GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, March 2, 2020

11:00 AM
in MRDC 4211

will be held the

DISSERTATION DEFENSE

for

Matthew G. Boebinger

“In Situ Examination of Nanoscale Reaction Pathways in Battery Materials”

Committee Members:

Prof. Matthew McDowell, Advisor, MSE

Prof. Josh Kacher, MSE

Prof. Meilin Liu, MSE

Prof. Gleb Yushin, MSE

Prof. Ting Zhu, ME/MSE

Raymond Unocic, Ph.D., Oak Ridge National Laboratory

Abstract:

In an effort to engineer less expensive and more energy-dense batteries, new materials must be developed to store and transport active ions reliably. However, the electrochemical reaction mechanisms of these materials must be understood and controlled to maximize reversibility during charge and discharge. This dissertation is focused on using in situ experiments, mainly the use of in situ transmission electron microscopy (TEM), to understand the nanoscale transformation pathways in different high-capacity electrode materials during reaction with Li⁺, Na⁺ and K⁺ ions. These materials, upon reacting with alkali-metal ions to form alloys or other compounds, often exhibit much higher specific storage capacities compared to conventional Li-ion battery electrode materials. In addition, these types of materials can also be used in lower-cost sodium- and potassium-based systems. They could therefore replace electrode materials in Li-ion batteries to enable higher specific energy batteries. However, the more substantial volumetric changes that these electrode materials undergo during reaction cause significant challenges, such as mechanical fracture of the active material and continuous growth of the solid-electrolyte interphase (SEI) on the surface of the anode particles leading to very low cyclability of these systems.

For the continued development of these battery systems, it is critical to understand both how the larger Na⁺ and K⁺ ions affect the nanoscale phase transformations during these reactions and how to engineer high capacity battery materials with high coulombic efficiency and longer cycle life. In the studies on the Cu₂S and Fe­S­­₂ active materials, the effect larger alkali metal ions have on the reaction mechanisms of large-volume-change materials was examined. After extensive in situ and ex situ experiments the larger volume changes associated with the sodium/potassium reactions indicated a more stable morphology for overall cycling behavior by demonstrating different reaction pathways and fracture behavior. In the study conducted on the Sb nanocrystals, it was demonstrated that small spherical particles naturally formed uniform internal voids that were easily filled and vacated during cycling. This was found to be due to the natural resilient oxide layer that formed after the first lithiation and prevents shrinkage during delithiation. Additionally, a model was developed that can serve as a tool to guide the creation of oxide or other types of shells that enable alloying materials to undergo voiding transformations in situ. All of these materials (Cu₂S, FeS₂ and Sb) demonstrated interesting and counter-intuitive phase evolution and mechanical degradation behavior when reacting with the alkali ions of different sizes. These findings all indicated that large-volume-change materials could enable stable cycling performance for next-generation batteries, whether they be Li-ion or another battery chemistry that undergoes complex morphological changes.