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

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

Ex-situ depth-sensing indentation measurements of

Electrochemistry Communications 13 (2011) 818–821Contents lists available at ScienceDirectElectrochemistry Communicationsjournal homepage: www.elsevier.com/locate/elecomEx-situ depth-sensing indentation measurements of electrochemically producedSi–Li alloy filmsBenjamin Hertzberg, Jim Benson, Gleb Yushin ⁎School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USAarticleinfoabstractArticle history:Received 19 April 2011Received in revised form 10 May 2011Accepted 11 May 2011Available online 18 May 2011Keywords:AnodeAlloyLithiumSiliconModulusHardnessWe report for the first time ex-situ measurements of the Young's modulus and hardness of Si and Si–Li alloythin film electrodes at various stages of lithium insertion via depth-sensing indentation experiments. Lithiumis electrochemically inserted into the Si electrode, after which its mechanical properties are investigated in ananoxic environment. The Young's modulus was found to decrease from an initial value of 92 GPa for pure Si to12 GPa at full lithium insertion (Li 15 Si 4 ). The hardness changes from an initial value of 5 GPa for Si to 1.5 GPafor Li 15 Si 4 .© 2011 Elsevier B.V. All rights reserved.1. IntroductionSilicon (Si) anodes have been the subject of extensive investigationin the last years [1–14]. The challenges of maintaining the integrity ofSi electrodes due to the dramatic expansion of the material during Liinsertion remain the major obstacle [6]. In a recent study Si placed in aconfined space was found to mechanically deform during Li insertion[7]. When Si nanoparticle anodes are produced with rigid binders[8,11,12], one may expect that a similar type of deformation may takeplace during the expansion of the particles into the differently shapedelectrode pores. The endurance of the Si-binder interface, critical forstable anode operation, must exceed the amplitude of the interfacialstresses caused by the initial expansion of Si particles, which are, inturn, dependent on the plasticity and elastic modulus of lithiated Si.Unfortunately, little experimental data are available on the mechanicalproperties of Si lithiated to various degrees. Such information is,however, crucial for the design and successful implementation ofstable Si-based anodes.By using density functional theory (DFT) Shenoy et al. calculated theYoung's modulus and Poisson ratios of both amorphous and crystallineLi–Si phases at a range of different concentrations [13]. Interestingly, forLi fraction changes from 0.5 to 0.76 (LiSi to Li 13 Si 4 ) the modulus wasfound to exhibit minimal changes [13]. Using electrochemical strainmicroscopy Kalinin et al. were able to directly map the lithiation/delithiation process with a nanometer-level resolution [14]. In the initial⁎ Corresponding author. Tel. +1 4043853261.E-mail address: yushin@gatech.edu (G. Yushin).charging process, the grain boundaries were observed to becomeheavily lithiated first; subsequently, the lithiated areas expandedinward into the higher-density areas as the anode approached fulllithiation [14]. Sethuraman et al. proposed a novel method to performin-situ measurements of the stress in thin Si–Li alloy film duringlithiation and delithiation by monitoring changes in the curvature ofsubstrate using a multi-beam optical sensor [15]. While the authorsconcluded that the biaxial modulus could be approximated by the rule ofmixtures, they observed rather small changes in the modulus when Lifraction changes in the range from 0.3 to 0.75 [15,16].In this study, we employed depth-sensing indentation (nanoindentation)measurements to acquire direct ex-situ experimentalvalues of the Young's modulus and hardness of nanocrystalline Sifilms at various stages of lithiation. The obtained results suggest somedeviation in the Young's modulus from the predictions of a simple ruleof mixing, which agrees with the actual data collected in priortheoretical and experimental studies [13,15], but disagrees with thesimplified conclusions of their authors.2. Experimental methodsIn order to avoid substrate effects in our nanoindentationmeasurements, a thick (~6 μm) Si film was used in our experiments.Si film was deposited using chemical vapor deposition (CVD) at750 °C. Silane (SiH 4 ) diluted to 5 wt.% with Ar was selected as theprecursor gas. The deposition was performed at a low-pressure(~1 Torr) to achieve high coating uniformity [7,9]. Carbon-coatednickel foils with a total thickness of ~120 um were used as substrates1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.elecom.2011.05.011

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