The escalating demand for large-scale energy storage systems has underscored the limitations of lithium-ion batteries (LIBs), primarily due to resource scarcity and cost concerns. In this context, sodium-ion batteries (SIBs) have emerged as a highly promising alternative, leveraging the natural abundance of sodium and exhibiting working principles analogous to LIBs. The cathode material is a pivotal component in SIBs, governing the cell voltage and providing the source of reversible Na+ ions. Among various candidates, layered transition metal oxides stand out due to their straightforward synthesis and high theoretical capacity. Specifically, O3-type NaNi0.5Mn0.5O2 has attracted significant attention as a premier cathode candidate for commercial SIBs, offering a stable Mn4+ framework and a high capacity derived from the two-electron redox of Ni2+/Ni4+. However, its practical application is hampered by severe structural degradation during cycling, originating from intricate phase transitions (O3hex. → O3′mon. → P3hex. → P’3mon. → P3′hex. → O3′hex.) upon Na+ extraction/insertion, leading to rapid capacity fading. Therefore, developing effective strategies to stabilize the structure of O3-NaNi0.5Mn0.5O2 is of paramount importance for advancing sodium-ion battery technology.
This work focuses on mitigating these challenges through cationic doping. While various active and inactive elements have been explored, the effect of bismuth (Bi) doping on the performance of O3-type Ni/Mn-based oxides remains unclear. Herein, we report a systematic investigation on Bi-doped O3-NaNi0.5-xMn0.5BixO2 (x = 0, 0.003, 0.005, 0.010, 0.030) cathode materials synthesized via a high-energy ball milling assisted solid-state reaction. The influence of Bi substitution on the crystal structure, morphology, and electrochemical properties is thoroughly examined, revealing that optimal Bi doping effectively refines grain size, expands Na+ diffusion pathways, suppresses detrimental phase transitions, and consequently enhances the cycling stability and rate capability of this layered oxide cathode for sodium-ion batteries.
Experimental Section
Material Synthesis
A series of Bi-doped layered oxides with the nominal composition O3-NaNi0.5-xMn0.5BixO2 (x = 0, 0.003, 0.005, 0.010, 0.030) were prepared. Stoichiometric amounts of NiO, MnO2, and Bi2O3 were mixed with a 3 wt% excess of Na2CO3·H2O (to compensate for sodium volatility during calcination). The mixture was subjected to high-energy ball milling in zirconia media with anhydrous ethanol at 500 rpm for 10 hours. The obtained slurry was dried and subsequently calcined in air at 900°C for 10 hours with a heating rate of 5°C/min to obtain the final cathode powders.
Material Characterization
The actual chemical composition was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The crystal structure was analyzed by X-ray diffraction (XRD) with Rietveld refinement performed using the GSAS software. The morphology and elemental distribution were observed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The surface chemical states were probed by X-ray photoelectron spectroscopy (XPS).
Electrochemical Measurements
Electrode slurries were prepared by mixing the active material, conductive carbon black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 8:1:1 within N-methyl-2-pyrrolidone (NMP) solvent. The slurry was cast onto aluminum foil, dried, and punched into circular electrodes. CR2032 coin cells were assembled in an argon-filled glove box using sodium metal as the counter/reference electrode, a glass fiber separator, and an electrolyte of 1 M NaClO4 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) with 1 vol% fluoroethylene carbonate (FEC). Galvanostatic charge/discharge tests were conducted within a voltage window of 2.0–4.0 V vs. Na+/Na on a LAND battery tester. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 100 kHz to 0.1 Hz. The Na+ diffusion coefficient was evaluated by the galvanostatic intermittent titration technique (GITT), applying a current pulse of 5 mA/g for 30 min followed by a 2-hour relaxation period.
Results and Discussion
Structural and Morphological Characteristics
The chemical compositions of all synthesized samples, as confirmed by ICP-OES, align well with their nominal formulas, indicating successful synthesis with precise control over Bi doping levels. The XRD patterns of all samples are presented below. All major diffraction peaks can be indexed to a hexagonal α-NaFeO2 structure (space group R$\bar{3}$m), confirming the formation of the desired O3-type phase. A closer inspection reveals that the (003) peak, representative of the interlayer spacing along the c-axis, systematically shifts to lower angles with increasing Bi content. This shift suggests an expansion of the lattice, which is attributed to the substitution of larger Bi3+ ions (ionic radius: 1.03 Å for CN=6) for smaller Ni2+ (0.69 Å) or Mn4+ (0.53 Å) ions in the transition metal (TM) layers.
To quantify the structural changes, Rietveld refinement was performed. The refined lattice parameters are summarized in Table 1. A clear trend of increasing lattice parameters a and c is observed upon Bi doping. Notably, for the optimally doped sample (x=0.005), the c-axis parameter increases from 1.60139 nm to 1.60648 nm. Consequently, the interslab distance (dNa), which is the spacing between adjacent oxygen layers encompassing the Na layer, also increases. This expansion creates wider diffusion channels for Na+ ions, which is expected to enhance ionic mobility—a critical factor for the rate performance of a sodium-ion battery cathode.
| Sample (x) | a = b (nm) | c (nm) | dNa (nm) | Rwp (%) |
|---|---|---|---|---|
| 0 (NaNM) | 0.296228 | 1.601390 | 0.324292 | 3.869 |
| 0.003 | 0.296720 | 1.601410 | 0.326960 | 7.498 |
| 0.005 | 0.296830 | 1.606480 | 0.326746 | 5.928 |
| 0.010 | 0.296850 | 1.608450 | 0.327476 | 5.232 |
| 0.030 | 0.296880 | 1.602006 | 0.327684 | 5.557 |
The morphological evolution induced by Bi doping was examined by SEM. The pristine NaNi0.5Mn0.5O2 (NaNM) exhibits large, plate-like particles with widths of 4–5 μm. In striking contrast, the Bi-doped sample (x=0.005, labeled NaNMB-0.5) shows significantly refined grains with widths of 2–3 μm, despite being synthesized under identical conditions. This pronounced grain refinement suggests that the incorporation of Bi alters the surface energy during the crystal growth process, thereby regulating the grain growth orientation. Smaller particle sizes shorten the diffusion path length for both Na+ and electrons, which is highly beneficial for achieving high-rate performance in a sodium-ion battery. EDS mapping confirms the homogeneous distribution of Na, Ni, Mn, Bi, and O elements throughout the NaNMB-0.5 particles.
Chemical State Analysis
XPS was employed to investigate the influence of Bi doping on the oxidation states of the transition metal ions. The Bi 4f spectrum displays two peaks at binding energies of 157.6 eV and 163.0 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively, confirming the presence of Bi3+. Detailed fitting of the Ni 2p and Mn 2p spectra reveals subtle but important changes in the Ni and Mn valence states, as quantified in Table 2. With Bi doping, the proportion of Ni3+ increases at the expense of Ni2+. Similarly, the concentration of Mn3+ shows a slight increase relative to Mn4+. This charge redistribution can be explained by charge compensation mechanisms and possible electron exchange between Ni and Mn sites. The presence of Ni3+ is known to promote more uniform Na+ distribution within the layers, facilitating smoother Na+ (de)intercalation. Meanwhile, Mn3+ can contribute additional capacity through the Mn3+/Mn4+ redox couple, although excessive amounts may trigger Jahn-Teller distortion.
| Sample (x) | Ni2+ (%) | Ni3+ (%) | Mn3+ (%) | Mn4+ (%) |
|---|---|---|---|---|
| 0 | 78.43 | 21.57 | 19.72 | 80.28 |
| 0.003 | 75.27 | 24.73 | 20.72 | 79.28 |
| 0.005 | 71.87 | 28.13 | 26.92 | 73.08 |
| 0.010 | 68.75 | 31.25 | 28.29 | 71.71 |
| 0.030 | 64.28 | 35.72 | 32.26 | 67.74 |
Electrochemical Performance Evaluation
The electrochemical performance of the materials as cathodes for sodium-ion batteries was evaluated. Figure 1 shows the initial charge/discharge profiles at 0.2 C (1 C = 100 mA/g). The undoped NaNM delivers a first-cycle discharge capacity of 128.1 mAh/g. Upon Bi doping, the capacity first increases, reaching a maximum of 138.1 mAh/g for the NaNMB-0.5 (x=0.005) sample, before decreasing with higher doping levels (x=0.01, 0.03). The charge/discharge curves of the Bi-doped samples, particularly NaNMB-0.5, appear smoother in the high-voltage region (>3.0 V) compared to the undoped material, which displays more pronounced voltage plateaus associated with complex phase transitions. This smoothing effect indicates that Bi doping effectively suppresses the sharp phase transitions in the O3-type structure during Na+ extraction, a key to improving structural reversibility.
The cycle stability was tested at 1 C. After 100 cycles, the capacity retention rates for the samples with x = 0, 0.003, 0.005, 0.010, and 0.030 are 63%, 85%, 91%, 87%, and 82%, respectively. The NaNMB-0.5 sample demonstrates superior cycling performance, retaining 91% of its initial capacity. This enhancement is attributed to the synergistic effects of Bi doping: the expanded lattice facilitates Na+ transport and reduces mechanical strain, while the suppressed multiphase evolution preserves the structural integrity of the layered cathode throughout repeated cycles.
The rate capability, a critical metric for high-power sodium-ion battery applications, is presented in Figure 2. The NaNMB-0.5 cathode exhibits outstanding performance across various current densities. It delivers 138.1 mAh/g at 0.2 C, and maintains 87.8 mAh/g even at a very high rate of 10 C. In contrast, the capacity of the undoped cathode decays rapidly with increasing current density. This remarkable rate capability stems from the refined particle size (shorter diffusion length) and widened Na+ diffusion channels (higher ionic conductivity). Furthermore, the long-term cycling performance at a high rate of 5 C was evaluated. The NaNMB-0.5 electrode demonstrates exceptional stability, with a capacity retention of 97% after 100 cycles, significantly outperforming the undoped material (88% retention). This result underscores the effectiveness of Bi doping in stabilizing the O3 structure under high-current operating conditions.
Kinetic Analysis
To gain deeper insight into the improved kinetics, EIS and GITT measurements were conducted. The Nyquist plots of cells after three cycles show that the NaNMB-0.5 electrode possesses a smaller charge-transfer resistance (Rct) compared to the undoped and other doped samples. This lower resistance correlates with faster electrode reaction kinetics, consistent with its superior rate performance. The Na+ chemical diffusion coefficient (DNa+) was calculated from GITT data using the following equation:
$$
D_{Na^+} = \frac{4}{\pi \tau} \left( \frac{m_B V_M}{M_B S} \right)^2 \left( \frac{\Delta E_s}{\Delta E_\tau} \right)^2
$$
where $\tau$ is the current pulse duration, $m_B$ and $M_B$ are the mass and molar mass of the active material, $V_M$ is the molar volume, $S$ is the electrode/electrolyte contact area, $\Delta E_s$ is the steady-state voltage change, and $\Delta E_\tau$ is the voltage change during the current pulse. The calculated DNa+ values for NaNMB-0.5 are consistently higher than those for the pristine NaNM throughout the tested potential range. For instance, in the mid-sodiation state, DNa+ for NaNMB-0.5 is on the order of 10-11 cm2/s, approximately one order of magnitude larger than that of the undoped sample (~10-12 cm2/s). This quantitatively confirms that Bi doping significantly enhances the Na+ diffusion kinetics within the layered oxide structure, which is fundamental to the improved electrochemical performance of this sodium-ion battery cathode.
Conclusion
In summary, a series of Bi-doped O3-type NaNi0.5-xMn0.5BixO2 layered oxides have been successfully synthesized and evaluated as high-performance cathode materials for sodium-ion batteries. The incorporation of Bi3+ induces several beneficial effects: (i) it refines the particle morphology by modifying surface energy during growth, shortening ionic/electronic transport paths; (ii) it expands the lattice parameters, especially the interslab spacing, thereby providing wider diffusion channels for Na+ ions; (iii) it modifies the local electronic structure, increasing the Ni3+ content which favors homogeneous Na+ (de)intercalation; and (iv) it effectively suppresses the detrimental multi-step phase transitions that plague the undoped material during cycling. As a result, the optimally doped sample (NaNi0.495Mn0.5Bi0.005O2) exhibits a high reversible capacity of 138.1 mAh/g at 0.2 C, excellent rate capability (87.8 mAh/g at 10 C), and outstanding cycling stability (91% retention at 1 C and 97% retention at 5 C over 100 cycles). This work elucidates the positive role of Bi doping and provides a simple yet effective strategy for developing stable, high-rate O3-type layered cathode materials, contributing to the advancement of practical sodium-ion battery technology.