Electrolytes for Dual‐Carbon Batteries

Crucial Roles of Ethyl Methyl Carbonate in Lithium-Ion and Dual-Ion Batteries: A Review

The essential role of electrolyte solutions in traditional electrochemical energy storage devices is crucial to enhancing their performance. Consequently, a wide array of electrolyte mixtures along with diverse electrodes have been extensively explored across different models of secondary batteries. Fascinatingly, the role of ethyl methyl carbonate (EMC) as a key cosolvent in the electrolyte mixture of commercial lithium-ion batteries with a graphite anode is garnering growing attention in alternative rechargeable dual-ion batteries utilizing graphite cathodes. In this context, the advancement and function of EMC as a solvent in electrolyte mixtures for lithium-ion and dual-ion batteries were extensively and thoroughly examined in this analysis, encompassing the genesis, synthesis process, and diverse characteristics for the practical uses of these batteries. Here, the review aims to guide readers in understanding EMC's function and impact as a cosolvent in electrolyte mixtures for both major secondary lithium-ion and dual-ion batteries, considering their distinct physicochemical characteristics.

LiNi0.5Mn1.5O4 Joins to Alleviate Self-Discharge of Anion-Graphite Intercalation Compounds

A straightforward hybrid approach of a blend cathode is put forward to alleviate self-discharged anion-graphite intercalation compounds in dual-ion batteries. The self-discharge mechanism of the LiNi0.5Mn1.5O4/graphite blend cathode is investigated by conventional electrochemical tests and in situ X-ray diffraction measurements. A charging behavior between electrode materials during self-discharge has been discovered. This work will contribute to advancing the practical implementation of dual-ion batteries and provide valuable theoretical support for the research of blend electrodes.

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.

Significant Strain Dissipation via Stiff-Tough Solid Electrolyte Interphase Design for Highly Stable Alloying Anodes

The pulverization of alloying anodes significantly restricts their use in lithium-ion batteries (LIBs). This study presents a dual-phase solid electrolyte interphase (SEI) design that incorporates finely dispersed Al nanoparticles within the LiPON matrix. This distinctive dual-phase structure imparts high stiffness and toughness to the integrated SEI film. In comparison to single-phase LiPON film, the optimized Al/LiPON dual-phase SEI film demonstrates a remarkable increase in fracture toughness by 317.8 %, while maintaining stiffness, achieved through the substantial dissipation of strain energy. Application of the dual-phase SEI film on an Al anode leads to a 450 % enhancement in cycling stability for lithium storage in dual-ion batteries. A similar enhancement in cycling stability for silicon anodes, which face severe volume expansion issues, is also observed, demonstrating the broad applicability of the dual-phase SEI design. Specifically, homogeneous Li-Al alloying has been observed in conventional LIBs, even when paired with a high mass loading LiNi0.5 Co0.3 Mn0.2 O2 cathode (7 mg cm-2 ). The dual-phase SEI film design can also accelerate the diffusion kinetics of Li-ions through interface electronic structure regulation. This dual-phase design can integrate stiffness and toughness into a single SEI film, providing a pathway to enhance both the structural stability and rate capability of alloying anodes.

Electrochemistry-Driven Interphase Doubly Protects Graphite Cathodes for Ultralong Life and Fast Charge of Dual-Ion Batteries

Dual-ion batteries (DIBs) with graphite as cathode material, show superiority in terms of sustainability, affordability, and environmental impact over Li-ion batteries that rely on transition-metal based cathodes. However, graphite cathodes severely suffer from poor structural stability during anion storage at high potentials because of the co-intercalation and oxidative decomposition of electrolytes. This work presents an in situ electrochemistry-driven route to create a bifunctional interphase through implantation of diethylenetriaminepenta(methylene-phosphonic acid) (DTPMP) on the surface of graphite particles. The reaction mechanisms and functions of DTPMP are investigated both experimentally and theoretically. The DTPMP-derived interphase not only improves the antioxidative stability of electrolytes but also benefits the desolvation of PF₆ ^- anions, which doubly protect the graphitic structure and give rise to fast-charge and ultralong cycling performance of graphite cathodes in DIBs.

Sodium Dual-Ion Batteries with Concentrated Electrolytes

Na-based dual-ion batteries (DIBs) are a class of post-lithium technology with advantages including extremely fast charging, cost-effectiveness, and high natural abundance of raw materials. Operating up to high voltages (≈5.0 V), the decomposition of classic carbonate-based electrolyte formulations and the subsequent fade of capacity continues to be a major drawback in the development of these systems. Here, the performance of a Na-DIB was investigated in different commonly employed electrolyte system, and a highly concentrated (3 m NaPF6 ) and fluorine-rich carbonate-based formulation was optimized to achieve a good performance when compared with literature (based on energy and power density, calculated at coin cell and only using the active mass of active materials).

Rational Design of Functional Electrolytes Towards Commercial Dual-Ion Batteries

Dual-ion batteries (DIBs) based on anion (de)intercalation into low-cost graphitic carbon cathodes hold great promise in grid-scale energy storage. Different from the electrolyte in rocking-chair batteries, which only serves as a charge transporter, both cations and anions in the electrolyte for DIBs participate in battery reactions. Hence, the impact of the electrolyte formulation on cycle life, energy density, as well as cost has become a subject of vital importance. This review discussed the challenges and recent progress of electrolytes for DIBs, with a particular focus on the exploration of electrolytes with high oxidation stability, high salt concentration, high ionic conductivity, and low cost. Moreover, the influence of varied ion concentrations at different state-of-charge levels on the electrolyte properties such as ionic conductivity and electrochemical stability is analyzed. Finally, perspectives on the current limitations and future research directions of electrolytes for DIBs are provided.

Influence of Strong Ionic Interaction on the Kinetics of Graphite Intercalation Compound Formation

Graphite intercalation compounds (GICs) of anions have attracted much attention as a positive electrode for use in dual-ion batteries. In this study, GICs of bis(trifluoromethanesulfonyl)amide (TFSA) anions were electrochemically synthesized in ionic liquids with lithium or magnesium salts to investigate the kinetics of GICs formation. By varying the concentrations of LiTFSA or Mg(TFSA)₂ , the activation energy of interfacial anion transfer resistance was discovered to be correlated with the viscosity of the ionic liquid electrolytes. Such a change in activation energy is unique to ionic liquids and not seen in common molecular solvent electrolytes. In addition, ionic liquids with nearly identical viscosities exhibited similar resistances and activation energies. The relationship between the viscosity of ionic liquids and the kinetic parameters of interfacial anion transfer found in this study provides important insights for the construction of dual-ion batteries that aim for both high capacity and high rate capability.

The Effects of Ultrasound Treatment of Graphite on the Reversibility of the (De)Intercalation of an Anion from Aqueous Electrolyte Solution

Low cycling stability is one of the most crucial issues in rechargeable batteries. Herein, we study the effects of a simple ultrasound treatment of graphite for the reversible (de)intercalation of a ClO₄^- anion from a 2.4 M Al(ClO₄)₃ aqueous solution. We demonstrate that the ultrasound-treated graphite offers the improved reversibility of the ClO₄^- anion (de)intercalation compared with the untreated samples. The ex situ and in situ Raman spectroelectrochemistry and X-ray diffraction analysis of the ultrasound-treated materials shows no change in the interlayer spacing, a mild increase in the stacking order, and a large increase in the amount of defects in the lattice accompanied by a decrease in the lateral crystallite size. The smaller flakes of the ultrasonicated natural graphite facilitate the improved reversibility of the ClO₄^- anion electrochemical (de)intercalation and a more stable electrochemical performance with a cycle life of over 300 cycles.

Interplay among Hexafluorophosphate, Difluoro(oxalato)borate Anions and Ethylene Carbonate during Their Insertion into a Graphite Electrode

Ethylene carbonate solutions dissolving mixed lithium salts composed of both difluoro(oxalato)borate (DFOB^-) and hexafluorophosphate (PF₆^-) anions are introduced into Li/graphite cells. The anions' intercalation procedures into the graphite positive electrode from these solutions are explored by charge/discharge, cyclic voltammetry, and impedance spectroscopic tests in combination with electrochemical in situ characterization including X-ray diffraction (XRD) and Raman spectroscopy. Furthermore, these solutions are characterized by ionic conductivity together with nuclear magnetic resonance measurements. The properties of the solutions are linked to the capacity values delivered by Li/graphite cells.

A PF6 - -Permselective Polymer Electrolyte with Anion Solvation Regulation Enabling Long-Cycle Dual-Ion Battery

Graphitic carbon that allows reversible anion (de)intercalation is a promising cathode material for cost-efficient and high-voltage dual-ion batteries (DIBs). However, one notorious but overlooked issue is the incomplete interfacial anion desolvation, which not only reduces the oxidative stability of electrolytes, but also results in solvent co-intercalation into graphite layers. Here, an "anion-permselective" polymer electrolyte with abundant cationic quaternary ammonium motif is developed to weaken the PF₆ ^- -solvent interaction and thus facilitates PF₆ ^- desolvation. This strategy significantly inhibits solvent co-intercalation as well as enhances the oxidation resistance of electrolyte, ensuring the structural integrity of graphite. As a result, the as-assembled graphite||Li cell achieves a superior cyclability with an average Coulombic efficiency of 99.0% (vs 95.7% for baseline electrolyte) and 87.1% capacity retention after 2000 cycles even at a high cutoff potential of 5.4 V versus Li⁺ /Li. Besides, this polymer also forms a robust cathode electrolyte interface, working together to enable a long-life DIB. This strategy of tuning anion coordination environment provides a promising solution to regulate solvent co-intercalation chemistry for DIBs.

Emerging trends in anion storage materials for the capacitive and hybrid energy storage and beyond

Electrochemical capacitors charge and discharge more rapidly than batteries over longer cycles, but their practical applications remain limited due to their significantly lower energy densities. Pseudocapacitors and hybrid capacitors have been developed to extend Ragone plots to higher energy density values, but they are also limited by the insufficient breadth of options for electrode materials, which require materials that store alkali metal cations such as Li⁺ and Na⁺. Herein, we report a comprehensive and systematic review of emerging anion storage materials for performance- and functionality-oriented applications in electrochemical and battery-capacitor hybrid devices. The operating principles and types of dual-ion and whole-anion storage in electrochemical and hybrid capacitors are addressed along with the classification, thermodynamic and kinetic aspects, and associated interfaces of anion storage materials in various aqueous and non-aqueous electrolytes. The charge storage mechanism, structure-property correlation, and electrochemical features of anion storage materials are comprehensively discussed. The recent progress in emerging anion storage materials is also discussed, focusing on high-performance applications, such as dual-ion- and whole-anion-storing electrochemical capacitors in a symmetric or hybrid manner, and functional applications including micro- and flexible capacitors, desalination, and salinity cells. Finally, we present our perspective on the current impediments and future directions in this field.

Locally Ordered Graphitized Carbon Cathodes for High-Capacity Dual-Ion Batteries

Dual-ion batteries (DIBs) inherently suffer from limited energy density. Proposed here is a strategy to effectively tackle this issue by employing locally ordered graphitized carbon (LOGC) cathodes. Quantum mechanical modeling suggests that strong anion-anion repulsions and severe expansion at the deep-charging stage raise the anion intercalation voltage, therefore only part of the theoretical anion storage sites in graphite is accessible. The LOGC interconnected with disordered carbon is predicted to weaken the interlaminar van der Waals interactions, while disordered carbons not only interconnect the dispersed nanographite but also partially buffer severe anion-anion repulsion and offer extra capacitive anion storage sites. As a proof-of-concept, ketjen black (KB) with LOGC was used as a model cathode for a potassium-based DIB (KDIB). The KDIB delivers an unprecedentedly high specific capacity of 232 mAh g^-1 at 50 mA g^-1 , a good rate capability of 110 mAh g^-1 at 2000 mA g^-1 , and excellent cycling stability of 1000 cycles without obvious capacity fading.

Nature of the Cathode-Electrolyte Interface in Highly Concentrated Electrolytes Used in Graphite Dual-Ion Batteries

Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li⁺/Li⁰ and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode-electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF₆, LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g^-1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF₆-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g^-1. The poor performance in LiPF₆-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in Li_xPF_y, PO_xF_y, Li_xPO_yF_z, and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF₆-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.

Dual-Carbon Batteries: Materials and Mechanism

Various carbon nanomaterials are being widely studied for applications in supercapacitors and Li-ion batteries as well as hybrid energy storage devices. Dual-carbon batteries (DCBs), in which both electrodes are composed of functionalized carbon materials, are capable of delivering high energy/power and stable cycles when they are rationally designed. This Review focuses on the electrochemical reaction mechanisms and energy storage properties of various carbon electrode materials in DCBs, including graphite, graphene, hard and soft carbon, activated carbon, and their derivatives. The interfacial chemistry between carbon electrodes and electrolyte is also discussed. The perspective for further development of DCBs is presented at the end.