Kei Kubota

Negative electrodes for Na-ion batteries

Research interest in Na-ion batteries has increased rapidly because of the environmental friendliness of sodium compared to lithium. Throughout this Perspective paper, we report and review recent scientific advances in the field of negative electrode materials used for Na-ion batteries. This paper sheds light on negative electrode materials for Na-ion batteries: carbonaceous materials, oxides/phosphates (as sodium insertion materials), sodium alloy/compounds and so on. These electrode materials have different reaction mechanisms for electrochemical sodiation/desodiation processes. Moreover, not only sodiation-active materials but also binders, current collectors, electrolytes and electrode/electrolyte interphase and its stabilization are essential for long cycle life Na-ion batteries. This paper also addresses the prospect of Na-ion batteries as low-cost and long-life batteries with relatively high-energy density as their potential competitive edge over the commercialized Li-ion batteries.

A new electrode material for rechargeable sodium batteries: P2-type Na2/3[Mg0.28Mn0.72]O2 with anomalously high reversible capacity

P2-type Na2/3[Mg0.28Mn0.72]O2 is prepared and electrode performance in Na cells is first provided. The sample surprisingly delivers a large reversible capacity (220 mA h g−1) even though electrochemically inactive magnesium ions are enriched in the host structure. This new electrode material is potentially utilized for rechargeable batteries made from only earth-abundant elements.

A novel K-ion battery: hexacyanoferrate(II)/graphite cell

Recently, research on novel and low cost batteries has been widely conducted to realize large-scale energy storage systems. However, few of the battery systems have delivered performance equal to that of Li-ion batteries. Herein, we propose non-aqueous K-ion batteries by developing hexacyanoferrate(II) compounds (so-called Prussian blue analogues), K1.75Mn[FeII(CN)6]0.93·0.16H2O and K1.64Fe[FeII(CN)6]0.89·0.15H2O, as affordable positive electrode materials. In particular, K1.75Mn[FeII(CN)6]0.93·0.16H2O prepared by a simple precipitation process delivers a high capacity of 141 mA h g−1 at 3.8 V as the average operating potential, resulting in a comparable energy density to that of LiCoO2, with excellent cyclability and rate performance in K half-cells. Operando X-ray diffraction measurements reveal that the excellent electrochemical performance of this material is attributed to its open and flexible framework, which can realize fully reversible K+ extraction/insertion and a structural change from monoclinic to tetragonal via cubic phases. For the first time, we demonstrate an inexpensive high-voltage K-ion full cell with a K1.75Mn[Fe(CN)6]0.93·0.16H2O/graphite configuration to prove its feasibility as a new promising battery system for an environmentally friendly future.

Synthesis of hard carbon from argan shells for Na-ion batteries

Hard carbon is an attractive material for negative electrodes in sodium-ion batteries. Herein, we report a new hard carbon synthesized via carbonization of argan shell biomass, which delivers an enhanced capacity higher than 330 mA h g−1 based upon reversible sodium insertion. We prepared hard carbon under different high-temperature treatment and biomass pretreatment conditions. The graphitization degree of the hard carbon increased as the carbonization temperature increased; simultaneously, the reversible capacity for sodium storage was significantly influenced by the carbonization temperature. Structural characterization revealed differences in the structures of the hard carbons synthesized at different carbonization temperatures, which elucidates the correlation between the increased capacity and the micropore size available for sodium storage. The composite electrodes containing the argan hard carbons with a sodium polyacrylate binder were tested in non-aqueous sodium half cells. The electrodes delivered reversible capacities as high as 300 mA h g−1 at a current density of 25 mA g−1 with superior reversibility and capacity retention of 94.1% after 70 cycles. By carbonization of argan shell biomass treated with HCl aqueous solution, we successfully demonstrated a higher reversible capacity of 333 mA h g−1 and an excellent capacity retention of 96.0% after 100 cycles.

Sodium and Manganese Stoichiometry of P2‐Type Na2/3MnO2

To realize a reversible solid-state Mn(III/IV) redox couple in layered oxides, co-operative Jahn-Teller distortion (CJTD) of six-coordinate Mn(III) (t2g (3) -eg (1) ) is a key factor in terms of structural and physical properties. We develop a single-phase synthesis route for two polymorphs, namely distorted and undistorted P2-type Na2/3 MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3 MnO2 delivers 216 mAh g(-1) as a 3 V class positive electrode, reaching 590 Wh (kg oxide)(-1) with excellent cycle stability in a non-aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na2/3 MnO2 . Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3 MnO2 , providing a new insight into utilization of the Mn(III/IV) redox couple for positive electrodes of Na-ion batteries.

Fc domain mediated self-association of an IgG1 monoclonal antibody under a low ionic strength condition

Recently, we reported that IgG1 monoclonal antibody A (MAb A) underwent liquid-liquid phase separation and separated into light and heavy phases under a low ionic strength condition. The liquid-liquid phase separation was induced due to self-association of MAb A in the heavy phase when the initial concentration of MAb A was between the two critical concentrations [Nishi et al., Pharm. Res., 27, 1348-1360 (2010)]. Here, we determined the interaction site of MAb A by using proteolytic Fab and Fc fragments of MAb A. The mean hydrodynamic diameter of the Fc fragment increased in a low ionic strength buffer, and furthermore the SPR measurement detected interactions of the Fc fragment with both whole MAb A and the Fc fragment, whereas the Fab fragment interacted with neither whole MAb A nor the Fc fragment. No binding was detected under an isotonic ionic strength condition. Zeta potential of MAb A was significant positive below pH 5.5 and negative above pH 6.5. Between pH 5.5 and 6.5 where the phase separation is significantly induced, MAb A had only a small positive or negative net charge. The isothermal titration calorimetry dilution method revealed that dissociation of MAb A accompanied endothermic heat changes, suggesting that intermolecular interactions among MAb A molecules were attributed to the enthalpically driven process. These results suggest that liquid-liquid phase separation of MAb A is mediated by a weak electrostatic intermolecular interaction among MAb A molecules mainly at Fc portions.

One-step preparation of amino-PEG modified poly(methyl methacrylate) microchips for electrophoretic separation of biomolecules

A simple method for a chemical surface modification of poly(methyl methacrylate) (PMMA) microchips with amino-poly(ethyleneglycol) (PEG-NH(2)) by nucleophilic addition-elimination reaction was developed to improve the separation efficiency and analytical reproducibility in a microchip electrophoresis (MCE) analysis of biomolecules such as proteins and enantiomers. In our procedure, the PEG chains were robustly immobilized only by introducing an aqueous solution of PEG-NH(2) into the PMMA microchannel. The electroosmotic mobilities on the modified chips remained almost constant during 35 days with 37 runs without any recoating. The PEG-NH(2) modified chip provided a fast, reproducible, efficient MCE separation of proteins with a wide variety of isoelectric points within 15s. Furthermore, the application of the modified chip to affinity electrophoresis using bovine serum albumin gave a good chiral separation of amino acids.

One-step immobilization of cationic polymer onto a poly(methyl methacrylate) microchip for high-performance electrophoretic analysis of proteins

Abstract One-step covalent immobilization of poly(ethyleneimine) (PEI) onto poly(methyl methacrylate) (PMMA) substrates was investigated to achieve an efficient separation of basic proteins in microchip electrophoresis (MCE). The PEI-treated PMMA chip showed the anodic electroosmotic flow and its rate was almost kept stable during 32 days with over 50 runs. This longer stability of the prepared microchip indicated that the loss of PEI was successfully suppressed by the immobilization through the covalent bond. Furthermore, the PEI modification onto the PMMA chip could apparently reduce the surface adsorption of cationic proteins. In the MCE analysis on the PEI-modified microchip, two proteins were successfully separated within 30 s only utilizing a separation length of 5 mm. While the migration time of the protein gradually increased during only four consecutive runs on an untreated PMMA chip, reproducible analyses were attained by using the PEI immobilized microchip. These results demonstrated that Coulombic repulsion force generated between cationic PEI and basic proteins could avoid the irreversible adsorption of the analytes onto the PMMA surface, which provided a high-performance analysis medium for biogenic compounds.

Towards K-Ion and Na-Ion Batteries as “Beyond Li-Ion”

Li-ion battery commercialized by Sony in 1991 has the highest energy-density among practical rechargeable batteries and is widely used in electronic devices, electric vehicles, and stationary energy storage system in the world. Moreover, the battery market is rapidly growing in the world and further fast-growing is expected. With expansion of the demand and applications, price of lithium and cobalt resources is increasing. We are, therefore, motivated to study Na- and K-ion batteries for stationary energy storage system because of much abundant Na and K resources and the wide distribution in the world. In this account, we review developments of Na- and K-ion batteries with mainly introducing our previous and present researches in comparison to that of Li-ion battery.

Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes

We examined the state of sodium electrochemically inserted in HC prepared at 700–2000 °C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The 23Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and −60 ppm. Each observed spectrum was ascribed to combinations of Na+ ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or non-extractable sodium ions in HC, and decomposed Na compounds such as Na2CO3. No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 °C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na3 triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li2 linear and Li4 square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage.