Prussian blue analogs have significant promise as active materials for the next generation of battery electrodes with improved cycle life and rate capability. Their useful electrochemical properties include two independent redox centers per unit cell; a nanoporous, open framework for rapid ion conduction; high stability during ion (de)insertion; and structural and electrochemical tunability for diverse applications. Here we share insights into how control of the five main crystallographic features (two transition-metal ions, the inserting ion, defects, and water) imparts control over the ion-insertion reaction. We then identify five key opportunities to expand our understanding of these materials, including the role of water in their ion conduction, modeling, synthesis methods, use as anode materials, and technoeconomics. Further research in these areas will accelerate the development of new, high-performing battery electrodes.

The application of transition metal fluorides as energy dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities/limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g −1 ) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr 1,3 FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 • C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semi-coherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF 2 .

Due to the rapid battery market expansion, and the limited and geographically concentrated lithium and cobalt resources, there is significant concern regarding the short-term supply and long-term sustainability of lithium-ion batteries (LIBs). Potassium-ion batteries (KIBs) are emerging as a promising complementary technology to LIBs due to the relative abundance of potassium. KIBs can also use graphite anodes providing a critical advantage over sodium-ion batteries (NIBs). In this perspective, we provide an overview of the most promising cathodes, anodes, and electrolytes to date for KIBs. We also present a concise techno-economic model to critically compare the most promising KIB chemistries and evaluate if they can compete with a leading NIB and LIBs. Finally, we identify five critical research challenges that need to be addressed for KIBs to become a viable technology.

Potassium-ion batteries (KIB) are a promising complementary technology to lithium-ion batteries because of the comparative abundance and affordability of potassium. Currently, the most promising KIB chemistry consists of a potassium manganese hexacyanoferrate (KMF) cathode, a Prussian blue analog, and a graphite anode (723 W h l −1 and 359 W h kg −1 at 3.6 V). No electrolyte has yet been formulated that is concurrently stable at the high operating potential of KMF (4.02 V vs K + /K) and compatible with K + intercalation into graphite, currently the most critical hurdle to adoption. Here, we combine a KMF cathode and a graphite anode with a KFSI in Pyr 1,3 FSI ionic liquid electrolyte for the first time and show unprecedented performance. We use high-throughput techniques to optimize the KMF morphology for operation in this electrolyte system, achieving 119 mA h g −1 at 4 V vs K + /K and a Coulombic efficiency of >99.3%. In the same ionic liquid electrolyte, graphite shows excellent electrochemical performance and we demonstrate reversible cycling by operando X-ray diffraction. These results are a significant and essential step forward toward viable potassium-ion batteries.

Prussian blue analogues (PBAs) have recently shown outstanding electrochemical properties ascribable to their unique open-framework crystal structure that allows the reversible insertion of alkali ions with negligible perturbation to the framework itself. Many hexacyanoferrate materials have shown excellent properties and are some of the most promising sodium-and potassiumion cathode materials in both aqueous and organic electrolytes. However, there is a distinct lack of candidate PBA materials that operate at low potentials as their characteristic crystalline framework shows instability. In this article we characterise the structure and electrochemical behavior of manganese hexacyanochromate which exhibits reversible sodium insertion at-0.86 V vs. SHE (1.84 V vs. Na + /Na), whilst maintaining the characteristic PBA cubic structure. This is the lowest redox potential of reported PBA materials and shows fast kinetics in a high voltage water-in-salt electrolyte. Further reduction in potential in an organic electrolyte shows decomposition of the crystalline structure