Opportunities and Limitations of Ionic Liquid- and Organic Carbonate Solvent-Based Electrolytes for Mg-Ion-Based Dual-Ion Batteries

Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI)2], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI)2], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Organic Electrode Materials for Dual-Ion Batteries

Organic materials are widely used in various energy storage devices due to their renewable, environmental friendliness and adjustable structure. Dual-ion batteries (DIBs), which use organic materials as the electrodes, are an attractive alternative to conventional lithium-ion batteries for sustainable energy storage devices owing to the advantages of low cost, environmental friendliness, and high operating voltage. To date, various organic electrode materials have been applied in DIBs. In this review, we present the development of DIBs with a following brief introduction of characteristics and mechanisms of organic materials. The latest progress in the application of organic materials as anode and cathode materials for DIBs is mainly reviewed. Finally, we also discussed the challenges and prospects of organic electrode materials for DIBs.

Sustainable Dual-Ion Batteries beyond Li

The limitations of resources used in current Li-ion batteries may hinder their widespread use in grid-scale energy storage systems, prompting the search for low-cost and resource-abundant alternatives. "Beyond-Li cation" batteries have emerged as promising contenders; however, they confront noteworthy challenges due to the scarcity of suitable host materials for these cations. In contrast, anions, the other crucial component in electrolytes, demonstrate reversible intercalation capacity in specific materials like graphite. The convergence of anion and cation storage has given rise to a new battery technology known as dual-ion batteries (DIBs). This comprehensive review presents the current status, advancements, and future prospects of sustainable DIBs beyond Li. Notably, most DIBs exhibit similar cathode reaction mechanisms involving anion intercalation, while the distinguishing factor lies in the cation types functioning at the anode. Accordingly, the review is organized into sections by various cation types, including Na-, K-, Mg-, Zn-, Ca-, Al-, NH4 + -, and proton-based DIBs. Moreover, a perspective on these novel DIBs is presented, along with proposed protocols for investigating DIBs and promising future research directions. It is envisioned that this review will inspire fresh concepts, ideas, and research directions, while raising important questions to further tailor and understand sustainable DIBs, ultimately facilitating their practical realization.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Cation Co-Intercalation with Anions: The Origin of Low Capacities of Graphite Cathodes in Multivalent Electrolytes

Dual-ion batteries involving anion intercalation into graphite cathodes represent promising battery technologies for low-cost and high-power energy storage. However, the fundamental origins regarding much lower capacities of graphite cathodes in earth abundant and inexpensive multivalent electrolytes than in Li-ion electrolytes remain elusive. Herein, we reveal that the limited anion-storage capacity of a graphite cathode in multivalent electrolytes is rooted in the abnormal multivalent-cation co-intercalation with anions in the form of large-sized anionic complexes. This cation co-intercalation behavior persists throughout the stage evolution of graphite intercalation compounds and leads to a significant decrease of sites practically viable for capacity contribution inside graphite galleries. Further systematic studies illustrate that the phenomenon of cation co-intercalation into graphite is closely related to the high energy penalty of interfacial anion desolvation due to the strong cation-anion association prevalent in multivalent electrolytes. Leveraging this understanding, we verify that promoting ionic dissociation in multivalent electrolytes by employing high-permittivity and oxidation-tolerant co-solvents is effective in suppressing multivalent-cation co-intercalation and thus achieving increased capacity of graphite cathodes. For instance, introducing adiponitrile as a co-solvent to a Mg²⁺-based carbonate electrolyte leads to 83% less Mg²⁺ co-intercalation and a ∼29.5% increase in delivered capacity of the graphite cathode.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

Confined WS2 Nanosheets Tubular Nanohybrid as High-Kinetic and Durable Anode for Sodium-Based Dual Ion Batteries

Sodium based dual-ion battery (SDIB) has been regarded as one of the promising batteries technologies thanks to its high working voltage and natural abundance of sodium source, its practical application yet faces critical issues of low capacity and sluggish kinetics of intercalation-type graphite anode. Here, a tubular nanohybrid composed of building blocks of carbon-film wrapped WS₂ nanosheets on carbon nanotube (WS₂ /C@CNTs) was reported. The expanded (002) interlayer and dual-carbon confined structure endowed WS₂ nanosheets with fast charge transportation and excellent structural stability, and thus WS₂ /C@CNTs showed highly attractive electrochemical properties for Na⁺ storage with high reversible capacity, fast kinetic, and robust durability. The full sodium-based dual ion batteries by coupling WS₂ /C@CNTs anode with graphite cathode full cell presented a high reversible capacity (210 mAh g^-1 at 0.1 A g^-1 ), and excellent rate performance with a high capacity of 137 mAh g^-1 at 5.0 A g^-1 .

Spreading the Landscape of Dual Ion Batteries: from Electrode to Electrolyte

The working mechanism of a dual-ion battery (DIB) differs from that of a lithium-ion battery (LIB) in that the anions in the electrolyte of the former can be intercalated as well. Researchers have been paying close attention to this device because of its high voltage, low price, and environmental friendliness. However, DIBs are still in their early research stages, and numerous issues need to be addressed and investigated further. Initially, this Review explains how DIBs work in principle and discusses the progress of electrode materials for cathode and anode. Furthermore, since the electrolytes used as the active material, as well as anion, solvent, and additives, have a significant impact on the DIB's capacity and voltage, the current status is also presented in terms of electrolytes, followed by an outlook on confronting the challenges. A comprehensive summary from electrode to electrolyte will guide the development of next-generation DIBs.

Negative sulfur-based electrodes and their application in battery cells: Dual-ion batteries as an example

In this work, a cell concept comprising of an anion intercalating graphite-based positive electrode (cathode) and an elemental sulfur-based negative electrode (anode) is presented as a transition metal- and in a specific concept even Li-free cell setup using a Li-ion containing electrolyte or a Mg-ion containing electrolyte. The cell achieves discharge capacities of up to 37 mAh g−1 and average discharge cell voltages of up to 1.9 V. With this setup, more than 100 cycles with a high capacity retention (> 90% of the highest achieved value) and Coulombic efficiencies up to 95% could be achieved, which opens a broad new field for energy storage approaches.

Sustainable and cost-effective ternary electrolyte Et3NHCl-AlCl3-Mg(DEP)2 for high-performance rechargeable magnesium batteries

Cost-effective and sustainable battery materials for large-scale batteries are the need of the hour to garner renewable energy with high-performance metal battery technologies. Here, we report the high-performance and long cycle life electrolyte prepared from low-cost triethylamine hydrochloride (Et₃NHCl) and aluminum chloride (AlCl₃) termed as (TA) with different concentrations of magnesium diethylphosphate (Mg(DEP)₂) salt. The optimized ratio of the 0.1 M Mg(DEP)₂ electrolyte has shown a high ionic conductivity of 4.5 × 10^-3 S cm^-1 at ambient temperature and good anodic stability of 2.41 V vs. Mg/Mg²⁺. The dissolution/deposition of magnesium (Mg) on a Pt working electrode was systematically analyzed in this electrolyte. Cyclic voltammetry (CV) of the Mg-graphite battery was used to probe the intercalation/de-intercalation of Mg-AlCl₄^- ions into/from the graphite layer structure. This was confirmed by various analytical techniques, such as energy dispersive X-ray spectroscopy, X-ray diffraction technique and X-ray photoemission spectroscopy. Notably, during the galvanostatic study analysis, the assembled Mg cell delivered a high discharge capacity of 115 mA h g^-1 at a high C/10 rate, with more than 180 cycles at >80% coulombic efficiency. This electrolyte will be helpful in grid-scale power storage systems in future generations.