Configurational Entropy Strategy Enhanced Structure Stability Achieves Robust Cathode for Aluminum Batteries.

Rechargeable aluminum batteries (RABs) are an emerging energy storage device owing to the vast Al resources, low cost, and high safety. However, the poor cyclability and inferior reversible capacity of cathode materials have limited the enhancement of RABs performance. Herein, a high configurational entropy strategy is presented to improve the electrochemical properties of RABs for the first time. The high-entropy (Fe, Mn, Ni, Zn, Mg)3 O4 cathode exhibits an ultra-stable cycling ability (109 mAh g-1 after 3000 cycles), high specific capacity (268 mAh g-1 at 0.5 A g-1 ), and rapid ion diffusion. Ex situ characterizations indicate that the operational mechanism of (Fe, Mn, Ni, Zn, Mg)3 O4 cathode is mainly based on the redox process of Fe, Mn, and Ni. Theoretical calculations demonstrate that the oxygen vacancies make a positive contribution to adjusting the distribution of electronic states, which is crucial for enhancing the reaction kinetics at the electrolyte and cathode interface. These findings not only propose a promising cathode material for RABs, but also provide the first elucidation of the operational mechanism and intrinsic information of high-entropy electrodes in multivalent ion batteries.

Engineering High Voltage Aqueous Aluminum-Ion Batteries.

The energy transition to renewables necessitates innovative storage solutions beyond the capacities of lithium-ion batteries. Aluminum-ion batteries (AIBs), particularly their aqueous variants (AAIBs), have emerged as potential successors due to their abundant resources, electrochemical advantages, and eco-friendliness. However, they grapple with achieving their theoretical voltage potential, often yielding less than expected. This perspective article provides a comprehensive examination of the voltage challenges faced by AAIBs, attributing gaps to factors such as the aluminum reduction potential, hydrogen evolution reaction, and aluminum's inherent passivation. Through a critical exploration of methodologies, strategies, such as underpotential deposition, alloying, interface enhancements, tailored electrolyte compositions, and advanced cathode design, are proposed. This piece seeks to guide researchers in harnessing the full potential of AAIBs in the global energy storage landscape.

Synergistic π-Conjugation Organic Cathode for Ultra-Stable Aqueous Aluminum Batteries.

Rechargeable aqueous aluminum batteries (AABs) are promising energy storage technologies owing to their high safety and ultra-high energy-to-price ratio. However, either the strong electrostatic forces between high-charge-density Al3+ and host lattice, or sluggish large carrier-ion diffusion toward the conventional inorganic cathodes generates inferior cycling stability and low rate-capacity. To overcome these inherent confinements, a series of promising redox-active organic materials (ROMs) are investigated and a π-conjugated structure ROMs with synergistic C═O and C═N groups is optimized as the new cathode in AABs. Benefiting from the joint utilization of multi-redox centers and rich π-π intermolecular interactions, the optimized ROMs with unique ion coordination storage mechanism facilely accommodate complex active ions with mitigated coulombic repulsion and robust lattice structure, which is further validated via theoretical simulations. Thus, the cathode achieves enhanced rate performance (153.9 mAh g-1 at 2.0 A g-1) and one of the best long-term stabilities (125.7 mAh g-1 after 4,000 cycles at 1.0 A g-1) in AABs. Via molecular exploitation, this work paves the new direction toward high-performance cathode materials in aqueous multivalent-ion battery systems.

Redox-Bipolar Polyimide Two-Dimensional Covalent Organic Framework Cathodes for Durable Aluminium Batteries.

Emerging rechargeable aluminium batteries (RABs) offer a sustainable option for next-generation energy storage technologies with low cost and exemplary safety. However, the development of RABs is restricted by the limited availability of high-performance cathode materials. Herein, we report two polyimide two-dimensional covalent organic frameworks (2D-COFs) cathodes with redox-bipolar capability in RAB. The optimal 2D-COF electrode achieves a high specific capacity of 132 mAh g-1 . Notably, the electrode presents long-term cycling stability (with a negligible ≈0.0007 % capacity decay per cycle), outperforming early reported organic RAB cathodes. 2D-COFs integrate n-type imide and p-type triazine active centres into the periodic porous polymer skeleton. With multiple characterizations, we elucidate the unique Faradaic reaction of the 2D-COF electrode, which involves AlCl2+ and AlCl4 - dual-ions as charge carriers. This work paves the avenue toward novel organic cathodes in RABs.

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.

An operando X-ray diffraction study of chloroaluminate anion-graphite intercalation in aluminum batteries.

We investigated rechargeable aluminum (Al) batteries composed of an Al negative electrode, a graphite positive electrode, and an ionic liquid (IL) electrolyte at temperatures down to -40 °C. The reversible battery discharge capacity at low temperatures could be superior to that at room temperature. In situ/operando electrochemical and synchrotron X-ray diffraction experiments combined with theoretical modeling revealed stable AlCl4-/graphite intercalation up to stage 3 at low temperatures, whereas intercalation was reversible up to stage 4 at room temperature (RT). The higher-degree anion/graphite intercalation at low temperatures affords rechargeable Al battery with higher discharge voltage (up to 2.5 V, a record for Al battery) and energy density. A remarkable cycle life of >20,000 cycles at a rate of 6C (10 minutes charge time) was achievable for Al battery operating at low temperatures, corresponding to a >50-year battery life if charged/discharged once daily.

Modulating Ion Diffusivity and Electrode Conductivity of Carbon Nanotube@Mesoporous Carbon Fibers for High Performance Aluminum-Selenium Batteries.

Selenium (Se)-based rechargeable aluminum batteries (RABs), known as aluminum-selenium (Al-Se) batteries, are an appealing new battery design that holds great promise for addressing the low-capacity problem of current RAB technology. However, their applications are hindered by mediocre high-rate capacity (≈100 mAh g-1 at 0.5 A g-1 ) and insufficient cycling life (50 cycles). Herein, the synthesis of mesoporous carbon fibers (MCFs) by coating mesoporous carbon with short-length mesopores and tunable mesopore sizes (2.7 to 8.9 nm) coaxially on carbon nanotubes (CNT) is reported. When compositing MCFs with Se for Al-Se batteries, a positive correlation between mesopore size and electrolyte ion diffusivity is observed, however when pore size is increased to 8.9 nm, large voids are created at the interface of CNT core and mesoporous carbon shell, leading to decreased electrode conductivity. The trade-off between ion diffusivity and interfacial connectivity/conductivity determines MCF with pore size of 7.1 nm as the best host material for Al-Se batteries. The composite cathode delivers high specific capacities (366 and 230 mAh g-1 at 0.5 and 1 A g-1 ), good rate performance, and excellent cycling stability (152 mAh g-1 after 500 cycles at 2 A g-1 ), superior over previously reported Se cathodes and other cathodes for RABs.

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.

ZCNC Beaded Heterostructure toward High-Performance Aluminum Batteries.

We designed and prepared the ZnSe/CoSe2@NC/CNTs (ZCNC) cathode material for aluminum batteries (ABs). The ZCN (ZnSe/CoSe2@NC) is connected by the interwoven carbon nanotube (CNT) conductive network to form a beaded structure. CNTs and the carbon formed by carbonization of organic ligands is beneficial to improving the electrical conductivity of the material and reducing structural damage during cycling. The internal electric field generated at the interface of heterostructures can promote the transfer of electrons/ions. This special structure promotes ZCNC excellent electrochemical properties. At 100 mA/g, the specific capacity of the first discharge reaches 338 mAh/g, while the specific capacity after 500 cycles still reaches 217 mAh/g. Compared with ZCN and CN(CoSe2@NC), it demonstrates a great advantage.

Modification of Al Surface via Acidic Treatment and its Impact on Plating and Stripping.

Amorphous Al2 O3 film that naturally exists on any Al substrate is a critical bottleneck for the cyclic performance of metallic Al in rechargeable Al batteries. The so-called electron/ion insulator Al oxide slows down the anode's activation and hinders Al plating/stripping. The Al2 O3 film induces different surface properties (roughness and microstructure) on the metal. Al foils present two optically different sides (shiny and non-shiny), but their surface properties and influence on plating and stripping have not been studied so far. Compared to the shiny side, the non-shiny one has a higher (~28 %) surface roughness, and its greater concentration of active sites (for Al plating and stripping) yields higher current densities. Immersion pretreatments in Ionic-Liquid/AlCl3 -based electrolyte with various durations modify the surface properties of each side, forming an electrode-electrolyte interphase layer rich in Al, Cl, and N. The created interphase layer provides more tunneling paths for better Al diffusion upon plating and stripping. After 500 cycles, dendritic Al deposition, generated active sites, and the continuous removal of the Al metal and oxide cause accelerated local corrosion and electrode pulverization. We highlight the mechanical surface properties of cycled Al foil, considering the role of immersion pretreatment and the differences between the two sides.

Nonsolvent-Induced Phase Separation pPAN Separators for Dendrite-Free Rechargeable Aluminum Batteries.

Rechargeable aluminum batteries (RAB) are a promising energy storage system with high safety, long cycle life, and low cost. However, the strong corrosiveness of chloroaluminate ionic liquid electrolytes (ILEs) severely limits the development of RAB separators. Herein, a nonsolvent-induced phase separation strategy was applied to fabricate the pPAN (poly(vinyl alcohol)-modified polyacrylonitrile) separator, which exhibits prominent chemical and electrochemical stability in ILEs. The pPAN separator, owing to its uniform pore size distribution and strong electronegativity with a zeta potential of about -10.20 mV, can effectively inhibit the growth of dendrites. Benefiting from the good ion conductivity (6.38 mS cm-1) and high ion migration number (0.133) of pPAN separator, the full cell with pPAN separator demonstrates stable operation for more than 500 cycles at 600 mA g-1, with a high capacity of 88.8 mAh g-1. When integrating into sodium-ion batteries, the pPAN separators also show an excellent electrochemical performance. This work provides a considerable approach for designing separators to address the issue of Al anode dendrite growth in RABs.

Unexpected Energy Applications of Ionic Liquids.

Ionic liquids and their various analogues are without doubt the scientific sensation of the last few decades, paving the way to a more sustainable society. Their versatile suite of properties, originating from an almost inconceivably large number of possible cation and anion combinations, allows tuning of the structure to serve a desired purpose. Ionic liquids hence offer a myriad of useful applications from solvents to catalysts, through to lubricants, gas absorbers, and azeotrope breakers. The purpose of this review is to explore the more unexpected of these applications, particularly in the energy space. It guides the reader through the application of ionic liquids and their analogues as i) phase change materials for thermal energy storage, ii) organic ionic plastic crystals, which have been studied as battery electrolytes and in gas separation, iii) key components in the nitrogen reduction reaction for sustainable ammonia generation, iv) as electrolytes in aluminum-ion batteries, and v) in other emerging technologies. It is concluded that there is tremendous scope for further optimizing and tuning of the ionic liquid in its task, subject to sustainability imperatives in line with current global priorities, assisted by artificial intelligence.

Optimization of an Artificial Solid Electrolyte Interphase Formed on an Aluminum Anode and Its Application in Rechargeable Aqueous Aluminum Batteries.

Electrochemical cells that incorporate aluminum (Al) as the active material have become increasingly popular due to the advantages of high energy density, cost-effectiveness, and superior safety features. Despite the progress made by research groups in developing rechargeable Al//MxOy (M = Mn, V, etc.) cells using an aqueous Al trifluoromethanesulfonate-based electrolyte, the reactions occurring at the Al anode are still not fully understood. In this study, we explore the artificial solid electrolyte interphase (ASEI) on the Al anode by soaking it in AlCl3/urea ionic liquid. Surprisingly, our findings reveal that the ASEI actually promotes the corrosion of Al by providing chloride anions rather than facilitating the transport of Al3+ ions during charge/discharge cycles. Importantly, the ASEI significantly enhances the cycling stability and activity of Al cells. The primary reactions occurring at the Al anode during the charge/discharge cycle were determined to be irreversible oxidation and gas evolution. Furthermore, we demonstrate the successful realization of urea-treated Al (UTAl)//AlxMnO2 cells (discharge operating voltage of ∼1.45 V and specific capacity of 280 mAh/g), providing a platform to investigate the underlying mechanisms of these cells further. Overall, our work highlights the importance of ASEI in controlling the corrosion of Al in aqueous electrolytes, emphasizing the need for the further development of electrolytic materials that facilitate the transport of Al3+ ions in rechargeable Al batteries.

Interface Engineering for Aqueous Aluminum Metal Batteries: Current Progresses and Future Prospects.

Aqueous aluminum metal batteries (AMBs) have attracted numerous attention because of the abundant reserves, low cost, high theoretical capacity, and high safety. Nevertheless, the poor thermodynamics stability of metallic Al anode in aqueous solution, which is caused by the self-corrosion, surface passivation, or hydrogen evolution reaction, dramatically limits the electrochemical performance and hampers the further development of AMBs. In this comprehensive review, the key scientific challenges of Al anode/electrolyte interface (AEI) are highlighted. A systematic overview is also provided about the recent progress on the rational interface engineering principles toward a relatively stable AEI. Finally, suggestions and perspectives for future research are offered on the optimization of Al anode and aqueous electrolytes to enable a stable and durable AEI, which may pave the way for developing high-performance AMBs.

A Carbon Aerogel Lightweight Al Battery for Fast Storage of Fluctuating Energy.

Al batteries have great potential for renewable energy storage owing to their low cost, high capacity, and safety. High energy density and adaptability to fluctuating electricity are major challenges. Here, a lightweight Al battery for fast storage of fluctuating energy is constructed based on a novel hierarchical porous dendrite-free carbon aerogel film (CAF) anode and an integrated graphite composite carbon aerogel film (GCAF) cathode. A new induced mechanism by the O-containing functional groups on the CAF anode is con-firmed for uniform Al deposition. The GCAF cathode possesses a higher mass utilization ratio due to the extremely high loading mass (9.5-10.0 mg cm-2 ) of graphite materials compared to conventional coated cathodes. Meanwhile, the volume expansion of the GCAF cathode is almost negligible, resulting in better cycling stability. The lightweight CAF‖GCAF full battery can adapt well to large and fluctuating current densities owing to its hierarchical porous structure. A large discharge capacity (115.6 mAh g-1 ) after 2000 cycles and a short charge time (7.0 min) at a high current density are obtained. The construction strategy of lightweight Al batteries based on carbon aerogel electrodes can promote the breakthrough of high-energy-density Al batteries adapted to the fast storage of fluctuating renewable energy.

Novel Insight into Rechargeable Aluminum Batteries with Promising Selenium Sulfide@Carbon Nanofibers Cathode.

Due to the unique electronic structure of aluminum ions (Al3+ ) with strong Coulombic interaction and complex bonding situation (simultaneously covalent/ionic bonds), traditional electrodes, mismatching with the bonding orbital of Al3+ , usually exhibit slow kinetic process with inferior rechargeable aluminum batteries (RABs) performance. Herein, to break the confinement of the interaction mismatch between Al3+ and the electrode, a previously unexplored Se2.9 S5.1 -based cathode with sufficient valence electronic energy overlap with Al3+ and easily accessible structure is potentially developed. Through this new strategy, Se2.9 S5.1 encapsulated in multichannel carbon nanofibers with free-standing structure exhibits a high capacity of 606 mAh g-1 at 50 mA g-1 , high rate-capacity (211 mAh g-1 at 2.0 A g-1 ), robust stability (187 mAh g-1 at 0.5 A g-1 after 3,000 cycles), and enhanced flexibility. Simultaneously, in/ex-situ characterizations also reveal the unexplored mechanism of Sex Sy in RABs.

Low-Dimensional and High-Crystallinity Carbonyl Cathodes Prepared by Physical Vapor Deposition for Green Aluminum Organic Batteries.

We report a low-cost, high theoretical specific capacity π-conjugated organic compound (PTCDA) with C═O active centers as the cathode material in aluminum organic batteries. In addition, in order to improve the electron transport rate of PTCDA, a new method is proposed in this paper, which uses physical vapor deposition (PVD) method to make PTCDA recrystallize and grow on stainless steel and quartz glass substrates to improve its crystallinity. The increase of crystallinity expands the PTCDA π-π-conjugated system, making electrons more delocalized, which is beneficial to the transmission rate of electrons and ions, thereby enhancing the conductivity of the material. The experimental results show that compared with pristine PTCDA, PTCDA(Ss) and PTCDA(G) with higher crystallinity have better cycling stability and rate capability. The DFT (density functional theory) results indicated that the electron-deficient carbonyl group in the PTCDA molecule could reversibly coordinate/dissociate with the positively charged Al complex ions (AlCl2+). This research work provides insights into the rational design of low-dimensional, high-crystallinity, high-performance cathode materials for green aluminum organic batteries.

Pencil-Drawing Graphite Nanosheets: A Simple and Effective Cathode for High-Capacity Aluminum Batteries.

Commercial graphite, as an attractive cathode material, has been extensively applied in rechargeable aluminum batteries. However, low capacity and complex cathode preparation procedures limit its further development. Herein, graphite nanosheets as cathode of aluminum battery have been prepared by a novel pencil-drawing strategy, which shows superior capacity of 96 mAh g-1 and excellent stability with almost 100% capacity retention after 2000 cycles at a current density of 0.5 A g-1 . By increasing charge-discharge current density to 2 A g-1 , the battery also exhibits a high capacity of 72 mAh g-1 and retains 90% after 6000 cycles. Furthermore, a stage 3 anion intercalation/deintercalation mechanism has been proposed according to in situ X-ray diffraction and ex situ characterization techniques. This work provides a controllable method for developing a graphite nanosheets cathode with a simplified process and contributes to the development of other kinds of energy storage devices.

Superlattice-Stabilized WSe2 Cathode for Rechargeable Aluminum Batteries.

Rechargeable aluminum batteries (RABs), with abundant aluminum reserves, low cost, and high safety, give them outstanding advantages in the postlithium batteries era. However, the high charge density (364 C mm-3 ) and large binding energy of three-electron-charge aluminum ions (Al3+ ) de-intercalation usually lead to irreversible structural deterioration and decayed battery performance. Herein, to mitigate these inherent defects from Al3+ , an unexplored family of superlattice-type tungsten selenide-sodium dodecylbenzene sulfonate (SDBS) (S-WSe2 ) cathode in RABs with a stably crystal structure, expanded interlayer, and enhanced Al-ion diffusion kinetic process is proposed. Benefiting from the unique advantage of superlattice-type structure, the anionic surfactant SDBS in S-WSe2 can effectively tune the interlayer spacing of WSe2 with released crystal strain from high-charge-density Al3+ and achieve impressively long-term cycle stability (110 mAh g-1 over 1500 cycles at 2.0 A g-1 ). Meanwhile, the optimized S-WSe2 cathode with intrinsic negative attraction of SDBS significantly accelerates the Al3+ diffusion process with one of the best rate performances (165 mAh g-1 at 2.0 A g-1 ) in RABs. The findings of this study pave a new direction toward durable and high-performance electrode materials for RABs.

Stable Quasi-Solid-State Aluminum Batteries.

Nonaqueous rechargeable aluminum batteries (RABs) of low cost and high safety are promising for next-generation energy storage. With the presence of ionic liquid (IL) electrolytes, their high moisture sensitivity and poor stability would lead to critical issues in liquid RABs, including undesirable gas production, irreversible activity loss, and an unstable electrode interface, undermining the operation stability. To address such issues, herein, a stable quasi-solid-state electrolyte is developed via encapsulating a small amount of an IL into a metal-organic framework, which not only protects the IL from moisture, but creates sufficient ionic transport network between the active materials and the electrolyte. Owing to the generated stable states at both positive-electrode-electrolyte and negative-electrode-electrolyte interfaces, the as-assembled quasi-solid-state Al-graphite batteries deliver specific capacity of ≈75 mA h g-1 (with positive electrode material loading ≈9 mg cm-2 , much higher than that in the conventional liquid systems). The batteries present a long-term cycling stability beyond 2000 cycles, with great stability even upon exposure to air within 2 h and under flame combustion tests. Such technology opens a new platform of designing highly safe rechargeable Al batteries for stable energy storage.