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Article pubs.acs.org/JPCC

Aging of Electrode/Electrolyte Interfaces in LiFePO4/Graphite Cells Cycled with and without PMS Additive Katarzyna Ciosek Högström,† Maria Hahlin,† Sara Malmgren,† Mihaela Gorgoi,‡ Håkan Rensmo,§ and Kristina Edström*,† †

Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489, Berlin, Germany § Department of Physics and Materials Science, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden ‡

ABSTRACT: LiFePO4/graphite cells are studied with hard X-ray photoelectron spectroscopy (HAXPES) in order to evaluate the influence of aging on the electrode/electrolyte interfaces. Cells cycled with the standard electrolyte, LiPF6 in ethylene carbonate/diethyl carbonate, are compared to cells cycled with addition of the filmforming additive propargyl methanesulfonate (PMS). Cycling is performed to accelerate aging at both 21 and 60 °C. The results show that PMS cells have better capacity retention at both temperatures. The thorough HAXPES analysis of the samples reveals that the PMS additive affects the interfaces on both negative and positive electrodes. The effects on the negative electrode show important variations in thicknesses and surface chemistry. Lower loss of cyclable lithium in the SEI and interface thicknesses that are stable during cycling are some reasons for the better capacity retention at 21 °C. The cycling fades quickly at 60 °C for both electrolytes, but the PMS electrolyte gives still a more stable cycling with thinner formed interface layers as probed by depth profiling characterization. PMS give a better long-term interface protection for Li-ion batteries, but capacity fading still cannot be fully prevented.

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INTRODUCTION

A well-known method to improve the performance of a Liion battery is implementation of electrolyte additives.7 Commercial cells contain a mixture of various additives; some examples are flame retardants,7−9 redox shuttles,7,8,10 shutdown additives7,8 and film-forming additives.7,11−14 Filmforming additives facilitate formation of a solid electrolyte interphase (SEI) on the graphite surface, ideally forming a more stable interface layer and therefore improving long-term cycling. Aging studies that have been performed on LiFePO4/ graphite cells indicate that the main process leading to capacity decline is loss of cyclable lithium,3,15−17 which is mostly caused by side reactions taking place at the negative electrode/ electrolyte interface. Film-forming additives have a great potential of minimizing the aging effects taking place at the interface. Vinylene carbonate (VC)11,12,18 is the most known SEI precursor. It improves the long-term performance of batteries and is usually added to commercial Li-ion cells. Recently, a new film-forming additive, propargyl methanesulfonate (PMS), was shown to improve cyclability of Li-ion batteries.13,14 Many studies describing the influence of filmforming additives on the SEI have been published;7,11−14,18

Li-ion batteries are a key technology for enabling the electrification of vehicles. One of the main challenges is the lifetime of the batteries manufactured today, which is shorter than that of the car. Also the cost of the battery is an obstacle, being the single most expensive component of the car. Improving the lifetime of Li-ion batteries is therefore a key to durability and cost-efficiency of the whole vehicle. In order to improve the lifetime of the Li-ion battery, all aging processes that occur within the battery during storage and operation have to be well-understood. Aging processes of Li-ion batteries are very complex.1 Aging can take place at different parts of the Li-ion battery, e.g., at electrode/electrolyte interfaces, within the active material or binder, and current collectors or conductive additives can corrode. Different degradation mechanisms are described in literature, and they vary for the positive and negative electrode.1,2 Furthermore, different operating conditions lead to different aging processes within the battery. For example, cycling at elevated temperature (around 60 °C, 140 °F) significantly accelerates capacity fading.1,3−6 Moreover, different processes can influence each other and lead to accelerated aging. Due to the complexity of the system, the interaction and progression of different aging processes are still poorly understood. © 2014 American Chemical Society

Received: March 19, 2014 Revised: May 20, 2014 Published: May 21, 2014 12649

dx.doi.org/10.1021/jp502732j | J. Phys. Chem. C 2014, 118, 12649−12660

The Journal of Physical Chemistry C

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Figure 1. SEM images of (a) graphite negative electrode and (b) LiFePO4 positive electrode.

KS6 graphite (Timcal), 2 wt % conductive carbon black (Super P, Erachem Comilog N.V.), and 10 wt % Kynar binder (vinylidene fluoride trifluoroethylene copolymer, Arkema). A high-resolution scanning electron microscope (SEM, Zeiss LEO 1550) was used to characterize the structure of uncycled electrode materials. The graphite had an average particle size of 20 μm (Figure 1a). LiFePO4 electrodes were made of 75 wt % hydrothermally synthesized carbon-coated LiFePO4 with a particle size around 200 nm (Figure 1b), 10 wt % conductive carbon black (Super P, Erachem Comilog N.V.), and 15 wt % Kynar binder. All electrodes were dried for at least 5 h at 120 °C in a vacuum oven within the glovebox and the battery cells were balanced with 20% extra capacity on the graphite negative electrode. The cells in this study were assembled with two different electrolytes. One hereafter called standard electrolyte was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) in a 2:1 volume ratio and one hereafter called PMS where 1 wt % of the film-forming additive PMS was added to the standard electrolyte. This is the same set of electrodes and electrolytes which have been used previously for studying the first cycles of graphite/LiFePO4 cell with PMS in the electrolyte.14 The graphite/LiFePO4 pouch cells were cycled galvanostatically at C/10 rate. In order to accelerate aging a wide voltage window of 2.7−4.2 V was chosen. The measurements were performed at room (21 °C) and elevated (60 °C) temperature. Hard X-ray Photoelectron Spectroscopy (HAXPES). Samples for interface analysis were collected after the 3rd, 50th, and 200th galvanostatic cycle in charged (with the graphite lithiated) or discharged state. The HAXPES spectra of cells stopped after the third cycle at 21 °C have previously been presented.14 The cells were opened in an argon-filled glovebox (≤1 ppm of H2O, ≤1 ppm of O2) and transferred to the spectrometer in a specially built portable transfer chamber in order to avoid exposure to air.22 The presented graphs show results from unwashed electrodes; however, some samples washed with DMC (dimethyl carbonate) were also analyzed. The HAXPES measurements were performed at the KMC-1 beamline using the HIKE end station at the BESSY II synchrotron (Helmholtz Zentrum Berlin, Germany). The excitation energy was generally 2300 eV. Some measurements with excitations energies 2624 and 6900 eV were also performed. The probing depth was defined as 3 times the inelastic mean free path of electrons in polyethylene and includes approximately 95% of the inelastically emitted electrons.21,26 The measurement time was kept as short as possible, while still discerning the shape of a spectrum, in order to avoid effects from radiation damage in the spectra. The electron takeoff angle was 80° defined relative to the surface plane of the sample, and the takeoff direction was collinear with

however, the differences in aging processes upon addition of an additive have not been studied in detail. The SEI is a multicomponent film containing both inorganic and organic compounds, which has to be measured and analyzed with caution due to its chemical sensitivity.19−22 The typical SEI thickness on graphite is only in the order of two tens of nanometers,14,21,23 which make it challenging to study. Various techniques have been used for electrode/electrolyte interface analysis;24 among them X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are the most commonly used.5,6,20,24,25 XPS is a powerful surface analysis technique, which provides information about the chemical structure and composition. However, the probing depth of commonly used in-house XPS is only about 10 nm,14 which is too small to enable the characterization of the entire electrode/electrolyte interface and track changes in SEI thickness on aging. Traditionally used depth profiling with argon etching has been shown to change the composition of the SEI,19 which may lead to misinterpretation of the results. Hard X-ray photoelectron spectroscopy (HAXPES) provides new interface characterization possibilities allowing for analysis deeper into the SEI material and further into the bulk of the electrode material. This nondestructive methodology has, therefore, the potential of giving a more reliable and reproducible interface characterization compared to a methodology based on sputtering.14,21 In the present work, graphite/LiFePO4 cells were cycled in a standard organic electrolyte as well as with an electrolyte containing the film-forming additive PMS. This article is a continuation of a previous study,14 where depth profiles of the same samples were shown after three cycles at 21 °C. Here, the earlier presented data is compared to our new results from long-term cycled samples. During cycling a wide voltage window was used in order to accelerate aging, and the tests were performed at 21 and 60 °C. Several samples were collected during cycling in order to perform interface analysis with HAXPES and follow the evolution of aging at the electrode/electrolyte interface of graphite and LiFePO4. This allowed a thorough analysis of the aging processes taking place at the cathode and anode interface and, for the first time, a detailed comparison study on the development of interface aging for samples cycled with and without the film-forming additive PMS.

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EXPERIMENTAL METHODS Full Cell Tests. Graphite/LiFePO4 pouch cells (vacuumsealed polymer-coated aluminum bags) were assembled in an argon-filled glovebox (≤1 ppm of H2O, ≤1 ppm of O2) as described in detail elsewhere.14,21 Graphite electrodes were made of 85 wt % potato-shaped graphite (Toyo Tanso), 3 wt % 12650



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Figure 2. LiFePO4/graphite cells cycled with the standard reference electrolyte and with the PMS electrolyte at 21 °C and at 60 °C: (a) capacity retention and (b) differential capacity during the first charge.

Figure 3. C 1s core level spectra for lithiated graphite cycled with the standard electrolyte (top) and with the PMS electrolyte (bottom) at 21 °C and at 60 °C. Calculated SEI thicknesses (d) are displayed in the spectra.

the e-vector of the incident photon beam. The spectra were energy calibrated using the C−H feature for the graphite negative electrode and the C−C/C−H feature for the LiFePO4 positive electrode, both set to 284.4 eV. The relative intensities of the different features Ij were determined from the following equation: Ij =

d = −λ sin(θ )ln(ILixC)

where θ is the electron takeoff angle defined relative to the surface plane of the sample and λ is the inelastic mean free path for polyethylene.26 The calculated thicknesses should be seen as average values due to the heterogeneous character of the interfaces and not as absolute values. Here they are merely used as a way of comparing the different cycled samples. Near-Edge X-ray Absorption Fine Structure (NEXAFS). S 1s NEXAFS spectra were recorded at the HIKE end station, KMC-1 beamline at the BESSY II synchrotron (HZB, Germany), in fluorescence yield mode (FY) using a Bruker XFlash 4010 and in total electron yield mode (TEY) using the sample current recorded via a Keithley electrometer. The energy scale was calibrated with measurements of a gold reference. Spectra were normalized to 1 at most intense resonance.

Aj /σj ∑i Ai /σi

(2)

(1)

where A is area of the feature and σ is the Scofield theoretical photoionization cross section.27 In each set of presented core level spectra, the intensities are normalized using the obtained relative atomic percentages of that element in the respective sample. Iron and lithium were excluded from quantitative analysis for the LiFePO4 positive electrode because of overlaps of Fe 2p core level peak with the fluorine plasmon and of Fe 3p with the Li 1s core level. The thickness calculations were based on a model described in our previous work.14,21 The SEI thickness d was calculated from C 1s LixC relative intensity ILixC, using the following equation:

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RESULTS AND DISCUSSION 1. Long-Term Cycling of LiFePO4/Graphite Cells. Figure 2a shows the capacity retention for LiFePO4/graphite 12651



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Figure 4. F 1s and P 2p core level spectra (hν = 2300 eV) for lithiated graphite cycled with the standard electrolyte (top) and with the PMS electrolyte (bottom) at 21 °C and at 60 °C.

a significant decrease in capacity retention compared to cells cycled at room temperature as shown in Figure 2a; however, the cell with PMS additive shows better long-term performance than the standard electrolyte cell. On the basis of these results HAXPES measurements were performed in order to better understand the aging mechanisms of the studied cells and to explain the processes taking place at the electrode/electrolyte interfaces. 2. Changes in the Graphite Electrode/Electrolyte Interface on Cycling with the Standard and PMS Electrolytes. 2.1. Changes in SEI Thickness on Cycling. Synchrotron-based HAXPES measurements were performed using an excitation energy of 2300 eV. For the measured core levels this results in a probing depth of around 18 nm, which thus enables more bulk-sensitive measurements than traditional in-house XPS.14,21 Reaching the carbon active material in the bulk is particularly important for aged electrodes as this provides a method for estimating changes in the SEI thickness. To achieve this, a set of measurements using a photon energy

cells cycled in the standard reference electrolyte and with the PMS electrolyte. It has previously been shown that at room temperature the PMS cell shows slightly higher irreversible capacity loss in the first cycle and that it leads to formation of a thicker SEI after three cycles.14 Figure 2a shows slightly higher capacity retention at the beginning of the cycling for the samples cycled in the standard electrolyte. However, after approximately the 50th cycle the capacity retention of the PMS cell is better than that of the standard electrolyte, which indicates formation of a more long-term stable SEI when the PMS additive is present. Figure 2b shows the differential capacity retention for the same set of cells during the first cycle. The peak at 2.7 V (0.8 vs Li+/Li) corresponds to solvent reduction and the broad peak at around 2.1 V (1.4 vs Li+/Li) to PMS reduction. At elevated temperature these peaks are shifted to slightly lower cell potentials, due to increase in reaction kinetics. Moreover, cells cycled at elevated temperature have bigger differential capacity peaks corresponding to electrolyte reduction, which suggests that thicker SEI is formed at 60 °C than at 21 °C. Both cells cycled at elevated temperature exhibit 12652



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Figure 5. Relative intensities of detected core level graphite peaks (hν = 2300 eV) for cells cycled with the standard electrolyte (top) and with PMS (bottom) at 21 °C and at 60 °C.

When comparing the impact of temperature after three cycles it is clear that a much thicker SEI is formed with both electrolytes during cycling at elevated temperature, as a significantly lower relative intensity of the LixC feature is observed in the C 1s spectra of these samples shown in Figure 3. The SEI thickness for the sample cycled three times with standard electrolyte at 60 °C was 34 nm, and for the PMS cell it was 29 nm, i.e., about 84% and 17% higher, respectively, than the thicknesses determined for the samples cycled at 21 °C. The SEI becomes even thicker after 50 cycles at elevated temperature, where the signal from the carbon active material is fully attenuated when using excitation energy of 2300 eV. Therefore, in order to determine its thickness a set of HAXPES measurements with higher excitation energy were performed. The measurements with 6900 eV excitation energy (∼47 nm probing depth14,21) showed that SEI after 50 cycles at 60 °C is thinner for the PMS cell than for the standard electrolyte cell. The thickness is estimated to be above 50 nm and above 60 nm for the PMS and standard electrolyte cells, respectively. Together the above results show that after aging at room and at elevated temperature the SEI on graphite electrodes cycled with PMS is thinner than on the ones cycled with standard electrolyte. The fact that the thickness of the PMS sample

of 6900 eV with a corresponding probing depth of around 47 nm was also used.14,21 Figure 3 shows C 1s core level spectra for lithiated graphite cycled with the two electrolytes at 21 °C and at 60 °C. The intensity of the carbon active material (at 282 eV in the C 1s spectra), attributed to the bulk material, LixC, is linked to the thickness of the SEI. In Figure 3 the intensity of the LixC peak is decreasing as a function of the number of cycles. It clearly shows that the SEI is growing in thickness on cycling, which confirms previous findings.5 Graphite cycled in the standard electrolyte at 21 °C has thinner SEI than those cycled with PMS after three cycles (19 and 25 nm, respectively), but its SEI becomes much thicker on cycling. After 200 cycles the SEI becomes about 89% and 28% thicker compared to thicknesses after three cycles for the standard electrolyte and PMS cell, respectively. This shows that the SEI on graphite in the PMScontaining cell is thinner and more stable after prolonged cycling, which has been suggested by Abe et al.13 This result is also in agreement with the results shown in Figure 2, where the capacity retention was slightly higher for the standard electrolyte after three cycles. Upon cycling the capacity retention decreased more for the graphite cycled with standard electrolyte than for that cycled with PMS, and after 200 cycles the PMS cell showed the better capacity retention. 12653



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the outermost parts of the interface followed by increased attenuation of the hydrocarbon signal. For the PMS samples the hydrocarbon feature instead increases on aging. The PMS additive is believed to polymerize on the surface of the electrode forming an SEI which is rich in hydrocarbons (Figures 3 and 5). The increase in the hydrocarbon feature thus indicates a continued decomposition of PMS upon aging. Another difference between the SEI formed from the different electrolytes is related to the total amount of lithium. For the standard electrolyte an increase in the content of lithium as a function of aging is observed, as shown in Figure 6, whereas the

varies less with cycling and temperature indicates that a more stable interface is formed in the presence of PMS. 2.2. Changes in SEI Chemical Composition at 21 °C. Beside the evolution of the SEI thickness upon aging there are other changes observed in the chemical composition of the electrode/electrolyte interfaces. Figure 4 shows the P 2p and F 1s core level spectra for the graphite electrodes cycled with the standard electrolyte or with the PMS electrolyte. In Figure 5 the relative amounts of detected elements in the graphite electrodes cycled with the standard and PMS electrolytes are depicted. Figures 4 and 5 show that for the standard electrolyte cells cycled at 21 °C the amount of salt decomposition products increases on aging. This is observed by the relative increase of the P−O/PO feature (∼133.7 eV in the P 2p 3/2 spectra), P−F feature (136.7 eV in the P 2p 3/2 spectra and the ∼687.3 in the F 1s feature), and LiF (684.7 eV in the F 1s spectra) in the figures. The relative increase of salt decomposition products indicates that these are formed to a higher extent on cycling. Generally, formation of salt decomposition products can be attributed to reduction of LiFP6, reactions between the salt and impurities or with other SEI components:20,28−31 LiPF6 ↔ PF5 + LiF LiPF6 + x e− + x Li+ → x LiF + LiPF6 − x

Figure 6. Relative intensities of the Li 1s feature (hν = 2300 eV) for graphite cells cycled with the standard electrolyte and with PMS at 21 °C.

LiPF6 + Li 2CO3 → 3LiF + POF3 + CO2

PF5 + Li 2CO3 → 2LiF + POF3 + CO2 PF5 + ROLi → POF3 + RF + LiF +

relative amount of lithium in the surface is fairly constant for the PMS cell. Moreover, the differences in the relative amount of lithium between the lithiated and delithiated state are much larger for the standard electrolyte samples, which could indicate lower stability of the SEI. The differences in both lithium content and SEI thickness show that a more stable SEI is formed in the case of PMS, which can be linked to the trend in capacity retention of these samples. The thicker SEI observed after three cycles for the PMS sample is accompanied by lower capacity retention, while after 200 cycles the thinner SEI in the case of PMS is accompanied by better capacity retention. Higher stability of the SEI and lower loss of cyclable lithium is suggested as the main reason for the better long-term capacity retention of the PMS cell. 2.3. Changes in Chemical Composition of the SEI at Elevated Temperature. In section 2.1 it was shown that a thicker SEI is formed at 60 °C than at 21 °C. This section will focus on the changes in composition of this SEI depending on the cycling temperature. The initial SEIs formed on cycling at elevated temperature for three cycles have higher relative amount of carbonates (CO3 in Figure 5) than samples cycled at 21 °C. The carbonate intensity increases by 40% and 20% for the standard electrolyte and the PMS cell, respectively. The P− O/PO feature is also more pronounced for samples cycled at 60 °C (Figures 4 and 5). This means that solvent and salt decomposition occur to a higher extent at elevated temperature. It is well-known that LiPF6 is in equilibrium with LiF and PF5.28,32 PF5, a strong Lewis acid,33 reacts instantaneously with solvent components and the equilibrium moves toward products, which is even more significant at elevated temperatures.28,32 The whole process leads to decomposition of salt and solvent components, with increasing amount of decomposition products formed at elevated temperature. Interestingly,

−

POF3 + 2x Li + 2x e → x LiF + LixPOF3 − x POF3 + x ROLi → x LiF + P(OR)x F3 − x

The increase of salt decomposition products is less pronounced for the PMS electrolyte samples. This is most apparent in the case of LiF, where there is a 7-fold increase in the amount in the case of the standard electrolyte and only a 2-fold increase for the PMS electrolyte. A significant increase in the amount of P−O/PO features on aging is observed for both cells. This increase is accompanied by a shift of the binding energy from 133.7 to 134.3 eV in the P 2p 3/2 spectra. The shift indicates that the P−O/PO feature is built up by a mixture of slightly different compounds overlapping in this binding energy region. A shift of the feature to a higher binding energy is a sign of a more dominant character of a compound in which phosphorus is more positively charged, e.g., fluorophosphates containing more fluorine functional groups as in OPF2OR. Figure 5 shows that the features of lithium alkoxide, LiOR (530.2 eV in O 1s spectra), and lithium oxide, Li2O (527.6 eV in O 1s spectra), decrease with increasing number of cycles irrespective of electrolyte used. Previously, it was shown that these compounds are formed in the inner parts of the SEI, close to the bulk graphite.14,21 Therefore, with a continuous buildup of the SEI on cycled samples, these signals become more attenuated. Beside many similarities there are also distinct differences in the composition of the electrode/electrolyte interfaces during aging for both the standard electrolyte cell and the PMS cell cycled at 21 °C. For the standard electrolyte, the hydrocarbon feature (284.4 eV in C 1s spectra in Figure 3) decreases on cycling, most likely due to a buildup of inorganic compounds in 12654



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Figure 7. P 2p core level spectra (hν = 2300 eV) for delithiated LiFePO4 electrodes cycled with the standard electrolyte (top) or with PMS (bottom) at room temperature and at 60 °C.

during cycling at 60 °C the inside of the pouch cell material of both cells (the standard electrolyte and the PMS cell) was covered with a brownish substance. Sloop et al. reported such a brown-colored precipitate during storage of the electrolyte at 85 °C for 2 days and identified it as a mixed carbonate ester from an EC ring-opening reaction.32 Similar SEI aging trends for both the standard electrolyte and the PMS cells cycled at 60 °C are shown in Figures 3−5. After 50 cycles there is significant increase of C−H and P−O/PO features as well as decrease of the amount of lithium. Moreover, aging at elevated temperature leads to a higher relative amount of P−F features, accompanied by lower relative amount of carbonates and LiF. These results show that aging at elevated temperature differs from aging at room temperature, which supports previous work published by us.6 Even though the SEI aging processes at elevated temperature are very similar for the standard electrolyte and PMS cells, thinner SEI with lower amounts of lithium is detected in the PMS cell, which is probably the reason for the better capacity retention. 3. Changes in the LiFePO4 Electrode/Electrolyte Interface on Cycling with the Standard and PMS Electrolytes. 3.1. Changes in the P 2p Core Level on Cycling. LiFePO4 electrodes were also analyzed with HAXPES in order to understand the changes in the electrode/electrolyte interface on the positive electrode during aging. Figure 7 shows P 2p core level spectra for delithiated LiFePO4 electrodes cycled with the different electrolytes at 21 °C and at 60 °C, respectively. For both the standard electrolyte and the PMScycled cells the phosphate feature (133.3 eV in P 2p 3/2 spectra), assigned to the bulk material, is slightly decreasing with the number of cycles, which indicates that an interface layer is continuously forming on LiFePO4 electrodes during aging. The thickness of this interface layer is, however, much thinner than on the graphite electrode. The buildup of an interface layer on the LiFePO4 electrode is much more pronounced for samples cycled at 60 °C, where a larger decrease of the bulk signal is observed. On cells aged at room temperature a thicker electrode/electrolyte interface is found on PMS-cycled electrodes, whereas at 60 °C it is the standard electrolyte cell which shows a thicker interface layer.

The feature assigned to P−F compounds increases during aging, and a new feature assigned to fluorophosphates, POxFy (134.8 eV in P 2p 3/2), is appearing. LiFePO4 electrodes cycled with VC additive were also reported to have fluorophosphates appearing on cycling.5 Interestingly, the amount of fluorophosphates is significantly reduced for all the samples washed with DMC (not shown here). Assuming that the solubilities of the fluorophosphates are similar in DMC and EC/DEC, the result suggests that they are mainly an electrolyte residue. Large amounts of similar features were observed to form on the graphite electrode. It could then be speculated that fluorophosphates that are formed on the negative electrode could dissolve, change the electrolyte composition as such, and therefore affect the positive electrode/electrolyte interface. 3.2. Changes of the LiFePO4 Electrode/Electrolyte Interface Composition at Room and Elevated Temperature. Figure 8 shows relative intensities of detected core level peaks of LiFePO4 electrodes cycled with standard and PMS electrolytes. Similar trends are observed during aging at room temperature for both standard electrolyte and PMS-cycled samples. Beside the previously mentioned decrease of the phosphate feature assigned to the bulk material, increase of the P−F feature, and the appearance and increase of a new feature assigned to fluorophosphates, additional changes occur. In general, we can detect an increase of C−O containing compounds and a decrease of LiF. The main difference for the two electrolytes when aging at room temperature is the feature assigned to C−C/C−H bonds. The relative amount of C−C/C−H is decreasing during cycling for the standard electrolyte electrodes while it is increasing for the PMS cells. For the standard electrolyte samples the C−C/C−H peak is mainly attributed to the carbon coating of the LiFePO4 particles and carbon black present in the bulk material; therefore, this feature decreases on cycling, as the thicker interface layer is formed. On the contrary, the LiFePO4 cathode cycled with PMS shows an increase in the C−C/C−H feature. It was previously shown that more hydrocarbons are formed with PMS additive on the negative electrode;14 therefore, higher amounts of these compounds on the positive electrode could either be attributed to the compounds primarily formed at the 12655



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Figure 8. Relative intensities of detected core level peaks (hν = 2300 eV) for LiFePO4 positive electrodes cycled with a standard electrolyte (top) and with PMS (bottom) at room temperature and at 60 °C.

transitions).34,35 Such excitations to bound states are sensitive toward the local bonding structure around the sulfur atom, and for sulfur a link between the main absorption line and the oxidation state has also been discussed.34,36 Parts a and b of Figure 9 show S K-edge NEXAFS spectra of graphite and LiFePO4 electrodes cycled with PMS electrolyte, together with measurements of uncycled graphite and LiFePO4 electrodes soaked in PMS electrolyte as reference samples. S Kedge NEXAFS measurements of both uncycled reference samples show a spectrum that resembles previously measured and theoretically calculated spectra of sulfur compounds with similar sulfur environments, i.e., methanesulfonate anion CH3SO3−.35 The main peak for the uncycled samples is located at 2475.8 eV. At higher energies, only small structures are observed at 2483 and 2890 eV and at lower energies there is a tiny pre-edge structure at about 2469 eV. The same features are observed for all the cycled electrodes. This supports the previously suggested decomposition mechanisms on graphite electrodes where the products have very similar sulfur environments as the PMS molecule.14 Distinguishing the main decomposition product from the PMS molecule itself has, however, proven to be difficult using NEXAFS.

negative electrode or it could be a product of PMS decomposition on the positive electrode. Similar aging trends are observed for the cathodes cycled at 60 °C as was shown for electrodes cycled at 21 °C. The main difference between the cells cycled with different electrolytes is that cells cycled with PMS have higher intensity of the C−C/ C−H feature and lower intensity of C−O containing compounds on aging at elevated temperature, whereas the standard electrolyte shows the opposite trend. 4. Near-Edge X-ray Absorption Fine Structure over the S K-Edge. The presented HAXPES results have shown that PMS has a substantial influence upon the aging of the SEI interphases in the battery cells, and in this section we thus specifically target the PMS molecule and its environment. Specific information about the PMS molecule and its decomposition products can be obtained through NEXAFS over the sulfur K-edge. The absorption process can be described by electronic excitations from the S 1s orbital to the unoccupied electronic states that at least partly are localized at the sulfur atom. For S K-edge NEXAFS spectra, the dominating feature originates from excitations to bound states with primarily S 3p character (e.g., 1s → π* and 1s → σ* 12656



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Figure 9. S K-edge NEXAFS spectra of graphite (a) and LiFePO4 (b) electrodes cycled with PMS electrolyte; from the top: uncycled, reference electrode (soaked in PMS electrolyte), electrodes cycled at 21 °C for 3, 50, and 200 cycles, and electrodes cycled at 60 °C for 3 and 50 cycles, respectively.

The interpretation that the main peak in the cycled samples to a large extent originates from decomposition products is based on previous combined electrochemical and spectroscopic results.14 For the graphite electrodes there is also a new peak at around 2472.9 eV observed in the spectra of the cycled electrodes. This clearly shows that some amounts of a second decomposition compound have been formed on the graphite electrode. As this peak appears at 2.9 eV lower energy than the main peak, it may correspond to absorption in an additional compound with a lower oxidation state of sulfur, e.g., +2.34,36 That is, on the basis of comparison to previously measured NEXAFS spectra,35 the spectra from the cycled graphite electrodes are interpreted to be a linear combination of two different sulfur compounds, i.e., compounds with sulfur oxidation state +4 and +2. Further support for such an interpretation can also be obtained from the S 2p/S 1s core level spectra. The S 2p and S 1s spectra of the graphite electrode cycled for three times with PMS electrolyte have previously been shown to contain a feature at lower binding energy.14 In the previous measurements the core levels indicated that a compound giving rise to the lower binding energy peak resides in the deeper parts of the SEI.14 A similar conclusion may be drawn from the HAXPES core level spectra in Figure 10. It shows a sulfur depth profile of the graphite electrode cycled for 200 times with PMS electrolyte, and also here two features are found in the most depth-sensitive measurement: the main peak, corresponding to sulfur oxidation state +4 (S 2p 3/2 168.5 eV, S 1s 2477.6 eV), and smaller feature that could be attributed to a sulfur in oxidation state +2

Figure 10. S 1s/S 2p depth profile of lithiated graphite electrode after 200 cycles at 21 °C. Probing depth corresponds to 2, 18, and 47 nm.

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new peak is smaller in surface-sensitive mode. The HAXPES and NEXAFS spectra thus support a similar depth profile during aging. The data strongly point toward multiple PMS decomposition products, where the oxidation state is either +4 or +2. In Figure 9a, it is observed that the relative intensity of the new absorption line is similar for all graphite samples, indicating that neither prolonged cycling nor elevated temperatures affect the relative amounts of different sulfur decomposition products. The observation of sulfur compounds on the surface of LiFePO4 (Figure 9b) is interesting, as the aging of the LiFePO4 electrode in section 3 showed differences between the standard and PMS electrolytes. The main sulfur absorption line overlaps with that of the reference sample; this suggests that these sulfur compounds have a very similar bonding structure as the PMS molecule. The compounds may thus be PMS itself and/or a PMS decomposition product formed either on the LiFePO4 surface or formed primarily at the anode and through SEI dissolution interacting with the cathode interface. The sulfur spectra do not change during aging, neither at 21 °C, nor at 60 °C.

(S 1s 2475.1 eV). The NEXAFS obtained in bulk- and surfacesensitive measurements on the graphite electrode cycled 200 times in PMS (Figure 11) show that the relative intensity of the

Figure 11. S K-edge NEXAFS FY (bulk-sensitive, black line) and TEY (surface-sensitive, gray dots) spectra of graphite electrode after 200 cycles at 21 °C.

Figure 12. Schematic picture of the evolution of the SEI on the carbon (LixC) negative electrode during cycling with standard electrolyte (top) and with PMS (bottom), at 21 (left) and 60 °C (right). The dashed line in the SEI represents the probing depth when using 2300 eV excitation energy (95% of total signal). Arrows show changes in the relative intensity of a feature on aging: ↑, small increase; ↑↑, large increase; ↓, small decrease; ↓↓, large decrease. 12658



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CONCLUSIONS In summary, Li-ion batteries cycled with the PMS film-forming additive have better long-term capacity retention than standard electrolyte cells, both at 21 °C and at 60 °C. The increased depth sensitivity of HAXPES compared to in-house XPS enabled performing a more detailed analysis of the processes taking place at the interface, tracking changes in SEI thickness on cycling and, for the first time, performing a detailed comparison study of the development of interface aging for samples cycled with and without a film-forming additive. Figure 12 summarizes processes that take place at the graphite electrode/electrolyte interface during aging. The gray area corresponds to the carbon active material, LixC, and the yellow area to SEI. The number of cycles and the SEI thicknesses are depicted in the schematic picture. At 21 °C the most pronounced signs of graphite interface aging are increased thickness of the SEI and higher amounts of lithium and salt decomposition products detected in the interface. These aging effects are less distinct for cells cycled with PMS, where thinner SEI and less lithium and salt decomposition products, such as P−O/PO and LiF, are found in the interface after 200 cycles. At 60 °C aging proceeds much faster, and the interface processes on graphite differ from those determined at room temperature. The SEI interface is significantly thicker after cycling at elevated temperature than at room temperature, and in contrary to aging at 21 °C, lower amounts of lithium are detected in the interface during long-term cycling. Instead, higher intensity of hydrocarbons and salt decomposition products, such as P−O and P−F, are detected. The HAXPES and NEXAFS results support the previously suggested PMS reduction reaction,14 although no conclusive results could be obtained. However, the results clearly show a second PMS decomposition product on the graphite electrode. This new decomposition product is suggested to have an oxidation state +2, which could originate from further reduction of previously suggested PMS decomposition products or from another decomposition mechanism. Similar interface aging processes are observed for the LiFePO4 positive electrode samples cycled at 21 and 60 °C. Long-term cycling leads to thicker interface layers. A thicker film is formed on the PMS sample on aging at 21 °C, whereas at 60 °C the standard electrolyte cell has a thicker interface. During long-term cycling fluorophosphates appear in the interface. The results suggest that they could be an electrolyte residue.

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and Patrik Johansson from Chalmers University of Technology are acknowledged for the PMS additive. We are thankful to Leif Nyholm, Torbjörn Gustafsson, Stevén Renault, David Rehnlund, and Bertrand Philippe at Uppsala University for useful discussions and support.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46 18 471 3713. E-mail: [email protected]. se. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Swedish Hybrid Vehicle Centre (SHC), the Swedish Energy Agency (STEM), StandUp for Energy, and Uppsala University. We thank HZB for the allocation of synchrotron radiation beam time at BESSYII. This research project has been supported by the European Commission under the seventh Framework Programme through the “Research Infrastructure” action of the “Capacities” Programme, NMI3-II Grant No. 283883. Frédéric Thébault 12659



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