Positive electrode active material and lithium-ion battery

Abstract

A positive electrode active material and a lithium-ion battery. The positive electrode active material is lithium iron phosphate coated with a carbon layer, and the ID / IG value of the positive electrode active material is 0.75-1.2. In the Raman spectrum of the positive electrode active material, the peak intensity at a wave number of 1360 cm-1 is used as ID, and the peak intensity at a wave number of 1580 cm-1 is IG.

Description

Positive electrode active material and lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 29, 2023, with application number 202311869845X. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the field of battery technology, and specifically relates to a positive electrode active material and a lithium-ion battery. Background Art

[0003] Lithium iron phosphate is widely used as the positive electrode material of lithium-ion batteries due to its wide source of raw materials, high theoretical specific capacity, stable voltage platform, good safety performance and low cost. However, the electronic conductivity of LiFePO4 is relatively low, about 10 -9 S / cm. And its olivine structure has only one-dimensional Li + Diffusion channel, when the material structure changes or the LiFePO4 surface is blocked by impurities, Li + The diffusion of Li + The diffusion coefficient is low (about 10 at room temperature) -14 ~10 -16 cm 2 / s), this phenomenon is particularly serious at low temperatures.

[0004] To improve the conductivity and low temperature performance of LiFePO4, the commonly used methods are: (1) doping with high-valent ions or metal oxides to increase the intrinsic conductivity of the material; (2) reducing the particle size and shortening the Li + (3) Use carbon coating or other conductive materials on the surface of LiFePO4 material to increase the surface conductivity of the material. + The increase in the diffusion rate of LiFePO4 does not significantly improve the low temperature performance of LiFePO4. + The transfer at the interface is the rate-determining step of the LiFePO4 electrode reaction. The poor low-temperature performance is due to the slow Li transfer at the interface between the electrode and the electrolyte. + Transfer process.

[0005] Therefore, there is an urgent need in the art to promote the + A technical solution to improve the low-temperature performance of lithium iron phosphate positive electrode materials by transferring at the interface between the electrode and the electrolyte, increasing the conductivity of LiFePO4, thereby improving the rate performance. Technical issues

[0006] This application aims to solve the following technical problems: how to increase the transfer rate of Li+ at the interface between the electrode and the electrolyte, increase the conductivity of lithium iron phosphate, thereby improving the rate performance and improving the low-temperature performance of lithium iron phosphate. Technical Solutions

[0007] In the first aspect, the present application provides a positive electrode active material, wherein the positive electrode active material is lithium iron phosphate coated with a carbon layer, and the positive electrode active material I D / I G The value is 0.75 to 1.2, wherein, in the Raman spectrum of the positive electrode active material, the wave number is 1360 cm -1 The peak intensity at I D , with a wave number of 1580 cm -1 The peak intensity at I G .

[0008] In a second aspect, the present application provides a lithium-ion battery comprising the above-mentioned positive electrode active material. Beneficial effects

[0009] The lithium-ion battery of the present application includes a positive electrode active material - lithium iron phosphate coated with a carbon layer with an ID / IG value of 0.75 to 1.2, which improves the Li + The desolvation rate at the interface between the electrode and the electrolyte is increased, and the desolvation ability is enhanced, thereby increasing the conductivity of the lithium iron phosphate and improving the rate performance and low-temperature performance of the lithium ion battery containing the positive electrode active material. Modes for Carrying Out the Invention

[0010] The positive electrode active material of this application D / I G The value is 0.75 to 1.2, for example, it can be 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1.0, 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, 1.2, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0011] The positive electrode active material of this application - lithium iron phosphate coated with carbon layer D / I G The value is 0.75~1.2, in order to increase Li + The desolvation rate at the interface between the electrode and the electrolyte enhances the desolvation ability, thereby improving the conductivity and rate performance of lithium iron phosphate and improving the low-temperature performance of lithium iron phosphate.

[0012] In one embodiment, the positive electrode active material I D / I GThe value is 0.8 to 1.0, for example, it can be 0.8, 0.82, 0.85, 0.88, 0.9, 0.92, 0.95, 0.98, 1.0, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0013] I of the positive electrode active material of this scheme D / I G The value is 0.8~1.0, I D / I G The value is within this range, which can further improve the Li + The desolvation rate at the interface between the electrode and the electrolyte enhances the desolvation ability, thereby improving the conductivity and rate performance of lithium iron phosphate and improving the low-temperature performance of lithium iron phosphate.

[0014] In one embodiment, the thickness of the carbon layer is 2-6 nm, for example, 2 nm, 3 nm, 4 nm, 5 nm, or 6 nm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0015] The thickness of the carbon layer in this scheme is 2~6nm, which can not only well cover the lithium iron phosphate, reduce the polarization phenomenon during the migration process, and play a certain shielding role, thereby improving the stability of the battery, but also shorten the lithium ion transmission channel, which is conducive to further improving the rate performance of the lithium-ion battery.

[0016] In one embodiment, the D50 of the positive electrode active material is 0.8-1.6 μm, for example, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, or 1.6 μm, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0017] The D50 of the positive electrode active material of this scheme is 0.8~1.6μm. In this range, the lithium ion migration channel is short, shortening the Li + The diffusion distance is shortened, which improves the rate performance of lithium-ion batteries. At the same time, it ensures a sufficient particle size so that the positive electrode active material is not easy to form soft agglomerates, avoiding network blockage during processing.

[0018] In one embodiment, the method for preparing the positive electrode active material comprises the following steps: mixing a FePO4 precursor, a lithium source, a carbon source and a dispersant, ball milling the mixture, and sintering the mixture to obtain the positive electrode active material.

[0019] In one embodiment, the FePO 4 precursor includes at least one of FePO 4 and FePO 4 coated with a carbon source.

[0020] In one embodiment, the carbon source includes at least one of glucose and polyethylene glycol.

[0021] The carbon source of this scheme includes at least one of glucose and polyethylene glycol. Glucose can form a liquid with a certain viscosity after melting, which is easy to coat the surface of lithium iron phosphate. After carbonization, a loose and porous carbon layer is formed, which is conducive to the infiltration of electrolyte and the migration of lithium ions, thereby improving the rate performance of lithium ion batteries; polyethylene glycol pyrolysis produces highly graphitized carbon, which is easy to control. D / I G The value is between 0.75 and 1.2. The synergistic effect of glucose and polyethylene glycol is beneficial to further improve the conductivity and rate performance of lithium iron phosphate and improve the low-temperature performance of lithium iron phosphate.

[0022] In one embodiment, the carbon source includes the glucose and the polyethylene glycol, and the mass ratio of the glucose to the polyethylene glycol is (0.6-1.2):1, for example, it can be 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0023] The carbon source of this solution includes glucose and polyethylene glycol, and the mass ratio of glucose to polyethylene glycol is (0.6-1.2):1, which can ensure that the carbon coating layer of the positive electrode active material has a sufficiently loose and porous structure, which is beneficial to the infiltration of the electrolyte and the migration of lithium ions, thereby improving the rate performance of the lithium ion battery; and it is easy to regulate I D / I G For I D / I G The value is between 0.75 and 1.2, which is conducive to further improving the conductivity and rate performance of lithium iron phosphate and improving the low-temperature performance of lithium iron phosphate.

[0024] In one embodiment, the sintering temperature is 680-720°C.

[0025] Example 1

[0026] 1. Preparation of carbon layer coated lithium iron phosphate

[0027] S1. Preparation of FePO4 precursor: 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) was dissolved in 100 mL of deionized water, and 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide, and 7.5 mL of furfuryl alcohol monomer were added. The mixture was reacted at 70°C for 3 h. After the reaction was completed, 0.1 mol / L NaOH solution was added to adjust the pH to 1.5. After stirring at room temperature for 2 h, the mixture was filtered, washed, dried at 90°C for 5 h, and then ground to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0028] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0029] 2. Preparation of lithium-ion batteries

[0030] (1) Preparation of positive electrode

[0031] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as a positive electrode active material. The carbon layer-coated lithium iron phosphate positive electrode active material, a binder PVDF (polyvinylidene fluoride), a dispersant PVP (polyvinyl pyrrolidone), and a conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly in a mass ratio of 97.4%:1.6%:0.2%:0.8% to obtain a positive electrode slurry. The positive electrode slurry is then coated on an aluminum foil through a coating process, and a positive electrode sheet is obtained after drying and cold pressing processes.

[0032] (2) Preparation of negative electrode sheet

[0033] The negative electrode active material graphite, binder SBR (styrene-butadiene latex), dispersant CMC (sodium carboxymethyl cellulose), and conductive agent SP (conductive carbon black Super-P) are mixed and stirred evenly in a mass ratio of 96.7%:1.5%:1.2%:0.6% to obtain a negative electrode slurry. The negative electrode slurry is then coated on a copper foil through a coating process, and a negative electrode sheet is obtained after drying and cold pressing processes.

[0034] (3) Selection of electrolyte

[0035] Ethylene carbonate electrolyte was selected to prepare lithium batteries.

[0036] (4) Selection of isolation membrane

[0037] PE (polyethylene) film is used as the isolation film.

[0038] (5) Preparation of lithium-ion batteries

[0039] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation layer, and then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum packaging, standing, forming, shaping, and other processes, a lithium-ion battery is obtained.

[0040] 3. Raman spectroscopy test

[0041] S1. Sample preparation: the obtained carbon layer-coated lithium iron phosphate positive electrode active material is evenly laid in a sample tank to prepare the sample, and it is important to keep the upper surface of the sample prepared in this way on the same horizontal plane;

[0042] S2, test, using the device model i-Raman ® The Prime instrument was connected to a computer and BWSpec software for Raman testing. The Raman spectrum was collected after adjusting the distance between the laser and the test sample. When the Raman peak intensity was maximized, the optimal test position was focused, the data was saved, and the Raman curve spectrum was obtained by subtracting the background baseline using BWSpec software. Data processing was performed and the wave number was read to be approximately 1360 cm -1 The peak intensity at D The wave number read is about 1580 cm -1 The peak intensity at G , calculate I D / I G value.

[0043] Example 2

[0044] 1. Preparation of carbon layer coated lithium iron phosphate

[0045] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0046] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, 200 mL of anhydrous ethanol was added as a dispersant, and the precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 8 h to prepare carbon layer coated lithium iron phosphate.

[0047] 2. Preparation of lithium-ion batteries

[0048] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0049] 3. Raman spectroscopy test

[0050] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0051] Example 3

[0052] 1. Preparation of carbon layer coated lithium iron phosphate

[0053] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0054] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6 h to prepare carbon layer coated lithium iron phosphate.

[0055] 2. Preparation of lithium-ion batteries

[0056] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0057] 3. Raman spectroscopy test

[0058] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0059] Example 4

[0060] 1. Preparation of carbon layer coated lithium iron phosphate

[0061] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0062] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 1.5 h to obtain a precursor slurry. The precursor slurry was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0063] 2. Preparation of lithium-ion batteries

[0064] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0065] 3. Raman spectroscopy test

[0066] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0067] Example 5

[0068] 1. Preparation of carbon layer coated lithium iron phosphate

[0069] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0070] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2.5 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0071] 2. Preparation of lithium-ion batteries

[0072] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0073] 3. Raman spectroscopy test

[0074] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0075] Example 6

[0076] 1. Preparation of carbon layer coated lithium iron phosphate

[0077] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 8.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0078] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0079] 2. Preparation of lithium-ion batteries

[0080] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0081] 3. Raman spectroscopy test

[0082] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0083] Example 7

[0084] 1. Preparation of carbon layer coated lithium iron phosphate

[0085] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 6.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0086] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0087] 2. Preparation of lithium-ion batteries

[0088] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0089] 3. Raman spectroscopy test

[0090] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0091] Example 8

[0092] 1. Preparation of carbon layer coated lithium iron phosphate

[0093] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0094] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 8 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0095] 2. Preparation of lithium-ion batteries

[0096] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0097] 3. Raman spectroscopy test

[0098] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0099] Example 9

[0100] 1. Preparation of carbon layer coated lithium iron phosphate

[0101] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0102] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4. The obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 6 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, 200 mL of anhydrous ethanol was added as a dispersant, and the mixture was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry. The precursor slurry was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0103] 2. Preparation of lithium-ion batteries

[0104] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0105] 3. Raman spectroscopy test

[0106] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0107] Example 10

[0108] 1. Preparation of carbon layer coated lithium iron phosphate

[0109] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0110] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 8 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0111] 2. Preparation of lithium-ion batteries

[0112] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0113] 3. Raman spectroscopy test

[0114] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0115] Example 11

[0116] 1. Preparation of carbon layer coated lithium iron phosphate

[0117] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0118] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 6 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0119] 2. Preparation of lithium-ion batteries

[0120] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0121] 3. Raman spectroscopy test

[0122] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0123] Example 12

[0124] 1. Preparation of carbon layer coated lithium iron phosphate

[0125] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0126] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 6 g glucose, and 6 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0127] 2. Preparation of lithium-ion batteries

[0128] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0129] 3. Raman spectroscopy test

[0130] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0131] Example 13

[0132] 1. Preparation of carbon layer coated lithium iron phosphate

[0133] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0134] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 10 g glucose, and 10 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0135] 2. Preparation of lithium-ion batteries

[0136] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0137] 3. Raman spectroscopy test

[0138] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0139] Example 14

[0140] 1. Preparation of carbon layer coated lithium iron phosphate

[0141] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 7.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0142] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 4 g glucose, and 4 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium iron phosphate.

[0143] 2. Preparation of lithium-ion batteries

[0144] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0145] 3. Raman spectroscopy test

[0146] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0147] Example 15

[0148] 1. Preparation of carbon layer coated lithium iron phosphate

[0149] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 8.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0150] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 1 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 680°C for 6 h to prepare carbon layer coated lithium iron phosphate.

[0151] 2. Preparation of lithium-ion batteries

[0152] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0153] 3. Raman spectroscopy test

[0154] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0155] Example 16

[0156] 1. Preparation of carbon layer coated lithium iron phosphate

[0157] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 6 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0158] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, and 7 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 3 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 710°C for 8 h to prepare carbon layer coated lithium iron phosphate.

[0159] 2. Preparation of lithium-ion batteries

[0160] The carbon layer-coated lithium iron phosphate prepared in this embodiment is used as the positive electrode active material, and the preparation of the lithium ion battery of this embodiment is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this embodiment are strictly consistent with the corresponding contents in Example 1.

[0161] 3. Raman spectroscopy test

[0162] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this example is strictly consistent with the corresponding content in Example 1.

[0163] Comparative Example 1

[0164] 1. Preparation of carbon layer coated lithium iron phosphate

[0165] S1. Preparation of FePO4 precursor: 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) was dissolved in 100 mL of deionized water, and 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide, and 5.5 mL of furfuryl alcohol monomer were added. The mixture was reacted at 70°C for 3 h. After the reaction was completed, 0.1 mol / L NaOH solution was added to adjust the pH to 1.5. After stirring at room temperature for 2 h, the mixture was filtered, washed, dried at 90°C for 5 h, and then ground to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0166] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 6 g glucose, and 10 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 680°C for 6 h to prepare carbon layer coated lithium iron phosphate.

[0167] The carbon layer-coated lithium iron phosphate prepared in this comparative example is used as the positive electrode active material, and the preparation of the lithium ion battery of this comparative example is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the different carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this comparative example are strictly consistent with the corresponding contents in Example 1.

[0168] 3. Raman spectroscopy test

[0169] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this comparative example is strictly consistent with the corresponding content in Example 1.

[0170] Comparative Example 2

[0171] 1. Preparation of carbon layer coated lithium iron phosphate

[0172] S1. Preparation of FePO4 precursor: 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) was dissolved in 100 mL of deionized water, and 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide, and 5 mL of furfuryl alcohol monomer were added. The mixture was reacted at 70°C for 3 h. After the reaction was completed, 0.1 mol / L NaOH solution was added to adjust the pH to 1.5. After stirring at room temperature for 2 h, the mixture was filtered, washed, dried at 90°C for 5 h, and then ground to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0173] S2. Preparation of carbon layer coated lithium iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 6 g glucose, and 10 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry, which was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 680°C for 6 h to prepare carbon layer coated lithium iron phosphate.

[0174] The carbon layer-coated lithium iron phosphate prepared in this comparative example is used as the positive electrode active material, and the preparation of the lithium ion battery of this comparative example is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the different carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this comparative example are strictly consistent with the corresponding contents in Example 1.

[0175] 3. Raman spectroscopy test

[0176] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this comparative example is strictly consistent with the corresponding content in Example 1.

[0177] Comparative Example 3

[0178] 1. Preparation of carbon layer coated lithium iron phosphate

[0179] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 8.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0180] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 10 g glucose, and 10 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2.5 h to obtain a precursor slurry. The precursor slurry was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 715°C for 8 h to prepare carbon layer coated lithium iron phosphate.

[0181] The carbon layer-coated lithium iron phosphate prepared in this comparative example is used as the positive electrode active material, and the preparation of the lithium ion battery of this comparative example is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the different carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this comparative example are strictly consistent with the corresponding contents in Example 1.

[0182] 3. Raman spectroscopy test

[0183] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this comparative example is strictly consistent with the corresponding content in Example 1.

[0184] Comparative Example 4

[0185] 1. Preparation of carbon layer coated lithium iron phosphate

[0186] S1. Preparation of FePO4 precursor: Dissolve 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) in 100 mL of deionized water, add 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide and 8.5 mL of furfuryl alcohol monomer, and react at 70°C for 3 h. After the reaction is completed, add 0.1 mol / L NaOH solution to adjust the pH to 1.5. After stirring at room temperature for 2 h, filter, wash, dry at 90°C for 5 h, and grind to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0187] S2. Preparation of carbon layer coated lithium iron phosphate: The polyfurfuryl alcohol-coated FePO4·2H2O was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4, and the obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 10 g glucose, and 9 g polyethylene glycol (PEG) were placed in a ball mill, and 200 mL of anhydrous ethanol was added as a dispersant. The precursor slurry was ground with 2 mm zirconium balls for 2.5 h to obtain a precursor slurry. The precursor slurry was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 720°C for 8 h to prepare carbon layer coated lithium iron phosphate.

[0188] The carbon layer-coated lithium iron phosphate prepared in this comparative example is used as the positive electrode active material, and the preparation of the lithium ion battery of this comparative example is completed with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the different carbon layer-coated lithium iron phosphate used as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this comparative example are strictly consistent with the corresponding contents in Example 1.

[0189] 3. Raman spectroscopy test

[0190] The Raman spectrum test of the carbon layer-coated lithium iron phosphate positive electrode active material prepared in this comparative example is strictly consistent with the corresponding content in Example 1.

[0191] Comparative Example 5

[0192] 1. Preparation of carbon layer coated lithium manganese iron phosphate

[0193] S1. Preparation of FePO4 precursor: 0.965 mol of ferrous sulfate heptahydrate (FeSO4·7H2O) was dissolved in 100 mL of deionized water, and 100 mL of 10 mol / L phosphoric acid solution, 100 mL of 30% (mass fraction) hydrogen peroxide, and 7.5 mL of furfuryl alcohol monomer were added. The mixture was reacted at 70°C for 3 h. After the reaction was completed, 0.1 mol / L NaOH solution was added to adjust the pH to 1.5. After stirring at room temperature for 2 h, the mixture was filtered, washed, dried at 90°C for 5 h, and then ground to obtain polyfurfuryl alcohol-coated FePO4·2H2O.

[0194] S2. Preparation of carbon layer coated lithium manganese iron phosphate: FePO4·2H2O coated with polyfurfuryl alcohol was dehydrated at 600°C in a box-type resistance furnace for 6 h to obtain polyfurfuryl alcohol-coated FePO4. The obtained polyfurfuryl alcohol-coated anhydrous FePO4, 0.308 mol lithium acetate, 0.708 mol lithium hydroxide, 7 g glucose, 7 g polyethylene glycol (PEG), and 20 g MnO2 were placed in a ball mill, 200 mL of anhydrous ethanol was added as a dispersant, and the mixture was ground with 2 mm zirconium balls for 2 h to obtain a precursor slurry. The precursor slurry was placed in a microwave oven to dry, and then transferred to a tubular resistance furnace protected by N2 atmosphere and sintered at 695°C for 6.5 h to prepare carbon layer coated lithium manganese iron phosphate.

[0195] The carbon layer coated lithium manganese iron phosphate prepared in this comparative example D / I G The value is 0.91, the carbon layer thickness is 3.6nm, and D50 is 1.22μm.

[0196] 2. Preparation of lithium-ion batteries

[0197] The carbon layer-coated lithium manganese iron phosphate prepared in the comparative example was used as the positive electrode active material, and the lithium ion battery of this example was prepared with reference to the scheme for preparing the lithium ion battery adopted in Example 1. Except for the carbon layer-coated lithium manganese iron phosphate used in this comparative example as the positive electrode active material, the other materials, proportions, and operations used to prepare the lithium ion battery in this comparative example were strictly consistent with the corresponding contents in Example 1.

[0198] 3. Raman spectroscopy test

[0199] The Raman spectrum test of the carbon layer coated lithium manganese iron phosphate positive electrode active material prepared in this comparative example is strictly consistent with the corresponding content in Example 1.

[0200] Carbon layer coated lithium iron phosphate prepared in Examples 1 to 16 and Comparative Examples 1 to 5 D / I G , carbon layer thickness and D50 are shown in Table 1 below.

[0201] Table 1

[0202] Group ID / IG of carbon layer coated lithium iron phosphate Carbon layer thickness of carbon layer coated lithium iron phosphate (nm) D50 of carbon layer coated lithium iron phosphate (μm) Example 1 0.90 3.5 1.20 Example 2 0.97 3.01.04 Example 3 0.85 3.81.28 Example 4 0.89 4.11.26 Example 5 0.91 3.21.14 Example 6 1.13 3.91.32 Example 7 0.81 2.91.15 Example 8 1.06 4.21.25 Example 9 0.82 2.71.09 Example 10 0.79 4.6 1.35 Example 11 1.05 3.10.98 Example 12 1.21 2.30.87 Example 13 0.96 7.21.56 Example 14 1.17 1.60.81 Example 15 1.02 5.11.67 Example 16 0.93 2.30.69 Comparative Example 10.69 4.31.43 Comparative Example 2 0.62 4.11.37 Comparative Example 3 1.29 3.91.52 Comparative Example 4 1.32 3.31.48 Comparative Example 5 0.91 3.61.22

[0203] Performance Testing

[0204] (1) Charge transfer resistance (Rct) test:

[0205] Initial electrochemical impedance spectroscopy (EIS) tests were performed using a Reference 600 electrochemical workstation manufactured by GAMRY Instruments, Inc., USA. Lithium metal was used as the counter and reference electrodes, and the active material electrode was used as the working electrode. The tests were conducted at 28°C, with a scan frequency of 100 kHz to 0.01 Hz and a scan amplitude of 5 mV. The EIS of the lithium iron phosphate cathode material was used to investigate its electrochemical behavior, including charge transfer impedance and lithium ion diffusion rate. Li . D Li The following formula can be used for calculation:

[0206]

[0207]

[0208] T--absolute temperature (K)

[0209] A - surface area of ​​positive electrode material (m 2 )

[0210] n--the number of electrons transferred per mole of active material

[0211] F--Faraday constant (96485.3383±0.0083C / mol)

[0212] C--lithium ion concentration (mol / L)

[0213] σ--Warburg impedance coefficient

[0214] The relationship between the real part of electrochemical impedance and σ is:

[0215]

[0216] where R Ω is the ohmic impedance, Rct is the charge transfer impedance, R w The Warburg impedance coefficient σ is numerically equal to the slope of the real part of the electrochemical impedance with respect to the inverse square root of the diagonal frequency.

[0217] (2) First discharge capacity test

[0218] The prepared button cells were then charged and discharged using a lithium-ion battery charge and discharge test system at -20±0.5°C. The charge cutoff voltage was 3.75V, the discharge cutoff voltage was 2.00V, and the charge and discharge current density was 2C (nominal specific capacity was 150mAh / g).

[0219] The performance test results are shown in Table 2.

[0220] Table 2

[0221] Group Rct (Ω) Li+ diffusion coefficient (cm2 / s) -20℃ 2C first discharge capacity in grams (mAh / g) Example 1 82.99.33×10-1188.7 Example 2 84.38.76×10-1187.4 Example 3 89.15.21×10-1183.4 Example 4 84.28.93×10-1187.8 Example 5 85.37.88×10-1186.3 Example 6 99.99.78×10-1276.7 Example 7 87.35.66×10-1184.9 Example 8 96.51.69×10-1178.3 Example 9 89.35.05×10-1182.9 Example 10 122.33.18×10-1267.2 Example 11 126. 51.31×10-1264.9 Example 12123.62.85×10-1266.5 Example 1394.53.11×10-1181.2 Example 14125.62.13×10-1265.7 Example 15133.69.52×10-1362.8 Example 1695.72.51×10-1179.5 Comparative Example 1143.47.72×10-1358.2 Comparative Example 2136.78.97×10-1361.9 Comparative Example 3227.11.04×10-1342.6 Comparative Example 4153.46.75×10-1354.4 Comparative Example 5177.14.13×10-1348.9

[0222] From Table 1 and Table 2, we can see that:

[0223] 1. Carbon layer-coated lithium iron phosphate prepared in Examples 1 to 5, 7, 9, 13 and 16 D / I G The value is in the more preferred range of 0.8 to 1.0, and its charge transfer resistance (Rct) and lithium ion diffusion rate D Li The 2C first discharge gram capacity at low temperature of -20℃ is better than that of other embodiments. The above results fully verify that the I D / I G The value is 0.8~1.0, I D / I G The value is within this range, which can further improve the Li + The desolvation rate at the interface between the electrode and the electrolyte enhances the desolvation ability, thereby improving the conductivity and rate performance of lithium iron phosphate and improving the low-temperature performance of lithium iron phosphate.

[0224] 2. Compared with Example 2, the carbon layer thickness of the carbon layer coated lithium iron phosphate prepared in Example 13 is 7.2 nm, which is thicker. The charge transfer resistance (Rct) and lithium ion diffusion rate D of Example 13 are Li The gram capacity of the first discharge at 2C at low temperature of -20°C is not as good as that of Example 2, but it can also achieve the beneficial effects of the present application. Compared with Example 6, the carbon layer thickness of the carbon layer coated lithium iron phosphate prepared in Example 14 is 1.6nm, the carbon layer thickness is thinner, and the carbon coating effect is not as good as that of Example 6. The charge transfer resistance (Rct) and lithium ion diffusion rate D of Example 14 are Li The gram capacity of the first discharge at 2C at a low temperature of -20°C is lower than that of Example 6, but the beneficial effects of the present application can also be achieved. The above results fully verify that the carbon layer thickness of 2-6nm can not only well coat the lithium iron phosphate, reduce the polarization phenomenon during the migration process, and play a certain shielding role, thereby improving the stability of the battery, but also shorten the lithium ion transmission channel, which is conducive to further improving the rate performance of the lithium ion battery.

[0225] 3. Compared with Example 11, the D50 of the carbon layer coated lithium iron phosphate prepared in Example 15 is 1.67 μm, which is larger. The charge transfer resistance (Rct) and lithium ion diffusion rate D of Example 15 are LiThe gram capacity of the first discharge at 2C at low temperature of -20°C is not as good as that of Example 11, but it can also achieve the beneficial effects of the present application. Compared with Example 5, the D50 of the carbon layer coated lithium iron phosphate prepared in Example 16 is 0.69μm, which is smaller. The charge transfer resistance (Rct) and lithium ion diffusion rate D of Example 15 are Li The gram capacity of the first discharge at 2C at low temperature -20℃ is not as good as that of Example 11, but it can also achieve the beneficial effects of the present application. The above results fully verify that when D50 is in the range of 0.8~1.6μm, the lithium ion migration channel is short, shortening the Li + The diffusion distance can further improve the rate performance of lithium-ion batteries, while ensuring a sufficient particle size so that the positive electrode active material is not easy to form soft agglomerates, avoiding network blockage during processing, and further improving the rate performance of lithium-ion batteries.

[0226] 4. Carbon layer coated lithium iron phosphate prepared in Comparative Examples 1 and 2 D / I G The value is less than the range of 0.75 to 1.2 in the present application. The carbon layer coated lithium iron phosphate prepared in Comparative Examples 3 to 4 has an I D / I G The value is greater than the range of 0.75 to 1.2 of the present application. The charge transfer resistance (Rct) and lithium ion diffusion rate D of Comparative Examples 1 to 4 Li The 2C first discharge capacity at low temperature of -20°C is significantly lower than that of Examples 1 to 16, and cannot achieve the performance of the present invention in promoting Li + The transfer of carbon layer at the interface between the electrode and the electrolyte improves the conductivity of LiFePO4, thereby improving the rate performance and improving the low temperature performance of the lithium iron phosphate cathode material. The above results fully verify that the carbon layer coated lithium iron phosphate of this application has the following advantages: D / I G is 0.75~1.2, which can increase Li + The desolvation rate at the interface between the electrode and the electrolyte enhances the desolvation ability, thereby improving the conductivity and rate performance of lithium iron phosphate and improving the low-temperature performance of lithium iron phosphate.

[0227] 5. Carbon layer coated lithium manganese iron phosphate prepared in Comparative Example 5 D / I G The value is 0.91, the thickness of the carbon layer is 3.6nm, and D50 is 1.22μm, which is similar to the carbon layer coated lithium iron phosphate prepared in Example 1. D / I G The value is 0.90, the carbon layer thickness is 3.5nm, and D50 is 1.20μm, which is similar. However, the charge transfer resistance (Rct) and lithium ion diffusion rate D of comparative example 5 are LiThe 2C first discharge capacity at low temperature of -20℃ is significantly lower than that of Example 1, and is the worst among all the examples and comparative examples. + In order to increase the transfer rate at the interface between the electrode and the electrolyte, improve the conductivity of lithium iron phosphate, thereby improving the rate performance and improving the low temperature performance of lithium iron phosphate, it is necessary to satisfy the requirement that the carbon layer is coated with lithium iron phosphate material, and its I D / I G The beneficial effects of the present application can be achieved only when the ratio is between 0.75 and 1.2.

Claims

1. A positive electrode active material, the positive electrode active material being lithium iron phosphate coated with a carbon layer, and the I D / I G value of the positive electrode active material is 0.75 to 1.2, wherein, In the Raman spectrum of the positive electrode active material, the peak intensity at a wavenumber of 1360 cm -1 is I D , and the peak intensity at a wavenumber of 1580 cm -1 is I G .

2. The positive electrode active material according to claim 1, wherein, The I of the positive electrode active material D / I G value is 0.8 to 1.

0.

3. The positive electrode active material according to claim 1, wherein The thickness of the carbon layer is 2-6 nm.

4. The positive electrode active material according to claim 1, wherein The D50 of the positive electrode active material is 0.8-1.6 μm.

5. The positive electrode active material according to claim 1, wherein The preparation method of the positive electrode active material includes the following steps: mixing and ball-milling an FePO4 precursor, a lithium source, a carbon source, and a dispersant, and sintering to obtain the positive electrode active material.

6. The positive electrode active material according to claim 5, wherein, The FePO4 precursor includes at least one of FePO4 and FePO4 coated with a carbon source.

7. The positive electrode active material according to claim 5, wherein The carbon source includes at least one of glucose and polyethylene glycol.

8. The positive electrode active material according to claim 7, wherein, The carbon source includes the glucose and the polyethylene glycol, and the mass ratio of the glucose to the polyethylene glycol is (0.6-1.2):

1.

9. The positive electrode active material according to claim 5, wherein The temperature of the sintering is 680-720 °C.

10. A lithium-ion battery, comprising the positive electrode active material according to any one of claims 1-9.

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