A combination of chemicals was created that could help electric cars power their way through the extreme temperatures that degrade the efficiency of current lithium-ion batteries. Combinations of compounds in electrolytes, in the laboratory, allow lithium-ion batteries to function very well both in sub-zero temperatures (Fahrenheit) — which are reached often during the winter in the northern United States and other parts of the world — and in very hot temperatures like those in the summer in the southwestern US. Those are temperatures where today's batteries — used to power electric cars, cellphones, laptops, and other devices — often retain little charge and expend it quickly.
The electrolytes allow lithium-ion batteries to work well below zero, even down to about -40 °C (-40 °F), and at temperatures of 60 °C (140 °F), making them attractive for commercial lithium-ion batteries. The compounds are found in a battery's electrolyte, the liquid material that bathes the electrodes, and plays a critical role shuttling around the lithium ions that make the battery work. The fluid has several important functions and is typically a mix of well-known compounds such as lithium hexafluorophosphate as the conducting salt, organic carbonates as the solvent, and closely held additives that boost performance.
The electrolyte serves an important function protecting the electrodes. As the battery operates, molecules from the electrolyte build up what is known as the SEI (solid electrolyte interface) on the anode or CEI (cathode electrolyte interface) on the cathode. These are the thin layers of materials that help protect those devices from chemical damage — from valuable lithium ions going into unwanted chemical reactions, for example, or from processes that eat away at the integrity of the electrodes.
Among the additives that were found effective when used in combination with others are cesium hexafluorophosphate (CsPF6), which helps build the ultrathin but strong protective structure on the anode; fluoroethylene carbonate (FEC), which does the same and improves battery performance at low temperatures; tris(trimethylsilyl) phosphite (TTMSPi), which helps form the protective structure on the cathode and helps prevent damage generated by some chemical activity; and 1,3-propane sultone (PS), which helps reduce the harmful effects of unwanted chemical reactions on the cathode.