Reversible Three-Electron Redox Behaviors of FeF3 Nanocrystals as

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Reversible Three-Electron Redox Behaviors of FeF3 Nanocrystals as High-Capacity Cathode-Active Materials for Li-Ion Batteries Ting Li, Lei Li, Yu L. Cao, Xin P. Ai, and Han X. Yang* Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: January 7, 2010

Three types of FeF3 nanocrystals were synthesized by different chemical routes and investigated as a cathode-active material for rechargeable lithium batteries. XRD and TEM analyses revealed that the as-synthesized FeF3 samples have a pure ReO3-type structure with a uniformly distributed crystallite size of ∼10 to 20 nm. Charge-discharge experiments in combination with cyclic voltammetric and XRD evidence demonstrated that the FeF3 in the nanocomposite electrode can realize a reversible electrochemical conversion reaction from Fe3+ to Fe0 and vice versa, enabling a complete utilization of its three-electron redox capacity (∼712 mAh · g-1). Particularly, the FeF3/C nanocomposites can be well cycled at very high rates of 1000-2000 mA · g-1, giving a considerably high capacity of ∼500 mAh · g-1. These results seem to indicate that the electrochemical conversion reaction can not only give a high capacity but also proceed reversibly and rapidly at room temperature as long as the electroactive FeF3 particles are sufficiently downsized, electrically wired, and well-protected from aggregation. The highrate capability of the FeF3/C nanocomposite also suggests its potential applications for high-capacity rechargeable lithium batteries. mnLi+ + MnXm + mne- T mLinX + nM0

Introduction Li-ion batteries (LIBs) represent the most advanced battery technology with the highest specific energy among all the rechargeable batteries currently commercialized, but they are still difficult to satisfy the fast-growing demand for lightweight and high-capacity electrical storage, such as in future wireless communications, electric vehicles, or power storage from renewable energy resources. Therefore, the search for new energetic materials for LIBs has been highlighted in battery chemistry and a wide range of intercalation compounds have been developed for high-capacity LIBs in the past decade.1-3 Though many types of metal oxides and phosphates have been tested as alternative cathode materials,4,5 no real breakthrough has been achieved in capacity, especially for intercalation cathodes, the capacity-determining electrode in the present LIBs systems. This embarrassment probably originates from the intrinsic chemistry of intercalation reactions that allows no more than one Li+ ion per formula unit to insert into the cathodic host lattice (i.e, less than oneelectron redox). Thus, it is necessary to develop new redox mechanisms and feasible materials in order to build future rechargeable lithium batteries with dramatically increased energy densities. Electrochemical conversion reactions seem to provide an alternative way to realize the significant breakthrough in the storage capacity of the cathode materials for LIBs by full utilization of all the oxidation states of a high-valence transition-metal compound. In recent years, a number of metal fluorides,6,7 oxides,8,9 sulfides,10 and nitrides11,12 have been demonstrated to produce a large multielectron redox capacity though reversible electrochemical conversion reactions: * To whom correspondence should be addressed. Tel: 086-27-68754526. Fax: 086-27-87884476. E-mail: [email protected].

(1)

Here, M stands for the transition-metal ions (M ) Fe+3, Ni+2, Cu+2, etc.) and X denotes fluoride, oxide, or sulfide anions (X ) F-, O-2, S-2, etc.). In such conversion reactions, MnXm is electrochemically reduced, with lithium uptake, to M/LinX at discharge, which reconverts to MnXm at subsequent charge, utilizing all the alterable valence states of the metal cations for reversible electrical storage. Particularly, many transition-metal fluorides have their metallic cations in high oxidation states and a strong ionic character of M-F bonds, which are expected to give a high reversible capacity and high redox voltage when used as cathode-active materials. However, metal fluorides have long been ignored as rechargeable cathode-active materials due to their insulating nature and apparent irreversibility in structural conversion. Until the late 1990s, Arai et al. first reported a high-voltage charge/discharge behavior of FeF3 with a reversible capacity of 80 mAh · g-1, which is even far below the theoretical 1e-transfer reaction capacity of the Fe3+/Fe2+ couple (237 mAh · g-1).13 Recently, Badway et al. revealed a reversible conversion process of FeF3, leading to a cycleable capacity of ∼600 mAh · g-1 at a very low current rate from carbon-FeF3 nanocomposites at 70 °C.14 At the same time, Li et al. reported the reversible phasetransition reactions of TiF3 and VF3 giving a Li-storage capacity of 500-600 mAh · g-1.6 It has been well-recognized from these pioneering work that the electrochemical conversion reactions of these metal fluorides are extremely rate- and temperaturesensitive and full capacity utilization of the valent electrons of the fluorides is only attainable at low rates and elevated temperatures.6,14-17 This phenomenon is understandable because these conversion reactions must involve structural decomposition and reconstruction of the metal fluorides along with reversible lithium insertion and transport in the bulk phases of metal fluorides. To improve the capacity utilization and rate capability,

10.1021/jp908741d  2010 American Chemical Society Published on Web 01/28/2010

Reversible High-Capacity FeF3 Cathode Materials it is favorable to downsize the metal fluorides so as to minimize the lithium insertion pathways and create the abundant active interfaces for reversible conversion of the metal fluorides. In recent studies of reversible metal fluoride conversion materials, a FeF3-based nanocomposite was selected as a model system because of its potentially high capacity and output voltage as a cathode material for rechargeable Li batteries. However, the FeF3 nanocomposites reported so far were mostly prepared by mechanical ball-milling with primary particle sizes on the scale of 100-1000 nm14,19 and their electrochemical performances are far from satisfaction in terms of cycleability and rate capability.18-20 In this work, we synthesized three kinds of FeF3 nanocrystals by different chemical routes and compared their electrochemical performances as cathode materials with our focus on the highrate utilization and cycleability at room temperature. Also, the three-eletron redox mechanism and structural evolution of the FeF3 nanocrystals during the discharge/charge process were also described. Experimental Section Three types of FeF3 nanocrystals were synthesized through different chemical routes. The first synthetic method was to precipitate the FeF3 nanoparticles from a solution reaction of Fe(NO3)3 ethanol solution (0.25 mol · L-1) and NH4HF2 aqueous solution (1.5 mol · L-1) by thoroughly stirring, using polyethylene glycol (PEG, 20 000) as a surfactant (0.005 mol · L-1). The powder sample so prepared was marked as P-FeF3. The second method was to synthesize the FeF3 nanoparticles by mixing a 0.048 mol · L-1 Fe(NO3)3 hexanol + H2O (1.06:1 mol %) solution with a 0.143 mol · L-1 NH4F hexanol + H2O (1.06:1 mol %) solution together, with a 0.327 mol · L-1 cetyltrimethyl ammonium bromide (CTAB) as a surfactant, with vigorous stirring for 2 h and then separating the powder sample by the centrifugation of the reaction solution. The sample so prepared was denoted as C-FeF3. The third synthetic route was so-called as a liquid-solid-solution phase-transfer reaction already used in the preparation of rare-earth fluoride nanocrystals.21,22 A typical experimental procedure is to mix 1 g of octadecylamine, 8 mL of linoleate acid, and 32 mL of ethanol together at stirring to form a homogeneous solution and then add aqueous Fe(NO3)3 solution (1.25 g/15 mL distilled water) and NH4HF2 solution (0.53 g/15 mL distilled water) one after the other into the mixed organic solution. This reaction mixture was stirred for about 10 min and then transferred to a 100 mL autoclave, sealed, and hydrothermally treated at 120 °C for about 6 h. After they cooled down naturally to room temperature, the products were deposited at the bottom of the vessel. The final products were purified with ethanol several times and denoted as L-FeF3. All the powder samples were dried under vacuum at 80 °C and then calcined at 400 °C for 2 h under high-purity argon to remove the organic residues. The FeF3/C nanocomposites were prepared by mechanical ball-milling of the as-prepared FeF3 nanopowders with graphite for 2 h (FeF3/graphite ) 1:1 by weight). The phase analysis of the synthesized samples was performed using a Shimadzu XRD-6000 system with Cu KR radiation. The electrode samples at different depths of charge and discharge for XRD analysis were made by disassembling the experimental cells in an Ar-filled glovebox and taking out and rinsing the electrode in dimethyl carbonate (DMC). The dried electrode was sealed in a polyethylene pouch and taken out immediately for XRD characterizations. The morphology and microstructure of the synthesized FeF3 nanocrystals were examined with a highresolution JEM-2010FEF transmission electron microscopy

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Figure 1. XRD patterns of the as-synthesized FeF3 samples: (a) P-FeF3, (b) C-FeF3, (c) L-FeF3, and (d) pure FeF3 (Alfa, ReO3-type).

system (HRTEM). The samples for HRTEM analysis were prepared by dispersing the powders in ethanol and releasing a few drops of the dispersed solution on a carbon film supported on a copper grid. For electrochemical evaluation, the FeF3/C cathode was prepared by mixing 80 wt % active material, 12 wt % acetylene black, and 8 wt % polytetrafluoroethylene (PTFE) into ca. ∼0.1 mm thick films and pressing the cathode films onto an aluminum net. The charge-discharge experiments were carried out on the test cells of a three-electrode design with a Li sheet as a counter electrode and a small piece of Li metal as a reference electrode. The cells were assembled in an argon-filled glovebox with a Celgard 2400 microporous membrane as the separator and 1 mol · L-1 LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylene methyl carbonate (EMC) (1:1:1 by wt) as the electrolyte. The cells were controlled by the BTS-55 Neware Battery Testing System (Shenzhen, China) in a voltage range of 4.5-1 V at different current densities. Cyclic voltammograms (CV) were measured using a CHI660a electrochemical station (Shanghai, China) at a scanning rate of 0.1 mV s-1 with the voltage ranges of 4.5-1 V and 4.5-2.5 V. Results and Discussion Structural Characterization. XRD patterns of the FeF3 nanocrystallites synthesized by different chemical routes, as compared with pure FeF3 powder, are shown in Figure 1. For all the as-prepared samples, the entire diffraction patterns are very similar to those of pure commercial FeF3 crystal and can be well-indexed to a ReO3-type structure (R3jc space group, JCPDS No.33-0647).14,20 Except for this similarity, there appeared a few very weak peaks (26.8° and 51.5°) in the XRD patterns of the synthesized samples corresponding to FeF2, which is possibly resulted from chemical reduction of a trace of FeF3 in the calcining process. By Lorentzian fitting of the XRD lines of the nanocrystal samples to obtain the 2θ and λ values of the FeF3 (012) peak and using the Scherrer equation (d ) 0.9λ/B cos θ), the average size of the as-synthesized FeF3 nanocrystals was calculated to be about 10-20 nm. This can also be clearly visualized from TEM images of the samples. Figure 2 shows the TEM images of the three FeF3 samples (P-FeF3, C-FeF3, and L-FeF3). Though slight aggregation appeared in the case of C-FeF3, a single crystallite with a diameter of ∼10 nm is evidently seen in the image for this sample. In contrast, the P-FeF3 and L-FeF3 samples were shown as homogeneously distributed crystallites with a uniform size of 10-20 nm, which agrees very well with the calculated values from the XRD data. Particularly, all the P-FeF3 crystallites

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Figure 2. TEM images of (a) P-FeF3, (b) magnified picture of (a), (c) C-FeF3, and (d) L-FeF3.

appear in a well-dispersed state and have an almost similar size distribution of ∼15 nm, suggesting that polyethylene glycol as a surfactant can effectively prevent the FeF3 nanocrystals from agglomeration. In comparison with the FeF3 nanocomposites previously reported,18-20 the chemically synthesized FeF3 nanocrystals have a considerably reduced size and appear in a betterdispersed state. These morphological features of the FeF3 nanocrystals may be beneficial to accelerate the phase-transform process, as well as lithium transport in the fluoride phase, during the electrochemical conversion reaction. Electrochemical Performance Characterization. The specific capacity and cycleability of the FeF3/C nanocomposites were directly evaluated by charge-discharge measurements at constant currents and at room temperature. Because carbon has no contribution to the specific capacity in this voltage range,7,14,19 the charge and discharge capacities observed from the composite electrodes can entirely be attributed to electroactive FeF3 nanocrystals. Figure 3 gives the voltage profiles of the FeF3/C electrodes cycled at a current density of 100 mA · g-1, which is a moderately high rate for conventional inserting cathodes in LIBs but ten times higher than those used for capacity evaluation of the ball-milled FeF3/C samples recently reported.14,20 Even though the charge and discharge were conducted at a quite high rate, the FeF3/C nanocomposites showed a well-defined twostaged discharge with a higher voltage plateau at 3.5-2.0 V, followed with a low voltage discharge at 1.7-1.0 V, demonstrating a two-step electrochemical reduction process. These discharge features in the V-I curves of Figure 3 are very similar

to those observed from the ball-milled FeF3/C samples at small current drain19,20 and at elevated temperature18 and are probably attributed to a two-step electrochemical conversion reaction, which proceeds through two successive steps: first, a oneelectron reduction of FeF3 with Li insertion to form LiFeF3 and, subsequently, a two-electron reduction of LiFeF3 to produce Fe and LiF, as revealed in recent studies.14,20 The total discharge capacities of these samples are all around 700 mAh · g-1, agreeing very well with the theoretical capacity (712 mAh · g-1) of FeF3 expected from a complete three-electron reduction. From the viewpoint of battery applications, only the discharge capacity of the FeF3 samples at the high voltage plateau is usable as a cathode-active material. In comparison, the P-FeF3 nanocrystals can deliver a quite high capacity of ∼300 mAh · g-1 in the high-voltage region of 4.5-2 V, obviously higher than those of the C-FeF3 (∼240 mAh · g-1) and L-FeF3 (∼150 mAh · g-1), possibly because the P-FeF3 crystallites are highly dispersed with an uniform size distribution, which are kinetically favorable for the lithium insertion and phase-transformation reactions. Even taking only the high-voltage capability into account, the P-FeF3 and C-FeF3 nanocrystals can still realize much higher capacities (∼300 and 240 mAh · g-1, respectively) than currently commercialized high-capacity LiCoO2 (∼150 mAh · g-1) and LiFePO4 (∼160 mAh · g-1). These data demonstrate a potential feasibility that a high redox capacity can be practically realized from the FeF3/C nanocomposite at room temperature and at a considerable high rate by an electrochemical conversion reaction.

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Figure 5. Cycling performance of the P-FeF3/C electrodes at various high rates, as labeled in the figure.

Figure 3. Discharge/charge profiles of the trifluoride electrodes: (a) P-FeF3/C, (b) C-FeF3/C, and (c) L-FeF3/C at a constant current of 100 mA · g-1. The inset in (a) displays a discharge curve of the P-FeF3/C at the high-voltage region of 4.5-2.5 V.

Figure 4. Comparison of the discharge capacities of the FeF3/C electrodes at the first ten cycles and at a constant cycling current of 100 mA · g-1.

Figure 4 shows the cycling performance of the FeF3/C nanocomposite electrodes. Compared with the A-FeF3/C sample made from commercial FeF3 (Alfa reagent), the chemically synthesized FeF3/C nanocrystals display good capacity retention

and give a reversible capacity of 742, 615, and 547 mAh · g-1, with slight capacity decay after 10 cycles. This good cycleability suggests that the electrochemical conversion reaction of FeF3 could occur very reversibly as long as the FeF3 particles are sufficiently downsized, electrically wired, and well-protected from aggregation. To test their practical availability as a cathode-active material, we cycled the FeF3 nanocomposites at different current rates. As an example, Figure 5 shows the high-rate performance of the P-FeF3/C electrode cycled at various current densities. At a considerable high rate of 500 mA · g-1, the P-FeF3/C electrode delivered an initial discharge capacity of 712 mAh · g-1 and remained ∼600 mAh · g-1 after 10 cycles. When the current density was increased to a very high value of 1000 mA · g-1, the discharge capacity of the P-FeF3/C electrode could still reach ∼500 mAh · g-1 during the cycles. In recent studies of the electrochemical conversion reaction of trifluorides, a full threeelectron redox capacity can only be observed from FeF3/C nanocomposites at a very low charge-discharge rate of ∼7.6 mA · g-1.14,20 The dramatic increase in the high-rate capability observed from the FeF3 nanocrystals chemically synthesized in this work implies that slow kinetics may not be an intrinsic nature of the conversion reaction, which could proceed rapidly and reversibly if appropriate nanodomains in the fluoride electrodes can be created to facilitate the Li insertion and electron conduction in the insulative fluoride particles. It should be pointed out that high loading of carbon (52% of the electrode mass), which may greatly dilute the active FeF3 within the conductive matrix, may also contribute greatly to the observed higher rate capability due to increased electroactive surface area. Electrochemical Conversion Mechanism. To further convince of the reversible conversion of the FeF3 nanocrystals electrochemically, we measured the cyclic voltammetric response (CV) of the FeF3/C nanocomposite by a powder microelectrode technique. Figure 6 shows a typical CV curve of the P-FeF3/C powder at a slow scan of 0.1 mV s-1 in the voltage range of 4.5-1.0 V. In the first negative scan, there are two obvious reduction peaks at 2.8 and 1.6 V, respectively, in accordance with the two discharge plateaus in Figure 3, corresponding to Li+ insertion into the FeF3 to form LiFeF3 and successive reductive decomposition of LiFeF3 into LiF and Fe0. In the reversed scan, three distinct oxidation peaks appeared at 2, 3.2, and 3.9 V. The former two peaks are reasonably attributed to the reverse oxidation reactions of the FeF3 formation through two steps: electrochemical conversion of the LiF/Fe0 nanomixture into a LiFeF3 phase and consecutive Li deintercalation from the LiFeF3 phase to regenerate FeF3 nanocrystals. The third oxidative peak at 3.9 V may be due to the surface film formation on the fluoride cathode by electrolyte

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Figure 6. Cyclic voltammograms of the P-FeF3/C sample in 1 mol · L-1 LiPF6 + EC-DMC-EMC. Scan rate ) 0.1 mV s-1.

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Figure 8. Ex situ XRD patterns of the P-FeF3/C electrodes at different discharge and charge states, as indicated in the figure.

discharged to 1.0 V, only the XRD signals of LiF and Fe are observed, while the XRD peaks characteristic of Fe3+ and Fe2+ vanish completely. In reversed charge, the XRD lines of the FeF3 phase reappear and become dominant at a complete charge of 4.5 V. This reversible change in the XRD pattern of the fluoride electrode evidently demonstrates the reversible structural conversion of the trifluoride during charge-discharge cycles. Conclusions Figure 7. Cyclic voltammograms of the P-FeF3/C sample in 1 mol · L-1 LiPF6 + EC-DMC-EMC in a narrow potential range of 4.5-2.5 V. Scan rate ) 0.1 mV s-1.

oxidation because this oxidation current peak appeared only in the first scan and disappeared since the second scan. In a very recent study of the structural conversion of the FeF3/C nanocomposite,20 it was found by high-resolution XRD and MAS NMR spectroscopic analysis that the first reduction step of Fe3+ to Fe2+ has comprised two processes: first, a half mole of Li-insertion reaction leading to a structural transformation of the ReO3-phase FeF3 to a lithiated rutile phase Li0.5FeF3 with the Fe oxidation state close to +2.5 and then a singlephase Li insertion to form LiFeF3. This detailed mechanism can also be confirmed electrochemically from the magnified voltammgrams of the as-prepared FeF3/C nanocrystals, scanning in the potential region of 4.5-2.5 V (Figure 7). As shown in Figure 7, two pairs of reversible redox peaks appear symmetrically in the potential region of 3.2-2.9 V, indicative of the stepwise Li-insertion/deinsertion processes for the first one-electron redox reaction of the FeF3 nanocrystals. To further confirm the assignments of the CV features and related structural conversions, we also performed an XRD analysis of the FeF3 electrode discharged and charged in different depths. As shown in Figure 8, the FeF3 electrode discharged to 2.4 V shows a very different XRD pattern from its initial pristine phase with the appearance of a number of new diffraction peaks at 32.5, 35, 53.2, and 63°, while the strongest XRD peak (012) of the FeF3 phase at 23.8° decreases its intensity considerably and other XRD signals at 33.4° (104), 34.4° (110), 48.8° (024) and 54.2° (116/211) disappear completely. Once discharged to 2.0 V, the main peak (012) at 23.8° from the electrode is no longer visible. Instead, there appear a group of new peaks at 38.7, 45.1, 65.6, 44.6, and 65°, which can be well-indexed to LiF and Fe0, indicating the onset of the conversion reaction of Fe2+ to Fe0. When the electrode is fully

In summary, we prepared the FeF3/C nanocomposites by chemical synthesis of FeF3 nanocrystals, followed by ballmilling the nanocrystals with graphite, and investigated these nanocomposites as cathode-active materials for high-capacity rechargeable lithium batteries. XRD and TEM analyses revealed that the as-synthesized FeF3 samples have a pure ReO3-type structure with a quite uniformly distributed crystallite size of ∼10 to 20 nm. Charge-discharge experiments demonstrated that all the FeF3/C samples can realize a reversible electrochemical conversion reaction from Fe3+ to Fe0 and vice versa, as confirmed by CV and XRD evidence, capable of utilizing completely a very high three-electron redox capacity (∼700 mAh · g-1). In addition, the FeF3/C nanocomposites can be well cycled at a very high rate of 1000-2000 mA · g-1, indicating that the electrochemical conversion reaction could also proceed very reversibly and rapidly as conventional electrochemical intercalation reactions. The experimental data given in this study suggest a potential feasibility to use metal fluorides as a high-capacity cathode material for lithium-ion batteries. Acknowledgment. The authors acknowledge the financial support by the 973 Program, China (Grant No. 2009CB220100). References and Notes (1) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (2) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652. (3) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem., Int. Ed. 2008, 47, 2930. (4) Ohzuku, T.; Brodd, R. J. J. Power Sources 2007, 174, 449. (5) Whittingham, M. S. Chem. ReV. 2004, 104, 4271. (6) Li, H.; Richter, G.; Maier, J. AdV. Mater. 2003, 15, 736. (7) Amatucci, G. G.; Pereira, N. J. Fluorine Chem. 2007, 128, 243. (8) Morcrette, M.; Rozier, P.; Dupont, L.; Mugnier, E.; Sannier, L.; Galy, J.; Tarascon, J. M. Nat. Mater. 2003, 2, 755. (9) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (10) Poizot, P.; Laruelle, S.; Grugeon, S.; Tarascon, J. M. J. Electrochem. Soc. 2002, 149, A1212. (11) Pereira, N.; Klein, L. C.; Amatucci, G. G. J. Electrochem. Soc. 2002, 149, A262.

Reversible High-Capacity FeF3 Cathode Materials (12) Pereira, N.; Balasubramanian, M.; Dupont, L.; McBreen, J.; Klein, L. C.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150, A1118. (13) Arai, H.; Okada, S.; Sakurai, Y.; Yamaki, J. J. Power Sources 1997, 68, 716. (14) Badway, F.; Cosandey, F.; Pereira, N.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150, A1318. (15) Doe, R. E.; Persson, K. A.; Meng, Y. S.; Ceder, G. Chem. Mater. 2008, 20, 5274. (16) Badway, F.; Mansour, A. N.; Pereira, N.; Al-Sharab, J. F.; Cosandey, F.; Plitz, I.; Amatucci, G. G. Chem. Mater. 2007, 19, 4129. (17) Li, H.; Balaya, P.; Maier, J. J. Electrochem. Soc. 2004, 151, A1878.

J. Phys. Chem. C, Vol. 114, No. 7, 2010 3195 (18) Plitz, I.; Badway, F.; Al-Sharab, J.; DuPasquier, A.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc. 2005, 152, A307. (19) Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150, A1209. (20) Yamakawa, N.; Jiang, M.; Key, B.; Grey, C. P. J. Am. Chem. Soc. 2009, 131, 10525. (21) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (22) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Inorg. Chem. 2006, 45, 6661.

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