Manganese in the SEI Layer of Li4Ti5O12 Studied by Combined

Article pubs.acs.org/JPCC

Manganese in the SEI Layer of Li4Ti5O12 Studied by Combined NEXAFS and HAXPES Techniques Tim Nordh,† Reza Younesi,*,† Maria Hahlin,‡ Roberto Félix Duarte,§ Carl Tengstedt,∥ Daniel Brandell,† and Kristina Edström† †

Department of Chemistry − Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden Department of Physics and Astronomy, Uppsala University, Box 516, 751 21 Uppsala, Sweden § Helmholz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany ∥ Scania CV AB, 151 87 Södertälje, Sweden ‡

ABSTRACT: A combination of hard X-ray photoelectron spectroscopy (HAXPES) and near edge X-ray absorption fine structure (NEXAFS) are here used to investigate the presence and chemical state of crossover manganese deposited on Li-ion battery anodes. The synchrotronbased experimental techniquesusing HAXPES and NEXAFS analysis on the same sample in one analysis chamberenabled us to acquire complementary sets of information. The Mn crossover and its influence on the anode interfacial chemistry has been a topic of controversy in the literature. Cells comprising lithium manganese oxide (LiMn2O4, LMO) cathodes and lithium titanate (Li4Ti5O12, LTO) anodes were investigated using LP40 (1 M LiPF6, EC:DEC 1:1) electrolyte. LTO electrodes at lithiated, delithiated, and open circuit voltage (OCV-stored) states were analyzed to investigate the potential dependency of the manganese oxidation state. It was primarily found that a solid surface layer was formed on the LTO electrode and that this layer contains deposited Mn from the cathode. The results revealed that manganese is present in the ionic state, independent of the lithiation of the LTO electrode. The chemical environment of the deposited manganese could not be assigned to simple compounds such as fluorides or oxides, indicating that the state of manganese is in a more complex form.

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known problem.4 A three-step mechanism for this process involving HF generation from LiPF6 followed by disproportion and subsequent dissolution of Mn by HF attack has been proposed,5,6 but the effect that the dissolved transition metal ionsparticularly manganesehave on the anode is still under debate. The investigations of manganese dissolution into the electrolyte, its crossover to the anode, and subsequent deposition started shortly after LMO emerged as a promising cathode material. In earlier studies, it was generally assumed that the manganese would be reduced to metallic Mn because of the difference in reduction potential, ca. 1.8 V (Mn2+/Mn) as compared to the 0.4 V of intercalating Li into graphite. However, the state of manganese in the anode solid electrolyte interphase (SEI) layer has over the years been a topic of discussion, where some researchers claim the existence of metallic Mn,7−12 others stat that it is ionic,13−16 and a third group suggests a two-step mechanism with reduction and subsequent oxidation.17,18 In these previous studies, a wide range of methods has been used to understand the role of manganese on graphite, but no consensus has yet been reached on the processes and the phases formed. To find the true mechanisms of capacity

INTRODUCTION As of today, many modern lithium-ion batteries (LIBs) still experience shortcomings in terms of power output and cell lifetime to be ideal, especially for heavy duty vehicles.1,2 For high current densities, the graphite anodeswell suited when high energy density is neededare associated with the risk of lithium plating and subsequent dendrite formation, which constitutes a battery safety hazard. In this context, lithium titanate (Li4Ti5O12, LTO) with an intercalating potential of 1.55 V vs Li/Li+ (hereafter all potentials are presented vs Li/ Li+) has been proposed as an alternative negative LIB electrode material. The higher intercalation potential would avoid lithium plating, and LTO has shown to have minimal volume expansion during cycling, making it a very robust anode material that allows for a very high number of cycles; e.g., cycling for over 1000 cycles with only small cell degradation in field tests with busses.3 The relative abundance of the required elements also makes LTO a good materials candidate due to production scalability and relative cost-effectiveness. The higher intercalation potential of LTO renders a safer battery, but at the cost of lost energy density. To mitigate this situation, one strategy would be the implementation of high voltage cathodes such as lithium manganese oxide (LMO) spinel and derivatives thereof, e.g., lithium manganese nickel oxide (LMNO) or lithium nickel manganese cobalt oxide (NMC). However, the manganese-based spinel materials exhibit transition metal dissolution, which constitutes a well© XXXX American Chemical Society

Received: December 1, 2015 Revised: January 21, 2016

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DOI: 10.1021/acs.jpcc.5b11756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Normalized capacity versus cycle number of cycled LTO|LMO samples. (b) Data collection points in the voltage profile of the cells.

foil current collector. Electrodes with a diameter of 20 mm were punched out of the foil and moved into an argon filled glovebox (H2O < 5 ppm, O2 < 1 ppm). The electrodes were then dried in a vacuum furnace for 12 h at 120 °C to remove water residuals. LMO electrodes were prepared in the same way, only substituting LTO for LiMn2O4. The electrodes were matched to generate a battery with 10% overcapacity of the anode with respect to the cathode. To a polymer separator (Solupor) with a diameter of 22 mm, 75 μL of an electrolyte consisting of 1 M LiPF6 (BASF) dissolved in ethylene carbonate (EC, BASF) and diethyl carbonate (DEC, BASF) in 1:1 volume ratio was added. The cell was vacuum-sealed in polyethylene-coated aluminum pouches, with aluminum current-collectors. Two cells were cycled galvanostatically for 30 cycles at a rate of C/5 using an Arbin BT-2043 battery testing system. One of the cells was then charged after the 30 cycles, making it a total of 30.5 cycles, thereby lithiating the LTO anode. The cells cycled for 30 cycles and 30.5 cycles are therefore called “delithiated” and “lithiated”, respectively. One reference cell was assembled in parallel but stored under open circuit voltage (OCV) in room temperature until spectroscopic analysis; this sample is referred to as the “OCV” sample. Also, a reference sample cell with graphite was prepared in the same manner, only substituting LTO for graphite (Toyo Tanso). HAXPES and NEXAFS measurements were conducted at the Helmholtz Zentrum Berlin, BESSY KMC-1 beamline, at the High Kinetic Energy Photoelectron Spectrometer (HIKE) end station. The HAXPES measurements were recorded using a Scienta R4000 electron analyzer and the NEXAFS by a Bruker XFlash 4010 fluorescence detector. The cells were opened in an argon-filled glovebox (H2O < 5 ppm, O2 < 1 ppm), and the electrodes were washed with dimethyl carbonate (DMC), cut and mounted onto a sample holder with copper tape. The sample holder was transferred from the glovebox to the loadlock of the end station using an airtight transfer-rod preventing the sample from any contacts with air. HAXPES measurements were conducted at 2005 and 6015 eV photon excitation energies and the NEXAFS measurements were swept between 6520 and 6730 eV. All HAXPES spectra were energy calibrated using the main carbon peak as a reference, assigning this to the C−C bond at 284.5 eV, and intensity normalized to the highest peak in the spectra. For spectra where no peak was observed, the noise level was adjusted to match the noise level in other spectra. The NEXAFS spectra were energy calibrated by reference measurement against the Au 4f peak. The data processing of the NEXAFS spectra, normalization to beam intensity, subtracting background, and normalizing the data were done using the Athena Demeter software version 0.9.24.20

degradation in the spinel-based battery systems, continuous research seems necessary to resolve this controversy. The two most popular suggestions so far have been that (i) Mn metal particles increase the conductivity of the SEI layer so that its passivating properties are lost, leading to continuous electrolyte degradation, and that (ii) ionic manganese species poison the anode surface, preventing Li intercalation and leading to loss of usable active material. Previous studies have been almost solely carried out on graphite, but the degradation of LMO is also observed in chemistries using the LTO anode material. It remains unclear, however, if this is because of a unique mechanism or reactions similar to those associated with graphite. LTO has previously been considered “free” of an SEI layer since it operates at a voltage inside the electrochemical stability window (ESW) of the electrolytes. In a previous paper,19 we challenged this assumption by surface-sensitive low-energy soft X-ray photoelectron spectroscopy (SOXPES) measurements and also presented a brief summary on other reports discussing similar observations. An SEI layer can indeed be observed on an LTO electrode surface, although to a lesser extent than on graphite. Therefore, all mechanisms proposed for Mn reduction and reoxidation on graphite could also be valid for LTO, not least since the operating voltage of LTO is lower than the reduction potential of manganese, 1.55 V compared to ca. 1.8 V. The effect of manganese dissolution and deposition in battery chemistries with LTO is poorly understood, and much less investigated than that on graphite. To the best of our knowledge, the only previous investigation is by Zhan et al.14 who used the LTO/LMO system as a reference in a broader study on Mn dissolution and deposition, but without analyzing this particular cell chemistry in depth. This study therefore aims to investigate the manganese in the SEI of LTO/LMO cell in detail. We used hard X-ray photoelectron spectroscopy (HAXPES) as well as near edge X-ray absorption fine structure (NEXAFS) to determine the oxidation state and composition of manganese on the LTO anode at different charge states. The experimental setup utilizes both detectors in one analysis chamber, thereby allowing a combination of NEXAFS and HAXPES data obtained from the same sample.

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EXPERIMENTAL SECTION A slurry consisting of micron-sized Li4Ti5O12 powder (∼9 μm, Life Power LTO•Phostech Lithium), carbon black powder (Super-P, Erachem Comilog) and Kynar 2801 (Handlapp, dissolved in a 5:95 wt % ratio with N-methyl-2-pyrrolidone) was prepared in a 75:10:15 weight ratio. The slurry was ballmilled for 2 h and then casted on a carbon-primed aluminumB


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Figure 2. HAXPES spectra depicting the C 1s, O 1s, P 2p, and Ti 2p core-levels of the four LTO samples: pristine, OCV, delithiated (after 30 cycles), and lithiated (after 30.5 cycles), acquired with the excitation photon energy of 2005 eV.

−CF2 bonds, respectively.23 There is also a minor contribution at 294 eV from CF3 groups in the Kynar binder. Compared to the pristine sample, the OCV-stored sample shows a slight increase in the relative intensity of the C−O peak at 286 eV in the C 1s spectra and the shoulder at about 533 eV in the O 1s spectra. Also, the P 2p spectra reveal the presence of phosphorus originated from the lithium salt anion on the OCV-stored sample. Therefore, the results suggest that a surface layer is formed on the OCV-stored LTO electrode in the LTO/LMO cell, which is in agreement with our previous results indicating the presence of a surface layer on uncycled LTO electrode in LTO half cells.19 Similar observations, but to a lesser extent, as discussed above for the OCV sample, can also be made when comparing the 30-cycles (delithiated) sample with the pristine sample. This indicates less SEI layer depositions on the cycled sample as compared to the OCV sample, again in accordance with previous half-cell studies.19 These results imply that there is a spontaneous degradation of the electrolyte in the cells that is independent of any externally applied potential/current, and that the formed surface layer is unstable and can be removed from the OCV electrode when electrochemically cycled, resulting in detection of lower amounts of SEI layer species on the 30-cycle cell electrode. Interestingly, there are some major differences between the delithiated and the lithiated LTO electrodes. In the Ti 2p spectra, an expected partial oxidation of titanium in the lithiated LTO sample can be observed, confirming that the LTO electrode is indeed lithiated. In the lithiated sample, the relative intensity of the C−O peak to the C−C peak is drastically increased as well as broadened as compared to the delithiated sample. The O 1s spectra of the lithiated sample also display a corresponding peak at about 534 eV, which is equivalent to the

A built-in linear combination method in Athena using the reference compounds was employed in order to find out the chemical environment of Mn in the samples.

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RESULTS AND DISCUSSION Electrochemistry. The LTO|LMO cells cycled for 30 cycles displayed a steady but declining capacity over the 30 cycles, as shown in Figure 1. These results are not unconventional for prototype full-cells fabricated for laboratory-scale, but can rather generally be expected.21,22 Naturally, improved performance can be obtained by tailoring the electrode formulation and cell assembly processes, but is considered outside the scope of the current study. The cells were limited with respect to the cathode, and displayed a capacity of 120 mAh/g in the first cycle, considering only the active cathode material. After 30 cycles, the cells retained 90.1% and 83.7% of the capacity of the first cycle and had a columbic efficiency of 99.8% and 99.4%. HAXPES. LTO has often been mentioned as an SEI-free electrode, but in our previous study19 we showed that SEI exists on LTO electrodes cycled in half-cells. To evaluate the presence of SEI on LTO in LTO/LMO full-cells and whether it contains Mn, we analyzed LTO electrodes at different charge states using synchrotron-based photoelectron spectroscopy. Figure 2 shows the HAXPES data for the different core levels of four different LTO samples: the pristine electrode, OCV-stored electrode, and two electrodes cycled for 30 and 30.5 cycles in LTO/LMO cells (delithiated and lithiated states for LTO electrodes). In the carbon spectra, there are three major peaks in the pristine sample at the binding energies of 284.5 eV, 286.8 and 291.6 eV, which are designated to C−C, −CH2−CF2, and C


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Figure 3. HAXPES spectra depicting the C 1s, O 1s, P 2p, and Ti 2p core-levels of the four LTO samples: pristine, OCV, delithiated (after 30 cycles), and lithiated (after 30.5 cycles), acquired with the excitation photon energies of 2005 and 6015 eV.

Figure 4. Mn 2p core-level spectra of the four LTIO samples: pristine, OCV, delithiated (after 30 cycles), and lithiated (after 30.5 cycles), acquired with the excitation photon energy of 2005 eV.

energy of the C−O bond.23 A similar behavior was observed by Song et al.24 and attributed to adsorption/desorption of SEI/ electrolyte decomposition species during cycling. That bulk material is clearly detected at a photon energy of 2005 eV, meaning that the SEI is not more than 20 nm thick (assuming that the SEI is a compact layer).19

Increasing the photon excitation energy increases the kinetic energy of the ejected photoelectron, thereby resulting in a longer inelastic mean-free path and increased probing depth. Figure 3 shows results from measurements done with a photon energy of 6015 eV superimposed on those from the 2005 eV measurements, generating a rough estimate of depth differences D


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Figure 5. (A) NEXAFS spectra at Mn K-edge of the delithiated (after 30 cycles) LTO sample and references of Mn, MnO, Mn2O3 and MnO2. (B) Same spectra as a, but scaled to better present the pre-edge feature. (C) Pre-edge features of the different spectra with the background removed.

structure. The pre-edge that appears in the spectra of ionic manganese comes from the transition between the core 1s to 3d orbitals.27 The 1s to 3d transition is less affected by neighboring atoms, and by analyzing the pre-edge, a more exact determination of the oxidation-state can therefore be made.28 Figure 5b shows the pre-edge feature and the background approximation of the peaks in more detail. Figure 5c show the pre-edge peaks with the background subtracted. The pre-edge peaks were integrated to extract information on the oxidationstate. The position of the centroid (i.e., the center of the integration area) is sensitive to the oxidation state (see Table 1). The fact that a pre-edge feature is visible for the sample in

in the samples. Less prominent peaks can be observed for all high-energy samples at positions associated with SEI formation products, indicating that the changes observed are all surface related. The difference seen between 6015 and 2005 eV excitation energies for the C 1s core level in the pristine sample merely suggest a slightly higher concentration of binder at the surface of the electrode as compared to the bulk. When examining the Mn 2p spectra, it is observed that there is manganese on all samples except the pristine (see Figure 4). The dissolution mechanism of manganese have previously been shown to be independent of electrochemical activity.25 In contrast to Zhan et al.,14 our results also confirm that the manganese deposition on the LTO electrode is independent of cell cycling, which can be clearly seen from the OCV sample. The binding energy measured for the Mn 2p peaks corresponds well to ionic manganese in the NIST-database,26 indicating that no metallic Mn was formed. However, the Mn 2p spectra cannot reveal oxidation states of different ionic Mn, thus, NEXAFs was used to analyze the oxidation state of Mn on the LTO electrodes. NEXAFS. The main adsorption-edge of NEXAFS is dependent on the oxidation state and can therefore be used to determine the oxidation number of different elements. In Figure 5a, the Mn K-edge of the delithiated sample is shown together with metallic manganese and different manganese oxides containing Mn+II, Mn+III, and Mn+IV, respectively. It is observed that the adsorption-edge of manganese on the cycled LTO sample appears between the signals of Mn+II- and Mn+IIIbased oxides, confirming that the manganese found on the sample is in at least the +II oxidation state. The main absorption edge is, however, dependent on long-range ordering, making the main edge able to shift some electronvolts in energy for samples with the same oxidation-sate but differing in

Table 1. Centering of the Pre-edge Peaks sample

centroid

MnO reference delithiated sample Mn2O3 reference MnO2 reference

6536.93 6537.44 6538.47 6539.60

itself further indicates that the manganese is ionic since this feature is absent in metallic manganese. As seen in Table 1 the positioning of the pre-edges shows that the sample is in between +II and +III, with a dominance toward +II since it is closer in position to the +II reference than to the +III reference. This finding would exclude metallic Mn poisoning as a reason for the accelerated capacity-loss in cells where LMO is coupled with LTO. The mechanism for manganese deposition on LIB anodes has not yet been fully understood. The most common theory proposed has involved an ion-exchange mechanism with lithium species in the cell. This would then yield manganese E


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best fit to the sample acquired with linear combination in Athena was a combination of MnF2 and MnCO3, but the fit was not able to match the sample signal well enough to be considered a good fit. The pre-edge feature and the intensity of the main absorption peak of MnF2 is a close match to those of the sample, suggesting that MnF2 could perhaps be present at the anode surface, but another component not investigated here must also be present to give rise to the sample response. It can therefore be suggested that the manganese on the LTO electrode sample is in a different chemical environment than merely simple fluorides, oxides, or carbonates. The NEXAFS spectra of the delithiated sample (red) together with the lithiated sample (blue) displayed in Figure 7a show that both samples have the same general shape but slightly different intensity. This minor difference is likely due to intensity calibration, but it could also originate in slightly different chemical environments; however, the similarities in the overall peak shape render this latter explanation less probable. Analogous to the HAXPES Mn 2p data (Figure 4), this suggests a passive role of manganese in the SEI layer, since neither its redox state nor its chemical environment varies over the charge/discharge cycle of the cell. In Figure 7b, the delithiated sample (red) is shown together with the graphite reference cell (green). The overall shape of the postedge tail in the graphite spectra is different than for the LTO sample, suggesting a different composition of manganese on the graphite electrode. Manganese on graphite would, however, still exist in the same oxidation state as on LTO since the adsorption-edge is the same. Both LTO and graphite has electrochemical reduction potentials below that of manganese, but neither display any presence of metallic manganese. Nevertheless, there still seems to be a different SEI-forming mechanism or at least a different manganese product on these two electrodes, suggesting an influence from the surface on the formation processes.

fluorides, oxides, and carbonates, as suggested in previous works.13−16 In Figure 6, the NEXAFS spectra of the delithiated sample is shown together with spectra of some expected reference

Figure 6. NEXAFS spectra at Mn K-edge of the delithiated (after 30 cycles) LTO sample and references of Mn, MnO, Mn2O3 and MnO2, MnF2, MnF3, and MnCO3.

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CONCLUSIONS Using a combination of HAXPES and NEXAFS techniques, where the same samples can be analyzed with both spectroscopic techniques, we have found that manganese deposits on LTO Li-ion battery anodes are in ionic state with a mixture of oxidation states +II and +III with a dominance toward +II, and that this is independent of any electrochemical

compounds. It is striking that none of the investigated references give a good fit to the sample spectra. A mixture of different compounds would appear as a linear combination of the different NEXAFS spectra, but interestingly, the references acquired could not be used to model the sample spectra. The

Figure 7. NEXAFS spectra at Mn K-edge of (a) delithiated and lithiated cycled LTO electrode samples, and (b) the delithiated LTO sample and a graphite sample. F


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(9) Amine, K.; Liu, J.; Kang, S.; Belharouak, I.; Hyung, Y.; Vissers, D.; Henriksen, G. Improved Lithium Manganese Oxide Spinel/graphite Li-Ion Cells for High-Power Applications. J. Power Sources 2004, 129, 14−19. (10) Xiao, X.; Ahn, D.; Liu, Z.; Kim, J.-H.; Lu, P. Atomic Layer Coating to Mitigate Capacity Fading Associated with Manganese Dissolution in Lithium Ion Batteries. Electrochem. Commun. 2013, 32, 31−34. (11) Gowda, S. R.; Gallagher, K. G.; Croy, J. R.; Bettge, M.; Thackeray, M. M.; Balasubramanian, M. Oxidation State of Cross-over Manganese Species on the Graphite Electrode of Lithium-Ion Cells. Phys. Chem. Chem. Phys. 2014, 16, 6898−6902. (12) Xiao, X.; Liu, Z.; Baggetto, L.; Veith, G. M.; More, K. L.; Unocic, R. R. Unraveling Manganese Dissolution/deposition Mechanisms on the Negative Electrode in Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2014, 16, 10398−10402. (13) Yang, L.; Takahashi, M.; Wang, B. A Study on Capacity Fading of Lithium-Ion Battery with Manganese Spinel Positive Electrode During Cycling. Electrochim. Acta 2006, 51, 3228−3234. (14) Zhan, C.; Lu, J.; Jeremy Kropf, A.; Wu, T.; Jansen, A. N.; Sun, Y.-K.; Qiu, X.; Amine, K. Mn(II) Deposition on Anodes and Its Effects on Capacity Fade in Spinel Lithium Manganate-Carbon Systems. Nat. Commun. 2013, 4, 2437. (15) Shkrob, I. A.; Kropf, A. J.; Marin, T. W.; Li, Y.; Poluektov, O. G.; Niklas, J.; Abraham, D. P. Manganese in Graphite Anode and Capacity Fade in Li Ion Batteries. J. Phys. Chem. C 2014, 118, 24335−24348. (16) Shin, H.; Park, J.; Sastry, A. M.; Lu, W. Degradation of the Solid Electrolyte Interphase Induced by the Deposition of Manganese Ions. J. Power Sources 2015, 284, 416−427. (17) Ochida, M.; Domi, Y.; Doi, T.; Tsubouchi, S.; Nakagawa, H.; Yamanaka, T.; Abe, T.; Ogumi, Z. Influence of Manganese Dissolution on the Degradation of Surface Films on Edge Plane Graphite Negative-Electrodes in Lithium-Ion Batteries. J. Electrochem. Soc. 2012, 159, A961−A966. (18) Delacourt, C.; Kwong, a.; Liu, X.; Qiao, R.; Yang, W. L.; Lu, P.; Harris, S. J.; Srinivasan, V. Effect of Manganese Contamination on the Solid-Electrolyte-Interphase Properties in Li-Ion Batteries. J. Electrochem. Soc. 2013, 160, A1099−A1107. (19) Nordh, T.; Younesi, R.; Brandell, D.; Edström, K. Depth Profiling the Solid Electrolyte Interphase on Lithium Titanate (Li4Ti5O12) Using Synchrotron-Based Photoelectron Spectroscopy. J. Power Sources 2015, 294, 173−179. (20) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (21) Li, S. R.; Sinha, N. N.; Chen, C. H.; Xu, K.; Dahn, J. R. A Consideration of Electrolyte Additives for LiNi0.5Mn1.5O4/ Li4Ti5O12 Li-Ion Cells. J. Electrochem. Soc. 2013, 160, A2014−A2020. (22) Li, S. R.; Chen, C. H.; Xia, X.; Dahn, J. R. The Impact of Electrolyte Oxidation Products in LiNi0.5Mn1.5O4/Li4Ti5O12 Cells. J. Electrochem. Soc. 2013, 160, A1524−A1528. (23) Younesi, R.; Christiansen, A.; Loftager, S.; García-Lastra, J. M.; Vegge, T.; Norby, P.; Holtappels, P. Charge Localization in the Lithium Iron Phosphate Li3Fe2(PO4)3 at High Voltages in LithiumIon Batteries. ChemSusChem 2015, 8, 3213−3216. (24) Song, M.-S.; Kim, R.-H.; Baek, S.-W.; Lee, K.-S.; Park, K.; Benayad, A. Is Li4Ti5O12 a Solid-Electrolyte-Interphase-Free Electrode Material in Li-Ion Batteries? Reactivity between the Li4Ti5O12 Electrode and Electrolyte. J. Mater. Chem. A 2014, 2, 631−636. (25) Choi, W.; Manthiram, A. Comparison of Metal Ion Dissolutions from Lithium Ion Battery Cathodes. J. Electrochem. Soc. 2006, 153, A1760−A1764. (26) Powell, C. J.; Jablonski, A. NIST Electron Inelastic-Mean-FreePath Database, version 1.; National Institute of Science and Technology: Gaithersburg, MD, 2010. (27) Chalmin, E.; Farges, F.; Brown, G. E. A Pre-Edge Analysis of Mn K-Edge XANES Spectra to Help Determine the Speciation of

activity in the cell. The pre-edge features and main absorption peak intensity suggests that MnF2 is a possible component on the anode surface. The chemical environment of the deposited Mn differs from that of graphite, even though the manganese on graphite also seems to be in a mixture of +II and +III oxidation states. There is an obvious SEI layer formation observed on the LTO electrodes, and it is also observed that manganese is present in this formed layer. However, apart from the manganese, the general appearance of the SEI is strikingly similar to that formed in half-cells.19 This highlights that Mn deposition does not control the chemical composition of the formed SEI in LTO-based LIBs. The results suggest that Mn has the same oxidation state and chemical environment in both charged and discharged states, indicating a rather passive role of manganese during cell cycling. However, the studies strongly imply that more studies are needed to investigate the unexpectedly complex nature of the Mn-containing compounds on the LTO surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +46 18 471 3769. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Swedish energy agency (STEM) for funding. We acknowledge HZB for the allocation of synchrotron radiation beam time for HAXPES/NEXAFS measurements. The authors would like to thank Mihaela Gorgoi, Franz Schäfers, Marcel Martin, and all the staff at BESSY for assistance during measurements and for keeping the facility functioning and available. Dr. William Brant is acknowledged for his help to use the Athena Demeter software.

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REFERENCES

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The Journal of Physical Chemistry C Manganese in Minerals and Glasses. Contrib. Mineral. Petrol. 2009, 157, 111−126. (28) Farges, F. Ab Initio and Experimental Pre-Edge Investigations of the Mn K-Edge XANES in Oxide-Type Materials. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 155109.

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Manganese in the SEI Layer of Li4Ti5O12 Studied by Combined

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