The pursuit of efficient and cost-effective energy storage systems has positioned sodium-ion battery technology as a frontrunner for grid-scale applications. The relative abundance of sodium compared to lithium offers a compelling path toward more sustainable and scalable electrochemical storage. Within this landscape, the development of high-performance, stable, and inexpensive cathode materials is a critical research frontier. Among the various cathode families, transition metal layered oxides, denoted as NaxTMO2 (where TM represents transition metals), stand out due to their high theoretical capacities, high energy densities, and relatively straightforward synthesis routes. The O3-type structure, an analog to the α-NaFeO2 configuration, is particularly attractive for its high sodium content and favorable initial capacity.
O3-type NaNi0.4Fe0.2Mn0.4O2 is a promising candidate material for sodium-ion battery cathodes. Its composition leverages the electrochemically active nickel (Ni) for capacity, the low-cost and stable iron (Fe), and the structurally supportive manganese (Mn). This combination aims to balance performance, cost, and stability. The synthesis of this material typically involves a two-step process: the preparation of a transition metal hydroxide precursor via co-precipitation, followed by a solid-state calcination step with a sodium source. The quality, morphology, and chemical homogeneity of the precursor profoundly influence the properties of the final cathode material, which in turn dictates the electrochemical performance of the sodium-ion battery.
The co-precipitation reaction is a complex process governed by several parameters, including pH, temperature, reactant concentration, and stirring speed. Among these, the pH of the reaction solution is a paramount factor. It directly controls the supersaturation level, which dictates the kinetics of nucleation and growth of the precursor particles. This study systematically investigates the effect of the co-precipitation pH on the physicochemical properties of the Ni0.4Fe0.2Mn0.4(OH)2 precursor and, subsequently, on the structural and electrochemical performance of the final O3-NaNi0.4Fe0.2Mn0.4O2 cathode in a sodium-ion battery. By understanding this relationship, we can tailor the synthesis to optimize for specific performance metrics such as rate capability or cycling stability.
Experimental Methodology: Synthesis and Characterization
The synthesis pathway involved a controlled co-precipitation followed by a high-temperature calcination. Aqueous solutions of NiSO4·6H2O, FeSO4·7H2O, and MnSO4·H2O were prepared with a cationic molar ratio of Ni:Fe:Mn = 4:2:4, resulting in a total metal ion concentration of 2 mol/L. This mixed salt solution was fed into a continuously stirred tank reactor (CSTR) along with separate streams of a 2 mol/L NaOH solution (for pH control) and a diluted NH4OH solution (acting as a complexing agent). The reaction was conducted under a protective nitrogen atmosphere at a constant temperature and stirring speed. The key variable was the pH of the reaction mixture, which was precisely controlled and maintained at four distinct values: 10.0, 10.2, 10.4, and 10.6. After the reaction and an aging period, the precipitated product was thoroughly washed, filtered, and dried at 110°C to obtain the Ni0.4Fe0.2Mn0.4(OH)2 precursor powders.
In the second step, each precursor was thoroughly mixed with a 5% molar excess of anhydrous Na2CO3 (n(Na):n(TM) = 1.05:1). The mixtures were then calcined in a tube furnace at 850°C for 24 hours in air. The resulting products were rapidly transferred to an argon-filled glovebox upon cooling to minimize air exposure, yielding the final O3-NaNi0.4Fe0.2Mn0.4O2 cathode materials.
The structural and morphological characterization was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The chemical composition of the precursors was verified by inductively coupled plasma optical emission spectrometry (ICP-OES). For electrochemical evaluation, cathode slurries were prepared by mixing the active material, carbon black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) solvent. The slurry was coated onto aluminum foil and dried to form the cathode electrode. CR2032-type coin cells were assembled inside an argon-filled glovebox using sodium metal as the counter/reference electrode, a glass fiber separator, and an electrolyte composed of 1 M NaClO4 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) (95:5 by volume). Galvanostatic charge-discharge tests were conducted within a voltage window of 2.0-4.0 V vs. Na+/Na. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry-derived differential capacity (dQ/dV) analysis were used to probe kinetic properties and phase transition behavior.
Influence of pH on Precursor Properties
The XRD patterns of all synthesized precursors confirmed the successful formation of a hydroxide phase with a layered structure, matching a Ni(OH)2-type pattern. A critical observation was the systematic shift of the diffraction peaks to slightly higher angles compared to pure Ni(OH)2. This shift can be explained by the Bragg equation:
$$2d\sin\theta = n\lambda$$
where \(d\) is the interplanar spacing, \(\theta\) is the diffraction angle, \(n\) is an integer, and \(\lambda\) is the wavelength. The incorporation of smaller Fe2+ and Mn2+ ions (compared to Ni2+) into the crystal lattice reduces the average ionic radius, leading to a contraction of the lattice parameters and a decrease in \(d\)-spacing. According to Bragg’s law, a smaller \(d\) results in a larger \(\theta\), which is consistent with the observed peak shift to the right. This confirms the effective co-precipitation of all three transition metals into a single, homogeneous hydroxide phase.
The impact of pH on the precursor morphology was profound and directly observable via SEM. The precursors consisted of secondary spherical particles formed by the aggregation of primary plate-like nanoparticles. As the pH increased from 10.0 to 10.6, a clear and consistent trend emerged: the size of the secondary spherical particles decreased significantly.
- At pH = 10.0, the particles were large and exhibited a relatively loose surface morphology.
- At pH = 10.6, the secondary particles were much smaller, denser, and more compact.
This phenomenon is governed by nucleation and growth kinetics. A higher pH leads to a greater supersaturation level in the solution. Supersaturation (\(\sigma\)) can be conceptually related to the driving force for precipitation:
$$\sigma = \frac{C – C^*}{C^*}$$
where \(C\) is the actual concentration and \(C^*\) is the equilibrium saturation concentration. At high supersaturation (high pH), the nucleation rate exceeds the crystal growth rate. Consequently, a larger number of nuclei form initially, consuming the available solute and leading to the formation of a greater quantity of smaller final particles. Conversely, at lower pH (lower supersaturation), growth dominates over nucleation, resulting in fewer but larger particles. Particle size distribution analysis quantitatively supported the SEM observations, showing a monotonic decrease in the median particle size (D50) with increasing pH.
Interestingly, the pH also influenced the precise chemical composition of the precursor, as detailed by ICP-OES analysis. The target stoichiometry was Ni0.4Fe0.2Mn0.4(OH)2. The results, summarized in the table below, show that while the compositions at pH 10.0 and 10.2 were closest to the target, a systematic trend was observed with increasing pH: the Ni content increased, and the Mn content decreased.
| Synthesis pH | Ni Content (wt.%) | Fe Content (wt.%) | Mn Content (wt.%) |
|---|---|---|---|
| 10.0 | 40.31 | 20.62 | 39.06 |
| 10.2 | 40.58 | 20.38 | 39.03 |
| 10.4 | 40.96 | 20.11 | 38.92 |
| 10.6 | 42.13 | 19.58 | 38.28 |
This compositional shift is attributable to the different solubility product constants (Ksp) of the constituent metal hydroxides. The Ksp for Ni(OH)2 is significantly lower than that for Mn(OH)2, meaning Ni2+ ions precipitate more readily at a given pH. Therefore, in a competitive co-precipitation environment at higher pH, the precipitation of Ni is further favored, leading to its relative enrichment in the solid phase, while the precipitation of the slower-precipitating Mn is slightly suppressed.
Structural and Morphological Evolution into the Final Cathode
The calcination of the precursors with sodium carbonate successfully yielded the desired O3-type layered oxide structure, as confirmed by XRD. All patterns were indexed to the α-NaFeO2 structure (space group \(R\overline{3}m\)). No impurity peaks were detected, indicating phase-pure materials. Rietveld refinement was used to extract the lattice parameters. A clear trend was observed: the unit cell volume contracted slightly but consistently with increasing synthesis pH. This lattice contraction correlates with the increased Ni content (and decreased Mn content) found in the precursors from higher pH syntheses. Since Ni3+ (in the charged state or due to air oxidation during calcination) is generally smaller than Mn4+, a higher Ni/Mn ratio leads to a smaller average ionic radius in the transition metal layer, thus reducing the unit cell volume.
| Cathode from pH | Lattice Parameter a (Å) | Lattice Parameter c (Å) | Unit Cell Volume V (Å3) |
|---|---|---|---|
| 10.0 | 2.97397 | 15.96750 | 122.30 |
| 10.2 | 2.97477 | 15.93821 | 122.15 |
| 10.4 | 2.97194 | 15.94268 | 121.95 |
| 10.6 | 2.97346 | 15.92604 | 121.90 |
The morphological characteristics of the precursors were largely inherited by the final cathode materials. SEM images revealed that the secondary spherical morphology was preserved after high-temperature calcination, although the primary particles sintered and grew into larger, well-defined polygonal blocks. Crucially, the size trend persisted: cathodes derived from precursors synthesized at higher pH (e.g., 10.6) consisted of significantly smaller secondary spheres than those from lower pH (e.g., 10.0) precursors. This inheritance is vital because the secondary particle size in the cathode directly influences the electrode packing density, the electrolyte-electrode contact area, and the solid-state diffusion path length for sodium ions—all key factors for sodium-ion battery performance.
Electrochemical Performance in Sodium-Ion Batteries
The electrochemical properties of the four cathode materials were evaluated in Na half-cells. The initial charge-discharge profiles at a low current rate (0.05 C) revealed the influence of pH. The initial discharge capacity showed a positive correlation with the synthesis pH: materials from higher-pH precursors delivered higher first-cycle capacity. This is directly linked to the compositional analysis. The capacity in the 2.0-4.0 V window primarily stems from the Ni2+/Ni3+ redox couple. Since the Ni content in the material increased with pH, a higher theoretical number of active Ni sites was available, translating to a higher achievable capacity. The initial Coulombic efficiency followed a non-monotonic trend, with the sample from pH 10.2 showing the highest value.
The rate capability tests, where the discharge current was progressively increased, unveiled a striking advantage for cathodes with smaller secondary particles. The material synthesized at pH 10.6 exhibited the best performance at high rates (e.g., 5 C), retaining a significantly higher capacity compared to the others. This superior rate capability is a direct consequence of the shortened sodium-ion diffusion path within the smaller secondary particles. The apparent chemical diffusion coefficient of sodium ions (\(D_{Na^+}\)) can be estimated from the low-frequency region of the EIS data using the following relationship derived from the Warburg impedance:
$$Z’ = R_e + R_{ct} + \sigma \omega^{-1/2}$$
where \(\sigma\) is the Warburg coefficient. The diffusion coefficient is inversely proportional to the square of \(\sigma\):
$$D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2}$$
In this equation, \(R\) is the gas constant, \(T\) is the absolute temperature, \(A\) is the electrode area, \(n\) is the number of electrons transferred per formula unit, \(F\) is Faraday’s constant, and \(C\) is the concentration of sodium ions. EIS measurements on fresh cells showed that the charge-transfer resistance (\(R_{ct}\)) decreased with increasing synthesis pH. The calculated \(D_{Na^+}\) values increased monotonically with pH, confirming faster ionic kinetics in the cathodes composed of smaller particles, which is essential for high-power sodium-ion battery applications.
| Electrode from pH | 1st Discharge Capacity (mAh/g) | Capacity at 5C (mAh/g) | Calc. \(D_{Na^+}\) (cm²/s) | Cycle-100 Capacity Retention at 1C (%) |
|---|---|---|---|---|
| 10.0 | 143.47 | ~65 | 3.29 × 10-13 | 75.75 |
| 10.2 | 147.52 | ~72 | 1.75 × 10-12 | ~70 |
| 10.4 | 149.35 | ~80 | 2.54 × 10-12 | ~65 |
| 10.6 | 153.83 | 87.07 | 2.91 × 10-12 | ~60 |
However, the long-term cycling stability at 1 C presented a trade-off. The cathode material derived from the pH 10.0 precursor, which had the largest secondary particles, demonstrated the best capacity retention over 100 cycles (75.75%). In contrast, the material from pH 10.6, despite its excellent rate capability, suffered from faster capacity fade. This inverse relationship between rate performance and cycling stability is a common dilemma in battery material design. The larger surface area of the smaller particles (from high pH) enhances kinetics but also increases the interfacial contact area with the electrolyte. This larger interface promotes more extensive side reactions, electrolyte decomposition, and possibly accelerated surface degradation and transition metal dissolution, all of which contribute to faster capacity fading during extended cycling in a sodium-ion battery.
Differential capacity (dQ/dV) analysis provided further insight into the structural evolution during cycling. All samples showed a main redox couple around 3.3/3.1 V, corresponding to the Ni2+/Ni3+ redox and the associated reversible O3 to P3 phase transition. The voltage polarization between the anodic and cathodic peaks increased with cycle number for all materials, indicating rising internal resistance and kinetic hindrance. Crucially, the voltage gap growth was least severe for the pH 10.0 sample and most severe for the pH 10.6 sample. This quantifies the superior structural reversibility and interfacial stability of the larger-particle cathode, aligning perfectly with its better cycling performance.
Conclusion and Outlook
This systematic investigation elucidates the critical and multifaceted role of co-precipitation pH in determining the properties of Ni-Fe-Mn hydroxide precursors and the subsequent O3-NaNi0.4Fe0.2Mn0.4O2 cathode materials for sodium-ion batteries. The pH acts as a powerful knob to control the nucleation vs. growth balance, thereby dictating the secondary particle size. Higher pH (10.4-10.6) promotes nucleation, yielding smaller, denser precursor and cathode particles. This morphology leads to shortened ion-diffusion paths, lower charge-transfer resistance, and superior rate capability. Concurrently, the higher pH also induces a compositional shift, enriching the material in electrochemically active Ni, which boosts the initial specific capacity.
However, this optimization for kinetics and capacity comes at the cost of cycling stability. The increased surface area of smaller particles exacerbates interfacial side reactions with the electrolyte, leading to faster capacity fade. Conversely, synthesis at a moderately lower pH (10.0-10.2) produces larger secondary particles. While this modestly sacrifices rate performance, it significantly enhances the cycling stability by minimizing detrimental electrode-electrolyte interactions.
The findings underscore a fundamental trade-off in material design for sodium-ion batteries. There is no universal “best” pH; the optimal condition depends on the targeted application profile. For applications requiring high power and fast charging (e.g., power tools, certain EVs), a higher synthesis pH (~10.6) may be preferred to leverage the excellent rate capability. For large-scale stationary energy storage, where long cycle life and longevity are paramount, a lower pH (~10.0) yielding superior cycling stability would be the more strategic choice. This work provides clear guidance for tailoring the co-precipitation process to engineer cathode materials with desired performance characteristics, advancing the practical development of reliable and high-performance sodium-ion battery technology.