Thermodynamics and Kinetics of Na-Ion Insertion into Hollandite-TiO2 and O3-Layered NaTiO2: An Unexpected Link between Two Promising Anode Materials for Na-Ion Batteries

First principle DFT calculations are used to study the thermodynamic and kinetic properties of Na-ion insertion in TiO2 hollandite, a potential anode for Na-ion batteries. The experimentally observed phase transformation from tetragonal TiO2 (I4/m) to monoclinic Na0.25TiO2 (I2/m) is confirmed. At high Na-ion concentrations the calculated formation energies predict a first-order phase transition toward the layered O′3-Na0.68TiO2 structure. Further sodiation initiates a solid solution reaction toward the layered O3-NaTiO2 phase, which was recently brought forward as a promising anode for Na-ion batteries. This transformation irreversibly transforms the one-dimensional hollandite tunnel structure into the layered structure and potentially brings forward an alternative route toward the preparation of the hard to prepare O3-NaTiO2 material. Energy barrier calculations reveal fast Na-ion diffusion at low concentrations and sluggish diffusion upon reaching the N0.25TiO2 phase, rationalizing why experimentally th...