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

Abstract

This invention discloses a high-rate in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material and its preparation method. The method involves mixing a solution containing a phosphorus source and an iron source, oxidizing it with H₂O₂, then adding a titanium source for doping. After dehydration, an in-situ titanium-doped sheet-like anhydrous lithium iron phosphate precursor is obtained. The precursor, along with a lithium source and a carbon source, is ball-milled, spray-dried to form pellets, and then calcined at high temperature to obtain the in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material. This invention, by selecting a titanium source capable of reaction transformation in the reaction system, achieves in-situ titanium doping at the precursor stage. This not only improves the uniformity and consistency of the composite material but also maintains the sheet-like morphology of the primary particles at higher sintering temperatures, significantly enhancing the rate performance of the composite material.

Description

Technical Field

[0001] This invention relates to a high-rate in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material and its preparation method, belonging to the field of lithium-ion cathode material preparation. Background Technology

[0002] The use of traditional fossil fuels has led to severe environmental pollution and consumed many non-renewable resources, resulting in a tight energy supply and an urgent need to shift from traditional energy sources to clean new energy sources. Lithium-ion batteries, as energy storage devices in this new energy sector, can promote energy transition. 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, their poor ionic and electronic conductivity results in unsatisfactory rate performance. Therefore, improvements are urgently needed.

[0003] A relatively simple method is to bulk dope the material with metal ions. Doping with high-valence metal ions can introduce a large number of defects, thereby improving the material's conductivity. However, cation doping generally involves mixing the dopant element into the solid-state sintering process or mixing it with the cathode material for secondary sintering. For example, Chinese patent CN202111185398.7 discloses a method for preparing titanium-doped lithium iron phosphate with nano-titanium dioxide. This method involves mixing iron phosphate, a lithium source, a carbon source, and nano-titanium dioxide, stirring until homogeneous, and then sintering the mixture at high temperature using a solid-state method before pulverizing it to synthesize titanium-doped lithium iron phosphate. Although this method has the potential for large-scale production, such doping methods suffer from uneven distribution and poor consistency, and the uniformity and accuracy of element doping cannot be guaranteed. Meanwhile, many researchers have also explored ways to control 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, which has certain safety risks. At the same time, it is highly dependent on production equipment, which is not conducive to large-scale production. Summary of the Invention

[0004] To address the problems in existing lithium iron phosphate / carbon composite materials, such as the inability to uniformly incorporate titanium sources and the inability to maintain a lamellar morphology after titanium incorporation, the first objective of this invention is to provide a high-rate in-situ titanium-doped lamellar lithium iron phosphate / carbon composite material. This material, with its uniformly incorporated titanium source and synergistic lamellar morphology, significantly improves the rate performance of the composite material. Furthermore, this material also possesses a porous, spherical structure, which facilitates sufficient contact between the material and the electrolyte when applied to batteries, providing more channels for lithium-ion insertion / extraction.

[0005] The second objective is to provide a method for preparing high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite materials. This method achieves in-situ titanium doping in the precursor stage by selecting a titanium source, which not only improves the uniformity and consistency of the composite material, but more importantly, it can also maintain the sheet-like morphology of the primary particles at a higher sintering temperature.

[0006] To achieve the above-mentioned technical objectives, this invention provides a method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material. The method involves mixing an aqueous solution containing a phosphorus source and an iron source, adding H2O2 for oxidation, then adding a titanium source for doping. After dehydration, an in-situ titanium-doped sheet-like anhydrous lithium iron phosphate precursor is obtained. The precursor, along with a lithium source and a carbon source, is ball-milled, spray-dried to form pellets, and then calcined at high temperature to obtain the in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material. The titanium source is an organic titanium that can be easily decomposed in the reaction system or transformed into a precipitate without introducing impurity anions.

[0007] The key to the technical solution of this invention lies in controlling the type of titanium source and achieving in-situ doping of titanium in the precursor stage. Specifically, the mechanism is as follows:

[0008] This invention first accelerates the reaction between iron powder and a phosphorus source using iron oxide, followed by a precipitation reaction under the action of hydrogen peroxide. During the doping reaction, the titanium source of this invention forms hydrophilic functional groups, which combine with phosphate ions through uniform coupling and are uniformly distributed in the FePO4·2H2O precursor in the form of precipitate. The dihydrate is then dehydrated to obtain iron phosphate, forming a titanium-doped iron phosphate solid solution. Simultaneously, during this process, the precursor preferentially grows on the (010) surface to form a lamellar morphology. Furthermore, this in-situ titanium doping method allows for bulk doping during precursor growth, stabilizing the lamellar morphology of the precursor. By directly employing a solid-state sintering process, the lamellar morphology of the precursor can be maintained at a higher sintering temperature, and combined with spray drying to form porous pellet structures.

[0009] As a preferred embodiment, the titanium source is at least one selected from isopropyl tris(diisooctylpyrophosphonoyloxy)titanate, bis(diisooctylpyrophosphonoyloxy)ethylene titanate, isopropyl tris(dioctylphosphonoyloxy)titanate, tetraisopropyl di(dioctylphosphite)titanate, ammonium oxalate, and metatitanic acid. The titanium sources selected in this invention can all simultaneously precipitate with ferric phosphate dihydrate at high temperature in an excess phosphoric acid reaction system to obtain a uniformly distributed precursor. Among them, ammonium oxalate is readily soluble in water and can mix more uniformly with the phosphorus and iron sources in aqueous solution, forming a precipitate in an excess phosphoric acid environment. Metatitanic acid can be converted into a precipitate in an excess phosphoric acid environment and at high temperature during the phosphoric acid reaction. The other four titanium sources all contain titanate bonds and are easily decomposed in water to form titanium precipitates.

[0010] As a preferred embodiment, the process of mixing the aqueous solution containing the phosphorus source and the iron source is as follows: the phosphorus source is added to the aqueous solution for dilution, and the diluted solution is heated in a water bath while being stirred. Then, the iron source is slowly added and mixed. The phosphorus source is 75-85 wt% concentrated phosphoric acid, the dilution volume ratio is 2-4 times, the water bath heating temperature is 40-50°C, and the iron source consists of iron powder and Fe3O4. The Fe3O4 is derived from magnetite flotation or from Fe3O4 obtained from chemicals.

[0011] As a preferred embodiment, the iron source is composed of iron powder and Fe3O4 in a ratio of (1-5):1. By adding a small amount of iron powder to the iron source of the present invention, Fe can be utilized... 0 Fe in Fe3O4 3+ It promotes the reaction of Fe3O4 and improves the precipitation efficiency of iron source to iron phosphate.

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

[0013] As a preferred embodiment, the amount of H2O2 used is 20% to 100% excess relative to the stoichiometric molar ratio of the reaction with the iron source. By using excess hydrogen peroxide, it can be ensured that Fe(H2PO4)2 is completely oxidized to form FePO4.

[0014] As a preferred embodiment, the oxidation time is 30–90 min and the oxidation temperature is 40–50 °C.

[0015] As a preferred embodiment, the amount of titanium source added is based on titanium, and the in-situ titanium-doped sheet-like anhydrous iron phosphate precursor is calculated according to Fe... 3+ The molar amount of titanium is 0.2% to 1.2%. In this invention, if the titanium doping amount is too low, the effect on maintaining the lamellar morphology and improving the rate performance is not obvious, while if the titanium doping amount is too high, it exceeds the solid solution limit. At the same time, since titanium is not an active material, it will affect the specific capacity of the final synthesized lithium iron phosphate / carbon composite material.

[0016] As a preferred embodiment, the doping reaction conditions are: temperature 90–99°C, time 2–6 h; and the dehydration temperature 500–650°C, dehydration time 3–4 h. If the doping reaction temperature is too low, it cannot be guaranteed that the iron source will be completely converted into ferric phosphate dihydrate.

[0017] As a preferred embodiment, the lithium source includes Li2CO3, and the carbon source is at least one selected from glucose, sucrose, cyclodextrin, polyethylene glycol, and starch; the ball milling time is 2-3 hours, and the ball-to-material ratio is (8-10):1.

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

[0019] As a preferred embodiment, the high-temperature calcination is carried out under an inert atmosphere. First, the temperature is increased to 450–500°C at a rate of 3°C / min and held for 4–5 hours. Then, the temperature is increased to 620–720°C at a rate of 3°C / min for 6–10 hours. This invention employs a two-stage calcination process, which controls the initial growth of particles in the low-temperature stage and completes the final sintering in the high-temperature stage. This method ensures complete crystal structure transformation while preventing excessive particle growth, resulting in a more uniform particle size distribution and a higher specific surface area. In this invention, the sintering temperature directly affects the crystal structure transformation of lithium iron phosphate. Theoretically, the crystal transformation of lithium iron phosphate occurs around 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, leading to a decrease in the electrochemical performance of the material. Excessively high sintering temperatures may lead to the formation of impurities. For example, when the sintering temperature reaches 820°C, impurities such as Li3PO4 are prone to appear in lithium iron phosphate, which can damage the electrochemical performance of the material. Meanwhile, temperature also affects the morphology of lithium iron phosphate. High-temperature sintering leads to increased particle fusion, resulting in larger particle size and reduced specific surface area. Furthermore, this invention maintains the compaction density of the material at a higher sintering temperature, yielding a low-carbon lithium iron phosphate / carbon composite material with a sheet-like morphology. A further preferred high-temperature sintering temperature is 680–720°C.

[0020] This invention provides a method for preparing high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite materials, specifically including the following steps:

[0021] 1) Weigh the phosphorus source and dilute it with deionized water. 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 phosphorus source evenly.

[0023] 3) Add an appropriate amount of H2O2 to the above solution for oxidation.

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

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

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

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

[0028] 8) Spray dry the above slurry to form pellets to obtain powder.

[0029] 9) Place the above powder in an argon furnace for high-temperature calcination to obtain the final product.

[0030] This invention also provides a high-rate in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material, obtained by the above-described preparation method. The composite material of this invention exhibits excellent electrochemical performance. The principle behind this is that by in-situ introducing titanium doping into the iron iron phosphate precursor, the uniformity and consistency of the precursor are significantly improved. Furthermore, since titanium has a smaller radius than iron, titanium at iron sites introduces lattice distortion, thereby introducing lattice defects into the crystal. These lattice defects enhance the lithium-ion diffusion rate of the composite material, resulting in a significant increase in the material's discharge specific capacity. In addition, the in-situ titanium doping method of this invention maintains the sheet-like morphology of the primary particles in the composite material. Compared to non-in-situ doping methods, which lead to particle size increase and agglomeration during sintering, this sheet-like morphology reduces the electron conduction path length, improves electron conduction efficiency, and thus significantly enhances the rate performance of the battery.

[0031] As a preferred embodiment, the secondary particles of the in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material have a porous spherical structure, while the primary particles retain a sheet-like morphology. This porous spherical structure allows the composite material of the present invention to have sufficient contact with the electrolyte when used in batteries, providing more channels for lithium ion insertion / extraction, and further improving rate performance and discharge specific capacity ratio.

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

[0033] (1) This invention utilizes low-cost raw materials and a simple liquid-phase synthesis method to synthesize the precursor and a streamlined process to synthesize lithium iron phosphate / carbon composite materials. Simultaneously, it incorporates in-situ titanium doping technology to achieve modification effects. The doped titanium element is uniformly doped into the precursor iron phosphate in the form of a precipitate, and the sample is free of impurity phases. Furthermore, the titanium doping introduces lattice defects, enhancing ion diffusion and significantly improving the material's discharge specific capacity. The doped titanium source is based on hydrophilic functional groups, and uniform coupling allows titanium to disperse among the precursor particles, improving uniformity and consistency. A solid-state sintering process is then used to prepare a plate-like in-situ titanium-doped lithium iron phosphate / carbon composite material with crystal orientation, synergistically improving the lithium-ion diffusion rate and morphological order of the material, thereby greatly enhancing its rate performance.

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

[0035] (3) The present invention synthesizes lithium iron phosphate / carbon composite cathode material by in-situ titanium doping of the precursor. Compared with undoped and titanium dioxide-doped lithium iron phosphate / carbon composite cathode materials, the present invention has better electrochemical performance and can better maintain the morphology of primary granular sheets when sintered at higher temperatures. When sintered at high temperatures, materials with low carbon content and better rate performance can be obtained.

[0036] (4) The composite material of the present invention has a porous spherical structure, which is conducive to the electrolyte to fully contact with it when applied to the battery, and can provide more channels for the insertion / extraction of lithium ions. Attached Figure Description

[0037] Figure 1 This is an XRD pattern of in-situ titanium-doped iron phosphate dihydrate from Embodiment 1 of the present invention.

[0038] Figure 2 This is an XRD diagram of the in-situ titanium-doped lithium iron phosphate / carbon composite material of Embodiment 1 of the present invention.

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

[0040] Figure 4 This is an EDS schematic diagram of in-situ titanium-doped iron phosphate dihydrate in Embodiment 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 This is a discharge curve of a half-cell assembled from 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 discharge specific capacity diagram of a half-cell assembled from 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 This is a discharge curve of a half-cell assembled from in-situ titanium-doped lithium iron phosphate / carbon composite material according to Example 2 of the present invention at different rates.

[0045] Figure 9 This is a discharge specific capacity diagram of a half-cell assembled from 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] Figure 10This is a discharge specific capacity diagram of a half-cell assembled from 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] Figure 11 This is a SEM image of the in-situ titanium-doped lithium iron phosphate / carbon composite material of Example 4 of the present invention.

[0048] Figure 12 This is a discharge specific capacity diagram of a half-cell assembled from 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] Figure 13 This is a SEM schematic diagram of ferric phosphate dihydrate from Comparative Example 1 of this invention.

[0050] Figure 14 This is a SEM image of the lithium iron phosphate / carbon composite material of Comparative Example 1 of the present invention.

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

[0052] Figure 16 This is a discharge specific capacity diagram of the half-cell assembled from lithium iron phosphate / carbon composite material in Comparative Example 1 of the present invention, after five cycles at different rates.

[0053] Figure 17 This is a SEM image of the lithium iron phosphate / carbon composite material of Comparative Example 2 of the present invention.

[0054] Figure 18 This is a discharge curve of the half-cell assembled from the lithium iron phosphate / carbon composite material of Comparative Example 2 of the present invention at different rates.

[0055] Figure 19 This is a discharge specific capacity diagram of the half-cell assembled from titanium-doped lithium iron phosphate / carbon composite material (Comparative Example 3) of the present invention, after five cycles at different rates.

[0056] Figure 20 This is a SEM image of the titanium-doped lithium iron phosphate / carbon composite material of Comparative Example 4 of this invention. Detailed Implementation

[0057] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Example 1

[0059] A method for preparing an in-situ titanium-doped sheet-like high-rate lithium iron phosphate / carbon composite material, comprising the following steps:

[0060] 0.25 mol of concentrated phosphoric acid (85 wt%) was diluted with three times its volume of water. The temperature of the constant-temperature heating mantle was raised to 40°C, and the mixture was stirred. Fe3O4 and iron powder (containing a total molar amount of 0.1 mol of Fe, with a molar ratio of Fe3O4 to iron powder of 1:1) were added in small, repeated additions, and stirred for 30 min. Then, H2O2 was added in 30% excess molar ratio relative to the stoichiometric ratio with the iron source for oxidation. After 1 h of oxidation, a sheet-like anhydrous iron phosphate precursor containing in-situ titanium doping was added according to the Fe... 3+ A molar amount of 1% tetraisopropyl di(dioctylphosphite)titanate was heated in a constant temperature water bath at 95°C for 3 hours to obtain a white turbid liquid. The white turbid liquid was filtered, washed three times with water, and the filter cake was dried in a 70°C forced-air drying oven to obtain in-situ titanium-doped iron phosphate dihydrate. The in-situ titanium-doped iron phosphate dihydrate was placed in a muffle furnace at 550°C for 4 hours for dehydration to obtain a pale yellow in-situ titanium-doped iron phosphate precursor.

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

[0062] The XRD pattern of the ferric phosphate dihydrate prepared in Example 1 is shown in Figure 1. Figure 1 As shown in the figure, 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 did not change the crystal structure of iron phosphate dihydrate. Figure 3 This is a SEM image of ferric phosphate dihydrate after in-situ titanium doping. The image shows that the primary particles are relatively small in size, have a small distribution range, and exhibit a stacked, lamellar structure. EDS analysis was also performed. Figure 4 EDS analysis shows that titanium was successfully incorporated into lithium iron phosphate dihydrate. XRD patterns of the prepared in-situ titanium-doped lithium iron phosphate / carbon composite material are shown in [Figure number missing]. Figure 2 As shown, comparison with the PDF card indicates that the prepared material is relatively pure, and no carbon peaks were observed, indicating that carbon exists in an amorphous form. The SEM images of the in-situ titanium-doped lithium iron phosphate / carbon composite material are shown below. Figure 5As shown, after spray drying, the secondary particles exhibit a porous microsphere shape while retaining the plate-like structure of the primary particles.

[0063] The in-situ titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, dispersed with NMP, and coated onto aluminum foil to form an electrode. The electrode was then cut into small discs with a diameter of 12 mm and assembled into a half-cell for electrochemical performance testing. The voltage range was 2.5-4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 7 As shown, the discharge specific capacity under 2C conditions is 140.36 mAh / g, and the discharge capacity under 5C conditions is 132.38 mAh / g. The capacity is significantly improved compared to the comparative example, indicating that titanium doping can enhance the lithium-ion diffusion characteristics and improve the electrochemical performance of the material.

[0064] Example 2

[0065] A method for preparing in-situ titanium-doped sheet-like high-ratio lithium iron phosphate / carbon composite material. The only difference between this embodiment and Example 1 is that the titanium source is replaced with an equal amount of tris(dioctylphosphoxy)titanate isopropyl ester. All other steps and conditions are the same, and in-situ titanium-doped lithium iron phosphate / carbon composite material powder is obtained.

[0066] The in-situ titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, dispersed with NMP, and coated onto aluminum foil to form an electrode. The electrode was then cut into small discs with a diameter of 12 mm and assembled into a half-cell for electrochemical performance testing. The voltage range was 2.5-4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 9 As shown, the discharge capacity is 138.88 mAh / g at 2C and as high as 129.83 mAh / g at 5C.

[0067] Example 3

[0068] A method for preparing an in-situ titanium-doped sheet-like high-rate lithium iron phosphate / carbon composite material, comprising the following steps:

[0069] 0.6 mol of 85% concentrated phosphoric acid was diluted with three times its volume of water. The temperature of the constant-temperature heating mantle was raised to 45°C, and the mixture was stirred. Fe3O4 containing a total molar amount of Fe (0.2 mol) and iron powder (with a molar ratio of Fe3O4 to iron powder of 1:1) were added in small, repeated additions, and stirred for 30 min. Then, H2O2 was added in 30% excess molar ratio relative to the stoichiometric ratio with the iron source for oxidation. After 1 h of oxidation, a sheet-like anhydrous iron phosphate precursor with in-situ titanium doping was added according to the Fe... 3+0.5% molar amount of titanium oxalate ammonium was heated in a constant temperature water bath at 97℃ for 3 hours to obtain a white turbid liquid. The white turbid liquid was filtered, washed three times with water, and the filter cake was dried in a 70℃ forced-air drying oven to obtain in-situ titanium-doped iron phosphate dihydrate. The in-situ titanium-doped iron phosphate dihydrate was placed in a muffle furnace at 600℃ for 4 hours for dehydration to obtain a pale yellow in-situ titanium-doped iron phosphate precursor.

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

[0071] The in-situ titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, dispersed with NMP, and coated onto aluminum foil to form an electrode. The electrode was then cut into small discs with a diameter of 12 mm and assembled into a half-cell for electrochemical performance testing. The voltage range was 2.5-4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 10 As shown, the discharge capacity is 145.72 mAh / g at 2C and as high as 132.18 mAh / g at 5C.

[0072] Example 4

[0073] A method for preparing in-situ titanium-doped sheet-like high-ratio lithium iron phosphate / carbon composite material by sintering at a higher temperature. The only difference between this example and Example 2 is that the sintering temperature is replaced by 450℃-4h followed by 720℃-6h, while the other steps and conditions are the same, to obtain in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0074] Figure 11 The image shows an SEM image of the in-situ titanium-doped lithium iron phosphate / carbon composite material obtained by sintering at high temperature in Example 4 of this invention, illustrating that in-situ titanium doping can maintain the lamellar morphology of the primary particles under high temperature conditions.

[0075] Example 5

[0076] A method for preparing in-situ titanium-doped sheet-like high-ratio lithium iron phosphate / carbon composite material is disclosed. The difference between this embodiment and Example 1 is that the titanium source is replaced with an equal amount of metatitanic acid, while the other steps and conditions are the same, resulting in in-situ titanium-doped lithium iron phosphate / carbon composite material powder.

[0077] The in-situ titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, dispersed with NMP, and coated onto aluminum foil to form an electrode. The electrode was then cut into small discs with a diameter of 12 mm and assembled into a half-cell for electrochemical performance testing. The voltage range was 2.5-4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 12 As shown, the discharge capacity is 136.13 mAh / g at 2C and as high as 132.50 mAh / g at 5C.

[0078] Comparative Example 1

[0079] A method for preparing a titanium-free lithium iron phosphate / carbon composite material is disclosed. The only difference between this comparative example and Example 1 is that no titanium source is added. All other steps and conditions are the same, and iron phosphate dihydrate precursor and lithium iron phosphate / carbon composite material are obtained sequentially.

[0080] Figure 13 This is a SEM image of ferric phosphate dihydrate in Comparative Example 1 of this invention. Ferric phosphate dihydrate exhibits a nanoscale lamellar morphology. Figure 14 The image shows a SEM image of the lithium iron phosphate / carbon composite material in the comparative example of this invention. After spray drying and pelletizing, the lithium iron phosphate particles exhibit a primary particle morphology of flakes and a secondary particle morphology of microspheres.

[0081] The lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, and NMP was added for dispersion. The mixture was then coated onto aluminum foil to form an electrode. Small circular pieces with a diameter of 12 mm were cut and assembled into half-cells for electrochemical performance testing. The voltage range was 2.5–4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 16 As shown, the specific capacity at 2C discharge is 123.16 mAh / g, and the discharge capacity at 5C is 110.46 mAh / g.

[0082] Comparative Example 2

[0083] A method for preparing lithium iron phosphate / carbon composite material synthesized by sintering at a higher temperature is disclosed. The only difference between this comparative example and Comparative Example 1 is that the sintering temperature is replaced by 450℃-4h followed by 720℃-6h. All other steps and conditions are the same, and the lithium iron phosphate dihydrate precursor and lithium iron phosphate / carbon composite material are obtained sequentially.

[0084] The lithium iron phosphate / carbon composite material prepared in Comparative Example 2, after sintering at a high temperature, showed that the lamellar morphology of the primary particles disappeared, and they exhibited an agglomerated state, such as... Figure 17 As shown.

[0085] The lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, and NMP was added for dispersion. The mixture was then coated onto aluminum foil to form an electrode, which was cut into small discs with a diameter of 12 mm and assembled into a half-cell. Electrochemical performance was tested in the voltage range of 2.5-4.1 V at room temperature. The isothermal charge-discharge curves under 2C conditions are shown below. Figure 18 As shown, the discharge specific capacity is only 114.59 mAh / g, indicating that the plate-like morphology 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 lithium iron phosphate / carbon composite material with non-in-situ titanium dioxide doping is disclosed. The only difference between this comparative example and Example 1 is that no titanium source is added in the precursor preparation stage to obtain a pale yellow iron phosphate precursor, while an equal amount of TiO2 is added in the ball milling stage to obtain titanium-doped lithium iron phosphate / carbon composite material powder.

[0088] The titanium-doped lithium iron phosphate / carbon composite material prepared above was mixed uniformly with acetylene black and PVDF in an 8 / 1 / 1 ratio, dispersed with NMP, and coated onto aluminum foil to form an electrode. The electrode was then cut into small discs with a diameter of 12 mm and assembled into a half-cell for electrochemical performance testing. The voltage range was 2.5-4.1 V, and the tests were conducted at room temperature. Rate performance is as follows: Figure 19 As shown, the discharge specific capacity is 133.44 mAh / g under 2C conditions and 116.23 mAh / g under 5C conditions.

[0089] Comparative Example 4

[0090] A method for preparing lithium iron phosphate / carbon composite material with non-in-situ titanium dioxide doping at a higher temperature. The only difference between this comparative example and Comparative Example 3 is that the sintering temperature is replaced by 450℃-4h followed by 720℃-6h. All other steps and conditions are the same, and titanium-doped lithium iron phosphate / carbon composite material powder is obtained.

[0091] Comparative Example 4, sintered at 720℃, also showed a relatively severe aggregation of primary particles, such as... Figure 20 As shown.

Claims

1. A method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material, characterized in that: An aqueous solution containing phosphorus and iron sources is mixed and oxidized by adding H2O2. Then, a titanium source is added for conversion and synthesis. After dehydration, an in-situ titanium-doped sheet-like anhydrous iron phosphate precursor is obtained. The precursor, along with a lithium source and a carbon source, is ball-milled, spray-dried to form pellets, and then calcined at high temperature to obtain an in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material. The titanium source is at least one of isopropyl tris(diisooctyl pyrophosphoryloxy) titanate, bis(diisooctyl pyrophosphoryloxy) ethylene titanate, isopropyl tris(dioctyl phosphoryloxy) titanate, tetraisopropyl di(dioctyl phosphite) titanate, titanium oxalate ammonium, and metatitanic acid. The iron source is composed of iron powder and Fe3O4, and the molar ratio of iron powder to Fe3O4 is (1~5):

1. The high-temperature calcination is carried out under an inert atmosphere. First, the temperature is raised to 450~500℃ at 3℃ / min and held for 4~5h. Then, the temperature is raised to 620~720℃ at 3℃ / min and held for 6~10h. The secondary particles of the in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material have a porous spherical structure, while the primary particles maintain a sheet-like morphology and have lattice defects.

2. The method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 1, characterized in that: The process of mixing the aqueous solution containing phosphorus source and iron source is as follows: the phosphorus source is added to the aqueous solution for dilution, and the diluted solution is heated in a water bath while being stirred. Then the iron source is slowly added for mixing. The phosphorus source is 75-85 wt% concentrated phosphoric acid, the dilution volume ratio is 2-4 times, and the water bath heating temperature is 40-50℃.

3. The method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 2, 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 molar amounts of iron; and the amount of H2O2 used is 20%~100% in excess relative to the stoichiometric molar ratio of the reaction with the iron source.

4. The method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 1, characterized in that: The oxidation time is 30-90 minutes, and the oxidation temperature is 40-50℃.

5. A method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 1 or 2, characterized in that: The amount of titanium source added is calculated based on titanium, and the in-situ titanium-doped plate-like anhydrous iron phosphate precursor is calculated according to Fe. 3+ The molar amount is 0.2% to 1.2%.

6. The method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 5, characterized in that: The conditions for the conversion and synthesis reaction are: temperature of 90~99℃ and time of 2~6h; the temperature for dehydration is 500~650℃ and the dehydration time is 3~4h.

7. A method for preparing a high-ratio in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material according to claim 1, 2, 3, 4 or 6, 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 2-3 hours, and the ball-to-material ratio is (8-10):

1.

8. A high-rate in-situ titanium-doped sheet-like lithium iron phosphate / carbon composite material, characterized in that: The secondary particles are obtained by the preparation method according to any one of claims 1 to 7, and the secondary particles have a porous spherical structure while the primary particles maintain a plate-like morphology.

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