One of the viable option to meet the stringent safety requirements of large-format LIBs is to replace the existing highly flammable/combustible, state-of-the-art organic based, electrolytes with safer ones. In this regard, the use of room temperature ionic liquids, RTILs, as advanced electrolytes in electrochemical energy storage devices has been one of the most enticing and emerging option. Though some imidazolium and chiefly pyrrolidinium based ILs are hailed as safer electrolytes, overcoming the limitations imposed by the highly volatile/combustible carbonate based electrolytes, the full scale and precise appraisal of their global (thermal stability, biodegradability, fire-induced) safety levels under abuse conditions remain to be fully addressed.

With the aim of providing the requested level of information on the safety aspects of ILs, we embarked on a detailed investigation of the short and long term thermal stability, combustion behavior and biodegradability tests of various pyrrolidinium-based ILs, synthesized in our laboratory. ILs enlisting the effect of alkyl chain length, [Pyr_1A] ⁺ (A=3-10), anion, Pyr₁₄ (TFSI, FSI and BETI), cation, [Pyr₁₄]⁺/ [Pyr_12O1]⁺ [TFSI] and salt addition (e.g. LiTFSI/Pyr₁₄TFSI) were methodically investigated. Through the use of myriad of techniques, i.e. ramped/isothermal thermogravimetric analysis, oxygen bomb calorimetry and multi-purpose fire propagation apparatus (FPA, ISO12136), key parameters governing the thermal and fire induced hazards such as ease of ignition, heat release rate, effective heat of combustion and fire induced toxicity (CO, Soot, THCs, SO₂, HF, NO, N₂O, and HCN) were determined. A number of biodegradability tests including Modified Sturm, Manometric Respirometry, and Zahn-Wellens/EMPA were conducted. The thermal stability,combustion and biodegradability tests of pyrrolidinium - based ILs are specific to the nature of the anion, cation, alkyl chain length as well the addition of lithium salt.

In general, our detailed study provides a new systematic approach towards understanding the thermal stability, biodegradability and fire-induced hazard evaluation of large families of pyrrolidinium based ILs as well as access to data needed for a performance based approach of the evaluation of their safety benefits.