Combined use of EPR and Na MAS NMR spectroscopy for assessing the properties of the mixed cobalt-nickel-manganese layers of P3-NaCoNiMnO.

Knowledge on the formation of mixed transition metal layers on lithium and sodium transition metal oxides, Li/Na(Co,Ni,Mn,)O, determines the ability to control their electrochemical properties as electrode materials in alkaline ion batteries. Taking this into account, herein we combine the EPR and Na MAS NMR spectroscopic techniques to gain insights into the structural peculiarities of the mixed cobalt-nickel-manganese layers of NaCoNiMnO with a three-layer stacking (P3-type) structure. Two types of compositions are examined where diamagnetic Co and paramagnetic Ni and Mn are stabilized: NaCoNiMnO and NaNiMnO. EPR spectroscopy operating in the X- and Q-band region is applied with an aim to improve the spectra resolution and, on the other hand, to provide straightforward information on the coordination of the transition metal ions inside the layers. The analysis of EPR spectra is based on the reference for the Mn and Ni ions occurring simultaneously in oxides with two layer stacking, P2-NaNiMnO. Complementary to EPR, Na MAS NMR spectroscopy at high spinning rates is undertaken to assess the local structure of the Na nucleus in the layered P3-NaCoNiMnO oxides. All results are discussed taking into account the EPR and NMR data for the well-known lithium analogues O3-LiCoNiMnO and O3-LiNiMnO. Finally, the structure peculiarities of the transition metal layers extracted from the EPR and NMR methods are demonstrated by electrochemical intercalation of Li ions into P3-NaCoNiMnO.