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

Modulating the interlayer H+ migration in MnO2 via W and K co-doping engineering to high-capacity aqueous zinc-ion batteries

MnO2 is a promising cathode material for high-energy-density aqueous zinc-ion batteries (AZIBs) due to its high voltage and abundance. However, its electrochemical activity is seriously damaged by the sediment of discharge by-product Zn4SO4(OH)6·nH2O (ZSH), which results from the pH change of electrolyte after "dead" H+ ions are trapped within the MnO2 lattice. Herein, MnO2 co-regulated by tungsten (W) and potassium (K) (denoted as WKMO) is proposed to address this issue. The addition of W and K significantly diminishes the resistance to H+ insertion/extraction and facilitates the migration of H+ within the MnO2 lattice, relieving the heavy accumulation of ZSH during the long cycle. Meanwhile, stronger W-O and K-O bonds stabilize the layered structure of WKMO and moderate oxygen defects endow WKMO with high conductivity and increased active sites. Benefiting from the effect of W and K co-doping, exceptional rate performance (150 mAh g-1 at 5 A g-1) and long-term cycling stability (238 mAh g-1 after 1000 cycles at 1 A g-1) are exhibited by WKMO, which are 42 % and 209 % higher than the original material, respectively. The reaction mechanism of H+/Zn2+ stepwise insertion/extraction is elucidated through physical and chemical characterization. The strategy of strengthening H+ migration and mitigating ZSH deposition via multi-element modulation offers a novel approach for fostering long-life Zn//MnO2 batteries.

Enhanced Kinetics and Stability of Zn-MnO2 Batteries with a Multifunctional TiO2 Coating

Zinc-ion batteries are a promising energy storage alternative, offering safety, cost-effectiveness, and environment-friendliness. MnO2 is appealing for its high capacity and output voltage, but it suffers from slow kinetics and poor stability due to severe Mn dissolution during cycling. Here, the performance of MnO2 is enhanced by coating it with a uniform TiO2 nanolayer that incorporates oxygen vacancies. The TiO2-MnO2 heterogeneous interface results in the formation of Ti─O─Mn bonds and a reduction in the interfacial valence state, thereby leading to the creation of an interface electron-enriched region that facilitates faster electron and ion transport. This multifunctional TiO2 coating not only promotes proton-dominated electrochemical reactions and ion diffusion but also acts as a protective barrier, preventing Mn dissolution and buffering volume changes during cycling. Consequently, the MnO2@TiO2 cathode demonstrates excellent specific capacity (299 mAh g-1 at 0.1 A g-1) and cycling stability, achieving 91.4% capacity retention after 2500 cycles at 1 A g-1 and 92.7% capacity retention after 600 cycles at a low current density of 0.2 A g-1. These results outperform many previously reported manganese-based cathodes, demonstrating MnO2@TiO2's potential as a high-performance and durable cathode material for zinc-ion batteries and advancing the development of efficient energy storage solutions.

Tunable Organic-Inorganic p-π-d Electron Conjugation Triggers d-π Hybridization in Quinonized MnO2 Superlattice toward Ultrastable and High-Rate Zn-MnO2 Batteries

Zn-MnO2 batteries with two-electron transfer harvest high energy density, high working voltage, inherent safety, and cost-effectiveness. Zn2+ as the dominant charge carriers suffer from sluggish kinetics due to the strong Zn2+-MnO2 coulombic interaction, which is also the origin of pestilent MnO2 lattice deformation and performance degradation. Current studies particularly involve H+ insertion-dominating chemistry, where the long-term cycle stability remains challenging due to the accumulative Zn2+ insertion and structural collapse. Herein, a simultaneously enhanced and stabilized Zn2+/H+ co-insertion chemistry is proposed by the quinone-hybridized MnO2 superlattice, a first-of-this-kind structure with a distinctive organic-inorganic-extended p-π-d conjugation, which enables a tunable interlayer d-π hybridization. Theoretical and experimental results substantiate that the interlayer d-π hybridization triggers the enhancement of polarons occupancy near Fermi level, the downward shift of O p-band center, the elevated Mn t2g occupation and thus improved [MnO6] stability upon unprecedentedly high Zn2+ contribution. The notable d-π hybridization endows MnO2 superlattice an ultrahigh specific capacity (435.9 mAh g-1 at 0.25 A g-1), state-of-the-art cycle stability (~100 % capacity retention after 30,000 cycles at 10 A g-1) with substantially enhanced rate performance. Our findings enlighten a new paradigm in the adjustment of Zn2+/H+ co-insertion chemistry towards high-performance rechargeable aqueous batteries.

MnO2 superstructure cathode with boosted zinc ion intercalation for aqueous zinc ion batteries

The simultaneous intercalation of protons and Zn2+ ions in aqueous electrolytes presents a significant obstacle to the widespread adoption of aqueous zinc ion batteries (AZIBs) for large-scale use, a challenge that has yet to be overcome. To address this, we have developed a MnO2/tetramethylammonium (TMA) superstructure with an enlarged interlayer spacing, designed specifically to control H+/Zn2+ co-intercalation in AZIBs. Within this superstructure, the pre-intercalated TMA+ ions work as spacers to stabilize the layered structure of MnO2 cathodes and expand the interlayer spacing substantially by 28 % to 0.92 nm. Evidence from in operando pH measurements, in operando synchrotron X-ray diffraction, and X-ray absorption spectroscopy shows that the enlarged interlayer spacing facilitates the diffusion and intercalation of Zn2+ ions (which have a large ionic radius) into the MnO2 cathodes. This spacing also helps suppress the competing H+ intercalation and the formation of detrimental Zn4(OH)6SO4·5H2O, thereby enhancing the structural stability of MnO2. As a result, enhanced Zn2+ storage properties, including excellent capacity and long cycle stability, are achieved.

Berlin Green with tunable iron content as ultra-high rate host for efficient aqueous ammonium ion storage

Prussian blue analogues (PBAs) are regarded as promising cathode materials for ammonium-ion batteries (AIBs) because of their low cost and superb theoretical capacity. However, its inherently poor conductivity and structural collapse can significantly limit the enhancement of rate property and cycling stability. In this work, Berlin Green (BG) electrode materials with similar wool-like clusters were constructed by direct precipitation method to accelerate the kinetic, which realizes outstanding cycling stability. Berlin Green with the appropriate amount of iron (BG-2) has a fast ion transport channel, enhanced structure stability, highly reversible insertion/extraction of NH4+, and fine electrochemical reaction activity. Benefiting from the unique architecture and component, the BG-2 electrode shows an excellent rate performance with a discharge/charge specific capacity of 60.1/59.3 mAh g-1 at 5 A g-1. Even at 5 A g-1, BG-2 exhibits remarkable cycling stability with an initial discharge capacity of 59.5 mAh g-1 and a capacity retention rate of approximately 76% after 30,000 cycles. The BG-2 reveals exceedingly good electrochemical reversibility during the process of NH4+ (de)insertion. BG materials indicate huge potential as a cathode material for the next generation of high-performance aqueous batteries.

Proton-selective coating enables fast-kinetics high-mass-loading cathodes for sustainable zinc batteries

The pressing demand for sustainable energy storage solutions has spurred the burgeoning development of aqueous zinc batteries. However, kinetics-sluggish Zn2+ as the dominant charge carriers in cathodes leads to suboptimal charge-storage capacity and durability of aqueous zinc batteries. Here, we discover that an ultrathin two-dimensional polyimine membrane, featured by dual ion-transport nanochannels and rich proton-conduction groups, facilitates rapid and selective proton passing. Subsequently, a distinctive electrochemistry transition shifting from sluggish Zn2+-dominated to fast-kinetics H+-dominated Faradic reactions is achieved for high-mass-loading cathodes by using the polyimine membrane as an interfacial coating. Notably, the NaV3O8·1.5H2O cathode (10 mg cm-2) with this interfacial coating exhibits an ultrahigh areal capacity of 4.5 mAh cm-2 and a state-of-the-art energy density of 33.8 Wh m-2, along with apparently enhanced cycling stability. Additionally, we showcase the applicability of the interfacial proton-selective coating to different cathodes and aqueous electrolytes, validating its universality for developing reliable aqueous batteries.

Advances of Zn Metal-Free "Rocking-Chair"-Type Zinc Ion Batteries: Recent Developments and Future Perspectives

Aqueous zinc ion battery (AZIBs) has attracted the attention of many researchers because of its safety, economy, environmental protection, and high ionic conductivity of electrolytes. However, the battery greatly suffers from zinc dendrite produced by zinc metal anode leading to poor cycle life and even unsafe problems, which limit its further development for various important applications. It is known that the success of the commercialization of lithium-ion batteries (LIBs) is mainly due to replacement of lithium metal anode with graphite, which avoids the formation of Li dendrite. Therefore, it is an important step to develop aqueous zinc ion anode to replace conventional zinc metal one with zinc-metal free anode material. In this review, the working principle and development prospect of "rocking-chair" AZIBs are introduced. The research progress of different types of zinc metal-free anode materials and cathode materials in "rocking-chair" AZIBs is reviewed. Finally, the limitations and challenges of the Zn metal-free "rocking-chair" AZIBs as well as solutions are deeply discussed, aiming to provide new strategies for the development of advanced zinc-ion batteries.

Recent development of manganese dioxide-based materials as zinc-ion battery cathode

The development of advanced cathode materials for zinc-ion batteries (ZIBs) is a critical step in building large-scale green energy conversion and storage systems in the future. Manganese dioxide is one of the most well-studied cathode materials for zinc-ion batteries due to its wide range of crystal forms, cost-effectiveness, and well-established synthesis processes. This review describes the recent research progress of manganese dioxide-based ZIBs, and the reaction mechanism, electrochemical performance, and challenges of manganese dioxide-based ZIBs materials are systematically introduced. Optimization strategies for high-performance manganese dioxide-based materials for ZIBs with different crystal forms, nanostructures, morphologies, and compositions are discussed. Finally, the current challenges and future research directions of manganese dioxide-based cathodes in ZIBs are envisaged.

Zn-ion Batteries: Charge Storing Mechanism and Development Challenges

Improving the energy share of renewable energy technologies is the only solution to reduce greenhouse gas emissions and air pollution. The high-performing green battery energy storage technologies are critical for storing energy to address the intermittent nature of renewable energy resources. In recent years, aqueous batteries, particularly Zn-ion batteries (ZIBs), have achieved and shown great potential for stationary energy storage systems owing to their low cost and safer operation. However, the practical applications of the ZIBs have significantly been impeded due to the gap between the breakthroughs achieved in academic research and industrial developments. The present review discusses the ZIB's advantages, possibilities, and shortcomings for stationary energy storage systems. The Review begins with a brief introduction to the ZIBs and their charge storage mechanisms based on the structural properties of cathode materials. The scientific and technical challenges that obstruct the commercialization of the ZIBs are discussed in detail concerning their impact on accelerating the utilization of the ZIBs for real-life applications. The final section highlights the outlook on research in this flourishing field.

Failure Mechanism, Electrolyte Design, and Electrolyte/Electrode Interface Regulation for Low-Temperature Zinc-Based Batteries

With more renewable energy developed to satisfy the human need in the energy crisis, electricity storage is critical in power utilization and storage. Due to its high safety, high nature reserve, and high energy density, the zinc-based battery is drawing increasing attention. Together with the expansion of human activities, the low-temperature battery is developed to satisfy the power demand in extreme environments, and as a critical component, electrolytes shall have a low freezing point and satisfying electrochemical properties in cold conditions. In this review, the development of low-temperature electrolytes for zinc-based batteries will be comprehensively introduced. First, the failure mechanism of zinc-based battery at low temperature will be illustrated. Second, five main types of low-temperature electrolytes will be introduced in detail. Finally, the regulation of electrolyte/electrode surface at low temperature will be discussed. This review aims to provide a guideline for low-temperature electrolyte design from the perspective of molecular behavior regulation.

An Artificial MnWO4 Cathode Electrolyte Interphase Enabling Enhanced Electrochemical Performance of δ-MnO2 Cathode for Aqueous Zinc Ion Battery

The dissolution of active material in aqueous batteries can lead to a rapid deterioration in capacity, and the presence of free water can also accelerate the dissolution and trigger some side reactions that affect the service life of aqueous batteries. In this study, a MnWO₄ cathode electrolyte interphase (CEI) layer is constructed on a δ-MnO₂ cathode by cyclic voltammetry, which is effective in inhibiting the dissolution of Mn and improving the reaction kinetics. As a result, the CEI layer enables the δ-MnO₂ cathode to produce a better cycling performance, with the capacity maintained at 98.2% (vs. activated capacity at 500 cycles) after 2000 cycles at 10 A g^-1. In comparison, the capacity retention rate is merely 33.4% for pristine samples in the same state, indicating that this MnWO₄ CEI layer constructed by using a simple and general electrochemical method can promote the development of MnO₂ cathodes for aqueous zinc ion batteries.

Challenges and Strategies in the Development of Zinc-Ion Batteries

Although promising, the practical use of zinc-ion batteries (ZIBs) remains plagued with uncontrollable dendrite growth, parasitic side reactions, and the high intercalation energy of divalent Zn²⁺ ions. Hence, much work has been conducted to alleviate these issues to maximize the energy density and cyclic life of the cell. In this holistic review, the mechanisms and rationale for the stated challenges shall be summarized, followed by the corresponding strategies employed to mitigate them. Thereafter, a perspective on present research and the outlook of ZIBs would be put forth in hopes to enhance their electrochemical properties in a multipronged approach.

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.