Reforming Magnet Waste to Prussian Blue for Sustainable Sodium-Ion Batteries

Abstract

Increasing generation of permanent magnet waste has resulted in an urgent need to preserve finite resources. Reforming these wastes as feedstock to produce renewables is an ideal strategy for addressing waste and energy challenges. Herein, our work reports a smart and sustainable strategy to convert iron in magnet wastes into Prussian blue analogues that can serve as cathode materials for sodium-ion batteries. Moreover, a method to control feed rates is proposed to generate high-quality materials with fewer [Fe(CN)6] vacancies at a feed rate of 3 mL min–1. The recycled Na1.46Fe[Fe(CN)6]0.85·□0.15 shows low vacancies and excellent cycling stability over 300 cycles (89% capacity retention at 50 mA g–1). In operando, evidence indicates that high-quality Prussian blue allows fast sodium-ion mobility and a high degree of reversibility over the course of cycling, although with a three-phase-transition mechanism. This study opens up a future direction for magnet waste created with the expectation of being environmentally reused.

Prussian Blue Analogues with Optimized Crystal Plane Orientation and Low Crystal Defects toward 450 Wh kg−1 Alkali‐Ion Batteries

“…The Fourier transforms of the Fe K‐edge EXAFS spectra are presented in Figure 1f. The spectrum shows three major peaks, with the peak positions at 1.24, 2.41, and 4.3 Å belonging to the Fe−C, Fe−N, and Fe−Fe shells, respectively [3a, 10] . A contour plot of the wavelet‐transformed XAS of NKPB‐3 is shown in Figure 1g.…”

Prussian Blue Analogues with Optimized Crystal Plane Orientation and Low Crystal Defects toward 450 Wh kg−1 Alkali‐Ion Batteries

“…Other peaks at 710.2 eV for Fe III 2p 3/2 and 723.5 eV for Fe III 2p 1/2 were also found. [10,19] These results indicate that Fe II is mainly involved in the valence state of Fe and that Fe III plays a small role in the framework. Because of the exposure to air during the washing process, Fe III was formed by the oxidation of Fe II .…”

“…The detailed crystal information, such as atom locations and occupations, can be found in Tables S1 and S2 in the Supporting Information. As shown in Figure 1a, b, both NKPB‐0 and NKPB‐3 have the cubic phase ( Fm ‐3 m ) with well‐crystallized structure and no impurities [10, 14] . After the introduction of potassium ions, the lattice parameter of NKPB‐3 ( a =10.0770 Å) was lower than that of NKPB‐0 ( a =10.2872 Å), meaning that the amount of [Fe(CN) 6 ] defects was decreased and a more stable structure was obtained after the suppressed volume expansion [15] .…”

“…[9] Last but not least, the crystal water may impede Na + movement to the lattice after occupying the interstitial spaces, and the coordinated water would then dissolve in the organic electrolyte, causing side reactions and bringing safety hazards. [9,10] Although adding large amounts of chelating agents is effective for slowing down the precipitation rate for a flawless framework, the amount of unwanted water in PBAs is larger than 10 wt % no matter which synthesis method (co-precipitation, self-decomposition, ball milling) is used. [6a, 7, 8, 11] More importantly, for practical production in pouch cells using PBAs as cathode, our recent work shows that pouch cells are swollen after cycling when using watercontaining PBAs, meaning that there are great safety hazards caused by water.…”

“…Furthermore, the electronic conduction is also be affected negatively by the [Fe(CN) 6 ] 4− vacancies, bringing higher Ohmic polarization [9] . Last but not least, the crystal water may impede Na + movement to the lattice after occupying the interstitial spaces, and the coordinated water would then dissolve in the organic electrolyte, causing side reactions and bringing safety hazards [9, 10] . Although adding large amounts of chelating agents is effective for slowing down the precipitation rate for a flawless framework, the amount of unwanted water in PBAs is larger than 10 wt % no matter which synthesis method (co‐precipitation, self‐decomposition, ball milling) is used [6a, 7, 8, 11] .…”