Enhancing the Structural Stability and Electrochemical Performance of δ-MnO₂ Cathodes via Fe³⁺ Doping for Aqueous Zinc-Ion Batteries

Abstract

Due to its unique layered structure that facilitates ion intercalation and deintercalation, δ-MnO₂ has emerged as a promising cathode material for aqueous zinc-ion batteries (ZIBs). However, its structural collapse and Mn dissolution during prolonged cycling significantly limit its practical application. In this study, we demonstrate that metal ion doping, particularly with Fe³⁺, can effectively stabilize the δ-MnO₂ structure and enhance its electrochemical performance. Through a hydrothermal synthesis approach, δ-MnO₂ materials with varying Fe³⁺ doping ratios are prepared and systematically investigated. Among them, the sample with a Mn:Fe molar ratio of 20:1 exhibits the best performance, maintaining the layered δ-MnO₂ phase while significantly increasing Mn³⁺ content and promoting the formation of oxygen vacancies. At a current density of 0.5 A·g⁻¹, the iron-doped sample exhibited an initial specific capacity of 116.24 mAh·g⁻¹, with a capacity retention rate of 41.7% after 200 cycles. In contrast, the undoped δ-MnO₂ showed an initial specific capacity of only 85.15 mAh·g⁻¹, with a capacity retention rate of merely 19.9% after 200 cycles. The results suggest that Fe³⁺ doping not only suppresses Mn dissolution but also improves structural stability and Zn²⁺ transport kinetics. This work provides new insights into the development of durable Mn-based cathode materials for aqueous ZIBs.

1. Introduction

The global transition toward sustainable energy systems demands the development of safe, low-cost, and high-performance energy storage technologies. While lithium-ion batteries (LIBs) currently dominate the market due to their high energy density and mature manufacturing infrastructure, their large-scale deployment is increasingly hindered by the scarcity of lithium resources, rising material costs, and safety concerns associated with flammable organic electrolytes [1,2,3]. These challenges have caused intensive research into alternative technologies based on abundant, environmentally benign elements, among which aqueous zinc-ion batteries (ZIBs) have emerged as a highly promising candidate [4,5,6].

ZIBs offer several compelling advantages: zinc metal is naturally abundant, inexpensive, non-toxic, and features a high theoretical capacity of 820 mAh·g⁻¹. Furthermore, ZIBs employ mild aqueous electrolytes, which not only improve ionic conductivity and manufacturing safety but also reduce environmental impact, making them suitable for large-scale, stationary energy storage applications [7,8,9]. In this system, metallic zinc is commonly used as the anode due to its low redox potential (−0.762 V vs. SHE), reversible stripping/plating behavior, and compatibility with aqueous media [10]. However, the commercial viability of ZIBs is largely constrained by the lack of cathode materials that simultaneously deliver high capacity, long-term structural integrity, and stable electrochemical cycling. Among the various explored cathodes—such as Prussian blue analogues, vanadium-based oxides, and organic materials—manganese dioxide (MnO₂) stands out due to its low cost, environmental friendliness, multiple redox states, and rich polymorphic diversity (α, β, γ, δ, etc.) [11,12,13]. In particular, δ-MnO₂, with its layered structure and large interlayer spacing (~7 Å), facilitates the intercalation and deintercalation of Zn²⁺ and H⁺ ions, and thus has attracted considerable interest as a ZIB cathode [14,15,16].

Nevertheless, δ-MnO₂ faces critical drawbacks during long-term cycling. The Jahn–Teller distortion associated with Mn³⁺ leads to structural instability and Mn dissolution, causing the collapse of its layered framework and a rapid decline in the capacity retention rate [17]. To mitigate these issues, recent studies have explored cation doping strategies—particularly with transition metal or rare-earth metal ions. Transition metal ions can stabilize the MnO₂ lattice and suppress Jahn–Teller distortion, while rare-earth metal ions, due to their larger ionic radii, expand the interlayer spacing upon incorporation, thereby facilitating ion transport during electrochemical processes. [18,19,20]. For example, Ding et al. investigated Cr-doped δ-MnO₂ and found that the incorporation of Cr enhanced the interaction between the δ-MnO₂ electrode material and Zn²⁺, thereby improving the material’s stability [21]. However, the effects of different dopants and doping levels on the crystal structure and electrochemical behavior of δ-MnO₂ remain underexplored, especially in terms of maintaining phase integrity while enhancing cycling stability.

In this work, we address these challenges by developing a series of metal-ion-doped δ-MnO₂ cathodes via a simple and scalable hydrothermal synthesis approach, with a particular focus on systematic Fe³⁺ doping. We investigate the influence of various Fe³⁺ doping ratios (Mn:Fe = 10:1, 20:1, 40:1) on the crystal structure, surface chemistry, and electrochemical performance of δ-MnO₂. For broader comparison, we also examine Cu²⁺ and Ru³⁺ dopants in the matrix of δ-MnO₂. Since Cu²⁺ exhibits better valence compatibility with Mn²⁺, its doping is expected to effectively modulate the structural stability of MnO₂ materials. Indeed, Liu et al. have systematically investigated Cu²⁺ doping in MnO₂-based systems [22]. In contrast, studies on Ru³⁺ incorporation into MnO₂ lattices remain scarce, though Zheng et al. reported that Ru doping in TiO₂ significantly alters the kinetic processes and structural stability [23]. Therefore, we selected these two dopants for comparative studies to elucidate their distinct effects. Structural characterizations (XRD, SEM, and XPS) confirm that moderate Fe³⁺ incorporation (20:1) preserves the δ-MnO₂ phase while increasing Mn³⁺ content and oxygen vacancies, both of which contribute to enhanced ion diffusion and cycling durability.

Notably, the Fe³⁺-doped δ-MnO₂ (b-FeMO) sample exhibits a specific capacity of 116.2 mAh·g⁻¹ and a capacity retention rate of 41.7% after 200 cycles at 0.5 A·g⁻¹, significantly outperforming undoped δ-MnO₂ (which retains only 19.9% under the same conditions). Post-cycling analyses confirm that Fe³⁺ doping mitigates Mn dissolution, preserves the layered morphology, and maintains a favorable redox environment. This work not only highlights the crucial role of Fe³⁺ in reinforcing structural stability and prolonging cycling life but also offers comparative insights into the roles of different dopants. More broadly, it provides practical design guidance for engineering robust Mn-based cathodes for aqueous zinc-ion batteries.

2. Materials and Methods

2.1. Materials Synthesis

Preparation of δ-MnO₂ (MO): The δ-MnO₂ was synthesized via a hydrothermal route. Briefly, 0.25 g of KMnO₄ and 0.066 g of Mn(NO₃)₂·4H₂O were dissolved separately in deionized water and stirred for 30 min. The Mn(NO₃)₂ solution was then slowly added to the KMnO₄ solution under continuous stirring. The mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 10 h. After natural cooling, the product was washed alternately with deionized water and ethanol, followed by drying at 60 °C overnight. The product was labeled as MO.

Preparation of Fe-doped δ-MnO₂ (FeMO): The synthesis followed the same hydrothermal procedure as MO, with additional Fe(NO₃)₃·9H₂O added to the KMnO₄ solution at different molar ratios of Mn:Fe (10:1, 20:1, and 40:1). The resulting products were labeled as a-FeMO, b-FeMO, and c-FeMO, respectively.

Preparation of Cu- and Ru-doped δ-MnO₂: Similarly, Cu(NO₃)₂ (0.017 g) and RuCl₃ (0.019 g) were introduced into the synthesis system, following the same hydrothermal process. The products were labeled as CuMO and RuMO, respectively.

2.2. Characterization

X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan) was used to determine the crystalline structure. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA) was conducted to analyze surface chemical states. Scanning electron microscopy (SEM, ZEISS Sigma 300, Oberkochen, Germany) along with energy-dispersive X-ray spectroscopy (EDS) and elemental mapping was used for morphology and composition analysis.

2.3. Electrochemical Measurements

The cathode slurry was prepared by mixing active material, acetylene black, and PVDF binder at a 7:2:1 weight ratio. The PVDF was dissolved in NMP and stirred at 45 °C. The active material and acetylene black were added and stirred for 30 min. The slurry was coated on iron foil and dried at 75 °C for 2 h under vacuum. CR2032 coin cells were assembled using zinc foil as the anode, glass fiber as the separator, and 2 M ZnSO₄ aqueous solution as the electrolyte. Cathodes were prepared with mass loadings of 0.39~0.78 mg/cm² for the active material.

Galvanostatic charge/discharge (GCD) and cycling tests were performed on a battery testing system (Neware, Shenzhen, China) within 0.7–1.8 V. Cyclic voltammetry (CV) was conducted using an electrochemical workstation (CHI760E, Shanghai, China) at a scan rate of 0.5 mV s⁻¹.

3. Results

3.1. Structural and Morphological Characterization

Figure 1a shows the X-ray diffraction (XRD) patterns of the pristine δ-MnO₂ (MO) and Fe³⁺-doped δ-MnO₂ (FeMO) samples with varying doping ratios. All major diffraction peaks of MO correspond well to the reported δ-MnO₂ phase (PDF#80-1098) [22], with apparent reflections at 12.5°, 25°, and 37°, corresponding to the (001), (002), and (111) crystal planes, respectively. The b-FeMO sample, with a Mn:Fe molar ratio of 20:1, retained these characteristic δ-MnO₂ peaks without the emergence of impurity phases, indicating that Fe³⁺ doping at this level does not disrupt the host lattice structure. Furthermore, the sharpness and intensity of the diffraction peaks in b-FeMO suggest good crystallinity. In contrast, the a-FeMO sample (10:1) exhibited an increased intensity of the (002) plane, implying preferential growth along specific crystal orientations induced by higher Fe³⁺ incorporation. Conversely, c-FeMO (40:1) shows weakened and broadened diffraction peaks, indicative of partial structural disruption or amorphization due to insufficient doping at a low level of Fe³⁺ concentration. For the Cu²⁺- and Ru³⁺-doped samples, the XRD patterns (Figure 1b) reveal that CuMO (Cu²⁺-doped) retained the δ-MnO₂ crystal structure, whereas RuMO (Ru³⁺-doped) shows distinct new peaks corresponding to an altered Mn/O stoichiometry, likely forming a different manganese oxide phase (PDF#15-0604), suggesting that Ru³⁺ doping significantly modifies the crystal lattice.

Figure 2 presents the SEM images of MO and b-FeMO, along with their energy-dispersive X-ray spectroscopy (EDS) elemental mapping. The MO sample consists of uniform flower-like microspheres assembled from interconnected nanosheets, in agreement with those reported in the literature findings [24]. Such a morphological structure can provide a high surface area beneficial for electrochemical activity. Similarly, the b-FeMO sample maintains the flower-like morphology, indicating that Fe³⁺ doping at 20:1 does not alter the microscale morphology of the δ-MnO₂ but helps stabilize the structure. The EDS elemental mapping of b-FeMO demonstrates a homogeneous distribution of Mn, O, and Fe throughout the structure, confirming the successful and uniform incorporation of Fe³⁺ into the δ-MnO₂ framework. In comparison, CuMO retained the nanosheet-assembled microsphere morphology (Figure S1), whereas RuMO displayed a different morphology, primarily composed of rod-like particles with residual flower-like structures (Figure S2). This morphological shift in RuMO is consistent with the structural changes observed in its XRD pattern, supporting the conclusion that Ru³⁺ doping affects both the crystal structure and the morphology [25].

Figure 3 shows the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of the Mn 2p and O 1s orbitals for all samples, with their corresponding full spectra shown in Figure S3. As was expected, the majority of the dopants went into the MO matrix with the following dopant: the Mn ratio close to 1:20 as revealed by the XPS elemental analysis (Table S1), indicating the successful doping approach. The Mn 2p spectra exhibit the characteristic Mn 2p_3/2 and Mn 2p_1/2 peaks at approximately 641.4 eV and 653.1 eV, respectively, with a spin-energy separation of ~11.7 eV, typical of MnO₂ [11]. The deconvolution of the Mn 2p_3/2 peak indicates a higher Mn³⁺ to Mn⁴⁺ ratio in b-FeMO compared to MO, with b-FeMO showing a Mn³⁺ content of 74.27%, significantly higher than MO (66.07%) and the other doped ones. This increase in Mn³⁺ content correlates with enhanced oxygen vacancy concentration with Fe³⁺ doping, which can facilitate improved electronic conductivity and ion diffusion kinetics. The O 1s spectra further support this observation: b-FeMO shows a stronger Mn–O–H peak, reflecting the formation of more oxygen-related defect sites due to Fe³⁺ doping. These features are beneficial for Zn²⁺ intercalation/deintercalation and are aligned with the improved electrochemical performance observed in b-FeMO [26].

Taken together, the structural (XRD), morphological (SEM/EDS), and chemical state (XPS) analyses consistently confirm the successful doping of Fe³⁺ into the δ-MnO₂ lattice without disrupting its crystal structure. The 20:1 Mn:Fe doping ratio (b-FeMO) was found to be optimal, preserving the structural integrity while introducing a higher Mn³⁺ content and enhanced oxygen vacancies. Although Cu or Ru could be also doped into the MO framework (CuMO or RuMO), the main structure or morphology was not persevered. These modifications in b-FeMO collectively contribute to the improved electrochemical performance and cycling stability of b-FeMO as a cathode material for aqueous zinc-ion batteries, validating the conclusions of this study.

3.2. Electrochemical Performance

Figure 4a–d presents the cyclic voltammetry (CV) curves of MO, b-FeMO, CuMO, and RuMO electrodes at a scan rate of 0.5 mV·s⁻¹. All electrodes exhibit two prominent redox peaks, corresponding to the reversible intercalation/deintercalation of H⁺ and Zn²⁺ into/from the MnO₂ layers [20]. For the pristine δ-MnO₂ (MO), the peaks are broad and less defined, suggesting relatively sluggish electrochemical kinetics and poor reversibility. In contrast, the b-FeMO sample shows sharper and more symmetric redox peaks, indicating increased redox kinetics and improved electrochemical reversibility after Fe³⁺ doping. The enhanced CV response can be attributed to the increased Mn³⁺ content (confirmed by XPS), which facilitates pseudocapacitive charge storage via oxygen vacancy-mediated charge transfer pathways.

The improved kinetics in b-FeMO are also reflected in the smaller peak potential separation (ΔE) [27], which decreased from ~190 mV in MO to ~135 mV in b-FeMO, highlighting lower electrochemical polarization. This trend is consistent with literature reports showing that Fe³⁺ doping in Mn-based oxides enhances structural stability and electrochemical kinetics by tuning electronic configurations and suppressing Jahn–Teller distortion [28].

GCD measurements (Figure 5a) further support the above conclusions. Notably, the b-FeMO electrode displayed much higher discharge capacities than other electrodes with different Fe loadings (a-FeMO and c-FeMO) (Figure S4), confirming the optimal doping content in the b-FeMO sample. As shown in Figure 5a, MO delivered an initial discharge capacity of 85.15 mAh·g⁻¹ at a current density of 0.5 A·g⁻¹, which degraded rapidly to 16.95 mAh·g⁻¹ after 200 cycles, corresponding to only a 19.9% retention rate. In comparison, b-FeMO exhibited an initial capacity of 116.24 mAh·g⁻¹ with 48.47 mAh·g⁻¹ remaining after 200 cycles—retaining 41.7% of its capacity, thus outperforming other Cu and Ru doped ones. This clearly demonstrates the superior cycling stability induced by Fe³⁺ doping.

Comparatively, similar MnO₂-based cathodes reported in the literature often have a lower retention rate or require complex synthesis procedures. For instance, Kamenskii et al. reported MnO₂ with 52% retention rate after 100 cycles at 0.3 A·g⁻¹ [29], while our b-FeMO achieved 61.4% at a higher rate of 0.5 A·g⁻¹, indicating better structural robustness.

The rate capability of the b-FeMO electrode (Figure 5b) was further evaluated at varying current densities ranging from 0.1 to 2.0 A·g⁻¹, with their GCD curves shown in Figure S5. b-FeMO exhibited average discharge capacities of 113.2, 98.7, 83.4, 71.2, and 59.5 mAh·g⁻¹ at current densities of 0.1, 0.2, 0.5, 1, and 2.0 A·g⁻¹, respectively. Notably, when the current density returned to 0.1 A·g⁻¹, 92.3% of the original capacity was recovered, suggesting excellent structural stability and rate adaptability. This rate performance significantly exceeds that of MO and other doped ones, which showed severe capacity degradation under similar conditions.

Table 1. Comparative electrochemical performance of materials.

Material	MO	b-FeMO	CuMO	RuMO
Initial discharge specific capacity (mAh/g)	85.15	116.24	146.53	210.57
Discharge specific capacity after 200 cycles (mAh/g)	16.95	48.47	51.67	44.58
Capacity retention rate (%)	19.9	41.70	35.23	21.17
Average Coulombic efficiency (%)	99.72	100	99.59	99.42

presents the comparative results of electrochemical performance among different materials. As evidenced by both the capacity retention rate and average Coulombic efficiency, b-FeMO demonstrates superior performance compared to the other three materials, exhibiting relatively slower performance degradation after 200 cycles.

In comparison, unmodified δ-MnO₂ cathodes typically suffer from fast capacity decay and limited rate response. For example, Feng et al. [30] and Ni et al. [31] reported the MnO₂ cathodes with a capacity drop of more than 50% when current density was increased from 0.1 to 1.0 A·g⁻¹. In contrast, our b-FeMO retains over 63% capacity at 2.0 A·g⁻¹, highlighting the kinetic superiority of Fe³⁺-doped δ-MnO₂.

3.3. Post-Cycling SEM and XPS Analysis:

To investigate the structural evolution and chemical composition of δ-MnO₂ electrodes during extended cycling, post-mortem SEM and XPS characterizations were conducted for both pristine MO and Fe³⁺-doped b-FeMO electrodes before and after 200 cycles.

As shown in Figure 6, the surface morphology of the MO electrode underwent significant degradation after 200 charge–discharge cycles at 0.5 A·g⁻¹. The pristine MO electrode originally exhibited spherical flower-like microspheres composed of thin nanosheets (Figure 6a,b), which are beneficial for Zn²⁺ diffusion and electrolyte penetration. However, after cycling (Figure 6c,d), these structures became fragmented, and extensive cracking and collapse of the microspheres was observed. This mechanical degradation likely results from Jahn–Teller distortions and Mn dissolution, both of which are known to cause lattice instability in δ-MnO₂ during repeated Zn²⁺ insertion/extraction [32].

In contrast, the b-FeMO electrode (Figure 6e–h) retained mostly its morphological integrity after 200 cycles. The nanosheet-based microspheres remained well-defined with minimal cracking, suggesting that Fe³⁺ doping effectively stabilizes the δ-MnO₂ layered structure. The improved mechanical robustness is likely due to the strengthened Mn–O bonding environment and reduction in lattice strain, which inhibit collapse and particle pulverization.

To further probe the chemical changes, high-resolution XPS spectra of Mn 2p and O 1s were collected before and after cycling for both MO and b-FeMO electrodes. For the MO electrode, post-cycling Mn 2p spectra (Figure 6i) revealed a reduced proportion of Mn³⁺, while the O 1s spectra (Figure 6j) showed a decrease in Mn–O–H species and an increase in lattice oxygen (Mn–O). This indicates a loss of oxygen vacancies, which are essential for facilitating Zn²⁺ diffusion and pseudocapacitive behavior. The reduction in Mn³⁺ and oxygen vacancies correlates with lower ionic conductivity and sluggish charge transfer, explaining the sharp capacity fading (from 85.15 to 16.95 mAh·g⁻¹) over 200 cycles.

In contrast, the b-FeMO electrode (Figure 6k,l) maintained a higher Mn³⁺ content and a relatively stable Mn–O–H signal in the O 1s region even after 200 cycles. The retention rate of Mn³⁺ not only reflects reduced Mn dissolution but also implies a greater density of oxygen vacancies, which enhance Zn²⁺ adsorption and facilitate ion diffusion. Furthermore, the appearance of Zn signals in the post-cycled EDS maps (Figures S6 and S7) confirms the effective and reversible Zn²⁺ intercalation in b-FeMO. In contrast, MO exhibited stronger Mn dissolution signals and reduced Zn content, supporting the notion that Fe³⁺ doping mitigates Mn loss and structural degradation.

Quantitative elemental analysis (Table S2) further confirmed that Mn loss in MO was significantly more severe than in b-FeMO. The enhanced retention rate of Mn in b-FeMO implies stronger framework stability, attributed to Fe³⁺ occupying interstitial or substitutional sites, reinforcing the layered structure and suppressing dissolution pathways.

4. Discussion

The combined CV, GCD, and rate performance analysis firmly supports that Fe³⁺ doping significantly improves Zn²⁺ storage kinetics, rate capability, and structural stability of δ-MnO₂. Compared to undoped and other doped samples (CuMO, RuMO), the optimized b-FeMO consistently outperforms in all aspects, with advantages that are further confirmed through comparison with recent literature. These findings validate the potential of Fe³⁺-doped δ-MnO₂ as a robust and high-performing cathode material for aqueous ZIBs [20].

The improved rate capability can be explained by the enhanced ion diffusion and electron transport stemming from the increased Mn³⁺ content and resulting oxygen vacancies. Additionally, the introduction of Fe³⁺ suppresses Mn dissolution, preserving the layered structure and avoiding pore clogging—a major issue in long-term Zn²⁺ intercalation/deintercalation.

Collectively, the post-cycling SEM and XPS results provide compelling evidence that Fe³⁺ doping significantly enhances both the structural and chemical stability of δ-MnO₂ during prolonged Zn²⁺ insertion/extraction. The b-FeMO electrode maintains its hierarchical structure, retains oxygen vacancies, suppresses Mn dissolution, and supports efficient Zn²⁺ reversibility—factors that are critical for achieving superior cycling performance (41.7% retention rate vs. 19.9% for MO after 200 cycles). These results reinforce the conclusion that Fe³⁺-doping is an effective and scalable strategy for improving the durability of manganese-based cathodes for aqueous zinc-ion batteries.

5. Conclusions

This study demonstrates that controlled Fe³⁺ doping significantly enhances the structural stability and electrochemical performance of δ-MnO₂ cathodes for aqueous ZIBs. Among the tested dopants, Fe³⁺ at a Mn:Fe ratio of 20:1 achieved the best balance, yielding a high specific capacity of 116.24 mAh·g⁻¹ and a retention rate of 41.7% after 200 cycles. The Fe³⁺ doping strategy effectively mitigates Mn dissolution, promotes oxygen vacancy formation, and stabilizes the layered structure. Comparative studies with Cu²⁺ and Ru³⁺ indicate that Cu²⁺ offers moderate improvements, while Ru³⁺ alters the crystal structure detrimentally. These findings provide a viable strategy for improving Mn-based cathodes for future zinc-ion battery development.