In situ X-ray absorption spectroscopic study on LiNi_0.5Mn _0.5O₂ cathode material during electrochemical cycling

We have investigated the local electronic and atomic structure of the LiMn_0.5Ni_0.5O₂ electrode during the first charge and discharge process using in situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges. The Ni K-edge structure in the XANES spectrum shifts to higher energy during charge and shifts back reversibly during discharge in the higher voltage region of ∼4 V, whereas the Mn K-edge structure does not appear to exhibit a rigid edge shift. Further Li-ion intercalation during extended discharge in the 1-V plateau leads to the reduction of Mn⁴⁺ ions suggesting that the charge compensation in this region is achieved via the reduction of Mn⁴⁺ ions to Mn²⁺. Mn K-edge EXAFS results indicate that a small amount of Li is found in the Ni²⁺/Mn ⁴⁺ layers. These Li ions in the transition metal layers are primarily present in the second coordination shell of Mn and not around Ni. Ni K-edge EXAFS fitting results suggest that the oxidation process upon Li deintercalation takes place in two steps: Ni²⁺ to Ni³⁺ first, and then Ni³⁺ to Ni⁴⁺.