Sodium-ion batteries (SIBs) have attracted much attention as a promising alternative to lithium-ion batteries due to the abundance and low cost of sodium resources. Among various cathode materials, layered oxide NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) offers some advantages such as environmental friendliness, low toxicity, and high theoretical capacity. However, its practical application is severely hindered by structural instability, poor cycling stability, and inferior rate capability, which originate mainly from undesirable

electrode-electrolyte interfacial side reactions, irreversible phase transitions during Na⁺ insertion/extraction, and sluggish Na⁺diffusion kinetics. Surface coating is considered as an effective strategy to address these issues. In particular, Al₂O₃ coatings can suppress parasitic reactions and stabilize the layered structure. Nevertheless, conventional coating methods often involve complex, multi-step processes such as atomic layer deposition (ALD) or post-annealing, which are costly and less scalable. In this work, we developed a facile one-step wet-chemical route combined with solid-state calcination to obtain an Al₂O₃ coating on NFM. The aim was to simultaneously enhance the cycling stability and rate capability via forming a protective layer, while enabling partial Al³⁺doping into the near-surface lattice. In addition, the underlying mechanism was also investigated.

The pristine NFM material was synthesized via mixing Ni_1/3Fe_1/3Mn_1/3(OH)₂ precursor with Na₂CO₃ (molar ratio 1.000∶0.525), followed by pre-sintering at 500 ℃ for 5 h, and then calcined in O₂ atmosphere at 850 ℃ for 15 h. For Al₂O₃ coating, different molar fractions (0.5%, 1.0%, and 2.0%) of Al(NO₃)₃·9H₂O were dissolved in ethanol, and the precursor was added. The mixture was stirred, ultrasonicated, and then dried at 70 ℃ and then at 120 ℃. The obtained Al-precoated precursor was calcined under the same conditions as the pristine NFM, obtaining the samples denoted as NFM-0.5A, NFM-1A, and NFM-2A, respectively.

CR2032 coin-type half-cells were assembled in an argon-filled glove box (Mikrouna Super 1220/750/900, China) using sodium metal as a counter electrode, glass fiber as a separator, and 1.0 mol/L NaClO₄ in ethylene carbonate (EC)/propylene carbonate (PC) (1∶1 by volume) with 5% (in mass fraction) fluoroethylene carbonate (FEC) as an electrolyte. The working electrode consisted of 80% (in mass fraction) active material, 10% (in mass fraction) carbon black, and 10% (in mass fraction) polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP, ca. 1.0-1.5 mL).

The crystal structure was characterized by X-ray diffraction (XRD) with Rietveld refinement. The surface morphology and element distribution were determined by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS). The electrochemical performance (i.e., the cycling stability at various rates (i.e., 1, 2, and 5 C) and the rate capability from 0.1 C to 10.0 C) were evaluated by a battery test system. The electrochemical impedance spectroscopy (EIS) was performed at 0.01-100.00 kHz with an amplitude of 5 mV. The Na⁺ diffusion coefficient was determined by galvanostatic intermittent titration technique (GITT).

The XRD patterns of pristine NFM and NFM-1A show no impurity peaks, both exhibiting an O3-type α-NaFeO₂ layered structure with a space group . The peaks (003) and (104) of NFM-1A shift slightly, and the Rietveld refinement reveals increased lattice parameters a, c, and cell volume (from 2.980 Å, 15.960 Å, and 122.755 ų for NFM to 2.983 Å, 15.988 Å, and 123.160 ų for NFM-1A), indicating partial Al³⁺ doping into the near-surface lattice, which expands the interlayer spacing and facilitates Na⁺ transport.

The SEM images show that NFM-1A and NFM-2A have a uniform coating layer. The TEM images and EDS spectra indicate a distinct Al₂O₃ coating layer with a thickness of approximately 15 nm on the NFM surface, and a small amount of Al³⁺ diffuses into the shallow lattice, forming strong Al-O bonds. The XPS analysis further shows the presence of Al₂O₃ on the surface, as evidenced by the Al 2p peak at 73.08 eV. The binding energies of Ni 2p, Fe 2p, and Mn 2p in NFM-1A shift slightly to lower values, compared with pristine NFM, which is attributed to a charge compensation from Al³⁺ incorporation, thus stabilizing the structure.

The results of electrochemical tests demonstrate that the NFM-1A electrode exhibits the optimum overall performance. The initial discharge capacities at 0.1 C are 140.53, 143.34, 145.99 mA·h·g^-1, and 140.48 mA·h·g^-1 for NFM, NFM-0.5A, NFM-1A, and NFM-2A, respectively, with the corresponding Coulombic efficiencies of 88.67%, 86.06%, 90.78%, and 87.62%. After 150 cycles at 1 C, the discharge capacities are 91.83, 104.46, 113.38 mA·h·g^-1, and 99.80 mA·h·g^-1, with the capacity retentions of 68.62%, 77.14%, 80.77%, and 77.16%, respectively. At a higher rate of 2 C, NFM-1A retains 82.77% of its initial capacity after 150 cycles (112.42 mA·h·g^-1), and at 5 C, it retains 73.31% after 300 cycles (92.20 mA·h·g^-1). The results of rate capability test show that NFM-1A delivers a discharge capacity of 98.0 mA·h·g^-1 at 10 C, far exceeding that of pristine NFM (56.26 mA·h·g^-1). When the rate is 0.1 C, NFM-1A recovers 92.1% of its initial capacity (137.16 mA·h·g^-1), demonstrating the excellent structural reversibility.

The GITT measurements reveal that the average Na⁺ diffusion coefficient of NFM-1A (1.20×10^-10 cm²/s) is approximately twice greater than that of pristine NFM (5.89×10^-11 cm²/s), confirming that the Al₂O₃ coating with partial Al³⁺ doping broadens the Na⁺transport channels. The EIS analysis shows that after 150 cycles, the charge transfer resistance (R_ct) of NFM-1A (347.40 Ω) is significantly lower than that of pristine NFM (544.48 Ω), and the conductive interface resistance (R_f) is also reduced (262.80 Ω vs. 344.39 Ω). The post-cycling XRD patterns and SEM images indicate that NFM-1A maintains better crystallinity and intact spherical particle morphology without cracks, whereas pristine NFM exhibits severe particle fragmentation. These results confirm that the Al₂O₃ coating effectively suppresses interfacial side reactions, stabilizes the layered structure, and preserves the integrity of secondary particles during prolonged cycling.

An Al₂O₃-coated NFM cathode material was prepared by a one-step wet-chemical method and subsequent solid-state calcination. The optimal coating amount was 1% (in mole fraction) (NFM-1A). The Al₂O₃ coating acted as a physical barrier to suppress electrode-electrolyte interfacial side reactions and, simultaneously, partial Al³⁺ doping into the near-surface lattice formed strong Al-O bonds, expanded the interlayer spacing, and enhanced Na⁺ diffusion kinetics. Consequently, NFM-1A exhibited significantly improved cycling stability (i.e., retention after 150 cycles of 80.77% at 1 C, 82.77% at 2 C, and 73.31% at 5 C) and outstanding rate capability (i.e., 98.0 mA·h·g^-1 at 10 C with capacity recovery of 92.1%). This work could provide a simple and scalable strategy for developing high-performance layered oxide cathodes for sodium-ion batteries.