Insights into the mechanism of electrode degradation and performance enhancing strategies for iron-ion batteries using X-ray absorption spectroscopy

Abstract

Rechargeable iron-ion (Fe-ion) batteries are gaining attention due to their unique characteristics, including earth abundance, cost-effectiveness, eco-friendly nature, and high electrochemical performance. However, capacity degradation during cycling hinders their effective use. To investigate the material's degradation in rechargeable Fe-ion batteries, two different coin cells are fabricated utilizing mild steel (MS) and ZnO-coated mild steel (ZnO@MS) as anodes. In both cases, V₂O₅ is used as the cathode, along with a non-aqueous electrolyte. Cyclic voltammetry and galvanostatic charge–discharge analyses are conducted at different cycling stages, viz. 20, 40, 60, and 80 cycles, for determining the electrochemical performance of these anode-based coin cell batteries. The coin cells are dismantled after cycling, and the post-cycled electrodes are subjected to ex situ scanning electron microscopy, X-ray diffraction, and X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements to probe the structural and chemical degradation mechanisms of the electrode materials. The results from the XANES and EXAFS measurements provide critical insights into the evolution of the electronic structure and local atomic environment, revealing degradation trends correlated with the cycling performance. The comparison between the MS and ZnO@MS anodes highlights the protective role of ZnO coating in mitigating degradation. In both cases, the V₂O₅ cathode exhibits significant transformation after cycling, possibly due to changes in the oxidation states due to the insertion of Fe ions in the cathode. Thus, these findings offer a deeper understanding of the stability of materials in Fe-ion batteries and anode modification possibilities, which are crucial for developing durable, cost-effective energy storage systems.

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1. Introduction

Electrochemical energy storage systems have become critical in transitioning towards renewable energy and sustainable mobility solutions.¹ These systems are essential for storing and delivering energy efficiently in applications ranging from portable electronics to electric vehicles and grid storage.² The global shift toward clean energy technologies has driven extensive research into advanced battery systems that offer higher energy densities, longer lifetimes, and lower environmental impacts.³ Conventional electrochemical energy storage solutions, such as supercapacitors and lead-acid batteries, have limitations in energy density, power performance, and long-term sustainability.⁴ As a result, next-generation electrochemical energy storage systems, particularly rechargeable batteries, are at the forefront of scientific innovation due to their usefulness in addressing the diverse demands of modern energy infrastructure.^5–7

For instance, lithium-ion batteries (LIBs) have emerged as the dominant technology in electrochemical energy storage due to their high energy density, relatively long cycle life, and established manufacturing processes. However, the widespread reliance on Li-ion batteries poses significant challenges.^8,9 The limited global lithium supply and the geopolitical concentration of lithium resources raise concerns about supply chain instability and cost accelerations. Moreover, the extraction and refining of lithium have substantial environmental consequences, including high water consumption and ecological disruption.¹⁰ It is also coupled with safety risks such as thermal runaway and flammability. The long-term sustainability of LIBs is increasingly being questioned, encouraging researchers to explore alternative battery chemistries that address these limitations while meeting growing global energy demands.^6,11,12

In this scenario, multivalent metal-ion batteries have gained attention as promising alternatives to traditional Li-ion systems. Unlike lithium, multivalent ions, such as magnesium (Mg²⁺), calcium (Ca²⁺), aluminum (Al³⁺), and iron (Fe²⁺/Fe³⁺), can carry multiple charges, potentially offering higher energy densities due to the increased charge transfer per ion.^13–15 Multivalent metal–air (Al–air, Zn–air, Mg–air, and Fe–air, etc.) batteries have also been widely explored due to their unique features and performances.^16–19 Among these, Fe-ion batteries are particularly attractive due to the earth-abundant nature of iron, its low toxicity, and the cost-effectiveness of using iron-based materials.^10,20 Furthermore, Fe-ion systems demonstrate the potential for long-term cycling stability, which is essential for large-scale applications such as grid storage and electric vehicles. However, one must address the challenges associated with the electrode material, such as stability, electrolyte compatibility, and ion diffusion, to realize the full potential of Fe-ion batteries.^21–32 Understanding the degradation mechanisms of these Fe-ion batteries is crucial to overcoming the challenges and enhancing electrochemical performance without degradation.³¹

In this work, we present a detailed study of the degradation mechanisms in rechargeable Fe-ion batteries by focusing on two different (i) MS and (ii) ZnO@MS anode materials, with V₂O₅ as the cathode and non-aqueous electrolyte. The ZnO coating was applied on mild steel using a spin-coating technique, and various coin cells were fabricated and tested using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopic (EIS) analyses over multiple cycles (20, 40, 60, and 80 cycles). The coin cells were disassembled after cycling measurements, and the electrodes were analyzed using ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) measurements to gain insights into the structural and chemical changes induced during multiple cycling. This study provides a comparative analysis of the degradation behavior of the MS and ZnO@MS anodes, offering insights into how the ZnO coating enhances the stability of the electrode material during cycling. Our findings contribute to the broader understanding of Fe-ion battery chemistry and its viability for future energy storage applications.

2. Experimental methods

2.1 Materials and accessories

Zinc acetate dihydrate (Zn(OOCCH₃)₂·2H₂O), vanadium oxide (V₂O₅), N-methylpyrrolidone (NMP), multiwalled carbon nanotubes (MWCNTs), ethanolamine, 2-methoxyethanol (2-ME), isopropyl alcohol (IPA), and polyvinylidene fluoride (PVDF) were purchased from Sigma-Aldrich. The Whatman filter paper (Cytiva, 1001-125), C22032 coin cell accessories (coin cane, coin cap, etc.), SS spacer, SS spring, and commercial 0.4 mm mild steel (MS) were used to fabricate the storage device for electrochemical evaluation.

2.2 Material synthesis, electrode, coin cell fabrication, and disassembling the coin cell

We investigated two different series of experiments based on rechargeable Fe-ion batteries. The first series consisted of vanadium oxide-based cathode materials, MS-based anode materials, a non-aqueous electrolyte based on tetraethylene glycol dimethyl ether (TEGDME), and iron perchlorate as the source of Fe²⁺ ions. Another series was based on the exact configuration, but the MS anode was replaced with a ZnO-coated mild steel (ZnO@MS) anode. Commercially available vanadium oxide was used as the cathode for the experiment.³¹

For the anode material in the first series, a 0.4 mm-thick MS sheet was polished with sandpaper before use in the coin cell. For another series of experiments, the ZnO solution was prepared using 25 mM zinc acetate as the primary precursor. Initially, 10 mL of 2-ME was used as the solvent, followed by adding zinc acetate to the solution, stirring for 10 minutes, and adding ethanolamine dropwise according to the desired molar ratio. In 10 minutes, all the precursors were dissolved, which was followed by 3 hours of continued stirring. After 3 hours of stirring, the solution was aged for 48 hours before use in spin coating.

The ZnO solution was spin-coated on the surface of the 0.4 mm MS sheet, which was polished with sandpaper. The first layer was coated on the surface of the MS using 10 µL of a ZnO solution for 30 seconds at 3000 rpm, followed by thermal treatment at 400 °C for 5 minutes under ambient conditions. This process was repeated twice to get a uniform ZnO layer on the MS sheet. This ZnO@MS was used as the anode to understand the impact of the ZnO layer on MS. The cathode material slurry ink was also synthesized using V₂O₅ as the active cathode, CNT as the conducting and PVDF as the binding agent. All the powders were mixed for 2 hours using a mortar and pestle. The mixed powder was poured into a 15 mL glass vial, and the powder was mixed in NMP and stirred for 24 hours using the drop-cast coating on the SS spacer. These coated spacers were dried before using in the coin cell at 80 °C for 90 minutes. The detailed fabrication of coin cells was reported by Yadav et al. in their previous work.^23,31 The fabricated CR2032 coin cells were used to test the fabricated electrode in the full-cell configuration.

Further, after complete GCD analysis at various cycling stages, the cell was opened with a plier using safety precautions. Both electrodes were removed from the coin cell accessories, and further, the separator that was stuck to the anode was removed, and the anode was cleaned with ethanol once and dried at room temperature for ex situ measurements.

2.3 Physical measurements

X-ray diffraction (XRD, Malvern Panalytical, Empyrean) was performed on both electrodes, MS and ZnO@MS, to ensure the coating of ZnO at MS using the Cu Kα radiation. A scanning electron microscope (SEM, Carl Zeiss EVO 18) was used to see the morphological characteristics of ZnO@MS along with the EDAX spectrum analysis. A Thermo Scientific (model: Apreo 2S) field-emission electron microscope (FESEM) was used to characterize the morphology of anodes and commercial V₂O₅ at higher magnifications.

The post-measurements (ex situ), near-edge X-ray absorption fine structure (NEXAFS) measurements on the cathode and anode materials were performed at the soft X-ray 10D X-ray absorption spectroscopy (XAS)-Korea Institute of Science and Technology (KIST) beamline located at Pohang Accelerator Laboratory, Pohang, Republic of Korea. This accelerator operates at 3.0 GeV with a maximum storage current of 400 mA. For NEXAFS, spectra were collected in total electron yield (TEY) mode at room temperature in a vacuum of ∼1.5 × 10⁻⁹ torr. These spectra were recorded with a step size of 0.1 eV. The grating with 1000 and 1800 grooves per mm with entrance and exit slits opening of 100 µm × 100 µm was used to collect the V L-edge, O K-, and Fe L-edge spectra. The energy resolution was ∼0.3 eV for recording the spectra at these edges. All spectra are background-subtracted by dividing by the incident photon flux and normalizing for post-edge height.^33,34

X-ray absorption spectroscopy (XAS) measurements were conducted at the 1D XRS KIST-PAL beamline that utilizes the bending magnets as a synchrotron source. This beamline utilizes a bending magnet as a synchrotron radiation source and is equipped with a Si(111) double-crystal monochromator, which enables a wide range of monochromatic energies ranging from 4 keV to 16 keV.³⁵ During the measurements, higher harmonics were effectively removed by detuning the incident beam to 60% of its maximum intensity. Two ionization chambers filled with He and N₂ gases were used to record the intensities of the incident and transmitted X-rays. A third chamber was also used to measure the intensity from the reference foil (Fe, in this case). For measurements, MS anodes and V₂O₅ cathodes were placed between the first and second ionization chambers so that the material surface makes an angle of 45° to the direction of the incident beam. The reference foil (Fe) for energy calibration was placed between the second and third ionization chambers. Due to the large thickness of cathodes (∼V₂O₅ coated SS), X-rays are unable to penetrate; hence, a fluorescence detector was placed at an angle of 45° from the material surface, enabling incident X-rays to make an angle of 90° from the detector. This enables the measurement of XAS in fluorescence mode. Under stationary conditions, XAS measurements were performed around the Fe K-edge (for MS anodes) and V K-edge (for V₂O₅) in the step-scanning mode.³⁶

The Athena program was used to obtain (i) the spectra by dividing the fluorescence current by the incident current, (ii) identify the absorption edge (E₀), (iii) fitting pre- and post-edge backgrounds, and (iv) obtain the normalized spectra.^37,38 Normalized spectra in the near-edge region (XANES) were used to get information on the oxidation state. The normalized spectra in extended range (EXAFS) were converted to the normalized absorbance χ as a function of the modulus of the photoelectron wave vector k using Athena. The k-weighted EXAFS spectra are Fourier transformed into R-space for qualitative analysis of the atomic structure and relative bond lengths with respect to the absorbing atoms.^36,39 For quantitative analysis, the fitting Fe-edge EXAFS spectra were obtained using ARTEMIS in the k range 3–11.5 Å. The theoretical structure for the anode was generated using the ATOM and FEFF from the crystallographic information file (CIF) of Fe.³⁸ For simulation of spectra, Debye–Waller factor and radial distance were kept free in order to determine the co-ordination number and bond-distance. The quality of fit was determined from the R-factor.

2.4 Electrochemical measurements

Various electrochemical measurements were performed to assess the performance of the cell using various equipment, including a Metrohm electrochemical workstation (AUTOLAB, PGSTAT302N) for cyclic voltammetry analysis and electrochemical impedance spectroscopy. ZnO@MS or MS was used as the counter electrode, along with V₂O₅ as the working electrode in the electrochemical workstation, and measurements were performed at lower and higher swipe rates. The NEWARE battery tester (5 V, 10 mA) was used to check the galvanostatic charge–discharge performance of the cell at various current rates.

3. Result and discussion

3.1 Physical characterization analysis of electrodes

The morphologies of pristine V₂O₅ and mild steel were investigated using the FESEM analysis. Fig. 1(a) shows a FESEM image at a scale of 1 µm for the pristine commercial V₂O₅ cathode material, which exhibits a flake-type morphology. The MS shows its lined-type characteristics due to polishing with fine sandpaper before use in the coin cell (Fig. 1(b)). Furthermore, the SEM was also performed on ZnO@MS to check the EDAX spectra. The SEM image of ZnO@MS is shown in Fig. 1(c), and the corresponding EDAX spectra are also shown in Fig. S1(a). These characteristics show the presence of Zn at the surface of the mild steel. Fig. 1(d) shows the cross-section FESEM image of ZnO@MS, where its thickness is around 285 nm with uniformity. The XRD pattern of MS typically shows its body-centered cubic (BCC) crystal structure, with key diffraction peaks appearing at the 2θ values of ∼44.7° (110), ∼65.0° (200), and ∼82.3° (211), corresponding to its characteristic crystallographic planes, as shown in Fig. 1(e). The XRD analysis of ZnO@MS reveals additional peaks corresponding to ZnO, with a prominent diffraction peak at 2θ = 35.24°, indicating the (101) crystallographic plane of the hexagonal wurtzite ZnO structure. These ZnO peaks, along with the characteristic MS peaks, confirm the successful coating of ZnO on the MS surface. Fig. 1(f) shows the XRD pattern of the commercial V₂O₅ cathode. The XRD analysis of vanadium pentoxide reveals its typical orthorhombic crystal structure, with characteristic diffraction peaks observed at 2θ values of ∼20.3° (001), and ∼26.1° (110). These peaks correspond to specific crystallographic planes and confirm the formation of V₂O₅, often referenced against JCPDS no. 41-1426. Thus, XRD patterns of MS, ZnO@MS and V₂O₅ are consistent with the literature for Fe-ion batteries.^22,26,31,40

Fig. 1 FESEM images of (a) V₂O₅ and (b) MS. (c) SEM images of ZnO@MS. (d) FESEM cross-section image of ZnO-coated MS with scale bar. XRD patterns of (e) MS and ZnO@MS and (f) V₂O₅ powder.

3.2 CV analysis

The CV analysis was conducted on the fabricated coin cells for both anodes. The measurements were investigated at the 03 and 15 mV s⁻¹ to confirm the redox behavior of both cells. Fig. 2(a) shows the CV curve at 03 mV s⁻¹, with peaks at 1.90 and 0.74 V, corresponding to the oxidation and reduction for MS-based coin cells, respectively. The ZnO@MS cell shows oxidation and reduction peaks at slightly lower and higher voltages, around 1.93 and 0.77 V, respectively. Fig. S1(b) displays the CV analysis at 15 mV s⁻¹, where the oxidation and reduction peaks shift to higher and lower potentials, respectively, at higher scan rates. These results confirm its redox mechanism, which is similar to that of rechargeable non-aqueous Fe-ion batteries.

Fig. 2 (a) CV curves for both MS and ZnO@MS-based cells at 03 mV s⁻¹, (b) GCD curves at 100 mA g⁻¹, (c) specific capacity with number of cycles, (d) coulombic efficiency with number of cycles at various current densities, (e) capacity retention with number of cycles at 100 mA g⁻¹ and (f) specific capacity with number of cycles.

3.3 GCD analysis

Galvanostatic charge–discharge analysis was performed to check the specific capacity, discharge voltage plateau, and cycling behavior of the battery at different cycle stages. Fig. 2(b) shows the GCD curve of voltage versus specific capacity at 100 mA g⁻¹. The specific capacity of ∼115 mAh g⁻¹ is measured for ZnO@MS, which is higher than the ∼95 mAh g⁻¹ for the pristine MS anode-based systems. Both curves show a discharge voltage plateau at an average potential of ∼1 V, which is higher for ZnO@MS, corresponding MS-based coin cell. Furthermore, the cell was investigated at various current densities to observe the electrochemical performance. Fig. 2(c) shows the specific capacity as a function of cycle number at various current densities, where the current densities varied from 100 to 300 mA g⁻¹. The specific capacity varies from 120 to 45 mAh g⁻¹ for ZnO@MS and from 100 to 40 mAh g⁻¹ for the MS-based anode across various cycles towards higher current density, as shown in Fig. 2(c). The results clearly indicate that the capacity retention at various current densities for ZnO@MS is more stable than that of MS for Fe-ion batteries. Fig. 2(d) shows the coulombic efficiency with the number of cycles, corresponding to a similar current density as mentioned in Fig. 2(c). The detailed charge–discharge curves are also shown in Fig. S1(c) and (d) at 100 mA g⁻¹ corresponding to ZnO@MS and MS. The data demonstrate that the ZnO@MS electrode consistently delivers higher specific capacity and exhibits significantly lower voltage polarisation compared to MS across all cycles. Notably, the ZnO@MS electrode retains over 80% of its capacity at the 50th cycle with stable charge–discharge plateaus, indicating better cycling stability, whereas the bare MS anode shows marked capacity decline and pronounced polarisation as cycling progresses, reflecting more degradation than the MS-based anode.

To investigate capacity degradation in detail, the analysis was conducted at a fixed current density of 100 mA g⁻¹. The cycling was investigated for various cycles, including 20, 40, 60, and 80 cycles. Fig. 2(e) shows a bar diagram of capacity retention versus cycle number, indicating that the coating effectively enhances capacity retention compared to the pristine MS anode. Fig. S2 shows a detailed comparison of the cyclability of these series of cells at different cycle stages, including specific capacities and cycle numbers. Fig. S2(a) compares the initial 20 cycles for both cells, showing higher capacity retention for the ZnO@MS anode material compared to pristine MS at 100 mA g⁻¹. Further, another cell was similarly cycled at 100 mA g⁻¹ for 40 cycles, as shown in Fig. S2(b). As shown, the degradation is higher in the pristine MS electrodes. The cell was also cycled for 60 cycles, as shown in Fig. S2(c), where degradation is higher for pristine MS, and ZnO@MS exhibits better capacity retention than the pure mild steel anode. In the final stage, the series of cells was subjected to 80 cycles, during which degradation steadily increased with the number of cycles, and capacity retention was better maintained with ZnO@MS.

Furthermore, this cell was also individually cycled at a higher current density of 300 mA g⁻¹ to assess the cyclic stability over a larger number of cycles. Fig. 2(f) shows that the coating effectively retains capacity for up to 250 cycles. These GCD results clearly show the difference in specific capacity and capacity retention for pristine MS and the ZnO@MS. These results were also consistent with the EIS analysis, ex situ SEM, XRD, and X-ray absorption spectra of the post-electrode after the complete cycling, which are discussed in the following sections.

3.4 EIS analysis

EIS measurements were performed before GCD cycling in both anode-based cells to analyze the different impedance components. It is an excellent technique to investigate and understand the charge dynamics of coin cells without dismantling them.^31,41 It provides bulk resistance, a solid electrode–electrolyte interface, and charge transfer resistance. The impedance measurements were performed at the OCP with an amplitude of 0.01 V. The frequency was varied from 0.1 Hz to 100 kHz with 50 points. After all the measurements, the equivalent circuit was fitted using the NOVA software. The EIS measurements were conducted for the Fe-ion cells before and after the GCD cycling to determine the impedance components of both pristine MS and ZnO@MS and to assess the effect of the coating on the pristine system. Fig. 3(a) shows the Nyquist plots for ZnO@MS and pristine MS-based cells, showing the apparent differences between them. These data were also fitted with the equivalent circuit, shown in Fig. S3, which suggests that the bulk resistance is lower for ZnO@MS, around 79 Ω, compared to 90 Ω for the pristine MS system. The solid electrode–electrolyte interface resistance was also analyzed using the fitted data, which suggests that ZnO@MS is 15 Ω, which is lower than that of pristine MS (106 Ω), as shown in Fig. S3. These results indicate that the coating initially affects the MS. The lower the electrode–electrolyte interface, the higher the specific capacity, as shown in the charge–discharge analysis. Fig. 3(b) shows the Nyquist plots after complete GCD cycling (80 cycles) of coin cells based on ZnO@MS and pristine MS. Both bulk and solid electrode–electrolyte resistance change with cycling. There is a significant change in both the bulk and solid electrode–electrolyte resistance for both electrodes, but the change in the case of the pristine system is more pronounced. This suggests that the ZnO@MS electrode performs more effectively during cycling, enhancing cyclic stability and capacity.

Fig. 3 Nyquist plots (a) before GCD cycling and (b) after the complete GCD cycling of the MS and ZnO@MS anodes.

3.5 Ex situ analysis

The following measurements were carried out after completing the electrochemical measurements on various coin cells. These were performed at various cycling stages. Ex situ characterization or the post-mortem results helped to understand the structural and morphological differences after the cycling of the coin cells at various cycling stages. This finding can be further correlated with the various electrochemical results. In ex situ characterization, mainly SEM, XRD, and XAS analyses were performed and are discussed in detail in the sections below.

3.5.1 SEM and XRD analyses. After opening the coin cells under ambient conditions, both electrodes were investigated using SEM and XRD measurements after 40 and 80 cycles (denoted as 40C and 80C) at 100 mA g⁻¹. The results were compared with those of the pristine electrode. Fig. 4(a) shows an SEM image of pristine MS with the corresponding EDAX mapping showing higher atomic percentage of Fe and lower oxygen content under ambient conditions. Fig. 4(b) and (c) show the MS after 40 and 80 cycles, respectively, with their corresponding EDAX mapping. The oxygen percentage after cycling increased due to the formation of the SEI layer, as shown in the SEM images. Furthermore, these electrodes were analyzed using XRD before cycling and were compared after cycling for 40 and 80 cycles, as shown in Fig. 4(d). Iron oxide additional peaks were observed after the 40 and 80 cycles and also marked for easy identification.^23,32 These results show the degradation of mild steel during cycling. Similarly, the ZnO@MS electrodes were also analyzed. Fig. 4(e–g) shows the SEM images corresponding to the EDAX mapping before cycling and after 40 and 80 cycles, respectively. The SEM image clearly shows no significant changes in the surface, compared to the pure MS. The EDAX results also show no significant changes in the atomic percentages of iron and oxygen. Further, these electrodes were also investigated by XRD, as shown in Fig. 4(h). The XRD pattern shows apparent differences between the cycled MS and cycled ZnO@MS. There are no changes in the ZnO@MS XRD pattern before and after cycling, indicating that ZnO@MS is resistant towards Fe oxidation. Probably, this makes the ZnO coating effective in improving electrochemical performance.

Fig. 4 SEM images of pristine MS (a) before cycling and (b) after 40 cycles and (c) 80 cycles, (d) corresponding XRD patterns of the cycled MS at various cycling stages. SEM images of ZnO@MS (e) before cycling and (f) after 40 cycles and (g) 80 cycles, and (h) corresponding XRD patterns of the cycled ZnO@MS at various cycling stages.

Further, Fig. S4(a) shows that the XRD pattern of the separator, which was stuck to the anode during cycling. It clearly indicates the iron oxide mixed phase after 40 and 80 cycles, and the color is visible in the camera photograph above the XRD pattern. Fig. S4(b) shows a camera photograph of the cycled anode based on mild steel and ZnO-coated mild steel at various cycling stages. The changes in the mild steel surface are also easily visible to the naked eye over cycling. In comparison, the ZnO@MS remains stable throughout cycling. The V₂O₅-based cathode was also investigated by XRD after cycling, similar to the anode materials for MS and ZnO@MS, as shown in Fig. S4(c) and (d). The XRD patterns show similar indexing over cycling in both cases, MS and ZnO@MS, due to the coating on the SS current collector. It also shows the Fe peaks as indexed, along with other peaks for V₂O₅ corresponding to the ICDD PDF #00-041-1426.

3.5.2 XAS analysis. The XAS technique is highly effective for post-cycling analysis of Fe-ion battery electrodes, providing insights into electrochemical changes after complete charge–discharge (GCD) cycling. After cycling, the coin cell is disassembled, and the electrode is characterized to study changes in oxidation states, local atomic environments, and structural degradation via anode analysis through Fe K-, L-edge, and O K-edge XAS spectra and cathode analysis through V K-edge, L-edge, and O K-edge.

Anode analysis through Fe K-, L-edge, and O K-edge: Fe K-edge XAS spectra corresponding to MS and ZnO@MS after various cycling stages and the respective pristine counterparts are shown in Fig. S5(a) and (b). The oxidation state changes at various cycling stages can be observed from the XANES spectra (XAS spectra from 7050 eV to 7200 eV), as shown in Fig. 5(a) and (b), respectively. Spectral features are indicated with vertical dotted lines (A₁, B₁, C₁, D₁) to highlight the exact differences caused by cycling. In the pristine MS anode, XANES spectra show a significant change in normalized intensity with cycling (Fig. 5(a)). However, in the ZnO@MS anode, the changes in normalized intensity are remarkably reduced, as shown in Fig. 5(b). This difference in the XANES spectra of both anodes may be associated with the dominant oxidation observed during cycling in the MS anode. Moreover, the main edge (energy at the half of step height) for ZnO@MS is 7113.7 eV, which remains almost the same throughout cycling, as shown in Table S1. It shows no changes in the oxidation state for the ZnO@MS anode. Thus, the Fe K-edge XANES analysis comparing the pristine MS and ZnO@MS anodes across cycling stages (20, 40, 60, and 80 cycles) demonstrates that the ZnO coating effectively stabilizes the structural characteristics and oxidation state of the anode due to the less significant changes in the (A₁, B₁, C₁, D₁) peak intensity and position for ZnO@MS. It suggests that the ZnO coating acts as a protective layer, minimizing iron oxidation and reducing interaction with the electrolyte. Since surface chemistry plays an important role in Fe oxidation, the Fe L-edge NEXAFS spectra are measured and shown in Fig. 5(c) and (d) for pristine MS and ZnO@MS anodes. Due to the relatively low probe depth of these measurements compared to XANES, the information is restricted to Fe ions and their interaction with the surface. The NEXAFS spectra were normalised with respect to the maximum peak intensity for clear comparison at various cycling stages. The spectral features in these spectra are marked with vertical black dotted lines, A₂, B₂, C₂, and D₂, to check the peak shift. The NEXAFS Fe L-edge analysis for MS and ZnO@MS at various stages of 20, 40, 60, and 80 cycles exhibits significant changes with respect to the respective anodes (Fig. 5(c) and (d)). This provides information on structural degradation under cycling conditions and may be associated with local coordination disorder, followed by chemical-state changes induced by cycling. Though it can be noticed from the dotted lines, A₂, B₂, C₂, and D₂, that the shift is much less in ZnO@MS than in the MS-based Fe-L edge; however, the shape and the shift of Fe L-edge NEXAFS after cycling are not significant. It may be due to Fe ions located on the surface of MS, in both cases, interacting with oxygen either from moisture (MS) or from the ZnO layer (in the case of ZnO@MS).

Fig. 5 Fe K-edge XANES spectra for the (a) pristine MS and (b) ZnO@MS anodes. Fe L-edge NEXAFS spectra for the (c) pristine MS and (d) ZnO@MS anodes.

Further, NEXAFS O K-edge spectra were analysed for MS and ZnO@MS-based cells. The NEXAFS O K-edge spectra for pristine MS cycled at different stages are shown in Fig. 6(a) and (b) in full range and pre-edge region as zoomed graphs, respectively. A dotted black line is used to distinguish different features, labelled as A₃, B₃, C₃, D₃, and E₃ in Fig. 6(a). Fig. 6(b) shows the magnified range, where the features corresponding to the cycles are easily traceable for MS anode degradation over cycling. The NEXAFS O K-edge spectra for ZnO@MS at different cycling stages are shown in Fig. 6(c) and (d) in full-range and pre-edge region as zoomed graphs, respectively. The ZnO@MS cycled anode at different stages is also shown in Fig. 6(c), and the respective features are marked as A₃, B₃, C₃, D₃, and E₃ using black dotted lines. The O K-edge NEXAFS spectra analysis for the pristine MS, cycled MS, and ZnO@MS anodes reveal distinct structural and chemical changes with cycling. The O K-pre-edge features are minimal for pristine MS, with an edge around ∼530 eV due to limited surface oxidation. However, the cycled MS anode exhibits stronger Fe–O features in this region,^42–46 with increased peak intensity (shown by A₃ in the figure) and shifts, indicating the formation of iron oxide layers due to repeated cycling and interaction with the electrolyte. This interaction may also lead to the presence of water⁴⁷ and carbon-like organic compounds on the MS surface, giving rise to a strong pre-edge region.⁴⁸ The ZnO@MS anode, in comparison with lines A₃, B₃, and C₃ in Fig. 6(b) and (d), shows stable Fe–O spectral features across cycles, with lower intensity changes and reduced shifts, suggesting that the ZnO coating effectively prevents significant oxidation by limiting Fe–electrolyte interaction. Surprisingly, there is a sharp peak (marked by the star) in the O K-edge spectra of cycled samples. This peak is possibly the V(L₂)-edge due to the presence of V ions at the anode surface, indicating that a small amount of vanadium dissolution from the V₂O₅ cathode and redeposition at the anode after cycling. Such crossover is well known in transition-metal oxides. Still, it typically occurs at very low concentrations, which explains why it is detectable in sensitive XAS but not visible in bulk techniques, such as XRD or SEM.^49,50

Fig. 6 O K-edge NEXAFS spectra for the (a) pristine MS-based anode and (c) ZnO@MS anodes at different cycle stages. (b) Zoomed plot corresponding to (a) and (d) zoomed plot corresponding to (c).

The k²-weighted EXAFS spectra of MS and ZnO@MS are shown in Fig. 7(a) and (b) for the qualitative analysis of local structural changes with cycling for both anodes. For the pristine MS anode, EXAFS data show considerable structural changes in both k-space and R-space with cycling. In k-space, oscillations gradually reduce and become irregular with increased cycling, as shown in Fig. 7(a). It indicates increased atomic disorder and structural changes. In R-space, the MS-based cycled anode shows noticeable shifts in Fe–O and Fe–Fe bond lengths, coordination loss, and continued oxidation, as Fe ions undergo repeated plating or stripping of the Fe ions, which destabilizes the anode structure. The cycled ZnO@MS anode shows far smaller changes in both k-space and R-space with cycling. In k-space, dampening of oscillations is minimized, suggesting that the ZnO coating limits atomic displacement and bond disorder, as shown in Fig. 7(b). Similarly, R-space spectra for ZnO@MS reveal smaller shifts in bond distances, which indicates that the ZnO layer acts as a protective barrier, reducing interaction with the electrolyte and maintaining a more stable local Fe environment over extended cycles. The corresponding simulated k-weighted EXAFS spectra and simulated non-phase corrected Fourier transform of EXAFS spectra are shown in Fig. S6(a) and (b) for the pristine anode and at various cycle stages, respectively. The simulated non-phase corrected Fourier transform of EXAFS spectra are shown in Fig. S7(a) and (b) for the ZnO@MS anode and at various cycle stages, respectively. Thus, the behavior of EXAFS oscillations infers that the ZnO coating reduces degradation and enhances capacity retention over cycling.

Fig. 7 k-Space EXAFS spectra (k²-weighted) for (a) MS-based anode and (b) ZnO@MS anodes.

Therefore, EXAFS spectra for both anodes are simulated to observe the quantitative changes in Fe coordination and bond length and these are provided in Table S2. The behaviour of coordination number and bond lengths of Fe–Fe_I, Fe–Fe_II and Fe–Fe_III shells as a function of cycles for MS and ZnO@MS is shown in Fig. 8. The coordination number of the 1st shell of the MS anode decreases after the initiation of cycling; however, in the case of ZnO@MS, deterioration sustains up to 20 cycles (Fig. 8(a)). Coordination numbers of 2nd and 3rd shells remain unaffected and no influence of cycling is observed for both cases (Fig. 8(a)). Bond distances of these shells are significantly influenced by cycling and generally decreases for both the MS and ZnO@MS anodes (Fig. 8(b)). However, it can be seen that this behavior is irregular for the MS anode and consistent for the ZnO@MS anode. Also, the slope of the linear fit is lower for ZnO@MS compared to the MS anode. This suggests less shell degradation for the ZnO@MS anode.

Fig. 8 Variation in the coordination number (N) (a) and bond distances (R) (b) for Fe–Fe (1st, 2nd and 3rd) shells. Standard values of these parameters are given in the respective subfigures.

Thus, Fe K-edge NEXAFS, Fe L-edge NEXAFS and Fe K-EXAFS analyses suggest that the ZnO@MS anode is more stable for Fe-ion batteries, which is also consistent with the GCD and EIS measurements. These findings emphasize the benefits of ZnO coating in Fe-ion batteries, highlighting how surface modifications can enhance the performance and cycle life of mild steel anodes during repeated cycling. We further investigated the impact of anode degradation behavior on the cathode using XANES and EXAFS analyses. These results are discussed in the forthcoming section.

Cathode analysis through V K-edge, L-edge, and O K-edge: We also investigated XANES and EXAFS measurements of the V₂O₅ cathode in both cases to understand the changes in the cathodes during and after cycling. The results are compared with those of the pristine V₂O₅ powder cathode to understand changes in the cycled V₂O₅ cathode material, which is coated on SS. The V₂O₅-coated electrodes are examined for the pristine MS and ZnO@MS anode-based cells after 20, 40, 60, and 80 cycles. This study reveals progressive changes in the electronic structure and atomic arrangement after cathode cycling. Fig. 9(a) shows the V K-edge XANES spectra for V₂O₅ powder and after 20, 40, 60, and 80 cycles with pristine MS as the anode in the full cell configuration. The vertical black-dotted lines (A₄, B₄, C₄, and D₄) were also included to analyse the spectral changes. Fig. 9(b) shows the V K-edge XANES spectra for V₂O₅ powder and after 20, 40, 60, and 80 cycles with ZnO@MS as the anode, and the corresponding changes are highlighted with similar black dotted lines (A₄, B₄, C₄, and D₄). In the XANES spectra of cycled V₂O₅ electrodes (20, 40, 60, 80 cycles), the observed shrinking and shift of the pre-edge peak to higher photon energy compared to the pristine bulk powder reflect changes in the local electronic environment around V due to cycling. The shift to higher energy suggests a partial oxidation of vanadium ions. Fe-ion insertion/extraction or intercalation/deintercalation during cycling alters the vanadium oxidation state, making the average oxidation state slightly higher than in the pristine material. Meanwhile, the shrinkage of the pre-edge peak indicates a loss of well-defined symmetry and increased distortion in the local structure. The changes are similar in (A₄, B₄, C₄, and D₄) for both systems, based on the MS anode with V₂O₅ and the ZnO@MS anode with V₂O₅. It shows that the cathode charge–discharge mechanism is not affected by the pristine or ZnO@MS anodes.

Fig. 9 V K-edge XANES spectra for (a) V₂O₅ cathode with the pristine MS anode and (b) V₂O₅ cathode with the ZnO@MS anode. V L-edge and O K-edge EXAFS spectra for (c) V₂O₅ cathode with the pristine MS anode and (d) V₂O₅ cathode with the ZnO@MS anode.

The V L-edge and O K-edge NEXAFS spectra of V₂O₅ after 20, 40, 60, and 80 cycles with the pristine MS anode and ZnO@MS are shown in Fig. 9(c) and (d), respectively. The highlighted blue area shows the V L-edge and the red area shows the O K-edge, corresponding to the A₅, B₅, C₅, and D₅ peaks with pristine MS at the anode side, as shown in Fig. 9(c). The highlighted blue area shows the V L-edge and red area shows the O K-edge, corresponding to similar A₅, B₅, C₅, and D₅ peaks with ZnO@MS at the anode side, as shown in Fig. 9(d). The observed decrease in normalized intensity and slight peak shrinkage indicates continuing changes (more significant for the 80 cycles) in the local electronic and atomic structure of the cathode material. This reduction in intensity suggests a loss of symmetry and increased disorder in the local bonding environment around V and O atoms due to slight cumulative strain from repeated Fe-ion insertion and extraction. The shrinking peak further indicates a less well-defined bonding environment, likely resulting from variations in V–O bond lengths and local coordination. EXAFS spectra shown in Fig. S8 were analysed to obtain information on this aspect.

The corresponding k-space spectra with pristine MS were also shown in Fig. 10(a). The k²-weighted EXAFS spectra are shown in Fig. 10(b) with ZnO@MS, corresponding to the simulated non-phase corrected Fourier transform of EXAFS spectra, Fig. 10(c) for the MS-based anode, and Fig. 10(d) for the ZnO@MS based anode. In k-space spectra, we observe oscillations corresponding to photoelectron scattering interactions with neighboring atoms. For cycled V₂O₅ electrodes, changes in the amplitude and frequency of these oscillations compared to the pristine sample indicate a change in local atomic structure with cycling. In R-space spectra (obtained by the Fourier transform of k-space in the range 3–8 Å⁻¹), peaks corresponding to specific bond distances, such as V–O and V–V interactions, are observed. For cycled V₂O₅ electrodes, significant changes are observed with respect to the pristine cathode. The peak corresponding to V–O bonds exhibits splitting for the pristine cathode. This splitting starts to diminish with cycling and disappears completely after 40 cycles in the non-ZnO-coated cathode. Also, shifts in the position of these peaks relative to the pristine sample indicate variations in bond lengths. This shift implies an elongation of the V–O bonds due to accumulated structural strain in the cathode.^49,50 These effects are ascribed to Fe ion insertion and extraction during cycling. The behaviour of V–O bonds is different for the cathode attached to ZnO@MS. In this case, partial splitting is clearly observed for ZnO@MS at 20, 40, and 60 cycles. Thus, V K-edge analysis indicates a dominant degradation of the cathode, which is attached to the MS anode. This ensures the stability of the ZnO@MS-based system. It may be contemplated that the stability of the anode not only affects the anode itself but also influences the nature of the cathode as well.

Fig. 10 k-Space EXAFS spectra (k²-weighted) and non-phase corrected Fourier transform of the EXAFS spectra (k-range 3–8 Å⁻¹) of V₂O₅ electrodes for the (a) MS anode and (b) ZnO@MS anode. Corresponding simulated non-phase corrected Fourier transform of the EXAFS spectra of the (c) MS anode and (d) ZnO@MS anode.

4. Conclusion

In this study, we successfully investigated the degradation mechanism of electrode materials in rechargeable Fe-ion batteries using two different anode types: mild steel and ZnO-coated mild steel, paired with a V₂O₅ cathode and non-aqueous electrolyte. The ZnO-coated MS, i.e., ZnO@MS anode, resulted in less or negligible changes on the anode surface and enhanced cyclic stability in comparison to the pristine MS anode. Cyclic voltammetry measurements explain the redox mechanism of cells. Galvanostatic cycling, impedance, ex situ XRD, SEM, and XANES/EXAFS measurements assisted in understanding the possible degradation pathways of the MS anode for iron ion batteries. More interestingly, the present study provides a way to improve the performance of the MS anode by integrating a protective ZnO layer and its potential for future Fe-ion battery applications.

Conflicts of interest

A patent has been filed on ‘Multivalent Iron-Polymer Rechargeable Battery Under Ambient Conditions and Method of Making Thereof’ with Jitendra Kumar Yadav, Bharti Rani, and Ambesh Dixit as inventors (patent application no. 202311059704) by the Indian Institute of Technology, Jodhpur. All other authors declare no conflict of interest.

Data availability

All the data used are provided in the manuscript and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5se01206f.

Acknowledgements

Ambesh Dixit acknowledge ANRF (formerly SERB), DST for project #CRG/2020/004023 and SERB/F/10090/2021-2022 for support of the work. Authors acknowledge AIoT Fab Facility and CRF, IIT Jodhpur, for providing physical characterizations support. J. K. Yadav acknowledge Bharti Rani and Priyanka Saini for their technical assistance and the Ministry of Education, Govt. of India, for the fellowship. K. H. C. acknowledges the financial support received from KIST2V10491. J. P. S. is thankful to ANRF for providing the Ramanujan Fellowship.

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