A global priority is the development of low-cost, efficient storage of off-peak electric power and of electrical energy generated by energy sources other than fossil fuels (e.g. wind, solar, nuclear).[1] The rechargeable battery offers efficient electrical energy storage (EES), but the Li-ion battery used in hand-held devices is too expensive for large-scale EES. Unlike Li, Na is readily available worldwide and therefore much less costly than Li. However, the existing Na/S [2] and Zebra (Na/NiCl 2 ) [3] batteries are operating on molten electrodes at 250 to 350 8C. Therefore, there is a huge incentive to develop a room-temperature rechargeable, low-cost Na-ion battery (SIB) of high energy density capable of discharge/ charge at a high rate.To date, room-temperature rechargeable batteries have used as cathode oxide hosts into/from which the working ion, H + or Li + , can be inserted/extracted reversibly over a limited solid-solution range. These two working ions are able to be mobile guests in a host having a close-packed oxide-ion array. However, Na + is too large to be sufficiently mobile in a host with close-packed oxide ions; it needs a host framework with a larger interstitial space. Although Na + is stable coordinated by 12 oxide ions in an oxoperovskite, for example, the Na x WO 3 bronze, too high an activation energy is required for Na + transfer between these sites across a shared face coordinated by four oxide ions. The O-2p orbitals that s bond to the Na impede its motion. Replacement of the O 2À ions by (C N) À ions weakens bonding to the Na; the activation energy for Na + transfer is, therefore, strongly reduced, which makes attractive exploration of hexacyanoperovskites as cathode hosts for a rechargeable SIB. We report a synthesis route for a sodium manganese hexacyanoferrate (NMHFC) containing low-cost cations, and we demonstrate that the NMHFC provides a promising highrate performance as the cathode of a SIB of high specific energy density and efficient EES.Prussian blue and its analogues were investigated as hosts for alkali ions several years ago, [4] but that work received little attention. Cui and co-workers have recently reported stable Na + cyclability into potassium copper hexacyanoferrate in an aqueous electrolyte, [5] but an aqueous electrolyte limits the stable voltage of a rechargeable battery to 1.5 V. Therefore, we have chosen to investigate the hexacyano-perovskites in a non-aqueous electrolyte. We have reported the activities versus Na + /Na 0 of KMFe(CN) 6 with M = Fe, Co, Ni, Mn, Zn; high-spin M = Mn II showed a reversible plateau at 3.82 and 3.56 V, [6] respectively, on charge and discharge. We report herein removal of the Na + ion for a SIB to obtain a 3.4 V cathode in a Na half-cell with rhombohedral Na 1.72 MnFe(CN) 6 (NMHFC-1) and compare the performance with that of a cubic Na 1.40 MnFe(CN) 6 (NMHFC-2).The molar Na:Fe:Mn ratios of the sodium manganese hexacyanoferrates were obtained by inductively coupled plasma (ICP) analysis. All the metal atoms were normalized to the Mn c...