Role of Oxide Ions in Thermally Activated Lithium Diffusion of Li[Li_1/3Ti_5/3]O₄: X‑ray Diffraction Measurements and Raman Spectroscopy

Although Li­[Li_1/3Ti_5/3]­O₄ (LTO) has been considered as an ideal electrode material for lithium-ion batteries (LIBs) because of its “zero-strain” character, initial LTO exhibits high Li conductivity (σ_Li) at high temperatures (<i>T</i>). In this paper, to clarify the inter-relation between LTO’s Li-diffusive nature and structural environment, we performed a systematic structural study on LTO using X-ray diffraction (XRD) measurements and Raman spectroscopy. The average and static information obtained by XRD measurements suggested that the bottleneck radius for Li conduction is limited to ∼0.41 even at 873 K, which is too small to explain the high σ_Li values in LTO. However, Raman spectroscopy demonstrated the dynamic structural changes of the LiO₆ octahedron with <i>T</i>; the bond interaction between Li and O atoms decreases with <i>T</i> because of its anharmonic potential energy. Because the Raman-active modes in LTO correspond to changes in oxide ion position, oxide ions are determined to play a crucial role in obtaining high σ_Li values

Template-Assisted Preferential Formation of a Syn Photodimer in a Pyrophosphate-Induced Self-Assembly of a Thymine-Functionalized Isothiouronium Receptor

The effect of anion templation is investigated for the photodimerization of a thymine-functionalized isothiouronium receptor. The receptor forms a photodimer at the thymine moiety in methanol upon UV irradiation, while the isothiouronium moiety works as an oxoanion binding site via a two-point hydrogen-bonding motif. As compared to the case of a free receptor, the presence of pyrophosphate (PPi) resulted in the preferential formation of the syn-type photodimer, which would be desirable for recognizing the templated PPi

Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li_1/3Ti_5/3]O₄: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy

Lithium titanium oxide Li­[Li_1/3Ti_5/3]­O₄ (LTO) is regarded as an ideal electrode material for lithium-ion batteries because of its “zero-strain” characteristic, high thermal stability, and structural stability. Here, the zero-strain means that the change in cubic lattice parameter is negligibly small during charge and discharge reactions. We performed <i>ex situ</i> Raman spectroscopy on Li_1+<i>x</i>[Li_1/3Ti_5/3]­O₄ samples with 0 ≤ <i>x</i> ≤ 0.94 to gain information about the relationship between a zero-strain reaction scheme and structural change at the atomic scale. The <i>x</i> = 0 (initial) sample exhibits three major Raman bands at 671, 426, and 231 cm^–1 and six minor Raman bands at 751, 510, 400, 344, 264, and 146 cm^–1. According to Raman spectroscopy results on other lithium titanium oxides such as Li₂TiO₃ and TiO₂, the Raman bands at 510, 400, and 146 cm^–1 are attributed to TiO₂ anatase, which is used as a starting material. As <i>x</i> increases from 0 to 0.94, the two major Raman bands at 426 and 231 cm^–1 show a blue shift, while the major Raman band at 671 cm^–1 maintains frequency. The three major Raman bands at 671, 423, and 231 cm^–1 are assigned to the <i>A</i>_1<i>g</i> mode of symmetric stretching vibration ν_sym(Ti–O), the <i>E</i>_<i>g</i> mode of asymmetric stretching vibration ν_asym(Li–O), and the <i>F</i>_2<i>g</i> mode of bending vibration δ­(Ti–O), respectively. Thus, the change in the Raman spectrum with <i>x</i> indicates that the bond length between the Ti and O atoms in the TiO₆ octahedron is independent of <i>x</i>, while that between the Li and O atoms in the LiO₆ octahedron and the bond angle between the Ti and O atoms in the TiO₆ octahedron change with <i>x</i>. Raman studies with decreasing <i>x</i> from 0.94 to 0.10 clarified that such local structural changes are reversible, as in the case for the electrochemical reaction. The zero-strain insertion scheme is discussed from the perspective of Raman spectroscopy

Electrochemical Film Formation on Magnesium Metal in an Ionic Liquid That Dissolves Metal Triflate and Its Application to an Active Material with Anion Charge Carrier

Irregular metallic growth at the anode during recharging of batteries can seriously influence the safety of batteries. To address this problem, we have attempted to design active anode materials with anion charge carriers and recently observed the formation and dissolution of an electrochemical film by triflate anions (CF₃SO₃^–) at the surface of magnesium in an ionic liquid (IL) electrolyte of Mg­(CF₃SO₃)₂, which represents a rare anode material. The effect of heterogeneous cations on film formation was examined in this work. In an IL that dissolves NaCF₃SO₃, sodium ions with a lower reduction potential than Mg²⁺/Mg would not be expected to assist film formation. However, to our surprise, we discovered that some sodium ions are involved in film formation. The sodium ions are believed to act as a cross-linking point for the formation of a film network, which resulted in fairly good reversibility for film formation. In a Ce­(CF₃SO₃)₃-IL electrolyte, an electrochemically formed film free of Ce³⁺ was obtained. The trivalent cerium cations were deactivated and transformed to an oxide on Mg metal. However, the reversibility of film formation in the Ce­(CF₃SO₃)₃ system did not meet the expected level. By coupling the film formation and dissolution behavior with a V₂O₅ cathode, a rechargeable battery was fabricated with dual ion transport species of Na⁺ or Ce³⁺ for the cathode and CF₃SO₃^– for the anode. The unique battery with NaCF₃SO₃ is demonstrated to exhibit good discharge/charge performance with long-term cyclability

Bifunctional Catalytic Activity of Iodine Species for Lithium–Carbon Dioxide Battery

Carbon dioxide (CO2) is a greenhouse gas, the emission of which is a concern due to its contribution to global warming. The lithium–CO2 battery has attracted attention as a means of CO2 reduction and its effective utilization. Li–CO2 batteries undergo discharge by the conversion of CO2 into lithium carbonate (Li2CO3), while charging is caused by the electrochemical decomposition of Li2CO3. Here, an iodine species was investigated as a bifunctional catalyst for both the discharge and charge processes. When the electrolyte in the Li–CO2 battery contains a small amount of iodine, lithium iodide (LiI) is first formed at the cathode during the initial stage of discharge and subsequently CO2 reduction occurs. The LiI that is formed accelerates CO2 reduction. Li2CO3 formed on the cathode during discharge is an insulator; therefore, the accumulation of Li2CO3 produces a passivation layer, which leads to charging at high overpotential (ca. 4.5 V vs Li+/Li). Iodine with a redox potential below 3.5 V vs Li+/Li cannot decompose Li2CO3, because the decomposition potential of Li2CO3 is 3.82 V. However, the redox potential of iodine in the trimethyl phosphate (TMP) electrolyte was greater than 3.8 V, so Li2CO3 could be chemically decomposed by the iodine in TMP. The iodine mediator (3I2/2I3–) in the Li salt–TMP electrolyte was confirmed to enhance the decomposition of Li2CO3 under a low charge voltage

Superior Low-Temperature Power and Cycle Performances of Na-Ion Battery over Li-Ion Battery

The most simple and clear advantage of Na-ion batteries (NIBs) over Li-ion batteries (LIBs) is the natural abundance of Na, which allows inexpensive production of NIBs for large-scale applications. However, although strenuous research efforts have been devoted to NIBs particularly since 2010, certain other advantages of NIBs have been largely overlooked, for example, their low-temperature power and cycle performances. Herein, we present a comparative study of spirally wound full-cells consisting of Li_0.1Na_0.7Co_0.5Mn_0.5O₂ (or Li_0.8Co_0.5Mn_0.5O₂) and hard carbon and report that the power of NIB at −30 °C is ∼21% higher than that of LIB. Moreover, the capacity retention in cycle testing at 0 °C is ∼53% for NIB but only ∼29% for LIB. Raman spectroscopy and density functional theory calculations revealed that the superior performance of NIB is due to the relatively weak interaction between Na⁺ ions and aprotic polar solvents

Insertion of Calcium Ion into Prussian Blue Analogue in Nonaqueous Solutions and Its Application to a Rechargeable Battery with Dual Carriers

We observed the first electrochemical insertion of Ca²⁺ into Prussian blue analogue, MnFe­(CN)₆, in nonaqueous solutions of Ca­(CF₃SO₃)₂ and various solvents including ionic liquid at 60 °C. The kinetics for the Ca²⁺ insertion reaction were studied by cyclic voltammetry, and were compared to those of Na⁺ intercalation. By coupling this phenomenon with metallic anodes, two energy storage devices were made. Ca anode produced a primary cell that operated at a voltage of around 2.0 V. When Mg plate was used as an anode, the negative active material associated with CF₃SO₃^–, which we have already reported was newly formed at the surface of Mg plate. By combining the negative active material, we have fabricated a novel rechargeable battery using dual ion transport species of Ca²⁺ for the cathode and CF₃SO₃^– for the anode, and demonstrated that the battery showed repeated discharge/charge performance

Driving Mechanisms of Ionic Polymer Actuators Having Electric Double Layer Capacitor Structures

Two solid polymer electrolytes, composed of a polyether-segmented polyurethaneurea (PEUU) and either a lithium salt (lithium bis­(trifluoromethanesulfonyl)­amide: Li­[NTf₂]) or a nonvolatile ionic liquid (1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­amide: [C₂mim]­[NTf₂]), were prepared in order to utilize them as ionic polymer actuators. These salts were preferentially dissolved in the polyether phases. The ionic transport mechanism of the polyethers was discussed in terms of the diffusion coefficients and ionic transference numbers of the incorporated ions, which were estimated by means of pulsed-field gradient spin–echo (PGSE) NMR. There was a distinct difference in the ionic transport properties of each polymer electrolyte owing to the difference in the magnitude of interactions between the cations and the polyether. The anionic diffusion coefficient was much faster than that of the cation in the polyether/Li­[NTf₂] electrolyte, whereas the cation diffused faster than the anion in the polyether/[C₂mim]­[NTf₂] electrolyte. Ionic polymer actuators, which have a solid-state electric-double-layer-capacitor (EDLC) structure, were prepared using these polymer electrolyte membranes and ubiquitous carbon materials such as activated carbon and acetylene black. On the basis of the difference in the motional direction of each actuator against applied voltages, a simple model of the actuation mechanisms was proposed by taking the difference in ionic transport properties into consideration. This model discriminated the behavior of the actuators in terms of the products of transference numbers and ionic volumes. The experimentally observed behavior of the actuators was successfully explained by this model

Highly Concentrated Electrolytes Containing a Phosphoric Acid Ester Amide with Self-Extinguishing Properties for Use in Lithium Batteries

Phosphoric acid ester amides were examined as a new self-extinguishing solvent for Li-ion batteries. The phosphoric acid ester amides used in this study contained two fluorinated alkyl groups and one amino group, (CF₃CH₂O)₂(NR₁R₂)­PO (PNR₁R₂). The thermal stability of the highly concentrated electrolyte of lithium bis­(fluorosulfonyl) amide (LiFSA) and PNR₁R₂ with a molar ratio of [LiFSA]/[PNR₁R₂] = 0.5 under overcharge depended on the modification of the amino substituent. Introduction of a phenyl group (R₁ = CH₃, R₂ = C₆H₅) was effective for improving thermal stability. The release of gases and heat that typically accompanied reaction of the solvent with the charged graphite anode was greatly suppressed. Density functional theory calculations indicated that PNR₁R₂ decomposed reductively near 0.5 V vs Li⁺/Li, suggesting poor Li ion insertion into the graphite. However, the highly concentrated electrolyte using LiFSA and PNR₁R₂ reduced the reductive potential of PNR₁R₂ and enabled not only the insertion of Li ions into the graphite but also reversible Li plating/stripping