Unraveling the Dissolution-Mediated Reaction Mechanism of α-MnO2 Cathodes for Aqueous Zn-Ion Batteries

Insights into the cycling stability of manganese-based zinc-ion batteries: from energy storage mechanisms to capacity fluctuation and optimization strategies

Manganese-based materials are considered as one of the most promising cathodes in zinc-ion batteries (ZIBs) for large-scale energy storage applications owing to their cost-effectiveness, natural availability, low toxicity, multivalent states, high operation voltage, and satisfactory capacity. However, their intricate energy storage mechanisms coupled with unsatisfactory cycling stability hinder their commercial applications. Previous reviews have primarily focused on optimization strategies for achieving high capacity and fast reaction kinetics, while overlooking capacity fluctuation and lacking a systematic discussion on strategies to enhance the cycling stability of these materials. Thus, in this review, the energy storage mechanisms of manganese-based ZIBs with different structures are systematically elucidated and summarized. Next, the capacity fluctuation in manganese-based ZIBs, including capacity activation, degradation, and dynamic evolution in the whole cycle calendar are comprehensively analyzed. Finally, the constructive optimization strategies based on the reaction chemistry of one-electron and two-electron transfers for achieving durable cycling performance in manganese-based ZIBs are proposed.

pH buffer KH2PO4 boosts zinc ion battery performance via facilitating proton reaction of MnO2 cathode

Zinc-ion batteries (ZIBs) are rapidly emerging as safe, cost-effective, nontoxic, and environmentally friendly energy storage systems. However, mildly acidic electrolytes with depleted protons cannot satisfy the huge demand for proton reactions in MnO2 electrodes and also cause several issues in ZIBs, such as rapidly decaying cycling stability and low reaction kinetics. Herein, we propose a pH-buffering strategy in which KH2PO4 is added to the electrolyte to overcome the problems caused by low proton concentrations. This strategy significantly improves the rate and cycle stability performance of zinc-manganese batteries, delivering a high capacity of 122.5 mAh/g at a high current density of 5 A/g and enabling 9000 cycles at this current density, with a remaining capacity of 70 mAh/g. Ex-situ X-ray diffraction and scanning electron microscopy analyses confirmed the generation/dissolution of Zn3PO4·4H2O and Zn4(OH)6(SO4)·5H2O, byproducts of buffer products and proton reactions. In-situ pH measurements and chemical titration revealed that the pH change during the electrochemical process can be adjusted to a low range of 2.2-2.8, and the phosphate distribution varies with the pH range. Those results reveal that H2PO4- provides protons to the cathode through the chemical balance of HPO42-, HPO42-, and Zn3PO4·4H2O. This study serves as a guide for studying the influences and mechanisms of buffering additives in Zn-MnO2 batteries.

Organic Intercalation Induced Kinetic Enhancement of Vanadium Oxide Cathodes for Ultrahigh-Loading Aqueous Zinc-Ion Batteries

Vanadium-based oxides have attracted much attention because of their rich valences and adjustable structures. The high theoretical specific capacity contributed by the two-electron-transfer process (V5+ /V3+ ) makes it an ideal cathode material for aqueous zinc-ion batteries. However, slow diffusion kinetics and poor structural stability limit the application of vanadium-based oxides. Herein, a strategy for intercalating organic matter between vanadium-based oxide layers is proposed to attain high rate performance and long cycling life. The V3 O7 ·H2 O is synthesized in situ on the carbon cloth to form an open porous structure, which provides sufficient contact areas with electrolyte and facilitates zinc ion transport. On the molecular level, the added organic matter p-aminophenol (pAP) not only plays a supporting role in the V3 O7 ·H2 O layer, but also shows a regulatory effect on the V5+ /V4+ redox process due to the reducing functional group on pAP. The novel composite electrode with porous structure exhibits outstanding reversible specific capacity (386.7 mAh g-1 , 0.1 A g-1 ) at a high load of 6.5 mg cm-2 , and superior capacity retention of 80% at 3 A g-1 for 2100 cycles.

Multi-Ion Engineering Strategies toward High Performance Aqueous Zinc-Based Batteries

As alternatives to batteries with organic electrolytes, aqueous zinc-based batteries (AZBs) have been intensively studied. However, the sluggish kinetics, side reactions, structural collapse, and dissolution of the cathode severely compromise the commercialization of AZBs. Among various strategies to accelerate their practical applications, multi-ion engineering shows great feasibility to maintain the original structure of the cathode and provide sufficient energy density for high-performance AZBs. Though multi-ion engineering strategies could solve most of the problems encountered by AZBs and show great potential in achieving practical AZBs, the comprehensive summaries of the batteries undergo electrochemical reactions involving more than one charge carrier is still in deficiency. The ambiguous nomenclature and classification are becoming the fountainhead of confusion and chaos. In this circumstance, this review overviews all the battery configurations and the corresponding reaction mechanisms are investigated in the multi-ion engineering of aqueous zinc-based batteries. By combing through all the reported works, this is the first to nomenclate the different configurations according to the reaction mechanisms of the additional ions, laying the foundation for future unified discussions. The performance enhancement, fundamental challenges, and future developing direction of multi-ion strategies are accordingly proposed, aiming to further accelerate the pace to achieve the commercialization of AZBs with high performance.

Suppressing Rampant and Vertical Deposition of Cathode Intermediate Product via PH Regulation Toward Large-Capacity and High-Durability Zn//MnO2 Batteries

Despite great prospects, Zn//MnO2 batteries suffer from rampant and vertical deposition of zinc sulfate hydroxide (ZSH) at the cathode surface, which leads to a significant impact on their electrochemical performance. This phenomenon is primarily due to the drastic increase in the electrolyte pH value upon discharging, which is closely associated with the electrodissolution of Mn-based active materials. Herein, the pH value change is effectively inhibited by employing an electrolyte additive with excellent pH buffering capability. As such, the formation of ZSH at the cathode is postponed, resulting in the deposition of ZSH in a horizontal arrangement. This strategy can significantly enhance the utilization efficiency of cathode active material, while also enabling a solid electrolyte interphase layer at the Zn anode to address low Zn stripping/plating reversibility. With the optimal electrolyte, the Zn//MnO2 battery realizes a 25.6% increase in the specific capacity at 0.2 A g-1 compared to that with the baseline electrolyte, great rate capability (161.6 mAh g-1 at 5 A g-1 ), and superior capacity retention (90.2% over 5,000 cycles). In addition, the pH buffering strategy is highly applicable in hydrogel electrolytes. This work underscores the importance of pH regulation for Zn//MnO2 batteries and provides enlightening insights.

Mechanism of Chalcophanite Nucleation in Zinc Hydroxide Sulfate Cathodes in Aqueous Zinc Batteries

Aqueous Zn-ion batteries with MnO2-based cathodes have seen significant attention owing to their high theoretical capacities, safety, and low cost; however, much debate remains regarding the reaction mechanism that dominates energy storage. In this work, we report our electron microscopy study of cathodes containing zinc hydroxide sulfate (Zn4SO4(OH)6·xH2O, ZHS) together with carbon nanotubes cycled in electrolytes containing ZnSO4 with varied amounts of MnSO4 incorporated. The primary Mn-containing phase is formed in situ in the cathode during cycling, where a dissolution-deposition reaction is identified between ZHS and chalcophanite (ZnMn3O7·3H2O). Mechanistic details of this reaction, in which the chalcophanite nucleates then separates from the ZHS flakes as the ZHS dissolves while acting as the primary Zn source for the reaction, are revealed using surface sensitive methods. These findings indicate the reaction is local to the ZHS flakes, providing new insight toward the importance of ZHS and the cathode microstructure.

An aqueous electrolyte densified by perovskite SrTiO3 enabling high-voltage zinc-ion batteries

The conventional weak acidic electrolyte for aqueous zinc-ion batteries breeds many challenges, such as undesirable side reactions, and inhomogeneous zinc dendrite growth, leading to low Coulombic efficiency, low specific capacity, and poor cycle stability. Here, an aqueous densified electrolyte, namely, a conventional aqueous electrolyte with addition of perovskite SrTiO3 powder, is developed to achieve high-performance aqueous zinc-ion batteries. The densified electrolyte demonstrates unique properties of reducing water molecule activity, improving Zn2+ transference number, and inducing homogeneous and preferential deposition of Zn (002). As a result, the densified electrolyte exhibits an ultra-long cycle stability over 1000 cycles in Zn/Ti half cells. In addition, the densified electrolyte enables Zn/MnO2 cells with a high specific capacity of 328.2 mAh g-1 at 1 A g-1 after 500 cycles under an extended voltage range. This work provides a simple strategy to induce dendrite-free deposition characteristics and high performance in high-voltage aqueous zinc-ion batteries.

Conversion-Type Cathode Materials for Aqueous Zn Metal Batteries in Nonalkaline Aqueous Electrolytes: Progress, Challenges, and Solutions

Aqueous Zn metal batteries are attractive as safe and low-cost energy storage systems. At present, due to the narrow window of the aqueous electrolyte and the strong reliance of the Zn2+ ion intercalated reaction on the host structure, the current intercalated cathode materials exhibit restricted energy densities. In contrast, cathode materials with conversion reactions can promise higher energy densities. Especially, the recently reported conversion-type cathode materials that function in nonalkaline electrolytes have garnered increasing attention. This is because the use of nonalkaline electrolytes can prevent the occurrence of side reactions encountered in alkaline electrolytes and thereby enhance cycling stability. However, there is a lack of comprehensive review on the reaction mechanisms, progress, challenges, and solutions to these cathode materials. In this review, four kinds of conversion-type cathode materials including MnO2 , halogen materials (Br2 and I2 ), chalcogenide materials (O2 , S, Se, and Te), and Cu-based compounds (CuI, Cu2 O, Cu2 S, CuO, CuS, and CuSe) are reviewed. First, the reaction mechanisms and battery structures of these materials are introduced. Second, the fundamental problems and their corresponding solutions are discussed in detail in each material. Finally, future directions and efforts for the development of conversion-type cathode materials for aqueous Zn batteries are proposed.

Protocol in Evaluating Capacity of Zn-Mn Aqueous Batteries: A Clue of pH

In the literature, Zn-Mn aqueous batteries (ZMABs) confront abnormal capacity behavior, such as capacity fluctuation and diverse "unprecedented performances." Because of the electrolyte additive-induced complexes, various charge/discharge behaviors associated with different mechanisms are being reported. However, the current performance assessment remains unregulated, and only the electrode or the electrolyte is considered. The lack of a comprehensive and impartial performance evaluation protocol for ZMABs hinders forward research and commercialization. Here, a pH clue (proton-coupled reaction) to understand different mechanisms is proposed and the capacity contribution is normalized. Then, a series of performance metrics, including rated capacity (C_r ) and electrolyte contribution ratio from Mn²⁺ (CfM), are systematically discussed based on diverse energy storage mechanisms. The relationship between Mn (II) ↔ Mn (III) ↔ Mn (IV) conversion chemistry and protons consumption/production is well-established. Finally, the concrete design concepts of a tunable H⁺ /Zn²⁺ /Mn²⁺ storage system for customized application scenarios, opening the door for the next-generation high-safety and reliable energy storage system, are proposed.

Bi-MOF Modulating MnO2 Deposition Enables Ultra-Stable Cathode-Free Aqueous Zinc-Ion Batteries

The Mn-based materials are considered as the most promising cathodes for zinc-ion batteries (ZIBs) due to their inherent advantages of safety, sustainability and high energy density, however suffer from poor cyclability caused by gradual Mn²⁺ dissolution and irreversible structural transformation. The mainstream solution is pre-adding Mn²⁺ into the electrolyte, nevertheless faces the challenge of irreversible Mn²⁺ consumption results from the MnO₂ electrodeposition reaction (Mn²⁺  → MnO₂ ). This work proposes a "MOFs as the electrodeposition surface" strategy, rather than blocking it. The bismuth (III) pyridine-3,5-dicarboxylate (Bi-PYDC) is selected as the typical electrodeposition surface to regulate the deposition reaction from Mn²⁺ to MnO₂ . Because of the unique less hydrophilic and manganophilic nature of Bi-PYDC for Mn²⁺ , a moderate MnO₂ deposition rate is achieved, preventing the electrolyte from rapidly exhausting Mn²⁺ . Simultaneously, the intrinsic stability of deposited R-MnO₂ is enhanced by the slowly released Bi³⁺ from Bi-PYDC reservoir. Furthermore, Bi-PYDC shows the ability to accommodate H⁺ insertion/extraction. Benefiting from these merits, the cathode-free ZIB using Bi-PYDC as the electrodeposition surface for MnO₂ shows an outstanding cycle lifespan of more than 10 000 cycles at 1 mA cm^-2 . This electrode design may stimulate a new pathway for developing cathode free long-life rechargeable ZIBs.

Complementary Operando Electrochemical Quartz Crystal Microbalance and UV/Vis Spectroscopic Studies: Acetate Effects on Zinc-Manganese Batteries

Zinc-ion batteries, in which zinc ions and protons do intercalation and de-intercalation during battery cycling with various proposed mechanisms under debate, have been studied. Recently, electrolytic zinc-manganese batteries, exhibiting the pure dissolution-deposition behavior with a large charge capacity, have been accomplished through using electrolytes with Lewis acid. However, the complicated chemical environment and mixed products hinder the investigation though it is crucial to understand the detailed mechanism. Here, cyclic voltammetry coupled electrochemical quartz crystal microbalance (EQCM) and ultraviolet-visible spectrophotometry (UV-Vis) are respectively, for the very first time, used to study the transition from zinc-ion batteries to zinc electrolytic batteries by the continuous addition of acetate ions. These complementary techniques operando trace the mass and the composition evolution. The observed formation and dissolution of zinc hydroxide sulfate (ZHS) and manganese oxides evince the effect of acetate ions on zinc-manganese batteries from an alternative perspective. Both the amount of acetate and the pH value have large impacts on the capacity and Coulombic efficiency of the MnO₂ electrode, and thus they should be optimized when constructing a full zinc-manganese battery with high rate capability and reversibility.

Interlayer Modulation of Layered Transition Metal Compounds for Energy Storage

Layered transition metal compounds are one of the most important electrode materials for high-performance electrochemical energy storage devices, such as batteries and supercapacitors. Charge storage in these materials can be achieved via intercalation of ions into the interlayer channels between the layer slabs. With the development of lithium-beyond batteries, larger carrier ions require optimized interlayer space for the unrestricted diffusion in the two-dimensional channels and effectively shielded electrostatic interaction between the slabs and interlayer ions. Therefore, interlayer modulation has become an efficient and promising approach to overcome the problems of sluggish kinetics, structural distortion, irreversible phase transition, dissolution of some transition metal elements, and air instability faced by these materials and thus enhance the overall electrochemical performance. In this review, we focus on the interlayer modulation of layered transition metal compounds for various batteries and supercapacitors. Merits of interlayer modulation on the charge storage procedures of charge transfer, ion diffusion, and structural transformation are first discussed, with emphasis on the state-of-art strategies of intercalation and doping with foreign species. Following the obtained insights, applications of modified layered electrode materials in various batteries and supercapacitors are summarized, which may guide the future development of high-performance and low-cost electrode materials for energy storage.

Balanced Interfacial Ion Concentration and Migration Steric Hindrance Promoting High-Efficiency Deposition/Dissolution Battery Chemistry

The solid-liquid transition reaction lays the foundation of electrochemical energy storage systems with high capacity, but realizing high efficiency remains a challenge. Herein, in terms of thermodynamics and dynamics, this work demonstrates the significant role of both interfacial H⁺ concentration and Mn²⁺ migration steric hindrance for the high-efficiency deposition/dissolution chemistry of zinc-manganese batteries. Specially, the introduction of formate anions can buffer the generated interfacial H⁺ to stabilize interfacial potential according to the Nernst equation, which stimulates high capacity. Compared with acetate and propionate anions, the formate anion also provides high adsorption density on the cathode surface to shield the electrostatic repulsion due to the small spatial hindrance. Particularly for the solvated Mn²⁺ , the formate-anion-induced lower energy barrier of the rate-determining step during the step-by-step desolvation process results in lower polarization and higher electrochemical reversibility. In situ tests and theoretical calculations verify that the electrolyte with formate anions achieve a good balance between ion concentration and ion-migration steric hindrance. It exhibits both the high energy density of 531.26 W h kg^-1  and long cycle life of more than 300 cycles without obvious decay.

Issues and Opportunities Facing Aqueous Mn2+ /MnO2 -based Batteries

Aqueous Mn²⁺ /MnO₂ -based batteries have attracted enormous attentions in aqueous energy storage fields, owing to their high working voltage and theoretical capacity (616 mAh g^-1 ) brought by the two-electron reaction (Mn²⁺ /Mn⁴⁺ ). However, there are currently several tricky challenges facing Mn²⁺ /MnO₂ -based batteries: their complicated working mechanisms, existing issues, and optimization strategies. This Perspective aims to provide a mechanistic understanding and an overview of the insufficiency, optimization, and future development for Mn²⁺ /MnO₂ -based batteries. The existing issues and deficiency in Mn²⁺ /MnO₂ -based batteries have been systematically analyzed, and optimization strategies have also been rationally summarized and discussed with deep insights. Also, the often-overlooked optimized objects and aspects have been highlighted with unique perspectives. The proposals of testing methods and performance assessment are presented, containing different degradation mechanisms. Based on the above points, this Perspective will provide guidance and contribute to the further development of aqueous Mn²⁺ /MnO₂ -based batteries.

High-Energy and Stable Subfreezing Aqueous Zn-MnO2 Batteries with Selective and Pseudocapacitive Zn-Ion Insertion in MnO2

One major challenge of aqueous Zn-MnO₂ batteries for practical applications is their unacceptable performance below freezing temperatures. Here the use of simple Zn(ClO₄ )₂ aqueous electrolytes is described for all-weather Zn-MnO₂ batteries even down to -60 °C. The symmetric, bulky ClO₄ ^- anion effectively disrupts hydrogen bonds between water molecules and provides intrinsic ion diffusion even while frozen, and enables ≈260 mAh g^-1 on MnO₂ cathodes at -30 °C . It is identified that subfreezing cycling shifts the reaction mechanism on the MnO₂ cathode from unstable H⁺ insertion to predominantly pseudocapacitive Zn²⁺ insertion, which converts MnO₂ nanofibers into complicated zincated MnO_x that are largely disordered and appeared as crumpled paper sheets. The Zn²⁺ insertion at -30 °C is faster and much more stable than at 20 °C, and delivers ≈80% capacity retention for 1000 cycles without Mn²⁺ additives. In addition, simple Zn(ClO₄ )₂ electrolyte also enables a nearly fully reversible and dendrite-free Zn anode at -30 °C with ≈98% Coulombic efficiency. Zn-MnO₂ prototypes with an experimentally verified unit energy density of 148 Wh kg^-1 at a negative-to-positive ratio of 1.5 and an electrolyte-to-capacity ratio of 2.0 are further demonstrated.

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.

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.

Highly Reversible Cycling of Zn-MnO2 Batteries Integrated with Acid-Treated Carbon Supportive Layer

Zn-MnO₂ battery with mild-acid electrolytes has been considered as a promising alternative to Li-ion battery for safe and cost-effective energy storage systems (ESSs), and for full electrification. However, the governing mechanism of MnO₂ electrochemistry has not been fully elucidated, hindering further advances in highly reversible MnO₂ cathodes. Eventual Mn²⁺ ion dissolution into the electrolyte adversely triggers the irreversible loss of Mn²⁺ ions and the excessive precipitation of zinc hydroxyl sulfate (Zn₄ SO₄ (OH)₆ ·xH₂ O, ZHS), leading to irreversible capacity loss upon prolonged cycling. To overcome these drawbacks, a rationally renovated cell structure is proposed by integrating an acid-treated carbon supportive layer (aCSL) in the MnO₂ cathode, which can play multifunctional roles rendering the additional reaction sites for the reversible formation/decomposition of ZHS and re-utilization of the dissolved Mn²⁺ ions. Furthermore, the improved affinity of the aCSL toward the electrolyte is beneficial for increasing active surface area and facilitating charge transfer at the cathode side. Benefiting from these features, compared to the conventional cell configuration, the aCSL-integrated Zn-MnO₂ cell exhibits superior cycling over 3000 cycles with negligible capacity decay (85.6% retention) at a current of 3 A g^-1 .

Hydroxyl Conducting Hydrogels Enable Low-Maintenance Commercially Sized Rechargeable Zn–MnO2 Batteries for Use in Solar Microgrids

Zinc (Zn)–manganese dioxide (MnO₂) rechargeable batteries have attracted research interest because of high specific theoretical capacity as well as being environmentally friendly, intrinsically safe and low-cost. Liquid electrolytes, such as potassium hydroxide, are historically used in these batteries; however, many failure mechanisms of the Zn–MnO₂ battery chemistry result from the use of liquid electrolytes, including the formation of electrochemically inert phases such as hetaerolite (ZnMn₂O₄) and the promotion of shape change of the Zn electrode. This manuscript reports on the fundamental and commercial results of gel electrolytes for use in rechargeable Zn–MnO₂ batteries as an alternative to liquid electrolytes. The manuscript also reports on novel properties of the gelled electrolyte such as limiting the overdischarge of Zn anodes, which is a problem in liquid electrolyte, and finally its use in solar microgrid applications, which is a first in academic literature. Potentiostatic and galvanostatic tests with the optimized gel electrolyte showed higher capacity retention compared to the tests with the liquid electrolyte, suggesting that gel electrolyte helps reduce Mn³⁺ dissolution and zincate ion migration from the Zn anode, improving reversibility. Cycling tests for commercially sized prismatic cells showed the gel electrolyte had exceptional cycle life, showing 100% capacity retention for >700 cycles at 9.5 Ah and for >300 cycles at 19 Ah, while the 19 Ah prismatic cell with a liquid electrolyte showed discharge capacity degradation at 100th cycle. We also performed overdischarge protection tests, in which a commercialized prismatic cell with the gel electrolyte was discharged to 0 V and achieved stable discharge capacities, while the liquid electrolyte cell showed discharge capacity fade in the first few cycles. Finally, the gel electrolyte batteries were tested under IEC solar off-grid protocol. It was noted that the gelled Zn–MnO₂ batteries outperformed the Pb–acid batteries. Additionally, a designed system nameplated at 2 kWh with a 12 V system with 72 prismatic cells was tested with the same protocol, and it has entered its third year of cycling. This suggests that Zn–MnO₂ rechargeable batteries with the gel electrolyte will be an ideal candidate for solar microgrid systems and grid storage in general.

Electrochemical reaction behavior of MnS in aqueous zinc ion battery

Rechargeable aqueous zinc ion battery (ZIB) is reviving as a promising energy storage device with respect to its distinguished safeness, cost-effectiveness, and eco-friendliness. However, its practical application is still challenged...

Discharging Behavior of Hollandite α-MnO2 in a Hydrated Zinc-Ion Battery

Hollandite, α-MnO₂, is of interest as a prospective cathode material for hydrated zinc-ion batteries (ZIBs); however, the mechanistic understanding of the discharge process remains limited. Herein, a systematic study on the initial discharge of an α-MnO₂ cathode under a hydrated environment was reported using density functional theory (DFT) in combination with complementary experiments, where the DFT predictions well described the experimental measurements on discharge voltages and manganese oxidation states. According to the DFT calculations, both protons (H⁺) and zinc ions (Zn²⁺) contribute to the discharging potentials of α-MnO₂ observed experimentally, where the presence of water plays an essential role during the process. This study provides valuable insights into the mechanistic understanding of the discharge of α-MnO₂ in hydrated ZIBs, emphasizing the crucial interplay among the H₂O molecules, the intercalated Zn²⁺ or H⁺ ions, and the Mn⁴⁺ ions on the tunnel wall to enhance the stability of discharged states and, thus, the electrochemical performances in hydrated ZIBs.

Electrolyte additives inhibit the surface reaction of aqueous sodium/zinc battery

In aqueous zinc-based batteries, the reaction by-product Zn₄SO₄(OH)₆·xH₂O is commonly observed when cycling vanadium-based and manganese-based cathodes. This by-product obstructs ion transport paths, resulting in enhanced electrochemical impedance. In this work, we report a hybrid aqueous battery based on a Na_0.44MnO₂ cathode and a metallic zinc foil anode. The surfactant sodium lauryl sulfate is added to the electrolyte as a modifier, and the performance before and after modification is compared. The results show that sodium lauryl sulfate can generate an artificial passivation film on the electrode surface. This passivation film reduces the generation of Zn₄SO₄(OH)₆·xH₂O and inhibits the dissolution of Na_0.44MnO₂ in the electrolyte. Therefore, the reaction kinetics and cycle stability of the battery are significantly enhanced. A battery with this electrolyte additive delivers an initial discharge capacity of 235 mA h g^-1 at a current density of 0.1 A g ^-1. At the same time, the battery has excellent rate performance. Under the high-rate condition of 1 A g^-1, the battery still maintains a capacity retention rate of 93% after 1500 cycles. Finally, the functional mechanism of by-product inhibition by the electrolyte additive is discussed.

Agar Acts as Cathode Microskin to Extend the Cycling Life of Zn//α-MnO2 Batteries

The Zn/MnO₂ battery is a promising energy storage system, owing to its high energy density and low cost, but due to the dissolution of the cathode material, its cycle life is limited, which hinders its further development. Therefore, we introduced agar as a microskin for a MnO₂ electrode to improve its cycle life and optimize other electrochemical properties. The results showed that the agar-coating layer improved the wettability of the electrode material, thereby promoting the diffusion rate of Zn²⁺ and reducing the interface impedance of the MnO₂ electrode material. Therefore, the Zn/MnO₂ battery exhibited outstanding rate performance. In addition, the agar-coating layer promoted the reversibility of the MnO₂/Mn²⁺ reaction and acted as a colloidal physical barrier to prevent the dissolution of Mn²⁺, so that the Zn/MnO₂ battery had a high specific capacity and exhibited excellent cycle stability.

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.