How General is the Conversion Reaction in Mg Battery Cathode: A Case Study of the Magnesiation of α-MnO2

alpha-MnO2 was previously reported as a Mg battery cathode with high discharge capacity but poor cyclability. Surprisingly, the discharge product did not contain any structurally invariant alpha-MgxMnO2, implying that the magnesiation of alpha-MnO2 does not follow the typical intercalation route. In the current work, we perform a comprehensive analysis about the magnesiation behavior of alpha-MnO2 by comparing the reactions following the intercalation and conversion routes. Our results unambiguously demonstrate that the conversion reaction that generates amorphous magnesium and manganese oxide is thermodynamically more preferable than the intercalation reaction. Even if the metastable intercalation could occur due to a possible kinetic barrier that prevents direct conversion, we predict that Mg can only be intercalated to a concentration below alpha-Mg0.125MnO2. Beyond that composition, the structure of the intercalated compound undergoes a strong tetragonal to orthorhombic distortion, which may destruct the integrity of the host framework. The thermodynamic driven force for the conversion reaction is attributed to the high stability of MgO. Therefore, we speculate that the conversion reaction may be a general phenomenon for oxide based Mg battery electrodes.