High-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material and preparation method thereof

Abstract

The invention discloses a high-rate in-situ titanium-doped sheet-shaped lithium iron phosphate / carbon composite material and a preparation method thereof.The preparation method comprises the steps that solutions containing a phosphorus source and an iron source are mixed, H2O2 is added for oxidation, then a titanium source is added for a doping reaction, and after dehydration, an in-situ titanium-doped sheet-shaped anhydrous iron phosphate precursor is obtained; the precursor, a lithium source and a carbon source are subjected to ball milling, spray drying and pelletizing and then subjected to high-temperature calcination, and the in-situ titanium-doped flaky lithium iron phosphate / carbon composite material is obtained. According to the method, the titanium source capable of being subjected to reaction conversion in a reaction system is selected, the in-situ titanium doping mode is achieved in the precursor stage, the uniformity and consistency of the composite material can be improved, the sheet-shaped morphology of primary particles can be maintained at the high sintering temperature, and the rate performance of the composite material is remarkably improved.

Description

Technical Field

[0001] The invention relates to a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material and a preparation method thereof, and belongs to the field of lithium ion positive electrode material preparation. Background Art

[0002] The use of traditional fossil energy has led to serious environmental pollution and consumed many non-renewable resources, resulting in a tight supply of energy resources. There is an urgent need to switch from traditional energy to clean new energy. Lithium-ion batteries are energy storage devices for new energy sources and can promote energy transformation. Lithium iron phosphate cathode materials are widely used in power batteries and electrochemical energy storage due to their high safety, long cycle life, and low cost. However, due to their poor ionic and electronic conductivity, the rate performance is not optimistic. Therefore, improvements are urgently needed.

[0003] A relatively simple method is to bulk-dope the material with metal ions. Doping with high-valent metal ions can bring a large number of defects, thereby improving the conductivity of the material. However, cation doping generally involves mixing doping elements into solid-phase sintering or mixing with positive electrode materials for secondary sintering. For example, Chinese patent CN202111185398.7 discloses a method for preparing nano-titanium dioxide-doped lithium iron phosphate, which is a method of mixing iron phosphate, a lithium source, a carbon source, and nano-titanium dioxide, stirring them evenly, sintering them at a high temperature solid phase method, and then crushing them to synthesize titanium-doped lithium iron phosphate. Although this method has the conditions for large-scale production, such a doping method will have the disadvantages of uneven distribution and poor consistency, and the uniformity and accuracy of element doping cannot be guaranteed. At the same time, many researchers have also explored controlling the morphology of lithium iron phosphate to improve its electrochemical performance. For example, Chinese patent CN201710327521.1 discloses a hydrothermal-calcination method for preparing lithium iron phosphate materials, which improves the electrochemical performance of lithium iron phosphate. However, the hydrothermal method requires high temperature and high pressure steps, has certain unsafety, and is highly dependent on production equipment, which is not conducive to large-scale production. Summary of the invention

[0004] In order to solve the problems in the prior art that the lithium iron phosphate / carbon composite material cannot be uniformly doped with a titanium source and cannot maintain a flaky morphology after the titanium source is doped, the first purpose of the present invention is to provide a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material, in which the titanium source is uniformly doped and the flaky morphology significantly improves the rate performance of the composite material. In addition, the material has a porous pellet structure, which is conducive to the full contact of the electrolyte with it when used in batteries, and can provide more channels for the insertion / extraction of lithium ions.

[0005] The second purpose is to provide a method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material. This method realizes in-situ titanium doping in the precursor stage by selecting a titanium source, which can not only improve the uniformity and consistency of the composite material, but more importantly, maintain the flaky morphology of the primary particles at a higher sintering temperature.

[0006] In order to achieve the above technical objectives, the present invention provides a method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material, which comprises mixing an aqueous solution containing a phosphorus source and an iron source and adding H 2 O 2 After oxidation, a titanium source is added for doping reaction, and after dehydration, an in-situ titanium-doped flaky anhydrous iron phosphate precursor is obtained; the precursor is ball-milled with a lithium source and a carbon source, spray-dried, and then calcined at high temperature to obtain an in-situ titanium-doped flaky lithium iron phosphate / carbon composite material; the titanium source is an organic titanium that can be easily decomposed in the reaction system or converted to form a precipitate without introducing impurity anions.

[0007] The key to the technical solution of the present invention is to control the type of titanium source and achieve in-situ doping of titanium element in the precursor stage. Specifically, the mechanism is:

[0008] The present invention firstly accelerates the reaction of ferroferric oxide and phosphorus source by iron powder, and then a precipitation reaction occurs under the action of hydrogen peroxide. During the doping reaction, the titanium source of the present invention forms a hydrophilic functional group, and the titanium and phosphate are combined by uniform coupling and uniformly distributed in the FePO4 in the form of precipitation. 4 ·2H 2 O precursor, and then the dihydrate is dehydrated to obtain iron phosphate to form a titanium-doped iron phosphate solid solution. At the same time, in this process, the precursor will preferentially grow on the (010) plane to form a lamellar morphology. In addition, through this in-situ titanium doping method, bulk doping can be performed during the growth of the precursor to stabilize the lamellar morphology of the precursor. Then, by directly adopting the solid phase sintering process, the lamellar morphology of the precursor can be maintained at a higher sintering temperature, and combined with spray drying to form a porous pellet structure.

[0009] As a preferred solution, the titanium source is at least one of isopropyl tris(diisooctyl pyrophosphate) titanate, bis(diisooctyl pyrophosphate) ethylene titanate, tris(dioctyl phosphoryloxy) isopropyl titanate, tetraisopropyl di(dioctyl phosphite) titanate, ammonium oxalate and metatitanic acid. The titanium sources selected by the present invention can be simultaneously precipitated with dihydrate iron phosphate at high temperature in an excess phosphoric acid reaction system to obtain a uniformly distributed precursor, wherein ammonium oxalate is easily soluble in water, can be more evenly mixed with phosphorus source and iron source in aqueous solution, and forms a precipitate in an excess phosphoric acid environment; metatitanic acid can be converted into a precipitate in an excess phosphoric acid environment and a high-temperature phosphoric acid reaction, and the remaining four titanium sources all contain titanate bonds, which are easily decomposed in water to form a titanium precipitate.

[0010] As a preferred solution, the process of mixing the aqueous solution containing the phosphorus source and the iron source is: adding the phosphorus source to the aqueous solution for dilution, heating the diluted solution in a water bath while stirring, and then slowly adding the iron source for mixing; wherein the phosphorus source is 75-85wt% concentrated phosphoric acid, the dilution volume multiple is 2-4 times, the water bath heating temperature is 40-50°C, and the iron source is iron powder and Fe 3 O 4 Composition. Among them, Fe 3 O 4 Fe from magnetite flotation or chemicals 3 O 4 .

[0011] As a preferred solution, the iron source is iron powder and Fe 3 O 4 According to (1-5): 1 composition. The iron source of the present invention can be used to utilize Fe by adding a small amount of iron powder. 0 with Fe 3 O 4 Fe 3+ Promote Fe 3 O 4 reaction and improve the precipitation efficiency of the iron source into iron phosphate.

[0012] As a preferred solution, the molar ratio of the iron source to the phosphorus source is 1:(2-4); wherein the iron source is measured in terms of the molar amount of the iron element.

[0013] As a preferred solution, the H 2 O 2 The amount of hydrogen peroxide used is 20% to 100% more than the stoichiometric molar ratio of the iron source reaction. By using excess hydrogen peroxide, Fe(H 2 PO 4 ) 2 All oxidized to form FePO 4 .

[0014] As a preferred solution, the oxidation time is 30 to 90 minutes and the oxidation temperature is 40 to 50°C.

[0015] As a preferred solution, the amount of the titanium source added is calculated as follows: the in-situ titanium-doped flaky anhydrous iron phosphate precursor is Fe 3+ When the titanium doping amount is too low, the effect of maintaining the flaky morphology and improving the rate performance is not obvious, while when the titanium doping amount is too high, it exceeds the solid solubility limit, and since titanium is not an active substance, it will affect the specific capacity of the final synthesized lithium iron phosphate / carbon composite material.

[0016] As a preferred solution, the conditions of the doping reaction are: temperature of 90-99°C, time of 2-6 hours; the temperature of the dehydration is 500-650°C, and the dehydration time is 3-4 hours. When the temperature of the doping reaction is too low, it is impossible to ensure that the iron source is completely converted into dihydrated iron phosphate.

[0017] As a preferred solution, the lithium source includes Li 2 CO 3 The carbon source is at least one of glucose, sucrose, cyclodextrin, polyethylene glycol and starch; the ball milling time is 2 to 3 hours, and the ball-to-material ratio is (8 to 10):1.

[0018] As a preferred solution, during the spray drying pelletizing process, the air inlet temperature is 200-260°C, and the air outlet temperature is 90-150°C.

[0019] As a preferred solution, the high-temperature calcination is carried out under an inert atmosphere, first heating to 450-500°C at 3°C / min, keeping warm for 4-5h, and then heating to 620-720°C at 3°C / min for 6-10h. The two-stage calcination process adopted in the present invention can control the initial growth of particles in the low-temperature section, and then complete the final sintering in the high-temperature section. In this way, while ensuring the complete transformation of the crystal structure, excessive growth of particles can be avoided, thereby obtaining a more uniform particle size distribution and a higher specific surface area. In the present invention, the sintering temperature directly affects the crystal structure transformation of lithium iron phosphate. In theory, the crystal transformation behavior of lithium iron phosphate occurs at about 450°C, and complete crystal transformation can be achieved by calcining at 550°C for 6 hours. If the sintering temperature is too low, the crystal structure may be incomplete, resulting in a decrease in the electrochemical properties of the material. Excessive sintering temperature may lead to the formation of impurities. For example, when the sintering temperature reaches 820°C, Li is prone to appear in lithium iron phosphate. 3 PO 4Impurities such as iodine and iodine will destroy the electrochemical properties of the material. At the same time, temperature will also affect the morphology of lithium iron phosphate. High-temperature sintering will lead to an intensification of particle fusion, increase the particle size, and reduce the specific surface area. In addition, the present invention can ensure the compaction density of the material at a higher sintering temperature to obtain a lithium iron phosphate / carbon composite material with a low carbon content and a flaky morphology. It is further preferred that the sintering temperature of the high-temperature section is 680-720°C.

[0020] The present invention provides a method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material, which specifically comprises the following steps:

[0021] 1) Weigh a phosphorus source and add deionized water to dilute it. Heat the diluted solution in a water bath to a certain temperature while stirring.

[0022] 2) Slowly add the iron source to the above solution and stir for a certain period of time to mix the iron source and the phosphorus source evenly.

[0023] 3) Add appropriate amount of H 2 O 2 Oxidation is performed.

[0024] 4) Add titanium source to the above solution and stir.

[0025] 5) Raise the reaction temperature to 90-99°C and react for 2-6 hours to obtain a white turbid liquid.

[0026] 6) The white turbid liquid is filtered and washed, and the filter cake is dried and then dehydrated to obtain anhydrous iron phosphate doped with in-situ titanium.

[0027] 7) The in-situ titanium-doped anhydrous ferric phosphate is ball-milled with a lithium source and a carbon source to obtain a mixed slurry.

[0028] 8) The slurry is spray-dried to obtain powder.

[0029] 9) The above powder is placed in an argon furnace for high temperature calcination to obtain the product.

[0030] The present invention also provides a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material obtained by the above-mentioned preparation method. The composite material of the present invention has excellent electrochemical properties, and its principle is: by in-situ introduction of titanium doping into the iron phosphate precursor, this practice significantly improves the uniformity and consistency of the precursor. And because the radius of titanium is smaller than that of iron, titanium in the iron position will cause lattice distortion, thereby introducing lattice defects into the crystal. This lattice defect enhances the lithium ion diffusion rate of the composite material, so that the discharge specific capacity of the material is significantly improved. In addition, the in-situ titanium doping method of the present invention allows the composite material to maintain the flaky morphology of the primary particles. Compared with the non-in-situ doping method, which causes the particle size to increase and agglomerate during the sintering process, this flaky morphology can reduce the path length of electron conduction and improve the electron conduction efficiency, thereby significantly improving the rate performance of the battery.

[0031] As a preferred solution, the in-situ titanium-doped flaky lithium iron phosphate / carbon composite secondary particles have a porous pellet structure, and the primary particles maintain a flaky morphology. This porous pellet structure facilitates full contact between the electrolyte and the composite material of the present invention when used in a battery, and can provide more channels for the insertion / ejection of lithium ions, further improving the rate performance and discharge specific capacity ratio.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention uses low-cost raw materials and a simple liquid phase synthesis method to synthesize precursors and a streamlined process to synthesize lithium iron phosphate / carbon composite materials, and combines the in-situ titanium doping technology to achieve the modification effect. The doped titanium element is uniformly doped in the precursor iron phosphate in the form of precipitation, and the sample does not contain impurity phases. At the same time, titanium doping introduces lattice defects, enhances the ion diffusion of the material, and significantly improves the discharge specific capacity of the material. The doped titanium source is based on a hydrophilic functional group, and the titanium is dispersed in the precursor particles through uniform coupling and bonding, thereby improving uniformity and consistency. Then, a sheet-like in-situ titanium-doped lithium iron phosphate / carbon composite material with crystal orientation is prepared through a solid phase sintering process, which synergistically improves the lithium ion diffusion rate and morphological order of the material, thereby greatly improving the rate performance.

[0034] (2) The technical solution of the present invention is simple to operate and is easy to mass produce high-rate lithium iron phosphate materials.

[0035] (3) The present invention performs in-situ titanium doping on the precursor to synthesize a lithium iron phosphate / carbon composite positive electrode material. Compared with the undoped and titanium dioxide-doped lithium iron phosphate / carbon composite positive electrode materials, the present invention has better electrochemical properties and can better maintain the flaky morphology of the primary particles when sintered at a higher temperature. When sintered at a high temperature, a material with a low carbon content and better rate performance can be obtained.

[0036] (4) The composite material of the present invention has a porous pellet structure, which is beneficial for the electrolyte to fully contact with the composite material when used in a battery, and can provide more channels for the insertion / ejection of lithium ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a schematic diagram of XRD of in-situ titanium-doped iron phosphate dihydrate according to Example 1 of the present invention.

[0038] Figure 2 This is a schematic diagram of XRD of the in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 1 of the present invention.

[0039] Figure 3 This is a SEM schematic diagram of in-situ titanium-doped iron phosphate dihydrate according to Example 1 of the present invention.

[0040] Figure 4 This is an EDS schematic diagram of in-situ titanium-doped iron phosphate dihydrate according to Example 1 of the present invention.

[0041] Figure 5 This is a SEM image of the in-situ titanium-doped lithium iron phosphate / carbon composite material of Example 1 of the present invention.

[0042] Figure 6 It is a discharge curve diagram of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 1 of the present invention at different rates.

[0043] Figure 7 This is a graph of the discharge specific capacity of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 1 of the present invention after five cycles at different rates.

[0044] Figure 8 It is a discharge curve diagram of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 2 of the present invention at different rates.

[0045] Fig. 9 This is a graph of the discharge specific capacity of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 2 of the present invention after five cycles at different rates.

[0046] Fig.10 This is a graph of the discharge specific capacity of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 3 of the present invention after five cycles at different rates.

[0047] Fig.11 This is a SEM image of the in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 4 of the present invention.

[0048] Fig.12This is a graph of the discharge specific capacity of a half-cell assembled with an in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 5 of the present invention after five cycles at different rates.

[0049] Fig.13 It is a SEM schematic diagram of ferric phosphate dihydrate of Comparative Example 1 of the present invention.

[0050] Fig.14 It is a SEM picture of the lithium iron phosphate / carbon composite material of Comparative Example 1 of the present invention.

[0051] Fig.15 It is a discharge curve diagram of a half-cell assembled with the lithium iron phosphate / carbon composite material of Comparative Example 1 of the present invention at different rates.

[0052] Fig.16 This is a graph of the discharge specific capacity of a half-cell assembled with a lithium iron phosphate / carbon composite material according to Comparative Example 1 of the present invention after five cycles at different rates.

[0053] Fig.17 It is a SEM picture of the lithium iron phosphate / carbon composite material of Comparative Example 2 of the present invention.

[0054] Fig.18 It is a discharge curve diagram of a half-cell assembled with a lithium iron phosphate / carbon composite material according to comparative example 2 of the present invention at different rates.

[0055] Fig.19 This is a discharge capacity diagram of a half-cell assembled with a titanium-doped lithium iron phosphate / carbon composite material according to Comparative Example 3 of the present invention after five cycles at different rates.

[0056] Fig. 20 This is a SEM image of the titanium-doped lithium iron phosphate / carbon composite material of Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0057] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] Example 1

[0059] A method for preparing an in-situ titanium-doped flaky high-rate lithium iron phosphate / carbon composite material, the steps of which are as follows:

[0060] Add 0.25 mol of concentrated phosphoric acid with a concentration of 85 wt% into water with a volume multiple of three times the volume to dilute it, raise the temperature of the constant temperature heating jacket to 40°C, and stir. Add Fe containing a total molar amount of 0.1 mol of Fe in small amounts and multiple times.3 O 4 and iron powder (Fe 3 O 4 The molar ratio of the mixture to the iron powder was 1 / 1, and the mixture was stirred for 30 min. Then, 30% excess H 2 O 2 After 1 h of oxidation, a flaky anhydrous iron phosphate precursor containing in-situ titanium doping was added according to Fe 3+ 1% of tetraisopropyl di(dioctylphosphite) titanate was added to the solution, and the temperature was raised to 95°C in a constant temperature water bath for heating. After the reaction for 3 hours, a white turbid liquid was obtained. The white turbid liquid was filtered, washed with water 3 times, and the filter cake was placed in a 70°C forced air drying oven to dry, and in-situ titanium-doped dihydrate iron phosphate was obtained; the in-situ titanium-doped dihydrate iron phosphate material was placed in a muffle furnace at 550°C-4h for dehydration, and a light yellow in-situ titanium-doped iron phosphate precursor was obtained.

[0061] The stoichiometric ratio of in-situ titanium-doped iron phosphate and lithium carbonate, cyclodextrin (10% wt), and polyethylene glycol (2% wt) were ball-milled for 2 hours (ball-to-material ratio was 10 / 1) to obtain a dark yellow slurry. The slurry was spray-dried and pelletized (the air inlet temperature was 220°C, and the air outlet temperature was 120°C) to obtain a dark yellow powder. The precursor powder was placed in an argon furnace with a heating rate of 3°C / min, a temperature regime of 450°C-4h, and then sintered at 650°C-6h to obtain an in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0062] The XRD pattern of the iron phosphate dihydrate prepared in Example 1 is shown in Figure 1 As shown in the figure, it can be seen that the iron phosphate dihydrate prepared by this method is a typical monoclinic crystal system, and the material is relatively pure. The in-situ doping of titanium does not change the crystal structure of iron phosphate dihydrate. Figure 3 This is the SEM image of ferrous phosphate dihydrate after in-situ doping with titanium. It can be seen from the figure that the size of the primary particles is relatively small, the distribution range is small, and it presents a stacked lamellar structure. EDS test was also carried out. Figure 4 It can be seen from the EDS that titanium element is successfully doped into the dihydrated iron phosphate. The XRD pattern of the prepared in-situ titanium-doped lithium iron phosphate / carbon composite material is shown in Figure 2 Compared with the PDF card, the prepared material is relatively pure, and no carbon peak is observed in the prepared material, indicating that carbon exists in an amorphous form. Figure 5 As shown, after spray drying, the secondary particles present a porous microsphere shape and maintain the lamellar structure of the primary particles.

[0063] The prepared in-situ titanium-doped lithium iron phosphate / carbon composite material was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1 V and the test was carried out at room temperature. Figure 7 As shown, the discharge capacity under 2C conditions is 140.36 mAh / g, and the discharge capacity under 5C conditions is 132.38 mAh / g. The capacity is greatly improved compared with the control example, indicating that titanium doping can enhance the diffusion characteristics of lithium ions and improve the electrochemical properties of the material.

[0064] Example 2

[0065] A method for preparing an in-situ titanium-doped flaky high-rate lithium iron phosphate / carbon composite material. The difference between this embodiment and embodiment 1 is only that the titanium source is replaced with an equal amount of tri(dioctylphosphoryloxy)titanate isopropyl ester, and the remaining steps and conditions are the same to obtain an in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0066] The prepared in-situ titanium-doped lithium iron phosphate / carbon composite material was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1 V and the test was carried out at room temperature. Fig. 9 As shown, the discharge capacity is 138.88 mAh / g at 2C, and the discharge capacity is as high as 129.83 mAh / g at 5C.

[0067] Example 3

[0068] A method for preparing an in-situ titanium-doped flaky high-rate lithium iron phosphate / carbon composite material, the steps of which are as follows:

[0069] Add 0.6 mol of concentrated phosphoric acid with a concentration of 85% to water with a volume multiple of three times the volume to dilute it, raise the temperature of the constant temperature heating mantle to 45°C, and stir. Add Fe containing a total molar amount of 0.2 mol of Fe in small amounts and multiple times. 3 O 4 and iron powder (Fe 3 O 4 The molar ratio of the mixture to the iron powder was 1 / 1, and the mixture was stirred for 30 min. Then, 30% excess H 2 O 2 After 1 h of oxidation, a flaky anhydrous iron phosphate precursor containing in-situ titanium doping was added according to Fe 3+The molar amount of 0.5% ammonium titanium oxalate was calculated, and the temperature was raised to 97°C in a constant temperature water bath for heating. After reacting for 3 hours, a white turbid liquid was obtained. The white turbid liquid was filtered, washed with water 3 times, and the filter cake was placed in a 70°C forced air drying oven to dry, and in-situ titanium-doped dihydrate iron phosphate was obtained; the in-situ titanium-doped dihydrate iron phosphate material was placed in a muffle furnace at 600°C-4h for dehydration, and a light yellow in-situ titanium-doped iron phosphate precursor was obtained.

[0070] The stoichiometric ratio of in-situ titanium-doped iron phosphate and lithium carbonate, glucose (10% wt), and polyethylene glycol (2% wt) were ball-milled for 2 hours (ball-to-material ratio of 10 / 1) to obtain a dark yellow slurry. The slurry was spray-dried and pelletized (inlet temperature of 200°C, outlet temperature of 110°C) to obtain a dark yellow powder. The precursor powder was placed in an argon furnace with a heating rate of 3°C / min, a temperature regime of 450°C-4h, and then sintered at 650°C-6h to obtain in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0071] The prepared in-situ titanium-doped lithium iron phosphate / carbon composite material was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1 V and the test was carried out at room temperature. Fig.10 As shown, the discharge capacity is 145.72 mAh / g at 2C, and the discharge capacity is as high as 132.18 mAh / g at 5C.

[0072] Example 4

[0073] A method for preparing an in-situ titanium-doped flaky high-rate lithium iron phosphate / carbon composite material synthesized by a sintering system at a relatively high temperature, wherein the difference between this embodiment and embodiment 2 is only that the sintering temperature is replaced with 450°C-4h and then 720°C-6h, and the remaining steps and conditions are the same, thereby obtaining an in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0074] Fig.11 This is a SEM image of the in-situ titanium-doped lithium iron phosphate / carbon composite material obtained by sintering under high temperature conditions in Example 4 of the present invention, indicating that in-situ titanium doping can maintain the flaky morphology of the primary particles under high temperature conditions.

[0075] Example 5

[0076] A method for preparing an in-situ titanium-doped flaky high-rate lithium iron phosphate / carbon composite material. The difference between this embodiment and embodiment 1 is only that the titanium source is replaced with an equal amount of metatitanic acid, and the remaining steps and conditions are the same to obtain an in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0077] The prepared in-situ titanium-doped lithium iron phosphate / carbon composite material was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1 V and the test was carried out at room temperature. Fig.12 As shown, the discharge capacity is 136.13 mAh / g at 2C, and the discharge capacity is as high as 132.50 mAh / g at 5C.

[0078] Comparative Example 1

[0079] A method for preparing a lithium iron phosphate / carbon composite material not doped with titanium element. The difference between this comparative example and Example 1 is only that no titanium source is added, and the remaining steps and conditions are the same, and a dihydrate iron phosphate precursor and a lithium iron phosphate / carbon composite material are obtained in sequence.

[0080] Fig.13 This is a SEM image of ferric phosphate dihydrate in Comparative Example 1 of the present invention. The ferric phosphate dihydrate exhibits a nanoscale flake morphology. Fig.14 This is a SEM picture of the lithium iron phosphate / carbon composite material in the comparative example of the present invention. After spray drying and balling, the lithium iron phosphate presents a primary particle with a flake morphology and a secondary particle with a microsphere morphology structure.

[0081] The lithium iron phosphate / carbon composite material prepared above was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1 V and the test was carried out at room temperature. Fig.16 As shown, the discharge capacity is 123.16 mAh / g at 2C, and 110.46 mAh / g at 5C.

[0082] Comparative Example 2

[0083] A method for preparing a lithium iron phosphate / carbon composite material synthesized by a sintering system at a higher temperature, wherein the difference between this comparative example and comparative example 1 is that the sintering temperature is replaced with 450°C-4h and then at 720°C-6h, and the remaining steps and conditions are the same, and a dihydrate iron phosphate precursor and a lithium iron phosphate / carbon composite material are obtained in sequence.

[0084] After the lithium iron phosphate / carbon composite material prepared in Comparative Example 2 was sintered at a high temperature, the flaky morphology of the primary particles disappeared and the particles were aggregated. Fig.17 shown.

[0085] The lithium iron phosphate / carbon composite material prepared above was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, NMP was added for dispersion, and the electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1V and the test was carried out at room temperature. The constant temperature charge and discharge curve under 2C conditions is shown in Figure 2. Fig.18 As shown, the discharge specific capacity is only 114.59 mAh / g, which indicates that the lamellar morphology structure of the primary particles of the precursor is destroyed under high temperature conditions, which is not conducive to the performance of electrical properties.

[0086] Comparative Example 3

[0087] A method for preparing a lithium iron phosphate / carbon composite material doped with titanium dioxide in situ. The difference between this comparative example and Example 1 is that no titanium source is added in the precursor preparation stage to obtain a light yellow iron phosphate precursor, and an equal amount of TiO is added in the ball milling stage. 2 , and obtain titanium-doped lithium iron phosphate / carbon composite material powder.

[0088] The titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed with acetylene black and PVDF in a ratio of 8 / 1 / 1, and NMP was added for dispersion. The electrode was coated on aluminum foil, cut into small discs with a diameter of 12 mm, and assembled into a half-cell to test the electrochemical performance. The voltage range was 2.5-4.1V and the test was carried out at room temperature. Fig.19 As shown, the discharge capacity under 2C conditions is 133.44 mAh / g, and under 5C conditions, the discharge capacity is 116.23 mAh / g.

[0089] Comparative Example 4

[0090] A method for preparing a lithium iron phosphate / carbon composite material doped with titanium dioxide non-in-situ at a higher temperature, wherein the difference between this comparative example and comparative example 3 is that the sintering temperature is replaced with 450°C-4h and then at 720°C-6h, and the remaining steps and conditions are the same, thereby obtaining a titanium-doped lithium iron phosphate / carbon composite material powder.

[0091] Comparative Example 4 was sintered at 720°C, and the primary particles also showed a relatively serious aggregation state, such as Fig. 20 shown.

Claims

1. A method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material, characterized in that: The aqueous solution containing phosphorus source and iron source is mixed and H2O2 is added for oxidation, and then a titanium source is added for conversion synthesis reaction, and after dehydration, an in-situ titanium-doped flaky anhydrous iron phosphate precursor is obtained; the precursor is ball-milled with a lithium source and a carbon source, spray-dried to form a ball, and then calcined at a high temperature to obtain an in-situ titanium-doped flaky lithium iron phosphate / carbon composite material; The titanium source is organic titanium that can be easily decomposed in the reaction system or converted to form a precipitate without introducing impure anions.

2. The method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 1, characterized in that: The titanium source is at least one of isopropyl tris(diisooctyl pyrophosphate) titanate, bis(diisooctyl pyrophosphate) ethylene titanate, isopropyl tris(dioctylphosphoryloxy) titanate, tetraisopropyl di(dioctylphosphite) titanate, ammonium oxalate and metatitanic acid.

3. A method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 1 or 2, characterized in that: The process of mixing the aqueous solution containing a phosphorus source and an iron source is as follows: adding a phosphorus source into the aqueous solution for dilution, heating the diluted solution in a water bath while stirring, and then slowly adding the iron source for mixing; wherein the phosphorus source is 75-85wt% concentrated phosphoric acid, the dilution volume multiple is 2-4 times, the water bath heating temperature is 40-50°C, the iron source is composed of iron powder and Fe3O4, and the molar ratio of iron powder to ferroferric oxide is (1-5):

1.

4. The method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 3, characterized in that: The molar ratio of the iron source to the phosphorus source is 1:(2-4); wherein the iron source is measured in terms of the molar amount of the iron element; The amount of H2O2 used is 20% to 100% in excess of the stoichiometric molar ratio of the reaction with the iron source.

5. The method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 1, characterized in that: The oxidation time is 30 to 90 minutes, and the oxidation temperature is 40 to 50°C.

6. The method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 1 or 2, characterized in that: The amount of the titanium source added is calculated as follows: the in-situ titanium-doped flaky anhydrous iron phosphate precursor is Fe 3+ 0.2 to 1.2% of the molar amount.

7. The method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 6, characterized in that: The conditions of the doping reaction are: temperature of 90-99° C., time of 2-6 hours; the temperature of the dehydration is 500-650° C., and the dehydration time is 3-4 hours.

8. A method for preparing a high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 1, 2, 4, 5 or 7, characterized in that: The lithium source includes Li2CO3, and the carbon source is at least one of glucose, sucrose, cyclodextrin, polyethylene glycol and starch; the ball milling time is 2 to 3 hours, and the ball-to-material ratio is (8 to 10): 1; The high-temperature calcination is carried out in an inert atmosphere, firstly heating to 450-500° C. at 3° C. / min, keeping the temperature for 4-5 hours, and then heating to 620-720° C. at 3° C. / min for 6-10 hours.

9. A high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material, characterized in that: The method is obtained by the preparation method according to any one of claims 1 to 8.

10. The high-rate in-situ titanium-doped flaky lithium iron phosphate / carbon composite material according to claim 9, characterized in that: The secondary particles have a porous pellet structure, and the primary particles maintain a flake morphology.

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