Published papers

A one-pot synthesis of three-dimensional carbon nanotube frameworks with bipyramidal sulfur particles and the application of these materials for a cathode in lithium-sulfur (Li-S) battery are reported. By simple mixing of multi-walled carbon nanotubes (MWCNTs), sulfur powder, and capping agents in water/tetrahydrofuran, micrometer bipyramidal sulfur particles enclosed with MWCNTs are synthesized. The MWCNTs spontaneously form a 3D conducting network inside and outside the sulfur particle. Along the edge of MWCNT framework, a sulfur particle-free region is present, which comprises approximate to 35 vol% based on the total volume. These sulfur-MWCNT bipyramidal particles are mixed with conductive carbon additive to prepare binder-free cathode for Li-S cells. The Li-S cells deliver a specific discharge capacity of approximate to 1600 mAh g(-1) at 0.05 C on the first cycle. In particular, these Li-S cells show high rate stability and Coulombic efficiency with deep discharge and charge (1.0-3.0 V vs Li/Li+). This resultant performance can arise from 1) homogeneous distribution of the conducting MWCNT framework and the carbon additive coating layer on the sulfur particle, which allow rapid Li+ ion/electrolyte diffusion and mitigation of polysulfide shuttle, respectively, and 2) the sulfur-free buffer space accommodating volume expansion. It is expected that this new cathode design with the simple synthetic process can reduce the number of preparation steps, thus allowing the construction of a low-cost Li-S battery.

Recent studies have suggested that a Li ion hopping or ligand- or anion-exchange mechanism is largely involved in Li ion conduction of highly concentrated liquid electrolytes. To understand the determining factors for the Li ion hopping/exchange dominant conduction in such liquid systems, ionic diffusion behavior and Li ion coordination structures of concentrated liquid electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and keto ester solvents with two carbonyl coordinating sites of increasing intramolecular distance (methyl pyruvate (MP), methyl acetoacetate (MA), and methyl levulinate (ML)) were studied. Diffusivity measurements of MP- and MA-based concentrated electrolytes showed faster Li ion diffusion than the solvent and FSA anion, demonstrating that Li ion diffusion was dominated by the Li ion hopping/exchange mechanism. A solvent-bridged, chain-like Li ion coordination structure and highly aggregated ion pairs (AGGs) or ionic clusters e.g. Li-x[FSA](y)((y-x)-) forming in the electrolytes were shown to contribute to Li ion hopping conduction. By contrast, ML, with greater intramolecular distance between the carbonyl moieties, is more prone to form a bidentate complex with a Li cation, which increased the contribution of the vehicle mechanism to Li ion diffusion even though similar AGGs and ionic clusters were also observed. The clear correlation between the unusual Li ion diffusion and the solvent-bridged, chain-like structure provides an important insight into the design principles for fast Li ion conducting liquid electrolytes that would enable Li ion transport decoupled from viscosity-controlled mass transfer processes.

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] 4 1. The corresponding glutaronitrile and adiponitrile mixtures have melting points below 60 degrees 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.

The phase behavior of binary mixtures of triglyme (G3) and Mg[TFSA]2 (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated, towards the development of a Mg2+-based room-temperature solvate ionic liquid (SIL) electrolyte. In a 1:1 molar ratio, G3 and Mg[TFSA]2 form a thermally stable complex (decomposition temperature, Td: 240 °C) with a melting point (Tm) of 70 °C, which is considerably lower than that of the analogous tetraglyme (G4) system (137 °C). X-ray crystallography of a single crystal of [Mg(G3)][TFSA]2 revealed that a single Mg2+ cation is coordinated by a single, distorted, tetradentate G3 molecule from one side, and two monodentate [TFSA]- anions, with transoid conformation, from the reverse side to form an ion pair. Raman spectra of [Mg(G3)][TFSA]2 in the molten state revealed the presence of different coordination structures, as the liquid exhibits changes in the vibrational modes corresponding to G3 and the [TFSA]- anion compared to those observed for the solid. Investigation of the ion pair stabilization energies by DFT calculations suggests that higher stability cation complexes and ion pairs co-exist in the molten state than those observed in the crystalline state. These results imply that the coordination structures of the ion pairs play a key role in providing SILs with low Tm. To decrease the Tm further, several asymmetric homologues of G3, which have higher conformational flexibility than G3, were investigated. Notably, a 1:1 mixture of Mg[TFSA]2 with G3Bu (where one of the terminal methyl groups of G3 is substituted for a butyl group) formed a thermally stable complex (Td: 251 °C) without any distinct Tm and showed reasonable ionic conductivity at room-temperature, indicating partial dissociation of ions. In this electrolyte, which showed high oxidative stability, quasi-reversible Mg deposition/dissolution was achieved, indicating that Mg2+-based room-temperature SILs can be utilized as a new class of Mg electrolyte.

The performance of current Liair batteries is greatly limited by critical obstacles such as electrolyte decomposition, high charging overpotentials, and limited cycle life. Thus, much effort is devoted to fundamental studies to understand the mechanisms of discharge/charge processes and overcome the above-mentioned obstacles. In particular, the search for new stable electrolytes is vital for long-lasting and highly cyclable batteries. The highly reactive lithium superoxide intermediate (LiO2) produced during discharge process can react with the electrolyte and produce a variety of byproducts that will shorten battery life span. To study this degradation mechanism, we investigated oxygen reduction reaction (ORR) in highly concentrated electrolyte solutions of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA])/dimethyl sulfoxide (DMSO). On the basis of rotating ring disk electrode measurements, we showed that LiO2 dissolution can be limited by increasing lithium salt concentration over 2.3 mol dm(-3). Our Raman results suggested that this phenomenon can be related to lack of free DMSO molecules and increasing DMSO-Li+ interactions with higher Li+ concentration. X-ray diffraction measurements for the products of ORR suggested that the side reaction of DMSO with Li2O2 and/or LiO2 could be suppressed by decreasing the solubility of LiO2 in highly concentrated electrolytes.

Organic ionic plastic crystals (OIPCs), which are soft crystals with plasticity and ionic conductivity, are expected to be applied as solid electrolytes in battery applications. Further improvement of ionic conductivity is necessary for practical use as an electrolyte for energy storage devices. Materials Informatics (MI) is a method of incorporating information science in materials development. In this research, MI is being used to develop OIPCs with high ionic conductivity. By using informatics science in addition to chemical knowledge, this research can be carried out efficiently and innovatively. The synthesis of eight new compounds resulted in six of them being solid at room temperature, while two of them were in a liquid state, namely, ionic liquids. We evaluated the phase transition temperatures and ionic conductivity for each compound. Notably, N-ethyl-N-methylpyrrolidinium trifluoro(trifluoromethyl)borate ([C₂mpyr][CF₃BF₃]) exhibited a high ionic conductivity of 1.75 × 10⁻⁴ S cm⁻¹ at 25 degrees C, which is one of the highest values reported among OIPCs to date. The combination of an experimental and MI based approach revealed an improved understanding of the relationship between ion size and ionic conductivity for a series of pyrrolidinium-based OIPCs, and it is expected that further improvements to this approach will yield greater understanding of structure-property relationships.

In recent years, the development of next-generation secondary batteries employing resource-abundant metals such as Na has garnered significant attention. However, the high reactivity of Na raises safety concerns, necessitating the development of safer devices. To address this, ionic liquids (ILs) and organic ionic plastic crystals (OIPCs) have emerged as promising novel electrolytes. Despite their potential, studies investigating the influence of cation structures on various properties remain scarce, particularly in composites where Na salts are introduced into OIPCs. This study focuses on the effects of cation species and Na-salt concentration in OIPCs, specifically in N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C₂epyr][FSA]) and N-ethyl-N-isopropylpyrrolidinium bis(fluorosulfonyl)amide ([Cᵢ₃epyr][FSA]), with the addition of sodium bis(fluorosulfonyl)amide (NaFSA). The phase transition behavior, dissociation state of Na salts, and electrochemical properties exhibited significant differences based on the cationic structure of the OIPCs. The combination of each OIPC with Na salt resulted in liquid mixtures, and the ionic conductivity increased significantly as the Na salt concentration increased. High ionic conductivities were achieved with [C₂epyr][FSA]/NaFSA (20 mol%) and [Cᵢ₃epyr][FSA]/NaFSA (10 mol%), showing values of 2.7 × 10⁻³ and 2.2 × 10⁻³ S cm⁻¹ at 25 degrees C, respectively. Linear sweep voltammetry results indicated superior oxidative stability in the [Ci₃epyr][FSA] system. Solvation numbers of Na⁺, influenced by differences in cationic side-chain structures, were determined to be 2.7 for the [C₂epyr]⁺ system and 2.9 for the [Cᵢ₃epyr]⁺ system. The results suggest that controlling solvation numbers is a critical factor in the molecular design of high-performance ionic conductors.

We report an investigation of the continuous oxidative dehydrogenation of 4-vinylcyclohexene (VCH) in high pressure steam. Oxidative dehydrogenation reactions such as this are often limited by unselective or total oxidation. We find that these side-reactions not only occur in this case, but can also have a major influence on the selectivity; a small increase in flow rate results in a complete switch in selectivity of the reaction. Our results suggest that styrene (ST) is formed as the initial product but that unless the H-2 is sequestered, ST may then be hydrogenated to yield ethylbenzene (EB). The observation of periodic temperature spikes near the surface of the catalyst bed indicate cycles of propagating flames occur in a relatively small volume of the reactor, leading to total oxidation of some VCH and removal of O-2, which would otherwise sequester the H-2. These flames give rise to the large variations in the observed product selectivity. We suggest that these observations may not be restricted to this reaction system and reactor configuration, and may occur in other situations where small-scale continuous flow oxidation reactors are used.

A perfluorinated alkyl chain grafted to carbon nanotubes is employed in the lithium-oxygen (Li-O-2) cell. We demonstrate a highly localized Li-O-2 electrochemical reaction in close proximity to the perfluorinated moiety owing to its high O-2 affinity. This hydrophobic modification can provide an enhancement in capacity for a very thin microbattery system.

The application of conventional solid polymer electrolyte (SPE) to lithium-oxygen (Li-O-2) batteries has suffered from a limited active reaction zone due to thick SPE and subsequent lack of O-2 gas diffusion route in the positive electrode. Here we present a new design for a three-dimensional (3-D) SPE structure, incorporating a carbon nanotube (CNT) electrode, adapted for a gas-based energy storage system. The void spaces in the porous CNT/SPE film allow an increased depth of diffusion of O-2 gas, providing an enlarged active reaction zone where Li+ ions, O-2 gas, and electrons can interact. Furthermore, the thin SPE layer along the CNT, forming the core/shell nanostructure, aids in the smooth electron transfer when O-2 gas approaches the CNT surface. Therefore, the 3-D CNT/SPE electrode structure enhances the capacity in the SPE-based Li-O-2 cell. However, intrinsic instability of poly(ethylene oxide) (PEO) of the SPE matrix to superoxide (O-2(center dot-)) and high voltage gives rise to severe side reactions, convincing us of the need for development of a more stable electrolyte for use in this CNT/SPE design.

It is of great interest to determine whether a relatively easily prepared and inexpensive protic ionic liquid (PIL) could be used in place of an aprotic ionic liquid (AIL) in a composite membrane for CO₂ separation. We prepared mechanically reliable sulfonated polyimide (SPI) composite membranes with a PIL, [N124][NTf₂] (N,N,N-butylethylmethylammonium bis(trifluoromethylsulfonyl)amide) by a solution casting method, and their mechanical, thermal, and gas separation (CO₂ and N₂) properties were compared with those of membranes incorporating an isomeric AIL, [N1123][NTf₂] ([N1123](+), N,N,N,N-ethyldimethylpropylammonium). We discussed the influence of the presence or absence of active protons in the ILs on CO₂ separation characteristics and the applicability of the PIL to a CO₂ separation membrane. The SPI/PIL membrane exhibited a slightly lower selectivity, but higher permeability, than the SPI/AIL membrane. It is supposed that these differences originate from the difference in membrane structure and CO₂ interactions between PIL and AIL, which was investigated through a combination of ab initio calculations, CO₂ solubility measurements, and dynamic mechanical analysis. The [N124][NTf₂](PIL)/SPI membranes showed a CO₂ permeability (PCO₂ = 240 Barrer) with a relatively high selectivity (alpha(CO₂/N₂) = 23) at 30 degrees C, which indicates that a PIL/SPI composite membrane can be employed for CO₂ separation. We also explored CO₂ separation under elevated temperatures and humidified conditions for confirming the applicability of the SPI/PIL membrane. Finally, the effect of PIL structures was studied by using [N122][NTf₂] ([N122](+), N,N,N-diethylmethylammonium) in comparison with [N124][NTf₂]. Composite [N122][NTf₂]/SPI membranes exhibited a comparable selectivity and an improved permeability. We anticipate these results will unveil a new paradigm in functional ion gels for critical CO₂ separation technology, toward utilization of effective, benign materials.

Cellulose is one of the main components of plant cell walls, abundant on earth, and can be acquired at a low cost. Furthermore, there has been increasing interest in its use in environmentally friendly, carbon‐neutral, sustainable materials. It is expected that the applications of cellulose will expand with the development of a simple processing method. In this study, we dissolved cellulose in aqueous N‐butyl‐N‐methylpyrrolidinium hydroxide solution ([C4mpyr][OH]/H2O) and investigated the cellulose regeneration process based on changes in solubility upon application of CO2 gas. We investigated the effect of transformation of the anion chemical structure on cellulose solubility by flowing CO2 gas into [C4mpyr][OH]/H2O and conducted pH, FT‐IR, and 13C NMR measurements. We observed that the changes in anion structure allowed for the modulation of cellulose solubility in [C4mpyr][OH]/H2O, thus establishing a simple and safe cellulose regeneration process. This regeneration process was also applied to enable the production of cellulose hydrogels. The hydrogel formed using this method was revealed to have higher mechanical strength than an analogous hydrogel produced using the same dissolution solvent with the addition of a cross‐linker. The ability to produce cellulose‐based hydrogels of different mechanical properties is expected to expand the possible applications.