Safety Appraisal of Lithium Bis(fluorosulfonyl) Imide (LiFSI) As Electrolyte Salt for Li-Ion Batteries

The success of Li-ion batteries technology relies on a careful evaluation of its safety and subsequently an ad hoc choice of battery key components such as electrolyte salts and solvents, which play a crucial role with respect to safety [1] and proper battery functioning, stabilizing interfacial reactions. The standard electrolyte consists of LiPF 6 salt dissolved in carbonates solvents. Thermal instability, moisture sensitivity, and release of HF via hydrolysis of PF 6 with protic species suggest a priority for the replacement of LiPF 6 with new solutes with improved thermal, chemical and/or electrochemical properties. Thereby, over the past two decades, great efforts have been made hunting for such compounds and lithium imide salts have been found to be of great interest despite some limitations. Within the imide family, there is now a rapidly growing interest for lithium bis(fluorosulfonyl) imide, Li[(FSO 2 ) 2 N] (LiFSI) to be used in liquid electrolyte owing to well balanced properties such as absence of release of HF under normal operation, low viscosity and high conductivity, high-rate and excellent low temperature performances. In addition storage of LiFSI salt based batteries at both high temperature and fully charged states is no more an issue [2]. The thermal stability comparative study of a fully lithiated graphite powder in presence of carbonates based electrolytes with LiPF 6 and LiFSI lithium salt is presented. DSC analysis revealed quite different behaviors. In the LiPF 6 case, a broad exotherm starts around 120°C, with the SEI breakdown, ending at 250°C whereas for LiFSI salt, a broad but weak peak starting around 80°C is followed by a sharp exotherm around 200°C. With the help of GC/MS, 19 F NMR and ESI-HRMS analyses, the reaction mechanisms involved in these different thermal events were studied and solutions are proposed to noticeably decrease the energy released in the LiFSI case. On the other hand, detailed investigations of the combustion behavior of LiPF 6 or LiFSI-based carbonate electrolytes and 1.3 Ah pouch cells were conducted with the objective of getting a better knowledge of lithium-ion battery system fire induced thermal and chemical threats. The well-controlled experimental conditions provided by the Tewarson calorimeter [3] have enabled the accurate evaluation of fire hazard rating parameters such as heat release rate and effective heat of combustion and the quantification of toxic effluents (HF, SO 2 , NOx…). The comparison of results of these two salts will be discussed. [1] D. Lisbona, T. Snee, Process Safety and Environmental Protection 89 (6)(2011) 434. [2] L. Li, S. Zhou, H. Han, H. Li, J. Nie, M. Armand, Z. Zhou, X. Haung, Journal of the Electrochemical Society 158 (2) (2011) A74. [3] P. Ribière, S. Grugeon, M. Morcrette, S. Boyanov, S. Laruelle, G. Marlair, Energy Environ. Sci. 5 (2012) 5271.