The measurements on the activity and solvation of H₂O in hydrate melts and aqueous electrolyte solutions containing various metal chlorides were carried out by a vapor pressure measurement using a transpiration method and ¹H quantitative NMR (¹H qNMR). The electrolyte concentration dependence of the detection rate of H₂O by ¹H qNMR reflected the change of the hydration structure of the first hydration shell, and the activity of H₂O by the vapor pressure measurement clearly showed the change of the network structure of water including the first and second hydration spheres. The correlation between the enthalpy and entropy of vaporization showed the existence of different kinds of water-cation interactions.

The electrochemical behaviour of Ti⁴⁺ (K₂TiF₆) at liquid tin and bismuth electrodes has been investigated by cyclic voltammetry, square wave voltammetry and chronoamperometry in the LiF–NaF–KF eutectic melt. Result shows that, on the liquid metal electrodes, the reduction reaction of the solvent occurs at more positive potential than that on the inert molybdenum electrode. There are two reduction steps including Ti⁴⁺ + e = Ti³⁺ and Ti³⁺ + 3e = Ti (Ti–Sn alloys) on the liquid tin electrode within the electrochemical window of the melt. However, only one redox process corresponding to the redox of the Ti⁴⁺/Ti³⁺ couple is observed at liquid bismuth electrode.

Full cells consisting of nanocrystalline Li₃V₂(PO₄)₃ (LVP) positive and standard commercial Li₄Ti₅O₁₂ (LTO) negative electrodes demonstrated outstanding cyclability: capacity retention of 77% over 10,000 cycles. We achieved this stable cycle performance by electrochemical preconditioning of LTO with Li prior to full-cell cycling. The strategy of Li preconditioning not only allows adjustment of the state of charge (SOC) between negative and positive electrodes, but also gives rise to the formation of a protective covering layer on the LTO surface. As we show, this covering layer plays an important role in preventing a key performance-limiting phenomenon—namely, the deposition of vanadium eluted from LVP onto LTO, which degrades the coulombic efficiency of Li⁺ intercalation/deintercalation into LTO crystals—yielding minimal SOC shifts and stable full-cell cycling.

Stable high-voltage operation of LiCoPO₄ (LCP) was successfully achieved via crystal-structure-matched surface coating using FePO₄ (FP) with an identical olivine structure to the LCP. The efficient formation of Fe³⁺-rich surface together with the partial dissolution of Fe³⁺ into LCP matrix yielded the excellent cycle performance with 99% of capacity retention at 100^th cycle, with a minimized Fe³⁺ dosage compared to the methods previously reported. This work confirms that the existence of the Fe³⁺ on the LCP surface is an important factor to bring about the stability of electrode/electrolyte interface.

In this work, IV characteristic and efficiency of protonic ceramic fuel cells and electrolysis cells were discussed based on the oxygen potential profile in mix conductive electrolyte. In protonic electrolyte such as barium zirconate, which exhibits partial hole conductivity in oxygen atmosphere, oxygen potential profile shows a steep slope where hole conductivity is low. As the result, hole blocking layer becomes extremely thin and the rest of the electrolyte shows a significant hole conductivity. Therefore, current leakage cannot be ignored even when the electrolyte material shows quite low electronic conductivity in one side of the electrolyte. When electronic leakage is significant, the analysis of electrochemical measurements becomes complicated. In such case, focusing on the net ionic current is a useful approach to evaluate the cell characteristics. Considering the effect of electrode polarization on the oxygen potential gap at electrode/electrolyte interface, electronic leakage becomes more serious in electrolysis mode. The simulation results showed that both improvement of ionic transportation number of electrolyte and reduction of polarization resistance of oxygen electrode are critical to achieve high efficiency in electrolysis cells.

We investigated the full-cell performance of sodium-ion batteries composed of a hard carbon (HC) negative electrode, a NaCrO₂ positive electrode, and an ionic liquid electrolyte Na[FSA]–[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) at 333 K. Before the full-cell tests, charge–discharge tests of the Na/HC and Na/NaCrO₂ half cells were conducted, from which the practical capacities were determined to be ca. 250 mAh (g-HC)⁻¹ and ca. 115 mAh (g-NaCrO₂)⁻¹, respectively. Using these capacities, the performance of HC/NaCrO₂ full cells with practical loading masses was evaluated by three-electrode cells with a sodium metal reference electrode, and the energy density was calculated to be 177 Wh (kg-(NaCrO₂ + HC))⁻¹. In particular, we focused on the effect of the sodium-ion concentration on the performance by varying the molar fraction of Na[FSA] (x(Na[FSA])) from 0.20 to 0.50. The best rate capability was obtained at a composition of x(Na[FSA]) = 0.50. The effect of the sodium-ion concentration was discussed in terms of the potential profiles of the positive and negative electrodes. The results were explained by the sodium-ion supplying capability of the electrolyte inside the electrode, where the sodium insertion reaction occurs.

To improve the performance of all-solid-state rechargeable batteries, it is important to understand the distribution behavior of carrier ions in electrolytes and electrodes. However, few methods are available for observing carrier ions directly inside an all-solid-state rechargeable battery because lithium, a common carrier ion, is a light element, making observing it directly difficult. In this study, the dynamic behavior of the reaction distribution of an all-solid-state rechargeable battery with a silver-ion solid electrolyte was investigated by using a high-energy X-ray radiography method. The use of silver ions improves the X-ray absorption contrast of carrier ion concentration because silver is a heavy element. In the solid electrolyte, no change in the concentration of carrier ions is detected. By contrast, in the composite electrode, a preferential reaction at the electrode/electrolyte interface is confirmed in the initial stages of charge and discharge. Although a change in the concentration of the solid electrolyte would be an advantage, reaction distribution in the composite electrode is one of the important issues from the viewpoint of practical application of high-energy-density, all-solid-state rechargeable batteries.

Capacity fading mainly caused by state-of-charge deviations between the positive and negative electrodes of lithium-ion batteries (LIBs) can be accurately predicted if the rates of side reactions occurring on these electrodes are known. Herein, we show that the rates of side reactions on LIB electrodes can be determined using an in-house-built high-current-precision battery cycler comprising galvanostat, charge/discharge control, and current switching units. An Li[Li_1/3Ti_5/3]O₄/Li[Li_1/3Ti_5/3]O₄ (LTO/LTO) symmetric cell is used to verify that the battery cycler provides currents accurate enough to determine side-reaction rates, and the rates of side reactions on LTO (negative) and LiNiMO (positive) electrodes in an LTO/LiNiMO cell are compared with intrinsic values obtained for the symmetric cell.

In this study, fine dispersion of LaCoO₃ nanoparticles on carbon via low-energy bead milling was attempted. X-ray diffraction and scanning transmission electron microscopy revealed that LaCoO₃ nanoparticle agglomerates formed by high-temperature calcination were broken up by milling with small beads at low rotation rate, while suppressing damage to the crystal structure. The low-energy bead-milled LaCoO₃/carbon exhibited 2.7 times higher kinetic current density for the oxygen reduction at −0.15 V vs. Hg/HgO than as-synthesized LaCoO₃/carbon; this enhancement may be attributed to the enlarged surface area and improved dispersion of the oxide on carbon.