Synergistic Effect of Partially Fluorinated Ether and Fluoroethylene

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Synergistic Effect of Partially Fluorinated Ether and Fluoroethylene Carbonate for High-Voltage Lithium-Ion Batteries with Rapid Chargeability and Dischargeability Choon-Ki Kim, Koeun Kim, Kyomin Shin, Jung-Je Woo, Saheum Kim, Sung You Hong, and Nam-Soon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12352 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Synergistic Effect of Partially Fluorinated Ether and Fluoroethylene Carbonate for High-Voltage Lithium-Ion Batteries with Rapid Chargeability and Dischargeability Choon-Ki Kim, ‡1 Koeun Kim, ‡1 Kyomin Shin,2 Jung-Je Woo,2 Saheum Kim,2 Sung You Hong,1 and Nam-Soon Choi*1 1

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Republic of Korea

2

Research & Development Division, Hyundai Motor Company, 772-1, Jangduk-dong, Hwaseong-si, Gyeonggi-do 445-706, Republic of Korea

‡These authors contributed equally. *Corresponding author : [email protected]

KEYWORDS : partially fluorinated ether, fluoroethylene carbonate, solid electrolyte interphase, 5V-class spinel-type cathode, lithium-ion battery

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

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: The roles of a partially fluorinated ether (PFE) based on a mixture of

1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane and 2-(difluoro(methoxy)methyl)-1,1,1,2,3,3,3heptafluoropropane on the oxidative durability of an electrolyte under high voltage conditions, the rate capability of the graphite and 5 V-class LiNi0.4Mn1.6O4 (LNMO) electrodes, and the cycling performance of graphite/LNMO full cells are examined. Our findings indicate that the use of PFE as a co-solvent in the electrolyte yields thermally stable electrolytes with selfextinguishing ability. Electrochemical tests confirm that the PFE combined with fluoroethylene carbonate (FEC) effectively alleviates the oxidative decomposition of the electrolyte at the highvoltage LNMO cathode and enables reversible electrochemical reactions of the graphite anodes and LNMO cathodes at high rates. Moreover, the combination of PFE, which mitigates electrolyte decomposition at high voltages, and FEC, which stabilizes the anode-electrolyte interface, enables the reversible cycling of high-voltage full cells (graphite/LNMO) with a capacity retention of 70.3% and a high Coulombic efficiency of 99.7% after 100 cycles at 1C rate at 30 °C.

1. INTRODUCTION Lithium-ion batteries (LIBs) have garnered a great deal of research attention in recent years as batteries with transportation applications.1–3 To attain good performance in terms of energy and power density, battery systems with high working potentials, high reversible capacities, and good rate capabilities must be developed.4 Spinel-type LiNixMn2-xO4 (x = 0.4–0.6) has recently gained recognition as a promising cathode material for high-energy-density LIBs, largely on the basis of its high operating potential of 4.7 V vs. Li/Li+, its low cost, and the high Li+-ion diffusivity within the three-dimensional channels of the spinel structure.5–7 However, high-


2

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voltage LiNixMn2-xO4 suffers from the severe oxidative decomposition of conventional carbonate-based electrolytes, limiting its window of electrochemical stability.8–10 The oxidative decomposition of the solvents in the electrolyte can be alleviated by decreasing their highest occupied molecular orbital (HOMO) energy levels. In this regard, fluorinated organic solvents have been proposed as promising high-voltage solvents owing to their low HOMO energy levels compared to those of non-fluorinated solvents.11–17 In addition, partially fluorinated ether solvents such as nonafluorobutyl methyl ether and 1,1,1,3,3,3,-hexafluoroisopropryl methyl ether can be used to reduce the flammability of organic liquid electrolytes because they have no flash point.11–13 The rate capability of batteries is highly affected by the electrode density (thickness and porosity), charge transport at the electrode-electrolyte interface, and electronic conduction at the electrode-current collector interface.18.19 The use of partially fluorinated ether solvents with low viscosities and high resistance to oxidation is expected to improve the wettability of the electrolyte toward the high-density electrode and the electrochemical performance of the highvoltage batteries at high rates. Herein, we demonstrate that the combination of the partially fluorinated ether (PFE) based on a mixture of 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane and 2-(difluoro (methoxy)methyl)1,1,1,2,3,3,3-heptafluoropropane and fluoroethylene carbonate (FEC) allows satisfactory anodic stability, fast charge transport at the interface between the high-density electrode and the electrolyte, and superior cycling stability for graphite anodes and LiNi0.4Mn1.6O4 (LNMO) cathodes at high rates. Furthermore, our investigation reveals that carbonate-based electrolytes combined with PFE have self-extinguishing ability.


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2. EXPERIMENTAL SECTION 2.1. Preparation of electrolytes and electrodes. Partially fluorinated ether (99%, a mixture of 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane

(PFE-1)

and

2-(difluoro(methoxy)-methyl)-

1,1,1,2,3,3,3-heptafluoropropane (PFE-2)) were obtained from Sigma-Aldrich. From the relative peak area in

19

F NMR spectra of Figure S1, we could estimate that the weight ratio between

PFE-1 and PFE-2 is 1:1.7~1.8. Structural stability of PFE-1 and PFE-2 during storing with and without the exposure to light for 10 days in a glove box was confirmed through 1H and

19

F

nuclear magnetic resonance (NMR) analysis (Figure S2). Ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and lithium hexafluorophosphate (LiPF6) were purchased from Soulbrain Co., Ltd. The electrolytes consisted of 1 M LiPF6 dissolved in FEC/EMC/PFE (FEC-PFE), FEC/EMC/DEC (FEC-DEC), FEC/EMC/DMC (FEC-DMC), and EC/EMC/DEC (EC-DEC) at a 1:6:3 volume ratio. A slurry was prepared by mixing 90 wt.% LNMO (supplied from Hyundai Motor Company) particles as an active material, 5 wt.% carbon black as a conducting material, and 5 wt.% polyvinylidene fluoride (PVDF) binder dissolved in anhydrous N-methyl-2-pyrrolidinone (NMP, 99.5%, Sigma-Aldrich). The resulting slurry was cast on an aluminum foil and then dried in a convection oven at 80 °C for 30 min. After drying, the electrode was pressed to a thickness of approximately 40 µm. The composite cathodes were dried under vacuum at 120 °C for 12 h prior to their assembly into cells. The specific capacity and mass loading of the cathodes for the halfcells were 1.4 mAh cm-2 and 11.7 mg cm-2, respectively. The specific capacity and mass loading of the thick cathode used in the full cells for cycling performance testing were 2.37 mAh cm-2 and 19.7 mg cm-2, respectively. The anode was composed of 94 wt.% graphite as the active material, 3 wt.% carbon black as the conducting material, and 3 wt.% binder (1.8 wt.% styrene–


4

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butadiene rubber + 1.2 wt.% sodium carboxymethyl cellulose). The specific capacity and active material mass loading of the anode were 2.95 mAh cm-2 and 8.20 mg cm-2, respectively. A 2032 coin-type cell was assembled in an argon-filled glove box with oxygen and moisture contents lower than 1.0 ppm. The thickness and porosity of the microporous polyethylene (SK Innovation Co., Ltd.) used as a separator were 20 µm and 38%, respectively. 2.2. Electrochemical tests. Li/LNMO half-cells were galvanostatically precycled at C/5 rate between 3.5 and 5.0 V at 30 °C using a computer-controlled battery testing apparatus (WonATech WBCS 3000). Thereafter, the charge rate capabilities of Li/LNMO, Li/graphite half-cells, and graphite/LNMO full cells with different electrolytes were evaluated at various charges from C/2 to 7C. A fixed C-rate of C/2 for the discharge process was applied to all cells. The discharge rate capabilities of the Li/LNMO, Li/graphite half-cells, and graphite/LNMO full cells with different electrolytes were measured at various discharge rates from C/2 to 7C. A fixed C-rate of C/2 for the charging process was applied to all the cells. Constant voltage conditions for the Li/graphite half-cells and graphite/LNMO full cells were applied at the end of charging at each cycle until the current was below C/20. Electrochemical floating tests for the Li/LNMO half-cells with three electrolytes at constant voltages of 5.0 and 5.3 V versus Li/Li+ were performed for 10 h. The graphite/LNMO full cells were galvanostatically precycled at a rate of C/10 between 3.0 and 4.9 V at 30 °C. Constant voltage conditions were applied at the end of the precycle charging until the current was below C/50. For the evaluation of cycling performance, following the precycle, the full cells were cycled at a rate of 1C between 3.0 and 4.9 V at 30 °C. Constant voltage conditions were applied at the end of charging following every cycle until the current was less than C/20.


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2.3. Characterization. For the ab initio calculation of the oxidation tendency of the organic solvents, the optimized geometries of the solvent molecules were obtained using Gaussian 09. Molecule optimization was carried out using density functional theory (DFT) at the B3LYP/6311+G level of theory. Self-extinguishing time of each electrolyte was measured. The electrolyte of 0.5 g was soaked into a 3 × 6 cm2 piece of porous glass filter paper. The flame source was removed after 2 s of ignition. The viscosity and ionic conductivity of the electrolytes were measured at 30 and 25 °C using a Brookfield viscometer (LVDV-ll+P) and an Oakton CON 11 standard conductivity meter, respectively. Contact angle measurements of the electrolytes on the LNMO cathodes and graphite anodes were carried out using Phoenix 300 and all snapshots were collected within 2 s after dropping a 10 µl volume of the electrolyte. To collect their electrodes for ex situ analysis, the full cells were carefully disassembled in a glove box after precycling. The electrodes were rinsed in dimethyl carbonate (DMC) to remove the residual LiPF6-based electrolyte and dried at room temperature. The X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher) measurements were performed with Al Kα (hν = 1486.6 eV) radiation under ultrahigh vacuum, using a 0.10 eV step size and 50 eV pass energy. All the samples were prepared in a glove box and sealed with aluminum pouch films under vacuum before use. All XPS spectra were calibrated by the hydrocarbon peak at a binding energy of 284.6 eV.

3. RESULTS AND DISCUSSION The oxidation and reduction tendencies of organic compounds can be theoretically predicted through a comparison of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels in a molecular orbital energy level diagram.


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Figure 1 shows the HOMO and LUMO energy levels of carbonate and the partially fluorinated ether (PFE) solvents. FEC, which is frequently used as a reducible additive in Li-ion cells, has a lower LUMO energy value than that of non-fluorinated carbonate solvents. This is the reason that FEC is electrochemically reduced before the reductive decomposition of non-fluorinated carbonate molecules at the anode. In addition, the FEC is expected to have a low oxidation tendency owing to its low HOMO energy compared with that of non-fluorinated carbonates. More fluorinated ether solvents (PFE-1 and PFE-2) have lower HOMO energy levels, indicating good oxidative durability. more reducible

more oxidative

-4

LUMO energy (eV)

-3

-2

-1

0

O

-8

EMC

O

O

DEC DMC

EC

-9

HOMO energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O F

-10 H3C

FEC

F

O

F

CF3

F F

F

PFE-1

O

O

F

PFE-2

F3C CF3

-11

H 3C

O

F

F

F

PFE : PFE-1+PFE-2

Figure 1. Chemical structures and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of carbonate solvents and partially fluorinated ether (a mixture of PFE-1 and PFE-2). PFE-1: 1,1,1,2,2,3,3,4,4-nonafluoro-4methoxybutane, PFE-2: 2-(difluoro(methoxy)methyl)-1,1,1,2,3,3,3-hepta-fluoropropane. The HOMO of EC, FEC, PFE-1, and PFE-2 is situated at the electron-rich motifs showing the donor character.


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Theoretical molecular orbital calculation confirmed that the introduction of fluorine atoms into organic molecules decreases their HOMO energy levels.20–22 EC has been widely exploited as an essential electrolyte solvent because of its high dissociation ability and its ability to form an appropriate solid electrolyte interface (SEI) layer on the carbonaceous anode.23 Unfortunately, because EC has a low affinity with fluorinated ether solvents, phase separation is observed for EC/EMC/partially fluorinated ether (PFE: a mixture of PFE-1 and PFE-2) (1/6/3, v/v/v) with 1 M LiPF6 (E1) (Figure 2a). PFE, which has higher density relative to carbonates (Table S1), is thought to be located at the bottom of the glass vial shown in the image because of its low affinity with the LiPF6-based electrolyte. The solubility of LiPF6, which is an indispensable electrolyte salt in commercialized Li-ion batteries, is limited in fluorinated ether-containing electrolytes

because

of

its

low

dissociation

constant.22,24

Thus,

lithium

bis(trifluoromethanesulfonyl)imide and lithium bis(perfluoroethylsulfonyl)imide, both of which have high dissociation abilities, have been used as Li salts for the study of fluorinated ethercontaining electrolytes.22–27 In the case of E1, the translucent top layer may be composed of EC/EMC with LiPF6 that is immiscible with PFE. Because EC, which has a high dielectric constant of 89.78 at 40 °C (Table S1), is responsible for the salt dissociation, most of the EC molecules preferentially coordinate with the LiPF6 salt in E1.23,28 This solvated EC seems to be immiscible with PFE in the LiPF6-based electrolyte (Figure 2a), and phase separation between the PFE and the LiPF6-based electrolyte occurs. Unlike E1, the E2 electrolyte, in which EC is replaced with FEC, is highly transparent. Notably, FEC, which has a high dielectric constant of 78.4 (Table S1), can coordinate with Li ions in E2 (Figure 2b).29,30 The effect of the Li salt on the miscibility of PFE with the electrolyte was confirmed (Figure S3).


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a

b

E1 O

O O LiPF6-based electrolyte immiscible with PFE

O

O O

O O

Immiscible CF3 F3C CF CF2

PFE layer

O

O

O

F3C

E2

O

O

O OCH3

O

O

O

O Li+ O O

O O

F

F2 C OCH 3

Miscible F3C CF CF 2 F 3C OCH3

F3 C

Phase separation

O

O

O

O

O

O

CF

CF

O

O

F3C

F2 C

F3C

O O

Li+

O

H3CO

O

O

O O

O

O

F2C F F3C

OCH3

CF CF3

No phase separation

E1 : 1M LiPF6 in EC/EMC/PFE (1/6/3, v/v/v), E2 (FEC-PFE) : 1M LiPF6 in FEC/EMC/PFE (1/6/3, v/v/v)

6.3

12.7

d

FEC-PFE

FEC-DEC

EC-DEC

EC-DEC

11.7

FEC-DEC

14 12 10 8 6 4 2 0

FEC-PFE

c

Self-extinguishing time (s g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Photographs showing a comparison of the compatibility of PFE with the electrolytes (a) without FEC and (b) with FEC. A schematic illustration of the phase separation induced by the immiscibility of PFE with the LiPF6-based electrolyte is also shown. (c) Self-extinguishing times of the electrolytes. (d) Photographs of combustion test.

PFE is completely miscible with the EC-EMC and FEC-EMC solvent mixtures in the absence of LiPF6. The solvated EC participating in the dissociation of the LiPF6 salt is thought to cause significant phase separation between the PFE and LiPF6-based electrolytes. Therefore, it is crucial that the chemical structure of the solvent species interacting with Li+ ions is properly selected to enable the successful application of fluorinated ether solvents to Li-ion cells. Because PFE, based on a partially fluorinated molecular architecture, has no flash point (Table S1), it is expected to mitigate the flammability of carbonate-based electrolytes. The action of PFE on the burning time of the electrolyte was evaluated by measuring the self-extinguishing time. Indeed,


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the burning time for the PFE-containing electrolyte (FEC-PFE) was drastically reduced, and it had relatively short normalized self-extinguishing time of 6.3 s g-1 compared with that of the PFE-free electrolytes (11.7 s g-1 for FEC-DEC and 12.7 s g-1 for EC-DEC). The flammability test result of electrolytes manifests that the replacement of the combustible DEC with PFE gives the electrolyte a self-extinguishing ability. To examine the oxidation durability of FEC/EMC/PFE (FEC-PFE), FEC/EMC/DEC (FECDEC), and EC/EMC/DEC (EC-DEC) electrolytes with 1 M LiPF6 in a highly oxidizing environments, electrochemical floating tests on Li/LNMO half-cells at constant voltages of 5.0 and 5.3 V vs. Li/Li+ were performed (Figure 3a and b). The currents evolved reflect the electrochemical decomposition of the electrolyte at a given voltage, and a sudden increase in the leakage current indicates the anodic limit of the electrolyte.14,31 Appreciable oxidative leakage currents at a constant voltage of 5.0 V were observed in the EC-DEC electrolyte (Figure 3a). This means that the EC-DEC electrolyte is prone to oxidative decomposition at the LNMO cathode. Clearly, the FEC-PFE electrolyte exhibits significantly reduced the normalized leakage currents compared with the EC-DEC electrolyte. The FEC-DEC electrolyte has similar leakage currents to those of FEC-PFE at a constant voltage of 5.0 V. A detailed comparison reveals that the replacement of DEC with the fluorinated ether solvent, PFE, leads to a further improvement in the oxidative stability of the electrolyte (Figure 3a). The FEC-DEC electrolyte, which produces a low leakage current at 5.0 V, generates significantly increased leakage currents at a constant voltage of 5.3 V (Figure 3b). The replacement of DEC with the PFE results in the generation of minimal leakage current at high voltage conditions. This is mostly because the HOMO energy level of PFE is much lower than that of DEC. DEC, which has a relatively high HOMO energy level, undergoes the cleavage of C-O bonds by losing one electron when the


10

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LNMO cathode is charged to 5.0 or 5.3 V and the formed radical cation (DEC•+) may produce various reactive compounds and CO2 gas, increasing the internal pressure in the cell (Figure 3c).32 The generated reactive species, such as the C2H5• radical, may attack the organic solvents (FEC, EMC, and DEC) and cause unwanted electrolyte decomposition, thus producing leakage currents. The decomposition of DEC in a highly oxidizing environment limits their application for high-voltage batteries. The superior oxidation stability of FEC-PFE can be explained by the absence of DEC and its high tendency to undergo oxidative decomposition.

Voltage (V)

5.3

Constant voltage region (monitoring leakage current)

5.0

80

5.0V

4.7

60

4.4

40

4.1

FEC-PFE FEC-DEC EC-DEC

3.8 3.5 0

3

20 0 6

9

12

b 5.6

Constant voltage region (monitoring leakage current) Constant current (charge)

5.3

Voltage (V)

100 Constant current (charge)

4.7

60

4.4

40

4.1


3.8 3.5

15

0

+

DEC •+ DEC

d 5.0

O O

O

DECH+, CO2↑, CH3CH2O, C2H5•

4.4

12

15

Mn3+→Mn4+

4.1 FEC-PFE FEC-DEC EC-DEC

3.5 0

PFE

9

Ni2+→ Ni3+→ Ni4+

3.8

DEC

6

4.7

Voltage (V)

5V-class LNMO cathode

O

0

3

+

Energy

O

-e-

20

Time (h) at 5.3V vs. Li/Li

Bond breaking by losing an electron O

Formation of radical cation

80

5.3V

5.0

Time (h) at 5.0V vs. Li/Li

c

100

Normalized leakage current

a 5.6

Normalized leakage current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

40

60

80 100 120 140 160 -1

Specific capacity (mAh g )

Figure 3. Charge voltage profiles and normalized leakage currents of Li/LNMO half-cells during electrochemical floating test at a charge voltage of (a) 5.0 and (b) 5.3 V vs. Li/Li+ at 30 °C. (c) Schematic drawing of the oxidative decomposition of DEC at the LNMO cathode. (d) Voltage profiles of Li/LNMO half-cells during precycling at a rate of C/5 at 30 °C.


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Figure 3d shows the charge and discharge profiles of the Li/LNMO half-cells during precycling at C/5 rate and 30 °C. The voltage plateaus at around 4.0 and 4.75 V correspond to the electrochemical oxidation of Mn3+ to Mn4+ and Ni2+ to Ni4+, respectively. The EC-DEC electrolyte delivers a relatively low discharge capacity of 118.5 mAh g-1 and unsatisfactory initial Coulombic efficiency (ICE) of 88.1% compared with those of the FEC-DEC and FECPFE electrolytes. This is most likely due to the electrochemical instability of the EC-DEC electrolyte at high voltages, as shown in Figure 3a and b, which consumes Li ions and electrons related to the reversible capacity of the cathode. In contrast, the cell containing the FEC-PFE electrolyte exhibited a higher discharge capacity of 127.6 mAh g-1 with a superior ICE of 94.7%. This increased discharge capacity of the LNMO cathode during the precycling may be ascribed to the superior oxidation durability of the FEC-PFE electrolyte. This outstanding oxidation durability of FEC-PFE maintained the long-term electrochemical reversibility of the LNMO cathode at 1C rate and 30 °C and led to excellent capacity retention of 94.2% with a high Coulombic efficiency (CE) of ~99.4% after 200 cycles (Figure S4). The FEC-DEC electrolyte, which had no significant oxidation currents at 5.0 V, achieved an improved capacity retention of 93.4%. On the other hand, EC-DEC electrolyte was detrimental to the cycling stability of the LNMO cathode at high voltage of 5.0V due to its severe electrolyte decomposition as shown in Figure 3a and b. To examine the rate capability of the LNMO cathode at 30 °C, the cathode was cycled at different charge and discharge C-rates, as shown in Figure 4a and e. The FEC-PFE electrolyte, which has vastly superior oxidation stability at 5.0 and 5.3 V compared to the other electrolytes, facilitated fast charge transport at high charge and discharge C-rates and, thus, delivered a high discharge capacity of more than 100 mAh g-1 at 5C rate. In particular, the FEC-PFE displayed


12

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outstanding quick chargeability even at high charge rates, forcing large amounts of Li+-ions out of the electrode, and the voltage rise by polarization was not significant at high charge C-rates, such as 5C, compared with those of FEC-DEC and EC-DEC electrolytes (Figure 4b-d). Furthermore, the electrolyte should remain in the cathode under the strong electric field from the cathode to the electrolyte induced by the high charge currents, and the uniform distribution of the electrolyte in the cathode will enable facile transport of Li ions. In contrast, the FEC-DEC and EC-DEC electrolytes showed poor chargeability, delivering a very low discharge capacity of less than 30 mAh g-1 at a discharge rate of C/2 after charging at 3C rate (Figure 4a). Notably, the potential of the Li/LNMO half-cells with FEC-DEC or EC-DEC rapidly reached the charge cutoff potential of 5.0 V because of the large polarization at a high rate (3C) (Figure 4c and d), and, consequently, the charging (delithiation) process of the LNMO cathode did not occur correctly. In the case of discharge rate capability, the EC-DEC electrolyte, which has inferior oxidation stability at high voltages (Figure 3a and b), caused abnormal charging at a relatively low charge rate of C/2 after discharge over five cycles at 1C rate (Figure 4h). The poor rate capability of the EC-DEC electrolyte at various charge and discharge rates may be attributed to the formation of a resistive surface film produced by its severe oxidative decomposition at the LNMO cathode. Unlike the EC-DEC electrolyte, the FEC-DEC electrolyte showed an improved discharge rate capability (Figure 4e and g). This finding suggests that when the cathode is charged in the FEC-DEC electrolyte at a low charging C-rate of C/2, the FEC-DEC electrolyte does not experience severe oxidative decomposition at the LNMO cathode. Accordingly, the FEC-DEC electrolyte forms a surface film that is less resistive to charge transport and, thus, delivers improved discharge capacity at high discharge C-rates compared to the EC-DEC electrolyte.


13


1C

C/2

140 120 100 80 60 40 20 0

3C

7C

5C

e 160

C/2

Discharge capacity (mAh g-1)


a 160


0

5

10

15

20

25

30

1C

C/2

140 120 100 80 60 40 20 0 0

Voltage (V)

C/2 1C 3C 5C 7C

Voltage (V)

f

FEC-PFE

0

20

40

60

80

5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

5

10

0

100 120 140

FEC-DEC

40

60

80

5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

C/2 1C 3C 5C 7C

EC-DEC

20

40

60

80

100 120 140 -1


40

60

80

100 120 140

20

40

60

80

100 120 140

Specific capacity (mAh g-1)

h 5.0 4.8 Voltage (V)

d 5.0 4.8

20

FEC-DEC

-1

0

30

C/2 1C 3C 5C 7C

0

100 120 140


4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

25

-1

Voltage (V)

Voltage (V)

C/2 1C 3C 5C 7C

20

20


g

0

15

FEC-PFE

Specific capacity (mAh g ) 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

C/2

C/2 1C 3C 5C 7C

-1

c

7C

5C

Cycle number

b 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

3C


Cycle number

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

C/2 1C 3C 5C 7C

4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

EC-DEC

0

20

40

60

80

100 120 140 -1


Figure 4. (a) Charge rate capability and (b–d) voltage profiles of the Li/LNMO half-cells from C/2 to 7C. A fixed discharge C-rate of C/2 was applied. (e) Discharge rate capabilities and (f–h) voltage profiles of the Li/LNMO half-cells from C/2 to 7C. The charge C-rate was fixed at C/2. Voltage profiles of half-cells were selected from the 5th cycle for each C-rate.


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In addition to the interfacial characteristics between the cathode and the electrolyte, the kinetics of the electrochemical reactions at the electrode at high C-rates is affected by the ionic conductivity of the electrolyte and the impregnation degree of the electrolyte throughout the pores of the electrode.18,33,34 Figure 5a gives an insight into the influence of the viscosity of electrolytes as a factor affecting the rate capability of a highly pressed LNMO cathode with high mass loading (11.7 mg cm-2) in the FEC-PFE, FEC-DEC, FEC-DMC and EC-DMC electrolytes. Although the FECDEC electrolyte has higher ionic conductivity relative to that of the FEC-PFE electrolyte, it delivers a noticeably reduced discharge capacity as a function of the applied C-rate. The use of PFE, which has the lowest viscosity of 0.52 cP in Table S1, led to a reduction in the viscosity of the FEC-PFE electrolyte (Figure 5b). These properties are expected to allow sufficient impregnation of the electrolyte into the high-density electrode and good performance in terms of the rate capability. Indeed, the cathode with the FEC-DMC electrolyte delivered a clearly improved discharge capacity compared to the FEC-DEC electrolyte (Figure 5a). When the voltage stability of the FEC-DMC electrolyte was evaluated with electrochemical floating test, it brings slightly higher oxidation current compared to the FEC-DEC electrolyte (Figure S5). Therefore, it is rationale that the enhanced discharge rate capability of the FEC-DMC electrolyte is mostly due to its low viscosity of 1.92 cP shown in Figure 5b. This finding indicates that in addition to the interfacial characteristic of the cathode, the viscosity of electrolytes is one of critical factors governing the rate capability of high-voltage LNMO cathodes. This lowered viscosity may help to improve the electrolyte distribution in the high-density LNMO cathode, thus achieving an excellent rate capability in the Li/LNMO half-cells at high discharge rates (Figure 5a and d).


15

5C

7C

C/2

b 3.5

8

3.0

7

2.5

6

2.0

5

1.5

4

Viscosity (cP)

120 100 80 60 40 20

1.0 0.5

0

0.0 0

5

10

15

20

25

3 2 1

-1

FEC-PFE FEC-DEC FEC-DMC EC-DMC

EC-DMC

3C

FEC-DMC

1C

C/2

FEC-DEC

140

FEC-PFE

-1

a

Page 16 of 33

Ionic conductivity (mS cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Discharge capacity (mAh g )


30

Cycle number

c

e

d

FEC-PFE

FEC-DEC

EC-DEC

FEC-DMC

~0o

8.0o

11.3o

6.8o

0o

14.7o

17.5o

12.2o

LNMO cathode

Graphite anode

Figure 5. (a) A comparison of the discharge rate capability of Li/LNMO half-cells with 1 M LiPF6 dissolved in various electrolyte solvents. The charge C-rate was fixed at C/2. (b) Viscosity and ionic conductivity of the electrolytes at room temperature. Schematic showing the distribution of (c) the carbonate-based electrolyte and (d) the PFE-containing electrolyte in the high-density electrodes. (e) Optical images showing contact angles of electrolytes on LNMO cathodes and graphite anodes.


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The FEC-DEC electrolyte, which has a high viscosity of 2.22 cP, delivered a reduced discharge capacity of 71.5 mAh g-1 at a 7C rate. This can be attributed to the non-uniform electrolyte distribution in the thick electrode structure and thick SEI layers formed by the severe electrolyte decomposition at high voltages (Figure 5c). Although the viscosity of FEC-DMC (1.92 cP) and EC-DMC (1.98 cP) was similar, very different discharge rate capability was observed. This indicates that FEC modulates the interface affecting the insertion kinetics of Li+ ions into LNMO at high discharge C-rates. In addition to the viscosity of electrolytes, the impregnation characteristics of electrolytes into electrodes can be considered as one of the important factors to enhance the electrochemical kinetics of electrodes. To explore the impregnation characteristics of electrolytes, the contact angle of the electrolyte on high-density LNMO electrode was compared as displayed in Figure 5e. The FEC-PFE electrolyte showed very low contact angle value of approximately 0° indicating an excellent wetting property toward high-density LNMO cathode thanks to its low viscosity and low surface tension.12,25 The FEC-PFE electrolyte showed superior wettability toward high-density graphite anode with a high mass loading level and a hydrophobic PE separator, as shown in Figure 5e and Figure S6. On the other hand, the carbonate-based electrolytes, FEC-DEC, EC-DEC, and FEC-DMC exhibited relatively high contact angle values on high-density electrodes and poor wettability toward hydrophobic PE separator. From this result, we confirmed that PFE possessing low viscosity assists in the enhancement of the impregnation characteristics of electrolytes into the high-density electrode and the wettability toward a PE separator. Before applying the FEC-PFE electrolyte to the full cell, the compatibility of PFE with the graphite anode was explored in Li/graphite half-cells.


17


b


1.5


-0.001

dQ/dV

Voltage (V)

a

1.0 0.5

-0.002 FEC reduction

-0.003 -0.004

0.0

EC reduction

-0.005

0

100

200

300

0.0

400

0.2

0.4

-1

3C

1C

C/2

5C

7C

C/2

300 200 FEC-PFE FEC-DEC EC-DEC

100

d Discharge capacity (mAh g-1)


c 400

0.6

0.8

1.0

1.2

1.4

Voltage (V)


3C

1C

C/2

400

5C

300

7C

C/2


200 100 0

0 0

5

10

15

20

25

30

0

5

e

1C

C/2

400

3C

5C

10

15

20

25

30

Cycle number

Cycle number

Discharge -1 capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

7C

C/2

300 200 FEC-PFE FEC-DEC EC-DEC

100 0 0

5

10

15

20

25

30

Cycle number

Figure 6. (a) Voltage profiles and (b) dQ/dV plots of Li/graphite half-cells during precycling between 0.01 and 2 V at a rate of C/10 at 30 °C. (c) Charge rate capability of Li/graphite halfcells from C/2 to 7C without applying the time cut-off condition. Constant voltage conditions were applied at the end of charging until the current was less than C/20. A fixed discharge C-rate of C/2 was applied. (d) Charge rate capability of Li/graphite half-cells from C/2 to 7C with a time cut-off condition corresponding to a given charge C-rate. A fixed discharge C-rate of C/2 was applied. (e) Discharge rate capability of Li/graphite half-cells from C/2 to 7C. The charge Crate was fixed at C/2.


18

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A comparison of the initial voltage profiles of the graphite anodes demonstrates that ICE (92.9%) in the FEC-PFE electrolyte is slightly lower than that in the FEC-DEC electrolyte (94.3%) (Figure 6a). This lowered ICE is mostly due to the capacity loss via high reduction tendency of PFE compared to FEC (Figure 1). During the initial lithiation of the graphite anode, a clear voltage plateau ascribed to the reductive decomposition of FEC and EC at the graphite anode appeared at around 0.88 and 0.52 V, respectively (Figure 6a). Their reductive decomposition potentials are shown more clearly in the dQ/dV plots (Figure 6b). The FEC-PFE and FEC-DEC electrolytes show a higher reduction potential with an onset of 1.0 V (peak potential of 0.88 V) induced by FEC decomposition compared to that of the EC-DEC electrolyte, which presents a reduction potential of 0.52 V owing to EC decomposition. This result is consistent with a previous report that the reduction of FEC occurs before that of EC owing to the relatively lower LUMO energy of FEC.35 The FEC-derived SEI on the graphite anode results in a superior rate capability, whereas the EC-derived SEI causes rapid capacity fading at high charge C-rates without applying time cut-off condition. End-condition of constant voltage (CV) was determined by current below C/20. (Figure 6c). Figure 6d shows the charge rate capability of the graphite anode with a time cut-off condition corresponding to a given C rate. In this algorithm, the FEC-PFE electrolyte delivered relatively high discharge capacity indicating good charging capability at high charge C-rates, compared to FEC-DEC and EC-DEC electrolytes. To realize fast chargeable graphite anodes, most of Li ions should be intercalated into graphite at the constant current (CC) mode of charging process at high C-rates. A clear disparity between these electrolytes is shown in voltage profiles at high charge C-rates of Figure S7. A comparison of CC-CV region of Li/graphite half-cells during charging at a charge rate of 1C confirms that the FEC-PFE electrolyte mitigates the increase of IR drop and enables relatively large amount of Li


19


Page 20 of 33

ions to be intercalated under CC condition (Figure S7a and b). Superior charge rate capability of graphite anodes with FEC-PFE was observed at a high charge rate of 3C (Figure S7c). Interestingly, the graphite with the FEC-DEC electrolyte exhibits no discharge capacity at discharge C-rates greater than 3C (Figure 6e). Conversely, a comparatively high discharge capacity of ca. 230 mAh g-1 was achieved in the FEC-PFE electrolyte at a high C-rate of 3C. This implies that the combination of PFE with FEC leads to the formation of more suitable interface for the deintercalation of Li+ ions from the lithiated graphite anode at high discharge rates. Although the FEC-derived SEI is very effective for the interfacial stabilization of the graphite anode, the control of the interface structure via the selection of an appropriate solvent is crucial for obtaining a good anode rate capability. Figure 7a shows the voltage profiles of graphite/LNMO full cells during precycling at a rate of C/10 at 30 °C. A higher ICE of 81.9% was achieved in the full cell with the FEC-PFE electrolyte compared to those for the FEC-DEC and EC-DEC electrolytes. Because similar ICEs were obtained for all three electrolytes in the Li/graphite half-cells (Figure 6a), the improved ICEs of the full cells are mostly due to the superior oxidation durability of the FEC-PFE electrolyte at the high-voltage LNMO cathode. The reduction potentials of around 3.05 V in the dQ/dV plots shown in Figure 7b can be attributed to FEC decomposition in the FEC-PFE and FEC-DEC electrolytes at the graphite anode. In comparison, the EC-DEC electrolyte displays a relatively high reduction potential of 3.28 V. This is in good agreement with the result that the reductive decomposition of FEC occurs at the graphite anode before that of EC (Figure 6b). Although an FEC-derived SEI is formed on the graphite anode, the significant capacity fading of the graphite/LNMO full cell with the FEC-DEC electrolyte starts to appear at a rate of 3C (Figure 7c and d).


20

Page 21 of 33

a

b 0.00010

5.0

0.00008 EC reduction

4.0

dQ/dV

Voltage (V)

4.5

3.5 3.0 FEC-PFE FEC-DEC EC-DEC

2.5 2.0 0

20

0.00006

FEC reduction


0.00002

40

60

0.00000 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

80 100 120 140 160 -1

Voltage (V)

Specific capacity (mAh g ) 1C

C/2

140

3C

7C

5C

d

C/2

120 100 80 60 FEC-PFE FEC-DEC EC-DEC

40 20

160


Discharge -1 capacity (mAh g )

c 160

1C

C/2

140

7C

5C

C/2

100 80 60 FEC-PFE FEC-DEC EC-DEC

40 20 0

0

5

10

15

20

25

0

30

5

10

100

140

98

120 100

96

80 94

60 FEC-PFE FEC-DEC EC-DEC

40 20

92

0 0

50

100

150

20

25

30

90 200

f

30 FEC-PFE FEC-DEC EC-DEC

25

-Z" (Ohm)

e 160

15

Cycle number

Cycle number Discharge capacity (mAh g-1)

3C

120

0

Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10 5 0 0

10

20

30

40

50

60

Z' (Ohm)

Cycle number

Figure 7. (a) Voltage profiles and (b) dQ/dV plots of graphite/LNMO full cells and voltage profiles during precycling at a rate of C/10 and 30 °C. Rate capability of graphite/LNMO full cells: (c) Charge rate capability of graphite/LNMO full-cells from C/2 to 7 C. The discharge C rate was fixed at C/2. (d) Discharge rate capability of graphite/LNMO full cells from C/2 to 7C. The charge C rate was fixed at C/2. (e) Discharge capacity retention and Coulombic efficiency of graphite/LNMO full-cells between 3.0 and 4.9 V at a rate of 1 C at 30 °C. (f) AC impedance spectra of graphite/LNMO full cells after precycling.


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Importantly, in the case of the FEC-PFE electrolyte, the PFE allows the FEC-derived SEI to act as a favorable interfacial layer for facile charge transport at high charge, and, thus, the full cell delivers an excellent discharge capacity of over 90 mAh g-1 at a high charge rate of 7C (Figure 7c). The use of FEC for the in situ formation of an artificial SEI layer on the graphite anode avoids the significant reductive decomposition of the electrolyte in the full cell. In addition, PFE, which can improve the oxidation durability of the electrolyte at the high-voltage LNMO cathode, allows the reversible electrochemical reaction of the cathode paired with the graphite anode upon prolonged cycling. A comparison of the cycling performance of the graphite/LNMO full cells at a rate of 1C and 30 °C is given in Figure 7e. Clearly, a substantial improvement in the cycling stability and CE of the full cells is achieved in the FEC-PFE electrolyte. Consequently, the full cell with the FEC-PFE electrolyte displayed an improved discharge capacity retention of 57.1% and maintained a very high CE of ~99.6% over 200 cycles. However, the discharge capacity of the full cell with the FEC-DEC electrolyte faded rapidly and was nearly identical to that of the full cell with the EC-DEC electrolyte (Figure 7e). Moreover, the FEC-DEC electrolyte does not maintain a CE of over 99.5%, which is essential for practical applications, after 80 cycles. In addition, FEC forms a stable protective layer on the graphite anode in the full cell with FECDEC. Nevertheless, the negative effect of DEC on the oxidation durability of the FEC-DEC electrolyte shown in Figure 3 results in the decrease of CE of the full cell with the FEC-DEC electrolyte via unwanted electrolyte decomposition at the high-voltage LNMO cathode, indicating the consumption of the limited Li+ sources in the full cell. A comparison of the resistance of the full cells after precycling indicates that the FEC-PFE electrolyte has relatively low interfacial resistance, including the SEI resistance and charge transfer resistance (Figure 7f).


22

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Graphite anode after precycle

a


Li-F

LNMO cathode after precycle

F1s C-F (binder)

LixPOyFz

CH-F LixPOyFz


Li-F

P2p P-O (P2p1/2) P-O (P2p3/2)

LixPOyFz

P-O (P2p1/2)

LixPFy

LixPOyFz

P-O (P2p3/2)

LixPFy


Graphite

C1s

C-H

CH-F

O-C-O Li2CO3 C-O-C

CH2-OCO2Li

CH2-CF2 (binder)

CH2-CF2 (binder)

Carbon black

CH2-OCO2Li

CH2-OCO2Li

b

Figure 8. (a) F 1s, P 2p and C 1s spectra of each electrode from graphite/LNMO full cells after precycling. (b) Schematic illustration for manipulation of FEC decomposition by PFE and preferential attack of radical cation (DEC•+) on FEC rather than EC.


23


Page 24 of 33

An additional important feature is that the semicircle of the full cell with the FEC-DEC electrolyte is much smaller than that of the EC-DEC electrolyte. This suggests that the FECderived SEI on the graphite anode is more ionically conductive than the EC-derived SEI. The FEC-derived SEI can assist Li ion migration to the anode and permits the reversible electrochemical reaction of graphite in the full cell. However, the FEC-DEC electrolyte imparts little improvement to the cycling performance of the full cells because it undergoes undesirable oxidative decomposition at the high-voltage LNMO cathode. These results confirm that the introduction of the PFE and the FEC, which lead to an improvement of the oxidation stability of the electrolyte at the high-voltage LNMO cathode and the formation of a stable SEI layer on the graphite anode, respectively, preserve the electrochemical properties of the graphite/LNMO full cell. The effect of electrolyte composition on the surface chemistry mechanisms of the graphite anode and LNMO cathode is presented in Figure 8. The F 1s core-level spectra acquired on the graphite anodes after the precycling show a relatively small peak at 687.2 eV, corresponding to the LixPOyFz formed by LiPF6 decomposition, and a pronounced peak at 684.8 eV is attributed to LiF created by LiPF6 and FEC decomposition at the graphite anode. Compared with the LiF signal from the graphite precycled in the EC-DEC electrolyte, a more intense LiF signal appeared for the FEC-DEC electrolyte. Importantly, despite the use of the same content of reducible FEC compounds, the use of the FEC-PFE electrolyte drastically reduced the LiF peak intensity. PFE, which has a good affinity for FEC molecules, seems to alleviate the reductive decomposition of FEC to form a resistive LiF compound as an SEI constituent (Figure 8a). Thanks to the interfacial architecture based on suitably controlled FEC-derived SEI by the presence of PFE, the graphite anodes exhibit good chargeability and dischargeability, which is a necessary requirement of batteries for electric vehicles (Figure 6c-e). In particular, the FEC-PFE


24

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electrolyte displayed markedly enhanced discharge rate capabilities of graphite/LNMO full cells that were not achieved in FEC-DEC and EC-DEC (Figure 7d). Because no significant LiF peak on the LNMO cathode appeared for the three electrolytes, we confirmed that FEC is responsible for the formation of LiF-based SEI on the graphite anode (Figure 8a). A comparison of P 2p core-level spectra of the graphite anodes clearly shows that the LiPF6 decomposition occurs more severely for the EC-DEC electrolyte. However, there was no significant difference in the P 2P spectra acquired on the LNMO cathode. In the case of the EC-DEC electrolyte, reductive decomposition of EC molecules mainly occurs at the graphite anode; thus, a slightly increased peak intensity corresponding to the ether linkages (-C-O-C) is observed in the C 1s spectra of the graphite anode. In addition, the peak intensity assigned to graphite was decreased for the ECDEC electrolyte. It is thought that uncontrollable decomposition of the EC-DEC electrolyte caused the formation of a relatively thick SEI layer, which blocked the C-C signal from the anode. A noticeable feature for the LNMO cathode precycled in the FEC-DEC electrolyte is the appearance of carbon singly bonded to hydrogen (C-H) at 285.2 eV. This is probably because the DEC•+ radical cations more easily attack the FEC molecules that are interacting with Li+-ions and, thereby, FEC decomposition leads to the formation of C-H containing species on the LNMO cathode surface (Figure 8a and b). FEC•+ radical cations36 may be formed by transferring one electron from the FEC molecule to high-voltage LNMO cathode and DEC•+ radical cations. Therefore, the binding energies of 686 eV in the F 1s spectrum and 290 eV in C 1s spectrum of the LNMO cathode with the FEC-DEC electrolyte are attributable to CH-F moiety that may be generated by further oxidative decomposition of FEC•+ with FEC molecules. This problematic decomposition behavior of FEC-DEC electrolyte at high-voltage LNMO cathode caused the decrease of carbon black (electrical conducting agent in the cathode) and CH2-CF2 binder signal.


25


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From the spectral studies, the FEC-PFE builds up a less resistive FEC-derived SEI via the manipulation of FEC decomposition at the graphite and effectually improves the cycling performance of high-voltage graphite/LNMO full cells thanks to the absence of DEC, which would otherwise create unwanted reactive species on the cathode.

CONCLUSIONS We have demonstrated that the addition of partially fluorinated ether with electronegative fluorine atoms improves the oxidation stability of electrolytes at high voltages and that partially fluorinated ether is responsible for the low flammability of the electrolytes. FEC, which has been widely used as a reducible additive for the formation of a stable SEI on graphite anodes, impeded the electrochemical kinetics of the high-density graphite anode at high discharge rates and the high-density LNMO cathode at high charge rates. Our investigation revealed that the combination of the partially fluorinated ether and FEC could substantially improve the cycling performance of graphite/LNMO full cells at high C-rates. It is believed that the partially fluorinated ether with low viscosity provides the environment facilitating the movement of a large number of lithium ions extracted from the electrode under high current densities. We hope that the results of this study will contribute to the search for a new class of electrolytes for highvoltage lithium-ion batteries with excellent rate capabilities and high thermal stability.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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Physicochemical properties of carbonate solvents and PFE; A photograph showing a comparison of the compatibility of PFE with the solvent mixture of EC-EMC or FEC-EMC; electrochemical performance of Li/LNMO half-cells, charge voltage profiles; normalized leakage currents of Li/LNMO half-cells during electrochemical floating test; Photographs showing the wetting property of electrolytes to a PE separator; Charge (lithiation) and discharge (delithiation) voltage profiles of Li/graphite half-cells for charge rate capability test with a time cut-off condition corresponding to a given charge C-rate AUTHOR INFORMATION Corresponding Author *E-mail : [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by Hyundai Motor Company and a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2013-C1AAA001-0030538). REFERENCES (1) Larcher, D.; Tarascon, J. M. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19-29.


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(2) Goodenough, J.B. Electrochemical Energy Storage in a Sustainable Modern Society. Energy Environ. Sci. 2014, 7, 14-18. (3) Choi, N.-S.; Chen, Z.; Freunberger, S.A.; Ji, X.; Sun, Y.-K.; Amine, K.; Yushin, G.; Nazar, L.F.; Cho, J.; Bruce, P.G. Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors. Angew. Chem. Int. 2012, 51, 9994-10024. (4) Xu, X.; Lee, S.; Jeong, S.; Kim, Y.; Cho, J. Recent Progress on Nanostructured 4 V Cathode Materials for Li-ion Batteries for Mobile Electronics. Mater. Today 2013, 16, 487-495. (5) Xiao, J.; Chen, X.; Sushko, P.V.; Sushko, M.L.; Kovarik, L.; Feng, J.; Deng, Z.; Zheng, J.; Graff, G.L.; Nie, Z.; Choi, D.; Liu, J.; Zhang, J.G.; Whittingham, M.S. High-Performance LiNi0.5Mn1.5O4 Spinel Controlled by Mn3+ Concentration and Site Disorder. Adv. Mater. 2012, 24, 2109-2116. (6) Yang, J.; Han, X.; Zhang, X.; Cheng, F.; Chen, J. Spinel LiNi0.5Mn1.5O4 Cathode for Rechargeable Lithium Ion Batteries: Nano vs Micro, Ordered Phase (P4332) vs Disordered Phase (Fd3m). Nano Res. 2013, 6, 679-687. (7) Santhanam, R.; Rambabu, B. Research Progress in High Voltage Spinel LiNi0.5Mn1.5O4 Material. J. Power Sources 2010, 195, 5442-5451. (8) Norberg, N.S.; Lux, S.F.; Kostecki, R. Interfacial Side-Reactions at a LiNi0.5Mn1.5O4 Electrode in Organic Carbonate-Based Electrolytes. Electrochem. Commun. 2013, 34, 29-32. (9) Wu, X.; Li, X.; Wang, Z.; Guo, H.; Yue, P. Capacity Fading Reason of LiNi0.5Mn1.5O4 with Commercial Electrolyte. Ionics 2013, 19, 379-383. (10) Demeaux, J.; Lemordant, D.; Galiano, H.; Caillon-Caravanier, M.; Claude-Montigny, B. Impact of Storage on the LiNi0.4Mn1.6O4 High Voltage Spinel Performances in AlkylcarbonateBased Electrolytes. Electrochim. Acta 2014, 116, 271-277.


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(11) Arai, J. A Novel Non-Flammable Electrolyte Containing Methyl Nonafluorobutyl Ether for Lithium Secondary Batteries. J. Appl. Electrochem. 2002, 32, 1071-1079. (12) Arai, J. Nonflammable Methyl Nonafluorobutyl Ether for Electrolyte Used in Lithium Secondary Batteries. J. Electrochem. Soc. 2003, 150, A219-A228. (13) Xia, L.; Xia, Y.; Wang, C.; Hu,H.; Lee, S.; Yu, Q.; Chen, H.; Liu, Z. 5 V-Class Electrolytes Based

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ChemElectroChem 2015, 2, 1707-1712. (14) Zhang, Z.; Hu, L.; Wu, H.; Weng, W.; Koh, M.; Redfern, P.C.; Curtiss, L.A.; Amine, K. Fluorinated Electrolytes for 5 V Lithium-Ion Battery Chemistry. Energy Environ. Sci. 2013, 6, 1806-1810. (15) Achiha, T.; Nakajima, T.; Ohzawa, Y.; Koh, M.; Yamauchi, A.; Kagawa, M.; Aoyama, H. Thermal Stability and Electrochemical Properties of Fluorine Compounds as Nonflammable Solvents for Lithium-Ion Batteries. J. Electrochem. Soc. 2010, 157, A707-A712. (16) Westhoff, K.; Bandhauer, T. A Multi-Functional Electrolyte for Lithium-Ion Batteries I. Non-Boiling Electrochemical Performance. J. Electrochem. Soc. 2016, 163, A1903-A1913. (17) Liu, Y.; Fang, S.; Shi, P.; Luo, D.; Yang, L.; Hirano, S.-i. Ternary Mixtures of NitrileFunctionalized Glyme, Non-Flammable Hydrofluoroether and Fluoroethylene Carbonate as Safe Electrolytes for Lithium-Ion Batteries. J. Power Sources 2016, 331, 445-451. (18) Buqa, H.; Goers, D.; Holzapfel, M.; Spahr, M.E.; Novák, P. High Rate Capability of Graphite Negative Electrodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2005, 152, A474A481. (19) Herstedt, M.; Fransson, L.; Edström, K. Rate Capability of Natural Swedish Graphite as Anode Material in Li-Ion Batteries. J. Power Sources 2003, 124, 191-196.


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Z. Carbonate-Based Electrolytes for High-Voltage Lithium Ion Batteries: A DFT Calculation and Experimental Study. ChemistrySelect 2017, 2, 7353-7361.


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A Table of Contents Entry

Discharge capacity -1 (mAh g ) @ C/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


160 140 120 100 80 60 40 20 0

Charge C rates 1C C/2 3C

7C

5C

C/2

HOMO of PFE LNMO


0

5

10

15

20

25

30

Cycle number


Synergistic Effect of Partially Fluorinated Ether and Fluoroethylene

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