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Electronic Supplementary Information

SnP nanocrystals as anode material for Na-ion battery

Junfeng Liu,‡a Shutao Wang,‡bc Kostiantyn Kravchyk,*bc Maria Ibáñez,bc Frank Krumeich,b Roland

Widmer,d Déspina Nasiou,e Michaela Meyns,*a Jordi Llorca,f Jordi Arbiol,eg Maksym V. Kovalenko,bc

Andreu Cabot*ag

a Catalonia Institute for Energy Research − IREC, 08930 Sant Adrià de Besòs, Barcelona, Spain b Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich,

CH-8093,Switzerland c EMPA-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, CH-

8600, Switzerland d Nanotech@surface Laboratory, EMPA-Swiss Federal Laboratories for Materials Science and Technology,

CH-8600, Switzerland e Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra,

08193 Barcelona, Catalonia, Spain f Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center

in Multiscale Science and Engineering. Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona,

Spain

g ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Catalonia, Spain

Table of Contents

1. Synthesis of tin phosphide using different tin precursors .................................................................... 2

2. Influence of reaction temperature ....................................................................................................... 3

3. Influence of the amount and type of phosphonic acid ......................................................................... 4

4. Influence of OAm .................................................................................................................................. 7

5. Influence of the reaction time .............................................................................................................. 8

6. Additional HRTEM characterization ...................................................................................................... 9

7. Additional XPS data ............................................................................................................................. 11

8. Additional electrochemical characterization ...................................................................................... 12

9. References ........................................................................................................................................... 15

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

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1. Synthesis of tin phosphide using different tin precursors

In a typical synthesis, 0.2 mmol of the selected tin precursor (bis[bis(trimethysilylamino]tin(II), tin(II)

strearate or tin(II) oxalate) and 10 mL of oleylamine (OAm) were added to a 50 mL three-necked flask

containing a stir bar. The reaction mixture was stirred moderately and was heated to 100 °C under

vacuum for 60 min to remove air, water and low-boiling point impurities. Then, the solution was placed

under argon and heated to 250 °C within 15 min. Then, 0.1 mL of hexamethylphosphorous triamide

(HMPT) was injected into the reaction flask and the reaction was kept at this temperature for 60 min.

The flask was allowed to cool to 200 °C by removing the heating mantle and then cooled rapidly to room

temperature with a water bath. The contents of the reaction mixture were transferred into centrifuge

tubes, followed by centrifugation at 5000 rpm (3200 g) for 5 min. The isolated powder was re-

suspended and precipitated using 1:3 (v:v) chloroform and ethanol and then centrifuged again. This

entire process was repeated twice.

For tin(II) acetylacetonate as precursor, 10 mL of OAm was added to a 50 mL three-necked flask

containing a stir bar. The mixture was stirred moderately and was heated to 100 °C under vacuum for 60

min. The solution was placed under argon and then 0.2 mmol of tin(II) acetylacetonate in 0.1 mL of

octadecene was injected into the flask. The temperature was increased to 250 °C within 15 min and then

0.1 mL of HMPT was injected to the reaction flask. The reaction was kept at this temperature for 60 min.

The flask was allowed to cool to 200 °C by removing the heating mantle and then cooled rapidly to room

temperature with a water bath. The washing steps were the same as with the syntheses utilizing other

tin precursors.

100 nm

20 30 40 50 60 70 80

Inte

nsi

ty (

a.u

.)

2 (degree)

SnP (JCPDS 65-9787)

tin(II) stearate

20 30 40 50 60 70 80

Sn(acac)2

Sn4P

3 (JCPDS 73-1820)

Inte

nsi

ty (

a.u

.)

2 (degree)

Sn (JCPDS 86-2265)

200 nm 200 nm 50 nm

a c

b d

e

f

20 30 40 50 60 70 80

bis[bis(trimethylsilyl)amino]tin(II)

Inte

nsi

ty (

a.u.)

2 (degree)

SnP (JCPDS 65-9787)

Sn4P

3 (JCPDS 73-1820)

Sn (JCPDS 04-0673)

Figure S1. TEM and XRD of as-synthesized NPs by bis[bis(trimethysilylamino]tin (a,b), tin acetylacetonate

(c,d), and tin strearate (e,f) as tin precursors.

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2. Influence of reaction temperature

200 nm

a b

Figure S2. TEM micrograph (a) and XRD pattern (b) of SnP nanocrystals (NCs) produced at 250 °C.

100 nm20 30 40 50 60 70 80

Inte

nsi

ty (

a.u.)

2 (degree)

SnP (JCPDS 65-9787)

a ba

Figure S3. TEM micrograph (a) and XRD pattern (b) of SnP NCs produced at 300 °C.

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3. Influence of the amount and type of phosphonic acid

20 30 40 50 60 70 80

Inte

nsi

ty (

a.u.)

2 (degree)

TDPA/Sn = 1

TDPA/Sn = 0.8

TDPA/Sn = 0.5

SnP (JCPDS 65-9787)

Sn4P

3 (JCPDS 73-1820)

Sn (JCPDS 04-0673)

TDPA/Sn = 0

a b

c d

e

Figure S4. TEM micrographs (a-d) and XRD patterns (e) of the particles produced from the reaction of tin

oxalate with HMPT at 270 °C. Reaction was carried out in octadecene and in the presence of OAm and

different amounts of TDPA: a) TDPA/Sn = 0; b) TDPA/Sn = 0.5; c) TDPA/Sn = 0.8; d) TDPA/Sn = 1. Scale

bars in TEM micrographs correspond to 100 nm.

Figure S5. XRD of NCs produced without TDPA at 270 °C before injecting HMPT (a) and after reaction

with HMPT for 1 h at 270 °C (b).

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a

b

c

dElement Weight% Atomic%

P 17 45

Sn 83 55

Element Weight% Atomic%

P 24 54

Sn 76 46

200 nm 200 nm

Figure S6. SEM micrograph and EDX spectrum of Sn-P NCs produced in the absence of TDPA (a, b) and in

the presence of TDPA (c, d)

100 nm

a b

Figure S7. TEM micrograph (a) and XRD pattern (b) of SnP NCs produced with ODPA.

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0 10 20 30 40 50 600

5

10

15

20

F

requen

cy (

%)

Size (nm)0 10 20 30 40 50 60

0

5

10

15

20

25

30

Fre

qu

ency

(%

)

Size (nm)

a b

Figure S8. Size distribution of SnP synthesized utilizing TDPA (a) and ODPA (b) as ligand.

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4. Influence of OAm

20 30 40 50 60 70 80

Inte

nsity (

a.u

.)

2 (degree)

SnP (JCPDS 65-9787)

100 nm

aa b

Figure S9. TEM micrograph (a) and XRD pattern (b) of SnP NCs produced without OAm.

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5. Influence of the reaction time

20 30 40 50 60 70 80

60 min

20 min

5 min

Inte

nsi

ty (

a.u

.)

2 (degrees)

Reaction time

1 min

Figure S10. XRD patterns of the SnP NCs obtained after different reaction times as displayed in the

graph.

100 nm 100 nm

aa

100 nm

ab ac

Figure S11. TEM micrographs of the SnP NCs obtained after different reaction times: a) 1 min; b) 5 min;

c) 20 min. Reaction temperature was 270 °C.

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6. Additional HRTEM characterization

Figure S12 shows a HRTEM micrograph taken from a SnP NC. The power spectrum (FFT) obtained from

this NC reveals that it has the SnP structural phase with a trigonal symmetry (S.G.: P-3m1) and lattice

parameters of a = 4.3922 Å, b = 4.3922 Å, c = 6.0400 Å, α = 90 °, β = 90 ° and γ = 120 °. The NC is

visualized along its [10-11] zone axis.

Figure S12. HRTEM micrograph of a SnP NC, (right) detail of the NC area squared in blue and its

corresponding power spectrum.

Figure S13 shows a HRTEM micrograph of a SnP NC. The FFT obtained from this NC reveals that it has

the SnP structural phase with a trigonal symmetry (S.G.: P-3m1) and lattice parameters of a = 4.3922 Å,

b = 4.3922 Å, c = 6.0400 Å, α = 90 °, β = 90 ° and γ = 120 °. The NC is visualized along its [11-23] zone axis.

Figure S13. HRTEM micrograph of a SnP NC, (right) detail of the NC squared in blue and its

corresponding power spectrum.

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The presence of non-streaky extra spots in Figures S12 and S13 may indicate the presence of a certain

ordered superstructure as suggested also by J.Gullman.1 This superstructure could be related to the

organized orientation of the diphosphorous pairs within the complex layers of filled octahedra. The

structure model for a compound of the SnP type with the atomic pairs all oriented parallel with the c-

axis was predicted by Hulliger.2 Every one of these 4 pair orientations in the Sn octahedra presents an

occupancy of about 25%, meaning that in the ideal material only one of the 4 diphosphorous pair

orientations would be occupied. If the occupied pair orientations were randomly distributed, the

expected electron diffractions and the power spectra should not present extra spots. However, when

the diphosphorous pair orientations are ordered, with certain orientations preferentially fixed, then a

superstructure is formed, resulting in extra spots (by following basic Bragg diffraction symmetry rules).

The observed reflections in the FFT (Figure S14) show some extra spots, circled in red, which do not exist

in the ideal SnP trigonal structure with a random distribution of the P2 pairs orientations. Only the

brightest spots from the FFT, circled in green, can be indexed (figure S14). A superstructure manifests

itself through additional reflections every half distance.

Figure S14. (a) Power spectrum (FFT) taken from the HRTEM image in Figure S13. (b) Calculated electron

diffraction pattern for the equivalent [11-23] zone axis in the pure SnP structure (random P2 pair

orientations distribution). (c) Atomic arrangement of the Sn (cyan) and the P (yellow) atoms in the SnP

structure along the [11-23] zone axis. (d) HRTEM experimental detail of a SnP NC along the [11-23]

zone axis. (e) 3D atomic modeling of a 10 nm diameter SnP NC along the [11-23] zone axis.

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7. Additional XPS data

Figure S15. XPS survey of SnP.

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8. Additional electrochemical characterization

Figure S16. SEM-EDX compositional maps of electrodes comprising SnP NCs measured before (pristine)

and after (1st, 2nd, 10th, 25th, 50th cycles) electrochemical cycling.

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0 20 40 60 80 1000

200

400

600

800

1000

Current: 0.1 A g-1

Capacity (

mA

h g

-1)

1M NaClO4

5M NaFSI

1M NaPF6

Cycle number

50

60

70

80

90

100

Coulo

mbic

effic

ien

cy (

%)

0 100 200 300 400 500 6000.0

0.4

0.8

1.2

1.6

Voltage (

V v

s. N

a+/N

a)

Capacity (mAh g-1)

Current: 0.1 A g-1

1M NaClO4

5M NaFSI

1M NaPF6

10th cycle

a b

Figure S17. Cycling performance (a) and galvanostatic charge-discharge voltage curves (b) of SnP NCs

measured with various sodium electrolytes. The batteries were cycled in the potential range of 5 mV-

1.5 V.

Figure S18. Cycling performance (a) and galvanostatic charge-discharge voltage curves (b) of SnP NCs at

various cut off ranges. The measurements were performed in 5 M NaFSI in DME electrolyte.

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Figure S19. Cycling performance of the SnP electrode at current densities of 500 mA g−1 (a) and

1000 mA g-1 (b).

Figure S20. Cyclic stability of a Na-ion half-cell employing SnP anode made from SnP NCs at 0.5 A g-1.

0 100 200 300 400 5000

200

400

600

800

1000

Ca

pa

city (

mA

h g

-1)

Cycle number

0

20

40

60

80

100

Current: 0.5 A g-1

Coulo

mbic

effic

ien

cy (

%)

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9. References

1. J. Gullman, J. solid state Chem., 1990, 87, 202-207.

2. F. Hulliger, Structural Chemistry of Layer-Type Phases. Dordrecht, Holland, 1976, pp. 145.