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

Abstract

The invention provides a preparation method of a high-power and high-energy-density lithium iron phosphate material. The preparation method comprises the following steps: (1) preparing a lithium iron phosphate precursor; (2) precursor pretreatment: carrying out ball milling on the lithium iron phosphate precursor and a liquefied paraffin / polyvinyl chloride binder in a solvent, and carrying out vacuum drying to obtain a pretreated precursor; (3) cerium doping: mixing a cerium salt solution with the pretreated precursor, and then performing 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 sintered intermediate; and second-stage sintering: adding a titanium source into the sintering intermediate to obtain a mixture, and continuously sintering 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 preparation method disclosed by the invention has relatively high energy density and power density.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high-power and high-energy-density lithium iron phosphate material, a preparation method thereof, a lithium iron phosphate positive electrode sheet and a preparation method and application thereof. BACKGROUND

[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 (weight and volume). Lithium iron phosphate (LFP) has become one of the most important positive electrode materials for lithium ion batteries due to its easy availability, low price and stable structure. However, the electrical conductivity of LFP is relatively low, and the Li+ movement path is one-dimensional, and the specific capacity is relatively low (compared with ternary materials).

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

[0004] However, energy density and power density are incompatible in battery design, and the trade-off is usually quantified by different battery chemical properties and manufacturing processes, i.e. sacrificing energy density to improve power density, replacing part of the active material with conductive filler, and improving power density by using large pores for ion transmission or using thinner electrodes, but this will reduce the proportion of active material inside the battery, resulting in a decrease in battery energy density.

[0005] To solve the above problems, the prior art such as patent CN103633289A increases the energy density of the battery by coating two layers of active material, and increases the conductive layer between the active material layers to improve the lithium ion diffusion concentration polarization, but this method is complex and increases the manufacturing cost; the patent CN109192935A improves the rate performance of the positive electrode by doping fluorine element and carbon coating in the positive electrode main material, which improves the rate performance to some extent, but the improvement effect is general for high surface density electrode sheets, the wettability between the electrolyte and the positive electrode sheet is general, and the capacity of the battery cell cannot be fully utilized.

[0006] Therefore, it is still a great technical challenge in the field to develop a high-power and high-energy-density positive electrode using a reasonable preparation process. SUMMARY

[0007] In view of the above deficiencies in the prior art, the present application provides a high-power and high-energy-density lithium iron phosphate material, a preparation method thereof, a lithium iron phosphate positive electrode sheet and a preparation method and application thereof. The lithium iron phosphate material provided by the present application has high energy density and power density.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: In a first aspect, the present application provides a preparation method of high-power and high-energy density lithium iron phosphate material, comprising the following steps: (1) Preparation of lithium iron phosphate precursor: mixing lithium source, iron source, phosphorus source and carbon source, and then spray drying to obtain spherical lithium iron phosphate precursor; (2) Pretreatment of precursor: ball milling the lithium iron phosphate precursor obtained in step (1) with liquefied paraffin / polyvinyl chloride binder in a solvent, and vacuum drying to obtain pretreated precursor; (3) Cerium doping: mixing cerium salt solution with the pretreated precursor obtained in step (2), and then spray drying to obtain cerium-doped precursor; (4) Two-stage sintering: First-stage sintering: sintering the cerium-doped precursor in an inert atmosphere to obtain a sintered intermediate; Second-stage sintering: adding titanium source to the sintered intermediate to obtain a mixture, and continuing to sinter the mixture 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] The present application 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, which can avoid the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single high-temperature sintering in the prior art; and the one-stage high-temperature sintering in the traditional process is prone to cause particle sintering agglomeration, forming large-size particles, reducing the compaction density of the material, and thus the compaction density of the lithium iron phosphate material is low.

[0010] The present application realizes in-situ forming of pore structure through cerium doping combined with liquefied paraffin / polyvinyl chloride binder system, thereby ensuring the compaction density of lithium iron phosphate. Through the combined action of binder pretreatment, spray cerium doping and two-stage sintering of titanium doping, the compaction density of the material is improved, and the ion diffusion efficiency under high surface density is also improved; spray drying ensures the nanoscale dispersion of Ce³ + , Ce³ + occupies Li sites to form oxygen vacancies, activates one-dimensional Li + channels, titanium doping widens the lattice spacing, Li + migration is easier, through binder pretreatment and cerium / titanium synergistic modification, breaking the traditional trade-off relationship between energy density and power density, providing a material basis for high-power power batteries.

[0011] Further, 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] Further, in step (1), the mass ratio of the carbon source to the iron source is (5-10):95.

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

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

[0015] Further, 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] Further, in step (2), the solvent is anhydrous ethanol.

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

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

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

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

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

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

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

[0024] Further, in step (4), the sintering time of the first-stage sintering is 2-4h.

[0025] Further, in step (4), the sintering intermediate is subjected to crushing and demagnetization, and the tap density of the sintering intermediate is 2.30-2.45g / cm 3 , D 50 is 2.0-3.5μm.

[0026] Further, in step (4), the sintering time of the second-stage sintering is 5-8h.

[0027] Further, in step (4), the titanium source is titanium white powder.

[0028] Further, in step (4), the mass ratio of the sintered intermediate to the titanium white powder is (93-97):(3-7).

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

[0030] In a second aspect, the present application provides a high-power and high-energy density lithium iron phosphate material, which is prepared by the method of the first aspect.

[0031] In a third aspect, the present application provides a preparation method of a lithium iron phosphate positive electrode sheet, comprising: (a) providing the sintered intermediate and the lithium iron phosphate material prepared by the preparation method of the first aspect; (b) mixing a first positive electrode active material, a conductive agent and a binder in a solvent to prepare a first slurry, wherein the first positive electrode active material is the sintered intermediate; mixing a second positive electrode active material, a conductive agent and a binder in a solvent to prepare a second slurry, wherein the second positive electrode active material is the lithium iron phosphate material; (c) coating the first slurry on a current collector to form a bottom layer, and coating the second slurry on the bottom layer to form a surface layer, and then drying and rolling to obtain a lithium iron phosphate positive electrode sheet, wherein the total area 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] Further, in the first slurry of step (b), the conductive agent comprises CNT and SP, and the binder comprises PVDF.

[0033] Further, in the first slurry of step (b), 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).

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

[0035] Further, 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] Further, in step (b), the viscosity of the first slurry is 4000-8000 mpa.s, and the solid content is 62-65%.​

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

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

[0039] Further, in step (c), when the coating speed is greater than or equal to 30 m / min, the thickness deviation is less than or equal to 1.5 μm.

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

[0041] In a fourth aspect, the present application provides a lithium iron phosphate positive electrode sheet, which is prepared by the method of the third aspect, and comprises a bottom layer and a surface layer; the porosity of the bottom layer is P1, and the porosity of the surface layer is P2; wherein P2

[0042] In a fifth aspect, the present application provides a lithium ion battery comprising the positive electrode sheet of the fourth aspect.

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

[0044] (2) The present application controls the grain size and carbon coating integrity through two-stage sintering, controls the sintering temperature of the second stage, promotes lattice reconstruction, reduces defects, and improves crystallinity and phase purity, which can avoid the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single high-temperature sintering in the prior art; and high-temperature sintering in the traditional process is easy to cause particle sintering agglomeration, forming large-size particles, reducing the compaction density of the material, so that the compaction density of the lithium iron phosphate material is low.

[0045] (3) The single cerium doping in the prior art can only improve the lithium ion conduction of lithium iron phosphate in the battery, but cannot improve the powder compaction density of the material. The present application realizes in-situ forming of pore structure by cerium doping combined with liquid paraffin / polyvinyl chloride bonding system, and the liquid paraffin / polyvinyl chloride bonding system pre-treats the lithium iron phosphate precursor, reduces the voids between the precursor particles, thereby improving the compaction density of the lithium iron phosphate, so that the lithium iron phosphate material has high energy density.

[0046] (4) The application improves the porosity of the pole piece by adopting different particle size lithium iron phosphate materials for layered coating, realizes the control of porosity gradient, thereby improving the wettability of electrolyte and positive active material under high surface density, and improving the lithium ion diffusion efficiency, thereby reducing the lithium ion concentration polarization, promoting the capacity of the battery, and making the lithium ion battery have high power density.

[0047] (5) The application improves the compaction density limit of the material and improves the ion diffusion efficiency under high surface density through the combined action of binder pretreatment, spray cerium doping and two-stage sintering of titanium doping; spray drying ensures the nanoscale dispersion of Ce 3+ , Ce 3+ occupies Li sites to form oxygen vacancies, inhibits oxygen release, activates one-dimensional Li + channels, promotes lithium ion flow, so that the lithium iron phosphate material has high power density, improves the electrochemical performance, and also stabilizes the material structure, and the cycle performance is obviously improved; titanium doping widens the lattice spacing, Li + migration is easier, and through binder pretreatment and cerium / titanium synergistic modification, the traditional trade-off relationship between energy density and power density is broken, providing a material basis for high-power power batteries. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical scheme and advantages of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Those skilled in the art should understand that the embodiments are only to help understand the present application, and should not be regarded as a specific limitation on the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. The process parameters not specified in the following embodiments are usually according to conventional conditions.

[0049] The endpoints of the ranges and any values disclosed in the present application are not limited to the precise values stated. These ranges and values should be construed as having a range of values around the stated values. For ranges, the endpoints are included within the range; for values, the endpoints and individual values are included within the range. These ranges and values are to be construed as specifically disclosed.

[0050] In a first aspect, the present application provides a preparation method of a high-power, high-energy-density lithium iron phosphate material, comprising the following steps: (1) Preparation of lithium iron phosphate precursor: mixing lithium source, iron source, phosphorus source and carbon source, and then spray drying 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, vacuum dried, and a pretreated precursor is obtained; (3) cerium doping: a cerium salt solution is mixed with the pretreated precursor obtained in step (2), and then spray dried to obtain a 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 sintered intermediate to obtain a mixture, and the mixture is continuously 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 the application has high energy density and power density.

[0052] The application controls the grain size and carbon coating integrity through two-stage sintering, controls the sintering temperature of the second stage, promotes lattice reconstruction, reduces defects, and improves crystallinity and phase purity, which can avoid the uneven growth of lithium iron phosphate grains and incomplete crystallization in some areas caused by single high-temperature sintering in the prior art; and high-temperature sintering in the traditional process is easy to cause sintering agglomeration of particles, forming large-size particles, reducing the compaction density of the material, so that the compaction density of the lithium iron phosphate material is low.

[0053] The application realizes in-situ forming of the pore structure by cerium doping combined with a liquefied paraffin / polyvinyl chloride binding system, thereby ensuring the compaction density of the lithium iron phosphate. Through the combined action of the binder pretreatment, spray cerium doping, and two-stage sintering of titanium doping, the compaction density limit of the material is improved, and the ion diffusion efficiency under high surface density is also improved; spray drying ensures the nanoscale dispersion of Ce 3+ , Ce 3+ occupies Li sites to form oxygen vacancies, activates one-dimensional Li + channels, titanium doping widens the lattice spacing, Li + migration is easier, through binder pretreatment and cerium / titanium synergistic modification, the traditional trade-off relationship 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 may be, for example, 760℃, 770℃, 780℃, 800℃, 820℃, 840℃, or 850℃.

[0055] As an optional embodiment, the sintering temperature of the second segment sintering can be, for example, 650℃, 660℃, 680℃, 700℃, 720℃, 730℃ or 740℃.

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

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

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

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

[0060] As an optional embodiment, in step (2), the mass ratio of the lithium iron phosphate precursor to the liquefied paraffin / polyvinyl chloride binder is (95-99):(1-5), which can be, for example, 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 embodiment, in step (2), the solvent is anhydrous ethanol.

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

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

[0064] The present application optimizes the filling of pores by further limiting the proportion of the binder, thereby avoiding the defect of large gaps between precursor particles in the traditional process and improving the compaction density of the material. Further limiting the ball milling parameters ensures uniform coating, and selecting vacuum drying avoids the influence of solvent residues on 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 0.5-1wt% of the mass of the pretreated precursor, for example, it can be 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt% or 1wt%.

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

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

[0069] The present application realizes nanoscale cerium dispersion by using the spray drying method, Ce 3+ occupies Li sites to form oxygen vacancies, avoids uneven cerium dispersion in traditional processes to cause local lattice defects, and affects ion conduction efficiency. By further controlling the addition mass of the cerium salt, the compaction density and ion conduction efficiency are balanced.

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

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

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

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

[0074] As an optional implementation, in step (4), the mass ratio of the sintering intermediate to the titanium white 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 2.58-2.62g / cm 3 , D 500.9-1.2 pm, for example, it can be 0.9 pm, 1.0 pm, 1.1 pm or 1.2 pm.

[0076] The present application limits the mass ratio of the sintering intermediate and the titanium source, so that the titanium doping (titanium white powder) better widens the lattice spacing, and the cerium further improves the Li + mobility.

[0077] In the second aspect, the present application provides a high-power and high-energy-density lithium iron phosphate material, which is prepared by the method of the first aspect.

[0078] In the third aspect, the present application provides a preparation method of a lithium iron phosphate positive electrode sheet, which comprises: (a) providing the sintering intermediate and the lithium iron phosphate material prepared by the preparation method of the first aspect; (b) adding a first positive electrode active material, a conductive agent and a binder into a solvent to mix to prepare a first slurry, wherein the first positive electrode active material is the sintering intermediate; adding a second positive electrode active material, a conductive agent and a binder into a solvent to mix to prepare a second slurry, wherein the second positive electrode active material is the lithium iron phosphate material; (c) coating the first slurry on a current collector to form a bottom layer, and coating the second slurry on the bottom layer to form a surface layer, and then drying and rolling to obtain a lithium iron phosphate positive electrode sheet, wherein the total area density of the lithium iron phosphate positive electrode sheet is 460-500 g / cm2, for example, it can be 460 g / cm2, 480 g / cm2or 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, it can be 2:8, 2.5:7.5 or 3:7.

[0079] The present application improves the porosity of the electrode sheet by the layered coating of the first slurry and the second slurry, realizes the porosity gradient control (the porosity decreases in the direction of the current collector→electrolyte), thereby improving the wettability of the electrolyte and the positive electrode active material under high area density, and improving the lithium ion diffusion efficiency, thereby reducing the lithium ion concentration polarization and promoting the capacity of the battery.

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

[0081] As an optional embodiment, 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 optional embodiment, in the second slurry, the conductive agent comprises CNT and SP; and the binder comprises PVDF.

[0083] As an optional embodiment, 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 embodiment, 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 embodiment, 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 embodiment, in step (c), a double-layer coating machine is used for coating.

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

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

[0089] In a fourth aspect, the present application provides a lithium iron phosphate positive electrode sheet, which is prepared by the method of the third aspect, and comprises a bottom layer and a surface layer; the porosity of the bottom layer is P1, and the porosity of the surface layer is P2; wherein P2

[0090] The lithium iron phosphate positive electrode sheet of the present application has low porosity (P1) in the bottom layer to ensure electron conduction, and has high porosity (P2) in the surface layer to improve electrolyte infiltration, and the porosity gradient (P2

[0091] In a fifth aspect, the present application provides a lithium ion battery comprising the positive electrode sheet of the fourth aspect.

[0092] The present application will be further described in detail below in combination with specific examples and comparative examples.

[0093] In the following examples and comparative examples: Example 1 Preparation of lithium iron phosphate material 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 (the mass ratio of liquefied paraffin to polyvinyl chloride is 1:1, which is used as a binder, liquefied paraffin is used to fill pores, and polyvinyl chloride is used to enhance structural bonding), and titanium dioxide (titanium source, second-stage sintering addition), all of which are commercially available.

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

[0095] (2) Pretreatment of precursor: The lithium iron phosphate precursor was mixed with liquefied paraffin / polyvinyl chloride at a mass ratio of 99:1, then placed in anhydrous ethanol and ball milled at a speed of 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: The cerium nitrate (cerium nitrate addition amount is 0.5wt% of the mass of the pretreated precursor) was mixed with the pretreated precursor obtained in step (2), and then spray dried at 240°C to obtain the cerium-doped precursor (D 50 = 38 μm).

[0097] (4) Two-stage sintering: First-stage sintering: calcination at 790°C for 3 h in an argon atmosphere, crushing and removing the magnet after sintering, to obtain a sintered intermediate, with a tap density of 2.38 g / cm 3 , D 50 = 2.8 μm.

[0098] Second-stage sintering: adding titanium dioxide to the sintered intermediate (mass ratio of sintered intermediate to titanium dioxide is 95:5), and treating at 700°C for 6 h under argon protection to obtain lithium iron phosphate material (tap density of 2.60 g / cm³, D 50 = 1.1 μm).

[0099] Preparation of lithium iron phosphate positive electrode sheet (a) providing the sintered intermediate and lithium iron phosphate material prepared by the above preparation method; (b) slurry preparation: First slurry: the sintered intermediate was used as the first positive electrode active material, and conductive agent (mass ratio of SP to CNT is 2:1) and binder PVDF (mass ratio is 96.5:1.5:2) were added to NMP solvent to prepare a first slurry, the viscosity of the first slurry was 6800 mPa·s, and the solid content was 63.5%. The second slurry: lithium iron phosphate material as the second positive active material, with conductive agent (mass ratio of SP and CNT is 2:1), binder PVDF is added into NMP solvent according to mass ratio 97:1.5:1.5 to mix, the viscosity of the second slurry is 7200 mPa·s, and the solid content is 62.3%; (c) Coating: coating is carried out by using a double-layer coating machine, the first slurry is coated on the carbon-coated aluminum foil current collector to form a bottom layer (thickness ratio is 30%), and the second slurry is coated on the bottom layer to form a surface layer (thickness ratio is 70%), and after drying and rolling, a lithium iron phosphate positive electrode sheet is obtained, wherein the total area density is 480 g / m 2 , the coating speed is 30 m / min, and the thickness deviation is ≤1.5 μm.

[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 the lithium iron phosphate precursor and the liquefied paraffin / polyvinyl chloride is adjusted to 97:3. The preparation method of the lithium iron phosphate positive electrode sheet is the same as that 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 addition amount of cerium nitrate accounts for 0.6 wt% of the mass of the pretreated precursor. The preparation method of the lithium iron phosphate positive electrode 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 positive electrode sheet is the same as that in Example 1.

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

[0104] The preparation method of the lithium iron phosphate positive electrode sheet provided in Example 5 is basically the same as that in Example 1, except that in the preparation method of the lithium iron phosphate positive electrode sheet, in step (c), the thickness ratio of the bottom layer and the surface layer is changed to 2:8, the first slurry is coated on the carbon-coated aluminum foil current collector to form a bottom layer (thickness ratio is 20%), and the second slurry is coated on the bottom layer to form a surface layer (thickness ratio is 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 addition amount of cerium nitrate accounts for 1.0 wt% of the mass of the pretreated precursor. The preparation method of the lithium iron phosphate positive electrode 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 the lithium iron phosphate precursor to the liquefied paraffin / polyvinyl chloride is adjusted to 95:5. The preparation method of the lithium iron phosphate positive electrode sheet is the same as that 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 positive electrode sheet is the same as that 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 in the second-stage sintering. The preparation method of the lithium iron phosphate positive electrode 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 in step (3) is not performed, and the pretreated precursor is directly subjected to two-stage sintering. The preparation method of the lithium iron phosphate positive electrode sheet 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., the liquefied paraffin / polyvinyl chloride binder is not added. The preparation method of the lithium iron phosphate positive electrode 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 only the first-stage sintering is performed in step (4), titanium dioxide is added to the cerium-doped precursor (mass ratio of the cerium-doped precursor to the titanium dioxide is 95:5), and the second-stage sintering is not performed. The preparation method of the lithium iron phosphate positive electrode 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 no titanium source doping, i.e., no titanium dioxide, is added in the second-stage sintering in step (4). The preparation method of the lithium iron phosphate positive electrode 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 completely the same as that in Example 1. The preparation method of the lithium iron phosphate positive plate is basically the same as that in Example 1, except that in the preparation method of the lithium iron phosphate positive plate, only the second slurry is used for coating in step (c), that is, the second slurry equal in amount to the first slurry in Example 1 is used for coating.

[0114] Comparative Example 6 The preparation method of the lithium iron phosphate material provided in Comparative Example 6 is basically the same as that in Example 1, except that in step (3), after the cerium nitrate solution is uniformly mixed with the pretreated precursor, drying is performed in an oven, and the drying temperature is the same as that 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-stage sintering is 700°C, and the sintering temperature of the second-stage sintering is 790°C.

[0116] Performance detection Test method: Full-electric test procedure, 1C rate charging, cut-off voltage 3.65V, 1C rate discharging, cut-off voltage 2.0V, 3 cycles for activation; rate discharge: 0.33C rate charging, cut-off voltage 3.65V, discharging at different rates (0.5C / 1C / 2C / 3C) respectively, cut-off voltage 2.0V, calculate the discharge capacity retention rate at different rates, 0.5C rate discharge capacity retention rate = 0.5C rate discharge capacity / 0.33C rate charging capacity*100; 1C rate discharge capacity retention rate = 1C rate discharge capacity / 0.33C rate charging capacity*100; 2C rate discharge capacity retention rate = 2C rate discharge capacity / 0.33C rate charging capacity*100; 3C rate discharge capacity retention rate = 3C rate discharge capacity / 0.33C rate charging capacity*100; Compaction density: take 2-3g powder, test its powder compaction density = powder mass (g) / compacted volume (cm 3 ) under 30KN pressure (pressure maintaining for 60s); Energy density: mass energy density = battery released energy (Wh) / battery mass (kg).

[0117] Lithium ion batteries were prepared by using the lithium iron phosphate positive plates provided in Examples 1-9 and Comparative Examples 1-7 and graphite as the active material of the negative plate. The batteries prepared in the examples and comparative examples were tested for performance according to the above test method, 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 positive plate prepared in Examples 1-9 can simultaneously improve the energy density and power density of the battery, the energy density is ≥183 Wh / kg, the 0.5 C discharge capacity rate is ≥99.3%, the 1 C discharge capacity rate is ≥98.4%, the 2 C discharge capacity rate is ≥96.9%, the 3 C discharge capacity rate is ≥96.1%, and the discharge capacity retention rate at high rate is high, indicating that the lithium iron phosphate positive plate prepared in Examples 1-9 has high power density.

[0120] Examples 3 and 6 have higher cerium doping amounts and exhibit better rate performance (3C≥98%), indicating that cerium doping significantly improves the power density.

[0121] (2) As can be seen from Examples 1, 2 and 7, the introduction of liquid paraffin / polyvinyl chloride improves the energy density of the battery, mainly filling the gap between the particles and improving the compaction density of lithium iron phosphate. The compaction densities of Examples 2 and 7 are higher (2.41 / 2.61 g / cm 3 , 2.42 / 2.62 g / cm 3 ), and the energy densities are also improved (185.0 Wh / kg, 184.7 Wh / kg). Comparative Example 2 does not add liquid paraffin / polyvinyl chloride binder, and the compaction density of Comparative Example 2 is significantly lower (2.23 / 2.56 g / cm³), and the energy density is also the lowest (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, activates the lithium ion channel, fundamentally improves the ionic conductivity of the material, and thus improves 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. Comparative Example 1 does not dope cerium, and the 3C discharge capacity retention rate is only 95.80%. The discharge capacity retention rate at high rate is high, indicating that the lithium iron phosphate positive plate prepared in Examples 1, 3 and 6 has high power density.

[0123] (4) As can be seen from Examples 1, 4 and 8, two-stage sintering controls the grain size and structural integrity, improves the compaction density while maintaining high energy density and rate performance. The second stage temperature rise can further improve the compaction density.

[0124] (5) As can be seen from Example 1 and Example 9, titanium doping during the secondary sintering process can better widen the lattice spacing, and synergize with cerium to further improve the Li + migration. The 3C capacity retention rate of Example 9 (97.36%) is higher than that of Example 1 (96.60%). This indicates that increasing the titanium doping amount further widens the lattice spacing, making it easier for Li + migration, thereby improving the power density of the electrode. The energy density of Example 9 (184.5 Wh / kg) is at the same level as that of Example 1 (184.0 Wh / kg), and even slightly higher, indicating that increasing the titanium content in this range does not negatively affect the energy density. Comparative Example 4 does not add titanium source doping, and the 3C capacity retention rate (95.10%) is low.

[0125] (6) As can be seen from Example 1, Example 5 and Comparative Example 5, the multi-layer coating technology effectively improves the power density of the electrode, and by changing the thickness ratio of the bottom layer to the surface layer, the proportion of the bottom layer in Example 5 is reduced, which may slightly weaken the electronic conduction ability and affect the performance at very high rate, and the power density of the electrode is slightly reduced. Comparative Example 5 uses the same active material as Example 1, but uses single-layer coating without porosity gradient design, and the 3C performance is significantly reduced, fully illustrating that the lack of porosity gradient leads to poor electrolyte wettability, large ion diffusion impedance, and significantly reduced 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 application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent substitutions for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a high-power, high-energy-density lithium iron phosphate material, characterized in that, The method comprises the following steps: (1) Preparation of lithium iron phosphate precursor: mixing lithium source, iron source, phosphorus source and carbon source, and then spray drying to obtain spherical lithium iron phosphate precursor; (2) Pretreatment of precursor: ball milling the lithium iron phosphate precursor obtained in step (1) with liquid paraffin / polyvinyl chloride binder in a solvent, and vacuum drying to obtain pretreated precursor; (3) Cerium doping: mixing cerium salt solution with the pretreated precursor obtained in step (2), and then spray drying to obtain cerium-doped precursor; (4) Two-stage sintering: First-stage sintering: sintering the cerium-doped precursor in an inert atmosphere to obtain a sintered intermediate; Second-stage sintering: adding titanium source to the sintered intermediate to obtain a mixture, and continuing to sinter the mixture in an inert atmosphere at a sintering temperature of 650-740°C 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 of claim 1, wherein: 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 drying temperature of the spray drying is 240-280°C; and / or, The lithium iron phosphate precursor D 50 50-60 μm.

3. The method of claim 1, wherein: In step (2), the mass ratio of the lithium iron phosphate precursor to the liquid paraffin / polyvinyl chloride binder is (95-99):(1-5); and / or, The solvent is anhydrous ethanol; and / or, The rotation speed of the ball milling is 300-500 rpm, and the time is 6-8 h; and / or, The drying temperature of the vacuum drying is 70-90°C, and the time is 4-5 h.

4. The method of claim 1, wherein: 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 temperature of the spray drying is 240-280°C; and / or, The cerium-doped precursor D 50 is 30-45 μm.

5. The method of claim 1, wherein: In step (4), the sintering temperature of the first-stage sintering is 760-850°C, and / or, The sintering time of the first-stage sintering is 2-4 h; and / or, The sintered intermediate is crushed and demagnetized, and the sintered intermediate has a compaction density of 2.30-2.45 g / cm 3 , D 50 2.0-3.5 μm; and / or, The sintering time of the second-stage sintering is 5-8 h; and / or, The titanium source is titanium white powder; Preferably, the mass ratio of the sintered intermediate to titanium white powder is (93-97):(3-7); and / or, The compaction density of the lithium iron phosphate material is 2.58-2.62 g / cm 3 , D 50 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 of any one of claims 1-5.

7. A method for preparing a lithium iron phosphate positive electrode sheet, comprising: (a) providing a sintered intermediate and a lithium iron phosphate material prepared by the preparation method of any one of claims 1-5; (b) mixing a first positive electrode active material, a conductive agent and a binder in a solvent to obtain a first slurry, the first positive electrode active material being the sintered intermediate; and mixing a second positive electrode active material, a conductive agent and a binder in a solvent to obtain a second slurry, the second positive electrode active material being the lithium iron phosphate material. (c) coating the first slurry on the current collector to form a bottom layer, and coating the second slurry on the bottom layer to form a surface layer, and after drying and rolling, a lithium iron phosphate positive electrode sheet is obtained, wherein the total area 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).

8. The method of claim 7, wherein: In the first slurry, the conductive agent comprises CNT and SP; the binder comprises PVDF; and / or, In the first slurry, the mass ratio of the first positive electrode active material, conductive agent, and 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), when the coating speed is ≥30 m / min, the thickness deviation is ≤1.5 μm. Preferably, in step (c), the coating speed is 30 m / min-60 m / min.

9. A lithium iron phosphate positive electrode sheet, characterized by, The lithium ion battery prepared by the method of claim 7 or 8 comprises a bottom layer and a surface layer; the porosity of the bottom layer is P1, and the porosity of the surface layer is P2; P2 10. A lithium-ion battery, characterized by, The lithium ion battery comprises the positive electrode sheet of claim 9.

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