Yoshitaka Tateyama

Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for Fast-Charging Lithium-Ion Batteries

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li-O2 systems). Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm(-3)). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li(+) cations to form a fluid polymeric network of anions and Li(+) cations.

First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers

Hysteresis in current-voltage curves has been an important issue for conversion efficiency evaluation and development of perovskite solar cells (PSCs). In this study, we explored the ion diffusion effects in tetragonal CH3NH3PbI3 (MAPbI3) and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calculations. The calculated activation energies of the anionic and cationic vacancy migrations clearly show that I(-) anions in both MAPbI3 and FAPbI3 can easily diffuse with low barriers of ca. 0.45 eV, comparable to that observed in ion-conducting materials. More interestingly, typical MA(+) cations and larger FA(+) cations both have rather low barriers as well, indicating that the cation molecules can migrate in the perovskite sensitizers when a bias voltage is applied. These results can explain the ion displacement scenario recently proposed by experiments. With the dilute diffusion theory, we discuss that smaller vacancy concentrations (higher crystallinity) and replacement of MA(+) with larger cation molecules will be essential for suppressing hysteresis as well as preventing aging behavior of PSC photosensitizers.

Termination Dependence of Tetragonal CH3NH3PbI3 Surfaces for Perovskite Solar Cells

We investigated the termination dependence of structural stability and electronic states of the representative (110), (001), (100), and (101) surfaces of tetragonal CH3NH3PbI3 (MAPbI3), the main component of a perovskite solar cell (PSC), by density functional theory calculations. By examining various types of PbIx polyhedron terminations, we found that a vacant termination is more stable than flat termination on all of the surfaces, under thermodynamic equilibrium conditions of bulk MAPbI3. More interestingly, both terminations can coexist especially on the more probable (110) and (001) surfaces. The electronic structures of the stable vacant and PbI2-rich flat terminations on these two surfaces largely maintain the characteristics of bulk MAPbI3 without midgap states. Thus, these surfaces can contribute to the long carrier lifetime actually observed for the PSCs. Furthermore, the shallow surface states on the (110) and (001) flat terminations can be efficient intermediates of hole transfer. Consequently, the formation of the flat terminations under the PbI2-rich condition will be beneficial for the improvement of PSC performance.

Superconcentrated electrolytes for a high-voltage lithium-ion battery

Finding a viable electrolyte for next-generation 5 V-class lithium-ion batteries is of primary importance. A long-standing obstacle has been metal-ion dissolution at high voltages. The LiPF6 salt in conventional electrolytes is chemically unstable, which accelerates transition metal dissolution of the electrode material, yet beneficially suppresses oxidative dissolution of the aluminium current collector; replacing LiPF6 with more stable lithium salts may diminish transition metal dissolution but unfortunately encounters severe aluminium oxidation. Here we report an electrolyte design that can solve this dilemma. By mixing a stable lithium salt LiN(SO2F)2 with dimethyl carbonate solvent at extremely high concentrations, we obtain an unusual liquid showing a three-dimensional network of anions and solvent molecules that coordinate strongly to Li+ ions. This simple formulation of superconcentrated LiN(SO2F)2/dimethyl carbonate electrolyte inhibits the dissolution of both aluminium and transition metal at around 5 V, and realizes a high-voltage LiNi0.5Mn1.5O4/graphite battery that exhibits excellent cycling durability, high rate capability and enhanced safety.

Sodium-Ion Intercalation Mechanism in MXene Nanosheets

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

Unusually stable ~100-fold reversible and instantaneous swelling of inorganic layered materials

Cells can swell or shrink in certain solutions; however, no equivalent activity has been observed in inorganic materials. Although lamellar materials exhibit increased volume with increase in the lamellar period, the interlamellar expansion is usually limited to a few nanometres, with a simultaneous partial or complete exfoliation into individual atomic layers. Here we demonstrate a large monolithic crystalline swelling of layered materials. The gallery spacing can be instantly increased ~100-fold in one direction to ~90 nm, with the neighbouring layers separated primarily by H2O. The layers remain strongly held without peeling or translational shifts, maintaining a nearly perfect three-dimensional lattice structure of >3,000 layers. First-principle calculations yield a long-range directional structuring of the H2O molecules that may help to stabilize the highly swollen structure. The crystals can also instantaneously shrink back to their original sizes. These findings provide a benchmark for understanding the exfoliating layered materials.

Preparation and Optical Properties of Fullerene/Ferrocene Hybrid Hexagonal Nanosheets and Large-Scale Production of Fullerene Hexagonal Nanosheets

The supramolecular nanoarchitectures, C(60)/ferrocene nanosheets, were prepared by a simple liquid-liquid interfacial precipitation method and fully characterized by means of SEM, STEM, HRTEM, XRD, Raman and UV-vis-NIR spectra. The highly crystallized C(60)/ferrocene hexagonal nanosheets had a size of ca. 9 microm and the formulation C(60)(ferrocene)(2). A strong charge-transfer (CT) band between ferrocene and C(60) was observed at 782 nm, indicating the presence of donor-acceptor interaction in the nanosheets. Upon heating the nanosheets to 150 degrees C, the CT band disappeared due to the sublimation of ferrocene from the C(60)/ferrocene hybrid, and C(60) nanosheets with an fcc crystal structure and the same shape and size as the C(60)/ferrocene nanosheets were obtained.

β-Cyclodextrin-crosslinked alginate gel for patient-controlled drug delivery systems: regulation of host–guest interactions with mechanical stimuli

Macroscopic mechanical force has attracted much attention as an existing yet novel energy source for modification of molecular interactions and molecular reactivity. Although various molecular level phenomena have been controlled by using macroscopic mechanical forces, control of host-guest inclusion has not been a research target. Here we report a novel controlled release system triggered by a mechanical stimulus. Release of ondansetron (ODN), an anti-emetic drug, from a designer hydrogel composed of a β-cyclodextrin (CyD) derivative and alginate (AL) can be controlled by mild mechanical compressions that mimic operation by a patients hand. ODN is released in response to mechanical compressions by changing the inclusion ability of CyD moieties, which may be conformationally distorted, and/or suppressed stabilization of the inclusion complex when an external stress is applied. It is revealed that the release can be induced even by the slight conformational restriction for β-CyD due to the mechanical stimulus. This controlled release technology provides a novel dosing strategy enabling on-demand administration of medicines through a mechanical stimulus generated intentionally by the patient.

Surface Properties of CH3NH3PbI3 for Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted considerable interest because of their high potential for solar energy conversion. Power conversion efficiencies of the PSCs have rapidly increased from 3.8 to over 20% only in the past few years. PSCs have several similarities to dye-sensitized solar cells in their device compositions; mesoporous TiO2 (mp-TiO2) is sensitized by light-absorbing components and placed into a medium containing hole transporting materials (HTMs). On the other hand, the perovskite materials for the light-harvesting, for example, CH3NH3PbI3 (MAPbI3), have a greater advantage for the photovoltaic applications; extremely long photocarrier diffusion lengths (over 1 μm) enable carrier transports without singnificant loss. In this respect, the surface states, that can be possible recombination centers, are also of great importance. Availability of solution processes is another important aspect in terms of low cost fabrication of PSCs. Two-step methods, where PbI2 is first introduced from solution onto a mp-TiO2 film and subsequently transformed into the MAPbI3 by the exposition of a solution containing MAI, suggest that use of such a high PbI2 concentration is crucial to obtain higher performance. The experiments also indicate that the PbI2-rich growth condition modifies TiO2/ or HTM/MAPbI3 interfaces in such a way that the photocarrier transport is improved. Thus, the characteristics of surfaces and interfaces play key roles in the high efficiencies of the PSCs. In this Account, we focus on the structural stability and electronic states of the representative (110), (001), (100), and (101) surfaces of tetragonal MAPbI3, which can be regarded as reasonable model HTM/MAPbI3 interfaces, by use of first-principles calculations. By examining various types of PbIx polyhedron terminations, we found that there are two major phases on all of the four surface facets. They can be classified as vacant- and flat-type terminations, and the former is more stable than the latter under thermodynamically equilibrium conditions. More interestingly, both terminations can coexist especially on the more probable (110) and (001) surfaces. Electronic states, that is, projected density of states, of the stable-vacant and PbI2-rich-flat terminations on the two surfaces are almost the same as that in bulk MAPbI3. These surfaces can contribute to the long carrier lifetime actually observed for the PSCs because they have no midgap surface states. Furthermore, the shallow surface states on the (110) and (001) flat terminations can be efficient intermediates for hole transport to HTMs. Consequently, the formation of the flat terminations under the PbI2-rich condition will be beneficial for the improvement of PSC performance.

Additive Effect on Reductive Decomposition and Binding of Carbonate-Based Solvent toward Solid Electrolyte Interphase Formation in Lithium-Ion Battery

The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and capability of a lithium-ion battery (LIB). Here we investigated the effects of adding vinylene carbonate (VC) to ethylene carbonate (EC) solvent, a typical electrolyte in LIBs, on the reductive decomposition. We focused on both thermodynamics and kinetics of the possible processes and used density functional theory-based molecular dynamics with explicit solvent and Blue-moon ensemble technique for the free energy change. We considered Li(+) in only EC solvent (EC system) and in EC solvent with a VC additive (EC/VC system) to elucidate the additive effects. In addition to clarifying the equilibrium properties, we evaluated the free energy changes along several EC or VC decomposition pathways under one-electron (1e) reduction condition. Two-electron (2e) reduction and attacks of anion radicals to intact molecules were also examined. The present results completely reproduce the gaseous products observed in the experiments. We also found a new mechanism involving the VC additive: the VC additive preferentially reacts with the EC anion radical to suppress the 2e reduction of EC and enhance the initial SEI formation, contrary to the conventional scenario in which VC additive is sacrificially reduced and its radical oligomerization becomes the source of SEI. Because our mechanism needs only 1e reduction, the irreversible capacity at the SEI formation will decrease, which is also consistent with the experimental observations. These results reveal the primary role of VC additive in the EC solvent.