Na-ion batteries (NIBs) are a technological analogue to Li-ion batteries (LIBs) but with several competitive benefits, including economic benefits stemming from the low cost of sodium resources and the possibility of employing Al as the anode current collector and chemical benefits of the lower desolvation energy of Na + in polar solvents and the better interfacial kinetics at the electrode/electrolyte interface, compared with Li + . The large size of Na + poses a challenge for developing NIB cathodes that allow reversible Na + intercalation and fast Na + diffusion. Prussian blue analogues (PBAs) have been explored as a family of NIB cathodes. The framework of Na + -filled PBAs (Na-PBAs), with a general formula NaxM[Fe(CN)6]1y•zH2O (0 < x ≤ 2, 0 < y ≤ 1, z < 2, M = transition metal), has large interstitial sites to accommodate Na + and directional channels for fast Na + diffusion, as well as the structural "openness" to realize small lattice strain during repetitive Na + intercalation. As a result, Na-PBAs have delivered some of the best cathode performance for NIBs so far. Interestingly, bigger K + -filled PBAs (K-PBAs), with a similar formula KxM[Fe(CN)6]1-y•zH2O, was initially studied as cathode materials for K-ion batteries (KIBs), but have also been demonstrated to be appealing cathode materials for NIBs, delivering a comparable capacity to Na-PBAs.