A kind of tricobalt tetroxide and its preparation method and application

Abstract

The present application belongs to the technical field of lithium ion battery cathode material precursor, and particularly relates to a kind of tricobalt tetroxide and its preparation method and application, the preparation method includes the following steps: (1) solution containing carbonate ion and ammonium ion is used as bottom liquid, and is added into cobalt metal liquid, magnesium and / or aluminum containing cation doping liquid, precipitant solution and complexing agent solution to carry out nucleation reaction, the pH of reaction is adjusted to 7.8-8.2, and seed slurry is obtained after reaction;(2) the slurry obtained in step (1) is used as bottom liquid, and the cobalt metal liquid, phosphate ester doping liquid and the precipitant solution are added into to carry out shell forming reaction, and the pH of reaction is adjusted to 6.8-7.1, to obtain cobalt carbonate slurry;(3) after solid-liquid separation of the cobalt carbonate slurry, the solid phase is taken and calcined to obtain.

Description

Technical Field

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

[0002] Lithium cobalt oxide, a cathode material for lithium-ion batteries, has long been the dominant cathode material in the consumer electronics field due to its high volumetric energy density and good processing performance. With the increasing demands for battery life and fast charging performance in 3C electronic products, the operating voltage of lithium cobalt oxide has gradually increased from 4.2V to 4.5V and even above 4.6V. However, under high voltage, lithium cobalt oxide faces severe challenges such as structural degradation, intensified interfacial side reactions, and a sharp decline in cycle life.

[0003] The electrochemical performance of lithium cobalt oxide is highly dependent on the microstructure of its precursor, cobalt tetroxide, and the morphology and density of cobalt tetroxide are directly inherited from the cobalt carbonate precursor. Therefore, structural design and element doping at the cobalt carbonate stage is one of the important ways to improve the overall performance of lithium cobalt oxide. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in related technologies. To this end, this invention proposes a cobalt tetroxide, its preparation method, and its application. The cobalt tetroxide prepared by this method, when applied to lithium-ion batteries, can significantly improve the electrochemical performance of lithium-ion batteries under high voltage.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for preparing cobalt tetroxide includes the following steps: (1) A solution containing carbonate ions and ammonium ions is used as the base liquid, and cobalt metal liquid, magnesium and / or aluminum cation doping liquid, precipitant solution and complexing agent solution are added to carry out the nucleation reaction. The pH of the reaction is adjusted to 7.8-8.2, and seed slurry is obtained after the reaction. (2) Using the seed slurry obtained in step (1) as the base liquid, the cobalt metal liquid, the phosphate dopant liquid and the precipitant solution are added to carry out the shell-forming reaction, and the pH of the reaction is adjusted to 6.8-7.1 to obtain cobalt carbonate slurry; (3) The cobalt carbonate slurry is obtained by solid-liquid separation and calcination of the solid phase.

[0006] In one embodiment, in step (1), the concentration of carbonate ions in the base solution is 0.5-2 mol / L, the concentration of ammonium ions is 0.05-0.10 mol / L, and the pH of the base solution is 8.0-8.5.

[0007] In one embodiment, the phosphate doping solution contains at least one of trimethyl phosphate, triethyl phosphate and tributyl phosphate; preferably, the phosphorus concentration in the phosphate doping solution is 0.1-1.0 mol / L.

[0008] In one embodiment, the cobalt metal liquid contains at least one of cobalt sulfate, cobalt chloride and cobalt nitrate; preferably, the cobalt ion concentration in the cobalt metal liquid is 1.2-1.8 mol / L; preferably, in step (1), the flow rate of the cobalt metal liquid is 20-30 L / h; preferably, in step (2), the flow rate of the cobalt metal liquid is 10-20 L / h.

[0009] In one embodiment, in step (1), the magnesium and / or aluminum-containing cationic doping solution includes at least one of a soluble aluminum salt and a soluble magnesium salt; preferably, the soluble aluminum salt includes at least one of aluminum chloride, aluminum sulfate and aluminum nitrate; preferably, the soluble magnesium salt includes at least one of magnesium sulfate, magnesium chloride and magnesium nitrate.

[0010] In one embodiment, in step (1), the magnesium and / or aluminum-containing cationic doping solution is a magnesium-containing cationic doping solution, and during the nucleation reaction, the molar ratio of Mg in the magnesium-containing cationic doping solution flowing into the cobalt metal solution flowing into the solution is (0.005-0.03):1.

[0011] In one embodiment, in step (1), the magnesium and / or aluminum-containing cationic doping solution is an aluminum-containing cationic doping solution, and during the nucleation reaction, the molar ratio of Al in the aluminum-containing cationic doping solution flowing into the cobalt metal solution flowing into the solution is (0.003-0.02):1.

[0012] In one embodiment, in step (1), the cation doping solution containing magnesium and / or aluminum is a cation doping solution containing magnesium and aluminum. During the nucleation reaction, the molar ratio of Mg in the cation doping solution flowing into the cobalt metal solution to Co in the cobalt metal solution flowing into the solution is (0.005-0.03):1, the molar ratio of Al in the cation doping solution flowing into the solution to Co in the cobalt metal solution flowing into the solution is (0.003-0.02):1, and the molar ratio of the sum of Mg and Al in the cation doping solution flowing into the solution to Co in the cobalt metal solution flowing into the solution is (0.005-0.04):1.

[0013] In one embodiment, in step (1), the flow rate of the magnesium and / or aluminum-containing cationic dopant is 2-15 L / h.

[0014] In one embodiment, in steps (1) and (2), the precipitant solution is a solution containing carbonate ions, and the concentration of carbonate ions in the precipitant solution is 1-3 mol / L.

[0015] In one embodiment, in steps (1) and (2), the precipitant solution includes a soluble carbonate, wherein the soluble carbonate is at least one of sodium carbonate, potassium carbonate, ammonium bicarbonate and sodium bicarbonate.

[0016] In one embodiment, in step (1), the pH value of the nucleation reaction is maintained at 7.8-8.2 by adjusting the flow rate of the precipitant solution.

[0017] In one embodiment, in step (1), the complexing agent solution is ammonia or ammonium citrate solution, the concentration of ammonium ions in the complexing agent solution is 0.15-6.0 mol / L, and the flow rate of the complexing agent solution is 1-10 L / h.

[0018] In one embodiment, in step (1), the nucleation reaction is carried out in a reactor. The volume of the bottom liquid accounts for 40%-50% of the reactor volume. After the liquid level in the reactor reaches 80%-85% of the total volume, the overflow concentration is started. During the concentration, cobalt metal liquid, magnesium and / or aluminum cation doping liquid, precipitant solution and complexing agent solution are continuously introduced to keep the liquid level in the reactor stable at 80%-85% of the total volume.

[0019] In one embodiment, in step (1), the temperature of the base liquid is 60-65°C.

[0020] In one embodiment, in step (1), the nucleation reaction takes 20-60 hours.

[0021] In one embodiment, in step (1), the nucleation reaction is accompanied by stirring at a speed of 200-400 rpm.

[0022] In one embodiment, in step (2), the slurry obtained in step (1) is divided into 2-5 portions and placed into different growth reactors. Then, cobalt metal liquid, phosphate ester dopant liquid and precipitant solution are added to the growth reactor in parallel to carry out the reaction.

[0023] In one embodiment, in step (2), the flow rates of the phosphate ester dopant and the cobalt metal liquid are controlled during the shell-forming reaction so that the molar feed ratio of phosphorus to cobalt is (0.001-0.02):1.

[0024] In one embodiment, in step (2), the flow rate of the phosphate dopant solution is 2-5 L / h.

[0025] In one embodiment, in step (2), the pH of the reaction is maintained at 6.8-7.1 by adjusting the flow rate of the precipitant solution.

[0026] In one embodiment, in step (2), the temperature of the shell-forming reaction is 35-40°C and the reaction time is 40-80h.

[0027] In one embodiment, in step (2), the shell-forming reaction is accompanied by stirring at a speed of 50-100 rpm.

[0028] In one embodiment, in step (3), the calcination refers to calcining at 700-750°C for 2-4 hours in an oxygen atmosphere.

[0029] In one embodiment, in step (3), the solid phase is washed, dried, and then calcined.

[0030] In one embodiment, in step (3), the washing is performed by washing the solid phase with water at a temperature of 50-70°C, and the amount of washing water used is 5-10 times the mass of the solid phase.

[0031] In one embodiment, in step (3), the drying temperature is 100-130°C, and the product is dried to constant weight before calcination.

[0032] A cobalt tetroxide is prepared by the preparation method described above.

[0033] A cathode material, wherein the raw materials for preparing the cathode material include cobalt tetroxide as described above.

[0034] A lithium-ion battery comprising the positive electrode material as described above.

[0035] The beneficial effects of this invention are: (1) The preparation method of this invention creatively proposes the design concept of "core-shell spatial functional partitioning". Through a staged synthesis process, cobalt tetroxide with a dense core and a radial shell structure is obtained, realizing the spatial decoupling of different elements inside the particles. In the nucleation reaction, aluminum / magnesium salts are added at the same time as complexing agents, and the reaction pH, reaction temperature and stirring speed are controlled at a high level. The doping of magnesium ions can promote grain fusion and maturation, which is beneficial to improving the density of the particles. The doping of aluminum ions can inhibit dendrite growth and make the primary particles more regular. When magnesium and aluminum co-doping is used, the interaction mechanisms of magnesium ions and aluminum ions are complementary. Magnesium ions promote densification and grain fusion and maturation. Aluminum ions are beneficial to lattice stability, inhibit dendrite growth and enhance lattice bond energy. The synergistic effect of the two further improves the density and structural stability of the core. In addition, the complexing agent can control the release rate of doped metal ions and prolong the crystal growth cycle. The ripening time of the particles provides kinetic auxiliary conditions for densification. The outer shell region is doped with phosphorus and the reaction temperature, stirring speed and pH value are controlled at a low level. For the first time, phosphate ester compounds are applied to the phosphorus doping of cobalt carbonate. Unlike inorganic phosphates, which immediately dissociate into a large number of free phosphate ions in water, leading to instantaneous supersaturation and surface enrichment, phosphate esters hydrolyze slowly under these reaction conditions, and phosphate ions are released gradually. This ensures that there is always a low concentration of phosphate ions at the crystal growth interface. The concentration is neither too high, which would lead to surface enrichment, nor too low, which would interrupt the induction. Thus, the uniform doping of phosphorus and the construction of a complete radial structure are achieved simultaneously. The strong selective adsorption of phosphate ions on specific crystal faces of cobalt carbonate is used to induce the radial preferential growth of the particles and construct a through-through radial open channel. Unlike the lattice substitution effect of core metal ion doping, this is a new approach to crystal growth regulation dominated by non-metallic anions. (2) The preparation method of the present invention prepares a cobalt carbonate structure with a "dense core-radial shell". During the sintering process of cobalt tetroxide, it plays the role of a directional exhaust channel. The open channel formed by radially arranged primary particles in the radial shell provides a smooth escape path for CO2 gas generated by the decomposition of cobalt carbonate. This fundamentally solves the problem of particle cracking caused by poor gas emission during the calcination and decomposition of high-density precursors, so that cobalt tetroxide particles have both high tap density and structural integrity. (3) The preparation method of the present invention constructs a synergistic enhancement system of bulk stability and interface protection through spatial partitioning doping, which solves the technical difficulties of "bulk phase needs to be strong and tough and surface needs to be protected" for high-voltage lithium cobalt oxide. In the core region, magnesium ions and cobalt ions have similar radii and are uniformly integrated into the lattice to form a solid solution, which enhances the lattice bond energy. Aluminum ions have a high valence state and strong Al-O bond, which inhibits the instability of the oxygen skeleton under high voltage. The two form a "double pillar" stabilizing effect in the bulk phase, which inhibits harmful phase transitions from the inside of the particles. In the outer shell region, phosphorus elements form a lithium-cobalt-phosphorus-oxygen composite surface layer in lithium cobalt oxide, which effectively isolates the direct contact between the electrolyte and the active material and reduces interfacial side reactions. Attached Figure Description

[0036] Figure 1 This is a cross-sectional view of cobalt tetroxide prepared in Example 1 of the present invention; Figure 2 This is a cross-sectional view of cobalt tetroxide prepared in Comparative Example 1 of the present invention; Figure 3 This is a cross-sectional view of cobalt tetroxide prepared in Comparative Example 2 of the present invention; Figure 4 This is a cross-sectional view of cobalt tetroxide prepared in Comparative Example 3 of the present invention. Detailed Implementation

[0037] The present invention will be further described below with reference to specific embodiments.

[0038] Example 1: A method for preparing cobalt tetroxide includes the following steps: (1) Preparation of solution: Cobalt sulfate with a cobalt ion concentration of 1.2 mol / L was prepared as a cobalt metal solution; magnesium sulfate with a magnesium ion concentration of 0.06 mol / L was prepared as a magnesium ion doping solution; trimethyl phosphate doping aqueous solution with a phosphorus element concentration of 0.1 mol / L was prepared as a phosphate doping solution; sodium bicarbonate solution with a carbonate ion concentration of 1 mol / L was prepared as a precipitant solution; and ammonium citrate solution with an ammonium ion concentration of 0.15 mol / L was prepared as a complexing agent solution. (2) Preparation of cobalt carbonate core Pure water, a precipitant solution, and a complexing agent solution were added to a 600L reactor as a base solution, which comprised 40% of the reactor volume. The base solution contained 0.5 mol / L carbonate ions and 0.05 mol / L ammonium ions, with a pH of 8.0 and a temperature of 60℃. The reactor was equipped with a three-layer, four-bladed oblique impeller. Cobalt metal solution, magnesium ion doping solution, precipitant solution, and complexing agent solution were added in parallel under stirring at 400 rpm to initiate a nucleation reaction. The flow rate of the cobalt metal solution was 20 L / h. The flow rate of the ion doping solution was 10 L / h, so that the molar ratio of magnesium ions to cobalt ions flowing in during the nucleation reaction was 0.025:1. The flow rate of the complexing agent solution was 1 L / h, and the flow rate of the precipitant was controlled to maintain the pH of the liquid phase at 7.8. When the liquid level in the reactor reached 80% of the total volume of the reactor, the overflow concentration was started. During the concentration period, the cobalt metal liquid, magnesium ion doping solution, precipitant solution, and complexing agent solution were continuously introduced and the liquid level in the reactor was kept stable at 80% of the total volume. After 25 h of reaction, the seed slurry was obtained. The seed slurry was a cobalt carbonate slurry with a dense core. (3) Preparation of radial shell doped with cobalt carbonate: The seed slurry obtained in step (2) was divided into two equal parts and transferred to two growth reactors respectively. The reactor blades were propeller-type paddles. The stirring speed of the reactor was 50 rpm and the temperature was 35°C. Cobalt metal liquid, phosphate ester dopant liquid and precipitant solution were added in parallel to carry out the shell-forming reaction. The flow rate of cobalt metal liquid was 10 L / h and the flow rate of phosphate ester dopant liquid was 2 L / h. The molar ratio of phosphorus element to cobalt element flowing in during the shell-forming reaction was 0.0167:1. The pH of the reaction was maintained at 6.8 by adjusting the flow rate of precipitant solution. The reaction was continued for 60 h to obtain a radially structured shell. (4) Washing, drying, and calcining of core-shell doped cobalt carbonate: The cobalt carbonate slurry obtained in step (3) was centrifuged and filtered to obtain a filter cake. The filter cake was washed with pure water at a temperature of 50°C, and the amount of water was 5 times the mass of the filter cake. The washed filter cake was placed in an oven and dried at 100°C to constant weight to obtain a core-shell cobalt carbonate precursor. The dried cobalt carbonate precursor was placed in a box furnace and calcined at 700°C for 2 hours in an air atmosphere to obtain a core-shell cobalt tetroxide product. The cross-sectional view of the obtained cobalt tetroxide product is shown below. Figure 1 As shown, the particles exhibit a clear core-shell structure, with a dense core and no obvious pores, while the outer shell particles are arranged radially, and the radial structure runs through the entire outer shell layer.

[0039] Example 2: A method for preparing cobalt tetroxide includes the following steps: (1) Preparation of solution: A cobalt nitrate solution with a cobalt ion concentration of 1.8 mol / L was prepared as the cobalt metal solution; a mixed solution of magnesium nitrate and aluminum nitrate with a magnesium ion concentration of 0.15 mol / L and an aluminum ion concentration of 0.09 mol / L was prepared as the cation doping solution; a triethyl phosphate aqueous solution with a phosphorus element concentration of 0.1 mol / L was prepared as the phosphate doping solution; a sodium carbonate solution with a carbonate ion concentration of 2 mol / L was prepared as the precipitant solution; and an ammonia aqueous solution with an ammonium ion concentration of 0.15 mol / L was prepared as the complexing agent solution. (2) Preparation of cobalt carbonate core Pure water, a precipitant solution, and a complexing agent solution were added to a 600L reactor as a base solution, which comprised 45% of the reactor volume. The base solution contained 1.0 mol / L carbonate ions, 0.08 mol / L ammonium ions, and had a pH of 8.3. The temperature was 62℃. The reactor was equipped with a three-layer, four-bladed oblique impeller. Under stirring at 300 rpm, cobalt metal liquid, a magnesium and aluminum cation doping solution, the precipitant solution, and the complexing agent solution were added in parallel flow to initiate a nucleation reaction. The flow rate of the cobalt metal liquid was 25 L / h, and the flow rate of the cation doping solution was 5 L / h, ensuring that magnesium ions flowed in during the nucleation reaction. The molar ratio of the inflowing cobalt ions to the inflowing aluminum ions is 0.0167:1, the molar ratio of the inflowing aluminum ions to the inflowing cobalt ions is 0.01:1, the molar ratio of the sum of the inflowing magnesium ions and aluminum ions to the inflowing cobalt ions is 0.0267:1, the flow rate of the complexing agent solution is 5 L / h, and the flow rate of the precipitant is controlled to maintain the pH of the liquid phase at 8.0. When the liquid level in the reactor reaches 85% of the total volume, the overflow concentration is started. During the concentration period, the cobalt metal liquid, the cation doping liquid, the precipitant solution, and the complexing agent solution are continuously introduced and the liquid level in the reactor is kept stable at 85% of the total volume. After reacting for 40 hours, the seed slurry is obtained. The seed slurry is a cobalt carbonate slurry with a dense core. (3) Preparation of radial shell doped with cobalt carbonate: The seed slurry obtained in step (2) was divided into three equal parts and transferred to the growth vessel. The impeller of the reaction vessel was a propeller-type impeller. The stirring speed of the reaction vessel was 80 rpm and the temperature was 38°C. Cobalt metal liquid, phosphate ester dopant liquid and precipitant solution were added in parallel to carry out the shell-forming reaction. The flow rate of cobalt metal liquid was 15 L / h and the flow rate of phosphate ester dopant liquid was 3.5 L / h. The molar ratio of phosphorus element to cobalt element flowing in during the shell-forming reaction was 0.0130:1. The pH of the reaction was maintained at 7.0 by adjusting the flow rate of precipitant solution. The reaction was continued for 60 h to obtain a radial shell structure. (4) Washing, drying, and calcining of core-shell doped cobalt carbonate: The cobalt carbonate slurry obtained in step (3) was centrifuged and filtered to obtain a filter cake. The filter cake was washed with pure water at a temperature of 60°C and the amount of water was 8 times the mass of the filter cake. The washed filter cake was placed in an oven and dried at 120°C to constant weight to obtain a core-shell cobalt carbonate precursor. The dried cobalt carbonate precursor was placed in a box furnace and calcined at 730°C for 3 hours in an air atmosphere to obtain a core-shell cobalt tetroxide finished product.

[0040] Example 3 A method for preparing cobalt tetroxide includes the following steps: (1) Preparation of solution: A cobalt chloride solution with a cobalt ion concentration of 1.5 mol / L was prepared as the cobalt metal solution; a mixed solution of magnesium chloride and aluminum chloride with a magnesium ion concentration of 0.15 mol / L and an aluminum ion concentration of 0.09 mol / L was prepared as the cation doping solution; a tributyl phosphate aqueous solution with a phosphorus element concentration of 0.2 mol / L was prepared as the phosphate doping solution; a potassium carbonate solution with a carbonate ion concentration of 1.5 mol / L was prepared as the precipitant solution; and an ammonium citrate solution with an ammonium ion concentration of 0.30 mol / L was prepared as the complexing agent solution. (2) Preparation of cobalt carbonate core Pure water, a precipitant solution, and a complexing agent solution were added to a 600L reactor as a base solution, which comprised 50% of the reactor volume. The base solution contained 1.5 mol / L carbonate ions, 0.10 mol / L ammonium ions, and had a pH of 8.5. The temperature was 65℃. The reactor was equipped with a three-layer, four-bladed oblique impeller. Under stirring at 200 rpm, cobalt metal liquid, a magnesium and aluminum cation dopant solution, the precipitant solution, and the complexing agent solution were added in parallel flow to initiate a nucleation reaction. The flow rate of the cobalt metal liquid was 30 L / h, and the flow rate of the cation dopant solution was 2 L / h, allowing the magnesium ions flowing in during the nucleation reaction to react with the magnesium ions in the base solution. The molar ratio of inflowing cobalt ions is 0.0067:1, the molar ratio of inflowing aluminum ions to inflowing cobalt ions is 0.004:1, the molar ratio of the sum of inflowing magnesium ions and aluminum ions to inflowing cobalt ions is 0.0107:1, the flow rate of the complexing agent solution is 10 L / h, and the flow rate of the precipitant is controlled to maintain the pH of the liquid phase at 8.2. When the liquid level in the reactor reaches 85% of the total volume, overflow concentration is started. During the concentration period, cobalt metal liquid, cation doping liquid, precipitant solution, and complexing agent solution are continuously introduced and the liquid level in the reactor is kept stable at 85% of the total volume. After 60 h of reaction, seed slurry is obtained. The seed slurry is a cobalt carbonate slurry with a dense core. (3) Preparation of radial shell doped with cobalt carbonate: The seed slurry obtained in step (2) was divided into 4 equal parts and transferred to the growth vessel. The impeller of the reaction vessel was a propeller type. The stirring speed of the reaction vessel was 100 rpm and the temperature was 40℃. Cobalt metal liquid, phosphate ester doping liquid and precipitant solution were added in parallel to carry out the shell-forming reaction. The flow rate of cobalt metal liquid was 20 L / h and the flow rate of phosphate ester doping liquid was 2 L / h. The molar ratio of phosphorus element to cobalt element flowing in during the shell-forming reaction was 0.0133:1. The pH of the reaction was maintained at 7.1 by adjusting the flow rate of precipitant solution. The reaction was continued for 80 h to obtain a shell with a radial structure. (4) Washing, drying, and calcining of core-shell doped cobalt carbonate: The cobalt carbonate slurry obtained in step (3) was centrifuged and filtered to obtain a filter cake. The filter cake was washed with pure water at a temperature of 70°C and the amount of water was 10 times the mass of the filter cake. The washed filter cake was placed in an oven and dried at 130°C to constant weight to obtain a core-shell cobalt carbonate precursor. The dried cobalt carbonate precursor was placed in a box furnace and calcined at 750°C for 4 hours in an air atmosphere to obtain a core-shell cobalt tetroxide finished product.

[0041] Comparative Example 1: A method for preparing cobalt tetroxide differs from Example 1 only in that: in step (1), the cation doping solution B is not prepared; in step (2), during the core preparation process, only cobalt metal liquid, precipitant solution, and complexing agent solution are added in parallel flow, without adding the cation doping solution, and the other conditions are the same as in Example 1. A cross-sectional view of the obtained cobalt tetroxide product is shown below. Figure 2 As shown, the core region has loose particle packing and obvious pores, and although the outer shell shows a radial morphology, the core structure is not very intact.

[0042] Comparative Example 2: The only difference from Example 1 is that: in step (1), the phosphate dopant solution C is not prepared; in step (3), during the shell preparation process, only the cobalt metal liquid and the precipitant solution are added in parallel, without adding the phosphate dopant solution, and the other conditions are the same as in Example 1. The cross-sectional view of the obtained cobalt tetroxide product is shown below. Figure 3 As shown, the core is dense with no obvious pores, but the outer shell has no radial structure features.

[0043] Comparative Example 3: The only difference from Example 1 is that in step (1), an aqueous solution of ammonium dihydrogen phosphate with a phosphorus concentration of 0.1 mol / L is used instead of an aqueous solution of trimethyl phosphate as the phosphorus source; in step (3), this inorganic phosphorus doping solution is added during the preparation of the outer shell, and the other conditions are the same as in Example 1. The cross-sectional view of the obtained cobalt tetroxide product is shown below. Figure 4 As shown, the outer shell only shows a partial radial tendency in a localized area near the core, and the radial structure is incomplete.

[0044] Table 1. Detection data of the examples and comparative examples

[0045] By comparing the SEM morphology and physical properties of the examples and comparative examples, it can be concluded that: Examples 1-3 have higher tap density and a more uniform radial structure extending to the outer layer of the particles; Comparative Example 1 has a lower tap density and a loose core; Comparative Example 2 has a higher tap density and a dense core but no radial structure in the outer shell; Comparative Example 3 has a higher tap density, but the radial structure is incomplete. Al / Mg co-doping of the core increases the tap density, while the slow-release doping of trimethyl phosphate in the outer shell allows the radial structure to completely penetrate the entire outer shell layer (Comparative Example 2 has no radial structure, and Comparative Example 3 only shows a localized radial structure).

[0046] Preparation and testing of lithium cobalt oxide and coin cells 1. Preparation of lithium cobalt oxide cathode materials: Cobalt tetroxide products obtained in Examples 1-3 and Comparative Examples 1-3 were respectively mixed with battery-grade lithium carbonate at a Li / Co molar ratio of 1.05:1. The mixtures were placed in a high-speed mixer and mixed at 800 rpm for 2 hours to obtain a mixture. The mixtures were placed in a corundum crucible and heated to 950°C at a heating rate of 5°C / min under air atmosphere. The mixtures were sintered at this temperature for 12 hours and then naturally cooled to room temperature. After crushing and passing through a 200-mesh sieve, lithium cobalt oxide cathode materials LCO-1 to LCO-6 (corresponding to Examples 1-3 and Comparative Examples 1-3, respectively) were obtained.

[0047] 2. Button Cell Assembly: The lithium cobalt oxide positive electrode materials LCO-1 to LCO-6 were used as the positive electrode active materials. The positive electrode active materials, conductive carbon black SuperP, and polyvinylidene fluoride (PVDF) binder were mixed at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added and ground into a uniform slurry. This slurry was coated onto the surface of an aluminum foil current collector and vacuum-dried at 120°C for 12 hours. After rolling, it was cut into positive electrode sheets with a diameter of 12 mm. Using a lithium metal sheet as the negative electrode, a Celgard 2400 polypropylene porous membrane as the separator, and a 1.0 mol / L LiPF6 solution of ethylene carbonate / diethyl carbonate / methyl ethyl carbonate (volume ratio 1:1:1) as the electrolyte, CR2032 type button cells were assembled in an argon-filled glove box, and designated as cells B1 to B6.

[0048] 3. Charge / discharge test method: The above-mentioned coin cells were subjected to constant current charge / discharge tests using the Blue Battery Testing System. The test voltage range was 3.0~4.6V, and the test temperature was 25±2℃. First, the cells were activated at a 0.1C rate for 3 weeks, and then a 200-cycle test was performed at a 1C rate. The discharge specific capacity in the first cycle and the capacity retention rate after 200 cycles were recorded.

[0049] 4. Test Results Table 2. Electrical performance data of the examples and comparative examples

[0050] Results Analysis: The lithium cobalt oxide cathode materials prepared in Examples 1-3 all achieved a capacity retention of over 93.5% after 1C / 200 cycles at a high voltage of 4.6V, significantly better than the comparative examples. Comparative Example 1, due to the lack of Al / Mg doping in the core, suffered from insufficient structural stability due to a porous core, resulting in a capacity retention of only 89.5%. Comparative Example 2, due to the lack of phosphorus doping in the outer shell and the absence of a radial structure to provide directional CO2 exhaust channels, experienced severe particle cracking during calcination, resulting in a capacity retention of only 85.0%. Comparative Example 3, due to the use of inorganic phosphate instead of phosphate ester, suffered from incomplete radial structure caused by instantaneous supersaturation of phosphate ions, resulting in a capacity retention of 88.0%. The above results indicate that the present invention significantly improves the cycling stability of lithium cobalt oxide at high voltage through the synergistic effect of core Al / Mg co-doping and shell phosphate ester slow-release doping.

Claims

1. A method for preparing cobalt tetroxide, characterized in that: Includes the following steps: (1) A solution containing carbonate ions and ammonium ions is used as the base liquid, and cobalt metal liquid, magnesium and / or aluminum cation doping liquid, precipitant solution and complexing agent solution are added to carry out the nucleation reaction. The pH of the reaction is adjusted to 7.8-8.2, and seed slurry is obtained after the reaction. (2) Using the seed slurry obtained in step (1) as the base liquid, the cobalt metal liquid, the phosphate dopant liquid and the precipitant solution are added to carry out the shell-forming reaction, and the pH of the reaction is adjusted to 6.8-7.1 to obtain cobalt carbonate slurry; (3) The cobalt carbonate slurry is obtained by solid-liquid separation and calcination of the solid phase.

2. The method for preparing cobalt tetroxide according to claim 1, characterized in that: In step (1), the concentration of carbonate ions in the bottom solution is 0.5-2 mol / L, the concentration of ammonium ions is 0.05-0.10 mol / L, and the pH of the bottom solution is 8.0-8.5; And / or, the phosphate ester doping solution contains at least one of trimethyl phosphate, triethyl phosphate and tributyl phosphate; preferably, the phosphorus concentration in the phosphate ester doping solution is 0.1-1.0 mol / L.

3. The method for preparing cobalt tetroxide according to claim 1, characterized in that: The cobalt metal liquid contains at least one of cobalt sulfate, cobalt chloride, and cobalt nitrate. Preferably, the cobalt ion concentration in the cobalt metal liquid is 1.2-1.8 mol / L; Preferably, in step (1), the flow rate of the cobalt metal liquid is 20-30 L / h; Preferably, in step (2), the flow rate of the cobalt metal liquid is 10-20 L / h.

4. The method for preparing cobalt tetroxide according to claim 1, wherein in step (1), the cation doping solution containing magnesium and / or aluminum includes at least one of soluble aluminum salt and soluble magnesium salt; preferably, the soluble aluminum salt includes at least one of aluminum chloride, aluminum sulfate and aluminum nitrate; preferably, the soluble magnesium salt includes at least one of magnesium sulfate, magnesium chloride and magnesium nitrate.

5. The method for preparing cobalt tetroxide according to claim 1, characterized in that: In steps (1) and (2), the precipitant solution is a solution containing carbonate ions, and the concentration of carbonate ions in the precipitant solution is 1-3 mol / L.

6. The method for preparing cobalt tetroxide according to claim 1, characterized in that: In step (2), the flow rates of the phosphate ester dopant and the cobalt metal liquid are controlled during the shell-forming reaction so that the molar feed ratio of phosphorus to cobalt is (0.001-0.02):

1.

7. The method for preparing cobalt tetroxide according to claim 1, characterized in that: In step (3), the calcination refers to calcining at 700-750℃ for 2-4 hours in an oxygen atmosphere.

8. A cobalt tetroxide, characterized in that: It is prepared by the preparation method according to any one of claims 1-7.

9. A positive electrode material, characterized in that, The raw materials for preparing the cathode material include cobalt tetroxide as described in claim 8.

10. A lithium-ion battery, characterized in that, Includes the cathode material as described in claim 9.

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