Having a high electrochemical stability novel ion-conductive electrolyte fluorinated ether

In recent years, energy-intensive and compact battery availability aroused the interest of the people into the grid and further electrification of transport renewable energy. Because of the high theoretical specific capacity (3860 mAh / gLi), the low reduction potential (-3.04 V), lithium metal batteries become a hot field of high energy density battery research. However, the lithium metal and the electrolyte resulting solid electrolyte interface (SEI) is porous in the deposition of lithium metal and the electrolyte during the continuous release can cause degradation of battery failure. In addition, the cathode electrolyte stability also caused great concern. For example, positive electrode materials such as high-voltage nickel-rich LiNi0.8Mn0.1Co0.1O2 (NMC 811) far exceeds the capacity of the commercial NMC111. However, to obtain high energy density of the positive electrode of the nickel-rich NMC 811, it requires a high charge cut-off voltage (at least 4.4 V or higher). Ether-based electrolytes, such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) having a high ion conductivity, and a lithium metal deposition and lift-high coulombic efficiency, is widely used a lithium metal battery. However, due to low oxidation stability ether (less than 4 V vs. Li / Li +), typically at a concentration of 0.1 M or 1 M ether can not be used with the nickel-rich NMC 811 or other high-voltage cathode. Thus, while the deposition of lithium metal / release and high ionic conductivity stable, improve the oxidative stability of the electrolyte is a huge challenge. Urgent need to design and synthesis of new electrolyte to support high voltage and lithium metal cathode. Currently, researchers generally following engineering strategies to improve the oxidative stability of the electrolyte: 1) a high salt solvent system. Higher salt concentration in the electrolyte can be reduced \”free\” concentration and mobility of the solvent, thereby effectively improving the oxidation stability of the electrolyte. Unfortunately, electrolyte salt soluble solvent has a high viscosity, high cost, low ionic conductivity and a slower reaction kinetics; 2) adding an electrolyte additives, such as hydrofluoroether (HFE). The hydrofluoroether any insoluble salt, nor having ion conductivity, but having high oxidative stability. However, the HFE is added directly into ether and the electrolyte does not significantly increase the oxidative stability of the electrolyte mixture. Although Balsara and DeSimone et al have reported some fluorinated ion-conducting electrolyte, the electrolyte system are all connected carbonate, lithium metal for stabilityPoor characterization. Accordingly, an urgent need for new methods of engineering beyond an electrolyte, the high oxidative stability of the ether solvent with high ionic conductivity combined hydrofluoroether. [The results show] To address this challenge, Yi Cui of Stanford University, Bao Zhenan, who designed a new method for the synthesis of novel fluorinated ethers electrolyte, that is, through the core and fluorinated ethers \”end group\” covalent bonding to achieve high ionic conductivity combined with high oxidation stability of a single electrolyte. Meanwhile, in a modular fashion to change the length and type of an ether group, a fluorinated segments and a length, a systematic study of this new electrolyte structure – nature of the relationship. When found, the new electrolyte prepared fluorinated ether ether having a longer and a shorter group segment fluorinated ionic conductivity of up to 2.7 × 10⁻4 S / cm (at 30 ° C), and having up to 5.6 V of anodic voltage (1.4 V higher than at least tetraethylene glycol dimethyl ether). Nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations show that with increasing shorter segments and ether segments fluorine, fluorinated ethers ion conductivity increases. Researchers such further electrolyte to the high voltage battery Li-NMC 811, the battery 100 can be stably cycle times at a magnification up to the C / 5. Related to the results of \”A new class of ionically conducting fluorinated ether electrolytes with high electrochemical stability\” in the title, published in the International Journal J. Am top. Chem. Soc. On. 具有高电化学稳定性的新型离子导电氟化醚电解质 [Detailed graphics]

First, the principle of molecular design, the electrolyte

The ether solvent having a high ion conductivity, but poor oxidation stability. In contrast, hydrofluoroether having high oxidative stability, but does not dissolve the lithium salt. Thus, the basic principle of molecular design is the need to combining conductive oxide ion stability ether hydrofluoroether (FIG. 1). However, not take into account the advantage of simple mixing of these two compounds, as the mixture continues to be low oxidation stability limit. Thus, researchers using a new electrolyte chemical methods, combined with the core through the fluorinated ether \”end group\” covalently fluorinated ether is obtained having a high ionic conductivity of the electrolyte. Wherein the nuclear fluorination can bring high oxidation stability, andCharge cutoff at a high rate to support rich Ni NMC 811 of the positive electrode, and an ether group are salts imparting high ionic conductivity and solubility of the electrolyte (FIG. 1). Meanwhile, researchers chose tetraglyme as a control, wherein the length of the synthesis ether fluorinated ether considerable, and changing the length of a modular fashion fluorinated segment and an ether segment, a systematic study of structure – property The relationship between.

具有高电化学稳定性的新型离子导电氟化醚电解质
Figure 1. principle of molecular design and an electrolyte novel fluorinated ether synthesis route.

Second, Synthesis and Thermal Properties of Novel fluorinated ether electrolyte Characterization

Researchers halide functional alkoxylated glycols to synthesize novel fluorinated ether fluoride (FIG. 1), specific process is as follows: first, using sodium hydride fluorinated tetraethylene glycol (FTEG) and fluorinated triethylene glycol (FTriEG) deprotonation. After deprotonation, to add various substituted alkoxy halides end group synthesized a series of fluorinated ether. Wherein, to explore the influence by the length of the transition from FTEG fluoride ion to FTriEG conductivity, oxidative stability and subsequent performance. At the same time, to explore the influence by the length of an ether methyl ether (MME) and dimethyl ether (DME). Finally, NMR and FTIR characterization confirmed the structure and purity of the product confirmation. Thermogravimetric analysis (TGA) indicated that fluorinated ethers, and the length of the segment determines the decomposition temperature of the fluorinated ether. DSC showed that, although the low molecular weight fluorinated ether, such as but not melting transition occurs tetraglyme, but the glass transition (Tg) appeared at about -21 ℃. Further, Tg does not change with an ether or fluorinated segments varies.

Third, the ion conductivity and transport mechanisms

Researchers select bis (fluorosulfonyl) amide salt (LiFSA) is a lithium salt, by electrochemical impedance spectroscopy (EIS) was dissolved in fluorinated ethers tested electrolyte the ionic conductivity (FIG. 2). The results show that with increased spacer group (-CH2CH2O-), the ionic conductivity at 30 ° C to 30-fold increase (FIG. 2a) DME-FTEG in from the MME-FTEG (-CH2O-). With the length of the ether from the DME-FTEG DEG-FTEG an increase of ionic conductivity only increased by 6 times. In addition, increased salt concentration from 0.1 M 1 M will lead to increased ionic conductivity. Meanwhile, with the fluorinated segment to be shortened from FT FTEGriEG, ionic conductivity also increased (FIG. 2b). At 30 ° C, the DEG-FTriEG was dissolved in 1 M LiFSA has the highest ion conductivity is 2.7 × 10-4 S / cm.

具有高电化学稳定性的新型离子导电氟化醚电解质
Figure 2. The novel fluorinated ethers ion transport ionic conductivity of the electrolyte and characterized.

The activation energy: researchers Arrhenius temperature dependence of the electrical conductivity form fit to obtain ion transport activation barrier. Found, FTriEG FTEG compound and compared with MME- and DEG-FTriEG and tetraethylene glycol dimethyl ether, DME-FTriEG electrolyte has a lower activation energy. Further, the activation energy does not change the trend of increasing salt concentration between and FTriEG FTEG compound. (PFG) NMR probe further lithium pulsed field gradient (Li + is 7Li) and anion (FSA⁻ to 19F) diffusion. The results show that in the compounds FTriEG, Li + and FSA⁻ highest diffusion rate in DME-FTriEG. Further, in the DME-FTriEG, the diffusion velocity is faster than FSA⁻ Li +, and in the MME, and DEG-FTriEG, Li + diffusion rate faster than FSA⁻. Thus, compared with the DME-FTriEG, higher transfer number of lithium DEG-FTriEG MME and the (tLi + = 0.4). Ion binding Environment: FTriEG in series, with the movement from the MME to the DEG DME, high-field chemical shift of the mobile 7Li (FIG. 2e) to, which may be due to a strong increase of ionic solvation of ions or due to . With the increase in the length of the ether, 19F chemical shift moved downfield (FIG. 2f). Compared with tetraethylene glycol dimethyl ether, is moved to the high field peak indicating the bond between the Li + ions with increasing FSA⁻ paired anion or anion fluorinated ether FSA⁻ stronger. Ion solvation structure: For a better understanding of the fluorinated ethers in the Li + ions with anionic FSA⁻ solvent environment, researchers performed molecular dynamics (MD) simulation. Figure 3 shows FTriEG tetraethylene glycol dimethyl ether and the concentration of compound in the 0.1 M lithium cation and anion FSA radial distribution function (RDFs). Wherein, tetraethylene glycol dimethyl ether ether groupThe oxygen (OCH2) (and the lone pair) adjacent to the highest likelihood with lithium cations, the same fluorinated ether ether group (the OCF2) is also adjacent to lithium cations. This indicates that the fluorinated ether ether segment coordinated with lithium, and facilitate the transport of lithium ions. Further, with the increase in the length of the ether, the OCF2 possibility solvated lithium shell is reduced, leading to a higher ion conductivity. Thus, these compounds fluorinated segments by \”fluoro\” effect with the fluorinated anionic significant interaction, thus inhibiting the migration of anions.

salts solvates structure 具有高电化学稳定性的新型离子导电氟化醚电解质
Figure 3. The fluorinated ether in the electrolyte.

Taking these factors can be seen, the fluorinated ether and the ether segment preferentially lithium ligand, and a fluorinated segment preferentially FSA anionic ligand. With the shortened period increased ethers and fluorine segments, fluorinated ethers ion conductivity increases. Compared with tetraethylene glycol dimethyl ether, fluorinated ethers all have higher transfer number, thanks to the fluorinated ether fluoro group with an anion interactions. Fourth, the oxidation stability of the redox voltage: LSV test results show that the oxidation voltage than all the fluorinated ether tetraethylene glycol dimethyl ether is at least 1.4 V (FIG. 4a). Further, the MME-FTriEG oxidation voltage and the MME-FTEG length linear ether, exhibits excellent stability. Researchers also tetraglyme and hydrofluoroether widely used simply mixing that, compared with the synthesis of fluorinated ether, and the mixture still has low oxidative stability. Thus, the methods further confirmed ethers and fluorinated segments can be covalently bound to achieve high ionic conductivity and high oxidation stability. The researchers also used to maintain a constant potential experiments to probe further oxidative stability fluoride (FIG. 4b-c). The results show, tetraethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether: TTE mixture oxidative stability between 3.8-4 V, and FTriEG compound, and oxidative stability is increased 5.4-5.6 V. Aluminum Corrosion: Typical concentrations for the next M LiFSA Condition 1 imide salt, the fluorinated ether may be passivated aluminum surface, stabilized at a high pressure, better than 1 M LiFSA tetraethylene glycol dimethyl ether (FIG. 4d-e). Meanwhile, the fluorinated ethers may support the deposition of lithium metal and release behavior, lithium metal effective to promote circulation. Oxide

具有高电化学稳定性的新型离子导电氟化醚电解质
Figure 4. fluorinated ether electrolyte reduction stability.

Fifth, the effect on voltage performance cathode material

To explore DEG-FTriEG electrolyte in voltage performance of the positive electrode material, the researchers used NMC 811 electrodes and assembled load of 4 mg / cm2 of cell . Found in tetraglyme as electrolyte battery cathode NMC 811 can not be charged at a C / 10 rate to a cutoff voltage. When an electrolyte to DEG-FTriEG, even if the cutoff voltage of 4.4 V, the cathode cycling performance was significantly improved. This indicates improved oxidative stability, can significantly improve the performance of the nickel-rich recycle NMC 811 cathodes. At a rate C / 5, the battery can run for at least 100 cycles and a coulombic efficiency of approximately 100%.

具有高电化学稳定性的新型离子导电氟化醚电解质
Figure 5. The fluorinated ether and the nickel-rich electrolyte battery performance NMC 811 of the cathode assembly.

[Summary] In conclusion, the researchers reported a new method for the high oxidation stability and high ionic conductivity of the fluorinated ether compound combined into a single compound. This new fluorinated ether electrolyte ion conductivity at 30 deg.] C up to 2.7 × 10-4 S / cm, oxidation voltage up to 5.6 V (tetraethylene glycol dimethyl ether is 4 V). Notably, at a typical concentration of 1 M LiFSA imide salt is present, the fluorinated ethers electrolyte does not corrode aluminum, oxidative stability similar to commercial carbonate electrolyte, while enables efficient lithium metal cycle. Moreover, tetraethylene glycol dimethyl ether with a simple mixture of hydrofluoroether still widely used with lower oxidation stability, and further confirmed that the ether or fluorinated segments can be covalently bound to achieve high ionic conductivity and high oxidation stability. Using NMR and MD simulation results indicate that with increasing shorter segments and ether segments fluorine, fluorinated ethers ion conductivity increases. Further, due to the high number of transfer FSA- specific interaction between the anionic fluorinated segment. Finally, researchers used as the cathode NMC 811 creates a load of 4 mg / cm2 of the battery, studies have shown that these cells can support the fluorinated cyclic ethers rate at C / 5, more than 100 times. This work provides valuable guidelines for the design and synthesis of novel electrolyte structure of the next-generation battery systems. Reference: A new class of ionically conducting fluorinated ether electrolytes with high electrochemical stability J. Am Chem Soc 2020. DOI:…. 10.1021 / jacs.9b11056 full text link: https: //pubs.acs.org/doi/pdf/10.1021/jacs.9b11056#

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