Role of the LiPF6 salt for the long-term stability of silicon electrodes in Li-ion batteries - A photoelectron spectroscopy study
Résumé
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.
Mots clés
Soft X-ray
Surface oxide
Theoretical capacity
Alloys
Cerium alloys
Electrochemical cells
Electrodes
Electrolytes
Lithium
Photoelectron spectroscopy
Potential energy surfaces
Seebeck effect
Surface reactions
Synchrotrons
X ray photoelectron spectroscopy
Silicon
Alloying process
At rests
Capacity fading
Charge/discharge
Depth-resolved analysis
Hard X ray
Li-ion batteries
Lithium-ion battery
Long term stability
Negative electrode
Non destructive
Partially fluorinated
Passivation layer
PES
SEI
Silicon electrode
Silicon nanoparticles
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