The cover art highlights the 73rd Special Feature of Electrochemistry entitled “Progress in Aqueous-Based Batteries,” introduced by Professor Hajime Arai et al., who served as chairpersons of the 4th International Zinc and other Aqueous Batteries Workshop (IZABW4), held in Kyoto, Japan, in September 2025.

The cover illustration symbolically represents aqueous battery technologies through cylindrical cell-like structures surrounded by an aqueous medium and O₂ molecules. It also incorporates structural motifs relevant to aqueous battery reactions, including layered MnO₂-based electrode materials and hexagonal-prism-like zinc-based reaction products. Together, these elements represent electrode reactions, electrolyte chemistry, and oxygen-electrode processes in zinc-based and other aqueous battery systems.

This Special Feature, organized in connection with the international workshop, brings together research achievements in aqueous batteries from fundamentals to applications, including electrode materials and catalysts, electrolytes, cell systems, analytical methods, and modeling. This cover art serves as the opening image for this collection of papers, beginning with the Editorial.

The cover art highlights the article by Yimin Lin et al., published as one of the papers in the 73rd Special Feature and selected as an Editor’s Choice article “Understanding Separator Properties Governing Zincate Crossover in Rechargeable Alkaline Zn–MnO₂ Batteries”

This study clarifies how separator properties govern zincate crossover in rechargeable alkaline Zn–MnO₂ batteries. By comparing six commercial separators, the authors showed that the anion-exchange membrane FAAM-75-PK effectively suppresses zincate diffusion while maintaining hydroxide ion transport, leading to improved cycling performance. The cover image schematically represents two alkaline Zn–MnO₂ battery systems with different separator functions. The transparent cells visualize ion transport, zincate crossover, and the role of the separator in controlling the chemical environment near the MnO₂ electrode, emphasizing the importance of separator design for durable rechargeable alkaline batteries.

The cover art highlights the article by Professor Masayuki Morita et al., published as one of the papers in the 73rd Special Feature “Progress in Aqueous-Based Batteries” and selected as an Editor’s Choice article “Research and Development of Zinc-based Rechargeable Batteries in RISING3”.

This article summarizes research and development on safe, resource-risk-free zinc-based rechargeable batteries conducted under the RISING, RISING2, and RISING3 national projects. Building on previous achievements in alkaline zinc–air systems, the study focuses on alkaline Zn–MnO₂ batteries using manganese dioxide as the positive electrode material. The authors demonstrate that the rechargeability and capacity of electrolytic manganese dioxide are strongly related to its structural water and microstructure. They further show that permanganate-derived manganese dioxide enables a reversible two-electron reaction even in alkaline electrolytes containing zinc species, providing an important approach toward higher-energy sealed zinc-anode rechargeable batteries. The cover image schematically represents the aqueous alkaline Zn–MnO₂ battery concept, in which zinc-based negative electrode reactions, MnO₂ redox processes, water-mediated proton transfer, and structural changes of manganese oxide are visualized across the cell. The bright ion-transport pathways and contrasting oxide domains emphasize the dynamic interfacial reactions underlying rechargeable zinc battery performance.

This cover art was created and published with financial support from the Committee of Battery Technology of the Electrochemical Society of Japan and JSPS KAKENHI Grant Number 25HP2006 under the Grant-in-Aid for Publication of Scientific Research Results.

The cover art is associated with the article entitled “Operando Monitoring of Rechargeable Zinc-Air Batteries with Acoustic Emission” by Valentin Rueß et al., which has been selected as an Editor’s Choice for the 73rd Special Feature, “Progress in Aqueous-Based Batteries,” jointly recommended by the guest editors from the Committee of Battery Technology and the editorial board.

This article demonstrates the applicability of acoustic emission (AE) analysis to rechargeable zinc–oxygen batteries as a non-invasive operando diagnostic method. By monitoring sound waves generated during battery operation (i.e., non-audible for the human ear), the study shows that AE can provide real-time insight. The study focuses on degradation-related phenomena and mechanically induced events in zinc–oxygen batteries. The cover visualizes this concept by depicting a cutaway zinc–oxygen button cell together with a semi-transparent ear, symbolizing the idea of “listening” to electrochemical and mechanical processes inside the battery. The internal crack-like features represent degradation and structural failure, while the surrounding wave patterns express the detection of acoustic signals during operation. The red diatomic particles represent oxygen molecules involved in the cathode, i.e. often referred to as air electrode. Together, these elements convey the central message of the article: that degradation processes in zinc–oxygen batteries can be monitored operando through acoustically detectable phenomena.

This cover art was created and published with financial support from the Committee of Battery Technology of the Electrochemical Society of Japan and JSPS KAKENHI Grant Number 25HP2006 under the Grant-in-Aid for Publication of Scientific Research Results.

The cover art highlights the article by Ryohei Tasaki et al., published in Electrochemistry and selected as an Editor’s Choice article, “Influence of CO₃^2– and C₂^2– on the Oxygen Evolution Performance of Perovskite La_0.7Sr_0.3FeO_3–δ Anode in Molten NaCl–CaCl₂”.

This study addresses the anodic reactions required for electrochemical conversion of CO₂ into calcium carbide in molten salts, a promising carbon capture and utilization route toward acetylene production. Using La_0.7Sr_0.3FeO_3–δ as a perovskite-type oxygen evolution anode, the authors clarify how the local anionic environment controls both oxygen evolution performance and electrode durability. In molten NaCl–CaCl₂ containing O^2– and CO₃^2–, selective oxygen evolution is achieved with high Faradaic efficiency and low corrosion. In contrast, dissolved C₂^2– is oxidized at lower potentials to form amorphous carbon, which accelerates anode degradation and suppresses oxygen evolution.

The cover image schematically represents this contrast. On the left, the perovskite oxide anode promotes oxygen evolution in the molten salt, supporting CO₂-derived calcium carbide production. On the right, carbide-related species and carbonaceous deposits illustrate the degradation pathway caused by C₂^2– crossover. The industrial background emphasizes the significance of stable molten-salt electrolysis for sustainable chemical production.

This special issue of Electrochemistry features content based on recent advances in aqueous battery research, chiefly consisting of papers presented in the 4th International Zinc and other Aqueous Batteries Workshop (IZABW4), held in September 2025 at Uji Campus of Kyoto University, Japan. There were 116 attendees from 7 countries with 26 oral and 36 poster presentations at IZABW4, sharing knowledge and promoting discussion on zinc, MnO₂ and air (oxygen) electrodes, electrolytes as well as cell/system fabrication. The local organizing committee members hope that this collection stimulates further innovation in the aqueous battery research.

Rechargeable zinc-air batteries (r-ZABs) are excellent candidates for next-generation secondary batteries due to their high energy densities and safety; however, they have issues of electrolyte volatilization, leakage, and side reactions at the air electrode caused by zincate ([Zn(OH)₄]²⁻) diffusion. A gel polymer electrolyte (GPE) consisting of a polystyrene-based densely crosslinked network structure bearing quaternary ammonium anion exchange groups at the crosslink points was developed. The GPE showed higher monovalent anion conductivity and lower divalent anion ([Zn(OH)₄]²⁻) diffusion coefficient than a conventional anion exchange membrane and a porous separator due to the network structure with high ion exchange capacity (IEC) of the novel GPE. The charge-discharge r-ZAB test using the GPE demonstrated higher cycle stability than a liquid electrolyte due to enhanced ion conductivity and suppressed diffusion of [Zn(OH)₄]²⁻, as well as sufficient mechanical toughness of the densely crosslinked network structure.

The polarity reversal operation of a symmetric battery —a strategy uniquely enabled by identical electrodes— using NASICON-type Na_2.5V_1.5Ti_0.5(PO₄)₃ electrodes in a highly concentrated aqueous sodium trifluoroacetate electrolyte was examined to gain insight into the inherent asymmetric properties that develop during unidirectional charge-discharge cycling. The asymmetrically grown electrode-electrolyte interfaces (EEIs) at both electrodes exerted persistent effects on subsequent reversely directed charge-discharge cycles following electrode switching, whereas the electrolyte pH split responded rapidly. A certain number of repeated charge-discharge cycles in the same direction was required to form a robust EEI tolerant to drastic pH changes. Polarity reversal operation combined with occasional electrode switching may promote quasi-symmetric EEI formation on both electrodes, which could be beneficial for improving cyclability and coulombic efficiency.

Measuring the local potential distributions in oxygen electrodes, such as bifunctional air electrodes for metal-air secondary batteries and cathodes for fuel cells, is crucial for understanding reaction processes and for developing electrode design guidelines. In this study, we explore indicators for optically visualizing the local potential in alkaline oxygen electrodes. Ni, Ag, and Cu are selected as potential indicators, and their brightness changes depending on applied potentials were measured by using a confocal optical microscope in aqueous solutions of 1.0 mol dm⁻³ and 8.0 mol dm⁻³ KOH. The brightness of Ni, Ag, and Cu foil electrodes (thickness: 30 µm) decreases/increases with oxidation/reduction at around the oxygen evolution reaction potential (1.22–1.42 V vs. RHE), the equilibrium potential (1.12–1.22 V vs. RHE), and the oxygen reduction reaction potential (0.62–0.77 V vs. RHE) regions, respectively, demonstrating that these materials can function as potential indicators for alkaline oxygen electrodes. Furthermore, Ni and Ag microwire electrodes (diameter: 50 µm), which are supposed to have less effect on mass transport than the metal foil electrodes, also show brightness changes similar to those of the metal foils in 8.0 mol dm⁻³ KOH.

Suppressing zincate crossover is necessary for rechargeable alkaline Zn-MnO₂ rechargeable batteries using electrolytic manganese dioxide (EMD). We benchmarked six commercial separators for their ability to block zincate ions while maintaining the transport of hydroxide ions. Among the separators, FAAM-75-PK (FP75), a commercial anion-exchange membrane, exhibited moderate conductivity (5.8 mS cm⁻¹) with the lowest diffusion coefficient of zincate ions (D_Zn = 2.0 × 10⁻⁸ cm² s⁻¹) and a moderate diffusion coefficient of hydroxide ions (D_OH = 3.5 × 10⁻⁵ cm² s⁻¹), leading to the highest permselectivity (D_OH/D_Zn = 1.7 × 10³). In the galvanostatic discharge–charge tests, FP75 exhibited the best capacity retention of 68 % at the 7th cycle. Characterization of the discharged EMD electrodes revealed that FP75 effectively suppressed ZnMn₂O₄ formation at the cathode, favoring Mn₃O₄ formation instead. The low Zn concentration of 6 ppm in the catholyte is consistent with the favored Mn₃O₄ formation. Beaker-cell tests identify a system-dependent zinc concentration threshold. When the Zn/Mn molar ratio exceeds ∼0.05–0.15, product formation shifts toward ZnMn₂O₄ from Mn₃O₄, and thus, the capacity decay accelerates.

Quinone-based molecules provide promising electrode materials for energy-storage devices; however, their redox reaction kinetics are typically slow because the process is governed by mass transfer. The micropore confinement of quinone-based molecules exhibits anomalous pseudocapacitive behavior owing to the characteristic adsorption potentials of micropores. In addition, a highly crystalline carbon surface enhances redox reversibility through strong π-π interactions. This study investigated the effects of surface crystallinity on redox reversibility utilizing a carbon mesosponge comprising single-layer graphene with a crystalline carbon surface. The carbon mesosponge improved the redox reaction despite its mesoporous nature. This result can be attributed to the large amount of adsorbed molecules on the micropore surface and the incorporation of single-layer graphene within the framework.

It is important to develop a rechargeable battery that is safe and free from resource risk to replace the current lithium–ion battery in various application fields. We have been conducting research and development of aqueous electrolyte batteries with zinc as the negative electrode (anode) through national programs, Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING, RISING2 and RISING3), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). In RISING2, we demonstrated a full cell that can achieve a target specific energy of 500 Wh kg⁻¹ for a zinc–air system through advancements in elemental technology and full-cell engineering. RISING3 aims to develop a sealed zinc-anode rechargeable battery by developing positive electrode (cathode) materials to replace air electrodes. A notable achievement of this study is proving that the manganese dioxide (MnO₂) prepared through electrolysis (EMD) has good rechargeability in alkaline electrolyte systems, in which the amount of structural water in the oxide determines the discharge capacity for the first one-electron reduction. We also found that the newly developed MnO₂ from the chemical reduction of permanganate (PMD) enables a reversible two-electron reaction even in the electrolyte containing zinc species, in which hetaerolite (ZnMn₂O₄) generally forms to inhibit further electrochemical reactions.

This study demonstrates that the acoustic emission (AE) methodology can be successfully applied to zinc-air battery systems. AE analysis provides real-time insights into mechanically induced events and enables qualitative correlation with electrochemical processes during different operating stages. The method shows high sensitivity to corrosion-driven hydrogen evolution under open-circuit conditions and to oxygen-evolution-related activity in catalyst-based gas diffusion electrodes during charging. In commercial cells, a pronounced increase in AE activity once the cell voltage exceeds 1.8 V indicates the onset of carbon-related degradation, while final AE spikes prior to failure reflect the mechanical manifestation of critical degradation events such as separator rupture or electrode collapse. Overall, the results highlight AE as a powerful diagnostic tool for identifying degradation phenomena in metal–air batteries. Consideration of the observed diffusion limitations and the future implementation of advanced analysis tools, including distribution of relaxation times, equivalent circuit models, and machine-learning approaches, will further refine the understanding of process contributions in next-generation rechargeable battery systems.

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.

Noncarbon-based air electrodes for metal–air batteries were investigated to enhance bifunctional activity for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as well as to improve durability during repeated OER–ORR cycling in alkaline aqueous electrolytes. A bilayer structure composed of a gas diffusion layer (GDL) and catalyst layer (CL) was constructed using antimony-doped tin oxide (ATO) as a substitute for carbon typically employed in conventional air electrodes. Optimizing the polytetrafluoroethylene content within the ATO-based GDL maximized air permeance and enhanced ORR performance while preserving OER activity. An air electrode incorporating an ATO-based GDL and a CL composed of a mixture of La_0.6Ca_0.4CoO₃ and ATO exhibited reversibility for both the ORR and OER. This noncarbon-based air electrode demonstrated superior stability under high anodic potentials during the OER compared with carbon-based electrodes, without inducing significant morphological or phase transformations. These findings demonstrate the noncarbon-based air electrode as a viable cathode candidate for secondary metal–air batteries, which require high reversibility and resistance to corrosion under high operating potentials.

Zinc secondary batteries is a promising candidate for large-scale energy storage facilities owing to their high specific energy capacity, safe aqueous electrolytes, abundant zinc resources, environmental sustainability, and cost-effectiveness. However, the application of zinc as a negative electrode in alkaline electrolytes has been hindered by shape change and dendrite growth during extended charge/discharge cycling. These dendrite growth issues significantly degrade the cycle characteristics and directly result in shortened battery lifetimes. This study addresses these challenges by demonstrating the effect of a small quantity of additive, 3,3-diaminopropylamine (Bis3), within a highly concentrated KOH aqueous electrolyte on dendrite formation during the cycling of zinc secondary batteries. The results indicate that Bis3 addition to a concentrated KOH electrolyte effectively inhibits the formation of zinc dendrites.

High-capacity cathode materials must be developed to improve the energy densities of Ni–Zn batteries. Ni-containing layered double hydroxide (LDH), with a similar structure to α-Ni(OH)₂, represents a promising candidate for developing such cathode materials. However, the detailed reaction mechanisms of this LDH system during overcharging until the Ni^3.5+ valence state and during discharging remain unknown. Here, the structural changes of β-Ni(OH)₂ and Ni–Fe LDH in their overcharged as well as subsequent discharged states were investigated by ex-situ XRD and XAFS analyses.

In the β-Ni(OH)₂ system, γ-NiOOH, which was formed during overcharging, re-transited into β-Ni(OH)₂ during discharging, with a drastic loss of its crystallinity. On the other hand, these drastic structural changes were not observed in the Ni–Fe LDH system even after repeated overcharge–discharge cycles, and its crystal structure was maintained during cycling. Moreover, the ex-situ XAFS measurements revealed that the valence number of Fe³⁺ in LDH did not change during the overcharge–discharge process. Conversely, the Fe K-edge extended XAFS spectrum indicated that only the Fe³⁺ sites were significantly distorted in the overcharged state. These results indicate that the redox-inactive Fe³⁺ sites absorbed the strain caused by Ni²⁺ oxidation up to 3.5 valences and maintained the structural stability of the LDH system.

Silicon oxide–based anode materials have attracted increasing attention for next-generation lithium-ion batteries because of their high theoretical capacity and improved cycling stability compared with silicon anodes. In addition, the use of low-cost and environmentally benign biomass resources as precursors for silicon oxide anodes has emerged as an attractive strategy. In this study, SiO₂/C nanocomposites were systematically investigated as anode materials using both a model system composed of commercially available SiO₂ and lignin and biomass-derived materials prepared from rice husks. The model SiO₂/C composites exhibited reversible capacities that increased with increasing SiO₂ content, while sodium-ion battery measurements revealed that the reversible capacity observed in lithium-ion cells predominantly originated from the electrochemical activity of SiO₂. Si K-edge X-ray absorption near-edge structure (XANES) analysis revealed that the extent of spectral changes for the rice-husk-derived SiO₂/C was smaller than that for the model SiO₂(50)/C. This result indicates a lower electrochemical activity of SiO₂ in the biomass-derived composite, which is responsible for the reduced reversible capacity. Nevertheless, rice husk-derived SiO₂/C delivered a reversible capacity exceeding 400 mAh g⁻¹ with good cycling stability, demonstrating that rice husks are a promising precursor for the preparation of SiO₂/C nanocomposites.

TiO₂/Pt/Au multilayer nanorods were fabricated by template-assisted electrodeposition and thermal conversion of an electrodeposited TiO₂ precursor. Layer thicknesses were tuned by charge and deposition time to yield reproducible multilayers. The nanorods autonomously propelled in 2 wt% H₂O₂ via self-electrophoresis, showing higher velocity under ultraviolet irradiation than in the dark. Reversible ON/OFF velocity modulation was observed. The enhanced propulsion is consistent with photocatalytic reactions at the TiO₂ segment that are suggested to increase the self-electrophoretic driving force, although direct phase identification of TiO₂ and full mechanistic assignment await further characterization.

The surface of magnesium (Mg) electrode was been modified by immersion pre-treatment of Mg in aqueous solution containing nano-sized inorganic particles, silica or layered double hydroxide (LDH), and a polymer for improving electrochemical phosphate ion removal from synthetic wastewater. Among the tested inorganics, polymers, and compositions, immersion pre-treatment in a solution containing nano-sized silica and poly(ethylene glycol) (PEG) (silica content of ca. 0.4 %) provided the highest electricity, the highest removal of phosphate ion, and the quickest formation of precipitate. Highly dispersed silica particles could enhance mass transfer at the interface region and increase phosphate ion conversion into magnesium ammonium phosphates (MAPs) through the enhancement of electrochemical reactions.

All-solid-state lithium-sulfur batteries (ASSLSBs) are promising next-generation high-energy-density storage systems. However, their practical application is hindered by sulfur’s insulating nature and the difficulty of establishing continuous ion-conducting pathways at solid-solid interfaces. In this study, we maximize the performance of rubber-derived sulfur composite cathodes, in which sulfur is effectively confined within a polymer network, by integrating them with a polyether-based solid polymer electrolyte (SPE) using a polymer-electrolyte impregnation technique. This method establishes a robust three-dimensional lithium-ion conducting network within the electrode and forms an adhesive SPE layer on the cathode surface, thereby ensuring intimate interfacial contact. The reduction of cathode particle size (D₅₀ < 1.5 µm) was found to be a governing factor for enhancing charge-discharge performance by increasing the effective contact area with the SPE. The optimized cell exhibited high discharge capacities of 600 mAh g⁻¹ at 60 °C, even with a high areal capacity of ca. 2 mAh cm⁻². Furthermore, a five-layer bipolar cell achieved 180 mAh with an average discharge voltage of 8.5 V. Notably, it showed a specific capacity of 800 mAh g⁻¹ (normalized by single-layer active mass), thereby overcoming the low-voltage limitation of Li-S systems. This work provides key design principles for high-energy-density polymer-based ASSLSBs.

Hydrogen production via alkaline water electrolysis (AWE) is a key approach for achieving large-scale, carbon-neutral hydrogen generation. However, its long-term durability is challenged by reverse-current events that occur under intermittent renewable power conditions. These events lead to repeated oxidation and reduction cycling of Ni-based anodes, which accelerates structural degradation, particularly in spinel-type NiCo₂O₄ catalysts that offer high oxygen evolution reaction (OER) activity but limited stability against Co leaching. In this study, we investigate lithium incorporation as a redox and interfacial buffering strategy to improve the electrochemical durability of NiCo₂O₄-based anodes subjected to repeated redox transitions under intermittent power. Two types of Li-modified catalysts, a (Li)NiO + NiCo₂O₄ composite and Li-doped NiCo₂O₄, were synthesized to examine whether the durability enhancement depends on a specific crystalline phase. Although the Li-containing catalysts exhibited distinct bulk structures, both showed significant improvements in reverse-current tolerance while maintaining OER Tafel slopes comparable to pristine NiCo₂O₄, indicating that the overall reaction mechanism was preserved. Enhanced durability is likely associated with Li-induced structural and electrochemical changes. Oxygen diffusion toward the substrate during Li migration may promote the formation of a Co-free NiO_x-rich layer, which in turn enhances adhesion and stabilizes the coating-substrate interface. In addition, Li contributed to the control of the active redox peak from Ni to appear over a longer ADT cycle. Therefore, lithium appears to regulate surface redox behavior and strengthen interfacial stability during repeated cycling. These results demonstrate that lithium incorporation provides an effective strategy to improve the operational reliability of NiCo₂O₄-based AWE anodes under fluctuating power conditions.

Electrochemical conversion of CO₂ into calcium carbide in molten salts has emerged as a promising route for carbon capture and utilization, enabling the production of acetylene, an important industrial chemical. However, continuous operation requires a stable oxygen evolution reaction (OER) at the anode while suppressing competing oxidation of carbonate and carbide species that regenerate CO₂. The anodic reaction mechanism in carbide-containing molten salts remains poorly understood. Here, the anodic behavior and durability of the perovskite oxide La_0.7Sr_0.3FeO_3−δ are investigated in molten NaCl–CaCl₂ at 873 K under controlled anionic environments containing O²⁻, CO₃²⁻, and C₂²⁻. Thermodynamic analysis using potential–pO²⁻ diagrams predicts that carbide oxidation occurs at the most negative potential, followed by carbon, oxide, and carbonate oxidation, which agrees with electrochemical measurements and gas analysis. When O²⁻ and CO₃²⁻ coexist, selective oxygen evolution is achieved with a Faradaic efficiency of 81.4 % and a low corrosion rate of 1.94 × 10⁻⁵ g cm⁻² h⁻¹. In contrast, dissolved C₂²⁻ undergoes anodic oxidation at lower potentials, producing amorphous carbon that accelerates electrode degradation and reduces OER efficiency. These results demonstrate that anodic stability is governed by the local anionic environment, highlighting the importance of maintaining O²⁻ and CO₃²⁻ coexistence while suppressing carbide transport to the anode.