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Rechargeable aqueous zinc-ion batteries (ZIBs) have resurged in large-scale energy storage applications due to their intrinsic safety, affordability, competitive electrochemical performance, and environmental friendliness. Extensive efforts have been devoted to exploring high-performance cathodes and stable anodes. However, many fundamental issues still hinder the development of aqueous ZIBs. Here, we critically review and assess the energy storage chemistries of aqueous ZIBs for both cathodes and anodes. First, this review presents a comprehensive understanding of the cathode charge storage chemistry, probes the existing deficiencies in mechanism verification, and analyzes contradictions between the experimental results and proposed mechanisms. Then, a detailed summary of the representative cathode materials and corresponding comparative discussion is provided with typical cases encompassing structural features, electrochemical properties, existing drawbacks, and feasible remedies. Subsequently, the fundamental chemical properties, remaining challenges, and improvement strategies of both Zn metal and non-Zn anodes are presented to thoroughly explore the energy storage chemistry of ZIBs and pursue the development of high-performance ZIBs. Furthermore, the progress of mechanistic characterization techniques and theoretical simulation methods used for ZIBs is timely reviewed. Finally, we provide our perspectives, critical analysis, and insights on the remaining challenges and future directions for development of aqueous ZIBs. Graphical Abstract

Rechargeable aqueous zinc‐ion batteries (ZIBs) show promise for use in energy storage. However, the development of ZIBs has been plagued by the limited cathode candidates, which usually show low capacity or poor cycling performance. Here, a reversible Zn//(Na,Mn)V8O20·nH2O system is reported, the introduction of manganese (Mn) ions in NaV8O20 to form (Na,Mn)V8O20 exhibits an outstanding electrochemical performance with a capacity of 377 mA h g−1 at a current density of 0.1 A g−1. Through experimental and theoretical results, it is discovered that the outstanding performance of (Na,Mn)V8O20·nH2O is ascribed to the Mn2+/Mn3+‐induced high electrical conductivity and Na+‐induced fast migration of Zn2+. Other cathode materials derived from (Na,Mn)V8O20·nH2O by substituting Mn with Fe, Co, Ni, Ca, and K are explored to confirm the unique advantages of transition metal ions. With an increase in Mn content in NaV8O20, (Na0.33,Mn0.65)V8O20 ·nH2O can deliver a reversible capacity of 150 mA h g−1 and a capacity retention of 99% after 1000 cycles, which may open new opportunities for the development of high‐performance aqueous ZIBs. Mn‐doped NaV8O20 is synthesized via a one‐step hydrothermal reaction. The Mott–Schottky plots, Tafel curves and calculation results are given to explain the improved cycling performance of Mn‐doped NaV8O20. Other cathode materials derived from (Na,Mn)V8O20·nH2O by substituting Mn with Fe, Co, Ni, Ca, and K are explored to confirm the unique advantages of transition metal ions in NaV8O20.

Hard carbon materials are characterized by having rich resources, simple processing technology, and low cost, and they are promising as one of the anode electrodes for commercial applications of sodium‐/potassium‐ion batteries. Simultaneously, exploring the alkali metal ion storage mechanism is particularly important for designing high‐performance electrode materials. However, the structure of hard carbon is more complex, and the description of energy storage behavior is quite controversial. In this study, the Magnolia grandiflora Lima leaf is used as a precursor, combined with simple pyrolysis and impurity removal processes, to obtain biomass‐derived hard carbon material (carbonized Magnolia grandiflora Lima leaf [CMGL]). When it is used as an anode for sodium‐ion batteries, it exhibits a high specific capacity of 315 mAh/g, and the capacity retention rate is 90.0% after 100 cycles. For potassium‐ion batteries, the charge specific capacity is 263.5 mAh/g, with a capacity retention rate of 85.5% at the same cycling. Furthermore, different electrochemical analysis methods and microstructure characterization techniques were used to further elucidate the sodium/potassium storage mechanism of the material. All the results indicate that the high potential slope region represents the adsorption/desorption characteristics on the surface active sites, whereas the low‐potential quasiplateau region belongs to the ion insertion/extraction in the graphitic microcrystallites interlayer. It is noteworthy that potassium ion is randomly intercalated between the graphitic microcrystallite layer without forming a segmented intercalation compound structure. To make full use of waste biomass resources, the Magnolia grandiflora Lima leaf is used as a precursor to obtain derived hard carbon material, which showed an excellent electrochemical performance as the sodium‐/potassium‐ion batteries anode, and the different electrochemical analysis methods and microstructure characterization techniques also further explain the corresponding energy storage mechanisms.

Biphasic and multiphasic compounds have been well clarified to achieve extraordinary electrochemical properties as advanced energy storage materials. Yet the role of phase boundaries in improving the performance is remained to be illustrated. Herein, we reported the biphasic vanadate, that is, Na1.2V3O8/K2V6O16·1.5H2O (designated as Na0.5K0.5VO), and detected the novel interfacial adsorption–insertion mechanism induced by phase boundaries. First‐principles calculations indicated that large amount of Zn2+ and H+ ions would be absorbed by the phase boundaries and most of them would insert into the host structure, which not only promote the specific capacity, but also effectively reduce diffusion energy barrier toward faster reaction kinetics. Driven by this advanced interfacial adsorption–insertion mechanism, the aqueous Zn/Na0.5K0.5VO is able to perform excellent rate capability as well as long‐term cycling performance. A stable capacity of 267 mA h g−1 after 800 cycles at 5 A g−1 can be achieved. The discovery of this mechanism is beneficial to understand the performance enhancement mechanism of biphasic and multiphasic compounds as well as pave pathway for the strategic design of high‐performance energy storage materials. A novel interfacial adsorption–insertion mechanism is observed in biphasic Na1.2V3O8/K2V6O16·1.5H2O cathode for aqueous Zn‐ion battery. The numerous phase boundaries in biphasic material could absorb H+ and Zn2+ ions and facilitate the subsequent ions insertion process. This advanced mechanism could bring about enhanced capacity as well as faster reaction kinetics, thus leading to brilliant capacity and remarkable long‐term cycling stability.

Sodium-ion batteries (SIBs) have been proposed as a potential substitute for commercial lithium-ion batteries due to their excellent storage performance and cost-effectiveness. However, due to the substantial radius of sodium ions, there is an urgent need to develop anode materials with exemplary electrochemical characteristics, thereby enabling the fabrication of sodium-ion batteries with high energy density and rapid dynamics. Carbon materials are highly valued in the energy-storage field due to their diverse structures, low cost, and high reliability. This review comprehensively summarizes the typical structure; energy-storage mechanisms; and current development status of various carbon-based anode materials for SIBs, such as hard carbon, soft carbon, graphite, graphene, carbon nanotubes (CNTs), and porous carbon materials. This review also provides an overview of the current status and future development of related companies for sodium-ion batteries. Furthermore, it offers a summary and outlook on the challenges and opportunities associated with the design principles and large-scale production of carbon materials with high-energy-density requirements. This review offers an avenue for exploring outstanding improvement strategies for carbon materials, which can provide guidance for future application and research.

Highlights Pourbaix diagram of Mn–Zn–H 2 O system was used to analyze the charge–discharge processes of Zn/MnO 2 batteries. Electrochemical reactions with the participation of various ions inside Zn/MnO 2 batteries were revealed. A detailed explanation of phase evolution inside Zn/MnO 2 batteries was provided. Aqueous rechargeable Zn/MnO 2 zinc-ion batteries (ZIBs) are reviving recently due to their low cost, non-toxicity, and natural abundance. However, their energy storage mechanism remains controversial due to their complicated electrochemical reactions. Meanwhile, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO 2 ZIBs, Mn 2+ is introduced in the electrolyte (e.g., ZnSO 4 solution), which leads to more complicated reactions inside the ZIBs systems. Herein, based on comprehensive analysis methods including electrochemical analysis and Pourbaix diagram, we provide novel insights into the energy storage mechanism of Zn/MnO 2 batteries in the presence of Mn 2+ . A complex series of electrochemical reactions with the co-participation of Zn 2+ , H + , Mn 2+ , SO 4 2− , and OH − were revealed. During the first discharge process, co-insertion of Zn 2+ and H + promotes the transformation of MnO 2 into Zn x MnO 4 , MnOOH, and Mn 2 O 3 , accompanying with increased electrolyte pH and the formation of ZnSO 4 ·3Zn(OH) 2 ·5H 2 O. During the subsequent charge process, Zn x MnO 4 , MnOOH, and Mn 2 O 3 revert to α-MnO 2 with the extraction of Zn 2+ and H + , while ZnSO 4 ·3Zn(OH) 2 ·5H 2 O reacts with Mn 2+ to form ZnMn 3 O 7 ·3H 2 O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn 2+ reversibly insert into/extract from α-MnO 2 , Zn x MnO 4 , and ZnMn 3 O 7 ·3H 2 O hosts; ZnSO 4 ·3Zn(OH) 2 ·5H 2 O, Zn 2 Mn 3 O 8 , and ZnMn 2 O 4 convert mutually with the participation of Mn 2+ . This work is believed to provide theoretical guidance for further research on high-performance ZIBs.

Hybrid supercapacitors (HSCs) comprising a battery‐type cathode and capacitive anode have recently become a research hotspot. Nevertheless, the low capacity utilization, poor kinetic behavior, and unstable structure of a single battery‐type oxide cathode restrict the overall performance of the device. Herein, the carbon quantum dots (CQDs) modified NiO/Co3O4 heterostructured flower‐like microspheres are constructed, and enhanced specific capacity, rate capability, and cycling performance are achieved when used as the cathode for HSCs. This is attributed to the fact that the modification of bifunctional CQDs as size regulators and conductive agents and the construction of heterostructure can not only improve the specific surface area and provide more electroactive sites, thereby enhancing the charge storage performance but also regulate the electronic structure and boost the interface charge transfer capability and electronic conductivity, thereby boosting the reaction kinetics and cycle stability. The enhanced electrochemical kinetic behavior is revealed by electrochemical kinetic analyses based on cyclic voltammetry, electrochemical impedance spectroscopy tests and density functional theory calculations. Meanwhile, the electrochemical reaction process and energy storage mechanism are illustrated by ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopy characterizations. Furthermore, an HSC is further constructed using the CQDs/NiO/Co3O4 heterostructured flower‐like microspheres as the cathode, simultaneously achieving high energy density (40.9 Wh kg−1), high power density (24 kW kg−1), and splendid cyclic stability (94.2% capacity retention after 5000 cycles at 10 A g−1). These synergistic modification strategies of bifunctional CQDs modification and heterostructure design provide a valuable direction for the design and development of HSCs with both high energy density and high power density. Carbon quantum dots (CQDs)/NiO/Co3O4 heterostructured flower‐like microsphere composite is constructed using bifunctional carbon quantum dots (CQDs) modification as size regulators and conductive agents and heterostructure design, achieving significantly enhanced rate property and cycling stability. The obtained CQDs/NiO/Co3O4 cathode matches with the high‐rate anode to assemble the hybrid supercapacitor, simultaneously delivering high energy density (40.9 Wh kg−1) and high power density (24 kW kg−1).

Currently, energy storage systems are of great importance in daily life due to our dependence on portable electronic devices and hybrid electric vehicles. Among these energy storage systems, hybrid supercapacitor devices, constructed from a battery-type positive electrode and a capacitor-type negative electrode, have attracted widespread interest due to their potential applications. In general, they have a high energy density, a long cycling life, high safety, and environmental friendliness. This review first addresses the recent developments in state-of-the-art electrode materials, the structural design of electrodes, and the optimization of electrode performance. Then we summarize the possible classification of hybrid supercapacitor devices, and their potential applications. Finally, the fundamental theoretical aspects, charge-storage mechanism, and future developing trends are discussed. This review is intended to provide future research directions for the next generation of high-performance energy storage devices.

Aqueous supercapacitors are a class of crucial high-power, long-life, safe and reliable energy storage devices, with their performance fundamentally dependent on electrode materials. Two-dimensional (2D) vanadium-based MXenes, possessing rich multivalent redox activity and tunable layered structures, have emerged as one of highly promising electrode candidates, exhibiting significantly superior specific capacitance and pseudocapacitive properties compared to conventional Ti3C2Tz. To overcome inherent limitations in conductivity and structural stability, this review summarizes strategies for regulating composition and microstructure through transition metal solid solution and medium-/high-entropy design. These approaches synergistically optimize electron conduction, expand ion migration pathways, and suppress electrode degradation, thereby comprehensively enhancing rate performance, cycle life, and energy density. This review systematically reveals the composition–structure–performance relationships, providing critical design insights and theoretical foundations for developing next-generation high-performance, long-life aqueous MXene-based supercapacitors.

Aqueous zinc‐ion batteries have been regarded as the most potential candidate to substitute lithium‐ion batteries. However, many serious challenges such as suppressing zinc dendrite growth and undesirable reactions, and achieving fully accepted mechanism also have not been solved. Herein, the commensal composite microspheres with α‐MnO2 nano‐wires and carbon nanotubes were achieved and could effectively suppress  ZnSO4·3Zn(OH)2·nH2O rampant crystallization. The electrode assembled with the microspheres delivered a high initial capacity at a current density of 0.05 A g−1 and maintained a significantly prominent capacity retention of 88% over 2500 cycles. Furthermore, a novel energy‐storage mechanism, in which multivalent manganese oxides play a synergistic effect, was comprehensively investigated by the quantitative and qualitative analysis for ZnSO4·3Zn(OH)2·nH2O. The capacity contribution of multivalent manganese oxides and the crystal structure dissection in the transformed processes were completely identified. Therefore, our research could provide a novel strategy for designing improved electrode structure and a comprehensive understanding of the energy storage mechanism of α‐MnO2 cathodes. Under the combined effect of surface catalysis and confined space for Carbon nanotube microsphere, α‐MnO2 nano‐wires are rapidly crystallized to form composite microspheres. These microspheres demonstrate excellent inhibitory ability for ZnSO4·3Zn(OH)2·nH2O. Moreover, a novel energy storage mechanism involving the multivalent manganese oxides (α‐MnO2, Mn3O4, and α‐MnO2·H2O) is demonstrated; meanwhile, the capacity contribution of each component is accurately determined.