Ultimately, batteries usually work best at room temperature. A higher ambient temperature may provide better short-term operation by making the charge and discharge reactions more efficient and reducing internal resistance as a result.
However, sustained operation at high temperatures also results in unwanted parasitic reactions between the electrodes and the electrolyte and a breakdown of the cell structure, leading to a long-term reduction in performance.
The international standard IEC 60601-1 that covers the general requirements for basic safety and essential performance requires that any part that can be touched by a person should not exceed 43 degrees Celsius. So, choosing a battery that heats up excessively could lead to non-compliance.
Temperature is also the reason why fast charging, despite the benefits it offers, is not conducive to the long-term health of the battery. In May 2017, it was reported that the electric vehicle giant Tesla was limiting the charging rate on vehicles being charged with its 120kW Superchargers to only 90kW if the vehicle had already accumulated too many DC fast-charge events. The company released a statement explaining the limitation.
"The peak charging rate possible in a li-ion cell will slightly decline after a very large number of high-rate charging sessions. This is due to physical and chemical changes inside of the cells.
"To maintain safety and retain maximum range, we need to slow down the charge rate when the cells are too cold, when the state of charge is nearly full, and also when the conditions of the cell change gradually with age and usage."