J. Phys. Chem. C 2007, 111, 11437-11444
11437
In Situ Conductivity, Impedance Spectroscopy, and Ex Situ Raman Spectra of Amorphous Silicon during the Insertion/Extraction of Lithium Elad Pollak,* Gregory Salitra, Valentina Baranchugov, and Doron Aurbach Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel ReceiVed: April 16, 2007; In Final Form: May 28, 2007
The electrical conductivity and the impedance behavior of thin layers of amorphous silicon (a-Si), which are promising anode materials for lithium-ion batteries, were monitored in situ during the insertion/extraction of lithium in 1 M of a LiBOB (Li-bioxalato borate) propylene carbonate solution. In addition, Raman spectra of the same electrodes were recorded in situ and ex situ during lithiation/delithiation processes in the abovementioned solutions. The conductivity of the a-Si electrode was increased by about 3.5 orders of magnitude during the course of lithium insertion. While the impedance response of these electrodes is complicated and cannot be resolved unambiguously, it is clear that the electrical conductivity influences strongly the electrodes’ impedance: a similar dependence of the electrical conductivity and the impedance of these electrodes on the potential are measured. The intensity of the Raman signal dropped significantly upon lithiation and recovered at a potential of 0.523 V vs Li/Li+. It is suggested that the drop in the intensity of the Raman signal of the silicon electrodes upon their lithiation is due to changes in the optical skin depth of the a-Si, which occur upon the formation of the Li-Si alloy.
Introduction
Experimental Section
Graphite electrodes are the most commonly used anodes in Li-ion batteries, mostly due to their relative good safety features and cycling ability. However, due to their relatively low charge capacity (372 mAh/g), alternative anodes should be considered. Silicon electrodes, which exhibit a theoretical charge capacity of ca. 4200 mAh/g (for a stoichiometry of Li4.2Si), are one of the most promising candidates that can replace graphite in rechargeable Li-ion batteries as the negative electrode.1 Recently, it was shown that amorphous silicon anodes behave reversibly at high capacity upon repeated lithiation/delithiation in ionic liquid solutions.2 However, as in the case of other metals, such as tin and aluminum,3,4 a huge volume change of the alloy material (approximately 300%) accompanies the insertion of lithium.5 This volume change leads to the formation of cracks, and eventually to the disintegration of the electrodes during repeated lithiation/delithiation processes.6 Nevertheless, when amorphous silicon (a-Si) is employed instead of crystalline silicon (c-Si), a dramatic improvement in the capacity retention is clearly obtained upon lithiation/delithiation cycling.7,8 It was suggested that the capacity fading observed in a-Si electrodes during the insertion/extraction of lithium stems from the contact loss that occurs due to the expansion/contraction of the electrode material and not to its pulverization. Despite the fact that numerous studies regarding the performance of silicon as a Li insertion compound have been published in recent years, the physical properties of the alloyed material formed upon the insertion of lithium is still quite obscure. This paper deals with some physical aspects of Li-Si electrodes based on in situ conductivity, in situ and ex situ Raman spectroscopy, and impedance spectroscopy measurements.
Ni substrates (99%, Goodfellow, U.K.) were used as current collectors for the silicon electrodes. Prior to the deposition of silicon, nickel foil current collectors were etched with a concentrated HNO3 solution in order to achieve a rough surface.9 This etching procedure was carried out in order to ensure the good adhesion of the silicon deposit to the surface of the nickel current collector. Silicon thin film electrodes were prepared by DC magnetron sputtering of silicon onto a roughened nickel foil, and on electrodes for in situ conductivity measurements with a gap of 5 and 10 µm, at a pressure of about 5 × 10-3 Torr of argon (99.995%). The base pressure of the sputtering chamber was 5 × 10-6 Torr. The target used was phosphorus-doped n-type silicon (99.999%, Kurt J. Lesker, USA), whose resistivity is 0.005-0.02 Ohm·cm. Layers of 1000 Å of silicon were deposited. The deposition process was controlled using a quartz crystal microbalance monitor. The electrolyte solution used throughout the work was 1 M lithium bis(oxalato)borate (LiBOB) (Chemetal) in propylene carbonate (PC) (Merck). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the silicon electrodes were measured in three-electrode cells, where a lithium wire and a lithium foil served as the reference and counter electrodes, respectively. Both CV and EIS were measured by a PGSTAT-30 Autolab electrochemical measuring system from Ecco Chemie, Inc., The Netherlands. The frequency range for the impedance measurements was between 1 MHz and 10 mHz. All impedance spectra were measured at equilibrium potentials, with an ac amplitude of 5 mV. In situ conductivity measurements of thin layers of silicon during the insertion/extraction of lithium were conducted by using an interdigitated array of a 100 Å Ti/1000 Å Pt on borosilicate glass, purchased from Abtech Scientific (VA).
* Address correspondence to this author. E-mail:
[email protected].
10.1021/jp0729563 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007
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Figure 1. Cell scheme for in situ Raman spectroscopy measurements.
The array consisted of 25 pairs of 3 mm long and 5 µm wide digits, which were separated by a 5 or 10 µm gap. This array was covered by a thin layer of silicon and served as the working electrode in these experiments. The conductivity was measured in situ during the electrochemical processes, using a bipotentiostat (Ecco Chemie, Inc., The Netherlands). The potential of the entire array was slowly scanned (10 µV/s) vs the Li/Li+ reference electrode, while maintaining a small potential difference (5 mV) between the two terminals of the electrode. Using this method, both faradaic and ohmic currents could be obtained simultaneously from the current at each terminal of the interdigitated micro array electrode using the following relation:10-12
IΩ ) (I1 - I2)/2
(1)
IF ) (I1 + I2)/2
(2)
therefore, the conductivity of the silicon coating could be obtained using the following relation:
σ ) IΩW/nl∆Vd (S)
(3)
where W, n, l, ∆V, and d are the gap width, number of digits (50), length of each digit, the potential difference applied between the terminals, and the thickness of the deposited layer of silicon, respectively. In order to calculate the specific conductivity of the thin film of silicon during its lithiation/ delithiation, one must consider the pronounced volume changes that accompany these processes (300% expansion upon full lithiation). However, as found by Beaulieu et al.,25 these volume changes are linear in the potential range of 0.5 V to 0 V vs Li/Li+, in which the lithiation process takes place. Thereby, these changes were taken into account when the specific conductivity was calculated. In addition, a series of conductivity measurements at steady states (i.e., constant electrochemical potentials of the electrodes) were carried out during both lithiation and delithiation processes. A current source (Yokogawa 7651) and a nanovoltmeter (Keithley 2182A) were attached to the terminals of the electrode for these measurements. In the first stage, the potential of both terminals of the gap electrode was slowly scanned (10 µV/s) until the desired potential was reached. The potential was held constant for a long time until equilibrium was reached, i.e., the electrochemical current dropped to zero. The resistance of the silicon film was measured by applying a current of 100 nA across the two terminals of the gap electrode and measuring the potential difference formed across the terminals. Raman spectra of the thin film silicon were measured in a back scattering configuration using a micro-Raman spectrometer,
HR 800 (Jobin Yvon Horiba), and holographic grating of 1800 grooves/mm with a He-Ne laser (excitation line 632.8 nm), objective 50× (numerical aperture 0.75). In situ Raman measurements were carried out using the cell presented in Figure 1, which allows simultaneous electrochemical and spectral measurements. It was found that there are similar Raman peaks of the a-Si and the solution species at the same wavenumber (around 480 cm-1). It was possible to overcome this problem by keeping the thickness of the solution layer between the electrode and the window to a minimum. In addition, we measured the electrode at different stages of lithiation (obtained during lithiation and delithiation), ex situ. We present herein the results obtained from ex situ Raman spectroscopy, since in this set of measurement, the peak located at 480 cm-1 is attributed to a-Si and there is no interference of the solution species that are present in the set of the in situ measurements. Ex situ Raman spectra were measured from electrodes at different lithiation/delithiation stages. The silicon electrodes were slowly scanned to the desired potentials, and then held at that potential for a long period of time. Electrodes containing different amounts of lithium were then taken from the electrochemical cell, washed thoroughly with γ-butyrolactone/DMC (dimethyl carbonate) 1:1 by solution weight and with neat DMC. This washing procedure ensured that no traces of the electrolyte or the solvent, which might interfere with the Raman spectrum of the silicon, were left on the surface of the electrodes.13 The electrodes were left to dry inside the glove box under argon atmosphere. The electrodes were then placed in a hermetically sealed cell with an optical glass window. Raman spectra of the silicon electrodes were obtained at the conditions mentioned above. XRD patterns were recorded using a BRUKER-AXS D8Advance diffractometer using Cu KR1 radiation. All electrochemical experiments and the assembly of the ex situ Raman spectroscopy cell were carried out under high-purity argon in a glove box (M. Braun, Inc.) in which the oxygen and water concentration were kept below 10 and 1 ppm, respectively. Results and Discussion Figure 2 shows the Raman spectra of a pristine a-Si thin film electrode deposited on a roughened nickel foil under ambient atmosphere. In general, a peak at 520 cm-1, which is indicative of crystalline silicon,14,15 was not detected. The bands centered around 155, 310, 475 cm-1, and the weak shoulder at 400 cm-1, are typical features of amorphous silicon vibration modes, associated with TA (transverse acoustic), LA (longitudinal optic), TO (transverse optic), and LO (longitudinal optic) phonons, respectively. The presence of these features and the absence of the 520 cm-1 peak leads to the conclusion that the
Properties of a-Si during Lithiation/Delithiation
Figure 2. Typical Raman spectrum of a pristine a-Si 1000 Å thin film on a roughened nickel foil.
Figure 3. Cyclic voltammograms of a 1000 Å a-Si at scan rates of 10 µV/s (A) and 300 µV/s (B).
sputtered thin Si films have an amorphous structure.16,17 The band observed at around 630 cm-1 corresponds to the 2LA second-order phonon Raman scattering and the TO + TA overtone.17,18 In addition, no diffractions, except those of the nickel foil, were detected by X-ray diffraction measurements of these Si film samples. Thus, the thin Si film prepared is considered to be totally amorphous. Figure 3 exhibits the CV of the silicon electrode at different scan rates. When a scan rate of 10 µV/s was employed (Figure 3a), three distinctive reduction peaks are observed in the cathodic branch of the voltammogram. Each peak can be attributed to the formation of a different composition of a LixSi alloy. The principal phases of the Si-Li alloy were already identified by the equilibrium titration method at 415 °C.19,20 The peaks at around 340, 210, and 70 mV vs Li/Li+ correspond to Li0f1.7Si, Li1.71f2.33Si or Li2.33f3.25Si, and Li3.25f4.2Si, respectively. The two peaks, located at about 310 mV and 495 mV vs Li/Li+ in the anodic branch, are attributed to the partial decomposition of the Li4.2Si and to the complete extraction of Li from the silicon host, respectively. As is clearly seen in Figure 3, the cathodic peak that is located at around 310 mV vs Li/Li+, which appears in the first cycle, has almost completely disappeared in the subsequent cycle. In addition, no oxidation peak is observed in the anodic branch, which might correspond to this reduction process. It is suggested that this irreversible peak might result from two factors: the first, as mentioned above, is of
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11439 course the insertion of lithium into the a-Si to form an alloy of Li-Si. However, unlike the two other potentials at which lithium is reversibly inserted into the silicon, at this potential the insertion of lithium is irreversible. This process of irreversible lithium insertion might be referred to as lithium trapping. A second process that might occur at this potential region is the reaction of silicon dangling bonds located at the entire volume of the a-Si electrode with lithium. Once these bonds have reacted with lithium, they become inactive; therefore, the intensity of the peak is weaker in the subsequent cycles. It should be emphasized that according to Takamura et al.,21 this reduction peak located at about 0.34 V vs Li/Li+ was absent when a scan rate of 1 mV/s was employed. Figure 3b shows the first and the 60th cycles of the a-Si electrode (1000 Å) at a scan rate of 300 µV/s. As a result of the higher scan rate used in this case, all reduction peaks are shifted to a lower potential and the two anodic peaks are shifted to higher potentials, due to kinetic limitations. The charge capacity of lithium extraction in the first cycle was 2942 mAh/g (active mass, 30 µg) with a faradaic efficiency of 81%. The charge capacity of lithium extraction in the 60th cycle was 2665 mAh/g with a faradaic efficiency of 96.5%. The capacity fading of the a-Si electrode was therefore 9.5% over 60 cycles. This relatively low capacity fading observed after 60 cycles is attributed to a number of factors such as the good adhesion of the a-Si to the roughened nickel current collector, relatively reversible volume changes of the a-Si in the course of the insertion/extraction of lithium, and a stable passivation layer formed on the a-Si electrode during the course of the first cathodic sweep. It is commonly known that the composition of the passivation layer formed on the anode during the first cathodic sweep is comprised of reduction products of both the solvent and the electrolyte molecules. It is known that in the case of LiBOB solutions in alkyl carbonates, the BOB- anion participates in the surface chemistry of Li or Li-graphite electrodes. The surface films thus formed provide a very good passivation, which characterizes both Li and Li-graphite electrodes in LiBOB solutions. Therefore, we assume that the reduction products of the BOB- anion comprise the solid electrolyte interphase (SEI) on the electrodes in the system described herein.22-24 Hence, we believe that the use of LiBOB solutions in the present study also contribute to the stable behavior of the a-Si electrodes observed upon repeated lithiation/ delithiation cycling and allow their reversible expansion/ contraction. However, a study of the nature of the surface films formed in this case is beyond the scope of the present work. Figure 4 presents the conductivity vs potential plot obtained by the microarray interdigitated electrodes of 5 µm and 10 µm gaps, calculated using eq 3. In order to confirm the potentiodynamic data seen in Figure 4, a series of open circuit measurements (Figure 5) were conducted at different potentials during the insertion/extraction of lithium, using a 10 µm gap electrode. It is quite clear that apart from slight differences (that will be discussed later on), the potentiodynamic and the open circuit measurements agree well with one another. The curves presented in Figures 4 and 5 were normalized to the changes in the thickness of the a-Si electrodes during lithiation/delithiation according to data obtained by Beaulieu et al., where the thickness of an a-Si electrode was measured during the insertion and extraction of lithium using atomic force microscopy measurements.25 The conductivity vs potential measurements indicate that a dramatic change in the conductivity occurs between 0.25 and 0.12 V vs Li/Li+. This huge leap in the conductivity of the a-Si
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Figure 6. Conductivity of the Li-Si electrodes as a function of potential in logarithmic scale (measured during the slow potential scan, as in Figure 4).
Figure 4. Potentiodynamic conductivity measurements: 5 µm gap electrode (A) and 10 µm gap electrode (B) in LiBOB/PC solutions. The scan rate during the measurements was 10 µV/s.
Figure 5. Conductivity vs potential, measured with a 5 µm gap electrode, at different levels of lithiation of a-Si in the course of a Li insertion-extraction cycle at equilibrium potentials.
electrode is attributed to the formation of a LixSi alloy, where x < 3.25. This peak in the conductivity vs potential curve does not correspond directly to the peaks obtained in the slow scan CV (Figure 3a). The maximum in the conductivity was measured around 0.12 V vs Li/Li+. Between 0.12 and 0.02 V vs Li/Li+, a decrease in the conductivity is measured. We attribute this decrease to the decomposition of the conductive Li3.25Si alloy and the formation of the less-conductive Li4.2Si alloy. The expansion of the a-Si electrode that occurs upon the insertion of lithium increases the overall conductivity measured in the experiments described herein, since during the expansion of silicon the cross section increases. However we find the effect of the volume changes on the conductivity of the silicon electrode during lithiation/delithiation to be quite negligible since the volume changes by 300%, while the conductivity changes by about 3.5 orders of magnitude. During the extraction of lithium, an increase in the conductivity is observed up to a potential of about 0.27 V vs Li/Li+. This increase in the conductivity is attributed to the decomposition of the Li4.2Si alloy and the reformation of the more conductive Li3.25Si alloy. Above potentials of 0.27 V vs Li/ Li+, the conductivity of the a-Si electrode drops as the rest of the lithium is being extracted from the silicon material. In order to complete the picture, Figure 6 shows a curve of the conductivity vs potential measured potentiodynamically, on a logarithmic scale. A small peak in the conductivity vs potential
curve appears at 0.34 V vs Li/Li+. Since this peak does not appear in the open circuit conductivity measurements (Figure 5), it may be identified as an artifact caused by the method of potentiodynamic measurements. Nevertheless, the increase in conductivity between OCV and 0.34 V vs Li/Li+ is quite substantial (about 1.5 orders of magnitude). This increase in the conductivity cannot be attributed to the insertion of lithium into the a-Si alone, since the amount of charge which passes between OCV and 0.34 V vs Li/Li+ is small in comparison with the charge, which is related to the two subsequent cathodic peaks (the lower potential range). We attribute this increase in the conductivity to two processes that occur simultaneously: the first is the reaction of dangling bonds of silicon over all its volume (not just on the surface area of the a-Si electrode) with lithium. During the course of this reaction, the dangling bonds are annealed, and therefore this process is irreversible. This process is believed to be the dominant factor, which is responsible for the increase in the conductivity. The second process is attributed to the insertion of lithium into the a-Si electrode. The lithium insertion that occurs around 0.34V vs Li/Li+ is irreversible. However, the effect of this process on the conductivity at potentials down to 0.34 V vs Li/Li+ is weak with respect to the former process. The conduction mechanism of the Li-Si alloy formed upon the insertion of lithium into a-Si is still unclear: on the one hand, the alloy thus formed might be a heavily doped semiconductor, but on the other hand, the Si-Li alloy might also possess metallic properties, which might explain the huge leap of the conductivity (ca. 3.5 orders of magnitude) during the insertion of lithium at potentials below 0.3 V vs Li/Li+. This topic is currently in an ongoing study and will be clarified in a later publication. The next step was to examine the possible correlation between the potential dependence of the electrical conductivity of these electrodes and their impedance behavior. Figure 7 shows impedance spectra presented as Nyquist plots measured during lithiation and delithiation of the thin a-Si electrodes at different equilibrium potentials (as indicated), in the first, second, and tenth cycles (Figure 7a-e). Figure 7c shows the typical Nyquist presentation of the high-medium-frequency part measured during the first cycle of lithium insertion/extraction. These spectra reflect the very complicated impedance behavior of the Li-Si electrodes, which cannot be resolved unambiguously. In general, the spectra exhibit three main features: a high frequency, opened (half) semicircle (i.e., the frequency employed was not high enough to reach the dc behavior at this end), a high-medium-frequency semicircle, which is potential dependent, and a low-frequency part in which Z′′ vs Z′ is a sloping line. On the basis of previous studies of Li, Li-graphite, and other Li insertion electrodes (e.g., LixMOy or LixMSy, M ) a transition
Properties of a-Si during Lithiation/Delithiation
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Figure 7. Impedance spectra (Nyquist plots) measured during the first cycle at high potentials (A) and during the first cycle at low potentials (B), and the expanded scale of the low-impedance region during the first cycle at low potentials (C), during the second cycle at low potentials (D), and during the tenth cycle at low potentials (E).
metal),26-29 we attribute the semicircles to charge transfer processes (coupled with interfacial and/or surface films capacitances), and the low-frequency part of the spectra to bulk transport processes, e.g., Li ion diffusion in the silicon bulk, balanced by the transfer of electrons. A rigorous analysis of the impedance behavior of these LiSi electrodes requires intensive measurements at different temperatures and different electrolyte solutions, coupled with some simulation and modeling and is beyond the scope of this paper. For the present study, it was interesting to find a possible connection between the potential dependence of the impedance and the electronic conductivity of these electrodes. For that purpose, Figure 8 shows the dependence of the magnitude of the impedance, |Z|, of these electrodes at ωf0, in a logarithmic scale, on the potential during in the course of Li insertion and extraction (indicated) for the first and second cycles (parts a and b of Figure 8, respectively). The impedance
behavior in subsequent cycles is quite similar to that measured during the second cycle. In Figure 8a (first cycle), the corresponding resistivity of the LixSi electrodes as a function of potential upon a lithiation-delithiation cycle is also presented (calculated from the conductivity data, Figure 6). The behavior in subsequent cycles is quite similar to that measured during the second cycle. Figure 9 presents the diameter of the main semicircle (∆Z′) in the Nyquist plots measured as a function of potential during lithiation and delithiation of the thin a-Si electrodes during the first and second cycles (indicated). The impedance of pristine Si electrodes, |Z|, is initially high and reflects very high interfacial charge transfer resistance, and in fact resembles very much the impedance behavior of nonactive metal electrodes upon their polarization in polar aprotic Li salt solutions from OCV (≈3V vs Li/Li+) to low potentials.30 At potentials below 0.5 V, the Si electrode’s impedance drops by 2 orders of
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Figure 8. log |Z′|ωf0 vs potential obtained from the EIS measurement during the course of the first (A) and the second (B) lithiationdelithiation cycles of a-Si electrodes. In Figure 8a, curve 1, the dependence of the electrode’s resistivity on the potential (calculated from the conductivity measurements) is also presented. (Curve 2 in Figure 8a is log |Z′| vs E.)
Figure 9. Diameter of the main circle (high-medium frequency) in the Nyquist plots (∆Z′) vs the electrode’s potentials. First cycle (A) and second cycle (B).
magnitude, and it remains stable during the course of their lithiation-delithiation, up to a potential >0.7 V, after which the impedance vs potential gradually and monotonously increases, in the course of delithiation (positive polarization). The impedance spectra measured at low potentials (
