Investigation of the local structure of the LiNi_0.5Mn_0.5O₂ cathode material during electrochemical cycling by X-ray absorption and NMR spectroscopy

In situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges and ⁶Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy have been carried out during the first charging and discharging process for the layered LiNi_0.5Mn_0.5O₂ cathode material. The Ni K-edge structure in the X-ray absorption near-edge structure (XANES) spectrum exhibits a rigid positive energy shift with increased Li deintercalation level, while the Mn XANES spectra do not show any substantial energy changes. The Ni edge shifts back reversibly during discharge. Further Li-ion intercalation at ∼1 V (vs. Li) could be accomplished by reduction of the Mn⁴⁺ ions. The ⁶Li MAS NMR results showed the presence of Li in the Ni²⁺/Mn⁴⁺ layers, in addition to the expected sites for Li in the lithium layers. All the Li ions in the transition metal layers are removed on the first charge, leaving residual lithium ions in the lithium layers.