Understanding the electrode/electrolyte interface reactivity in Li-Ion batteries via post mortem and operando XPS, XAS, and XPEEM investigations

The increasing need for light and long lasting Li-ion batteries demands the continuous search for suitable high voltage cathode active materials (CAM). One bottleneck is the electrode-electrolyte interface instability at high operating potentials (above 4.3 V vs. Li⁺/Li) and, in this context, there is an urgent need to understand the surface chemical reactivity and the surface structural changes of the CAM in order to gain more fundamental insights into the degradation mechanisms leading to the capacity and voltage fading of Li-ion batteries.

In this work, we combine X-ray photoelectron spectroscopy (XPS), X-ray photoemission electron microscopy (XPEEM), and X-ray absorption spectroscopy (XAS) to obtain a full picture of the electrode-electrolyte interfacial reactivity in several battery systems of current technological and scientific interest.

In the first part of my talk, I will present the post mortem investigation of LiNi (NCA) cycled vs. Li (LTO) in a carbonate-based liquid electrolyte. The analysis of the cathode reveals (i) the strong correlation between the overpotential observed during the first (de-)lithiation and the surface aging of the pristine particles covered by adventitious Li₂CO₃ and reduced Ni and Co, and (ii)the surface structural degradation and the formation of an inactive layer. Simultaneously, we demonstrate the poisoning of the anode by Ni, Co, and binder components caused by their migration/diffusion from the cathode at high voltage.

In the second part, I will show the recent development of the operando XPS, XAS and XPEEM spectroscopy for all-solid-state batteries. In particular, I will focus on the complex reactivity during the (de-)lithiation of SnO₂ as anode material by employing operando XPS and XAS, allowing the identification of intermediate species, which could not be observed in post mortem analyses. I will also present the recent results obtained with the operando XPEEM cell investigating a graphite working electrode, validating the feasibility of such experiments.