The escalating global demand for energy storage has driven intensive research into electrochemical systems beyond the conventional lithium-ion battery. While lithium-ion technology offers high energy density, concerns regarding lithium scarcity and cost volatility necessitate the exploration of alternative chemistries. In this context, sodium-ion battery technology emerges as a highly promising candidate due to the natural abundance and low cost of sodium resources, coupled with electrochemical principles similar to those of lithium-ion systems. The development of high-performance cathode materials remains a central challenge for realizing the full potential of sodium-ion battery technology. Among various candidates, P2-type layered transition metal oxides (NaxTMO2) attract significant attention for their relatively high capacity, ease of synthesis, and favorable two-dimensional pathways for Na+ diffusion.
Specifically, P2-Na2/3Ni1/3Mn2/3O2 is a benchmark material that delivers a high working voltage (~3.7 V) and a specific capacity approaching its theoretical value. However, its practical application is severely hindered by an irreversible phase transition from the P2 to the O2 structure when charged to high voltages (above ~4.2 V vs. Na+/Na). This transformation involves a gliding of transition metal (TM) layers and is accompanied by a drastic unit cell volume change (≈23%), leading to rapid structural degradation and capacity fade upon cycling. Therefore, stabilizing the P2 structure against this detrimental phase transition is crucial for developing durable cathode materials for sodium-ion battery applications.
Elemental substitution in the TM layer is a widely adopted strategy to modulate the structural and electrochemical properties of layered oxides. Substitutions can be categorized into electrochemically active (e.g., Cu, Fe, Co) and inactive (e.g., Mg, Zn, Li, Ti) elements. Active substituents like Cu can participate in redox reactions, helping to maintain capacity while potentially altering the phase transition pathway. Inactive substituents like Mg primarily act as structural pillars, stabilizing the host framework by mitigating TM layer sliding and reducing volume strain, often at the expense of some reversible capacity. The synergistic combination of active and inactive elements presents a compelling route to engineer cathode materials that simultaneously exhibit high capacity, structural resilience, and excellent cycling stability for sodium-ion battery systems.
In this work, we propose and investigate a co-substitution strategy employing both copper and magnesium in the TM layer of P2-Na2/3Ni1/3Mn2/3O2. The rationale is to harness the complementary benefits of both elements: Cu2+/Cu3+ redox activity to contribute to capacity and modify high-voltage behavior, and Mg2+ as a stabilizing pillar to suppress layer gliding and minimize structural strain. A series of P2-type Na0.67Ni0.33-x-yCuxMgyMn0.67O2 materials were synthesized via a solid-state method. Comprehensive characterization reveals that the optimal composition effectively inhibits the irreversible P2-O2 transition, instead promoting a more reversible transformation to an OP4-type phase. This results in significantly enhanced cycling stability and rate capability. The underlying mechanisms are elucidated through a combination of in situ X-ray diffraction and density functional theory calculations, providing fundamental insights into the role of co-substitution in stabilizing the cathode structure for high-performance sodium-ion battery applications.
Material Synthesis and Structural Characterization
The target P2-type layered oxides with the general formula Na0.67Ni0.33-x-yCuxMgyMn0.67O2 (where 0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.10) were prepared using a conventional solid-state reaction. Stoichiometric amounts of Na2CO3, NiO, CuO, MgO, and MnO2 precursors were thoroughly mixed by ball-milling, pressed into pellets, and calcined at 900°C for 15 hours in air. The as-synthesized materials were stored in an argon-filled glovebox to minimize exposure to moisture and CO2. The specific compositions investigated in this study are designated as shown in Table 1.
| Sample Designation | Chemical Formula | Nominal Composition (Ni:Cu:Mg:Mn) |
|---|---|---|
| NCMM-00-00 | Na0.67Ni0.33Mn0.67O2 | 0.33 : 0 : 0 : 0.67 |
| NCMM-05-05 | Na0.67Ni0.23Cu0.05Mg0.05Mn0.67O2 | 0.23 : 0.05 : 0.05 : 0.67 |
| NCMM-10-05 | Na0.67Ni0.18Cu0.10Mg0.05Mn0.67O2 | 0.18 : 0.10 : 0.05 : 0.67 |
| NCMM-15-05 | Na0.67Ni0.13Cu0.15Mg0.05Mn0.67O2 | 0.13 : 0.15 : 0.05 : 0.67 |
| NCMM-05-10 | Na0.67Ni0.18Cu0.05Mg0.10Mn0.67O2 | 0.18 : 0.05 : 0.10 : 0.67 |
X-ray diffraction (XRD) patterns confirmed that all synthesized materials crystallized in a pure P2-type structure with the space group P63/mmc. No impurity phases were detected, indicating successful incorporation of Cu and Mg into the crystal lattice. Rietveld refinement was performed on the pristine and co-substituted samples to obtain detailed structural parameters. The refined lattice parameters are summarized in Table 2.
| Sample | a (Å) | c (Å) | Unit Cell Volume (Å3) | Na Layer Spacing (dO-Na-O, Å)* |
|---|---|---|---|---|
| NCMM-00-00 | 2.8870 | 11.1550 | 80.52 | ~2.36 |
| NCMM-10-05 | 2.8922 | 11.1783 | 80.98 | ~2.35 |
* Estimated from c-axis parameter and oxygen z-coordinate.
The results show that co-substitution with Cu and Mg leads to a slight expansion in both the a-axis and, more notably, the c-axis parameters. The increase in the c-parameter suggests an enlarged interlayer spacing, which is beneficial for Na+ ion transport. This expansion can be attributed to the larger ionic radii of Cu2+ (0.73 Å) and Mg2+ (0.72 Å) compared to Ni2+ (0.69 Å) in octahedral coordination. Morphological analysis by scanning electron microscopy revealed that all samples consisted of irregular platelet-like particles with sizes ranging from 2 to 5 micrometers, with the co-substituted sample showing a more uniform particle size distribution.
Electrochemical Performance Evaluation
The electrochemical properties of the prepared cathodes were evaluated in half-cell configurations versus sodium metal within a voltage window of 2.00–4.35 V. The galvanostatic charge-discharge profiles of the pristine material, NCMM-00-00, exhibit multiple voltage steps during the first cycle, indicative of complex Na+/vacancy ordering and phase transitions. A prominent plateau appears above 4.18 V, corresponding to the detrimental P2-to-O2 phase transformation. This process is highly irreversible, as evidenced by the significant capacity loss and voltage profile changes in subsequent cycles.
In striking contrast, the charge-discharge curves of the co-substituted material, particularly the optimal NCMM-10-05 sample, are much smoother. The multiple voltage steps below 4.0 V are significantly suppressed, and the high-voltage plateau above 4.18 V is replaced by a more sloping profile. This immediate observation suggests that Cu/Mg co-substitution effectively mitigates both the Na+/vacancy ordering and the irreversible P2-O2 phase transition, which is paramount for achieving stable cycling in a sodium-ion battery.
The electrochemical performance data for key samples are quantitatively compared in Table 3. The NCMM-10-05 cathode delivered a high initial reversible capacity of 113 mAh g-1 at 0.2C (1C = 100 mA g-1). More importantly, it demonstrated superior rate capability and cycling stability compared to the unsubstituted counterpart.
| Sample | Initial Discharge Capacity @ 0.2C (mAh g-1) | Capacity @ 8C (mAh g-1) | Capacity Retention after 200 cycles @ 1C (%) | Capacity Retention after 500 cycles @ 5C (%) |
|---|---|---|---|---|
| NCMM-00-00 | 150 | 20.9 | 69.8 | 20.5 |
| NCMM-10-05 | 113 | 64.1 | 88.9 | 74.0 |
The NCMM-10-05 material retained 64.1 mAh g-1 even at a very high rate of 8C, and its capacity recovered to 107.6 mAh g-1 when the rate was returned to 0.2C, demonstrating excellent reversibility and kinetics. In long-term cycling tests at 1C, NCMM-10-05 maintained 88.9% of its capacity after 200 cycles, significantly outperforming NCMM-00-00 (69.8%). Even under a strenuous 5C rate for 500 cycles, NCMM-10-05 showed a remarkable capacity retention of 74.0%, with Coulombic efficiency consistently above 99%. These results unequivocally establish that synergistic Cu and Mg substitution drastically improves the structural integrity and electrochemical robustness of the P2-type cathode, making it highly suitable for demanding sodium-ion battery applications.
Kinetics and Phase Transition Analysis
To gain deeper insight into the enhanced performance, we investigated the Na+ diffusion kinetics using galvanostatic intermittent titration technique (GITT). The apparent chemical diffusion coefficient of Na+ (DNa+) was calculated using the following equation derived from Fick’s second law for a short-time potentiostatic transient:
$$ D_{Na^+} = \frac{4}{\pi \tau} \left( \frac{m_B V_m}{M_B A} \right)^2 \left( \frac{\Delta E_s}{\Delta E_\tau} \right)^2 $$
where τ is the duration of the current pulse, mB, MB, and Vm are the active mass, molar mass, and molar volume of the electrode material, A is the electrode/electrolyte contact area, ΔEs is the steady-state voltage change after the pulse, and ΔEτ is the voltage change during the pulse. The calculated DNa+ values as a function of voltage are compared in Table 4.
| Voltage Region / Sample | NCMM-00-00 DNa+ (cm2 s-1) | NCMM-10-05 DNa+ (cm2 s-1) | Remarks |
|---|---|---|---|
| Below 4.0 V (Solid Solution) | ~10-10 – 10-11 | ~10-10 – 10-11 | Comparable kinetics |
| ~3.3 & 3.7 V (Ordering Regions) | Drops sharply to ~10-12 | Remains stable ~10-11 | Suppressed ordering in NCMM-10-05 |
| Above 4.18 V (Phase Transition) | ~10-13 – 10-14 | ~10-10 – 10-11 | P2-O2 vs. P2-OP4 transition |
The GITT analysis reveals a dramatic improvement in Na+ transport kinetics for the co-substituted material, especially in the high-voltage region. For NCMM-00-00, DNa+ plummets by several orders of magnitude during the Na+/vacancy ordering steps and the P2-O2 transition, creating significant kinetic barriers. In contrast, DNa+ for NCMM-10-05 remains relatively high and stable throughout the entire charging process, indicating facile Na+ (de)intercalation without major kinetic bottlenecks. Electrochemical impedance spectroscopy (EIS) further supported these findings, showing significantly reduced charge-transfer resistance (Rct) and surface film resistance (Rsf) for NCMM-10-05 compared to NCMM-00-00 after cycling.
The most critical analysis comes from in situ X-ray diffraction (XRD), which directly probes the structural evolution during operation. For NCMM-00-00, the in situ patterns confirm the irreversible disappearance of P2-phase peaks and the emergence of O2-phase peaks above 4.2 V. For the NCMM-10-05 cathode, the structural evolution is entirely different. During charging to 4.35 V, the P2 structure is preserved until about 4.24 V, where a new set of diffraction peaks emerges. These new peaks are indexed to an OP4-type phase, characterized by a periodic alternation of octahedral (O) and prismatic (P) stacking of oxygen layers. This OP4 phase is structurally related to both P2 and O2 but involves a much smaller unit cell volume change. Upon discharge, the transformation from OP4 back to P2 is fully reversible. The evolution of lattice parameters extracted from the in situ XRD data is summarized in Table 5.
| State (Voltage) | Phase | c-axis (Å) | Volume Change from Pristine (%) |
|---|---|---|---|
| Pristine (2.00 V) | P2 | 11.18 | 0 |
| Charged to 4.35 V | OP4 (Z-phase) | ~10.7-11.1* | ≈ -7.1 |
| Discharged to 2.00 V | P2 | ~11.18 | Near 0 |
*The OP4 phase has a different unit cell; this value represents an effective c-spacing.
The key finding is that the volume change associated with the P2-OP4 transition (≈7.1%) is less than one-third of that for the P2-O2 transition (≈23%) observed in the unsubstituted material. This drastically reduced mechanical strain is the fundamental reason for the superior cycling stability of the co-substituted cathode in a sodium-ion battery.
Theoretical Insights from Density Functional Theory (DFT) Calculations
To unravel the atomic-scale mechanisms behind the stabilization effect, we performed spin-polarized DFT+U calculations. Supercell models of Na0.67Ni0.33Mn0.67O2 and Na0.67Ni0.18Cu0.10Mg0.05Mn0.67O2 were constructed, and their structures were optimized in both discharged and charged (Na-deficient) states.
1. Structural Stability and Volume Change: Geometry optimization confirmed the experimental observations. Upon charging (Na removal), the model for the unsubstituted compound showed a significant contraction along the c-axis, consistent with TM layer sliding towards an O-type configuration. In contrast, the model for the co-substituted compound exhibited a slight expansion along the c-axis upon charging, indicating that the P2-type interlayer spacing is effectively “pinned” and resistant to collapse. The calculated overall volume change upon charging was 3.0% for the unsubstituted model but only 1.1% for the co-substituted model, aligning with the in situ XRD data.
2. Role of Mg – Local Na+ Retention: A pivotal role of Mg2+ is to trap Na+ ions in its local environment at high states of charge. We calculated the formation energy (Ef) for different local configurations of Na+ ions around a Mg site in a highly desodiated structure. The formation energy is defined as:
$$E_f = E_{total} – E_{pre} – \frac{E_{unit}}{N}$$
where Etotal is the total energy of the system with a specific Na configuration, Epre is the energy of the preconditioned host, Eunit is the energy per formula unit, and N is the number of formula units. Two plausible configurations were evaluated (Table 6): (A) Two Na+ ions residing on opposite sides of the TM layer containing Mg, and (B) Two Na+ ions residing on the same side of the Mg-containing TM layer, creating an adjacent empty Na layer which facilitates the local formation of an O-type stacking sequence.
| Configuration | Description | Calculated E_f (eV) | Implication |
|---|---|---|---|
| A | Na+ ions on opposite sides (maintains local P-stacking) | -6.26 | Energetically favorable |
| B | Na+ ions on same side (induces local O-stacking) | -6.41 | More favorable |
The calculation reveals that Configuration B is energetically more stable. This indicates that at high voltage, Mg promotes the retention of Na+ in a specific arrangement that locally induces a transition from P to O coordination. The periodic alternation of such Mg-stabilized regions (with local O-stacking) and other regions (retaining P-stacking) across the crystal explains the formation of the long-period OP4 (Z) phase observed experimentally, rather than a complete conversion to the O2 phase.
3. Role of Cu – Electronic Structure and Redox Activity: Projected density of states (PDOS) analysis showed that in the co-substituted material, Cu 3d states contribute significantly near the Fermi level. Upon charging, holes are introduced primarily into the hybridized O 2p – Cu 3d bands, indicating the participation of Cu2+/Cu3+ redox couples alongside the conventional Ni2+/Ni4+ redox. This additional redox activity helps maintain capacity despite the reduction of Ni content. Furthermore, Bader charge analysis confirmed a smaller average increase in the Ni oxidation state during charging for the co-substituted material compared to the pristine one, implying a more moderate structural perturbation.
4. Na+ Diffusion Barrier: The energy barrier for Na+ migration through the prismatic sites was calculated using the nudged elastic band method. The barrier was found to be significantly lower in the co-substituted structure (1.67 eV) than in the unsubstituted one (4.78 eV). This reduction is attributed to the expanded interlayer spacing and the weakened electrostatic interaction between Na+ and the more covalent (Cu/Mg, Mn)-O bonds, as visualized by electron localization function maps. This theoretical finding directly correlates with the enhanced rate capability and higher DNa+ values measured by GITT.
Air and Moisture Stability
A practical challenge for many Mn/Ni-based layered oxides is their sensitivity to ambient air, where they react with H2O and CO2 to form surface species like Na2CO3 and NaOH, degrading electrochemical performance. We evaluated the environmental stability of the optimal NCMM-10-05 material by exposing it to humid air for 10 days and immersing it in water for 24 hours. XRD patterns after these treatments showed no detectable phase changes or the formation of crystalline impurities, indicating remarkable structural robustness. Electrochemical testing of the water-soaked sample showed only a minor initial capacity loss, with stable cycling thereafter, retaining 82.6% capacity after 100 cycles at 1C. This excellent air and moisture stability, attributed to the stabilizing effect of Mg and possibly Cu in the structure, is a highly desirable feature for practical sodium-ion battery manufacturing and storage.
Conclusion
In summary, this work demonstrates a highly effective strategy for engineering high-voltage stable P2-type cathode materials for sodium-ion battery applications. The synergistic co-substitution of electrochemically active Cu and inactive Mg into the TM layer of P2-Na2/3Ni1/3Mn2/3O2 achieves multiple beneficial effects:
- Phase Transition Modulation: It completely suppresses the irreversible, high-strain P2-O2 phase transition. Instead, it promotes a reversible transformation to an OP4-type phase, which involves a significantly smaller volume change (~7% vs. ~23%).
- Enhanced Kinetics: The substitution expands the lattice, weakens Na+-O interactions, and lowers the Na+ diffusion barrier, leading to superior rate performance (64.1 mAh g-1 at 8C).
- Improved Cycling Stability: The mitigated structural strain results in excellent long-term cyclability, with 88.9% capacity retention after 200 cycles at 1C and 74% after 500 cycles at 5C.
- Air Stability: The modified structure exhibits remarkable resistance to moisture and air, a critical practical advantage.
The underlying mechanisms have been thoroughly elucidated: Mg acts as a structural anchor, locally retaining Na+ and guiding the formation of a hybrid OP4 phase, while Cu contributes to redox capacity and modifies the electronic structure. This synergistic approach, validated by combined experimental and theoretical analysis, provides a fundamental blueprint for designing high-performance, stable layered oxide cathodes. It addresses key challenges of capacity retention and structural integrity at high voltages, paving a clear path forward for the development of advanced, durable sodium-ion battery technology for large-scale energy storage.