A high-energy-density lithium iron phosphate cathode material and a preparation method thereof

Abstract

This invention relates to a high-energy-density lithium iron phosphate cathode material and its preparation method. The method includes: thoroughly mixing anhydrous iron phosphate A and B, containing different amounts of doping elements, with a carbon source, a lithium source, a modifying additive, and a solvent, and then grinding them separately to obtain slurries A' and B' of different particle sizes; mixing slurries A' and B' in a certain proportion and spray-drying them to obtain powder; sintering the powder under an inert atmosphere, followed by grinding and sieving to obtain the lithium iron phosphate cathode material. This invention prepares slurries of different particle sizes from iron phosphate raw materials with different doping amounts and grain sizes, and mixes them in a specific proportion, which can form a good particle size distribution after sintering. The final lithium iron phosphate cathode material has a high compaction density, and the preparation method is simple and easy to operate, suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to a high-energy-density lithium iron phosphate cathode material for lithium-ion batteries and its preparation method, belonging to the field of lithium-ion batteries. Background Technology

[0002] In recent years, the new energy vehicle market has experienced explosive growth, leading to a surge in demand for lithium-ion batteries, the core power source for these vehicles. As replacements for traditional gasoline-powered cars, the driving range of new energy vehicles has always been a key concern. Recent rapid advancements in new energy battery technology, such as BYD's Blade Battery technology and CATL's CTP technology, have significantly improved the energy density of lithium iron phosphate (LFP) batteries, resulting in a substantial increase in the driving range of electric vehicles equipped with LFP batteries, thus meeting consumer demands for longer driving range. Furthermore, compared to lithium nickel manganese cobalt oxide (LCO) batteries, LFP batteries offer longer cycle life, better safety, and lower cost. Consequently, their application in new energy vehicles is becoming increasingly prevalent, making them the most popular cathode material for lithium-ion batteries.

[0003] The inherent properties of lithium iron phosphate (LFP) materials result in poor ionic and electronic conductivity. To meet practical application requirements, LFP cathode materials are typically carbon-coated to improve electronic conductivity, while LFP particles are nano-sized to enhance ionic conductivity. This leads to a generally low compaction density for LFP materials. Higher driving range remains a constant pursuit for electric vehicles, thus requiring LFP cathode materials to possess higher energy density. Patent CN104752716B describes the synthesis of LFP materials with small, uniform particle size and good electrochemical performance by preparing a mixed aqueous solution of phosphorus, iron, and lithium sources, adding a specific surfactant, and adjusting the pH in conjunction with a specific sintering process. This resulted in a high energy density. However, the complex liquid-phase synthesis process makes large-scale industrial production difficult, and the relatively small particle size also limits its compaction density, hindering the achievement of higher energy density requirements. In patent CN106602061B, high-density lithium iron phosphate material was prepared by spheroidizing particles. However, the resulting lithium iron phosphate spherical particles are composed of numerous nano-secondary particles aggregated together. The particles are not tightly bound. In actual battery manufacturing, the spherical particles will break after the electrode is rolled. The spherical particles cannot fully contact the conductive agent and binder inside, which leads to performance degradation during use. Summary of the Invention

[0004] To overcome the problem of low compaction density in existing lithium iron phosphate cathode materials, this invention provides a high compaction density lithium iron phosphate cathode material and its preparation method. This lithium iron phosphate material exhibits high compaction density, and the preparation method is simple and easy to operate, making it suitable for large-scale industrial production.

[0005] The technical solution adopted in this invention is as follows:

[0006] A method for preparing a lithium iron phosphate cathode material includes the following steps:

[0007] Anhydrous iron phosphate A and B, containing different amounts of doped elements, were thoroughly mixed with carbon source, lithium source, modifying additive and solvent, and then ground separately to obtain slurries A' and B' with different particle sizes.

[0008] Slurries A' and B' are mixed in a certain proportion and then spray-dried to obtain powder;

[0009] The powder was sintered under an inert atmosphere, and then ground and sieved to obtain lithium iron phosphate cathode material.

[0010] Furthermore, the doping elements mentioned above can be one or more of Al, Mg, Zr, Ti, V, etc.

[0011] Furthermore, the content (mass fraction) of doping elements in anhydrous iron phosphate A is 0-0.2%, and the content (mass fraction) of doping elements in anhydrous iron phosphate B is 0.2%-2%.

[0012] Furthermore, the crystallite size of anhydrous iron phosphate A is 100–200 nm, and the crystallite size of anhydrous iron phosphate B is 50–100 nm. These crystallite sizes were obtained through XRD refinement and fitting calculations.

[0013] Furthermore, the grinding particle size of the above-mentioned slurry A' is 3 to 5 times the crystal size of the anhydrous iron phosphate A used, the grinding particle size of slurry B' is 1 to 3 times the crystal size of the anhydrous iron phosphate B used, and the particle size ratio of slurry B' to slurry A' does not exceed 0.8.

[0014] Furthermore, in the above-mentioned mixed slurry, the proportion of ferric phosphate in slurry A' to the total ferric phosphate in slurries A' and B' does not exceed 40%.

[0015] Furthermore, the lithium source is one of lithium carbonate, lithium hydroxide, lithium bicarbonate, lithium acetate, and lithium oxalate. The carbon source is one or more of glucose, sucrose, fructose, polyethylene glycol, polyethylene, and polypropylene. The solvent is water, methanol, ethanol, or acetone.

[0016] Furthermore, the modified additives mentioned above are nano-oxides or hydroxides containing elements such as Al, Mg, Zr, Ti, and V, or they can be inorganic salts or organic compounds containing the above elements.

[0017] Further, the molar ratio of the lithium source to iron phosphate is (0.49–0.55):1; the carbon source accounts for 2%–20% of the mass of the iron phosphate; the mass ratio of the modifying additive to iron phosphate does not exceed 5%; and the ratio of the solvent to the total mass of the iron phosphate, lithium source, carbon source, and modifying additive is (0.2–3):1.

[0018] Furthermore, the sintering conditions are sintering at 650–850°C for 5–30 hours.

[0019] Furthermore, the inert atmosphere includes a nitrogen atmosphere.

[0020] The present invention also provides a lithium iron phosphate cathode material prepared by the above method.

[0021] This invention discovered in experiments that doping iron phosphate with a certain amount of other metal elements can alter its reactivity during sintering. Furthermore, it was found that iron phosphate with different grain sizes exhibits significant differences in reactivity during sintering. Based on these findings, this invention prepares slurries of different particle sizes from iron phosphate raw materials with varying doping amounts and grain sizes, and mixes them in a specific ratio. This results in a well-balanced particle size distribution after sintering. On one hand, the combination of large and small particles improves the compaction density of the lithium iron phosphate material; on the other hand, the presence of a certain proportion of small particles provides good capacity and rate performance. The final lithium iron phosphate cathode material has a compaction density higher than 2.6 g / cm³. 3 It has a 0.1C amplification capacity of over 155mAh / g and a 1C capacity of over 140mAh / g, and exhibits good cycling performance. Attached Figure Description

[0022] Figure 1 This is a SEM image of the lithium iron phosphate material in Example 1 of this invention.

[0023] Figure 2 This is a SEM image of the lithium iron phosphate material in Comparative Example 1 of this invention.

[0024] Figure 3 These are the 0.1C discharge curves of the lithium iron phosphate materials in Embodiment 1 and Comparative Example 1 of the present invention.

[0025] Figure 4 These are the 1C discharge curves of the lithium iron phosphate materials in Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0027] Example 1

[0028] 950g of anhydrous iron phosphate (containing 0.36% Ti and a grain size of 96nm), 240g of lithium carbonate, 80g of sucrose, and 0.28g of nano-titanium dioxide additive were mixed evenly with 1000g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of approximately 280nm. 50g of anhydrous iron phosphate (containing 0.06% Al and a grain size of 180nm), 15g of lithium carbonate, 5g of sucrose, and 0.02g of nano-titanium dioxide additive were mixed evenly with 100g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 550nm. The two slurries were then mixed and dried using a spray dryer. The resulting powder was sintered in a tube furnace at 770℃ for 20 hours under a nitrogen atmosphere, then ground and sieved through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The compaction density of this lithium iron phosphate material is 2.64 g / cm³. 3 The 0.1C discharge capacity is 158 mAh / g, and the 1C discharge capacity is 142 mAh / g. The morphology of the lithium iron phosphate material prepared in this embodiment is as follows. Figure 1 As shown.

[0029] Example 2

[0030] 800g of anhydrous iron phosphate (containing 0.35% Zr, with a grain size of 66nm), 200g of lithium carbonate, 150g of fructose, 1.6g of nano-magnesium hydroxide additive, and 1600g of methanol were mixed evenly. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 120nm. 200g of anhydrous iron phosphate (containing 0.15% Mg, with a grain size of 108nm), 32g of lithium hydroxide, 8g of sucrose, 0.4g of nano-magnesium hydroxide additive, and 350g of methanol were mixed evenly. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 505nm. The two slurries were then mixed and dried using a spray dryer. The resulting powder was sintered in a tube furnace at 780℃ for 25 hours under a nitrogen atmosphere, then ground and sieved through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The compaction density of this lithium iron phosphate material was 2.62g / cm³. 3 The 0.1C discharge capacity is 157mAh / g, and the 1C discharge capacity is 141mAh / g.

[0031] Example 3

[0032] 700g of anhydrous iron phosphate (containing 2% Ti and a grain size of 52nm), 175g of lithium carbonate, 100g of glucose, 0.7g of tetrabutyl titanate additive, and 400g of pure water were mixed evenly. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 150nm. 300g of anhydrous iron phosphate (containing 0.03% V and a grain size of 138nm), 75g of lithium carbonate, 30g of polyethylene glycol, 0.3g of tetrabutyl titanate additive, and 500g of pure water were mixed evenly. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 430nm. The two slurries were then mixed evenly and dried using a spray dryer. The resulting powder was sintered in a tube furnace at 800℃ for 30 hours under a nitrogen atmosphere, then ground and sieved through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The compaction density of this lithium iron phosphate material is 2.67 g / cm³. 3 The 0.1C discharge capacity is 156mAh / g, and the 1C discharge capacity is 143mAh / g.

[0033] Example 4

[0034] 900g of anhydrous iron phosphate (containing 0.65% Ti and a grain size of 72nm), 224g of lithium carbonate, 100g of polyethylene glycol, and 2.3g of nano-vanadium pentoxide additive were mixed evenly with 1000g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 210nm. 100g of anhydrous iron phosphate (containing 0.17% Ti and a grain size of 190nm), 25g of lithium carbonate, 5g of glucose, and 0.2g of nano-vanadium pentoxide additive were mixed evenly with 180g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 650nm. The two slurries were then mixed and dried using a spray dryer. The resulting powder was sintered in a tube furnace at 785℃ for 28 hours under a nitrogen atmosphere, then ground and sieved through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The compaction density of this lithium iron phosphate material is 2.65 g / cm³. 3 The 0.1C discharge capacity is 155mAh / g, and the 1C discharge capacity is 141mAh / g.

[0035] Example 5

[0036] 850g of anhydrous iron phosphate (containing 0.25% Ti and a grain size of 58nm), 210g of lithium carbonate, 50g of sucrose, and 0.08g of nano-alumina additive were mixed evenly with 1200g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 80nm. 150g of anhydrous iron phosphate (without doping elements and a grain size of 135nm), 37g of lithium carbonate, 6g of glucose, and 0.01g of nano-alumina additive were mixed evenly with 250g of pure water. The mixture was first ground in a basket mill for 30 minutes, then ground in a sand mill to form a slurry with a particle size of 420nm. The two slurries were then mixed evenly and dried using a spray dryer. The resulting powder was sintered in a tube furnace at 650℃ for 30 hours under a nitrogen atmosphere, then ground and sieved through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The lithium iron phosphate material has a compaction density of 2.60 g / cm3, a 0.1C discharge capacity of 158 mAh / g, and a 1C discharge capacity of 143 mAh / g.

[0037] Comparative Example 1

[0038] 300g of anhydrous iron phosphate was weighed, and then 74g of lithium carbonate, 20g of sucrose, and 0.1g of nano-titanium dioxide were added. The mixture was first ground in a basket mill for 30 minutes, then ball-milled, and finally spray-dried. The resulting powder was sintered in a tube furnace under nitrogen atmosphere at 760℃ for 18 hours. Afterward, it was ground and passed through a 200-mesh sieve to obtain the lithium iron phosphate cathode material. The compaction density of this lithium iron phosphate material is 2.38g / cm³, the 0.1C discharge capacity is 155mAh / g, and the 1C discharge capacity is 128mAh / g.

[0039] The morphology of the lithium iron phosphate material prepared in Comparative Example 1 is as follows: Figure 2 As shown. The 0.1C discharge curves of the lithium iron phosphate materials in Example 1 and Comparative Example 1 are shown below. Figure 3 As shown. The 1C discharge curves of the lithium iron phosphate materials in Example 1 and Comparative Example 1 are as follows. Figure 4 As shown.

[0040] The comparison between the examples and the comparative examples is shown in Table 1.

[0041] Table 1

[0042]

[0043] Comparing the lithium iron phosphate materials obtained in the examples and comparative examples, it can be found that the lithium iron phosphate material in the examples has a higher compaction density and a higher 1C high-rate discharge capacity. The main reason is that the examples use a mixture of slurries with two particle sizes, and by controlling the grain size and dopant content of iron phosphate, different iron phosphates possess different reactivity; thus, a good morphology of large and small particles is obtained after sintering. This morphology of lithium iron phosphate not only has a higher compaction density but also better electrical performance. The main reason is that the small particles effectively fill the gaps between the large particles, increasing the compaction density of the lithium iron phosphate material; in addition, since the small particles have better electrochemical activity, the presence of a certain proportion of small particles can also provide good capacity and rate performance.

[0044] The specific embodiments of the present invention disclosed above are intended to help understand the content of the present invention and to implement it accordingly. Those skilled in the art will understand that various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present invention. The present invention should not be limited to the content disclosed in the embodiments of this specification; the scope of protection of the present invention is defined by the claims.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The method comprises the following steps: The anhydrous iron phosphate A and B containing different contents of doping elements are mixed with a carbon source, a lithium source, a modified additive and a solvent respectively, and then are ground to obtain slurries A' and B' of different particle sizes; The slurries A' and B' are mixed in a certain proportion and are dried by spray drying to obtain a powder; The powder is sintered in an inert atmosphere, and then is ground and sieved to obtain a lithium iron phosphate positive electrode material; The mass fraction of the doping elements in the anhydrous iron phosphate A is 0-0.2%, and the mass fraction of the doping elements in the anhydrous iron phosphate B is 0.2%-2%; The crystallite size of the anhydrous iron phosphate A is 100-200 nm, and the crystallite size of the anhydrous iron phosphate B is 50-100 nm; The grinding particle size of the slurry A' is 3-5 times the crystallite size of the anhydrous iron phosphate A, the grinding particle size of the slurry B' is 1-3 times the crystallite size of the anhydrous iron phosphate B, and the particle size ratio of the slurry B' to the slurry A' is not more than 0.

8.

2. The method of claim 1, wherein, The doping elements are at least one of Al, Mg, Zr, Ti and V.

3. The method of claim 1, wherein, The proportion of the iron phosphate in the slurry A' in all the iron phosphates in the slurries A' and B' is not more than 40%.

4. The method of claim 1, wherein, The lithium source is one of lithium carbonate, lithium hydroxide, lithium bicarbonate, lithium acetate and lithium oxalate; the carbon source is at least one of glucose, sucrose, fructose, polyethylene glycol, polyethylene and polypropylene; the solvent is water, methanol, ethanol or acetone; and the modified additive is a nano-oxide or a nano-hydroxide containing Al, Mg, Zr, Ti and V, or is an inorganic salt or an organic compound containing the above elements.

5. The method of claim 1, wherein, The molar ratio of the lithium source to the iron phosphate is (0.49-0.55):1; The mass of the carbon source accounts for 2%-20% of the mass of the iron phosphate; the mass ratio of the modified additive to the iron phosphate is not more than 5%; and the proportion of the solvent relative to the total mass of the iron phosphate, the lithium source, the carbon source and the modified additive is (0.2-3):

1.

6. The method of claim 1, wherein, The sintering condition is sintering at 650-850 ℃ for 5-30 h.

7. A lithium iron phosphate positive electrode material prepared by the method according to any one of claims 1-6.

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