Processing Rusty Metals into Versatile Prussian Blue for Sustainable Energy Storage

To reach a closed‐loop material system and meet the urgent requirement of sustainable energy storage technologies, it is essential to incorporate efficient waste management into designing new energy storage materials. Here, a “two birds with one stone” strategy to transform rusty iron products into Prussian blue as high‐performance cathode materials, and recover the rusty iron products to their original status, is reported. Owing to the high crystalline and Na+ content, the rusty iron derived Prussian blue shows a high specific capacity of 145 mAh g−1 and excellent cycling stability over 3500 cycles. Through the in situ X‐ray diffraction and in situ Raman spectra, it is found that the impressive ion storage capability and stability are strongly related to the suppressed structure distortion during the charge/discharge process. The ion migration mechanism and the possibility to serve as a universal host for other kinds of ions are further illuminated by density functional theory calculations. This work provides a new strategy for recycling wasted materials into high value‐added materials for sustainable battery systems, and is adaptable in the nanomedicine, catalysis, sensors, and gas storage applications.


Introduction
Battery technologies, with the most famous members being the lead-acid and lithium-ion batteries (LIBs), have revolutionized whole industries and our lifestyles over the past century. [1] Along www.advenergymat.de www.advancedsciencenews.com Iron, one of the most abundant and non-toxic 3d elements, has been widely used in our daily lives, which benefits from the improved strength, versatility, cost-effectiveness, and heat resistance of iron metal products. [6] While enjoying the convenience of these products, humans suffer from huge waste due to the inevitable rusting of iron products in moist atmosphere, which is close to one-quarter of the total iron production per year. [5b,7] The recycling of iron scrap is widely carried out today, but we are currently far away from an effective strategy to deal with the large amount of iron rust. [4b] In principle, iron rust could be recycled via reducing and melting, but this is not cost-efficient compared to the production of iron or steel products from iron ore. [8] Inspired by the redox-active nature of iron in battery electrodes (e.g., Fe in LiFePO 4 cathode for LIBs), if a large amount of iron rust could be directly converted into valuable battery products in an eco-friendly and facile way, we could close the loop in turning waste into wealth for a green and sustainable society.
Herein, as a proof-of-concept, we demonstrate a facile "two birds with one stone" method to process the rusty iron into Prussian blue cathode materials for sodium-ion batteries (SIBs), while recovering the wasted iron products to their original status. Due to the slow crystal growth rate in the acid environment, the as-prepared rust-derived Prussian blue (R-PB) shows high crystallinity and high sodium-ion content. This R-PB cathode displays excellent Na-storage properties, even when compared with the PBs synthesized from analytically pure chemicals, which could deliver a high specific capacity of 145 mAh g −1 at 0.1 C, excellent rate performance up to 10 C with a capacity of 97.8 mAh g −1 , and an ultra-long cycle life of over 3500 cycles at 5 C with capacity retention of 70.3%. In situ X-ray diffraction (XRD) and in situ Raman spectroscopy further revealed the excellent reversibility of R-PB during sodium ion insertion and extraction. Density functional theory (DFT) calculations showed that the R-PB framework is capable of Li + /Na + /K + ion storage, demonstrating its versatility for battery electrodes. This work provides an efficient strategy to recycle waste chemicals into battery electrodes, which is also applicable in other potential fields such as nanomedicine, catalysis, sensors, gas storage, and more.

Results and Discussion
Rusty nails are chosen as an attractive example of such raw materials because they are both abundant and low-cost ( Figure S1, Supporting Information). They possess unknown complexities in either compositions or phases, giving rise to great difficulties in recycling via traditional methods. In attempts to reuse the iron source in rusty nails, we carried out a simple method to recycle the non-soluble iron rust into soluble ferrous/ferric ions for preparing high-performance cathode materials. Figure 1 illustrates the sustainable synthesis procedure for processing rusty iron metals into Prussian blue for rechargeable alkaliion batteries. As shown in the flow chart in Figure S1, Supporting Information, the rusty nails are successfully restored to their original bright metal status by removing the surface rusty. During this procedure, the rusty is dissolved by the acid, forming ferrous or ferric ions, which are substantially utilized for preparing R-PB. This procedure is also easy to scale up, because of the fast reaction kinetics of the dissolution reaction as well as the high solubility of ferrous or ferric ions in water. This treatment represents a new strategy that offers low cost, strong feasibility, and sustainability for industrial large-scale energy storage systems, when compared to the conventional high-temperature sintering method for battery electrodes. Figure 2a shows the Rietveld refined neutron diffraction pattern of R-PB, where the diffraction peaks match well with the single Prussian blue phase. Table S1, Supporting Information, exhibits the refinement results, demonstrating that the crystal structure of R-PB is cubic phase with space group Fm−3m and the lattice parameters a = b = c = 10.3354 Å with the unit cell volume of V = 1104.032406 Å 3 . A high-spin FeN 6 octahedron is bonded with a low-spin FeC 6 octahedron by the CN bonding with the Na + occupying the 8c and 24d positions in the 3D open cubic structure. [9] Soft X-ray absorption near edge structure (XANES) spectroscopy ( Figure 2d) was carried out to investigate the local structure and chemical environment. The Fe L-edge XANES spectra reveal two peaks that have originated from the electron transitions to anti-bonding π* states and unoccupied e g orbitals. [10]. The Fourier transform infrared spectroscopy (FT-IR) spectrum of R-PB (Figure 2e) shows sharp absorptions at 3472, 2075, and 1617 cm −1 which could be attributed to OH, CN, and HOH bonds, respectively. [11] From comparing these with the spectra of the standard reference samples, it is found that the Fe cations are a mixture of Fe 2+ and Fe 3+ ions (Fe 2+ :Fe 3+ = 1.03:0.81). Thermogravimetric analysis (TGA) was conducted to test the content of absorbed water and interstitial water from room temperature up to 500 °C in Ar. The weight loss of R-PB was 1.7% and 22.4% before 150 °C and 310 °C, respectively, which is due to the water molecules adsorbed on the surface and the lattice water in the vacancies. [5a] According to the obtained atomic occupation and TGA shown in Table S1, Supporting Information; Figure 2f, the chemical formula for R-PB could be represented as Na 1.57 Fe[Fe(CN) 6 ] 0.836 ⋅3.91H 2 O. Compared with the PB prepared by the typical co-precipitation method (>25% defects and <1 Na + per unit), the high Na + content and low amount of [Fe(CN) 6 ] vacancies in R-PB could significantly enhance the initial Coulombic efficiency and suppress the side-reactions as cathode for SIBs. [12] .The morphology www.advenergymat.de www.advancedsciencenews.com of the R-PB is shown in Figure 2g, where the blue powder sample has particles 50-150 nm in size with a nanocubic shape, and the scanning transmission electron microscopy (STEM) in Figure 2h shows that all the elements are uniformly distributed in the particles. Due to the filtering process and highly selective reaction possibilities of NaFe(CN) 4 , the STEM energy-dispersive X-ray spectroscopy line analysis ( Figure S4, Supporting Information) confirms that there is no impurity in the particles, demonstrating the feasibility of processing complex waste to pure PB.
The electrochemical performance of the R-PB was tested between 2.0 and 4.2 V using sodium metal as counter electrode and 1 m NaClO 4 in ethylene carbonate and diethyl carbonate (EC/DEC: v/v = 1:1) with 4% fluoroethylene carbonate additive as the electrolyte. The cyclic voltammetry (CV) curves (Figure 3a) clearly show the well-defined and symmetric redox couples of Fe 2+ /Fe 3+ during the first five cycles. As we reported previously, [9] the redox pairs near 3.0, 3.3, and 3.8 V are due to three different sodium-ion storage sites at 8c (Na1, interstitial), 24d (Na2, edge), and 32f (Na3, face), respectively, in the 3D open structure of R-PB. The high plateaus of 4.05 V are related to the side-reactions between the electrolyte and the electrode caused by the interstitial water as well as the formation of the solid-electrolyte interphase (SEI) film, while the charge and discharge curves became highly reversible after the first cycle. [9] It is worth noting that the 2c position contributes more than 60% of the Na + storage sites and the whole capacity, while the other two positions contribute the high potential plateau. The first five charge/discharge curves of R-PB at the current density of 0.1 C (1 C = 150 mA g −1 ) are exhibited in Figure 3b, and the first discharge specific capacity of 145 mAh g −1 can be obtained with initial Coulombic efficiency (ICE) of ≈90%. The plateaus at 3.0, 3.3, and 3.8 V are related to the different Na + storage energy barriers, and the irreversible plateaus above 4.0 V disappeared after the second cycle, which is consistent with the CV analysis. The rate performance of R-PB was investigated at different current densities from 0.2-10 C (Figure 3c), and it is worth noting that the R-PB shows excellent fast charging performance with the discharge specific capacities of 111.8, 108.6, 103.5, and 97.8 mAh g −1 at the rates of 0.5, 1, 2, and 5 C, respectively. Even when the current density was further increased to 10 C, which means that the batteries could be fully charged/ discharged within 6 min, the capacity was still maintained at 93.8 mAh g −1 , corresponding to a capacity retention of ≈83.8%. The capacity could recover to its initial value when the current density was returned to its initial value. The long-term cycling stability of the R-PB was studied at the current densities of 1 and 5 C after activation at 0.1 C for 5 cycles (Figure 3d,f). The R-PB shows better cycling performance at a high current density of 5 C, in which there was 59.1% capacity retention, even after 3500 cycles, corresponding to a capacity fading rate of only 1.17% per 100 cycles compared with 5.96% per 100 cycles at 1 C. Benefiting from the well-controlled co-precipitation process in the acid solution, the R-PB shows a low vacancy concentration and suppressed crystal water content, leading to high initial Coulombic efficiency (ICE), and enhanced cycling and rate performances. [12a] Compared to previously reported iron-based PB synthesized from analytically pure chemicals as cathode material for organic SIBs (Figure 3e,g), the R-PB is one of the most competitive candidates regarding both rate performance and cycling stability. [13] To further investigate the structural evolution of the R-PB sample, in situ powder XRD (PXRD) and in situ Raman spectroscopy were carried out during the charging and discharging process. Figure 4a shows the in situ PXRD patterns of R-PB during the first cycle at the current density of 25 mA g −1 between 2.0 and 4.2 V. The main strong reflections of the (200), (220), (400), and (420), planes of R-PB were selected and enlarged individually for further study. All the peaks gradually shifted to higher angles during the charging process, while the intensity and the half-peak breadth at ≈17.1° and 24.5° became larger during the charging progress, indicating the lattice shrinkage and the internal structural strain [14] caused by the extraction of Na + . The peak shifts are ascribed to the phase transition from cubic to tetragonal during the Na + extraction process, and a mixture of cubic and tetragonal phases was formed. [15] The peaks at ≈38.8° and 47.3° are well matched to the Al current collector, which remained unchanged during the whole charge and discharge process. During the discharge process, all the peaks shifted toward lower degree, although could not return to their initial positions after discharge to 2.0 V, indicating irreversible structural evolution. The irreversible lattice shrinkage and expansion process might be due to the side reactions between the electrolyte and R-PB, in which the lattice water reacted with Na + and formed [NaH 2 O] + . [16] In situ heated XRD was employed (Figure. S10, Supporting Information), which showed the structural evolution process. The R-PB could maintain its cubic crystal structure up to 200 °C and then started transforming to tetragonal when the temperature increased above 200 °C due to the removal of crystal water, which caused the lattice to shrink. [16a] The structural evolution is consistent with the irreversible phase transition from cubic to tetragonal during the charging process. In consideration of the high sensitivity of the (CN) − vibration stretching mode to the oxidation state of the bonding iron, in situ Raman spectroscopy (Figure 4b) was further employed to determine the sodium content and the valence state variation of the iron ions during the charge and discharge processes during the second cycle. [12a] Starting from the fully charged state, only a strong peak at 2149 cm −1 and a weak peak at 2172 cm −1 were observed, which could be ascribed to the ν (CN) bonding with Fe 2+/3+ . During the discharge process, the main peak shifted to a low wavenumber, demonstrating that the Fe 3+ (Na content = 0) had been reduced to Fe 2+ (Na content = 2) during the sodium insertion process. During the charging process, the peaks shifted in reverse toward high wave numbers and finally resume their original positions after full charging, indicating that the Fe 2+ had been oxidized to Fe 3+ , accompanied by the extraction of Na + . The above results indicated that the structural change of R-PB is highly reversible during the sodium-ion insertion and extraction process after the first cycle activation process, which further contributes to the impressive long cycle life of R-PB as cathode material for SIBs.
DFT calculations were further performed to deeply understand the intrinsic properties of the crystal structure that are responsible for the outstanding performance. As shown in Figure 4c, the Na + diffusion energy barriers along the a, b, and c axes are only 0.41 eV (Figure 4e), which means that the PB material possibly possesses fast 3D pathways for Na + ion diffusion. The relatively low energy barriers can be ascribed to the sodium superionic conductor (NASICON)-type structure, where the sodium can be accessed expediently, which greatly contributes to the enhanced rate performance of SIBs. As shown in Figure 4d, the charge interaction between Na + ions and the PB framework is negligible, which favors Na + migration. Furthermore, the average voltages of the Fe-ions as host for Li + /Na + /K + have been provided in Figure 4f, and one can see that, with increasing Li + /Na + /K + concentration, the average voltage exhibits a decreasing tendency, and the computational voltages closely trace the experimental, where K + possesses the highest voltage platform, indicating that R-PB could also be applied as cathode material for LIBs and potassium-ion batteries (KIBs). The corresponding charge and discharge curves of the R-PB for LIBs and KIBs are shown in Figure S11 and S12, Supporting Information. Considering that another raw material, sodium ferrocyanide, could be prepared from waste sodium cyanide and iron salt, it is possible to synthesis R-PB fully based on waste materials, offering a more sustainable choice for closed loop waste recycling. Moreover, our strategy can be extended to the recycling of other rusty metals, such as nickel, forming Ni-based Prussian blue analog for various applications ( Figures S13 and S14, Supporting Information). The existence of the lattice water has been proved that would have adverse effect on the electrochemical performance of PB and PBAs as cathode for organic SIBs, therefore, the removal of the interstitial water might further enhance the electrochemical performance of the R-PB as cathode material for organic SIBs. [11b,17]

Conclusion
In summary, we have developed a facile synthesis strategy to directly convert waste materials into a universal alkali metal ion host for high-performance rechargeable batteries. Owing to the intact crystal structure and negligible structural distortion during the Na + insertion/extraction process, the waste-derived R-PB exhibits a high Na storage capability of 145 mAh g −1 and unprecedented cycling stability with ≈59.1% capacity retention over 3500 cycles when applied as cathode material for SIBs. The DFT calculations further validate the potential of R-PB as cathode material for multiple kinds of rechargeable batteries. This proof-of-concept work provides new routes for the recycling of waste metal into high value-added products, which are of great importance for building a resource-sustainable environment and opening up a promising prospect for large-scale energy storage applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.