First-Principles Prediction of Na Diffusivity in Doped NaCrO₂Layered Cathode Materials with van der Waals Interactions

The O3-layered NaCrO2 is a promising Na-ion cathode material with a good thermal stability and specific capacity, but it suffers from a poor rate capability. To develop high-rate Na-ion cathodes, we present a first-principles study of the stability and Na diffusion in pure and doped NaCrO2 with PBE and various functionals that explicitly include van der Waals (vdW) interactions. The interlayer distances in partially desodiated NaCrO2 are significantly reduced with the inclusion of vdW interactions, which directly affects the prediction of Na diffusion barriers. A linear relation between the interlayer distance and the diffusion barrier is established with different functionals. We notice that the increased diffusion barriers are mostly due to the reduced interlayer distances predicted by the vdW-inclusive functionals rather than the inclusion of vdW interactions in the transition state calculations. Other factors such as the charge density change introduced by different dopants also have effects on the Na diffusion barriers. We find low-concentration metal doping (Al, Zn, Mn, and Co) in NaCrO2 has minor effects on its thermodynamic stability but the Na diffusivity can be significantly promoted. The Co-doped NaCrO2 outperforms other doped NaCrO2 with lower Na diffusion barriers and can be considered as a potential candidate for high-rate Na-ion cathode materials. This study highlights the importance of vdW interactions in layered transition metal oxides and offers strategies to improve first-principles predictions for these structures.