Performance enhancement at low temperatures and in situ X-ray analyses of discharge reaction of Li/(CFx)n cells

In the Sandia National Laboratories internally funded Laboratory Directed Research and Development (LDRD) project we are studying the fundamental limitation(s) of the discharge reaction that reduces the operating voltage of the Li/(CFx)n cells at moderate discharge rates. As a subset of this effort, we are evaluating the electrochemical properties of (CFx)n electrodes prepared with materials from different vendors at different temperatures and in two different electrolytes in order to provide an optimized system to the above study. The temperatures studied span the range −51 to 72 °C. The electrolytes consist of EC:EMC (3:7 wt.%)–1.2 M LiPF6 denoted as HCE (Highly Conductive Electrolyte) and EC:PC:EMC (1:1:3 wt.%)–1 M LiBF4 denoted as SNL-E (Sandia National Laboratories Electrolyte). The four different (CFx)n materials studied showed comparable capacity at 0 °C and above in the two electrolytes. However, at sub-ambient temperatures the SNL-E performed better than the HCE. The performance improvement with SNL-E comes mainly from a lower interfacial resistance compared to HCE.The different (CFx)n materials performed differently in the SNL-E especially at sub-zero temperatures. For example, at sub-zero temperatures the delivered capacity varied between 6% and >60% of the room temperature capacity. In order to rationalize this observation, SEM photographs of the powder particles were taken that showed that the particle size of the best performer(s) is smaller than that of the others. Finally, these data seem to indicate that optimization of the properties of the electrolyte as well as the electrode is critical to maximizing delivered capacity especially at sub-ambient temperatures.Our X-ray results shall be presented in more detail in an upcoming manuscript. We have elected to present a brief discussion of these results in this present paper as the findings lend support to the formation of intermediate species in the discharge reaction of Li/(CFx)n cell.