The Li-ion technology still needs improvements to fulfil the requirements for electric mobility, especially in terms of energy density. LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode material thanks to its high energy density, outperforming current commercialized cathode materials [1]. However, it suffers from interfacial stability problems in long-term applications, especially in full-cell configuration against a graphite electrode. To understand the underlying problem of stability, Raman spectroscopy was selected to study the “near-surface” region of the electrode materials (~30 to ~300 nm) [2]. Raman spectroscopy has been used to characterize pristine LNMO (e.g. ordered or disordered LNMO type [3]) but difficulties were encountered to properly assign the observed vibration modes. To make such assignment, an assumption of discernible Ni-O and Mn-O bond vibrations in the Raman spectrum is usually postulated [4]. However, no proof has yet been given to support this approach.

In order to properly assign the Raman signals and to use an original approach to determine the transition metal oxidation states of disordered LNMO during (de)lithiation, we combined operando X-ray diffraction (Figure 1a) and operando Raman spectroscopy measurements (Figure 1b). During cycling, (de)lithiation of the spinel occurs with changes in the oxidation states of Ni²⁺↔Ni³⁺↔Ni⁴⁺ at ca. 4.7 V vs. Li⁺/Li, respectively correlating with three different cubic phases (all Fd-3m, Figure 1a). It can be seen that for each phase, different Raman signatures are obtained (Figure 1b) that are characterized by changes in peak intensities/positions. Unfortunately, no distinct vibration signal could be specifically attributed to only Ni-O vibration modes.

We resolve the last bottleneck for understanding the Raman spectra by simulating and coupling the Raman vibration modes of the LNMO spinel to their calculated intensities by means of density functional theory calculations (using CPKS method with CRYSTAL14 code). The results of the simulated Raman intensities prove for the first time the distinct major contributions of Ni-O and Mn-O vibrations to different Raman peaks and confirm the assumption of discernible Ni-O and Mn-O bond vibrations.

All these results, combining information from bulk and surface, will be discussed to demonstrate that Raman spectroscopy combined with calculated Raman intensities is the tool of choice for investigating LNMO as a future cathode material for Li-ion batteries.

Aknowlegdement

The authors thank SAFT Company for its financial support.

References

[1] A. Manthiram, K. Chemelewski, and E.-S. Lee, “A perspective on the high-voltage LiMn1.5Ni0.5O4 spinel cathode for lithium-ion batteries,” Energy Environ. Sci., vol. 7, no. 4, p. 1339, 2014.

[2] R. Baddour-hadjean and J.-P. Pereira-Ramos, “Raman Microspectrometry Applied to the Study of Electrode Materials for Lithium Batteries,” Chem. Rev., vol. 110, pp. 1278–1319, 2010.

[3] N. Amdouni, K. Zaghib, F. Gendron, a. Mauger, and C. M. Julien, “Structure and insertion properties of disordered and ordered LiNi0.5Mn1.5O4 spinels prepared by wet chemistry,” Ionics (Kiel)., vol. 12, no. 2, pp. 117–126, Jun. 2006.

[4] D. H. Park, S. T. Lim, S.-J. Hwang, J.-H. Choy, J. H. Choi, and J. Choo, “Influence of nickel content on the chemical bonding character of LiMn2−xNixO4 spinel oxides,” J. Power Sources, vol. 159, no. 2, pp. 1346–1352, Sep. 2006.