Phosphate positive electrode material, preparation method and application thereof

Abstract

The application belongs to the field of lithium ion batteries, and particularly relates to a phosphate positive electrode material and a preparation method and application thereof. The phosphate positive electrode material comprises phosphate particles and a carbon layer coated on the surfaces of the phosphate particles, and lithium phosphate quantum dots with a particle size of 1nm to 5nm are distributed in primary particles of the phosphate particles. The key of the application is to construct lithium phosphate quantum dots with a particle size of 1nm to 5nm in the interior of the phosphate positive electrode material, so as to endow the phosphate positive electrode material (LMFP) with an extremely high lithium ion diffusion coefficient (DLi + ), so that the lithium ion battery has high rate performance and cycle performance.

Description

Technical Field

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

[0002] In lithium-ion battery cathode materials, olivine-type phosphate materials such as lithium iron phosphate (LiFePO4, LFP) and lithium manganese iron phosphate (LiMn) are commonly used. x Fe 1-x PO4 (LMFP) has been widely studied and applied due to its high thermal stability, long cycle life, and abundant raw material resources. However, the low intrinsic electronic conductivity and ion diffusion coefficient of this type of material severely limit its high-rate charge-discharge performance, which is a core technical problem that has long needed to be solved in this field.

[0003] To address these issues, existing technologies have proposed various improvement schemes, primarily including carbon coating, nano-sizing and ion doping, and quantum dot surface modification. Carbon coating refers to constructing an electronic conductive network by coating the surface of the cathode material with a carbon layer, which is the most common strategy for improving the conductivity of cathode materials. However, this method has inherent limitations: First, the carbon coating layer mainly affects electron transport on the particle surface and between particles, without substantially improving the ion diffusion capability within the bulk phase of the material. The migration of lithium ions within the olivine structure remains slow, leading to increased polarization voltage at high rates. Second, an excessively thick carbon layer reduces the material's compaction density and volumetric energy density, while uneven coating may result in some active materials not being covered, leading to incomplete electron transport paths. Nano-sizing and ion doping shorten the ion diffusion path by nano-sizing the material or performing bulk ion doping to improve intrinsic conductivity. However, nano-sizing easily leads to poor material processing performance and increased side reactions; while conventional metal ion doping has limited effect on improving ionic conductivity and may introduce new lattice defects. Quantum dot surface modification aims to optimize the interfacial properties of cathode materials by utilizing the special effects of quantum dots. For example, CN118851118A discloses a lithium iron phosphate composite material modified with nitrogen-carbon quantum dots. CN115360342A discloses a cathode material using carbon quantum dots containing amino groups to coat a lithium iron phosphate matrix. However, the mechanism of quantum dot surface modification still focuses on surface modification and interface optimization. The quantum dots exist as external additives on the particle surface and do not form an intrinsic bulk phase fusion with the phosphate matrix. This non-in-situ modification method may have problems such as weak interfacial bonding and insufficient stability of the coating layer during long-term cycling, and it also cannot fundamentally solve the problem of slow diffusion kinetics of lithium ions within the bulk phase of the material. In addition, some existing technologies directly use lithium phosphate (Li3PO4) to coat phosphate cathode materials. For example, CN118738326A discloses a method for preparing lithium manganese iron phosphate co-coated with nano-sized lithium phosphate and a carbon layer. This method mixes nano-sized lithium phosphate with a carbon source and an antioxidant together with a lithium manganese iron phosphate precursor, and then sintersties to form a surface coating layer. The core of this technology lies in using lithium phosphate to improve interfacial ion conduction, but lithium phosphate, as an externally added coating component, only exists on the particle surface. CN117361486A discloses a method for preparing a composite precursor, the key of which is the addition of nano-sized lithium phosphate with a D50 of less than 50 nm in the later stage. This technology, through a complex co-precipitation process, aims to distribute lithium phosphate in the pores and surface of the ferromanganese phosphate precursor, as a nanoscale filling or coating phase, and further coat ferrophosphate to form a core-shell structure.

[0004] Despite the numerous efforts made by existing technologies to improve the performance of phosphate cathode materials, the following significant defects and unresolved problems still exist, mainly reflected in: (1) The bulk ion transport bottleneck has not been effectively resolved: Existing improvement schemes focus on electronic conductivity or surface interface engineering, lacking effective means to construct and optimize lithium-ion diffusion channels within the bulk phase of the material. This results in the diffusion of ions within the particles becoming the rate-controlling step during high-rate charging and discharging, generating huge polarization resistance, causing the battery voltage plateau to drop and the capacity to decay sharply. (2) Fast charging performance is limited under high SOC (State of Charge): Especially in lithium manganese iron phosphate (LMFP) materials, when charged to the high-voltage plateau (SOC>80%), the change in the material lattice parameters leads to a further increase in ion transport resistance, resulting in a severe "ion transport blockage" phenomenon, making high-current charging impossible and significantly extending the total charging time. (3) Limitations of surface modification technology: Technologies including carbon coating and quantum dot surface modification have limited improvement effects to the interface, and have problems such as stability of binding with the matrix, coating uniformity, and possible sacrifice of energy density. They cannot achieve bulk synergistic effect of electron transport and ion transport. The modification methods are limited to the surface and interface and cannot build bulk ion channels. Among them, lithium phosphate coating (CN118738326A) or pore / surface distribution (CN117361486A) mention that their scope of action is concentrated on the external surface, interface or pores of material particles. They fail to reach and optimize the lithium ion diffusion path inside the primary particles of the material. During high-rate charge and discharge, the slow diffusion of ions inside the particles is still a performance bottleneck, resulting in severe polarization voltage drop and capacity decay. In addition, the size and coarse bonding method of lithium phosphate limit its maximum performance. The lithium phosphate used in the prior art is tens to hundreds of nanometers in size (e.g., <100nm, D50 <50nm). This relatively large lithium phosphate phase has a small specific surface area and limited contact interface with the matrix, failing to provide a sufficient number of low-barrier ion transition sites, resulting in inefficient and insufficient utilization of its ionic conductivity enhancement capabilities. Furthermore, for LMFP materials, manganese dissolution remains a significant problem at high voltages, which existing surface coating technologies struggle to fundamentally suppress, leading to rapid capacity decay during cycling and challenging cycle stability, especially lifetime under high-rate cycling. Therefore, there is an urgent need in this field for a new technological solution that can fundamentally optimize electron and ion transport dynamics simultaneously by addressing the bulk structure of the material, while also possessing excellent structural stability, to overcome the performance bottleneck of olivine phosphate cathode materials in high-power applications. Summary of the Invention

[0005] The primary objective of this invention is to provide a novel phosphate cathode material that exhibits high rate performance and cycle stability.

[0006] The second objective of this invention is to provide a method for preparing a phosphate cathode material.

[0007] A third objective of this invention is to provide a phosphate cathode material prepared by the above method.

[0008] The fourth objective of this invention is to provide the application of the above-mentioned phosphate cathode material in lithium-ion batteries.

[0009] The phosphate cathode material provided by the present invention includes phosphate particles and a carbon layer coated on the surface of the phosphate particles. The primary particles of the phosphate particles are distributed with lithium phosphate quantum dots with a particle size of 1nm to 5nm.

[0010] The method for preparing the phosphate cathode material provided by this invention includes the following steps:

[0011] S1. Synthesis of precursor: A precursor is obtained by hydrothermal reaction of a lithium source, a metal source and a phosphorus source, wherein the metal source is selected from at least one of an iron source, a manganese source and a cobalt source, and the hydrothermal reaction is performed under conditions that make the obtained precursor rich in amorphous lithium phosphate clusters.

[0012] S2. Add lithium phosphate and carbon source: Add nano-sized lithium phosphate and carbon source to the precursor to obtain carbon source precursor;

[0013] S3. In-situ quantum dot precipitation: The carbon source precursor is granulated and then subjected to low-temperature pre-sintering and high-temperature sintering to obtain phosphate cathode material; the heating rate of the low-temperature pre-sintering is 3~5℃ / min, the temperature is 300℃~400℃, and the time is 2h~5h; the heating rate of the high-temperature sintering is 1~2℃ / min, the temperature is 600℃~700℃, and the time is 3h~10h.

[0014] The key to this invention lies in constructing lithium phosphate quantum dots with a particle size of 1nm to 5nm inside the phosphate cathode material, which endows the phosphate cathode material with an extremely high lithium-ion diffusion coefficient (DLi). +Its 18s internal resistance can reach less than 176 Ω·ms, and the 3.2V plateau retention rate can reach over 94.20%, exhibiting excellent rate performance and cycle durability. The reasons for this are speculated to be: firstly, the construction of lithium phosphate quantum dots with a particle size of 1nm~5nm within the phosphate cathode material allows these quantum dots to be distributed at bulk grain boundaries and defects, acting as nanoscale "ion highways." This provides numerous additional, low-barrier diffusion paths for the rapid migration of lithium ions within the olivine phase particles, significantly reducing bulk ion transport impedance and thus achieving synergistic optimization of ion / electron transport; secondly, lithium phosphate quantum dots, acting as robust "nanopeg points," can effectively suppress grain boundary slip and microcrack formation caused by repeated lattice expansion and contraction during charging and discharging, delaying particle pulverization; for LMFP materials, quantum dots can act as manganese ions (Mn... 2+ The "capture trap" for manganese leaching significantly reduces the damage to the electrolyte and the negative electrode SEI film caused by manganese leaching through surface adsorption or lattice anchoring, thereby improving the cycle life of the full cell. Attached Figure Description

[0015] Figure 1 This is a transmission electron microscope (TEM) image of the phosphate cathode material obtained in Example 1.

[0016] Figure 2 This is a TEM image of the reference phosphate cathode material obtained in Comparative Example 2;

[0017] Figure 3 This is a TEM image of the phosphate cathode material obtained in Comparative Example 3. Detailed Implementation

[0018] The phosphate cathode material provided by the present invention includes phosphate particles and a carbon layer coated on the surface of the phosphate particles, wherein lithium phosphate quantum dots are distributed inside the primary particles of the phosphate particles.

[0019] In the aforementioned phosphate cathode material, the lithium phosphate quantum dots have a particle size of 1 nm to 5 nm, such as 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, and 5 nm. When the particle size of lithium phosphate quantum dots is less than 1 nm, the ultrasmall quantum dots are prone to excessive reaction with the matrix or embedding in lattice defects, affecting the structural integrity of the phosphate main phase. They are also partially dissolved or consumed, unable to maintain a uniform distribution over a long period, leading to the failure of ion diffusion channels. Furthermore, due to their extremely small size, the quantum dots are easily deactivated and difficult to prepare. When the particle size of lithium phosphate quantum dots is greater than 5 nm, the larger lithium phosphate particles may act as physical barriers, blocking the inherent diffusion channels of lithium ions in the olivine structure. Additionally, the surface activity of the quantum dots decreases, the number of adsorption sites decreases, and the ability to capture manganese ions weakens, failing to effectively alleviate capacity decay during cycling. The inhibitory effect on manganese dissolution decreases sharply, and the quantum dots lose their nanoscale advantages, even introducing negative effects. This invention strictly controls the particle size range of lithium phosphate quantum dots to 1nm~5nm, which is the key to balancing stability, ion conductivity and structural enhancement, ensuring efficient construction and long-term cycling stability of bulk ion channels.

[0020] In the aforementioned phosphate cathode material, the content of lithium phosphate quantum dots is preferably 0.7% to 3.5% of the total mass of the phosphate cathode material. Controlling the content of lithium phosphate quantum dots within this preferred range achieves an optimal balance among key performance parameters such as ionic conductivity, electronic conductivity, structural stability, and energy density. This is because controlling the content of lithium phosphate quantum dots within this preferred range allows the nanoscale lithium phosphate quantum dots to form a continuous and efficient lithium-ion "penetration network" or "highway" within the primary particles. This significantly reduces the energy barrier and impedance of lithium-ion diffusion in the olivine bulk phase, effectively forming continuous ion transport paths and solving the problem of slow bulk ion diffusion in the material. This, in turn, more effectively improves the specific capacity and energy density, giving lithium-ion batteries better rate performance and cycle performance. Specifically, the content of lithium phosphate quantum dots can be 0.7%, 1.4%, 2.1%, 2.8%, 3.5%, etc.

[0021] In the aforementioned phosphate cathode material, the type of phosphate particles is not particularly limited and can be at least one of various existing olivine-type phosphate material particles, such as lithium iron phosphate, lithium manganese iron phosphate, and lithium cobalt phosphate. In a preferred embodiment, the general formula of the phosphate particles is LiMPO4•xLi3PO4; M is at least one of Fe, Mn, and Co; and x is 0.01~0.05, such as 0.01, 0.02, 0.03, 0.04, 0.05, etc.

[0022] In the aforementioned phosphate cathode material, the purpose of the carbon layer is primarily to improve conductivity, suppress volume expansion and structural collapse, prevent side reactions, and promote lithium-ion transport. The preferred mass of the carbon layer is 1% to 3% of the total mass of the phosphate cathode material, such as 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0023] The method for preparing the phosphate cathode material provided by this invention includes the following steps:

[0024] S1. Synthesis of precursor: A precursor is obtained by hydrothermal reaction of a lithium source, a metal source and a phosphorus source, wherein the metal source is selected from at least one of an iron source, a manganese source and a cobalt source;

[0025] S2. Add lithium phosphate and carbon source: Add nano-sized lithium phosphate and carbon source to the precursor to obtain carbon source precursor;

[0026] S3. In-situ quantum dot precipitation: The carbon source precursor is granulated and then subjected to low-temperature pre-sintering and high-temperature sintering to obtain phosphate cathode material; the heating rate of the low-temperature pre-sintering is 3~5℃ / min, the temperature is 300℃~400℃, and the time is 2h~5h; the heating rate of the high-temperature sintering is 1~2℃ / min, the temperature is 600℃~700℃, and the time is 3h~10h.

[0027] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the amount of lithium source and phosphorus source is preferably such that the total lithium-phosphorus molar ratio Li / P in the obtained precursor is 3.05~3.15, such as 3.05, 3.08, 3.1, 3.12, 3.15, etc.

[0028] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the molar ratio of the metal source to the phosphorus source is preferably (0.96~1.02):1, such as 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, etc.

[0029] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the lithium source can be at least one of lithium hydroxide, lithium carbonate, lithium nitrate and lithium fluoride.

[0030] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the phosphorus source may be at least one of phosphoric acid, monoammonium phosphate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, diammonium dihydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, and diammonium hydrogen phosphate.

[0031] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the metal source is selected from at least one of an iron source, a manganese source, and a cobalt source. The iron source may include at least one of ferrous sulfate, ferrous chloride, and ferrous nitrate. The manganese source may include at least one of manganese sulfate, manganese chloride, manganese nitrate, manganese carbonate, and manganese oxalate. The cobalt source may include at least one of cobalt sulfate, cobalt chloride, cobalt nitrate, and cobalt oxalate.

[0032] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the hydrothermal reaction conditions are designed to enrich the resulting precursor with amorphous lithium phosphate clusters. Specifically, the hydrothermal reaction conditions preferably include a temperature of 160℃~200℃, such as 160℃, 165℃, 170℃, 175℃, 180℃, 185℃, 190℃, 195℃, 200℃, etc.; a pH value of 7~8.5, such as 7, 7.2, 7.5, 7.8, 8, 8.2, 8.5, etc.; and a time of 10h~15h, such as 10h, 11h, 12h, 13h, 14h, 15h, etc. Under the above conditions, a small amount of amorphous lithium phosphate clusters can be formed in situ. These lithium phosphate clusters are uniformly attached to the surface and voids of the precursor particles, providing conditions for the subsequent formation of quantum dots.

[0033] In the preparation process of the above-mentioned phosphate cathode material, in step S1, the hydrothermal reaction is preferably carried out in the presence of a reducing agent. This avoids the oxidation of metal ions during the hydrothermal reaction, thereby giving the phosphate cathode material better performance. Examples of reducing agents include at least one of ascorbic acid, triethyl phosphite, and phosphorus pentasulfide.

[0034] In the preparation process of the aforementioned phosphate cathode material, step S2, adding nano-sized lithium phosphate and a carbon source to the precursor can be achieved by uniformly mixing the precursor with nano-sized lithium phosphate and / or lithium- and phosphorus-containing compounds and a carbon source. Preferably, this mixing is carried out in a low-temperature environment of 5°C to 15°C, specifically 5°C, 8°C, 10°C, 12°C, 15°C, etc. When the mixing is carried out in the above-preferred low-temperature environment, not only can the agglomeration of cathode material particles be effectively avoided, but it is also easier to make them uniformly dispersed, allowing the nano-lithium phosphate or precursor to effectively embed into the defects and crystal interfaces of the precursor particles, giving the lithium-ion battery better rate performance and cycle stability. Furthermore, the mixing is preferably carried out in a ball mill. The preferred mixing conditions include a rotation speed of 450 rpm to 550 rpm, such as 450 rpm, 480 rpm, 500 rpm, 520 rpm, 550 rpm, etc.; and a time of 4 h to 8 h, such as 4 h, 5 h, 6 h, 7 h, 8 h, etc.

[0035] In the preparation process of the above-mentioned phosphate cathode material, in step S2, the particle size of the nano-sized lithium phosphate is preferably 50nm~100nm, such as 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 100nm, etc. The addition of the nano-sized lithium phosphate can directly supplement lithium phosphate into the precursor. The lithium- and phosphorus-containing compound can be lithium dihydrogen phosphate or a mixture of phosphoric acid and lithium hydroxide. The lithium- and phosphorus-containing compound can generate nano-sized lithium phosphate in situ to supplement the nano-sized lithium phosphate. Furthermore, the amount of the nano-sized lithium phosphate and / or the lithium- and phosphorus-containing compound is preferably such that Li3PO4 accounts for 0.7%~3.5% of the total mass of the final product, specifically 0.7%, 1.4%, 2.1%, 2.8%, 3.5%, etc.

[0036] In the preparation process of the above-mentioned phosphate cathode material, the purpose of adding the carbon source in step S2 is to form a carbon coating layer. The type of carbon source is not particularly limited, but examples include at least one of glucose, sucrose, polyvinyl alcohol, and polyvinylidene fluoride. The amount of carbon source used is preferably 5-10% of the total mass of the precursor, such as 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, etc.

[0037] In the preparation process of the above-mentioned phosphate cathode material, the granulation method in step S3 is not particularly limited, but spray drying granulation is preferred.

[0038] In the preparation process of the above-mentioned phosphate cathode material, step S3, the sintering method for the in-situ precipitation of quantum dots includes sequential low-temperature pre-sintering and high-temperature crystallization. The heating rate of the low-temperature pre-sintering is 3~5℃ / min, such as 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5℃ / min, etc. The temperature of the low-temperature pre-sintering is 300℃~400℃, such as 300℃, 320℃, 340℃, 360℃, 380℃, 400℃, etc. The time of the low-temperature pre-sintering is 2h~5h, such as 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, etc. The heating rate of the high-temperature sintering is 1~2℃ / min, such as 1, 1.2, 1.4, 1.6, 1.8, 2℃ / min, etc. The high-temperature sintering temperature is 600℃~700℃, such as 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, etc. The high-temperature pre-sintering time is 3h~10h, such as 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, etc. The low-temperature pre-sintering mainly aims to remove moisture and carbonize the carbon source to form an initial carbon network, while some nano-lithium phosphate begins to diffuse into the matrix. Slow heating and long holding time are key in the high-temperature sintering process. During this process, the olivine-type main crystal phase gradually forms and grows. Since the lattice constant of Li3PO4 does not match that of the olivine-type main crystal phase, it cannot be completely dissolved. Lithium phosphate accumulates at defects such as grain boundaries and dislocations. By precisely controlling the heating rate and holding time, these substances can be controlled to nucleate and grow into 1-5 nm lithium phosphate quantum dots.

[0039] In a preferred embodiment, the method for preparing the phosphate cathode material further includes washing the precursor with water before adding nano-sized lithium phosphate and carbon source to the precursor. This process can remove impurities (such as lithium sulfate) from the precursor, thus avoiding the influence of these impurities on the performance of the phosphate cathode material and giving the lithium-ion battery better rate performance and cycle stability.

[0040] The present invention also provides a phosphate cathode material prepared by the above method.

[0041] The present invention also provides the application of the phosphate cathode material in lithium-ion batteries.

[0042] The present invention will be described in detail below through embodiments.

[0043] Example 1

[0044] This embodiment is used to illustrate the phosphate cathode material and its preparation method provided by the present invention.

[0045] S1. Synthesis of the precursor: 7.80 kg (186 mol) of lithium hydroxide monohydrate, 16.65 kg (60 mol) of ferrous sulfate heptahydrate, 6.91 kg (60 mol) of 85% phosphoric acid aqueous solution, 50 g of ascorbic acid, and 150 kg of water were added to a stirred tank and mixed thoroughly. The final pH of the mixture was precisely adjusted to 7.5 using LiOH solution. The resulting suspension was transferred to a 5L high-pressure reactor, and the air was purged with nitrogen three times. The reaction was carried out at 190℃ for 5 hours. After the reaction was completed, the mixture was cooled, and the mother liquor was removed by plate and frame filtration. The mixture was then washed with 100 kg of pure water to obtain the washed slurry (precursor).

[0046] S2. Add lithium phosphate and carbon source: Add 0.19 kg phosphoric acid (85%), 0.21 kg lithium hydroxide monohydrate, 0.75 kg glucose and 0.37 kg polyethylene glycol 4000 to the washed slurry and stir until uniform. Add pure water to control the solid content of the slurry to 25%. In a low temperature environment of 10℃, use a high-energy ball mill at a speed of 500 rpm for 4 hours to obtain a mixed slurry (carbon source precursor).

[0047] S3. In-situ precipitation of quantum dots: The above mixed slurry is spray-dried and granulated, and then subjected to two-stage programmed temperature rise sintering. Specifically, under a nitrogen atmosphere, the temperature is first raised to 350°C at 4°C / min and held for 3 hours, and then raised to 700°C at a slow rate of 2°C / min and held for 3 hours. After cooling and gas crushing, a phosphate cathode material doped with bulk lithium phosphate quantum dots is obtained.

[0048] The transmission electron microscope (TEM) image of the phosphate cathode material is shown below. Figure 1 ,from Figure 1 As can be seen, it comprises phosphate particles and a carbon layer coating the surface of the phosphate particles. The primary particles of the phosphate particles contain densely distributed white lithium phosphate quantum dots. The lithium phosphate quantum dots have a particle size of 1 nm to 5 nm and account for 2.1% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe and x is 0.03. The carbon layer accounts for 2.0% of the total mass of the phosphate cathode material.

[0049] Example 2

[0050] This embodiment is used to illustrate the phosphate cathode material and its preparation method provided by the present invention.

[0051] S1. Synthesis of the precursor: Similar to Example 1, specifically, 7.80 kg (186 mol) of lithium hydroxide monohydrate, 16.65 kg (60 mol) of ferrous sulfate heptahydrate, 6.91 kg (60 mol) of 85% phosphoric acid aqueous solution, 50 g of ascorbic acid, and 150 kg of water were added to a stirred tank and mixed thoroughly. The final pH of the mixture was precisely adjusted to 7.5 using LiOH solution. The resulting suspension was transferred to a 5L high-pressure reactor, and the air was purged with nitrogen three times. The reaction was carried out at 190°C for 5 hours. After the reaction was completed, the mixture was cooled, and the mother liquor was removed by plate and frame filtration. The mixture was then washed with 100 kg of pure water to obtain the washed slurry (precursor).

[0052] S2. Add lithium phosphate and carbon source: Add 0.06 kg phosphoric acid (85%), 0.07 kg lithium hydroxide monohydrate, 0.75 kg glucose and 0.37 kg polyethylene glycol 4000 to the washed slurry and stir until uniform. Add pure water to control the solid content of the slurry to 25%. In a low temperature environment of 10℃, use a high-energy ball mill at a speed of 500 rpm for 4 hours to obtain a mixed slurry (carbon source precursor).

[0053] S3. In-situ precipitation of quantum dots: The above mixed slurry is spray-dried and granulated, and then subjected to two-stage programmed temperature rise sintering. Specifically, under a nitrogen atmosphere, the temperature is raised to 300°C at 3°C / min and held for 5 hours, and then raised to 600°C at a slow rate of 1°C / min and held for 10 hours. After cooling and gas crushing, a phosphate cathode material doped with bulk lithium phosphate quantum dots is obtained.

[0054] The phosphate cathode material includes phosphate particles and a carbon layer coating the surface of the phosphate particles. The primary particles of the phosphate particles contain lithium phosphate quantum dots with a particle size of 1 nm to 3 nm. The content of lithium phosphate quantum dots accounts for 0.7% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe and x is 0.01. The mass of the carbon layer is 2.0% of the total mass of the phosphate cathode material.

[0055] Example 3

[0056] This embodiment is used to illustrate the phosphate cathode material and its preparation method provided by the present invention.

[0057] S1. Synthesis of the precursor: 7.80 kg (186 mol) of lithium hydroxide monohydrate, 16.65 kg (60 mol) of ferrous sulfate heptahydrate, 6.91 kg (60 mol) of 85% phosphoric acid aqueous solution, 50 g of ascorbic acid, and 150 kg of water were added to a stirred tank and mixed thoroughly. The final pH of the mixture was precisely adjusted to 7.5 using LiOH solution. The resulting suspension was transferred to a 5L high-pressure reactor, and the air was purged with nitrogen three times. The reaction was carried out at 190℃ for 5 hours. After the reaction was completed, the mixture was cooled, and the mother liquor was removed by plate and frame filtration. The mixture was then washed with 100 kg of pure water to obtain the washed slurry (precursor).

[0058] S2. Add lithium phosphate and carbon source: Add 0.317 kg phosphoric acid (85%), 0.35 kg lithium hydroxide monohydrate, 0.75 kg glucose and 0.37 kg polyethylene glycol 4000 to the washed slurry and stir until uniform. Add pure water to control the solid content of the slurry to 25%. In a low temperature environment of 10℃, use a high-energy ball mill at a speed of 500 rpm for 4 hours to obtain a mixed slurry (carbon source precursor).

[0059] S3. In-situ precipitation of quantum dots: The above mixed slurry is spray-dried and granulated, and then subjected to two-stage programmed temperature rise sintering. Specifically, under a nitrogen atmosphere, the temperature is raised to 400°C at 5°C / min and held for 2 hours, and then raised to 650°C at a slow rate of 1.5°C / min and held for 5 hours. After cooling and gas crushing, a phosphate cathode material doped with bulk lithium phosphate quantum dots is obtained.

[0060] The phosphate cathode material includes phosphate particles and a carbon layer coating the surface of the phosphate particles. The primary particles of the phosphate particles contain lithium phosphate quantum dots with a particle size of 1 nm to 3 nm. The content of lithium phosphate quantum dots accounts for 3.5% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe and x is 0.05. The mass of the carbon layer accounts for 2.0% of the total mass of the phosphate cathode material.

[0061] Example 4

[0062] Phosphate cathode materials were prepared according to the method of Example 1, except that 16.65 kg (60 mol) of ferrous sulfate heptahydrate in step S1 was replaced with 6.672 kg (24 mol) of ferrous sulfate heptahydrate and 6.084 kg (36 mol) of manganese sulfate monohydrate. The remaining conditions were the same as in Example 1, resulting in a bulk lithium phosphate cathode material doped with lithium phosphate quantum dots. This phosphate cathode material comprises phosphate particles and a carbon layer coating the surface of the phosphate particles. The primary particles of the phosphate particles contain lithium phosphate quantum dots with a particle size of 4 nm to 5 nm. The content of lithium phosphate quantum dots accounts for 2.1% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe and Mn, x is 0.03, and the mass of the carbon layer is 2.0% of the total mass of the phosphate cathode material.

[0063] Example 5

[0064] Phosphate cathode materials were prepared according to the method of Example 1, except that 0.19 kg of phosphoric acid (85%) and 0.21 kg of lithium hydroxide monohydrate in step S2 were replaced with 0.191 kg of nano-sized lithium phosphate (primary particle size of 30 nm to 50 nm). The remaining conditions were the same as in Example 1, resulting in a bulk lithium phosphate cathode material doped with lithium phosphate quantum dots. This phosphate cathode material comprises phosphate particles and a carbon layer coating the surface of the phosphate particles. The primary particles of the phosphate particles contain lithium phosphate quantum dots with a particle size of 4 nm to 5 nm. The content of lithium phosphate quantum dots accounts for 2.1% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe, x is 0.03, and the mass of the carbon layer is 2.0% of the total mass of the phosphate cathode material.

[0065] Comparative Example 1

[0066] The phosphate cathode material was prepared according to the method of Example 1, except that step S1 did not include the step of precisely adjusting the final pH of the mixture to 7.5 using LiOH solution, at which point the pH of the system was 6.0, and phosphoric acid and lithium hydroxide monohydrate were not added in step S2. The remaining conditions were the same as in Example 1, resulting in a reference phosphate cathode material. This reference phosphate cathode material comprises phosphate particles and a carbon layer coating the surface of the phosphate particles, wherein no quantum dots are present in the phosphate particles.

[0067] Comparative Example 2

[0068] Phosphate cathode materials were prepared according to the method of Example 1, except that the heating rate of high-temperature sintering in step S3 was controlled at 8°C / min, while the other conditions were the same as in Example 1, to obtain a reference phosphate cathode material. This reference phosphate cathode material comprises phosphate particles and a carbon layer coating the surface of the phosphate particles. A TEM image of this reference phosphate cathode material is shown below. Figure 2 ,from Figure 2 It can be seen that the primary particles of the phosphate particles contain lithium phosphate quantum dots with a particle size of 15nm~30nm. The content of lithium phosphate quantum dots accounts for 2.1% of the total mass of the phosphate cathode material. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is Fe and x is 0.03. The mass of the carbon layer is 2.0% of the total mass of the phosphate cathode material.

[0069] Comparative Example 3

[0070] Phosphate cathode materials were prepared according to the method of Example 4, except that step S1 did not include the step of precisely adjusting the final pH of the mixture to 7.5 using LiOH solution, at which point the pH of the system was 6.0, and phosphoric acid and lithium hydroxide monohydrate were not added in step S2. The remaining conditions were the same as in Example 1, resulting in a reference phosphate cathode material. This reference phosphate cathode material comprises phosphate particles and a carbon layer coating the surface of the phosphate particles. A TEM image of this reference phosphate cathode material is shown below. Figure 3 ,from Figure 3 It can be seen that no quantum dots are present in the phosphate particles.

[0071] Battery fabrication example

[0072] S1. Preparation of the positive electrode sheet for lithium-ion batteries: The phosphate positive electrode materials obtained in the above embodiments and the reference phosphate positive electrode materials obtained in the comparative examples, the conductive agent Super-P, and the binder PVDF are added to N-methylpyrrolidone at a mass ratio of 96:2:2 and mixed evenly to prepare a positive electrode slurry. The positive electrode slurry is coated on the current collector aluminum foil with a surface density of 18 mg / cm², dried at 85°C, and then cold-pressed. After trimming, cutting, and slitting, it is dried under vacuum at 85°C for 4 hours and then the tabs are welded to prepare the positive electrode sheet for lithium-ion batteries.

[0073] S2. Preparation of negative electrode sheet for lithium-ion battery: Active material graphite, conductive agent Super-P, thickener CMC, and binder SBR are added to deionized water at a mass ratio of 96.5:1.0:1.0:1.5 and mixed evenly to form a negative electrode slurry. The negative electrode slurry is coated on current collector copper foil with a surface density of 18 mg / cm², dried at 85°C, and then cold-pressed. After trimming, cutting, and slitting, it is dried under vacuum at 110°C for 4 hours. The tabs are then welded to form the negative electrode sheet for lithium-ion battery.

[0074] S3. Preparation of electrolyte for lithium-ion batteries: The electrolyte for lithium-ion batteries uses 1M LiPF6 as the lithium salt and a mixture of dimethyl carbonate (DMC), ethylene carbonate (EC), and propylene carbonate (PC) as the non-aqueous organic solvent, with the mass ratio of each carbonate being DEC:EC:PC = 1:1:1.

[0075] S4. Preparation of lithium-ion batteries: Positive electrode, negative electrode and separator polyethylene (PE film) are wound into cells with a thickness of 4mm, a width of 40mm and a length of 80mm. The cells are placed in a packaging bag and sealed. They are vacuum baked at 75℃ for 10h, injected with electrolyte, sealed and left to stand for 24h to form and remove excess gas, thus obtaining lithium-ion batteries.

[0076] Battery Test Example 1

[0077] The performance of lithium-ion batteries corresponding to the phosphate cathode materials obtained in Examples 1-3 and 5 and the reference phosphate cathode materials obtained in Comparative Examples 1-2 were tested according to the following methods, and the results are shown in Table 1.

[0078] (1) 0.5C coulombic efficiency / 0.5C charging capacity / 0.5C discharging capacity: At 25°C, five lithium-ion batteries corresponding to the phosphate cathode materials obtained in the above embodiments and the reference phosphate cathode materials obtained in the comparative examples were taken. The lithium-ion batteries were first charged to 4.2V with a constant current of 0.5C (1.275mA) and then charged to 0.05C (0.128mA) at a constant voltage of 4.2V to obtain the charging capacity. The charging capacity was divided by the weight of the cathode material to obtain the charging capacity. The average value of the charging capacity of the five lithium-ion batteries was taken as the 0.5C charging capacity. Then, the lithium-ion batteries were discharged to 2.5V with a constant current of 0.5C (1.275mA) to obtain the discharging capacity. The discharging capacity was divided by the weight of the cathode material to obtain the discharging capacity. The average value of the discharging capacity of the five lithium-ion batteries was taken as the 0.5C discharging capacity. 0.5C coulombic efficiency = (0.5C discharge capacity / 0.5C charge capacity) × 100%.

[0079] (2) 18s internal resistance: At 25°C, the battery at 50% SOC was discharged with a constant current of 1C for 18 seconds, and the voltage difference ΔV (mV) before and after discharge was recorded. The DC internal resistance (mΩ) was calculated according to the formula R=ΔV / I, and then the value was divided by the mass (g) of the positive electrode active material in the battery to obtain the internal resistance (mΩ / g).

[0080] (3) 3.2V plateau retention rate: Under constant temperature (e.g., 25°C), the battery is subjected to its first charge-discharge test at a current of 0.5C. The capacity released in the 3.2V ± 0.05V voltage range of the discharge curve (plateau capacity) is recorded. Plateau retention rate = plateau capacity / total capacity over the entire voltage range × 100%.

[0081] (4) 1C discharge capacity: At 25°C, take 3 lithium-ion batteries corresponding to the phosphate cathode materials obtained in each of the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.2V with a constant current of 0.5C. Then, charge them to 0.05C at a constant voltage of 4.2V. Finally, discharge the lithium-ion batteries to 2.5V with a constant current of 1C to obtain the 1C discharge capacity.

[0082] (5) 5C discharge capacity: At 25°C, take 3 lithium-ion batteries corresponding to the phosphate cathode materials obtained in each of the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.2V with a constant current of 0.5C. Then, charge them to 0.05C at a constant voltage of 4.2V. Finally, discharge the lithium-ion batteries to 2.5V with a constant current of 5C to obtain the 5C discharge capacity.

[0083] (6) -10℃ discharge capacity retention rate: At 25℃, take 5 lithium-ion batteries corresponding to the phosphate cathode materials obtained in the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.2V with a constant current of 0.5C. Then charge them to 0.05C at a constant voltage of 4.2V. Then place the batteries in a -10℃ constant temperature chamber for 12 hours. Discharge the lithium-ion batteries to 2.5V with a constant current of 1C at -10℃ to obtain the -10℃ discharge capacity. Then divide the -10℃ discharge capacity by the discharge capacity of 0.5C at 25℃ in step (1) to obtain the -10℃ discharge capacity retention rate.

[0084] Battery Test Example 2

[0085] The performance of lithium-ion batteries corresponding to the phosphate cathode material obtained in Example 4 and the reference phosphate cathode material obtained in Comparative Example 3 was tested according to the following methods, and the results are shown in Table 1.

[0086] (1) 0.5C coulombic efficiency / 0.5C charging capacity / 0.5C discharging capacity: At 25°C, five lithium-ion batteries corresponding to the phosphate cathode materials obtained in the above embodiments and the reference phosphate cathode materials obtained in each comparative example were taken. The lithium-ion batteries were first charged to 4.4V with a constant current of 0.5C (1.275mA) and then charged to 0.05C (0.128mA) at a constant voltage of 4.4V to obtain the charging capacity. The charging capacity was divided by the weight of the cathode material to obtain the charging capacity. The average value of the charging capacity of the five lithium-ion batteries was taken as the 0.5C charging capacity. Then, the lithium-ion batteries were discharged to 2.5V with a constant current of 0.5C (1.275mA) to obtain the discharging capacity. The discharging capacity was divided by the weight of the cathode material to obtain the discharging capacity. The average value of the discharging capacity of the five lithium-ion batteries was taken as the 0.5C discharging capacity. 0.5C coulombic efficiency = (0.5C discharge capacity / 0.5C charge capacity) × 100%.

[0087] (2) 18s internal resistance: At 25°C, the battery at 50% SOC was discharged with a constant current of 1C for 18 seconds, and the voltage difference ΔV (mV) before and after discharge was recorded. The DC internal resistance (mΩ) was calculated according to the formula R=ΔV / I, and then the value was divided by the mass (g) of the positive electrode active material in the battery to obtain the internal resistance (mΩ / g).

[0088] (3) 3.9V plateau retention rate: Under constant temperature (e.g., 25°C), the battery is subjected to its first charge-discharge test at a current of 0.5C. The capacity released in the 3.9V ± 0.05V voltage range of the discharge curve (plateau capacity) is recorded. Plateau retention rate = plateau capacity / total capacity in the entire voltage range × 100%.

[0089] (4) 1C discharge capacity: At 25°C, take 3 lithium-ion batteries corresponding to the phosphate cathode materials obtained in each of the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.4V with a constant current of 0.5C, charge them to 0.05C at a constant voltage of 4.4V, and then discharge them to 2.5V with a constant current of 1C to obtain the 1C discharge capacity.

[0090] (5) 5C discharge capacity: At 25°C, take 3 lithium-ion batteries corresponding to the phosphate cathode materials obtained in each of the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.4V with a constant current of 0.5C, charge them to 0.05C at a constant voltage of 4.4V, and then discharge them to 2.5V with a constant current of 5C to obtain the 5C discharge capacity.

[0091] (6) -10℃ discharge capacity retention rate: At 25℃, take 5 lithium-ion batteries corresponding to the phosphate cathode materials obtained in the above embodiments and the reference phosphate cathode materials obtained in each comparative example. First, charge the lithium-ion batteries to 4.4V with a constant current of 0.5C. Then charge them to 0.05C at a constant voltage of 4.4V. Then place the batteries in a -10℃ constant temperature chamber for 12 hours. Discharge the lithium-ion batteries to 2.5V with a constant current of 1C at -10℃ to obtain the -10℃ discharge capacity. Then divide the -10℃ discharge capacity by the discharge capacity of 0.5C at 25℃ in step (1) to obtain the -10℃ discharge capacity retention rate.

[0092] Table 1

[0093]

[0094] As can be seen from the results in Table 1, the 18s internal resistance of Examples 1-3 and Example 5 is significantly lower than that of Comparative Examples 1-2, and the 18s internal resistance of Example 4 is significantly lower than that of Comparative Example 3. This demonstrates that the phosphate cathode material provided by this invention exhibits high rate performance. Furthermore, the 3.2V plateau retention rate and -10℃ discharge capacity retention rate of Examples 1-3 and Example 5 are higher than those of Comparative Examples 1-2, and the 3.9V plateau retention rate and -10℃ discharge capacity retention rate of Example 4 are higher than those of Comparative Example 3. This indicates that the phosphate cathode material provided by this invention possesses good cycle stability.

[0095] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A phosphate cathode material, characterized in that, The phosphate cathode material includes phosphate particles and a carbon layer coated on the surface of the phosphate particles. The primary particles of the phosphate particles have lithium phosphate quantum dots with a particle size of 1 nm to 5 nm distributed inside. The general formula of the phosphate particles is LiMPO4•xLi3PO4, where M is at least one of Fe, Mn and Co, and x is 0.01 to 0.

05.

2. The phosphate cathode material according to claim 1, characterized in that, The content of lithium phosphate quantum dots accounts for 0.7% to 3.5% of the total mass of the phosphate cathode material.

3. The phosphate cathode material according to claim 1 or 2, characterized in that, The mass of the carbon layer is 1% to 3% of the total mass of the phosphate cathode material.

4. A method for preparing the phosphate cathode material according to any one of claims 1 to 3, characterized in that, The method includes the following steps: S1. Synthesis of precursor: A precursor is obtained by hydrothermal reaction of a lithium source, a metal source and a phosphorus source, wherein the metal source is selected from at least one of an iron source, a manganese source and a cobalt source, and the hydrothermal reaction is performed under conditions that make the obtained precursor rich in amorphous lithium phosphate clusters. S2. Add lithium phosphate and carbon source: Add nano-sized lithium phosphate and carbon source to the precursor to obtain carbon source precursor; S3. In-situ quantum dot precipitation: The carbon source precursor is granulated and then subjected to low-temperature pre-sintering and high-temperature sintering to obtain phosphate cathode material; the heating rate of the low-temperature pre-sintering is 3~5℃ / min, the temperature is 300℃~400℃, and the time is 2h~5h; the heating rate of the high-temperature sintering is 1~2℃ / min, the temperature is 600℃~700℃, and the time is 3h~10h.

5. The method for preparing the phosphate cathode material according to claim 4, characterized in that, In step S1, the amount of lithium source and phosphorus source used is such that the total lithium-phosphorus molar ratio Li / P in the obtained precursor is 3.05~3.15; The molar ratio of the metal source to the phosphorus source is (0.96~1.02):1; The lithium source is selected from at least one of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium fluoride; The phosphorus source is selected from at least one of phosphoric acid, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, and diammonium hydrogen phosphate. The iron source is selected from at least one of ferrous sulfate, ferrous chloride and ferrous nitrate; The manganese source is selected from at least one of manganese sulfate, manganese chloride, manganese nitrate, manganese carbonate, and manganese oxalate; The cobalt source is selected from at least one of cobalt sulfate, cobalt chloride, cobalt nitrate, and cobalt oxalate; The conditions for the hydrothermal reaction include a temperature of 160℃~200℃, a pH value of 7~8.5, and a time of 5h~15h. The hydrothermal reaction is carried out in the presence of a reducing agent; The reducing agent is selected from at least one of ascorbic acid, triethyl phosphite, and phosphorus pentasulfide.

6. The method for preparing the phosphate cathode material according to claim 4, characterized in that, In step S2, the method of adding nano-sized lithium phosphate and carbon source to the precursor is to mix the precursor with nano-sized lithium phosphate and / or lithium- and phosphorus-containing compounds and carbon source evenly. The amount of the nano-sized lithium phosphate and / or lithium- and phosphorus-containing compounds used is such that Li3PO4 accounts for 0.7% to 3.5% of the total mass of the final product; The lithium- and phosphorus-containing compounds are lithium dihydrogen phosphate or a mixture of phosphoric acid and lithium hydroxide; The carbon source is selected from at least one of glucose, sucrose, polyvinyl alcohol, and polyvinylidene fluoride; The amount of carbon source used is 5-10% of the total mass of the precursor; The mixing is carried out in a low-temperature environment of 5℃~15℃; The mixing method involves ball milling at a speed of 450-550 rpm for 4-8 hours.

7. The method for preparing the phosphate cathode material according to claim 4, characterized in that, The method also includes washing the precursor with water before adding nanoscale lithium phosphate and carbon source to the precursor.

8. The phosphate cathode material prepared by the method according to any one of claims 4 to 7.

9. The application of the phosphate cathode material according to any one of claims 1 to 3 and 8 in lithium-ion batteries.

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