7Li NMR Knight Shifts in Li−Sn Compounds: MAS NMR

J. Phys. Chem. C 2010, 114, 6749–6754

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Li NMR Knight Shifts in Li-Sn Compounds: MAS NMR Measurements and Correlation with DFT Calculations Emilie Bekaert,† Florent Robert,‡ Pierre Emmanuel Lippens,‡ and Michel Me´ne´trier*,† CNRS, UniVersite´ de Bordeaux, ICMCB, 87 aVenue du Dr. A. Schweitzer, Pessac, F-33608, France, and Institut Charles Gerhardt, Equipe Agre´gats, Interfaces et Mate´riaux pour l’Energie, UMR 5253 (CNRS), UniVersite´ Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France ReceiVed: January 14, 2010; ReVised Manuscript ReceiVed: March 2, 2010

Several Li-Sn crystalline phases, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, were prepared by ballmilling and studied by 7Li MAS NMR spectroscopy with silica as a diluting agent to avoid field penetration limitations. All phases except for LiSn exhibit exchanged NMR signals at room temperature for the various types of Li present in the unit cells, in the 10 to 100 ppm range. Electronic structure calculations based on first-principles method led to a rather good correlation between the participation of the Li 2s orbital to the density of states (DOS) at the Fermi level and the corresponding NMR Knight shift for the two Li crystallographic types in the case of LiSn, and for the weighted average of the different crystallographic types in the case of the NMR-exchanged signals for the other compounds. 1. Introduction Increasing the specific energy and the cell cycle life of lithium-ion batteries is a continuous challenge. Among the negative electrode materials currently used, graphite is a standard that provides a theoretical specific capacity of 372 mAh g-1 corresponding to the formation of the LiC6 graphite intercalation compound.1–3 Carbonaceous materials show good cycling performances and little volume change during the cycling. Many anode materials that form alloys with lithium have been investigated to increase the limited capacity and improve the cycling life of graphite.4,5 Particular attention has been paid in recent years to tin-based anodes that can store up to 4.4 Li per Sn atom, which is considerably higher than graphite and coke in terms of mass and volume capacity.6–11 Unfortunately, they suffer from poor cyclability due to large volume changes during lithium insertion and extraction. Indeed, the volume of the highly lithiated compound, Li22Sn5, increases by 300% compared to that of β-Sn, which leads to the formation of microcracks within the electrode. To solve this problem, several authors used composite materials in which an electrochemically inactive matrix can reduce the effects of volume changes.12–15 Conversion-type reactions can also lead to the formation of Li-Sn nanocompounds with different compositions embedded in an electrochemically inactive matrix.16–19 For these different types of electrode materials, the Li-Sn alloys are formed reversibly and an accurate characterization is required to understand the electrochemical mechanisms and improve the performances. The Li-Sn system was recently examined by 119Sn Mo¨ssbauer spectroscopy.20 Analysis of the isomer shift (δ) and quadrupole splitting (∆) in terms of crystal structure allowed two types of Li-Sn compounds to be distinguished: the Snrichest compounds (Li2Sn5, LiSn) and the Li-richest ones (Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, Li22Sn5). From these results, a ∆-δ correlation diagram was generated and used to identify * To whom correspondence should be addressed. E-mail: menetrier@ icmcb-bordeaux.cnrs.fr. † Universite´ de Bordeaux. ‡ Universite´ Montpellier II.

the Li-Sn phases during the electrochemical process of new Sn-based materials. 7 Li NMR shifts have been reported for a series of Li-Sn alloys by Furuya et al. from static echo experiments, and the nature of the relaxation mechanism has been studied.21 Although the spectra are not shown in that report, one can consider that MAS should lead to much better precision in the determination of the signal position. Furthermore, echo-type experiments are required in static mode due to the width of the signals, but can be hampered if the T2 relaxation time is short. In MAS conditions, the signals become narrow enough so that single pulse experiments can be used, which eliminates such difficulties. Only Li22Sn5 was, to our knowledge, analyzed with MAS NMR.22,23 The objective of the present work is therefore first to establish a database for the Li NMR (Knight) shifts in the Li-Sn system using high-resolution (MAS) NMR, and try to establish a correlation between these and the participation of Li to the DOS at the Fermi level for the corresponding Li species. These data will then be useful to identify such alloys formed in situ as nanocompounds in the conversion-type negative electrode materials based on intermetallics. 2. Experimental Section 2.1. Synthesis. There are 7 Li-Sn phasessLi2Sn5, LiSn, Li7Sn3 Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, according to the Li-Sn binary diagram6swhose structures were characterized by X-ray diffraction (XRD).24–31 Mechanical alloying was used to synthesize these samples.32 The materials were prepared with a SPEX 8000 vibratory mill. The vial was shaken at a 20 Hz frequency in the three orthogonal directions. The impact speed of the balls was several meters per second and the shock frequencies were several hundred hertz. A pure Sn rod (99.9% Aldrich) and a Li foil (99.9% FMC) were used for the synthesis. The starting materials were placed in stoichiometric amounts and were sealed into a stainless steel milling container (25 cm3) with stainless steel balls for 48 h under argon atmosphere. The experiments were performed in a glovebox under argon atmosphere. The ball to powder weight ratio was 50:1. The mechanically alloyed powder was annealed into a sealed

10.1021/jp100365u  2010 American Chemical Society Published on Web 03/12/2010

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TABLE 1: Annealing Temperature (Tan) Used to Synthesize the Li-Sn Samples

Bekaert et al. TABLE 2: Composition and Purity of the Li-Sn Samples (from ref 20)

compd

Tan (°C)

samples

composition

percent

Li2Sn5 LiSn Li7Sn3 Li5Sn2 Li13Sn5 Li7Sn2 Li22Sn5

150 250 400 420 400 600 150

Li2Sn5

Li2Sn5 Sn/Li LiSn Li7Sn3 Li2Sn5 Li2Sn5 Li7Sn3 Li13Sn5 Li7Sn2 Li22Sn5 Sn/Li

91 9 100 94.8 5.2 86.8 13.2 100 100 94 6

stainless steel tube in a vacuum furnace at a temperature Tan for 168 h (Table 1). The different annealing temperatures were selected from the Li-Sn binary diagram by considering a value lower than the melting or peritectic temperature, to avoid crystalline growth. The purity and crystallinity of the phases were examined by XRD. During the analysis, the powders were maintained in a hermetically closed aluminum sample holder with a beryllium window to prevent oxygen contamination. 2.2. Analyses. 7Li MAS NMR spectra were recorded on a Bruker Avance 300 spectrometer (7.05 T magnet) at 116 MHz, with a standard 2.5-mm Bruker MAS probe. The spinning speed used was 30 kHz. A single pulse sequence (preacquisition delay 10 µs) was used, with a 90° pulse duration of 3.2 µs. The recycle delay was checked to lead to full relaxation and varied in the 0.5-500 s range. The sample was placed into a 2.5 mm diameter zirconia rotor in the drybox. The (external) reference used is a 1 M LiCl aqueous solution set at 0 ppm. Variable-temperature NMR spectra were recorded with a standard 4-mm Bruker MAS probe (WVT design). The sample was placed into a 4 mm rotor in the same condition as described before. Fitting of NMR spectra was achieved by using the DmFit program.33 The electronic structures of the Li-Sn phases were evaluated from the full-potential linearized augmented plane-wave method (FLAPW) as implemented in the WIEN code.34 This method is based on the density functional theory (DFT);35,36 the generalized gradient approximation (GGA) with the exchange-correlation potential by Perdew, Burke, and Ernzerhof was used.37 The unit cell is partitioned into atomic spheres centered at the atomic positions and an interstitial region. The atomic muffin-tin radii were Rmt(Sn) ) 2.6 au and Rmt(Li) ) 2.4 au for all the compounds. To improve the energy linearization, the basis set was extended with Sn 4d and Li 1s local orbitals. In the interstitial region, the wave functions were expanded in plane waves with wavenumbers K such as min(Rmt) × max(K) ) 8 and the charge density was expanded in a Fourier series with Gmax ) 15 Ry-1/2.

LiSn Li7Sn3 Li5Sn2 Li13Sn5 Li7Sn2 Li22Sn5

minimizing heating effects,38 considerably improves the resolution. This suggests that the whole sample now experiences the same field, and therefore that the skin depth is comparable to, or larger than, the size of the alloy particles, that is in the micrometer range. LiSn has a monoclinic structure: S.G. P2/m, a ) 51.7 Å, b ) 7.74 Å, c ) 3.18 Å, b ) 104.5°.25 In this structure, Li atoms occupy two crystallographic sites (4b and 8e). The fit of the 7 Li NMR spectrum of LiSn/SiO2 mixture in Figure 2 shows four narrow Lorentzien-type signals. The first one consists of a small peak at 2.5 ppm, which corresponds to diamagnetic phases

Figure 1. 7Li MAS NMR spectra (116 MHz, single pulse, 30 kHz spinning) for LiSn and an LiSn/SiO2 mixture (50 wt %). Arbitrary scale; * ) spinning sidebands.

3. Results and Discussion The XRD patterns confirm the high crystallinity of our samples. Table 2 shows purity and sample compositions from chemical analysis as already given in ref 20. LiSn and Li13Sn5 are phase-pure, while the XRD patterns of Li2Sn5 and Li22Sn5 show the presence of β-Sn and Li. Alloy impurities were only detected for Li5Sn2 and Li7Sn3 (Li7Sn3 contained some Li5Sn2 impurity and vice versa). 7 Li MAS NMR spectra for LiSn are shown in Figure 1. The spectrum for the pure sample contains four broad signals, even under relatively fast MAS conditions. However, if the alloy is mixed with dry silica (50 wt %), the observed signals are considerably narrower. This broadening is mostly due to the metallic character that prevents the diffusion of the radio frequency field to the heart of the material. Diluting with insulating silica, in addition to improving probe tuning and

Figure 2. Decomposition of the LiSn/SiO2 MAS NMR spectrum into individual Lorentzien-type signals. The sum of the four individual components is not shown, being fully coincident with the experimental spectrum.

7Li

NMR Knight Shifts in Li-Sn Compounds

Figure 3. 7Li MAS NMR spectra (116 MHz, single pulse, 30 kHz spinning) for LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5. These phases were mixed with SiO2 (50 wt %). Arbitrary intensity scale; (a) full ppm scale; (b) expended ppm scale.

(note that for comparison the chemical shift for Li2O is 2.9 ppm and that for LiOH is 1.3 ppm). The second and third signals, considerably narrower than the first one, and appearing at +32 and +42 ppm, can be assigned to the two crystallographic types of Li. Comparison between the relative intensity of the signals and the multiplicity of the sites occupied by Li allows the peaks at + 32 and +42 ppm to be assigned to Li atoms in 4b and 8e sites, respectively. Another signal appears at +78 ppm, which is assumed to correspond to an impurity phase. Although the Li2Sn5 sample, even when dispersed in silica, was too heterogeneous for safe spinning of the rotor and could consequently not be analyzed with MAS NMR, static experiments suggest a main signal close to 80 ppm for this alloy. We therefore assume that the corresponding impurity signal observed for LiSn corresponds to traces of Li2Sn5. The other Li-Sn phases (Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5) were also mixed with silica and the corresponding 7Li MAS NMR spectra are shown in Figure 3a. In agreement with XRD, impurities were detected for Li5Sn2 and Li7Sn3; Li7Sn3 contained some Li5Sn2 impurity and vice versa. If we look more attentively at the spectra (Figure 3b), 7Li MAS NMR shows that none of the Li-Sn phases were actually pure: Li7Sn3, presence of Li5Sn2, LiSn, and diamagnetic phases (Li2O-LiOH); Li5Sn2, presence of Li7Sn3, LiSn, and diamagnetic phases (LiOH); Li13Sn5, presence of Li7Sn2, LiSn, and diamagnetic phases (LiOH-Li2CO3); Li7Sn2, presence of Li13Sn5; and Li22Sn5, presence of Li7Sn3, Li5Sn2, and diamagnetic phases (Li2O-LiOH).

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6751 Although some samples were not pure, the shifts can be given for the different phases, as shown in Table 3. These results are globally in agreement with those reported by Furuya et al., but we can notice that the static experiments performed by these authors did not allow them to identify the two signals for LiSn.21 The salient feature from our data is that, except for LiSn, the number of Li NMR signals does not correspond to the number of crystallographic sites for Li, based on the crystal structures of the alloys.20 For the Li7Sn2, Li5Sn2, and Li7Sn3 phases, the NMR spectra are composed of 1 signal instead of 6, 3, and 7, respectively. This suggests either that the difference in the position for each signal is too small to be resolved in our conditions or that an exchange of the expected NMR signals occurs due to fast hopping of lithium ions between the crystallographic sites (with respect to the time scale of the NMR experiment). We have therefore recorded the spectrum of Li7Sn2 (the phase that presents less impurity) at variable temperature, using a cooling device for the drive airflow in a 4-mm Bruker WVT probe. Figure 4 shows the spectra in the range (260 K < T < 323 K), using a 15 kHz spinning speed. The single-pulse line becomes slightly narrower upon heating, which suggests that very fast movement allows the residual dipolar interactions to be further decreased, and the signal is therefore fully exchanged at higher temperatures. Although the part of the spectrum assigned to the Li13Sn5 impurity also becomes narrower at elevated temperature, no change in the Li7Sn2 signal other than a narrowing occurs between room temperature and 323 K; we conclude that the room temperature signal is already exchanged. The NMR spectrum recorded at lower temperature is different from that recorded at 323 K with a new signal appearing at 5 ppm. This suggests a slowing down of the exchange phenomenon, although the temperature range explored clearly does not allow observation of all the individual signals. Since we could not reach the temperature where individual signals can be identified, it is unfortunately not possible to undertake 2D exchange experiments to prove the exchanged character of the room temperature signals. However, we consider that this is strongly suggested by our experiments. We will come back to this point with DFT calculations in the second part of this paper. Besides, the shifts did not show significant changes upon heating, which is in agreement with the metallic character of the compound (Knight shift). The cases of Li13Sn5 and Li22Sn5 are more complex. The fit of the former in Figure 5 shows one main signal at 17.5 ppm and a minority one at 22 ppm in addition to the 0 ppm one assigned to Li2CO3, the (very weak) 3 ppm one assigned to Li2O, the 9.5 ppm one assigned to Li7Sn2, and the two signals at 42.5 and 32 ppm due to traces of LiSn as impurities; instead, 6 signals are expected for Li13Sn5 based on the number of crystallographic sites. The question arises as to whether the minority signal is due to an unidentified impurity, or to a type of Li that would not undergo an exchange at room temperature. Considering the crystal structure for this alloy, it does not appear that any of the crystallographic Li site is isolated from the others, since all the sites are interconnected by shared faces. We therefore assume that the minority 22 ppm signal present in the Li13Sn5 is due to some unidentified impurity. As concerns Li22Sn5, although the width of the NMR lines leads to considerable imprecision in the fitting process, four signals seem to be present. Two 7Li MAS NMR spectra for this compound have been published,22,23 with slight differences, and are globally similar to our result. There is also some debate about the actual

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TABLE 3: Number of Crystallographic Sites for Li and Assignment of the Signals Resulting from the Fit of 7Li MAS NMR Spectra for the Li-Sn Samples alloy

Li sites

Li2Sn5

1

LiSn

2

isotropic chemical shift (ppm) ∼80 (static) 77.5 (Li2Sn5)

42.5 (8e)

32 (4b)

Li7Sn3

7

42.5 (LiSn)

32 (LiSn)

Li5Sn2

3

42.3 (LiSn)

32 (LiSn)

Li13Sn5

7

42.5 (LiSn)

32 22 ?? (LiSn)

Li7Sn2

6

Li22Sn5

16

95.5 79

59

37

structure of this compound,39 so that we will not analyze further the spectrum for this material. In Li-Sn materials, metallic-type electronic configuration leads to so-called hyperfine paramagnetic interactions of two

2 17.5 7 Li sites Exchanged 17.3 (Li7Sn3)

14.5 (Li5Sn2) 14.3 3 Li sites exchanged

Figure 5. Decomposition of the Li13Sn5/SiO2 MAS NMR spectrum into individual Lorentzien-type signals. The experimental spectrum is shifted from the fitted one (dashed line) for clarity. * ) spinning sidebands.

3 and 0 (Li2O and Li2CO3)

9.5 6 Li sites exchanged 14.5 (∼Li5Sn2)

7 ?? 2.4 (Li2O -LiOH)

kinds. First, the dipolar interaction between the nuclear spins of the probed Li ions and the polarized conduction electron spins at the Fermi level (Pauli-type susceptibility) in their environment leads to some broadening of the NMR signals. It is, at least partly, averaged out by fast spinning of the sample at the magic angle. Second, these polarized electron spins may induce an “extra” effective field at the nuclear site if an s orbital of Li participates to the DOS at the Fermi level, leading to a Knight shift of the NMR signal, given by:38

K)

Figure 4. Variable-temperature 7Li MAS NMR spectra (116 MHz, single pulse, 15 kHz spinning) for Li7Sn2/SiO2 mixture (50 wt %).

1.3 (LiOH) 9.5 (Li7Sn2)

16 7 Li sites exchanged 16 (Li13Sn5) 17 0.5 (∼Li7Sn3)

(Li2O -LiOH) 1.3 (LiOH)

8π 〈|Ψr)0 | 2〉χse 3

(1)

where 〈|Ψr)0|2〉 is the electronic density of conduction electrons at the Li nucleus averaged over the Fermi surface and χse is the Pauli susceptibility. The latter term is proportional to the Li s-type DOS at the Fermi level. The observed small values of K for the Li-Sn compounds, lower than 100 ppm, are related to the small fraction of electrons at the Fermi level that contribute to the paramagnetism. The Li s partial DOS, nLis(E), were evaluated for βSn, Li, and the different Li-Sn compounds from the projection of the wave functions onto the spherical harmonic of s symmetry in Li muffin-tin spheres. This provides the contribution of each crystallographic site to nLis(EF), at the Fermi level. For example, we found nLis(EF) ) 0.0117 and 0.0135 states/eV/atom for the 4b and 8e Li crystallographic sites of LiSn, respectively. Although the difference between these two values of nLis(EF) is small, it is consistent with the observed increase in the experimental values of K from 32 to 42 ppm. For the other compounds, the values of nLis(EF) were averaged over the different Li crystallographic sites, which should represent the single exchanged NMR signals observed, for a comparison with the observed experimental values of the Knight shift given in Table 3. Experimental values of K are reported against the calculated values of nLis(EF) in Figure 6. The measured value for metallic Li is also included for comparison. The data were linearly fitted and the observed good correlation confirms that variations of K are mainly related to those of nLis(EF) as expected from relation 1. This clearly indicates that variations of the Knight shift for the Li-Sn compounds reflect those of the Li s

7Li

NMR Knight Shifts in Li-Sn Compounds

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Figure 6. Experimental values of the NMR shift as a function of the calculated value of the density of Li s states at the Fermi level. The linear correlation is shown by the solid line. Figure 7. Calculated total densities of states of Li, βSn, and the Li-Sn compounds. The origin of energy is taken at the Fermi level (vertical line).

DOS at the Fermi level, as was also discussed, in a less quantitative manner, by Wu et al.38 for ternary Li(TM)Sn4 stannides (M ) Ru, Rh, or Ir). Departure from linear trend, especially for very small values of K, could be due to the effect of Li p electrons at the Fermi level. The value of the Li p DOS at the Fermi level is about five times that of Li s DOS for Li7Sn2, Li13Sn5, Li5Sn2, and Li7Sn3 and could slightly contribute to the Knight shift via a weaker and opposite polarization of the s core electron spins. Orbital effects leading to screening of the magnetic field (chemical shift) can also become significant when the resulting Knight shift becomes small; taking them into account in DFT calculations requires much more involved strategies as discussed by d’Avezac et al.40 However, to our knowledge, the chemical shifts for 7Li in diamagnetic compounds remain in the [5; -5] ppm range. Let us also mention here that the values of nLis(EF) for the different crystallographic types of Li in any given Li-Sn compound (except for Li2Sn5 that contains only one) differ from each other more strongly than Li(1) from Li(2) in the case of LiSn as shown in Table 4. Since the two NMR signals of the latter are well-resolved (and correlate well with the two values of nLis(EF)), this suggests that the individual NMR signals for the other compounds should also be well-resolved. This, and the fact that the observed NMR signals correlate with the averaged values, further supports the hypothesis that these individual signals (except for LiSn) are not observed due to an exchange phenomenon because of mobility. The variations of nLis(EF) as a function of the composition reflect changes in the hybridization of the Li s states. The observed linear trend between K and nLis(EF) allows different groups of Li-Sn compounds to be distinguished depending on

the ranges of values for K. Metallic Li gives the highest value (K ≈ 260 ppm), which first decreases with increasing Sn amount, K ≈ 100 ppm for Li22Sn5 and K ≈ 10-20 ppm for Li7Sn2, Li13Sn5, Li5Sn2, and Li7Sn3, and then increases for LiSn (K ≈ 40 ppm) and Li2Sn5 (K ≈ 80 ppm). Such variations can be related to the 3 types of electronic structures for these materials as obtained from the calculated total DOS (Figure 7). The high value of the DOS of metallic βSn at the Fermi level is mainly due to interactions between Sn 5p orbitals. The structures of the two Sn-rich compounds, Li2Sn5 and LiSn, are also based on covalent Sn-Sn interactions, which gives a high DOS around the Fermi level as in the case of βSn. However, there are also a significant number of Li-Sn bonds that explain the contribution of the Li s states and the rather high values of K. The four Li-rich compounds show low DOS around the Fermi level that can be mainly related to the decrease in the number of Sn-Sn bonds and, to a less extent, in that of Li-Sn bonds. This could explain the low Li s DOS at the Fermi level and the low values of K. Finally, metallic Li has a BCC structure and the DOS is characterized by high Li s and p states at the Fermi level. This could also explain the high value of K for Li22Sn5, which has a similar structure with dispersed Sn atoms at the Li sites. 4. Conclusion Diluting the metallic powders with dry silica allowed 7Li MAS NMR spectra to be recorded for several Li-Sn crystalline

TABLE 4: Calculated Value of the Density of Li s States at the Fermi Level for the Various Crystallographic Types of Li in Each Li-Sn Compounda nLis(EF) Li2Sn5 LiSn Li7Sn3 Li5Sn2 Li13Sn5 Li7Sn2 a

Li(1)

Li(2)

Li(3)

Li(4)

Li(5)

Li(6)

Li(7)

avg

0.01902 0.01170 (×1) 0.00618 (×1) 0.00329 (×2) 0.00520 (×1) 0.00249 (×1)

0.01351 (×2) 0.00467 (×1) 0.01044 (×2) 0.00304 (×2) 0.00631 (×1)

0.00888 (×1) 0.00942 (×1) 0.00709 (×2) 0.00563 (×2)

0.00659 (×1)

0.00683 (×1)

0.01199 (×1)

0.00818 (×1)

0.01065 (×2) 0.00614 (×2)

0.00854 (×2) 0.00492 (×4)

0.00309 (×2) 0.00784 (×4)

0.00611 (×2)

0.00762 0.00738 0.00633 0.00596

The multiplicity of each is given in parentheses, and the weighted averaged value (plotted in Figure 6) is given in the last column.

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phases (LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5), and to assign the signals observed. Traces of impurities corresponding to other alloy compositions were detected for samples that appeared phase-pure from analysis of XRD data. All phases except for LiSn exhibit exchanged NMR signals at room temperature for the various types of Li present in the unit cells but the individual signals could not be revealed in the [263-323 K] temperature range explored. Electronic structure calculations with DFT led to a rather good correlation between the participation of the Li s states to the DOS at the Fermi level (or the weighted average for the case of exchanged NMR signals) and the corresponding NMR Knight shift. Acknowledgment. This work was carried out in the framework of the European Community funded Network of Excellence ALISTORE (contract no. SES6-CT-2003-503532). The authors are also grateful to Re´gion Aquitaine for financial support. References and Notes (1) Nagaura, T.; Tazawa, K. Prog. Batteries Sol. Cells 1990, 9, 209. (2) Disma, F.; Aymard, L.; Dupont, L.; Tarascon, J.-M. J. Electrochem. Soc. 1996, 143, 3959–3972. (3) Salver-Disma, F.; Lenain, C.; Beaudoin, B.; Aymard, L.; Tarascon, J. M. Solid State Ionics 1997, 98, 145–158. (4) Yang, J.; Takeda, Y.; Imanishi, N.; Ichikawa, T.; Yamamoto, O. Solid State Ionics 2000, 135, 175–180. (5) Besenhard, J. O.; Yang, J.; Winter, M. J. Power Sources 1997, 68, 87–90. (6) Wen, C. J.; Huggins, R. A. J. Electrochem. Soc. 1981, 128, 1181– 1187. (7) Dahn, J. R.; Courtney, I. A.; Mao, O. Solid State Ionics 1998, 111, 289–294. (8) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045– 2052. (9) Whitehead, A. H.; Elliott, J. M.; Owen, J. R. J. Power Sources 1999, 81-82, 33–38. (10) Yu, A.; Frech, R. J. Power Sources 2002, 104, 97–100. (11) Nuli, Y.-N.; Zhao, S.-L.; Qin, Q.-Z. J. Power Sources 2003, 114, 113–120. (12) Yoshio, I.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395–1397. (13) Fuji Photo Film Co. Ltd., European Patent, EP 0704 921 A1, 1995. (14) Tanizaki, H.; Miyagi, O.; Fukushima, A. U.S. patent 0053131, 2004. (15) Aboulaich, A.; Mouyane, M.; Robert, F.; Lippens, P. E.; OlivierFourcade, J.; Willmann, P.; Jumas, J. C. J. Power Sources 2007, 174, 1224– 1228. (16) Kepler, K. D.; Vaughey, J. T.; Thackeray, M. M. Electrochem. Solid-State Lett. 1999, 2, 307–309.

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