Operando APXPS for direct probing of Li ion battery LCO electrode/electrolyte interface chemistry during lithiation/delithiation†

Abstract

The real-time interface chemistry between the lithium cobalt oxide (LCO) working electrode and the LiClO₄/propylene carbonate (PC) electrolyte is investigated during lithiation/delithiation using dip-and-pull ambient pressure photoelectron spectroscopy (APXPS). The APXPS results appear to exhibit the seldom discussed Co²⁺ state in the LCO structure, where the operando measurements indicate electron transfer among Co²⁺, Co³⁺, and Co⁴⁺ states. Specifically, the lithiation of LCO reduces the Co⁴⁺ state to both Co³⁺ and Co²⁺ states, where, as a function of voltage, reduction to the Co²⁺ state is initially more pronounced followed by Co³⁺ formation. In addition, a delay in surface delithiation is observed during the reverse potential steps. This is discussed in terms of overpotential at the interface measurement position as a consequence of the dip-and-pull setup for this experiment. Finally, the shifts in the apparent binding energies of the spectral features corresponding to the electrolyte and LCO at their interface show that the electrochemical potentials at delithiation voltage steps are different from the lithiation steps at the same applied voltages. This further explains the non-responsive delithiation. The BE shift observed from the LCO surface is argued to be dominantly due to the semi-conductive nature of the sample. Overall, this article shows the importance of operando APXPS for probing non-equilibrium states in battery electrodes for understanding electron transfer in the reactions.