Decoupling interfacial processes in the formulation of fluorine-free lithium metal batteries

Fluorine additives in lithium metal batteries are often correlated with improved performance, yet lead to challenges associated with cost, health, and safety. In this work, we formulate a series of electrolytes that allow us to interrogate the physical and electrochemical properties of non-fluorinated and fluorinated anions (nitrate, bis(oxalato)borate, and hexafluorophosphate) on the reversible electrodeposition of Li metal via traditional surface characterization methods, spectroscopy, and electroanalytical techniques. When the salts are used in the moderate concentration regime (0.5 M Li), the solid electrolyte interphases (SEIs) generated during cycling are primarily solvent-derived and are statistically similar in terms of ionic transport properties. Nuclear magnetic resonance (NMR) in conjunction with transient voltammetry measurements indicate that Li metal CE is correlated with charge transfer, which is strongly influenced by Li⁺ solvation structure. Specifically, we find that the nitrate anion displaces solvent molecules in the Li⁺ solvation sphere, leading to tunable solvation/desolvation dynamics. We demonstrate that these metrics can be utilized to construct fluorine-free electrolytes containing lithium nitrate and lithium bis(oxalato)borate for high performance lithium metal anodes.