STUDY OF THE REACTIVITY OF ELECTROLYTE SOLVENTS AND ADDITIVES IN LI-ION CELLS AND DESIGN OF NEW ELECTROLYTE BLENDS

Abstract

While electrolyte additives have a great impact on Li-ion cell lifetime, the ways they work are still poorly understood. In an effort to better understand how additives function, two gas chromatography-based methods were developed. One of the methods allows the neutral and organic components of the electrolyte to be determined, while the other allows the composition of the gas produced during cell use to be determined. These two methods were used in order to determine the concentration-time profiles of some additives as well as the gas produced in Li-ion cells charged to low voltage (i.e. 3.5 V) and high voltage (i.e. 4.7 V). Some of the measurements allowed the reduction and oxidation pathways of solvents and additives to be determined and other measurements allowed the relationship between impedance and additive consumption to be determined. Using the information gathered on ethylene carbonate-based electrolytes, new ethylene carbonate-free electrolyte systems were also developed. These consist of ester or linear alkyl carbonate-based electrolytes containing small amounts of passivating agents. Once the passivating agent loading was optimized, cells with ester-based electrolytes presented good capacity retention when cycled up to 40°C and 4.2 V and good low temperature rate capability. Cells with the linear alkyl-based electrolytes on the other hand presented better capacity retention than cells with state of the art ethylene carbonate-based electrolytes when cycled at high voltage (i.e. > 4.3 V), even at 55°C.