A Joint Computational and Experimental Evaluation\nof CaMn₂O₄ Polymorphs as Cathode Materials for\nCa Ion Batteries

Abstract

The\nidentification of potential cathode materials is a must for\nthe development of a new calcium-ion based battery technology. In\nthis work, we have first explored the electrochemical behavior of\nmarokite–CaMn₂O₄ but the experimental\nattempts to deinsert Ca ion from this compound failed. First-principles\ncalculations indicate that in terms of voltage and capacity, marokite–CaMn₂O₄ could sustain reversible Ca deinsertion reactions;\nhalf decalciation is predicted at an average voltage of 3.7 V with\na volume variation of 6%. However, the calculated barriers for Ca\ndiffusion are too high (1 eV), in agreement with the observed difficulty\nto deinsert Ca ion from the marokite structure. We have extended the\ncomputational investigation to two other CaMn₂O₄ polymorphs, the spinel and the CaFe₂O₄ structural\ntypes. Full Ca extraction from these CaMn₂O₄ polymorphs is predicted at an average voltage of 3.1 V, but with\na large volume variation of around 20%. Structural factors limiting\nCa diffusion in the three polymorphs are discussed and confronted\nwith a previous computational investigation of the virtual-spinel\n[Ca]_T[Mn₂]_OO₄. Regardless\nthe potential interest of [Ca]_T[Mn₂]_OO₄ as cathode for Ca ion batteries, calculations suggests\nthat the synthesis of this compound would hardly be feasible. The\npresent results unravel the bottlenecks associated with the design\nof feasible intercalation Ca electrode materials, and allow proposing\nguidelines for future research.