(146a) Online Electrochemical Mass Spectrometry for the Investigation of Electrolyte Degradation in High Voltage Li-Ion Pouch Cells

Transportation is a key sector of the US economy while accounting for 27% of total US greenhouse gas emissions during 2020. The ongoing wide scale adoption of electric vehicles (EV) enables electrification of the transportation sector and the opportunity for significant reduction in greenhouse gas emissions. High voltage (>4.2V) battery development is critical as car manufacturers strive to develop high power, extended range EVs. One key issue surrounding high voltage battery performance is electrolyte stability. While some initial electrolyte degradation is desirable during formation to develop the SEI, overactive electrolyte degradation throughout extended cycling can lead to poor capacity retention and coulombic efficiency (CE), as well as excessive gas evolution and pouch pressurization. Electrolyte degradation typically results from oxidation and reduction reactions occurring at the interface between electrolyte and anode or cathode, drive by voltage extremes. To explore the high voltage stability of electrolyte and battery chemistries, we leverage online electrochemical mass spectrometry (OLEMS) to characterize online gas evolution in batteries. OLEMS is a powerful tool that we have used to identify gaseous and volatile electrolyte degradation products throughout battery cycling, as well as quantify gaseous species production rates as a function of time and voltage.

While OLEMS analysis of battery systems has predominantly employed coin cell-type cells, we have developed an OLEMS system that continually samples 2.5 Ah pouch cells using Ar carrier gas. By characterizing gas evolution in pouch cell samples, we have designed our DEMS system and experiments with an applications-based approach. We leverage our system to characterize gas evolution behavior during formation and initial C/3 cycling for ethylene carbonate/ethyl methyl carbonate (EC/EMC) and fluoroethylene carbonate/dimethyl carbonate (FEC/DMC) electrolyte systems under standard and elevated upper cutoff voltages of 4.2 and 4.5 V. We find that both electrolyte solvent and upper cutoff voltage condition greatly impact gas product species, production profiles during cycling, and gas quantity. Gas evolution of oxidation products, such as CO2 and CO, is exacerbated by the elevated upper voltage cutoff position, while reduction products are only minimally affected. Overall, we find the EC/EMC electrolyte is more stable against gas evolution at both upper and lower upper cutoff voltage conditions. Our reported work serves to inform future electrolyte development for high voltage Li-ion batteries, to demonstrate OLEMS as a powerful tool for gas evolution analysis, and to offer a fundamental understanding of gas evolution behavior for applications-based battery pouch cells.