Gaussian smearing method with a width of 0.1 eV was employed. A gamma-centered k-point grid of 5×5×3 was employed to sample the Brillouin zone. The atomic structures underwent optimization until the energy difference in the electronic self-consistent field (SCF) fell below 10 -6 eV, and all forces were less than 10 -2 eV/ Å. The initial structure containing three formula units and exhibiting R 3 symmetry was adapted from the structural parameters published by Wang et al. The NEB calculations have been carried out using the PBE functional with four distinct images in a charge-neutral fashion to evaluate the Na-ion migration minimum energy