Polypyrrole-coated triple-layer yolk–shell Fe₂O₃ anode materials with their superior overall performance in lithium-ion batteries

1.   Introduction

The increasingly prevailing electric vehicles market developed a heightened demand for advances in lithium-ion batteries (LIBs) [1–4]. However, the theoretical capacity of current commercial anode materials such as graphite substantially hampers advancements of high-density LIBs [5–6]. Therefore, developing anode materials with elevated capacities is necessary as an immediate strategic imperative for the rapidly increasing market [7–8].

Transition metal oxides and their alloys have recently garnered significant interest owing to the exceptional theoretical capacities, among which iron oxide (Fe₂O₃), abundant in natural resources and cost-effective, is particularly favored. The Fe₂O₃ electrode exhibits a substantial theoretical capacity of 1007 mAh⋅g⁻¹, which is more than twice that of commercial graphite (372 mAh⋅g⁻¹) [9]. Furthermore, compared to conventional graphite anodes, the lithiation potential of Fe₂O₃, ranging between 0.5–1.0 V, effectively prevents the formation of lithium dendrites, significantly enhancing the safety of the battery, Fe₂O₃ electrode boasts a significant theoretical capacity [10–11].

However, its uses as an anode for LIBs are hindered by considerable volume changes during charge-discharge, leading to irreversible capacity decay and a limited commercialization process [12]. In recent studies, researchers have broadly categorized solutions to this volume change issue into two approaches: 1) Developing nanostructures capable of accommodating volume changes and increasing reactive surface area, thereby enhancing long-lasting charge transfer capabilities and mitigating volume changes during charge-discharge cycles; 2) Combining Fe₂O₃ with materials that are relatively stable during charge-discharge processes to constrain the volume changes of the matrix effectively [13]. For instance, Xia et al. [14] developed a pod-like nanostructure that accommodates the volumetric expansion of Fe₂O₃ and provides a conductive framework for fast electron transfer. Jeong et al. [15] synthesized urchin-like hollow Fe₂O₃ nanostructures using a one-step method involving in-situ polymerization and chemical etching. Chen et al.[16] utilized MnO₂-modified Fe-MOF (metal–organic framework), inert in electrochemical reactions, as a precursor to create a composite Fe₂O₃ anode. In a recent report, Park et al. [17] integrated carbon nanotubes (CNTs) with Fe₂O₃ to design a Fe₂O₃/C composite material, where the addition of CNTs significantly enhanced the cyclic stability.

However, these developed synthesis methods for the anodic Fe₂O₃ nanomaterials are generally complex and require stringent conditions. More importantly, long-term cyclic stability cannot be maintained in most cases even after creating the specific nanostructure constructions. In response to these challenges, the primary objective of this work has been to streamline the synthesis procedure without compromising the exceptional and long-lasting capacity.

The nanostructure for Fe₂O₃ anode materials was meticulously designed in this work, where an effective and straightforward “shell-to-core” synthesis approach was demonstrated. A stable structure of a three-layer hollow yolk–shell was successfully constructed. Subsequently, each layer of the Fe₂O₃ was uniformly coated with polypyrrole (Ppy) through simple steps. The uniform coating of polypyrrole boosts the anode conductivity and effectively maintains nanostructures of Fe₂O₃ during charging and discharging. Significant improvements in cycling efficiency and enduring robustness have been realized for the resultant Fe₂O₃ anodes, as validated through extensive assessments. The proposed advances in Fe₂O₃ anode materials provide insights into the nanostructured anode materials with the durability and stability for their energy storage uses.

2.   Experimental

2.1   Sample preparation

L-(-)-glucose (99%, biological reagent), iron(III) chloride hexahydrate (FeCl₃·6H₂O) (≥99%, analytical reagent), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) (98%, guaranteed reagent), and pyrrole (99%, guaranteed reagent) were acquired from Sigma-Aldrich for the fabrication of composite materials. Polyvinylidene difluoride (PVDF), serving as a binder, and N-methylpyrrolidone (NMP), employed as a solvent, were also procured from Sigma-Aldrich.

The sample preparation began by fabricating triple-layer yolk–shell Fe₂O₃ structures (Fe₂O₃-TLY) using a carbon sacrifice template (CST). Specifically, a 200 mL glucose solution (0.5 mol⋅L⁻¹) was hydothermally reacted in an autoclave for 120 min at 180°C, yielding approximately 2.5 g of CST. This CST was then mixed with a 50 mL of Fe(NO₃)₃·9H₂O (5 mol⋅L⁻¹) through ultrasonication for 30 min, followed by a 4 h stirring in a water bath (30°C). The Fe³⁺ coated CST was subsequently calcined under 550°C for 2 h to produce Fe₂O₃-TLY.

For the polypyrrole-coated triple-layer yolk–shell Fe₂O₃ (Fe₂O₃@Ppy-TLY) composite formation, a straightforward physical vapor deposition (PVD) technique is employed. Initially, the 1 mol⋅L⁻¹ solution of FeCl₃·6H₂O was prepared, from which 1 mL was gradually added to 500 mg of Fe₂O₃-TLY. This mixture underwent ultrasonic treatment for 30 min to achieve uniform adsorption of Fe³⁺ on the material’s surface. The resultant materials were oven-dried at 80°C for 6 h. Following this, 2 mL of pyrrole and the dried material were sealed and heated at 40°C for 36 h, allowing pyrrole to deposit on each shell layer of Fe₂O₃ and polymerize in situ, catalyzed by Fe³⁺ ions, forming a polypyrrole layer. Finally, the temperature was elevated to 550°C in the argon atmosphere for 2 hand then cooled slowly with the furnace to yield the polypyrrole-coated triple-layer yolk–shell Fe₂O_3, denoted as Fe₂O₃@Ppy-TLY composite. The overall synthesis procedure of Fe₂O₃@Ppy-TLY composites is depicted in Fig. 1.

Fig. 1.  Synthetic route to produce Fe₂O₃@Ppy-TLY.

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2.2   Physicochemical characterization

The morphology of the prepared materials was observed using scanning electron microscopy (SEM, JSM-7800F) and transmission electron microscopy (TEM, FEI Talos F200X). Structural characterization of the samples was conducted using X-ray diffraction (XRD) with a Rigaku Smartlab SE and Cu K_α radiation (wavelength λ = 0.15418 nm), operating at a tube voltage of 40 kV and a current of 40 mA. Thermal analysis was performed in an ambient atmosphere from 30°C to 650°C at a heating rate of 10°C⋅min⁻¹ using a HITACHI STA200 thermogravimetric analyzer (TGA). The molecular vibrations and rotations in organic materials within the synthesized materials were qualitatively analyzed using a Raman microscope (Renishaw inVia) under laser excitation at 532 nm. X-ray photoelectron spectroscopy (Thermo Scientific Nexsa) analyzed the surface chemical state and composition.

2.3   Electrochemical tests

Electrochemical tests were conducted using a CR2032 coin half-cell. Such a procedure involved mixing 80wt% prepared material, 10wt% conductive carbon black, and 10wt% PVDF, followed by adding an appropriate amount of NMP for uniform grinding. This mixture was then uniformly coated onto a 10 µm thick copper foil. After drying, the foil was cut into circular shapes with an area of 1.1 cm², each piece carrying approximately 0.8 mg⋅cm⁻² of the mixture. Lithium metal was used as both the counter and reference electrode. The electrolyte comprised 1 mol⋅L⁻¹ LiPF₆ dissolved in a 1:1 volume ratio of ethylene carbonate and diethyl carbonate, and the separator was a Celgard 2004 membrane. The cells were assembled in a glove box (Etelux Lab2000) with water and oxygen volume contents below 1 × 10⁻⁶. After assembly, the cells were left to stand in air for 8 h to allow the electrolyte to wet the electrode materials. Electrochemical testing, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), was performed using a CHI660E electrochemical workstation. Battery cycling tests were carried out on a Land CT2001A battery measurement system. In this study, the CV tests were conducted between 0.01–3 V at a scan rate of 0.1 mV⋅s⁻¹, and EIS was analyzed in the frequency range of 10⁻²–10⁵ Hz with an applied voltage amplitude of 10 mV.

3.   Results and discussion

3.1   Structural characteristics

The prepared Fe₂O₃-TLY and the polypyrrole-coated Fe₂O₃@Ppy-TLY are comprehensively compared in Fig. 2. Phase and elemental constituents for as-prepared Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY powders are presented in Fig. 2(a). The XRD result demonstrates that all peaks of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY powders align well with pure rhombohedral hematite (JCPDS PDF No. 33-0664), confirming the high crystallinity and purity of Fe₂O₃. The pronounced peaks at 24.1°, 33.2°, and 35.6° correspond to the (012), (104), and (110) crystal planes of Fe₂O₃ [17–18]. Interestingly, the peaks of Fe₂O₃@Ppy-TLY exhibit a decreased intensity compared to Fe₂O₃-TLY due to the outer layer of amorphous polypyrrole [12]. Raman spectrum in Fig. 2(b) reveals the presence of carbon in Fe₂O₃@Ppy-TLY, with two distinct principal peaks at 1370 and 1566 cm⁻¹ as disordered D band and the graphitic G band of carbon, respectively [19–20].

Fig. 2.  (a) XRD patterns of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY powders and (b) Raman spectra of Fe₂O₃@Ppy-TLY; (c) TGA plot of the Fe₂O₃@Ppy-TLY powders in air atmosphere; (d) FTIR curves of the Fe₂O₃@Ppy-TLY.

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Fig. 2(c) illustrates the TGA analysis on Fe₂O₃@Ppy-TLY to ascertain the content of Fe₂O₃ in the synthesized composite. The notable weight declines during 200–400°C is correlated with polypyrrole decomposition that commences at 200°C and then undergoes complete oxidation around 400°C [21]. The residual Fe₂O₃ corresponds to 61.9wt% in the composites. Further insights into the alterations after the polypyrrole coating are provided by FTIR spectroscopy (Fig. 2(d)). A peak near 543 cm⁻¹ can be seen, resulting from bending vibrations of O–Fe–O [22]. The other peaks align with the infrared spectrum of polypyrrole, strengthening the successful coating of polypyrrole in Fe₂O₃@Ppy-TLY.

The XPS analysis of Fe₂O₃@Ppy-TLY proves the existence of iron, oxygen, and carbon elements (Fig. 3(a)). More specifically, the Fe 2p spectrum (Fig. 3(b)) exhibits prominent peaks at 711.2 and 724.4 eV, corresponding to the Fe 2p_3/2 and Fe 2p_1/2. The peaks at 710.8 and 713.6 eV can be attributed to Fe²⁺ 2p_3/2 and Fe³⁺ 2p_3/2, while those at 724.8 and 727.4 eV correspond to Fe²⁺ 2p_1/2 and Fe³⁺ 2p_1/2. Notably, the presence of Fe²⁺ in both Fe 2p_3/2 and Fe 2p_1/2 is a common occurrence in iron–carbon composite materials. This phenomenon could be attributed to certain reduction of iron elements by carbon materials at high temperatures, which is also reported in previous research [23–24]. Additional weak binding energy peaks at 733.5 and 718.7 eV are attributed to the product’s satellite peaks [16–17]. The C 1s spectrum shows three peaks with binding energies of 284.6, 285.5, and 288.6 eV (Fig. 3(c)), corresponding to C–C, C–N, and C–O bonds, respectively [25–26]. The presence of the C–N bond suggests nitrogen incorporation within the carbon shell, likely due to incomplete pyrolysis of polypyrrole [12]. Fig. 3(d) displays the high-resolution N 1s spectrum with peaks at 398.2 eV (pyridinic-N), 399.8 eV (pyrrolic-N), and 400.8 eV (graphitic-N) [27]. Pyrrolic nitrogen dominates the peak area compared to graphitic nitrogen, indicating successful nitrogen doping into the material. Pyrrolic-N and pyridinic-N constructs new Lewis base sites over sample surface, thereby improving material’s conductivity [28–30].

Fig. 3.  XPS spectra of Fe₂O₃@Ppy-TLY: (a) overall spectra and the specific spectra of (b) Fe 2p, (c) C 1 s, and (d) N 1 s.

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Morphological and structural observations were also conducted for samples before and after being coated with polypyrrole. The sucrose-derived carbon microspheres show a diameter of ~800 nm, which were then served as templates in the following hydrothermal synthesis (Fig. S1, see the Supplementary information). Fig. 4(a) presents the Fe₂O₃-TLY morphology, presenting spherical shapes (average diameter of ~400 nm) with non-smooth surfaces comprising numerous dense particles (shown in the magnified view). Conversely, Fe₂O₃@Ppy-TLY (Fig. 4(b)) features a uniform polypyrrole coating, resulting in a denser and smoother surface compared to the uncoated Fe₂O₃. Broken Fe₂O₃-TLY spheres (Fig. 4(c)) show a multi-layer hollow yolk–shell internal structure. In Fig. 4(d), Fe₂O₃@Ppy-TLY exhibits a uniform polypyrrole coated sample with denser and smoother surface.

Fig. 4.  Morphologic pictures for (a) Fe₂O₃-TLY, (b) Fe₂O₃@Ppy-TLY, (c) one broken Fe₂O₃-TLY sphere, and (d) magnified view of Fe₂O₃@Ppy-TLY spheres; TEM images of (e) Fe₂O₃-TLY, (f) Fe₂O₃@Ppy-TLY, and (g) individual Fe₂O₃@Ppy-TLY spheres; (h) TEM image of a single Fe₂O₃@Ppy-TLY sphere, and (h1–h4) element mapping of an individual Fe₂O₃@Ppy-TLY. The inserted in (a) shows the SEM image of one individual Fe₂O₃-TLY sphere.

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Additional observations by TEM provide further information. Fig. 4(e) displays perfectly triple-layer yolk–shell structures for Fe₂O₃-TLY. Notably, the structures endured undamaged during the polypyrrole coating process, shown in Fig. 4(f). The uniform polypyrrole layer on each Fe₂O₃ shell (Fig. 4(g)) reveals an even thickness (h) of ~50 nm aligned with both inner and outer surfaces of original spheres. Furthermore, the elemental mapping of Fe₂O₃@Ppy-TLY confirms the presence of polypyrrole’s principal elements—C, N, and O—on each shell layer, in addition to Fe (Fig. 4(h1)–(h4)). This observation is consistent with XPS data and further validates the uniform polypyrrole coating for Fe₂O₃@Ppy-TLY.

3.2   Lithium storage performance

The half-cell testing has investigated the electrochemical performance of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY anode materials. As depicted in Fig. 5(a), during the cathodic sweeps, Fe₂O₃@Ppy-TLY reveals a pronounced reduction peak at 0.70 V during the first sweep, attributing to the Fe generation from the reduction reaction of Li₂Fe₂O₃ [31–32]. An irreversible reduction peak at 1.33 V corresponds to the formation of a solid electrolyte interface (SEI) under electrolyte decomposition [12,33]. At 1.58 V, the reversible reduction peak signifies the incorporation of Li⁺ into the Fe₂O₃ lattices[31], which is also shown in the CV curves of Fe₂O₃-TLY electrode (Fig. S2). In subsequent cycles, the pronounced reduction peak shifts to 0.79 V, and remains stable with a decreased intensity afterwards. This phenomenon results from the structural changes and resultant SEI film growth following the initial irreversible reactions [34–35]. During the anodic sweeps, two reversible oxidation peaks at 1.66 and 1.89 V can be corresponding to Fe³⁺/Fe oxidation reactions [31,33]. The oxidation peak at 2.44 V can be ascribed to the reaction between Li⁺ and the oxygen-containing functional groups in polypyrrole, as well as the electrostatic adsorption of Li⁺ [36] . The overlapping of the three pairs of redox peaks during the subsequent sweeps suggests great redox reversibility for Fe₂O₃@Ppy-TLY electrodes.

Fig. 5.  (a) CV curve of Fe₂O₃@Ppy-TLY electrodes at 0.1 mV⋅s⁻¹; (b) initial charge and discharge curves of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY at 0.05 C; (c) long cycling performance of both electrodes at 1 C; differential capacity dQ/dV curves for (d) Fe₂O₃-TLY and (e) Fe₂O₃@Ppy-TLY. Q is the amount of charge generated during charging and discharging, and V is the voltage during charging and discharging.

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Fig. 5(b) illustrates the 1st galvanostatic charge-discharge (GCD) curve recorded on the Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes at 0.05 C (1 C = 1007 mA⋅g⁻¹). Discharge plateaus can be observed at 0.7, 1.6, and 2.4 V for Fe₂O₃@Ppy-TLY electrodes. The initial discharge specific capacity of Fe₂O₃@Ppy-TLY is 2421.89 mAh⋅g⁻¹, surpassing that of Fe₂O₃-TLY at 2224.24 mAh⋅g⁻¹. It should be noted that both electrodes reveal great specific discharge capacity due to the well-designed nanostructures. Fe₂O₃@Ppy-TLY anode presents a higher discharge capacity after being coated with polypyrrole, which is supported by previous literature [12]. The relatively low initial coulombic efficiency (ICE = 58.4%) can be explained by the formation of irreversible lithium compounds and SEI films [31,37].

The performance characteristics for Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes running 500 cycles at 1 C are presented in Fig. 5(c). Both electrodes show remarkable stability, maintaining a charge-discharge specific capacity of over 1000 mAh⋅g⁻¹ for the first 50 cycles. The capacity of Fe₂O₃@Ppy-TLY gradually increases during the subsequent 200 cycles, owing to the activation process induced by electrolyte penetration into the nanospheres, and the enlarged active surface participating in redox reactions brought by the fully activated three-layer nanostructure [10,38–42]. Furthermore, during the charge-discharge process, the continuous activation of the carbon layer within polypyrrole provides more insertion/extraction and electrostatic adsorption sites for Li⁺ on one hand, while exposing more oxygen-containing functional groups for reversible electrochemical reactions with lithium ions on the other [36]. These factors collectively enhance the reversible specific capacity of the Fe₂O₃@Ppy-TLY electrode material.

After 200 cycles, the reversible specific capacity of Fe₂O₃@Ppy-TLY stabilizes at approximately 1400 mAh⋅g⁻¹ (Figs. S3(a) and 5(c)), markedly exceeding the commercial graphite (372 mAh⋅g⁻¹). The Fe₂O₃-TLY electrode experiences considerable capacity diminishing as the cycling process, reducing to 228.5 mAh⋅g⁻¹ after 500 cycles (Fig. S3(b)). This divergence in performance between Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY reveals that the polypyrrole coating effectively mitigates the volume change of Fe₂O₃ during charge-discharge process, awarding exceptional structural stability to the anode materials [12,15,43]. Notably, the specific capacities of both electrode matrials exceed the theoretical value of Fe₂O₃ (1006 mAh⋅g⁻¹), attributable to the pseudocapacitive effect and reversible reactions of LiOH/Li during charge-discharge processes [10,44–45].

We employed the dQ/dV analysis method to investigate the performance variations of two electrode materials during the capacity degradation process. As shown in Fig. 5(d) and (e), the dQ/dV curves for Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY are presented respectively. The characteristic peaks observed in these curves are induced by the redox reactions between the electrode materials and lithium ions. The electrochemical reaction process between Li⁺ and Fe₂O₃ can be described by the follows.

In the dQ/dV curves of Fe₂O₃-TLY, two significant characteristic peaks were observed. Peak A during the lithiation process corresponds to the insertion of Li⁺ into the Fe₂O₃ lattice to form Li₂Fe₂O₃ and the subsequent conversion reaction of Li₂Fe₂O₃ to Fe and Li₂O [15–16]. Peak B during the delithiation process can be attributed to the two-step oxidation of Fe to Fe²⁺ and Fe²⁺ to Fe³⁺[17]. The capacity of the Fe₂O₃-TLY electrode remains stable before 100 cycles. However, in subsequent cycles, the two characteristic peaks broaden significantly and even disappear by 500 cycles, indicating irreversible and substantial capacity degradation. This is due to the volume expansion and contraction of the iron oxide matrix during lithiation/delithiation, leading to structural damage and eventual pulverization and shedding [12].

In sharp contrast, the dQ/dV curves of Fe₂O₃@Ppy-TLY exhibit five prominent characteristic peaks. Peaks B and D are similar to the electrochemical reactions of the Fe₂O₃-TLY electrode. Peak C can be attributed to the insertion of Li⁺ into the polypyrrole matrix. Peak A and E arise from the reaction of Li⁺ with oxygen-containing functional groups within the polypyrrole matrix and electrostatic adsorption phenomena [36]. Notably, the five significant characteristic peaks in the dQ/dV curves of Fe₂O₃@Ppy-TLY extend throughout the 500 cycles. The increased intensity of peaks A, B, and D indicates that the iron oxide matrix maintains its microstructural integrity before and after cycling, without suffering irreversible structural damage. Conversely, the exposure of more active sites during the expansion and contraction of the iron oxide matrix during cycling enhances electrochemical reactions, leading to increased capacity. The increasing values of peaks C and E with cycling further confirm that the polypyrrole coating layer contributes to the capacity enhancement of the Fe₂O₃@Ppy-TLY electrode through three pathways: increased Li⁺ insertion/extraction and electrostatic adsorption sites, and the introduction of functional groups that react with Li⁺.

Fig. 6(a) investigates the charge-discharge performance of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes at varied rates. At current densities of 0.25, 1, 2, 4, 6, 8, and 10 C, the reversible specific capacities of Fe₂O₃@Ppy-TLY were respectively 1373.45, 1173.04, 1069.4, 951.33, 885.96, 837.52, and 796.19 mAh⋅g⁻¹ (Fig. S4(a)), consistently surpassing those of Fe₂O₃-TLY (Fig. S4(b)). Notably, at 10 C, Fe₂O₃@Ppy-TLY maintains a reversible capacity of approximately 800 mAh⋅g⁻¹, demonstrating superb high-current cycling stability. When the current density reduces to 0.25 C, the reversible capacity of Fe₂O₃@Ppy-TLY rapidly reaches 1460.35 mAh⋅g⁻¹, much exceeding the initial reversible capacity. After 1000 cycles at a current density of 6 C (Fig. S5), the Fe₂O₃@Ppy-TLY electrode still retains a reversible capacity of 498.27 mAh·g⁻¹, with a coulombic efficiency over 100%. This further underscores the exceptional rapid charge-discharge capability and outstanding cyclic stability for Fe₂O₃@Ppy-TLY electrodes.

Fig. 6.  (a) Rate performance of the Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes; (b) alternating current impedance spectra of both electrodes before cycling and after 250 cycles at 1 C; long cycling performance of Fe₂O₃@Ppy-TLY electrodes at (c) 10 C and (d) 15 C. $\mathbf{Z}'$and$\mathbf{Z}''$are the real and imaginary components of the impedance; CPE1 represents capacitance, and CPE2 is the degree of similarity to a pure capacitor; R_s is the electrolyte resistance; R_ct is the charge transfer impedance; R_SEI is the charge transfer impedance of the SEI film; W_diff is the concentration polarization impedance.

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The EIS characterization recording the charge transfer processes is presented in Fig. 6(b). Nyquist plots contrasting the half-cells of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes are illustrated for the as-prepared samples and the tested samples after 250 cycles at 1 C. A larger radius in high-frequency semicircles suggesting a greater charge transfer impedance [22,25–26,46]. The fitting circuit is depicted in the inset, with fitted data available in Table S1. The charge transfer resistance of Fe₂O₃@Ppy-TLY (51.35 and 22.01 Ω before and after cycling) is lower than that of Fe₂O₃-TLY (74.74 and 193.7 Ω before and after cycling). The significant enhancement in the charge transfer capability of the Fe₂O₃@Ppy-TLY electrode is due to the increased conductivity brought by polypyrrole coating [12]. Furthermore, after 250 cycles, the charge transfer resistance (R_ct) of the Fe₂O₃-TLY electrode notably increased, whereas that of the Fe₂O₃@Ppy-TLY electrode decreased, aligns with Fig. 4(c). The decrease in R_ct for the Fe₂O₃@Ppy-TLY electrode is attributed to the maintained integrity of its nanostructure without deterioration caused by volumetric expansion and the formation of SEI film in between [12].

Long-term cycling tests were conducted on the Fe₂O₃@Ppy-TLY at 10 C (Fig. 6(c)). A gradual increase in reversible capacity is observed in 750 cycles, which is consistent with findings in previous studies [10,39,44,47]. After 6000 cycles, the Fe₂O₃@Ppy-TLY maintains a high capacity of 544.33 mAh⋅g⁻¹. A similar trend is noted for the tests conducted at 15 C (Fig. 6(d)), where the capacity gradually increases and then declines to 156.75 mAh⋅g⁻¹ after 10000 full charge-discharge cycles. This indicates that nanostructures remain intact despite high currents, exhibiting exceptional stability [12].

The above results indicate that the conductivity of the iron-oxide composite coated with Ppy improved, and the three-dimensional structure could also be maintained during the charging and discharging process. The results show that Fe₂O₃@Ppy-TLY prepared in this study has excellent cyclic stability and outstanding reversible specific capacity. As in Table 1, This study reports the highest stable lithium reversible capacity for iron oxide anode materials currently documented in the literature, demonstrating the outstanding high-current stability of Fe₂O₃@Ppy-TLY.

Table  1.  Comparison of representative publications for iron oxide anode materials

Ref.	Anode	Initial charge/discharge capacity / (mAh·g−1)	Reversible capacity / (mAh·g−1)	Reversible capacity at high current density / (mAh·g−1)
This work	Fe2O3@Ppy-TLY	1416.05, 2421.89	1377.9 (1.0 C, 500)	544 (10 C, 6000)
[17]	Fe2O3/C/CNT	1390, 1598	1230 (0.1 C, 100)	985 (1 C, 1000)
[21]	Fe3O4/carbon	999, 1795	1317 (0.1 C, 130)	525 (5 C, 300)
[31]	Fe3O4@N-HPCNs	1415, 1725	1240 (0.1 C, 400)	290 (10 C, rate)
[11]	FPCNs	554, 633	544 (1.0 C, rate)	332 (2 C, 300)
[48]	SFO	927, 1306	968 (0.1 C,100)	653 (1.6 C, rate)
[15]	Fe2O3@PANI	958, 1208	893 (0.1 C, 100)	724 (10 C, rate)
[34]	Fe3O4/CNTs/rGO	1458, 1863	1080 (1.0 C,450)	540 (10 C, rate)
[49]	γ-Fe2O3/CNT	709, 972	1187 (0.2 C, 400)	518 (4 C, 300)
[50]	CNT/α-Fe2O3/C	1201, 1773	1213 (0.1 C, 60)	611 (4 C, rate)
[51]	3D net-like FeOx/C		851 (0.2 C, 50)	714 (1 C, 300)
[32]	MWCNT/γ-Fe2O3	857, 1160	642.2 (0.1 C, 310)	205 (5 C, rate)
[22]	fabric-like α-Fe2O3	1478, 2264	1028 (0.5 C, 100)	495 (1 C, rate)
Note: The fourth and fifth columns from the left in the table represent the reversible capacity of the anode material after cycling a certain number of times at low or high current densities, current density and cycle number for material cycling tests are in parentheses respectively. It should be noted that the “rate” symbol in the cycle number position indicates that the reversible capacity data was obtained from the original author's rate capability tests, as long-cycle performance data at low or high current densities was not provided. For example, “1377.9 (1.0 C, 500) ” indicates that the anode material maintains a reversible capacity of 1375.11 mAh·g−1 after 500 cycles at 1 C. “544 (1.0 C, rate) ” indicates that the anode material maintains a reversible capacity of 544 mAh·g−1 at 1 C under rate testing. CNT: carbon nanotube; rGO: reduced graphene oxide; 3D: three dimensions.

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The variable scan rate CV tests on Fe₂O₃@Ppy-TLY electrode are conducted to further investigate the kinetic in the electrochemical processes. In Fig. 7(a), despite variations in scan rate, the CV curves of Fe₂O₃@Ppy-TLY electrode material exhibit similar peaks, despite shifts can be seen in the corresponding peak positions. These shifts are attributed to polarization effects. The contributions of surface-induced capacitive processes (SCP) and diffusion-controlled intercalation processes (DIP) in the charging-discharging of electrode materials can be calculated using Eqs. (5) and (6), as described below [25–26,46].

Fig. 7.  (a) Variable scan rate CV curves of Fe₂O₃@Ppy-TLY anodes recorded from 0.1 to 2 mV⋅s⁻¹ in 0.01–3.0 V; (b) fitted lines for logarithm cathodic peak current and logarithm scan rates for peak 1, peak 2, peak 3, and peak 4; (c) normalized contribution ratio of SCP and DIP at different scan rates of 0.1–2.0 mV⋅s⁻¹; (d) SCP contribution at 1.0 mV⋅s⁻¹ showed by the fitted green area.

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where i denotes the current density, and v stands for the scanning speed; a and b are variable coefficients. k₁ and k₂ are the constants representing the contributions of SCP and DIP respectively [52]. The values of k₁ and k₂ can be ascertained by evaluating the peak current (I_p) at identical scan rates, thereby obtaining the contribution from SCP and DIP models.

In Eq. (5), when the value of b is 0.5 or 1, it indicates that lithium ions’ charge and discharge process is solely controlled by either SCP (b = 0.5) or DIP (b = 1) [25]. The peak current at various scan rate for the same characteristic peak can be used to calculate the value of b.

Through fitting calculations using these equations, the b values for the four characteristic peaks of the Fe₂O₃@Ppy-TLY electrode material are 0.94, 0.77, 0.98, and 0.97 (Fig. 7(b)). These values lie between 0.5 and 1.0, and are close to 1, suggesting the charge storage occurring on Fe₂O₃@Ppy-TLY anodes is predominantly controlled by DIP in conjunction with a certain extent of SCP.

Fig. 7(c) presents the calculated contribution of SCP for the Fe₂O₃@Ppy-TLY at the scan rate increasing from 0.1 to 2 mV⋅s⁻¹, which are 66.9%, 70.7%, 73.5%, 77.6%, 80.3%, 83.9%, 86.8%, 92.7%, and 97.5% respectively. The contribution of SCP increases with increasing scan rate. This increase is attributable to the prolonged diffusion time of Li⁺ ions to the electrode material under low current conditions, which accentuates the influence of the diffusion process and makes the contribution of DIP more pronounced. Fig. 7(d) presents a straightforward view regarding the contribution of SCP at 1 mV⋅s⁻¹. These indicates that SCP mainly controls the lithium storage behavior of Fe₂O₃@Ppy-TLY electrode material at high scan rate [26].

To elucidate the stability of the polypyrrole-coated shell structure, the morphology of lithium-ion half-cells composed of Fe₂O₃ and polypyrrole after 50 cycles at a current density of 0.2 C was observed. After 50 cycles, Fe₂O₃-TLY exhibited a pronounced convex spherical shape (Fig. 8(a)). In contrast, the Fe₂O₃@Ppy-TLY electrode maintained a regular spherical structure (Fig. 8(b)). Fig. S6 revealed that the iron oxide shell structure of Fe₂O₃-TLY had fractured, whereas the triple-layer shell structure of Fe₂O₃@Ppy-TLY remained intact after 50 cycles. This observation is consistent with the results of electrochemical cycling tests. Fig. 8(c) and (e) shows cross-sectional scanning electron microscopy images of Fe₂O₃-TLY and Fe₂O₃@Ppy-TLY electrodes before and after cycling, respectively. Both electrodes initially exhibited flat surfaces. The thickness variation rate of Fe₂O₃@Ppy-TLY before and after cycling was only 20.5%, significantly lower than that of Fe₂O₃-TLY (45.9%). These observations clearly demonstrate that the Ppy coating significantly enhances the stability of the iron oxide triple-layer structure. Furthermore, it reveals that the triple-layer shell structure coated with polypyrrole effectively mitigates the volumetric expansion and contraction of the substrate material during lithiation/delithiation (Fig. 8(g)). During lithiation, each polypyrrole coating layer, forming a complete thin-shell structure, disperses the expansion stress of each iron oxide nanoparticle and directly limits the outward expansion of iron oxide particles, forcing the substrate particles to expand only into the internal space. This significantly reduces the expansion of iron oxide during charge and discharge processes. Additionally, the outer shell layer accommodates the volume changes of the inner structure, further dispersing the expansion/contraction stress, thereby maintaining the integrity of the designed microstructure.

Fig. 8.  SEM images after 50 cycles of (a) Fe₂O₃-TLY and (b) Fe₂O₃@Ppy-TLY, with the corresponding TEM images as insets; cross-sectional SEM images of Fe₂O₃-TLY (c) before and (d) after 50 cycles; cross-sectional SEM images of Fe₂O₃@Ppy-TLY (e) before and (f) after 50 cycles; (g) schematic illustration of the mechanism by which Fe₂O₃@Ppy-TLY mitigates volume expansion.

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4.   Conclusion

The proposed novel synthesis method for preparing uniform polypyrrole-coated triple-layer yolk–shell Fe₂O₃ structures (Fe₂O₃@Ppy-TLY) by combining the CST and PVD was studied. The uniformly coated polypyrrole outer layer enhances the material’s conductivity and effectively mitigates volume expansion during charge-discharge cycles, thus maintaining the integrity of Fe₂O₃@Ppy-TLY nanostructures. The polypyrrole coating further enhances the capacity of the Fe₂O₃@Ppy-TLY electrode by increasing the Li⁺ reaction sites and electrostatic adsorption sites. During the uses as lithium-ion battery electrodes (LIBs), Fe₂O₃@Ppy-TLY electrode material exhibits a surprisingly high specific capacity (2421.89 mAh⋅g⁻¹ for the first discharge at 0.05 C). Moreover, It also shows superb rate performance (1373.45 mAh⋅g⁻¹ at 0.25 C, 951.33 mAh⋅g⁻¹ at 4 C, and 796.19 mAh⋅g⁻¹ at 10 C) when charging and discharging at different rates. Even after high current rapid charge-discharge process, the specific capacity for Fe₂O₃@Ppy-TLY can still be maintained at a high level. This investigation provides critical insights into the material design and structural construction of superior performance anodes for LIBs.