Multinuclear NMR Study of the Solid Electrolyte Interface on the Li

Article pubs.acs.org/JPCC

Multinuclear NMR Study of the Solid Electrolyte Interface on the Li-FeSn2 Negative Electrodes for Li-Ion Batteries Hua Huo,†,§ Mohamad Chamas,‡,§ Pierre-Emmanuel Lippens,‡,§ and Michel Ménétrier*,†,§ †

CNRS, Université de Bordeaux, ICMCB, 87 Avenue du Dr. A. Schweitzer, 33608 F-Pessac Cedex, France Institut Charles Gerhardt, UMR 5253 CNRS, Equipe Agrégats, Interfaces et Matériaux pour l’Energie, Université Montpellier 2, 34095 Montpellier Cedex, France § Alistore European Research Institute, 33 Rue Saint Leu, 80039 Amiens Cedex, France ‡

ABSTRACT: The composition of the solid electrolyte interface (SEI) layer on the Li-FeSn2 negative electrode has been determined by a multinuclear solid state NMR study. The strong e-nuclear dipolar and contact interactions between the as-formed superparamagnetic Fe nanoparticles and the SEI layer allows a layer wise observation highly depending on the proximity. Li+ from LiPF6, which is traditionally considered as a postdeposited product from the electrolyte solution, has been observed as presenting different solvation states by 19 F → 7Li cross polarization magic angle spinning (CPMAS) NMR. Direct contact between LiOH and LixSn or Fe nanoparticles is suggested by Fermi-Contact shifts of −11 and −2 ppm observed in 7Li and 1H NMR spectra, respectively. The LixSn alloy signal remains invisible to 7Li NMR due to the close proximity to the Fe nanoparticles. Effects of different discharging rates and aging at the discharge state are discussed as well.

1. INTRODUCTION As an important energy storage source, rechargeable Li-ion batteries (LiB) have successfully accelerated the development of portable electronics. However, when further extending the applications of LIB to electric vehicles (EV) and aerospace, which require high energy density and safety, the low theoretical specific capacity of the carbonaceous negative electrode (372 mA h/g) in the currently commercialized LIB is no doubt a major limitation. Extensive efforts have been made seeking for alternatives to the carbonaceous negative electrode materials. It has been demonstrated that metal tin can accommodate Li reversibly by forming LixSn alloys with different compositions. The value of x can be up to 4.4 (Li22Sn5), corresponding to a theoretical specific capacity of 993 mA h/g.1,2 The major drawback of metal tin as a negative electrode material is the severe volume variation upon Li+ insertion/extraction, which leads to electrode disintegration. A promising solution to this issue is to use tin−transition metal (Fe, Co, Ni, Cu, and Mn) intermetallics instead of pure metal tin. During the Li intercalation, Sn can react with lithium to form a LixSn alloy, while the transition metal can act as an inactive matrix to buffer the volume change and maintain the cyclability of the electrode.3,4 Despite the high energy density of the Sn-based intermetallics, the electrochemical mechanism, especially the formation of the solid electrolyte interface (SEI) on this type of negative electrodes still remains obscure. The concept of SEI was first applied to the lithium metal negative electrode, which is thermodynamically unstable against the electrolyte. Upon contact with the charged negative electrode, electrolyte decomposes to form a “passivation” film on the surface of the electrode, which can protect the electrode © 2012 American Chemical Society

from further corruption. The importance of SEI lies in not only the function of protection but also a media of Li+ diffusion and transportation. The existence of similar SEI layers on the surfaces of graphitic and Sn-based intermetallic negative electrodes had been proven.5,6 SEI layers on the graphitic negative electrodes have been studied by various experimental methods such as Fourier transform infrared (FTIR),7 X-ray photoelectron spectroscopy (XPS),8,9 electrical impedance spectroscopy (EIS),10,11 and nuclear magnetic resonance (NMR).12,13 However, to date only a few papers have been published investigating the SEI formation at the surface of intermetallic electrodes.14−16 In the present paper, we will focus on the understanding of the Li−Sn alloying and the SEI formation processes during the lithiation of the FeSn2 negative electrode by solid state NMR. Solid state NMR experiments performed on metallic or magnetic samples are always disturbed by some practical problems, i.e., unstable spinning, heating effect, probe tuning, and unresolved broad signals overlapping with spinning sidebands.17,18 It has been reported that diluting the samples with inert materials, for instance, dry silica can effectively solve some of the problems listed above.19 Positive electrode materials containing magnetic ions have been studied extensively by using solid state NMR. The NMR active nuclear spins (e.g., 6Li, 7Li, etc.) interact with the unpaired electron spins of the magnetic ions through bond (Fermi-contact interaction) or through space (dipolar interaction). Chemical information about local environments can be extracted by analyzing the Received: April 20, 2011 Revised: January 4, 2012 Published: January 4, 2012 2390

dx.doi.org/10.1021/jp210017b | J. Phys. Chem. C 2012, 116, 2390−2398

The Journal of Physical Chemistry C

Article

NMR spectra.20 For example, Ménétrier et al. investigated the 7Li NMR of a series of Li2CO3/active material mixtures and illustrated that the shape of the envelope of the spinning sideband manifolds is sensitive to the proximity of Li2CO3 in the SEI layer to the paramagnetic active materials.21 A series of papers by Dupré et al. have elaborated further on this aspect.22−26 The solid state NMR study on the Sn-based intermetallic negative electrode materials is still at the beginning stage. A database of 7Li NMR (Knight) shifts in the Li−Sn system, including LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, was established by Bekaert et al. This can help to identify Li−Sn alloys formed during the Li-intercalation process.19 In the study of tin composite oxide glass (TCO) electrodes, Goward et al. also brought forward a hypothesis that the 7Li NMR shifts of Li−Sn nanodomains formed after lithiation may not be real Knight shifts if the motion of free electrons is restricted by the size of such domains and/or the influence of surface oxygen species.27 In this paper, a complete collection of samples prepared under different conditions (discharge rate, discharge state, rinsing with DMC or not, and aging time) have been characterized using multinuclear NMR. This will allow us to understand how each factor affects the SEI formation. Figure 1 shows the first discharge

According to the results obtained by 119Sn and 57Fe Mössbauer spectroscopy from the same starting material as that used in the present paper, Li7Sn2 and Fe nanoparticles (∼7 nm) are formed at the end of the discharging process.16 It is worth noting that formation of Li7Sn2 instead of Li22Sn5 at the end of discharge was also observed for other tin intermetallic compounds.28,29 XRD characterization showed the appearance of a broad peak at ∼23° (2θ Cu Kα) corresponding to the range of Li-rich alloys.30 Using the Sherrer formula on this XRD line, the authors concluded that the particle size of the Li−Sn alloyed formed is about 5 nm. Magnetic susceptibility measurement indicated that the electrochemically formed Fe nanoparticles possess a superparamagnetic property.30

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of FeSn2. FeSn2 was prepared from stoichiometric amounts of Sn (Sigma-Aldrich, 99.5% purity) and Fe (Sigma-Aldrich, 99.5% purity) as in refs 30 and 31 in an alumina crucible under a controlled Ar/H2 (5%) atmosphere, heated to 470 °C for 5 h before being air-quenched. The purity and the crystallinity of the powder materials were controlled by X-ray diffraction (XRD) with a Philips X’Pert MPD diffractometer equipped with the X’celerator detector, and the morphology of the samples was characterized by scanning electron microscopy (SEM). Pure and well-crystallized FeSn2 particles with an averaged size of several micrometers were obtained. FeSn2 lithiation (first discharge) was carried out with Swagelok-type twoelectrode cells assembled inside an argon-filled glovebox. A lithium foil was used as the counter electrode, and the working electrode was made up of 80 wt % pristine material, 10 wt % polytetrafluoroethylene (PTFE) binder, and 10 wt % carbon black (SP) as a conductive additive. The composite was pressed into pellets (7 mm diameter). The electrolyte was composed of 1 M LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), EC/PC/DMC 1:1:3, v/v/v. A glass microfiber felt (Whatman) was used as the separator. Electrochemical discharge/ charge curves were recorded on a Maccor series 4000 battery test system under galvanostatic conditions at different C-rates between 0.01 and 1.2 V vs Li+/Li0. The nonrinsed electrode materials were extracted from the cells in an argon-filled drybox before NMR measurements. For the rinsed samples, the electrode materials were, moreover, washed three times with DMC and dried under vacuum. 2.2. Solid-State NMR Spectroscopy. 1H, 7Li, and 31P magic angle spinning (MAS) NMR spectra were obtained with a Bruker Avance 100 spectrometer with wide-bore 2.35 T superconducting magnet. 19F and 19F → 7Li cross-polarization (CP) MAS NMR spectra were recorded with a Bruker Avance 300 spectrometer with wide-bore 7.05 T superconducting magnet. The samples were spun at the magic angle with a spinning speed of 30 kHz for all experiments. 1H, 7Li, 31P, and 19F chemical shifts are referenced to tetramethylsilane (TMS) using secondary reference H2O (4.7 ppm), 1 M LiCl aqueous solution, 85% H3PO4 using secondary reference Al(PO3)3 (−50.8 ppm), and CFCl3 using secondary reference LiF (−204.0 ppm), respectively. Single pulse and/or rotor-synchronized EchoMAS (90°−τ−180°) sequences were utilized with 90° pulse durations of 1.1, 1.2, 1.0, and 1.2 μs for 1H, 7Li, 19F, and 31P, respectively. The Hartmann−Hahn condition for the 19F → 7Li CPMAS NMR experiments was set on LiF and optimized on the samples. Silica (Prolabo) was dehydrated under vacuum in a Buchi furnace at 200 °C for 12 h. Samples mixed with dehydrated silica

Figure 1. The first discharge curves for the FeSn2 versus Li cells with discharge rates of C/10, C/20, and C/40.

curves for the FeSn2 versus Li cells at different discharge rates. In order to make the paper easily understood by the reader, the following sample labeling is adopted through the paper: MSnx-C/ n-FD (or PD)-NR (or R)-F (or A)-(m)-(SiO2), where M stands for the transition metal and MSnx indicates the composition of the starting intermetallic compound; C/n is the discharge rate (C/n corresponds to the reaction of 1 Li in n hours); “FD” and “PD” stand for fully and partially discharged, respectively; “NR” and “R” stand for nonrinsed and rinsed, respectively; “F” and “A” stand for fresh and aged at discharge state, respectively. In the case of comparison between/among samples prepared under an identical condition is needed, an additional number m (m = 1, 2, 3....) will be added. “SiO2” indicates that the sample is mixed with dry silica (50 wt %). For example, the information conveyed by the sample label “FeSn2-C/40-PD-NR-F-1-SiO2” is a mixture of dry silica (50 wt %) and a fresh partially discharged FeSn2 sample with a discharge rate of C/40; no rinsing was performed; more than one sample have been prepared under this condition, while this one is labeled as “1”. 2391



Article

Figure 2. (a) 7Li single pulse MAS NMR of selected samples. The zoomed-in view of the dotted area is shown on the right. (b) The decompositions of the 7Li NMR spectra.

3. RESULTS AND DISCUSSION 3.1. Results. 3.1.1. 7Li NMR. The full scale 7Li NMR single pulse spectra of selected samples are shown in Figure 2a on the left, showing the line shape of the “envelope” of the spinning sideband manifolds. The zoom-in view of the region

were prepared by gently grinding dehydrated silica (50 wt %) and the sample (50 wt %) with a mortar and pestle. The 2.5 mm rotors are packed in an argon-filled glovebox prior to the NMR experiments. Decomposition of the 7Li NMR spectra was performed using DMfit, a software developed by Massiot.32 2392



Article

spectra obtained on samples discharged at the rates of C/10 and C/20 without mixing with silica. Rinsing with DMC may partially reduce this signal, while mixing with dry silica removes this signal from the spectra. The −11.0 ppm signal is an unusually negative shift, which is out of the range of diamagnetic species. A rough estimation can be made that the contribution of this −11.0 ppm signal to the spectrum decreases when the discharge rate changes from C/10 to C/20 and disappears when C/40 is adopted, which in addition to its very small magnitude (as discussed in the previous paragraph) strongly suggests that this signal is not due to possibly amorphous Li− Sn regions. We therefore propose to assign this signal to a type of Li-O-Fe or Li-O-LixSn connection, which gives rise to Fermi contact shift. Nevertheless, this interaction is so weak that it can be easily disturbed by mixing with dry silica. The −4.9 ppm signal with a 440 Hz line width can be observed in all nonrinsed samples. In freshly discharged samples, this signal is removed upon rinsing. However in FeSn2-C/10-FD-R-A, a sample aged in the cell at discharge state for 2 months, this −4.9 ppm medium-broad (440 Hz) signal can survive from the rinsing process. The information provided by 7Li NMR so far is not adequate to assign this signal; hence, we will leave this for a later discussion. The sharp signal (60 Hz) due to solvated LiPF6 salt is around −2.1 ppm in most of the nonrinsed samples. FeSn2-C/40-FD-NR-F, a sample discharged at a rate of C/40 is an exception in the spectrum of which this sharp signal (60 Hz) appears shifted to −4.9 ppm. When the same sample is mixed with silica, the salt signal is back to the original position at −2.1 ppm. This illustrates that the shift value of this signal is presumably affected by the proximity to the superparamagnetic center, which can be changed by mixing with silica. The nature of this shift could be either a pseudocontact shift due to dipolar interaction between nuclear spins and unpaired electrons or a bulk magnetic susceptibility (BMS) shift. In the decomposition of the spectrum obtained on the partially discharged sample, FeSn2-C/20-PD-NR-F (4.4 Li/ FeSn2), two more signals both centered at 14.3 ppm with different line widths (850 and 250 Hz) have been revealed in addition to the four signals mentioned above. This shift can be assigned to the Li5Sn2 alloy.19 However, as suggested by the result of 119Sn Mossbauer spectroscopy, in the conversion reaction, FeSn2 (or CoSn2) undergoes direct transformation to Li7Sn2 and metallic Fe (Co) nanoparticles without any intermediate.35 Noticing that the intensity of this signal is rather low compared to the diamagnetic 7Li signal, one possible explanation is that this Li5Sn2 comes from a small amount of βSn present as an impurity in the pristine FeSn2. To further explore this signal due to Li5Sn2 and the “invisible” LixSn signal, experimental conditions have been varied. A rotorsynchronized spin−echo experiment with a long refocus delay between the 90 and 180 degree pulses was adopted to suppress signals with short T2 relaxation times. As shown in Figure 3, signals due to Li−Sn alloys can be revealed with a refocus delay time of 20 rotor-cycles (667 μs). For the Li-CoSn2 sample, a broad signal associated with intense spinning sidebands is observed, which indicates that the LixSn alloy is in close proximity to the superparamagnetic Co nanoparticles. The LixSn alloy signal is still missing in the fully discharged Li-FeSn2 sample, which is consistent with the results obtained by magnetic susceptibility measurements showing that the saturation magnetization of Fe nanoparticles formed at the end of the first discharge is higher than that of Co.4 The decompositions of these four spectra are shown in Figure 3b.

circled by the dotted line is displayed on the right-hand side showing the isotropic resonances. When comparing the full scale spectra of the same sample with and without dry silica, a decrease of intensity of the spinning sidebands upon mixing with dry silica can be observed. As mentioned in the Introduction, the line shape of the sidebands envelope reflects the dipolar interaction between the nuclear spins and the unpaired electron, which is extremely sensitive to distance. Hence this observation illustrates that mixing with silica can separate some lithium-containing species from the superparamagnetic Fe nanoparticles. FeSn2-C/10-FD-NR-F-1 and FeSn2-C/10-FDNR-F-2 are two nonrinsed samples prepared under identical conditions. However, the overall line shapes of the envelope shown by the two are quite different. A closer examination on the isotropic resonances shows that this difference is mainly due to a sharp signal at about −2.1 ppm, which does not contribute much to the intensity of spinning sidebands. This signal is assigned to solvated Li+ from the LiPF6 salt in the electrolyte postdeposited on the surface of the sample during the drying process of the electrode once recovered from the electrochemical cell. The intensity of this −2.1 ppm sharp signal varies in the nonrinsed samples implying that the content of this postdeposited species in different samples can vary. The absence of this signal in all the 7Li NMR spectra of rinsed samples indicates that the residual salt is removed by rinsing with DMC. One important point that should be noted here is the absence of the 7Li signal due to LixSn alloy (10−100 ppm19) in all spectra. An estimation of the theoretical 7Li NMR intensity is made by taking the integration of a baseline corrected 7Li NMR single pulse spectrum obtained on a weighted LiCoO2 sample. The amounts of lithium in the Li-FeSn2 samples are roughly calculated based on the Faradays from the electrochemical curves by assuming the FeSn2 in the starting materials fully converted to Li7Sn2. Similarly, integrations are taken on the baseline corrected spectra of Li-FeSn2 samples; they are considered as the observed 7 Li NMR intensities. The result of this rough estimation suggests an intensity loss of approximately 70−80% in every 7Li MAS NMR spectrum. It is plausible to conclude that the 7Li NMR signal due to LixSn alloy is invisible in a single pulse experiment. Similar intensity losses in 31P MAS NMR due to both short nuclear-electron distances and strong magnetic effects have been reported.33,34 In order to observe the LixSn signal, reducing the magnetic effect seems necessary. According to the magnetic susceptibility measurement, the as-formed Co nanoparticles appear to be less superparamagnetic than the Fe nanoparticles.4 Therefore sample CoSn2-C/ 10-FD-R-F was prepared for a comparison of the observability of the LixSn signal versus the magnetic susceptibility of transition metal nanoparticles. However, no obvious signal due to LixSn can be observed on either Li-FeSn2 or Li-CoSn2 samples by using a single pulse experiment. In order to extract more detailed information from the 7Li NMR spectra, decompositions have been performed on the LiFeSn2 series of samples as shown in Figure 2b. The spectrum of each fully discharged sample can be decomposed to up to 4 signals, including two broad signals at −1.9 and −11.0 ppm (line width, 840−860 Hz), one medium-broad signal at −4.9 ppm (line width, 440 Hz), and the sharp signal (line width, 60 Hz) mentioned earlier, which is located around −2.1 ppm in most cases. The −1.9 ppm broad signal appears constantly in all the spectra and is close to the diamagnetic region (between −1 and 5 ppm). Hence it can be assigned to a sum of all the diamagnetic lithium-containing species (e.g., LiF, Li2CO3, Li2O, etc.) and cannot be further decomposed due to insufficient resolution. The −11.0 ppm broad signal shows up in all the 2393



Article

Figure 3. (a) 7Li MAS NMR spectra obtained with a rotor-synchronized spin−echo (90−τ−180) sequence, τ = 20 rotor-cycles (666.67 μs). The zoom-in view of the dotted area is shown on the right. (b) The decompositions of the 7Li NMR spectra.

The 14.3 ppm narrower signal can be observed in both rinsed and nonrinsed partially discharged Li-FeSn2 samples. The T2 relaxation of the 14.3 ppm broader signal may be a bit faster and leads to a decrease in observability. 3.1.2. 19F → 7Li CPMAS NMR. 19F → 7Li CPMAS NMR was performed to detect species with close lithium−fluorine proximity, for instance, LiF. Having been reported by many authors as one of the major components of SEI layers, the LiF signal was expected. The Hartmann−Hahn condition of cross-polarization was first set on the LiF model compound. The build-up of the LiF signal intensity versus contact time is fast and reaches the maximum at a contact time of 50 μs; however, the retention delay is long (200 s) due to the long T1 relaxation time of F in LiF. As shown in Figure 4a, the 19F → 7Li CPMAS NMR signal of LiF is at −0.4 ppm. It is not very surprising that no 19F → 7Li CPMAS NMR signal can be obtained on the paramagnetic Li-FeSn2 samples using the same Hartmann−Hahn condition and retention delay. Cross-polarization performed on magnetic materials is always difficult, sometime even impossible, due to the extremely short T1 relaxation time in the rotating frame (T1ρ). The contact time and retention delay were further optimized on the Li-FeSn2 samples. The best optimized 19F → 7Li CP signal can be obtained with the same match condition but with an extended contact time of 2 ms; a short retention delay of 2 s is adequate. The 19F → 7Li CP signal observed on the sample discharged at C/10 is a broad signal at −4.9 ppm. While the signal obtained from the C/40 sample is presumably a combination of the sharp and the broad signals at −4.9 ppm. The result of 19F → 7Li CP is consistent with the observations of 7Li NMR and illustrates that both the sharp (60 Hz) and the broad (440 Hz) −4.9 ppm signals are due to species with lithium−fluorine proximity.

Figure 4. 19F → 7Li CPMAS NMR spectra of (a) LiF, contact time = 50 μs, pulse delay = 200 s and (b, c) discharged Li-FeSn2 samples, contact time = 2 ms, pulse delay = 2 s.

3.1.3. 1H NMR. The full scale 1H NMR single pulse spectra of the Li-FeSn2 sample discharged at C/10 with and without dry silica are shown in Figure 5 on the left, showing the envelopes of the spinning sideband manifolds. The zoom-in view of the region circled by the dotted line is displayed on the right-hand side showing the isotropic resonances. Two well resolved signals at 0.3 and 3.4 ppm, which appear to be shifted to 0.9 and 4.0 ppm, respectively, upon mixing with silica, can be assigned to −CH3 and −CH2 groups in the solvent molecules, which keep the Li+ ions solvated. These two signals do not contribute to the intensity of 2394



Article

Figure 5. 1H MAS NMR of selected samples. The zoom-in view of the dotted area is shown on the right.

spinning sidebands, consistent with the nature of solvated species in the postdeposition layer. The broadening due to chemical shift anisotropy and magnetic effect is presumably averaged out by the local dynamics of the solvent molecules. The change of shift values of these two signals upon mixing with dry silica indicates that the magnetic effect experienced by these species can be affected by diluting. Different from the 7Li NMR in which a similar effect can only be observed on the slow discharged sample (C/40), 1H NMR shows sensitivity to this magnetic effect even on the fast discharged (C/10) sample. As discussed in the 7Li NMR part, the solvated species should be closer to the electrode in the slow discharged sample. Assuming this change of NMR shift can reflect the proximity to the electrode within a certain range of distance, 1 H NMR can probably probe a longer distance than 7Li NMR. This difference is presumably related with the high gyro-magnetic ratio of the proton. The dependency on distance and gyromagnetic ratio implies the nature of this magnetic effect can be a nuclear−electron dipolar interaction. Another important observation is the absence of a broad signal at around −2 ppm in the spectrum obtained on the sample mixed with dry silica. This is quite similar to the phenomena observed in the 7Li NMR that the −11.0 ppm disappeared when the same sample is mixed with silica. Therefore we assign this −2 ppm broad signal to the Fermi contact shift due to H−O−Fe or H−O−LixSn weak interaction. It is tempting to speculate that the −11 ppm Li and −2 ppm H may share some similar environment. One reasonable explanation is that LiOH, as an inorganic decomposition product of the electrolyte, is deposited on the surface of the LixSn or Fe nanoparticles. There is some weak interaction between the O in LiOH and the LixSn/Fe surface which probably leads to a partial electron transfer from O to Li and H. Hence, Fermi contact shifts are observed in both 7Li and 1H NMR. However it must be emphasized that the nature of the interaction is so weak that it can be easily disturbed upon mixing with dry silica. This effect may result from a simple mechanical effect (separation of the species from the surface of the LixSn/Fe particles), and/or the SiOH species that can be expected to be present on the surface of (even) dry silica may react with the LixSn surface and break this interaction. 3.1.4. 19F NMR. A static PTFE background signal of the probe is seen in 19F NMR at about −120 ppm. For comparison, the spectra of selected samples with and without background

Figure 6. 19F MAS NMR of selected samples (a) with and (b) without background subtraction. Isotropic resonances are labeled with dotted lines. 2395



Article

Figure 7. 31P MAS NMR of selected samples. The zoom-in view of the dotted area is shown on the right.

the sample drying process required for ex-situ characterizations, such as MAS NMR, when recovering the electrode material from a battery. The real situation seems much more complicated than that. In the particular case of the Li-FeSn2 electrode, superparamagnetic Fe nanoparticles are formed in the electrode upon discharging. The dipolar interaction between the unpaired electrons in Fe and the 1H/7Li/19F/31P nuclei in the SEI species is highly sensitive about distance; hence, determining the proximity of different species to the electrode is possible. As discussed in the literature,37,38 the SEI layer can be divided into two parts: inorganic decomposition products (LiF, Li2O, LiOH, LiCO3...) in direct contact with the electrode (inner layer SEI) and organic decomposition products, alkyl carbonates, on top of the inorganic layer and probably semisoluble in the electrolyte solution (outer layer SEI). The XPS study on the LiFeSn2 negative electrode material also confirms the existence of organic species at the outer surface of the electrode.16 Being the interface between the electrode and electrolyte, the SEI plays not only the role of protection but also Li+ conduction/diffusion. Hence understanding to the whole Li+ diffusion process through the SEI layer is essential. LiPF6 salt exists in the electrolyte as solvated ions (Li+ and PF6−). This type of solvated ion deposits on the surface of the sample during the drying process and gives rise to the sharp −2.1 ppm signal, −74 ppm doublet, and −146 ppm multiplet in 7Li, 19F, and 31P NMR, respectively. Desolvation of the solvated Li+ at the surface of the SEI layer is an important sometime rate-determine step of Li+ diffusion.39,40 When the Li+ is half-desolvated and half-inserted to the outer layer of SEI, a change of the 7Li NMR shift from −2.1 to −4.9 ppm due to the magnetic effect indicates this process. The signal remains sharp and can be shifted back to −2.1 ppm by mixing with silica. These facts suggest that the desolvation is incomplete and the insertion is not deep. The reader should keep in mind that this phenomenon of shift value change was only observed on the slow discharged (C/40) sample, which is consistent with the rate-determining feature of this desolvation process. The broader −4.9 ppm signal in 7Li NMR can be assigned to completely desolvated Li+, which may exist in the organic layer of SEI in a freshly discharged sample. It is only when the Li+ ions are semi- or fully-desolvated that the signal can be detected by 19F → 7Li CPMAS NMR. A longer contact time (2 ms) compared to the short one (50 μs) optimized for the LiF model compound implies that the 19F source is spatially separated from the Li+ ions, presumably the PTFE binder or desolvated/semidesolvated PF6− in the outer layer of SEI, since the solvation can give rise to large spatial separation and effectively prevents the cross-polarization process. In a freshly discharged sample,

subtraction are shown in parts a and b of Figure 6, respectively. A large first order phasing is required for spectra with such broad spinning sideband manifolds. The presence of the background signal dramatically distorts the baseline when first order phase correction is applied. Applying background subtraction before first order phase correction can help to reduce the line distortion. In the meanwhile, background subtraction also helps to resolve the MAS PTFE (binder) signal from the static PTFE signal due to probe background. In the nonrinsed sample discharged with the rate of C/10, the isotropic signal at −74 ppm can be taken as the sum of one sharp doublet, which has only some contribution on the first two spinning sidebands, and a broader signal which contributes to the spinning sideband manifolds. The doublet is due to the P−F Jcoupling in solvated PF6−, while the broader signal with intense spinning sidebands can be some PF6− either less solvated or closer to the electrode. In the nonrinsed sample with a discharge rate C/40, only the broader −74 ppm can be observed. Both types of −74 ppm signals can be easily removed by rinsing with DMC. The −204 ppm broad signal with intense spinning sidebands is assigned to LiF close to the Fe nanoparticles. The −125.6 ppm signal is assigned to PTFE binder.36 The shape of the envelope of the strong spinning sideband manifolds indicates that the PTFE binder is closely involved in the SEI layer on the surface of electrode. In the spectrum of the aged sample, the −204 ppm LiF signal shows lower relative intensity compared to the PTFE signal. This can be due to either an increase of the “NMR-visible” PTFE amount or a decrease of the “NMR-visible” LiF amount in the aged SEI layer. 3.1.5. 31P NMR. As shown in Figure 7, the 31P NMR spectra nonrinsed samples contain two signals: the multiplet at around −146 ppm due to PF6− and a signal at −7 ppm, which is assigned to phosphate species. No 31P NMR signal can be observed on the rinsed samples (not shown here) indicating that both PF6− and phosphate can be removed by rinsing with DMC. Similar to the 7Li NMR spectra, the shape of the envelope shows less nuclei−electron dipolar interaction when diluted by dry silica. The relative content of the phosphate species seems to increase with a slower discharge rate. 3.2. Discussion. In the following several paragraphs, the authors would like to make some further discussion about the SEI formation process in this Li-FeSn2 series of samples by summarizing the information extracted from multinuclear NMR spectroscopy. 3.2.1. LiPF6. The LiPF6 salt is generally considered by most LIB researchers as a postdeposition from the electrolyte during 2396



Article

affected by rinsing with DMC or mixing with dry silica. Nevertheless, this is not the case for the interaction between LixSn alloy and Fe nanoparticles, since the signal remains invisible under either condition. The original idea of using FeSn2 intermetallic instead of pure Sn is to use the as-formed Fe nanoparticles as a matrix to buffer the volume variation due to lithium insertion and extraction. Aggregation of particles is the most undesired mechanism fighting against the long-term cyclability. The invisibility of LixSn alloy signal in this system can be taken as a proof of homogeneous dispersion of LixSn nanoparticles in the matrix of Fe nanoparticles and the integrity of the electrode formed by these two species. The 7Li NMR signal due to Li5Sn2 alloy observed on partially discharged samples is likely to originate from β Sn impurity in the pristine compound, since the alloy formed from such impurity will not be in contact with the Fe nanoparticles. However, the reason why the alloy from this impurity is no longer observable by NMR after the full discharge is not understood. The number of superparamagnetic Fe nanoparticles indeed increases, but there is no obvious reason for a stronger proximity with the alloy from the impurity. Preliminary 7Li NMR result of the fully discharged Li-MnSn2 negative electrode is shown in figure 8.

all the salt signals discussed above can be removed by rinsing with DMC. In the sample aged at discharge state for 2 months, the −4.9 ppm broad signal in 7Li NMR due to fully desolvated Li+ ions can survive from the rinsing process. However, in the same aged sample, the −74 ppm signal with broad spinning sideband manifolds in 19F NMR due to desolvated PF6− still can be completely rinsed off. This gives a hint that when aged at discharge state, the Li+ can diffuse into the inorganic (inner) layer of SEI, while the PF6− probably stays in the organic (outer) layer. 3.2.2. LiOH. The hypothesis of LiOH on the surface of LixSn or Fe nanoparticles with some weak Li (H)−O−Fe interaction is supported by the experimental results of negatively shifted 7Li (−11.0 ppm) and 1H (−2.0 ppm) NMR signals. As discussed above, the contribution of this −11.0 ppm signal to the spectrum decreases when the discharge rate changes from C/10 to C/20 and disappears when C/40 is adopted. This dependence on the discharge rate makes a careful examination on the discharge curves (Figure 1) necessary, especially the curves of SEI formation part. An obvious conclusion is that, at the FeSn2 SEI formation stage, discharging at a slower rate consumes more lithium. LiOH is probably formed by a fast reaction between a trace amount of H2O in the electrolyte and desolvated Li+ ion at the active surface of the electrode. This reaction can continue to form Li2O by taking one more desolvated Li+ ion. Also Li2O can further react with some phosphorus oxy-fluoride species to form phosphates. Given the fact that the desolvation of solvated Li+ ions is a slow step, it appears reasonable to speculate that the reaction stops at the stage of LiOH during a fast discharge process, while a slow discharge process allows it to continue to form Li2O and phosphate. 31P NMR also illustrates an increasing signal due to phosphate species upon slower discharge. 3.3.3. LixSn Alloy. As already mentioned above, 119Sn Mossbauer spectroscopy indicated that the conversion product in this Li-FeSn2 system is Li7Sn2.31 However the 7Li NMR signal due to Li7Sn2 alloy is invisible. For comparison, a Li-CoSn2 sample was studied. Due to the less superparamagnetic property of Co nanoparticles, a very broad LixSn signal can be seen in a rotor-synchronized echo experiment with a long refocus delay time of 667 μs. Generally speaking, the signal intensity obtained from Hahn spin−echo experiment decays with the increasing refocus delay time τ at a rate of exp (−2τ/T2). For spin−echo experiment with long refocus delay time, the signals associated with short T2 relaxation times decrease to almost 0, while the decay of the signals associated with longer T2 relaxation times is relatively slow, and such signals can still be observed. Besides, strong nuclear-electron dipolar interactions usually lead to rapid T2* relaxation (the apparent transversal relaxation), corresponding to strong line broadenings for nuclei close to magnetic centers. In the present case of the Co system, the (real) T2 relaxation time of Li in the Li−Sn alloy is obviously long enough for it to be observed using Hahn echo with long refocus delay. This is due to the fact that the (real) T2 for the Hahn echo experiment is rather governed by nuclear dipolar interactions, since this experiment refocuses the e-nucleus dipolar interaction.41 Detailed investigation of the relaxation times for the various signals would be interesting, but fall beyond the scope of the present report. For the Fe system, the magnetic effect of Fe nanoparticles is apparently so strong that all the LixSn alloy remains invisible by Li NMR. This may be due to the fact that too strong e-nucleus dipolar interactions lead to too broad spectra even under MAS, and/or that this induces T1 limitations, which obviously cannot be overcome. In another aspect of view, the invisibility can also be taken as a characteristic feature. Some weak interaction or attachment on the surface of Fe nanoparticles or LixSn can be

Figure 8. 7Li single pulse MAS NMR of MnSn2-C/10-FD-R-F sample.

Large amount of β Sn impurity (∼ 20 w.t. %) has been detected in the pristine MnSn2 sample by XRD. In this case, a signal around 16 − 17 ppm due to Li13Sn5 or Li7Sn3 alloy formed by lithium alloying reaction with β Sn impurity can be clearly resolved in the 7Li NMR spectrum. The as formed Li−Sn alloy is separated from the magnetic Mn nanoparticles and observable in 7Li NMR.

4. CONCLUSIONS The SEI layer on top of lithiated FeSn2 microparticles is an interesting system for the understanding of SEI formation on a conversion-type negative electrode by using NMR. The dipolar interaction between the NMR active nuclear spins and the unpaired electrons in superparamagnetic Fe is anisotropic and highly depends on distance. This mechanism allows a layer-wise observation of different types of signals. This system is thus believed to contain a subtle layered structure: postdeposited solvated Li+ and PF6−/semi- and fully desolvated Li+ and PF6− (presumably in the outer layer of SEI)/desolvated Li+ having diffused in the inner SEI layer toward the electrode/LiOH on the surface of Fe or LixSn nanoparticles/tightly packed LixSn and Fe nanoparticles. Mixing with dry silica can also help to 2397



Article

(29) Naille, S.; Mouyane, M.; El Amraoui, M.; Lippens, P. E.; Jumas, J. C.; Olivier-Fourcade, J. Hyperfine Interact. 2008, 187, 19. (30) Chamas, M. Electrodes negatives pour batteries Li-ion a base de FeSn2. Ph.D. Thesis, Universite Montpellier 2, Montpellier, France, 2010. (31) Chamas, M.; Lippens, P. E.; Jumas, J. C.; Boukerma, K.; Dedryvere, R.; Gonbeau, D.; Hassoun, J.; Panero, S.; Scrosati, B. J. Power Sources 2011, 196, 7011. (32) 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. (33) Canesson, L.; Boudeville, Y.; Tuel, A. J. Am. Chem. Soc. 1997, 119, 10754. (34) Flambard, A.; Ruhlmann, L.; Canny, J.; Thouvenot, R. C. R. Chim. 2008, 11, 415. (35) Ionica-Bousquet, C. M.; Lippens, P. E.; Aldon, L.; OlivierFourcade, J.; Jumas, J. C. Chem. Mater. 2006, 18, 6442. (36) Katoh, E.; Sugimoto, H.; Kita, Y.; Ando, I. J. Mol. Struct. 1995, 355, 21. (37) Peled, E.; Golodnitsky, D.; Ardel, G. J. Electrochem. Soc. 1997, 144, L208. (38) Winter, M. Z. Phys. Chem. (Muenchen, Ger.) 2009, 223, 1395. (39) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, A1120. (40) Yamada, Y.; Iriyama, Y.; Abe, T.; Ogumi, Z. Langmuir 2009, 25, 12766. (41) Chazel, C.; Menetrier, M.; Croguennec, L.; Delmas, C. Magn. Reson. Chem. 2005, 43, 849.

determine some weak contacts/interactions. However, lithium species located in the same layer with a similar distance to the superparamagnetic center still remain irresolvable, for example, the −1.9 ppm broad signal in 7Li NMR. Finally, the tight contact between the Fe and LixSn nanoparticles makes the latter not observable by Li NMR, at least in our conditions.

■

AUTHOR INFORMATION

■

REFERENCES

Corresponding Author

*E-mail: [email protected]. Phone: +33 5 40 00 66 39. Fax: +33 6 77 08 75 56. (1) Robert, F.; Lippens, P. E.; Olivier-Foureade, J.; Jumas, J. C.; Gillot, F.; Morcrette, M.; Tarascon, J. M. J. Solid State Chem. 2007, 180, 339. (2) Wen, C. J.; Huggins, R. A. J. Electrochem. Soc. 1981, 128, 1181. (3) Winter, M.; Besenhard, J. O. Electrochim. Acta 1999, 45, 31. (4) Naille, S.; Dedryvere, R.; Zitoun, D.; Lippens, P. E. J. Power Sources 2009, 189, 806. (5) Dey, A. N.; Sullivan, B. P. J. Electrochem. Soc. 1970, 117, 222. (6) Wachtler, M.; Besenhard, J. O.; Winter, M. J. Power Sources 2001, 94, 189. (7) Aurbach, D.; Chusid, O. J. Power Sources 1997, 68, 463. (8) Peled, E.; Tow, D. B.; Merson, A.; Gladkich, A.; Burstein, L.; Golodnitsky, D. J. Power Sources 2001, 97-8, 52. (9) Naille, S.; Dedryvere, R.; Martinez, H.; Leroy, S.; Lippens, P. E.; Jumas, J. C.; Gonbeau, D. J. Power Sources 2007, 174, 1086. (10) Wang, C. S.; Appleby, A. J.; Little, F. E. J. Electroanal. Chem. 2001, 497, 33. (11) Wang, C. S.; Appleby, A. J.; Little, F. E. J. Electroanal. Chem. 2002, 519, 9. (12) Meyer, B. M.; Leifer, N.; Sakamoto, S.; Greenbaum, S. G.; Grey, C. P. Electrochem. Solid State Lett. 2005, 8, A145. (13) Xu, K.; Zhuang, G. R. V.; Allen, J. L.; Lee, U.; Zhang, S. S.; Ross, P. N.; Jow, T. R. J. Phys. Chem. B 2006, 110, 7708. (14) Ehinon, K. K. D.; Naille, S.; Dedryvere, R.; Lippens, P. E.; Jumas, J. C.; Gonbeau, D. Chem. Mater. 2008, 20, 5388. (15) Menkin, S.; Golodnitsky, D.; Peled, E. Electrochem. Commun. 2009, 11, 1789. (16) Chamas, M.; Lippens, P. E.; Jumas, J. C.; Boukerma, K.; Dedryvere, R.; Gonbeau, D.; Hassoun, J.; Panero, S.; Scrosati, B. J. Power Sources 2011, 196, 7011. (17) Kittel, C. Introduction to Solid State Physics, 8th ed.; John Wiley: Hoboken, NJ, 2005. (18) Wu, Z. Y.; Eckert, H.; Senker, J.; Johrendt, D.; Kotzyba, G.; Mosel, B. D.; Trill, H.; Hoffmann, R. D.; Pottgen, R. J. Phys. Chem. B 2003, 107, 1943. (19) Bekaert, E.; Robert, F.; Lippens, P. E.; Menetrier, M. J. Phys. Chem. C 2010, 114, 6749. (20) Grey, C. P.; Dupre, N. Chem. Rev. 2004, 104, 4493. (21) Menetrier, M.; Vaysse, C.; Croguennec, L.; Delmas, C.; Jordy, C.; Bonhomme, F.; Biensan, P. Electrochem. Solid State Lett. 2004, 7, A140. (22) Dupre, N.; Martin, J. F.; Guyomard, D.; Yamada, A.; Kanno, R. J. Mater. Chem. 2008, 18, 4266. (23) Dupre, N.; Martin, J. F.; Oliveri, J.; Guyomard, D.; Yamada, A.; Kanno, R. Electrochem. Commun. 2008, 10, 1897. (24) Dupre, N.; Oliveri, J.; Degryse, J.; Martin, J. F.; Guyomard, D. Ionics 2008, 14, 203. (25) Dupre, N.; Martina, J. F.; Guyomard, D.; Yamada, A.; Kanno, R. J. Power Sources 2009, 189, 557. (26) Dupre, N.; Martin, J. F.; Oliveri, J.; Soudan, P.; Guyomard, D.; Yamada, A.; Kanno, R. J. Electrochem. Soc. 2009, 156, C180. (27) Goward, G. R.; Nazar, L. F.; Power, W. P. J. Mater. Chem. 2000, 10, 1241. (28) Naille, S.; Ionica-Bousquet, C. M.; Robert, F.; Morato, F.; Lippens, P. E.; Olivier-Fourcade, J. J. Power Sources 2007, 174, 1091. 2398


Multinuclear NMR Study of the Solid Electrolyte Interface on the Li

Recommend Documents