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 technology reduces CO₂ emissions by shutting off the engine when the vehicle is at a stop, such as traffic lights, and restarts the engine instantly when the driver accelerates the vehicle to proceed. Fuel consumption is also reduced, which is desirable from the vehicle owner’s perspective. Start-stop technology is not new, with initial usage traced back to 1980’s by VW in Fiat. However, this application has been greatly limited by the low energy density and poor cycle life of lead acid batteries, which have been primarily used for this application. Batteries intended for start-stop applications must start the engine a high number of times, must survive limited or extended engine-off periods, and must be able to operate over a wide temperature range (-40° C to 60 °C).

Start-stop automobiles have gained momentum recently because of more stringent carbon emission regulations, but also due to technology enhancements achieved in lithium ion batteries. Compared to lead acid, lithium ion batteries have higher energy density as well as longer cycle life. However, the power performance of lithium ion batteries is limited at the 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 viscosities to the formulation which keep the electrolyte 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. 

Using its unique High Throughput Battery Workflow, Wildcat has developed promising electrolyte formulations with significantly improved low temperature power performance while improving or maintaining high temperature stability relative to a baseline electrolyte formulation.  Wide temperature range formulations were developed for both NMC//graphite and NMC//LTO electrode chemistries and over 30% improvement on both low temperature power performance as well as high temperature stability has been successfully achieved.

Figure 1. Solvent composition has strong effect on reducing low temperature impedance (blue/purple is best result, gray/red is worst)