Thermal stability of pyrrolidinium-FSI ionic liquid electrolyte and lithium-ion electrodes at elevated temperatures

Ionic liquids are an alternative electrolyte for lithium-ion batteries which are expected to increase thermal stability of the device. The thermal stability of lithium bis(fluorosulfonyl)imide salt (LiFSI) in 1-methyl-3-propylpyrrolidinium bis(fluorosulfonyl)imide (P13FSI) ionic liquid electrolyte was investigated at elevated temperatures after cycling in contact with commercially available electrode materials. Six commercial electrodes were investigated; LiFePO ₄ , Li _1.2 Ni _0.15 Co _0.1 Mn _0.55 O ₂ , LiMn _1.5 Ni _0.5 O ₄ , Li ₄ Ti ₅ O ₁₂ , LiCoO ₂ and graphite. Differential scanning calorimetry was performed on electrode | electrolyte combinations in high pressure hermetically sealed crucibles from 25 to 600 ^◦ C. Of the electrodes studied here, LiFePO ₄ showed the lowest heat release and Li ₄ Ti ₅ O ₁₂ showed the highest onset temperature under the conditions used. Additionally, the ionic liquid electrolyte showed superior thermal stability as compared to the conventional electrolyte when used with LiMn _1.5 Ni _0.5 O ₄ and Li ₄ Ti ₅ O ₁₂ electrodes. Incremental state of charge investigations with both LiFePO ₄ and Li ₄ Ti ₅ O ₁₂ half cells showed that thermal stability varies with state of charge of the electrode. The thermal stability advantage provided by P13FSI electrolyte with each electrode should be considered to determine if the benefits of higher decomposition temperatures are outweighed by the higher energy released during ionic liquid decomposition.