Tris(2,2,2-trifluoroethyl) Phosphite Research Articles

Elevated Electrochemical Performance of LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ Cathodes with Tris(2,2,2-trifluoroethyl) Phosphite as an Efficient Electrolyte Additive.

The significant role of Tris(2,2,2-trifluoroethyl) phosphite (TTFP) as an efficient additive during cycling of the layered nanostructured LiNi0.1Mg0.1Co0.8O₂ and olivine LiFePO₄ cathode materials in EC/DMC and 1M LiPF6 electrolyte for Li-ion battery are extensively investigated in this work. The electrochemical characterization techniques such as cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy show that TTFP improves cycling stability and reduces the irreversible capacity of LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ electrodes. Also, the presence of TTFP in electrolyte solution reduces the impedance in LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ cathode materials at room temperature. A family of Nyquist plots was obtained from LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ electrodes for various potentials during the course of charging. The addition of TTFP in the electrolyte reduces the surface impedance of lithiated LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ which can be attributed to the reaction of the additive on the electrode's surface. Also, the presence of the additive TTFP in LiNi0.1Mg0.1Co0.8O₂ and LiFePO₄ cell enhances the lithium diffusion rate and improves the electronic conductivity of the cathode material.

Thermal stability of modified lithium-ion battery electrolyte by flame retardant, tris (2,2,2-trifluoroethyl) phosphite

With the increasing awareness of green energy, electric vehicles have become the future trend, with lithium-ion batteries (LIBs) regarded as the most suitable energy storage carrier. Therefore, more and more research topics are focused on LIBs, and all parties are working hard to improve the performance of LIBs. Yet, the safety concerns caused by the failure of LIBs cannot be ignored. LIBs themselves are energetic materials, and the causes of accidents often go through multistage irreversible reactions. Several studies have also pointed out that the electrolyte has a significant correlation with the response characteristics because, in the process of LIBs thermal runaway, the electrolyte participating in the oxidation of the entire battery leads to a considerable amount of heat and even runaway reaction as well. Accordingly, it is necessary to obtain a safer electrolyte by modification. In this study, a significant flame retardant (FR) additive, tris (2,2,2-trifluoroethyl) phosphite (TTFP), is used to suppress lithium-ion battery fires or even explosions and maintain typical battery performance. The performance of the electrolyte was tested by differential scanning calorimetry and thermogravimetric analyzer, and the electrolysis was examined on liquid flash point (FP), self-extinguishing time (SET), and conductivity. During the heating process, adding TTFP to the electrolyte effectively delayed the exothermic peak, reduced the amount of heat, improved the FP, and curtailed the SET. The hazard degree of the electrolyte under high-temperature environment was much lower than before adding the additives, and the additives were finally obtained. It can conclusively prove the safety of lithium batteries without lessening the practical performance of the batteries.

Screening electrolytes designed for high voltage electrochemical capacitors

Standard acetonitrile/tetraalkylammonium-tetrafluoroborate-based electrochemical capacitor electrolytes were modified with both fluorinated additives (including fluorinated analogs of ethylmethyl carbonate and diethyl carbonate, lithium difluoro(oxalato)borate, and tris(2,2,2-trifluoroethyl) phosphite) and spiro(1,1′)-bipyrrolidinium tetrafluoroborate salt, to improve stability at higher voltages (>2.7 V). Compounds investigated included (2,2,2-trifluoroethyl) methyl carbonate (TFEMC), ethyl(2,2,2-trifluoroethyl) carbonate (ETFEC), bis(2,2,2-trifluoroethyl) carbonate (bTFEC), lithium difluoro(oxalato)borate (LiDFOB), and tris(2,2,2-trifluoroethyl) phosphite (TTFEPi). Screening of modified electrolytes in prototype coin cells using constant current cycling and voltage hold test protocols revealed good retention of capacitance and a reduced cell internal resistance growth when operating at maximum voltages between 3.5 and 4.0 V. These results suggest new strategies for increasing the operating limits of electric double-layer type supercapacitors, to increase stored energy and improve their compatibility with lithium-ion battery cells for use in hybrid configurations.

Effect of fluorinated additives or co-solvent on performances of graphite//LiMn2O4 cells cycled at high potential

Herein, a comprehensive study of electrochemical performances of the combined effect of fluorinated additives; fluoroethylene carbonate (FEC); and tris(2,2,2-trifluoroethyl) phosphite (TTFP) or the 2,2,2-trifluoroethyl methyl carbonate (TFEMC) as co-solvent, on Graphite//LiMn2O4 cells cycled at high potential is reported. On one side, each additive has a specific function, the FEC is dedicated to the negative electrode and the TTFP to the opposite one. The electrolyte mixture with (4% FEC + 1% TTFP) additive has shown the best ability to reduce fading of the LiMn2O4 electrode, especially at high rates. On the other side, by studying the comparative thermal and transport properties of the formulated electrolytes with different proportions of TFEMC, we demonstrate that the difference in charge distribution of EMC and TFEMC molecules induced by the presence of fluorine atoms, modifies the solvation model of the Li+ cation, and changes its behavior at the CEI interface and impact strongly the electrochemical performances. Finally, the EIS investigation of the LMO/electrolyte interfaces in the presence of TFEMC demonstrates that despite a spontaneous chemical reactivity of the TFEMC at the cathode interface over time, the conductive and good quality CEI is formed, which positively impact the cyclability. This study shows that against LMO surface phenomena, the combination in adequate proportions of fluorinated additives or solvent can be a solution not only to avoid the oxidative reactivity of LMO-cathode, but also to prevent its harmful consequences on the Li-metal or graphite-anode by controlling the solvation of lithium-ion.

Revisiting the Mechanism Behind Transition-Metal Dissolution from Delithiated LiNixMnyCozO2 (NMC) Cathodes

Dissolution of transition metals (TMs) from lithium-ion battery cathodes under high-voltage conditions is a major issue affecting battery performance that is not well understood mechanistically. Here, this phenomenon is studied by chemically aging pristine and charged LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes in the presence of different solutions. The solution composition was varied by 1) adding water to a standard electrolyte, 2) replacing LiPF6 salt with lithium acetylacetonate (Li-acac), 3) and/or adding oxidatively unstable tris(2,2,2-trifluoroethyl) phosphite (TTFP) as an electrolyte additive. Our results demonstrate that while TM dissolution from pristine NMC532 cathodes is dominated by HF-attack, TM dissolution from charged NMC532 cathodes is affected by many other factors apart from HF-attack. We suggest that reduction of TMs due to chemical/electrochemical oxidation of the electrolyte at cathode/electrolyte interface, followed by formation of soluble TM-complexes with concomitant Li+ intercalation into the cathode, is the dominant mechanism of TM-dissolution at high voltage.

Safety optimization enabled by tris(2,2,2-trifluoroethyl)phosphite additive for advanced pouch lithium ion batteries

The safety issue challenges the application of lithium ion batteries. To solve this problem, organophosphous additives are applied to construct a non-inflammable electrolyte. Herein an enhanced performance is obtained with tris(2,2,2-trifluoroethyl)phosphite (TTFP) additive for pouch lithium ion batteries. With comparable electrochemical performance, cells with optimized electrolyte come to thermal runaway at higher temperature, which indicates more stable thermal stability. On the other hand, cells with optimized electrolyte exhibit low self-discharge performance. XPS results confirm a better SEI layer, which contributes to the enhancements of the thermal stability and self-discharge. All these results show that TTFP will bring better safety performance and will be a strong candidate for applicable electrolyte.

Comparative Performance Evaluation of Flame Retardant Additives for Lithium Ion Batteries – I. Safety, Chemical and Electrochemical Stabilities

AbstractWithin this 1st part of a comparative study, flame retardant electrolyte additives (FRs), as candidates for lithium ion battery electrolytes, from four different phosphorous‐containing molecule classes, are investigated. The five FRs (tris(2,2,2‐trifluoroethyl)phosphate (TFP), tris(2,2,2‐trifluoroethyl)phosphite (TTFPi), bis(2,2,2 trifluoroethyl)methylphosphonate (TFMP), (ethoxy)pentafluorocyclotriphosphazene (PFPN) and (phenoxy)pentafluoro‐cyclotriphosphazene (FPPN)) are investigated in a comparative manner to conclude structure‐property relationships according to their self‐extinguishing time (SET), onset temperature of the thermal runaway, chemical and electrochemical stability. SET experiments using standard electrolyte (1 M LiPF6 in ethylene carbonate : dimethyl carbonate 1 : 1 wt%) confirm high reproducibility of the used SET device. The results reveal a strong dependency of the ignition time on the SET. Therefore it is suggested, to investigate the SET for various ignition times. All FR additives remain chemically stable for weeks after added to the standard electrolyte during storage (approved by nuclear magnetic resonance). Cyclophosphazenes show superior results concerning their first time of inflammation and thermal electrolyte stability (approved by adiabatic reaction calorimetry). All additives show partial electrolyte decomposition only during the first charge in voltammetric experiments.

Improved high-voltage performance of LiNi1/3Co1/3Mn1/3O2 cathode with Tris(2,2,2-trifluoroethyl) phosphite as electrolyte additive

Tris(2,2,2-trifluoroethyl) phosphite (TTFEP) is investigated as an electrolyte additive to improve the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode at high operating voltage (4.6V). Charge/discharge measurements demonstrate that TTFEP is effective to improve the cycling stability and rate capability of LiNi1/3Co1/3Mn1/3O2 cathode. The capacity retention of LiNi1/3Co1/3Mn1/3O2/Li cell with 1% TTFEP-containing electrolyte reaches up to 85.4% after 100 cycles at 0.5C (1C=160mAg−1), while that of the cell with the baseline electrolyte (1M LiPF6 in EC/DMC electrolyte) only remains 74.2%. Moreover, the discharge capacity of the cathode with 1% TTFEP-containing electrolyte could maintain around 112.0mAhg−1 at 4C. Based on the characterization of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), a protective interphase film formed on the cathode surface can be found due to the preferential oxidation of TTFEP, which inhibits the electrolyte decomposition and mitigates the cathode structural destruction, leading to the improved electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode at high voltage.

Effect of Phosphorus-Based Additives on Cathode-Electrolyte Interface of Charged Ncm Surface through Potentiostatic Holds

Nickel-rich layered oxide cathode materials have seen widespread deployment due to their high gravimetric capacity and high average discharge voltage. However, achieving this high capacity requires charging voltages above the potential at which the electrolytes decompose at the cathode surface (>4.5 V vs. Li/Li+). This interfacial instability leads to impedance rise and capacity loss. One strategy to stabilize this interface is by the addition of additives to the electrolyte, in which small amounts of a new component change the interactions of the interface. Potentiostatic holds with LiNi0.5Mn0.3 Co0.2O2 (NMC) as the working electrode and Li4Ti5O12 (LTO) as the counter/reference allow a quantitative measurement of the oxidation reactions occurring at cathode. A 60 hour hold allows for concentration polarizations to relax and oxidative side reactions to dominate the current. A more stable electrolyte at the cathode/electrolyte interface would lead to a lower current during the potentiostatic hold. In this work, the performance of five phosphorus-based additive electrolyte formulations (1% wt. ratio) is evaluated against a baseline organic carbonate electrolyte in NMC (LiNi0.5Mn0.3 Co0.2O2 ) / LTO (Li4Ti5O12) full cells. These additives include triethylphosphite (TEPi), tris(2,2,2-trifluoroethyl)phosphite (TTFPi), tris(trimethylsilyl) phosphite (TMSPi), and tris(trimethyl silyl) phosphate (TMSPa). The electrochemical protocol used to evaluate these additives comprises multiple cycles up to 4.4 V vs. Li+/Li, a potentiostatic hold at 4.6 V vs. Li+/Li, followed by AC impedance spectroscopy and extended cycling. Of the formulations surveyed, only electrolytes containing TMSPa and TMSPi (aged for one week) increased the coulombic efficiency during the initial cycling. For the potentiostatic hold, only the aged TMSPi electrolyte lowered the terminal oxidation current compared to the baseline electrolyte, indicating increased stability of the cathode/electrolyte interface. AC impedance spectra indicated the aged TMSPi electrolyte had a slightly elevated high-frequency semicircle (traditionally associated with phenomena at the solid-electrolyte interphase) and a vastly decreased mid-frequency semicircle (traditionally associated with charge-transfer phenomena), compared with all other electrolytes tested. Characterization of cycled electrodes was performed with XPS and SEM will be presented. Acknowledgements The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No. DE-AC02-06CH11357. The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable, worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Communication—A Phosphorus Pentafluoride Scavenger to Suppress Solid Electrolyte Interphase Damage at Moderately Elevated Temperature

We demonstrate that lithium/silicon monoxide (SiO) cell failure can be hindered by utilizing a Lewis-basic tris(2,2,2-trifluoroethyl) phosphite (TTFP), as an electrolyte additive. In the absence of TTFP, the solid electrolyte interphase (SEI) on the SiO electrode is attacked by Lewis-acidic phosphorus pentafluoride (PF5) generated by thermal decomposition of lithium hexafluorophosphate (LiPF6) at moderately elevated temperature. The SEI damage is followed by electrolyte decomposition/film deposition on the damaged electrode surface causing film growth, which leads to severe cell polarization and eventual failure. Conversely, effective PF5 scavenging by TTFP through Lewis acid-base adduct formation decreases SEI damage and thus hinders cell failure.

Degradation of Solid Electrolyte Interphase By Phosphorus Pentafluoride Generated By Thermal Decomposition of Lithium Hexafluorophosphate

Introduction The solid electrolyte interphase (SEI) on the negative electrode is known to play a key role on cell safety. Namely, if the SEI layer is damaged at high temperatures, electrode/electrolyte decomposition is induced along with cell heating, which can trigger thermal runaway. Lithium hexafluorophosphate (LiPF6) is known be thermally decomposed (LiPF6 → LiF + PF5) over 70oC. Here, the as-generated phosphorus pentafluoride (PF5) is known to react with organic solvents such as ethylene carbonate (EC) at high temperatures.1 It is thus very likely that PF5 can attack the organic and/or inorganic components of SEI layer to deteriorate the passivating ability of SEI layer. In this work, to confirm the SEI damage by PF5 attack, the open-circuit voltage (OCV) was monitored while the Li/SiO cells, wherein the concentration of PF5 precursor (LiPF6) was varied, were stored at 85oC. A more serious SEI damage is expected with a larger LiPF6 loading. As a PF5 scavenger, tris(2,2,2-trifluoroethyl) phosphite (TTFP) was added into the electrolyte solution. The former (PF5) is a strong Lewis acid, while the latter is Lewis base, such that a strong complex formation is expected from the two.2 Experimental Two different experiments were performed. (1) The 2032-type coin cells with SiO working electrode, Li counter electrode and electrolyte (1.3 M LiPF6 EC:DEC (3:7, v/v)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The cells were then dismantled and re-assembled by adding different electrolyte (0.0 M, 0.7 M and 1.3 M LiPF6 in EC:DEC (3:7, (v/v)). The re-assembled cells were stored at 85oC for 1 day, while the OCV was monitored. (2) The 2032-type coin cells with SiO working electrode, Li counter electrode and two different electrolyte solution (1.3 M LiPF6 in EC:DEC (3:7, v/v) and 1.3 M LiPF6 in EC:DEC (3:7, v/v) + 5% TTFP)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The same storage experiment was performed. The passivation ability of SEI layer was assessed by monitoring the OCV values and the chemical composition of SEI layer was analyzed by using XPS. Results and Discussion Fig. 1 (a) shows the OCV profiles obtained from the Li/SiO cells during the 85oC storage. As is seen, the OCV values increase at a faster rate with an increase in LiPF6 concentration. The OCV increase signifies the oxidation (self-discharge) of SiO electrode, which in turn indicates the damage and loss of passivating ability of SEI layer.3 The SEI damage is resulted from PF5 attack, which is generated by thermal decomposition of LiPF6. Fig. 1 (b) compares the OCV change for two Li/SiO cells. The cell containing TTFP shows a much slower OCV increase, illustrating that the SEI layer is rather intact even upon 85oC exposure. The SEI damage by PF5 attack was marginal due to the scavenging action of TTFP for PF5. Reference 1. S. E. Sloop, J. B. Kerr, and K. Kinoshita, J. Power Sources, 119-121, 330 (2003). 2. S. S. Zhang, K. Xu, and T. R. Jow, Electrochemical and Solid-state Letters, 5, A206 (2002). 3. H. Park, T. Yoon, J. Mun, J. H. Ryu, J. J. Kim, and S. M. Oh, J. Electrochem. Soc., 160, A1539 (2013). Fig ure 1 . The OCV profiles obtained from the Li/SiO cells that were stored at 85oC. (a) The concentration of LiPF6 was varied in the electrolyte solutions, (b) TTFP was added into the electrolyte solution in one cell. Figure 1

Mechanistic Insight in the Function of Phosphite Additives for Protection of LiNi0.5Co0.2Mn0.3O2 Cathode in High Voltage Li-Ion Cells.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

Exploiting chemically and electrochemically reactive phosphite derivatives for high-voltage spinel LiNi0.5Mn1.5O4 cathodes

A family of organophosphorus compounds including triphenyl phosphite (TPP), trimethyl phosphite (TMP), tris(2,2,2-trifluoroethyl) phosphite (TFEP), and tris(trimethylsilyl) phosphite (TMSP) is investigated as additives for the stabilization of high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode-electrolyte interface. Our investigation reveals that the cycling performance of Li/LNMO half cells with the TMP, TFEP or TMSP additive is drastically improved at 60 °C compared to the baseline electrolyte. Among the various phosphite-based additives tested, TMSP additive enables facile Li ion transport at high C rates and significantly enhances the storage performance of the Li/LNMO cells at 60 °C. To understand the effects of the phosphite-based additives on electrolyte oxidative decomposition at high voltages, the surface chemistry of the cathode after precycling is investigated via ex-situ X-ray photoelectron spectroscopy (XPS). Additionally, the roles of phosphite-based additives to suppress LiPF6 hydrolysis and to remove HF are examined via 19F and 31P NMR spectroscopies.

Tris(2,2,2-trifluoroethyl) phosphite as an electrolyte additive for high-voltage lithium-ion batteries using lithium-rich layered oxide cathode

In this paper, we report positive effect of Tris(2,2,2-trifluoroethyl) phosphite (TTFP) as additive during initial activation and cycling of Li-rich-NMC xLi2MnO3–(1 − x)LiMO2 (x >> 1; M = Ni, Co, Mn) cathode in EC/DMC + 1 M LiPF6 electrolyte. Firstly conductivity and viscosity of electrolyte with x wt.% TTFP; 0 wt.% <x < 20 wt.% were studied. From the electrochemical characterization using cyclic voltammetry, galvanostatic charge/discharge and EIS we show that TTFP passives aluminum current collector and the acts as oxygen scavenger during the extended activation plateau up 4.7 V vs. Li+/Li (up to 301 mAh g−1 in charge). The beneficial effect of TTFP improves the long-term cycling stability of the cathode with more than 95% of initial capacity in Gr//Li-Rich -NMC full cell is maintained after 50 cycles at C rate. Finally, XPS analysis of Li-rich -NMC electrodes show presence of TTFP on the surface during cycling and confirm the presence of Mn3+ at the end of discharge. The convergence of all characterizations indicates that TTFP should act as a catalyst to several surface reactions which are beneficial to long cycling cell performances.

Trialkyl Phosphites and Diaryliodonium Salts as Co-initiators in a System for Radical-Promoted Visible-Light-Induced Cationic Polymerization

Trialkyl phosphites ((RO)3P) can act as co-initiators for the diaryliodonium-induced cationic polymerization of cyclohexene oxide (CHO) or THF. A radical initiation step is also required, consistent with the essential role of a radical chain reaction of the phosphite with the iodonium salt to form polymerization-starting aryltrialkoxyphosphonium salts (ArP(+)(OR)3). We used the visible photolysis of phenylazoisobutyronitrile (PAIBN) as the radical initiation step. The presence of multiple fluorine substituents on the phosphite, as in tris(2,2,2-trifluoroethyl) phosphite (TFP), allows polymerization to proceed with a minimal amount of chain transfer from nucleophilic attack by the phosphite. In a typical experiment, a CHO solution of bis(4-tert-butylphenyl)iodonium hexafluorophosphate (0.05 M), TFP (0.1 M), and PAIBN (0.02 M) was illuminated with a 65-W compact fluorescent bulb for 1 h, resulting in a 78% conversion to poly(cyclohexene oxide) with an average molecular weight (MW) of 25000. We also used competition experiments to determine approximate rate constants for reactions of phenyl radicals with CHO (k = 2 × 10(6) M(-1) s(-1)) and with TFP (k = 2 × 10(8) M(-1) s(-1)).

Tris(4-fluorophenyl) Phosphine and Tris(2,2,2-trifluoroethyl) Phosphite as Flame-Retarding Additives in Li-Ion Batteries

Tris(4-fluorophenyl) phosphine (TFPP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) were used as flame-retarding additives in Li-ion battery electrolytes. The cell performance of the TFPP- and TTFP-containing electrolytes was examined by cyclic voltammetry, differential scanning calorimetry, electrochemical impedance spectroscopy and scanning electron microscopy. The flammability of the electrolytes was also examined by measuring the self-extinguishing time of the electrolytes. The results showed that the TTFP additive suppressed the flammability of the electrolyte, improved the thermal stability of the battery and its electrochemical cell performance. On the other hand, the TFPP additive only improved the thermal stability of the electrolyte at the expense of a slight decrease in electrochemical cell performance. The results show that TTFP is a better flame-retarding additive in the electrolyte of Li-ion batteries than TFPP.

Partially fluorinated solvent as a co-solvent for the non-aqueous electrolyte of Li/air battery

In this work we study methyl nonafluorobutyl ether (MFE) and tris(2,2,2-trifluoroethyl) phosphite (TTFP), respectively, as a co-solvent for the non-aqueous electrolyte of Li–air battery. Results show that in certain solvent ratios, both solvents are able to increase the specific capacity of carbon in Li/O2 and Li/air cells. More interestingly, the improvement in discharge performance of the Li/air cells increases with discharge current density. These results cannot be explained by the ionic conductivity and viscosity data of the electrolytes since the participation of fluorinated co-solvents hardly changes viscosity of the solvent blends while reversely reduces ionic conductivity of the electrolyte. In particular, we find that a 30wt.% (vs. solvent) addition of TTFP into a 0.2m (molality) LiSO3CF3 PC electrolyte can significantly improve the discharge performance of Li/air cells, and that the resultant electrolyte is able to support long-term operation of Li/air cells in dry ambient environments due to its low volatility. We believe that the observed performance improvement is associated with the increased dissolution kinetics and solubility of oxygen in fluorinated solvent containing electrolyte.

Diphenyloctyl phosphate and tris(2,2,2-trifluoroethyl) phosphite as flame-retardant additives for Li-ion cell electrolytes at elevated temperature

The effect of diphenyloctyl phosphate (DPOF) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) as flame-retardant (FR) additives in the liquid electrolyte of Li-ion cells is evaluated at both elevated temperature (40 °C) and room temperature (RT, 25 °C). The tested cells use mesocarbon microbeads (MCMB) and LiCoO 2 as the anode and cathode materials, respectively. Cell characteristics are investigated by means of electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results of the cycle performance tests demonstrate the superior discharge capacity and capacity retention of the DPOF-containing cell compared will TTFP after cycling at both RT and 40 °C. Therefore, these results confirm the promising potential of DPOF as an FR additive for improving the electrochemical performance of Li-ion batteries.

Tris(2,2,2-trifluoroethyl) phosphite as a co-solvent for nonflammable electrolytes in Li-ion batteries

In this work, we used tris(2,2,2-trifluoroethyl) phosphite (TTFP), in which the oxidization number of phosphorus was three (III), to formulate nonflammable electrolytes of the Li-ion batteries. Using 1 m (mole solute per kilogram solvent) LiPF 6 3:3:4 (w) propylene carbonate (PC)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte as a baseline, the effect of TTFP on the flammability and conductivity of the electrolytes, as well as the cell performance was evaluated. It is observed that the addition of TTFP can substantially reduce flammability of the electrolytes at a small expense in the ionic conductivity. When the TTFP content reaches 15 wt.% versus the solvent, the electrolyte becomes nonflammable. In Li/graphite half-cell, TTFP not only suppresses PC decomposition and graphite exfoliation but also increases Coulombic efficiency (CE) of the lithiation and delithiation cycle. In Li/cathode (a lithium nickel-based mixed oxide cathode) half-cell, TTFP has negligible adverse impact on the cycling performance when the cells are cycled between 2.7 and 4.2 V. In graphite/cathode Li-ion cell using PC-based electrolytes, TTFP can improve cycling performance, especially at high temperature (60 °C), since its presence favors the formation of solid electrolyte interface (SEI) film on the graphite electrode and increases thermal stability of LiPF 6-based electrolytes.

A Thermal Stabilizer for LiPF[sub 6]-Based Electrolytes of Li-Ion Cells

We report that tris(2,2,2-trifluoroethyl) phosphite (TTFP), in which the oxidization number of phosphor is three (III), can effectively improve thermal stability of LiPF 6 -based electrolytes of Li-ion cells. Effect of TTFP on the performance of Li/graphite, Li/cathode, and Li-ion cells is estimated using cyclic voltammetry and galvanostatic cycling. Results show that TTFP is electrochemically stable in Li-ion chemistry. We also found that TTFP can prevent propylene carbonate (PC) decomposition and graphite exfoliation in Li/graphite half-cell. We propose a mechanism of TTFP protecting LiPF 6 and discuss the effect of TTFP on cell performance.