Modified positive electrode material and preparation method and application thereof

Abstract

The application provides a modified positive electrode material and a preparation method and application thereof, and the preparation method comprises the following steps: (1) mixing a nickel source, a cobalt source and a manganese source with a solvent to obtain a first mixed salt solution, mixing an aluminum source and an alkali liquor to obtain an aluminum source solution, and adding the first mixed salt solution, the aluminum source solution, ammonia water and the alkali liquor into a bottom liquor in parallel flow to control pH and perform one-step reaction; (2) mixing the nickel source, the cobalt source and the manganese source with the solvent to obtain a second mixed salt solution, stopping feeding of the first mixed salt solution and the ammonia water, and passing in oxalic acid solution and the second mixed salt solution in parallel flow to perform two-step reaction and obtain a precursor; and (3) mixing the precursor obtained in the step (2) with a lithium source, and performing sintering treatment to obtain the modified positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a modified cathode material, its preparation method, and its application. Background Technology

[0002] High energy density, long cycle life, and safety have become the most important indicators for ternary cathode materials in lithium-ion batteries. There are two methods to improve the specific capacity of ternary cathode materials: one is to increase the nickel content, and the other is to increase the charging cut-off voltage to release more lithium ions. While increasing the nickel content can significantly improve the specific capacity, it can introduce safety concerns for the battery cell. Currently, some mainstream cathode material manufacturers in China prefer the high-voltage approach, believing it makes it easier to achieve a balance between high energy density and safety. However, this approach also has significant drawbacks: severe irreversible capacity loss during the first discharge, and excessively high cut-off voltages can damage the material's structure, thus affecting its cycle performance.

[0003] CN112993258A discloses a method for doping and coating ternary cathode materials, ternary cathode materials, and lithium-ion batteries. The method includes: 1) preparing a ternary cathode material precursor using a co-precipitation method with silicates, a doped metal source, a nickel source, a cobalt source, and a manganese source; 2) using a saturated silicate solution as a base solution to clean the ternary cathode material core; during the cleaning process, adding metal salts for precipitation and coating; dehydration; and a second sintering to obtain the doped and coated ternary cathode material.

[0004] CN104882589A discloses a method for preparing a carbon-coated ternary cathode material, the steps of which are as follows: preparing a ternary cathode material precursor; preparing a suspension of the ternary cathode material precursor, wherein the mass percentage concentration of the ternary cathode material precursor in the suspension is 5% to 30%; adding lithium acrylate to the suspension of the ternary cathode material precursor at a molar ratio of Li:(Ni+Co+Mn) of 1.03 to 1.05:1; and adding lithium acrylate to the suspension at a temperature of 50℃ to 80℃. Ammonium persulfate is added to a suspension of a ternary cathode material precursor to induce a polymerization reaction of lithium acrylate, thereby obtaining a suspension of a ternary cathode material precursor coated with lithium polyacrylate. The mass of the ammonium persulfate is 2% to 10% of the mass of the lithium acrylate, and the reaction time is 5 to 8 hours. The suspension of the ternary cathode material precursor coated with lithium polyacrylate is dried to obtain spherical particles. The ternary cathode material precursor particles coated with lithium polyacrylate are then sintered to obtain a carbon-coated ternary cathode material.

[0005] To improve capacity and cycle stability under high voltage, existing technologies typically involve coating and doping the cathode material to enhance the stability of the cathode / electrolyte interface and the layered structure, thereby achieving a balance between high energy density and safety. However, this approach increases material cost to some extent, and uneven doping can also affect material consistency and electrochemical performance. Summary of the Invention

[0006] The purpose of this invention is to provide a modified cathode material, its preparation method, and its application. This invention improves the cycling stability of the material under high voltage by changing the type of complexing agent during the co-precipitation process at the precursor end and designing the composition and structure of the inner and outer layers of the material.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a modified cathode material, the method comprising the following steps:

[0009] (1) A first mixed salt solution is obtained by mixing a nickel source, a cobalt source and a manganese source with a solvent, and an aluminum source solution is obtained by mixing an aluminum source solution with an alkaline solution. The first mixed salt solution, the aluminum source solution, ammonia water and the alkaline solution are added to the bottom liquid in parallel, and the pH is controlled to carry out a one-step reaction.

[0010] (2) Nickel source, cobalt source and manganese source are mixed with solvent to obtain second mixed salt solution. The first mixed salt solution and ammonia water are stopped from being fed and oxalic acid solution and second mixed salt solution are introduced to carry out two-step reaction to obtain precursor.

[0011] (3) The precursor obtained in step (2) and the lithium source are mixed and sintered to obtain the modified cathode material.

[0012] This invention, at the precursor end, modifies the type of complexing agent during co-precipitation, utilizing the complexation difference between ammonia and oxalic acid complexing agents to design a core-shell structure with a low external nickel content and a high internal nickel content, exhibiting a loose internal structure and a compact external structure. After lithium sintering, the cathode material inherits the composition and structural characteristics of the precursor, resulting in a specially designed cathode material with good cycle stability and high energy density under high voltage. The preparation method of this invention is simple, low-cost, and produces minimal environmental pollution, making it suitable for industrial production.

[0013] Preferably, the nickel source in step (1) includes any one or a combination of at least two of nickel sulfate, nickel chloride, or nickel nitrate.

[0014] Preferably, the cobalt source includes any one or a combination of at least two of cobalt sulfate, cobalt chloride, or cobalt nitrate.

[0015] Preferably, the manganese source includes any one or a combination of at least two of manganese sulfate, manganese chloride, or manganese nitrate.

[0016] Preferably, the aluminum source comprises aluminum sulfate octadecahydrate.

[0017] Preferably, the alkaline solution comprises a sodium hydroxide solution.

[0018] Preferably, the molar ratio of Ni, Co and Mn in the first mixed salt solution is (80-96):(1-10):(1-10), for example: 80:10:10, 85:8:7, 90:5:5, 92:3:5 or 96:2:2, etc.

[0019] Preferably, the total mass concentration of metal ions in the first mixed salt solution is 95-105 g / L, for example: 95 g / L, 98 g / L, 100 g / L, 102 g / L or 105 g / L, etc.

[0020] Preferably, the molar concentration of aluminum ions in the aluminum source solution is 2 to 4 g / L, for example: 2 g / L, 2.5 g / L, 3 g / L, 3.5 g / L or 4 g / L, etc.

[0021] Preferably, the mass concentration of the ammonia water is 25-30%, for example: 25%, 26%, 27%, 28%, 29% or 30%, etc.

[0022] Preferably, the mass concentration of the alkaline solution is 16-20%, for example: 16%, 17%, 18%, 19% or 20%, etc.

[0023] Preferably, in step (1), the flow rate of the first mixed salt solution is 7 to 10 L / h, for example: 7 L / h, 8 L / h, 9 L / h or 10 L / h.

[0024] Preferably, the flow rate of the aluminum source solution is 1 to 3 L / h, for example: 1 L / h, 1.5 L / h, 2 L / h, 2.5 L / h or 3 L / h, etc.

[0025] Preferably, the flow rate of the ammonia water is 0.8 to 1.5 L / h, for example: 0.8 L / h, 0.9 L / h, 1 L / h, 1.2 L / h or 1.5 L / h, etc.

[0026] Preferably, the flow rate of the alkaline solution is 1.2 to 2.5 L / h, for example: 1.2 L / h, 1.5 L / h, 1.8 L / h, 2 L / h or 2.5 L / h, etc.

[0027] Preferably, the base liquid includes ammonia and an alkaline solution.

[0028] Preferably, the mass concentration of ammonia in the base solution is 2 to 7 g / L, for example: 2 g / L, 3 g / L, 4 g / L, 5 g / L or 7 g / L.

[0029] Preferably, the pH of the base solution is 11.5 to 11.8, for example: 11.5, 11.6, 11.7 or 11.8.

[0030] Preferably, the pH of the one-step reaction is 11 to 11.5, for example: 11, 11.1, 11.2, 11.3, 11.4 or 11.5, etc.

[0031] Preferably, the endpoint of the one-step reaction is that the median particle size D50 of the particles in the reaction system is 2 to 12 μm, for example: 2 μm, 5 μm, 8 μm, 10 μm or 12 μm, etc.

[0032] Preferably, the nickel source in step (2) includes any one or a combination of at least two of nickel sulfate, nickel chloride, or nickel nitrate.

[0033] Preferably, the cobalt source includes any one or a combination of at least two of cobalt sulfate, cobalt chloride, or cobalt nitrate.

[0034] Preferably, the manganese source includes any one or a combination of at least two of manganese sulfate, manganese chloride, or manganese nitrate.

[0035] Preferably, the molar ratio of Ni, Co and Mn in the second mixed salt solution is (20-60):(10-30):(10-30), for example: 33:33:34, 40:30:30, 50:20:30 or 60:20:20, etc.

[0036] Preferably, the total mass concentration of metal ions in the second mixed salt solution is 95–105 g / L.

[0037] Preferably, the solute in the oxalic acid solution includes any one or a combination of at least two of sodium oxalate, ammonium oxalate, or oxalic acid.

[0038] Preferably, the oxalic acid solution has a mass concentration of 50–70 g / L, for example: 50 g / L, 55 g / L, 60 g / L, 65 g / L, or 70 g / L.

[0039] Preferably, the flow rate of the oxalic acid solution in step (2) is 2 to 3 L / h, for example: 2 L / h, 2.2 L / h, 2.5 L / h, 2.8 L / h or 3 L / h, etc.

[0040] Preferably, the flow rate of the second mixed salt solution is 8 to 12 L / h, for example: 8 L / h, 9 L / h, 10 L / h, 11 L / h or 12 L / h.

[0041] Preferably, the median particle size D50 of the precursor is 3 to 15 μm, for example: 3 μm, 5 μm, 8 μm, 10 μm or 15 μm, etc.

[0042] Preferably, the lithium source in step (3) includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, lithium oxalate, lithium chloride, or lithium nitrate.

[0043] Preferably, the sintering process includes solid-state sintering.

[0044] Preferably, the sintering process includes one-step sintering and two-step sintering.

[0045] Preferably, the temperature of the one-step sintering is 450-550℃, for example: 450℃, 480℃, 500℃, 520℃ or 550℃.

[0046] Preferably, the sintering time in the first step is 4 to 6 hours, for example: 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0047] Preferably, the temperature of the two-step sintering is 600-800℃, for example: 600℃, 650℃, 700℃, 750℃ or 800℃.

[0048] Preferably, the two-step sintering time is 10 to 16 hours, for example: 10 hours, 11 hours, 12 hours, 14 hours or 16 hours.

[0049] Secondly, the present invention provides a modified cathode material, which is prepared by the method described in the first aspect. The modified cathode material includes a core and a shell disposed on the surface of the core, wherein the core has the chemical formula LiNi. a Co b Mn c Al d O2, wherein 0.8 < a ≤ 0.9, 0.05 ≤ b < 0.1, 0.05 ≤ c < 0.1, 0.01 ≤ d < 0.03, a + b + c + d = 1, and the chemical formula of the outer shell is LiNi. x Co y Mn z O2, where 0.2 < x ≤ 0.6, 0.1 < y ≤ 0.3, 0.1 < z ≤ 0.3, and x + y + z = 1.

[0050] Thirdly, the present invention provides a positive electrode sheet comprising the modified positive electrode material as described in the second aspect.

[0051] Fourthly, the present invention provides a lithium-ion battery comprising a positive electrode as described in the third aspect.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] (1) In this invention, the type of complexing agent is changed during the co-precipitation process at the precursor end, and the composition and structure of the inner and outer layers of the material are designed to improve the cycling stability of the material under high voltage.

[0054] (2) The initial specific capacity of the battery made from the modified cathode material described in this invention can reach 200 mAh g. -1 The above results show that the capacity retention rate can reach over 93% after 50 laps. Attached Figure Description

[0055] Figure 1 This is a SEM image of the precursor described in Example 1.

[0056] Figure 2 This is a cross-sectional SEM image of the precursor described in Example 1. Detailed Implementation

[0057] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0058] Example 1

[0059] This embodiment provides a modified cathode material, and the preparation method of the modified cathode material is as follows:

[0060] (1) Prepare a solution A with a metal ion concentration of 100 g / L by dissolving nickel sulfate, cobalt sulfate, and manganese sulfate in a Ni:Co:Mn molar ratio of 80:10:10. Prepare an aluminum solution with a concentration of 2.8 g / L by dissolving aluminum sulfate octahydrate in alkali and water. Add a 27% ammonia solution (solution C) and an 18% sodium hydroxide solution (solution D) at 400 rpm and 60°C to prepare a bottom solution with an ammonia concentration of 5 g / L and a pH of 11.6. Add solutions A, B, C, and D to the reactor in parallel at flow rates of 8 L / h, 1.3 L / h, 1.0 L / h, and 1.7 L / h, respectively. Control the pH of the reaction system to 11.2. Stop feeding solutions A and C when the particle size reaches 6 μm.

[0061] (2) A prepared oxalic acid solution E with a concentration of 60 g / L and a ternary solution F with a Ni, Co, Mn molar ratio of 5:2:3 and a metal ion concentration of 100 g / L were fed into the reactor at feed rates of 8 L / h and 2.2 L / h, respectively. After the particles grew to 10 μm, they were washed, filtered, and dried to obtain a core composition of Ni. 0.8 Co 0.1 Mn 0.1 Al0.01 (OH)2, the shell is composed of Ni 0.5 Co 0.2 Mn 0.3 The precursor of (OH)2, and the SEM image of the precursor are shown below. Figure 1 As shown, the cross-sectional SEM image of the precursor is as follows: Figure 2 As shown;

[0062] (3) The precursor was mixed with lithium hydroxide at a lithium ratio of 1.05 and heated to 520°C at a heating rate of 3°C / min and held for 5 h. Then, it was heated to 700°C at a heating rate of 3°C / min and held for 14 h to obtain a LiNi core. 0.8 Co 0.1 Mn 0.1 Al 0.01 O2, shell is LiNi 0.5 Co 0.2 Mn 0.3 O2 cathode material.

[0063] Example 2

[0064] This embodiment provides a modified cathode material, and the preparation method of the modified cathode material is as follows:

[0065] (1) Prepare a solution A with a metal ion concentration of 100 g / L by dissolving nickel sulfate, cobalt sulfate, and manganese sulfate in a Ni:Co:Mn molar ratio of 88.89:6.06:5.05. Prepare an aluminum solution with a concentration of 2.8 g / L by dissolving aluminum sulfate octadeca in alkali and water. Add a 28% ammonia solution (solution C) and an 18% sodium hydroxide solution (solution D) at 380 rpm and 60°C to prepare a bottom solution with an ammonia concentration of 5 g / L and a pH of 11.7. Add solutions A, B, C, and D to the reactor in parallel at flow rates of 8 L / h, 1.3 L / h, 1.0 L / h, and 1.7 L / h, respectively. Control the pH of the reaction system to 11.3. Stop feeding solutions A and C when the particle size reaches 7.5 μm.

[0066] (2) A prepared oxalic acid solution E with a concentration of 60 g / L and a ternary solution F with a Ni, Co, Mn molar ratio of 5:2:3 and a metal ion concentration of 100 g / L were fed into the reactor at feed rates of 8 L / h and 2.2 L / h, respectively. After the particles grew to 10 μm, they were washed, filtered, and dried to obtain a core composition of Ni. 0.88 Co 0.06 Mn 0.05 Al 0.01 (OH)2, the shell is composed of Ni 0.5 Co 0.2 Mn 0.3(OH)2 precursor;

[0067] (3) The precursor was mixed with lithium hydroxide at a lithium ratio of 1.03 and heated to 500°C at a heating rate of 3°C / min and held for 5 h. Then, it was heated to 750°C at a heating rate of 3°C / min and held for 12 h to obtain a LiNi core. 0.88 Co 0.06 Mn 0.05 Al 0.01 O2, shell is LiNi 0.5 Co 0.2 Mn 0.3 O2 cathode material.

[0068] Example 3

[0069] The only difference between this embodiment and embodiment 1 is that when the particle size in step (1) reaches 1.5 μm, the feeding of solutions A and C is stopped. All other conditions and parameters are exactly the same as in embodiment 1.

[0070] Example 4

[0071] The only difference between this embodiment and embodiment 1 is that when the particle size in step (1) reaches 15 μm, the feeding of solutions A and C is stopped. Other conditions and parameters are exactly the same as in embodiment 1.

[0072] Example 5

[0073] The only difference between this embodiment and Example 1 is that the oxalic acid concentration is 40 g / L, while the other conditions and parameters are exactly the same as in Example 1.

[0074] Example 6

[0075] The only difference between this embodiment and Example 1 is that the oxalic acid concentration is 80 g / L, while the other conditions and parameters are exactly the same as in Example 1.

[0076] Example 7

[0077] The only difference between this embodiment and Embodiment 1 is that the oxalic acid feed rate is 1 L / h, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0078] Example 8

[0079] The only difference between this embodiment and Embodiment 1 is that the oxalic acid feed rate is 4 L / h, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0080] Example 9

[0081] The only difference between this embodiment and Embodiment 1 is that the molar ratio of Ni, Co, and Mn in solution A is 8:1:1, and the molar ratio of Ni, Co, and Mn in solution F is 5:2:3. All other conditions and parameters are exactly the same as in Embodiment 1.

[0082] Example 10

[0083] The only difference between this embodiment and Embodiment 1 is that one-step sintering is used only at 600°C; all other conditions and parameters are exactly the same as in Embodiment 1.

[0084] Comparative Example 1

[0085] The only difference between this comparative example and Example 1 is that solutions E and F are not introduced, and LiNi is ultimately prepared. 0.88 Co 0.06 Mn 0.05 Al 0.01 O2 cathode material.

[0086] Comparative Example 2

[0087] The only difference between this comparative example and Example 1 is that no oxalic acid solution was added; all other conditions and parameters are exactly the same as in Example 1.

[0088] Performance testing:

[0089] The positive electrode materials prepared in the comparative example and Example 1 were mixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 90:5:5 and placed in a high-speed stirrer mold. An appropriate amount of N-methylpyrrolidone was added, and the mixture was stirred at 3000 rpm for 10 minutes to obtain a slurry of suitable viscosity. This slurry was then coated onto a clean current collector and dried in a vacuum drying oven for 24 hours. The dried electrode sheet was rolled to a suitable thickness, punched into a 10 mm diameter sheet, and dried with a separator and battery casing in a vacuum environment for 12 hours. Finally, a 2025 coin cell was assembled in a glove box using a lithium sheet as the counter electrode. Charge-discharge tests were performed using a Blue Electric CT2001A electrochemical tester, with a voltage range of 2.8-4.5V, a test current density of 0.2C, and 50 cycles. The test results are shown in Table 1.

[0090] Table 1

[0091]

[0092]

[0093] As shown in Table 1, and based on Examples 1-2, the initial specific capacity of the battery made from the modified cathode material of this invention can reach 200 mAh g⁻¹. -1 The above results show that the capacity retention rate can reach over 93% after 50 laps.

[0094] A comparison of Examples 1 and 3-4 shows that the timing of feeding oxalic acid and the second mixed salt solution into the modified cathode material of the present invention affects the performance of the modified cathode material. Feeding the material after the particle size of the one-step reaction reaches 2-12 μm results in better performance of the cathode material. If the feeding time is too early, the discharge capacity of the material will be reduced. If the feeding time is too late, the cycle performance of the material will be affected.

[0095] Comparing Examples 1 and 5-8, it can be seen that the concentration of oxalic acid and the feeding rate in step (2) affect the performance of the modified cathode material. The modified cathode material with better performance is obtained by controlling the concentration of oxalic acid solution at 50-70 g / L and the feeding rate at 2-3 L / h. If the range is exceeded, the capacity and cycle life of the material will be affected.

[0096] A comparison of Examples 1 and 9 shows that the present invention uses a high-nickel material for the core and a low-nickel material for the outer shell, which is beneficial to improving the cycle performance of the materials.

[0097] A comparison of Examples 1 and 10 shows that the present invention can improve the capacity and cycle performance of materials through stepwise sintering.

[0098] As can be seen from the comparison between Example 1 and Comparative Examples 1-2, the present invention utilizes the complexation difference between ammonia complexing agent and oxalic acid complexing agent to design a core-shell structure with low external nickel content and high internal nickel content, and a loose internal structure and a compact external structure. After lithium sintering, the cathode material inherits the composition and structural characteristics of the precursor, so that the specially designed cathode material has good cycle stability and high energy density under high voltage.

[0099] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a modified cathode material, characterized in that, The preparation method includes the following steps: (1) A first mixed salt solution is obtained by mixing a nickel source, a cobalt source and a manganese source with a solvent, and an aluminum source solution is obtained by mixing an aluminum source solution with an alkaline solution. The first mixed salt solution, the aluminum source solution, ammonia water and the alkaline solution are added to the bottom liquid in parallel, and the pH is controlled to carry out a one-step reaction. (2) Nickel source, cobalt source and manganese source are mixed with solvent to obtain second mixed salt solution. The first mixed salt solution and ammonia water are stopped from being fed and oxalic acid solution and second mixed salt solution are introduced to carry out two-step reaction to obtain precursor. The oxalic acid solution has a mass concentration of 50–70 g / L; the flow rate of the oxalic acid solution is 2–3 L / h. (3) The precursor obtained in step (2) and the lithium source are mixed and sintered to obtain the modified cathode material.

2. The preparation method according to claim 1, characterized in that, The nickel source in step (1) includes any one or a combination of at least two of nickel sulfate, nickel chloride, or nickel nitrate.

3. The preparation method according to claim 1, characterized in that, The cobalt source in step (1) includes any one or a combination of at least two of cobalt sulfate, cobalt chloride, or cobalt nitrate.

4. The preparation method according to claim 1, characterized in that, The manganese source in step (1) includes any one or a combination of at least two of manganese sulfate, manganese chloride, or manganese nitrate.

5. The preparation method according to claim 1, characterized in that, The aluminum source in step (1) includes aluminum sulfate octadecahydrate.

6. The preparation method according to claim 1, characterized in that, The alkaline solution in step (1) includes a sodium hydroxide solution.

7. The preparation method according to claim 1, characterized in that, Step (1) The molar ratio of Ni, Co and Mn in the first mixed salt solution is (80-96):(1-10):(1-10).

8. The preparation method according to claim 1, characterized in that, Step (1) The total mass concentration of metal ions in the first mixed salt solution is 95-105 g / L.

9. The preparation method according to claim 1, characterized in that, The molar concentration of aluminum ions in the aluminum source solution in step (1) is 2-4 g / L.

10. The preparation method according to claim 1, characterized in that, The mass concentration of the ammonia water in step (1) is 25-30%.

11. The preparation method according to claim 1, characterized in that, The mass concentration of the alkaline solution in step (1) is 16-20%.

12. The preparation method according to claim 1, characterized in that, In step (1), the flow rate of the first mixed salt solution is 7-10 L / h.

13. The preparation method according to claim 1, characterized in that, The flow rate of the aluminum source solution in step (1) is 1 to 3 L / h.

14. The preparation method according to claim 1, characterized in that, The flow rate of the ammonia water in step (1) is 0.8 to 1.5 L / h.

15. The preparation method according to claim 1, characterized in that, The flow rate of the alkaline solution in step (1) is 1.2 to 2.5 L / h.

16. The preparation method according to claim 1, characterized in that, The base liquid in step (1) includes ammonia and alkaline solution.

17. The preparation method according to claim 1, characterized in that, The mass concentration of ammonia in the bottom solution in step (1) is 2-7 g / L.

18. The preparation method according to claim 1, characterized in that, The pH of the base solution in step (1) is 11.5 to 11.

8.

19. The preparation method according to claim 1, characterized in that, The pH of the one-step reaction is 11 to 11.

5.

20. The preparation method according to claim 1, characterized in that, The endpoint of the one-step reaction is when the median particle size D50 of the particles in the reaction system is 2–12 μm.

21. The preparation method according to claim 1, characterized in that, The nickel source in step (2) includes any one or a combination of at least two of nickel sulfate, nickel chloride, or nickel nitrate.

22. The preparation method according to claim 1, characterized in that, The cobalt source in step (2) includes any one or a combination of at least two of cobalt sulfate, cobalt chloride, or cobalt nitrate.

23. The preparation method according to claim 1, characterized in that, The manganese source in step (2) includes any one or a combination of at least two of manganese sulfate, manganese chloride, or manganese nitrate.

24. The preparation method according to claim 1, characterized in that, In step (2), the molar ratio of Ni, Co and Mn in the second mixed salt solution is (20-60):(10-30):(10-30).

25. The preparation method according to claim 1, characterized in that, In step (2), the total mass concentration of metal ions in the second mixed salt solution is 95-105 g / L.

26. The preparation method according to claim 1, characterized in that, The solute in the oxalic acid solution in step (2) includes any one or a combination of at least two of sodium oxalate, ammonium oxalate, or oxalic acid.

27. The preparation method according to claim 1, characterized in that, In step (2), the flow rate of the second mixed salt solution is 8-12 L / h.

28. The preparation method according to claim 1, characterized in that, The median particle size D50 of the precursor in step (2) is 3 to 15 μm.

29. The preparation method according to claim 1, characterized in that, The lithium source in step (3) includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, lithium oxalate, lithium chloride, or lithium nitrate.

30. The preparation method according to claim 1, characterized in that, The sintering process described in step (3) includes solid-state sintering.

31. The preparation method according to claim 1, characterized in that, The sintering process described in step (3) includes one-step sintering and two-step sintering.

32. The preparation method according to claim 31, characterized in that, The sintering temperature for the first step is 450–550°C.

33. The preparation method according to claim 31, characterized in that, The sintering time for the first step is 4 to 6 hours.

34. The preparation method according to claim 31, characterized in that, The temperature for the two-step sintering is 600–800°C.

35. The preparation method according to claim 31, characterized in that, The two-step sintering time is 10-16 hours.

36. A modified cathode material, characterized in that, The modified cathode material is prepared by the method according to any one of claims 1-35, the modified cathode material comprising a core and a shell disposed on the surface of the core, wherein the core has the chemical formula LiNi. a Co b Mn c Al d O2, wherein 0.8 < a ≤ 0.9, 0.05 ≤ b < 0.1, 0.05 ≤ c < 0.1, 0.01 ≤ d < 0.03, a + b + c + d = 1, and the chemical formula of the outer shell is LiNi. x Co y Mn z O2, where 0.2 < x ≤ 0.6, 0.1 < y ≤ 0.3, 0.1 < z ≤ 0.3, and x + y + z = 1.

37. A positive electrode plate, characterized in that, The positive electrode sheet comprises the modified positive electrode material as described in claim 36.

38. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode as described in claim 37.

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