UT Austin team synthesizes molybdenum-antimony composite material for Li-ion anode; 3x the volumetric capacity of a graphite anode with better safety, but poor cycle life
Researchers at the University of Texas at Austin report the synthesis of a molybdenum-antimony composite (Mo3Sb7–C) for use as anode material for a lithium-ion battery in a paper in the ACS Journal of Physical Chemistry C. 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 Mo3Sb7–C composite anodes offer three times higher volumetric capacity than the graphite anode.
|Comparing the cyclability of Mo3Sb7, Mo3Sb7-C, and graphite. Credit: ACS, Applestone et al.Click to enlarge.|
Also, with an operating voltage (0.8 V) well above that of Li/Li+, the Mo3Sb7–C composite anodes offer better safety, they say.
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%, 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. To alleviate this problem, antimony-containing intermetallic compounds with different lithium reaction mechanisms have been pursued over the years, e.g., Cu2Sb, CoSb, CrSb, and MnSb, in which only Sb is electrochemically active, and SnSb, InSb, Zn4Sb3, and AlSb, 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 Mo3Sb7-C composites offer the following 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.—Applestone et al.
The material was 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, sub-micrometer 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 charge–discharge cycling, they found.
Although the material showed higher discharge capacity than graphite, they team also found that the Mo3Sb7-C composite begins to exhibit capacity fade at around 70 cycles. They suggested that future work by incorporating oxides such as Al2O3 or MoO2 to obtain composites and optimizing the ratios of Mo3Sb7, C, and Al2O3 or MoO2 could improve the cycle life at higher number of cycles.
Danielle Applestone, Sukeun Yoon and Arumugam Manthiram (2011) Mo3Sb7–C Composite Anodes for Lithium-Ion Batteries. The Journal of Physical Chemistry C 115 (38), 18909-18915 DOI: /10.1021/jp206012