Origin of the High Specific Capacity in Sodium Manganese Hexacyanomanganate

Sodium manganese hexacyanomanganate, NaxMn[Mn(CN)6], is an electrochemically active Prussian blue analogue (PBA) that has been studied experimentally as electrode material in rechargeable sodium-ion batteries. It a reversible specific capacity of 209 mA h g–1, which substantially higher than the theoretical 172 g–1 expected for two reduction events conventional PBAs. suggested high originates from this compound’s unique ability to insert third sodium ion per formula unit. However, plausibility mechanism remained ambiguous. Here, we use density functional theory (DFT) with hybrid calculate formation energies various oxidation states and magnetic phases NaxMn[Mn(CN)6] system. We confirm compound Na3MnII[MnI(CN)6] is, indeed, thermodynamically stable. contains manganese(I), ions occupy interfacial position lattice subcubes. also provide strong evidence phase fully oxidized Mn[Mn(CN)6] charge-disproportionated, containing manganese(II) manganese(IV). proceed show presence crystalline water increases potential system hydrated compounds have crystal geometries potentials closely match experiment. This work clarifies charge-storage well-known but less-understood PBA.