Lithium iron phosphate composite material and preparation method thereof, positive plate and secondary battery

Abstract

The invention provides a lithium iron phosphate composite material and a preparation method thereof, a positive plate and a secondary battery. The lithium iron phosphate composite material comprises a lithium iron phosphate matrix doped with a magnesium element and a titanium element, and a carbon coating layer coating at least part of the surface of the lithium iron phosphate matrix, the doping amount of the magnesium element is 0.5-2.0% of the molar weight of the iron element in the lithium iron phosphate matrix, and the doping amount of the titanium element is 0.5-1.5% of the molar weight of the iron element in the lithium iron phosphate matrix. According to the lithium iron phosphate composite material provided by the invention, the magnesium element and the titanium element are doped, the magnesium element optimizes the lattice stability, and the titanium element enhances the conductivity; and collaborative optimization of the lattice structure and the conductive network is realized by accurately controlling the doping amount, so that the cycle performance and the rate capability of the secondary battery are improved.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, and particularly relates to a lithium iron phosphate composite material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] Lithium-ion batteries are the core power source for new energy vehicles, energy storage systems, and portable electronic devices, and their performance directly affects the competitiveness of end products. However, with the continuous expansion of application scenarios, existing lithium-ion batteries are increasingly revealing many bottlenecks in their overall performance.

[0003] Currently, commercial lithium-ion batteries, in pursuit of high energy density, generally face significant degradation in cycle performance and rate capability. Electrode materials in high-energy-density systems exhibit insufficient structural stability during long-term cycling, leading to accelerated capacity decline. Simultaneously, the battery's internal resistance increases with the number of uses, making it difficult to support continuous high-rate charging and discharging. Furthermore, existing technologies often focus on improving single performance parameters, making it difficult to achieve synergistic optimization between cycle life and fast-charging capability.

[0004] Therefore, how to simultaneously improve the cycle life and rate performance of lithium-ion batteries has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] This invention provides a lithium iron phosphate composite material, which has a stable crystal structure and an excellent conductive network, and can improve the cycle performance and rate performance of secondary batteries.

[0006] This invention provides a method for preparing lithium iron phosphate composite materials. The lithium iron phosphate composite materials prepared by this method have a stable crystal lattice structure and an excellent conductive network, which can improve the cycle performance and rate performance of secondary batteries.

[0007] The present invention also provides a positive electrode sheet, comprising the above-described lithium iron phosphate composite material or the lithium iron phosphate composite material prepared by the above-described preparation method. Therefore, this positive electrode sheet can improve the rate performance and cycle performance of secondary batteries.

[0008] The present invention also provides a secondary battery comprising the above-described lithium iron phosphate composite material, or the lithium iron phosphate composite material prepared by the above-described preparation method, or the above-described positive electrode sheet. Therefore, this secondary battery exhibits excellent rate performance and cycle performance.

[0009] The first aspect of the present invention provides a lithium iron phosphate composite material, comprising a lithium iron phosphate matrix doped with magnesium and titanium, and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate matrix.

[0010] The magnesium doping amount is 0.5-2.0% of the molar amount of iron in the lithium iron phosphate matrix, and the titanium doping amount is 0.5-1.5% of the molar amount of iron in the lithium iron phosphate matrix.

[0011] The lithium iron phosphate composite material as described above, wherein the molar ratio of lithium, composite metal elements and phosphorus in the lithium iron phosphate matrix is ​​(1.01-1.05):(0.96-0.99):1; the composite metal elements include a mixture of iron, magnesium and titanium.

[0012] In the lithium iron phosphate composite material described above, the carbon coating layer is formed by carbonizing a raw material including a reducing carbon source and a high-conductivity carbon source, wherein the conductivity of the high-conductivity carbon source is not less than 10. -9 S / cm; and / or,

[0013] The thickness of the carbon coating layer is 1-5 nm.

[0014] The lithium iron phosphate composite material as described above, wherein the compacted density of the lithium iron phosphate composite material is not less than 2.65 g / cm³; and / or,

[0015] The average particle size of the lithium iron phosphate composite material is 0.6-1.5 μm.

[0016] A second aspect of the present invention provides a method for preparing the aforementioned lithium iron phosphate composite material, comprising the following steps:

[0017] Raw materials including lithium source, phosphorus source, iron source, carbon source, magnesium source and titanium source are mixed, dried at 150-300℃, and then subjected to low-temperature sintering treatment and high-temperature sintering treatment in sequence to obtain the lithium iron phosphate composite material; wherein, the low-temperature sintering treatment temperature is 300-400℃, and the high-temperature sintering treatment temperature is 650-750℃; the magnesium content in the magnesium source is 0.5-2.0% of the total molar amount of iron in the iron source, and the titanium content in the titanium source is 0.5-1.5% of the total molar amount of iron in the iron source.

[0018] In the preparation method described above, the holding time for the low-temperature sintering treatment is 2-4 hours; and / or,

[0019] The heating rate for the low-temperature sintering treatment is 3-5℃ / min; and / or,

[0020] The holding time for the high-temperature sintering treatment is 8-15 hours; and / or,

[0021] The heating rate for the high-temperature sintering treatment is 2-4℃ / min; and / or,

[0022] The amount of carbon source added is 7-26% of the mass of the lithium iron phosphate composite material.

[0023] In the preparation method described above, the lithium source includes lithium triphosphate; and / or,

[0024] The iron source includes a mixture of ferric phosphate and ferric oxide; and / or,

[0025] The carbon source includes a mixture of a reducing carbon source and a highly conductive carbon source, wherein the highly conductive carbon source includes polyethylene glycol.

[0026] In the preparation method described above, the mass ratio of the reducing carbon source to the highly conductive carbon source is (5-18):(2-8); and / or,

[0027] The high conductivity carbon source also includes at least one of graphene and carbon nanotubes.

[0028] A third aspect of the present invention provides a positive electrode sheet comprising the lithium iron phosphate composite material described in the first aspect above, or the lithium iron phosphate composite material prepared by the preparation method described in the second aspect above.

[0029] A fourth aspect of the present invention provides a secondary battery comprising the lithium iron phosphate composite material of the first aspect, or the lithium iron phosphate composite material prepared by the preparation method of the second aspect, or the positive electrode sheet provided in the third aspect.

[0030] The lithium iron phosphate composite material provided by this invention uses magnesium and titanium doping. Magnesium optimizes lattice stability, while titanium enhances conductivity. By precisely controlling the doping amount, the synergistic optimization of the lattice structure and conductive network is achieved, thereby improving the cycle performance and rate performance of the secondary battery. Attached Figure Description

[0031] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0032] Figure 1 The image shows a SEM image of the lithium iron phosphate composite material provided in Example 1 of this invention.

[0033] Figure 2 The image shows the XRD pattern of the lithium iron phosphate composite material provided in Example 1 of this invention.

[0034] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] The inventors studied lithium iron phosphate (LFP) materials in existing lithium-ion batteries and found that current technologies dope LFP with a specific metal element (such as manganese) to improve its compaction density. While this method increases the compaction density, it cannot simultaneously improve the cycle life and rate performance of lithium-ion batteries. Further research revealed that this incompatibility stems from the difficulty of simultaneously optimizing the crystal lattice structure and electron conduction pathways with a single metal doping.

[0037] Based on this, the first aspect of the present invention provides a lithium iron phosphate composite material, comprising a lithium iron phosphate matrix doped with magnesium and titanium elements, and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate matrix.

[0038] The magnesium doping amount is 0.5-2.0% of the molar amount of iron in the lithium iron phosphate matrix, and the titanium doping amount is 0.5-1.5% of the molar amount of iron in the lithium iron phosphate matrix.

[0039] It should be noted that the lithium iron phosphate composite material has an olivine-type structure, with magnesium and titanium elements dissolved in the crystal lattice. Magnesium occupies the iron sites, and titanium occupies the lithium sites.

[0040] For example, the amount of magnesium doping is 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9% or 2.0% of the molar amount of iron in the lithium iron phosphate matrix, or a range of any two of these values.

[0041] For example, the amount of titanium doping is 0.5%, 0.7%, 0.9%, 1.1%, 1.3% or 1.5% of the molar amount of iron in the lithium iron phosphate matrix, or a range of any two of these values.

[0042] The lithium iron phosphate composite material provided by this invention possesses a stable crystal lattice structure and an excellent conductive network, which can improve the cycle performance and rate performance of secondary batteries. The reason is as follows:

[0043] On the one hand, magnesium occupies iron sites, suppressing volume changes during charge and discharge through lattice regulation and improving cycle stability; titanium occupies lithium sites, enhancing electronic conductivity and lithium-ion diffusion rate by widening lithium-ion migration channels. Their synergistic effect achieves dual optimization of lattice structure and conductive network, thereby improving the structural stability and intrinsic ion and electron transport capabilities of the lithium iron phosphate composite material, ultimately enhancing the cycle performance and rate performance of the secondary battery. On the other hand, precise control of the doping amounts of magnesium and titanium ensures lattice stability while further enhancing the rate performance and cycle life of the lithium iron phosphate composite material. Furthermore, the carbon coating layer forms a three-dimensional conductive network, further reducing interfacial impedance and improving the rate performance of the secondary battery.

[0044] In one specific embodiment, the molar ratio of lithium, composite metal elements, and phosphorus in the lithium iron phosphate matrix is ​​(1.01-1.05):(0.96-0.99):1; the composite metal elements include a mixture of iron, magnesium, and titanium. By precisely controlling the molar ratio of lithium, composite metal elements, and phosphorus in the lithium iron phosphate matrix, the uniform distribution of each element during crystal growth is ensured while maintaining the stability of the material structure, thereby reducing lattice distortion.

[0045] For example, the molar ratio of lithium, composite metal elements and phosphorus in the lithium iron phosphate matrix is ​​1.01:0.96:1, 1.01:0.97:1, 1.01:0.99:1, 1.03:0.96:1, 1.03:0.96:1, 1.03:0.96:1, 1.05:0.96:1, 1.05:0.97:1 or 1.05:0.99:1, or a range consisting of any two of these values.

[0046] To further balance the conductivity and compaction density of lithium iron phosphate composite materials, the thickness of the carbon coating layer and the raw materials used in its preparation can also be controlled.

[0047] In one specific embodiment, the carbon coating layer is formed by carbonizing a raw material comprising a reducing carbon source and a highly conductive carbon source, wherein the conductivity of the highly conductive carbon source is not less than 10. -9 S / cm. A reducing carbon source provides reducing groups to achieve Fe... 3+ To Fe 2+ The high efficiency of reduction, coupled with the high conductivity of the carbon source, further enhances the conductivity of the lithium iron phosphate composite material.

[0048] For example, the conductivity of the high-conductivity carbon source is 10. -9 S / cm, 10 -8 S / cm, 10 -7 S / cm, 10 -6 S / cm, 10 -5 S / cm, 10 -4 S / cm, 10 -3 S / cm or higher, or a range consisting of any two of these values.

[0049] In one specific embodiment, the thickness of the carbon coating layer is 1-5 nm. This thickness balances the conductivity of the lithium iron phosphate composite material with the energy density of the secondary battery.

[0050] For example, the thickness of the carbon coating is 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm or 5 nm, or a range of any two of these values.

[0051] In one specific embodiment, the compaction density of the lithium iron phosphate composite material is not less than 2.65 g / cm³. Within this compaction density range, the energy density of the secondary battery is further improved.

[0052] For example, the compaction density of the lithium iron phosphate composite material is 2.65 g / cm³, 2.66 g / cm³, 2.67 g / cm³, 2.68 g / cm³, 2.69 g / cm³ or above, or a range consisting of any two of these values.

[0053] In this invention, the test method for the compacted density of lithium iron phosphate composite material is as follows: Weigh 1.0000g of lithium iron phosphate composite material powder, pour it into a standard cylindrical mold and level it. Place the mold in the center of a press and pre-compact it to 20MPa at a constant rate of 0.1-0.3mm / min. Then, increase the pressure to 1 ton at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Next, increase the pressure to 2 tons at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Finally, increase the pressure to 3 tons at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Record the density of the last test result, which is the compacted density. Test three parallel samples and calculate the compacted density by taking the average value.

[0054] In one specific embodiment, the average particle size of the lithium iron phosphate composite material is 0.6-1.5 μm. This average particle size range further improves the compaction density of the lithium iron phosphate composite material.

[0055] For example, the average particle size of the lithium iron phosphate composite material is 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm or 1.5 μm, or a range of any two of these values.

[0056] The method for testing the average particle size of the lithium iron phosphate composite material in this invention adopts ISO 14887:2000 "Particle size analysis - Sample preparation and dispersion procedures".

[0057] A second aspect of this invention provides a method for preparing a lithium iron phosphate composite material, comprising the following steps:

[0058] Raw materials including lithium, phosphorus, iron, carbon, magnesium, and titanium sources are mixed, dried at 150-300℃, and then subjected to low-temperature sintering and high-temperature sintering treatments in sequence to obtain lithium iron phosphate composite materials. The low-temperature sintering temperature is 300-400℃, and the high-temperature sintering temperature is 650-750℃. The magnesium content in the magnesium source is 0.5-2.0% of the total molar amount of iron in the iron source, and the titanium content in the titanium source is 0.5-1.5% of the total molar amount of iron in the iron source.

[0059] For example, the drying temperature is 150°C, 170°C, 190°C, 210°C, 230°C, 250°C, 270°C, 290°C, or 300°C, or a range of any two of these values.

[0060] For example, the temperature of the low-temperature sintering treatment is 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C or 400°C, or a range of any two of these values.

[0061] For example, the sintering temperature is 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, or 750°C, or a range of any two of these values.

[0062] This invention achieves magnesium and titanium doping in lithium iron phosphate composite materials by adding magnesium and titanium sources to the raw materials. Simultaneously, it significantly reduces internal defects in the material by controlling crystal growth and carbon coating in stages through a segmented sintering process. Specifically, low-temperature pre-sintering (300-400℃) promotes the reduction of Fe by the carbon source. 3+ For Fe 2+ It is initially carbonized; then high-temperature sintering (650-750℃) is carried out to complete crystal growth and solid solution of doping elements, avoiding lattice defects and particle agglomeration caused by direct high-temperature sintering.

[0063] In this invention, the phosphorus source refers to a raw material that provides phosphorus, the magnesium source refers to a raw material that provides magnesium, and the titanium source refers to a raw material that provides titanium.

[0064] This invention does not specifically limit the types of phosphorus sources, magnesium sources, and titanium sources, and they can be conventional materials in the art. For example, phosphorus sources include, but are not limited to, iron phosphate; magnesium sources include, but are not limited to, magnesium oxide and magnesium hydroxide; and titanium sources include, but are not limited to, titanium oxide and titanium hydroxide.

[0065] This invention does not impose specific limitations on the mixing method of the raw materials, including but not limited to wet grinding. Preferably, the particle size D50 of the slurry after wet grinding is ≤ 0.5μm.

[0066] This invention does not specifically limit the solvent used in wet grinding, including but not limited to deionized water. To further improve the effect of wet grinding, deionized water can be replaced with an ethanol-water mixed solvent (volume ratio 1:1-3), and a small amount of surfactant (such as sodium dodecyl sulfate) can be added to improve the dispersibility of the raw materials. The low surface tension of ethanol and the wetting effect of surfactants can significantly reduce the viscosity of the slurry, improve grinding efficiency, ensure uniform mixing of raw materials at the nanoscale, and reduce agglomeration during subsequent sintering.

[0067] To further improve the uniformity of heating during sintering, the product after wet grinding can also be desolvated.

[0068] This invention does not specifically limit the drying method, including but not limited to spray drying.

[0069] In one embodiment, the inlet temperature of the spray dryer is 200-300°C, and the outlet temperature is 100-150°C.

[0070] To further improve the purity of lithium iron phosphate composite materials, sintering can be performed under inert gas protection.

[0071] The present invention does not specifically limit the type of inert gas. In one embodiment, the inert gas includes at least one of nitrogen, argon, and carbon dioxide.

[0072] To further improve the structural uniformity of lithium iron phosphate composite materials, microwave irradiation treatment was applied during both low-temperature and high-temperature sintering processes. Microwave irradiation treatment directly acts on the interior of the material through electromagnetic waves, significantly shortening the sintering time while reducing grain boundary defects and improving the material's structural uniformity and conductivity.

[0073] In one specific embodiment, the holding time for low-temperature sintering is 2-4 hours. During this holding time, the carbon source is pre-decomposed, ensuring sufficient reduction of ferric ions and forming a uniform coating layer on the particle surface. This forms an amorphous conductive network for subsequent carbonization, improving the electronic conductivity and interparticle connectivity, and further increasing the compaction density of the lithium iron phosphate composite material.

[0074] For example, the holding time for low-temperature sintering is 2h, 2.5h, 3h, 3.5h or 4h, or a range of any two of these values.

[0075] In one specific embodiment, the heating rate of the low-temperature sintering treatment is 3-5°C / min. This heating rate further improves the structural uniformity of the lithium iron phosphate composite material.

[0076] For example, the heating rate of the low-temperature sintering treatment is 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min or 5℃ / min, or a range of any two of these values.

[0077] In one specific embodiment, the holding time for high-temperature sintering is 8-15 hours. This holding time helps to promote full crystal growth, complete the crystal phase reaction, and ensure the full implementation of magnesium and titanium doping and the carbothermic reduction process.

[0078] For example, the holding time for high-temperature sintering is 8h, 9h, 10h, 11h, 12h, 13h, 14h or 15h, or a range of any two of these values.

[0079] In one specific embodiment, the heating rate of the high-temperature sintering process is 2-4 °C / min. This heating rate further reduces grain boundary defects.

[0080] For example, the heating rate of the high-temperature sintering treatment is 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min or 4℃ / min, or a range of any two of these values.

[0081] In one specific embodiment, the amount of carbon source added is 7-26% of the mass of the lithium iron phosphate composite material. Within this range, the amount of carbon source added is more conducive to balancing the conductivity and energy density of the secondary battery.

[0082] For example, the amount of carbon source added is 7%, 9%, 11%, 13%, 15%, 17%, 19%, 21%, 23%, 25% or 26% of the mass of the lithium iron phosphate composite material, or a range of any two of these values.

[0083] In one specific embodiment, the lithium source includes lithium triphosphate. Using lithium triphosphate as the lithium source eliminates gas generation during sintering, further optimizing the particle morphology and crystal structure of the lithium iron phosphate composite material, and increasing its compaction density, thereby improving the energy density of the secondary battery.

[0084] In one specific embodiment, the iron source includes a mixture of iron phosphate and iron oxide. By using iron phosphate and iron oxide as the iron source and precisely controlling the molar ratio of iron to phosphorus, the phase purity of the lithium iron phosphate composite material is further improved.

[0085] In one embodiment, the carbon source comprises a mixture of a reducing carbon source and a highly conductive carbon source, wherein the highly conductive carbon source comprises polyethylene glycol. The reducing carbon source provides reducing groups to promote Fe... 3+ Reduction and high-conductivity carbon sources enhance the conductivity of lithium iron phosphate composites; polyethylene glycol controls particle morphology and forms a uniform carbon coating layer through molecular chain dispersion.

[0086] This invention does not specifically limit the types of reducing carbon sources, including but not limited to glucose.

[0087] When polyethylene glycol is used as a carbon source with high conductivity, the present invention does not specifically limit the degree of polymerization of polyethylene glycol (PEG), but the degree of polymerization is preferably 1000-8000.

[0088] The reducing groups of glucose and the dispersing effect of PEG are synergistically optimized to ensure both Fe 3+ Efficient reduction to Fe 2+ Furthermore, by controlling the particle morphology through PEG molecular chains, a uniform carbon coating layer is formed, which improves the conductivity and rate performance of the secondary battery.

[0089] In one specific embodiment, the mass ratio of the reducing carbon source to the highly conductive carbon source is (5-18):(2-8). Within this range, the mass ratio of the reducing carbon source to the highly conductive carbon source further balances the conductivity and compaction density of the lithium iron phosphate composite material.

[0090] For example, the mass ratio of the reducing carbon source to the highly conductive carbon source is 5:2, 5:5, 5:8, 10:2, 10:5, 10:8, 15:2, 15:5, 15:8, 18:2, 18:5, or 18:8, or a range of any two of these values.

[0091] In one specific embodiment, the highly conductive carbon source further includes at least one of graphene and carbon nanotubes. The high conductivity and large specific surface area of ​​graphene and carbon nanotubes significantly enhance the rate performance of the material, while their sheet-like structure can inhibit particle agglomeration and improve compaction density.

[0092] A third aspect of the present invention provides a positive electrode sheet comprising the lithium iron phosphate composite material described in the first aspect, or the lithium iron phosphate composite material prepared by the preparation method described in the second aspect. Because this positive electrode sheet comprises the aforementioned lithium iron phosphate composite material, it can improve the rate performance and cycle performance of the secondary battery.

[0093] The present invention does not specifically limit the structure of the positive electrode sheet. In one embodiment, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least a portion of the surface of the positive current collector. The positive active layer includes the above-mentioned lithium iron phosphate composite material, conductive agent and binder.

[0094] This invention does not specifically limit the material of the positive electrode current collector; it can be any material conventional in the art. For example, the material of the positive electrode current collector can be either aluminum foil or nickel foil.

[0095] This invention does not specifically limit the type of conductive agent; it can be any material conventional in the art. For example, the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0096] This invention does not specifically limit the type of adhesive, which can be a conventional material in the art. For example, the adhesive can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0097] This invention does not specifically limit the preparation method of the positive electrode sheet. In one embodiment, the positive electrode sheet can be prepared by a method including the following steps:

[0098] The lithium iron phosphate composite material of the present invention is dispersed with a conductive agent and a binder in an N-methylpyrrolidone (NMP) solvent and thoroughly stirred to form a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on a positive electrode current collector, and after drying, rolling and slitting, a positive electrode sheet is obtained.

[0099] This invention does not impose specific limits on the amount of positive electrode active material, conductive agent and binder, and can be adjusted according to the actual situation.

[0100] A fourth aspect of this invention provides a secondary battery comprising the lithium iron phosphate composite material described in the first aspect, or the lithium iron phosphate composite material prepared by the preparation method described in the second aspect, or the positive electrode sheet provided in the third aspect. Therefore, this secondary battery exhibits excellent rate performance and cycle performance.

[0101] It is conceivable that, in addition to the aforementioned positive electrode, the secondary battery of the present invention also includes a negative electrode, an electrolyte, and a separator.

[0102] The present invention does not specifically limit the structure of the negative electrode sheet. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least a portion of the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent.

[0103] This invention does not specifically limit the material of the negative electrode current collector; it can be any conventional material in the art. For example, the negative electrode current collector can be any of copper foil, nickel foam, or copper foam.

[0104] This invention does not specifically limit the type of negative electrode active material, and it can be any negative electrode active material commonly used in batteries. For example, the negative electrode active material can be selected from at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrodes), and tin-based negative electrode materials (mainly including tin and tin alloys).

[0105] This invention does not specifically limit the type of binder; it can be any binder commonly used in battery negative electrodes. For example, the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0106] This invention does not specifically limit the type of conductive agent; it can be any conductive agent commonly used in the negative electrode of batteries. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0107] This invention does not specifically limit the preparation method of the negative electrode sheet. In one embodiment, the negative electrode sheet can be prepared by a method including the following steps:

[0108] The negative electrode active material, conductive agent, and binder are dispersed in deionized water and thoroughly stirred to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector, and after drying, rolling, and slitting, a negative electrode sheet is obtained.

[0109] This invention does not impose specific limits on the amount of negative electrode active material, conductive agent, and binder, and these amounts can be adjusted according to actual conditions.

[0110] This invention does not specifically limit the composition of the electrolyte, which may include one or more solvents commonly used in current battery electrolytes, as well as lithium salts commonly used in current battery electrolytes. For example, the solvent may include at least one of ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, and γ-butyrolactone; the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0111] This invention does not specifically limit the material of the separator; it can be any separator material commonly used in batteries. For example, the separator can be selected from any of the following: polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven fabric separator, and separator with ceramic coating.

[0112] This invention does not specifically limit the preparation method of secondary batteries. In one embodiment, they can be prepared by a method including the following steps:

[0113] The above-mentioned positive electrode sheet, separator, and negative electrode sheet are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried, electrolyte is injected into the dried battery. After the battery is placed, formed, and resealed, the secondary battery is completed.

[0114] The present invention will be further described below through specific embodiments.

[0115] Example 1

[0116] The preparation method of the lithium iron phosphate composite material in this embodiment includes the following steps:

[0117] (1) Weigh out trilithium phosphate (Li3PO4), iron phosphate (FePO4·2H2O), and iron oxide red (Fe2O3) according to the stoichiometric ratio of Li, Fe, and P of 1.01:0.97:1. Weigh out 10% glucose and 5% PEG-4000 by mass of the lithium iron phosphate composite material. Weigh out magnesium oxide and titanium oxide, such that the doping amounts of magnesium and titanium are 1.0% and 1.0% of the molar amount of iron, respectively.

[0118] (2) Mix all the above raw materials with deionized water, add zirconia balls, and grind in a sand mill for 4 hours to obtain a slurry with D50=0.4μm.

[0119] (3) The slurry is spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a dried precursor powder.

[0120] (4) The precursor powder was placed in a tube furnace with high-purity nitrogen gas. The temperature was first raised to 350°C at 5°C / min and held for 3 hours, and then raised to 700°C at 3°C / min and held for 12 hours. After natural cooling, the powder was subjected to air jet pulverization and passed through a 400-mesh sieve to obtain lithium iron phosphate composite material.

[0121] Example 2

[0122] This embodiment is basically the same as Embodiment 1, except that:

[0123] (1) The stoichiometric ratio of Li, Fe, and P is 1.03: 0.955: 1. Weigh 8% glucose and 2% PEG-4000 by mass of the lithium iron phosphate composite material. Weigh magnesium hydroxide and titanium hydroxide so that the doping amounts of magnesium and titanium are 0.5% and 0.5% of the molar amount of iron, respectively.

[0124] (2) Mix all the above raw materials with deionized water, add zirconia balls, and grind in a sand mill for 4 hours to obtain a slurry with D50=0.4μm.

[0125] (3) The slurry is spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a dried precursor powder.

[0126] (4) The precursor powder was placed in a tube furnace with high-purity nitrogen gas. The temperature was first raised to 300°C at 3°C / min and held for 2 hours, and then raised to 650°C at 2°C / min and held for 8 hours. After natural cooling, the powder was subjected to air jet pulverization and passed through a 400-mesh sieve to obtain lithium iron phosphate composite material.

[0127] Example 3

[0128] This embodiment is basically the same as Embodiment 1, except that:

[0129] (1) The stoichiometric ratio of Li, Fe, and P is 1.05 : 0.956 : 1. Weigh 18% glucose and 8% PEG-4000 by mass of the lithium iron phosphate composite material. Weigh magnesium carbonate and titanic acid so that the doping amounts of magnesium and titanium are 2.0% and 1.5% of the molar amount of iron, respectively.

[0130] (2) Mix all the above raw materials with deionized water, add zirconia balls, and grind in a sand mill for 4 hours to obtain a slurry with D50=0.4μm.

[0131] (3) The slurry is spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a dried precursor powder.

[0132] (4) The precursor powder was placed in a tube furnace with high-purity nitrogen gas. The temperature was first raised to 400℃ at 4℃ / min and held for 4h, then raised to 750℃ at 4℃ / min and held for 15h. After natural cooling, the powder was subjected to air jet pulverization and passed through a 400-mesh sieve to obtain lithium iron phosphate composite material.

[0133] Example 4

[0134] This embodiment is basically the same as Embodiment 1, except that:

[0135] Polyethylene glycol was replaced with carbon nanotubes.

[0136] Example 5

[0137] This embodiment is basically the same as Embodiment 1, except that:

[0138] Polyethylene glycol was replaced with graphene.

[0139] Example 6

[0140] This embodiment is basically the same as Embodiment 1, except that:

[0141] Replace lithium triphosphate with lithium carbonate and iron oxide red with ferrous oxalate.

[0142] Comparative Example 1

[0143] This comparative example is basically the same as Example 1, except that:

[0144] (1) Weigh lithium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate according to the stoichiometric ratio of Li, Fe, and P of 1.01:1:1. Weigh sucrose equivalent to 20% of the theoretical product (lithium iron phosphate composite material). Do not add magnesium oxide or titanium oxide.

[0145] (4) The precursor powder was placed in a tube furnace with high-purity nitrogen gas. The temperature was first raised to 350°C at 5°C / min and held for 3 hours, and then raised to 750°C at 3°C / min and held for 12 hours. After natural cooling, the powder was subjected to air jet pulverization and passed through a 400-mesh sieve to obtain lithium iron phosphate composite material.

[0146] Comparative Example 2

[0147] This comparative example is basically the same as Example 1, except that:

[0148] Weigh out titanium oxide so that the amount of titanium doping is 2% of the molar amount of iron.

[0149] Comparative Example 3

[0150] This comparative example is basically the same as Example 1, except that:

[0151] Weigh out magnesium oxide so that the amount of magnesium doping is 2.5% of the molar amount of iron.

[0152] Comparative Example 4

[0153] This comparative example is basically the same as Example 1, except that:

[0154] (4) The precursor powder was placed in a tube furnace with high-purity nitrogen gas, heated to 700°C at 3°C / min and kept at that temperature for 12 hours. After natural cooling, it was subjected to air jet pulverization and passed through a 400-mesh sieve to obtain lithium iron phosphate composite material.

[0155] Experimental Example 1

[0156] 1. The compacted density, tapped density, average particle size, percentage of magnesium doping in the lithium iron phosphate matrix (abbreviated as magnesium doping), percentage of titanium doping in the lithium iron phosphate matrix (abbreviated as titanium doping), carbon coating thickness, and percentage of carbon coating mass in the lithium iron phosphate matrix (abbreviated as carbon coating content) of the lithium iron phosphate composite materials of the examples and comparative examples were tested. The results are shown in Table 1. The preparation parameters of the lithium iron phosphate composite materials of the examples and comparative examples are shown in Table 1. The lithium iron phosphate composite material of Example 1 was characterized by SEM and XRD. The results are shown in Table 1. Figure 1 , 2 .

[0157] Depend on Figure 1 It can be seen that the lithium iron phosphate composite material of Example 1 has a smooth surface and no obvious defects.

[0158] Depend on Figure 2 It can be seen that the lithium iron phosphate composite material of Example 1 has a peak close to that of the theoretical card, indicating that it completely corresponds to the theoretical lithium iron phosphate phase. The lithium iron phosphate composite material of the present invention has high purity and good crystallinity.

[0159] 2. Testing Methods

[0160] 1) Test method for tap density

[0161] The general method for determining the tap density of powder products is GB / T 21354-2008.

[0162] 2) Test method for average particle size

[0163] ISO 14887:2000 "Particle size analysis - Sample preparation and dispersion procedures" was adopted.

[0164] 3) Test methods for magnesium and titanium doping levels

[0165] The tests were performed using ICP-MS (inductively coupled plasma mass spectrometry).

[0166] 4) Test method for carbon coating thickness

[0167] The thickness of the carbon coating was measured using TEM (transmission electron microscopy).

[0168] 5) Test method for compacted density

[0169] Weigh 1.0000g of lithium iron phosphate composite powder, pour it into a standard cylindrical mold, and level it. Place the mold in the center of a press and pre-compact it to 20MPa at a constant rate of 0.1-0.3mm / min. Then, increase the pressure to 1 ton at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Next, increase the pressure to 2 tons at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Finally, increase the pressure to 3 tons at a constant rate of 8-12mm / min, hold the pressure for 30 seconds, and then release the pressure to 20MPa. Record the density of the final test using the system; this is the compacted density. Test three parallel samples and calculate the compacted density by taking the average value.

[0170] Table 1

[0171]

[0172] Experimental Example 2

[0173] 1. The lithium iron phosphate composite materials of the examples and comparative examples were prepared into secondary batteries and their electrochemical performance was tested. The test results are shown in Table 2.

[0174] Methods for preparing secondary batteries:

[0175] A lithium iron phosphate composite material was mixed with conductive carbon black (SP) and polyvinylidene fluoride (PVDF) at a weight ratio of 95:5:5, and dispersed to obtain a positive electrode slurry. This slurry was then coated onto an aluminum foil current collector and rolled to obtain an electrode with an areal density of 17 g / cm³. 2 The positive electrode is then punched into a small disc with a diameter of 12mm using a membrane die. After drying and weighing, the positive electrode is assembled into a CR2032 coin cell in a glove box under an Ar protective atmosphere, using a Li metal disc as the negative electrode, a polypropylene microporous membrane as the separator, and 1M LiPF6 / EC (ethylene carbonate) + DMC (dimethyl carbonate) as the electrolyte.

[0176] Test method:

[0177] 1) Test method for discharge specific capacity

[0178] 0.2C Discharge Specific Capacity: The battery was charged and discharged at 25℃ using a battery charge and discharge tester. The charge and discharge regime was as follows: constant current charging at 0.2C to 4.25V, then constant voltage charging at 4.25V until the current decreased to 0.02C, and after resting for 5 minutes, constant current discharging at 0.2C to 2.5V. The discharge capacity Q was recorded. 0.2c 0.2C discharge specific capacity = Q 0.2c / M, where M is the mass of the lithium iron phosphate composite material, and in this invention, M=10g.

[0179] 1C discharge specific capacity Q 1c The testing method is the same.

[0180] 2) Test methods for rate performance

[0181] 1C discharge rate capacity retention rate = Q 1c / Q 0.2c ×100%.

[0182] 3) Cyclic performance testing methods

[0183] At 25°C, the capacitor was charged at a constant current rate of 1C to 4.50V, then charged at a constant voltage rate of 0.05C to 4.50V, and finally discharged at a discharge rate of 1C to 3.0V. This charge-discharge cycle was repeated 500 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 500th cycle were measured. 500 .

[0184] The capacity retention rate Q after 500 cycles is calculated using the following formula.

[0185] Capacity retention rate Q = Q 500 / Q1*100%.

[0186] Table 2

[0187]

[0188] As shown in Table 2, compared with the comparative example, the secondary battery made from the lithium iron phosphate composite material of the present invention has excellent rate performance and cycle performance.

[0189] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A lithium iron phosphate composite material, characterized in that, The lithium iron phosphate matrix includes a doped magnesium and titanium element, and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate matrix. The magnesium doping amount is 0.5-2.0% of the molar amount of iron in the lithium iron phosphate matrix, and the titanium doping amount is 0.5-1.5% of the molar amount of iron in the lithium iron phosphate matrix.

2. The lithium iron phosphate composite material according to claim 1, characterized in that, The molar ratio of lithium, composite metal elements and phosphorus in the lithium iron phosphate matrix is ​​(1.01-1.05):(0.96-0.99):1; the composite metal elements include a mixture of iron, magnesium and titanium.

3. The lithium iron phosphate composite material according to claim 1 or 2, characterized in that, The carbon coating layer is formed by carbonizing raw materials including a reducing carbon source and a highly conductive carbon source, wherein the conductivity of the highly conductive carbon source is not less than 10. -9 S / cm; and / or, The thickness of the carbon coating layer is 1-5 nm.

4. The lithium iron phosphate composite material according to claim 1, characterized in that, The compaction density of the lithium iron phosphate composite material is not less than 2.65 g / cm³; and / or, The average particle size of the lithium iron phosphate composite material is 0.6-1.5 μm.

5. A method for preparing the lithium iron phosphate composite material according to any one of claims 1-4, characterized in that, Includes the following steps: Raw materials including lithium source, phosphorus source, iron source, carbon source, magnesium source and titanium source are mixed, dried at 150-300℃, and then subjected to low-temperature sintering treatment and high-temperature sintering treatment in sequence to obtain the lithium iron phosphate composite material; wherein, the low-temperature sintering treatment temperature is 300-400℃, and the high-temperature sintering treatment temperature is 650-750℃; the magnesium content in the magnesium source is 0.5-2.0% of the total molar amount of iron in the iron source, and the titanium content in the titanium source is 0.5-1.5% of the total molar amount of iron in the iron source.

6. The preparation method according to claim 5, characterized in that, The holding time for the low-temperature sintering treatment is 2-4 hours; and / or, The heating rate for the low-temperature sintering treatment is 3-5℃ / min; and / or, The holding time for the high-temperature sintering treatment is 8-15 hours; and / or, The heating rate for the high-temperature sintering treatment is 2-4℃ / min; and / or, The amount of carbon source added is 7-26% of the mass of the lithium iron phosphate composite material.

7. The preparation method according to claim 5, characterized in that, The lithium source includes lithium triphosphate; and / or, The iron source includes a mixture of ferric phosphate and ferric oxide; and / or, The carbon source includes a mixture of a reducing carbon source and a highly conductive carbon source, wherein the highly conductive carbon source includes polyethylene glycol.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the reducing carbon source to the highly conductive carbon source is (5-18):(2-8); and / or, The high conductivity carbon source also includes at least one of graphene and carbon nanotubes.

9. A positive electrode plate, characterized in that, The lithium iron phosphate composite material according to any one of claims 1-4, or the lithium iron phosphate composite material prepared by the preparation method according to any one of claims 5-8.

10. A secondary battery, characterized in that, It includes the lithium iron phosphate composite material according to any one of claims 1-4, or the lithium iron phosphate composite material prepared by the preparation method according to any one of claims 5-8, or the positive electrode sheet according to claim 9.

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