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Journal of Power Sources 301 (2016) 35e40

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Li4Ti5O12 and LiMn2O4 thin-film electrodes on transparent conductingoxides 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, Germanyb Institut de recherche d’Hydro-Qu�ebec, 1800 Boul. Lionel-Boulet, Varennes, QC J3X 1S3, Canadac Department of Mining and Materials Engineering, McGill University, M.H. Wong Building, 3610 rue University, Montr�eal, 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 2015Received in revised form31 August 2015Accepted 16 September 2015Available online xxx

Keywords:Li4Ti5O12

LiMn2O4

BatteryThin filmHigh powerElectrochromic

* Corresponding author.E-mail address: manuel.roeder@isc.fraunhofer.de (

http://dx.doi.org/10.1016/j.jpowsour.2015.09.0630378-7753/© 2015 Elsevier B.V. All rights reserved.

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 animportant step towards all-solid-state batteries that are attractivefor portable devices [1], but also next generation batteries for fastcharging electrical vehicles [2]. Thin-film electrodes exposesignificantly higher charge and discharge rates than standardcomposite electrodes, thus have the potential to increase the powerdensity of thin-film batteries [1e4].

Thin-film electrodes are often prepared via physical vapor

M. Roeder).

deposition methods such as pulsed laser deposition [5], sputteringdeposition [6], or electrostatic spray deposition [7]. These methodsensure control over uniformity and thickness of the films, but arelimited with respect to substrate sizes and pricing. Wet chemicalsolegel techniques are low cost, flexible with regard to substratesize and geometry, and can be modified easily for stoichiometrycontrol [8]. Solegel procedures are established in battery researchfor synthesizing active material powders with controlled particlesizes [9e12]. Here, we applied the dip-coating method for coatingconducting transparent substrates with LiMn2O4 thin films ascathode 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

Fig. 1. Colorless Li4Ti5O12 (A) and green to brownish LiMn2O4 (B) transparent thin filmelectrodes prepared on FTO glass slides. Li4Ti5O12 and LiMn2O4 films were heat-treatedat 600C and 400 �C (after each coating procedure), respectively. (For interpretation ofthe references to colour in this figure legend, the reader is referred to the web versionof this article.)

M. Roeder et al. / Journal of Power Sources 301 (2016) 35e4036

is known for its low cost and high cycling stability. Both electrodesystems have spinel type crystal structure and relatively low elec-tronic and ionic conductivity, which can be compensated for bylowering the particle size down to the nanometer scale. A particularchallenge is the use of transparent conducting oxides as substratesfor optical devices. The temperature for heat-treatment of thoseoxides is limited, mainly due to the underlying commonly usedsoda-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 commerciallybroadly available transparent electrode systems. However, ITOshows decreasing conductivity by heat treatment exceeding 300 �C[14,15]. On the other hand, FTO was found to be suitable for theapplied heat procedures as it keeps its initial optical and electricalproperties, even at higher temperatures. This enabled us to use thinelectrode films prepared by solegel in optical devices. In pre-liminary tests on the optical properties of such devices, both thinfilm electrode materials showed intense color changes uponcharging/discharging paving the way for electrochromicapplications.

2. Experimental

Both Li4Ti5O12 and LiMn2O4 thin films were prepared by a sol-egel dip-coating process. If not mentioned otherwise, all chemicalsused were supplied by SigmaeAldrich. The Li4Ti5O12 solution (sol)was synthesized according to Mosa et al.. Briefly, a lithium-richsolution with a molar ratio Li/Ti ¼ 1.2/1 was prepared by addingtitanium isopropoxide and lithium acetate as precursors to amixture of absolute ethanol, acetic acid, water, and hydrochloricacid. The sol was stirred at room temperature for 2 h [16]. ForLiMn2O4, stoichiometric amounts of lithium acetylacetonate andmanganese(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 ashigh-boiling additive.

Li4Ti5O12 and LiMn2O4 films were deposited onto transparentTEC7 FTO glass substrates (Pilkington Glass Inc.) which have a sheetresistance of 7 U/sq. The dip-coating procedure was done in acontrolled atmosphere in air at 25 �C and 20% humidity with ahome-built dip-coater (Fraunhofer ISC). The coater has a sampleholder attached to a carriage movable over a guide rail. Controlledby a computer-operated stepping motor, the carriage can be movedup and down with defined constant speed and neglectable initialacceleration step. The program further allows the programming ofdip-duration, which is important for letting the solution calm downbefore the actual coating starts by withdrawing the dipped sub-strate from the sol.

As the coating solutions of Li4Ti5O12 and LiMn2O4 differ, thecoating and heating process was adjusted for each active materialsolution. For Li4Ti5O12, different film thicknesses were obtained byalternation of the withdrawal speed which was varied between 3and 50 cm/min. The wet Li4Ti5O12 single-layer films were heat-treated at 600 �C for 6 h in air. For LiMn2O4 films, withdrawalspeeds were chosen between 5 and 15 cm/min. Compared toLi4Ti5O12, film thickness variation by just altering the withdrawalspeed was limited. To gain film thickness and capacity, multi-layerfilms were prepared by repeating the coating procedure and thesubsequent thermal treatment for each layer at 400 �C for 1 h in air.

The crystallinity of the thin film electrodes was determined byx-ray diffraction (Siemens XRD D5000). Surface and cross-sectionsamples were also investigated by scanning electron microscopy(Carl Zeiss Microscopy SUPRA FE-SEM). The porosity of Li4Ti5O12films 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 wasused as counter and reference electrode. The electrolyte was 1 MLiTFSI salt dissolved in 3:7 EC:DEC. LiPF6-typed electrolyte was notused, 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 anappropriate electrode size of 1 cm2.

For the assessment of the electrodes suitability for electro-chromic applications, transmittance spectra of uncharged and fullycharged thin film samples of Li4Ti5O12 and LiMn2O4 were acquiredusing a UVevis spectrometer (Avantes, AvaSpec).

3. Results and discussion

3.1. Thin-film micro-batteries

Thin films of Li4Ti5O12 were produced as single-layer electrodeswith various film thicknesses and LiMn2O4 as multi-layer elec-trodes on transparent conducting FTO-coated glass slides. First, thestability of the sols and the homogeneity of the deposited filmswere evaluated. Both LiMn2O4 and Li4Ti5O12 sols were transparentand homogeneous, and did not show any signs of precipitation overat least 6 months storage time. The Li4Ti5O12 sol appeared slightlyyellow while the LiMn2O4 sol exhibited an amberish color. Afterheat 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 andLiMn2O4 films. In the diffractogram of the Li4Ti5O12 film, the peaksof a typical face-centered cubic spinel structure (JCPDS # 49e0207)are observed. The peaks of the LiMn2O4 film are less intense due tothe lower film thickness and the semi-crystallized phase comparedto the Li4Ti5O12 film. However, the coated films can be clearlyidentified as the target spinel (JCPDS # 35e0782). Among the peaksof the coated active material, additional peaks are found to beattributed to the transparent conducting FTO layer on the glasssubstrate.

Fig. 3 shows SEM images of the surfaces and cross-sections ofthe heat-treated samples. Surface images of Li4Ti5O12 (Fig. 3A) andof LiMn2O4 (Fig. 3B) electrodes reveal smooth and crack-free ho-mogeneous films with evenly distributed pores. Single-layerLi4Ti5O12 films were prepared with various film thicknessesranging from about 100 nm up to 800 nm by variation of thewithdrawal rate established during dip-coating. A representativecross-section image of a Li4Ti5O12 film shows a 600 nm thick

Fig. 2. XRD diffractograms of Li4Ti5O12 (A) and LiMn2O4 (B) thin film electrodes on FTOglass after heat treatment at 600 �C and 400 �C, respectively. Both graphs show purephases of the respective electrode material. The FTO coating on the glass substrate wasfound 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

deposit with uniform grain size indicating a successful film for-mation and densification took place (Fig. 3C). Fig. 3D and E show acomparison of cross-sectional SEM images of a 3-layer (D) and asingle-layer (E) LiMn2O4 film with the same final film thickness of150 nm. Both electrodes showuniformity in particle sizing and poredistribution after successful film preparation. The multi-layer na-ture of the 3-layer electrode is not observed (Fig. 3E). This is knownin solegel research as dip-coated layers of the same materialappear as one in SEM images [19]. It indicates, that the quality ofthe surface after each heat-treatment allows seamless layer-stacking. However, layer induced effects will be observed in latterelectrochemical measurements (Fig. 5).

Using ellipsometric porosimetry, a film porosity of 17% wasdetermined for the Li4Ti5O12. The method is based on the change of

the refraction index and the film thickness during the adsorptionand desorption of water. This alteration is calculated model-basedfrom the variation of the polarization of the incident light [17].Due to the color of the LiMn2O4, ellipsometric porosimetry couldnot be performed at the cathode films.

For electrochemical characterization, thin-film electrodes wereinvestigated via potentiodynamic and galvanostatic measurements.Fig. 4 shows cyclovoltamograms (CVs) of selected Li4Ti5O12 andLiMn2O4 thin film electrodes. The scan rate was varied between 0.1and 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 lithiumions into the Li4Ti5O12 structure forming Li7Ti5O12 while chargingfollowed by highly reversible deintercalation of Li7Ti5O12 back toLi4Ti5O12 during discharging, due to the Ti4þ/Ti3þ redox couple. Wekept the Li4Ti5O12 transparent thin film electrode cycling for a totalof 100 CVs at 1 mV/s. The very stable Ti4þ/Ti3þ redox couple re-actions of Li4Ti5O12 causes the electrode to performwith very goodreversibility and demonstrates a good long-term stability by hardlyaltering 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 thecharacteristic behavior of two-step extraction (peaks at 3.98 and4.14 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 atdifferent sweep rates of 0.1, 0.5 and 1mV/s. Also here, the increasedsweep rates cause almost no polarization of the redox peak posi-tions and show perfectly reversible cycling behavior at each scanrate, which affirms the good kinetics of the LiMn2O4 thin filmelectrodes. The fast Li/Li þ ion insertion and deinsertion of both,anode and cathode thin film electrodes is related to the nanometerscale of the films, which enables short lithium ion diffusion path-ways in the electrode layer compensating the lack of binder andconductive 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. 4. CVs of Li4Ti5O12 (A) and LiMn2O4 (B) samples at various scan rates of 0.1 (bluelines), 0.5 (red lines), and 1 mV/s (blue lines). Of each scan rate, at least five cycles wereacquired 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 thisarticle.)

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) preparedvia variation of the drawing speed during the coating process. (B) LiMn2O4 single-layerwith a thickness of 50 nm (black) and 150 nm (blue) and multi-layer electrodes withthicknesses of 150 nm (3 � 50 nm) (red) and 400 nm (8 � 50 nm) (green). Thedischarge capacity is plotted as a function of the C-rate. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

M. Roeder et al. / Journal of Power Sources 301 (2016) 35e4038

discharged at various C-rates between 1C and 100C (Fig. 5). 1Crepresents the current density needed for charging or dischargingthe whole capacity of the electrode in 1 h. Carbon-coated Li4Ti5O12anodes [20e22] and LiMn2O4 cathodes [23,24] have already beenevaluated earlier as potential high power electrode materials, butsignificant decrease in discharge capacity was usually observedwhen facing discharge currents higher than 10C. This can beexplained by the binder and carbonaceous additives in compositeelectrodes adding more solid/solid-interfaces to the composite filmdue to the particle mix. Also, standard composite electrodes areconsiderably thicker compared to thin film electrodes reaching100 mm andmore. Here, the prepared thin film electrodes consist ofpure active material without additives. This excludes unwantedside reactions between active material and additives [25].Furthermore, thin film thicknesses below 1 mm allow the alreadymentioned short and fast lithium ion movement inside the activematerial coating.

The discharge capacity of both Li4Ti5O12 and LiMn2O4 single-layer samples remained on the same level up to very high chargeand discharge rates of 100C. Nearly constant capacity for all C-ratesup 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 differentbehavior (Fig. 5B). At 1C discharge current, the obtained capacityscales 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 resistanceof the electrode as the film thickness grows with each additionallayer. This was ruled out by comparing the 3-layer electrode withthe 150 nm single-layer electrode. Both provide the same final filmthickness of 150 nm and show similar constant capacity up to 8C.

Increasing the discharge current leads to the observed capacity fadeof 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 dropstowards the single-layer capacity. This indicates that at fast dis-charging speed mainly the top layers are used. We explain thisobservation with the generation of layer interfaces within theelectrode films due to the multiple deposition and heat-treatmentprocesses. Thus, the multi-layer interfaces interrupt the fast ionpathways in the final electrode.

3.2. Optical characteristics

We deposited Li4Ti5O12 and LiMn2O4 thin films to FTO-glasstransparent current collectors in order to see potential electro-chromic behavior and color changes at the charged and dischargedstates of the electrodes. Many standard electrode materials inlithium ion battery research show a color change upon ion (de)intercalation and there are many similarities between thin filmbatteries and certain types of electrochromic devices [26]. Elec-trochromic windows based on metal oxide layers follow the sameworking principle as lithium ion batteries [27]. Thin films of elec-trochromic materials change their optical absorption in the visiblerange as charge is inserted or extracted. Generally, these interca-lation processes have to be very fast for the device to be attractivefor electrochromic applications.

While performing the electrochemical tests, electrochromic ef-fects and color changes were noticed. The produced transparentLi4Ti5O12 and LiMn2O4 electrodes showed very attractive colorchange capability. The optical changes are shown on representative

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-glasssubstrate (grey). (For interpretation of the references to colour in this figure legend,

M. Roeder et al. / Journal of Power Sources 301 (2016) 35e40 39

electrodes of Li4Ti5O12 and LiMn2O4 in Fig. 6. Fig. 6A shows acompletely colorless and highly transparent 600 nm single-layerLi4Ti5O12 electrode in its discharged as-prepared state. After beingcharged to 1.2 V vs. Li/Liþ, the Li4Ti5O12 electrode switched to anattractively dark grey/blue color (Fig. 6B). The 150 nm LiMn2O4electrode 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 theircharged and discharged states. Fig. 7A compares the transmittancespectra of the discharged/colorless-transparent and the charged/blue-colored Li4Ti5O12 electrode. In the visible light spectrumfrom 380 nm to 780 nm, this results in an impressive DT of~40e65%, which is similar to the commercially established andmore expensive electrochromic WO3 coatings [28]. On the otherhand, the LiMn2O4 electrode shows a red shift from a light-greentint (discharged) to an intensive orange color (charged) whilekeeping its transmittance level at about 50e55% in the visible lightspectrum (Fig. 7B). These features are considered very interestingwith regard to the rising demand for neutrally tintingwindows. In atwo-electrode assembly, Li4Ti5O12 will turn blue while the LiMn2O4will add its orange color, which should enable grey tints to beachieved. 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. Furtherinvestigations on the promising electrochromic properties ofLiMn2O4 and Li4Ti5O12 thin film electrodes will be subject of aseparate publication.

the reader is referred to the web version of this article.)

4. Conclusion

Li4Ti5O12 and LiMn2O4 thin film electrodes were successfullyprepared 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 upto 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 filmelectrodes. As single-layers, both active materials showed almostno capacity drop up to 100C charge and discharge rate and superiorcycle stability. However, multi-layer electrodes are lacking high

Fig. 6. Optical photographs of a Li4Ti5O12 thin film electrode in its colorless discharged (A) athe uncharged state (C) changing to orange when being charged (D). (For interpretation of ththis article.)

power performance presumably due to induced higher internalresistances. The electrochromic properties of Li4Ti5O12 and LiMn2O4thin film electrodes were demonstrated in first measurements.Li4Ti5O12 showed highly reversible changes in visible transmittancefrom T ¼ 30e75% switching between a colorless and a dark-bluestate, while LiMn2O4 was changing its colors between light-greenand orange tints, which could be useful for the design of grey-switching windows. Using standard lithium ion battery activematerials may be a low-cost alternative to establish inexpensiveelectrochromic materials. In future studies, the electrochromic

nd dark-blue colored charged state (B). The LiMn2O4 electrode showed a green color ine references to colour in this figure legend, the reader is referred to the web version of

M. Roeder et al. / Journal of Power Sources 301 (2016) 35e4040

properties will be investigated in more detail to see whether usingthese 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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