ARTICLE pubs.acs.org/JPCC
Mo3Sb7C Composite Anodes for Lithium-Ion Batteries Danielle Applestone, Sukeun Yoon, and Arumugam Manthiram* Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: Mo3Sb7C composite has been synthesized by first firing a mixture of Mo and Sb metals and then ball-milling the resultant material with carbon. X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) data reveal that these composites are composed of uniformly dispersed, submicrometer sized, crystalline Mo3Sb7 in a conductive carbon matrix. The presence of carbon in the composite drastically improves the cycle life of Mo3Sb7 as the carbon buffers the volume changes occurring during chargedischarge cycling. With a discharge capacity of 518 mAh/g and 907 mAh/cm3 and a tap density of 1.75 g/cm3 for the composite, the Mo3Sb7C composite anodes offer three times higher volumetric capacity than the graphite anode. Also, with an operating voltage (∼0.8 V) well above that of Li/Li+, the Mo3Sb7C composite anodes offer better safety.
1. INTRODUCTION Lithium-ion batteries are being widely used as power sources for portable electronic devices such as cellphones and laptops. They are also pursued intensively for plug-in hybrid electric vehicle applications. However, the currently used layered LiCoO2 cathode and the carbon anode have the drawbacks of limited energy density and safety problems. Particularly, chemical instability arising from an overlap of the Co:3d band with the top of the O:2p band in the LiCoO2 cathode, the formation of solid-electrolyte interfacial (SEI) layer by a reaction of the carbon anode surface with the electrolyte, and lithium plating in the carbon anode arising from a charge/discharge potential close to that of Li/Li+ pose serious safety concerns. These difficulties have created enormous interest in the development of alternative cathode and anode materials for lithium-ion batteries.13 With respect to alternative anodes, antimony alloys are appealing because they offer high theoretical capacity (gravimetric and volumetric) and an operating voltage well above that of metallic lithium. Unfortunately, the reaction of antimony with lithium to form LiSb is accompanied by a large volume change of 137%,48 which results in cracking and crumbling of the alloy particles, disconnection of the electrical contact between the particles and current collectors, and consequent capacity fade during cycling.9,10 To alleviate this problem, antimony-containing intermetallic compounds with different lithium reaction mechanisms have been pursued over the years, e.g., Cu2Sb,11 CoSb,12 CrSb,13 and MnSb,14 in which only Sb is electrochemically active, and SnSb,1517 InSb,18 Zn4Sb3,19 and AlSb,20 in which both the metals are electrochemically active. However, most of these intermetallic alloy anodes still exhibit capacity fade. With an aim to improve the cycle life of Sb-containing intermetallics, composites consisting of Mo3Sb7 and C were explored. The Mo3Sb7C composites offer the following r 2011 American Chemical Society
advantages as an anode material: (i) active antimony particles are constrained in the crystal structure of Mo3Sb7, which suppresses the agglomeration responsible for much of the capacity fade with antimony alloy electrodes and (ii) the carbon matrix surrounding the Mo3Sb7 particles acts as a buffer to alleviate the volume expansion. The Mo3Sb7C composites are prepared by heating first antimony and molybdenum metal in a furnace to obtain Mo3Sb7 and then high-energy mechanical milling (HEMM) of the resulting Mo3Sb7 with carbon. The ultrafine Mo3Sb7 particles dispersed in the carbon matrix are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and electrochemical chargedischarge measurements including impedance analysis.
2. EXPERIMENTAL SECTION The Mo3Sb7C composite was prepared as described below. First, the Mo3Sb7 alloy powders were obtained by heating a mixture of required amounts of Sb (99.9%, Aldrich) and Mo (99.8%, Aldrich) powders at 780 °C in a flowing 5% H2 atmosphere for 18 h. The Mo3Sb7 alloy obtained was then ground and sieved to eliminate particles over 100 μm. The Mo3Sb7 powder with particle size
