Highly Concentrated Electrolytes Containing a Phosphoric Acid Ester

Apr 20, 2018 - Phosphoric acid ester amides were examined as a new self-extinguishing solvent for Li-ion batteries. The phosphoric acid ester amides u...
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Cite This: J. Phys. Chem. C 2018, 122, 9738−9745

Highly Concentrated Electrolytes Containing a Phosphoric Acid Ester Amide with Self-Extinguishing Properties for Use in Lithium Batteries Tohru Shiga,* Chika-aki Okuda, Yuichi Kato, and Hiroki Kondo Toyota Central Research & Development Laboratories Inc., Yokomichi, Nagakute-city, Aichi-ken 480-1192, Japan S Supporting Information *

ABSTRACT: Phosphoric acid ester amides were examined as a new self-extinguishing solvent for Li-ion batteries. The phosphoric acid ester amides used in this study contained two fluorinated alkyl groups and one amino group, (CF3CH2O)2(NR1R2)PO (PNR1R2). The thermal stability of the highly concentrated electrolyte of lithium bis(fluorosulfonyl) amide (LiFSA) and PNR1R2 with a molar ratio of [LiFSA]/[PNR1R2] = 0.5 under overcharge depended on the modification of the amino substituent. Introduction of a phenyl group (R1 = CH3, R2 = C6H5) was effective for improving thermal stability. The release of gases and heat that typically accompanied reaction of the solvent with the charged graphite anode was greatly suppressed. Density functional theory calculations indicated that PNR1R2 decomposed reductively near 0.5 V vs Li+/Li, suggesting poor Li ion insertion into the graphite. However, the highly concentrated electrolyte using LiFSA and PNR1R2 reduced the reductive potential of PNR1R2 and enabled not only the insertion of Li ions into the graphite but also reversible Li plating/stripping.

1. INTRODUCTION Lithium batteries using volatile and flammable electrolytes are excellent power sources for vehicles of the future but pose serious safety issues.1,2 For example, fire or explosion upon overcharge may occur in some cases. 3−5 The use of incombustible compounds, such as phosphazenes with a −NP− group, and trialkyl phosphates, (CnH2n+1O)3PO, can eliminate this risk and have been investigated in this application.6−8 However, Li salts are not very soluble in phosphazenes.9−12 Phosphates can attack the graphite anode and break down its laminar structure due to cointercalation of Li ions and solvent. Moreover, under overcharged conditions, phosphates generate large amounts of gases at a temperature lower than that of organic electrolytes.13 One strategy for the suppression of graphite destruction is the use of a high concentration of lithium salt.14−16 Highly concentrated electrolytes using lithium bis(fluorosulfonyl)amide [LiFSA; Li(FSO 2 ) 2 N)] and trimethyl phosphate (TMP) or tris(trifluoroethyl) phosphate [(CF3CH2O)3PO, TFEP] have been investigated.17 The TFEP is a fluorinated alkyl phosphate with self-extinguishing properties. A highly concentrated electrolyte with a molar ratio of [LiFSA]/[solvent] = 0.5 enabled the insertion of Li ions into the graphite anode. In addition, an electrolyte with [LiFSA]/[TFEP] = 0.5 showed good thermal stability in the presence of charged graphite, and the gas generation was reduced dramatically. The present study describes nonaqueous electrolytes of phosphoric acid ester amide as a new class of self-extinguishing solvent. The phosphoric acid ester amides in this study have a chemical formula of (CF3CH2O)2(NR1R2)PO (R1, R2: alkyl or phenyl group, abbreviation: PNR1R2), in which one trifluoroethyl group in TFEP is exchanged by one amino substituent. © 2018 American Chemical Society

Therefore, the phosphoric acid ester amide involves a P−N single bond similar to phosphazene compounds but was expected to provide better thermal stability and incombustibility. Highly concentrated electrolytes composed of LiFSA and PNR1R2 were prepared up to a molar ratio of [LiFSA]/ [PNR1R2] = 0.5, and the Li-ion solvation and electrochemical properties were examined. The results indicated that a highly concentrated electrolyte employing PNR1R2 is an excellent candidate as a safe electrolyte for lithium batteries.

2. EXPERIMENTAL SECTION Materials. The two types of phosphoric acid ester amides used were dimethylamino-di(trifluoroethyl) phosphate (PNMeMe) and methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) (Tohso Finechem Corporation) (Figure 1). The physical properties of PNMeMe and PNMePh were listed in Table S1. The water contents in the liquids were 12 ppm (PNMeMe) and 37 ppm (PNMePh), respectively. Lithium

Figure 1. Chemical structures of the phosphoric acid ester amides. Received: December 26, 2017 Revised: April 10, 2018 Published: April 20, 2018 9738

DOI: 10.1021/acs.jpcc.7b12461 J. Phys. Chem. C 2018, 122, 9738−9745

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The Journal of Physical Chemistry C

Electrochemical Measurements. The anode was prepared by mixing graphite powder (90% w/w, Osaka Gas) with polyvinylidene difluoride (#1120 Kureha) and N-methylpyrrolidone (NMP; Wako Chemicals) in a kneading machine (ARE310, Thinky Co. Ltd.) at 2200 rpm for 5 min. The anode slurry was spread onto a copper current collector (20 μm thick) using a doctor-blade technique and dried at 150 °C under vacuum for 5 h. The anode sheet was pounded to obtain a 14 mm diameter disk electrode with a thickness of 35 μm. The active material loading was 2.79 mg/cm2. Half coin cells (Figure S2) were fabricated using the 14 mm diameter disk electrodes, one filter paper (25 μm thick, Kiriyama 5C), a Li metal disk 18 mm in diameter and 0.4 mm thick (Honjo Metal), and the highly concentrated electrolyte. The coin cell was connected with a charge−discharge unit (Hokuto Denko). The charge−discharge current was 0.01−0.1 mA/cm2 without using constantvoltage mode. The cutoff for the graphite disk was set at 0.03− 2.2 V. Charge−discharge tests were performed at temperatures up to 60 °C. Electrochemical impedance spectroscopy (EIS) was conducted with an LF impedance analyzer (HewlettPackard, 4192A) with an amplitude of 100 mV over a frequency range from 5 Hz to 12 MHz to investigate the formation of a passive layer on graphite.

bis(fluorosulfonyl) amide (LiFSA) and propylene carbonate (PC) were obtained from Kishida Chemicals. Vinylene carbonate (VC), an electrolyte additive, was available from Aldrich. Electrolytes. Highly concentrated electrolytes were prepared by mixing LiFSA and the phosphoric acid ester amide with molar ratios of [Li salt]/[solvent] of 0.125 to 0.5. For reference, a 1 mol/L LiPF6/EC+DMC (1:1 v/v) electrolyte [battery grade, ethylene carbonate (EC), dimethyl carbonate (DMC), Kishida Chemicals] was used. Computational Details. The Gaussian program 09E01 was used to calculate one-electron oxidation and one-electron reduction potentials of the two phosphoric acid ester amides. The geometries of the solvents were fully optimized at the B3LYP/6-311++G(d,p) level. DSC Measurements. The charged anode material was prepared as a pellet by compressing graphite (Osaka Gas, average particle diameter D50: 6 μm). A coin cell composed of the anode pellet, Li metal, and an electrolyte of 1 mol/L of LiPF6/EC + DMC was fabricated. The electrolyte was added to the cell in an argon-filled glovebox, and the cell was connected to a charge/discharge measurement apparatus (Hokuto Denko) and charged up to 0.01 V to obtain a sample with a 100% SOC using constant-current mode. The temperature was maintained at 25 °C. The charged anode pellet was removed from the cell and washed several times with dimethyl carbonate. The charged anode pellet (2.5 mg) was wetted with 1.35 μL of highly concentrated electrolyte at a molar ratio of [Li salt]/[solvent] = 0.5, and DSC measurements (Thermo Plus TG8120, Rigaku) were obtained under an argon flow of 10 mL/min and a heating rate of 5 °C/s. The thermal stability of 1 mol/L of LiPF6/EC + DMC was also determined. The charged cathode material was prepared by compressing LiNiO2 powder (Sakai Chemical Industry, LiNi0.8Co0.15Al0.05O2, D50: 7 μm) to obtain a disk electrode. A coin cell composed of the cathode disk, Li metal, and an electrolyte of 1 mol/L of LiPF6/EC + EMC then was fabricated. The cell was connected to a charge/discharge measurement apparatus (Hokuto Denko) and charged up to 4.2 V under constant-current mode and continuous constantvoltage mode at 4.2 V to obtain a sample with 100% SOC. The composition of the charged cathode was Li0.52Ni0.8Co0.15Al0.05O2 (145 mAh/g). After washing the charged cathode disk, 2.5 mg of the cathode disk was wetted with 1.35 μL of the electrolyte for DSC analysis. Ignition Test. The ignition tests were conducted using glass fiber filter paper strips (Whatman, 8 × 50 mm2) saturated with 0.15 mL of electrolyte at a molar ratio of [Li salt]/[solvent] = 0.5; a gas burner with a flame at ca. 700 °C was used to attempt ignition. Raman Analysis. Some electrolytes were prepared by mixing LiFSA and PNMePh at [LiFSA]/[PNMePh] ratios of 0.125, 0.25, and 0.5. The electrolytes and pure PNMePh solvent were poured into glass capillaries. The LiFSA powder was placed in a quartz cell filled with argon (Figure S1). Raman measurements were made through the capillaries or quartz cell using a Raman spectrophotometer (NR-3300, Jasco). The wavelength of the excitation laser was 532 nm, and the beam diameter in the excitation region was 1 mm. FTIR Analysis. The FTIR measurements were made under attenuated total reflection mode to study the composition of the SEI on graphite using a Nicolet FTIR spectrophotometer (Avatar360).

3. RESULTS AND DISCUSSION 3.1. Electrochemical Stability of the Phosphoric Acid Ester Amide. One-electron oxidation and reduction potentials of phosphoric acid ester amides were calculated using DFT calculations with the (B3LYP/6-311++G(d,p)) basis set.18−20 The optimized geometries of PNMeMe and PNMePh were determined to minimize their free energy, and then the potentials were calculated. The one-electron oxidation potential of PNMeMe was calculated to 5.130 V vs Li+/Li, whereas PNMePh had a one-oxidation potential of 4.744 V vs Li+/Li (Figure 2). In contrast, one-electron reduction potentials of the

Figure 2. One-electron oxidation and reduction potentials of phosphoric acid ester amides.

two phosphoric acid ester amides were 0.436 and 0.547 V, suggesting decomposition of the phosphoric acid ester amides at the surface of the graphite anode, leading to passivation layer formation. Each potential window of PNMeMe or PNMePh, which was the difference between the oxidative and reductive potentials, was 4.694 or 4.197 V. In addition, Mulliken electric charge distributions on each element in PNMeMe and PNMePh were provided by the DFT calculations (Figure S3). The electric charges on some elements are summarized in Table 1. The results showed a characteristic difference for nitrogen; i.e., the nitrogen in PNMeMe was negatively charged (−0.263), whereas PNMePh had a positive charge of +0.124. This indicated the effect of the electron-donating nature of the 9739

DOI: 10.1021/acs.jpcc.7b12461 J. Phys. Chem. C 2018, 122, 9738−9745

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The Journal of Physical Chemistry C Table 1. Mulliken Electric Charge Distribution of Phosphoric Ester Amides solvent

P in P−N

N in P−N

O in P0

0 in P−O−C

0 in P−O−C

PNMeMe PNMePh

0.330 0.002

−0.263 0.124

−0.169 −0.116

−0.161 −0.131

−0.176 −0.139

the reductive decomposition of PNMePh. The superconcentrated electrolyte of PNMePh had a peak at 1.95 V, reflecting lithium stripping. The efficiency of lithium plating/stripping increased with cycle number (Figure S5). Therefore, the high concentration enabled elongation of the electrochemical window and improved electrochemical stability.21,22 The CV curves of the dilute and superconcentrated electrolytes in the high potential region are displayed in Figure S6. The rise in the oxidative current, which began close to 4.2 V at 25 °C, in the two electrolytes was unchanged. The CV test was also done for the PNMeMe electrolytes, and the results were the same as those using the PNMePh electrolytes. As shown in Figure S7, partially reversible Li plating/stripping was observed in the electrolyte with [LiFSA]/[PNMeMe] = 0.5. The oxidative current was observed near 5.1 V vs Li+/Li. 3.2. Thermal Stability of Highly Concentrated Electrolyte. Thermal stability of the electrolyte in the presence of charged active materials was determined using differential scanning calorimetry (DSC) under an argon flow. Graphite was examined with a 100% state of charge (SOC). Previous DSC measurements indicated an exothermic reaction of the charged graphite with a typical nonaqueous electrolyte that occurred between 120 and 140 °C.23,24 Figure 5a shows DSC profiles for charged graphite with an SOC of 100% immersed in various electrolytes. In the 1 mol/L LiPF6-EC + DMC electrolyte, three peaks were observed at 140, 265, and 298 °C. The first small signal was due to decomposition of the passivation layer on the charged graphite, and the two latter peaks reflected reactions between LiPF6 and the alkyl carbonate solvents.25,26 The heat flow of the main peak observed at 298 °C was 10.4 W/g (watts per g of graphite). The amount of heat generated in the total reaction ΔHf was 2800 J/g. The superconcentrated electrolyte with PNMeMe ([LiFSA]/[PNMeMe] = 0.5) exhibited three exothermic peaks between 200 and 300 °C: 220 °C, 257 °C, and 288 °C. The heat flow of the main peak observed at 288 °C was 7.78 W/g (watts per g of graphite). The amount of heat generated in the total reaction ΔHf was 3100 J/g. Thus, a reduction in the thermal stability of the anode is a serious disadvantage when using PNMeMe against a standard electrolyte. In contrast, the PNMePh system had smaller exothermic peaks and a much lower ΔHf of 2350 J/g. The first two signals for the PNMePh system were smaller than those for the two

phenyl substituent, leading to reduction in the oxidative potential for PNMePh. We calculated an optimized geometry of LiFSA solvated by two PNMePh molecules, [LiFSA]/ [PNMePh] = 0.5, to estimate Mulliken electric charge distribution. The solvation structure is shown in Figure 3.

Figure 3. Optimized geometry of Li+-solvated structure for [LiFSA]/ [PNMePh] = 0.5. The symbols represent phosphorus (orange), nitrogen (blue), oxygen (red), fluorine (light blue), carbon (dark gray), sulfur (yellow), and hydrogen (light gray).

The coordination of the solvent to the Li ion occurs through the oxygen atom in the OCH2CF3 group of PNMePh. The electric charges on phosphorus and nitrogen in one PNMePh of the solvated structure were +0.002 and +0.330, respectively (Figure S4). The other PNMePh molecule had a negatively charged phosphorus (−0.434) and a positively charged nitrogen (+0.252). The results denoted the similar tendency as pure PNMePh solvent. Cyclic voltammetry for the phosphoric acid ester amide electrolytes ([LiFSA]/[PNMePh] = 0.125 and 0.5) was performed at 25 °C using a three-electrode cell with Ag+/Ag reference electrode [BAS, RE-7 electrolyte: acetonitrile solution containing 0.01 M AgNO3 and 0.1 M (C4H9)4NClO4]. The working electrode for potential greater than 3.0 V vs Li+/Li was a platinum plate, whereas a glassy carbon electrode was used as the working electrode to avoid Li−Pt alloys. As shown in Figures 4a and 4b, large cathodic currents due to lithium plating were observed below 0.5 V (vs Li+/Li) in the PNMePh electrolytes. In dilute electrolyte, the current corresponding to Li stripping was not observed, and no stripping was caused by

Figure 4. CV profiles for the PNMePh electrolytes: (a) [LiFSA]/[PNMePh] = 0.125 and (b) [LiFSA]/[PNMePh] = 0.5. Cycle number was 1 (black), 2 (pink), 3 (orange), 4 (blue), and 5 (green). 9740

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Figure 5. DSC profiles for various electrolytes with immersed (a) graphite and (b) LiNi0.8Co0.15Al0.05O2 with 100% SOC; highly concentrated electrolytes of [LiFSA]/[PNMeMe] = 0.5 (green), [LiFSA]/ [PNMePh] = 0.5 (red), and 1 mol/L LiPF6/EC + DMC (black).

phosphoric acid ester amides, and the main peak was shifted to a higher temperature (5.39 W/g, at 339 °C). To investigate reactivity of the charged cathode material with the electrolyte, DSC profiles were recorded for LiNi0.8Co0.15Al0.05O2 powders with 100% SOC in the presence of several electrolytes (Figure 5b). In this study, the LiPF6/EC + DMC electrolyte also had two exothermic peaks with calorific values of 6.12 W/g (watts per g of Li0.52Ni0.8Co0.15Al0.05O2) at 240 °C and of 1.74 W/g at 285 °C, with a ΔHf of 2050 J/g. The former signal was due to the main reaction with the electrolyte, attributed to a structural change in the delithiated cathode accompanied by oxygen liberation and combustion of the electrolyte with the liberated oxygen. The subsequent peak observed at 290 °C was caused by reaction of the remaining electrolyte with the cathode.27 The DSC profiles for the highly concentrated electrolytes with PNMeMe or PNMePh are shown as green and red lines, respectively, in Figure 5b. A small signal observed near 180 °C for both electrolytes was identified as decomposition of the passivation film on the charged cathode. Compared to the standard electrolyte, the two DSC peaks for the PNMeMe electrolyte shifted to a higher temperature by 22 °C. The PNMeMe system first showed a peak at 262 °C, with a heat flow of 6.47 W/g, and a second peak with a heat flow of 2.21 W/g at 329 °C (ΔHf = 2350 J/g). In contrast, the [LiFSA]/[PNMePh] = 0.5 electrolyte had two large signals between 250 and 360 °C with respective ΔHf values of 3.11 and 1.49 J/g. The total amount of heat in the reactions was ΔHf = 1600 J/g. The separation between the exothermic peaks of the PNMePh and standard systems was 45 °C (285 °C vs 240 °C). Therefore, the highly concentrated electrolyte using PNMePh had good thermal stability against the charged LiNi0.8Co0.15Al0.05O2 cathode material. Thus, introduction of a phenyl group into the amide improved thermal stability. 3.3. Ignition Test. Ignition behavior of the highly concentrated electrolytes was investigated with a [LiFSA]/ [solvent] of 0.5. For comparison, highly concentrated electrolyte with PC as the solvent ([LiFSA]/[solvent] = 0.5), as well as the typical LiPF6-EC + DMC electrolyte, were also prepared. Ignition tests were performed using glass fiber filter papers saturated with the electrolytes. After the electrolyte was applied to the paper, a flame at 700 °C was introduced using a gas burner. Figure 6 shows photographs of the glass fiber filter papers during the ignition test. The typical LiPF6/EC + DMC electrolyte and the superconcentrated electrolyte with PC were flammable (time until the ignition: within 0.2 s), whereas the highly concentrated electrolytes with the phosphoric acid ester amides (PNMeMe, PNMePh) did not burn in the test over 15 s, indicating that they were self-extinguishing.

Figure 6. Photographs of ignition tests using (a) highly concentrated electrolytes of [LiFSA]/ [PNMeMe] = 0.5, (b) [LiFSA]/[PNMePh] = 0.5, (c) [LiFSA]/[PC] = 0.5, and (d) 1 mol/L LiPF6-EC + DMC. Glass fiber filter papers were saturated with electrolyte, and ignition was attempted using a gas burner.

3.4. Raman Studies of Electrolytes Employing Phosphoric Acid Ester Amide. The electrochemical and thermal stabilities and ignition test led to a focus on PNMePh in subsequent experiments. Raman spectroscopic measurements for phosphoric acid ester amide electrolytes with various molar ratios of LiFSA to PNMePh were conducted first to elucidate the coordination of Li ions with the solvent molecules. Phosphoric acid ester amides have characteristic Raman signals for PO and P−N stretching. The spectra span the regions of 680−860 cm−1 and 1200−1350 cm−1 (Figures 7a and 7b). Raman profiles of LiFSA powder and pure PNMePh solvent are also shown in Figures 7a and 7b. The signal near 710 cm−1 was due to the stretching vibration of P−O−C, adjacent to the P O bond, in PNMePh, and the peak located at 840 cm−1 was assigned to the stretching vibration of P−N.28−30 As the concentration of LiFSA increased, the former signal shifted to higher Raman wavenumber by 6 cm−1, which suggests coordination of PNMePh molecules with Li ions. However, the signal for the P−N stretching vibration was impervious to high concentrations. The waveform separation for the 710 cm−1 peak represented free and solvated PNMePh molecules. The peak was fitted by two components with peaks of 710 and 716 cm−1. The proportion of the former decreased with [LiFSA]/ [PNMePh] molar ratio, whereas the proportion of the latter increased (Table 1), which reflect free and solvated PNMePh molecules, respectively.31−33 Upon addition of LiFSA salt, a new Raman band appeared near 730 cm−1, assigned to the S−N vibration in LiFSA. Upon an increase in Li+ ion concentration, this band was intensified and red-shifted. This spectral behavior was reported by Umebayashi et al.34 The phosphoric acid ester amide electrolytes produced three peaks between 1200 and 9741

DOI: 10.1021/acs.jpcc.7b12461 J. Phys. Chem. C 2018, 122, 9738−9745

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Figure 7. Raman spectra of the LiFSA-PNMePh electrolytes. Ratios indicate the molar ratio of LiFSA to PNMePh; [LiFSA]/[PNMePh] = 0.125 (pink), [LiFSA]/[PNMePh] = 0.25 (green), and [LiFSA]/[PNMePh] = 0.5 (red). Blue and black lines represent Raman profiles of LiFSA powder and pure PNMePh solvent, respectively.

1350 cm−1. The peak located near 1260 cm−1 was assigned to the PO stretching vibration in PNMePh. As the concentration of LiFSA was increased, the peak shift at 1260 cm−1 became more significant, up to 6 cm−1 in the superconcentrated solution. According to the waveform separation, two components with peaks of 1260 and 1267 cm−1 were obtained, which corresponded to free and solvated PNMePh molecules, respectively (Table 2). The signals near 1220 and 1290 cm−1 Table 2. Ratio of free and solvated solvents Low Wavenumber

High Wavenumber

electrolyte

710 cm−1

716 cm−1

1260 cm−1

1267 cm−1

[LiFSA]/[PNMePh]

free solvent

solvated solvent

free solvent

solvated solvent

0 0.125 0.25 0.50

100 76 65 46

0 24 35 54

100 82 67 61

0 18 33 39

Figure 8. Charge/discharge curves of graphite/Li half cells with electrolytes composed of LiFSA and PNMePh at 60 °C. Ratios indicate the molar ratio of LiFSA to PNMePh. The charge/discharge current was 0.03 mA/cm2. The light green line represents the charge/ discharge curve for the cell using [LiFSA]/[PNMePh] = 0.5 electrolyte with 2% dissolved vinylene carbonate.

were due to the SO stretching vibration in LiFSA35 and deformation mode of P−O−CH2CF3 in PNMePh. The former signal was intensified and red-shifted with increasing LiFSA concentration, which reflected bulk (free) FSA− and that bound to Li+ ions. The signal on the deformation mode was uninfluenced. The results shown in Figures 7a and 7b support the solvation of Li ions by the PNMePh solvent and the structures and conformations of solvates such as contact ion pairs and aggregates.36−38 3.5. Intercalation of Li Ions into Active Materials. Although Li ion insertion into graphite is unlikely to occur in 1 mol/L of LiPF 6 −PC and LiFSA−trimethyl phosphate solutions, a high concentration of Li salt in the electrolyte improves Li-ion-insertion behavior.14−17 Nonaqueous electrolytes were prepared with various concentrations of LiFSA in PNMePh solvent, followed by application to the graphite/Li cell. Figure 8 shows charge−discharge curves for the graphite/ Li half cells at 60 °C. The test started at charging (i.e., insertion of Li ions into graphite). The cell using the electrolyte with a molar ratio of [LiFSA]/[PNMePh] = 0.125 charged at high potentials; however, it could not discharge sufficiently (charge capacity = 315 mAh/g, discharge capacity = 8 mAh/g). The charge plateau near 1 V was due to the cointercalation of PNMePh molecules with Li ions and subsequent destruction of the graphite structure.39,40 The electrolyte with [LiFSA]/ [PNMePh] = 0.25 enabled Li-ion intercalation into graphite. The charge and discharge capacities were 352 mAh/g and 92 mAh/g, respectively. In contrast, the electrolyte with [LiFSA]/

[PNMePh] = 0.5, i.e., highly concentrated electrolyte, had a profile similar to that of the conventional electrolyte system (1 mol/L LiPF6/EC + DMC). The three potential plateaus, characterized as Li+ insertion into graphite, appeared clearly at 0.175, 0.091, and 0.056 V for the [LiFSA]/[PNMePh] = 0.5 electrolyte,41 and charging up to 360 mAh/g was possible. When the polarity of the current was changed, three discharge plateaus corresponding to the charge behavior were fuzzy. Total discharge capacity was 124 mAh/g. Thus, the highly concentrated LiFSA in PNMePh enabled repeated Li-ion insertion and extraction and enhanced the reversible capacity. The mechanism of Li-ion insertion in the highly concentrated electrolyte currently is not well understood. Insertion may be affected by the passivation layer (solid electrolyte interphase, SEI) at the surface of the graphite anode. The SEI formation is a factor that made high irreversible capacity (236 mAh/g). Low ionic conductivity of [LiFSA]/[PNMePh ] = 0.5 electrolyte (0.07 mS/cm at 25 °C) is an additional factor of the poor discharge behavior. It is incomparable with that of the conventional electrolyte (10.46 mS/cm for 1 mol/L LiPF6EC + DMC). We described temperature dependence of ionic conductivity for both electrolytes in the Supporting Information. The DFT calculations and CV measurements for the PNMePh electrolyte revealed reductive decomposition near 0.5 V vs Li+/Li, indicating SEI formation. To enhance discharge capacity for the cell using [LiFSA]/[PNMePh ] = 0.5 electrolyte, vinylene carbonate (VC) was added to the 9742

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The Journal of Physical Chemistry C electrolyte.42,43 Since decomposition of VC occurs at 1.0 V, film formation occurs before decomposition of PNMePh. The effect of VC addition affected discharge behavior of the cell (Figure 8). Discharge capacity increased 2-fold (from 124 mAh/g to 245 mAh/g). The cycle performance up to the 50th cycle is shown in Figure S11. Figure 9 shows the Cole−Cole plots for the graphite/Li cells before and after the first charge/discharge cycle at 60 °C. Two

SO2 asymmetric vibrations, respectively.44−46 For PNMePh, the P−O symmetric vibration was located at 763 cm−1. The symmetric and asymmetric vibrations of CF3 appeared at 960 and 1162 cm−1, respectively. The peak at 1060 cm−1 reflected deformation of P−O−CH2CF3. Two signals at 1265 cm−1and 1288 cm−1 were assigned to stretching vibrations of PO and P−N, respectively. Two strong signals between 1400 and 1500 cm−1 were due to amino groups. Several FTIR signals were observed for SEI on graphite without VC additive and were assigned to PNMePh: 862 cm−1, 961 cm−1, 1087 cm−1, 1278 cm−1, 1417 cm−1, and 1483 cm−1. The peak at 1151 cm−1 assigned to the symmetric stretching of SO2 was not clear. No stretching modes of CH2CF3 groups were observed at 2846 cm−1, 2958 cm−1, or 2917 cm−1. These signals were also detected in spectra of SEI on graphite with VC additives. In addition, a peak shift near 848 cm−1 was observed, which suggests the inclusion of FSA ions (S−N−S unit). The absorbances between 1400 and 1500 cm−1, due to a decreased number of amino units in cells with VC addition, indicate a small amount of PNMePh degradation. Cell performance for LiNi0.8Co0.15Al0.05O2/Li cells using the electrolytes [LiFSA]/[PNMePh] = 0.125 and 0.5 is shown in the Supporting Information (Figures S13−S14). A large capacity drop-off in dilute electrolyte ([LiFSA]/[PNMePh] = 0.125) was detected at 60 °C. When the cell was dismantled after the test, the transparent electrolyte was found to have changed to a purple liquid (Figure S15). A similar phenomenon was observed in a storage test of LiNi0.8Co0.15Al0.05O2 cathode dipped into a dilute solution of the [LiFSA]/[PNMePh] = 0.125 electrolyte at 60 °C. Therefore, the coloring of the electrolyte may be caused by release of Co or Ni from LiNi 0.8 Co 0.15 Al 0.05 O 2 . The Cole−Cole plots for LiNi0.8Co0.15Al0.05O2/Li cells using the [LiFSA]/[PNMePh] = 0.5 electrolyte were shown in Figure S16. A large semicircle and another small one were obtained in the pristine sample. The former semicircle at the high-frequency region increased its diameter during the cycle. It indicates that the reductive decomposition of PNMePh at the surface of Li metal proceeded fairly. The latter semicircle at the low-frequency region was unchanged before and after the first charge/ discharge cycle at 60 °C, suggesting that a passive layer at the cathode was not thicker.

Figure 9. Cole−Cole plots for graphite/Li cells using electrolytes of [LiFSA]/[PNMePh] = 0.5 with no additive (red) and 2% VC (green). The open and solid symbols represent the cells before and after the first charge/discharge cycle at 60 °C, respectively.

semicircles were observed in all samples. The semicircle between 5 kHz and 12 MHz reflects interface resistance at Li metal. The semicircle appearing at the low-frequency region should correspond to the interface between electrolyte and graphite. The two semicircles became clearly larger after the charge/discharge process. Therefore, the large polarization of graphite/Li cells at the first cycle was caused by resistance increase at both electrodes. When comparing the samples with and without VC after the charge/discharge test, the diameter of the semicircle at the low-frequency region significantly increased in no additive system. Thus, the addition of VC into the electrolyte is effective to suppress the reductive decomposition of the PNMePh solvent at graphite. FTIR spectra were obtained to analyze the SEI on graphite in graphite/Li cells using the [LiFSA]/[PNMePh] = 0.5 electrolytes (Figure 10). The FTIR profiles of LiFSA powder and PNMePh in the region of 600−1600 cm−1 are shown in the Supporting Information (Figure S12). In the FTIR spectrum for LiFSA powder, signals at 759 cm−1, 854 cm−1, 1176 cm−1, and 1365 cm−1 were assigned to symmetric S−N−S vibrations, asymmetric S−N−S vibrations, symmetric SO2 vibrations, and

4. CONCLUSIONS Alkyl phosphates have been investigated because of their nonflammability characteristics. However, their poor Li-ion insertion capability and their instability under overcharge have limited their application to Li-ion battery technologies. A previous study demonstrated that a superconcentrated fluorinated alkyl phosphate electrolyte possessed excellent Liion insertion capability and good thermal stability under overcharge. The present study examined phosphoric acid ester amides having two fluorinated alkyl groups and one amino group as a new class of self-extinguishing solvent. Results showed that introduction of a phenyl substituent into the amino group (PNMePh) provided greater thermal stability than that of the fluorinated phosphate. Although the reductive potential of PNMePh was 0.61 V vs Li+/Li, it was lowered by a superconcentrated solution of PNMePh. Therefore, Li-ion insertion was observed in the PNMePh electrolyte. Although the Li-ion intercalation was largely irreversible, it could be reversed by addition of vinylene carbonate. Partially reversible Li plating−stripping was also observed in the superconcen-

Figure 10. FTIR spectra of the graphite anode after the first charge/ discharge cycle of the graphite/Li cell using electrolytes of [LiFSA]/ [PNMePh] = 0.5 with no additive (red) and 2% VC (green). 9743

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trated electrolyte. Thus, two major issues with alkyl phosphates were resolved by highly concentrated electrolyte of LiFSA and PNMePh.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12461. Test apparatus, DFT calculation data, typical CV profiles, DSC curves, Raman and FTIR profiles, and cell performance of graphite/Li and LiNiO2Li cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-561-71-7607. ORCID

Tohru Shiga: 0000-0001-7331-3380 Author Contributions

T.S. conceived and conducted the experiments, analyzed the data, and wrote the paper. C.O. analyzed the DSC data; H.K. provided the DFT calculations; and Y.K performed the Raman analysis. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Shigeharu Kawauchi of Toyota CRDL for ionic conductivity measurement. REFERENCES

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