Article pubs.acs.org/JPCC
Electrochromism of LixFePO4 Induced by Intervalence Charge Transfer Transition Sho Furutsuki, Sai-Cheong Chung, Shin-ichi Nishimura, Yusuke Kudo, Koichi Yamashita, and Atsuo Yamada* Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: The lithium-ion battery is the most advanced energy storage system, which utilizes an electrode reaction involving reversible lithium intercalation into a solid matrix. The structural and transport properties of these battery materials have been extensively studied as a function of lithium content, and structural/electronic phase diagrams have been revealed for a wide variety of lithium intercalation compounds. Here, we focused on the electrochromic response upon lithium intercalation and discovered distinctive color changes of LixFePO4, which is recognized as a promising cathode material for large-scale lithium-ion batteries. The emergence of a broad optical absorption band over the visible spectrum with coloring from pale gray to dark green was observed in accordance with the increase of the Fe3+/Fe2+ mixed-valence state and hence the solid−solution compositional domain in the phase diagram of LixFePO4. The color changes were analyzed using ab initio computational methods and rationalized to the intervalence charge transfer (IVCT) transition and its kinetic activation energy based on Marcus−Hush theory.
1. INTRODUCTION To assist the worldwide commercialization of electric vehicles and establish more efficient energy storage systems in the upcoming smart grid society, large-scale application of Li-ion batteries is becoming crucially important, because their energy densities are higher than any other conventional rechargeable battery systems. The present lithium-ion battery technology is based on the reversible intercalation reaction discovered in the early 1980s for both the cathode1 (LixCoO2: 0 < x < 1) and the anode2 (LixC6: 0 < x < 1). Since the first commercialization of this system by Sony Corp. in 1991, combination of these materials has remained basically unchanged. However, the solid host−guest reaction based on the reversible intercalation scheme has a long research history prior to commercialization of the lithium-ion battery and offers a wide variety of host− guest combinations. General scientific interest has been the structural and electronic phenomena induced by guest incorporation, such as phase separation,3 the Jahn−Teller effect,4 and metal−insulator transitions.5 A change of the electronic state is often induced by introduction of mobile electrons or holes that can absorb photon energy with visible wavelength and cause a distinct color change, called “electrochromism” in the electrochemistry community, as typically observed with guest intercalation into d0 transition metal compounds such as LixMO3 (M = Mo, W),6 H3+xPM12O40 (M = Mo. W),7 and Li4+xTi5O12,8 where even a slight amount of guest intercalation leads to significant optical absorption. Progressive color change upon lithium intercalation into graphite, LixC, is also well known.9 © 2012 American Chemical Society
Here we focus on the electrochromic response from localized polarons induced by the intervalence charge transfer (IVCT) transition in the Fe3+/Fe2+ (d5/d6) mixed-valence state. The importance of the Fe3+/Fe2+ redox reaction in solids has rapidly increased, because all of the electrode reactions in recently discovered promising iron-based cathode materials for lithiumion batteries, such as LiFePO4,10 Li2FePO4F,11 Li2FeP2O7,12 Li2FeSiO4,13 and LiFeSO4F,14 are based on the Fe3+/Fe2+ reaction. Lithium iron phosphate LixFePO4 (0 < x < 1), proposed by Padhi et al.10 as a new class of cathode materials in 1997, has already enabled production of large-scale lithium batteries. Problems related to cost and safety in the present LiCoO2/graphite system can be solved at minimum expense to energy density by use of abundantly available iron as a full Fe3+/Fe2+ one-electron redox center and fixing all oxygen atoms within the olivine framework via strong P−O covalent bonds.15 It is well known that mixed-valence states in various materials causes IVCT transition between neighboring metal ions and is one of the major factors that account for the colors of minerals.16 For compounds containing both Fe3+ and Fe2+, such as Fe3O4 and rockbridgeite (Fe2+Fe43+(PO4)3(OH)5), the corresponding optical transition of Fe2+ (site 1) + Fe3+ (site 2) → Fe3+ (site 1) + Fe2+ (site 2) is reported to occur at ca. 1.6 eV.17 The energy required for IVCT transition depends on the framework surrounding Fe2+ or Fe3+ and on the distance Received: May 2, 2012 Revised: June 13, 2012 Published: June 21, 2012 15259
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between Fe2+ and Fe3+. In most cases, IVCT transition occurs with an energy ranging from 12 000 to 16 000 cm−1, which corresponds to wavelengths of 600−900 nm. 16 Light absorption in this wavelength region suggests that the sample colors turn blue or green and provides a possible quantitative diagnosis for the presence of the Fe3+/Fe2+ mixed-valence state. Thereby, we decided to investigate the relationship between the proportion of solid−solution and the optical properties in LixFePO4, which has not been documented before.
collected at 0.020−0.028° steps over the 2θ range of 15−120°. Structural parameters were refined by Rietveld analysis using the program Topas ver. 3.1. For samples with the composition of x = 0.6, the accurate atomic Li/Fe ratio was confirmed by atomic absorption spectroscopy (Hitachi, Z-2300). Transmission electron microscopy (JEOL, JEM-2100) was used to observe SAED patterns. Diffuse reflectance measurements were performed on each Li0.6FePO4 composition with different proportions of the solid−solution phase using a UV−vis spectrometer (JASCO, V670) equipped with an integrating sphere accessory. Patterns were collected at 1 nm steps over the wavelength range of 200−900 nm (6.2−1.4 eV). Ab Initio Calculations. Density functional theory within the GGA+U approximation and the VASP code17 was employed. The effective U value (U-J) of 4.3 eV18 was adopted and used for calculation of the Fe2+ and Fe3+ states, although one may expect a larger value for the more oxidized state. Spinpolarized calculations with the electronic spin orderings in the ferromagnetic and antiferromagnetic forms were considered. The magnetic ground states of LiFePO4 and FePO4 are experimentally reported to be antiferromagnetic with Neel temperatures at ca. 52 and 125 K, respectively.19 However, the influence of magnetic ordering on energy has been reported to be quite small.20 We observed an antiferromagnetic ground state for the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution below 80 K and paramagnetism at room temperature similar to LiFePO4 or FePO4.19 The small magnetic interactions have an insignificant influence on the positions of these band states (as revealed by comparing the density of states, DOS, plots of the ferromagnetic and antiferromagnetic calculations); therefore, without loss of generality we will use the results of the ferromagnetic calculations for the following discussion. Orthorhombic unit cells were assumed for LiFePO4 and FePO4. For Li0.6FePO4, a monoclinic unit cell approximately three times the size of LiFePO4 was employed; this size of the unit cell is consistent with that determined by the spectroscopic methods described above. The energy cutoff for the plane wave basis set was 500 eV, and a k-point mesh with dimensions of 2 × 4 × 2 (for the monoclinic cell) or 2 × 4 × 4 (for the orthorhombic cell) were used.
2. EXPERIMENTAL AND CALCULATION METHODS Sample Preparation. LiFePO4 with different particle sizes were synthesized by solid-state reaction using stoichiometric amounts of Li2CO3 (99.9%, Wako), FeC2O4·2H2O (99%, Junsei), and (NH4)2HPO4 (>99%, Wako) as starting materials. Particle sizes were controlled to ca. 200, 100, and 50 nm by altering the milling conditions, the presence/absence of carbon coating, and the sintering temperature. Precursors corresponding to a 5 g yield of final product were mixed and ground thoroughly using a conventional planetary milling apparatus (Fritsch) for 12 h. The LiFePO4 phase was formed by sintering at 873 K for 6 h under a purified Ar gas flow. Self-assembled LiFePO4/C composites of 100 nm mean size particles were synthesized by an initial addition of 10 wt % (in final product) porous carbon black, and sintering was performed at 873 K for 6 h under Ar gas flow. For production of 50 nm mean size particles, only the precursors were mixed and ground for 6 h with addition of 50 mL of acetone first followed by removal of acetone under vacuum and a further 12 h milling under dry condition with addition of 10 wt % (in final product) porous carbon black. This was sintered at 673 K for 6 h. Chemical oxidation of LiFePO4 was performed with tetrafluoroborate NO2BF4 (Aldrich, >95%) to obtain FePO4. The redox potential of NO2+/NO2 is ca. 5.1 V vs Li/Li+ and effective for LiFePO4 oxidation where the Fe3+/Fe2+ redox potential is 3.4 V vs Li/ Li+. Twice the amount of NO2BF4 necessary for the estimated reaction was dissolved in acetonitrile, and LiFePO4 was then added. The mixture was stirred with bubbling of purified Ar gas for 24 h at room temperature to complete the following reaction LiFePO4 + NO2 BF4 → FePO4 + LiBF4 + NO2 (1)
3. RESULTS AND DISCUSSIONS 3.1. Electrochromism. 3.1.1. Mixed Fe3+/Fe2 Valence State in the Phase Diagram. The mixed-valence state of Fe3+/ Fe2+ in LixFePO4 is closely related to its phase diagram.22,23 The phase diagram of LixFePO4 and its particle size dependency have been the focus of intensive research over the past 5 years,24,25 and it has been revealed that the phase separation into FePO4(Fe3+)/LiFePO4(Fe2+) and formation of the LixFe3+1−xFe2+xPO4 (Fe3+/Fe2+ mixed-valence state) solid− solution are thermodynamically competitive.23 Figure 1a shows the overall scheme of the phase diagram for different lithium concentrations and temperatures. The cathode reaction of LixFePO4 at room temperature is dominated by the mixture of LiαFePO4 and Li1−βFePO4 phases (α < x < 1 − β) with narrow single-phase regions (0 < x < α and 1 − β < x < 1) close to the stoichiometric end members of LiFePO4 and FePO4. The miscibility gap, α < x < 1 − β, shrinks with increasing temperature,23 and the system becomes a complete solid− solution over the entire compositional domain (0 < x < 1) at temperatures above 520 K.22,26 The phase diagram resembles a eutectoid system, with the eutectoid point at around x = 0.6 and 473 K, which provides the strong metastable nature of the
The products were washed several times with acetonitrile to remove impurities before being dried thoroughly. For FePO4 with particle sizes of 100 and 50 nm containing carbon black, decarbonization was conducted by annealing the carboncomposite FePO4 at 773 K for 6 h under dried air flow. Chemical reduction was performed to obtain Li0.6FePO4 by reacting FePO4 with an estimated amount of LiI (>99.9%, Aldrich) in acetonitrile. The mechanism for reduction by LiI in acetonitrile proceeds according to the following equations, where oxidation of I− to I2 occurs in two reversible steps 6I− − 4e− → 2I3− + E10 ≈ 3.0 V vs Li/Li+
(2)
2I3− − 2e− → 3I 2 + E10 ≈ 3.7 V vs Li/Li+
(3)
+
Only reaction 2 is effective for Li insertion into FePO4 to form LixFePO4 (0 < x < 1) according to the following equation FePO4 + 3x /2LiI → LixFePO4 + x /2LiI3
(4)
Sample Characterization. X-ray diffraction patterns were obtained with Co Kα (Bruker, D8) and Cu Kα (Rigaku, RintTTR III) radiation as the X-ray source. The patterns were 15260
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1a and 1b. Powder X-ray diffraction (XRD) profiles of these samples are summarized in Figure S1, Supporting Information. A size-dependent miscibility gap and the solubility limits α and β were confirmed from the XRD data measured for samples B− D and are consistent with the previous report.24 XRD patterns measured for LiFePO4 (200 nm), FePO4 (200 nm), and samples A and E are given in Figure 2a. After annealing sample
Figure 1. Phase diagram and color of LixFePO4. (a) Lithium composition vs temperature. (b) Room-temperature size-dependency diagram. Prepared samples are denoted as A−E in the diagrams: (A) simply mixed phase of LiFePO4 and FePO4 (particle size 200 nm) with a molar ratio of 3:2, (B−D) Li-inserted FePO4 (Li0.6FePO4) with particle sizes of 200, 100, and 50 nm, respectively, and (E) metastable Li0.6(Fe2+0.6Fe3+0.4)PO4 solid solution. (c) Photoimages of Li0.6FePO4 samples A−E showing the color change, i.e., electrochromic response.
Figure 2. Formation and structure of metastable Li0.6(Fe2+0.6Fe3+0.4)PO4 solid−solution. (a) Powder XRD patterns showing formation of metastable solid solution. From top to bottom, LiFePO4, FePO4, mixture of LiFePO4 and FePO4 (molar ratio of 3: 2), and Li0.6(Fe2+0.6Fe3+0.4)PO4 solid−solution obtained after quenching from 350 °C. (b) Whole pattern fitting for the metastable solid−solution phase with monoclinic superlattice. (Inset) Corresponding SAED pattern of the [110]ortho zone axis. (c) Optimized structure of Li(Fe3+0.4Fe2+0.6)PO4, where every two Li vacancies are next to each other, as observed from the b axis. Parallelogram frame in the center indicates the unit cell of the monoclinic superlattice.
Li0.6(Fe2+0.6Fe3+0.4)PO4 solid−solution at room temperature.27 The solid solubility limits α and β in LiαFePO4 and Li1−βFePO4 at room temperature systematically increase with a reduction in the particle size, as shown in Figure 1b.24 3.1.2. Test Samples with Different Proportions of Mixed Fe3+/Fe2+ Valence State. Such an established phase diagram and its size dependency indicate that various intermediate LixFePO4 samples with different proportions of the mixed Fe3+/Fe2+ valence state in solid−solution can be prepared by controlling the particle size, sintering temperatures, and quenching protocols and be rationalized according to their optical properties. Thereby, we have set the lithium composition to x = 0.6 for systematic studies and synthesized five types of Li0.6FePO4: (A) mixture of LiFePO4:FePO4 = 3:2 with a mean particle size of 200 nm, (B−D) chemically Liinserted Li0.6FePO4 into FePO4 with mean particle sizes of 200, 100, and 50 nm, respectively, and (E) metastable solid− solution Li0.6(Fe3+0.4Fe2+0.6)PO4 at room temperature obtained by annealing sample A at 623 K for 6 h followed by quenching to room temperature, as indicated by the red markers in Figure
A, a simple mixture of LiFePO4:FePO4 = 3:2 at 623 K and quenching to room temperature the unique metastable Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution is formed, as confirmed by the XRD profile shown in Figure 2a(E), which was fully indexed in a monoclinic cell (a = 11.8646(7) Å, b = 4.7552(1) Å, c = 15.6291(6) Å, α = γ = 90°, β = 100.373(2)°, and V = 867.35(6) Å3), which is a distorted modification of the original Pnma unit cell, as shown in Figure 2b. This corresponds to a supercell with a volume three times that of the unit cell of the original olivine framework. The inset of Figure 2b shows a selected area electron diffraction (SAED) pattern of the [110]ortho zone axis. The extra spots in the single-crystalline SAED pattern also confirm the existence of a supercell and are consistent with the powder XRD results. The optimized structure at the ground state for the monoclinic 15261
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Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution obtained by ab initio calculations with the generalized gradient approximation + U (GGA + U) method is shown in Figure 2c. The nonstoichiometry of Li was investigated by studying a hierarchy of configurations for Li ions and vacancies. The configuration with the lowest energy was found to be that with pairing of Li vacancies, probably due to lower strain energy (see Figure S3, Supporting Information, for more details). 3.1.3. Optical Properties. Systematic color changes of these Li0.6FePO4 samples were observed, as summarized in Figures 1 and 3. Figure 1c shows photographs of the prepared samples.
insertion into 200 nm FePO4 shows little difference from that of sample A because α and β are small when the particle size is large (>200 nm) (Figure 1a and 1b). As the particle size is decreased to 100 nm (sample C) or 50 nm (sample D), the absorption become larger over the entire visible region. This gradation is in a good agreement with the enlargement of α and β in the intermediate LiαFePO4/Li(1−β)FePO4 solid−solution and is consistent with the color change observed. We suggest that these optical properties can be a diagnostic indicator of the size-dependent phase diagram of LixFePO4 at room temperature. Note that the color change is reversible upon lithium insertion/extraction as shown in Figure S2, Supporting Information. 3.2. Analysis with ab Initio Studies. 3.2.1. Density of States. Ab initio calculations were performed to obtain further insight into the nature of the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid− solution phase. Bader analysis29 of charge density revealed two types of Fe ions with charges of +1.56 and +2.02, which are close to those of +1.54 and +2.02 in LiFePO4 and FePO4, respectively. This demonstrates that there are distinct Fe2+ and Fe3+ ions in the metastable Li0.6(Fe3+0.4Fe2+0.6)PO4 solid− solution. Figure 4 shows DOS plots for LiFePO4, FePO4, and
Figure 3. Diffuse reflectance spectra plotted as Kubelka−Munk function Φ = (1 − R)2/2R (where R is the reflectance) for different Li0.6FePO4 samples. Large absorption below λ = 400 nm is derived from typical transitions between the bandgaps. In the visible region around 650 nm, the absorption intensity increases as the proportion of solid−solution is increased, which is consistent with the color changes (electrochromism) of the powder samples.
Sample A has the lightest color, which is consistent with the insulating nature of the single-valent end members, FePO4 and LiFePO4. The color of sample B, the product of chemical lithiation of 200 nm FePO4, is almost the same color as sample A. The obvious color change of the lithiated product emerges when the particle size becomes smaller at 100 and 50 nm for samples C and D, where the sample becomes greener. The color change was highly pronounced for the metastable monoclinic Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution (sample E), which exhibits a very deep green, regardless of the particle size. For a quantitative assessment of these apparent colors, diffuse reflectance spectra over the wavelength range of 300−900 nm (1.4−4.1 eV) were measured for these samples. The results were consistent with the direct color observations and are summarized in Figure 3. The Kubelka−Munk (KM) function Φ(R) = (1 − R)2/2R, which is converted from diffuse reflectance, gives the optical absorbance of a powder sample.28 The simply mixed sample composed of end members LiFePO4 and FePO4 (A) has little absorption in this region. The metastable solid−solution phase (E) obtained by quenching from 623 K shows a distinctly large and broad absorption peak (KM > 1.5) in the visible region around 650 nm (ca. 2 eV). Such a distinctive color change indicates that the two separated phases became a single solid-solution phase, which leads to IVCT transition in the mixed-valence state of Fe2+ and Fe3+. The absorbance of sample B obtained by chemical lithium
Figure 4. Total and projected density of states for ferromagnetic LixFePO4 calculated with the GGA + U (U = 4.3 eV) method. In the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution, the Fe3+ d orbital lies between the two Fe2+ orbitals, forming a new bandgap at the Fermi level (dotted box), which indicates that the Fe2+ → Fe3+ IVCT occurs in the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution.
Li0.6FePO4. The general features of the DOS plots for LiFePO4 and FePO 4 agree very well with those of previous calculations.18,30−32 The calculated bandgaps for LiFePO4 and FePO4 are ca. 3.7 and 2.0 eV, respectively. The electronic configuration of Fe in LiFePO4 is high-spin t2g4eg2, where the majority d states spread from −5 to −0.5 eV and the singly occupied minority t2g state is localized just below the Fermi 15262
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level (Figure 4a). For FePO4, the Fe ion is in the high-spin t2g3eg2 configuration, so that the exchange interaction pushes the majority of d states deep below the Fermi level at ca. −7 eV. The unoccupied spin-up states are localized at ca. 2−4 eV above the Fermi level (Figure 4b). The Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution contains charge-separated Fe2+ and Fe3+ ions, so that the DOS for the localized d states of Fe in Li0.6(Fe3+0.4Fe2+0.6)PO4 has a combination of the features of LiFePO4 and FePO4. Here, the unoccupied minority d states of Fe3+ lie between the “bandgap” formed by the d states of Fe2+ (Figure 4c). As a result, the highest occupied state of the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution is the minority state due to the d orbitals of Fe2+ while the lowest unoccupied states are the minority states due to the d orbital of Fe3+. Therefore, the lowest electronic excitation is a charge transfer from Fe2+ to Fe3+ as framed in Figure 4c. 3.2.2. Assignment for Excitations. The experimental and computed absorption spectra are shown in Figur. 5 on an electronvolt scale. The computed optical spectra were determined by calculating the transition matrix involving the Kohn−Sham states. The complex part of the dielectric function (ε2) was calculated according to
ε2, αβ =
4π 2e 2 1 lim Ω q → 0 q2
∑ 2ωkδ(εck − εvk − ω)⟨uck+ e
αq
|uvk⟩
c ,v ,k
⟨uck + eβq|uvk⟩
(5)
where v and c are indices for the valence and conduction band states, respectively, and uck is the state wave function at a particular k point. The real part of the dielectric function (ε1) was obtained by Kramer−Kronig transformation. As a result the absorbance was obtained by I(ω) = ( 2 )ω( ε1(ω)2 + ε2(ω)2 ) − ε1(ω))1/2
(6)
Density functional theory usually underestimates the bandgap; therefore, we shifted the computed spectra for LiFePO4, FePO4, and Li0.6FePO4 to higher energy by 1.0, 1.0, and 0.7 eV, respectively, which is a procedure generally accepted as scissors operator. Otherwise, the other features of the computed and experimental spectra are in good agreement. The experimental absorption of LiFePO4 starts to increase at ca. 4.8 eV but not significantly before 5.8 eV, which is consistent with the largest bandgap (3.7 eV) calculated for this composition. For FePO4, the absorption in the range of 3−6 eV can be attributed to charge transfer transitions from the oxygendominated valence bands to the empty d states of Fe3+ based on our calculated DOS (Figure 4b). Although it is difficult to extrapolate a bandgap from the measured spectrum, we consider that the bandgap is close to 3 eV, which is larger than the 2 eV suggested by Zaghib et al.33 The larger bandgap is more consistent with the slightly yellowish inherent color of the sample. An absorption edge at ca. 2 eV (600−650 nm) is likely to result in a sample colored vibrant orange or red, which is not consistent with the color of this material. In contrast to the spectra for LiFePO4 and FePO4, the spectrum of mixedvalence state Li0.6(Fe3+0.4Fe2+0.6)PO4 significantly shows a small but broad absorption peak at ca. 2 eV (Figure 5). We attribute this absorption to the Fe2+ to Fe3+ IVCT transition, which can be assigned to the transition from the occupied, minority spin d state of Fe2+ just below the Fermi level to the empty, minority spin d states of Fe3+ at ca. 1 eV above the Fermi level on the DOS (Figure 4). The peaks at 3−6 eV are attributed to the other d−d transitions of the Fe2+ ion. Thereby, the IVCT absorption accounts for the color of the solid−solution phase 3.2.3. Kinetic Implications. Finally, we would like to discuss the implications of the present study to the electronic transport properties of the Li0.6(Fe3+0.4Fe2+0.6)PO4 solid−solution phase. It is well accepted that the electronic conduction in LixFePO4 is due to the diffusion of small polarons involving localized, selftrapped charge carriers on Fe ions.33 However, there is controversy regarding the activation energy of the polaron diffusion. Zaghib et al.33 suggested a mechanism involving magnetic polarons, where electron transfer occurs between the localized ferromagnetic-configured Fe ions (t2g4eg2 of Fe2+ to t2g3eg2 of Fe3+). Furthermore, they reported an activation energy of 0.65 ± 0.05 eV for the electrical conductivity of LiFePO 4 . 33 Mö s sbauer spectroscopic measurements of Li0.65FePO4 at 500−675 K provided an estimate of the activation energy at 0.775 ± 0.108 eV.34 First-principles calculations probing the adiabatic pathway give lower values of ca. 0.2 eV.35 In order to rationalize the difference, lithium binding to polarons (binding energy > 0.37 eV) is suggested to be involved in the measured activation energy.34 However, Zaghib et al. argues that iron phosphate glasses, which have a
Figure 5. Experimental Kubelka−Munk function Φ(R) and calculated absorbance I(ω) for LiFePO 4 , FePO 4 , and metastable Li0.6(Fe2+0.6Fe3+0.4)PO4 solid−solution. Calculated spectra (dotted lines) have been shifted to the right by 1.0, 1.0, and 0.7 eV, respectively (scissors operator). (Inset) General transition model proposed by Hush,36 showing the intersection of diabatic surfaces for the mixed-valence state of Fe. Activation energy for the transition (red arrow) is estimated to be one-quarter of the energy for optical excitation (black arrow), which is approximately 2 eV, as observed from the visible absorption in the dotted box. 15263
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similar Fe2+/Fe3+ small polaron mechanism for electrical conduction but without the involvement of Li+ ions, possess an activation energy of ca. 0.6 eV, similar to that for LixFePO4.33 According to Marcus−Hush36 and related theories, we can estimate the diabatic activation energy (Ea) for thermal polaron transport from the optical electron transfer energy (Eλ). Using the theoretical equation Ea = Eλ/4, as schematically shown in the inset of Figure 5, and our experimental value Eλ ≈ 2.0 eV (Figure 5) results in Ea ≈ 0.5 eV obtained for the diabatic polaron diffusion. This value agrees well with those of Zaghib et al.33 and Ellis et al.34
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4. CONCLUSIOINS We established a relationship between the proportion of solid− solution phase in a sample and the electrochromism (color) of nonstoichiometric LixFePO4. The increased coloring from pale gray to green as the particles become smaller for nanosized LixFePO4 was therefore accounted for. This provides a simple tool to determine the amount of solid−solution phase in a sample. The particle size-dependent phase diagrams can then become directly “visible”. Through diffuse reflectance spectroscopy and first-principles calculations we determined that the coloration is due to an IVCT transition between the localized d states of Fe2+ and Fe3+ ions. Furthermore, the energy for the IVCT peak at 2 eV enables us to roughly estimate the activation energy for the diabatic polaron transfer of 0.5 eV, which is in good agreement with the most accurate literature values. Emerging static/kinetic information regarding the electrochromism of the important battery electrode materials is thus revealed. Such exotic features have been obscured so far, because these electrode materials based on polaronic transport are typically used in a sealed system with metallic additives as conducting agents. The present findings may open up a new direction of research with a sense of optoelectrochemistry for investigation of the thermodynamics and kinetics of intercalation electrode systems.
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ASSOCIATED CONTENT
* Supporting Information S
Description of the material included. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the cabinet office, government of Japan, the Funding Program for World-Leading Innovative R&D on Science and Technology. Helpful discussions with Professors K. Domen, J. Kubota, H. Zhou, and Dr. P. Barpanda are very much appreciated.
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REFERENCES
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dx.doi.org/10.1021/jp304221z | J. Phys. Chem. C 2012, 116, 15259−15264
