The performance of lithium-ion batteries have been substantially improved in almost all aspects since the initial commercialization.[1] However, cycling stability and safety is still one of the main concerns. In this regard, electrolytes play an important role. Although carbonate electrolytes are reduced at the anode electrode, they are able to form a stable passivating layer called solid electrolyte interphase (SEI). This layer is able to suppress continuous electrolyte degradation improving the cyclability of the batteries. SEI-forming additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are found to be particularly effective to stabilise this SEI layer.[2,3] However, K. Kim et al. have recently reported the thermal instability of FEC in LiPF₆-based electrolyte for Li-ion batteries.[4] They show that FEC can be defluorinated by Lewis acids such as PF₅, generating HF and other acids that are detrimental to the battery.

Herein, we have studied the effect of FEC in LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/Li cells at room- and elevated-temperatures. In addition, we have investigated the possible mechanism of such fast degradation of LiPF₆- and FEC-based electrolytes and approaches to inhibit it.

At room temperature, NMC/Li cells containing FEC additive in the electrolyte presented longer cycle life than the FEC-free electrolytes. However, both electrolytes showed poor electrochemical performance when cycled at 55°C (Fig. 1a). Interestingly, we observed that the LiPF₆-based electrolytes which also contains FEC undergo color change when stored at 55°C precipitating a brown solid (Fig. 1b). Shelf life of LiPF₆ in FEC solution was investigated by nuclear magnetic resonance (NMR) confirming the hydrolysis of LiPF₆ to phosphoric acid. Furthermore, the obtained solid was identified as a fluorine-based crosslinked polymer derived from FEC. Incorporation of a moisture scavenger compound, such as lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), in the electrolyte formulation can slow down the degradation process (Fig. 1b) and improve the battery performance at 55°C (Fig. 1c).

[1] J. Electrochem. Soc. 2017, 164, A5019-A5025. [2] ACS Energy Lett. 2017, 2, 1337−1345. [3] J. Power Sources, 2018, 400, 147–156. [4] Electrochim. Acta, 2017, 225, 358–368.