Hydrolysis of ethylene carbonate with water and hydroxide under battery operating conditions

This study deals with the decomposition of ethylene carbonate (EC) by H₂O in the absence and presence of catalytically active hydroxide ions (OH^- ) at reaction conditions close to lithium-ion battery operation. We use On-line Electrochemical Mass Spectrometry (OEMS) to quantify the CO₂ evolved by these reactions, referred to as H₂O-driven and OH-driven EC hydrolysis. By examining both reactions at various temperatures (10 -80°C) and water concentrations (<20 ppm or 200, 1000, and 5000 ppm H₂O) with or without catalytically active OH-ions in EC with 1.5 M LiClO4, we determine an Arrhenius relationship between the CO2 evolution rate and the cell temperature. While the apparent activation energy for the base electrolyte (<20 ppm H₂O) is very large (app. E_a ≈153 kJ/mol), substantially lower values are obtained in the presence of H₂O (app. E_a ≈99 ± 3 kJ/mol), which are even further decreased in the presence of catalytically active OH-(app. E_a ≈43 ± 5 kJ/mol). Our data show that OH^--driven EC hydrolysis is relevant already at room temperature, whereas H₂O-driven EC hydrolysis (i.e., without catalytically active OH-) is only relevant at elevated temperature (≥40°C), as is the case for the base electrolyte. Thus, catalytic quantities of OH-, e.g., from hydroxide contaminants on the surface of transition metal oxide based active materials, would be expected to lead to considerable CO₂ gassing in lithium-ion cells.