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Ex-situ depth-sensing indentation measurements of ...

Ex-situ depth-sensing indentation measurements of ...

B. Hertzberg et al. /

B. Hertzberg et al. / Electrochemistry Communications 13 (2011) 818–821819for Si deposition and as current collectors for electrochemicalmeasurements.For electrochemical Li insertion experiments, Si film-coated Ni foilelectrodes were assembled into 2016 lithium-ion coin cells inside anAr filled glovebox. 1 M lithium hexafluorophosphate (LiPF 6 ) salt,dissolved in ethylene, dimethyl, and diethyl carbonate (1:1:1 wt. %solution), was used as an electrolyte. Li foil (electrochemical grade,Alfa Aesar) of 1 mm in thickness was used as a counter electrode. Liinsertions into Si films were performed in a series of two steps. First, aconstant current was applied at a slow C/40 rate until the potentialreached a pre-selected value. This was followed by a constantpotential step, at which the selected potential was applied to thecell until the current dropped to 1–5% of the C/40 current appliedduring the constant current step.To prepare samples for mechanical measurements, the lithiated Sielectrodes were removed from the coin cell in an Ar box and mountedon a stainless steel substrate using a thermoplastic polymer adhesive.Great care was taken in order to avoid a reaction of lithiated Si with airor moisture during the time between cell disassembly and depthsensingindentation measurements inside a nanoindenter chamberfilled with Ar. After mounting on a rigid stainless steel substrate, thesamples were dipped into an anhydrous dimethyl carbonate (DMC)filled vial, placed inside a double-walled Ar-filled container andsealed. Before indentation, ultra high purity Ar (99.9999%, Air Gas)was purging the sealed indentation chamber at 0.5 scfm for 15 minand continued for the duration of the experiments. During the short(~10 s) transfer of the samples from the container to the Ar-fillednanoindenter chamber, the samples were coated with an ~1 mm layerof DMC, which evaporated in the chamber in about 30 min during acontinuous Ar purge.Depth-sensing indentation measurements were performed on aHysitron TI 900 TriboIndenter. A diamond cube corner tip (Northstar,radius b40 nm) was used for the measurements. Calibration wasperformed using a polycarbonate standard with Young's modulus3.1 GPa and a hardness of 200 MPa, and a 6 coefficient fitting curve.Indentations were done using max loads of 6000–8500 μN. Theloading and unloading rates ranged from 300 to 350 μN/s and werethe same for loading and unloading. 16 indentations were made ineach sample using the same experimental parameters and spaced10 μm apart. The average values for the modulus and hardness arereported.Reduced modulus values were calculated from the unloadingcurve using the methods developed by Oliver and Pharr [17]. Area wasdetermined by calibration of the tip geometry as a function ofpenetration depth. Stiffness measurements were made by fitting theunloading of an indent to a power law function using a regressionanalysis. This allows the slope at the initial unloading to be calculatedanalytically and reduces the dependence of the method on theamount of creep occurring in the samples. The hardness wascalculated using the calibrated area function and the maximum loadof the indent. While localized stresses produced during nanoindentationmay create concentration gradients and local variations inmechanical properties [18], these effects were not considered in ourcalculations.3. Results and discussionFig. 1a shows the cross-sectional micrograph of the deposited Sifilm. The films' thicknesses obtained from SEM were in a goodagreement with what was obtained from the mass changes observedin the sample after CVD deposition, assuming the density of thedeposited Si to be 2.3 g/cm 3 . XRD confirmed the formation ofnanostructured Si. In contrast to Si CVD deposited at 500 °C, whichshowed XRD-amorphous microstructure [7,9], the deposition performedfor the current studies at 750 °C revealed distinct Si peakformation. Fig. 1b shows a typical high-resolution portion of the XRDpattern for the as-deposited unlithiated Si thin film. The broad Si peakvisible at 28.7 corresponds to diffraction from the {111} family ofplanes. Using an instrumental standard and the Scherrer method, theaverage grain size was calculated as 74 nm.Fig. 2a shows an example of the galvanostatic Li insertion profilefor the produced Si film anodes. The shape of the profile is similar tothe profiles previously reported in literature for other Si electrodes(see e.g. Refs. [3,9,15]). In contrast to intercalation-type electrodematerials, these profiles do not exhibit horizontal plateaus and cover alarger potential range. The following constant potentials wereselected for the electrochemical alloying: 0.174 V (for LiSi), 0.137 V(for Li 1.68 Si), 0.078 V (for Li 2.61 Si) and 0.010 V (for Li 3.75 Si). Due to thelarge Si film thickness and high Si capacity, Li losses due to SEIformation (no more than 1%, estimated) were neglected in ourcalculations of the alloys' composition.Examples of typical load vs. displacement curves for nanoindentationson lithium-silicon thin films are presented in Fig. 2b. Thesegraphs show the load-unload behavior for pure Si films and for Si filmslithiated to different final compositions. We can see the progressivetransformation of the material as more lithium is inserted. At the samevalue of the applied load the nanoindenter tip penetrates to largerdepth for higher Li content, indicating a systematic reduction inhardness upon Li insertion into Li–Si alloy. The slope of the unloadingcurve also decreases with increasing degree of lithiation, indicating areduction in Young's modulus. Occasionally, an “elbow” in theunloading curves of lithiated samples (Fig. 2c) was observed. Inpure Si this feature is associated with pressure induced crystalline-toamorphousphase transformations [19]. If this is also true for Li–Sialloys, then this observation suggests the presence of nanocrystallinegrains in all the lithiated films.Fig. 2d shows the load vs. displacement curves for the Li 1.08 Sisample. While some variation in the loading curves is observed, theslopes of the unloading curves are commonly very similar, indicatinghigh degree of homogeneity of the Young's modulus throughout theFig. 1. (a) Cross-sectional SEM micrograph of a CVD-deposited Si film on C-coated Ni foil substrate, (b) high-resolution X-ray diffraction pattern of the Si film.

820 B. Hertzberg et al. / Electrochemistry Communications 13 (2011) 818–821aPotential (V) 0.6 SiLi 1.08 SiLi 2.06 SiVolume Fraction of LiSi 0.00Li 0.6 Si 0.39Li 1.08 Si 0.53Li 2.06 Si 0.67Li 3.75 Si 0.78Li 1Li 3.75 SibLoad (µN)100008000600040002000as-deposited SiLi 0.6 SiLi 1.08 SiLi 2.06 SiLi 3.75 Si0.000500 1000 1500 2000 2500 3000 3500 4000 0 100 200 300 400 500 600 700 800 900 10001100Capacity (mAh/g)Depth (nm)cLoad (uN)90008000700060005000400030002000100000Li 2.06 Sielbow100 200 300 400 500 600 700 800 900 10001100Depth (nm)dLoad (uN)90008000700060005000400030002000100000Li 1.08 Si100 200 300 400 500 600 700 800 900 10001100Depth (nm)eYoung's Modulus (GPa)12011010090807060504030201000.0fDFT calculationsadopted from [13] for5.5new data crystalline and amorphous phases5.04.54.0new data3. of mixtures1.0adopted from [15]0.5rule of mixtures0.00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Volume fraction of LiVolume Fraction of LiHardness (GPa)Fig. 2. (a) Electrochemical profile for lithium insertion into nanocrystalline Si film; (b) load-displacement curves for Si film samples at various stages of lithiation; (c) a loaddisplacementcurve with an “elbow” in the unloading curve; (d) load-displacement curves for Si film lithiated to Li 1.08 Si; (e) Young's modulus of Li–Si alloys as function of volumefraction of Li; (f) hardness of Li–Si alloys as function of volume fraction of Li.sample. The slopes of the unloading curves were used to determinethe average Young's modulus of our samples. Fig. 2e shows thechanges in the Young's modulus as a function of the volume fraction oflithium in the film, calculated assuming theoretical values for thedensity and molar mass of Li and Si. For comparison purposes, wehave also included a theoretical measurement of the Young's modulusbased on a linear rule of mixtures:E = E Li V Li + E Si V Siwhere V Li and V Si are the volume fractions of Li and Si, respectively, inthe film, and E Li and E Si are the experimentally determined modulus ofmicrocrystalline Li and nanocrystalline Si films, respectively. Note thatthe atomic and volume fractions are very similar (b4% difference).The Young's modulus of nanocrystalline Si film measured from theload-unload curves was found to be 92 GPa, close to the valuesreported for amorphous (90 GPa) silicon [13]. The Young's modulus ofLi was determined to be 5 GPa, consistent with the reported values[15]. Our experiments show when the thin film is lithiated to Li 0.6 Si,the Young's modulus decreases linearly to a value of 58 GPa, as wouldbe predicted by the rule of mixtures. Interestingly, however, higherdegree of lithiation to ~Li 1.08 Si and Li 2.06 Si leads to the increase in theYoung's modulus. A similar increase was also observed in Ref. [15],although the absolute values of the Young's modulus are higher in ourmeasurements. Specifically, in this range the Young's modulusremains constant at ~70 GPa, significantly exceeding the linear ruleof mixtures. This value, however, is an average between the DFTpredictedYoung's modulus for amorphous and crystalline phases ofsimilar compositions [13]. The value of the Young's modulus is related tothe average stiffness of inter-atomic bonds. However, the charge state ofindividual atoms, the ionic character and stiffness of individual Si–Libonds (and to a smaller degree, the stiffness of Si–Si and Li–Li bonds) as

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