The understanding of the electrode-electrolyte interface in general and of the degradation reaction mechanisms at the surface of the active materials in particular remain crucial topics in the search for high energy density and longevity next generation of Li-ion batteries. For both liquid and solid-state electrolytes, the interface stability challenges associated with the electrolytes and the surface structure of the active materials arises when the cell operates at high working voltages.
In this thesis, I highlight how the combination of X-ray photoelectron spectroscopy (XPS), X-ray photoemission electron microscopy (XPEEM) and X-ray absorption spectroscopy (XAS) can provide a reliable platform to investigate the electrode-electrolyte interface and the surface modifications of the active materials. The complementarity of these techniques allows an in-depth analysis from the surface (few nm) to the bulk (few hundreds of nm) of the active material particles and with different lateral resolutions, thus probing from single particles to the overall electrode.
The first part of the thesis is dedicated to the combination of post mortem measurements to investigate electrodes cycled in liquid electrolytes. The LiNi0.8Co0.15Al0.05O2 (NCA) cathode is thoroughly investigated at different states of charge, clarifying the role during cycling of the adventitious carbonate on the pristine NCA powder and the charge compensation mechanisms occurring during (de-)lithiation. In particular, at the high potential of 4.9 V vs. Li+/Li, oxygen redox activity is observed on the NCA surface, accompanied with partial dissolution of the transition metals, which are subsequently detected after long-term cycling on the Li4Ti5O12 (LTO) counter electrode.
In the second part of the thesis, I discuss the development of electrochemical cells for operando and in-situ analysis, using a solid-state electrolyte to adhere to the vacuum requirement of XPS, XAS and XPEEM techniques. The combination of in-situ XAS and operando XPS analysis during (de )lithiation of SnO2 conversion-alloy anode material allows us to identify with remarkable precision the potential at which the different redox reactions take place. The versatility of the electrochemical cell for operando XPS measurements allows also the in-situ XAS monitoring of the transition metals oxidation state in the case of the cathode LiNi1/3Co1/3Mn1/3O2 (NCM111). Finally, a newly developed electrochemical cell for in-situ XPEEM is presented, permitting one to localise with high lateral resolution the reactions occurring on the surface of the active materials and the solid-electrolyte.
The establishment and validation of this platform of techniques opens the door to the investigation of complex materials for the next generation of Li-ion batteries for both liquid and solid-state electrolytes, with a particular focus on the surface reactions often responsible for the Li-ion battery capacity fading during prolonged cycling
