Unveiling Local Solvation Structures of Local High-Concentration Electrolytes (LHCEs)

Local high-concentration electrolytes (LHCEs) with fluorinated ethers acting as diluents can extend the lifespan of lithium metal batteries (LMBs) through the reduction of the cation-solvent correlation and the promotion of the Li + -FSI - coordination. However, information about the local solvation structure of LHCEs remains limited, leaving a knowledge gap regarding their microstructure characteristics. Here, we investigate the impact of diluents on the local solvation structure through a series of HCEs and LHCEs with various fluorinated ether diluents and solvent-diluent ratios. Based on SAXS measurements, we can observe that the q peak’s intensity at 0.9 Å -1 (d spacing ≈ 6.98 Å) decreases with the increase of the TTE concentration, indicating that the local solvation structure of LHCEs has transformations relating to solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs), and aggregates (AGGs). 2D 1 H- 19 F heteronuclear overhauser spectroscopy (HOESY) NMR technique is employed to investigate the interaction between different components in the electrolyte via hydrogen-fluorine intermolecular bonding. With the addition of TTE into the HCEs, the solvent-diluent correlation can be discovered but the interaction between anions and solvent/diluent molecules is negligible. To verify the coordination between DME and Li + ions, internally referenced diffusion-ordered spectroscopy (IR-DOSY) NMR is applied. Compared to D DME (1.093 x 10 -9 m 2 /s) in the solvent-diluent solution (i.e. DME:TTE 1.2:3), D DME (0.149 x 10 -9 m 2 /s) in LHCE (i.e. LiFSI:DME:TTE 1:1.2:3) is smaller 7.3 times, contributing to the strong coordination between DME and Li + cations. Meanwhile, the transference number of Li + ions is hindered from the HCE regime to the LHCE regime despite the enhancement of the diffusion coefficient of DME and TTE. These discoveries allow us to evaluate DME⋅⋅⋅Li + ⋅⋅⋅FSI - as a system rather than separate components, providing insight into the relationships between the microstructure properties of LHCEs and electrolyte formulation, aiming to enhance LMB performance.