Ion-exchange surface modification enhances cycling stability and kinetics of sodium manganese hexacyanoferrate cathode in sodium-ion batteries

High Capacity and Fast Kinetics Enabled by Metal-Doping in Prussian Blue Analogue Cathodes for Sodium-Ion Batteries

Prussian blue analogues (PBAs) have gained tremendous attention as promising low-cost electrochemically-tunable electrode materials, which can accommodate large Na+ ions with attractive specific capacity and charge-discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal-doping strategy has been demonstrated in FeNiHCF, Na1.40Fe0.90Ni0.10[Fe(CN)6]0.85 ⋅ 1.3H2O, delivering a capacity as high as 148 mAh g-1 at 10 mA g-1. At an exceptionally high rate of 25.6 A g-1, a reversible capacity of ~55 mAh g-1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal-doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long-term cyclability. In situ synchrotron X-ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low-spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe2+C6/Fe3+C6 redox couple is responsible for stable high-rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high-performance electrode materials for sustainable sodium-ion batteries.

Review and New Perspectives on Non-Layered Manganese Compounds as Electrode Material for Sodium-Ion Batteries

After more than 30 years of delay compared to lithium-ion batteries, sodium analogs are now emerging in the market. This is a result of the concerns regarding sustainability and production costs of the former, as well as issues related to safety and toxicity. Electrode materials for the new sodium-ion batteries may contain available and sustainable elements such as sodium itself, as well as iron or manganese, while eliminating the common cobalt cathode compounds and copper anode current collectors for lithium-ion batteries. The multiple oxidation states, abundance, and availability of manganese favor its use, as it was shown early on for primary batteries. Regarding structural considerations, an extraordinarily successful group of cathode materials are layered oxides of sodium, and transition metals, with manganese being the major component. However, other technologies point towards Prussian blue analogs, NASICON-related phosphates, and fluorophosphates. The role of manganese in these structural families and other oxide or halide compounds has until now not been fully explored. In this direction, the present review paper deals with the different Mn-containing solids with a non-layered structure already evaluated. The study aims to systematize the current knowledge on this topic and highlight new possibilities for further study, such as the concept of entatic state applied to electrodes.

How Does Ion Exchange Construct Binary Hexacyanoferrate? A Case Study

In this work, using electrochemical active Fe as an ion-exchange element (attack side) and the Na _x MnFe(CN)₆ slurry with a high solid content (MnHCF) as a template (defensive side), a series of binary hexacyanoferrates are prepared via a simple Mn/Fe ion-exchange process, in which Na _x FeFe(CN)₆ (FeHCF) and solid solution Na _x (FeMn)Fe(CN)₆ are concentrated on the shell and the core, respectively. The proportions of the two structures are mainly controlled by the competition between the ion-exchange rate in the bulk material and dissolution-reprecipitation rate. Slowing down the attacking rate, such as the use of a chelating agent complexed with the attacker Fe, is advantageous to form a thermodynamically metastable state with homogeneous distribution of elements since the diffusion of Fe²⁺ in the solid MnHCF is relatively fast. The shell FeHCF could be adjusted by the dissolution-reprecipitation rate, which is driven by the solubility difference. Adding the chelating agent in the defensive side will promote the dissolution of MnHCF and reprecipitation of FeHCF on the surface. Meanwhile, with the increase of Fe sources, the thickness of the shell FeHCF increases, and correspondingly the content of solid solution decreased due to FeHCF is more stable than solid solutions in thermodynamics. Finally, such a design principle in this case study could also be generalized to other ion-exchange processes. Considering the difference of two components in solubility, the larger difference can make the core/shell structure more clear due to the enhancement of dissolution-reprecipitation route.