DOI: 10.1002/adfm.200600937 Towards a Fundamental …btinnovations.org/shanebeattie/Papers/Towards_a_Fundamental... · David Talaga, Michel Couzi, and Jean-Marie Tarascon 1. Introduction

DOI: 10.1002/adfm.200600937

Towards a Fundamental Understanding of the ImprovedElectrochemical Performance of Silicon–Carbon Composites**

By Juliette Saint, Mathieu Morcrette, Dominique Larcher,* Lydia Laffont, Shane Beattie, Jean-Paul Pérès,David Talaga, Michel Couzi, and Jean-Marie Tarascon

1. Introduction

Over recent years, it has become critically important to finda new negative electrode material with a higher capacity thancommercial graphite to facilitate the fabrication of improvedlithium-ion batteries. An important argument for the use ofgraphite in batteries pertains to the relatively low volumechange upon the intercalation of Li into the host during charg-ing of the battery; the total volume change for LiC6 is onlyabout 10 %. However, due to the low electrochemical capacity(theoretical value of 372 mA h g–1) of graphite, a lot of atten-tion has been focused on finding alternative negative electrodematerials, especially in the area of Li–metal alloys such asLi–Al and Li–Si. Si is of great interest because it has the high-est theoretical electrochemical capacity for alloying with Li(3572 mA h g–1 for Li3.75Si);[1–8] however, the incorporation of

Li is accompanied by large volume changes and structuralmodifications, which lead to the generation of strong internalmechanical stresses and cracking of the particles, finally result-ing in the loss of electrical conductivity.[9–14] As a result, the Lialloys exhibit poor cycling behavior and their use as negativeelectrodes has not been pursued since the end of the 1980s.However, work on amorphous tin oxide materials (ATCO)[15]

by Fuji led to a renewal of interest in Li alloys at the end of thelast decade, and recently several approaches have been devel-oped to improve the retention capacity of Si-based negativeelectrodes. Many studies have been devoted to examining themechanism of formation of Li alloys, and much effort has beenexpended on characterizing the cracking phenomenon in orderto minimize it in the case of Si.[16–21] It is expected that a reduc-tion of the particle and crystallite size to the nanometer regime(for powders and thin films) will have a positive influence onthe cycling behavior,[11,13,14] since the mechanical stress devel-oped during the lithiation process might perhaps be reduced oreven avoided.[22–24] Furthermore, for Si thin films, the retentioncapacity is enhanced because of the strong adhesion of the ac-tive material to the conductive support (thus maintaining elec-trical contact) during cycling. Another approach to resolve theSi cycling issue is the fabrication of composite microstructuresin which the Si particles are finely dispersed in a conductivematrix.[5] The small expansions in volume, relative lightness,good electronic conductivity, and most importantly, good ionicconductivity, make carbon the ideal matrix material for this ap-plication. Several different methods have been used to synthe-size Si–C composites including ball milling,[25–27] carbon coatingfrom the gas phase (chemical vapor deposition (CVD) andthermal vapor deposition (TVD)),[28–32] and the pyrolysis of in-timately mixed precursors.[33–41] Composites synthesized by the

Adv. Funct. Mater. 2007, 17, 1765–1774 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1765

–[*] Prof. D. Larcher, Dr. J. Saint, Dr. M. Morcrette, Prof. L. Laffont,

Dr. S. Beattie, Prof. J.-M. TarasconLaboratoire de Réactivité et Chimie des SolidesUniversité de Picardie—Jules Verne33, rue Saint Leu, 80039 Amiens Cedex (France)E-mail: [email protected]. J.-P. PérèsSAFT—Direction de la Recherche111, Boulevard Alfred Daney 33074 Bordeaux (France)D. Talaga, Dr. M. CouziISM Université Bordeaux I351, Cours de la Libération 33405 Talence Cedex (France)

[**] The authors are indebted to A. de Guibert (SAFT), C. Jehoulet(SAFT), B. Simon (SAFT), S. Hamelet (LRCS), and P. Roca i Cabar-rocas (Ecole Polytechnique) for helpful discussions, sharing skills,and synthesizing samples.

Silicon–carbon composites consisting of Si particles embedded in a dense and non-porous carbon matrix are prepared by thepyrolysis of intimate mixtures of poly(vinyl chloride) (PVC) and Si powder at 900 °C under a flow of N2. In contrast to baremicrometer-sized (1–10 lm) and nanometer-sized (10–100 nm) Si powders, which show poor cycling behavior with almost nocapacity remaining after 15 cycles, the texture of the composite is seen to greatly enhance the reversibility of the alloying reac-tion of Si with Li. For instance, a capacity of ca. 1000 mA h g–1 is achieved for 20 cycles (0–2.0 V vs. Li+/Li) for a silicon–carboncomposite containing nanometer-sized Si particles. We also demonstrate that a mild manual grinding treatment degrades thecycling performance of the composites to levels as low as the parent Si, even though free Si is not released. The electrochemicalmeasurements in conjunction with Raman spectroscopy data indicate that a huge stress is exerted on the Si domains by thein situ formed carbon. This carbon-induced stress is found to disappear during the milling of the composites, indicating thatthe carbon-induced pressure, along with the accompanying improvement in electrical connectivity, are the key parameters forthe improved cycling behavior of Si versus Li.

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latter process show a significant improvement in the Si cyclingbehavior, especially when the carbon precursor is poly(vinylchloride) (PVC). Here, the Si particles are finely dispersed in aPVC matrix, which is first dissolved in propylene oxide. Thepyrolysis reaction is carried out after evaporation of the sol-vent. The as-obtained composites exhibit a capacity as high as750 mA h g–1 over 40 cycles.[33] The origin of this electrochemi-cal behavior has not been explained so far. Here, we report afundamental study of the properties of these Si–C composites.In this work, all of the composites have been synthesized viathe pyrolysis of Si–PVC mixtures. The effect of the Si precursoris discussed in terms of the size and texture. Finally, we presenta systematic study of the Si/C interface in the composites; Ra-man spectroscopy and electron energy loss spectroscopy(EELS) studies help to provide a better understanding of theelectrochemical behavior of these systems.

2. Results and Discussion

Two kinds of Si particles are used. Micrometer-sized Si (SiM)particles are purchased from Aldrich (325 mesh) and nanome-ter-sized Si particles (SiN) are synthesized by the CO2-laser py-rolysis of SiH4 (Commissariat à l’Energie Atomique, Saclay,France). For the sake of clarity, powders obtained by the pyrol-ysis of Si–PVC blends are labeled by adding a “Cp” suffix toSi. For instance, Si–C composites obtained by the pyrolysis of aSiM–PVC mixture are labeled SiMCp.

Two forms of carbon are compared: Ketjen Black KJ and pyr-olyzed PVC. Given the high carbon content of our electrodes(56 wt %) and the large contribution from carbon to the electro-chemical signature, the two carbonaceous materials need to be

characterized before testing the electrochemical properties ofthe Si. Both the carbonaceous materials have been found to bedisordered carbons by X-ray diffraction (XRD), and no residualchlorine is detected by energy-dispersive X-ray spectroscopy(EDS). Brunauer–Emmett–Teller (BET) specific surface areasalong with scanning electron microscopy (SEM) and transmis-sion electron microscopy (TEM) observations reveal the highlydivided nature of KJ carbon (700 m2 g–1, 60 % porosity), whichis in sharp contrast to the very dense and bulky texture of pyro-lyzed PVC (< 1 m2 g–1, no porosity). The electrochemical re-sponses of the two materials are also very different, with a veryhigh first discharge capacity (2500 mA h g–1) and very limitedreversibility for KJ carbon, and a much lower initial dischargecapacity (365 mA h g–1) but good reversibility for pyrolyzedPVC. Since only the KJ carbon shows a high first discharge ca-pacity, it’s electrochemical trace will be superimposed on thegalvanostatic cycling plots for bare Si discussed in Section 2.1.

2.1. Bare Crystallized Si Powders

As shown in Table 1, the SiM and SiN samples exhibit refinedcubic cell parameters very close to the literature value(5.430 Å, Joint Committee on Powder Diffraction Standards(JCPDS) file # 27-1402), and no impurities are detected in theXRD patterns.

For SiN, the calculated crystallite size (6–7 nm) is indepen-dent of the crystallographic direction, indicating isotropic crys-tallites. The SiM particles range in size from 1 to 10 lm,whereas the rod-like SiN particles exhibit a size distributionfrom 10 to 100 nm. The good match between the expected geo-metric surface area calculated from the average particle sizeand the BET specific surface area derived from the nitrogen

1766 www.afm-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 1765–1774

Table 1. Particle/crystallite sizes, BET specific surface areas, SEM/TEM images with selected area electron diffraction (SAED) patterns, and XRD patternsfor the different Si and Si–C composite samples used in this study.

Sample Particle sizeCrystallite size

[nm]

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SiM 1�10 lm > 100 2.7 (111)

(220)

(311)

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SiN 10�100 nm 6.6 ± 0.8 70

a=5.425(2) Å

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(311)

SiNCp 100�500 lm 7 < 1

Si

Si

Si

Carbon

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J. Saint et al./Improved Electrochemical Performance of Silicon–Carbon Composites

adsorption data is an indication of the non-porous and densenature of these particles. The SiN particles are completely oxi-dized during a thermogravimetric analysis (TGA) run in air, asverified by XRD. The measured weight gain is found to be low-er (80 %) than expected for the complete oxidation of Si ac-cording to Si + O2 → SiO2 (114 %). Although the presence oflarge amounts of hydrogen cannot be ruled out in these sam-ples since they are synthesized from SiH4, this discrepancy isunlikely to originate from remnant hydrogen because of thelight weight of hydrogen. The observed discrepancy is insteadmore likely to result from the presence of an oxide layer on thesurface of the particles; from the TGA results, the thickness ofthis layer is estimated to be 12 % of the particle radius. Fromthese assumptions, the oxide layer is calculated to range from0.6 to 6 nm in thickness for the SiN particles.

The electrochemical behavior of these crystallized Si parti-cles towards metallic Li is compared in Figure 1. Although thefirst discharge capacity (at a similar rate of 1 Li per 30 h) ob-viously increases as the particle size is decreased (Li1.7Si forSiM, Li2.1Si for SiN), the full electrochemical room-temperaturelithiation of Si (i.e., Li3.75Si)[8] is not achieved, even after takinginto account the oxide content as being inactive. In both cases,the cell capacity drops very quickly to almost zero after 15 cy-cles (Fig. 1, inset). Also, it is worth noting that most of the firstcycle irreversibility of the cells is due to the KJ additive carbon.Under our testing conditions (Swagelok cells, 50 wt % KJ con-ducting carbon, 1 Li per 30 h), going from micrometer-sized tonanoscale samples is therefore not the critical parameter toachieve long cycle life for Si–Li cells. Nevertheless, these testsand the corresponding ∂x/∂V = f(V) curves (Fig. 1, inset), where

Adv. Funct. Mater. 2007, 17, 1765–1774 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 1767

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x is the extent of lithiation and V is the measured voltage, re-veal some interesting characteristics: i) a discharge peak at1.0 V versus Li+/Li due to the reaction of KJ with Li, ii) thepresence of a sharp charging feature at around 0.4 V versusLi+/Li for SiM–Li cells, which is different from the two broadsignals observed at around 0.3 and 0.5 V versus Li+/Li for theSiN–Li cells, and iii) the distinct splitting of the first sharp low-voltage plateau into two broad regions in subsequent dis-charges. The well-defined Si charge plateau has been recentlyattributed to the removal of Li from a crystallized Li15Si4 cubicphase.[8] This plateau is only observed when dealing with largeparticles or thick films, and can therefore be used as a probefor changes in the textural/structural organization within thematerials during cycling or thermal treatment.

2.2. Crystallized Si–C Composites

The Si samples are mixed with PVC and the resulting plasticblends are pyrolyzed as described in the Experimental section,resulting in a final carbon content of ca. 56 wt %. The XRDpatterns and Bragg peak widths of the pyrolyzed powders (Ta-ble 1) do not indicate any change in structure or crystallite size,but there is a drastic decrease in the BET specific surface areas(from 70 m2 g–1 for SiN to less than 1 m2 g–1 for SiNCp); more-over, SEM and TEM images reveal the formation of very large(50–500 lm) dense particles consisting of unmodified Si parti-cles embedded in a dense carbon matrix derived from PVC(Fig. 2a). Backscattered electron (BSE) images reveal that theSi particles are homogeneously dispersed within the carbonmatrix (Fig. 2b). Control experiments using bare SiN annealed

by the same procedure as used for the Si–PVC blends indicateno modification in the texture (reflection width, specific sur-face area) or structure of the powders. Thus it is clear that thethermal treatment does not trigger any sintering of the Si pow-ders.

The electrochemical response of the as-prepared pyrolyzedsamples varies significantly from that of the pristine samples(Fig. 3). Firstly, in contrast to bare Si, in the as-pyrolyzed sam-ples, the smaller the Si particles, the lower the lithiation levelof Si: Li2.6Si for SiMCp, Li2.4Si for SiNCp. Secondly, embeddingthe particles in the carbon matrix enhances the first Si lithia-tion for both SiM (Li1.7Si → Li2.6Si) and SiN (Li2.1Si → Li2.4Si).Thirdly, due to the changes in characteristics of the carbon ma-trix, the first cycle irreversibility is considerably lowered fromat least 1000 mA h g–1 (KJ) to 200–300 mA h g–1 (pyrolyzedPVC). Notably, the different cycling rates used for bare Si(1 Li per 30 h) and Si–C composites (1 Li per 10 h) cannot ac-count for the differences in the first discharge and irreversibleloss observed for KJ and pyrolyzed-PVC carbons. Cycling ex-periments (not shown here) conducted on both carbons at simi-lar rates reveal the same dramatic differences. The final andmost important difference is the clear and systematic improve-ment of the retention capacity with cycling; notably, the SiNCpcell exhibits a capacity of around 1000 mA h g–1 for 20 cycles.Whereas the Coulombic efficiency is nearly 100 % for this cell,it is worth noting that there is a loss in capacity of 0.8 % per cy-cle, which is still too high for practical applications. The reac-tion of Si with Li in the SiMCp and SiNCp systems is clearlyidentifiable on the ∂x/∂V = f(V) derivative curves; these alsoindicate that the SiN particles do not show any drastic structur-al and/or textural changes during the formation of the compos-ite.

With an eye towards the integration of these systems in com-mercial Li-ion batteries, the large composite particles havebeen ground and sieved to obtain a particle size smaller than40 lm. This is easily achieved by the simple manual grinding ofthe samples with a mortar and pestle (Fig. 4). Surprisingly, thismild milling process has a dramatic effect on the reversibilityof the electrochemical reactions regardless of the size of the Siparticles (Fig. 3a). A complete loss of capacity is observed aftera few cycles for the milled samples, similar to the behavior seenfor bare Si. As deduced from HRTEM observations and fromthe particles size of the ground Si/C itself (Fig. 4b), a destruc-tion of the composites with complete separation of the Si do-mains from the carbon matrix is not the right scenario to ex-plain such behavior The low BET specific surface areas of theground SiNCp composites (around 5 m2 g–1) quantitatively cor-roborate this point. The electrochemical performance of theSi–C composites is therefore intimately linked to the fragile in-terfacial interactions between carbon and Si, and is greatly sen-sitive to the grinding process. These results underline the needfor the accurate characterization of the Si/C interface; betterunderstanding of this interface is expected to lead to improvedcontrol of the electrochemical properties of these composites.It is likely that both the chemical composition and physical in-teractions need to be considered in studying the interface.


Figure 2. SEM a) secondary electron and b) backscattered electron im-ages of as-prepared SiMCp composite particles.

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2.3. Characterization of the Si/C Interface

As previously shown by TGA, Si has a strong tendency toundergo surface oxidation, and thus the Si/C interface couldconceivably be simply composed of silicon oxide. Another pos-sibility is the reduction of this oxide by carbon and the forma-tion of SiC at the interface during pyrolysis. The interface hasbeen studied by EELS measurements of both bare SiN andSiNCp composite samples.

EEL spectra collected at the edge and center of a SiN parti-cle (Fig. 5) exhibit important variations in the energy positions,intensities, and shapes of the Si L2,3 fine structures, indicatingsignificant variations in the chemical bonding within theprobed zones. At the center of the particle, the EEL spectrummatches that of Si with a Si L2,3-edge characterized by a strongpeak located at 101 eV, followed by a small plateau at102.9 eV, and then a broadened region.[42] The EEL spectrumacquired at the edge of the particle corresponds to the Si L2,3-edge of SiO2 with a sharper onset strongly shifted to higher en-ergy (107.5 eV),[43] followed by two features with maxima lo-cated at around 115 and 120 eV, and finally by a broad region.The EELS measurements indicate that the Si particles are sur-rounded by a SiO2 layer varying in thickness from 1 to 5 nm,which agrees well with the values deduced from the TGA re-sults. A legitimate issue is the influence of this oxide layer onthe electrochemical performance of the Si-based compositeelectrodes, even though no extra peaks corresponding to the


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Figure 3. a) Voltage–composition galvanostatic curves versus Li for as-prepared (left panels) and ground (right panels) Si–C composites (1 Liper 10 h, 20 °C). The LixSi formulae indicate the extent of Si lithiation atthe end of the first discharge, and the thick vertical bars denote the ex-pected capacity for the complete reaction with both C and Si. b) Derivative∂x/∂V = f(V) plots for the first (solid) and second (dashed) galvanostaticcycles (top); changes in the charge/discharge capacity as a function of thecycle number (middle); and Coulombic efficiencies (bottom) for as-madeSiMCp (right column) and SiNCp (left column) samples.

Figure 4. SEM images showing the particle size and morphology of SiNCpparticles a) before and b) after manual grinding.

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reduction of nanoscale SiO2 by Li are observed in the deriva-tive plots (Fig. 3b). Nevertheless, to ensure the electrochemicaltransparency of the SiO2-coated Si particles towards Li, wehave fully oxidized our 10–100 nm sized particles and testedtheir reactivity versus Li. Notably, no capacity is observed forthe fully oxidized particles. From thermodynamic calculations,we have determined that the DG0 of the reaction

SiO2 + 4 Li → Si + 2 Li2O (1)

is negative (–72 kJ mole–1 of SiO2), corresponding to a DE0 val-ue of 0.19 V versus Li+/Li. Therefore, given that potentialsgreater than equilibrium potentials of about 1 V are necessaryto trigger these types of conversion reactions, the chances ofthe SiO2 being reduced by Li through a conversion reactionare extremely slim.

To further understand the Si/C interface in the composites,several EELS line-spectra have been acquired across the inter-face of the SiNCp particles (both Si L2,3- and C K-edges). Fig-ure 6b shows spectra recorded along the line indicated on thescanning transmission electron microscopy (STEM) bright-field image (Fig. 6a) of the analyzed particle. All the scanshave been collected with a maximum total acquisition time of1 min in order to minimize irradiation and instability effects.The evolution of the Si L2,3 fine structure and the C K-edgealong the line indicates that the core of the particle is com-posed of Si, which is surrounded by a SiO2 layer; the thickness

of this layer (5–10 nm) is strongly dependent on theparticle size. Outside this oxide layer, only the C K-edge can be detected, indicating that the carbon ma-trix surrounds the particles without any trace of SiC.Indeed, the fine structure of SiC is characterized by adistinct peak located at 103 eV.[44] Therefore, the na-ture of the surface of the Si particles is not changedduring the pyrolysis treatment, and no evidence isfound for chemical bonding at the (Si/SiO2)/C inter-face. This absence of strong chemical bonding couldaccount for the weakness of the interface, as evi-denced by the degradation in performance after thegrinding step. However, loose physical contact be-tween the components can also be ruled out since theground composites showing poor cycling perfor-mance are still composed of dense micrometer-sized(< 40 lm) composite particles, each containing nu-merous embedded Si clusters. We believe that thegrinding step results in breakages of the particles;each of these breaks resulting in a partial unshieldingof the Si particles lying along the cracks. This lets theparticles with newly exposed surfaces free to fully re-act with Li, therefore triggering further cracks, pul-verization of the electrode and progressive capacitydecay.

Other aspects, such as the physical nature of the in-teractions between the Si particles and the carbonmatrix need to be explored. For Si thin films showing


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excellent cycling behavior, it has been demonstrated that elec-trical contacts are maintained due to the strong adhesion of thefilm to the collector, which implies the existence of a hugestress at the Si/substrate interface as a result of the volumechanges during cycling. Raman spectroscopy is known to be anexcellent probe for the detection and estimation of these stress-es; a compressive stress is characterized by a shift of the Ramanpeak to higher frequencies.[45] Here, we have performed Ra-man measurements on Si powders and Si–C composites beforeand after grinding.

The Raman spectra of the SiM and SiN particles (Fig. 7) arecharacterized by an intense peak located between 500 and520 cm–1, corresponding to first-order optical phonons (trans-

verse optic (TO) and longitudinal optic (LO)). The peak posi-tion, shape, and full width at half-maximum (FWHM) are verysensitive to the size, crystallinity, and presence of strains in theSi domains. For the SiM samples, the Raman peak is located at520 cm–1 and is symmetric with a FWHM of 5.5 cm–1. For theSiN particles, the Raman peak is shifted to lower frequency(501 cm–1), and is asymmetric and much broader (FWHM of17.5 cm–1). This is undoubtedly related to the disorder at thecrystallite boundaries, which decreases the phonon decay time.In an ideal crystal, the correlation length is infinite and there-fore the phonon eigenstates are plane waves. Therefore, theusual k = 0 momentum selection rules for first-order Ramanare satisfied. As the crystallite is reduced to nanoscale dimen-sions, the most important effect on the Raman spectra is thatthe crystal momentum conservation rules are relaxed. The pho-non scattering is no longer limited to the center of the Brillouinzone. Indeed, scattering near the zone center also needs to beconsidered. As a result, symmetry-forbidden modes are ob-served in addition to shifting of the first-order optical phonon.

The size-dependent Raman shift observed for Si nanoparticlesis generally explained by the quantum confinement model;however, the experimentally observed shifts for SiN as com-pared to SiM cannot be explained by this model alone. It is rea-sonable to assume that the observed differences in the Ramanspectra also arise from surface and disorder effects in the nano-meter-sized Si powders.

Before determining the change in the Raman signal afterPVC pyrolysis, we have examined the effect of the annealingtreatment. After treatment at 900 °C without PVC, the Ramanpeak for the SiN sample shows a slight shift towards higherfrequencies (501 → 508 cm–1); however, the asymmetry andbroadening of the peak remain very pronounced (Fig. 8).XRD, BET, and electrochemical measurements do not suggestany crystallite growth or change in the specific surface area,thus it is clear that textural/structural modifications are not re-sponsible for this slight shift of the Raman peak. Finally, an-nealed SiN shows higher polarization and worse cycling behav-ior as compared to the pristine powder. In contrast, the SiNCpcomposite surprisingly shows a sharp and symmetric Ramansignal that is shifted to higher frequencies (519 cm–1) (Fig. 8).Thus, the carbon coating unambiguously exerts a compressivestress on the SiN particles. Since this compressive stress acts asan opposing force against the volumetric expansion of the par-ticles during the uptake of Li, it should be able to limit the pul-verization of the composite if the matrix is strong enough.Moreover, the existence of this tight Si/C interface enhancesthe electrical connectivity around and between the Si domains,thus improving the level of lithiation, as for instance observedbetween SiM and SiMCp (x = 1.7 → x = 2.6).

The most impressive result shown in Figure 8 is the completereversibility of the stress-induced Raman changes after themanual grinding of the composites. The Raman spectrum ofthe milled SiNCp composite indeed perfectly matches that ofannealed SiN in terms of the peak position, symmetry, and peakwidth; the very poor retention capacity of this sample furthersuggests that the presence of compressive strains is essentialfor enhancing the electrochemical performance of the compos-ites. We believe that the grinding process generates cracks, re-leasing the stress induced by the in situ formation of the carbonmatrix around the Si particles without totally freeing the Si do-mains. Therefore, the ground samples exhibit long-term elec-trochemical performance that is similar to that of bare Si. Thisis indeed the case for both the micrometer- and nanometer-sized samples (Fig. 3a).

3. Conclusions

We have demonstrated that the pyrolysis of a mixture of Sinanoparticles and PVC produces Si–C composites where the Siparticles are finely dispersed in a very dense and bulky tex-tured carbon matrix. This process makes it possible to greatlyenhance the electrochemical performance of Si, especially interms of the retention capacity. Here, we have tried to deter-mine the underlying reasons for this dramatic improvement.


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By systematically studying the influence of the carbon-coat-ing process on the Si particles we have determined the impor-tance of the Si/C interface and the interposed thin SiO2 layer(as deduced by EELS). We have also established the presenceof compressive stress on the Si particles (as measured by Ra-man spectroscopy). This compressive stress is induced by thecarbon coating and not by the SiO2 layer, since the main shiftin the Raman peak position is observed between Si nanoparti-cles coated by only SiO2 and those coated by both SiO2 andcarbon (Fig. 8). Therefore, this compressive stress is related tothe pressure exerted on the Si particles by the carbon matrix

derived from PVC because of its very dense texture. We be-lieve that this pressure enables the electrode particles to re-main in good electrical contact during cycling, and thus limitsthe pulverization of the electrode. The beneficial effects of car-bon coating have recently been reported by Ng et al., whereinnegative electrodes based on carbon-coated Si particles fabri-cated by a spray drying process sustain very large reversible ca-pacities over tens of cycles.[46]

These results are clearly related to the nature of the inter-face. The chemical nature of the bonding at the interface is dif-ficult to determine, but we believe that the SiO2 layer promotes


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Figure 8. Left panels: Voltage–composition galvanostatic curves versus Li for SiN, annealed (900 °C) SiN (50 wt % Ketjen Black, 1 Li per 30 h, 20 °C),SiNCp, and ground SiNCp (56 wt % carbon derived from pyrolyzing PVC, 1 Li per 10 h, 20 °C). The equivalent contribution of the Ketjen Black KJ conduct-ing carbon has been computed and superimposed (bold dashed line) to demonstrate its contribution to the overall capacity. The insets show the dis-charge capacity as a function of the cycle number. The middle and right panels show selected Raman spectra and mapping of the Raman signal over20 lm × 20 lm regions.

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the growth of a dense surface carbon layer upon heating. Thisis not surprising given that the surfaces of metallic particles areoften functionalized after first depositing a SiO2 layer. Deter-mination of the chemical nature of the interface is quite com-plex for bulk materials, and becomes practically impossible atthe nanoscale level because of the lack of suitable analyticaltools. So we are left with either thermal annealing or an empiri-cal chemical approach to spot the importance of the natureof the interface. The latter approach consists in changing thenature of the coating polymer. Polymers other than PVC, suchas poly(vinylidene fluoride) (PVDF) and poly(methyl methac-rylate) (PMMA), have been used. However, of all the poly-mers tested so far only PVC is able to form a dense and uni-form coating around the Si particles. The interface can also bemodified by annealing at higher temperatures. At higher tem-peratures, we note the formation of a SiC interface at theexpense of SiO2 (as deduced from EELS observations); how-ever, the amount of SiC formed is far too high for a valid com-parison to be made between SiC and SiO2 interfaces. Thus,although we do not fully understand the mechanism, our ex-perimental results suggest that there is something unique aboutthe nature of the SiO2/Si interface in the presence of PVC interms of its ability to build up a compressive stress on the parti-cles.

Ryu et al. have previously pointed out the importance ofpressure on Si for achieving a decent retention capacity and forallowing the LixSi alloying reaction to proceed to a good ex-tent.[47] We have confirmed their predictions by conductingsimilar measurements on bare 100 nm Si powders, thus sug-gesting a valuable strategy for engineering Si-containing elec-trodes with confined Si particles. Such confinement can beachieved either by hard templating methods or by the additionof appropriate chemicals. We are currently pursuing these ap-proaches. Although, dramatic improvements in cycling perfor-mance have been obtained for chemically prepared carbon-coated Si particles, as compared to manually mixed Si–C com-posites, one remaining issue is our present inability to exceed50 cycles without a rapid decrease in capacity. This decreasecan be explained by the release of pressure as the coating dete-riorates. It is worth noting that chemical and mechanical coat-ing techniques have been extensively used in the Li-ion batterycommunity since the pioneering work by Amatucci et al. onLiMn2O4; the coatings serve to prevent direct contact betweenthe electrode and the electrolyte and thus minimize electrodedegradation by the formation of a passivation layer.[48] De-tailed SEM studies of Si–C composite electrodes fabricatedfrom both chemically and mechanically prepared carbon-coat-ed Si particles will allow the elucidation of the role of the car-bon shell. The electrodes will be studied during cycling, andwith and without the application of physical pressure, whichwill allow the correlation (if any) between the electrolyte deg-radation and the carbon-driven pressure effects to be deter-mined. The results of these studies are expected to help ac-count for recent results claiming that the use of specificpolymeric electrode binders is more important than carboncoatings.[49]

4. Experimental

Micrometer-sized silicon (SiM) particles were purchased from Al-drich (325 mesh). Nanometer-sized Si particles (SiN) were synthesizedby the CO2-laser pyrolysis of SiH4 (Commissariat à l’Energie Atomi-que, Saclay, France).

Typically, a solution of PVC ((CH2–CHCl)n, Aldrich,42 000 g mole–1) was prepared by dissolving 2 g of the polymer in about50 mL of propylene oxide (Acros Organics, 99 %). Subsequently,180 mg of powdered Si was dispersed in the PVC solution and the re-sulting suspension was stirred in a fume-hood until the solvent com-pletely evaporated. Owing to the very high vapor pressure of propyleneoxide (400 mmHg (1 mmHg = 133 Pa) at 17.8 °C [50]), this step re-quired several hours at ambient temperature and resulted in the forma-tion of a solvent-free blended elastic composite precursor. This com-posite precursor was placed in an alumina boat and heated at1 °C min–1 to 900 °C under a constant flow of N2 (> 99.9 % purity) in aquartz tube furnace. The furnace was maintained at 900 °C for 3 h. Sub-sequently, the furnace was turned off and allowed to cool naturally.Control experiments were also performed on Si-free PVC samples,yielding a gray and compact carbon powder (yield of 15 wt %). Thisyield was used to estimate the initial Si/PVC ratio and was used to engi-neer the desired Si/C ratios assuming no loss of Si during synthesis; thefinal carbon content was obtained by weight difference before and afterpyrolysis. The Si and carbon contents of selected SiM–C pyrolyzed com-posites were also confirmed by TGA (Mettler, Toledo, alumina cruci-ble, static air, 1 °C min–1), assuming that the weight loss is due to theevolution of CO2 and that there is no significant gain in weight due tothe formation of SiO2, as confirmed for bulk SiM.

The specific surface area and porous volume were determined fromthe results of N2 adsorption–desorption experiments at 77 K usingMicromeritics 2375 Gemini and ASAP 2020 analyzers and the BETmulti-point and Barrett–Joyner–Halenda (BJH) methods [51,52]. Thesamples were preheated for 1 h under a flow of argon at 150 °C (BET)or under vacuum at 100 °C (BJH) before testing.

The morphology of the powders was investigated by SEM using aPhilips XL 30 field emission gun microscope equipped with a Link IsisEDS system and a Quanta F20 BSE probe. TEM and HRTEM investi-gations were performed on a FEI Tecnai F20 S-Twin instrument alsoequipped with an EDS detector.

For EELS investigations, the powders were dispersed in dimethylcar-bonate (DMC) and deposited on Cu grids coated with a lacey-carbonfilm. An argon-filled dry box and a vacuum transfer holder were usedto prepare air-sensitive samples. High-energy spectra were recorded ona FEI Tecnai microscope operated at 200 keV with a Wien filter mono-chromator (Delft University, The Netherlands) operating in STEMmode with a ca. 2 nm probe size. The energy positions of the Si L2,3-edges were accurately determined using a coarse internal calibrationsystem based on the electrostatic drift tube of the EELS spectrometer.All the line scans were recorded using a computer drift correction pro-cedure operating every five spectra. Based on the energy rangerecorded, the spectrometer entrance aperture defining the collectionefficiency was 2 mm, and the corresponding collection angle was5.84 mrad.

Raman spectra were collected using a Jobin-Yvon T64000 spectrom-eter (LPMC, Bordeaux, France) using a 514.5 nm argon-ion lasersource (Coherent Innova 305) working in a confocal geometry with aspot diameter of about 1 lm. In order to limit heat-induced sampledegradation due to laser irradiation, the power of the incident beamwas limited to 0.4–0.7 mW, corresponding to an analysis depth of about50 nm. The system was controlled and the data was acquired usingLabspec software.

Electrochemical analyses were performed in Swagelok-type cells as-sembled in an argon-filled dry box. The Si–C samples were tested aspositive electrodes, either as-made or by mixing with 15 wt % KetjenBlack (KJ, EC-300, Akzo Nobel) additive carbon. The positive elec-trode was separated from the negative one (Li metal disk) by two disksof Whatman GF/D boron silicate glass fiber; the whole assembly was


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soaked in a 1 M solution of LiPF6 in a 1:1 (v/v) ethylene carbonate/di-methyl carbonate EC/DMC mixture. Electrochemical tests were per-formed either on the as-prepared pyrolyzed materials or after grindingand sieving the materials to an average size of 40 lm. For pure Si pow-ders, the electrodes were prepared by mixing with 50 wt % KJ carbon.The cells were cycled between 0 and 2 V versus Li+/Li in galvanostaticmode at a rate of 1 Li per 10 h for the composite electrodes and 1 Liper 30 h for the bare Si powders. The cells were monitored with Mac-Pile and VMP controllers (Biologic, S.A., Claix, France) operating ingalvanostatic mode. The extent of Si lithiation (i.e., x in LixSi) was cal-culated assuming a full reaction with carbon (either KJ or from the py-rolysis of PVC); the gravimetric capacities and cycling performance ofthese carbon materials were obtained from carbon–Li cells cycled with-in the same voltage windows with equivalent current densities. For thecomposites, all capacities are reported with respect to the total compos-ite mass (i.e., Si + C).

Retention capacity tests were performed on ground and sieved mate-rials using coin cells with the same electrolyte, a Li disk, and separators.Positive electrodes were prepared by mixing the solvent 1-methyl-2-pyrrolidinone (NMP) with the active materials (80 %); PVDF was usedas the binder (6 wt %) and KJ carbon as the electronic conductor(14 wt %). The suspension was spread on stainless-steel disks whichserved as current collectors and NMP was evaporated at 100 °C for2–3 h.

Received: October 10, 2006Revised: February 1, 2007

Published online: June 20, 2007

–[1] S.-C. Lai, J. Electrochem. Soc. 1976, 123, 1196.[2] R. A. Sharma, R. N. Seefurth, J. Electrochem. Soc. 1976, 123, 1763.[3] R. N. Seefurth, R. A. Sharma, J. Electrochem. Soc. 1977, 124, 1207.[4] R. N. Seefurth, R. A. Sharma, J. Electrochem. Soc. 1980, 127, 1101.[5] B. A. Boukamp, G. C. Lesh, R. A. Huggins, J. Electrochem. Soc. 1981,

127, 725.[6] C. J. Wen, R. A. Huggins, J. Solid State Chem. 1981, 37, 271.[7] W. J. Weydanz, M. Wohlfahrt-Mehrens, R. A. Huggins, J. Power

Sources 1999, 81–82, 237.[8] M. N. Obrovac, L. Christensen, Electrochem. Solid-State Lett. 2004, 7,

93.[9] A. N. Dey, J. Electrochem. Soc. 1971, 118, 1547.

[10] R. A. Huggins, J. Power Sources 1988, 22, 341.[11] J. Yang, M. Winter, J. O. Besenhard, Solid State Ionics 1996, 90, 281.[12] J. O. Besenhard, J. Yang, M. Winter, J. Power Sources 1997, 68, 87.[13] M. Winter, J. O. Besenhard, M. E. Spahr, P. Novak, Adv. Mater. 1998,

10, 725.[14] M. Winter, J. O. Besenhard, Electrochim. Acta 1999, 45, 31.[15] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science

1997, 276, 1395.[16] H. Li, X. Huang, L. Chen, G. Zhou, Z. Zhang, D. Yu, Y. J. Mo, N. Pei,

Solid State Ionics 2000, 135, 281.[17] L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause, J. R.

Dahn, Electrochem. Solid-State Lett. 2001, 4, 137.[18] L. Y. Beaulieu, K. C. Hewitt, R. L. Turner, A. Bonakdarpour, A. A.

Abdo, L. Christensen, K. W. Eberman, L. J. Krause, J. R. Dahn, J.Electrochem. Soc. 2003, 150, 149.

[19] L. Y. Beaulieu, S. D. Beattie, T. D. Hatchard, J. R. Dahn, J. Electro-chem. Soc. 2003, 150, 419.

[20] L. Y. Beaulieu, T. D. Hatchard, A. Bonakdarpour, M. D. Fleischauer,J. R. Dahn, J. Electrochem. Soc. 2003, 150, 1457.

[21] P. Limthongkul, Y. I. Jang, N. Dudney, Y. M. Chiang, Acta Mater.2003, 51, 1103.

[22] J. Graetz, C. C. Ahn, R. Yazami, B. Fultz, Electrochem. Solid-StateLett. 2003, 6, 194.

[23] J. Wolfenstine, J. Power Sources 1999, 79, 111.[24] J. Wolfenstine, D. Foster, J. Read, W. K. Behl, W. Luecke, J. Power

Sources 2000, 87, 1.[25] C. S. Wang, G. T. Wu, X. B. Zhang, Z. F. Qi, W. Z. Li, J. Electrochem.

Soc. 1997, 145, 2751.[26] I. S. Kim, G. E. Blomgren, P. N. Kumta, J. Power Sources 2004, 130,

275.[27] G. X. Wang, J. Yao, H. K. Liu, Electrochem. Solid-State Lett. 2003, 7,

250.[28] A. M. Wilson, B. M. Way, T. Van Buuren, J. R. Dahn, J. Appl. Phys.

1995, 77, 2363.[29] A. M. Wilson, J. R. Dahn, J. Electrochem. Soc. 1995, 142, 326.[30] M. Yoshio, H. Wang, K. Fukuda, T. Umeno, N. Dimov, Z. Ogumi, J.

Electrochem. Soc. 2002, 149, 1598.[31] N. Dimov, S. Kugino, M. Yoshio, J. Electrochem. Soc. 2003, 149, 1579.[32] W. R. Liu, J. H. Wang, H. C. Wu, D. T. Shieh, M. H. Yang, N. L. Wu,

J. Electrochem. Soc. 2005, 152, 1719.[33] J. Yang, B. F. Wang, K. Wang, Y. Liu, J. Y. Xie, Z. S. Wen, Electro-

chem. Solid-State Lett. 2003, 6, 154.[34] Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano, Y. Takeda, Electro-

chem. Solid-State Lett. 2004, 7, 369.[35] Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano, Y. Takeda, Solid

State Ionics 2004, 168, 61.[36] I. S. Kim, P. N. Kumta, J. Power Sources 2004, 136, 145.[37] X.-W. Zhang, P. K. Patil, C. Wang, A. J. Appleby, F. E. Little, D. L.

Cocke, J. Power Sources 2004, 125, 206.[38] Z. S. Wen, J. Yang, B. F. Wang, K. Wang, Y. Liu, Electrochem. Com-

mun. 2003, 5, 165.[39] B. C. Kim, H. Uono, T. Sato, T. Fuse, T. Ishihara, M. Senna, Solid

State Ionics 2004, 172, 33.[40] B. C. Kim, H. Uono, T. Sato, T. Fuse, T. Ishihara, M. Ue, M. Senna, J.

Electrochem. Soc. 2005, 152, 523.[41] Y. Liu, K. Hanai, T. Matsumura, N. Imanishi, A. Hirano, Y. Takeda,

Electrochem. Solid-State Lett. 2004, 7, 492.[42] P. E. Batson, Microsc. Microanal. Microstruct. 1991, 2, 395.[43] B. Fultz, C. C. Ahn, M. M. Disko, in Transmission Electron Loss Spec-

trometry in Materials Science and the EELS Atlas (Ed: C. C. Ahn),Wiley-VCH, Weinheim 2004.

[44] D. P. Yu, X. J. Xing, M. Tencé, M. Y. Pan, Y. Leprince-Wong, PhysicaE 2002, 15, 1.

[45] V. Paillard, P. Puech, R. Sirvin, S. Hamma, P. Roca i Cabarrocas, J.Appl. Phys. 2001, 90, 3276.

[46] S. H. Ng, J. Wang, D. Wexler, K. Konstantinov, Z. P. Guo, H. K. Liu,Angew. Chem. Int. Ed. 2006, 45, 6896.

[47] J. H. Ryu, J. W. Kim, Y. E. Sung, S. M. Oh, Electrochem. Solid-StateLett. 2004, 7, 306.

[48] G. G. Amatucci, A. Blyr, C. Sigala, P. Alfonse, J.-M. Tarascon, SolidState Ionics 1997, 104, 13.

[49] J. Li, J. R. Dahn, presented at the Int. Meeting on Lithium Batteries,Biarritz, France, June 2006.

[50] Handbook of Chemistry and Physics (Ed: D. R Lide), 73rd ed., CRCPress, Boca Raton, FL 1992–1993.

[51] S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 1938, 60, 309.[52] E. P. Barrett, L. G. Joyner, P. C. Halenda, J. Am. Chem. Soc. 1951, 73,

373.

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DOI: 10.1002/adfm.200600937 Towards a Fundamental …btinnovations.org/shanebeattie/Papers/Towards_a_Fundamental... · David Talaga, Michel Couzi, and Jean-Marie Tarascon 1. Introduction