and 400 °C. Gaussian peak fitting of the time-resolved NPD data collected under isothermal heating conditions (Figure 8) shows two steps in the structural evolution: a rapid, relatively large structural change from 0 to 30 min (S1), followed by a more steady, subtle change from 30 to 240 min (S2). During S1, the γ/β-MnO2 structure goes through the same changes as in the nonisothermal experiment (Figure 7). That is, the structure of γ/β-MnO2 formed after isothermal heating at 250 °C is similar to that formed by nonisothermal heat treatment to 250 °C, and the structural evolution follows a similar pathway. Under isothermal conditions, however, the structural rearrangement takes place over a much shorter time frame (