Fig. 5

Crystal structure, Na+ ion diffusion paths, different types of Na+ ions in a single unit, and corresponding migration energy barriers. a, c, d Various images of the crystal structure of Na4Fe3(PO4)2(P2O7) material with three different types of Na+ ions. b The migration energy barriers within the same Na+ ion groups. e The migration energy barriers between different Na+ ion groups (equivalent to three-dimensional diffusion pathways)