Dissolution Of MnO2 Research Articles

Enhancing the efficiency of two-electron zinc-manganese batteries enabled by Glycine complexation of manganese ions and compatibility with zinc anodes

Aqueous Zn//MnO2 batteries, leveraging the Mn2+/MnO2 conversion reaction, are gaining significant interest for their high redox potential and cost-effectiveness. However, they typically require a highly acidic environment to initiate this redox process. Herein, Glycine (Gly), a gentle and safe amino acid, is employed to enhance the effectiveness of depositing and dissolving Mn2+/MnO2. On one hand, Gly can complex with Mn2+, reducing its concentration and releasing H+. This not only promotes the dissolution of MnO2 but also increases the electrode potential. On the other hand, the Gly attaches to the Zn metal anode’s surface, shielding it from unwanted reactions. Hence, the assembled aqueous Zn//MnO2 battery exhibits an elevated output voltage during the discharge of 1.5 V with high coulombic efficiency (0.5 mAh cm−2 capacity), a long cycling life and excellent rate. This work showcases an efficient approach to enable the two-electron process of MnO2 cathode materials in aqueous zinc batteries.

Stable Cycling of Ultrahigh Mass Loaded MnO2 for Low-Cost Sodium-Ion Batteries

Achieving cost-effective and sustainable solutions for large-scale energy storage is critical for advancing the global clean energy transition. The intermittent nature of green energy underscores the inadequacy of current large scale energy storage technologies like pumped hydroelectricity which is susceptible to geographic and water resource constraints, making batteries a prime candidate for this role. Although lithium-ion batteries (LIBs) with high energy and power density are a viable solution, the high cost and uneven distribution of lithium reserves limits the widespread application of LIBs for grid energy storage. Considering these developments and the challenges posed by limited lithium reserves, sustainable and low-cost sodium-ion batteries (SIBs) emerge as a promising alternative.Among the various battery electrode materials, manganese dioxide (MnO2) stands out as a promising choice for large-scale energy storage applications due to its earth abundance, cost-effectiveness and non-toxic nature. Although MnO2 is known as a pseudocapacitive material with superior cycling stability in aqueous electrolytes, its dissolution in non-aqueous electrolytes has restricted its widespread use in long lifetime batteries. In this study, we demonstrate that an ether-based electrolyte solution (1M NaClO4 in diglyme) is able to achieve stable cycling of electrodeposited ε-MnO2 as a cathode for SIBs, reaching a over 75% capacity retention over 1000 cycles. This response surpasses conventional ester-based electrolytes (58% of 1M NaClO4 in EC/PC). A comparative cycling stability study, coupled with electrochemical quartz crystal microbalance characterization, validates the efficacy of the ether-based electrolyte in mitigating MnO2 dissolution. Furthermore, the integration of 3D printed graphene aerogel (3D GA) as a scaffold for electrodeposited MnO2 leads to enhanced electrochemical performance of the MnO2 electrode (3D MnO2/GA), resulting in a high areal capacity of 4.4 mAh cm-2 at a current density of 10 mA cm-2. The 3D MnO2/GA electrode possesses scalable electrochemical performance with mass loadings from 20 mg cm-2 to 80 mg cm-2. Using this electrode, a high mass loaded sodium-ion battery (SIB) device was fabricated by using utilizing 58 mg cm-2 3D MnO2/GA as the cathode and 16 mg cm-2 dip-coated TiO2@Cu foam as the anode. This MnO2 | TiO2 device delivered a notable areal capacity of 6.2 mWh cm-2 and a high areal power density of 70.7 mW cm-2. Moreover, the SIB device demonstrates 85% capacity retention after 500 cycles, underscoring the stable cycling of this high-energy and power-density SIB device.

Regulated Interfacial Proton and Water Activity Enhances Mn2+/MnO2 Platform Voltage and Energy Efficiency

Aqueous zinc battery (AZB) is one of the most promising energy storage systems for large-scale applications due to its high safety, low cost, and environmental friendliness. To match the high capacity of zinc metal (~820 mAh g-1), various cathode materials, such as oxides, open structure analogues, chalcogens, and halogens, have been explored for AZBs. Among them, manganese dioxide (MnO2) has been considered to be one of the most attractive candidates due to its high capacity (~308 mAh g-1 for one-electron transfer) and adequate redox potential (~0.70 V vs. SHE). Recently, the MnO2 dissolution/deposition charge storage mechanism has been demonstrated to further increase the voltage and capacity of Mn-based AZBs. The dissolution/deposition (Mn2+/MnO2) is a two-electron transfer process, which doubles the theoretical capacity of MnO2 cathode to ~616 mAh g-1 and considerably increases the redox potential (~1.229 V vs. SHE). The charge storage via the dissolution/deposition mechanism pushes the aqueous-based energy density even higher, to over 1000 Wh kg-1.However, this charge storage mechanism involves liquid-solid transition, as well as the thermodynamics and kinetics of this reaction are highly dependent on the acidity and interfacial environment. Yet, the opposite effect of interfacial H+ on the dissolution/deposition processes and the role of interfacial H2O are rarely discussed. Here we introduce tetrafluoroborate anion (BF4 -) into the sulfate-based electrolyte to regulate the interfacial H+ and H2O activity. First, BF4 - hydrolysis increases the electrolyte’s acidity, promoting MnO2 dissolution. Second, BF4 - forms an H-bond network with interfacial H2O that assists H+ diffusion while retaining sufficient H2O supply to facilitate MnO2 deposition. As a result, the cathode-free Zn//MnO2 electrolytic cell achieves a high plateau of ~1.92 V and energy efficiency of ~84.23 % in the BF4 - containing electrolyte. Significantly, the cell delivers 1000 cycles at 1 C with ~100 % Coulombic efficiency and a high energy efficiency retention of 93.65 %. Our findings disclose a new strategy to promote Mn2+/MnO2 platform voltage and energy efficiency for future large-scale energy storage systems.

The mechanisms of ·OH formation in MnO2 and oxalate system: Implication for ATZ removal

Manganese oxides (MnO2) are commonly prevalent in groundwater, sediment and soil. In this study, we found that oxalate (H2C2O4) dissolved MnO2, leading to the formation of Mn(II)/(III), CO2(aq) and reactive oxygen species (·CO2-/O2·-/H2O2/·OH). Notably, CO2(aq) played a crucial role in ·OH formation, contributing to the degradation of atrazine (ATZ). To elucidate underneath mechanisms, a series of reactions with different gas-liquid ratios (GLR) were conducted. At the GLR of 0.3, 3.76, and + ∞ 79.4 %, 5.32 %, and 5.28 % of ATZ were eliminated, in which the cumulative ·OH concentration was 39.6 μM, 8.11 μM, and 7.39 μM and the cumulative CO2(aq) concentration was 11.2 mM, 4.7 mM, and 2.8 mM, respectively. The proposed reaction pathway was that CO2(aq) participated in the formation of a ternary complex [C2O4-Mn(II)-HCO4·3 H2O]-, which converted to a transition state (TS) as [C2O4-Mn(II)-CO3-OH·3 H2O]-, then decomposed to a complex radical [C2O4-Mn(II)-CO3·3 H2O]·- and ·OH after electron transfer within TS. It was novel to discover the role of CO2(aq) for ·OH yielding during MnO2 dissolution by H2C2O4. This finding helps revealing the overlooked processes that CO2(aq) influenced the fate of ATZ or other organic compounds in environment and providing us ideas for new technique development in contaminant remediation. Environmental implicationManganese oxides and oxalate are common in soil, sediment and water. Their interactions could induce the formation of Mn(II)/(III), CO2(aq) and ·CO2-/O2·-/H2O2. This study found that atrazine could be effectively removed due to ·OH radicals under condition of high CO2(aq) concentration. The concentrations of Mn (0.0002–8.34 mg·L−1) and CO2(aq) (15–40 mg·L−1) were high in groundwater, and the surface water or rainfall seeps into groundwater and bring organic acids, which might promote the ·OH formation. The results might explain the missing steps of herbicides transformation in these environments and be helpful in developing new techniques in remediation in future.

MnO2-based dual channel surface plasmon resonance fiber sensor for trace glutathione and refractive index detection.

Glutathione (GSH) plays vital role in human biological systems, so its rapid and sensitive detection is necessary for health condition monitoring. In this work, a simple structure for dual channel GSH and refractive index (RI) detection is proposed. By introducing Au-MnO2 thin film coating on the fiber surface for the first time, GSH solution would lead to the dissolution of MnO2, the change in GSH levels could be monitored over a short period in channel 2. For channel 1, ITO-Ag thin film is applied for RI change detection. After optimization, the GSH detection sensitivity reached about -2.361 nm/mM in the range of 0.005-50 mM, and the RI sensitivity reached 1704.252 nm/RIU in the range of 1.331-1.3895 RIU. Channel 1 could also put into GSH detection in the high concentration scale to enlarge the sensor's range and 0.095 nm/mM of sensitivity is acquired within the range of 50-600 mM. With the presence of MnO2 film, the detection sensitivity increased 25.663 times. Neither channel interferes with the operation of the other. Proposed sensor provides stability, high selectivity and elevation in GSH detection sensitivity, which shows great potential for environmental and biological detection field and their applications.

Promoting the reversibility of electrolytic MnO2-Zn battery with high areal capacity by VOSO4 mediator

Electrolytic MnO2-Zn batteries possess high energy density due to the high reduction potential and capacity of the cathode Mn2+/MnO2. However, the low reversibility of the Mn2+/MnO2 conversion results in a limited lifespan. In this study, we propose the utilization of VOSO4 as a redox mediator in the MnO2-Zn battery to facilitate the dissolution of MnO2. Through various techniques such as electrochemical measurements, ex-situ UV-visible spectroscopy, X-ray diffraction, and scanning electron microscopes, we validate the interaction between VO2+ and MnO2, which effectively mitigates the accumulation of MnO2. The introduction of the redox mediator results in exceptional redox reversibility and outstanding cycling stability of the MnO2/VOSO4-Zn battery at high areal capacities, with 900 cycles at 5 mAh cm-2 and 500 cycles at 10 mAh cm-2. Notably, even in the flow battery device, the battery exhibits a stable cycling performance over 300 cycles at 20 mAh cm-2. These research findings shed light on the potential large-scale application of electrolytic MnO2-Zn batteries.

Tracking Mn and Zn in Rechargeable Aqueous Zn-MnO2 Batteries By Operando X-Ray Absorption

Zn-MnO2 batteries with mildly acidic electrolytes are a promising chemistry for large scale storage thanks to their remarkable energy density, low cost, and high safety. This is mainly obtained thanks to the high capacity of the Zn metal anode, and the nonflammable character of the aqueous electrolyte. MnO2 is one of the most common cathode of choice, not only for being Earth-abundant, but also because it can undergo a two-electron mechanism, which is however complex and still not fully understood. There is currently agreement in considering for discharge a MnO2 dissolution, leading to soluble Mn2+ and simultaneous precipitation of Zinc Hydroxide Sulfate (ZHS, ZnSO4[Zn(OH2)]3·xH2O). When charging the process is not simply reverted. In fact, a distinct electrochemical profile is observed, with at least two distinct plateaus and a third, apparently pseudocapacitive stage (Figure 1a). A similar multistage profile is observed during the second discharge. Although such profile is characteristic and observed with different MnO2 phases and architectures, the underlying mechanism remains elusive, as it seems to involve mainly poorly crystallized phases.We studied the mechanism by operando X-ray absorption (XAS) at the Mn and Zn K-edges to follow speciation simultaneously and quantitatively in the cathode and in the electrolyte via principal component analysis. Beam intensity needed appropriate regulation to avoid interference with the experiment.Simultaneous X-ray diffraction allowed precise correlation with the MnO2 dissolution and ZHS formation. We found evidence of Mn(III) intermediate occurring during local bond reorganization, which is inferred by the significant evolution of the absorption fine structure region (EXAFS) of the Mn K-edge (Figure 1b). In contrast, minor Zn spectral changes reflect primarily processes of precipitation and dissolution, suggesting that no Zn-Mn mixed phases form during cycling. Figure 1

One-pot synthesis of MnO2/coconut shell-derived activated carbon composite with high zinc storage performance

Herein, the MnO2/coconut shell-derived activated carbon composite (MnO2/AC) has been prepared via a facile hydrothermal process. To prepare AC, coconut shell and K2CO3 have been utilized as the raw material and the activator, respectively. In the composite, MnO2 nanorods are well dispersed on AC, which demonstrates combination effect, and hence inhibiting the dissolution of MnO2 and improving the conductivity of MnO2 during the charging and discharging process. The as-prepared MnO2/AC possesses good cyclic performance and superior rate capability when applied in zinc ion battery. The discharge capacity of MnO2/AC remains 204.1 mAh g−1 at 0.2 A g−1 after 200 cycles.

Dual-modification of oxygen vacancies and PEDOT coating on MnO2 nanowires for high-performance zinc ion battery

MnO2 has outstanding potential for application in cathode materials of zinc ion batteries (ZIBs). However, the poor electrical conductivity, poor ion diffusion kinetics, and structural instability greatly limit its rate performance, specific capacity, and cycling stability. Herein, we prepared PEDOT-coated MnO2 cathode material with oxygen vacancy defects, namely Vö-MnO2@PEDOT, by Dual-modification of MnO2 nanowires. The PEDOT coating optimizes the electron transfer capability of the material while inhibiting the dissolution of MnO2. The oxygen vacancies accelerate ion diffusion. Vö-MnO2@PEDOT possesses a joint energy storage mechanism, containing the insertion/extraction of H+/Zn2+ and dissolution/precipitation of MnO2. This joint energy storage mechanism explains the high specific capacity and the increase in capacity. The Vö-MnO2@PEDOT displays a high specific capacity of 400 mAh/g at 0.2 A/g. Besides, at 2 A/g, Vö-MnO2@PEDOT outputs a high capacity of 213 mAh/g after 2000 cycles. This research provides a feasible research idea for the performance optimization of manganese oxide cathode materials.

Deciphering the electrochemical behavior of Mn-based electrode-electrolyte coupling system toward advanced electrochemical energy storage devices

Mn-based aqueous electrochemical energy storage devices (AEESDs) are promising candidates for sustainable and flexible energy applications due to their environmental benignity, high theoretical capacity and versatile architecture. One of the effective strategies to boost their electrochemical performance is to introduce Mn2+ ions into the electrolyte, which can trigger a reversible solid/liquid reaction process of Mn2+/MnO2 deposition/dissolution with a high capacity and an ideal electrochemical kinetics. However, the complex energy storage mechanism that involves the Mn2+/MnO2 deposition/dissolution and the intrinsic insertion/extraction of proton and metal ions remains elusive and poses a great challenge for the rational design of Mn-based AEESDs. Moreover, the insufficient dissolution of MnO2 can lead to the deterioration of performance, which hinders their practical applications. To address these issues, we systematically investigate the Mn2+ ions added Mn-based AEESDs by employing a novel quasi-steady electrochemical measurement technique, and establish a kinetic evolution model to elucidate the solid/liquid reaction at different interface conditions. Furthermore, a full cell is assembled and measured to explore the real electrochemical process of Mn-based electrode with additive Mn2+ ions, which is influenced by the potential windows and charge-discharge rate. This study may provide new insights for the development of advanced AEESDs.

Wet Spinning of Graphene Oxide Fibers with Different MnO2 Additives.

We present the fabrication of graphene oxide (GO) and manganese dioxide (MnO2) composite fibers via wet spinning processes, which entails the effects of MnO2 micromorphology and mass loading on the extrudability of GO/MnO2 spinning dope and on the properties of resulted composite fibers. Various sizes of rod and sea-urchin shaped MnO2 microparticles have been synthesized via hydrothermal reactions with different oxidants and hydrothermal conditions. Both the microparticle morphology and mass loading significantly affect the extrudability of the GO/MnO2 mixture. In addition, the orientation of MnO2 microparticles within the fibers is largely affected by their microscopic surface areas. The composite fibers have been made electrically conductive via chemical or thermal treatments and then applied as fiber cathodes in Zn-ion battery prototypes. Thermal annealing under an argon atmosphere turns out to be an appropriate method to avoid MnO2 dissolution and leaching, which have been observed in the chemical treatments. These rGO/MnO2 fiber cathodes have been assembled into prototype Zn-ion batteries with Zn wire as the anode and xanthan-gum gel containing ZnSO4 and MnSO4 salts as the electrolyte. The resulted electrochemical output depends on the annealing temperature and MnO2 distribution within the fiber cathodes, while the best performer shows stable cycling stability at a maximum capacity of ca. 80 mA h/g.

Visualization the fixation of cadmium on manganese dioxide in sulfur reduction environments.

Manganese oxides as common soil components were considered as an important sink for the cadmium pollution, which, however, would be affected by the reductive sulfide introduced during the flooding period of paddy soil. In this study, the phase transitions caused by the reactions among S2-, MnO2 and Cd2+ were visualized by atomic force microscopy (AFM). The dissolution of MnO2 was in-situ studied by AFM in the S2-containing environments. Moreover, in the ternary system (S2-, MnO2 and Cd2+), the pre-adsorption of Cd2+ by the MnO2 nanosheets would promote the subsequent precipitation of CdS on the surface of MnO2, while the pre-formed CdS nanoparticles in the aquatic phase would tend to suspense rather than precipitating on MnO2. The kinetic study results indicated that the CdS crystallite generation rate was faster than the MnO2 dissolution rate in the aquatic environments with different sulfide contents. In the macroscopic Cd2+ fixation test, the introduction of S2- dramatically improved the fixation of the pre-adsorbed Cd2+ on the MnO2 nanosheets by forming the CdS precipitate. This study provided a fundamental understanding of the interactions among the S2-, MnO2 and Cd2+ ternary system and shed light on the development of Cd pollution remediation methods for paddy soils.

Synergistic combination of a Co-doped σ-MnO2 cathode with an electrolyte additive for a high-performance aqueous zinc-ion battery

The key challenges in aqueous zinc-manganese dioxide batteries (MnO2//Zn) are their poor electrochemical kinetics and stability, which are mainly due to low conductivity and the inevitable dissolution of MnO2. A synergistic combination of a Co-doped σ-MnO2 electrode (Co-MnO2) and a Co(CH3COO)2•4H2O (CoAc) electrolyte additive is here developed to design a high-performance aqueous MnO2//Zn battery (denoted as a Co-MnO2//Zn battery with CoAc). The introduction of Co ions (Co3+/Co2+) into the σ-MnO2 electrode is achieved via a facile one-step electrodeposition method. Benefitting from the synergistic coupling effect of the Co-MnO2 electrode and the CoAc electrolyte additive, the fabricated Co-MnO2//Zn battery with CoAc shows a commendable discharge capacity of 313.8 mAh g−1 at 0.5 A g−1, excellent rate performance, excellent durability over 1000 cycles (∼ 92% capacity retention at 1.0 A g−1) and admirable energy density (439.3 Wh kg−1), which is a significant improvement compared with an un-doped σ-MnO2//Zn battery.

A dual-optical sensor for mancozeb by UCNP@PVP@MnO2 nanozyme

In this work, a fluorescence/colorimetric dual-mode detection method based on MnO2 nanoflower-decorated upconversion nanoparticles: NaYF4:Yb/Er@polyvinylpyrrolidone@MnO2 (UCNP@PVP@MnO2) was proposed to detect the presence of mancozeb (MB). In this detection system, the MnO2 nanoflowers in the nanocomplex of UCNP@PVP@MnO2 would quench the fluorescence of the UCNP. With the addition of H2O2 and 3,3′,5,5′-tetramethylbenzidine (TMB), the reaction between MnO2 and H2O2 resulted in the dissolution of MnO2 and the dissolution of the MnO2 layer contributed to the fluorescence recovery of UCNP. Simultaneously, MnO2 oxidized the colorless TMB to a blue product oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB). The blue solution was able to quench the recovered fluorescence of UCNP due to the fluorescence inter filter effect (IFE) between the UCNP and blue oxTMB. Finally, with the addition of MB, the oxTMB was reduced to TMB by MB and the color of the solution became lighter while the fluorescence intensity of the solution increased. The detection method had a good linear range of 5–120 μM and 0.5–60 μM for fluorescence and colorimetric detection, respectively, and the limits of detection (LOD) were 2.34 and 0.245 μM, respectively.

In-situ regulated competitive proton intercalation and deposition/dissolution reaction of MnO2 for high-performance flexible zinc-manganese batteries

Due to the limitation of energy density caused by the one-electron reaction and capacity loss caused by the Mn(Ⅲ) disproportionation reaction, it is difficult to realize the synchronous improvement of energy density and cycle performance of Zn//MnO2 secondary batteries. Here, a competition mechanism is designed for the zinc-manganese battery to achieve both improved energy density and cycling stability by coupling a high crosslinking density hydrogel electrolyte (HCH) with MnO2 deposition and dissolution reaction. Compared to conventional aqueous electrolytes (2 M ZnSO4, 0.2 M MnSO4), the ultrathin HCH (101 μm) widens the battery's voltage window to 2.4 V without significantly increasing the weight of the electrolyte. HCH not only broadens the voltage window but also effectively delays the proton shuttle from cathode to anode, providing a favorable environment for MnO2/Mn2+ reversible cyclic reaction. The Zn//MnO2 battery achieved an energy density of 858 Wh/kgα-MnO2 and a stable cycle of 7000 cycles due to the limitation of disproportionated reactants by the in situ deposition and dissolution of MnO2 at the cathode.

Mn2+/I- Hybrid Cathode with Superior Conversion Efficiency for Ultrahigh-Areal-Capacity Aqueous Zinc Batteries.

Aqueous Zn-Mn2+ electrolysis batteries utilizing the two-electron-transfer reaction between Mn2+ and MnO2 attract great attention because of their superior theoretical capacity (616 mAh g-1). However, the low conductivity of deposited MnO2 and the poor conversion efficiency of Mn2+/MnO2 inevitably result in limited areal capacity and unsatisfied cycling stability, which have become the main hurdles of aqueous Zn-Mn2+ batteries' applications. Herein, we propose a novel Mn2+/I- hybrid cathode that couples the triiodide/iodide redox with the Mn2+/MnO2 redox to optimize the electrolysis kinetics. Because of the synergistically enhanced conversion reaction between Mn2+/I- and the promoter effect of I- to the dissolution of MnO2, this hybrid cathode not only exhibits fast reaction kinetics, thus demonstrating ultrahigh rate capability (100 mA cm-2), but also displays observably enhanced conversion efficiency up to 96.0% with excellent reversibility of 2000 cycles. Especially the superhigh areal capacity of 20 mAh cm-2 with more than 100 cycles among the reported static Zn-Mn2+ electrolysis batteries is demonstrated. The excellent battery performance as well as the facile electrode hybridization approach designed here paves the way for the practical applications of aqueous Zn batteries.

A Sub‐Square‐Millimeter Microbattery with Milliampere‐Hour‐Level Footprint Capacity

AbstractAs substantial progress has been made to miniaturize intelligent microsystems to the sub‐square‐millimeter scale, there is a desperate need to move beyond existing microbattery technologies to offer adequate energy at the same footprint. A micro‐origami technology able to wind up a flat layer stack into a Swiss roll presents a promising approach in this regard because it mimics the most successful way to make energy‐dense full‐sized batteries. Here, an on‐chip Swiss‐roll current collector made via the micro‐origami process is developed and it is infused with a MnO2 slurry comprising a zincophilic binder. The zincophilic binder layer enhances zinc ion transportability and suppresses MnO2 dissolution. The MnO2 Swiss‐roll microelectrode is used to create an on‐chip microbattery with a small electrode footprint area of 0.75 mm2, which shows a footprint capacity up to 3.3 mAh cm–2. The microbattery demonstrates a reversible capacity of more than 1 mAh cm–2 for 150 cycles. The battery stability can be improved to over 600 cycles at a 50% depth of discharge. An on‐chip integration of a microsystem with the microbattery is demonstrated. The microbatteries set foot in the untrodden sub‐square‐millimeter area providing adequate energy for ever more miniaturized intelligent microsystems.

Interface regulated MnO2/Mn2+ redox chemistry in aqueous Zn ion batteries

Due to the rapid development of energy storage systems from the goal of carbon neutrality, rechargeable aqueous Zn-MnO2 batteries have attracted growing attention. While the controversy surrounding various mechanisms, such as Zn2+ insertion, Zn2+/H+ co-insertion and Mn dissolution/deposition, leads to limitation of MnO2 cathodes, especially for the β-MnO2. Though showing promising performance in recent works, the reaction process and influence factors of β-MnO2 cathodes are still under debate. In this work, a complete MnO2/Mn2+ redox chemistry is proposed to explain the abnormal electrochemical behaviors of β-MnO2 cathodes, which indicates the reversible dissolution/deposition of layered manganese oxides appears after the initial MnO2 dissolution reaction. The revealed effects of interfacial environment in regulating the electrochemical reactions also provide a new perspective for studying the working mechanism of aqueous Zn-MnO2 batteries. Besides, the high specific capacity shown in the MnO2/Mn2+ redox chemistry also contributes to designing high-performance cathodes for AZIBs.

An in-depth mechanistic insight into the redox reaction and degradation of aqueous Zn-MnO2 batteries

Rechargeable aqueous Zn/MnO2 batteries raise massive research activities in recent years. However, both the working principle and the degradation mechanism of this battery chemistry are still under debate. Herein, we provide an in-depth electrochemical and structural investigation on this controversial issue based on α-MnO2 crystalline nanowires. Mechanistic analysis substantiates a two-electron reaction pathway of Mn2+/Mn4+ redox couple from part of MnO2 accompanying with a reversible precipitation/dissolution of flaky zinc sulfate hydroxide (ZSH) during the discharge/charge processes. The formation of the ZSH layer is double-edged, which passivates the deep dissolution of MnO2 upon discharging, but promotes the electrochemical deposition kinetics of active MnO2 upon charging. The cell degradation originates primarily from the corrosion failure of metallic zinc anode and the accumulation of irreversible ZnMn2O4 phases on the cathode. The addition of MnSO4 to the electrolyte could afford supplementary capacity contribution via electro-oxidation of Mn2+. However, a high MnSO4 concentration will expedite the cell failure by corroding the metallic zinc anodes. The present study will shed a fundamental insight on developing new strategies toward practically viable Zn/MnO2 batteries.

Dual metal ions and water molecular pre-intercalated δ-MnO2 spherical microflowers for aqueous zinc ion batteries

Layered δ-MnO2 is a promising cathode material for aqueous zinc ion batteries (AZIBs) due to its high theoretical capacity, high operating voltage and low cost. However, the dissolution of MnO2 and the disproportionation of Mn3+ will lead to irreversible reaction and serious structural degradation of the material during cycling process. In this work, the Al3+ pre-intercalated K0.27MnO2·0.54H2O was prepared by a one-step hydrothermal method with citric acid as the complexing agent and weak reducing agent. Based on the pillars of bimetallic ions K+, Al3+ and water, the framework and interlayer of δ-MnO2 is stabilized. Besides, a certain amount of Al3+ facilitates the increase of crystal water compared with the pure K0.27MnO2·0.54H2O, which is not only conducive to promote the construction of porous and loose 3D morphology, but also beneficial to improve the stability of layered structure and accelerate the migration rate of zinc ions. Contributed to the dissolution/deposition reaction mechanism combined with H+/Zn2+ co-insertion/co-extraction mechanism, it has achieved the high capacity with the maximum reversible specific capacity of 269.5 mAh g−1 at 0.5 A g−1 and excellent stability with 205.8 mAh g−1 even after 300 cycles in Zn//Al-KMO battery.