Solvent–Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes

The latest developments of localized high-concentration electrolytes (LHCEs) shed light on stabilizing the high-energy-density lithium (Li) metal batteries. It is generally considered that the nonsolvating diluents introduced into the LHCEs improve the viscosity and wettability of high-concentration electrolytes (HCEs) without changing their inner solvation structures, thus maintaining the highly coordinated contact ion pairs (CIPs) and ionic aggregates (AGGs) of the precursor HCEs with limited free solvent numbers and high Coulombic efficiency (CE) of Li metal anodes. Herein, we show an unexpected effect of the diluent amount on the solvation structures of the LHCEs: as the diluent amount increases, the proportions of free solvent molecules and CIPs rise up simultaneously. The latter is probably due to the partial splits of the AGGs via the dipole-dipole interactions between the diluent and solvent molecules. Accordingly, a moderately diluted LHCE shows the best Coulombic efficiency of Li metal anodes (99.6%), compared with the precursor HCE (97.4%) or highly diluted LHCE (99.0%). This work reveals a new criterion of the LHCE chemical formulation for the designing of advanced electrolytes for high-energy-density batteries.