Origin of 5 V Electrochemical Activity Observed in Non-Redox Reactive Divalent Cation Doped LiM0.5 − xMn1.5 + xO4 ( 0 ⩽ x ⩽ 0.5 ) Cathode Materials: In Situ XRD and XANES Spectroscopy Studies

Abstract

Divalent cation doped lithiated Mn spinel with Zn and Mg as cathode materials for a lithium battery are investigated and partial reversible behavior is observed at the 5 V region. The electrochemical charge and discharge potential profiles of the Zn-doped materials indicate a close relationship between the lattice energy and lattice parameters in the Zn-doped spinel system. Lithium ions extracted from octahedral sites at the 5 V plateau during the charge cycle are partially reinserted back into the tetrahedral sites during the discharge step, which contributes to the partial reversible 5 V behavior. The significant findings reported here are that the strong tetrahedral site preference of divalent nonreactive cations such as Zn and Mg force Li cations onto octahedral sites in these materials, thus resulting in electroactivity at 5 V. In situ X-ray absorption spectroscopy measurements show that the Mn edge is shifted to higher energy at the 4 V plateau during charge cycle and remains unchanged at the 5 V plateau. In situ Zn -edge X-ray absorption near-edge structure measurements reveal that the valence state of zinc ions is unchanged at the 5 V plateau region. In situ Mn -edge extended X-ray absorption fine structure studies suggest that ions in the Zn-spinel lattice are partially oxidized to at the 5 V plateau during the anodic process and ions are reduced back to during the cathodic process at the 5 V plateau. The oscillations of the lattice parameters observed at the 5 V plateau region during the anodic charge step are attributed to chemical instability of ions.