During the charging and discharging of lithium-ion-battery cathodes through the de- and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of (18)O-labelled Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2, which demonstrates that oxygen is extracted from the lattice on charging a Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2 cathode, although we detected no O2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li(+) removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn(4+) and Li(+) ions, which serve to promote the localization, and not the formation, of true O2(2-) (peroxide, O-O ~1.45 Å) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.

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Largely based on its very high rechargeable capacity, silicon appears as an ideal candidate for the next generation of negative electrodes for Li-ion batteries. However, a crucial problem with silicon is the large volume expansion undergone upon alloying with lithium, which results in stability problems. Means to avoid such problems are largely linked to the understanding of the interfacial chemistry during charging/discharging. This is especially of great importance when using nanometric silicon particles. In this work, the interfacial mechanisms (reaction of surface oxide, Li−Si alloying process, and passivation layer formation) accompanying lithium insertion/ extraction into Si/C/CMC composite electrodes have been scrutinized by Xray photoelectron spectroscopy (XPS). A thorough nondestructive depthresolved analysis was carried out by using both soft X-rays (100−800 eV) and hard X-rays (2000−7000 eV) from two different synchrotron facilities compared with in-house XPS (1487 eV). The unique combination utilizing hard and soft X-ray photoelectron spectroscopy accompanied with variation of the analysis depth allowed us to access interfacial phase transitions at the surface of silicon particles as well as the composition and thickness of the SEI (electrode/electrolyte interface layer).

Silicon as a negative electrode material for lithium-ion batteries has attracted tremendous attention due to its high theoretical capacity, and fluoroethylene carbonate (FEC) was used as an electrolyte additive, which significantly improved the cyclability of silicon-based electrodes in this study. The decomposition of the FEC additive was investigated by synchrotron-based X-ray photoelectron spectroscopy (PES) giving a chemical composition depth-profile. The reduction products of FEC were found to mainly consist of LiF and −CHF–OCO2-type compounds. Moreover, FEC influenced the lithium hexafluorophosphate (LiPF6) decomposition reaction and may have suppressed further salt degradation. The solid electrolyte interphase (SEI) formed from the decomposition of ethylene carbonate (EC) and diethyl carbonate (DEC), without the FEC additive present, covered surface voids and lead to an increase in polarization. However, in the presence of FEC, which degrades at a higher reduction potential than EC and DEC, instantaneously a conformal SEI was formed on the silicon electrode. This stable SEI layer sufficiently limited the emergence of large cracks and preserved the original surface morphology as well as suppressed the additional SEI formation from the other solvent. This study highlights the vital importance of how the chemical composition and morphology of the SEI influence battery performance.

The relationship between the elevated temperature performance of Li/graphite half-cells and the composition and morphology of the solid electrolyte interphase (SEI) formed on the graphite surface has been investigated for two electrolyte systems: 1MLiPF6 in ethylene carbonate/dimethyl carbonate EC/DMC (2:1) and 1MLiBF4 in EC/DMC (2:1). Precycled cells were stored at different temperatures up to 80°C, and the graphite electrodes were analyzed chemically (by X-ray photoelectron spectroscopy) and electrochemically under continued cycling. Loss of charge (for both salts) and of intercalation capacity (for LiBF4false) occurred after elevated temperature storage. The charge loss could be coupled to disappearance of the R­OCO2normalLi phase from the surface, with subsequent exposure of the graphite surface. The amount of LiF increased with increased storage temperature, but the LiF morphology differed between the two electrolyte systems. A model for the morphological changes of the SEI layer on storage at elevated temperature is proposed. © 2001 The Electrochemical Society. All rights reserved.

In the race for better Li-ion batteries, research on anode materials is very intensive as there is a strong desire to find alternatives to carbonaceous negative electrodes. A large part of these studies is devoted to alloying reactions, which have been known for more than thirty years but that have regained great interest by downsizing particle sizes, moving to nano-textured/ nanostructured composites, or designing new electrode concepts. It is not the scope of this review to retrace twenty-five years of research, but rather to highlight recent advances that have been made in the use of Sn or Si-based electrodes together with the remaining challenges to be addressed and issues to be solved prior to such electrodes being commercially implemented in Li-ion cells.

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Silicon presents a very high theoretical capacity (3578 mAh/g) and appears as a promising candidate for the next generation of negative electrodes for Li-ion batteries. An important issue for the implementation of silicon is the understanding of the interfacial chemistry taking place during charge/discharge since it partly explains the capacity fading usually observed upon cycling. In this work, the mechanism for the evolution of the interfacial chemistry (reaction of surface oxide, Li–Si alloying process, and passivation layer formation) upon long-term cycling has been investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100–800 eV) and hard X-rays (2000–7000 eV) from two different synchrotron facilities. The results are compared with those obtained with an in-house spectrometer (1486.6 eV). The important role played by the LiPF6 salt on the stability of the silicon electrode during cycling has been demonstrated in this study. A partially fluorinated species is formed upon cycling at the outermost surface of the silicon nanoparticles as a result of the reaction of the materials toward the electrolyte. We have shown that a similar species is also formed by simple contact between the electrolyte and the pristine electrode. The reactivity between the electrode and the electrolyte is investigated in this work. Finally, we also report in this work the evolution of the composition and covering of the SEI upon cycling as well as proof of the protective role of the SEI when the cell is at rest.