Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by

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Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by ToF-SIMS Imaging Performed on In Situ FIB Cross Section Arnaud Bordes, Eric De Vito, Cédric Haon, Adrien Boulineau, Alexandre Montani, and Philippe Marcus Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00155 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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Chemistry of Materials

Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by ToF-SIMS Imaging Performed on In Situ FIB Cross Section Arnaud Bordes †,‡, Eric De Vito*,†, Cédric Haon†, Adrien Boulineau†, Alexandre Montani† and Philippe Marcus‡.

† CEA, LITEN, 17 rue des Martyrs, F-38054 Grenoble, France. Univ. Grenoble Alpes, F-38000, Grenoble, France. ‡ Groupe de Physico-Chimie des Surfaces, Institut de Recherche de Chimie Paris, CNRS-Chimie ParisTech, 75005 Paris, France.

ABSTRACT: Considering its specific capacity, silicon is one of the most promising materials to replace graphite in lithium ion batteries anodes. However its rapid capacity fading prevents its use in current batteries. Understanding lithiation and degradation mechanisms of silicon is important for improving its cyclability. In this work a novel approach is developed by using a focused ion beam (FIB) implemented in the analysis chamber of a state-of-the-art secondary ion mass spectrometer (ToF-SIMS). Detailed mapping of elements distribution, including lithium, inside a silicon particle or in the entire depth of the electrode, can thus be performed. During the first lithiation, a core-shell mechanism is observed and its evolution upon electrochemical cycling was examined. This mechanism is observed for all particles in the electrode, independently of their position. Cross analysis with Auger spectroscopy allowed Li concentration in the entire shell to be quantified. Fast lithiation paths getting through the pure silicon core have been evidenced by complementary SEM and TEM analysis. Defects observed by TEM are supposed to contribute significantly in the Li diffusion inside the particle. This approach also provided evidence of lithium progressively trapped in Si particles after aging, in close relationship with capacity loss found for silicon anodes along cycling.

or using additives in the electrolyte to stabilize the SEI.11-12 Despite these efforts, the silicon cyclability still needs to be improved through the finding of new solutions. In this way, (de)lithiation mechanisms have to be clearly understood. Many works have been achieved in that direction. In-situ TEM has been extensively applied to understand (de)lithiation mechanisms on crystalline silicon (cr-Si) nanowires,13-15 crystalline and amorphous silicon (a-Si) nanoparticles,15-17 and a-Si thin films.18-19 This ended up suggesting a two-phase mechanism with a sharp reaction front dividing amorphous lithiated silicon (a-LixSi) from cr-Si 20 during the first lithiation. To go further, several groups quantified lithium in the lithiated phase by using various instruments such as TEM-EELS 21, NMR 22 or Auger Electron Spectroscopy (AES) 23. They converged to a value of x in the a-LixSi phase around 3. Nonetheless, many observations questioned the core-shell model: a preferential lithiation front has been identified on nanowires 13, 15, 24 depending on the crystalline orientation; also, the existence of 10

1. Introduction Lithium ion batteries have made possible the emergence of many modern portable electronic devices.1-2 The development of electrical energy demanding systems like cars or renewable intermittent energy storage unit require improvement of energy density, cost and safety of current batteries. Silicon is a material of choice as anode material because of its abundance and high theoretical capacity of 3579 mAh g-1, considering Li15Si4.3-4 In spite of these advantages, silicon presents a severe drawback concerning its cyclability. After a few cycles, the specific capacity decreases dramatically under an acceptable range. Such fading is mainly attributed to the tremendous expansion during silicon lithiation.5 This expansion leads to continuous solid electrolyte interface (SEI) formation, particle pulverization and loss of contact.6-7 In order to minimize these phenomena, several solutions have been proposed like: reducing the silicon particle size to nanometer scale,6 developing architectured materials (e.g. Si@C, yolk shell, pomegranate),8-

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electron gun was preferred to the ion gun to carry out the deposition. To minimize amorphization and damages caused to the cross section, all FIB cuts were carried out by using 30 kV / 700 pA conditions for the rough raster followed by a final ionic polishing step using a 30 kV / 80 pA ion beam.

regions inside particles where lithiation is favored have been reported. 17, 19, 25 In this study, an innovative way to explore silicon reaction mechanisms is used, by carrying out an in-situ Focused Ion Beam (FIB) cut in the analysis chamber of a Time of Flight Secondary Ion Mass Spectrometer (ToF-SIMS). To circumvent limitations of ToF-SIMS in terms of lateral resolution and quantitative accuracy, FIB-SEM, TEM and Auger Electron Spectroscopy (AES) are also considered.

TEM HRTEM images and electron diffraction patterns were recorded using a FEI Tecnai G2 microscope operating at an accelerating voltage of 200 kV. Prior to observation, the pristine electrode was embedded within an epoxy resin and then prepared by ultramicrotomy.

2. Experimental part Cell assembly and electrochemistry A Si-based slurry made of an aqueous mixture of microsilicon (82 wt%), carbon fibers (12 wt%) and carboxymethyl cellulose (6 wt%,) was deposited on a copper foil current collector. CR2032 half coin cells were assembled using a lithium metal foil as counter electrode, a Celgard® 2400 as separator and a Viledon® propylene foil. 1 M LiPF6 in ethylene carbonate (EC)–diethyl carbonate (DEC) (wt% of 1:1) was used as electrolyte. Galvanostatic discharges were carried out at C/20 rate based on a 3579 mAh g-1 capacity 3 at 25 °C using an Arbin® battery tester.

3. Results and discussion 3.1. Evolution of the structure of Si particles upon the first lithiation In order to study the evolution of the lithiation of Si particles, analyses were carried out along the first galvanostatic discharge at different states of lithiation, as shown in Figure 1. Lithiation states correspond to 5%, 10%, 25%, 50% and 70% of the theoretical capacity and were chosen to represent the whole lithiation process. Figure 1 shows a typical discharge curve of a micron-sized Si composite electrode. At the beginning, the potential decreases quickly, reaching temporarily negative values (because of polarization of the cell) before rapidly regaining positive values and stabilizing on a plateau at 50 mV. This plateau indicates the occurrence of the twophase mechanism during silicon lithiation.26 Three points were chosen on this plateau in order to monitor the silicon particle structure changes during the first lithiation. When the electrode reaches approximatively 60% of its maximal theoretical capacity, the potential slowly decreases, marking the end of the two-phase mechanism. In order to reach the Li15Si4 phase corresponding to the full lithiation, a constant voltage step at 5 mV was performed, until the current reaches the equivalent of a C/100 rate.27

FIB-ToF-SIMS imaging After the cell opening, electrodes were gently washed twice with pure dimethyl carbonate (DMC). The electrodes were mounted on a sample holder with a double sided carbon adhesive tape. Samples were transferred directly from the glove box to the ION-TOF ToF-SIMS5 preparation chamber by using a sealed transfer vessel to avoid exposure to air. Electrodes were investigated by using a 60 keV Bi3++ analysis beam and a 30 keV Ga FIB. The FIB cut was milled with a 20 nA current and then “polished” with a 5 nA current. Measurement current was set to 0.12 pA. Depending on the elements of interest, positive or negative secondary ions were analysed. The vacuum during the experiments was below 5.10−9 mbar. Auger characterization After ToF-SIMS characterization, the same airtight vessel was used to avoid air exposure during the transfer to the Physical Electronics 700Xi scanning Auger nanoprobe introduction chamber. The analyses were done at 5 kV/5 nA. In these conditions spatial resolution as low as 20 nm can be reached while maintaining an energy resolution of 0.5%. SEM images were acquired by using a scintillator as the secondary electron detector. AES depth profiles were obtained by using argon ion sputtering (1 keV/1 µA). AES intensities are estimated from the peak-to-peak height of the derivative spectra. FIB-Microscopy Electrodes were transferred from the glove box to the FIB/Scanning electron Microscope (SEM) ZEISS NVISION 40 using a transfer vessel to avoid any exposure to air. A layer of few nanometers of tungsten was deposited to protect the first atomic layers on the surface of the sample during the FIB cut. Because of the high sensitivity of lithiated materials, the

Figure 1. Electrochemical profile of the first lithiation of silicon anode vs Li/Li+. Red dots mark the different states of charge studied. The cells were cycled at a C/20 rate.

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Chemistry of Materials overlay of the chemical mapping in positive polarity for Li+, Si+ and F+ is presented whereas for the 10% lithiated sample an overlay of Li+, Si+, Cu+ is shown in order to better distinguish the copper current collector. For the 5% lithiated electrode, a surface particle is shown (figure 3a). The particle surface is covered by a mixture of Li and F (purple), and can be explained by the formation of a SEI layer. However, some parts of the surface of the particle are not yet covered by SEI and bare silicon is visible (green). The bulk of the particle also appears in green indicating that at these early stages of lithiation, lithium does not penetrate in silicon and does not cover the entire silicon surface: all ions have been consumed in parasitic reactions leading to SEI formation. In figure 3b, a particle close to the current collector, lithiated at 10%, is displayed. The regions close to the surface of the particle are rich in lithium (red) whereas in the inner part, the level of lithium detected is significantly lower. This can reasonably be considered as noise and the core free of lithium, consisting therefore of pure silicon (green), in agreement with previous studies.14, 16 This structure could be interpreted as the early stage of a core-shell mechanism. In fact the shell is very thin and the core occupies the major part of the particle. For the 25% and 50% lithiated electrodes (figure 3c and d), chemical mappings of the particles show the evolution of the core shell structure. The lithiated shell becomes thicker and the pure silicon core becomes smaller. After 50% of lithiation the core appears very small and a crack (indicated by a white arrow) in the particle starts to appear.

Figure 2. a)Secondary electron imaging of the FIB cut performed in the ToF-SIMS analysis chamber. b)The same FIB cut imaged from another angle. Silicon particles are clearly identified.

In situ FIB cut was then performed for each sample in order to analyze the cross section. Figure 2 shows a typical FIB cut performed on a 25% lithiated electrode in the ToF-SIMS analysis chamber, captured with the secondary electron detector. The crater is fairly large in order to maximize the field of view. The shape of the crater has been optimized to avoid contamination of the region of interest by redeposition. On the cross section (Figure 2b), Si particles are clearly identified as well as the current collector. By achieving the ToF-SIMS analysis directly after the FIB cut, any contamination or oxidation of lithiated silicon can be avoided. In fact, even in the best transfer conditions or under ultra-high vacuum, as shown in supporting data S1, lithiated silicon is still very sensitive to oxidation. In such conditions, a chemical mapping of Si particles can be carried out for each sample, and is displayed in figure 3. For 5%, 25%, 50% and 70% lithiated samples, an

Figure 3. ToF-SIMS chemical mapping of Li+(red), Si+(green) and F+(or Cu+)(blue) of silicon particles cross sections for different states of lithiation: a) surface particle lithiated at 5% of theoretical capacity; b) particle close to the current collector lithiated at 10% of theoretical capacity; c) surface particle lithiated at 25% of theoretical capacity; d) bulk particle lithiated at 50% of theoretical capacity; e) bulk particle lithiated at 70% of theoretical capacity. The evolution of the core/shell structure over lithiation is visible.

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Figure 4. SEM imaging of FIB cross sections of silicon particles lithiated at: a) 10% of theoretical capacity; b) 25% of theoretical capacity. In these pictures, inclusions of lithiated silicon in the cr-Si core are identified.

Finally on the 70% lithiated particle, no more core shell structure is visible; the entire particle appears homogeneous. As far as ToF-SIMS imaging goes, lithiation happens in a single phase. This result is consistent with previous studies22, 27 claiming the end of the two-phase mechanism starting at around 70% of lithiation. Figure 3e shows a silicon particle freshly fractured: at the top of the particle, the SEI layer is visible and indicated by the presence of fluorine (blue) whereas the amount of fluorine lying on the surface of the fractured area (indicated by a white arrow), is notably lower.

then alloy with silicon. Nonetheless, these defects are randomly distributed and are too circumscribed to explain the very specific direction observed previously. The high resolution images of pristine particle presented in figure 5 b,c,d should allow a better insight in the material. Figure 5b shows a defect free part of the crystalline particle. On the surface, around 2 nm of amorphous phase corresponding to silicon oxide is identified. Figure 5c shows the presence of a linear defect that can be interpreted as a sub-grain boundary resulting from very slight misorientations occurring between the neighboring grains around the direction. The electron diffraction pattern of the whole crystal presented in insert of figure 5c shows subdivided spots. These spots subdivisions result from multiple diffractions that occur when the incident beam goes through the sub-grains and the boundary successively. Figure 5d offers an enlarged view of the boundary (white frame). These defects where silicon is badly crystallized are presumed to accelerate the lithium diffusion that results in such particular fast alloying paths. In other systems, Wang et al. suggested the presence of fast diffusion of lithium in defects within nanocrystalline silicon particles.17 Our group reported as well a similar phenomenon happening in 300 nm amorphous silicon thin film for very different type of defects19. It seems that in the presence of defects, lithium tends to diffuse rapidly in it, no matter the particles scale. Also, Lee et al. showed that anisotropic silicon lithiation and thus swelling creates mechanical stress leading to silicon fracture. In line with this result, the fast diffusion paths within the particles observed in this study are probably contributing to active material pulverization.13 The lithiation of micron-sized Si particles starts with a twophase mechanism, resulting from the combination of two phenomena: 1) a core shell mechanism with a pure Si core, shrinking along lithiation, 2) fast diffusion paths favoring diffusion of lithium through the particle and creating multiple core and broadening during lithiation. In a second stage, according to literature18, the Si particles become homogeneous and keep being lithiated like a solid solution.

3.2. Structure of the core-shell In addition to the observation of the silicon particles lithiation evolution, figure 3 also gives information about the structure of the Si core. In particular, figure 3c and d show that Li penetrates the Si core of the particles, turning a single pure Si core into multiple ones. In order to complete the crucial chemical information brought by ToF-SIMS, a detailed morphological study of similar samples was also achieved by using SEM. Figure 4 shows the SEM image of a FIB cut performed on particles in 10% (figure 4a) and 25% (figure 4b) lithiated electrodes. The three particles presented in figure 4a exhibit the same structure: a thin light layer around the particle and a dark core separated by multiple light contrasted paths. Considering ToF-SIMS imaging, the thin light layer surrounding the particle can be associated with lithiated silicon and the dark core with pure silicon. The paths appear with the same contrast as the shell and are also attributed to the same lithiated phase. No such paths could be observed in ToF-SIMS experiments at this state of charge because their thicknesses were below the ToF-SIMS lateral resolution. The cross section of the particle presented in figure 4b presents exactly the same characteristics with a thicker lithiated silicon shell and broader paths. This observation is consistent with previous ToF-SIMS results. SEM permits not only to follow the thickening of the lithiated phases but also to observe that the paths are roughly straight and follow specific directions. In order to understand the origin of these paths, pristine particles were observed by TEM along the crystalline direction. A low resolution picture as presented in Figure 5a shows that particles contain many defects, which can be identified as dislocations. These defects corresponding to a deformation in the crystalline lattice may help lithium to diffuse and

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Figure 5. TEM imaging of a pristine silicon particle. a) Low resolution image exhibiting dislocations; b) high resolution image of a defect free area; c) high resolution picture and diffraction pattern of an area presenting a planar defect identified as a subgrain boundary; d) zoom in the defect identified in c. A frame has been added in order to identify the defect

3.3 Quantification of lithium in Si particles ToF-SIMS imaging allowed us to obtain essential qualitative information. However, a quantitative approach is much more complex: ToF-SIMS analysis is subject to matrix effects, which makes the calibration on reference samples very difficult. Moreover in the case of cross section imaging, other artifacts like shadowing effect, make the quantification intricate. Nonetheless, for a single particle it is possible to give some insights on quantitative approach as presented in Figure 6a by assuming that, in a given composition and spatial range, the concentration of species is proportional to the corresponding secondary ion signal.28 To obtain the intensity plotted on the y-axis, the average area under the peak associated with Li+ is calculated over 6 pixels on the same vertical line and normalized to Si+. For the 25% lithiated particle, the profile contains three different intensity levels. The higher level corresponds to the outer part of the particle and is due to SEI. Then, in the lithiated shell, the intensity remains quite stable before drastically dropping in the core. This profile supports the hypothesis of a constant composition in the lithiated shell, which is separated from the pure silicon core by an interface as sharp as the ToF-SIMS lateral resolution allows to estimate (~200 nm). To obtain an accurate quantification of the lithium content in the lithiated phase, a cross analysis with Auger electron spectroscopy was carried out. Previous works in our group

proved this approach to be efficient to quantify Li concentration in the lithiated silicon phase.23 The same methodology based on the Alloy Reference Relative Sensitivity Factors (ARRSFs) determined from lithium silicide model compounds is used. The lithium content in the silicon alloy (LixSi) is then given by the equation 1:

∗

Equation (1)

Signal intensities ( and ) are obtained by measuring the peak to peak intensity of derivate Auger spectra.

is obtained for k = 3.25 (Li13Si4) and is equal to 0.12 which is consistent with values calculated elsewhere.23 The evolution of x value in the shell of a particle is shown in Figure 6b. In the first minutes of sputtering, corresponding to the extreme surface of the particle, lithium quantification is not possible because of the presence of lithium oxides and impurities such as carbonaceous species. After 6 minutes of sputtering, lithium and silicon signals become free of impurities and the quantification of the lithiated phase is thus possible. A plateau at x=3.1 is reached, in accordance with previous studies.21, 23

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Figure 6. a) ToF-SIMS chemical mapping and intensity of lithium signal normalized to silicon over the width of the cross section for a 25% lithiated silicon particle. The two horizontal lines on the chemical mapping show the analysis area. b) Secondary electron imaging and Auger quantification depth profile of the amount of lithium present in the LiSi alloy of the same 25% lithiated silicon particle. The white rectangle on the secondary electron imaging shows the analysis area.

in the schematic view of the proposed lithiation mechanism represented in Figure 7. 3.4 Homogeneity of silicon lithiation upon cycling FIB associated with ToF-SIMS allows us also to monitor the lithiation of Si particles at different locations in the electrode. Hence it is possible to study the homogeneity of the lithiation mechanism in the whole depth of the electrode. Figure 8 shows the chemical mapping of the whole cross section for 10% and 25% lithiated electrodes. On the 10% lithiated electrode (figure 8a), core-shell structures with thin lithiated shells are visible over the complete depth of the electrode. For the 25% lithiated electrode (figure 8b), the cores are smaller and their sizes are comparable between particles close to the surface and others located in the depth. The structure of the core of bulk particles also presents fast diffusion paths. In addition, these observations are similar over the entire depth of the electrode during the first lithiation, in good correlation with previous analyses. To push the study further, and investigate the lithiation homogeneity with aging, limited lithiation capacity cycling has been performed. The cut-off capacity was set to 1000 mAh g-1 in order to expand the lifespan of the cycled cells. Figure 9 shows selected cycles of a typical cell. During the first lithiation, the electrochemical profile exhibits the same characteristics as the one shown above (figure 1), apart from the cut-off capacity, which makes the cell stop while the potential is stable on the 0.05 V plateau. The first delithiation capacity is clearly inferior to 1000 mAh g-1 because of the first irreversible capacity loss. In all subsequent anode lithiations, the curve exhibits a quite different shape; it does not show a plateau anymore.

Figure 7. Schematic view of a particle during the first lithiation.

FIB-ToF-SIMS imaging, when associated with Auger, allows to extrapolate the quantitative information to the entire shell of the particle. The quantification is no more restricted to a specific region of the particle corresponding with the surface of the electrode, as it was the case in our previous work based on Auger depth profiling analysis.23 Also, thanks to the FIB cut, buried particles are now accessible. This was also unconceivable with the Ar sputtering approach. Finally a description of the silicon particles during the first lithiation stage is given

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Figure 8. ToF-SIMS chemical mapping of Li+(red), Si+(green) and Cu+(blue) of a full cross section for a 10% lithiated electrode. b) Secondary electron imaging of a full cross section for a 25% lithiated electrode and ToF-SIMS chemical mapping of Li+(red), Si+(green) and Na+(blue) of selected particles.

potential plot (figure 9) does not exhibit a plateau at 0.05 V, excluding a two-phase mechanism. Particles visible on figure 10b do not present a core shell structure anymore; it seems that all the silicon has already been lithiated. Once again observations are consistent in the whole depth of the electrode. Contrary to lithiation voltage plots, delithiation curves are almost identical from cycles 1 to 10, except for a shift due to irreversible lithium losses. This discrepancy is the result of a low coulombic efficiency of the cell. In order to identify what happens to this “lost” lithium, chemical mappings of cross sections were performed on delithiated electrodes after respectively 5 and 10 cycles (Figure 11). On Figure 11a (5 cycles), most of the visible silicon particles, reveal a good delithiation. Few particles exhibit nonetheless the presence of lithium and are partially or totally lithiated, probably because they are no longer electrically connected. After 10 cycles (figure 11b) the proportion of poorly delithiated particles (or fragments of particles coming from pulverization) increases and is not negligible. These poorly delithiated particles are likely disconnected from the conductive grid, making the extraction of lithium impossible.

Figure 9. Electrochemical profile of selected charge/discharge of silicon electrode vs Li/Li+. Cells were cycled at C/20 rate and the cut-off potential for delithiation was 1.2 V vs Li/Li+. The initial cell potential (OCV) is 2.9 V vs Li/Li+. An inlet is proposed to better identify differences occurring in the end of lithiation between the different cycles.

According to the mechanism proposed by McDowell et al.,16.this would be related to a solid solution, not a two-phase mechanism. In fact lithium alloys with amorphous silicon, which has already been lithiated in the previous lithiation.27 However, when the potential reaches 0.05 V, the curve presents a sharp break and the potential stabilizes on a plateau at 0.05 V. This potential stabilization indicates that the twophase mechanism happens again, related to the lithiation of remaining pristine crystalline silicon. In fact, the silicon in the core of the particles, not consumed during the first lithiation, is now being lithiated to compensate the first irreversible capacity loss. Figure 10a confirms this interpretation by offering a view of a particle in the bulk of the electrode after the third lithiation. This particle reveals the remaining pristine silicon exhibiting a clear core shell structure, including the fast lithiation paths discussed previously. After 10 cycles the discharge

Figure 10. ToF-SIMS chemical mapping of Li+ (red) and Si+ (green) of a) the FIB cross section of a lithiated silicon particle from the bulk of an electrode after 3 cycles performed at limited capacity of 1000 mAh/g; b) the FIB cross section of a lithiated electrode after10 cycles performed at limited capacity of 1000 mAh/g.

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Figure 11. ToF-SIMS chemical mapping of Li+(red), Si+(green) and Cu+(blue) of the FIB cross section of a delithiated electrode after a) 5 cycles performed at limited capacity of 1000 mAh/g; b) 10 cycles performed at limited capacity of 1000 mAh/g.

It is known that volume changes induce loss of electronic contacts and pulverization of active material leading to an increase of inactive particles.4, 29-30 These particles are detected near the electrode surface as well as close to the current collector or in the electrode depth: no specific patterns or preferential areas can be identified. This phenomenon of lithium trapping added to the continuous SEI formation explains the poor efficiency of the cell. A similar phenomenon was previously reported on Si thin films.31

ASSOCIATED CONTENT Supporting Information available : Oxydation of lithiated silicon under ultra-high vacuum

AUTHOR INFORMATION Corresponding Author

4. Conclusion The combination of FIB-ToF-SIMS imaging with AES quantitative analysis allowed us to describe precisely lithiation mechanisms of silicon particles during the first cycle. At the very beginning, corresponding to the first 5% of lithiation, lithium is consumed to form the SEI layer, therefore no lithium is detected in the particle itself. Then a two-phase mechanism takes place, exhibiting a Li3.1Si shell separated from a pure silicon core by a sharp interface. SEM observations point out the presence of fast lithiation paths penetrating this core. They are supposed to arise from subgrain boundaries defects favoring Li diffusion inside the particle as underlined by TEM analysis. From 10% to 70%, the shell is getting thicker; the core smaller and fast lithiation paths start to be visible in ToFSIMS confirming the nature of the content of these fast diffusion paths. After that point the core shell structure is no longer observed and the particles start to show severe damages. These fast diffusion paths are presumed to favor active material pulverization. The lithiation mechanism seems to be homogeneous over the depth of the electrode. If the capacity is sufficiently limited, the two-phase mechanism is maintained until the progressive lithiation upon cycling consumes all pristine cr-Si. After complete amorphization, silicon is being lithiated along a solid solution mechanism. The continuous consumption of the pristine silicon core compensates the loss of active material due to particle disconnection from the conductive network over cycles and continuous SEI formation. This phenomenon related to large volume changes is detrimental to battery lifespan. Also the novel method developed here, consisting of coupling in situ FIB, ToF-SIMS and Auger spectroscopy could enhance the advanced characterization of many materials, in particular the very air sensitive ones.

*E-mail: [email protected]

ACKNOWLEDGEMENTS All the measurements were performed at the Nanocharacterization Platform (PFNC) of Minatec (Grenoble, France). This work has been achieved in the scope of the EU-funded BACCARA project (http://project-baccara.eu/). The authors are very thankful to Jolanta Swiatowska and Antoine Seyeux (Chimie ParisTech) for helpful discussion and to Renaud Godefroy for scientific illustration.

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REFERENCES

(1) Tarascon, J. M.; Armand, M., Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414 (6861), 359-367. (2) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D., Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4 (9), 3243-3262. (3) Obrovac, M. N.; Christensen, L., Structural Changes in Silicon Anodes During Lithium Insertion/Extraction. Electrochem. Solid-State Lett. 2004, 7 (5), A93-A96. (4) Zhang, W.-J., A Review of the Electrochemical Performance of Alloy Anodes for Lithium-Ion Batteries. J. Power Sources 2011, 196 (1), 13-24. (5) Wu, H.; Cui, Y., Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nano Today 2012, 7 (5), 414-429. (6) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y., Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6 (2), 1522-1531. (7) Pereira-Nabais, C.; Światowska, J.; Chagnes, A.; Gohier, A.; Zanna, S.; Seyeux, A.; Tran-Van, P.; Cojocaru, C.-S.; Cassir, M.; Marcus, P., Insight into the Solid Electrolyte Interphase on Si Nanowires in Lithium-Ion Battery: Chemical and Morphological Modifications Upon Cycling. J. Phys. Chem. C 2014, 118 (6), 2919-2928. (8) Liu, N.; Lu, Z.; Zhao, J.; McDowell, M. T.; Lee, H.-W.; Zhao, W.; Cui, Y., A Pomegranate-Inspired Nanoscale Design for Large-Volume-Change Lithium Battery Anodes. Nat. Nanotechnol. 2014, 9 (3), 187-192. (9) Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y., A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett. 2012, 12 (6), 3315-3321. (10) Sourice, J.; Quinsac, A.; Leconte, Y.; Sublemontier, O.; Porcher, W.; Haon, C.; Bordes, A.; De Vito, E.; Boulineau, A.; Jouanneau Si Larbi, S.; Herlin-Boime, N.; Reynaud, C., One-Step Synthesis of Si@C Nanoparticles by Laser Pyrolysis: High-Capacity Anode Material for Lithium-Ion Batteries. ACS Appl. Mater. & Interfaces 2015, 7 (12), 6637-6644. (11) Xu, C.; Lindgren, F.; Philippe, B.; Gorgoi, M.; Björefors, F.; Edström, K.; Gustafsson, T., Improved Performance of the Silicon Anode for Li-Ion Batteries: Understanding the Surface Modification Mechanism of Fluoroethylene Carbonate as an Effective Electrolyte Additive. Chem. Mater. 2015, 27 (7), 2591-2599. (12) Bordes, A.; Eom, K.; Fuller, T. F., The Effect of Fluoroethylene Carbonate Additive Content on the Formation of the Solid-Electrolyte Interphase and Capacity Fade of Li-Ion Full-Cell Employing Nano Si–Graphene Composite Anodes. J. Power Sources 2014, 257 (0), 163-169. (13) Lee, S. W.; McDowell, M. T.; Berla, L. A.; Nix, W. D.; Cui, Y., Fracture of Crystalline Silicon Nanopillars During Electrochemical Lithium Insertion. Proceedings of the National Academy of Sciences 2012, 109 (11), 4080-4085. (14) Liu, X. H.; Wang, J. W.; Huang, S.; Fan, F.; Huang, X.; Liu, Y.; Krylyuk, S.; Yoo, J.; Dayeh, S. A.; Davydov, A. V.; Mao, S. X.; Picraux, S. T.; Zhang, S.; Li, J.; Zhu, T.; Huang, J. Y., In Situ AtomicScale Imaging of Electrochemical Lithiation in Silicon. Nat. Nanotechnol. 2012, 7 (11), 749-756. (15) Liu, X. H.; Liu, Y.; Kushima, A.; Zhang, S.; Zhu, T.; Li, J.; Huang, J. Y., In Situ Tem Experiments of Electrochemical Lithiation and Delithiation of Individual Nanostructures. Adv. Energy Mater. 2012, 2 (7), 722-741. (16) McDowell, M. T.; Lee, S. W.; Harris, J. T.; Korgel, B. A.; Wang, C.; Nix, W. D.; Cui, Y., In Situ Tem of Two-Phase Lithiation of Amorphous Silicon Nanospheres. Nano Lett. 2013, 13 (2), 758-764. (17) Wang, F.; Wu, L.; Key, B.; Yang, X.-Q.; Grey, C. P.; Zhu, Y.; Graetz, J., Electrochemical Reaction of Lithium with Nanostructured Silicon Anodes: A Study by in-Situ Synchrotron X-Ray Diffraction and Electron Energy-Loss Spectroscopy. Adv. Energy Mater. 2013, 3 (10), 1324-1331.


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(18) Wang, J. W.; He, Y.; Fan, F.; Liu, X. H.; Xia, S.; Liu, Y.; Harris, C. T.; Li, H.; Huang, J. Y.; Mao, S. X.; Zhu, T., Two-Phase Electrochemical Lithiation in Amorphous Silicon. Nano Lett. 2013, 13 (2), 709-715. (19) Bordes, A.; De Vito, E.; Haon, C.; Secouard, C.; Montani, A.; Marcus, P., Investigation of Lithium Insertion Mechanisms of a Thin-Film Si Electrode by Coupling Time-of-Flight Secondary-Ion Mass Spectrometry, X-Ray Photoelectron Spectroscopy, and Focused-Ion-Beam/Sem. ACS Appl. Mater. & Interfaces 2015, 7 (50), 27853-27862. (20) Chon, M. J.; Sethuraman, V. A.; McCormick, A.; Srinivasan, V.; Guduru, P. R., Real-Time Measurement of Stress and Damage Evolution During Initial Lithiation of Crystalline Silicon. Phys. Rev. Lett. 2011, 107 (4), 045503. (21) Danet, J.; Brousse, T.; Rasim, K.; Guyomard, D.; Moreau, P., Valence Electron Energy-Loss Spectroscopy of Silicon Negative Electrodes for Lithium Batteries. Phys. Chem. Chem. Phys. 2010, 12 (1), 220-226. (22) Key, B.; Bhattacharyya, R.; Morcrette, M.; Seznéc, V.; Tarascon, J.-M.; Grey, C. P., Real-Time Nmr Investigations of Structural Changes in Silicon Electrodes for Lithium-Ion Batteries. J. Am. Chem. Soc. 2009, 131 (26), 9239-9249. (23) Radvanyi, E.; De Vito, E.; Porcher, W.; Danet, J.; Desbois, P.; Colin, J.-F.; Larbi, S. J. S., Study of Lithiation Mechanisms in Silicon Electrodes by Auger Electron Spectroscopy. J. Mater. Chem. A 2013, 1 (16), 4956-4965. (24) Lee, S. W.; McDowell, M. T.; Choi, J. W.; Cui, Y., Anomalous Shape Changes of Silicon Nanopillars by Electrochemical Lithiation. Nano Lett. 2011, 11 (7), 3034-3039. (25) Robert, D., Etude multi-échelledes mécanismes de dé(lithiation)et de dégradation d'électrodes à base de LiFePO4 et de silicium pour accumulateur Li-ion. Material chemistry. Université de Grenoble, 2013. French. . (26) Huggins, R. A., Lithium Alloy Negative Electrodes. J. Power Sources 1999, 81–82 (0), 13-19. (27) Li, J.; Dahn, J. R., An in Situ X-Ray Diffraction Study of the Reaction of Li with Crystalline Si. J. Electrochem. Soc. 2007, 154 (3), A156-A161. (28) Cheng, J.; Wucher, A.; Winograd, N., Molecular Depth Profiling with Cluster Ion Beams. J. Phys. Chem. B 2006, 110 (16), 8329-8336. (29) Chen, Z.; Christensen, L.; Dahn, J. R., Large-Volume-Change Electrodes for Li-Ion Batteries of Amorphous Alloy Particles Held by Elastomeric Tethers. Electrochem. Commun. 2003, 5 (11), 919-923. (30) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y., HighPerformance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3 (1), 31-35. (31) Pereira-Nabais, C.; Światowska, J.; Rosso, M.; Ozanam, F.; Seyeux, A.; Gohier, A.; Tran-Van, P.; Cassir, M.; Marcus, P., Effect of Lithiation Potential and Cycling on Chemical and Morphological Evolution of Si Thin Film Electrode Studied by Tof-Sims. ACS Appl. Mater. & Interfaces 2014, 6 (15), 13023-13033.

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Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by

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