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Preparation of a Self-Supported SiO 2 Membrane as a
Separator for Lithium-Ion Batteries
Neil Amponsah Kyeremateng,* [a, d] Dion Gukte, [b] Marc Ferch, [a] Jan Buk, [c] Tomas Hrebicek, [c]
and Robert Hahn* [b]
For the first time, electrophoretic deposition (EPD) has been
employed to prepare a self-supported, inorganic membrane
consisting of SiO 2 nano-fibers, as a separator for lithium-ion
batteries. The SiO 2 nano-fibers that were fabricated by a low-
cost force spinning technique were deposited by EPD directly
onto LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode material. Citric acid charging
agent and anhydrous acetone solvent were used. The resulting
porosity and tortuosity of the EPD SiO 2 separator were 71.42 %,
and 1.70, respectively. The slightly higher tortuosity of the EPD-
SiO 2 -fiber separator (60 μ m) led to a lower rate capability in
comparison to commercial GF/A glass fiber separator (260 μ m).
On the other hand, the latter exhibited lower self-discharge
than the former in full-cells with a graphite anode; this is
proposed to be related to the different purities of the two
materials that impart different electronic properties or the
presence of 20 wt % PVDF in the EPD-SiO 2 separator. Indeed,
the deposited membrane has good characteristics as a battery
separator and the EPD process is extremely feasible for the
fabrication of miniaturized lithium-ion batteries on wafer level.
1. Introduction
Lithium-ion (Li-ion) batteries started to boost the marketing of
portable electronic devices in 1991. [1] Owing to their high
energy density and efficiency, they have now instigated the
mass commercialization of electric vehicles with good
autonomy. [2] Miniaturized forms of Li-ion batteries have also
been successfully adopted for micro-electronic applications. [3]
The great success of the Li-ion technology achieved, so far, is
because of the significant research and development efforts
dedicated to it since the past three decades. [4] However, Li-ion
batteries truly perform far below their theoretical capabilities. It
is still believed that the performance of Li-ion or lithium-based
batteries can be drastically improved if significant progress is
made in electrode and electrolyte materials. [5]
Apart from the focus on electrode and electrolyte materials,
various approaches are also being investigated to develop
better separator materials for Li-ion batteries. [6] Actually,
separators account for about 20 % of the Li-ion battery cost,
and much of that cost relates to the separator’s production
alone; hence, there is still significant interest in cost-effective
fabrication methods for Li-ion battery separators. [7] The separa-
tor is an indispensable component of the Li-ion battery as it
prevents direct contact between the anode and cathode of the
cells to minimize self-discharge. It also ensures effective trans-
port of electro-active species (i. e. lithium ions) from one
electrode to another via the absorbed electrolyte. Convention-
ally, polyolefins, especially polypropylene materials (Celgard®)
are used as the separator for commercial Li-ion batteries. [1b,8]
The principal requirements of a battery separator include ( i )
good chemical and electrochemical stability, ( ii ) good porosity,
( iii ) high electrical resistance, ( iv ) good wettability, ( v ) good
thermal stability and ( vi ) good mechanical strength. Though
the Celgard® separators sufficiently meet most of the above
requirements, their polymeric nature undermines their thermal
and mechanical stabilities which are very necessary for over-
coming the thermal runaway issues of Li-ion batteries.
In fact, tri-layered polyolefin separators, consisting of a
polyethylene (PE) layer sandwiched between two polypropy-
lene (PP) layers, exhibit the “shut-down’’ effect: that is in case
of overheating, the PE layer melts, losing its porosity and
mechanically blocking Li + ion movement (to shut down the
cell, and avoid thermal runaway); though the PP layer can
prevent some dimensional changes, it will also eventually
melt. [6a,7a,9] Thus, such polymeric separators have a serious
thermal limitation: either significant shrinkage at 100 ° C, or
definite irreversible melting at T � 135 ° C). [6a,9] Although compo-
site separators with polymers and inorganic particles offer
excellent wettability and better thermal stability simultane-
ously, they are not mechanically well-adapted to withstand
handling in cell winding and assembly. [6e,7a,10] As a consequence,
bi-layer separators called Separion® which consist of polymeric
[a] Dr. N. A. Kyeremateng, M. Ferch
Micro Energy Storage Group, Research Center for Microperipheric Technol-
ogies, Technical University of Berlin, 13355 Berlin, Germany
[b] D. Gukte, Dr. R. Hahn
Fraunhofer Institut für Zuverlässigkeit und Mikrointegration (IZM), Micro
Energy Storage Group, 13355 Berlin, Germany
E-mail: [email protected]
[c] J. Buk, T. Hrebicek
Pardam, Žižkova 2759, 413 01, Roudnice nad Labem, Czech Republic
[d] Dr. N. A. Kyeremateng
Center for Process Innovation (CPI), Nanomaterials and Composites, The
Coxon Building, John Walker Road, Sedgefield, County Durham, TS21 3FE,
England, United Kingdom
Fax:
+
49 3046403-123
E-mail: [email protected]
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This
is an open access article under the terms of the Creative Commons Attri-
bution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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non-woven mats coated with a thin layer of inorganic (SiO 2 ,
CeO 2 , Al 2 O 3 , ZrO 2 etc.) nano-particles have also emerged. [7a] The
Separion® performs better than all the aforementioned separa-
tor types as the polyethylene terephthalate (PET) mat – which
has a high melting point of 260 ° C – imparts better thermal
stability, and the anchored inorganic nanoparticles also impart
better mechanical properties; it is thus receiving attention for
cells for automotive applications. [11]
Recently, much attention is being given to free-standing,
inorganic separators consisting mostly of SiO 2 , CeO 2 , Al 2 O 3 , or
ZrO 2 and being engineered for flexibility to withstand
winding. [6d,12] Shi et al. [6b] have succeeded to prepare a flexible,
free-standing, polymer-free, inorganic separator based on SiO 2
fibers; the polymeric content was removed via a thermal
treatment in air. He et al. [13] have also prepared a free-standing,
inorganic separator based on Al 2 O 3 nano-wires without using
any polymer additives. Indeed, the traditional insulating
inorganic materials can be exploited to develop better
separators for safer Li-ion batteries; however, significant
advances in processing methods are still required. Furthermore,
as demonstrated by Honma and co-workers, [14] the traditional,
insulating inorganic materials can even be exploited to develop
free-standing quasi-solid-state electrolytes for bi-polar lithium
batteries.
Although the free-standing inorganic separators are good,
for some applications (especially those requiring miniaturized
Li-ion batteries), it will be ideal to fabricate the separator as a
self-supported layer on one electrode; that can even allow to
fabricate much thinner separators, for instance, for wafer-level
fabrication of Li-ion micro-batteries. [15] In this work, electro-
phoretic deposition (EPD) was investigated – for the first time –
as a cost-effective method for preparing a self-supported
inorganic membrane, consisting of SiO 2 nano-fibers, as a
separator for Li-ion batteries. Physico-chemical characterization
of the obtained membrane has been done. A thorough
electrochemical characterization has also been done to fully
evaluate the suitability of the membrane as a separator for Li-
ion batteries.
Experimental Section
Electrode preparation
LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA), purchased from Toda, was mixed with
C65 carbon black and PVDF binder in the ratio of 92 : 4 : 4 wt % in
NMP solvent. A homogeneous slurry (agglomerate size below
25 μ m) was achieved by processing several times in the mixer
(SpeedMixer ™ DAC 150 SP) in combination with ultrasonic treat-
ment (ultrasonic sonotrode UP2000ST, Hielscher Ultrasonics
GmbH). The cathode film was then made by tape casting with TQC
(AB3220) automatic film applicator.
Production of SiO 2 nanofibers
Highly, cost-efficient, force spinning was used for the production of
SiO 2 nanofibers. Pardam has developed SiO 2 nanofibers from a new
precursor, which is thousand-fold cheaper than TEOS, normally
used for production of inorganic nanofibers. An industrial line
Cyclone TOWER was used for the fiber production. Ethanol that is
typically used has been replaced by water in the solution system,
so the whole process is much cheaper and characterized by higher
yield. After spinning, calcination was done in an industrial
calcination furnace and, finally, ball milling was used to shorten the
fiber length to ca. 40 μ m. The milling was required to facilitate the
stability of the fibers in the EPD suspension.
Electrophoretic deposition
The suspension for the EPD consisted of milled SiO 2 nanofibers
(1 g/l), citric acid (0.2 g/l) and PVDF (0.4 g/l) using acetone as
solvent. Stirring and sonication were alternated to achieve a very
stable suspension. The acetone was dried with molecular sieves for
2 days before use. A constant potential EPD was carried out with
an electric field of 50 V/cm, with the cathode film as the working
electrode, and a stainless plate as the counter electrode.
Physico-chemical characterization
Scanning electron microscopy (SEM) was done using a Zeiss Leo
1530 scanning electron microscopy. For cross-section, the samples
were cut with scissors; for observing the GF/A separator, Pt was
sputtered (for 30s) onto the separator.
Electrochemical measurements
All electrochemical experiments were done with PAT cells in a
climatic chamber at a constant temperature of 25 ° C. Half-cells and
full-cells were assembled with GF/A glass fiber separator soaked
with LP30 electrolyte (1 M LiPF 6 in EC : DMC 1 : 1 vol. UBE). Cell
testing was done employing a Basytec CTS-XL multichannel battery
tester. The cut-off voltages for the half-cells and full cells were
3.0 � U(V vs Li/Li + ) � 4.2 and 3.0 � U(V) � 4.1, respectively, with a
CC-CV program consisting of charge at 0.1 C and discharge at
variable currents.
2. Results and Discussion
2.1. Morphology and Composition
Electrophoretic deposition is a simple, versatile, scalable and
relatively inexpensive route for preparing thick films; and it
requires only an electric field and a suspension of charged
particles. [16] EPD is receiving significant attention, recently, for
the processing of battery [17] and supercapacitor [18] materials.
The main challenge of the EPD process is the difficulty in
choosing the right charging agent for a stable suspension. [16b,19]
Citric acid was chosen as the charging agent for this work; it
has been previously reported as a good charging agent with
acetone solvent for EPD. [20] Indeed, the combination of the citric
acid charging agent and the ball-milled SiO 2 nano-fibers
(diameter: 100–300 nm, length: 2–40 μ m) gave a stable suspen-
sion in acetone. Zeta potential ( ζ ) of  10 mV was measured
with Zetasizer nanoseries from Malvern Instruments; the charge
on the fibers is probably higher than it was measured,
considering that the technique is mostly suitable for spherical
particles. Moreover, a BET surface area of 97 m 2 /g was
measured for the starting SiO 2 powder, confirming the high
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surface area of the nanostructured material. Citric acid, as a
weak acid, gets ionized upon dissolution in acetone according
to the reaction given in Eq. (1); and the anion is adsorbed
around the SiO 2 nano-fibers. This is consistent with the
negative zeta potential measured.
C 6 H 8 O 7 ! C 6 H 5 O 3 
7 þ 3H þ (1)
The constant potential EPD gave very uniform thick films of
SiO 2 nano-fibers on LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathode (ca. 70 μ m
thick) prepared beforehand by tape casting. Representative
cross-sectional and top-view SEM images of the deposited SiO 2
are presented in Figure 1, together with the surface view of a
commercially available GF/A glass fiber separator at different
magnifications. It is worth emphasizing that the solvent and
charging-agent system used was very effective as very homo-
geneous films were obtained with the constant electric field
EPD, without the need for pulse-field EPD. The evolution of
deposited SiO 2 thickness with deposition time, as well as
deposited mass with thickness is showcased in Figure 2; it can
be noticed from Figure 2a that, as expected, a direct linear
relationship exists for the deposition time and thickness until
t = 15 min – this is in agreement with the deviation from
Hamaker’s law, as a consequence of the reduction of the
electric field strength during the deposition as a thick
insulating SiO 2 layer is grown on the electrode’s surface. The
elemental analysis (energy dispersive spectroscopy) results
obtained for the starting powder and the EPD materials are
shown in Table 1. From the EDS results, the fluorine proportion
was estimated to be approx. 21 % of the Si content, which is in
good agreement with the initial PVDF proportion (20 wt % with
respect to the SiO 2 powder) added to the suspension. More-
over, a very small contamination (1 at %) of sodium was
identified in the starting powder and in the EPD layer; this
contamination comes from the precursor of the water/glass
colloid suspension used to produce the SiO 2 nano-fibers.
No other metallic impurities were identified, which is an
indication of the high-quality of the ball milling tools used to
Figure 1. SEM images of (a–b) EPD SiO 2 membrane deposited onto LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) electrode (in top-view and cross-section), and (c–d) GF/A glass
fiber separator at different magnifications.
Table 1. EDS elemental analysis of starting SiO 2 powder and EPD
deposited SiO 2 .
Element Powder SiO 2 EPD SiO 2
at % at %
Si 36.36 27.25
Na 1.0 0.6
O 57.25 50.5
C 5.39 15.68
F 0 5.97
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shorten the lengths of the fibers. While the small carbon
proportion (5.39 at %) in the powder sample is actually
emanating from the carbon tape used for the SEM, the higher
amount (15.68 at %) of carbon in the EPD SiO 2 can be ascribed
to the binder in the deposited layer, considering that the EPD
SiO 2 layer is deposited on the cathode material. Actually, the
EDS of the EPD SiO 2 was done on the surface of a sample with
a thick EPD SiO 2 layer, to avoid any signal from the underlying
cathode composite.
Furthermore, it was imperative to determine the porosity of
the SiO 2 membrane made by EPD in order to properly bench-
mark its electrochemical performance. The porosity ( ɛ ) of the
membrane was estimated to be 71.42 % using the relation
given in Eq. 2; where 1 b corresponds to the bulk density of the
membrane – which is defined by its apparent mass and volume
( 1 b = 0.69 g/cm 3 ) –, and 1 p corresponds to the particle density
of the material. As SEM revealed that the binder content in the
suspension was kept in the deposit, the particle density of the
membrane was estimated to be 2.414 g/cm 3 considering a
composite comprising 20 wt % PVDF ( 1 = 1.78 g/cm 3 ) and
80 wt % SiO 2 ( 1 = 2.65 g/cm 3 ), and using the relation given in
Eq. (3); where 1 c is the density of the composite EPD-SiO 2
separator, and w i and 1 i correspond to the weight fraction and
density of the components, respectively. Indeed, the relation
given in Eq. (2) is well reported to give reasonable values of
porosity. [6e] Moreover, a porosity of 92.2 % was estimated for
the GF/A glass fiber separator (please see Figure 1c–d) which is
very consistent with the experimental porosity (91 %) provided
by the supplier. Thus, the GF/A glass fiber separator is more
porous than the EPD-SiO 2 separator, which is significantly
obvious from the morphology shown in the SEM images of
Figure 1.
e ¼ 1  1 b
1 p
� � � 100 % (2)
1 c ¼ 1
w 1
1 1 þ w 2
1 2
! � 100 % (3)
2.2. Electrochemical Performance
The performance of the EPD SiO 2 , self-supported on
LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) was evaluated as a separator for
lithium-ion batteries in comparison to a commercially available
GF/A glass fiber separator. The voltage profiles of the
galvanostatic cycling performance of the NCA half-cell with
only GF/A glass fiber separator is presented in Figure 3a; the
first discharge capacity was 168 mAh/g at 0.1 C, and it did not
fade for up to 50 charge/discharge cycles. It can be noted that
the charge is always followed by a short potential plateau due
to the constant current-constant potential (CC-CV) program
used to cycle the cells.
The rate capability of NCA half-cell with only GF/A glass
fiber separator is presented in Figure 3b together with those of
NCA electrodes coated with EPD-SiO 2 layers of different
thicknesses. Due to the use of lithium counter electrode, the
NCA electrodes coated with EPD-SiO 2 were additionally covered
with two GF/A glass fiber separators (each of 260 μ m) as was
done for the non-coated electrodes. Actually, without the GF/A
glass fiber separators, the half cells could not be properly
charged due to the formation of dendrites. It can be noted
from Figure 3b that the rate capability is nearly the same from
0.1 C to 3 C, but from 3 C to 10 C, the rate capability decreases
with increasing SiO 2 layer thickness; this is consistent with
pronounced diffusion limitation as a consequence of increased
ionic transport length for the charge/discharge reactions.
Furthermore, the effect of the EPD process on the perform-
ance of the NCA cathode in full-cells with graphite anode was
evaluated by measuring the discharge capacities of electrodes
with varying EPD SiO 2 separator thickness on the cathode. As
Figure 2. Evolution of deposition time with deposited SiO 2 thickness (a), and deposited mass with thickness (b).
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shown in Figure 4a, the discharge capacity after formation (at
0.1 C) is higher with the GF/A glass fiber separator (no EPD
process), and it decreases quite linearly with increasing EPD-
SiO 2 separator thickness or increasing deposition time. Though
Figure 3. Charge/discharge voltage profile of NCA half-cell with a 520 μ m GF/A glass fiber separator (a); effect of the presence of EPD-SiO 2 on the rate
capability of NCA half-cells with GF/A glass fiber separator (b).
Figure 4. Effect of the EPD process on the attainable capacity of NCA full cells at 0.1 C (a), and rate capability of NCA/ graphite full cells (NCA FIZM : made in-
house; NCA cc : purchased from custom cells) with 40 μ m EPD-SiO 2 or 260 μ m GF/A glass fiber separator (b). Voltage profiles of the NCA FIZM /graphite (c) and
NCA cc /graphite (d) full-cells at different discharge rates.
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40, 60, and 140 μ m were the main thicknesses studied, 20, 47
and 80 μ m films were made to verify the observed trend. The
employed acetone was dried, but it might have still absorbed
some moisture during the deposition or it can just be direct
humidity interaction with the cathode material that caused
some lithium leach-out. It is also suspected that the applied
deposition voltage (50 V) might induce some structural
changes in the material or exacerbate the lithium leach-out in
the humid environment. As the 40 μ m EPD-SiO 2 membrane
gave the best rate capability in half-cells than the 60 and
140 μ m membranes, that thickness was chosen for the
subsequent electrochemical studies.
What is more, the effect of EPD-SiO 2 on the rate capability
of NCA-graphite full-cells without any GF/A glass fiber separator
is presented in Figure 4b. It can be noticed that irrespective of
the NCA cathode material type (one made in-house, as
described, and another commercial) the thinner EPD-SiO 2
separator (40 μ m) imparts pronounced poor rate capability
(especially from 3 C) to the full-cell in comparison to the GF/A
glass fiber separator. This behavior is in agreement with the
trend observed in the half-cells of Figure 3b, and it can be
related to the different tortuosities of the EPD SiO 2 and the GF/
A glass fiber separators. Also, the voltage profiles of the full-
cells at varied discharge rates are presented in Figure 4c–d. It
can be noticed that the commercial cathode (NCA cc ) shows
much polarization and poorer rate capability as it is thicker,
and significantly calendared.
The tortuosities ( τ ) of the EPD SiO 2 (40 μ m) and the GF/A
glass fiber (260 μ m) were estimated to be 1.70 and 1.38,
respectively, using the relation given in Eq. (4); [8] where ɛ is the
porosity, R e is the electrolyte resistance determined from the
ionic conductivity of the electrolyte (11.19 × 10  3 S cm  1 ) and
the cell constant of each separator, and R s is the resistance of
each separator material placed between 2 stainless steel plates
and soaked with liquid electrolyte in the PAT cell. The R s
resistances were determined with the help of electrochemical
impedance spectroscopy. For this purpose, EPD of the SiO 2 was
done on a stainless steel plate, whilst the glass fiber separator
was just sandwiched between two stainless steel foils. The
tortuosities of the two materials are consistent with their
morphologies; according to literature, τ > 1 is indicative of a
hindered system which consists of non-cylindrical and non-
parallel pores. [6a] The poor rate capability of the EPD SiO 2 -based
full-cells, in comparison to the GF/A glass fiber separator, can
indeed be corroborated by its higher tortuosity and lower
porosity.
t ¼ ffiffiffiffiffiffiffiffiffiffiffiffi ffi
R s
R e � e
r (4)
Furthermore, to characterize the effectiveness of the EPD-
SiO 2 as a separator material for LIBs, the self-discharge of the
Graphite/NCA full-cell with the EPD-SiO 2 separator was esti-
mated and compared to that of a GF/A separator. To achieve
that, after the formation at 0.1 C, one galvanostatic charge/
discharge cycle followed by one charge at 0.1 C were done,
and the state-of-charge (at open circuit potential) was sub-
sequently monitored for 336 hrs (14 days) followed by two
galvanostatic discharge/charge cycles, as showcased in Fig-
ure 5a. Consequently, as shown in Figure 5b, the self-discharge
(SD) of the graphite/NCA cells with EPD-SiO 2 or GF/A separator
were estimated to be 6.0 % and 7.4 %, respectively, using the
relation given in Eq. (5). [21] As shown in Figure 5a, the open-
circuit-potential drop during the storage was more with the
GF/A separator cell than with the EPD-SiO 2 separator cell. It can
thus be asserted that the GF/A separator has some impurities
that cause the potential drop during the storage, and
consequently, the slightly higher self-discharge or it is the
presence of the 20 wt % PVDF binder that improves the self-
discharge characteristics of the much thinner EPD-SiO 2 separa-
tor.
SD ¼ ð D 1  D 2 Þ
D 1 � 100 % (5)
Figure 5. Comparison of (a) voltage profiles of galvanostatic cycling with 14 days state-of-charge evolution, and (b) estimated self-discharge, for Graphite/NCA
full-cell with GF/A or EPD-SiO 2 separators.
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56
57
Where D 1 corresponds to the last discharge capacity before
storage, and D 2 corresponds to the discharge capacity directly
after storage.
3. Conclusions
The suitability of electrophoretic deposition (EPD) to prepare
self-supported SiO 2 as a separator membrane for Li-ion
batteries has been demonstrated. The EPD separator made
with SiO 2 nanofibers is less porous and more tortuous than
commercial GF/A glassfiber separator; this led to poor rate
capability in full cells with the former even though it was much
thinner. However, the thinner EPD-SiO 2 separator exhibited
lower self-discharge than the thicker GF/A glassfiber separator
– which is believed to be more related to the presence of
20 wt % PVDF in the EPD-SiO 2 separator. The proposed method
cannot only be extremely useful for preparing self-supported
separator membranes for miniaturized LIBs with liquid electro-
lytes, but also for all-solid-state batteries where the self-
standing SiO 2 membrane can be infiltrated with a solution of a
soluble sulphide solid-state electrolyte and dried – as demon-
strated by Kim et al. [22] Future works will be aimed at
determining the experimental conditions that are causing the
NCA cathode electrodes subjected to the EPD process to
deliver lower capacities than those not subjected to EPD but
directly tested with the GF/A separator. Additional works will
also investigate EPD of the SiO 2 separator onto anode materials
such as graphite and Li 4 Ti 5 O 12 .
Acknowledgements
We are grateful to Karla Kern and Denis Bernsmeier of TU Berlin
for the Zeta potential and BET measurements, respectively. This
work was supported by the BMBF hearing-aid battery project
(Project #: 10043013). Open access funding enabled and
organized by Projekt DEAL.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: electrophoretic deposition · silica nanofibers ·
separator · batteries · energy storage
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Manuscript received: November 2, 2019
Revised manuscript received: November 24, 2019
Accepted manuscript online: December 9, 2019
Version of record online: February 3, 2020
Batteries & Super caps

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