Transition metal dichalcogenide-based materials for rechargeable aluminum-ion batteries: A mini-review

Rechargeable aluminum-ion batteries (AIBs) have emerged as a promising candidate for energy storage applications and have been extensively investigated over the past few years. Due to their high theoretical capacity, nature of abundance, and high safety, AIBs can be considered an alternative to lithium-ion batteries. However, the electrochemical performance of AIBs for large-scale applications is still limited due to the poor selection of cathode materials. Transition metal dichalcogenides (TMDs) have been regarded as appropriate cathode materials for AIBs due to their wide layer spacing, large surface area, and distinct physiochemical characteristics. This mini-review provides a succinct summary of recent research progress on TMD-based cathode materials in non-aqueous AIBs. The latest developments in the benefits of utilizing 3D-printed electrodes for AIBs are also explored.

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 Se_2.9 S_5.1 -based cathode with sufficient valence electronic energy overlap with Al³⁺ and easily accessible structure is potentially developed. Through this new strategy, Se_2.9 S_5.1 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 Se_x S_y in RABs.

Additive-Driven Interfacial Engineering of Aluminum Metal Anode for Ultralong Cycling Life

Rechargeable Al batteries (RAB) are promising candidates for safe and environmentally sustainable battery systems with low-cost investments. However, the currently used aluminum chloride-based electrolytes present a significant challenge to commercialization due to their corrosive nature. Here, we report for the first time, a novel electrolyte combination for RAB based on aluminum trifluoromethanesulfonate (Al(OTf)3) with tetrabutylammonium chloride (TBAC) additive in diglyme. The presence of a mere 0.1 M of TBAC in the Al(OTf)3 electrolyte generates the charge carrying electrochemical species, which forms the basis of reaction at the electrodes. TBAC reduces the charge transfer resistance and the surface activation energy at the anode surface and also augments the dissociation of Al(OTf)3 to generate the solid electrolyte interphase components. Our electrolyte's superiority directly translates into reduced anodic overpotential for cells that ran for 1300 cycles in Al plating/stripping tests, the longest cycling life reported to date. This unique combination of salt and additive is non-corrosive, exhibits a high flash point and is cheaper than traditionally reported RAB electrolyte combinations, which makes it commercially promising. Through this report, we address a major roadblock in the commercialization of RAB and inspire equivalent electrolyte fabrication approaches for other metal anode batteries.

Fast-charging aluminium-chalcogen batteries resistant to dendritic shorting

Although batteries fitted with a metal negative electrode are attractive for their higher energy density and lower complexity, the latter making them more easily recyclable, the threat of cell shorting by dendrites has stalled deployment of the technology^1,2. Here we disclose a bidirectional, rapidly charging aluminium-chalcogen battery operating with a molten-salt electrolyte composed of NaCl-KCl-AlCl₃. Formulated with high levels of AlCl₃, these chloroaluminate melts contain catenated Al_nCl_3n+1^- species, for example, Al₂Cl₇^-, Al₃Cl₁₀^- and Al₄Cl₁₃^-, which with their Al-Cl-Al linkages confer facile Al³⁺ desolvation kinetics resulting in high faradaic exchange currents, to form the foundation for high-rate charging of the battery. This chemistry is distinguished from other aluminium batteries in the choice of a positive elemental-chalcogen electrode as opposed to various low-capacity compound formulations^3-6, and in the choice of a molten-salt electrolyte as opposed to room-temperature ionic liquids that induce high polarization^7-12. We show that the multi-step conversion pathway between aluminium and chalcogen allows rapid charging at up to 200C, and the battery endures hundreds of cycles at very high charging rates without aluminium dendrite formation. Importantly for scalability, the cell-level cost of the aluminium-sulfur battery is projected to be less than one-sixth that of current lithium-ion technologies. Composed of earth-abundant elements that can be ethically sourced and operated at moderately elevated temperatures just above the boiling point of water, this chemistry has all the requisites of a low-cost, rechargeable, fire-resistant, recyclable battery.

Exceptional Sodium-Ion Storage by an Aza-Covalent Organic Framework for High Energy and Power Density Sodium-Ion Batteries

Redox-active covalent organic frameworks (COFs) are a new class of material with the potential to transform electrochemical energy storage due to the well-defined porosity and readily accessible redox-active sites of COFs. However, combining both high specific capacity and energy density in COF-based batteries remains a considerable challenge. Herein, we demonstrate the exceptional performance of Aza-COF in rechargeable sodium-ion batteries (SIBs). Aza-COF is a microporous 2D COF synthesized from hexaketocyclohexane and 1,2,4,5-benzenetetramine by a condensation reaction, which affords phenazine-decorated channels and a theoretical specific capacity of 603 mA h g^-1. The Aza-COF-based electrode exhibits an exceptional average specific capacity (550 mA h g^-1), energy density (492 W h kg^-1) at 0.1 C, and power density (1182 W kg^-1) at 40 C. The high capacity and energy density are attributed to swift surface-controlled redox processes and rapid sodium-ion diffusion inside the porous electrode. Rate capability studies showed that the battery also performs well at high current rates: 1 C (363 mA h g^-1), 5 C (232 mA h g^-1), 10 C (161 mA h g^-1), and 20 C (103 mA h g^-1). In addition, the long-term cycling stability test revealed very good capacity retention (87% at 5 C) and Coulombic efficiencies near unity over 500 cycles.

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.

Comparing Computational Predictions and Experimental Results for Aluminum Triflate in Tetrahydrofuran

Exploring reliable electrolytes for aluminum ion batteries requires an in-depth understanding of the behavior of aluminum ions in ethereal-organic solvents. Electrolytes comprised of aluminum trifluoromethanesulfonate (Al-triflate) in tetrahydrofuran (THF) were investigated computationally and experimentally. Optimized geometries, redox potentials, and vibrational frequencies of species likely to be present in the electrolyte were calculated by density functional theory and then measured spectroscopically and electrochemically. Aluminum appears to be electrochemically active in THF with a reduction onset near 0 V versus Al/Al³⁺. Spectroscopic measurements reveal explicit evidence for the presence of two concentration-dependent ionic environments for the triflate anions, namely, outer-shell ligands and Al-bound triflates. Additionally, ionic conductivities of ∼2.5 mS/cm were measured for these electrolytes ∼0.8M.

The Compensation Effect Mechanism of Fe-Ni Mixed Prussian Blue Analogues in Aqueous Rechargeable Aluminum-Ion Batteries

An aluminum-ion battery was assembled with potassium nickel hexacyanoferrate (KNHCF) as a cathode and Al foil as an anode in aqueous electrolyte for the first time, based on Al³⁺ intercalation and deintercalation. A combination of ex situ XRD, X-ray photoelectron spectroscopy (XPS), galvanostatic intermittent titration technique (GITT), and differential capacity analysis was used to unveil the crystal structure changes and the insertion/extraction mechanism of Al³⁺ . Al³⁺ could reversibly insert/extract into/from KNHCF nanoparticles through a single-phase reaction with reduction/oxidation of Fe and Ni. Over long-term cycling, it was Fe rather than Ni that contributed to more capacity owing to the dissolution of Ni from the KNHCF structure, which could be expressed as a compensation effect of mixed redox centers in KNHCF. KNHCF delivered an initial discharge capacity of 46.5 mAh g^-1 . The capacity decay could be attributed to the unstable interface between Al foil and the aqueous electrolyte owing to the catalytic activity of the Ni transferring from Ni dissolution of KNHCF to the Al foil anode, rather than KNHCF structure collapse; KNHCF maintained its 3 D framework structure for 500 cycles. This work is expected to inspire more exhaustive investigations of the mechanisms that occur in aluminum-ion batteries.

Challenges and benefits of post-lithium-ion batteries

Post-Li-ion batteries based on Na, Mg, and Al offer substantial electrochemical and economic advantages in comparison with Li-ion batteries.

Water-in-Salt Electrolyte Promotes High-Capacity FeFe(CN)6 Cathode for Aqueous Al-Ion Battery

Prussian blue analogues (PBAs) are considered to be ideal multivalent cation host materials due to their unique open-framework structure. In aqueous solution, however, the PBAs' cathodes have a low reversible capacity limited by the single electrochemical group Fe(CN)₆^3- and high crystal water content. They also suffer from fast cycle fading, resulting from significant oxygen/hydrogen evolution and cathode dissolution. In this work, a high-capacity PBA-type FeFe(CN)₆ cathode with double transition metal redox sites is successfully demonstrated in 5 m Al(CF₃SO₃)₃ Water-in-Salt electrolyte (Al-WISE). Due to Al-WISE having a wide electrochemical window (2.65 V) and low dissolution of the cathode, our PBA cathode exhibits a high discharge capacity of 116 mAh/g and the superior cycle stability >100 cycles with capacity fading of 0.39% per cycle.

Two-dimensional boron nitride as a sulfur fixer for high performance rechargeable aluminum-sulfur batteries

Aluminum-ion batteries (AIBs) are regarded as promising candidates for post-lithium-ion batteries due to their lack of flammability and electrochemical performance comparable to other metal-ion batteries. The lack of suitable cathode materials, however, has hindered the development of high-performing AIBs. Sulfur is a cost-efficient material, having distinguished electrochemical properties, and is considered an attractive cathode material for AIBs. Several pioneering reports have shown that aluminum-sulfur batteries (ASBs) exhibit superior electrochemical capacity over other cathode materials for AIBs. However, a rapid decay in the capacity is a huge barrier for their practical applications. Here, we have demonstrated systematically for the first time that the two-dimensional layered materials (e.g. MoS2, WS2, and BN) can serve as fixers of S and sulfide compounds during repeated charge/discharge processes; BN/S/C displays the highest capacity of 532 mAh g-1 (at a current density of 100 mA g-1) compared with the current state-of-the-art cathode material for AIBs. Further, we could improve the life-span of ASBs to an unprecedented 300 cycles with a high Coulombic efficiency of 94.3%; discharge plateaus at ~1.15 V vs. AlCl4-/Al was clearly observed during repeated charge/discharge cycling. We believe that this work opens up a new method for achieving high-performing ASBs.

Realization of Lithium-Ion Capacitors with Enhanced Energy Density via the Use of Gadolinium Hexacyanocobaltate as a Cathode Material

Li-ion storage devices having superior energy density are critical for one-time-charge long-term applications. Currently, much research endeavor is directed at enhancing the energy density of hybrid Li-ion capacitors, which incorporate the high energy of conventional Li-ion batteries with the elevated power density of Li-ion supercapacitors. Herein, we prepare orthorhombic GdCo(CN)₆ as a new Prussian blue analogue (PBA), showing that this compound offers excellent energy/power densities (605 W·h kg^-1 and 174 W kg^-1, respectively) and features Li-ion storage capacities (352 and 258 mA·h g_electrode^-1 at 100 and 1000 mA g^-1, respectively) that are almost twice higher than those of other cathode materials utilized in hybrid Li-ion capacitors. Thus, this study not only opens a new path for the exploration of new-type PBAs, but also provides insights on the use of lanthanides in energy storage applications.

PBA@POM Hybrids as Efficient Electrocatalysts for the Oxygen Evolution Reaction

To realize the effective conversion of renewable energy through water decomposition, efficient electrocatalysts for the oxygen evolution reaction (OER) are essential. In this article, PBA@POM was successfully prepared with a Prussian blue analogue (PBA) as the initial structure. A facile hydrothermal process is reported for obtaining PBA@POM by etching the cubic PBA with a strong Brønsted acid, H₃ PMo₁₂ O₄₀ (HPMo). The hollow cube structure not only exposes more active sites but also promotes electron transport, which results in excellent electrocatalytic activity for the OER. Compared with the PBA, which initially simply adhered to POM, the optimum PBA@POM hybrids display remarkably enhanced OER catalytic activity, with an almost constant overpotential of 440 mV at a current density of 10 mA cm^-2 and a small Tafel slope (23.45 mV dec^-1 ). The facilely prepared PBA@POM with good electrochemical activity and stability promises great potential for the OER.

Paving the Path toward Reliable Cathode Materials for Aluminum-Ion Batteries

Aluminum metal is a high-energy-density carrier with low cost, and thus endows rechargeable aluminum batteries (RABs) with the potential to act as an inexpensive and efficient electrochemical device, so as to supplement the increasing demand for energy storage and conversion. Despite the enticing aspects regarding cost and energy density, the poor reversibility of electrodes has limited the pursuit of RABs for a long time. Fortunately, ionic-liquid electrolytes enable reversible aluminum plating/stripping at room temperature, and they lay the very foundation of RABs. In order to integrate with the aluminum-metal anode, the selection of the cathode is pivotal, but is limited at present. The scant option of a reliable cathode can be accounted for by the intrinsic high charge density of Al³⁺ ions, which results in sluggish diffusion. Hence, reliable cathode materials are a key challenge of burgeoning RABs. Herein, the main focus is on the insertion cathodes for RABs also termed aluminum-ion batteries, and the recent progress and optimization methods are summarized. Finally, an outlook is presented to navigate the possible future work.

Electrochemically activated spinel manganese oxide for rechargeable aqueous aluminum battery

Aluminum is a naturally abundant, trivalent charge carrier with high theoretical specific capacity and volumetric energy density, rendering aluminum-ion batteries a technology of choice for future large-scale energy storage. However, the frequent collapse of the host structure of the cathode materials and sluggish kinetics of aluminum ion diffusion have thus far hampered the realization of practical battery devices. Here, we synthesize Al_xMnO₂·nH₂O by an in-situ electrochemical transformation reaction to be used as a cathode material for an aluminum-ion battery with a configuration of Al/Al(OTF)₃-H₂O/Al_xMnO₂·nH₂O. This cell is not only based on aqueous electrolyte chemistry but also delivers a high specific capacity of 467 mAh g^-1 and a record high energy density of 481 Wh kg^-1. The high safety of aqueous electrolyte, facile cell assembly and the low cost of materials suggest that this aqueous aluminum-ion battery holds promise for large-scale energy applications.

Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn–Co PBAs as highly selective catalysts for A3 coupling

The occurrence of metal ion exchange in Zn₃[Co(CN)₆]₂ and Cu₃[Co(CN)₆]₂ Prussian blue analogues (Zn–Co and Cu–Co PBAs) was demonstrated for the first time.

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 Al_mNi_n(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.

Emerging Nonaqueous Aluminum-Ion Batteries: Challenges, Status, and Perspectives

Aluminum-ion batteries (AIBs) are regarded as viable alternatives to lithium-ion technology because of their high volumetric capacity, their low cost, and the rich abundance of aluminum. However, several serious drawbacks of aqueous systems (passive film formation, hydrogen evolution, anode corrosion, etc.) hinder the large-scale application of these systems. Thus, nonaqueous AIBs show incomparable advantages for progress in large-scale electrical energy storage. However, nonaqueous aluminum battery systems are still nascent, and various technical and scientific obstacles to designing AIBs with high capacity and long cycling life have not been resolved until now. Moreover, the aluminum cell is a complex device whose energy density is determined by various parameters, most of which are often ignored, resulting in failure to achieve the maximum performance of the cell. The purpose here is to discuss how to further develop reliable nonaqueous AIBs. First, the current status of nonaqueous AIBs is reviewed based on statistical data from the literature. The influence of parameters on energy density is analyzed, and the current situation and existing problems are summarized. Furthermore, possible solutions and concerns regarding the construction of reliable nonaqueous AIBs are comprehensively discussed. Finally, future research directions and prospects in the aluminum battery field are proposed.

Prussian Blue Analogs for Rechargeable Batteries

Non-lithium energy storage devices, especially sodium ion batteries, are drawing attention due to insufficient and uneven distribution of lithium resources. Prussian blue and its analogs (Prussian blue analogs [PBAs]), or hexacyanoferrates, are well-known since the 18th century and have been used for hydrogen storage, cancer therapy, biosensing, seawater desalination, and sewage treatment. Owing to their unique features, PBAs are receiving increasing interest in the field of energy storage, such as their high theoretical specific capacity, ease of synthesis, as well as low cost. In this review, a general summary and evaluation of the applications of PBAs for rechargeable batteries are given. After a brief review of the history of PBAs, their crystal structure, nomenclature, synthesis, and working principle in rechargeable batteries are discussed. Then, previous works classified based on the combination of insertion cations and transition metals are analyzed comprehensively. The review includes an outlook toward the further development of PBAs in electrochemical energy storage.

Effects of the variation of metal substitution and electrolyte on the electrochemical reaction of metal hexacyanoferrates

Metal hexacyanoferrates (MHCFs), also called Prussian blue analogs, are known as electrochemical electrodes and are ion-adsorbent. To investigate the effect of the ionic radius of the adsorbate (cations adsorbed upon reduction) and the pore size of the adsorbent (porous electrode that stores cations upon reduction), we investigated the electrochemical reactions with various alkali cations and by changing the metal sites of the MHCFs. First, we succeeded in controlling the pore sizes of the MHCFs, where the lattice constant a could be estimated as a = 0.98D_sum + 7.21, where D_sum represented the sum of the ionic diameters of the metal M and Fe. Concerning the electrochemical reaction, the redox potential increased when the hydration energy of the adsorbate decreased, implying that the hydration energy of the adsorbate affected the stability of the reduced state. With cadmium hexacyanoferrate, which has a large pore size, the variation of the redox potential was suppressed in comparison to that with copper hexacyanoferrate, which has a small pore size. With Fourier transform-infrared (FT-IR) analysis before and after the redox reactions, Na⁺ insertion accompanied by H₂O was presumed in the reduced state.

The redox potential of metal hexacyanoferrates (MHCFs), also called Prussian blue analogs, is qualitatively understood with the hydration energy of the cations in the supporting electrolyte.

Two-Dimensional Vanadium Carbide (MXene) as a High-Capacity Cathode Material for Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (Al batteries) can potentially be safer, cheaper, and deliver higher energy densities than those of commercial Li-ion batteries (LIBs). However, due to the very high charge density of Al³⁺ cations and their strong interactions with the host lattice, very few cathode materials are known to be able to reversibly intercalate these ions. Herein, a rechargeable Al battery based on a two-dimensional (2D) vanadium carbide (V₂CT_x) MXene cathode is reported. The reversible intercalation of Al³⁺ cations between the MXene layers is suggested to be the mechanism for charge storage. It was found that the electrochemical performance could be significantly improved by converting multilayered V₂CT_x particles to few-layer sheets. With specific capacities of more than 300 mAh g^-1 at high discharge rates and relatively high discharge potentials, V₂CT_x MXene electrodes show one of the best performances among the reported cathode materials for Al batteries. This study can lead to foundations for the development of high-capacity and high energy density rechargeable Al batteries by showcasing the potential of a large family of intercalation-type cathode materials based on MXenes.

Rechargeable Aluminum-Ion Batteries Based on an Open-Tunnel Framework

Rechargeable batteries based on an abundant metal such as aluminum with a three-electron transfer per atom are promising for large-scale electrochemical energy storage. Aluminum can be handled in air, thus offering superior safety, easy fabrication, and low cost. However, the development of Al-ion batteries has been challenging due to the difficulties in identifying suitable cathode materials. This study presents the use of a highly open framework Mo_2.5 + y VO_9 + z as a cathode for Al-ion batteries. The open-tunnel oxide allows a facile diffusion of the guest species and provides sufficient redox centers to help redistribute the charge within the local host lattice during the multivalent-ion insertion, thus leading to good rate capability with a specific capacity among the highest reported in the literature for Al-based batteries. This study also presents the use of Mo_2.5 + y VO_9 + z as a model host to develop a novel ultrafast technique for chemical insertion of Al ions into host structures. The microwave-assisted method employing diethylene glycol and aluminum diacetate (Al(OH)(C₂ H₃ O₂ )₂ ) can be performed in air in as little as 30 min, which is far superior to the traditional chemical insertion techniques involving moisture-sensitive organometallic reagents. The Al-inserted Al _x Mo_2.5 + y VO_9 + z obtained by the microwave-assisted chemical insertion can be used in Al-based rechargeable batteries.

Investigation of the Reversible Intercalation/Deintercalation of Al into the Novel Li3VO4@C Microsphere Composite Cathode Material for Aluminum-Ion Batteries

The Li₃VO₄@C microsphere composite was first reported as a novel cathode material for rechargeable aluminum-ion batteries (AIBs), which manifests the initial discharge capacity of 137 mAh g^-1 and and remains at 48 mAh g^-1 after 100 cycles with almost 100% Coulombic efficiency. The detailed intercalation mechanism of Al into the orthorhombic Li₃VO₄ is investigated by ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) of Li₃VO₄@C electrodes and the nuclear magnetic resonance aluminum spectroscopy (²⁷Al NMR) of ionic liquid electrolytes in different discharge/charge states. First-principle calculations are also carried out to investigate the structural change as Al inserts into the framework of Li₃VO₄. It is revealed that the Al/Li₃VO₄@C battery goes through electrochemical dissolution and deposition of metallic aluminum in the anode, as well as the insertion and deinsertion of Al³⁺ cations in the cathode in the meantime. The rechargeable AIBs fabricated in this work are of low cost and high safety, which may make a step forward in the development of novel cathode materials based on the acidic ionic liquid electrolyte system.

Potassium Secondary Batteries

Potassium may exhibit advantages over lithium or sodium as a charge carrier in rechargeable batteries. Analogues of Prussian blue can provide millions of cyclic voltammetric cycles in aqueous electrolyte. Potassium intercalation chemistry has recently been demonstrated compatible with both graphite and nongraphitic carbons. In addition to potassium-ion batteries, potassium-O₂ (or -air) and potassium-sulfur batteries are emerging. Additionally, aqueous potassium-ion batteries also exhibit high reversibility and long cycling life. Because of potentially low cost, availability of basic materials, and intriguing electrochemical behaviors, this new class of secondary batteries is attracting much attention. This mini-review summarizes the current status, opportunities, and future challenges of potassium secondary batteries.

Challenges and Prospect of Non-aqueous Non-alkali (NANA) Metal-Air Batteries

Non-aqueous non-alkali (NANA) metal-air battery technologies promise to provide electrochemical energy storage with the highest specific energy density. Metal-air battery technology is particularly advantageous being implemented in long-range electric vehicles. Up to now, almost all the efforts in the field are focused on Li-air cells, but other NANA metal-air battery technologies emerge. The major concern, which the research community should be dealing with, is the limited and rather poor rechargeability of these systems. The challenges we are covering in this review are related to the initial limited discharge capacities and cell performances. By comprehensively reviewing the studies conducted so far, we show that the implementation of advanced materials is a promising approach to increase metal-air performance and, particularly, metal surface activation as a prime achievement leading to respectful discharge currents. In this review, we address the most critical areas that need careful research attention in order to achieve progress in the understanding of the physical and electrochemical processes in non-aqueous electrolytes applied in beyond lithium and zinc air generation of metal-air battery systems.

Electrochemical Film Formation on Magnesium Metal in an Ionic Liquid That Dissolves Metal Triflate and Its Application to an Active Material with Anion Charge Carrier

Irregular metallic growth at the anode during recharging of batteries can seriously influence the safety of batteries. To address this problem, we have attempted to design active anode materials with anion charge carriers and recently observed the formation and dissolution of an electrochemical film by triflate anions (CF₃SO₃^-) at the surface of magnesium in an ionic liquid (IL) electrolyte of Mg(CF₃SO₃)₂, which represents a rare anode material. The effect of heterogeneous cations on film formation was examined in this work. In an IL that dissolves NaCF₃SO₃, sodium ions with a lower reduction potential than Mg²⁺/Mg would not be expected to assist film formation. However, to our surprise, we discovered that some sodium ions are involved in film formation. The sodium ions are believed to act as a cross-linking point for the formation of a film network, which resulted in fairly good reversibility for film formation. In a Ce(CF₃SO₃)₃-IL electrolyte, an electrochemically formed film free of Ce³⁺ was obtained. The trivalent cerium cations were deactivated and transformed to an oxide on Mg metal. However, the reversibility of film formation in the Ce(CF₃SO₃)₃ system did not meet the expected level. By coupling the film formation and dissolution behavior with a V₂O₅ cathode, a rechargeable battery was fabricated with dual ion transport species of Na⁺ or Ce³⁺ for the cathode and CF₃SO₃^- for the anode. The unique battery with NaCF₃SO₃ is demonstrated to exhibit good discharge/charge performance with long-term cyclability.

High-Voltage and Noncorrosive Ionic Liquid Electrolyte Used in Rechargeable Aluminum Battery

As a promising post-lithium battery, rechargeable aluminum battery has the potential to achieve a three-electron reaction with fully use of metal aluminum. Alternative electrolytes are strongly needed for further development of rechargeable aluminum batteries, because typical AlCl₃-contained imidazole-based ionic liquids are moisture sensitive, corrosive, and with low oxidation voltage. In this letter, a kind of noncorrosive and water-stable ionic liquid obtained by mixing 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) with the corresponding aluminum salt (Al(OTF)₃) is studied. This ionic liquid electrolyte has a high oxidation voltage (3.25 V vs Al³⁺/Al) and high ionic conductivity, and a good electrochemical performance is also achieved. A new strategy, which first used corrosive AlCl₃-based electrolyte to construct a suitable passageway on the Al anode for Al³⁺, and then use noncorrosive Al(OTF)₃-based electrolyte to get stable Al/electrolyte interface, is put forward.

Advanced High Energy Density Secondary Batteries with Multi-Electron Reaction Materials

Secondary batteries have become important for smart grid and electric vehicle applications, and massive effort has been dedicated to optimizing the current generation and improving their energy density. Multi-electron chemistry has paved a new path for the breaking of the barriers that exist in traditional battery research and applications, and provided new ideas for developing new battery systems that meet energy density requirements. An in-depth understanding of multi-electron chemistries in terms of the charge transfer mechanisms occuring during their electrochemical processes is necessary and urgent for the modification of secondary battery materials and development of secondary battery systems. In this Review, multi-electron chemistry for high energy density electrode materials and the corresponding secondary battery systems are discussed. Specifically, four battery systems based on multi-electron reactions are classified in this review: lithium- and sodium-ion batteries based on monovalent cations; rechargeable batteries based on the insertion of polyvalent cations beyond those of alkali metals; metal-air batteries, and Li-S batteries. It is noted that challenges still exist in the development of multi-electron chemistries that must be overcome to meet the energy density requirements of different battery systems, and much effort has more effort to be devoted to this.