Origin of the High Specific Capacity in Sodium Manganese Hexacyanomanganate

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