A solvation-compressed electrolyte is introduced, consisting of 2 M Li bis(fluorosulfonyl)imide (LiFSI) dissolved in a mixture of dimethoxyethane (DME) and 2,2-dichlorodiethylether (ClDEE) with a volume ratio of 1:7 (denoted as Cl 7 electrolyte). The solubility of the Li salt in various solvents is initially examined (Supplementary Fig. 1). It shows that ClDEE alone has a solubility for LiFSI of less than 0.05 mol/L, which can be considered a non-solvating diluent. Additionally, when DME is mixed with 16 mol/L LiFSI, there are clearly many undissolved precipitates in the electrolyte, indicating that DME alone cannot fully dissociate such a large amount of Li salts. Yet, when DME is combined with ClDEE at a 1:7 volume ratio, 2 M LiFSI can dissolve entirely and shows a smooth flow. This signifies the attainment of a record-breaking 16 mol L−1 concentration of Li salts within the localized solvation phase (here, within the DME phase). Given such a high concentration of FSI− around Li+, inorganic-rich robust SEI derived by decomposition of anion can be anticipated. In addition, ClDEE features a boiling point of up to 178 °C38, exceeding that of the majority of ether solvents reported, revealing its considerable potential for practical exploitation.
Since the composition of electrode/electrolyte interphases as well as the dynamic behavior of Li ion at the interface are intimately linked to the solvation structure, we undertook an in-depth analysis of the solvation structures of the electrolytes. Molecular dynamics (MD) simulations were carried out to inquire into the distribution scenario of anions and solvents around Li ions at the microscopic scale (Fig. 2a–d, Supplementary Fig. 2 and Supplementary Data 1). In conventional high-concentration electrolyte that is nearly saturated at room temperature (10 M LiFSI in DME, donated as 10 M electrolyte), the coordination numbers of ODME at ~2.06 Å and OFSI at ~2.19 Å are 2.02 and 1.99 respectively, reflecting a significant concentration of anions occupies the primary solvation shell of Li+. In the Cl 7 electrolyte, the coordination of FSI− has become markedly stronger, with the coordination number of OFSI rising to 2.51, significantly higher than 1.13 observed for ODME. Moreover, the oxygen atoms from ClDEE are closely aligned beyond the primary solvation shell of Li+ at around 3 Å, resembling a compressed outer layer that encapsulates the primary solvation shell of Li+. Results from the statistical evaluation of various solvation clusters also reveal that the Cl 7 electrolyte contains a markedly greater proportion of FSI−-dominated solvation clusters (Supplementary Fig. 3). Additionally, the presence of a significant amount of ClDEE enhances the wettability of the electrolyte with tight ion aggregation and ensures acceptable Li+ transport properties (Supplementary Figs. 4, 5, Supplementary Note 1).
Radical distribution functions (g(r), solid lines) and coordination number (n(r), dashed lines) in (a) 10 M electrolyte and (b) Cl 7 electrolyte. Snapshots of the MD simulation boxes of (c) 10 M and (d) Cl 7 electrolytes. e 7Li NMR spectra of different electrolytes. f WAXS reveals the presence of a shorter interatomic correlation distance within the Cl 7 electrolyte, forming more compact solvation clusters.
The results of the simulation calculations are further confirmed by experimental characterization. From the results of 7Li nuclear magnetic resonance (NMR) spectra (Fig. 2e), as the concentration of LiFSI in DME rises from 2 M to 10 M, the 7Li peak experiences a progressive downward shift. This is attributed to the fact that FSI− anions, which have a weaker electron cloud density compared to DME, are progressively occupying the solvation shell of Li+ and the solvation structure evolves from solvent-dominated to anion-dominated gradually39,40. However, the electron cloud density surrounding Li+ in the Cl 7 electrolyte is noticeably higher compared to typical high-concentration electrolytes. We attribute this anomalous phenomenon to the reduced distance between Li+ and FSI− anions within the compressed solvation structure of the Cl 7 electrolyte, leading to the formation of smaller solvation clusters and a denser electron cloud around the Li+. Wide-angle X-ray scattering (WAXS) was further employed to probe the information for solvation clusters. In Fig. 2f, the WAXS results show that in traditional single-phase DME-based electrolytes, there are two typical solvation clusters corresponding to the scattering vector (
