Carbon layer coated lithium iron phosphate as well as preparation method and application thereof

Abstract

The invention provides carbon layer coated lithium iron phosphate as well as a preparation method and application thereof, and relates to the technical field of modification of lithium iron phosphate materials. LiFePO4 crystal nucleus growth and surface functionalization are achieved through a microwave-hydrothermal method, preliminary morphology control is conducted on LiFePO4 through trace ascorbic acid and polyethylene glycol, then hydrogen bonds are formed through sugar alcohol hydroxyl and the surface of LiFePO4 through a microwave reactor, and surface modification is achieved. Through dynamic low-temperature carbon layer construction, particle sintering caused by traditional high temperature is avoided by utilizing the low-temperature thermal corresponding characteristic of sugar alcohol, then carbonization is completed through a gradient heating method, and finally a graphene-like porous carbon layer is formed. Compared with a traditional coating mode, the method has the advantages of lower carbonization temperature, thinner carbon layer thickness, more excellent carbon layer conductivity, lower process energy consumption, lower battery internal resistance and more stable cycle period.

Description

Technical Field

[0001] The present invention relates to the technical field of modification of lithium iron phosphate materials, and particularly relates to a carbon-coated lithium iron phosphate, a preparation method thereof, and an application thereof. Background Art

[0002] Since the first generation of commercial lithium-ion batteries was developed in the 1990s, lithium-ion batteries have been widely used in various electronic devices, such as energy storage and conversion carriers. In particular, the application of lithium-ion batteries in automobiles has had a huge development. As the most expensive component in lithium-ion batteries, the cathode material has also received extensive attention due to its strong influence on battery capacity, cycle life, and safety. For example, LiCoO2, LiNiO2, LiMn2O4, among which LiFePO4 is a promising cathode material for rechargeable batteries, having outstanding advantages such as high theoretical capacity, long cycle life, excellent thermal stability, environmental friendliness, and low cost. However, due to the limitations of its structure, compared with most traditional cathode materials, the LiFePO4 cathode material exhibits poor ionic conductivity, resulting in initial capacity loss and poor rate capability, which hinders its application in hybrid electric vehicles and pure electric vehicles.

[0003] Traditional cathode materials usually have a layered or spinel structure, which can form two-dimensional channels or large one-dimensional channels between layers to promote the diffusion of lithium ions and have good charge and discharge performance. For example, in the structure of LiFePO4, phosphate tetrahedrons are located between LiO6 octahedrons and FeO6 octahedrons, and the diffusion of lithium ions only forms narrow one-dimensional "pores", which limits the insertion and extraction of Li during charge and discharge. In addition, due to the lack of a continuous FeO6 octahedron network, electrons are restricted in the Fe-O-Fe path during conduction, resulting in poor conductivity of LiFePO4. The above disadvantages greatly limit the application of LiFePO4 under high-rate conditions, especially vehicle power. In order to improve the electronic conductivity and ionic conductivity of the LiFePO4 cathode material, methods such as surface modification and doping are adopted. As one of the most important modification methods, surface coating has been widely studied because it can improve the dispersion and thermal stability of active material particles, increase the surface activity of particles, isolate the electrolyte solution and the cathode material to avoid interaction, and promote the physical, chemical, and mechanical properties of the cathode material.

[0004] Carbon coating is a common method to improve the electrochemical performance of LiFePO4. The benefits of carbon coating are that it can not only improve the electrical conductivity of the sample, but also inhibit the growth of grains at high temperatures, reduce the agglomeration effect, thereby refining the grains, achieving the dual effects of improving its structural properties and enhancing its electrochemical performance. The method of carbon coating is very convenient and feasible. Generally, pure LiFePO4 has a low specific capacity due to the limitations of its structure. Traditional carbon coatings are mainly divided into in-situ carbon coating and non-in-situ carbon coating. Among them, in-situ carbon coating refers to adding a carbon source during the preparation of the LiFePO4 precursor. The advantage of in-situ carbon coating is that during the hydrothermal preparation process, the carbonization of the carbon source can be used to inhibit crystal growth, reduce particle agglomeration, and improve the electrical conductivity between particles. Non-in-situ carbon coating refers to synthesizing LiFePO4 from lithium source, phosphorus source, and iron source by hydrothermal method, supplemented by ball milling to synthesize LiFePO4 / C, so that the carbon source adheres to the surface of the sample, and the electrical conductivity of the carbon source is improved by sintering. The advantage of non-in-situ carbon coating is that the carbon source is directly added to the LiFePO4 powder, and the operation is simple and convenient. However, no matter which method is used, there will be high-temperature sintering, resulting in lattice distortion, affecting its high crystallinity, and the thickness of the formed carbon layer varies significantly. An overly thick carbon layer hinders the diffusion of lithium ions, while an overly thin layer has insufficient electrical conductivity, and it is difficult to control the thickness. During the carbon coating process, the carbon layer distribution may be uneven, resulting in insufficient electrical conductivity of some active materials, insignificant improvement in electrical conductivity, high process energy consumption, low improvement in cycle life, and insignificant improvement in rate performance. Summary of the Invention

[0005] In view of the above problems, the present invention adopts sorbitol-assisted microwave-hydrothermal method to achieve the growth of LiFePO4 crystal nuclei and surface functionalization in one step, and uses dynamic low-temperature carbon layer construction. Utilizing the low-temperature thermal response characteristics of sorbitol (decomposes at 150 - 250 °C), it avoids particle sintering caused by traditional high temperatures, and then completes carbonization through gradient heating, retaining the high crystallinity of lithium iron phosphate, and greatly improving its cycling performance compared with traditional coating methods.

[0006] One of the purposes of the present invention is to provide a preparation method of carbon-coated lithium iron phosphate.

[0007] Another purpose of the present invention is to provide carbon-coated lithium iron phosphate prepared by this preparation method.

[0008] The third purpose of the present invention is to provide an application of this carbon-coated lithium iron phosphate in lithium-ion batteries.

[0009] In order to achieve the above purposes of the present invention, the following technical solutions are specifically adopted:

[0010] The present invention provides a preparation method of carbon-coated lithium iron phosphate, comprising the following steps:

[0011] S1. Mix a lithium source, an iron source, and a phosphorus source and dissolve them in an aqueous solution containing sorbitol. Add ascorbic acid and polyethylene glycol to obtain a mixed solution.

[0012] S2. Place the mixed solution obtained in step S1 into a microwave reactor for reaction. After cooling, centrifuge to obtain a lithium iron phosphate precursor.

[0013] S3. Disperse the precursor obtained in step S2 in ethanol, add sodium borohydride, and perform ultrasonic treatment. Remove the solvent to form sorbitol-coated lithium iron phosphate.

[0014] S4. In a tube furnace, perform gradient temperature-raising carbonization on the sorbitol-coated lithium iron phosphate obtained in step S3 to obtain lithium iron phosphate coated with graphene-like porous carbon.

[0015] The preparation process schematic diagram of the present invention is as Figure 1 shown. First, lithium iron phosphate was synthesized by the microwave hydrothermal method. The lithium source, iron source, and phosphorus source were dissolved in an aqueous solution of sorbitol (carbon source). Trace ascorbic acid was added to inhibit the oxidation of Fe 2+ , and polyethylene glycol (PEG-400) was introduced as a morphology-directing agent. React in a microwave reactor. The hydroxyl groups of sorbitol form hydrogen bonds with the surface of lithium iron phosphate to achieve in-situ surface modification. After rapid cooling, centrifuge to obtain a precursor. Then disperse the precursor in ethanol, add sodium borohydride (NaBH4) as a mild reducing agent (although ascorbic acid has been used as the main reducing agent, NaBH4 can provide an additional reducing environment to ensure that Fe on the surface of LiFePO4 2+ is not oxidized to Fe 3+ ) during the carbon coating process. Perform ultrasonic treatment and remove the solvent in a vacuum constant temperature oven to form a lithium iron phosphate film uniformly coated with sorbitol (highly uniformly coated lithium iron phosphate). In a tube furnace, complete carbonization by the gradient temperature-raising method to finally form a graphene-like porous carbon layer.

[0016] In some embodiments, in step S1, the lithium source is lithium hydroxide and / or lithium carbonate; the iron source is one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, etc.; the phosphorus source is one or more of phosphoric acid, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate.

[0017] In some embodiments, in step S1, the mass ratio of the lithium source, iron source, and phosphorus source is 3:1:1; the concentration of sorbitol in the aqueous solution containing sorbitol is 0.3 - 0.8 mol / L, preferably 0.5 mol / L; the total concentration of the lithium source, iron source, and phosphorus source in the mixed solution is 0.4 - 0.6 g / mL, preferably 0.5 g / mL.

[0018] In some embodiments, the concentration of ascorbic acid in the mixed solution in step S1 is 0.01 - 0.03 wt%, preferably 0.01 wt%, and the concentration of polyethylene glycol in the mixed solution is 0.5 - 1.5 wt%, preferably 1 wt%.

[0019] In some embodiments, in step S2, the reaction temperature is 180 - 220 °C, preferably 200 °C, the reaction pressure is 1.8 - 2.2 MPa, preferably 2 MPa, the reaction time is 25 - 45 minutes, preferably 30 minutes, and the microwave frequency is 2 - 3 GHz, preferably 2.45 GHz.

[0020] In some embodiments, in step S3, the dispersion concentration of the precursor in ethanol is 6 - 10 wt%, and the addition concentration of sodium borohydride is 0.03 - 0.05 mol / L, preferably 0.05 mol / L.

[0021] In some embodiments, in step S3, the ultrasonic treatment time is 45 minutes - 75 minutes, preferably 1 hour.

[0022] In some embodiments, in step S4, the gradient temperature increase sequentially includes: maintaining the temperature at 130 - 170 °C for 45 minutes - 1 hour 15 minutes, maintaining the temperature at 230 - 270 °C for 1 hour 30 minutes - 2 hours 30 minutes, and maintaining the temperature at 330 - 370 °C for 45 minutes - 1 hour 15 minutes, preferably maintaining the temperature at 150 °C for 1 hour, maintaining the temperature at 250 °C for 2 hours, and maintaining the temperature at 350 °C for 1 hour.

[0023] The low-temperature process (highest 350 °C vs. conventional 800 °C) avoids lattice distortion and retains the high crystallinity of lithium iron phosphate. Sorbitol molecules form an ultrathin and highly adhesive carbon film through hydrogen bonding. The sorbitol-assisted microwave-hydrothermal method realizes the growth of LiFePO4 crystal nuclei and surface functionalization in one step. It inhibits the growth of LiFePO4 grains, can increase the specific surface area, enhance the conductivity of electrons between particles and on the surface, reduce the occurrence of battery polarization, adsorb and maintain the stability of the electrolyte. It can improve the specific capacity, rate performance, and cycle performance of LiFePO4. Due to its excellent structure and high conductivity, this material can conduct current and ions quickly and efficiently during charge and discharge, significantly improving the energy density of the battery. This material exhibits good stability during long-term cyclic charge and discharge, reducing the phenomenon of capacity attenuation. This enables lithium-ion batteries to maintain high performance during long-term use and adapt to various complex application environments.

[0024] In a second aspect, the present invention provides a carbon-coated lithium iron phosphate prepared by the above preparation method.

[0025] In a third aspect, the present invention provides an application of the carbon-coated lithium iron phosphate in a lithium-ion battery.

[0026] The positive electrode slurry of the lithium-ion battery comprises a solid component and a solvent. The solid component includes a positive electrode material (lithium iron phosphate coated with a carbon layer), a binder, and a conductive agent. Among them, the mass percentages are respectively the positive electrode material (96-98%), the binder (1-2%), and the conductive agent (1-2%).

[0027] The negative electrode slurry of the lithium-ion battery comprises a solid component and a solvent. The solid component includes a negative electrode material, a binder, a thickening agent, and a conductive agent. Among them, the mass percentages are respectively the negative electrode material (94-97%), the binder (1-2%), the thickening agent (1-2%), and the conductive agent (1-2%).

[0028] The positive electrode sheet of the lithium-ion battery is prepared by coating the positive electrode slurry of the lithium-ion battery on a positive electrode aluminum foil current collector, followed by rolling, slitting, die-cutting, and baking. Among them, the positive electrode slurry comprises a solid component and a solvent. The solid component includes a positive electrode material, a binder, and a conductive agent.

[0029] The negative electrode sheet of the lithium-ion battery is prepared by coating the negative electrode slurry of the lithium-ion battery on a negative electrode copper foil current collector, followed by rolling, slitting, die-cutting, and baking. Among them, the negative electrode slurry comprises a solid component and a solvent. The solid component includes a negative electrode material, a binder, a thickening agent, and a conductive agent.

[0030] The lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, a separator, and a housing. The positive electrode is prepared by coating the positive electrode slurry on an aluminum foil current collector. Among them, the positive electrode slurry comprises a solid component and a solvent. The solid component includes a positive electrode material substance, a binder, and a conductive agent. Among them, the negative electrode slurry comprises a solid component and a solvent. The solid component includes a negative electrode material substance, a binder, a thickening agent, and a conductive agent.

[0031] Beneficial effects:

[0032] The present invention proposes to realize the growth of LiFePO4 crystal nuclei and surface functionalization through the microwave-hydrothermal method. mainly through the preliminary morphology control of LiFePO4 by trace ascorbic acid and polyethylene glycol, and then through the microwave reactor to form hydrogen bonds between the sorbitol hydroxyl group and the surface of LiFePO4 to achieve surface modification. Through the construction of a dynamic low-temperature carbon layer, using the low-temperature thermal response characteristics of sorbitol (decomposing at 150-250 °C), the particle sintering caused by traditional high temperature is avoided, and then carbonization is completed by the gradient heating method to finally form a graphene-like porous carbon layer.

[0033] Through sorbitol-assisted microwave-hydrothermal method and dynamic low-temperature carbon layer construction, utilizing the low-temperature thermal response characteristics of sorbitol, high crystallinity retention of LiFePO4 is achieved and an ultrathin, highly adherent carbon layer is formed through hydrogen bonding. Compared with traditional coating methods, this method has a lower carbonization temperature, a thinner carbon layer thickness, better carbon layer conductivity, lower process energy consumption, lower battery internal resistance, and a more stable cycle period.

[0034] The present invention has been described in detail above, but the above embodiments are essentially illustrative only and are not intended to limit the present invention. In addition, the present invention is not limited by any theory described in the foregoing prior art or the summary of the invention or the following examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the preparation process of lithium iron phosphate coated with a carbon layer according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present invention will be further described below in conjunction with embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection required by the present invention.

[0037] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the embodiments are all conventional raw materials, reagents, methods in the art.

[0038] Example 1

[0039] (1) First, 30 g of LiOH, 10 g of FeSO4, and 10 g of H3PO4 were mixed. 9.11 g of sorbitol and 100 mL of deionized water were weighed to prepare a 0.5 mol / L aqueous solution. The powder materials were added to the solution and ultrasonically stirred evenly. Subsequently, 10 mg of ascorbic acid and 1 g of polyethylene glycol were added and ultrasonically stirred. The mixed solution was placed in a microwave reactor (200 °C, 2 MPa, 2.45 GHz) and reacted for 30 minutes, then quickly cooled and centrifuged to obtain a precursor. The precursor was dispersed in ethanol (concentration 10%), and 0.5 g of sodium borohydride (NaBH4) (0.05 mol / L) was added as a mild reducing agent, and ultrasonically treated for 1 hour. The solvent was removed in a vacuum constant temperature oven (100 °C). In a tubular furnace, carbonization was completed by gradient heating (holding at 150 °C for 1 hour → holding at 250 °C for 2 hours → holding at 350 °C for 1 hour), and finally, lithium iron phosphate coated with graphene-like porous carbon (LiFePO4@C) was formed.

[0040] (2) Dissolve 100 g of polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) solvent, and then add the LiFePO4@C cathode material and carbon black conductive agent in corresponding proportions and mix evenly. Among them, for the lithium-ion battery cathode slurry, by weight percentage, it is LiFePO4@C (98%), PVDF (1%), and conductive agent (1%). Dissolve 100 g of sodium carboxymethyl cellulose (CMC) in deionized water, and then add the anode material graphite, carbon black conductive agent, and styrene-butadiene rubber (SBR) and mix evenly. Among them, for the lithium-ion battery anode slurry, by weight percentage, it is graphite anode material (97%), carbon black (1%), CMC (1%), and SBR (1%).

[0041] (3) Coating the obtained lithium-ion battery cathode slurry on an aluminum foil current collector, and coating the anode slurry on a copper foil current collector. Finally, the positive and negative electrode plates are obtained through rolling, slitting, die-cutting, and baking.

[0042] (4) Assemble the positive and negative electrode plates in step (3) with a separator, electrolyte, and housing into a lithium-ion battery.

[0043] Example 2

[0044] The preparation method of this example is the same as that of Example 1, only changing the concentration of sorbitol to 0.8 mol / L.

[0045] The preparation method of the lithium-ion battery is the same as that of Example 1.

[0046] Example 3

[0047] The preparation method of this example is the same as that of Example 1, only changing the concentration of sorbitol to 0.3 mol / L.

[0048] The preparation method of the lithium-ion battery is the same as that of Example 1.

[0049] Comparative Example 1

[0050] Adopt traditional non-in-situ carbon-coated lithium iron phosphate:

[0051] First, mix 30 g of LiOH, 10 g of FeSO4, and 10 g of H3PO4 and stir ultrasonically. Wash and precipitate the obtained turbid solution with ethanol and deionized water multiple times, and obtain a powder sample through vacuum drying. Then, using sorbitol as the carbon source, while keeping other experimental conditions unchanged, weigh 0.5 g of sorbitol, mix and ball-mill for 3 h at 300 r / min rotation, and dry the mixed powder sample in a test tube. Nitrogen (protective gas) is pre-sintered at 300 °C for 3 h, and then sintered at 650 °C for 6 h to carbonize the sugar alcohol and attach it to the surface of LiFePO4 to obtain the final product (LiFePO4 / C).

[0052] The preparation method of the lithium-ion battery is the same as that of Example 1.

[0053] Comparative Example 2

[0054] The difference between the preparation method of the lithium-ion battery and that of Example 1 is that the cathode material is lithium iron phosphate.

[0055] Test Example:

[0056] Test the specific surface area, internal resistance of the lithium-ion battery, 2C rate charging, 3C rate discharging, room temperature cycle retention rate, and high temperature cycle retention rate of the cathode materials in the test examples and comparative examples.

[0057] Test method:

[0058] The specific surface area is in accordance with the national standard GBT 30835-2014. The voltage range for 2C rate and 3C rate discharging, room temperature cycle and high temperature cycle is: 2.8 - 3.65V, and the current for room temperature cycle and high temperature cycle is: 1C.

[0059] As shown in Table 1.

[0060] Table 1

[0061]

[0062] When the cathode materials of Example 1, Example 2, Example 3, Comparative Example 1, and Comparative Example 2 are applied to lithium-ion batteries, test the specific surface area of the cathode material, internal resistance of the lithium-ion battery, 2C rate charging, 3C rate discharging, room temperature cycle retention rate, and high temperature cycle retention rate. The performance of several test items such as the specific surface area of the lithium-ion battery, internal resistance of the lithium-ion battery, 2C rate charging, 3C rate discharging, room temperature cycle retention rate, and high temperature cycle retention rate in Example 1 is higher than that in Example 2, Example 3, Comparative Example 1, and Comparative Example 2. Synthesizing in a low-temperature manner avoids lattice distortion and retains the high crystallinity of lithium iron phosphate. Sugar alcohol molecules are cross-linked by hydrogen bond induction to form an ultra-thin and highly adherent carbon film, which can achieve a larger specific surface area compared with the carbon layer formed at high temperature, enhance the conductivity of electrons between particles and on the surface, reduce the occurrence of battery polarization, reduce the internal resistance of the lithium-ion battery, and improve the cycle stability of the lithium-ion battery. Carbon coating is carried out on lithium iron phosphate using sugar alcohol as the carbon source.

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: within the scope of the spirit and essence defined by the claims of the present invention, the technical solutions recorded in the foregoing embodiments can be modified, or some or all of the technical features can be equivalently replaced; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A method for preparing carbon-layer-coated lithium iron phosphate, characterized in that: The following steps are involved: S1, dissolving a lithium source, an iron source and a phosphorus source in an aqueous solution containing sorbitol, and adding ascorbic acid and polyethylene glycol to obtain a mixed solution; S2, placing the mixed solution obtained in step S1 into a microwave reactor for reaction, cooling and centrifuging to obtain a lithium iron phosphate precursor; S3, dispersing the precursor obtained in step S2 in ethanol, adding sodium borohydride for ultrasonic treatment, removing the solvent, and forming sorbitol-coated lithium iron phosphate; S4. In a tubular furnace, the sorbitol-coated lithium iron phosphate obtained in step S3 is subjected to gradient temperature carbonization to obtain graphene-like porous carbon-coated lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that: In step S1, the lithium source is lithium hydroxide and / or lithium carbonate; the iron source is one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate; and the phosphorus source is one or more of phosphoric acid, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate.

3. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of the lithium source, the iron source and the phosphorus source is 3:1:1; the concentration of sorbitol in the aqueous solution containing sorbitol is 0.3-0.8 mol / L; and the total concentration of the lithium source, the iron source and the phosphorus source in the mixed solution is 0.4-0.6 g / mL.

4. The preparation method according to claim 1, characterized in that: In step S1, the concentration of ascorbic acid in the mixed solution is 0.01-0.03 wt %, and the concentration of polyethylene glycol in the mixed solution is 0.5-1.5 wt %.

5. The preparation method according to claim 1, characterized in that: In step S2, the reaction temperature is 180-220° C., the reaction pressure is 1.8-2.2 MPa, the reaction time is 25-45 min, and the microwave frequency is 2.45 GHz.

6. The preparation method according to claim 1, characterized in that: In step S3, the concentration of the precursor dispersed in ethanol is 6-10 wt %, and the concentration of sodium borohydride added is 0.03-0.05 mol / L.

7. The preparation method according to claim 1, characterized in that: The ultrasonic treatment time in step S3 is 45 min-75 min.

8. The preparation method according to claim 1, characterized in that: The gradient heating in step S4 includes: keeping warm at 130-170°C for 45-75 min, keeping warm at 230-270°C for 90-150 min, and keeping warm at 330-370°C for 45-75 min.

9. A carbon layer-coated lithium iron phosphate, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the carbon layer-coated lithium iron phosphate according to claim 9 in a lithium ion battery.

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