Investigation into the ability of langbeinite-type K2M2(SO4)3 (M = Mn, Fe, Co and Ni) materials to accommodate Na: The importance of the electronegativity of the framework cation

In this work we report a study of the langbeinite-type K2-xNaxM2(SO4)3 materials (M ​= ​Mn, Fe, Co and Ni) in order to evaluate the level of Na incorporation possible. The work showed that the level of Na incorporation decreased across the transition metal series, with the highest level of Na incorporation (up to x=1.3) observed for M=Mn. This trend does not appear to be related to the ionic radius of the transition metal, but rather its’ electronegativity. We illustrate this relationship with the inclusion of our prior work on K2-xNaxMg2(SO4)3 where even higher levels of Na were possible, and demonstrate that through co-doping with Mg, higher Na levels can be achieved for K2-xNaxM2(SO4)3 materials (M ​= ​Mn, Fe). The dependence on the electronegativity of the divalent cation is attributed to greater electronegativity leading to enhanced polarization of the framework, thus allowing for greater stabilization of the smaller Na cation as it ‘moves’ towards the edge of the framework cage to better fulfil its’ coordination requirements. Given the interest in related systems for Na ion battery applications, this work provides a new factor to consider when designing new materials for such applications.