Synthesis and characterization of diazonium salts with polyethylene glycol appendages and resulting films afforded by electrodeposition for use as a battery separator material

Article abstract of DOI:10.1021/cm501482h

The coating of three-dimensional nanostructured electrodes is a significant challenge for the future of many energy storage devices and, if successful, could profoundly increase battery power. The synthesis of a new class of monomers that can be electroch

Full text of DOI:10.1021/cm501482h

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Article
Synthesis and Characterization of Diazonium Salts
with PEG Appendages and Resulting Films Afforded by
Electrodeposition for use as a Battery Separator Material
Daniel J Bates, C. Michael Elliott, and Amy L. Prieto
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm501482h • Publication Date (Web): 18 Aug 2014
Downloaded from http://pubs.acs.org on September 1, 2014
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Chemistry of Materials
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Synthesis and Characterization of Diazonium Salts with PEG
Appendages and Resulting Films Afforded by Electrodeposi-
tion for use as a Battery Separator Material
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Daniel J. Bates, C. Michael Elliott, and Amy L. Prieto*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80524
KEYWORDS: Electrodeposition, Diazonium, Lithium Ion Battery, Separator, Solid State, Nanoscale
ABSTRACT: The coating of three dimensional nanostructured electrodes is a significant challenge for the future of many
energy storage devices and, if successful, could profoundly increase battery power. The synthesis of a new class of mono-
mers that can be electrochemically polymerized is a key first step to affording a conformally coated, nanoscale lithium-ion
battery separator and is presented herein. Characterization of the monomers was accomplished through NMR and IR
spectroscopy. Planar films electrodeposited from the monomers were characterized using redox probe experiments and
impedance spectroscopy. The films are chemically grafted to the underlying substrate, conformal, pinhole free, less than
10 nm thick and exhibit electrical resistivity values as high as 28,000 MΩ/cm.
INTRODUCTION
er. In fact, 3D nanostructured cathodes and anodes have
already been fabricated.4
Energy demands are steadily increasing worldwide. Alt-
hough fossil fuels have been used to supply the demand
via combustion, the effects on our environment cannot be
overlooked. This has led to a demand for greener energy
sources, although many of these sources (such as wind
and solar) are intermittent. Because of this, there is a
parallel need for better energy storage devices that can
charge and discharge rapidly. Lithium-ion batteries are
energy storage systems that are currently used to power
mobile electronic devices such as cell phones and laptops.
High power density lithium-ion batteries are even used to
power electric vehicles and power tools, but higher power
densities are still desired in order to reduce charge time
and increase possible use in high power applications. Fur-
thermore, with enough electric vehicles with high power
density batteries, the idea of a wide spread smart grid
would become economically feasible.
The major challenge in this field is the development of a
conformally coated, nanoscale material that can perform
as a separator and an electrolyte between high surface
area electrodes in an energy storage device. This problem
has been a major challenge for several reasons. First, na-
noscale materials are fragile and hard to work with. The
separator cannot be made independently of the anode or
cathode since assembly would be essentially impossible
with such a thin material; therefore, the separator needs
to be fabricated in situ. Second, the separator needs to
conformally coat the 3D nanostructure. Promising work
has been done using atomic layer deposition to coat elec-
trodes with a thin ceramic layer (Al2O3) to minimize sol-
vent decomposition and solid electrolyte interface (SEI)
formation, but the method is time-intensive and not yet
cost effective for commercial use.5,6 Third, controlling the
thickness of the separator can be difficult, especially for
films only 10s of nanometers thick. For polymerizations,
this usually requires a self-limiting polymerization mech-
anism to avoid incomplete or non-uniform film coverage.
Fourth, regardless of thickness, the separator must be
able to handle the physical stresses that occur during
Lithium-ion batteries store 2-3 times more energy per
weight than nickel-metal-hydride rechargeable batteries.1
Although lithium-ion batteries are revolutionizing tech-
nology, improvements are needed to keep up with the
world’s ever increasing energy storage demands. Current
battery technology uses layered thin films, which have
low surface areas due to their two dimensional geometry.
The films are microns thick, enabling modest power den-
sities at best.2,3 To maximize the power density, nanoscale
three-dimensional (3D) battery structures need to be fab-
ricated. A nanoscale 3D structure would greatly increase
the interfacial surface area of the battery, and therefore
increase charge/discharge rates (current) and thus, pow-
charging/discharging
wherein
significant
expan-
sion/contraction processes in the electrodes can occur. If
the separator cannot withstand these stresses, electrical
shorts between the electrodes in the device will develop.
Fifth, the separator has to be electronically insulating
when less than 100 nm thick, but also must be ionically
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conducting. These challenges are great and progress in To test the hypothesis that a diazonium-based film could
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the field of nanoscale separators has been slow.
be grafted, be nanoscale in thickness, and function as a
lithium-ion battery separator material, four monomers
were synthesized that all had diazanium functionalities.
The secondary hypothesis of this research is that mono-
mer structure, specifically, PEG tail length and position
on the benzene ring will influence ionic conductivity and
electrical resistivity of the resulting polymer film.
Electrodeposition of a self-limiting polymer-based separa-
tor can, in principle, address all of these issues, assuming
the monomer is chosen such that it affords an ionically
conducting and electronically insulating film. Electrodep-
osition as a synthetic technique is ideal because it allows
for conformal coating onto 3D structures employing sim-
ple equipment. Using either the cathode or anode as the
active electrode, the separator can be deposited directly
onto the electrode surface, minimizing handling and the
possibility of physical damage. Choosing a self-limiting
electrodeposition polymerization process affords a con-
formal coating with a very thin and uniform film.7 As the
polymerization occurs, the electrode becomes electroni-
cally insulating, leading to self-limiting behavior. In other
words, polymerization occurs more rapidly over regions
of highest electrical conductivity (i.e. bare surfaces),
which results in a complete, uniform modification of the
conducting surface.
9
To evaluate the deposited films, thickness measurements
were attempted using profilometry and scanning electron
microscopy imaging. The extent of grafting was evaluated
by impedance spectroscopy on sonicated samples. Im-
pedance spectroscopy was further used to determine ionic
and electronic conductivities across thin film (2D) sam-
ples. Table 1 summarizes criteria that were used to judge
the diazonium-based films along with the best reported
values found in the literature for comparison.13–15
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Three of the four monomers have never been reported in
the literature and none have been used in lithium-ion
battery separator applications. The monomers are: 4-(2-
The use of a polymer-based separator has appeal because
viscoelastic polymers can accommodate the stresses asso-
ciated with expansion and contraction of the electrodes,
as previously mentioned.8 Polymers with appropriate vis-
coelastic properties can be viewed simultaneously as vis-
cous liquids and elastic solids, allowing for movement of
the polymer when a stress is applied. This behavior is in
stark contrast to ceramic based separators which are hard
and brittle.9 In theory, employing a viscoelastic polymer
should allow the separator to maintain a physical separa-
tion between cathode and anode, while maintaining good
separator interfacial contact to each, even during the ex-
pansion/contraction processes.
(2-(2-methoxyethoxy)ethoxy)ethyl)benzene
tetrafluoroborate (p-triethoxy, synthesized by Bahr et
al.16),
2-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)benzene
diazonium tetrafluoroborate (o-triethoxy), 4-
diazonium
(polyethylene glycol)benzene diazonium tetrafluorobo-
rate (p-PEG), and 2-(polyethylene glycol)benzene diazo-
nium tetrafluoroborate (o-PEG).16 The p-PEG, o-PEG, p-
triethoxy, and o-triethoxy short hand notation will be
used for the remainder of this paper. The structures are
provided in Figure 1. We demonstrate that the monomer
structure influences the electrical resistivity of the result-
ing films through PEG tail length and position on the
benzene ring. The films described herein are extremely
thin (3-10 nm), conformally coat the electrodes, and ex-
hibit resistivity values as high as 28000 MΩ/cm.
In terms of designing polymer films with the desired
properties, the monomer choice is a critical factor to con-
sider. Films must conduct ions, specifically Li+, and must
also be electronically insulating. Ideally, the monomer
should graft to the electrode surface, increasing the inti-
mate contact between the polymer and the electrode, as
well as the mechanical robustness of the film, thus de-
creasing the likelihood of shorts. Herein we describe an
approach focused on achieving nanoscale polymer films
that can be grafted to the electrode surface. Diazonium
salts were chosen because they have been shown to graft
to electrode surfaces and exhibit great self-limiting depo-
sition characteristics.10–12 In addition, diazonium salts can
be deposited out of acidic aqueous conditions (pH ≤ 3) if
the monomers are soluble.12 The grafting of diazonium
salts involves a 1e- reduction of the diazonium functional
group followed by loss of N2(g) and phenyl radical for-
mation.12 The phenyl radical then reacts with the elec-
trode surface to form a chemical bond. Diazonium salts
have been electrografted onto many electrode materials
such as carbon, metals, oxides, semiconductors, and pol-
ymers.12
EXPERIMENTAL
CHEMICALS
Dichloromethane was purged with nitrogen and dried
over 4 Å molecular sieves before storing in a glove box. All
other reagents were obtained commercially and used as
received.
APPARATUS
Cyclic voltammetry (CV) and chronoamperometry exper-
iments were carried out using a CH Instruments Electro-
chemical Workstation Model 750 D. Impedance meas-
urements were carried out using a GAMRY Instruments
Reference 3000 potentiostat/galvanostat. Acetonitrile
(99.9 %) was purchased from Macron Chemicals and used
as received. Nuclear magnetic resonance spectroscopy
was carried out using a Varian Mercury-Inova 300 NMR
Spectrometer Systems. Infrared spectroscopy (IR) meas-
urements were performed using Nicolet 380 FT-IR spec-
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Chemistry of Materials
2-(Polyethylene glycol)nitrobenzene (6) was synthe-
trophotometer and the samples were prepared in KBr
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pellets.
sized following the method published by Bahr et al.; ex-
cept polyethylene glycol was used instead of tri(ethylene
glycol)monomethyl ether, o-nitrophenol was used instead
of p-nitrophenol, and 5 % methanol in dichloromethane
(by volume) was used as the eluent for the column chro-
SYNTHESIS
All syntheses were modified from a publication by Bahr et
al. who reported the synthesis of p-PEG.16 For the diazoti-
zation reaction, the method published by Tang et al. was
found to be more repeatable, yielded more product, and
the products were of a higher purity according to NMR
than the method published by Bahr et al.16,17 For increased
purity, a silica column was employed on the o-PEG salt
(14, vide infra) with 5 % MeOH in CH2Cl2 (by volume) as
the eluent. Each diazonium salt synthesis involves 4 steps
starting from an alcohol, see Scheme 1. This reaction
scheme is therefore useful for many starting materials as
long as an alcohol group is present.
1
matography.16 A yellow oil was afforded at 63 % yield. H
NMR (CDCl3) δ: 7.84 (d, 1H), 7.52 (t, 1H), 7.12 (d, 1H), 7.03
(t, 1H), 4.27 (t, 2H), 3.91 (t, 2H), 3.74 (m, 4H), 3.74 to 3.60
(m, 20H).
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4-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)aniline
(7)
was synthesized using the method published by Bahr et
al.16 “Acidic ethanol” used in the synthesis was afforded by
adding 2 mL conc. HCl to 50 mL EtOH which was the
same for all the aniline syntheses. A reddish-brown oil
1
was afforded at 92 % yield. H NMR (CDCl3) δ: 6.78 (d,
2H), 6.73 (d, 2H), 4.08 (t, 2H), 3.85 (t, 2H), 3.79-3.63 (m,
6H), 3.59 (t, 2H), 3.41 (s, 3H). IR (neat) 3432, 3356, 3233,
2876, 1630, 1513, 1456, 1351, 1238, 1107, 943, 826 cm-1.
2-(2-(2-Methoxyethoxy)ethoxy)ethyl
toluenesulfonate (1) was synthesized using the method
published by Bahr et al.16 A clear and colorless oil was
afforded at 93 % yield. H NMR (CDCl3) δ: 7.80 (d, 2H),
7.35 (d, 2H), 4.14 (t, 2H), 3.68 (t, 2H), 3.64 to 3.54 (m, 6H),
3.52 (t, 2H), 3.37 (s, 3H), 2.49 (s, 3H).
p-
1
2-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)aniline
(8)
was synthesized following the method published by Bahr
et al.; except compound 4 was used as the starting materi-
al instead of compound 3.16 A reddish-brown oil was af-
forded at 94 % yield. 1H NMR (CDCl3) δ: 6.85 (m, 4H), 4.17
(t, 2H), 3.84 (t, 2H), 3.73 (t, 2H), 3.68 (m, 4H), 3.58 (t, 2H),
3.40 (s, 3H). IR (neat) 3470, 3360, 3202, 2877, 1619, 1511,
1461, 1352, 1281, 1223, 1118, 947, 851, and 747 cm-1.
Poly(ethylene glycol) p-toluenesulfonate (2) was syn-
thesized following the method published by Bahr et al.16
Polyethylene glycol (Mn = 400 g/mol) was used instead of
tri(ethylene glycol)monomethyl ether. A clear and color-
less oil was afforded at 82 % yield. 1H NMR (CDCl3) δ: 7.61
(d, 2H), 7.15 (d, 2H), 3.95 (t, 2H), 3.33 to 3.53 (m, 16H),
2.24 (s, 3H).
4-(Polyethylene glycol)aniline (9) was synthesized fol-
lowing the method published by Bahr et al.; except com-
pound 5 was used as the starting material instead of com-
pound 3.16 A reddish-brown oil was afforded at 88 % yield.
1H NMR (CDCl3) δ: 6.80 (m, 4H), 4.04 (t, 2H), 3.82 (m,
4H), 3.75 to 3.55 (m, 30H). IR (neat) 3440, 3356, 2872,
1636, 1511, 1465, 1356, 1294, 1244, 1118, 943, 834, and 730 cm-
4-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)nitrobenzene
(3) was synthesized using the method published by Bahr
et al.16 One modification was made, that being 5 % meth-
anol in dichloromethane (by volume) was used as the
eluent for the column chromatography. A clear and light
yellow oil was afforded at 82 % yield. H NMR (CDCl3) δ:
8.18 (d, 2H), 6.99 (d, 2H), 4.23 (t, 2H), 3.90 (t, 2H), 3.80 to
3.60 (m, 6H), 3.55 (t, 2H), 3.38 (s, 3H).
1
1
.
2-(Polyethylene glycol)aniline (10) was synthesized
following the method published by Bahr et al.; except
compound 6 was used as the starting material instead of
compound 3.16 A light brown oil was afforded at 98 %
yield. 1H NMR (CDCl3) δ: 6.83 (m, 4H), 4.16 (t, 2H), 3.85 (t,
2H), 3.73 (m, 4H), 3.72 to 3.60 (m, 28H). IR (neat) 3465,
3361, 2872, 1619, 1507, 1461, 1352, 1281, 1223, 1123, 947, 843,
and 742 cm-1.
2-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)nitrobenzene
(4) was synthesized following the method published by
Bahr et al.; except o-nitrophenol was used instead of p-
nitrophenol and 5 % methanol in dichloromethane (by
volume) was used as the eluent for the column chroma-
tography.16 A yellow oil was afforded at 82 % yield. H
NMR (CDCl3) δ: 7.81 (d, 1H), 7.51 (t, 1H), 7.12 (d, 1H), 7.03
(t, 1H), 4.27 (t, 2H), 3.91 (t, 2H), 3.76 (t, 2H), 3.65 (m, 4H),
3.55 (t, 2H), 3.38 (s, 3H).
1
4-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)benzene dia-
zonium tetrafluoroborate (11; p-triethoxy) was synthe-
sized using the method published by Tang et al.17 A red-
1
dish-brown tar was afforded at 95 % yield. H NMR (d-
4-(Polyethylene glycol)nitrobenzene (5) was synthe-
sized following the method published by Bahr et al.; ex-
cept polyethylene glycol was used instead of tri(ethylene
glycol)monomethyl ether and 5 % methanol in dichloro-
methane (by volume) was used as the eluent for the col-
umn chromatography.16 A yellow oil was afforded at 56 %
acetone) δ: 8.75 (d, 2H), 7.55 (d, 2H), 4.55 (t, 2H), 3.92 (t,
2H), 3.67 (t, 2H), 3.58 (m, 4H), 3.46 (t, 2H), 3.27 (s, 3H). IR
(neat) 3115, 3060, 2893, 2250, 1582, 1490, 1455, 1340, 1286,
1072, 938, 847, and 742 cm-1.
2-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)benzene dia-
zonium tetrafluoroborate (12; o-triethoxy) was syn-
thesized following the method published by Tang et al.17 A
1
yield. H NMR (CDCl3) δ: 8.25 (d, 2H), 7.02 (d, 2H), 4.26
(t, 2H), 3.93 (t, 2H), 3.75 (m, 4H), 3.75 to 3.62 (m, 14H).
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reddish-brown tar was afforded at 84 % yield. 1H NMR (d-
The “tail” of connecting ethylene glycol repeating units
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acetone) δ: 8.59 (d, 1H), 8.31 (t, 1H), 7.82 (d, 1H), 7.54 (t,
1H), 4.75 (t, 2H), 4.02 (t, 2H), 3.72 (t, 2H), 3.62 (m, 4H),
3.51 (t, 2H), 3.32 (s, 3H). IR (neat) 3110, 2885, 2267, 1594,
1569, 1490, 1452, 1352, 1310, 1085, 939, 851, and 768 cm-1.
was chosen to incorporate ion mobility into the resulting
polymer film. Two different tail lengths (triethoxy and
PEG) were synthesized to evaluate if tail length affected
ionic conductivity or electrical resistivity. The position of
the tail on the benzene ring was also investigated for the
same reasons. One hypothesis was that if the tail and dia-
zonium functional groups are in the para-position relative
to each other, the benzene rings might pack more tightly
onto the electrode surface leading to a film with higher
density. Steric hindrance has been shown to influence
packing density in a predictable matter for self-assembled
monolayers.20,21 A denser film should afford a lower ionic
conductivity and a greater electrical resistivity. Further-
more, with a PEG-type tail in the para-position to the
grafting site, lithium ion transport into the PEG region
might be difficult. In contrast, with a tail in the ortho-
position, the PEG moiety would be directly adjacent to
the electrode surface and not blocked by the benzene
rings, potentially allowing for better ion mobility very
near the electrode surface. Having the PEG tail in the or-
tho-position should also lead to a less dense film because
the benzene rings should not be able to pack as tightly
onto the electrode surface for steric reasons. To summa-
rize, the secondary hypothesis of this research is that
monomer structure, specifically, PEG tail length and posi-
tion on the benzene ring will influence ionic conductivity
and electrical resistivity of the resulting polymer film.
4-(Polyethylene glycol)benzene diazonium tetra-
fluoroborate (13; p-PEG) was synthesized following the
method published by Tang et al.17 A reddish-brown tar
was afforded at 78 % yield. 1H NMR (d-acetone) δ: 8.80 (d,
2H), 7.60 (d, 2H), 4.58 (t, 2H), 3.93 (t, 2H), 3.67 to 3.47 (m,
30H). IR (neat) 3110, 2885, 2258, 1586, 1511, 1452, 1344, 1290,
1089, 955, 847, and 734 cm-1.
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2-(Polyethylene glycol)benzene diazonium tetra-
fluoroborate (14; o-PEG) was synthesized following the
method published by Tang et al.17 A reddish-brown tar
was afforded at 27 %, the low final yield was due to col-
umn purification. 1H NMR (d-acetone) δ: 8.61 (d, 1H), 8.30
(t, 1H), 7.82 (d, 1H), 7.54 (t, 1H), 4.76 (t, 2H), 3.98 (t, 2H),
3.72 to 3.52 (m, 32H). IR (neat) 3110, 2885, 2267, 1590, 1569,
1494, 1356, 1297, 1089, 951, 838, 767, and 734 cm-1
ELECTROCHEMICAL TECHNIQUES
Cyclic voltammetry and film depositions (chronoam-
perometry) were performed using a glass cyclic voltam-
metry cell composed of a working and reference com-
partment connected by a Luggin capillary. The working
compartment was degassed with nitrogen for 5 minutes
to remove oxygen from the compartment prior to taking a
CV or performing a deposition. The electrolyte solution
was composed of 0.1 M TBAPF6 in acetonitrile. The mon-
omer concentration was chosen to be 10 mM, which was
determined to be an appropriate concentration for the
complete modification of the electrode surface. At low
concentrations, ca. 1 mM, it has been reported (for other
monomers) that surface modification does not occur.18
After each deposition, the modified glassy carbon (GC)
electrode was sonicated for 15 minutes in neat acetonitrile
to remove any non-grafted components in the film. Im-
pedance spectroscopy was performed using a soft contact
method published by Rhodes et al.7 The diazonium modi-
fied GC electrode was lowered into a pool of In/Ga. Gold
evaporated onto glass was used as a plate to hold the
In/Ga and provide good electrical contact to the eutectic.
The frequency range used was 1 Hz to 1 MHz with a volt-
age amplitude of 10 mV.
Infrared spectroscopy was used to identify key functional
groups during the synthesis of the diazonium monomers.
The nitro stretching peak (ca. 1500 cm-1) was difficult to
identify prior to the hydrogenation step due to multiple
peaks near 1500 cm-1 (the peak nearest to 1500 cm-1 was
present for each intermediate product).22 IR was success-
fully used to identify the amine stretching frequency
(3250-3500 cm-1) after hydrogenation of the nitrobenzene
confirming successful reduction to the aromatic amine.23
After the diazotizing reaction, the amine peaks were
greatly suppressed and a sharp peak at 2260 cm-1 was pre-
sent, consistent with the presence of the diazonium func-
tional group.24
Cyclic voltammetry was used to determine the reduction
potentials of the diazonium salts (Figure 2). All diazoni-
um salts exhibit a broad irreversible reduction peak,
common for diazonium salts.11,25 All peaks occur in the
range of -0.5 to -0.9 V vs SSCE, therefore the depositions
were performed at -1.0 V vs SSCE. As seen in Figure 2, the
o-PEG does show an additional wave at ca. -1.6 V vs SSCE,
most likely due to an impurity present that we were not
able to identify. This was surprising since o-PEG was the
only diazonium salt purified by column chromatography;
however, by depositing at -1.0 V electrochemical reactions
with the impurity were likely avoided. Ortho substituted
diazonium salts have more negative reduction potentials
likely due to the proximity of the electron-donating ether
group to the diazonium. In general, electron-donating
groups are expected to shift the reduction potential to
more negative values. The concentration of diazonium
RESULTS AND DISCUSSION
Monomer structure is an important consideration for the
physical properties of the resulting polymer film. All of
the monomers chosen for this study have a diazonium
“head” that can undergo grafting to multiple types of sur-
faces.12,19 Also, the diazonium functionality allows for di-
rect reduction of the monomer onto the electrode surface
without the use of a radical initiator, and should result in
self-limiting characteristics which produce very thin
films.10–12
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Chemistry of Materials
type films reported by Rhodes et al.7 Para-substituted
salt was the same for all the cyclic voltammograms, but
1
2
3
4
5
6
7
the peak current in the CV varied significantly for the
monomers. Differences in the self-limiting nature of the
reduction for each of the monomers is likely the reason
for the different peak heights. Comparing the four diazo-
nium salts, there is a strong correlation between lower
initial current in the voltammogram and lower final cur-
rent at the end of the deposition.
films exhibit higher resistances than their ortho counter-
part consistent with the hypothesis that the benzene rings
should pack more tightly. Polymers formed from the
shorter tail monomers in each pair also had the higher
resistance, consistent with less steric hindrance and tight-
er packing onto the electrode surface.
8
9
The thickness of the films was calculated using the capac-
itance values obtained from the impedance spectroscopy.
Using the equation below for capacitance, the thickness
was calculated.
Chronoamperometry was used to deposit the diazonium
based films onto a GC electrode. Deposition curves can be
seen in Figures 3-6. Insets show the current decrease on a
log scale for better comparison. After 15 minutes the cur-
rent dropped at least 2 orders of magnitude in all cases
and as much as 5 orders of magnitude for p-PEG. This
demonstrates the remarkable self-limiting nature of dia-
zonium-based films. As expected, para substituted diazo-
nium salts showed better shutoff current (vide infra) as
well as the best self-limiting reduction. Again, this is like-
ly due to the ability of the benzene rings of the diazonium
to pack more tightly onto the electrode surface for steric
reasons. The PEG based monomers also exhibited better
self-limiting behavior (relative to the shorter, methoxy-
terminated monomers), likely due to steric exclusion of
fresh monomer to the electrode surface by the longer
chain tails.
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In the equation above, C = capacitance (F), κ = dielectric
constant, ε0 = permittivity of free space (F/cm), A = area of
electrode (cm2), and D = film/dielectric thickness (cm).
The dielectric constant of the films was estimated to be
2.7 ± 0.2, which is an average of 4 dielectric constants of
polymer films with similar molecular architectures to the
ones prepared here: poly(phenylene oxide), 2.59; polycar-
bonate, 2.92; an azopolymer with aliphatic and aromatic
fragments, 2.5; poly(phenylene oxide), 2.77.7,28,29 The area
of the electrode was constant throughout all experiments
and was equal to 0.71 cm2. The capacitance values were
determined using the same fit as the resistance values,
given in Table 1. All films were extremely thin, as expected
based on their self-limiting property. Based on the above
calculation, three films (o-triethoxy, p-triethoxy, and p-
PEG) were estimated to be 3 nm thick while o-PEG was
estimated to be 10 nm thick. Attempts to measure the
thickness of the films using profilometry or SEM imaging
were unsuccessful because the film thicknesses were be-
low the accurate detection limits of the instruments.
After each deposition, the coated GC electrode was soni-
cated in neat acetonitrile for 15 minutes to remove any
non-grafted components from the film before any charac-
terization. Similar treatment of non-grafted films typically
removes the film or at least introduces "bare patches"
which were not evident in the impedance tests or redox
shut-off experiments (vide infra).
Impedance spectroscopy was used to determine the elec-
tronic resistance and ionic conductivity of the films. Open
circuit potential (OCP) was measured for 30 seconds prior
to impedance measurements and impedance measure-
ments were taken about OCP. Over 30 seconds, sample
OCP varied less than 50 μV and was considered stable.
None of the films exhibit a diffusional (Warburg) tail at
low frequency in the Nyquist plot (Figure 7); hence, none
of the films exhibited ionic conductivity.26,27 This is not
surprising since the as-deposited films should be void of
any ions. Attempts to measure impedance with plasti-
cized films (1.0 M LiClO4 in ethyl carbonate/dimethyl
carbonate (70/30 by volume) were unsuccessful because
an oxide layer formed instantly on the In/Ga eutectic up-
on contact with the plasticizer. However, ion permeation
through the film is suggested by redox probe experiments
(vide infra, Figures 8-11). A depressed semicircle at medi-
um to high frequency can be modeled with a resistor in
parallel with a capacitor, a simple Randles circuit. The
model can be attributed to the film resistance that is in
parallel with the film capacitance. The semicircle is de-
pressed which is likely due to leaky or imperfect interfa-
cial capacitance.27 Resistance values for the films can be
found in Table 2. The values are all in the kΩ range,
which is comparable to nanoscale poly(phenylene oxide)
Using the calculated thicknesses and the measured re-
sistances, the resistivities were calculated for all four films
(Table 2). All films exhibit high resistivity values (1200-
28000 MΩ/cm), consistent with being insulators.30 It is
difficult to compare these values with those of commer-
cially available separators because those resistivity values
are typically reported as a Gurley number, which is the
permeability of the separator to air.31 Air permeability is
proportional to the resistivity of a separator, but for a giv-
en morphology (which must be known).31 Since the mor-
phology of the diazonium based films reported here is not
known, a Gurley number could not be calculated. Howev-
er, for this application the more relevant number is the
breakdown voltage of the films, Attempts to perform sol-
id-state linear sweep voltammetry on the films (not
shown) resulted in a breakdown of all of the films near 1.5
V (using the same experimental procedures as impedance
measurements described previously, however with the
reference electrode shorted to the auxiliary electrode).
This implies that although the films described here have
high electrical resistivities, they are not thick enough to
hold up to the voltages of typical lithium cells (ca. 3.2 V).
Future work will involve modifying the electrochemical
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polymerization conditions to afford significantly thicker ionic conductivity will be very low as the polymer loses
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4
films, while maintaining good grafting and high electrical
resistivities.
chain mobility. Due to the extremely thin nature of the
diazonium-based films, as well as the fact that they are
robustly grafted to the electrode surface, it was not possi-
ble to isolate a large enough sample size for differential
scanning calorimetry.
In order to determine if the deposited films contained
pinhole defects or exhibited incomplete coverage, ferro-
cene and 1, 1’, 3, 3’-tetra-t-butyl ferrocene were used as
probe molecules in a redox shut-off experiment. The fer-
rocene probes showed a small irreversible anodic peak for
all four films (Figures 8-11). This data alone was inconclu-
sive with regards to the presence or absence of pinholes,
because the small current measured could also be at-
tributed to the diffusion of ferrocene through the polymer
film. Consequently, 1, 1’, 3, 3’-tetra-t-butyl ferrocene was
also examined. In contrast to the unsubstituted ferro-
cene, the bulkier 1, 1’, 3, 3’-tetra-t-butyl ferrocene yielded
essentially no current on the modified GC with the excep-
tion of the o-triethoxy film (Figure 11). If the two ferro-
cenes were accessing the electrode surface through pin-
holes significantly larger than molecular dimensions, the
ferrocene and 1, 1’, 3, 3’-tetra-t-butyl ferrocene probe mol-
ecules would give similar electrochemical responses. The
differences in the voltammetry of the two differently sized
ferrocene molecules is clear evidence that they are per-
meating the films via pathways that are of molecular di-
mensions, such as diffusion through the films, as opposed
to directly accessing the electrode surface through pin-
holes. Interestingly, although o-triethoxy films were
more electrically resistive than o-PEG, the probe experi-
ment suggests o-triethoxy is better at allowing molecules,
and possibly ionic species, to transport to the electrode
surface. Further evidence for this is seen in the deposition
curves, which show worse shut-off/blocking effects for o-
triethoxy. The data suggests that o-triethoxy-like poly-
mers may well be more suitable as a separator material for
ionic transport reasons, but would also require the thick-
est film due to its lower electrical resistivity. The fact that
ferrocene was able to reach the electrode surface and oxi-
dize, for all films, is noteworthy. This suggests that
transport of molecules (and possibly ions) through the
film is possible and that there is a size dependency to the
transport that could be of some importance in applica-
tions that require ionic permeability of these films, such
as a separator in a lithium-ion battery.
5
6
7
8
9
CONCLUSION
A new class of monomers with a diazonium functional
group was successfully synthesized and characterized.
The monomers have a diazonium head group that can
undergo reductive grafting to electrode surfaces and also
leads to self-limiting polymerization.
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The original hypothesis of this research was that diazoni-
um-based monomers could be electrochemically deposit-
ed onto an electrode surface and would be chemically
grafted to the surface. Due to the self-limiting behavior
of such processes, the expectation was that the films
would be nanoscale in thickness and could serve as a plat-
form coating to function as a lithium-ion battery separa-
tor and electrolyte material. Each monomer proposed
was successfully electrochemically polymerized to give
very uniform, conformal and pin-hole-free films. The fact
that these films were not removed by post-polymerization
sonication suggests that the films are indeed grafted to
the electrode. Impedance spectroscopy has provided elec-
tronic conductivities of dry films in air. The monomer
structure influences electrical resistivity through PEG tail
length and position on the benzene ring. The films were
extremely thin (3-10 nm) and exhibited resistivity values
as high as 28000 MΩ/cm. Films afforded from para-
substituted diazonium salts exhibit higher resistivity val-
ues than ortho substituted salts, and blocked the elec-
trode better during redox shut-off experiments. Therefore
para substitution decreases transport (probe molecules or
monomer) to the electrode surface but increases electrical
resistivity. In other words, ortho substitution appears to
facilitate mobility through the film while para substitu-
tion increases electrical resistivity of the film. While it
was not possible to obtain ionic conductivities of plasti-
cized, ion-doped films, redox-shutoff experiments con-
firm the lack of pinholes larger than molecular dimen-
sions and the ability of small molecules (i.e., ferrocene) to
permeate through the films. Table 3 summarizes the cri-
teria used to evaluate these diazonium-based films in the
context of how they might perform as lithium-ion battery
separators.
Comparing ortho vs. para substitution in Figures 8-11, the
data suggests that para substituted films are better at
blocking the electrode from probe molecules than the
ortho substituted films are. As anticipated, with the tail in
the ortho position transport of molecules through the
film to the electrode is facile, which is consistent with our
hypothesis about the relative ability of the benzene heads
to pack. Furthermore, assuming the site of attachment is
at the original position of the diazonium group, the ortho
position forces the PEG tail to be directly adjacent to the
electrode surface, possibly facilitating molecule and ion
movement very near the electrode surface.
Finally, increasing the thickness of the films is still de-
sired as this would increase the voltage window for opera-
tion. The films described here constitute a first step to-
ward developing a nanoscale lithium-ion battery separa-
tor material that undergoes grafting to the electrode sur-
face. These are critical components in the conformal
coating of a three-dimensional electrode surface.
Finally, glass transition temperature (Tg) is also an im-
portant consideration for a separator material because the
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Chemistry of Materials
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FIGURES
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Figure 3. Deposition curve for 2-(polyethylene gly-
col)benzene diazonium tetrafluoroborate (o-PEG). Inset
shows current on a log scale.
Figure 1. Monomer structures. From top to bottom: p-
triethoxy, o-triethoxy, p-PEG, o-PEG.
Figure 4. Deposition curve for 4-(polyethylene gly-
col)benzene diazonium tetrafluoroborate (p-PEG). Inset
shows current on a log scale. The noise is due to very low
current, which was near the limitations of the potenti-
ostat.
Figure 2. Cyclic voltammograms for all four diazonium
salts; CVs were taken in a solution of 0.1 M tetrabu-
tylammonium hexafluorophosphate in acetonitrile, scan
rate = 100 mV/sec, the glassy-carbon electrode was pol-
ished using 0.3 μm β-alumina on a polishing pad before
each scan.
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Figure
5.
Deposition
curve
for
2-(2-(2-(2-
Figure 7. Nyquist plotted impedance data with fits for all
four diazonium-based films deposited.
methoxyethoxy)ethoxy)ethyl)benzene diazonium tetra-
fluoroborate (o-triethoxy). Inset shows current on a log
scale.
Figure
6.
Deposition
curve
for
4-(2-(2-(2-
methoxyethoxy)ethoxy)ethyl)benzene diazonium tetra-
fluoroborate (p-triethoxy). Inset shows current on a log
scale.
Figure 8. Cyclic voltammograms from ferrocene and
1,1’,3,3’-tetra-t-butyl ferrocene probe experiments for p-
PEG film.
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Chemistry of Materials
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Figure 9. Cyclic voltammograms from ferrocene and
1,1’,3,3’-tetra-t-butyl ferrocene probe experiments for o-
PEG film.
Figure 10. Cyclic voltammograms for ferrocene and
1,1’,3,3’-tetra-t-butyl ferrocene probe experiments for p-
triethoxy film.
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SCHEMES
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Scheme 1. Reaction scheme used to synthesize diazo-
nium salts starting from an alcohol. The reaction is
applicable to many alcohols.16, 17
Figure 11. Cyclic voltammograms for ferrocene and
1,1’,3,3’-tetra-t-butyl ferrocene probe experiments for o-
triethoxy film.
TABLES
Table 1. A summary of the physical properties that were used as criteria to
judge diazonium-based films. Included are ideal values along with some of
the best reported values in literature.
Physical Property
Ideal
Reported in Literature
Reductively Polymerized
yes
yes (polymers in general) (Ref 15)
Electronic Conductivity
Ionic Conductivity
zero
7 x 10-12 S/cm (Ref 14)
≥ 10-2 S/cm
1.6 x 10-4 S/cm (Ref 13)
21 ± 2 nm (Ref 14)
21-43 nm (Ref 14)
yes (Ref 15)
Conformal/Uniform Coating yes
Thickness
Grafted
Tg
10-50 nm
yes
-100 °C
-80 °C (Ref 13)
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Chemistry of Materials
Table 2. Values obtained by impedance spectroscopy
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Resistance
(kΩ)
Capacitance
(nF)
Thickness
(nm)
Film
Resistivity (kΩ*cm)
p-triethoxy
o-triethoxy
p-PEG
11.5 ± 0.1
4.34 ± 0.02
9.40 ± 0.05
1.66 ± 0.01
59.0 ± 0.3
54.0 ± 0.3
53.0 ± 0.2
17.0 ± 0.1
2.9 ± 0.2
2.8 ± 0.2 E+06
9.9 ± 0.6 E+05
2.1 ± 0.1 E+06
1.2 ± 0.1 E+05
3.1 ± 0.2
3.2 ± 0.2
10 ± 1
o-PEG
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Table 3. A summary of the physical properties that were used as criteria to judge diazonium-
based films. Included are ideal values along with some of the best reported values in literature.
Physical Property
Diazo-film
Ideal
Reported in Lit.
Reductively Polymerized
yes
yes
yes (polymers in general) (Ref 15)
Electronic Conductivity
Ionic Conductivity
3.6 x 10-10 S/cm
none
zero
7 x 10-12 S/cm (Ref 14)
≥ 10-2 S/cm
yes
1.6 x 10-4 S/cm (Ref 13)
21 ± 2 nm (Ref 14)
21-43 nm (Ref 14)
yes (Ref 15)
Conformal/Uniform Coating Conformal
Thickness
Grafted
Tg
3-10 nm
yes
10-50 nm
yes
unknown
-100 °C
-80 °C (Ref 13)
(1)
Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Angew.
Chem. Int. Ed. 2008, 47, 2930–2946.
AUTHOR INFORMATION
(2) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.;
Kim, B.; Hammond, P. T.; Shao-Horn, Y. Nat. Nano.
2010, 116, 531–537.
(3) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S.
Chem.l Rev. 2004, 104, 4463–4492.
Corresponding Author
* Amy.Prieto@colostate.edu
Author Contributions
(4) Arthur, T. S.; Bates, D. J.; Cirigliano, N.; Johnson, D.
C.; Malati, P.; Mosby, J. M.; Perre, E.; Rawls, M. T.;
Prieto, A. L.; Dunn, B. MRS Bull. 2011, 36, 523–531.
(5) Leung, K.; Qi, Y.; Zavadil, K. R.; Jung, Y. S.; Dillon,
A. C.; Cavanagh, A. S.; Lee, S.-H.; George, S. M. J.
Am. Chem. Soc. 2011, 133, 14741–14754.
(6) Woo, J. H.; Trevey, J. E.; Cavanagh, A. S.; Choi, Y. S.;
Kim, S. C.; George, S. M.; Oh, K. H.; Lee, S.-H. J.
Electrochem. Soc. 2012, 159, A1120 –A1124.
All authors have given approval to the final version of the
manuscript.
Funding Sources
Partial support for D. J. Bates through NSF grant CHE-
1058121, remaining support for D. J. Bates through Prieto Bat-
tery Inc.
ACKNOWLEDGMENT
We would like to thank Dr. Matthew Rawls, Daniel DiRocco,
Don Heyse, and Prieto Battery, Inc. for assistance with in-
strumentation as well as useful suggestions. We would also
like to thank Kyle Schachinger for donation of the 1,1’,3,3’
tetra t-butyl ferrocene. In memory of C.M.E.
(7) Rhodes, C. P.; Long, J. W.; Rolison, D. R. Electro-
chem.l Solid-State Lett. 2005, 8, A579–A584.
(8) Painter, P. C.; Coleman, M. M. Fundamentals of
Polymer Science an Introductory Text, Second Edi-
tion; CRC Press: Boca Raton, 1997; pg 416.
(9) Tilley, R. Understanding Solids The Science of Mate-
rials; John Wiley & Sons, Inc.: West Sussex PO19
8SQ, England, 2004; pg 159.
(10) Adenier, A.; Combellas, C.; Kanoufi, F.; Pinson, J.;
Podvorica, F. I. Chem. Mater. 2006, 18, 2021–2029.
(11) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume,
O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem.
Soc. 1997, 119, 201–207.
ABBREVIATIONS
EC/DMC = ethyl carbonate/dimethyl carbonate, GC = glassy
carbon, PEG = poly(ethylene glycol), TBAPF6 = tetrabu-
tylammonium hexafluorophosphate, CV = cyclic voltamme-
try.
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