Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes

Abstract

We demonstrate that Li + hopping conduction, which cannot be explained by conventional models i.e., Onsager's theory and Stokes' law, emerges in highly concentrated liquid electrolytes composed of LiBF 4 and sulfolane (SL). Self-diffusion coefficients of Li + (D Li ), BF 4 − (D BF 4 ), and SL (D SL ) were measured with pulsed-field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF 4 ≤ 3, the ratios D SL /D Li and D BF 4 /D Li become lower than 1, suggesting faster diffusion of Li + than SL and BF 4 − , and thus the evolution of Li + hopping conduction. X-ray crystallographic analysis of the LiBF 4 /SL (1:1) solvate revealed that the two oxygen atoms of the sulfone group are involved in the bridging coordination of two different Li + ions. In addition, the BF 4 − anion also participates in the bridging coordination of Li + . The Raman spectra of the highly concentrated LiBF 4 −SL solution suggested that Li + ions are bridged by SL and BF 4 − even in the liquid state. Moreover, detailed investigation along with molecular dynamics simulations suggests that Li + exchanges ligands (SL and BF 4 − ) dynamically in the highly concentrated electrolytes, and Li + hops from one coordination site to another. The spatial proximity of coordination sites, along with the possible domain structure, is assumed to enable Li + hopping conduction. Finally, we demonstrate that Li + hopping suppresses concentration polarization in Li batteries, leading to increased limiting current density and improved rate capability compared to the conventional concentration electrolyte. Identification and rationalization of Li + ion hopping in concentrated SL electrolytes is expected to trigger a new paradigm of understanding for such unconventional electrolyte systems.

Ionic transport in highly concentrated lithium bis(fluorosulfonyl)amide electrolytes with keto ester solvents: structural implications for ion hopping conduction in liquid electrolytes

“…more rapidly than solvent molecules and anions) in sulfolane (SL)-based highly concentrated electrolytes. 19 Here we note that another group also reported the fastest diffusion of Li ions in SL-based concentrated electrolytes. 20 This provides clear experimental evidence to suggest that Li ion hopping or exchange mechanisms make a significant contribution to Li ion diffusion.…”

“…29 In contrast to these examples, the diffusion behavior observed for the highly concentrated Li[FSA]/keto ester systems is more akin to our recently published observations for SL-based concentrated electrolytes. 19 , marked a high value ranging from 0.54 to 0.60 for MP-and MAbased electrolytes, while t Li was found to be lower than 0.5 for the ML-based electrolytes (see the ESI, † Fig. S1).…”

Concentrated Ionic‐Liquid‐Based Electrolytes for High‐Voltage Lithium Batteries with Improved Performance at Room Temperature

“…Additionally, the cell employing DEMEFTFSI 0.6 LiFTFSI 0.4 as the electrolyte fully recovered its capacity when the C‐rate was brought back to C/10. In contrast, the cell employing DEMEFTFSI 0.9 LiFTFSI 0.1 delivered an extremely low capacity at 5 C, which was probably owing to the lower diffusion limiting current density . The selected voltage profiles of the cells employing the 10 and 40 mol % electrolytes are presented in Figure c and d, respectively.…”

“…However, the formation of aggregate species seems to have a rather different influence on the transport of Li + ions. It has been proposed that, in a concentrated electrolyte, the Li + transport mechanism changes from a diffusion‐controlled vehicular transport of small Li + complexes to hopping‐type ion transport through the exchange of anions in the Li + ‐containing aggregate species . This must be true to explain the exceptional performance of the Li/LMR cell employing the DEMEFTFSI 0.6 LiFTFSI 0.4 at high rates, despite its substantially lower conductivity and higher viscosity than the electrolytes containing 10 and 20 mol % LiFTSFI.…”

“…In contrast, the cell employing DE-MEFTFSI 0.9 LiFTFSI 0.1 delivered an extremelyl ow capacity at 5C, which was probablyo wing to the lower diffusion limiting current density. [36,37] The selected voltage profiles of the cells employingt he 10 and4 0mol %e lectrolytes are presented in Figure 4c and d, respectively.T he cell polarization of the concentrated electrolyte increased gradually with the cycling rate whereas an abrupt polarization increase was observed for the dilutede lectrolyte above C/2.…”