High-power and high-energy-density lithium iron phosphate material, preparation method thereof, lithium iron phosphate positive electrode sheet and preparation method and application thereof

Abstract

This invention provides a method for preparing high-power, high-energy-density lithium iron phosphate material, comprising the following steps: (1) preparation of lithium iron phosphate precursor; (2) precursor pretreatment: ball milling the lithium iron phosphate precursor with liquefied paraffin / polyvinyl chloride binder in a solvent, followed by vacuum drying to obtain a pretreated precursor; (3) cerium doping: mixing a cerium salt solution with the pretreated precursor, followed by spray drying to obtain a cerium-doped precursor; (4) two-stage sintering: first-stage sintering: sintering the cerium-doped precursor in an inert atmosphere to obtain a sintering intermediate; second-stage sintering: adding a titanium source to the sintering intermediate to obtain a mixture, and continuing to sinter the mixture in an inert atmosphere to obtain the lithium iron phosphate material; wherein the sintering temperature of the first-stage sintering is higher than that of the second-stage sintering. The lithium iron phosphate material prepared by the method of this invention has high energy density and power density.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to high-power, high-energy-density lithium iron phosphate materials and their preparation methods, lithium iron phosphate cathode sheets and their preparation methods and applications. Background Technology

[0002] In recent years, lithium-ion batteries have been widely used in many automotive and transportation industries due to their high energy density and power density (both by weight and volume). Lithium iron phosphate (LFP) has become one of the most important cathode materials for lithium-ion batteries due to its readily available raw materials, low price, and stable structure. However, LFP has low conductivity and a one-dimensional Li+ movement path, resulting in relatively low specific capacity (compared to ternary materials).

[0003] Therefore, it is urgent to improve the efficiency of ion and electron conduction in lithium iron phosphate batteries and improve their rate performance.

[0004] However, energy density and power density are incompatible in battery design. They are usually balanced by quantifying different battery chemical properties and manufacturing processes. This means that power density is increased at the expense of energy density. This can be achieved by replacing some active materials with conductive fillers, using large pores for ion transport, or using thinner electrodes. However, all of these methods reduce the proportion of active materials inside the battery, which in turn reduces the battery's energy density.

[0005] In response to the above problems, existing technologies, such as patent CN103633289A, increase the energy density of the battery by coating two layers of active material, and then add a conductive layer between the active material layers to improve the lithium-ion diffusion concentration polarization. However, this method has a complex process and increases manufacturing costs. Patent CN109192935A improves the rate performance of the positive electrode by doping the positive electrode material with fluorine and coating it with carbon. Although this method improves the performance to a certain extent, the improvement effect is generally limited for electrodes with high areal density. The wettability between the electrolyte and the positive electrode is also generally limited, and the cell capacity is not fully utilized.

[0006] Therefore, developing high-power, high-energy-density cathodes using appropriate fabrication processes remains a major technical challenge in this field. Summary of the Invention

[0007] To address the aforementioned shortcomings in the existing technology, the present invention aims to provide a high-power, high-energy-density lithium iron phosphate material and its preparation method, a lithium iron phosphate cathode sheet and its preparation method and application. The lithium iron phosphate material provided by the present invention has high energy density and power density.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing high-power, high-energy-density lithium iron phosphate materials, comprising the following steps: (1) Preparation of lithium iron phosphate precursor: The lithium source, iron source, phosphorus source and carbon source are mixed and spray dried to obtain spherical lithium iron phosphate precursor; (2) Precursor pretreatment: The lithium iron phosphate precursor obtained in step (1) is ball-milled with liquefied paraffin / polyvinyl chloride binder in a solvent and vacuum dried to obtain a pretreated precursor; (3) Cerium doping: The cerium salt solution is mixed with the pretreated precursor obtained in step (2), and then spray-dried to obtain the cerium-doped precursor; (4) Two-stage sintering: First stage sintering: The cerium-doped precursor is sintered in an inert atmosphere to obtain a sintered intermediate; Second stage sintering: A titanium source is added to the sintering intermediate to obtain a mixture, and the mixture is sintered in an inert atmosphere at a sintering temperature of 650-740℃ to obtain the lithium iron phosphate material; wherein, the sintering temperature of the first stage sintering is higher than that of the second stage sintering.

[0009] This invention controls grain size and carbon coating integrity through a two-stage sintering process. By controlling the sintering temperature of the second stage, it promotes lattice reconstruction, reduces defects, and improves crystallinity and phase purity. This avoids the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single-stage high-temperature sintering in existing technologies. Furthermore, the traditional one-stage high-temperature sintering process easily leads to particle agglomeration, forming large-sized particles and reducing the compaction density of the material, resulting in a lower compaction density of lithium iron phosphate materials.

[0010] This invention achieves in-situ molding of the porous structure by using a cerium-doped liquefied paraffin / polyvinyl chloride bonding system, thereby ensuring the compaction density of lithium iron phosphate. Through the combined effects of binder pretreatment, spray-drying cerium doping, and two-stage sintering with titanium doping, the compaction density of the material is improved while simultaneously enhancing its ion diffusion efficiency at high areal density; spray drying ensures Ce³… + Nanoscale dispersion, Ce³ + Occupying Li sites creates oxygen vacancies, activating one-dimensional Li + Channels, titanium doping widens lattice spacing, Li + Migration is easier, and through binder pretreatment and cerium / titanium synergistic modification, the traditional trade-off between energy density and power density is broken, providing a material basis for high-power power batteries.

[0011] Furthermore, in step (1), the lithium source includes lithium carbonate, the phosphorus source includes iron phosphate, the iron source includes iron phosphate, and the carbon source is selected from sucrose, glucose, or polyethylene glycol (PEG).

[0012] Furthermore, in step (1), the mass ratio of the carbon source to the iron source is (5-10):95.

[0013] Furthermore, in step (1), the drying temperature of the spray drying is 240-280℃.

[0014] Furthermore, in step (1), the lithium iron phosphate precursor D 50 It is 50-60μm.

[0015] Furthermore, in step (2), the mass ratio of the lithium iron phosphate precursor to the liquefied paraffin / polyvinyl chloride binder is (95-99):(1-5).

[0016] Furthermore, in step (2), the solvent is anhydrous ethanol.

[0017] Furthermore, in step (2), the ball milling speed is 300-500 rpm and the time is 6-8 h.

[0018] Furthermore, in step (2), the vacuum drying temperature is 70-90℃ and the time is 4-5h.

[0019] Furthermore, in step (3), the cerium salt is cerium nitrate.

[0020] Furthermore, in step (3), the mass of the cerium salt is 0.5-1 wt% of the mass of the pretreated precursor.

[0021] Furthermore, in step (3), the spray drying temperature is 240-280℃.

[0022] Furthermore, in step (3), the cerium-doped precursor D 50 It is 30-45μm.

[0023] Furthermore, in step (4), the sintering temperature of the first sintering stage is 760-850℃.

[0024] Furthermore, in step (4), the sintering time for the first sintering stage is 2-4 hours.

[0025] Furthermore, in step (4), the sintering intermediate is pulverized and demagnetized, and the compacted density of the sintering intermediate is 2.30-2.45 g / cm³. 3 D 50 It is 2.0-3.5μm.

[0026] Furthermore, in step (4), the sintering time for the second sintering stage is 5-8 hours.

[0027] Furthermore, in step (4), the titanium source is titanium dioxide.

[0028] Furthermore, in step (4), the mass ratio of the sintering intermediate to titanium dioxide is (93-97):(3-7).

[0029] Furthermore, in step (4), the compaction density of the lithium iron phosphate material is 2.58-2.62 g / cm³. 3 D 50 The range is 0.9-1.2 μm.

[0030] In a second aspect, the present invention provides a high-power, high-energy-density lithium iron phosphate material, said lithium iron phosphate material being prepared by the method described in the first aspect.

[0031] Thirdly, the present invention provides a method for preparing a lithium iron phosphate cathode sheet, comprising: (a) Providing a sintering intermediate and a lithium iron phosphate material prepared by the preparation method described in the first aspect; (b) A first positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a first slurry, wherein the first positive electrode active material is the sintering intermediate; a second positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a second slurry, wherein the second positive electrode active material is the lithium iron phosphate material; (c) The first slurry is coated on the current collector to form a bottom layer, and the second slurry is coated on the bottom layer to form a surface layer. After drying and rolling, a lithium iron phosphate positive electrode sheet is obtained, wherein the total areal density of the lithium iron phosphate positive electrode sheet is 460-500 g / cm², and the thickness ratio of the bottom layer to the surface layer of the lithium iron phosphate positive electrode sheet is (2-3):(7-8).

[0032] Furthermore, in step (b), the conductive agent in the first slurry comprises CNT and SP; the binder comprises PVDF.

[0033] Furthermore, in step (b), the mass ratio of the first positive electrode active material, conductive agent, and binder in the first slurry is (96.5-97.5):(1.2-2):(1.5-2).

[0034] Furthermore, in step (b), the conductive agent in the second slurry comprises CNT and SP; the binder comprises PVDF.

[0035] Furthermore, in the second slurry, the mass ratio of the second positive electrode active material, the conductive agent, and the binder is (96.5-97.5):(1.2-2):(1.5-2).

[0036] Furthermore, in step (b), the viscosity of the first slurry is 4000-8000 mPa·s, and the solid content is 62-65%.

[0037] Furthermore, in step (b), the viscosity of the second slurry is 3000-7200 mPa·s, and the solid content is 61-64%.

[0038] Furthermore, in step (c), a double-layer coating machine is used for coating.

[0039] Furthermore, in step (c), the thickness deviation is ≤1.5μm when the coating speed is ≥30m / min.

[0040] Furthermore, in step (c), the coating speed is 30 m / min to 60 m / min.

[0041] Fourthly, the present invention provides a lithium iron phosphate cathode sheet, which is prepared by the method described in the third aspect, comprising: a bottom layer and a top layer; the porosity of the bottom layer is P1, and the porosity of the top layer is P2; wherein, P2 < P1.

[0042] Fifthly, the present invention provides a lithium-ion battery comprising a positive electrode as described in the fourth aspect.

[0043] Compared with the prior art, the beneficial effects of the present invention include at least one of the following: (1) The lithium iron phosphate material provided by the present invention has high energy density and power density.

[0044] (2) The present invention controls the grain size and carbon coating integrity through two-stage sintering; controls the sintering temperature of the second stage to promote lattice reconstruction, reduce defects, and improve crystallinity and phase purity. This avoids the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single high-temperature sintering in the prior art. Furthermore, the high-temperature sintering of the traditional process easily leads to particle agglomeration, forming large-sized particles, reducing the compaction density of the material, thus resulting in a lower compaction density of lithium iron phosphate material.

[0045] (3) Existing technologies using single cerium doping can only improve lithium-ion conduction in batteries, but cannot improve the powder compaction density of the material. This invention achieves in-situ molding of the pore structure by combining cerium doping with a liquefied paraffin / polyvinyl chloride bonding system. The liquefied paraffin / polyvinyl chloride bonding system pre-treats the iron phosphate precursor, reducing the voids between precursor particles, thereby improving the compaction density of lithium iron phosphate and giving the lithium iron phosphate material a higher energy density.

[0046] (4) This invention improves the porosity of the electrode by using lithium iron phosphate materials of different particle sizes for layered coating, thereby achieving porosity gradient control, which improves the wettability of electrolyte and positive electrode active material under high areal density, and increases lithium ion diffusion efficiency, thereby reducing lithium ion concentration polarization, promoting the utilization of cell capacity, and making the lithium ion battery have a high power density.

[0047] (5) This invention improves the compaction density limit of the material and enhances its ion diffusion efficiency at high areal density through the combined effects of binder pretreatment, spray cerium doping, and two-stage sintering with titanium doping; spray drying ensures that Ce 3+ Nanoscale dispersion, Ce 3+ Occupying Li sites creates oxygen vacancies, inhibits oxygen release, and activates one-dimensional Li. + The presence of channels promotes lithium-ion flow, resulting in higher power density in lithium iron phosphate materials. This not only improves electrochemical performance but also stabilizes the material structure, significantly enhancing cycle performance. Titanium doping widens the lattice spacing, allowing Li... + Migration is easier, and through binder pretreatment and cerium / titanium synergistic modification, the traditional trade-off between energy density and power density is broken, providing a material basis for high-power power batteries. Detailed Implementation

[0048] 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. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0049] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0050] In a first aspect, the present invention provides a method for preparing high-power, high-energy-density lithium iron phosphate materials, comprising the following steps: (1) Preparation of lithium iron phosphate precursor: The lithium source, iron source, phosphorus source and carbon source are mixed and spray dried to obtain spherical lithium iron phosphate precursor; (2) Precursor pretreatment: The lithium iron phosphate precursor obtained in step (1) is ball-milled with liquefied paraffin / polyvinyl chloride binder in a solvent and vacuum dried to obtain a pretreated precursor; (3) Cerium doping: The cerium salt solution is mixed with the pretreated precursor obtained in step (2), and then spray-dried to obtain the cerium-doped precursor; (4) Two-stage sintering: First stage sintering: The cerium-doped precursor is sintered in an inert atmosphere to obtain a sintered intermediate; Second stage sintering: A titanium source is added to the sintering intermediate to obtain a mixture, and the mixture is sintered in an inert atmosphere at a sintering temperature of 650-740℃ to obtain the lithium iron phosphate material; wherein, the sintering temperature of the first stage sintering is higher than that of the second stage sintering.

[0051] The lithium iron phosphate material provided by this invention has high energy density and power density.

[0052] This invention controls grain size and carbon coating integrity through a two-stage sintering process. By controlling the sintering temperature of the second stage, it promotes lattice reconstruction, reduces defects, and improves crystallinity and phase purity. This avoids the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single high-temperature sintering in existing technologies. Furthermore, the high-temperature sintering of traditional processes easily leads to particle agglomeration, forming large-sized particles and reducing the compaction density of the material, resulting in a lower compaction density of lithium iron phosphate materials.

[0053] This invention achieves in-situ molding of the porous structure by using a cerium-doped liquefied paraffin / polyvinyl chloride bonding system, thereby ensuring the compaction density of lithium iron phosphate. Through the combined effects of binder pretreatment, spray-drying cerium doping, and two-stage sintering with titanium doping, the compaction density limit of the material is improved while enhancing its ion diffusion efficiency at high areal densities; spray drying ensures the preservation of cerium... 3+ Nanoscale dispersion, Ce 3+ Occupying Li sites creates oxygen vacancies, activating one-dimensional Li + Channels, titanium doping widens lattice spacing, Li + Migration is easier, and through binder pretreatment and cerium / titanium synergistic modification, the traditional trade-off between energy density and power density is broken, providing a material basis for high-power power batteries.

[0054] As an optional implementation, the sintering temperature of the first stage sintering is 760-850℃, and the sintering temperature of the first stage sintering can be, for example, 760℃, 770℃, 780℃, 800℃, 820℃, 840℃ or 850℃.

[0055] As an alternative implementation, the sintering temperature of the second stage sintering can be, for example, 650°C, 660°C, 680°C, 700°C, 720°C, 730°C, or 740°C.

[0056] As an optional implementation, in step (1), the lithium source includes lithium carbonate, the phosphorus source includes iron phosphate, the iron source includes iron phosphate, and the carbon source is selected from sucrose, glucose, or polyethylene glycol (PEG).

[0057] As an optional implementation, in step (1), the mass ratio of the carbon source to the iron source is (5-10):95, for example, it can be 5:95, 6:95, 7:95, 8:95, 9:95 or 10:95.

[0058] As an optional implementation, in step (1), the drying temperature of the spray drying is 240-280℃, for example, it can be 240℃, 250℃, 260℃, 270℃ or 280℃.

[0059] As an optional implementation, in step (1), the lithium iron phosphate precursor D 50 It is 50-60μm, for example, it can be 50μm, 55μm or 60μm.

[0060] As an optional implementation, in step (2), the mass ratio of the lithium iron phosphate precursor to the liquefied paraffin / polyvinyl chloride binder is (95-99):(1-5), for example, it can be 95:5, 96:4, 97:3, 98:2 or 99:1. The mass ratio of liquefied paraffin to polyvinyl chloride is 1:(0.8-1.2).

[0061] As an optional implementation, in step (2), the solvent is anhydrous ethanol.

[0062] As an optional implementation, in step (2), the ball milling speed is 300-500 rpm, for example, 300 rpm, 400 rpm or 500 rpm, and the time is 6-8h, for example, 6h, 7h or 8h.

[0063] As an optional implementation, in step (2), the drying temperature of vacuum drying is 70-90℃, for example, 70℃, 80℃ or 90℃, and the time is 4-5h, for example, 4h, 4.5h or 5h.

[0064] This invention improves the compaction density of the material by further limiting the binder ratio and optimizing the filling of pores, thereby avoiding the defect of large interparticle gaps in the precursor in traditional processes. Further limiting the ball milling parameters ensures uniform coating, and vacuum drying avoids solvent residue affecting conductivity.

[0065] As an optional implementation, in step (3), the cerium salt is cerium nitrate.

[0066] As an optional implementation, in step (3), the mass of the cerium salt is 0.5-1 wt% of the mass of the pretreated precursor, for example, it can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt% or 1 wt%.

[0067] As an optional implementation, in step (3), the cerium-doped precursor D 50 It can be 30-45μm, for example, it can be 30μm, 35μm, 40μm or 45μm.

[0068] As an optional implementation, in step (3), the spray drying temperature is 240-280℃.

[0069] This invention employs spray drying to achieve nanoscale cerium dispersion. 3+ Occupying Li sites creates oxygen vacancies, avoiding the localized lattice defects caused by uneven cerium dispersion in traditional processes, which affect ion conductivity. Further control of the cerium salt addition mass balances the compaction density and ion conductivity.

[0070] As an optional implementation, in step (4), the sintering time of the first sintering stage is 2-4 hours, for example, 2 hours, 3 hours or 4 hours.

[0071] As an optional implementation, in step (4), the sintering intermediate is pulverized and demagnetized, and the compaction density of the sintering intermediate is 2.30-2.45 g / cm³. 3 D 50 The thickness ranges from 2.0 to 3.5 μm, for example, it can be 2.0 μm, 2.5 μm, 3.0 μm or 3.5 μm.

[0072] As an optional implementation, in step (4), the sintering time of the second sintering stage is 5-8h, for example, 5h, 6h, 7h or 8h.

[0073] As an optional implementation, in step (4), the titanium source is titanium dioxide.

[0074] As an optional implementation, in step (4), the mass ratio of the sintering intermediate to titanium dioxide is (93-97):(3-7), for example, it can be 93:7, 94:6, 95:5, 96:4 or 97:3.

[0075] As an optional implementation, in step (4), the compaction density of the lithium iron phosphate material is 2.58-2.62 g / cm³. 3 D 50The value is 0.9-1.2μm, for example, it can be 0.9μm, 1.0μm, 1.1μm or 1.2μm.

[0076] This invention limits the mass ratio of the sintering intermediate to the titanium source, enabling titanium doping (titanium dioxide) to better widen the lattice spacing and synergistically enhance Li... + Migration rate.

[0077] In a second aspect, the present invention provides a high-power, high-energy-density lithium iron phosphate material, wherein the lithium iron phosphate material is prepared by the method described in the first aspect.

[0078] Thirdly, the present invention provides a method for preparing a lithium iron phosphate cathode sheet, comprising: (a) Providing a sintering intermediate and a lithium iron phosphate material prepared by the preparation method described in the first aspect; (b) A first positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a first slurry, wherein the first positive electrode active material is the sintering intermediate; a second positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a second slurry, wherein the second positive electrode active material is the lithium iron phosphate material; (c) The first slurry is coated onto the current collector to form a bottom layer, and the second slurry is coated onto the bottom layer to form a surface layer. After drying and rolling, a lithium iron phosphate positive electrode sheet is obtained. The total areal density of the lithium iron phosphate positive electrode sheet is 460-500 g / cm², for example, 460 g / cm², 480 g / cm² or 500 g / cm². The thickness ratio of the bottom layer to the surface layer of the lithium iron phosphate positive electrode sheet is (2-3):(7-8), for example, 2:8, 2.5:7.5 or 3:7.

[0079] This invention improves the porosity of the electrode by layering the first slurry and the second slurry, thereby achieving porosity gradient control (porosity decreases in the direction of current collector → electrolyte). This improves the wettability of the electrolyte and the positive electrode active material under high areal density, and increases the lithium-ion diffusion efficiency, thereby reducing lithium-ion concentration polarization and promoting the full utilization of the cell capacity.

[0080] As an alternative embodiment, in the first slurry, the conductive agent includes CNT and SP; the binder includes PVDF.

[0081] As an optional implementation, in the first slurry, the mass ratio of the first positive electrode active material, the conductive agent, and the binder is (96.5-97.5):(1.2-2):(1.5-2).

[0082] As an alternative embodiment, in the second slurry, the conductive agent includes CNT and SP; the binder includes PVDF.

[0083] As an optional implementation, in the second slurry, the mass ratio of the second positive electrode active material, the conductive agent, and the binder is (96.5-97.5):(1.2-2):(1.5-2).

[0084] As an optional implementation, in step (b), the viscosity of the first slurry is 4000-8000 mPa·s, and the solid content is 62-65%.

[0085] As an optional implementation, in step (b), the viscosity of the second slurry is 3000-7200 mPa·s, and the solid content is 61-64%.

[0086] As an optional implementation, in step (c), a double-layer coating machine is used for coating.

[0087] As an optional implementation, in step (c), when the coating speed is ≥30m / min, the thickness deviation is ≤1.5μm.

[0088] As an optional implementation, the coating speed in step (c) is 30m / min-60m / min, for example, it can be 30m / min, 40m / min, 50m / min or 60m / min.

[0089] Fourthly, the present invention provides a lithium iron phosphate cathode sheet, which is prepared by the method described in the third aspect, comprising: a bottom layer and a top layer; the porosity of the bottom layer is P1, and the porosity of the top layer is P2; wherein, P2 < P1.

[0090] The lithium iron phosphate cathode of the present invention has a low porosity (P1) at the bottom layer to ensure electron conduction and a high porosity (P2) at the surface layer to enhance electrolyte wetting. The porosity gradient (P2 < P1) guides the electrolyte to penetrate from the surface to the interior, reducing concentration polarization.

[0091] Fifthly, the present invention provides a lithium-ion battery comprising the positive electrode sheet described in the fourth aspect.

[0092] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0093] In the following embodiments and comparative examples: Example 1 Preparation of lithium iron phosphate materials Raw materials and functions: The raw materials used in the preparation process include lithium carbonate (lithium source), iron phosphate (phosphorus source, iron source), sucrose (carbon source), cerium nitrate solution (cerium salt) (mass percentage concentration of 1.5%), liquefied paraffin / polyvinyl chloride binder with a mass ratio of liquefied paraffin to polyvinyl chloride (as binder, liquefied paraffin is used to fill pores, and polyvinyl chloride is used to enhance structural bonding) of 1:1, and titanium dioxide (titanium source, added in the second stage of sintering). All of the above raw materials are commercially available.

[0094] Preparation method: (1) Preparation of lithium iron phosphate precursor: 23.5g lithium carbonate, 95.0g iron phosphate and 6.0g sucrose were dissolved in deionized water, mixed and ground, and then spray-dried at 260℃ to obtain spherical lithium iron phosphate precursor (D 50 =55μm).

[0095] (2) Precursor pretreatment: The lithium iron phosphate precursor was mixed with liquefied paraffin / polyvinyl chloride at a mass ratio of 99:1, placed in anhydrous ethanol, ball-milled at 400 rpm for 7 h, and then vacuum-dried at 80 °C for 4.5 h to obtain the pretreated precursor.

[0096] (3) Cerium doping: Cerium nitrate (the amount of cerium nitrate added is 0.5 wt% of the mass of the pretreated precursor) is mixed with the pretreated precursor obtained in step (2), and then spray-dried at 240°C to obtain the cerium-doped precursor (D). 50 =38μm).

[0097] (4) Two-stage sintering: First stage sintering: Calcination at 790℃ for 3 hours in an argon atmosphere, followed by pulverization and demagnetization, yielded a sintered intermediate with a compacted density of 2.38 g / cm³. 3 D 50 =2.8μm.

[0098] Second-stage sintering: Titanium dioxide (mass ratio of sintering intermediate to titanium dioxide 95:5) was added to the sintering intermediate, and the mixture was treated at 700℃ for 6 hours under argon protection to obtain lithium iron phosphate material (compacted density 2.60 g / cm³, D... 50 =1.1μm).

[0099] Preparation of lithium iron phosphate cathode sheet (a) Provide the sintering intermediate and lithium iron phosphate material prepared by the above preparation method; (b) Slurry preparation: First slurry: Using a sintered intermediate as the first positive electrode active material, it is mixed with a conductive agent (SP to CNT in a mass ratio of 2:1) and a binder PVDF in a mass ratio of 96.5:1.5:2 in NMP solvent. The viscosity of the first slurry is 6800 mPa·s, and the solid content is 63.5%. The second slurry was prepared by mixing lithium iron phosphate material as the second positive electrode active material with conductive agent (SP to CNT in a mass ratio of 2:1) and binder PVDF in an NMP solvent at a mass ratio of 97:1.5:1.5. The viscosity of the second slurry was 7200 mPa·s and the solid content was 62.3%. (c) Coating: A double-layer coating machine is used for coating. The first slurry is coated onto the carbon-coated aluminum foil current collector to form the underlayer (thickness ratio 30%), and the second slurry is coated onto the underlayer to form the surface layer (thickness ratio 70%). After drying and rolling, the lithium iron phosphate positive electrode sheet is obtained, wherein the total areal density is 480 g / m³. 2 The coating speed is 30m / min, and the thickness deviation is ≤1.5μm.

[0100] Example 2 The preparation method of the lithium iron phosphate material provided in Example 2 is basically the same as that in Example 1, except that the mass ratio of lithium iron phosphate precursor to liquefied paraffin / polyvinyl chloride is adjusted to 97:3. The preparation method of the lithium iron phosphate cathode sheet is the same as in Example 1.

[0101] Example 3 The preparation method of the lithium iron phosphate material provided in Example 3 is basically the same as that in Example 1, except that the amount of cerium nitrate added accounts for 0.6 wt% of the mass of the pretreated precursor. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0102] Example 4 The preparation method of the lithium iron phosphate material provided in Example 4 is basically the same as that in Example 1, except that the second-stage sintering temperature is adjusted to 740°C. The preparation method of the lithium iron phosphate cathode sheet is the same as in Example 1.

[0103] Example 5 The preparation method of the lithium iron phosphate material provided in Example 5 is exactly the same as that in Example 1.

[0104] The preparation method of the lithium iron phosphate cathode sheet provided in Example 5 is basically the same as that in Example 1. The difference is that in the preparation method of the lithium iron phosphate cathode sheet, in step (c), the thickness ratio of the bottom layer to the surface layer is changed to 2:8. The first slurry is coated on the carbon-coated aluminum foil current collector to form the bottom layer (thickness ratio of 20%); the second slurry is coated on the bottom layer to form the surface layer (thickness ratio of 80%).

[0105] Example 6 The preparation method of the lithium iron phosphate material provided in Example 6 is basically the same as that in Example 1, except that the amount of cerium nitrate added accounts for 1.0 wt% of the mass of the pretreated precursor. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0106] Example 7 The preparation method of the lithium iron phosphate material provided in Example 7 is basically the same as that in Example 1, except that the mass ratio of lithium iron phosphate precursor to liquefied paraffin / polyvinyl chloride is adjusted to 95:5. The preparation method of the lithium iron phosphate cathode sheet is the same as in Example 1.

[0107] Example 8 The preparation method of the lithium iron phosphate material provided in Example 8 is basically the same as that in Example 1, except that the second-stage sintering temperature is adjusted to 650°C. The preparation method of the lithium iron phosphate cathode sheet is the same as in Example 1.

[0108] Example 9 The preparation method of the lithium iron phosphate material provided in Example 9 is basically the same as that in Example 1, except that titanium dioxide (mass ratio 93:7) is added to the sintering intermediate during the second sintering stage. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0109] Comparative Example 1 The preparation method of the lithium iron phosphate material provided in Comparative Example 1 is basically the same as that in Example 1, except that the cerium doping step (3) is not performed, and the pretreated precursor is directly sintered in two stages. The preparation method of the lithium iron phosphate cathode is the same as that in Example 1.

[0110] Comparative Example 2 The preparation method of the lithium iron phosphate material provided in Comparative Example 2 is basically the same as that in Example 1, except that the precursor pretreatment in step (2) is not performed, i.e., liquefied paraffin / polyvinyl chloride binder is not added. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0111] Comparative Example 3 The preparation method of the lithium iron phosphate material provided in Comparative Example 3 is basically the same as that in Example 1, except that step (4) only involves the first stage of sintering, in which titanium dioxide is added to the cerium-doped precursor (the mass ratio of cerium-doped precursor to titanium dioxide is 95:5), and the second stage of sintering is not performed. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0112] Comparative Example 4 The preparation method of the lithium iron phosphate material provided in Comparative Example 4 is basically the same as that in Example 1, except that in the second stage of sintering in step (4), no titanium source doping is added, that is, no titanium dioxide is added. The preparation method of the lithium iron phosphate cathode sheet is the same as that in Example 1.

[0113] Comparative Example 5 The preparation method of the lithium iron phosphate material provided in Comparative Example 5 is exactly the same as that in Example 1. The preparation method of the lithium iron phosphate cathode is basically the same as that in Example 1, except that in the preparation method of the lithium iron phosphate cathode, step (c) only uses the second slurry for coating, that is, the second slurry with an equal amount as the first slurry and the second slurry in Example 1 is used for coating.

[0114] Comparative Example 6 The preparation method of lithium iron phosphate material provided in Comparative Example 6 is basically the same as that in Example 1, except that in step (3), the cerium nitrate solution is uniformly mixed with the pretreated precursor and then dried in an oven at the same temperature as in Example 1.

[0115] Comparative Example 7 The preparation method of the lithium iron phosphate material provided in Comparative Example 7 is basically the same as that in Example 1, except that the sintering temperature of the first sintering stage is 700°C and the sintering temperature of the second sintering stage is 790°C.

[0116] Performance testing Test method: The full-electric testing process involves three cycles of activation: 1C charging at a cutoff voltage of 3.65V, followed by 1C discharging at a cutoff voltage of 2.0V. For rate discharging, a 0.33C charging rate with a cutoff voltage of 3.65V is followed by discharging at different rates (0.5C / 1C / 2C / 3C) with a cutoff voltage of 2.0V. The discharge capacity retention rate at different rates is calculated as follows: 0.5C discharge capacity retention rate = (0.5C discharge capacity / 0.33C charging capacity) * 100; 1C discharge capacity retention rate = (1C discharge capacity / 0.33C charging capacity) * 100; 2C discharge capacity retention rate = (2C discharge capacity / 0.33C charging capacity) * 100; 3C discharge capacity retention rate = (3C discharge capacity / 0.33C charging capacity) * 100. Compacted density: Weigh 2-3g of powder and test its compacted density under a pressure of 30KN (holding pressure for 60s) = powder mass (g) / compacted volume (cm³) 3 ); Energy density: Mass energy density = Energy released by the battery (Wh) / Mass of the battery (kg).

[0117] Lithium-ion batteries were prepared using the lithium iron phosphate positive electrode sheets provided in Examples 1-9 and Comparative Examples 1-7, and the negative electrode sheet with graphite as the active material. The batteries prepared in the examples and comparative examples were subjected to performance tests according to the above-described test methods, and the test results are shown in Table 1.

[0118] Table 1

[0119] As can be seen from Table 1: (1) The lithium iron phosphate cathode prepared in Examples 1-9 of the present invention can simultaneously improve the energy density and power density of the battery. The energy density is ≥183Wh / kg; the discharge capacity rate at 0.5C is ≥99.3%, the discharge capacity rate at 1C is ≥98.4%, the discharge capacity rate at 2C is ≥96.9%, and the discharge capacity rate at 3C is ≥96.1%. The discharge capacity retention rate at high rates is relatively high, indicating that the lithium iron phosphate cathode prepared in Examples 1-9 has a high power density.

[0120] Examples 3 and 6, with higher cerium doping levels, exhibited superior rate performance (3C≥98%), indicating that cerium doping significantly improves power density.

[0121] (2) As can be seen from Examples 1, 2, and 7, the introduction of liquefied paraffin / polyvinyl chloride improves the energy density of the battery cell, mainly by filling the gaps between particles and increasing the compaction density of lithium iron phosphate. The compaction densities of Examples 2 and 7 are even higher (2.41 / 2.61 g / cm³). 3 2.42 / 2.62 g / cm 3 The energy density also increased accordingly (185.0 Wh / kg, 184.7 Wh / kg). Comparative Example 2, without the addition of liquefied paraffin / polyvinyl chloride binder, had a significantly lower compaction density (2.23 / 2.56 g / cm³) and the lowest energy density (183.0 Wh / kg).

[0122] (3) As can be seen from Examples 1, 3, and 6, cerium doping significantly improves the power density of the electrode. The main reason is that cerium doping introduces oxygen vacancies, which activate lithium-ion channels and fundamentally improve the ionic conductivity of the material, thereby improving the rate performance, especially the 3C discharge capacity retention rate. The 3C discharge capacity retention rates of Examples 1, 3, and 6 are 96.60%, 98.02%, and 98.13%, respectively. In Comparative Example 1, which was not doped with cerium, the 3C discharge capacity retention rate is only 95.80%. The high discharge capacity retention rate at high rates indicates that the lithium iron phosphate cathode sheets prepared in Examples 1, 3, and 6 have high power densities.

[0123] (4) As can be seen from Examples 1, 4 and 8, the two-stage sintering process maintains high energy density and rate performance while improving compaction density by controlling grain size and structural integrity. The increased temperature in the second stage can further improve compaction density.

[0124] (5) As can be seen from Examples 1 and 9, titanium doping during the secondary sintering process can better widen the lattice spacing and synergistically enhance the Li-10 ... + Mobility. The 3C capacity retention of Example 9 (97.36%) was higher than that of Example 1 (96.60%). This indicates that increasing the titanium doping amount further widens the lattice spacing, making Li... + Migration is easier, thus increasing the power density of the electrode. The energy density of Example 9 (184.5 Wh / kg) is basically at the same level as, or even slightly higher than, that of Example 1 (184.0 Wh / kg), indicating that increasing the titanium content within this range does not have a negative impact on the energy density. Comparative Example 4, without the addition of titanium source doping, has a 3C capacity retention rate (95.10%) and a low electrode power density.

[0125] (6) As can be seen from Examples 1, 5 and Comparative Example 5, the multilayer coating technology effectively improves the power density of the electrode. However, by changing the thickness ratio of the bottom layer to the top layer, the reduced bottom layer ratio in Example 5 compared to Example 1 may slightly weaken the electron conduction capability, affecting the performance at extremely high rates, and the power density of the electrode is slightly reduced. Comparative Example 5 uses the same active material as Example 1, but adopts a single-layer coating without a porosity gradient design, resulting in a significant decrease in 3C performance. This fully demonstrates that the lack of a porosity gradient leads to poor electrolyte wettability, high ion diffusion resistance, and a significant reduction in the power density of the electrode.

[0126] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-power, high-energy-density lithium iron phosphate material, characterized in that, Includes the following steps: (1) Preparation of lithium iron phosphate precursor: The lithium source, iron source, phosphorus source and carbon source are mixed and spray dried to obtain spherical lithium iron phosphate precursor; (2) Precursor pretreatment: The lithium iron phosphate precursor obtained in step (1) is ball-milled in a solvent with a binder composed of liquefied paraffin and polyvinyl chloride, and then vacuum dried to obtain a pretreated precursor; (3) Cerium doping: The cerium salt solution is mixed with the pretreated precursor obtained in step (2), and then spray-dried to obtain the cerium-doped precursor; (4) Two-stage sintering: First stage sintering: The cerium-doped precursor is sintered in an inert atmosphere to obtain a sintered intermediate; Second stage sintering: A titanium source is added to the sintering intermediate to obtain a mixture, and the mixture is sintered in an inert atmosphere at a sintering temperature of 650-740℃ to obtain the lithium iron phosphate material; wherein, the sintering temperature of the first stage sintering is higher than that of the second stage sintering.

2. The method according to claim 1, characterized in that: In step (1), the lithium source includes lithium carbonate, the phosphorus source includes iron phosphate, the iron source includes iron phosphate, and the carbon source is selected from sucrose, glucose, or polyethylene glycol; and / or, The mass ratio of the carbon source to the iron source is (5-10):95; and / or, The spray drying temperature is 240-280℃; and / or, The lithium iron phosphate precursor D 50 It is 50-60μm.

3. The method according to claim 1, characterized in that: In step (2), the mass ratio of the lithium iron phosphate precursor to the binder composed of liquefied paraffin and polyvinyl chloride is (95-99):(1-5); and / or, The solvent is anhydrous ethanol; and / or, The ball milling speed is 300-500 rpm, and the time is 6-8 hours; and / or, The vacuum drying temperature is 70-90℃, and the drying time is 4-5 hours.

4. The method according to claim 1, characterized in that: In step (3), the cerium salt is cerium nitrate; and / or, The mass of the cerium salt is 0.5-1 wt% of the mass of the pretreated precursor; and / or, The spray drying temperature is 240-280℃; and / or, The cerium-doped precursor D 50 It is 30-45μm.

5. The method according to claim 1, characterized in that: In step (4), the sintering temperature of the first sintering stage is 760-850℃, and / or, The sintering time for the first stage of sintering is 2-4 hours; and / or, The sintering intermediate is pulverized and demagnetized, and its compacted density is 2.30-2.45 g / cm³. 3 D 50 2.0-3.5 μm; and / or, The sintering time for the second stage of sintering is 5-8 hours; and / or, The titanium source is titanium dioxide; The mass ratio of the sintering intermediate to titanium dioxide is (93-97):(3-7); and / or, The compaction density of the lithium iron phosphate material is 2.58-2.62 g / cm³. 3 D 50 The range is 0.9-1.2 μm.

6. A high-power, high-energy-density lithium iron phosphate material, characterized in that, The lithium iron phosphate material is prepared by the method described in any one of claims 1-5.

7. A method for preparing a lithium iron phosphate cathode, comprising: (a) Providing a sintering intermediate and a lithium iron phosphate material prepared by the preparation method according to any one of claims 1-5; (b) A first positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a first slurry, wherein the first positive electrode active material is the sintering intermediate; a second positive electrode active material, a conductive agent, and a binder are added to a solvent and mixed to prepare a second slurry, wherein the second positive electrode active material is the lithium iron phosphate material; (c) The first slurry is coated onto the current collector to form a bottom layer, and the second slurry is coated onto the bottom layer to form a surface layer. After drying and rolling, a lithium iron phosphate cathode sheet is obtained, wherein the total areal density of the lithium iron phosphate cathode sheet is 460-500 g / cm³. 2 The thickness ratio of the bottom layer to the surface layer of the lithium iron phosphate cathode is (2-3):(7-8).

8. The method according to claim 7, characterized in that: In step (b), in the first slurry, the conductive agent comprises CNTs and SPs; the binder comprises PVDF; and / or, In the first slurry, the mass ratio of the first positive electrode active material, the conductive agent, and the binder is (96.5-97.5):(1.2-2):(1.5-2); and / or, In the second slurry, the conductive agent comprises CNT and SP; the binder comprises PVDF; and / or, In the second slurry, the mass ratio of the second positive electrode active material, conductive agent, and binder is (96.5-97.5):(1.2-2):(1.5-2); and / or, In step (b), the viscosity of the first slurry is 4000-8000 mPa·s, and the solid content is 62-65%; and / or, In step (b), the viscosity of the second slurry is 3000-7200 mPa·s, and the solid content is 61-64%; and / or, In step (c), a double-layer coating machine is used for coating; and / or, In step (c), the thickness deviation is ≤1.5μm when the coating speed is ≥30m / min; In step (c), the coating speed is 30 m / min-60 m / min.

9. A lithium iron phosphate cathode sheet, characterized in that, The substrate is prepared by the method of claim 7 or 8, comprising: a bottom layer and a top layer; the porosity of the bottom layer is P1, and the porosity of the top layer is P2; Where P2 < P1.

10. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the positive electrode sheet as described in claim 9.

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