Defect‐Healing Induced Monoclinic Iron‐Based Prussian Blue Analogs as High‐Performance Cathode Materials for Sodium‐Ion Batteries

Abstract

Prussian blue analogs (PBAs) have attracted wide interest as a class of ideal cathodes for rechargeable sodium‐ion batteries due to their low cost, high theoretical capacity, and facile synthesis. Herein, a series of highly crystalline Fe‐based PBAs (FeHCF) cubes, where HCF stands for the hexacyanoferrate, is synthesized via a one‐step pyrophosphate‐assisted co‐precipitation method. By applying this proposed facile crystallization‐controlled method to slow down the crystallization process and suppress the defect content of the crystal framework of the PBAs, the as‐prepared materials demonstrate high crystallization and a sodium‐rich induced rhombohedral phase. As a result, the as prepared FeHCF can deliver a high specific capacity of up to 152.0 mA h g−1 (achieving ≈90% of its theoretical value) and an excellent rate capability with a high‐capacity retention ratio of 88% at 10 C, which makes it one of the most competitive candidates among the cathodes reported regarding both capacity and rate performance. A highly reversible three‐phase‐transition sodium‐ion storage mechanism has been revealed via multiple in situ techniques. Furthermore, the full cells fabricated with as‐prepared cathode and commercial hard carbon anode exhibit excellent compatibility which shows great prospects for application in the large‐scale energy storage systems.

Designing CoHCF@FeHCF Core–Shell Structures to Enhance the Rate Performance and Cycling Stability of Sodium‐Ion Batteries

“…[43] The material has excellent rate performance, with a specific capacity of 39 mAh g −1 at a high multiplicity of 41.7 C and essentially no capacity degradation after 5000 cycles. As the research progresses, the PBAs studied as cathode materials for SIBs sodium-ion batteries mainly include A x FeFe(CN) 6 (FeHCF), [44,45] A x MnFe(CN) 6 (MnHCF), [46,47] A x NiFe(CN) 6 (NiHCF), [48,49] A x CoFe(CN) 6 (CoHCF), [50,51] A x VFe(CN) 6 (VHCF). [52] Due to the high-spin Fe, Co, and Mn coordination with N to form M A −N, FeHCF, MnHCF, and CoHCF these PBAs have two-electron redox-active centers with a theoretical specific capacity of 170 mAh g −1 .…”

K-Ion Slides in Prussian Blue Analogues

“…The “tilt-engineering” approach − responsible for stabilizing simultaneous electric polarization and bulk magnetism at room temperature in the layered perovskites (Ca y Sr 1– y ) 1.15 Tb 1.85 Fe 2 O 7 is a high-profile example . It is in this context that there has been substantial recent interest in the relevance of structural distortions for the physical and chemical properties of PBAs. ,− …”

“…One domain of PBA chemistry in which distortions appear particularly important is the application of PBAs as cathode materials for Na- and especially K-ion batteries. ,, PBAs are arguably the most promising cathode material for K-ion batteries, identified for their high operating voltages, favorable charge rates, and inexpensive solution-phase synthesis from earth-abundant elements. , The specific capacity of PBAsgeneral formula A x B­[B′(CN) 6 ] y depends not only on the atomic weight and redox properties of the transition-metal ions B and B′ but also on the concentration of hexacyanometallate vacancies. Efforts to increase the specific capacity of PBA cathodes by reducing vacancy concentrations (i.e., y → 1) have resulted in materials that exhibit structural phase changes on electrochemical cycling .…”

In Situ Atomic‐Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na3V2(PO4)3‐Based All‐Solid‐State Sodium Batteries

“…[25] In situ Raman analysis revealed a highly reversible three-phase transition as the sodium-ion storage mechanism of Na 2 FeFe(CN) 6 during sodiation/desodiation processes. [26] Among various in situ techniques, in situ TEM possesses the capability of resolving the microstructural evolution and a providing diffraction information of the electrode materials. It can create a variety of external fields, including electric and thermal fields, and can observe the dynamic structure evolutions at the same time.…”