In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues

Aluminum intercalation behaviours of {[Fe(Tp)(CN)3]2[M(H2O)2]} cyanido-bridged chain compounds in an ionic liquid electrolyte

As the development of aluminum-ion batteries is still in its infancy, researchers are still dedicated to exploring suitable host materials and investigating their aluminum intercalation behaviours. Here, a series of cyanido-bridged chain compounds with the formula {[FeIII(Tp)(CN)3]2[MII(H2O)2]}n (M = Ni, Co, Mn, Zn, Cu) are studied as cathode electrodes for aluminum-ion batteries with [EMIm]Cl-AlCl3 (1-ethyl-3-methylimidazolium chloride-AlCl3) ionic liquid as the electrolyte. The electrochemical properties suggested Fe3+/Fe2+ to be the redox-active couple during the aluminum intercalation and deintercalation processes of these compounds, and the observed maximum specific capacity obtained by the Fe-Co compound is 200 mA h g-1 despite the rapid specific capacity fading. To gain a deeper understanding of the capacity decay suffered by these compounds, further investigation was conducted to explore the evolution of compounds during the electrochemical measurements. It has been attributed to the following reasons: 1. thermodynamic instability results in the transformation/damage of two of the chain structures (for the Fe-Ni and Fe-Co compounds) during heat treatment on electrodes, a crucial step in electrode preparation; 2. the acidic nature of the electrolyte triggers the destruction of the chain structure, with the appearance of partial reduction of Fe3+ to Fe2+, and a new interaction of the cyano group with aluminum; 3. the high charge density of inserted Al ions makes the chain structure suffer from structural damage during both the charging and discharging processes. The progressive accumulation of trapped intercalated ions hampers their involvement in the reaction, consequently decreasing electrochemical reversibility.

Improved Reversible Capacity and Cycling Stability by Linear (N=O) Anions in Fe[Fe(CN)5 NO] as Sodium-Ion Battery Cathode

Prussian blue analogues (PBAs) are promising cathode materials for sodium-ion batteries (SIBs) due to their tunable chemistry, open channel structure, and low cost. However, excessive crystal water and volume expansion can negatively impact the lifetime of actual SIBs. In this study, a novel iron nitroprusside: Fe[Fe(CN)5 NO] (PBN) was synthesized to effectively eliminate the detrimental effects of crystal water on the reversible capacity and cycling stability of PBA materials. Experiments and DFT calculations demonstrated that PBN has lower crystal water and volume expansion compared to Fe[Fe(CN)6 ] (PB). Also, the N=O bond in PBN significantly reduces the diffusion potential of Na+ in the skeleton. Without any modification, the cathode material exhibited a capacity of up to 148.6 mAh g-1 at 50 mA g-1 as well as maintained 102.9 mAh g-1 after 200 cycles. This work expands our knowledge of the crystal structure of PBA cathode materials and facilitates the rational design of high-quality PBA cathodes for SIBs.