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

Abstract

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.