Pseudoconcentrated Electrolyte with High Ionic Conductivity and

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Pseudo-Concentrated Electrolyte with High Ionic Conductivity and Stability Enables High Voltage Lithium-Ion Battery Chemistry Guoqiang Ma, Li Wang, Xiangming He, Jianjun Zhang, Huichuang Chen, Weiguo Xu, and Yuansheng Ding ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01020 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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ACS Applied Energy Materials

Pseudo-Concentrated Electrolyte with High Ionic Conductivity and Stability Enables High Voltage Lithium-Ion Battery Chemistry Guoqiang Ma1,2, Li Wang1*, Xiangming He1*, Jianjun Zhang2, Huichuang Chen2, Weiguo Xu2, Yuansheng Ding2 Corresponding authors' email: [email protected]; [email protected] 1

Institute of Nuclear & New Energy Technology, Tsinghua University, Beijing 100084, China

2

Zhejiang Chemical Industry Research Institute Co. Ltd., Hangzhou 310023, China

Keywords: Li-ion battery; High voltage electrolyte; pseudo-concentrated electrolyte; solvation; heterogeneous liquid. Abstract: Electrolytes play a decisive role in determining the energy density, cycling life, safety and temperature adaptability of lithium ion batteries or any advanced battery chemistries. Herea“pseudo-concentrated electrolyte” combining advantages of both concentrated and diluted electrolytes is described as a brand-new approach for high performance lithium-ion batteries. By designing a heterogeneous liquid structure for electrolyte, we made it possible for Li+ to form a solvation sheath structure that is only attainable in concentrated electrolytes while using low salt concentration. Such pseudo-concentrated electrolyte, though with lithium bis(trifluoromethanesulfonyl imide) as lithium salt and carbonates as solvents, demonstrates high electrochemical oxidation resistance, high anti-corrosion capability towards Al current collector that are usually the characteristics of concentrated electrolytes, as well as high ionic conductivity, and low viscosity that are only available from diluted electrolytes. Thus,this innovative approach provides a brand new avenue to tailor electrolyte properties for advanced battery chemistry applications.

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1. Introduction With the rapid development of smart portable devices, electric vehicles, and energy-storage systems, lithium-ion batteries (LIBs) with high energy density, high power density, high safety, and long cycle life have been highly demanded.1-2 A viable electrolyte is of primary importance for the high performance Li-ion battery since it controls the operation voltage, rate capability, life spun and safety of LIBs.3-5The LiPF6/EC-based electrolytes have dominated the electrolyte market of 4 V-class LIBs over the past 25 years due to its stable electrochemical properties.6-8However, trace amount of HF will generate in LiPF6 based electrolyte, which can result in the dissolution of metal ions and the corrosion. And ionic conductivity and stability still need to be improved to meet the demand of future LIBs3, 5. Recently,lithium bis(trifluoromethanesulfonyl imide) (LiTFSI) and lithium bisfluorosulfonyl imide (LiFSI) salt based electrolytes receive great attention due to their higherionic conductivity, better thermal safety, and higher stability against electrodes at high charge state than the LiPF6/EC-based electrolytes. However, the operation voltage of LIBs using LiTFSI- or LiFSI-based electrolytes is limited below 3.7 V vs. Li/Li+ due to its corrosivity against Al current collector.9-10Therefore, it is of vital importance, but extremely challenging to explore an electrolyte with high ionic conductivity, large electrochemical potential window, good thermal safety and good stability against the current collectors.Solvation can expand the electrochemical window of battery electrolytes, providing new possibilities to use LiTFSI- or LiFSI-based electrolytes with high operation voltage of LIBs.3, 11 Very recently, by 2 ACS Paragon Plus Environment

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developing a “water in salt” electrolyte with large amount of LITFSI salt and trace of H2O, Suo et al. successfully expanded the electrochemical window of aqueous solution from 1.23 V to 3.0 Vdue to the strong bonding between H2O and Li+, where almost all H2O molecules are involved in solvation sheath for Li+ ions.12-13Besides tuning oxidation resistance, the concentrated electrolytes using LiTFSI and LiFSI salts showed superior capability in suppressing corrosion of Al foil, and stabilizing the anode by the formation of stable SEI on Li metal/graphite.14-18Despitethe advantages of large electrochemical potential window and good stability against the current collectors, concentrated electrolytes using excessive lithium salts, however, causes high viscosity, low conductivity and high cost, which hinder their wide application in industry.19-20

Figure.1 Schematic diagrams of the electrolyte solution structure for13, 15 (a) the conventional dilute electrolyte, (b) the concentrated electrolyte and (c) the pseudo-concentrated electrolyte. Anions and most solvent molecules are free in the dilute electrolyte (Figure. 1a), while almost all



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the anions and solvent molecules solvate with Li+ ions, forming a unique solvation sheath structure of Li+ ions and the reinforced three-dimensional network in the concentrated electrolyte (Figure. 1b). By introducing the non-solvent liquid, the reinforced three-dimensional network is separated into small clusters, while retains the solvation sheath structure of Li+ ions (Figure. 1c).

Here, we propose a dilute electrolyte with unique solvation sheath structure to achieve the combined advantages of diluted electrolyte (e.g., high conductivity and low cost) and concentrated electrolyte (e.g., large electrochemical stability window and good stability against Al current collector). It is essentially important to understandthe different electrochemical and physical properties between the concentrated and dilute electrolytes for the development of advanced electrolyte systems.The schematic diagrams of all the species and their 3-dimensional distribution in conventional dilute electrolyte and concentrated electrolyte are shown in Figure. 1a and 1b.15, 21 In the dilute electrolyte (around 1 mol L-1, the molar ratio of Li+ to solvent is less than 0.1), one bare Li+ ion is solvated with several solvent molecules (around 4-6), while anions and the extra solvent molecules are free (Figure. 1a).11,

15, 22

However, in the

concentrated electrolyte (more than 3 mol L-1, the ratio of Li+ to solvent is more than 0.5), almost all the solvents are bonded round Li+ ions, and anions are also involved in the formation of solvation sheath of Li+ions.13-15The strong bonding to Li+ ions reduces their electron density, making both the solvent and anions difficult in providing electrons but more easily to accept electrons. The bonded solvent and anions become inert in oxidation and complexation, but are active in reduction, thus extending the electrochemical window and reducing corrosion kinetics to Al current 4 ACS Paragon Plus Environment

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collector. Besides, the SEI composition is dominated by the reduction of TFSI– rather than carbonate solvents in the highly concentrated electrolytes, which is good for long-term battery cycling. Despite of these advantages, concentrated electrolyte usually exhibits much lower ionic conductivity than dilute electrolyte (less than one tenth, Figure. S1). For a given electrolyte with fixed salt and solvent, ionic Li+ concentration

conductivity depends on electrolyte viscosity,

and Li+

mobility.23For the concentrated electrolyte, the bonds between anions and cations will form a three-dimensional (3D) network (Figure. 1b).15The as-formed 3D network reduces mobility of all the species in the electrolytes and increases the viscosity, andthe bonding to the network also reduce the Li+ mobility, which leads to limited ionic conductivity of concentrated electrolytes. Moreover, the wettability of the concentrated electrolyte to the electrode and the separator is also lower than that of dilute electrolytes due to the 3D network structure (Figure. S2). Thus, the ideal concentrated electrolytes should have strongly bonded solvents and anions in solvation sheath structure of Li+ ions but without three-dimensional network, allowing to achieve a low viscosity and a high conductivity, while still maintaining the superior electrochemical and chemical performances. Recently, Linda Nazar et al. and Masayoshi Watanabe introduced a highly fluorinatedether (HFE) as diluent in both concentrated (ACN)2:LiTFSI and [Li(G4)1][TFSA] electrolytes to reduce the viscosity,increase the ionic conductivity of the electrolytes, and suppressthe polysulfides

for Li-sulfur

high-concentration

batteries.21,

electrolytes

to

24

And

improve

Zhang the

proposed

performance


a

localized

lithium-metal


Batteries.25In this work, we reported a new concept called ‘pseudo-concentrated electrolyte’, in which a non-solvent was introduced into a concentrated electrolyte to break three-dimensional network into small clusters, while Li+ solvation sheath structure remains similar to what in the concentrated electrolyte.

2. Results and Discussions Non-flammable hexafluoroisopropyl methyl ether (HFME) is selected to be added to the DMC (Dimethyl carbonate): LiTFSI based-concentrated electrolyte. As shown in Figure. S3, LiTFSIcannot be dissolved in HFME. However, HFME can be miscible in the commonly used carbonate-based solvent and has an extended electrochemical window. In addition, HFME shows low viscosity and good wettability with separator. All these chemical and physical properties show that HFME is an ideal “assistant solvent” to realize the proposed ‘pseudo-concentrated electrolyte’. In fact, the prepared pseudo-concentrated electrolyte is a stable, transparent and uniform solution (Figure. S3), indicating it is feasible in terms of appearance.


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a

b

(50, 1.32)

1.4

100

0.8

25oC

0.6 10

0.4

(0, 0.41)

0.2

2.5 2.0 1.5

Dilute electrolyte Concentrated electrolyte Pseudo-concentrated electrolyte

1.0 0.5

0.0 -0.2

0.0 3. 3

1 0

20

40

60

80

o 0 C

3.0

-1 lgσ/µ S.cm

1.0

Viscosity/mpa.S

Co nductivity /S.cm -1

4.0 3.5

1.2

100

3. 4

3.5

3.6

30

20

Dilute electrolyte Concentrated electrolyte Pseudo -concentrated electrolyte

1800

50

1600

40

2nd

30

3rd

20

1st 2nd 3rd

1400 1200

15 10

1000

4.0

10

3rd

0 2.0

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2 .5

3.0

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5.0

Po tential (V) vs Li/Li+ 600

5

400

0

200

-5 3.5

3.9

1st

Current/ µA

d

35

25

3.8

1000/T(K )

Current/µΑ

c

3.7 -1

Mass for the co ntent o f HFME/%

Current/uA

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


Dilute electrolyte DMC:LiTFSI=3 :1 Concentrated electrolyte Pseudo-concentrated electro lyte

1st

0

4.0

4.5

5.0

5.5

6.0

6.5

2.0

2.5

3.0

3.5

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4.5

5.0

Voltage/V

Voltage/V

Figure.2 (a) The conductivity and viscosity of the pseudo concentrated electrolyte (LiTFSI: DMC=1:1.5, n:n) with different content of HFME (in mass). The conductivity reaches maximum when the content of HFME is around 50%. It is concluded by proper viscosity and mobile Li+ ions concentration. (b) Arrhenius plots of the ionic conductivity as a function of 1000/T for the dilute electrolyte (LiTFSI: DMC=1:12, n:n, about 1M), the concentrated electrolyte (LiTFSI: DMC=1:1.5, n:n, about 5mol L-1) and the pseudo-concentrated electrolyte (LiTFSI: DMC=1:1.5, n:n, the concentration of HFME in the pseudo-concentrated electrolyte is 50%, about 2.5 mol L-1).The pseudo-concentrated electrolyte delivers much higher conductivity than the concentrated electrolyte. (c) Linear sweep voltammetry (LSV) profiles for different electrolytes to determine their oxidationresistanceusing Pt electrode. The pseudo-concentrated electrolyte shows high stability up to 5.6 V. (d) Cyclic voltammetric (CV) profiles for the Al foil electrodes in the pseudo concentrated electrolyte, the concentrated electrolyte, the conventional dilute electrolyte, and the



moderate electrolyte with equivalent apparent concentration to the pseudo-concentrated electrolyte (DMC: LiTFSI=3:1, n:n). Al foil shows the best stability in the pseudo-concentrated electrolyte.

The physical properties of the pseudo-concentrated electrolyte with different ratios of HFME are illustrated in Figure. 2. With the increase of salt concentration, more Li+–DMC complex pairs form, thus the viscosity is increased and ion conductivity at 20 oCis reduced due to tapering off of free DMC (as shown in Figure. S1). However, as shown in Figure. 2a, when the content of HFME goes up from zero to around 50%, the Li+ ion conductivity of 5M LiTFSI-DMC concentrated electrolyte increases from 0.41 mScm-1 to the maximum value of 1.39 mScm-1 and the viscosity is largely reduced from 356.2 mPa s to only 10 mPa s. Therefore, the optimum content of HFME in the pseudo-concentrated electrolyte is determined as 50%, where the “apparent” salt concentration is around 2.5 mol L-1. In Figure. 2b, the ionic conductivities of the dilute electrolyte, the concentrated electrolyte and the pseudo-concentrated electrolyte all follow Arrhenius formula in a temperature range between -15 oC and 25 oC. In the concentrated electrolyte, the ionic conductivity is quickly reduced with the decrease of temperature. The high viscosity and low mobility of Li+ in the concentrated electrolyte limits it application in LIBs especially at a low temperature. Owing to the low viscosity and low melting point of HFME, the ionic conductivity of the pseudo-concentrated electrolyte is significantly improved compared to those of the concentrated electrolyte. The similar activation energy (obtained from the slop of the fitted curves) for ion transfer in both the dilute electrolyte and pseudo-concentrated electrolyte demonstrates the same ion transport 8 ACS Paragon Plus Environment

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mechanism in these twoelectrolytes. In addition, pseudo-concentrated electrolyte shows good wettability to Celgard separator. As shown in Figure. S2, the contact angle

of

electrolyte

to

separator

reduces

from

88.4o

for

the

5

mol

L-1concentratedelectrolyte to 16.7o for 2.5 mol L-1pseudo-concentrated electrolyte, which is even smaller than the contact angle of 46.3o for the conventional dilute electrolyte. Moreover, the pseudo-concentrated electrolyte is incombustible (as shown in Figure. S4c), while both the concentrated electrolyte and the diluted electrolyte a flammable (Figure. S4a and Figure. S4b), though the concentrated electrolyte does not burn as fiercely as the dilute electrolyte owing to the much lower content of flammable organic solvents. In short, in addition to improve the electrochemical property, the introduction of non-solvent HFME also ameliorates the electrolyte in terms of viscosity, conductivity, wettability, and thermal stability.

The physical properties of pseudo-concentrated electrolyte are similar to a dilute electrolyte, while the electrochemical behavior of pseudo-concentrated electrolyte is more like a concentrated electrolyte. The electrochemical property and structure of the pseudo-concentrated electrolyte are comprehensively examined. Figure. 2c shows the electrochemical oxidation potentials of three electrolytes measured using linear sweep voltammetry (LSV) with Pt electrode. The onset of oxidation for the dilute electrolyte occurs at 4.55 V, while it extends to 5.65 V for the concentrated electrolyte and the pseudo-concentrated electrolyte. It has been reported that the elimination of free solvents of electrolyte decreases the dissolution of metal ions in the electrolyte. And the solvent activity is reduced when coordinated to Li+ at high concentration as well 9 ACS Paragon Plus Environment


as an inner Helmholtz layer increasingly populated by TFSI anions. Therefore, concentrated/pseudo-concentrated electrolyte can improve the high voltage stability and suppress the Al corrosion. In addition, severe corrosion of aluminum current collector above 3.7 V vs. Li/Li+, which is normally observed in the dilute LiTFSI-based electrolyte, is effectively suppressed in the pseudo-concentrated electrolyte, and the corrosion current in pseudo-concentrated electrolyte is even smaller than that in the concentrated electrolyte (Figure. 2d). The surface morphologies of Al foils after polarization in three different electrolytes at 4.5 V for 12 h are compared in Figure. S5. A lot of large pitting on the surface of Al foil polarized in the dilute electrolyte can be observed, confirming a severe corrosion of Al current collector. While no obvious pitting can be found on Al foils polarized in concentrated electrolyte or pseudo-concentrated electrolyte. To demonstrate the unique capability of HFME on reducing corrosion against Al foil, a moderate electrolyte that has equivalent apparent concentration (DMC: LiTFSI=3:1, n:n) to the pseudo-concentrated electrolyte was prepared. As shown in Figure. S5c, some pitting still can be observed on Al foil polarized in the moderate electrolyte. Solvation DMC

Intensity/a.u.

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

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Solvation TFSI-

Free DMC

Pseudo-concentrated Pseuod concentrated electrolyte electrolyte

HFME

Free TFSI-

Concentrated electrolyte Dilute electrolyte Pure HFME Pure DMC

960 950 940 930 920 910 900 890 880 770 760 750 740 730 720 710 700

Wave number 10 ACS Paragon Plus Environment

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Figure. 3 Raman spectra of different solutions in the range of 890–900cm-1 and 700– 780cm-1.There is no free DMC and TFSI-can be detected in the concentrated electrolyte, which is consistent with the references. The pseudo-concentrated electrolyte retains the solvation structure of DMC and TFSI- in the concentrated electrolyte, while the viscosity is decreased, and the mobility of Li+ is increased, leading to the improved Li+ conductivity.

The solvation structures of Li+ ions in the three electrolytes are characterized using Raman spectroscopy. As shown in Figure. 3, free DMC molecules exhibit an O-CH3 stretching vibration band at 904 cm-1. This band shifts to 936 cm-1 when DMC is participated in Li+ solvation.15In the dilute LiTFSI/DMC electrolyte, most DMC molecules are in free state, which is in accordance with the fact that the molar ratio of solvent-to-salt in dilute electrolyte (10) is higher than typical solvation numbers (4~6) for solvated Li+ ions.3, 11, 26 For the concentrated electrolyte (LiTFSI: DMC=1:1.5, n:n), Raman band for free DMC completely disappears, suggesting that the majority of DMC molecules are solvated with Li+ ions. Moreover, the band for free TFSI- also disappears, suggesting all the TFSI- anions are also solvated with Li+. Therefore, there is almost no free DMC and TFSI- in the concentrated electrolyte, which is consistent with the previous reported results. Hence, TFSI- in the concentrated solution connects with each other through Li+ ions, leading to a reinforced three-dimensional network (Figure.1b).15 The special solvation structure of Li+ ion is responsible for the outstanding properties such as the suppressed Al corrosion and the improved oxidation resistance. Furthermore, Raman spectroscopy of the pseudo-concentrated electrolyte is almost the same compared with the concentrated electrolyte, except for 11 ACS Paragon Plus Environment


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the band corresponding to HFME at 737 cm-1. Therefore, adding HFME into concentrated electrolyte does not change the solvation structure of Li+ ions. However, the reinforced three-dimensional network structure of the concentrated solution is cut into clusters by the non-solvent HFME (As shown in Figure.1c). Therefore, the solvated Li+ ions in pseudo-concentrated electrolyte are more mobile than that in the concentrated electrolyte, and the free HFME molecules in the pseudo-concentrated electrolyte facilitate low viscosity. Meanwhile, the pseudo-concentrated electrolyte also retains the concentrated electrolyte’s superior electrochemical properties. Additionally, HOMO of HFME is lower than the commonly used carbonate-based solvent, thus the oxidation of the pseudo-concentrated electrolyte at high voltage can be further reduced.27 Therefore, the pseudo-concentrated electrolyte shows improved electrochemical stability and even better anti-corrosion capability to Al foil than the concentrated electrolyte.

LiNi1/3Co1/3Mn1/3O4|Li

half-cell

(Li

excess)

is

used

to

evaluate

the

electrochemical performances of pseudo-concentrated electrolyte, as shown in Figure.

S6. The LiNi1/3Co1/3Mn1/3O4 cathode cannot be charged in the dilute 1 mol L-1LiTFSI/DMC electrolyte, owing to the continuous Al corrosion at around 3.7 V, which is consistent with CV in Figure. 2. In contrast, the concentrated electrolyte enables a reversible Li+ deintercalation/ intercalation reaction on LiNi1/3Co1/3Mn1/3O4 cathode even at a high cut-off voltage of 4.4 V.


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Figure.

4

(a)

The

first

and

(b)

the

300th

charge–discharge

voltage

curve

of

LiNi1/3Co1/3Mn1/3O4|graphite pouch cell in different electrolyte. (c) The first 30 cycles and (d) the whole 300 cycles of cycling performance of LiNi1/3Co1/3Mn1/3O4|graphitefull-cell in the different electrolytes.

Figure. S7 and Figure. S8 show the charge/discharge cycling stability of LiNi1/3Co1/3Mn1/3O4|Li half-cell using different electrolytes. The cell using moderate electrolyte displays a poor cycle performance owing to Al corrosion. However, the capacity retention of the cell with pseudo-concentrated electrolyte after 50 cycles is over 95%, which is the best among the cells with three electrolytes due to supper electrochemical oxidation resistance of pseudo-concentrated electrolyte. The reaction kinetics of the cells with three electrolytes was evaluated by electrochemical impedance spectroscopy using LiNi1/3Co1/3Mn1/3O4|Li half-cell (Figure.S9). Owing 13 ACS Paragon Plus Environment


the high viscosity and low conductivity, the charge transfer resistance is as high as 550 Ω for the cell with the concentrated electrolyte, which is three times higher than that of the cell with 1 mol L-1LiPF6 based electrolyte (150 Ω). However, the resistance of the cell with the pseudo-concentrated electrolyte is only 150 Ω too, which can be attributed to the good wettability and low viscosity of the pseudo-concentrated electrolyte.

Additionally, the behavior of three electrolytes is compared using graphite|Li half-cells (Figure. S10). According to the results on rate performance, the pseudo-concentrated electrolyte is better than the concentrated electrolyte. This can be attributed to its higher conductivity, lower viscosity and the improved wettability with the electrode.

The pseudo-concentrated electrolyte is further applied in the high-voltage LiNi1/3Co1/3Mn1/3O4|graphite pouch full cell (around 1200 mAh, as shown in Figure. 4). Fullcell has a much harsher condition and causes higher sensitivity to parasitic reactions than halfcell, owing to the limited Li+ ions and electrolyte. As shown in Figure.4a, the reversible capacities during the first cycle of the full cell using the concentrated electrolyte is very low compared to that of the cell using the dilute electrolyte. One main reason may be that the electrodes and separator cannot be well infiltrated with the electrolyte due to its high viscosity (Figure.1 and Figure. S3), resulting in low utilization of the active material. While, the first reversible capacities of the cell using pseudo-concentrated electrolyte is close to the cell using dilute 14 ACS Paragon Plus Environment

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electrolyte, showing considerable wettability and active material utilization. Encouragingly, the charge/discharge profiles 500 cyclesshow smaller polarization for the cell using concentrated/pseudo-concentrated electrolyte (Figure. 4b). Moreover, the columbic efficiency reaches over 99% after 2 cycles for the cell in theconcentrated/pseudo-concentrated electrolyte (Figure. 4c). Generally, high columbic efficiency indicates inhibitedparasitic reactions between electrolyte and cathode/Al current collector, then the higher electrochemical stability of pseudo-concentrated electrolyte can be deduced. Additionally, the capacity retentions of the full-cell after 500 cyclesin dilute electrolyte, concentrated electrolyte and pseudo-concentrated electrolyte are 79.2%, 86.2% and 89.2%, respectively (Figure. 4d). The better cycling performance of the full-cell in the pseudo-concentrated electrolyte can be attributed to the high ionic conductivity, good wettability, high resistance against oxidation and negligible corrosion on Al current collector. 3. Conclusion In summary, by considering the physical property and chemical property separately, a new strategy for electrolyte design named ‘pseudo-concentrated’ is proposed and verified successfully. In detail, the solvation, which can be designed by adjusting the ratio of LiTFSI to solvent, is responsible greatly for the electrochemical properties of the electrolyte. However, the free solvent is generally responsible for the physical properties, and a non-solvent which miscibility with the solvent help decouple the physical properties and chemical properties. Then, a new LiTFSI/carbonate based pseudo-concentrated electrolyte is prepared by introducing a 15 ACS Paragon Plus Environment


non-solvent HFME. It shows normal apparent concentration, but behaves both like concentrated electrolyte in chemistry and electrochemistry and like dilute electrolyte in physics. It is similar in electrochemical window and Al foil friendly to the contrast concentrated electrolyte, while obviously higher nonflammability, higher ionic conductivity and lower viscosity than the contrast concentrated electrolyte (3.4 and 0.028 times, respectively). When applied to LIBs, the pseudo-concentrated electrolyte endows LiNi1/3Co1/3Mn1/3O4|graphite pouch cell with promising cycle ability, rate capability and safety. This shows that as long as the non-solvent is of good electrochemical, chemicaland physical properties compatible with the battery chemistry, it can retain both the superiorities of the concentrated electrolyte and the diluted electrolyte, and its features like nonflammability and high wettability may endows the battery chemistry with enhanced safety, rate capability and low temperature endurance additionally. The new strategy for electrolyte design paves a facile way for future efforts to optimize electrolytes for the advanced energy storageapplications, which is not limited to the lithium-ion batteries. 4. Experimental Section

Materials: Battery-grade DMC, LiTFSI and LiPF6 were obtained from ZhangjiagangGuotaiHuarong New Chemical Materials Co. Ltd, China, and used without further treatment. As seen in Fig.S1, hexafluoroisopropyl methyl ether(HFME, purity 99.99%) was supplied by Zhejiang Research Institute of Chemical Industry, Ltd. (Hangzhou, China). The fluoroether was dried using 3 Å molecular sieves until the H2O content was negligible(

Pseudoconcentrated Electrolyte with High Ionic Conductivity and

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