Mg-Doped O3-Na[Ni_0.6Fe_0.25Mn_0.15]O₂ Cathode for Long-Cycle-Life Na-Ion Batteries

Abstract

The O3-type layered oxide materials have the advantage of high specific capacity, which makes them more competitive in the practical application of cathode materials for sodium-ion batteries (SIBs). However, the existing reported O3-type layered oxide materials still have a complex irreversible phase transition phenomenon, and the cycle life of batteries needs, with these materials, to be further improved to meet the requirements. Herein, we performed structural characterization and electrochemical performance tests on O3-NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂ (x = 0, 0.025, 0.05, and 0.075, denoted as NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg). The optimized NFM-2.5Mg has the largest sodium layer spacing, which can effectively enhance the transmission rate of sodium ions. Therefore, the reversible specific capacity can reach approximately 148.1 mAh g⁻¹ at 0.2C, and it can even achieve a capacity retention of 85.4% after 100 cycles at 1C, demonstrating excellent cycle stability. Moreover, at a low temperature of 0 °C, it also can keep capacity retention of 86.6% after 150 cycles at 1C. This study provides a view on the cycling performance improvement of sodium-ion layered oxide cathodes with a high theoretical specific capacity.

1. Introduction

Lithium-ion batteries (LIBs) possess advantages such as a low self-discharge rate, high energy density, long cycle life, low memory effect, and wide operating temperature range. They are currently one of the main methods of electrochemical energy storage and have been widely applied in the commercial sector (such as smartphones, laptops, electric vehicles, and electric motorcycles, etc.) [1,2,3,4]. However, the promotion and development of LIBs are hindered by serious resource problems. Firstly, from a global perspective on resource reserves, lithium resources are notably scarce, constituting only 0.0065% of the total. Furthermore, the distribution of lithium resources is uneven, and approximately 70% of the lithium reserve is located in the “Lithium Triangle” region in South America (Chile, Argentina, and Bolivia) [5,6]. The price of lithium carbonate has fluctuated wildly, soaring from CNY/TON 40,000 to 600,000, which highlights the fragility of the resource supply chain over the past five years. This situation not only drives up the cost of end batteries but also hinders the promotion of megawatt-level energy storage scenarios such as wind-solar-storage power stations. Therefore, it is necessary to seek alternative, more economical, and sustainable battery technologies or energy storage solutions [7]. In contrast, sodium resources are more abundant (2.83%, approximately 435 times that of lithium resources) [8,9,10]. Based on the original cost, the raw material cost of sodium-based compounds (such as NaCl) is less than 1/20 of lithium salts. Moreover, from a geographical perspective, sodium resources are widely distributed globally, and the technology for extracting sodium from rock salt mines and seawater is mature at the industrial level [8,9,10]. Therefore, the relatively low-cost advantage of sodium-ion batteries (SIBs) makes them one of the candidates for energy storage devices, which have a similar working principle to LIBs [11]. It is particularly worth noting that SIBs have unique advantages over LIBs in terms of low-temperature performance and fast charging capabilities. This is of great significance for energy storage and emergency power supply scenarios in cold regions. In addition, SIBs boast diversified cathode materials that show a minimal dependence on nickel and the strategic resource cobalt [12]. Generally, the cathode materials of SIBs are mainly classified into three categories based on the technical methods: layered transition metal oxides [13,14,15,16,17], Prussian blue/white analogs [18,19,20], and polyanionic compounds [21,22,23,24]. More specifically, whereas Prussian blue analogs (PBAs) offer economic benefits, their residual crystalline water content induces deleterious side reactions, which may progress to potentially culminating in thermal runaway and cyanide gas (such as HCN) emissions. Polyanionic compounds exhibit outstanding cyclability and low specific capacity, but their material cost is relatively high due to containing vanadium-based components. Furthermore, vanadium-based materials (Na₃V₂(PO₄)₃) are expensive and pollute the environment. In contrast, layered oxide materials perform the best among cathode materials for sodium-ion batteries, owing to their low cost (raw material cost < 80 CNY/kg), high vibration density (>3.2 g cm⁻³), and compatibility with the production line of lithium-ion ternary cathode materials (directly reusing the sintering kilns and coating equipment).

According to the different structures, Delmas classified these layered oxide materials into two types: P2-type and O3-type [25]. The characteristic of the P2 structure is that sodium ions exist at the P (prismatic) position in the MO₂ layer. This is a relatively stable structure, but it has a relatively low content of sodium ions (theoretical capacity < 140 mAh g⁻¹). Additionally, the material has the following drawbacks: (1) initial sodium content is low, so pre-treatment with sodium is needed; (2) P2→O2 phase transformation under high pressure; and (3) poor air stability. The O3-type structure in the MO₂ layer displays Na ions at the O (octahedral) position, with high sodium-ion content, but its structure is prone to damage. In pursuit of a high sodium storage capacity, the O3-type structure with high sodium-ion content shows superior competitiveness. The α-NaFeO₂ cathode is a well-researched O3 layered oxide. However, in order to maintain the TM-O bonds, the TMO₂ layer slides during the sodium ion extraction process, leading to a complex structural evolution of the O3-O′3-P3-P′3-P3′-O3′ in the O3-type layered material [26]. In situ synchrotron radiation studies have confirmed that this multi-step phase transformation is accompanied by a volume change in the crystal unit as high as 6.8%, resulting in the initiation of microcracks in the particles, an increase in interfacial side reactions, and poor cycling stability of the material. To suppress irreversible phase transitions, researchers proposed constructing multivalent sodium ion layered oxides, aiming to enhance the capacity and cycle life of batteries. Following this research direction, a large number of new binary/ternary O3 layered oxides have been reported in recent years, such as Na_5/6Fe_1/2Mn_1/2O₂ [27], NaNi_1/2Mn_1/2O₂ [28], and NaNi_1/3Fe_1/3Mn_1/3O₂ [29], which have improved the battery capacity and cycle life to some extent. Furthermore, metal-ion doping is an effective method that can stabilize the structure of sodium ions, thereby ensuring the cycling stability of SIBs. Metal-ion doping refers to the process of introducing some electrochemically inert ions, such as Zn²⁺, Cu²⁺, Ca²⁺, and Mg²⁺, into the TMO₂ layer [30,31,32,33]. The doping of Cu²⁺ can enhance conductivity, which is prone to causing incomplete reactions or leading to the reduction of copper to metallic copper, thereby causing structural instability. Cu²⁺ doping can enhance conductivity; similarly, Zn²⁺ doping can inhibit phase transformation. Although doping with Zn²⁺ can suppress phase transitions, its strong Jahn-Teller distortion is prone to causing local lattice distortion, and high concentrations of Zn²⁺ can significantly hinder the diffusion kinetics of sodium ions. Ca²⁺ doping can enhance the material’s cycle life. Nonetheless, it may block the diffusion channels of sodium ions as the radius of Ca²⁺ is relatively large (approximately 1.00 Å), resulting in a decrease in rate performance. Studies have shown that the capacity decline is more significant at low temperatures (−10 °C). Among these, doping with Mg²⁺ has significant advantages because it possesses unique physical and chemical properties. The main reasons are as follows: (1) The ionic radius of Mg²⁺ (0.72 Å) closely matches that of Ni²⁺/Ni³⁺ (~0.69 Å), thereby minimizing lattice distortion to the greatest extent; (2) it effectively maintains the charge balance due to its stable +2 oxidation state. Moreover, Mg has abundant resources (shell abundance 2.33%) and a significantly lower cost than Cu and Zn, making it more economically viable for industrialization. However, electrochemically inactive-ion doping reduces the capacity of the material, which is often overlooked in research. Therefore, exploring metal-ion-doped nickel-rich materials that can improve cycling stability while maintaining a high reversible specific capacity is of great significance for promoting the practical application of sodium positive electrode materials.

In this study, we proposed to enhance the rate performance of layered cathode materials by doping with Mg²⁺ and successfully synthesized Mg-doped cathode materials (NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂) using industrially feasible coprecipitation methods. In situ X-ray diffraction (XRD) and electrochemical tests indicated that Mg doping expanded the sodium layer spacing (from 3.128 to 3.188 Å), effectively suppressing the structural distortion during the sodium-ion insertion/extraction process, thereby significantly improving the cycling stability. Particularly, the 2.5% Mg-doped sample (NFM-2.5Mg) exhibited the best performance (capacity retention of 85.4% after 100 cycles at 1C and reversible capacity of 116.4 mAh g⁻¹ at 10C). The above results indicate that NFM-2.5Mg obtained a good balance between the reversible capacity and electrochemical performance. It provides insights and guidance for the improvement of electrochemical properties and practical application of SIBs cathodes with a high theoretical specific capacity.

2. Results and Discussions

In order to investigate the effect of magnesium doping on the structure of O3-type layered materials, X-ray diffraction (XRD) analysis was first conducted on the samples. Figure 1a illustrates the XRD patterns of the NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂ (x = 0, 0.025, 0.05, and 0.075, denoted as NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg) cathode materials. All the obtained XRD patterns show diffraction peaks corresponding to the standard α-NaFeO₂ structure, which belongs to the rhombohedral R-3m space group. The Rietveld refinement experiment was carried out to investigate the structural information of O3-type layered NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg (Figure S1 and Tables S1–S4). Because the size of Mg²⁺ (γ = 0.72 Å) is larger than Ni²⁺ (γ = 0.69 Å), as the doping concentration of magnesium in the crystal structure increases, the lattice parameters (a and c) and the related unit volume continue to increase (Figure 1b) [34]. The quantitative refinement results of the lattice parameter spacing of the sodium layer (S_NaO2) are shown in Figure S2. As shown in Figure 1c, the value of S_NaO2 increases from 3.128 to 3.188 Å for 2.5% Mg substitution. Furthermore, S_NaO2 exhibits a gradual reduction trend when the Mg substitution amount is greater than 2.5%, but it is still greater than the NFM sample. Similarly to the Sb-doped O3-NaNM [35], the spacing of the TM layer (S_TMO2) increases when transition metal-ion doping exceeds a certain concentration, resulting in a gradual decrease of S_NaO2. The higher sodium layers tend to promote the migration of sodium ions.

We conducted an elemental content analysis of the samples using the high-precision and high-sensitivity inductively coupled plasma optical emission spectrometry (ICP-OES) method to accurately determine the elemental content of the samples, aiming to rigorously verify whether the actual doping concentration of magnesium (Mg) is consistent with the preset theoretical design value. The actual mole ratios of Mg in NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg are 0.0247, 0.0483, and 0.0741, respectively, as shown in Table S5. The deviations from the theoretical values (0.025, 0.050, and 0.075) are all less than 5%, indicating that the coprecipitation method can achieve uniform doping of Mg elements and precise stoichiometric control. What is particularly crucial is that the actual elemental ratios of Ni, Fe, and Mn also match the theoretical design values. This result conclusively confirms that during the entire material synthesis process, the key transition metal elements did not exhibit significant segregation or loss, thereby ensuring the integrity and consistency of the chemical composition of the target material.

Additionally, the images of the synthesized NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg cathode materials captured by scanning electron microscopy (SEM) are shown in Figure 2. It can be seen that all the samples exhibit a dense spherical structure with a size of ~5 μm. These spherical particles are formed by the close packing of smaller sheet-like original particles. It is worth noting that plate-like primary particles size of all the doped samples synthesized in this study has been significantly reduced to ~1 μm, which is unlike the primary particle size (~2 μm) of similar materials reported in the literature [36]. This micro–nano structure design offers multiple advantages: (1) The smaller primary particles significantly shorten the diffusion path of lithium ions within the particles; (2) the densely packed spherical structure effectively increases the material’s tap density. Therefore, this compact structure is expected to significantly enhance the volumetric energy density of the final SIBs.

Furthermore, the specific surface areas of the NFM, NFM-2.5Mg, NFM-5.0Mg, and MFM-7.5Mg were measured using the Brunauer–Emmett–Teller (BET) instrument, and the results are shown in Figure 3. The specific surface areas of the NFM, NFM-2.5Mg, NFM-5.0Mg, and MFM-7.5Mg cathode materials are 0.34654, 0.35566, 1.77181, and 2.20602 m² g⁻¹, respectively. The specific surface area of the material shows an upward trend as the doping amount of Mg increases, which is consistent with the observed reduction in particle size by SEM (Figure 2).

The electrochemical performances of the prepared cathodes were evaluated through half-cells (CR 2032). As shown in Figure 4a, the NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg electrodes have an obvious potential plateau during the first two charge/discharge cycles at a current density of 0.2C, indicating that the cathode material undergoes a classic reversible phase transition [37,38]. The initial reversible capacities of the NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg samples are 149.6, 148.1, 144.6, and 142.4 mAh g⁻¹, respectively, with corresponding initial coulombic efficiencies of 92.5%, 96.2%, 93.2%, and 92.2%. During Na⁺ extraction/insertion, Ni provides the reversible capacity through redox Ni²⁺/Ni⁴⁺ reactions, while Mg has no electrochemical activity. Therefore, the excessive doping of Mg can have a negative impact on the reversible capacity. When the doping amount of Mg is 2.5%, the reversible capacity of the NFM-2.5Mg cathode hardly decreases. In addition, the NFM-2.5Mg sample possesses the highest initial coulombic efficiency, indicating that appropriate Mg substitution can improve the reversibility of this electrode within the potential range of 2.0–4.0 V (vs. Na⁺/Na). After the initial two cycles of activation, the cycling stability of NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg samples is examined at a current density of 150 mA g⁻¹ (equivalent to 1C rate) conditions, as shown in Figure 4b.

Although the NFM sample exhibits a high first specific discharge capacity (141.5 mAh g⁻¹), after 100 cycles the specific discharge capacity and capacity retention are only 109.8 mAh g⁻¹ and 77.6%, which is attributed to structural damage caused by irreversible phase transitions. The specific discharge capacities after 100 cycles of NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg are 121.4, 119.1, and 115.6 mAh g⁻¹, and the retention rates are 85.4%, 85.5%, and 85.6%, respectively. When the doping concentration of Mg is greater than 2.5%, the improvement in cycling stability is no longer significant.

As shown in Figure 4c, the rate capabilities of the NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg samples were tested at different densities ranging from 0.2C to 10C. The NFM-2.5Mg electrode showed the best rate capacity with the reversible specific capacities. The reversible specific capacity at different current rates (0.2C, 0.5C, 1C, 2C, 5C, and 10C) was 147.0, 139.7, 135.3, 130.0, 123.2, and 116.4 mAh g⁻¹, respectively. Moreover, the capacity retention rate of NFM-2.5Mg at 10C is 79.2% of that at 0.2C, but the NFM capacity retention was just 57.3%. The specific discharge capacity of NFM-2.5Mg rapidly recovered to 138.8 mAh g⁻¹, showing the excellent reversible performance of NFM-2.5Mg when the current density returned to 0.2C.

Furthermore, the cyclic voltammetry (CV) tests were conducted within the voltage range of 2.0–4.2 V (vs. Na⁺/Na) at a scan rate of 0.1 mV s⁻¹. The oxidation peak appears around 3.0 V (vs. Na⁺/Na), indicating the structural transition from the O3 phase to P3 phase during the extraction of Na ions, whereas the oxidation peak within in the range of 4.0–4.2 V (vs. Na⁺/Na) represents the irreversible transformation of P3 to the O3′′ phase [39]. As displayed in Figure 5a, with the increase in the content of Mg doping, the oxidation peak around 3.0 V (vs. Na⁺/Na) shifts towards a higher potential; meanwhile, the peak intensity within the range of 4.0–4.2 V (vs. Na⁺/Na) decreases. This indicates that the electrochemical inertness of magnesium enables the layered structure of the material to be stabilized, slows down the irreversible phase transformation process from the P3 phase to the O3″ phase, and thereby provides a higher possibility for transformation from the O3 phase to the P3 phase.

The differences in cyclic stability and rate capability between NFM and NFM-Mg electrodes are attributed to the varying doping concentrations of Mg ions. The Mg ions in the TMO₂ layer can enlarge the cell parameters, reduce the concentration of S_NaO2, while also lowering the diffusion barrier of sodium ions [35,40]. This indicates that an appropriate amount of Mg ions is beneficial for enhancing cyclic stability and rate capability. However, the sodium layer spacing will decrease when the concentration of magnesium ions exceeds the appropriate range, thereby reducing the transmission rate of sodium ions and leading to a decline in electrochemical performance [41]. This effect is particularly significant at a high current rate.

The performance of electrode materials in a low-temperature environment is one of the crucial factors in determining their practical application. The cycling stability and rate performance of NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg electrodes were tested at 0 °C. The NFM-2.5Mg electrode exhibited the best cycle stability, with an initial specific discharge capacity after 150 cycles of up to 128.3 mAh g⁻¹ at 1C and a capacity retention of 86.6%, as shown in Figure 5b. However, the capacity retentions for the NFM, NFM-2.5Mg, and NFM-5.0Mg electrodes are only 66.6%, 83.3%, and 83.1%, respectively. The NFM-2.5Mg electrode still had a better rate performance at 0 °C; the reversible specific capacities of NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg were 98.6, 109.1, 93.8, and 91.2 mAh g⁻¹ at 5C, respectively (Figure 5c). Additionally, the rate performance differences between NFM and NFM-2.5 mg were explored using the constant current intermittent titration technique (GITT), as shown in Figure S3. The testing methods and calculation formulas are detailed in the Supporting Information. In Figure S3, it can be observed that the sodium-ion diffusion coefficient variation laws of NFM and NFM-2.5Mg cathode materials are significantly different in the voltage range of 2.0–4.0 V (vs. Na⁺/Na). The sodium-ion diffusion coefficient (D_Na+) of the NFM-2.5Mg is significantly higher than that of the NFM, which is 10⁻¹⁰–10⁻¹² cm⁻² s⁻¹ during the charging process. However, the D_Na+ of the NFM fluctuates greatly, indicating that its structure is unstable. This instability has a greater impact at high current rates, thereby resulting in a poor rate performance.

Furthermore, the rate performance and cycle performance of NFM-2.5Mg are superior to those of some previously reported doped O3 layered oxide cathode materials [27,32,35,41,42,43,44]. It is worth noting that we also conducted a low-temperature test, and this was rarely mentioned in the previously reported literature (Table S3). This confirms the crucial role of expanding the interlayer spacing in enhancing the ion’s transport kinetics.

During the initial cycling process (with the voltage states of 2.0–4.0 V (V vs. Na⁺/Na) at 0.1C), in situ XRD analysis was conducted on the NFM-2.5Mg cathode, leading to a deeper understanding of, during the discharge/charge process, the influence of Mg substitution on structural evolution. The O3-(003) peak (2θ = 16.8°) and O3-(006) peak (2θ = 33.9°) gradually shifted to lower angles with the initial Na extraction, as shown in Figure 6a. The O3 phase completely transforms into the P3 phase as the O3-(104) peak disappears. During the subsequent charging processes, the P3 phase peak continued to shift until it reached 4.0 V (vs. Na⁺/Na) without any new peaks. When the voltage further dropped to 2.0 V (vs. Na⁺/Na), the NFM-2.5Mg returned to its original O3 electrode state. This was in sharp contrast to the charging process, indicating that the phase transition process is highly reversible. The absence of the O3″ phase during charging and discharging indicates that irreversible P3-O3″ phase transformation can be suppressed by Mg doping [36,45]. These findings demonstrate that Mg substitution can reinforce the layered structure and impede the irreversible phase transformation of the O3 cathode material throughout the discharging and charging process.

The O3-type materials are susceptible to oxidation in humid air, leading to the delocalization of Na ions to react with H₂O and CO₂, forming Na₂CO₃·nH₂O on the material surface, thereby disrupting their layered structure [46]. The aging experiments (the material was subjected to a 10 min immersion in water, followed by a 24 h drying process at 110 °C) were performed on NFM and NFM-2.5Mg to test the structural stability in humid air.

It can be seen from the XRD patterns that both NFM and NFM-2.5Mg undergo structural changes after aging experiments (Figure 6b,c). In the layered structure, Na⁺ escaped to the surface of the material and reacted with CO₂ and H₂O, thereby forming the Na-deficient O′3-Na_1-yNi_0.6Fe_0.25Mn_0.15O₂ and O′3-Na_1-yNi_0.575Fe_0.25Mn_0.15Mg_0.025O₂ phase with sodium deficiency, as well as the dolomite hydration phase and Na₂CO₃·nH₂O [47]. After the aging experiment, the intensity of the O’3-(003) peak in NFM-2.5Mg was higher and the peak shape was clearer, while the intensity of the dolomite peak was weaker. This indicates that less sodium ions were removed from NFM-2.5Mg. During the aging process, the substitution of Mg significantly hindered the extraction of Na⁺, which contributed to the air stability of the O3-type layered oxide materials.

3. Experimental Section

3.1. Material Preparation

The stoichiometric ratios of nickel sulfate hexahydrate (NiSO₄·6H₂O, Shanghai McLyn Biochemical Technology Co., Ltd., Shanghai, China), manganese sulfate monohydrate (MnSO₄·H₂O, Shanghai McLyn Biochemical Technology Co., Ltd.), iron sulfate heptahydrate (FeSO₄·7H₂O, Shanghai McLyn Biochemical Technology Co., Ltd.), magnesium sulfate hexahydrate (MgSO₄·6H₂O, Shanghai McLyn Biochemical Technology Co., Ltd.), sodium hydroxide (NaOH, Aladdin Biochemical Technology Co., Ltd., Shanghai, China), and ammonium hydroxide (NH₃·H₂O, Aladdin Biochemical Technology Co., Ltd.) were utilized as the starting materials to synthesize the Ni_0.6−xFe_0.25Mn_0.15Mg_x(OH)₂ (x = 0, 0.025, 0.05, and 0.075) precursor through a hydroxide coprecipitation method. The stoichiometrically achieved precursor is thoroughly mixed with an excess of 2 mol% NaOH (to compensate for sodium loss due to the volatilization at elevated temperatures, serving as the essential sodium source) and preheated for 6 h at 450 °C. Then, it was calcined at 750 °C under the flow of O₂ for 12 h and subsequent slow cooling.

This was followed by calcination at 750 °C for 12 h under flowing O₂. The resulting NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂ (x = 0, 0.025, 0.05, and 0.075, corresponding materials NFM, NFM-2.5Mg, NFM-5.0Mg, and NFM-7.5Mg) materials were stored in an Ar-filled glove box.

3.2. Material Characterization

The X-ray powder diffraction (XRD) analysis was conducted using copper Cu Kα radiation (λ = 1.54184 Å, BRUKER, D8 ADVANCE) within a scanning range of 10° to 80° to determine the structure and phases of the O3-NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂ cathode materials. To obtain detailed structural information, the XRD data were refined to the Rietveld refinement analysis using the Fullprof program. The prepared powders were examined by scanning electron microscopy (SEM, S-4800, Hitachi (China), Ltd., Shanghai, China). In situ X-ray diffraction (XRD) tests were carried out on cells with Be window using the D8 ADVANCE(Bruker AXS, Beijing, China) diffractometer. Element content analysis was carried out using inductively coupled plasma optical emission spectrometry (ICP-OES, Avio 500, PerkinElmer Instruments Co., Ltd. Shenzhen, China).

3.3. Electrochemical Measurements

The cathodes consisted of 80 wt% NFM or NFM-Mg sample, 10 wt% conductive agent (Super-P carbon), and 10 wt% binder (PVDF, polyvinylidene fluoride). All materials were combined with organic solvent (N-methyl-2-pyrrolidinone, NMP) to create a uniform slurry. Then, it was coated onto the Al foil (12 μm) and subjected to vacuum drying for approximately 16 h at 110 °C. The loading amount of the active material was within the range of 2.0–2.5 mg cm⁻².

All cells (CR 2032) were prepared in an Ar-filled glove box. The electrolyte comprised 1 M NaPF₆ mixed with three types of organic solvents (ethylene glycol carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:1). Additionally, 5% (volume ratio) of additive (4-fluoro-1,3-dioxolane) was added to the aforementioned electrolyte. The diaphragm is made of glass fiber that separates the cathode and sodium metal anode.

The constant current charge and discharge process was measured using a LAND test system (CT3001AU, Wuhan Blue Power Electronics Co., Ltd., Wuhan, China). The cyclic voltammogram curves were collected through the electrochemical multi-channel workstation (Autolab, PGSTAT302N, Maoqiang Technology (Shanghai) Co., Ltd., Chengdu, China).

The low-temperature performance was evaluated in a test chamber (GZ-THERMOTEX, MH150A-40, Shanghai Yiheng Technology Co., Ltd.,Shanghai, China) with a constant temperature of 0 °C.

4. Conclusions

In conclusion, we have prepared Mg-doped O3-NaNi_0.6−xFe_0.25Mn_0.15Mg_xO₂ cathode materials using industrially feasible coprecipitation methods. Through the meticulous synthesis of the rich nickel cathode material, the optimized NFM-2.5Mg demonstrated an excellent high specific discharge capacity (148.1 mAh g⁻¹ under 0.2C conditions), excellent cycle stability (after 100 cycles, capacity retention of 85.4% at 1C), and good rate performance (116.4 mAh g⁻¹ at 10C). Even at 0 °C, the NFM-2.5Mg electrode demonstrates good cycling stability (after 150 cycles, capacity retention of 86.6% at 1C) and outstanding rate performance (reversible capacity of 109.1 mAh g⁻¹ at 5C). Notably, Rietveld refinement, electrochemical, and in situ XRD tests suggest that by introducing the alkaline earth metal Mg²⁺ with lower covalency and a larger ionic radius to replace Ni²⁺, the Na layer spacing can be expanded (from 3.128 to 3.188 Å), and the trigger potential for anion redox reactions can be enhanced, thereby suppressing irreversible phase transitions during charge/discharge processes (no O3″ phase formation). Furthermore, the aging experiments indicate that the NFM-2.5Mg electrode exhibits better water resistance than the original NFM cathode. These results provide theoretical and technical support for designing and preparing the high specific capacity, long cycle life, and air-stable sodium-ion O3-type cathode materials, promoting the practical application of SIBs.