Operando analysis of graphite intercalation compounds with fluoride-containing polyatomic anions in aqueous solutions

The formation of graphite intercalation compounds (GICs) in aqueous solutions has attracted much attention, but reversibility in the formation/deformation of GICs is a challenging issue to construct highly safe rechargeable batteries. In this study, we used an operando analysis (X-ray diffraction and Raman spectroscopy) to discuss the feasibility of using fluoride-containing polyatomic anions in the formation of GICs in aqueous highly concentrated solutions. We found that the intercalation of anions containing a C₂F₅ moiety (such as [N(SO₂CF₃)(SO₂CF₂CF₃)]⁻ or [N(SO₂CF₂CF₃)₂]⁻) does not occur in the bulk of graphite, but only in the surface region. In addition, anions containing a CF₃ moiety show different behaviors: SO₃CF₃⁻ shows greater reversibility and larger stage-number than N(SO₂CF₃)₂⁻ in the formation of GICs. These results provide design guidelines for the reversible intercalation and de-intercalation of anions and their application as a cathode material in aqueous rechargeable batteries

Kinetic properties of sodium-ion transfer at the interface between graphitic materials and organic electrolyte solutions

Graphitic materials cannot be applied for the negative electrode of sodium-ion battery because the reversible capacities of graphite are anomalously small. To promote electrochemical sodium-ion intercalation into graphitic materials, the interfacial sodium-ion transfer reaction at the interface between graphitized carbon nanosphere (GCNS) electrode and organic electrolyte solutions was investigated. The interfacial lithium-ion transfer reaction was also evaluated for the comparison to the sodium-ion transfer. From the cyclic voltammograms, both lithium-ion and sodium-ion can reversibly intercalate into/from GCNS in all of the electrolytes used here. In the Nyquist plots, the semi-circles at the high frequency region derived from the Solid Electrolyte Interphase (SEI) resistance and the semi-circles at the middle frequency region owing to the charge-transfer resistance appeared. The activation energies of both lithium-ion and sodium-ion transfer resistances were measured. The values of activation energies of the interfacial lithium-ion transfer suggested that the interfacial lithium-ion transfer was influenced by the interaction between lithium-ion and solvents, anions or SEI. The activation energies of the interfacial sodium-ion transfer were larger than the expected values of interfacial sodium-ion transfer based on the week Lewis acidity of sodium-ion. In addition, the activation energies of interfacial sodium-ion transfer in dilute FEC-based electrolytes were smaller than those in concentrated electrolytes. The activation energies of the interfacial lithium/sodium-ion transfer of CNS-1100 in FEC-based electrolyte solutions were almost the same as those of CNS-2900, indicating that the mechanism of interfacial charge-transfer reaction seemed to be the same for highly graphitized materials and low-graphitized materials each other

Sodium/Lithium-Ion Transfer Reaction at the Interface between Low-Crystallized Carbon Nanosphere Electrodes and Organic Electrolytes

Carbon nanosphere (CNS) electrodes are the candidate of sodium-ion battery (SIB) negative electrodes with small internal resistances due to their small particle sizes. Electrochemical properties of low-crystallized CNS electrodes in dilute and concentrated sodium bis(trifluoromethanesulfonyl) amide/ethylene carbonate + dimethyl carbonate (NaTFSA/EC + DMC) were first investigated. From the cyclic voltammograms, both lithium ion and sodium ion can reversibly insert into/from CNSs in all of the electrolytes used here. The cycling stability of CNSs in concentrated electrolytes was better than that in dilute electrolytes for the SIB system. The interfacial charge-transfer resistances at the interface between CNSs and organic electrolytes were evaluated using electrochemical impedance spectroscopy. In the Nyquist plots, the semicircles at the middle-frequency region were assigned to the parallel circuits of charge-transfer resistances and capacitances. The interfacial sodium-ion transfer resistances in concentrated organic electrolytes were much smaller than those in dilute electrolytes, and the rate capability of CNS electrodes in sodium salt-concentrated electrolytes might be better than in dilute electrolytes, suggesting that CNSs with concentrated electrolytes are the candidate of SIB negative electrode materials with high rate capability. The calculated activation energies of interfacial sodium-ion transfer were dependent on electrolyte compositions and similar to those of interfacial lithium-ion transfer

Black Phosphorus-Graphite Material Composites with a Low Activation Energy of Interfacial Conductivity

The reaction kinetics of electrochemical lithiation/delithiation in a composite consisting of black phosphorus and cup-stacked carbon nanotube (BP-CSCNT) were investigated. Two semicircles were observed in Nyquist plots at the high and low frequency regions, which were attributed to the resistance of lithium-ion transport through the surface film and the resistance of the alloying/dealloying reaction (charge-transfer resistance), respectively. The activation energy using the charge transfer reaction was evaluated from the temperature-dependence of the interfacial conductivity. Even with the use of ethylene carbonate-based and propylene carbonate electrolytes, the activation energy was calculated to be 25–26 kJ mol⁻¹, which is much smaller than that obtained with graphite electrodes and cathode materials. These results indicate that the ionic charge transfer process in BP-CSCNT composite electrodes is not coupled with the desolvation process and suggest that the charge transfer in BP-CSCNT is exceptionally fast compared to that in other insertion materials

Kinetics of Interfacial Lithium-ion Transfer between a Graphite Negative Electrode and a Li₂S-P₂S₅ Glassy Solid Electrolyte

All-solid-state lithium-ion batteries that use sulfide solid electrolytes have attracted much attention due to their high safety and wide electrochemical window. In this study, highly oriented pyrolytic graphite (HOPG) and 75Li₂S-25P₂S₅ (mol%) glass were used as a model graphite negative electrode and a sulfide solid electrolyte, respectively. Interfacial lithium-ion transfer between 75Li₂S-25P₂S₅ glass and the HOPG electrode was studied by AC impedance spectroscopy measurements. The activation energy of the interfacial lithium-ion transfer was estimated to be around 37 kJ mol⁻¹, which was much smaller than that at the interface between organic liquid electrolytes and HOPG electrode, indicating that the lithium-ion transfer at the interface between 75Li₂S-25P₂S₅ glass and HOPG electrode proceeded quite rapidly. Furthermore, surface deposition of TiO₂ and surface oxidation on HOPG electrodes were performed using the atomic layer deposition (ALD) method. Interfacial lithium-ion transfer between 75Li₂S-25P₂S₅ glass and ALD-modified-HOPG electrodes was also investigated. The activation energies of the interfacial lithium-ion transfer were slightly higher than that of HOPG, but the resistance of the charge-transfer process was lower, indicating that the affinity of the HOPG electrode for the glass electrolyte was improved by surface modification

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss

ペロブスカイト構造を有する二機能性空気極触媒に関する研究

京都大学0048新制・課程博士博士(工学)甲第20397号工博第4334号新制||工||1672(附属図書館)京都大学大学院工学研究科物質エネルギー化学専攻(主査)教授 安部 武志, 教授 作花 哲夫, 教授 陰山 洋学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDGA

Electrochemical intercalation of bis(fluorosulfonyl)amide anions into graphite from aqueous solutions

Graphite intercalation compounds of bis(fluorosulfonyl)amide (FSA-GICs) are electrochemically synthesized in a highly concentrated aqueous solution. While only water decomposition occurs at the graphite electrode in a dilute aqueous solution (1 mol kg−1 NaFSA), redox peaks clearly appear in a highly concentrated aqueous solution (19 mol kg−1 NaFSA). Under the application of a constant current, the electrode potential reaches 1.7 V (vs. Ag/AgCl), which is far beyond the upper limit of the potential window, in 19 mol kg−1 NaFSA aq., and the formation of FSA-GIC is confirmed by X-ray diffraction patterns. Acceptor-type GICs using organic anions are observed for the first time in highly concentrated aqueous solutions of NaFSA