New Concepts in Electrolytes

Small-Dipole-Molecule-Containing Electrolytes for High-Voltage Aqueous Rechargeable Batteries

High-voltage aqueous rechargeable batteries are promising competitors for next-generation energy storage systems with safety and high specific energy, but they are limited by the absence of low-cost aqueous electrolytes with a wide electrochemical stability window (ESW). The decomposition of aqueous electrolytes is mainly facilitated by the hydrogen bond network between water molecules and the water molecules in the solvation sheath. Here, three types of small dipole molecules (small molecules containing a dipole; glycerol (Gly), erythritol (Et), and acrylamide (AM)) are reported to develop aqueous electrolytes with high safety and wide ESW (over 2.5 V) for aqueous lithium-, sodium-, and zinc-ion batteries, respectively. The solvation-sheath structures are explored by ab initio molecular dynamics (MD) simulations, demonstrating that three types of dipole molecules deplete the water molecules in the solvation sheath of the charge carrier and break the hydrogen bond network between the water molecules, thus effectively expanding the ESW. A battery constructed from lithium titanate and lithium manganate in Gly-containing electrolyte exhibits an output voltage of 2.45 V and retains a specific capacity of 119.6 mAh g^-1 after 400 cycles. This work provides another strategy for exploiting low-cost high-voltage electrolytes for aqueous energy-storage systems.

Solid electrolytes for solid-state Li/Na–metal batteries: inorganic, composite and polymeric materials

This feature article presents the electrolyte synthetic approaches, design strategies, and merging materials that may address the critical issues of solid electrolytes for solid-state Li/Na–metal batteries.

A Dynamic and Self-Adapting Interface Coating for Stable Zn-Metal Anodes

The zinc (Zn)-ion battery has attracted much attention due to its high safety and environmental protection. At present, the critical issues of the generation of dendrites and the accumulation of dead Zn on the surface will lead to a sharp decline of the battery life. Zn dendrites can be inhibited to some extent by constructing an interface protective coating. However, the existing rigid coating method cannot maintain conformal contact with Zn due to the volume change of Zn deposition and will cause fracture irreversibly during the cycle. Here, a highly self-adaptable poly(dimethylsiloxane) (PDMS)/TiO_{2- x} coating is developed that can dynamically adapt to volume changes and inhibit dendrites growth. PDMS has high dynamic and self-adaptability due to the crosslinking of the B-O bond. In addition, the rapid and uniform transfer of Zn²⁺ is induced by the oxygen-vacancy-rich TiO_{2- x} . The assembled cells still achieve 99.6% coulombic efficiency after 700 cycles at a current density of 10 mA cm^-2 . The adaptive interface coating constructed provides a sufficient guarantee for the stable operation of the Zn anode.

Liquid-Phase Speed of Sound and Vapor-Phase Density of Difluoromethane

Difluoromethane (HFC-32, DFM), with a global warming potential (GWP) of 677, is of interest as a pure refrigerant and as a component in low-GWP refrigerant mixtures. Additionally, difluoromethane has recently been identified as a safe, liquefied-gas electrolyte material in batteries. Using state-of-the-art instruments for measurements, this paper presents new liquid-phase speed of sound and vapor-phase density data for difluoromethane. Two hundred and nine liquid-phase speed of sound values were measured using a dual-path pulse-echo instrument at temperatures from 230 to 345 K and pressures from 2.1 to 70 MPa. Accounting for all sources of uncertainty, the relative expanded combined uncertainty (k = 2) in the speed of sound ranged from 0.035 to 0.17%. One hundred and thirty-eight vapour-phase density values were measured using a two-sinker densimeter at temperatures from 240 to 340 K and pressures from 0.1 to 1.61 MPa with an uncertainty of 0.011 to 0.12%. These experimental data will be valuable in the ongoing development of a new fundamental thermodynamic equation of state for difluoromethane.

A low-concentration all-fluorinated electrolyte for stable lithium metal batteries

A novel low-concentration all-fluorinated electrolyte was designed to stabilize lithium metal batteries with excellent wettability and safety.

Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes

Acetonitrile has emerged as a solvent candidate for novel electrolyte formulations in metal-ion batteries and supercapacitors. It features a bright local C≡N stretch vibrational mode whose infrared (IR) signature is sensitive to battery-relevant cations (Li⁺, Mg²⁺, Zn²⁺, Ca²⁺) both in pure form and in the presence of water admixture across a full possible range of concentrations from the dilute to the superconcentrated regime. Stationary and time-resolved IR spectroscopy thus emerges as a natural tool to study site-specific intermolecular interactions from the solvent perspective without introducing an extrinsic probe that perturbs solution morphology and may not represent the intrinsic dynamics in these electrolytes. The metal-coordinated acetonitrile, water-separated metal-acetonitrile pair, and free solvent each have a distinct vibrational signature that allows their unambiguous differentiation. The IR band frequency of the metal-coordinated acetonitrile depends on the ion charge density. To study the ion transport dynamics, it is necessary to differentiate energy-transfer processes from structural interconversions in these electrolytes. Isotope labeling the solvent is a necessary prerequisite to separate these processes. We discuss the design principles and choice of the CD₃¹³CN label and characterize its vibrational spectroscopy in these electrolytes. The Fermi resonance between ¹³C≡N and C-D stretches complicates the spectral response but does not prevent its effective utilization. Time-resolved two-dimensional (2D) IR spectroscopy can be performed on a mixture of acetonitrile isotopologues and much can be learned about the structural dynamics of various species in these formulations.

Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations

Salt-in-ionic liquid electrolytes have attracted significant attention as potential electrolytes for next generation batteries largely due to their safety enhancements over typical organic electrolytes. However, recent experimental and computational studies have shown that under certain conditions alkali cations can migrate in electric fields as if they carried a net negative effective charge. In particular, alkali cations were observed to have negative transference numbers at small mole fractions of alkali-metal salt that revert to the expected net positive transference numbers at large mole fractions. Simulations have provided some insights into these observations, where the formation of asymmetric ionic clusters, as well as a percolating ion network, could largely explain the anomalous transport of alkali cations. However, a thermodynamic theory that captures such phenomena has not been developed, as ionic associations were typically treated via the formation of ion pairs. The theory presented herein, based on the classical polymer theories, describes thermoreversible associations between alkali cations and anions, where the formation of large, asymmetric ionic clusters and a percolating ionic network are a natural result of the theory. Furthermore, we present several general methods to calculate the effective charge of alkali cations in ionic liquids. We note that the negative effective charge is a robust prediction with respect to the parameters of the theory and that the formation of a percolating ionic network leads to the restoration of net positive charges of the cations at large mole fractions of alkali metal salt. Overall, we find excellent qualitative agreement between our theory and molecular simulations in terms of ionic cluster statistics and the effective charges of the alkali cations.

Self-Healing of a Covalently Cross-Linked Polymer Electrolyte Membrane by Diels-Alder Cycloaddition and Electrolyte Embedding for Lithium Ion Batteries

Thermally reversible self-healing polymer (SHP) electrolyte membranes are obtained by Diels-Alder cycloaddition and electrolyte embedding. The SHP electrolytes membranes are found to display high ionic conductivity, suitable flexibility, remarkable mechanical properties and self-healing ability. The decomposition potential of the SHP electrolyte membrane is about 4.8 V (vs. Li/Li⁺) and it possesses excellent electrochemical stability, better than that of the commercial PE film which is only stable up to 4.5 V (vs. Li/Li⁺). TGA results show that the SHP electrolyte membrane is thermally stable up to 280 °C in a nitrogen atmosphere. When the SHP electrolyte membrane is used as a separator in a lithium-ion battery with an LCO-based cathode, the SHP membrane achieved excellent rate capability and stable cycling for over 100 cycles, and the specific discharge capacity could be almost fully recovered after self-healing. Furthermore, the electrolyte membrane exhibits excellent electrochemical performance, suggesting its potential for application in lithium-ion batteries as separator material.

Reconstructing Vanadium Oxide with Anisotropic Pathways for a Durable and Fast Aqueous K-Ion Battery

Aqueous potassium-ion batteries are long-term pursued, due to their excellent performance and intrinsic superiority in safe, low-cost storage for portable and grid-scale applications. However, the notorious issues of K-ion battery chemistry are the inferior cycling stability and poor rate performance, due to the inevitably destabilization of the crystal structure caused by K-ions with pronouncedly large ionic radius. Here, we resolve such issues by reconstructing commercial vanadium oxide (α-V₂O₅) into the bronze form, i.e., δ-K_0.5V₂O₅ (KVO) nanobelts, as cathode materials with layered structure of enlarged space and anisotropic pathways for K-ion storage. Specifically, it can deliver a high capacity as 116 mAh g^-1 at the 1 C-rate, an outstanding rate capacity of 65 mAh g^-1 at 50 C, and a robust cyclic stability with 88.2% capacity retention after 1,000 cycles at 1 C. When coupled with organic anode in a full-cell configuration, the KVO electrodes can output 95 mAh g^-1 at 1 C and cyclic stability with 77.3% capacity retention after 20,000 cycles at 10 C. According to experimental and calculational results, the ultradurable cyclic performance is assigned to the robust structural reversibility of the KVO electrode, and the ultrahigh-rate capability is attributed to the anisotropic pathways with improved electrical conductivity in KVO nanobelts. In addition, applying a 22 M KCF₃SO₃ water-in-salt electrolyte can impede the dissolving issues of the KVO electrode and further stabilize the battery cyclic performance. Lastly, the as-designed AKIBs can operate with superior low-temperature adaptivity even at -30 °C. Overall, the KVO electrode can serve as a paradigm toward developing more suitable electrode materials for high-performance AKIBs.

Steric Effect Tuned Ion Solvation Enabling Stable Cycling of High-Voltage Lithium Metal Battery

1,2-Dimethoxyethane (DME) is a common electrolyte solvent for lithium metal batteries. Various DME-based electrolyte designs have improved long-term cyclability of high-voltage full cells. However, insufficient Coulombic efficiency at the Li anode and poor high-voltage stability remain a challenge for DME electrolytes. Here, we report a molecular design principle that utilizes a steric hindrance effect to tune the solvation structures of Li⁺ ions. We hypothesized that by substituting the methoxy groups on DME with larger-sized ethoxy groups, the resulting 1,2-diethoxyethane (DEE) should have a weaker solvation ability and consequently more anion-rich inner solvation shells, both of which enhance interfacial stability at the cathode and anode. Experimental and computational evidence indicates such steric-effect-based design leads to an appreciable improvement in electrochemical stability of lithium bis(fluorosulfonyl)imide (LiFSI)/DEE electrolytes. Under stringent full-cell conditions of 4.8 mAh cm^-2 NMC811, 50 μm thin Li, and high cutoff voltage at 4.4 V, 4 M LiFSI/DEE enabled 182 cycles until 80% capacity retention while 4 M LiFSI/DME only achieved 94 cycles. This work points out a promising path toward the molecular design of non-fluorinated ether-based electrolyte solvents for practical high-voltage Li metal batteries.

Fast Li-ion Conductor of Li3HoBr6 for Stable All-Solid-State Lithium-Sulfur Battery

Rare-earth (RE) solid-state halide electrolytes have been extensively studied recently in the field of lithium (Li) ion all-solid-state batteries (ASSBs) due to their excellent electrochemical performances. Herein, a new RE-based solid halide electrolyte Li₃HoBr₆ (LHB) has been synthesized and exhibits high Li ion conductivity up to mS cm^-1 at room temperature. Theoretical calculations have identified four different Li ion migration pathways, in which the out-of-plane pathways are much more favorable than the direct in-plane pathways. In addition, LHB has a wider electrochemical window in comparison to a sulfide solid electrolyte and good deformability. The LHB-based Li-sulfur ASSB assembled by cold pressing can exhibit good cycling stability with high Coulombic efficiency, which shows that LHB has potential application in ASSBs.

MAS-NMR of [Pyr13][Tf2N] and [Pyr16][Tf2N] Ionic Liquids Confined to Carbon Black: Insights and Pitfalls

Electrolytes based on ionic liquids (IL) are promising candidates to replace traditional liquid electrolytes in electrochemical systems, particularly in combination with carbon-based porous electrodes. Insight into the dynamics of such systems is imperative for tailoring electrochemical performance. In this work, 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide and 1-Hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide were studied in a carbon black (CB) host using spectrally resolved Carr-Purcell-Meiboom-Gill (CPMG) and 13-interval Pulsed Field Gradient Stimulated Echo (PFGSTE) Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). Data were processed using a sensitivity weighted Laplace inversion algorithm without non-negativity constraint. Previously found relations between the alkyl length and the aggregation behavior of pyrrolidinium-based cations were confirmed and characterized in more detail. For the IL in CB, a different aggregation behavior was found compared to the neat IL, adding the surface of a porous electrode as an additional parameter for the optimization of IL-based electrolytes. Finally, the suitability of MAS was assessed and critically discussed for investigations of this class of samples.

Gram-Scale Synthesis of Nanosized Li3 HoBr6 Solid Electrolyte for All-Solid-State Li-Se Battery

Rare earth (RE) based halide solid electrolytes (HEs) are recently considered as research hotspots in the field of all-solid-state batteries (ASSBs). The RE-based HEs possess high ionic conductivity, credible deformability, and good stability, which can bring excellent electrochemical performances for ASSBs. However, the conventional synthetic methods of RE HEs are a mechanochemical process and co-melting strategy, both approaches require expensive raw materials and sophisticated equipment. Therefore, a lot of research work is required to promote the preparation methods for these promising SSEs in ASSBs. Thus, a vacuum evaporation-assisted synthesis method is developed for the massive synthesis of HEs. The as-prepared Li₃ HoBr₆ (LHB) has a high lithium-ion conductivity close to the mS cm^-1 level and the LHB-based Li-Se ASSBs can be assembled by cold pressing. Theoretical calculations have revealed that the Li migrations are highly preferred in Li₃ HoBr₆ owing to the low energy cost and high tolerance of stable structure. The tetrahedral and octahedral pathways are responsible for Li migrations in short and long ranges, respectively. The results show that the LHB-based Li-Se battery has good stability and rate performance, indicating that LHB has potential application in the field of ASSBs.

Robust Solid Electrolyte Interphases in Localized High Concentration Electrolytes Boosting Black Phosphorus Anode for Potassium-Ion Batteries

Black phosphorus (BP) shows superior capacity toward K ion storage, yet it suffers from poor reversibility and fast capacity degradation. Herein, a BP-graphite (BP/G) composite with a high BP loading of 80 wt % is synthesized and stabilized via the utilization of a localized high concentration electrolyte (LHCE), i.e., potassium bis(fluorosulfonyl)imide in trimethyl phosphate with a fluorinated ether as the diluent. We reveal the benefits of high concentration electrolytes rely on the formation of an inorganic component rich solid electrolyte interphase (SEI), which effectively passivates the electrode from copious parasite reactions. Furthermore, the diluent increases the electrolyte's ionic conductivity for achieving attractive rate capability and homogenizes the elemental distribution in the SEI. The latter essentially improves the SEI's maximum elastic deformation energy for accommodating the volume change, resulting in excellent cyclic performance. This work promotes the application of advanced potassium-ion batteries by adopting high-capacity BP anodes, on the one hand. On the other hand, it unravels the beneficial roles of LHCE in building robust SEIs for stabilizing alloy anodes.

On the crystallography and reversibility of lithium electrodeposits at ultrahigh capacity

Lithium metal is a promising anode for energy-dense batteries but is hindered by poor reversibility caused by continuous chemical and electrochemical degradation. Here we find that by increasing the Li plating capacity to high values (e.g., 10–50 mAh cm⁻²), Li deposits undergo a morphological transition to produce dense structures, composed of large grains with dominantly (110)_Li crystallographic facets. The resultant Li metal electrodes manifest fast kinetics for lithium stripping/plating processes with higher exchange current density, but simultaneously exhibit elevated electrochemical stability towards the electrolyte. Detailed analysis of these findings reveal that parasitic electrochemical reactions are the major reason for poor Li reversibility, and that the degradation rate from parasitic electroreduction of electrolyte components is about an order of magnitude faster than from chemical reactions. The high-capacity Li electrodes provide a straightforward strategy for interrogating the solid electrolyte interphase (SEI) on Li —with unprecedented, high signal to noise. We find that an inorganic rich SEI is formed and is primarily concentrated around the edges of lithium particles. Our findings provide straightforward, but powerful approaches for enhancing the reversibility of Li and for fundamental studies of the interphases formed in liquid and solid-state electrolytes using readily accessible analytical tools.

Lithium metal batteries offer high-capacity electrical energy storage but suffer from poor reversibility of the metal anode. Here, the authors report that at very high capacities, lithium deposits as dense structures with a preferred crystallite orientation, yielding highly reversible lithium anodes.

Stable Anion-Derived Solid Electrolyte Interphase in Lithium Metal Batteries

High-energy-density lithium (Li) metal batteries are severely hindered by the dendritic Li deposition dictated by non-uniform solid electrolyte interphase (SEI). Despite its unique advantages in improving the uniformity of Li deposition, the current anion-derived SEI is unsatisfactory under practical conditions. Herein regulating the electrolyte structure of anions by anion receptors was proposed to construct stable anion-derived SEI. Tris(pentafluorophenyl)borane (TPFPB) anion acceptors with electron-deficient boron atoms interact with bis(fluorosulfonyl)imide anions (FSI^- ) and decrease the reduction stability of FSI^- . Furthermore, the type of aggregate cluster of FSI^- in electrolyte changes, FSI^- interacting with more Li ions in the presence of TPFPB. Therefore, the decomposition of FSI^- to form Li₂ S is promoted, improving the stability of anion-derived SEI. In working Li | LiNi_0.5 Co_0.2 Mn_0.3 O₂ batteries under practical conditions, the anion-derived SEI with TPFPB undergoes 194 cycles compared with 98 cycles of routine anion-derived SEI. This work inspires a fresh ground to construct stable anion-derived SEI by manipulating the electrolyte structure of anions.

Solvate electrolytes for Li and Na batteries: structures, transport properties, and electrochemistry

Polar solvents dissolve Li and Na salts at high concentrations and are used as electrolyte solutions for batteries. The solvents interact strongly with the alkali metal cations to form complexes in the solution. The activity (concentration) of the uncoordinated solvent decreases as the salt concentration is increased. At extremely high salt concentrations, all the solvent molecules are involved in the coordination of the ions and form the solvates of the salts. In this article, we review the structures, transport properties, and electrochemistry of Li/Na salt solvates. In molten solvates, the activity of the uncoordinated solvent is negligible; this is the main origin of their peculiar characteristics, such as high thermal stability, wide electrochemical window, and unique ion transport. In addition, the solvent activity greatly influences the electrochemical reactions in Li/Na batteries. We highlight the attractive features of molten solvates as promising electrolytes for next-generation batteries.

Commercialization-Driven Electrodes Design for Lithium Batteries: Basic Guidance, Opportunities, and Perspectives

Current lithium-ion battery technology is approaching the theoretical energy density limitation, which is challenged by the increasing requirements of ever-growing energy storage market of electric vehicles, hybrid electric vehicles, and portable electronic devices. Although great progresses are made on tailoring the electrode materials from methodology to mechanism to meet the practical demands, sluggish mass transport, and charge transfer dynamics are the main bottlenecks when increasing the areal/volumetric loading multiple times to commercial level. Thus, this review presents the state-of-the-art developments on rational design of the commercialization-driven electrodes for lithium batteries. First, the basic guidance and challenges (such as electrode mechanical instability, sluggish charge diffusion, deteriorated performance, and safety concerns) on constructing the industry-required high mass loading electrodes toward commercialization are discussed. Second, the corresponding design strategies on cathode/anode electrode materials with high mass loading are proposed to overcome these challenges without compromising energy density and cycling durability, including electrode architecture, integrated configuration, interface engineering, mechanical compression, and Li metal protection. Finally, the future trends and perspectives on commercialization-driven electrodes are offered. These design principles and potential strategies are also promising to be applied in other energy storage and conversion systems, such as supercapacitors, and other metal-ion batteries.

A synergistic exploitation to produce high-voltage quasi-solid-state lithium metal batteries

The current Li-based battery technology is limited in terms of energy contents. Therefore, several approaches are considered to improve the energy density of these energy storage devices. Here, we report the combination of a heteroatom-based gel polymer electrolyte with a hybrid cathode comprising of a Li-rich oxide active material and graphite conductive agent to produce a high-energy "shuttle-relay" Li metal battery, where additional capacity is generated from the electrolyte's anion shuttling at high voltages. The gel polymer electrolyte, prepared via in situ polymerization in an all-fluorinated electrolyte, shows adequate ionic conductivity (around 2 mS cm^-1 at 25 °C), oxidation stability (up to 5.5 V vs Li/Li⁺), compatibility with Li metal and safety aspects (i.e., non-flammability). The polymeric electrolyte allows for a reversible insertion of hexafluorophosphate anions into the conductive graphite (i.e., dual-ion mechanism) after the removal of Li ions from Li-rich oxide (i.e., rocking-chair mechanism).

Preparation and Properties of an Ultrahigh-Energy-Density Aqueous Supercapacitor with a Superconcentrated Electrolyte and a Sr-Modified Lanthanum Zirconate Flexible Electrode

Although supercapacitors are considered to play a vital role in flexible electronic devices, there are still some problems that need to be overcome, such as low energy density and narrow electrochemical stability windows in aqueous electrolytes. Herein, we have successfully synthesized a series of Sr-modified La₂Zr₂O₇ (LZO-x) nanofibers as a new electrode material by a facile electrospinning technique. To determine the best doping sample, the changes in structures and electrochemical performances of La₂Zr₂O₇ (LZO-x) nanofibers with various Sr contents are investigated carefully. Then, the LZO-0.2 sample shows the highest capacitance (1445 mF·cm^-2). Furthermore, we also develop a low-cost superconcentrated electrolyte, which achieves a wide electrochemical stability window of 2.7 V using a working electrode (LZO-0.2). Finally, we use the LZO-0.2 electrode and the superconcentrated electrolyte to fabricate a flexible supercapacitor device, which shows an excellent capacitance of 175 F·g^-1 at a current density of 1.15 A·g^-1. Moreover, the aqueous device has excellent cycle stability and outstanding flexibility, and the energy density of this device is 177.2 Wh·kg^-1 and the corresponding power density is 1557.7 W·kg^-1.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Electrolyte Design for Lithium Metal Anode-Based Batteries Toward Extreme Temperature Application

Lithium anode-based batteries (LBs) are highly demanded in society owing to the high theoretical capacity and low reduction potential of metallic lithium. They are expected to see increasing deployment in performance critical areas including electric vehicles, grid storage, space, and sea vehicle operations. Unfortunately, competitive performance cannot be achieved when LBs operating under extreme temperature conditions where the lithium-ion chemistry fail to perform optimally. In this review, a brief overview of the challenges in developing LBs for low temperature (<0 °C) and high temperature (>60 °C) operation are provided followed by electrolyte design strategies involving Li salt modification, solvation structure optimization, additive introduction, and solid-state electrolyte utilization for LBs are introduced. Specifically, the prospects of using lithium metal batteries (LMBs), lithium sulfur (Li-S) batteries, and lithium oxygen (Li-O2 ) batteries for performance under low and high temperature applications are evaluated. These three chemistries are presented as prototypical examples of how the conventional low temperature charge transfer resistances and high temperature side reactions can be overcome. This review also points out the research direction of extreme temperature electrolyte design toward practical applications.

Exchange of Li and AgNO3 Enabling Stable 3D Lithium Metal Anodes with Embedded Lithophilic Nanoparticles and a Solid Electrolyte Interphase Inducer

Three-dimensional (3D) current collectors can effectively mitigate the volumetric expansion of working lithium metal anodes (LMAs). However, the practical utilization of 3D current collectors for lithium metal batteries remains unsatisfactory because of inhomogeneous deposition of lithium ions and an unstable solid electrolyte interphase (SEI). Herein, a facile method based on the exchange reaction between Li and AgNO₃ is exploited to embed Ag nanoparticles (NPs) and LiNO₃ in a carbon paper (ALCP@Li). The Ag NPs act as a seed for even lithium deposition inside the carbon matrix by virtue of their excellent lithiophilicity. Simultaneously, LiNO₃ plays an effective role in stabilizing LMAs by evolving a robust N-rich SEI. As a result, such 3D LMAs show a high Coulombic efficiency in half-cells (200 cycles, 99% at 1 mA cm^-2, 1 mAh cm^-2) and a low overpotential (60 mV). When paired with commercial thick NCM622 and LiFePO₄ cathodes, the 3D LMA-based full cells exhibit stable cycling in carbonate electrolytes.

Bifunctional Metal Meshes Acting as a Semipermeable Membrane and Electrode for Sensitive Electrochemical Determination of Volatile Compounds

The monitoring of toxic inorganic gases and volatile organic compounds has brought the development of field-deployable, sensitive, and scalable sensors into focus. Here, we attempted to meet these requirements by using concurrently microhole-structured meshes as (i) a membrane for the gas diffusion extraction of an analyte from a donor sample and (ii) an electrode for the sensitive electrochemical determination of this target with the receptor electrolyte at rest. We used two types of meshes with complementary benefits, i.e., Ni mesh fabricated by robust, scalable, and well-established methods for manufacturing specific designs and stainless steel wire mesh (SSWM), which is commercially available at a low cost. The diffusion of gas (from a donor) was conducted in headspace mode, thus minimizing issues related to mesh fouling. When compared with the conventional polytetrafluoroethylene (PTFE) membrane, both the meshes (40 μm hole diameter) led to a higher amount of vapor collected into the electrolyte for subsequent detection. This inedited fashion produced a kind of reverse diffusion of the analyte dissolved into the electrolyte (receptor), i.e., from the electrode to bulk, which further enabled highly sensitive analyses. Using Ni mesh coated with Ni(OH)₂ nanoparticles, the limit of detection reached for ethanol was 24-fold lower than the data attained by a platform with a PTFE membrane and placement of the electrode into electrolyte bulk. This system was applied in the determination of ethanol in complex samples related to the production of ethanol biofuel. It is noteworthy that a simple equation fitted by machine learning was able to provide accurate assays (accuracies from 97 to 102%) by overcoming matrix effect-related interferences on detection performance. Furthermore, preliminary measurements demonstrated the successful coating of the meshes with gold films as an alternative raw electrode material and the monitoring of HCl utilizing Au-coated SSWMs. These strategies extend the applicability of the platform that may help to develop valuable volatile sensing solutions.

Advanced Electrolyte Design for High-Energy-Density Li-Metal Batteries under Practical Conditions

Given the limitations inherent in current intercalation-based Li-ion batteries, much research attention has focused on potential successors to Li-ion batteries such as lithium-sulfur (Li-S) batteries and lithium-oxygen (Li-O₂ ) batteries. In order to realize the potential of these batteries, the use of metallic lithium as the anode is essential. However, there are severe safety hazards associated with the growth of Li dendrites, and the formation of "dead Li" during cycles leads to the inevitable loss of active Li, which in the end is undoubtedly detrimental to the actual energy density of Li-metal batteries. For Li-metal batteries under practical conditions, a low negative/positive ratio (N/P ratio), a electrolyte/cathode ratio (E/C ratio) along with a high-voltage cathode is prerequisite. In this Review, we summarize the development of new electrolyte systems for Li-metal batteries under practical conditions, revisit the design criteria of advanced electrolytes for practical Li-metal batteries and provide perspectives on future development of electrolytes for practical Li-metal batteries.

Electrochemical Surface Plasmon Resonance Spectroscopy for Investigation of the Initial Process of Lithium Metal Deposition

The initial process of Li-metal electrodeposition on the negative electrode surface determines the charging performance of Li-metal secondary batteries. However, minute depositions or the early processes of nucleation and growth of Li metal are generally difficult to detect under operando conditions. In this study, we propose an optical diagnostic approach to address these challenges. Surface plasmon resonance (SPR) spectroscopy coupled with electrochemical operation is a promising technique that enables the ultrasensitive detection of the initial stage of Li-metal electrodeposition. The SPR is excited in a thin copper film deposited on a glass substrate, which also serves as a current collector enabling electrochemical Li-metal deposition. For a propylene carbonate (PC)-based Li-ion battery electrolyte, under both cyclic voltammetry and constant-current operation, Li-metal deposition is readily detected by changes in the SPR absorption dip in the reflectance spectrum. Electrochemical SPR is highly sensitive to metal deposition, with a demonstrated capability of detecting an average thickness of approximately 0.1 nm, corresponding to a few atomic layers of Li. To identify the growth mechanism, the SPR reflectance spectra of various possible Li-metal deposition processes were simulated. Comparison of the simulated spectra with the experimental data found good agreement with the well-known nucleation and growth model for Li-metal deposition from PC-based electrolytes. The demonstrated operando electrochemical SPR measurement should be a valuable tool for basic research on the initial Li-metal deposition process.

Characterising lithium-ion electrolytes via operando Raman microspectroscopy

Knowledge of electrolyte transport and thermodynamic properties in Li-ion and beyond Li-ion technologies is vital for their continued development and success. Here, we present a method for fully characterising electrolyte systems. By measuring the electrolyte concentration gradient over time via operando Raman microspectroscopy, in tandem with potentiostatic electrochemical impedance spectroscopy, the Fickian "apparent" diffusion coefficient, transference number, thermodynamic factor, ionic conductivity and resistance of charge-transfer were quantified within a single experimental setup. Using lithium bis(fluorosulfonyl)imide (LiFSI) in tetraglyme (G4) as a model system, our study provides a visualisation of the electrolyte concentration gradient; a method for determining key electrolyte properties, and a necessary technique for correlating bulk intermolecular electrolyte structure with the described transport and thermodynamic properties.

Structural Effects of Solvents on Li-Ion-Hopping Conduction in Highly Concentrated LiBF4/Sulfone Solutions

Li-ion-hopping conduction is known to occur in certain highly concentrated electrolytes, and this conduction mode is effective for achieving lithium batteries with high rate capabilities. Herein, we investigated the effects of the solvent structure on the hopping conduction of Li ions in highly concentrated LiBF₄/sulfone electrolytes. Raman spectroscopy revealed that a Li⁺ ion forms complexes with sulfone and anions, and contact ion pairs and ionic aggregates are formed in the highly concentrated electrolytes. Li⁺ exchanges ligands (sulfone and BF₄^-) rapidly to produce unusual hopping conduction in highly concentrated electrolytes. The structure of the solvent significantly influences the hopping conduction process. We measured the self-diffusion coefficients of Li⁺ (D_Li), anions (D_anion), and sulfone solvents (D_sol) in electrolytes. The ratio of the self-diffusion coefficients (D_Li/D_sol) tended to be higher for cyclic sulfones (sulfolane and 3-methylsulfolane) than for acyclic sulfones, which suggests that cyclic sulfone molecules facilitate Li-ion hopping. The hopping conduction increases the Li⁺-transference number (tLi+abc) under anion-blocking conditions, and tLi+abc of [LiBF₄]/[cyclic sulfone] = 1/2 is as high as 0.8.

Sustainable Lithium-Metal Battery Achieved by a Safe Electrolyte Based on Recyclable and Low-Cost Molecular Sieve

As one typical clean-energy technologies, lithium-metal batteries, especially high-energy-density batteries which use concentrated electrolytes hold promising prospect for the development of a sustainable world. However, concentrated electrolytes with aggregative configurations were achieved at the expense of using extra dose of costly and environmental-unfriendly salts/additives, which casts a shadow over the development of a sustainable world. Herein, without using any expensive salts/additives, we employed commercially-available low-cost and environmental-friendly molecular sieves (zeolite) to sieve the solvation sheath of lithium ions of classic commercialized electrolyte (LiPF₆ -EC/DMC), and resulting in a unique zeolite sieved electrolyte which was more aggregative than conventional concentrated electrolytes. Inspiringly, the new-designed electrolyte exhibited largely enhanced anti-oxidation stability under high-voltage (4.6 volts) and elevated temperature (55 °C). NCM-811//Li cells assembled with this electrolyte delivered ultra-stable rechargeabilities (over 1000 cycles for half-cell; 300 cycles for pouch-cell). More importantly, sustainable NCM-811//Li pouch-cell with negligible capacity decay can also be obtained through using recyclable zeolite sieved electrolyte. This conceptually-new way in preparing safe and highly-efficient electrolyte by using low-price molecular sieve would accelerate the development of high-energy-density lithium-ion/lithium-metal batteries.

Electrolyte Structure of Lithium Polysulfides with Anti-Reductive Solvent Shells for Practical Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) battery is regarded as a promising secondary battery. However, constant parasitic reactions between the Li anode and soluble polysulfide (PS) intermediates significantly deteriorate the working Li anode. The rational design to inhibit the parasitic reactions is plagued by the inability to understand and regulate the electrolyte structure of PSs. Herein, the electrolyte structure of PSs with anti-reductive solvent shells was unveiled by molecular dynamics simulations and nuclear magnetic resonance. The reduction resistance of the solvent shell is proven to be a key reason for the decreased reactivity of PSs towards Li. With isopropyl ether (DIPE) as a cosolvent, DIPE molecules tend to distribute in the outer solvent shell due to poor solvating power. Furthermore, DIPE is more stable than conventional ether solvents against Li metal. The reactivity of PSs is suppressed by encapsulating PSs into anti-reductive solvent shells. Consequently, the cycling performance of working Li-S batteries was significantly improved and a pouch cell of 300 Wh kg^-1 was demonstrated. The fundamental understanding in this work provides an unprecedented ground to understand the electrolyte structure of PSs and the rational electrolyte design in Li-S batteries.

Constructing nitrided interfaces for stabilizing Li metal electrodes in liquid electrolytes

Traditional Li ion batteries based on intercalation-type anodes have been approaching their theoretical limitations in energy density. Replacing the traditional anode with metallic Li has been regarded as the ultimate strategy to develop next-generation high-energy-density Li batteries. Unfortunately, the practical application of Li metal batteries has been hindered by Li dendrite growth, unstable Li/electrolyte interfaces, and Li pulverization during battery cycling. Interfacial modification can effectively solve these challenges and nitrided interfaces stand out among other functional layers because of their impressive effects on regulating Li⁺ flux distribution, facilitating Li⁺ diffusion through the solid-electrolyte interphase, and passivating the active surface of Li metal electrodes. Although various designs for nitrided interfaces have been put forward in the last few years, there is no paper that specialized in reviewing these advances and discussing prospects. In consideration of this, we make a systematic summary and give our comments based on our understanding. In addition, a comprehensive perspective on the future development of nitrided interfaces and rational Li/electrolyte interface design for Li metal electrodes is included.

In this perspective, we make a systematic summary and give out our comments on constructing nitrided interfaces for stabilizing Li metal electrodes.

Neural Network Analysis of Electron Microscopy Video Data Reveals the Temperature-Driven Microphase Dynamics in the Ions/Water System

Real-time field-emission scanning electron microscopy (FE-SEM) measurements and neural network analysis were successfully merged to observe the temperature-induced behavior of soft liquid microdomains in mixtures of different ionic liquids with water. The combination of liquid FE-SEM and in situ heating techniques revealed temperature-driven solution restructuring for ions/water systems with different water states and their critical point behavior expressed in a rapid switch between thermal expansion and shrinkage of liquid microphases at temperatures of ≈100-130 °C, which was directly recorded on electron microscopy videos. Automation of FE-SEM video analysis by a neural network approach allowed quantification of the morphological changes in ions/water systems during heating on the basis of thousands of images processed with a speed almost equal to the frame rate of original electron microscopy videos. Tracking and evolution of the micro-heterogeneous domains, hypothesized in the Ioliomics concept, was mapped and quantified for the first time. The present study describes the concept for quick acquisition of big data in electron microscopy, develops rapid neural network analysis and shows how to link microscopic data to fundamental molecular properties.

Porous Composite Gel Polymer Electrolyte with Interfacial Transport Pathways for Flexible Quasi Solid Lithium-Ion Batteries

The growing demand for safer energy storage devices leads to wide research on solid-state lithium-ion batteries. However, as an important component in the solid-state battery, the solid-state electrolyte often encounters problems, especially the low conductivity at room temperature, inhibiting the development of solid-state batteries. Here, improved electrochemical performances of lithium-ion batteries are obtained by designing a composite gel polymer electrolyte with a sponge-like structure. The porous composite gel polymer electrolyte (PCGPE) is developed by a facile phase inversion process of poly(vinylidiene fluoride-hexafluoropropylene) (PVdF-HFP) and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). The solid-state nuclear magnetic resonance test proves the continuous porous structure constructs fast Li-ion transport pathways on internal interfaces. As a result, the ionic conductivity of PCGPE is up to 5.45 × 10^-4 S cm^-1 at room temperature. Moreover, an initial capacity of 142.2 mAh g^-1 and 82.6% capacity retention at 1 C after 350 cycles are successfully achieved in flexible LiFPO₄//PCGPE//Li batteries.

Manipulating anion intercalation enables a high-voltage aqueous dual ion battery

Aqueous graphite-based dual ion batteries have unique superiorities in stationary energy storage systems due to their non-transition metal configuration and safety properties. However, there is an absence of thorough study of the interactions between anions and water molecules and between anions and electrode materials, which is essential to achieve high output voltage. Here we reveal the four-stage intercalation process and energy conversion in a graphite cathode of anions with different configurations. The difference between the intercalation energy and hydration energy of bis(trifluoromethane)sulfonimide makes the best use of the electrochemical stability window of its electrolyte and delivers a high intercalation potential, while BF₄⁻ and CF₃SO₃⁻ do not exhibit a satisfactory potential because the graphite intercalation potential of BF₄⁻ is inferior and the graphite intercalation potential of CF₃SO₃⁻ exceeds the voltage window of its electrolyte. An aqueous dual ion battery based on the intercalation behaviors of bis(trifluoromethane)sulfonimide anions into a graphite cathode exhibits a high voltage of 2.2 V together with a specific energy of 242.74 Wh kg⁻¹. This work provides clear guidance for the voltage plateau manipulation of anion intercalation into two-dimensional materials.

The interactions between water molecules, electrode materials and anions are essential yet challenging for aqueous dual ion batteries. Here, the authors demonstrate the voltage manipulation of dual ion batteries through matching intercalation energy and solvation energy of different anions.

Fick Diffusion Coefficient in Binary Mixtures of [HMIM][NTf2] and Carbon Dioxide by Dynamic Light Scattering and Molecular Dynamics Simulations

Dynamic light scattering (DLS) experiments and equilibrium molecular dynamics (EMD) simulations were performed in the saturated liquid phase of the binary mixture of 1-hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide ([HMIM][NTf₂]) and carbon dioxide (CO₂) to access the Fick diffusion coefficient (D₁₁). The investigations were performed within or close to saturation conditions at temperatures between (298.15 and 348.15) K and CO₂ mole fractions (x_CO2) up to 0.81. The DLS experiments were combined with polarization-difference Raman spectroscopy (PDRS) to simultaneously access the composition of the liquid phase. For the first time in an electrolyte-based system, D₁₁ was directly calculated from EMD simulations by accessing the Maxwell-Stefan (MS) diffusion coefficient and the thermodynamic factor. Agreement within combined uncertainties was found between D₁₁ from DLS and EMD simulations for CO₂ mole fractions up to 0.5. In general, an increasing D₁₁ with increasing x_CO2 could be observed, with a local maximum present at a CO₂ mole fraction of about 0.75. The local maximum could be explained by an increasing MS diffusion coefficient with increasing x_CO2 over the entire studied composition range and a decreasing thermodynamic factor at x_CO2 above 0.7. Finally, PDRS and EMD simulations were combined to investigate the influence of the fluid structure on the diffusive process.

Structural Transitions During Formation and Rehydration of Proton Conducting Polymeric Membranes

Knowledge of the transitions occurring during the formation of ion-conducting polymer films and membranes is crucial to optimize material performances. The use of non-destructive scattering techniques that offer high spatio-temporal resolution is essential to investigating such structural transitions, especially when combined with complementary techniques probing at different time and spatial scales. Here, a simultaneous multi-technique study is performed on the membrane formation mechanism and the subsequent hydration of two ion-conducting polymers, the well-known commercial Nafion and a synthesized sulfonated poly(phenylene sulfide sulfone) (sPSS). The X-ray data distinguish the multi-stage processes occurring during drying. A sol-gel-membrane transition sequence is observed for both polymers. However, while Nafion membrane evolves from a micellar solution through the formation of a phase-separated gel, forming an oriented supported membrane, sPSS membrane evolves from a solution of dispersed polyelectrolyte chains via formation of an inhomogeneous gel, showing assembly and ionic phase separation only at the end of the drying process. Impedance spectroscopy data confirm the occurrence of the sol-gel transitions, while gel-membrane transitions are detected by optical reflectance data. The simultaneous multi-technique approach presented here can connect the nanoscale to the macroscopic behavior, unraveling information essential to optimize membrane formation of different ion-conducting polymers.