1. Li4Ti5O12 and LiMn2O4 thin-film electrodes on transparent conducting
oxides for all-solid-state and electrochromic applications
Manuel Roeder a, *
, Alexis B. Beleke b, c
, Uwe Guntow a
, Johanna Buensow a
,
Abdelbast Guerfi b
, Uwe Posset a
, Henning Lorrmann a
, Karim Zaghib b
, Gerhard Sextl a
a
Center for Applied Electrochemistry, Fraunhofer Institute for Silicate Research, Neunerplatz 2, 97083 Würzburg, Germany
b
Institut de recherche d’Hydro-Quebec, 1800 Boul. Lionel-Boulet, Varennes, QC J3X 1S3, Canada
c
Department of Mining and Materials Engineering, McGill University, M.H. Wong Building, 3610 rue University, Montreal, QC H3A 2B2, Canada
h i g h l i g h t s
First preparation of lithium titanate transparent electrode via solegel dip coating.
First preparation of lithium manganate transparent cathode via solegel dip coating.
New synthesis for lithium manganate sol.
Transparent electrodes provide excellent rate capabilities.
Electrochromic studies on prepared electrodes.
a r t i c l e i n f o
Article history:
Received 23 July 2015
Received in revised form
31 August 2015
Accepted 16 September 2015
Available online xxx
Keywords:
Li4Ti5O12
LiMn2O4
Battery
Thin film
High power
Electrochromic
a b s t r a c t
Transparent Li4Ti5O12 and LiMn2O4 thin films were prepared on transparent conducting oxide substrates
via solegel dip coating technique. The resulting films were highly uniform and transparent after
annealing at 600
C and 400
C for Li4Ti5O12 and LiMn2O4, respectively. Different film thicknesses ranging
from 50 nm to about 800 nm were attained and the films were characterized by scanning electron
microscopy (SEM) for surface morphology and X-ray diffraction (XRD) for evaluating the crystallization of
the active materials. Ellipsometric porosimetry was used to measure the porosity of the anode material.
Furthermore, electrochemical techniques such as cyclovoltametry and galvanostatic charging/discharg-
ing confirm the successful synthesis of electrochemically active LiMn2O4 and Li4Ti5O12 films. Single-layer
electrodes show stable capacities at discharge rates up to 100C indicating the high rate capability and
therefore are classified as materials for power. Furthermore, the electrochromic effects were observed for
both thin-film systems and evaluated in preliminary tests.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Establishing thin-film technologies for lithium ion batteries is an
important step towards all-solid-state batteries that are attractive
for portable devices [1], but also next generation batteries for fast
charging electrical vehicles [2]. Thin-film electrodes expose
significantly higher charge and discharge rates than standard
composite electrodes, thus have the potential to increase the power
density of thin-film batteries [1e4].
Thin-film electrodes are often prepared via physical vapor
deposition methods such as pulsed laser deposition [5], sputtering
deposition [6], or electrostatic spray deposition [7]. These methods
ensure control over uniformity and thickness of the films, but are
limited with respect to substrate sizes and pricing. Wet chemical
solegel techniques are low cost, flexible with regard to substrate
size and geometry, and can be modified easily for stoichiometry
control [8]. Solegel procedures are established in battery research
for synthesizing active material powders with controlled particle
sizes [9e12]. Here, we applied the dip-coating method for coating
conducting transparent substrates with LiMn2O4 thin films as
cathode material. LiMn2O4 is a non-toxic, environmentally friendly,
and inexpensive active material for lithium ion batteries. As anode,
Li4Ti5O12 thin films were prepared as this promising active material* Corresponding author.
E-mail address: manuel.roeder@isc.fraunhofer.de (M. Roeder).
Contents lists available at ScienceDirect
Journal of Power Sources
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0378-7753/© 2015 Elsevier B.V. All rights reserved.
Journal of Power Sources 301 (2016) 35e40

2. is known for its low cost and high cycling stability. Both electrode
systems have spinel type crystal structure and relatively low elec-
tronic and ionic conductivity, which can be compensated for by
lowering the particle size down to the nanometer scale. A particular
challenge is the use of transparent conducting oxides as substrates
for optical devices. The temperature for heat-treatment of those
oxides is limited, mainly due to the underlying commonly used
soda-lime glass substrate which holds a glass transition tempera-
ture of 573 C [13]. Indium-doped tin oxide (ITO) and fluorine-
doped tin oxide (FTO) are the most prominent and commercially
broadly available transparent electrode systems. However, ITO
shows decreasing conductivity by heat treatment exceeding 300 C
[14,15]. On the other hand, FTO was found to be suitable for the
applied heat procedures as it keeps its initial optical and electrical
properties, even at higher temperatures. This enabled us to use thin
electrode films prepared by solegel in optical devices. In pre-
liminary tests on the optical properties of such devices, both thin
film electrode materials showed intense color changes upon
charging/discharging paving the way for electrochromic
applications.
2. Experimental
Both Li4Ti5O12 and LiMn2O4 thin films were prepared by a sol-
egel dip-coating process. If not mentioned otherwise, all chemicals
used were supplied by SigmaeAldrich. The Li4Ti5O12 solution (sol)
was synthesized according to Mosa et al.. Briefly, a lithium-rich
solution with a molar ratio Li/Ti ¼ 1.2/1 was prepared by adding
titanium isopropoxide and lithium acetate as precursors to a
mixture of absolute ethanol, acetic acid, water, and hydrochloric
acid. The sol was stirred at room temperature for 2 h [16]. For
LiMn2O4, stoichiometric amounts of lithium acetylacetonate and
manganese(II) acetate tetrahydrate as metal precursors were dis-
solved in ethanol and 2-(2-methoxyethoxy) acetic acid. After stir-
ring for 1 h at room temperature, 1,5-pentane-diol was added as
high-boiling additive.
Li4Ti5O12 and LiMn2O4 films were deposited onto transparent
TEC7 FTO glass substrates (Pilkington Glass Inc.) which have a sheet
resistance of 7 U/sq. The dip-coating procedure was done in a
controlled atmosphere in air at 25 C and 20% humidity with a
home-built dip-coater (Fraunhofer ISC). The coater has a sample
holder attached to a carriage movable over a guide rail. Controlled
by a computer-operated stepping motor, the carriage can be moved
up and down with defined constant speed and neglectable initial
acceleration step. The program further allows the programming of
dip-duration, which is important for letting the solution calm down
before the actual coating starts by withdrawing the dipped sub-
strate from the sol.
As the coating solutions of Li4Ti5O12 and LiMn2O4 differ, the
coating and heating process was adjusted for each active material
solution. For Li4Ti5O12, different film thicknesses were obtained by
alternation of the withdrawal speed which was varied between 3
and 50 cm/min. The wet Li4Ti5O12 single-layer films were heat-
treated at 600 C for 6 h in air. For LiMn2O4 films, withdrawal
speeds were chosen between 5 and 15 cm/min. Compared to
Li4Ti5O12, film thickness variation by just altering the withdrawal
speed was limited. To gain film thickness and capacity, multi-layer
films were prepared by repeating the coating procedure and the
subsequent thermal treatment for each layer at 400 C for 1 h in air.
The crystallinity of the thin film electrodes was determined by
x-ray diffraction (Siemens XRD D5000). Surface and cross-section
samples were also investigated by scanning electron microscopy
(Carl Zeiss Microscopy SUPRA FE-SEM). The porosity of Li4Ti5O12
films was measured by means of ellipsometric porosimetry [17].
All electrochemical measurements were carried out in a three-
electrode set-up at room temperature. Lithium metal foil was
used as counter and reference electrode. The electrolyte was 1 M
LiTFSI salt dissolved in 3:7 EC:DEC. LiPF6-typed electrolyte was not
used, as potential HF formation attacks glass [18] and, therefore,
would be harmful to the electrode substrate.
A Biologics VMP-300 was used for cyclic voltammetry and gal-
vanostatic measurements of the thin film electrodes cut to an
appropriate electrode size of 1 cm2
.
For the assessment of the electrodes suitability for electro-
chromic applications, transmittance spectra of uncharged and fully
charged thin film samples of Li4Ti5O12 and LiMn2O4 were acquired
using a UVevis spectrometer (Avantes, AvaSpec).
3. Results and discussion
3.1. Thin-film micro-batteries
Thin films of Li4Ti5O12 were produced as single-layer electrodes
with various film thicknesses and LiMn2O4 as multi-layer elec-
trodes on transparent conducting FTO-coated glass slides. First, the
stability of the sols and the homogeneity of the deposited films
were evaluated. Both LiMn2O4 and Li4Ti5O12 sols were transparent
and homogeneous, and did not show any signs of precipitation over
at least 6 months storage time. The Li4Ti5O12 sol appeared slightly
yellow while the LiMn2O4 sol exhibited an amberish color. After
heat treatment, transparent and homogeneous thin films of color-
less Li4Ti5O12 and greenish LiMn2O4 were obtained (Fig. 1).
The crystallinity of the coatings was determined by means of X-
ray diffraction. Fig. 2 shows the XRD diffractograms of Li4Ti5O12 and
LiMn2O4 films. In the diffractogram of the Li4Ti5O12 film, the peaks
of a typical face-centered cubic spinel structure (JCPDS # 49e0207)
are observed. The peaks of the LiMn2O4 film are less intense due to
the lower film thickness and the semi-crystallized phase compared
to the Li4Ti5O12 film. However, the coated films can be clearly
identified as the target spinel (JCPDS # 35e0782). Among the peaks
of the coated active material, additional peaks are found to be
attributed to the transparent conducting FTO layer on the glass
substrate.
Fig. 3 shows SEM images of the surfaces and cross-sections of
the heat-treated samples. Surface images of Li4Ti5O12 (Fig. 3A) and
of LiMn2O4 (Fig. 3B) electrodes reveal smooth and crack-free ho-
mogeneous films with evenly distributed pores. Single-layer
Li4Ti5O12 films were prepared with various film thicknesses
ranging from about 100 nm up to 800 nm by variation of the
withdrawal rate established during dip-coating. A representative
cross-section image of a Li4Ti5O12 film shows a 600 nm thick
Fig. 1. Colorless Li4Ti5O12 (A) and green to brownish LiMn2O4 (B) transparent thin film
electrodes prepared on FTO glass slides. Li4Ti5O12 and LiMn2O4 films were heat-treated
at 600C and 400 C (after each coating procedure), respectively. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
M. Roeder et al. / Journal of Power Sources 301 (2016) 35e4036

3. deposit with uniform grain size indicating a successful film for-
mation and densification took place (Fig. 3C). Fig. 3D and E show a
comparison of cross-sectional SEM images of a 3-layer (D) and a
single-layer (E) LiMn2O4 film with the same final film thickness of
150 nm. Both electrodes show uniformity in particle sizing and pore
distribution after successful film preparation. The multi-layer na-
ture of the 3-layer electrode is not observed (Fig. 3E). This is known
in solegel research as dip-coated layers of the same material
appear as one in SEM images [19]. It indicates, that the quality of
the surface after each heat-treatment allows seamless layer-
stacking. However, layer induced effects will be observed in latter
electrochemical measurements (Fig. 5).
Using ellipsometric porosimetry, a film porosity of 17% was
determined for the Li4Ti5O12. The method is based on the change of
the refraction index and the film thickness during the adsorption
and desorption of water. This alteration is calculated model-based
from the variation of the polarization of the incident light [17].
Due to the color of the LiMn2O4, ellipsometric porosimetry could
not be performed at the cathode films.
For electrochemical characterization, thin-film electrodes were
investigated via potentiodynamic and galvanostatic measurements.
Fig. 4 shows cyclovoltamograms (CVs) of selected Li4Ti5O12 and
LiMn2O4 thin film electrodes. The scan rate was varied between 0.1
and 1 mV/s Fig. 4A shows peaks at 1.54 V (reduction) and 1.59 V
(oxidation) which are typical for the lithium intercalation of lithium
ions into the Li4Ti5O12 structure forming Li7Ti5O12 while charging
followed by highly reversible deintercalation of Li7Ti5O12 back to
Li4Ti5O12 during discharging, due to the Ti4þ
/Ti3þ
redox couple. We
kept the Li4Ti5O12 transparent thin film electrode cycling for a total
of 100 CVs at 1 mV/s. The very stable Ti4þ
/Ti3þ
redox couple re-
actions of Li4Ti5O12 causes the electrode to perform with very good
reversibility and demonstrates a good long-term stability by hardly
altering shape, size, and position of its anodic and cathodic peaks.
The sharp and well-defined peaks indicate fast ion de- and inser-
tion. The CVs in Fig. 4B of LiMn2O4 thin film electrodes show the
characteristic behavior of two-step extraction (peaks at 3.98 and
4.14 V) and insertion (peaks at 4.13 and 3.97 V) of the lithium ions in
the cubic spinel, due to the reversible Mn4þ
/Mn3þ
redox couple.
Analogous to Li4Ti5O12, the LiMn2O4 electrode was also cycled at
different sweep rates of 0.1, 0.5 and 1 mV/s. Also here, the increased
sweep rates cause almost no polarization of the redox peak posi-
tions and show perfectly reversible cycling behavior at each scan
rate, which affirms the good kinetics of the LiMn2O4 thin film
electrodes. The fast Li/Li þ ion insertion and deinsertion of both,
anode and cathode thin film electrodes is related to the nanometer
scale of the films, which enables short lithium ion diffusion path-
ways in the electrode layer compensating the lack of binder and
conductive carbon additives and keeping the internal layer resis-
tance at a minimum.
In view of the use of thin-film electrodes in high power appli-
cations, Li4Ti5O12 and LiMn2O4 samples were charged and
Fig. 2. XRD diffractograms of Li4Ti5O12 (A) and LiMn2O4 (B) thin film electrodes on FTO
glass after heat treatment at 600 C and 400 C, respectively. Both graphs show pure
phases of the respective electrode material. The FTO coating on the glass substrate was
found to be inert during the electrode film preparation process.
Fig. 3. SEM pictures of the surface Li4Ti5O12 (A) and LiMn2O4 (B) films on FTO glass substrates. (C) Shows the cross-section of a 600 nm thick Li4Ti5O12 thin film electrode assembly.
(D) Shows a 150 nm thick 3-layer LiMn2O4 compared with a single-layer LiMn2O4 electrode with similar thickness (E).
M. Roeder et al. / Journal of Power Sources 301 (2016) 35e40 37

4. discharged at various C-rates between 1C and 100C (Fig. 5). 1C
represents the current density needed for charging or discharging
the whole capacity of the electrode in 1 h. Carbon-coated Li4Ti5O12
anodes [20e22] and LiMn2O4 cathodes [23,24] have already been
evaluated earlier as potential high power electrode materials, but
significant decrease in discharge capacity was usually observed
when facing discharge currents higher than 10C. This can be
explained by the binder and carbonaceous additives in composite
electrodes adding more solid/solid-interfaces to the composite film
due to the particle mix. Also, standard composite electrodes are
considerably thicker compared to thin film electrodes reaching
100 mm and more. Here, the prepared thin film electrodes consist of
pure active material without additives. This excludes unwanted
side reactions between active material and additives [25].
Furthermore, thin film thicknesses below 1 mm allow the already
mentioned short and fast lithium ion movement inside the active
material coating.
The discharge capacity of both Li4Ti5O12 and LiMn2O4 single-
layer samples remained on the same level up to very high charge
and discharge rates of 100C. Nearly constant capacity for all C-rates
up to 100C was observed for all Li4Ti5O12 measured samples,
independently on film thickness (Fig. 5A).
Whereas LiMn2O4 single-layer electrodes show excellent ca-
pacity retention, multi-layer LiMn2O4 films exhibit a different
behavior (Fig. 5B). At 1C discharge current, the obtained capacity
scales well with the additional amount of applied layers. However,
a significant decrease of capacity with increasing C-rate is observed.
The first obvious reason could be an increasing internal resistance
of the electrode as the film thickness grows with each additional
layer. This was ruled out by comparing the 3-layer electrode with
the 150 nm single-layer electrode. Both provide the same final film
thickness of 150 nm and show similar constant capacity up to 8C.
Increasing the discharge current leads to the observed capacity fade
of the 3-layer electrode whereas the corresponding 150 nm single-
layer electrode shows again excellent capacity retention up to 100C.
It is noticed that the capacity of the multi-layer electrodes drops
towards the single-layer capacity. This indicates that at fast dis-
charging speed mainly the top layers are used. We explain this
observation with the generation of layer interfaces within the
electrode films due to the multiple deposition and heat-treatment
processes. Thus, the multi-layer interfaces interrupt the fast ion
pathways in the final electrode.
3.2. Optical characteristics
We deposited Li4Ti5O12 and LiMn2O4 thin films to FTO-glass
transparent current collectors in order to see potential electro-
chromic behavior and color changes at the charged and discharged
states of the electrodes. Many standard electrode materials in
lithium ion battery research show a color change upon ion (de)
intercalation and there are many similarities between thin film
batteries and certain types of electrochromic devices [26]. Elec-
trochromic windows based on metal oxide layers follow the same
working principle as lithium ion batteries [27]. Thin films of elec-
trochromic materials change their optical absorption in the visible
range as charge is inserted or extracted. Generally, these interca-
lation processes have to be very fast for the device to be attractive
for electrochromic applications.
While performing the electrochemical tests, electrochromic ef-
fects and color changes were noticed. The produced transparent
Li4Ti5O12 and LiMn2O4 electrodes showed very attractive color
change capability. The optical changes are shown on representative
Fig. 4. CVs of Li4Ti5O12 (A) and LiMn2O4 (B) samples at various scan rates of 0.1 (blue
lines), 0.5 (red lines), and 1 mV/s (blue lines). Of each scan rate, at least five cycles were
acquired with up to 100 cycles at 1 mV/s for Li4Ti5O12. (For interpretation of the ref-
erences to colour in this figure legend, the reader is referred to the web version of this
article.)
Fig. 5. (A) Li4Ti5O12 single-layer thin film electrodes with film thicknesses of 200 nm
(black), 280 nm (red), 440 nm (green), 600 nm (blue) and 840 nm (light blue) prepared
via variation of the drawing speed during the coating process. (B) LiMn2O4 single-layer
with a thickness of 50 nm (black) and 150 nm (blue) and multi-layer electrodes with
thicknesses of 150 nm (3 Â 50 nm) (red) and 400 nm (8 Â 50 nm) (green). The
discharge capacity is plotted as a function of the C-rate. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
M. Roeder et al. / Journal of Power Sources 301 (2016) 35e4038

5. electrodes of Li4Ti5O12 and LiMn2O4 in Fig. 6. Fig. 6A shows a
completely colorless and highly transparent 600 nm single-layer
Li4Ti5O12 electrode in its discharged as-prepared state. After being
charged to 1.2 V vs. Li/Liþ
, the Li4Ti5O12 electrode switched to an
attractively dark grey/blue color (Fig. 6B). The 150 nm LiMn2O4
electrode provides a light green color in its as-prepared state
(Fig. 6C) changing to orange when charged to 4.3 V vs. Li/Liþ
(Fig. 6D).
Transmittance measurements via UVevis spectroscopy where
carried out to take a closer look at the electrode changes in their
charged and discharged states. Fig. 7A compares the transmittance
spectra of the discharged/colorless-transparent and the charged/
blue-colored Li4Ti5O12 electrode. In the visible light spectrum
from 380 nm to 780 nm, this results in an impressive DT of
~40e65%, which is similar to the commercially established and
more expensive electrochromic WO3 coatings [28]. On the other
hand, the LiMn2O4 electrode shows a red shift from a light-green
tint (discharged) to an intensive orange color (charged) while
keeping its transmittance level at about 50e55% in the visible light
spectrum (Fig. 7B). These features are considered very interesting
with regard to the rising demand for neutrally tinting windows. In a
two-electrode assembly, Li4Ti5O12 will turn blue while the LiMn2O4
will add its orange color, which should enable grey tints to be
achieved. In accordance with the subtractive color mixing model,
this would result in a grey-turning transparent battery assembly.
Moreover, it may be possible to apply new non-destructive mea-
surement methods for lithium ion batteries by linking the elec-
trochromic behavior to the electrochemical processes. Further
investigations on the promising electrochromic properties of
LiMn2O4 and Li4Ti5O12 thin film electrodes will be subject of a
separate publication.
4. Conclusion
Li4Ti5O12 and LiMn2O4 thin film electrodes were successfully
prepared as single- and multi-layer films on transparent con-
ducting oxides, namely FTO glass substrates, via a solegel dip-
coating procedure. The ability of exploiting their full capacity up
to 100C was demonstrated, which makes them excellent candi-
dates for thin film micro-batteries. The solegel dip-coating tech-
nique is a cost-effective and viable route for preparing thin film
electrodes. As single-layers, both active materials showed almost
no capacity drop up to 100C charge and discharge rate and superior
cycle stability. However, multi-layer electrodes are lacking high
power performance presumably due to induced higher internal
resistances. The electrochromic properties of Li4Ti5O12 and LiMn2O4
thin film electrodes were demonstrated in first measurements.
Li4Ti5O12 showed highly reversible changes in visible transmittance
from T ¼ 30e75% switching between a colorless and a dark-blue
state, while LiMn2O4 was changing its colors between light-green
and orange tints, which could be useful for the design of grey-
switching windows. Using standard lithium ion battery active
materials may be a low-cost alternative to establish inexpensive
electrochromic materials. In future studies, the electrochromic
Fig. 6. Optical photographs of a Li4Ti5O12 thin film electrode in its colorless discharged (A) and dark-blue colored charged state (B). The LiMn2O4 electrode showed a green color in
the uncharged state (C) changing to orange when being charged (D). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 7. (A) Transmittance spectra of a charged (blue) and discharged (black) Li4Ti5O12
transparent thin film electrode compared to the pure FTO-glass substrate (grey). (B)
Shows the transmittance spectra of a charged (orange) and uncharged (green) LiMn2O4
electrode, respectively, also compared to the transmittance of the pure FTO-glass
substrate (grey). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
M. Roeder et al. / Journal of Power Sources 301 (2016) 35e40 39

6. properties will be investigated in more detail to see whether using
these electrode systems could be a viable concept for smart win-
dows and other electrochromic devices.
Acknowledgments
This research was supported by the Bayerisches Wirt-
schaftsministerium and the Bavarian Research Alliance.
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