Nanospace Control of SnO₂ Nanocrystallites-Embedded Nanoporous Carbon for Reversible Electrochemical Charge–Discharge Reactions

The electrochemical charge–discharge reactions of SnO₂ with Li ions, which are composed of SnO₂–Sn conversion and Sn–Li alloying/dealloying reactions, are generally irreversible, causing severe capacity fading during cycling. As a novel material overcoming the problem, SnO₂ nanocrystallites-embedded nanoporous carbons, of which SnO₂ loading amount, nanopore size and nanopore volume were systematically tuned, were successfully synthesized by <i>in situ</i> synthesis using vaporized SnCl₂. Effectiveness of carbon nanospace confinement as well as key factors of appropriate pore filling fraction and dispersive deposition of SnO₂ nanocrystallites in carbon nanospace for reversible electrochemical reactions were revealed by a detailed investigation on the relation between the structural parameters and charge–discharge properties. Consequently, the nanoporous composites with SnO₂ loading amount below 69 wt % showed high capacities of 734–855 mAh g^–1 and high capacity retentions of 70–86% at the 30th charge–discharge cycle. The high capacity retention was preserved even after 100 cycles. The appropriate embedding of SnO₂ nanocrystallites in carbon nanospace also resulted in the enhancement of rate capability as well as the decrease in reaction resistance, which was demonstrated by electrochemical impedance spectroscopy

Direct Observation of Electrochemical Lithium–Sulfur Reaction inside Carbon Nanotubes

An ongoing challenge for next-generation energy storage systems is to maximize the battery performance of lithium–sulfur (Li–S) systems, which exhibit high theoretical capacity and high energy density. Despite the outstanding effects observed by nanoconfinement of sulfur within conductive porous media, few studies have elucidated the ideal nanospace for the Li–S reaction because nanoscale characterization of lithiated sulfur molecules is difficult. We present direct evidence of electrochemically lithiated sulfur molecules confined inside carbon nanotubes (CNTs) using Cs-corrected high-resolution scanning transmission electron microscopy with electron energy loss spectroscopy. For a certain diameter of CNTs, short sulfur chains were stabilized inside CNTs via the charge transfer interaction, exhibiting a unique electrochemical activity and stable cycle performance compared to those of long sulfur chains. Our findings reveal that optimal CNTs have the one-dimensional channels for smooth progress of the lithiation reaction

Enhanced Electric Double-Layer Capacitance by Desolvation of Lithium Ions in Confined Nanospaces of Microporous Carbon

Carbon electrodes with specific microporous structures are strongly desired to improve the performance of electric double-layer capacitors (EDLCs). We report solvated states of Li ions in confined carbon micropores affecting specific capacitance. The average Li⁺ solvation number of 1 M LiClO₄/propylene carbonate (PC) electrolyte introduced into porous carbon electrodes was determined using Raman spectroscopy and ⁷Li NMR. Micropores with slightly larger pore size against the solvated molecules and the narrow two-dimensional spaces decreased the solvation number, enhancing specific capacitance. Hence, specific carbon morphology may be related to high EDL capacitance, and micropore structure is important in obtaining highly capacitive EDLC materials

Experimental Information on the Adsorbed Phase of Water Formed in the Inner Pore of Single-Walled Carbon Nanotube Itself

Thus far, nobody has successfully obtained the accurate information on the properties of the adsorbed phases of gases or vapors formed inside a cylindrical micropore of single-walled carbon nanotube (SWCNT) itself based on the experimental procedure. In this work, we succeeded in analyzing experimentally the properties of adsorbed nitrogen and water confined in the inner pore of SWCNT itself by opening the pore composed of close-ended SWCNT without any changes in the surface state and also by applying the unique method for characterization; both the amounts, as well as properties, of surface functional groups and the bundle structure are the same even after the treatments for introducing an open-ended structure to a close-ended one. As a result, the average pore sizes, as well as characteristic adsorption behavior, on the two types of sample were available from the analysis of respective difference adsorption isotherms of nitrogen measured at 77 K between the adsorbed amounts on the open-ended SWCNT and that on the close-ended one. The evaluated pore sizes well coincide with the results estimated by Raman data. These results strongly support that we could analyze the adsorbed phases formed only in the inner pore of SWCNTs by applying the present method. Furthermore, we could analyze the adsorbed phase of water formed inside the cylindrical micropore of SWCNTs, showing the difference in the densities of adsorbed water depending on the pore sizes from the value of bulk water; the densities of the adsorbed water were evaluated to be 0.62 and 0.71 g mL^–1 for SWCNTs having average pore sizes of 1.3 and 1.7 nm, respectively, which were in harmony with those obtained by the theoretical calculations reported by other researchers. The proposed analysis method makes it possible to recognize the focused states of the adsorbed water formed inside the cylindrical micropore of SWCNT more precisely and correctly. The method proposed will shed light on the discussion related to the detailed nature of various adsorbed gases into SWCNT, to the detailed role of adsorbed species formed inside pore in various phenomena, and to the designing the useful materials based on the gained knowledge

Enhanced CO₂ Adsorptivity of Partially Charged Single Walled Carbon Nanotubes by Methylene Blue Encapsulation

We prepared a partially charged single walled carbon nanotube (SWCNT) by charge transfer-mediated encapsulation of methylene blue (MB) molecules, which enhances the CO₂ adsorptivity. The liquid phase adsorption of MB molecules on SWCNT could give the MB-encapsulated SWCNT, which was evidenced by the remarkable depression of the X-ray diffraction intensity from the ordered bundle structure, the decrease of N₂ and H₂ adsorption in the internal tube spaces of SWCNT, and the high-resolution transmission electron microscopic observation. The molecular spectroscopic examination revealed the charge transfer interaction between the encapsulated MB molecules and SWCNT. The electrical conductivity increased by the encapsulation of MB suggested the electron transfer from SWCNT to MB molecules, giving rise to positively charged SWCNT. The enhancement of CO₂ adsorption by the MB-encapsulation coincided with the positively charged SWCNT