Multi-length-scale x-ray spectroscopies for determination of surface reactivity at high voltages of LiNi_0.8Co_0.15Al_0.05O₂ vs Li₄Ti₅O₁₂

The surface evolution of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) and Li₄Ti₅O₁₂ (LTO) electrodes cycled in a carbonate-based electrolyte was systematically investigated using the high lateral resolution and surface sensitivity of x-ray photoemission electron microscopy combined with x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. On the cathode, we attest that the surface of the pristine particles is composed of adventitious Li₂CO₃ together with reduced Ni and Co in a +2 oxidation state, which is directly responsible for the overpotential observed during the first de-lithiation. This layer decomposes at 3.8 V vs Li⁺/Li, leaving behind a fresh surface with Ni and Co in a +3 oxidation state. The charge compensation upon Li⁺ extraction takes place above 4.0 V and is assigned to the oxidation of both Ni and oxygen, while Co remains in a +3 oxidation state during the whole redox process. We also identified the formation of an inactive surface layer already at 4.3 V, rich in reduced Ni and depleted in oxygen. However, at 4.9 V, NiO-like species are detected accompanied with reduced Co. Despite the highly oxidative potential, the surface of the cathode after long cycling is free of oxidized solvent byproducts but contains traces of LiPF₆ byproducts (LiF and PO_xF_y). On the LTO counter electrode, transition metals are detected only after long cycling vs NCA to 4.9 V as well as PVdF and LiPF₆ byproducts originating from the cathode. Finally, harvested cycled electrodes prove that the influence of the crosstalk on the electrochemical performance of LTO is limited.