New Concepts in Electrolytes

Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes

To understand and control key electrochemical processes-metal plating, corrosion, intercalation, etc.-requires molecular-scale details of the active species at electrochemical interfaces and their mechanisms for desolvation from the electrolyte. Using free energy sampling techniques we reveal the interfacial speciation of divalent cations in ether-based electrolytes and mechanisms for their delivery to an inert graphene electrode interface. Surprisingly, we find that anion solvophobicity drives a high population of anion-containing species to the interface that facilitate the delivery of divalent cations, even to negatively charged electrodes. Our simulations indicate that cation desolvation is greatly facilitated by cation-anion coupling. We propose anion solvophobicity as a molecular-level descriptor for rational design of electrolytes with increased efficiency for electrochemical processes limited by multivalent cation desolvation.

Recent Advances in Living Cationic Polymerization with Emerging Initiation/Controlling Systems

While the conventional living cationic polymerization (LCP) provided opportunities to synthesizing well-defined polymers with predetermined molecular weights, desirable chemical structures and narrow dispersity, it is still important to continuously innovate new synthetic methods to meet the increasing requirements in advanced material engineering. Consequently, a variety of novel initiation/controlling systems have be demonstrated recently, which have enabled LCP with spatiotemporal control, broadened scopes of monomers and terminals, more user-friendly operations and reaction conditions, as well as improved thermomechanical properties for obtained polymers. In this work, recent advances in LCP is summarized with emerging initiation/controlling systems, including chemical-initiated/controlled cationic reversible addition-fragmentation chain transfer (RAFT) polymerization, photoinitiated/controlled LCP, electrochemical-controlled LCP, thionyl/selenium halide-initiated LCP, organic acid-assisted LCP, and stereoselective LCP. It is hoped that this summary will provide useful knowledge to people in related fields and stimulate new ideas to promote the development and application of LCP in both academia and industry.

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.

Water-in-Salt LiTFSI Aqueous Electrolytes. 1. Liquid Structure from Combined Molecular Dynamics Simulation and Experimental Studies

The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy density in aqueous Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform infrared spectra measurements over a wide range of temperatures and salt concentrations are reported for the LiTFSI (lithium bis(trifluoromethane sulfonyl)imide)-based water-in-salt electrolyte. Classical molecular dynamics simulations are validated against the experiments and used to gain additional information about the electrolyte structure. Based on our analyses, a new model for the liquid structure is proposed. Specifically, we demonstrate that at the highest LiTFSI concentration of 20 m the water network is disrupted, and the majority of water molecules exist in the form of isolated monomers, clusters, or small aggregates with chain-like configurations. On the other hand, TFSI^- anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies of this system.

Computing the frequency fluctuation dynamics of highly coupled vibrational transitions using neural networks

The description of frequency fluctuations for highly coupled vibrational transitions has been a challenging problem in physical chemistry. In particular, the complexity of their vibrational Hamiltonian does not allow us to directly derive the time evolution of vibrational frequencies for these systems. In this paper, we present a new approach to this problem by exploiting the artificial neural network to describe the vibrational frequencies without relying on the deconstruction of the vibrational Hamiltonian. To this end, we first explored the use of the methodology to predict the frequency fluctuations of the amide I mode of N-methylacetamide in water. The results show good performance compared with the previous experimental and theoretical results. In the second part, the neural network approach is used to investigate the frequency fluctuations of the highly coupled carbonyl stretch modes for the organic carbonates in the solvation shell of the lithium ion. In this case, the frequency fluctuation predicted by the neural networks shows a good agreement with the experimental results, which suggests that this model can be used to describe the dynamics of the frequency in highly coupled transitions.

Simultaneous Stabilization of the Solid/Cathode Electrolyte Interface in Lithium Metal Batteries by a New Weakly Solvating Electrolyte

So far, the practical application of Li metal batteries has been hindered by the undesirable formation of Li dendrites and low Coulombic efficiencies (CEs). Herein, 1,2-diethoxyethane (DEE) is proposed as a new electrolytic solvent for lithium metal batteries (LMBs), and the performances of 1.0 m LiFSI in DEE are evaluated. Because of the low dielectric constant and dipole moment of DEE, the majority of the FSI^- exists in associated states like contact ion pairs and aggregates, which is similar to the highly concentrated electrolytes. These associated complexes are involved in the reduction reaction on the Li metal anode, forming sound solid electrolyte interphase layers. Furthermore, free FSI^- ions in DEE are observed to participate in the formation of cathode electrolyte interphase layers. These passivation layers not only suppress dendrite growth on the Li anode but also prevent unwanted side-reactions on the LiFePO₄ cathode. The average CE of the Li||Cu cells in LiFSI-DEE is observed to be 98.0%. Moreover, LiFSI-DEE also plays an important role in enhancing the cycling stability of the Li||LiFP cell with a capacity retention of 93.5% after 200 cycles. These results demonstrate the benefits of LiFSI-DEE, which creates new possibilities for high-energy-density rechargeable LMBs.

Experimental Thermal Hazard Investigation of Pressure and EC/PC/EMC Mass Ratio on Electrolyte

Electrolytes are involved in the thermal runaway (TR) process of cells, which is a potential hazard in lithium-ion batteries (LIBs). Therefore, the effects of different mass ratio of carbonate solvents (ethylene carbonate (EC)/propylene carbonate (PC)/ethyl methyl carbonate (EMC)) with LiBF4 and different environmental pressure on the combustion characteristics of electrolyte such as flame centerline temperature, mass loss rate (MLR) and heat release rate (HRR) were analyzed. The combustion process could be divided into four stages: ignition, stable combustion stage, stable combustion with flame color change stage and extinguishing; with the decrease of pressure, the MLR of electrolyte declined and the combustion time prolonged, while the temperature of flame centerline increased.

Organosulfide-Based Deep Eutectic Electrolyte for Lithium Batteries

Deep eutectic electrolytes (DEEs) are a new class of electrolytes with unique properties. However, the intermolecular interactions of DEEs are mostly dominated by Li⋅⋅⋅O interactions, limiting the diversity of chemical space and material constituents. Herein, we report a new class of DEEs induced by Li⋅⋅⋅N interactions between 2,2'-dipyridyl disulfide (DpyDS) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The strong ion-dipole interaction triggers the deep eutectic phenomenon, thus liberating the Li⁺ from LiTFSI and endowing the DEEs with promising ionic conductivity. These DEEs show admirable intrinsic safety, which cannot be ignited by flame. The DEE at the molar ratio of DpyDS:LiTFSI=4:1 (abbreviated as DEE-4:1) is electrochemically stable between 2.1 and 4.0 V vs. Li/Li⁺ , and exhibits an ionic conductivity of 1.5×10^-4  S cm^-1 at 50 °C. The Li/LiFePO₄ half cell with DEE-4:1 can provide a reversible capacity of 130 mAh g^-1 and Coulombic efficiency above 98 % at 50 °C.

High-Energy Aqueous Sodium-Ion Batteries

Water-in-salt electrolytes (WISE) have largely widened the electrochemical stability window (ESW) of aqueous electrolytes by formation of passivating solid electrolyte interphase (SEI) on anode and also absorption of the hydrophobic anion-rich double layer on cathode. However, the cathodic limiting potential of WISE is still too high for most high-capacity anodes in aqueous sodium-ion batteries (ASIBs), and the cost of WISE is also too high for practical application. Herein, a low-cost 19 m (m: mol kg^-1 ) bi-salts WISE with a wide ESW of 2.8 V was designed, where the low-cost 17 m NaClO₄ extends the anodic limiting potential to 4.4 V, while the fluorine-containing salt (2 m NaOTF) extends the cathodic limiting potential to 1.6 V by forming the NaF-Na₂ O-NaOH SEI on anode. The 19 m NaClO₄ -NaOTF-H₂ O electrolyte enables a 1.75 V Na₃ V₂ (PO₄ )₃ ∥Na₃ V₂ (PO₄ )₃ full cell to deliver an appreciable energy density of 70 Wh kg^-1 at 1 C with a capacity retention of 87.5 % after 100 cycles.

Correlated Ion Transport and the Gel Phase in Room Temperature Ionic Liquids

Here we present a theory of ion aggregation and gelation of room temperature ionic liquids (RTILs). Based on it, we investigate the effect of ion aggregation on correlated ion transport-ionic conductivity and transference numbers-obtaining closed-form expressions for these quantities. The theory depends on the maximum number of associations a cation and anion can form and the strength of their association. To validate the presented theory, we perform molecular dynamics simulations on several RTILs and a range of temperatures for one RTIL. The simulations indicate the formation of large clusters, even percolating through the system under certain circumstances, thus forming a gel, with the theory accurately describing the obtained cluster distributions in all cases. However, based on the strength and lifetime of associations in the simulated RTILs, we expect free ions to dominate ionic conductivity despite the presence of clusters, and we do not expect the percolating cluster to trigger structural arrest in the RTIL.

Porous Polyamide Skeleton-Reinforced Solid-State Electrolyte: Enhanced Flexibility, Safety, and Electrochemical Performance

The growing demand for safer lithium-ion batteries draws researchers' attention to solid-state electrolytes. In general, a desired electrolyte should be flexible, mechanically strong, and with high ionic conductivity. A solid-state electrolyte with a polymer as a matrix seems to be able to meet these demands. However, a pure polymer electrolyte lacks sufficient strength to suppress Li dendrites, and hybrids with ceramic components often lead to poor flexibility, both far from satisfactory. Herein, a solid-state electrolyte is designed by employing a mass-produced porous polyamide (PA) film infiltrated with polyethylene oxide (PEO)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The PA/PEO/LiTFSI electrolyte is flexible but robust with a Young's modulus of up to 1030 MPa, ensuring steady Li//Li cycling without short circuit for more than 400 h. Also, the porous structure of the PA film decreases the crystalline regions and effectively enhances the ionic conductivity (2.05 × 10^-4 S cm^-1 at 30 °C). When cycled at 1C, solid-state LiFePO₄//Li batteries assembled with the PA/PEO/LiTFSI electrolyte retain 82% capacity after 300 cycles (60 °C). In addition, a flexible LiFePO₄//PA/PEO/LiTFSI//Li pouch cell can also work well in harsh operating environments, such as being folded, crimped, and pierced.

Effects of fluorinated solvents on electrolyte solvation structures and electrode/electrolyte interphases for lithium metal batteries

Electrolyte is very critical to the performance of the high-voltage lithium (Li) metal battery (LMB), which is one of the most attractive candidates for the next-generation high-density energy-storage systems. Electrolyte formulation and structure determine the physical properties of the electrolytes and their interfacial chemistries on the electrode surfaces. Localized high-concentration electrolytes (LHCEs) outperform state-of-the-art carbonate electrolytes in many aspects in LMBs due to their unique solvation structures. Types of fluorinated cosolvents used in LHCEs are investigated here in searching for the most suitable diluent for high-concentration electrolytes (HCEs). Nonsolvating solvents (including fluorinated ethers, fluorinated borate, and fluorinated orthoformate) added in HCEs enable the formation of LHCEs with high-concentration solvation structures. However, low-solvating fluorinated carbonate will coordinate with Li⁺ ions and form a second solvation shell or a pseudo-LHCE which diminishes the benefits of LHCE. In addition, it is evident that the diluent has significant influence on the electrode/electrolyte interphases (EEIs) beyond retaining the high-concentration solvation structures. Diluent molecules surrounding the high-concentration clusters could accelerate or decelerate the anion decomposition through coparticipation of diluent decomposition in the EEI formation. The varied interphase features lead to significantly different battery performance. This study points out the importance of diluents and their synergetic effects with the conductive salt and the solvating solvent in designing LHCEs. These systematic comparisons and fundamental insights into LHCEs using different types of fluorinated solvents can guide further development of advanced electrolytes for high-voltage LMBs.

Electrolytes and Interphases in Potassium Ion Batteries

Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost-effectiveness, high-voltage, and high-power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid-state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.

Improvement of Electrochemical Stability Using the Eutectic Composition of a Ternary Molten Salt System for Highly Concentrated Electrolytes for Na-Ion Batteries

The increase in the concentration of electrolytes for secondary batteries has significant advantages in terms of physicochemical and electrochemical performance. This study aims to explore a highly concentrated electrolyte for Na-ion batteries using a ternary salt system. The eutectic composition of the Na[N(SO₂F)₂]-Na[N(SO₂F)(SO₂CF₃)]-Na[SO₃CF₃] ternary molten salt system increases solubility into an organic solvent, enabling the use of highly concentrated electrolytes for Na-ion batteries. The ternary salt system achieved concentrations of 5.0 m (m = mol kg^-1) with propylene carbonate (PC), 2.9 m with dimethoxyethane, 2.0 m with ethylene carbonate/dimethyl carbonate, and 3.9 m with ethylene carbonate/diethyl carbonate. The highly concentrated electrolyte of 5.0 m in PC suppressed Al corrosion and exhibited better oxidative stability. Stable electrochemical performance using hard carbon/NaCrO₂ in the full-cell configuration introduces a new strategy to explore highly concentrated electrolytes for secondary batteries.

Synthesis and properties of poly(1,3-dioxolane) in situ quasi-solid-state electrolytes via a rare-earth triflate catalyst

We report a rare-earth triflate catalyst Sc(OTf)3 for the ring-opening polymerization of 1,3-dioxolane and the in situ production of a quasi-solid-state poly(1,3-dioxolane) electrolyte, which not only demonstrates a superior ionic conductivity of 1.07 mS cm-1 at room temperature, but achieves dendrite-free lithium deposition and a high Coulombic efficiency of 92.3% over 200 Li plating/striping cycles.

Advanced liquid electrolytes enable practical applications of high-voltage lithium-metal full batteries

High-voltage lithium metal batteries (HVLMBs) have received widespread attention as next generation high-energy-density batteries to meet the urgent demands of modern life. However, the unstable interphase between electrolytes and highly reactive electrodes is still an important threshold for practical applications. In this feature article, we review the formation mechanism of the electrode-electrolyte interphase in terms of cathodes and the Li metal anode, respectively, and summarize the surface modification methods to stabilize the interphase of HVLMBs. Electrolyte regulation strategies especially those using electrolyte additives are introduced, and the relationship between liquid electrolyte formulation, interphase engineering and the electrochemical performance of HVLMBs is analyzed. Finally, an industry-level evaluation is carried out and the remaining challenges are discussed for advanced electrolytes to guarantee the practical applications and commercialization of HVLMBs.

An electrolyte- and catalyst-free electrooxidative sulfonylation of imidazo[1,2-a]pyridines

An electrolyte- and catalyst-free electrooxidative C–H activation reaction is developed to afford 3-sulfonylated imidazo[1,2-a]pyridines in good to excellent yields.

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest. New research interests have reemerged in a subgroup of these phenomena-electrochemical growth of metals in battery anodes. In this Review, the geometry of the building blocks and their mode of assembly are defined as key descriptors to categorize deposition morphologies. To control Zn electrodeposit morphology, we consider fundamental electrokinetic principles and the associated critical issues. It is found that the solid-electrolyte interphase (SEI) formed on Zn has a similarly strong influence as for alkali metals at low current regimes, characterized by a moss-like morphology. Another key conclusion is that the unique crystal structure of Zn, featuring high anisotropy facets resulting from the hexagonal close-packed lattice with a c/a ratio of 1.85, imposes predominant influences on its growth. In our view, precisely regulating the SEI and the crystallographic features of the Zn offers exciting opportunities that will drive transformative progress.

Low-Temperature Multielement Fusible Alloy-Based Molten Sodium Batteries for Grid-Scale Energy Storage

The sustainable future of modern society relies on the development of advanced energy systems. Alkali metals, such as Li, Na, and K, are promising to construct high-energy-density batteries to complement the fast-growing implementation of renewable sources. The stripping/deposition of alkali metals is compromised by serious dendrite growth, which can be intrinsically eliminated by using molten alkali metal anodes. Up to now, most of the conventional molten alkali metal-based batteries need to be operated at high temperatures. To decrease the operating temperature, we extended the battery chemistry to multielement alloys, which provide more flexibility for wide selection and rational screening of cost-effective and fusible metallic electrodes. On the basis of an integrated experimental and theoretical study, the depressed melting point and enhanced interfacial compatibility are elucidated. The proof-of-concept molten sodium battery enabled by the Bi-Pb-Sn fusible alloy not only circumvents the use of costly Ga and In elements but also delivers attractive performance at 100 °C, holding great promise for grid-scale energy storage.

A Non-aqueous H3 PO4 Electrolyte Enables Stable Cycling of Proton Electrodes

A non-aqueous proton electrolyte is devised by dissolving H₃ PO₄ into acetonitrile. The electrolyte exhibits unique vibrational signatures from stimulated Raman spectroscopy. Such an electrolyte exhibits unique characteristics compared to aqueous acidic electrolytes: 1) higher (de)protonation potential for a lower desolvation energy of protons, 2) better cycling stability by dissolution suppression, and 3) higher Coulombic efficiency owing to the lack of oxygen evolution reaction. Two non-aqueous proton full cells exhibit better cycling stability, higher Coulombic efficiency, and less self-discharge compared to the aqueous counterpart.

Hopping in High Concentration Electrolytes - Long Time Bulk and Single-Particle Signatures, Free Energy Barriers, and Structural Insights

Although ion-hopping is believed to be a significant mode of transport for small ions in liquid high concentration electrolytes (HCE), its bulk signatures over sufficiently long time intervals are yet to be shown. We computationally establish the long and short time imprints of hopping in HCEs using LiBF₄-in-sulfolane mixtures as models. The high viscosity of this electrolyte leads to significant dynamic heterogeneity in Li-ion transport. Li-ions exhibit a preference to transit to previously occupied Li-ion-sites, bridged through anion or solvent molecules. Hopping in the liquid matrix was found to be an activated process, whose free energy barrier and transition state structure have been determined. Evidence for nanoscale compositional heterogeneity at high salt concentrations is also presented. The simulations shed light on the composition, stiffness, and lifetime of the solvation shell of Li ions. The understanding of HCEs gleaned from this study will spearhead the choice, engineering and applicability of this class of electrolytes.

Lithium Metal Anodes with Nonaqueous Electrolytes

High-energy rechargeable lithium (Li) metal batteries (LMBs) with Li metal anode (LMA) were first developed in the 1970s, but their practical applications have been hindered by the safety and low-efficiency concerns related to LMA. Recently, a worldwide effort on LMA-based rechargeable LMBs has been revived to replace graphite-based, Li-ion batteries because of the much higher energy density that can be achieved with LMBs. This review focuses on the recent progress on the stabilization of LMA with nonaqueous electrolytes and reveals the fundamental mechanisms behind this improved stability. Various strategies that can enhance the stability of LMA in practical conditions and perspectives on the future development of LMA are also discussed. These strategies include the use of novel electrolytes such as superconcentrated electrolytes, localized high-concentration electrolytes, and highly fluorinated electrolytes, surface coatings that can form a solid electrolyte interphase with a high interfacial energy and self-healing capabilities, development of "anode-free" Li batteries to minimize the interaction between LMA and electrolyte, approaches to enable operation of LMA in practical conditions, etc. Combination of these strategies ultimately will lead us closer to the large-scale application of LMBs which often is called the "Holy Grail" of energy storage systems.

Probing the electrode-solution interfaces in rechargeable batteries by sum-frequency generation spectroscopy

An in-depth understanding of the electrode-electrolyte interaction and electrochemical reactions at the electrode-solution interfaces in rechargeable batteries is essential to develop novel electrolytes and electrode materials with high performance. In this perspective, we highlight the advantages of the interface-specific sum-frequency generation (SFG) spectroscopy on the studies of the electrode-solution interface for the Li-ion and Li-O₂ batteries. The SFG studies in probing solvent adsorption structures and solid-electrolyte interphase formation for the Li-ion battery are briefly reviewed. Recent progress on the SFG study of the oxygen reaction mechanisms and stability of the electrolyte in the Li-O₂ battery is also discussed. Finally, we present the current perspective and future directions in the SFG studies on the electrode-electrolyte interfaces toward providing deeper insight into the mechanisms of discharging/charging and parasitic reactions in novel rechargeable battery systems.

The Features and Progress of Electrolyte for Potassium Ion Batteries

Nowadays, Li-ion batteries have achieved great success and are widely used in various fields. However, the scarcity and uneven distribution of lithium resources together with the increasing cost may hamper the sustainable development of Li-ion batteries in the future. Hence, many researchers have turned to potassium ion batteries due to their abundant raw materials, low price, and high energy density. Although great progress has been made in recent years, there are still existing many challenges, especially the severe side reaction between electrolyte and K metal, which leads to an unstable solid-liquid interface and low coulombic efficiency. Hence, an excellent electrolyte may be the key to development of K-ion batteries in the future. Unfortunately, no systematic research has been conducted to study the electrolyte and its role on the performance yet. In order to compensate for this limitation, in this paper, the status and progress of electrolytes for K-ion batteries are reviewed, the issues and challenges existing in the development of electrolyte are clarified, and the future development is prospected.

Thermal runaway of Lithium-ion batteries employing LiN(SO2F)2-based concentrated electrolytes

Concentrated electrolytes usually demonstrate good electrochemical performance and thermal stability, and are also supposed to be promising when it comes to improving the safety of lithium-ion batteries due to their low flammability. Here, we show that LiN(SO₂F)₂-based concentrated electrolytes are incapable of solving the safety issues of lithium-ion batteries. To illustrate, a mechanism based on battery material and characterizations reveals that the tremendous heat in lithium-ion batteries is released due to the reaction between the lithiated graphite and LiN(SO₂F)₂ triggered thermal runaway of batteries, even if the concentrated electrolyte is non-flammable or low-flammable. Generally, the flammability of an electrolyte represents its behaviors when oxidized by oxygen, while it is the electrolyte reduction that triggers the chain of exothermic reactions in a battery. Thus, this study lights the way to a deeper understanding of the thermal runaway mechanism in batteries as well as the design philosophy of electrolytes for safer lithium-ion batteries.

Eutectic Electrolytes as a Promising Platform for Next-Generation Electrochemical Energy Storage

ConspectusThe rising global energy demand and environmental challenges have spurred intensive interest in renewable energy and advanced electrochemical energy storage (EES), including redox flow batteries (RFBs), metal-based rechargeable batteries, and supercapacitors. While many researchers focus on the design of new chemistry and structures for high-capacity and stable electrode materials, the electrolyte also plays a significant role in enabling the successful function of these new electrode materials and chemistries. Discovery of new electrolytes is urgently needed to keep up with the rapid growth of EES. Benefiting from the strong intermolecular interaction between different components, eutectic electrolytes possess various specific functionalities that conventional electrolytes do not have, such as highly concentrated systems, non-flammability, high degrees of structural flexibility, and good thermal and chemical stability, thereby leading researchers to consider them as a new class of ionic fluids for EES applications.In this Account, we aim to provide a mechanistic understanding of this energy chemistry and an overview of recent progress in the development of eutectic electrolytes for next-generation EES. First, we describe different mechanisms that guide the formation of eutectic electrolytes and discuss the structure-property relations, electron transfer and ion transport mechanisms, and interfacial chemistry in eutectic electrolytes. Generally, three main intermolecular interactions, namely hydrogen-bond interactions, Lewis acid-base interactions, and van der Waals interactions, control the formation of eutectic electrolytes and determine their unique characters in terms of electrochemical, thermal, ion transport, and interfacial properties. These versatile intermolecular interactions can be further modified by tailoring the functional moieties of organic molecules and/or selecting suitable compositions of mixtures. The solvent-free eutectic electrolyte can maximize the molar ratio of redox-active materials, thus increasing the energy density of RFBs. We discuss the relationships between eutectic parameters (viscosity, polarity, ionic conductivity, surface tension, and coordination environment) and the molar ratio, stability, utilization, and electrochemical reversibility of redox-active materials, RFB power, and energy density. We then introduce the application of both metal- and organic-based eutectic electrolytes in the RFB field, along with the relevant perspective for future study in this field. The highly concentrated eutectic electrolytes show attractive features at electrolyte/electrode interfaces to expand the electrochemical window and meanwhile inhibit metal dendrite formation in metal-based rechargeable batteries, supercapacitors, and hybrids of these. The remaining challenges and potential research directions in these areas are also discussed. Eutectic electrolytes offer enormous opportunities and open appealing prospects as redox reaction and charge transport media for EES. We hope this Account provide guidance for the future design of advanced eutectic electrolytes toward next-generation EES systems.

Tailoring Solution-Processable Li Argyrodites Li6+xP1-xMxS5I (M = Ge, Sn) and Their Microstructural Evolution Revealed by Cryo-TEM for All-Solid-State Batteries

Owing to their high Li⁺ conductivities, mechanical sinterability, and solution processability, sulfide Li argyrodites have attracted much attention as enablers in the development of high-performance all-solid-state batteries with practicability. However, solution-processable Li argyrodites have been developed only for a composition of Li₆PS₅X (X = Cl, Br, I) with insufficiently high Li⁺ conductivities (∼10^-4 S cm^-1). Herein, we report the highest Li⁺ conductivity of 0.54 mS cm^-1 at 30 °C (Li_6.5P_0.5Ge_0.5S₅I) for solution-processable iodine-based Li argyrodites. A comparative investigation of three iodine-based argyrodites of unsubstituted and Ge- and Sn-substituted solution-processed Li₆PS₅I with varied heat-treatment temperature elucidates the effect of microstructural evolution on Li⁺ conductivity. Notably, local nanostructures consisting of argyrodite nanocrystallites in solution-processed Li_6.5P_0.5Ge_0.5S₅I have been directly captured by cryogenic transmission electron microscopy, which is a first for sulfide solid electrolyte materials. Specifically, the promising electrochemical performances of all-solid-state batteries at 30 °C employing LiCoO₂ electrodes tailored by the infiltration of Li_6.5P_0.5Ge_0.5S₅I-ethanol solutions are successfully demonstrated.

A review on energy chemistry of fast-charging anodes

Fundamentals, challenges, and solutions towards fast-charging graphite anodes are summarized in this review, with insights into the future research and development to enable batteries suitable for fast-charging application.

Emerging interfacial chemistry of graphite anodes in lithium-ion batteries

Emerging interfacial chemistry of the graphite anode in today's lithium-ion batteries paves the way to next-generation, high-performance energy storage devices.

Covalently cross-linked polymer stabilized electrolytes with self-healing performance via boronic ester bonds

This article reported a facile fabrication of self-healing solid polymer electrolytes via boronic ester bonds.