Abstract
In this work, we investigate the influence of lithium salt concentration (LiFSI, lithium bis(fluorosulfonyl)imide) and ionic liquid (IL) composition on the performance and interfacial stability of high-rate energy storage devices employing lithiated graphite anodes. By varying the Li salt molarity (1-4 M LiFSI), a clear concentration-dependent differences in electrochemical response and interfacial behavior were observed, with higher LiFSI concentrations showing improved cycling stability, longer lifetimes, and attenuated growth of interfacial resistances. As the 4 M IL systems exhibited the best electrochemical performance, they were selected for a detailed investigation of the effects of IL cation identity on decomposition pathways and SEI (solid-electrolyte interphase) chemistry, particularly under rapid cycling. Electrochemical and surface analyses reveal that the SEI chemistry of 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (P13FSI)-based systems diverges significantly depending on the presence or absence of vinylene carbonate (VC) as an SEI-preserving additive. When VC is added, a chemically distinct, organic-rich SEI forms, attributed to the ring-opening reduction of the P13(+) cation, generating vinyl-like compounds that create a self-preserving interfacial film. This suppresses LiFSI decomposition and maintains SEI integrity. In contrast, without VC, the SEI becomes thicker and more heterogeneous, predominantly composed of inorganic byproducts. Conversely, 1-ethyl-3-methylimidazolium bis(fluoromethanesulfonyl)imide (EmimFSI)-based systems undergo progressive SEI degradation due to Emim(+) decomposition into low-molecular-weight fragments that react with carbon and oxygen to form an increasingly inorganic SEI. This leads to the loss of the protective organic layer and reduced long-term performance. These findings emphasize the critical role of IL cation structure and salt concentration in shaping SEI chemistry, highlighting the need for targeted additive strategies to also suppress cation decomposition and enable durable, high-rate energy storage systems.