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. Ea ≈153 kJ/mol), substantially lower values are obtained in the presence of H2O (app. Ea ≈99 ± 3 kJ/mol), which are even further decreased in the presence of catalytically active OH− (app. Ea ≈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.