A preparation method of a lithium iron phosphate positive electrode material co-modified by phosphoric acid pretreatment and ion doping

Abstract

The application provides a preparation method of a lithium iron phosphate positive electrode material modified by phosphoric acid pretreatment and ion doping. Lithium source, iron source, phosphorus source, carbon source, titanium source and vanadium source are mixed with water as a solvent and then sand milling is performed; the slurry is taken out after sand milling; the slurry is subjected to spray drying to obtain yellow material; the yellow material is subjected to primary calcination under an inert atmosphere to obtain a lithium iron phosphate precursor; the lithium iron phosphate precursor is aged in a phosphoric acid solution and then subjected to suction filtration and drying to obtain black powder; and the black powder is subjected to secondary calcination under an inert atmosphere to obtain the lithium iron phosphate positive electrode material. By dispersing the lithium iron phosphate precursor in a phosphoric acid solution with a certain concentration, defects are generated on the surface of the precursor after acidification, which is beneficial to the growth of particles after secondary calcination, improves the compaction density, and the residual phosphate groups are attached to the surface of the precursor and penetrate into the surface layer, which can promote the diffusion of lithium ions between particles and the transmission of lithium ions and electrons between particles.

Description

Technical Field

[0001] This invention relates to a lithium-ion battery cathode material and its preparation method in the field of electrochemical energy storage, specifically to a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is widely used in portable electronic devices and new energy vehicles due to its high safety, long cycle life, environmental friendliness, and low production cost. However, its inherently low conductivity has always limited its further development. Currently, the widely used carbon coating relies on a conductive carbon layer to coat the surface of lithium iron phosphate particles to improve the electronic conductivity of the material. However, the carbon coating layer does not facilitate the passage of ions and cannot improve the ionic conductivity of the material. Chinese invention patent CN111403710A discloses a method for preparing ternary doped lithium manganese oxide cathode material coated with aluminum trifluoride (AlF3). After coating with aluminum trifluoride (AlF3), it can effectively alleviate capacity decay, prevent electrolyte corrosion, and reduce the dissolution of manganese ions. Chinese invention patent CN107591532B uses a double-layer coating of silver and aluminum fluoride on lithium nickel cobalt manganese oxide cathode material, with aluminum fluoride as the inner shell and silver as the outer shell. This can simultaneously improve the electronic and ionic conductivity of the material. However, silver is expensive and the preparation process is complex. Summary of the Invention

[0003] The purpose of this invention is to improve the compaction density and conductivity of lithium iron phosphate. This invention improves the ion diffusion rate within lithium iron phosphate by doping with two metal ions and increases the compaction density through phosphoric acid pretreatment. This method has a simple process flow, produces environmentally friendly synthesized materials, and is easily industrialized.

[0004] The method for solving the problem in this invention is achieved through the following technical solution:

[0005] Step (1): Place the lithium source, iron source, phosphorus source, carbon source, titanium source, and vanadium source into a sand mill with deionized water as the solvent for sand milling;

[0006] Step (2): After grinding to a certain particle size, remove the slurry;

[0007] Step (3): Spray dry the slurry from step (2) to obtain yellow material;

[0008] Step (4): The yellow material from step (3) is placed in an inert atmosphere tube furnace for one calcination to obtain the lithium iron phosphate precursor;

[0009] Step (5): The lithium iron phosphate precursor from step (4) is aged in a phosphoric acid solution for a certain period of time, then filtered and dried to obtain a black powder.

[0010] Step (6): The black powder obtained in step (5) is calcined a second time under an inert atmosphere to obtain lithium iron phosphate cathode material.

[0011] Specifically, the iron source used in step (1) is one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, and ferric phosphate;

[0012] Specifically, the lithium source used in step (1) is one of lithium hydroxide, lithium carbonate, lithium bicarbonate, and lithium dihydrogen phosphate, and is fed in a lithium:iron ratio of 1.02-1.06:1.

[0013] Specifically, in step (1), the carbon source is one or more of glucose, sucrose, fructose, polyethylene glycol, and tannic acid, and the feed is carried out according to the ratio of carbon to iron = 0.3-0.5:1;

[0014] Specifically, in step (1), the titanium source is one or more of titanium dioxide and tetrabutyl titanate, and the materials are fed in a molar ratio of titanium:iron = 0.008-0.03:1;

[0015] Specifically, in step (1), the vanadium source is one or more of ammonium metavanadate, vanadium pentoxide, and vanadium oxalate, and is fed in a molar ratio of vanadium:iron = 0.001-0.006:1;

[0016] Specifically, in step (1), deionized water is added at a solid content of 35%-50%.

[0017] Specifically, in step (2), the particle size D50 is 300-500nm.

[0018] Specifically, the drying temperature in step (3) is 75-85℃.

[0019] Specifically, in step (4), the calcination conditions are 300-400℃, the heating rate is 2-5℃ / min, the holding time is 1-2h, and the protective gas is either N2 or Ar.

[0020] Specifically, in step (5), the phosphoric acid concentration is 50%-75%, which is prepared by mixing 85% phosphoric acid with a certain amount of water; the quality of the lithium iron phosphate precursor added is controlled according to the solid content of 20%-60%.

[0021] Specifically, in step (6), the stirring and aging time is 1-7 hours, and the rotation speed is 500-800 r / min;

[0022] Specifically, in step (7), the calcination temperature is 600-800℃, the heating rate is 2-5℃ / min, the holding time is 7-10h, and the protective gas is either N2 or Ar.

[0023] Compared with other lithium iron phosphate modification methods, the beneficial effects of this invention are:

[0024] 1. Co-doping of titanium and vanadium is beneficial to improving the intrinsic conductivity of the material. Since the inside of the particles does not come into contact with the electrolyte, a higher ion diffusion rate is required to transfer lithium ions. Meanwhile, the surface layer is wetted by the electrolyte, and some vanadium is converted into lithium vanadium phosphate. Both can promote the transfer of ions between particles and improve the utilization rate of the doped metal.

[0025] 2. By dispersing the lithium iron phosphate precursor in a phosphoric acid solution of a certain concentration, certain defects are generated on the surface of the precursor after acidification. After secondary calcination, it is conducive to particle growth and improves the compaction density. At the same time, the residual phosphate ions adhere to the precursor surface and penetrate into the surface layer. After secondary calcination, a small amount of lithium vanadium phosphate is generated and coated on the surface layer together with carbon. Lithium vanadium phosphate is an excellent fast ion conductor, which can promote the diffusion of lithium ions between particles and promote the transport of lithium ions and electrons between particles. Attached Figure Description

[0026] Figure 1 The images are scanning electron microscope images of Example 1 at different magnifications.

[0027] Figure 2 The images are scanning electron microscope (SEM) images of Comparative Example 1 at different magnifications.

[0028] Figure 3 The charge-discharge curves for Example 1 at 0.1C are shown.

[0029] Figure 4 The charge-discharge curves for Comparative Example 1 at 0.1C are shown. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] This invention discloses a phosphoric acid pretreatment and ion doping co-modified lithium iron phosphate cathode material, its preparation method, and the method thereof. The inventive concept is further illustrated below with specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. It should be understood that, based on the content of this invention, those skilled in the art can make various modifications or alterations to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0032] Example 1

[0033] Ferrous sulfate, lithium dihydrogen phosphate, glucose, titanium dioxide, and ammonium metavanadate were mixed with deionized water in a molar ratio of 1:1.02:0.34:0.008:0.001 and then milled in a sand mill to maintain a solid content of 36%. After milling for a certain period, the particle size D50 of the slurry was measured to be 330 mm. The slurry was then spray-dried at 75°C. A yellow material was obtained after spray drying. 100 g of this yellow material was placed in an inert atmosphere tube furnace, heated to 350°C, and held for 60 minutes under nitrogen atmosphere. Sintering yielded the lithium iron phosphate precursor for later use. 70g of lithium iron phosphate precursor was aged in a 75% phosphoric acid solution at a rotation speed of 500 r / min for 2 hours. After aging, the precursor was filtered and dried to obtain powder. The powder was placed in a tube furnace under a nitrogen atmosphere and heated to 650℃ for 200 min, held at that temperature for 420 min, and then cooled to room temperature to obtain a phosphoric acid pretreated and ion-doped co-modified lithium iron phosphate cathode material and its preparation method.

[0034] Example 2

[0035] Ferrous oxalate, lithium dihydrogen phosphate, glucose, titanium dioxide, and ammonium metavanadate were mixed with deionized water in a molar ratio of 1:1.03:0.36:0.01:0.002 and then milled in a sand mill. The solid content was controlled at 38%. After milling for a certain period of time, the particle size D50 of the slurry was measured to be 390. The slurry was then spray-dried at a temperature of 75℃. After spray drying, a yellow material was obtained. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60 to 300℃, and held for 70min under nitrogen as the protective gas. The sintering yielded a lithium iron phosphate precursor for later use. 50g of the lithium iron phosphate precursor was placed in a 60% phosphoric acid solution for aging at 500r / min for 3h. After aging, it was filtered and dried to obtain powder. The powder was placed in a nitrogen atmosphere tube furnace, heated to 700℃ for 200min, and held for 560min. After cooling to room temperature, a phosphoric acid pretreated and ion-doped co-modified lithium iron phosphate cathode material and its preparation method were obtained.

[0036] Example 3

[0037] Iron phosphate, lithium bicarbonate, glucose, tetrabutyl titanate, and vanadium pentoxide were mixed with deionized water in a molar ratio of 1:1.04:0.34:0.03:0.006 and milled in a sand mill with a solid content controlled at 40%. After milling for a certain period, the particle size D50 of the slurry was measured to be 450. The slurry was then spray-dried at 75℃. A yellow material was obtained after spray drying. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-300℃, and held for 70min under nitrogen atmosphere. Sintering yielded a lithium iron phosphate precursor for later use. 100g of the lithium iron phosphate precursor was aged in a 50% phosphoric acid solution at 500r / min for 5h. After aging, it was filtered and dried to obtain powder. The powder was placed in a nitrogen atmosphere tube furnace, heated to 700℃ for 200min, and held for 560min. After cooling to room temperature, the lithium iron phosphate cathode material was obtained.

[0038] Comparative Example 1

[0039] Iron phosphate, lithium carbonate, glucose, tetrabutyl titanate, and vanadium pentoxide were mixed with deionized water in a molar ratio of 1:1.04:0.34:0.01:0.002 and milled in a sand mill with a solid content controlled at 40%. After milling for a certain period, the particle size D50 of the slurry was measured to be 450. The slurry was then spray-dried at 75℃. A yellow material was obtained after spray drying. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-300℃, and held for 70min under nitrogen atmosphere. Sintering yielded a lithium iron phosphate precursor for later use. 70g of the lithium iron phosphate precursor was aged in an 85% phosphoric acid solution at 500r / min for 1h. After aging, it was filtered and dried to obtain powder. The powder was placed in an inert atmosphere tube furnace, heated to 700℃ for 200min, and held for 560min. After cooling to room temperature, the lithium iron phosphate cathode material was obtained.

[0040] Comparative Example 2

[0041] Iron phosphate, lithium bicarbonate, fructose, tetrabutyl titanate, and ammonium metavanadate were mixed with deionized water in a molar ratio of 1:1.04:0.34:0.01:0.002 and milled in a sand mill with a solid content controlled at 40%. After milling for a certain period, the particle size D50 of the slurry was measured to be 450. The slurry was then spray-dried at 75℃. After spray drying, a yellow material was obtained. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-300℃, and held for 70min under nitrogen atmosphere. Sintering was then performed to obtain a lithium iron phosphate precursor for later use. 70g of the lithium iron phosphate precursor was aged in an 85% phosphoric acid solution at a rotation speed of 500r / min for 1h. After aging, the solution was filtered and dried to obtain a powder. The powder was placed in an inert atmosphere tube furnace, heated to 700℃ for 200min, and held for 560min. After cooling to room temperature, the lithium iron phosphate cathode material was obtained.

[0042] Comparative Example 3

[0043] Iron phosphate, lithium carbonate, glucose, tetrabutyl titanate, and vanadium pentoxide were mixed with deionized water in a molar ratio of 1:1.04:0.34:0.01:0.002 and milled in a sand mill with a solid content controlled at 40%. After milling for a certain period, the particle size D50 of the slurry was measured to be 450. The slurry was then spray-dried at 75℃. A yellow material was obtained after spray drying. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-300℃, and held for 70min under nitrogen atmosphere. Sintering yielded a lithium iron phosphate precursor for later use. 70g of the lithium iron phosphate precursor was placed in 100ml of deionized water and stirred at 500r / min. After aging for 1h, it was filtered and dried to obtain powder. The powder was placed in an inert atmosphere tube furnace, heated to 700℃ for 200min, and held for 560min. After cooling to room temperature, the lithium iron phosphate cathode material was obtained.

[0044] Comparative Example 4

[0045] Iron phosphate, lithium bicarbonate, glucose, titanium dioxide, and vanadium pentoxide were mixed with deionized water in a molar ratio of 1:1.04:0.34:0.04:0.002 and then milled in a sand mill. The solid content was controlled at 40%. After milling for a certain period of time, the particle size D50 of the slurry was measured to be 450. The slurry was then spray-dried at 75℃. After spray drying, a yellow material was obtained. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-300℃, and held for 70 minutes under nitrogen as the protective gas. Sintering yielded the lithium iron phosphate precursor for later use. 70g of lithium iron phosphate precursor was aged in a 75% phosphoric acid solution at a rotation speed of 500 r / min for 5 hours. After aging, the precursor was filtered and dried to obtain powder. The powder was placed in an inert atmosphere tube furnace, heated to 700℃ for 200 min, held at that temperature for 560 min, and then cooled to room temperature to obtain a phosphoric acid pretreated and ion-doped co-modified lithium iron phosphate cathode material and its preparation method.

[0046] Comparative Example 5

[0047] Ferrous sulfate, lithium dihydrogen phosphate, glucose, titanium dioxide, and ammonium metavanadate were mixed with deionized water in a molar ratio of 1:1.02:0.34:0.01:0.007 and then milled in a sand mill to maintain a solid content of 36%. After milling for a certain period, the particle size D50 of the slurry was measured to be 330 mm. The slurry was then spray-dried at 75°C. A yellow material was obtained after spray drying. 100 g of this yellow material was placed in an inert atmosphere tube furnace, heated to 350°C, and held for 60 minutes under nitrogen atmosphere. Sintering yielded the lithium iron phosphate precursor for later use. 70g of lithium iron phosphate precursor was aged in a 75% phosphoric acid solution at a rotation speed of 500 r / min for 5 hours. After aging, the precursor was filtered and dried to obtain powder. The powder was placed in an inert atmosphere tube furnace, heated to 650℃ for 200 min, held at that temperature for 420 min, and then cooled to room temperature to obtain a phosphoric acid pretreated and ion-doped co-modified lithium iron phosphate cathode material and its preparation method.

[0048] Comparative Example 6

[0049] Ferrous sulfate, lithium dihydrogen phosphate, glucose, titanium dioxide, and ammonium metavanadate were mixed with deionized water in a molar ratio of 1:1.02:0.34:0:0 and milled in a sand mill. The solid content was controlled at 36%. After milling for a certain period of time, the particle size D50 of the slurry was measured to be 330. The slurry was then spray-dried at 75℃. After spray drying, a yellow material was obtained. 100g of the yellow material was placed in an inert atmosphere tube furnace, heated to 60-350℃, and held for 60min under nitrogen atmosphere. Sintering was then performed to obtain a lithium iron phosphate precursor for later use. 70g of the lithium iron phosphate precursor was aged in a 75% phosphoric acid solution at 500r / min for 2h. After aging, it was filtered and dried to obtain powder. The powder was placed in an inert atmosphere tube furnace, heated to 650℃ for 200min, held for 420min, and then cooled to room temperature to obtain a phosphoric acid pretreated and ion-doped co-modified lithium iron phosphate cathode material and its preparation method.

[0050] Table 1: Parameter changes and results comparison between the examples and comparative examples

[0051]

[0052] 1. Table 1 shows the parameter changes and results comparison between the examples and comparative examples. It can be seen that within the optimal ratio range, lithium iron phosphate has the best compaction density and 0.1C discharge specific capacity. Vanadium and titanium co-doping can improve the intrinsic conductivity and electrical performance of the material by replacing some iron sites. On the other hand, vanadium, phosphoric acid, and lithium can generate lithium vanadium phosphate at high temperature. Due to the aging of the lithium iron phosphate precursor in phosphoric acid solution, the surface of the material is in full contact with phosphoric acid, and then lithium vanadium phosphate is generated at high temperature, while the deep layer of the material is still lithium iron phosphate doped with vanadium and titanium. At the same time, after the lithium iron phosphate precursor is aged by phosphoric acid, some defects are formed on the surface, which can promote the growth of some grains in the subsequent secondary calcination process. The appropriate increase of large particles is conducive to the filling of small particles, which increases the compaction density of the material.

[0053] 2. During the charging and discharging process, the improved deep insertion and extraction efficiency of lithium iron phosphate is due to the co-doping of vanadium and titanium. However, the contact between lithium iron phosphate particles is achieved through a carbon coating layer, which is not conducive to the transport of lithium ions. After coating with lithium vanadium phosphate, the surface of lithium iron phosphate is co-coated with carbon and lithium vanadium phosphate. As an excellent fast ion conductor, lithium vanadium phosphate can accelerate the lithium ion transport efficiency between particles and improve the electrochemical performance of the material. At the same time, an appropriate amount of carbon content ensures the smooth progress of the high-temperature reaction and the realization of high compaction density.

[0054] 3. Through the electrochemical performance of a phosphoric acid pretreatment and ion doping co-modified lithium iron phosphate cathode material and its preparation method in specific embodiments, it can be found that the present invention can simultaneously improve the electrical properties and compaction density of the material. In addition, the lithium iron phosphate obtained by the preparation method provided by the present invention under different combinations of lithium source, iron source, phosphorus source and vanadium source all have good electrochemical performance, indicating that the preparation method has good universality and is conducive to large-scale use.

Claims

1. A method for preparing a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping, characterized in that, Includes the following steps: Step (1): Mix lithium source, iron source, phosphorus source, carbon source, titanium source and vanadium source with water as solvent and then perform sand milling; feed lithium source, carbon source, titanium source, vanadium source and iron source in a molar ratio of lithium:carbon:titanium:vanadium:iron = 1.02-1.06:0.3-0.5:0.008-0.03:0.001-0.006:

1. Step (2): After grinding to a certain particle size, remove the slurry; Step (3): Spray dry the slurry from step (2) to obtain yellow material; Step (4): The yellow material from step (3) is calcined once under an inert atmosphere to obtain the lithium iron phosphate precursor; Step (5): The lithium iron phosphate precursor from step (4) is placed in a phosphoric acid solution of a certain concentration and aged for a certain period of time, then filtered and dried to obtain a black powder. The phosphoric acid concentration is 50%-75%, and the mass of the lithium iron phosphate precursor added is controlled according to the solid content of 20%-60%. Step (6): The black powder obtained in step (5) is calcined a second time under an inert atmosphere to obtain lithium iron phosphate cathode material.

2. The method for preparing a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping according to claim 1, characterized in that, The iron source used in step (2) is one of ferrous sulfate, ferrous oxalate, ferrous chloride, and ferric phosphate; The lithium source used is one of lithium hydroxide, lithium carbonate, lithium bicarbonate, and lithium dihydrogen phosphate. The carbon source is one of glucose, sucrose, fructose, polyethylene glycol, or tannic acid. The titanium source is one or more of titanium dioxide and tetrabutyl titanate; The vanadium source is one or more of ammonium metavanadate, vanadium pentoxide, and vanadium oxalate.

3. The method for preparing a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping according to claim 1, characterized in that, In step (2), the particle size D50 is 300-500 nm.

4. The method for preparing a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping according to claim 1, characterized in that, In step (4), the calcination conditions are 300-400℃, the heating rate is 2-5℃ / min, the holding time is 1-2h, and the protective gas is either N2 or Ar.

5. The method for preparing a lithium iron phosphate cathode material co-modified by phosphoric acid pretreatment and ion doping according to claim 1, characterized in that, In step (6), the calcination temperature is 600-800℃, the heating rate is 2-5℃ / min, the holding time is 7-10h, and the inert atmosphere is either N2 or Ar.

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