A composite additive for a lithium iron phosphate positive electrode material, a positive electrode material, and a preparation method and application thereof

Abstract

This invention relates to the field of lithium-ion battery cathode material technology, and discloses a composite additive for lithium iron phosphate cathode materials, the cathode material itself, its preparation method, and its application. The composite additive, by mass parts, comprises 1-20 parts of an ammonium salt compound, 15-60 parts of component A, and 0.01-1 parts of component B. Component A includes one or more of anatase nano-titanium dioxide and nano-zirconia, and component B includes one or more of multi-walled carbon nanotubes, carbon black, and hard carbon. This invention, through the synergistic compounding of the ammonium salt compound, component A, and component B, can improve the dispersibility, conductivity, and structural stability of lithium iron phosphate cathode materials, facilitating the construction of more complete electron transport pathways and enhancing lithium-ion migration capabilities, thereby improving the rate performance and cycle performance of the material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a composite additive for lithium iron phosphate cathode material, the cathode material itself, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, due to their advantages such as high energy density, long cycle life, and environmental friendliness, have been widely used in electric vehicles, energy storage systems, and portable electronic devices. As a crucial component of lithium-ion batteries, the performance of the cathode material directly impacts the battery's energy density, rate performance, cycle stability, and safety. Lithium iron phosphate (LFP) has become a widely used cathode material in lithium-ion batteries due to its high safety, low cost, good thermal stability, and environmental friendliness.

[0003] However, lithium iron phosphate (LFP) still has certain limitations. Its poor electronic conductivity and low lithium-ion diffusion rate restrict its application under high-rate charge-discharge and high-compact-density conditions, thus affecting the material's rate performance and cycle performance. Meanwhile, in actual preparation, it is necessary to consider the material's conductivity, structural stability, compaction density, and electrochemical performance, which places high demands on the preparation and modification of LFP cathode materials.

[0004] To improve the performance of lithium iron phosphate cathode materials, existing technologies typically employ modification methods such as nano-sizing, surface carbon coating, elemental doping, and surfactant-based particle morphology control. For example, materials with smaller particle sizes can be prepared through solid-state methods, hydrothermal methods, sol-gel methods, or co-precipitation methods to shorten the lithium-ion diffusion path; electron transport capabilities can also be improved by coating the material surface with a conductive carbon layer, or structural stability and cycle performance can be improved by introducing specific additives, dopants, and auxiliary agent systems.

[0005] However, existing technologies still have shortcomings in practical applications. On the one hand, existing modification schemes often need to consider multiple functional requirements, making it difficult to simultaneously achieve a balance between material nanostructuring, conductivity modification, structural stability, and high compaction density. This can easily lead to problems such as decreased compaction density, insufficient structural stability, or unsatisfactory overall electrochemical performance. On the other hand, some preparation processes are characterized by long procedures, numerous control points, and high costs, which are not conducive to stable industrial implementation. For certain additives or modification systems, there are also issues such as uneven mixing and dispersion with precursors, limited effects, and unstable performance improvements.

[0006] In addition, while existing technologies improve the high-rate performance and long-cycle stability of lithium iron phosphate materials, they often fail to balance conductivity and structural stability under high-temperature conditions, which limits the application of materials in high-temperature environments or extreme working conditions. Summary of the Invention

[0007] This invention provides a composite additive for lithium iron phosphate cathode materials, cathode materials, their preparation methods, and applications, in order to solve the problems of poor electronic conductivity, low lithium-ion diffusion rate, difficulty in simultaneously achieving nano-sizing, conductivity modification, structural stability, and compaction density in existing lithium iron phosphate cathode materials, and the relatively complex, costly, and limited performance improvement of existing modification processes.

[0008] In a first aspect, the present invention provides a composite additive for lithium iron phosphate cathode materials, comprising, by mass parts, 1 to 20 parts of an ammonium salt compound, 15 to 60 parts of component A, and 0.01 to 1 part of component B; Component A includes one or more of anatase nano-titanium dioxide and nano-zirconia, while component B includes one or more of multi-walled carbon nanotubes, carbon black, and hard carbon.

[0009] In this invention, the ammonium salt compound refers to a compound containing ammonium ions or a quaternary ammonium salt structure that can play a dispersing and regulating role in the composite additive system. The ammonium salt compound can improve the dispersion uniformity of component A and component B in ball milling and subsequent precursor slurry, reduce the agglomeration of inorganic particles or conductive components, and enable the composite additive to act more uniformly on the lithium iron phosphate precursor system.

[0010] In one optional embodiment, the composition comprises, by mass parts, 5 to 15 parts of ammonium salt compound, 15 to 60 parts of component A, and 0.01 to 1 part of component B.

[0011] By rationally controlling the amount of ammonium salt compounds, this invention can balance the dispersing and regulating effects with the material structure stability during the subsequent calcination process, thereby giving the resulting lithium iron phosphate cathode material superior overall performance.

[0012] In one optional embodiment, the ammonium salt compound includes one or more of hexadecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, dodecyldimethylbenzylammonium chloride, ammonium sulfate, and dodecylammonium sulfate.

[0013] In one optional embodiment, the anatase nano-titanium dioxide has a particle size of 20nm to 300nm, preferably 50nm to 150nm. For example, the particle sizes of the anatase nano-titanium dioxide are 20nm, 30nm, 50nm, 100nm, 150nm, 200nm, and 300nm.

[0014] Anatase nano-titanium dioxide can participate in regulating the particle structure and interface state of lithium iron phosphate materials, which is beneficial to improving the phase stability and cycle stability of the materials.

[0015] In one optional embodiment, the particle size of the nano-zirconia is 10nm~300nm, preferably 100nm~300nm. For example, the particle size of nano-zirconia is 10nm, 30nm, 50nm, 100nm, 150nm, 200nm, or 300nm.

[0016] Nano-zirconia has good structural stability and can play a role in stabilizing the particle structure and inhibiting abnormal grain growth during cycling in composite additive systems.

[0017] In one optional embodiment, the total content of magnetic metal substances in component A is less than or equal to 50 ppm. By controlling the content of magnetic metal substances in component A, the adverse effects of magnetic metal impurities on the phase stability and electrochemical performance of lithium iron phosphate cathode materials can be reduced.

[0018] In one optional embodiment, when component B includes both hard carbon and multi-walled carbon nanotubes, the mass ratio of hard carbon to multi-walled carbon nanotubes is (0.5-2):1, for example, the mass ratio of hard carbon to multi-walled carbon nanotubes is 0.5:1, 1:1, or 2:1; preferably 1:1.

[0019] Secondly, the present invention provides a method for preparing a composite additive as described in any of the above technical solutions, comprising the following steps: S1. The ammonium salt compound and component A are ball-milled and mixed in the presence of a dispersion medium to obtain a mixed slurry; S2. Add component B to the mixed slurry and continue ball milling to obtain a ball-milled slurry; S3. Remove the dispersion medium from the ball mill slurry and dry it to obtain the final product; preferably, the drying temperature is 80℃~100℃ and the drying time is 2h~4h. In the preparation method of the composite additive of the present invention, ammonium salt compound and component A are first ball-milled and mixed in the presence of a dispersion medium, and then component B is added and ball-milling is continued. This is beneficial for the ammonium salt compound to first disperse and surface-regulate component A, forming a relatively uniform inorganic stable component dispersion system. Subsequently, component B is added and ball-milling is continued, which is beneficial for the conductive component to be further uniformly dispersed and compounded with component A, thereby improving the overall dispersion uniformity and conductive network construction ability of the composite additive.

[0020] Compared to the method of directly mixing and ball milling ammonium salt compounds, component A and component B in one step, the present invention adopts a step-by-step ball milling method, which can reduce the agglomeration between inorganic nanoparticles and conductive components, making it easier for the composite additive to be evenly dispersed in the subsequent lithium iron phosphate precursor slurry, thereby improving its effect on improving the structural stability, conductivity and cycle performance of lithium iron phosphate cathode materials.

[0021] In one optional embodiment, in step S1, the ammonium salt compound and component A are dried before ball milling; In one optional embodiment, in step S1, the ball-to-material ratio of the ball mill is (10~20):1, the ball milling speed is 200 r / min~300 r / min, and the ball milling time is 2h~4h; In this invention, the ball-to-material ratio refers to the mass ratio of grinding balls to solid materials added to the ball mill.

[0022] In one optional embodiment, in step S2, component B is dried before being added to the mixed slurry; In one optional embodiment, in step S2, the ball milling speed is 200 r / min to 300 r / min, and the ball milling time is 1 h to 2 h; Preferably, the drying temperature of the drying process is 80℃~120℃, and the drying time is 4h~6h; Preferably, the dispersion medium includes one or more of ethanol, ethylene glycol, isopropanol, and acetonitrile; Preferably, the amount of the dispersion medium is 5 to 15 times the total mass of the ammonium salt compound and component A; for example, the amount of the dispersion medium is 5, 10, or 15 times the total mass of the ammonium salt compound and component A.

[0023] Thirdly, the present invention provides a lithium iron phosphate material containing a composite additive as described in any of the above technical solutions, wherein the lithium iron phosphate material is prepared from raw materials including a lithium source, an iron source, a phosphorus source and the composite additive; and the amount of the composite additive added is 0.2wt% to 3wt% based on the total dry basis mass of the lithium source, iron source and phosphorus source.

[0024] In this invention, the total dry basis mass of the lithium source, iron source and phosphorus source refers to the total mass of the solid materials of the lithium source, iron source and phosphorus source after deducting water or solvent.

[0025] Fourthly, the present invention provides a method for preparing lithium iron phosphate material as described in the above technical solution, comprising the following steps: (1) Lithium source, iron source, phosphorus source and water are mixed to obtain lithium iron phosphate precursor slurry; (2) The composite additive is mixed with the lithium iron phosphate precursor slurry and then dried to obtain precursor powder; (3) The precursor powder is first pre-calcined at a temperature of 300℃~400℃ for 2h~3h, then heated to 600℃~850℃ for 6h~8h, cooled, crushed, and sieved to obtain the product.

[0026] In an optional embodiment, in step (1), lithium source, iron source, and phosphorus source are weighed according to the molar ratio of Li, Fe, and P of (1.00-1.05):1:(0.95-1.02), and the lithium source, iron source, and phosphorus source are mixed with water to obtain lithium iron phosphate precursor slurry. Preferably, the solid content of the lithium iron phosphate precursor slurry is 20wt%-30wt%.

[0027] In this invention, the total molar ratio of Li, Fe, and P elements refers to the total molar ratio of Li, Fe, and P elements in the lithium source, iron source, and phosphorus source, respectively. When the same raw material contains two or more of Li, Fe, and P, the corresponding elements in the raw material are all included in the total molar amount of the corresponding element. For example, when iron phosphate is used as the iron source, Fe in iron phosphate is included in the total molar amount of Fe element, and P in iron phosphate is included in the total molar amount of P element; when lithium dihydrogen phosphate is used as the lithium source, Li in lithium dihydrogen phosphate is included in the total molar amount of Li element, and P in lithium dihydrogen phosphate is included in the total molar amount of P element.

[0028] In one optional embodiment, in step (2), the composite additive is added to the lithium iron phosphate precursor slurry, and ultrasonic dispersion and mechanical stirring are performed sequentially to obtain a mixed slurry; then the mixed slurry is spray-dried to obtain precursor powder.

[0029] In one optional embodiment, in step (2), the composite additive is added to the lithium iron phosphate precursor slurry, ultrasonically dispersed for 30 min to 60 min, and then mechanically stirred for 1 h to 2 h to obtain a mixed slurry; then the mixed slurry is spray-dried to obtain precursor powder.

[0030] In one optional embodiment, in step (2), the inlet temperature of the spray dryer is 200-240°C and the outlet temperature is 80-100°C.

[0031] In one optional embodiment, the lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; In one optional embodiment, the iron source includes one or more of ferric phosphate, ferrous oxalate, and ferric oxide; In one alternative embodiment, the phosphorus source includes one or more of ammonium dihydrogen phosphate, phosphoric acid, or diammonium hydrogen phosphate.

[0032] In one optional embodiment, in step (3), the precursor powder is first pre-calcined at a temperature of 300℃~400℃ for 2h~3h, and then heated to 700℃~800℃ for calcination. In one optional implementation, in step (3), the heating rate is 5℃ / min to 10℃ / min; In one alternative implementation, in step (3), calcination is carried out under an inert atmosphere; Preferably, the gas flow rate of the inert atmosphere is 50 mL / min to 100 mL / min.

[0033] Fifthly, the application of a lithium iron phosphate material as described in any of the above technical solutions or a lithium iron phosphate material prepared by any of the above technical solutions in a lithium-ion battery.

[0034] The lithium iron phosphate material can be used as a positive electrode active material for lithium-ion batteries. Because this lithium iron phosphate material has good conductivity, rate performance, and cycle stability, it is suitable for lithium-ion battery systems that require high rate charge / discharge performance, high cycle life, or high-temperature stability.

[0035] In one optional embodiment, the lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a current collector and a positive active material layer disposed on the surface of the current collector. The positive active material layer includes the aforementioned lithium iron phosphate material.

[0036] In one optional embodiment, the positive electrode active material layer further includes a conductive agent and a binder. The conductive agent may include one or more of conductive carbon black, acetylene black, carbon nanotubes, and graphene; the binder may include one or more of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0037] When lithium-ion batteries are prepared using the aforementioned lithium iron phosphate material, the capacity retention performance of the battery under high-rate discharge, long-cycle use, and high-temperature conditions can be improved.

[0038] The technical solution of this invention has the following advantages: The composite additive of this invention, through the combination of ammonium salt compounds, component A, and component B, can synergistically improve the overall performance of lithium iron phosphate cathode materials. The ammonium salt compounds are beneficial for improving system dispersibility and interfacial interactions. The anatase nano-titanium dioxide and nano-zirconia in component A help enhance the structural stability of the material and improve the interfacial state during cycling. The multi-walled carbon nanotubes, carbon black, and hard carbon in component B are beneficial for constructing conductive pathways and improving electron transport capabilities. Therefore, this composite additive can, to a certain extent, balance conductivity, lithium-ion diffusion performance, and structural stability, thereby improving the rate performance, cycle performance, and high-temperature stability of lithium iron phosphate cathode materials. The resulting material using this composite additive can achieve higher 1C initial discharge specific capacity, 5C discharge specific capacity, and cycle retention rate, and exhibits superior performance in terms of compaction density, conductivity, and high-temperature cycle stability.

[0039] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0040] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0041] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0042] The method for testing the content of magnetic metal substances in this invention is as follows: a magnetic rod with a magnetic field strength of 6000-8000 GS is used to adsorb magnetic foreign matter in the sample. Then, the non-magnetic powder adsorbed on the surface of the magnetic rod is washed with water, while the magnetic foreign matter is retained. The obtained magnetic foreign matter is then dissolved in aqua regia, and the contents of Fe, Cr, Ni and Zn in the solution are tested by ICP. The sum of the contents of the above four elements is taken as the content of magnetic foreign matter in component A.

[0043] The raw materials involved in the embodiments and comparative examples of this invention are as follows: Hexadecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, dodecyldimethylbenzylammonium chloride, ammonium sulfate, and dodecylammonium sulfate were purchased from Inokai. Multi-walled carbon nanotubes, purchased from Tiannai Technology; Carbon black, purchased from Cabot; Hard carbon, ethylene glycol, and butanediol were purchased from Wanhua Chemical. Propylene glycol, phosphoric acid, and ammonium dihydrogen phosphate were purchased from Inokai. Ferric phosphate was purchased from Anhui Nayuan; ferrous oxalate was purchased from Hefei Yalong. Lithium carbonate, purchased from Tianqi Lithium.

[0044] Example 1 A. Preparation of composite additives; By mass parts, the composite additive consists of the following components: Five parts of ammonium salt compound, 60 parts of component A, and 1 part of component B. The ammonium salt compound is hexadecyltrimethylammonium chloride; component A is anatase nano-titanium dioxide (purchased from Guizhou Shengwei), with a particle size of 50 nm and a magnetic metallic content of 10 ppm; component B is multi-walled carbon nanotubes.

[0045] The preparation method of the composite additive includes the following steps: S1. Weigh out hexadecyltrimethylammonium chloride, component A anatase nano-titanium dioxide, and component B multi-walled carbon nanotubes according to the above mass proportions, place them in a vacuum drying oven, and dry them at 80°C for 6 hours to remove moisture. The dried anatase nano-titanium dioxide (component A) and hexadecyltrimethylammonium chloride (HMC) were added to a high-energy ball mill and mixed using ethanol as the dispersion medium. The amount of ethanol used was 10 times the total mass of component A and the ammonium salt compound; the ball-to-material ratio was 10:1, where the ball-to-material ratio is the mass ratio of the grinding balls to the solid material added to the ball mill; the ball milling speed was 200 r / min, and the milling time was 4 h, yielding a mixed slurry.

[0046] S2. Add the dried multi-walled carbon nanotubes of component B to the mixed slurry obtained in step S1, and continue ball milling at 200 r / min for 2 h to disperse the multi-walled carbon nanotubes in the mixed slurry, thus obtaining the ball-milled slurry.

[0047] S3. The ball mill slurry obtained in step S2 is filtered to remove the ethanol dispersion medium. The resulting filter cake is then placed in a vacuum drying oven and dried at 80°C for 4 hours to obtain the composite additive.

[0048] B. Preparation of lithium iron phosphate materials; The preparation method of lithium iron phosphate cathode material includes the following steps: (1) Weigh lithium carbonate, iron oxide and ammonium dihydrogen phosphate in a molar ratio of Li, Fe and P of 1.02:1:1, add them to deionized water, stir with a magnetic stirrer for 30 min to obtain a uniform lithium iron phosphate precursor slurry with a solid content of 30%.

[0049] (2) Add the composite additive prepared in step A to the lithium iron phosphate precursor slurry obtained in step (1), wherein the amount of the composite additive added is 0.2 wt%, based on the total dry basis mass of the lithium source, iron source and phosphorus source in step (1); then disperse using an ultrasonic disperser for 60 min, and then stir with a mechanical stirrer for 1 h to uniformly disperse the composite additive in the lithium iron phosphate precursor slurry to obtain a mixed slurry. The mixed slurry is fed into a spray dryer for spray drying, wherein the inlet temperature of the spray dryer is 200℃ and the outlet temperature is 80℃, to obtain dried precursor powder.

[0050] (3) The precursor powder was placed in an argon-protected calcination furnace with an argon flow rate of 50 mL / min and heated to 300℃ at a heating rate of 5℃ / min for 3 h. Then, the temperature was increased to 700℃ at a heating rate of 5℃ / min and calcined for 8 h. After calcination, the temperature was naturally cooled to room temperature of 25℃, and then pulverized by an air jet mill and passed through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0051] Example 2 A. Preparation of composite additives; By mass parts, the composite additive consists of the following components: The mixture comprises 10 parts of ammonium salt compound, 35 parts of component A, and 0.5 parts of component B. The ammonium salt compound is a mixture of dodecyltrimethylammonium bromide and ammonium sulfate in a 1:1 mass ratio. Component A is a mixture of anatase nano-titanium dioxide (purchased from Anhui Jingrui New Materials) and nano-zirconia (purchased from Anhui Jingrui New Materials) in a 2:1 mass ratio, wherein the anatase nano-titanium dioxide has a particle size of 150 nm, the nano-zirconia has a particle size of 100 nm, and the total content of magnetic metals in component A is 30 ppm. Component B is carbon black.

[0052] The preparation method of the composite additive includes the following steps: S1. Weigh out the ammonium salt compound, component A, and component B according to the above mass proportions, place them in a vacuum drying oven, and dry them at 100°C for 5 hours to remove moisture; The dried component A and ammonium salt compound were added to a high-energy ball mill and mixed using ethanol as the dispersion medium. The amount of ethanol used was 10 times the total mass of component A and the ammonium salt compound; the ball-to-material ratio was 15:1, where the ball-to-material ratio is the mass ratio of the grinding balls to the solid material added to the ball mill; the ball mill speed was 250 r / min, and the ball milling time was 3 h, yielding a mixed slurry.

[0053] S2. Add the dried component B to the mixed slurry obtained in step S1, and continue ball milling at 250 r / min for 1.5 h to disperse component B in the mixed slurry and obtain the ball-milled slurry.

[0054] S3. Filter the ball mill slurry obtained in step S2 to remove the ethanol dispersion medium, and then place the obtained filter cake in a vacuum drying oven and dry it at 90°C for 3 h to obtain the composite additive.

[0055] B. Preparation of lithium iron phosphate materials; The preparation method of lithium iron phosphate cathode material includes the following steps: (1) Weigh lithium hydroxide, ferrous oxalate and phosphoric acid in a molar ratio of Li, Fe and P of 1:1:1, add them to deionized water, stir with a magnetic stirrer for 40 min to obtain a uniform lithium iron phosphate precursor slurry with a solid content of 20%.

[0056] (2) Add the composite additive prepared in step A to the lithium iron phosphate precursor slurry obtained in step (1), wherein the amount of the composite additive added is 1.5 wt%, based on the total dry basis mass of the lithium source, iron source and phosphorus source in step (1); then disperse using an ultrasonic disperser for 45 min, and then stir with a mechanical stirrer for 1.5 h to uniformly disperse the composite additive in the lithium iron phosphate precursor slurry to obtain a mixed slurry. The mixed slurry is fed into a spray dryer for spray drying, wherein the inlet temperature of the spray dryer is 220°C and the outlet temperature is 90°C, to obtain dried precursor powder.

[0057] (3) The precursor powder was placed in a nitrogen-protected calcination furnace with a nitrogen flow rate of 75 mL / min and heated to 350°C at a heating rate of 8°C / min for 2.5 h. Then, the temperature was increased to 750°C at a heating rate of 8°C / min and calcined for 7 h. After calcination, the powder was naturally cooled to room temperature of 25°C and then pulverized by an air jet mill and passed through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0058] Example 3 A. Preparation of composite additives; By mass parts, the composite additive consists of the following components: The mixture contains 15 parts of an ammonium salt compound, 15 parts of component A, and 0.01 parts of component B. The ammonium salt compound is dodecyl dimethyl benzyl ammonium chloride; component A is nano-zirconia (purchased from Shanghai Shenhua Titanium Dioxide Co., Ltd.), with a particle size of 300 nm and a total magnetic metal content of 50 ppm; component B is a mixture of hard carbon and multi-walled carbon nanotubes in a 1:1 mass ratio.

[0059] The preparation method of the composite additive includes the following steps: S1. Weigh out the ammonium salt compound, component A, and component B according to the above mass proportions, place them in a vacuum drying oven, and dry them at 120°C for 4 hours to remove moisture; The dried component A and ammonium salt compound were added to a high-energy ball mill and mixed using ethanol as the dispersion medium. The amount of ethanol used was 10 times the total mass of component A and the ammonium salt compound; the ball-to-material ratio was 20:1, where the ball-to-material ratio is the mass ratio of the grinding balls to the solid material added to the ball mill; the ball mill speed was 300 r / min, and the ball milling time was 2 h, yielding a mixed slurry.

[0060] S2. Add the dried component B to the mixed slurry obtained in step S1, and continue ball milling at 300 r / min for 1 h to disperse component B in the mixed slurry and obtain the ball-milled slurry.

[0061] S3. Filter the ball mill slurry obtained in step S2 to remove the ethanol dispersion medium, and then place the obtained filter cake in a vacuum drying oven and dry it at 100°C for 2 h to obtain the composite additive.

[0062] B. Preparation of lithium iron phosphate materials; The preparation method of lithium iron phosphate cathode material includes the following steps: (1) Weigh lithium hydroxide, iron oxide and diammonium hydrogen phosphate according to the molar ratio of Li, Fe and P of 1.05:1:1.02, add them to deionized water, stir with a magnetic stirrer for 50 min to obtain a uniform lithium iron phosphate precursor slurry with a solid content of 30%.

[0063] (2) Add the composite additive prepared in step A to the lithium iron phosphate precursor slurry obtained in step (1), wherein the amount of the composite additive added is 3 wt%, based on the total dry basis mass of the lithium source, iron source and phosphorus source in step (1); then disperse it using an ultrasonic disperser for 30 min, and then stir it with a mechanical stirrer for 2 h, so that the composite additive is uniformly dispersed in the lithium iron phosphate precursor slurry to obtain a mixed slurry. The mixed slurry is fed into a spray dryer for spray drying, wherein the inlet temperature of the spray dryer is 240℃ and the outlet temperature is 100℃, to obtain dried precursor powder.

[0064] (3) The precursor powder was placed in an argon-protected calcination furnace with an argon flow rate of 100 mL / min and heated to 400℃ at a heating rate of 10℃ / min for 2 hours. Then, the temperature was increased to 800℃ at a heating rate of 10℃ / min and calcined for 6 hours. After calcination, the powder was naturally cooled to room temperature of 25℃ and then pulverized by an air jet mill and passed through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0065] Example 4 The only difference between this embodiment and Embodiment 2 is that the composition of the composite additive is different. By mass, the composite additive consists of the following components: 1 part of ammonium salt compound, 35 parts of component A, and 0.5 parts of component B.

[0066] Example 5 The only difference between this embodiment and Embodiment 2 is that the composition of the composite additive is different. By mass, the composite additive consists of the following components: 5 parts of ammonium salt compound, 35 parts of component A, and 0.5 parts of component B.

[0067] Example 6 The only difference between this embodiment and Embodiment 2 is that the composition of the composite additive is different. By mass, the composite additive consists of the following components: 15 parts of ammonium salt compound, 35 parts of component A, and 0.5 parts of component B.

[0068] Example 7 The only difference between this embodiment and Embodiment 2 is that the composition of the composite additive is different. By mass, the composite additive consists of the following components: 20 parts of ammonium salt compound, 35 parts of component A, and 0.5 parts of component B.

[0069] Example 8 The only difference between this embodiment and Embodiment 2 is the preparation method of the composite additive.

[0070] Specifically, according to the mass proportions described in Example 2, ammonium salt compound, component A, and component B were weighed and dried separately. The dried ammonium salt compound, component A, and component B were then simultaneously added to a high-energy ball mill, and ball milling was performed using ethanol as the dispersion medium at a speed of 250 r / min for 4.5 h to obtain a ball-milled slurry. Subsequently, the ball-milled slurry was filtered to remove the ethanol dispersion medium, and the resulting filter cake was dried to obtain the comparative composite additive. Except for the different order of addition in the preparation process of the composite additive, the types and amounts of the ammonium salt compound, component A, and component B, the amount of dispersion medium, the filtration and drying conditions, the amount of composite additive added, and the preparation process of the lithium iron phosphate cathode material were all the same as in Example 2.

[0071] Example 9 The only difference between this embodiment and embodiment 2 is the calcination temperature. Specifically, in step (3), the precursor powder is placed in a nitrogen-protected calcination furnace with a nitrogen flow rate of 75 mL / min and heated to 350°C at a heating rate of 8°C / min for 2.5 h. Then, the temperature is increased to 600°C at a heating rate of 8°C / min and calcined for 7 h. After calcination, the powder is naturally cooled to room temperature of 25°C and then pulverized using an air jet mill and passed through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0072] Example 10 The only difference between this embodiment and embodiment 2 is the calcination temperature. Specifically, in step (3), the precursor powder is placed in a nitrogen-protected calcination furnace with a nitrogen flow rate of 75 mL / min and heated to 350°C at a heating rate of 8°C / min for 2.5 h. Then, the temperature is increased to 850°C at a heating rate of 8°C / min and calcined for 7 h. After calcination, the powder is naturally cooled to room temperature of 25°C and then pulverized using an air jet mill and passed through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0073] Comparative Example 1 The only difference between this comparative example and Example 2 is that no composite additive is added to the lithium iron phosphate precursor slurry in step (2); except for the absence of composite additive, the lithium iron phosphate precursor slurry is still processed according to the ultrasonic dispersion, mechanical stirring and spray drying conditions in step (2) of Example 2. The types and amounts of lithium source, iron source and phosphorus source, preparation conditions of lithium iron phosphate precursor slurry, pre-calcination conditions, calcination conditions, cooling, crushing and sieving conditions are the same as in Example 2, and the blank control group lithium iron phosphate cathode material is obtained.

[0074] Comparative Example 2 The only difference between this comparative example and Example 2 is that no ammonium salt compounds are added when preparing the composite additive in this comparative example. That is, the composite additive consists only of component A and component B. The types and amounts of components A and B, the ball milling, filtration and drying conditions of the composite additive, and the preparation conditions of the lithium iron phosphate cathode material are all the same as in Example 2.

[0075] Specifically, the composite additive consists of the following components: 35 parts of component A and 0.5 parts of component B; component A is a mixture of anatase nano-titanium dioxide and nano-zirconium dioxide in a mass ratio of 2:1, and component B is carbon black; components A and B are prepared according to the conditions described in Example 2 to obtain a comparative composite additive that does not contain ammonium salt compounds. Subsequently, according to the amount of composite additive added in Example 2 and the preparation method of lithium iron phosphate cathode material, the comparative composite additive is added to the lithium iron phosphate precursor slurry to obtain the lithium iron phosphate cathode material.

[0076] Test Example 1 This test example demonstrates the testing of lithium iron phosphate cathode materials prepared according to the embodiments and comparative examples of the present invention. The test methods are as follows: (1) Compacted density: Weigh 1.000±0.005 g of the lithium iron phosphate cathode material sample to be tested, and use a Shenzhen Sansi compaction density meter to test the compaction density. The test pressure is 3 t. The sample is compacted under the test pressure, and the compaction density of the material is calculated based on the mass and volume of the compacted sample. The unit is g / cm³.

[0077] (2) Iron phosphide content: XRD analysis was performed on the lithium iron phosphate cathode material samples using an X-ray diffractometer, with a test range of 2θ = 10° to 80°. Phase identification was performed based on the obtained XRD patterns, and the iron phosphide phase in the samples was quantitatively analyzed using XRD refinement analysis to obtain the iron phosphide content in the samples. The iron phosphide content is based on the total mass of the lithium iron phosphate cathode material being tested, and the unit is wt%.

[0078] (3) Grain size and grain growth rate test: High-magnification morphology images of the lithium iron phosphate cathode material before and after 1000 cycles were obtained using scanning electron microscopy (SEM). The obtained SEM images were imported into ThermoFisher's AVIZO software for image modeling and particle size statistical analysis to obtain the average grain size of the sample before and after cycling.

[0079] For each sample, at least three different fields of view were selected for statistical analysis, and at least 100 grains were counted in each field of view. The average value was taken as the grain size test result of the sample.

[0080] Grain growth amplitude is calculated using the following formula: Grain growth amplitude = Grain size after 1000 cycles - Grain size before 1000 cycles; where the units for both grain size and grain growth amplitude are nm.

[0081] (4) Electronic conductivity test: The electronic conductivity of the lithium iron phosphate cathode material under test was measured using a four-probe resistivity meter with an instrument accuracy of ±0.5%FS and a probe spacing of 1 mm.

[0082] The sample preparation method is as follows: The lithium iron phosphate cathode material powder to be tested was dried under vacuum at 85℃ for 2 h. After cooling to room temperature, 0.75 g of the dried powder was weighed and pressed into a disc sample under a pressure of 15 MPa. The disc sample had a diameter of 13 mm and a thickness of 0.75 mm.

[0083] Before testing, the disc samples were kept at the test temperature for 30 minutes. When testing the room temperature electronic conductivity, the test temperature was 25℃; when testing the high temperature electronic conductivity at 60℃, the test temperature was 60℃.

[0084] During testing, the four-probe resistivity meter was preheated for 30 minutes and calibrated using a standard resistance gauge. The circular sample was placed flat on the testing stage, with the four probes perpendicularly contacting the center region of the sample. A current I was applied to the two outer probes, and the voltage U was measured on the two inner probes. Each sample was tested at three different locations. After each test, the sample was rotated 90°, and the average of the three test results was taken as the final result.

[0085] The resistivity ρ is calculated using the following formula: ρ = 2π×l×U / I; where ρ is resistivity in Ω·cm; l is probe spacing in cm; U is voltage in V; and I is current in A.

[0086] The electronic conductivity σ is calculated using the following formula: σ = 1 / ρ; where σ is the electronic conductivity, in units of S / cm.

[0087] (5) Electrochemical performance testing: The lithium iron phosphate cathode material to be tested, PVDF binder and conductive carbon black were weighed and mixed in a mass ratio of 95:2:3, and an appropriate amount of solvent was added to prepare a cathode slurry. The cathode slurry was uniformly coated on the surface of an aluminum foil current collector, and the cathode sheet was obtained after drying, rolling and cutting.

[0088] A coin cell battery was assembled using the aforementioned positive electrode as the positive electrode and a lithium sheet as the negative electrode. After assembly, charge-discharge tests were performed, with the charge-discharge cutoff voltage range being 2.0–3.75 V. The test rates included 1C and 5C, with 1C calculated at 170 mAh / g.

[0089] The test current is calculated using the following formula: I = 170 × m × C; In the formula, I is the test current in mA; m is the mass of lithium iron phosphate cathode material in the cathode sheet in g; C is the charge / discharge rate, C is 1 for 1C test and C is 5 for 5C test.

[0090] The 1C initial discharge specific capacity and 5C discharge specific capacity of the samples were tested respectively, and the units are mAh / g.

[0091] (6) High-temperature rate performance and cycle performance test at 60℃: The assembled coin cells were placed in a constant temperature environment of 60℃ for high-temperature electrochemical performance testing. The test conditions were: charge / discharge cutoff voltage range of 2.0–3.75 V, discharge test at 5C rate, and the specific capacity at 60℃ and 5C was obtained, in mAh / g.

[0092] The battery was subjected to 1000 charge-discharge cycles at 60℃. The initial discharge capacity before the cycles and the discharge capacity after the 1000 cycles were recorded. The capacity retention rate after 1000 cycles at 60℃ was calculated using the following formula: Capacity retention = Discharge capacity after 1000 cycles / Initial discharge capacity × 100%.

[0093] (7) Room temperature cycling performance test The assembled coin cells were subjected to cyclic charge-discharge tests at room temperature, with a charge-discharge cutoff voltage range of 2.0–3.75 V. The initial discharge capacity before cycling and the discharge capacity after 1000 cycles were recorded, and the capacity retention after 1000 cycles was calculated using the following formula: Capacity retention after 1000 cycles = Discharge capacity after 1000 cycles / Initial discharge capacity × 100%.

[0094] The test results are shown in Table 1 below; Table 1: Performance test results of lithium iron phosphate cathode materials prepared in each example and comparative example

[0095] As shown in Table 1, compared with Comparative Examples 1 and 2, the lithium iron phosphate cathode materials prepared in Examples 1 to 10 exhibit superior overall compaction density, lower iron phosphide content, smaller grain growth amplitude before and after cycling, and better room temperature and high temperature electronic conductivity. Furthermore, they demonstrate superior performance in high-temperature rate capability, initial discharge specific capacity, rate discharge specific capacity, and cycle capacity retention. Therefore, the composite additive formed by the compounding of ammonium salt compounds, component A, and component B in this invention can improve the dispersion state, phase stability, conductivity, and cycle structure stability of lithium iron phosphate cathode materials, thereby enhancing their overall electrochemical performance.

[0096] Further comparison of Examples 2, 4 to 7 shows that when the amount of ammonium salt compound added is in the range of 1 to 20 parts, the resulting lithium iron phosphate cathode material has an improved effect compared with Comparative Example 1 without the addition of composite additives and Comparative Example 2 without the addition of ammonium salt compound. This indicates that the ammonium salt compound can cooperate with component A and component B to have a positive impact on the dispersibility, conductivity, phase stability and cycle stability of lithium iron phosphate cathode material.

[0097] Furthermore, in Examples 5, 2, and 6, the amount of ammonium salt compound added was in the range of 5–15 parts. The resulting lithium iron phosphate cathode material exhibited superior overall performance, better balancing compaction performance, conductivity, rate performance, and cycle stability. It is evident that when the amount of ammonium salt compound added is controlled within the range of 5–15 parts, its dispersion regulation effect on components A and B is more sufficient, which is beneficial for the synergistic effect of the structural stabilizing effect of component A and the conductivity modification effect of component B, thereby achieving a better overall modification effect.

[0098] In Example 4, the amount of ammonium salt compound added was less than 5-15 parts. Compared with Comparative Example 2, Example 4 still added ammonium salt compound, so its overall performance was improved to some extent compared with Comparative Example 2 without ammonium salt compound, indicating that even with a relatively low amount of ammonium salt compound, it can still play a role in dispersion regulation and interface improvement to a certain extent. However, due to the low amount of ammonium salt compound, its dispersion promoting effect on components A and B was insufficient, making it difficult to fully improve the dispersion uniformity of the composite additive in the lithium iron phosphate precursor slurry. Therefore, its overall performance was lower than that of Examples 5, 2, and 6.

[0099] In Example 7, the amount of ammonium salt compound added was higher than 5-15 parts. Compared with Comparative Example 2, Example 7 still showed some performance improvement, indicating that even at higher addition levels, ammonium salt compounds could still work together with components A and B to exert a modifying effect. However, compared with Examples 5, 2, and 6, the overall performance of Example 7 decreased, indicating that higher addition levels of ammonium salt compounds are not necessarily better. When the amount of ammonium salt compound added is too high, it will affect the uniform composite state of the inorganic and conductive components, and may adversely affect the particle structure or interface state of the material during subsequent calcination, thereby limiting the improvement in rate performance, high-temperature performance, or cycle stability.

[0100] Compared to Comparative Example 1, the embodiments of the present invention incorporated a composite additive, resulting in a significant improvement in the overall performance of the obtained lithium iron phosphate cathode material. This demonstrates that the composite additive of the present invention can effectively enhance the conductivity, structural stability, and electrochemical performance of the lithium iron phosphate cathode material. Compared to Comparative Example 2, the embodiments of the present invention further introduced ammonium salt compounds, resulting in materials with superior overall performance in terms of impurity phase control, grain stability, conductivity, and cycle performance. This indicates that ammonium salt compounds are not optional conventional components in the composite additive system of the present invention, but rather important components that can synergistically interact with components A and B.

[0101] Comparing Example 8 and Example 2, it can be seen that, with the same types and amounts of components in the composite additive, different feeding sequences and ball milling methods affect the overall performance of the resulting lithium iron phosphate cathode material. Example 2 employs a stepwise composite method, first ball milling and mixing the ammonium salt compound with component A, then adding component B and continuing ball milling. This method facilitates the formation of a more uniform inorganic modification system before introducing the conductive component for composite formation, thereby improving the modification effect of the composite additive in the lithium iron phosphate material. Example 8 did not use the above stepwise composite method, and its overall performance decreased, indicating that the preparation sequence of the composite additive in this invention has a positive effect on improving material performance.

[0102] Comparing Examples 9, 10, and 2, it is evident that the lithium iron phosphate cathode materials obtained at calcination temperatures of 600℃ and 850℃ still exhibit improved performance compared to the comparative examples. This indicates that the calcination temperature range of 600℃ to 850℃ can meet the basic requirements for material preparation in this invention. Furthermore, a calcination temperature of 700℃ to 800℃ is preferred for better overall performance. Calcination temperatures that are too low or too high can affect the phase formation, grain state, or interface structure of the material; a moderate calcination temperature is more conducive to the effective functioning of the composite additives.

[0103] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A composite adjuvant for a lithium iron phosphate cathode material, characterized in that, By mass, it includes 1-20 parts of ammonium salt compound, 15-60 parts of component A, and 0.01-1 parts of component B; Component A includes one or more of anatase nano-titanium dioxide and nano-zirconia, while component B includes one or more of multi-walled carbon nanotubes, carbon black, and hard carbon.

2. The composite additive for a lithium iron phosphate cathode material according to claim 1, characterized in that, By mass, it includes 5-15 parts of ammonium salt compound, 15-60 parts of component A, and 0.01-1 parts of component B; And / or, the ammonium salt compound includes one or more of hexadecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, dodecyldimethylbenzylammonium chloride, ammonium sulfate, and dodecylammonium sulfate. 3.The composite additive for a lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The particle size of the anatase nano-titanium dioxide is 20nm~300nm, preferably 50nm~150nm; And / or, the particle size of the nano-zirconia is 10nm~300nm, preferably 100nm~300nm; And / or, the total content of magnetic metal substances in component A is less than or equal to 50 ppm.

4. A method of producing the complex adjuvant according to any one of claims 1 to 3, characterized by, Includes the following steps: S1. The ammonium salt compound and component A are ball-milled and mixed in the presence of a dispersion medium to obtain a mixed slurry; S2. Add component B to the mixed slurry and continue ball milling to obtain a ball-milled slurry; S3. Remove the dispersion medium from the ball mill slurry and dry it to obtain the final product.

5. The preparation method according to claim 4, characterized in that, In step S1, the ammonium salt compound and component A are dried before ball milling. And / or, in step S1, the ball-to-material ratio of the ball mill is (10~20):1, the ball mill speed is 200 r / min~300 r / min, and the ball milling time is 2h~4h; And / or, in step S2, component B is dried before being added to the mixed slurry; And / or, in step S2, the ball milling speed is 200 r / min to 300 r / min, and the ball milling time is 1 h to 2 h; Preferably, the drying temperature of the drying process is 80℃~120℃, and the drying time is 4h~6h; Preferably, the dispersion medium includes one or more of ethanol, ethylene glycol, isopropanol, and acetonitrile.

6. A lithium iron phosphate material containing the composite additive as described in any one of claims 1-3, characterized in that, The lithium iron phosphate material is prepared from raw materials including lithium source, iron source, phosphorus source and the composite additive; based on the total dry basis mass of the lithium source, iron source and phosphorus source, the amount of the composite additive added is 0.2wt% to 3wt%.

7. A method for preparing lithium iron phosphate material as described in claim 6, characterized in that, Includes the following steps: (1) Lithium source, iron source, phosphorus source and water are mixed to obtain lithium iron phosphate precursor slurry; (2) The composite additive is mixed with the lithium iron phosphate precursor slurry and then dried to obtain precursor powder; (3) The precursor powder is first pre-calcined at a temperature of 300℃~400℃ for 2h~3h, then heated to 600℃~850℃ for 6h~8h, cooled, crushed, and sieved to obtain the product.

8. The preparation method according to claim 7, characterized in that, The lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate. And / or, the iron source includes one or more of ferric phosphate, ferrous oxalate, and ferric oxide; And / or, the phosphorus source includes one or more of ammonium dihydrogen phosphate, phosphoric acid, or diammonium hydrogen phosphate.

9. The preparation method according to claim 7 or 8, characterized in that, In step (3), the precursor powder is first pre-calcined at a temperature of 300℃~400℃ for 2h~3h, and then heated to 700℃~800℃ for calcination. And / or, in step (3), the heating rate is 5℃ / min~10℃ / min; And / or, in step (3), calcination is carried out under an inert atmosphere; Preferably, the gas flow rate of the inert atmosphere is 50 mL / min to 100 mL / min.

10. The application of the lithium iron phosphate material as described in claim 6 or the lithium iron phosphate material prepared by any of the preparation methods described in claims 7-9 in lithium-ion batteries.

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