Chemical State Analysis of Li_1−xNi_0.5Mn_1.5O₄ by Soft X-ray Absorption Spectroscopy

Abstract

Soft X-ray absorption spectroscopy measurements were carried out in order to clarify the chemical states of Li_1−xNi_0.5Mn_1.5O₄ for the cathode in lithium ion batteries. XANES spectra exhibited that Ni valence varies from divalent to tetravalent via an intermediate state of trivalent although Mn valence remains tetravalent. These phenomena originate from the formation of the occupied d-orbital of Ni in the vicinity of the Fermi level in the valence band of Li_1−xNi_0.5Mn_1.5O₄, which was supported by the first-principles calculations. Thus, it is concluded that Ni plays an important role in maintaining charge neutrality of Li_1−xNi_0.5Mn_1.5O₄ releasing Li⁺ ions by the charging of the battery.