Highly stable lithium metal batteries enabled by nanometric anion aggregates reinforced solvation structure in locally concentrated ionic liquid electrolytes

The practical application of lithium metal batteries (LMBs) requires electrolytes that simultaneously ensure high safety and interfacial stability. Although locally concentrated ionic liquid electrolytes (LCILEs) exhibit exceptional electrochemical stability and compatibility with electrode electrolyte interfaces (EEIs), two major challenges persist: (i) safety risks caused by excessive low-flash-point diluents, and (ii) insufficient understanding of how diluents modulate solvation structures. Herein, we introduce a low-diluent-content LCILE system composed of lithium bis(fluorosulfonyl)imide (LiFSI) salt, N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₃FSI) ionic liquid, and trifluoromethanesulfonate (TFS) diluent. The TFS diluent strengthens ion-ion interactions by lowering the dielectric constant of the electrolyte, resulting in the formation of a unique nanometric anion aggregates (N-AGGs) reinforced solvation structure. These large anionic clusters exhibit accelerated redox decomposition kinetics, facilitating the rapid formation of a thin, dense, and low-impedance EEI. Consequently, the Li/LiNi_0.6Co_0.2Mn_0.2O₂ coin cell achieves 87.8 % capacity retention over 300 cycles at 4.3 V, while a practical 1.4 Ah Li/NCM622 pouch cell retains 84.5 % capacity after 80 cycles at 4.5 V. Furthermore, the electrolyte demonstrates exceptional safety, and 2 Ah Li metal pouch cells successfully pass rigorous nail penetration tests without any ignition or explosion. This work not only provides a design strategy for intrinsically safe and high-performance electrolytes but also highlights the critical role of anion cluster decomposition kinetics in shaping EEI formation.