Linear ether-based highly concentrated electrolytes for Li-sulfur batteries.

Li-S batteries have attracted attention as next-generation rechargeable batteries owing to their high theoretical capacity and cost-effectiveness. Sparingly solvating electrolytes hold promise because they suppress the dissolution and shuttling of polysulfide intermediates to increase the coulombic efficiency and extend the cycle life. This study investigated the solubility of polysulfide (Li2S8) in a range of liquid electrolytes, including organic electrolytes, highly concentrated electrolytes, and ionic liquids. The Li2S8 solubility was well correlated with the donor number (DNNMR), estimated via23Na-NMR, and was lower than 100 mM_(elemental sulfur) in electrolytes with DNNMR < 14, regardless of the type of electrolyte. Highly concentrated electrolytes comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and linear chain dialkyl ethers such as methyl propyl ether (MPE), n-butyl methyl ether (BME), and ethyl propyl ether (EPE) were studied as sparingly solvating electrolytes for Li-S batteries. Monomethyl ethers, such as BME, showed more pronounced Li-ion coordination and higher ionic conductivity, whereas the steric hindrance of the longer alkyl chains in EPE lowered the solvation number, enhanced ion association, and lowered the ionic conductivity despite the solvents having similar dielectric constants. The charge-discharge rate capabilities of Li-S cells with dialkyl ether-based electrolytes were more impressive than those of cells with a localized high-concentration electrolyte using sulfolane (SL) and hydrofluoroether (HFE), [Li(SL)2][TFSA]-2HFE. The higher rate performance was attributed to the superior Li-ion transport properties of the dialkyl ether-based electrolytes. A pouch-type cell using lightweight [Li(BME)3][TFSA] demonstrated an energy density exceeding 300 W h kg-1 under lean electrolyte conditions.

Evolving better solvate electrolytes for lithium secondary batteries.

The overall performance of lithium batteries remains unmatched to this date. Decades of optimisation have resulted in long-lasting batteries with high energy density suitable for mobile applications. However, the electrolytes used at present suffer from low lithium transference numbers, which induces concentration polarisation and reduces efficiency of charging and discharging. Here we show how targeted modifications can be used to systematically evolve anion structural motifs which can yield electrolytes with high transference numbers. Using a multidisciplinary combination of theoretical and experimental approaches, we screened a large number of anions. Thus, we identified anions which reach lithium transference numbers around 0.9, surpassing conventional electrolytes. Specifically, we find that nitrile groups have a coordination tendency similar to SO2 and are capable of inducing the formation of Li+ rich clusters. In the bigger picture, we identified a balanced anion/solvent coordination tendency as one of the key design parameters.

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.

Ion Transport in Glyme- and Sulfolane-Based Highly Concentrated Electrolytes.

Highly concentrated electrolytes (HCEs) have a similarity to ionic liquids (ILs) in high ionic nature, and indeed some of HECs are found to behave like an IL. HCEs have attracted considerable attention as prospective candidates for electrolyte materials in future lithium secondary batteries owing to their favorable properties both in the bulk and at the electrochemical interface. In this study, we highlight the effects of the solvent, counter anion, and diluent of HCEs on the Li+ ion coordination structure and transport properties (e. g., ionic conductivity and apparent Li+ ion transference number measured under anion-blocking conditions, t L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ ). Our studies on dynamic ion correlations unveiled the difference in the ion conduction mechanisms in HCEs and their intimate relevance to t L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ values. Our systematic analysis of the transport properties of HCEs also suggests the need for a compromise to simultaneously achieve high ionic conductivity and high t L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ values.

Li+ transference number and dynamic ion correlations in glyme-Li salt solvate ionic liquids diluted with molecular solvents.

Highly concentrated electrolytes (HCEs) have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to various beneficial properties both in the bulk and at the electrode/electrolyte interface. One particular class of HCEs consists of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and oligoethers that behave like ionic liquids. [Li(G4)][TFSA], which comprises an equimolar mixture of LiTFSA and tetraglyme (G4), is an example. In our previous works, the addition of low-polarity molecular solvents to [Li(G4)][TFSA] was found to effectively enhance the conductivity while retaining the unique Li-ion solvation structure. However, it remains unclear how the diluents affect another key electrolyte parameter-the Li+ transference number-despite its critical importance for achieving the fast charging/discharging of Li secondary batteries. Thus, in this study, the effects of diluents on the extremely low Li+ transference number under anion-blocking conditions in [Li(G4)][TFSA] were elucidated, with a special focus on the polarity of the additional solvents. The concentration dependence of the dynamic ion correlations was further studied in the framework of the concentrated electrolyte theory. The results revealed that a non-coordinating diluent is not involved in the modification of the ion transport mechanism, and therefore the low Li+ transference number is inherited by the diluted electrolytes. In contrast, a coordinating diluent effectively reduces the anti-correlated ion motions of [Li(G4)][TFSA], thereby improving the Li+ transference number. This is the first time that the significant effects of the coordination properties of the diluting solvents on the dynamic ion correlations and Li+ transference numbers have been reported for diluted solvate ionic liquids.