Organic framework in-situ derived low-energy-gap lithium iron phosphate material and application thereof

Abstract

The invention discloses an organic framework in-situ derived low-energy-gap lithium iron phosphate material and application thereof, and relates to the technical field of lithium batteries. The lithium iron phosphate (101) interplanar spacing d, crystal ion vacancy V, lithium iron phosphate normal-temperature band gap E1 and lithium iron phosphate low-temperature band gap E2 are utilized to construct an expression of the optimization degree kappa, the value range of the optimization degree kappa is controlled, and the lithium iron phosphate material meeting the value range has relatively good conductivity, rate capability and low-temperature performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a low-energy-gap lithium iron phosphate material derived in situ from an organic framework and applications thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4, LFP) is one of the mainstream cathode materials for lithium-ion batteries. Its high safety, long cycle life, and low cost have led to its widespread use in electric vehicles, energy storage systems, and other fields. However, LFP suffers from poor rate performance and conductivity, which have significantly limited its further development.

[0003] To address the issues of poor rate performance, poor conductivity, and poor low-temperature performance of lithium iron phosphate, the industry has focused on optimizing performance through technologies such as metal ion doping and carbon coating, but the improvements remain to be further improved. Therefore, there is an urgent need to provide lithium iron phosphate materials with significant advantages in both rate performance and conductivity.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a low-bandgap lithium iron phosphate material and a preparation method thereof, aiming to provide a lithium iron phosphate material with significantly improved rate performance and conductive properties.

[0006] Another object of the present invention is to provide a positive electrode sheet and a lithium battery, aiming to significantly improve the electrochemical performance of the battery.

[0007] The present invention is achieved in that:

[0008] In a first aspect, the present invention provides a low-bandgap lithium iron phosphate material, the lithium iron phosphate material comprising a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix, and the lithium iron phosphate material is doped with titanium;

[0009] The optimization degree κ of lithium iron phosphate material satisfies:

[0010]

[0011] 2nm -2 .eV -1 <κ≤100nm -2 .eV -1 , the unit of κ is nm -2 .eV -1 ;

[0012] Wherein, d represents the (101) interplanar spacing of lithium iron phosphate calculated from XRD test results, in nm;

[0013] V represents the crystal ion vacancy obtained from XRD test results, the unit is nm -3 ;

[0014] E1 represents the band gap of lithium iron phosphate treated at 25°C obtained by UV-visible diffuse reflectance test, in eV;

[0015] E2 represents the band gap of lithium iron phosphate treated at a temperature of -52°C obtained by UV-visible diffuse reflectance testing, in eV.

[0016] In an optional embodiment, the lithium iron phosphate material satisfies at least one of features A1 to G1:

[0017] Feature A1: 50nm -2 .eV -1 <κ<97nm -2 .eV -1 ;

[0018] Feature B1: the value range of d is 0.40nm-0.45nm;

[0019] Feature C1: V range is 0.5nm -3 -1.5nm -3 ;

[0020] Feature D1: The value range of E1 is 1.5eV-3.0eV;

[0021] The value range of characteristics E1:E2 is 1.5eV-3.5eV;

[0022] Feature F1: The doping amount of titanium is 0.01%-5%;

[0023] Feature G1: The mass percentage of the carbon coating layer ranges from 0.1% to 5%.

[0024] In a second aspect, the present invention provides a method for preparing the low-bandgap lithium iron phosphate material of the aforementioned embodiment, comprising: mixing an iron source, a titanium source, an organic gel raw material, and organic phosphoric acid to obtain a gel;

[0025] The gel is dried and mixed with a lithium source and calcined.

[0026] In an optional embodiment, the process for preparing the gel satisfies at least one of features A2 to E2:

[0027] Feature A2: The organic gel raw material is selected from at least one of polyacrylic acid, gelatin and cellulose derivatives;

[0028] Feature B2: The organic phosphoric acid is at least one selected from 1-ethylimidazole ammonium dihydrogen phosphate, 1-butylimidazole ammonium dihydrogen phosphate, and 1-methylimidazole ammonium dihydrogen phosphate;

[0029] Characteristic C2: The molar ratio of the iron element in the iron source to the organic gel raw material is 1:(0.05-0.30);

[0030] Feature D2: The molar ratio of iron element to organophosphoric acid in the iron source is 1:(1.00-1.05);

[0031] Feature E2: The iron source is at least one selected from ferric chloride, ferric nitrate, and ferric sulfate.

[0032] In an optional embodiment, the process of preparing the gel includes: first mixing an iron source, a titanium source and an organic gel raw material to obtain a mixture, mixing the mixture with organic phosphoric acid, adjusting the pH value to 6-8, and heating to 60°C-90°C to react until a gel is generated.

[0033] In an optional embodiment, the titanium source is an organic titanium source, and the molar ratio of the iron element in the iron source to the titanium element in the organic titanium source is 1:(0.005-0.10).

[0034] In an optional embodiment, the process of drying the gel satisfies at least one of the following characteristics:

[0035] Feature A3: Control the drying temperature to 110℃-130℃;

[0036] Feature B3: Control the drying time to 4h-8h;

[0037] Characteristic C3: The gel is left to stand for 1-3 hours before being dried.

[0038] In an optional embodiment, at least one of features A4 to F4 is satisfied during the calcination process of mixing with a lithium source:

[0039] Feature A4: Calcination temperature is 500°C-800°C;

[0040] Feature B4: calcination time is 2h-6h;

[0041] Characteristic C4: The molar ratio of the iron element in the iron source to the lithium element in the lithium source is 1:(1.00-1.05);

[0042] Feature D4: The lithium source is selected from at least one of lithium carbonate, lithium chloride and lithium hydroxide;

[0043] Feature E4: The mixed material is ground before calcination;

[0044] Feature F4: Calcination is carried out under an inert atmosphere.

[0045] In a third aspect, the present invention provides a positive electrode plate comprising a current collector and a positive electrode active layer loaded on the current collector, wherein the positive electrode active layer contains the lithium iron phosphate material of any one of the aforementioned embodiments or the lithium iron phosphate material prepared by any one of the preparation methods of the aforementioned embodiments.

[0046] In a fourth aspect, the present invention provides a lithium battery comprising the positive electrode sheet of the aforementioned embodiment.

[0047] The present invention has the following beneficial effects: the design of titanium element doping and in-situ carbon coating changes the microstructure and electrical conductivity of lithium iron phosphate, the doping of tetravalent titanium ions changes the interplanar spacing during crystal growth, and because titanium ions are doped into the divalent iron position, ion vacancies are generated. The (101) interplanar spacing and vacancies affect the lithium ion migration rate, thereby affecting the rate performance of the lithium ion battery, while the doping of titanium ions improves the electronic conductivity of lithium iron phosphate, reduces the band gap of lithium iron phosphate at room temperature and low temperature, and affects the low-temperature electrochemical performance of the lithium iron phosphate material. The present invention uses the lithium iron phosphate (101) interplanar spacing d, crystal ion vacancies V, lithium iron phosphate room temperature band gap E1, and lithium iron phosphate low temperature band gap E2 to construct an expression for the optimization degree κ. By regulating the optimization degree κ to meet a specific range, the conductivity and rate performance of the lithium iron phosphate material can be significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0049] Figure 1 This is the XRD pattern of the lithium iron phosphate material of Example 1;

[0050] Figure 2 This is the UV-visible diffuse reflection test diagram of the lithium iron phosphate material in Example 1. DETAILED DESCRIPTION

[0051] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0052] The inventors integrated the XRD test results and the UV-visible diffuse reflectance test results, and creatively used the lithium iron phosphate (101) crystal plane spacing d, the crystal ion vacancy V, the lithium iron phosphate room temperature band gap E1, and the lithium iron phosphate low temperature band gap E2 to construct an expression for the optimization degree κ, and controlled the value range of the optimization degree κ. The lithium iron phosphate material that meets this range has excellent conductivity and rate performance.

[0053] An embodiment of the present invention provides a low-bandgap lithium iron phosphate material, comprising a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix, wherein the lithium iron phosphate material is doped with titanium, and the optimization degree κ of the lithium iron phosphate material is defined as satisfying:

[0054]

[0055] 2nm -2 .eV -1 <κ≤100nm -2 .eV -1 , the unit of κ is nm -2 .eV -1 ;

[0056] Wherein, d represents the (101) interplanar spacing of lithium iron phosphate calculated from XRD test results, in nm;

[0057] V represents the crystal ion vacancy obtained from XRD test results, the unit is nm -3 ;

[0058] E1 represents the band gap of lithium iron phosphate treated at room temperature (25°C) obtained by UV-visible diffuse reflectance test, in eV;

[0059] E2 represents the band gap of lithium iron phosphate treated at low temperature (-52°C) obtained by UV-visible diffuse reflectance test, in eV;

[0060] It should be noted that the interplanar spacing d of lithium iron phosphate (101) affects the lithium ion diffusion channel. The larger the interplanar spacing, the more favorable the lithium ion diffusion, and vice versa. Crystal ion vacancies V, vacancies reduce the obstacles to lithium ion diffusion channels. The formation of certain vacancies is conducive to lithium ion diffusion. Lithium iron phosphate has a room temperature band gap E1 and a low temperature band gap E2. The band gap affects the conductivity of lithium iron phosphate. The lower the band gap, the better the electrochemical performance of lithium iron phosphate.

[0061] The optimization degree κ of the lithium iron phosphate material provided in the embodiment of the present invention satisfies: 2nm -2 .eV -1 <κ≤100nm -2 .eV -1 , such as 2nm -2 .eV-1 , 5nm -2 .eV -1 , 10nm -2 .eV -1 , 20nm -2 .eV -1 , 30nm -2 .eV -1 , 40nm -2 .eV -1 , 50nm - 2 .eV -1 , 60nm -2 .eV -1 , 70nm -2 .eV -1 , 80nm -2 .eV -1 , 90nm -2 .eV -1 , 95nm -2 .eV -1 , 100nm -2 .eV -1 The inventors found that when the optimization degree κ of lithium iron phosphate material meets the above range, the (101) interplanar spacing, vacancy concentration and band gap of lithium iron phosphate are in a relatively excellent state, which significantly improves the conductivity, rate performance and low temperature performance of lithium iron phosphate material. Preferably, the optimization degree κ meets: 50nm -2 .eV -1 <κ<97nm -2 .eV -1 By optimizing the optimization degree κ, it is beneficial to further improve the rate performance and low-temperature performance of lithium iron phosphate materials.

[0062] In order to further improve the performance of lithium iron phosphate materials, the inventors optimized each parameter in the optimization degree κ expression:

[0063] In a preferred embodiment, the interplanar spacing d of the lithium iron phosphate (101) ranges from 0.40 nm to 0.45 nm, such as 0.40 nm, 0.41 nm, 0.42 nm, 0.43 nm, 0.44 nm, 0.45 nm, etc. If the interplanar spacing of the lithium iron phosphate (101) is too large, the crystal structure of the lithium iron phosphate is affected; if the interplanar spacing of the lithium iron phosphate (101) is too small, the migration rate of lithium ions is reduced.

[0064] In a preferred embodiment, the value range of the crystal ion vacancy V is 0.5 nm. -3 -1.5nm -3 , such as 0.5nm -3 , 0.8nm -3, 1.0nm -3 , 1.2nm -3 , 1.5nm -3 If the crystal ion vacancy is too large, it will affect the structural strength of lithium iron phosphate and the electrochemical cycle stability; if the crystal ion vacancy is too small, it will have little effect on the lithium ion migration rate.

[0065] In a preferred embodiment, the band gap E1 of lithium iron phosphate ranges from 1.5eV to 3.0eV, such as 1.5eV, 1.8eV, 2.0eV, 2.3eV, 2.5eV, 2.8eV, 3.0eV, etc.; the band gap E2 ranges from 1.5eV to 3.5eV, such as 1.5eV, 1.8eV, 2.0eV, 2.3eV, 2.5eV, 2.8eV, 3.0eV, 3.3eV, and 3.5eV. If the band gap of lithium iron phosphate is too large, its conductivity will be affected. Considering the structural stability of lithium iron phosphate, titanium doping should not be too high, so it is difficult to reduce the band gap of lithium iron phosphate below 1.5eV.

[0066] In a preferred embodiment, the carbon coating layer accounts for 0.1% to 5% by weight, such as 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, etc. If the carbon coating layer accounts for too much by weight, the electrochemical performance of the lithium iron phosphate will be reduced; if the carbon coating layer accounts for too little by weight, it will be difficult to enhance the conductive performance.

[0067] In some embodiments, the lithium iron phosphate material is doped with titanium at a concentration of 0.01% to 5%. A specific titanium doping concentration is beneficial for improving the material's conductivity. Specifically, the titanium doping concentration refers to the percentage by mass of titanium, which can be 0.01%, 0.05%, 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, etc.

[0068] Existing modification methods for lithium iron phosphate materials mostly use metal ion doping and carbon coating. Most of them first synthesize iron phosphate and then mix it with lithium carbonate, titanium compounds, and carbon sources, resulting in titanium doping being limited to the surface of lithium iron phosphate and low carbon coating uniformity, resulting in unsatisfactory improvement in the electrochemical properties of lithium iron phosphate materials.

[0069] An embodiment of the present invention provides a method for preparing a low-bandgap lithium iron phosphate material, comprising the following steps:

[0070] S1. Preparation of gel

[0071] The iron source, titanium source, organic gel raw material and organic phosphoric acid are mixed and reacted, the phosphate in the organic phosphoric acid forms a precipitate with metal ions such as iron and titanium, and the positively charged organic matter part in the organic phosphoric acid gradually forms a gel with the added organic gel raw material to obtain a gel.

[0072] In some embodiments, the iron source is selected from at least one of ferric chloride, ferric nitrate, and ferric sulfate, and the iron source can be any one or more of the above. The organogel raw material is selected from at least one of polyacrylic acid, gelatin, and a cellulose derivative, and the organogel raw material can be any one or more of the above, and can be combined with an organophosphoric acid to form a gel. The organophosphoric acid is selected from at least one of 1-ethylimidazole ammonium dihydrogen phosphate, 1-butylimidazole ammonium dihydrogen phosphate, and 1-methylimidazole ammonium dihydrogen phosphate, and the organophosphoric acid can be any one or more of the above.

[0073] Furthermore, the molar ratio of the iron element in the iron source to the organic gel raw material is 1:(0.05-0.30), such as 1:0.05, 1:0.10, 1:0.20, 1:0.30, etc.; the molar ratio of the iron element in the iron source to the organic phosphoric acid is 1:(1.00-1.05), such as 1:1.00, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, etc. Excessive amounts of the organic gel raw material and organic phosphoric acid result in an excessively thick carbon coating layer, affecting subsequent mixing with lithium carbonate. Excessive amounts make it difficult for the carbon coating layer to form a gel. The amounts of the organic gel raw material and organic phosphoric acid are preferably within the above ranges so that the mass ratio of the carbon coating layer formed after calcination meets the requirements.

[0074] In some embodiments, the process for preparing the gel comprises: first mixing an iron source and an organic gel raw material to obtain a mixture, gradually adding organic phosphoric acid to the mixture with stirring, adjusting the pH to 6-8, and heating to 60°C-90°C until the solution forms a gel, and then stopping heating. Specifically, the pH value before heating can be adjusted to 6.0, 6.5, 7.0, 7.5, 8.0, etc., and the temperature of the solution after heating can be 60°C, 70°C, 80°C, 90°C, etc. If the pH value of the reaction is too low, it will affect the precipitation of iron phosphate, resulting in too low an iron phosphate content in the gel; if the pH value of the reaction is too high, iron hydroxide impurities will be produced.

[0075] The mixing is performed in a solvent, and the mass ratio of the organogel raw material to the solvent is 1:0.5-5. In some embodiments, the solvent may be water.

[0076] In some embodiments, an organic titanium source is also added during the preparation of the mixture, and titanium doping is formed after subsequent calcination. The molar ratio of the iron element in the iron source to the titanium element in the organic titanium source is 1: (0.005-0.10), such as 1: 0.005, 1: 0.01, 1: 0.03, 1: 0.05, 1: 0.08, 1: 0.10, etc. Controlling the amount of the organic titanium source within the above range can make the titanium doping amount more appropriate, which can further improve the electronic / ionic conductivity. Titanium doping significantly improves the conductivity and structural stability of the lithium iron phosphate material through lattice modification and interface optimization, thereby improving the rate performance of the product.

[0077] S2. Calcination

[0078] The gel is dried, mixed with a lithium source, and then calcined to prepare a lithium iron phosphate material.

[0079] In some embodiments, during the drying process of the gel, the drying temperature is controlled to be between 110°C and 130°C, such as 110°C, 120°C, or 130°C; and the drying time is controlled to be between 4 hours and 8 hours, such as 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours. By regulating the drying temperature and time, the gel dries and shrinks, completing the coating of the iron phosphate. Before drying, the gel is allowed to stand for 1 hour to 3 hours, such as 1 hour, 2 hours, or 3 hours, to ensure uniform distribution of the gel.

[0080] In some embodiments, after drying and before calcining, the mixed material is ground to mix the raw materials evenly, thereby obtaining a lithium iron phosphate material with better uniformity after calcination.

[0081] Furthermore, calcination can be performed under an inert atmosphere, which may be any type of inert atmosphere, such as nitrogen or argon. The calcination temperature is 500°C to 800°C, such as 500°C, 600°C, 700°C, or 800°C, and the calcination time is 2 hours to 6 hours, such as 2 hours, 3 hours, 4 hours, 5 hours, or 6 hours. By regulating the calcination temperature and time, the reaction proceeds fully, resulting in a uniform lithium iron phosphate material.

[0082] Furthermore, the lithium source is selected from at least one of lithium carbonate, lithium chloride, and lithium hydroxide, and the lithium source can be any one or more of the above. The molar ratio of the iron element in the iron source to the lithium element in the lithium source is 1:(1.00-1.05), such as 1:1.00, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, etc. The amount of the lithium source is controlled within the above range to fully react the iron and improve the product yield.

[0083] In summary, the low-energy-gap lithium iron phosphate material derived from the organic framework in situ of the present application refers to the low-energy-gap (also known as band gap) lithium iron phosphate material of the present application that can be prepared by the preparation method of the present application. The in-situ derivatization of the organic framework refers to the reaction of phosphate radicals in the organic phosphoric acid with iron and titanium ions to form a precipitate in the preparation method, and the positively charged organic part in the organic phosphoric acid is complexed with the added organic gel raw material to form an in-situ carbon coating. As the reaction proceeds, the organic matter gradually forms a gel to become an organic framework. However, those skilled in the art will know that the preparation method of the low-energy-gap lithium iron phosphate material of the present application is not limited to this.

[0084] Embodiments of the present invention also provide a positive electrode sheet comprising a current collector and a positive electrode active layer supported on the current collector. The positive electrode active layer contains the low-bandgap lithium iron phosphate material provided by embodiments of the present invention. The improved lithium iron phosphate material enhances the performance of the positive electrode sheet. Specifically, the current collector is not limited to a common aluminum foil and can be used, but is not limited to, this material.

[0085] The present invention also provides a lithium battery comprising the positive electrode sheet provided in the present invention, and further comprising a negative electrode sheet, an electrolyte, a separator, etc., to form a complete battery structure. Due to improvements in the lithium iron phosphate material, the lithium battery provided in the present invention has significantly improved rate performance.

[0086] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0087] The present invention provides a lithium iron phosphate material as shown in Table 1. The lithium iron phosphate materials in the embodiment and comparative example in Table 1 were tested. The specific items and methods are as follows:

[0088] (1) Testing method for the interplanar spacing d of lithium iron phosphate (101): XRD was used for testing.

[0089] (2) Test method for crystal ion vacancy V: XRD is used for testing, and the ion vacancies in lithium iron phosphate are calculated by the lattice expansion method.

[0090] (3) Testing method for the room temperature band gap E1 and low temperature band gap E2 of lithium iron phosphate: testing is performed using a UV-visible spectrometer.

[0091] The lithium iron phosphate materials provided in the embodiments of the present invention and the comparative examples were assembled into a lithium battery. The assembly method was as follows: lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 8:1:1 and added to NMP solvent for magnetic stirring to form a uniform slurry. The slurry was coated on aluminum foil and vacuum dried at 100°C for 12 hours. The prepared electrode sheet was cut into 12 mm circular pieces using a slicer and weighed. The load of each electrode was controlled at 1.5 mg / cm 2 -3mg / cm 2 The electrode sheet was assembled into a button cell in the glove phase, with metallic lithium as the negative electrode, Celgrad 2400 as the separator, and a 1 mol / L LiPF6 organic solution (the solvent was a mixture of EC, DEC, and DMC in a volume ratio of EC:DEC:DMC=1:1:1) as the electrolyte to form a CR2032 button cell.

[0092] Performance test method: Use a blue battery test cabinet to perform constant current charge and discharge tests.

[0093] Table 1 Performance parameters of lithium iron phosphate materials provided in this embodiment and comparative example

[0094]

[0095] It can be seen from Table 1 that the optimization degree κ of the lithium iron phosphate material provided in the embodiment meets 2nm -2 .eV -1 <κ≤100nm -2 .eV -1 The battery prepared by it has good electrochemical performance at room temperature and low temperature.

[0096] The preparation methods of the lithium iron phosphate materials of the above examples and comparative examples are described below.

[0097] Method Example 1

[0098] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material, the steps of which are as follows:

[0099] Ferric chloride, butyl titanate, and polyacrylic acid were prepared in a molar ratio of 1:0.01:0.1 (iron in ferric chloride, titanium in butyl titanate, and polyacrylic acid). The mixture was then mixed in water (polyacrylic acid to water ratio of 1:4). 1-Ethylimidazole ammonium dihydrogen phosphate (1-Ethylimidazole ammonium dihydrogen phosphate) was gradually added with stirring. The molar ratio of 1-Ethylimidazole ammonium dihydrogen phosphate to iron in ferric chloride was 1.02:1. The pH was adjusted to 7 with aqueous ammonia. The mixture was transferred to a water bath and heated to 80°C with slow stirring until a gel formed (approximately 60 minutes). The mixture was then allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (molar ratio of iron in the iron source to lithium in the lithium source of 1:1.05), ground for 6 hours, and calcined at 700°C under a nitrogen atmosphere for 2 hours to obtain carbon-coated titanium-doped lithium iron phosphate.

[0100] Method Example 2

[0101] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material, the steps of which are as follows:

[0102] Ferric nitrate, butyl titanate, and industrial gelatin were prepared in a molar ratio of 1:0.02:0.1 (iron in ferric nitrate, titanium in butyl titanate, and industrial gelatin). The mixture was then mixed in water (mass ratio of gelatin to water was 1:5). 1-Butyl imidazole ammonium dihydrogen phosphate was gradually added with stirring. The molar ratio of 1-Butyl imidazole ammonium dihydrogen phosphate to iron in ferric nitrate was 1.02:1. The pH was adjusted to 7 with aqueous ammonia. The mixture was transferred to a water bath, heated to 90°C, and stirred slowly until a gel formed. The mixture was allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (molar ratio of iron in the iron source to lithium in the lithium source was 1:1.02), ground for 6 hours, and calcined at 600°C under a nitrogen atmosphere for 2 hours to obtain carbon-coated titanium-doped lithium iron phosphate.

[0103] Method Example 3

[0104] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material, the steps of which are as follows:

[0105] Ferric sulfate, butyl titanate, and a cellulose derivative were prepared in a molar ratio of 1:0.03:0.2 (iron from ferric sulfate, titanium from butyl titanate, and cellulose derivative). The mixture was then mixed in water (cellulose derivative:water ratio of 1:4 by mass). 1-Methylimidazole ammonium dihydrogen phosphate (1-Methylimidazole ammonium dihydrogen phosphate) was gradually added with stirring. The molar ratio of 1-Methylimidazole ammonium dihydrogen phosphate to iron from ferric sulfate was 1.02:1. The pH was adjusted to 8 with aqueous ammonia. The mixture was transferred to a water bath, heated to 70°C, and stirred gently until a gel formed. The mixture was allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (iron from the iron source: lithium from the lithium source in a molar ratio of 1:1.05), ground for 8 hours, and calcined at 600°C under a nitrogen atmosphere for 4 hours to produce carbon-coated titanium-doped lithium iron phosphate.

[0106] Method Example 4

[0107] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material, the steps of which are as follows:

[0108] Ferric sulfate, butyl titanate, and polyacrylic acid were prepared in a molar ratio of 1:0.04:0.2 (iron from ferric sulfate, titanium from butyl titanate, and polyacrylic acid). The mixture was then mixed in water (polyacrylic acid to water ratio of 1:3.5). 1-Methylimidazole ammonium dihydrogen phosphate (1-Methylimidazole ammonium dihydrogen phosphate) was gradually added with stirring. The molar ratio of 1-Methylimidazole ammonium dihydrogen phosphate to iron from ferric sulfate was 1.05:1. The pH was adjusted to 7 with aqueous ammonia. The mixture was transferred to a water bath, heated to 70°C, and stirred slowly until a gel formed. The mixture was allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (molar ratio of iron from the iron source to lithium from the lithium source of 1:1.05), ground for 8 hours, and calcined at 700°C under a nitrogen atmosphere for 2 hours to produce carbon-coated titanium-doped lithium iron phosphate.

[0109] Method Example 5

[0110] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material derived in situ from an organic framework, the steps of which are as follows:

[0111] Ferric chloride, butyl titanate, and industrial gelatin were prepared in a molar ratio of 1:0.05:0.1 (iron in ferric chloride, titanium in butyl titanate, and industrial gelatin). The mixture was then mixed in water (mass ratio of industrial gelatin to water was 1:4). 1-Ethylimidazole ammonium dihydrogen phosphate (1-Ethylimidazole ammonium dihydrogen phosphate) was gradually added with stirring. The molar ratio of 1-Ethylimidazole ammonium dihydrogen phosphate to iron in ferric chloride was 1:1. The pH was adjusted to 8 with aqueous ammonia. The mixture was transferred to a water bath, heated to 70°C, and stirred slowly until a gel formed. The mixture was allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (molar ratio of iron in the iron source to lithium in the lithium source was 1:1.05), ground for 8 hours, and calcined at 700°C under a nitrogen atmosphere for 2 hours to obtain carbon-coated titanium-doped lithium iron phosphate.

[0112] Method Example 6

[0113] This embodiment provides a method for preparing a low-bandgap lithium iron phosphate material derived in situ from an organic framework, the steps of which are as follows:

[0114] Ferric nitrate, butyl titanate, and a cellulose derivative were prepared in a molar ratio of 1:0.005:0.1 (iron in ferric nitrate, titanium in butyl titanate, and cellulose derivative). The mixture was then mixed in water (cellulose derivative:water ratio of 1:1). 1-Ethylimidazole ammonium dihydrogen phosphate (1-Ethylimidazole ammonium dihydrogen phosphate) was gradually added with stirring. The molar ratio of 1-Ethylimidazole ammonium dihydrogen phosphate to iron in ferric nitrate was 1.02:1. The pH was adjusted to 7 with aqueous ammonia. The mixture was transferred to a water bath, heated to 80°C, and stirred gently until a gel formed. The mixture was allowed to stand for 2 hours. The gel was vacuum dried at 120°C for 6 hours. The dried gel was then mixed with lithium carbonate (iron in the iron source: lithium in the lithium source, a molar ratio of 1:1.05), ground for 8 hours, and calcined at 650°C under a nitrogen atmosphere for 4 hours to produce carbon-coated titanium-doped lithium iron phosphate.

[0115] Method Comparative Example 1

[0116] This comparative example provides a method for preparing a lithium iron phosphate material, and the steps are as follows:

[0117] Ferric sulfate, butyl titanate, and glucose were prepared in a molar ratio of 1:0.02:0.2 (iron from ferric sulfate, titanium from butyl titanate, and glucose). The mixture was then mixed in water (iron sulfate to water ratio of 1:8). Diammonium hydrogen phosphate was added at a molar ratio of 1.02:1 (to the iron from ferric sulfate). The pH was adjusted to 7 with aqueous ammonia. The mixture was heated to 80°C in a water bath, stirred gently, and allowed to stand for 2 hours. The resulting precipitate was vacuum dried at 120°C for 6 hours. The dried material was then mixed with lithium carbonate (iron from the iron source: lithium from the lithium source in a molar ratio of 1:1.05), ground for 8 hours, and calcined at 700°C under a nitrogen atmosphere for 2 hours to produce carbon-coated titanium-doped lithium iron phosphate.

[0118] Method Comparative Example 2

[0119] This comparative example provides a method for preparing a lithium iron phosphate material, and the steps are as follows:

[0120] Ferric sulfate and glucose were prepared in a molar ratio of 1:0.2 and mixed in water (the mass ratio of ferrous sulfate to water was 1:8). Diammonium hydrogen phosphate was added at a molar ratio of 1.02:1 to the iron in the ferrous sulfate. The pH was adjusted to 7 with aqueous ammonia. The mixture was transferred to a water bath, heated to 80°C, stirred gently, and allowed to stand for 2 hours. The resulting precipitate was vacuum dried at 120°C for 6 hours. The dried material was mixed with lithium carbonate (the molar ratio of iron in the iron source to lithium in the lithium source was 1:1.05), ground for 8 hours, and then calcined at 700°C under a nitrogen atmosphere for 2 hours to obtain carbon-coated titanium-doped lithium iron phosphate.

[0121] Method Comparative Example 3

[0122] This comparative example provides a method for preparing a lithium iron phosphate material, and the steps are as follows:

[0123] Ferric nitrate and polyacrylamide were mixed in a 1:0.1 molar ratio in water (the mass ratio of polyacrylamide to water was 1:4). Diammonium hydrogen phosphate was added at a molar ratio of 1.02:1 to the iron in the ferric nitrate. The pH was adjusted to 5, and the mixture was transferred to a water bath, heated to 70°C, stirred slowly, and allowed to stand for 2 hours. The resulting precipitate was vacuum dried at 120°C for 6 hours. The dried material was mixed with lithium carbonate (the molar ratio of the iron in the iron source to the lithium in the lithium source was 1:1.05), ground for 8 hours, and then calcined at 800°C under a nitrogen atmosphere for 2 hours to obtain carbon-coated titanium-doped lithium iron phosphate.

[0124] The XRD pattern of the lithium iron phosphate material provided in Example 1 is as follows: Figure 1 As shown, the calculation is performed according to the Bragg equation 2dsinθ=nλ. Where λ=0.15406nm, θ is the X-ray incident angle, n is an integer (n=1,2,3┄), and the vacancy is calculated according to the lattice expansion. The calculation formula is Where V is the vacancy concentration in nm -3 , a is the change in the sample lattice constant, a0 is the lattice constant calculated from the standard card, Ω is the volume expansion caused by each vacancy, the value is (0.4-0.6)*atomic volume, unit is nm 3 .

[0125] The UV-visible diffuse reflection test diagram of the lithium iron phosphate material provided in Example 1 is as follows: Figure 2 As shown, the band gap is where λ g is the intercept value of the absorption spectrum of the lithium iron phosphate sample and the wavelength. E1 and E2 were both tested according to the above method. When testing E1, the lithium iron phosphate material was placed at room temperature (25°C) for 12 hours, and when testing E2, the lithium iron phosphate material was placed at low temperature (-52°C) for 12 hours.

[0126] Combined with Table 1, lithium iron phosphate titanium doping affects the (101) crystal plane spacing. With the doping of titanium ions, the (101) crystal plane increases, the crystal ion vacancy concentration increases with the doping of titanium ions, and the band gap decreases with the doping of titanium, thereby enhancing the conductivity.

[0127] Specifically, as the amount of titanium doping increases, the interplanar spacing of lithium iron phosphate (101) increases, and the vacancies in the crystal also increase, thereby increasing the lithium ion conductivity of the lithium iron phosphate material. The incorporation of titanium reduces the band gap of lithium iron phosphate and increases the electronic conductivity of lithium iron phosphate. Increasing titanium ion doping and in-situ carbon coating improve the 1C electrochemical performance of the lithium iron phosphate battery. As the amount of titanium increases, the low-temperature performance of the lithium iron phosphate battery changes more significantly, and the -20°C 1C discharge specific capacity can reach 84% of that at room temperature, significantly improving the discharge performance of lithium iron phosphate at high rate and low temperature conditions.

[0128] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A low energy gap lithium iron phosphate material, characterized in that: The lithium iron phosphate material includes a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix, and the lithium iron phosphate material is doped with titanium; The optimization degree κ of lithium iron phosphate material satisfies: 2nm -2 .eV -1 <κ≤100nm -2 .eV -1 , the unit of κ is nm -2 .eV -1 ; Wherein, d represents the (101) interplanar spacing of lithium iron phosphate calculated from XRD test results, in nm; V represents the crystal ion vacancy obtained from XRD test results, the unit is nm -3 ; E1 represents the band gap of lithium iron phosphate treated at 25°C obtained by UV-visible diffuse reflectance test, in eV; E2 represents the band gap of lithium iron phosphate treated at a temperature of -52°C obtained by UV-visible diffuse reflectance testing, in eV.

2. The low energy gap lithium iron phosphate material according to claim 1, characterized in that: The lithium iron phosphate material satisfies at least one of features A1 to G1: Feature A1: 50nm -2 .eV -1 <κ<97nm -2 .eV -1 ; Feature B1: the value range of d is 0.40nm-0.45nm; Feature C1: V range is 0.5nm -3 -1.5nm -3 ; Feature D1: The value range of E1 is 1.5eV-3.0eV; The value range of characteristics E1:E2 is 1.5eV-3.5eV; Feature F1: The doping amount of titanium is 0.01%-5%; Feature G1: The mass percentage of the carbon coating layer ranges from 0.1% to 5%.

3. A method for preparing the low energy gap lithium iron phosphate material according to claim 1 or 2, characterized in that: include: The iron source, the titanium source, the organic gel raw material and the organic phosphoric acid are mixed and reacted to obtain a gel; The gel is dried and mixed with a lithium source and then calcined.

4. The preparation method according to claim 3, characterized in that The process for preparing the gel satisfies at least one of features A2 to E2: Feature A2: The organic gel raw material is selected from at least one of polyacrylic acid, gelatin and cellulose derivatives; Feature B2: The organic phosphoric acid is at least one selected from 1-ethylimidazole ammonium dihydrogen phosphate, 1-butylimidazole ammonium dihydrogen phosphate and 1-methylimidazole ammonium dihydrogen phosphate; Feature C2: The molar ratio of the iron element in the iron source to the organogel raw material is 1:(0.05-0.30); Feature D2: The molar ratio of the iron element in the iron source to the organophosphoric acid is 1:(1.00-1.05); Feature E2: The iron source is at least one selected from ferric chloride, ferric nitrate and ferric sulfate.

5. The preparation method according to claim 4, characterized in that The process of preparing the gel includes: first mixing the iron source, the titanium source and the organic gel raw material to obtain a mixture, mixing the mixture with the organic phosphoric acid, adjusting the pH value to 6-8, and heating to 60-90°C to react until a gel is generated.

6. The preparation method according to claim 5, characterized in that The titanium source is an organic titanium source, and the molar ratio of the iron element in the iron source to the titanium element in the organic titanium source is 1:(0.005-0.10).

7. The preparation method according to claim 3, characterized in that During the drying of the gel, at least one of features A3 to C3 is satisfied: Feature A3: Control the drying temperature to 110℃-130℃; Feature B3: Control the drying time to 4h-8h; Characteristic C3: The gel is allowed to stand for 1-3 hours before being dried.

8. The preparation method according to claim 3, characterized in that During the calcination process of mixing with the lithium source, at least one of features A4 to F4 is satisfied: Feature A4: Calcination temperature is 500°C-800°C; Feature B4: calcination time is 2h-6h; Feature C4: The molar ratio of the iron element in the iron source to the lithium element in the lithium source is 1:(1.00-1.05); Feature D4: The lithium source is selected from at least one of lithium carbonate, lithium chloride and lithium hydroxide; Feature E4: The mixed material is ground before calcination; Feature F4: Calcination is carried out under an inert atmosphere.

9. A positive electrode plate, characterized in that: The invention comprises a current collector and a positive electrode active layer loaded on the current collector, wherein the positive electrode active layer contains the low-energy-gap lithium iron phosphate material according to any one of claims 1 to 2 or the low-energy-gap lithium iron phosphate material prepared by the preparation method according to any one of claims 3 to 8.

10. A lithium battery, characterized in that: Including the positive electrode sheet according to claim 9.

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