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A Structural Battery and its Multifunctional Performance
Leif E. Asp,* Karl Bouton, David Carlstedt, Shanghong Duan, Ross Harnden,
Wilhelm Johannisson, Marcus Johansen, Mats K. G. Johansson, Göran Lindbergh,
Fang Liu, Kevin Peuvot, Lynn M. Schneider, Johanna Xu, and Dan Zenkert
1. Introduction
Lightweight electrical energy-storage systems are required to
meet the ever-growing needs of electrification across transport
modes and consumer goods.
[1,2]
Current battery systems add
weight with no contribution to the system’s structural
performance. For instance, the battery
of the Tesla model S (85 kWh) weighs
25% of the total vehicle weight.
[3]
For
electric vehicles to be more efficient, and
for all-electric aircraft to evolve, total energy
storage must be increased while maintain-
ing or reducing weight.
[4]
This article
addresses an alternative approach to realize
efficient electrically powered systems.
Here, the electrical energy storage is
integrated in the structural material of
the vehicle—via multifunctional materials
coined as “structural battery composites
or structural power composites.”
[5–8]
Electrical energy storage in structural load
paths has been shown to offer large mass
savings for cars, aircraft, consumer elec-
tronics, etc.
[9–15]
Due to their multifunc-
tionality, structural battery composites are often referred to as
“mass-less energy storage”and have the potential to revolution-
ize the future design of electric vehicles and devices.
The first attempt to make a laminated structural battery
composite was by the US Army Research Laboratory (ARL) in
2007.
[16]
They used a carbon fiber (CF) lamina as a negative elec-
trode and a metal mesh coated with a cathode material as positive
electrode, separated by a glass fiber (GF) fabric. The structural
battery composite showed promising mechanical performance
but could not store electrochemical energy due to poor electrical
insulation. Liu et al. suggested a structural battery composite
using short CF-reinforced electrodes combined with a solid-state
polymer electrolyte matrix.
[17]
The CFs were electrochemically
inactive and only used as reinforcement. However, they were
not able to manufacture the short fiber electrodes as intended,
nor were they able to identify a solid-state electrolyte with suffi-
ciently high ionic conductivity. Instead they used a gel electrolyte,
resulting in a battery with a low tensile modulus, of 3 GPa. The
battery demonstrated an energy density of 35 Wh kg
1
. Inspired
by these works, the authors of the current study engaged in the
development of structural battery composites. In a first attempt,
Ekstedt et al. made a functioning laminated structural battery
using a gel electrolyte reinforced with a CF weave negative elec-
trode, a glass weave separator, and a lithium–iron–phosphate
(LFP)/aluminum fiber weave positive electrode.
[18]
No experi-
mental data on the electrochemical capacity or mechanical prop-
erties were reported. However, the mechanical properties are
expected to have been poor, given the use of a very soft matrix
material. In fact, electrolytes with a low Young’s modulus (few
MPa or less) cannot be used to realize structural batteries as they
do not allow efficient mechanical load transfer between fibers.
Prof. L. E. Asp, D. Carlstedt, S. Duan, M. Johansen, Dr. F. Liu, Dr. J. Xu
Department of Industrial and Materials Science
Chalmers University of Technology
Gothenburg SE-412 96, Sweden
E-mail: leif.asp@chalmers.se
K. Bouton, R. Harnden, Dr. W. Johannisson, Prof. D. Zenkert
Department of Engineering Mechanics
KTH Royal Institute of Technology
Stockholm SE-100 44, Sweden
Prof. M. K. G. Johansson, L. M. Schneider
Department of Fibre and Polymer Technology
KTH Royal Institute of Technology
Stockholm SE-100 44, Sweden
Prof. G. Lindbergh, K. Peuvot
Department of Chemical Engineering
KTH Royal Institute of Technology
Stockholm SE-100 44, Sweden
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aesr.202000093.
© 2021 The Authors. Advanced Energy and Sustainability Research pub-
lished by Wiley-VCH GmbH. This is an open access article under the terms
of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited.
DOI: 10.1002/aesr.202000093
Engineering materials that can store electrical energy in structural load paths can
revolutionize lightweight design across transport modes. Stiff and strong bat-
teries that use solid-state electrolytes and resilient electrodes and separators are
generally lacking. Herein, a structural battery composite with unprecedented
multifunctional performance is demonstrated, featuring an energy density of
24 Wh kg
1
and an elastic modulus of 25 GPa and tensile strength exceeding
300 MPa. The structural battery is made from multifunctional constituents,
where reinforcing carbon fibers (CFs) act as electrode and current collector.
A structural electrolyte is used for load transfer and ion transport and a glass fiber
fabric separates the CF electrode from an aluminum foil-supported lithium–iron–
phosphate positive electrode. Equipped with these materials, lighter electrical
cars, aircraft, and consumer goods can be pursued.
RESEARCH ARTICLE
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Load transfer between reinforcing fibers is a key feature of the
matrix material in any structural composite.
To move away from soft electrolytes, Carlson
[19]
made a lami-
nated structural battery component using a bicontinuous multi-
functional polymer/ionic liquid electrolyte matrix, developed for
structural supercapacitors by Shirshova et al.
[20]
The structural
battery was made from an IMS65 CF-woven fabric negative elec-
trode and an LFP-coated metal foil positive electrode, separated
by a glass fabric. The stack was impregnated with the multifunc-
tional polymer/ionic liquid electrolyte system, which phase sep-
arated during cure. The structural battery was used to light an
LED, but no multifunctional material data were reported.
[19]
A similar approach was taken by Yu et al. to make structural
battery negative half cells.
[21]
The laminated structural battery
half cells were made from T700 CF electrodes in a bicontinuous
epoxy/ionic liquid structural electrolyte. The half cells were
made into coin cells and electrochemically cycled versus lithium
(Li) metal. A first discharge capacity in the range of
12–25 mAh g
1
was reported and the samples experienced
significant capacity losses after continued cycling. Stiffness data
presented for the structural battery half cells were along the fiber
direction and hence dominated by the CFs.
Recently, a bicontinuous polymer electrolyte system, referred
to as a structural battery electrolyte (SBE), was developed for
structural battery composites by Ihrner et al.
[22]
and later opti-
mized for manufacturing by Schneider et al.
[23]
The SBE consists
of a porous methacrylate polymer (for mechanical load transfer)
impregnated with a liquid electrolyte mixture containing Li salt
for ionic conductivity. The SBE has a Young’s modulus
around 0.5 GPa and an ionic conductivity of 2 10
4
Scm
1
.
Johannisson et al. demonstrated highly promising electrochemi-
cal and mechanical performance using this SBE in CF electrode
half cells.
[7]
Moreover, the SBE matrix was found to sustain the
volume change of CF as the mechanical properties were unaf-
fected by electrochemical cycling. A similar SBE is used in
the current study.
In a recent study, Moyer et al. reported on yet another type of
CF-reinforced structural Li-ion battery composite.
[24]
The battery
did not exploit the electrochemical capability of CFs. Instead, the
active material in the negative electrode was graphite and LFP
was used as active material in the positive electrode. The CF
composite-supported electrode laminae were separated by a
Whatman GF separator soaked in a liquid electrolyte.
Consequently, no mechanical loads can be transferred in the
electrochemically active region of the CF-reinforced Li-ion bat-
tery composite. Moyer et al. reported an energy density surpass-
ing 35 Wh kg
1
. They further demonstrate a low Young’s
modulus of around 2 GPa (they report 2 MPa but the data suggest
that this is a misprint).
[24]
The work on structural battery composites to date either
presents good electrochemical or mechanical performance. No
study has yet demonstrated good combined properties.
Furthermore, no structural battery with an elastic modulus
greater than that of a glassy polymer has been reported. Also,
mechanical data provided in previous work only present stiffness
and strength data in the fiber direction (which is the stiff and
strong direction) and not in the direction perpendicular to the
fibers. Finally, most previous studies do not fully exploit
multifunctional material constituents. For example, fiber
reinforcement adds stiffness and strength and acts as a current
collector, but is not electrochemically active, or a liquid electrolyte
that cannot transfer mechanical loads is used.
In this article, we propose a structural battery composite mate-
rial made from multifunctional material constituents and dem-
onstrate its multifunctional performance. The structural battery
composite consists of a CF negative electrode and an aluminum
film-supported positive electrode separated by a GF separator in a
SBE matrix material. Consequently, the CFs act as host for Li
(i.e., active electrode material), conduct electrons, and reinforce
the material. Similarly, the positive electrode foil provides com-
bined mechanical and electrical functionality. The SBE facilitates
Li-ion transport and transfers mechanical loads between fibers,
particles, and plies. Two types of GF fabric separators, a
Whatman GF/A and a GF plain weave, are used as model mate-
rials to investigate the effects of separator thickness and architec-
ture, as well as material anisotropy, on the multifunctional
performance. The structural battery composite full cells are
fabricated, as shown in Figure 1.
2. Results and Discussion
2.1. Structural Battery Microstructures
The structural battery composites were studied in a scanning elec-
tron microscope (SEM). Micrographs of the cells with the two dif-
ferent separators are shown in Figure 2. Due to the applied
pressure during thermal curing, the thickness of both separators
is significantly thinner than their original thickness. The average
thickness of the Whatman GF/A separator is 185 μm, whereas it
is 70 μm for the GF plain weave separator. Unlike the relatively
constant separator thickness, the CF electrode thickness varies
dramatically across the width of the structural battery cell. The
fiber volume fraction varies within the negative electrode but
is on average 20%. The variation in the CF electrode thickness
is inherent to the manual manufacturing process.
Moreover, it is evident from Figure 2 that the appearance of
the CF electrode differs between the two separator solutions.
This will affect the local properties of the electrode lamina
(e.g., conductivities) which will influence the electrochemical
performance of the cell.
2.2. Electrochemical Performance
The specific capacities and energy densities of the tested struc-
tural battery cells are shown in Table 1.
The nominal voltage during discharge was 2.8 V for both cell
types. The energy densities at 0.05 C (i.e., a discharge time of
20 h) of the battery cells utilizing the Whatman GF/A and
GF plain weave separator are 11.6 and 23.6 Wh kg
1
, respec-
tively. These numbers are based on the total mass of the battery
cell (i.e., accounting for the mass of the electrodes, separator,
SBE, and current collectors in the cell). The corresponding
energy densities, when only accounting for mass of the active
electrode materials (i.e., CFs and LFP particles), are 90.1 and
106 Wh kg
1
, respectively. In Table 1 the calculated maximum
energy densities for the two cells are reported. The maximum
energy density with respect to the active material is the same
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for the two cell types. The difference in measured energy density
with respect to active material mass for the two structural battery
cells is related to the dissimilar separator thickness and is
explained by the higher internal resistance of the structural bat-
tery with the thicker Whatman GF/A separator, resulting in a
lower utilization of the electrodes’maximum available capacities.
Furthermore, the specific powers for the two cells at 3 C are 5.94
and 9.56 W kg
1
, respectively. With respect to the active electrode
materials, the specific power is 34.7 W kg
1
. All tested battery
cells showed similar electrochemical performances.
Figure 1. Structural battery composite fabrication, showing the steps: battery component manufacture, pouch-cell manufacture, and curing of the SBE.
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The electrochemical performance of a structural battery com-
posite with a Whatman GF/A separator is shown in Figure 3.
Figure 3a shows the typical charge/discharge voltage profiles
of the structural battery cells during galvanostatic cycling at dif-
ferent C rates. The voltage profiles indicate a stable charge/dis-
charge process at the different C rates and a relatively balanced
cell. Moreover, in Figure 3b, the energy density for a structural
battery cell with the Whatman GF/A separator is presented for
different C rates. It is evident that the capacity fade is negligible
within the given cycling process and stable energy densities at
different C rates are demonstrated. Furthermore, good capacity
retention is observed over long-term cycling at 1 C (Figure 3c). It
should be noted that the battery cell has been cycled for 35 cycles
(i.e., preconditioning and cycle scheme shown in Figure 3b) prior
to the cycling procedure, as shown in Figure 3c. Hence, the cell
shows high capacity retention even after more than 60 charge/
discharge cycles. Finally, Figure 3d shows the energy density
versus the C rate for both separator alternatives. Notably, the
energy density of the structural battery with the GF plain weave
separator is significantly higher than that of the cell with the
thicker Whatman GF/A separator. In general, the electrochemi-
cal performance is limited by the mass of the active materials
relative to the total mass. For these structural batteries, the per-
formance is limited by the excessive amount of SBE. This can be
mitigated by an improved manufacturing process that allows for
more evenly distributed CFs and higher fiber volume fraction,
i.e., increased volume fraction of active material, in the negative
electrode.
2.3. Mechanical Performance
The mechanical properties of the structural batteries were char-
acterized under tensile loading in both x- and y-directions, as
shown in Figure 4. The GF plain weave separator was placed with
the fibers extending either in 45or 0/90directions, where
the 0direction is parallel to the x-direction. The average elastic
moduli and the tensile strengths of the laminated full cells are
shown in Table 2.
Representative load–displacement curves for all three speci-
men types and loading directions are shown in Figure 4. The
modulus is highest in the x-direction for all structural battery
composites, see Exin Table 2. Furthermore, a linear force–
displacement relationship in the x-direction is found for all
samples, see Figure 4d. The high modulus and linear response
are expected as the stiff CFs in the negative electrode are oriented
parallel to the loading direction. The highest modulus, Ex,is
found for the 0/90GF plain weave separator structural battery
composite. Again, this is explained by the beneficial orientation
of the reinforcing fibers in the separator, i.e., the GFs extending
Figure 2. SEM micrographs of the structural batteries’cross sections, as indicated by the inscribed box in the schematic. Structural battery cross sections
with a) Whatman GF/A separator and b) GF plain weave separator. Average thicknesses in the regions depicted in the micrographs: a) CF electrode,
65 μm; Whatman GF/A separator, 185 μm; and positive (LFP) electrode, 50 μm; b) CF electrode, 125 μm; GF separator, 70 μm; and positive (LFP) elec-
trode, 50 μm.
Table 1. Representative specific capacities and energy densities of the
tested structural battery cells at 0.05 C (i.e., a discharge time of 20 h),
as well as the calculated maximum energy densities, with a nominal
voltage during discharge of 2.8 V. Specific power at 3 C with a nominal
voltage during discharge of 2.48 V. “Battery cell”corresponds to the
case where the total mass of battery cell is accounted for (i.e.,
accounting for all constituents) whereas “Active mat.”represents
values where only the mass of the active electrode materials is
accounted for (i.e., CFs and LFP particles).
Separator Whatman GF/A GF plain weave
Battery cell Active mat. Battery cell Active mat.
Specific capacity [Ah kg
1
] 4.13 32.2 8.55 38.4
Energy density [Wh kg
1
] 11.6 90.1 23.6 106
Calculated maximum
energy density [Wh kg
1
]
37.7 220 60.6 220
Specific power [W kg
1
] 5.94 34.7 9.56 34.7
Total mass of cell [g cm
2
] 0.074 0.046
Cell thickness [mm] 0.40 0.27
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in the 0direction. Hence, the CFs in the negative electrode and
the GFs extending in the 0direction both reinforce the battery
composite to their maximum in this particular composite and for
this load direction.
In contrast, the modulus in the y-direction, Ey, is generally
lower. This is explained by the matrix-dominated response of
the composite to tensile loads in the y-direction. The moduli
in the y-direction for the structural batteries made from the
45-oriented GF plain weave and the Whatman GF/A separa-
tors are quite low compared with 0/90, which is a much more
favorable configuration. In Figure 4e, a conspicuous nonlinear
force–displacement response is observed. This is particularly
pronounced for the 45-oriented GF plain weave separator
structural battery as this composite sustains substantial shear
loads (i.e., scissoring of the weave).
As discussed in the Experimental Section, the tensile strength
results may be impaired by premature failure inherent to surface
defects or stress concentrations in the clamping region. In fact,
most of the specimens failed at the grips. Consequently, the
strength values reported in Table 2 should be considered as lower
bounds of strength for the tested structural battery composites.
The strengths of the specimen with the Whatman GF/A separa-
tor are typically half of those for the specimens with GF plain
weave separator, both in the x- and y-directions. The highest
recorded strength exceeds 312 MPa and is for the structural bat-
tery composite with the 45-oriented GF plain weave separator.
2.4. Multifunctional Performance
The structural battery composite demonstrates excellent multi-
functional performance. The thinner GF plain weave separator
results in a lighter structural battery cell, as the amount of sepa-
rator material and SBE is greatly reduced. As a consequence, the
energy density is more than doubled—it increases from 11.6 to
23.6 Wh kg
1
. Moreover, the improvement in electrochemical
performance comes in hand with enhanced mechanical perfor-
mance as the elastic modulus is increased by almost 40% (from
18.3 to 25.4 GPa) and 360% (from 2.9 to 13.3 GPa) in the x- and
y-directions, respectively. Thus, both the electrochemical and
mechanical performance can be dramatically improved replacing
the Whatman GF/A separator with the significantly thinner and
stiffer GF plain weave fabric.
Further improvements can likely be achieved. First, using thin-
ner separators will reduce the mass of the structural battery as the
amount of SBE is also reduced. This will immediately result in
increased energy density and elastic modulus. The increased elas-
tic modulus results from the increase in the relative thickness of
the stiffer CF lamina. The current structural battery composite
designs are generally less stiff than expected for a CF-reinforced
composite. In particular, elastic modulus in the x-direction
appears to be on the low side. There are at least three reasons
for this. First, the fiber volume fraction in the negative electrode
is lower than desired and limited by the manufacturing method
and can probably be improved significantly using, e.g., a vacuum-
infusion manufacturing method.
[25]
Second, due to the manual
process, the CFs in the negative electrode are not ideally straight,
which again may be remedied using fixtures to hold the spread
tow fiber bundles during manufacturing. Finally, the stiff carbon
fiber electrode constitutes only a part of the total laminate
thickness, which is shown in the micrograph of the structural bat-
tery composite with the Whatman GF/A separator shown in
Figure 2a. Thinner separators is the key approach to improve this.
Figure 3. Results from electrochemical characterization based on the total weight of the battery cell. a) Voltage profile at different C rates. b) Energy
density at different C rates. c) Long-term cycling (at 1 C). d) Energy density versus C rate for the two separator solutions. Note that the C rates are defined
with respect to the capacity of the tested battery cells.
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The elastic modulus of the structural batteries presented here
ranges over an order of magnitude at maintained electrochemical
performance. Within this selection of materials, the GF plain
weave separators oriented in 45and 0/90demonstrate
lower and upper bounds for the possible variation in modulus
without affecting the electrochemical performance.
Consequently, requirements on mechanical performance for
given applications can be met by clever selection of fiber
orientations and lay-up.
Until now only a few studies reporting the multifunctional
performance of structural batteries have been published.
Hopkins et al. presented an overview of the work to date.
[26]
Among the studies cited only a handful consider structural
battery composites, which they refer to as coupled structural
batteries. Furthermore, almost all of these demonstrate poor
multifunctional performance, where either the electrical or the
mechanical performance is prioritized. For example, Thakur
and Dong
[27]
reported an energy density of 24 Wh kg
1
at an elas-
tic modulus of only 0.29 GPa, whereas Meng et al.
[28]
demon-
strated a structural battery material with an elastic modulus of
Figure 4. The structural battery full cell laminates and their mechanical response. Schematics showing the orientation of the separator fabrics relative to
the loading directions: a) Whatman GF/A; b) GF plain weave with its warp and weft yarns oriented in 0/90; and c) GF plain weave with its warp and weft
yarns oriented in 45. Representative load–displacement curves from tensile tests. d) Loading in the x-direction and e) loading in y-direction.
Table 2. Mechanical properties from tensile tests of the structural battery
laminates. Exand Eyare the elastic moduli in the x- and y-directions,
respectively. Xand Yare the tensile strengths in the x- and y-
directions, respectively. Note that the reported strength values are
considered lower bounds due to limitations in the sample preparation
process, as discussed in Experimental Section.
Property Separator type
Whatman GF/A GF plain weave 45GF plain weave 0/90
Ex[GPa] 18.3ð0.9Þ14.6ð0.6Þ25.4ð3.3Þ
Ey[GPa] 2.9ð0.5Þ2.8ð0.2Þ13.3ð0.7Þ
X[MPa] >163 >312 >287
Y[MPa] >16 >34 >72
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7.0 GPa but a low energy-storage capability of 1.4 Wh kg
1
.
Moyer et al. reported a multifunctional device, exploiting CFs
as collectors.
[24]
Use of a liquid electrolyte generated a high
energy density (35 Wh kg
1
) but resulted in an inferior elastic
modulus (2 GPa). Liu et al. reported an energy density of
35 Wh kg
1
and an elastic modulus of 3 GPa.
[17]
The relatively
high energy density can be partly explained by the fact that they
used LiCoO
2
, which has a higher specific capacity than LFP, as
the active material in the positive electrode. It is also not clear,
however, if the energy density reported is relative to the mass of
the entire cell or the active materials, nor is the charging history
for the cell given.
The multifunctional properties of structural battery compo-
sites made to date are shown in Figure 5. It is evident that no
previous structural battery has been made that matches the mul-
tifunctional performance of the structural battery composite pre-
sented in the current study.
The structural battery composite is contained in a pouch
bag as described in the Experimental Section. To further
illustrate the electrochemical and mechanical functions, the
structural battery composite is extracted from the pouch bag
inside the glovebox and connected to an LED. A video
demonstrating the electrochemical function under handling
and gentle mechanical bending is presented in Supporting
information. Two still shots from the video are shown in
Figure 6.
3. Conclusion
Structural battery composite materials, exploiting multifunc-
tional constituents, have been realized and demonstrate an
energy density of 24 Wh kg
1
and an elastic modulus of
25 GPa. Their combined electrochemical and mechanical prop-
erties outperform all previous structural battery materials
reported in the literature. From the relationships found between
the constituents and multifunctional performance of the struc-
tural battery composite, we have confirmed the importance of
having thin and stiff separators and to increase the relative thick-
ness of, as well as the fiber volume fraction in, the negative CF
electrode. Armed with this understanding of the complementary,
and sometimes counteracting, electrochemical and mechanical
functions, future structural battery composites can be
designed—where electrochemical energy density and elastic
stiffness and mechanical strength can be tuned as desired.
4. Experimental Section
Materials: Ultrathin spread tow UD tapes of T800SC-12k-50C PAN-
based CF with a linear tow weight of 0.52 g m
1
were supplied by
Oxeon AB, Sweden. The spread tow CF tapes had a width of 15 mm
and were used as negative electrodes. The multifunctional properties of
the T800 fiber are reported by, e.g., Kjell et al.
[29]
and Fredi et al.
[30]
Moreover, a battery-grade single-side LFP (LiFePO
4
)-coated aluminum foil
(82 μm thick; rated capacity of 1 mAh cm
2
) positive electrode was
purchased from Custom Cells Itzehoe GmbH, Germany. Two types of
GF separators were used: 1) Whatman glass microfiber separator
(Whatman GF/A, 260 μm thick) supplied by Sigma Aldrich and 2) a 0/90
woven GF fabric (GF plain weave, style 1086), with a surface weight of
53 g m
2
(50 μm thick), manufactured by Gividi Fabrics s.r.l., Italy,
and supplied by Isola Group, Europe. The bicontinuous SBE included
the following constituents: for the polymer material part Bisphenol A
ethoxylate dimethacrylate (M
n
: 540 g mol
1
) was supplied by Sartomer
Europe; for the liquid electrolyte part, propylene carbonate (PC)
(PC ≥99%, acid <10 ppm, H
2
O<10 ppm) and ethylene carbonate (EC)
(99% anhydrous) supplied by Sigma Aldrich were used. Furthermore, for
the SBE lithium trifluoromethanesulfonate (LiTf ) (99.99%), lithium bis(ox-
alato)borate (LiBoB) and 2,2 0-azobis(2-methylpropionitrile) (AIBN) pur-
chased from Sigma Aldrich were used. All materials were used as received.
Structural Battery Full Cell Preparation: An illustrative overview of the
structural battery composite full cells manufacture is shown in
Figure 1. The negative electrode was made from a CF spread tow and
the positive electrode was a commercially available LFP electrode foil.
Both electrodes were cut in dimensions of 30 15 mm
2
. This corre-
sponded to 19 mg of CFs and 38 mg of LFP particles in the complete
Figure 5. Elastic modulus and cell level energy density of reported struc-
tural battery composites, numbered by their references.
Figure 6. Images from video (available as Supporting Information) showing the structural battery cell without the pouch bag lighting an LED inside the
glovebox while exposed to mechanical loading. a) Before the cell is connected to the circuit (LED light off ) and b) when connected to circuit (LED light on).
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battery cell. A copper foil current collector was adhered to the CFs using
silver conductive paint. An aluminum current collector was placed on the
aluminum side of the LFP foil. Two types of separators were used: 1) one
made from Whatman GF/A and 2) one made from a stack of two 0/90
woven GF fabrics (denoted GF plain weave). Both separators were cut
slightly larger than the electrodes (50 20 mm
2
) to ensure that the elec-
trodes did not come in direct contact (to avoid short circuit). The battery
cell stack was then placed inside a pouch laminate bag (PET/Al/PE, 12 μm/
9μm/75 μm thick) to protect the electrochemical cell from air and mois-
ture. The battery cell stack was then impregnated with an SBE mixture
using a pipet before the pouch bag was vacuum heat sealed. The SBE mix-
ture was prepared in accordance with the procedure described in the study
by Schneider et al.,
[23]
with the only differences being that DMMP in the
electrolyte was replaced with PC, the mixing ratio of monomer versus liq-
uid electrolyte was taken, and that a LiBoB salt was added to prevent
unwanted side reactions induced by the exclusive use of LiTf, as discussed
in Supporting information. The SBE solution was made by mixing 50:50 wt
% of 1) a liquid electrolyte solution made from the mixture of LiBoB and
LiTf at concentrations of 0.4 and 0.6 M, respectively, in EC:PC 1:1 w/w
(50:50 wt%) and 2) monomer bisphenol A ethoxylate dimethacrylate
and the thermal initiator AIBN (1 wt% of the monomer weight). To achieve
a homogeneous solution, the SBE mixture was stirred using a vortex
before it was added to the battery cell. Once the pouch bag was vacuum
heat sealed, it was transferred to a preheated oven outside the glovebox
and thermally cured at 90 C for 60 min. During thermal curing pressure
was applied on the cells using clamps.
In total, 16 structural battery composite cells were manufactured and
characterized for their multifunctional performance. Among these, 12
were made using a Whatman GF/A separator and 4 with the GF plain
weave separator.
Electrochemical Testing: The specific capacity of the structural battery full
cells was measured by means of repeated galvanostatic charge and dis-
charge cycles using a Neware CT-4008-5V10mA-164 battery cycler. The
cells were cycled between 2.00 and 3.55 V using a series of current densi-
ties or C rates. Prior to the electrochemical characterization, the battery
cells were cycled for ten complete charge/discharge cycles to precondition
the cells. During this stage, the applied current density was set to
1.76 mA g
1
with respect to the mass of the active electrode materials
(i.e., CFs and LFP particles) available in the battery cell. The electrochemi-
cal characterization was conducted after the preconditioning stage and the
cells were cycled at current densities applied as follows: five complete
charge/discharge cycles for each current step using 1.76, 3.51, 7.02,
and 14.0 mA g
1
relative to the active electrode materials in the battery
cell. This was found to be approximately equivalent to 0.05, 0.15, 0.5,
and 3 C, with respect to the capacity of the tested battery cells.
Between each charge and discharge cycle, a resting time of 60 min was
used to allow ion-concentration gradients to relax. In total, eight battery
cells were characterized electrochemically using the Whatman GF/A sep-
arator and two with the GF plain weave separator.
Mechanical Testing: Tensile tests were conducted to characterize the
elastic properties of the laminate in x- and y-directions, as shown in
Figure 4. Due to the dimensions of the full cell, measuring mechanical
properties of the laminate presents certain limitations. For these speci-
mens, the challenge lies in the strain measurement without the possibility
of using conventional techniques such as digital image correlation or
strain gauges. The microtester used for mechanical characterization, a
Deben 2 kN tensile stage, further prevented use of conventional test stand-
ards, such as ASTM D3039. Instead, test specimens were adapted to fit the
test equipment, with the dimensions 30 3.3 mm
2
(length width).
Slender test specimens were cut from the manufactured structural bat-
tery cells with great caution to allow for precise measurement of the elastic
modulus. Accurate measurement of composite strength, however,
requires comprehensive sample preparation, involving polishing free
edges and tabbing clamping regions to prevent premature failure and
hence underestimated material strength. Such sample preparations were
not possible here. Tensile tests were made on a minimum of five samples
per specimen type. Mechanical tests were conducted on pristine structural
battery cells, only, i.e., no electrochemical cycling was performed on the
samples prior to the mechanical test. Tensile tests were conducted under
displacement control at a rate of 0.1 mmmin
1
. Applied strain was calcu-
lated from the crosshead displacement of the microtester compensating
for the machine compliance according to ASTM D3379 and is further
described in Supporting Information.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors would like to thank the following sources for funding this
research: Swedish Energy Agency grant #37712-1; the Swedish
Research Council, projects 2017-03898 and 621-2014-4577; the strategic
innovation program SIP LIGHTer (funding provided by VNNOVA, the
Swedish Energy Agency and Formas); H2020 Clean Sky II project
SORCERER no. 738085 and work supported by the Air Force Office of
Scientific Research under award numbers FA9550-17-1-0338 and
FA9550-17-1-0244.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
biomimetics, carbon fiber composites, fibrous materials, lithium-ion
batteries, multifunctional materials, self-sustaining materials, solid states
Received: December 10, 2020
Revised: December 15, 2020
Published online: January 27, 2021
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