Fluorinated Ether Research Articles

Enhanced performance of lithium metal batteries via cyclic fluorinated ether based electrolytes

To address the challenges associated with applying high-voltage cathodes in lithium metal batteries (LMBs) there is a need for new electrolytes enabling stable interphases at both electrodes. Here we attack this by using a dioxolane-derived cyclic fluorinated ether, 2,2-bis(trifluoromethyl)-1,3-dioxolane (BTFD), as a fluorinated diluent to a 1,2-dimethoxyethane (DME) based electrolyte. The cells using the resulting BTFD-based electrolytes exhibit higher Coulombic efficiencies for lithium stripping and plating as compared to those using the non-fluorinated ether-based electrolyte. This originates from the reduced formation of ‘dead Li’ at the anode, as shown by using electrochemical impedance spectroscopy (EIS). In practice, the BTFD-based electrolytes are shown to improve the performance of Li||NMC cells, which is due to the formation of a predominantly inorganic cathode electrolyte interphase (CEI) that suppresses the cathode degradation during cycling. We used X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) to characterize the CEIs’ overall composition and structure. To obtain more details on the CEI speciation, Raman and nuclear magnetic resonance (NMR) spectroscopies were employed, assisted by molecular level computations. Overall, we demonstrate how the very design of the electrolyte composition influences the performance of LMBs.

Unveiling the Role of Fluorination in Hexacyclic Coordinated Ether Electrolytes for High-Voltage Lithium Metal Batteries.

Fluorinated ethers have become promising electrolyte solvent candidates for lithium metal batteries (LMBs) because they are endowed with high oxidative stability and high Coulombic efficiencies of lithium metal stripping/plating. Up to now, most reported fluorinated ether electrolytes are -CF3-based, and the influence of ion solvation in modifying degree of fluorination has not been well-elucidated. In this work, we synthesize a hexacyclic coordinated ether (1-methoxy-3-ethoxypropane, EMP) and its fluorinated ether counterparts with -CH2F (F1EMP), -CHF2 (F2EMP), or -CF3 (F3EMP) as terminal group. With lithium bis(fluorosulfonyl)imide as single salt, the solvation structure, Li-ion transport behavior, lithium deposition kinetics, and high-voltage stability of the electrolytes were systematically studied. Theoretical calculations and spectra reveal the gradually reduced solvating power from nonfluorinated EMP to fully fluorinated F3EMP, which leads to decreased ionic conductivity. In contrast, the weakly solvating fluorinated ethers possess higher Li+ transference number and exchange current density. Overall, partially fluorinated -CHF2 is demonstrated as the desired group. Further full cell testing using high-voltage (4.4 V) and high-loading (3.885 mAh cm-2) LiNi0.8Co0.1Mn0.1O2 cathode demonstrates that F2EMP electrolyte enables 80% capacity retention after 168 cycles under limited Li (50 μm) and lean electrolyte (5 mL Ah-1) conditions and 129 cycles under extremely lean electrolyte (1.8 mL Ah-1) and the anode-free conditions. This work deepens the fundamental understanding on the ion transport and interphase dynamics under various degrees of fluorination and provides a feasible approach toward the design of fluorinated ether electrolytes for practical high-voltage LMBs.

Unveiling decaying mechanism of non-flammable all-fluorinated carbonate electrolytes in lithium metal batteries with 4.6-V LiCoO2 cathodes at elevated temperatures

Highly fluorinated electrolytes have attracted much attention in lithium metal batteries (LMBs) because of their unique properties, such as increased anodic stability and altered coordination state of Li+ ions. However, the complex decaying mechanism of fluorinated electrolytes at elevated temperatures is not well understood, which is essential to assess their suitability for practical applications in harsh environments. Herein the influence of elevated temperatures on the stability of fluorinated electrolytes and the formation of interphases between the 4.6-V LiCoO2 cathode and electrolyte are systematically investigated. The results reveal that elevated temperatures significantly affect the stability of the electrolyte and the formation of interphases. In particular, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether undergoes decomposition catalyzed by reactive oxygen from the delithiated cathode at high temperatures and 4.6 V. Meanwhile, the degradation of fluorinated electrolytes is primarily caused by the temperature-induced decomposition of fluoroethylene carbonate. Although methyl (2,2,2-trifluoroethyl) carbonate has the highest stability among the solvents in this fluorinated system, it still experiences degradation in harsh environments. These findings highlight the importance of considering highly fluorinated ethers and the synergistic effects between fluorinated carbonate solvents, lithium salts, and solvation structures when designing fluorinated electrolytes, especially for applications at elevated temperatures. This work sheds light on the design and optimization of highly fluorinated electrolytes, enabling the development of 4.6-V LiCoO2-based LMBs capable of withstanding challenging environments and operating conditions.

Unique double-layer solid electrolyte interphase formed with fluorinated ether-based electrolytes for high-voltage lithium metal batteries

Li metal batteries using high-voltage layered oxides cathodes are of particular interest due to their high energy density. However, they suffer from short lifespan and extreme safety concerns, which are attributed to the degradation of layered oxides and the decomposition of electrolyte at high voltage, as well as the high reactivity of metallic Li. The key is the development of stable electrolytes against both high-voltage cathodes and Li with the formation of robust interphase films on the surfaces. Herein, we report a highly fluorinated ether, 1,1,1-trifluoro-2-[(2,2,2-trifluoroethoxy) methoxy] ethane (TTME), as a co-solvent, which not only functions as a diluent forming a localized high concentration electrolyte (LHCE), but also participates in the construction of the inner solvation structure. The TTME-based electrolyte is stable itself at high voltage and induces the formation of a unique double-layer solid electrolyte interphase (SEI) film, which is embodied as one layer rich in crystalline structural components for enhanced mechanical strength and another amorphous layer with a higher concentration of organic components for enhanced flexibility. The Li||Cu cells display a noticeably high Coulombic efficiency of 99.28% after 300 cycles and Li symmetric cells maintain stable cycling more than 3200 h at 0.5 mA/cm2 and 1.0 mAh/cm2. In addition, lithium metal cells using LiNi0.8Co0.1Mn0.1O2 and LiCoO2 cathodes (both loadings ∼3.0 mAh/cm2) realize capacity retentions of >85% over 240 cycles with a charge cut-off voltage of 4.4 V and 90% for 170 cycles with a charge cut-off voltage of 4.5 V, respectively. This study offers a bifunctional ether-based electrolyte solvent beneficial for high-voltage Li metal batteries.

Highly fluorinated co-solvent enabling ether electrolyte for high-voltage lithium ion batteries with graphite anode

Conventional ether electrolytes are generally considered unsuitable for use with graphite anodes and high-voltage cathodes due to their co-intercalation with graphite and poor oxidation stability, respectively. In this work, a highly fluorinated ether molecule, 1,1,1-trifluoro-2-[(2,2,2-trifluoroethoxy) methoxy] ethane (TTME), is introduced as a co-solvent into the conventional ether system to construct a fluorinated ether electrolyte, which not only avoids the co-intercalation with graphite but also is compatible with high-voltage cathodes. Li||graphite half-cells using the fluorinated ether electrolyte deliver stable cycling with a capacity retention of 91.7% for 300 cycles. Moreover, LiNi0.8Co0.1Mn0.1O2 (NCM811)||graphite and LiCoO2 (LCO)||graphite full-cells (cathode loadings are ≈3 mAh/cm2) with the fluorinated ether electrolyte show capacity retentions of > 90% over 200 cycles with a charge cut-off voltage of 4.4 V and > 97% for 100 cycles with a charge cut-off voltage of 4.5 V, respectively. The dense and firm solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed by the fluorinated ether electrolyte on the anode and cathode, respectively, are key to excellent cell performance. These results have significance for the subsequent application of ether electrolytes for high-voltage lithium ion batteries (up to 4.5 V) with graphite anodes.

Tunning solvation structure in non-flammable, localized high-concentration electrolytes with enhanced stability towards all aluminum substrate-based K batteries

Although metallic potassium or graphite anode with low redox potentials are desirable for potassium-ion batteries (PIBs), the severe capacity decay caused by consecutive reduction reactions on the aggressively reactive anode surface is inevitable because of the scarcity of an effective passivation layer. Additionally, the development of PIBs faces potential safety hazards due to the usage of reactive anodes with flammable electrolytes. Herein, a fire-retardant localized high-concentration electrolyte (LHCE) is formulated by adding a low-polarity co-solvent of highly fluorinated ether into the concentrated potassium bis(fluorosulfonyl)imide/trimethyl phosphate. A unique electrolyte solvation environment with the enhanced anion-cation interaction in an LHCE facilitates a durable anion-derived solid electrolyte interphase (SEI) with self-accelerated interfacial kinetics and minimizes the SEI dissolution compared to the conventional carbonate-based electrolyte. Consequently, the unmodified Al substrate could sustain stable potassium metal cycling above 3400 h (over 800 cycles) with high reversibility (98%), and the graphite anode retains prolonged cyclic stability (over 300 cycles) with a high reversible capacity (257.6 mAh g−1) and capacity retention (92.6 % at 0.2 C). Meanwhile, Al corrosion at high voltage is significantly suppressed, and the electrochemical window is extended due to less free solvents in an LHCE, enabling good electrolyte compatibility with high-voltage cathodes. Coupling non-flammable solvents and the concept of LHCE could facilitate a better understanding of electrolyte chemistry and offer more opportunities to use K or graphite anode for all Al substrate-based PIBs highlighted with high performance and superb safety.

Selecting the Optimal Fluorinated Ether Co-Solvent for Lithium Metal Batteries.

To guide the selection of a suitable fluorinated ether (FE) co-solvent for lithium metal batteries, it is crucial to understand the relationship between the organic structures of the FEs and the electrochemical performance of an FE-containing electrolyte. In this work, 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane (FEE), 1,1,2,2-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)propane (TTE), and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane (OFDEE) were chosen as representative FE co-solvents because of their distinct structural properties. The structure-activity relationship between the FEs and the electrochemical performance of Li||LiNi0.6Mn0.2Co0.2O2 (Li||NMC622) cells was correlated and quantified by Fourier-transform infrared and multi-dimensional nuclear magnetic resonance techniques. Sand's model was also employed to assess the extent of lithium dendrite formation in the cells using various FE electrolytes. The cycling performance of Li||NMC622 cells using different FE co-solvents follows the order FEE > TTE > OFDEE. Since the direct measurement of Sand's time is difficult, we introduced relative Sand's time to probe the diffusion behavior of each electrolyte, and the results showed that the best performance was obtained in the electrolyte with the longest relative Sand's time. Moreover, the lithium metal cell using the electrolyte with FEE co-solvent showed similar capacity retention compared with the baseline electrolyte at room temperature, but it demonstrated significantly improved low-temperature performance. The results indicate that FEE is a promising co-solvent candidate for improving the low-temperature performance of lithium metal batteries because it possesses not only non-solvating behavior but also very low viscosity and non-flammability. The advanced electrolyte LiPF6-FEC-DMC-FEE enables very stable cycling of lithium metal batteries at various temperatures.

Fluorinated ether decomposition in localized high concentration electrolytes

Localized high concentration electrolytes (LHCEs) are a promising class of battery materials to enable stable cycling of the lithium metal anode. Here, we report the use of operando nuclear magnetic resonance (NMR) spectroscopy to observe electrolyte decomposition during Li stripping/plating and identify the influence of individual components in LHCEs on Li metal battery performance. Data from operando19F solution NMR indicates that both bis(fluorosulfonyl)imide (FSI−) salt and bis(2,2,2-trifluoroethyl) ether (BTFE) diluent molecules play a key role in solid electrolyte interphase (SEI) formation. Three-electrode electrochemical impedance spectroscopy (EIS) of commercial pouch cells also shows differences in interfacial resistances between LHCE and standard high concentration electrolytes (HCEs) that may be explained by incorporation of BTFE and BTFE reaction products into the SEI. Based on solid-state NMR and X-ray photoelectron spectroscopic characterization, we find that BTFE diluents decompose to form CF2- and CF3-containing fragments within a LiF-rich SEI deposited on the anode surface. The CEI on the cathode (here, LiNi0.8Mn0.1Co0.1O2) side of the battery also contains higher quantities of LiF and trapped LiFSI after cycling in the LHCE compared to a HCE that are attributed to diluent decomposition and correlated with lower impedance at the cathode. Overall, this work provides a new framework to consider highly fluorinated ether molecules—instead of functioning purely as diluting agents in LHCEs, these fluorinated ethers exhibit tunable interfacial reactivity that can be leveraged to control Li deposition behavior.

(Invited) A Micelle Electrolyte Enabled By Fluorinated Ether Additives for Polysulfide Suppression, High Voltage Cathode, and Li Metal Anode Stabilization

The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh/kg) and low active material cost. However, poor cycling stability and coulombic efficiency caused by polysulfide dissolution have proven to be major obstacles for a practical Li-S battery implementation. In this work, we develop a novel strategy to suppress polysulfide dissolution using hydrofluoroethers (HFEs) with bi-functional, amphiphlic surfactant-like design: a polar lithiophilic “head” attached to a fluorinated lithiophobic “tail.” A unique solvation mechanism is proposed for these solvents whereby dissociated lithium ions are readily coordinated with lithiophilic “head” to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by changing the additive structure and concentration using small angle X-ray and neutron scattering. These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal: Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium. Li-S cells are demonstrated with 1395 mAh/g initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency—evidence that the micelle structure of the amphiphilic additives in HFEs can prohibit polysulfide dissolution while enabling facile Li+ transport and anode passivation. In addition, this type of electrolytes is also stable toward high voltage cathode to enable a lithium metal anode and high voltage cathode high-energy density rechargeable lithium battery.

Pushing Lithium–Sulfur Batteries towards Practical Working Conditions through a Cathode–Electrolyte Synergy

AbstractThe commercialization of lithium–sulfur (Li−S) batteries is still hindered by the unsatisfactory cell performance under practical working conditions, which is mainly caused by the sluggish cathode redox kinetics, severe polysulfide shuttling, and poor Li stripping/plating reversibility. Herein, we report an effective strategy by combining Se‐doped S hosted in an ordered macroporous framework with a highly fluorinated ether (HFE)‐based electrolyte to simultaneously address the aforementioned issues in both cathode and anode. A reversible and stable high areal capacity of >5.4 mAh cm−2 with high Coulombic efficiency >99.2 % can be achieved under high areal Se/S loading (5.8 mg cm−2), while the underlying mechanism was further revealed through synchrotron X‐ray probes and Time‐of‐Flight Secondary Ion Mass Spectrometry (ToF‐SIMS). The practical application potential was further evaluated at low (0 °C) and high (55 °C) temperatures under high areal Se/S loading (>5.0 mg cm−2) and thin Li metal (40 μm).

Pushing Lithium-Sulfur Batteries towards Practical Working Conditions through a Cathode-Electrolyte Synergy.

The commercialization of lithium-sulfur (Li-S) batteries is still hindered by the unsatisfactory cell performance under practical working conditions, which is mainly caused by the sluggish cathode redox kinetics, severe polysulfide shuttling, and poor Li stripping/plating reversibility. Herein, we report an effective strategy by combining Se-doped S hosted in an ordered macroporous framework with a highly fluorinated ether (HFE)-based electrolyte to simultaneously address the aforementioned issues in both cathode and anode. A reversible and stable high areal capacity of >5.4 mAh cm-2 with high Coulombic efficiency >99.2 % can be achieved under high areal Se/S loading (5.8 mg cm-2 ), while the underlying mechanism was further revealed through synchrotron X-ray probes and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The practical application potential was further evaluated at low (0 °C) and high (55 °C) temperatures under high areal Se/S loading (>5.0 mg cm-2 ) and thin Li metal (40 μm).

Localized High‐Concentration Electrolytes Boost Potassium Storage in High‐Loading Graphite

AbstractReversible intercalation of potassium‐ion (K+) into graphite makes it a promising anode material for rechargeable potassium‐ion batteries (PIBs). However, the current graphite anodes in PIBs often suffer from poor cyclic stability with low coulombic efficiency. A stable solid electrolyte interphase (SEI) is necessary for stabilizing the large interlayer expansion during K+ insertion. Herein, a localized high‐concentration electrolyte (LHCE) is designed by adding a highly fluorinated ether into the concentrated potassium bis(fluorosulfonyl)imide/dimethoxyethane, which forms a durable SEI on the graphite surface and enables highly reversible K+ intercalation/deintercalation without solvent cointercalation. Furthermore, this LHCE shows a high ionic conductivity (13.6 mS cm−1) and excellent oxidation stability up to 5.3 V (vs K+/K), which enables compatibility with high‐voltage cathodes. The kinetics study reveals that K+ intercalation/deintercalation does not follow the same pathway. The potassiated graphite exhibits excellent depotassiation rate capability, while the formation of a low stage intercalation compound is the rate‐limiting step during potassiation.

Improving Cell Resistance and Cycle Life with Solvate/Thiophosphate Hybrid Electrolytes in Lithium Batteries

Solid electrolytes (SEs) have become a practical option for lithium ion and lithium metal batteries due to their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates whose excellent ionic conductivities at room temperature are comparable to those of the commercially-utilized ionic liquids. Hybrid solid-liquid electrolytes exhibit higher ionic conductivities than their bare solid electrolyte counterparts due to decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface. In this study, we add lithium bis(triflouromethane sulfonyl)imide (LiTFSI) and a highly fluorinated ether (HFE) solvate electrolyte to the surface of Li7P3S11 (LPS) and Li10GeP2S12 (LGPS) pellets and evaluate their overall cell resistance in Li-Li symmetric cells relative to their bare Li/SE/Li counterparts. Time-resolved electrochemical impedance spectroscopy (EIS) shows an order of magnitude lower cell resistance for the LGPS-solvate than for the bare LGPS. Cyclic voltammetry (CV) of the bare and hybrid SE cells shows an order of magnitude higher current density for the LGPS-solvate cell over the bare LGPS. Interestingly, hybrid electrolytes made by combining HFE-modified solvates and SE exhibit all the benefits of the interlayer modified SEs, without the necessity of pellet manufacture. This study suggests that solvates can be used to improve the cell resistance and current density of solid electrolytes by altering the grain boundary structures and the interphase between electrode and electrolyte.

Improving Cell Resistance and Cycle Life with Solvate-Coated Thiophosphate Solid Electrolytes in Lithium Batteries.

Solid electrolytes (SEs) have become a practical option for lithium ion and lithium metal batteries because of their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates, whose excellent ionic conductivities at room temperature are comparable to those of commercially utilized ionic liquids. Hybrid solid-liquid electrolytes exhibit higher ionic conductivities than their bare SE counterparts because of decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface. In this study, we add lithium bis(trifluoromethane sulfonyl)imide and a highly fluorinated ether solvate electrolyte to the surface of Li7P3S11 (LPS) and Li10GeP2S12 (LGPS) pellets and evaluate their overall cell resistance in Li-Li symmetric cells relative to their bare Li/SE/Li counterparts. Time-resolved electrochemical impedance spectroscopy shows an order of magnitude lower cell resistance for LGPS-solvate than for bare LGPS. In contrast, the LPS-solvate system exhibits a higher cell resistance than bare LPS. Scanning electron microscopy and energy dispersive X-ray spectroscopy show that LGPS allows for the total permeation of the solvate into the bulk SE. Although LPS has smaller grain sizes and higher porosity, it has a higher solubility in 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which results in an LPS-TTE interlayer on the surface of the pellet, thereby increasing overall cell resistance. Cyclic voltammetry of the bare and hybrid SE cells shows an order of magnitude higher current density for the LGPS-solvate cell over the bare LGPS. Bare LPS shorts after two cycles, whereas the LPS-solvate cell does not short within the timeframe of the experiment (100 cycles). This study suggests that solvates can be used to improve the cell resistance and current density of SEs by altering the grain boundary structures and the interphase between electrode and electrolyte.

Aqueous Li-Ion Batteries: Now in Striking Distance

In the inaugural issue of Joule , Chunsheng Wang, Kang Xu, and colleagues demonstrate a groundbreaking advance in Li-ion battery electrolytes with their design and demonstration of 4 V aqueous Li-ion batteries, which in theory could eliminate several environmental, safety, and manufacturing restrictions and concerns raised by non-aqueous electrolytes.

Development and Mechanism Investigation of Novel Fluorinated Ethers As Secondary Solvent in Electrolyte for Lithium-Sulfur Batteries

Sulfur (S) has been identified as one of the most promising cathode material due to its high theoretical specific capacity (1675 mAh/g), good environment compatibility, and low cost. These features make lithium-sulfur battery a potential candidate for the next-generation battery, especially in electric vehicles and portable devices. However, the application of lithium-sulfur (Li-S) battery is still impeded by some limitations, including the low electrical and ionic conductivity of the elemental sulfur and the discharge product (Li2S), and the shuttle effect due to the presence the dissolved polysulfide (PS) species in electrolyte. 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) has been identified as an effective secondary solvent in electrolyte for lithium sulfur batteries. In this study, fluorinated ethers (FEs) other than TTE were attempted in 1,3-dioxolane (DOL) based electrolyte, and comparison was made over cell performance, viscosity/conductivity of electrolyte, and aggregation states of lithium PS species. 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl Ether (HFE), ethyl 1,1,2,2-tetrafluoroethyl ether (ETE), and 1,1,2,3,3,3-hexafluoropropyl ethyl ether (HEE) were identified as promising alternatives to TTE to accommodate wider range of applications of Li-S batteries. However, amphiphilic FEs, like 1,1,1,2,2,3,3,4,4-Nonafluoro-6-(pentyloxy)hexane (NHPE) and 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluoro-8-(pentyloxy)octane (TDPE) were not effective at all. Depending on formula weight, viscosity and conductivity, cell performance of Li-S batteries containing electrolytes with different FEs also varied. And FEs with coulombic efficiency higher than that of TTE were discovered. In DOL/FE based electrolyte, the aggregation states of lithium PS species were investigated via small-angle X-ray scattering (SAXS), and revealed a variation in dimension of assemblies of lithium PS species in the presence of different FEs. The difference of cell performance can therefore be concluded as the variation in transport behavior of lithium PS species in electrolyte. In addition, this would offer some insights into the failure mechanisms of Li-S batteries, and promote further development of electrolyte for Li-S batteries. Figure 1

Effect of Using Fluorinated Phosphate Ester and Fluorinated Ether As Electrolyte Solvents for Lithium Ion Batteries with LiNi0.5Mn1.5-XTixO4 Cathode

Introduction LiNi0.5Mn1.5O4 1) is one of the candidates for next generation cathode active material of Li ion batteries, because of large capacity of around 130 mAh/g and the high charge-discharge potential of around 4.65V versus Li. On the other hand, because of the higher potential than conventional cathode active materials, 5V class spinel has the problem of the capacity fading due to decomposition of electrolyte and Mn dissolution. We have studied the improvement of lifetime by Ti substitution for Mn of LiNi0.5Mn1.5O4 2) 3) 4) 5). To suppress electrolyte decomposition, it was necessary to develop electrolyte solvents which have higher resistance to oxidation. The decomposition reaction of electrolyte may cause gas generation, and it can cause swelling of cells. At room temperature, amount of gas was not large even in conventional electrolyte, but in the case of operation at elevated temperature large amount of gas generation was a problem. We have reported the decrease of gas emission by using fluorinated phosphate (FPE) as an electrolyte solvent6). In this report, we evaluated the composition dependence of electrolytes with ethylene carbonate, FPE and fluorinated ether (FE). By results of HOMO value calculation, FE was expected to have higher resistance to oxidation than conventional solvents. Experimental LiNi0.5Mn1.5-xTixO4 was used as the cathode active material, and graphite was used as the active anode material. The discharge capacity as a function of cycle number was tested using stack type laminated cells with these active materials. 1 M solution of LiPF6 in 40:60(v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) or 0.8 M solution of LiPF6in 20:(80-x):x (v/v) (x = 0 to 50) mixture of EC, FPE and FE were used as electrolytes. Cycle tests were performed at 45 ºC. The volume of the cells was measured by the Archimedes method. Results Figure 1 shows discharge capacity retention as a function of cycle number for LiNi0.5Mn1.5-xTixO4/graphite cells with 0.8M-LiPF6EC/FPE/FE=20/(80-x)/x (v/v) (x=0,10,20,30, 50) as electrolytes. Capacity retention after charge-discharge cycles increased with FE content in electrolyte. By evaluating impedance tests, the increase of resistance after cycles was suppressed with increase of FE content. Existence of FE is thought to have the effect of suppress of forming SEI during charge-discharge cycles. Figure 2 shows the increase rate of cell volumes after 300 charge-discharge cycles at 45 °C. The volume increase ratio of the cell with EC/DMC=40/60 was 59%, and those of EC/FPE/FE=20/80/0 and EC/FPE/FE=20/50/30 were less than 1%. Amount of gas generation was found to be significantly reduced by using EC/FPE/FE. These results show that FE also had high oxidation resistance around 4.7V versus Li. References Qiming Zhong, Arman Bonakdarpour, Meijie Zhang, Yuan Gao, and J.R.Dahn, J.Electrochem.Soc., 144, 205, January 1997T.Noguchi, I.Yamazaki, T.Numata, M.ShirakataThe 44th Battery Symposium in Japan,2C12(2003)J.-H. Kim, S.-T. Myung, C. S. Yoon, I.-H. Oh, and Y.-K. Suna, J.Electrochem.Soc., 151, A1911 (2004)T.Noguchi, I.Yamazaki, T.Numata, M.Shirakata The 45th Battery Symposium in Japan,1B29(2004)T.Noguchi, I.Yamazaki, T.Numata, K.Utsugi #1378, 220th ECS MeetingT.Noguchi, M.Uehara, Y.Katoh, K.Utsugi #824, PRiMe 2012 Figure 1

Addressing Self-Discharge in Lithium-Sulfur Batteries By Electrolyte Modification

Despite recent advances in lithium-ion batteries, the limits of modern energy storage still place bounds on other up-and-coming technologies such as renewable energy generation and electric vehicles. In an effort to overcome these limitations, many researchers have turned to lithium-sulfur (Li-S) batteries as the next big step in energy storage technology. Given the high theoretical capacity (1675 mAh/g) and high energy density (2600 Wh/kg) of sulfur, along with its low cost and eco-friendliness compared with traditional Li-ion cathode materials, the advantages of Li-S batteries are clear.1,2 Despite these advantages, there are several major barriers that must still be addressed: low practical capacity, poor cycling stability, and low efficiency are endemic to Li-S batteries. These problems are largely due to the dissolution and side-reaction of soluble lithium polysulfides in the electrolyte.1–3 Similarly critical, this behavior can lead to high self-discharge. Compared to the many attempts to improve efficiency and cycling stability of Li-S batteries,1,4–10 many fewer efforts have focused on characterizing and mitigating the self-discharge.11–15 In this work, the self-discharge of Li-S cells was studied and found to be quite severe. Using coin cells with high-sulfur-loading cathodes (>5 mg S/cm2) and a conventional electrolyte of 0.5M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane/1,3-dioxolane (DOL/DME, 1:1 v/v), self-discharge after resting for two weeks at 45°C averaged around 33%. Addition of 0.2M LiNO3 did not significantly decrease self-discharge. Taking a cue from work on fluorinated additives for improving solid electrolyte interphase (SEI) stability in silicon anodes for Li-ion batteries,16 we tested the self-discharge of cells with a fluorinated ether (FE) containing electrolyte. Although cells with FE co-solvent alone did not show lower self-discharge, those with both FE and LiNO3had a lower average self-discharge of 25%, as shown in Fig. 1a. To understand the mechanism by which these additives function, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were carried out on the lithium anodes after cycling. Anodes from cells with FE and LiNO3had a significant amount of surface film, as shown in Fig. 1b. FTIR indicates that new components formed at the surface of electrodes with these combined additives. These traits are believed to indicate formation of a more robust SEI layer, which can mitigate the shuttle effect and thus decrease self-discharge. This work identifies a direction for future efforts to mitigate the self-discharge of Li-S batteries by tuning the choice of FE and the electrolyte composition. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0005475.1. X. Ji, S. Evers, R. Black, and L. F. Nazar, Nat. Commun., 2, 1–7 (2011).2. S. S. Zhang, J. Power Sources, 231, 153–162 (2013).3. H. Yamin, A. Gorenshtein, J. Penciner, Y. Sternberg, and E. Peled, J. Electrochem. Soc., 135, 1045–1048 (1988).4. S.-R. Chen, Y.-P. Zhai, G.-L. Xu, Y.-X. Jiang, D.-Y. Zhao, J.-T. Li, L. Huang, and S.-G. Sun, Electrochim. Acta, 56, 9549–9555 (2011).5. T. Xu, J. Song, M. L. Gordin, H. Sohn, Z. Yu, S. Chen, and D. Wang, ACS Appl. Mater. Interfaces, 5, 11355–11362 (2013).6. J. Song, T. Xu, M. L. Gordin, P. Zhu, D. Lv, Y.-B. Jiang, Y. Chen, Y. Duan, and D. Wang, Adv. Funct. Mater.(2013).7. J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein, and L. F. Nazar, Angew. Chemie Int. Ed., 51, 3591–3595 (2012).8. Y. V. Mikhaylik. Electrolytes For Lithium Sulfur Cells. US Patent 0193835 A1. (2008).9. S. S. Zhang, Electrochim. Acta, 70, 344–348 (2012).10. D. Aurbach, E. Pollak, R. Elazari, G. Salitra, C. S. Kelley, and J. Affinito, J. Electrochem. Soc., 156, A694–A702 (2009).11. Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 151, A1969–A1976 (2004).12. H.-S. Ryu and H.-J. Ahn, Mater. Sci. Forum, 486-487, 630–633 (2005).13. H. S. Ryu, H. J. Ahn, K. W. Kim, J. H. Ahn, K. K. Cho, and T. H. Nam, Electrochim. Acta, 52, 1563–1566 (2006).14. H. S. Ryu, H. J. Ahn, K. W. Kim, J. H. Ahn, J. Y. Lee, and E. J. Cairns, J. Power Sources, 140, 365–369 (2005).15. N. Azimi, W. Weng, C. Takoudis, and Z. Zhang, Electrochem. commun., 37, 96–99 (2013).16. V. Etacheri, O. Haik, Y. Goffer, G. a Roberts, I. C. Stefan, R. Fasching, and D. Aurbach, Langmuir, 28, 965–76 (2012).

Theoretical Study of the O···Cl Interaction in Fluorinated Dimethyl Ethers Complexed with a Cl Atom: Is It through a Two-Center–Three-Electron Bond?

Theoretical investigations are carried out on the interaction between fluorinated dimethyl ethers (FDME, nF = 0-4) and the Cl atom. Short intermolecular O···Cl distances between 2.401 and 2.938 Å reveal the formation of a new class of complexes. The interaction energies calculated with the G2(MP2) method range between -9.1 (nF = 4) and -26.0 (nF = 0) kJ/mol. The charge transfer occurring from the ethers to atomic Cl is moderate and ranges between 0.012 e (nF = 4) to 0.188 e (nF = 0). The binding energies are linearly related to the proton affinity, to the charge transfer (CT) occurring in the molecular system and inversely proportional to the ionization potential and electron affinity (IP-EA) values. The CT and spin density data indicate substantial two-center-three-electron O···Cl interaction in CH3OCH3···Cl and CH3OCH2F···Cl systems, whereas for highly fluorinated ethers the interaction is predominantly electrostatic in nature. The formation of the complex results in a contraction of the CH bonds, especially in the gauche position. The blue shifts of the C-H stretching vibrations calculated in the partially deuterated isotopomers range between 2 and 54 cm(-1) and are correlated to the variation of the CH distances.

3P239 高度含フッ素エーテル型リン脂質液晶膜の構造安定性(生体膜・人工膜 A) 構造・物性))

3P239 高度含フッ素エーテル型リン脂質液晶膜の構造安定性(生体膜・人工膜 A) 構造・物性))