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

Abstract

Prussian blue analogs are well suited for sodium‐ion battery cathode materials due to their cheap cost and high theoretical specific capacity. NaxCoFe(CN)6 (CoHCF), one of the PBAs, has poor rate performance and cycling stability, while NaxFeFe(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.

Elevating the Performance of the Prussian Blue Analogue Cathode for Sodium-Ion Batteries by Adjusting the Low Spin Fe Redox

“…The methodology proposed in this work can tackle the challenge of eliminating interstitial water and uplifting the average potential by tuning the low-spin redox of PBA. CoFeHCF outperforms various SIB cathodes reported earlier such as CoHCF@FeHCF (118 mAh g –1 @ 17 mA g –1 ), Na x Zn y Fe 1– y [Fe(CN) 6 ] (132 mAh g –1 @ 15 mA g –1 ), Na 1.73 Fe[Fe(CN) 6 ]·3.8H 2 O(116 mAh g –1 @ 10 mA g –1 ), Na 1.58 Fe[Fe(CN) 6 ] 0.87 ·2.38H 2 O (123 mAh g –1 @ 15 mA g –1 ), Na 1.59 Ni 0.65 Fe 0.35 [Fe(CN) 6 ] 0.97 ·2.28H 2 O (128 mAh g –1 @ 10 mA g –1 ), etc . (A detailed comparison of various reports is shown in Table S2.)…”

“…In this regard, Kong et al designed a core–shell structure of cobalt hexacyanoferrate (CoHCF) and FeHCF, significantly enhancing composited PBA cathode’s rate performance and cycling stability. The CoHCF is used as a core that offers two-electron redox reactions with high reversible capacity but lacks capacity retention at high current rates; hence, FeHCF is used as a shell in the composite, which provides excellent stability at high current rates and facilitates high electrical conductivity for easy transfers of electrons . Similar studies of composite formation and structural modifications are reported, such as HCS-PBMN and CoNi-HCF@Ni-HCF which indicates the electrochemical properties of PBA can be tuned by adopting element substitution and structural modifications.…”

“…These peaks were further deconvoluted in three subpeaks correlated to Co 2+ (782.82 and 798.88 eV), Co 3+ (780.83 and 796.93 eV), and satellite peaks of Co 2p (788.03 and 803.12 eV). 40,41 Similarly, Fe 2p spectra show two intense peaks, which are further deconvoluted in two subpeaks corresponding to Fe 2+ 2p 3/2 at 708.2 eV and Fe 2+ 2p 1/2 at 721 eV, while other peaks can be assigned to the satellite peaks of Fe 2p. 42 This indicates that the only oxidation state of Fe atoms is +2, which has occurred due to the synthesis of CoFeHCF under N 2 .…”

“…The redox peaks observed at 3.16/2.75 V and 3.74/3.55 V correspond to the redox activity of low-spin Fe 2+ → Fe 3+ couples bonded to C atom and high-spin Co 2+ /Fe 2+ → Co 3+ /Fe 3+ bonded to N atom, respectively. 40 The CV of the FeHCF and CoHCF samples shows only single redox peaks corresponding to Fe 2+ / Fe 3+ and Co 2+ /Co 3+ (Figure S12).…”

Magnetic Properties of the Thin Films of Prussian Blue Analogues Obtained by Ion-Exchange Synthesis

“…The Prussian blue analogues (PBAs) A j M k [M(CN) 6 ] l • n H 2 O, where M' and M are 3d transition metal ions and A is an interstitial cation, attract great attention due to their rich palette of properties such as room-temperature ferromagnetism [1,2], photomagnetism [3, [4], magnetic sensitivity to external pressure [5,6], and the negative thermal expansion [7]. Thin films of PBAs are therefore considered as interesting materials for power supply and storage [8][9][10][11], ion-sieving membranes [12], or future molecule-based spintronic devices [13,14]. One of the most important advantages of PBAs is their highly symmetrical structure.…”