Extremely Stable Sodium Metal Batteries Enabled by Localized High

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Extremely Stable Sodium Metal Batteries Enabled by Localized High Concentration Electrolytes Jianming Zheng, Shuru Chen, Wengao Zhao, Junhua Song, Mark Engelhard, and Ji-Guang Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01213 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Energy Letters

Extremely Stable Sodium Metal Batteries Enabled by Localized High Concentration Electrolytes Jianming Zheng,† Shuru Chen,† Wengao Zhao,† Junhua Song,† Mark H. Engelhard,‡ and JiGuang Zhang*,† †

Energy and Environment Directorate, Pacific Northwest National Laboratory, 902 Battelle

Boulevard, Richland, WA 99354, USA ‡

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902

Battelle Boulevard, Richland, WA 99354, USA

ABSTRACT: Sodium (Na) metal is a promising anode for Na ion batteries. However, the high reactivity of Na metal with electrolytes and the low Na metal cycling efficiency have limited its practical application in rechargeable Na metal batteries. High concentration electrolytes (HCE, ≥4 M) consisting of sodium bis(fluorosulfonyl)imide (NaFSI) and ether solvent could ensure the stable cycling of Na metal with high coulombic efficiency, but at the cost of high viscosity, poor wettability, and high salt cost. Here, we report that the salt concentration could be significantly reduced (≤1.5 M) by a hydrofluoroether as an “inert” diluent, which maintains the solvation structures of HCE, thereby forming a localized high concentration electrolyte (LHCE). An LHCE (2.1 M NaFSI/1,2-dimethoxyethane (DME) - bis(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2)) enables dendrite-free Na deposition with a high coulombic efficiency

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ACS Energy Letters

of >99%, fast charging (20C) and stable cycling (90.8% retention after 40,000 cycles) of Na||Na3V2(PO4)3 batteries.

solvent Na+ anion

X

solvent + Na anion

"inert" diluent diluent

Na metal

Na3V2(PO4)3

TOC GRAPHICS

Capacity (mAh 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

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80 60 40

20C

2.1 M NaFSI/DME-BTFE(1:2) 3.1 M NaFSI/DME-BTFE(1:1) 5.2 M NaFSI/DME

20 0 0

8000

16000 24000 Cycle number

32000

40000

Sodium (Na)-ion batteries (SIBs) are being considered as a promising alternative to lithium ion battery (LIBs), owing to the natural abundance and uniform distribution of sodium resources in the earth’s crust.1-2 Sharing a working principle similar to LIBs, a typical SIB operates by adopting non-graphitic carbonaceous anode materials such as hard carbon,3 intercalation-based layered materials,4 Na-alloying type materials such as Sb@C,5 BiSb,6 and conversion-type materials such as Fe2O3.7 However, a significant penalty in energy density and battery working voltage of corresponding SIBs exists with these anodes owing to their low specific capacity as well as operating voltage well above that of the Na/Na+ redox couple.1 Therefore, direct use of Na metal as anode shows clear advantages because it has the highest specific capacity (1165 mAh g-1) and the lowest redox potential (−2.714 V vs. standard hydrogen electrode). However, the practical utilization of Na metal as anode is hindered by its high reactivity with electrolyte.811

The parasitic reactions between Na metal and electrolytes is the primary reason for a low

plating/stripping coulombic efficiency (CE) (typically ≤95%), accounting for a rapid capacity


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fading of sodium metal batteries (SMBs).12 To date, representative strategies to address the stability issue of Na metal anode include the surface modification via atomic layer deposition (Al2O3)13-14 and molecular layer deposition (alucone),15 application of a free-standing composite protective layer16 or graphene film,17 fabrication of Na@r-GO composite,18 and the liquid/solid electrolyte modulation.19-22 Analogous to the Li metal battery system,23 novel electrolyte composition is regarded as one of the most pragmatic approaches to enhance the interfacial stability of Na metal anode.20-21 Recently, high concentration electrolytes (HCEs) have attracted great research interest for Libased batteries,24-28 ascribed to their unique solvation structures and functionalities, which also work well for SMBs,21 and potassium (K) metal batteries.29 As reported earlier, the reversible Na deposition/stripping was achieved by using an HCE (≥4 M) based on ether solvents (e.g., 1,2dimethoxyethane, DME) and the sodium bis(fluorosulfonyl)imide (NaFSI) salt.21 Long-term cycling with a high CE of 99% was obtained when Na||Cu cells were cycled at 1.0 mA cm-2, proving the concentrated NaFSI/DME a promising electrolyte for developing long cycle life SMBs. However, HCEs involve high salt cost, high viscosity, and poor wettability toward the separator and thick cathode electrode, posing a great challenge to its large scale applications. An effective approach to overcome the shortcomings of HCEs is to dilute it with an “inert” solvent, forming a localized high concentration electrolyte (LHCE), as schematically illustrated in Figure 1a. The “inert” diluent has minimal or no effect on the solvation structure of cationanion aggregates (AGGs) that exist in concentrated electrolyte. Instead, it can significantly lower the sodium salt concentration, reduce the viscosity, increase the conductivity, and improve the wettability of the electrolyte. Hydrofluoroethers are identified to meet the “inert” requirements due to their low dielectric constant and low donor number,30 and have been used to dilute the


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HCEs

for

Li-based

batteries,

including

bis(trifluoromethanesulfonyl)imide/tetraglyme)31

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ether-based and

(e.g.,

carbonate-based

concentrated (e.g.,

lithium

concentrated

LiBF4/propylene carbonate),30 which however is often accompanied with compromised stability towards metal anodes.30 Recently, Chen et al. demonstrated a high-voltage lithium metal battery enabled by LHCE (unpublished results). Here, we demonstrate for the first time that the use of a hydrofluoroether, i.e., bis(2,2,2-trifluoroethyl) ether (BTFE), to dilute an HCE of 5.2 M NaFSI/DME electrolyte (NaFSI:DME 1:1 molar ratio) could further enhance the stability of Na metal anodes. Particularly, the BTFE-diluted electrolytes 2.1 M NaFSI/DME-BTFE (DME:BTFE 1:2 by mol) and 1.5 M NaFSI/DME-BTFE (DME:BTFE 1:3 by mol) result in dendrite-free Na plating at high current densities with high CE (>99%), and enable long-term cycling of Na||Na3V2(PO4)3 batteries. This excellent performance could not be achieved by using the traditional dilute ~1.7 M NaFSI/DME electrolyte or HCE of 5.2 M NaFSI/DME.

Physicochemical properties of HCE and LHCEs Raman spectroscopy was employed to characterize the solvation structure of HCE (5.2 M NaFSI/DME) and three LHCEs: (NaFSI/DME-BTFE) with DME:BTFE molar ratios of 1:1 (3.1 M NaFSI), 1:2 (2.1 M NaFSI), and 1:3 (1.5 M NaFSI). As illustrated in Figure 1b, free DME molecules exhibit absorption bands at 825 and 853 cm-1, which are assigned to the -CH2-O-CH3 stretching vibration of DME.32 The dilute 1.7 M NaFSI/DME (NaFSI:DME molar ratio 1:5) electrolyte shows obvious presence of DME molecules, as evidenced by the broad peaks between 820~860 cm-1, in addition to its vibration band at ~869 cm-1 that is assigned to Na+-coordinated DME. This dilute electrolyte also exhibits an absorption band at 722 cm-1, indicating the


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presence of a large amount of uncoordinated FSI− anion. In the HCE of 5.2 M NaFSI/DME, the free DME disappears to form the contact ion pairs (CIPs, FSI− coordinates with one Na+ ion) and AGGs (FSI− coordinates with two or more Na+ ions), which is further corroborated by the distinct upshift of the FSI− Raman band to 751 cm-1.

c

a

HCE LHCE

b

DME:BTFE molar ratio

1:1

10

1.7 M

1.5 M

2.1 M

3.1 M

1.0 M

(1:5)

1:3

1

1 M NaPF6/ EC-DEC

NaFSI:DME

1:2 5.2 M

Viscosity (mPa S)

100

Electrolye composition Coordinated DME DME:BTFE = 1:3

BTFE

AGGs

Free FSI

-

1.5 M

6

DME:BTFE = 1:2

2.1 M DME:BTFE = 1:1

3.1 M HCE NaFSI:DME = 1:1

5.2 M

Dilute NaFSI:DME = 1:5

1.7 M

BTFE DME

950

900

850 800 750 -1 Raman shift (cm )

d Conductivity (mS cm-1)

LHCE

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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700

1 M NaPF6/EC-DEC DME:BTFE molar ratio

4

1:3 1:2 1:1

2

LHCE HCE

0 -10

0

10

20

30

40

50

Temperature (oC)

Figure 1. (a) Schematic illustration of dilution from an HCE to an LHCE. (b) Raman spectra of HCE and LHCEs with different NaFSI concentrations. (c) Viscosity and (d) ionic conductivity of dilute electrolyte, HCE, and LHCEs with different NaFSI concentrations. Detailed compositions of the electrolytes are provided in Table S1.

After dilution with BTFE, besides the absence of a DME absorption band at 825 cm-1, the LHCEs (NaFSI/DME-BTFE) exhibit the same vibration band at ~751 cm-1 for Na+-FSI−-solvent


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AGGs, confirming that the Na+-coordinated DME solvation structure is well preserved. In addition, the consistent presence of the ~835 cm-1 vibration band indicates that BTFE does not engage in the solvation of Na+ cations, confirming the LHCE solvation structure (as schematically shown in Figure 1a). With the well maintained Na+-FSI−-solvent AGGs solvation structure, the LHCE (NaFSI/DME-BTFE) is expected to possess unique functionalities similar to those of HCE (5.2 M NaFSI/DME) in stabilizing the Na metal anode.21 Furthermore, dilution with BTFE solvent greatly reduces the viscosity and improves the ionic conductivity of the electrolyte (Figure 1c, d), which could promptly wet the separator and highly loaded cathode electrodes, signifying enhanced electrochemical performance of SMBs.


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Na metal CE and deposition morphology

a

100

Efficiency (%)

J = 1 mA cm-2 1.5 2.1 3.1 5.2

1.7 M NaFSI/DME

50

M M M M

NaFSI/DME-BTFE(1:3) NaFSI/DME-BTFE(1:2) NaFSI/DME-BTFE(1:1) NaFSI/DME

1.0 M NaPF6/EC-DEC 0 0

+

Voltage (V vs. Na/Na )

b

50

1.0 0.8

1.5 M 2.1 M 3.1 M 5.2 M

0.1

100

150

d


200 250 Cycle number

300

e

5.2 M NaFSI/DME

350

400

3.1 M NaFSI/DME-BTFE(1:1)

J = 0.2 mA cm-2

0.0

-0.1 0.0

0.2

0.4

0.6

0.8

10 µm

1.0

10 µm

Areal capacity (mAh cm-2)

c

f

0.0


g


J = 1 mA cm-2

Polarization

+

Voltage (V vs. Na/Na )

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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-0.2 1.5 2.1 3.1 5.2

-0.4 0.0

0.5

M M M M


1.0

1.5

2.0

10 µm

10 µm

Areal capacity (mAh cm-2)

Figure 2. (a) CE of Na deposition/stripping using Cu electrodes at 1 mA cm-2 after 2 formation cycles at 0.2 mA cm-2 with an areal capacity of 1 mAh cm-2. (b) Initial deposition/stripping profiles at 0.2 mA cm-2. (c) Initial deposition profiles at 1 mA cm-2. (d-g) SEM images of Na deposited on Cu electrodes at 1 mA cm-2 with an areal capacity of 2 mAh cm-2: (d) HCE (5.2 M NaFSI/DME); (e) LHCE (3.1 M NaFSI/DME-BTFE (1:1)); (f) LHCE (2.1 M NaFSI/DMEBTFE (1:2)); (g) LHCE (1.5 M NaFSI/DME-BTFE (1:3)).

Na||Cu cells were used to investigate the CE of the Na metal deposition/stripping using HCE and LHCEs (NaFSI/DME-BTFE) (Figure 2a, b). The “zoom-in” version of Figure 2a is further


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shown in Figure S1 for a more clear comparison. With regular low concentration electrolytes, the Na||Cu cells show very low CE, which is only ca. 20% for 1 M sodium hexafluorophosphate (NaPF6)/ethylene carbonate (EC) – diethyl carbonate (DEC) and ca. 50% for 1.7 M NaFSI/DME. This is ascribed to the existence of a large amount of free solvent molecules and aggressive parasitic reactions with the highly reactive Na metal. Using HCE of 5.2 M NaFSI/DME, the significant decrease in free solvent molecules noticeably improves the CE of the Na||Cu cell to 89.3% in the first cycle and the CE quickly stabilizes at 98–99%, consistent with results reported previously.21 The LHCEs (NaFSI/DME-BTFE) of decreased NaFSI salt concentrations show similar or even better stability with Na metal anodes, exhibiting CEs of >99.0% during long-term cycling. The average CE of LHCE 2.1 M NaFSI/DME-BTFE (1:2) in 400 cycles is determined to be 98.95%, which is even higher than the 97.80% obtained for HCE of 5.2 M NaFSI/DME. The superior stability of LHCE with Na metal anode is further evidenced by the stable deposition/stripping profiles during cycling (Figure S2). The result demonstrates the exceptional stability of the LHCEs at reduced salt concentrations, ascribed to the well maintained locally concentrated solvation structures and the stability between BTFE and Na metal. Furthermore, the LHCEs show much less voltage hysteresis for Na deposition at an elevated current density of 1 mA cm-2 (Figure 2c), confirming their enhanced kinetics for Na metal deposition. The morphology of the Na deposited in different electrolytes at 1 mA cm-2 was examined by scanning electron microscopy (SEM). As shown in Figure 2d-g, nodule-like Na deposits of 5~10 um particle size are observed in HCE and LHCEs, which are significantly larger than those deposited in the dilute electrolytes, e.g., 1 M NaPF6/EC-DEC (Figure S3) and 1.7 M NaFSI/DME (Figure S4). In comparison, the Na nodules deposited in the LHCEs appear to be slightly larger than those deposited in the HCE. A similar trend is observed when Na is deposited


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at a lower current density of 0.2 mA cm-2 (Figure S5). A possible reason for the large particle size of Na deposited in LHCEs is that particle growth (rather than nucleation) dominates the deposition process because of the lower Na+ ion concentration. The larger particle size of Na deposits readily reduces the contact area with the electrolytes and is beneficial for reducing the parasitic reactions with the electrolyte and enhancing long-term cycling stability of SMBs.

Na metal deposition/stripping cycling stability a Voltage (V)

0.4

Na||Na cell

J = 1.0 mA cm-2

0.2 0.0 -0.2

HCE (5.2 M NaFSI/DME) LHCE (2.1 M NaFSI/DME-BTFE(1:2))

-0.4 0

100

200

300

400

500

600

700

800

900

1000

Time (h)

b Voltage (V)

0.4

Na||Na cell

J = 2.0 mA cm-2

0.2 0.0 -0.2

HCE (5.2 M NaFSI/DME) LHCE (2.1 M NaFSI/DME-BTFE(1:2))

-0.4 0

100

c 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

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200

300

J = 2.0 mA cm-2

400

500

600

700

Time (h)

d

J = 2.0 mA cm-2

0.2

0.2

0.2

0.0

0.0

0.0

-0.2

-0.2 HCE

100

105

110 Time (h)

120

300

900

1000

J = 2.0 mA cm-2

-0.2 HCE

LHCE

115

800

e

305

LHCE

310 315 Time (h)

320 500

HCE

505

LHCE

510 515 Time (h)

520

Figure 3. (a, b) Long-term cycling of Na deposition/stripping of Na||Na cells using HCE (5.2 M NaFSI/DME) and LHCE (2.1 M NaFSI/DME-BTFE (1:2)) at (a) 1.0 mA cm-2, and (b) 2.0 mA cm-2 after two initial cycles at 0.2 mA cm-2; (c-e) magnified sections at different stages of


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cycling of the Na deposition/stripping voltage profiles of Na||Na cells at 2.0 mA cm-2 shown in panel (b). The areal capacity is 1 mAh cm-2 for each deposition or stripping process.

The stability and long-term compatibility of the HCE or LHCEs against the Na metal anode was further investigated using Na||Na symmetric cells. Figure 3 shows the cycling performance of Na||Na cells with HCE of 5.2 M NaFSI/DME and LHCE of 2.1 M NaFSI/DME-BTFE (1:2) at a current density of 1 mA cm-2 (Figure 3a) and 2 mA cm-2 (Figure 3b-e). The Na||Na cell with HCE shows stable cycling, but exhibits large electrode overpotential for Na metal deposition/stripping. The voltage response is >0.06 V at 1 mA cm-2 and >0.2 V at 2 mA cm-2. Moreover, the voltage polarization increases during cycling, especially at 2 mA cm-2, indicating a relatively poor interfacial stability of HCE against the Na metal anode. On the contrary, the Na||Na cell with LHCE of 2.1 M NaFSI/DME-BTFE (1:2) exhibits very stable voltage profiles of much lower electrode overpotential (Figure 3c-e). The Na metal deposition/stripping profiles demonstrate favorable Na metal exchange with an ideal cell voltage response of 0.025 V at 1 mA cm-2 and 0.05 V at 2 mA cm-2. The excellent cycling along with limited polarization indicates that the LHCE with BTFE diluent improves the interfacial compatibility, enabling the reversible deposition/stripping of Na metal. The cycling performance of Na||Na cells using HCE and different LHCEs are compared in Figures S6 (1.0 mA cm-2), S7 (2.0 mA cm-2). In comparison, the LHCE of 2.1 M NaFSI/DME-BTFE (1:2) is more favorable for reversible Na metal cycling at high current densities (≥2.0 mA cm-2), which allows for fast charging/discharging in real SMBs.


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Electrochemical performance of Na||Na3V2(PO4)3 (NVP) batteries b1000

NVP||NVP cell HCE LHCE

10 5

c 3000

Na||Na cell

HCE LHCE (DME-BTFE(1:2))

2000

500

1000

0

0 0

20

C/10

100

0

1C

C/5

HCE LHCE

0 0

1000 2000 Zre (ohm)

2C

5C

1000 2000 Zre (ohm)

3000

C/5

10C

-1

Capacity (mAh g )

d

10 Zre (ohm)

Na||NVP cell

-Zim (ohm)

-Zim (ohm)

15

-Zim (ohm)

a

75

Na||NVP 1.5 M NaFSI/DME-BTFE(1:3) 2.1 M NaFSI/DME-BTFE(1:2) 3.1 M NaFSI/DME-BTFE(1:1) 5.2 M NaFSI/DME

50 25 0

Capacity (mAh g )

e -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

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5

10

C/10

15 20 Cycle number

25

30

35

20C

80 60 40

1.5 M 2.1 M 3.1 M 5.2 M

20 0 0

NaFSI/DME-BTFE(1:3) NaFSI/DME-BTFE(1:2) NaFSI/DME-BTFE(1:1) NaFSI/DME 10000

20000 Cycle number

30000

40000

Figure 4. (a-c) Nyquist plots of (a) NVP||NVP cells, (b) Na||Na cells, and (c) Na||NVP cells using HCE (5.2 M NaFSI/DME) and LHCE (2.1 M NaFSI/DME-BTFE (1:2)) prior to electrochemical cycling. (d) Rate capacity of Na||NVP batteries using HCE and different LHCEs. (e) Cycling performance of Na||NVP batteries at 20C after 3 formation cycles at C/10.

Interfacial reaction kinetics of cells using HCE and LHCE was investigated by electrochemical impedance spectroscopy (EIS). The impedance spectra for NVP||NVP cells, Na||Na cells, and


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Na||NVP cells using HCE (5.2 M NaFSI/DME) and LHCE (2.1 M NaFSI/DME-BTFE (1:2)) before cycling are presented in Figure 4a-c. The cells using LHCE showed lower electrolyte resistance and interfacial resistance than those using HCE. The interfacial resistances of NVP||NVP cells, Na||Na cells, and Na||NVP cells using LHCE (2.1 M NaFSI/DME-BTFE (1:2)) (1.4 Ω, 385 Ω, 218 Ω, respectively) are much lower than those using HCE (5.2 M NaFSI/DME) (10.2 Ω, 1982 Ω, 985 Ω, respectively) (Table S2). The result suggests that the dilution with BTFE significantly improves the interfacial reaction kinetics, which benefits to enhance the power performance of SMBs. The rate performance of Na||NVP batteries using HCE and LHCEs was investigated and result is shown in Figure 4d. Due to the high viscosity of HCE, the Na||NVP battery using HCE shows poor rate capability, as reflected by the relatively fast capacity decline at increasing C rates. The Na||NVP batteries using LHCEs with different content of BTFE diluent show superior rate capability, delivering higher capacities at ascending C rates, as compared to the one with HCE (5.2 M NaFSI/DME). In particular, the Na||NVP battery with 2.1 M NaFSI/DME-BTFE (1:2) is capable of delivering a high discharge capacity of 92 mAh g-1 at 10C, corresponding to 94.6% of its initial capacity at C/10. The enhanced rate capability of the Na||NVP battery with LHCEs coincides with the improved interfacial reaction kinetics that is facilitated by the reduced viscosity (Figure 1c), as well as the improved ionic conductivity (Figure 1d) and wettability of the LHCEs (Figure S8). Figure 4e shows the cycling performance of Na||NVP batteries using HCE and different LHCEs at a harsh 20C rate for both charge and discharge. All the batteries deliver similar initial discharge capacity of 95–98 mAh cm-2 upon formation cycles at C/10. During subsequent cycling at 20C, the Na||NVP batteries exhibit different cycling stability depending on the


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electrolytes used. The battery using 1.7 M NaFSI in DME electrolyte shows fast capacity fading, ascribed to the poor stability between Na metal and the dilute electrolyte (Figure S9). Using HCE, the cycling stability of the Na||NVP battery is greatly improved because the parasitic reactions between the Na metal anode and the concentrated electrolyte are mitigated. However, the Na||NVP battery with HCE yet shows continuous increase of cell polarization, especially at the end of discharge, indicating an increase of kinetic barrier and a loss of battery energy density after cycling (Figure S10). In contrast, the Na||NVP batteries with different BTFE-diluted LHCEs display superior long-term cycling as compared to the one with HCE. Using 2.1 M NaFSI/DME-BTFE (1:2), the Na||NVP battery delivers a discharge capacity of 66.4 mAh g-1 after 40,000 cycles (90.8% capacity retention), significantly higher than the 24.0 mAh g-1 (48.0% capacity retention) for the battery with HCE. To the best of our knowledge, the extremely fast charging and long-term cycling performance in LHCE of 2.1 M NaFSI/DME-BTFE (1:2) is the best ever reported for Na||NVP batteries.33-35 More importantly, the Na||NVP batteries with BTFE-diluted LHCEs show very stable charge/discharge voltage profiles, with very limited increase of cell polarization (Figure S10). The superior cycling stability of Na||NVP batteries with LHCEs is also achieved during cycling at other C rates, including C/3, 5C, and 10C (Figures S10, S11). All of these results substantiate that with the “inert” BTFE diluent, the LHCEs with low NaFSI salt concentration further promote the electrochemical performances as compared to HCE, validating the LHCEs as promising electrolytes for constructing high CE, fast charging, and long cycle life SMBs.


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70

b

at 0 nm

60 50

Na

40

Na

Na

O

O

O

30

Atomic ratio (%)

a Atomic ratio (%)

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 10 0

C

N

F

S

C

F N

S

C

CN

F

LHCE Dilute HCE Na SEI in different Electrolytes

S

70

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at 10 nm

60

Na

50 40

Na

O

30

0

O

O

20 10

Na

F CN

F

S

F

S CN

S CN

LHCE Dilute HCE Na SEI in different Electrolytes

Figure 5. Atomic ratios of C/N/O/F/Na/S elements detected at (a) the surface (0 nm) and (b) 10 nm depth of SEI layer after the 10th stripping of Na||Cu cells using dilute (1.7 M NaFSI/DME), HCE (5.2 M NaFSI/DME), and LHCE (2.1 M NaFSI/DME-BTFE (1:2)).

To further ascertain the functioning mechanism of HCE/LHCEs, XPS analysis was performed to study to the chemical composition of the solid electrolyte interphase (SEI) layer formed on Na metal anodes. The XPS data reveal that the SEI layers formed in HCE (5.2 M NaFSI/DME) and LHCE (2.1 M NaFSI/DME-BTFE (1:2)) contain a larger quantity of F and S elements, while the one formed in dilute 1.7 M NaFSI/DME shows higher content of C and O elements (Figure 5 and Figures S12, S13). This result discloses that more FSI− anions participate in the formation of a more robust SEI layer when Na metal is cycled in either an HCE or an LHCE, thereby mitigating the consumption of solvent molecules during repeated Na deposition/stripping process, which agrees well with the unique functionalities of HCEs reported for Li-based battery systems.25-28 The chemical composition of an SEI layer formed in LHCE (2.1 M NaFSI/DME-BTFE (1:2)) closely resembles that formed in the HCE of 5.2 M NaFSI/DME, indicating the preservation of concentrated salt solvation structures in LHCEs and their unique functionalities in forming a Fenriched SEI layer (Figures S12, S13). In addition to the ability to form a stabilized SEI layer,


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LHCE electrolytes show improved wettability, lower viscosity, and higher conductivity, therefore outperforming the HCE. This diluting strategy could be generalized to the use of other more cost-effective hydrofluoroethers, e.g., 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) (Figure S14, Table S3), and may inspire further research in identifying more sustainable “inert” diluents.

To summarize, we discovered that an LHCE with low overall salt concentration (2.1 M NaFSI/DME-BTFE (1:2)) enables the dendrite-free Na plating with high CE of >99%, and greatly enhance the fast charging (20C) and stable cycling (90.8% after 40,000 cycles) of Na||NVP batteries. This is due to the “inert” nature of BTFE diluent that does not break the localized Na+-FSI−-DME solvation structures, but plays an important role in improving the interfacial reaction kinetics and interfacial stability of Na metal anode. This excellent performance is even superior to that achieved in HCE of 5.2 M NaFSI/DME. The fundamental findings reported in this work open a new avenue to tailor electrolyte chemistry with low salt concentration for developing high performance SMBs. They could also be widely applied to promote the development of other battery systems, including Li metal batteries, K metal batteries, and aqueous-based batteries, to reduce the electrolyte cost and enhance the fast charging/discharging capability of these rechargeable metal-anode–based batteries.

ASSOCIATED CONTENT Supporting Information.


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Experimental details, Additional electrochemical data, SEM images, and XPS data, physicochemical properties of electrolytes, fitted results of EIS data, price and supplier information for BTFE, TTE, and NaFSI (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies (VTO) of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) program under Contract DE-AC0205CH11231. Microscopy as well as spectroscopy characterizations were performed in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.


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REFERENCES (1)

Oh, S.-M.; Myung, S.-T.; Yoon, C. S.; Lu, J.; Hassoun, J.; Scrosati, B.; Amine, K.; Sun,

Y.-K. Advanced Na[Ni0.25Fe0.5Mn0.25]O2/C–Fe3O4 Sodium-Ion Batteries Using EMS Electrolyte for Energy Storage. Nano Lett. 2014, 14, 1620–1626. (2)

Li, X.; Wu, D.; Zhou, Y.-N.; Liu, L.; Yang, X.-Q.; Ceder, G. O3-type

Na(Mn0.25Fe0.25Co0.25Ni0.25)O2: A Quaternary Layered Cathode Compound for Rechargeable Na Ion Batteries. Electrochem. Commun. 2014, 49, 51–54. (3)

Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-Ion Batteries. Adv. Funct. Mater.

2013, 23, 947–958. (4)

Wang, Y.; Yu, X.; Xu, S.; Bai, J.; Xiao, R.; Hu, Y.-S.; Li, H.; Yang, X.-Q.; Chen, L.;

Huang, X. A Zero-Strain Layered Metal Oxide as the Negative Electrode for Long-Life SodiumIon Batteries. Nat. Commun. 2013, 4, 2365. (5)

Song, J.; Yan, P.; Luo, L.; Qi, X.; Rong, X.; Zheng, J.; Xiao, B.; Feng, S.; Wang, C.; Hu,

Y.-S.; et al. Yolk-Shell Structured Sb@C Anodes for High Energy Na-Ion Batteries. Nano Energy 2017, 40, 504–511. (6)

Zhao, Y.; Manthiram, A. High-Capacity, High-Rate Bi–Sb Alloy Anodes for Lithium-Ion

and Sodium-Ion Batteries. Chem. Mater. 2015, 27, 3096–3101. (7)

Jiang, Y.; Hu, M.; Zhang, D.; Yuan, T.; Sun, W.; Xu, B.; Yan, M. Transition Metal

Oxides for High Performance Sodium Ion Battery Anodes. Nano Energy 2014, 5, 60–66.


17


(8)

Page 18 of 21

Yoon, H.; Zhu, H.; Hervault, A.; Armand, M.; MacFarlane, D. R.; Forsyth, M.

Physicochemical Properties of N-propyl-N-methylpyrrolidinium Bis(fluorosulfonyl)imide for Sodium Metal Battery Applications. Phys. Chem. Chem. Phys. 2014, 16, 12350–12355. (9)

Matsumoto, K.; Hosokawa, T.; Nohira, T.; Hagiwara, R.; Fukunaga, A.; Numata, K.;

Itani, E.; Sakai, S.; Nitta, K.; Inazawa, S. The Na[FSA]–[C2C1im][FSA] (C2C1im+:1-ethyl-3methylimidazolium and FSA−:bis(fluorosulfonyl)amide) Ionic Liquid Electrolytes for Sodium Secondary Batteries. J. Power Sources 2014, 265, 36–39. (10) Mohd Noor, S. A.; Yoon, H.; Forsyth, M.; MacFarlane, D. R. Gelled Ionic Liquid Sodium Ion Conductors for Sodium Batteries. Electrochim. Acta 2015, 169, 376–381. (11) Bayley, P. M.; Trease, N. M.; Grey, C. P. Insights into Electrochemical Sodium Metal Deposition as Probed with in Situ 23Na NMR. J. Am. Chem. Soc. 2016, 138, 1955–1961. (12) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503–11618. (13) Zhao, Y.; Goncharova, L. V.; Lushington, A.; Sun, Q.; Yadegari, H.; Wang, B.; Xiao, W.; Li, R.; Sun, X. Superior Stable and Long Life Sodium Metal Anodes Achieved by Atomic Layer Deposition. Adv. Mater. 2017, 29, 1606663. (14) Luo, W.; Lin, C.-F.; Zhao, O.; Noked, M.; Zhang, Y.; Rubloff, G. W.; Hu, L. Ultrathin Surface Coating Enables the Stable Sodium Metal Anode. Adv. Energy Mater. 2017, 7, 1601526. (15) Zhao, Y.; Goncharova, L. V.; Zhang, Q.; Kaghazchi, P.; Sun, Q.; Lushington, A.; Wang, B.; Li, R.; Sun, X. Inorganic–Organic Coating via Molecular Layer Deposition Enables Long Life Sodium Metal Anode. Nano Lett. 2017, 17, 5653–5659.


18

Page 19 of 21 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

ACS Energy Letters

(16) Kim, Y.-J.; Lee, H.; Noh, H.; Lee, J.; Kim, S.; Ryou, M.-H.; Lee, Y. M.; Kim, H.-T. Enhancing the Cycling Stability of Sodium Metal Electrodes by Building an Inorganic–Organic Composite Protective Layer. ACS Appl. Mater. Interfaces 2017, 9, 6000–6006. (17) Wang, H.; Wang, C.; Matios, E.; Li, W. Critical Role of Ultrathin Graphene Films with Tunable Thickness in Enabling Highly Stable Sodium Metal Anodes. Nano Lett. 2017, 17, 6808– 6815. (18) Wang, A.; Hu, X.; Tang, H.; Zhang, C.; Liu, S.; Yang, Y.-W.; Yang, Q.-H.; Luo, J. Processable and Moldable Sodium-Metal Anodes. Angew. Chem. Int. Ed. 2017, 56, 11921– 11926. (19) Zhou, W.; Li, Y.; Xin, S.; Goodenough, J. B. Rechargeable Sodium All-Solid-State Battery. ACS Cent. Sci. 2017, 3, 52–57. (20) Seh, Z. W.; Sun, J.; Sun, Y.; Cui, Y. A Highly Reversible Room-Temperature Sodium Metal Anode. ACS Cent. Sci. 2015, 1, 449–455. (21) Cao, R.; Mishra, K.; Li, X.; Qian, J.; Engelhard, M. H.; Bowden, M. E.; Han, K. S.; Mueller, K. T.; Henderson, W. A.; Zhang, J.-G. Enabling Room Temperature Sodium Metal Batteries. Nano Energy 2016, 30, 825–830. (22) Schafzahl, L.; Hanzu, I.; Wilkening, M.; Freunberger, S. A. An Electrolyte for Reversible Cycling of Sodium Metal and Intercalation Compounds. ChemSusChem 2017, 10, 401–408. (23) Zheng, J.; Engelhard, M. H.; Mei, D.; Jiao, S.; Polzin, B. J.; Zhang, J.-G.; Xu, W. Electrolyte Additive Enabled Fast Charging and Stable Cycling Lithium Metal Batteries. Nat. Energy 2017, 2, 17012.


19


Page 20 of 21

(24) Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (25) Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J.-G. High Rate and Stable Cycling of Lithium Metal Anode. Nat. Commun. 2015, 6, 6362. (26) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. “Water-in-Salt” Electrolyte Enables High-Voltage Aqueous Lithium-Ion Chemistries. Science 2015, 350, 938–943. (27) Yamada, Y.; Furukawa, K.; Sodeyama, K.; Kikuchi, K.; Yaegashi, M.; Tateyama, Y.; Yamada, A. Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for FastCharging Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5039–5046. (28) Zheng, J.; Lochala, J. A.; Kwok, A.; Deng, Z. D.; Xiao, J. Research Progress towards Understanding the Unique Interfaces between Concentrated Electrolytes and Electrodes for Energy Storage Applications. Adv. Sci. 2017, 4, 1700032. (29) Xiao, N.; McCulloch, W. D.; Wu, Y. Reversible Dendrite-Free Potassium Plating and Stripping Electrochemistry for Potassium Secondary Batteries. J. Am. Chem. Soc. 2017, 139, 9475–9478. (30) Doi, T.; Shimizu, Y.; Hashinokuchi, M.; Inaba, M. Dilution of Highly Concentrated LiBF4/Propylene Carbonate Electrolyte Solution with Fluoroalkyl Ethers for 5-V LiNi0.5Mn1.5O4 Positive Electrodes. J. Electrochem. Soc. 2017, 164, A6412–A6416.


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(31) Dokko, K.; Tachikawa, N.; Yamauchi, K.; Tsuchiya, M.; Yamazaki, A.; Takashima, E.; Park, J.-W.; Ueno, K.; Seki, S.; Serizawa, N.; et al. Solvate Ionic Liquid Electrolyte for Li–S Batteries. J. Electrochem. Soc. 2013, 160, A1304–A1310. (32) Fukushima, K.; Chibahara, H. Conformational Difference due to Types of Cation in 1,2dimethoxyethane-cation Complexes as Studied by Raman and FT-IR Spectroscopy. J Mol. Struct. 1993, 291, 145–150. (33) Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3, 444–450. (34) Jian, Z.; Han, W.; Lu, X.; Yang, H.; Hu, Y.-S.; Zhou, J.; Zhou, Z.; Li, J.; Chen, W.; Chen, D.; et al. Superior Electrochemical Performance and Storage Mechanism of Na3V2(PO4)3 Cathode for Room-Temperature Sodium-Ion Batteries. Adv. Energy Mater. 2013, 3, 156–160. (35) Zhu, C.; Kopold, P.; van Aken, P. A.; Maier, J.; Yu, Y. High Power–High Energy Sodium Battery Based on Threefold Interpenetrating Network. Adv. Mater. 2016, 28, 2409– 2416.


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Extremely Stable Sodium Metal Batteries Enabled by Localized High

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