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.

Following our recent study demonstrating predominant Li-ion hopping conduction in sulfolane (SL)-based highly concentrated electrolytes with LiBF4, LiClO4, and lithium bis­(fluorosulfonyl)­amide, herein a systematic study on transport properties and Li-ion coordination of SL-based electrolytes with lithium bis­(trifluoromethanesulfonyl)­amide was performed. In the highly concentrated region, Li ions clearly diffuse faster than SL and TFSA anions. The two oxygen atoms of the SL sulfonyl group tend to coordinate to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions, verifying the previous observation of the unusual Li-ion conduction and its relevance to the SL- and anion-bridged, chainlike Li-ion coordination structure for the SL-based concentrated systems with other Li salts. Moreover, addition of hydrofluoroether (HFE) to the SL-based concentrated electrolytes greatly enhances diffusion coefficients but fragments the chainlike Li-ion coordination to smaller clusters, leading to a reduced contribution of Li-ion hopping to the overall Li-ion conduction. The SL-based concentrated electrolyte and its mixtures with HFE showed lower lithium polysulfide solubility and higher rate capability for lithium–sulfur (Li–S) cells compared with previously reported tetraglyme-based electrolytes. The SL-based electrolytes were found to manifest a significant improvement in Li-ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li–S batteries.

Li+ ion hopping conduction through ligand (solvent and anion) exchange emerges in solvent-deficient liquid electrolytes of [Li salt]/[dinitrile] > 1.

Polar solvents dissolve Li and Na salts at high concentrations and are used as electrolyte solutions for batteries. The solvents interact strongly with the alkali metal cations to form complexes...

Sulfones are polar molecules that can be used as thermally stable electrolyte solvents for Li-ion batteries (LIBs). Li salts form stoichiometric solvates with sulfones in the electrolytes. The melting points of the solvates tend to be higher than room temperature, thereby limiting the operating temperature range of the batteries. In this study, the applicability of ternary eutectic mixtures of LiN­(SO2F)2 (LiFSA), sulfolane (SL), and dimethyl sulfone (DMS) as LIB electrolytes was assessed. Relative to the binary LiFSA–sulfone electrolytes, the ternary eutectic electrolytes remained liquid over a wide temperature range due to the increased entropy of mixing. Sulfone-bridged Li+–sulfone–Li+ and anion-bridged Li+–FSA––Li+ network structures were formed in the eutectic electrolyte with a composition of [LiFSA]/[SL]/[DMS] = 1/1.5/1.5. Pulsed-field gradient NMR measurements revealed that the Li+ ion dynamically exchanges sulfones and anions and diffuses more rapidly than these ligands, resulting in the relatively high Li+ transference number of the electrolyte. Highly reversible charge–discharge processes of the LiCoO2 and graphite electrodes were attained using the ternary eutectic electrolyte. The rate capability of the Li/LiCoO2 cell in the eutectic electrolyte was comparable to that of the cell in the conventional 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate solution despite its lower ionic conductivity.

Li-ion-hopping conduction is known to occur in certain highly concentrated electrolytes, and this conduction mode is effective for achieving lithium batteries with high rate capabilities. Herein, we investigated the effects of the solvent structure on the hopping conduction of Li ions in highly concentrated LiBF 4 /sulfone electrolytes. Raman spectroscopy revealed that a Li + ion forms complexes with sulfone and anions, and contact ion pairs and ionic aggregates are formed in the highly concentrated electrolytes. Li + exchanges ligands (sulfone and BF 4 − ) rapidly to produce unusual hopping conduction in highly concentrated electrolytes. The structure of the solvent significantly influences the hopping conduction process. We measured the selfdiffusion coefficients of Li + (D Li ), anions (D anion ), and sulfone solvents (D sol ) in electrolytes. The ratio of the self-diffusion coefficients (D Li /D sol ) tended to be higher for cyclic sulfones (sulfolane and 3-methylsulfolane) than for acyclic sulfones, which suggests that cyclic sulfone molecules facilitate Li-ion hopping. The hopping conduction increases the Li + -transference number (t Li abc + ) under anion-blocking conditions, and t Li abc + of [LiBF 4 ]/[cyclic sulfone] = 1/2 is as high as 0.8.

Li deposition and dissolution in highly concentrated electrolytes consisting of sulfolane (SL) and two amide-type Li salts, LiN(SO 2 CF 3 ) 2 (LiTFSA) and LiN(SO 2 F) 2 (LiFSA), were investigated. The chronopotentiometry test of Li/Cu cells containing these two electrolytes demonstrated that the reversibility of Li deposition/dissolution and the cycling performance were better in the LiFSA/SL electrolyte than in the LiTFSA/SL electrolyte. Gas analysis with electrochemical mass spectroscopy revealed that the SL molecules were reduced to form tetrahydrothiophene (THT) and butane in the LiTFSA/SL electrolyte during Li deposition. In contrast, these side reactions were significantly suppressed in the LiFSA/SL electrolyte. The X-ray photoelectron spectroscopy analysis for the deposited Li in the LiTFSA/SL electrolyte suggests that Li 2 O and sulfurous compounds were formed on the Li surface by the reductive decomposition of SL. For the LiFSA/SL electrolyte, the LiF-rich passivation layer derived from the FSA anion could effectively suppress further decomposition of SL, resulting in highly reversible Li deposition and dissolution.

Liquid structures, transport properties, and electrochemical properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and dinitrile solvents [succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN)] were investigated. In the LiTFSA/SN and LiTFSA/ADN systems, the stable crystalline solvates of LiTFSA–(SN)1.5 [melting point (Tm): 59 °C] and LiTFSA–(ADN)1.5 (Tm: 50 °C) were formed, respectively. In contrast, the LiTFSA/GN mixtures of a wide range of compositions were found to be glass-forming liquids at room temperature. Raman spectroscopy of LiTFSA/GN liquid mixtures revealed that increasing the LiTFSA concentration results in the formation of the solvent-bridged network structure Li+–GN–Li+. In addition, the considerable formation of contact ion pairs and ionic aggregates was observed in highly concentrated electrolytes. In the liquids, the Li+ ion dynamically exchanged ligands (GN and TFSA) and higher LiTFSA concentrations led to an increase in the ratio of the self-diffusion coefficients of Li+ and TFSA−, DLi/DTFSA, as determined by pulsed field gradient NMR spectroscopy. The Li+ transference number (tLi+) of the [LiTFSA]/[GN] = 1/1.5 electrolyte in an electrochemical cell under anion-blocking conditions was estimated to be as high as 0.74. Furthermore, electrochemical measurements revealed that the reductive stability of the LiTFSA/GN electrolyte increases with increasing LiTFSA concentration. A [LiTFSA]/[GN] = 1/1.5 electrolyte is stable against the Li metal electrode, provided that the polarization is relatively small. Owing to high tLi+, a Li–S battery with the [LiTFSA]/[GN] = 1/1.5 electrolyte showed a high rate discharge capability despite its low ionic conductivity (0.21 mS cm−1) at room temperature.