Molecular Level Origin of Ion Dynamics in Highly Concentrated Electrolytes.

Single-ion conducting liquid electrolytes are key to achieving rapid charge/discharge in Li secondary batteries. The Li+ transference (or transport) numbers are the defining properties of such electrolytes and have been discussed in the framework of concentrated solution theories. However, the connection between macroscopic transference and microscopic ion dynamics remains unclear. Molecular dynamics simulations were performed to obtain direct information regarding the microscopic behaviors in highly concentrated electrolytes, and the relationships between these behaviors and the transference number were determined under anion-blocking conditions. Various solvents with different donor numbers (DNs) were used along with a Li salt of the weakly Lewis basic bis(fluorosulfonyl)amide anion for electrolyte preparation. Favorable ordered Li+ structuring and a continuous Li+ conduction pathway were observed for the fluoroethylene carbonate-based electrolyte due to its low DN. The properties were less pronounced at higher DNs, e.g., for the dimethyl sulfoxide-based electrolyte. The τLi-solventlife/τdipolerelax ratio was introduced as a factor for ion dynamics, and the two mechanisms of ion transport were considered an exchange mechanism (τLi-solventlife/τdipolerelax < 1) and a vehicle mechanism (translational motion of solvated Li+) (τLi-solventlife/τdipolerelax ≥ 1). Vehicle-type transport was dominant with high DNs, while exchangeable transport was preferable at lower DNs. These findings should aid the further selection of solvents and Li salts to prepare single-ion conducting electrolytes.

High-concentration LiPF6/sulfone electrolytes: structure, transport properties, and battery application.

Non-flammable and oxidatively stable sulfones are promising electrolyte solvents for thermally stable high-voltage Li batteries. In addition, sulfolane-based high-concentration electrolytes (HCEs) show high Li+ ion transference numbers. However, LiPF6 has not yet been investigated as the main salt in sulfone-based HCEs for Li batteries. In this study, we investigated the phase behaviors, solvate structures, and transport properties of binary and ternary mixtures of LiPF6 and the following sulfone solvents: sulfolane (SL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and 3-methyl sulfolane (MSL). The stable crystalline solvates Li(SL)4PF6 and Li(DMS)2.5PF6 with high melting points were formed in the LiPF6/SL and LiPF6/DMS mixtures, respectively. In contrast, LiPF6/EMS, LiPF6/MSL, and LiPF6/SL/another sulfone mixtures remained liquids over a wide temperature range. Raman spectroscopy revealed that SL and another sulfone are competitively coordinated to Li+ ions to dissociate LiPF6 in the ternary mixtures. Although the ionic conductivity decreased with increasing LiPF6 concentration due to an increase in viscosity, Li+ ions diffused faster than PF6-via exchanging ligands in the HCE [LiPF6]/[SL]/[DMS] = 1/2/2, resulting in a higher Li ion transference number than that in conventional Li battery electrolytes.

Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers.

We demonstrate that tetra-arm poly(ethylene glycol) gels containing highly concentrated sulfolane-based electrolytes exhibit high Li+ transference numbers. The low polymer concentration and homogeneous polymer network in the gel electrolyte are useful in achieving both mechanical reliability and high Li+ transport ability.

Does Li-ion transport occur rapidly in localized high-concentration electrolytes?

The ionic conductivity and lithium-ion transference number of electrolytes significantly influence the rate capability of Li-ion batteries. Highly concentrated Li-salt/sulfolane (SL) electrolytes exhibit elevated Li+ transference numbers due to lithium-ion hopping via a ligand exchange mechanism within their -Li+-SL-Li+- network. However, highly concentrated electrolytes (HCEs) are extremely viscous and have an ionic conductivity that is one order of magnitude less than that of conventional electrolytes. Dilution of HCEs with a non-coordinating hydrofluoroether (HFE) lowers the viscosity and produces localized high-concentration electrolytes (LHCE). However, the mechanism of Li+ transport in LHCEs is unclear. This study investigated the transport properties of LHCEs prepared by diluting a SL-based HCE with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Electrolyte viscosity decreases dramatically upon dilution, whereas ionic conductivity increases only slightly. Ion diffusivity increases with increasing HFE content due to the decrease in electrolyte viscosity. However, the Li+ transference number declines, because the HFE interferes with conduction via the Li+ hopping mechanism. The resulting decrease in the product of ionic conductivity and Li+ transference number indicates superior lithium-ion transport in the parent HCE compared with LHCEs.

Solvate electrolytes for Li and Na batteries: structures, transport properties, and electrochemistry.

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 in the solution. The activity (concentration) of the uncoordinated solvent decreases as the salt concentration is increased. At extremely high salt concentrations, all the solvent molecules are involved in the coordination of the ions and form the solvates of the salts. In this article, we review the structures, transport properties, and electrochemistry of Li/Na salt solvates. In molten solvates, the activity of the uncoordinated solvent is negligible; this is the main origin of their peculiar characteristics, such as high thermal stability, wide electrochemical window, and unique ion transport. In addition, the solvent activity greatly influences the electrochemical reactions in Li/Na batteries. We highlight the attractive features of molten solvates as promising electrolytes for next-generation batteries.

Structural Effects of Solvents on Li-Ion-Hopping Conduction in Highly Concentrated LiBF4/Sulfone Solutions.

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 LiBF4/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 BF4-) rapidly to produce unusual hopping conduction in highly concentrated electrolytes. The structure of the solvent significantly influences the hopping conduction process. We measured the self-diffusion coefficients of Li+ (DLi), anions (Danion), and sulfone solvents (Dsol) in electrolytes. The ratio of the self-diffusion coefficients (DLi/Dsol) 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 (tLi+abc) under anion-blocking conditions, and tLi+abc of [LiBF4]/[cyclic sulfone] = 1/2 is as high as 0.8.

Highly concentrated LiN(SO2CF3)2/dinitrile electrolytes: Liquid structures, transport properties, and electrochemistry.

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.

Li-ion hopping conduction in highly concentrated lithium bis(fluorosulfonyl)amide/dinitrile liquid electrolytes.

Li+ ion hopping conduction in highly concentrated solutions of lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in dinitrile solvents, namely succinonitrile, glutaronitrile, and adiponitrile, was investigated. Phase behaviors of the LiFSA/dinitrile binary mixtures assessed by differential scanning calorimetry suggested that LiFSA and the dinitriles form stable solvates in a molar ratio of 1 : 2. For succinonitrile, a glass forming room temperature liquid is formed when [LiFSA]/[succinonitrile] > 1. The corresponding glutaronitrile and adiponitrile mixtures have melting points below 60 °C. The self-diffusion coefficients of Li+, FSA-, and dinitrile measured with pulsed field gradient NMR suggested that Li+ ion diffuses faster than anion and dinitrile in the liquids of composition [LiFSA]/[dinitrile] = 1/0.8, indicating emergence of Li+ ion hopping conduction. X-ray crystallography for the LiFSA-(dinitrile)2 solvates and Raman spectroscopy for the liquids with composition [LiFSA]/[dinitrile] > 1 revealed that the two cyano groups of the dinitrile coordinate to two different Li+ ions and form solvent-bridged structures of (Li+-dinitrile-Li+). In addition, the Raman spectra suggested that ionic aggregates (Li+-FSA--Li+) are formed in the liquids with composition [LiFSA]/[dinitrile] > 1. Although there is frequent ligand (dinitrile and/or anion) exchange for each Li+ ion in the liquid state, the polymeric network structures (solvent-bridged structure and ionic aggregates) restrict the facile motion of ligands because each ligand is interacting with multiple Li+ ions in the highly concentrated electrolytes. This induces the faster diffusion of the Li+ ion than that of the ligands, i.e., hopping conduction of Li+ through ligand exchange. Electrochemical measurements clarified that the [LiFSA]/[succinonitrile] = 1/0.8 electrolyte possesses a relatively high Li+ transport ability (limiting current density > 7 mA cm-2) thanks to the Li+ hopping conduction, regardless of its extremely high viscosity (3142 mPa s) and relatively low conductivity (0.26 mS cm-1) at room temperature. Furthermore, this electrolyte was shown to have a high Li+ transference number (>0.6), exhibited reversible Li metal deposition/dissolution i.e. suppression of reductive decomposition of the solvent, and could be successfully applied to graphite and LiNi1/3Mn1/3Co1/3O2 half-cells.

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

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 LiBF4 and sulfolane (SL). Self-diffusion coefficients of Li+ ( DLi), BF4- ( DBF4), and SL ( DSL) were measured with pulsed-field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF4 ≤ 3, the ratios DSL/ DLi and DBF4/ DLi become lower than 1, suggesting faster diffusion of Li+ than SL and BF4-, and thus the evolution of Li+ hopping conduction. X-ray crystallographic analysis of the LiBF4/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 BF4- anion also participates in the bridging coordination of Li+. The Raman spectra of the highly concentrated LiBF4-SL solution suggested that Li+ ions are bridged by SL and BF4- even in the liquid state. Moreover, detailed investigation along with molecular dynamics simulations suggests that Li+ exchanges ligands (SL and BF4-) 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.