Materials with metastable structures often exhibit high stability up to a certain temperature, despite not being thermodynamically stable. Therefore, it is important to use the excellent physical properties generated by metastable structure. Secondary batteries are an energy device that stores chemical energy by producing thermodynamically metastable materials. Metastable materials can also be directly utilized as the electrode active materials and the solid electrolytes. The rapid quenching and mechanochemical processes are useful to obtain room temperature metastable materials. This review provides an overview of our research progress on glassy and metastable crystalline materials for all-solid-state batteries.

This study evaluated the influence of halide anion of phenoxyimine–magnesium halide complex on magnesium deposition–dissolution in Mg(TFSA)₂/triglyme electrolyte containing the magnesium halide complex. The chloride complex provides rather high Coulombic efficiency of reversible magnesium deposition compared with bromide analogue. This difference appears to be in relation to the different complexation equilibrium by Mg²⁺, Cl⁻, and solvent molecules.

Recently, electrolytes in which lithium polysulfides (PS) is sparingly soluble, such as solvate ionic liquids (SILs), were proposed and used as electrolytes for lithium-sulfur (Li-S) batteries. Li-S batteries with the SIL electrolyte showed good cycle stability and Coulombic efficiency; however, the actual energy density of preliminary tested cells was much lower than the theoretical value due to the low sulfur loading and excess electrolyte. In this study, we employed Al foam sheets as the cathode current collector in order to three-dimensionally collect the current, which resulted in increased sulfur loading and enabled us to control the cathode porosity by mechanically pressing the Al foam sheets. High sulfur loading on the cathodes (up to 6 mg_-sulfur cm⁻²) was achieved with an initial discharge capacity of 1000 mAh g⁻¹. Simultaneously, in order to reduce the amount of electrolyte, the cathode porosity was controlled; the discharge capacity in the SIL electrolyte was maintained at ∼1000 mAh g⁻¹ when the cathode porosity was no less than 30%. At sufficiently high porosity, the discharge capacity was independent of the electrolyte amount and was 1000 mAh g⁻¹ for electrolyte volume/total void volume ratios higher than 3 in a coin cell configuration.

Pre-lithiation of graphene-like graphite (GLG) was conducted and its effects on structural and electrochemical properties of GLG were investigated. When lithium naphthalenide was used for the pre-lithiation, a large amount of the solution was required for the intercalation of lithium ions into GLG. Moreover, binder in the composite electrode was degraded by the pre-lithiation and the discharge capacity considerably decreased. On the other hand, pre-lithiation of GLG with lithium metal resulted in the increase of interlayer distance to similar value to that of electrochemically full-charged GLG, even though the amount of the added lithium metal was equivalent to only 28% or 38% of SOC of the GLG. In addition, the decreases in the charge capacity were more than those expected from the amounts of lithium metal. The full and immediate interlayer expansion by the pre-lithiation of GLG suppressed the repeated exfoliation and re-formation of SEI unlike electrochemically charged GLG. Since the discharge capacities were almost identical to that of the pristine GLG, the coulombic efficiency was greatly improved. The issue of low coulombic efficiency of GLG at the initial cycle can be conquered by this method, and it would increase the probability of the practical application of GLG.

In this work, we investigated electrical conduction properties, crystal structures, and electron-density distributions of LaBaGaO₄-based protonic conductors; i.e., La_0.95M_0.05BaGaO_4−δ (M = Ba, Sr, and Ca) and La_0.9Ba_1.1Ga_0.9M′_0.1O_4−δ (M′ = Ga, Al, and In). It was demonstrated from the conductivity measurements that the Sr-substituted sample showed the highest conductivity among La_0.95M_0.05BaGaO_4−δ and the partial substitutions of Al and In for Ga deteriorated the conductivity. To clarify effects of the substitutions on crystal structures and electron-density distributions, we performed a Rietveld refinement and a maximum-entropy method. As a result, it was found that a distortion of the crystal structure was the lowest in the Sr-substituted sample among La_0.95M_0.05BaGaO_4−δ and became larger by the partial substitution for Ga. It was also suggested that both the Al and In substitutions made (Ga, M′)-O bond stronger and thus (Ga, M′)O₄ tetrahedra rigid. Such structural changes by the partial substitutions could be considered to affect the electrical conduction properties.

The structures of polymer electrolyte membranes and catalyst layer binders and the distribution of water therein are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. To aid the understanding of the in-plane water distribution, neutron reflectometry (NR) was carried out on a Nafion^® film with a thickness of 150 nm formed on a 20-nm Pt layer deposited on Si(100) with a native SiO₂ layer. By means of ambient pressure X-ray absorption spectroscopy at room temperature in air, the Pt substrate was found to be metallic. For NR, the temperature was set at 80°C and the relative humidity at 30, 50 and 80%, simulating the conditions for power generation. Clear NR modulation was obtained under each condition. NR data were fit very well with a 3-sublayered model parallel to the substrate with different densities of Nafion and water. The influence of the Pt substrate was observed not only at the Nafion/Pt interface, but also on the thin-film structure. The water uptake in a Nafion film on Pt also differed from that on SiO₂. At 80°C, the surface of the Pt substrate was proposed to be oxidized, and the Nafion/Pt interface was found to contain water, in contrast to the interface observed at room temperature.

Impact of structural disorder induced by mechanical milling on electrochemical properties of carbonaceous materials is systematically examined. Carbonaceous materials with different structural disorder are prepared by mechanical milling with different rotation speeds. Thus prepared carbons show sloping voltage profiles in Li/Na cells, which resemble those of soft carbons. The disordered carbon material prepared at 600 rpm shows a reversible capacity of >500 mA h g⁻¹ in a Li cell. In addition, the disordered carbon also delivers 200 mA h g⁻¹ in a Na cell with high Coulombic efficiency for continuous cycles. Heat treatment of the disordered carbon results in the formation of hard carbon with nanopores, and thus Li/Na storage properties are significantly changed.

Mg(Mg_0.5V_1.4Ni_0.1)O₄ was synthesized by a solid-phase method under high vacuum. The product was assigned to a spinel structure with space group Fd3m based on the powder X-ray diffraction spectrum. Synthesized materials of uniform composition were confirmed from elemental mapping by STEM-EDS. Charge and discharge cycle tests showed a discharge capacity exceeding 60 mAh/g at 90°C. Cyclability was maintained for 19 cycles of charging and discharging. The electronic structure was analyzed by MEM using the result of Rietveld analysis, which showed that Mg could be easily inserted into the Mg(Mg_0.5V_1.4Ni_0.1)O₄ and that the host structure was stable due to the high covalency of M(16d)-O. MO₆ octahedral strain was alleviated by increasing the amount of substitution of Ni. The valence of the transition metals was examined by XAFS; the spectra at the V K-edge for all samples showed that V existed in the trivalent and tetravalent states. It was suggested that V could be converted to a higher oxidation state after the 1st charge.

Na-Sb alloy was synthesized as an advanced negative electrode material for all-solid-state sodium batteries by a mechanochemical process. An all-solid-state symmetric cell using a composite of an Na-Sb alloy and Na₃PS₄ solid electrolyte operated reversibly with a high reversible capacity of 370 mAh g⁻¹ at room temperature under a current density of 0.064 mA cm⁻². The composite electrode showed an effective ionic conductivity of 3.4 × 10⁻⁵ S cm⁻¹. The cell also operated at 60°C under the same current density with low polarization compared to room temperature operation. The sodiation process from NaSb to Na₃Sb was examined by ex situ X-ray diffraction (XRD) measurements. An all-solid-state half-cell using a TiS₂ composite electrode and a Na₃Sb composite electrode showed a high reversible capacity of 200 mAh g⁻¹ at room temperature under a constant current density of 0.064 mA cm⁻². Na-Sb alloys are a suitable negative electrode material for all-solid-state sodium batteries.

Electrical conductivity of LiClO₄ solution in binary solvent of propylene carbonate (PC) and 1,2-dimethoxyethane (DME) coexisting with LiCoO₂ powder was measured. Maximum value of conductivity in LiClO₄/PC-DME solution observed at ca. 1:1 of volume fraction shifted toward lower content of DME. In higher LiClO₄ concentration reached to 3 mol L⁻¹, the conductivity of LiClO₄-PC_1−xDME_x increased with DME content even if LiCoO₂ was added. The results of FT-IR measurement support the results of conductivity measurement for PC solvation of the slurry sample of LiCoO₂/LiClO₄/PC-DME system.

To identify an electrocatalyst that can effectively generate power and reduce CO₂, we measured the power generation characteristics of a polymer electrolyte fuel cell fed with H₂ and CO₂ to the anode (Pt/C) and cathode (Pt_(1−x)Ru_x/C), respectively. The obtained power generation characteristics were compared to their corresponding CO₂ reduction characteristics. A trade-off relationship between the CO₂ reduction and power generation performances was observed. Overall, the Pt_0.8Ru_0.2/C electrocatalyst has the potential to be an efficient cathodic catalyst in a H₂-CO₂ fuel cell that reduces CO₂ while generating power.