Correlating Interfacial Structure and Dynamics to Performance in Lithium Metal Batteries

While the process of electrifying transportation is already underway, competing with fossil fuels in applications such as long-range vehicles and aircrafts will require energy densities that are beyond what is achievable using conventional Li-ion battery chemistries. Li metal batteries are promising candidates for such applications, yet meeting cycle life, power density, and safety demands while utilizing the unmatched specific capacity of Li metal anodes is a formidable challenge. It is well known that the interfacial layer of electrolyte decomposition products which forms on the Li surface during electrochemical cycling (i.e. the solid electrolyte interphase (SEI)) is critical in dictating Li deposit morphology and subsequent performance. However, both the composition and arrangement of the SEI are difficult to study because the SEI is just nanometers-thin, air-sensitive, and evolves as a function of electrochemical cycling protocol. Thus, it is important to develop in situ and operando techniques which are capable of characterizing the SEI in its native environment. Here, we study interphase formation in carbonate, ether, solid ceramic, and highly concentrated electrolytes to develop a framework for the general design of electrolytes and SEIs for Li metal batteries. In the first chapter, we broadly motivate electrochemical energy storage devices and define the metrics which make them attractive compared to alternative forms of energy storage. We then describe Li-based batteries, outline the differences between Li-ion and Li metal batteries, and present some of the key advantages and challenges that Li metal chemistries face. After, we provide a classical description of electrodeposition frameworks, focusing on the effects of charge-transfer kinetics and ion transport on deposition morphology. Then, we present the SEI as a factor which convolutes this process in Li metal anodes and describe how the SEI is formed and arranged on the electrode surface. Finally, we describe common tools used to characterize the SEI and how these may be used to design future electrolytes. The second chapter focuses on the effect of potassium additives on conventional carbonate electrolytes. Recent work has shown that alkali metal additives can lead to smooth Li deposits, yet the underlying mechanisms are not well understood. In this work, we demonstrate that alkali metal additives (here, K+) alter SEI composition, thickness, and solubility. Through post-mortem elemental analyses, we find that K+ ions do not deposit, but instead modify the reactivity of the electrode-electrolyte interface. Using quantitative nuclear magnetic resonance (NMR) and density functional theory (DFT), we show that K+ mitigates solvent decomposition at the Li metal surface. These findings suggest that alkali metal additives can be leveraged to suppress the formation of undesired SEI components (e.g., Li2CO3, soluble organic species), serving as an alternative approach for SEI modification compared to sacrificial additives. We believe that our work will spur further interest in the underexplored area of cation engineering. In the third chapter, we examine both chemical structure and ion dynamics in the SEI, correlating these properties to electrochemical performance to guide the design of new electrolytes. We use a combination of NMR spectroscopy and X-ray photoelectron spectroscopy (XPS) to show that fast Li transport, well-ordered SEI architectures, and low solubility at the electrode/SEI interface in 0.5 M LiNO3 + 0.5 M LiTFSI electrolyte bi-salt in 1,3-dioxolane:dimethoxyethane (DOL:DME, 1:1, v/v) are responsible for the formation of low-surface-area Li deposits and high Coulombic efficiency (CE). This improved performance in the presence of LiNO3 is observed despite the fact that there are higher quantities and more types of compounds in the SEI than in LiTFSI alone, suggesting that the identity of the electrolyte decomposition products, rather than the amount, alters plating. SEI design strategies that increase SEI stability and Li interfacial exchange rate are thus expected to lead to more even current distribution, ultimately providing a new framework to generate smooth Li morphologies during plating/stripping. The fourth chapter describes the dynamic behavior of the interface between a lithium metal electrode and a solid-state electrolyte, lithium lanthanum zirconium oxide (Li7La3Zr2O12 or LLZO). The evolution of this interface throughout cycling involves multiscale mechanical and chemical heterogeneity at the micro- and nano-scale and plays a critical role in all-solid-state battery performance. These features are dependent on operating conditions such as current density and stack pressure. Here we report the coupling of operando acoustic transmission measurements with NMR and magnetic resonance imaging (MRI) to correlate changes in interfacial mechanics (such as contact loss and crack formation) with the growth of lithium microstructures during cell cycling. Together, the techniques reveal the chemo-mechanical behavior that governs lithium metal and LLZO interfacial dynamics at various stack pressure regimes and with voltage polarization. In the fifth chapter, we redefine the premise of a class of Li metal battery electrolytes known as localized high concentration electrolytes (LHCE). LHCEs operate on the assumption that high concentration electrolytes (HCEs) may be augmented using a “diluent,” which interacts scarcely with both the ionic species and the Li metal surface, forming pockets of localized high concentration Li+ which have advantageous bulk and interfacial properties. We report on the use of operando NMR spectroscopy to observe electrolyte decomposition during Li stripping/plating and identify the influence of individual components in LHCEs on Li metal battery performance. Data from operando 19F solution NMR indicates that both bis(fluorosulfonyl)imide (FSI–) salt and bis(2,2,2-trifluoroethyl)ether (BTFE) diluent molecules play a key role in SEI formation, in contrast to prior reports that suggest diluents are inert. Using solution 17O NMR, we assess differences in solvation between LHCEs and low concentration electrolytes (LCEs). We find that BTFE diluents are reduced during Li metal battery operation, which can be detected with operando NMR, but not conventional electrochemical methods. Solid-state NMR (SSNMR) and XPS measurements confirm that LHCEs decompose to form an SEI on Li metal that contains organic BTFE reduction products (CF2, CF3), trapped BTFE, and high quantities of lithium fluoride, likely due to both BTFE and FSI– reduction. These chemical characterizations are correlated with changes in interfacial impedance measured separately at the anode and cathode using three-electrode electrochemical impedance spectroscopy (EIS). Insight into the mechanisms of SEI and CEI formation in LHCEs suggests that fluorinated ethers exhibit tunable reactivity that can be leveraged to control Li deposition behavior. To conclude, we reflect on some of the broad guidelines for electrolyte and SEI engineering that we gleaned from the previous chapters. Finally, we highlight recent notable works which we think will enable major advances in interfacial characterization of Li metal batteries (focusing on in situ and operando techniques which can be applied to study both structure and dynamics in commercial setups)