Committee Members:
Prof. Matthew McDowell, Advisor, MSE/ME
Prof. Rosario Gerhardt, MSE
Prof. Faisal Alamgir, MSE
Prof. Seung Soon Jang, MSE
Prof. Marta Hatzell, ME
Investigating Reaction Mechanisms of Next-Generation Active Materials for Low-Temperature Lithium-Ion Batteries
Abstract:
The goal of this work is to understand electrochemical reactions and their resulting structural and morphological changes in alloying and conversion type battery active materials when charging/discharging at low temperatures. Hindered kinetics, electrolyte incompatibility, and lithium dendrite formation limit the use of conventional lithium-ion battery chemistries at temperatures below -20 ºC unless bulky thermal management systems are used. The differing reaction mechanisms of next-generation active materials may enable materials that can be cycled at temperatures below -20 ºC. However, the electrochemical and structural behavior of these materials are not well understood at these temperatures. This work aims to characterize the kinetic phenomena and structural changes in such materials at low temperature in alloying and conversion type electrodes to improve their electrochemical performance.
Conventional graphite anodes are a large contributor to the poor low temperature behavior of LIBs, due to slow lithium diffusion and particular electrolyte requirements. Many studies have improved the performance through specialized electrolytes, surface coatings, and electrode modifications, but poor performance and low capacity persist at low temperatures. The work presented here investigates the electrochemical and material evolution behavior of anode materials that alloy with lithium. We find that alloy anodes such as antimony are promising materials at low temperature and can be cycled at temperatures down to -40 ºC, while exhibiting high first cycle specific capacity. Galvanostatic intermittent titration technique (GITT) experiments reveal large contributions to the low temperature overpotential from kinetic processes for antimony and tin, like charge transfer and solid-state diffusion, while the low temperature behavior of silicon is dominated by the thermodynamic hysteresis.
Both high rate and low temperature cycling suffer from kinetic limitations, and it is of interest to understand whether the limiting processes when cycling at high rate and room temperature are similar to those occurring with moderate rate but at low temperature. If so, the extensive understanding of high rate cycling at room temperature and strategies for improvement could be applied to moderate rate, low temperature operation. In the case that the processes are different, it will be important to determine why high rate and low temperature cycling are mechanistically different, despite both being kinetically limited. Electrochemical data indicates that graphite experiences similar lithiation processes at both high rate and low temperature, but the formation of SEI is hindered at low temperatures. Antimony demonstrates significant electrochemical differences between the high rate and low temperature cases, with large overpotentials required for lithiation to begin at high rates. Further work will characterize the changes in the SEI morphology and composition of graphite when formed at high rate and low temperature and well as understanding changes to the antimony particle structure and morphology at high rates.
Additional proposed work with further investigate the chemical and structural changes that occur in alloy anodes at low temperature and reveal if conversion cathodes can replace intercalation cathodes for high energy batteries that can be operated at low temperature. First, antimony exhibits uncharacteristic rapid capacity decay when cycled at -40 ºC. Understanding the mechanism of degradation at low temperature will enable engineering of the electrode to improve material stability. This work will involve electrochemical experiments to determine changes in impedance with cycling and extensive ex-situ characterization of the electrodes and well as in-situ x-ray diffraction experiments to dynamically track phase changes in these materials down to -40 ºC. Finally, high-capacity alloy anodes will require high-capacity cathodes, beyond the typical intercalation materials. Therefore, I will investigate the electrochemical properties and structural evolution of iron fluoride, a high-capacity conversion type cathode, down to -40 ºC and fabricate full cells using these next-generation materials for low-temperature operation.