Bendable Inorganic Thin-Film Battery for Fully Flexible Electronic
Systems
Min Koo,† Kwi-Il Park,† Seung Hyun Lee,† Minwon Suh,† Duk Young Jeon,† Jang Wook Choi,‡
Kisuk Kang,§ and Keon Jae Lee*,†
†Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro,
Yuseong-gu, Daejeon 305-701, Republic of Korea
‡Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu,
Daejeon 305-701, Republic of Korea
§Department of Materials Science and Engineering, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of
Korea
*S Supporting Information
ABSTRACT: High-performance flexible power sources have gained attention, as they enable the realization of next-generation
bendable, implantable, and wearable electronic systems. Although the rechargeable lithium-ion battery (LIB) has been regarded
as a strong candidate for a high-performance flexible energy source, compliant electrodes for bendable LIBs are restricted to only
a few materials, and their performance has not been sufficient for them to be applied to flexible consumer electronics including
rollable displays. In this paper, we present a flexible thin-film LIB developed using the universal transfer approach, which enables
the realization of diverse flexible LIBs regardless of electrode chemistry. Moreover, it can form high-temperature (HT) annealed
electrodes on polymer substrates for high-performance LIBs. The bendable LIB is then integrated with a flexible light-emitting
diode (LED), which makes an all-in-one flexible electronic system. The outstanding battery performance is explored and well
supported by finite element analysis (FEA) simulation.
KEYWORDS: Bendable thin-film battery, all-solid-state, rechargeable LIB, flexible electronic system
The advent of a fully flexible electronic system will be agreat leap in technology, as it will open the door to the
next-generation electronic environment based on bendable,
implantable, and wearable devices. These next-generation
electronic devices are marked by unprecedented advantages
of excellent portability, lightweight, and conformal contact on
curvilinear surfaces.1,2 Although the remarkable development of
mechanically flexible electronic devices has been widely
reported, their feasibility has been restricted in unit
components, such as light-emitting diodes (LEDs),3,4 sensing
electrodes,5,6 circuit elements,7−9 and radio frequency identi-
fication (RFID) antennas.10 Toward all-in-one flexible systems,
the development of a bendable high-power source that can be
applied to consumer electronics has been an obstacle to
overcome.
Rechargeable lithium ion batteries (LIBs) have shown great
promise as flexible power sources due to their high operating
voltage, high energy capacity, and long-term cyclability.2,11 In
recent years, compliant materials on curvilinear surfaces, such as
carbon nanotubes,12−15 carbon nanofibers,16 graphene,17,18
metal oxide-based nanowires,19 and slurry-typed mixtures of
nanostructured active materials,20,21 have been explored as
flexible LIB electrodes. Although they have shown advanced
performance for flexible LIBs, the combination of these as
anode or cathode has only been accessible to a few electrode
materials that are synthesizable in certain nanostructures or
carbon templates.22 Moreover, the use of liquid-type electro-
lytes has added more complexities in the realization of a fully
flexible LIB, and their thermal stability should be carefully
considered.23,24 In addition, the lightweight thin-film shape of
Received: June 15, 2012
Revised: July 27, 2012
Published: July 30, 2012
Letter
pubs.acs.org/NanoLett
© 2012 American Chemical Society 4810 dx.doi.org/10.1021/nl302254v | Nano Lett. 2012, 12, 4810−4816
flexible LIBs required for nano/microelectromechanical
systems (NEMS/MEMSs) cannot be formed by slurry-type
composite materials.
In this paper, we developed a flexible LIB based on all-solid-
state materials with an energy density, 2.2 × 103 μWh/cm3 at a
rate 46.5 μA/cm2 (0.5 C) under polymer sheet wrapping, the
highest energy density ever achieved for flexible batteries. The
LIB properties as a function of the bending radius (Rc) show
suitability for a high-performance flexible energy source. The
stable energy density delivered on flexible substrates is well
supported by theoretical studies and finite element analysis
(FEA) simulation. Finally, a high-performance bendable thin-
film LIB is then integrated into a flexible LED display system
on a plastic substrate. As far as we know, this is the first
prototype of a fully functional all-flexible electronic system.
Figure 1a shows the robustness of a flexible LIB turning on a
blue LED in bent condition. The employment of high
electrochemical potential materials, as depicted in the inset,
leads to the maximum charging voltage of 4.2 V and the specific
capacity of 106 μAh/cm2 (discharge capacity of 683 μAh) at a
rate of 46.5 μA/cm2 under polydimethylsiloxane (PDMS)
polymer wrapping (2.54 × 2.54 × 0.2 cm3), which indicates
higher performance than that of previously reported flexible
LIBs based on nanosized materials.12−21 The construction of
the bendable thin-film battery starts with a standard fabrication
process upon a brittle mica substrate. A cathode current
collector (CCC), a lithium cobalt oxide cathode (LiCoO2;
high-temperature (HT) annealing of LiCoO2 at 700 °C), a
lithium phosphorus oxynitride electrolyte (LiPON), a lithium
(Li) metal anode, and protective encapsulation multilayers were
sequentially deposited on the substrate (Figure 1b-i). LiCoO2
has received great interest from many researchers due to its
high operating potential of ∼4 V and high reversible capacity,
and is currently the most widely used cathode in LIB.25−27
Although advanced cathode materials having high capacity and
potential have been recently developed, LiCoO2 is still regarded
as the most reliable cathode material. As is the case for any
other electrode materials, it should be noted that the HT
annealing process for the crystallinity of LiCoO2 material is a
crucial step to realize the high- performance solid-state LIB. It is
because the solid-state lithium diffusion is critically hindered by
any imperfections in the crystal.
The next step is physical delamination of the mica substrate
using sticky tapes (Figure 1b-ii). The mica material with weak-
layered crystalline structure can be delaminated into thin sheets
Figure 1. (a) Photograph of a bendable LIB turning on a blue LED in
bent condition. The inset shows the stacked layers in the flexible LIB.
(b) Schematic illustration of the process for fabricating flexible LIBs. A
cultivated LIB on brittle mica substrate (i) is followed by mica
substrate delamination using sticky tapes (ii), and then, the flexible
LIB is transferred onto a PDMS polymer substrate (iii). Finally, the
flexible LIB is covered with another PDMS sheet to enhance
mechanical stability (iv). (c) Schematic image of the mechanical
neutral plane generated from the counterbalance between tensile and
compressive strain.
Figure 2. (a) Cross-sectional scanning electron microscope (SEM)
image of a thin-film LIB on a mica substrate before substrate
delamination. (b) A glancing incident X-ray diffraction pattern of the
LiCoO2 cathode material on a CCC/mica substrate.
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by feeble peeling forces. Moreover, the intermolecular forces
between layers of the mica substrate are weakened during the
high-temperature annealing process. Therefore, the mica
substrate can be easily removed even by the sticky tape
without damaging the thin film LIB. The flexible LIB from
substrate delamination is then transferred onto a PDMS
polymer sheet (Figure 1b-iii), where the surface bondability of
the PDMS helps the stable settlement of the flexible LIB. The
final step is capping of the fabricated flexible LIB with another
thin PDMS sheet to enhance its mechanical stability (Figure
1b-iv). The details of this process are presented in Figure S1 in
the Supporting Information.
In this work, our transfer approach using mica substrate
delamination enables the highly crystalline LiCoO2 cathode
thin film from HT annealing (Tanneal ≥ 700 °C) for high-
performance LIBs to be utilized on flexible polymer substrates
(Tglass ≤ 150 °C). Moreover, the thin-film active material in
LIBs can be uniformly deposited upon mica substrates due to
its ultraflatness property,28,29 which can considerably increase
the possibility of LiCoO2’s preferred orientation growth,
resulting in reduced charge transfer resistance30 of Li ions
from the anode into the cathode and vice versa.
Stable electrochemical activity under mechanical flexibility is
demonstrated in Figure 1a. This is due to the electrochemically
active parts being located at the mechanical neutral plane
(Figure 1c) formed by 1 mm thick PDMS capping shown in
Figure 1b-iv. This protocol is similar to that of a stretchable and
foldable electronic device previously reported by D.-H. Kim et
al.31 While the film is bent as illustrated in Figure 1c, tensile
strain arises on one outer side and compressive strain on the
other inner side. Accordingly, the counterbalance between
these opposite strains develops a mechanical neutral space. The
effects of the external PDMS sheet capping were analyzed by
FEA simulation modeling (Figure S2, Supporting Information).
The bending deformations correspond to uniaxial stresses σxx
defined along the x-direction; σyy = σzz = 0.
32 In this calculation,
the PDMS capping can play a role in shifting of the mechanical
neutral plane (Figure S2, Supporting Information), which the
inorganic thin-film battery settles in. The reduction of induced
stress σxx under 10 N bending forces reaches a maximum of
6.13% (i.e., 0.14 MPa) at (x, z) = (0, 5) between LiPON and
LiCoO2 layers, where a contact resistance
30 may develop
(Figure S3, Supporting Information).
A cross-sectional image of the thin-film LIB on a mica
substrate is shown in Figure 2a. The employment of well-
crystalline LiCoO2 has been the focus of many studies on
flexible LIBs based on plastic substrates with low thermal
stability.33−35 However, the crystalline properties of LiCoO2
produced by previously reported non-HT methods have not
reached that of HT-annealed LiCoO2 (≥700 °C). In Figure 2b,
the glancing incident X-ray diffraction pattern of our LiCoO2
cathode material shows the well-defined crystallinity from the
optimization of the HT annealing process. The HT phase
formation of the LiCoO2 material through sufficient HT
annealing treatment is especially important in order to avoid
poor cycling and high self-discharge. The HT phase of the
layered LiCoO2 structure (space group R3m̅) can be verified
with (006), (012), (018), and (110) peak orientations.36
Moreover, although the LIB with Li metal anode has a higher
energy density than those with a carbon insertion anode,37
recent studies of flexible LIBs have reported difficulty in
carefully employing Li as an active material due to the danger of
Figure 3. (a) Robustness tests of a flexible LIB on a bending stage machine. (b) Charge and discharge behavior of thin-film LIBs at the 1st cycle. (c)
Charge and discharge curves at the 100th cycle.
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explosion. In our work, stable incorporation with Li anode
using our transfer method after deposition in vacuum
conditions enables the production of high-performance flexible
LIBs on polymer substrates.
Figure 3a demonstrates robustness testing at various fixed
bending radius conditions on a bending stage machine. The
flexible LIB wrapped with two PDMS polymer sheets (1 mm in
thickness) was bent from 2.5 mm (Rc = 16.0 mm) to 12.5 mm
(Rc = 3.1 mm) compared with nonbending status (Rc = ∞).
Even in the strain over a bending radius of 3.1 mm, we cannot
observe any external damages (see the video S1 in the
Supporting Information). At various bending radius values,
Figure 3b and c show the charge and discharge profile during
galvanostatic cycling tests at a rate of 46.5 μA/cm2 between 3.0
and 4.2 V. At the first cycle of the discharge curve in Figure 3b,
it is interesting to note that the specific capacities of the flexible
LIBs are enhanced as compared to 94 μAh/cm2 for a mica LIB
before substrate delamination, denoted as “on mica”. Along with
further increasing the degree of bending deformation on the
flexible batteries, the specific capacity of 106 μAh/cm2 for the
nonbending unit on a flexible substrate is gradually decreased
from the maximum value to 99 μAh/cm2 at Rc = 3.1 mm.
Figure 3c shows that the same tendency holds even at the
100th cycle.
This well-consistent reduction in specific capacity can be
understood in terms of the charge transfer resistance of Li ions
from and into the electrode including the contact resistance
existing between LiPON and LiCoO2 layers,
30 which are greatly
Figure 4. (a) Nyquist plots of AC impedance test measured at 5.0 V. An electrode had 18 mm × 18 mm area. (b) Coulombic efficiencies of a flexible
LIB bent to Rc = 3.1 mm and a mica LIB. (c) Capacity retention as a function of bending state at a constant current rate of 46.5 μA/cm
2 during 100
cycles. (d) Capacity retention at various rates: 46.5, 93, and 139.5 μA/cm2. (e) Voltage retention during fatigue tests. (f) Demonstration of 1 cycle
fatigue test: flat (left) and bent (right) state of the flexible LIB.
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affected by the residual internal stresses of the flexible LIBs.
Both of the resistances certainly give rise to overvoltage during
charge and discharge, as shown in Figure 3b and c. The
maximum of the overvoltage is observed for the composite on
mica, while the minimum is given to the nonbending unit on
the flexible substrate. The overvoltage thereafter increases with
increased bending deformation. Note that the generation of
overvoltage with the number of cycles and degree of bending
deformation has the same tendency as that of charge density as
seen in Figure 3b and c.
To further investigate the correlation between overvoltage
and the degree of bending deformation, AC impedance was
measured at 3.9 V by applying an AC-amplitude of 5 mV over
the frequency range from 10−3 to 7 × 105 Hz at room
temperature. The results are presented with various substrate
conditions in Figure 4a. On the Nyquist plots of the AC
impedance test, the high-frequency semicircles indicate the
charge transfer resistance of Li ions including the contact
resistance in LIBs. The highest impedance on the brittle mica
substrate is considerably decreased by substrate delamination.
However, the degree of bending deformation on the flexible
polymer substrate further increases the impedance in arc size.
Therefore, it is deduced that the all-inclusive charge transfer
resistance induced from residual stresses causes losses in the
specific capacity of LIBs.38 The residual stresses of sputter-
deposited films on a mica substrate are gradually released upon
the delamination of the mica layers, and then, the bending
deformation can further increase the internal stresses.
This presumption can be developed from the analysis of
residual stresses σf across a deposited film originating from
Stoney’s formula39 given by
σ δ=
−
E t
v t3(1 ) rf
s s
2
s f
2
where Es is the elastic modulus, ts is the thickness, and νs is the
Poisson ratio of the substrate; tf is the thickness of the
deposited film, and δ is the bending deflection at a distance r
from the center of the substrate (see the Supporting
Information for details).
In this analysis, the residual stress σf is parabolically and
linearly increased with ts and δ, respectively. In other words, the
residual stresses can be considerably reduced by the mica
substrate delamination, but they can also be enhanced by the
degree of the substrate bending.
To evaluate the cyclability performance, the Coulombic
efficiency of our flexible LIB being bent to Rc = 3.1 mm is
compared with that of the mica LIB. As shown in Figure 4b, the
Coulombic efficiency properties of the mica LIB and of the
bent flexible LIB maintain their original capability over 99.8%
even during 100 cycles. Figure 4c shows the capacity retention
as a function of the degree of bending deformation at the
constant current rate of 46.5 μA/cm2. After 100 cycles, the
capacity retention for a nonbending condition on the flexible
substrate is 98.4% of the original value, and those for Rc = 16.0
and 3.1 mm show a similar capacity retention of 94.5%. As
shown in Figure 4d, the retention was measured at various
Figure 5. (a) FEA simulation geometry (i) for the correlation between
substrate conditions and internal molar volume change stress. The
stress distributions on mica (ii) and PDMS polymer (iii) substrates are
compared, and the quantitative analysis of defined normal stress σxx
(iv) is given. (b) Change of induced stresses as a function of applied
bending force.
Figure 6. (a) Schematic diagram of an all-flexible LED system. (b)
Picture of an all-in-one flexible LED system integrated with a bendable
LIB.
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rates: 46.5 (0.5 C), 93 (1 C), and 139.5 μA/cm2 (1.5 C). The
discharge capacity for a nonbending condition retains 97.3%
after 100 cycles. Those for Rc = 16.0 and 3.1 mm are 93.0 and
88.2%, respectively.
From Figure 4c and d, it is readily seen that the capacity
retention is gradually aggravated by further bending progressing
during 100 cycles of charge and discharge. It can be confirmed
here that both contributions of the residual stresses caused by
the degree of bending as well as the molar volume change in
Li1‑dCoO2 resulting during charge and discharge accelerate the
degradation of the specific capacity of the LIB. Figure 4e shows
the fatigue endurance of a flexible LIB under 2 × 104 bending
cycles to Rc = 3.1 mm with a bending speed of 0.2 m/s. The
endurance is evaluated with the voltage retention of the LIB.
The initial delivered cell voltage of the flexible LIB of 4.071 V
only decreases to 4.060 V, which just corresponds to 0.27%
after 2 × 104 bending tests (Figure 4e). Figure 4f demonstrates
the voltage retention during one bending cycle. The tested LIB
is represented by two red dotted circles.
Detailed interpretation of the correlation between substrate
conditions and molar volume change stress during 100 cycles of
charge and discharge was carried out with FEA simulation. This
calculation assumes that the lattice parameters of LiCoO2 as a
cathode material are a = b = 0.282 nm and c = 1.41 nm,40 and
then during charge and discharge cycling of a LIB, reversible Li
extraction from and insertion into the cathode could lead to a
change in composition to Li0.62CoO2, whose lattice parameters
are a = b = 0.28 nm and c = 2.93 nm.41 The internal stresses
arising from molar volume change are marked with red arrows
in Figure 5a-i. Consequently, the stress distributions on mica
and PDMS polymer substrates are depicted in Figure 5a-ii and
a-iii. These results show that the capacities of LIBs transferred
onto polymer substrates can be increased due to the prompt
release of stresses by the molar volume change, which directly
relates to the all-inclusive charge transfer resistance of Li ions.
In addition, Figure 5a-iv presents the quantitative analysis of
defined normal stress σxx along the x-direction. As shown in the
results, the reduction of induced stresses due to the transfer
onto PDMS substrate occurs at x-axis positions A, B, and C as
well. The change in induced residual stresses is analyzed as a
function of bending force in Figure 5b. The compressive