Unravelling redox processes of Li₇MnN₄ upon electrochemical Li extraction-insertion using operando XAS

A large data set of XAS (X-ray Absorption Spectroscopy) Manganese K-edge spectra has been collected operando and studied upon the electrochemical oxidation of the promising Li-ion battery anode material Li₇MnN₄. Using chemometric tools such as PCA (Principal Component Analysis) and MCR-ALS (Multivariate Curve resolution - Alternating Least Squares), three independent environment spectra were insulated. Based on the faradaic yield and well-chosen comparison of absorption spectrum energies within the frame of the coordination charge model, these environments were ascribed to unusual oxidation states allowed by nitride chemistry at a low potential (∼1.2 V vs. Li⁺/Li), i.e. Mn⁵⁺ (3d²), Mn⁶⁺ (3d¹) and Mn⁷⁺ (3d⁰). Also, their relative amounts are discussed with regard to the long-range structural variation which can be simply described by two successive biphasic domains followed by a solid-solution behaviour. Gathering this long-range and local structure information provides a complete picture of the redox mechanisms occurring in Li₇MnN₄.