Operando Ambient Pressure X Ray Photoelectron Spectroscopy for Li-Ion Battery Electrode/Electrolyte Interface Probing
Authors: Liu Qianhui , Ericson Tove , Kallquist Ida , Lindgren Fredrik , Hahlin Maria 
Published: February 2024

Electrochemical reactions occur at the electrode/electrolyte interface. In lithium-ion batteries, the reversible Li ion insertion/desertion from the interfaces of electrodes gives the capacity of the battery. The surface phase transition and component chemical states change during the lithiation/delithiation in turn hugely effect the further battery capacity and reversibility. In the past decades, many ex-situ characterizations gave some insights of electrode surface behavior after cycling. However, the surface chemistry of electrode relies decisively on the real battery operation condition, e.g. the passive layer formed on the electrode surface changes dramatically with the absence of electrolyte. Therefore it is emerging and necessary to develop operando study for electrode/electrolyte interface probing during battery cycling.
X-ray photoelectron spectroscopy (XPS) as a surface sensitive technique detects the photoelectrons ejected from core level with certain pass energy. The binding energy of the electrons give the information of chemical composition and chemical states of the materials. Ambient pressure XPS, developed rapidly in recent years after since it gives possibility of the participation of gas and/ liquid phase, enabling the observation of structure dynamics during chemical reactions, thereby unprecedentedly extended the application of the technique. A method called dip-and-pull is used for operando liquid/solid interface probing. With a typical three-electrode system, the electrode is pulled up from the electrolyte with still contact to liquid to maintain the circuit. A thin film of electrolyte formed with a shape of meniscus at the interface of electrode, electrolyte and atmosphere is probed through. The probing of the electrochemical active surface through the liquid can be achieved with an optimized liquid layer and surface condition.
In this study Lithium cobalt oxide (LCO), as the most widely used positive electrode for Li-ion battery with rock salt layered structure optimized for reversible Li intercalation/deintercalation, is used for interface dynamics investigation. As Li is deintercalated from LCO structure, the charge transfer from Li+ to Co-O environment causes a phase change. It was commonly assumed that the charge due to the deintercalation of Li from structure is fully transferred to Co3+, i.e. the loss of one electron contributes to one Co4+ from Co3+ oxidation. However, it is recently reported that the real condition maybe more complicated. The charge transfer from Li+ to Oa- (a<2) might take place especially during high voltage due to the hybrid fermi level of O and Co which might increase the mobility of Oa- and further results in cobalt reduction and oxygen evolution reaction. But the mechanism of structure evolution under corresponding potentials are very unclear. In this study, with the operando probing with APXPS, it is discovered that a native phase of Co2+ may play an unignorable role in the charge transfer, giving some new understandings of phase transition during lithiation/delithiation (see figure 1). With the development of APXPS for battery probing, combining with software data treatment, a clearer picture of phase evolution is possible to be given by time-resolved probing with APXPS. Potentially hidden intermediate states during electrochemical reactions which cannot be given from ex-situ probing are resolved.

Figure 1