6Li MAS NMR Investigation of Electrochemical Lithiation of RuO2

Feb 2, 2009 - “Job-Sharing” Storage of Hydrogen in Ru/Li2O Nanocomposites. Lijun Fu , Kun Tang , Hyunchul Oh , Kandavel Manickam , Thomas Bräunig...
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Chem. Mater. 2009, 21, 856–861

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Li MAS NMR Investigation of Electrochemical Lithiation of RuO2: Evidence for an Interfacial Storage Mechanism

Emilie Bekaert,† Palani Balaya,‡ Sevi Murugavel,§ Joachim Maier,§ and Michel Me´ne´trier*,† CNRS, UniVersite´ de Bordeaux, ICMCB, 87 AV. Schweitzer, 33608 Pessac, France, Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany, Department of Mechanical Engineering, National UniVersity of Singapore, Singapore ReceiVed October 14, 2008. ReVised Manuscript ReceiVed December 19, 2008

Nanocrystalline RuO2 was electrochemically lithiated using a 6Li-enriched negative electrode, and selected samples at various states of lithiation-delithiation were characterized ex situ by 6Li magic-angle spinning nuclear magnetic resonance (6Li MAS NMR). In the first plateau (up to one Li per RuO2), a signal with considerable shift and loss of intensity is observed, showing a strongly paramagnetic character for the LiRuO2 phase. A signal due to solid electrolyte interphase (SEI) appears at ∼0 ppm on this first plateau, but significantly grows only on the subsequent conversion plateau (from 1 to 4 Li/RuO2). Li2O is detected only at the very end of the latter plateau. On further lithiation (4 to 5.5 Li/RuO2), the magnitude of the Li2O signal remains constant, and a new signal at 4 ppm appears, that we can assign to interfacial Li hypothesized earlier in this system. Upon subsequent delithiation, NMR shows that the interfacial Li first disappears, then Li2O also disappears, and the reconstructed Li-RuO2 phase is clearly different from the one formed during the initial lithiation of RuO2. Besides, the SEI signal slightly changes but does not decrease in magnitude upon delithiation. NMR results are in satisfactory agreement with the characteristic features of the proposed “job-sharing” mechanism.

Introduction +

In the classical Li-ion battery, the Li cation is transferred from a positive electrode, such as LixCoO2 (0.5 < x < 1), to a graphite electrode during charging while the reverse process occurs when discharging. The high reversibility of the electrochemical process is caused by the soft insertion/ extraction of Li+ in these host lattices.1,2 A drawback is the limited chemical capacity. Even if the intercalation limit in these oxides is reached during discharge, the entire chemical driving force may not be used; indeed, upon further incorporation of lithium, the oxide can be further reduced to the metal or even to an alloy of lithium with the metal. In such cases, however, an extraction of lithium seems to be difficult, because of sluggish kinetics. Surprisingly, it turns out that these multiphase reactions are partially reversible and may be used for battery purposes.3 The reason is that the multiphase mixture is nanocrystalline and/or partially amorphous. Specifically, Poizot et al.3 achieved far-reaching reversible lithium storage recently in several transition-metal oxides (CoO, NiO, FeO, etc.) by conversion reaction, resulting in a reduction to the respective metal and the formation of Li2O. * Author to whom correspondence should be addressed. E-mail address: [email protected]. † CNRS, Universite´ de Bordeaux, ICMCB. ‡ Department of Mechanical Engineering, National University of Singapore. § Max Planck Institute for Solid State Research.

(1) Armand, M. B. In Materials for AdVanced Batteries; Murphy, D. W., Broadhead, J., Steele, B. C. H., Eds.; Plenum Press: New York, 1980; pp 145-161. (2) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783–789. (3) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496.

Such a heterogeneous reaction for CoO could uptake 2 Li atoms, leading to the formation of Co/Li2O composites in which cobalt metal grains 2-5 nm in size are embedded in an amorphous Li2O matrix. Such a conversion reaction favors a storage capacity in the range of 800-1200 mAh/g, with reasonably good cyclic performance. The process of lithium incorporation/removal reverses in a voltage range of 0-3 V, with ∼75% Coulombic efficiency at its first cycle and ∼100% Coulombic efficiency in the subsequent cycles. The Co/Li2O to CoO reversibility upon removal of lithium can be explained by the extremely small grain sizes of Li2O and Co metal. For the purpose of practical application, however, the performance of almost all reported materials has remained less satisfactory, because the reversibility at the first cycle is ,100%. RuO2 was found to be an exceptional material,4 in that 98% Coulombic efficiency was achieved during the first cycle, with a reversible capacity of 1110 mAh/g (corresponding to the storage of 5.5 Li+ in the voltage window of 0.02-4.3 V), as shown in Figure 1. However, the RuO2/Li half-cell could operate reversibly for only four cycles without a loss in capacity; beyond this point, the cell fails, because of a large volume expansion (∼100%) during the conversion reaction. Despite this disadvantage and the high cost, this material is still considered to be a model material for understanding the physical and chemical reasons for achieving this unique favorable combination of high capacity and high Coulombic efficiency. (4) Balaya, P.; Li, H.; Kienle, L.; Maier, J. AdV. Funct. Mater. 2003, 13, 621–625.

10.1021/cm8028005 CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

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Li MAS NMR InVestigation of Lithiated RuO2

Figure 1. Charge/discharge curve for the RuO2 versus Li cell; the curves in the range of 0.02-1.2 V refer to low-potential extra storage, and the asterisk symbols (*) indicate the composition of the samples used in the present magic-angle spinning nuclear magnetic resonance (MAS NMR) study.

Apart from the small spacing of interfaces, which leads to strongly reduced transport lengths, and modified conductivities making thus insulating materials such as Li2O or LiF electrochemically active,3-7 the appearance of nanocrystallinity leads to further exciting features in the context of lithium batteries. One example is the influence of the Gibbs-Kelvin term in the chemical potential, which leads to modifications of the cell voltage, as well as in the shape of the discharge curves.8 The most striking feature is a new lithium storage mechanism that was proposed for lithium batteries, which relies on heterogeneous interfacial accommodation of the Li+ and e- species. As shown in Figure 1, at low potential (0.05-1.2 V), a sloping behavior with a quite reversible lithium storage capacity of 120 mAh/g is observed in RuO2. Similar storage effects can be observed in most of the transition-metal oxides and fluorides.3-7 In addition, extra storage has been reported recently by Liao et al.9 in Fe/LiF composite films operated within a high-voltage window. To understand the extra storage of Li+, two “classical” explanations have been given in the literature: one refers to alloy reaction at the grain boundaries/interfaces10 and the other refers to a reaction with the liquid electrolyte at the solid/liquid interphase.11 In the present case, there is no clear evidence of any alloy reaction between the ruthenium and lithium metals, although the nanosized forms may behave somewhat differently. The latter possibility of a formation and decomposition of a passivation layer or a Li-storage therein is not compatible with the high reversibility12 within this potential window (0.02-1.2 V). Recently, an alternative explanation for this lithium storage in the low-potential sloped regime, was presented, viz, an (5) Li, H.; Richter, G.; Maier, J. AdV. Mater. 2003, 15, 736–739. (6) Li, H.; Balaya, P.; Maier, J. J. Electrochem. Soc., A 2004, 151 (11), A1878–A1885. (7) Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc., A 2003, 150, A1209–A1218. (8) Jamnik, J.; Maier, J. Phys. Chem. Chem. Phys. 2003, 5, 5215–5220. (9) Liao, P.; MacDonald, B. L.; Dunlap, R. A.; Dahn, J. R. Chem. Mater. 2008, 20, 454–461. (10) Beaulieu, L. Y.; Larcher, D.; Dunlap, R. A.; Dahn, J. R. J. Electrochem. Soc. 2000, 147, 3206–3212. (11) Laruelle, S.; Grugeon, S.; Poizot, P.; Dolle, M.; Dupont, L.; Tarascon, J. M. J. Electrochem. Soc. 2002, 149, A627-A634. (12) Zhukovskii, Y. F.; Balaya, P.; Dolle, M.; Kotomin, E. A.; Maier, J. Phys. ReV. B 2007, 76, 736–741.

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interfacial charge storage mechanism that is, in fact, thermodynamically required.8 According to this model, when in contact with a metal, Li+ can be accommodated at the Li2O side of the boundary while the electrons are restricted to the metal side. Hence, storage is possible through a contact of two phases, none of which possesses a solubility by itself. In the limit of overlapping space charges, the difference between battery and supercapacitor is becoming blurred.13 Furthermore, this mechanism was supported by firstprinciples calculations on the atomic and electronic structure of polar Ti/Li2O (111) and nonpolar Cu/LiF (001) interfaces with extra Li atoms inserted inside both two-dimensional (2D) interfaces.12,14 Unlike bulk LiX (where X ) O, F) and transition metals (M), an M/LiX interface saturated with extra neutral lithium can store a few monolayers (1-3) of inserted Li atoms with electrons being transferred largely to the transition-metal surface, in accordance with the mechanism proposed.8 While LiX surface layers or interfacial core serve as hosts for extra Li+, adatoms of the transition metal serve as electron sinks, depending on its electronegativity. Diffusion of the extra Li atoms along the M/LiX interface is observed to be energetically much easier than Li-atom penetration into the bulk. In this paper, we provide 6Li magic-angle spinning nuclear magnetic resonance (6Li MAS NMR) experimental results on nanocrystalline RuO2 samples that are electrochemically lithiated (discharged) and/or delithiated (charged) to different potentials, corresponding to different levels of incorporation or removal of Li ions, using 6Li-enriched lithium metal as the counterelectrode, as proposed in ref 15. The NMR signals obtained are discussed in the light of existing mechanistic details8,11 for the extra lithium storage beyond the stoichiometric limits (4 Li+ in the case of RuO2) at low potential. Experimental Section The electrochemical measurements were performed using twoelectrode Swagelock-type cells. The working electrode was composed of commercially available RuO2 (Alfa) with a grain size of ∼100 nm and poly(vinylene difluoride) (PVDF), in a weight ratio of 10:1. The mixture was pasted onto titanium foil (99.6%, GoodFellow) prior to the electrochemical measurements. Pure lithium foil (enriched 6Li, Aldrich) was used as the counterelectrode and the complete cell was assembled in a glovebox. The discharge and charge cycling was performed using an Arbin MSTAT system. The electrochemically reacted electrodes were rinsed using anhydrous dimethyl carbonate (DMC) in a glovebox, to remove traces of liquid electrolytes, and then was recovered for the NMR measurements. NMR. 6Li MAS NMR spectra were recorded using a Bruker Avance 300 solid-state spectrometer at 44.16 MHz while spinning the sample at the magic angle (MAS) at 30 kHz. A single-pulse sequence was utilized with a 90° pulse duration of 3.2 µs and a repetition time of 20 s, to allow full relaxation of all of the signals. The deadtime of the probe is ∼30 µs under such conditions. The 2.5-mm rotors were packed in an argon-filled glovebox. Decom(13) Maier, J. Nat. Mater. 2005, 4, 805–815. (14) Zhukovskii, Y. F.; Balaya, P.; Kotomin, E. A.; Maier, J. Phys. ReV. Lett. 2006, 9605, 792–795. (15) Armstrong, A. R.; Paterson, A. J.; Dupre, N.; Grey, C. P.; Bruce, P. G. Chem. Mater. 2007, 19, 1016–1023.

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Figure 2. 6Li MAS spectra for the various Li-RuO2 samples prepared electrochemically (single-pulse, 30 kHz spinning, arbitrary intensity scale). The x value shown in the figure denotes the amount of Li per mole of RuO2.

Figure 3. Decomposition of the 6Li MAS NMR spectrum for x ) 0.5.

position of the spectra is achieved using the DMfit software,16 with Lorentzian contributions, unless otherwise specified.

Results and Discussion 6

Li MAS NMR spectra for the various Li-RuO2 samples with x values corresponding to the asterisk symbols (*) in Figure 1 are plotted in Figure 2, using an arbitrary intensity scale. In all cases, no spinning sidebands (which would have been 258 ppm away, from the isotropic position under our conditions) were observed for any spectrum. For x ) 0.5, a rather broad signal, centered at ∼9 ppm, is observed, as shown by the decomposition in Figure 3. At the first plateau, the insertion of Li into the RuO2 network is assumed to be due to a two-phase mechanism.17 Rutile RuO2 exhibits a metallic conductivity, because of the overlap of partially filled t2g 4d orbitals of LS Ru4+ (t2 g4) via the common edge of RuO6 octahedra, as does (as expected) the lithiated phase LiRuO2 with LS Ru3+ (t2g5) (actually, the Li1.3RuO2), for which DiSalvo et al. reported a Pauli-type paramagnetism.18 However, the quasi-loss of Li NMR observation (as shown in the following discussion) suggests paramagnetic behavior, rather than true Pauli-type paramagnetism, which might originate from defects in the tiny (16) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70–76. (17) Ohzuku, T.; Sawai, K.; Hirai, T. J. Electrochem. Soc. 1990, 137, 3004– 30010. (18) Di Salvo, F. J.; Murphy, D. W.; Waszczak, J. V. Synth. Met. 1979, 1, 29–34.

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Figure 4. Decomposition of the 6Li MAS NMR spectrum for x ) 4.

electrochemically intercalated particles. This makes the situation complex for the interactions felt by the Li nuclei in the nearby 4d site that is also sharing edges with the RuO6 octahedra.19 Note also that another broad and weak signal is observed in the vicinity of 0 ppm, which might be due to the beginning of the formation of the solid electrolyte interphase (SEI) (although it was not formally identified by scanning transmission electron microscopy (STEM) at this point).4 It contains diamagnetic Li species (causing a very small chemical shift), but it is in very intimate contact with the paramagnetic compound (causing the large width of the signal, because of through-space dipolar interactions). For x ) 1 (2 V discharge cutoff), the 6Li NMR spectrum is quite similar, with a slight increase in the intensity of the shifted signal, which confirms the nature of the LiRuO2 phase previously discussed, and the two-phase mechanism. The x ) 2 spectrum in Figure 2 is again very similar to the first two spectra. For x ) 4 (that is, at the end of the conversion reaction plateau), decomposition of the signal in Figure 4 shows a very well-defined signal, corresponding to Li2O (2.8 ppm), together with a signal centered at 0 ppm, which we again assign to the SEI. Note that this signal, although at the same position as that assigned to the SEI in the less-lithiated samples, is considerably narrower, which can be explained by the change in the nature of the material itself. For lower values of x, the SEI forms on the LiRuO2 phase. Because the latter has previously been suggested to be strongly paramagnetic, close proximity can induce strong dipolar interactions on the Li nuclei of the SEI. A similar effect was reported for surface layers on paramagnetic layered transition-metal oxides.20,21 For x ) 4, nanoparticles of ruthenium metal are present, which should exert much weaker paramagnetic dipolar interactions on the Li atom in SEI. We will comment on the magnitude of the SEI signal in the following discussion. Decomposition of the spectrum (Figure 4) also reveals significant signal intensity at 4 ppm, which is a position that does not correspond, to the best of our (19) Davidson, I. J.; Greedan, J. E. J. Solid State Chem. 1984, 51, 104– 117. (20) Me´ne´trier, M.; Vaysse, C.; Croguennec, L.; Delmas, C.; Jordy, C.; Bonhomme, F.; Biensan, P. Electrochem. Solid State Lett. 2004, 7 (6), A140–A143. (21) Me´ne´trier, M.; Bains, J.; Croguennec, L.; Flambard, A.; Bekaert, E.; Jordy, C.; Biensan, P.; Delmas, C. J. Solid State Chem. 2008, 181, 3303–3307.

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Li MAS NMR InVestigation of Lithiated RuO2

Figure 5. Decomposition of the 6Li MAS NMR spectrum for x ) 5.6.

knowledge, to any diamagnetic lithium-containing phase. We consider this signal to be very well in agreement with the interfacial Li hypothesized, that starts to appear in this voltage range. Indeed, this NMR shift is slightly too high for a shielding mechanism in a diamagnetic compound (chemical shift), and could correspond to a Li ion that resides at a surface site of Li2O, but with a chemical bond with the Ru nanoparticles to which it has “transferred” its electron. A paramagnetic shift, even minor, indeed requires an orbital overlap between the atom carrying the spin and a spherical (s-type) orbital of the nucleus studied by NMR. The presence of electron spins, as for the SEI species at the very beginning of the discharge discussed previously, also leads to a certain width of the NMR line. However, the exact nature of the magnetism of the Ru nanoparticles is not known (real metal or paramagnetism), so that we cannot go any further in this analysis. Let us emphasize here that the nature of this signal does not support the possible formation of Li-Ru alloys during the final sloped regime. Because the voltage continuously changes during this period, this would necessarily correspond to the formation of an alloy with a continuously changing Li composition, rather than to the formation of definite alloys with a two-phase mechanism. Although one cannot exclude that Li-Ru alloys may have Li NMR (Knight) shifts in the range of the value observed, this shift would then necessarily change continuously with the composition during discharge. This is clearly not the case, although only two compositions (actually, the two extreme ones) were characterized by NMR. For x ) 5.6 (20 mV discharge cutoff), a rather similar spectrum is obtained (see Figure 2 and decomposition in Figure 5). Upon charge to x ) 5, decomposition of the signal again leads to similar components (see Figure 6), whereas for x ) 3.1, a new strongly shifted component (14.5 ppm) appears (see Figure 7). At this point, a discussion of the magnitude of the signals is important. Indeed, NMR is a quantitative technique, but this requires considerable care. For similar types of signals with small width, the present type of experiments should be quantitative, if one takes into account all the spinning sidebands of the MAS spectra. However, as already mentioned, the entire spectra are contained in the isotropic signal for all our samples. A

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Figure 6. Decomposition of the 6Li MAS NMR spectrum for x ) 5 (obtained upon first charge).

Figure 7. Decomposition of the 6Li MAS NMR spectrum for x ) 3.1 (obtained upon first charge).

condition for quantitative observation is that the quality factor of the probe is similar; this was checked by the tuning process for each experiment, and no detrimental effect of the metallic particles was observed, regardless of the MAS condition, most probably because these particles are embedded in a diamagnetic Li2O matrix. Another prerequisite for quantitative observation is, of course, that the recycling time allows full relaxation. As indicated in the Experimental Section, the 20 s recycling time used was checked to fulfill this criterion. This appears rather short for diamagnetic species such as the SEI and Li2O, but the immediate proximity of both species from the paramagnetic material (either “LiRuO2” or the metallic particles) clearly leads to a decrease of the T1, as was observed in ref 20. The same dipolar interaction also leads to considerable width of the signal of the SEI, as discussed previously. Finally, because the 6Li isotope was used, quantitative analysis also relies on the fact that the abundance of this isotope is identical in all the samples. In that respect, it is worth mentioning that the amount of lithium salt present in the electrolyte (with natural abundance, i.e., 7.4% 6Li isotope) corresponds to ∼0.2 Li in the RuO2 material, based on the relative amount of electrolyte (0.1 cm3) and RuO2 (100 mg). Therefore, we have constructed graphs from the decomposition of the central signal of the spectra recorded for the various samples analyzed. Figure 8 gives, in arbitrary units, the total amount of Li observed by NMR, relative to the amount of Li expected to be present in the NMR sample (on a Faradays basis) from the electrochemical curve

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Figure 8. Total amount of Li observed by NMR per mole of RuO2 and per Faraday for the various samples.

Figure 9. Amount of Li observed by NMR per mass of sample in each signal for the various samples.

leading to each sample (on an arbitrary scale). Figure 9 gives the amount of Li observed by NMR in each signal, relative to the mass of the sample present in the rotor. This allows the following global comment on the series of spectra. As already suggested, the total amount of Li observed by NMR and scaled on a Faradays basis is very low in the first plateau (2 V) range, whereas it is approximately constant in the conversion/interfacial region upon further discharge, for which we therefore assume that all the Li present in the sample is observed by NMR. This initial loss of observation is too large to be caused by a possible reaction of 7Li from the electrolyte solution, and it is furthermore clearly correlated to the presence of the strongly shifted signal (close to 9 ppm on the plateau of discharge that we assigned to the LiRuO2 phase). The strong paramagnetic interactions previously suggested in this phase must lead to a very broad signal, which, consequently, is largely lost in the relatively long deadtime of the probe at 44 MHz. Upon charging, a signal (even more shifted at 14.4 ppm) appears at ∼2.5 V, which is again correlated with a decrease in the total amount of Li observed. This suggests that a paramagnetic lithiumcontaining compound is formed, but one that is different from the LiRuO2 phase. Indeed, the charge curve is quite different from the discharge one, and the higher voltage measured upon charging (and upon the following discharge) was explained by the amorphous character of the RuO2 formed during charging.22 These results further suggest that a transitory “LiRuO2” phase is formed upon charging after a full discharge (that is conversion-reconstruction), and that it is significantly different from that obtained during the first discharge, even in its local crystallographic and electronic structure, as probed by NMR. The signal that is assigned to the SEI appears but remains weak in the first plateau region. It clearly grows during the conversion regime and the interfacial regime. It is also striking that it continues to increase during the first charge, although the total amount of Li in the sample indeed decreases. This means that cycling in the range of 20 mV to 1.2 V does not involve decomposition of the SEI upon charging. It is also important to note that the actual shift of

this signal gradually changes from -0.6 ppm to +0.4 ppm during the discharge-charge process (see the decomposition of the spectra). Note also that the best fits of the experimental spectra were obtained using different Lorentzian/Gaussian characters for the different samples for the SEI signal, whereas all other signals were fitted using pure Lorentzian components. This means that the SEI probably contains a distribution of Li environments, and that its nature changes for the different charge/discharge states, but in a way that we cannot analyze further solely based on NMR characterization. However, it clearly does not appear simultaneously (that is, at the same voltage) with the interfacial Li. It is reasonable to consider that the SEI grows at the contact of the (initial) grains of RuO2 with the electrolyte, whereas the interfacial Li develops inside the nanostructure that has formed in these grains, so that the two processes do not interfere. The SEI fraction increases for x ) 3.1 upon charging. At this potential, as explained earlier, domains of an unidentified “LiRuO2” phase are formed, resulting in fresh surfaces that, when in contact with liquid electrolytes, can form additional SEI layers. The signal of Li2O clearly appears at the end of the conversion plateau. It is indeed striking that it is not observed at approximately one-third of the conversion plateau for x ) 2, where one-third of the final amount could have been expected. During the slope region following the plateau, the amount of Li2O does not increase further very significantly, which shows that it is created during the conversion plateau, but observed only and totally at the end of this process. A possible explanation is that the native Li2O being formed during the conversion remains so intimately close to the original LiRuO2 and/or the native Ru metal nanoparticles that it cannot be observed by NMR, again because of paramagnetic interactions. Note that, in that respect, the lack of Li NMR observation per transferred Faraday is highest for x ) 2, which supports this explanation. While charging to 1.2 V, the amount of Li2O does not decrease, but it has completely disappeared at 2.5 V. This, in good agreement with the absence of a plateau during charging and during the second discharge in the voltage curve and with the previous comments about the “LiRuO2”-type signal upon

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charging, again shows that the conversion process is not reversible in the sense that it does not involve the original LiRuO2 intermediate phase. The disappearance of Li2O confirms that a lithiated RuO2-type phase forms upon reconstruction during charge, but this phase is different from that formed via the lithiation of RuO2 during discharge. The signal at 4 ppm assigned to the interfacial lithium based on its shift (see above) appears rather “early”, because the discharge cutoff at 800 mV corresponds to the very end of the plateau (∼4 Li) where the sloped regime has barely started. It is striking that a major quantity of interfacial Li is already formed at this point, as compared to the amount observed after full discharge (20 mV cutoff). Besides, approximately one-half of the amount formed upon discharge is still present after charging to 1.2 V, while the entire amount has disappeared after charging to 2.5 V. Thus, based on our (ex situ) NMR study, it seems that the formation of Li2O and the interfacial lithium do not occur as sequentially along the lithiation process as one would expect from the shape of the discharge curve. However, it is very important, in that respect, to mention that the latter illustrates a dynamical process, while relaxation of the electrode indeed occurs after the various cutoffs leading to the NMR samples. Conclusion The present 6Li magic-angle spinning nuclear magnetic resonance (6Li MAS NMR) investigation of the lithiation

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process of rutile RuO2 strongly confirms earlier reports of that system and, in particular, the reversible appearance of a specific type of Li species after the conversion process, assigned to the so-called “interfacial” Li that has not actually been “seen” by any experimental method previously. It is clearly distinct from the Li species within the SEI layer that, in addition, do not disappear upon charging. As often observed in such systems, the complexity of the possible NMR interactions from electron spins and more or less delocalized electrons present in these poorly otherwise characterized nanometric materials somewhat restricts the degree of sophistication to which a reasonable analysis can be brought. However, it does highlight the differences in equilibrated samples used in such (necessarily) ex situ MAS NMR experiments and the perception that one has of the electrochemical processes from the dynamic charge-discharge voltage curves. Acknowledgment. This work was performed in the framework of the ALISTORE Network of Excellence (Contract No. SES6-CT-2003-503532), funded by the EC. The authors thank J. Jamnik, J.-M. Tarascon, M. Dolle´, and Y. Hu for useful discussions. P.B. also thanks MPI-FKF for the award of Max Planck Fellowship during the period of 2001-2006. CM8028005 (22) Delmer, O.; Balaya, P.; Kienle, L.; Maier, J. AdV. Mater. 2008, 20, 501–505.