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

In the past decade, the most sought-after cathode materials for SIBs and PIBs have included transition metal oxides (TMOs), polyanionic phosphates and fluorophosphates, NASICON-type polyanionic compounds, and alkali metal hexacyanoferrates (AMHCFs/PBAs). From the commercialization perspective, Novasis Energies, Inc. and Faradion Ltd. have successfully used AMHCFs and TMOs as cathode materials, respectively, for large-scale energy storage in sodium-ion batteries (SIBs). The open framework voids (∼3.4 Å) in AMHCFs are ideal for the insertion of the larger K+/Na+ ions into PIBs and SIBs. The lower cost/performance ratio due to cheaper raw materials and sustainability due to coulombically efficient cycling stability make AMHCFs ideal cathode materials for sustainable future batteries. In terms of commercial prospects, we review the AMHCFs as cathode materials for both SIBs and PIBs with the focus on optimizing AMHCF cathodes through morphology control, reducing [Fe(CN)6] vacancies, and minimizing interstitial water, while maintaining high crystallinity and Na+/K+ content in the AMHCFs to enhance the electrochemical performance. Surface engineering, gradient compounds, graphene-based composites, and multiple cation substitutions can create efficient Na+/K+ ion diffusion pathways. The storage mechanisms and optimizing the electrolyte systems in AMHCFs are critical for efficient Na+/K+ ion batteries for stationary storage.