In recent years, the rapid growth of energy consumption has spurred an urgent demand for efficient, long-lasting, safe, and low-cost renewable energy storage systems. Among various energy storage technologies, rechargeable batteries stand out as the most effective means to store electrical energy and power electronic devices. While lithium-ion batteries (LIBs) have become a mature commercial technology, their widespread application in large-scale energy storage is hindered by the uneven distribution, scarcity, and high cost of lithium resources. Consequently, there is a pressing need for alternatives that are abundant, affordable, and capable of meeting the demands of large-scale deployment. Sodium-ion batteries (SIBs) have emerged as a promising candidate due to the abundance of sodium, its low cost, and a “rocking-chair” working mechanism similar to that of LIBs. The performance of SIBs largely depends on the cathode materials, which include layered transition metal oxides, polyanionic compounds, and Prussian blue analogues. Among these, layered transition metal oxides, particularly P2-type NaxTMO2 (where TM represents transition metals such as Ni, Mn, Cu, etc.), have attracted significant attention due to their high theoretical capacity, simple structure, and excellent ionic conductivity. However, P2-type materials like Na0.67Ni0.33Mn0.67O2 (NNM) face challenges such as phase transitions (e.g., P2 to O2) at high voltages, leading to rapid capacity decay. To address these issues, strategies like ion doping have been explored. In this study, we focus on Cu doping as a means to enhance the electrochemical performance of NNM cathodes for sodium-ion batteries. We prepared a series of Cu-doped P2-type Na0.67Ni0.33−xCuxMn0.67O2 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25) materials via a sol-gel method and investigated their structural, morphological, and electrochemical properties. Our goal is to develop high-performance cathode materials that offer improved cycling stability and rate capability for sodium-ion battery applications.
The sol-gel method was employed to synthesize the Cu-doped materials. Starting materials, including sodium acetate trihydrate (with a 5% excess to compensate for sodium loss during high-temperature sintering), nickel acetate tetrahydrate, copper acetate monohydrate, and manganese acetate tetrahydrate, were dissolved in deionized water according to the stoichiometric ratios. Citric acid monohydrate was used as a chelating agent. The mixture was stirred, evaporated at 80°C to form a wet gel, dried at 120°C for 24 hours, and then pre-calcined at 500°C for 5 hours. The resulting powder was ground and sintered at 900°C for 12 hours in air to obtain the final products. The materials were labeled as NNMCux, where x represents the Cu doping amount. For characterization, we used field-emission scanning electron microscopy (FESEM) to examine morphology, X-ray diffraction (XRD) to analyze crystal structure, energy-dispersive X-ray spectroscopy (EDS) to determine elemental composition, and X-ray photoelectron spectroscopy (XPS) to assess oxidation states. Electrochemical tests were conducted by assembling CR2032 coin cells with the prepared materials as cathodes, sodium metal as the anode, and a glass fiber separator. The electrolyte consisted of 1 M NaClO4 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 5% fluoroethylene carbonate (FEC). Galvanostatic charge-discharge tests were performed within a voltage range of 2.0–4.3 V, and cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out to evaluate reaction kinetics and interfacial properties.
The XRD patterns of the NNMCux materials are shown in Figure 1. All samples exhibited diffraction peaks consistent with the standard P2-type Na0.67Ni0.33Mn0.67O2 (JCPDS No. 54-0894), confirming a hexagonal crystal system with space group P63/mmc. The sharp peaks indicated high crystallinity. With increasing Cu doping, the (002) diffraction peak gradually shifted to lower angles, suggesting an expansion of the interlayer spacing. This can be attributed to the larger ionic radius of Cu2+ (0.73 Å) compared to Ni2+ (0.69 Å). The lattice parameters were refined using Rietveld analysis, and the results are summarized in Table 1. For NNMCu0.15, the lattice constants a and c increased to 2.8909 Å and 11.1747 Å, respectively, compared to 2.8873 Å and 11.1609 Å for the undoped NNMCu0. The expansion of the lattice provides wider ion channels for Na+ diffusion, which is beneficial for enhancing the kinetics in sodium-ion battery electrodes. The interplanar spacing dhkl can be calculated using the formula for hexagonal crystals:
$$ \frac{1}{d_{hkl}^2} = \frac{4}{3} \left( \frac{h^2 + hk + k^2}{a^2} \right) + \frac{l^2}{c^2} $$
where h, k, and l are Miller indices. The increase in d-spacing with Cu doping facilitates faster Na+ transport, a critical factor for improving the rate performance of sodium-ion batteries.
| Sample | a (Å) | c (Å) | Volume (Å3) | Rwp (%) | Gof |
|---|---|---|---|---|---|
| NNMCu0 | 2.8873 | 11.1609 | 80.5803 | 6.84 | 1.69 |
| NNMCu0.15 | 2.8909 | 11.1747 | 80.8782 | 5.46 | 1.56 |
Table 1: Lattice parameters from Rietveld refinement for undoped and Cu-doped materials.
Morphological analysis via FESEM revealed that all materials consisted of hexagonal platelet-like particles with smooth surfaces. The undoped NNMCu0 showed sharp edges, while Cu doping led to slightly rounded particles, indicating modified crystal growth. The NNMCu0.15 sample exhibited the most uniform particle size distribution, whereas excessive doping (x ≥ 0.20) caused aggregation. EDS mapping confirmed the homogeneous distribution of Na, Ni, Mn, Cu, and O elements in the doped materials, verifying successful Cu incorporation into the NNM structure. This uniform distribution is crucial for ensuring consistent electrochemical performance across the electrode in sodium-ion batteries.
XPS was used to investigate the oxidation states of transition metals. The survey spectra confirmed the presence of Na, Ni, Mn, O, and Cu in the doped samples. High-resolution spectra for Ni 2p showed peaks at binding energies of 855.31 eV (Ni 2p3/2) and 872.89 eV (Ni 2p1/2), corresponding to Ni2+. The Mn 2p spectra exhibited peaks at 642.48 eV (Mn 2p3/2) and 654.0 eV (Mn 2p1/2), indicating a mixture of Mn3+ and Mn4+. For Cu-doped samples, the Cu 2p3/2 peak at 933.3 eV confirmed the presence of Cu2+. The introduction of Cu2+ not only alters the electronic structure but also contributes additional redox activity (Cu2+/Cu3+), which can enhance the capacity of sodium-ion battery cathodes.
The electrochemical performance of the NNMCux materials was evaluated through galvanostatic charge-discharge tests. Figure 2 presents the initial charge-discharge curves at 0.1 C (1 C = 170 mA g−1) within 2.0–4.3 V. The undoped NNMCu0 delivered an initial discharge capacity of 149.6 mAh g−1, but it showed rapid capacity fading due to the P2-O2 phase transition at high voltages. With Cu doping, the discharge capacities decreased slightly: 138.4, 130.5, 125.7, 116.5, and 101.6 mAh g−1 for x = 0.05, 0.10, 0.15, 0.20, and 0.25, respectively. However, the voltage plateaus associated with phase transitions became smoother, indicating that Cu doping suppressed the undesirable P2-O2 transformation. This suppression is vital for improving the cycling stability of sodium-ion battery electrodes.
The cycling performance at 0.1 C over 100 cycles is summarized in Table 2. The capacity retention rates after 100 cycles were 28.12%, 43.85%, 48.70%, 79.10%, 75.86%, and 55.76% for x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25, respectively. NNMCu0.15 exhibited the best performance, retaining 100.25 mAh g−1 (79.10% retention) from an initial 126.74 mAh g−1. In contrast, the undoped material retained only 42.1 mAh g−1. The enhanced cycling stability can be attributed to the stabilization of the P2 structure by Cu2+ doping, which inhibits Na+/vacancy ordering and reduces volume changes during charge-discharge cycles. This makes Cu-doped materials promising for long-life sodium-ion batteries.
| Cu Doping (x) | Initial Discharge Capacity (mAh g−1) | Capacity after 100 Cycles (mAh g−1) | Capacity Retention (%) |
|---|---|---|---|
| 0 | 149.6 | 42.1 | 28.12 |
| 0.05 | 138.4 | 60.7 | 43.85 |
| 0.10 | 130.5 | 63.5 | 48.70 |
| 0.15 | 125.7 | 100.25 | 79.10 |
| 0.20 | 116.5 | 88.4 | 75.86 |
| 0.25 | 101.6 | 56.7 | 55.76 |
Table 2: Electrochemical performance of Cu-doped NNMCux materials over 100 cycles at 0.1 C.
Rate capability tests were conducted at various current densities from 0.1 C to 2.0 C (Figure 3). NNMCu0.15 demonstrated superior rate performance, delivering capacities of 126.74, 115.32, 98.45, 80.12, and 65.34 mAh g−1 at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively. When the current was returned to 0.1 C, the capacity recovered to 101.98 mAh g−1, showing a retention of 80.46%. This excellent rate capability is linked to the expanded interlayer spacing and improved Na+ diffusion kinetics due to Cu doping. The diffusion coefficient of Na+ (DNa+) can be estimated from EIS data using the formula:
$$ D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where R is the gas constant, T is temperature, A is electrode area, n is the number of electrons, F is Faraday’s constant, C is Na+ concentration, and σ is the Warburg coefficient. EIS spectra showed that NNMCu0.15 had lower charge-transfer resistance (Rct = 266.26 Ω) compared to NNMCu0 (Rct = 408.16 Ω), indicating faster electrochemical reactions. The reduced impedance contributes to the enhanced rate performance, which is critical for high-power applications of sodium-ion batteries.
Cyclic voltammetry (CV) curves at a scan rate of 0.5 mV s−1 revealed redox peaks corresponding to Ni2+/Ni4+, Mn3+/Mn4+, and Cu2+/Cu3+ couples. For NNMCu0.15, the peaks were more stable over cycles, with minimal shift, confirming better reversibility. The integration of CV curves allows calculation of the specific capacity using the equation:
$$ Q = \frac{1}{mv} \int I \, dV $$
where Q is capacity, m is mass, v is scan rate, and I is current. The results aligned with galvanostatic data, further validating the improved electrochemical stability. The suppression of phase transitions in Cu-doped materials was evident from the diminished peaks at high voltages (above 4.1 V), which are associated with the P2-O2 transformation. This suppression is key to prolonging the cycle life of sodium-ion battery cathodes.
To understand the impact of Cu doping on structural stability, we performed ex-situ XRD on cycled electrodes. The NNMCu0.15 electrode maintained the P2 structure after 100 cycles, while the undoped NNMCu0 showed significant peak broadening and additional phases, indicating degradation. This structural resilience is attributed to the presence of Cu2+ in the transition metal layers, which strengthens the framework and mitigates strain during Na+ insertion/extraction. The average volume change (ΔV) per cycle can be expressed as:
$$ \Delta V = \frac{\Delta c}{c_0} \times 100\% $$
where Δc is the change in lattice parameter c, and c0 is the initial value. For NNMCu0.15, ΔV was less than 2% over 100 cycles, compared to over 5% for NNMCu0, highlighting the stabilizing effect of Cu doping. Such minimal volume change is advantageous for maintaining electrode integrity in sodium-ion batteries.
In summary, we successfully synthesized Cu-doped P2-type Na0.67Ni0.33−xCuxMn0.67O2 materials via a sol-gel method. Cu doping expanded the interlayer spacing, suppressed the P2-O2 phase transition, and enhanced Na+ diffusion kinetics. The optimal doping level was x = 0.15, which delivered a balance between capacity and stability. NNMCu0.15 exhibited an initial discharge capacity of 126.74 mAh g−1 at 0.1 C, with 79.10% capacity retention after 100 cycles, along with excellent rate performance. These improvements are attributed to the synergistic effects of structural stabilization, additional redox activity from Cu2+/Cu3+, and reduced electrochemical impedance. Our findings demonstrate that Cu doping is an effective strategy for developing high-performance cathode materials for sodium-ion batteries. Future work could explore co-doping with other elements or surface coatings to further enhance the energy density and longevity of sodium-ion battery systems. The advancement of such materials is crucial for the commercialization of sodium-ion batteries in large-scale energy storage applications.