Unraveling the Role of Doping in Selective Stabilization of NaMnO2 Polymorphs: Combined Theoretical and Experimental Study

Dopants are known to modify structural, electronic, chemical, and other properties of materials; therefore, analysis of doping effects is of great interest in the fields of fundamental and applied science. However, in many functional materials, particularly transition metal (TM) compounds, such analysis could be quite complex owing to subtle interplay between possible oxidation states of various types of TM, which is hard to elucidate experimentally and difficult to model theoretically. In this work, we performed a study of the role of 3d TM and some non-TM dopants in stabilization of structural polymorphs of NaMnO2, a highly promising material for electrocatalysis and Na-ion battery applications. Our X-ray diffraction experiments and DFT+U modeling revealed the exclusive formation of α- or β-NaMnO2 polymorphs via substitutional doping of NaMnO2 by Ti or Cu cations, respectively, whereas doping with other elements results in formation of several structural polymorphs. In the most important case of stabilization of β-NaMnO2 by Cu cations, we find that geometry of this structure allows 2+ oxidation state of Cu, unlike α-NaMnO2, where Cu adopts a more artificial 3+ oxidation state, which explains lower stability of α-type polymorph.