In the realm of energy storage, the development of advanced battery technologies is paramount to meet the growing demands for renewable energy integration and portable electronics. Among these, sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries due to the abundance and low cost of sodium resources. However, sodium-ion batteries often suffer from lower energy density compared to their lithium counterparts, primarily due to limitations in cathode materials. Layered oxides, such as P2-type NaxTM O2 (where TM represents transition metals), are widely studied for sodium-ion battery cathodes because of their simple synthesis, good air stability, and high ionic diffusion rates. Nonetheless, the specific capacity of these materials is typically restricted to around 100 mAh g−1 when relying solely on transition metal redox reactions. To overcome this, recent research has focused on activating anion redox, specifically involving oxygen ions (O2−/(O2)n−), to provide additional capacity. While Mg doping has been shown to trigger such anion redox processes, it often leads to irreversible phase transitions and severe voltage hysteresis, degrading cycling performance. In this study, we explore the co-doping of Mg and Co to modify the crystal and electronic structures, aiming to enhance specific capacity, reduce voltage hysteresis, and improve cycle stability in sodium-ion battery cathodes.
The sodium-ion battery represents a cost-effective and sustainable energy storage solution, particularly for large-scale applications where resource availability is critical. The cathode material plays a pivotal role in determining the overall performance of a sodium-ion battery, including its capacity, voltage, and longevity. Traditional cathode materials rely on transition metal redox, but this approach has inherent limitations due to structural stability constraints. By harnessing anion redox, we can potentially double the capacity, but this often comes at the expense of cycle life and voltage efficiency. Our work addresses these challenges by designing a series of P2-type layered oxides with the general formula Na0.67MgxCoyMn1-x-yO2 (0 ≤ x ≤ 0.34, 0 ≤ y ≤ 0.22). We investigate how Mg and Co co-doping influences the anion redox process, with a focus on optimizing electrochemical properties for sodium-ion battery applications. Through comprehensive characterization and testing, we identify Na0.67Mg0.22Co0.22Mn0.56O2 as a superior cathode material, offering high reversible capacity, improved rate capability, and mitigated voltage hysteresis.
To synthesize the cathode materials, we employed both solid-state and sol-gel methods. For compositions with x ≤ 0.22 and y ≤ 0.22, solid-state reactions were used: stoichiometric amounts of Na2CO3, MgO, Co2O3, and Mn2O3 were mixed with 5% excess Na2CO3, ball-milled in ethanol, dried, pressed into pellets, and calcined at 900°C for 16 hours in air. For Na0.67Mg0.34Mn0.66O2, which could not be obtained as a pure phase via solid-state synthesis, we utilized a sol-gel approach: acetates of sodium, magnesium, and manganese were dissolved in deionized water with citric acid as a chelating agent, stirred at 80°C to form a gel, dried at 60°C, pre-calcined at 400°C for 5 hours, and then sintered at 900°C for 16 hours. The phase purity and structure were analyzed using X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.54056 Å) over a 2θ range of 10° to 80°. Morphology was examined via scanning electron microscopy (SEM). Elemental composition was verified by inductively coupled plasma (ICP) analysis. For electrochemical evaluation, cathode slurries were prepared by mixing active material, Super P carbon, and polyvinylidene fluoride (PVDF) in a 7:2:1 weight ratio, coated onto carbon-coated aluminum foil, and dried at 120°C overnight. CR2032 coin cells were assembled in an argon-filled glovebox with sodium metal as the counter electrode, glass fiber separator, and electrolyte of 1 M NaClO4 in propylene carbonate (PC). Galvanostatic charge-discharge tests were conducted between 1.5 V and 4.5 V vs. Na/Na+ at various rates, and cyclic voltammetry (CV) was performed at a scan rate of 0.1 mV s−1 over the same voltage range.
The XRD patterns of the synthesized materials, including Na0.67Mg0.34Mn0.66O2 (NMM), Na0.67Mg0.2Co0.1Mn0.7O2 (NMC10M), Na0.67Mg0.15Co0.15Mn0.7O2 (NMC15M), and Na0.67Mg0.22Co0.22Mn0.56O2 (NMC22M), confirmed the formation of pure P2-type layered structures with the space group P63/mmc. All patterns exhibited characteristic peaks consistent with hexagonal symmetry, indicating successful incorporation of Mg and Co into the lattice. The SEM image of NMC22M revealed irregularly shaped particles with uniform distribution and sizes ranging from 1 to 3 μm, which facilitates sodium ion transport during cycling. The electrochemical performance of these materials was evaluated in sodium-ion battery half-cells. The initial charge-discharge curves at 0.1C rate showed that NMM delivered a reversible capacity of 205 mAh g−1, while NMC10M, NMC15M, and NMC22M offered 191.4, 168, and 198 mAh g−1, respectively. The capacity below 4 V increased with Co content, from 32 mAh g−1 for NMC10M to 50 mAh g−1 for NMC22M, attributed to the Co3+/Co4+ redox couple introduced by Co doping.
A critical issue in anion-redox-based sodium-ion battery cathodes is voltage hysteresis, which arises from irreversible structural changes and sluggish kinetics. We compared the voltage hysteresis between NMM and NMC22M by examining their discharge curves at a capacity of 125 mAh g−1. NMM exhibited a hysteresis of 1.4 V, whereas NMC22M showed a reduced hysteresis of 0.8 V, indicating improved reversibility. Moreover, the average operating voltage of NMC22M was higher than that of NMM, maintaining above 2.5 V at 0.5C rate, which enhances the energy density of the sodium-ion battery. Cyclic voltammetry further elucidated the redox processes. For NMM, the CV curves displayed significant changes between the first and second cycles, suggesting poor reversibility. In contrast, NMC22M showed smoother and more symmetric peaks, with minimal variation across cycles, confirming enhanced structural stability and faster sodium ion diffusion. The reduction peak around 2.75 V, associated with Mn4+ to Mn3+ reduction, was less pronounced in NMC22M, implying that Co doping suppresses the reduction of Mn4+, thereby stabilizing the crystal framework.
To quantify the electrochemical performance, we summarize the key parameters in Table 1. The table includes specific capacities, voltage hysteresis, and capacity retention for the different compositions, highlighting the superiority of NMC22M as a cathode material for sodium-ion batteries.
| Material | Initial Reversible Capacity (mAh g−1) | Voltage Hysteresis at 125 mAh g−1 (V) | Capacity Retention after 50 Cycles at 0.5C (%) | Capacity at 2C Rate (mAh g−1) |
|---|---|---|---|---|
| NMM | 205 | 1.4 | 70 | 110 |
| NMC10M | 191.4 | 1.2 | 57 | 95 |
| NMC15M | 168 | 1.0 | 75 | 120 |
| NMC22M | 198 | 0.8 | 80 | 140 |
The rate capability and long-term cycling stability are crucial for practical sodium-ion battery applications. As shown in Figure 1 (not included here, but described in text), NMC22M demonstrated excellent rate performance, delivering 140 mAh g−1 at a high rate of 2C, while NMM only managed 110 mAh g−1. After 50 cycles at 0.5C, NMC22M retained 80% of its initial capacity, compared to 70% for NMM. This improvement is ascribed to the enhanced ionic and electronic conductivity imparted by Co doping, which mitigates kinetic limitations during charge-discharge processes. The electrochemical impedance spectroscopy (EIS) data further supported these findings. We modeled the EIS spectra using an equivalent circuit with components for solution resistance (RΩ), solid-electrolyte interphase resistance (RSEI), charge transfer resistance (Rct), and Warburg impedance (Zw). The fitted values, summarized in Table 2, reveal that NMC22M had lower RSEI and Rct compared to NMM, indicating reduced interfacial and charge transfer resistances, which contribute to better rate performance and lower voltage hysteresis in the sodium-ion battery.
| Material | RSEI (Ω) | Rct (Ω) |
|---|---|---|
| NMM | 24.38 | 360.8 |
| NMC22M | 12.78 | 219.3 |
The role of Mg and Co co-doping can be understood through electronic structure modifications. Mg doping, with its +2 oxidation state, reduces the average oxidation state of manganese, creating oxygen holes that activate anion redox. However, this process can destabilize the lattice, leading to oxygen release and phase transformations. Co doping introduces Co3+/Co4+ redox, which overlaps with the oxygen band, enhancing electronic conductivity and stabilizing the structure. The combined effect is expressed in the following formula for the overall capacity contribution:
$$ Q_{\text{total}} = Q_{\text{TM}} + Q_{\text{O}} $$
where \( Q_{\text{total}} \) is the total specific capacity, \( Q_{\text{TM}} \) is the capacity from transition metal redox (primarily Co in this case), and \( Q_{\text{O}} \) is the capacity from anion redox. For NMC22M, the high capacity of 198 mAh g−1 suggests significant contributions from both sources. The voltage hysteresis reduction can be modeled by considering the overpotentials associated with sodium ion diffusion and charge transfer. The overpotential \( \eta \) is given by:
$$ \eta = \frac{RT}{F} \ln \left( \frac{i}{i_0} \right) + i R_{\text{ct}} $$
where \( R \) is the gas constant, \( T \) is temperature, \( F \) is Faraday’s constant, \( i \) is current density, and \( i_0 \) is exchange current density. Lower \( R_{\text{ct}} \) values for NMC22M, as seen in Table 2, result in smaller overpotentials, thereby reducing voltage hysteresis. This is critical for improving the energy efficiency of sodium-ion batteries.
Furthermore, we analyzed the structural stability during cycling using in-situ XRD (not shown, but discussed). The P2 structure of NMC22M remained largely intact upon sodium extraction and insertion, with minimal phase transitions compared to NMM, which underwent irreversible P2-to-O2 transformations. This stability is attributed to the Co doping, which strengthens the metal-oxygen bonds and suppresses cation migration. The capacity fading mechanism was also investigated. For NMM, rapid capacity decay was linked to oxygen loss and structural degradation, whereas NMC22M exhibited slower degradation due to preserved lattice integrity. The capacity retention after 100 cycles was estimated using the equation:
$$ \text{Retention} = \frac{C_n}{C_1} \times 100\% $$
where \( C_n \) is the capacity at cycle n and \( C_1 \) is the initial capacity. For NMC22M, retention exceeded 75% after 100 cycles at 1C rate, underscoring its durability as a sodium-ion battery cathode.
The optimization of Mg and Co ratios is essential for balancing capacity and stability. We conducted a series of experiments with varying x and y in Na0.67MgxCoyMn1-x-yO2. The results, compiled in Table 3, illustrate the trade-offs: higher Mg content increases anion redox capacity but exacerbates voltage hysteresis, while higher Co content improves kinetics but may reduce overall capacity if it replaces too much Mn. NMC22M represents an optimal compromise, offering high capacity, low hysteresis, and good cycling performance.
| Composition (x, y) | Anion Redox Contribution (mAh g−1) | Transition Metal Redox Contribution (mAh g−1) | Voltage Hysteresis (V) | Cycling Stability Index |
|---|---|---|---|---|
| (0.34, 0) | ~150 | ~55 | 1.4 | Low |
| (0.2, 0.1) | ~130 | ~61.4 | 1.2 | Medium |
| (0.15, 0.15) | ~110 | ~58 | 1.0 | High |
| (0.22, 0.22) | ~140 | ~58 | 0.8 | Very High |
The cycling stability index is a qualitative measure based on capacity retention over 50 cycles, with “Very High” indicating >80% retention. These findings emphasize the importance of co-doping strategies in advancing sodium-ion battery technology. Beyond electrochemical performance, we evaluated the practical implications for sodium-ion battery manufacturing. The synthesis methods used here are scalable, and the materials exhibit good air stability, reducing processing costs. The enhanced rate capability of NMC22M makes it suitable for high-power applications, such as electric vehicles and grid storage, where fast charging and discharging are required. Additionally, the reduced voltage hysteresis improves the round-trip efficiency of the sodium-ion battery, which is crucial for energy management systems.
In conclusion, we have demonstrated that Mg and Co co-doping effectively modifies the anion redox process in P2-type layered oxide cathodes for sodium-ion batteries. The optimized material, Na0.67Mg0.22Co0.22Mn0.56O2, delivers a high reversible capacity of 198 mAh g−1, improved rate performance (140 mAh g−1 at 2C), and mitigated voltage hysteresis (0.8 V at 125 mAh g−1). These enhancements are attributed to the synergistic effects of Mg triggering anion redox and Co enhancing electronic conductivity and structural stability. Our work provides a new design strategy for high-performance sodium-ion battery cathodes, paving the way for more efficient and cost-effective energy storage solutions. Future research could explore other co-doping combinations or surface modifications to further boost performance and longevity of sodium-ion batteries.
The development of sodium-ion batteries is rapidly progressing, and cathode materials like NMC22M hold great promise for commercial adoption. By addressing key challenges such as voltage hysteresis and cycle life, we can unlock the full potential of sodium-ion batteries as a sustainable alternative to lithium-ion systems. Continued innovation in material science will be essential to meet the evolving demands of the energy storage market, and studies like this contribute to that ongoing effort. Ultimately, the goal is to create sodium-ion batteries that are not only affordable and abundant but also high-performing and reliable for a wide range of applications.