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Jun 16, 2016 - After glovebox assembly in a home-built Teflon cell, monitoring of the diffraction efficiency of these gratings during the lithiation/d...
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Monitoring Volumetric Changes in Silicon Thin-Film Anodes through In Situ Optical Diffraction Microscopy Jonathon Duay,† Kjell W. Schroder,†,‡ Sankaran Murugesan,†,§ and Keith J. Stevenson*,†,⊥ †

Department of Chemistry, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States ⊥ Center for Electrochemical Energy Storage, Skolkovo Institute of Science and Technology, 3 Nobel Street, Moscow 143026, Russia S Supporting Information *

ABSTRACT: A high-resolution in situ spectroelectrochemical optical diffraction experiment has been developed to understand the volume expansion/contraction process of amorphous silicon (a-Si) thin-film anodes. Electrodes consisting of 1D transmissive gratings of silicon have been produced through photolithographic methods. After glovebox assembly in a home-built Teflon cell, monitoring of the diffraction efficiency of these gratings during the lithiation/delithiation process is performed using an optical microscope equipped with a Bertrand lens. When the diffraction efficiency along with optical constants obtained from in situ spectroscopic ellipsometry is utilized, volume changes of the active materials can be deduced. Unlike transmission electron microscopy and atomic force microscopy characterization methods of observing silicon’s volume expansion, this experiment allows for real-time monitoring of the volume change at charge/discharge cycles greater than just the first few along with an experimental environment that directly mimics that of a real battery. This technique shows promising results that provide needed insight into understanding the lithium alloying reaction and subsequent induced capacity fade during the cycling of alloying anodes in lithium-ion batteries. KEYWORDS: silicon anode, lithium-ion batteries, optical diffraction, refractive index, volume expansion, spectroscopic ellipsometry, amorphous silicon, transmissive grating

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INTRODUCTION Lithium-ion batteries are proven to be great devices for mobile applications such as cell phones and laptops; however, applications such as electric vehicles as well as other future applications will require much higher gravimetric capacity in order to compete with the present well-established combustionbased engines.1 Current research is focused on replacing the graphitic carbon anode with silicon, which can theoretically provide a greater than 10-fold increase in both the gravimetric and volumetric capacity.2,3 However, this increase in the capacity comes with a large increase in the volume of the silicon material upon charging. This volume expansion is a major issue because it can cause strain and eventual pulverization of the electrode.4,5 This process results in very low cycle retention and thus limits silicon’s application to future energy storage devices. Currently, there is a large emphasis in the literature to mitigate this pulverization through a variety of techniques including © XXXX American Chemical Society

nanosizing the silicon particles as well as combining these nanosized materials with softer materials or voids in order to buffer this expansion.6−11 Although these methods are promising, it is important first to understand the magnitude of this volume change during multiple cycling events. Here amorphous silicon (a-Si) is used because it has advantages over crystalline silicon (c-Si) due to its homogeneous volume expansion along with shorter diffusional length and a smaller charge-transfer resistance.4,12−19 Recent theoretical studies have shown that diffusion of lithium through a-Si is more facile and its energy barrier is lower than that of c-Si.20 Also, an experimental effort toward building nanosized a-Si thin films showed enhanced lithiation behavior.21 Recent in situ Received: March 30, 2016 Accepted: June 16, 2016

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DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic Indicating the Process Used To Prepare the a-Si Gratingsa

a

(a) Fabrication of PPF on quartz, (b) PECVD, (c) spin coating of the photoresist, (d) exposure and development of a spin-coated photoresist, (e) RIE of a-Si:H and PPF through the photoresist, and (f) removal of the photoresist by sonication in acetone.

patterned on an appropriate length scale can act as a diffraction grating as long as it is differentiated from its surroundings by either the optical component n, index of refraction, or k, absorptivity. For instance, monochromatic light propagating through a 1D-patterned array (e.g., a periodically spaced set of parallel features) will be diffracted, forming a row of bright spots. The intensity of these diffracted spots depends on both the thickness of the pattern and the degree of contrast between the refractive index and absorptivity of the pattern and surroundings. As a consequence, any electrochemical or volumetric change occurring at the grating material or its surroundings will result in detectable modulation of the intensity of diffracted light due to changes in the volume (including spacing between diffracting elements) as well as the refractive index and absorptivity of the system. The figure of merit used to describe the degree of diffraction is called the diffraction efficiency (DE). Here we have used this technique to understand the volume expansion/contraction of electrochemical lithium alloying by monitoring the DE of an a-Si:H thin-film grating.

transmission electron microscopy (TEM) studies have demonstrated22 that a-Si undergoes a two-phase process of lithiation, forming an amorphous−amorphous interface between the a-Si reactant and the amorphous LixSi. Such a phase formation is attributed to the need for a high local lithium concentration at the phase boundary to break the covalently bonded Si−Si atoms. This has also been shown by Yassar et al., who found that a-Si has two different diffusion paths present (longitudinal and radial) and a phase transition from the pristine a-Si to Li22Si5 during lithiation.16 Along these lines, our group has shown that hydrogenated amorphous silicon (aSi:H) particles are better for the lithium alloying reaction,23 and Cui et al. showed that the stress intensification is lower in isotropic a-Si expansion compared to anisotropic C−Si expansion.24 Considerable interest has been devoted to understanding the volume change during electrochemical lithiation of these silicon electrodes. Researchers have devised suitable analytical techniques to study this lithiation/delithiation process through in situ methods. Earlier efforts by Dahn et al. focused on understanding the volume expansion in an a-Si-based system through in situ atomic force microscopy (AFM) measurements.25 These measurements revealed insight into the percentage of volume expansion and contraction in subsequent cycles of the lithium alloying reaction. In situ TEM experiments26 have also been designed to study the volume expansion in anode materials such as silicon and tin as well as other oxidebased anodes; however, the measurements are aimed at a specific region and a small part of the sample. Furthermore, it is not possible to do multiple cycles because of electrochemical amorphorization (loss of crystal orientation). Therefore, these results may not resolve the whole picture of silicon’s volume expansion. In addition, these experimental techniques are very expensive and require specialized experimental setup. Herein, the volume expansion/contraction behavior of a-Si:H thin-film electrodes at intermediate cycle numbers is characterized by in situ high-resolution spectroelectrochemical optical diffraction based microscopy. Diffraction-based imaging is an emerging optical-based sensing technique that employs the diffraction of visible light as a signal for the detection of receptor/analyte interactions.27−33 Earlier our group showed diffraction-based imaging of electrochemically induced Li+ insertion/deinsertion in gratings of metal oxides and mixedmetal oxides.34 Accordingly, any spatially periodic material

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EXPERIMENTAL SECTION

Reagents. All chemicals were used as received. 1-Methoxy-2propyl acetate (PGMEA) was obtained from Sigma-Aldrich. Photoresist AZ 1518 was purchased from Microchemicals, while the positive photoresist S1818 and the MF321 developer were obtained from Shipley Co. LiPF6 (1 M) in a 1:1 volume ratio of ethylene carbonate to diethyl carbonate with less than 200 ppm of water was obtained from BASF. Lithium foil was obtained from Sigma-Aldrich. Fabrication of Opaque and Transparent Pyrolyzed Photoresist Film (PPF) Electrodes. Opaque and transparent PPFs were prepared by a previously reported procedure.35−37 Briefly, quartz microscopic slides (6.45 cm2 and 1 mm thickness, Technical Glass Products) were heated at 800 °C in air to remove organic contaminations and further placed in piranha (3:1 H2SO4/30% H2O2) to remove any additional residual organics. (Caution! Piranha is a strong oxidizing solution and must be prepared in a f ume hood with proper protection. Always add H2O2 to H2SO4.) An undiluted AZ1518 photoresist for opaque PPFs and a diluted AZ1518 photoresist [diluted to 25% (v/v) with PGMEA] for transparent PPFs were spun onto the piranha-cleaned quartz slides at 6000 rpm for 60 s. After spin-coating, the photoresist slides were soft-baked for 10 min at 90 °C on a hot plate and then transferred to a tube furnace. After purging with 5% H2/95% N2 (∼100 mL/min) for 15 min, the photoresist slides were pyrolyzed by heating to 1000 °C at 5 °C/min and holding at that temperature for 1 h before allowing them to cool B

DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces slowly back to room temperature at 5 °C/min. The PPFs were then removed from the furnace and stored for 3 days prior to use to allow for the oxide layer to stabilize.38 As detailed elsewhere,39 opaque and transparent PPFs prepared by this method are 250 ± 20 and 11 ± 1 nm thick, display root-mean-square roughnesses of 0.39 ± 0.07 and 1.01 ± 0.06 nm, and have sheet resistances of 97 ± 3 and 1850 ± 70 Ω/□, respectively. Preparation of a-Si:H Gratings. a-Si:H gratings were fabricated by photolithography onto the PPF electrodes using the following steps, as described in Scheme 1. (a) After the fabrication of PPFs by the method described above, (b) an Oxford Instruments Plasmalab 80 plus instrument was used for the deposition of a-Si:H thin films over the transparent and opaque PPF electrodes. The conditions followed for this deposition included flow rates of 50 sccm argon and 4 sccm SiH4, a power of 100 W, and a temperature of 200 °C. The total deposition time is 10 min, which results in a film thickness of around 200 nm, measured by AFM and spectroscopic ellipsometry (SE). Assuming a density of ≃2.2 g/cm3 for a-Si:H,40 the mass loading was calculated to be 44 μg/cm2, which corresponds to a total silicon loading amount of 22 μg for each electrode. (c) Next, a Microposit S1818 photoresist was spin-coated onto the a-Si:H-coated PPF at 3000 rpm for 60 s. Following spin coating, the PPF/a-Si:H/photoresist is soft-baked on a hot plate for 60 s at 110 °C to remove solvent from the resist layer and make it photosensitive. (d) Next, the sample, together with the mask, was mounted for UV irradiation in a mask aligner (SUSS MicroTec Lithography, GhbH MA6/BA6) and exposed for 19 s to 385 nm light (mercury lamp, I-line) with an intensity of 10 mW/cm2. This step was performed in soft contact mode, where the resist-coated sample was brought into physical contact with the mask. This soft mask was designed in house but fabricated externally by Advanced Reproductions, Inc. After exposure, the photoresist-coated sample was developed in a MF321 developer for 45 s to remove the UV-exposed portions of the photoresist. (e) Next, reactive ion etching (RIE) was performed using a March Plasma 170IF RIE etching system. The exposed a-Si:H was first etched at 100 W using 10.5 sccm SF6 and 2 sccm O2 flow rates for 60 s, followed by etching of the PPF at 100 W using 25 sccm O2 flow rates of 60 and 360 s for the transparent and opaque PPFs, respectively. (f) Finally, the remaining photoresist was removed by sonicating the sample in acetone, resulting in 1D gratings of a-Si:H on PPFs. Spectroelectrochemical Studies. All electrochemical measurements were performed using a home-built Teflon cell with a fixed working electrode area of 0.45 cm2, a path length of 1 cm, a cell volume of ∼1 mL, and metallic lithium as both the counter and reference electrodes using a CHI 440 potentiostat (CH Instruments). Cyclic voltammetry was done using a 5 mV/s scan rate between 0.01 and 2.01 V versus Li/Li+, while chronoamperometry was done by stepping the potential between 0.01 and 2.01 V versus Li/Li+ for 1000 s each. All potentials discussed below were versus Li/Li+ unless otherwise denoted. The cells were assembled in an argon-filled glovebox (Unilab 2000, MBraun). Optical Diffraction Measurement. Diffraction experiments were performed utilizing a home-built CCD microscope setup, as described in our earlier publication and schematically shown in Scheme 2.34 The electrochemical cell was positioned on the microscope stage of a Nikon Eclipse E600 optical microscope. Light from a tungsten lamp (100 W, USHIO Inc.) was passed through the electrochemical cell into the microscope objective (Nikon 10X ELWD, 0.60 NA). Periodic spacing of the sample resulted in a diffraction pattern that is located at the back focal plane of the objective. A telescopic lens known as a Bertrand lens28,41,42 (Micronon) allows a view of this back focal plane and thus this diffraction pattern. Images of the 1D diffraction patterns at three different wavelengths (475, 540, and 630 nm) were acquired every 10 s during the chronoamperometry and cyclic voltammetry experiments using a Photometrics CoolSNAP HQ 12-bit CCD camera (Roper Scientific) controlled with MetaMorph imaging software (Universal Imaging Corp., version 5.0r4). Materials Characterization. Scanning electron microscopy (SEM) was performed with a Quanta 650 microscope operated at 30.00 kV. The 1D gratings over PPF electrodes were mounted on the

Scheme 2. Schematic Diagram (Not to Scale) of the Experimental Analysis Setup To Monitor the DE during Lithiation/Delithiation of a-Si:H Gratings

aluminum stub with double-sided carbon tape (Ted Pella) for SEM analysis. The topography of the 1D gratings was measured by AFM performed using an Asylum Research MFP-3D instrument. All measurements were obtained in tapping mode with aluminum-coated monolithic scanning probe microscopy tips with alignment grooves (BudgetSensors; cantilever length, 125 μm; resonance frequency, ∼300 kHz; tip radius, 10 nm). In situ SE measurements utilizing a home-built Teflon cell were taken from 200 to 1000 nm at an angle of 75° using 100 revolutions per measurement by means of a J. A. Woollam M-2000 variable-angle spectroscopic ellipsometer. WVASE32 software was used to perform the fittings of Ψ and Δ.

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RESULTS Figure 1 represents microscopy images obtained from AFM and SEM of the a-Si:H grating on transparent PPF fabricated by the method detailed in the Experimental Section. AFM and SEM data indicate 10-μm-wide and 200-nm-thick bars of a-Si:H with 10-μm-wide slit widths. It should be noted that the 3D AFM image is not an ideal representation because the aspect ratio (height to width) of these bars is much less; however, a real representation of the true aspect ratio would result in an image with little to no visual relief. The figure of merit used here to describe the diffraction from these gratings is called the DE and can be calculated by the following equation: DE(λ) =

∑ Idiff Iinc

(1)

where Idiff is the intensity of the diffracted light and Iinc is the incident intensity, which is determined by summing the intensity of the diffracted spots and the intensity of the undiffracted center spot (Figure S3 demonstrates visually how the DE was calculated for the experiment presented here). Equation 1 is employed to examine the changes in the diffraction pattern during charging/discharging of the a-Si:H grating. The actual physical parameters that describe this DE are represented by eq 2.43 C

DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) 90 × 90 μm 3D AFM image of an a-Si:H grating on transparent PPF. (b) SEM image of the same a-Si:H grating.

Figure 2. (a) DEs at three different wavelengths, (i) 475, (ii) 540, and (iii) 630 nm, during cyclic voltammetry of 10.5 cycles at 5 mV/s and the corresponding transposed current versus time curve. (b) Expanded view of the 10th cycle found in part a.

⎡ −4πTkavg ⎤ ⎡ π Δk(λ) T ⎥ ⎢sinh2 DE(λ) = exp⎢ λ cos θ ⎣ λ cos θ ⎦ ⎣ + sin 2

π Δn(λ) T ⎤ ⎥ λ cos θ ⎦

475 nm light but negative for the 540 and 630 nm light. This demonstrates the inherent complexity of this measurement because it would be expected that if there were only a thickness change affecting this metric, all DEs, regardless of wavelength, would modulate in the same direction upon lithiation and delithiation. To breakdown the complexity of this measurement, first using a grating made on opaque PPF, the lateral expansion of the silicon thin film was determined to be negligible and thus contributes little to the DE (see the Supporting Information). Next, transparent PPF was used in the experiment performed in Figure 2 because a-Si:H is a transparent semiconductor and interacts strongly with the incident light. This allows quick determination of its thickness modulation through utilization of eq 2. However, in order to perform this calculation, the optical constants n and k must be determined at each charged state. This measurement was achieved through in situ SE. SE is wellknown as an accurate and nondestructive optical characterization technique for thin films. SE works by simply reflecting an elliptically polarized beam of light off a sample at oblique

(2)

where T is the thickness of the grating, kavg is the average absorptivity of the grating, λ is the wavelength of light analyzed, θ is the Bragg angle, and Δk and Δn are the differences between the absorptivity and refractive index of the grating and electrolyte. This DE was monitored utilizing different wavelength filters (475, 540, and 630 nm) while cycling the grating on transparent PPF between 2.01 and 0.01 V versus Li/Li+ using cyclic voltammetry at a 5 mV/s scan rate. Figure 2 demonstrates that there is clearly a modulation in the DE upon lithiation and delithiation of the a-Si:H grating. However, a closer look at the modulation at different wavelengths, as seen in Figure 2b, which represents the 10th cycle, reveals that the direction of this modulation during lithiation is positive for the D

DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. SE parameters Δ (a) and Ψ (b) measured for three potential stepping cycles between a lithiation potential of 0.01 V and a delithiation potential of 2.01 V versus Li/Li+. (c and d) Δ and Ψ experimental and fit results obtained from the physical model used in part e whose layers use the coefficients found in part f.

oscillators in parallel are used for the opaque PPF layer (1) and its thickness is held constant at 250 nm. Lorentz oscillators are chosen here because the PPF here acts as a metallic film. Next, the unlithiated a-Si:H layer (2) is modeled as a Tauc−Lorentz layer because this particular model is excellent for semiconductors because it forces the absorbance to zero below the band gap and better describes k above it.47 Above the a-Si:H layer is a geometric mixing layer or an effective medium approximation (EMA) layer (3), which incorporates both the aSi:H layer below it and the LixSi layer above it and is characterized by the percentage of each. This EMA layer is added as a solid solution between the unlithiated a-Si:H and fully lithiated LixSi layers. The LixSi layer (4), which represents the fully lithiated silicon and is characterized as a metal alloy, is modeled as a single Lorentz oscillator. Above this is a solid electrolyte interphase (SEI) layer (5), which is modeled with a Lorentz oscillator rather than a Cauchy dispersion model because this layer consists of a complicated mixture of organic and optically absorbing inorganic products.48,49 Finally, a Cauchy dispersion model is used for 1 M LiPF6 in 1:1 (v/v) EC/DEC solvent (ambient) used for the experiments here. Parts c and d of Figure 3 indicate good fits using this model with mean-squared errors (MSEs) of less than 10. For both the

angles and measuring the changes in the amplitude (Ψ) and phase (Δ) of this reflected light. Here, a special electrochemical cell is used that allows transmission and reflection of the ellipsometer light source through the electrolyte and off of a planar thin-film electrode. The angle of incidence for this cell is fixed at 75°. Parts a and b of Figure 3 demonstrate the changes in Δ and Ψ upon lithiation/delithiation during three cycles of a continuous aSi:H thin film deposited on opaque PPF. Here, chronoamperometry is used, and the electrodes were stepped between a lithiation potential of 0.01 and a delithiation potential of 2.01 V versus Li/Li+ for 1000 s each. It is shown that this process is highly reversible with little hysteresis for all three cycles. During lithiation, the spectrum transitions from an oscillating wave, characteristic of an a-Si:H transparent semiconductor,44 to a near featureless spectrum, characteristic of the reflective LixSi metallic alloy.45 Parts c and d of Figure 3 show the model fits for these curves. The physical model represented by Figure 3e utilizes the model parameters for each layer found in the table in Figure 3f. Starting from the bottommost quartz substrate layer (0), SiO2’s optical constants, which are commonly found in the literature, are used and the thickness is set to 1 mm.46 Three Lorentz E

DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Bar graph indicating the changes in the layer thickness obtained from SE at different times during lithiation and delithiation via chronoamperometry. Chronoamperometric plots for the (b) lithiation step potential of 0.01 V and (c) delithiation step potential 2.01 V versus Li/ Li+.

Figure 5. Effective optical components, (a) n and (b) k, calculated using weighted averages based on the thicknesses for layers 1−5 (Figure 3e) from the SE results at a-Si:H’s fully lithiated and delithiated states.

lithiated and delithiated fittings, all of the optical constants for each layer were held constant and the only parameters that were fit were the thicknesses of layers 2−5 and the percentages pertaining to the EMA layer. To support the layered structure, we used a TEM to image a focused ion beam (FIB)-milled cross section of a lithiated aSi:H film (Figure S4 in the Supporting Information). The resulting layers imaged are in good agreement with those modeled by ellipsometry. Figure 4a demonstrates the changes in layers 2−5 using the above model to fit the ellipsometry data during the potential stepping between the lithiation and delithiation potentials. All fits had MSEs between 1 and 10 with no trend in MSE upon lithiation or delithiation. The EMA layer (3) is divided by the percentages obtained in the model to better visualize the lithiation front. This figure demonstrates that during lithiation there is a fast initial formation of the solid solution (EMA) layer. The percentage of this layer that represents the unlithiated a-Si:H slowly decreases as the experiment progresses. This is seen to correlate well with the chronoamperometry graph during lithiation, shown in Figure 4b, which demonstrates an initial increase in the current followed by a low, slowly decreasing current throughout the experiment. In contrast, delithiation shows a fast transition to the fully delithiated state. Again, this correlates well with the chronoamperometric graph in Figure 4c (time steps are

decreased for better visualization), which shows an initial large current that very rapidly decreases to the background. When the optical constants of each layer (1−5) are combined with their thicknesses, effective n and k values can be obtained using a thickness-weighted average of these optical constants for all five layers. These effective n and k values for the lithiated and delithiated states are found in Figure 5. These five layers are chosen for these effective n and k values because these are the layers that are found in the grating but not in the slit. Therefore, Δn and Δk found in eq 2 will be the difference between these values and the values of n and k for the electrolyte. When Figure 5 is analyzed in detail, it is found that the absorptivity component, k, increases for all wavelengths after lithiation, while for the index of refraction component, n, below ≃500 nm, there is a decrease upon lithiation and, above ≃500 nm, there is an increase upon lithiation. This compares well with the DE results shown in Figure 2, where the 475 nm wavelength results show an increase in the DE but the 540 and 630 nm wavelengths show a decrease, indicating that the change in the refractive index has a major effect on the DE. In fact, Schanze et al. previously reported that the DE is much more dependent on n than on k.27 When these effective changes in the optical constants and the previous lateral expansion results upon lithiation are utilized, the calculation for the volume change during lithiation/ F

DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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different colors were noticed, which is most likely due to differences in the thickness, density, and hydrogen content of aSi:H. Although care was taken to only analyze electrodes that visually had similar hue, minor changes in the composition may still exist. These changes can cause slight discrepancies in the value obtained for the optical components through ellipsometry with those actually found in the grating because these SE measurements are very sensitive to changes in the composition. Therefore, it is concluded that this analytical technique shows a lot of promise but is limited to only qualitative measurements by the lack of understanding of the SEI layer as well as the lack of reproducibility of the a-Si:H films.

delithiation using eq 2 is realized. For all calculations, the charge density accumulated during the experiment is used to assign the appropriate optical constants and lateral dimensions. Because this is the first method that is able to analyze the volume change at cycle numbers above the first few, Figure 6a

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CONCLUSION The volume change of a-Si:H thin-film gratings during cycling has been characterized through an in situ optical diffraction experiment. In order to obtain expansion in three dimensions, two diffraction experiments were performed: one to obtain information about the lateral expansion requiring a-Si:H to be synthesized on opaque PPF gratings (see the Supporting Information) and one to obtain thickness changes requiring synthesis on transparent PPF gratings. The lateral expansion of these gratings is shown to be less than 1% of their initial width. In order to acquire thickness changes using the transparent PPF gratings, optical components (refractive index, n, and absorptivity, k) of the a-Si:H thin film from in situ SE were needed and obtained. Using these components and lateral expansion measurements, it is shown that during cyclic voltammetry the volume change could be monitored well beyond the first cycle. For example, during the 10th cycle of aSi:H gratings, volume expansion begins during the cathodic sweep and continues through the first part of the anodic sweep until the value of the current recovers to its positive oxidative values, at which point the volume begins to contract back to its original value. This in situ optical diffraction method is presented here as a novel and relatively inexpensive way of obtaining volume change information beyond the first few cycles of a-Si:H thin films in a genuine battery environment. Currently, we are exploring ways to make this method more quantitative by trying to gain a good understanding of the dynamics of the SEI layer during cycling. Additionally, the quality of the a-Si films deposited by the current PECVD method adds variation in the data that needs to be better understood before the method is further extended and modeled.

Figure 6. (a) Calculated change in the thickness of an a-Si:H grating during its 10th cyclic voltammetry cycle. (b) 5 mV/s cyclic voltammetry curve transposed as current versus time for the 10th cyclic voltammetry cycle.

shows the thickness change calculated for the 10th cycle found in Figure 2 for the three different wavelengths. The quantitative thickness value increases with increasing wavelength, with the 475 nm wavelength showing a maximum increase of 22.8 nm (11.4% volume increase) and the 630 nm wavelength showing a maximum increase of 29.6 nm (14.8% volume increase). Although we observe these different quantitative thickness values, the qualitative changes in thickness during the potential sweeps are the same. Regardless of the wavelength analyzed, the increase in the calculated thickness does not begin until the onset of the lithiation current below 0.51 V. Interestingly, the thickness continues to increase even after the voltage sweep changes from cathodic to anodic, with an increase in the thickness continuing to occur until the current becomes anodic at around 0.51 V. The material then begins to contract back toward its initial thickness (although never quite reaching it) at potentials above 0.51 V. Therefore, it can be concluded that during cyclic voltammetry the a-Si:H’s volume expands at potentials below and contracts at potentials above 0.51 V versus Li/Li+ regardless of the sweep direction. Although these results are compelling, more quantitative results are still desired. It is speculated that the quantitative differences in the calculated thickness change versus wavelength analyzed may be associated with the SE analytical technique used here for the SEI layer. The dynamics of formation of the SEI layer and its resultant structure and composition is currently a large research topic in the literature and has previously been modeled with SE using multilayers.45,48,49 In addition, the apparent variation in the thickness change with wavelength can also be due to the inconsistencies of a-Si:H deposited by plasma-enhanced chemical vapor deposition (PECVD), where, depending on the position in the chamber,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03822. Evaluation of the lateral expansion of the grating, an annotated diffraction pattern demonstrating how the DE is determined, and a TEM image of a FIB-milled cross section of a cycled aSi:H film (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.J.S.). Present Addresses ‡

K.W.S.: Big Delta Systems, Inc., Houston, TX 77058. S.M.: Baker Hughes, Houston, TX 77019.

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DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the program “Understanding Charge Separation and Transfer at Interfaces in Energy Materials (EFRC:CST)”, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001091.

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DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b03822 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX