Effect of water molecules on the diffusion of alkali metal ions in CuHCF cathodes.

Prussian blue analogues have shown promise as cathode materials competing with sodium ion batteries, while the presence of water molecules severely affects their performance in batteries. Based on density functional theory, the effect of water molecules on the diffusion of alkali ions is revealed by employing CuHCF (copper hexacyanoferrate) as a cathode. The diffusions of alkali ions in CuHCF with or without inserted water molecules are investigated. It was found that Li/Na forms hydrated ions with water molecules in the interstitial voids, while K cannot form hydrated ions due to spatial constraints. It would lead to a dramatic increase of diffusion barriers when Li/Na ions carrying water molecules diffuse. The number of inserted water molecules significantly impacts the diffusion of Li/Na ions.