Optical diagnostics of lithium-sulfur and lithium-ion battery electrolytes using attenuated total reflection infrared spectroscopy

Abstract

Operando investigations of batteries allow direct monitoring of intrinsic chemical and physical processes governing performance. FTIR spectroscopy is a powerful technique for real time species identification and quantification. ATR FTIR allows investigations of battery electrolytes in realistic geometries and at high C-rates. A novel operando spectro-electrochemical cell has been demonstrated. ATR FTIR spectroscopy was used for operando measurements of polysulfide dissolution in Li-S batteries, which leads to loss of active material and lowers lifetimes. The cell was also used to study electrolyte decomposition in Li-ion batteries, which can lead to thermal runaway and catastrophic failure. An optical diagnostic technique to determine the order and concentration of polysulfides in Li-S battery electrolytes was developed from theoretical models and experimental measurements. The polysulfide diagnostic was used to study the evolution of polysulfides during normal battery operation. The trends observed in the polysulfide order and concentration with respect to state of charge are consistent with prevailing understanding of the electrochemical mechanisms of Li-S battery operation. Long-term evolution of polysulfides was observed over 7 discharge/charge cycles. Capacity fading is evident in the decay of polysulfide order and concentration at the same state of charge between cycles. Sulfur is not recovered by charging the cell in latter cycles, and the active material is lost as solid Li2S. The improvement on capacity retention by adding LiNO3 to the electrolyte was observed in terms of polysulfide order and concentration. The operando cell was modified for operation at elevated temperatures to investigate decomposition of the electrolyte LiPF6 EC/DEC in Li-ion cells. As the cell geometry constraints do not allow for direct measurement of internal temperature, an optical thermometry technique was developed to relate the IR spectrum of the electrolyte to its temperature. Operando FTIR spectra provided insight into the decomposition of the EC solvent. SEI formation leads to a double-bond polymerization of EC, and bulk thermal decomposition leads to a ring-opening polymerization of EC. The observed SEI formation follows a parabolic growth rate. The thermal decomposition mechanism is independent of electrode species and is catalyzed by the formation of PF5, a product of the electrolyte salt, LiPF6.