On the operando study of lithium concentration in the graphite anode of a Li-ion battery

Abstract

The capability to rapidly charge a Li-ion battery is important for widespread electricvehicle adoption, but fast charging can cause battery performance issues. The degradation of Li-ion batteries during fast charging is not well understood; however, operando research on Li-ion cells and batteries in general has rapidly grown in popularity in the past few years to study fast charging among other battery-related phenomena, including chemical reactions in the electrode, SEI layer formation, electrolyte transport, and other phenomena in real-time. Operando research has proven to be valuable for elucidating the electrochemical mechanisms at play inside cycling batteries. Fourier transform infrared spectroscopy (FTIR) used in conjunction with attenuated total reflection (ATR) enables operando measurements of liquid electrolytes, including the measurement of phenomena such as molecular degradation and the transport of Li-ions through the cell. This research focused on solvation shifting of solvent infrared absorption bands in the presence of lithium ions. The lithium-shifted and nonlithium-shifted infrared absorption peaks of ethyl methyl carbonate and ethylene carbonate were compared to infer ion concentration changes during cycling. Lithium concentrations were calibrated using EC/EMC/LiPF6 electrolytes with known lithium concentrations from 0.5 mol/liter to 1.75 mol/liter. A custom Li-ion liquid half-cell with a graphite anode and EC/EMC/LiPF6 electrolyte was assembled in an argon-filled glove box and observed with FTIR-ATR. The results showed a rapid and abrupt change in lithium concentration at various charging rates. These measurements were the first operando measurements of lithium ions using FTIR/ATR and the first measurements of this kind under fast-charge conditions. The ability to observe lithium concentration within a Li-ion cell has significant implications for the utility of future operando studies, as well as the potential to link lithium movement to battery performance, especially at high charging rates.