A Ni-doping-induced phase transition and electron evolution in cobalt hexacyanoferrate as a stable cathode for sodium-ion batteries

High Capacity and Fast Kinetics Enabled by Metal-Doping in Prussian Blue Analogue Cathodes for Sodium-Ion Batteries

Prussian blue analogues (PBAs) have gained tremendous attention as promising low-cost electrochemically-tunable electrode materials, which can accommodate large Na+ ions with attractive specific capacity and charge-discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal-doping strategy has been demonstrated in FeNiHCF, Na1.40Fe0.90Ni0.10[Fe(CN)6]0.85 ⋅ 1.3H2O, delivering a capacity as high as 148 mAh g-1 at 10 mA g-1. At an exceptionally high rate of 25.6 A g-1, a reversible capacity of ~55 mAh g-1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal-doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long-term cyclability. In situ synchrotron X-ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low-spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe2+C6/Fe3+C6 redox couple is responsible for stable high-rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high-performance electrode materials for sustainable sodium-ion batteries.

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

Prussian blue analogs are well suited for sodium-ion battery cathode materials due to their cheap cost and high theoretical specific capacity. Nax CoFe(CN)6 (CoHCF), one of the PBAs, has poor rate performance and cycling stability, while Nax FeFe(CN)6 (FeHCF) has better rate and cycling performance. The CoHCF@FeHCF core-shell structure is designed with CoHCF as the core material and FeHCF as the shell material to enhance the electrochemical properties. The successfully prepared core-shell structure leads to a significant improvement in the rate performance and cycling stability of the composite compared to the unmodified CoHCF. The composite sample of core-shell structure has a specific capacity of 54.8 mAh g-1 at high magnification of 20 C (1 C = 170 mA g-1 ). In terms of cycle stability, it has a capacity retention rate of 84.1% for 100 cycles at 1 C, and a capacity retention rate of 82.7% for 200 cycles at 5 C. Kinetic analysis shows that the composite sample with the core-shell structure has fast kinetic characteristics, and the surface capacitance occupation ratio and sodium-ion diffusion coefficient are higher than those of the unmodified CoHCF.

Na1.51 Fe[Fe(CN)6 ]0.87 ·1.83H2 O Hollow Nanospheres via Non-Aqueous Ball-Milling Route to Achieve High Initial Coulombic Efficiency and High Rate Capability in Sodium-Ion Batteries

Prussian blue analogues (PBAs) have attracted extensive attention as cathode materials in sodium-ion batteries (SIBs) due to their low cost, high theoretical capacity, and facile synthesis process. However, it is of great challenge to control the crystal vacancies and interstitial water formed during the aqueous co-precipitation method, which are also the key factors in determining the electrochemical performance. Herein, an antioxidant and chelating agent co-assisted non-aqueous ball-milling method to generate highly-crystallized Na_{2- x} Fe[Fe(CN)₆ ]_y with hollow structure is proposed by suppressing the speed and space of crystal growth. The as-prepared Na_{2- x} Fe[Fe(CN)₆ ]_y hollow nanospheres show low vacancies and interstitial water content, leading to a high sodium content. As a result, the Na-rich Na_1.51 Fe[Fe(CN)₆ ]_0.87 ·1.83H₂ O hollow nanospheres exhibit a high initial Coulombic efficiency, excellent cycling stability, and rate performance via a highly reversible two-phase transition reaction confirmed by in situ X-ray diffraction. It delivers a specific capacity of 124.2 mAh g^-1 at 17 mA g^-1 , presenting ultra-high rate capability (84.1 mAh g^-1 at 3400 mA g^-1 ) and cycling stability (65.3% capacity retention after 1000 cycles at 170 mA g^-1 ). Furthermore, the as-reported non-aqueous ball-milling method could be regarded as a promising method for the scalable production of PBAs as cathode materials for high-performance SIBs.

Acid-Assisted Ball Mill Synthesis of Carboxyl-Functional-Group-Modified Prussian Blue as Sodium-Ion Battery Cathode

Prussian blue attracts the attention of many researchers as a promising candidate for use in sodium-ion battery cathodes due to its open frameworks and high working potential. However, the interstitial water in its crystal structure and its poor electronic conductivity limits its performance in practical sodium-ion batteries. Here, acid-assisted ball milling synthesis was employed as a versatile method for the production of surface-modified Prussian blue. With (CH₃COO)₂Fe being used as the raw material, the Prussian blue produced using ball milling synthesis was modified by the carboxyl functional group on its surface, which resulted in lower interstitial water content and enhanced electrochemical cycling performance. In addition, ball milling synthesis provided the as-prepared Prussian blue with a large surface area, improving its electrochemical rate performance. When used as the cathode of sodium-ion batteries, as-prepared Prussian blue delivered a specific capacity of 145.3 mAh g⁻¹ at 0.2 C and 113.7 mAh g⁻¹ at 1 C, maintaining 54.5% of the initial capacity after 1000 cycles at 1 C (1 C = 170 mA g⁻¹). Furthermore, a solid-state sodium-ion battery was mounted, with as-prepared Prussian blue being employed as the cathode and Na metal as the anode, which delivered a high specific capacity of 128.7 mAh g⁻¹ at 0.2 C. The present study put forward an effective solution to overcome the limitations of Prussian blue for its commercial application.

High-Performance Fe-Based Prussian Blue Cathode Material for Enhancing the Activity of Low-Spin Fe by Cu Doping

Iron-based Prussian blue (FeHCF) has great application potential in the large-scale production of sodium-ion (Na⁺) batteries because of its high theoretical capacity and abundant Fe ore resources. However, the Fe(CN)₆ vacancies and crystal water seriously affect the electrochemical performance. Herein, a Cu-doped FeHCF (Cu-FeHCF) cathode material is successfully prepared directly by a coprecipitation method. After Cu doping, the monoclinic structure and the quasi-cubic morphology are retained, but the electrochemical performance is significantly improved. In addition to few Fe(CN)₆ vacancies and low crystal water, the improved performance is also related to the enhanced electrochemical activity of low-spin Fe and the stabilizing effect of Cu on the crystal structure. Moreover, Cu doping also controls the side reaction to a certain extent. As a result, after Cu doping, the initial discharge capacity is enhanced from 107.9 to 127.4 mA h g^-1 at 100 mA g^-1, especially the capacities contributed by low-spin Fe increase from 30.0, 21.7, and 16.7 mA h g^-1 to 48.8, 45.4, and 43.7 mA h g^-1 for the first three cycles, respectively. Even at 2 A g^-1, Cu-FeHCF still has a promising initial capacity of 82.3 mA h g^-1 and only a 0.047% capacity decay rate for each cycle over 500 cycles. Therefore, Cu-FeHCF shows excellent application potential in the field of Na⁺ energy storage batteries.