Hexacyanoferrate‐Type Prussian Blue Analogs: Principles and Advances Toward High‐Performance Sodium and Potassium Ion Batteries

Abstract

Na‐ion batteries (NIBs) and K‐ion batteries (KIBs) are promising candidates for next‐generation electric energy storage applications due to their low costs and appreciable energy/power density compared to Li‐ion batteries. In the search for viable electrode materials for NIBs and KIBs, Prussian blue analogs (PBAs) with inherent rigid and open frameworks and large interstitial voids have shown an impressive ability to accommodate big alkali‐metal ions without structure collapse. In particular, hexacyanoferrates (HCFs) utilizing abundant Fe(CN)6 resources are the most interesting subgroup of PBAs, being able to deliver a specific capacity of 70–170 mAh g‒1 and a voltage of 2.5‒3.8 V in NIBs/KIBs. In this Review, a comprehensive discussion of the HCF‐type cathode materials in terms of their structural features, redox mechanisms, synthesis control, and modification strategies based on research advances over the last ten years. The methodologies and achievements in improving the material properties of HCFs including the compositional stoichiometry, crystal water, crystallinity, morphology, and electrical conductivity are outlined, with the aim to promote understanding of these materials and provide new insights into future design of PBAs for advanced rechargeable batteries.

The Rise of Prussian Blue Analogs: Challenges and Opportunities for High‐Performance Cathode Materials in Potassium‐Ion Batteries

“…The chemical composition and crystal structure of Na‐containing PBAs are suitable for Na + de/intercalation, while the 3D channels overcome the sluggish diffusion kinetics. [ 48–51 ] Owing to their open and robust framework structure, the structural collapse has been largely alleviated during electrochemical cycling. [ 52–54 ] Despite impressive achievements have been made toward the goal of better cathode materials for SIBs, the overall electrochemical performance of PBAs has rendered it less competitive compared with other popular cathode materials.…”

“…[ 58,59 ] In addition, high‐quality PBAs are often synthesized via a facile and mild chemical coprecipitation method without requiring further calcination, which is more environmentally friendly and efficient for mass production. [ 17,49 ] 2) 3D open framework with large interstitial voids. PBAs have the typical perovskite‐type structure bridged by four C ≡ N anions with large interstitial sites of >3.5 Å, which can theoretically accommodate the large‐sized K + (1.38 Å) with minimal volume change.…”

“…The better compatibility between K + and Prussian blue crystal structure contributes to the exceptional cycle lifespan as cathode materials for PIBs. [ 15,49 ] 3) Good mutual compatibility between Prussian blue and K + leads to a lower Gibbs free energy and, therefore, a higher redox potential. [ 17 ] Taking MnFe‐based PBAs as an example, KMnFe‐PBA, NaMnFe‐PBA, and LiMnFe‐PBA show average discharge voltages of 3.7, 3.4, and 3.5 V in K, Na, and Li half‐cells, corresponding to specific energy densities of 521, 476, and 479 W h kg −1 , respectively.…”

“…On the contrary, some critical structural parameters, such as the lattice integrity and symmetry, are highly dependent on the portion of interstitial water and of great significance to the structural stability. [ 17,46,49 ] Second, due to the fast reaction kinetics of chemical coprecipitation routes, the nucleation and growth of PBAs crystal grains happen immediately and produce low crystallinity and small grains containing numerous coordinated water and Fe(CN) 6 vacancies. The presence of certain levels of interstitial/coordinated water, as well as vacancies, shows adverse impacts on their electrochemical performance.…”

“…[ 48,52 ] Moreover, PBAs suffer from low bulk density and poor electrical conductivity, which further degrades their power density. [ 49,66 ]…”