A phosphoric acid-based cathode active material, its preparation method and application

Abstract

The present invention discloses a phosphoric acid-based cathode active material, a preparation method thereof, and an application thereof, belonging to the field of secondary battery materials. Among them, the molecular formula of the phosphoric acid-based cathode active material is L a Fe x1 Nb x2 M x3 (PO 4 ) y (P 2 O 7 ) z (TiO 3 ) e / BN@C, and the boron and nitrogen-doped carbon layer forms a conductive coating layer on the surface of L a Fe x1 Nb x2 M x3 (PO 4 ) y (P 2 O 7 ) z (TiO 3 ) e Calculated by mass fraction, the boron and nitrogen-doped carbon layer accounts for 1-5% of the cathode active material, and the boron and nitrogen element content accounts for 0.05-0.2% of the coating layer. Boron nitride compounds can improve the electrochemical performance of the cathode active material, while reducing the temperature during the manufacturing process and ensuring the capacity of the active material; in addition, Nb doping can reduce the internal resistance of the battery, improve the cycle stability and rate performance.

Description

Technical Field

[0001] The present invention relates to the field of secondary batteries, and particularly to a phosphate-based cathode active material for secondary batteries, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium / sodium ion batteries play an important role as secondary energy storage devices in the fields of new energy vehicles, mobile intelligent devices, large-scale energy storage, etc. With the rapid expansion of the new energy vehicle and large-scale energy storage markets, the demand for power / storage batteries is also rapidly increasing. Therefore, it is extremely important to develop high-performance batteries and materials. Phosphate has the advantages of environmental friendliness, low cost, and stable structure, and has been applied in the fields of lithium-ion batteries and sodium-ion batteries, and the market demand is also continuously increasing.

[0003] Currently, commercially available phosphate cathode materials such as lithium iron phosphate, lithium manganese iron phosphate, sodium iron pyrophosphate, sodium iron phosphate, and sodium vanadium phosphate have been widely used in the field of power / storage batteries. However, although phosphate cathode materials have a stable structure and long cycle life, all phosphate-based cathode materials have the problem of low ionic and electronic conductivity, which greatly limits the performance of the battery, especially the high-rate and low-temperature performance. How to improve the electrochemical performance of phosphate cathode materials is one of the hot research directions in the industry.

[0004] As a new type of porous crystalline hybrid material, metal-organic framework materials have the characteristics of adjustable structure, long-range order, and controllable porosity. Making them as precursors to be compounded with phosphates to prepare structure-controllable and high-performance electrode materials can greatly improve the electrochemical properties of cathode materials. In view of this, the present invention provides a technical solution to solve the insufficient electrochemical performance of phosphate electrode materials. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a phosphate cathode active material. By doping with boron-nitrogen hybrid crystals, a composite phosphate cathode material is prepared, which effectively shortens the ion migration distance, improves the ionic and electronic conductivity of the cathode material, and enhances the electrochemical performance of the cathode active material, especially the cycle performance and rate performance. By forming a boron-containing hybrid crystal material with an organic ligand and compounding it with the phosphate cathode material, the material has a high content of boron and nitrogen elements, and the contained B-H functional group has good reducibility, which is beneficial to reducing the transition metal ions in the phosphate cathode material to a low valence state, reducing the temperature during the manufacturing process of the cathode material, and thus reducing the manufacturing cost. In addition, niobium and titanate-doped phosphate-based cathode materials can reduce the internal resistance of the battery, which is of great significance for designing high-rate power batteries.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A phosphate electrode active material, the chemical general formula of the active material is L a Fe x1 Nb x2 M x3 (PO 4 ) y (P 2 O 7 ) z (TiO 3 ) e / BN@C, the boron and nitrogen doped carbon layer forms a conductive coating layer on the surface of L a Fe x1 Nb x2 M x3 (PO 4 ) y (P 2 O 7 ) z (TiO 3 ) e ; wherein 0.5 < a ≤ 1.1, 0 < x1 < 1, 0 < x2 < 0.05, 0 < x3 < 1, 0 < y ≤ 1, 0 ≤ z ≤ 1, 0 < e ≤ 0.05, L is one or both of Na and Li, and M is one or more of Mn, Ni, V, Ti, Mg, Cu, Zn, Zr. Preferably, 0.8 < a ≤ 1, 0 < x1 < 1, 0 < x2 < 0.03, 0 < x3 < 0.8, 0 < y ≤ 1, 0 ≤ z ≤ 0.8, 0 < e ≤ 0.05.

[0008] Among them, calculated by mass fraction, the content of the boron and nitrogen doped carbon layer on the surface of the positive electrode active material accounts for 1 to 5% or less of the positive electrode active material, and the content of boron and nitrogen elements accounts for 0.05 to 0.2% of the coating layer.

[0009] The second object of the present invention is to provide a preparation method of the above-mentioned phosphate electrode active material, comprising the following steps:

[0010] S1. Add sodium borohydride or lithium borohydride, a nitrogen-containing organic precursor, and glucose into a flask, reflux and stir and heat at 100 - 250 °C for 1 - 5 h under nitrogen conditions, and cool it to room temperature to obtain carbon source powder A;

[0011] S2. Select several of carbon source powder A, L source compound, Fe source compound, M source compound, Nb source compound, and phosphorus source compound as raw materials according to the structure of the phosphate active material, and weigh an appropriate amount of the above raw materials for premixing to obtain a premixed powder;

[0012] S3. Add the premixed powder, a solvent, and a dispersant into a nano sand mill for ball milling to obtain a slurry, and the primary particle size range of the slurry is 100 - 400 nm;

[0013] S4. Spray-dry and granulate the slurry to obtain precursor powder; then pre-sinter the precursor powder in an inert atmosphere, cool it, and perform double-planet mechanical ball milling with sodium metatitanate or lithium metatitanate to mix evenly. Sinter the evenly mixed powder in an inert atmosphere, cool it, and obtain the above-mentioned composite phosphate cathode active material L coated with a boron and nitrogen-doped carbon layer a Fe x1 Nb x2 M x3 (PO 4 ) y (P 2 O 7 ) z (TiO 3 ) e / BN@C

[0014] Among them, in step S2, the L source compound is selected from one or more of sodium carbonate, sodium hydroxide, sodium oxalate, sodium nitrate, lithium carbonate, lithium hydroxide, lithium oxalate, and lithium nitrate; the Fe source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the M source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the Nb source compound is selected from one or more of niobium pentoxide, niobium tetroxide, niobium trioxide, niobium oxide, niobium sulfide, niobium pentachloride, niobium tetrachloride, and niobium nitride; the phosphorus source compound is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the nitrogen-containing organic precursor is selected from one or more of 2-methylimidazole, 2-ethylimidazole, 3-propylimidazole, and imidazole

[0015] Among them, in step S3, the dispersant is selected from one or more of ammonium polyacrylate, polyvinylpyrrolidone, and polyethylene glycol; the solvent is deionized water; the mass ratio of the pre-mixed powder to the solvent is 1:(2 - 5); the mass of the dispersant is 0.1 - 2% of the mass of the solvent; the ball milling time is 5 - 15 h

[0016] Among them, in step S4, the inert atmosphere is nitrogen or argon; the particle size of sodium metatitanate or lithium metatitanate is 0.2 - 0.6 µm; the pre-sintering temperature is 300 - 400 °C, and the sintering time is 1 - 3 h; the sintering temperature is 500 - 800 °C, and the sintering time is 5 - 15 h

[0017] Among them, in step S4, calculated by mass fraction, the boron and nitrogen-doped carbon layer accounts for 1 - 5% of the active material, and the boron and nitrogen element content accounts for 0.05 - 0.2% of the coating layer

[0018] A third object of the present invention is to provide a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the above-mentioned positive electrode active material.

[0019] A fourth object of the present invention is to provide a sodium-ion battery or a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet is the above-mentioned positive electrode sheet.

[0020] Beneficial effects:

[0021] The boron-nitrogen composite phosphate-based positive electrode active material provided by the present invention improves the electrochemical performance of the positive electrode material by compounding a boron-nitrogen compound with a phosphate-based active material. The composite material contains a certain content of boron and nitrogen elements, and the contained B-H functional group has good reducibility, which is beneficial to the reduction of transition metal ions in the phosphate-based positive electrode material to a low valence state, reduces the temperature during the manufacturing process of the positive electrode material, and reducing the crystallization temperature is of great significance for the phosphate-based active material used in sodium-ion batteries, because too high a crystallization temperature will cause the formation of a sodium iron phosphate structure positive electrode material without electrochemical activity in the phosphate-based positive electrode material, resulting in extremely low capacity of the active material. Therefore, the precursor rich in boron-nitrogen compounds can well improve this defect. In addition, during the heat treatment process, the hybrid crystal material is cracked into a carbon layer rich in boron and nitrogen elements, which effectively improves the disadvantages of poor ionic and electronic conductivity of the phosphate-based positive electrode material and improves the electrochemical performance of the positive electrode active material. In addition, Nb doping can reduce the internal resistance of the battery, improve the cycle stability and rate performance. Specific embodiments

[0022] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0023] Example 1

[0024] A composite phosphate positive electrode active material, with the molecular formula NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C.

[0025] The preparation method of the positive electrode active material comprises the following steps:

[0026] S1. Add sodium borohydride, 2-methylimidazole (mass ratio 1:5), and glucose into a flask. Under nitrogen atmosphere, reflux and stir at 220 °C for 2 h, then cool it to room temperature to obtain carbon source powder A.

[0027] S2. Weigh carbon source powder A, sodium carbonate, ferrous oxalate, niobium oxide, magnesium oxide, and ammonium dihydrogen phosphate and premix them to obtain a premixed powder; where the molar ratio of Na:Fe:Nb:Mg:P is 1:0.7:0.01:0.01:1.

[0028] S3. Add the premixed powder, deionized water at a mass ratio of 1:2, and ammonium polyacrylate (ammonium polyacrylate is 0.2% of the mass of deionized water) into a nano sand mill for high-energy ball milling for 8 h to obtain a slurry, and the primary particle size of the slurry is about 200 nm.

[0029] S4. Spray-dry and granulate the slurry to obtain a precursor powder; then pre-sinter the precursor powder at 350 °C for 2 h under nitrogen atmosphere, cool it, and perform double planetary mechanical ball milling and mixing with sodium metatitanate until evenly mixed. Sinter the evenly mixed powder at 550 °C for 10 h under nitrogen atmosphere and naturally cool it to room temperature to obtain a cathode active material with the molecular formula NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C. By mass fraction, the boron and nitrogen-doped carbon layer accounts for 2% of the active material, and the boron and nitrogen element content accounts for 0.1% of the coating layer.

[0030] Apply the obtained cathode active material to a cathode plate and a button cell. The preparation process is as follows:

[0031] Cathode plate: Weigh 1.9 g of the obtained cathode active material, add 0.05 g of carbon black and 0.05 g of polyvinylidene fluoride dissolved in N, N'-methylpyrrolidone, homogenize and coat it on an aluminum foil to make a cathode plate.

[0032] Button cell: In a glove box under argon atmosphere, use metallic sodium as the counter electrode, glass fiber as the separator, and 1 M / NaPF 6 / PC:EMC:EC (volume ratio 1:1:1) as the electrolyte to assemble a 2032 button cell.

[0033] Example 2

[0034] A composite phosphate cathode active material with the molecular formula Li 0.1 Na 0.9Fe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C.

[0035] The preparation method of the positive electrode active material includes the following steps:

[0036] S1. Add sodium borohydride, 2-ethylimidazole (mass ratio 1:5), and glucose into a flask, reflux and stir at 180 °C for 5 h under nitrogen conditions, and then cool it to room temperature to obtain carbon source powder A.

[0037] S2. Weigh carbon source powder A, lithium carbonate, sodium carbonate, ferrous oxalate, niobium oxide, magnesium oxide, and ammonium dihydrogen phosphate for premixing to obtain a premixed powder; where the molar ratio of Li:Na:Fe:Nb:Mg:P is 0.1:0.9:0.7:0.01:0.01:1.

[0038] S3. Add the premixed powder, deionized water in a mass ratio of 1:2, and ammonium polyacrylate (ammonium polyacrylate is 0.5% of the mass of deionized water) into a nano sand mill for high-energy ball milling for 15 h to obtain a slurry, and the primary particle size of the slurry is about 300 nm.

[0039] S4. Spray-dry and granulate the slurry to obtain a precursor powder; then pre-sinter the precursor powder at 300 °C for 3 h in a nitrogen atmosphere, cool it, and perform double planetary mechanical ball milling and mixing with sodium metatitanate uniformly. Sinter the uniformly mixed powder at 500 °C for 15 h in a nitrogen atmosphere and naturally cool it to room temperature to obtain the positive electrode active material with the molecular formula Li 0.1 Na 0.9 Fe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C. By mass fraction, the boron and nitrogen doped carbon layer accounts for 1.8% of the active material, and the boron and nitrogen element content accounts for 0.05% of the coating layer.

[0040] Apply the above-obtained positive electrode active material to the positive electrode sheet and button battery, and the preparation process is as follows:

[0041] Positive electrode sheet: Weigh 1.9 g of the obtained positive electrode active material, add 0.05 g of carbon black and 0.05 g of polyvinylidene fluoride dissolved in N, N'-methylpyrrolidone, and after homogenization, coat it on an aluminum foil to make a positive electrode sheet.

[0042] Button battery: In a glove box under an argon atmosphere, using metallic sodium as the counter electrode, glass fiber as the separator, and 1 M / NaPF 6 / PC: EMC: EC (volume ratio 1:1:1) as the electrolyte, assemble a 2032 button battery.

[0043] Example 3

[0044] A composite phosphate positive electrode active material with the molecular formula LiFe 0.38 Nb 0.02 Mn 0.6 PO 4 (TiO 3 ) 0.01 / BN@C.

[0045] The preparation method of this positive electrode active material includes the following steps:

[0046] S1. Add lithium borohydride, 2-methylimidazole (mass ratio 1:5), and glucose to a flask, reflux and stir at 200 °C for 5 h under nitrogen conditions, and then cool it to room temperature to obtain carbon source powder A.

[0047] S2. Weigh carbon source powder A, lithium carbonate, ferrous oxalate, niobium oxide, manganese oxalate, and ammonium dihydrogen phosphate for premixing to obtain a premixed powder; where the molar ratio of Li:Fe:Nb:Mn:P is 1:0.38:0.02:0.6:1.

[0048] S3. Add the premixed powder, deionized water at a mass ratio of 1:2, and ammonium polyacrylate (ammonium polyacrylate is 0.5% of the mass of deionized water) to a nano sand mill for high-energy ball milling for 10 h to obtain a slurry, and the primary particle size of the slurry is about 150 nm.

[0049] S4. Spray-dry and granulate the slurry to obtain a precursor powder; then pre-sinter the precursor powder at 350 °C for 3 h under a nitrogen atmosphere, cool it, and perform double planetary mechanical ball milling and mixing with lithium metatitanate uniformly. Sinter the uniformly mixed powder at 700 °C for 12 h under a nitrogen atmosphere, and naturally cool it to room temperature to obtain the product with the molecular formula LiFe 0.38 Nb 0.02 Mn 0.6 PO 4 (TiO 3 ) 0.01The positive electrode active material of / BN@C. By mass fraction, the boron nitride-doped carbon layer accounts for 3% of the active material, and the boron and nitrogen element content accounts for 0.15% of the coating layer.

[0050] Apply the obtained positive electrode active material to the positive electrode sheet and button battery. The preparation process is as follows:

[0051] Positive electrode sheet: Weigh 1.9 g of the obtained positive electrode active material, add 0.05 g of carbon black and 0.05 g of polyvinylidene fluoride dissolved in N, N'-methylpyrrolidone, and after homogenization, coat it on the aluminum foil to make a positive electrode sheet.

[0052] Button battery: In a glove box under an argon atmosphere, using metallic lithium as the counter electrode, glass fiber as the separator, and 1 M / LiPF 6 / PC: EMC: EC (volume ratio 1:1:1) as the electrolyte, assemble a 2032 button battery.

[0053] Example 4

[0054] A composite phosphate positive electrode active material with the molecular formula LiFe 0.98 Nb 0.01 Mg 0.01 PO 4 (TiO 3 ) 0.01 / BN@C.

[0055] The preparation method of this positive electrode active material includes the following steps:

[0056] S1. Add lithium borohydride, 2-methylimidazole (mass ratio 1:5), and glucose to a flask, reflux and stir at 200 °C for 3 h under nitrogen conditions, and cool it to room temperature to obtain carbon source powder A.

[0057] S2. Weigh carbon source powder A, lithium carbonate, iron oxalate, niobium oxide, magnesium oxide, and ammonium dihydrogen phosphate for premixing to obtain a premixed powder; where the molar ratio of Li:Fe:Nb:Mg:P is 1:0.98:0.01:0.01:1.

[0058] S3. Add the premixed powder, deionized water in a mass ratio of 1:2, and ammonium polyacrylate (ammonium polyacrylate is 0.4% of the mass of deionized water) to a nano sand mill for high-energy ball milling for 10 h to obtain a slurry, and the primary particle size of the slurry is about 250 nm.

[0059] S4. Spray-dry and granulate the slurry to obtain a precursor powder; then pre-sinter the precursor powder at 300 °C for 3 h under a nitrogen atmosphere, cool it, and perform double planetary mechanical ball milling and mixing with lithium metatitanate evenly. Sinter the uniformly mixed powder at 700 °C for 8 h under a nitrogen atmosphere, and naturally cool it to room temperature to obtain the molecular formula LiFe0.98 Nb 0.01 Mg 0.01 PO 4 (TiO 3 ) 0.01 The positive electrode active material of / BN@C. By mass fraction, the boron nitride-doped carbon layer accounts for 2% of the active material, and the boron and nitrogen element content accounts for 0.1% of the coating layer.

[0060] Apply the obtained positive electrode active material to the positive electrode sheet and button battery. The preparation process is as follows:

[0061] Positive electrode sheet: Weigh 1.9 g of the obtained positive electrode active material, add 0.05 g of carbon black and 0.05 g of polyvinylidene fluoride dissolved in N, N'-methylpyrrolidone, and coat it on the aluminum foil after homogenization to make the positive electrode sheet.

[0062] Button battery: In a glove box under an argon atmosphere, use metallic lithium as the counter electrode, glass fiber as the separator, and 1 M / LiPF 6 / PC:EMC:EC (volume ratio 1:1:1) as the electrolyte to assemble a 2032 button battery.

[0063] Example 5

[0064] The difference from Example 1 is the content of each raw material of this positive electrode active material. By changing the amount of sodium metatitanate, the molecular formula of the positive electrode active material for sodium-ion batteries obtained is NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.05 / BN@C.

[0065] The rest is the same as Example 1 and will not be elaborated here.

[0066] Example 6

[0067] The difference from Example 1 is the content of each raw material of this positive electrode active material. Among them, the molar ratio of Na:Fe:Nb:Mg:P is 1.05:0.75:0.01:0.01:1, and the molecular formula of the positive electrode active material for sodium-ion batteries obtained is Na 1.05 Fe 0.75 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO3 ) 0.01 / BN@C。

[0068] The rest is the same as in Example 1 and will not be elaborated here.

[0069] Example 7

[0070] Different from Example 1, the boron and nitrogen doped carbon layer accounts for 5% of the active material, and the boron and nitrogen element content accounts for 0.1% of the coating layer.

[0071] The rest is the same as in Example 1 and will not be elaborated here.

[0072] Example 8

[0073] Different from Example 3 is the content of each raw material of the positive electrode active material, where the molar ratio of Li:Fe:Nb:Mn:P is 1:0.26:0.04:0.7:1, and the molecular formula of the positive electrode active material for sodium ion batteries obtained is LiFe 0.26 Nb 0.04 Mn 0.7 PO 4 (TiO 3 ) 0.01 / BN@C。

[0074] The rest is the same as in Example 3 and will not be elaborated here.

[0075] Example 9

[0076] Different from Example 4 is the content of each raw material of the positive electrode active material, where the molar ratio of Li:Fe:Nb:Mg:P is 1:0.96:0.02:0.02:1, and the molecular formula of the positive electrode active material for sodium ion batteries obtained is LiFe 0.96 Nb 0.02 Mg 0.02 PO 4 (TiO 3 ) 0.01 / BN@C

[0077] The rest is the same as in Example 4 and will not be elaborated here.

[0078] Comparative Example 1

[0079] Different from Example 1 is that the powder A in the raw materials for preparing the positive electrode active material only contains glucose. The molecular formula of the positive electrode active material in this comparative example is NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3) 0.01 / C.

[0080] The rest is the same as in Example 1 and will not be elaborated here.

[0081] Comparative Example 2

[0082] Differing from Example 1 is the setting of the positive electrode active material. The molecular formula of the positive electrode active material in this comparative example is NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 / BN@C.

[0083] The rest is the same as in Example 1 and will not be elaborated here.

[0084] Comparative Example 3

[0085] Differing from Example 1 is the setting of the positive electrode active material. The molecular formula of the positive electrode active material in this comparative example is NaFe 0.7 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 / C.

[0086] The rest is the same as in Example 1 and will not be elaborated here.

[0087] Comparative Example 4

[0088] Differing from Example 1 is that the boron-nitrogen doped carbon layer accounts for 8% of the active material, and the boron-nitrogen element content accounts for 0.1% of the coating layer.

[0089] The rest is the same as in Example 1 and will not be elaborated here.

[0090] Comparative Example 5

[0091] Differing from Example 1 is that the boron-nitrogen doped carbon layer accounts for 2% of the active material, and the boron-nitrogen element content accounts for 0.5% of the coating layer.

[0092] The rest is the same as in Example 1 and will not be elaborated here.

[0093] The button cells obtained from the above Examples 1 to 11 and Comparative Examples 1 to 5 were subjected to cyclic performance and rate performance tests in the charge-discharge range of 2.0 - 4.2V.

[0094] The cyclic performance test was carried out on a Land battery tester, and the test voltage range was 2.0 - 4.2V. The initial discharge specific capacity at 0.1C, the capacity retention rate at 10C, and the capacity retention rate after 100 cycles at 1C rate of the battery were recorded.

[0095] The test results are shown in Table 1

[0096] Table 1

[0097] Serial number Positive electrode active material Mass ratio of boron nitride doped carbon layer to active material / % Mass ratio of boron and nitrogen element content to coating layer / % First discharge specific capacity mAh / g Capacity retention rate at 10C / % Capacity retention rate after 100 cycles / % Example 1 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 2 0.1 103 91 97 Example 2 <![CDATA[Li 0.1 Na 0.9 Fe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 1.8 0.05 105 90 98 Example 3 <![CDATA[LiFe 0.38 Nb 0.02 Mn 0.6 PO 4 (TiO 3 ) 0.01 / BN@C]]> 3 0.15 151 86 96 Example 4 <![CDATA[LiFe 0.98 Nb 0.01 Mg 0.01 PO 4 (TiO 3 ) 0.01 / BN@C]]> 2 0.1 156 88 99 Example 5 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.05 / BN@C]]> 2 0.1 100 92 95 Example 6 <![CDATA[Na 1.05 Fe 0.75 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 2 0.1 102 90 96 Example 7 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 5 0.1 102 88 94 Example 8 <![CDATA[LiFe 0.26 Nb 0.04 Mn 0.7 PO 4 (TiO 3 ) 0.01 / BN@C]]> 3 0.15 150 85 97 Example 9 <![CDATA[LiFe 0.96 Nb 0.02 Mg 0.02 PO 4 (TiO 3 ) 0.01 / BN@C]]> 2 0.1 155 87 98 Comparative example 1 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / C]]> 2 0 99 79 89 Comparative example 2 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 / BN@C]]> 2 0.1 97 83 84 Comparative example 3 <![CDATA[NaFe 0.7 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 / C]]> 2 0 98 78 85 Comparative example 4 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 8 0.1 96 88 98 Comparative example 5 <![CDATA[NaFe 0.7 Nb 0.01 Mg 0.01 (PO 4 ) 0.5 (P 2 O 7 ) 0.25 (TiO 3 ) 0.01 / BN@C]]> 2 0.5 95 82 80

[0098] From the test results of Examples 1 to 9 and Comparative Examples 1 to 5, it can be seen that when elements such as Nb and Mg are doped, the cycling performance and rate performance of the battery are both relatively high. The doping of Nb element can significantly reduce the internal resistance of the battery and improve the electrochemical performance of the positive electrode active material, especially the rate performance. Among them, according to Example 1 and Comparative Example 1, boron-nitrogen doping can also improve the ionic and electronic conductivities of the material, which is of great significance for improving the rate performance and cycling performance. In addition, the fusion of metatitanate is also beneficial to improving the cycling stability. According to Examples 1, 7 and Comparative Examples 1, 5, it can be seen that boron-nitrogen doping can improve the electrochemical performance, but too high a proportion will have the opposite effect. The imidazole heterocyclic complex carbonizes during the sintering process. During carbonization, boron-nitrogen elements hybridize with carbon, having better reducibility, which can reduce the sintering temperature of the positive electrode active material and is beneficial to reducing the manufacturing cost. At the same time, if the boron-nitrogen doping proportion is too high, it will lead to uneven distribution of carbon materials on the surface of the active material, resulting in the influence on the battery capacity and rate. The research results show that it is better that the mass proportion of boron-nitrogen doping in the carbon coating layer does not exceed 0.2%. According to Examples 1, 2 and Comparative Example 4, when the carbon coating amount on the surface of the positive electrode active material is too high, although the cycling performance and rate performance of the battery are significantly improved, however, too much carbon coating layer reduces the proportion of the active material in the positive electrode material, and the discharge specific capacity of the positive electrode material decreases significantly, which is not conducive to improving the energy density of the phosphate positive electrode material and reducing the battery cost. Therefore, calculated by mass fraction, it is better that the content of the boron-nitrogen doped carbon layer coating on the surface of the composite phosphate positive electrode active material does not exceed 5% of the positive electrode active material, and the battery can obtain better discharge specific capacity, cycling performance and rate performance.

[0099] According to the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions or variations made by those skilled in the art on the basis of the present invention all fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A phosphoric acid-based positive electrode active material, characterized in that: The chemical general formula of the positive electrode active material is L a Fe x1 Nb x2 M x3 (PO4) y (P2O7) z (TiO3) e / BN@C, the boron and nitrogen doped carbon layer forms a conductive coating layer on the surface of L a Fe x1 Nb x2 M x3 (PO4) y (P2O7) z (TiO3) e ; where 0.8 < a ≤ 1, 0 < x1 < 1, 0 < x2 < 0.03, 0 < x3 < 0.8, 0 < y ≤ 1, 0 ≤ z ≤ 0.8, 0 < e ≤ 0.05, L is one or both of Na and Li, M is one or more of Mn, Ni, V, Ti, Mg, Cu, Zn, Zr; calculated by mass fraction, the boron and nitrogen doped carbon layer accounts for 1-5% of the positive electrode active material, and the boron and nitrogen content in the boron and nitrogen doped carbon layer accounts for 0.05-0.2% of the carbon layer; The positive electrode active material is prepared by the following steps: S1, adding sodium borohydride or lithium borohydride, a nitrogen-containing organic precursor, and glucose into a flask, refluxing and stirring and heating at 100-250° C. for 1-5 hours under nitrogen conditions, and cooling to room temperature to obtain a carbon source powder A; S2. Select several of carbon source powder A, L source compound, Fe source compound, M source compound, Nb source compound and phosphorus source compound as raw materials according to the product structure, and weigh appropriate amounts of the above raw materials for premixing to obtain premixed powder; S3, adding the premixed powder, solvent and dispersant into a nano sand mill for ball milling to obtain a slurry, wherein the primary particle size of the slurry is in the range of 100 to 400 nm; S4. The slurry is spray-dried and granulated to obtain a precursor powder, and then the precursor powder is pre-sintered in an inert atmosphere. After cooling, the precursor powder is mixed evenly with sodium metatitanate or lithium metatitanate by double planetary mechanical ball milling, and the evenly mixed powder is sintered in an inert atmosphere and cooled to obtain a positive electrode active material.

2. A method for preparing a positive electrode active material as claimed in claim 1, characterized in that: The preparation method comprises the following steps: S1. Sodium borohydride or lithium borohydride, a nitrogen-containing organic precursor and glucose are added to a flask, heated under reflux stirring at 100-250° C. for 1-5 h under nitrogen conditions, and cooled to room temperature to obtain a carbon source powder A; S2. Select several of carbon source powder A, L source compound, Fe source compound, M source compound, Nb source compound and phosphorus source compound as raw materials according to the product structure, and weigh appropriate amounts of the above raw materials for premixing to obtain premixed powder; S3, adding the premixed powder, solvent and dispersant into a nano sand mill for ball milling to obtain a slurry, wherein the primary particle size of the slurry is in the range of 100 to 400 nm; S4, spray-drying and granulating the slurry to obtain a precursor powder, and then pre-sintering the precursor powder under an inert atmosphere, and mixing it with sodium metatitanate or lithium metatitanate by double planetary mechanical ball milling after cooling, and sintering the mixed powder under an inert atmosphere, and cooling to obtain the boron-nitrogen-doped carbon-coated phosphate positive electrode active material L a Fe x1 Nb x2 M x3 (PO4) y (P2O7) z (TiO3) e / BN@C.

3. The preparation method according to claim 2, characterized in that: In step S2, the L source compound is selected from one or more of sodium carbonate, sodium hydroxide, sodium oxalate, sodium nitrate, lithium carbonate, lithium hydroxide, lithium oxalate, and lithium nitrate; the Fe source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the M source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the Nb source compound is selected from one or more of niobium oxide, niobium sulfide, niobium pentachloride, niobium tetrachloride, and niobium nitride; the phosphorus source compound is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; and the nitrogen-containing organic precursor is selected from one or more of imidazoles.

4. The preparation method according to claim 3, wherein the niobium oxide is selected from one or more of niobium pentoxide and niobium trioxide; and the imidazole is selected from one or more of 2-methylimidazole and 2-ethylimidazole.

5. The preparation method according to claim 2, characterized in that: In step S3, the dispersant is selected from one or more of ammonium polyacrylate, polyvinyl pyrrolidone, and polyethylene glycol; the solvent is deionized water; the mass ratio of the premixed powder to the solvent is 1:(2-5); the mass of the dispersant is 0.1-2% of the mass of the solvent; and the ball milling time is 5-15 hours.

6. The preparation method according to claim 2, characterized in that: In step S4, the particle size of the sodium metatitanate or lithium metatitanate is 0.2~0.6µm; the pre-sintering temperature is 300~400°C, and the sintering time is 1~3h; the sintering temperature is 500~800°C, and the sintering time is 5~15h; the inert atmosphere is nitrogen or argon.

7. A positive electrode plate for a battery, characterized in that: The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to claim 1 or the positive electrode active material obtained by the preparation method according to any one of claims 2-6.

8. A sodium ion battery or a lithium ion battery, characterized in that: The sodium ion battery or lithium ion battery comprises a positive electrode sheet, a negative electrode sheet and a separator separated between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet is the positive electrode sheet according to claim 7.