Understanding migration barriers for monovalent ion insertion in transition metal oxide and phosphate based cathode materials: A DFT study

Abstract High ionic conductivity is a prerequisite requirement for materials used in monovalent metal-ion rechargeable batteries. The extensive search of new electrode materials for Na-ion and K-ion monovalent metal-ion batteries requires a deep understanding of structural and chemical details of cation migration through the crystal lattice. In the paper, we consider three classes of transition metal oxide and phosphate cathode materials: AMn2O4 spinels, AMPO4 olivines and AVPO4F tavorites (A = Li, Na, K, □ ; M = Fe, Mn), used for pragmatic applications for secondary (rechargeable) batteries. Herein we examine Na+ and K+ migration characteristics in comparison with that of Li+ by means of DFT+U, local energy calculations, empirical potentials, and bond valence energy landscape (BVEL). It is found that despite larger radii of Na+ and K+, the migration barriers are comparable with that of Li+. In several cases, we reveal that the migration barrier of K+ can be even lower than that of Li+. This behavior is explained through the interplay of site and lattice energies during cation migration. For automation of screening of migration properties via DFT calculations, a new Python-based framework (SIMAN) is developed and benchmarked across three cathode materials structures.