Improving the energy density and power density of electrode materials for Li-ion batteries (LIBs) is highly important to further develop electric and hybrid-electric vehicles. For the improvements, understanding the charge-discharge mechanisms from a viewpoint of the electronic structure is indispensable. Recently, electronic-structure analyses of the electrode materials using soft x-ray spectroscopy have been of particular importance.

In this study we demonstrate operando Mn L₃ soft x-ray emission spectroscopy (XES) for LiMn₂O₄ [1] which is a typical cathode material of LIB. We developed an operando cell consisting of the LiMn₂O₄ cathode, a counter electrode and an electrolyte solution [2] by modifying the in situ cell for fuel cell catalysts [3]. The charge-discharge experiments were performed by cyclic voltammetry. The operando XES experiments were carried out using ultrahigh-resolution x-ray emission spectrometer [4] at BL07LSU of SPring-8. The XES spectra were analyzed by theoretical analyses based on the configuration-interaction full-multiplet (CIFM) calculation [5-7].

In the operando Mn L₃ XES study for LiMn₂O₄, we revealed that the open-circuit voltage (OCV) state is almost the same as the initial state consisting of the Mn³⁺ and Mn⁴⁺ states. For the charged state, the Mn L₃ XES spectrum largely changed corresponding to the oxidation of Mn³⁺ to Mn⁴⁺ state at the Mn³⁺ site for the OCV. The spectrum for the discharged state almost returned to the spectrum for the OCV, while the small difference indicates that the Mn³⁺ state is slightly enhanced for the discharged state. Thus, the Mn 3d electronic states were reversibly changed for the charge-discharge process [2]. Moreover, charge-transfer (CT) effects between the Mn 3d and O 2p orbitals for each valence state were clarified. For the Mn⁴⁺ state, a negative CT energy was determined by the CIFM calculation, suggesting a very strong CT effect from the O 2p to Mn 3d orbitals.

In the presentation, XES and soft X-ray absorption spectroscopy studies for other electrode materials will also be reported, and the relationship between the electronic structure and electrochemical performance will be discussed.

References

[1] M. M. Thackeray, Prog. Solid St. Chem. 25, 1 (1997).

[2] D. Asakura et al., Electrochem. Commun. 50, 93 (2015).

[3] H. Niwa et al., Electrochem. Commun. 35, 57 (2013).

[4] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).

[5] Y. Nanba et al., J. Phys. Chem. C 116, 24896 (2012).

[6] Y. Nanba et al., Phys. Chem. Chem. Phys. 16, 7031 (2014).

[7] D. Asakura et al., J. Phys. Chem. Lett. 5, 4008 (2014).