Electrochromic effect of indium tin oxide in lithium iron phosphate

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Electrochromic effect of indium tin oxide in lithium iron phosphate battery cathodes for state of charge determination Valentin Roscher, Florian Rittweger, and Karl-Ragmar Riemschneider ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16439 • Publication Date (Web): 17 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Applied Materials & Interfaces

Electrochromic Effect of Indium Tin Oxide in Lithium Iron Phosphate Battery Cathodes for State of Charge Determination Valentin Roscher, Florian Rittweger, and Karl-Ragmar Riemschneider∗ Department of Information and Electrical Engineering, Hamburg University of Applied Sciences, 20099 Hamburg, Germany E-mail: [email protected]*

Abstract

Keywords

In this paper, we discuss the origin of an optical effect in lithium iron phosphate (LFP) battery cathodes, which depends on the electrical charge transferred into the battery. Utilizing indium tin oxide (ITO) as an electrode additive, we were able to observe a change in reflectivity of the cathode during charging and discharging with lithiation and delithiation being clearly visible in the form of lithiation fronts. Further investigations using in situ video microscopy and in situ Raman spectroscopy on test cells with an optical window indicates that ITO additionally acts as an electrochromic marker within the LFP cathode. This enhances the optical effect due to local potentials around the lithiation fronts, which enables the voltagedependent reflectivity of the ITO to be visible in the LFP cathode. Structural analysis with scanning electron microscopy (SEM) and X-ray crystallography (XRD) are presented as well. The observed effect allows for novel battery research methods and for a possible commercial application as a sensor for state of charge (SOC) estimation similar to the optical fiber approach reported by Ghannoum et al. for a graphite anode. 1

lithium ion batteries, electrochromic marker, battery state determination, state of charge, video microscopy, lithium iron phosphate, indium tin oxide, raman spectroscopy

1

Introduction

The high energy density of lithium ion batteries makes them favorable for use in mobile consumer electronics. Recently, the increased interest in e-mobility has also driven battery production capacities. Current research aimed at increasing battery capacity uses two approaches: improving battery cells and making battery usage more efficient. While the former is pursued by developing novel storage materials, the latter is based on improving information on the cell state. As the storage of energy in batteries mostly happens in the electrodes, this primarily concerns knowledge of the electrode state. Battery research currently employs two distinct types of methods for electrode analysis. Post-mortem methods refer to techniques that are used on cells after disassembly. They may yield information on chemical aging processes such as lithium plating, delamination and decomposition of the current collectors. Common examples include scanning electron microscopy, 2 X-ray diffractometry, 3 mass spec-

ACS Paragon Plus Environment

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troscopy 4 and visual inspection. In contrast, in situ methods can be performed during cell operation. Electrical methods such as voltage and current measurements are practically always used, as they are needed for cell operation. More sophisticated methods include electrochemical impedance spectroscopy, 5,6 in situ neutron scattering, 7,8 electrochemical-acoustic time of flight analysis, 9 in situ inside-out magnetic resonance imaging 10 and in situ Raman spectroscopy. 11 Optical observation of battery electrodes during cycling has already been reported in the literature. For this, battery test cells with an optical window are usually used in combination with digital image recording or video microscopy. 12 For lithium ion batteries, research has only been published on the anode, where a distinct color change is observable during charge/discharge cycles. 13 Here, the intercalation of lithium ions into the graphite layer leads to a color change that spans black, blue, red and gold. 14

an optical window, we already proposed the use of an optical fiber embedded in a regular LFP cathode and covered with an LFP/ITO mixture to measure a light signal, which depends on the transferred charge into and from the battery. Expanding the charge interpretation, a SOC determination is easily feasible. Nevertheless, the physical and chemical origin of the effect was not yet clearly determined. Therefore, in this paper we explicitly address the electrochemical behavior of ITO, performing in a double role as a conductive agent and an electrochromic marker in the LFP electrodes. Finally, we think that the correlation between the charge and the optical signal obtained while observing the cathode through an optical test cell or via an optical fiber share at least a similar origin and does not depend on the actual investigation method. The latter enables the opportunity to use the presented optical effect for state of charge determination.

Problem to be Solved. So far, similar investigations for cathodes are not published in the literature, as additives in the cathodes impede optical observation. This is particularly true for lithium iron phosphate (LFP) cathodes, where carbon black is commonly added as a conductive agent to mitigate LFP’s low conductivity. As the carbon is optically black, in situ observation using video microscopy is hindered. In a limited sense, this is also true for Raman spectroscopy. 11 In order to overcome this problem, we replaced carbon black with a conductive agent that is also transparent. Within the class of transparent conductive oxides, indium tin oxide (ITO) was chosen for this task. ITO is commonly used as conductive layer for electrochromic windows and for transparent wiring matrices in flat displays, 15 but is not currently used in energy storage materials. Our approach of replacing carbon with ITO was presented in a previous publication. 16 There, we focused on potential applications of the correlation between transferred electrical charge and optical reflectivity of the electrode. Since a commercial cell will not be covered with

Materials Preparation. The cathodes described in this paper were created from a slurry consisting of LFP nanoparticles (≈200 nm) with a thin carbon coating (Südchemie), ITO nanoparticles (≈50 nm), as well as PVDF dissolved in NMP (all three Sigma-Aldrich) as a binder. The slurry was then coated onto an aluminum foil using a doctor blade setup set to 200 µm slurry thickness. The LFP/ITO cathodes were then left to dry and heated. Ratios for different LFP/ITO mixtures described later on, i. e. 75% LFP and 15% ITO, are given in weight percent. Finally, the cathodes were introduced into a glovebox for cell assembly. A 1 M lithium hexafluoride EC/DMC solution (Sigma-Aldrich) was used as the electrolyte. In our setups, lithium metal was used as the anode to remove potential effects from the intercalation stages of graphite electrodes. The chemical reaction in the cathode during charge and discharge is described as

2

Experimental Section

LiFe(II)PO4 Fe(III)PO4 + Li+ + e−

. (1)

This means that during charge, lithium ions are


2

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carried out by in-house developed hard- and software solutions. The electrical charge was then obtained via current integration known as coulomb counting. The test cell was illuminated with white LEDs offering a broad distribution of wavelengths in the visible spectra but also showing a peak in the blue part of the spectrum. The reflected light from the cathode was then measured with a USB microscope camera in terms of intensities of the color channels red, green and blue of the camera sensor. Additionally, the whole measurement setup were placed in a box to suppress the influence of ambient light. Post-processing of the images was performed with MATLAB. 18

removed from the cathode. During discharge, lithium ions are inserted into the cathode. In preliminary tests, the optical changes in the graphite electrode during charge described in the literature 1,13,14 were also reproduced in our test cell setup. Test Cells and Electrode Setup. The battery test cell was placed in an ECC-Opto-Std (EL-Cell GmbH) with a window diameter of 6 mm. Common borosilicate glass was used as window glass, which allowed for an optical observation of the battery test cell during cycling with visible light. In contrast to the common cell geometry, our electrodes are slightly differently arranged to directly investigate the LFP/ITO cathode (see Figure 1). Although the cathode becomes anode during charging, we will refer the LFP/ITO electrode to be the cathode as it is usually done throughout the battery material literature. Apart from the chosen geometry, a more regular geometry could be investigated as well, where ITO-coated glass has to be used as electrode current collector instead of aluminum. 16 Finally, various other electrode arrangements are possible, which have been discussed previously. 17

Illumination source

3 3.1

In Situ Electrode Observation

Video Microscopy. In situ video microscopy was a central method for investigating the LFP/ITO cathode. Image analyses was mainly performed in two ways. First, integral image analysis was used, where the reflectivity of the whole electrode was averaged and compared against the transferred charge. As a result, a very good correlation between reflectivity and charge was found (see Figure 2). Second, spatially resolved image analysis was applied to determine the paths of lithium intrusion into the cathode. Image processing was used to find the resulting intrusion fronts (see Figure 3). In the window test cells, a characteristic intrusion of lithium ions into the electrodes is observable during cell discharge. This intrusion generally works its way from the cathode-separator interface towards the cathode center (gray arrows). During charge, this process is reversed starting again from the interface (white arrows). Information on the ion pathways within the electrode could be collected by observing several material combinations in various geometric setups (not shown). In the most simple setup, an electrode (anode or cathode) is directly observed from the top as shown in Figure 1. It is also possible to observe two electrodes simul-

USB microscope camera (resp. Raman microscope)

Cell lid

Results and Discussion

Glass window LFP cathode Current collector Separator Lithium anode Current collector

Electrode contacts

Figure 1: Left: Arrangement of the test cell and measurement setup for electrode observation using a camera or Raman microscope. Right: Electrode setup within the test cell. The cathode is observable through a glass window from the top of the test cell.

Electrical and Optical Measurement Setup. The monitoring of the cycle plan as well as measuring voltage and current were both


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taneously side-by-side. In this case, the two electrodes can be anode and cathode, i. e. an LFP/ITO electrode and a graphite electrode, or both of them can be either anode or cathode, i. e. testing two cathode mixtures simultaneously. If this is the case, both electrodes are electrically connected and cycled with a common counter electrode. The latter case will be used to investigate the influence of ITO in the LFP/ITO cathode.

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Figure 2: Left: Charge and reflectivity of the optically observed LFP/ITO cathode during charge and discharge cycles. Right: Different illustration of the data from the left figure to show the strong dependency between the charge and electrode reflectivity.

Figure 3: Illustration of the observed optical effect in the window test cell. Top figures show contrast enhanced real images from the LFP/ITO cathode during discharge (a-c) and charge (d-f) while the bottom figures show related sketches. The discharged electrode area (dark gray, 1), the area that is not yet discharged (gray, 2) and the aluminum current collector (light gray, 3) are marked with numbers. The evolving dark and light gray areas at each step, which are related to lithium intercalation and deintercalation, are highlighted with arrows and reveal that the intercalation and deintercalation process starts from the border of the cathode.

Separation of LFP and ITO Effects. To examine the origin of the reflectivity change of the LFP/ITO cathode during cycling we built a battery test cell with two cathodes (top images Figure 4). In this cell, an LFP/ITO cathode and an cathode made from pure ITO were placed side-by-side sharing the same current collector and hence, having the same electrical potential. As seen in Figure 4, the LFP/ITO side (green rectangle) shows the well known optical effect during cycling as found for the single LFP/ITO cathode in Figure 3. Due to the electrode geometry with the underlying current collector, the ion intercalation and deintercalation is mostly confined to the electrode-electrolyte interface. The strong correlation between the reflectivity and the transferred charge is also clearly found as shown in the bottom figure and the reflectivity is stable during rest times (a, c). The optical behavior of the pure ITO side (red rectangle) is completely different. Starting in

a rest period (a) with the cell voltage being above ≈ 2.5 V the electrode is almost transparent with a metallic glance originating from the underlying aluminum foil, which results in a high reflectivity. When the cell is discharged and the voltage drops below ≈ 2.5 V the electrode suddenly changes from clear transparency to a distinctive black color leading to a reduced reflectivity (b). Upon resting, i. e. increasing cell voltage, the color change is reverted (c). In addition and different from the LFP/ITO side, the color change happens at the whole ITO electrode side simultaneously rather than solely at the electrode border. Therefore, we think that the pure ITO side itself obeys almost no ion storage capacity since a lithium insertion into the ITO would take longer and the full cell voltage is dominated by the LFP/ITO side only. Although no relation between the reflectivity of the pure ITO side and the cell charge was found, a dependency between the reflectivity and the cell voltage could be clearly resolved. To determine the role of ITO within the


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eas and differ also from the overall cell potential. Nevertheless, they are stable at each cycle step, which allows for the voltage-dependent reflectivity change of the ITO to take place. This enables the observation of the distinctive lithium intercalation fronts that progress into the cathode during charge and which are dissolved during discharge. The observed behavior of the lithiation fronts is in accordance with a microscopic description of the transport process of lithium ions between LFP nano crystals where the ions are redistributed during relaxation times to yield only completely lithiated and delithiated LFP crystals. 19–21 Raman Spectroscopy. To perform in situ Raman spectroscopy, the window test cells were placed inside a Bruker Senterra Raman microscope using a 534 nm green laser. The laser was aimed at a border region of the LFP/ITO cathode close to the separator. The cell was then charged and discharged, and Raman spectra were taken every five minutes (see Figure 5). To eliminate the baseline for each measurement and enhance the visibility of the peaks a differential spectra was calculated (not shown). In Figure 5, pink lines correspond to battery charging while blue lines refer to discharging and gray lines highlight the rest period, respectively. As one can see several reversible processes occur during cycling. The wave numbers 952 cm−1 , 1059 cm−1 and 1079 cm−1 were assigned to characteristic vibration modes of LFP, 11 while the wave numbers 175 cm−1 , 240 cm−1 and 307 cm−1 were assigned to ITO. 22 The lines in the gray marked area, i. e. in the range 1250 cm−1 to 1500 cm−1 , belong to the carbon components in the LFP/ITO cathode. 23 All assignments are summed up in Table 1. To comment on the evolving and vanishing signals during cycling, one characteristic wave number of each component, ITO (175 cm−1 ) and LFP (952 cm−1 ), is selected and shown separately in Figure 6. During discharge, the intensity of the LFP signal (red line) increases because of the formation of additional LFP when lithium ions are inserted into the electrode during the process (see Equation 1). The signal is reduced to the baseline during charge, when

Figure 4: Top: Images of two different cathodes consisting of pure ITO (red rectangle) and an LFP/ITO mixture (green rectangle) at selected cycle steps marked in the center and bottom figure according to (a), (b) and (c). Both cathodes share the same current collector. Center: Charge and reflectivity of the ITO cathode. Bottom: Voltage and reflectivity of the LFP/ITO electrode. The ITO case differs from the LFP/ITO case, as the reflectivity of the pure ITO electrode does not correlate with the estimated charge but is clearly influenced from the applied voltage. LFP/ITO cathode we state that replacing the common additive carbon black with the conductive agent ITO a optical observation of the LFP cathode is possible in general due to the substitution of a non-transparent material by a transparent one. Furthermore, we think that ITO enhances the optical effect due to the found electrochromic behavior and the strong voltage-dependent reflectivity of the ITO is locally affected within the LFP/ITO cathode. This means that during cycling the electrical potential varies within the cathode, especially in the regions where the lithium ions are intercalated into the ion storing LFP. Around the intercalation boundaries the potentials are different between charged and discharged ar-


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8 Intensity [a. u.]

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6 4 2 0

Charge Rest Discharge Rest Charge Rest Discharge Rest Charge Rest

0

200

400

600

800

1,000

1,200

1,400

1,600

Raman shift [cm-1]

Indium tin oxide

Lithium iron phosphate

Carbon

Figure 5: Raman spectra of the LFP/ITO cathode taken in 5 minute intervals during cycling. Regions that correspond to signals from ITO (yellow), LFP (red) and carbon (gray) are marked in the plot. Spectral lines related to charging and discharging processes as well as rest periods are highlighted (pink, blue and gray lines). Table 1: Active Raman modes visible in the spectra (compare Figure 5). Cause ITO LiFePO4 Figure 6: Charge and selected wave numbers from the differential Raman spectrum of the LFP/ITO cathode based on the spectra in Figure 5. Shown are the wave numbers assigned to ITO (175 cm−1 ) and LFP (952 cm−1 ) according to literature. 11,22 The ITO and LFP signals are clearly opposite in phase with each other. The reason for the instability of both signals during rest periods (constant charge) are addressed in the text.

Carbon

Wave number 175 cm−1 , 240 cm−1 (Sn-O), 307 cm−1 (In2 O2 ) 22 952 cm−1 (FePO4 ν1 (PO4 )), 1059 cm−1 , 1079 cm−1 (FePO4 ν3 (PO4 )) 11 1250 cm−1 to 1500 cm−1 23

and decreases during discharge. Unlikely, this is not expected and we were not able to resolve this behavior. Showing anti-cyclic behavior with respect to LFP (952 cm−1 ), the intensity of the ITO signal (blue line) is increased during charge indicating that In2 O3 is build in the process according to In2 O3 + 6Li+ + 6e− 2In + 3Li2 O as supposed by Bressers and Meulenkamp. 24 This interaction between lithium and ITO further implies the formation of metallic indium. The latter can explain the observed change of the pure ITO electrode to metallic black during discharge (compare Figure 4b) and adds to the given explanation for the reflectivity change in the LFP/ITO cathode before. We assume the unstable ITO and LFP Raman signals during the rest period after charging originate from a starting relaxation process that involves both

the chemical reaction is reverted and LFP is oxidized to iron phosphate. In the cycle phases where the battery is at rest and no current is applied, i. e. the charge is constant, the LFP signal after discharge is stable as expected. Unlikely, it is unstable during the rest periods after charge. Additionally, the signal is anti-cyclic with respect to the ITO signal, which might explain the behavior due to an interaction between ITO and LFP. The remaining two wave numbers of LFP show an opposite behavior, where the LFP signal increases during charge


6

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ITO as well as LFP. Unfortunately, we were not able to investigate this up to now.

3.2

Post Mortem Analysis

Scanning Electron Microscopy. To investigate the macroscopic makeup of the material, scanning electron microscopy was used. The images in Figure 7 show the individual components LFP (top left) and ITO (top right) as well as the LFP/ITO mixture (bottom). The LFP particle size is roughly homogeneous and shows approx. 200 nm on the long axis and 100 nm on the short axis. The nanoscale ITO crystals range in size from below 50 nm up to 100 nm. Mixing both components has almost no effect on the particle sizes (bottom) and the smaller ITO crystals (green/white) are covering the larger LFP particles (red). This leads to an increase in conductivity similar to the known increase of conductivity due to the usual addition of carbon black to LFP as conducting agent, which fills the space between the LFP particles. 25,26 Finally, the binder can also be discerned in the bottom image (gray).

Figure 7: Top left: LFP nanoparticles in PVDF binder. Top right: Indium tin oxide particles in PVDF binder. Crystal structures of various sizes are visible. Bottom: Mixture of 71% LFP and 26% ITO (rest is binder).

X-Ray Diffraction. XRD analysis was performed on pure ITO and LFP electrodes as well as on an LFP/ITO cathode (see Figure 8). An additional XRD measurement was done on a single aluminum foil to take into account signals from the current collector. The LFP/ITO cathode was disassembled at an intermediate charged cell state being not completely charged or discharged. Hence, a comparison of the spectra with results from in situ XRD measurements, 27–29 which spans the whole range from charged to discharged state, shows that signals from the FP as well as from the LFP phase are found. Composing the peaks for aluminum (gray), ITO (blue) and LFP (red), all major signals found in the LFP/ITO cathode (black) are reproduced showing that no further substances besides the intended ones are present in the investigated LFP/ITO cathode.

3.3

Variation of Cathode Composition. Plausibility tests were carried out to further verify the use of the marker material ITO in LFP battery cathodes for comparability. This includes a variation of the ratio of marker and storage material. The effect of different compositions on cell capacity and internal resistance was investigated. For this purpose, cells with different mixing ratios of ITO, LFP and carbon were setup and cycled with the ECC PAT-Cell (manufacturer EL-Cell GmbH) instead of the optical test cell. We examined square samples with an area of one square centimeter. Discharge capacity and internal resistance were recorded in two cells with two discharges each. The averaged result is displayed together with the sample spread in Table 2. The increase in the discharge capacity with a reduction in the ITO content and thus the use of more LFP active material is clear from the data. Remarkably, the capacity of the material with 75% LFP and 15% ITO is very close to the

Root Cause Discussion


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charge process. The measurements with pure ITO and pure LFP cathodes show a discharge capacity close to zero and almost no energy could be stored and retrieved from these test cells. In the case of the pure ITO cathode, this is due to the low storage capacity for lithium ions as discussed before. The discharge capacity of the LFP cathode made without ITO or carbon is similarly small because the internal resistance is very high even though the ionic storage capacity is large.

Figure 8: XRD spectra of the aluminum foil substrate (gray), ITO cathodes (blue) and LFP cathodes (red) as well as a mixture of 71% LFP and 26% ITO. Except for the substrate, all samples include PVDF binder. Symbols correspond to ITO (), LFP (♦) and Aluminum (∗).

Effect Cause Hypotheses. To sum up, the cell setups used an unconventional electrode arrangement in which the direct and spatial contact of a large part of the active material is disjoined from the separator by the current collector and the movement of lithium ions is only possible through the border of the cathode. Scanning electron microscopy and X-ray diffraction yielded plausible results, but offer no contributions to explain the observed optical effect. Raman spectroscopy was used in situ revealing that the intensities of the LFP and the ITO Raman shift are both dependent on the electrical charge transferred into the battery. Together with the results from the video microscopy measurements the cause of the optical effect in the LFP cathode with the ITO marker is discussed by the following hypotheses: First. A change in electrolyte state or passive cell components such as the binder or current collector as a result of applied voltage or chemical side reactions could be ruled out. Comparative measurements on blank materials (bare aluminum, pure binder on aluminum) have not shown any measurable optical changes during cycling. Second. Apart from our investigations a difference in color between LFP (gray) and iron phosphate (yellow-brown) is well known in the literature. 30 As charge and discharge in the battery correspond to a transformation between LiFePO4 and FePO4 , a change in color is expected. In fact, measurements on a commercial LFP+C cathode without ITO showed an optical effect, which, however, was very weak. This is presumed to be due to the carbon ad-

Table 2: Parameters of several cathodes with selected compositions of ITO, LFP and C, which are given in weight percent. 10% of each composition is PVDF binder. Composition LFP/ITO/C % /% /% 90 / 0 / 0a 55 / 35 / 0 65 / 25 / 0 75 / 15 / 0 0 / 80 / 10b 0 / 90 / 0c

Capacity mAh 0.001 0.647 1.061 1.379 1.583 0.001

Coulomb efficiency % 55.9 96.2 94.8 95.4 57.2

Resistance Ω 45.26 68.89 53.28 96.56 64.73 207.06

a

No ion storing LFP was used. For that reason, the discharge capacity is negligible. b Commercial LFP material with added carbon. c No additional conducting agent was added, leading to severely limited electron transport and very low usable capacity.

capacity of the commercial electrode material (80% LFP and 10% C), in which carbon is used as a conducting additive. The mixtures with 15% and 25% ITO have Coulomb efficiencies similar to values reported in the literature for commercial cells. The coulombic efficiency CE of a battery is calculated as CE =

Qout Qin

,

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(2)

where Qin and Qout are the electrical charges being stored in and extracted from the battery during a charge/discharge cycle. Hence, CE yields information about losses during the


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ditives hindering optical observation. We conclude that this effect contributes partially to the observed change in reflectivity. Third. We confirmed that ITO possesses a voltage-dependent optical reflectivity in the presence of lithium ions, even if no ion storing LFP is present in the electrode. The chemical reaction In2 O3 + 6Li+ + 6e− 2In + 3Li2 O supposed by Bressers and Meulenkamp 24 involves the appearance of metallic indium, which can explain the observed change of the ITO electrode from almost transparent to metallic black at a certain voltage threshold level while the battery is cycled. Considering the results from the Raman spectroscopy of the LFP/ITO cathode, which show an enhanced signal of ITO during charging, we suppose that similar reactions take place within the LFP/ITO cathode at those regions, where the electrical potential locally enables the switching of the discovered voltage-dependent reflectivity. Again, those regions correspond to the lithium ion intercalation fronts in the LFP/ITO cathode. Based on the given hypotheses, we assume that the discovered effect has its origin in the combination of the voltage-dependent reflectivity of the ITO in the presence of lithium ions and an interaction between ITO and LFP, which allows for the local potential environment in the cathode where the reflectivity change can take place. As ITO plays a crucial role in this chemical setup, it has to be understood as a marker material for the transferred electrical charge into the LFP cathode.

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the cathode components. In situ Raman spectroscopy was used to analyze chemical processes within the LFP/ITO cathode during cycling. Overall, we found a strong voltage-dependent reflectivity of ITO, which transfers locally into the LFP/ITO cathode enabling the measurement of a changing reflectivity and additionally allows for a clear observation of lithiation fronts in the battery cathode. The presented effect can aid future materials research by means of direct and macroscopic access to information on ionic transport within the electrode. We further think that the effect might be reproducible with other common cathode materials and the use of the much cheaper fluorine doped tin oxide replacing ITO is also conceivable. Finally, observing the cathode with an optical fiber instead of video microscopy should allow for a commercial state of charge determination in lithium ion batteries. Acknowledgement The work presented was part of the German-Chinese project "SinoGerman Electromobility Research" (SINGER) funded by the German ministry BMVI (grant 03EM0204E). It was partially conducted at the Peking University Graduate School Shenzhen. Initial concept investigations were supported within the graduate school ’Key Technologies for Sustainable Energy Systems in Smart Grids’ lead by the University of Hamburg and the Hamburg University of Applied Sciences.

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Conclusions

In this paper, we present the discovery and explanation of an optical effect in a lithium iron phosphate cathode with added indium tin oxide by means of a reflectivity change of the cathode while the battery is charged and discharged. Optical observation with video microscopy was made possible using the transparent conductive agent ITO, which surprisingly revealed to act as an electrochromic marker in addition. SEM and XRD were used to measure the sizes of the LFP and ITO nanoparticles and to clarify

(2) Chen, G.; Song, X.; Richardson, T. J. Electron Microscopy Study of the LiFePO4 to FePO4 Phase Transition. Electrochemical and Solid-State Letters 2006, 9, A295–A298. (3) Liu, X.; Wang, D.; Liu, G.; Srinivasan, V.;


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Electrochromic effect of indium tin oxide in lithium iron phosphate

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