A Joint Computational and Experimental Evaluation of CaMn2O4 Polymorphs as Cathode Materials for Ca Ion Batteries

The identification of potential cathode materials is a must for the development of a new calcium-ion based battery technology. In this work, we have first explored the electrochemical behavior of marokite-CaMn2O4 but the experimental attempts to deinsert Ca ion from this compound failed. First-principles calculations indicate that in terms of voltage and capacity, marokite-CaMn2O4 could sustain reversible Ca deinsertion reactions; half decalciation is predicted at an average voltage of 3.7 V with a volume variation of 6%. However, the calculated barriers for Ca diffusion are too high (1 eV), in agreement with the observed difficulty to deinsert Ca ion from the marokite structure. We have extended the computational investigation to two other CaMn2O4 polymorphs, the spinel and the CaFe2O4 structural types. Full Ca extraction from these CaMn2O4 polymorphs is predicted at an average voltage of 3.1 V, but with a large volume variation of around 20%. Structural factors limiting Ca diffusion in the three polymorphs are discussed and confronted with a previous computational investigation of the virtual-spinel [Ca](T)[Mn-2](O)O-4. Regardless the potential interest of [Ca](T)[Mn-2](O)O-4 as cathode for Ca ion batteries, calculations suggests that the synthesis of this compound would hardly be feasible. The present results unravel the bottlenecks associated with the design of feasible intercalation Ca electrode materials, and allow proposing guidelines for future research.