Insights into the electronic structure and vibrational dynamics of Li7MnN4 anode material for Li-ion battery: A combined experimental and computational study

Abstract

The electronic structure of the promising Li-ion battery anode material Li7MnN4 synthesized by a solid-state reaction is studied using ab initio calculations completed by Raman spectroscopy experiments. The structural optimization reliably reproduces the experimental one, hence validating the accuracy of the chosen Density Functional Theory method. The theoretical analysis of the electronic structure reveals the nature of the valence band as composed from band filled by electrons with spin-up states only, which allows refuting literature data about the claimed electronic character of Li7MnN4. Actually, the calculated electronic band gap Eg = 0.95 eV is found to be in good agreement with available experimental data. A careful experimental approach provides the first experimental Raman spectra of hygroscopic Li7MnN4 at 293 K and 130 K. The analysis of the phonon states in the Γ-point of the Brillouin zone, completed by the computation of the Raman scattering intensities of the vibrational modes of the Li7MnN4 structure give a remarkable agreement between simulated and experimental Raman spectra. With such a good matching, a reliable assignment of all the observed Raman peaks to the vibrations of specific structural units in the Li7MnN4 lattices is proposed. In particular, the most intense band in the Raman spectrum is ascribed to a totally symmetric MnN4 breathing mode. We also show that, using different wavelengths of exciting radiation, the transition from off-resonance to resonance Raman scattering process can be observed. Furthermore, Raman spectroscopy is revealed as an efficient in situ diagnostic tool to control the degradation of the Li7MnN4 powder in open air through the observation of extra bands in the Raman spectra. Results of this study shed a light on the understanding of the fundamental properties of Li7MnN4 and pave a way for the upcoming operando Raman spectroscopy investigation of the atomic-scale induced structural changes of this negative electrode material for Li-ion battery.