Method for preparing single-crystal lithium-rich manganese-based positive electrode material by synergic regulation and control of deep eutectic solvent and aluminum gradient doping

Abstract

The invention relates to a method for preparing a single-crystal lithium-rich manganese-based positive electrode material by synergistically regulating and controlling a deep eutectic solvent and aluminum gradient doping, which comprises the following steps of: carrying out multi-stage aluminum gradient doping to prepare a single-crystal lithium-rich manganese-based positive electrode material by synergistically regulating and controlling the deep eutectic solvent and the aluminum gradient doping; multi-level regulation and control on the structure of the positive electrode material from the inside to the surface are realized; the formation of an inducible single-crystal structure is assisted by the deep eutectic solvent; through addition of phosphorus / boron-containing anions and microwave strengthening treatment, oxygen vacancy lattice repair is promoted; carbon in-situ coating treatment is realized by adding a carbon source, so that the conductivity and interface stability of the material are improved; and lithium defects caused by other steps can be compensated through pre-lithiation treatment, and the first coulombic efficiency is improved. And five parts are integrated, so that the structural stability, the electrochemical performance and the safety of the material are multiply improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material preparation, and in particular to a method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping. Background Art

[0002] With the increasing global demand for renewable energy and electric vehicles, lithium-ion batteries are becoming increasingly important as mainstream energy storage devices. Cathode materials are a key determinant of lithium-ion battery performance, directly impacting the overall battery performance through their energy density, cycle life, and safety. Lithium-rich manganese-based cathode materials have become a research hotspot in recent years due to their high specific capacity, abundant resources, and low cost. However, traditional lithium-rich manganese-based cathode materials generally suffer from poor structural stability, rapid capacity decay, low initial coulombic efficiency, and susceptibility to volume expansion and microcracks during cycling, limiting their practical application and promotion.

[0003] Currently, lithium-rich manganese-based cathode materials mainly exist in a polycrystalline form. In polycrystalline structures, defects and side reactions easily arise at the grain interfaces, leading to accelerated structural damage during charge and discharge, further affecting electrochemical performance and cycle life. In contrast, single-crystal materials, due to their continuous and complete crystal structure, can effectively suppress interfacial reactions and the formation of intergrainary cracks, significantly improving the material's cycle stability and rate performance. Furthermore, controlling the local electronic structure and crystal defects through elemental doping is widely considered an important approach to improving material performance.

[0004] Traditional methods for preparing lithium-rich manganese-based single-crystal materials mainly include high-temperature solid-state methods, sol-gel methods, and hydrothermal methods. However, these methods generally suffer from high reaction temperatures, complex processes, high energy consumption, and insufficient product uniformity. In recent years, deep eutectic solvents (DES) have attracted widespread attention in the field of materials synthesis as an emerging green synthesis medium due to their low toxicity, low cost, and excellent solubility. Furthermore, gradient doping technology can optimize the local structure and stability of materials by controlling the distribution of dopant elements, thereby improving the electrochemical performance and cycle life of the materials. Therefore, developing a novel preparation method based on a low-melting-point DES solvent system, combined with gradient doping and single-crystal structure induction, has significant scientific and application value for improving the performance of lithium-rich manganese-based cathode materials. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a method for synergistically controlling the preparation of single-crystal lithium-rich manganese-based cathode materials using a deep eutectic solvent and aluminum gradient doping, combined with Al... 3+Gradient doping, in-situ carbon coating, and pre-lithiation treatment of cathode materials have achieved multiple improvements in the structural stability, electrochemical performance, and safety of cathode materials.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] A method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping includes the following steps:

[0008] (1) Dissolve nickel salt, manganese salt and low-concentration aluminum salt in deionized water to form a metal salt solution. Add alkaline precipitant and complexing agent to the solution to carry out a co-precipitation reaction to obtain a low-concentration aluminum-doped Ni-Mn hydroxide precursor. After washing and drying, precursor A is obtained.

[0009] (2) Add precursor A and medium-concentration aluminum salt to deep eutectic solvent A, ultrasonically disperse and stir and heat to allow aluminum ions to fully penetrate into the surface of precursor particles and partially diffuse into the interior of the crystal lattice to form a medium-concentration pre-doped layer. Then, perform low-temperature preheating to fix the aluminum distribution, wash away residual solvent components, and obtain precursor B.

[0010] (3) After mixing and grinding the precursor B with lithium source and high-concentration aluminum salt evenly, it is added to the deep eutectic solvent B, then ultrasonically dispersed and microwave-assisted heating is used to adsorb and enrich aluminum ions on the particle surface to obtain substance C with obvious gradient doping structure.

[0011] (4) Calcine the substance C in an argon or nitrogen atmosphere at a calcination temperature of 500-600℃, a heating rate of 3-7℃ / min, and a holding time of 20-30 hours. Then cool it to room temperature, wash and dry it to obtain a calcined product with a carbon coating layer generated by the pyrolysis of the carbon source.

[0012] (5) The calcined product is immersed in a lithium salt solution, centrifuged and dried to achieve prelithiation;

[0013] Among them, the deep eutectic solvent A is prepared by mixing choline chloride and urea; the deep eutectic solvent B is prepared by mixing choline chloride, urea, phosphorus / boron anions and carbon source; the doping amount of aluminum salt in step (1) is 0.5-3% of the amount of Ni element; the doping amount of aluminum salt in step (2) is 3-5% of the amount of Ni element; the doping amount of aluminum salt in step (3) is 5-10% of the amount of Ni element.

[0014] Preferably, in step (1), the coprecipitation reaction conditions are: temperature 20-30℃, stirring speed 400-600rpm, and reaction time 3-5h;

[0015] In step (2), when the aluminum doping concentration is moderate, ultrasonic dispersion is performed for 25-40 minutes, followed by stirring at 50-100℃ for 1 hour to allow a small amount of Al to be absorbed. 3+ It slowly penetrates into the surface and interior of the precursor particles; then it is preheated in a muffle furnace at 150-200℃ for 30-80 minutes, so that Al 3+ It is fixed to the surface of the precursor and partially diffuses into the crystal lattice;

[0016] In step (3), the mixture is ultrasonically dispersed for 25-40 minutes and then heated to 130-180°C under microwave conditions, followed by heat preservation for 60-120 minutes to promote Al... 3+ It is rapidly adsorbed and enriched on the outer surface of the precursor particles.

[0017] Preferably, the aluminum salt is selected from one or more of Al(NO3)3·9H2O, AlCl3·6H2O, and Al2(SO4)3·18H2O.

[0018] Preferably, the phosphorus- or boron-containing anion is one or more of Na3PO4, KH2PO4, Na2HPO4, NaH2PO4, NaBO2, Na2B2O4, and Na2B4O7; the carbon source is one or more of glucose, sucrose, and cyclodextrin; the doping amounts of the two components in the deep eutectic solvent are as follows: on a molar ratio basis, phosphorus-containing / boron-containing anion: choline chloride = 1:5-1:10; carbon source: choline chloride = 1:5-1:10.

[0019] Preferably, in step (5), the lithium salt molten salt is selected from one or more of the following: lithium trifluoromethyl sulfinate acetonitrile solution, lithium hexafluorophosphate carbonate solution, lithium hexafluorosilicate carbonate solution, and lithium tetrafluoroborate acetonitrile solution with a molar concentration of 0.3-0.7 mol / L, and the soaking time is 8-16 hours.

[0020] Preferably, the nickel salt is selected from one or more of NiSO4·6H2O, Ni(NO3)2·6H2O, and NiCl2·6H2O; and the manganese salt is selected from one or more of MnSO4·7H2O, Mn(NO3)2·4H2O, and MnCl2·4H2O.

[0021] Preferably, the alkaline precipitant is selected from one or more of sodium hydroxide, potassium hydroxide, and ammonium hydroxide; the complexing agent is selected from one or more of ammonia, ethylenediaminetetraacetic acid, citric acid, or their salts; and the alkaline precipitant and the complexing agent are mixed at a volume ratio of 0.5:1 to 2:1.

[0022] Preferably, in step (3), the lithium source is selected from one or more of Li2CO3, LiOH·H2O, LiNO3, and Li2O, and the precursor B is mixed with the lithium source in a molar ratio of 1:3 to 3:1.

[0023] Preferably, in step (2), the co-solvent used is 3-8 times the mass of precursor A; in step (3), the co-solvent used is 3-8 times the mass of precursor B.

[0024] Preferably, in the deeply eutectic solvent, the molar ratio of choline chloride to urea is 1:1.5 to 1:2.5.

[0025] Preferably, the preparation method of deep eutectic solvent A or deep eutectic solvent B is as follows: weigh each component in a certain molar ratio and mix them; heat and stir at about 60–80°C until the two gradually melt to form a transparent and uniform liquid, and then cool to room temperature to obtain the product.

[0026] The present invention employs the above-described structure and has the following advantages:

[0027] 1. This invention achieves multi-level control of the cathode material structure from the interior to the surface through multi-stage aluminum gradient doping; single-crystal structure formation can be induced by the deep eutectic solvent DES; oxygen vacancy lattice repair is promoted by the addition of phosphorus / boron-containing anions and microwave strengthening treatment; in-situ carbon coating is achieved by adding a carbon source, which helps to improve the conductivity and interface stability of the material; and pre-lithiation treatment can compensate for lithium defects caused by other steps, improving the first coulombic efficiency. Specifically:

[0028] (1) This invention achieves multi-level control of the cathode material structure from the inside to the surface by constructing a gradient concentration distribution of aluminum ions. The low concentration of aluminum ions inside can effectively stabilize the crystal structure, alleviate the volume change and stress concentration of the material during calcination and charge-discharge processes, and reduce the risk of particle cracking; the medium concentration aluminum doping layer, through deep eutectic solvent-assisted treatment, enables aluminum ions to diffuse more uniformly into the crystal lattice, which helps to improve the overall crystal quality and ion migration path of the material; while the high concentration of aluminum enriched region on the surface can suppress the dissolution of transition metals and side reactions during high-voltage cycling, enhance the stability of the electrode / electrolyte interface, and thus extend the cycle life.

[0029] (2) This invention introduces a deep eutectic solvent composed of choline chloride and urea to replace the traditional molten salt system. It has a low melting point, high ionic conductivity, and excellent designability. Structural reconstruction and assisted calcination can be achieved in the medium temperature range, significantly reducing the heat treatment temperature and saving energy and protecting the environment. The added phosphorus or boron-containing anions can slowly release oxygen sources during calcination, promoting the self-repair of oxygen vacancy lattice defects; the carbon source is carbonized in situ at high temperature to form a coating layer, further improving the conductivity and interface stability of the material.

[0030] (3) The lithium solution pre-lithiation is adopted to effectively compensate for lithium defects in the material, and the coulombic efficiency is increased to more than 89% for the first time.

[0031] 2. The single-crystal lithium-rich manganese-based cathode material prepared by this invention has the characteristics of complete structure, uniform particle size, gradient doping, and surface carbon coating, which effectively improves the electrochemical performance and cycle life of the material and is suitable for industrial applications of high energy density lithium-ion batteries. Attached Figure Description

[0032] Figure 1 These are SEM comparison images of the single-crystal lithium-rich manganese-based cathode materials prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention.

[0033] Figure 2 This is a comparison of the first charge-discharge curves of the single-crystal lithium-rich manganese-based cathode materials prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention.

[0034] Figure 3 This is a comparison of the cycle performance curves of the single-crystal lithium-rich manganese-based cathode materials prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention.

[0035] Figure 4 This is a comparison chart of the 50-cycle voltage drop curves of the single-crystal lithium-rich manganese-based cathode materials prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention. Detailed Implementation

[0036] To clearly illustrate the technical features of this solution, the invention will be described in detail below through specific implementation methods and in conjunction with the accompanying drawings.

[0037] 1. Instruments and reagents

[0038] The reagents and chemicals used in the experiments of this invention are shown in Table 1, and the instruments and equipment used are shown in Table 2.

[0039] Table 1. Experimental reagents and chemicals

[0040]

[0041]

[0042] Table 2 Experimental Equipment and Instruments

[0043]

[0044] 2. Examples and Comparative Examples

[0045] 2.1 Example 1

[0046] (1) Dissolve 0.05 mol NiSO4·6H2O, 0.15 mol MnSO4·7H2O, and 0.0005 mol Al(NO3)3·9H2O in 1 L of deionized water to form solution A; mix 500 mL of 1 mol / L NaOH solution with 500 mL of 1 mol / L NH3·H2O solution to form solution B; quickly add solution B to solution A, and stir at 500 rpm for 4 hours at 25 °C to obtain aluminum-doped Ni. 0.25 Mn 0.75 (OH)2 precipitate, washed and dried to obtain precursor A.

[0047] (2) Precursor A was mixed with 0.0015 mol of Al(NO3)3·9H2O in a deep eutectic solvent A (5 times the mass of precursor A) based on choline chloride and urea (molar ratio of 1:2), ultrasonically dispersed (40 kHz, 30 min), and stirred at 80 °C for 1 hour to promote the absorption of a small amount of Al. 3+ Slowly penetrating into the surface and interior of precursor particles, forming a medium concentration of Al 3+ A pre-doped layer was formed; subsequently, the Al was preheated in a muffle furnace at 180°C for 1 hour to allow it to harden. 3+ The precursor is fixed on the surface of the precursor and partially diffuses into the crystal lattice. After cooling to room temperature, the precursor is filtered and separated from the precursor by washing with anhydrous ethanol to obtain precursor B.

[0048] The preparation process of deep eutectic solvent A is as follows: weigh out a certain molar ratio of choline chloride and urea and mix them; heat and stir at about 60–80℃, and the two gradually melt to form a transparent and uniform liquid; cool to room temperature, and the liquid remains liquid at room temperature, which is the deep eutectic solvent.

[0049] (3) After weighing the precursor B and Li2CO3 in a molar ratio of 1:3, add 0.004 mol Al(NO3)3·9H2O, mix the three together and grind them thoroughly. Then add eutectic solvent B (5 times the mass of precursor B). In eutectic solvent B, the molar ratio of choline chloride: urea: Na2B4O7: glucose is 1:2:0.1:0.2. After ultrasonic dispersion (40 kHz, 30 min), microwave-assisted heating to 150 °C and holding for 90 minutes promotes Al 3+ Material C rapidly adsorbs and accumulates on the outer surface of precursor particles, forming a high-concentration doped region on the surface, exhibiting an overall obvious gradient doped structure.

[0050] The preparation process of the deep eutectic solvent B is as follows: weigh out a certain molar ratio of choline chloride, urea, Na2B4O7 and glucose and mix them; heat and stir at about 60-80℃, and gradually melt to form a transparent and uniform liquid; cool to room temperature, and the liquid remains liquid at room temperature.

[0051] (4) Transfer substance C to an alumina crucible, heat it to 550℃ at 5℃ / min under an inert atmosphere, and hold it at that temperature for 24 hours to induce the formation of a single-crystal Li. 1.2 Ni 0.2 Mn 0.6 O2 materials;

[0052] (5) The calcined product is washed and dried, and then soaked in 0.5 mol / L lithium trifluoromethyl sulfinate (LiCF3SO2) acetonitrile solution for 12 hours for pre-lithiation treatment. After that, it is centrifuged and dried to obtain the product.

[0053] 2.2 Example 2

[0054] (1) Dissolve 0.05 mol NiSO4·6H2O, 0.15 mol MnSO4·7H2O, and 0.0015 mol Al(NO3)3·9H2O in 1 L of deionized water to form solution A; mix 500 mL of 1 mol / L NaOH solution with 500 mL of 1 mol / L NH3·H2O solution to form solution B; quickly add solution B to solution A, and stir at 500 rpm for 4 hours at 25 °C to obtain aluminum-doped Ni. 0.25 Mn 0.75 (OH)2 precipitate, washed and dried to obtain precursor A.

[0055] (2) Precursor A was mixed with 0.0025 mol of Al(NO3)3·9H2O in a deep eutectic solvent A (5 times the mass of precursor A) based on choline chloride and urea (molar ratio of 1:2), ultrasonically dispersed (40 kHz, 30 minutes), and stirred at 80 °C for 1 hour to promote the addition of a small amount of Al. 3+ Slowly penetrating into the surface and interior of precursor particles, forming a medium concentration of Al 3+ A pre-doped layer was formed; subsequently, the Al was preheated in a muffle furnace at 180°C for 1 hour to allow it to harden. 3+ The precursor is fixed on the surface of the precursor and partially diffuses into the crystal lattice. After cooling to room temperature, the precursor is filtered and separated from the precursor by washing with anhydrous ethanol to obtain precursor B.

[0056] (3) After weighing the raw materials, precursor B and Li2CO3 in a molar ratio of 1:3, add 0.005 mol Al(NO3)3·9H2O, mix the three together and grind them thoroughly. Then add eutectic solvent B (5 times the mass of precursor B). In eutectic solvent B, the molar ratio of choline chloride:urea:Na2B4O7:glucose is 1:2:0.1:0.2. Then disperse by ultrasonication (40kHz, 30min) and heat to 160℃ with microwave assistance for 100 minutes to promote Al 3+ Material C rapidly adsorbs and accumulates on the outer surface of precursor particles, forming a high-concentration doped region on the surface, exhibiting an overall obvious gradient doped structure.

[0057] (4) Transfer substance C to an alumina crucible, heat it to 550℃ at 5℃ / min under an inert atmosphere, and hold it at that temperature for 24 hours to induce the formation of a single-crystal Li. 1.2 Ni 0.2 Mn 0.6 O2 materials;

[0058] (5) The calcined product is washed and dried, and then soaked in 0.5 mol / L lithium trifluoromethyl sulfinate (LiCF3SO2) acetonitrile solution for 12 hours for pre-lithiation treatment. After that, it is centrifuged and dried to obtain the product.

[0059] 2.3 Example 3

[0060] Steps (1), (2), and (3) are the same as in Example 1.

[0061] (4) Transfer substance C to an alumina crucible, heat it to 580℃ at 5℃ / min under an inert atmosphere, hold it at that temperature for 20 hours, and induce the formation of a single-crystal Li. 1.2 Ni 0.2 Mn 0.6 O2 materials;

[0062] (5) The calcined product is washed and dried, and then soaked in 0.4 mol / L lithium trifluoromethyl sulfinate (LiCF3SO2) acetonitrile solution for 8 hours for pre-lithiation treatment. After that, it is centrifuged and dried to obtain the product.

[0063] 2.4 Example 4

[0064] (1) Dissolve 0.05 mol Ni(NO3)2·6H2O, 0.15 mol Mn(NO3)2·4H2O, and 0.0015 mol Al(NO3)3·9H2O in 1 L of deionized water to form solution A; mix 500 mL of 1 mol / L NaOH solution with 1 L of 1 mol / L NH3·H2O solution to form solution B; quickly add solution B to solution A, and stir at 550 rpm for 5 hours at 25 °C to obtain aluminum-doped Ni. 0.25 Mn 0.75 (OH)2 precipitate, washed and dried to obtain precursor A.

[0065] Steps (2) and (3) are the same as in Example 1, except that the aluminum salt is replaced with AlCl3·6H2O and the lithium source is replaced with LiNO3.

[0066] Steps (4) and (5) are the same as in Example 1.

[0067] 2.5 Example 5

[0068] Step (1) is the same as in Example 1.

[0069] (2) Precursor A was mixed with 0.0015 mol of Al(NO3)3·9H2O in a deep eutectic solvent A (5 times the mass of precursor A) based on choline chloride and urea (molar ratio of 1:2), ultrasonically dispersed (40 kHz, 30 min), and stirred at 80 °C for 1 hour to promote the absorption of a small amount of Al. 3+ Slowly penetrating into the surface and interior of precursor particles, forming a medium concentration of Al 3+ A pre-doped layer was formed; subsequently, the Al was preheated in a muffle furnace at 180°C for 1 hour to allow it to harden. 3+ The precursor is fixed on the surface of the precursor and partially diffuses into the crystal lattice. After cooling to room temperature, the precursor is filtered and separated from the precursor by washing with anhydrous ethanol to obtain precursor B.

[0070] (3) After weighing the raw materials, precursor B and Li2CO3 in a molar ratio of 1:3, add 0.004 mol Al(NO3)3·9H2O, mix the three together and grind them thoroughly. Then add eutectic solvent B (5 times the mass of precursor B). In eutectic solvent B, the molar ratio of choline chloride:urea:NaH2PO4:cyclodextrin is 1:2:0.1:0.1. Then, disperse by ultrasonication (40 kHz, 30 min) and heat to 150 ℃ with microwave assistance for 90 minutes to promote Al 3+ Material C rapidly adsorbs and accumulates on the outer surface of precursor particles, forming a high-concentration doped region on the surface, exhibiting an overall obvious gradient doped structure.

[0071] Steps (4) and (5) are the same as in Example 1.

[0072] 2.6 Comparative Example 1

[0073] The difference between this embodiment and Example 1 is as follows: no aluminum was added; the solvent system did not use eutectic solvents A and B; the solvent system used was a LiCl+KCl monolayer molten salt (molar ratio 2:1); no microwave pretreatment of the solvent was performed; during sintering, the mixture was placed in a muffle furnace and heated to 900°C at a rate of 5°C / min, held at that temperature for 12 hours, and then naturally cooled; no pre-lithiation was performed.

[0074] 2.7 Comparative Example 2

[0075] The difference between this embodiment and Embodiment 1 is that: no aluminum was added, and deep eutectic solvents A and B were not used; during sintering, material C was placed in a muffle furnace and heated to 900°C at a rate of 5°C / min, held at that temperature for 12 hours, and then naturally cooled; no pre-lithiation was performed.

[0076] 2.8 Comparative Example 3

[0077] The difference between this embodiment and Example 1 is that eutectic solvents A and B were not used, and the solvent system was KCl:LiCl:Na2B4O7:glucose = 1:2:0.1:0.2 (molar ratio).

[0078] 2.9 Comparative Example 4

[0079] The difference between this embodiment and Example 1 is that: deep eutectic solvents A and B were not used, and the traditional molten salt system KCl:LiCl = 1:2 (molar ratio) was used; Na2B1O7 and carbon source were not added.

[0080] 2.10 Comparative Example 5

[0081] The difference between this embodiment and Embodiment 1 is that only step (1) involves low-concentration aluminum doping, while steps (2) and (3) do not involve aluminum doping.

[0082] 3. Tests and Results

[0083] 3.1 The electrochemical performance of the lithium manganese oxide cathode materials prepared in Examples 1-5 and Comparative Examples 1-5 was tested, and the test results are shown in Table 3.

[0084] Table 3. Electrochemical performance test results of lithium manganese oxide cathode material

[0085]

[0086] As shown in Table 3, the embodiments of the present invention significantly outperform the comparative examples in terms of initial coulombic efficiency, cycle stability, and voltage retention. This indicates that the five-in-one design of multi-stage gradient aluminum doping + DES-assisted induced single-crystal structure + microwave strengthening and oxygen vacancy lattice repair + in-situ coating + pre-lithiation treatment in this application can effectively improve the comprehensive electrochemical performance of single-crystal lithium-rich manganese-based cathode materials.

[0087] Specifically, the initial coulombic efficiency of the material obtained in Example 1 in Table 3 reached 85.2%, which is a significant improvement compared to Comparative Example 1 (74.1%) and Comparative Example 2 (68.7%) without aluminum doping and without deep eutectic solvent treatment, with improvements of 11.1% and 16.5%, respectively. This improvement is mainly attributed to: low-concentration co-precipitation doping of aluminum ions improving the bulk structural stability; and medium-concentration deep eutectic solvent treatment promoting Al... 3+ Uniform penetration optimizes crystal quality; high-concentration surface enrichment doping and in-situ carbon coating enhance interfacial stability; and the pre-lithiation step effectively compensates for initial lithium loss. These multi-level designs work synergistically to ensure effective lithium ion insertion and extraction during the first cycle, improving initial efficiency.

[0088] Comparing Comparative Example 3 with Example 1, it can be seen that Comparative Example 3 did not use the deep eutectic solvent (DES) composed of choline chloride and urea in the solvent system, but used a mixed system of KCl:LiCl:Na2B4O7:glucose. Although Na2B4O7 and the reducing carbon source glucose were introduced, the low-melting-point blend system with a stable and homogeneous molecular level was not formed, which led to uneven aluminum doping distribution and a lack of effective surface enrichment gradient control ability, thus affecting the integrity of single crystal particle boundaries and structural stability. As can be seen from Table 3, the electrochemical performance of the material obtained in Comparative Example 3 is significantly worse than that in Example 1.

[0089] Comparative Example 4 further simplified the solvent system compared to Examples 1 and 3, using only the traditional KCl:LiCl system without introducing Na2B4O7 and carbon source auxiliaries. This resulted in a material with inferior electrochemical performance compared to Comparative Example 3. This is because: in a high-melting-point salt medium without hydrogen bond network support, the aluminum source has poor mobility, and due to the lack of Na2B4O7 for repair and carbon source coating, it is prone to the accumulation of oxygen defects in the crystals during calcination, leading to severe surface side reactions and affecting the material's structural stability.

[0090] Compared to Example 1, Comparative Example 5 only underwent a low-concentration aluminum doping treatment once, and did not form a multi-gradient progressive enrichment structure. This resulted in the aluminum being too dispersed in the cathode particles, lacking surface structure regulation function, and still being unable to effectively suppress oxygen release from the crystal structure and interfacial side reactions under high voltage, leading to poor electrochemical performance.

[0091] 3.2 Furthermore, to more fully demonstrate the advantages of the single-crystal lithium-rich manganese-based cathode material prepared in this application, SEM comparison images of the single-crystal lithium-rich manganese-based cathode materials prepared in Examples 1-2 and Comparative Examples 1-2 are also provided. Figure 1 ), Comparison of first-cycle charge-discharge curves ( Figure 2 ), Comparison of cycle performance curves ( Figure 3 Comparison of voltage drop curves over 50 cycles () Figure 4 ).

[0092] Figure 1 These are SEM comparison images of the cathode materials, from... Figure 1 As can be seen, Examples 1, 2, and Comparative Example 1 all basically exhibit single primary particles with an octahedral morphology. The grains in Examples 1 and 2 are slightly larger than those in Comparative Example 1, and have sharper octahedral boundaries, indicating better crystallinity. Comparative Example 2, on the other hand, shows secondary particles with a polycrystalline morphology. Micron-sized single-crystal materials have higher load-bearing capacity than polycrystalline materials, exhibiting higher strength and toughness under severe deformation forces. Examples 1, 2, and Comparative Example 1 were prepared using deep eutectic solvent and molten salt assisted solvents, respectively, and the resulting particles had different morphologies. This indicates that different assisted solvents can have a certain influence on the change in surface energy, thereby affecting the growth of crystal faces during calcination.

[0093] Figure 2 This is a comparison chart of the first charge-discharge curves of the cathode material, from... Figure 2 It can be seen that, Figure 2 The initial coulombic efficiency of the samples from Examples 1 and 2 is compared with that of Comparative Examples 1 and 2. It can be seen that the initial coulombic efficiency of the sample from Example 1 is as high as 85.2%, significantly better than that of Comparative Example 1 (74.1%) and Comparative Example 2 (68.7%), with improvements of 11.1% and 16.5%, respectively. Example 2 also achieves 84.6%, showing equally excellent performance. These results indicate that the gradient structure constructed through multi-stage aluminum doping and deep eutectic solvent-assisted treatment can effectively enhance the structural stability and reversible lithium-ion insertion / extraction efficiency of the material. In contrast, the comparative samples, without aluminum doping or DES treatment, resulted in more crystal defects, poor interface stability, severe initial lithium loss, and significantly lower coulombic efficiency.

[0094] Figure 3 This is a comparison chart of the cycle performance curves of the cathode materials, from... Figure 3It can be seen that, at a 0.2C rate, Example 1 maintained a high discharge capacity of 246.5 mAh / g after 200 cycles, with a capacity retention rate as high as 92.2%, while Comparative Examples 1 and 2 only achieved 88.7% and 65.8%, respectively. Comparative Example 2, which did not use any auxiliary solvent, exhibited severe capacity decay, indicating that the eutectic solvent played a crucial role in reducing defects and enhancing structural stability. Furthermore, the capacity retention rates of Examples 2–5 were all above 88.9%, demonstrating that this technical route has good repeatability and stability.

[0095] Figure 4 This is a comparison chart of the voltage drop curves of the positive electrode material over 50 cycles. Figure 4 It can be seen that the voltage decay in the first 50 cycles of Example 1 is only 0.04V, which is significantly better than that of Comparative Example 1 (0.12V) and Comparative Example 2 (0.27V). This fully demonstrates that the deep eutectic solvent and aluminum gradient doping process ensure good structural stability of the crystal; the boron / phosphorus anions release oxygen sources during calcination, promoting the self-repair of lattice defects; and the surface carbon coating reduces polarization and stabilizes high-voltage cycling.

[0096] In summary, this invention achieves multi-level control over the cathode material structure from the interior to the surface by constructing a gradient concentration distribution of aluminum ions. The low-concentration aluminum ion doping in the interior effectively stabilizes the crystal structure, mitigating volume changes and stress concentration during calcination and charge / discharge processes, and reducing the risk of particle cracking. The medium-concentration aluminum doping layer, assisted by a deep eutectic solvent, allows aluminum ions to diffuse more uniformly into the crystal lattice, contributing to improved overall crystallinity and ion migration pathways. Meanwhile, the high-concentration aluminum enrichment region on the surface suppresses transition metal dissolution and side reactions during high-voltage cycling, enhancing electrode / electrolyte interface stability and extending cycle life. Furthermore, this invention introduces a deep eutectic solvent composed of choline chloride and urea, replacing the traditional molten salt system. This solvent has a low melting point, high ionic conductivity, and excellent designability, enabling structural reconstruction and assisted calcination within a medium temperature range, significantly reducing heat treatment temperature and promoting energy conservation and environmental protection. The added phosphorus or boron-containing anions can slowly release oxygen during calcination, promoting the self-repair of lattice defects; the carbon source is carbonized in situ at high temperature to form a coating layer, further improving the conductivity and interfacial stability of the material. Furthermore, the pre-lithiation treatment with lithium solution after calcination can effectively compensate for lithium defects in the material, increasing the initial coulombic efficiency to over 89%.

[0097] The single-crystal lithium-rich manganese-based cathode material prepared by this invention has the characteristics of complete structure, uniform particle size, gradient doping, and surface carbon coating, which effectively improves the electrochemical performance and cycle life of the material.

[0098] The specific embodiments described above should not be construed as limiting the scope of protection of this invention. Any alternative modifications or variations made to the embodiments of this invention by those skilled in the art will fall within the scope of protection of this invention. All aspects not detailed in this invention are well-known to those skilled in the art.

Claims

1. A method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping, characterized in that, Includes the following steps: (1) Dissolve nickel salt, manganese salt and low-concentration aluminum salt in deionized water to form a metal salt solution. Add alkaline precipitant and complexing agent to the solution to carry out a co-precipitation reaction to obtain a low-concentration aluminum-doped Ni-Mn hydroxide precursor. After washing and drying, precursor A is obtained. (2) Add precursor A and medium-concentration aluminum salt to deep eutectic solvent A, ultrasonically disperse and stir and heat to allow aluminum ions to fully penetrate into the surface of precursor particles and partially diffuse into the interior of the crystal lattice to form a medium-concentration pre-doped layer. Then, perform low-temperature preheating to fix the aluminum distribution, wash away residual solvent components, and obtain precursor B. (3) After mixing and grinding the precursor B with lithium source and high-concentration aluminum salt evenly, it is added to the deep eutectic solvent B, then ultrasonically dispersed and microwave-assisted heating is used to adsorb and enrich aluminum ions on the particle surface to obtain substance C with obvious gradient doping structure. (4) Calcine substance C in an argon or nitrogen atmosphere at a calcination temperature of 500-600℃, a heating rate of 3-7℃ / min, and a holding time of 20-30 hours. After cooling to room temperature, wash and dry to obtain a calcined product with a carbon coating. (5) The calcined product is immersed in a lithium salt solution, centrifuged and dried to achieve prelithiation; Among them, deep eutectic solvent A is prepared by mixing choline chloride and urea; deep eutectic solvent B is prepared by mixing choline chloride, urea, phosphorus / boron-containing anions, and a carbon source. In step (1), the amount of aluminum salt doped is 0.5-3% of the amount of Ni element; in step (2), the amount of aluminum salt doped is 3-5% of the amount of Ni element; in step (3), the amount of aluminum salt doped is 5-10% of the amount of Ni element.

2. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, In step (1), the coprecipitation reaction conditions are: temperature 20-30℃, stirring speed 400-600rpm, and reaction time 3-5h; In step (2), when the aluminum doping concentration is moderate, ultrasonic dispersion is performed for 25-40 minutes, followed by stirring at 50-100℃ for 1 hour to allow a small amount of Al to be absorbed. 3+ It slowly penetrates into the surface and interior of the precursor particles; then it is preheated in a muffle furnace at 150-200℃ for 30-80 minutes, so that Al 3+ It is fixed to the surface of the precursor and partially diffuses into the crystal lattice; In step (3), the mixture is ultrasonically dispersed for 25-40 minutes and then heated to 130-180°C under microwave conditions, followed by heat preservation for 60-120 minutes to promote Al... 3+ It is rapidly adsorbed and enriched on the outer surface of the precursor particles.

3. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, The aluminum salt is selected from one or more of Al(NO3)3·9H2O, AlCl3·6H2O, and Al2(SO4)3·18H2O.

4. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, The phosphorus or boron-containing anions are one or more of Na3PO4, KH2PO4, Na2HPO4, NaH2PO4, NaBO2, Na2B2O4, and Na2B4O7; the carbon source is one or more of glucose, sucrose, and cyclodextrin; the doping amounts of the two components in the deep eutectic solvent B are as follows: on a molar ratio basis, phosphorus / boron-containing anion: choline chloride = 1:5-1:10; carbon source: choline chloride = 1:5-1:

10.

5. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, In step (5), the lithium salt molten salt is selected from one or more of the following: lithium trifluoromethyl sulfinate acetonitrile solution, lithium hexafluorophosphate carbonate solution, lithium hexafluorosilicate carbonate solution, and lithium tetrafluoroborate acetonitrile solution with a molar concentration of 0.3-0.7 mol / L, and the soaking time is 8-16 hours.

6. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, The nickel salt is selected from one or more of NiSO4·6H2O, Ni(NO3)2·6H2O, and NiCl2·6H2O; the manganese salt is selected from one or more of MnSO4·7H2O, Mn(NO3)2·4H2O, and MnCl2·4H2O.

7. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, The alkaline precipitant is selected from one or more of sodium hydroxide, potassium hydroxide, and ammonium hydroxide; the complexing agent is selected from one or more of ammonia, ethylenediaminetetraacetic acid, citric acid, or their salts; the alkaline precipitant and the complexing agent are mixed in a volume ratio of 0.5:1 to 2:

1.

8. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, In step (3), the lithium source is selected from one or more of Li2CO3, LiOH·H2O, LiNO3, and Li2O, and the precursor B is mixed with the lithium source in a molar ratio of 1:3 to 3:

1.

9. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, In step (2), the mass of the deep eutectic solvent A used is 3-8 times that of the precursor A; in step (3), the mass of the deep eutectic solvent B used is 3-8 times that of the precursor B.

10. The method for preparing single-crystal lithium-rich manganese-based cathode materials by synergistic regulation of deep eutectic solvent and aluminum gradient doping according to claim 1, characterized in that, In both eutectic solvent A and eutectic solvent B, the molar ratio of choline chloride to urea is 1:1.5 to 1:2.

5. The preparation method of deep eutectic solvent A or deep eutectic solvent B is as follows: weigh out each component in a certain molar ratio and mix them; heat and stir at about 60–80℃ until the two gradually melt to form a transparent and uniform liquid, and then cool to room temperature to obtain the final product.

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