Mun Sek Kim PhD Thesis Defense

Mun Sek Kim
PhD Candidate
Chemical Engineering
Academic advisor: Professor Yi Cui

Abstract: Investigating the effects of solid-electrolyte interphase inorganic compounds via suspension electrolytes for lithium metal batteries

Lithium metal batteries (LMBs) provide a significant increase in energy density (exceeding 500 Wh kg-1) compared to traditional lithium-ion batteries (around 260 Wh kg-1). Yet, their progress towards widespread adoption is hampered by limited rechargeability and safety concerns tied to lithium metal anodes. The primary issues are traced back to the suboptimal formation of solid-electrolyte interphases (SEIs) on lithium metal anodes that result in uneven electro-deposition/dissolution of lithium during repeated cell cycling. Strategies aimed at constructing desirable SEIs have identified the formulation of inorganic-rich SEIs through electrolyte engineering as a particularly successful approach in enhancing the longevity and safety of LMBs. Nevertheless, understanding the role that interphasial inorganic constituents play in enhancing the electrochemical reversibility of lithium metal anodes remained under-discussed. Therefore, the focal point of my thesis work is to reveal the distinctive features of inorganic SEI compounds, namely Li2O, Li3N, and LiF, by employing suspension electrolytes for lithium metal anodes. In-depth examinations of Li2O, Li3N, and LiF as the suspension matrix in electrolytes led to pivotal scientific discoveries that deepened our understanding of SEIs and electrolytes for lithium metal anodes. First, it was revealed that these inorganic SEI compounds uniquely modify the Li+ solvation environment near their surfaces, clarifying their role in shaping the solvation dynamics, which is critical for the formation and spatiotemporal evolution of SEIs on lithium metal anodes. Second, all these inorganic SEI compounds create a weakly solvating environment but in a different way, in which Li2O, Li3N, and LiF promote anion-derived, organic-poor, and Li2O-rich SEIs on lithium metal anodes, respectively. Their unique attributes contribute to the rationale for the enhanced electrochemical performances observed with inorganic-rich SEIs on lithium metal anodes. Lastly, the systematic revealment of the working mechanism of these inorganic SEI compounds offers the innovation of heterogeneous electrolytes that deviate from conventional electrolytes used in lithium batteries. Taken together, these scientific findings and insights serve as the cornerstone for devising optimal SEI chemistries/structures and electrolyte engineering, advancing the development of high-energy and efficient LMBs.