Localized High-Concentration Sulfone Electrolytes for High-Efficiency Lithium-Metal Batteries

An inorganic-rich but LiF-free interphase for fast charging and long cycle life lithium metal batteries

Li metal batteries using Li metal as negative electrode and LiNi1-x-yMnxCoyO2 as positive electrode represent the next generation high-energy batteries. A major challenge facing these batteries is finding electrolytes capable of forming good interphases. Conventionally, electrolyte is fluorinated to generate anion-derived LiF-rich interphases. However, their low ionic conductivities forbid fast-charging. Here, we use CsNO3 as a dual-functional additive to form stable interphases on both electrodes. Such strategy allows the use of 1,2-dimethoxyethane as the single solvent, promising superior ion transport and fast charging. LiNi1-x-yMnxCoyO2 is protected by the nitrate-derived species. On the Li metal side, large Cs+ has weak interactions with the solvent, leading to presence of anions in the solvation sheath and an anion-derived interphase. The interphase is surprisingly dominated by cesium bis(fluorosulfonyl)imide, a component not reported before. Its presence suggests that Cs+ is doing more than just electrostatic shielding as commonly believed. The interphase is free of LiF but still promises high performance as cells with high LiNi0.8Mn0.1Co0.1O2 loading (21 mg/cm2) and low N/P ratio (~2) can be cycled at 2C (~8 mA/cm2) with above 80% capacity retention after 200 cycles. These results suggest the role of LiF and Cs-containing additives need to be revisited.

Green Ether Electrolytes for Sustainable High-voltage Potassium Ion Batteries

Ether-based electrolytes are promising for secondary batteries due to their good compatibility with alkali metal anodes and high ionic conductivity. However, they suffer from poor oxidative stability and high toxicity, leading to severe electrolyte decomposition at high voltage and biosafety/environmental concerns when electrolyte leakage occurs. Here, we report a green ether solvent through a rational design of carbon-chain regulation to elicit steric hindrance, such a structure significantly reducing the solvent's biotoxicity and tuning the solvation structure of electrolytes. Notably, our solvent design is versatile, and an anion-dominated solvation structure is favored, facilitating a stable interphase formation on both the anode and cathode in potassium-ion batteries. Remarkably, the green ether-based electrolyte demonstrates excellent compatibility with K metal and graphite anode and a 4.2 V high-voltage cathode (200 cycles with average Coulombic efficiency of 99.64 %). This work points to a promising path toward the molecular design of green ether-based electrolytes for practical high-voltage potassium-ion batteries and other rechargeable batteries.

From Liquid to Solid-State Lithium Metal Batteries: Fundamental Issues and Recent Developments

The widespread adoption of lithium-ion batteries has been driven by the proliferation of portable electronic devices and electric vehicles, which have increasingly stringent energy density requirements. Lithium metal batteries (LMBs), with their ultralow reduction potential and high theoretical capacity, are widely regarded as the most promising technical pathway for achieving high energy density batteries. In this review, we provide a comprehensive overview of fundamental issues related to high reactivity and migrated interfaces in LMBs. Furthermore, we propose improved strategies involving interface engineering, 3D current collector design, electrolyte optimization, separator modification, application of alloyed anodes, and external field regulation to address these challenges. The utilization of solid-state electrolytes can significantly enhance the safety of LMBs and represents the only viable approach for advancing them. This review also encompasses the variation in fundamental issues and design strategies for the transition from liquid to solid electrolytes. Particularly noteworthy is that the introduction of SSEs will exacerbate differences in electrochemical and mechanical properties at the interface, leading to increased interface inhomogeneity-a critical factor contributing to failure in all-solid-state lithium metal batteries. Based on recent research works, this perspective highlights the current status of research on developing high-performance LMBs.

Localized high-concentration electrolytes get more localized through micelle-like structures

Liquid electrolytes in batteries are typically treated as macroscopically homogeneous ionic transport media despite having a complex chemical composition and atomistic solvation structures, leaving a knowledge gap of the microstructural characteristics. Here, we reveal a unique micelle-like structure in a localized high-concentration electrolyte, in which the solvent acts as a surfactant between an insoluble salt in a diluent. The miscibility of the solvent with the diluent and simultaneous solubility of the salt results in a micelle-like structure with a smeared interface and an increased salt concentration at the centre of the salt-solvent clusters that extends the salt solubility. These intermingling miscibility effects have temperature dependencies, wherein a typical localized high-concentration electrolyte peaks in localized cluster salt concentration near room temperature and is used to form a stable solid-electrolyte interphase on a Li metal anode. These findings serve as a guide to predicting a stable ternary phase diagram and connecting the electrolyte microstructure with electrolyte formulation and formation protocols of solid-electrolyte interphases for enhanced battery cyclability.

Data-Driven Insight into the Reductive Stability of Ion-Solvent Complexes in Lithium Battery Electrolytes

Lithium (Li) metal batteries (LMBs) are regarded as one of the most promising energy storage systems due to their ultrahigh theoretical energy density. However, the high reactivity of the Li anodes leads to the decomposition of the electrolytes, presenting a huge impediment to the practical application of LMBs. The routine trial-and-error methods are inefficient in designing highly stable solvent molecules for the Li metal anode. Herein, a data-driven approach is proposed to probe the origin of the reductive stability of solvents and accelerate the molecular design for advanced electrolytes. A large database of potential solvent molecules is first constructed using a graph theory-based algorithm and then comprehensively investigated by both first-principles calculations and machine learning (ML) methods. The reductive stability of 99% of the electrolytes decreases under the dominance of ion-solvent complexes, according to the analysis of the lowest unoccupied molecular orbital (LUMO). The LUMO energy level is related to the binding energy, bond length, and orbital ratio factors. An interpretable ML method based on Shapley additive explanations identifies the dipole moment and molecular radius as the most critical descriptors affecting the reductive stability of coordinated solvents. This work not only affords fruitful data-driven insight into the ion-solvent chemistry but also unveils the critical molecular descriptors in regulating the solvent's reductive stability, which accelerates the rational design of advanced electrolyte molecules for next-generation Li batteries.

Electron Delocalization Enables Sulfone-based Single-solvent Electrolyte for Lithium Metal Batteries

Li-metal batteries (LMB), although providing high energy density, face the grand challenge of identifying good electrolyte solvents for cycling. Common solvents are either only stable against lithium metal anode or only stable against LiNix Mny Co1-x-y O2 (NMC) cathode. There is significant effort trying to increase the cathode stability for ether electrolytes, which are in general stable against lithium metal anode. In comparison, there is much less effort trying to increase the anode stability of electrolytes that are stable against NMC cathode. One example is the sulfone-based electrolyte. It has good cathode stability but is hindered from practical application because of (1) high viscosity and poor wetting capability and (2) poor anode stability. Here, we solve these issues by modifying the sulfone molecules using resonance and electron withdrawing effect. The viscosity is significantly reduced by delocalizing the electrons through introducing additional oxygen on the molecular backbone and applying appropriate fluorination. The resulting molecule 2,2,2-trifluoroethyl mesylate (TFEM) has decreased Lewis basicity and less reactivity toward Li+ . The electrolyte based on TFEM as single solvent enables cycling of LMB under harsh conditions of low N/P ratio (21 mg/cm2 NMC811 and 50 μm Li) with 90 % capacity retention after 160 cycles at C/3 discharge rate.

Ultrastable Dendrite-Free Potassium Metal Batteries Enabled by Weakly-Solvated Electrolyte

Potassium (K) metal is considered one of the most promising anodes for potassium metal batteries (PMBs) because of its abundant and low-cost advantages but suffers from serious dendritic growth and parasitic reactions, resulting in poor cyclability, low Coulombic efficiency (CE), and safety concerns. In this work, we report a localized high-concentration electrolyte (LHCE) consisting of potassium bis(fluorosulfonyl)imide (KFSI) in a cosolvent of 1,2-dimethoxyethane (DME) and 1,1,2,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) to solve the problems of PMBs. TTE as a diluent not only endows LHCE with advantages of low viscosity, good wettability, and improved conductivity but also solves the dendrite problem pertaining to K metal anodes. Using the formulation of LHCE, a CE of 98% during 800 cycles in the K||Cu cell and extremely stable cycling of over 2000 h in the K||K symmetric cell are achieved at a current density of 0.1 mA cm-2. In addition, the LHCE shows good compatibility with a Prussian Blue cathode, allowing almost 99% CE for the K||KFeIIFeIII(CN)6 full cell during 100 cycles. This promising electrolyte design realizes high-safety and energy-dense PMBs.

High-voltage lithium-metal batteries enabled by ethylene glycol bis(propionitrile) ether-LiNO3 synergetic additives

The employment of Li metal anodes is a key to realizing ultra-high energy batteries. However, the commercialization of lithium metal batteries (LMBs) remains challenging partially due to the thermodynamic instability and competitive oxidative decomposition of the solvent. Herein, a bi-functional electrolyte for stabilizing the interfaces of both the Li metal anode and LiCoO2 (LCO) cathode is designed by introducing lithium nitrate (LiNO3) through Ethylene Glycol Bis(Propionitrile) Ether (DENE). For the anode, the C8H12N2O2-LiNO3 coordination-solvation contributes to forming a stable Li3N-enhanced solid electrolyte interphase (SEI), which increases the average Li coulombic efficiency (CE) up to 98.5%. More importantly, in situ electrochemical dilatometry further reveals that the highly reversible behavior and a low volume expansion of lithium deposition are related to the stable Li3N-enhanced SEI. The designed electrolyte enables the Li‖LCO cell to achieve an average CE of 99.2% and a high capacity retention of 88.2% up to 4.6 V after 100 cycles. This work provides a strategic guidance in developing high-voltage Li‖LCO batteries with dual electrolyte additives.

Probing the Origin of Viscosity of Liquid Electrolytes for Lithium Batteries

Viscosity is an extremely important property for ion transport and wettability of electrolytes. Easy access to viscosity values and a deep understanding of this property remain challenging yet critical to evaluating the electrolyte performance and tailoring electrolyte recipes with targeted properties. We proposed a screened overlapping method to efficiently compute the viscosity of lithium battery electrolytes by molecular dynamics simulations. The origin of electrolyte viscosity was further comprehensively probed. The viscosity of solvents exhibits a positive correlation with the binding energy between molecules, indicating viscosity is directly correlated to intermolecular interactions. Salts in electrolytes enlarge the viscosity significantly with increasing concentrations while diluents serve as the viscosity reducer, which is attributed to the varied binding strength from cation-anion and cation-solvent associations. This work develops an accurate and efficient method for computing the electrolyte viscosity and affords deep insight into viscosity at the molecular level, which exhibits the huge potential to accelerate advanced electrolyte design for next-generation rechargeable batteries.

Liquid electrolyte chemistries for solid electrolyte interphase construction on silicon and lithium-metal anodes

Next-generation battery development necessitates the coevolution of liquid electrolyte and electrode chemistries, as their erroneous combinations lead to battery failure. In this regard, priority should be given to the alleviation of the volumetric stress experienced by silicon and lithium-metal anodes during cycling and the mitigation of other problems hindering their commercialization. This review summarizes the advances in sacrificial compound-based volumetric stress-adaptable interfacial engineering, which has primarily driven the development of liquid electrolytes for high-performance lithium batteries. Besides, we discuss how the regulation of lithium-ion solvation structures helps expand the range of electrolyte formulations and thus enhance the quality of solid electrolyte interphases (SEIs), improve lithium-ion desolvation kinetics, and realize longer-lasting SEIs on high-capacity anodes. The presented insights are expected to inspire the design and synthesis of next-generation electrolyte materials and accelerate the development of advanced electrode materials for industrial battery applications.

Tuning and Balancing the Donor Number of Lithium Salts and Solvents for High-Performance Li Metal Anode

The low reversibility of Li deposition/stripping in conventional carbonate electrolytes hinders the development of lithium metal batteries. Herein, we proposed a combination of solvents with a moderate donor number (DN) and LiNO3 as the sole salt, which has rarely been attempted due to its low solubility or dissociation degree in common solvents. It is found that the DN value of solvents is highly correlated to the reversibility of Li deposition behavior when LiNO3 is applied as the sole salt. The combination of LiNO3 and solvents with moderate DN behaves like a quasi-concentrated electrolyte even at a common or moderate concentration, while neither the solvents with poor solubility and low dissociation for LiNO3 (which usually corresponds to a low DN) nor the solvents with high dissociation for LiNO3 (which usually corresponds to an overly high DN) can achieve a high reversibility for low conductivity or excessive solvent decomposition. As a result, a Coulombic efficiency as high as 99.6% for Li deposition/stripping is achieved with the optimized combination. We believe this work will give a better understanding of the role of anions and solvents in the regulation of the solvation structure, and DN can be utilized as an important guideline to sieve suitable solvents for LiNO3 as the main salt to exhibit intriguing properties beyond traditional cognition.

40 Years of Low-Temperature Electrolytes for Rechargeable Lithium Batteries

Rechargeable lithium batteries are one of the most appropriate energy storage systems in our electrified society, as virtually all portable electronic devices and electric vehicles today rely on the chemical energy stored in them. However, sub-zero Celsius operation, especially below -20 °C, remains a huge challenge for lithium batteries and greatly limits their application in extreme environments. Slow Li+ diffusion and charge transfer kinetics have been identified as two main origins of the poor performance of RLBs under low-temperature conditions, both strongly associated with the liquid electrolyte that governs bulk and interfacial ion transport. In this review, we first analyze the low-temperature kinetic behavior and failure mechanism of lithium batteries from an electrolyte standpoint. We next trace the history of low-temperature electrolytes in the past 40 years (1983-2022), followed by a comprehensive summary of the research progress as well as introducing the state-of-the-art characterization and computational methods for revealing their underlying mechanisms. Finally, we provide some perspectives on future research of low-temperature electrolytes with particular emphasis on mechanism analysis and practical application.

Proximity Matters: Interfacial Solvation Dictates Solid Electrolyte Interphase Composition

The composition of the solid electrolyte interphase (SEI) plays an important role in controlling Li-electrolyte reactions, but the underlying cause of SEI composition differences between electrolytes remains unclear. Many studies correlate SEI composition with the bulk solvation of Li ions in the electrolyte, but this correlation does not fully capture the interfacial phenomenon of SEI formation. Here, we provide a direct connection between SEI composition and Li-ion solvation by forming SEIs using polar substrates that modify interfacial solvation structures. We circumvent the deposition of Li metal by forming the SEI above Li+/Li redox potential. Using theory, we show that an increase in the probability density of anions near a polar substrate increases anion incorporation within the SEI, providing a direct correlation between interfacial solvation and SEI composition. Finally, we use this concept to form stable anion-rich SEIs, resulting in high performance lithium metal batteries.

Ion Transport in Glyme- and Sulfolane-Based Highly Concentrated Electrolytes

Highly concentrated electrolytes (HCEs) have a similarity to ionic liquids (ILs) in high ionic nature, and indeed some of HECs are found to behave like an IL. HCEs have attracted considerable attention as prospective candidates for electrolyte materials in future lithium secondary batteries owing to their favorable properties both in the bulk and at the electrochemical interface. In this study, we highlight the effects of the solvent, counter anion, and diluent of HCEs on the Li+ ion coordination structure and transport properties (e. g., ionic conductivity and apparent Li+ ion transference number measured under anion-blocking conditions,t L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ ). Our studies on dynamic ion correlations unveiled the difference in the ion conduction mechanisms in HCEs and their intimate relevance tot L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ values. Our systematic analysis of the transport properties of HCEs also suggests the need for a compromise to simultaneously achieve high ionic conductivity and hight L i a b c ${{t}_{{\rm L}{\rm i}}^{{\rm a}{\rm b}{\rm c}}}$ values.

Electrolytes with moderate lithium polysulfide solubility for high-performance long-calendar-life lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with high energy density and low cost are promising for next-generation energy storage. However, their cycling stability is plagued by the high solubility of lithium polysulfide (LiPS) intermediates, causing fast capacity decay and severe self-discharge. Exploring electrolytes with low LiPS solubility has shown promising results toward addressing these challenges. However, here, we report that electrolytes with moderate LiPS solubility are more effective for simultaneously limiting the shuttling effect and achieving good Li-S reaction kinetics. We explored a range of solubility from 37 to 1,100 mM (based on S atom, [S]) and found that a moderate solubility from 50 to 200 mM [S] performed the best. Using a series of electrolyte solvents with various degrees of fluorination, we formulated the Single-Solvent, Single-Salt, Standard Salt concentration with Moderate LiPSs solubility Electrolytes (termed S6MILE) for Li-S batteries. Among the designed electrolytes, Li-S cells using fluorinated-1,2-diethoxyethane S6MILE (F4DEE-S6MILE) showed the highest capacity of 1,160 mAh g-1 at 0.05 C at room temperature. At 60 °C, fluorinated-1,4-dimethoxybutane S6MILE (F4DMB-S6MILE) gave the highest capacity of 1,526 mAh g-1 at 0.05 C and an average CE of 99.89% for 150 cycles at 0.2 C under lean electrolyte conditions. This is a fivefold increase in cycle life compared with other conventional ether-based electrolytes. Moreover, we observed a long calendar aging life, with a capacity increase/recovery of 4.3% after resting for 30 d using F4DMB-S6MILE. Furthermore, the correlation between LiPS solubility, degree of fluorination of the electrolyte solvent, and battery performance was systematically investigated.

Toward High-Performance Metal-Organic-Framework-Based Quasi-Solid-State Electrolytes: Tunable Structures and Electrochemical Properties

Metal-organic frameworks (MOFs) have been reported as promising materials for electrochemical applications owing to their tunable porous structures and ion-sieving capability. However, it remains challenging to rationally design MOF-based electrolytes for high-energy lithium batteries. In this work, by combining advanced characterization and modeling tools, a series of nanocrystalline MOFs is designed, and the effects of pore apertures and open metal sites on ion-transport properties and electrochemical stability of MOF quasi-solid-state electrolytes are systematically studied. It isdemonstrated that MOFs with non-redox-active metal centers can lead to a much wider electrochemical stability window than those with redox-active centers. Furthermore, the pore aperture of MOFs is found to be a dominating factor that determines the uptake of lithium salt and thus ionic conductivity. The ab initio molecular dynamics simulations further demonstrate that open metal sites of MOFs can facilitate the dissociation of lithium salt and immobilize anions via Lewis acid-base interaction, leading to good lithium-ion mobility and high transference number. The MOF quasi-solid-state electrolyte demonstrates great battery performance with commercial LiFePO4 and LiCoO2 cathodes at 30 °C. This work provides new insights into structure-property relationships between tunable structure and electrochemical properties of MOFs that can lead to the development of advanced quasi-solid-state electrolytes for high-energy lithium batteries.

Insights into the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries

Lithium-based rechargeable batteries have dominated the energy storage field and attracted considerable research interest due to their excellent electrochemical performance. As indispensable and ubiquitous components, electrolytes play a pivotal role in not only transporting lithium ions, but also expanding the electrochemical stable potential window, suppressing the side reactions, and manipulating the redox mechanism, all of which are closely associated with the behavior of solvation chemistry in electrolytes. Thus, comprehensively understanding the solvation chemistry in electrolytes is of significant importance. Here we critically reviewed the development of electrolytes in various lithium-based rechargeable batteries including lithium-metal batteries (LMBs), nonaqueous lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs), and aqueous lithium-ion batteries (ALIBs), and emphasized the effects of interactions between cations, anions, and solvents on solvation chemistry, and functions of solvation chemistry in different types of electrolytes (strong solvating electrolytes, moderate solvating electrolytes, and weak solvating electrolytes) on the electrochemical performance and redox mechanism in the abovementioned rechargeable batteries. Specifically, the significant effects of solvation chemistry on the stability of electrode-electrolyte interphases, suppression of lithium dendrites in LMBs, inhibition of the co-intercalation of solvents in LIBs, improvement of anodic stability at high cut-off voltages in LMBs, LIBs and ALIBs, regulation of redox pathways in LSBs and LOBs, and inhibition of hydrogen/oxygen evolution reactions in LOBs are thoroughly summarized. Finally, the review concludes with a prospective outlook, where practical issues of electrolytes, advanced in situ/operando techniques to illustrate the mechanism of solvation chemistry, and advanced theoretical calculation and simulation techniques such as "material knowledge informed machine learning" and "artificial intelligence (AI) + big data" driven strategies for high-performance electrolytes have been proposed.

Suppression of Interphase Dissolution Via Solvent Molecule Tuning for Sodium Metal Batteries

Solvent molecule tuning is used to alter the redox potentials of solvents or ion-solvent binding energy for high-voltage or low-temperature electrolytes. Herein, an electrolyte design strategy that effectively suppresses solid electrolyte interphase (SEI) dissolution and passivates highly-reactive metallic Na anode via solvent molecule tuning is proposed. With rationally lengthened phosphate backbones with ─CH2 ─ units, the low-solvation tris(2-ethylhexyl) phosphate (TOP) molecule effectively weakens the solvation ability of carbonate-based electrolytes, reduces the free solvent ratio, and enables an anion-enriched primary Na+ ion solvation sheath. The decreased free solvent and compact lower-solubility interphase established in this electrolyte prevent electrodes from continuous SEI dissolution and parasitic reactions at both room temperature (RT) and high temperature (HT). As a result, the Na/Na3 V2 (PO4 )3 cell with the new electrolyte achieves impressive cycling stability of 95.7% capacity retention after 1800 cycles at 25 °C and 62.1% capacity retention after 700 cycles at 60 °C. Moreover, the TOP molecule not only maintains the nonflammable feature of phosphate but also attains higher thermal stability, which endows the electrolyte with high safety and thermal stability. This design concept for electrolytes offers a promising path to long-cycling and high-safety sodium metal batteries.

Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers

We demonstrate that tetra-arm poly(ethylene glycol) gels containing highly concentrated sulfolane-based electrolytes exhibit high Li⁺ transference numbers. The low polymer concentration and homogeneous polymer network in the gel electrolyte are useful in achieving both mechanical reliability and high Li⁺ transport ability.

Three-dimensional heterogeneity in liquid electrolyte structures promotes Na ion transport and storage performance in Na-ion batteries

Unlike solid materials, the molecular structure and chemical distribution in electrolyte solutions have been considered in isotropic states. Herein, we reveal controllable regulation of solution structures in electrolytes by manipulating solvent interactions for Na-ion batteries. Low-solvation fluorocarbons as diluents in concentrated phosphate electrolytes induce adjustable heterogeneity in electrolyte structures through variable intermolecular forces between high-solvation phosphate and diluents. An optimal trifluorotoluene (PhCF3) diluent weakens the solvation strength around Na+ and spontaneously leads to a locally enlarged Na+ concentration and global 3D continuous Na+ transport path thanks to the appropriate electrolyte heterogeneity. Besides, strong correlations between the solvation structure and the Na+ storage performance and interphases are demonstrated. PhCF3 diluted concentrated electrolyte enables superior operations of Na-ion batteries at both room temperature and a high temperature of 60 °C. A hard carbon anode exhibits a reversible capacity of 300 mA h g-1 at 0.2C and excellent life over 1200 cycles without decay.

Solvation Structure Modulation of High-Voltage Electrolyte for High-Performance K-Based Dual-Graphite Battery

Due to the advantages of dual-ion batteries (DIBs) and abundant resources, potassium-based dual-carbon batteries (K-DCBs) have wide application prospects. However, conventional carbonate ester-based electrolyte systems have obvious drawbacks such as poor oxidation resistance and difficulty in sustaining the anion intercalation process at high voltages, which seriously affect the capacity and cycle performance of K-DCBs. Therefore, a rational design of more efficient novel electrolyte systems is urgently required to realize high-performance K-DCBs. Herein, a solvation structure modulation strategy for the K-DCB electrolyte systems is reported. Consequently, substantial K⁺ ion storage improvement at the graphite anode and enhanced bis(fluorosulfonyl)imide anion (FSI^- ) intercalation capacity at the graphite cathode are successfully realized simultaneously. As a proof-of-concept, the assembled K-DCB exhibited a discharge capacity of 103.4 mAh g^-1 , and after 400 cycles, ≈90% capacity retention is observed. Moreover, the energy density of the K-DCB full cell reached 157.6 Wh kg^-1 , which is the best performance in reported K-DCBs till date. This study demonstrates the effectiveness of solvation modulation in improving the performance of K-DCBs.

Structurally integrated asymmetric polymer electrolyte with stable Janus interface properties for high-voltage lithium metal batteries

Solid-state polymer electrolytes are outstanding candidates for next-generation lithium metal batteries in the realm of high specific energy densities, high safeties and tight contact with electrodes. However, their applications are still hindered by the limitations that no single polymer is electrochemically stable with the oxidizing high-voltage cathode and the highly reductive Li anode, simultaneously. Herein, a bilayer asymmetric polymer electrolyte (SL-SPE) without accessional interface resistance that using poly (ethylene glycol) diacrylate (PEGDA) as a "bridge" to connect the sulfonyl (OS = O)-contained oxidation-tolerated layer and polyether-derived reduction-tolerated layer (SPE), is proposed and synthesized by sequential two-step UV polymerizations. SL-SPE can provide widened electrochemical stability window up to 5 V, while simultaneously deploying a stable Janus interface property. Eventually, the superior high-voltage (4.4 V) cycling durability can be displayed in LiNi_0.6Co_0.2Mn_0.2O₂|SL-SPE|Li batteries. This finding provides a bran-new idea for designing multifunctional polymer electrolytes in the application of solid-state batteries.

Origin of dendrite-free lithium deposition in concentrated electrolytes

The electrolyte solvation structure and the solid-electrolyte interphase (SEI) formation are critical to dictate the morphology of lithium deposition in organic electrolytes. However, the link between the electrolyte solvation structure and SEI composition and its implications on lithium morphology evolution are poorly understood. Herein, we use a single-salt and single-solvent model electrolyte system to systematically study the correlation between the electrolyte solvation structure, SEI formation process and lithium deposition morphology. The mechanism of lithium deposition is thoroughly investigated using cryo-electron microscopy characterizations and computational simulations. It is observed that, in the high concentration electrolytes, concentrated Li⁺ and anion-dominated solvation structure initiate the uniform Li nucleation kinetically and favor the decomposition of anions rather than solvents, resulting in inorganic-rich amorphous SEI with high interface energy, which thermodynamically facilitates the formation of granular Li. On the contrary, solvent-dominated solvation structure in the low concentration electrolytes tends to exacerbate the solvolysis process, forming organic-rich mosaic SEI with low interface energy, which leads to aggregated whisker-like nucleation and growth. These results are helpful to tackle the long-standing question on the origin of lithium dendrite formation and guide the rational design of high-performance electrolytes for advanced lithium metal batteries.

Multifunctional solvent molecule design enables high-voltage Li-ion batteries

Elevating the charging cut-off voltage is one of the efficient approaches to boost the energy density of Li-ion batteries (LIBs). However, this method is limited by the occurrence of severe parasitic reactions at the electrolyte/electrode interfaces. Herein, to address this issue, we design a non-flammable fluorinated sulfonate electrolyte by multifunctional solvent molecule design, which enables the formation of an inorganic-rich cathode electrolyte interphase (CEI) on high-voltage cathodes and a hybrid organic/inorganic solid electrolyte interphase (SEI) on the graphite anode. The electrolyte, consisting of 1.9 M LiFSI in a 1:2 v/v mixture of 2,2,2-trifluoroethyl trifluoromethanesulfonate and 2,2,2-trifluoroethyl methanesulfonate, endows 4.55 V-charged graphite||LiCoO₂ and 4.6 V-charged graphite||NCM811 batteries with capacity retentions of 89% over 5329 cycles and 85% over 2002 cycles, respectively, thus resulting in energy density increases of 33% and 16% compared to those charged to 4.3 V. This work demonstrates a practical strategy for upgrading the commercial LIBs.

Improved Cycling of Li||NMC811 Batteries under Practical Conditions by a Localized High-Concentration Electrolyte

Li||NMC811 battery, with lithium-metal (high specific capacity and low redox potential) as anode and LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NMC811) as cathode, has been widely accepted to be a good candidate as one of the high-energy-density batteries. However, its cyclability needs improvement to fulfill the requirement for its future commercial use, especially under practical conditions. Electrolyte plays a key role in improving the cycling performance of Li||NMC811 batteries, where a high voltage/electrochemical window and good stability with the electrodes of the electrolyte are required. Herein, a localized high-concentration electrolyte with an additive of lithium difluoro(oxalate)borate (LiDFOB) is reported that improves the cycling performance of Li||NMC811 cells under crucial conditions with Li foil thickness of 50 µm, cathode areal loading of 4 mAh cm^-2 , the areal capacity ratio between the negative and positive electrodes (N/P ratio) of 2.6 and the electrolyte/cell capacity ratio (E/C ratio) of 3.0 g (Ah)^-1 . These cells can maintain 80% of the capacity after 195 cycles.

High Performance Low-Temperature Lithium Metal Batteries Enabled by Tailored Electrolyte Solvation Structure

The electrochemical performances of lithium metal batteries are determined by the kinetics of interfacial de-solvation and ion transport, especially at low-temperature environments. Here, a novel electrolyte that easily de-solvated and conducive to interfacial film formation is designed for low-temperature lithium metal batteries. A fluorinated carboxylic ester, diethyl fluoromalonate (DEFM), and a fluorinated carbonate, fluoroethylene carbonate (FEC) are used as solvents, while high concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is served as the solute. Through tailoring the electrolyte formulation, the lithium ions in the high concentrated fluorinated carboxylic ester electrolyte are mainly combined with anions, which weakens the bonding strength of lithium ions and solvent molecules in the solvation structure, beneficial to the de-solvation process at low temperature. The fluorinated carboxylic ester (FCE) electrolyte enables the LiFePO₄ (LFP) | Li half-cell achieves a high capacity of 91.9 mAh g^-1 at -30 °C, with high F content in the interface. With optimized de-solvation kinetics, the LFP | Li full cell remains over 100 mAh g^-1 at 0 °C after cycling 100 cycles. Building new solvents with outstanding low-temperature properties and weaker solvation to match with Li metal anode, this work brings new possibilities of realizing high energy density and low temperature energy storage batteries.

Solvent versus Anion Chemistry: Unveiling the Structure-Dependent Reactivity in Tailoring Electrochemical Interphases for Lithium-Metal Batteries

Electrolytes are critical for the reversibility of various electrochemical energy storage systems. The recent development of electrolytes for high-voltage Li-metal batteries has been counting on the salt anion chemistry for building stable interphases. Herein, we investigate the effect of the solvent structure on the interfacial reactivity and discover profound solvent chemistry of designed monofluoro-ether in anion-enriched solvation structures, which enables enhanced stabilization of both high-voltage cathodes and Li-metal anodes. Systematic comparison of different molecular derivatives provides an atomic-scale understanding of the unique solvent structure-dependent reactivity. The interaction between Li⁺ and the monofluoro (-CH₂F) group significantly influences the electrolyte solvation structure and promotes the monofluoro-ether-based interfacial reactions over the anion chemistry. With in-depth analyses of the compositions, charge transfer, and ion transport at interfaces, we demonstrated the essential role of the monofluoro-ether solvent chemistry in tailoring highly protective and conductive interphases (with enriched LiF at full depths) on both electrodes, as opposed to the anion-derived ones in typical concentrated electrolytes. As a result, the solvent-dominant electrolyte chemistry enables a high Li Coulombic efficiency (∼99.4%) and stable Li anode cycling at a high rate (10 mA cm^-2), together with greatly improved cycling stability of 4.7 V-class nickel-rich cathodes. This work illustrates the underlying mechanism of the competitive solvent and anion interfacial reaction schemes in Li-metal batteries and offers fundamental insights into the rational design of electrolytes for future high-energy batteries.

An Amphiphilic Molecule-Regulated Core-Shell-Solvation Electrolyte for Li-Metal Batteries at Ultra-Low Temperature

Lithium metal batteries hold great promise for promoting energy density and operating at low temperatures, yet they still suffer from insufficient Li compatibility and slow kinetic, especially at ultra-low temperatures. Herein, we rationally design and synthesize a new amphiphilic solvent, 1,1,2,2-tetrafluoro-3-methoxypropane, for use in battery electrolytes. The lithiophilic segment is readily to solvate Li⁺ to induce self-assembly of the electrolyte solution to form a peculiar core-shell-solvation structure. Such unique solvation structure not only largely improves the ionic conductivity to allow fast Li⁺ transport and lower the desolvation energy to enable facile desolvation, but also leads to the formation of a highly robust and conductive inorganic SEI. The resulting electrolyte demonstrates high Li efficiency and superior cycling stability from room temperature to -40 °C at high current densities. Meanwhile, anode-free high-voltage cell retains 87 % capacity after 100 cycles.

Rate-Dependent Failure Mechanisms and Mitigating Strategies of Anode-Free Lithium Metal Batteries

Anode-free lithium (Li) metal batteries (AFLMBs) could provide a specific energy over 500 Wh/kg, but their cycle life requires improvement. In this work, we propose a new method to calculate the real Coulombic efficiency (CE) of the Li metal during the cycling of AFLMBs. Through this approach, we find low rate discharging unfavorable for Li CE, which is mitigated through electrolyte optimization. In contrast, high rate discharging boosts Li reversibility, indicating AFLMBs to be intrinsically suited for high power use cases. However, AFLMBs still fail rapidly, due to the Li stripping overpotential buildup, which is mitigated by a zinc coating that enables a better electron/ion transferring network. We believe well-targeted strategies need to be better developed to synergize with the intrinsic features of AFLMBs to enable their commercialization in the future.

Strongly Solvating Ether Electrolytes for High-Voltage Lithium Metal Batteries

Ethers are promising electrolytes for lithium (Li) metal batteries (LMBs) because of their unique stability with Li metal. Although intensive research on designing anion-enriched electrolyte solvation structures has greatly improved their electrochemical stabilities, ether electrolytes are approaching an anodic bottleneck. Herein, we reveal the strong correlation between electrolyte solvation structure and oxidation stability. In contrast to previous designs of weakly solvating solvents for enhanced anion reactivities, the triglyme (G3)-based electrolyte with the largest Li⁺ solvation energy among different linear ethers demonstrates greatly improved stability on Ni-rich cathodes under an ultrahigh voltage of 4.7 V (93% capacity retention after 100 cycles). Ether electrolytes with a stronger Li⁺ solvating ability could greatly suppress deleterious oxidation side reactions by decreasing the lifetime of free labile ether molecules. This study provides critical insights into the dynamics of the solvation structure and its significant influence on the interfacial stability for future development of high-efficiency electrolytes for high-energy-density LMBs.

Tailored Solvation and Interface Structures by Tetrahydrofuran-Derived Electrolyte Facilitates Ultralow Temperature Lithium Metal Battery Operations

Ineffectiveness of Li-ion batteries (LIBs) in cold climates hinders electronics to work in various conditions including frigid environments, despite high demands. Given that intrinsic properties of LIB materials cause this problem, optimized cell chemistries ultimately are required for low-temperature usage. In this study, Li-metal batteries (LMBs) composed of a Li-metal anode (LMA) stabilized by a localized high-concentration electrolyte (LHCE) are found to significantly enhance low-temperature performance. The LHCE allows the LMA to have compact and regular deposition and excellent plating/stripping efficiency at sub-zero temperatures. The LHCE produces an inorganic-rich solid-electrolyte interphase with larger amounts of Li₂ O/LiF interfaces, dominance of ion aggregates in Li⁺ solvation, and enhanced Li⁺ transport, which can greatly improve the LMA stability. LMB full cells based on LiNi_0.8 Co_0.1 Mn_0.1 O₂ cathodes with the tailored electrolyte show high retentions of 75 and 64 % at -20 and -40 °C, respectively. Furthermore, the LMB configuration retains its charge-discharge capability even at -60 °C.

Low Concentration Sulfolane-Based Electrolyte for High Voltage Lithium Metal Batteries

Electrolyte engineering is crucial for the commercialization of lithium metal batteries. Here, lithium metal is stabilized in the highly reactive sulfolane-based electrolyte under low concentration (0.25 M) for the first time. Inorganic-polymer hybrid solid electrolyte interphase (SEI) with high ionic conductivity, low bonding with lithium and high flexibility enables dense chunky lithium deposition and high plating/stripping efficiency. Low concentration electrolyte (LCE) also enables excellent cycling stability of LiNi_0.5 Co_0.2 Mn_0.3 O₂ (NCM523)/Li cells at 1 C (90.7 % retention after 500 cycles) and 0.3 C (83.3 % retention after 1000 cycles). With a low N/P ratio (≈2), the capacity retention for NCM523/Li cells can achieve 94.3 % after 100 cycles at 0.3 C. Exploring the LCE is of paramount significance because it provides more possibilities of the lithium salt selections, especially reviving some lithium salts that are excluded before due to their low solubility. More importantly, LCE has the significant advantage of commercialization due to its cost-effectiveness.

A Systematic Study on the Effects of Solvating Solvents and Additives in Localized High-Concentration Electrolytes over Electrochemical Performance of Lithium-Ion Batteries

Localized high-concentration electrolytes (LHCEs) based on five different types of solvents were systematically studied and compared in lithium (Li)-ion batteries (LIBs). The unique solvation structure of LHCEs promotes the participation of Li salt in forming solid electrolyte interphase (SEI) on graphite (Gr) anode, which enables solvents previously considered incompatible with Gr to achieve reversible lithiation/delithiation. However, the long cyclability of LIBs is still subject to the intrinsic properties of the solvent species in LHCEs. Such issue can be readily resolved by introducing a small amount of additive into LHCEs. The synergetic decompositions of Li salt, solvating solvent and additive yield effective SEIs and cathode electrolyte interphases (CEIs) in most of the studied LHCEs. This study reveals that both the structure and the composition of solvation sheaths in LHCEs have significant effect on SEI and CEI, and consequently, the cycle life of energetically dense LIBs.

A monofluoride ether-based electrolyte solution for fast-charging and low-temperature non-aqueous lithium metal batteries

The electrochemical stability window of the electrolyte solution limits the energy content of non-aqueous lithium metal batteries. In particular, although electrolytes comprising fluorinated solvents show good oxidation stability against high-voltage positive electrode active materials such as LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), the ionic conductivity is adversely affected and, thus, the battery cycling performance at high current rates and low temperatures. To address these issues, here we report the design and synthesis of a monofluoride ether as an electrolyte solvent with Li-F and Li-O tridentate coordination chemistries. The monofluoro substituent (-CH₂F) in the solvent molecule, differently from the difluoro (-CHF₂) and trifluoro (-CF₃) counterparts, improves the electrolyte ionic conductivity without narrowing the oxidation stability. Indeed, the electrolyte solution with the monofluoride ether solvent demonstrates good compatibility with positive and negative electrodes in a wide range of temperatures (i.e., from -60 °C to +60 °C) and at high charge/discharge rates (e.g., at 17.5 mA cm^-2). Using this electrolyte solution, we assemble and test a 320 mAh Li||NCM811 multi-layer pouch cell, which delivers a specific energy of 426 Wh kg^-1 (based on the weight of the entire cell) and capacity retention of 80% after 200 cycles at 0.8/8 mA cm^-2 charge/discharge rate and 30 °C.

An Integrated Design of Electrodes for Flexible Dual-Ion Batteries

Due to the widespread employment of carbon materials in novel dual-ion batteries (DIBs) with high energy density, they possess the potential for large-scale energy storage and are inexpensive and environmentally friendly. However, drawbacks such as Al current collector corrosion and significant self-weight, as well as lithium metal abuse and poor deposition reversibility, impair the energy density and cycle performance of lithium-graphite DIBs (Li-G DIBs), severely limiting their application potential. Therefore, an integrated electrode structure design was proposed. That is, the flexible graphite and single-walled carbon nanotubes (SWCNTs) composite cathode (GSC), which is light-weight and self-supporting, and the self-supporting lithium metal anode, which is loaded on the flexible carbon cloth (CC) derived from waste mask (Li@CC), were prepared. Not only were the impacts of current collector corrosion and active material exfoliation avoided on the electrochemical performance, but the areal loading of Li metal was also regulated and its reversibility of deposition enhanced. At a current density of 200 mA g^-1 , the constructed Li@CC//GSC full cell could release a specific capacity of 100.5 mAh g^-1 , and the capacity retention rate after 300 cycles was greater than 80 %. Moreover, the fabricated flexible Li@CC//GSC full cell is not only recyclable and produces less environmental pollution but also has potential applications in wearable devices.

Regulating Frozen Electrolyte Structure with Colloidal Dispersion for Low Temperature Aqueous Batteries

Electrolyte freezing under low temperatures is a critical challenge for the development of aqueous batteries (ABs). While lowering the freezing point of the electrolyte has caught major research efforts, limited attention has been paid to the structural evolution during the electrolyte freezing process and regulating the frozen electrolyte structure for low temperature ABs. Here, we reveal the formation process of interconnected liquid regions for ion transport in frozen electrolytes with various in situ variable-temperature technologies. More importantly, the low-temperature performance of ABs was significantly improved with the colloidal electrolyte design using graphene oxide quantum dots (GOQDs), which effectively inhibits the growth of ice crystals and expands the interconnected liquid regions for facial ion transport. This work provides new insights and a promising strategy for the electrolyte design of low-temperature ABs.

Revealing the Multifunctions of Li3N in the Suspension Electrolyte for Lithium Metal Batteries

Inorganic-rich solid-electrolyte interphases (SEIs) on Li metal anodes improve the electrochemical performance of Li metal batteries (LMBs). Therefore, a fundamental understanding of the roles played by essential inorganic compounds in SEIs is critical to realizing and developing high-performance LMBs. Among the prevalent SEI inorganic compounds observed for Li metal anodes, Li₃N is often found in the SEIs of high-performance LMBs. Herein, we elucidate new features of Li₃N by utilizing a suspension electrolyte design that contributes to the improved electrochemical performance of the Li metal anode. Through empirical and computational studies, we show that Li₃N guides Li electrodeposition along its surface, creates a weakly solvating environment by decreasing Li⁺-solvent coordination, induces organic-poor SEI on the Li metal anode, and facilitates Li⁺ transport in the electrolyte. Importantly, recognizing specific roles of SEI inorganics for Li metal anodes can serve as one of the rational guidelines to design and optimize SEIs through electrolyte engineering for LMBs.

High-Voltage Cyclic Ether-Based Electrolytes for Low-Temperature Sodium-Ion Batteries

Cyclic ethers are promising solvents for low-temperature electrolytes, but they still suffer from intrinsic poor antioxidant abilities. Until now, ether-based electrolytes have been rarely reported for high-voltage sodium-ion batteries (SIBs) operated under a low-temperature range. Herein, a novel ether-based electrolyte consisting of tetrahydrofuran as the main solvent is proposed and it could be utilized for a high-voltage Na2/3Mn2/3Ni1/3O2 (MN) cathode in a wide-temperature range from -40 to 25 °C. Meanwhile, a thin and robust inorganic component-rich cathode electrolyte interface layer is elaborately introduced on the MN cathode by this tailored electrolyte, resulting in excellent cycle life of MN cathode. Specifically, a capacity retention of 97.2% after 140 cycles could be delivered by MN at 0.3 C at room temperature (RT). Especially at an ultra-low temperature of -40 °C, the initial discharge capacity of MN could still approach 89.3% of that at RT, and the capacity retention is 94.1% at 0.2 C after 100 cycles. This work provides a new insight into the rational design of ether-based electrolytes for high-voltage and stable SIBs operated in a wide-temperature range.

Locally Concentrated Ionic Liquid Electrolytes for Lithium-Metal Batteries

Non-flammable ionic liquid electrolytes (ILEs) are well-known candidates for safer and long-lifespan lithium metal batteries (LMBs). However, the high viscosity and insufficient Li⁺ transport limit their practical application. Recently, non-solvating and low-viscosity co-solvents diluting ILEs without affecting the local Li⁺ solvation structure are employed to solve these problems. The diluted electrolytes, i.e., locally concentrated ionic liquid electrolytes (LCILEs), exhibiting lower viscosity, faster Li⁺ transport, and enhanced compatibility toward lithium metal anodes, are feasible options for the next-generation high-energy-density LMBs. Herein, the progress of the recently developed LCILEs are summarised, including their physicochemical properties, solution structures, and applications in LMBs with a variety of high-energy cathode materials. Lastly, a perspective on the future research directions of LCILEs to further understanding and achieve improved cell performances is outlined.

Does Li-ion transport occur rapidly in localized high-concentration electrolytes?

The ionic conductivity and lithium-ion transference number of electrolytes significantly influence the rate capability of Li-ion batteries. Highly concentrated Li-salt/sulfolane (SL) electrolytes exhibit elevated Li⁺ transference numbers due to lithium-ion hopping via a ligand exchange mechanism within their -Li⁺-SL-Li⁺- network. However, highly concentrated electrolytes (HCEs) are extremely viscous and have an ionic conductivity that is one order of magnitude less than that of conventional electrolytes. Dilution of HCEs with a non-coordinating hydrofluoroether (HFE) lowers the viscosity and produces localized high-concentration electrolytes (LHCE). However, the mechanism of Li⁺ transport in LHCEs is unclear. This study investigated the transport properties of LHCEs prepared by diluting a SL-based HCE with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Electrolyte viscosity decreases dramatically upon dilution, whereas ionic conductivity increases only slightly. Ion diffusivity increases with increasing HFE content due to the decrease in electrolyte viscosity. However, the Li⁺ transference number declines, because the HFE interferes with conduction via the Li⁺ hopping mechanism. The resulting decrease in the product of ionic conductivity and Li⁺ transference number indicates superior lithium-ion transport in the parent HCE compared with LHCEs.

Dual-Salt Localized High-Concentration Electrolyte for Long Cycle Life Silicon-Based Lithium-Ion Batteries

Silicon-based materials are considered the most promising anodes for next-generation lithium-ion batteries (LIBs) owing to their high specific capacity. However, poor interfacial stability due to enormous volume changes severely restricts their mass application in LIBs. Here, we design a fluoroethylene carbonate (FEC)-containing dual-salt (LiFSI-LiPF₆) ether-based localized high-concentration electrolyte (D-LHCE-F) for enhancing the interfacial stability of silicon-based electrodes. It is revealed that the dominating LiFSI salt of superior chemical and thermal stability prevents the formation of corrosive HF, while the addition of FEC improves the interface stability by promoting the formation of protective LiF-rich SEI and increasing the flexibility of the interface. This robust and flexible SEI layer can adapt to substantial variations in the volume of silicon electrodes while preserving the integrity of the interface. The SiO_x/C electrode using the unique D-LHCE-F retains up to 78.5% of its initial capacity after 500 cycles at 0.5C, well surpassing that of the control electrolyte (3.4% capacity retention). More notably, the cycle life of the SiO_x/C||NCM90 (LiNi_0.9Co_0.05Mn_0.05O₂) full batteries is effectively enhanced thanks to the stabilized electrode/electrolyte interfaces. The key findings of this work offer crucial knowledge for rationally designing electrolyte chemistry to enable the practical application of high-energy-density LIBs adopting silicon-based anodes.

Structure-Dynamics Interrelation Governing Charge Transport in Cosolvated Acetonitrile/LiTFSI Solutions

Concentrated ionic solutions present a potential improvement for liquid electrolytes. However, their conductivity is limited by high viscosities, which can be attenuated via cosolvation. This study employs a series of experiments and molecular dynamics simulations to investigate how different cosolvents influence the local structure and charge transport in concentrated lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI)/acetonitrile solutions. Regardless of whether the cosolvent's dielectric constant is low (for toluene and dichloromethane), moderate (acetone), or high (methanol and water), they preserve the structural and dynamical features of the cosolvent-free precursor. However, the dissimilar effects of each case must be individually interpreted. Toluene and dichloromethane reduce the conductivity by narrowing the distribution of Li⁺-TFSI^- interactions and increasing the activation energies for ionic motions. Methanol and water broaden the distributions of Li⁺-TFSI^- interactions, replace acetonitrile in the Li⁺ solvation, and favor short-range Li⁺-Li⁺ interactions. Still, these cosolvents strongly interact with TFSI^-, leading to conductivities lower than that predicted by the Nernst-Einstein relation. Finally, acetone preserves the ion-ion interactions from the cosolvent-free solution but forms large solvation complexes by joining acetonitrile in the Li⁺ solvation. We demonstrate that cosolvation affects conductivity beyond simply changing viscosity and provide fairly unexplored molecular-scale perspectives regarding structure/transport phenomena relation in concentrated ionic solutions.

Room-Temperature Pseudo-Solid-State Iron Fluoride Conversion Battery with High Ionic Conductivity

Li-metal batteries (LMBs) employing conversion cathode materials (e.g., FeF₃) are a promising way to prepare inexpensive, environmentally friendly batteries with high energy density. Pseudo-solid-state ionogel separators harness the energy density and safety advantages of solid-state LMBs, while alleviating key drawbacks (e.g., poor ionic conductivity and high interfacial resistance). In this work, a pseudo-solid-state conversion battery (Li-FeF₃) is presented that achieves stable, high rate (1.0 mA cm^-2) cycling at room temperature. The batteries described herein contain gel-infiltrated FeF₃ cathodes prepared by exchanging the ionic liquid in a polymer ionogel with a localized high-concentration electrolyte (LHCE). The LHCE gel merges the benefits of a flexible separator (e.g., adaptation to conversion-related volume changes) with the excellent chemical stability and high ionic conductivity (∼2 mS cm^-1 at 25 °C) of an LHCE. The latter property is in contrast to previous solid-state iron fluoride batteries, where poor ionic conductivities necessitated elevated temperatures to realize practical power levels. The stable, room-temperature Li-FeF₃ cycling performance obtained with the LHCE gel at high current densities paves the way for exploring a range of architectures including flexible, three-dimensional, and custom shape batteries.

Inner Lithium Fluoride (LiF)-Rich Solid Electrolyte Interphase Enabled by a Smaller Solvation Sheath for Fast-Charging Lithium Batteries

Fast-charging lithium-ion batteries (LIBs) have been severely hampered by the slow development of their electrolytes. Herein, we demonstrate that the size effect of solvent sheath would pose a great effect on the fast-charging performance of LIBs. Three similar ethers, including diethyl ether (DEE), dipropyl ether (DPE), and dibutyl ether (DBE), were mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and lithium bis(fluorosulfonyl)imide (1 M) to form an electrolyte for fast-charging LIBs, respectively. The results showed that it is more difficult to form ternary graphite intercalation compounds (GICs) in the electrolyte with a larger solvation sheath. The DEE electrolyte can form stable GICs and generate an inner LiF-rich solid electrolyte interphase (SEI), lowering the diffusion barrier of Li⁺. Therefore, the graphite anode powered by the DEE electrolyte can maintain a capacity of 190 mAh g^-1 at 4 C after 500 cycles. This kind of size effect of solvation sheath is also applicable to lithium metal batteries.

Structural and Chemical Evolutions of Li/Electrolyte Interfaces in Li-Metal Batteries: Tracing Compositional Changes of Electrolytes under Practical Conditions

Despite the promises in high-energy-density batteries, Li-metal anodes (LMAs) have suffered from extensive electrolyte decomposition and unlimited volume expansion owing to thick, porous layer buildup during cycling. It mainly originates from a ceaseless reiteration of the formation and collapse of solid-electrolyte interphase (SEI). This study reveals the structural and chemical evolutions of the reacted Li layer after different cycles and investigates its detrimental effects on the cycling stability under practical conditions. Instead of the immediately deactivated top surface of the reacted Li layer, the chemical nature underneath the reacted Li layer can be an important indicator of the electrolyte compositional changes. It is found that cycling of LMAs with a lean electrolyte (≈3 g Ah^-1 ) causes fast depletion of salt anions, leading to the dynamic evolution of the reacted Li layer structure and composition. Increasing the salt-solvent complex while reducing the non-solvating diluent retards the rate of depletion in a localized high-concentration electrolyte, thereby demonstrating prolonged cycling of Li||NMC622 cells without compromising the Li Coulombic efficiencies and high-voltage stability.