The pursuit of sustainable and large-scale energy storage systems has intensified the search for alternatives to the dominant lithium-ion technology. While commercially successful, concerns regarding lithium’s limited geographical distribution and long-term cost viability have prompted significant research into sodium-ion batteries. Sodium, belonging to the same alkali metal group as lithium, shares similar physicochemical properties, including a comparable standard electrode potential ($E^0_{Na+/Na} = -2.71$ V vs. SHE, $E^0_{Li+/Li} = -3.04$ V vs. SHE). Its abundance and lower cost make sodium-ion batteries a highly attractive candidate for stationary storage applications. Among various configurations, aqueous sodium-ion batteries, employing water-based electrolytes, offer enhanced safety, lower manufacturing costs, and higher ionic conductivity compared to their organic electrolyte counterparts, despite a narrower operational voltage window.
The performance of an aqueous sodium-ion battery is critically dependent on the cathode material. Manganese-based oxides, particularly those with tunnel structures, have shown great promise due to manganese’s low toxicity, natural abundance, and rich redox chemistry. One standout material is orthorhombic Na0.44MnO2 (or Na4Mn9O18), which possesses a unique crystal framework featuring large S-shaped tunnels and smaller pentagonal tunnels. This robust structure facilitates reversible insertion and extraction of sodium ions while mitigating detrimental phase transitions and structural strain during cycling, which is crucial for achieving long-term stability in an aqueous sodium-ion battery.
The synthesis pathway plays a definitive role in determining the material’s morphology, particle size, crystallinity, and ultimately, its electrochemical properties. While traditional methods like solid-state reaction and hydrothermal synthesis are common, the sol-gel technique offers distinct advantages for preparing cathode materials for sodium-ion batteries. It ensures excellent stoichiometric control and homogeneous mixing at the molecular level. However, a persistent challenge in conventional sol-gel processing is the severe agglomeration of particles during the thermal drying stage, where capillary forces pull particles together as the solvent evaporates. This agglomeration can hinder electrolyte penetration and sodium-ion diffusion, limiting the rate capability and cycle life of the sodium-ion battery.
In this work, we present a modified sol-gel synthesis strategy for Na0.44MnO2 that addresses this issue. We successfully integrated freeze-drying (lyophilization) technology to replace the conventional thermal drying step. The fundamental principle involves rapidly freezing the wet gel, thereby immobilizing the nascent particle network in solid ice. Subsequent sublimation of the ice under vacuum removes the solvent without creating the liquid-vapor interfaces that cause capillary stress. This process is anticipated to preserve a more open and less agglomerated morphology. We conducted a comprehensive comparative study between materials synthesized via this freeze-drying-assisted sol-gel method (FD-SG) and those from the traditional thermal-drying-assisted sol-gel method (TD-SG). The structural, morphological, and electrochemical properties were rigorously characterized to elucidate the impact of the drying technique on the performance of this promising cathode material for aqueous sodium-ion batteries.
Experimental Methodology: Synthesis and Cell Assembly
The synthesis of Na0.44MnO2 commenced with a citric acid-assisted sol-gel process. Stoichiometric amounts of sodium nitrate (NaNO3) and manganese(II) acetate tetrahydrate (Mn(CH3COO)2·4H2O) were dissolved in deionized water. This solution was then added dropwise into a 2 mol/L aqueous solution of citric acid, which acts as a complexing agent, under continuous stirring. The pH of the mixture was adjusted to approximately 6.5 using aqueous ammonia. The resulting solution was heated at 80°C with constant stirring for several hours until a viscous gel formed.
Critical Drying Step:
For the Freeze-Drying (FD-SG) sample, the wet gel was rapidly frozen in liquid nitrogen and then transferred to a freeze-dryer. The ice was sublimated under a vacuum (below 10 Pa) for over 48 hours to obtain a dry, fluffy precursor powder.
For the Thermal-Drying (TD-SG) sample, the wet gel was simply placed in a conventional oven at 120°C for 12 hours to evaporate the water, resulting in a denser, crusted precursor.
Both dried precursors were then subjected to a two-stage calcination process in a muffle furnace. They were first pre-calcined at 500°C for 4 hours to decompose organic components and nitrates, followed by grinding and final sintering at 850°C for 10 hours in static air to crystallize the Na0.44MnO2 phase.
The electrochemical evaluation was performed using a hybrid aqueous electrolyte. The cathode slurry was prepared by mixing the active material (Na0.44MnO2), conductive carbon (acetylene black), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1, using N-methyl-2-pyrrolidone (NMP) as the solvent. The slurry was coated onto an aluminum foil current collector and dried. CR2032-type coin cells were assembled in an ambient atmosphere using a zinc metal foil as the anode, a glass fiber separator, and an aqueous electrolyte comprising 1 mol/L Na2SO4, 0.5 mol/L ZnSO4, and 0.05 mol/L MnSO4. The MnSO4 additive is known to suppress manganese dissolution from the cathode in aqueous systems. Galvanostatic charge-discharge tests were conducted within a voltage window of 0.5–2.0 V versus Zn2+/Zn. Cyclic voltammetry (CV) was performed at a scan rate of 0.1 mV/s over the same voltage range. The specific capacity was calculated based on the mass of the Na0.44MnO2 cathode active material.
Structural and Morphological Characterization
X-ray diffraction (XRD) analysis confirmed the successful synthesis of the target phase. The XRD patterns for both the FD-SG and TD-SG samples are presented below. All major diffraction peaks can be indexed to the orthorhombic structure of Na0.44MnO2 (space group Pbam, PDF#27-0750). No detectable impurity peaks were observed, indicating high phase purity. The sharp and intense peaks suggest good crystallinity for both materials. A subtle but notable difference lies in the relative intensity and full width at half maximum (FWHM) of certain peaks. The FD-SG sample often exhibits slightly sharper peaks, which may be correlated with more uniform crystallite size or reduced micro-strain.
The most striking differences between the two synthesis methods are revealed through electron microscopy. Scanning Electron Microscopy (SEM) images provide a clear visual contrast. The TD-SG sample exhibits a typical morphology of rod-like particles, but with a high degree of agglomeration and fusion at the contacts. The rods are not well-separated, forming dense clusters. In contrast, the FD-SG sample displays remarkably well-dispersed, uniform nanorods. The individual rods are clearly separated, with minimal point contact between them, creating a highly porous network. This open structure is a direct consequence of the freeze-drying process, which prevents the collapse of the gel network and the coalescence of particles during solvent removal.
Transmission Electron Microscopy (TEM) offers further insight. The FD-SG nanorods show a regular, rectangular cross-section with smooth surfaces and uniform width along their length, typically around 400 nm in diameter. Selected Area Electron Diffraction (SAED) patterns confirm their single-crystalline nature. The TD-SG rods, however, often appear less uniform, with irregular surfaces, visible cracks, or fused segments, attributable to the stresses experienced during conventional thermal drying.
| Sample | Primary Morphology | Particle Size (Width) | Agglomeration Level | Structural Defects (from TEM) |
|---|---|---|---|---|
| FD-SG (Freeze-Dried) | Well-dispersed, uniform nanorods | ~400 nm | Very Low | Minimal; smooth surfaces |
| TD-SG (Thermal-Dried) | Agglomerated, fused rod clusters | Broad distribution | High | Present; cracks, fused segments |
Electrochemical Performance Analysis
The electrochemical behavior of the two materials as cathodes in an aqueous sodium-ion battery was systematically investigated. Cyclic voltammetry (CV) curves for the initial cycles provide information on redox processes and reversibility. Both samples exhibit characteristic redox peaks corresponding to the insertion/extraction of Na+ ions within the tunnel structure of Na0.44MnO2. During the anodic scan, a major oxidation peak appears around 1.57 V (vs. Zn2+/Zn), associated with the extraction of Na+ ions. The cathodic scan shows two reduction peaks at approximately 1.38 V and 1.20 V, corresponding to the stepwise re-insertion of Na+ ions. The FD-SG sample demonstrates superior peak sharpness and higher peak current intensities compared to the TD-SG sample, indicating faster reaction kinetics and higher electrochemical activity. Furthermore, the CV profiles for the FD-SG sample show better overlap from the 2nd cycle onward, suggesting excellent cycling reversibility, a critical attribute for a durable sodium-ion battery.
Galvanostatic charge-discharge profiles at various current rates (C-rates) reveal significant differences in performance. The theoretical capacity based on one electron transfer per Mn (for Na0.44MnO2 ≈ Na0.44MnO2) is approximately 121 mAh/g. However, in practice, higher capacities can be accessed through additional mechanisms. The initial discharge capacity at a low rate (e.g., 0.1C) for the FD-SG sample often exceeds this value, demonstrating its high utilization. The voltage profiles feature the characteristic plateaus consistent with the CV peaks.
The rate capability, a key metric for high-power applications of a sodium-ion battery, is dramatically enhanced for the FD-SG material. As the current density is progressively increased, the FD-SG cathode retains a much higher fraction of its low-rate capacity compared to the TD-SG cathode. For instance, at a rate of 2C (where 1C is defined as the current required to charge/discharge the theoretical capacity in one hour), the FD-SG sample delivered 125 mAh/g, while the TD-SG sample delivered only 101.6 mAh/g. This enhanced rate performance is a direct benefit of the open, non-agglomerated morphology, which ensures short solid-state diffusion lengths for Na+ ions and excellent liquid-phase electrolyte access to the entire active surface area. The relationship between capacity retention and current density can be modeled using empirical formulas for capacity fade, often observing a logarithmic dependence:
$$ C_{rate} = C_0 – k \cdot \log(i) $$
where $C_{rate}$ is the capacity at a given current $i$, $C_0$ is the low-rate capacity, and $k$ is a kinetics-related parameter that is smaller for the FD-SG material.
| Performance Metric | FD-SG Sample | TD-SG Sample |
|---|---|---|
| Initial Discharge Capacity (1C) | 170.0 mAh/g | 123.7 mAh/g |
| Capacity at 4C | 71.4 mAh/g | 47.6 mAh/g |
| Capacity Retention after 30 cycles (1C) | 94.5% | 69.3% |
| Approx. Na-ion Diffusion Coefficient (DNa+)* | ~10-12 cm2/s | ~10-13 cm2/s |
*Estimated from CV or EIS data, indicating faster kinetics for FD-SG.
The long-term cycling stability at a fixed current rate further underscores the advantage of the freeze-drying approach. The FD-SG cathode exhibits outstanding capacity retention over extended cycling. After 30 cycles at 1C, it retained 163.5 mAh/g, corresponding to a high capacity retention of 94.5% from its maximum stabilized value. In stark contrast, the TD-SG cathode suffered from rapid capacity decay, retaining only 87.6 mAh/g (69.3% retention). The stable framework of the Na0.44MnO2 structure, combined with the robust, strain-tolerant morphology of the FD-SG rods, effectively buffers the volume changes associated with Na+ (de)insertion. This minimizes mechanical degradation and particle isolation over repeated cycles, which is a common failure mode in sodium-ion battery electrodes. The capacity (C) as a function of cycle number (N) for the TD-SG sample often follows a faster decay trend, which can be approximated by:
$$ C_N = C_1 \cdot e^{-\alpha N} $$
where $\alpha$ is the fade coefficient, which is significantly larger for the TD-SG material.
Discussion: Mechanism of Performance Enhancement
The superior electrochemical performance of the FD-SG synthesized Na0.44MnO2 can be attributed to a synergistic combination of favorable morphological and structural factors, all stemming from the freeze-drying process.
1. Enhanced Ionic and Electronic Transport: The open, porous network of well-separated nanorods drastically reduces the tortuosity for ion transport within the electrode. The short diffusion length within the nanorods themselves (small particle size) and the unimpeded pathways between them lower the overall resistance for Na+ diffusion. This is quantified by a higher apparent diffusion coefficient ($D_{Na+}$), as inferred from electrochemical impedance spectroscopy (EIS) or CV analysis. The improved electronic percolation due to better mixing with conductive carbon in a less agglomerated structure also contributes to lower polarization.
2. Maximized Electroactive Surface Area and Full Electrolyte Access: The non-agglomerated state ensures that nearly the entire geometric surface area of the active material is accessible to the aqueous electrolyte. This maximizes the interface where the charge-transfer reaction occurs, leading to higher specific capacity, especially at high rates. The relationship between accessible surface area (A) and current (I) in a kinetically mixed-controlled process can be described by:
$$ I = nFAk^0 C^{1-\alpha} (C’)^\alpha $$
where $n$ is electron number, $F$ is Faraday’s constant, $k^0$ is the standard rate constant, $C$ and $C’$ are concentrations, and $\alpha$ is the charge transfer coefficient. A larger effective $A$ for FD-SG directly results in a higher current (capacity) for the same overpotential.
3. Mechanical Integrity and Strain Accommodation: The individual, crystalline nanorods possess intrinsic strength. Their independent nature allows them to expand and contract slightly during cycling without generating large shear stresses that would occur in a densely sintered agglomerate. This local strain accommodation prevents crack propagation and the loss of electrical contact, which is the primary reason for the exceptional cycling stability of the FD-SG cathode in this aqueous sodium-ion battery system.
4. Mitigation of Manganese Dissolution: While common in aqueous systems, manganese dissolution from the cathode is a major degradation mechanism. The highly crystalline nature and low defect concentration of the FD-SG rods, as observed by TEM, provide a more stable surface that is less prone to Mn2+ leaching into the electrolyte. The added MnSO4 in the electrolyte further helps suppress this by common-ion effect, but a robust starting material is fundamental.
Conclusion and Perspectives
In summary, this work demonstrates a highly effective synthesis strategy for optimizing the performance of tunnel-structured Na0.44MnO2 as a cathode material for aqueous sodium-ion batteries. By integrating freeze-drying technology into the sol-gel process, we successfully circumvent the agglomeration problem inherent in conventional thermal drying. The resulting material exhibits a well-defined, porous architecture composed of uniform, single-crystalline nanorods. This tailored morphology confers exceptional electrochemical properties, including high specific capacity, remarkable rate capability, and outstanding cycling stability, surpassing the performance of its thermally-dried counterpart.
The findings highlight the critical importance of controlling nano/microstructure through innovative synthesis in advancing electrode materials for next-generation energy storage. The freeze-drying-assisted sol-gel method presented here is a versatile and scalable technique that can be extended to other oxide-based electrodes for sodium-ion batteries and beyond. Future work could focus on further optimizing the freeze-drying parameters (freezing rate, annealing temperature), exploring cation doping (e.g., Ti, Mg, Al) to enhance the intrinsic electronic conductivity and structural stability of Na0.44MnO2, and assembling full aqueous sodium-ion cells with compatible anode materials to evaluate practical energy density. The continued development of such high-performance, safe, and low-cost aqueous sodium-ion battery systems holds significant promise for grid-scale energy storage and other large-scale applications.