Proton Insertion Promoted a Polyfurfural/MnO2 Nanocomposite Cathode for a Rechargeable Aqueous Zn-MnO2 Battery

An efficient electrolyte additive of quaternized hardwood kraft lignin enabling dendrite-free aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have great application potential in large-scale grid energy storage systems. However, the practical application of AZIBs is hindered by several serious issues arising from zinc anodes, such as hydrogen evolution, corrosion, and the formation of zinc dendrites and byproducts. In this study, renewable and low-cost hardwood kraft lignin was modified by the grafting of quaternary ammonium groups. Quaternized hardwood kraft lignin (QHKL) was used as an electrolyte additive for the AZIBs. The addition of 1 wt% of QHKL to the reference electrolyte (RE) strongly contributed to the suppression of hydrogen evolution and zinc electrode self-corrosion. Moreover, it promoted even and homogeneous zinc ion deposition. The Zn//Zn symmetric battery with the electrolyte containing the QHKL additive had remarkable cycling s\ility at various current densities and exhibited highly reversible processes in zinc ion plating and stripping. Thus, the Zn//MnO2 battery with the QHKL-modified electrolyte had outstanding rate performance and remarkable cyclability and maintained a discharge specific capacity of 55.4 mAh g-1 after 3000 cycles at a high current density of 1.5 A g-1. This value was approximately 2.5 times greater than the discharge specific capacity of the battery with the RE under the same test conditions. More importantly, the XRD, SEM and EDS analyses revealed that the zinc anode after cycling had barely any dendrites or byproducts (3Zn(OH)2‧ZnSO4‧xH2O).

Ramsdellite-MnO2 Regeneration via Acid-Mediated Redox Tuning toward Rechargeable Aqueous Zinc-Ion Batteries

The mounting accumulation of spent alkaline batteries (SABs) elicits concerns over both environmental threats and the recycling industry's profitability, closely tied to the chemical reactions in manganese-based waste treatment. Herein, we design an acid-modulated phase-reconstruction strategy for sustainable recovery of manganese oxides from SABs, where moderate proton participation facilitates the preformation of MnOOH intermediates before the initial transformation to ramsdellite-MnO2 (RM-R, orthorhombic) and the final conversion to pyrolusite-MnO2 (RM-β, tetragonal) nanomaterials. This rarely reported metastable RM-R phase features a unique tunneled framework (1 × 2 edge-shared MnO6 octahedra) enabling reversible H+/Zn2+ (de)intercalation, though its traditional synthesis remains challenging due to thermodynamic instability. First-principles calculations reveal that RM-R possesses lower Zn2+ diffusion barriers (0.44 eV) than RM-β (0.99 eV), consistent with its superior Zn2+ storage performance. Moreover, the higher specific surface area enables the RM-R cathode with a battery-supercapacitor hybrid storage behavior, which delivers remarkable capacity (214.9 mA h g-1 at 0.1 A g-1) and long cycling stability (98% retention after 1000 cycles), outperforming most reported MnO2-based cathodes. This low-acid regeneration protocol (4 mL of HCl/1.85 g of waste) paves a sustainable way for closed-loop battery recycling and clarifies the structure-property relationships in metastable manganese oxides.

Proton Storage Chemistry in Aqueous Zinc-Inorganic Batteries with Moderate Electrolytes

The proton (H+) has been proved to be another important energy storage ion besides Zn2+ in aqueous zinc-inorganic batteries with moderate electrolytes. H+ storage usually possesses better thermodynamics and reaction kinetics than Zn2+, and is found to be an important addition for Zn2+ storage. Thus, understanding, characterizing, and modulating H+ storage in inorganic cathode materials is particularly important. In this review, recent advances regarding the proton storage chemistry in aqueous zinc-inorganic batteries with moderate electrolytes are systematically reviewed. First, the four proton storage reaction patterns of H+ insertion, H+/Zn2+ co-insertion, H+-dependent conversion, and H+-dependent dissolution/deposition reaction are explicitly presented. Meanwhile, the proton storage processes of multi-sites and multi-steps, and the Hopping and Grotthuss proton transport mechanisms are carefully introduced. Second, the characterization techniques of proton storage are systematically classified into four types of electrochemical characterization techniques of batteries, structural characterization techniques of inorganic cathodes, pH characterization technique of electrolyte, and quantitative analysis technologies of H+ storage contribution. Third, the structural engineering of proton storage modulation is preliminarily refined to be interlayer engineering, doping engineering, defect engineering, composite engineering, and other engineering. Finally, the emerging challenges and perspectives about future directions of proton storage chemistry are proposed.

Revisiting the Charging Mechanism of α-MnO2 in Mildly Acidic Aqueous Zinc Electrolytes

In recent years, there have been extensive debates regarding the charging mechanism of MnO2 cathodes in aqueous Zn electrolytes. The discussion centered on several key aspects including the identity of the charge carriers contributing to the overall capacity, the nature of the electrochemical process, and the role of the zinc hydroxy films that are reversibly formed during the charging/discharging. Intense studies are also devoted to understanding the effect of the Mn2+ additive on the performance of the cathodes. Nevertheless, it seems that a consistent explanation of the α-MnO2 charging mechanism is still lacking. To address this, a step-by-step analysis of the MnO2 cathodes is conducted. Valuable information is obtained by using in situ electrochemical quartz crystal microbalance with dissipation (EQCM-D) monitoring, supplemented by solid-state nuclear magnetic resonance (NMR), X-ray diffraction (XRD) in Characterization of Materials, and pH measurements. The findings indicate that the charging mechanism is dominated by the insertion of H3O+ ions, while no evidence of Zn2+ intercalation is found. The role of the Mn2+ additive in promoting the generation of protons by forming MnOOH, enhancing the stability of Zn/α-MnO2 batteries is thoroughly investigated. This work provides a comprehensive overview on the electrochemical and the chemical reactions associated with the α-MnO2 electrodes, and will pave the way for further development of aqueous cathodes for Zn-ion batteries.

Se-dopant Modulated Selective Co-Insertion of H+ and Zn2+ in MnO2 for High-Capacity and Durable Aqueous Zn-Ion Batteries

MnO2 is commonly used as the cathode material for aqueous zinc-ion batteries (AZIBs). The strong Coulombic interaction between Zn ions and the MnO2 lattice causes significant lattice distortion and, combined with the Jahn-Teller effect, results in Mn2+ dissolution and structural collapse. While proton intercalation can reduce lattice distortion, it changes the electrolyte pH, producing chemically inert byproducts. These issues greatly affect the reversibility of Zn2+ intercalation/extraction, leading to significant capacity degradation of MnO2. Herein, we propose a novel method to enhance the cycling stability of δ-MnO2 through selenium doping (Se-MnO2). Our work indicates that varying the selenium doping content can regulate the intercalation ratio of H+ in MnO2, thereby suppressing the formation of ZnMn2O4 by-products. Se doping mitigates the lattice strain of MnO2 during Zn2+ intercalation/deintercalation by reducing Mn-O octahedral distortion, modifying Mn-O bond length upon Zn2+ insertion, and alleviating Mn dissolution caused by the Jahn-Teller effect. The optimized Se-MnO2 (Se concentration of 0.8 at.%) deposited on carbon nanotube demonstrates a notable capacity of 386 mAh g-1 at 0.1 A g-1, with exceptional long-term cycle stability, retaining 102 mAh g-1 capacity after 5000 cycles at 3.0 A g-1.

Polymer hetero-electrolyte enabled solid-state 2.4-V Zn/Li hybrid batteries

The high redox potential of Zn0/2+ leads to low voltage of Zn batteries and therefore low energy density, plaguing deployment of Zn batteries in many energy-demanding applications. Though employing high-voltage cathode like spinel LiNi0.5Mn1.5O4 can increase the voltages of Zn batteries, Zn2+ ions will be immobilized in LiNi0.5Mn1.5O4 once intercalated, resulting in irreversibility. Here, we design a polymer hetero-electrolyte consisting of an anode layer with Zn2+ ions as charge carriers and a cathode layer that blocks the Zn2+ ion shuttle, which allows separated Zn and Li reversibility. As such, the Zn‖LNMO cell exhibits up to 2.4 V discharge voltage and 450 stable cycles with high reversible capacity, which are also attained in a scale-up pouch cell. The pouch cell shows a low self-discharge after resting for 28 days. The designed electrolyte paves the way to develop high-voltage Zn batteries based on reversible lithiated cathodes.

Conformal poly 3,4-ethylene dioxythiophene skin stabilized ε-type manganese dioxide microspheres for zinc ion batteries with high volumetric energy density

Manganese dioxide (MnO2) is an important active material for energy storage. Constructing microsphere-structured MnO2 is key for practical application due to the high tapping density for high volumetric energy density. However, the unstable structure and poor electrical conductivity hinder the development of MnO2 microspheres. Herein, Poly 3,4-ethylene dioxythiophene (PEDOT) is painted conformally on ε-MnO2 microspheres to stabilize the structure and enhance the electrical conductivity via in-situ chemical polymerization. When used for Zinc ion batteries (ZIBs), the obtained material (named MOP-5) with high tapping density (1.04 g cm-3) delivers a superior volumetric energy density (342.9 mWh cm-3) and excellent cyclic stability (84.5% after 3500 cycles). Moreover, we find the structure transformation of ε-MnO2 to ZnMn3O7 during the initial few cycles of charge and discharge, and the ZnMn3O7 provides more reaction sites for Zinc ions from analysis of the energy storage mechanism. The material design and theoretical analysis of MnO2 in this work may provide a new idea for future commercial applications of aqueous ZIBs.

Enhanced H+ Storage of a MnO2 Cathode via a MnO2 Nanolayer Interphase Transformed from Manganese Phosphate

The MnO₂ cathode has attracted extensive attention in aqueous zinc ion battery research due to its environmental benignity, low cost, and high capacity. However, sluggish kinetics of hydrated zinc ion and manganese dissolution lead to insufficient rate and cycle performances. In this study, a manganese phosphate nanolayer synthesized in situ on a MnO₂ cathode can be transformed into a δ-MnO₂ nanolayer interphase after activation upon cycling, endowing the interphase with abundant interlayer water. As a result, the δ-MnO₂ nanolayer interphase with the function of H⁺ topochemistry significantly enhances H⁺ (de)insertion in the MnO₂ cathode, which leads to a kinetics conversion from Zn²⁺-dominated (de)insertion to H⁺-dominated (de)insertion, thus endowing the MnO₂ cathode with superior rate and cycle performances (85.9% capacity retention after 1000 cycles at 10 A g^-1). This strategy can be highly scalable for other manganese-based cathodes and provides an insight for developing high-performance aqueous zinc ion batteries.

A Static Tin-Manganese Battery with 30000-Cycle Lifespan Based on Stabilized Mn3+/Mn2+ Redox Chemistry

High-potential Mn³⁺/Mn²⁺ redox couple (>1.3 V vs SHE) in a static battery system is rarely reported due to the shuttle and disproportionation of Mn³⁺ in aqueous solutions. Herein, based on reversible stripping/plating of the Sn anode and stabilized Mn²⁺/Mn³⁺ redox couple in the cathode, an aqueous Sn-Mn full battery is established in acidic electrolytes. Sn anode exhibits high deposition efficiency, low polarization, and excellent stability in acidic electrolytes. With the help of H⁺ and a complexing agent, a reversible conversion between Mn²⁺ and Mn³⁺ ions takes place on the graphite surface. Pyrophosphate ligand is initially employed to form a protective layer through a complexation process with Sn⁴⁺ on the electrode surface, effectively preventing Mn³⁺ from disproportionation and hindering the uncontrollable diffusion of Mn³⁺ to electrolytes. Benefiting from the rational design, the full battery delivers satisfied electrochemical performance including a large capacity (0.45 mAh cm^-2 at 5 mA cm^-2), high discharge plateau voltage (>1.6 V), excellent rate capability (58% retention from 5 to 30 mA cm^-2), and superior cycling stability (no decay after 30 000 cycles). The battery design strategy realizes a robustly stable Mn³⁺/Mn²⁺ redox reaction, which broadens research into ultrafast acidic battery systems.

Highly Flexible K-Intercalated MnO2 /Carbon Membrane for High-Performance Aqueous Zinc-Ion Battery Cathode

The layered MnO₂ is intensively investigated as one of the most promising cathode materials for aqueous zinc-ion batteries (AZIBs), but its commercialization is severely impeded by the challenging issues of the inferior intrinsic electronic conductivity and undesirable structural stability during the charge-discharge cycles. Herein, the lab-prepared flexible carbon membrane with highly electrical conductivity is first used as the matrix to generate ultrathin δ-MnO₂ with an enlarged interlayer spacing induced by the K⁺ -intercalation to potentially alleviate the structural damage caused by H⁺ /Zn²⁺ co-intercalation, resulting in a high reversible capacity of 190 mAh g^-1 at 3 A g^-1 over 1000 cycles. The in situ/ex-situ characterizations and electrochemical analysis confirm that the enlarged interlayer spacing can provide free space for the reversible deintercalation/intercalation of H⁺ /Zn²⁺ in the structure of δ-MnO₂ , and H⁺ /Zn²⁺ co-intercalation mechanism contributes to the enhanced charge storage in the layered K⁺ -intercalated δ-MnO₂ . This work provides a plausible way to construct a flexible carbon membrane-based cathode for high-performance AZIBs.

High-Performance Aqueous Zinc-Ion Batteries Enabled by Binder-Free and Ultrathin V2O5-x@Graphene Aerogels with Intercalation Pseudocapacitance

As a result of the absence of solid-state diffusion limitation, intercalation pseudocapacitance behavior is emerging as an attractive charge-storage mechanism that can greatly facilitate the ion kinetics to boost the rate capability and cycle stability of batteries; however, related research in the field of zinc-ion batteries (ZIBs) is still in the initial stage and only found in limited cathode materials. In this study, a novel V₂O_5-x@rGO hybrid aerogel consisting of ultrathin V₂O₅ nanosheets (∼1.26 nm) with abundant oxygen vacancies (Vö) and a three-dimensional (3D) graphene conductive network was specifically designed and used as a freestanding and binder-free electrode for ZIBs. As expected, the ideal microstructure of both the material and the electrode enable fast electron/ion diffusion kinetics of the electrode, which realize a typical intercalation pseudocapacitance behavior as demonstrated by the simulation calculation of cyclic voltammetry (CV), ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and first-principles density functional theory (DFT) calculation. Thanks to the elimination of solid-state diffusion limitation, the V₂O_5-x@rGO electrode delivers a high reversible rate capacity of 153.9 mAh g^-1 at 15 A g^-1 and 90.6% initial capacity retention at 0.5 A g^-1 after 1050 cycles in ZIBs. The intercalation pseudocapacitance behavior is also realized in the assembled soft-pack battery, showing promising practical application prospects.

Advances in the regulation of kinetics of cathodic H+/Zn2+ interfacial transport in aqueous Zn/MnO2 electrochemistry

Rechargeable aqueous Zn-MnO₂ energy storage systems have attracted extensive attention owing to their high theoretical capacity and non-flammable mild aqueous electrolytes. Nevertheless, the complicated reaction mechanism of a MnO₂-based cathode severely restricts its further development. Therefore, it is crucial to clarify the kinetics of H⁺/Zn²⁺ interfacial transport in the MnO₂ cathode for realizing controllable regulation of interfacial ion transport and then realizing high capacity and long lifespan. Recently, based on different reaction mechanisms, various strategies have been employed to improve the performance of aqueous Zn/MnO₂ cells, such as surface modifications and structural engineering. Herein, we systematically summarize the recent advances in the modulation of interfacial H⁺/Zn²⁺ transport and related redox kinetics to effectively improve the electrochemical responses. Furthermore, the challenges of designing novel MnO₂ cathodes have also been prospected in detail to provide possible guidelines for the development of Zn/MnO₂ batteries.

Structural Regulation of Oxygen Vacancy-Rich K0.5 Mn2 O4 Cathode by Carbon Hybridization for Enhanced Zinc-Ion Energy Storage

High-voltage manganese-based materials are considered as promising cathode materials for aqueous zinc-ion batteries (AZIBs). Herein, oxygen vacancy-rich K_0.5 Mn₂ O₄ sheets were anchored uniformly onto honeycomb-like interconnected carbon nanoflakes (CNF@K_0.5 Mn₂ O₄ ) for AZIB cathode applications. In the composite, the CNFs provided excellent intergranular electron transport capability, while the oxygen vacancies enhanced the electron transport efficiency inside crystals, and the embedded K ions expanded the interlayer spacing and stabilized the layered crystal structure. A reversible specific capacity of 241 mAh g^-1 could be maintained by the composite at 0.5 A g^-1 for 400 cycles. A combination of ex-situ analytical methods and density functional theory calculations was carried out to elucidate the electrochemical mechanism of reversible zinc storage. In addition, flexible quasi-solid-state batteries of Zn//CNF@K_0.5 Mn₂ O₄ were constructed by substituting the traditional aqueous electrolyte for a quasi-solid-state gel electrolyte, which worked efficiently and exhibited high bending durability.

Highly Sensitive and Selective Gas Sensors Based on NiO/MnO2 @NiO Nanosheets to Detect Allyl Mercaptan Gas Released by Humans under Psychological Stress

NiO nanosheets are synthesized in situ on gas sensor chips using a facile solvothermal method. These NiO nanosheets are then used as gas sensors to analyze allyl mercaptan (AM) gas, an exhaled biomarker of psychological stress. Additionally, MnO₂ nanosheets are synthesized onto the surfaces of the NiO nanosheets to enhance the gas-sensing performance. The gas-sensing response of the NiO nanosheet sensor is higher than that of the MnO₂ @NiO nanosheet sensor. The response value can reach 56.69, when the NiO nanosheet sensor detects 40 ppm AM gas. Interestingly, a faster response time (115 s) is obtained when the MnO₂ @NiO nanosheet sensor is exposed to 40 ppm of AM gas. Moreover, the selectivity toward AM gas is about 17-37 times greater than those toward confounders. The mechanism of gas sensing and the factors contributing to the enhance gas response of the NiO and MnO₂ @NiO nanosheets are discussed. The products of AM gas oxidized by the gas sensor are identified by gas chromatography-mass spectrometry (GC/MS). AM gas detection is an unprecedented application for semiconductor metal oxides. From a broader perspective, the developed sensors represent a new platform for the identification and monitoring of gases released by humans under psychological stress, which is increasing in modern life.

Mechanical Insights into the Electrochemical Properties of Thornlike Micro-/Nanostructures of PDA@MnO2@NMC Composites in Aqueous Zn Ion Batteries

As emerging energy storage devices, aqueous zinc ion batteries (AZIBs) with outstanding advantages of high safety, high energy density, and environmental friendliness have attracted much research interest. Herein, the favorable thornlike MnO₂ micro-/nanostructures (PDA@MnO₂@NMC) are rationally constructed by the incorporation of both carbon substrates (NMC) and polydopamine (PDA) surface modifications. Ex situ X-ray diffraction and Raman characteristics show the formation of MnOOH and ZnMn₂O₄ products, corresponding to H⁺ and Zn²⁺ insertions in two discharge platforms. Density functional theory (DFT) calculations also demonstrate that PDA can firmly anchor onto MnO₂ surfaces and prevent the dissolution of MnOOH. In addition, PDA with more hydrophilic groups can capture more H⁺ together with the increased surface capacitance and the extension of the first discharge platform, while the NMC carbon substrate can provide abundant active sites for the overgrown MnO₂ nanowires, improve the conductivity, and promote fast ion and electron transportations. Further, electrochemical impedance spectroscopy (EIS) and GITT results show that the ohmic resistance of PDA@MnO₂@NMC decreases to almost half and, in particular, the ion diffusion coefficient increases more than 30 times of pure MnO₂. As such, PDA@MnO₂@NMC in the AZIB cathode exhibits excellent electrochemical performance compared to the pure MnO₂, which is expected to have favorable competitiveness in energy storage devices.

Boosted H+ Intercalation Enables Ultrahigh Rate Performance of the δ-MnO2 Cathode for Aqueous Zinc Batteries

H+ intercalation, as a critical battery chemistry, enables electrodes' high rate performance due to the fast diffusion kinetics of H+. In this work, more water molecules are introduced into δ-MnO2 by the protonation of δ-MnO2 with abundant oxygen vacancies. Benefiting from the structure with a close arrangement of water molecules in interlayers, the Grotthuss transport of proton is achieved in the energy storage of the δ-MnO2 cathode. As a result, the δ-MnO2 cathode exhibits an ultrahigh rate performance with a capacity of 368.1 mAh g-1 at 0.5 A g-1 and 83.4 mAh g-1 at 50 A g-1, which has a capacity retention of 73% after 1100 cycles at 10 A g-1. The study of the storage mechanism reveals that the Grotthuss intercalation of proton predominates the storage process, which empowers the cathode with high rate performance.

Boosting the Cycling Stability of Aqueous Zinc-Ion Batteries through Nanofibrous Coating of a Bead-like MnOx Cathode

Rechargeable aqueous zinc-ion batteries (AZIBs) are close complements to lithium-ion batteries for next-generation grid-scale applications owing to their high specific capacity, low cost, and intrinsic safety. Nevertheless, the viable cathode materials (especially manganese oxides) of AZIBs suffer from poor conductivity and inferior structural stability upon cycling, thereby impeding their practical applications. Herein, a facile synthetic strategy of bead-like manganese oxide coated with carbon nanofibers (MnO_x-CNFs) based on electrospinning is reported, which can effectively improve the electron/ion diffusion kinetics and provide robust structural stability. These benefits of MnO_x-CNFs are evident in the electrochemical performance metrics, with a long cycling durability (i.e., a capacity retention of 90.6% after 2000 cycles and 71% after 5000 cycles) and an excellent rate capability. Furthermore, the simultaneous insertion of H⁺/Zn²⁺ and the Mn redox process at the surface and in the bulk of MnO_x-CNFs are clarified in detail. Our present study not only provides a simple avenue for synthesizing high-performance Mn-based cathode materials but also offers unique knowledge on understanding the corresponding electrochemical reaction mechanism for AZIBs.

Reunderstanding the Reaction Mechanism of Aqueous Zn-Mn Batteries with Sulfate Electrolytes: Role of the Zinc Sulfate Hydroxide

Rechargeable aqueous Zn-Mn batteries have garnered extensive attention for next-generation high-safety energy storage. However, the charge-storage chemistry of Zn-Mn batteries remains controversial. Prevailing mechanisms include conversion reaction and cation (de)intercalation in mild acid or neutral electrolytes, and a MnO₂ /Mn²⁺ dissolution-deposition reaction in strong acidic electrolytes. Herein, a Zn₄ SO₄ ·(OH)₆ ·xH₂ O (ZSH)-assisted deposition-dissolution model is proposed to elucidate the reaction mechanism and capacity origin in Zn-Mn batteries based on mild acidic sulfate electrolytes. In this new model, the reversible capacity originates from a reversible conversion reaction between ZSH and Zn_x MnO(OH)₂ nanosheets in which the MnO₂ initiates the formation of ZSH but contributes negligibly to the apparent capacity. The role of ZSH in this new model is confirmed by a series of operando characterizations and by constructing Zn batteries using other cathode materials (including ZSH, ZnO, MgO, and CaO). This research may refresh the understanding of the most promising Zn-Mn batteries and guide the design of high-capacity aqueous Zn batteries.

Enabling Reversible MnO2/Mn2+ Transformation by Al3+ Addition for Aqueous Zn-MnO2 Hybrid Batteries

Aqueous rechargeable Zn-manganese dioxide (Zn-MnO₂) hybrid batteries based on dissolution-deposition mechanisms exhibit ultrahigh capacities and energy densities due to the two-electron transformation between MnO₂/Mn²⁺. However, the reported Zn-MnO₂ hybrid batteries usually use strongly acidic and/or alkaline electrolytes, which may lead to environmental hazards and corrosion issues of the Zn anodes. Herein, we propose a new Zn-MnO₂ hybrid battery by adding Al³⁺ into the sulfate-based electrolyte. The hybrid battery undergoes reversible MnO₂/Mn²⁺ transformation and exhibits good electrochemical performances, such as a high discharge capacity of 564.7 mAh g^-1 with a discharge plateau of 1.65 V, an energy density of 520.8 Wh kg^-1, and good cycle life without capacity decay upon 2000 cycles. Experimental results and theoretical calculation suggest that the aquo Al³⁺ with Brønsted weak acid nature can act as the proton-donor reservoir to maintain the electrolyte acidity near the electrode surface and prevent the formation of Zn₄(OH)₆(SO₄)·0.5H₂O during discharging. In addition, Al³⁺ doping during charging introduces oxygen vacancies in the oxide structure and weakens the Mn-O bond, which facilitates the dissolution reaction during discharge. The mechanistic investigation discloses the important role of Al³⁺ in the electrolyte, providing a new fundamental understanding of the promising aqueous Zn-MnO₂ batteries.

Modulating residual ammonium in MnO2 for high-rate aqueous zinc-ion batteries

Manganese dioxide (MnO₂), as a promising cathode candidate, has attracted great attention in aqueous zinc ion batteries (ZIBs). However, the undesirable dissolution of Mn²⁺ and the sluggish kinetic reaction are still two challenges to overcome before achieving good cycling stability and high-rate performance of ZIBs. Herein, β-MnO₂ with chemically residual NH₄⁺ (NMO) was successfully fabricated by controlling the washing condition and utilized as a cathode in a ZIB. Interestingly, NH₄⁺, as a layer pillar in the tunnel structure of NMO, could enhance its conductivity by changing the chemical structure, contributing to accelerating the kinetics of the charge carrier. Moreover, the residual NH₄⁺ in NMO could stabilize the chemical microstructure through the cationic electrostatic shielding effect and the formation of Mn-O⋯H bonds in NMO, promoting the reversible and successive insertion/extraction of H⁺/Zn²⁺ in the ZIB. As a result, the Zn/NMO battery exhibits excellent rate performance (up to 8.0 A g^-1) and cycling stability (10 000 cycles). This work will pave the way for the design of cathode materials with nonmetallic doping for high-performance ZIB systems.

Mg ion pre-intercalated MnO2 nanospheres as high-performance cathode materials for aqueous Zn-ion batteries

Rechargeable Zn-MnO2 batteries with mild and nearly neutral aqueous electrolytes have shown great potential for large-scale energy storage because of their high safety, low cost, environmental friendliness and high energy...

Ultralong-Life Cathode for Aqueous Zinc-Organic Batteries via Pouring 9,10-Phenanthraquinone into Active Carbon

Organic carbonyl electrode materials have shown a great potential in various rechargeable batteries but limited by the problems of poor cycling and rate performance owing to their high solubility in aqueous electrolytes and low conductivity. To address these problems, the 9,10-phenanthraquinone (PQ)@active carbon (AC) composite fabricated by melting PQ molecules into porous AC is considered as a superstable cathode material for aqueous zinc batteries. The introduction of AC improves the structural stability and restrains the PQ dissolution in an aqueous electrolyte. As a result, the PQ@AC composite electrode delivers a reversible discharge capacity of 150.0 mA h g^-1 at a current density of 0.1 A g^-1, and it also features an unprecedented cycling performance of 36 000 cycles with a capacity retention of 96.3% at 5 A g^-1. Moreover, the Zn²⁺ and H⁺ in an aqueous electrolyte are verified to co-insert into the PQ@AC composite electrode using various ex situ characterizations and electrochemical test. This strategy provides a new avenue for organic carbonyl compounds with quinone substructures to improve their electrochemical performance of other batteries.

From spent Zn-MnO2 primary batteries to rechargeable Zn-MnO2 batteries: A novel directly recycling route with high battery performance

For the first time, spent Zn-MnO₂ primary batteries are recycled to directly build rechargeable Zn-MnO₂ batteries with a mixed solution of sulfuric acid and hydrogen peroxide as the leachate, which aimed to the efficient recovery of spent Zn-MnO₂ primary batteries and the realization of high-powered rechargeable Zn-MnO₂ batteries. After simple purification, the leached liquid is directly used as the working solution to prepare an electrolytic rechargeable Zn-MnO₂ battery. The experimental results show that the performance of the recycling solution of the neutral Zn-MnO₂ primary battery was better than that of the alkaline Zn-MnO₂ primary battery, and both performed better than the solution prepared with chemically pure reagents. After optimizing the pH of the working solution and charging current, the obtained rechargeable Zn-MnO₂ battery can provide an energy efficiency of 72.33 % ± 0.55, a coulombic efficiency of 90.17 % ± 0.71, and excellent cycle stability. These experimental results show that spent Zn-MnO₂ primary batteries can be successfully recycled to prepare rechargeable Zn-MnO₂ batteries, demonstrating very good application potential.

2,3-diaminophenazine as a high-rate rechargeable aqueous zinc-ion batteries cathode

Organic materials are attracting extensive attention as promising cathodes for rechargeable aqueous zinc-ion batteries (ZIBs). However, most of them fail to implement the requirement of batteries with combined high-rate and long-cycle performance. Herein, we report a flexible organic molecule 2,3-diaminophenazine (DAP) which exhibits ultrahigh rate performance up to 500C and high capacity retention of 80% after 10,000 cycles at 100C (25.5 A g^-1). Moreover, the Zn²⁺ storage mechanism in the DAP electrode is revealed by ex-situ characterization technologies and theoretical calculation, and the redox active centers CN participate in the reversible electrochemical reaction process. Furthermore, electrochemical analyses show that surface-controlled electrochemical behavior contributes to the high-rate performance of DAP cathodes. Besides, its excellent long-cycle performance can be ascribed to the suppressed DAP dissolubility by using a modified glass fiber separator with carbon nanotubes (CNT) film. Our work provides useful insight into the design of high-rate and long-life ZIBs.

Oxygen-Deficient β-MnO2@Graphene Oxide Cathode for High-Rate and Long-Life Aqueous Zinc Ion Batteries

Recent years have witnessed a booming interest in grid-scale electrochemical energy storage, where much attention has been paid to the aqueous zinc ion batteries (AZIBs). Among various cathode materials for AZIBs, manganese oxides have risen to prominence due to their high energy density and low cost. However, sluggish reaction kinetics and poor cycling stability dictate against their practical application. Herein, we demonstrate the combined use of defect engineering and interfacial optimization that can simultaneously promote rate capability and cycling stability of MnO₂ cathodes. β-MnO₂ with abundant oxygen vacancies (V_O) and graphene oxide (GO) wrapping is synthesized, in which V_O in the bulk accelerate the charge/discharge kinetics while GO on the surfaces inhibits the Mn dissolution. This electrode shows a sustained reversible capacity of ~ 129.6 mAh g^-1 even after 2000 cycles at a current rate of 4C, outperforming the state-of-the-art MnO₂-based cathodes. The superior performance can be rationalized by the direct interaction between surface V_O and the GO coating layer, as well as the regulation of structural evolution of β-MnO₂ during cycling. The combinatorial design scheme in this work offers a practical pathway for obtaining high-rate and long-life cathodes for AZIBs.

Coordinately Unsaturated Manganese-Based Metal-Organic Frameworks as a High-Performance Cathode for Aqueous Zinc-Ion Batteries

The slow Zn²⁺ intercalation/deintercalation kinetics in cathodes severely limits the electrochemical performance of aqueous zinc-ion batteries (ZIBs). Herein, we demonstrate a new kind of coordinately unsaturated manganese-based metal-organic framework (MOF) as an advanced cathode for ZIBs. Coordination unsaturation of Mn is performed with oxygen atoms of two adjacent -COO^-. Its proper unsaturated coordination degree guarantees the high-efficiency Zn²⁺ transport and electron exchange, thereby ensuring high intrinsic activity and fast electrochemical reaction kinetics during repeated charging/discharging processes. Consequently, this MOF-based electrode possesses a high capacity of 138 mAh g^-1 at 100 mA g^-1 and a long life span (93.5% capacity retention after 1000 cycles at 3000 mA g^-1) due to the above advantages. Such distinct Zn²⁺ ion storage performance surpasses those of most of the recently reported MOF cathodes. This concept of adjusting the coordination degree to tune the energy storage capability provides new avenues for exploring high-performance MOF cathodes in aqueous ZIBs.

Saccharin Anion Acts as a "Traffic Assistant" of Zn2+ to Achieve a Long-Life and Dendritic-Free Zinc Plate Anode

Due to the great advantages of low cost, high capacity, and excellent safety, the Zn metal is a promising candidate material for rechargeable aqueous battery systems. However, its practical applications have been restricted by the uncontrollable dendrite growth and electrode side reactions (such as corrosion, passivation, and hydrogen evolution reactions) during the plating process. Herein, we reveal that the dendrite growth would expose the electrode to more highly active tips, exacerbating the passivation of the electrode and the decomposition of the electrolyte by in situ optical microscopies. We propose a low-cost, nontoxic, low-concentration (less than 1 g/L), and effective electrolyte additive, saccharin sodium, which can guide an even Zn deposition without obvious electrode side reactions in the charge/discharge process. The saccharin anion acts as a "traffic assistant" of Zn²⁺ and demonstrates its great potential for practical application. The assembled Zn symmetrical battery shows an excellent cycling performance at a high current density and capacity (an extremely long cycle life over 3800 h is obtained at 5 mA/cm² and 8 mA h/cm², and 20 mA/cm² and 5 mA h/cm² show a lifetime over 800 h), and the full cell (coupled to an AC electrode) presents a stable cycle life with a capacity retention of 86.4% even after 8000 cycles at 5 mA/cm². The saccharin sodium proposed in this work is promising to solve the anode problems in advanced Zn batteries.

Advances and Perspectives of Cathode Storage Chemistry in Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) have captured a surge of interest in recent years as a promising alternative for scalable energy storage applications owing to the intrinsic safety, affordability, environmental benignity, and impressive electrochemical performance. Despite the facilitated development of this technology by many investigations, however, its smooth implementation is still plagued by inadequate energy density and undesirable life span, which calls for an efficient and controllable cathode storage chemistry. Here, this review focuses on the key bottlenecks by offering a comprehensive summary of representative cathode materials and comparatively analyzing their structural features and electrochemical properties. Then, we critically present several feasible electrode design strategies to guide future research activities from a fundamental perspective for high-energy-density and durable cathode materials mainly in terms of interlayer regulation, defect engineering, multiple redox reactions, activated two-electron reactions, and electrochemical activation and conversion. Finally, we outline the remaining challenges and future perspectives of developing high-performance AZIBs.

Modulating MnO2 Interface with Flexible and Self-Adhering Alkylphosphonic Layers for High-Performance Zn-MnO2 Batteries

Metal oxides are essential electrode materials for high-energy-density batteries, but it remains highly challenging to modulate their interfacial charge-transfer process and improve their cycling stability. Here, using MnO₂ nanofibers as an example, we describe the application of self-assembled alkylphosphonic modification layers for significantly improved cycling stability and high-rate performance of Zn-MnO₂ batteries. Two modifier organic molecules with the same phosphonic functional group but different alkyl tail lengths were employed and systematically compared, including butylphosphonic acid (BPA) and decylphosphonic acid (DPA). The phosphonic groups form strong interfacial covalent bonding and assist the generation of conformal and flexible coatings with few nanometers thickness on a MnO₂ surface. The intertwined alkylphosphonic molecules in the modulation layers have interconnected phosphonic groups, which improve interfacial charge transfer of H⁺ ions for fast conversion of MnO₂ to MnOOH without compromising electrolyte wetting. Importantly, the coating layers effectively reduce dissolutive loss of Mn²⁺ from MnO₂ during battery cycling since diffusion of both water molecules and divalent Mn²⁺ cations was inhibited across the modification layers. The flexible coatings could readily adapt to the morphological changes of MnO₂ during battery cycling and provide long-lasting protection. Overall, we identified that BPA has the optimal balance of hydrophobic-hydrophilic components and enabled modified MnO₂ cathodes with >30% improved discharge capacity compared with unmodified MnO₂ cathodes, together with substantially improved long-term cycling stability with >60% capacity retention for 400 cycles in aqueous ZnSO₄ electrolytes without any Mn²⁺ additive. This work provides new insights into tuning electrochemical pathways that move away from the prevailing rigid, ceramic coating-based surface modifications.

Progressive "Layer to Hybrid Spinel/Layer" Phase Evolution with Proton and Zn2+ Co-intercalation to Enable High Performance of MnO2-Based Aqueous Batteries

Manganese oxides are promising host materials in rechargeable aqueous batteries due to their low cost and high capacity; however, their practical applications have long been restricted by their sluggish reaction kinetics and poor cycling stability. Herein, the layered K_0.36H_0.26MnO₂·0.28H₂O (K36) with a proton and Zn²⁺ cointercalation mechanism leads to a progressive phase evolution from layer-type K36 to hybrid layer-type K_xH_yZn_zMnO₂·nH₂O and spinel-type ZnMn₂O₄ nanocrystal after a long-term cycle. Accordingly, K36 shows a high specific capacity (∼329.8 mAh g^-1 at 0.1C), a superior rate performance (∼100.1 mAh g^-1 at 10C), and a remarkable cycling stability (capacity retention of ∼93.4% over 3000 cycles at 4C). This work provides a new viewpoint of enhancing electrode performance via generating hybrid phases under electrochemical driving and will be a benefit to developing the next-generation aqueous batteries.

A carbon-coated spinel zinc cobaltate doped with manganese and nickel as a cathode material for aqueous zinc-ion batteries

Here, a new amorphous material composed of carbon-coated zinc cobaltate doped with manganese and nickel ZNMC@C (ZnNi_0.5Mn_0.5CoO₄@C) with a spinel structure is proposed as a cathode material for use in aqueous zinc-ion batteries. This cathode material exhibits a high charge/discharge capacity with an initial capacity of about 160 mA h g^-1 and its capacity retention rate remains at 60% after 500 cycles at 0.2 A g^-1, which is higher than that of some reported spinel cathode materials. This superior electrochemical performance can be ascribed to the synergistic effect of the co-doping of manganese and nickel, which produces reversible multivalence redox transition activity (Co⁴⁺/Co³⁺, Ni⁴⁺/Ni³⁺/Ni²⁺, and Mn⁴⁺/Mn³⁺) that facilitates the insertion and migration of zinc ions and the existence of an outer amorphous carbon coating that effectively inhibits the dissolution of the cathode structure and stabilizes the cathode structure. In addition, the cycling mechanism of ZNMC@C was analyzed in detail through electrochemical measurements of the different cycling stages, including the kinetic behavior based on cyclic voltammetry and electrochemical impedance spectroscopic analysis and the reaction mechanism from X-ray photoelectron spectroscopy, ex situ X-ray diffractometry and ex situ scanning electron microscopy analysis. These research results suggest that the ZNMC@C composite material could be a competitive cathode material for Abs (aqueous rechargeable batteries).

Naphthalene dianhydride organic anode for a 'rocking-chair' zinc-proton hybrid ion battery

Rechargeable batteries consisting of a Zn metal anode and a suitable cathode coupled with a Zn²⁺ ion-conducting electrolyte are recently emerging as promising energy storage devices for stationary applications. However, the formation of high surface area Zn (HSAZ) architectures on the metallic Zn anode deteriorates their performance upon prolonged cycling. In this work, we demonstrate the application of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), an organic compound, as a replacement for the Zn-metal anode enabling the design of a 'rocking-chair'zinc-proton hybrid ion battery. The NTCDA electrode material displays a multi-plateau redox behaviour, delivering a specific discharge capacity of 143 mA h g^-1 in the potential window of 1.4 V to 0.3 V vs. Zn|Zn²⁺. The detailed electrochemical characterization of NTCDA in various electrolytes (an aqueous solution of 1 M ZnOTF, an aqueous solution of 0.01 M H₂SO₄, and an organic electrolyte of 0.5 M ZnOTF/acetonitrile) reveals that the redox processes leading to charge storage involve a contribution from both H⁺ and Zn²⁺. The performance of NTCDA as an anode is further demonstrated by pairing it with a MnO₂ cathode, and the resulting MnO₂||NTCDA full-cell (zinc-proton hybrid ion battery) delivers a specific discharge capacity of 41 mA h g_total^-1 (normalized with the total mass-loading of both anode and cathode active materials) with an average operating voltage of 0.80 V.