Application of In Operando UV/Vis Spectroscopy in Lithium Sulfur Batteries

Transcription

1 DOI: /cssc Application of In Operando UV/Vis Spectroscopy in Lithium Sulfur Batteries Manu U. M. Patel and Robert Dominko* [a] Application of UV/Vis spectroscopy for the qualitative and quantitative determination of differences in the mechanism of lithium sulfur battery behavior is presented. With the help of catholytes prepared from chemically synthesized stoichiometric mixtures of lithium and sulfur, calibration curves for two different types of electrolyte can be constructed. First-order derivatives of UV/Vis spectra show five typical derivative peak positions in both electrolytes. In operando measurements show a smooth change in the UV/Vis spectra in the wavelength region between l =650 and 400 nm. Derivatives are in agreement with derivative peak positions observed with catholytes. Recalculation of normalized reflections of UV/Vis spectra obtained in operando mode enable the formation of polysulfides and their concentrations to be followed. In such a way, it is possible to distinguish differences in the mechanism of polysulfide shuttling between two electrolytes and to correlate differences in capacity fading. Introduction The advantages and disadvantages of sulfur as a cathode material in lithium sulfur batteries have been thoroughly discussed. [1] In the last decade, there has been a significant improvement in lithium sulfur battery performance owing to the development of new types of cathode materials, [2] electrolytes, [3] anodes, [4] and sophisticated separators. [5] There has been an overall development of the whole system rather than the development of just one individual component of the system, which has led to better performing lithium sulfur batteries. [6] In addition to the known problems of lithium sulfur batteries, there has been poor understanding of the system as a result of the lack of proper analytical techniques, and this has slowed down the development of lithium sulfur batteries. However, much credit has to be given to recently developed analytical techniques for the latest developments in lithium sulfur batteries. Some important in situ analytical techniques that have led to better understanding of the system are synchrotron sulfur K-edge X-ray absorption spectroscopy, [7] UV/Vis spectroscopy, [8] X-ray diffraction and transmission X-ray microscopy, [9] Raman spectroscopy, [10] and the four-electrode Swagelok cell. [11] There has also been a considerable contribution from some semi-in situ or ex situ analytical techniques, including solid-state NMR spectroscopy, [7] X-ray photoelectron spectroscopy (XPS), [12] and UV/Vis spectroscopy, [13] towards lithium sulfur battery development. However, even after the development of such advanced analytical techniques, it has not been possible to reach the full potential of the lithium sulfur system. One of the main reasons behind this is the absence of a universal procedure for making [a] M. U. M. Patel, Dr. R. Dominko Laboratory for Materials Chemistry National Institute of Chemistry Hajdrihova 19, SI-1000 Ljubljana (Slovenia) Robert.Dominko@ki.si lithium sulfur batteries. Each research group contributing towards the development of lithium sulfur batteries has its own method of preparing composites, electrolytes, and separators, and using them as cell components. We need to have analytical techniques that can easily detect even small differences in battery performance as a result of the changes made to its components. Furthermore, the development of analytical techniques has to be in this direction to obtain a lithium sulfur battery with favorable performance. Lithium polysulfide intermediates, which are formed during battery cycling, can be characterized by using UV/Vis spectroscopy. [8, 14] The basic or general interaction between UV/Vis electromagnetic radiation and molecules of polysulfides coming in contact can give rise to a number of phenomena, including fluorescence/phosphorescence (absorption and re-emission), reflection, scattering, absorbance, and photochemical reactions, depending upon the kinds of molecules under investigation. [15] Using this interaction as a foundation, we can use UV/ Vis spectroscopy as an analytical tool to monitor polysulfides in lithium sulfur batteries. The polysulfides formed are colored in nature and can be detected by UV/Vis spectroscopy in the battery separator based on the principle of Beer Lambert s law. [15] In our recent work, we studied interactions between UV/Vis radiation and lithium polysulfides in different catholyte solutions during cycling of the battery. [8] From the measured set of spectra obtained from catholytes with chemically synthesized stoichiometric mixtures of polysulfides, we showed that the polysulfides could be qualitatively and quantitatively determined. To be able to perform such measurements, we built an electrochemical cell based on the pouched cell configuration with a sealed glass cover. This configuration enabled us to detect polysulfides in the separator of the battery and to determine the analytical contents. [8] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

2 Herein, we report a procedure for the quantitative and qualitative determination of polysulfides in lithium sulfur batteries by using in operando UV/Vis spectroscopy. A well-developed procedure is presented and explained in detail. It is based on the recalculation of UV/Vis reflections at selected wavelengths into polysulfide concentrations. To do this, we performed measurements of chemically synthesized stoichiometric mixtures of polysulfides to obtain calibration curves. We applied the developed procedure to the study of capacity fading in two different types of electrolyte. Results and Discussion Electrochemical characteristics of the lithium sulfur cells (capacity retention and degradation) are largely influenced by the type of electrolyte. The observed differences are related to the solubility and mobility of polysulfides in electrolytes. [14] Herein, we selected two standard electrolytes that were studied with cathodes prepared by infiltrating sulfur into Printex XE2 carbon black. The capacity retention during cycling is presented in Figure 1. Batteries cycled with 1 m LiTFSI in sulfolane electrolyte showed a higher initial capacity, which was followed by faster capacity fading. A much slower capacity drop was observed with a battery cycled with 1 m LiTFSI in TEGDME:DOL electrolyte. This battery showed a higher coulombic efficiency. Because the sulfur cathodes are from the same batch and both batteries contain the same amount of electrolyte normalized per amount of sulfur, we can ascribe the observed difference to the mechanism during charge and discharge. To understand this mechanism, we used UV/Vis spectroscopy in operando mode for the determination of processes leading to different coulombic efficiency and capacity-fading characteristics. As shown in our previous work, polysulfides can be quantitatively and qualitatively determined in the separator by UV/ Vis spectroscopy, and by using a proper calibration we can distinguish between long-, mid-, and short-chain polysulfides, as well as determine their concentrations. [8] Interactions between polysulfide molecules and UV/Vis electromagnetic radiation depend on the length of the polysulfide chain, on the alkali metal, and on the solvent in which polysulfides are dissolved. This is demonstrated with two sets of photographs taken on the catholytes prepared by dissolving different stoichiometric mixtures of sulfur and lithium (Figure 2). The photographs in Figure 2 show a visible difference, namely, long-chain polysulfides absorb light at higher wavelengths (towards the IR region) and short-chain polysulfides adsorb light closer to the UV part of the spectrum. Interactions between polysulfides and the electrolyte can easily be monitored by using UV/Vis spectroscopy, the results of which generally show broad absorption bands due to the presence of p bands in the range between l = 185 and 1000 nm. [16] Based on the observed differences in the color of catholytes, changes in the UV/Vis spectra in the range of l =650 to 400 nm can be expected. Figure 1. Cycling behavior of a lithium sulfur composite in two different electrolyte systems. TEGDME:DOL=tetraethylene glycol dimethyl ether:1,3- dioxolane. Figure 2. Photographs of six different catholyte solutions in both electrolytes used in this study (the stoichiometric ratio between lithium and sulfur is presented at the top of each picture column). Measurements of catholyte solutions For the purpose of calibration, we synthesized polysulfides with different stoichiometric mixtures of lithium and sulfur. We used these stoichiometric mixtures of polysulfides to prepare a set of catholyte solutions with six different concentrations for both electrolyte systems. Glass-fiber separators were wetted with 0.1 ml of each catholyte solution, sealed in a cell with a window and UV/Vis spectroscopy was performed. Figures 3 and 4 show the UV/Vis spectra and their first-order derivatives for two different concentrations, namely, 10 and 50 mm. Other concentrations showed a similar trend. As expected from the colors of the prepared catholytes, dissolved lithium polysulfides with longer chains (mixtures with a higher stoichiometric ratio between sulfur and lithium) showed reflectance towards the longer wavelengths of the UV/Vis spectrum. With lower ratios between lithium and sulfur, the positions of the UV/Vis spectra were shifted towards shorter wavelengths. To find a methodology to discriminate between different catholyte solutions, we focused on the position of the first derivative of the UV/Vis spectra. As presented in Figures 3 c and d and 4c and d, the derivatives show a characteristic position for each stoichiometric composition for both concentrations presented (similar trends were also observed for other concentrations). We determined five characteristic wavelengths for both analyzed electrolyte systems. The catho Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

3 Figure 3. UV/Vis spectra of different stoichiometric mixtures between lithium and sulfur for a) 10 and b) 50 mm concentrations in 1m LiTFSI in sulfolane electrolyte and the corresponding first-order derivative curves (c and d). lyte solution in 1 m LiTFSI in sulfolane electrolyte showed the maxima of derivatives at l =570 nm, which could be ascribed to long-chain polysulfides because the stoichiometric ratio between sulfur and lithium was 4:1. Maxima at l = 550 and 510 nm were ascribed to mid-chain polysulfides (the stoichiometric ratio between sulfur and lithium was between 3:1 and 2:1) and maxima at l =490 and 470 nm could be ascribed to short-chain polysulfides (the stoichiometric ratio between sulfur and lithium was 3:2 or 1:1). A similar trend was observed with catholyte solutions in 1 m LiTFSI in TEGDME:DOL electrolyte with a difference in the position of derivative peaks. We found the derivative for long-chain polysulfides at l = 620 nm, derivatives for the mid-chain polysulfides at l = 580 and 550 nm, and derivatives for short-chain polysulfides at l = 520 and 450 nm. For simplicity of data management, we selected the closest point measured by the spectrometer, and so typical measurements in all our experiments were performed every 10 nm. As we demonstrated in our recent work, [8] the measured reflections of polysulfides were linearly correlated with the natural logarithm of the concentration of the polysulfides. It should be noted that the intensity of the measured reflectance in the UV/Vis spectra is a function of the position of the cell in the UV/Vis spectrometer, the wettability and quantity of the electrolyte in the cell, the type of cathode composite, and so forth; thus all measured spectra have to be normalized for comparison. At the selected wavelengths (determined from the firstorder derivatives in Figure 3c and d and Figure 4 c and d), we obtained normalized values of reflectance for six different concentrations (1, 2.5, 5, 10, 25, and 50 mm). The obtained values were correlated to the natural logarithm of the concentration and a linear correlation was observed for each selected wavelength. Figures 5 and 6 present the dependences of the measured normalized reflectance with concentration; the equations represent the basis for the recalculation of the concentration within the in operando measurements. In operando measurements The configuration of the battery enabled simultaneous measurements of UV/Vis spectra without stopping the battery and even without taking any part of the electrolyte out of the battery. [8] Spectra were measured in the range between l =2000 and 250 nm during battery operation. Figures 7 and 8 show the measured UV/Vis spectra from the lithium sulfur batteries in the two different types of electrolytes used herein. The first spectra taken during discharge showed almost no change in the reflection up to l = 350 nm; this is in agreement with 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

4 Figure 4. UV/Vis spectra of different stoichiometric mixtures between lithium and sulfur for a) 10 and b) 50 mm concentrations in 1m LiTFSI in TEGDME:DOL electrolyte and the corresponding first-order derivative curves (c and d). measurements of the cell with pure electrolyte. Owing to the formation of soluble polysulfides during discharge, the absorption is shifted to higher wavelengths, which is in agreement with the predicted formation of long-chain polysulfides. Continuous discharging shifted the absorption curve towards shorter wavelengths; this indicates the formation of short polysulfides (shift of colors in Figure 7a and 8 a from red to green to blue). The opposite shift of the absorption curve could be observed during charging, as seen in Figures 7 b and 8 b; again, a change of colors from red to green to blue indicates a shift of the spectra during the charging process. An even clearer picture of the dynamics within the separator can be obtained from the first-order derivatives (Figures 7 c and d and 8 c and d) of the measured UV/Vis spectra. A continuous shift of the derivative peaks from long to short wavelengths can be observed in both electrolytes. We focused on the wavelength range between l = 650 and 400 nm because we observed changes in the derivative peak position in that region. The major positions of derivative peaks were in agreement with peaks observed from the derivatives of catholyte solutions (Figures 3c and d and 4 c and d). Although the measured UV/Vis spectra showed a significant difference between the two electrolytes, our focus was on the qualitative and quantitative determination of polysulfides in the separator during in operando measurements. Deconvolution of in operando mode UV/Vis spectra Correlations between concentrations and normalized reflectances obtained from measurements with different stoichiometric equilibria (Figures 5 and 6) were used for the development of a simple procedure that allowed a direct comparison between different sets of measurements. The procedure was as follows: 1) From the measured spectra, we collected the intensity of the reflections for each UV/Vis spectrum at preselected wavelengths (as determined in Figures 3 and 4). We focused on the five different wavelengths obtained from the derivatives of catholyte solutions. 2) The collected intensities were normalized according to the reflectance of the first spectrum (the intensity of the normalized first spectrum is normalized equal to one). 3) The normalized intensities at shorter wavelengths were then subtracted by longer wavelengths (for example, the column with the normalized intensities at l = 570 nm was taken as that measured). Intensities at l = 550 nm were subtracted from the normalized intensities at l =570 nm (I* 550nm =I 570nm I 550nm ). Furthermore, intensities at l = 510 nm were subtracted from the normalized intensities at 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

5 Figure 5. Linear fits of the normalized intensities measured from catholyte solutions in the 1m LiTFSI in sulfolane electrolyte with different concentrations of short- (l=470 (a) and 490 nm (b)), mid- (l=510 (c) and 550 nm (d)), and long-chain polysulfides (l=570 nm (e)). l = 570 nm and the recalculated intensities at l = 550 nm (I* 510nm =I 570nm I* 550nm I 510nm ). Similar calculations were performed for the calculation of intensities at l =490 (I* 490nm = I 570nm I* 550nm I* 510nm I 490nm ) and 470 nm (I* 470nm = I 570nm I* 550nm I* 510nm I* 490nm I 470nm ). Wavelengths used in the explanation are for 1 m LiTFSI in sulfolane electrolyte. 4) Values obtained as explained under point 3) were used for the calculation of concentrations of different types of polysulfides by using the relationship between the normalized reflectance and the concentration, as determined in Figures 5 and 6. Figure 9 shows a comparison of the polysulfide evolution in the separator for both electrolyte systems. Batteries were measured at a cycling rate of C/20 during the first discharge and charge cycle. Typical electrochemical curves with capacities of around 800 mah g 1 were measured. Deconvolution of the UV/ Vis spectra measured in the 1 m LiTFSI in sulfolane electrolyte showed saturation of the electrolyte with long-chain polysulfides (recalculated concentration from the intensity at l = 570 nm). A maximum concentration of about 30 mm was achieved at the beginning of the low-voltage plateau. With a continuous discharge, we observed a decrease in the concentration of long-chain polysulfides and an increase in concentration of mid-chain polysulfides (recalculated concentrations from the intensities at l = 550 and 510 nm). The concentration of mid-chain polysulfides reached a maximum value ( 5 mm) in the middle of the low-voltage plateau, probably due to conversion of long-chain polysulfides into shorter chains. With the maximum of mid-chain polysulfides, we noticed an increase of short-chain polysulfides (recalculated concentrations from the intensities at l =490 and 470 nm), reaching a maximum value 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

6 Figure 6. Linear fits of the normalized intensities measured from catholyte solutions in the 1m LiTFSI in TEGDME:DOL electrolyte with different concentrations of short- (l=450 (a) and 520 nm (b)), mid- (l = 550 (c) and 580 nm (d)), and long-chain polysulfides (l=620 nm (e)). of about 2 mm at the end of the discharge. We observed a mirror-like evolution of polysulfides in the separator during the charging process with a difference in the concentration of long-chain polysulfides at the end of the charge. Remaining long- and mid-chain polysulfides in the separator indicated incomplete reoxidation of polysulfides into sulfur and they could be correlated to the difference between charge used for reduction and charge used for oxidation. Deconvolution of the UV/Vis spectra measured in the 1 m LiTFSI in TEGDME:DOL electrolyte showed different evolution of polysulfides than that of evolution in sulfolane-based electrolytes. We observed an increase in concentration of longchain polysulfides (recalculated from the intensity at l = 620 nm) at the beginning of the discharge cycle, reaching a maximum at the end of the high-voltage plateau. The recalculated concentration was 15 mm. By decreasing the concentration of long-chain polysulfides, we detected a very small amount of mid-chain polysulfides in the separator (intensities at l = 580 and 550 nm). A much higher concentration of shortchain polysulfides was observed (intensities at l = 520 and 450 nm). An increase in the short-chain polysulfide concentration recalculated from the intensities at l = 450 nm started with a decrease in the concentration of long-chain polysulfides. The maximum concentration of short-chain polysulfides during discharge was equivalent to the concentration of longchain polysulfides. With a continuous discharge at the lowvoltage plateau, polysulfides were consumed and at the end of discharge we detected a concentration of approximately 15 mm of short-chain polysulfides in the separator. During charging, the concentration of polysulfides detected in the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

7 Figure 7. UV/Vis spectra measured in operando mode during the first cycle of the lithium sulfur battery with 1m LiTFSI in sulfolane electrolyte: a) all spectra measured during discharge, b) all spectra measured during charge, and c,d) the corresponding first-order derivatives of the UV/Vis spectra (color in all figures changes from red to green to blue). separator was not exactly the same as that during discharging; however, we were able to detect the higher formation of short-chain polysulfides, which was accompanied by the formation of long-chain polysulfides. At the end of charging, short-chain polysulfides were consumed completely and the concentration of long-chain polysulfides was approximately the same as that in the experiment in which the battery was cycled in sulfolane-based electrolyte. Based on the different mode of polysulfide formation in the electrolytes used in this study, we can correlate capacity fading with differences in the formation of long-chain polysulfides. The higher solubility found in the sulfolane-based electrolyte can lead to higher irreversible loss of active material on the surface of metallic lithium. We correlate this with faster capacity degradation in the battery assembled with 1m LiTFSI in sulfolane electrolyte. To our surprise, we detected a relatively high concentration of short-chain polysulfides in the separator when 1 m LiTFSI in TEGDME:DOL electrolyte was used. This phenomenon could influence non-homogenous distribution of polysulfides on the surface of the electrode. Conclusions A procedure for the quantitative and qualitative determination of polysulfides from the measured UV/Vis spectra during in operando measurements was presented. The procedure relied on measurements of catholyte solutions prepared from polysulfides with different stoichiometric ratios of lithium and sulfur, which enabled us to recalculate intensities of in operando measurements. We systematically studied two different electrolytes and compared the evolution of polysulfides during the first discharge and charge processes in the studied electrolytes. Two major differences were observed. The electrode cycled in the 1 m LiTFSI in sulfolane electrolyte showed a higher concentration of long-chain polysulfides than that of the electrode cycled in the 1 m LiTFSI in TEGDME:DOL electrolyte. We attributed this higher concentration of long-chain polysulfides in the separator to the faster capacity degradation in the battery with 1 m LiTFSI in sulfolane electrolyte, which was connected with the irreversible reaction of long-chain polysulfides on the surface of lithium. The second difference between the studied systems was in the formation of soluble short-chain polysulfides, which was much higher in the case of the battery cycled with 1 m LiTFSI in TEGDME:DOL electrolyte. The appearance of short-chain polysulfides in the separator could lead to uncontrolled precipitation of the end discharge product on the surface of the electrode, and consequently, leave active material without efficient wiring Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

8 Figure 8. UV/Vis spectra measured in operando mode during the first cycle of the lithium sulfur battery with 1m LiTFSI in TEGDME:DOL electrolyte: a) all spectra measured during discharge, b) all spectra measured during charge, and c,d) corresponding first-order derivatives of the UV/Vis spectra (color in all figures changes from red to green to blue). Experimental Section Preparation of carbon black/sulfur composite: A ratio of 6:4 carbon black (Printex XE2) and sulfur was taken and ball-milled for 30 min at 300 rpm. The obtained mixture was then heated at 1558C for 6 h in an argon atmosphere to encapsulate sulfur in the carbon black (Printex XE2, Degusa). After cooling to room temperature, the samples were recovered and the content of sulfur was confirmed by elemental analysis (CHNS). Figure 9. Recalculated concentrations of short-, mid-, and long-chain polysulfides detected in the separator for both types of electrolytes compared herein. The left column corresponds to measurements in 1m LiTFSI in sulfolane electrolyte and the right column corresponds to measurements in 1m LiTFSI in TEGDME:DOL electrolyte. Preparation of electrodes and battery assembly: Electrodes from the composite were prepared by making a slurry of composite, polytetrafluoroethylene (PTFE), and carbon black in a ratio of 8:1:1 in isopropanol solvent. The slurry was then casted on the surface of aluminum foil by using the doctor blade technique. Electrodes with surface areas of 2 cm 2 contained 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7,

9 approximately 1 mg of sulfur per cm 2. Dried electrodes were used to assemble the battery. Lithium metal foil, with a hole that was half the size of the cathode, acted as the counter electrode and 1 m LiTFSI in sulfolane and/or 1 m LiTFSI in TEGDME:DOL were used as the electrolytes. A pouched polymer bag with a sealed glass cover was used to prepare the battery. [8] The configuration of the assembled cell was as follows: a glass-fiber separator was placed onto the surface of the cathode, and this separator was wetted with 60 ml of electrolyte per 1 mg of sulfur in the electrode. The lithium foil was placed on the separator and the sandwich was then sealed in a vacuum sealer in such a way that there was no lithium present between the glass cover and the cathode, which was present below the separator; therefore, the UV/Vis spectra could be obtained without any interference. The same settings were used for the calibration with standard catholyte solutions; however, in this case, only the separator wetted with 0.1 ml of catholyte was sealed in the cell. For the in operando measurements, the cell was attached to a UV/Vis spectrometer (PerkinElmer Lambda 950) in such a way that the UV light was directly focused on the glass cover of the cell; the whole setup was completely covered by a thick black plastic bag to avoid interference of light from the surroundings. After taking the initial spectra of the batteries, cycling of the batteries was started by using a SP-200 potentiostat/galvanostat (BioLogic). The batteries were run at a charging rate of one electron C/20 for one complete cycle. Meanwhile, UV/ Vis spectra were recorded every 15 min from the beginning of the discharge until the end of the charge cycle for both batteries with 1 m LiTFSI in sulfolane and 1m LiTFSI in TEGDME:DOL electrolytes. 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