CHAPTER 8: Electrolyte Evolution and SEI Interfaces

Key properties of electrolytes dictate the interfacial chemistry, thereby heavily influencing battery performance. Extensive research effort has been devoted to optimizing electrolyte formulations to achieve wide operational temperature, extended voltage window, and fast charging capabilities. Such functionalities hinge on the interfacial layer formed between the electrolyte and electrode, which is intrinsically difficult to characterize. In-depth knowledge is required to understand electrolyte evolution and the nature of solid electrolyte interphase (SEI) within rechargeable batteries. This chapter focuses on the application of nuclear magnetic resonance (NMR) spectroscopy in characterizing electrolytes and the SEI layer. First, we focus on multi-nuclei solution NMR spectroscopy, which has been used to reveal ion solvation structure and electrolyte decomposition products. Then we detail recent studies using various solid-state NMR techniques to understand the chemical compositions of the SEI and the functionality of electrolyte additives. Lastly, an emerging technique – dynamic nuclear polarization (DNP) is introduced, which enhances the interphase signals via the hyperpolarization method and holds great promise in revealing surface species. Applying various NMR techniques to electrolyte systems provides exciting opportunities for studying the chemical compositions, chemical structures, and ion dynamics of electrolytes as well as the interfacial species that are critical to the battery performance.