Delineating the Triphasic Side Reaction Products in High‐Energy Density Lithium‐Ion Batteries

Abstract Unwanted cathode‐electrolyte side reactions remain a major challenge to the stability of lithium‐ion batteries, especially at high temperatures. This study presents a quantitative analysis on gaseous, soluble, and solid byproducts generated by reactions between LiNiO 2 and carbonate electrolytes. Online electrochemical mass spectrometry (OEMS) outgassing profiles reveal the emergence of a pre‐plateau region as temperature increases, characterized by two small plateaus at 3.7 and 4.0 V, which are attributed to catalytic ethyl methyl carbonate (EMC) decomposition and ethylene carbonate (EC) hydrolysis, respectively. Activation energies for electrolyte oxidation reactions are quantified with OEMS, confirming that the main outgassing event is driven by direct oxygen‐electrolyte chemical reactions. Nuclear magnetic resonance and X‐ray photoelectron spectroscopy analyses show that EC decomposition yields both soluble and solid byproducts, while EMC produces mainly solid species, leading to a thicker, organic‐rich cathode‐electrolyte interphase; the presence of LiPF 6 further accelerates solvent degradation. Additionally, the effectiveness of various dopants in suppressing gas evolution is evaluated across a broad temperature range, with magnesium‐doped cathodes exhibiting the most effective suppression by stabilizing lattice oxygen. Overall, this work establishes a clear correlation among triphasic side‐reaction products, guiding rational design of cathode materials and electrolyte formulations for high‐energy lithium‐ion batteries.