Basic electrochemical characteristics of CaMO3 perovskites (M = Mo, Cr, Mn, Fe, Co, and Ni) as cathode materials for Ca ion batteries are investigated using first principles calculations at the Density Functional Theory level (DFT). Calculations have been performed within the Generalized Gradient Approximation (GGA) and GGA+U methodologies, and considering cubic and orthorhombic perovskite structures for CaxMO3 (x = 0, 0.25, 0.5, 0.75 and 1). The analysis of the calculated voltage-composition profile and volume variations identifies CaMoO3 as the most promising perovskite compound. It combines good electronic conductivity, moderate crystal structure modifications, and activity in the 2-3 V region with several intermediate CaxMoO3 phases. However, we found too large barriers for Ca diffusion (around 2 eV) which are inherent to the perovskite structure. The CaMoO3 perovskite was synthesized, characterized and electrochemically tested, and results confirmed the predicted trends.

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 firstly explored the electrochemical behaviour of marokite-CaMn 2 O 4 but the experimental attempts to deinsert Ca ion from this compound failed. First principles calculations indicate that in terms of voltage and capacity, marokite-CaMn 2 O 4 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 CaMn 2 O 4 polymorphs, the spinel and the CaFe 2 O 4 structural types. Full Ca extraction from these CaMn 2 O 4 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 forCa ion batteries, calculations suggests that the synthesis of this compound would hardly be feasible. The present results unravel the bottlenecks associated to the design of feasible intercalation Ca electrode materials, and allow proposing guidelines for future research. IntroductionSecondary (i.e. rechargeable) batteries are a power source widely used in portable devices (such as personal computers, camcorders and cellular phones) and are also increasingly present in transport and stationary applications. While the current state-of-the-art technology is Li-ion, research efforts are intensified towards the development of alternative technologies to satisfy the ever increasing demands for enhanced energy density 1 . Indeed, the use of lithium metal anodes with high electrochemical capacities (3860 mAh/g) is only possible under certain specific conditions to avoid safety issues and the most used negative electrode material is graphite (372 mAh/g). Efforts to develop successful magnesium anodes (2210 mAh/g) have culminated in proof of concept of this technology. Yet, the high charge-to-radius ratio for magnesium ions has resulted only in very covalent hosts such as Mo 6 S 8-y Se y (y=1,2) Chevrel phases fulfilling the requirements to be used as cathode materials. 2 We have recently reported on the feasibility and reversibility of calcium plating in conventional alkyl carbonate electrolytes at moderate temperatures 3 which opens the way to the development of a new rechargeable battery technology using calcium anodes. This is especially attractive given the abundance of calcium on the earth crust 4 , the high capacity of calcium anodes (1340 mAh/g), its negative reduction potential (only 170 mV above that of lithium metal) and the lower charge to radius ratio for calcium ions which holds promise of better kin...

A combination of experiments and calculations allows grasping more information on the capacity fading upon cycling of the Na2Ti3O7 electrode material in Na batteries.

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