Development of low temperature Li-ion electrolytes for NASA and DoD applications

Both NASA and the US Army have interest in developing secondary energy storage devices with improved low temperature performance to meet the demanding requirements of space missions and man-portable applications. Lithium-ion systems have been identified as having the most promise due to their high specific energy density and wide operational temperature ranges from the use of organic solvent-based electrolytes, rather than aqueous-based systems. Initially, the SOA lithium-ion technology was limited to operation above −10°C, due primarilly to the fact that the electrolytes employed had high melting points and were highly viscous at low temperatures, resulting in low ionic conductivity. However, due to recent developments in electrolyte formulations at the Army and at JPL, improved low temperature performance of lithium-ion cells have been demonstrated, with efficient cell operation to temperatures as low as −30°C. This was achieved by developing multi-component solvent systems, based on mixtures of cyclic and aliphatic alkyl carbonates. In the course of investigating the viability of a number of advanced electrolyte systems, it was identified that the protective surface films which form on the carbonaceous-based anodes can strongly influence the low temperature capabilities of lithium-ion cells, in addtion to the ionic conductivity of the electrolyte. Thus, in order to optimize an electrolyte for low temperature applications, it is necessary to balance the inherent physical properties of the formulations (i.e. freezing point, viscosity, and ionic conductivity) with the observed compatibility with the chosen cell chemistry (i.e. the nature of the passivating films formed on the electrodes).