Article Cite This: ACS Appl. Energy Mater. 2019, 2, 5133−5139
www.acsaem.org
Understanding the Mechanism of Enhanced Cycling Stability in Sn− Sb Composite Na-Ion Battery Anodes: Operando Alloying and Diffusion Barriers W. Peter Kalisvaart,*,†,‡ Hezhen Xie,†,‡ Brian C. Olsen,†,‡ Erik J. Luber,†,‡ and Jillian M. Buriak*,†,‡ †
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, AB T6G 2G2, Canada Nanotechnology Research Center, National Research Council Canada, 11421 Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada
Downloaded via BUFFALO STATE on August 1, 2019 at 12:23:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Sn−Sb composites are of great interest for high capacity sodium-ion batteries due to their high stability, but because multiple phases and alloys are formed during cycling, the roles of each are challenging to deduce. In this work, two approaches were taken to investigate the importance of β-SnSb formation on the cycling stability of Sn-rich Sn−Sb composite sodium-ion battery (SIB) anodes. First, to tease out the role of each component, thin layers of amorphous silicon, with thicknesses ranging from 0.5 to 10 nm, were incorporated between Sn and Sb layers, of equal thicknesses. Silicon has low solubility in both tin and antimony, and thus acts as a barrier layer that can interfere with the formation of Sn−Sb alloys. The equivalent composition of this sandwich structure was Sn53Sb47. Upon electrochemical cycling, a clear correlation between capacity retention and Si thickness was observed, and it was found that a 1 nm thick Si layer was sufficient to inhibit the formation of the β-SnSb intermetallic, resulting in loss of the capacity of the tin layer after a few tens of cycles. The second approach involved capping a Sn film with increasingly thicker Sb layers. Thicker antimony layers were found to have a large positive influence on cycling stability with a marked drop-off in the capacity retention when there is not enough Sb to fully convert the bilayer into β-SnSb. These results point to the necessity of the Sn and Sb being in intimate contact prior to cycling for the β-SnSb phase to form in operando, which is necessary for the excellent capacity retention of the Sn−Sb system. KEYWORDS: sodium-ion batteries, silicon, antimony, multilayers, cycling, tin
■
upon deposition. Surprisingly, the β-SnSb in the bilayers does not undergo the same drastic reduction in grain size upon cycling. After 150 cycles, both bilayer films consisted of βSnSb, with small amounts of Sn. All evidence of elemental Sb disappeared from the diffraction patterns. The evolution of the diffraction patterns combined with that of the voltage profiles and dQ/dV curves strongly suggests a progressive reaction between Sn and Sb to β-SnSb, induced by sodiation/ desodiation cycling.13 In the present paper, we further test the strength of the interaction between Sn and Sb during sodiation cycling and the positive effects of β-SnSb formation on stability during sodiation cycling. Successively thinner barrier layers of Si, between 10 and 0.5 nm, were introduced between the Sn (top) and Sb (bottom) layers in Sn(50−x/2 nm)/Si(x nm)/Sb(50− x/2 nm) trilayers, which are referred to as Sn/Si(x nm)/Sb
INTRODUCTION Sodium-ion battery (SIB) anode materials based on alloying reactions are being intensively investigated due to their very high specific capacities.1−6 Although it has one of the highest theoretical reversible capacities among potential sodium-ion battery (SIB) anode materials (847 mAh/g), elemental tin is typically observed to have very low capacity retention, on the order of tens of cycles.7−11 However, antimony and the tincontaining intermetallic, β-SnSb, perform quite well with respect to capacity retention, both as thin films and as powders and composites.7,12−19 In situ transmission electron microscopy (TEM) experiments elucidating the reaction mechanisms during sodiation−desodiation cycling of β-SnSb showed that, after the first cycle, the β-SnSb intermetallic assumes a nanocrystalline13 (nc) or amorphous20 structure that persists through 100−150 cycles. Sodiation induces phase segregation, with nc-Na3Sb being formed first, followed by Na15Sn4. Sn/Sb bilayers, regardless of the order of deposition, also exhibited improved capacity retention compared to elemental Sn, where β-SnSb intermetallic forms at the interface between the layers © 2019 American Chemical Society
Received: April 25, 2019 Accepted: June 7, 2019 Published: June 7, 2019 5133
DOI: 10.1021/acsaem.9b00819 ACS Appl. Energy Mater. 2019, 2, 5133−5139
Article
ACS Applied Energy Materials
carbonate (DEC) with 10 vol % fluoroethylene carbonate (FEC) as additive. Electrochemical measurements were performed on an Arbin BT2000 battery testing system. Each trilayer sample was subjected to 100 sodiation/desodiation cycles and the bilayers were cycled 200 times at 100 mA/g in the first and 200 mA/g in subsequent cycles. The lower and upper cutoff potentials were 0.01 and 2 V vs Na/Na+, respectively. Characterization. X-ray diffraction (XRD) analysis on asdeposited and cycled films was performed on a Bruker AXS diffractometer (Discover 8, Bruker, Madison, WI) or a Rigaku Ultima IV multipurpose diffractometer. The Bruker diffractometer was equipped with a Histar general-area two-dimensional detection system (GADDs) with a sample−detector distance of 220 mm. The Rigaku diffractometer has a 285 mm goniometer radius and a linear detector. Both use Cu Kα radiation (λ = 1.5406 Å). Phases were identified using the database of EVA software. Cycled films were rinsed with DEC and dried prior to analysis. Scanning electron microscopy (SEM) was performed on a ZEISS Sigma field-emission SEM operating at 5 kV and 20 μA.
throughout. Both Sn and Sb possess primarily substitutional solubility in Si, with very low solubility limits of less than 0.15%,21,22 attributed to their large atomic size mismatches causing significant atomic strains upon substitution in the Si lattice. As such, Si is expected to block, or at the very least hinder interdiffusion of Sn and Sb and limit the formation of the β-SnSb intermetallic. However, evidence for interdiffusion between amorphous Si and Sb in Si/Sb bi- and multilayer films was found, but only when the Sb thickness was less than or equal to the thickness of Si.23,24 A series of Sb(x nm)/ Sn(100−x nm) bilayers is also tested where the Sb thickness is varied from 0 to 65 nm. Capping layers of Sb lead to dramatic improvements in capacity retention for Sb/Sn bilayers even below 50 nm Sb thickness, which is not enough to fully convert the Sn to β-SnSb. A strong correlation between the amount of β-SnSb and capacity retention is found in Sn/Si/Sb trilayers, which both decrease with increasing Si thickness, emphasizing β-SnSb’s central role in improving stability of Sn during sodiation cycling.
■
■
RESULTS AND DISCUSSION XRD patterns of as-deposited trilayer films are shown in Figure 2, which were recorded at an incident angle of 1°. For trilayer
EXPERIMENTAL METHODS
Solvents and Reagents. Dichloromethane and 2-propanol were purchased from Fisher Scientific. Fluoroethylene carbonate (99%) and metallic sodium (99.9%) were purchased from Sigma-Aldrich. Sodium perchlorate (>98% purity), ethylene carbonate (99% purity), and diethyl carbonate (>99%) were purchased from Alfa Aesar. Film Deposition. Boron-doped Si with resistivity
