Wide Temperature Range Electrolyte Formulations for Start-Stop Vehicle Applications

Abstract

Over the last decade, many governments have implemented more stringent regulations on vehicle fuel economy and CO2 emissions. For example, European targets for new passenger cars reduce emissions to 130g CO2 per kilometer by 2015, with further reduction to 95g by 2021. Start-stop vehicle engines, which shut off during stops for traffic or at a light, play an important role in achieving these targets. These vehicles require a battery with sufficient power to re-start the engine over a wide range of conditions and sufficient energy to run the lights, air conditioning, etc. on the vehicle when stopped. Lithium ion batteries are promising candidates to replace currently used lead acid batteries due to their high energy density as well as longer cycle life. However, the power performance of lithium ion batteries is limited at low temperatures required for automotive applications due to low conductivities of the electrolyte and the solid electrolyte interphase (SEI). Solutions to the low temperature problem generally consist of adding solvents with very low melting points and/or low viscosity to the formulation which keep the formulation from freezing or reduce the viscosity at low temperature. However, these solvents tend to be detrimental to high temperature stability, required to meet the lifetime requirements of the vehicle batteries. Wildcat has developed electrolyte formulations that improve the power performance at low temperature, but improve or maintain the high temperature stability relative to baseline electrolyte formulation. Wide temperature range formulations were developed for both NMC//graphite and NMC//LTO electrode chemistries.