Electrochemical Response of Nanocrystalline Tetragonal Manganese

9644

J. Phys. Chem. C 2007, 111, 9644-9651

Electrochemical Response of Nanocrystalline Tetragonal Manganese Dioxides Prepared by Spray Vapor Pyrolysis and Ball Milling Christiane Poinsignon,*,† Holger Klein,‡ Pierre Strobel,‡ Claude Roux,† and Christine Surcin§ LEPMI Laboratoire de Physico-Chimie des Mate´ riaux et des Interfaces INPG ENS Electrochimie Electrometallurgie de Grenoble UMR CNRS 5631, BP 75 F-38402 St Martin d’He` res Cedex, France, Institut Ne´ el, CNRS et UniVersite´ J. Fourier, BP 166, F-38042 Grenoble Cedex 9, France, and Laboratoire de ReactiVite´ et de Chimie des Solides CNRS UMR 6007, 100 rue Saint Leu 80039 Amiens Cedex ReceiVed: October 5, 2006; In Final Form: February 18, 2007

The electrochemical and microstructural properties of nanometric tetragonal β-MnO2 (MD) prepared by highenergy ball milling (BM) and spray pyrolysis (nano-β) were studied in comparison with the original volumetric phase (micro-β). The microstructure was characterized by X-ray diffraction and transmission electron microscopy. Electrochemical properties were studied by step linear voltametry in 1 M KOH. Both the equilibrium potential and the incremental capacity at high potential of the MnO2 electrode increase when the crystallite size decreases. Ball-milled MD and nano-β both become insertion materials but only the later presents a good cyclability. The increase in electrode potential in 1 M KOH measures the decrease in Gibbs free energy due to the surface energy contribution (1.3 J/m2) generated by the nanometric size of the particles at the solid-liquid interface. Thermogravimetric analysis measurements in the 300-1200 °C range show for all samples a decrease in MnO2-Mn2O3 transition temperature which also leads to a calculation of the surface energy of nanometric samples, according to the Gibbs-Thomson relationship. At the solid-gas interface, nano-β has a surface energy σ three times larger than BM samples: 0.6 J/m2 for BM samples and 1.9 J/m2 for nano-β ones. That illustrates the main differences between both synthesis processes: crystal growth under equilibrium for nano-β, size decrease by striking, and lamination leading to unstable nanometric BM samples.

Introduction Electrochemical and surface properties of a micrometric β-MnO2 (micro-β) and a homodispersed nanometric tetragonal β-MnO2 (nano-β) prepared by spray pyrolysis were studied by electrochemical potential spectroscopy and acid-base titration.1 Through the measurement of the electrode potential E in a quasiequilibrium state during slow reduction in aqueous electrolyte, it was possible to determine and quantify the Gibbs free energy variation (∆G) induced by the nanometric size and magnification of surface energy. Equation 1 expresses the contribution of surface energy to the Gibbs free energy variation measured by the potential E of the nanometric material electrode

∆G ) ∆H - T∆S + γ∆A ) - nFE

(1)

where A is the specific surface area, γ is the surface strain, ∆H and ∆S are the enthalpy and entropy variations, F is the Faraday constant, and n the number of exchanged electrons in the redox reaction during reduction of MnO2 into MnOOH:

MnO2 + H+ + e- r f MnOOH In this study, the electrochemical properties of six β-MnO2 samples of same stoichiometry differing only by grain size and preparation process are compared during their reduction by proton insertion in 1 M KOH and by lithium insertion in LiAsF6propylene carbonate electrolytic solution. These samples are a * Corresponding author. † LEPMI INPG ENS. ‡ CNRS et Universite ´ J. Fourier. § Laboratoire de Reactivite ´ et de Chimie des Solides.

commercial micrometric phase (micro-β), a nanocrystalline phase (nano-β) obtained from manganese nitrate by spray pyrolysis,1 and new nanometric samples prepared by high-energy ball milling (BM) from the micrometric phase. The first two materials present a high chemical homogeneity and uniform particle size, especially the nanocrystalline phase. Is this still valid for ball-milled samples? The aim of this study is to compare the microstructure and electrochemical properties of these different nanometric β-MnO2 during reduction by H+ and Li+ insertion in order to probe the effect of grain size and strain induced by BM on the electrode potential and cyclability of the nanometric oxides. Experimental Section Materials. Three kinds of β-MnO2 samples were used in this study: (i) a commercial sample prepared by thermal decomposition of manganese nitrate at 300 °C purchased by Touzart and Matignon company, named “micro-β,” (ii) the nanocrystalline sample (“nano-β”) synthesized by spray pyrolysis as described in ref 1, and (iii) two series of nanometric β-MnO2 prepared by high-energy BM of micro-β in a Spex 8000 mill with 2 ball/ sample weight ratios (r) of 3.5 and 7 and different milling times (t; 1, 2, 4, 8, 16 h and 1, 2, and 12 h, respectively). The samples are then named “spr-t”, typically sp3.5-16 for a 3.5 ball/ sample weight ratio milled for 16 h and sp7-12 for a 7 ball/ sample weight ratio milled for 12 h. The sample names used throughout this paper and conditions of milling are given in Table 1. Microstructure. Crystallographic characterization was carried out by X-ray diffraction (XRD). An average crystallite size was determined from XRD peak broadening analysis.2 XRD patterns

10.1021/jp0665473 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007

Nanocrystalline Tetragonal Manganese Dioxides

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9645

TABLE 1: Nomenclature and Milling Conditions of Samples name

origin

ball/sample weight ratio

milling time (h)

micro-β nano-β sp3.5,1 sp3.5,16 sp7,2 sp7,12

commercial ref. 1 micro-β ibid. ibid. ibid.

3.5 3.5 7 7

1 16 2 12

TABLE 2: Cell Parameters of BM Samples milling time (h)

a

c

line number

R

1 2 12 16 nano-β1 pyrolusite6

4.402(2) 4.400(3 4.397(7) 4.394(90!) 4.4004(4) 4.3983

2.872(1) 2.872(2) 2.870(4) 2.865(10) 2.8707(1) 2.873

5 5 5 4 5

0.0042 0.0066 0.016 0.063

were collected on a Bruker D8 powder diffractometer equipped with monochromatized Cu KR radiation and operated in transmission geometry. The recording conditions were 2θ step 0.02° and counting time 80 s/step. The instrumental width was estimated from quartz diffraction peaks measured in similar conditions. Individual peaks were fitted using the Siemens Diffrac-AT software package to extract experimental full widths at half-maximum (FWHM) and integral widths (L). Strain and size components were extracted from the line widths using the Williamson-Hall (WH) analysis.3 This method uses the fact that the crystallite size contribution varies as (cos θ)-1, whereas the strain contribution varies as tan θ. The equation used is

L cos θ ) λ/D + k sin θ

Figure 1. X-ray diffraction pattern of nano-β MnO2 samples showing an enlarged part of the X-ray diffraction patterns of ball-milled nano-β MnO2 samples with reflections 110, 101, and 111.

(2)

where L is the integral width, λ is the wavelength used, D is the size of coherent diffraction domains, k is a near-unity constant, and is the microstrain term. As a result, a plot of (L cos θ) as a function of (sin θ) yields D from the constant term and from the slope. Infrared (IR) spectra were recorded in transmission geometry with a Nicolet 710 FTIR on pellets made from less than 1 mg of sample powder dispersed in a 150 mg BrTl matrix to gain a sufficient transparency of the semi-metallic sample.4 Weight loss and thermal effects were investigated by thermogravimetry on a SETARAM TAG 24 microbalance apparatus using a 8°/min heating rate up to 1200 °C under Ar flow. Morphology. A transmission electron microscopy (TEM) study was conducted using a Philips CM300ST electron microscope operated at 300 kV in order to get a more detailed view of the morphology of individual crystallites of nanometric samples. A suspension of powders in ethanol was created by ultra-sound agitation, and a drop of the suspension was deposited on a copper grid covered by a holey carbon film. Electrochemical Measurements. The electrochemical study was performed in aqueous electrolyte using the same electrochemical setup as in ref 1. The electrochemical response of manganese dioxide samples was studied in a three-electrode cell with a Hg/HgO reference electrode, a nickel wire as the counter electrode, and a mixture of oxide and graphite (1:3) as the working electrode in a 1 M KOH electrolyte; the graphiteMnO2 mixture is shaped into a circular waffle of 16 mm diameter. The cell was monitored using a MacPile computercontrolled system (Biologic, Claix, France). For an accurate determination of I(V) and x(V) curves, reduction and oxidation

Figure 2. IR spectra of BM samples (sp3.5-1 and sp3.5-16 ) recorded in transmission geometry.

Figure 3. TGA curves of micro-β, nano-β, sp3.5-16, and sp7-12 samples recorded with 8 °C/min heating rate from ambient temperature to 1200 °C under argon flow.

were carried out under potentiostatic control with a slow voltage step scanning rate: 10 mV per 2 h. Li insertion was studied in a button cell with lithium as negative and comparison electrode, a 1 M LiPF6 ethylene carbonate-dimethyl carbonate (EC-DMC) electrolyte and a

9646 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Poinsignon et al.

Figure 4. TEM images of the micrometer scale morphology of samples nano-β (a), sp3.5-16 (b), and sp7-12 (c).

TABLE 3: Weight Losses and Reaction Temperatures Recorded by Thermogravimetry from 278 to 1300 K under Ar Flow on Micro-β and Nanometric Ball-Milled Samples sample

∆pH2O(300 °C) (%)

T (K) MnO2 f Mn2O3

∆p02 (%)

∆ptotal(700 °C) (%)

T (K) Mn2O3 f Mn3O4

D (nm) (coherence length)

micro-β nano-β sp3.5-1 sp7-2 sp3.5-08 sp3.5-16 sp7-12

0.57 1.3 1.8 2 2.1 3.1 3.1

918 813 886 873 870 878 860

8.43 7.6 7.644 7.16 6.524 6. 5.13

9 8.94 9,44 9.16 8,52 9.1 8.23

(957 °C) 1230 (966 °C) 1239 (985 °C) 1258 (977 °C) 1250 (980 °C) 1253 (982 °C) 1255 (990 °C) 1263

500 88 89 48 30 18 15

TABLE 4: Microstructural Properties of β-MnO2 Samples technique sp3.5, 1 sp7,2 sp7,12 sp3.5,16 nano-β particle size/nm TEM XRD: hk0 hkl (l*0) microstrain term XRD: hk0 hkl (l*0) a

89 a 0.0026 0.035

48 a 0.0041 0.039

5-20 15 ≈27 a 0.047

10 18 ≈50 a 0.057

40-70 88 a 0.0046 0.045

Poorly defined (see text).

composite cathode prepared according to the protocol given in ref 5. Cells were cycled under galvanostatic control with C/50 regime. Results 1. Chemical and Structural Properties of the Samples. XRD patterns of micro-β, nano-β, and ball-milled samples were found to be fully consistent with the expected pyrolusite (rutiletype) structure, space group P42/mnm (Figure 1). Refined cell parameters for the nano-β sample are a ) 4.4004(4) and c ) 2.8707(1), in excellent agreement with ref 6. Table 2 presents the cell parameters of the different samples. They seem to decrease with increasing milling time, but at the same time, the residue of refinement R increases considerably because of the decrease in peak maximum accuracy related to the broadening effect of high milling time. IR spectra of BM samples are shown in Figure 2. They are very similar to those of micro-β and nano-β, which were presented in Figure 2 of ref 1. The main differences concern the intensity of Mn-O vibration modes and their resolution, which decrease with increasing milling time. The two lowfrequency modes at 400 and 337 cm-1 confirm the rutile structure of all samples.7 In the high-frequency range, a small signature of OH stretching modes is observed at 3300-3500 cm-1, indicating the presence of OH groups or adsorbed molecular water: they are more important for the 12 and 16 h

milled samples than for samples milled for short times, because of the hydration of the large surface generated by BM. The presence of H2O or OH groups was investigated further by thermogravimetry (TG) in the 25-300 °C range. Effects of the nanometric size on chemical reactions with oxygen loss occurring at higher temperature were investigated as well by TG up to 1200 °C. These reactions are the reduction of MnO2 to Mn2O3 (eq 3) then to Mn3O4 (eq 4) which are observed at 645 and 960 °C, respectively, for bulk pyrolusite-type MnO2:

2MnO2 f Mn2O3 + 1/202

at 645 °C for micro-β

(3)

3Mn2O3 f 2Mn3O4 + 1/2O2

at 960 °C for micro-β

(4)

Figure 3 presents the thermograms of micro-β, nano-β, sp712, and sp3.5-16 samples in the 25-1200 °C range. Values of transition temperatures and associated weight loss are given in Table 3. Micro-β loses less than 0.6% weight on heating up to 350 °C, whereas larger weight losses are observed on nanometric samples: from 1.3% for nano-β to 3.1% for the 12 and 16 h milled samples. At 645 °C, the mass loss observed for micro-β (8.43%) is fairly close to the theoretical value for reaction 3 (9.2%). For nano-β, this reaction occurs at 540 °C with a lower oxygen loss (7.6%), as expected from the 1.3% weight loss due to the elimination of OH surface groups below 300 °C. This also holds for ball-milled samples, with weight losses of 5.13% and 6.0% for sp7-12 and sp3.5-16, respectively, and decomposition temperatures intermediate between those of micro-β and nanoβ. Upon further heating, reaction 4 is observed for all of the nanometric samples at a slightly higher temperature (990 °C) than for micro-β (960 °C). 2. Morphology and Microstructure. The morphology of samples has to be investigated at different length scales. Figure 4 gives an example of images of three representative samples at relatively low magnification. The nano-β sample is composed



Figure 5. (a) TEM image showing individual grains of the agglomerates of sample nano-β. Well-defined facets can clearly be observed. (b) Single crystal seen along its long dimension. The lateral facets are perpendicular to the [110] directions of the tetragonal cell. The darker contrast in the upper and left part of the crystal is due to structural defects. (c) TEM micrograph of the sp7-12 sample. The smallest grains have a diameter of about 5 nm, but bigger grains can be observed as well. (d) TEM micrograph of the sp3.5-16 sample. Smallest grains are about 10 nm in diameter.

Figure 6. Williamson-Hall plot of integral linewidth L for various nanometric MnO2 samples. Closed symbols: hk0 reflections; open symbols: hkl (l * 0).

of round aggregates of nanosized particles (Figure 4a). The size of the aggregates is rather uniform between 600 and 700 nm in diameter. In the sp3.5-16 (Figure 4b) and sp7-12 (Figure 4c) samples, nanosized particles are also agglomerated, but the agglomerates do not show any particular state of aggregation.

Figure 7. Variation of the size of coherent diffraction domains D(hk0) with milling time.

In addition to the type of particles shown here, some larger grains with diameters of a few microns were also observed in both samples. At a larger magnification, the morphology of the particles constituting the agglomerates of the nano-β sample can be observed (Figure 5a). They are clearly facetted in the shape of


Poinsignon et al.

Figure 8. Variation of incremental capacity with potential during potentiostatic reduction by step of 10 mV per 2 h for: micro-β sp7-1, sp3,2, sp7-12, sp3,5-16 and V(x) curves for the 5 samples. Note that ∆q is written instead of for (dq/dV).

parallelepipeds capped with square pyramids on each end. This morphology is consistent with that expected for tetragonal crystallites with a rutile-type structure (crystal class 4/mmm). The particle size varies little. Their square section has edges of about 40 nm while the distance between the summits of the pyramid caps is about 70 nm. One of these particles seen along its long direction is shown in Figure 5b. Again, the well-defined facets of the crystal are visible. The lattice fringes observable in this image show that the particle is a single crystal and that its lateral surfaces are perpendicular to the [110] directions of the tetragonal unit cell. The darker contrast in the upper and left parts of the crystal are due to a high density of planar defects perpendicular to the [100] direction.

Figure 5c shows the details of the morphology of sample sp7-12. Very small crystallites which can be identified by their different orientations of lattice plane fringes can be observed. Their mean size is about 5 nm, and no particular facets are visible. These extremely small and thin grains are agglomerated around a larger one in the center of the image. Here, the same lattice plane fringes can be observed for about 20 nm. A similar image of sample sp3.5-16 shows that here the smallest grains are approximately twice as large as those in the previous sample and do not show any facets. The average crystallite size (or more exactly the size of coherent diffraction domains D) was also calculated from the broadening of XRD reflections. This effect can be appreciated



Figure 9. Variation of OCV of the nanometric samples with particle size.

from Figure 1, where line broadening is clearly observed when going from the bottom diagram (1 h milling) to the top one (16 h milling). Strain generated by BM was simultaneously determined from the Williamson-Hall relationship (see eq 2). Plots of (L cos θ) as a function of (sin θ) are given in Figure 6. They show a remarkable difference in angular dependence of the line width for different families of inter reticular planes: the hkl (l * 0) planes yield an important contribution of microstrains (high slope), while this effect is almost negligible in hk0 planes. This shows evidence that this material behaves rather anisotropically on milling. Quantitative results are summarized in Table 4; note that because of the high slope and the high inaccuracy of extrapolating to sin θ ) 0, grain size values for hkl planes are poorly defined. TEM image presented in Figure 4a in ref 1 allows for the estimation of the average grain size of micro-β sample isles different than 500 nm. Both TEM and XRD show that BM decreases considerably the crystallite size and that the dominant factor is milling time. For the hk0 planes family, the crystallite size D is reduced from approximately 90 to 15 nm after 12 h milling; the evolution of D (hk0) with milling time is given in Figure 7, showing a dramatic decrease in size in the first hours of milling; the maximum milling seems to be reached before 12 h. Milling seems to increase microstrains but in limited proportions: values of the microstrain term are in the range 0.035-0.039 for 1-2 h milling and reach 0.057 after 16 h milling along the hkl planes; it is ten times lower along the hk0 planes. Note that the nano-β sample obtained by pyrosol route has also a rather high value (0.045). 3. Electrochemical Properties. The variation of incremental capacity dq/dV with potential during the reduction process in 1 M KOH of micro-β, sp7-1, sp3.5-2, sp7-12, sp3.5-16, and nano-β is presented in Figure 8. As previously observed,1,8 electrochemical reduction of micro-β gives rise to a unique, wide current wave with maximum intensity at -0.52 V versus HgHgO. Compared with this very simple shape, the following voltammograms present a new reduction wave at much higher potential (ca. -0.15 V vs Hg-HgO); its area (hence associated electrochemical capacity) clearly increases considerably with increasing milling time: this modification is complete for sp3.5-16 which delivers 50% of its capacity above -0.35 V in two waves peaking at -0.2 V and -0.1 V. For sp7-12, the capacity above the same potential value is 45%. The smaller the crystallite size, the higher the potential at which the capacity is recovered and the higher the related OCV (open circuit potential). The OCV varies linearly with D (see Figure 9a): it increases from -0.09 V (/Hg-HgO) for micro-β to +0.045 V

Figure 10. I(t) curves recorded during the reduction in 1 M KOH by steps of 10 mV per 2 h; (above) sp3.5-2, (middle) sp7-12, (below) sp3.5-16. Abscissa, time in hours; left ordinate, potential of the working electrode (Uw) versus Hg-HgO reference electrode; right ordinate, intensity in milliamperes per gram.

for sp3.5-16. In addition, Figure 9b shows that the incremental capacity recovered over -0.35 V varies linearly with 1/D. Chrono-amperograms of sp3.5-2, sp7-12, and sp3.5-16 samples are presented in Figure 10. They show the variation of reduction current intensity with time during each potential step: current relaxes during the main part of the step and confirms the observation made for nano-β in ref 1, namely, that these samples behave now as insertion materials. A comparison of the three chrono-amperograms shows a better resolution of the potential of Mn sites with milling time. The 16 h milled sample delivers intense current during the first 60 h of the reduction process at high potential because of the reduction of Mn4+ situated in two types of sites at -0.1 and -0.2 V. Figure 11 presents the V(Q) curves recorded during Li insertion in micro-β, nano-β, and sp7-12 samples: they all present a potential plateau at 2.8 V versus Li/Li+, whatever the crystallite size is. The recovered capacity seems to increase with decreasing particle size; only nano-β presents a good cyclability (more than 20 cycles in C/50 regime with low capacity loss).


Poinsignon et al.

Figure 11. V(Q) curves recorded during the reduction in EC-DMC LiPF6 electrolyte of micro-β (left), nano-β (middle), and sp3.5-16 (right).

Discussion The main difference in microstructure between the two types of nanometric particles under study is well-illustrated by the TEM images presented in Figure 5: nano-β prepared by spray pyrolysis is made of well-defined and dense particles (Figure 5a) of relatively large size (70 over 40 nm), whereas BM particles are globular, very thin and aggregated. XRD pattern as well as IR spectra confirm that all samples are of rutile type, whereas the broadening of XRD lines and IR vibration modes account for decreasing crystallite size and loss of well-defined particle shape. Thermograms of micro-β, nano-β, sp7-12, and sp3.5-16 in Figure 3 give evidence to a decrease in the MnO2 decomposition temperature with nanometric size. Surprisingly, in spite of a larger crystallite size (88 nm), the homogeneously dispersed nano-β has the lowest reaction temperature; this decomposition reaction also occurs in a narrower temperature range than for heterogeneously dispersed BM samples. This observation, in addition to the TEM images analysis, leads to consider the BM samples and the couple micro-β and nano-β separately. GibbsThomson9 relationship (eq 5) relates the lowering of the decomposition temperature T to crystallite size r ) D/2; it allows therefore for the determination of the surface energy σ (joule/ m2):

(T - T°)/T°) -2Vβ/∆H‚σ‚1/r

(5)

where T° is the decomposition temperature of bulk β-MnO2, Vβ is its molar volume (3.35 10-5 m3/mol), and ∆H is the enthalpy of the MnO2-Mn2O3 reaction (-146.8 kJ/mol at 298 K).10 A plot of (T-T°)/T° versus 1/D (see Figure 12) provides evidence of one straight line going through the ”sp” samples points and a second line connecting micro-β and nano-β points. From their slope, surface energies of 0.55 and 1.95 J/m2 are obtained for BM and nano-β, respectively. Such a higher surface energy for the self-generated free particles was previously observed by Nanda et al.11 Moreover, the intercept of the “sp” straight line with the y axis does not coincide with the micro-β point: that reveals the out of equilibrium state induced of BM samples. It must be emphasized that the weight loss due to water departure in the 25-350 °C range varies linearly with D and that the weight loss due to oxygen elimination in the 350800 °C range (a phenomenon concerning the bulk material) varies linearly with 1/D. They confirm that the assignment we did to surface water elimination before pyrolusite to bixbyite transition is not a parasitic effect generated by metallic pollutant. This large difference in surface energy between samples illustrates the influence of preparation processes on the cohesion energy.12 BM provides hetero-dispersed aggregates of nanometric particles by laminating and splitting micro-β crystals, depending on milling times (Figure 5). A contamination by metallic impurities (less than 0.3% from previous XPS measure-

Figure 12. Variation of (T - T°)/T° with 1/D for all samples with T° transition temperature of micro-β to bixbyite. (D is the coherence length deduced from WH XRD pattern analysis).

ments on BM samples) is possible. Spray vapor pyrolysis, on the other hand, generates at 300 °C individual, dense, homodispersed crystals of nano-β of great purity. The first factor which controls the decomposition reaction at the solid-gas interface is the surface energy. Nanda et al.11 show that it is related to cohesion energy which controls the oxygen departure, higher for nano-β than for metastable BM samples. The variation of the equilibrium potential of electrodes in aqueous electrolyte Eeq with the crystallite size of manganese dioxide is presented in Figure 9. Eeq varies roughly linearly with D whatever the synthesis process of nanometric MnO2. The Eeq measurement concerns the liquid electrolyte-MnO2 interface controlled by the Mn4+/Mn3+ couple, the potential of which is influenced by the average coordinence number of Mn on the surface of the crystallite:1 the smaller the crystallite, the lower the Mn average coordinence and the higher the value of the Mn4+/Mn3+ potential. Equation 1 given in Introduction relates the increase in electrode potential to the increase in surface energy γ∆A in aqueous electrolyte. The specific surface area of nano-β and micro-β was determined by nitrogen adsorption to be13 4 and 61.5 m2 g-1, respectively. The increase in Eeq inferred by the nanometric size to the β-MnO2 phase, 0.068 V (Table 5), is related to an increase of surface area ∆A of 57.5 m2 g-1, (the electron number n exchanged in the redox relation between MnO2 and MnOOH is 1). The calculated surface energy is 6562 J/mol or 75.4 J/g which leads to a surface energy of 1.31 J m-2, at the solid-liquid interface, in relatively good agreement with



TABLE 5: Electrochemical and Microstructural Data of the Different β-MnO2 Samples

a

sample

micro-β

nano-β

sp7-1

sp3.5-2

sp3.5-16

sp7-12

OCV(V) (Hg-HgO 1 M KOH) OCV (SHE) (V) X ) Q/Q0 at -0.35 V D (nm) S0 (m2 g-1)

-0.09 0.73 0 500 4

-0. 022 0.798 0.15 88 61.5

0.024 0.844 0.225 89 n.m.a

0.039 0.859 0.3 48 n.m.

0.045 0.865 0.5 18 n.m.

0.035 0.855 0.45 15 n.m.

Abbreviation n.m ) not measured.

1.9 J/m2 obtained from the decrease in decomposition temperature in air. The surface tension γ ) 0.023 J can be determined separately at the liquid-solid interface by the electrode potential measurement.1

∆E ) - ∆G/nF ) γ∆A ) σ If the cyclability of nano-β was improved in comparison with micro-β,1 this is no longer valid for BM samples: nanometric size improves their thermodynamic and kinetic properties during the reduction process, but the oxidation process of the reduced samples is poor, likely because of the nonequilibrium state of the nanometric material generated by BM. The reduction of these samples by lithium insertion in organic electrolyte does not manifest the influence of the nanometric size on the potential of the electrode material: it is clear in Figure 11 that the reduction potential of micro-β, nano-β, and sp3.5-16 is the same, 2.8 V versus Li. Any significant potential variation is not observed for lithium insertion. That can be explained by the large potential range of the Li/Li+ couple and the presence of the solid electrolyte interface (SEI). In lithium batteries, the organic electrolyte is immediately degraded in contact with lithium and inserted compounds and generates the solid electrolyte interface. When a nano-β electrode performed with success several redox cycles, it was impossible to reoxidize the BM electrode after the first reduction. Thus, BM destroys the cyclability of the electrode material for lithium insertion as well as for proton insertion in aqueous electrolyte. Our observations are not in agreement with those of Kang et al. in their study of BM LiMn2O4,14 but these authors used BM to coat cubic LiMn2O4 with carbon. They observed that strain generated by BM neutralizes the Jahn Teller distortion during discharge, blocks the transition from cubic to tetragonal phase, and improves significantly the cyclability of Mn spinels for lithium insertion. Anyway, they did not notice any increase in potential of the nanometric phase during its reduction by lithium insertion, in agreement with our observations. Here, TEM images (Figure 5) and microstructural properties given in Table 4 show that tetragonal β-MnO2 reacts very anisotropically to BM. The different thermodynamic responses of H and Li inserted MnO2 come from the different interactions of Li+ and H+ and the oxygen network of MnO2. For H-MnO2, XPS measurements showed15 that oxygen anionic network carries the charge variation generated by the variation in coordinence of Mn4+ (MnO2) and its reduction into Mn3+ (MnOOH), then a proton covalently bound to oxygen probes immediately the charge

variation of the anionic network. For Li-MnO2 in organic electrolyte, immediate degradation of the electrolyte in contact with lithium and inserted dioxide generates the SEI2 which screens and levels the surface energy variations induced by the nanometric size of material in remaining permeable to Li+. Conclusions This study of electrochemical and microstructural properties of nanometric β-MnO2 prepared by spray pyrolysis and BM shows the main difference between both synthesis processes: crystal growth under equilibrium for spray pyrolysis (nano-β), size decrease by striking, and lamination leading to unstable nanometric samples for BM. In aqueous electrolyte, the equilibrium potential Eeq increases regularly when the crystallite size of the electrode material decreases because of the contribution of a 1.3 J/m2 surface energy, whatever the synthesis process. That is no longer valid with regard to cyclability and thermal transformations. The decrease of the MnO2-Mn2O3 reaction temperature with decreasing crystallite size provides evidence, thanks to the Gibbs-Thomson relationship, of an important difference between both types of nanometric dioxides. The surface energy of nanometric crystallites obtained by spray pyrolysis is three times larger than that of the BM samples, 1.95 J/m2 and 0.55 J/m2, respectively. The lower surface energy of BM dioxide manifests the degraded cohesion energy of the nanometric material prepared in an unstable state by high-energy BM. References and Notes (1) Poinsignon, C.; Djurado, E.; Klein, H.; Strobel, P.; Thomas, F. Electrochim. Acta 2006, 51, 3076. (2) Guinier, J. The´ orie et technique de la radiocristallographie; Dunod Ed.; 1964, p 462. (3) Williamson, G. K. and Hall, W. H. Acta Metall. 1953, 1, 22. (4) Sato, H.; Enoki, T. Phys. ReV B 2000, 61, 3563. (5) Holzapfel, M.; Alloin, F.; Yazami, R. J. Phys. Chem. B 2002, 106, 13165. (6) Baur, W. H. Acta Crystallogr. 1976, 32, 2200. (7) Ocana, M.; Cerna, C. J. Spectrochim. Acta 1991, 47A, 765. (8) Poinsignon, C.; Amarilla, J. M.; Tedjar, F. J. Mater. Chem. 1993 3, 1227. (9) Thompson, W. Philos. Mag. 1871, 42, 448. (10) Fritsch, S.; Post, J. E.; Suib, S. L.; Navrotsky, A. Chem. Mater. 1998, 474. (11) Nanda, K.; Maisels, A.; Kruis, F. E.; Fissan, H.; Stappert, V. Phys. ReV. Lett. 2003, 91, 106102. (12) Nanda, K. Appl. Phys. Lett. 2005, 87, 0213909. (13) Pre´lot, B.; Poinsignon, C.; Thomas, F.; Villie´ras, F. J. Colloid Interface Sci. 2003, 257, 77. (14) Kang, S.-H.; Goodenough, J. B.; Rabenberg, L. K. Chem. Mater. 2001, 13, 1758. (15) Poinsignon, C.; Berthome´, G.; Pre´lot, B.; Thomas, F.; Villie´ras, F. J. Electrochem. Soc. 2004, 151, 1.

Electrochemical Response of Nanocrystalline Tetragonal Manganese

Recommend Documents