The Reverse of Electrostatic Interaction Force for Ultrahigh-Energy Al-Ion batteries.

The two-dimensional (2D) MXenes with sufficient interlayer spacing are promising cathode materials for aluminum-ion batteries (AIBs), yet the electrostatic repulsion effect between the surface negative charges and the active anions (AlCl4 - ) hinders the intercalation of AlCl4 - and is usually ignored. Here, we propose a charge regulation strategy for MXene cathodes to overcome this challenge. By doping N and Co, the zeta potential is gradually transformed from negative (Ti3 C2 Tx ) to near-neutral (Ti3 CNTx ), and finally positive (Ti3 CNTx @Co). Therefore, the electrostatic repulsion force can be greatly weakened between Ti3 CNTx and AlCl4 - , or even formed a strong electrostatic attraction between Ti3 CNTx @Co and AlCl4 - , which can not only accommodate more AlCl4 - ions in the Ti3 CNTx @Co interlayers to increase the capacity, but also solve the stacking and expansion problems. As a result, the optimized Al-MXene battery exhibits an ultrahigh capacity of up to 240 mAh g-1 (2-4 times the capacity of graphite cathode, 60-120 mAh g-1 ) and a potential ultrahigh energy density (432 Wh kg-1 , 2-4 times the value of graphite, 110-220 Wh kg-1 ) based on the mass of cathode materials, comparable to LiFePO4 -based lithium-ion batteries (350-450 Wh kg-1 , based on the mass of LiFePO4 ).

Nonaqueous Rechargeable Aluminum Batteries: Progresses, Challenges, and Perspectives.

For significantly increasing the energy densities to satisfy the growing demands, new battery materials and electrochemical chemistry beyond conventional rocking-chair based Li-ion batteries should be developed urgently. Rechargeable aluminum batteries (RABs) with the features of low cost, high safety, easy fabrication, environmental friendliness, and long cycling life have gained increasing attention. Although there are pronounced advantages of utilizing earth-abundant Al metals as negative electrodes for high energy density, such RAB technologies are still in the preliminary stage and considerable efforts will be made to further promote the fundamental and practical issues. For providing a full scope in this review, we summarize the development history of Al batteries and analyze the thermodynamics and electrode kinetics of nonaqueous RABs. The progresses on the cutting-edge of the nonaqueous RABs as well as the advanced characterizations and simulation technologies for understanding the mechanism are discussed. Furthermore, major challenges of the critical battery components and the corresponding feasible strategies toward addressing these issues are proposed, aiming to guide for promoting electrochemical performance (high voltage, high capacity, large rate capability, and long cycling life) and safety of RABs. Finally, the perspectives for the possible future efforts in this field are analyzed to thrust the progresses of the state-of-the-art RABs, with expectation of bridging the gap between laboratory exploration and practical applications.

Design Strategies of High-Performance Positive Materials for Nonaqueous Rechargeable Aluminum Batteries: From Crystal Control to Battery Configuration.

Rechargeable aluminum batteries (RABs) have been paid considerable attention in the field of electrochemical energy storage batteries due to their advantages of low cost, good safety, high capacity, long cycle life, and good wide-temperature performance. Unlike traditional single-ion rocking chair batteries, more than two kinds of active ions are electrochemically participated in the reaction processes on the positive and negative electrodes for nonaqueous RABs, so the reaction kinetics and battery electrochemistries need to be given more comprehensive assessments. In addition, although nonaqueous RABs have made significant breakthroughs in recent years, they are still facing great challenges in insufficient reaction kinetics, low energy density, and serious capacity attenuation. Here, the research progresses of positive materials are comprehensively summarized, including carbonaceous materials, oxides, elemental S/Se/Te and chalcogenides, as well as organic materials. Later, different modification strategies are discussed to improve the reaction kinetics and battery performance, including crystal structure control, morphology and architecture regulation, as well as flexible design. Finally, in view of the current research challenges faced by nonaqueous RABs, the future development trend is proposed. More importantly, it is expected to gain key insights into the development of high-performance positive materials for nonaqueous RABs to meet practical energy storage requirements.

Enhancing Electrochemical Performance of Aluminum-Ion Batteries with Fluorinated Graphene Cathode.

In pursuit of high-performance aluminum-ion batteries, the selection of a suitable positive electrode material assumes paramount importance, and fluorinated graphene (FG) nanostructures have emerged as an exceptional candidate. In the scope of this study, a flexible tantalum foil is coated with FG to serve as the positive electrode for aluminum-ion batteries. FG positive electrode demonstrates a remarkable discharge capacity of 109 mA h g-1 at a current density of 200 mA g-1, underscoring its tremendous potential for energy storage applications. Concurrently, the FG positive electrode exhibits a discharge capacity of 101 mA h g-1 while maintaining an impressive coulombic efficiency of 95 % over 300 cycles at a current density of 200 mA g-1, which benefiting from the significant structure of FG. The results of the in-situ Raman spectroscopy signified the presence of intercalation/de-intercalation processes of AlCl4 - behavior within the FG layers.

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.

Alternate Storage of Opposite Charges in Multisites for High-Energy-Density Al-MOF Batteries.

The limited active sites of cathode materials in aluminum-ion batteries restrict the storage of more large-sized Al-complex ions, leading to a low celling of theoretical capacity. To make the utmost of active sites, an alternate storage mechanism of opposite charges (AlCl4 - anions and AlCl2 + cations) in multisites is proposed herein to achieve an ultrahigh capacity in Al-metal-organic framework (MOF) battery. The bipolar ligands (oxidized from 18π to 16π electrons and reduced from 18π to 20π electrons in a planar cyclic conjugated system) can alternately uptake and release AlCl4 - anions and AlCl2 + cations in charge/discharge processes, which can double the capacity of unipolar ligands. Moreover, the high-density active Cu sites (Cu nodes) in the 2D Cu-based MOF can also store AlCl2 + cations for a higher capacity. The rigid and extended MOF structure can address the problems of high solubility and poor stability of small organic molecules. As a result, three-step redox reactions with two-electron transfer in each step are demonstrated in charge/discharge processes, achieving high reversible capacity (184 mAh g-1 ) and energy density (177 Wh kg-1 ) of the optimized cathode in an Al-MOF battery. The findings provide a new insight for the rational design of stable high-energy Al-MOF batteries.

Enhanced storage behavior of quasi-solid-state aluminum-selenium battery.

The current aluminum batteries with selenium positive electrodes have been suffering from dramatic capacity loss owing to the dissolution of Se2Cl2 products on the Se positive electrodes in the ionic liquid electrolyte. For addressing this critical issue and achieving better electrochemical performances of rechargeable aluminum-selenium batteries, here a gel-polymer electrolyte which has a stable and strongly integrated electrode/electrolyte interface was adopted. Quite intriguingly, such a gel-polymer electrolyte enables the solid-state aluminum-selenium battery to present a lower self-discharge and obvious discharging platforms. Meanwhile, the discharge capacity of the aluminum-selenium battery with a gel-polymer electrolyte is initially 386 mA h g-1 (267 mA h g-1 in ionic liquid electrolyte), which attenuates to 79 mA h g-1 (32 mA h g-1 in ionic liquid electrolyte) after 100 cycles at a current density of 200 mA g-1. The results suggest that the employment of a gel-polymer electrolyte can provide an effective route to improve the performance of aluminum-selenium batteries in the first few cycles.

The effect of graphitization degree of carbonaceous material on the electrochemical performance for aluminum-ion batteries.

Aluminum-ion batteries are currently regarded as the most promising energy storage batteries. The recent development of aluminum-ion batteries has been greatly promoted based on the use of graphitic carbon materials as a positive electrode. However, it remains unclear whether all carbonaceous materials can achieve excellent electrochemical behaviour similar to graphite. In this study, the correlation between the graphitization degree and capacity of a graphite electrode is systematically investigated for aluminum-ion batteries. The results show that the higher the graphitization degree, the larger the charge/discharge capacity and the better the cycling stability. Moreover, graphite nanoflakes with the highest graphitization degree deliver an initial discharge capacity of 66.5 mA h g-1 at a current density of 100 mA g-1, eventually retaining 66.3 mA h g-1 after 100 cycles with a coulombic efficiency of 96.1% and capacity retention of 99.7%, exhibiting an ultra-stable cycling performance. More importantly, it can be concluded that the discharge capacity of different kinds of graphite materials can be predicted by determining the graphitization degree.

Sb2Se3 nanorods with N-doped reduced graphene oxide hybrids as high-capacity positive electrode materials for rechargeable aluminum batteries.

For substantially promoting the unexpected rechargeable capability induced from the dissolution of Sb2Se3 positive electrode materials in aluminum batteries, here a novel prototype of a cell assembled by a hybrid of single-crystalline Sb2Se3 nanorods and N-doped reduced graphene oxide (SNG) coupled with a modified separator has been developed. With this specific cell design, the hybrid positive electrode material exhibits a high discharge potential (∼1.8 V) with a considerably high initial discharge capacity of up to 343 mA h g-1 at a current density of 500 mA g-1. Owing to the alleviation of active material loss from the positive electrode, the cell having the modified separator exhibits much enhanced cycling stability. Besides, nitrogen-doping on the reduced graphene oxide is found to boost the active sites in SNG, and thus the cycling performance has been largely improved. The strategy used in this work offers a universal plateau for designing long-life cycle aluminum batteries.

Ordered WO3-x nanorods: facile synthesis and their electrochemical properties for aluminum-ion batteries.

In this work, we have synthesized ordered WO3 nanorods via a facile hydrothermal process. And the series WO3-x nanorods with oxygen vacancies are obtained via a subsequent thermal reduction process. The formation mechanisms of WO3-x nanorods with different oxygen vacancies are proposed. And the electrochemical results reveal that the WO3-x nanorods exhibit the improved specific capacity due to the oxygen vacancies caused by the thermal reduction. More importantly, the reaction mechanism of the WO3-x nanorods as cathodes for aluminum-ion batteries has been proved.

Facile synthesis of Ni11(HPO3)8(OH)6/rGO nanorods with enhanced electrochemical performance for aluminum-ion batteries.

The electrochemical behaviors of the ultrashort nickel phosphite nanorods supported on reduced graphene oxide (Ni11(HPO3)8(OH)6/rGO nanorods), as a candidate for cathodic applications in aluminum-ion batteries, are firstly investigated. Ni11(HPO3)8(OH)6/rGO nanorods are synthesized by a facile solvothermal process. Ni11(HPO3)8(OH)6 and Ni11(HPO3)8(OH)6/rGO cathodes both possess very high initial discharge capacities of 132.4 and 182.0 mA h g-1 at a current density of 200 mA g-1, respectively. In addition, the long-term cycling stability of the Ni11(HPO3)8(OH)6/rGO cathode is further evaluated, exhibiting a discharge capacity of 49.2 mA h g-1 even over 1500 cycles. More importantly, the redox reaction mechanism of the Ni11(HPO3)8(OH)6 cathode for aluminum-ion batteries revealed that Ni11(HPO3)8(OH)6 is partially substituted with Al3+ to form AlmNin(HPO3)8(OH)6 and metallic Ni in the nanorod-like Ni11(HPO3)8(OH)6 cathodes during the discharge process. These findings are of great significance for the further development of novel materials for aluminum-ion batteries.

Exfoliation Mechanism of Graphite Cathode in Ionic Liquids.

Graphene has been successfully electrochemically exfoliated by electrolysis of cathode graphite in the aluminum-ion battery with ionic liquid electrolyte comprising AlCl3 and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl). The AlCl4-, Al2Cl7-, etc., intercalation into graphite flakes in ionic liquid of the aluminum-ion battery by different electrolysis processes to exfoliate graphite has been researched in detail. As a result of the enhanced structural flexibility, the intercalant gallery height increases in the less than five-layer graphene film, providing more free space for AlCl4-, Al2Cl7-, etc. transport. Therefore, a quantity of 3-5 layers rather than 1-2 layers of graphene can be obtained. The results clearly demonstrate that graphene has been produced in the graphite cathode in AlCl3/EMImCl ionic liquids, which is significantly meaningful for accelerating the theoretical research and industrialized application of graphene. Meanwhile, it has a vitally important role for promoting the recycling Al-ion batteries.

High-Performance Aluminum-Ion Battery with CuS@C Microsphere Composite Cathode.

On the basis of low-cost, rich resources, and safety performance, aluminum-ion batteries have been regarded as a promising candidate for next-generation energy storage batteries in large-scale energy applications. A rechargeable aluminum-ion battery has been fabricated based on a 3D hierarchical copper sulfide (CuS) microsphere composed of nanoflakes as cathode material and room-temperature ionic liquid containing AlCl3 and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) as electrolyte. The aluminum-ion battery with a microsphere electrode exhibits a high average discharge voltage of ∼1.0 V vs Al/AlCl4-, reversible specific capacity of about 90 mA h g-1 at 20 mA g-1, and good cyclability of nearly 100% Coulombic efficiency after 100 cycles. Such remarkable electrochemical performance is attributed to the well-defined nanostructure of the cathode material facilitating the electron and ion transfer, especially for chloroaluminate ions with large size, which is desirable for aluminum-ion battery applications.