Phase Stability of Post-spinel Compound AMn₂O₄ (A = Li, Na, or Mg) and Its Application as a Rechargeable Battery Cathode

At high pressures, spinel compounds can transform to CaFe₂O₄, CaMn₂O₄, or CaTi₂O₄ phases, which are regarded as post-spinel phases. Here, first-principles calculations are used to systematically study the stability of post-spinel LiMn₂O₄, NaMn₂O₄, and MgMn₂O₄, as well as their potential application as rechargeable battery cathodes. Thermodynamically, the stability of the post-spinel phase is highly related to the electronic configuration of transition-metal ions. By changing the concentration of Jahn–Teller active Mn³⁺, the relative stabilities of post-spinel phases can be easily monitored. It provides a practical way to obtain post-spinel compounds with desirable structures. Kinetically, post-spinel phases can be stable under ambient conditions, because of the high barrier that must be overcome to rearrange MnO₆ octahedrons. The most spectacular finding in this work is the high cationic mobility in post-spinel compounds. The activation energy barrier of the migration of Mg²⁺ in CaFe₂O₄-type MgMn₂O₄ is 0.4 eV, suggesting that the mobility of Mg²⁺ in this compound is comparable to that of Li⁺ in typical Li-ion battery cathodes. To explore the potential application of post-spinel compounds as rechargeable battery cathodes, the voltage profile for the electrochemical insertion/removal of Mg in CaFe₂O₄-type MgMn₂O₄ is predicted. Its theoretical energy density is 1.3 times greater than that of typical Li-ion battery cathodes. These outstanding properties make CaFe₂O₄-type MgMn₂O₄ an attractive cathode candidate for rechargeable Mg batteries.