Kristina Edström, Uppsala University, Uppsala, Sweden
Important and critical reactions take place on the electrode surfaces and at interfaces to the electrolyte in Li-ion and Na-ion batteries. The operation voltage of these electrodes is typically close to or even outside the thermodynamic stability window of the electrolyte, which leads to electrolyte decomposition even with the use of organic electrolyte solvents. For Li-ion batteries, this process is self-limiting on the negative electrode as reduction of the electrolyte solvents and salts forms a passivating layer. This is the layer is called the solid electrolyte interphase (SEI). This interphase is at a certain thickness passivating for electrons to tunnel through it but still ionically conducting for lithium or sodium ions. It is hence essential for the prevention of extensive and continuous electrolyte decomposition. On the positive electrode, oxidation of solvents, decomposition of the electrolyte salt, and dissolution of metal ions from the electrode material may decrease battery performance. Altogether, the properties of interfaces and interphase layers in Li-ion and Na-ion batteries influence to a large extent, parameters such as safety, capacity loss, rate capability, and cycle life. Photoelectron spectroscopy (PES) is a widely used technique to study surfaces and interfaces in batteries because of its sensitivity to chemical environments/chemical shifts and as a technique to study the redox state. PES provides information on elemental compositions, relative amounts, and the depth distribution of the species analyzed. In this presentation, the possibility to follow the formation of the SEI is discussed as new techniques such as ambient pressure XPS are developed. Model systems are investigated and as a background to understand the results a description of how to assess XPS peak positions which shift due to SEI formation. The impact on peak positions caused by changing electrode potentials were using a model system consisting of a mixture of Li₄Ti₅O₁₂, carbon coated Li₂FeSiO₄, carbon coated LiFePO₄, LiMn₂O₄, Carbon black, and PVdF-HFP binder. Also, standard LiMn₂O₄ and nano-Si electrodes were analyzed to demonstrate changes to the spectra caused by changing Li content (state of charge). The results show that components in the SEI have a significantly different binding energy reference point relative to the bulk electrode material (i.e., up to 2 eV). It is also shown that electrode components with electronically insulating/semiconducting nature are shifted as a function of electrode potential relative to highly conducting materials. Further, spectral changes due to lithiation/sodiation are highly depending on the nature of the active material and its lithiation/sodiation mechanism. Correct interpretations of peak shifts are necessary for correct data interpretation but even more important is the information the peak shifts can provide about double layers at buried interfaces within the electrode. Considering battery operation, it should be noted that the presence of a double-layer potential drop at an interface between different battery components will influence the (de)lithiation kinetics because it may act as a barrier for Li-ion or Na-ion insertion/extraction depending on the dipole orientation.