Carbon-coated lithium iron phosphate, preparation method, lithium battery positive electrode material and electrode sheet

Abstract

The present application relates to the field of battery, especially to a carbon-coated lithium iron phosphate, a preparation method, a lithium battery positive electrode material and an electrode sheet, the preparation method comprising mixing an iron-containing compound, a phosphorus-containing compound, a lithium-containing compound and a carbon-iron-containing compound, carrying out a hydrothermal reaction and calcination to obtain the carbon-coated lithium iron phosphate. The preparation method provided by the present application can uniformly coat the glucose acid root on the surface of the lithium iron phosphate under high-temperature conditions in the reaction kettle through the chemical bond effect of ferrous gluconate itself, and the ferrous ions in the ferrous gluconate can also completely enter the synthesis reaction of the lithium iron phosphate, which can make the lithium iron phosphate have higher crystallinity to some extent, and help to improve the specific capacity and the conductivity of the material.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more particularly to a carbon-coated lithium iron phosphate, its preparation method, a lithium battery cathode material, and an electrode sheet. Background Technology

[0002] In recent years, with the phasing out of government subsidies for new energy vehicles, battery manufacturers have shifted their focus from pursuing high-energy-density lithium-ion batteries to those offering better overall cost-effectiveness. This has led to stricter cost control, coupled with consumers' increasing emphasis on the safety performance of new energy vehicles. In summary, the market demand for lithium-ion battery materials has exploded, while the requirements for overall performance have also become increasingly stringent.

[0003] Cathode materials are an important component of lithium batteries. Although the performance of cathode materials has been greatly improved, their cycle performance, rate performance, and safety still need further improvement.

[0004] Currently, the industrial process for preparing lithium iron phosphate involves ball milling a mixture of iron phosphate and phosphorus sources, lithium carbonate as the lithium source, and glucose as the carbon source. The milled slurry is then spray-dried and granulated, followed by high-temperature calcination in the presence of an inert gas to obtain lithium iron phosphate material. However, this method is complex, involves expensive production equipment and safety issues related to high-temperature and high-pressure equipment, and has high synthesis costs for the raw material iron phosphate, presenting a technological barrier.

[0005] This invention aims to improve the performance of lithium iron phosphate cathodes by optimizing the raw materials for lithium iron phosphate synthesis and improving the process. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing carbon-coated lithium iron phosphate, which involves mixing an iron-containing compound, a phosphorus-containing compound, a lithium-containing compound, and a carbon-iron-containing compound, followed by a hydrothermal reaction and calcination to obtain carbon-coated lithium iron phosphate.

[0007] Furthermore, the molar ratio of the iron-containing compound, phosphorus-containing compound, lithium-containing compound, and carbon-iron-containing compound is 0.8-1:0.8-1:1-1.2:0-0.2.

[0008] Furthermore, the iron-containing compound is one or more of ferrous sulfate, ferrous hydroxide, or ferrous oxalate;

[0009] Furthermore, the phosphorus-containing compound is one or more of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate;

[0010] Furthermore, the lithium-containing compound is one or more of lithium carbonate, lithium hydroxide, or lithium phosphate;

[0011] Furthermore, the carbon-iron compound is ferrous gluconate.

[0012] Furthermore, the hydrothermal reaction is carried out at a temperature of 180-220°C for a duration of 6-12 hours.

[0013] Furthermore, the calcination is carried out in stages.

[0014] Furthermore, the first calcination temperature is 300-400℃, the first calcination time is 2-4h, and the first heating rate is 2-105℃ / min; the second calcination temperature is 700-900℃, the second calcination time is 6-8h, and the second heating rate is 2-105℃ / min.

[0015] Furthermore, vacuum drying is included before sintering, with the drying temperature being 80℃-90℃ and the time being 12-24h.

[0016] The present invention also provides a carbon-coated lithium iron phosphate, obtained by the preparation method described above.

[0017] The present invention also provides a lithium battery cathode material, comprising the above-mentioned carbon-coated lithium iron phosphate.

[0018] The present invention also provides a positive electrode sheet for a lithium battery, comprising the above-mentioned positive electrode material.

[0019] The beneficial effects of this patent are:

[0020] (1) The present invention uses ferrous gluconate as a carbon source to realize the in-situ coating synthesis of lithium iron phosphate material. The carbon source coating layer can effectively improve the electronic conductivity of lithium iron phosphate, thereby improving the electrochemical performance of lithium iron phosphate.

[0021] (2) The experimental operation process of this invention is simple, the raw materials are simple and environmentally friendly, and no impurities are introduced during the experiment. The product is singular and suitable for mass production.

[0022] (3) This invention provides a method for preparing carbon-coated lithium iron phosphate cathode material. Through the chemical bond action of ferrous gluconate itself, gluconate ions under high temperature conditions can be uniformly coated on the surface of lithium iron phosphate in the reaction vessel. In addition, ferrous ions in ferrous gluconate can also completely enter the synthesis reaction of lithium iron phosphate. This can make lithium iron phosphate more crystallizable to a certain extent, which helps to improve the specific capacity and conductivity of the material.

[0023] (4) By adjusting the concentration of the carbon source and the hydrothermal temperature (changing the molar ratio of ferrous ions), the particle size of the generated lithium iron phosphate can be improved, thereby preparing a high-density lithium iron phosphate cathode material (ferrous gluconate hydrolyzes to release ferrous ions under high temperature and heat; by controlling the solution temperature through heating, the hydrolysis rate is affected, effectively reducing the growth rate of lithium iron phosphate particles during the reaction, thus refining the lithium iron phosphate particles). This invention adopts;

[0024] (5) The gluconate used in this invention contains gluconate ions and Fe. 2+ Intermolecular interactions exist, forming a complex structure between the two. The particle size of the synthesized material can be effectively controlled by adding the amount of ferrous gluconate.

[0025] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0026] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of the present invention and do not constitute a limitation on the technical solutions of the present invention.

[0027] Figure 1 This is a preparation process diagram of Embodiment 1 of the present invention.

[0028] Figure 2 This is the XRD pattern of the product of Embodiment 1 of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0030] An embodiment of the present invention provides a method for preparing carbon-coated lithium iron phosphate, wherein an iron-containing compound, a phosphorus-containing compound, a lithium-containing compound and a carbon-iron-containing compound are mixed, subjected to a hydrothermal reaction and calcination, to obtain carbon-coated lithium iron phosphate.

[0031] The experimental operation of this invention is simple, the raw materials are simple and environmentally friendly, and no impurities are introduced during the experiment. The product is singular and suitable for mass production. The carbon-iron compound is used as a carbon source to achieve in-situ coating of the material. The coating layer can isolate the active material and avoid direct contact with the electrolyte to prevent side reactions. At the same time, it can effectively inhibit the aggregation of the material and improve the electrochemical performance of the material.

[0032] In another embodiment of the present invention, the molar ratio of the iron-containing compound, the phosphorus-containing compound, the lithium-containing compound and the carbon-iron-containing compound is 0.8-1:0.8-1:1-1.2:0-0.2.

[0033] When the ratio is outside this range, the reaction will be incomplete, byproducts will be generated, and the electrical performance will be affected.

[0034] The iron-containing compound is one or more of ferrous sulfate, ferrous hydroxide, or ferrous oxalate.

[0035] Specifically, ferrous oxalate dihydrate is preferred because it has a lower cost and is more conducive to industrial production.

[0036] In another embodiment of the present invention, the phosphorus-containing compound is one or more of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

[0037] In another embodiment of the present invention, the lithium-containing compound is one or more of lithium carbonate, lithium hydroxide, or lithium phosphate.

[0038] In another embodiment of the present invention, the carbon-iron compound is ferrous gluconate.

[0039] This invention uses ferrous gluconate as a supplementary iron source, and enhances the carbon source coating effect based on the complexation between gluconate and ferrous ions, thereby improving the electrochemical performance of lithium iron phosphate. In this invention, ferrous gluconate is introduced into the preparation of lithium iron phosphate cathode materials as both a carbon source and a ferrous source, and is finally used as a supplementary ferrous source to synthesize lithium iron phosphate crystal materials. The ferrous ion-complexed gluconate can uniformly coat the surface of lithium iron phosphate, refining the particle size of the synthesized lithium iron phosphate, thus improving the electrochemical performance of the material. Furthermore, this invention reduces process complexity and improves product purity.

[0040] In another embodiment of the present invention, the hydrothermal reaction is carried out at a temperature of 180-220°C for a duration of 6-12 hours.

[0041] If the conditions for the hydrothermal reaction are not within this range, the reaction will be incomplete, affecting the uniformity of particle size, the purity and crystallinity of the product, and consequently its electrochemical performance.

[0042] In another embodiment of the present invention, the calcination is carried out in stages.

[0043] Specifically, the first calcination temperature is 300-400℃, the first calcination time is 2-4h, and the first heating rate is 2-10℃ / min; the second calcination temperature is 700-900℃, the second calcination time is 6-8h, and the second heating rate is 2-10℃ / min.

[0044] This invention employs segmented calcination. The first stage of calcination can achieve pre-decomposition of each component, while the second stage involves carbon coating. Using this temperature range can improve the graphitization of the carbon layer and increase the crystallinity of lithium iron phosphate. Using this heating rate can improve the microstructure of the product, thereby enhancing its electrochemical performance.

[0045] In another embodiment of the present invention, vacuum drying is further included before sintering, wherein the drying temperature is 80℃-90℃ and the time is 12-24h.

[0046] The present invention also provides a carbon-coated lithium iron phosphate, obtained by the above preparation method.

[0047] The present invention also provides a lithium battery cathode material, comprising the above-mentioned carbon-coated lithium iron phosphate.

[0048] The present invention also provides a positive electrode sheet for a lithium battery, comprising the above-mentioned positive electrode material.

[0049] Specific embodiments of the present invention are as follows:

[0050] Example 1:

[0051] (1) Weigh 7.2098g of ferrous oxalate dihydrate, 4.5989g of lithium carbonate, 3.7101g of diammonium hydrogen phosphate and 4.4612g of ferrous gluconate respectively, dissolve them in deionized water, and mechanically stir and mix for 2 hours to obtain a mixed solution;

[0052] (2) Transfer the above mixture into a reaction vessel, seal the hydrothermal vessel, place it in an oven at 180°C, and perform a hydrothermal reaction for 12 hours. After the hydrothermal reaction is completed, carbon-coated lithium iron phosphate precursor can be obtained.

[0053] (3) After cooling to room temperature, remove the product and place it in a vacuum oven to dry at 90°C for 12 hours;

[0054] (4) The precursor was placed in an alumina pot under an inert gas protective atmosphere to sinter and obtain lithium iron phosphate / carbon composite material (sintered at 300℃ for 3h, then heated to 800℃ for 7h; the heating rate was 5℃ / min).

[0055] The obtained material was subjected to XRD testing to obtain... Figure 2 For detailed preparation procedures, please refer to [link / reference]. Figure 1 .

[0056] Example 2:

[0057] (1) Weigh 8.1103 g of ferrous oxalate dihydrate, 5.751 g of lithium carbonate, 4.0911 g of diammonium hydrogen phosphate and 2.2413 g of ferrous gluconate respectively, dissolve them in deionized water, and mechanically stir and mix for 2 h to obtain a mixed solution;

[0058] (2) Transfer the above mixture to a reaction vessel, seal the hydrothermal vessel, and place it in an oven at 220℃. The hydrothermal reaction time is 6 hours. After the hydrothermal reaction is complete, carbon-coated lithium iron phosphate precursor can be obtained;

[0059] (3) After cooling to room temperature, remove the product and place it in a vacuum oven to dry at 85°C for 18 hours;

[0060] (4) The precursor was placed in an alumina pot under an inert gas protective atmosphere to sinter and obtain lithium iron phosphate / carbon composite material (sintered at 350℃ for 3h, then heated to 800℃ for 6h; the heating rate was 2℃ / min).

[0061] Example 3:

[0062] (1) Weigh 9.0030g of ferrous oxalate dihydrate, 5.1731g of lithium carbonate, 4.4487g of diammonium hydrogen phosphate and 1.1168g of ferrous gluconate respectively, dissolve them in deionized water, and mechanically stir and mix for 2 hours to obtain a mixed solution;

[0063] (2) Transfer the above mixture to a reaction vessel, seal the hydrothermal vessel, and place it in an oven at 200℃. The hydrothermal reaction time is 10 hours. After the hydrothermal reaction is complete, carbon-coated lithium iron phosphate precursor can be obtained;

[0064] (3) After cooling to room temperature, remove the product and place it in a vacuum oven to dry at 80°C for 12 hours;

[0065] (4) The precursor was placed in an alumina pot under an inert gas protective atmosphere to sinter and obtain lithium iron phosphate / carbon composite material (sintered at 400℃ for 2h, then heated to 900℃ for 6h, with a heating rate of 10℃ / min).

[0066] Example 4

[0067] The iron source, phosphorus source, lithium source, and carbon source were replaced with 1 mol ferrous sulfate, 0.8 mol lithium phosphate, 1.2 mol diammonium hydrogen phosphate, and 0.2 mol ferrous gluconate, respectively, while the rest remained the same as in Example 2.

[0068] Example 5

[0069] The iron source, phosphorus source, lithium source, and carbon source were replaced with 0.8 mol ferrous hydroxide, 0.9 mol lithium hydroxide, 1 mol phosphoric acid, and 0.1 mol ferrous gluconate, respectively, while the rest remained the same as in Example 2.

[0070] Example 6

[0071] The iron source, phosphorus source, lithium source, and carbon source were replaced with 0.4 mol ferrous hydroxide and 0.5 mol ferrous sulfate, 0.3 mol lithium hydroxide and 0.6 mol lithium carbonate, 0.5 mol ammonium dihydrogen phosphate and 0.5 mol diammonium hydrogen phosphate, and 0.1 mol ferrous gluconate, respectively, while the rest remained the same as in Example 2.

[0072] Comparative Example 1:

[0073] According to the molar ratio in Example 2, ferrous oxalate dihydrate, lithium carbonate, diammonium hydrogen phosphate, and glucose were weighed and dissolved in deionized water and mixed.

[0074] The rest is the same as in Example 2.

[0075] Comparative Example 2

[0076] The hydrothermal method was replaced with the ball milling method, and other conditions were the same as in Example 2.

[0077] Comparative Example 3

[0078] The calcination conditions were 800℃, sintering for 10h, and a heating rate of 15℃ / min (i.e., no segmented sintering was performed). Other conditions were the same as those in Comparative Example 2.

[0079] Comparative Example 4

[0080] The amounts of ferrous oxalate dihydrate, lithium carbonate, diammonium hydrogen phosphate, and ferrous gluconate were replaced with 0.5 mol, 1.5 mol, 0.2 mol, and 0.3 mol, respectively, while the rest remained the same as in Example 2.

[0081] Comparative Example 5

[0082] The hydrothermal conditions were changed to 130℃ for 5 hours, and everything else was the same as in Example 2.

[0083] Comparative Example 6

[0084] A method for preparing carbon-coated / lithium iron phosphate includes the following steps:

[0085] (1) Weigh 9.0122g of ferrous oxalate dihydrate, 4.4507g of lithium carbonate and 5.7541g of diammonium hydrogen phosphate respectively, dissolve them in 60g of deionized water, and mechanically stir and mix for 2h to obtain a mixed solution;

[0086] (2) Transfer the mixture in (1) into the reactor, seal the hydrothermal reactor, place it in the oven at 180°C, and the hydrothermal reaction time is 12h. After the hydrothermal reaction is completed, carbon-coated lithium iron phosphate precursor can be obtained.

[0087] (3) After cooling to room temperature, remove the product and place it in a vacuum oven to dry at 80°C for 24 hours;

[0088] (4) The precursor was placed in an alumina pot under an inert gas protective atmosphere to sinter and obtain lithium iron phosphate / carbon composite material (sintered at 350℃ for 2h, then heated to 700℃ for 8h, with a heating rate of 5℃ / min).

[0089] The carbon-coated metal-doped lithium iron phosphate materials obtained in the various embodiments and comparative examples were used as positive electrode materials, acetylene black as conductive agents, and polyvinylidene fluoride as binders to form electrode sheets. Metallic lithium was used as the negative electrode, and the sheets were assembled into coin cells. Tests were conducted under different charge / discharge current conditions at room temperature, ranging from 2.0 to 4.0 V. The specific preparation method is as follows: ① Prepare an N-methylpyrrolidone solution of polyvinylidene fluoride with a concentration of 0.02 g / mL; ② Weigh the lithium iron phosphate positive electrode material and polyvinylidene fluoride prepared in Example 1 in a ratio of 8:1:1. ③ The PVDF N-methylpyrrolidone solution and acetylene black are ground and mixed in an agate mortar; ④ After mixing evenly, the slurry is uniformly coated on aluminum foil with a coating thickness of 75μm; ⑤ The coated electrode is dried in a vacuum drying oven at 110℃ for 12h; ⑥ The dried electrode is cut into 12mm diameter round pieces using a slicer and used as working electrodes to assemble 2032R coin cells in a glove box for testing (assembly sequence: negative electrode shell, spring, gasket, lithium sheet, separator, electrolyte, working electrode sheet, positive electrode shell).

[0090] The specific electrochemical test results are shown in Table 1.

[0091] Table 1. Electrochemical test results of sodium iron phosphate doping in each example and comparative example.

[0092]

[0093]

[0094] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and changes to the form and details of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection of this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for preparing carbon-coated lithium iron phosphate, characterized in that, Ferrous oxalate dihydrate 8.1103g, lithium carbonate 5.7510g, diammonium hydrogen phosphate 4.0911g and ferrous gluconate 2.2413g were dissolved in deionized water, and then hydrothermal reaction was carried out at 220℃ for 6h, and sintering was carried out at 350℃ for 3h, and then sintering was carried out at 800℃ for 6h; The temperature rising rate was 2℃ / min, and carbon-coated lithium iron phosphate was obtained, wherein the size of the carbon-coated lithium iron phosphate was 200-400nm, the 0.1C capacity was 160.6mAh / g, and the capacity retention rate after 100 cycles was 96.9%.

2. The production method according to claim 1, characterized by, Before sintering, vacuum drying was further included, wherein the drying temperature was 80-90℃, and the time was 12-24h. 3.A carbon-coated lithium iron phosphate obtained by the preparation method of any one of claims 1-2. 4.A lithium battery positive electrode material, comprising the carbon-coated lithium iron phosphate of claim 3. 5.A lithium battery positive electrode sheet, comprising the positive electrode material of claim 4.

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