Ca / Mg co-doped sodium nickel manganese oxide positive electrode material and preparation method thereof, pole piece and battery

Abstract

The invention discloses a Ca / Mg co-doped sodium nickel manganese oxide positive electrode material, a preparation method thereof, a pole piece and a battery, and belongs to the field of batteries. According to the Ca / Mg co-doped sodium nickel manganese oxide positive electrode material, three different metal cations of nickel, manganese and magnesium are contained in a transition metal layer, two different metal cations of sodium and calcium are contained in a sodium layer, and the chemical formula of the Ca / Mg co-doped sodium nickel manganese oxide positive electrode material is Na < 0.61 > Ca < 0.03 > Ni < 0.23 > Mg < 0.10 > Mn < 0.67 > O2; the Ca / Mg co-doped sodium nickel manganese oxide positive electrode material is of a P2 type; the problem that an existing P2 type Na < 0.67 > Ni < 0.33 > Mn < 0.67 > O < 2 > positive electrode material is poor in cycling stability and structural reversibility in a high-voltage interval is solved.

Description

Technical Field

[0001] This invention relates to the field of batteries, specifically to a Ca / Mg co-doped sodium nickel manganate cathode material, its preparation method, electrode sheet, and battery. Background Technology

[0002] The rapid development of renewable energy infrastructure has driven an urgent demand for advanced energy storage technologies and smart grid systems. Although lithium-ion batteries currently dominate the market, their long-term sustainability is challenged by limited lithium resources and rising costs. This context has accelerated the exploration of alternative energy storage systems that combine cost advantages and performance competitiveness. Sodium-ion batteries, due to their abundant sodium reserves, low cost, and similar electrochemical principles to mature lithium batteries, are emerging as a highly promising alternative technology. The energy density of sodium-ion batteries primarily depends on the cathode material. Among them, layered oxide cathodes are considered ideal due to their high theoretical capacity, fast two-dimensional sodium-ion transport channels, and good manufacturability. Among various layered oxides, P2-type and O3-type Na... x The TMO2 architecture, with its excellent balance between performance and practical application potential, is leading the commercialization process of this system.

[0003] The P2 type structure has an open prismatic Na + The site and stable lattice framework exhibit superior air stability and more efficient Na₂O₃ extraction. + It exhibits improved migration ability and enhanced structural stability. Among these, the manganese-rich component, Na... x Ni y Mn z O2 materials are known for their high energy density potential (derived from Ni). 2+ / Ni 4+ The high-voltage redox activity and the economic advantages of manganese have become current research hotspots. However, P2-type Na... x Ni y Mn z O2 cathodes still face a series of challenges in practical applications: their inherently low sodium content, susceptibility to Na⁺ / vacancy ordering, significant volume changes accompanying phase transitions, and the material's inherent hygroscopicity. These issues collectively restrict their electrochemical performance and the feasibility of large-scale production. Existing technology one discloses a Mg-doped P2-type cathode material Na… 0.67 Ni 0.33-x Mn 0.67 Mg xO2 (Existing technology 1: Feng J, Luo S, Wang J, et al. Stable electrochemical properties of magnesium-doped Co-free layered P2-type Na) 0.67 Ni 0.33 Mn 0.67 O2 cathode material for sodium ion batteries[J]. ACSSustainable Chemistry & Engineering, 2022, 10(15): 4994-5004.), in which Mg was successfully produced by ball milling. 2+ Introducing Na 0.67 Ni 0.33 Mn 0.67 O2 crystal lattice and form solid solution, Mg 2+ Partially replaced Ni 2+ / Ni 3+ Ni redox reaction 2+ During the charging process, more Na is promoted. + Retained at the triangular prism sites, thus maintaining the overall charge balance of the system. This doping effectively suppresses the P2 to O2 phase transition during charge and discharge, significantly improving the cycling stability of the material. However, single-element doping often fails to achieve ideal comprehensive electrochemical performance under high voltage conditions. The existing technology only provides limited improvement in the 2.0-4.3V voltage range, and its long-cycle testing is only conducted at a low rate of 0.1C, which is insufficient to meet the requirements of practical applications for high power and long lifespan.

[0004] Existing technology 2 successfully prepared P2-type Na by simultaneously introducing Zn and Ti into the transition metal layer through ball milling. 0.66 Ni 0.27 Zn 0.06 Mn 0.61 Ti 0.06O2 materials (Prior technology 2: Huang J, Xu L, Ye D, et al. Suppressing the P2–O2 phase transition of P2-type Ni / Mn-based layered oxide by synergistic effect of Zn / Ti co-doping for advanced sodium-ion batteries[J]. Journal of Alloys and Compounds, 2024, 976: 173397.). The Zn / Ti co-doping strategy effectively reduces lattice strain and suppresses the harmful P2→O2 phase transition under high voltage, thereby significantly enhancing the cycle durability of the electrode; however, although this technical solution introduces two doping elements at the same time, they are both concentrated in the transition metal layer, and only the structure of this layer is modified, so the improvement on the overall electrochemical performance of the material is still relatively limited. Furthermore, the improvement in electrode performance in this study is limited to the 4.3V cutoff voltage range and cannot be extended to higher voltage (>4.3V) operating ranges. All electrochemical tests were conducted only at room temperature, lacking systematic characterization over a wider temperature range (such as low or high temperatures), thus failing to comprehensively assess its practical environmental adaptability. In addition, P2-type layered oxides are generally sensitive to moisture and are prone to structural degradation and electrochemical performance decay due to H2O intrusion. This technical solution did not conduct relevant tests on the material's moisture resistance and environmental stability, failing to verify its reliability under real storage and operating conditions, thus limiting its practical application potential.

[0005] Although prior art patent CN 118315567 A discloses a dual-site metal ion-doped sodium nickel iron manganate cathode material, the chemical formula of the dual-site metal ion-doped sodium nickel iron manganate cathode material is: [Na 1-x M x / n ](Ni a Fe b Mn c ) 1-y N y O2, where M is one or both of K and Ca metal ions, N is one or more of Mg, Al, Ti, Zn, Zr, Cu, Nb, and W metal ions, n is the valence state of the M metal ion, 0 < x, y ≤ 0.1, and the values ​​of a, b, and c depend on charge balance. The transition metal layer of the sodium cathode material is doped with transition metals Mg, Al, Ti, Zn, Zr, Cu, Nb, and W, whose ionic radii are close to those of nickel, iron, and manganese. The stronger binding energy between the doped atoms and oxygen suppresses the slippage of the transition metal layer during sodium ion insertion / extraction. Simultaneously, K, whose ionic radius is close to and larger than that of sodium ions, is utilized. + or Ca2+ Doping into the sodium layer of the cathode material broadens the transport path of sodium ions, thereby improving material stability while ensuring the preservation of Na+. + Normal diffusion allows the material to maintain its original discharge specific capacity and improve cycle stability. However, the practical application of O3-type layered oxides is still limited by several inherent defects, including narrow Na+ diffusion. + Problems such as diffusion channels, poor environmental stability, and irreversible phase transitions lead to its Na + Transmission dynamics are sluggish and capacity retention is low.

[0006] Therefore, achieving a balance among multiple performance characteristics, designing a P2-type cathode material with strong cycle stability and structural reversibility under high voltage range, suppressing its structural degradation caused by moisture erosion in humid environments, expanding its operating temperature range, and enhancing its electrochemical adaptability in low and high temperature environments are all of great significance for promoting the transformation of this material system into practical application scenarios and developing durable and commercially viable sodium-ion batteries. Summary of the Invention

[0007] This solution provides Ca / Mg co-doped sodium nickel manganate cathode material, its preparation method, electrode sheet, and battery, solving the problem of P2-type Na 0.67 Ni 0.33 Mn 0.67 This study addresses the poor cycle stability and structural reversibility of O2 cathode materials in the high-voltage range (>4.3V) and suppresses structural degradation caused by moisture erosion in humid environments. Simultaneously, it expands the operating temperature range and enhances the electrochemical adaptability under both low and high temperature environments, thereby promoting the transformation of this material system into practical applications.

[0008] To alleviate or partially alleviate the above-mentioned technical problems, the solution of the present invention is as follows:

[0009] The Ca / Mg co-doped sodium nickel manganate cathode material contains three different metal cations (nickel, manganese, and magnesium) in the transition metal layer and two different metal cations (sodium and calcium) in the sodium layer. The chemical formula of the Ca / Mg co-doped sodium nickel manganate cathode material is Na. 0.61 Ca 0.03 Ni 0.23 Mg 0.10 Mn 0.67 O2;

[0010] The Ca / Mg co-doped sodium nickel manganate cathode material is of type P2.

[0011] The synergistic effect of Ca and Mg stabilized Ni 2+ / Ni 3+ The redox couple expanded the range of Na.+ Interlayer spacing, optimized Na + The vacancy ordering inhibits cracking of the active material and mitigates structural degradation during high-voltage cycling. The synergistic effect of these effects enables the material to achieve excellent capacity retention and superior rate performance at room temperature, while also exhibiting improved electrochemical performance at high cutoff voltages. Furthermore, the Ca / Mg-doped cathode demonstrates excellent environmental adaptability, maintaining stable electrochemical performance under humid conditions and over a wide temperature range.

[0012] This solution also provides a method for preparing the above-mentioned Ca / Mg co-doped sodium nickel manganate cathode material, including the following steps:

[0013] Step S1: Dissolve nickel salt, manganese salt and magnesium salt in deionized water in stoichiometric proportions to prepare metal salt solutions; at the same time, dissolve Na2C2O4 in deionized water to prepare sodium oxalate solutions.

[0014] Step S2: The metal salt solution and sodium oxalate solution are stirred in a constant temperature water bath. The metal salt solution is added dropwise to the sodium oxalate solution at a constant dropping rate. After the addition is complete, stirring continues to complete the co-precipitation. The resulting precipitate is filtered, washed, and dried to obtain (Ni) 0.23 Mg 0.10 Mn 0.67 C2O4·2H2O precursor;

[0015] Step S3: The obtained precursor is thoroughly mixed with stoichiometric CaCO3 and an excess of 5% stoichiometric Na2CO3, heated to 900°C in air atmosphere and calcined, and then cooled in the furnace to obtain Ca / Mg co-doped sodium nickel manganate cathode material.

[0016] Preferably, the nickel salt in step S1 is selected from NiSO4·6H2O; the manganese salt is selected from MnSO4·H2O; and the magnesium salt is selected from MgSO4·7H2O.

[0017] The total concentration of the metal salt solution in step S1 is 1.5 mol L. -1 .

[0018] Preferably, the concentration of the sodium oxalate solution in step S1 is 0.45 mol / L. -1 .

[0019] Preferably, the constant temperature water bath temperature in step S2 is 70 °C.

[0020] Preferably, the dropping rate of the metal salt solution in step S2 is 2 mL / min.

[0021] Preferably, the stirring time in step S2 is 4 hours; the drying is vacuum drying at a temperature of 120°C for 12 hours.

[0022] Preferably, the heating rate in step S3 is 5°C / min; the calcination time is 13 h.

[0023] This solution also provides an electrode, which comprises the aforementioned Ca / Mg co-doped sodium nickel manganate cathode material.

[0024] This solution also provides a battery, which includes the aforementioned electrode, counter electrode, and separator, wherein the separator is disposed between the electrode and the counter electrode.

[0025] The technical solution of this invention has the following beneficial technical effects:

[0026] (1) The multi-site synergistic doping strategy can achieve multi-dimensional control of the electronic structure, ion channels and interface stability of materials by precisely introducing different elements into different lattice sites (alkali metal sites and transition metal sites). This strategy helps to rationally design sodium-ion layered oxide cathode materials with high energy density, excellent structural integrity and long cycle life, and provides a more feasible material basis for their practical application.

[0027] (2) The core innovation of this invention lies in the use of a targeted doping strategy, that is, precise doping is performed in the sodium layer and the transition metal layer respectively. Specifically, a Ca / Mg co-doped P2 type material Na was successfully prepared by combining co-precipitation and heat treatment. 0.61 Ca 0.03 Ni 0.23 Mg 0.10 Mn 0.67 O2. Wherein, Ca 2+ The sodium and Mg layers were positioned and introduced into the sodium layer. 2+ It is then precisely incorporated into the transition metal layer, enabling independent control of different structural sites.

[0028] (3) Improve the cycling stability and structural reversibility in the high voltage range (>4.3V) and suppress structural degradation caused by moisture erosion in humid environments. At the same time, expand its operating temperature range and enhance its electrochemical adaptability in low and high temperature environments, thereby promoting the transformation of this material system into practical application scenarios. Attached Figure Description

[0029] Figure 1 It is the XRD pattern of the material;

[0030] Figure 2 This is a Rietveld refinement image of the material's XRD pattern;

[0031] Figure 3These are SEM images of undoped pristine and Ca / Mg-doped pristine, as well as elemental distribution diagrams of Ca / Mg-doped pristine.

[0032] Figure 4 This is an HRTEM image of a Ca / Mg-doped sample;

[0033] Figure 5 It is the XPS spectrum of the material;

[0034] Figure 6 This is the XPS spectrum of the Ca / Mg-doped sample;

[0035] Figure 7 These are the Ni 2p and Mn 2p spectra of the material;

[0036] Figure 8 The images show the in-situ XRD patterns and corresponding charge-discharge curves of the pristine sample during the first cycle at 0.1 C.

[0037] Figure 9 The images show the in-situ XRD patterns and corresponding charge-discharge curves of the Ca / Mg-doped sample during the first cycle at 0.1 C.

[0038] Figure 10 This is the XPS spectrum of the Ca / Mg-doped sample during the first cycle;

[0039] Figure 11 These are the initial charge-discharge curves of the material in the voltage ranges of 2.0-4.3 V and 2.0-4.5 V;

[0040] Figure 12 The performance of the materials of Example 1 and Comparative Examples 1-3 after cycling for 200 cycles in voltage ranges of 2.0-4.3 V and 2.0-4.5 V;

[0041] Figure 13 These are the charge-discharge curves of the materials in Example 1 and Comparative Examples 1-3 in the voltage range of 2.0-4.3 V;

[0042] Figure 14 These are the charge-discharge curves of the materials in Example 1 and Comparative Examples 1-3 in the voltage range of 2.0-4.5 V;

[0043] Figure 15 This refers to the rate performance of the material in the voltage ranges of 2.0-4.3 V and 2.0-4.5 V.

[0044] Figure 16 These are the charge-discharge curves of the material at different rates within a voltage range of 2.0-4.3 V;

[0045] Figure 17 These are the charge-discharge curves of the material at different rates within a voltage range of 2.0-4.5 V;

[0046] Figure 18 The tests included cyclic stability testing of the material at 8 C and cyclic stability testing of the Ca / Mg-doped sample at 20 C.

[0047] Figure 19 This is a comparison of the cycle performance of Ca / Mg-doped cathode and electrode materials reported in the literature;

[0048] Figure 20 It is the dQ / dV curve of the material in the voltage range of 2.0-4.3 V;

[0049] Figure 21 It shows the dQ / dV curve of the sample in the voltage range of 2.0-4.5 V;

[0050] Figure 22 It is at 0.1 mV s -1 CV curves of pristine and Ca / Mg-doped samples at scan rates;

[0051] Figure 23 The CV contour plots and CV curves of pristine and Ca / Mg-doped electrodes at different scan rates are shown.

[0052] Figure 24 It represents the linear relationship and corresponding fitting, as well as the contribution ratio of capacitive behavior and diffusion control behavior;

[0053] Figure 25 These are GITT plots for Pristine, Ca-doped, Mg-doped, and Ca / Mg-doped.

[0054] Figure 26 The EIS spectra of pristine and Ca / Mg-doped samples after different cycles are shown.

[0055] Figure 27 It is a comparative analysis of the in-situ electrochemical impedance spectroscopy after 50 cycles and the evolution of charge transfer resistance during the cycling process;

[0056] Figure 28 These are SEM images of the Ca / Mg-doped electrode and the pristine electrode after 0 and 50 cycles.

[0057] Figure 29The results show the XRD Rietveld refinement of the electrode after 50 cycles at 1 C and a comparison of the XRD patterns of the Ca / Mg-doepd and pristine electrodes after 50 cycles.

[0058] Figure 30 The XRD patterns of pristine and Ca / Mg doped electrodes after different immersion times are shown.

[0059] Figure 31 At high loading (5 mg cm -2 Electrochemical performance of pristine and Ca / Mg-doped samples under the following conditions;

[0060] Figure 32 This is a graph showing the electrochemical performance at -15 °C.

[0061] Figure 33 This is a graph showing the electrochemical performance at 55 °C.

[0062] Figure 34 This is a schematic diagram of the full cell's operation and its electrochemical performance.

[0063] Figure 35 It shows the cycle performance of the full battery at 1 C rate for 150 cycles and its corresponding charge-discharge curves. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0065] Example 1

[0066] First, stoichiometric amounts of NiSO4·6H2O (1.84 g), MnSO4·H2O (3.38 g), and MgSO4·7H2O (0.74 g) were dissolved in 20 mL of deionized water to prepare a metal salt solution; simultaneously, Na2C2O4 (4.02 g) was dissolved in 70 mL of deionized water to prepare another solution. Both solutions were stirred in a 70 °C constant temperature water bath, and the metal salt solution was added dropwise to a sodium oxalate solution at a rate of 2 mL / min. Stirring continued for 4 h to complete co-precipitation. The resulting precipitate was filtered, washed, and vacuum dried at 120 °C for 12 h to obtain (NiSO4·6H2O) solution. 0.23 Mg 0.10 Mn 0.67C2O4·2H2O precursor. This precursor was thoroughly mixed with stoichiometric CaCO3 (0.014 g) and an excess of 5% stoichiometric Na2CO3 (0.21 g), and calcined in air at a rate of 5°C / min to 900°C for 13 h. The mixture was then cooled in the furnace to obtain the final target product, Na. 0.61 Ca 0.03 Ni 0.23 Mg 0.10 Mn 0.67 O2 (denoted as Ca / Mg-doped).

[0067] Comparative Example 1

[0068] First, NiSO4·6H2O and MnSO4·H2O in stoichiometric proportions were dissolved in deionized water to prepare a metal salt solution; simultaneously, Na2C2O4 was dissolved in deionized water to prepare another solution. Both solutions were stirred in a 70 °C constant temperature water bath, and the metal salt solution was added dropwise to the sodium oxalate solution at a rate of 2 mL / min. Stirring continued for 4 h to complete co-precipitation. The resulting precipitate was filtered, washed, and vacuum dried at 120 °C for 12 h to obtain the precursor. This precursor was thoroughly mixed with an excess of 5% stoichiometric Na2CO3, and calcined in air at a temperature increased to 900 °C at 5 °C / min for 13 h. The calcined product, Na2C2O4, was obtained by furnace cooling. 0.67 Ni 0.33 Mn 0.67 O2 (denoted as Pristine).

[0069] Comparative Example 2

[0070] First, stoichiometric amounts of NiSO4·6H2O and MnSO4·H2O were dissolved in 20 mL of deionized water to prepare a metal salt solution; simultaneously, Na2C2O4 was dissolved in 70 mL of deionized water to prepare another solution. Both solutions were stirred in a 70 °C constant temperature water bath, and the metal salt solution was added dropwise to a sodium oxalate solution at a rate of 2 mL / min. Stirring was continued for 4 h to complete co-precipitation. The resulting precipitate was filtered, washed, and vacuum dried at 120 °C for 12 h to obtain the precursor. This precursor was thoroughly mixed with stoichiometric amounts of CaCO3 and an excess of 5% stoichiometric amount of Na2CO3, and calcined in air at a temperature of 5 °C / min to 900 °C for 13 h. The calcined product, Na2C2O4, was obtained by furnace cooling. 0.61 Ca 0.03 Ni 0.33 Mn 0.67 O2 (denoted as Ca-doped).

[0071] Comparative Example 3

[0072] First, NiSO4·6H2O, MnSO4·H2O, and MgSO4·7H2O in stoichiometric proportions were dissolved in 20 mL of deionized water to prepare a metal salt solution; simultaneously, Na2C2O4 was dissolved in 70 mL of deionized water to prepare another solution. Both solutions were stirred in a 70 °C constant temperature water bath, and the metal salt solution was added dropwise to a sodium oxalate solution at a rate of 2 mL / min. Stirring was continued for 4 h to complete co-precipitation. The resulting precipitate was filtered, washed, and vacuum dried at 120 °C for 12 h to obtain the precursor. This precursor was thoroughly mixed with an excess of 5% stoichiometric Na2CO3, and calcined in air at a temperature increased to 900 °C at 5 °C / min for 13 h. The calcined product, Na2C2O4, was obtained by furnace cooling. 0.67 Ni 0.23 Mg 0.10 Mn 0.67 O2 (denoted as Mg-doped).

[0073] Example 2

[0074] The materials obtained in Examples 1 and Comparative Examples 1-3 were mixed with polyvinylidene fluoride (PVdF) and conductive carbon black (SuperP) at a mass ratio of 85:8:7, dispersed in N-methylpyrrolidone (NMP) to form a uniform slurry, and then uniformly coated onto aluminum foil with a doctor blade and dried at 120°C for 6 hours. The prepared electrode sheets were pressed by two rollers and then cut into discs with a diameter of 14 mm and an active material loading of 2-3 mg, which were used as working electrodes. Sodium metal was rolled and cut into discs with a diameter of 16 mm, which were used as counter electrodes. In an argon-filled glove box, using GF / D glass fiber filter paper as the separator, a 1M sodium perchlorate (NaClO4) solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1) containing 5 wt% fluoroethylene carbonate (FEC) as the electrolyte, a CR2032 button cell was assembled. The CT-4008T battery testing system (Shenzhen Xinwei Electronics Co., Ltd.) was used for galvanostatic charge-discharge (GCD) testing (charge-discharge tests were performed in the ranges of 2.0-4.3 V and 2.0-4.5 V, respectively) and galvanostatic intermittent titration (GITT) testing. The PARSTAT4000A electrochemical workstation (Princeton University, USA) was used for electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) testing.

[0075] The electrode sheet with high active material loading uses an active material loading of 5 mg cm⁻¹ -2 The electrode plates were tested.

[0076] Example 3

[0077] Based on Example 2, commercial hard carbon (HC) was used as the negative electrode, and pristine and Ca / Mg-doped positive electrodes and a separator were assembled into a full cell, and its electrochemical performance was systematically tested and evaluated.

[0078] Figure 1 (a) shows the XRD patterns of the four samples from Example 1 and Comparative Examples 1-3. All diffraction peaks are sharp and clear, consistent with the P2-type layered oxide structure (space group P63 / mmc, PDF #27-0751). Compared with the undoped samples, Ca... 2+ or Mg 2+ The absence of impurity phase diffraction peaks after doping indicates that the doping did not alter the P2-type bulk structure of the material. Figure 1 (b) shows that Ca 2+ / Mg 2+ The (002) diffraction peak of the co-doped sample shifts to a higher angle, indicating that Ca 2+ With Mg 2+ It has successfully entered the crystal lattice, causing a contraction in the interplanar spacing.

[0079] Figure 2 The XRD Rietveld refinement results for pristine, Ca-doped, Mg-doped, and Ca / Mg-doped samples are shown, where (a) Na 0.67 Ni 0.33 Mn 0.67 O2、(b)Na 0.61 Ca 0.03 Ni 0.33 Mn 0.67 O2、(c)Na 0.67 Ni 0.23 Mg 0.10 Mn 0.67 O2 and (d)Na 0.61 Ca 0.03 Ni 0.23 Mg 0.10 Mn 0.67 O2. R values ​​for all spectra. wp The values ​​were all below 10%, and the χ² value was less than 2, indicating good finishing quality and confirming that Ca... 2+ With Mg 2+ Successful crystal inclusion occurred, and the actual composition of the resulting material was consistent with the expected stoichiometry. Analysis of the refined data showed that, compared to the pristine sample, the Ca / Mg-doped sample exhibited a higher concentration of Mg. 2+ The introduction of replacement part Ni 2+ The interlayer spacing of the transition metals shrank from 2.1113 Å to 1.9409 Å, while the interlayer spacing of the sodium layer increased due to Ca. 2+Partially replaced Na + The ion Å increased from 3.4742 Å to 3.6307 Å. This structural change helps to lower the sodium ion migration barrier, thereby improving the material's Na+ ion mobility. + Diffusion kinetics.

[0080] In addition, the results of the metal element composition and ratio of the Ca / Mg-doped samples determined by inductively coupled plasma optical emission spectrometry (ICP-OES) are shown in Table 1.

[0081] Table 1

[0082] Na Ca Ni Mg Mn 0.61 0.03 0.23 0.10 0.67

[0083] Figure 3 SEM images of pristine and Ca / Mg-doped samples, respectively. Figure 3 As shown in (a), the pristine samples exhibit a wide particle size distribution (0.2-3 μm) and show irregular aggregation and agglomeration; while Figure 3 (b) The Ca / Mg-doped material, while maintaining the original morphological characteristics, achieved a more uniform particle size distribution (1-2 μm), and its particle uniformity and orderly arrangement were improved. Figure 3 (c) shows the EDS elemental distribution of the Ca / Mg doped sample. Na, Ni, Mn, O, Ca, and Mg are uniformly distributed across all observation areas, indicating that Ca... 2+ With Mg 2+ It has been successfully incorporated into the crystal lattice, and there is no local elemental segregation.

[0084] Figure 4 High-resolution transmission electron microscopy (HRTEM) images of the Ca / Mg-doped sample. The results show that the sample particles have good crystallinity, and its lattice fringe spacing is 2.429 Å as measured by Digital Micrograph software, which is consistent with the interplanar spacing of P2-type layered oxides (101).

[0085] Figure 5 The full spectrum of each sample is shown. Compared with pristine, the Ca-doped, Mg-doped and Ca / Mg-doped samples show characteristic peaks of Ca, Mg or both, respectively. The other elemental signal peaks are basically the same, indicating that doping did not cause significant changes in other surface components.

[0086] Figure 6 In (a), the Ca 2p energy spectrum shows two main peaks at 347.4 eV and 350.9 eV, corresponding to Ca 2p, respectively. 3 / 2 and Ca 2p 1 / 2This confirms that Ca exists in the +2 valence state. Figure 6 The Mg 1s spectrum of the Ca / Mg-doped sample in (b) further confirms that Mg is also in the +2 valence. Figure 6 The binding energy in the (c)Na 1s spectrum is 1071.1 eV, indicating that Na has a +1 valence. Figure 6 The (d)Ni 2p spectrum can be decomposed into Ni 2+ (854.7 eV, 871.9 eV) and Ni 3+ The two sets of peaks (856.6 eV, 873.9 eV) have satellite peaks at 861.1 eV and 879.3 eV, respectively. Figure 6 The (e)Mn 2p spectrum shows Mn 2p 3 / 2 It can be fitted to 641.8 eV (Mn 3+ ) and 643.2eV (Mn 4+ Two components, Mn 2p 1 / 2 This also corresponds to two sub-peaks at 653.6 eV and 654.3 eV, indicating that Mn exists in a mixed valence state. Figure 6 The peaks at 528.9 eV, 530.4 eV and 533.1 eV in the (f)O 1s spectrum are attributed to lattice oxygen, adsorbed oxygen and hydroxyl oxygen, respectively.

[0087] according to Figure 7 XPS fitting results show that Ni in all samples 2+ / Ni 3+ ( Figure 7 (a) and Mn 3+ / Mn 4+ ( Figure 7 The initial surface states of (b) are basically the same, indicating that doping mainly affects the bulk structure of the material, rather than the surface chemical bonding state. To evaluate Ca... 2+ With Mg 2+ The effect of doping on Ni was quantitatively analyzed in each sample using Gaussian fitting. 2+ with Ni 3+ Relative content: Ni in pristine sample 2+ with Ni 3+ The proportions were 58.18% and 41.82% respectively; for Ca-doped samples, they were 60.27% and 39.73% respectively; for Mg-doped samples, they were 69.96% and 30.04% respectively; and for Ca / Mg-doped samples, they were 74.08% and 25.92% respectively. The results indicate that Ni in the doped samples... 3+ The content of all samples decreased, with the Ni content in the Ca / Mg-doped sample decreasing. 3+ Lowest content. Due to Ni 3+As Jahn-Teller active ions, excessive amounts of Ca / Mg can induce irreversible structural distortions in materials, leading to the distortion of two-dimensional ion channels in layered oxides and consequently impairing the rate performance of the material. Therefore, Ca / Mg co-doping effectively reduces Ni... 3+ The ratio helps maintain structural stability and improve electrochemical kinetic performance.

[0088] To investigate the structural evolution of electrode materials during charge and discharge, in-situ XRD tests were performed on Pristine and Ca / Mg-doped samples during the first cycle (2.0-4.3 V, 0.1 C). Figure 8 The charge-discharge curves and corresponding in-situ XRD patterns of the Pristine sample are shown. During charging, as the voltage increases to 4.0 V, the (002) and (004) diffraction peaks shift to lower angles, while the (100) and (102) peaks shift to higher angles and their intensity decreases, indicating c-axis expansion and a-axis contraction. This structural change originates from Na. + After extraction, the electrostatic repulsion of OO increases, leading to slippage of the TMO2 layer. When the voltage reaches 4.0 V, hydrated phase diffraction peaks appear near 2θ ≈ 12.5° and 25°, indicating that the change in unit cell volume promotes the embedding of water molecules into the sodium layer. During subsequent discharge, as the voltage decreases, the hydrated phase diffraction peaks gradually weaken and disappear completely at 2.0 V, while the remaining diffraction peaks return to their original P2 phase positions, indicating that the structural evolution is somewhat reversible.

[0089] In contrast, the Ca / Mg-doped sample exhibited a completely reversible solid solution reaction during the first charge-discharge cycle in the voltage range of 2.0–4.3 V. Figure 9 The peak shift was smaller across the entire voltage range, with only a trace amount of hydrated phase observed near 4.3 V, and the phase recovered to its initial position after discharging to 2.0 V. This effective suppression of the phase transition and harmful interfacial side reactions can be attributed to Ca. 2+ / Mg 2+ The result of dual-site synergistic modification, Ca 2+ Sodium layer and Mg 2+ Together, they act as "structural pillars" in the transition metal layers, effectively enhancing interlayer bonding and suppressing Jahn-Teller distortion. Simultaneously, doping also alleviates Na… + The ordered rearrangement of vacancies during the insertion / extraction process causes lattice instability, which significantly improves the structural stability of the material. This structural advantage provides a reasonable explanation for its higher discharge plateau voltage, better capacity retention, higher discharge specific capacity, and better rate performance.

[0090] The charge compensation mechanism of Ca / Mg-doped samples was systematically studied using in-situ X-ray photoelectron spectroscopy (XPS). Figure 10 As shown, the Ni 2p XPS spectrum of the Ca / Mg-doped sample exhibits a significant peak shift during charge and discharge, indicating a reversible change in the valence state of Ni, which is key evidence of its participation in redox reactions. In contrast, the Mn 2p spectrum does not show a significant shift throughout the electrochemical process, confirming that the Jahn-Teller distortion was effectively suppressed. The Na1s peak intensity in the uncharged state is significantly higher than that in the sample charged to 4.3 V, indicating that Na... + A large amount of sodium was extracted from the electrode, resulting in a significant decrease in sodium content. In contrast, the Ca 2p and Mg 1s spectra did not show significant intensity changes before and after charge and discharge, indicating that the Ca in the sodium layer... 2+ With the transition metal layer Mg 2+ All of them are electrochemically inert and do not participate in redox reactions during cycling. Therefore, this synergistic doping strategy enhances the mechanical stability of the layered structure through the "pillar effect," suppresses lattice collapse during cycling, and thus effectively improves the electrochemical performance of the material.

[0091] The sodium storage performance of the synthesized material was systematically evaluated using a sodium half-cell at room temperature. To investigate the effect of the voltage window, tests were conducted in the ranges of 2.0–4.3 V and 2.0–4.5 V. Figure 11 (a) shows the results of each sample in the 2.0–4.3 V window and at a 0.2 C ratio (1 C = 100 mA g). -1 Typical charge-discharge curves under ( ).

[0092] The undoped sample exhibited the highest initial capacity (139.80 mAh g⁻¹). -1 However, it had the lowest coulombic efficiency (85.01%); the Ca-doped sample capacity was the second highest (132.65 mAh g⁻¹). -1 Coulomb efficiency remains limited (89.27%), mainly due to Ni 2 + / Ni 3+ The dominant role of redox reactions, while Mn 4+ / Mn 3+ Its contribution to capacity is negligible. Meanwhile, the slight decrease in discharge capacity compared to the original material can be attributed to Ca. 2+ Occupying sodium sites reduces available Na vacancies and limits active Na. + The participation of Ca. As compensation, Ca 2+ The embedding widens the interlayer spacing and promotes Na +The extraction and insertion of ions accelerate ion diffusion kinetics, thereby improving the coulombic efficiency of the Ca-doped cathode, and this advantage becomes increasingly significant during long-term cycling. In contrast, the Ca / Mg-doped sample, despite having a lower capacity (119.97 mAh g⁻¹), exhibits superior performance. -1 However, the Coulomb efficiency significantly improved to 94.06%, which may be due to Na + Over-embedded behavior, Na + Vacancies ordering and Mn 4+ / Mn 3+ The combined effect of redox reactions. Notably, the Mg-doped sample exhibited the highest coulombic efficiency (95.1%) and a high capacity (128.19 mAh g⁻¹). -1 The decrease in capacity of these two materials is mainly attributed to the partial substitution of Ni by Mg doping, which reduces the concentration of electrochemically active Ni, further confirming the role of Ni in this process. 2+ / Ni 3+ The redox couple plays a dominant role in the contributions of capacity and voltage. In addition, the introduction of Mg effectively improves cycling stability by suppressing Jahn-Teller distortion and inducing reversible structural transitions, and the smoothing of the voltage curve in the Mg-containing electrode also contributes to improved cycling stability.

[0093] Within a wider voltage range of 2.0–4.5 V, all samples exhibited higher charge-discharge capacity, such as Figure 11 (b) is mainly attributed to Ni 2+ / Ni 4+ Further activation of the redox pair and Na + Improved ionic conductivity. Thanks to the regulatory effect of Ca / Mg co-doping, the Ca / Mg-doped sample provides 127.85 mAh g⁻¹. -1 While achieving reversible capacity, a peak coulombic efficiency of 98.63% was achieved. The Mg-doped sample also exhibited a high efficiency of 98.1%, while the coulombic efficiencies of the Ca-doped and pristine samples were 94.17% and 92.82%, respectively, both below 95%. The gradually sloping charge-discharge plateau above 4.1 V clearly demonstrates that the substitution of Mg at Ni sites effectively suppressed Na... + Vacancy ordering and phase transition together contribute to the improvement of electrochemical reversibility.

[0094] Ca 2+ and Mg 2+ Co-doping stabilizes Ni 2+ / Ni 3+The redox pair promotes the formation of the Na-O-Mg local configuration. This structure induces ionic O 2p orbital characteristics, activating oxidation reactions in the high-voltage range, thereby simultaneously improving the coulombic efficiency and discharge voltage of the material. Furthermore, the partial substitution of Ni by Mg effectively suppresses phase transitions and transition metal layer slip during cycling, resulting in a smoother charge-discharge profile.

[0095] Cyclic test results within the 2.0-4.3 V voltage range at a 1 C rate are as follows: Figure 12 As shown in (a), after 200 cycles, the remaining capacities of the pristine, Ca-doped, Mg-doped, and Ca / Mg-doped samples were 9.37, 76.35, 64.38, and 92.87 mAh g, respectively. -1 The corresponding capacity retention rates were 6.58%, 87.17%, 50.51%, and 82.32%, respectively. Among them, the Ca-doped sample exhibited the best cycling stability, mainly due to the presence of Ca... 2+ The channel broadening effect resulting from sodium embedding promotes Na + Reversible migration.

[0096] Figure 12 (b) demonstrates cycling performance over a wider voltage range of 2.0–4.5 V. When the charge cutoff voltage is increased to 4.5 V, the capacity retention of all samples is lower than their performance in the 2.0–4.3 V range. The pristine and Ca-doped samples exhibit particularly significant capacity decay after 200 cycles, with retention rates of only 5.04% and 17.32%, respectively. Under these harsh conditions, Mg doping plays a crucial role in mitigating irreversible phase transitions and stabilizing the transition metal layer. In contrast, the Ca / Mg-doped sample retains 73.25% of its capacity after 200 cycles, significantly higher than the 44.43% of the Mg-doped sample, highlighting the superior performance of Ca-doped samples. 2+ With Mg 2+ Synergistic effect of co-doping in improving structural integrity and cycling stability. Figure 13 and Figure 14 The charge-discharge behavior of each electrode in two voltage ranges was demonstrated. Although all samples exhibited similar electrochemical characteristics, the Ca / Mg-doped electrode showed highly overlapping charge-discharge curves in both voltage windows, further confirming its excellent structural reversibility.

[0097] The rate performance of the material was systematically evaluated within voltage windows of 2.0–4.3 V and 2.0–4.5 V, and current ranges from 0.2 C to 15 C. Figure 15Compared to the pristine sample, all doped samples (Ca-doped, Mg-doped, and Ca / Mg-doped) exhibited significantly improved rate performance, as shown in the corresponding charge-discharge curves. Figure 16 and Figure 17 .

[0098] Despite Mg substituting some of the electrochemically active Ni and Ca occupying some Na sites, the Ca / Mg-doped cathode still exhibits excellent rate performance. Within the voltage range of 2.0–4.3 V, its discharge capacities at rates from 0.2 C to 15 C are 119.97, 113.44, 100.55, 90.78, 80.58, 72.78, and 64.62 mAh g⁻¹, respectively. -1 This performance is comparable to that of the Ca-doped sample, which, under the same conditions, had capacities of 132.65, 86.17, 77.40, 75.43, 70.78, 64.47, and 57.69 mAh g, respectively. -1 This is mainly due to Ca 2+ The channel widening effect.

[0099] In contrast, despite having higher active Ni and sodium content, the pristine samples exhibited significantly poorer rate performance, with corresponding capacities of 139.80, 96.95, 74.22, 63.12, 51.71, 39.43, and 27.91 mAh g⁻¹. -1 The charge-discharge curves show three characteristic voltage regions: ~2.2 V corresponds to Mn 3+ / Mn 4+ Redox reactions, 2.5-4.1 V corresponding to Ni 2+ / Ni 4+ Redox activity, 4.1-4.3 V, is related to the redox activity of oxygen.

[0100] When the charging cutoff voltage is increased to 4.5 V, the high-voltage phase transition behavior becomes more pronounced. This is thanks to Mg... 2+ Despite the suppression of structural phase transitions, Mg-doped and Ca / Mg-doped cathodes still exhibit superior rate performance compared to other samples, with this stabilizing effect reflected in smoother charge-discharge curves above 4.1 V. Figure 17 As shown in (d), the discharge capacities of the Ca / Mg-doped samples in the rate range from 0.2 C to 15 C are 127.85, 123.05, 112.33, 100.79, 84.58, 68.99 and 57.92 mAh g, respectively. -1 , Figure 17The capacities of the (c) Mg-doped samples under the same conditions were 115.68, 104.38, 97.40, 89.26, 73.42, 57.73, and 46.28 mAh g⁻¹. -1 , Figure 17 The capacities of (b) Ca-doped samples were slightly lower, at 125.26, 88.6, 85.59, 80, 68.97, 54.18, and 38.35 mAh g, respectively. -1 The pristine samples, however, exhibited a sharp decrease in capacity at the same rate, with values ​​of only 142.65, 100.98, 79.09, 63.86, 43.09, 23.33, and 10.83 mAh g⁻¹. -1 As current density increases, the capacity of the pristine sample decreases most significantly, while the capacity decay of the Ca-doped and Mg-doped samples is relatively moderate. The Ca / Mg-doped cathode exhibits the slowest capacity decay, maintaining relatively stable performance output at all rate powers. These results fully validate the synergistic effect of Ca / Mg co-doping in improving the rate performance and structural integrity of the material.

[0101] In the 8 C high-rate cycling test, the discharge capacity variation trends of the pristine, Ca-doped, Mg-doped, and Ca / Mg-doped electrodes were highly consistent with the results at the 1 C rate. Figure 18 As shown in (a), the Ca / Mg-doped sample exhibits the best high-rate performance, with a capacity of 58.40 mAh g⁻¹ after 400 cycles. -1 The capacity retention rate reached 80.61%, while the capacities of pristine, Ca-doped, and Mg-doped samples were 18.87 mAh g⁻¹. -1 (23.28%), 44.09 mAh g -1 (66.19%) and 42.32 mAh g -1 (55.40%). Even at ultra-high rates of 20 C, the Ca / Mg-doped cathode still exhibits excellent cycling stability, with a capacity retention of up to 83.94% after 400 cycles. Figure 18 As shown in (b), it is significantly superior to other comparative samples. The performance comparison of each electrode is as follows: Figure 19 As shown. This performance improvement is mainly attributed to Ca. 2+ With Mg 2+ Co-doping effect of Ca 2+ The interlayer spacing of sodium and Mg was increased. 2+The transition metal layer skeleton was stabilized, which enhanced the interlayer electrostatic interaction, suppressed material cracking and structural degradation under high pressure, and thus significantly improved the long-term cycling stability of the material.

[0102] Figure 20 and Figure 21 The dQ / dV curves show that above 4.0 V, both pristine and Ca-doped materials exhibit significant attenuation of redox peak intensity and increased voltage polarization, indicating a gradual loss of electrochemical reversibility. Notably, the redox peak overlap of the Ca-doped sample is improved (especially below 4.0 V), suggesting enhanced cycle reversibility in this voltage range. Meanwhile, the redox peak shape of the Mg-containing electrode is flatter than that of the original and Ca-doped samples, indicating that the partial substitution of Ni by Mg in the P2 phase structure helps to moderate the electrochemical reaction kinetics during cycling.

[0103] Within the voltage range of 2.0–4.5 V, the dQ / dV curves of both Mg-doped and Ca / Mg-doped samples exhibit a smoother redox characteristic in the 4.0–4.5 V range. Notably, the redox reaction in this voltage range dominates the contribution to the total capacity above 4.0 V, while its contribution to capacity above 4.3 V is relatively limited. Compared to the pristine sample, the Ca / Mg-doped cathode shows significantly broadened redox peaks near 3.6 V and 4.1 V during charging, with the corresponding reduction peak even merging into a single broad peak. The Mg-doped sample also exhibits a similar evolution pattern.

[0104] Comprehensive dQ / dV analysis shows that Ca substitution primarily improves the cycling reversibility of the material below 4.0 V, while Mg doping at Ni sites in the P2 structure effectively modulates the electrochemical reaction kinetics and mitigates phase transition behavior above 4.0 V. Therefore, the Ca / Mg co-doping strategy not only enhances the electrochemical stability in the low-voltage region but also improves the overall performance of the material. + Diffusion kinetics also effectively mitigated the degradation of high-voltage structures caused by phase transitions.

[0105] Figure 22 The figure shows the cyclic voltammetry curves of the pristine sample. Multiple pairs of redox peaks can be observed at 3.25 / 3.05 V, 3.38 / 3.26 V, 3.67 / 3.55 V, 3.73 / 3.62 V, and 4.27 / 3.98 V. The first four sets of peaks correspond to Ni. 2+ / Ni 3+ / Ni 4+ The stepwise redox process and Na +The vacancy ordering behavior is observed, while the peak pair at 4.27 / 3.98 V is attributed to the P2-O2 phase transition reaction. The CV curves show poor overlap in continuous cycling, especially in the voltage range above 4.0 V where phase transitions typically occur, reflecting the Na... + Irreversible structural evolution and side reactions during the insertion / extraction process. In contrast, the Ca / Mg-doped sample showed highly overlapping CV curves in the first three cycles, with redox peaks exhibiting fusion characteristics and smoother curves, indicating that its sodium ion insertion / extraction process has significantly improved reversibility and established a more stable redox environment. This result is consistent with the dQ / dV analysis conclusions.

[0106] Based on 0.2-1.0 mV s -1 Cyclic voltammetry test data at scan rate ( Figure 23 The electrochemical response characteristics of the two materials were compared using contour plots. With increasing scan rate, the peak current of the Ca / Mg-doped cathode material gradually increased, and the separation of the redox peaks also showed a systematic expansion, indicating highly reversible sodium ion insertion / extraction behavior. Notably, under the same testing conditions, the current response of the pristine sample significantly decreased at higher scan rates (especially around 3.98 V), indicating the presence of irreversible side reactions.

[0107] The b-value is an important parameter for determining the kinetic mechanism of electrode processes: the closer its value is to 1, the greater the contribution of capacitive behavior; the closer it is to 0.5, the more dominant the diffusion process is. For example... Figure 24 As shown in (c), the b values ​​of the O1 and R1 peaks in the Ca / Mg-doped sample are 0.80 and 0.76, respectively, which are significantly higher than the corresponding peaks of 0.65 and 0.64 in the pristine material. Figure 24 As shown in (a). This illustrates that in Na + During the insertion / extraction process, pseudocapacitive behavior predominates, and Ca... 2+ and Mg 2+ The introduction of the Na layer and the transition metal layer, respectively, further enhanced the ion diffusion kinetics.

[0108] The calculated capacitance contribution ratio shows that when the scan rate increases from 0.2 mV / s... -1 Rise to 1.0 mV s -1 At that time, the capacitance contribution rate of both samples increased with increasing scan rate. Specifically, the capacitance contribution rates of the Ca / Mg-doped samples were 68.84%, 75.89%, 81.07%, 84.03%, and 85.12%, respectively. Figure 24As shown in (b), it is significantly higher than that of the undoped electrode material under the same conditions, which are 52.81%, 64.59%, 66.79%, 69.86%, and 71.33%, respectively. Figure 24 As shown in (d). This result fully confirms that the Ca / Mg co-doping strategy can effectively optimize Na + Diffusion behavior, thereby improving overall electrochemical performance.

[0109] Furthermore, we analyzed the sodium ion kinetics of pristine, Ca-doped, Mg-doped, and Ca / Mg-doped samples using GITT testing. Figure 25 The GITT curves of each sample during the charge-discharge process and the corresponding Na values ​​are shown. + Diffusion coefficient (D) Na⁺ The Mg-doped and Ca / Mg-doped samples exhibited the highest D values ​​near 4.3 V. Na⁺ The value indicates that the introduction of Mg into Ni 3+ / Ni 4+ The redox reaction effectively promoted the growth of Na. + It facilitates transport and suppresses the occurrence of harmful phase transitions.

[0110] In contrast, the undoped pristine sample exhibits two distinct D peaks near 4.3 V. Na⁺ The valley values ​​correspond to the redox reaction stages and structural phase transition processes of different elements, respectively, followed by their D values. Na⁺ The sample recovered to its initial level, while the Ca-doped sample only observed D once. Na⁺ Decline. This difference indicates that Ca... 2+ The presence of sodium in the layer helps maintain a favorable environment for sodium ion migration, thereby improving the material's kinetic stability. The original sample, lacking a sustained and efficient ion conduction mechanism, exhibited relatively poor electrochemical performance.

[0111] After 200 cycles at 1C rate, the R of the Ca / Mg-doped cathode... ct The Ω is 292.17, significantly lower than the 391.20 Ω of the original material. Figure 26 R ct The decrease in R indicates that Ca / Mg co-doping effectively enhances the interfacial and structural stability of the electrode material. Meanwhile, during cycling... ct The continuous decline also reflects the gradual formation of a stable cathode-electrolyte interface layer during electrochemical activation.

[0112] To further investigate the interaction between pristine and Ca / Mg-doped samples in Na +To assess the differences in charge transfer dynamics, we employed in-situ EIS technology to study the charge transfer resistance (R) during a charge-discharge process after 50 cycles. ct The evolution was monitored. For example... Figure 27 As shown, under open-circuit voltage conditions, the R of the Ca / Mg-doped sample ct The R0 value was significantly lower than that of the pristine sample, consistent with its higher Na⁺ diffusion coefficient and superior high-rate performance. At 4.3 V, the R0 value of the pristine sample was significantly lower. ct The Ro dropped sharply from 254.40 Ω to 119.77 Ω, while the Ro of the Ca / Mg-doped sample... ct The RΩ only gradually decreased from 140.97 Ω to 106.16 Ω. Within the 4.3 V plateau range, the RΩ of the Ca / Mg-doped sample... ct It remains stable, while the pristine electrode exhibits R during discharge. ct The rapid increase in this trend closely matches the GITT test results.

[0113] To further investigate the effect of Ca / Mg doping on the structural stability of the material, we performed XRD and SEM characterization on the electrode after 50 cycles at 1C rate. Figure 28 SEM images of Ca / Mg-doped and pristine samples after cycling are shown. It is clearly observed that the Ca / Mg-doped cathode has a smaller particle size and more uniform particle dispersion, which helps maintain structural stability during long-term cycling. In stark contrast, the doped cathode maintained its morphology intact before and after cycling, with no obvious cracks appearing on the surface or inside, while the original material showed surface cracks that propagated and penetrated the particles after 50 cycles, leading to crystal structure breakage. Particle cracking disrupts the structural integrity of the cathode, exacerbates side reactions, and promotes the migration of transition metal ions from the layered structure into the electrolyte. Transition metal dissolution further degrades the solid electrolyte interface (SEI) on the cathode surface, causing capacity decay. In the Ca / Mg-doped cathode, strong TM-O bonds effectively suppress irreversible slippage of the transition metal layer, while Ca and Mg act as structural pillars to alleviate lattice collapse, thus synergistically improving the mechanical stability of the material, suppressing crack initiation, and maintaining morphological integrity, consistent with COHP calculations. In addition, the complete secondary particle structure effectively blocks electrolyte penetration and reduces the occurrence of side reactions.

[0114] Figure 29 The XRD pattern shows the material after cycling. It is evident that the intensity of the (002) diffraction peak in the pristine cathode is significantly reduced, indicating that its main structure was severely damaged during cycling. In contrast, the (002) peak in the Ca / Mg-doped cathode returns to a low-angle position after cycling, which helps to improve the performance of Na in subsequent cycles.+ Migration kinetics. To further analyze the crystal structure evolution during cycling, we performed XRD Rietveld refinement on the two cathode materials. The results showed that compared with before cycling, the lattice parameters a and c of the pristine cathode changed by approximately 0.25% ((2.88735-2.88010) / 2.88735 = 0.251%) and 0.27% ((11.1709-11.1410) / 11.1709 = 0.267%), respectively, while the corresponding changes for the Ca / Mg-doped cathode were only 0.21% ((2.88649-2.88037) / 2.88649 = 0.212%) and 0.23% ((11.16910-11.14334) / 11.14334 = 0.231%). This more stable cell parameter can be attributed to the enhanced TM-O bond strength and Ca. 2+ The "pillar effect" of the two components synergistically enhances the anti-slip capability of the layered framework. Meanwhile, the lower Ni content in the material... 3+ The content also effectively suppressed the damage to the structure caused by Jahn-Teller distortion.

[0115] To evaluate the air stability of pristine and Ca / Mg-doped cathode materials, we used XRD to analyze the phase changes of the samples after 24 and 48 hours of water immersion treatment. The results are as follows: Figure 30 After 24 hours of immersion, the diffraction peak intensity of the pristine sample decreased significantly, while the Ca / Mg-doped sample maintained its original crystalline state without significant structural degradation. Extending the immersion to 48 hours, the original sample showed obvious impurity peaks near 12° and 25°, indicating the formation of a hydrated phase, and its diffraction intensity further decreased. In contrast, the Ca / Mg-doped sample did not show obvious hydration peaks under all treatment conditions, and the positions and intensities of each diffraction peak remained stable, demonstrating excellent resistance to water and oxygen and crystallinity retention. These results indicate that Ca / Mg doping can effectively improve the structural stability of materials in humid air, thereby helping to extend the cycle life of battery materials after long-term storage in practical use.

[0116] To evaluate the practical application potential of Ca / Mg doped materials, materials with high active material loading (5 mg cm⁻¹) were selected. -2 Electrode sheets were tested within a voltage range of 2.0-4.3 V. Figure 31 The initial capacity of the Ca / Mg-doped cathode was 115.44 mAh g. -1 Its performance is slightly lower than that of pristine. However, its rate performance is significantly improved, maintaining 57.77 mAh g⁻¹ at 5 C rate.-1 Its capacity exceeds that of the unmodified electrode, which has a capacity of 46.69 mAh g. -1 Furthermore, this co-doped cathode exhibits excellent cycle durability, maintaining a capacity retention of 71.03% after 100 cycles at 1 C rate, significantly higher than the 30.58% of the undoped sample. This confirms its superior performance under high load conditions and demonstrates its application potential as a cathode material for sodium-ion batteries.

[0117] To evaluate the temperature adaptability of the materials, pristine and Ca / Mg-doped samples were systematically tested under low-temperature (-15 °C) and high-temperature (55 °C) conditions. At -15 °C and a 0.1 C rate, the coulombic efficiency of the Ca / Mg-doped sample was 95.42%, significantly higher than the 87.38% of the pristine sample, and its initial discharge capacity was 122.30 mAh g⁻¹. -1， like Figure 32 As shown in (a), the discharge capacities of the Ca / Mg-doped samples were 103.56, 84.86, 74.99, 65.44, and 52.75 mAh g⁻¹ as the current density gradually increased from 0.2 C to 5 C. -1 At high current densities, it exceeded the pristine sample's values ​​of 117.13, 88.65, 64.35, 58.56, and 48.92 mAh g⁻¹ under the same conditions. -1 ,like Figure 32 (b) The Ca / Mg-doped sample retained 90.73% of its capacity after 200 cycles at 1 C rate. Figure 32 Even after 800 cycles, the retention rate (f) remained at 81.05%, far superior to the 65.33% retention rate of the pristine sample after 200 cycles. Figure 32 (c) shows excellent low-temperature cycling stability.

[0118] At a high temperature of 55 °C ( Figure 33 The improved Na⁺ kinetics facilitated the intercalation reaction in the high-voltage region, extending the charging plateau near 4.3 V. The initial capacities of pristine and the Ca / Mg-doped sample at 0.1 C were 158.53 mAh g⁻¹, respectively. -1 and 143.01 mAh g -1 As the discharge rate increased from 0.2 C to 5 C, the discharge capacities of the Ca / Mg-doped samples were 131.45, 105.15, 91.01, 87.78, and 60.93 mAh g⁻¹, respectively. -1All of these values ​​were higher than those of the pristine sample (99.46, 74.55, 65.56, 57.83, and 48.78 mAh g⁻¹). -1 High temperatures promote sodium ion diffusion, increasing specific capacity, but also exacerbate interfacial side reactions, leading to accelerated capacity decay. This characteristic is evident in the rapid capacity decay of the pristine cathode: after 150 cycles at 1 C, its capacity retention is only 18.15%. In contrast, the Ca / Mg-doped cathode retains 65.69% of its capacity under the same conditions, more than three times that of pristine. This temperature adaptability across a wide operating temperature range (-15 to 55 °C) significantly broadens the potential application scenarios for this material.

[0119] Commercial hard carbon (HC) was used as the negative electrode, and pristine and Ca / Mg-doped positive electrodes were assembled into full cells, and their electrochemical performance was systematically evaluated. Figure 34 (a) illustrates the charge-discharge mechanism of a full-cell assembly using P2-type cathode materials. Figure 34 (b) The initial charge-discharge curves of the pristine / / HC and Ca / Mg-doped / / HC full cells at a rate of 0.2 C and a voltage range of 2.0–4.3 V show that their specific capacities based on the mass of the positive electrode active material are 128.05 and 114.29 mAh g, respectively. -1 The corresponding first-week Coulomb efficiencies were 77.58% and 86.65%, respectively. Figure 34 Rate performance tests (c)-(e) show that the Ca / Mg-doped / / HC full cell performs better, with capacities of 114.29, 107.75, 100.98, 91.28, and 70.38 mAh g at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C rates, respectively. -1 It significantly outperforms the capacity of pristine / / HC full cells at the same rate (128.05, 95.77, 69.08, 42.91 and 23.91 mAh g⁻¹). -1 ).

[0120] After 150 cycles at 1 C rate, the Ca / Mg-doped / / HC full cell retained 79.36% of its capacity, significantly higher than the 11.25% of the pristine / / HC full cell. Figure 35 From a practical application perspective, energy density is a key indicator for measuring battery performance. Based on the mass of the positive electrode active material, the initial energy density of the Ca / Mg-doped / / HC full cell is 368.32 Wh·kg⁻¹. -1After 150 cycles, it still maintains 259.32 Wh·kg. -1 The energy retention rate of the Ca / Mg-doped material reached 70.41%, while that of the Pristine / / HC full cell was only 7.85%. Notably, the latter's energy retention rate was lower than its capacity retention rate, indicating that it experienced both capacity decay and voltage drop during cycling. Furthermore, the Ca / Mg-doped material exhibited a high compaction density of 3.16 g / cm³ at 260 MPa. -1 This further confirms that the Ca / Mg co-doping strategy has better overall performance in the full-cell system, and together with the previous tests, it demonstrates the practical application potential of this material in sodium-ion batteries.

[0121] To better illustrate the present invention, numerous specific details have been provided in the detailed embodiments described above. Those skilled in the art should understand that the present invention can be practiced even without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of the present invention.

[0122] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A Ca / Mg co-doped sodium nickel manganate cathode material, characterized in that, The Ca / Mg co-doped sodium nickel manganate cathode material contains three different metal cations—nickel, manganese, and magnesium—in the transition metal layer, and two different metal cations—sodium and calcium—in the sodium layer. The chemical formula of the Ca / Mg co-doped sodium nickel manganate cathode material is Na. 0.61 Ca 0.03 Ni 0.23 Mg 0.10 Mn 0.67 O2; The Ca / Mg co-doped sodium nickel manganate cathode material is of type P2.

2. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material as described in claim 1, characterized in that, Includes the following steps: Step S1: Dissolve nickel salt, manganese salt and magnesium salt in deionized water in stoichiometric proportions to prepare metal salt solutions; at the same time, dissolve Na2C2O4 in deionized water to prepare sodium oxalate solutions. Step S2: The metal salt solution and sodium oxalate solution are stirred in a constant temperature water bath. The metal salt solution is added dropwise to the sodium oxalate solution at a constant dropping rate. After the addition is complete, stirring continues to complete the co-precipitation. The resulting precipitate was filtered, washed, and dried to obtain (Ni) 0.23 Mg 0.10 Mn 0.67 C2O4·2H2O precursor; Step S3: The obtained precursor is thoroughly mixed with stoichiometric CaCO3 and an excess of 5% stoichiometric Na2CO3, heated to 900°C in air atmosphere and calcined, and then cooled in the furnace to obtain Ca / Mg co-doped sodium nickel manganate cathode material.

3. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 2, characterized in that, The nickel salt in step S1 is selected from NiSO4·6H2O; the manganese salt is selected from MnSO4·H2O; and the magnesium salt is selected from MgSO4·7H2O. The total concentration of the metal salt solution in step S1 is 1.5 mol L. -1 .

4. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 2, characterized in that, The concentration of the sodium oxalate solution in step S1 is 0.45 mol / L. -1 .

5. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 2, characterized in that, In step S2, the constant temperature water bath temperature is 70 °C.

6. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 2, characterized in that, The dropping rate of the metal salt solution in step S2 is 2 mL / min.

7. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 2, characterized in that, In step S2, the stirring time is continued for 4 hours; the drying is vacuum drying, the drying temperature is 120°C, and the drying time is 12 hours.

8. The method for preparing Ca / Mg co-doped sodium nickel manganate cathode material according to claim 1, characterized in that, The heating rate in step S3 is 5°C / min; the calcination time in step S3 is 13 h.

9. An electrode sheet, characterized in that, The electrode comprises the Ca / Mg co-doped sodium nickel manganate cathode material as described in claim 1.

10. A battery, characterized in that, The battery includes an electrode, a counter electrode, and a separator as described in claim 9, wherein the separator is disposed between the electrode and the counter electrode.

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