Satoshi Kajiyama

Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.

High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

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.

Aragonite Nanorods in Calcium Carbonate/Polymer Hybrids Formed through Self‐Organization Processes from Amorphous Calcium Carbonate Solution

Nanostructured inorganic/polymer hybrid thin films comprising aragonite nanorods derived from aqueous suspensions of amorphous calcium carbonate (ACC) are prepared. For the formation of calcium carbonate (CaCO₃)/polymer hybrids, spincoated and annealed films of poly(vinyl alcohol) (PVA) that function as polymer matrices are soaked in aqueous colloidal solutions dispersing ACC stabilized by poly(acrylic acid) (PAA). In the initial stage, calcite thin films form on the surface. Subsequently, aragonite crystals start to form inside the PVA matrix that contains PVA crystallites which induce aragonite nucleation. Nanostructured hybrids composed of calcite thin films consisting of nanoparticles and assembled aragonite nanorods are formed in the matrices of PVA.

Selective synthesis and thin-film formation of α-cobalt hydroxide through an approach inspired by biomineralization

An approach inspired by biomineralization leads to the selective formation of highly crystalline α-Co(OH)2 and its oriented thin films in the presence of organic molecules under mild conditions.

Formation of Helically Structured Chitin/CaCO3 Hybrids through an Approach Inspired by the Biomineralization Processes of Crustacean Cuticles.

Chitin/CaCO3 hybrids with helical structures are formed through a biomineralization-inspired crystallization process under ambient conditions. Liquid-crystalline chitin whiskers are used as helically ordered templates. The liquid-crystalline structures are stabilized by acidic polymer networks which interact with the chitin templates. The crystallization of CaCO3 is conducted by soaking the templates in the colloidal suspension of amorphous CaCO3 (ACC) at room temperature. At the initial stage of crystallization, ACC particles are introduced inside the templates, and they crystallize to CaCO3 nanocrystals. The acidic polymer networks induce CaCO3 crystallization. The characterization of the resultant hybrids reveals that they possess helical order and homogeneous hybrid structures of chitin and CaCO3 , which resemble the structure and composition of the exoskeleton of crustaceans.

Phase separation of a hexacyanoferrate-bridged coordination framework under electrochemical na-ion insertion.

Phase separation and transformation induced by electrochemical ion insertion are key processes in achieving efficient energy storage. Exploration of novel insertion electrode materials/reactions is particularly important to unravel the atomic/molecular-level mechanism and improve the electrochemical properties. Here, we report the unconventional phase separation of a cyanide-bridged coordination polymer, Eu[Fe(CN)6]·4H2O, under electrochemical Na-ion insertion. Detailed structural analyses performed during the electrochemical reaction revealed that, in contrast to conventional electrochemical phase separation induced by the elastic interaction between nearest neighbors, the phase separation of NaxEu[Fe(CN)6]·4H2O is due to a long-range interaction, namely, cooperative rotation ordering of hexacyanoferrates. Kolmogorov-Johnson-Mehl-Avrami analysis showed that the activation energy for the phase boundary migration in NaxEu[Fe(CN)6]·4H2O is lower than that in other conventional electrode materials such as Li(1-x)FePO4.

Rapid and topotactic transformation from octacalcium phosphate to hydroxyapatite (HAP): a new approach to self-organization of free-standing thin-film HAP-based nanohybrids

Biomineralization-inspired processing is attractive for the preparation of functionalized inorganic/organic polymer hybrid materials because the materials are obtained under mild conditions and by using organic templates. As for the formation processes of ordered nanocrystalline hydroxyapatite (HAP), the preparation of self-standing hybrid films based on HAP has not yet been established. In the present study, self-standing thin-film hybrids composed of HAP and poly(vinyl alcohol) (PVA) are obtained by rapid and topotactic transformation of thin films based on octacalcium phosphate (OCP) as a precursor in the organic polymer matrix. Bioinspired crystallization of calcium phosphate on the PVA matrix in the presence of poly(acrylic acid) leads to the formation of nanocomposite structures with oriented OCP nanorod crystals 2–4 nm in width and 10–30 nm in length. The nanostructures allow the composites to transform rapidly into a HAP/PVA hybrid thin film in water. The transformation proceeds without a change in the original OCP/PVA nanostructures, resulting in the formation of a HAP/PVA hybrid thin film with oriented HAP nanorod crystals 5–6 nm in width and 20–50 nm in length. The HAP/PVA hybrids have been obtained as self-standing films with submicrometer scale thickness. The ratio of organic to inorganic components in the self-standing hybrid thin films is similar to that in bones of vertebrates.

Noncovalent Approach to Liquid-Crystalline Ion Conductors: High-Rate Performances and Room-Temperature Operation for Li-Ion Batteries

We report advanced liquid-crystalline (LC) electrolytes for use in lithium-ion batteries (LIBs). We evaluated the potential of LC electrolytes with a half cell composed of Li metal and LiFePO4 which is a conventional positive electrode for LIBs. Low-molecular-weight carbonates of ethylene carbonate or propylene carbonate were incorporated into the two-dimensional (2D) nanostructured electrolyte composed of mesogen-containing carbonate and lithium bis(trifluoromethylsulfonyl)imide. The incorporation of low-molecular-weight carbonates increased the ionic conductivity with maintaining 2D nanostructures in the LC state. High-power performances at relatively high current densities induced by higher ionic conductivities have been achieved by LC electrolytes with low-molecular-weight carbonates. Furthermore, room-temperature operation of LIBs using LC electrolytes is reported for the first time. In the research field of electrolytes for LIBs, we demonstrate the progress of a new category of LC electrolytes.

MXene as a Charge Storage Host

The development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., Ti2CT x, Ti3C2T x, V2CT x, and Nb2CT x, where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins. MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes for advanced EES applications is warranted.

One-dimensional supramolecular hybrids: self-assembled nanofibrous materials based on a sugar gelator and calcite developed along an unusual axis

One-dimensional (1D) supramolecular hybrid structures composed of self-assembled fibers and CaCO3 calcite crystals have been developed through an approach inspired by biomineralization. The supramolecular 1D template consisting of a low-molecular-weight gelator induces the crystal growth of calcite along an unusual axis.