Divó-Matos1 P. Acevedo-Peña2, E. Reguera1
1) National Polytechnic Institute, Center for Applied Science and Advanced Technogy, Legaria Unit, Mexico City, Mexico
2) CONACyT- National Polytechnic Institute, Center for Applied Science and Advanced Technogy, Legaria Unit, Mexico City, Mexico
Prussian blue analogs (PBAs) form a relatively large family of coordination polymers, (Tn)i (Ak)j[(Mm+(CN)6) (m-6)]L where (6-m) x L = (n x i) + j x k) corresponds to the charge balance in the formula unit. T and M are transition metals, with M being restricted to metals with a maximum of six d electrons, and A is a charge balance ion, usually an alkaline or alkaline earth metal. Within the context of battery materials, those compositions with M = Ti, V, Cr, and Mn have received little attention because their PBAs are unstable during redox cycles and aging. For M = Co, Rh, Rh, Pd, and Pt, their coordination polymers are inactive within the potential windows of interest, in addition to the high cost, particularly the last four ones. Within the remaining metals, Fe, Ru, and Os, only Fe appears to be technologically attractive considering its relatively high relative abundance in the earth’s crust and their Fe(3+/2+) pair redox together with the stability of the related hexacyanoferrate(II,III) salts. This explains that, with rare exceptions, the study of PBAs as cathode materials has been limited to the iron atom as inner metal. Regarding the outer metal (T), Mn, Fe, Co, Ni, and Cu have been considered. Mn has a large number of accessible oxidation states but, the Jahn-Teller in Mn(3+) and related structural distortion is a limitation for the cathode stability. The same is valid for Cu(2+). Ni is inactive and the formed PBA usually has a small crystallite size due to the high polarizing power of the Ni(2+) ion. But Ni(2+) hexacyanoferrates have received certain attention as cathode materials. This explains why the remaining 3d metals, Fe and Co, have received the greatest attention. Their 3+/2+ redox pair is accessible within the potential window of interest and the corresponding PBAs have a cubic framework. Certain attention has also received Zn3A2[Fe(CN)6]2, which crystallizes with a hexagonal unit cell, related to tetrahedral coordination for the Zn atom, with an easy extraction and insertion for the exchangeable A metal.
In a typical PBA, the -T-NC-M-CN-T- chains form a 3D framework related to the octahedral coordination of T and M metals to the N and C ends of the CNs, respectively. When the T:M atomic ratio is equal to 1, we have uninterrupted chains with the exchangeable metal (A) occupying the interstitial voids. The windows size, of about 4 Å, is enough to facilitate the extraction and insertion of Na and K cations. In Zn3A2[Fe(CN)6]2, the window size is about 6.8 x 8 Å. For T >M, related to the charge balance, systematic vacancies for the building block, [M(CN)6], appear in the structure. This facilitates the ionic transport in the solid but reduces the battery charge density. In the as-synthesized PBAs, the metal (T) found at the surface of a given cavity generated by a vacancy, has a mixed coordination sphere formed by N ends of CNs ligands and water molecules, (NCN)6-n(H2O)n. The filling of the cavity volume is completed by weakly bonded water molecules, which form a hydrogen-bonding network through the solid framework. Since these metal centers behave as Lewis acid sites, a proton of these water molecules is available to be transported through the solid by jumping between weakly-bonded water molecules (Grotthuss mechanism). This is an attractive feature of PBAs for proton-based batteries. In that sense, compositions involving Ni, and Cu are particularly attractive related to their high polarizing power.
This contribution discusses these and other structural features of PBAs and their impact on the reported performance of PBAs as cathode materials for Na and K-ions-based batteries, including those results obtained by our research group during the last three years. Regarding the ions transport through the solid framework, the understanding of structural features is discussed from a recently developed percolation model which helps to visualize the expected effects within the cathode material.
Acknowledgments. The studies herein summarized have been performed in the National Laboratory for Energy Conversion and Storage, which resulted from a joint project between CONACyT- UNAM and IPN.