XPS investigations of electrolyte/electrode interactions for various Li-ion battery materials
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- Oswald, S., Mikhailova, D., Scheiba, F. et al. Anal Bioanal Chem (2011) 400: 691. doi:10.1007/s00216-010-4646-z
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For future Li-ion battery applications the search for both new design concepts and materials is necessary. The electrodes of the batteries are always in contact with electrolytes, which are responsible for the transport of Li ions during the charging and discharging process. A broad range of materials is considered for both electrolytes and electrodes so that very different chemical interactions between them can occur, while good cycling behavior can only be obtained for stable solid-electrolyte interfaces. X-ray photoelectron spectroscopy (XPS) was used to study the most relevant interactions between various electrode materials in contact with different electrolyte solutions. It is shown how XPS can provide useful information on reactivities and thus preselect suitable electrode/electrolyte combinations, prior to electrochemical performance tests.
KeywordsLi ion batteryElectrolyteSurface interactionXPS
Li-ion batteries are of growing interest, beside their application in many mobile devices, especially for future use in electric and hybrid vehicles. The increase of the specific energy density is a major challenge. Therefore, new materials and new working principles are under investigation. Basically all batteries consist of two electrodes (cathode and anode) coupled by an ionically conductive electrolyte medium. The electrolyte enables the charge carrier transport (Li+ ions) during charging and discharging, but must guarantee high stability and low self-discharge. Therefore, a key requirement for an electrode/electrolyte combination is its chemical and electrochemical stability.
X-ray photoelectron spectroscopy (XPS) is a well-accepted method for the investigation of the chemical behavior of thin films and surfaces. For example, surface-near changes of the electronic structure of LixCrMnO4 as the cathode material in a Li-ion battery were investigated. Therefore, a “quasi in situ” method was established by the use of a dedicated transfer chamber with Ar atmosphere to protect the samples in different states of charge and fatigue against changes/contamination due to contact with air during the transfer from glove box into spectrometer . It was revealed that the poor cycle stability of LixCrMnO4 in the highly charged state obtained during Li-extraction up to a cut-off voltage around 5.2 V is connected with the irreversible formation of Cr(6+)-like species, which are electrochemically inactive in the following reduction process .
The same method was now applied to investigate the interaction between several electrolytes and various electrode materials. This work was motivated by the observation that the capacity of some materials drops already in the first cycles, depending on the specific electrolyte used. With the XPS measurements, reaction products of both the electrode and (dried) electrolyte residuals can be observed. In this paper, results are presented for LiCrMnO4, LiMoO2, and Li2O2 electrode materials after contact with electrolytes based on LiPF6, LiBOB (lithium bis(oxalato)borate) or LiClO4 dissolved in ethylene carbonate/dimethyl carbonate (EC/DMC).
All the active or reference materials were used in powder form either prepared by ourselves or commercially available. LiCrMnO4 was synthesized following a modified Pechini process described already in [1, 2] leading to a single-phase material as confirmed by X-ray diffraction. LiMoO2 was prepared in an evacuated sealed silica tube at 950 °C from stoichiometric amounts of Li2MoO4 (Alfa Aesar, 99%) and Mo (Alfa Aesar, 99.99%). X-ray powder diffraction revealed a presence of two different modifications of LiMoO2: the rhombohedral one R-3 m with a = 2.801 Å, c = 15.507 Å, and the monoclinic one C2/m with a = 10.589 Å, b = 2.867 Å, c = 4.984 Å, β = 99.04°, which have very similar crystal structures. Li2O2 was a commercial product (Acros Organics); the considered reference materials LiF (Alfa Aesar), Li2CO3 (Fluka), and LiBOB (Chemetall), too.
The materials were either investigated as pure powders (Li2O2, reference materials) or as electrode mixtures (LiCrMnO4, LiMoO2). Such mixtures are necessary to ensure reasonable mechanical stability and sufficient electrical conductivity and typically consist of 80% active material, 10% carbon black, and 10% PVDF (polyvinylidene fluoride). After intensive mixing, the materials were dried in vacuum at 100 °C. The materials (the pure ones and the cathode mixtures) were then pressed on an aluminum grid, which is also used in electrochemical experiments as current collector. The reference materials were also measured as pure powder filled in a special sample holder with molds.
Electrolyte–electrode interaction experiments
All experiments to study the electrolyte-materials interaction were performed in an argon filled glove box. The oxygen and moisture contents in the glove box were always maintained below 1 ppm. The electrolytes consist of a solution of the active conducting salt dissolved in EC and DMC in a typical weight ratio of EC:DMC = 2:1. The concentration of the active components (LiPF6, LiBOB, LiClO4) was 1 M in all cases. The cathode materials, fixed on Al grids, were soaked in the electrolyte solutions for specified periods, washed with DMC and dried in dynamic vacuum after that.
The XPS measurements were carried out at a PHI 5600 CI (Physical Electronics) spectrometer which is equipped with a hemispherical analyzer operated with a typical pass energy of 29 eV and an analysis area of 800 μm in diameter. Monochromatic Al-Kα excitation (350 W) was used, additionally applying a low-energy electron charge neutralizer. To avoid any contact of the samples with air and moisture, a dedicated transfer chamber (Physical Electronics) was used to transfer the samples from the glove box to the XPS spectrometer. Unless the use of the charge neutralizer source different peak positions and/or peak shape changes induced by residual surface charging can be observed. To minimize these effects for all the shown spectra, the binding energy scale was corrected based on the C1s peak from contaminations (around 284.8 eV) or from the amorphous carbon (around 284.3 eV) as internal binding energy standard. However, this is only a first approximation, since differential charging  in the samples cannot be completely avoided. Otherwise, following the complex chemistry and the delicate sample handling (glove box, transfer chamber, ultra-high vacuum) products from side reaction are often found. Because of these reasons, the quantification of exact peak positions and chemical shifts is also difficult. Nevertheless, for the main peaks under discussion, peak fitting using the PHI-MULTIPAK software  was used to estimate the changes in chemical portions. Concentrations were estimated with the same software using standard single element sensitivity factors and assuming a homogeneous distribution of the elements in the surface-near region.
Sputter depth profiling was done using Ar+ ions (3.5 keV, scan size 2 mm × 2 mm) with conditions leading to a 1 nm/min sputter rate, determined for a Ta-oxide reference.
Results and discussion
LiCrMnO4 as a candidate for high-voltage (>5 V against Li/Li+) cathode material was already successfully tested using the standard LiPF6-containing electrolyte. The cycle stability during charging up to 4.8 V was sufficiently good, however, only half of the expected specific capacity of the material can be reached with this low end-of-charge voltage. For complete charging up to 5.2 V, a pronounced degradation was found, which is probably caused by irreversible structural changes of the material due to the different oxidation states of the electrochemically active Cr-ions. At such high voltages, electrolyte degradation cannot be ruled out completely, but a LixCrMnO4 electrode should be stable in contact with the LiPF6-containing electrolyte.
In fact, after contact of the LiCrMnO4 material with the electrolyte for 30 min no significant changes in the spectra of Mn and Cr were found. Li at the surface converts partially to LiF, as some residual humidity in the chemicals always leads to enough HF for such a surface reaction. However, the ratio of the metal concentrations Li:Cr:Mn remained nearly constant, confirming that the LiF formation is only a very minor and neglectable side reaction.
Results of peak fit of the Mo(4+/3+) and Mo(6+) species at the Mo3d5/2 peak for the LiMoO2 samples after storage in different electrolytes
Preliminary XPS investigations of Mo electronic states in LixMoO2 upon lithium deinsertion in electrochemical test cells with LiPF6 electrolyte solution have shown a side reaction between lithium molybdate and the electrolyte resulting in a formation of a thick solid-electrolyte-interface(SEI)-layer, mainly containing LiF and Li carbonate, while almost no Mo can be detected anymore within the escape depths of the photo electrons. The targeted studies of the electrolyte/electrode contact were therefore performed for three different electrolyte salts. The contact with LiPF6 electrolyte (Fig. 1 second spectra from bottom) revealed an enrichment of the surface with lithium and fluorine: a LiF layer is formed fast; Li is further enriched relative to Mo at the surface. The average oxidation state of Mo was slightly shifted to Mo(6+). A further experiment with LiClO4 as electrolyte salt (Fig. 1—third spectra from bottom) revealed a similar behavior: a thick Li–Cl–O layer was formed, and not any Mo (Fig. 1b) could be observed at the interface. Also the carbonate and C–O species are enhanced (Fig. 1c).
Only the electrolyte containing LiBOB showed no formation of a Li-containing overlayer (Fig. 1—top spectra). The oxidation state of Mo after contact with LiBOB was reduced to Mo(4+/3+), because the Mo3d7/2 peak at 236.5 eV corresponding to Mo(6+) has disappeared (Fig. 1b). Moreover, the Li excess on the surface was also reduced, approaching the expected LiMoO2 stoichiometry, and only traces of remaining boron were found.
Li2O2 is not an intercalation-type electrode material like LiCrMnO4 or LiMoO2, but formed during discharge on the cathode of a Li-air battery. In such a battery, oxygen is reduced on discharge at the cathode and reacts with lithium ions to lithium peroxide [7, 8]. Depending on the electrode kinetics of the oxygen reduction either lithium peroxide Li2O2 or lithium oxide Li2O can be formed . However, on carbon-based electrodes it has been shown by ex-situ Raman  and X-ray diffraction  that the main discharge product is Li2O2. On charge lithium peroxide is decomposed again into molecular oxygen and lithium ions.
Unlike classical insertion type materials, where Li ions are extracted or inserted from or into a bulk crystal, the electrochemical reaction in a Li-air battery is an electrocatalytic process which takes place only at the electrode interface. Therefore, the interface formed between the electrocatalytically active electrode surface and lithium peroxide might strongly influence the electrode kinetics for the peroxide oxidation leading to increased overpotentials for charging the battery. Hence, the chemical stability of bulk Li2O2 was tested against different electrolytes and the resulting interfaces were analyzed using XPS.
Results of peak fit of the Li1s peak with respect to the portions attributed to Li–F and Li–O for Li2O2 after storage in LiPF6
2 min LiPF6
2 h LiPF6
18 h LiPF6
XPS analysis of the Li2O2 surface after exposure to a LiBOB electrolyte solution showed an enrichment of boron oxide and carbonate species. This indicates an oxidative decomposition of the LiBOB salt by Li2O2 leading to the formation of boron oxide and Li2CO3.
In contrast, the XPS signature of the perchlorate anion is largely preserved even after an exposure time of 18 h, when only a weak chloride signal at 208.5 eV can be observed. While this may indicate a slow decomposition of LiClO4, it is most likely that the chloride signal originates from LiCl, already contained in the electrolyte solution as an impurity. Despite the rather good stability of the perchlorate salt, significant changes can be observed in the Li1s, O1s, and C1s spectra. This can be attributed to the formation of carbonates, which are clearly visible in the C1s spectrum. Unlike in the case of LiBOB, these carbonates cannot be a decomposition product of the electrolyte salt, but must be due to an oxidative decomposition of the electrolyte solvent.
The XPS results showed that LiPF6 and LiBOB react with Li2O2 and revealed that decomposition products of Li2O2 and the electrolyte salt can be found on the surface of the Li2O2 particles. This reaction does not only lead to a loss of lithium and electrolyte salt by forming electrochemically inactive decomposition products, but may also impact the electrocatalytic oxidation of Li2O2 by altering the interface between Li2O2 and the electrocatalytic active electrode surface. Therefore, LiPF6 and LiBOB are not suitable for the use in Li-air batteries. LiClO4 shows only weak or no decomposition and may be used as electrolyte salt. However, carbonate formation of the LiClO4 electrolyte indicates that the commonly used electrolyte solvents may also be chemically unstable in the presence of Li2O2.
The different stability of the electrolyte salt agrees well with electrochemical measurements, which showed almost no oxidation current in the LiPF6-based electrolyte, whereas in the LiClO4 electrolyte oxidation was observed above 3 V vs. Li/Li+.
Conclusions and outlook
It is shown that XPS as a standard method for examining chemical states at surfaces is also a very suitable tool for the study of electrode–electrolyte interactions in Li-ion battery research. Nevertheless, the application on real complex electrode composites is challenging by side-effects such as: surface roughness, differential charging/energy referencing, residuals of chemicals at the surfaces despite washing, residual surface alteration despite protection in a glove box and Ar-filled transfer chamber. Thus, quantitative results such as calculations of absolute surface concentrations are often not meaningful, however, the identification of the surface species resulting from the chemical reactions gives strong hints to elucidate the underlying reaction mechanisms. It can be concluded that the results for this class of materials are not predictable; the interactions between the conducting salts, the organic solvents and the oxide materials are too complex. However, conclusions about an appropriate electrolyte selection can be drawn based on XPS data in combination with results from electrochemical experiments on the same electrode/electrolyte combination.
Systematic studies on variations of electrode materials in combination with studies on suitable reference materials with XPS are planned and will be supported by spatially resolved surface investigations with high-resolution Auger-electron spectroscopy. However, stronger charging effects and electron-radiation-induced damages will be new challenges.
This work is financially supported by the BMBF (grant no. 03KP801 “Elektrochemie für Elektromobilität - Verbund Süd”)