Synthesis and Electrochemical Performance of Low Surface Area Alloy Anodes for Lithium-Ion Battery
Alloy negative-electrode materials could significantly increase the energy density of commercial Li-ion batteries . From a practical point of view, alloys prepared by ball milling are inexpensive lithium hosts and easily scalable. However, such alloys typically have small particle sizes and high surface area, increasing their reactivity with electrolyte. Here, we describe a composite alloy material which has been post treated to significantly reduce surface area. The resulting composites have significantly reduced surface area and improved cycling performance over ball milled alloys of similar composition.
Alloys were prepared ball milling stoichiometric amounts of Si and Fe. Post-treated alloys were ground and sieved to a 53 μm particle size. Electrodes comprising active material, carbon black and polyimide binder were cycled in 2325 coin-type cells with two Celgard 2300 separators, a lithium foil counter/reference electrode and 1M LiPF6 in EC/DEC/FEC 60/30/10 by volume electrolyte at 30 °C. Cells were cycled at a C/4 rate and trickled discharged to C/20 in a voltage range of 0.005 V-0.9 V. X-ray diffraction (XRD) measurements were collected using a Rigaku Ultima IV diffractometer with a Cu Kαsource. A Phenom G2-pro Scanning Electron Microscope (SEM, Nanoscience, Arizona) was used to study the particle size and morphology of the samples. Surface area was determined by single-point Brunauer, Emmett, and Teller (BET) method using a Micromeritics Flowsorb II 2300 surface area analyzer.
Figure 1 shows the XRD pattern of ball-milled Si/Fe alloy, which consists of XRD peaks from Si, Fe and FeSi2 phases. Figure 2 shows the SEM images of the Si/Fe and post-treated alloy particles. For Si/Fe, many fine particles with a diameter less than 3 μm can be observed along with a few large particles. The particle size range for the post-treated particles is between 10 and 40 μm. BET surface area measurement shows that the post treated particles have a significantly reduced surface area compared to the Si/Fe ball milled alloy (2.6 m2/g for Si/Fe, 0.9 m2/g for post-treated particles), which is consistent with the SEM results.
Figure 3 shows the cycling performance of Si/Fe and the post-treated particles. The post-treated particles have a reversible capacity of 600 mAh/g after the first cycle and good capacity retention. By contrast, the capacity of the ball milled Si/Fe alloy electrode fades gradually during cycling.
The authors acknowledge funding from NSERC and 3M Canada, Co. Xiuyun Zhao acknowledges the support from the DREAMS program.