Rusty nails are chosen as the raw material because they are easily available (Figure S1). They possess unknown complexities in either compositions or phases, showing great difficulties in the purpose of recycling. In attempts to re-use the iron source in rusty nails, we carry out a simple method to circulate the non-soluble iron rust into soluble ferrous/ferric ions for preparing high-performance cathode materials. Figure 1 illustrates the sustainable synthesis procedure of processing rust iron metals into the Prussian blue for rechargeable alkali-ion batteries. As shown in the flow chart and Figure S1, the rusty nails are successfully getting back to their original bright nail by removing the rust. During this procedure, the rust is dissolved by the acid, forming ferrous or ferric ions, which are substantially utilized for preparing R-PB. This treatment put forward a new strategy of low-cost, strong feasibility and sustainability for the industrialized large-scale energy storage system, compared to conventional high-temperature sintering method for battery electrodes.
Figure 2a shows the Rietveld refined neutron diffraction pattern of R-PB and the diffraction peaks match well with the single Prussian blue phase. Table S1 and Figs. 2b-c exhibit the refinement results representing the crystal structure of R-PB shows cubic phase with a space group of Fm-3m and the lattice parameters of a = b = c = 10.3354 Å with the volume of V = 1104.032406 Å3. A high-spin FeN6 octahedral is bonding with a low-spin FeC6 by the octahedral by the C≡N with the Na+ occupied the positions of 8c and 24d in the 3-dimension open cubic structure.12 Soft X-ray absorption near edge structure (XANES) spectroscopy (Fig. 2d) was carried out to measure 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 eg orbitals13. The Fourier transforming infrared spectrum (FT-IR) spectra of R-PB 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.14, 15 From comparing these with the spectra of the standard reference samples, the Fe cations are a mixture of Fe2+ and Fe3+. Thermogravimetric analysis (TGA) was measured to test the content of absorbed water and interstitial water from room temperature to 500°C in Ar. The weight loss of R-PB is 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.6 According to the obtained atomic occupation and TGA shown in Table S1 and Fig. 2f, the chemical formula for R-PB could be represented as Na1.57Fe[Fe(CN)6]0.836⋅3.91H2O. The high Na+ content and low [Fe(CN)6] vacancies amount of R-PB could significantly enhance the initial coulombic efficiency and suppress the side-reaction as cathode for SIBs.16, 17 The morphologies of the R-PB are shown in Fig. 2g, the blue powder sample has nanocubic shape with the particle size of 50–150 nm and the scanning transmission electron microscopy (STEM) shows that all elements are uniformly distributed in the particles. The STEM energy-dispersive X-ray spectroscopy line analysis (Figure S4) confirms that there is no impurity in the particles, demonstrating the feasibility from complex waste to pure PBs.
The electrochemical performance of the R-PB was tested between 2.0-4.2 V using metal sodium as counter electrode. The cyclic voltammetry (CV) curves (Fig. 3a) clearly show the well-defined and symmetric redox couples of Fe2+/Fe3+ during the first five cycles. As we reported previously12, the redox pairs near 3.0, 3.3 and 3.8 V are due to three different sodium-ion storage sites at 2c (Na1, interstitial), 3d (Na2, edge) and 3e (Na3, face) in the 3-dimension open structure of R-PB.12 The high plateaus of 4.05 V are related to the side-reaction between the electrolyte and the electrode caused by the interstitial water as well as the formation of SEI film, while the charge and discharge curves become highly reversible after the first cycle. It is worth noticing that the 2c position contributes more than 60% of Na+ storage sites and 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 (1C = 150 mA g− 1) are exhibited in Fig. 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 performances of R-PB were carried at different current densities from 0.2 C-10 C (Fig. 3c), 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, 97.8 mAh g− 1 at the rate of 0.5, 1, 2, 5 C. Even when the current density further increased to 10 C, which means the batteries could be fully charged/discharged within 6 mins, the capacity still maintain 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 returns. The long-term cycling stability of the R-PB was studied at the current densities of 1 C and 5 C after activated at 0.1 C for 5 cycles. The R-PB shows better cycling performance at a high current density of 5 C that the capacity maintains at 70.25 % even after 3500 cycles, corresponding to the capacity fading rate of only 0.85 % per 100 cycles and 5.96 % per 100 cycles at 1 C. Benefiting from a well-controlled co-precipitation process in the acid solution, the R-PB shows low-vacancy concentration and suppressed crystal water content, leading to high ICE, enhanced cycle and rate performance16. Compared to previously reported iron-based PB synthesized by analytically pure chemicals as cathode materials for organic SIBs, the R-PB is one of the most competitive candidates regarding both rate performance and cycling stability18, 19, 20.
To further investigate the structural evolution of the R-PB sample, in situ PXRD and in situ Raman spectra analyses 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 2.0-4.2 V. The main strong reflections of the (200), (220), (400) and (420) planes of R-PB are selected and enlarged individually for further study. All of 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 shrink and the internal structural strain21 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 formed22, 23. The peaks at ~ 38.8° and 47.3° are well matched to the Al current collector, which remain unchanged during the whole charge and discharge process. During the discharge process, all peaks shifted towards lower degree, however, could not return to their initial positions after discharged to 2.0 V, indicating the irreversible structural evolution. The irreversible lattice shrink and expansion process might due to the side reactions between the electrolyte and R-PB, in which the lattice water reacted with Na+ and formed the [NaH2O]+, consistent with the first charge/discharge cycle and our previous work24, 25. In consideration of the highly sensitivity of (C≡N)− vibration stretching mode to the oxidation state of the bonding iron, in situ Raman spectra was further employed to determine the sodium content and the valance state variation of the iron ions during the charge and discharge during the second cycle16. 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 Fe2+/3+. During the discharge process, the main peak shifts to a low wavenumber, demonstrating that the Fe3+ (Na content = 0) has been reduced to Fe2+ (Na content = 2) during the sodium insertion process. During the charging process, the peaks shift in reverse toward high wave number and finally resume their original positions after fully charged, indicating that the Fe2+ has been oxidized to Fe3+ accompanied with the extraction of Na+. The above results indicate that the structure change of R-PB is highly reversible during the sodium-ion insertion and extraction process after the first cycle activation, which further contributes to the impressive long cycle life of R-PB as cathode materials for SIBs.
DFT calculations were further performed to deeply understand the intrinsic properties of the crystal structure that are responsible for the outstanding performance. The Na+ diffusion energy barriers along with the a, b, and c axes are only 0.41 eV, which means that material-PB possibly possesses fast three-dimensional pathways for Na+ ion diffusion. The relatively low energy barriers can be ascribed to the NASICON-type structure where the sodium can be accessed expediently, which greatly contributes to the enhanced rate performance of SIBs. As shown in Fig. 4e, the charge interaction between Na+ ions and the framework is negligible, which is in favor of Na+ migration. Furthermore, the average voltage of Fe− as host for Li+/Na+/K+ have been provided in Fig. 4f, one can see that with the increases of Li+/Na+/K+ concentration, the average voltage exhibits a decreasing tendency, and the computational voltage traces the experimental well where K+ possess the highest voltage platform, indicating R-PB could also be applied as cathode materials for LIBs and potassium-ion batteries. Considering another raw material, sodium ferrocyanide, could be prepared from the wasted sodium cyanide and iron salt, it is possible to synthesis R-PB fully based on wasted materials, offering a more sustainable choice for close-of-loop waste recycle.