Progression of Solid Electrolyte Interphase Formation on

Apr 3, 2012 - Gnanaraj , J. S.; Zinigrad , E.; Asraf , L.; Gottlieb , H. E.; Sprecher , M.; .... Handan Yildirim , Alper Kinaci , Maria K. Y. Chan , a...
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Progression of Solid Electrolyte Interphase Formation on Hydrogenated Amorphous Silicon Anodes for Lithium-Ion Batteries David E. Arreaga-Salas,¶ Amandeep K. Sra,*,¶ Katy Roodenko, Yves J. Chabal, and Christopher L. Hinkle* Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: Silicon anodes based on an alloy reaction with lithium have a large theoretical specific capacity making them an appealing candidate for use in lithium-ion batteries. A major factor influencing the power cyclability and cycle life of the battery is the formation of the solid electrolyte interphase (SEI) layer. In this work, the progression of SEI formation on hydrogenated amorphous Si (a-Si:H) anodes is determined as a function of applied electrochemical potential during the first charging cycle by combining cyclic voltammetry measurements with detailed surface chemical analysis. During this first lithiation cycle, the SEI layer begins to form at 1.8 V by decomposition of the LiPF6 electrolyte to LiF, LixPFy, and PFy. The SEI layer, with LiF as the major species, continues to form upon further charging and forms a nonuniform layer on the surface of the electrode. At 0.4 V the Li atoms begin to penetrate the a-Si:H network, and upon full charging at 0.0 V, the anode itself is comprised in part by Si−Li, Si−F, and a network of F−Si−Lin. During the second lithiation cycle, Li causes significant scission of the Si−Si bonds resulting in the formation of high concentrations of LixSi.

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INTRODUCTION Li-ion batteries (LIBs) containing silicon anodes have been the subject of many recent investigations due to the extremely high capacity of silicon (3580 mAh/g for alloy Li15Si4) in comparison with currently used graphite anodes (372 mAh/g for LiC6).1−5 While graphite is lithiated by Li+ through an intercalation process, the dynamics of Li in the Si-based anodes are more complicated and still under investigation.6−9 Though Si anodes seem to be the ideal candidate for anodes, they are however associated with a massive structural change during the lithiation and delithiation processes leading to mechanical fracture and thus resulting in limited cycle life.1,2,4,10−13 In addition, the electrochemical side reactions which affect the performance, safety, and cycle life of the LIBs are closely linked to the complex reactions occurring at the electrode−electrolyte interface. These reactions lead to the formation of a passive solid electrolyte interphase (SEI) layer at the surface of the anode. The SEI layer is formed through the decomposition of the electrolyte into an insoluble solid film and is a critical factor in power cyclability and cycle life of LIBs.3,9,14−17 In order to maintain repeatable lithiation reactions, the SEI layer formation must be fully understood as it controls the passivation, stability, and impedance of the anode. In this investigation, the step by step formation of the SEI layer as a function of applied electrochemical potential during the first charging cycle is determined by employing cyclic voltammetry (CV) measurements of the electrode and correlating them to surface analysis techniques including X© 2012 American Chemical Society

ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and atomic force microscopy (AFM). By stopping the reaction at intermediate applied voltages and extracting the anode for chemical and morphological analysis, detailed information of the surface chemical composition and structure is obtained.

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EXPERIMENTAL SECTION The anodes used in this study are hydrogenated amorphous Si (a-Si:H) electrodes fabricated to exclude the effects of the other electrode components (binding material and conductor) on the SEI formation. The use of a-Si:H anodes also excludes any additional stress induced by crystalline-Si undergoing amorphization upon first reaction with Li.18 The a-Si:H electrodes were prepared by a standard technique using radio frequency plasmaenhanced chemical vapor deposition (rf PECVD) of 2% silane diluted with He gas at 200 °C and 50 W onto a thin stainless steel (SST) foil substrate. The as-deposited undoped a-Si:H electrodes were ∼200 nm thick and of low porosity, and similar deposition conditions have shown the H-content to be around 5%.19−21 Electrochemical reactions were performed using a Teflon beaker half-cell with 1 M LiPF6 in a 1:1 mixture of ethylene carbonate and diethyl carbonate (EC/DEC) as the electrolyte (purchased from MTI Corp). Three electrode halfReceived: January 24, 2012 Revised: March 12, 2012 Published: April 3, 2012 9072

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Figure 1. (a) Cyclic voltammetry curves of the first lithiation, first delithiation, and second lithiation cycles. (b) Si 2p XPS spectra of the electrode at the lithiation−delithiation stages of the first and second CV cycles.

Figure 2. AFM images recorded from (a) OCP to 1.8 V and (b) OCP to 0.8 V and SEM images of (c) OCP to 0.6 V and (d) OCP to 0.0 V showing the progressive roughening of the electrodes due to the formation of the SEI layer.

Raman/IR. Samples were measured with a Nicolet Almega XR dispersive Raman spectrometer from Thermofisher. The spectra were obtained with a 780 nm laser at 2.5 mW power as measured at the sample plane. The Raman spectrometer was equipped with a microscope where the 10× objective (NA 0.25) was utilized. IR absorption spectra are recorded in reflection mode using a Nicolet 6700 FTIR spectrometer from Thermo Scientific equipped with a DTGS detector. Reference spectra were obtained using a-Si:H anodes prior to the electrochemical treatment. An IR spectrum obtained from LiPF6 material was obtained in transmission, using LiPF6 powder pressed in a KBr pellet (the spectrum was referenced to the spectrum obtained from the pure KBr pellet). This procedure was performed under ambient air conditions.

cells were fabricated out of the a-Si:H/SST working electrode, and Li metal foil was used as both the counter and reference electrode. Cyclic voltammetry (CV) measurements were performed in the dark using an EG & G Princeton Applied Research potentiostat/galvanostat model 273 A. All potentials are quoted with regard to the Li/Li+ redox potential. For lithiation (charge), the voltage of the a-Si:H electrode was swept from the open circuit potential (∼2.5 V) to 0.0 V at a scan rate of 0.5/0.2 mV/s. For surface studies during cycling, the anodes were carefully separated, washed with DEC, and dried under vacuum. All the operation was done in a glovebox under argon atmosphere. XPS. XPS measurements were done on a PHI 5800 spectrometer equipped with an Al Kα monochromated X-ray source (1486.7 eV). A takeoff angle of 45° from the surface was employed. Spectra were recorded with 29.35 eV pass energy, at 0.125 eV/step. XPS peak assignments were made based on detailed, self-consistent curve fitting of the recorded spectra using Voigt line shapes in conjunction with a Shirley background subtraction utilizing “AAnalyzer” software.22

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RESULTS AND DISCUSSION Before discussion of the detailed progression of the SEI formation, an overview of the chemical composition of the SEI formed on the a-Si:H electrode needs to be determined. Figure 1a shows the CV curves obtained upon cycling the a-Si:H electrode for the first lithiation, first delithiation, and second 9073

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lithiation cycles at a scan rate of 0.5 mV/s. Si 2p XPS spectra recorded at each of these end points are shown in Figure 1b. After the first lithiation, broad shoulders are detected at both lower and higher binding energies around the deposited Si−Si peak at 99.3 eV. The lower binding energy shoulder corresponds to the formation of Si−Li bonds, while the shoulder on the high binding energy side relates to the formation of Si−F bonding. This Si−F linkage includes a Li component as well and will be discussed in more detail later in this paper. These new chemical states indicate the beginning of Si−Si bond scission and the insertion of Li and F into the aSi:H matrix. During the first delithation, the Si−Li peak intensity is significantly reduced, almost to the limit of detection, indicating removal of the Li from the a-Si:H anode. The Si−F peak also greatly reduces in intensity. After the second lithitation, the Si−Li peak intensity significantly increases indicating greater incorporation of the Li in the electrode as compared to the first lithiation. The Si−Li and Si− F peak positions, in comparison to the peak positions during the first lithiation cycle, are nearly identical with both of the two components broadened due to extensive lithiation causing more bond disorder. These results suggest that the first lithiation cycle primarily forms an SEI layer with a small amount of Li incorporated into the electrode itself while the second lithiation cycle results in significant Li insertion into the a-Si:H anode. This is confirmed in the cyclic voltammetry measurements seen in Figure 1a that indicate considerably higher current during the second lithiation cycle. Many experimental procedures for Li-ion batteries include a precycling step for the reasons just discussed.23,24 This precycle forms the SEI and allows for the next lithiation cycle to insert Li to levels consistent with the theoretical capacity of the Si. During the CV measurements, going from open circuit potential (OCP) to 0.0 V a characteristic peak is observed during the first lithiation stage (Figure 1a), around 1.3−1.5 V, and disappears for the subsequent cycles. This electrochemical reaction is believed to set the conditions for the upcoming cycles and drive the formation of the SEI. The morphological changes of the a-Si:H electrodes during the first lithiation cycle were also monitored by SEM and AFM at each applied bias (Figure 2). At 1.8 V the surface appears relatively flat with evidence of grain formation indicating the onset of SEI formation. As the applied bias continues to 0.6 V, the surface of the Si electrode becomes fully covered with small “clumps” and roughens indicating the growth of the SEI layer. In the fully charged state at 0.0 V, the top down SEM image indicates that the entire surface of the electrode is covered with a nonuniform SEI layer. This full coverage is confirmed by the significant attenuation of the Si 2p XPS spectrum as discussed later. Figure 3 shows Raman data obtained from the as-deposited (prior to lithiation) a-Si:H (dashed lines) and electrochemically cycled (solid line) samples. Raman data obtained from the sample during the first lithiation cycle ramped to 0.0 V shows the same line shapes as detected for the as-deposited a-Si:H samples suggesting minimal Li insertion. The Raman data obtained from the second lithitation sample show an asymmetric peak, which indicates the breakage of the shortdistance order that is typical for a-Si:H (Figure 3c). This finding is in accordance with molecular-dynamics simulations that predict the breakage of Si−Si covalent bonds due to the insertion of Li.25 Moreover, these results show that the insertion is gradual. Although the XPS data clearly indicate a small amount of Si−Li formation at the end of the first

Figure 3. Raman data obtained from unmodified (dashed lines) and electrochemically treated (solid line) samples. (a) Unmodified PECVD a-Si:H deposited on SST; electrochemically modified PECVD-Si (on SST) (b) after first lithiation and (c) after second lithiation.

lithiation cycle, the Raman data suggest that the a-Si:H network is mostly preserved during the first cycle and extensive Si−Si bond breakage only takes place after subsequent cycles. Again, this is consistent with the CV curves shown in Figure 1a. In order to study the progression of the initial SEI layer formation, the first lithiation cycle was studied in more detail. Cyclic voltammetry was performed on the electrodes by applying potential from OCP toward 0.0 V (lithiation) at a slower sweep rate of 0.2 mV/s. The anodes were extracted from the cell after each voltage step of 0.2 V, washed with DEC, and used for surface characterization. A fresh electrode was used for each data point, to avoid any contamination due to even minimal air exposure. In Figure 4a, for example, sample A was prepared by sweeping the voltage from OCP to 1.8 V, sample B was prepared by sweeping the voltage from OCP to 1.6 V, and the last sample J was swept from OCP to 0.0 V. The Si 2p spectra of the pristine a-Si:H electrode shows the dominant Si− Si peak at 99.3 eV binding energy. Following the electrochemical reaction, this peak intensity weakens (Figure 4b) due to attenuation caused by the electrolyte decomposition and the formation of the SEI overlayer. At 0.6 V the Si−Si peak is still dominant and exhibits no change in the chemical environment of the a-Si:H electrode, indicating that there is still no reaction between the solvent decomposition products and the underlying anode up to this point. Upon complete charging (0.0 V), the Si 2p peak is significantly broadened due to the formation of new chemical states (inset of Figure 4b). The shoulder at lower binding energies corresponds to the insertion of Li into the Si matrix, while the broadening at higher binding energies corresponds to the insertion of F in the Si matrix as discussed next.16,23 The process of decomposition of the electrolyte is best evidenced by the F 1s and Li 1s spectra (Figure 5). In these regions, at an applied bias of 1.8 V, the dominant peak at 687.8 eV is due to the presence of LiPF6 from the electrolyte, and the relatively smaller peak at 685.6 eV is due to the formation of LiF which is known as a main decomposition product of LiPF6.3,26,27 Continuing the lithiation reaction results in a steady decrease of the LiPF6 peak intensity and a concomitant increase in the LiF peak intensity. 9074

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Figure 4. (a) CV cures for the first lithiation cycle from OCP (2.5 V) to 0.0 V at a sweep rate of 0.2 mV/s halted at voltage steps of 0.2 V as indicated (samples A−J); (b) Si 2p XPS spectra of selected samples (C, D, F, G, and J) at various applied biases during the first lithiation cycle. The inset shows the expanded spectrum of sample G (OCP to 0.6 V) and sample J (OCP to 0.0 V) indicating that the Si−Si network, still prevalent at 0.6 V, is severely disrupted upon complete charging to 0.0 V.

Figure 5. (a) F 1s XPS spectra showing the decomposition of LiPF6 (at 687.8 eV) to LiF (at 685.6 eV) as the bias is applied and in sample H (OCP to 0.4 V) the change in the slope indicating the formation of F−Si−Lin networks. (b) Li 1s XPS spectra and (c) P 2p XPS spectra confirming the decomposition of LiPF6 to LiF.

Figure 6. (a) F 1s XPS spectra obtained by applying biases from OCP to 0.2 V (sample I) and OCP to 0.0 V (sample J), indicating that LiF is the major component of the SEI layer and confirms the formation of F−Si−Lin. (b) Li 1s spectra obtained from sample I and sample J confirming the LiF as a major component of the SEI layer along with formation of Si−Li and (c) Si 2p spectrum obtained by applying biases from OCP to 0.0 V (sample J) corroborating the formation of Li−Si and F−Si−Lin species formed in the a-Si:H electrode. 9075

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To gain an understanding of the chemical reactions occurring during the insertion of the Li and F into the Si electrode, the F 1s and Li 1s spectra at charging states 0.2 V (sample I) and 0.0 V (sample J) were compared to the Si 2p spectra (Figure 6). The Si 2p spectrum of sample J at 0.0 V (Figure 6c) shows a broad peak consistent with the presence of Si−Li and Si−F chemical environments, with intensities attenuated by the SEI overlayer. The F 1s spectra show a very strong peak at 685.6 eV attributable to LiF that increases upon further lithiation in addition to very weak contributions due to residual LiPF6 (687.8 eV), LixPFy (688 eV), and PFy (689.4 eV).6 This is also evident in the Li 1s spectra, where the peaks due to LixPFy are weakening while the peak due to LiF strengthens confirming that LiF is the major component of the SEI layer. The F 1s deconvoluted spectrum at 0.2 V (sample I) (Figure 6a) also indicates the appearance of a peak in the lower binding energy region (684.6 eV) as well, which increases in intensity and broadens upon further charging to 0.0 V. This peak is attributed to the formation of F−Si surrounded by a network of Li (F−Si−Lin) and is correlated with the appearance of the broadened Si−F peak in the Si 2p spectra at 0.0 V.16 In the absence of this Li network, the peak due to only Si−F would appear in the higher binding energy region of the F 1s spectrum. The significant reduction in this F−Si−Lin peak intensity following delithiation (as seen previously in Figure 1b) provides further evidence of this type of chemical bonding in the electrode. This Lin−Si−F type of network is also detected in the Li 1s region. The Li 1s deconvoluted spectrum at 0.2 V (sample I) and 0.0 V (sample J) shows a peak in the lower binding energy region at 53.8 eV, indicating the formation of LixSi in the surface region of the electrode (Figure 6b). A corresponding peak also appears in the deconvoluted Si 2p spectrum at 97.5 eV, consistent with the insertion of Li in the Si network (Figure 6c). Importantly, the concentrations of these chemical assignments are consistent across each of the Si 2p, F 1s, and Li 1s spectral regions when considering the evolution of the peak shapes and intensities throughout the reaction providing confirmation that these peak assignments are indeed accurate. For completeness and further chemical confirmation, the P 2p spectra were also analyzed (Figure 5c). At 1.8 V the major species present are LixPFy (136.7−137.8 eV) which includes LiPF6 (137.8 eV) and PFy (137.5−137.8). Upon charging to 1.6 and 0.4 V, the LixPFy peaks weaken and peaks due to LixPFyO appear in the region of 133−135 eV.28 The presence of these compounds is also supported by the FTIR analysis indicating that PO is formed as discussed below. Figure 7 shows FTIR spectra measured for each reacted anode, referenced to the spectra recorded on the same sample prior to the electrochemical treatment. Table 1 (Supporting Information) shows possible assignments along with the referenced literature sources. All the data in Figure 7 contain peaks in the lower wavenumber region, 450−680 cm−1. These peaks originate from the residual LiPF6 material (major peaks at 550−565 and 820−860 cm−1), superimposed on the background originating from LiF species (662 cm−1).29,30 The absorption bands in the 780−1140 cm−1 spectral range are due to the residual LiPF6 material (820−860 cm−1), P−F bonds (850 cm−1), and possibly some P−O−C bonds (740−830 cm−1).14,15,31−33 The spectrum obtained from sample J (OCP to 0.0 V) shows strong peaks in the 1066−1300 cm−1 region, which are also present (although much weaker) in spectra obtained from the other samples. These peaks are assigned to

Figure 7. FTIR data obtained in reflection (82° to surface normal) and referenced to the data obtained from the PECVD Si sample that was not subjected to the electrochemical process. The electrochemical potential (vs OCP) is specified for each curve in the figure. The spectral range above 3000 cm−1 is shown in order to highlight the lack of hydroxyl-related species formation. An interference pattern (above 3000 cm−1) occurs due to a small thickness difference between the sample that contains SEI films and the reference (PECVD a-Si:H deposited on SST). The spectrum of a LiPF6 powder compound obtained by mixing in a KBr pellet (measurement recorded in transmission) is given for comparison.

the PO stretching mode in PF3O type of species that are generated in LiPF6 + H2O → PF5 + LiF, PF5 + H2O → HF + PF3O type of reactions.11,31 This indicates that the Li−F and P−F containing compounds are the major species present on the surface. This spectral range is also associated with absorption bands due to Si−F bonds, in accordance with the XPS data that show the existence of Si in high oxidation states (see Table 1 in Supporting Information). Finally, absorption bands in the spectral range at 1350−1600 cm−1 are detected only for the sample isolated after the second lithiation cycle. These features are assigned to ROCO2Li, Li2CO3, and R−CO2−Mn+ (R = alkyl; M = Si/Li) carboxylate salts, as confirmed by the existence of the peaks due to C−O, C−C−O, and C−O−O groups with the typical frequencies in the 1000−1250 cm−1 region.15,31−34 These species correspond to typical decomposition products of carbonate-based solvents. The IR analysis therefore indicates that the SEI layer is mainly composed of organic compounds with functionalities of alkyl, carboxylate metal salt, and inorganic compounds including Li− F and P−F containing compounds. Based on all of the surface analysis data, the SEI composition and formation mechanisms can be proposed as follows. During the first charging cycle at 1.8 V, the electrolyte and solvent begin to decompose on the electrode surface. Between 1.8 and 0.6 V, the decomposed electrolyte and solvent contribute to the formation of the SEI layer on the electrode. After the first lithiation, the SEI layer is primarily composed of LiF as the major species along with lower concentrations of LiPF6, LixPFy, and PFy. There is not yet any interaction of the Li and F ions with the underlying Si electrode at 0.6 V. Si does not take part in the reaction until late in the first lithiation step. At biases lower than 0.4 V, the lithiation of the a-Si:H anode commences as Li and F pass through the SEI layer resulting in the formation of Si−Li, Si−F, and a network of F−Si−Lin. When Si begins to react with the electrolyte byproducts, it does so with F and Li simultaneously (not just high-capacity Li−Si). During the second lithiation cycle, significant scission of the Si−Si 9076

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(19) Cull, T. S.; Kernan, M. J.; Chan, P. H.; Fedders, P. A.; Leopold, D. J.; Norberg, R. E.; Wickboldt, P.; Paul, W. Amorphous Microcryst. Silicon Technol.1997 1997, 467, 123. (20) Haage, T.; Schmidt, U. I.; Fath, H.; Hess, P.; Schroder, B.; Oechsner, H. J. Appl. Phys. 1994, 76, 4894. (21) Remes, Z.; Vanecek, M.; Torres, P.; Kroll, U.; Mahan, A. H.; Crandall, R. S. J. Non-Cryst. Solids 1998, 227, 876. (22) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (23) Lee, Y. M.; Lee, J. Y.; Shim, H. T.; Lee, J. K.; Park, J. K. J. Electrochem. Soc. 2007, 154, A515. (24) Choi, N.-S.; Yew, K. H.; Choi, W.-U.; Kim, S.-S. J. Power Sources 2008, 177, 590. (25) Johari, P.; Qi, Y.; Shenoy, V. B. J. Am. Chem. Soc. 2011, 11, 5494. (26) Chen, L. B.; Xie, J. Y.; Yu, H. C.; Wang, T. H. J. Appl. Electrochem. 2009, 39, 1157. (27) Park, C. M.; Kim, J. H.; Kim, H.; Sohn, H. J. Chem. Soc. Rev. 2010, 39, 3115. (28) Leroy, S.; Blanchard, F.; Dedryvere, R.; Martinez, H.; Carre, B.; Lemordant, D.; Gonbeau, D. Surf. Interface Anal. 2005, 37, 773. (29) Glowacki, E. D.; Marshall, K. L.; Tang, C. W.; Sariciftci, N. S. Appl. Phys. Lett. 2011, 99, 043305. (30) Dalavi, S.; Xu, M.; Ravdel, B.; Zhou, L.; Lucht, B. L. J. Electrochem. Soc. 2010, 157, A1113. (31) Ota, M.; Izuo, S.; Nishikawa, K.; Fukunaka, Y.; Kusaka, E.; Ishii, R.; Selman, J. R. J. Electroanal. Chem. 2003, 559, 175. (32) Gnanaraj, J. S.; Zinigrad, E.; Asraf, L.; Gottlieb, H. E.; Sprecher, M.; Schmidt, M.; Geissler, W.; Aurbach, D. J. Electrochem. Soc. 2003, 150, A1533. (33) Gnanaraj, J. S.; Zinigrad, E.; Levi, M. D.; Aurbach, D.; Schmidt, M. J. Power Sources 2003, 119, 799. (34) Chen, L. B.; Wang, K.; Xie, X. H.; Xie, J. Y. J. Power Sources 2007, 174, 538.

bonds by the Li occurs resulting in formation of high concentrations of LixSi.

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ASSOCIATED CONTENT

S Supporting Information *

Possible FTIR peak assignments along with the referenced literature sources (Table 1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.L.H.); amandeep.sra@ utdallas.edu (A.K.S.). Author Contributions ¶

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Mathew Halls for his valuable discussions and Kui Tan for KBr pellet preparation. The authors acknowledge the full financial support of the U.S. Department of Energy, from the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-SC001951 for K.R. and the Office of Energy Efficiency & Renewal Energy under award DE-EE0004186 for C.L.H. D.E.A. thanks the Mexican Council of Science and Technology (CONACYT) for the support under the graduate scholarship program for studies abroad.

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