A multi-stage composite cathode material, its preparation method and application

Abstract

This invention relates to the field of new energy technology, and particularly to a multi-stage composite cathode material, its preparation method, and its application. The composite cathode material comprises a three-stage blend of phosphate spherical materials, phosphate single particles, and ternary materials. The phosphate spherical materials are formed by primary particle agglomeration, with a uniform particle size distribution ((D90-D10) / D50) = 1.2~2.0, and a closed-pore porosity ≤ 5 vol.%. The phosphate single particles are obtained by breaking down and dissociating the phosphate spherical materials. This invention uses hydrothermal synthesis to prepare phosphate spherical materials and single particles. The former, formed by nanoparticle agglomeration, exhibits good rate performance and nitrogen-containing porosity (excellent thermal resistance), while the latter is a ternary material that fills the gaps and provides thermal insulation. This synergistic effect of the ternary and phosphate materials optimizes the battery's energy density, thermal safety, and lifespan.

Description

Technical Field

[0001] This invention generally relates to the field of lithium-ion batteries, and more specifically, to a multi-stage composite cathode material, its preparation method, and its application. Background Technology

[0002] In the current lithium-ion power battery field, the core contradiction of mainstream cathode materials lies in the difficulty of balancing high energy density and high safety: Ternary materials (NCM, NCA) have high specific capacity and high voltage platform, with an energy density of 200-300Wh / kg, meeting the long range requirements of electric vehicles, but they have poor thermal stability and low thermal runaway temperature (130-200℃), which can easily lead to safety accidents; Phosphate materials, such as lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP), rely on PO bonds, have excellent thermal stability (thermal runaway temperature >270℃), long life (>3000 cycles), and low cost, but due to their low specific capacity (~170mAh / g) and low voltage platform, their energy density is only 140-180Wh / kg, making it difficult to achieve fast charging and unable to meet high-end requirements.

[0003] This "seesaw effect" makes it difficult for a single material to simultaneously meet the combined requirements of high safety and long range. Therefore, the industry has explored a technical route that combines ternary materials with lithium iron phosphate (LFP / LMFP) to attempt to combine the advantages of both. However, existing hybrid technology solutions all have significant limitations and have failed to effectively solve key bottleneck problems.

[0004] First, the spread of thermal runaway in composite materials remains difficult to suppress effectively: Ternary materials, especially high-nickel ternary materials, have low thermal runaway temperatures, releasing large amounts of combustible materials and heat when it occurs. Existing composite technologies struggle to achieve effective thermal management at the micro- and nano-scale. Current composite technologies typically involve simple physical mixing; at the electrode micro-scale, the heat released by ternary particles can be rapidly conducted to adjacent areas through contact points or conductive networks between particles. While the thermal conductivity of phosphate materials (1~2 W / mK) is not significantly lower than that of ternary materials (2~4 W / mK), it is still insufficient to promptly block heat spread and cannot effectively improve battery-level safety.

[0005] Second, there is a mismatch in the electrochemical performance between the composite components: ternary materials and phosphate materials exhibit significant differences in rate performance and low-temperature performance. The ion diffusion coefficient within phosphate materials is (10⁻¹⁴~10⁻¹³ cm⁻¹). 2The rate of change ( / s) is 2-3 orders of magnitude higher than that of ternary materials. During high-rate charge / discharge of the composite electrode material, the phosphate component undergoes severe polarization, becoming a bottleneck region for Li ion insertion / extraction, thus reducing overall rate performance. Simultaneously, it forces the ternary component to bear excess current, accelerating material degradation. On the other hand, the Li ion diffusion barrier of phosphate materials increases sharply at low temperatures, resulting in significant capacity deterioration (LFP capacity can decay to below 50% at -20℃). In this composite material, the phosphate component is almost "deactivated" at low temperatures, leading to a sharp drop in the overall output power of the battery pack and safety risks such as lithium plating due to localized overload.

[0006] For example, patent WO2023046066A1, concerning battery cathode materials and their applications, mentions a three-stage hybrid cathode material, with particles ranging from largest to smallest: lithium manganese iron phosphate, ternary materials / lithium-rich manganese-based / lithium cobalt oxide / lithium manganese oxide, and ternary materials / lithium-rich manganese-based / lithium cobalt oxide / lithium manganese oxide. In this patent, lithium manganese iron phosphate, as the largest particle, amplifies its inherent shortcomings in capacity, rate performance, and low-temperature performance, directly reducing the overall rate performance of the electrode. Simultaneously, the secondary and tertiary smaller particles (ternary / lithium-rich manganese-based materials, etc.), which account for only 10-30% of the mass, are forced to bear most of the current during high-rate charge and discharge, further accelerating material lifespan degradation. Furthermore, as smaller particle components, ternary / lithium cobalt oxide materials have close contact between particles, allowing for unimpeded heat diffusion, resulting in a persistently high risk of thermal runaway.

[0007] Patent CN114520312A, concerning positive electrode active materials, positive electrode slurry, positive electrode sheets, and batteries, mentions a five-level particle blending system. It specifies the five particle levels from largest to smallest as lithium manganese iron phosphate, ternary materials, ternary materials / lithium manganese iron phosphate, ternary materials / lithium manganese iron phosphate, and lithium manganese iron phosphate, with a particle size ratio of 1.00 / (0.35-0.50) / (0.20-0.27) / (0.17-0.18) / (0.15-0.16). This patent also uses lithium manganese iron phosphate as the largest particle, failing to address its performance shortcomings and still reducing the overall electrode performance, thus failing to solve the core bottleneck of existing blending technologies.

[0008] Therefore, there is currently a lack of an effective solution that can deeply synergize the high energy density of ternary materials with the high safety of phosphate materials. The market urgently needs an innovative material or structural design that can achieve complementary advantages and seamless synergy between the two materials at the nano / micro scale, rather than a simple physical superposition or layered isolation, thereby truly achieving synergistic optimization of energy density, thermal safety, and cycle life to meet the stringent requirements of next-generation power batteries.

[0009] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0010] The summary section introduces a series of simplified concepts, which will be further explained in detail in the detailed description section. This summary section is not intended to limit the key features and essential technical features of the claimed technical solution, nor is it intended to determine the scope of protection of the claimed technical solution.

[0011] The primary objective of this invention is to provide a multi-level composite cathode material that can synergize the advantages of ternary materials and phosphate materials, thereby optimizing the energy density, thermal safety, and lifespan of the battery.

[0012] The second objective of this invention is to provide a method for preparing phosphate spherical materials and phosphate single particles in a multi-stage compound cathode material.

[0013] The third objective of this invention is to provide an application of a multi-stage composite cathode material in lithium-ion batteries.

[0014] Specifically, the multi-stage composite cathode material provided by the present invention includes a three-stage mixed composite of phosphate spherical material, phosphate single particle and ternary material.

[0015] This invention provides a method for preparing phosphate spherical materials and phosphate single particles in multi-stage composite cathode materials, comprising the following steps:

[0016] S1. Lithium source, transition metal element source, phosphorus source and solvent are mixed, and then hydrothermally synthesized and purified to obtain pure phase phosphate single particle precursor.

[0017] S2. Take the pure phase phosphate single particle precursor obtained in step S1 and mix it with the carbon source aqueous solution. After spray granulation, sinter it. The spray drying temperature during spray granulation is 140~170℃ to obtain phosphate spherical material.

[0018] S3. The phosphate spherical material obtained in step S2 is crushed and dissociated to obtain phosphate single particles.

[0019] As can be seen from the above technical solution, the advantages and positive effects of the present invention are as follows:

[0020] The phosphate spherical materials and phosphate single particles prepared by the hydrothermal synthesis method of this invention have higher rate performance than traditional phosphate materials. Under the high rate use conditions of batteries, the current is relatively uniformly dispersed among the components in the compound material, avoiding polarization deterioration and accelerated attenuation of the positive electrode.

[0021] In this invention, the third-stage small particles are phosphate single particles, which fill the gaps in the second-stage ternary material and provide steric hindrance, hindering the heat transfer between the ternary material particles, confining some of the heat within a single particle, and delaying the occurrence of thermal runaway.

[0022] In this invention, the first-stage large particles are made of phosphate spherical material. The spherical material is formed by the agglomeration of phosphate nanoparticles, which has good rate performance and avoids dragging down the overall rate performance of the positive electrode. On the other hand, the spherical material has a certain number of pores, which are mainly filled with nitrogen gas. Its thermal conductivity of 0.024 W / mK is two orders of magnitude lower than that of phosphate materials or ternary materials, making it an excellent thermal resistance material. This will suppress heat transfer within the microscale of the electrode and delay the occurrence of thermal runaway.

[0023] Specifically, 1. The phosphate single particles of this invention are synthesized hydrothermally, with a primary particle size of LFP ≤150nm or LMFP ≤100nm. The phosphate spherical materials and phosphate single particles prepared by the hydrothermal synthesis method exhibit higher rate performance compared to traditional phosphate materials due to their nanoscale primary particle size. The materials obtained by the hydrothermal synthesis method have a higher (010) crystal plane exposure ratio, further improving rate performance. When used in combination with ternary materials, under high-rate battery conditions, the components in the composite material distribute the current relatively uniformly, avoiding polarization deterioration and accelerated attenuation of the positive electrode.

[0024] 2. The particle size uniformity (SPAN) of the phosphate spherical material in this invention is 1.2~2.0. ① The particle size of the spherical material determines the path length for lithium ions to diffuse from the inside of the spherical material outward. Relatively uniform particle size is beneficial for the uniform distribution of current within the electrode during charging and discharging. ② Both phosphate spherical materials and ternary materials are usually relatively rounded spherical large particles. According to Furnas's spherical particle filling theory, when the particle size distribution (SPAN) ≤ 2.0, small particles can effectively fill the gaps between large particles, with a porosity < 25%, which is beneficial for improving the compaction density of the electrode. ③ Relatively uniform phosphate spherical materials can have higher spatial distribution uniformity when compounded with secondary and tertiary particles within the electrode, which is beneficial for forming a relatively continuous thermal resistance network within the electrode and delaying the thermal runaway diffusion of ternary material particles.

[0025] 3. The closed-cell porosity of the phosphate spherical material of this invention is ≤5 vol.%. The spherical material contains a certain number of pores, mainly filled with nitrogen gas. Its thermal conductivity (0.024 W / mK) is two orders of magnitude lower than that of phosphate materials or ternary materials, making it an excellent thermal resistance material. This will suppress heat transfer within the electrode microscale and delay the occurrence of thermal runaway in ternary materials. Furthermore, it will, to a certain extent, block the oxygen released by ternary materials, inhibiting chain combustion during thermal runaway.

[0026] 4. The three-stage compound particle size D50 ratio of this invention: phosphate spherical material / ternary material / phosphate single particle = 1.00 / (0.35~0.45) / (0.10~0.25). ① The larger-sized phosphate spherical material acts as a thermal resistance framework, while the nano-sized phosphate single particles fill the gaps between the ternary materials to form local thermal resistance nodes, effectively hindering direct contact between ternary material particles. ② Both the phosphate spherical material and the single particles have nano-sized primary particles, with the ternary material providing a fast channel for lithium-ion transport for the phosphate material, synergistically improving the electrode rate performance. ③ According to Horsfield's theory of the densest packing of multi-stage particles, the powder achieved with the above D50 ratio has theoretically the smallest porosity, which is beneficial for achieving a higher compaction density. Detailed Implementation

[0027] The exemplary embodiments will now be described more fully. However, the exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art.

[0028] The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a full understanding of embodiments of the invention. However, those skilled in the art will recognize that the invention can be practiced without one or more of the specific details described, or other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring various aspects of the invention.

[0029] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid obscuring the invention.

[0030] Firstly, embodiments of this application provide a multi-level composite cathode material, which includes a three-level blend of phosphate spherical materials, phosphate single particles, and ternary materials. It is understood that "multi-level blending" refers to achieving complementary cathode material performance by combining phosphate spherical materials and phosphate single particles with different structural morphologies in a specific ratio; "phosphate spherical materials" refers to phosphate-based cathode materials with a spherical structure; "phosphate single particles" refers to phosphate-based cathode materials with a single particle morphology; and "ternary materials" refers to cathode materials composed of three metal elements, such as lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide.

[0031] According to one embodiment of the present invention, the phosphate spherical material is formed by primary particle agglomeration, with a particle size distribution uniformity ((D90-D10) / D50) = 1.2~2.0, and a closed-cell porosity ≤ 5 vol.%. It is understood that "particle size distribution uniformity" is an indicator measuring the uniformity of particle size distribution. "Closed-cell porosity" refers to the percentage of the volume of closed pores within the material relative to the total volume of the material; the closed-cell porosity affects the ion transport performance of the material.

[0032] According to one embodiment of the present invention, the D50 ratio of each component in the three-stage mixed compound is: phosphate spherical material: ternary material: phosphate single particle = 1.00: (0.35-0.45): (0.10-0.25). It is understood that the D50 ratio of each component refers to the proportion of each component weighed according to its median particle size D50, which meets the requirements. In this embodiment, the D50 can be 10.0 μm for the lithium manganese iron phosphate spherical material (Mn / Fe=70 / 30), 1.5 μm for the lithium iron phosphate, and approximately 4.0 μm for the ternary material.

[0033] According to one embodiment of the present invention, the multi-stage compound cathode material is prepared by mixing phosphate spherical material, phosphate single particles and ternary material.

[0034] According to one embodiment of the present invention, both phosphate spherical materials and phosphate single particles are prepared by a hydrothermal synthesis method. The phosphate spherical materials are lithium manganese iron phosphate spherical materials, and the phosphate single particles are lithium iron phosphate single particles. The average diameter of their primary particles is ≤100 nm for lithium manganese iron phosphate and ≤150 nm for lithium iron phosphate. It is understood that "hydrothermal synthesis method" refers to a synthesis method in which water is used as a solvent in a closed container, and a high-temperature, high-pressure environment is created by heating to cause the reactants to undergo a chemical reaction to generate the target product. "Primary particles" refers to the most basic, non-agglomerated, independent particle units constituting the cathode material.

[0035] Secondly, embodiments of this application provide a method for preparing phosphate spherical materials and phosphate single particles, comprising the following steps:

[0036] S1. Lithium source, transition metal element source, phosphorus source and solvent are mixed, and then hydrothermally synthesized and purified to obtain pure phase phosphate single particle precursor.

[0037] S2. Take the pure phase phosphate single particle precursor obtained in step S1 and mix it with the carbon source aqueous solution. After spray granulation, sinter it. The spray drying temperature during spray granulation is 140~170℃ to obtain phosphate spherical material.

[0038] S3. The phosphate spherical material obtained in step S2 is crushed and dissociated to obtain phosphate single particles.

[0039] Understandably, this preparation method first obtains pure-phase phosphate single particles through hydrothermal synthesis, then obtains phosphate spherical materials through spray granulation and high-temperature sintering, and finally breaks down and dissociates the spherical materials to obtain more single particles. This achieves efficient preparation of single particles and spherical materials, and the preparation process is continuous, which is beneficial for controlling the purity and performance of the materials.

[0040] According to one embodiment of the present invention, in step S1, at least two single-particle precursors are obtained depending on the transition metal element source used, including a first pure-phase phosphate single-particle precursor and a second pure-phase phosphate single-particle precursor; the phosphate spherical material in step S2 is obtained from the corresponding first pure-phase phosphate single-particle precursor, and the phosphate single particles in step S3 are obtained from the corresponding second pure-phase phosphate single-particle precursor.

[0041] According to one embodiment of the present invention, in step S1, the lithium source is selected from one or more of lithium carbonate, lithium sulfate, lithium hydroxide, and lithium phosphate; the M element source is selected from one or more of the sulfate, nitrate, and oxalate corresponding to the M element; the phosphorus source is phosphoric acid; and the solvent is one or more of water, ethylene glycol, ethanol, and diethylene glycol. The hydrothermal synthesis temperature is 180~210℃, and the holding time is 0.5~3h. It is understood that the "lithium source" is the raw material providing lithium ions, the "M element source" is the raw material providing the metal element (such as iron, manganese, etc.) in the phosphate, the "phosphorus source" is the raw material providing phosphorus, and the "solvent" is used to dissolve the raw materials to ensure the reaction proceeds fully. The hydrothermal synthesis temperature and holding time affect the degree of reaction completion and the structural properties of the product.

[0042] According to one embodiment of the present invention, in step S2, the carbon source is one or more of sucrose, glucose, polyethylene glycol, oxalic acid, citric acid, hexadecyltrimethylammonium bromide, thioacetamide, and phenolic resin, and the amount of carbon source added is 2.0~15.0 wt. based on the mass of a single pure phase phosphate particle.

[0043] According to one embodiment of the present invention, in step S2, the equipment used for spray granulation of the spherical material is a multi-fluid spraying device, the fluid pressure before the nozzle is 0.4~0.7MPa, and the spray drying chamber temperature, i.e., the spray drying temperature, is 140~170℃. The sintering temperature of the spherical material is 650~700℃, and the sintering time is 1~4h. The sintering temperature of the phosphate single-particle material is 690~760℃, and the sintering time is 2~8h.

[0044] Thirdly, embodiments of this application provide a lithium-ion battery, which includes a multi-stage composite cathode material of any one of the above or a combination of phosphate spherical material and phosphate single particles prepared by any one of the above methods.

[0045] Example 1

[0046] In this embodiment, the preparation steps of phosphate spherical material include: Step S1, using lithium carbonate as the lithium source, ferrous sulfate and manganese sulfate as transition metal element sources, and phosphoric acid as the phosphorus source, and mixing with water as the solvent to obtain a first mixture, wherein the molar ratio of ferrous sulfate to manganese sulfate is 3:7. The first mixture is hydrothermally synthesized at 190~200℃ for 1h and purified, with a hydrothermal heating rate of 25~70℃ / min, to obtain a pure phase lithium manganese iron phosphate single-particle precursor; Step S2, the pure phase lithium manganese iron phosphate single-particle precursor is mixed with glucose and polyethylene glycol aqueous solution (the addition amounts are 6.4wt.% and 3.0wt.% respectively based on the mass of the single-particle precursor), spray granulated, and then sintered at 700℃ for 5h to obtain lithium manganese iron phosphate spherical material with a particle size distribution uniformity of 2 and a closed-pore porosity of 4vol.%. The spray granulation process utilizes a multi-fluid spray system with a nozzle pressure of 0.6–0.7 MPa and a spray drying chamber temperature of 170°C. Sintering is then performed in an atmosphere furnace at 680°C for 2 hours. The primary particle size of the spherical lithium manganese iron phosphate material is 50–60 nm.

[0047] The preparation steps of lithium iron phosphate single particles include: Step S1', using lithium carbonate as the lithium source, ferrous sulfate as the transition metal element source, and phosphoric acid as the phosphorus source, and mixing them with water as the solvent to obtain a second mixture. The second mixture is hydrothermally synthesized at 180-200℃ for 2-4 hours and then purified to obtain a pure-phase lithium iron phosphate single particle precursor; Step S2', the pure-phase lithium iron phosphate single particle precursor is mixed with sucrose and polyethylene glycol aqueous solution (the addition amounts are 5.5 wt.% and 2.5 wt.% respectively, based on the mass of the single particle precursor), spray-granulated, and then sintered at 750℃ for 5-8 hours to obtain a sintered product; Step S3', the sintered product is dissociated by airflow crushing to obtain lithium iron phosphate single particles.

[0048] Subsequently, multi-stage composite cathode materials were prepared: raw materials were weighed according to the D50 ratio of each component (lithium manganese iron phosphate spherical material: NCM523 ternary material: lithium iron phosphate single particle = 10:4:1.5), and mixed with a dual planetary mixer at 200 r / min for 1 h to obtain multi-stage composite cathode materials.

[0049] In this embodiment, the D50 of the spherical lithium manganese iron phosphate material (Mn / Fe=70 / 30) is 10.0 μm, the D50 of lithium iron phosphate is 1.5 μm, and the D50 of the single-crystal ternary material is approximately 4.0 μm. The mass ratio of the spherical lithium manganese iron phosphate material (Mn / Fe=70 / 30), the single-crystal ternary material, and the lithium iron phosphate is 70%:20%:10%.

[0050] Example 2

[0051] The difference between Example 2 and Example 1 lies in that, during spray granulation, the spray pressure is reduced to 0.4 MPa to decrease the generation of small-sized spherical particles, thereby reducing the particle size distribution (SPAN). Simultaneously, the spray drying chamber temperature is reduced to 140°C to extend the drying time, allowing sufficient time for the spherical particles to shrink, thus reducing porosity. Therefore, the material differences are due to the control and alteration of the particle size distribution consistency (SPAN) of the phosphate spherical particles, the closed-cell porosity, and the particle size distribution ratio (D50) of the (phosphate spherical particles: ternary material: phosphate single particles), as shown in Table 1.

[0052] Comparative Example 1

[0053] The main difference between Comparative Example 1 and Example 1 is that the hydrothermal synthesis temperature was reduced to 175°C, which resulted in a decrease in the primary particle size and an increase in the viscosity of the slurry obtained before spray granulation in step S2, ultimately leading to a particle size distribution uniformity (SPAN) of 2.5 for the phosphate spherical material.

[0054] Comparative Example 2

[0055] The main difference between Comparative Example 2 and Example 1 is that the phosphate single particle preparation method adopts solid-phase synthesis, and the resulting primary particle size is 244 nm.

[0056] Comparative Example 3

[0057] The main difference between Comparative Example 3 and Example 1 is that, during spray granulation, the temperature of the spray drying chamber was increased to 200°C for rapid drying. This rapid drying and hardening of the spherical material surface resulted in an increase in internal porosity. Therefore, the material difference was that the closed-cell porosity of the phosphate spherical material was 5.9%.

[0058] Comparative Example 4

[0059] The main difference between Comparative Example 4 and Example 1 is that the hydrothermal synthesis time was extended to 3 hours. The primary growth of phosphate particles led to an increase in the solid content of the slurry obtained in step S2, which in turn resulted in an increase in the particle size of the spherical material. The particle size distribution (phosphate spherical material: ternary material: phosphate single particle) D50 was 14:4:1.5.

[0060] Comparative Example 5

[0061] The main difference between Comparative Example 5 and Example 1 is that the multi-stage compound cathode material does not use phosphate single particles, but only phosphate spherical materials and ternary materials to form a two-stage compound system.

[0062] The different process parameters and test results of Examples 1-2 and Comparative Examples 1-5 are shown in Table 1 below.

[0063] Table 1

[0064]

[0065] In Example 1, spherical lithium manganese iron phosphate material (phosphate spherical material), single lithium iron phosphate particles (phosphate single particles), and single-crystal ternary materials were used. The resulting composite material had a 5C discharge capacity to 0.1C discharge capacity ratio (power performance) of 87.8%. After fully charging the coin cell, it was disassembled, and the positive electrode powder was scraped off for DSC testing, showing that its exothermic peak temperature was 225.4℃ and the highest exothermic rate was 0.700mW. Similarly, Example 2 achieved similar results.

[0066] In Example 2, the particle size distribution width of the phosphate spherical material was narrowed and the number of internal pores was reduced. Compared with Example 1, the compaction density was slightly increased, and the 5C discharge capacity was also increased due to the enhanced charge transfer caused by the reduction in the number of pores. However, the decrease in thermal runaway initiation temperature and the increase in heat release rate indicate that its thermal stability was slightly reduced.

[0067] In Comparative Example 1, the SPAN value of the spherical lithium manganese iron phosphate particle size distribution increased to 2.5. The wider particle size distribution caused the particle size distribution of the compound material to deviate from the design, resulting in a decrease in the compaction density of the material to 2.23 g / cm³. 3 Furthermore, due to the slight increase in interparticle porosity, the charge transport impedance within the material increases, resulting in a decrease in power performance to 77.3%.

[0068] In Comparative Example 2, the lithium iron phosphate single-particle material produced using a solid-state method had an average primary particle size of 250 nm. Due to the larger primary particles, the compaction density of the composite material was significantly increased to 2.33 g / cm³. 3 However, this was accompanied by a severe deterioration in power performance (73.8%); the size of the primary particles had no significant impact on the thermal stability of the material.

[0069] In Comparative Example 3, spherical lithium manganese iron phosphate with higher porosity was used. Due to the deterioration of the conductive network between the primary lithium manganese iron phosphate particles, the power performance of the composite material deteriorated to 75.6%. Due to the more voids inside the spherical material, heat transfer within the material was effectively blocked, and the temperature of the exothermic peak increased significantly to 227.2℃, while the exothermic rate also decreased.

[0070] In Comparative Example 4, when lithium manganese iron phosphate was made into larger spherical particles, the power performance of the composite material did not deteriorate. However, since the larger spherical particles could not effectively disperse the ternary material particles, the thermal stability of the composite material decreased, and the temperature at which the exothermic peak occurred dropped to 222.8℃.

[0071] In Comparative Example 5, the removal of the phosphate single-particle component reduced the material's compaction density due to the loss of small particles filling the voids in the material. Increased voids worsened charge transport, thus reducing the 5C discharge capacity, but increased the resistance to heat transfer within the material, resulting in improvements in both thermal runaway temperature and heat release rate.

[0072] It should be understood that the various examples described above can be utilized in multiple directions (e.g., tilted, inverted, horizontal, vertical, etc.) and in multiple configurations without departing from the principles of the invention. The illustrated embodiments are shown and described merely as examples of effective application of the principles of the invention, and the invention is not limited to any specific details of these embodiments.

[0073] Of course, upon careful consideration of the above description of the representative embodiments, those skilled in the art will readily understand that various modifications, additions, substitutions, deletions, and other changes can be made to these specific embodiments, and that such changes are within the scope of the principles of the invention. Therefore, the foregoing detailed description should be clearly understood as being given by way of illustration and example only, and the spirit and scope of the invention are defined solely by the appended claims and their equivalents.

Claims

1. A multi-stage composite cathode material, characterized in that, The multi-stage compound cathode material includes a three-stage blend of phosphate spherical material, phosphate single particles, and ternary material; the phosphate spherical material is formed by primary particle agglomeration with a uniform particle size distribution ((D90-D10) / D50) = 1.2~2.0, and the closed-cell porosity in the spherical material is 2.9 vol.% ≤ 5 vol.%; the phosphate single particles are obtained by breaking down and dissociating the phosphate spherical material; the D50 ratio of each component in the three-stage blend is: phosphate spherical material : ternary material : phosphate single particle = 1.00 : (0.35-0.45) : (0.10-0.25).

2. The multi-stage composite cathode material according to claim 1, characterized in that, The multi-stage composite cathode material is prepared by mixing phosphate spherical material, phosphate single particles and ternary material.

3. A method for preparing a multi-stage composite cathode material as described in any one of claims 1 to 2, characterized in that, The preparation methods for phosphate spherical materials and phosphate single particles include the following steps: S1. Lithium source, transition metal element source, phosphorus source and solvent are mixed, and then hydrothermally synthesized and purified to obtain pure phase phosphate single particle precursor. S2. Take the pure phase phosphate single particle precursor obtained in step S1 and mix it with the carbon source aqueous solution. After spray granulation, sinter it. The spray drying temperature during spray granulation is 140~170℃ to obtain phosphate spherical material. S3. The phosphate spherical material obtained in step S2 is crushed and dissociated to obtain phosphate single particles.

4. The method for preparing the multi-stage composite cathode material according to claim 3, characterized in that, In step S1, depending on the different transition metal element sources used, at least two single-particle precursors are obtained, including a first pure-phase phosphate single-particle precursor and a second pure-phase phosphate single-particle precursor; the phosphate spherical material in step S2 is obtained from the corresponding first pure-phase phosphate single-particle precursor, and the phosphate single particles in step S3 are obtained from the corresponding second pure-phase phosphate single-particle precursor.

5. The method for preparing the multi-stage composite cathode material according to claim 3, characterized in that, In step S1, the lithium source is selected from one or more of lithium carbonate, lithium sulfate, lithium hydroxide, and lithium phosphate; the transition metal element source is selected from one or more of the sulfate, nitrate, and oxalate corresponding to the transition metal element; the phosphorus source is phosphoric acid; and the solvent is one or more of water, ethylene glycol, ethanol, and diethylene glycol. The hydrothermal synthesis temperature is 180~210℃, the holding time is 0.5~4h, and the hydrothermal heating rate is 25~70℃ / min.

6. The method for preparing the multi-stage composite cathode material according to claim 3, characterized in that, In step S2, the carbon source is one or more of sucrose, glucose, polyethylene glycol, oxalic acid, citric acid, hexadecyltrimethylammonium bromide, thioacetamide, and phenolic resin. Based on the mass of a single pure phase phosphate particle, the amount of carbon source added is 2.0 wt.% to 15.0 wt.%.

7. The method for preparing the multi-stage composite cathode material according to claim 3, characterized in that, In step S2, the spherical material spray granulation is carried out using a multi-fluid spraying device with a fluid pressure of 0.4~0.7MPa in front of the nozzle.

8. The method for preparing the multi-stage composite cathode material according to claim 3, characterized in that, In step S2, the sintering temperature of the spherical material is 650~700℃ and the sintering time is 1~4h; the sintering temperature of the phosphate single particles is 690~760℃ and the sintering time is 2~8h.

9. A lithium-ion battery, characterized in that, The lithium-ion battery comprises a combination of phosphate spherical material and phosphate single particles prepared by the preparation method of the multi-stage composite cathode material according to any one of claims 1 to 2 or any one of claims 3 to 8.

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