Hajime Arai

While Bi ₂ O ₃ -modified MnO ₂ electrodes in alkaline electrolytes are promising for large scale applications with 2-electron discharge-charge processes, the discharge potential decrease with cycling needs to be clarified, together with the role of the bismuth redox mediator. In this study, we employed reaction analysis of Bi ₂ O ₃ -modified birnessite electrodes using operando X-ray diffraction and X-ray absorption fine structure measurements to detail the behavior of the reaction products and intermediates, in comparison with the behavior of the birnessite electrode without modification. The discharge potential decrease in the Bi ₂ O ₃ -modified birnessite from the 1st to 9th discharge (still higher than that of the non-modified sample) was correlated with the evolution of the interlayer spacing of the birnessite host as well as the formation of the Mn ³⁺ intermediate and the final Mn(OH) ₂ . The results suggest that modification of a birnessite-structured MnOOH intermediate phase ( δ -MnOOH) leads to the low redox potential of the bismuth mediator, probably owing to the local pH increase at the electrode surface, indicating the importance of controlling the local conditions of the redox mediator for better electrode potential stability.

The use of solid electrolytes (SEs) allows for the construction of solid‐state cells with bilayer‐to‐multilayer architectures. By appropriate choice of electrolytes for the positive‐ and negative‐electrode sides, electrolyte degradation can be mitigated, thus considerably increasing the design flexibility. However, certain electrolyte combinations lead to nonideal chemical compatibility, increasing the heteroelectrolyte interfacial resistance. Precise measurement of the interfacial resistance is necessary for quantitatively evaluating the compatibility of solid electrolytes. In this work, we employed a solid‐state four‐electrode cell to investigate the compatibility between three chloride‐based SEs—(Li ₃ InCl ₆ (LIC), Li ₃ ScCl ₆ (LSC), and Li ₃ YCl ₆ (LYC))—and the sulfide‐based Li ₇ P ₂ S ₈ I (LPSI). Electrochemical impedance spectroscopy (EIS) analysis reveals that the growth rate of the interfacial resistance is the highest in LIC, but the activation energies of the interfacial resistances are identical in all three SEs. Temperature‐dependent synchrotron X‐ray diffraction and density functional theory calculations strongly suggest that Li ₄ PS ₄ I, whose activation energy is similar to those measured by EIS, is the common interfacial product among the three cells, and that its relative degree of formation is consistent with the difference in interfacial resistance growth. This study demonstrates that electrochemical measurements using solid‐state four‐electrode cells constitute a promising technique for precisely investigating unstable heteroelectrolyte interfaces.

All-solid-state batteries are promising as safe and high energy density power sources but often suffer from short cycle life due to insufficient solid|solid contact at the electrode|electrolyte interface. It has been reported that the conventional polycrystalline (PC) positive electrode materials need coating to suppress the interfacial reaction with sulfide-based solid electrolytes, while single-crystal-like (SC) materials can be cycled without coating. In this study, the behavior of uncoated SC and PC NCM523 electrodes was comparatively examined in sulfide-based all-solid-state cells using three-electrode cells to resolve the interfacial resistive components and relate them to cycling performance. By employing a chemically reduced Li4Ti5O12 reference electrode, four resistive components were separated, allowing the charge-transfer resistance and the interparticle contact-related resistance to be evaluated independently. The SC cell showed high cycling stability, whereas the capacity of the PC cell once decreased and partially recovered afterward, which was confirmed to be reproducible. With detailed impedance analysis, the capacity loss of the PC cell was chiefly assigned to an increase in the charge-transfer resistance, and the partial recovery was correlated with a contact-related resistance among the NCM523 particles. Both resistive components were kept low in the SC cell, leading to the higher cycling stability of the SC cell.

In recent years, the rapidly growing demand for lithium-ion batteries (LIBs) has intensified concerns over resource risks associated with the critical elements required for these systems. Consequently, the development of practical alternatives to LIBs has become increasingly important. In our research, we focus on battery chemistries employing highly concentrated aqueous electrolytes. Among these, highly concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) aqueous electrolytes—known as water-in-salt electrolytes (WISEs)—offer several advantages, including an expanded electrochemical stability window (ESW) and compatibility with cost-effective aluminum current collectors, while maintaining the intrinsic safety, low cost, and carbon-neutrality of batteries using conventional aqueous electrolytes.¹^,² Since the first report on WISEs, research into highly concentrated aqueous electrolytes has advanced rapidly. However, WISEs, particularly those based on amine-type salts, are prone to salt crystallization with even modest decreases in temperature, which severely impedes ion transport and degrades battery performance.

Morphological engineering of platinum-based electrocatalysts has been widely explored to enhance anodic alcohol oxidation in direct alcohol fuel cells. Recently, three-dimensional structures incorporating micro- and mesopores have been developed as promising anode electrocatalyst structures for direct alcohol fuel cells. However, the influence of their pore size on the catalytic activity and stability remains unclear. In this study, platinum model electrodes with cylindrical pores of 1.3, 1.8, and 3.0 nm were used to systematically evaluate the influence of platinum micro- and mesopore size on the electrooxidation of ethylene glycol, methanol, and ethanol in 1.0 mol dm ^–3 KOH aqueous solution. The oxidation kinetics were evaluated using voltammograms, Tafel plots, and chronoamperograms with current densities normalized to electrochemical surface area. The observed relationship between the platinum pore size and alcohol oxidation activity as well as durability is discussed based on the d-band center downshift associated with a decrease in pore size, providing fundamental insights into catalyst nanostructure–reaction performance relationships.

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The degradation behavior of bifunctional air electrodes featuring with a bilayer design with an oxygen reduction reaction (ORR) catalytic layer and an oxygen evolution reaction (OER) catalytic layer was investigated using electrochemical impedance spectroscopy (EIS). The impedance spectra were analyzed using an equivalent circuit model incorporating two finite diffusion terms: resistance to ionic diffusion and resistance to oxygen diffusion. The bilayer electrode demonstrated excellent stability for over 100 hours at ambient temperature, even when subjected to severe anodic treatment at 200 mA cm−2. However, analysis of the EIS signals revealed that oxygen diffusion progressively emerged as the limiting factor as the testing time was prolonged. The degradation analysis of the bilayer electrodes yields critical insights and actionable recommendations for subsequent electrode optimization and design improvements.

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Alkaline batteries using zinc electrodes are attractive to be used in large-scale energy storage systems owing to their low cost, high safety standards and high energy density. On the other hand, the zinc electrodes suffer from shape change to lead to capacity decrease. Such shape change has been clarified by the combination of operand x-ray diffraction and operando optical microscopy to find that inhomogeneous current distribution promotes zinc deposition at the edge and the resultant loss of zinc oxide [1]. This study shows whether the reported rechargeability enhancement by the hydrophilic-polymer additives [2] can be ascribed to the mitigation of the reaction inhomogeneity. In this study, we used butendiol vinyl alcohol co-polymer (BVOH) as the additive to enhance the turnover number of zinc (total cycled capacity divided by the installed zinc capacity). The operando x-ray diffraction revealed that ZnO was mostly lost after the 2 ^nd cycle at the edge of the electrode without BVOH, while broad diffraction of ZnO was captured with BVOH. The operando optical analysis showed that course uneven Zn deposition was formed at the vicinity of the current collectors without BVOH, whereas fine homogeneous Zn deposition was observed with BVOH, suggesting ZnO fixation during the oxidation process. It is deduced that the higher the BVOH concentration is, the more viscose the electrolyte becomes to inhibit zincate diffusion, to enhance ZnO fixation, and to improve the rechargeability. [Acknowledgment] This work is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). [References] [1] A. Ikezawa et al., J. Power Sources , 507 , 230291 (2021). [2] T. Kakeya et al., J. Power Sources , 407 , 180 (2018). Figure 1