2, FZ Jülich, Jülich, , Germany
For state-of-the-art rechargeable batteries, the working principle is commonly based on the reversible insertion of alkali metal ions into a host structure, usually a layered transition metal oxide for the positive electrode (cathode). The nature of the alkali insertion reaction into the host structure is a key issue for the performance of the electrode material, such as electrode potential and reversible capacity. Both are intimately coupled to the electronic structure of the material and its evolution upon deintercalation. In the past, the electronic structure of intercalation materials has been inferred from the performance of the material such as its electrode potential at different state of charge (SOC), or has been experimentally determined using electrodes which were electrochemically deintercalated using liquid electrolyte and then analyzed after emersion by XPS and XAS (“post mortem analysis”). The results of these studies are subject to uncertainties due to the large surface sensitivity of the measurement techniques. For post mortem studies of cathode materials that were in contact with liquid electrolytes, surface contamination by electrolyte residuals and the solid electrolyte interface (SEI) layer are inevitable and oppose an unambiguous interpretation of the data.
In this contribution we present an in operando XPS study of the cathode material NaxCoO2, which to our knowledge is the first study of this kind dealing with a layered oxide cathode material. An all-solid-state-battery with a NaxCoO2 cathode was assembled under UHV conditions in such a way that it was possible to measure XPS of the bare cathode surface in operando while cycling the battery. In addition, batteries were precharged in situ and transferred under UHV conditions to the synchrotron facility BESSY II to measure XAS at the cathode surfaces at various charge states.
In the XPS and XAS measurements using the all-solid-state-battery approach we were able to follow the charge process in the core levels (Co2p, O1s, Na1s) as well as in the valence band region without uncertainties caused by electrode-electrolyte interactions. During charging a decrease in alkali content and change in cobalt oxidation state of the cathode material is clearly visible. Already at comparably low deintercalation states -within the reversible region- changes in the O2p orbitals were measured. Correlation of XPS/XAS and electrochemical data indicate a reflection of the electronic structure evolution by the charge curve.