Hydrofluoroether Research Articles

(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.

A computational drop-in assessment of hydrofluoroethers in Organic Rankine Cycles

Organic Rankine Cycles (ORCs) are experiencing a growing interest due to their ability to generate electricity from residual low waste heat sources. HFC-245fa is a representative working fluid for ORC applications, but it has recently been phased-out in new equipment because of its high global warming potential (GWP). In this work, the soft-SAFT molecular-based equation of state is used to evaluate the capacity of nine promising low-GWP hydrofluoroethers (HFEs) as alternative working fluids in ORC applications using different key performance indicators focused on energy efficiency and service fluids consumption. The thermodynamic model has been employed to characterize these fluids by describing saturated densities, vapor pressure, surface tension, temperature-enthalpy, and temperature-entropy diagrams, including further validation with binary mixtures. Then, based on technical criteria focused on the thermal efficiency and working and service fluids consumption, the soft-SAFT model has been used to conduct a feasibility study of HFEs as direct substitutes for HFC-245fa in such applications. Although pure fluids can not reach the same efficiency as the benchmark, HFE-356mmz, HFE-7000, and HFE-7100 appear as promising replacements, capable of approaching system requirements operating at low pressure with low cooling water and heating fluid flow rates, while exhibiting lower GWP values.

Hole Transport Layer Microroughening for Waveguide Mode Extraction in Organic Light‐Emitting Diodes

Herein, simple and scalable approach to enhance the out‐coupling efficiency in organic light‐emitting diodes (OLEDs) is presented. By exposing the 4,4′‐bis[N‐(1‐naphthyl‐1‐)‐N‐phenylamino]‐biphenyl (NPB) hole transport layer to hydrofluoroether (HFE) solvent for several minutes, a controllable microroughening of the film is achieved, which results in the formation of random scattering centers in the OLED structure. Optimized NPB microroughening for phosphorescent OLED with Ir(ppy)3 emitter leads to luminous efficacy improvement of 23% (from 64 to 79 lm W−1 at 1000 cd m−2) without additional external out‐coupling. Angular resolved spectroscopy of the OLED with an internal scattering layer reveals constant color almost independent of the viewing angle and Lambertian intensity distribution.

An Ionic Liquid Electrolyte with Enhanced Li+ Transport Ability Enables Stable Li Deposition for High-Performance Li-O2 Batteries.

Bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte could endow Li-O2 batteries with low charging overpotential. However, their weak Li+ transport ability (LTA) leads to non-uniform Li deposition. Herein, guided by Sand formula, the LTA of TFSI-IL electrolyte is greatly enhanced to realize robust Li deposition through introducing hydrofluoroether (HFE) and optimizing electrolyte component ratios to regulate solvation environment. The solvation environment changes from Li(TFSI)2 - ion pair into ionic aggregate clusters in the optimal electrolyte thanks to the slicing function of HFE toward ionic aggregate network. The transport parameters of Sand formula are synchronously enhanced, resulting in highly robust Li deposition behavior with greatly improved Coulombic efficiency (ca. 97.5 %) and cycling rate (1 mA cm-2 ). Cycling stability of Li-O2 batteries was greatly improved (a tiny overpotential rise of 64 mV after 75 cycles).

An Ionic Liquid Electrolyte with Enhanced Li+ Transport Ability Enables Stable Li Deposition for High‐Performance Li‐O2 Batteries

AbstractBis(trifluoromethanesulfonyl)imide‐based ionic liquid (TFSI‐IL) electrolyte could endow Li‐O2 batteries with low charging overpotential. However, their weak Li+ transport ability (LTA) leads to non‐uniform Li deposition. Herein, guided by Sand formula, the LTA of TFSI‐IL electrolyte is greatly enhanced to realize robust Li deposition through introducing hydrofluoroether (HFE) and optimizing electrolyte component ratios to regulate solvation environment. The solvation environment changes from Li(TFSI)2− ion pair into ionic aggregate clusters in the optimal electrolyte thanks to the slicing function of HFE toward ionic aggregate network. The transport parameters of Sand formula are synchronously enhanced, resulting in highly robust Li deposition behavior with greatly improved Coulombic efficiency (ca. 97.5 %) and cycling rate (1 mA cm−2). Cycling stability of Li‐O2 batteries was greatly improved (a tiny overpotential rise of 64 mV after 75 cycles).

Surface tension and density of dielectric heat transfer fluids of HFE type-experimental data at 0.1 MPa and modeling with PC-SAFT equation of state and density gradient theory

Hydrofluoroethers (HFEs) belong to a family of promising fluids that find application potential both in scientific research and in many industrial areas. These include electronics cooling, cascade refrigeration, heat transfer fluids, lubricant carriers or even rinsing agents. However, the thermophysical properties of HFEs required for an effective design of technical applications are only vaguely described and the available experimental data for thermodynamic and transport properties are still limited. In order to improve the description of HFEs, new experimental measurements of liquid density and surface tension at pressure of 0.1 MPa and temperatures from 260 to 338 K were carried out with a series of five HFEs; namely HFE-7000, HFE-7100, HFE-7200, HFE-7300, and HFE-7500. Liquid density was determined from a single sinker buoyancy method with an expanded uncertainty of 0.50 kg•m−3. The Wilhelmy plate method and the du Noüy ring method were used for the measurement of surface tension with expanded uncertainties of 0.068 mN•m−1 and 0.18 mN•m−1, respectively. In the modelling part, a consistent fluid property model was developed based on the physically based equation of state PC-SAFT. Additionally, regarding polarity of HFEs, empirical correlation for dipole moment was addressed. In case of larger HFE molecules, it was determined that the polar modification named PCP-SAFT, provides only marginal correction. The equations were combined with the density gradient theory (DGT) in order to model vapor-liquid phase interface and predict the surface tension. DGT+PC-SAFT was found to provide good predictions for the surface tension of the five selected HFEs.

Hydrofluoroether-assisted dilution of Na-ion concentrated ionic liquid electrolyte for safe, stable cycling of high-voltage Na-metal batteries

Sodium (Na)-powered rechargeable batteries (NRBs) are promising as sustainable energy storage systems. To overcome an inherent energy density handicap of NRBs, increasing cell voltage is necessary by building a Na metal battery (NMB), which simultaneously features coveted high-voltage stability and efficient Na dendrite protection. Although ionic liquids (ILs) are eligible to provide superior oxidative stability, their practical uses are challenging due to high viscosity and sluggish ionic transport at a higher Na concentration. Here, a localized Na+ ion concentrated ionic liquid (LNCIL) electrolyte consisting of an IL and a hydrofluoroether (HFE) as cosolvents is developed. The addition of a non-solvating HFE lowers the viscosity and improves separator wettability, thereby facilitating Na+ ion transport. Furthermore, HFE dilution promotes the involvement of dual anions (FSI/TFSI) in the development of a protective solid-electrolyte interphase, leading to Na dendrite suppression. A Na||Na3V2(PO4)3 cell incorporating the LNCIL electrolyte demonstrates excellent cyclability (~96.6% capacity retention over 500 cycles). Moreover, with oxidative stability up to 4.9 V (vs. Na/Na+) and non-flammability, the LNCIL electrolyte ensures the safe operation of high-voltage NMBs.

A Self-Assemble Micelle Electrolyte for Polysulfide Suppression and Li Stabilization in Li-S Battery

Li-S battery, a promising next-generation technology, has received dramatic attention recently due to high theoretical energy density (2600Wh kg-1 ) and low materials cost. However, its application has been impeded by notorious polysulfide dissolution/shuttling which causes poor cycling stability and coulombic efficiency. Herein, I will introduce a novel strategy to suppress the polysulfide dissolution by using hydrofluoroethers (HFEs) electrolyte additive. A bi-functional, surfactant-like molecule structure with a polar lithiophilic “head” attached to a fluorinated lithiophobic “tail”, was designed for these HFE solvents whereby a unique solvation mechanism is proposed: dissociated lithium ions are readily coordinated with lithiophilic “head” to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by small angle X-ray scattering (SAXS) and is studied as a function of additive structure and concentration. The modeling work is underway to further investigate coordination between molecules and micelle structure evolution under different conditions. These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal, which is evidenced by superior battery performance: Li-S cells are demonstrated with 1395 mAh g-1 initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency; Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium.

Effects of Polysulfide Solubility and Li Ion Transport on Li–S Batteries Using Sparingly Solvating Electrolytes

Sparingly solvating electrolytes are an emerging class of electrolytes used in Li–S batteries. In this type of electrolytes, polysulfide dissolution and shuttling can be suppressed, leading to high Coulombic efficiency and cycle life. To optimize the electrolytes for high energy density Li–S cells, effects of polysulfide solubility and Li ion transport properties on Li–S battery performance were investigated for tetraglyme (G4)- and sulfolane (SL)-based electrolytes diluted with a hydrofluoroether (HFE). The Li2S8 solubility was found to be very low (1 mM in atomic S concentration) in [Li(G4)0.8][TFSA]-4.3HFE and [Li(SL)2][TFSA]-4.0HFE. Cells with [Li(SL)2][TFSA]-4.0HFE exhibited better rate capability compared with cells with [Li(G4)0.8][TFSA]-4.3HFE despite lower ionic conductivity of [Li(SL)2][TFSA]-4.0HFE. With regard to the Li ion coordination structure in [Li(SL)2][TFSA]-4.0HFE, the two oxygen atoms of the SL sulfonyl group coordinates to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions. The unique SL- and anion-bridged, chain-like Li ion coordination structure was considered to be involved in the unusual Li ion self-diffusion behavior and Li ion tranport for [Li(SL)2][TFSA]-4.0HFE. The higher Li ion transference number measured under anion-blocking condition (t Li +) of [Li(SL)2][TFSA]-4.0HFE was suggested to contribute to the better rate performance, rather than the polysulfide solubility and ionic conductivity. Furthermore, [Li(SL)2][TFSA]-4.0HFE demonstrated an initial discharge capacity of 1130 mAh g−1 at a low electrolyte volume to sulfur weight ratio of 4, whereas a typical organic electrolyte failed to achieve such a high capacity owing to limitations of the redox mechanism mediated by dissolved polysulfides. In addition to the low solubility of polysulfides, the high t Li + is crucial for achieving high energy density Li–S batteries by reducing the electrolyte amount. The SL-based electrolytes were found to manifest a significant improvement in Li ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li-S batteries. Acknowledgement This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). References 1) A. Nakanishi. et al, J. Phys. Chem. C, 2019, 123, 14229.2) M. Yanagi et al, J. Electrochem. Soc., 2020, 167, 070531. Figure 1

300Wh/Kg Class High-Energy Li-S Laminated Batteries Using High-Sulfur Loading Cathode on 3D Mesh Structured Aluminum Foam

[Introduction] In recent years, lithium-sulfur batteries can be expected as a promising candidate for next generation energy storage devices because of its high energy density. Generally, the cathode is fabricated by infiltration of sulfur into Ketjen black (KB) with aluminum foil current collector. However, it is not easy to increase the sulfur loading amounts and maintain the rate performances for electric vehicles which are required more than 3C rate, due to the lack of Li ion and electron paths. To solve this issue, we have reported the new type sulfur electrode prepared using 3D mesh structured Al foam, resulting in increased sulfur loading amounts which are 5 - 10 times or more compared to conventional sulfur cathode fabricated using 2D structured aluminum foil current collector [1], [2], [3]. In this study, we optimized the fabrication method of sulfur cathode for high sulfur loading on 3D mesh structured Al foam for a high energy density Li-S laminated battery.[Experimental] S/KB composite was obtained by mixing sulfur and KB with weight ratio of 6:4, by heating at 155 °C for 12 h under N2 atmosphere. Afterward obtained S/KB composite was dispersed by ball milling for 12h. S/KB cathode was fabricated by S/KB slurry (weight ratio of S/KB (6/4) : Binder = 96.5 : 3.5). The high sulfur loaded cathode (15.3 mg/cm2) was obtained using 3D mesh structured Al foam (1mm in thickness). To assemble the cell, the cathode was pressed (0.4 mm in thickness). 5 Ah class laminated cell (70 x 70 mm) was prepared using the S/KB cathodes (6 stacked), Li anodes (7 stacked), separators (7 stacked), and the electrolyte (LiTFSI: G1(monoglyme)/G3(triglyme): HFE(hydrofluoroether)= 1: 1: 4 (molar ratio)). Electrochemical performances were tested with voltage range between 1.0 V – 3.3 V after activation process.In addition, coin cells were prepared using above materials to evaluate the ionic conductivity by DC polarization method.[Results and Discussion] To design a high energy density cell such as laminated type cell, it is important to optimize the ratio between using amount of electrolyte and solid materials in electrode. Fig. 1 shows the relationship between the calculated E/S ratio (electrolyte amount (µℓ)/sulfur loading amounts (mg)), porosity filling rate of electrolyte (%), and the gravimetric energy density (Wh/kg). It was possible to reduce the electrolyte filling rate to 100% which is corresponding to E/S = 1.7, achieving the 340 Wh/kg theoretically. It is important to secure an ion path in whole electrode to optimize utilization of E/S ratio. In our previous report, we fabricated the S/KB cathode using 3D mesh structured Al foam regardless of the porosity, implying that it has bad ion conductivity. To improve the ion conductivity of S/KB cathode which has high sulfur loading amounts, we prepared a cathode that loaded S/KB very uniformly and retained the high porosity of the 3D mesh structured Al foam.As a result, we confirmed that the S/KB cathode/3D Al mesh which has good porosity shows improved ionic conductivity than control sample fabricated by conventional method.Fig. 2 shows the charge/discharge curve of Li-S laminated battery prepared by a new method with optimized E/S ratio of 2.3 (conventional method requires high E/S ratio of 3.6 or more). High capacity of 5.2 Ah, and energy density (301 Wh/kg and 435 Wh/ℓ, excluding laminate and tab) was achieved with high sulfur loaded laminated cell (sulfur loading amount of 15.4 mg/cm2, 1156 mAh/g-S).

Towards High Performance Lithium Sulfur Batteries: Surface Oxidation of Micro Porous Carbon for Better Sulfur Positive

1. IntroductionElemental sulfur has extensive attentions because of its high theoretical specific capacity (1672 mA h g-1), natural abundance and low law-material cost. Therefore, sulfur is a promising candidate for a cathode of next generation batteries. However, there are many issues in fully demonstrating the potential of sulfur-based cathode for Li-S cells: intrinsically insulating of sulfur, large volume change during cycles and dissoluble lithium polysulfide intermediates (Li2Sn, 4 ≤ n ≤ 8) in an electrolyte. In our previous study, we solved these problems by using micro porous carbon (AC) to accommodate sulfur and demonstrate its stable cycling.1 Moreover, we proved that oxidation of AC with dilute nitric acid is useful to enhance sulfur-utilization.2 In this work, we apply hydrogen peroxide (H2O2), concentrated nitric acid (HNO3) and potassium permanganate (KMnO4) to oxidizing agent for surface functionalization of AC. Oxygen atomic percentage of ACs increases by using stronger oxidizing agent. Electrochemical performance of KMnO4-oxidized AC-sulfur composite cathode shows the highest specific capacity (614.04 mAh g-1 at the 50th cycle).2. Method2.1. preparation of H2O2 ACTo prepare H2O2 AC, AC was added into 30 wt.% H2O2 and stirred for 48 hrs. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight.2.2. preparation of HNO3 ACTo prepare HNO3 AC, AC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 hrs. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight.2.3. preparation of KMnO4 ACTo prepare KMnO4 AC, AC and KMnO4 powder were added into 98 wt.% H2SO4 and stirred for 2 hrs. Next, deionized water and citric acid were added into the solution. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight.2.4. preparation of 1000ºC ACTo prepare reduced AC for comparison, AC was thermally annealed at 1000ºC for 1 hr under Ar atmosphere.2.5. CharacterizationTo determine surface oxygen loading of each AC, X-ray photoelectron spectroscopy (XPS) was carried out. To prepare AC-sulfur (S) composites, each AC was mixed with S at a weight ratio of AC : S = 48 : 52. The mixture was thermally annealed at 155ºC for 5 hr, so that AC-S, H2O2 AC-S, HNO3 AC-S, KMnO4 AC-S, 1000ºC AC-S were obtained. Each AC-S cathode was prepared by mixing the AC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89 : 5 : 3 : 3 and then coating on an Al foil current collector. The cells with the AC-S electrode and Li metal foil as a counter electrode were assembled in a glove box filled with Ar. The solution, lithium bis(trifluorosulfonyl)imide (LiTFSI) : tetraglyme (G4) : hydrofluoroether (HFE) = 10 : 8 : 40 (by mol), was used for the electrolyte. A charge-discharge cycling test was carried out at a current density of 167.2 mA g-1 (0.1 C) with charge and discharge cutoff voltages of 3.0 and 1.0 V at 25ºC.3. Major results and conclusionTable 1 shows the surface element ratio of each AC based on the area of C1s and O1s spectra. Each oxidized AC had increased surface oxygen loading compared with pristine AC. For oxidized ACs, surface oxygen increased in the order of H2O2 AC, HNO3 AC, KMnO4 AC. This order is consistent with the order of strength of oxidizing agents. Thus, oxygen atomic percentage of ACs was found to be higher by using stronger oxidizing agent. Contrary to such oxidation, 1000ºC AC had decreased surface oxygen loading compared with pristine AC. This is because surface oxygen-containing functional groups of AC are reduced by thermal treatment under an inert atmosphere. Fig. 1 shows discharge capacity for each AC-S cathode. The cathode with higher oxygen-containing AC exhibited higher discharge capacity. This suggests that affinity between Li2Sn and AC surface was enhanced by introducing polar functional groups.We will also report results of further characterization for oxidized ACs. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteies (ALCA-SPRING [JPMJAL1301])” from JST.(1) T. Takahashi et al., Prog. Nat. Sci.: Mater. Int., 25, 612 (2015).(2) S. Okabe et al., Electrochemistry, 85, 671 (2017). Figure 1

Vapor-phase catalytic methylation of 1,1,1,3,3,3-hexafluoroisopropanol for the mass production of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether

With the phasing-out of chlorofluorocarbons and hydrochlorofluorocarbons required by the Montreal and Kyoto Protocols, hydrofluoroethers (HFEs) are now considered to be promising alternatives due to their zero ozone-depletion and low global-warming potentials, and their significant capacities for use in heat-pump and cleaning-agent applications. However, the pollution-free and large-scale synthesis of HFEs has been a long-standing challenge. To address the issue, we previously reported a novel synthetic method for the large-scale production of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether (HFE-356mmz), a representative HFE, through the vapor-phase methylation of 1,1,1,3,3,3-hexafluoroisopropanol using metal fluorides as catalysts. In this work, mixed oxides of Mg and Al with various Mg/Al2 ratios were employed as alternative catalysts; their abilities to promote the reaction were determined and the methylation mechanism was explored. All Mg-Al mixed oxides promoted the production of HFE-356mmz, albeit with different efficiencies, which were found to be determined by the surface acid-base properties of the catalysts. The results agree well with those obtained using metal fluorides as catalysts and provide new mechanistic evidence. Our study not only offers further evidence of the reaction mechanism, but also affords a more universal and operable process that uses more-common and less-expensive catalysts.

Vapor-liquid equilibria for binary systems carbon dioxide + 1,1,1,2,3,3-hexafluoro-3-(2,2,2-trifluoroethoxy)propane or 1-ethoxy-1,1,2,2,3,3,4,4,4-nonafluorobutane at 303.15–323.15 K

The phase behavior of binary mixtures of carbon dioxide (CO2) and hydrofluoroethers (HFEs) has been studied. In particular, experimental vapor-liquid equilibrium (VLE) data for CO2 + 1,1,1,2,3,3-hexafluoro-3-(2,2,2-trifluoroethoxy)propane (HFE-449mec-f) and 1-ethoxy-1,1,2,2,3,3,4,4,4-nonafluorobutane (HFE-7200) at temperatures of 303.15, 313.15, and 323.15 K are reported. The VLE data were measured using a static-type apparatus and then correlated using the Peng-Robinson equation of state with the van der Waals one fluid and Wong-Sandler-NRTL mixing rules. Reasonable correlation results were obtained from the Peng-Robinson equation of state with both the van der Waals one fluid and the Wong-Sandler-NRTL mixing rules. The GC-SAFT-VR equation also gave good predictions of the phase behavior. Additionally, the group contribution SAFT-VR (GC-SAFT-VR) equation was used to predict the experimental VLE in good agreement with the experimental data, as well as the full p,T phase diagram for both systems.

Safe, Stable Cycling of Lithium Metal Batteries with Low‐Viscosity, Fire‐Retardant Locally Concentrated Ionic Liquid Electrolytes

AbstractIonic liquid (IL) electrolytes with concentrated Li salt can ensure safe, high‐performance Li metal batteries (LMBs) but suffer from high viscosity and poor ionic transport. A locally concentrated IL (LCIL) electrolyte with a non‐solvating, fire‐retardant hydrofluoroether (HFE) is presented. This rationally designed electrolyte employs lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1‐methyl‐1‐propyl pyrrolidinium bis(fluorosulfonyl)imide (P13FSI) and 1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropyl ether (TTE) as the IL and HFE, respectively (1:2:2 by mol). Adding TTE enables a Li‐concentrated IL electrolyte with low viscosity and good separator wettability, facilitating Li‐ion transport to the Li metal anode. The non‐flammability of TTE contributes to excellent thermal stability. Furthermore, synergy between the dual (FSI/TFSI) anions in the LCIL electrolyte can help modify the solid electrolyte interphase, increasing Li Coulombic efficiency and decreasing dendritic Li deposition. LMBs (Li||LiCoO2) employing the LCIL electrolyte exhibit good rate capability (≈89 mAh g−1 at 1.8 mA cm−2, room temperature) and long‐term cycling (≈80% retention after 400 cycles).

A Micelle Electrolyte Enabled by Fluorinated Ether Additives for Polysulfide Suppression and Li Metal Stabilization in Li-S Battery

The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh kg−1) 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 scattering (SAXS). 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−1 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.

A New Class of Ionically Conducting Fluorinated Ether Electrolytes with High Electrochemical Stability.

Increasing battery energy density is greatly desired for applications such as portable electronics and transportation. However, many next-generation batteries are limited by electrolyte selection because high ionic conductivity and poor electrochemical stability are typically observed in most electrolytes. For example, ether-based electrolytes have high ionic conductivity but are oxidatively unstable above 4 V, which prevents the use of high-voltage cathodes that promise higher energy densities. In contrast, hydrofluoroethers (HFEs) have high oxidative stability but do not dissolve lithium salt. In this work, we synthesize a new class of fluorinated ether electrolytes that combine the oxidative stability of HFEs with the ionic conductivity of ethers in a single compound. We show that conductivities of up to 2.7 × 10-4 S/cm (at 30 °C) can be obtained with oxidative stability up to 5.6 V. The compounds also show higher lithium transference numbers compared to typical ethers. Furthermore, we use nuclear magnetic resonance (NMR) and molecular dynamics (MD) to study their ionic transport behavior and ion solvation environment, respectively. Finally, we demonstrate that this new class of electrolytes can be used with a Ni-rich layered cathode (NMC 811) to obtain over 100 cycles at a C/5 rate. The design of new molecules with high ionic conductivity and high electrochemical stability is a novel approach for the rational design of next-generation batteries.

Experimental studies on thermosyphon using low global warming potential refrigerant HFE7000 and nanorefrigerant HFE7000/Al2O3

Thermosyphon is used in numerous applications such as permafrost, cooling building and structures, Alaska pipeline, electronic cooling, and other applications. Improving the performance of thermosyphon is essential for technology advancement. Therefore, experimentation is conducted to improve the efficiency of thermosyphon with the natural refrigerant hydrofluoroether (HFE) and Al2O3/HFE7000 nanorefrigerant. The Al2O3 nanoparticle is chosen based on its economic feasibility and better thermo-physical properties with the refrigerants. Firstly, the preparation of Al2O3/HFE7000 nanorefrigerant is carried out specifically at different volume concentrations of the nanoparticle to check the long-term stability. Secondly, the heat transfer characteristics of the thermosyphon charged Al2O3/HFE7000 nanorefrigerant of 0.025%, 0.05%, and 0.075% volume concentration and pure HFE7000 is investigated experimentally. The nanorefrigerant charged thermosyphon experimented for different inclinations and different volume concentrations as the working fluid. It was observed that the two-phase closed thermosyphon charged with Al2O3/HFE7000 nanorefrigerant enhanced its evaporator heat transfer performance also decreased the thermal resistance of 57.5% compared with the pure HFE7000 and was at its peak for 0.05% volume concentration. The heat transfer of nanorefrigerant Al2O3/HFE7000 0.025%, 0.05%, and 0.075% volume concentration is increases 41.61%, 88.414%, and 74.362% than HFE7000. In conclusion, the results of the experiments suggest that the use of Al2O3/HFE7000 nanofluid produce a significant thermal enhancement in thermosyphon. This research also discloses the effect of dimensionless parameters such as the Bond number of the boiling phenomenon, Prandtl and condensation number of conduction phenomenon, and Ohensorge number of buoyancy phenomenon in thermosyphon with Al2O3/HFE7000 nanorefrigerant. It is identified that the volume concentration of 0.05% Al2O3/HFE7000 has a considerable effect on nondimensional parameters.

Effects of Polysulfide Solubility and Li Ion Transport on Performance of Li–S Batteries Using Sparingly Solvating Electrolytes

Sparingly solvating electrolytes are an emerging class of electrolytes used in Li–S batteries. In this type of electrolytes, polysulfide dissolution and shuttling can be suppressed, resulting in high Coulombic efficiency and cycle life. To optimize the electrolytes for high energy density cells, effects of polysulfide solubility and Li ion transport properties on Li–S battery performance were investigated for tetraglyme (G4)-based solvate ionic liquids [Li(G4)x][TFSA] and a sulfolane (SL)-based concentrated electrolyte [Li(SL)2][TFSA], which are both diluted with a hydrofluoroether (HFE). The Li2S8 solubility is low (1 mM in atomic S concentration) in [Li(G4)0.8][TFSA]−4.3HFE and [Li(SL)2][TFSA]−4.0HFE. Cells with [Li(SL)2][TFSA]−4.0HFE exhibited better rate capability despite their lower ionic conductivity. The higher transference number (tLi+) of [Li(SL)2][TFSA]−4.0HFE may predominantly contribute to the rate performance, rather than polysulfide solubility and ionic conductivity. Furthermore, [Li(SL)2][TFSA]−4.0HFE demonstrated an initial discharge capacity of 1130 mAh g−1 at a low electrolyte volume to sulfur weight ratio of 4, whereas a typical organic electrolyte failed to achieve such a high capacity owing to limitations of the redox mechanism mediated by dissolved polysulfides. In addition to the low solubility of polysulfides, the high tLi+ is crucial for achieving high energy density Li–S batteries by reducing the electrolyte amount.

Hydrofluoroether Impurities—Chemical Detection Using a Deep Learning Laser Speckle Contrast Evolving Spiking Neural Network

Hydrofluoroether (HFE) impurities detection is an issue related to detecting chemical contamination within a high volume manufacturing (HVM) chiller caused by a rapid emulated environmental attack. The aftereffects of the cycle emulated attack may eventually create a micro-crack in the heat-exchanger. This event eventually causes the contamination of HFE due to multiple chemical interactions. This study proposes a new classification methodology to detect HFE chemical impurities using induction by a 532nm laser. The purpose of the laser induction is to leverage its laser speckle contrast attributes and amplify its detection. One of the reasons for choosing the 532nm laser spectrum is its highest quantum efficiency. Once amplification of detection is achieved, the detector tends to be in a highly sensitive mode due to low marginality to differentiate between two different conditions. This mode is prone to be stochastic. Thus, a new form of architecture known as Deep Learning Laser Speckle Contrast Evolving Spiking Neural Network (DL-LSC-ESNN) is proposed. The architecture utilizes speckle contrast domain conversion, dimensional additivity of the receptive neuron of evolving spiking neural network (ESNN), followed by the strength of Convolution Neural Network (CNN) feature extraction (FE) capability. Ultimately, the evolving and adaptive ability of ESNN is assimilated and integrated seamlessly. CNN acted to extract only an important spike train, and the result is its essences of important spikes or spike feature maps. The spike feature maps are then fed into the ESNN neuron repository, which either assimilates or creates a new neuron repository. The proposed methods show significant improvement in the accuracy of detection against multiple baseline state of the art CNN architecture and ultimately demonstrated its capability to detect real-time contamination of HFE thus improved the detection rate significantly for the HVM environment.

Highly fluorinated organic–inorganic nanocomposites with improved refractive index and photopatterning properties

We developed a highly fluorinated photocrosslinkable nanocomposite that can be applied from solutions in fluorous solvents and has a higher refractive index (RI) compared to the conventional amorphous fluoropolymers. Its two main components are an aromatic fluoroalkylated epoxy binder (RF-FDP-E) and fluoroalkyl ether-type phosphonic acid-substituted anatase titania (TiO2-PEPA) nanoparticles. RF-FDP-E had an RI of about 1.48 at 589 nm and the transmittance of its 270 nm thick film was around 92% in the visible light region. Thin films of RF-FDP-E mixed with small amounts of a photoacid generator (PAG) could be photolithographically patterned by using commercially available hydrofluoroether (HFE) solvents, forming structures with a 10 μm size. The TiO2-PEPA nanoparticles with an average diameter of 18 nm could also be transparently dissolved into the HFE solvents; the RI at 589 nm and transmittance in the visible light region of the resulting 280 nm thick films were 1.65 and 88%, respectively. The thin film (510 nm) obtained from a dissolved mixture of RF-FDP-E and TiO2-PEPA (50% in weight) exhibited RI and transmittance of 1.50 and 87%, respectively, and the original photopatterning properties were maintained.