The development of the next generation high-energy density Li-ion batteries requires a deep understanding on the interface between high voltage cathode and electrolyte. When cathode materials are cycled to high potential, especially above 4.3 V vs. Li⁺/Li, different reactions can take place at the cathode-electrolyte interface: electrolyte oxidation¹, surface structural changes, dissolution of the transition metals (TMs)², oxygen oxidation³ and release⁴, and binder degradation⁵. Moreover, all those parasitic reactions can also affect the counter electrode via a "cross-talk" process of organic/inorganic species and TMs. To elucidate these complex reaction mechanisms, a combination of different spectroscopy techniques is required in order to have full picture of the interface reactivity. Although X-ray photoelectron spectroscopy (XPS) provides valuable knowledge concerning the binder and electrolyte stability, information about the TMs oxidation states and the surface structure are more difficult to obtain. On the other hand, X-ray absorption spectroscopy (XAS) of TM L-edges, C K and O K-edges provides very important knowledge on the oxidation states and structural evolution during cycling. By combining the high lateral resolution and surface sensitivity of X-ray photoemission electron microscopy (XPEEM)⁶ with total electron yield (TEY) and total fluorescence yield (TFY), one can gain a better understanding of the evolution of both the surface and the bulk of the particles.

In this contribution, we show the successful combination of XPS, XPEEM and XAS in TEY and TFY modes to elucidate the complex surface reactivity of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) when it is cycled versus Li₄Ti₅O₁₂ (LTO), especially at 4.9 V vs. Li⁺/Li in LP30 electrolyte. Our results show the instability at high potential of the organic binder contained in the electrodes and the LiPF₆ salt in the electrolyte, while EC:DMC oxidation byproducts are not observed on neither the cathode nor the anode despite the high potential. Reduced Ni is detected on the NCA surface and, due to the limited oxygen release observed from the NCA material⁷, reversible oxygen oxidation is observed in the O K-edge spectra at the NCA surface, proving the participation of O together with Ni in the redox process of NCA at high potential (Figure 1a,b). The cubic-like NiO structure is also visible in the O K-edge caused by structural change. Despite the presence of Ni, Co and LiF on the LTO surface (Figure 1c), due to the instability of reduced TMs and the LiPF₆/binder at the cathode-electrolyte interface, their influence on the electrochemical performances of LTO is limited. Instead, the overall fading of the NCA vs. LTO cell is mainly attributed to the structural degradation of the NCA particles.

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