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

Abstract

The identification of potential cathode materials \nis a must for the development of a new calcium-ion based \nbattery technology. In this work, we have first explored the \nelectrochemical behavior of marokite−CaMn2O4 but the \nexperimental attempts to deinsert Ca ion from this compound \nfailed. First-principles calculations indicate that in terms of \nvoltage and capacity, marokite−CaMn2O4 could sustain \nreversible Ca deinsertion reactions; half decalciation is \npredicted at an average voltage of 3.7 V with a volume \nvariation of 6%. However, the calculated barriers for Ca \ndiffusion are too high (1 eV), in agreement with the observed \ndifficulty to deinsert Ca ion from the marokite structure. We \nhave extended the computational investigation to two other \nCaMn2O4 polymorphs, the spinel and the CaFe2O4 structural types. Full Ca extraction from these CaMn2O4 polymorphs is \npredicted at an average voltage of 3.1 V, but with a large volume variation of around 20%. Structural factors limiting Ca diffusion \nin the three polymorphs are discussed and confronted with a previous computational investigation of the virtual-spinel \n[Ca]T[Mn2]OO4. Regardless the potential interest of [Ca]T[Mn2]OO4 as cathode for Ca ion batteries, calculations suggests that \nthe synthesis of this compound would hardly be feasible. The present results unravel the bottlenecks associated with the design of \nfeasible intercalation Ca electrode materials, and allow proposing guidelines for future research.