Localized High-concentration Electrolytes Research Articles

Suppression of potasside reaction in localized high concentration electrolytes utilizing fluorine-substituted benzene as bifunctional additive for potassium-metal batteries

Localized high-concentration electrolytes (LHCEs) have unique solvation structures that do not affect the original salt–solvent coordination from highly concentrated electrolytes. LHCEs enable a wide electrochemical voltage window and mitigate the extensive dendritic growth of metallic anodes. However, in K metal batteries, LHCE undergoes undesirable side reactions because of potasside (K−), triggering aggressive chemistry with a diluent, represented as hydrofluoroethers (HFEs), eventually resulting in poor cycle life. In this study, 1, 3, 5-trifluorobenzene (TFB) was introduced as a functional additive to prevent the parasitic K− reaction. The three symmetrically substituted fluorines prevented TFB from disrupting the as-formed solvation structure of LHCE located in the outer sphere. This characteristic increases the reaction energy barrier between K− and HFE, suppressing the deterioration of the metal anode by adding only 3 wt.% of TFB in LHCE. Moreover, TFB preferentially decomposes at each electrode because of its molecular energy level and increases the reversibility of the cell, reducing unnecessary consumption of electrolytes with a stable interface. This study discusses a novel method to prevent the K− reaction at the electrolyte level and the utilization of LHCE in K batteries to pursue higher energy densities, sustaining the advantage of using a metal anode.

Empowering the Potassium-Sulfur Battery with Commendable Reaction Kinetics and Capacity Output by Localized High-Concentration Electrolytes.

Potassium-sulfur (K-S) batteries are one of the promising high-energy-density candidates beyond current lithium-ion batteries. Nevertheless, in practice, the utilization of K-S batteries is largely hindered due to the dissolution and shuttle effect of the cathode redox intermediates and the scarcity of an effective anode protection layer in conventional electrolytes. Herein, electrolyte engineering is applied to formulate an ether-based localized high-concentration electrolyte (LHCE) for the first time in a K-S cell with the mitigated parasitic effect of polysulfide dissolution and shuttle and the tuned anode-electrolyte interface property. A nonsolvating and polysulfide-stable fluoroether is sieved as a cosolvent in such an LHCE, which possesses the ultralow polysulfides solubility due to less roaming solvents and thus alleviates the polysulfides shuttle effect. The anion-derived solid electrolyte interphase enriched in inorganic components is constructed due to the strengthened cation-anion interplay in the primary solvation sheath and highlighted with accelerated interfacial kinetics in a K-S cell. It is validated that the proposed LHCE unlocks the theoretical capacity of the K-S cell based on the conversion between S and K2S3. It is further revealed that the lifespan is limited to the anode corrosion with severe cosolvent degradation caused by limited solvating solvent compatibility with metallic K, and the inevitable byproduct accumulation at the S cathode. The K-S cell based on the designed LHCE could achieve a prolonged lifespan with a reversible capacity of 448 mA h/gs after 80 cycles with an elaborate cathode design. This work shines a light on the electrolyte design perspective for full utilization and an in-depth mechanistic understanding of high-energy-density K-S batteries.

Anion‐Derived Solid Electrolyte Interphase Enabled by Diluent Modulated Dimethyl Carbonate‐Based Localized High Concentration Electrolytes for Lithium Metal Batteries

Carbonate‐based electrolytes generally suffer from low Coulombic efficiency and poor cycling stability in lithium metal batteries. In this work, localized high concentration electrolytes (LHCEs) based on dimethyl carbonate (DMC) with varying diluent additions are designed. LHCEs demonstrate higher Li+ transference numbers and a greater proportion of contact ion pairs (CIPs) and ion pair aggregates (AGGs) in the solvation structures, facilitating the formation of anion‐derived solid electrolyte interphase (SEI). Furthermore, LHCEs enhance the Coulombic efficiency of Li||Cu cells and improve the anodic stability against lithium. One of these LHCEs, prepared with appropriate diluent addition, exhibits excellent capacity retention in Li||NCM622 cells at 0.5 C after 150 cycles, thus presenting promising possibilities for the development of high energy density lithium metal batteries.

Tailoring Solvation Solvent in Localized High-Concentration Electrolytes for Lithium||Sulfurized Polyacrylonitrile.

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for lithium-sulfur (Li-S) batteries due to its significantly reduced polysulfide (PS) dissolution compared to that of elemental S cathodes. Although conventional carbonate-based electrolytes are stable with SPAN electrodes, they are unstable with Li metal anodes. Recently, localized high-concentration electrolytes (LHCEs) have been developed to improve the stability of Li anodes. Here, we report a new strategy to further improve the performance of Li||SPAN batteries by replacing the conventional solvating solvent 1,2-dimethoxyethane (DME) in LHCEs with a new solvating solvent, 1,2-diethoxyethane (DEE). The new optimal DEE-LHCE exhibits less reactivity against Li2S2, alleviates PS dissolution, forms a better cathode-electrolyte interphase layer on the SPAN cathode, and enhances SPAN structural reversibility even at elevated temperatures (45 °C). Compared to DME-LHCE, DEE-LHCE with the same salt and diluent leads to better performance in Li||SPAN batteries (with 82.9% capacity retention after 300 cycles at 45 °C), preservation of the SPAN cathode structure, and suppression of volume change of the Li metal anode. A similar strategy on tailoring the solvating solvents in LHCEs can also be used in other rechargeable batteries to improve their electrochemical performances.

An Ultralow-concentration and Moisture-resistant Electrolyte of Lithium Difluoro(oxalato)borate in Carbonate Solvents for Stable Cycling in Practical Lithium-ion Batteries.

The electrolyte concentration not only impacts the battery performance but also affects the battery cost and manufacturing. Currently, most studies focus on high-concentration (>3 M) or localized high-concentration electrolytes (~1 M); however, the expensive lithium salt imposes a major concern. Most recently, ultralow concentration electrolytes (<0.3 M) have emerged as intriguing alternatives for battery applications, which feature low cost, low viscosity, and extreme-temperature operation. However, at such an early development stage, many works are urgently needed to further understand the electrolyte properties. Herein, we introduce an ultralow concentration electrolyte of 2 wt % (0.16 M) lithium difluoro(oxalato)borate (LiDFOB) in standard carbonate solvents. This electrolyte exhibits a record-low salt/solvent mass ratio reported to date, thus pointing to a superior low cost. Furthermore, this electrolyte is highly compatible with commercial Li-ion materials, forming stable and inorganic-rich interphases on the lithium cobalt oxide (LiCoO2) cathode and graphite anode. Consequently, the LiCoO2-graphite full cell demonstrates excellent cycling performance. Besides, this electrolyte is moisture-resistant and effectively suppresses the generation of hydrogen fluoride, which will markedly facilitate the battery assembly and recycling process under ambient conditions.

The Solvation Structure of Localized High Concentration Electrolytes

AbstractThe development of liquid electrolytes receives significant attention within the field of battery research. New concepts are emerging, and one of these groundbreaking ideas is localized high concentration electrolytes (LHCEs). The fundamental characteristic of this type of electrolyte is related to its solvation structure. However, despite the progress made, the solvation process and its relationship towards physicochemical and electrochemical properties are not yet fully understood. A comprehensive understanding of LHCEs and their solvation structure requires further dedicated research and analysis. This concept review offers a thorough examination of the design principles governing LHCEs, elucidates methodologies for investigating solvation structures, connects these insights to interphase chemistry, and explores potential applications in future battery technology.

Stabilizing cathode-electrolyte interphase by localized high-concentration electrolytes for high-voltage sodium-ion batteries

Sodium-ion batteries (SIBs) operating under high-voltages suffer from unstable cathode-electrolyte interphase (CEI) formed on the cathode surface, resulting in continuous electrolyte decomposition, surface reconstruction, transition metal dissolution, and eventually capacity decay. Therefore, designing high-voltage electrolyte and constructing the robust CEI are critical for high-energy SIBs. Herein, a localized high-concentration electrolyte (LHCE) is fabricated by dissolving sodium hexafluorophosphate in methyl ethyl carbonate, fluorinated ethylene carbonate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, which demonstrates oxidative stability exceeding 6 V relative to Na+/Na. The dense and homogeneous inorganics-rich CEIs are generated by the preferential decomposition of PF6− anions during the electrochemical charge to 4.4 V, which lower interface impedance, facilitate Na+ transportation and suppress subsequent side reactions. Consequently, the cell integrating high-voltage P2-Na0.7Li0.03Mg0.03Ni0.27Mn0.6Ti0.07O2 cathode and Na anode exhibits a capacity retention of 87.3% after 200 cycles and a high-rate discharge capacity of 84 mA h g−1 at 20 C under a high cut-off voltage of 4.4 V. This work offers a feasible pathway to tailor electrolyte chemistry and interphase properties for practical next generation high-voltage SIBs.

Highly Stable Lithium Metal Batteries Enabled by Tuning the Molecular Polarity of Diluents in Localized High-Concentration Electrolytes.

Electrolyte plays a crucial role in ensuring stable operation of lithium metal batteries (LMBs). Localized high-concentration electrolytes (LHCEs) have the potential to form a robust solid-electrolyte interphase (SEI) and mitigate Li dendrite growth, making them a highly promising electrolyte option. However, the principles governing the selection of diluents, a crucial component in LHCE, have not been clearly determined, hampering the advancement of such a type of electrolyte systems. Herein, the diluents from the perspective of molecular polarity are rationally designed and developed. A moderately fluorinated solvent, 1-(1,1,2,2-tetrafluoroethoxy)propane (TNE), is employed as a diluent to create a novel LHCE. The unique molecular structure of TNE enhances the intrinsic dipole moment, thereby altering solvent interactions and the coordination environment of Li-ions in LHCE. The achieved solvation structure not only enhances the bulk properties of LHCE, but also facilitates the formation of more stable anion-derived SEIs featured with a higher proportion of inorganic species. Consequently, the corresponding full cells of both Li||LiFePO4 and Li||LiNi0.8 Co0.1 Mn0.1 O2 cells utilizing Li thin-film anodes exhibit extended long-term stability with significantly improved average Coulombic efficiency. This work offers new insights into the functions of diluents in LHCEs and provides direction for further optimizing the LHCEs for LMBs.

Interfacial polymerization mechanisms assisted flame retardancy process of low-flammable electrolytes on lithium anode

Thermal runaway is a hazardous risk, occurring more readily in high-energy–density lithium-ion batteries (LIBs), which leads to a rapid temperature rise and even combustion or explosion when using flammable electrolyte systems. Flame retardants (FRs), such as trimethyl phosphate (TMPa) and triethyl phosphate (TEP), are commonly utilized due to their effective flame suppression, low toxicity, and excellent thermal stability. However, the lack of in-depth understanding of the flame retardancy mechanism and solid electrolyte interphase (SEI) formation process has made the development of functional electrolytes difficult at present. In this study, we clarified the flame retardancy and interfacial reaction mechanisms of low-flammable TMPa localized high-concentration electrolytes (LHCE) using hybrid ab initio and reactive force field (HAIR) schemes. Long-term HAIR simulation reveals that phosphorous radicals produced by the decomposition of TMPa capture carbon radicals, encouraging their polymerization into low-flammable oligomers, while fluorine-containing solvents in the electrolyte capture hydrogen radicals and produce nonflammable hydrofluoric acid (HF). This synergistic flame retardancy mechanism provides essential atomic-level insights for the rational design of high-safety electrolytes in the future.

Active Diluent-Anion Synergy Strategy Regulating Nonflammable Electrolytes for High-Efficiency Li Metal Batteries.

High-energy Li metal batteries (LMBs) consisting of Li metal anodes and high-voltage cathodes are promising candidates of the next generation energy-storage systems owing to their ultrahigh energy density. However, it is still challenging to develop high-voltage nonflammable electrolytes with superior anode and cathode compatibility for LMBs. Here, we propose an active diluent-anion synergy strategy to achieve outstanding compatibility with Li metal anodes and high-voltage cathodes by using 1,2-difluorobenzene (DFB) with high activity for yielding LiF as an active diluent to regulate nonflammable dimethylacetamide (DMAC)-based localized high concentration electrolyte (LHCE-DFB). DFB and bis(fluorosulfonyl)imide (FSI- ) anion cooperate to construct robust LiF-rich solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), which effectively stabilize DMAC from intrinsic reactions with Li metal anode and enhance the interfacial stability of the Li metal anodes and LiNi0.8 Co0.1 Mn0.1 O2 (NCM811) cathodes. LHCE-DFB enables ultrahigh Coulombic efficiency (98.7 %), dendrite-free, extremely stable and long-term cycling of Li metal anodes in Li || Cu cells and Li || Li cells. The fabricated NCM811 || Li cells with LHCE-DFB display remarkably enhanced long-term cycling stability and excellent rate capability. This work provides a promising active diluent-anion synergy strategy for designing high-voltage electrolytes for high-energy batteries.

Active Diluent‐Anion Synergy Strategy Regulating Nonflammable Electrolytes for High‐Efficiency Li Metal Batteries

AbstractHigh‐energy Li metal batteries (LMBs) consisting of Li metal anodes and high‐voltage cathodes are promising candidates of the next generation energy‐storage systems owing to their ultrahigh energy density. However, it is still challenging to develop high‐voltage nonflammable electrolytes with superior anode and cathode compatibility for LMBs. Here, we propose an active diluent‐anion synergy strategy to achieve outstanding compatibility with Li metal anodes and high‐voltage cathodes by using 1,2‐difluorobenzene (DFB) with high activity for yielding LiF as an active diluent to regulate nonflammable dimethylacetamide (DMAC)‐based localized high concentration electrolyte (LHCE‐DFB). DFB and bis(fluorosulfonyl)imide (FSI−) anion cooperate to construct robust LiF‐rich solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), which effectively stabilize DMAC from intrinsic reactions with Li metal anode and enhance the interfacial stability of the Li metal anodes and LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes. LHCE‐DFB enables ultrahigh Coulombic efficiency (98.7 %), dendrite‐free, extremely stable and long‐term cycling of Li metal anodes in Li || Cu cells and Li || Li cells. The fabricated NCM811 || Li cells with LHCE‐DFB display remarkably enhanced long‐term cycling stability and excellent rate capability. This work provides a promising active diluent‐anion synergy strategy for designing high‐voltage electrolytes for high‐energy batteries.

Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries.

High concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) have emerged as promising candidates to enable higher energy density Li-ion batteries due to their advantageous interfacial properties that result from their unique solvent structures. Using electrophoretic NMR and electrochemical techniques, we characterize and report full transport properties, including the lithium transference numbers (t+) for electrolytes ranging from the conventional ∼1 M to HCE regimes as well as for LHCE systems. We find that compared to conventional electrolytes, t+ increases for HCEs; however the addition of diluents to LHCEs significantly decreases t+. Viscosity effects alone cannot explain this behavior. Using Onsager transport coefficients calculated from our experiments, we demonstrate that there is more positively correlated cation-cation motion in HCEs as well as fast cation-anion ligand exchange consistent with a concerted ion-hopping mechanism. The addition of diluents to LHCEs results in more anticorrelated motion indicating a disruption of concerted cation-hopping leading to low t+ in LHCEs.

Micelle-like bulk structure of localized high-concentration electrolytes

Micelle-like bulk structure of localized high-concentration electrolytes

(Invited) Extending Cycle and Calendar Life of Si Based Li-Ion Batteries By Localized High Concentration Electrolytes

Si is one of the most promising anode materials for the next generation of lithium (Li) ion batteries (LIBs). Recently, we have developed a porous Si with a unique carbon coating structure (“carbon on Si” instead of “Si on carbon”). The nano-porosity of the materials absorbed most of volume expansion so the particles (between 3-5 mm) can retain their integrity during cycling instead of breakdown observed in bulk Si particles. Although single layer pouch cells (Si/NMC622, 62 mAh) with this porous Si anode and baseline electrolyte (1.2 M LiPF6 in EC-EMC (3:7 by wt.) +10 wt.% FEC) can retain 91% capacity after 800 cycles, their calendar life is still far shorter from 10-year target required by electrical vehicles.In this work, we present a systematic investigation of the application of localized high concentration electrolyte (LHCE) to extend the cycle life and calendar life of Si-LIBs. A typical LHCE contains a Li salt (typically LiFSI) with high solubility in a common solvent (such as DMC), and a diluent (such as TTE) with minimal solubility of the Li salt. Compared with other electrolyte design, one distinct advantage of LHCE is that solvent and diluent in LHCE can be largely decoupled from each other: change in once components will have not significantly affect the functionality of other components. This characteristic of LHCE enables much more freedom in the electrolyte designs to satisfy different practical application needs (such as long cycle life, long calendar life, high rate, high temperature, and high voltage). For example, Si/NMC622 cells with a composition of LiFSI-2.15DMC-0.17VC-0.69FEC-4.31TTE (in molar ratio) have increased cycle life of Si/NMC622 pouch cells to 2000 cycles with 80% capacity retention at 25°C as compared with 1000 cycles when baseline electrolyte was used. However, this electrolyte is less stable when LiCoO2 is used as the cathode, leading to rapid capacity loss at elevated temperatures (45°C). One possible reason for this is that cobalt in the cathode leads to more rapid decomposition of FEC than in the case of NMC622 at 45°C. On the other hand, the stability of LHCE is significantly affected by the amount of free (unbound) solvent (including DMC, VC, and FEC in the above example) in the electrolyte. By minimizing the amount of total free solvents, an electrolyte with a composition of LiFSI-2DMC-0.1EC-0.1FEC-3TTE has led to a long cycle life of Si/NMC622 cells at 25°C. Increasing the amount of main solvent in the electrolyte can further increase the conductivity of the electrolyte and significantly increase the charge/discharge rate of the cells.The long calendar life of Si-LIBs can be measured by accelerated tests at elevated temperatures (up to 55°C). Although FEC-containing electrolytes can increase the cycle life of Si-LIBs at room temperature, most Si-LIBs with FEC-containing electrolytes exhibit a rapid increase in impedance at elevated temperature, leading to a short projected-calendar-life. We designed a series of experiments to investigate the effect of FEC on the calendar life of Si-LIBs. The results clearly indicate that the formation of a high impedance interphase layer on the anode and cathode surfaces due to the decomposition of FEC at high temperatures is one of the fundamental reasons for the short calendar life of Si-LIBs. By replacing FEC with an alternative additive, a new LHCE electrolyte has been designed that led to much better cycling stability of Si-LIBs at 45°C (less than 5% capacity loss in 500 cycles) and much longer calendar life. More details of these investigations will be reported in this presentation.

Enhancing Cycle Life of Lithium Metal Batteries By Regulating Solid-Electrolyte Interphase Using Gel Polymer Electrolyte

Lithium metal with high theoretical capacity and low redox potential is one of the promising anode materials for high energy density batteries. However, the lithium metal has safety problems and poor cycling performance, due to the growth of Li dendrite and side reactions between Li and electrolytes. One of the most effective strategies to stabilize Li metal is forming robust solid-electrolyte interphase (SEI). Recently, anion-derived and inorganic-rich SEI is known to be stable and ionic conductive, leading to good cycling stability. One of the most popular strategies to form durable SEI is to use the localized high-concentration electrolytes (LHCE) which construct an anion-derived SEI layer on Li metal. In this work, we synthesized LHCE-based gel polymer electrolytes to form a stable SEI layer and enhance cycling stability. Small amount of cross-linking agent was added into dimethoxyethane (DME)-based LHCE when synthesizing gel polymer electrolyte. The functional groups in cross-linking agents increase Li+ and anion interaction in the gel polymer electrolytes, resulting in formation of the stable SEI layer. The solvation structure and chemical composition of the SEI layer were investigated by density functional theory (DFT) calculation and spectroscopic analyses such as FT-IR, Raman, and XPS. The Li/NCM811 cells with LHCE-based gel polymer electrolyte were assembled for the cycling test, and they exhibited superior cycling performance than liquid electrolyte-based cells.

(Invited) Modeling of the Electric Double Layer (EDL) at Li/SEI/Electrolyte Interfaces

Liquid electrolytes, consisting of salts, solvents, and additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium(Li)-ion batteries. However, the electric double layer (EDL) structure near charged surfaces is still unsolved, despite its importance in dictating the species being reduced for SEI formation near a negative electrode. Recently, we have developed an interactive Molecular Dynamics -Density Functional Theory -data statistics (MD-DFT-data) model to investigate the reduction reactions of multicomponent electrolytes within the EDL. We will illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. Our results reveal that the role of fluoroethylene carbonate (FEC) additive differs drastically in the two electrolytes as an SEI modifier to form the beneficial F-containing SEI component (e.g., LiF). The competition among the cations, anions, and various species in the solvents with a charged surface at different temperatures can all jointly determine the EDL structure and therefore the SEI compositions. [1]While the classical Poisson–Boltzmann EDL model developed for fully solvated ions face new challenges in high-concentration liquid electrolytes (HCE), localized high-concentration liquid electrolytes (LHCE) as well as solid electrolytes (SE), new theoretical developments are required. We will introduce a new DEL model in SE and SEI by solving the DFT-informed Poisson–Fermi–Dirac equation and demonstrate how it can be used for interlayer thickness design. [2][1] Wu, Q.S, McDowell, M.T., & Qi, Y., Journal of the American Chemical Society 145 (4), 2473-2484 (2023)[2] Swift, M.W., Swift, J.W. & Qi, Y. Modeling the electrical double layer at solid-state electrochemical interfaces. Nat Comput Sci 1, 212–220 (2021)

Flame‐Retardant Nonaqueous Electrolytes for High‐Safety Potassium‐Ion Batteries

AbstractPotassium‐ion batteries (PIBs) with conventional organic‐based flammable electrolytes suffer from serious safety issues with a high risk of ignition and burning especially under harsh conditions, which significantly limits their widespread applications. Flame‐retardant electrolytes (FREs) are considered as one of the most effective strategies to address these safety issues. Therefore, it's much necessary to summarize the challenges, recent progress, and design principles of flame‐retardant nonaqueous electrolytes for PIBs to guide their development and future applications. In this review, an in‐depth introduction and explanation of the origins of electrolyte flammability are first presented. Particularly, the state‐of‐the‐art design principles of FREs for PIBs are extensively summarized and emphasized, including the electrolyte flame‐retardant solvents/additives, highly concentrated electrolytes (HCEs), localized high‐concentration electrolytes (LHCEs), ionic liquids‐based electrolytes and solid‐state electrolytes. Moreover, the advantages and drawbacks of each approach are systematically presented and discussed, following by proposed perspectives to guide the rational development of next‐generation high‐safety PIBs for practical applications.

A Semisolvated Sole-Solvent Electrolyte for High-Voltage Lithium Metal Batteries.

Lithium metal batteries (LMBs) coupled with a high-voltage Ni-rich cathode are promising for meeting the increasing demand for high energy density. However, aggressive electrode chemistry imposes ultimate requirements on the electrolytes used. Among the various optimized electrolytes investigated, localized high-concentration electrolytes (LHCEs) have excellent reversibility against a lithium metal anode. However, because they consist of thermally and electrochemically unstable solvents, they have inferior stability at elevated temperatures and high cutoff voltages. Here we report a semisolvated sole-solvent electrolyte to construct a typical LHCE solvation structure but with significantly improved stability using one bifunctional solvent. The designed electrolyte exhibits exceptional stability against both electrodes with suppressed lithium dendrite growth, phase transition, microcracking, and transition metal dissolution. A Li||Ni0.8Co0.1Mn0.1O2 cell with this electrolyte operates stably over a wide temperature range from -20 to 60 °C and has a high capacity retention of 95.6% after the 100th cycle at 4.7 V, and ∼80% of the initial capacity is retained even after 180 cycles. This new electrolyte indicates a new path toward future electrolyte engineering and safe high-voltage LMBs.

(Invited) Modeling of the Bulk and Electrical Double-Layer Structures of Localized High-Concentration Electrolytes

A new class of electrolytes, localized high-concentration electrolytes (LHCE), have shown many benefits to high-capacity electrodes (lithium (Li)-metal, silicon, sodium, zinc, and potassium).[1] These electrolytes combine high-concentration electrolytes (HCE) with salt and solvent with low-viscosity diluents. The dilutes were added to increase ionic conductivity while the locally highly concentrated salt-solvent clusters will facilitate stable solid-electrolyte interphase (SEI) formation while preventing metallic-dendrite formation. For example, it was demonstrated that an LHCE based on lithium bis(fluorosulfonyl)imide (LiFSI) salt, Dimethoxyethane (DME) solvent, and tris(2,2,2-trifluoroethyl)orthoformate (TFEO) diluent (e.g., LiFSI-1.2DME-2TFEO by mol) forms inorganic-rich, monolithic SEI and shows excellent cycling performance for Li-metal batteries under practical conditions.[2] Here, the interactions among cations, anions, solvent molecules, and diluent molecules create a complex design space, making the trial-and-error empirical approach less efficient.Using density functional theory and classical molecular dynamics simulations, we reveal several important design principles:a) The dielectric constant of the pure solvent or diluent is not a good descriptor to describe the solubility of the salt in solvent and diluent.b) The single ion–solvent (diluent) binding energy is insufficient to predict the solubility, which also depends on the molecular structures and the corresponding solvation shell structure.c) The salt-solvent, salt-diluent, and diluent-solvent solubilities jointly determine the solvation structures in LHCE. For example, at certain concentrations, the Li+ solvation shells contain both anions and solvent molecules with minimal participation of the diluent molecules. The local salt concentration can be even higher than its counterpart in the HCE. These simulated solvation structures in bulk electrolytes compare well with Raman spectrum and SAXS data.d) Moreover, the solvation structures are temperature-sensitive. We revealed why the local salt concentration is higher at a certain temperature by MD simulations, which is further validated through Raman deconvolution analyses.e) The SEI formation is sensitive to the electrolyte structure near a charged surface. Therefore, the electrical double-layer (EDL) structure of LHCE needs to be resolved. Here we give a statistical representation of the LHCE in the EDL and show how it differs from the bulk structure and its impact on SEI formation.Reference:[1] J. Electrochem. Soc., 2021, 168, 010522[2] Nat. Energy 2019, 4, 796-805

Non-Fluorinated Diluent in Localized High Concentration Electrolytes Enabling Superior Performance of Lithium Metal Negative Electrode Battery

Localized high-concentration electrolytes (LHCEs) are very promising strategies for the high-energy-density lithium (Li) metal batteries (LMBs). Nonsolvating diluents introduced in the LHCEs plays a critical role in physicochemical properties of LHCE and the overall LMB performance. However, there is a lack of design strategies for ideal nonsolvating diluents, and the reported cases are limited to fluorinated nonsolvating diluents (FNDs). FNDs suffer from accelerated decomposition at lithium metal, leading to electrolyte dry-up and ultimately battery failure. Furthermore, the high cost and potential environmental hazards of FNDs necessitate the development of non-fluorinated nonsolvating diluents (NFNDs). Here, we present a design rule for the ideal NFNDs by spectroscopically characterizing the Li+ solvation ability and miscibility. Our design rule identifies the ideal NFNDs, based on the superior cycling performane of candidate diluents over 350 cycles (99.0% ethoxybezene), 500 cycles (98.5% anisole), and 1400 cycles (99.0%, furan). NMR spectra revealed that the designed NFNDs were highly stable in electrolytes during extended cycles. Raman spectroscopy and theoretical calculation reveal that resonance of an electron pair on the oxygen atom of NFND molecules decreases the lithium-ion solvation ability, thereby achieving desirable non-solvating characteristics while maintaining good miscibility, superior cathodic stability, and low prices. Figure 1