Kelsey Anne Cavallaro
Advisor: Prof. Matthew T. McDowell
will defend a doctoral thesis entitled,
Investigating Reaction Mechanisms of Next-Generation Active Materials for Low-Temperature Lithium-Ion Batteries
On
Tuesday, November 19 at 12:00 p.m.
Pettit 102 A&B
and
Virtually via MS Teams
Committee
Prof. Matthew T. McDowell – School of Materials Science and Engineering and George W. Woodruff School of Mechanical Engineering (advisor)
Prof. Faisal Alamgir – School of Materials Science and Engineering
Prof. Rosario Gerhardt – School of Materials Science and Engineering
Prof. Marta Hatzell – George W. Woodruff School of Mechanical Engineering
Prof. Seung Soon Jang – School of Materials Science and Engineering
Abstract
Lithium-ion batteries have become ubiquitous in most energy storage applications because of their high energy density and long cycle life, enabled by the intercalation chemistry of the electrodes that are used. However, interest in novel lithium-based chemistries has increased in recent years, as engineering improvements have reached the theoretical limits the intercalation materials. In particular, current lithium-ion battery chemistries only operate with ideal performance in a narrow temperature range around room temperature, preventing effective energy storage in many environments. At low temperature, transport and kinetic limitations prevent intercalation of lithium ions in electrode materials, and many commercial electrolytes freeze below 0 ºC. This necessitates the use of extensive heating systems in low temperature applications, such as space exploration and electric aviation.
Materials that electrochemically alloy with lithium, known as alloy electrodes, have been studied since the inception of lithium-ion batteries because of their high specific capacity. However, they were abandoned early in the development of commercial batteries in favor of intercalation materials due to their rapid mechanical degradation. Despite this, research on these materials continues and they show significant potential to expand the low temperature range of batteries. The vast majority of research on these materials focuses on their room temperature electrochemical behavior. This dissertation aims to fill that gap by investigating the electrochemical, morphological, and mechanistic evolution of alloy anodes at low temperatures to guide future engineering of low temperature batteries.
First, the electrochemical behavior of three common alloy anodes was investigated to determine if lithium alloys are successful replacements for graphite in lithium-ion batteries, as well as to better understand the low temperature processes that influence performance. I showed that these lithium alloys could be charged and discharged down at temperatures down to -40 ºC with ten times higher specific capacity than graphite on the first cycle. Antimony, in particular, demonstrates improved low temperature performance and cycling stability compared to other alloy materials, in part due to its high electrode potential. Three-electrode cells with a lithium metal reference electrode were used to determine the influence of the counter electrode on low temperature measurements, showing that lithium plating and stripping at the counter electrode can obscure the effects of the working electrode of interest on the voltage profile. Finally, I used the galvanostatic intermittent titration technique to elucidate the processes which dominate overpotential at low temperatures and found that kinetic and thermodynamic limitations are alloy-dependent and can change significantly from one cycle to the next and between charge and discharge, indicating mechanistic changes during early stages of cycling.
Next, I investigated morphological and mechanistic changes of alloy foil anodes when lithiated and delithiated at various temperatures. Ex-situ characterization revealed temperature dependent fracture behavior for both indium and tin alloy anodes, with increased degree of surface fracture occurring at low temperatures. Unexpectedly, larger Coulombic efficiencies were achieved in cells with tin electrodes cycled at -20 ºC compared to 60 or 20 ºC, despite the extreme morphological evolution of the tin anode at low temperatures and the high thermal energy available at higher temperatures to drive lithium diffusion and phase propagation. Cryogenic focused ion beam milling revealed that both tin and indium demonstrate similar trends in lithiation and delithiation behavior as a function of temperature. Both tin and indium exhibit homogeneous lithiation and delithiation with planar reaction fronts between unreacted, lithiated, and delithiated phases, which likely contributes to both the high Coulombic efficiency and increase in extent of fracture seen at low temperature. At high temperatures, inhomogeneous lithiation and delithiation behavior occurs with indium forming distinct lithiated nuclei separated by unreacted regions, while tin shows clear evidence of lithium trapping, which results in low Coulombic efficiency. Chronoamperometry and optical microscopy were used to reveal temperature- and composition-dependent phase nucleation phenomena, which explain the changes in homogeneity of lithiation and delithiation as a function of temperature.
Overall, the work completed in this dissertation indicates that lithium alloy anodes cam be an effective replacement for graphite to enable low temperature batteries. Furthermore, these findings provide fundamental understanding of the temperature-dependency of the reaction mechanisms and morphological evolution of alloy materials.