CN105810911A - High-rate preparation method of lithium iron phosphate/graphene composite cathode material - Google Patents

Abstract

The invention relates to a preparation method of a lithium iron phosphate/graphene composite positive electrode material. The lithium iron phosphate/graphene composite is mainly obtained through a polymer-assisted one-step solvothermal reaction. The preparation method includes: (1) dispersing graphene and auxiliary polymer in a solvent to form a suspension, phosphoric acid and lithium hydroxide are separately prepared into solutions, and sequentially adding phosphoric acid and lithium hydroxide to the suspension. solution to obtain a lithium phosphate suspension attached to the surface of graphene; (2) add soluble ferrous salt and antioxidant to the above suspension, and then transfer it to the reaction kettle for solvothermal reaction; (3) after separation and washing After drying, the compound is mixed with an organic carbon source, and after high-temperature heat treatment in an inert atmosphere, a lithium iron phosphate-graphene composite positive electrode material is obtained. Through this method, lithium iron phosphate and graphene in the composite positive electrode material are fully composited, and the material has excellent rate performance, and can be used as a positive electrode material for lithium-ion power batteries.

Description

A high rate lithium iron phosphate / Preparation method of graphene composite cathode material

technical field

The invention belongs to the field of positive electrode materials for lithium ion batteries, and in particular relates to a preparation method for a high-rate lithium iron phosphate/graphene composite positive electrode material. The obtained lithium iron phosphate/graphene composite positive electrode material has high rate performance and can be widely used in lithium Ion power battery materials.

Background technique

Lithium-ion batteries are a new generation of energy storage devices, which have the advantages of high energy density, long cycle life, small self-discharge, and no memory effect. They have been widely used in portable electronic products such as notebook computers, mobile phones, and digital cameras. Lithium-ion batteries are composed of positive electrodes, negative electrodes, separators and electrolytes, among which the positive electrode material plays a decisive role in battery performance. Currently used cathode materials include layered LiCoO ₂ , spinel LiMnO ₂ and olivine LiFePO ₄ .

Compared with LiCoO ₂ and LiMnO ₂ , LiFePO ₄ has the advantages of high theoretical specific capacity (170 mAh/g), excellent cycle stability, high temperature thermal stability, low cost, and no environmental pollution, but its rate performance and Low temperature performance is relatively poor. The reasons for these problems are, on the one hand, the poor conductivity of LiFePO ₄ itself (~10 ^-9 S/cm), on the other hand, the crystal structure of LiFePO ₄ has obvious anisotropy, and lithium ions can only pass through one-dimensional along the b-axis direction. Channel migration and weak diffusion ability (the lithium ion migration rate in LiFePO ₄ is 1.8×10 ^-14 cm ² /S at room temperature, and the lithium ion migration rate in FePO ₄ is 2.2×10 ^-16 cm ² /S). These severely limit the application of LiFePO4 lithium _- ion batteries.

By adjusting the size and morphology of LiFePO ₄ particles, the diffusion rate of lithium ions can be increased, thereby improving the performance of LiFePO ₄ lithium-ion batteries. Patent CN101007630A discloses a method for preparing LiFePO ₄ with adjustable morphology. Submicron and nanoscale LiFePO ₄ particles can be prepared by solvothermal reaction by adding a crystal growth inhibitor. Patent CN101327920A discloses a flaky LiFePO ₄ nanocrystal powder and its preparation method, and obtains a flaky LiFePO ₄ nanocrystal with (020) orientation, which is beneficial to the extraction and insertion of lithium ions in the crystal. However, as the particle size of LiFePO ₄ decreases, on the one hand, the volumetric energy density of the battery will decrease (decrease in bulk density), and on the other hand, the conductivity and adhesion between LiFePO ₄ particles and the current collector will deteriorate, so it is necessary to use More conductive agents and binders.

Graphene is a two-dimensional crystal composed of sp2 hybridized carbon atoms, which has good electrical conductivity and large specific surface area. The excellent electrical properties of graphene make it suitable as a conductive additive. Patent CN101794874A discloses an electrode with graphene as a conductive additive and its application in lithium-ion batteries. The positive electrode sheet of the battery is prepared by directly adding graphene conductive agent, and the high-power charge-discharge performance, charge-discharge efficiency and cycle life of the battery are improved. Significantly increased. Patent CN101752561A improves battery performance by mixing graphene and LiFePO ₄ particles to obtain a compound, but direct mixing makes the combination of LiFePO ₄ particles and graphene weak, and the agglomeration of graphene itself cannot be avoided. Patents CN102044666A, CN102104143A, CN102299326A, CN103094564A and CN104779395A etc. grow LiFePO ₄ particles in situ on the surface of graphene by solvothermal method, which also cannot solve the problem of graphene self-aggregation under high temperature solvothermal conditions. Patent CN102751496A prepares Fe ₂ O ₃ /graphene composite as a precursor, and then obtains LiFePO ₄ /graphene composite through solvothermal reaction, but the process is cumbersome.

Contents of the invention

The purpose of the present invention is to provide a kind of preparation method of high rate lithium iron phosphate/graphene composite positive electrode material, this method can realize LiFePO ₄ and fully compound with graphene, the obtained lithium iron phosphate/graphene composite positive electrode material has higher rate performance. Specific steps are as follows:

(1) Disperse graphene and auxiliary polymer in a solvent to obtain a graphene dispersion, make phosphoric acid and lithium hydroxide into solutions respectively; add phosphoric acid solution and lithium hydroxide solution dropwise to the graphene dispersion to form Lithium phosphate suspension attached to the surface of graphene; the mass ratio of auxiliary polymer to graphene is (0.5~3):1; the mass ratio of phosphoric acid to graphene is (10~100):1, lithium hydroxide and phosphoric acid The molar ratio is (2.5~3): 1;

(2) Add soluble ferrous salts and antioxidants to the above-mentioned lithium phosphate suspension attached to the surface of graphene, and then transfer it to a solvothermal reaction kettle for reaction to obtain a lithium iron phosphate solution attached to the surface of graphene; control the solvent The thermal reaction temperature is 140-200°C, and the reaction time is 0.5-20 hours; the molar ratio of soluble ferrous salt to phosphoric acid described in step (1) is 1:1;

(3) The product obtained in step (2) is separated, washed and dried to obtain a lithium iron phosphate/graphene composite, and the obtained lithium iron phosphate/graphene composite is mixed with an organic carbon source, and heat-treated at high temperature under an inert atmosphere After that, a lithium iron phosphate/graphene composite positive electrode material is obtained.

In the present invention, the graphene is single-layer or few-layer graphene obtained by graphite exfoliation.

In the present invention, the auxiliary polymer is a polymer capable of accepting protons to form a salt, which is any one of linear (branched) polyethyleneimine or polyacrylamide.

In the present invention, the solvent described in step (1) is a mixed solvent of water and alcohol.

In the present invention, the alcohol is ethylene glycol or glycerol, and the volume ratio of water to alcohol is 1:0.1-10.

In the present invention, the soluble ferrous salt is any one of ferrous sulfate, ferrous chloride, ferrous acetate and their corresponding crystalline hydrates.

In the present invention, the antioxidant is any one of L-ascorbic acid, citric acid or glucose, and the dosage is 0-20 wt% of the mass of soluble ferrous salt.

In the present invention, the organic carbon source is any one of glucose, sucrose, polystyrene or phenolic resin, and the dosage is 0-20 wt% of the mass of the lithium iron phosphate/graphene composite.

In the present invention, the high-temperature heat treatment temperature in step (3) is 550-750° C., and the heat treatment time is 1-10 hours.

The present invention uses the auxiliary polymer to react with H ₃ PO ₄ to form a polyelectrolyte adsorbed on the surface of graphene, and then reacts with LiOH to form a Li ₃ PO ₄ precipitation on the surface of graphene, which hinders the self-aggregation of graphene under solvothermal conditions, and at the same time Facilitate in-situ growth of LiFePO ₄ particles. The assisted polymer decomposition under high _- temperature hydrothermal conditions further fully combined LiFePO particles with graphene.

_The present invention obtains LiFePO4 nanosheets with uniform size by means of solvothermal reaction, which has excellent lithium ion migration performance, and at the same time combines the excellent electrical conductivity of graphene and the full composite of LiFePO4 _nanosheets and graphene, and the obtained composite material has high The rate performance has broad application prospects in the field of lithium-ion power batteries.

Description of drawings

Fig. 1 is the XRD spectrum of the lithium iron phosphate/graphene composite that embodiment 1 obtains.

Figure 2 is the XRD spectrum of the compound obtained in Example 2.

Fig. 3 is the XRD spectrum of the lithium iron phosphate obtained in Example 3.

Fig. 4 is the FESEM picture of the lithium iron phosphate/graphene composite that embodiment 1 obtains.

FIG. 5 is a FESEM image of the complex obtained in Example 2.

FIG. 6 is a FESEM image of lithium iron phosphate obtained in Example 3.

Fig. 7 is the rate discharge curve obtained from the test of the battery made of the composite positive electrode material in Example 1.

Fig. 8 is the cycle discharge curve obtained under the test condition of 10 C rate of the battery made of composite cathode material in Example 1.

Fig. 9 is the rate discharge curve obtained from the test of the battery made of the compound of Example 2.

Figure 10 is the rate discharge curve obtained from the test of the battery made of lithium iron phosphate in Example 3.

Fig. 11 is the rate discharge curve obtained from the test of the battery made of the composite positive electrode material in Example 4.

detailed description

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific embodiments. The following examples are intended to illustrate the present invention, without any limitation to the content of the invention itself. It should be understood that the one or more steps mentioned in the present invention do not exclude the existence of other methods and steps before and after the combined steps, or other methods and steps may be inserted between the explicitly mentioned steps. It should also be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Unless otherwise stated, the numbering of each method step is only for the purpose of identifying each method step, rather than limiting the sequence of each method or limiting the scope of implementation of the present invention, the change or adjustment of its relative relationship, without substantial technical content Under the conditions of change, it should also be regarded as the scope that the present invention can be implemented.

Example 1

60 mL of ethylene glycol and 20 mL of deionized water were prepared as a mixed solvent, and 60 mg of graphene was dispersed into a 30 mL mixed solvent, and 60 mg of branched polyethyleneimine PEI with a molecular weight of 600 was added thereto, and ultrasonicated for 30 minutes. With stirring, a solution of 1.46 g of 85% H ₃ PO ₄ dissolved in 10 mL of solvent was added dropwise, and the mixture was stirred for 30 minutes to obtain a mixed solution A. Prepare solution B in which 1.44 g LiOH·H ₂ O is dissolved in 30 mL solvent. The mixed solution A was gradually dropped into the solution B, and the remaining solvent was added. After stirring for 30 minutes, 3.52 g FeSO ₄ ·7H ₂ O and 0.40 g L-ascorbic acid were added quickly. After 30 minutes of airtight stirring, it was transferred to a solvothermal reaction kettle and reacted at 200 ° C for 12 h. After cooling, wash three times with deionized water and ethanol respectively, centrifuge and dry. The obtained powder was mixed with 20 wt% glucose and sintered in a nitrogen-protected sealed tube furnace at 700 °C for 5 hours to obtain a lithium iron phosphate/graphene composite.

The lithium iron phosphate in the obtained composite can be seen as an olivine structure by powder X-ray diffraction, as shown in FIG. 1 . Field emission scanning electron microscopy (FESEM) shows that lithium iron phosphate in the composite is attached to the surface of graphene. The size of the particles is hundreds of nanometers in length and width, and tens of nanometers in thickness, as shown in Figure 4. The obtained composite is made into a CR2016 battery, the positive electrode is composed of active material: conductive agent SP: binder PVDF=8:1:1, and the metal lithium sheet is used as the negative electrode. The test voltage range is 2.5-3.8 V, and the temperature is 25°C. Figure 7 shows the rate discharge curve. At a low rate of 0.2 C, it has a high specific capacity of 153 mAh/g. As the rate increases, 10 C has a specific capacity of 138 mAh/g, and 20 C has a specific capacity of 135 mAh. /g, which is equivalent to being able to maintain 88% of the capacity at a low rate. At the same time, it has good cycle stability under high rate conditions. The discharge cycle at 10 C rate is shown in Figure 8, and 96% of the capacity can be retained after 300 cycles.

Example 2

60 mL of ethylene glycol and 20 mL of deionized water were prepared as a mixed solvent, and 60 mg of graphene was dispersed into 30 mL of the mixed solvent, to which 60 mg of branched polyethyleneimine PEI with a molecular weight of 600 was added, and ultrasonicated for 30 minutes. Add a solution of 1.44 g LiOH·H ₂ O dissolved in 30 mL of mixed solvent, stir and mix well. Add 1.46 g of 85% H ₃ PO ₄ dissolved in 10 mL of mixed solvent drop by drop under stirring, and add the remaining solvent. After stirring for 30 minutes, quickly add 3.52 g FeSO ₄ 7H ₂ O and 0.40 g L-ascorbic acid . After 30 minutes of airtight stirring, it was transferred to a solvothermal reaction kettle and reacted at 200 ° C for 12 h. After cooling, wash three times with deionized water and ethanol respectively, centrifuge and dry. The obtained powder was mixed with 20 wt% glucose and sintered in a nitrogen-protected sealed tube furnace at 700 °C for 5 hours to obtain a lithium iron phosphate/graphene composite.

The X-ray diffraction pattern of the composite is shown in Fig. 2 . The field emission scanning electron microscope is shown in Fig. 5. As a comparative example, compared with Example 1, the combination of graphene and lithium iron phosphate is not very tight, and the two will separate during the centrifugation process. It can also be seen from FESME that the lithium iron phosphate on the surface of graphene is relatively loose. The obtained compound was made into a CR2016 battery, and the conditions were the same as in Example 1. Figure 9 shows the rate discharge curve. The specific capacity is 80 mAh/g at 10 C, and 65 mAh/g at 20 C, which is not as good as the compound obtained in Example 1.

Example 3

60 mL of ethylene glycol and 20 mL of deionized water were used as a mixed solvent. Dissolve 1.44 g LiOH·H ₂ O in 30 mL of mixed solvent. Add a solution of 1.46 g of 85% H ₃ PO ₄ dissolved in 10 mL of mixed solvent drop by drop under stirring, and add the remaining solvent. After stirring for 30 minutes, quickly add 3.52 g of FeSO ₄ 7H ₂ O and 0.40 g of L-ascorbic acid . After 30 minutes of airtight stirring, it was transferred to a solvothermal reaction kettle and reacted at 200 ° C for 12 h. After cooling, wash with deionized water and ethanol three times, centrifuge and dry. The obtained powder was mixed with 20 wt% glucose and sintered in a nitrogen-protected sealed tube furnace at 700 °C for 5 hours to obtain lithium iron phosphate.

The X-ray diffraction pattern of lithium iron phosphate is shown in Figure 3. The field emission scanning electron microscope is shown in Fig. 6. As a comparative example, Figure 10 shows the rate discharge curve. The specific capacity is 97mAh/g at 10 C and 84 mAh/g at 20 C. Compared with Example 1, the prepared lithium iron phosphate/graphene composite has a significant improvement in the rate performance.

Example 4

The steps are the same as in Example 1, and the amount of glucose added is 10 wt% of the powder. The rate discharge curve is shown in Figure 11. By improving the carbon content, the performance of the composite was further significantly improved, with a specific capacity of 148 mAh/g at 10 C and 142 mAh/g at 20 C.

Claims (9)

1. A method for preparing a high-rate lithium iron phosphate/graphene composite positive electrode material, characterized in that the nano-sized flake-shaped lithium iron phosphate is prepared in situ on the graphene surface by hydrothermal reaction, and the specific steps are as follows: (1) ) Disperse graphene and auxiliary polymer in a solvent to obtain a graphene dispersion, and prepare phosphoric acid and lithium hydroxide into solutions respectively; add phosphoric acid solution and lithium hydroxide solution dropwise to the graphene dispersion to form an attached Lithium phosphate suspension on the surface of graphene; the mass ratio of auxiliary polymer to graphene is (0.5~3):1; the mass ratio of phosphoric acid to graphene is (10~100):1, and the molar ratio of lithium hydroxide and phosphoric acid The ratio is (2.5~3): 1; (2) Add soluble ferrous salts and antioxidants to the above-mentioned lithium phosphate suspension attached to the surface of graphene, and then transfer it to a solvothermal reaction kettle for reaction to obtain a lithium iron phosphate solution attached to the surface of graphene; control the solvent The thermal reaction temperature is 140-200°C, and the reaction time is 0.5-20 hours; the molar ratio of soluble ferrous salt to phosphoric acid described in step (1) is 1:1; (3) The product obtained in step (2) is separated, washed and dried to obtain a lithium iron phosphate/graphene composite, and the obtained lithium iron phosphate/graphene composite is mixed with an organic carbon source, and heat-treated at high temperature under an inert atmosphere After that, a lithium iron phosphate/graphene composite positive electrode material is obtained. 2. preparation method according to claim 1, is characterized in that, described Graphene is the monolayer or few-layer Graphene that obtains by graphite exfoliation. 3. The preparation method according to claim 1, characterized in that, the auxiliary polymer is a polymer capable of accepting protons to form a salt, which is any one of linear (branched) polyethyleneimine or polyacrylamide. 4. The preparation method according to claim 1, characterized in that the solvent in step (1) is a mixed solvent of water and alcohol. 5. The preparation method according to claim 4, wherein the alcohol is ethylene glycol or glycerol, and the volume ratio of water to alcohol is 1:0.1-10. 6. The preparation method according to claim 1, wherein the soluble ferrous salt is any one of ferrous sulfate, ferrous chloride, ferrous acetate and their corresponding crystalline hydrates. 7. preparation method according to claim 1, is characterized in that, described antioxidant is any one in L-ascorbic acid, citric acid or glucose, and consumption is 0～20 of soluble ferrous salt quality. wt%. 8. preparation method according to claim 1, is characterized in that, described organic carbon source is any one in glucose, sucrose, polystyrene or phenolic resin, and consumption is 0% of lithium iron phosphate/graphene composite quality ~20 wt%. 9. The preparation method according to claim 1, characterized in that the high temperature heat treatment temperature in step (3) is 550-750°C, and the heat treatment time is 1-10 hours.