Epitaxial Nickel Ferrocyanide Stabilizes Jahn–Teller Distortions of Manganese Ferrocyanide for Sodium‐Ion Batteries

Abstract

Manganese‐based Prussian Blue, Na2−δMn[Fe(CN)6] (MnPB), is a good candidate for sodium‐ion battery cathode materials due to its high capacity. However, it suffers from severe capacity decay during battery cycling due to the destabilizing Jahn–Teller distortions it undergoes as Mn2+ is oxidized to Mn3+. Herein, the structure is stabilized by a thin epitaxial surface layer of nickel‐based Prussian Blue (Na2−δNi[Fe(CN)6]). The one‐pot synthesis relies on a chelating agent with an unequal affinity for Mn2+ and Ni2+ ions, which prevents Ni2+ from reacting until the Mn2+ is consumed. This is a new and simpler synthesis of core–shell materials, which usually needs several steps. The material has an electrochemical capacity of 93 mA h g−1, of which it retains 96 % after 500 charge–discharge cycles (vs. 37 % for MnPB). Its rate capability is also remarkable: at 4 A g−1 (ca. 55 C) it can reversibly store 70 mA h g−1, which is also reflected in its diffusion coefficient of ca. 10−8 cm2 s−1. The epitaxial outer layer appears to exert an anisotropic strain on the inner layer, preventing the Jahn–Teller distortions it normally undergoes during de‐sodiation.

Structural complexity in Prussian blue analogues

“…61 Coating the particles could certainly be a promising solution since it also prevents dissolution of Mn ions. 68,69 Vacancy network correlations may also play a role since they appear to improve ionic conductivity. 28,53…”

“…61 Coating the particles could certainly be a promising solution since it also prevents dissolution of Mn ions. 68,69 Vacancy network correlations may also play a role since they appear to improve ionic conductivity. 28,53 9 Concluding remarks If there is one take-home message from our brief survey it is surely that PBAs support an extraordinarily rich diversity of structural complexity.…”

Formation of Mn–Ni Prussian Blue Analogue Spheres as a Superior Cathode Material for Potassium-Ion Batteries

“…Herein, we synthesize Ni-substituted manganese hexacyanoferrate (denoted MnNi-PBA) spheres by a self-templating strategy. The Ni substitution can stabilize the crystal structure and further enhance the cycling performance. − Moreover, the designed porous spherical architecture can not only furnish abundant active sites but also shorten the K + diffusion path, thus improving the ion diffusion kinetics during the charge and discharge process. As a result, when applied as a cathode material for PIBs, the synthesized MnNi-PBA spheres exhibit an optimized reversible capacity, enhanced rate performance, and long cycling life.…”

Opportunities in Na/K [hexacyanoferrate] frameworks for sustainable non-aqueous Na+/K+ batteries

“…Pure Ni-based AMHCFs are disadvantageous due to economic reasons; however, the Ni incorporation into the AMHCF structure can stabilize the structure and balance the manufacturing cost. 194 The startup company Novasis Energies, Inc. successfully commercialized Na-MHCF cathode-based sodium-ion batteries operating under 0–5 V, delivering an energy density of 100–130 W h kg −1 , which confirms the commercial prospects for AMHCF based Na/K ion batteries. 122…”