(Invited) On the Balance of Intercalation and Conversion Reactions in Battery Cathodes

Batteries based on the intercalation of multivalent ions, such as magnesium, offer substantial improvements in volumetric energy density owing to non-dendritic plating/stripping on high-energy density metal anodes. However, the stability of magnesium oxide (MgO) creates a strong driving force for reactions between Mg 2+ ions and transition metal oxide cathodes to follow a conversion pathway (resulting in MgO and reduced transition metal oxides) rather than the desired intercalated pathway. The formation of MgO is particularly problematic: In addition to destroying intercalation cathode material, Mg 2+ conductivity in MgO is low enough that MgO is effectively passivating with regard to further insertion/removal of Mg. Such conversion reactions have been observed in several materials and postulated to occur in many others. However, it is not presently known a priori which materials will follow an intercalation (or conversion) reaction pathway: Some materials predicted to be unstable upon magnesiation nevertheless exhibit reversible intercalation, while other seemingly-stable materials undergo conversion reactions. Understanding which materials favor intercalation (and why) will guide future cathode discovery efforts and can provide insight about which chemical properties govern the balance between conversion and intercalation, aiding the rational design of new materials. In this presentation, I will discuss our efforts to analyze this balance across chemical and structural space (including extension to sulfides, selenides, and other multivalent working ions) using density functional theory (DFT) calculations. I will also describe the trends and insights that emerge from the resulting data, including the impact of polymorph selection and transition metal chemistry on the driving forces for intercalation/conversion. Figure 1