The imperative shift towards sustainable energy systems, driven by global energy scarcity and environmental degradation, necessitates the integration of renewable sources like wind and solar. However, their inherent intermittency underscores the critical need for advanced large-scale energy storage solutions. While lithium-ion batteries have dominated the energy storage landscape for decades, concerns over lithium resource availability and cost have intensified the search for viable alternatives. Sodium-ion batteries emerge as a compelling candidate, leveraging the elemental abundance and low cost of sodium, which shares similar physicochemical properties with lithium.
Within the architecture of a sodium-ion battery, the cathode material is a primary determinant of performance parameters such as energy density, cycle life, and cost. Among the various cathode families, layered transition metal oxides (NaxTMO2, where TM = transition metal) are particularly promising due to their high theoretical capacity and relatively straightforward synthesis. The O3-type NaNi1/3Fe1/3Mn1/3O2 (often denoted as NFM111) composition has attracted significant attention. It utilizes earth-abundant elements (Ni, Fe, Mn) and exhibits a favorable balance of energy density and stability. However, challenges such as structural degradation during cycling, air sensitivity, and sluggish sodium-ion kinetics can hinder its practical application.
To address these challenges, material engineering strategies focusing on synthesis optimization and cation doping are essential. Conventional synthesis methods like solid-state reaction often suffer from inhomogeneous mixing, leading to irregular particle morphology and inconsistent performance. Co-precipitation offers better homogeneity but can still result in particle agglomeration. In contrast, spray drying is a versatile and scalable technique that enables the production of uniformly mixed, spherical precursor particles with controlled morphology directly from a solution. Concurrently, strategic doping with alien cations (e.g., Ti4+) can stabilize the crystal structure, expand the Na+ layer spacing, and enhance ionic conductivity. This article details a comprehensive study on the synthesis of NFM111 via a spray-drying method, followed by TiO2 doping, and systematically investigates the influence of final calcination temperature on the material’s structural and electrochemical properties.
Experimental Methodology: Synthesis and Processing
The synthesis pathway was designed to ensure atomic-scale homogeneity and controlled final morphology. The process flowchart and key chemical reactions are summarized below.
1. Precursor Solution Preparation and Spray Drying
A stoichiometric aqueous solution was prepared using acetates and citrates as metal sources to facilitate good solubility and mixing. The following reactants were dissolved in deionized water under constant stirring at 60°C:
- Nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O)
- Iron(III) citrate (FeC6H5O7)
- Manganese(II) acetate tetrahydrate (Mn(CH3COO)2·4H2O)
- Sodium acetate trihydrate (NaCH3COO·3H2O) – used with a slight excess (Na/Transition Metals = 1.05) to compensate for sodium volatility during high-temperature treatment.
- Anhydrous citric acid (C6H8O7) – added as a chelating agent (Citric Acid/Transition Metals = 1:1 molar ratio) to prevent selective precipitation and promote the formation of a homogeneous mixed-metal complex.
The clear solution was then fed into a spray dryer. The operating parameters were: inlet temperature of 180°C, nitrogen as the drying gas, and a controlled feed rate. The rapid evaporation of solvent in the hot chamber resulted in the formation of dry, fine precursor powder consisting of mixed metal citrates/acetates.
2. Thermal Treatment and Doping Process
The as-sprayed precursor powder was subjected to a two-step thermal treatment process:
Step 1: Pre-oxidation. The precursor was heated at 550°C for 3 hours under a pure oxygen atmosphere. This step decomposes the organic components (citrates/acetates) and initiates the formation of an intermediate oxide phase while maintaining a oxidizing environment to control the transition metal oxidation states.
Step 2: Doping and High-Temperature Calcination. The pre-oxidized powder was thoroughly mixed with 1 mol% of nano-sized TiO2 via ball milling for 30 minutes. The doped mixture was then calcined in a muffle furnace at different final temperatures—850°C, 900°C, and 950°C—for 10 hours in air. The resulting materials are labeled as NFM111@TiO2-850, NFM111@TiO2-900, and NFM111@TiO2-950, respectively. An undoped NFM111 sample, calcined at 900°C, was also prepared for comparison.
The overall chemical transformation from precursors to the final layered oxide can be conceptually represented by the following reaction, although the actual process involves multiple intermediate steps:
$$ \text{Ni,Fe,Mn,Na (Citrates/Acetates)} \xrightarrow[\text{Spray Drying}]{\text{550°C, O}_2} \text{Amorphous Intermediate} \xrightarrow[\text{Ball Mill + TiO}_2]{\text{850-950°C, Air}} \text{NaNi}_{1/3}\text{Fe}_{1/3}\text{Mn}_{1/3}\text{O}_2 $$
3. Material Characterization and Electrochemical Testing
The crystal structure was analyzed using X-ray diffraction (XRD). Rietveld refinement was performed to extract lattice parameters. Morphology and elemental distribution were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). High-resolution transmission electron microscopy (HR-TEM) was used to observe lattice fringes and confirm doping. Electrodes were fabricated by mixing active material, conductive carbon, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1. CR2032 coin-type half-cells were assembled using sodium metal as the anode, a glass fiber separator, and 1 M NaClO4 in EC:DEC (1:1 v/v) electrolyte. Electrochemical tests included galvanostatic charge-discharge, rate capability, long-term cycling, and cyclic voltammetry (CV).
Structural and Morphological Characterization
1. Crystal Structure Analysis (XRD)
The XRD patterns of all synthesized samples, both doped and undoped, confirmed the formation of a well-crystallized O3-type layered structure (space group R-3m), characteristic of α-NaFeO2. The distinct splitting of the (006)/(012) and (018)/(110) doublets indicates a highly ordered layered configuration. A critical observation was the systematic shift of the (003) diffraction peak to lower angles for the TiO2-doped samples compared to the undoped NFM111.
According to Bragg’s law, $$ n\lambda = 2d_{(hkl)}\sin\theta $$, a decrease in the diffraction angle (2θ) corresponds to an increase in the interplanar spacing (d). Therefore, the leftward shift of the (003) peak directly signifies an expansion of the interlayer spacing (d003), which is the pathway for sodium-ion diffusion. The lattice parameters (a, c), c/a ratio, d003 spacing, and unit cell volume (V) were calculated and are tabulated below.
| Sample | a (Å) | c (Å) | c/a ratio | d003 (Å) | V (Å3) |
|---|---|---|---|---|---|
| NFM111 (Undoped) | 2.9772 | 16.1922 | 5.432 | 5.3994 | 124.48 |
| NFM111@TiO2-850 | 3.0758 | 16.1285 | 5.244 | 5.4325 | 132.14 |
| NFM111@TiO2-900 | 3.1168 | 16.8363 | 5.402 | 5.4125 | 141.64 |
| NFM111@TiO2-950 | 2.9861 | 16.1257 | 5.400 | 5.6542 | 124.53 |
The data reveals that the sample calcined at 900°C (NFM111@TiO2-900) exhibits the most significant lattice expansion, boasting the largest c-axis parameter (16.84 Å), the largest interlayer spacing d003 (5.41 Å), and consequently the greatest unit cell volume (141.64 Å3). This expansion is attributed to the successful incorporation of Ti4+ ions (ionic radius ~0.605 Å for VI coordination) into the transition metal layers. The substitution of larger Ti4+ for smaller Mn4+/Fe3+/Ni2+ ions, coupled with charge compensation mechanisms, effectively “pillars” the layered structure, widening the sodium layer. Furthermore, a c/a ratio greater than 4.9 typically indicates a well-defined layered structure, and values exceeding 5.4, as seen for the 900°C and 950°C samples, are associated with better reversibility of sodium (de)intercalation. The abnormal parameters for the 850°C sample suggest incomplete crystallization or different phase composition at this lower temperature.
2. Morphology and Elemental Distribution (SEM/TEM/EDS)
SEM imaging showed that the spray-drying-derived precursors yielded spherical secondary particles. After high-temperature calcination, the samples retained a generally spherical morphology but were composed of densely packed primary crystallites. Notably, the surface texture differed: NFM111@TiO2-900 displayed a slightly rougher, “fog-like” surface, while the 850°C and 950°C samples appeared smoother. This surface characteristic for the 900°C sample is consistent with optimal integration of the TiO2 dopant.
EDS mapping confirmed a uniform spatial distribution of Na, Ni, Fe, Mn, and O elements throughout the particles, validating the effectiveness of the spray-drying process in achieving atomic-scale homogeneity. The quantitative EDS analysis yielded transition metal ratios very close to the nominal 1:1:1 (Ni:Fe:Mn), confirming successful synthesis of the target composition.
HR-TEM analysis provided direct evidence of TiO2 incorporation. Lattice fringes corresponding to the (003) planes of the layered NFM111 structure were clearly observed. In regions, these fringes were interfaced with crystalline domains exhibiting different lattice spacing, identified as TiO2 (likely anatase or rutile phases). This intimate contact and potential partial substitution at the interface support the XRD findings of structural modification.
Electrochemical Performance Evaluation
The electrochemical properties of the synthesized cathode materials were evaluated in sodium-ion battery half-cells.
1. Galvanostatic Charge-Discharge Profiles
The initial charge-discharge curves at 0.1C (13 mA g-1) within 2.0-4.0 V are shown conceptually below. All doped samples exhibited characteristic voltage plateaus corresponding to phase transitions typical of O3-type layered oxides. The most important metrics are summarized in the following table:
| Sample | 1st Discharge Capacity (mAh g-1) | 1st Coulombic Efficiency (%) | Remarks |
|---|---|---|---|
| NFM111@TiO2-850 | 132.6 | 84.6 | Lower capacity, moderate efficiency. |
| NFM111@TiO2-900 | 143.2 | 89.7 | Highest capacity and efficiency. |
| NFM111@TiO2-950 | 135.6 | 87.1 | Capacity and efficiency lower than 900°C sample. |
The superior initial performance of NFM111@TiO2-900 is directly correlated with its optimal structural parameters. The expanded interlayer spacing reduces the energy barrier for Na+ extraction/insertion, leading to higher reversible capacity. The high first-cycle Coulombic efficiency indicates minimal irreversible side reactions and efficient formation of a stable solid-electrolyte interphase (SEI).
2. Rate Capability and Long-Term Cycling Stability
Rate Capability: Cells were cycled at increasing current densities from 0.1C to 5C. NFM111@TiO2-900 consistently delivered the highest discharge capacities at every rate. For instance, at a high rate of 2C (260 mA g-1), it retained a significantly higher capacity than the other two samples. This excellent rate performance is a direct consequence of the enhanced Na+ diffusion kinetics facilitated by the enlarged d-spacing and stabilized structure from optimal TiO2 doping at 900°C. The relationship between diffusion coefficient (DNa+) and structural parameters can be approximated by models considering the activation barrier for hopping, which is lower in expanded structures:
$$ D_{\text{Na}^+} \propto \exp\left(-\frac{E_a}{k_B T}\right) $$
where the activation energy \( E_a \) is influenced by the interlayer distance.
Long-Term Cycling: After initial activation at 0.1C, cells were cycled at 0.5C (65 mA g-1) for 100 cycles. The performance metrics are compared below:
| Sample | Discharge Capacity at 0.5C (mAh g-1, ~5th cycle) | Capacity after 100 cycles (mAh g-1) | Capacity Retention (%) |
|---|---|---|---|
| NFM111@TiO2-850 | ~118 | ~88 | ~74.9 |
| NFM111@TiO2-900 | ~143 | ~115 | ~80.3 |
| NFM111@TiO2-950 | ~125 | ~82 | ~65.9 |
NFM111@TiO2-900 demonstrated the best cycling stability with a high capacity retention of 80.3% after 100 cycles. The TiO2 doping serves a dual purpose: it pillars the structure to mitigate irreversible layer gliding and phase transitions during deep cycling, and it may also stabilize the surface against electrolyte decomposition. The poor retention of the 950°C sample suggests that excessive temperature may lead to sodium loss, cation disorder, or detrimental particle growth, offsetting the benefits of doping.
3. Electrochemical Kinetics (Cyclic Voltammetry)
CV curves at a slow scan rate of 0.1 mV s-1 provided insights into the redox behavior and phase evolution. All samples exhibited pairs of oxidation/reduction peaks corresponding to the reversible oxidation of Ni2+/Ni3+/Ni4+ and Fe3+/Fe4+, with Mn remaining largely electrochemically inactive as Mn4+ in this voltage window. The CV profile for NFM111@TiO2-900 showed the sharpest and most symmetric peaks with the largest integrated area (corresponding to capacity). This indicates the fastest Na+ diffusion kinetics and the smallest polarization (\( \Delta E_p = E_{pa} – E_{pc} \)) among the series. The well-defined peaks also confirm the highly reversible phase transition sequence: O3 → (O3+P3) → P3 upon charging, and its reversal upon discharging.
Conclusion and Perspective
This work successfully demonstrates an integrated materials engineering approach to enhance the performance of O3-type NaNi1/3Fe1/3Mn1/3O2 cathode material for sodium-ion batteries. The combination of spray-drying synthesis and strategic TiO2 doping proves highly effective.
Key Findings:
- Synthesis Advantage: The spray-drying method produced chemically homogeneous precursor powders, which translated into well-crystallized final materials with uniform particle morphology.
- Optimal Calcination Temperature: A final calcination temperature of 900°C was identified as optimal. At this temperature, the TiO2 dopant effectively integrates into the host structure, leading to the most significant lattice expansion (largest c-axis, d003 spacing, and unit cell volume).
- Structure-Property Correlation: The expanded interlayer spacing in NFM111@TiO2-900 directly facilitated faster sodium-ion transport, as evidenced by its superior rate capability. The structural stabilization imparted by doping resulted in excellent cycling stability.
- Electrochemical Superiority: The NFM111@TiO2-900 sample delivered the highest initial discharge capacity (143.2 mAh g-1), the best rate performance, and the highest capacity retention (80.3% after 100 cycles at 0.5C) among the series.
Future Outlook: This study highlights the critical importance of fine-tuning synthesis parameters (like calcination temperature) in conjunction with doping strategies. Future work could explore:
- The precise mechanism of Ti incorporation (substitution vs. surface coating) using advanced spectroscopy.
- The impact of different doping concentrations and other dopants (Al, Mg, Zn) on air stability and high-voltage performance.
- Scalability of the spray-drying process for commercial production of high-performance sodium-ion battery cathodes.
In conclusion, the engineered NFM111@TiO2 material synthesized via the optimized spray-drying and calcination route presents a promising, cost-effective cathode candidate, contributing to the advancement of sustainable energy storage technologies based on abundant sodium resources.