A structural study on a specific Li-ion ordered complex in dimethyl carbonate-based dual-cation electrolytes.

Dimethyl carbonate (DMC) is a linear carbonate solvent commonly used as an electrolyte for electric double-layer capacitors (EDLCs) and Li-ion batteries. However, there are serious problems with the use of DMC as an electrolyte solvent: (1) low ionic conductivity when using Li salts (e.g. LiBF4) and (2) liquid-liquid phase separation when using spiro-type quaternary ammonium salts (e.g. SBPBF4). Dual-cation electrolytes, i.e., bi-salt (SBPBF4 and LiBF4) in DMC, are promising candidates to avoid the phase separation issue and to enhance the total and Li+ conductivities. Herein, we reported a specific Li-ion structure in DMC-based dual-cation electrolytes by combining high-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations. Quantitative radial distribution function analysis based on experimental and simulation results revealed that the phase-separated SBPBF4/DMC (i.e., the bottom phase of 1 M SBPBF4/DMC) forms long-range ion ordering based on the structured SBP+-BF4- ion pairs. When adding LiBF4 salt into SBPBF4/DMC (i.e., dual-cation electrolyte), the ordered SBP+-BF4- structure disappeared owing to the formation of Li-ion solvation complexes. We found that in the dual-cation electrolyte Li ions form multiple Li+-Li+ ordered complexes in spite of relatively low Li-salt concentration (1 M), being a promising Li+-conducting medium with reduced Li salt usage and low viscosity.

Ionic Liquids for the Chemical Recycling of Polymeric Materials and Control of Their Solubility.

Plastics are wonderful materials that have modernized our daily life; however, importance of effective recycling of plastics is gradually recognized widely. In this account, we describe our discovery of new and efficient methods for the chemical recycling of plastics using ionic liquids (ILs). Since the chemical recycling usually requires high temperature conditions to breakdown chemical bonds in polymeric materials, we thought that less-flammability and non-volatility of ionic liquids are the most suitable physical properties for this purpose. Ionic liquids successfully depolymerized polyamides and unsaturated polyesters smoothly and corresponding monomeric materials were obtained in good yields. To the best of our knowledge, this was the first use of Ionic liquids for such reactions. However, we encountered another difficult problem-separation. To solve the problem, we developed solubility-switchable ionic liquids, a new type of ionic liquids in which solubility is readily changed using the chemistry of protective groups. Conversion between hydrophilic and lipophilic forms was readily achieved using a simple chemical treatment under mild conditions, and the complete separation of products was achieved by liquid-liquid-extraction. The robustness of either form unlocks their wide use as reaction solvents.

Controlling mechanical properties of ultrahigh molecular weight ion gels by chemical structure of ionic liquids and monomers.

A new class of ion gels, termed ultrahigh molecular weight (UHMW) gels, formed by physical entanglement of ultrahigh molecular weight polymers in ionic liquids, are synthesised using facile one step radical polymerisation with significantly low initiator conditions, and exhibit superior mechanical characteristics such as stretchability, recyclability, and room temperature self-healing ability. In this study, UHMW gels are synthesised using various combinations of monomer and IL structures, and the effect of their chemical structures on the physicochemical properties of UHMW gels are thoroughly investigated. UHMW polymers are prepared in situ for all combinations of ILs and monomers used in this study, indicating the wide applicability of this fabrication strategy. The structure-property relationships between chemical structures and mechanical properties of UHMW gels are investigated in detail. Furthermore, the differences in self-healing efficiency of UHMW gels depending on the chemical structure is discussed in terms of individual polymer conformation and polymer-polymer interaction based on molecular dynamics simulations.

Polyether-based solid electrolytes with a homogeneous polymer network: effect of the salt concentration on the Li-ion coordination structure.

We report a solid polymer electrolyte with an ideal polyether network that was synthesized by using tetra-functional poly(ethylene glycol) (TetraPEG) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt. The solid TetraPEG electrolyte had few network defects (<5%) and exhibited high mechanical toughness by enduring approximately 11-fold elongation at a 1 : 10 ratio of Li salt to O atoms of PEG (Li/OPEG). We found that the mechanical properties strongly depend on the Li/OPEG ratio, which mainly contributes to the density of crosslinking points in the electrolyte. Raman spectroscopy and high-energy X-ray total scattering were used with all-atom molecular dynamics simulations to visualize the structural effects of Li-ion coordination in the TetraPEG network. At lower salt contents (Li/OPEG = 1 : 10), Li ions were found to preferentially coordinate with OPEG atoms rather than the TFSA anions to form crown ether-like Li+-PEG complexes as ion pair-free species. With increasing salt content, the TFSA anions partially coordinated with Li ions through O atoms of TFSA (OTFSA) to afford contact ion pairs surrounded by both OPEG and OTFSA atoms. Finally, the ion pairing enhanced mononuclear ion pairs as well as multinuclear ionic aggregates when more Li salt was added. This structural change in the Li-ion complexes was directly reflected by the ion-conducting properties of the electrolyte. The TetraPEG electrolyte composed of the ion pair-free Li+ species (Li/OPEG = 1 : 10) exhibited higher ionic conductivity, and the conductivity gradually decreased with increasing salt content because of extensive ion pairing for both mononuclear contact ion pairs and multinuclear aggregates. Regarding the electrochemical properties, the optimum electrolyte composition to realize a reversible Li deposition/dissolution reaction for a negative electrode was found to be Li/OPEG = 1 : 4.

A structural and electrochemical study of lithium-ion battery electrolytes using an ethylene sulfite solvent: from dilute to concentrated solutions.

We report the structural and electrochemical characteristics of lithium (Li)-ion battery (LIB) electrolyte solutions using an ethylene sulfite (ES) solvent that is used as an electrolyte additive for LIBs. From dilute to highly concentrated ES solutions with lithium bis(fluorosulfonyl)amide (LiFSA), the formation of Li-ion complexes was investigated using a combined Raman and infrared spectroscopy study with the aid of density functional theory (DFT) calculations to quantitatively determine their solvation and ion-pair structures depending on the Li salt concentration (cLi). The results reveal that, in the dilute solutions (<1.0 mol dm-3), Li-ions are fully solvated with ES molecules to form a tetrahedral-like [Li(ES)4]+ complex; however, with the increasing cLi (up to 2.5 mol dm-3), the Li-ion complex changes in structure to form contact ion-pairs coordinated with both ES and FSA anions. It also reveals that further increasing cLi to approximately 3.0 mol dm-3 leads to the ionic aggregate formation, i.e., multiple Li-ion complexes linked via several FSA anions. LiFSA/ES electrolyte solutions exhibited a reversible Li-ion insertion/deinsertion reaction into/from the graphite anode irrespective of cLi. This is due to the high-grade ES-derived passivation films on the electrode as a result of the preferential reductive decomposition of the ES molecules trapped within the Li-ion coordination sphere. According to the charge-discharge test, the concentrated LiFSA/ES solutions exhibited the high C-rate performance, which is superior to the concentrated electrolyte solutions using conventional organic solvents.

Polymer network formation mechanism of multifunctional poly(ethylene glycol)s in ionic liquid electrolyte with a lithium salt.

We report a controlled polymer network gel electrolyte based on a multifunctional poly(ethylene glycol) (PEG) prepolymer (herein, tetrafunctional PEGs (tetra-PEGs) and bisfunctional linear PEGs (linear-PEGs)) and an ionic liquid (IL)-based electrolyte solution containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSA) salt. The gel electrolyte was obtained via a gelation reaction, i.e., the Michael addition reaction between maleimide (MA)-terminated tetra-PEGs and thiol (SH)-terminated tetra- or linear-PEGs (termed tetra/tetra-PEG gel or tetra/linear-PEG gel systems), in a LiTFSA/IL solution under noncatalytic conditions at room temperature. For the tetra/linear-PEG system, the gelation reaction depended on the ratio of tetra-PEG-MA and linear-PEG-SH; an optimum terminal MA/SH ratio of 1 : 1 yielded a reaction efficiency (p) of ∼98% (an ideal polymer network structure). The tetra/tetra-PEG system with an MA/SH ratio of 1 : 1 also achieved a reaction efficiency of ∼98%. Time-resolved rheological measurements revealed that the network formation process can be categorized into three steps: (I) oligomer formation at an early stage of the reaction, (II) formation of a roughly linked polymer network with a large mesh size as the reaction proceeded, and (III) full network formation also at the local scale near the gelation completion time. The resulting tetra/linear-PEG ion gel with an optimum MA/SH ratio of 1 : 1 exhibited high stretchability, enduring approximately 10-fold elongation, and superior ion-conducting properties compared with the corresponding IL-based electrolyte solution.

Anion effects on Li ion transference number and dynamic ion correlations in glyme-Li salt equimolar mixtures.

To achieve single-ion conducting liquid electrolytes for the rapid charge and discharge of Li secondary batteries, improvement in the Li+ transference number of the electrolytes is integral. Few studies have established a feasible design for achieving Li+ transference numbers approaching unity in liquid electrolytes consisting of low-molecular-weight salts and solvents. Previously, we studied the effects of Li+-solvent interactions on the Li+ transference number in glyme- and sulfolane-based molten Li salt solvates and clarified the relationship between this transference number and correlated ion motions. In this study, to deepen our insight into the design principles of single-ion conducting liquid electrolytes, we focused on the effects of Li+-anion interactions on Li ion transport in glyme-Li salt equimolar mixtures with different counter anions. Interestingly, the equimolar triglyme (G3)-lithium trifluoroacetate (Li[TFA]) mixture ([Li(G3)][TFA]) demonstrated a high Li+ transference number, estimated via the potentiostatic polarization method (tPPLi = 0.90). Dynamic ion correlation studies suggested that the high tPPLi could be mainly ascribed to the strongly coupled Li+-anion motions in the electrolytes. Furthermore, high-energy X-ray total scattering measurements combined with all-atom molecular dynamics simulations showed that Li+ ions and [TFA] anions aggregated into ionic clusters with a relatively long-range ion-ordered structure. Therefore, the collective motions of the Li ions and anions in the form of highly aggregated ion clusters, which likely diminish rather than enhance ionic conductivity, play a significant role in achieving high tPPLi in liquid electrolytes. Based on the dynamic ion correlations, a potential design approach is discussed to accomplish single-ion conducting liquid electrolytes with high ionic conductivity.

Correction: Local structure of a highly concentrated NaClO4 aqueous solution-type electrolyte for sodium ion batteries.

Correction for 'Local structure of a highly concentrated NaClO4 aqueous solution-type electrolyte for sodium ion batteries' by Ryo Sakamoto et al., Phys. Chem. Chem. Phys., 2020, 22, 26452-26458, DOI: 10.1039/D0CP04376A.

A Bilayer Structure Composed of Mg|Co-MnO2 Deposited on a Co(OH)2 Film to Realize Selective Oxygen Evolution from Chloride-Containing Water.

A fluorine-doped tin oxide-coated glass electrode modified with a bilayer film of underlying α-Co(OH)2 and overlying Mg-intercalated and Co-doped δ-type (layered) MnO2 (Mg|Co-MnO2) preferentially yielded oxygen with a Faradaic efficiency as high as 79% in the presence of chloride ions at high concentration (0.5 M). This noble metal-free electrode was fabricated by cathodic electrolysis of aqueous Co(NO3)2 followed by anodic electrolysis of a mixture of Mn2+, Co2+, and cetyltrimethylammonium (CTA+) ions in water. The CTA+ ions accommodated in the interlayer spaces of Co-doped δ-MnO2 were replaced with Mg2+ by ion exchange. The upper Mg|Co-MnO2 could effectively block the permeation of Cl- ions and allow only H2O and O2, while the under α-Co(OH)2 acted as an oxidation catalyst for the H2O penetrated through the upper coating. Thus, the oxygen evolution reaction (OER) was preferred to the chlorine evolution reaction (CER). In artificial seawater (pH 8.3), the blocking effect against Cl- decreased because of ion exchange of the intercalated Mg2+ ions with Na+ in solution, but the OER efficiency still remained at 57%, much higher than that (28%) without the upper Mg|Co-MnO2. This demonstrates that the interlayer spaces between MnO2 layers acted as pathways for H2O molecules to reach the active sites of the underlying Co(OH)2. Density functional theory (DFT) calculations revealed that the most stable structure of hydrated Mg2+ ion, in which a part of coordinated H2O molecules is hydrolyzed, has less affinity toward Cl- ion than that of hydrated Na+ ion.

Lithium-ion coordination-induced conformational change of PEG chains in ionic-liquid-based electrolytes.

We report the structure of poly(ethylene glycol) (PEG) in a imidazolium-based ionic liquid (IL) electrolyte containing lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt, as determined using Raman spectroscopy, high-energy X-ray total scattering (HEXTS), and molecular dynamics (MD) simulations. The Raman spectral study indicated that the TFSA anions bound to Li ions are desolvated when PEG is added to the LiTFSA/IL solution to form stable Li+-PEG complexes. Via quantitative analysis of the obtained Raman spectra, the desolvation number of the TFSA [nd, per one oxygen atom of the ethylene glycol unit (Opeg)] was determined to be ∼0.4, irrespective of the shape (star or linear) and molecular weight of the polymer. On the basis of radial distribution functions obtained from the HEXTS experiments and MD simulations, we demonstrated that the Li+-PEG complexation induces a conformational change of the PEG chain from gauche/anti-conformers to a syn conformer. This Li+-coordination-induced conformation resulted in a decrease in the radius of gyration (Rg) of the PEG chain, implying a folding behavior of polymer chains through multiple OpegLi+Opeg interactions.

Local structure of a highly concentrated NaClO4 aqueous solution-type electrolyte for sodium ion batteries.

Aqueous Na-ion batteries with highly concentrated NaClO4 aq. electrolytes are drawing attention as candidates for large-scale rechargeable batteries with a high safety level. However, the detailed mechanism by which the potential window in 17 m NaClO4 aq. electrolyte was expanded remains unclear. Therefore, we investigated the local structure around a Na+ ion or a ClO4- ion using X-ray diffraction combined with empirical potential structure refinement (EPSR) modelling and Raman spectroscopy. The results showed that in 17 m NaClO4 aq. electrolyte, most of the water molecules were coordinated to Na+ ions and few free water molecules were present. The 17 m NaClO4 aq. electrolyte could be interpreted as widening the potential window because almost all water molecules participated in hydration of the Na+ ions.

Fluorinated alkyl-phosphate-based electrolytes with controlled lithium-ion coordination structure.

Herein, we propose Li-ion solvation-controlled electrolytes based on non-flammable organic solvent TFEP and an LiFSA salt [TFEP: tris(2,2,2-trifluoroethyl)phosphate, LiFSA: lithium bis(fluorosulfonyl)amide] to allow Li-ion insertion into a graphite electrode for Li-ion batteries. Comprehensive structural study based on (1) infrared (IR)/Raman spectroscopy, (2) high-energy X-ray total scattering (HEXTS), and (3) molecular dynamics (MD) simulation revealed the solvation (or coordination) structures of Li ions in TFEP-based electrolytes at the molecular level. In binary LiFSA/TFEP with a Li salt concentration (cLi) < 1.0 mol dm-3, Li ions are coordinated with both TFEP and FSA components; in detail, two TFEP molecules coordinate in an O-donating monodentate manner and one FSA in an O-donating bidentate manner to form [Li(TFEP)2(bi-FSA)] as the major species. We demonstrated that adding acetonitrile (AN) to the LiFSA/TFEP electrolytes caused structural changes in the Li-ion complexes. The bi-FSA bound to the Li ion changed its coordination mode to mono-FSA, which was induced by solvating AN molecules to Li ions. The redox reaction corresponding to insertion/deinsertion of Li ions into/from the graphite electrode successfully occurred in 1.0 mol dm-3 LiFSA/TFEP with an AN electrolyte system, while there was no or reduced Li-ion insertion in the electrolyte without AN. We discussed the relationship between the structure and electrode reaction of the Li-ion complexes based on the FSA-coordination characteristics; i.e., in LiFSA/TFEP with the AN system, the mono-FSA bound to the Li ion is easier to decoordinate due to weaker Li+mono-FSA- interactions rather than the Li+bi-FSA- interactions, which mainly contribute to charge-transfer at the electrode/electrolyte interface to allow Li-ion insertion/deinsertion in the graphite anode.

Small-angle X-ray scattering study on nano-scale structures controlled by water content in a binary water/ionic liquid system.

We report the water-in-ionic-liquid microemulsions (ME) formed in a binary water/ionic liquid system, without organic solvents, using a surfactant ionic liquid (SAIL) based on 1-butyl-3-methylimidazolium (C4mIm+) as the cation and dioctyl sulfosuccinate (AOT-) as the anion. Small-angle X-ray scattering (SAXS) revealed that MEs were stably formed in the binary water/SAIL solutions in the low water content region (water volume fraction, φw < 0.1), and the ME size systematically increased with increasing φw. We further investigated the nanostructures of the high φw region using a combination of SAXS and rheological measurements and found that the MEs changed to a stacked lamellar structure comprising SAIL bilayers and water phases at φw > 0.12. At the largest water content, φw = 0.99, vesicle structures were obtained.

Solvation-controlled lithium-ion complexes in a nonflammable solvent containing ethylene carbonate: structural and electrochemical aspects.

The structural and electrochemical properties of lithium-ion solvation complexes in a nonflammable organic solvent, tris(2,2,2-trifluoroethyl)phosphate (TFEP) containing ethylene carbonate (EC), were investigated using vibrational spectroscopic and electrochemical measurements. Based on quantitative Raman and infrared (IR) spectral analysis of the Li bis(trifluoromethanesulfonyl)amide (TFSA) salt in TFEP + EC electrolytes, we successfully evaluated the individual solvation numbers of EC (nEC), TFEP (nTFEP), and TFSA- (nTFSA) in the first solvation sphere of the Li-ion. We found that the nEC value linearly increased with increasing EC mole fraction (xEC), whereas the nTFEP and nTFSA values gradually decreased with increasing nEC. The ionic conductivity and viscosity (Walden plots) indicated that mainly Li+TFSA- ion pairs formed in neat TFEP (xEC = 0). This ion pair gradually dissociated into positively charged Li-ion complexes as xEC increased, which was consistent with the Raman/IR spectroscopy results. The redox reaction corresponding to an insertion/desertion of Li-ion into/from the graphite electrode occurred in the LiTFSA/TFEP + EC system at xEC ≥ 0.25. The same was not observed in the lower xEC cases. We discussed the relation between Li-ion solvation and electrode reaction behaviors at the molecular level and proposed that nEC plays a crucial role in the electrode reaction, particularly in terms of solid electrolyte interphase formation on the graphite electrode.

Microbial and genomic characterization of Geobacillus thermodenitrificans OS27, a marine thermophile that degrades diverse raw seaweeds.

Seaweeds are a nonlignocellulosic biomass, but they are often abundant in unique polysaccharides that common microbes can hardly utilize; therefore, polysaccharide degradation is key for the full utilization of seaweed biomass. Here, we isolated 13 thermophiles from seaweed homogenates that had been incubated at high temperature. All of the isolates were Gram-positive and preferentially grew at 60-70 °C. Most formed endospores and were tolerant to seawater salinity. Despite different sources, all isolates were identical regarding 16S rRNA gene sequences and were categorized as Geobacillus thermodenitrificans. Their growth occurred on seaweed polysaccharides with different profiles but required amino acids and/or vitamins, implying that they existed as proliferative cells by utilizing nutrients on seaweed viscous surfaces. Among 13 isolates, strain OS27 was further characterized to show that it can utilize a diverse range of seaweed polysaccharides and hemicelluloses. Notably, strain OS27 degraded raw seaweeds while releasing soluble saccharides. The degradation seemed to depend on enzymes that were extracellularly produced in an inducible manner. The strain could be genetically modified to produce heterologous endoglucanase, providing a transformant that degrades more diverse seaweeds with higher efficiency. The draft sequences of the OS27 genome contained 3766 coding sequences, which included intact genes for 28 glycoside hydrolases and many hypothetical proteins unusual among G. thermodenitrificans. These results suggest that G. thermodenitrificans OS27 serves as a genetic resource for thermostable enzymes to degrade seaweeds and potentially as a microbial platform for high temperature seaweed biorefinery via genetic modification.

Experimental Observation of Two Features Unexpected from the Classical Theories of Rubber Elasticity.

Although the elastic modulus of a Gaussian chain network is thought to be successfully described by classical theories of rubber elasticity, such as the affine and phantom models, verification experiments are largely lacking owing to difficulties in precisely controlling of the network structure. We prepared well-defined model polymer networks experimentally, and measured the elastic modulus G for a broad range of polymer concentrations and connectivity probabilities, p. In our experiment, we observed two features that were distinct from those predicted by classical theories. First, we observed the critical behavior G∼|p-p_{c}|^{1.95} near the sol-gel transition. This scaling law is different from the prediction of classical theories, but can be explained by analogy between the electric conductivity of resistor networks and the elasticity of polymer networks. Here, p_{c} is the sol-gel transition point. Furthermore, we found that the experimental G-p relations in the region above C^{*} did not follow the affine or phantom theories. Instead, all the G/G_{0}-p curves fell onto a single master curve when G was normalized by the elastic modulus at p=1, G_{0}. We show that the effective medium approximation for Gaussian chain networks explains this master curve.

Effect of protonation on the solvation structure of solute N-butylamine in an aprotic ionic liquid.

We report on the acid-base reaction of an amine solute in an aprotic ionic liquid from a structural point of view. Thus, the solvation structures of n-butylamine and n-butylammonium (BuNH2 and BuNH3+, respectively, acid-base reaction: BuNH2 + H+ ⇄ BuNH3+) in 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]) were investigated by high-energy X-ray total scattering combined with molecular dynamics simulations. We found that the solvation structure drastically changed as a result of the protonation reaction in [C2mIm][TFSA]. The NH3+ group was preferentially solvated by TFSA- anions in the protonated BuNH3+ system, whereas the neutral NH2 group was surrounded by both TFSA- and C2mIm+ ions in the BuNH2 system. With regard to the nearest neighbor solute-TFSA interaction, the solvation number for TFSA- anions (nTFSA) increased upon protonation (i.e. BuNH2:nTFSA = 2 and BuNH3+:nTFSA = 3). Both BuNH2 and BuNH3+ interacted with O atoms within TFSA- through hydrogen bonding (i.e. N-HO) interactions, and the N-HO distance was appreciably shorter for positively charged BuNH3+ as compared to neutral BuNH2. These results revealed that the solvation is more stable in energy for the protonated BuNH3+ group because of its stronger hydrogen bonding as compared to neutral BuNH2. This is the origin of the large Gibbs energy for the protonation reaction (ΔG = -94.7 kJ mol-1) and the high acid dissociation constant (pKa = 16.6) of BuNH2 in [C2mIm][TFSA] solution.

Role of polar side chains in Li+ coordination and transport properties of polyoxetane-based polymer electrolytes.

Lithium ion conducting polymer electrolytes (PEs) have been the subject of intense research for lithium metal battery applications. Here, we investigate the effects of polar side chains on Li+ coordination and ionic transport properties to gain insights for improving the insufficient conductivity of traditional ether-based solid PEs. Poly(trimethyleneoxide)-based (or polyoxetane-based) polymers with ether or nitrile groups were synthesized by ring-opening polymerization. The thermal, ionic transport, and electrochemical properties and the local structure of Li+ coordination were studied in the presence of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA). The glass transition temperature (Tg) of the PEs with ether side chains increased with increasing LiTFSA content, whereas the PEs with the nitrile functionality showed the opposite trend at higher salt concentrations. In addition to the unique trend for the Tg values of the PEs in the presence of LiTFSA, the nitrile groups played pivotal roles as coordination sites for Li+ ions in the first coordination shell and as a polar medium to increase the permittivity of the PEs. These characteristics of the nitrile groups can endow PEs with improved ionic transport properties.

Defect-free network formation and swelling behavior in ionic liquid-based electrolytes of tetra-arm polymers synthesized using a Michael addition reaction.

The gelation mechanism of tetra-arm poly(ethylene glycol) (TetraPEG) prepolymers via a Michael addition reaction was investigated from the viewpoint of chemical reaction kinetics. The polymer network was formed by mixing two different TetraPEGs functionalized with maleimide and thiol terminal groups (TetraPEG-MA and TetraPEG-SH) in aqueous solutions, and the gelation rate was strongly dependent on the solution pH. We found that the gelation reaction can be a second-order reaction when the acid-base equilibrium of the terminal SH groups (-SH ⇆ -S- + H+) was taken into account, resulting in a quantitative estimation of the rate constant (kgel) in the current polymer solution system. Based on the kgel value, the network connectivity (p), which corresponds to efficiency at the linking point, was evaluated to be p > 95% at the end of the reaction; thus, the resulting TetraPEG hydrogels have a homogeneous polymer network without network defects. We used the TetraPEG network as a polymer matrix in a lithium-ion battery gel electrolyte: dried TetraPEG gels were swollen with ionic liquid-based electrolytes containing Li salts to prepare TetraPEG ion gel electrolytes. Swelling behaviors of the TetraPEG network were characterized from the swelling rate and the equilibrium swelling ratio, and we found that these swelling behaviors were significantly affected by the Li-ion component. We concluded that an intermolecular interaction between Li-ions and the polymer (Li-ion coordination with the O atoms within the PEG chains) plays a key role in the fundamental physical properties of the gel electrolyte.

Ion-solvation structure and battery electrode characteristics of nonflammable organic electrolytes based on tris(trifluoroethyl)phosphate dissolving lithium salts.

The structure and properties of lithium salt solutions based on tris(2,2,2-trifluoroethyl)phosphate (TFEP) solvent have been studied to design a safer electrolyte system for large-sized lithium-ion battery applications. Influences of the ionic structure on the polarization behavior of the LiCoO2 (LCO) positive electrode were investigated. The ionic conductivity and viscosity of the solution consisting of lithium salts dissolved in TFEP, LiX/TFEP (X = PF6, BF4 and TFSA) (TFSA = (CF3SO2)2N), were measured. The results suggest that the ion-solvation structure greatly depends on the anionic species in the salt. Spectroscopic measurements also support the conclusion that the Li+-solvation structure varies with the lithium salts. The differences in the ionic structure of LiX/TFEP influence the electrochemical oxidation potential of the solution and the polarization behavior of the LCO electrode. The overvoltage for Li-desertion/insertion from/into LCO in LiX/TFEP, being much higher than that observed in conventional LIB electrolyte solutions, shows the order of BF4 < PF6 < TFSA. The addition of ethylene carbonate (EC) to LiX/TFEP increases the ionic conductivity, which is probably caused by changes in the Li+-solvation structure in TFEP. The overvoltage for the Li-desertion/insertion of LCO is much lowered by the addition of EC to LiX/TFEP.