Exploring Quasi-1D Oxides As Ca2+ Hosts for Calcium Battery Applications

Abstract

The study of intercalation compounds [1] led to the identification of redox reactions in host transition metal compounds involving reversible (usually topotactic) insertion and deinsertion of Mn+ ions. Studies with Li+ turned out to be a cornerstone in the development of the Li-ion battery technology [2] that has synergistically boosted portable electronics, while intercalation of multivalent cations (n>1) remained an academic curiosity. Yet, the current energy needs for batteries require significant improvement in energy density, beyond the capabilities of the traditional Li-ion technology, and multivalent cation chemistries represent a thrilling alternative in terms of the amount of energy that can be delivered. Ca metal based batteries have been almost unexplored as reversible plating and stripping of Ca with conventional organic electrolytes at moderate temperature was reported only recently [3,4]. The alkylcarbonate based electrolytes used in this work offer a wide electrochemical stability window, thus inciting the exploration of possible high voltage cathode materials with reliable electrochemical set-ups [5]. Development of potential multivalent host structures is challenging and the choice of cathode materials extremely limited, as divalent cation insertion/extraction is associated with sluggish solid state diffusion kinetics and high activation energies for the charge transfer kinetics. Layered intercalation compounds, with expandable interlayer space like TiS2, V2O5, MoO3, etc., are typically the most studied, but fundamental understanding of the reaction mechanisms in these phases is limited [6,7]. In this work we have explored the possibility of using Ca3Co2O6 cobaltite as Ca-based battery cathode [8]. The pristine compound has a 1D structure of CoO6 octahedra and prisms alternating along chains. By means of ex-situ synchrotron X-ray powder diffraction we have realized that Ca can be extracted by electrochemical methods driving to an incommensurate structure of the type Can+2Con+1O3n+3 (or, equivalently Ca1+xCoO3), characterized by the alternation of n octahedra and one prism along the Co-chain. The reversibility of the redox process seems to be rather limited in the present experimental conditions, indicating some kinetic or thermodynamic issues. Structurally related phases have been also explored as strategy for identifying alternative Ca2+ hosts.