Optimized Nonflammable Concentrated Electrolytes by Introducing a Low-Dielectric Diluent

Design of Localized High-Concentration Electrolytes from the Perspective of Physicochemical Properties

The physicochemical properties of electrolytes profoundly impact the energy density, rate performance, and manufacturability of rechargeable lithium batteries. Localized high-concentration electrolytes (LHCEs), a novel electrolyte class, have attracted considerable interest, yet the impact of diluents on their physicochemical properties remains unclear, as most reports involve only a few samples. Here we prepared 345 electrolyte samples using 21 diluents and systematically investigated the effect of diluent type and content on the miscibility, density, viscosity, and ion conductivity of LHCEs. We found that the physicochemical properties of LHCEs are mainly affected by the diluents' density and viscosity, regardless of type. Notably, the ionic conductivity exhibits two typical variance trends, "volcano" and "descending," both correlating strongly with diluents' viscosity rather than dielectric constant, a parameter commonly employed in electrolyte design. This anomaly can be explained by the "plum pudding" solvation model, providing essential insights for developing lightweight, highly fluid, and conductive LHCEs.

Interacted Ternary Component Ensuring High-Security Eutectic Electrolyte for High Performance Sodium-Metal Batteries

Due to the intrinsic flame-retardant, eutectic electrolytes are considered a promising candidate for sodium-metal batteries (SMBs). However, the high viscosity and ruinous side reaction with Na metal anode greatly hinder their further development. Herein, based on the Lewis acid-base theory, a new eutectic electrolyte (EE) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), succinonitrile (SN), and fluoroethylene carbonate (FEC) is reported. As a strong Lewis base, the ─C≡N group of SN can effectively weaken the interaction between Na+ and TFSI-, achieving the dynamic equilibrium and reducing the viscosity of EE. Moreover, the FEC additive shows a low energy level to construct thicker and denser solid electrolyte interphase (SEI) on the Na metal surface, which can effectively eliminate the side reaction between EE and Na metal anode. Therefore, EE-1:6 + 5% FEC shows high ionic conductivity (2.62 mS cm-1) and ultra-high transference number of Na+ (0.96). The Na||Na symmetric cell achieves stable Na plating/stripping for 1100 h and Na||Na3V2(PO4)3/C cell shows superior long-term cycling stability over 2000 cycles (99.1% retention) at 5 C. More importantly, the Na||NVP/C pouch cell demonstrates good cycling performance of 102.1 mAh g-1 after 135 cycles at 0.5 C with an average coulombic efficiency of 99.63%.

Solvation Effect: The Cornerstone of High-Performance Battery Design for Commercialization-Driven Sodium Batteries

Sodium batteries (SBs) emerge as a potential candidate for large-scale energy storage and have become a hot topic in the past few decades. In the previous researches on electrolyte, designing electrolytes with the solvation theory has been the most promising direction is to improve the electrochemical performance of batteries through solvation theory. In general, the four essential factors for the commercial application of SBs, which are cost, low temperature performance, fast charge performance and safety. The solvent structure has significant impact on commercial applications. But so far, the solvation design of electrolyte and the practical application of sodium batteries have not been comprehensively summarized. This review first clarifies the process of Na+ solvation and the strategies for adjusting Na+ solvation. It is worth noting that the relationship between solvation theory and interface theory is pointed out. The cost, low temperature, fast charging, and safety issues of solvation are systematically summarized. The importance of the de-solvation step in low temperature and fast charging application is emphasized to help select better electrolytes for specific applications. Finally, new insights and potential solutions for electrolytes solvation related to SBs are proposed to stimulate revolutionary electrolyte chemistry for next generation SBs.

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.

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.

High-safety and high-efficiency electrolyte design for 4.6 V-class lithium-ion batteries with a non-solvating flame-retardant

Nonflammable electrolytes are critical for the safe operation of high-voltage lithium-ion batteries (LIBs). Although organic phosphates are effective flame retardants, their poor electrochemical stability with a graphite (Gr) anode and Ni-rich cathodes would lead to the deterioration of electrode materials and fast capacity decay. Herein, we develop a safe and high-performance electrolyte formulation for high-voltage (4.6 V-class) LIBs using flame-retarding ethoxy(pentafluoro) cyclotriphosphazene (PFPN) as a non-solvating diluent for the high-concentration carbonate-ether hybrid electrolyte. In contrast to conventional nonflammable additives with restricted dosage, the high level of PFPN (69% mass ratio in our electrolyte design) could significantly increase the electrolyte flash point and protect the favored anion-rich inner solvation sheath because of its non-solvating feature, thus preventing solvent co-intercalation and structural damage to the Gr anode. The nonflammable electrolyte could also form a stable LiF-rich cathode electrolyte interphase (CEI), which enables superior electrochemical performances of Gr‖LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) full cells at high voltages (∼82.0% capacity retention after 1000 cycles at 4.5 V; 89.8% after 300 cycles at 4.6 V) and high temperatures (50 °C). This work sheds light on the electrolyte design and interphase engineering for developing practical safe high-energy-density LIBs.

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.

Nonflammable Localized High-Concentration Electrolytes with Long-Term Cycling Stability for High-Performance Li Metal Batteries

High-concentration electrolytes (HCEs) can effectively enhance interface stability and cycle performance of Li metal batteries (LMBs). However, HCEs suffer from low ionic conductivity, high viscosity, high cost, and high density. Herein, fluorobenzene (FB) diluted localized high-concentration electrolytes (LHCEs) consisting of lithium bis(fluorosulfonyl)imide (LiFSI)/triethyl phosphate (TEP)/FB are developed. 2.3 M LHCE can reserve concentrated Li⁺-FSI^--TEP solvation structures. Diluent FB possesses low density, low viscosity, low cost, low dielectric constant, low LUMO, and a good fluorine-donating property, which can significantly reduce viscosity, improve ionic conductivity, promote the formation of LiF-rich SEI, and enhance interaction of Li⁺-TEP and Li⁺-FSI^- ion-pairs of the electrolytes. 2.3 M LHCE is a highly safe nonflammable electrolyte. 2.3 M LHCE can effectively inhibit dendrite growth on Li metal anode. 2.3 M LHCE endows LiFePO₄ cells with good rate capability (discharge capacity of 112.7 mAh g^-1 at 5 C rate) and excellent cycling performance (capacity retention of 95.4% after 1000 cycles). 2.3 M LHCE also shows good compatibility with LiNi_0.8Co_0.1Mn_0.1O₂ and exhibits outstanding cycle stability (capacity retention of 86.4% after 500 cycles). Therefore, 2.3 M LHCE is a promising electrolyte for practical applications in LMBs.

Significance of Antisolvents on Solvation Structures Enhancing Interfacial Chemistry in Localized High-Concentration Electrolytes

Localized high-concentration electrolytes (LHCEs) provide a new way to expand multifunctional electrolytes because of their unique physicochemical properties. LHCEs are generated when high-concentration electrolytes (HCEs) are diluted by antisolvents, while the effect of antisolvents on the lithium-ion solvation structure is negligible. Herein, using one-dimensional infrared spectroscopy and theoretical calculations, we explore the significance of antisolvents in the model electrolyte lithium bis(fluorosulfonyl)imide/dimethyl carbonate (LiFSI/DMC) with hydrofluoroether. We clarify that the role of antisolvent is more than dilution; it is also the formation of a low-dielectric environment and intensification of the inductive effect on the C=O moiety of DMC caused by the antisolvent, which decrease the binding energy of the Li⁺···solvent and Li⁺···anion interactions. It also has beneficial effects on interfacial ion desolvation and Li⁺ transport. Furthermore, antisolvents also favor reducing the lowest unoccupied molecular orbital (LUMO) energy level of the solvated clusters, and FSI^- anions show a decreased reduction stability. Consequently, the influence of antisolvents on the interfacial chemical and electrochemical activities of solvation structures cannot be ignored. This finding introduces a new way to improve battery performance.

Multiscale Simulation of Solid Electrolyte Interface Formation in Fluorinated Diluted Electrolytes with Lithium Anodes

Lithium metal batteries (LMBs) hold great promise in facilitating high-energy batteries due to their merits such as high specific capacity, low reduction potential, and so forth. However, the realizations of practical LMBs are hindered by severe problems such as undesirable dendrite growth, poor Coulombic efficiency, and so forth. A recently proposed fluorinated electrolyte based on 1 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in designed fluorinated 1,4-dimethoxybutane (FDMB) solvent has attracted significant attention because of its excellent electrochemical performance that origins from its superior physical and chemical properties, especially its unique ability in forming a robust, stable solid electrolyte interphase (SEI). However, the detailed structure and reaction mechanism of the SEI formation in such a novel electrolyte remains unclear. In this work, we carry out a hybrid ab initio and reactive molecular dynamics (HAIR) simulation to investigate the elementary reactions that regulate the formation of the primitive SEI, paying special attention to the process that involves FDMB, the fluorinated solvent. HAIR simulation reveals that both FSI^- anion and FDMB provide F that is adequate to form a uniformed LiF layer that resembles the inorganic inner layer (IIL) of the SEI. N and S radicals from the FSI^- anion, which do not deposit on the electrode interface to form lithium-containing inorganic substances, promote the polymerization reaction of unsaturated carbon chains produced by FDMB defluorination, forming the organic outer layer (OOL) of the SEI. The combination of the LiF-rich IIL and polymer-rich organic OOL explains the superior performance of the FDMB-based electrolyte in the device. The detailed reaction mechanism and SEI observed in this work provide insights into the atomic scale for the rational design of F-rich electrolytes in the near future.

High-Safety and Dendrite-Free Lithium Metal Batteries Enabled by Building a Stable Interface in a Nonflammable Medium-Concentration Phosphate Electrolyte

Lithium metal anodes are promising for their high energy density and low working potential. However, high reactivity and dendrite growth of lithium metal lead to serious safety issues. Lithium dendrite may form "dead lithium" or pierce the separator, which will cause low efficiency and short-circuit inside the battery. A nonflammable phosphate-based electrolyte can effectively solve the flammability problem. Also, it shows poor compatibility with lithium metal anodes, resulting in an unstable solid electrolyte interface (SEI), which leads to dendrite growth and poor electrochemical performance. In this study, trimethyl phosphate is used to ensure the safety of lithium metal batteries. By adjusting the concentration of lithium salt and introducing fluoroethylene carbonate, a stable SEI layer is formed on the surface of the lithium metal anode and dendrite growth of the lithium metal anode is inhibited. Lithium metal batteries with a modified electrolyte achieved stable electrochemical plating/stripping, and the full cell has 93.4% capacity left and the coulombic efficiency is nearly 100%. In addition, the modified electrolyte can also enable reversible intercalation and de-intercalation of Li⁺ in the commercial graphite anode. This work may provide an alternative direction for the development of lithium metal batteries with high safety and high energy density.

Extending insertion electrochemistry to soluble layered halides with superconcentrated electrolytes

Insertion compounds provide the fundamental basis of today's commercialized Li-ion batteries. Throughout history, intense research has focused on the design of stellar electrodes mainly relying on layered oxides or sulfides, and leaving aside the corresponding halides because of solubility issues. This is no longer true. In this work, we show the feasibility of reversibly intercalating Li⁺ electrochemically into VX₃ compounds (X = Cl, Br, I) via the use of superconcentrated electrolytes (5 M LiFSI in dimethyl carbonate), hence opening access to a family of Li_xVX₃ phases. Moreover, through an electrolyte engineering approach, we unambiguously prove that the positive attribute of superconcentrated electrolytes against the solubility of inorganic compounds is rooted in a thermodynamic rather than a kinetic effect. The mechanism and corresponding impact of our findings enrich the fundamental understanding of superconcentrated electrolytes and constitute a crucial step in the design of novel insertion compounds with tunable properties for a wide range of applications including Li-ion batteries and beyond.

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

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

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

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

Non-Flammable and Highly Concentrated Carbonate Ester-Free Electrolyte Solutions for 5 V-Class Positive Electrodes in Lithium-Ion Batteries

Non-flammable and highly concentrated electrolyte solutions were designed using tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a main solvent toward a radical improvement in the safety and energy density of lithium-ion batteries. Unlike conventional carbonate ester-based solutions, simple TFEP-based electrolyte solutions were not intrinsically compatible with 5 V-class LiNi_0.5 Mn_1.5 O₄ positive electrodes, even at high concentrations. Based on the degradation mechanism that was analyzed by Raman spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, a fluorinated diluent of methyl 3,3,3-trifluoropropionate (FMP) was introduced to suppress the decomposition of LiBF₄ and TFEP at high potentials. A nearly saturated LiBF₄ /TFEP+FMP electrolyte solution with a specific composition improved the charge and discharge performance of a LiNi_0.5 Mn_1.5 O₄ electrode, and the solution structure was studied by pulsed-field-gradient NMR spectroscopy.

Advanced Low-Flammable Electrolytes for Stable Operation of High-Voltage Lithium-Ion Batteries

Despite being an effective flame retardant, trimethyl phosphate (TMP_a ) is generally considered as an unqualified solvent for fabricating electrolytes used in graphite (Gr)-based lithium-ion batteries as it readily leads to Gr exfoliation and cell failure. In this work, by adopting the unique solvation structure of localized high-concentration electrolyte (LHCE) to TMP_a and tuning the composition of the solvation sheaths via electrolyte additives, excellent electrochemical performance can be achieved with TMP_a -based electrolytes in Gr∥LiNi_0.8 Mn_0.1 Co_0.1 O₂ cells. After 500 charge/discharge cycles within the voltage range of 2.5-4.4 V, the batteries containing the TMP_a -based LHCE with a proper additive can achieve a capacity retention of 85.4 %, being significantly higher than cells using a LiPF₆ -organocarbonates baseline electrolyte (75.2 %). Meanwhile, due to the flame retarding effect of TMP_a , TMP_a -based LHCEs exhibit significantly reduced flammability compared with the conventional LiPF₆ -organocarbonates electrolyte.

Theoretically predicting the feasibility of highly-fluorinated ethers as promising diluents for non-flammable concentrated electrolytes

The practical application of nonflammable highly salt-concentrated (HC) electrolyte is strongly desired for safe Li-ion batteries. Not only experimentalists but also theoreticians are extensively focusing on the dilution approach to address the limitations of HC electrolyte such as low ionic conductivity and high viscosity. This study suggests promising highly-fluorinated ethers to dilute the HC electrolyte based on non-flammable trimethyl phosphate (TMP) solvent. According to the quantum mechanical and molecular dynamics calculations, the fluorinated ether diluents showed a miscibility behavior in HC TMP-based electrolyte. While such miscibility behavior of the diluent with TMP solvent has been significantly enhanced by increasing its degree of fluorination, i.e., the “fluorous effect”, it is remarkable that the self-diffusion constant of Li⁺ and the ionic conductivity should be significantly improved by dilution with bis(1,1,2,2-tetrafluoro ethyl) ether (B2E) and bis(pentafluoro ethyl) ether (BPE) compared to other common hydrofluoroether diluents. In addition, the fluorinated-ether diluents have high ability to form a localized-concentrated electrolyte in HC TMP-based solution, leading to high expectation for the formation of a stable and a compact inorganic SEI film.

Cycling a Lithium Metal Anode at 90 °C in a Liquid Electrolyte

Stable operation at elevated temperature is necessary for lithium metal anode. However, Li metal anode generally has poor performance and safety concerns at high temperature (>55 °C) owing to the thermal instability of the electrolyte and solid electrolyte interphase in a routine liquid electrolyte. Herein a Li metal anode working at an elevated temperature (90 °C) is demonstrated in a thermotolerant electrolyte. In a Li|LiFePO₄ battery working at 90 °C, the anode undergoes 100 cycles compared with 10 cycles in a practical carbonate electrolyte. During the formation of the solid electrolyte interphase, independent and incomplete decomposition of Li salts and solvents aggravate. Some unstable intermediates emerge at 90 °C, degenerating the uniformity of Li deposition. This work not only demonstrates a working Li metal anode at 90 °C, but also provides fundamental understanding of solid electrolyte interphase and Li deposition at elevated temperature for rechargeable batteries.