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

Polyoxomolybdate-Based Metal-Organic Framework-Derived Cu-Embedded Molybdenum Dioxide Hybrid Nanoparticles as Highly Efficient Electrocatalysts for Al-S Batteries

High-performance rechargeable aluminum-sulfur batteries (RASB) have great potential for various applications owing to their high theoretical capacity, abundant sulfur resources, and good safety. Nevertheless, the practical application of RASB still faces several challenges, including the polysulfide shuttle phenomenon and low sulfur utilization efficiency. Here, we first developed a synergistic copper heterogeneous metal oxide MoO2 derived from polymolybdate-based metal-organic framework as an efficient catalyst for mitigating polysulfide diffusion. This composite enhances sulfur utilization and electrical conductivity of the cathode. DFT calculations and experimental results reveal the catalyst Cu/MoO2@C not only effectively anchors aluminum polysulfides (AlPSs) to mitigate the shuttle effect, but also significantly promotes the catalytic conversion of AlPSs on the sulfur cathode side during charging and discharging. The unique nanostructure contains abundant electrocatalytic active sites of oxide nanoparticles and Cu clusters, resulting in excellent electrochemical performance. Consequently, the established RASB exhibits an initial capacity of 875 mAh g-1 at 500 mA g-1 and maintains a capacity of 967 mAh g-1 even at a high temperature of 50 °C.

Locally Concentrated Ionic Liquid Electrolytes for Wide-Temperature-Range Aluminum-Sulfur Batteries

Aluminum-sulfur (Al-S) batteries are promising energy storage devices due to their high theoretical capacity, low cost, and high safety. However, the high viscosity and inferior ion transport of conventionally used ionic liquid electrolytes (ILEs) limit the kinetics of Al-S batteries, especially at sub-zero temperatures. Herein, locally concentrated ionic liquid electrolytes (LCILE) formed via diluting the ILEs with non-solvating 1,2-difluorobenzene (dFBn) co-solvent are proposed for wide-temperature-range Al-S batteries. The addition of dFBn effectively promotes the fluidity and ionic conductivity without affecting the AlCl4 - /Al2 Cl7 - equilibrium, which preserves the reversible stripping/plating of aluminum and further promotes the overall kinetics of Al-S batteries. As a result, Al-S cells employing the LCILE exhibit higher specific capacity, better cyclability, and lower polarization with respect to the neat ILE in a wide temperature range from -20 to 40 °C. For instance, Al-S batteries employing the LCILE sustain a remarkable capacity of 507 mAh g-1 after 300 cycles at 20 °C, while only 229 mAh g-1 is delivered with the dFBn-free electrolyte under the same condition. This work demonstrates the favorable use of LCILEs for wide-temperature Al-S batteries.

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 .

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.

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.

A cobalt-based metal-organic framework and its derived material as sulfur hosts for aluminum-sulfur batteries with the chemical anchoring effect

With the urgent need to explore high-performance electrochemical energy storage systems, rechargeable Al-ion batteries (AIBs) have attracted attention from researchers and engineers due to their traits, such as abundance and safety. Among all the issues waiting to be solved, the development of a reliable positive electrode material with high specific capacity is an absolute priority for the commercialization of AIBs. Sulfur has a natural advantage when used as the active material, and its theoretical specific capacity is as high as 1675 mA h g-1. MOFs and MOF-derived materials have been proved to be promising hosts for Li-S batteries. Herein, we report a novel Al-S battery system employing MOF (ZIF-67) and MOF-derived materials as sulfur host materials. After being chemically combined with sulfur, the composite still maintains its unique well-defined polyhedron morphology. The voltage hysteresis phenomenon is effectively alleviated with the aid of the host matrix. DFT calculations confirm that ZIF-67 and carbonized ZIF-67-700 polyhedrons can act as an anchor point towards sulfur (S8) and polysulfides (Al2S3, Al2S6, Al2S12, and Al2S18), preventing the detrimental dissolution and shuttle effect. These findings can enlighten future researchers regarding Al-S batteries and broaden the application of MOFs in the field of electrochemical energy storage systems.