Combined use of EPR and 23Na MAS NMR spectroscopy for assessing the properties of the mixed cobalt–nickel–manganese layers of P3-NayCo1−2xNixMnxO2

Knowledge on the formation of mixed transition metal layers on lithium and sodium transition metal oxides, Li/Na(Co,Ni,Mn,)O2, 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 23Na MAS NMR spectroscopic techniques to gain insights into the structural peculiarities of the mixed cobalt–nickel–manganese layers of NayCo1−2xNixMnxO2 with a three-layer stacking (P3-type) structure. Two types of compositions are examined where diamagnetic Co3+ and paramagnetic Ni3+ and Mn4+ are stabilized: Na2/3Co1/3Ni1/3Mn1/3O2 and Na1/2Ni1/2Mn1/2O2. 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 Mn4+ and Ni2+ ions occurring simultaneously in oxides with two layer stacking, P2-Na2/3Ni1/3Mn2/3O2. Complementary to EPR, 23Na MAS NMR spectroscopy at high spinning rates is undertaken to assess the local structure of the Na nucleus in the layered P3-NayCo1−2xNixMnxO2 oxides. All results are discussed taking into account the EPR and NMR data for the well-known lithium analogues O3-LiCo1/3Ni1/3Mn1/3O2 and O3-LiNi1/2Mn1/2O2. 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-NayCo1−2xNixMnxO2.