Ultrahigh-nickel single-crystal positive electrode material and preparation method thereof

Abstract

This invention discloses an ultra-high nickel single-crystal cathode material and its preparation method, belonging to the field of lithium battery technology. The preparation method includes: preparing Ni and Mn continuous gradient precursors in three reactors in series; achieving tungsten / molybdenum and zirconium / titanium dual-element gradient doping through layered spraying; completing crystallization and surface reconstruction through four-stage oxygen-controlled sintering; constructing fast ion channels through directional pore formation; and constructing a three-dimensional conductive network through CVD deposition and nitrogen doping. The resulting cathode material exhibits a gradient structure that alleviates phase transition stress, directional mesopores that improve rate capability, and a three-dimensional network that stabilizes the interface. It maintains >85% capacity retention after 1000 cycles at 4.5V, >95% capacity retention at 5C, and <0.5mL / g of gas generation at high temperatures. The particles show no cracks after cycling, combining high capacity, fast charging, long cycle life, and high safety, making it suitable for high-energy-density power batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to an ultra-high nickel single crystal cathode material and its preparation method. Background Technology

[0002] Currently, the main technical approach to improving the energy density of lithium-ion batteries is to increase the nickel content of the cathode material. Existing technologies mainly include the following categories: Polycrystalline aggregate materials: Although spherical secondary particles prepared by co-precipitation can achieve high capacity, microcracks are easily generated at the grain boundaries between primary particles under high-pressure cycling, and electrolyte infiltration leads to severe side reactions and poor cycle life.

[0003] Conventional single-crystal materials: The particles are grown into micron-sized single crystals through high-temperature sintering, eliminating grain boundaries. However, they suffer from long lithium-ion diffusion paths, poor rate performance, and transgranular microcracks that still exist under high voltage.

[0004] Simple coating / doping modification: Usually, single element (such as Al, Zr, Mg) doping or surface coating (such as Al2O3, TiO2) can improve the interface stability to a certain extent, but cannot solve the anisotropic stress concentration problem of the bulk structure in the deep delithiation state, and cannot simultaneously take into account capacity, rate capability and lifetime.

[0005] Therefore, existing technologies for ultra-high nickel cathode materials have the following technical challenges: Bulk structure instability: The H2-H3 phase transition in the deep delithiation state causes a sharp contraction of the c-axis of the cell parameter, generating microcracks (including intergranular cracks and transgranular cracks), resulting in rapid capacity decay.

[0006] Insufficient kinetic performance: Although single crystallization eliminates grain boundaries, Li + The diffusion path is long and the rate performance is poor, which cannot meet the requirements of fast charging.

[0007] Existing technologies mostly employ single doping or simple coating, which makes it difficult to systematically solve the aforementioned coupling problems. Therefore, developing an ultra-high nickel cathode material that can simultaneously address structural stability, interface stability, and kinetic performance is of great significance. Summary of the Invention

[0008] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide an ultra-high nickel single crystal cathode material and its preparation method. The resulting cathode material has a gradient structure to alleviate phase transformation stress, directional mesopores to improve rate capability, and a three-dimensional network to stabilize the interface. It retains >85% of its capacity after 1000 cycles at 4.5V, >95% of its capacity at 5C, and has <0.5mL / g of gas generation at high temperature. The particles are free of cracks after cycling. It has high capacity, fast charging, long cycle life and high safety, and is suitable for high energy density power batteries.

[0009] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing an ultra-high nickel single crystal cathode material, comprising the following steps: Preparation of S1 gradient precursor: A three-reactor series co-precipitation system was used to control the pH, ammonia concentration, and feed rates of Ni, Co, and Mn salts in the three reactors. The pH of the three reactors was increased sequentially in the range of 10.2-11.6, the ammonia concentration was increased sequentially in the range of 0.3-0.8 mol / L, and the molar ratios of Ni, Co, and Mn were 93:5:2, 91:5:4, and 89:5:6, respectively, to prepare a hydroxyl oxide precursor with a continuous gradient distribution of Ni and Mn. S2 Dual-Element Gradient Doping: The precursor and lithium source are mixed at a Li:Me molar ratio of (1.02-1.08):1 to obtain a mixture. Using a layered spray granulation technique, a solution containing at least one of the M source and N source is sprayed onto the surface of the mixture in 3-4 layers. The M source is a tungsten or molybdenum source, and the N source is a zirconium or titanium source. The first layer is sprayed with only the M source solution, and the second to fourth layers are sprayed with both M and N source solutions. During spraying, the concentration of the M source increases and the concentration of the N source decreases. After each spraying, the mixture is dried to form a gradient doped layer. The concentration of the M element increases from the inside to the outside, and the concentration of the N element decreases from the inside to the outside, forming a "strong outside and tough inside" mechanical distribution. The shell layer has high strength and corrosion resistance, and the core layer has high toughness and crack resistance. S3 four-stage sintering: The first stage (pre-sintering) is sintered at 450-500℃ for 4-8h with an oxygen concentration of 21%; the second stage (nucleation) is sintered at 700-750℃ for 3-6h with an oxygen concentration of 50%, and 0.2-1wt.% LiF mineralizer is added to promote the preferential growth of (003) crystal faces; the third stage (crystallization) is sintered at 820-880℃ for 10-15h with an oxygen concentration ≥95%; the fourth stage (surface reconstruction) is sintered at 750-800℃ for 1-3h, and a mixture of CO and CO2 gas is introduced to induce the formation of oxygen vacancies on the surface and reconstruct it into a spinel / rock salt phase; S4 Directional Pore Formation: The sintered product is dispersed in a mixed solution containing organic acid and oxidant, pH=4-5, ultrasonically treated for 15-60 min, and selectively etched (104) crystal planes to form mesopores oriented along the

[001] direction; then filtered, washed and dried. S5 Three-Dimensional Conductive Network Construction S5-1: The material is placed in a fluidized bed reactor, with ferrocene and thiophene as catalyst precursors and methane and ethylene as carbon sources, and chemical vapor deposition (CVD) is carried out at 600-700℃ for 30-90 min. S5-2: Mix the product of step S5-1 with a nitrogen source at a mass ratio of 1:(5-15), and heat-treat at 700-800℃ under a nitrogen atmosphere for 1-3 hours to form an N-doped amorphous carbon outer layer. S5-3 (interfacial bonding): Annealing for 0.5-2 hours at 500-600℃ and oxygen partial pressure of 0.05-0.5% induces the formation of CNM and MOC covalent bonds, resulting in ultra-high nickel single crystal cathode material.

[0010] Preferably, in step S1, the reaction temperature is 50-60℃.

[0011] Preferably, in step S2, the lithium source is LiOH·H2O or Li2CO3.

[0012] Preferably, in step S3, the volume ratio of CO to CO2 in the CO and CO2 mixture is 1:(5-15).

[0013] Preferably, in step S4, the organic acid is at least one of oxalic acid and maleic acid, and the oxidant is H2O2.

[0014] Preferably, in step S4, the concentration of the organic acid is 0.2-0.3 mol / L and the concentration of the oxidant is 0.05-0.1 mol / L.

[0015] Preferably, in step S5-1, the ferrocene feed rate is 0.1-0.3 g / min, the thiophene feed rate is 0.05-0.1 mL / min, and the volume ratio of methane to ethylene is (3-5):1.

[0016] Preferably, in step S5-2, the nitrogen source is at least one of melamine and urea.

[0017] On the other hand, the present invention provides an ultra-high nickel single crystal cathode material, which is prepared by the above-mentioned preparation method of ultra-high nickel single crystal cathode material.

[0018] This invention achieves the following collaborative mechanism through multi-scale structural design: (1) Structural stability mechanism The four-layer gradient structure alleviates volumetric stress during lithiation / delithiation and suppresses microcrack formation; the high (003) crystal plane exposure reduces the contact between the highly active (104) crystal plane and the electrolyte, thus reducing interfacial side reactions; in the three-phase symbiotic interface, the spinel phase provides three-dimensional Li + The channel, the rock salt phase acts as a chemical barrier to prevent electrolyte erosion, the layered phase ensures capacity output, and the coherent interface of the three eliminates phase interface impedance.

[0019] (2) Ion diffusion mechanism Mesoporous channels oriented along the

[001] direction are Li+ It provides a short-range, fast diffusion path, solving the problem of poor rate performance caused by the large size of single-crystal materials; the three-dimensional channel of the spinel phase further enhances the Li + Diffusion coefficient.

[0020] (3) Interface stabilization mechanism The three-dimensional network not only improves electronic conductivity, but more importantly, it anchors surface oxygen atoms through CNM and MOC covalent bonds, suppressing the release of reactive oxygen species under high voltage; the nitrogen-doped carbon layer improves electrolyte wettability and promotes interfacial Li + Transport; the surface rock salt phase acts as a physical barrier, effectively inhibiting the dissolution of transition metals.

[0021] This invention systematically addresses the core defects of ultra-high nickel single-crystal cathode materials from three dimensions: bulk structure, ion conduction, and interface stability, through a multi-scale synergistic design involving gradient precursors, dual-element gradient doping, four-segment sintering, directional pore formation, and three-dimensional conductive network construction. Compared with existing technologies, it possesses the following outstanding advantages: 1. This invention employs a three-reactor series process to prepare a Ni / Mn continuous gradient precursor, combined with dual-element gradient doping to form a "strong on the outside, tough on the inside" gradient structure, effectively alleviating volumetric stress and anisotropic deformation during lithium insertion / extraction. Four-segment sintering induces preferential growth of the (003) crystal plane and surface spinel / rock salt phase reconstruction, fundamentally suppressing the generation of intergranular / transgranular microcracks. At a high cutoff voltage of 4.5V, the capacity retention after 1000 cycles at 1C is >85%, reaching a maximum of 90.1%, far superior to conventional polycrystalline and single-crystal materials.

[0022] 2. The directional pore-forming process of the present invention constructs directional mesoporous channels along the

[001] direction, significantly shortening the Li + Diffusion pathway; surface spinel phase provides three-dimensional Li + The rapid conduction channel, combined with a three-dimensional carbon-nitrogen conductive network, significantly improves ion and electron conduction efficiency. Capacity retention is >95% at 5C high rates, addressing the industry pain points of poor rate performance and inability to fast charge traditional single-crystal materials.

[0023] 3. The gradient doped layer and nitrogen-doped carbon layer of this invention form a dual interface barrier, effectively reducing the residual alkali content on the material surface; CNM and MOC covalent bonds anchor lattice oxygen, suppressing oxygen release and electrolyte decomposition under high voltage. Gas production after 7 days of storage at 85℃ is <0.5mL / g, far lower than conventional ultra-high nickel materials, and the battery thickness expansion rate is ≤3.5%, significantly improving high-temperature safety and storage performance.

[0024] 4. The cathode material of this invention has an initial charge-discharge efficiency of >91%, and can reach up to 94.2%; the interface coherent structure and three-dimensional conductive network significantly reduce the interface impedance and charge transfer impedance, achieving synergistic optimization of high capacity, high rate, long cycle life and low gas production, which fully meets the engineering application requirements of high voltage and high energy density power batteries. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1 The preparation method of the ultra-high nickel single crystal cathode material in this embodiment includes the following steps: S1 gradient precursor preparation A three-reactor co-precipitation system was used. The molar ratio of Ni:Co:Mn in the first reactor was controlled at 93:5:2, pH at 10.8, and ammonia concentration at 0.4 mol / L; the molar ratio in the second reactor was 91:5:4, pH at 11.2, and ammonia concentration at 0.6 mol / L; and the molar ratio in the third reactor was 89:5:6, pH at 11.6, and ammonia concentration at 0.8 mol / L. The reaction temperature was controlled at 50℃, and the stirring speed at 400 rpm to prepare a hydroxyl oxide precursor with a continuous gradient distribution of Ni and Mn.

[0027] S2 bi-element gradient doping The precursor and LiOH·H2O were mixed at a Li:Me molar ratio of 1.06:1 to obtain a mixture. A layered spray granulation technique was used to spray ammonium metatungstate solution and zirconium nitrate solution onto the surface of the mixture in three layers: First layer: Ammonium metatungstate solution (W content 0.05 mol%) was sprayed and dried; Second layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.1 mol%, Zr content 0.08 mol%) was sprayed and dried; Third layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.15 mol%, Zr content 0.04 mol%) was sprayed and dried, forming a gradient doped layer.

[0028] S3 Four-Stage Sintering First stage: sintering at 480℃ for 6 hours with an oxygen concentration of 21%; Second stage: sintering at 720℃ for 4 hours with an oxygen concentration of 50%, while adding 0.5wt.% LiF; Third stage: sintering at 860℃ for 12 hours with an oxygen concentration of 95%; Fourth stage: sintering at 780℃ for 2 hours, while introducing a mixture of CO and CO2 (volume ratio CO CO2 = 1:10).

[0029] S4 directional drilling The sintered product was dispersed in a mixed solution containing 0.2 mol / L oxalic acid and 0.05 mol / L H2O2 at pH 4.5, sonicated for 30 min, filtered, washed with deionized water until neutral, and vacuum dried at 120 °C.

[0030] S5 Three-Dimensional Conductive Network Construction S5-1: The material was placed in a fluidized bed reactor, using ferrocene and thiophene as catalyst precursors, and methane and ethylene as carbon sources, and chemical vapor deposition was performed at 650℃ for 45 min. The feed rate of ferrocene was 0.1 g / min, the feed rate of thiophene was 0.05 mL / min, and the volume ratio of methane to ethylene was 3:1.

[0031] S5-2: Mix the product of step S5-1 with melamine at a mass ratio of 1:10, and heat-treat at 750°C under a nitrogen atmosphere for 2 hours to form an N-doped amorphous carbon outer layer.

[0032] S5-3: Annealing at 550℃ and oxygen partial pressure of 0.1% for 1 hour induces the formation of CNM and MOC covalent bonds, resulting in ultra-high nickel single crystal cathode material.

[0033] Example 2 The preparation method of the ultra-high nickel single crystal cathode material in this embodiment includes the following steps: S1 gradient precursor preparation A three-reactor co-precipitation system was used. The molar ratio of Ni:Co:Mn in the first reactor was controlled at 93:5:2, pH at 10.8, and ammonia concentration at 0.4 mol / L; the molar ratio in the second reactor was 91:5:4, pH at 11.2, and ammonia concentration at 0.6 mol / L; and the molar ratio in the third reactor was 89:5:6, pH at 11.6, and ammonia concentration at 0.8 mol / L. The reaction temperature was controlled at 55℃, and the stirring speed at 400 rpm to prepare a hydroxyl oxide precursor with a continuous gradient distribution of Ni and Mn.

[0034] S2 bi-element gradient doping The precursor and Li₂CO₃ were mixed at a Li:Me molar ratio of 1.02:1 to obtain a mixture. A layered spray granulation technique was used to spray ammonium metatungstate solution and zirconium nitrate solution onto the surface of the mixture in three layers: First layer: Ammonium metatungstate solution (W content 0.05 mol%) was sprayed and dried; Second layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.1 mol%, Zr content 0.08 mol%) was sprayed and dried; Third layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.15 mol%, Zr content 0.04 mol%) was sprayed and dried, forming a gradient doped layer.

[0035] S3 Four-Stage Sintering First stage: Sintering at 450℃ for 8 hours with an oxygen concentration of 21%; Second stage: Sintering at 700℃ for 6 hours with an oxygen concentration of 50%, while adding 0.2wt.% LiF; Third stage: Sintering at 820℃ for 15 hours with an oxygen concentration of 95%; Fourth stage: Sintering at 750℃ for 3 hours, while introducing a mixture of CO and CO2 (volume ratio CO CO2 = 1:5).

[0036] S4 directional drilling The sintered product was dispersed in a mixed solution containing 0.25 mol / L maleic acid and 0.08 mol / L H2O2 at pH 4, sonicated for 15 min, filtered, washed with deionized water until neutral, and vacuum dried at 120 °C.

[0037] S5 Three-Dimensional Conductive Network Construction S5-1: The material was placed in a fluidized bed reactor, using ferrocene and thiophene as catalyst precursors, and methane and ethylene as carbon sources, and chemical vapor deposition was performed at 600℃ for 30 min. The feed rate of ferrocene was 0.2 g / min, the feed rate of thiophene was 0.08 mL / min, and the volume ratio of methane to ethylene was 4:1.

[0038] S5-2: Mix the product of step S5-1 with urea at a mass ratio of 1:5, and heat-treat at 700℃ under a nitrogen atmosphere for 3 hours to form an N-doped amorphous carbon outer layer.

[0039] S5-3: Annealing at 500℃ and oxygen partial pressure of 0.05% for 2 hours induces the formation of CNM and MOC covalent bonds, resulting in ultra-high nickel single crystal cathode material.

[0040] Example 3 The preparation method of the ultra-high nickel single crystal cathode material in this embodiment includes the following steps: S1 gradient precursor preparation A three-reactor co-precipitation system was used. The molar ratio of Ni:Co:Mn in the first reactor was controlled at 93:5:2, pH at 10.8, and ammonia concentration at 0.4 mol / L; the molar ratio in the second reactor was 91:5:4, pH at 11.2, and ammonia concentration at 0.6 mol / L; and the molar ratio in the third reactor was 89:5:6, pH at 11.6, and ammonia concentration at 0.8 mol / L. The reaction temperature was controlled at 60℃, and the stirring speed at 400 rpm to prepare a hydroxyl oxide precursor with a continuous gradient distribution of Ni and Mn.

[0041] S2 bi-element gradient doping The precursor and LiOH·H2O were mixed at a Li:Me molar ratio of 1.08:1 to obtain a mixture. A layered spray granulation technique was used to spray ammonium metatungstate solution and zirconium nitrate solution onto the surface of the mixture in three layers: First layer: Ammonium metatungstate solution (W content 0.05 mol%) was sprayed and dried; Second layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.1 mol%, Zr content 0.08 mol%) was sprayed and dried; Third layer: Ammonium metatungstate + zirconium nitrate solution (W content 0.15 mol%, Zr content 0.04 mol%) was sprayed and dried, forming a gradient doped layer.

[0042] S3 Four-Stage Sintering First stage: Sintering at 500℃ for 4 hours with an oxygen concentration of 21%; Second stage: Sintering at 750℃ for 3 hours with an oxygen concentration of 50%, while adding 1 wt.% LiF; Third stage: Sintering at 880℃ for 10 hours with an oxygen concentration of 95%; Fourth stage: Sintering at 800℃ for 1 hour, while introducing a mixture of CO and CO2 (volume ratio CO CO2 = 1:15).

[0043] S4 directional drilling The sintered product was dispersed in a mixed solution containing 0.3 mol / L oxalic acid and 0.1 mol / L H2O2 at pH 5, sonicated for 60 min, filtered, washed with deionized water until neutral, and vacuum dried at 120 °C.

[0044] S5 Three-Dimensional Conductive Network Construction S5-1: The material was placed in a fluidized bed reactor, using ferrocene and thiophene as catalyst precursors, and methane and ethylene as carbon sources, and chemical vapor deposition was performed at 700℃ for 30 min. The feed rate of ferrocene was 0.3 g / min, the feed rate of thiophene was 0.1 mL / min, and the volume ratio of methane to ethylene was 5:1.

[0045] S5-2: Mix the product of step S5-1 with melamine at a mass ratio of 1:15, and heat-treat at 800℃ under a nitrogen atmosphere for 1 hour to form an N-doped amorphous carbon outer layer.

[0046] S5-3: Annealing at 600℃ and oxygen partial pressure of 0.5% for 2 hours induces the formation of CNM and MOC covalent bonds, resulting in ultra-high nickel single crystal cathode material.

[0047] Comparative Example 1 Ni was prepared by conventional co-precipitation. 0.9 Co 0.05 Mn 0.05 For the polycrystalline precursor, 1000 g of the precursor and LiOH·H2O were weighed out at a molar ratio of Li:Me = 1.05, and 0.2 mol% ZrO2 (relative to the total metal content) was added. The three components were placed in a high-speed mixer and mixed at 1000 rpm for 30 min to obtain a homogeneous mixture. The mixture was then placed in an alumina crucible and placed in a tube furnace. Under an oxygen atmosphere, the temperature was increased to 780 °C at 5 °C / min, and sintered for 15 h, followed by natural cooling to room temperature. The sintered product was crushed and passed through a 200-mesh sieve to obtain the conventional polycrystalline ultra-high nickel cathode material.

[0048] Comparative Example 2 Ni was prepared by conventional co-precipitation. 0.9 Co 0.05 Mn 0.05 A polycrystalline precursor was prepared by weighing 1000 g of precursor and LiOH·H2O at a molar ratio of Li:Me = 1.08. The two were placed in a high-speed mixer and mixed at 1000 rpm for 30 min to obtain a homogeneous mixture. The mixture was then placed in an alumina crucible and placed in a tube furnace. Under an oxygen atmosphere, the temperature was increased to 920 °C at 5 °C / min, and sintered for 15 h, followed by natural cooling to room temperature. The sintered product was crushed and passed through a 200-mesh sieve to obtain irregular blocky single-crystal particles. 100 g of the above single-crystal material was dispersed in 200 mL of anhydrous ethanol and ultrasonically dispersed for 30 min to obtain a suspension. Aluminum isopropoxide (Al2O3 coating weight 1 wt.%) was dissolved in 50 mL of anhydrous ethanol and added dropwise to the above suspension while stirring. After the addition was complete, the mixture was heated to 80 °C and stirred until dry. The dried product was then heat-treated at 450 °C in air for 2 h. After natural cooling and passing through a 200-mesh sieve, Al2O3-coated conventional single-crystal material LiNi was obtained. 0.9 Co 0.05 Mn 0.05 O2@Al2O3.

[0049] Comparative Example 3 The difference from Example 1 is that in step S3, the fourth stage of sintering is not performed; and steps S4 and S5 are not performed.

[0050] Comparative Example 4 Ni was prepared by conventional co-precipitation. 0.9 Co 0.05 Mn 0.05 A polycrystalline precursor was prepared by weighing 1000g of precursor and LiOH·H2O at a molar ratio of Li:Me = 1.08. The two were placed in a high-speed mixer and mixed at 1000rpm for 30min to obtain a homogeneous mixture. The mixture was then placed in an alumina crucible and placed in a tube furnace. Under an oxygen atmosphere, the temperature was increased to 920℃ at 5℃ / min and sintered for 15h, followed by natural cooling to room temperature. The sintered product was crushed and passed through a 200-mesh sieve to obtain a single-crystal material. Subsequently, the single-crystal material was subjected to the directional pore formation process in step S4 of Example 1 and the three-dimensional conductive network construction process in step S5.

[0051] Comparative Example 5 The difference from Example 1 is that in step S2, when spraying, all three layers are sprayed with ammonium metatungstate + zirconium nitrate solution (W content is 0.1 mol%, Zr content is 0.08 mol%).

[0052] Electrochemical performance testing The cathode materials of Examples 1-3 and Comparative Examples 1-5 were assembled into coin cells: cathode (96% active material, 2% PVDF, 2% SP), lithium cathode, electrolyte 1M LiPF6, solvent EC / DMC / EMC (volume ratio 1:1:1) + 2% VC.

[0053] Voltage range: 2.8-4.5V. Rate testing: After activation at 0.33C, tests are performed at 0.33C, 1C, 3C, 5C, and 8C rates. Cyclic testing: 1C charge / discharge, test temperature 25℃.

[0054] The test results are shown in Table 1: Table 1. Electrochemical performance test results of the assembled batteries in Examples 1-3 and Comparative Examples 1-5 As shown in Table 1, Comparative Example 1, a traditional polycrystalline ultra-high nickel cathode material, exhibits severe side reactions and significant irreversible capacity loss during the first charge-discharge cycle due to its numerous grain boundaries and large specific surface area, resulting in extremely low initial efficiency. High grain boundary impedance restricts electron / ion conduction, leading to severe polarization under high current and extremely poor 5C rate retention. Numerous intergranular microcracks are generated during cycling, causing continuous electrolyte erosion and rapid structural collapse, resulting in a 500-cycle retention of only 68.5% and failure to complete 1000 cycles.

[0055] Comparative Example 2 used conventional single crystals coated with Al2O3. High-temperature sintering led to increased lattice defects, and the Al2O3 coating reduced the proportion of active material, resulting in low capacity. High residual alkali on the single crystal surface and significant interfacial side reactions resulted in mediocre first-efficiency performance. Large-size single crystals lacked directional channels, and Li...+ The diffusion path is long, resulting in weak fast charging capability and poor rate performance. Deep delithiation H2-H3 phase transition induces transgranular cracks, and single coating cannot suppress structural instability, with a retention rate of only 60.2% after 1000 cycles.

[0056] Comparative Example 3 only has a gradient structure, without oriented mesoporous and spinel phases, Li + Diffusion remains limited, resulting in a significantly lower rate capability compared to the previous example. Without surface reconstruction and a carbon-nitrogen network, high interfacial impedance and unsuppressed oxygen release lead to a substantial decrease in 500 / 1000 cycle retention.

[0057] Although Comparative Example 4 has a conductive network, it lacks gradients and directional holes, limiting bulk conduction. While its scaling factor is better, it is still lower than that of the Example. Without gradient internal stress buffering, cracks still appear during deep cycling, indicating structural instability and significant cycle decay.

[0058] In Comparative Example 5, uniform doping failed to create a "strong on the outside, tough on the inside" distribution, resulting in insufficient shell protection and mediocre rate performance. It could not effectively suppress phase transformation stress and cracks, exhibited insufficient interface stability, and showed significantly worse cycling performance than the examples.

[0059] High-temperature storage gas generation test Test conditions: 100% SOC, stored at 85℃ for 7 days, gas production was measured using the water displacement method. Test results are shown in Table 2. Table 2. Gas generation test results during high-temperature storage As shown in Table 2, Comparative Example 1 exhibits numerous grain boundaries, a large specific surface area, and a high residual lithium content on the surface, leading to rapid gas generation through rapid reaction with the electrolyte and an extremely high gas production rate. During cycling and high-temperature storage, numerous intergranular cracks are generated, electrolyte continuously invades, side reactions become uncontrolled, and the thickness expansion rate is large. The interface is severely damaged, the structure is pulverized, the active material is deactivated, and the capacity recovery rate is low.

[0060] Comparative Example 2 showed high residual alkali on the single crystal surface, and the single Al2O3 coating was not dense or firm, failing to completely isolate the electrolyte, resulting in high gas production. At high temperatures, the coating layer easily detached, transgranular cracks appeared, electrolyte penetration intensified, and thickness expansion was significant. Interface stability was insufficient, oxygen release and side reactions remained severe, and the capacity recovery rate was generally poor.

[0061] Comparative Example 3, lacking the fourth-stage sintering surface reconstruction and carbon coating, exhibits highly active crystal faces directly contacting the electrolyte, resulting in relatively high gas production. However, the absence of directional pore formation and a three-dimensional conductive network leads to insufficient stress release and electron conduction, resulting in localized cracking and excessive thickness expansion. While the bulk phase is relatively stable, insufficient interface protection leads to uncontrollable high-temperature side reactions and a generally low capacity recovery rate.

[0062] Although Comparative Example 4 has a carbon network, it lacks gradient internal stress buffering, making its structure prone to distortion at high temperatures. It exhibits high oxygen activity and gas production is still higher than in the Example. Without gradient doping and a surface spinel / rock salt phase barrier, electrolyte erosion is exacerbated, and thickness expansion is greater than in the Example. The bulk structure is unstable, and lattice distortion is significant after high-temperature storage, resulting in a lower capacity recovery rate than in the Example.

[0063] Comparative Example 5, with its uniform doping, failed to form a "dense on the outside, tough on the inside" shell-core structure, exhibited weak surface oxygen suppression and significantly higher gas production than the Example. Without gradient stress buffering, it showed a significant high-temperature volume effect, insufficient interface stability, and greater thickness expansion than the Example. At high temperatures, its structural integrity and interface stability were weaker than the gradient-doped system, and its capacity recovery rate was lower than that of the Example.

[0064] Loop structure representation After 500 cycles, the battery was disassembled for TEM analysis, and the results are shown in Table 3: Table 3. TEM analysis results after 500 battery cycles As can be seen from Table 3, Comparative Example 1 has severe intergranular cracks. This is due to the inherent defects of the polycrystalline structure: there are a large number of grain boundaries between primary particles, and stress is concentrated at the grain boundaries during cycling, directly generating through cracks; there is no stress buffer design: there is no precursor gradient and no doping gradient, which cannot alleviate the volume deformation caused by the H2-H3 phase transition; there is no interface strengthening: there is no coating, no conductive network, and no phase reconstruction. Once cracks are generated, they propagate rapidly and the structure is completely destroyed.

[0065] Comparative Example 2 exhibits transgranular microcracks because the large single-crystal particles lack stress buffering: the coarse single-crystal particles grown at high temperatures cannot release anisotropic stress during deep delithiation; there is no gradient structure: the bulk composition is uniform, the phase transformation stress is concentrated, and transgranular cracks are directly generated; single coating is ineffective: Al2O3 only has a physical coating on the surface, which cannot stabilize the lattice, cannot suppress phase transformation, and cannot prevent crack generation and propagation.

[0066] Comparative Example 3 exhibits localized microcracks because it has a bulk gradient but lacks surface strengthening: the precursor gradient can alleviate some internal stress, but it lacks surface reconstruction and spinel / rock salt phase protection; there is no directional pore formation: there is a lack of stress release channels, and localized stress concentration still occurs; there is no three-dimensional conductive network: there is no covalent bond anchoring and interface support, and localized microcracks still appear after cycling.

[0067] Comparative Example 4 exhibits significant transgranular cracks because of the lack of a bulk phase gradient: the homogeneous composition of the single crystal prevents the release of phase transformation internal stress, which is the core reason for crack formation; the conductive network only acts on the surface: the CVD carbon layer can only improve conductivity, but cannot change the bulk stress distribution, and cannot prevent lattice cracking; and there is no surface phase reconstruction: lacking the mechanical support of the rock salt / spinel phase, the cracks directly penetrate the particles.

[0068] Comparative Example 5 exhibits slight localized cracks due to the lack of gradient stress distribution: uniform doping cannot form a "strong outside and tough inside" structure, resulting in insufficient mechanical strength of the outer shell; it cannot suppress localized stress concentration: under deep cycling, slight deformation and cracks still appear at the particle edges and crystal planes; and it lacks sufficient interface protection: the absence of a gradient shell to block the electrolyte exacerbates interface erosion and localized cracks.

Claims

1. A method for preparing an ultra-high nickel single-crystal cathode material, characterized in that, Includes the following steps: Preparation of S1 gradient precursor: A three-reactor series co-precipitation system was used to control the pH, ammonia concentration, and feed rates of Ni, Co, and Mn salts in the three reactors. The pH of the three reactors was increased sequentially in the range of 10.2-11.6, the ammonia concentration was increased sequentially in the range of 0.3-0.8 mol / L, and the molar ratios of Ni, Co, and Mn were 93:5:2, 91:5:4, and 89:5:6, respectively, to prepare a hydroxyl oxide precursor with a continuous gradient distribution of Ni and Mn. S2 Dual-Element Gradient Doping: The precursor and lithium source are mixed at a Li:Me molar ratio of (1.02-1.08):1 to obtain a mixture. Using a layered spray granulation technique, a solution containing at least one of the M source and N source is sprayed onto the surface of the mixture in 3-4 layers, wherein the M source is a tungsten or molybdenum source, and the N source is a zirconium or titanium source. The first layer is sprayed with only the M source solution, and the second to fourth layers are sprayed with both the M source and N source solutions, with the concentration of the M source increasing and the concentration of the N source decreasing during spraying. After each spraying, the mixture is dried to form a gradient doped layer. S3 four-stage sintering: The first stage is sintered at 450-500℃ for 4-8 hours with an oxygen concentration of 21%; the second stage is sintered at 700-750℃ for 3-6 hours with an oxygen concentration of 50%, while adding 0.2-1wt.% LiF mineralizer; the third stage is sintered at 820-880℃ for 10-15 hours with an oxygen concentration ≥95%; the fourth stage is sintered at 750-800℃ for 1-3 hours, while introducing a mixture of CO and CO2 gas; S4 directional pore formation: The sintered product is dispersed in a mixed solution containing organic acid and oxidant, pH=4-5, ultrasonically treated for 15-60 min, filtered, washed and dried; S5 Three-Dimensional Conductive Network Construction S5-1: The material is placed in a fluidized bed reactor, with ferrocene and thiophene as catalyst precursors and methane and ethylene as carbon sources, and chemical vapor deposition is carried out at 600-700℃ for 30-90 min. S5-2: Mix the product of step S5-1 with a nitrogen source at a mass ratio of 1:(5-15) and heat-treat at 700-800℃ under a nitrogen atmosphere for 1-3 hours. S5-3: Annealing at 500-600℃ and oxygen partial pressure of 0.05-0.5% for 0.5-2h yields ultra-high nickel single crystal cathode material.

2. The method for preparing ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S1, the reaction temperature is 50-60℃.

3. The method for preparing the ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S2, the lithium source is LiOH·H2O or Li2CO3.

4. The method for preparing the ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S3, the volume ratio of CO to CO2 in the CO and CO2 mixture is 1:(5-15).

5. The method for preparing ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S4, the organic acid is at least one of oxalic acid and maleic acid, and the oxidant is H2O2.

6. The method for preparing the ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S4, the concentration of the organic acid is 0.2-0.3 mol / L, and the concentration of the oxidant is 0.05-0.1 mol / L.

7. The method for preparing ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S5-1, the ferrocene feed rate is 0.1-0.3 g / min, the thiophene feed rate is 0.05-0.1 mL / min, and the volume ratio of methane to ethylene is (3-5):

1.

8. The method for preparing ultra-high nickel single-crystal cathode material as described in claim 1, characterized in that, In step S5-2, the nitrogen source is at least one of melamine and urea.

9. A high-nickel single-crystal cathode material, characterized in that, It is prepared by the preparation method of ultra-high nickel single crystal cathode material as described in any one of claims 1-8.

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