Resolving Electrolyte Decomposition Products in Gas, Liquid, and Solid Phases in Lithium–Metal Batteries

Lithium (Li)-metal batteries with high-voltage cathodes are promising next-generation, high-energy automotive batteries. While ether-based electrolytes are known for their high reductive stability, their limited oxidative stability against high-voltage cathodes remains a key barrier to long-term service life. Here, we present a methodology enabling a comprehensive, quantitative assessment of cathode–electrolyte reactions, based on a model fluorinated 1,2-diethoxyethane-based electrolyte and LiNiO₂ cathode. Online electrochemical mass spectroscopy at varying temperatures reveals both the thermodynamic and kinetic features of the electrolyte oxidative decomposition by quantifying gaseous byproducts and the reaction activation energy. Nuclear magnetic resonance spectroscopic results unveil alcohol and alkoxy acetic acid species as soluble decomposition products of ether electrolytes. Time-of-flight secondary ion mass spectrometry, combined with region-of-interest and spatial normalized standard deviation analyses, quantitatively determines the thickness and spatial and chemical homogeneity of the cathode–electrolyte interphase. This work establishes a quantitative methodology to assess gaseous, soluble, and solid cathode–electrolyte decomposition products.