Combining highly concentrated electrolytes with a polymer network is a valid approach to simultaneously achieve fast Li + ion transport, high thermal stability, and a wide electrochemical window in a quasi-solid-state form. In this work, flexible gel electrolytes comprising commercially available poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and highly concentrated electrolytes of Li salts/sulfolane (SL) were prepared by a simple solution casting method. The anionic effects of the gel electrolytes on the Li-ion conductivity and charge transfer kinetics at the gel/electrode interface were investigated. The SL-based gel electrolyte with lithium bis(fluorosulfonyl)amide (LiFSA) showed an ionic conductivity of 0.7 mS cm −1 and a high Li transference number (> 0.5) at 30 °C. The charge transfer resistance in a [Li/gel/LiCoO2] cell with LiFSA was lower than that of the cells with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or LiBF4, indicating faster interfacial charge transfer kinetics in the gel electrolyte with FSA. The Li/LiCoO2 cell with the LiFSA/SL gel electrolyte exhibited a higher capacity than that of the cells with the LiTFSA/SL and LiBF4/SL gel electrolytes. Hence, rationally designed gel electrolytes containing highly concentrated SL-based electrolytes enable the high rate performance of Li batteries.

Herein, distinct differences in Li-deposition/dissolution reversibility were found in sulfolane (SL)-based electrolytes, depending on the Li-salt species and their solvation structures, owing to changes in the composition and nature of the solid-electrolyte interphase (SEI) and in the Li-deposit morphology. For this purpose, two lithium salts, lithium bis(trifluoromethanesulfonyl)amide (Li [TFSA]) and lithium bis(fluorosulfonyl)amide (Li[FSA]) were selected. Relatively low-concentration electrolytes (1 mol dm −3 , LCEs), high-concentration electrolytes (∼3 mol dm −3 , HCEs), and localized high-concentration electrolytes (HCEs diluted by a noncoordinating solvent at 1 mol dm −3 , LHCEs) were prepared to alter the solvation structures. The Coulombic efficiency (CE) for Li deposition/dissolution was better in the Li[FSA] solutions than in the Li[TFSA] solutions. Particularly, the CE of the Li[FSA] HCE and LHCE solutions reached 98−99%. The reduction potentials of the chemical species in these solutions followed the order E Li/Li + < E SL < E anion . Reflecting on the change in the solvation structures, E Li/Li + and E anion increased in the following order: LCE < HCE < LHCE, which was established by both experiments and DFT-MD calculations. The anion reduction current for the formation of the SEI was much larger than the SL reduction current and was the largest in the LHCEs for both Li[TFSA] and Li[FSA] solutions. Thus, SEI formation may be mainly attributed to anion reduction, which was accelerated in the HCEs and LHCEs. However, the compositions of the formed SEI were different; the SEI for the Li[TFSA] solutions was rich in anion fragments and organic compounds, whereas that for the Li[FSA] solutions was rich in LiF and inorganic compounds. The difference in the SEI formation process was also supported by DFT-MD calculations. The Li-deposit morphology increased in the order LCE < HCE < LHCE in accordance with the increasing CE. However, the nature and composition of the SEI were the most critical factors for enhancing Li-deposition/dissolution reversibility.

Certain highly concentrated electrolytes (HCEs) enhance the charge-transfer reaction rate at the electrode/electrolyte interface in Li-ion batteries. The solvation structure of Li+ in HCEs significantly affects the electrochemical interfacial reaction kinetics. However, the effects of different anions on these kinetics have not yet been fully understood. Therefore, in this study, we investigated the effects of anionic species on the liquid structure and electrochemical reactions of HCEs composed of various Li salts and propylene carbonate (PC). Raman spectra revealed that both PC and anions were coordinated to Li+ ions in HCEs and that the concentration of uncoordinated (free) PC changed depending on the anionic species. Consequently, the activity of Li+ in the electrolyte changed depending on the anionic species. The use of Li salts with weakly Lewis basic anions increased the activity of Li+ and decreased the concentration of free PC in HCEs. The activity of Li+ in the electrolyte significantly affected the Li+ intercalation/deintercalation reaction rate of the LiCoO2 thin-film electrode. Electrochemical impedance spectroscopy revealed that the interfacial reaction rate of LiCoO2 was enhanced in the HCEs with anions having weaker Lewis basicity owing to the higher Li+ ion activity.

The design and fabrication of lithium secondary batteries with a high energy density and shape flexibility are essential for flexible and wearable electronics. In this study, we fabricated a high-voltage (5 V class) flexible lithium polymer battery using a lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ) cathode. A LiNi 0.5 Mn 1.5 O 4 -hybridized gel polymer cathode (GPC) and a gel polymer electrolyte (GPE) membrane, both containing a sulfolane (SL)-based highly concentrated electrolyte (HCE), enabled the fabrication of a polymer battery by simple lamination with a metallic lithium anode, where the injection of the electrolyte solution was not required. GPC with high flexibility has a hierarchically continuous three-dimensional porous architecture, which is advantageous for forming continuous ion-conduction paths. The GPE membrane has significant ionic conductivity enough for reliable capacity delivery. Therefore, the fabricated lithium polymer pouch cells demonstrated excellent capacity retention under continuous deformation conditions. This study provides a promising strategy for the fabrication of scalable and flexible 5 V class batteries using GPC and GPE containing SL-based HCE.

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