Computational study of Li+ solvation structures in fluorinated ether, non-fluorinated ether, and organic carbonate-based electrolytes at low and high salt concentrations

Abstract

Understanding the solvation structure of electrolytes is crucial for optimizing the performance and stability of lithium-ion batteries. Novel electrolytes are essential for enhancing electrolyte structure and ensuring better integration with modern electrode systems. Herein, we report a novel weakly solvated ether electrolyte (WSEE) composed of a pure fluorinated ether solvent, which results in an anion-rich solvation structure even at a low salt concentration of 1 M. To explore this, we selected the advanced fluorinated solvent 2,2-difluoroethyl methyl ether (FEME) and compared it with dipropyl ether (DPE), ethylene carbonate (EC), and diethyl carbonate (DEC). The prepared electrolyte systems include DPE with 1 M, 1.8 M, and 4 M LiFSI; FEME with 1 M, 1.8 M, and 4 M LiFSI; and a 1 : 1 vol% EC/DEC mixture containing 1 M LiPF₆. In this work, we comprehensively investigate the Li⁺ solvation structures using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Our computational findings indicate the presence of large ion aggregates (AGGs) in each DPE- and FEME-based electrolyte, while SSIPs (68%) are the dominant species in the mixed EC/DEC electrolyte. Notably, the formation of large ion aggregates is more pronounced in FEME-based electrolytes. The dominant solvation structures in the ether-based electrolytes are the anion-rich complexes Li⁺(FSI⁻)₃(DPE)₁ and Li⁺(FSI⁻)₃(FEME)₁. We find that, similar to DPE, the FEME solvent also exhibits weak solvating power across all examined salt concentrations. More specifically, we find that FEME has weaker solvating power than DPE. This behavior is predicted by MD simulations, which indicate a strong preference for Li⁺ ions to coordinate with FSI⁻ anions within the primary solvation shell. We also observe that the number of unique solvation structures in the ether-based electrolytes increases with salt concentration, with FEME + LiFSI showing slightly more unique solvation structures than DPE + LiFSI. Furthermore, the quantum mechanical features of the Li⁺ solvation structures in DPE + 1.8 M LiFSI, FEME + 1.8 M LiFSI, and EC/DEC + 1 M LiPF₆ electrolytes are analyzed in detail using DFT calculations. We anticipate that this study will provide valuable insights into the Li⁺ solvation structures in DPE, FEME, and EC/DEC electrolytes, where the ether-based electrolytes exhibit closely similar properties.