Journal of Power Sources 196 (2011) 7747–7749 Contents lists available at ScienceDirect Journal of Power Sources jo ur nal homep age: www.elsevier.com/locate/jpowsour Short communication Young’s modulus of polycrystalline Li 22 Si 5 J.B. Ratchford a,∗ , B.E. Schuster b , B.A. Crawford c , C.A. Lundgren a , J.L. Allen a , J. Wolfenstine a a U.S. Army Research Laboratory, Adelphi Laboratory Center, 2800 Powder Mill Road, Adelphi, MD 20783, United States b U.S. Army Research Laboratory, Aberdeen Proving Grounds, MD 21005, United States c Nanomechanics Incorporated, Analytical Services Laboratory, 105 Meco Lane, Suite 100, Oak Ridge, TN 37830, United States a r t i c l e i n f o Article history: Received 15 February 2011 Received in revised form 18 April 2011 Accepted 19 April 2011 Available online 23 April 2011 Keywords: Lithium Silicon Li22Si5 Young’s modulus Nanoindentation a b s t r a c t In order for Li–Si alloys to be used in Li-ion batteries as anodes, knowledge of their mechanical proper- ties, such as Young’s moduli, is crucial. Young’s modulus of polycrystalline Li 22 Si 5 was determined from nanoindentation testing. The value of Young’s modulus was 35.4 ± 4.3 GPa. This value is approximately one-half of the predicted value based on density functional theory calculations. This difference was not a result of the testing procedure or microstructural variables. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Many researchers have investigated alternative electrode mate- rials to graphite that would significantly increase the anode capacity of lithium-ion batteries [1–5]. Li–Si alloys are potential alternatives, in particular Li 22 Si 5 , which has a capacity 10 times (4200 mAh g -1 ) greater than graphite (372 mAh g -1 ) [1–5]. How- ever, the capacities of Li–Si alloys decrease with cycling as a result of substantial volume changes with Li-ion addition/removal that cause the alloy to fracture. In order to solve the fracture problem, knowledge of the mechanical properties for the alloy is required; in particular, Young’s modulus [6,7]. For example, Cheng and Ver- brugge [6] have stated “Experiments and theoretical calculations are also urgently needed to provide material parameters, such as Young’s modulus E and effective surface energy  eff that are seldom available, for quantitative predictions of fracture and decripitation of lithium-ion battery electrodes.” Values of Young’s moduli for the Li–Si alloys were unknown until the very recent predictions of Shenoy et al. [7] that were based on density functional theory (DFT). For example, Shenoy et al. [7] predicted that Young’s modulus for a polycrystalline Li 22 Si 5 alloy was ∼78 GPa. However, no experimentally measured value exists to verify this predicted value. Therefore, the objective of this study was to experimentally determine the value of Young’s ∗ Corresponding author. Tel.: +1 301 394 0476; fax: +1 301 394 0273. E-mail address: joshua.ratchford@us.army.mil (J.B. Ratchford). modulus for polycrystalline Li 22 Si 5 using nanoindentation test- ing. 2. Experimental To synthesize polycrystalline Li 22 Si 5 , stoichiometric amounts of silicon powder and lithium granules were mixed and then pel- letized. The pellet was placed inside a molybdenum crucible. The crucible containing the pellet was heated from 20 to 800 ◦ C over the course of 40 min and then held at 800 ◦ C for an additional 30 min. The sample was then heated at 450 ◦ C for 16 h to ensure its homo- geneity before it was slowly cooled to 20 ◦ C [9]. All syntheses were performed in glove box filled with argon, which contained less than 1 ppm oxygen and 1 ppm water. To determine the phase purity of Li 22 Si 5 , ∼100 mg of the sample was removed from the center of the crucible, ground into fine pow- der, and analyzed by X-ray diffraction. Because the alloy is sensitive to ambient moisture, the sample was hermetically sealed with Kap- ton film. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to confirm the weight percentage of lithium in the syn- thesized Li 22 Si 5 . To prepare samples of polycrystalline Li 22 Si 5 for mechanical testing and microstructural analysis, granules of the sample were removed from the center of the crucible, then cold mounted and polished using standard metallographic techniques. Because the alloy is water reactive, the papers/clothes were lubri- cated with mineral oil instead of water. The surfaces of the granules were fine polished until a mirror finish was obtained. To determine the grain size of the alloy, an etching solution was developed. The composition of this solution was 0.2% by mass water and 0.3% by 0378-7753/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2011.04.042