High-rate long-cycle positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a high-rate long-cycle positive electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The preparation method provided by the invention comprises the following steps: adjusting a mixed solution of nickel salt, cobalt salt and manganese salt to be alkaline, carrying out primary precipitation, sequentially carrying out at least two precipitation steps after the primary precipitation is completed, and adding a solution containing Ni, Co, Mn and modified elements each time; a precursor is obtained; mixing, sintering and crushing the precursor material and a lithium salt to obtain an intermediate material; and mixing the intermediate material with a single-walled carbon nanotube to prepare mixed slurry, and then performing spray drying to obtain the composite material. The capacity retention ratio of a lithium ion battery assembled by the prepared high-rate long-cycle positive electrode material is still higher than 90% after 500 cycles of discharge at the high rate of 5C, the DCIR growth rate is lower than 80%, and the lithium ion battery has excellent high-rate long-cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a high-rate long-cycle cathode material, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] The information disclosed in the background art of the present invention is only intended to enhance the overall understanding of the present invention and is not necessarily regarded as an admission or an indication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Since the commercial application of lithium-ion batteries in 1990, they have occupied a core position in the fields of consumer electronics, electric vehicles, etc. due to advantages such as high voltage, high energy density, and no memory effect. With the continuous expansion of application scenarios, the market has put forward higher requirements for the energy density, power density, and cycle life of lithium-ion batteries.

[0004] As a key component of lithium-ion batteries, the performance of the cathode material directly determines the energy density, power output, and cycle stability of the battery. Among them, ternary cathode materials have become the current mainstream choice due to their high theoretical capacity and relatively reasonable cost. However, ternary materials face two core challenges in practical applications: Firstly, the material itself has a relatively high impedance (including electronic impedance and ionic impedance), especially during high-rate charge and discharge, the impedance increases significantly, severely limiting the power performance of the battery; Secondly, ternary materials show strong oxidizing properties at high voltages, easily reacting with the electrolyte to cause surface structure damage and metal ion dissolution, which in turn leads to continuous impedance growth during the cycle, ultimately resulting in rapid capacity decay of the battery and shortened cycle life.

[0005] To improve the performance of ternary materials, the prior art mainly reduces impedance and improves stability through two directions: particle structure design and surface modification coating. For example, optimizing the internal lithium-ion migration channels through core-shell structures or gradient element distributions can reduce the ionic transport resistance to a certain extent; while using carbon-based materials (such as graphene) for surface coating can improve electronic conductivity and reduce surface side reactions, but traditional two-dimensional sheet-like carbon materials (such as graphene) are prone to stacking on the particle surface, hindering lithium-ion diffusion and instead increasing ionic impedance. In addition, some structure designs or coating processes cannot synergistically optimize the electronic and ionic transport paths, resulting in limited overall impedance reduction effect. Especially under high-rate cycling conditions, the impedance increases significantly, making it difficult to meet the requirements of long-life high-power scenarios. Therefore, developing a ternary cathode material that can simultaneously reduce electronic impedance and ionic impedance and inhibit impedance growth during the cycle to improve its high-rate cycling performance has become an urgent technical problem in this field. Summary of the Invention

[0006] In view of this, the present invention provides a high-rate long-cycle cathode material, a preparation method thereof, and a lithium-ion battery. The high-rate long-cycle cathode material provided by the present invention realizes the dual reduction of electron impedance and ion impedance through gradient core-shell modification and single-walled carbon nanotube coating, can significantly inhibit the impedance growth during the cycle, and has good high-rate long-cycle performance at the same time.

[0007] In the first aspect, the present invention provides a preparation method of a high-rate long-cycle cathode material, including the following steps: Adjust the mixed solution of nickel salt, cobalt salt, and manganese salt to be alkaline for the first precipitation. After the first precipitation is completed, at least two precipitation steps are carried out in sequence, and a solution containing Ni, Co, Mn, and a modification element is added each time; a precursor is obtained. Mix and sinter the precursor material with a lithium salt, and then crush it to obtain an intermediate material. Mix the intermediate material with single-walled carbon nanotubes to prepare a mixed slurry, and then carry out spray drying to obtain the product.

[0008] Preferably, in the mixed solution of nickel salt, cobalt salt, and manganese salt, the molar ratio of Ni, Co, and Mn is (6-9.5):(0.3-3):(0.3-3); in the solution containing Ni, Co, Mn, and a modification element, the total molar amount of Ni, Co, and Mn to the molar amount of the modification element is 1:(0.2-1.2).

[0009] Furthermore, each modification element added each time is independently selected from one or more of Ag, Cu, W, Sr, Zr, Co, Ti, Mg, Al, or F.

[0010] Furthermore, after the first precipitation is completed, a solution containing Ni, Co, Mn, and a first modification element is added for the second precipitation; the total molar amount of Ni, Co, and Mn to the molar amount of the first modification element is 1:(0.3-0.7); then a solution containing Ni, Co, Mn, and a second modification element is added for precipitation, and the total molar amount of Ni, Co, and Mn to the molar amount of the second modification element is 1:(0.8-1.2) to obtain a precursor; wherein, the first modification element is Co, and the second modification element is Ti, Zr, and Co; among the second modification elements, the molar ratio of Ti, Zr, and Co is (0.003-0.007):(0.003-0.007):(8-12).

[0011] Preferably, in the step of mixing and sintering the precursor material with a lithium salt, the total molar amount of Ni, Co, and Mn in the precursor material to the molar amount of the Li element of the lithium salt is 1:(1.02-1.08); the D50 particle size of the intermediate material is 2-10 μm.

[0012] Preferably, the mass fraction of the single-walled carbon nanotubes in the total mass of the single-walled carbon nanotubes and the intermediate material is 0.01-0.5 wt%.

[0013] Preferably, in the mixed slurry, the solvent is one or more of N-methylpyrrolidone, acetone, N,N-dimethylformamide or dimethyl sulfoxide; the solid content of the mixed slurry is 60-80 wt%.

[0014] Preferably, the inlet temperature of the spray drying is 180-220 °C, and the outlet temperature is 50-80 °C.

[0015] In a second aspect, the present invention provides a high-rate long-cycle cathode material prepared by the above preparation method.

[0016] In a third aspect, the present invention provides a lithium-ion battery comprising the above high-rate long-cycle cathode material.

[0017] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) In the present invention, a gradient-distributed modified element core-shell structure is formed through at least two precipitation steps, and combined with the surface coating process of single-walled carbon nanotubes (SWCNT), the non-uniform gradient distribution of the modified elements inside the ternary particles is realized, optimizing the Li + migration channels inside the particles and reducing the migration impedance; single-walled carbon nanotubes (SWCNT) as the coating material, its excellent electronic conductivity can greatly reduce the electronic impedance of the ternary material, and at the same time, the network structure does not affect Li + diffusion, avoiding the problem of increased ionic impedance caused by the sheet structure of traditional graphene coatings, etc.; in addition, the SWCNT coating layer can also effectively protect the surface of the ternary cathode, reduce side reactions with the electrolyte and metal ion dissolution, and inhibit the continuous growth of impedance during the cycle, thus significantly improving the cycle life of the material.

[0018] (2) The lithium-ion battery assembled with the high-rate long-cycle cathode material prepared by the present invention still has a capacity retention rate higher than 90% after discharging 500 cycles at a high rate of 5C, and the DCR growth rate is lower than 80%. It has excellent high-rate long-cycle performance and can effectively inhibit the capacity decay caused by the increase of electron / ion impedance and the aggravation of surface side reactions during high-rate discharging, thus meeting the continuous performance requirements of scenarios such as electric vehicles and electric tools that require high-power output. Description of the Drawings

[0019] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments and descriptions thereof of the invention are used to explain the invention and shall not unduly limit the invention. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] Figure 1 is a schematic structural diagram of the high-rate long-cycle cathode material of Embodiment 1 of the present invention; Figure 2 is a scanning electron microscope image of the high-rate long-cycle cathode material of Embodiment 1 of the present invention. Detailed Embodiments

[0021] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0022] The present invention provides a method for preparing a high-rate long-cycle cathode material, comprising the following steps: Adjust the mixed solution of nickel salt, cobalt salt and manganese salt to be alkaline for the first precipitation. After the first precipitation is completed, at least two precipitation steps are sequentially carried out, and a solution containing Ni, Co, Mn and a modifying element is added each time; a precursor is obtained; Mix and sinter and crush the precursor material with a lithium salt to obtain an intermediate material; Mix the intermediate material with single-walled carbon nanotubes to prepare a mixed slurry, and perform spray drying after mixing to obtain the product.

[0023] The present invention constructs a gradient-modified core-shell structure precursor through multi-stage precipitation, and combines the surface coating process of single-walled carbon nanotubes (SWCNT) to synergistically optimize the internal ion migration and surface electron transport properties of the ternary cathode material, thereby achieving long cycle stability at high rates. Specifically, in the preparation process of the high-rate long-cycle cathode material of the present invention, first, based on a mixed solution of nickel, cobalt, and manganese salts, an initial particle core structure is formed through the first alkaline precipitation; subsequently, at least two precipitation steps are carried out in sequence, and a solution containing Ni, Co, Mn, and a modifying element is added each time. The key to this design lies in controlling the distribution of the modifying element inside the particle through multi-stage precipitation - from the inner core to the outer layer, the type or concentration of the modifying element gradually changes, forming a core-shell structure with a gradient distribution. This gradient design can specifically optimize the lithium-ion migration channels in different regions inside the particle: in the inner core region, the crystal structure is stabilized by the modifying element, reducing the phase change during the cycle; in the outer layer region, the interfacial energy barrier for lithium-ion deintercalation is reduced by the high-concentration modifying element, while suppressing the surface oxidation side reaction at high voltages, thereby overall reducing the internal migration impedance. After the preparation of the precursor is completed, during the process of sintering with the lithium salt to form the ternary cathode material, the lithium source fully reacts with the metal elements in the precursor to generate an active substance with a layered structure (such as LiNi x Co y Mn z O2), and the crushing step ensures that the material particles are of uniform size, providing a good dispersion basis for subsequent coating. Finally, the intermediate material is mixed with SWCNT to prepare a mixed slurry and spray-dried to construct a uniform carbon coating layer on the material surface.

[0024] As a one-dimensional nanomaterial, the high aspect ratio of SWCNT enables it to interweave and form a network conductive network on the particle surface. This structure can significantly reduce the surface electron impedance through the high electron conductivity of SWCNT, and at the same time avoid the problem of lithium-ion diffusion hindrance caused by the stacking of traditional two-dimensional sheet-like carbon materials (such as graphene), thus balancing the electron and ion transport efficiencies. Therefore, the present invention combines the gradient-modified core-shell structure by multi-stage precipitation with the low-temperature network coating process of SWCNT, and synergistically reduces the material impedance from two dimensions of "optimizing internal ion migration" and "enhancing surface electron transport", and finally achieves a long cycle performance with a capacity retention rate higher than 90% after 500 cycles under high-rate (such as 5C) discharge conditions.

[0025] In the mixed solution of nickel salt, cobalt salt, and manganese salt described in the present invention, the molar ratio of Ni, Co, and Mn is (6~9.5) : (0.3~3) : (0.3~3); Ni is the main electroactive element, providing Li +The deintercalation site, and a high nickel content is beneficial to improving the theoretical capacity of the material. In the solution containing Ni, Co, Mn and the modifying element, the ratio of the total molar amount of Ni, Co and Mn to the molar amount of the modifying element is 1: (0.2 to 1.2). By controlling the doping amount of the modifying element, this ratio ensures that it can effectively optimize the material properties (such as suppressing side reactions on the surface, improving ion migration, etc.) without damaging the layered structure of the main ternary material (excessive modifying elements may cause lattice distortion and reduce the number of electroactive sites).

[0026] The present invention places no special restrictions on the selection of nickel salts, cobalt salts and manganese salts, and common nickel salts, cobalt salts and manganese salts in the coprecipitation method in the art can be used.

[0027] In the present invention, each time the modifying element added is independently selected from one or more of Ag, Cu, W, Sr, Zr, Co, Ti, Mg, Al or F. Different modifying elements have different property modification effects. For example, Ag and Cu can improve local electron conductivity, and the addition of F can reduce the surface reaction activity by forming a stable LiF interface layer, and so on. Those skilled in the art can select specific modifying elements according to different property requirements. The modifying element is usually added in the form of a soluble salt, and the present invention places no special restrictions on this.

[0028] In one or more embodiments of the present invention, after the first precipitation is completed, a solution containing Ni, Co, Mn and the first modifying element is added for the second precipitation; the ratio of the total molar amount of Ni, Co and Mn to the molar amount of the first modifying element is 1: (0.3 to 0.7); then a solution containing Ni, Co, Mn and the second modifying element is added for precipitation, and the ratio of the total molar amount of Ni, Co and Mn to the molar amount of the second modifying element is 1: (0.8 to 1.2) to obtain a precursor; wherein, the first modifying element is Co, and the second modifying element is Ti, Zr and Co; among the second modifying elements, the molar ratio of Ti, Zr and Co is (0.003 to 0.007):(0.003 to 0.007):(8 to 12). In the above steps, by enriching Co elements on the outer layer of the particles, the high electron conductivity of Co is used to enhance the surface electron transport ability. At the same time, the octahedral coordination stability of Co can inhibit the collapse of the surface layered structure during the cycling process, and the gradient distribution of Co between the inner layer and the outer layer forms a continuous electron transport channel; and the high ionic radius of Ti and Zr is used to form a "physical barrier" on the outermost layer of the particles to hinder the direct contact between the electrolyte and the active material and reduce the dissolution of metal ions.

[0029] In the present invention, in the step of mixing and sintering the precursor material with a lithium salt, the ratio of the total molar amount of Ni, Co, and Mn in the precursor material to the molar amount of Li element in the lithium salt is 1: (1.02 - 1.08), more preferably 1: (1.03 - 1.05), and most preferably 1:1.04. During the sintering process, Li will partially volatilize, and an appropriate excess of Li can ensure the formation of a complete layered structure. It should be noted that the total molar amount of Ni, Co, and Mn here does not include the molar amount of the modifying elements. The present invention does not impose special restrictions on the type of lithium salt, and common lithium salt types in the art can be used, such as lithium hydroxide, lithium nitrate, lithium carbonate, etc. The sintering process of the present invention is carried out under oxygen-rich conditions. The present invention does not impose special restrictions on the specific sintering and crushing processes, and the sintering and crushing methods in the preparation process of common ternary cathode materials in the art can be used.

[0030] In the present invention, the D50 particle size of the intermediate material is 2 - 10 μm, more preferably 4 - 6 μm. This particle size range optimizes the performance by balancing the specific surface area and the ion diffusion path length: a smaller particle size shortens the diffusion distance of Li + inside the particles, which is beneficial to the rapid deintercalation and intercalation of Li + at high rates; at the same time, it avoids too large a specific surface area caused by too small a particle size (which will increase the side reactions with the electrolyte) and a decrease in the tap density (which will affect the energy density), and also avoids an increase in the ion diffusion resistance caused by too large a particle size (capacity decay will occur at high rates).

[0031] In the present invention, the mass fraction of the single-walled carbon nanotubes in the total mass of the single-walled carbon nanotubes and the intermediate material is 0.01 - 0.5 wt%, more preferably 0.03 - 0.2 wt%, and can be, for example, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.08 wt%, 0.1 wt%, 0.12 wt%, 0.15 wt%. The one-dimensional high aspect ratio characteristic of SWCNT enables it to form a continuous network conductive network on the particle surface at a relatively low addition amount, reducing the electron impedance, and at the same time, it also avoids the ion diffusion hindrance and cost increase caused by agglomeration at too high an addition amount.

[0032] In the mixed slurry of the present invention, the solvent is one or more of N-methylpyrrolidone, acetone, N,N-dimethylformamide, or dimethyl sulfoxide. The above solvents are beneficial to the uniform mixing of the single-walled carbon nanotubes and the intermediate material. The present invention does not impose special restrictions on the preparation process of the mixed slurry. The present invention preferably uses high-pressure homogenization for preparation. The solid content of the mixed slurry is preferably 60 - 80 wt%, more preferably 65 - 75 wt%.

[0033] In the present invention, the inlet temperature of the spray drying is 180 - 220 °C, and the outlet temperature is 50 - 80 °C. The relatively low spray drying temperature avoids the oxidation of SWCNT caused by sudden heat (oxidation will lead to a decrease in conductivity, thereby reducing the coating effect), and finally forms a uniform and stable SWCNT coating layer.

[0034] The present invention also provides a high-rate long-cycle cathode material prepared by the above preparation method.

[0035] The present invention provides a lithium-ion battery including the above high-rate long-cycle cathode material.

[0036] The lithium-ion battery provided by the present invention includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes the above high-rate long-cycle cathode material. The present invention does not impose any special restrictions on the preparation methods of the positive electrode and the negative electrode, nor on the types of the separator and the electrolyte. Those skilled in the art can make selections according to actual needs. The present invention also does not impose any special restrictions on the assembly method of the lithium-ion battery. The lithium-ion battery can be assembled by using the commonly used preparation methods in the art.

[0037] The lithium-ion battery assembled with the high-rate long-cycle cathode material of the present invention still has a capacity retention rate higher than 90% after discharging 500 cycles at a high rate of 5C, and the DCR growth rate is lower than 80%. It has excellent high-rate long-cycle performance and can effectively inhibit the capacity decay caused by the increase in electron / ion impedance and the aggravation of surface side reactions during high-rate discharging, thereby meeting the continuous performance requirements of scenarios such as electric vehicles and power tools that require high-power output.

[0038] The technical solution of the present invention will be further elaborated below in conjunction with specific embodiments. The present invention does not impose any special restrictions on the sources of the reagents used in the following embodiments. Commercially available products well-known to those skilled in the art can be used.

[0039] Example 1 This example provides a preparation method of a high-rate long-cycle cathode material and the obtained high-rate long-cycle cathode material.

[0040] (1) Solution preparation: Solution A: Nickel sulfate, cobalt sulfate, and manganese sulfate are prepared into an aqueous solution with a total molar concentration of 2 mol / L according to the molar ratio of Ni∶Co∶Mn = 90∶5∶5, which is Solution A.

[0041] Solution B: Take an appropriate amount of Solution A, add cobalt sulfate with Co as the first modification element, and control the molar amount of the modification element Co to the total molar amount of Ni, Co, and Mn in Solution A to be 5:10 to obtain Solution B.

[0042] Solution C: Take an appropriate amount of Solution A, add cobalt sulfate, titanyl sulfate, and zirconium sulfate with Co, Ti, and Zr as the second modification elements, and control the molar ratio of the modification elements Ti, Zr, and Co to the total molar amount of Ni, Co, and Mn in Solution A to be 0.005:0.005:10:10 to obtain Solution C.

[0043] (2) Coprecipitation reaction: Take a part of Solution A, adjust the pH to about 12 with an aqueous NaOH solution and ammonia water for precipitation. After the precipitation is completed, add Solution B thereto, adjust the pH to about 12 with an aqueous NaOH solution and ammonia water for precipitation. After the precipitation is completed, add Solution C thereto and adjust the pH to about 12 with an aqueous NaOH solution and ammonia water for precipitation. After the precipitation is completed, wash and dry to obtain a precursor.

[0044] (3) Sintering: Mix the precursor with lithium hydroxide, and control the molar ratio of the total molar amount of Ni, Co, and Mn (excluding modification elements) in the precursor to the molar amount of Li in lithium hydroxide to be 1:1.04; heat up to 720 °C in an oxygen atmosphere and sinter for 11 h, and obtain an intermediate material after crushing, and its D50 particle size is 4.5 μm.

[0045] (4) Surface coating: Mix the intermediate material in step (3) with single-walled carbon nanotubes in N-methylpyrrolidone, and prepare a mixed slurry by high-pressure homogenization. Among them, control the mass of the single-walled carbon nanotubes to be 0.05 wt% of the total mass of the intermediate material and the single-walled carbon nanotubes, and control the solid content of the mixed slurry to be 72 wt%. Then spray-dry the mixed slurry, with the inlet temperature being 200 °C and the outlet temperature being 60 °C. After the spray drying is completed, a high-rate long-cycle cathode material with surface carbon coating is obtained, and its material structure schematic diagram is as Figure 1 shown, and the material scanning electron microscope picture is as Figure 2 shown. It can be seen that a coating layer appears on the surface of the ternary material.

[0046] Example 2 Compared with Example 1, the difference in this example is that in the sintering process of step (3), an intermediate material with a D50 particle size of 5.6 μm is obtained after crushing.

[0047] Example 3 Compared with Example 1, the difference in this example is that in the surface coating process of step (4), the mass of the single-walled carbon nanotubes is controlled to be 0.1 wt% of the total mass of the intermediate material and the single-walled carbon nanotubes.

[0048] Comparative Example 1 Compared with Example 1, the difference in this comparative example is that gradient precipitation is not carried out. The specific steps are as follows: (1)Co-precipitation: Nickel sulfate, cobalt sulfate and manganese sulfate were prepared into an aqueous solution with a total molar concentration of 2 mol / L according to the molar ratio of Ni∶Co∶Mn = 90∶5∶5. Then, the pH was adjusted to about 12 with an aqueous NaOH solution and ammonia water for precipitation. After precipitation, it was washed and dried to obtain a precursor.

[0049] (2)Sintering: The precursor was mixed with lithium hydroxide, and the ratio of the total molar amount of Ni, Co and Mn in the precursor to the molar amount of Li in lithium hydroxide was controlled to be 1:1.04; it was heated to 720 °C and sintered for 11 h in an oxygen atmosphere, and after crushing, an intermediate material was obtained, and its D50 particle size was 4.5 μm.

[0050] (3)Surface coating: The intermediate material in step (3) was mixed with single-walled carbon nanotubes in N-methylpyrrolidone, and a mixed slurry was prepared by high-pressure homogenization. Among them, the mass of single-walled carbon nanotubes was controlled to be 0.05 wt% of the total mass of the intermediate material and single-walled carbon nanotubes, and the solid content of the mixed slurry was controlled to be 72 wt%. Then, the mixed slurry was spray-dried, the inlet temperature was 200 °C, the outlet temperature was 60 °C, and a ternary cathode material with surface carbon coating was obtained after spray-drying.

[0051] Comparative Example 2 Compared with Example 1, the difference in this comparative example is that single-walled carbon nanotubes are not added in step (4) of this comparative example.

[0052] Comparative Example 3 Compared with Example 1, the difference in this comparative example is that single-walled carbon nanotubes are replaced by graphene in step (4) of this comparative example.

[0053] Comparative Example 4 Compared with Example 1, the difference in this comparative example is that single-walled carbon nanotubes are replaced by multi-walled carbon nanotubes in step (4) of this comparative example.

[0054] Test Example Preparation of pouch lithium-ion batteries: The cathode materials of Examples 1-3 and Comparative Examples 1-4 were respectively dispersed in NMP together with conductive agents SuperP, CNT, and binder polyvinylidene fluoride (PVDF) according to a mass ratio of 96.5:1.5:0.5:1.5, and stirred evenly to obtain electrode slurries. The electrode slurries were coated on the surface of aluminum foil and dried at 95°C to obtain positive electrode sheets. The negative electrode was a commercial graphite negative electrode, the electrolyte was a commercial electrolyte, and the separator was a three-layer PP / PE / PP separator (thickness 14 μm). Pouch batteries of about 3 Ah were fabricated and subjected to full-cell performance tests in the voltage range of 2.75 V - 4.3 V. The test conditions were 25°C and cycling 500 times at a 2C charge / 5C discharge rate. The test results are summarized in Table 1.

[0055] Rate performance test: After obtaining the reference capacity by charging and discharging at 0.5C, discharge at 5C, and calculate the ratio of the two, which is the 5C / 0.5C discharge capacity retention rate.

[0056] DCR (direct current internal resistance) test method: Let the battery stand at 25°C ± 2°C for 1 h, discharge to the lower limit voltage at 0.1C, and stand for 10 min; charge to the upper limit voltage at 0.1C with a cut-off current of 0.05C, and stand for 10 min; discharge to the lower limit voltage at 0.1C (for initial capacity), and stand for 10 min; charge to the upper limit voltage at 0.1C with a cut-off current of 0.05C; discharge at 1C for 30 min and stand for 30 min; discharge at 0.1C for 10 s (sampling at the 200th ms) and discharge at the standard discharge current for 30 s (sampling at the 200th ms); DCR = (V1 - V2) / (1C - 0.1C); where V1 is the terminal voltage corresponding to discharging at 0.1C for 10 s, and V2 is the voltage corresponding to discharging at 1C for 5 s.

[0057] Table 1 Test data of lithium-ion batteries in examples and comparative examples

[0058] As can be seen from the data in Table 1, the synergistic effect of gradient modification and single-walled carbon nanotube (SWCNT) coating is remarkable in controlling the impedance of the cathode material and improving the high-rate cycling performance. Taking Example 1 as the basic scheme, the initial DCR is 17 mΩ. After 500 cycles, the DCR increases to 29 mΩ (growth rate of 71%). The discharge capacity retention rate at 5C / 0.5C is 91%, and the cycling capacity retention rate is 94%. In Example 2, due to the increase in the D50 particle size of the intermediate material to 5.6 μm, the initial DCR rises to 18 mΩ, and after cycling, the DCR increases to 32 mΩ (growth rate of 78%). However, the cycling capacity retention rate still remains at 94%, indicating that although the increase in particle size within a certain range slightly increases the initial impedance, it has little impact on the long-term cycling stability. In Example 3, by increasing the SWCNT coating amount to 0.1 wt%, the initial DCR is further reduced to 16 mΩ, and after cycling, the DCR only increases to 24 mΩ (growth rate of 50%). The discharge capacity retention rate at 5C / 0.5C and the cycling capacity retention rate are respectively increased to 92% and 95%, indicating that increasing the SWCNT coating amount within a reasonable range can more effectively reduce the initial impedance and inhibit the impedance growth during cycling, thereby enhancing the high-rate cycling performance.

[0059] Comparing with the data of the comparative examples, in Comparative Example 1, gradient precipitation was not adopted, and the initial DCR was as high as 23 mΩ. After cycling, the DCR increased sharply by 101 mΩ (growth rate of 339%). The discharge capacity retention rate at 5C / 0.5C was only 79%, and the cycling capacity retention rate was 81%, showing a significant gap compared with Example 1, indicating that gradient modification plays a crucial role in improving the rate performance, cycling stability of the ternary cathode material, and inhibiting the growth of cycling impedance. In Comparative Example 2, only gradient precipitation was adopted without SWCNT coating. The initial DCR was 22 mΩ, and after cycling, the DCR increased to 82 mΩ (growth rate of 273%). The discharge capacity retention rate at 5C / 0.5C was 82%, and the cycling capacity retention rate was 85%, indicating that SWCNT coating can reduce the initial impedance, and at the same time can significantly improve the rate performance and cycling stability of the material and inhibit the impedance growth. In Comparative Examples 3 and 4, graphene and multi-walled carbon nanotubes were used to replace the SWCNT coating. Although the DCR growth and cycling stability were better than those of Comparative Example 1 without gradient precipitation modification and Comparative Example 2 without carbon coating, compared with Example 1, the effect of inhibiting impedance growth and cycling stability were still insufficient, further verifying the unique advantages of SWCNT in balancing electron / ion transport and inhibiting the growth of cycling impedance under high-rate charge and discharge conditions.

[0060] In summary, the embodiments of the present invention adopt the collaborative design of the gradient modified core-shell structure and the SWCNT coating. By optimizing the internal ion migration channels and the surface electron transport network, the initial impedance of the cathode material is effectively reduced, and the continuous increase of impedance during the cycling process is significantly suppressed. Finally, a long cycling performance with a capacity retention rate of over 90% after 500 cycles at a high discharge rate of 5C and a high rate performance with a 5C / 0.5C discharge capacity retention rate higher than 90% are achieved, which are technical effects difficult to achieve by single gradient modification or the coating of carbon materials such as graphene or carbon nanotubes.

[0061] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a high-rate and long-cycle cathode material, characterized in that, It includes the following steps: Adjust the mixed solution of nickel salt, cobalt salt and manganese salt to be alkaline for the first precipitation. After the first precipitation is completed, at least two precipitation steps are carried out in sequence, and a solution containing Ni, Co, Mn and a modifying element is added each time; Obtain a precursor; Mix and sinter the precursor material with a lithium salt and then crush it to obtain an intermediate material; Mix the intermediate material with single-walled carbon nanotubes to prepare a mixed slurry, and then carry out spray drying to obtain the product.

2. The preparation method according to claim 1, wherein, In the mixed solution of nickel salt, cobalt salt and manganese salt, the molar ratio of Ni, Co and Mn is (6~9.5) : (0.3~3) : (0.3~3); in the solution containing Ni, Co, Mn and a modifying element, the total molar amount of Ni, Co and Mn to the molar amount of the modifying element is 1: (0.2~1.2).

3. The preparation method according to claim 2, characterized in that, Each modifying element added each time is independently selected from one or more of Ag, Cu, W, Sr, Zr, Co, Ti, Mg, Al or F.

4. The preparation method according to claim 3, characterized in that After the first precipitation is completed, a solution containing Ni, Co, Mn and a first modifying element is added for the second precipitation; the total molar amount of Ni, Co and Mn to the molar amount of the first modifying element is 1: (0.3~0.7); then a solution containing Ni, Co, Mn and a second modifying element is added for precipitation, and the total molar amount of Ni, Co and Mn to the molar amount of the second modifying element is 1 : (0.8~1.2) to obtain a precursor; wherein, the first modifying element is Co, and the second modifying element is Ti, Zr and Co; in the second modifying element, the molar ratio of Ti, Zr and Co is (0.003~0.007):(0.003~0.007) : (8~12).

5. The preparation method according to claim 1, characterized in that, In the step of mixing and sintering the precursor material with a lithium salt, the total molar amount of Ni, Co and Mn in the precursor material to the molar amount of the Li element of the lithium salt is 1 : (1.02~1.08); the D50 particle size of the intermediate material is 2~10μm.

6. The preparation method according to claim 1, characterized in that, The mass fraction of the single-walled carbon nanotubes in the total mass of the single-walled carbon nanotubes and the intermediate material is 0.01~0.5wt%.

7. The preparation method according to claim 1, characterized in that, In the mixed slurry, the solvent is one or more of N-methylpyrrolidone, acetone, N,N-dimethylformamide or dimethyl sulfoxide; the solid content of the mixed slurry is 60~80wt%.

8. The preparation method according to claim 1, wherein, The inlet temperature of the spray drying is 180~220°C, and the outlet temperature is 50~80°C. A high-rate long-cycle cathode material prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, It includes the high-rate long-cycle cathode material according to claim 9.

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