Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Surfaces, Interfaces, and Applications
Mechanical Properties and Chemical Reactivity of LixSiOy Thin Films Yun Xu, Caleb Stetson, Kevin Wood, Eric Sivonxay, Chun-Sheng Jiang, Glenn Teeter, Svitlana Pylypenko, Sang-Don Han, Kristin Persson, Anthony K. Burrell, and Andriy Zakutayev ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10895 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Mechanical Properties and Chemical Reactivity of LixSiOy Thin Films Yun Xu a, Caleb Stetson b,c, Kevin Wood a, Eric Sivonxay d , Chunsheng Jiang a, Glenn Teeter a, Svitlana Pylypenko c, Sang-Don Han b, Kristin A. Persson d,e, Anthony Burrell b, Andriy Zakutayev a* a
Materials Science Center, National Renewable Energy Laboratory, Golden, CO, 80401,
USA b
Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden,
CO, 80401, USA c
Department of Chemistry, Colorado School of Mines, Golden, CO, 80401, USA
d
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
e
Department of Materials Science & Engineering, University of California, Berkeley, CA
94720-1760, USA KEYWORDS: mechanical properties, chemical reactivity, solid electrolyte interphases, LixSiOy, lithium ion batteries
ACS Paragon Plus Environment
1
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 23
ABSTRACT: Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO2 on Si is often inevitable. However, it is not clear if this layer has positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties of the SiO2 lithiation products, and by convolution of chemical and electrochemical effects during the anode lithiation process. In this study, LixSiOy thin films as model materials for lithiated SiO2 were deposited by magnetron sputtering at ambient temperature, with the goal of 1) decoupling chemical reactivity from electrochemical reactivity, and 2) evaluating the physical and electrochemical properties of LixSiOy. XPS analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO2, can be achieved through sputtering. Our density functional theory calculations also confirm that possible phases formed by lithiating SiO2 are very close to the measured film compositions. Scanning probe microscopy measurements show the mechanical properties of the film are strongly dependent on lithium concentration, with ductile behavior and higher Li content, and brittle behavior at lower Li content. Chemical reactivity of the thin films was investigated by measuring AC impedance evolution, suggesting that LixSiOy continuously reacts with electrolyte, in part due to high electronic conductivity of the film determined from solid state impedance measurements. Electrochemical cycling data of sputter deposited LixSiOy/Si films also suggest that LixSiOy is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.
ACS Paragon Plus Environment
2
Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Introduction Silicon (Si) is considered to be one of the leading candidates for the next generation anode materials in Li-ion batteries, as it features a theoretical capacity more than 10 times higher than graphite.1 However, one of the major challenges of silicon is the large volume change during the lithiation and delithiation processes. This volume change eventually leads to the breakdown of the solid electrolyte interphase (SEI), and further reactions that cause increased electrolyte consumption, higher interphase resistance, and the ultimate failure of the cell.2 There have been extensive studies on the SEI formation on the surface of silicon.3 While various reports on the overall composition of the SEI are reaching some consensus, the understanding of its physical properties and chemical reactivity is still limited. Most work is focused on the SEI composition through characterization by Fourier Transform Infrared Reflectance (FTIR), and X-ray Photoemission Spectroscopy (XPS),4-9 while less research has been done to separately study SEI properties independent of the electrode10. Theoretically, a good SEI should be flexible or soft enough to accommodate the large volumetric change of the silicon. In addition to the mechanical properties of the SEI, high ionic conductivity and low electronic conductivity are also important factors in determining if a certain phase is beneficial to electrode stability. It has been hypothesized, that one of the most important SEI components for Si anodes in Li-ion batteries is LixSiOy, which originates from the lithiation of the native oxide on Si (SiO2). The role of SiO2 on a Si anode is controversial: on one hand, it confines the volume expansion of Si, while on the other hand it induces interfacial reactions that
ACS Paragon Plus Environment
3
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 23
consume lithium in the electrolyte. Lithiation of SiO2 is a complicated process, and the products depend on the charge state of the battery. 11-13 General findings are SiO2 can be reduced to possible products of Li2Si2O5, Li4SiO4, Li2O, Si, and LixSi.10 Nevertheless, most of the recent model system studies are focused on the fully oxidized lithium silicates such as Li4SiO4, Li2SiO3, Li2Si2O5.7, 14-16 In this work, the mechanical and electrochemical properties, as well as the chemical reactivity of the composite LixSiOy film are investigated. For this purpose, a special combinatorial LixSiOy composite film with composition close to the lithiated SiO2 was synthesized by reactive sputtering, allowing to study its mechanical properties without any effect from electrolyte decomposition products. According to XPS results, both the lithium-rich and silicon-rich areas consist of lithium silicates, some LixSiy and very little SiO2. The film compositions agree well with the phase diagram predicted by DFT calculations. It is found that the composite LixSiOy film exhibits relatively-high electronic conductivity and low hardness, based on scanning probe microscopy results and impedance measurements. Computed bulk moduli shows the same trend as the experimental results, specifically that higher Li content leads to lower moduli. To evaluate the chemical reactivity of these materials, a multilayer thin film LixSiOy on Si was measured using impedance spectroscopy in a coin cell followed by electrochemical charge/discharge cycling. The results indicate that the composite film is unable to fully passivate the Si surface, due to its high electronic conductivity, and despite its ductile mechanical properties.
ACS Paragon Plus Environment
4
Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Methods LixSiOy thin films were synthesized by RF magnetron sputtering (13.56MHz) using lithium target (Lesker, 99.9%, 2" diameter) and silicon target (Lesker, 99.999%, 3" diameter). The lithium target was cleaned with hexane (99.9%) before use to remove residual mineral oil, which protects lithium target from being oxidized during the shipment. Furthermore, oxidized film on top of lithium target was periodically removed by Dremel tool with copper brush. During the deposition, 30 W RF power was applied to both lithium and silicon targets. Copper foil and Eagle XG glass were used as substrate for electrochemical measurement and scanning probe microscopy measurement respectively. To generate plasma, Ar (99.999% purity) was introduced at a constant pressure of 3 mTorr into a vacuum chamber with 3x10-8 Torr base pressure. The film was deposited at room temperature, so it is expected to be amorphous (also confirmed by xray diffraction). After the synthesis, the samples were removed from a chamber to a glovebox. Airless transfer in a vacuum vessel to all characterization instruments was adopted because of the high reactivity of LixSiOy. As a reference for comparison, thin films of Li-free Si and LixSiOy/Si were made using the same method. The XPS measurements were performed using a glovebox-integrated Phi 5600 XPS system. Glovebox conditions were better than
