Potassium, magnesium, and calcium diffusion in Na2Ti3O7

Post-lithium (Li)-ion batteries are presently being investigated by the science community to improve materials for energy storage applications. The use of alternative ions such as sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca) as charge carriers has potential benefits that will revolutionize the field. Fast diffusion is a key advantage of Li-ions, and these alternative ions need to compete in this regard, particularly in anode materials. Here, we use density functional theory calculations to study the diffusion properties of K, Mg, and Ca dopants in Na2Ti3O7. It is calculated that Na self-diffusion is faster. Codoping with trivalent or tetravalent cations can lower the migration energies of K, Mg, and Ca dopants in Na2Ti3O7 significantly. Nevertheless, Na self-diffusion is prevalent, and only K codoped with Ga leads to a lower migration energy. When codoping Na with Al, the migration energy is reduced from 0.69 to 0.39 eV, representing a significant reduction of ∼43.5%. These findings provide a clear understanding of ion migration mechanisms and offer practical guidance for lattice engineering strategies and doping to optimize ion transport in layered titanates for energy storage applications.