First-principles analysis of the interplay between electronic structure and volume change in colquiriite compounds during Li intercalation

A main source of capacity fading in lithium-ion batteries is the degradation of the active cathode materials caused by the series of volume changes during charge and discharge cycles. The quaternary colquiriite-type fluorides LixCaFeF6 and LixCaCoF6 were reported to have negligible volume changes in specific Li concentration ranges, making the underlying colquiriite structure a promising candidate for so-called zero-strain behavior. Using first-principles electronic structure calculations based on density functional theory with a Hubbard-U correlation correction on the transition-metal ions, we systematically investigate the equilibrium volumes of the colquiriite-type fluorides LixCaMF6 with M = Ti, V, Cr, Mn, Fe, Co, and Ni at the Li concentrations x = 0, 1, and 2. We elucidate the connection between the total volume of the structures and the local volumes of fluorine coordinated octahedra around the cations, and we find trends along the series of the 3d transition-metal elements. In the lithiation step from x =1 to x = 2 we find volume changes of about 10%, and we discuss the discrepancy to the experimentally reported smaller value for LixCaFeF6. The suitability as cathode material was furhter investigated by calculating the theoretical voltages and capacities. From x = 0 to x = 1 we describe the compensating structural mechanisms that lead to an exceptionally small volume change of LixCaMnF6, which posseses a high theoretical voltage and moderate capacity. This compound is therefore a particularly promising zero-strain cathode material.