Electroactive Polymer Fiber Separators for Stable and Reversible Overcharge Protection in Rechargeable Lithium Batteries

Abstract

An approach was developed to fabricate functionalized separators capable of providing long-term overcharge protection for secondary lithium batteries. Free-standing non-woven fiber membranes consisting of an electroactive polymer and a supporting polymer were prepared by an inexpensive and scalable electrospinning technique. The membranes sustained large shunt current densities despite the presence of an inert polymer component that dilutes the electroactive polymer. A bilayer fiber separator prepared by this method provided a reversible voltage-regulated current shunt for a Li1.05Mn1.95O4/Li cell for more than 1000 135% overcharge cycles at a 2/3 C rate, which is the most stable overcharge protection yet reported. This approach enables better distribution of the electroactive polymer which should reduce the cost of overcharge protection separators.

Full text

Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014) A1039
Electroactive Polymer Fiber Separators for Stable and Reversible
Overcharge Protection in Rechargeable Lithium Batteries
Bin Wang,aThomas J. Richardson, and Guoying Chen∗,z
Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, USA
An approach was developed to fabricate functionalized separators capable of providing long-term overcharge protection for secondary
lithium batteries. Free-standing non-woven fiber membranes consisting of an electroactive polymer and a supporting polymer were
prepared by an inexpensive and scalable electrospinning technique. The membranes sustained large shunt current densities despite
the presence of an inert polymer component that dilutes the electroactive polymer. A bilayer fiber separator prepared by this method
provided a reversible voltage-regulated current shunt for a Li1.05Mn1.95O4/Li cell for more than 1000 135% overcharge cycles at a
2/3 C rate, which is the most stable overcharge protection yet reported. This approach enables better distribution of the electroactive
polymer which should reduce the cost of overcharge protection separators.
© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in
any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.061406jes]
All rights reserved.
Manuscript submitted January 13, 2014; revised manuscript received March 24, 2014. Published May 2, 2014.
As the battery industry moves toward higher energy density cells
and larger packs for vehicular and aviation applications, there is a
great need to address the safety hazards associated with cell over-
charge/overdischarge. A variety of conditions may be responsible for
overcharging in secondary lithium batteries, including charging at
normal rates but beyond rated capacity, overvoltage excursions for
varying periods, charging at a rate too high for one electrode without
exceeding the maximum voltage, and other more complex scenarios.1
The effectiveness of overcharge protection by the commonly used
redox shuttle method2is limited by its rate capability, stability, and
operating voltage window. Most of the studies performed so far were
focused on the LiFePO4chemistry which operates at a relatively low
3.45 V vs. Li+/Li. The best result to date is 300 cycles at 100%
overcharging of a LiFePO4-Li4/3Ti5/3 O4cell at C/10 charging rate.3
We previously developed a novel approach using electroactive
polymers for overcharge protection,4which has subsequently been
investigated by other groups.5–7When incorporated into a microp-
orous separator membrane, the elecroactive polymer remains insulat-
ing during normal cell charge and discharge, but it creates a reversible,
resistive internal short upon overcharging.8The polymer limits the cell
potential and protects the cell components from damage without re-
stricting ion transport in the electrolyte or any significant leakage
current. A bilayer configuration, in which a polymer with a higher
oxidation potential is placed in contact with the cathode to set the pro-
tection voltage and a lower voltage polymer is placed next to the anode
to protect the high voltage polymer from degradation at the anode po-
tential, was later introduced to expand the operating voltage window
in high-energy cells.9The ability of the polymers in providing over-
charge protection for LiFePO4and LiNi0.8Co0.15 Al0.05O2cells was
clearly demonstrated on the comparison studies of protected and un-
protected half cells. Although overcharge protection at both high rates
and low temperatures were achieved using the bilayer configuration,
the protected cells suffered from poor stability, delivering only tens of
cycles.10 Significant improvement was achieved when the composite
membranes were prepared on highly porous glass-fiber substrates that
allowed more uniform distribution and higher utilization of the elec-
troactive polymers.11 Reversible overcharge protection for hundreds
of deep overcharge cycles was demonstrated in LiNi1/3Co1/3Mn1/3 O2,
LiFePO4,andspinelLi
1.05Mn1.95 O4cells. Some non-uniformity, how-
ever, remained due to the nature of solution casting, as the polymer
∗Electrochemical Society Active Member.
aPresent address: New Materials R&D Center, Institute of Chemical Materials, China
Academy of Engineering Physics, Chengdu, Sichuan 621900, People’s Republic of
China.
zE-mail: gchen@lbl.gov
distribution is often dominated by the process of solvent evaporation.
The additional step of impregnating the polymer into conventional
membrane substrates also increases the cost of the composites. We
therefore explored ways to directly prepare membranes incorporating
well-dispersed electroactive fibers.
Electrospinning is a simple, non-mechanical technique that has
gained much attention due to its ability to prepare large quantities of
fibers at a relatively low cost.12,13 A variety of polymer fibers with di-
ameters ranging from tens of nanometers to microns, including those
of the electroactive polymers, have been produced by this approach
and used in various industries.14 In recent years, the technique has also
been adapted to prepare polymer mats suitable for use as separators
in secondary batteries.15–18 By controlling the processing parameters,
interconnected fiber separators with open pore structures, large porosi-
ties ranging from 30 to 90%, and varying pore sizes from submicron to
a few microns were obtained. It has been shown that the high porosity
and open pore structure in these separators greatly improve the rate
capability and power density of rechargeable lithium batteries.19–22
Here we report the fabrication, characterization and performance of
novel electrospun non-woven battery separators capable of providing
overcharge protection for rechargeable lithium batteries. For the first
time, stable protection for well over 1000 deep overcharge cycles was
demonstrated over a period of more than one year.
Experimental
Regioregular poly(3-butylthiophene) (P3BT) with a polydispersity
index of 2.3 and a mean molecular weight (Mw) of 54,000; poly(9,9-
dioctylfluorene) end-capped with dimethylphenyl groups (PFO-DMP)
with a polydispersity index of 3.7 and Mw58,200; poly(methyl
methacrylate) (PMMA) with Mw350,000; and polyethylene oxide
(PEO) with Mw600,000 were obtained from Sigma-Aldrich. Chloro-
form solutions containing the desired ratios of an electroactive poly-
mer (PFO-DMP or P3BT) and a supporting polymer (PMMA or PEO)
were prepared with rigorous stirring at room temperature.
To prepare the fibers and mats, polymer solutions were loaded
into the syringe (Fig. 1) and electrospun at 20 kV onto aluminum
foil substrates. The feeding rate of the polymer solution was typ-
ically 0.75 mL/h and the needle-to-collector distance was 25 cm.
After the deposition, freestanding fiber mats could be peeled from
the substrates. Bilayer membranes were prepared by first elec-
trospinning PFO-DMP/PEO fibers onto the Al collector and then
spinning P3BT/PEO polymer fibers directly onto the surface of the
PFO-DMP/PEO mat. The thickness of the layers was adjusted by
controlling the concentration of the solution and the time of electro-
spinning. A small amount of PEO was used to adjust solution viscosity

A1040 Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014)
Figure 1. Schematics of the electrospinning apparatus.
and to enhance the morphology of polymer fibers. When desired, the
PEO component was removed by sonication in deionized water for 3
h, washing with water and ethyl alcohol, and then drying overnight
under vacuum.
Scanning electron microscopy (SEM) images and energy disper-
sive X-ray analysis (EDX) maps of the fiber membranes were collected
using a field emission microscope (JEOL JSM-7500F) operating at
25 kV accelerating voltage.
Visual monitoring of the charge propagation through the fibers
was performed on a neutral PFO-DMP/PEO mat 25 mm long, 15 mm
wide, and 5 μm thick. The mat was placed on a glass slide with Au
electrical contacts applied by vacuum sputtering. Strips of lithium foil
(Alfa-Aesar) were placed on both sides of the polymer film to mini-
mize the IR drop in the electrolyte. 1 M LiPF6in ethylene carbonate
(EC): diethyl carbonate (DEC) (1:1 v/v, Novolyte Technologies Inc.)
Figure 2. SEM images of the electrospun-fiber composites: a) PFO-
DMP/PMMA (1:5) and b) P3BT/PEO (3:1). Polymer weight ratios as indi-
cated.
was immobilized in a 150 um thick polyvinylidene difluoride mem-
brane (Gelman FP Vericel), which was transparent and colorless when
wetted. This membrane was placed on top of the polymer mat to make
sure that the film and lithium foils were fully in contact with the elec-
trolyte. Another glass slide was placed on top of the membrane, and
the cell was sealed using low vapor pressure epoxy (Torr-seal, Varian
Associates) before removing it from an argon-filled glove box (O2<1
ppm, H2O<1 ppm). The mat, with an estimated cross-section of 7.5
×10−4cm2, was charged galvanostatically from one end. A current
of 40 μA, equivalent to 50 mA cm−2, was passed for 560 min and
the potential was recorded at each end of the film. Optical images of
the film during the oxidation process were recorded at 1 min intervals
using an Axis Communications 2120 network camera.
The cathodes for half-cell studies were prepared by mixing the
active material, acetylene carbon black (Denka) and polyvinylidene
difluoride (PVdF, Kynar 2801) in an N-methyl-2-pyrrolidone
(NMP) solution. The weight ratios between the components were
80:10:10 for the Li1.05Mn1.95 O4electrodes (Toda) and 84:8:8 for the
Figure 3. SEM image a) and EDS maps of oxygen b) and sulfur c) of a
P3BT/PEO fiber composite prepared by electrospinning.

Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014) A1041
LiNi0.8Co0.15 Al0.05O2electrodes. The slurry was uniformly coated
onto an aluminum foil using a doctor blade, dried overnight in air,
and then dried under vacuum at 120◦C for 10 h. The mass loadings
for Li1.05Mn1.95 O4and LiNi0.8Co0.15 Al0.05 O2electrodes were 4.5 and
8.1 mg/cm2, respectively. Cathode disks with an area of 0.78 cm2
were cut from the electrode sheets and assembled into 2023-type coin
cells in the glove box. Lithium foil was used as counter electrode,
electrospun-fiber composites with an area of 1.98 cm2as separators,
and1MLiPF
6in 1:1 EC: DEC as electrolyte. The cells were galvano-
statically cycled using a Maccor multichannel battery testing system
(Model 4200). All experiments were carried out at room temperature.
Figure 4. a) Schematics of the in situ optical cell and images of a fiber film
(PFO-DMP/PEO in 3:1 weight ratio) during its galvanostatic oxidation in 1 M
LiPF6in 1:1 EC: PC. Images were taken under an optical microscope (100x
magnification) after passing current for the indicated amount of time; b) optical
image of the boundary between the oxidized and the neutral part of the film
after passing current for 120 min.
Figure 5. Digital images of electrospun-composite membranes on an Al sub-
strate: a) after the deposition of PFO-DMP/PEO layer and b) after the deposi-
tion of both PFO-DMP/PEO and P3BT/PEO layers.
Figure 6. Top-view SEM images of an electrospun PFO-DMP/P3BT com-
posite membrane after the removal of Al substrate and PEO component:
a) PFO-DMP side and b) P3BT side.

A1042 Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014)
a)
3950 3960 3970 3980 3990
2.8
3.2
3.6
4.0
4.4
800th
Voltage (V)
Time (h)
4950 4960 4970 4980 4990
1000th
Time (h)
1950 1960 1970 1980 1990
2.8
3.2
3.6
4.0
4.4
400th
Voltage (V)
2950 2960 2970 2980 2990
600th
950960970980990
200th
010203040
2.8
3.2
3.6
4.0
4.4
10th
Voltage (V)
b)
0 200 400 600 800 1000
0
50
100
150
Overcharge
Discharge
Cycle number
Specific capacities (mAh/g)
Figure 7. a) Charge-discharge cycling profiles and b) specific capacities as a function of the cycle number of a Li1.05Mn1.95O4half-cell overcharge protected by
the electrospun PFO-DMP/P3BT composite separator.

Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014) A1043
Results and Discussion
Synthesis and properties of electroactive-fiber-membranes.—
Electrospinning is a relatively mature technology that is commonly
used to produce a variety of polymer fibers for commercial applica-
tions. Figure 1shows a schematic diagram of the basic setup for the
technique. A polymer solution is pumped from a syringe and a high
voltage is applied between the needle tip and a conductive collec-
tor. This leads to a conical deformation of the solution as it exits the
needle, which is referred to as the “Taylor cone”.23,24 When the volt-
age reaches a critical value, electrostatic forces overcome the surface
tension of the polymer solution and a liquid jet is ejected from the
nozzle. This electrically charged jet is further elongated by electro-
static forces to form long and thin threads. The rapid evaporation of
the solvent reduces the jet diameter from hundreds of microns to tens
of nanometers. The fibers accumulate on the surface of the collector to
form a nonwoven, ultrafine porous mat. The aspect ratio of the fibers
is readily tuned by the adjustment of various conditions, including the
applied voltage, solvents, solution concentration, viscosity and feed-
ing speed. The duration of the electrospinning process controls the
thickness of the fiber mats.
Figure 2shows the SEM images of the electrospun fiber com-
posites consisting of PFO-DMP/PMMA, obtained by electrospinning
aCHCl
3solution containing the polymers in a weight ratio of 1:5
(Fig. 2a), and a P3BT/PEO composite obtained from spinning a CHCl3
solution containing 3 wt% of P3BT and 1 wt% of PEO (Fig. 2b). In
both cases, the addition of the supporting polymer was critical to
controlling the viscosity of the solutions that allows the formation
of polymer fibers instead of droplets. The average diameter of the
fibers was 1 μm, with 100 nm pores evident on the fiber surfaces
resulting from solvent evaporation. The porous nature of the fibers
may be beneficial for electrolyte absorption and wettability. In Fig.
3, the oxygen and sulfur-EDX maps of the P3BT/PEO fiber com-
posite show the distribution of PEO and P3BT, respectively. The two
polymers were intimately mixed at the individual fiber level. The
preparation of composites with varying degrees of fiber-level mixing
between the electroactive and supporting polymers is possible through
the adjustment of fabrication conditions.
The presence of the second polymer phase reduces the concen-
tration of the electroactive polymer in the composite fibers but does
not compromise its electronic continuity. To examine charge prop-
agation in the composites, an in situ cell was fabricated that al-
lowed visual monitoring of the fiber mat during the oxidation process.
Figure 4a shows the cell configuration and the images taken during
the passage of a steady state current of 40 μA (50 mA/cm2) through a
PFO-DMP and PEO fiber-film in a weight ratio of 3:1. The oxidation
front progressed at an average speed of 0.75 μm/s. The color of the
fibers changed from the light yellow of the neutral state to black upon
oxidation, with a distinct and nearly straight boundary between the
oxidized and the neutral regions (Fig. 4b). This shows that there is
sufficient contact to allow charge propagation between the fibers as
well as within individual fibers.
Electroactive-fiber-separators for overcharge protection.— Bi-
layer PFO-DMP/PEO and P3BT/PEO membranes prepared by elec-
trospinning were used for overcharge protection. After the initial
deposition of PFO-DMP/PEO fibers on an aluminum collector, the
P3BT/PEO polymer fibers were electrospun onto the surface of the
PFO-DMP/PEO film. The approximate thicknesses of the layers were
40 and 10 um, respectively. The weight ratios of both PFO/PEO and
P3BT/PEO were 3:1 and the total mass loading of the polymers was
4.6 mg/cm2. Optical images of the membranes, collected after de-
position of the PFO-DMP/PEO layer (Fig. 5a) and the subsequent
deposition of the red P3BT/PEO layer (Fig. 5b), show that smooth
mats with uniform coverage were obtained in both cases. Since PEO
is unstable above 4 V,25 the PEO component was removed before cell
assembly. SEM images of the membrane observed from the PFO-
DMP side (Fig. 6a) and P3BT side (Fig. 6b) indicate that removal of
the PEO had minimal impact on the fiber morphology, as it is clear
that both types of electroactive fibers were intact and well-connected
in a highly porous structured membrane.
Cells were assembled with the high-voltage PFO-DMP side facing
the cathode and the low-voltage P3BT side facing the anode. The cy-
cling performance of a spinel Li1.05Mn1.95 O4/Li cell protected by such
a separator is shown in Fig. 7a. When cycled at 2/3 C rate with 135%
overcharge, the cell repeatedly reached and maintained a steady state
potential of 4.2 V. The discharge capacity was retained under severe
overcharge conditions for well over 1000 cycles over 5000 h or 7
months of cycling with some rest periods (Fig. 7b). The separator re-
covered from the overcharged cell was black in color, providing strong
evidence that it created an internal shunt for the overcharge current.
This is the most stable overcharge protection reported in recharge-
able lithium batteries so far, and the results demonstrate the ability
a)
b)
380 400 420 440 460 480
2.8
3.2
3.6
4.0
4.4
Voltage (V)
30th
1460 1480 1500 1520 1540
2.8
3.2
3.6
4.0
4.4
Voltage (V)
Time (h)
145th
0 20 40 60 80 100 120 140 160
0
50
100
150
200
Overcharge
Discharge
Specific capacities (mAh/g)
Cycle number
Figure 8. a) Charge-discharge cycling profiles and b) specific capacities as a
function of the cycle number of a LiNi0.8Co0.15 Al0.05O2half-cell overcharge
protected by the electrospun PFO-DMP/P3BT composite separator.

A1044 Journal of The Electrochemical Society,161 (6) A1039-A1044 (2014)
of electroactive fiber separators in providing prolonged overcharge
protection.
The performance of a protected LiNi0.8Co0.15 Al0.05O2/Li cell is
showninFig.8. At C/4 rate and 50% overcharge, a constant volt-
age was reached at 4.2 V and the cell delivered steady performance
for more than 160 cycles (Fig. 8a). The upper limiting voltage is
significantly lower compared to that of the previous cells protected
by a similar PFO-DMP/P3BT composite prepared with either a mi-
croporous or glass-fiber membrane.11 This is likely due to improved
polymer distribution and reduced internal resistance in the electro-
spun fiber separator. The discharge capacity of this cell gradually
decayed during cycling (Fig. 8b), a phenomenon previously observed
in LiNi0.8Co0.15 Al0.05O2cells charged to 4.2 V. As a result, the ex-
tent of overcharge protection provided by the electroactive separator
gradually increased, reaching nearly 100% at 160 cycles.
Conclusions
Novel electroactive-fiber-composite membranes with a uniform
distribution of electroactive polymer were prepared by a simple elec-
trospinning process. When used as battery separators, the membranes
provided stable overcharge protection with a significant improvement
in cycle life over that achieved with redox shuttles. Electrospinning is
a cost-effective and scalable way to produce lithium-ion battery sepa-
rators with reliable voltage-regulated shunting. The rate capability of
the fiber composites, as well as the performance of fiber-composite
separators including a supporting polymer that is stable in the cell
operating window, such as PVdF and PMMA, will be the subjects of
future reports.
Acknowledgments
We thank Drs. Yuegang Zhang and Liwen Ji for the assistance in
electrospinning, and Dr. Wei Zhang for the assistance in electrode
fabrication. This work was supported by the Assistant Secretary for
Energy Efficiency and Renewable Energy, Office of FreedomCAR
and Vehicle Technologies of the U. S. Department of Energy under
Contract No. DE-AC02-05CH11231.
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