Effect of cation (Li+, Na+, K+, Rb+, Cs+) in aqueous electrolyte on the electrochemical redox of Prussian blue analogue (PBA) cathodes

Prussian blue analogue (PBA) material is a promising cathode for applications in Na-ion and K-ion batteries which can support high c-rates for charge and discharge. In this study, the material of composition [K2CuIIFeII(CN)6] was synthesized and its structural and electrochemical redox behavior was investigated with 5 different alkali insertion cations (Li+, Na+, K+, Rb+, Cs+). Galvanostatic measurements indicate that the redox potential strongly depends on the ionic radius of the inserted cation. The redox potential varies by ∼400 mV between using Li+ (0.79 Å) or Cs+ (1.73 Å) in the electrolyte. The underlying modification of the Fe2+/Fe3+ redox potential in PBA is proposed to be due to the weakening of the Fe–C bond in the material. This hypothesis is supported by XRD measurements which reveal that the lattice parameter of the de-intercalated host structure follows the same trend of monotonic increase with the cation size. The relatively minor volume changes accompanying the redox (1.2%–2.4%) allow the PBA to accommodate differently sized cations, although the structural hindrances are quite pronounced at high c-rates for the larger ones (Rb+ and Cs+). Cycle aging studies indicate that the minimum capacity fade rate is observed in case of K+ and Rb+ containing electrolyte. The peak intensity corresponding to the [220] crystallographic plane varies depending on the state of charge of PBA, since this plane contains the insertion cations. Owing to the sensitivity of the redox potential to the insertion cation coupled with the observed fast ion-exchange ability, the PBA material may find additional analytical applications such as ion sensing or filtration devices.