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

Area of Science:

• Materials Science
• Electrochemistry
• Spectroscopy

Background:

• Lithium-ion batteries are crucial for energy storage.
• Developing advanced anode materials is key to improving battery performance.
• Li₇MnN₄ shows promise as a next-generation anode material.

Purpose of the Study:

• To investigate the electrochemical oxidation mechanisms of Li₇MnN₄.
• To identify the oxidation states of manganese during battery operation.
• To correlate local and long-range structural changes with redox processes.

Main Methods:

• Operando X-ray Absorption Spectroscopy (XAS) at the Manganese K-edge.
• Chemometric analysis including Principal Component Analysis (PCA) and Multivariate Curve Resolution - Alternating Least Squares (MCR-ALS).
• Coordination charge model for analyzing absorption spectrum energies.

Main Results:

• Three distinct manganese oxidation states (Mn⁵⁺, Mn⁶⁺, Mn⁷⁺) were identified, enabled by nitride chemistry at low potentials (~1.2 V vs. Li⁺/Li).
• Faradaic yield and spectral analysis confirmed these unusual oxidation states.
• Relative amounts of these states correlate with biphasic domains and solid-solution behavior during structural changes.

Conclusions:

• The study provides a comprehensive understanding of the redox mechanisms in Li₇MnN₄.
• Unusual high oxidation states of manganese are accessible in this nitride material.
• Combining local and long-range structural data offers a complete picture of battery material behavior.