Long cycle life doped modified ternary positive electrode material and application thereof

Abstract

This invention relates to the field of ternary cathode material technology, specifically to a long-cycle-life doped modified ternary cathode material and its applications. The preparation process of a long-cycle-life doped modified ternary cathode material includes: preparing precursor powder; preparing cathode powder; and preparing the ternary cathode material. This invention, by mixing and sintering vanadium pentoxide and lithium selenate with the precursor powder to prepare vanadium-selenium co-doped cathode powder, achieves the following: Firstly, vanadium can solve the problem of bulk structure decay, and selenium can solve the problem of interfacial side reaction decay. The combination of the two can achieve complementary functions of bulk structure stability and interfacial passivation, synergistically improving the first-cycle coulombic efficiency. Secondly, after vanadium and selenium co-doping, a V-O-TM-Se cross-linked network can be formed, significantly improving the overall lattice rigidity and synergistically suppressing irreversible structural phase transitions during charge and discharge, thereby greatly improving the cycle capacity retention rate.

Description

Technical Field

[0001] This invention relates to the field of ternary cathode material technology, specifically to a long-cycle-life doped modified ternary cathode material and its applications. Background Technology

[0002] Lithium-ion batteries are efficient and convenient energy storage devices that have been widely used in electric vehicles, portable electronic devices, and large-scale energy storage systems. Currently, commercially available lithium-ion batteries mainly use cathode materials such as LiCoO2, LiFePO4, and LiMnO2. LiCoO2 has stable electrochemical performance and is easy to synthesize, but its structure is prone to collapse under high delithiation conditions, resulting in an actual capacity that cannot approach the ideal state, and Co resources are scarce and expensive. While LiMnO2 is inexpensive, the Jahn-Teller distortion effect leads to a significant decrease in capacity and poor conductivity. Although LiFePO4 has good safety and stability, it suffers from poor rate performance. Compared to these materials, ternary lithium nickel cobalt manganese oxide (LiCO) materials can overcome the shortcomings of single-component materials, possessing advantages such as structural stability, high specific capacity, and low cost, thus showing greater research potential.

[0003] Nickel-cobalt-manganese ternary cathode materials, especially high-nickel ternary materials, have become the mainstream cathode materials for current power lithium-ion batteries due to their high reversible specific capacity, moderate production cost, and good processing adaptability. With the increase of nickel content in the material, the reversible specific capacity of ternary materials is significantly improved, which can effectively improve the energy density of power battery systems and meet the core requirements of long range for new energy vehicles. However, at the same time, during charge-discharge cycles, high-nickel ternary materials undergo an irreversible H2-H3 phase transition, which triggers a violent contraction and expansion of the c-axis of the crystal lattice. This leads to the continuous accumulation of internal stress inside the secondary particles, which in turn induces the initiation and propagation of intergranular microcracks, and may even cause the secondary particles to break apart. The electrolyte will continue to penetrate into the particles through the microcracks, exacerbating the dissolution of transition metal ions and the irreversible transformation of the layered structure to the spinel phase and rock salt phase, causing permanent deactivation of active sites, resulting in rapid capacity decay and a significant reduction in cycle life.

[0004] Furthermore, during low-temperature, high-current charging and discharging, the severe polarization of the ternary cathode leads to a significant increase in the charging platform, causing irreversible precipitation of metallic lithium on the surface of the anode, forming lithium dendrites. The precipitated lithium dendrites will continue to react with the electrolyte, consuming active lithium, and may even result in "dead lithium" that can no longer participate in electrochemical reactions. Since the active lithium in the entire battery is entirely provided by the cathode, irreversible lithium loss will directly lead to the continuous decay of the cathode's discharge capacity.

[0005] Therefore, there is a need to propose a long-cycle-life doped modified ternary cathode material and its application to improve battery cycle stability and extend its service life. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a long-cycle-life doped modified ternary cathode material and its applications.

[0007] This invention provides a process for preparing a long-cycle-life doped modified ternary cathode material, comprising the following steps: S1: Preparation of precursor powder After hydroxylation of multi-walled carbon nanotubes, they are dispersed in a polyvinylpyrrolidone solution, then mixed with a mixed salt solution formed by dissolving nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate. The mixture is then reacted with sodium hydroxide solution and ammonia water in the base solution. After aging, washing, and drying, the precursor powder is obtained. S2: Preparation of cathode powder The above precursor powder, lithium hydroxide monohydrate, vanadium pentoxide, lithium selenate, magnesium hydroxide and LiNO3-LiF were mixed and air-jet pulverized, and then sintered in argon and oxygen atmospheres respectively to obtain cathode powder. S3: Preparation of ternary cathode materials S3.1: Add polyN-vinylcaprolactam to anhydrous ethanol at a ratio of 1g:(90-100)mL, heat and stir at 40-50℃ until completely dissolved to obtain a polyN-vinylcaprolactam solution; S3.2: Add the above positive electrode powder to anhydrous ethanol at a ratio of 1g:(3-5)mL, and ultrasonically disperse for 20-30min to obtain a powder suspension; S3.3: Add the above polyN-vinylcaprolactam solution to the above powder suspension, stir at 500-600 rpm for 2-3 h, then vacuum filter, wash with anhydrous ethanol and vacuum dry at 80℃ for 10-12 h to obtain dry powder. S3.4: Heat the above-mentioned dry powder in air at a rate of 2℃ / min to 150-160℃ and heat-treat for 2-3 hours. After naturally cooling to room temperature, grind and pass through a 200-300 mesh sieve to obtain the ternary cathode material.

[0008] Furthermore, S1 includes the following steps: S1.1: Add multi-walled carbon nanotubes to a mixed acid solution at a ratio of 1g:(10-20)mL, heat and reflux at 70-80℃ for 4-5h, and then wash until neutral to obtain hydroxylated carbon nanotubes. S1.2: Add polyvinylpyrrolidone to deionized water, stir thoroughly to dissolve, then add hydroxylated carbon nanotubes, ultrasonically disperse at 1-3℃ for 30-40 min, centrifuge at 3000-4000 r / min for 10-20 min, collect the upper dispersion to obtain a carbon nanotube dispersion with a concentration of 4-5 mg / mL. S1.3: Dissolve nickel sulfate hexahydrate, cobalt sulfate heptahydrate and manganese sulfate monohydrate in deionized water, stir thoroughly to dissolve, and obtain a mixed salt solution. Then add the above carbon nanotube dispersion, stir thoroughly to mix, and obtain a premixed solution. S1.4: Add the above premixed solution, 4 mol / L sodium hydroxide solution and 6 mol / L ammonia water to the base solution, keep warm at 50-55℃ and stir for 8-10 h, then let stand and age for 10-12 h, centrifuge and wash until the pH of the filtrate reaches 7 and vacuum dry at 120℃ for 10-12 h to obtain the precursor powder.

[0009] Furthermore, S2 includes the following steps: S2.1: Add the precursor powder obtained in step S1.4, lithium hydroxide monohydrate and lithium selenate to a double planetary mixer and premix for 15-20 min. Then add vanadium pentoxide and magnesium hydroxide and continue mixing for 20-30 min. Then add LiNO3-LiF and mix at high speed of 2000-3000 rpm for 20-30 min. After air jet milling, a mixture is obtained. S2.2: Place the above mixture in an atmosphere roller kiln, introduce high-purity argon gas, and heat it to 400-450℃ at 3℃ / min. Hold it at this temperature for 4-5 hours for pre-sintering. Then introduce high-purity oxygen and heat it to 500-550℃ at 2℃ / min. Hold it at this temperature for 2-3 hours for sintering. Then maintain the oxygen atmosphere and heat it to 730-750℃ at 2℃ / min. Hold it at this temperature for 7-8 hours for sintering. After cooling to room temperature in the furnace, grind it through a 400-mesh sieve to obtain the positive electrode powder.

[0010] Furthermore, the mixed acid solution is prepared by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 3:1.

[0011] Furthermore, the amount of polyvinylpyrrolidone added is 70-80% of the mass of the multi-walled carbon nanotubes.

[0012] Furthermore, the mass ratio of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate is (12.4-12.5):(1.6-1.7):1, and the total metal ion concentration in the mixed salt solution is 2 mol / L.

[0013] Furthermore, the volume ratio of the carbon nanotube dispersion to the mixed salt solution is 1:(2-2.5), and the volume ratio of the premix, 4mol / L sodium hydroxide solution, 6mol / L ammonia solution and the base solution is (9-10):(2.2-2.3):(1.5-1.6):1, with the base solution being a 0.3mol / L dilute ammonia solution.

[0014] Furthermore, the mass ratio of the precursor powder, lithium hydroxide monohydrate, magnesium hydroxide, and LiNO3-LiF is (338-342):(110-112):1:(14.1-14.5), and the molar amount of vanadium doped in vanadium pentoxide is 1-1.2% of the total molar amount of nickel, cobalt, and manganese metals, the molar amount of selenium doped in lithium selenate is 0.8-1% of the total molar amount of nickel, cobalt, and manganese metals, and the molar ratio of LiNO3 to LiF in LiNO3-LiF is 4:1.

[0015] Furthermore, the mass ratio of poly(N-vinylcaprolactam) to cathode powder is 1:(45-50).

[0016] Application of the long cycle life doped modified ternary cathode material described in any of the above in the preparation of lithium-ion batteries.

[0017] The present invention has the following advantages: 1. In this invention, vanadium pentoxide and lithium selenate are mixed with precursor powder and sintered to form vanadium-selenium co-doped cathode powder. Vanadium doping can strengthen the lattice framework of nickel-cobalt-manganese ternary cathode powder and suppress irreversible phase transitions and cation mixing. Selenium doping can passivate the grain boundaries and surfaces of particles and suppress transition metal dissolution and interfacial side reactions. When the two are co-doped, on the one hand, vanadium anchors the framework inside the particles, suppresses phase transitions, and reduces mixing, solving the problem of bulk structure decay. Selenium passivates the interface at grain boundaries and the surface, resists corrosion, and suppresses metal dissolution, solving the problem of interfacial side reaction decay. This achieves complementary functions of bulk structure stability and interface passivation, and synergistically improves the first-cycle coulombic efficiency. On the other hand, after vanadium and selenium are co-doped, a VO-TM-Se cross-linked network can be formed, which significantly improves the overall lattice rigidity and synergistically suppresses irreversible structural phase transitions during charging and discharging, thereby greatly improving the cycle capacity retention rate.

[0018] 2. In this invention, an organic coating layer is constructed in situ on the outer surface of the particles and the inner wall of the three-dimensional channels through the strong coordination bond between poly(N-vinylcaprolactam) and the surface of nickel-cobalt-manganese cathode powder. After obtaining the ternary cathode material, the poly(N-vinylcaprolactam) coating layer can anchor the highly active Ni on the surface of nickel-cobalt-manganese through coordination bonds. 3+ / Ni 4+ This eliminates the catalytic oxidation effect of lithium on the electrolyte, inhibits irreversible oxidative decomposition of the electrolyte during the first charge-discharge cycle, and reduces the ineffective consumption of active lithium, thereby effectively improving the first-cycle Coulomb effect of the ternary cathode material. Furthermore, the lactam polar groups of poly(N-vinylcaprolactam) can react with Li... + Solvation coordination occurs, resulting in Li +It provides continuous hopping transport sites, while the coordination coating of polyN-vinylcaprolactam can suppress the continuous increase of interfacial impedance during cycling, thereby significantly reducing the electrode polarization of ternary cathode materials in low-temperature environments, improving its discharge capacity retention rate in low-temperature environments, and thus improving the reliability and range of power batteries in cold environments.

[0019] 3. In this invention, hydroxylated carbon nanotubes are co-precipitated and uniformly embedded inside the precursor secondary spherical particles to form precursor powder. Then, sintering is performed in an oxygen atmosphere to oxidize and remove the multi-walled carbon nanotubes, thereby leaving three-dimensional interconnected nanopores with the same size and morphology as the original multi-walled carbon nanotube network in situ. High-temperature sintering is then continued to allow the matrix material to form phases, crystallize, and grow. After obtaining the cathode powder, the nanopores left by the multi-walled carbon nanotubes can absorb and buffer the stress caused by the expansion and contraction of the lattice volume during charging and discharging. This fundamentally inhibits the initiation and propagation of microcracks, maintains the mechanical integrity of the secondary spherical particles, reduces side reactions, transition metal dissolution, irreversible consumption of active lithium, and irreversible transformation of the layered structure to the spinel / rock salt phase. This fundamentally curbs the continuous decay of cycle capacity and thus significantly improves cycle life. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the preparation process of the long cycle life doped modified ternary cathode material used in the embodiments of the present invention. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this invention.

[0022] Example 1: A preparation process for a long-cycle-life doped modified ternary cathode material, such as... Figure 1 As shown, it includes the following steps: S1: Preparation of precursor powder S1.1: Add multi-walled carbon nanotubes to a mixed acid solution at a ratio of 1g:10mL, heat and reflux at 70℃ for 4h, and then wash until neutral to obtain hydroxylated carbon nanotubes. The mixed acid solution is prepared by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 3:1. S1.2: Polyvinylpyrrolidone was added to deionized water at a ratio of 1g:200mL, stirred thoroughly to dissolve, and then hydroxylated carbon nanotubes were added. The mixture was ultrasonically dispersed at 1℃ for 30min, and centrifuged at 3000r / min for 10min. The supernatant was collected to obtain a carbon nanotube dispersion with a concentration of 4mg / mL. The amount of polyvinylpyrrolidone added was 70% of the mass of the multi-walled carbon nanotubes. S1.3: Dissolve nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate in deionized water and stir thoroughly to obtain a mixed salt solution. Then add the above carbon nanotube dispersion and stir thoroughly to obtain a premixed solution. The mass ratio of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate is 12.4:1.6:1, the total metal ion concentration in the mixed salt solution is 2 mol / L, and the volume ratio of carbon nanotube dispersion to mixed salt solution is 1:2. S1.4: Add the above premixed solution, 4 mol / L sodium hydroxide solution and 6 mol / L ammonia water to the base liquid, keep warm at 50℃ and stir for 8 h, then let stand and age for 10 h, centrifuge and wash until the pH of the filtrate reaches 7 and vacuum dry at 120℃ for 10 h to obtain the precursor powder. The volume ratio of the premixed solution, 4 mol / L sodium hydroxide solution, 6 mol / L ammonia water and base liquid is 9:2.2:1.5:1, and the base liquid is a 0.3 mol / L dilute ammonia water solution. S2: Preparation of cathode powder S2.1: Add the precursor powder obtained in step S1.4, lithium hydroxide monohydrate, and lithium selenate to a double planetary mixer and premix for 15 min. Then add vanadium pentoxide and magnesium hydroxide and continue mixing for 20 min. Then add LiNO3-LiF and mix at high speed of 2000 rpm for 20 min. After air jet milling, a mixture is obtained. The mass ratio of precursor powder, lithium hydroxide monohydrate, magnesium hydroxide, and LiNO3-LiF is 338:110:1:14.1. The molar amount of vanadium doped in vanadium pentoxide is 1% of the total molar amount of nickel, cobalt, and manganese. The molar amount of selenium doped in lithium selenate is 0.8% of the total molar amount of nickel, cobalt, and manganese. The molar ratio of LiNO3 to LiF in LiNO3-LiF is 4:1. S2.2: Place the above mixture in an atmosphere roller kiln, introduce high-purity argon gas, and heat to 400°C at 3°C / min. Hold for 4 hours for pre-sintering. Then introduce high-purity oxygen and heat to 500°C at 2°C / min. Hold for 2 hours for sintering. Then maintain the oxygen atmosphere and heat to 730°C at 2°C / min. Hold for 7 hours for sintering. After cooling to room temperature in the furnace, grind through a 400-mesh sieve to obtain positive electrode powder. S3: Preparation of ternary cathode materials S3.1: Add polyN-vinylcaprolactam to anhydrous ethanol at a ratio of 1g:90mL, heat and stir at 40℃ until completely dissolved to obtain a polyN-vinylcaprolactam solution; S3.2: Add the above positive electrode powder to anhydrous ethanol at a ratio of 1g:3mL, and ultrasonically disperse for 20min to obtain a powder suspension; S3.3: The above polyN-vinylcaprolactam solution was added to the above powder suspension, stirred at 500 rpm for 2 h, and then vacuum filtered, washed with anhydrous ethanol and vacuum dried at 80 °C for 10 h to obtain a dry powder, wherein the mass ratio of polyN-vinylcaprolactam to positive electrode powder is 1:45. S3.4: The above-mentioned dry powder is heated to 150°C in air at a rate of 2°C / min and heat-treated for 2 hours. After naturally cooling to room temperature, it is ground and passed through a 200-mesh sieve to obtain the ternary cathode material.

[0023] Example 2: A preparation process for a long-cycle-life doped modified ternary cathode material, such as... Figure 1 As shown, it includes the following steps: S1: Preparation of precursor powder S1.1: Add multi-walled carbon nanotubes to a mixed acid solution at a ratio of 1g:15mL, heat and reflux at 75℃ for 4.5h, and then wash until neutral to obtain hydroxylated carbon nanotubes. The mixed acid solution is prepared by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 3:1. S1.2: Polyvinylpyrrolidone was added to deionized water at a ratio of 1g:210mL, stirred thoroughly to dissolve, and then hydroxylated carbon nanotubes were added. The mixture was ultrasonically dispersed at 2℃ for 35min, and centrifuged at 3500r / min for 15min. The supernatant was collected to obtain a carbon nanotube dispersion with a concentration of 4.5mg / mL. The amount of polyvinylpyrrolidone added was 75% of the mass of the multi-walled carbon nanotubes. S1.3: Dissolve nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate in deionized water and stir thoroughly to obtain a mixed salt solution. Then add the above carbon nanotube dispersion and stir thoroughly to obtain a premixed solution. The mass ratio of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate is 12.45:1.65:1, the total metal ion concentration in the mixed salt solution is 2 mol / L, and the volume ratio of carbon nanotube dispersion to mixed salt solution is 1:2.25. S1.4: Add the above premixed solution, 4 mol / L sodium hydroxide solution and 6 mol / L ammonia water to the base liquid, keep warm at 55℃ and stir for 9 h, then let stand and age for 11 h, centrifuge and wash until the pH of the filtrate reaches 7 and vacuum dry at 120℃ for 11 h to obtain the precursor powder. The volume ratio of the premixed solution, 4 mol / L sodium hydroxide solution, 6 mol / L ammonia water and base liquid is 9.5:2.25:1.55:1, and the base liquid is a 0.3 mol / L dilute ammonia water solution. S2: Preparation of cathode powder S2.1: Add the precursor powder obtained in step S1.4, lithium hydroxide monohydrate, and lithium selenate to a double planetary mixer and premix for 17.5 min. Then add vanadium pentoxide and magnesium hydroxide and continue mixing for 25 min. Then add LiNO3-LiF and mix at high speed of 2500 rpm for 25 min. After air jet milling, a mixture is obtained. The mass ratio of precursor powder, lithium hydroxide monohydrate, magnesium hydroxide, and LiNO3-LiF is 340:111:1:14.3. The molar amount of vanadium doped in vanadium pentoxide is 1.1% of the total molar amount of nickel, cobalt, and manganese. The molar amount of selenium doped in lithium selenate is 0.9% of the total molar amount of nickel, cobalt, and manganese. The molar ratio of LiNO3 to LiF in LiNO3-LiF is 4:1. S2.2: Place the above mixture in an atmosphere roller kiln, introduce high-purity argon gas, and heat to 425°C at 3°C / min, hold for pre-sintering for 4.5 hours, then introduce high-purity oxygen, heat to 525°C at 2°C / min, hold for sintering for 2.5 hours, then maintain the oxygen atmosphere, heat to 740°C at 2°C / min, hold for sintering for 7.5 hours, cool to room temperature with the furnace, and grind through a 400-mesh sieve to obtain positive electrode powder; S3: Preparation of ternary cathode materials S3.1: Add polyN-vinylcaprolactam to anhydrous ethanol at a ratio of 1g:95mL, heat and stir at 45℃ until completely dissolved to obtain a polyN-vinylcaprolactam solution; S3.2: Add the above positive electrode powder to anhydrous ethanol at a ratio of 1g:4mL, and ultrasonically disperse for 25min to obtain a powder suspension; S3.3: The above polyN-vinylcaprolactam solution was added to the above powder suspension, stirred at 550 rpm for 2.5 h, and then vacuum filtered, washed with anhydrous ethanol and vacuum dried at 80 °C for 11 h to obtain a dry powder, wherein the mass ratio of polyN-vinylcaprolactam to positive electrode powder is 1:47.5. S3.4: The above-mentioned dried powder is heated to 155°C in air at a rate of 2°C / min and heat-treated for 2.5 hours. After being naturally cooled to room temperature, it is ground and passed through a 300-mesh sieve to obtain the ternary cathode material.

[0024] Example 3: A preparation process for a long-cycle-life doped modified ternary cathode material, such as... Figure 1 As shown, it includes the following steps: S1: Preparation of precursor powder S1.1: Add multi-walled carbon nanotubes to a mixed acid solution at a ratio of 1g:20mL, heat and reflux at 80℃ for 5h, and then wash until neutral to obtain hydroxylated carbon nanotubes. The mixed acid solution is prepared by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 3:1. S1.2: Polyvinylpyrrolidone was added to deionized water at a ratio of 1g:220mL, stirred thoroughly to dissolve, and then hydroxylated carbon nanotubes were added. The mixture was ultrasonically dispersed at 3℃ for 40min, and centrifuged at 4000r / min for 20min. The supernatant was collected to obtain a carbon nanotube dispersion with a concentration of 5mg / mL. The amount of polyvinylpyrrolidone added was 80% of the mass of the multi-walled carbon nanotubes. S1.3: Dissolve nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate in deionized water and stir thoroughly to obtain a mixed salt solution. Then add the above carbon nanotube dispersion and stir thoroughly to obtain a premixed solution. The mass ratio of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate is 12.5:1.7:1, the total metal ion concentration in the mixed salt solution is 2 mol / L, and the volume ratio of carbon nanotube dispersion to mixed salt solution is 1:2.5. S1.4: Add the above premixed solution, 4 mol / L sodium hydroxide solution and 6 mol / L ammonia water to the base liquid, keep warm at 55℃ and stir for 10 h, then let stand and age for 12 h, centrifuge and wash until the pH of the filtrate reaches 7 and vacuum dry at 120℃ for 12 h to obtain the precursor powder. The volume ratio of the premixed solution, 4 mol / L sodium hydroxide solution, 6 mol / L ammonia water and base liquid is 10:2.3:1.6:1, and the base liquid is a 0.3 mol / L dilute ammonia water solution. S2: Preparation of cathode powder S2.1: Add the precursor powder obtained in step S1.4, lithium hydroxide monohydrate, and lithium selenate to a double planetary mixer and premix for 20 min. Then add vanadium pentoxide and magnesium hydroxide and continue mixing for 30 min. Then add LiNO3-LiF and mix at high speed of 3000 rpm for 30 min. After air jet milling, a mixture is obtained. The mass ratio of precursor powder, lithium hydroxide monohydrate, magnesium hydroxide, and LiNO3-LiF is 342:112:1:14.5. The molar amount of vanadium doped in vanadium pentoxide is 1.2% of the total molar amount of nickel, cobalt, and manganese. The molar amount of selenium doped in lithium selenate is 1% of the total molar amount of nickel, cobalt, and manganese. The molar ratio of LiNO3 to LiF in LiNO3-LiF is 4:1. S2.2: Place the above mixture in an atmosphere roller kiln, introduce high-purity argon gas, and heat to 450°C at 3°C / min. Hold for pre-sintering for 5 hours. Then introduce high-purity oxygen and heat to 550°C at 2°C / min. Hold for sintering for 3 hours. Then maintain the oxygen atmosphere and heat to 750°C at 2°C / min. Hold for sintering for 8 hours. After cooling to room temperature in the furnace, grind through a 400-mesh sieve to obtain positive electrode powder. S3: Preparation of ternary cathode materials S3.1: Add polyN-vinylcaprolactam to anhydrous ethanol at a ratio of 1g:100mL, heat and stir at 50℃ until completely dissolved to obtain a polyN-vinylcaprolactam solution; S3.2: Add the above positive electrode powder to anhydrous ethanol at a ratio of 1g:5mL, and ultrasonically disperse for 30min to obtain a powder suspension; S3.3: The above polyN-vinylcaprolactam solution was added to the above powder suspension, stirred at 600 rpm for 3 h, and then vacuum filtered, washed with anhydrous ethanol and vacuum dried at 80 °C for 12 h to obtain a dry powder, wherein the mass ratio of polyN-vinylcaprolactam to positive electrode powder is 1:50. S3.4: The above-mentioned dried powder is heated to 160°C in air at a rate of 2°C / min and heat-treated for 3 hours. After naturally cooling to room temperature, it is ground and passed through a 300-mesh sieve to obtain the ternary cathode material.

[0025] Comparative Example 1 differs from Example 1 in that vanadium pentoxide and lithium selenate are not added in step S2.1.

[0026] Comparative Example 2 differs from Example 1 in that lithium selenate is not added in step S2.1, and the amount of vanadium pentoxide added is increased, wherein the molar amount of vanadium doping is 1.8% (relative to the total molar amount of nickel, cobalt and manganese metals).

[0027] Comparative Example 3 differs from Example 1 in that: vanadium pentoxide is not added in step S2.1, and the amount of lithium selenate added is increased, wherein the molar amount of selenium doping is 1.8% (relative to the total molar amount of nickel, cobalt and manganese metals).

[0028] Comparative Example 4 differs from Example 1 in that the molar amount of vanadium doping in step S2.1 is adjusted to 0.9% (relative to the total molar amount of nickel, cobalt, and manganese), and the molar amount of selenium doping is adjusted to 0.7%.

[0029] Comparative Example 5 differs from Example 1 in that the molar amount of vanadium doping in step S2.1 is adjusted to 1.3% (relative to the total molar amount of nickel, cobalt, and manganese), and the molar amount of selenium doping is adjusted to 1.1%.

[0030] Comparative Example 6 differs from Example 1 in that: no poly-N-vinylcaprolactam solution is added in step S3.3.

[0031] Comparative Example 7 differs from Example 1 in that hydroxylated carbon nanotubes are not added in step S1.2.

[0032] Test example: Test 1: CR2032 coin cells were prepared using the same process with the ternary cathode materials obtained in Examples 1-3 and Comparative Examples 1-5. The cathode material ratio was ternary cathode material: conductive carbon black: PVDF = 8:1:1, and the single-sided coating surface density was 10 mg / cm³. 2 The compacted density after roller pressing is 3.5 g / cm³. 3 The cells were vacuum dried at 120°C for 12 hours, then assembled into button cells in an argon-protected glove box, and tested after standing for 12 hours.

[0033] First-cycle coulombic efficiency test: Under 25℃, 2.8-4.3V voltage range, 0.1C constant current charge and discharge, the first-cycle charging capacity and discharge capacity were tested, and the first-cycle coulombic efficiency was calculated. Each group was tested 3 times, and the average value was taken. The results are shown in Table 1.

[0034] Cyclic performance test: Under 25℃ environment, 2.8-4.3V voltage range, 1C constant current charge and discharge, 200 cycles, calculate capacity retention rate, each group is tested 3 times, and the average value is taken. The results are shown in Table 1.

[0035] Table 1: Results of Coulomb efficiency and capacity retention after 200 cycles As shown in Table 1, when no vanadium and selenium were added to Comparative Example 1, the first-cycle coulombic efficiency and cycle capacity retention of the ternary cathode material were lower than those of Example 1. When only vanadium or selenium was added to Comparative Examples 2 and 3, the first-cycle coulombic efficiency and cycle capacity retention of the ternary cathode material were higher than those of Comparative Example 1, but still lower than those of Example 1. It can be seen that by mixing vanadium pentoxide and lithium selenate with precursor powder and sintering them to prepare vanadium and selenium co-doped cathode powder, not only can the first-cycle coulombic efficiency be synergistically improved, but the cycle capacity retention can also be synergistically improved. Furthermore, in Comparative Examples 4 and 5, when the molar amount of vanadium doping was not in the range of 1-1.2 mol%, and the molar amount of selenium doping was not in the range of 0.8-1%, the first-cycle coulombic efficiency and cycle capacity retention of the ternary cathode materials were lower than those in Example 1. Therefore, it can be seen that when the molar amount of vanadium doping is 1-1.2 mol% and the molar amount of selenium doping is 0.8-1%, the co-doping effect is optimal.

[0036] Test 2: The ternary cathode material prepared in Comparative Example 6 was made into a button cell according to the method of Test 1. The first-cycle coulombic efficiency was then tested according to the first-cycle coulombic efficiency test method in Test 1. The test was repeated 3 times and the average value was taken. The results are shown in Table 2.

[0037] Low-temperature discharge capacity retention test: The coin cells prepared in Examples 1-3 and Comparative Example 6 were fully charged at 0.2C at room temperature, and then discharged at a rate of 0.2C to the cutoff voltage (voltage range of 2.8-4.3V), which was recorded as the room temperature discharge capacity. Then, after being fully charged at 0.2C at room temperature, they were discharged at a rate of 0.2C to the cutoff voltage (voltage range of 2.8-4.3V) at -20℃, which was recorded as the low-temperature discharge capacity. The discharge capacity retention rate was calculated using the following formula: Low-temperature discharge capacity retention rate = low-temperature discharge capacity / room temperature discharge capacity × 100%. Each group was tested three times, and the average value was taken. The results are shown in Table 2.

[0038] Table 2: Test results of first-cycle coulombic efficiency and low-temperature discharge capacity retention As shown in Table 2, the first-cycle coulombic efficiency and -20℃ discharge capacity retention of the ternary cathode material prepared in Comparative Example 6 without the use of poly-N-vinylcaprolactam coating are both lower than those in Example 1. This shows that by constructing an organic coating layer in situ on the outer surface of the particles and the inner wall of the three-dimensional channels through the strong coordination bond between poly-N-vinylcaprolactam and the surface of the nickel-cobalt-manganese cathode powder, the first-cycle coulombic effect of the ternary cathode material can be effectively improved, and its discharge capacity retention under low temperature conditions can be improved, thereby improving the reliability and range of the power battery in cold environments.

[0039] Test 3: Following the method of Test 1, the ternary cathode material prepared in Comparative Example 7 was made into a coin cell. Then, the coin cells prepared in Examples 1-3 and Comparative Example 7 were subjected to constant current charge and discharge at 1C in a 25°C environment with a voltage range of 2.8-4.3V for 1500 cycles. The cycle capacity retention rate was calculated. Each group was tested three times, and the average value was taken. The results are shown in Table 3.

[0040] Table 3: Capacity retention test results after 1500 cycles As shown in Table 3, when no carbon nanotubes were added to create pores in Comparative Example 7, the ternary cathode material obtained after 1500 charge-discharge cycles had a much lower cycle capacity retention rate than that of Example 1. This shows that by uniformly embedding hydroxylated carbon nanotubes into the precursor secondary spherical particles through co-precipitation to form precursor powder, and then sintering it in an oxygen atmosphere to oxidize and remove the multi-walled carbon nanotubes, three-dimensional interconnected nanopores with the same size and morphology as the original multi-walled carbon nanotube network are left in situ. After further high-temperature sintering, the matrix material is phase-formed, crystallized, and grown to obtain cathode powder, which can fundamentally curb the continuous decay of cycle capacity and thus significantly improve cycle life.

[0041] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims. Parts not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A long-cycle-life doped modified ternary cathode material, characterized in that, Its preparation process includes the following steps: S1: Preparation of precursor powder After hydroxylation of multi-walled carbon nanotubes, they are dispersed in a polyvinylpyrrolidone solution, then mixed with a mixed salt solution of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate. The mixture is then reacted with sodium hydroxide solution and ammonia water in the base solution. After aging, washing, and drying, the precursor powder is obtained. S2: Preparation of cathode powder The above precursor powder, lithium hydroxide monohydrate, vanadium pentoxide, lithium selenate, magnesium hydroxide and LiNO3-LiF were mixed and air-jet pulverized, and then sintered in argon and oxygen atmospheres respectively to obtain cathode powder. S3: Preparation of ternary cathode materials S3.1: Add polyN-vinylcaprolactam to anhydrous ethanol at a ratio of 1g:(90-100)mL, heat and stir at 40-50℃ until completely dissolved to obtain a polyN-vinylcaprolactam solution; S3.2: Add the above positive electrode powder to anhydrous ethanol at a ratio of 1g:(3-5)mL, and ultrasonically disperse for 20-30min to obtain a powder suspension; S3.3: Add the above polyN-vinylcaprolactam solution to the above powder suspension, stir at 500-600 rpm for 2-3 h, then vacuum filter, wash with anhydrous ethanol and vacuum dry at 80℃ for 10-12 h to obtain dry powder. S3.4: Heat the above-mentioned dry powder in air at a rate of 2℃ / min to 150-160℃ and heat-treat for 2-3 hours. After naturally cooling to room temperature, grind and pass through a 200-300 mesh sieve to obtain the ternary cathode material.

2. The long-cycle-life doped modified ternary cathode material according to claim 1, characterized in that, S1 includes the following steps: S1.1: Add multi-walled carbon nanotubes to the mixed acid solution at a ratio of 1g:(10-20)mL, heat and reflux at 70-80℃ for 4-5h, and then wash until neutral to obtain hydroxylated carbon nanotubes; S1.2: Add polyvinylpyrrolidone to deionized water, stir thoroughly to dissolve, then add hydroxylated carbon nanotubes, ultrasonically disperse at 1-3℃ for 30-40 min, centrifuge at 3000-4000 r / min for 10-20 min, collect the upper dispersion to obtain a carbon nanotube dispersion with a concentration of 4-5 mg / mL. S1.3: Dissolve nickel sulfate hexahydrate, cobalt sulfate heptahydrate and manganese sulfate monohydrate in deionized water, stir thoroughly to dissolve, and obtain a mixed salt solution. Then add the above carbon nanotube dispersion, stir thoroughly to mix, and obtain a premixed solution. S1.4: Add the above premixed solution, 4 mol / L sodium hydroxide solution and 6 mol / L ammonia water to the base solution, keep warm at 50-55℃ and stir for 8-10 h, then let stand and age for 10-12 h, centrifuge and wash until the pH of the filtrate reaches 7 and vacuum dry at 120℃ for 10-12 h to obtain the precursor powder.

3. The long-cycle-life doped modified ternary cathode material according to claim 2, characterized in that, S2 includes the following steps: S2.1: Add the precursor powder obtained in step S1.4, lithium hydroxide monohydrate and lithium selenate to a double planetary mixer and premix for 15-20 min. Then add vanadium pentoxide and magnesium hydroxide and continue mixing for 20-30 min. Then add LiNO3-LiF and mix at high speed of 2000-3000 rpm for 20-30 min. After air jet milling, a mixture is obtained. S2.2: Place the above mixture in an atmosphere roller kiln, introduce high-purity argon gas, and heat it to 400-450℃ at 3℃ / min. Hold it at this temperature for 4-5 hours for pre-sintering. Then introduce high-purity oxygen and heat it to 500-550℃ at 2℃ / min. Hold it at this temperature for 2-3 hours for sintering. Then maintain the oxygen atmosphere and heat it to 730-750℃ at 2℃ / min. Hold it at this temperature for 7-8 hours for sintering. After cooling to room temperature in the furnace, grind it through a 400-mesh sieve to obtain the positive electrode powder.

4. The long-cycle-life doped modified ternary cathode material according to claim 2, characterized in that, The mixed acid solution is prepared by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 3:

1.

5. The long-cycle-life doped modified ternary cathode material according to claim 2, characterized in that, The amount of polyvinylpyrrolidone added is 70-80% of the mass of multi-walled carbon nanotubes. 6.The long cycle life doped modified ternary cathode material of claim 2, characterized in that, The mass ratio of nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate is (12.4-12.5):(1.6-1.7):1, and the total metal ion concentration in the mixed salt solution is 2 mol / L. 7.The long cycle life doped modified ternary cathode material of claim 2, characterized in that, The volume ratio of carbon nanotube dispersion to mixed salt solution is 1:(2-2.5), and the volume ratio of premix, 4mol / L sodium hydroxide solution, 6mol / L ammonia solution and base solution is (9-10):(2.2-2.3):(1.5-1.6):

1. The base solution is 0.3mol / L dilute ammonia solution. 8.The long cycle life doped modified ternary cathode material of claim 3, characterized in that, The mass ratio of precursor powder, lithium hydroxide monohydrate, magnesium hydroxide, and LiNO3-LiF is (338-342):(110-112):1:(14.1-14.5), and the molar amount of vanadium doped in vanadium pentoxide is 1-1.2% of the total molar amount of nickel, cobalt, and manganese metals, the molar amount of selenium doped in lithium selenate is 0.8-1% of the total molar amount of nickel, cobalt, and manganese metals, and the molar ratio of LiNO3 to LiF in LiNO3-LiF is 4:

1. 9.The long cycle life doped modified ternary cathode material of claim 1, wherein, The mass ratio of poly(N-vinylcaprolactam) to cathode powder is 1:(45-50).

10. The application of a long cycle life doped modified ternary cathode material according to any one of claims 1-9 in the preparation of lithium-ion batteries.

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