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As a new type of secondary ion battery, aqueous zinc‐ion battery has a broad application prospect in the field of large‐scale energy storage due to its characteristics of low cost, high safety, environmental friendliness, and high‐power density. In recent years, manganese dioxide (MnO2)‐based materials have been extensively explored as cathodes for Zn‐ion batteries. Based on the research experiences of our group in the field of aqueous zinc ion batteries and combining with the latest literature of system, we systematically summarize the research progress of Zn−MnO2 batteries. This article first reviews the current research progress and reaction mechanism of Zn−MnO2 batteries, and then respectively expounds the optimization of MnO2 cathode, Zn anodes, and diverse electrolytes and their effects on battery performance. Additionally, primary challenges related to different components and their respective strategies for mitigating them are discussed, with the ultimate objective of offering comprehensive guidance for the design and fabrication of high‐performance Zn−MnO2 batteries. Finally, the future research and development direction of aqueous Zn−MnO2 batteries with high energy density, high safety and long life is envisioned. In recent years, Zn−MnO2 batteries have attracted more and more attention. This review not only summarizes the battery mechanism under different pH, but also discusses the main challenges encountered and latest developments in anode and cathode materials and various electrolyte materials (liquid, solid and gel), which are crucial for enabling the design of high‐performance batteries. In the end, prospects of the sustainable development of Zn−MnO2 batteries are summarized.

Zinc (Zn)-based batteries have attracted significant interest for applications ranging from electric bikes to grid storage because of its advantageous properties like high abundance, non-toxicity and low-cost. Zn offers a high theoretical capacity of two electrons per atom, resulting in 820 mAh/g, making it a promising anode material for the development of highly energy dense batteries. However, the advancement of Zn-based battery systems is hindered by the limited availability of cathode materials that simultaneously offer high theoretical capacity, long-term cycling stability, and affordability. In this work, we present a new mixed material cathode system, comprising of a mixture of manganese dioxide (MnO2) and copper oxide (Cu2O) as active materials, that delivers a high theoretical capacity of ~280 mAh/g (MnO2 + Cu2O active material) (based on the combined mass of MnO2 and Cu2O) and supports stable cycling for >200 cycles at 1C. We further demonstrate the scalability of this novel cathode system by increasing the electrode size and capacity, highlighting its potential for practical and commercial applications.

Rechargeable zinc–manganese dioxide batteries that use mild aqueous electrolytes are attracting extensive attention due to high energy density and environmental friendliness. Unfortunately, manganese dioxide suffers from substantial phase changes (e.g., from initial α-, β-, or γ-phase to a layered structure and subsequent structural collapse) during cycling, leading to very poor stability at high charge/discharge depth. Herein, cyclability is improved by the design of a polyaniline-intercalated layered manganese dioxide, in which the polymer-strengthened layered structure and nanoscale size of manganese dioxide serves to eliminate phase changes and facilitate charge storage. Accordingly, an unprecedented stability of 200 cycles with at a high capacity of 280 mA h g −1 (i.e., 90% utilization of the theoretical capacity of manganese dioxide) is achieved, as well as a long-term stability of 5000 cycles at a  utilization of 40%. The encouraging performance sheds light on the design of advanced cathodes for aqueous zinc-ion batteries. Zn-MnO 2 batteries offer high energy density, but phase changes that lead to poor cathode stability hinder development of rechargeable versions. Here the authors report structurally reinforced polyaniline-intercalated MnO 2 nanolayers that boost performance by eliminating phase transformation.

Manganese oxide (MnO 2 ) is one of the most promising intercalation cathode materials for zinc ion batteries (ZIBs). Specifically, a layered type delta manganese dioxide (δ-MnO 2 ) allows reversible insertion/extraction of Zn 2+ ions and exhibits high storage capacity of Zn 2+ ions. However, a poor conductivity of δ-MnO 2 , as well as other crystallographic forms, limits its potential applications. This study focuses on δ-MnO 2 with nanoflower structure supported on graphite flake, namely MNG, for use as an intercalation host material of rechargeable aqueous ZIBs. Pristine δ-MnO 2 nanoflowers and MNG were synthesized and examined using X-ray diffraction, electron spectroscopy, and electrochemical techniques. Also, performances of the batteries with the pristine δ-MnO 2 nanoflowers and MNG cathodes were studied in CR2032 coin cells. MNG exhibits a fast insertion/extraction of Zn 2+ ions with diffusion scheme and pseudocapacitive behavior. The battery using MNG cathode exhibited a high initial discharge capacity of 235 mAh/g at 200 mA/g specific current density compared to 130 mAh/g which is displayed by the pristine δ-MnO 2 cathode at the same specific current density. MNG demonstrated superior electrical conductivity compared to the pristine δ-MnO 2 . The results obtained pave the way for improving the electrical conductivity of MnO 2 by using graphite flake support. The graphite flake support significantly improved performances of ZIBs and made them attractive for use in a wide variety of energy applications.

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.

HighlightsThe flower-like δ-MnO2 nanostructures with controlled oxygen vacancies as an extraordinary ZIBs cathode are innovatively developed.The cathode can present large capacity and high-rate zinc-ion storage.DFT analysis substantially unveils the effects of various vacancy concentrations on their electrochemical performances.In recent years, manganese-based oxides as an advanced class of cathode materials for zinc-ion batteries (ZIBs) have attracted a great deal of attentions from numerous researchers. However, their slow reaction kinetics, limited active sites and poor electrical conductivity inevitably give rise to the severe performance degradation. To solve these problems, herein, we introduce abundant oxygen vacancies into the flower-like δ-MnO2 nanostructure and effectively modulate the vacancy defects to reach the optimal level (δ-MnO2−x−2.0). The smart design intrinsically tunes the electronic structure, guarantees ion chemisorption–desorption equilibrium and increases the electroactive sites, which not only effectively accelerates charge transfer rate during reaction processes, but also endows more redox reactions, as verified by first-principle calculations. These merits can help the fabricated δ-MnO2−x−2.0 cathode to present a large specific capacity of 551.8 mAh g−1 at 0.5 A g−1, high-rate capability of 262.2 mAh g−1 at 10 A g−1 and an excellent cycle lifespan (83% of capacity retention after 1500 cycles), which is far superior to those of the other metal compound cathodes. In addition, the charge/discharge mechanism of the δ-MnO2−x−2.0 cathode has also been elaborated through ex situ techniques. This work opens up a new pathway for constructing the next-generation high-performance ZIBs cathode materials.

Structure deterioration and side reaction, which originated from the solvated H 2 O, are the main constraints for the practical deployment of both cathode and anode in aqueous Zn-ion batteries. Here we formulate a weakly solvating electrolyte to reduce the solvating power of H 2 O and strengthen the coordination competitiveness of SO 4 2− to Zn 2+ over H 2 O. Experiment results and theoretical simulations demonstrate that the water-poor solvation structure of Zn 2+ is achieved, which can (i) substantially eliminate solvated-H 2 O-mediated undesirable side reactions on the Zn anode. (ii) boost the desolvation kinetics of Zn 2+ and suppress Zn dendrite growth as well as structure aberration of the cathode. Remarkably, the synergy of these two factors enables long-life full cells including Zn/NaV 3 O 8 ·1.5H 2 O, Zn/MnO 2 and Zn/CoFe(CN) 6 cells. More importantly, practical rechargeable AA-type Zn/NVO cells are assembled, which present a capacity of 101.7 mAh and stability of 96.1% capacity retention after 30 cycles at 0.66 C. The practical deployment of aqueous zinc-ion batteries is hindered by the structure deterioration and side reactions at electrodes. Here, the authors introduce a weakly solvating electrolyte with butanone as an electrolyte additive to stabilize both the cathode and anode of aqueous zinc-ion batteries simultaneously.

Large-scale energy storage is of significance to the integration of renewable energy into electric grid. Despite the dominance of pumped hydroelectricity in the market of grid energy storage, it is limited by the suitable site selection and footprint impact. Rechargeable batteries show increasing interests in the large-scale energy storage; however, the challenging requirement of low-cost materials with long cycle and calendar life restricts most battery chemistries for use in the grid storage. Recently we introduced a concept of manganese-hydrogen battery with Mn2+/MnO₂ redox cathode paired with H⁺/H₂ gas anode, which has a long life of 10,000 cycles and with potential for grid energy storage. Here we expand this concept by replacing Mn2+/MnO₂ redox with a nickel-based cathode, which enables ∼10× higher areal capacity loading, reaching ∼35 mAh cm−2. We also replace high-cost Pt catalyst on the anode with a low-cost, bifunctional nickel-molybdenum-cobalt alloy, which could effectively catalyze hydrogen evolution and oxidation reactions in alkaline electrolyte. Such a nickel-hydrogen battery exhibits an energy density of ∼140 Wh kg−1 (based on active materials) in aqueous electrolyte and excellent rechargeability with negligible capacity decay over 1,500 cycles. The estimated cost of the nickel-hydrogen battery based on active materials reaches as low as ∼$83 per kilowatt-hour, demonstrating attractive characteristics for large-scale energy storage.

Aqueous battery systems feature high safety, but they usually suffer from low voltage and low energy density, restricting their applications in large-scale storage. Here, we propose an electrolyte-decoupling strategy to maximize the full potential of Zn–MnO 2 batteries by simultaneously enabling the optimal redox chemistry of both the Zn and MnO 2 electrodes. The decoupled Zn–MnO 2 battery exhibits an open-circuit voltage of 2.83 V (in contrast to the typical voltage of 1.5 V in conventional Zn–MnO 2 batteries), as well as cyclability with only 2% capacity fading after deep cycling for 200 h. Benefiting from the full utilization of MnO 2 , the Zn–MnO 2 battery is also able to maintain approximately 100% of its capacity at various discharge current densities. We also demonstrate the feasibility of integrating the Zn–MnO 2 battery with a wind and photovoltaic hybrid power generating system. This electrolyte-decoupling strategy is shown to be applicable for other high-performance zinc-based aqueous batteries such as Zn–Cu and Zn–Ag batteries. Low energy density and limited cyclability are preventing the commercialization of aqueous Zn–MnO 2 batteries. Here, the authors combine the merits of operating Zn anodes in alkaline conditions and MnO 2 cathodes in acidic conditions, via an electrolyte-decoupling strategy, to realize high-performance batteries.

Direct electrochemical reduction of CO 2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO 2 -consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO 2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–N x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–N x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–N x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO 2 reduction catalysts. Inexpensive and selective electrocatalysts for CO 2 reduction hold promise for sustainable fuel production. Here, the authors report N-coordinated, non-noble metal-doped porous carbons as efficient and selective electrocatalysts for CO 2 to CO conversion.