Lithium iron phosphate material and preparation method thereof, and lithium ion battery

Abstract

The present invention belongs to the technical field of lithium-ion battery positive electrode materials, and specifically relates to a lithium iron phosphate material, a preparation method thereof, and a lithium-ion battery. The preparation method of the lithium iron phosphate material comprises the following steps: S1, adding graphene oxide, activated carbon, and cassava flour to water to react; after the reaction is complete, filtering, drying, and sintering to obtain a reduced graphene oxide-carbon composite material; S2, mixing the lithium iron phosphate with the reduced graphene oxide-carbon composite material to obtain a reduced graphene oxide-carbon-coated lithium iron phosphate material. Using the reduced graphene oxide-carbon-coated lithium iron phosphate material prepared by the present invention as the positive electrode material of a lithium iron phosphate battery effectively improves the conductivity, discharge specific capacity, and cycle performance of the lithium iron phosphate battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery positive electrode materials, and specifically relates to a lithium iron phosphate material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] With the development of new energy technologies, there is an urgent need for lithium-ion batteries with high capacity, high cycle life, high energy density, and high rate capability. Lithium iron phosphate (LIFP) has attracted considerable attention as a cathode material due to its environmental friendliness, cost-effectiveness, excellent cycle life, inherent stability, and relatively high capacity. However, LFP, with its olivine structure, has a low practical specific capacity and poor electron and ion conductivity, significantly limiting its application.

[0003] In the existing technology, the performance of lithium iron phosphate is improved by carbon coating. Carbon coating of lithium iron phosphate can increase the electron mobility during the battery charging and discharging process, and improve the discharge capacity and cycle performance of lithium iron phosphate batteries. However, the carbon coating technology used in traditional processes has no obvious effect on improving the conductivity, discharge capacity and cycle performance of lithium iron phosphate batteries. Summary of the Invention

[0004] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a lithium iron phosphate material and a preparation method thereof, and a lithium-ion battery. The reduced graphene oxide-carbon-coated lithium iron phosphate material prepared by the present invention is used as the positive electrode material of the lithium iron phosphate battery, which effectively improves the conductivity, discharge specific capacity and cycle performance of the lithium iron phosphate battery.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A first aspect of the present invention provides a method for preparing a lithium iron phosphate material, comprising the following steps:

[0007] S1, adding graphene oxide, activated carbon and cassava flour to water to react, and after the reaction is completed, filtering, drying and sintering to obtain a reduced graphene oxide-carbon composite material;

[0008] S2. Mixing lithium iron phosphate with the reduced graphene oxide-carbon composite material to obtain a reduced graphene oxide-carbon-coated lithium iron phosphate material.

[0009] The reduced graphene oxide-carbon-coated lithium iron phosphate material prepared by the present invention is used as the positive electrode material of the lithium iron phosphate battery, which effectively improves the conductivity, discharge specific capacity and cycle performance of the lithium iron phosphate battery.

[0010] The applicant speculates that the reason may be that cassava flour can cause stronger intermolecular cross-linking between the small polar functional groups in graphene oxide and activated carbon, so that the activated carbon and graphene oxide are better connected, and a complete reduced graphene oxide-carbon material is formed on the surface of lithium iron phosphate. As a result, when the reduced graphene oxide-carbon-coated lithium iron phosphate material is used as the positive electrode material of the lithium iron phosphate battery, the conductivity, discharge specific capacity and cycle performance of the lithium iron phosphate battery are effectively improved.

[0011] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, the water is deionized water.

[0012] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, in step S2, the mixing adopts a dual planetary stirring device, the revolution speed of the dual planetary stirring device is 20 to 40 rpm (for example, it can be 20 rpm, 22 rpm, 24 rpm, 26 rpm, 28 rpm, 30 rpm, 32 rpm, 36 rpm or 40 rpm, etc.), and the dispersion speed is 450 to 550 rpm (for example, it can be 450 rpm, 480 rpm, 510 rpm, 530 rpm or 550 rpm, etc.).

[0013] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, in step S2, the mixing time is 20 to 40 minutes, for example, the mixing time can be 20 minutes, 22 minutes, 24 minutes, 26 minutes, 28 minutes, 30 minutes, 35 minutes or 40 minutes.

[0014] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, the mass ratio of the graphene oxide, activated carbon, and cassava flour is (0.3-0.4): (100-110): (2-8), for example, it can be 0.3:100:2, 0.38:100:6, 0.38:100:5 or 0.38:100:7, etc.

[0015] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, the particle size of the activated carbon is 50-75 μm (screened through a 200-300 mesh sieve), for example, it can be 50 μm, 55 μm, 60 μm, 65 μm, 70 μm or 75 μm, etc.

[0016] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, in step S1, the reaction temperature is 60°C to 100°C, for example, the reaction temperature can be 60°C, 70°C, 80°C, 90°C or 100°C, etc. At this temperature, it is conducive to the uniform dispersion of graphene oxide.

[0017] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, in step S1, the drying temperature is 80℃~120℃, for example, it can be 80℃, 90℃, 100℃, 110℃ or 120℃, etc., and the drying time is 5~20h, for example, it can be 5h, 10h, 12h, 14h, 16h, 18h or 20h, etc.

[0018] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, the sintering temperature is 600-900°C, for example, the sintering temperature can be 600°C, 700°C, 800°C or 900°C, etc., the sintering time is 1 to 5h, for example, it can be 1h, 2h, 3h, 4h or 5h, etc., and the sintering atmosphere is an inert gas atmosphere.

[0019] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, the inert gas is nitrogen or argon.

[0020] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, in step S2, the mass ratio of the reduced graphene oxide-carbon composite material to the lithium iron phosphate is (0.5-2):100, for example, it can be 0.5:100, 1:100, 1.5:100 or 2:100, etc.

[0021] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, the graphene oxide is prepared by Hummers' modified method, which includes the following steps: pre-oxidizing and oxidizing graphite, centrifuging, washing, and drying to obtain the graphene oxide.

[0022] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, the pre-oxidation step is to add graphite to concentrated H2SO4 under ice-water bath conditions to react and obtain pre-oxidized graphite.

[0023] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, in the pre-oxidation step, the volume ratio of the mass of the graphite to the concentrated H2SO4 is 2: (80~100) g / mL, for example, it can be 2:80g / mL, 2:90g / mL or 2:100g / mL, etc.

[0024] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, the oxidation step is to add an oxidant to the pre-oxidized graphite under ice-water bath conditions, mix them evenly, and then react the pre-oxidized graphite with the oxidant at 40-60°C (for example, it can be 40°C, 45°C, 50°C, 55°C or 60°C, etc.). After the reaction is completed, a terminator is added to terminate the reaction, wherein the oxidant is potassium permanganate and the terminator is deionized water and hydrogen peroxide.

[0025] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, the mass ratio of the graphite to the oxidant is 2:(10-15), for example, it can be 2:10, 2:11, 2:12, 2:13 or 2:15.

[0026] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, in the oxidation step, the pre-oxidized graphite is reacted with the oxidant at 40-60°C (for example, 40°C, 45°C, 50°C, 55°C or 60°C, etc.) for 1-3h (for example, 1h, 2h or 3h, etc.).

[0027] In the above-mentioned method for preparing lithium iron phosphate material, as a preferred embodiment, in the oxidation step, after the reaction is completed, a terminator is added at 80-100° C. to terminate the reaction.

[0028] In the above-mentioned preparation method of lithium iron phosphate material, as a preferred embodiment, the washing is ultrasonic cleaning, and the washing step is washing with deionized water and HCL (5% volume fraction) until neutral.

[0029] A second aspect of the present invention provides a lithium iron phosphate material, which is prepared by the preparation method of the lithium iron phosphate material provided by the first aspect.

[0030] A third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet is the lithium iron phosphate material provided in the second aspect. The preparation method of the positive electrode sheet is as follows:

[0031] A binder (such as polyvinylidene fluoride (PVDF)), a conductive agent (such as conductive carbon black SP) and an organic solvent (N-methylpyrrolidone) are mixed together and stirred to obtain a mixed slurry. Subsequently, reduced graphene oxide-carbon-coated lithium iron phosphate material is added to the mixed slurry, stirred and dispersed to obtain a positive electrode slurry. The positive electrode slurry is coated on the surface of an aluminum foil current collector and rolled to obtain a positive electrode sheet.

[0032] The positive electrode sheet prepared by the present invention has low resistance and excellent conductivity. The lithium ion battery provided by the present invention has high discharge specific capacity and excellent cycle performance.

[0033] Compared with the prior art, the present invention has at least one of the following advantages:

[0034] 1) The reduced graphene oxide-carbon-coated lithium iron phosphate material prepared by the present invention is used as the positive electrode material of the lithium iron phosphate battery, which effectively improves the conductivity, discharge specific capacity and cycle performance of the lithium iron phosphate battery.

[0035] 2) The present invention prepares reduced graphene oxide-carbon-coated lithium iron phosphate material through a reduced graphene oxide-carbon-coated lithium iron phosphate process, and uses the prepared material to prepare a positive electrode slurry, which is then prepared into a positive electrode sheet, thereby optimizing the conductivity of the slurry and the electrode sheet and increasing the migration rate of lithium ions.

[0036] 3) By modifying the lithium iron phosphate positive electrode and using graphene as the conductive network structure between the lithium iron phosphate particles, the overall conductivity of the lithium iron phosphate is improved, thereby improving the subsequent battery cycle capacity and capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a schematic flow chart of the preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided by the present invention. DETAILED DESCRIPTION

[0038] In order to further understand the present invention, the preferred embodiments of the present invention are described below in conjunction with examples. The scope of protection of this application includes but is not limited to the following examples. The following examples are only used to illustrate the advantages and effects of the technical solutions of this application and do not constitute a limitation on the scope of protection of this application. Equivalent substitutions made by those skilled in the art based on this application are all within the scope of protection of this application.

[0039] Unless otherwise defined, the technical terms used in the following examples have the same meanings as commonly understood by those skilled in the art to which this invention belongs. The experimental reagents used in the following examples, unless otherwise specified, are conventional biochemical reagents; the experimental reagent amounts, unless otherwise specified, are those used in routine experimental procedures; and the experimental methods, unless otherwise specified, are conventional methods.

[0040] Figure 1 The schematic diagram of the process for preparing the reduced graphene oxide-carbon-coated lithium iron phosphate material provided by the present invention is as follows: Figure 1 As shown, a first aspect of an embodiment of the present invention provides a method for preparing a reduced graphene oxide-carbon-coated lithium iron phosphate material, comprising the following steps:

[0041] S0. Graphene oxide is prepared by a Hummers improved method, the Hummers improved method comprising the following steps: pre-oxidizing and oxidizing graphite, centrifugally separating, washing, and drying to obtain the graphene oxide, wherein the pre-oxidation step is to add graphite to concentrated H2SO4 under ice-water bath conditions to react to obtain pre-oxidized graphite, and the volume ratio of the mass of the graphite to the concentrated H2SO4 is 2:(80-100) g / mL; the oxidation step is to add an oxidant to the pre-oxidized graphite under ice-water bath conditions, mix them uniformly, and then react the pre-oxidized graphite with the oxidant at 40-60°C for 1-3h. After the reaction is completed, a terminator is added at 80-100°C to terminate the reaction, wherein the oxidant is potassium permanganate, the terminator is deionized water and hydrogen peroxide, the mass ratio of the graphite to the oxidant is 2:(10-15), the washing is ultrasonic cleaning, and the washing step is washing with deionized water and HCL (5% volume fraction) until neutral.

[0042] S1. Add graphene oxide, activated carbon and cassava flour to deionized water for reaction. After the reaction is completed, filter, dry and sinter to obtain a reduced graphene oxide-carbon composite material, wherein the mass ratio of the graphene oxide, activated carbon and cassava flour is (0.3-0.4): (100-110): (2-8), the particle size of the activated carbon is 50-75 μm, the reaction temperature is 60°C-100°C, the drying temperature is 80°C-120°C, the drying time is 5-20 h, the sintering temperature is 600-900°C, the sintering time is 1-5 h, and the sintering atmosphere is nitrogen or argon atmosphere.

[0043] S2. Mixing lithium iron phosphate with the reduced graphene oxide-carbon composite material for 20 to 40 minutes to obtain a reduced graphene oxide-carbon-coated lithium iron phosphate material, wherein the mixing adopts a dual planetary stirring device, the revolution speed of the dual planetary stirring device is 20 to 40 rpm, and the dispersion speed is 450 to 550 rpm; the mass ratio of the reduced graphene oxide-carbon composite material to the lithium iron phosphate is (0.5-2):100.

[0044] In the embodiment of the present invention, graphene oxide is prepared by a modified Hummers method. The graphene oxide is mixed with activated carbon and cassava flour, cooled, and sintered to obtain a reduced graphene oxide-carbon composite material. The composite material is mixed with lithium iron phosphate and used as a positive electrode active material to prepare a slurry, which can improve the conductivity, capacity, and cycle performance of the battery.

[0045] In the embodiment of the present invention, cassava flour is added, and the cassava flour interacts with graphene oxide and activated carbon to form a complete graphene-carbon material on the surface of lithium iron phosphate. As a result, when the reduced graphene oxide-carbon-coated lithium iron phosphate material is used as the positive electrode material of the lithium iron phosphate battery, the conductivity, discharge specific capacity and cycle performance of the lithium iron phosphate battery are effectively improved.

[0046] A second aspect of an embodiment of the present invention provides a reduced graphene oxide-carbon-coated lithium iron phosphate material, which is prepared by the preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided by the first aspect.

[0047] A third aspect of an embodiment of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet is the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in the second aspect. The preparation method of the positive electrode sheet is as follows:

[0048] A binder (such as polyvinylidene fluoride (PVDF)), a conductive agent (such as conductive carbon black SP) and an organic solvent (N-methylpyrrolidone) are mixed together and stirred to obtain a mixed slurry. Subsequently, reduced graphene oxide-carbon-coated lithium iron phosphate material is added to the mixed slurry, stirred and dispersed to obtain a positive electrode slurry. The positive electrode slurry is coated on the surface of an aluminum foil current collector and rolled to obtain a positive electrode sheet.

[0049] In order to further understand the present invention, the lithium iron phosphate material and its preparation method, and the lithium ion battery provided by the present invention are described in detail below with reference to the embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0050] In the following examples and comparative examples:

[0051] The graphene oxide solution was prepared by adding graphene oxide to deionized water to prepare a graphene oxide solution with a concentration of 0.5% (g / mL) (0.5 g of graphene oxide in 100 mL of the solution).

[0052] The preparation method of the cassava flour solution is as follows: 10 g of cassava flour is added into 100 mL of deionized water, dissolved, and then filtered to obtain a filtrate, wherein the filtrate is a cassava flour solution with a concentration of 0.1 g / mL.

[0053] Example 1

[0054] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this embodiment specifically includes the following steps:

[0055] S0, preparation of graphene oxide using Hummers improved method

[0056] 2 g of graphite was placed in a beaker containing 90 mL of concentrated sulfuric acid and stirred for five minutes. Subsequently, 12 g of potassium permanganate was gradually added to the beaker and the mixture was stirred for 15 minutes. The entire process was carried out in an ice-water bath with the temperature controlled to be no higher than 4° C. The beaker was then moved to a 45° C. constant temperature water bath and stirred for 1 hour. After the reaction was completed, 50 mL of deionized water was added and the mixture was placed in a 95° C. water bath and stirred. After 15 minutes, another 50 mL of deionized water was added. 20 mL of H2O2 (hydrogen peroxide, concentration of 30%) was then added to the beaker, the mixture was stirred thoroughly, and the mixture was reacted for 30 minutes to obtain a mixture.

[0057] The mixture was centrifuged, and ultrasonically cleaned with HCl (5% volume fraction) and deionized water to remove impurities. The pH value of the mixture was adjusted to 7, and then dried at room temperature to obtain dry graphene oxide.

[0058] S1. Preparation of reduced graphene oxide and activated carbon composite materials

[0059] A 0.5% graphene oxide solution (3.8 mL) was mixed in 200 mL of deionized water, heated in a water bath at 90°C, stirred for 10 min, 5 g (200 mesh powder) of activated carbon was added, and stirred for 10 min; then, 3 mL of cassava flour solution (concentration of 0.1 g / mL) was added, and a black precipitate was formed after 1 min. After the precipitate was formed, it was cooled to room temperature. After the reagent was cooled to room temperature, it was filtered, and the collected filtrate (solid) was dried at 100°C for 10 h, and finally sintered under nitrogen conditions at 800°C for 2 h to obtain a reduced graphene oxide-carbon composite material.

[0060] S2. Preparation of reduced graphene oxide-carbon-coated lithium iron phosphate material

[0061] The reduced graphene oxide-carbon composite material prepared in step S1 was mixed with lithium iron phosphate for 30 minutes to obtain a reduced graphene oxide-carbon-coated lithium iron phosphate material. The mass ratio of the reduced graphene oxide-carbon composite material to the lithium iron phosphate was 1.5:100. The mixing was performed using a dual planetary stirring device with an orbital speed of 30 rpm and a dispersion speed of 500 rpm.

[0062] Example 2

[0063] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this embodiment is basically the same as that in Example 1, except that, in step S2, the mass ratio of the reduced graphene oxide-carbon composite material to the lithium iron phosphate is 0.5:100.

[0064] Example 3

[0065] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this embodiment is basically the same as that in Example 1, except that 0.3 g of cassava flour is added in step S1, and specifically comprises the following steps:

[0066] S0, preparation of graphene oxide using Hummers improved method

[0067] Exactly the same as step S0 in Example 1.

[0068] S1. Preparation of reduced graphene oxide and activated carbon composite materials

[0069] A 0.5% graphene oxide solution (3.8 mL) was mixed in 200 mL of deionized water, heated in a water bath at 90°C, stirred for 10 min, 5 g (200 mesh powder) of activated carbon was added, and stirred for 10 min; then, 0.3 g of cassava flour was added, and a black precipitate was formed after 1 min. After the precipitate was formed, it was cooled to room temperature. After the reagent was cooled to room temperature, it was filtered, and the collected filtrate (solid) was dried at 100°C for 10 h, and finally sintered under nitrogen conditions at 800°C for 2 h to obtain a reduced graphene oxide-carbon composite material.

[0070] S2. Preparation of reduced graphene oxide-carbon-coated lithium iron phosphate material

[0071] Exactly the same as step S2 in Example 1.

[0072] Comparative Example 1

[0073] Lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate were weighed and prepared in a molar ratio of 1:1:1 between lithium, iron, and phosphorus. Glucose (20% of the theoretical mass of the lithium iron phosphate) was then weighed and mixed. The resulting solid powder was placed in a ball mill, and an appropriate amount of anhydrous ethanol was added as an additive. The milling speed was set at 3600 rpm and the milling time was 8 hours. The milled rheological body was dried in a vacuum drying oven at 80°C for 10 hours. The dried powder was ground into a uniform fine powder in a mortar and pestle. The powder was then placed in a high-temperature tube furnace and heated at 300°C for 3 hours under a nitrogen atmosphere. The temperature was then raised to 750°C and sintered for 10 hours to produce carbon-coated lithium iron phosphate.

[0074] Comparative Example 2

[0075] No treatment was done on the lithium iron phosphate.

[0076] Comparative Example 3

[0077] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this comparative example is basically the same as that in Example 1, except that "5g (200 mesh powdered) activated carbon" is replaced by "13g glucose".

[0078] Comparative Example 4

[0079] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this comparative example is basically the same as that of comparative example 3, except that no cassava flour is added in step S1.

[0080] Comparative Example 5

[0081] The preparation method of the reduced graphene oxide-carbon-coated lithium iron phosphate material provided in this comparative example is basically the same as that in Example 1, except for the timing of adding cassava flour, and specifically comprises the following steps:

[0082] S0, preparation of graphene oxide using Hummers improved method

[0083] Exactly the same as step S0 in Example 1.

[0084] S1. Preparation of reduced graphene oxide and activated carbon composite materials

[0085] A 0.5% graphene oxide solution (3.8 mL) was mixed in 200 mL of deionized water, heated in a water bath at 90 ° C, stirred for 10 min, 5 g (200 mesh powder) of activated carbon was added, and stirred for 10 min. After the reagent was cooled to room temperature, it was centrifuged and filtered. The collected filtrate (solid) was dried at 100 ° C for 10 h, and then sintered under nitrogen conditions at 800 ° C for 2 h. Finally, the sintered product was added to 200 mL of deionized water, heated in a water bath at 90 ° C, stirred for 10 min, and then 3 mL of cassava flour solution (concentration of 0.1 g / ml) was added. After standing for 5 min, it was cooled with room temperature (no black precipitate was produced). After the reagent was cooled to room temperature, it was centrifuged and filtered. The collected filtrate (solid) was dried at 100 ° C for 10 h to obtain a reduced graphene oxide-carbon composite material.

[0086] S2. Preparation of reduced graphene oxide-carbon-coated lithium iron phosphate material

[0087] Exactly the same as step S2 in Example 1.

[0088] Performance Testing

[0089] The lithium iron phosphate materials prepared in the above Examples 1-3 and Comparative Examples 1-5 were used as positive electrode materials to prepare lithium iron phosphate positive electrode sheets, and their membrane resistance was measured. Half-cells were made with metal lithium negative electrodes, and their discharge specific capacities and cycle performances were tested and compared.

[0090] PVDF, SP and N-methylpyrrolidone are mixed together and stirred to obtain a mixed slurry, and then the above-mentioned lithium iron phosphate (LFP) material is added to the mixed slurry, stirred and dispersed to obtain a positive electrode slurry. The solid content of the positive electrode slurry is ensured to be about 48%. The positive electrode slurry is coated on the surface of the aluminum foil current collector and rolled to obtain a lithium iron phosphate positive electrode sheet, wherein the mass ratio of each raw material is: LFP (the above-mentioned lithium iron phosphate material): SP: PVDF = 94:3:3.

[0091] A lithium iron phosphate positive electrode and a metallic lithium negative electrode are made into a half-cell.

[0092] The battery was prepared using lithium iron phosphate as the positive electrode, metallic lithium as the negative electrode, 1 mol / L LiPF6 as the electrolyte, a 1:1 (volume ratio) mixed solvent of EC and DMC, and Celgard 2400 as the separator.

[0093] Lithium iron phosphate positive electrode sheet resistance test: The four-probe test principle is used to test the diaphragm resistance with a two-probe resistance tester. The electrode is cut into a square size of 40*80mm, and then the electrode is placed under the two probes. The two probes are connected to the resistance meter through two poles. The handle of the test device is turned, and the probe is subjected to a stable pressure to squeeze the electrode. The pressure is controlled by the pressure gauge. After reaching a certain pressure (about 2N of force), the resistance data of the resistance meter is read. This data is the diaphragm resistance.

[0094] The battery is tested for discharge specific capacity. The test conditions and steps are as follows:

[0095] Constant current charging, current intensity: 0.1C. Charging cut-off potential: 4.2V;

[0096] Constant current discharge, current intensity: 0.1C. Discharge cut-off potential: 2.5V.

[0097] The battery cycle performance is tested. The test conditions and steps are as follows:

[0098] (1) Constant current discharge, current intensity: 0.1C, discharge cut-off potential: 2.5V;

[0099] (2) Let stand: 5 min;

[0100] (3) Constant current charging, current intensity: 0.1C, charging cut-off potential: 4.2V;

[0101] (4) Let stand: 5 min;

[0102] (5) Return to step (1) and repeat the cycle 150 times.

[0103] The test results of the resistance of the positive electrode sheet, the initial discharge specific capacity of the battery, and the cycle performance are shown in Table 1. The capacity retention rate in Table 1 is the discharge specific capacity retention rate after 150 cycles.

[0104] Table 1

[0105]

[0106] It can be seen from Table 1 that using the lithium iron phosphate materials prepared in Examples 1 to 3 as the positive electrode materials of lithium iron phosphate batteries reduces the membrane resistance of the positive electrode sheet, and can significantly improve the conductivity, discharge specific capacity (the theoretical value is 170 mAh / g, and the higher the capacity, the more difficult it is to improve) and cycle performance of the lithium iron phosphate battery.

[0107] Comparative Example 1 uses a chemical synthesis method to modify lithium iron phosphate. Although this comparative example can improve the electrical performance of the battery, it is still inferior to Examples 1 to 3; the preparation method of Comparative Example 1 is complex, energy-intensive, and takes a long time to prepare. The present application achieves modification of lithium iron phosphate by physically mixing reduced graphene oxide-carbon composite materials with lithium iron phosphate. The operation method is simple, the preparation time is short, and the performance is even better in subsequent lithium-ion battery applications.

[0108] It can be seen from Comparative Example 3 that when "activated carbon" is replaced with "glucose" which can also serve as a carbon source, and the prepared lithium iron phosphate material is used as the positive electrode material of the lithium iron phosphate battery, the conductivity of the electrode, the discharge capacity and cycle capacity of the battery are not significantly improved, indicating that only by mixing the reduced graphene oxide-carbon composite material generated by the interaction of cassava flour, graphene oxide and activated carbon with lithium iron phosphate can the performance of the battery be significantly improved.

[0109] It can be seen from Comparative Example 4 that when cassava flour is not added to Comparative Example 3, although the performance of the battery cannot be significantly improved compared with Comparative Example 2, the performance is slightly improved compared with Comparative Example 3, indicating that cassava flour cannot play a role in the system where glucose and graphene oxide coexist to improve battery performance.

[0110] It can be seen from Comparative Example 5 that when the reduced graphene oxide-carbon composite material prepared by reducing graphene oxide and then adding cassava flour is mixed with lithium iron phosphate, the performance of the battery cannot be significantly improved.

[0111] The lithium iron phosphate material and its preparation method, as well as the lithium ion battery provided by the present invention are described in detail above. The description of the above embodiments is only intended to help understand the method and core concept of the present application.

[0112] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The following steps are involved: S1, adding graphene oxide, activated carbon and cassava flour to water to react, and after the reaction is completed, filtering, drying and sintering to obtain a reduced graphene oxide-carbon composite material; S2, mixing lithium iron phosphate with the reduced graphene oxide-carbon composite material to obtain a reduced graphene oxide-carbon-coated lithium iron phosphate material; The mass ratio of the graphene oxide, activated carbon, and cassava flour is (0.3-0.4): (100-110): (2-8); in step S1, the reaction temperature is 60°C to 100°C; in step S2, the mixing adopts a dual planetary stirring device, the revolution speed of the dual planetary stirring device is 20-40 rpm, and the dispersion speed is 450-550 rpm.

2. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S2, the mixing time is 20 to 40 minutes.

3. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The particle size of the activated carbon is 50-75 μm.

4. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S1, the drying temperature is 80° C. to 120° C., and the drying time is 5 to 20 hours.

5. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The sintering temperature is 600-900° C., the sintering time is 1-5 hours, and the sintering atmosphere is an inert gas atmosphere.

6. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S2, the mass ratio of the reduced graphene oxide-carbon composite material to the lithium iron phosphate is (0.5-2):

100.

7. The method for preparing lithium iron phosphate material according to claim 1, characterized in that: The graphene oxide is prepared by a Hummers improved method, which includes the following steps: pre-oxidizing and oxidizing graphite, centrifugally separating, washing, and drying to obtain the graphene oxide.

8. A lithium iron phosphate material, characterized in that: The lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material according to any one of claims 1 to 7.

9. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that: The active material of the positive electrode sheet is the lithium iron phosphate material according to claim 8.

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