Preparation Method of Cathode Material for Secondary Battery with In-situ Interface Residual Alkali Conversion and Stabilization

Abstract

The present invention relates to the field of energy materials and secondary batteries, and discloses a preparation method for stabilizing the cathode material of a secondary battery by in-situ interfacial residual alkali conversion. The chemical formula is AM x TMO2, which is a polymer / lamellar structure. Among them, the lamellar oxide cathode material with x>0.5; AM is one or more of Li and Na; TM is one or more of the transition metals Ni, Co, Mn, Fe, Cu, Zn, Cr, and Ti elements. The process of the present invention includes precursor material preparation, cathode material sintering, coating treatment, and low-temperature calcination. Among them, the coating treatment is divided into: 1. Inorganic fluoride treatment; 2. Organic solvent treatment. The process of the present invention in-situ converts the residual alkali on the surface of the lamellar oxide cathode material to form a heteroepitaxial organic polymer coating, thereby obtaining a lamellar oxide cathode material with a stable interface for a secondary battery. The process of the present invention is simple and controllable, and can effectively improve the air stability of the cathode material after in-situ conversion, promoting the actual production and application of the lamellar oxide cathode material.

Description

Technical Field

[0001] The present invention belongs to the field of energy materials and secondary batteries, and particularly relates to a preparation method for stabilizing a cathode material of a secondary battery by in-situ interfacial residual alkali conversion. Background Art

[0002] In the field of secondary batteries, lithium-ion batteries dominate due to their advantages of high energy density and long life. With the continuous development of technology, sodium-ion batteries, with their cost advantages and safety characteristics, have gradually shown unique competitiveness in the field of secondary batteries, and together with lithium-ion batteries, have jointly promoted the development and application of secondary battery technology.

[0003] Among the cathode materials of lithium-ion batteries and sodium-ion batteries, layered oxides have become attractive cathode candidate materials for rechargeable secondary batteries due to their high theoretical capacity. In scenarios such as transportation and electrode preparation, various alkaline substances, such as hydroxides and carbonates, will be generated and accumulated on the surface of the layered oxide cathode material, which will cause problems such as gelation of the cathode material slurry, corrosion of the current collector, poor mechanical properties, gas generation in the battery, and decline in electrical performance, restricting their practical applications. Among them, constructing surface coatings (such as metal oxides, fluorides, phosphates, and spinel oxides) is a common strategy to solve the physical and chemical problems of layered oxide cathode materials.

[0004] In the existing technology, metal oxides (Al 2 O 3 , TiO 2 and MgO) and fluorides (LiF, AlF 3 ) as inert coating materials can improve the cycling performance by reducing surface side reactions, while their poor ion transport ability will hinder the transport kinetics of Li + , Na + and other ions. In addition, metal phosphates (Li 3 PO 4 , AlPO 4 ) have excellent ionic conductivity, but poor electronic conductivity and structural stability; spinel oxides have a three-dimensional skeleton and good lattice matching with layered oxides, and are considered to be a stable phase surface layer, but will sacrifice a large amount of the original capacity.

[0005] Based on this, the present invention proposes to in-situ transform the surface residual alkali through treatment with inorganic fluoride and organic solvent, and simultaneously form a heteroepitaxial ion-conducting organic polymer coating. The polymer coating on the surface of the positive electrode material reduces the surface residual alkali, and has multifunctionality and effectiveness in controlling the residual alkali and enhancing the interfacial stability of the layered oxide positive electrode of rechargeable secondary batteries. The polymer coating formed by the transformation of the interfacial residual alkali is an ion conductor, which can enhance the electron and ion transport at the interface of the positive electrode material, and simultaneously exhibits a specific capacity comparable to that of the original material and a more stable capacity output during charging and discharging, reducing voltage hysteresis and capacity decay, and having excellent cycle stability and structural stability. In addition, this polymer coating does not contain hydrophilic groups and has hydrophobicity, meeting the requirements of transportation and storage in large-scale production applications. The present invention provides a solution to overcome the problems of surface residual alkali and interfacial instability of layered oxide positive electrode materials in actual production applications, and effectively promotes the practical application of positive electrode materials. Summary of the Invention

[0006] In order to solve the deficiencies in the prior art, the present invention proposes a method for in-situ transforming surface residual alkali through treatment with inorganic fluoride and organic solvent, and simultaneously forming a heteroepitaxial ion-conducting organic polymer coating to solve the problems of surface residual alkali and interfacial instability of layered oxide positive electrode materials in actual applications. This material has the advantages of excellent interfacial stability and cycling performance. Another advantage of the present invention is that the synthesis process is simple and controllable, which can improve air stability and promote the actual production application of layered oxide positive electrode materials.

[0007] A preparation method for stabilizing the positive electrode material of a secondary battery by in-situ interfacial residual alkali transformation, and the technical solution is as follows:

[0008] S1 Preparation of precursor material: Weigh the alkali metal source and transition metal source according to the stoichiometric ratio, place them in a natural agate mortar and grind them thoroughly for 0.5 - 1 h until evenly ground. Subsequently, use a tablet press and the corresponding mold (with a diameter of 10 mm), and press the uniformly mixed precursor material into a cylindrical thin sheet (with a diameter of 10 mm and a height of 3 - 5 mm) under a pressure of 16 - 20 Mpa.

[0009] S2 Sintering of the positive electrode material: Put the cylindrical thin sheet obtained in step S1 into a corundum boat, and then place it in a muffle furnace. Sinter it in air at a heating rate of 2 - 5 °C / min to 800 - 1100 °C, calcine for 10 - 16 h, and naturally cool to about 100 °C. Then, the sintered cylindrical thin sheet can be taken out. Place the sintered cylindrical thin sheet in an agate mortar and grind it thoroughly for 10 - 20 min until evenly ground to obtain a powdered positive electrode material.

[0010] S3 Coating Treatment: Mix the sample obtained in step S2 with an inorganic fluoride of a certain concentration in an alcohol solution, then place it on a stirrer and stir at a speed of 300 - 500 rpm for 2 h. Dry the stirred solution to finally obtain a powdered fluorinated cathode material. Mix the fluorinated cathode material with an organic mixed solvent, place it on a stirrer again at a speed of 300 - 500 rpm, stir for 0.5 - 1 h, and then dry it.

[0011] S4 Low-temperature Calcination: Place the dried material in a corundum boat, then place it in a tube furnace, and sinter it to 200 - 400 °C at a heating rate of 2 - 5 °C / min in an inert gas, calcine for 2 - 5 h, and naturally cool to room temperature to obtain the final polymer@layered cathode material.

[0012] A cathode material with in-situ interfacial residual alkali conversion, having a polymer / layered structure, and a chemical formula of AM x TMO 2 , where the layered oxide cathode material has x > 0.5. The coating is a polymer formed by converting interfacial residual alkali and is coated on the surface of the layered oxide. Among them, AM is one or more of Li and Na; the transition metal TM is one or more of the elements Ni, Co, Mn, Fe, Cu, Zn, Cr, and Ti. The coated layer thickness of the polymer-coated layered oxide cathode material is 2 - 10 nm, preferably 2 - 5 nm.

[0013] Furthermore, the alkali metal source in step S1 is one or more of a sodium source and a lithium source. The sodium source is selected from one or more of sodium sulfate, sodium acetate, sodium formate, sodium carbonate, sodium chloride, disodium hydrogen phosphate, and sodium dihydrogen phosphate. The lithium source is selected from one or more of lithium carbonate, lithium oxalate, lithium acetate, lithium sulfate, lithium hydroxide, and lithium chloride. Among them, the transition metal TM is at least one of the elements Ni, Co, Mn, Fe, Cu, Zn, Cr, and Ti, and the transition metal TM source comes from one or more of the corresponding oxides, nitrates, phosphates, carbonates, oxalates, and acetates of TM. Among them, since some alkali metal sources will volatilize during high-temperature sintering, 3 - 10 wt% of the alkali metal source should be appropriately increased.

[0014] Furthermore, the inorganic fluoride in step S3 is selected from one or more of anhydrous hydrogen fluoride, ammonium fluoride, ammonium bifluoride, potassium bifluoride, sodium bifluoride, aluminum fluoride, sodium fluoride, lithium fluoride, and calcium fluoride.

[0015] Furthermore, the organic solvent in step S3 is selected from one or a mixture of diethyl ether, acetone, propylene oxide, tetrahydrofuran, epichlorohydrin, ethylene oxide, and 1,2-epoxybutane.

[0016] Further, in step S3, the drying method is one or more of blowing drying, water bath drying, oil bath drying, rotary evaporation drying, and vacuum drying.

[0017] Further, in step S3, the alcohol solution is one or more of absolute ethanol, methanol, and propanol.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] (1) The synthesis process of the present invention is simple and controllable. It can be applied to the layered oxide cathode materials for lithium-ion batteries and can also be further extended to the layered oxide cathode materials for sodium-ion batteries, with wide applicability.

[0020] (2) By treating with inorganic fluoride and organic solvents, the present invention method in-situ converts the surface residual alkali and simultaneously forms a heteroepitaxial ion conductor organic polymer coating, indicating its versatility and effectiveness in controlling residual alkali and enhancing the interface stability of the layered oxide cathode for rechargeable secondary batteries. In addition, the polymer coating formed by the conversion of the interface residual alkali enhances the electron and ion transport at the interface and shows more stable capacity output during charging and discharging, reducing voltage hysteresis and capacity decay, and having excellent cycle stability.

[0021] (3) The organic skeleton in this polymer coating formed by the conversion of the interface residual alkali does not contain hydrophilic groups. According to the requirements of transportation and storage in large-scale production applications of the material, the present invention conducts an exposure air experiment on the material. By using characterization techniques and electrochemical performance tests, its excellent air stability is proved, which is beneficial to large-scale actual production applications. Description of the Drawings

[0022] Figure 1 Scanning electron microscope photograph (SEM) of the cathode material prepared in Example 1.

[0023] Figure 2 Transmission electron microscope image (TEM) of the cathode material prepared in Example 1.

[0024] Figure 3 Scanning electron microscope photograph (SEM) of the cathode material prepared in Comparative Example 3.

[0025] Figure 4 Transmission electron microscope image (TEM) of the cathode material prepared in Comparative Example 3.

[0026] Figure 5 O 1s X-ray photoelectron spectroscopy (XPS) of the cathode materials prepared in Example 1 and Comparative Example 3.

[0027] Figure 6The first-cycle voltage drop and Coulombic efficiency diagrams of the cathode materials prepared in Example 1 and Comparative Example 3 in the voltage range of 2.0 - 4.8 V at a current density of 0.1 C.

[0028] Figure 7 The rate performance of the cathode material prepared in Example 1 in the voltage range of 2.0 - 4.8 V.

[0029] Figure 8 The rate performance of the cathode material prepared in Comparative Example 3 in the voltage range of 2.0 - 4.8 V.

[0030] Figure 9 The cycling performance and energy density diagrams of the cathode materials of Example 1 and Comparative Example 3 in the voltage range of 2.0 - 4.8 V at a current density of 0.5 C.

[0031] Figure 10 The long-term cycling performance of the cathode materials prepared in Example 1 and Comparative Example 3 in the voltage range of 2.0 - 4.8 V at a current density of 1 C.

[0032] Figure 11 The transmission electron microscope image (TEM) of the cathode material prepared in Example 2.

[0033] Figure 12 The Fourier transform infrared spectroscopy (FT-IR) diagrams of the cathode materials prepared in Example 2 and Comparative Example 4.

[0034] Figure 13 The pH value comparison of the cathode materials prepared in Example 2 and Comparative Example 4.

[0035] Figure 14 The first-cycle charge-discharge curves of the cathode materials prepared in Example 2 and Comparative Example 4 in the voltage range of 2.0 - 4.0 V at a current density of 0.1 C.

[0036] Figure 15 The comparison of the first-cycle voltage drop and Coulombic efficiency diagrams of the cathode materials prepared in Example 2 and Comparative Example 4 in the voltage range of 2.0 - 4.0 V at a current density of 0.1 C.

[0037] Figure 16 The charge-discharge curves at different rates of the cathode materials prepared in Example 2 and Comparative Example 4 in the voltage range of 2.0 - 4.0 V.

[0038] Figure 17 The rate performance comparison of the cathode materials prepared in Example 2 and Comparative Example 4 in the voltage range of 2.0 - 4.0 V.

[0039] Figure 18Comparison chart of median voltage and energy efficiency performance of the cathode materials prepared in Example 2 and Comparative Example 4 at different current densities in the voltage range of 2.0 - 4.0 V.

[0040] Figure 19 Comparison of the long - cycle performance of the cathode materials prepared in Example 2 and Comparative Example 4 at a current density of 1 C in the voltage range of 2.0 - 4.0 V.

[0041] Figure 20 Contact angle test results of the cathode materials prepared in Example 2 and Comparative Example 4.

[0042] Figure 21 Comparison of the rate performance of the cathode materials prepared in Example 2 and Comparative Example 4 in the voltage range of 2.0 - 4.0 V after exposure to air experiment.

[0043] Figure 22 Comparison of the long - cycle performance of the cathode materials prepared in Example 2 and Comparative Example 4 at a current density of 1 C in the voltage range of 2.0 - 4.0 V after exposure to air experiment. Detailed implementation mode

[0044] The preparation method of the in - situ interface residual alkali conversion - stabilized secondary battery cathode material described in the present invention will be further described below in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Any simple modification, equivalent change and modification made to the following examples based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

[0045] In the following examples, the experimental methods are all conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0046] Example 1, 1. Preparation of polymer@Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 layered cathode material,

[0047] S1. Preparation of precursor material: Lithium carbonate (excess 3 - 10 wt%), nickel oxide, cobalt sesquioxide and manganese sesquioxide were weighed according to the stoichiometric ratio of 1.2:0.13:0.13:0.54, placed in a natural agate mortar and ground thoroughly for 45 min until evenly ground. Subsequently, a tablet press and the corresponding mold (diameter 10 mm) were used to press the evenly mixed precursor material into a cylindrical thin sheet (diameter 10 mm, height 3 - 5 mm) with a pressure of 18 Mpa.

[0048] S2, Sintering of the positive electrode material: Place the cylindrical flakes obtained in step S1 into a corundum boat, then place it in a muffle furnace. Sinter in air at a heating rate of 5 °C / min to 1000 °C, calcine for 15 h, and naturally cool to about 100 °C. Then, the sintered cylindrical flakes can be taken out. Place the sintered cylindrical flakes in an agate mortar and grind them thoroughly for 15 min until they are evenly ground, and the powdered positive electrode material can be obtained.

[0049] S3, Coating treatment: Mix the sample obtained in step S2 with 3 wt% inorganic fluoride in an ethanol solution, then place it on a stirrer and stir at a speed of 300 - 500 rpm for 2 h. Place the stirred solution in an oil bath at 80 °C for oil bath drying, and finally obtain powdered fluorinated Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 . Mix the fluorinated Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 with an organic mixed solvent with a volume ratio of 3:1 = tetrahydrofuran:propylene oxide, and place it on a stirrer again at a speed of 300 - 500 rpm and stir for 0.5 - 1 h. Then place the solution in a water bath at 60 °C to evaporate the solvent and obtain a dry material.

[0050] S4, Low-temperature calcination: Place the dried material in a corundum boat, then place it in a tube furnace. Sinter in an inert gas at a heating rate of 2 - 3 °C / min to 300 °C, calcine for 3 h, and naturally cool to room temperature. Then the final polymer@Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 .

[0051] 2. Battery assembly part

[0052] Mix the sample prepared above, polyvinylidene fluoride (PVDF), and conductive agent (Super P) in a mass ratio of 7:2:1. Using N-methylpyrrolidone (NMP) as the solvent, place them in a shaking tube and shake with an oscillator for 30 min to prepare a uniform slurry. Then, evenly coat the slurry onto an aluminum foil current collector, and then place it in a vacuum drying oven at 80 °C for vacuum drying for 12 h. Wait for the vacuum oven to cool to room temperature and then take out the positive electrode sheet. Use a cutting machine to punch the positive electrode sheet into a positive electrode disc with a diameter of 12 mm, and the loading amount of the positive electrode active material on the disc is 1 - 1.2 mg / cm -2, stored in a glove box under an argon atmosphere for later use. Using the above-mentioned positive electrode sheet, lithium metal as the negative electrode material, Whatman glass fiber membrane as the separator, and 1 mol / L NaClO 4 (the solvent is a mixed solution of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1 added with 5% fluoroethylene carbonate) as the electrolyte, assembled into a coin-type half-cell in a glove box under argon protection. After dropping 100 - 120 μL of the electrolyte, it was sealed and the electrical performance was tested at room temperature. The voltage range for the battery test was 2.0 - 4.8 V.

[0053] Example 2, 1. Preparation of polymer@NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 layered positive electrode material,

[0054] S1, Preparation of precursor materials: Weigh sodium carbonate (excess 3 - 10 wt%), nickel oxide, iron oxide, and manganese trioxide according to the stoichiometric ratio of 1:1 / 3:1 / 3:1 / 3, place them in a natural agate mortar and grind thoroughly for 45 min until evenly ground. Subsequently, use a tablet press and the corresponding mold (diameter 10 mm), and press the evenly mixed precursor materials into a cylindrical thin sheet (diameter 10 mm, height 3 - 5 mm) with a pressure of 18 Mpa.

[0055] S2, Sintering of positive electrode materials: Place the cylindrical thin sheet obtained in step S1 into a corundum boat, then place it in a muffle furnace, sinter in air at a heating rate of 5 °C / min to 1000 °C, calcine for 15 h, and naturally cool to about 100 °C, then the sintered cylindrical thin sheet can be taken out. Place the sintered cylindrical thin sheet in an agate mortar and grind thoroughly for 15 min until evenly ground to obtain a powdery positive electrode material.

[0056] S3, Coating treatment: Mix the sample obtained in step S2 with 3 wt% inorganic fluoride in an ethanol solution, then place it on a stirrer and stir at a speed of 300 - 500 rpm for 2 h. Place the stirred solution in an oil bath at 80 °C for oil bath drying to finally obtain a powdery fluorinated NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 . Mix the fluorinated NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 with an organic mixed solvent with a volume ratio of 3:1 = tetrahydrofuran:propylene oxide, and place it on a stirrer again at a speed of 300 - 500 rpm and stir for 0.5 - 1 h. Then place the solution in a water bath at 60 °C to evaporate the solvent to obtain a dried material.

[0057] S4, Low-temperature calcination: Place the dried material in a corundum boat, then place it in a tube furnace, and sinter it to 300 °C at a heating rate of 2-3 °C / min in an inert gas, calcine for 3 h, and naturally cool to room temperature to obtain the final polymer@NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 。

[0058] 2. Battery assembly part

[0059] The assembly of the coin-type half-cell is the same as that in Example 1. The negative electrode material uses sodium metal, and the voltage range for battery testing is 2.0-4.0 V.

[0060] Example 3

[0061] The experimental steps and the battery assembly part are the same as those in Example 1, except that the proportion of NH 4 F is changed to 4 wt%.

[0062] Example 4, the experimental steps and the battery assembly part are the same as those in Example 2, except that the proportion of NH 4 F is changed to 4 wt%.

[0063] Example 5

[0064] The experimental steps and the battery assembly part are the same as those in Example 1, except that the proportion of NH 4 F is changed to 5 wt%.

[0065] Example 6, the experimental steps and the battery assembly part are the same as those in Example 2, except that the proportion of NH 4 F is changed to 5 wt%.

[0066] Example 7

[0067] The experimental steps and the battery assembly part are the same as those in Example 1, except that 3 wt% NH 4 F is changed to 3 wt% ammonium bifluoride.

[0068] Example 8, the experimental steps and the battery assembly part are the same as those in Example 2, except that 3 wt% NH 4 F is changed to 3 wt% ammonium bifluoride.

[0069] Example 9

[0070] The experimental steps and the battery assembly part are the same as those in Example 1, except that 3 wt% NH 4 F is changed to 3 wt% aluminum fluoride.

[0071] Example 10. The experimental procedure and the battery assembly part are the same as those in Example 2, except that 3 wt% NH 4 F is changed to 3 wt% aluminum fluoride.

[0072] Example 11

[0073] The experimental procedure and the battery assembly part are the same as those in Example 1, except that the volume ratio of tetrahydrofuran to propylene oxide is changed to 4:1.

[0074] Example 12. The experimental procedure and the battery assembly part are the same as those in Example 2, except that the volume ratio of tetrahydrofuran to propylene oxide is changed to 4:1.

[0075] Example 13

[0076] The experimental procedure and the battery assembly part are the same as those in Example 1, except that the volume ratio of tetrahydrofuran to propylene oxide is changed to 5:1.

[0077] Example 14. The experimental procedure and the battery assembly part are the same as those in Example 2, except that the volume ratio of tetrahydrofuran to propylene oxide is changed to 5:1.

[0078] Example 15

[0079] The experimental procedure and the battery assembly part are the same as those in Example 1, except that the organic solvent is changed to ether:propylene oxide = 3:1.

[0080] Example 16. The experimental procedure and the battery assembly part are the same as those in Example 2, except that the organic solvent is changed to ether:propylene oxide = 3:1.

[0081] Example 17

[0082] The experimental procedure and the battery assembly part are the same as those in Example 1, except that the organic solvent is changed to acetone:propylene oxide = 3:1.

[0083] Example 18. The experimental procedure and the battery assembly part are the same as those in Example 2, except that the organic solvent is changed to acetone:propylene oxide = 3:1.

[0084] Example 19

[0085] The experimental procedure and the battery assembly part are the same as those in Example 1, except that the organic solvent is changed to tetrahydrofuran:ethylene oxide = 3:1.

[0086] Example 20. The experimental procedure and the battery assembly part are the same as those in Example 2, except that the organic solvent is changed to tetrahydrofuran:ethylene oxide = 3:1.

[0087] Example 21

[0088] The experimental procedures and the battery assembly part are the same as those in Example 1, except that the organic solvent is changed to tetrahydrofuran: epichlorohydrin = 3:1.

[0089] Example 22, the experimental procedures and the battery assembly part are the same as those in Example 2, except that the organic solvent is changed to tetrahydrofuran: epichlorohydrin = 3:1.

[0090] Example 23

[0091] The experimental procedures and the battery assembly part are the same as those in Example 1, except that the organic solvent is changed to tetrahydrofuran: 1,2-epoxybutane = 3:1.

[0092] Example 24, the experimental procedures and the battery assembly part are the same as those in Example 2, except that the organic solvent is changed to tetrahydrofuran: 1,2-epoxybutane = 3:1.

[0093] Comparative Example 1

[0094] The experimental procedures and the battery assembly part are the same as those in Example 1, except that in step (S3): the sample obtained in step (S2) is mixed with 3 wt% NH 4 F in an ethanol solution, and then placed on a stirrer and stirred at a speed of 300 - 500 rpm for 2 h. The stirred solution is placed in an oil bath at 80 °C for oil bath drying, and finally a powdered fluorinated Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 .

[0095] Comparative Example 2, the experimental procedures and the battery assembly part are the same as those in Example 2, except that in step (S3): the sample obtained in step (S2) is mixed with 3 wt% NH 4 F in an ethanol solution, and then placed on a stirrer and stirred at a speed of 300 - 500 rpm for 2 h. The stirred solution is placed in an oil bath at 80 °C for oil bath drying, and finally a powdered fluorinated NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 .

[0096] Comparative Example 3

[0097] 1. Prepare Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 layered cathode material,

[0098] Preparation of S1 precursor material: Lithium carbonate (3 - 10 wt% in excess), nickel oxide, cobalt(III) oxide, and manganese(III) oxide were weighed according to the stoichiometric ratio of 1.2:0.13:0.13:0.54, placed in a natural agate mortar, and ground thoroughly for 45 min until evenly ground. Subsequently, a tablet press and corresponding mold (with a diameter of 10 mm) were used to press the evenly mixed precursor material into a cylindrical thin sheet (with a diameter of 10 mm and a height of 3 - 5 mm) under a pressure of 18 Mpa.

[0099] S2 Sintering of the positive electrode material: The cylindrical thin sheet obtained in step S1 was placed in a corundum boat and then placed in a muffle furnace. It was sintered to 1000 °C in air at a heating rate of 5 °C / min, calcined for 15 h, and naturally cooled to about 100 °C. Then, the sintered cylindrical thin sheet could be taken out, placed in a natural agate mortar, and ground thoroughly for 15 min until evenly ground to obtain the powdery positive electrode material.

[0100] 2. Battery assembly

[0101] The battery assembly part is the same as that in Example 1

[0102] Comparative Example 4, 1. Preparation of NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 Layered positive electrode material

[0103] (S1) Preparation of precursor material: Sodium carbonate (3 - 10 wt% in excess), nickel oxide, iron oxide, and manganese(III) oxide were weighed according to the stoichiometric ratio of 1:1 / 3:1 / 3:1 / 3, placed in a natural agate mortar, and ground thoroughly for 45 min until evenly ground. Subsequently, a tablet press and corresponding mold (with a diameter of 10 mm) were used to press the evenly mixed precursor material into a cylindrical thin sheet (with a diameter of 10 mm and a height of 3 - 5 mm) under a pressure of 18 Mpa.

[0104] (S2) Sintering of the positive electrode material: The cylindrical thin sheet obtained in step S1 was placed in a corundum boat and then placed in a muffle furnace. It was sintered to 1000 °C in air at a heating rate of 5 °C / min, calcined for 15 h, and naturally cooled to about 100 °C. Then, the sintered cylindrical thin sheet could be taken out, placed in a natural agate mortar, and ground thoroughly for 15 min until evenly ground to obtain the powdery positive electrode material.

[0105] 2. Battery assembly

[0106] The battery assembly part is the same as that in Example 2

[0107] The results of the electrochemical performance test are shown in Table 1.

[0108] Table 1 Electrical performance results of the prepared positive electrode materials

[0109]

[0110] The process method provided by the present invention utilizes the reaction of inorganic fluoride with surface residual alkaline compounds, and further polymerizes and reacts with organic solvents to realize the in-situ conversion of surface residual alkali into a polymer coating, thereby preparing a high-performance layered oxide cathode material. Table 1 shows the comparison of the initial charge specific capacity, initial discharge specific capacity of the layered oxide cathode material prepared by the present invention at 0.1C, and the capacity retention rate after 100 cycles at 1C rate.

[0111] Based on these results, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) tests were carried out on Example 1 and Comparative Example 3 to obtain detailed information about their morphological and structural characteristics. Example 1 ( Figure 1 ) and Comparative Example 3 ( Figure 3 ) SEM images show that the samples are composed of spherical secondary particles, about 10 µm in size, aggregated by sub-nanometer primary particles, and the interface treatment did not destroy this spherical morphology. Figure 4 The TEM of 2 CO 3 shows that there are irregular impurities on the surface of Comparative Example 3, mainly composed of amorphous LiOH and Li Figure 2 In contrast, Example 1 of Figure 5 a) shows a smooth and flat surface with a uniform and thin coating, which may be due to the effective removal of surface residual alkaline compounds during the interface treatment and subsequent surface reconstruction process. At the same time, a uniform amorphous heteroepitaxial polymer coating with a thickness of about 5 nm can be observed. The surface chemical properties of Example 1 and Comparative Example 3 were analyzed using X-ray photoelectron spectroscopy (XPS). The O 1s XPS spectra of Example 1 ( Figure 5 b) and Comparative Example 3 show peaks at 529.4 eV, 531.6 eV and 533.9 eV, corresponding to lattice oxygen, OH − and oxygen in carbonate respectively. In Example 1, the peak intensities of OH − and carbonate oxygen are significantly reduced, indicating a significant reduction in residual alkaline substances. The organic polymer coating has no hydrophilic groups and has hydrophobic properties, which can inhibit the adsorption of water and hydroxyl groups, and reduce surface OH − and CO 3 2− . Figures 6 - 10 The electrochemical tests of Figure 11 show that the performance of Example 1 is significantly better than that of Comparative Example 3. These results indicate that Example 1 after interface treatment has significant advantages in cycle stability, specific capacity and rate performance. Figure 11 The TEM image of Example 2 ofFigure 12 The Fourier transform infrared spectroscopy (FT-IR) in [Example 2] shows that there is a characteristic peak at 2362 cm⁻¹, which is attributed to the stretching vibration of the C-H bond, and this peak is not detected in Comparative Example 4. In addition, the sample of Example 2 shows a peak of HCO3⁻ at 1155 cm⁻¹, while the signals corresponding to CO at 1429 cm⁻¹ and 883 cm⁻¹ 3 2⁻ weaken, indicating that the surface residual alkali decreases and a polymer coating is formed. As Figure 13 shown, the measured pH value of Example 2 is 13.542, which is lower than the pH value of 14.014 of Comparative Example 4. From Figure 14 Example 2 and Figure 22 Comparative Example 4 of the SEM images, both samples are composed of irregular micron-scale plate-like particles, without obvious differences. Similarly, after the electrochemical performance test and the exposure to air test ( Figures 14 - 22 ), it shows that the performance of Example 2 is significantly better than that of Comparative Example 4. Based on these results, our strategy solves the problems of poor air stability, poor ion transport, and interfacial side reactions by implementing interfacial treatment to remove surface residual alkali and simultaneously construct a polymer coating, while showing strong versatility and universality, and is applicable to the actual production application of secondary battery systems.

Claims

1. A method for preparing a secondary battery positive electrode material by in-situ interface residual alkali conversion stabilization, characterized in that: The following steps are involved: S1, preparation of precursor materials: sodium carbonate or lithium carbonate and the corresponding oxide are weighed according to the stoichiometric ratio, and are fully ground in an agate mortar for 0.5-1h until the ground is uniform; then a tablet press and a corresponding mold with a diameter of 10mm are used to press the mixed precursor materials into cylindrical sheets with a diameter of 10mm and a height of 3-5mm using a pressure of 16-20Mpa; S2, sintering of positive electrode material: placing the cylindrical slice obtained in step S1 into a corundum porcelain boat, and then placing it in a muffle furnace, sintering it to 800-1100°C at a heating rate of 2-5°C / min in air, calcining for 10-16h, and naturally cooling it to about 100°C, then taking out the sintered cylindrical slice, placing the sintered cylindrical slice in an agate mortar, and grinding it thoroughly for 10-20min until it is evenly ground, thereby obtaining a powdered positive electrode material; S3, coating treatment: the sample obtained in step S2 is mixed with ammonium fluoride or ammonium bifluoride in an alcohol solution, and then placed on a stirrer, stirred at 300-500 rpm for 2 hours, and the stirred solution is dried to finally obtain a powdered fluorinated sample material, and the fluorinated sample material is mixed with one or more of propylene oxide, tetrahydrofuran, and ethylene oxide, and placed on a stirrer at 300-500 rpm again, stirred for 0.5-1 hour, and then dried; S4, low-temperature calcination: the dried material is placed in a corundum porcelain boat, and then placed in a tube furnace, sintered to 200-400°C at a heating rate of 2-5°C / min in an inert gas, calcined for 2-5h, and naturally cooled to room temperature to obtain the final polymer @ layered cathode material; The layered oxide cathode material converted from in-situ interfacial residual alkali is a polymer / layered structure, and the chemical expression is AMxTMO2 , wherein the layered oxide positive electrode material has 1≤X≤1.2; AM is one or more of Li and Na; TM is at least one of transition metal elements Ni, Co, Mn, Fe, Cu, Zn, and Ti; the coating is a polymer formed by the conversion of interfacial residual alkali and coated on the surface of the layered oxide, and the coating thickness is 2-10nm.

2. The method for preparing a secondary battery positive electrode material by in-situ interfacial residual alkali conversion stabilization according to claim 1, characterized in that: The drying method in step S3 is one or more of air drying, water bath drying, oil bath drying, rotary evaporation drying, and vacuum drying.

3. The method for preparing a secondary battery positive electrode material by in-situ interfacial residual alkali conversion stabilization according to claim 1, characterized in that: The alcohol solution in step S3 is one or more of anhydrous ethanol, methanol, and propanol.

4. A secondary battery, characterized in that: The raw material for preparing the positive electrode includes the positive electrode material obtained by any preparation method in claims 1-3.

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