Adsorption of Dimethyl Sulfoxide on LiCoO2 Thin Films: Interface Formation Studied by Photoemission Spectroscopy

The formation of electrode–electrolyte interfaces is of key importance for the stability and performance of lithium-ion batteries. To increase efficiency and lifetime, a detailed understanding of the processes at these interfaces is necessary. Both chemical reactions resulting in the formation of interface layers as well as double layer formation, i.e., band bending, influence the batteries’ properties. In this contribution, we investigate the interface formation between a thin film LiCoO 2 cathode material and a solvent adsorbate. Using a surface-science approach, dimethyl sulfoxide (DMSO, (CH 3 ) 2 SO) was used as molecular probe and adsorbed stepwise onto LiCoO 2 at low temperature. After each step, the interface was analyzed by synchrotron-based soft X-ray photoemission spectroscopy (SXPS). We established the energy level alignment of the LiCoO 2 −DMSO interface, showing a band bending downward in the cathode material attributed to Li-ion transport into the adsorbate phase. Lithium transport results in the formation of a reaction layer containing Li 2 CO 3 and Li 2 O, constituting a solid electrolyte interphase layer (SEI).