Electrochemical anion intercalation/de-intercalation into the graphite electrode generally takes place at high electrode potential of over 4.5 V (Li⁺/Li), so it is attractive as the positive electrode reaction for the post lithium-ion batteries. In this study, the kinetic behavior of the anion intercalation reaction in the organic solution is studied. The interfacial activation energy of anion species is as low as about 25 kJ mol⁻¹, which is much lower than that of Li⁺-ion. The result means the anion intercalation reaction is essentially fast and therefore, it is suitable for the high power batteries. In addition, the activation energy is not influenced by the solvation ability in the solution and from the result, we conclude that the interfacial activation barrier will exist on the graphite electrode side, contrary to the Li⁺-ion case.

The crystal structure and Li storage properties of Cu-doped rutile TiO₂ after a phase change caused by lithiation were investigated for the first time. Structural analysis results confirmed that undoped rutile TiO₂ was transformed to a distorted layered rock-salt Li_xTiO₂ structure with a small volume expansion of only 1 % when cycled in a potential range of 1.0–3.0 V vs. Li⁺/Li. A substitutional solid solution of Cu²⁺ was formed in layered Li_xTiO₂. The Cu doping increased both the interlayer distance and electronic conductivity of the layered Li_xTiO₂. As an Li-ion battery anode, a Cu-doped TiO₂ electrode exhibited a long cycle life, maintaining a reversible capacity of 120 mAh g⁻¹ over 10000 cycles at 5C and an excellent rate capability of 108 mAh g⁻¹ at 50C. Furthermore, this electrode could also be potentially used as a Na storage material. These attractive properties demonstrate high applicability of Cu-doped rutile TiO₂ as a novel anode material.

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.

Analysis of the differential capacity profile (dQ/dV vs. V), which can capture the changes in the electrode structure inside a battery, is an effective electrochemical method for investigating the degradation behavior and mechanism of Li-ion cells. However, the electrode reactions corresponding to the peaks in a dQ/dV vs. V curve are undefined. Hence, it is difficult to qualitatively analyze the mechanism of cell degradation using this curve. In this work, we propose an original method for attributing the peaks in a dQ/dV vs. V curve. Peak attribution is implemented using a three-electrode Li-ion laminate cell with a LiCoO₂ cathode, graphite anode, and lithium reference electrode. During charge and discharge, the potential differences (dE) of the LiCoO₂-Li and graphite-Li sides and voltage difference (dV) of the LiCoO₂-graphite full cell are measured simultaneously. Meanwhile, the Coulomb amounts (dQ) of the LiCoO₂-Li and graphite-Li sides are equivalent to that of the LiCoO₂-graphite full cell at a certain time. By comparing the dQ/dV vs. V curve of the full cell to the dQ/dE vs. E curves of the LiCoO₂-Li and graphite-Li sides, the peaks in the differential capacity curve can be attributed to specific structural changes in the electrodes. Importantly, this information is acquired without disassembling the cell, making our proposed analytical method a convenient means of studying cell degradation under various conditions, such as low temperature, high temperature, or high-rate charging.

A Li-excess cation-disordered rocksalt oxide, Li_1.2Nb_0.2V_0.6O₂, with higher theoretical capacity than traditional stoichiometric and layered oxides, is synthesized and tested as positive electrode materials for battery applications. Although Li_1.2Nb_0.2V_0.6O₂ cannot be synthesized by conventional calcination method, a single phase and metastable oxide is successfully synthesized by high-energy mechanical milling. Electrode performance of metastable and nanosized Li_1.2Nb_0.2V_0.6O₂ is significantly improved by heat treatment at 600 °C. Heat treated Li_1.2Nb_0.2V_0.6O₂ with a partial cation ordered layered structure delivers a high reversible specific capacity of 320 mAh g⁻¹ on the basis of highly reversible two-electron redox of V ions. Moreover, inferior cyclability originating from the dissolution of V ions is successfully improved by using concentrate electrolyte solution, and over 90 % capacity retention is achieved after 50 cycles. This finding opens a new way to design high-capacity metastable Li-excess oxides for advanced Li-ion batteries with higher energy density.

Room temperature ionic liquids (ILs) are the appealing research target as electrolytes for lithium batteries. The cation and anion structure of ILs influences their physical and chemical properties. In this study, an electron-donating branched substituent was introduced into imidazolium cations to promote charge delocalization and to improve the reduction stability of ILs. The ILs with branched substituent group at the third position of imidazolium cation, 1-allyl-3-isobutylimidazolium bis(fluorosulfonyl)amide ([AB_isoIm][FSA]) and 1-allyl-3-tert-butylimidazolium bis(fluorosulfonyl)amide ([AB_tertIm][FSA]), exhibited lower cathodic potential (−2.54 and −2.79 V vs. Fc/Fc⁺, respectively). [AB_isoIm][FSA] and [AB_tertIm][FSA] showed sufficiently high ionic conductivity (4.85 and 3.66 mS/cm at 298 K, respectively), although the viscosity increased with the introduction of bulky branched substituent. Lithium secondary batteries composed of electrolytes using [AB_isoIm][FSA] and [AB_tertIm][FSA] showed stable charge-discharge behavior.

An LiCoO₂ (LCO) phase is prepared on a perovskite type Li_0.29La_0.57TiO₃ (LLTO) solid electrolyte by heating mixed lithium salts of LiNO₃ and LiCl with Co(NO₃)₂·6H₂O at 700 °C for 1 h. The resultant LCO is evaluated as a positive electrode in an all-solid-state Li secondary battery. Liquid-phase sintering using molten salts has been effective for the formation of a favorable interface between oxides in which lithium ions migrate electrochemically with reversibility. The fracture surface revealed by field emission scanning electron microscopy observation shows that the microscopic texture of the LCO consists of a dense 1 to 2 µm thick layer closely attached to the solid electrolyte over a wide area, as well as LCO spherical particles with sizes of several micrometers. The former growth has superior electrochemical activity compared to the latter. Additionally, a preferential growth plane of the LCO on LLTO is analyzed by transmission electron microscopy and the process of formation with heating is described.

A combinatorial synthesis system consisting of co-sputtering multiple radio frequency (RF) cathodes was developed to investigate the presence of new amorphous solid electrolytes in a pseudo-ternary Li₃PO₄-Li₄SiO₄-LiAlO₂ system. Oxynitride solid-electrolyte films were synthesized under an N₂ atmosphere by reactive RF sputtering with cathodes made of different materials, such as Li₃PO₄, Li₄SiO₄, and LiAlO₂ installed at different positions in the chamber. The formation of amorphous films with no grains and a continuous atomic distribution was confirmed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. In a single synthesis, we fabricated an oxynitride solid-electrolyte film, in which the composition range of the components was approximately one-eighth that in the corresponding pseudo-ternary system. The highest Li-ion conductivity of 3.1 × 10⁻⁶ S cm⁻¹ was obtained for the LiPSiAlON film with a composition ratio of LiPON : LiSiON : LiAlON = 48.5 : 33.9 : 17.6. The different bonding states of O and N in the LiPSiAlON film doped with P, Si, and Al were examined using X-ray photoemission spectroscopy. The ionic conductivity was improved by the mixed anion effect. The combinatorial synthesis enables the efficient optimization of the chemical composition and facilitates the development of highly conductive amorphous solid electrolytes.

Electrochemical CO₂ reduction is crucial for developing a sustainable closed-carbon-cycle society; however, the factors determining product selectivity have not been fully established, especially when the reaction is carried out over metal cathodes. The selectivity of the CO₂ reduction pathway can be improved by modifying the cathode’s catalyst surface. This can be achieved by using specific surface of single crystal or by alloying through the addition of an impurity. In this study, the products of electrochemical CO₂ reduction were evaluated in an aqueous KHCO₃ system at various applied currents and with various metal cathodes. The metal cathodes were coated by Ag nanoparticles, as the Ag cathode was determined to yield the best Faradaic efficiency for CO production in an aqueous KHCO₃ system. Our results suggest that some Ag coated cathodes enhance CO₂ reduction, and thus the reduction products are tunable through surface modification.

The relationship between reductive decomposition of anion and the efficiency of magnesium deposition was investigated in magnesium bis(trifluoromethane sulfonyl)imide (Mg(TFSI)₂) salt and triglyme (G3) solvent without and with magnesium ethoxy chloride (EtOMgCl) salt. Cyclic voltammetry, energy dispersive X-ray spectroscopy analysis, in situ infrared spectroscopy during deposition–dissolution of magnesium and Raman spectroscopy were conducted. Coulombic efficiency for magnesium deposition was also calculated by chrono amperometry. In Mg(TFSI)₂/G3 (molar ratio of 1/15), magnesium was hardly deposited. In EtOMgCl/Mg(TFSI)₂/G3 (molar ratio of 2/1/15), magnesium was deposited efficiently. The reduction of TFSI⁻ anion occurred in Mg(TFSI)₂/G3 during magnesium deposition, while the reduction was suppressed in EtOMgCl/Mg(TFSI)₂/G3. The addition of EtOMgCl salt can suppress the reduction of TFSI⁻ anion, leading to highly efficient magnesium deposition.

All-solid-state sodium secondary batteries are expected to be low-cost, next-generation batteries. NiS₂ and FeS₂ are potential candidates as positive electrode materials owing to their high theoretical capacities. However, it is difficult to achieve sufficient capacity with bulk FeS₂. In this study, pyrite Ni_1−xFe_xS₂ (x = 0, 0.3, 0.5, 0.7, 0.9, and 1) electrodes are prepared by a mechanochemical process. The all-solid-state sodium cells with Ni_1−xFe_xS₂ show higher discharge–charge potentials than those with NiS₂, and higher capacities than those with FeS₂. In addition, Ni_1−xFe_xS₂ exhibits a higher rate performance than those of NiS₂ and FeS₂. The all-solid-state cells using Ni_1−xFe_xS₂ (x = 0.3, 0.5, and 0.7) are discharged and charged with a high capacity of approximately 390 mAh g⁻¹, without significant capacity fading for at least 30 cycles. The solid-solution formation of NiS₂ and FeS₂ results in lower material cost, higher rate performance, higher discharge–charge potential than those of NiS₂, and higher capacity than that of FeS₂. Pyrite Ni_1−xFe_xS₂ is a promising positive electrode material for all-solid-state sodium secondary batteries.

Toward the development of all-solid-state batteries with enhanced performance, this study describes the investigation of the degeneration mechanism under low-humid conditions of an argyrodite-type sulfide-based solid electrolyte. The degeneration of the electrolyte with moisture occurs even under the condition of super-low humidity in a dry room with a dew point (dp) as low as −50 °Cdp. Formation of hydrogen sulfide is detected when the electrolyte is exposed to dry air with −20 °Cdp. The results of impedance measurements suggest that the grain surface of the electrolyte is degenerated with moisture, resulting in a decrease in the lithium-ion conductivity at the grain boundary. The degenerated electrolyte surface can be partially recovered by heating at 170 °C in vacuo, although a small degeneration in bulk may occur in the heating process.