The relentless pursuit of sustainable and cost-effective energy storage solutions has intensified the search for alternatives to the dominant lithium-ion battery technology. While lithium-ion batteries offer high energy density and excellent cycle life, concerns regarding the geographical concentration, long-term supply security, and rising cost of lithium resources have spurred significant research into post-lithium systems. Among these, the sodium-ion battery stands out as a highly promising successor due to the natural abundance, low cost, and widespread distribution of sodium, coupled with its physicochemical similarities to lithium. The sodium-ion battery holds immense potential for large-scale grid storage and distributed energy systems, where cost and sustainability are paramount over ultra-high energy density.
The performance of a sodium-ion battery is critically dependent on the properties of its cathode material. Layered transition metal oxides with the general formula NaxMeO2 (Me = transition metals) are particularly attractive due to their high theoretical capacity, suitable operating voltage, and relatively simple synthesis. These materials crystallize in various polymorphs, primarily distinguished as P2- or O3-type structures, where the letter denotes the prismatic (P) or octahedral (O) coordination of sodium ions, and the number represents the number of transition metal oxide layers within the unit cell’s stacking sequence. The P2-type layered oxides, specifically those based on manganese, have garnered substantial attention for sodium-ion battery cathodes because of manganese’s low cost, environmental friendliness, and high operational voltage stemming from the Mn3+/Mn4+ redox couple.
However, practical deployment of P2-type manganese-based oxides in sodium-ion battery systems is hampered by several intrinsic challenges. The Jahn-Teller distortion associated with Mn3+ ions leads to structural deformation and irreversible capacity fade during cycling. Furthermore, these materials often undergo detrimental phase transitions (e.g., from P2 to O2 or OP4 phases) upon deep sodium extraction/insertion, especially at high voltages, causing rapid capacity decay and poor rate capability. Structural instability remains a key bottleneck for the long-term cyclability of manganese-based cathodes in sodium-ion battery applications.
To mitigate these issues, strategic cationic doping has proven to be an effective approach. The partial substitution of manganese with other electrochemically active or inert metal ions can enhance structural stability, suppress phase transitions, and improve ionic conductivity. Iron is an exceptionally appealing dopant due to its low cost, natural abundance, and the electrochemically active Fe3+/Fe4+ redox couple which operates at a relatively high potential. Incorporating iron into the manganese oxide framework can not only participate in charge compensation but also strengthen the metal-oxygen bonds, thereby stabilizing the layered structure during the cycling of a sodium-ion battery. This study focuses on the design, synthesis, and comprehensive evaluation of iron-doped P2-type sodium manganese oxides as advanced cathode materials for sodium-ion battery technology. We systematically investigate the impact of the iron-to-manganese ratio on the crystal structure, morphology, and electrochemical properties to establish a correlation between composition, structure, and performance in a sodium-ion battery.
1. Experimental Methodology
1.1. Materials Synthesis via Sol-Gel Process
The target cathode materials, Na0.7FexMn(1-x)O2 (with x = 0.2 and 0.35), were synthesized using a citric acid-assisted sol-gel method, a technique renowned for producing homogeneous precursors with excellent stoichiometric control at the molecular level. For the synthesis of Na0.7Fe0.2Mn0.8O2, stoichiometric amounts of sodium carbonate (Na2CO3, 2.5 mmol), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 1.0 mmol), and manganese carbonate (MnCO3, 4.0 mmol) were precisely weighed. These precursors were dissolved/dispersed in deionized water under constant magnetic stirring. A 10 mmol portion of citric acid, acting as a chelating agent and fuel, was added to the mixture. The pH of the solution was carefully adjusted to approximately 6 using dilute nitric acid to promote complexation. The solution was then continuously stirred and heated at 70°C for several hours until a viscous gel formed. This gel was transferred to an oven and dried at 100°C for 36 hours to obtain a dry, porous precursor. The dried gel was ground into a fine powder using an agate mortar and subjected to a two-step calcination process: first at 450°C for 3 hours to decompose organic components and nitrates, followed by a final high-temperature treatment at 700°C for 12 hours in air to crystallize the desired P2-phase oxide. The same procedure was repeated for Na0.7Fe0.35Mn0.65O2, adjusting the Fe and Mn precursor amounts accordingly. The final products were reground before characterization and electrode fabrication.
1.2. Material Characterization Techniques
The crystal structure and phase purity of the synthesized powders were examined using X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.5406 Å). Data was collected over a 2θ range of 10° to 80° with a step size of 0.02°. Rietveld refinement was performed using specialized software to extract precise lattice parameters. The morphological features and particle size of the materials were observed by field-emission scanning electron microscopy (FE-SEM). The elemental composition and doping ratio were verified using X-ray fluorescence (XRF) spectroscopy to confirm the actual Na:Fe:Mn ratio matched the intended stoichiometry.
1.3. Electrochemical Evaluation in Sodium-Ion Battery Cells
To evaluate the performance of the materials in a sodium-ion battery configuration, CR2032-type coin cells were assembled in an argon-filled glovebox. The working electrode was prepared by blending the active material (Na0.7FexMn(1-x)O2), conductive carbon (Keijen Black), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 7.5:1.5:1. N-methyl-2-pyrrolidone (NMP) was added to form a homogeneous slurry, which was then coated onto an aluminum foil current collector and dried at 120°C under vacuum for 12 hours. The mass loading of active material was controlled to be approximately 1.5-2.0 mg cm-2. Sodium metal foil was used as both the counter and reference electrode. A glass fiber membrane served as the separator, and 1 M NaClO4 in a mixture of ethylene carbonate and propylene carbonate (EC:PC, 1:1 by volume) with 5% fluoroethylene carbonate (FEC) additive was employed as the electrolyte. Galvanostatic charge-discharge tests were conducted within a voltage window of 2.0-4.0 V (vs. Na+/Na) at various current rates (where 1C is defined based on the theoretical capacity of the material). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were also performed to investigate the redox behavior and electrode kinetics.
2. Results and Discussion
2.1. Structural and Morphological Evolution with Iron Doping
The XRD patterns of the synthesized Na0.7Fe0.2Mn0.8O2 and Na0.7Fe0.35Mn0.65O2 powders are presented below. All diffraction peaks for both samples can be indexed to a hexagonal lattice with the P63/mmc space group, characteristic of the P2-type layered structure. No impurity peaks were detected, confirming the successful formation of phase-pure materials through the sol-gel route. The sharp and well-defined peaks indicate a high degree of crystallinity. A closer inspection reveals subtle peak shifts upon increasing the iron content, suggesting modifications in the unit cell dimensions.
Rietveld refinement of the XRD data was employed to quantify the structural changes. The refined lattice parameters (a, c) and unit cell volume (V) are summarized in Table 1, alongside the reported values for undoped P2-Na0.7MnO2+δ for comparison.
| Sample | a (Å) | c (Å) | V (Å3) | c/a ratio |
|---|---|---|---|---|
| Na0.7MnO2+δ (Reference) | 2.876 | 11.12 | 79.70 | 3.867 |
| Na0.7Fe0.2Mn0.8O2 | 2.884 | 11.14 | 80.21 | 3.864 |
| Na0.7Fe0.35Mn0.65O2 | 2.871 | 11.17 | 79.76 | 3.890 |
The incorporation of iron into the transition metal layer has a pronounced effect. For Na0.7Fe0.2Mn0.8O2, both the a-parameter and, more significantly, the c-parameter increase compared to the undoped material. The c-parameter, which corresponds to the interlayer spacing (the distance between transition metal oxide slabs where sodium ions reside), shows a clear expansion. This expansion can be attributed to the stronger ionic character and different bond length of Fe–O compared to Mn–O. Iron (Fe3+) has a higher electronegativity than manganese (Mn3+/Mn4+), leading to a more covalent Fe–O bond. While this strengthens the transition metal layer, the ionic radius differences and local charge balance requirements can lead to an overall expansion of the sodium layer to minimize electrostatic repulsion. The relationship governing the interlayer spacing can be conceptually linked to the ionic radii (r) and bond strength:
$$ \Delta d_{(Na-layer)} \propto \frac{1}{E_{Me-O}} \cdot \left( r_{Me^{n+}} – r_{Mn^{n+}} \right) $$
where $E_{Me-O}$ is the average Me-O bond energy. The increased interlayer spacing is highly beneficial for sodium-ion battery electrodes as it facilitates easier (de)intercalation of the larger Na+ ions, potentially enhancing rate capability. For the sample with higher iron doping (x=0.35), the a-parameter contracts slightly while the c-parameter expands further, leading to a higher c/a ratio. This indicates a more anisotropic distortion of the unit cell, which may be related to a different local coordination environment and the suppression of Jahn-Teller active Mn3+ ions, thereby stabilizing the P2 structure against distortion during sodium-ion battery operation.
SEM analysis reveals the morphological impact of the synthesis process and doping. Both materials exhibit plate-like or flaky particle morphologies, which is typical for layered oxides synthesized via high-temperature calcination. The primary particle thickness is around 100 nm. Na0.7Fe0.2Mn0.8O2 consists of particles with a larger aspect ratio (thinner plates), whereas Na0.7Fe0.35Mn0.65O2 particles appear as thicker plates or blocks with a smaller aspect ratio. This change in morphology suggests that the iron doping level influences the crystal growth kinetics during the high-temperature annealing step. The plate-like morphology is advantageous as it provides a large interfacial area for electrolyte contact and shortens the diffusion path for sodium ions within the solid phase, a critical factor for achieving good performance in a sodium-ion battery. XRF analysis confirmed that the actual elemental composition of the final products was in excellent agreement with the designed stoichiometry, validating the precision of the sol-gel synthesis method.
2.2. Electrochemical Performance in Sodium-Ion Battery Configuration
The electrochemical properties of the synthesized materials were evaluated as cathodes in a sodium-ion battery versus sodium metal. The galvanostatic charge-discharge profiles at a moderate rate of 0.5C (where C-rate is defined based on the theoretical capacity) within the 2.0-4.0 V window are shown in Figure 3. Both materials exhibit the characteristic sloping voltage profiles of P2-type layered oxides, indicative of solid-solution (single-phase) reactions during most of the sodium (de)intercalation process. No obvious voltage plateaus associated with two-phase reactions are observed in this voltage range, suggesting good structural stability for the initial cycle.
The initial charge/discharge specific capacities for Na0.7Fe0.2Mn0.8O2 are approximately 88.5 mA h g-1 and 63.7 mA h g-1, respectively, yielding an initial Coulombic efficiency (ICE) of about 72%. For Na0.7Fe0.35Mn0.65O2, the initial capacities are 74.0 mA h g-1 (charge) and 49.0 mA h g-1 (discharge), with an ICE of ~66%. The lower initial Coulombic efficiency is common for oxide cathodes and can be attributed to side reactions at the electrode/electrolyte interface, possible electrolyte decomposition, and the irreversible extraction of sodium ions from sites that cannot be re-occupied during discharge. The higher initial charge capacity of the x=0.2 sample suggests more accessible sodium ions from the structure, possibly due to its more open morphology and optimal interlayer spacing.
The long-term cycling stability, a critical metric for any practical sodium-ion battery, is presented in Figure 3. After 100 consecutive charge-discharge cycles at 0.5C, Na0.7Fe0.2Mn0.8O2 retains discharge/charge capacities of 51.4/51.9 mA h g-1, corresponding to a capacity retention of approximately 80.6% relative to its 2nd cycle discharge capacity (to exclude initial irreversible loss). More impressively, Na0.7Fe0.35Mn0.65O2 exhibits exceptional capacity retention, maintaining 51.6/52.2 mA h g-1 after 100 cycles, which represents a retention of over 105% compared to its 2nd cycle. This apparent increase in capacity over the first few cycles, sometimes observed in nanostructured transition metal oxides, can be attributed to a gradual activation process of the electrode material, improved electrolyte wetting, and the progressive formation of a stable solid-electrolyte interphase (SEI). The high iron content in this sample appears to significantly stabilize the P2 structure against detrimental phase transitions and Mn dissolution, leading to superior cycle life in the sodium-ion battery. The average Coulombic efficiency over the 100 cycles exceeds 99.5% for both samples, indicating highly reversible electrochemical reactions.
The rate capability of a cathode material determines the power performance of the sodium-ion battery. Figure 4 depicts the rate performance of both materials, tested at progressively increasing current densities from 0.2C to 2C, and then returning to 0.5C. The delivered discharge capacities at each rate are summarized in Table 2.
| Current Rate | Na0.7Fe0.2Mn0.8O2 Discharge Capacity (mA h g-1) | Na0.7Fe0.35Mn0.65O2 Discharge Capacity (mA h g-1) |
|---|---|---|
| 0.2C | ~134 | ~72 |
| 0.5C | ~104 | ~45 |
| 1C | ~73 | ~23 |
| 2C | ~35 | ~4 |
| Return to 0.5C | ~110 | ~51 |
A clear trend is observed: Na0.7Fe0.2Mn0.8O2 demonstrates significantly better rate performance. It delivers substantial capacity even at 2C (35 mA h g-1), and upon returning to 0.5C, it swiftly recovers a high capacity of ~110 mA h g-1, close to its initial value at that rate. This indicates minimal structural damage or kinetic hindrance from high-rate cycling. In contrast, the capacity of Na0.7Fe0.35Mn0.65O2 drops sharply with increasing rate, nearly fading at 2C. However, its ability to recover to ~51 mA h g-1 at 0.5C confirms that the structure remains largely intact, but the sodium-ion diffusion kinetics become severely limited at high currents. This difference can be explained by a trade-off: higher iron doping (x=0.35) provides exceptional structural stability for long cycle life but may increase the electronic resistivity or create diffusion bottlenecks for Na+ ions. The sample with moderate doping (x=0.2) strikes a better balance, offering both good cyclability and superior rate capability, making it more suitable for sodium-ion battery applications requiring power.
The enhanced performance can be analyzed through the lens of crystal chemistry and electrochemistry. The partial substitution of Mn by Fe achieves multiple goals:
1. Structural Pillaring Effect: The stronger Fe–O bonds act as pillars within the transition metal layer, reinforcing the structure against collapse during sodium extraction, which mitigates layer gliding and phase transitions.
2. Suppression of Jahn-Teller Distortion: Fe3+ (d5, high-spin) is not Jahn-Teller active. Replacing a portion of Jahn-Teller active Mn3+ (d4, high-spin) reduces the overall lattice distortion, leading to a more isotropic and stable framework.
3. Additional Redox Activity: The Fe3+/Fe4+ couple contributes to charge compensation at high voltages (above 3.5 V vs. Na+/Na), supplementing the Mn3+/Mn4+ redox. This participation can be described by the overall redox reaction during charge:
$$ \text{Na}_{0.7}\text{Fe}^{3+}_{x}\text{Mn}^{3+}_{y}\text{Mn}^{4+}_{z}\text{O}_2 \rightarrow \text{Na}_{0.7-\delta}\text{Fe}^{3+}_{x-\alpha}\text{Fe}^{4+}_{\alpha}\text{Mn}^{4+}_{y+z}\text{O}_2 + \delta\text{Na}^+ + \delta e^- $$
where $y+z = 1-x$.
4. Expanded Sodium Layer: As evidenced by the increased c-parameter, the interlayer spacing is widened, reducing the energy barrier for Na+ ion hopping. The diffusion coefficient (DNa+) can be empirically related to the structural parameters and is expected to be higher in the doped samples, following an Arrhenius-type relationship influenced by the activation energy ($E_a$) for diffusion:
$$ D_{Na^+} \propto \exp\left(-\frac{E_a}{k_B T}\right) $$
A larger interlayer spacing typically correlates with a lower $E_a$ for ion migration.
3. Conclusion
This study successfully demonstrates the design and synthesis of iron-doped P2-type layered oxides, Na0.7FexMn(1-x)O2 (x = 0.2, 0.35), as high-performance cathode materials for sodium-ion battery applications. The sol-gel method proved effective in producing phase-pure materials with controlled stoichiometry and plate-like morphology. Systematic investigation reveals that iron doping plays a multifaceted role: it stabilizes the P2 crystal structure, expands the interlayer spacing to facilitate sodium-ion transport, and introduces an additional redox-active couple. Electrochemical evaluation within a sodium-ion battery configuration shows that the doping level critically influences the performance trade-off between cycling stability and rate capability.
Na0.7Fe0.35Mn0.65O2 exhibits outstanding long-term cycling stability with minimal capacity fade over 100 cycles, attributed to its highly stabilized structure. On the other hand, Na0.7Fe0.2Mn0.8O2 delivers a more balanced and superior overall performance, combining good specific capacity, excellent rate capability (retaining usable capacity at 2C), and stable cycling. This material, with its moderate iron content, presents an optimal composition that enhances kinetic properties without compromising structural integrity. The findings underscore the efficacy of strategic cation doping as a powerful tool for engineering advanced electrode materials. The developed Na0.7Fe0.2Mn0.8O2 cathode is a highly promising candidate for the next generation of cost-effective, durable, and high-power sodium-ion battery systems, paving the way for their practical implementation in large-scale energy storage.