Self-Assembled Molecular Fibers Aligned by Compression in Water.

Molecular self-assembly has attracted much attention as a potential approach for fabricating nanostructured functional materials. To date, energy-efficient fabrication of nano-objects such as nanofibers, nanorings, and nanotubes is achieved using well-designed self-assembling molecules. However, the application of molecular self-assembly to industrial manufacturing processes remains challenging because regulating the positions and directions of self-assembled products is difficult. Non-covalent molecular assemblies are also too fragile to allow mechanical handling. The present work demonstrates the macroscopic alignment of self-assembled molecular fibers using compression. Specifically, the macroscopic bundling of self-assembled nanofibers is achieved following dispersion in water. These fiber bundles can also be chemically crosslinked without drastic changes in morphology via trialkoxysilyl groups. Subsequently, vertically oriented porous membranes can be produced rapidly by slicing the bundles. This technique is expected to be applicable to various functional self-assembled fibers and can lead to the development of innovative methods of producing anisotropic nanostructured materials.

Sintering Metal-Organic Framework Gels for Application as Structural Adhesives.

Metal-organic frameworks (MOFs)/coordination polymers are promising materials for gas separation, fuel storage, catalysis, and biopharmaceuticals. However, most applied research on MOFs is limited to these functional materials thus far. This study focuses on the potential of MOFs as structural adhesives. A sintering technique is applied to a zeolitic imidazolate framework-67 (ZIF-67) gel that enables the joining of Cu substrates, resulting in a shear strength of over 30 MPa, which is comparable to that of conventional structural adhesives. Additionally, systematic experiments are performed to evaluate the effects of temperature and pressure on adhesion, indicating that the removal of excess 2-methylimidazole and the by-product (acetic acid) from the sintered material by vaporization results in a microstructure composed of large spherical ZIF-67 crystals that are densely aggregated, which is essential for achieving a high shear strength.

Direct Sintering Behavior of Metal Organic Frameworks/Coordination Polymers.

In this study, we investigate the sintering behavior and mechanisms of metal-organic frameworks/coordination polymers (CPs) through physical and microstructural characterization of [Zn(HPO4)(H2PO4)2]·2H2Im (ZPI; a melting CP, Im = imidazole) and ZIF-8 (a non-melting CP). By performing simple compaction and subsequent sintering, a bulk body of CPs was obtained without losing the macroscopic crystallinity. The sintering behavior was found to be dependent on the temperature, heating rate, and physical properties of the CPs and, in particular, their meltability. During sintering, shrinkage occurred in both the CPs, but the observed shrinkage rate of the ZPI was in the 10-20% range, whereas that of the ZIF-8 was less than 1%. Additionally, the sintering mechanisms of the ZPI and ZIF-8 varied between low and high temperatures, and in the case of ZPI, localized melting between the primary particles was the dominant mechanism on the high-temperature side. However, substantial shrinkage did not correspond to an increase in density; on the contrary, a decrease in the apparent density of ZPI was observed as the sintering temperature was increased. The sintering technique is well established and commercially available; thus, the results obtained in this study can be utilized for optimizing the manufacturing conditions of melting CPs.

Theoretical and Experimental Studies of KLi6TaO6 as a Li-Ion Solid Electrolyte.

We study a hexagonal oxide KLi6TaO6 (KLTO), proposed as a Li-ion solid electrolyte, by using a recently developed screening method. First-principles calculations predict that KLTO presents a good Li-ion conductivity (σLi) and a low activation energy (Ea). Li migration is enhanced by the presence of excess Li ions in the interstitial region via a kick-out mechanism. Our experimental results demonstrate that Sn-doped KLTO presents a conductivity of 1 × 10-5 S cm-1, a σLi of 6 × 10-6 S cm-1, and a relatively low Ea of 36 kJ mol-1, which confirm the validity of the proposed screening method. Conversely, detailed analyses of the microstructure and X-ray diffraction patterns of KLTO samples indicate that a stable Li-excess condition is not achieved, therefore leaving potential improvement of the performance of KLTO as a Li-ion solid electrolyte by optimizing extrinsic doping and fabrication processes.

Lithium-ion attack on yttrium oxide in the presence of copper powder during Li plating in a super-concentrated electrolyte.

Li plating/stripping on Cu and Y2O3 (Cu + Y2O3) electrodes was examined in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide and methylphenylamino-di(trifluoroethyl) phosphate. In principle, Li+ ions cannot intercalate into a Y2O3 crystal because its intercalation potential obtained from first-principles calculations is -1.02 V vs. Li+/Li. However, a drastic decrease in the electrode potential and a subsequent constant-potential region were observed during Li plating onto a Cu + Y2O3 electrode, suggesting that Li+ interacted with Y2O3. X-ray diffraction (XRD) patterns and X-ray absorption fine structure (XAFS) spectra of the Cu + Y2O3 electrodes after the Li plating were recorded to verify this phenomenon. The XRD and XAFS results indicated that the crystallinity of Y2O3 crystals was lowered because of attack by Li+ ions or that the Y2O3 crystal structure was broken while the +3 valence state of Y was maintained.

Observation of lithium stripping in super-concentrated electrolyte at potentials lower than regular Li stripping.

Lithium plating/stripping was investigated under constant current mode using a copper powder electrode in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) with methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) and vinylene carbonate (VC) as additives. Typical Li plating/stripping for Cu electrodes in organic electrolytes of conventional lithium batteries proceeds at potentials of several millivolts versus a Li counter electrode. In contrast, a large overpotential of hundreds of millivolts was observed for Li plating/stripping with the super-concentrated electrolyte. When Li stripping started immediately after Li plating and with no rest time between plating and stripping, two potential plateaus, i.e., two-step Li stripping, was observed. The potential plateau for the 1st stripping step appeared at -0.2 V versus a Li metal counter electrode. The electrical capacity for the 1st stripping step was 0.04 mA h cm-2, which indicates irregular Li stripping. Two-step Li stripping was also recorded using cyclic voltammetry. The electrochemical impedance spectroscopy (EIS) studies indicated that the two-step Li stripping behaviour reflected two different solid electrolyte interphases (SEIs) on electrodeposited Li in a Cu electrode. The SEI for the 1st-step stripping was in a transition period of the SEI formation. The open circuit voltage (OCV) relaxation with an order of tens of hours was detected after Li plating and before Li stripping. The in operando EIS study suggested a decrease of the charge transfer resistance in the Cu powder electrode during the OCV relaxation. Since the capacitance for the voltage relaxation was a dozen microfarads, it had a slight contribution to the 1st-step Li stripping behaviour. The voltage relaxation indicated the possibility that it is difficult for Li ions to be electrodeposited or that the Li plating is in a quasi-stable state.

Competition between Conversion Reaction with Cerium Dioxide and Lithium Plating in Superconcentrated Electrolyte.

Li-ion insertion into cerium dioxide (CeO2) and its subsequent conversion reaction were studied using a CeO2/copper composite electrode in a superconcentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) and methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) under conditions promoting Li plating/stripping. Since the conversion reaction potential with CeO2 generally lies above the Li plating/stripping level, the conversion ideally occurs first in the cathodic scan. However, the conversion reaction was delayed until after the Li plating in the superconcentrated electrolyte contrary to expectations, whereas this phenomenon was unobserved in a dilute LiFSA/PNMePh electrolyte. Energy-dispersive X-ray spectroscopy and electrochemical impedance analysis indicated that the reversed order of the electrochemical behaviors was caused by the solid electrolyte interphase (SEI) on the CeO2, which had a different material composition and a higher interfacial resistance than the SEI on electrodeposited metallic lithium.