High-Voltage Potassium Hexacyanoferrate Cathode via High-Entropy and Potassium Incorporation for Stable Sodium-Ion Batteries

Inhibiting the Jahn-Teller Effect of Manganese Hexacyanoferrate via Ni and Cu Codoping for Advanced Sodium-Ion Batteries

Manganese (Mn)-based Prussian blue analogs (PBAs) are of great interest as a prospective cathode material for sodium-ion batteries (SIBs) due to their high redox potential, easy synthesis, and low cost. However, the Jahn-Teller effect and low electrical conductivity of Mn-based PBA cause poor structure stability and unsatisfactory performance during the cycling. Herein, a novel nickel- and copper-codoped K2Mn[Fe(CN)6] cathode is developed via a simple coprecipitation strategy. The doping elements improve the electrical conductivity of Mn-based PBA by reducing the bandgap, as well as suppress the Jahn-Teller effect by stabilizing the framework, as verified by the density functional theory calculations. Simultaneously, the substitution of sodium with potassium in the lattice is beneficial for filling vacancies in the PBA framework, leading to higher average operating voltages and superior structural stability. As a result, the as-prepared Mn-based cathode exhibits excellent reversible capacity (116.0 mAh g-1 at 0.01 A g-1) and superior cycling stability (81.8% capacity retention over 500 cycles at 0.1 A g-1). This work provides a profitable doping strategy to inhibit the Jahn-Teller structural deformation for designing stable cathode material of SIBs.

High-Entropy Prussian Blue Analogues Enable Lattice Respiration for Ultrastable Aqueous Aluminum-Ion Batteries

Aqueous aluminum ion batteries (AAIBs) hold significant potential for grid-scale energy storage owing to their intrinsic safety, high theoretical capacity, and abundance of aluminum. However, the strong electrostatic interactions and delayed charge compensation between high-charge-density aluminum ions and the fixed lattice in conventional cathodes impede the development of high-performance AAIBs. To address this issue, this work introduces, for the first time, high-entropy Prussian blue analogs (HEPBAs) as cathodes in AAIBs with unique lattice tolerance and efficient multipath electron transfer. Benefiting from the intrinsic long-range disorder and robust lattice strain field, HEPBAs enable the manifestation of the lattice respiration effect and minimize lattice volume changes, thereby achieving one of the best long-term stabilities (91.2% capacity retention after 10 000 cycles at 5.0 A g-1) in AAIBs. Additionally, the interaction between the diverse metal atoms generates a broadened d-band and reduced degeneracy compared with conventional Prussian blue and its analogs (PBAs), which enhances the electron transfer efficiency with one of the best rate performance (79.2 mAh g-1 at 5.0 A g-1) in AAIBs. Furthermore, exceptional element selectivity in HEPBAs with unique cocktail effect can facile tune electrochemical behavior. Overall, the newly developed HEPBAs with a high-entropy effect exhibit promising solutions for advancing AAIBs and multivalent-ion batteries.

Prussian Blue Analogues for Sodium-Ion Battery Cathodes: A Review of Mechanistic Insights, Current Challenges, and Future Pathways

Prussian blue analogues (PBAs) have emerged as highly promising cathode materials for sodium-ion batteries (SIBs) due to their affordability, facile synthesis, porous framework, and high theoretical capacity. Despite their considerable potential, practical applications of PBAs face significant challenges that limit their performance. This review offers a comprehensive retrospective analysis of PBAs' development history as cathode materials, delving into their reaction mechanisms, including charge compensation and ion diffusion mechanisms. Furthermore, to overcome these challenges, a range of improvement strategies are proposed, encompassing modifications in synthesis techniques and enhancements in structural stability. Finally, the commercial viability of PBAs is examined, alongside discussions on advanced synthesis methods and existing concerns regarding cost and safety, aiming to foster ongoing advancements of PBAs for practical SIBs.

A P2/P3 Biphasic Layered Oxide Composite as a High-Energy and Long-Cycle-Life Cathode for Potassium-Ion Batteries

Layered transition metal oxides are extensively considered as appealing cathode candidates for potassium-ion batteries (PIBs) due to their abundant raw materials and low cost, but their further implementations are limited by slow dynamics and impoverished structural stability. Herein, a layered composite having a P2 and P3 symbiotic structure is designed and synthesized to realize PIBs with large energy density and long-term cycling stability. The unique intergrowth of P2 and P3 phases in the obtained layered oxide is plainly characterized by X-ray diffraction refinement, high-angle annular dark field and annular bright field-scanning transmission electron microscopy at atomic resolution, and Fourier transformation images. The synergistic effect of the two phases of this layered P2/P3 composite is well demonstrated in K+ intercalation/extraction process. The as-prepared layered composite can present a large discharge capacity with the remarkable energy density of 321 Wh kg-1 and also manifest excellent capacity preservation after 600 cycles of K+ uptake/removal.

Capacitive-Controlled Prussian White with a Nickel Iron Hexacyanoferrate Composite Cathode for Rapid Sodium Diffusion

Prussian blue analogues receive tremendous attention owing to their spacious three-dimensional skeleton, high theoretical specific capacity, facile synthesis procedure, and high cost-effectiveness as among the most promising candidates for cathode materials in sodium-ion batteries (SIBs). Nonetheless, the practical specific capacity, especially under high current, is particularly frail due to the sluggish ion diffusion. In this study, the strategy of Ni substitution and formation of water-coordinated Fe is applied to lower the crystal field energy and elevate the active low-spin (LS) Fe content, which leads to a capacitive sodium storage mechanism, resulting in a substantial specific capacity under high current density. The delivered specific capacity of PW-325@2NiFe-55 is 95 mAh g-1 at 50 C, which is 72.5% capacity retention of the one at 0.5 C. Also, it maintains 80.2% of its initial specific capacity after 500 cycles at 5 C. Furthermore, a hypothesis of a joint diffusion-controlled and capacitive mechanism for high-spin (HS) Fe and a mere capacitive mechanism for LS Fe is put forward and verified through potentiastatic tests, operando 57Fe Mössbauer spectroscopy, and ex situ XRD, which provides a new horizon to enhance the electrochemical performance for SIBs.

Entropy-Mediated Stable Structural Evolution of Prussian White Cathodes for Long-Life Na-Ion Batteries

The high-entropy approach is applied to monoclinic Prussian White (PW) Na-ion cathodes to address the issue of unfavorable multilevel phase transitions upon electrochemical cycling, leading to poor stability and capacity decay. A series of Mn-based samples with up to six metal species sharing the N-coordinated positions was synthesized. The material of composition Na1.65 Mn0.4 Fe0.12 Ni0.12 Cu0.12 Co0.12 Cd0.12 [Fe(CN)6 ]0.92 □0.08  ⋅ 1.09H2 O was found to exhibit superior cyclability over medium/low-entropy and conventional single-metal PWs. We also report, to our knowledge for the first time, that a high-symmetry crystal structure may be advantageous for high-entropy PWs during battery operation. Computational comparisons of the formation enthalpy demonstrate that the compositionally less complex materials are prone to phase transitions, which negatively affect cycling performance. Based on data from complementary characterization techniques, an intrinsic mechanism for the stability improvement of the disordered PW structure upon Na+ insertion/extraction is proposed, namely the dual effect of suppression of phase transitions and mitigation of gas evolution.