Effect of the external metal on the electrochemical behavior of M3[Co(CN)6]2 (M: Co, Ni, Cu, Zn), towards their use as anodes in potassium ion batteries

A series of crystalline-cubic Prussian blue analogues (PBA) M-3[Co(CN)(6)](2) (M=Co, Ni, Cu, Zn) were straightforwardly synthesized via the coprecipitation method as confirmed by X-ray diffraction and FTIR spectroscopy. These transition metal hexacyanocobaltates show electrochemical activity related to reversible K-ion storage between 0.05-2.6 V vs K-o/K+, that makes them suitable as anode-materials in K-ion batteries. The non-steady open circuit potential (OCP) observed in cells containing Cu-3[Co(CN)(6)](2) and Zn-3[Co(CN)(6)](2) electrodes suggests the presence of spontaneous redox reactions at the electrolyte/PBA interface, which leads to the loss of electrochemical activity with the time. On the other hand, the Co-3[Co(CN)(6)](2) and Ni-3[Co(CN)(6)] show extended -stability periods and multiple reduction/oxidation reactions which lead to the reversible charge storage mechanism. From an in-depth electrochemical study on the Co-3[Co(CN)(6)](2)-PBA, used as reference, an improvement on capacity retention is observed when the electrode is discharged at high negative potentials, 0.2 V vs K-o/K+, possibly due to the beneficial effect of a different electrochemical reaction on the PBA surface. The Ni-3[Co(CN)(6)](2)-PBA does not follow this trend, suggesting a different kinetics for this reaction on the particle surface, avoiding to observe its favorable effect as in the case of the Co-3[Co(CN)(6)](2). This is the first time that transition metal-modified hexacyanocobaltates are reported as anode materials for K-ion batteries; particularly the Ni-3[Co(CN)(6)](2) PEA exhibits specific capacities as high as similar to 310 mA h g(-1 )in the first discharge making it a promising material for applications in novel energy-storage technologies as K-ion batteries. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

A series of crystalline-cubic Prussian blue analogues, including Co-3[Co(CN)(6)](2) and Ni-3[Co(CN)(6)](2), show high stability and long-lasting electrochemical activity, suitable for anode materials in K-ion batteries. Through in-depth electrochemical study, it is found that high negative potential discharge can improve capacity retention.