Synthesis of Metal Nanoparticles and Metal Fluoride Nanoparticles from Metal Amidinate Precursors in 1-Butyl-3-Methylimidazolium Ionic Liquids and Propylene Carbonate

Article abstract of DOI:10.1002/open.201600105

Decomposition of transition-metal amidinates [M{MeC(NiPr)₂}_n] [M(AMD)_n; M=Mn^II, Fe^II, Co^II, Ni^II, n=2; Cu^I, n=1) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (triflate) ([BMIm][TfO]), and 1-butyl-3-methylimidazolium tosylate ([BMIm][Tos]) or in propylene carbonate (PC) gives transition-metal nanoparticles (M-NPs) in non-fluorous media (e.g. [BMIm][Tos] and PC) or metal fluoride nanoparticles (MF₂-NPs) for M=Mn, Fe, and Co in [BMIm][BF₄]. FeF₂-NPs can be prepared upon Fe(AMD)₂ decomposition in [BMIm][BF₄], [BMIm][PF₆], and [BMIm][TfO]. The nanoparticles are stable in the absence of capping ligands (surfactants) for more than 6 weeks. The crystalline phases of the metal or metal fluoride synthesized in [BMIm][BF₄] were identified by powder X-ray diffraction (PXRD) to exclusively Ni- and Cu-NPs or to solely MF₂-NPs for M=Mn, Fe, and Co. The size and size dispersion of the nanoparticles were determined by transmission electron microscopy (TEM) to an average diameter of 2(±2) to 14(±4) nm for the M-NPs, except for the Cu-NPs in PC, which were 51(±8) nm. The MF₂-NPs from [BMIm][BF₄] were 15(±4) to 65(±18) nm. The average diameter from TEM is in fair agreement with the size evaluated from PXRD with the Scherrer equation. The characterization was complemented by energy-dispersive X-ray spectroscopy (EDX). Electrochemical investigations of the CoF₂-NPs as cathode materials for lithium-ion batteries were simply evaluated by galvanostatic charge/discharge profiles, and the results indicated that the reversible capacity of the CoF₂-NPs was much lower than the theoretical value, which may have originated from the complex conversion reaction mechanism and residue on the surface of the nanoparticles.

Full text of DOI:10.1002/open.201600105

DOI: 10.1002/open.201600105
Synthesis of Metal Nanoparticles and Metal Fluoride
Nanoparticles from Metal Amidinate Precursors in 1-Butyl-
3-Methylimidazolium Ionic Liquids and Propylene
Carbonate
Kai Schꢀtte,[a] Juri Barthel,[b] Manuel Endres,[a] Marvin Siebels,[a] Bernd M. Smarsly,[c]
Junpei Yue,[c] and Christoph Janiak*[a]
Decomposition of transition-metal amidinates [M{MeC(NiPr)2}n]
[M(AMD)n; M=MnII, FeII, CoII, NiII, n=2; CuI, n=1) induced by
microwave heating in the ionic liquids (ILs) 1-butyl-3-methyl-
imidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([BMIm][PF6]), 1-butyl-3-
methylimidazolium trifluoromethanesulfonate (triflate) ([BMIm]
[TfO]), and 1-butyl-3-methylimidazolium tosylate ([BMIm][Tos])
or in propylene carbonate (PC) gives transition-metal nanopar-
ticles (M-NPs) in non-fluorous media (e.g. [BMIm][Tos] and PC)
or metal fluoride nanoparticles (MF2-NPs) for M=Mn, Fe, and
Co in [BMIm][BF4]. FeF2-NPs can be prepared upon Fe(AMD)2
decomposition in [BMIm][BF4], [BMIm][PF6], and [BMIm][TfO].
The nanoparticles are stable in the absence of capping ligands
(surfactants) for more than 6 weeks. The crystalline phases of
the metal or metal fluoride synthesized in [BMIm][BF4] were
identified by powder X-ray diffraction (PXRD) to exclusively Ni-
and Cu-NPs or to solely MF2-NPs for M=Mn, Fe, and Co. The
size and size dispersion of the nanoparticles were determined
by transmission electron microscopy (TEM) to an average di-
ameter of 2(ꢀ2) to 14(ꢀ4) nm for the M-NPs, except for the
Cu-NPs in PC, which were 51(ꢀ8) nm. The MF2-NPs from
[BMIm][BF4] were 15(ꢀ4) to 65(ꢀ18) nm. The average diameter
from TEM is in fair agreement with the size evaluated from
PXRD with the Scherrer equation. The characterization was
complemented by energy-dispersive X-ray spectroscopy (EDX).
Electrochemical investigations of the CoF2-NPs as cathode ma-
terials for lithium-ion batteries were simply evaluated by galva-
nostatic charge/discharge profiles, and the results indicated
that the reversible capacity of the CoF2-NPs was much lower
than the theoretical value, which may have originated from
the complex conversion reaction mechanism and residue on
the surface of the nanoparticles.
1. Introduction
Soft wet-chemical synthesis in organic solvents from metal–or-
ganic complexes is an important method to obtain metal or
metal alloy nanoparticles.[1–4] Metal amidinates are important
precursors for metal nanoparticles[5,6] for thin metal films from
low-pressure chemical vapor deposition (CVD) and atomic
layer deposition (ALD).[7,8]
Herein, we report the use of metal amidinates [M{MeC-
(NiPr)2}n] of manganese, iron, cobalt, nickel, and copper as pre-
cursors for the synthesis of nanomaterials in ionic liquids (ILs)
or in propylene carbonate (PC) to yield selectively transition-
metal nanoparticles (M0-NPs) or metal fluoride nanoparticles
(MF2-NPs) as stable colloids (Scheme 1). Depending on the
used IL (fluorous or non-fluorous) and the used metal amidi-
nates, we obtained phase-pure metal M0- or MF2-NPs.
Metal fluoride MFx-NPs are of significant importance in
modern chemistry and materials science, as they are applied,
for example, as cathode materials in lithium-ion batteries in
mobile phones, laptops, electric and hybrid vehicles, and other
mobile devices.
[a] Dr. K. Schꢀtte, M. Endres, M. Siebels, Prof. Dr. C. Janiak
Institut fꢀr Anorganische Chemie und Strukturchemie
Heinrich-Heine-Universitꢁt Dꢀsseldorf
40204 Dꢀsseldorf (Germany)
E-mail: janiak@uni-duesseldorf.de
[b] Dr. J. Barthel
Gemeinschaftslabor fꢀr Elektronenmikroskopie RWTH-Aachen
Ernst Ruska-Centrum fꢀr Mikroskopie und Spektroskopie mit Elektronen
52425 Jꢀlich (Germany)
[c] Prof. Dr. B. M. Smarsly, J. Yue
Physikalisch-Chemisches Institut
Justus-Liebig-Universitꢁt Gießen
35392 Gießen (Germany)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
open.201600105.
Modification of lithium transition-metal electrodes (e.g.
oxides, phosphates etc.) is a key issue to improve the per-
formance of lithium-ion batteries and has therefore been iden-
tified as an important aspect of research in this field.[9–11] As-
sembled batteries based on nanosized materials would result
in interesting performance (e.g. short charging time, long life-
time, high capacity).[12] Grey et al. showed that the use of FeF2-
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ILs are alternatives to aqueous and organic solvents[17,18] and
are regarded as a new liquid medium[19,20] for the preparation
of inorganic materials, including M-NPs.[21–29] ILs have negligible
vapor pressure, high thermal stability, high ionic conductivity,
a broad liquid-state temperature range, and the ability to dis-
solve a variety of materials.[30,31] M-NPs in ILs have been pre-
[32–34]
pared from metal salts reduced by H2
or without a reduc-
tant,[35] from organometallic metal complexes,[36] including
metal carbonyls,[37] through thermal or photochemical[38] de-
composition or by electroreduction/electrodeposition.[21,39,40]
The electrostatic and steric properties of the ILs stabilize the
M-NPs without the need for additional capping ligands, surfac-
tants, or polymers.[21,40,41] PC is an aprotic, highly dipolar sol-
vent with low viscosity,[42,43] low flammability, low volatility, and
low toxicity.[44] PC also has a large liquid temperature range
(m.p. ꢁ498C, b.p. 2438C), is considered a “green solvent” of
only low (eco)toxicity, and is completely biodegradable.[45]
There are only a few reports on the synthesis of M-NPs in or-
ganic carbonates.[46–48] Whereas ILs are expensive solvents, or-
ganic carbonates, such as PC, are available at low cost.
Scheme 1. Synthesis and structural formula of the metal amidinates
[M{MeC(NiPr)2}n], [M(AMD)n] and their microwave-assisted thermal decompo-
sition into M-NPs or MF2-NPs in IL or PC.
NPs instead of macroscopic LiFeF3 led to a significant improve-
ment in performance of the batteries.[13] Li et al. presented
a straightforward method to synthesize FeF3-NPs by the de-
composition of Fe(NO3)3·9H2O in 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIm][BF4]). The [BMIm][BF4] IL was de-
scribed as a stabilization medium for the NPs and the fluoride
source for the FeF3-NPs.[14] Pinna, Kemnitz, and co-workers and
Wickleder and co-workers independently also reported the
simple syntheses of MFx-NPs from metal salts in conventional
solutions. In contrast to our work, they used diethylene glycol
and methanol as the solvents and HF in methanol solution and
ammonium fluoride as the fluorine sources. Similar to this
work, the MFx-NPs were obtained under microwave
heating.[15,16]
2. Results and Discussion
2.1. Thermal Decomposition of M-Amidinates
The transition-metal amidinates [M{MeC(NiPr)2}n] [M(AMD)n;
M=MnII, FeII, CoII, NiII, n=2; CuI, n=1) (Scheme 1) were dis-
solved/suspended under a nitrogen atmosphere in dried and
deoxygenated [BMIm][BF4], [BMIm][PF6], [BMIm][TfO], [BMIm]
[Tos], or PC. Quantitative decomposition of the metal amidi-
nates was then achieved by microwave heating after only 10
(IL) or 40 min (PC) by using a low power of 50 W to give an ap-
proximate temperature of 2408C in the reaction mixture
(Scheme 1).
Dispersions of the NPs in the ILs or PC were reproducibly
obtained by this microwave decomposition route. Quantitative
decomposition of the metal amidinates by microwave heating
1.1. ILs and PC as Nonconventional Solvents in Nanoparticle
Synthesis
1
was verified by H NMR spectroscopy through the disappear-
For the synthesis of the NPs, we used the 1-butyl-3-methylimi-
dazolium tetrafluoroborate ([BMIm][BF4]) and 1-butyl-3-methyli-
midazolium tosylate ([BMIm][Tos]) ILs or the organic carbonate
propylene carbonate (PC). For the Fe-containing NPs, [BMIm]
ILs with [PF6]ꢁ and [TfO]ꢁ anions (Scheme 2) were also
considered.
ance of the signal for the N-C(CH3)-N methyl group. Thermog-
ravimetric analysis (TGA) decomposition measurements also
showed that the amidinates fully decomposed at temperatures
between 120 and 1958C (Table 1 and Figure S2 in the Support-
ing Information).[49]
Table 1. Thermogravimetric analysis of the metal amidinates.[a]
Amidinate
Decomposition temperature [8C]
DmTGA (exptl/calcd)
Mn(AMD)2
Fe(AMD)2
Co(AMD)2
Ni(AMD)2
Cu(AMD)
120
130
180
135
185
81/74
83/75
83/78
83/79
69/69
[a] See thermogravimetric diagrams in Figure S2.
Scheme 2. Liquid media used here for NP synthesis and stabilization.
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2.2. NPs from M(AMD)n in [BMIm][BF4]
Microwave-induced thermal decomposition of the metal amidi-
nates in [BMIm][BF4] gave M0- or MF2-NPs depending on the
used M-amidinates. The resulting NPs were analyzed by trans-
mission electron microscopy (TEM) and powder X-ray diffrac-
tion (PXRD) for their morphology, size, and size dispersion as
well as their crystalline phase (MF2- or M-NPs) (Figure 1–6).
Metal/fluorine ratios and oxidation states were determined by
energy-dispersive X-ray spectroscopy (EDX, by combination
with TEM) (Figure 1–4) and by X-ray photoelectron spectrosco-
Figure 2. From top to bottom: HAADF-STEM, particle-size histogram, EDX,
SAED, and PXRD (FeF2 reference peaks in red from COD 9007536) of
1.0 wt% FeF2-NPs in [BMIm][BF4] from Fe(AMD)2. Space group of FeF2; P42/
mnm, tetragonal, rutile structure.
Figure 1. From top to bottom: HAADF-STEM, particle-size histogram, EDX
and PXRD (MnF2 reference peaks in red from COD 9009076) of 1.0 wt%
MnF2-NPs in [BMIm][BF4] from Mn(AMD)2.
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Figure 4. From top to bottom: TEM, particle-size histogram, EDX, and PXRD
(Ni-hexagonal close packing reference peaks in red from COD 9008509) of
1.0 wt% Ni-NPs in [BMIm][BF4] from Ni(AMD)2.
Figure 3. From top to bottom: HAADF-STEM, particle-size histogram, EDX,
and PXRD (CoF2 reference peaks in red from COD 9009073) of 1.0 wt% CoF2-
NPs in [BMIm][BF4] from Co(AMD)2. Space group of CoF2: P42/mnm (136), tet-
ragonal, rutile structure.
Table 2. MF2 composition determination in [BMIm][BF4].[a]
py (Figure S5). The determined M/F ratios confirmed the for-
mation of MF2 nanoparticles (Table 2).
Amidinate
Phase[b]
Molar ratio M/F
EDX[c]
XPS
The crystalline part of the nanoparticle product was identi-
fied by PXRD as metal fluoride nanoparticles (MF2-NPs) for the
middle-transition-metals Mn, Fe, and Co, whereas only metal
nanoparticles (M-NPs) were found for the late-transition-metals
Ni and Cu. For FeF2, selected area electron diffraction (SAED)
also confirmed the formation of FeF2 (Figure 2). For the MF2-
NPs, the [BF4]ꢁ anion in the [BMIm][BF4] IL must be the fluorine
source, perhaps in combination with a very small amount
(<10 ppm) of residual water. The [BF4]ꢁ anion is also known to
Mn(AMD)2
Co(AMD)2
Fe(AMD)2
MnF2
CoF2
FeF2
1:1.7
1:1.8
1:1.9
–
1:2.0[d]
1:2.0[d]
[a] 1.0 wt% MFx-NP/[BMIm][BF4] dispersions obtained by microwave irradi-
ation with 50 W for 10 min at 2408C; see matching entries for size deter-
minations in Table 3. [b] Precipitated and washed with acetonitrile, refer-
ence: Crystallography Open Database (COD). [c] k-factor Mn: 1.375, k-
factor F: 1.601, k-factor Fe: 1.403, k-factor Co: 1.495; estimated minimum
errorsꢀ1–2%. [d] See Figure S5.
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be temperature labile.[50] We are not aware of any other easy
or straightforward methods for the synthesis of MnF2, FeF2, or
CoF2 nanoparticles. At present, metal difluoride nanoparticles
are prepared by reduction of metal trifluoride or alkali-transi-
tion-metal fluorides.[51,52]
High-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images (Figure 1–3) yield particle
sizes of 8–29 nm for MnF2, 20–77 nm for CoF2, and 25–98 nm
for FeF2 (Figure 1–3). The average particle sizes are 15ꢀ4 nm
for MnF2, 43ꢀ11 nm for CoF2, and 65ꢀ18 nm for FeF2. For the
elongated FeF2 particles, a longer dimension was measured.
The MF2-NPs are air stable for several days, as assessed by
PXRD, which was measured several days after the synthesis of
the NPs and after storing the product in air.
The microwave-induced decomposition of Ni(AMD)2 and
Cu(AMD) in [BMIm][BF4] gives black and deep-red 1.0 wt% dis-
persions of elemental Ni- and Cu-NPs. The PXRD patterns pres-
ent only diffractions for Ni and Cu metal (Figures 5 and 6).
Spherical M-NPs with narrow size distributions are obtained
(Table 3). The average diameter for the microwave-synthesized
Table 3. M/MF2-NP size and size distribution.[a]
Precursor
[BMIm][BF4][a]
M/MFx-NPs
TEM Ø (s) [nm][b]
PXRD Ø (s) [nm][b,c]
Mn(AMD)2
Fe(AMD)2
Co(AMD)2
Ni(AMD)2
Cu(AMD)
MnF2
FeF2
CoF2
Ni
15(ꢀ4)
65(ꢀ18)
43(ꢀ11)
11(ꢀ3)
14(ꢀ4)
14(ꢀ3)
73(ꢀ3)
47(ꢀ5)
12(ꢀ3)
17(ꢀ3)
Cu
[BMIm][Tos][a]
[d]
Mn(AMD)2
Fe(AMD)2
Co(AMD)2
Ni(AMD)2
Cu(AMD)
–
–
–
–
–
Fe
Co
Ni
11(ꢀ2)
7(ꢀ4)
12(ꢀ4)
–
13(ꢀ2)
[d]
–
–
PC[a]
[e]
Mn(AMD)2
Fe(AMD)2
Co(AMD)2
Ni(AMD)2
Cu(AMD)
Mn
Fe
Co
Ni
4(ꢀ2)
2(ꢀ2)
4(ꢀ2)
8(ꢀ5)
51(ꢀ8)
–
–
–
[e]
12(ꢀ2)
55(ꢀ7)
Cu
Figure 5. From top to bottom: HAADF-STEM, particle-size histogram, EDX,
and PXRD (Cu reference peaks in red from COD 9013014) of 1.0 wt% Cu-NPs
in [BMIm][BF4] from Cu(AMD).
[a] 1.0 wt% M-NP/[BMIm][BF4], M-NP/[BMIm][Tos], or M-NP/PC dispersion
obtained by microwave irradiation with 50 W for 10 min (40 min for PC)
at 2408C. [b] Average diameter (Ø) and standard deviation (s). See the Ex-
perimental Section for TEM measurement conditions. [c] From the Scher-
rer equation, Scherrer-factor: 0.95.[53] [d] No particles were obtainable.
[e] No precipitation possible.
nanoparticle sizes. The M-NPs dispersions are stable with non-
agglomerated NPs present 6 weeks after the synthesis accord-
ing to high-resolution (HR)-STEM measurements performed at
that time (Figures 1–6).
Ni- and Cu-NPs at 1.0 wt% metal dispersions in [BMIm][BF4] is
between 11(ꢀ3) and 14(ꢀ4) nm, respectively, as observed in
previous reports for M-NPs in this IL.[35–37,40,48] The size and size
dispersions of the M- and MF2-NPs from [BMIm][BF4] are sum-
marized in Table 3. The M- and MF2-NP sizes agree with the
size evaluations from the PXRD patterns by using the Scherrer
equation with Scherrer factor k=0.95.[53] No extra stabilizers or
capping molecules were added to achieve and stabilize these
The literature describes the synthesis of FeF3 nanoparticles
starting by the decomposition of iron(III) nitrate in fluorine-
containing ILs. Maier et al. used metal fluoride nanoparticles as
an electrode material in lithium-ion batteries.[10,14] A direct syn-
thesis of phase-pure FeF2 nanoparticles, which are also dis-
cussed as a promising electrode material for lithium-ion batter-
ies, has not yet been described.
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2.4. NPs from M(AMD)n in [BMIm][Tos]
The non-fluorous [BMIm][Tos] IL was investigated for compari-
son to the fluorous [BMIm][BF4] IL. The metal amidinates were
dissolved/suspended in [BMIm][Tos] and heated by microwave
irradiation up to 2408C for 10 min.
The microwave-induced decomposition of the amidinates
M(AMD)n with M=Fe, Co, Ni in [BMIm][Tos] gave small M0-NPs.
For M=Mn and Cu, no particles were obtained. The M-NP/IL
dispersions were stable without extra stabilizers or capping
molecules with non-agglomerated NPs present 6 weeks after
the synthesis according to the TEM measurements performed
at that time. The average diameter of the M-NPs at 1.0 wt% of
M-NP/[BMIm][Tos] was between 7 and 12 nm, as determined
by TEM (Table 3; Figures 9–11, top). Elemental analysis by EDX
spectroscopy (Figure 9–11, bottom) gave the expected bands
for Fe, Co, and Ni.
Figure 6. PXRD (FeF2 reference peaks in red from COD 9007536) of 1.0 wt%
FeF2-NPs in [BMIm][PF6] from Fe(AMD)2.
2.3. FeF2-NPs from Fe(AMD)2 in [BMIm][PF6] and [BMIm][TfO]
The decomposition of Fe(AMD)2 in the fluorine-containing
[BMIm][PF6] and [BMIm][TfO] ILs also gave phase-pure and
crystalline FeF2 nanoparticles (Figures 6 and 7). The [PF6]ꢁ and
Figure 7. PXRD (FeF2 reference peaks in red from COD 9007536) of 1.0 wt%
FeF2-NPs in [BMIm][TfO] from Fe(AMD)2.
[TfO]ꢁ anions served as fluorine sources for the FeF2-NPs.
From [BMIm][PF6] and [BMIm][TfO], prismatic FeF2 nanoparti-
cles were synthesized with a typical size distribution of 68
(ꢀ14) nm for the former and 72(ꢀ12) nm for the latter IL
(Figure 8).
In contrast to the [BF4]ꢁ and [PF6]ꢁ anions, the formation of
FeF2 nanoparticles in [BMIm][TfO] is surprising because of the
thermally stability and expected insensitivity to hydrolytic
cleavage of the triflate anion, [CF3SO3]ꢁ.[54–56] However, another
fluorine source can be excluded according to the low fluorine
content in the IL (<50 ppm), which was determined by ion
chromatographic analysis.
Figure 9. TEM (top) and EDX (bottom) of 1.0 wt% Fe-NPs in [BMIm][Tos]
from Fe(AMD)2.
2.5. NPs from M(AMD)n in Propylene Carbonate (PC)
The organic solvent PC served as a comparative media for the
ionic liquids. The corresponding metal amidinates were dis-
solved/suspended in PC and heated in the microwave reactor
up to 2408C for 40 min.
The microwave-induced decomposition of the metal amidi-
nates in PC required a longer time of 40 min to complete in
comparison to 10 min in the ILs. After attempting to irradiate
Fe- and Co(AMD)2 for a short period of 10 min, very small
metal clusters were seen in the HR-TEM images, which ag-
glomerate to bigger particles in the electron beam of the
transmission electron microscope (see Figures S3 and S4). After
40 min of microwave irradiation, the synthesis in PC gave very
small M0-NPs, except for Cu. The median diameter for the mi-
Figure 8. TEM images of 1.0 wt% FeF2-NPs in [BMIm][TfO].
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Figure 10. TEM (top) and EDX (bottom) of 1.0 wt% Co-NPs in [BMIm][Tos]
from Co(AMD)2.
crowave-synthesized M-NPs at 1.0 wt% M-NPs in PC was be-
tween 2 and 8 nm for M=Mn, Fe, Co, and Ni, as determined
by HR-TEM (Figures 12–15, top; Table 3). The Cu-NPs were
somewhat larger than those seen before (Figure 16).[5] Again,
no extra stabilizers or capping molecules were needed to ach-
ieve and stabilize the NPs, that is, to prevent agglomeration of
these particle for up to at least 6 weeks after synthesis. At that
time, the non-agglomerated NPs were still present, as seen by
HR-STEM measurements. Elemental analysis by EDX spectros-
copy (Figures 12–15, bottom) gave the expected bands for Mn,
Fe, Co, and Ni. The formation of Fe and Co-NPs was supported
by X-ray photoelectron spectroscopy (XPS, Figures 10 and
11).[57] No oxygen was detected by EDX and no shift from M0
was observed in the X-ray photoelectron spectra; thus, it can
be assumed that elemental cobalt and iron nanoparticles were
obtained. We can, of course, not exclude the formation of sur-
face metal oxide or slightly charged metal nanoparticles.
Only the larger Ni-NPs proved of sufficient crystallinity to
afford a PXRD pattern (Figure 12).
Figure 11. From top to bottom: TEM, EDX, and PXRD (Ni-hexagonal close
packing reference peaks in red from COD 9008509) of 1.0 wt% Ni-NPs in
[BMIm][Tos] from Ni(AMD)2.
CoF2 þ 2 Liþ þ 2 e ! 2 LiF þ Co
E ¼ 2:85 V, C ¼ 553 mAh gꢁ1
ð1Þ
in which E is the potential and C is the specific capacity.
The galvanostatic charge/discharge profiles of the CoF2
nanoparticles from decomposition of Co(AMD)2 in [BMIm][BF4]
are presented in Figure 17. The redox plateau at the first dis-
charge is 1.8 V, which is much lower than the theoretical value
but comparable to that reported in the literature.[58–60] Two oxi-
dation plateaus in the charge process are in the potential
ranges of 3.5 to 4.2 V and 4.2 to 4.8 V, which correspond to the
processes of reconverting back into CoxF and CoF2, respective-
ly.[58] In the second discharge process, the slope process before
the plateau at 1.5 V can be assigned to the reduction of CoxF
with lithium ions. It is noticeable that the redox process of
CoxF disappears after several cycles, which is one of main rea-
sons for the reduced capacity. If the current density is in-
creased to 20 mAgꢁ1, the charge and discharge plateaus are
4.3 and 1.55 V, respectively. The specific capacity stabilizes at
70 to 80 mAhgꢁ1. However, the specific capacity is much lower
2.6. Electrochemical Measurements
The possible electrochemical performance of CoF2 as a cathode
material for lithium-ion batteries was evaluated by the galva-
nostatic charge/discharge profiles on a half-cell.
Fluorides as electrode materials for lithium-ion batteries nor-
mally undergo a conversion reaction and deliver higher energy
density than the current cathode materials (such as LiCoO2 and
LiFePO4). The overall conversion reaction for CoF2 can be de-
scribed by [Eq. (1)]:
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Figure 12. HR-TEM (top) and EDX (bottom) of 1.0 wt% Mn-NPs in PC from
Mn(AMD)2.
than the theoretical value, even at such a low discharge rate,
for which the reasons can be concluded to result from the
complex conversion mechanism (low reversible capacity), resi-
due functional groups on the surface of the nanoparticles, and
so on.
3. Conclusions
Figure 13. From top to bottom: HR-TEM, EDX, and XPS (Fe reference in red
We described herein the simple, reproducible, and broadly ap-
plicable microwave-induced decomposition of metal amidi-
nates for the selective synthesis of small, uniform, and phase-
pure metal fluoride nanoparticles (MF2-NPs) (M=Mn, Fe, Co)
and transition-metal nanoparticles (M-NPs) (M=Ni, Cu) in the
and Fe3O4 in dashed-green) of 1.0 wt% Fe-NPs in PC from Fe(AMD)2.
Synthesis in [BMIm][Tos] and PC media seemed to yield
smaller NPs than in [BMIm][BF4], with the only exception of Cu.
Decomposition of metal amidinates in fluorous ILs could pro-
vide easy and novel access to metal fluoride nanoparticles as
shown by the phase-pure formation of MF2 for M=Mn, Fe,
and Co. The phase purity was based on powder X-ray diffrac-
tion, energy-dispersive X-ray spectroscopy, and X-ray photo-
electron spectroscopy analysis of the derived product. CoF2
nanoparticles as a cathode material for lithium-ion batteries
presented a less reversible capacity than the theoretical one,
which was believed to be caused by the complex conversion
reaction mechanism.
fluorous
1-butyl-3-methylimidazolium
tetrafluoroborate
([BMIm][BF4]) ionic liquid (IL). Fe amidinate was also decom-
posed in fluorous [BMIm] ILs with hexafluorophosphate ([PF6])
and trifluoromethanesulfonate ([TfO]) anions to FeFe2 nanopar-
ticles. Decomposition in the non-fluorous [BMIm][Tos] (Tos=to-
sylate) ionic liquid or in propylene carbonate (PC) yielded M-
NPs. Thus, MF2-NPs were obtained in fluorous media, that is, in
ILs with [BF4]ꢁ, [PF6]ꢁ, and [TfO]ꢁ counterions for M=Mn, Fe,
and Co. In non-fluorous [BMIm][Tos] and PC media as well as
for M=Ni and Cu, the metal nanoparticles were obtained from
decomposition of the transition-metal amidinates. The results
nicely showed the advantages of the metal–organic precursor
concept based on metal amidinates together with unconven-
tional solvents and microwave-heating assisted pyrolysis.[61] No
additional surfactants or other reducing agents (e.g. dihydro-
gen, NaBH4, etc.) were necessary for soft chemical nanomateri-
al formation. M-NP dispersions were stable, and nonagglomer-
ated NPs were still present 6 weeks after synthesis according
to transmission electron microscopy measurements performed
at that time.
Experimental Section
All synthesis experiments were performed with Schlenk techniques
under a nitrogen or argon atmosphere, as the amidinates are hy-
groscopic and air sensitive. Propylene carbonate (PC) was dried
under high vacuum (1.0 Pa) for several days. 1,3-Diisopropylcarbo-
diimide (>99%), manganese(II) chloride (>98%), iron(II) chloride
(>98%), cobalt(II) chloride (>99%), nickel(II) chloride (>99%),
copper(I) chloride (>99%), methyllithium, 1-chlorobutane (>99%),
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Figure 14. From top to bottom: HR-TEM, EDX, and XPS (Co reference peaks
in red and CoO in dashed-green) of 1.0 wt% Co-NPs in PC from Co(AMD)2.
Figure 15. From top to bottom: HR-TEM, EDX, and PXRD (Ni-hexagonal close
packing reference peaks in red from COD 9008509) of 1.0 wt% Ni-NPs in PC
from Ni(AMD)2.
and 1-methylimidazole (>99%) were obtained from Sigma–Aldrich
and were used without further purification; racemic propylene car-
bonate was obtained from Sigma–Aldrich (purity 99.7%, H2O free).
The metal amidinates were synthesized by deprotonation and
methylation of 1,3-diisopropylcarbodiimide with methyllithium.
The resulting lithium amidinate was treated with metal halides
(Scheme 1) according to literature procedures.[62,63]
The [BMIm][BF4] and [BMIm][Tos] ionic liquids were synthesized by
treating 1-methylimidazole with 1-chlorobutane to yield first
[BMIm][Cl], which was further treated with HBF4 and sodium tosy-
late to give [BMIm][BF4] and [BMIm][Tos]. The ILs were dried under
high vacuum (0.10 mPA) at 808C for several days. Anion exchange
was assessed by ion chromatography (Dionex ICS-1100, with
IonPac AS14, 4ꢂ250 mm column) to be >99%. The water content
by coulometric Karl Fischer titration (ECH/ANALYTIK JENA AQUA
40.00) was less than 10 ppm.
Thermogravimetric analysis (TGA) was performed with a Netzsch
TG 209 F3 Tarsus equipped with an Al crucible by using a heating
rate of 10 Kminꢁ1 under an inert atmosphere (N2).
Figure 16. HAADF-STEM of 1.0 wt% Cu-NPs in PC from Cu(AMD).
a sample angle of 308. Spectra were recorded by using polychro-
matic AlKa excitation (14 kV, 20 mA) and an emission angle of 08.
Calibration of the spectra was performed by recording spectra
X-ray photoelectron spectroscopy (XPS-ESCA) measurements were
performed with a Fisons/VG Scientific ESCALAB 200X xp-spectrom-
eter, operating at room temperature, a pressure of 1.0 mPa, and
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always showed trace amounts of oxygen. Preparation on the grids
was done in a glove box. However, there was an unavoidable 30 s
air contact upon bringing the grid in the TEM chamber before
evacuation. A low intensity oxygen signal at 0.52 eV was also
always present on noble metal samples and pure carbon grids
(from surface oxidation).
In a typical procedure for the synthesis of the metal nanoparticles
(M-NPs), decomposition by microwave heating was performed
under a nitrogen atmosphere. The metal amidinate powders (see
Table 4) were dissolved/suspended (for ꢂ12 h) at room tempera-
Table 4. Mass of the amidinates in 2.0 g [BMIm][BF4], [BMIm][Tos] or PC
for ꢂ1.0 wt% total metal dispersion.
Precursor[a]
mg [mmol] [M{MeC(NiPr)2}n][b]
{Mn(AMD)2}2
{Fe(AMD)2}2
Co(AMD)2
Ni(AMD)2
{Cu(AMD)}2
122.8 (0.18)
121.1 (0.18)
113.3 (0.33)
115.9 (0.34)
64.5 (0.16)
[a] see Scheme 1 for structural formula. [b] Molar mass of {Mn(AMD)2}2:
674.81 gmolꢁ1 {Fe(AMD)2}2: 676.64 gmolꢁ1 Co(AMD)2: 340.42 gmolꢁ1
Ni(AMD)2: 340.18 gmolꢁ1 {Cu(AMD)}2: 409.56 gmolꢁ1
density [BMIm]
[BF4]: 1.21 gmLꢁ1, 1 mL, 1.21 g; density PC: 1.19 gmLꢁ1, 1 mL, 1.19 g.
;
;
;
;
;
ture in the dried and deoxygenated IL or PC. The mass of the
metal amidinates was set for 1.0 wt% M-NP in IL or PC dispersion.
The mixture was placed in a microwave reactor (CEM, Discover)
and irradiated for 10 min (IL) or 40 min (PC) at a power of 50 W to
an indicated temperature of 2408C. Each decomposition reaction
was performed at least twice.
Figure 17. Galvanostatic charge/discharge profiles of a half-cell with CoF2-
NPs as the cathode material at different current densities. CoF2-NPs from de-
composition of Co(AMD)2 in [BMIm][BF4].
For the electrochemical measurements, the CoF2 nanoparticles
were precipitated from the IL dispersion by the addition of dry
acetonitrile (10 mL), which was followed by centrifugation, decant-
ation, and redispersing (3ꢂ) by supersonic treatment in acetonitrile
(10 mL), centrifugation, and finally drying under high vacuum. The
electrochemical performance of CoF2 as a cathode material for lith-
ium-ion batteries was evaluated by galvanostatic charge/discharge
profiles on a half-cell. The working electrode was prepared by coat-
ing the slurry composed of 90 wt% CoF2 and 10 wt% PVDF on an
aluminum foil. The half-cell with lithium foil as the counter elec-
trode was assembled in an argon-filled glove box with [O2]
<0.5 ppm and [H2O]<0.5 ppm. The electrolyte was 1m LiPF6 in
ethylene carbonate/dimethyl carbonate (EC/DMC) (50:50 v/v) from
Sigma–Aldrich, and Whatmann glass microfiber membranes were
used as the separator. The cut-off potentials were controlled to 1.0
and 4.8 V versus Li+/Li by an electrochemical workstation from
Metrohm Autolab PGSTAT 302. LiPF6 can decompose even at room
temperature following the path outlined in Equation (2):
with AlKa X-rays from clean samples of copper, silver, and gold at
20 and 10 eV pass energies and comparison with reference values.
PXRD data were measured at ambient temperature with a Bruker
D2 Phaser by using a flat sample holder and CuKa radiation (l=
1.54182 ꢃ, 35 kV). Samples were precipitated with acetone from
the NP/IL and NP/PC dispersion and were washed with acetonitrile.
PXRD patterns were measured for 2–12 h. Small shifts in the PXRD
patterns are not uncommon for nanoparticles. A number of effects
can be considered for such shifts, including range of stoichiometric
composition, partly inhomogeneous elemental distribution, defects
such as stacking and twin faults, and nanosized crystalline domains
being much smaller than the bulk reference material, which cause
lattice contraction or expansion and strain.[64,65]
NMR spectra were recorded with a Bruker Avance DRX 200 (1H,
200 MHz) and Avance DRX 500 (13C, 125.57 MHz) at 298 K in C6D6,
and the chemical shifts are referenced to the solvent peaks against
tetramethylsilane.
*
LiPF6ðsÞ)ꢁLiFðsÞ þ PF5ðgÞ
ð2Þ
High-angle annular dark-field scanning transmission electron mi-
croscopy (HAADF-STEM) micrographs were taken at room tempera-
ture with an FEI Tecnai G2 F20 TEM[66] operated at an accelerating
voltage of 200 kV. Samples were deposited on 50 mm carbon-
coated gold or copper grids. The size distribution was calculated
from a manual diameter determination over a minimum of 60 iso-
lated particles.
Besides, LiPF6 can hydrolyze with moisture to form HF [Eq. (3)]:
LiPF6ðsolÞ þ H2O ! LiFðsÞ þ 2 HFðsolÞ þ POF3ðsolÞ
ð3Þ
However, in commercial electrolytes [1 molLꢁ1 LiPF6 in EC/DMC
(1:1 v/v)], the impurities are negligible (HF content less than
10 ppm), and the decomposition temperature of LiPF6 in such elec-
trolytes is believed to be as high as 808C, which is far above the
TEM-EDX was performed with an FEI Tecnai G2 F20, 136 eV, and an
exposure time for individual EDX spectra of 3 min. EDX analysis
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and HF. Besides, the electrochemical stability window of such an
electrolyte is regarded to be as high as 5.0 V versus Li+/Li due to
a passivation reaction, which is much larger than the potential ap-
plied in our case. Therefore, LiF may form but should be in a very
minor amount.
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Published online on && &&, 2016
&
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ÝÝ These are not the final page numbers!

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ChemistryOpen 2016, 00, 0 – 0
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ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ