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

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.

A Crystalline-Water Electrolyte Enabled High Depth-of-Discharge Anodes in Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries are regarded as a promising energy storage solution for a green and sustainable society in the future. However, the practical application of metallic zinc anode is plagued by the thermodynamic instability issue of water molecules in conventional electrolytes, which leads to severe dendrite growth and side reactions. In this work, an ultra-thin and high areal capacity metallic zinc anode is achieved by utilizing crystalline water with a stable stoichiometric ratio. Unlike conventional electrolytes, the designed electrolyte can effectively suppress the reactivity of water molecules and diminish the detrimental corrosion on the metallic zinc anode, while preserving the inherent advantages of water molecules, including great kinetic performance in electrolytes and H+ capacity contribution in cathodes. Based on the comprehensive performance of the designed electrolyte, the 10 µm Zn||10 µm Zn symmetric cell stably ran for 1000 h at the current density of 1 mA cm-2, and the areal capacity of 1 mAh cm-2, whose depth-of-discharge is over 17.1%. The electrochemical performance of the 10 µm Zn||9.3 mg cm-2 polyaniline (PANI) full-cell demonstrates the feasibility of the designed electrolyte. This work provides a crucial understanding of balancing activity of water molecules in aqueous zinc metal batteries.

Superhalide-Anion-Motivator Reforming-Enabled Bipolar Manipulation toward Longevous Energy-Type Zn||Chalcogen Batteries

Neutrophilic superhalide-anion-triggered chalcogen conversion-based Zn batteries, despite latent high-energy merit, usually suffer from a short lifespan caused by dendrite growth and shuttle effect. Here, a superhalide-anion-motivator reforming strategy is initiated to simultaneously manipulate the anode interface and Se conversion intermediates, realizing a bipolar regulation toward longevous energy-type Zn batteries. With ZnF2 chaotropic additives, the original large-radii superhalide zincate anion species in ionic liquid (IL) electrolytes are split into small F-containing species, boosting the formation of robust solid electrolyte interphases (SEI) for Zn dendrite inhibition. Simultaneously, ion radius reduced multiple F-containing Se conversion intermediates form, enhancing the interion interaction of charged products to suppress the shuttle effect. Consequently, Zn||Se batteries deliver a ca. 20-fold prolonged lifespan (2000 cycles) at 1 A g-1 and high energy/power density of 416.7 Wh kgSe-1/1.89 kW kgSe-1, outperforming those in F-free counterparts. Pouch cells with distinct plateaus and durable cyclability further substantiate the practicality of this design.

Electrochemical Failure Mechanism of δ-MnO2 in Zinc Ion Batteries Induced by Irreversible Layered to Spinel Phase Transition

Phase transitions of Mn-based cathode materials associated with the charge and discharge process play a crucial role on the rate capability and cycle life of zinc ion batteries. Herein, a microscopic electrochemical failure mechanism of Zn-MnO2 batteries during the phase transitions from δ-MnO2 to λ-ZnMn2O4 is presented via systematic first-principle investigation. The initial insertion of Zn2+ intensifies the rearrangement of Mn. This is completed by the electrostatic repulsion and co-migration between guest and host ions, leading to the formation of λ-ZnMn2O4. The Mn relocation barrier for the λ-ZnMn2O4 formation path with 1.09 eV is significantly lower than the δ-MnO2 re-formation path with 2.14 eV, indicating the irreversibility of the layered-to-spinel transition. Together with the phase transition, the rearrangement of Mn elevates the Zn2+ migration barrier from 0.31 to 2.28 eV, resulting in poor rate performance. With the increase of charge-discharge cycles, irreversible and inactive λ-ZnMn2O4 products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO2 battery.

A salt-concentrated electrolyte for aqueous ammonium-ion hybrid batteries

The development of aqueous ammonium-ion batteries (AAIBs) is currently attracting great attention because of the interesting electrochemical features induced by the charge carrier NH4+. One possible way to improve the performance of AAIBs is increasing the salt concentration in the electrolyte. Yet, few studies focus on the complex electrode-electrolyte interface behaviors in highly concentrated electrolytes, which affect the electrochemical performance of AAIBs significantly. Herein, we aim to understand the impact of CH3COONH4 electrolyte concentration on the NH4+ storage performance of a bimetallic hydroxide material. Experimental and theoretical simulation results indicate that the acetate anion will participate in the construction of the solvated NH4+ in a highly concentrated electrolyte, facilitating the adsorption of the solvated NH4+ cluster on the electrode surface. Besides, a new partial de-solvation model is also proposed, demonstrating an energy favorable de-solvation process. Finally, an ammonium-ion hybrid battery is designed, which provides a high average discharge voltage of 1.7 V and good energy density of 368 W h kg(cathode)-1, outperforming most of the state-of-the-art aqueous batteries. This work provides new understanding about the electrode's interfacial chemistry in different concentrated CH3COONH4 electrolytes, establishes a correlation between the electrolyte concentration and the electrode's performances, and demonstrates the superiority of the hybrid ammonium-ion battery design.

Regulating interfacial reaction through electrolyte chemistry enables gradient interphase for low-temperature zinc metal batteries

In situ formation of a stable interphase layer on zinc surface is an effective solution to suppress dendrite growth. However, the fast transport of bivalent Zn-ions within the solid interlayer remains very challenging. Herein, we engineer the SEI components and enable superior kinetics of Zn metal batteries under harsh conditions through regulating the sequence of interfacial chemical reaction. With the differences in chemical reactivity of trimethyl phosphate co-solvent and trifluoromethanesulfonate anions in the Zn2+-solvation shell, Zn3(PO4)2 and ZnF2 are successively generated on Zn metal surface to form a gradient ZnF2-Zn3(PO4)2 interphase. Mechanistic studies reveal the outer ZnF2 facilitates Zn2+ desolvation and inner Zn3(PO4)2 serves as channels for fast Zn2+ transport, contributing to long-term cycling at subzero temperatures. Impressively, the gradient SEI enables a high lifespan over 7000 hours in Zn symmetric cell and a capacity retention of 86.1% after 12000 cycles in Zn-KVOH full cell at -50 °C.

Supramolecular Engineering of Cathode Materials for Aqueous Zinc-ion Energy Storage Devices: Novel Benzothiadiazole Functionalized Two-Dimensional Olefin-Linked COFs

Two-dimensional covalent organic frameworks (COFs) have emerged as promising materials for energy storage applications exhibiting enhanced electrochemical performance. While most of the reported organic cathode materials for zinc-ion batteries use carbonyl groups as electrochemically-active sites, their high hydrophilicity in aqueous electrolytes represents a critical drawback. Herein, we report a novel and structurally robust olefin-linked COF-TMT-BT synthesized via the aldol condensation between 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4'-(benzothiadiazole-4,7-diyl)dibenzaldehyde (BT), where benzothiadiazole units are explored as novel electrochemically-active groups. Our COF-TMT-BT exhibits an outstanding Zn²⁺ storage capability, delivering a state-of-the-art capacity of 283.5 mAh g^-1 at 0.1 A g^-1 . Computational and experimental analyses reveal that the charge-storage mechanism in COF-TMT-BT electrodes is based on the supramolecularly engineered and reversible Zn²⁺ coordination by the benzothiadiazole units.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

In Situ Electrochemically Oxidative Activation Inducing Ultrahigh Rate Capability of Vanadium Oxynitride/Carbon Cathode for Zinc-Ion Batteries

As a promising candidate for large-scale energy storage, aqueous zinc-ion batteries (ZIBs) still lack cathode materials with large capacity and high rate capability. Herein, a spherical carbon-confined nanovanadium oxynitride with a polycrystalline feature (VN_xO_y/C) was synthesized by the solvothermal reaction and following nitridation treatment. As a cathode material for ZIBs, it is interesting that the electrochemical performance of the VN_xO_y/C cathode is greatly improved after the first charging process viain situ electrochemically oxidative activation. The oxidized VN_xO_y/C delivers a greatly enhanced reversible capacity of 556 mAh g^-1 at 0.2 A g^-1 compared to the first discharge capacity of 130 mAh g^-1 and a high capacity of 168 mAh g^-1 even at 80 A g^-1. The ex situ characterizations verify that the insertion/extraction of Zn²⁺ does not affect the crystal structure of oxidized VN_xO_y/C to promise a stable cycle life (retain 420 mAh g^-1 after 1000 cycles at 10 A g^-1). The experimental analysis further elucidates that charging voltage and H₂O in the electrolyte are curial factors to activate VN_xO_y/C in that the oxygen replaces the partial nitrogen and creates abundant vacancies, inducing a conversion from VN_xO_y/C to VN_x-mO_y+2m/C and then resulting in considerably strengthened rate performance and improved Zn²⁺ storage capability. The study broadens the horizons of fast ion transport and is exceptionally desirable to expedite the application of high-rate ZIBs.

Proton-Reservoir Hydrogel Electrolyte for Long-Term Cycling Zn/PANI Batteries in Wide Temperature Range

Advanced aqueous batteries are promising for next generation flexible devices owing to the high safety, yet still requiring better cycling stability and high capacities in wide temperature range. Herein, a polymeric acid hydrogel electrolyte (PAGE) with 3 M Zn(ClO₄ )₂ was fabricated for high performance Zn/polyaniline (PANI) batteries. With PAGE, even at -35 °C the Zn/Zn symmetrical battery can keep stable for more than 1 500 h under 2 mA cm^-2 , and the Zn/PANI battery can provide ultra-high stable specific capacity of 79.6 mAh g^-1 for more than 70 000 cycles at 15 A g^-1 . This can be mainly ascribed to the -SO₃ ^- H⁺ function group in PAGE. It can generate constant protons and guide the (002) plane formation to accelerate the PANI redox reaction kinetics, increase the specific capacity, and suppress the side reaction and dendrites. This proton-supplying strategy by polymeric acid hydrogel may further propel the development of high performance aqueous batteries.

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.

Synergistic Chaotropic Effect and Cathode Interface Thermal Release Effect Enabling Ultralow Temperature Aqueous Zinc Battery

Although rechargeable zinc-ion batteries are promising candidates for next-generation energy storage devices, their inferior performance at subzero temperatures limits their practical application. Here, a strategy to destroy the H-bond network by adding synergistic chaotropic regents is reported, thus reducing the freezing point of the aqueous electrolyte below -90 °C. Owing to the synergistic chaotropic effect between urea and Zn(ClO₄ )₂ and the thermal release effect on the cathode interface during charging, Zn//VO₂ batteries feature a specific capacity of 111.4 mAh g^-1 and stability after ≈1000 cycles with 81.9% capacity retention at -40 °C. This work demonstrates that the synergistic chaotropic effect and the thermal effect on the interface can effectively widen the operation range of temperature of aqueous electrolytes and maintain fast kinetics, which provides a new design strategy for all-weather aqueous zinc batteries.

Theory-Driven Design of a Cationic Accelerator for High-Performance Electrolytic MnO2 -Zn Batteries

Aqueous electrolytic MnO₂ -Zn batteries are considered as one of the most promising energy-storage devices for their cost effectiveness, high output voltage, and safety, but their electrochemical performance is limited by the sluggish kinetics of cathodic MnO₂ /Mn²⁺ and anodic Zn/Zn²⁺ reactions. To overcome this critical challenge, herein, a cationic accelerator (CA) strategy is proposed based on the prediction of first-principles calculations. Poly(vinylpyrrolidone) is utilized as a model to testify the rational design of the CA strategy. It manifests that the CA effectively facilitates rapid cations migration in electrolyte and adequate charge transfer at electrode-electrolyte interface, benefiting the deposition/dissolution processes of both Mn²⁺ and Zn²⁺ cations to simultaneously improve kinetics of cathodic MnO₂ /Mn²⁺ and anodic Zn/Zn²⁺ reactions. The resulting MnO₂ -Zn battery regulated by CA exhibits large reversible capacities of 455 mAh g^-1 and 3.64 mAh cm^-2 at 20 C, as well as a long lifespan of 2000 cycles with energy density retention of 90%, achieving one of the best overall performances in the electrolytic MnO₂ -Zn batteries. This comprehensive work integrating theoretical prediction with experimental studies provides opportunities to the development of high-performance energy-storage devices.

Surface Transformation Enables a Dendrite-Free Zinc-Metal Anode in Nonaqueous Electrolyte

Significant challenges remain in developing rechargeable zinc batteries mainly because of reversibility problems on zinc-metal anodes. The dendritic growth and hydrogen evolution on zinc electrodes are major obstacles to overcome in developing practical and safe zinc batteries. Here, a dendrite-free and hydrogen-free Zn-metal anode with high Coulombic efficiency up to 99.6% over 300 cycles is realized in a newly designed nonaqueous electrolyte, which comprises an inexpensive zinc salt, zinc acetate, and a green low-cost solvent, dimethyl sulfoxide. Surface transformation on Cu substrate plays a critical role in facilitating the dendrite-free deposition process, which lowers the diffusion energy barrier of the Zn atoms, leading to a uniform and compact thin film for zinc plating. Furthermore, in situ electrochemical atomic force microscopy reveals the plating process via a layer-by-layer growth mechanism and the stripping process through an edge-dissolution mechanism. In addition, Zn||Mo₆ S₈ full cells exhibit excellent electrochemical performance in terms of cycling stability and rate capability. This work presents a new opportunity to develop nonaqueous rechargeable zinc batteries.