The pursuit of sustainable and cost-effective energy storage solutions has led to intensified research into sodium-ion battery technology. While lithium-ion batteries dominate the market, concerns regarding lithium resource scarcity and cost have prompted the exploration of sodium-ion battery alternatives, particularly for large-scale grid storage. Sodium offers advantages due to its natural abundance and geographical distribution. Among the various cathode materials for sodium-ion battery systems, layered transition metal oxides with the general formula NaxTMO2 (TM = transition metal, 0 < x ≤ 1) are highly promising due to their high theoretical capacity, relatively simple synthesis, and good air stability.
P2-type layered oxides, characterized by prismatic Na+ coordination and ABBA oxygen stacking, are particularly attractive. A prominent member of this family is P2-Na0.67Ni0.33Mn0.67O2. This material is appealing for sodium-ion battery applications because it offers a high specific capacity (theoretically ~173 mAh g-1), a high operating voltage, and stability in ambient air. However, its practical implementation in a reliable sodium-ion battery is hampered by significant challenges. During high-voltage charging (>4.2 V vs. Na/Na+), it undergoes a detrimental P2 to O2 phase transition involving slab gliding, which induces large volume strain and mechanical degradation. At lower voltages, the ordering of Na+/vacancies and transition metal ion dissolution further contribute to rapid capacity fade and poor rate capability, limiting the cycle life of the sodium-ion battery.
A widely adopted strategy to mitigate these issues in sodium-ion battery cathodes is cationic doping. The substitution of a fraction of the redox-active transition metal ions with inert or differently charged ions can suppress phase transitions, enhance structural stability, and improve Na+ diffusion kinetics. Elements such as Mg2+ and Li+ have been investigated individually. Mg2+ doping, due to its stable +2 oxidation state and similar ionic radius to Ni2+, can act as a “pillar” in the transition metal layer, stabilizing the structure against collapse during deep desodiation. Li+ doping, on the other hand, can influence the local electronic structure and Na+ diffusion pathways. However, the synergistic effects of co-doping with both Mg and Li in the specific P2-Na0.67Ni0.33Mn0.67O2 system for sodium-ion battery application warrant deeper investigation to find the optimal balance between capacity, rate performance, and cycle life.
In this work, we systematically investigate the effect of Mg and Mg-Li co-doping on the structure and electrochemical performance of P2-type layered oxide cathodes for sodium-ion battery. We first optimize the Mg doping level in Na0.67Ni0.33-xMgxMn0.67O2 and then further tune the performance by introducing Li at the optimal Mg content, resulting in a series of Na0.67Ni0.26-x-yMgxLiyMn0.67O2 materials. A comprehensive analysis combining structural characterization, electrochemical measurements, and kinetic studies reveals that the Mg-Li co-doped cathode exhibits superior performance, marking a significant step towards viable high-energy sodium-ion battery technology.
Experimental Synthesis and Characterization
All cathode materials were synthesized via a liquid-phase co-precipitation method followed by high-temperature calcination, a scalable route suitable for sodium-ion battery material production. For the parent compound P2-Na0.67Ni0.33Mn0.67O2 (denoted as NNM), stoichiometric amounts of nickel acetate and manganese acetate were dissolved in deionized water (Solution A). A separate NaOH solution (Solution B) was prepared as the precipitating agent. Solution A and B were simultaneously dripped into a reaction vessel under vigorous stirring. The resulting precipitate was collected, dried, and then calcined in air at 850°C for 25 hours.
The Mg-doped series, Na0.67Ni0.33-xMgxMn0.67O2 (x = 0.06, 0.07, 0.08), denoted as NNM-6Mg, NNM-7Mg, and NNM-8Mg, were prepared by adding magnesium acetate to Solution A in the appropriate molar ratio, keeping the total cation stoichiometry constant. Based on the optimal performance from the Mg-doped series, the Mg-Li co-doped series, Na0.67Ni0.26-yMg0.07LiyMn0.67O2 (y = 0.03, 0.04, 0.05), denoted as NNML-3Li, NNML-4Li, and NNML-5Li, were synthesized by further incorporating lithium acetate into the precursor solution.
The crystal structure was characterized by X-ray diffraction (XRD). Morphology and elemental distribution were examined using field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The surface chemical state was analyzed via X-ray photoelectron spectroscopy (XPS).
Electrodes for the sodium-ion battery half-cells were fabricated by mixing the active material, conductive carbon (acetylene black), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) solvent. The slurry was cast onto aluminum foil and dried. CR2032 coin cells were assembled in an argon-filled glovebox using the prepared cathode, a sodium metal anode, a glass fiber separator, and an electrolyte consisting of 1 M NaClO4 in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume) with 5% fluoroethylene carbonate additive.
Electrochemical performance of the sodium-ion battery cells was evaluated using a battery testing system. Galvanostatic charge-discharge tests were conducted between 2.0 and 4.3 V at various C-rates (1 C = 173 mA g-1). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation. Galvanostatic intermittent titration technique (GITT) was employed to investigate Na+ diffusion kinetics. Ex-situ XRD was conducted on electrodes at different states of charge/discharge to monitor structural evolution.
| Sample Notation | Chemical Formula | Doping Strategy |
|---|---|---|
| NNM | Na0.67Ni0.33Mn0.67O2 | Parent (Undoped) |
| NNM-6Mg | Na0.67Ni0.27Mg0.06Mn0.67O2 | Mg-doped |
| NNM-7Mg | Na0.67Ni0.26Mg0.07Mn0.67O2 | Mg-doped (Optimal) |
| NNM-8Mg | Na0.67Ni0.25Mg0.08Mn0.67O2 | Mg-doped |
| NNML-3Li | Na0.67Ni0.23Mg0.07Li0.03Mn0.67O2 | Mg-Li co-doped |
| NNML-4Li | Na0.67Ni0.22Mg0.07Li0.04Mn0.67O2 | Mg-Li co-doped (Optimal) |
| NNML-5Li | Na0.67Ni0.21Mg0.07Li0.05Mn0.67O2 | Mg-Li co-doped |
Structural and Morphological Analysis
The XRD patterns of all synthesized materials are shown in Figure 1. All major diffraction peaks for every sample, including the parent and doped variants, can be indexed to the hexagonal P2-type structure with the space group P63/mmc (PDF#54-0894). No detectable impurity phases are observed, confirming that Mg and Li have been successfully incorporated into the layered lattice without altering the primary P2 structure. The sharp and intense peaks indicate high crystallinity, which is beneficial for the electrochemical performance of the sodium-ion battery cathode. Lattice parameters were refined via Rietveld analysis. A general trend of slight contraction in the c-axis lattice parameter is observed with Mg doping, attributed to the stronger ionic interaction between Mg2+ and O2- compared to Ni2+. The Mg-Li co-doped samples show further subtle adjustments in lattice parameters, suggesting a synergistic effect on the crystal structure.
The morphology of the materials, as revealed by SEM, consists of well-defined hexagonal platelet-like particles with smooth surfaces and clear edges, typical of layered oxides synthesized via high-temperature solid-state reactions. The particle size distribution ranges from approximately 0.5 to 3 μm. Doping does not significantly alter the primary particle morphology. EDS mapping for the representative NNML-4Li sample confirms the homogeneous distribution of Na, Ni, Mn, O, Mg, and Li throughout the particles, validating the effectiveness of the co-precipitation synthesis for creating uniform doped materials for sodium-ion battery cathodes.
Raman spectroscopy provides further insight into the local structure. The spectra for NNM, NNM-7Mg, and NNML-4Li show characteristic modes. The peak around 391 cm-1 is associated with the bending vibration of oxygen in the transition metal layer. The intensity of this peak increases upon Mg and Mg-Li doping, which is often correlated with enhanced structural stability in layered oxides for sodium-ion battery applications. Furthermore, a slight shift of the Raman peaks to lower wavenumbers is observed for the doped samples, which may be linked to changes in Na+ distribution and local bonding environments.
XPS analysis was performed on the NNML-4Li sample to investigate the surface chemical states. The survey spectrum confirms the presence of all expected elements. The Li 1s peak is identified at a binding energy of approximately 52.4 eV, verifying the successful incorporation of Li into the near-surface region of the sodium-ion battery cathode material.
Electrochemical Performance in Sodium-Ion Battery
The electrochemical performance was evaluated in Na half-cells. First, the rate capability of the Mg-doped series was investigated. The results are summarized in Table 2. While the undoped NNM sample delivers the highest initial capacity at low rate (0.1C), its capacity drops dramatically as the current density increases, indicating poor kinetics and significant polarization. Among the Mg-doped samples, NNM-7Mg (x=0.07) exhibits the best balance between initial capacity and rate retention. At 1C, NNM-7Mg delivers a discharge capacity of 133.9 mAh g-1, significantly higher than its counterparts at higher Mg contents.
| Sample | Discharge Capacity @ 0.1C (mAh g-1) | Discharge Capacity @ 1C (mAh g-1) | Capacity Retention after 200 cycles @ 1C |
|---|---|---|---|
| NNM | 160.5 | ~93.6 (at 0.5C) | 9.2% |
| NNM-6Mg | 144.4 | ~97.8 (at 0.5C) | ~45% |
| NNM-7Mg | 146.6 | 133.9 | 48.1% |
| NNM-8Mg | 145.7 | ~94.8 (at 0.5C) | ~42% |
Building upon the optimal Mg content (x=0.07), Li was co-doped to form the NNML series. The rate performance is dramatically improved, as detailed in Table 3. The NNML-4Li (y=0.04) sample stands out. Although its initial capacity at 0.1C (136.3 mAh g-1) is slightly lower than that of NNM, it demonstrates exceptional capacity retention at high rates. Notably, when the current density returns to 0.1C after high-rate cycling, NNML-4Li recovers 125.2 mAh g-1, showing excellent structural resilience and minimal degradation—a critical feature for a practical sodium-ion battery.
| Sample | Discharge Capacity @ 0.1C (mAh g-1) | Discharge Capacity @ 1C (mAh g-1) | Capacity Retention after 200 cycles @ 1C | Recovered Capacity @ 0.1C after Cycling |
|---|---|---|---|---|
| NNM-7Mg | 146.6 | 133.9 | 48.1% | 111.1 mAh g-1 |
| NNML-3Li | 134.1 | ~125 | ~70% | 117.1 mAh g-1 |
| NNML-4Li | 136.3 | 130.1 | 77.2% | 125.2 mAh g-1 |
| NNML-5Li | 135.3 | ~122 | ~72% | 114.1 mAh g-1 |
The long-term cycling stability at 1C is the most striking improvement. The undoped NNM cathode suffers from catastrophic failure, retaining only 9.2% of its capacity after 200 cycles. The optimized Mg-doped sample (NNM-7Mg) improves the retention to 48.1%. Remarkably, the Mg-Li co-doped NNML-4Li cathode exhibits outstanding cyclability, maintaining 100.4 mAh g-1 after 200 cycles, corresponding to a high capacity retention of 77.2%. This represents a major advancement in the cycling life of this class of cathode materials for sodium-ion battery applications.
Reaction Mechanism and Kinetic Analysis
To understand the origin of the enhanced performance, we employed several electrochemical and structural probes. Cyclic voltammetry (CV) curves provide insight into the redox reactions. For the undoped NNM, distinct redox couples are visible: pairs around 3.0 V are associated with the Mn3+/Mn4+ redox, while the prominent pairs at ~3.25/3.37 V and ~3.53/3.72 V correspond to the Ni2+/Ni3+ and Ni3+/Ni4+ transitions, respectively. A broad redox feature above 4.0 V is attributed to anionic (oxygen) redox activity. In the doped samples, the intensity of the Ni-related peaks decreases proportionally with the amount of electrochemically inert Mg2+ substitution. For the NNML-4Li cathode, the CV profile shows better-defined peaks and higher reversibility, especially in the high-voltage region, indicating suppressed parasitic reactions and more stable anionic redox, which is crucial for the energy density of the sodium-ion battery.
The Na+ diffusion coefficient (DNa+) is a key kinetic parameter governing the rate capability of a sodium-ion battery cathode. We calculated DNa+ from GITT measurements using the following equation derived from Fick’s second law for a semi-infinite diffusion system:
$$ D_{GITT} = \frac{4}{\pi \tau} \left( \frac{m_B V_M}{M_B S} \right)^2 \left( \frac{\Delta E_s}{\Delta E_t} \right)^2 $$
where \(\tau\) is the constant-current pulse time, \(m_B\) is the active material mass, \(M_B\) is its molar mass, \(V_M\) is the molar volume, \(S\) is the electrode/electrolyte contact area, \(\Delta E_s\) is the change in steady-state voltage, and \(\Delta E_t\) is the total voltage change during the pulse. The calculated DNa+ values across the entire sodiation/desodiation voltage window are plotted in Figure 2. The NNML-4Li cathode exhibits consistently higher Na+ diffusion coefficients, ranging from ~4.9 × 10-12 to 1.6 × 10-9 cm2 s-1, compared to the undoped and singly Mg-doped materials. This enhanced ionic conductivity is a direct contributor to the superior rate performance of the Mg-Li co-doped sodium-ion battery cathode.
Electrochemical impedance spectroscopy (EIS) was conducted on cells at a charged state. The Nyquist plots consist of a high-frequency intercept related to bulk resistance (Rb), one or two semicircles in the mid-frequency range, and a low-frequency Warburg tail. The mid-frequency semicircle is typically assigned to the charge-transfer resistance (Rct) at the electrode/electrolyte interface. The fitted values are presented in Table 4. The Mg-Li co-doped NNML-4Li electrode shows a significantly lower Rct (32.3 Ω) compared to NNM-7Mg (144.7 Ω) and NNM (541.1 Ω). Furthermore, the EIS spectrum of NNML-4Li reveals an additional semicircle at very high frequencies, which can be attributed to a stable passivating layer (SEI-like layer) on the cathode surface with a resistance of ~98 Ω. This layer likely protects the cathode from continuous side reactions with the electrolyte, contributing to the excellent cycling stability observed in the sodium-ion battery test.
| Sample | Bulk Resistance, Rb (Ω) | Surface Layer Resistance, Rsf (Ω) | Charge-Transfer Resistance, Rct (Ω) |
|---|---|---|---|
| NNM | 4.8 | Not resolved | 541.1 |
| NNM-7Mg | 4.2 | Not resolved | 144.7 |
| NNML-4Li | 3.4 | 98.1 | 32.3 |
To directly observe structural evolution, ex-situ XRD was performed on the NNML-4Li electrode at various states of charge and discharge. The results confirm that the P2 structure is maintained throughout the cycling process within the 2.0-4.3 V window. No new diffraction peaks corresponding to an O2 phase appear, even at the fully charged state (4.3 V). This indicates that the Mg-Li co-doping effectively suppresses the deleterious P2-O2 phase transition, which is a primary failure mechanism for the undoped material in a high-voltage sodium-ion battery. The (002) and (004) peaks shift smoothly to lower angles during charging (Na+ extraction) and back to higher angles during discharging (Na+ insertion), corresponding to the expansion and contraction of the interlayer spacing. This reversible “breathing” of the crystal structure, without phase transformation, underpins the exceptional cycling stability of the co-doped cathode.
Discussion on the Synergistic Effect of Mg-Li Co-doping
The superior performance of the Mg-Li co-doped sodium-ion battery cathode can be attributed to a combination of structural, electronic, and interfacial stabilization effects, which we summarize with the following conceptual model.
1. Structural Pillaring and Bond Strength: The incorporation of Mg2+ (ionic radius ~0.72 Å for 6-coordination) into the transition metal layer, partially replacing Ni2+ (~0.69 Å), introduces stronger Mg-O bonds due to the higher charge density of Mg2+. This acts as a “pillar” that strengthens the TM-O bond network, increasing the energy barrier for the slab gliding required for the P2-O2 transition. This is reflected in the stabilized high-voltage plateau and the absence of O2 phase peaks in ex-situ XRD.
2. Modulation of Na Layer and Diffusion Paths: The co-doping of Li+ (a small cation with high mobility) is hypothesized to occupy sites within or adjacent to the Na layer. Li+ can act as a “lubricant” or disrupt the long-range ordering of Na+/vacancies that typically occurs in the undoped material at specific voltages. This disorder facilitates smoother and faster Na+ (de)intercalation. The enhanced Na+ diffusion coefficient (DGITT) calculated from GITT data provides quantitative evidence for this effect. The improvement can be modeled by considering a reduced activation energy (Ea) for Na+ hopping:
$$ D = D_0 \exp\left(-\frac{E_a}{k_B T}\right) $$
where \(D_0\) is the pre-exponential factor, \(k_B\) is Boltzmann’s constant, and \(T\) is temperature. The Mg-Li co-doping likely reduces the effective \(E_a\) for Na+ migration, leading to higher \(D\) values.
3. Electronic Structure and Redox Activity: Mg2+ is electrochemically inactive in this voltage window, which reduces the total Ni content and thus the absolute capacity from cationic redox. However, this substitution also mitigates the excessive oxidation of Ni and the associated large local structural distortions. The preserved Li may participate in or modulate the anionic (O2-/On-) redox activity at high voltages, making it more reversible, as seen in the stabilized CV peaks above 4.0 V. This trade-off results in a slightly lower initial capacity but a vastly more stable one over many cycles, which is more valuable for a long-life sodium-ion battery.
4. Interfacial Stabilization: The formation of a more stable interface layer, as inferred from the EIS data, is critical. The modified surface chemistry due to Mg and Li may promote the formation of a denser, more ionically conductive, and mechanically robust cathode-electrolyte interphase (CEI). This layer protects the bulk cathode particles from crack propagation induced by strain and from parasitic reactions with the electrolyte, significantly reducing impedance growth over time (lower Rct).
In summary, the optimization of the sodium-ion battery cathode performance can be expressed as a multi-variable function where co-doping parameters (x, y) are key variables:
$$ \text{Performance}(x, y) = f\left(\text{Structural Stability}(x, y),\, D_{Na+}(x, y),\, R_{ct}(x, y),\, \text{CEI Stability}(x, y)\right) $$
Our experimental data identifies (x=0.07, y=0.04) as a local optimum in this performance landscape for the P2-Na0.67Ni0.33Mn0.67O2 system.
Conclusion
We have successfully developed a high-performance P2-type layered oxide cathode for sodium-ion battery through rational Mg-Li co-doping engineering. Starting from P2-Na0.67Ni0.33Mn0.67O2, an initial optimization established Na0.67Ni0.26Mg0.07Mn0.67O2 as the optimal Mg-doped composition. Subsequent Li co-doping yielded the champion material, Na0.67Ni0.22Mg0.07Li0.04Mn0.67O2. This cathode exhibits an outstanding combination of properties: a high reversible capacity of 130.1 mAh g-1 at 1C, exceptional rate capability with excellent capacity recovery, and remarkable cycling stability with 77.2% capacity retention after 200 cycles—a dramatic improvement over the undoped material’s 9.2% retention.
Through a combination of GITT, EIS, CV, and ex-situ XRD analyses, we elucidated the mechanisms behind this enhancement. The synergistic effect of Mg-Li co-doping (i) suppresses the harmful P2-O2 phase transition, (ii) disrupts Na+/vacancy ordering to facilitate faster ionic diffusion (higher DNa+), (iii) lowers charge-transfer resistance (Rct), and (iv) promotes a stable protective interface layer. This work demonstrates that multi-element co-doping is a powerful and effective strategy to address the intrinsic challenges of high-capacity layered oxide cathodes, paving the way for the development of more durable and high-power sodium-ion battery systems for future energy storage applications.