Polyoxometalates@Metal-Organic Frameworks Derived Bimetallic Co/Mo2 C Nanoparticles Embedded in Carbon Nanotube-Interwoven Hierarchically Porous Carbon Polyhedron Composite as a High-Efficiency Electrocatalyst for Al-S Batteries

Al-S battery (ASB) is a promising energy storage device, notable for its safety, crustal abundance, and high theoretical energy density. However, its development faces challenges due to slow reaction kinetics and poor reversibility. The creation of a multifunctional cathode material that can both adsorb polysulfides and accelerate their conversion is key to advancing ASB. Herein, a composite composed of polyoxometalate nanohybridization-derived Mo2 C and N-doped carbon nanotube-interwoven polyhedrons (Co/Mo2 C@NCNHP) is proposed for the first time as an electrochemical catalyst in the sulfur cathode. This composite improves the utilization and conductivity of sulfur within the cathode. DFT calculations and experimental results indicate that Co enables the chemisorption of polysulfides while Mo2 C catalyzes the reduction reaction of long-chain polysulfides. X-ray photoelectron spectroscopy (XPS) and in situ UV analysis reveal the different intermediates of Al polysulfide species in Co/Mo2 C@NCNHP during discharging/charging. As a cathode material for ASB, Co/Mo2 C@NCNHP@S composite can deliver a discharge-charge voltage hysteresis of 0.75 V with a specific capacity of 370 mAh g-1 after 200 cycles at 1A g-1 .

High Performance and Long-cycle Life Rechargeable Aluminum Ion Battery: Recent Progress, Perspectives and Challenges

The rising energy crisis and environmental concerns caused by fossil fuels have accelerated the deployment of renewable and sustainable energy sources and storage systems. As a result of immense progress in the field, cost-effective, high-performance, and long-life rechargeable batteries are imperative to meet the current and future demands for sustainable energy sources. Currently, lithium-ion batteries are widely used, but limited lithium (Li) resources have caused price spikes, threatening progress toward cleaner energy sources. Therefore, post-Li, batteries that utilize highly abundant materials leading to cost-effective energy storage solutions while offering desirable performance characteristics are urgently needed. Aluminum-ion battery (AIB) is an attractive concept that uses highly abundant aluminum while offering a high theoretical gravimetric and volumetric capacity of 2980 mAh g^-1 and 8046 mAh cm^-3 , respectively. As a result, intensified efforts have been made in recent years to utilize numerous electrolytes, anodes, and cathode materials to improve the electrochemical performance of AIBs, and potentially create high-performance, low-cost, and safe energy storage devices. Herein, recent progress in the electrolyte, anode, and cathode active materials and their utilization in AIBs and their related characteristics are summarized. Finally, the main challenges facing AIBs along with future directions are highlighted.

Multifunctional Quasi-Solid-State Zinc-Sulfur Battery

The introduction of structural energy storage devices into emerging markets, such as electric vehicles, is predominately hindered by weak energy density, safety concerns, and immaturity of the field in materials. Herein, fabrication and testing of a freeze-resistant, multifunctional quasi-solid-state zinc-sulfur battery (ZnS) are reported. To this end, an electrostatic spray coating technique was used to deposit a thin layer of sulfur on the highly porous, unidirectional activated carbon nanofibers (A-CNFs) as a load-bearing cathode. This technique could fill micro- and mesopores, and microsized channels with sulfur, achieving an extensive sulfur loading of 60 wt %. Several drawbacks of structural energy storage devices (applicability under varied climate conditions, poor electrochemical performance and mechanical properties) are addressed by initiating an antifreezing hydrogel electrolyte with a failure strain of over 200%. This electrolyte possesses ethylene glycol and an I2 additive as an antifreezing agent and redox mediator, respectively. The as-assembled ZnS battery offers a high energy density of 283 Wh/kg based on the CNF-S cathode (149 Wh/kg based on the ZnS cell) and mechanical properties beyond state-of-the-art structural energy storage devices with a tensile strength of 377 MPa, Young's modulus of 16.7 GPa, and energy-to-failure of 4.5 MJ/m3. The electrochemomechanical properties of the ZnS battery were also investigated to elucidate the effects of electrochemical energy storage on mechanical properties and vice versa. Overall, the ZnS battery outperforms state-of-the-art structural energy storage devices in terms of energy storage and load-bearing capabilities.

The Negative-Charge-Triggered "Dead Zone" between Electrode and Current Collector Realizes Ultralong Cycle Life of Aluminum-Ion Batteries

Typically, volume expansion of the electrodes after intercalation of active ions is highly undesirable yet inetvitable, and it can significantly reduce the adhesion force between the electrodes and current collectors. Especially in aluminum-ion batteries (AIBs), the intercalation of large-sized AlCl₄ ^- can greatly weaken this adhesion force and result in the detachment of the electrodes from the current collectors, which seems an inherent and irreconcilable problem. Here, an interesting concept, the "dead zone", is presented to overcome the above challenge. By incorporating a large number of OH^- and COOH^- groups onto the surface of MXene film, a rich negative-charge region is formed on its surface. When used as the current collector for AIBs, it shields a tiny area of the positive electrode (adjacent to the current collector side) from AlCl₄ ^- intercalation due to the repulsion force, and a tiny inert layer (dead zone) at the interface of the positive electrode is formed, preventing the electrode from falling off the current collector. This helps to effectively increase the battery's cycle life to as high as 50 000 times. It is believed that the proposed concept can be an important reference for future development of current collectors in rocking chair batteries.

Atomically Dispersed Iron Active Sites Promoting Reversible Redox Kinetics and Suppressing Shuttle Effect in Aluminum-Sulfur Batteries

Rechargeable aluminum-sulfur (Al-S) batteries have been considered as a highly potential energy storage system owing to the high theoretical capacity, good safety, abundant natural reserves, and low cost of Al and S. However, the research progress of Al-S batteries is limited by the slow kinetics and shuttle effect of soluble polysulfides intermediates. Herein, an interconnected free-standing interlayer of iron single atoms supported on porous nitrogen-doped carbon nanofibers (FeSAs-NCF) on the separator is developed and used as both catalyst and chemical barrier for Al-S batteries. The atomically dispersed iron active sites (Fe-N4) are clearly identified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption near-edge structure. The Al-S battery with the FeSAs-NCF shows an improved specific capacity of 780 mAh g-1 and enhanced cycle stability. As evidenced by experimental and theoretical results, the atomically dispersed iron active centers on the separator can chemically adsorb the polysulfides and accelerate reaction kinetics to inhibit the shuttle effect and promote the reversible conversion between aluminum polysulfides, thus improving the electrochemical performance of the Al-S battery. This work provides a new way that can not only promote the conversion of aluminum sulfides but also suppress the shuttle effect in Al-S batteries.

Electrocatalysis for Continuous Multi-Step Reactions in Quasi-Solid-State Electrolytes Towards High-Energy and Long-Life Aluminum-Sulfur Batteries

Aluminum-sulfur (Al-S) batteries of ultrahigh energy-to-price ratios are a promising energy storage technology, while they suffer from a large voltage gap and short lifespan. Herein, we propose an electrocatalyst-boosting quasi-solid-state Al-S battery, which involves a sulfur-anchored cobalt/nitrogen co-doped graphene (S@CoNG) positive electrode and an ionic-liquid-impregnated metal-organic framework (IL@MOF) electrolyte. The Co-N₄ sites in CoNG continuously catalyze the breaking of Al-Cl and S-S bonds and accelerate the sulfur conversion, endowing the Al-S battery with a shortened voltage gap of 0.43 V and a high discharge voltage plateau of 0.9 V. In the quasi-solid-state IL@MOF electrolytes, the shuttle effect of polysulfides has been inhibited, which stabilizes the reversible sulfur reaction, enabling the Al-S battery to deliver 820 mAh g^-1 specific capacity and 78 % capacity retention after 300 cycles. This finding offers novel insights to design Al-S batteries for stable energy storage.

An Efficient Rechargeable Aluminium-Amine Battery Working Under Quaternization Chemistry

Rechargeable aluminium (Al) batteries (RABs) have long-been pursued due to the high sustainability and three-electron-transfer properties of Al metal. However, limited redox chemistry is available for rechargeable Al batteries, which restricts the exploration of cathode materials. Herein, we demonstrate an efficient Al-amine battery based on a quaternization reaction, in which nitrogen (radical) cations (R3 N.+ or R4 N+ ) are formed to store the anionic Al complex. The reactive aromatic amine molecules further oligomerize during cycling, inhibiting amine dissolution into the electrolyte. Consequently, the constructed Al-amine battery exhibits a high reversible capacity of 135 mAh g-1 along with a superior cycling life (4000 cycles), fast charge capability and a high energy efficiency of 94.2 %. Moreover, the Al-amine battery shows excellent stability against self-discharge, far beyond conventional Al-graphite batteries. Our findings pave an avenue to advance the chemistry of RABs and thus battery performance.

Understanding the Oxidation and Reduction Reactions of Sulfur in Rechargeable Aluminum-Sulfur Batteries with Deep Eutectic Solvent and Ionic Liquid Electrolytes

Al-based batteries are promising next-generation rechargeable batteries owing to the abundance of raw materials and their high potential energy density. The Al-S system has attracted considerable attention because of its high energy density and low cost. However, its low discharge voltage plateau (0.6-1.2 V) hampers its practical application. Herein, eight ionic liquids or deep eutectic solvents were studied as electrolyte candidates for an Al-S cell. This was the first study to demonstrate that an Al-S cell based on an AlCl₃ /acetamide electrolyte (1.3 molar ratio) showed high discharge voltage plateaus (1.65-1.95 V) and a charging cut-off voltage of 2.5 V in Al-S cells. An Al-S cell of 0.33 mAh capacity with the AlCl₃ /acetamide electrolyte successfully lit up a red LED (forward voltage 1.6-2.0 V) for around 2 h. This work may help in promoting the development of high-performance and low-cost Al-S cells.

Challenges and Strategies of Low-Cost Aluminum Anodes for High-Performance Al-Based Batteries

The ever-growing market of electric vehicles and the upcoming grid-scale storage systems have stimulated the fast growth of renewable energy storage technologies. Aluminum-based batteries are considered one of the most promising alternatives to complement or possibly replace the current lithium-ion batteries owing to their high specific capacity, good safety, low cost, light weight, and abundant reserves of Al. However, the anode problems in primary and secondary Al batteries, such as, self-corrosion, passive film, and volume expansion, severely limit the batteries' practical performance, thus hindering their commercialization. Herein, an overview of the currently emerged Al-based batteries is provided, that primarily focus on the recent research progress for Al anodes in both primary and rechargeable systems. The anode reaction mechanisms and problems in various Al-based batteries are discussed, and various strategies to overcome the challenges of Al anodes, including surface oxidation, self-corrosion, volume expansion, and dendrite growth, are systematically summarized. Finally, future research perspectives toward advanced Al batteries with higher performance and better safety are presented.

Facilitating Charge Reactions in Al-S Batteries with Redox Mediators

The Al-S battery is a promising next-generation battery candidate due to high abundance of both aluminium and sulfur. However, the sluggish kinetics of the Al-S battery reactions produces very high overpotentials. Here, for the first time, it was demonstrated that the incorporation of redox mediators could dramatically improve the kinetics of Al-S batteries. On the example of iodide redox mediators, it was shown that the charging voltage of Al-S batteries could be decreased by about 0.23 V with as little as 2.3 wt% of redox mediator added as electrolyte additive. Control electrochemical measurements, without prior discharge of the battery, demonstrated that >97 % of the charge capacity was due to the desired oxidation of Al2 S3 and polysulfides, and X-ray diffraction experiments confirmed the formation of sulfur as the final charge product. The beneficial role of redox mediators was demonstrated with cheap and environmentally friendly electrolytes made of urea and AlCl3 . This work showed that dramatic performance improvements could be achieved with low concentration of electrolyte additives, and therefore, much further performance improvements could be sought by combining multiple additives.

Ionic Liquid Electrolytes for Electrochemical Energy Storage Devices

For decades, improvements in electrolytes and electrodes have driven the development of electrochemical energy storage devices. Generally, electrodes and electrolytes should not be developed separately due to the importance of the interaction at their interface. The energy storage ability and safety of energy storage devices are in fact determined by the arrangement of ions and electrons between the electrode and the electrolyte. In this paper, the physicochemical and electrochemical properties of lithium-ion batteries and supercapacitors using ionic liquids (ILs) as an electrolyte are reviewed. Additionally, the energy storage device ILs developed over the last decade are introduced.

Aluminum and lithium sulfur batteries: a review of recent progress and future directions

Advanced materials with various micro-/nanostructures have attracted plenty of attention for decades in energy storage devices such as rechargeable batteries (ion- or sulfur based batteries) and supercapacitors. To improve the electrochemical performance of batteries, it is uttermost important to develop advanced electrode materials. Moreover, the cathode material is also important that it restricts the efficiency and practical application of aluminum-ion batteries. Among the potential cathode materials, sulfur has become an important candidate material for aluminum-ion batteries cause of its considerable specific capacity. Two-dimensional materials are currently potential candidates as electrodes from lab-scale experiments to possible pragmatic theoretical studies. In this review, the fundamental principles, historical progress, latest developments, and major problems in Li-S and Al-S batteries are reviewed. Finally, future directions in terms of the experimental and theoretical applications have prospected.

Rechargeable Aluminium-Sulfur Battery with Improved Electrochemical Performance by Cobalt-Containing Electrocatalyst

The rechargeable aluminium-sulfur (Al-S) battery is regarded as a potential alternative beyond lithium-ion battery system owing to its safety, promising energy density, and the high earth abundance of the constituent electrode materials, however, sluggish kinetic response and short life-span are the major issues that limit the battery development towards applications. In this article, we report Co^II,III as an electrochemical catalyst in the sulfur cathode that renders a reduced discharge-charge voltage hysteresis and improved capacity retention and rate capability for Al-S batteries. The structural and electrochemical analysis suggest that the catalytic effect of Co^II,III is closely associated with the formation of cobalt sulfides and the changes in the valence states of the Co^II,III during the electrochemical reactions of the sulfur species, which lead to improved reaction kinetics and sulfur utilization in the cathode. The Al-S battery, assembled with the cathode consisting of Co^II,III decorated carbon matrix, demonstrates a considerably reduced voltage hysteresis of 0.8 V, a reversible specific capacity of ≈500 mAh g^-1 at 1 A g^-1 after 200 discharge-charge cycles and of ≈300 mAh g^-1 at 3 A g^-1 .

Electrochemical Performance of Graphitic Multi-walled Carbon Nanotubes with Different Aspect Ratios as Cathode Materials for Aluminum-ion Batteries

Graphitic multi-walled carbon nanotubes (MWCNTs) can function as high-performance cathode materials for rechargeable Al-ion batteries with well-defined discharging plateaus and reasonable charge/discharge C-rates. However, the main intercalation/deintercalation or adsorption/desorption path of AlCl4- anions into or onto G-MWCNTs has not been elucidated. Herein, we used battery cells comprised of G-MWCNTs with different aspect ratios, Al metal, and AlCl3/1-ethyl-3-methylimidazolium chloride ionic liquid as the cathode, anode, and electrolyte, respectively. The electrochemical performance of the Al||G-MWCNT cell increased as the aspect ratio of the G-MWCNT cathode increased (i. e., longer and thinner). The degree of defects of the G-MWCNTs was similar (0.15-0.22); hence, the results confirm that the main and alternate paths for the AlCl4- intercalation/de-intercalation or adsorption/desorption into/from or onto/from the G-MWCNT are the basal and edge planes, respectively. The step-like structures of defects on the basal plane provide the main reaction site for AlCl4- anions.

A Critical Evaluation of the Effect of Electrode Thickness and Side Reactions on Electrolytes for Aluminum-Sulfur Batteries

The high abundance and low cost of aluminum and sulfur make the Al-S battery an attractive combination. However, significant improvements in performance are required, and increasing the thickness and sulfur content of the sulfur electrodes is critical for the development of batteries with competitive specific energies. This work concerns the development of sulfur electrodes with the highest sulfur content (60 wt %) reported to date for an Al-S battery system and a systematic study of the effect of the sulfur electrode thickness on battery performance. If low-cost electrolytes made from acetamide or urea are used, slow mass transport of the electrolyte species is identified as the main cause of the poor sulfur utilization when the electrode thickness is decreased, whereas complete sulfur utilization is achieved with a less viscous ionic liquid. In addition, the analysis of very thin electrodes reveals the occurrence of degradation reactions in the low-cost electrolytes. The new analysis method is ideal for evaluating the stability and mass transport limitations of novel electrolytes for Al-S batteries.

Controlled growth and ion intercalation mechanism of monocrystalline niobium pentoxide nanotubes for advanced rechargeable aluminum-ion batteries

Rechargeable aluminum-ion batteries (RAIBs) have attracted increasing attention owing to their high theoretical volumetric capacity, high resource abundance, and good safety performance. However, the existing RAIB systems usually exhibit relatively low specific capacities limited by the cathode materials. In this study, we developed a one-step chemical vapor deposition method to prepare single-crystal orthogonal Nb2O5 nanotubes for serving as high-performance electrode materials for RAIBs, showing a high reversible capability of 556 mA h g-1 at 25 mA g-1 and good thermal endurability at elevated temperatures (50 °C). A combination of a series of detailed ex situ structural characterization studies verified the reversible intercalation/deintercalation of chloroaluminate anions (AlCl4-) into/from the (001) planes of monocrystalline Nb2O5 nanotubes. It also revealed that the nanoarchitecture of Nb2O5 nanotubes with thin tube walls, hollow inner space and a short ion transport distance is conducive to the rapid kinetics of the insertion/extraction process. This work provides a promising route to design high-performance electrode materials based on transition metal compounds for RAIBs via the rational modulation of their structure and morphology.

Two-dimensional composite of D-Ti3C2Tx@S@TiO2 (MXene) as the cathode material for aluminum-ion batteries

MXenes, the two-dimensional layered materials, are widely used in electrochemical storage devices and exhibit excellent electrochemical performances. Aluminum-ion batteries are favored by researchers around the world due to our urgent need for emerging clean energy; the study of cathode materials is most important in aluminum-ion battery research. Herein, we have prepared a two-dimensional layered composite of D-Ti₃C₂T_x@S@TiO₂ by a simple method, which shows excellent electrochemical performance in aluminum-ion batteries. The discharge specific capacity is 151.3 mA h g^-1 after 120 cycles, which is about 80.0 mA h g^-1 higher than that of Ti₃C₂T_x in aluminum-ion batteries, and the capacity retention rate is 72.3%. The Coulomb efficiency is about 85% during steady cycling. The reasons for its excellent electrochemical performances are explored herein, and it is proved by DFT calculations that Ti₃C₂S is more stable than Ti₃C₂OH in aluminum-ion batteries. The elemental sulfur reacted with the exposed Ti-ions to form some Ti-S bonds, which play a supporting role in maintaining the original structure of the material and preventing the superposition of layers between the materials. Some TiO₂ nanoparticles were grown in situ on the surface of the material by further oxidation treatment, which improved the stability of the material. In addition, we studied the charge/discharge mechanism of the aluminum-ion battery using Ti₃C₂T_x as the cathode material based on the changes in the contents of Al and Cl, and the change in the valence of Ti-ions. The results show that [AlCl₄]^- was intercalated/de-intercalated in the layers of the cathode material during the charge/discharge processes.

A rechargeable aqueous aluminum–sulfur battery through acid activation in water-in-salt electrolyte

A water-in-salt electrolyte is employed to give rechargeable aqueous Al–S batteries.

A Four-Electron Sulfur Electrode Hosting a Cu2+ /Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu₂ S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g^-1 based on the sulfur mass or 609 mAh g^-1 based on the mass of Cu₂ S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg^-1 based on the mass of the Cu₂ S/carbon composite cathode, and stable cycling over 110 cycles.

The Rechargeable Aluminum Battery: Opportunities and Challenges

Aluminum battery systems are considered as a system that could supplement current lithium batteries due to the low cost and high volumetric capacity of aluminum metal, and the high safety of the whole battery system. However, first the use of ionic liquid electrolytes leading to AlCl₄ ^- instead of Al³⁺ , the different intercalation reagents, the sluggish solid diffusion process and the fast capacity fading during cycling in aluminum batteries all need to be thoroughly explored. To provide a good understanding of the opportunities and challenges of the newly emerging aluminum batteries, this Review discusses the reaction mechanisms and the difficulties caused by the trivalent reaction medium in electrolytes, electrodes, and electrode-electrolyte interfaces. It is hoped that the Review will stimulate scientists and engineers to develop more reliable aluminum batteries.

Polyacrylonitrile Separator for High-Performance Aluminum Batteries with Improved Interface Stability

Herein we report, for the first time, an overall evaluation of commercially available battery separators to be used for aluminum batteries, revealing that most of them are not stable in the highly reactive 1-ethyl-3-methylimidazolium chloride:aluminum trichloride (EMIMCl:AlCl₃) electrolyte conventionally employed in rechargeable aluminum batteries. Subsequently, a novel highly stable polyacrylonitrile (PAN) separator obtained by the electrospinning technique for application in high-performance aluminum batteries has been prepared. The developed PAN separator has been fully characterized in terms of morphology, thermal stability, and air permeability, revealing its suitability as a separator for battery applications. Furthermore, extremely good compatibility and improved aluminum interface stability in the highly reactive EMIMCl:AlCl₃ electrolyte were discovered. The use of the PAN separator strongly affects the aluminum dissolution/deposition process, leading to a quite homogeneous deposition compared to that of a glass fiber separator. Finally, the applicability of the PAN separator has been demonstrated in aluminum/graphite cells. The electrochemical tests evidence the full compatibility of the PAN separator in aluminum cells. Furthermore, the aluminum/graphite cells employing the PAN separator are characterized by a slightly higher delivered capacity compared to those employing glass fiber separators, confirming the superior characteristics of the PAN separator as a more reliable separator for the emerging aluminum battery technology.