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

Abstract

α-MnO 2 was previously reported as Mg battery cathode with high discharge capacity but poor cyclability. Surprisingly, the discharge product did not contain any structurally invariant α-Mg x MnO 2 , implying that the magnesiation of α-MnO 2 does not follow the typical intercalation route. In current work we perform a comprehensive analysis about the magnesiation behavior of α-MnO 2 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 possible kinetic barrier that prevents direct conversion, we predict that Mg can only be intercalated to a concentration below α-Mg 0.125 MnO 2 . Beyond that composition the structure of 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.

On the Balance of Intercalation and Conversion Reactions in Battery Cathodes

“…[1,14,15] Additionally, most cathodes in modern Li-ion batteries follow an intercalation pathway during electrochemical discharge, while MV battery cathodes have been shown in a number of cases to undergo conversion reactions, [6,[16][17][18][19][20] wherein a reduced transition metal oxide is formed alongside a thermodynamically stable alkaline earth metal oxide, such as MgO. [6,21,22] Because conversion reactions of the type described above are reduction reactions, they occur only upon battery discharge, when the cathode material is being reduced. [6,21,22] Because conversion reactions of the type described above are reduction reactions, they occur only upon battery discharge, when the cathode material is being reduced.…”

Manganese Dioxide As Rechargeable Magnesium Battery Cathode

“…In addition to unveiling complicated mechanism in the magnesiation of rMB cathode (Gautam et al, 2015Ling et al, 2015), computational work helps find suitable cathode candidate for experimental studies. Liu et al (2015) found that among various spinel compounds MnO2 is particularly interesting due to the stability especially at charged state.…”

Sponge-Like Porous Manganese(II,III) Oxide as a Highly Efficient Cathode Material for Rechargeable Magnesium Ion Batteries

“…The shell was identi- fied as amorphous magnesium oxide mixed with reduced manganese oxide, while the core remains crystalline. 54,55 The thermodynamic driving force for this conversion reaction is attributed to the high stability of MgO.…”