Danielle Applestone is a Ph.D. candidate completing her dissertation this semester under the supervision of Dr. Arumugam Manthiram. Her dissertation is entitled,”Synthesis and Characterization of Nanocomposite Alloy Anodes for Lithium-Ion Batteries.”
Lithium-ion batteries are most commonly employed as power sources for portable electronic devices. Limited capacity, high cost, and safety problems associated with the commercially used graphite anode materials are hampering the use of lithium-ion batteries in larger-scale applications such as the electric vehicle. Nanocomposite alloys have shown promise as new anode materials because of their better safety due to higher operating potential, increased energy density, low cost, and straightforward synthesis as compared to graphite. The purposes of this research were to investigate and understand the electrochemical properties of several types of nanocomposite alloys and to assess their viability as replacement anode materials for lithium-ion batteries.
Tin and antimony are two elements that are active toward lithium. This dissertation research was focused on tin-based and antimony-based nanocomposite alloy materials. Tin and antimony each have larger theoretical capacities for lithium insertion than commercially available anodes, but the capacity fades dramatically in the first few cycles when metallic tin or antimony is used as the anode material in a lithium-ion battery. This capacity fade is largely due to the agglomeration of particles in the anode material and the formation of a barrier layer between the surface of the anode and the electrolyte. In order to suppress agglomeration, the active anode material can be constrained by an inactive matrix of material that makes up the nanocomposite. By controlling the surface of the particles in the nanocomposite via methods such as the addition of additives to the electrolyte, the detrimental effects of the solid-electrolyte interphase layer (SEI) can be minimized, and the capacity of the material can be maintained. The nanocomposite alloys described in this dissertation can be used above the voltage where lithium plating occurs, thereby enhancing the safety of the anode.
High-energy mechanical milling and furnace heating were the synthesis methods used for the materials in this work. XRD, SEM, STEM, TEM, and XPS were used for characterization of the materials. Electrochemical cells were made with the materials in order to study the performance at various temperatures, potential ranges, and charge rates. The lithiation/delithiation reaction mechanisms for these nanocomposite materials were explored using ex-situ XRD.
Three different nanocomposite alloy anode materials were developed over the course of this research: Mo3Sb7-C, Cu2Sb-Al2O3-C, and Cu6Sn5-TiC-C. Mo3Sb7-C is a material with high gravimetric capacity and a reaction mechanism whereby crystalline Mo3Sb7 disappears and is reformed during each cycle. Cu2Sb-Al2O3-C is a material with small particle size (2 – 10 nm) and long cycle life (+ 500 cycles), and it is made through a one-step synthesis process. The reversibility of the reaction of Cu2Sb-Al2O3-C with lithium is improved when longer milling times are used for synthesis. The reaction mechanism for Cu2Sb-Al2O3-C appears to be dependent upon the size of the crystalline Cu2Sb particles. The coulombic efficiency of Cu2Sb-Al2O3-C cells was improved through the addition of 2 % vinylethylene carbonate to the electrolyte. Cu6Sn5-TiC-C is a material with high tap density (2.2 g/cm3), which also makes it a material of high volumetric capacity. The reversibility of the reaction of Cu6Sn5-TiC-C with lithium is improved when the material is cycled above 0.2 V vs. Li/Li+.