Diperovskite (NH4)3FeF6/Graphene Nanocomposites towards

Superior Na-Ion Storage

Zhanghui Hao,a Masayoshi Fuji b,* and Jisheng Zhou a,*

a. State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China.b. Advanced Ceramics Research Center, Nagoya Institute of Technology, 3-101-1, Honmachi, Tajimi, Gifu 507-0033, Japan

* Corresponding authors: zhoujs@mail.buct.edu.cn (J Zhou) fuji@nitech.ac.jp (M Fuji)

Supporting Data:

0.30 0.45 0.60 0.75 0.900.0

0.1

0.2

0.3

0.4

Per

cent

age

Average diameter (μm)

0.0 1.0 2.0 3.0 4.00.0

0.1

0.2

0.3

0.4

Per

cent

age

Average diameter (μm)

(a) (b)

Figure S1. The particles size distribution of the (a) (NH4)3FeF6 and (b) (NH4)3FeF6/GNS prepared at 200 oC for 2 h.

0.0 0.2 0.4 0.6 0.8 1.00

35

70

105

140

Qua

ntity

ads

orbe

d (c

m3 g

-1)

Relative pressure (P/Po)

/ (NH4)3FeF6/GNS / (NH4)3FeF6

Electronic Supplementary Material (ESI) for Sustainable Energy & Fuels.This journal is © The Royal Society of Chemistry 2019

Figure S2. N2 adsorption-desorption isotherms of the pure (NH4)3FeF6 and (NH4)3FeF6/GNS.

25 200 400 600 80020

40

60

80

100

Temperature (oC)

Mas

s (%

)

-10

0

10

20

30

40

33.2%

39.0% D

SC

(μv/mg)

Figure S3. TG and DSC curves of the (NH4)3FeF6/GNS obtained in air atmosphere from room temperature to 800 oC at a rate of 10 oC min-1.

The TG curve of (NH4)3FeF6/GNS can be divided into two stages: (I) at 25-400 oC, there is rapid weight loss of 39.0% at ca. 200 oC, which should be attributed to the transformation from (NH4)3FeF6 phase to FeF3 phase. There is an endothermic peak on the corresponding temperature range of differential scanning calorimetry (DSC) line; (II) at 400-800 oC, when the temperature up to 410 oC, the TG curve also display a rapid decline corresponding to the oxidations of graphene into CO2/CO and FeF3 into Fe2O3. Therefore, there is an obvious exothermal peak on the corresponding temperature range of DSC line. The (NH4)3FeF6 and GNS content in (NH4)3FeF6/GNS can be calculated to be ca. 78.0 wt% and 22.0 wt%, respectively.

10 20 30 40 50 60 70 80

FeF2/GNS

Two-theta(degree)

Inte

nsity

(Cou

nts)

FeF2-PDF#79-0745

(a) (b)

140 210 280 3500.0

0.1

0.2

0.3

0.4

Perc

enta

ge

Average diameter (μm)

Figure S4. (a) XRD pattern, (b) crystal structure and (c, d) SEM images of FeF2/GNS obtained by annealing (NH4)3FeF6/GNS at 400 oC for 3 h.

10 20 30 40 50 60 70 80

(300

)(2

20)

(211

)

(200

)

(110

)

(100

)

(420

)

(400

)

(220

)

(200

)

Inte

nsity

(Cou

nts)

Two-theta(degree)

NH4FeF3

(111

)

(NH4)3FeF6

4:1

6:1

8:1

NH4FeF3-PDF#54-1071

(NH4)3FeF6-PDF#77-0147

Figure S5. XRD pattern of the samples obtained at various NH4F/Fe(acac)3 molar ratios of 4:1, 6:1 and 8:1 at 200 oC for 2 h.

Figure S6. SEM images of the samples obtained at various NH4F/Fe(acac)3 molar ratios of (a and d) 4:1, (b and e) 6:1 and (c and f) 8:1 at 200 oC for 2 h.

10 20 30 40 50 60 70 80

80 oC

150 oC200 oC

300 oC

400 oC

NH4FeF3

(NH4)3FeF6

Inte

nsity

(Cou

nts)

Two-theta(degree)

NH4FeF3-PDF#54-1071

(NH4)3FeF6-PDF#77-0147

Figure S7. XRD pattern of the samples obtained at different reaction temperatures of 80, 150, 200, 300 and 400 oC with a NH4F/Fe(acac)3 molar ratio of 8:1.

0.0 1.0 2.0 3.0 4.00.00

0.15

0.30

0.45

0.60

Perc

enta

ge

Average diameter (μm)

0.0 1.0 2.0 3.00.00

0.15

0.30

0.45

0.60

Perc

enta

ge

Average diameter (μm)

0.0 1.0 2.0 3.0 4.0 5.00.00

0.15

0.30

0.45

0.60

Perc

enta

ge

Average diameter (μm)

Figure S8. SEM images of the samples obtained at different reaction temperatures (a and f) 80, (b and g) 150, (c and h) 200, (d and i) 300 and (e and j) 400 oC with a NH4F/Fe(acac)3 molar ratio of 8:1. (Inset: corresponding to the particles size distribution of the samples.)

1200 1000 800 600 400 200 0

Na 1s

Na KLL

F 1s

N 1sO 1s

pristine

1st charge

1st discharge

Binding Energy (eV)

C 1sFe 2P

690 687 684 681

Binding Energy (eV)

Fe3+-Fpristine685.0

0.1 VNa+-F

684.5

PVDF

685.7

F 1s2.8 V

Fe3+-F

PVDF

408 405 402 399 396Binding Energy (eV)

pristine

401.4

399.9

398.8

N doped in GNS

(NH4+)

(NH4+)

400.7398.5

399.7N doped in GNS

0.1 V

398.8

399.9N doped in GNS

(NH4+)

401.4

N1s

2.8 V

732 726 720 714 708

711.5

satellite peak

Fe3+

Fe 2P3/2Fe 2P1/2

pristine

Binding Energy (eV)

710.1

708.0

712.1 Fe3+satellite peak

Fe0

Fe2+

Fe 2P3/2

Fe 2P1/2

0.1 V

711.5

Fe 2p3/2

2.8 V Fe 2p

Fe 2p1/2 Fe3+

satellite peak

1077 1074 1071 1068 1065

0.1 V

2.8 V

Na 1s

Binding Energy (eV)

1071.0

1072.7(a) (b)

(c) (d) (e)

Figure S9. (a) XPS survey spectra, (b) Na1s, (c) N1s, (d) Fe2p and (e) F1s spectra of pristine (NH4)3FeF6/GNS, and (NH4)3FeF6/GNS electrodes after discharging to 0.1 V and charging to 2.8 V in the first cycle, respectively.

ⅰ ⅱ ⅲ

ⅳ ⅴ ⅵ

Figure S10. (i) STEM images of (NH4)3FeF6/GNS electrode after discharging to 0.1 V in the first cycle and corresponding elemental mappings of C (ii), Fe (iii), N (iv), F (v), Na (vi).

0 100 200 300 4000.0

0.6

1.2

1.8

2.4

3.0

GNS 0.1 A g-1

1st 2nd 3rd

Specific capacity (mAh/g)

Vol

tage

(V)

Figure S11. The initial three charge/discharge curves of GNS at 0.1 A g-1.

The first reversible capacities of (NH4)3FeF6/GNS and GNS are ca. 487.8 mA h g-1 and 282.5 mA h g-1 (Fig. S11). Therefore, the specific capacity contribution of GNS in the (NH4)3FeF6/GNS composite is 62.1 mA h g-1 (calculated by 282.5×22.0%), and the specific capacity contribution of (NH4)3FeF6 in the (NH4)3FeF6/GNS should be 425.7 mA h g-1 (calculated by 487.8 – 62.1). Therefore, the actual capacity of (NH4)3FeF6 is ca. 545.7 mA h g-1 (calculated by 425.7 / 78.0%).

0 150 300 450 6000.0

0.6

1.2

1.8

2.4

3.0

0.1 A g-1

1st 2nd 3rd

Specific capacity (mAh/g)

Volta

ge(V

)

FeF2/GNS

0 20 40 60 80 1000

100

200

300

400

Spec

ific

Cap

acity

(mA

h/g)

Cycle Number

FeF2/GNS

1 A g-1

5 A g-1

10 A g-1

(a) (b)

Figure S12. (a) initial three galvanostatic charge/discharge curves and (b) cycling performances of FeF2/GNS.

Figure S13. (a, b) Electrochemical impedance spectroscopy (EIS) after 2 cycles and 5 cycles at 0.1 A g-1, (c, d) Warburg coefficient σ after 2 cycles and 5 cycles at low-frequency of pure (NH4)3FeF6, FeF2/GNS and (NH4)3FeF6/GNS. (Inset: corresponding to equivalent circuit model for SIBs.)

Table S1. The fitting results of the EIS curves obtained by equivalent circuit.

The sampleRe

(Ω)

Rf

(Ω)

Rct

(Ω)

σ

(Ω·S-1/2)

D

(cm2·S-1)

(NH4)3FeF6

After 2-cycles 14.13 8.35 5.98 801.311 3.21×10-17

After 5-cycles 15.42 11.76 26.41 803.124 3.21×10-17

FeF2/GNS After 2-cycles 10.07 3.49 6.00 288.74 4.05×10-16

After 5-cycles 10.09 8.97 15.57 171.32 1.15×10-15

(NH4)3FeF6/GNS After 2-cycles 11.33 4.53 3.01 47.211 3.13×10-13

After 5-cycles 11.63 4.72 3.26 32.043 6.80×10-13

As shown in Fig. S13, the EIS curves of the pure (NH4)3FeF6, FeF2/GNS and (NH4)3FeF6/GNS were fitted by the equivalent circuit, which consist of a semicircle in high frequency region and a line in low frequency region. Re is the electrolyte impedance, and Rf and Cf are the resistance and capacitance of the SEI layer formed on the surface of the electrodes, respectively. Rct and Cdl are the charge-transfer resistance and double-layer capacitance. Zw is the Warburg impedance. The sodium ion diffusion coefficient (D) can be calculated at low frequency with the Equation S1:

Equation S1 𝐷=

𝑅2𝑇2

2𝐴2𝑛4𝐹4𝐶2𝜎2

where R is the gas constant, T is the absolute temperature in experiment, A is the electrode surface area, n is the number of transferred electrons, F is Faraday's constant, C is the concentration

of sodium ion in anode electrode materials. σ is the Warburg coefficient, which was determined as the slope of Z' vs. ω-1/2 plots in the low-frequency region. The value of σ can be got by the Equation S2:

Equation S2 𝑍`= 𝑅𝑒+ 𝑅𝑐𝑡+ 𝜎𝜔‒ 1 2

The EIS simulation results of the pure (NH4)3FeF6 and (NH4)3FeF6/GNS are presented in Fig. S13 and Table S1.

The sodium ion diffusion coefficient (D) of pure (NH4)3FeF6, FeF2/GNS and (NH4)3FeF6/GNS are 3.21×10-17, 4.05×10-16 cm2 s−1 and 3.13×10-13 cm2 s−1 after the 2th cycle, and 3.21×10-17, 1.15×10-15 cm2 s−1 and 6.80×10-13 cm2 s−1 after 5th cycle, respectively.

Table S2. Electrochemical performance comparison for Na-ion storage performances of TMFs reported in the references.

Materials Crystalstructure

Voltage range

Initial reversible capacity Cycle performance Rate performance Ref

CoF2 Rutile-type 0.01-3.0 V 190 mA h g-1

at 553 mAg-140.7 mA h g-1 at 553 mAg-1

after 30 cycles — 1

TiO0.9(OH)0.9F12·0.59H2O

Hexagonal-tungsten-

bronze-type0.5-2.9 V 150 mA h g-1

at 25 mAg-1100 mA h g-1 at 25 mAg-1

after 115 cycles

160 mA h g-1 at 25 mA g-1

130 mA h g-1 at 50 mA g-1

120 mA h g-1 at 75 mA g-1

100 mA h g-1 at 125 mA g-1

2

SnF2@C Like-rutile-type 0.01-2.0 V 563 mA h g-1

at 0.05 C337 mA h g-1 at 0.05 C

after 50 cycles

490 mA h g-1 at 0.05 C384 mA h g-1 at 0.1 C329 mA h g-1 at 0.2 C288 mA h g-1 at 0.5C191 mA h g-1 at 1C

3

NaF-Ti — 0.01-2.5 V 45 μAh cm-2 at 2 μA cm-2 —

4.1 μAh cm-2 at 2 μA cm-2

1.7 μAh cm-2at 8 μA cm-2

1.2 μA cm-2at 12 μA cm-2

0.7 μA cm-2at 20 μA cm-2

4

KNb2O5FTetragonal-tungsten-

bronze-type0.1-3.0 V 170-180 mA h g-1

at 0.1 C100-110 mA h g-1 at 0.1 C

after 140 cycles

~90 mA h g-1 at 0.5 C60 mA h g-1 at 1 C~30 mA h g-1 at 5 C

5

NH4FeF3/GNS Perovskite 0.1-2.8 V 504 mA h g-1

at 0.05 A g-1

175 mA h g-1 at 0.2 A g-1

after 100 cycles117 mA h g-1 at 0.5 A g-1

after 100 cycles

211 mA h g-1 at 0.2 A g-1

153 mA h g-1 at 0.5 A g-1

113 mA h g-1 at 1.0 A g-1

76 mA h g-1 at 2.0 A g-1

6

FeF2/GNS Rutile-type 0.1-2.8 V 319.3 mA h g-1

at 0.1 A g-1

160.2 mA h g-1 at 1.0 A g-1

after 100 cycles130.3 mA h g-1 at 5.0 A g-1

after 100 cycles

— This work

(NH4)3FeF6/GNS Diperovskite 0.1-2.8 V 487.8 mA h g-1

at 0.1 A g-1

276.9 mA h g-1 at 1.0 A g-1

after 100 cycles177.4 mA h g-1 at 5.0 A g-1

after 100 cycles

287.3 mA h g-1 at 1.0 A g-1

257.8 mA h g-1 at 2.0 A g-1

219.4 mA h g-1 at 5.0 A g-1

180.6 mA h g-1 at 10.0 A g-1

This work

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