Graphene-modified lithium iron phosphate and preparation method, battery and application thereof

Abstract

The present application relates to the technical field of positive electrode active materials, and specifically to a graphene-modified lithium iron phosphate and its preparation method, battery, and application. The preparation method comprises the following steps: subjecting an aqueous solution containing phosphoric acid grafted graphene and lithium iron grafted graphene to a hydrothermal reaction and then calcining to obtain graphene-modified lithium iron phosphate. The preparation method of the graphene-modified lithium iron phosphate of the present application adopts a method of in-situ generation of lithium iron phosphate on graphene, and generates lithium iron phosphate and graphene doping in one step during the hydrothermal reaction. The preparation steps are simple, the cost is low, the preparation conditions are mild, and it is easy to implement.

Description

Technical Field

[0001] The present application relates to the technical field of positive electrode active materials, and in particular to a graphene-modified lithium iron phosphate and a preparation method, a battery, and an application thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4), the cathode active material of lithium batteries, has the characteristics of high thermal stability and long cycle life, but low rate performance. For lithium iron phosphate, improving its rate performance is certainly the key direction of improvement, but as a cathode active material, its cycle stability is also particularly important for batteries.

[0003] Existing methods to improve the cycle stability of lithium iron phosphate include optimizing battery materials (such as doping modification, surface coating, particle size control, etc.), optimizing electrolytes (such as selecting electrolytes with high chemical stability, optimizing electrolyte ratios, controlling electrolyte acidity and moisture content, etc.), and improving battery processes (such as optimizing electrode porosity and compaction density, using highly conductive current collectors, introducing conductive agents and binders, etc.). Among them, optimizing battery materials is a more common method. Although doping with metals (such as manganese, cobalt, nickel, aluminum, titanium, etc.) or non-metals (such as nitrogen, sulfur, fluorine, etc.) can effectively improve the conductivity and ion diffusion coefficient of lithium iron phosphate, thereby improving its rate performance, its cycle stability and service life will be reduced due to element doping. Summary of the invention

[0004] The present application provides a graphene-modified lithium iron phosphate and a preparation method, a battery, and an application thereof. The preparation method can generate lithium iron phosphate and graphene doping in one step, and the preparation steps are simple, the cost is low, the preparation conditions are mild, and it is easy to implement.

[0005] This application is implemented through the following technical solutions:

[0006] In a first aspect, the present application provides a method for preparing graphene-modified lithium iron phosphate, comprising the following steps:

[0007] The reaction solution is subjected to a hydrothermal reaction and then calcined to obtain graphene-modified lithium iron phosphate; the reaction solution contains a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene, and lithium iron-grafted graphene;

[0008] The mass ratio of the phosphoric acid grafted graphene, the lithium iron grafted graphene and the phosphoric acid is (0.1-0.2):(0.7-0.9):1.

[0009] In some possible implementations, preparing the lithium iron grafted graphene includes the following steps:

[0010] The first solution is subjected to a first ball milling, and then washed and dried to obtain lithium-grafted graphene oxide; the first solution contains graphene oxide and a lithium source;

[0011] The second solution is subjected to a second ball milling, and then washed and dried to obtain the lithium iron grafted graphene; the second solution contains the lithium grafted graphene oxide and ferrous cysteine.

[0012] In some possible implementations, in the first ball milling step, the ball-to-material ratio is 1:(2-5), the ball milling time is 5h-15h, and the ball milling speed is 100rpm-300rpm.

[0013] In some possible implementations, in the second ball milling step, the ball-to-material ratio is 1:(3-6), the ball milling time is 4h-10h, and the ball milling speed is 100rpm-300rpm.

[0014] In some possible implementations, preparing the phosphoric acid grafted graphene comprises the following steps:

[0015] The graphene oxide and phosphoric acid are mixed and ball-milled, and then washed and dried to obtain the phosphoric acid-grafted graphene.

[0016] In some possible implementations, in the step of preparing the phosphoric acid grafted graphene: the ball-to-material ratio of ball milling is 1:(3-8), the ball milling time is 3h-12h, the ball milling speed is 200rpm-400rpm, and the drying temperature is 100°C-130°C.

[0017] In some possible implementations, the calcination temperature is 500°C to 700°C.

[0018] In a second aspect, the present application provides a graphene-modified lithium iron phosphate prepared by the above-mentioned method for preparing graphene-modified lithium iron phosphate, comprising lithium iron phosphate and graphene doped in the lithium iron phosphate.

[0019] In some possible implementations, the powder resistivity of the graphene-modified lithium iron phosphate is 5Ω·cm~12Ω·cm.

[0020] In a third aspect, the present application provides a battery, wherein the positive electrode active material of the battery includes the above-mentioned graphene-modified lithium iron phosphate.

[0021] In a fourth aspect, the present application provides an application of the above-mentioned battery in household appliances, electric vehicles, ships, satellites or energy storage systems.

[0022] The graphene-modified lithium iron phosphate and the preparation method thereof provided in the present application have at least the following beneficial technical effects compared with the prior art:

[0023] (1) The preparation method of graphene-modified lithium iron phosphate of the present application is to subject a reaction solution containing a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene and lithium iron-grafted graphene to a hydrothermal reaction. The phosphoric acid-grafted graphene and the lithium iron-grafted graphene are not only used as raw materials for graphene, but also as "seeds" for the hydrothermal reaction to generate graphene-modified lithium iron phosphate, which can promote the reaction of the lithium source, the iron source and phosphoric acid to generate lithium iron phosphate, thereby improving the reaction rate and yield; the graphene-modified lithium iron phosphate improves the structural stability and electrical conductivity of the lithium iron phosphate. The graphene-modified lithium iron phosphate prepared by this preparation method has excellent electrical conductivity, cycle stability and rate performance.

[0024] (2) In the preparation method of graphene-modified lithium iron phosphate of the present application, during the hydrothermal reaction, ferrous cysteine ​​can not only play a role in slowly releasing ferrous ions and controlling the reaction rate, but the reducibility of cysteine ​​can also prevent ferrous ions from being oxidized to ferric ions, thereby reducing the occurrence of side reactions and improving the yield. In addition, ferrous cysteine ​​contains S and C elements. After calcination, S and C elements are also doped in lithium iron phosphate to improve the diffusion rate of lithium ions, thereby improving the cycle performance and charge and discharge efficiency of the battery.

[0025] (3) The preparation method of graphene-modified lithium iron phosphate of the present application generates lithium iron phosphate and graphene doping in one step in a hydrothermal reaction. The preparation steps are simple, the cost is low, the preparation conditions are mild, and it is easy to implement.

[0026] (4) The graphene-modified lithium iron phosphate of the present application adopts graphene-doped lithium iron phosphate and doped with S and C elements, thereby improving the conductivity, cycle stability and rate performance of lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present drawings or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present drawings. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0028] Figure 1 This is the SEM image of graphene-modified lithium iron phosphate in Example 1 of the present application.

[0029] The purpose, features and advantages of this figure will be further described in conjunction with the embodiments with reference to the accompanying drawings. DETAILED DESCRIPTION

[0030] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0031] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0032] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, a~b (i.e. a and b), a~c, b~c, or a~b~c, where a, b, c can be single or multiple, respectively.

[0033] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. For example, without departing from the scope of the implementation regulations of this application, the first XX can also be called the second XX, and similarly, the second XX can also be called the first XX. Therefore, the features defined as "first" and "second" can explicitly or implicitly include one or more of the features.

[0034] The terms used in the embodiments of the present application are only for the purpose of describing specific implementation rules, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the implementation rules of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0035] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation regulations of this application.

[0036] The weight of the relevant components mentioned in the embodiment description of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the embodiment description of the present application, it is within the scope disclosed in the embodiment description of the present application. Specifically, the mass described in the embodiment description of the present application can be a mass unit known in the chemical industry such as μg, mg, g, kg, etc.

[0037] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0038] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0039] All phosphoric acid involved in the embodiments of the present application is concentrated phosphoric acid with a concentration of about 85%.

[0040] The cycle stability of lithium iron phosphate is an important indicator of high capacity, long life and safety of batteries, and the cycle stability is closely related to the structural stability of lithium iron phosphate.

[0041] At present, the rate performance is improved by doping lithium iron phosphate with metal elements (such as manganese, cobalt, nickel, aluminum, titanium, etc.) and / or non-metals (such as nitrogen, sulfur, phosphorus, fluorine, etc.). However, the cycle stability and service life of lithium iron phosphate are reduced due to the doped heteroatoms.

[0042] To this end, the present application provides a graphene-modified lithium iron phosphate and a preparation method and a battery. The graphene-modified lithium iron phosphate prepared by the preparation method improves the structural stability and electrical conductivity of the lithium iron phosphate. The graphene-modified lithium iron phosphate has excellent electrical conductivity, cycle stability and rate performance.

[0043] The following is a detailed description of a graphene-modified lithium iron phosphate and a preparation method and a battery according to an embodiment of the present application.

[0044] The present invention provides a method for preparing graphene-modified lithium iron phosphate, comprising the following steps:

[0045] S1. The reaction solution is subjected to a hydrothermal reaction and then calcined to obtain graphene-modified lithium iron phosphate; the reaction solution comprises a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene and lithium iron-grafted graphene;

[0046] The mass ratio of phosphoric acid grafted graphene, lithium iron grafted graphene and phosphoric acid is (0.1~0.2):(0.7~0.9):1.

[0047] The preparation method of the graphene-modified lithium iron phosphate of the embodiment of the present application is to hydrothermally react the reaction solution containing lithium source, iron source, phosphoric acid, phosphoric acid-grafted graphene and lithium iron-grafted graphene. Phosphoric acid-grafted graphene and lithium iron-grafted graphene are not only used as raw materials for graphene, but also as "seeds" for hydrothermal reaction to generate graphene-modified lithium iron phosphate, which can promote the reaction of lithium source, iron source and phosphoric acid to generate lithium iron phosphate, thereby improving the reaction rate and yield; graphene-modified lithium iron phosphate improves the structural stability and conductivity of lithium iron phosphate. The graphene-modified lithium iron phosphate prepared by the preparation method has excellent conductivity, cycle stability and rate performance. It should be noted that the ratio of lithium source, iron source and phosphoric acid is a conventional technology in the art, and there is no need to make special restrictions in the embodiments of the present application, but as an example, the molar ratio of lithium source, iron source and phosphoric acid can be (0.95~1.05): (0.95~1.05): 1.

[0048] In the embodiment, the mass ratio of phosphoric acid grafted graphene, lithium iron grafted graphene and phosphoric acid is (0.1-0.2):(0.7-0.9):1, which can be a typical but non-limiting mass ratio such as 0.15:0.8:1, 0.2:0.7:1, 0.1:0.9:1, or a range between any two mass ratios.

[0049] In some embodiments, in the above step S1, the iron source includes at least one of ferrous sulfate, ferrous chloride, and ferrous oxalate.

[0050] In some embodiments, in the above step S1, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium acetate.

[0051] In some embodiments, in the above step S1, preparing phosphoric acid grafted graphene comprises the following steps:

[0052] S1.1. The first graphene oxide and the first phosphoric acid are mixed and ball-milled, and then washed and dried to obtain phosphoric acid-grafted graphene.

[0053] In the above step of preparing phosphoric acid grafted graphene, the first graphene oxide and the first phosphoric acid are mixed and ball-milled to make the phosphate group easy to graft with the first graphene oxide; washing is performed to remove excess phosphoric acid, and the phosphoric acid grafted graphene is obtained after drying. It should be noted that the amount of the first phosphoric acid is not included in the amount of phosphoric acid used in the hydrothermal reaction.

[0054] In some embodiments, in the above step S1.1, the mass ratio of the first graphene oxide to the first phosphoric acid is (0.03-0.08):1. In an exemplary embodiment, it can be a typical but non-limiting mass ratio such as 0.054:1 or a range between any two mass ratios.

[0055] In some embodiments, in the above step S1.1, in the ball milling step, the ball to material ratio is 1: (3-8), the ball milling time is 3h-12h, and the ball milling rate is 200rpm-400rpm. In this case, the graphene oxide is fully contacted and grafted with phosphoric acid to obtain phosphoric acid-grafted graphene. It should be noted that the ball to material ratio, time and rate of ball milling can be adjusted according to the actual preparation situation and needs, and are not limited to the range shown in the embodiments of the present application.

[0056] In some embodiments, in the above step S1.1, in the washing step, pure water is used for immersion for 5 minutes to 10 minutes, and suction filtration is performed after each immersion, and the immersion and suction filtration steps are repeated until the washing water is neutral. In this case, the use of pure water immersion can remove excess impurities such as phosphoric acid and prevent the impurities from affecting the subsequent reaction.

[0057] In some embodiments, in the above step S1.1, the drying temperature is 100°C to 130°C. In the exemplary embodiment, it can be a typical but non-limiting temperature such as 100°C, 110°C, 120°C, 130°C, or a range between any two temperatures. In this case, low-temperature drying removes moisture without destroying the structure of the phosphoric acid-grafted graphene. It should be noted that the drying time can be determined according to the actual production volume and is not particularly limited in this embodiment.

[0058] In some embodiments, in the above step S1, preparing lithium iron grafted graphene comprises the following steps:

[0059] S1.2 After the first solution is subjected to a first ball milling, it is washed and dried to obtain lithium-grafted graphene oxide; the first solution contains second graphene oxide and lithium hydroxide.

[0060] S1.3 After the second solution is subjected to a second ball milling, it is washed and dried to obtain lithium iron grafted graphene; the second solution contains lithium grafted graphene oxide and ferrous cysteine.

[0061] In the above-mentioned step of preparing lithium iron grafted graphene, the radius of lithium ion is less than the radius of ferrous ion, and lithium ion is first grafted onto the second graphene oxide, so that lithium ion can be fully grafted, and then ferrous cysteine ​​is grafted onto lithium grafted graphene oxide, avoiding the situation that lithium ion grafting is difficult or cannot be grafted due to the presence of ferrous cysteine ​​with large radius. In addition, ferrous cysteine ​​is grafted onto lithium grafted graphene oxide, and during hydrothermal reaction, ferrous cysteine ​​can not only play a slow release of ferrous ions, control the reaction speed, and the reducibility of cysteine ​​can also prevent ferrous ions from being oxidized to iron ions, thereby reducing the occurrence of side reactions and improving yield. In addition, the S element and C element contained in ferrous cysteine, after roasting, the S element and C element are doped in lithium iron phosphate, improve the lithium ion diffusion rate, thereby improving the cycle performance and charge and discharge efficiency of the battery. Furthermore, the low decomposition temperature of cysteine ​​allows the hydrothermal reaction product to be calcined at a relatively low temperature (500°C~600°C) to obtain graphene-modified lithium iron phosphate.

[0062] In some embodiments, in the above steps S1.2 and S1.3, the mass ratio of the second graphene oxide, lithium hydroxide and ferrous cysteine ​​is 1: (2.5-5): (18-43), and in the exemplary embodiment, it can be a typical but non-limiting mass ratio such as 1: 4.7: 34.7 or a range between any two mass ratios. In this case, lithium hydroxide and ferrous cysteine ​​are in excess, so that lithium and iron can be fully grafted onto the second graphene oxide.

[0063] In some embodiments, in the above step S1.2, the total mass of the second graphene oxide and the first graphene oxide is the total mass of the graphene in the graphene-modified lithium iron phosphate.

[0064] In some embodiments, the mass ratio of the first graphene oxide to the second graphene oxide is 1:(1.2-2), and in exemplary embodiments, it can be a typical but non-limiting mass ratio such as 1:1.2, 1:1.5, 1:2, or a range between any two mass ratios. In this case, the amount of the second graphene oxide is sufficient to graft lithium ions and ferrous ions and promote the hydrothermal reaction to generate lithium iron phosphate.

[0065] In some embodiments, in the above step S1.2, in the first ball milling step, the ball-to-material ratio is 1: (2-5), the ball milling time is 5h-15h, and the ball milling rate is 100rpm-300rpm. In this case, the second graphene oxide and the lithium source fully react, so that the lithium ions are fully grafted onto the graphene oxide to obtain lithium-grafted graphene oxide. It should be noted that the ball-to-material ratio, time and rate of ball milling can be adjusted according to the actual preparation situation and needs, and are not limited to the range shown in the embodiments of the present application.

[0066] In some embodiments, in the above step S1.2, in the washing step, pure water is used for 5 min to 10 min, and suction filtration is performed after each immersion. In some embodiments, in the above step S1.2, the number of washings is 3 to 5 times. In this case, pure water immersion can remove excess reaction impurities and prevent impurities from affecting the subsequent reaction.

[0067] In some embodiments, in the above step S1.2, the drying temperature is 100°C to 130°C, and in the exemplary embodiment, it can be a typical but non-limiting temperature such as 100°C, 110°C, 120°C, 130°C, or a range between any two temperatures. In this case, low-temperature drying removes moisture without destroying the structure of lithium-grafted graphene oxide.

[0068] In some embodiments, in the above step S1.2, the first solution also contains water. It should be noted that water is only used as a ball milling liquid, which plays a lubricating role in the ball milling process, reduces the friction and wear between the grinding balls and the materials being ground, and cools the grinding balls and the mill to prevent overheating and damage; the amount of water is added in real time according to the actual ball milling needs, and is not particularly limited in the embodiments of the present application.

[0069] In some embodiments, in the above step S1.3, in the second ball milling step, the ball-to-material ratio is 1: (3-6), the ball milling time is 4h-10h, and the ball milling rate is 100rpm-300rpm. In this case, the lithium-grafted graphene oxide and the ferrous cysteine ​​react fully, so that the ferrous cysteine ​​is fully hydrogen-bonded to the graphene oxide to obtain lithium-iron grafted graphene. It should be noted that the ball-to-material ratio, time and rate of ball milling can be adjusted according to the actual preparation situation and needs, and are not limited to the range shown in the embodiments of the present application.

[0070] In some embodiments, in the above step S1.3, in the washing step, pure water is used for 5 min to 10 min, and suction filtration is performed after each immersion. In some embodiments, in the above step S1.4, the number of washings is 3 to 5 times. In this case, pure water immersion can remove excess reaction impurities and prevent impurities from affecting the subsequent reaction.

[0071] In some embodiments, in the above step S1.3, the drying temperature is 100°C to 130°C, and in the exemplary embodiment, it can be a typical but non-limiting temperature such as 100°C, 110°C, 120°C, 130°C, or a range between any two temperatures. In this case, low-temperature drying removes moisture without destroying the structure of the lithium iron grafted graphene.

[0072] In some embodiments, in the above step S1.3, the second solution also contains water. It should be noted that water is only used as a ball milling liquid, which plays a lubricating role in the ball milling process, reduces the friction and wear between the grinding balls and the materials being ground, and cools the grinding balls and the mill to prevent overheating and damage; the amount of water is added in real time according to the actual ball milling needs, and is not particularly limited in the embodiments of the present application.

[0073] In some embodiments, in the above step S1, the temperature of the hydrothermal reaction is 150°C to 200°C, and in the exemplary embodiment, it can be a typical but non-limiting temperature such as 150°C, 170°C, 180°C, 200°C, or a range between any two temperatures. In this case, at this temperature, the hydrothermal reaction promotes the in-situ reaction of phosphoric acid, lithium, and iron on graphene to generate lithium iron phosphate, and at the same time, the graphene is coated to form a "seed" of the lithium iron phosphate reaction, which promotes the reaction.

[0074] In some embodiments, in the above step S1, the time of the hydrothermal reaction is 1 h to 5 h. In exemplary embodiments, it can be a typical but non-limiting time such as 1 h, 2 h, 3 h, 4 h, 5 h, or a range between any two times.

[0075] In some embodiments, in the above step S1, the calcination temperature of the calcination step is 500°C to 600°C. In the exemplary embodiment, it can be a typical but non-limiting temperature such as 500°C, 580°C, 600°C, or a range between any two temperatures. It should be noted that it is well known in the art that the calcination is carried out under a hydrogen atmosphere, so it is not particularly limited in the embodiments of the present application; the calcination time is related to the amount of the hydrothermal reaction product, and can be adjusted conventionally according to the knowledge in the art, and is not particularly limited in the embodiments of the present application.

[0076] A second aspect of an embodiment of the present application provides a graphene-modified lithium iron phosphate prepared by the preparation method of graphene-modified lithium iron phosphate provided in an embodiment of the present application, comprising lithium iron phosphate and graphene doped in the lithium iron phosphate.

[0077] The graphene-modified lithium iron phosphate of the embodiment of the present application includes lithium iron phosphate and graphene doped in the lithium iron phosphate (doped with S element and C element), thereby improving the conductivity, cycle stability and rate performance of the lithium iron phosphate.

[0078] In some embodiments, the powder resistivity of the graphene-modified lithium iron phosphate is 5Ω·cm~12Ω·cm. In an exemplary embodiment, it can be a typical but non-limiting powder resistivity such as 5Ω·cm, 8Ω·cm, 10Ω·cm, 12Ω·cm, or a range between any two powder resistivities. In this case, the graphene-modified lithium iron phosphate has a very high lithium ion transfer rate, thereby improving the energy density, cycle stability and safety of the battery.

[0079] A third aspect of an embodiment of the present application provides a battery, wherein the positive electrode active material of the battery includes the graphene-modified lithium iron phosphate provided in an embodiment of the present application.

[0080] The following is explained in conjunction with specific embodiments. All phosphoric acids involved in the following embodiments are concentrated phosphoric acid with a concentration of about 85%; all roastings involved in the following embodiments are carried out under a hydrogen atmosphere; the water used in the ball milling in the following embodiments is only used as a ball milling liquid, and the amount is added in real time according to the actual ball milling needs, and is not particularly limited in the embodiments of this application.

[0081] Example 1

[0082] Example 1 provides a method for preparing graphene-modified lithium iron phosphate, the steps are as follows:

[0083] (1) The mass ratio of phosphoric acid grafted graphene, lithium iron grafted graphene and phosphoric acid is 0.15:0.8:1, and the molar ratio of lithium acetate, ferrous sulfate and phosphoric acid is 0.95:0.95:1, obtaining lithium acetate, ferrous sulfate, phosphoric acid and graphene oxide as reaction raw materials;

[0084] The graphene oxide is divided into two parts: a first graphene oxide and a second graphene oxide, and the mass ratio of the first graphene oxide to the second graphene oxide is 1:1.5.

[0085] (2) Preparation of phosphoric acid grafted graphene

[0086] ① The mass of the first phosphoric acid is obtained by taking the mass ratio of the first graphene oxide to the first phosphoric acid as 0.054:1, and the first graphene oxide and the first phosphoric acid are mixed and ball-milled (the ball-to-material ratio is 1:4, the ball-milling time is 5 hours, and the ball-milling speed is 300 rpm), and then washed to neutrality (pure water immersion for 6 minutes, filtration after each immersion, and the immersion and filtration steps are repeated until the washing water is neutral).

[0087] ② Dry at 110°C until the water is completely removed to obtain phosphoric acid grafted graphene.

[0088] (3) Lithium iron grafted graphene is prepared with a mass ratio of second graphene oxide, lithium hydroxide and ferrous cysteine ​​of 1:4.7:34.7:

[0089] ① Ball mill the mixture of the second graphene oxide, lithium hydroxide and water (ball milling solution): the ball-to-material ratio is 1:3, the ball milling time is 8 hours, and the ball milling speed is 120 rpm.

[0090] ② After ball milling, soak in pure water for 6 minutes, filter after each soaking, and repeat the washing 3 times.

[0091] ③ Dry the washed filter residue at 120° C. until the water is completely removed to obtain lithium-grafted graphene oxide.

[0092] ④ Ball mill the mixture of lithium-grafted graphene oxide, ferrous cysteine ​​and water (ball milling solution): the ball-to-material ratio is 1:3, the ball milling time is 4 hours, and the ball milling speed is 120 rpm.

[0093] ⑤ After ball milling, soak in pure water for 6 minutes, filter after each soaking, and repeat the washing 3 times.

[0094] ⑥ Dry the washed filter residue at 120° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0095] (4) The mass of each component is obtained according to the mass ratio of phosphate grafted graphene, lithium iron grafted graphene and phosphoric acid being 0.15:0.8:1 and the molar ratio of lithium acetate, ferrous sulfate and phosphoric acid being 1:1:1. Lithium acetate, ferrous sulfate, phosphoric acid, phosphate grafted graphene, lithium iron grafted graphene and water (ball milling solution) are mixed into a reaction solution, and then a hydrothermal reaction is carried out at 180°C for 2 hours.

[0096] (5) The product after the hydrothermal reaction is filtered and calcined at 550° C. for 12 h to obtain graphene-modified lithium iron phosphate.

[0097] Example 1 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 6Ω·cm.

[0098] Example 2

[0099] Example 2 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0100] In step (2), phosphoric acid grafted graphene is prepared:

[0101] ① After the first graphene oxide and the first phosphoric acid are mixed and ball-milled (the ball-to-material ratio is 1:5, the ball-milling time is 6 hours, and the ball-milling speed is 200 rpm), they are washed to neutrality (pure water is soaked for 5 minutes, and filtered after each soaking, and the soaking and filtering steps are repeated until the washing water is neutral).

[0102] ② Dry at 100°C until the water is completely removed to obtain phosphoric acid grafted graphene.

[0103] Example 2 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 10Ω·cm.

[0104] Example 3

[0105] Example 3 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0106] In step (2), phosphoric acid grafted graphene is prepared:

[0107] ① After the first graphene oxide and the first phosphoric acid are mixed and ball-milled (the ball-to-material ratio is 1:3, the ball-milling time is 3 hours, and the ball-milling speed is 400 rpm), they are washed to neutrality (pure water is soaked for 8 minutes, and filtration is performed after each soaking, and the soaking and filtration steps are repeated until the washing water is neutral).

[0108] ② Dry at 130°C until the water is completely removed to obtain phosphoric acid grafted graphene.

[0109] Example 3 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 12Ω·cm.

[0110] Example 4

[0111] Example 4 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0112] In step (2), phosphoric acid grafted graphene is prepared:

[0113] ① After the first graphene oxide and the first phosphoric acid are mixed and ball-milled (the ball-to-material ratio is 1:8, the ball-milling time is 12 h, and the ball-milling speed is 400 rpm), they are washed to neutrality (pure water is soaked for 10 min, and filtered after each soaking, and the soaking and filtering steps are repeated until the washing water is neutral).

[0114] ② Dry at 120°C until the water is completely removed to obtain phosphoric acid grafted graphene.

[0115] Example 4 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 11Ω·cm.

[0116] Example 5

[0117] Example 5 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0118] In step (3), lithium iron grafted graphene is prepared:

[0119] ① Ball mill a mixture of the second graphene oxide, lithium carbonate and water (ball milling solution): the ball-to-material ratio is 1:2, the ball milling time is 5 hours, and the ball milling speed is 250 rpm.

[0120] ② After ball milling, soak in pure water for 5 minutes, filter after each soaking, and repeat the washing 5 times.

[0121] ③ Dry the washed filter residue at 100° C. until the water is completely removed to obtain lithium-grafted graphene oxide.

[0122] ④ Ball mill the mixture of lithium-grafted graphene oxide, ferrous chloride and water (ball milling solution): the ball-to-material ratio is 1:4, the ball milling time is 6 hours, and the ball milling speed is 150 rpm.

[0123] ⑤ After ball milling, soak in pure water for 5 minutes, filter after each soaking, and repeat the washing 4 times.

[0124] ⑥ Dry the washed filter residue at 100° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0125] Example 5 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 6Ω·cm.

[0126] Example 6

[0127] Example 6 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those in Example 1, except that:

[0128] In step (3), lithium iron grafted graphene is prepared:

[0129] ① Ball mill the mixture of the second graphene oxide, lithium carbonate and water (ball milling solution): the ball-to-material ratio is 1:5, the ball milling time is 15 h, and the ball milling speed is 300 rpm.

[0130] ② After ball milling, soak in pure water for 10 minutes, filter after each soaking, and repeat the washing 4 times.

[0131] ③ Dry the washed filter residue at 130° C. until the water is completely removed to obtain lithium-grafted graphene oxide.

[0132] ④ Ball mill the mixture of lithium-grafted graphene oxide, ferrous chloride and water (ball milling solution): the ball-to-material ratio is 1:5, the ball milling time is 8 hours, and the ball milling speed is 280 rpm.

[0133] ⑤ After ball milling, soak in pure water for 9 minutes, filter after each soaking, and repeat the washing 5 times.

[0134] ⑥ Dry the washed filter residue at 130° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0135] Example 6 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 9Ω·cm.

[0136] Example 7

[0137] Example 7 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0138] In step (3), lithium iron grafted graphene is prepared:

[0139] ① Ball mill the mixture of the second graphene oxide, lithium carbonate and water (ball milling solution): the ball-to-material ratio is 1:4, the ball milling time is 12 h, and the ball milling speed is 280 rpm.

[0140] ② After ball milling, soak in pure water for 8 minutes, filter after each soaking, and repeat the washing 5 times.

[0141] ③ Dry the washed filter residue at 110° C. until the water is completely removed to obtain lithium-grafted graphene oxide.

[0142] ④ Ball mill the mixture of lithium-grafted graphene oxide, ferrous chloride and water (ball milling solution): the ball-to-material ratio is 1:6, the ball milling time is 10 h, and the ball milling speed is 130 rpm.

[0143] ⑤ After ball milling, soak in pure water for 8 minutes, filter after each soaking, and repeat the washing 3 times.

[0144] ⑥ Dry the washed filter residue at 110° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0145] Example 7 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 7Ω·cm.

[0146] Example 8

[0147] Example 8 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0148] In step (1), the mass ratio of the first graphene to the second graphene is 1:1.2.

[0149] In step (4), the temperature of the hydrothermal reaction is 150° C. and the hydrothermal reaction time is 5 h.

[0150] Example 8 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 5Ω·cm.

[0151] Example 9

[0152] Example 9 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0153] In step (1), the mass ratio of the first graphene to the second graphene is 1:2.

[0154] In step (4), the temperature of the hydrothermal reaction is 180° C. and the hydrothermal reaction time is 1 h.

[0155] Example 9 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 8Ω·cm.

[0156] Example 10

[0157] Example 10 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those in Example 1, except that:

[0158] In step (1), the mass ratio of phosphoric acid grafted graphene, lithium iron grafted graphene and phosphoric acid is 0.2:0.7:1.

[0159] Example 10 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 9Ω·cm.

[0160] Embodiment 11

[0161] Example 11 provides a method for preparing graphene-modified lithium iron phosphate, the steps are basically the same as those in Example 1, except that:

[0162] In step (1), the mass ratio of phosphoric acid grafted graphene, lithium iron grafted graphene and phosphoric acid is 0.1:0.9:1.

[0163] Example 11 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this example: the powder resistivity of the graphene-modified lithium iron phosphate is 10Ω·cm.

[0164] Comparative Example 1

[0165] Comparative Example 1 provides a method for preparing graphene-modified lithium iron phosphate, and the steps are basically the same as those in Example 1, except that:

[0166] The steps for preparing lithium iron grafted graphene in step (3) are as follows:

[0167] ① Ball mill the mixture containing the second graphene oxide, lithium carbonate, ferrous cysteine ​​and water: the ball-to-material ratio is 1:3, the ball milling time is 8 hours, and the ball milling speed is 120 rpm.

[0168] ② After ball milling, soak in pure water for 6 minutes, filter after each soaking, and repeat the washing 3 times.

[0169] ③ Dry the washed filter residue at 120° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0170] Comparative Example 1 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this embodiment: the powder resistivity of the graphene-modified lithium iron phosphate is 25Ω·cm.

[0171] Comparative Example 2

[0172] Comparative Example 2 provides a method for preparing graphene-modified lithium iron phosphate, and the steps are basically the same as those in Example 1, except that:

[0173] The steps for preparing lithium iron grafted graphene in step (3) are as follows:

[0174] ① Ball mill the mixture of the second graphene oxide, ferrous cysteine ​​and water (ball milling solution): the ball-to-material ratio is 1:3, the ball milling time is 4 hours, and the ball milling speed is 120 rpm.

[0175] ② After ball milling, soak in pure water for 6 minutes, filter after each soaking, and repeat the washing 3 times.

[0176] ③ Dry the washed filter residue at 120° C. until the water is completely removed to obtain iron-grafted graphene oxide.

[0177] ④ Ball mill the mixture of iron-grafted graphene oxide, lithium carbonate and water (ball milling solution): the ball-to-material ratio is 1:3, the ball milling time is 8 hours, and the ball milling speed is 120 rpm.

[0178] ⑤ After ball milling, soak in pure water for 6 minutes, filter after each soaking, and repeat the washing 3 times.

[0179] ⑥ Dry the washed filter residue at 120° C. until the water is completely removed to obtain lithium iron grafted graphene.

[0180] Comparative Example 2 also provides a graphene-modified lithium iron phosphate, which is prepared by the preparation method of this embodiment: the powder resistivity of the graphene-modified lithium iron phosphate is 30Ω·cm.

[0181] Comparative Example 3

[0182] Comparative Example 3 provides a method for preparing graphene-modified lithium iron phosphate, the steps are as follows:

[0183] (1) lithium acetate, ferrous sulfate and phosphoric acid are obtained as reaction raw materials with a molar ratio of lithium acetate, ferrous sulfate and phosphoric acid of 0.95:0.95:1 and a doping amount of graphene oxide of 10% of the mass of lithium iron phosphate;

[0184] (2) After lithium acetate, ferrous sulfate, phosphoric acid and water (ball milling solution) are mixed into a reaction solution, a hydrothermal reaction is carried out at 300°C for 2 hours.

[0185] (3) The product after the hydrothermal reaction is filtered and calcined at 800°C for 12 hours to obtain graphene-modified lithium iron phosphate.

[0186] Comparative Example 4

[0187] Comparative Example 4 provides a method for preparing a graphene-modified lithium iron phosphate positive electrode material, the steps are as follows:

[0188] (1) Graphene oxide was dissolved in water and ultrasonically dispersed at 80 kHz for 3 h to prepare a solution with a mass fraction of 0.8%. Then, a 35% mass fraction HCl solution was added, and the volume ratio of the graphene oxide solution to the HCl solution was 4:1. Then, aluminum powder (particle size less than 10 μm) with a mass of 5 times that of the graphene oxide and 0.1 g / L of a surfactant, sodium dodecyl sulfate, were added and reacted for 45 min to obtain a reduced graphene oxide-aluminum solution.

[0189] (2) Add ferrous oxalate, phosphoric acid and lithium carbonate to the reduced graphene oxide-aluminum solution. The molar ratio of ferrous oxalate, phosphoric acid and lithium carbonate is 1:1:1, and the ratio of the total mass of ferrous oxalate, phosphoric acid and lithium carbonate to the mass of the mixed solution is 1:1. Mix well, add 0.3% by mass of ascorbic acid, heat to 70°C and react for 3 hours.

[0190] (3) In a nitrogen atmosphere, the temperature was increased to 150°C at a heating rate of 5°C / min and dried for 12 h. The temperature was then increased to 800°C at a heating rate of 5°C / min and calcined for 10 h to obtain a graphene-modified lithium iron phosphate positive electrode material.

[0191] In order to verify the progress of the graphene-modified lithium iron phosphate and its preparation method provided in the embodiments of the present application, taking Embodiment 1 as an example, the graphene-modified lithium iron phosphate prepared in Embodiment 1 was scanned by a scanning electron microscope to obtain the corresponding Figure 1 The graphene-modified lithium iron phosphate of the embodiment and the comparative example was then made into button batteries, and then the button batteries were subjected to electrical performance tests including 0.1C discharge, initial charge and discharge efficiency, 5C discharge and 1C cycle 500 times capacity retention rate in the voltage range of 2.0V~3.8V using a blue electric device. The test results are shown in Table 1 below.

[0192] in,

[0193] The preparation steps of button cells are as follows:

[0194] (1) Preparation of positive electrode slurry: Graphene-modified lithium iron phosphate: conductive carbon black: polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and solvent N-methylpyrrolidone (NMP) was added to adjust the viscosity to 4000 mPa·s~8000 mPa·s to prepare positive electrode slurry.

[0195] (2) The positive electrode slurry is evenly coated on one surface (perpendicular to the thickness direction) of the aluminum foil (positive electrode current collector) and dried, and then rolled and sliced ​​to obtain a positive electrode sheet containing a positive electrode film layer.

[0196] (5) Using lithium hexafluorophosphate-ethyl methyl carbonate as the electrolyte and polypropylene as the isolation membrane, the prepared positive electrode sheet and negative lithium sheet are combined to make a button half-cell.

[0197]

[0198] From Table 1 above and the instructions attached Figure 1 At least the following conclusions can be drawn:

[0199] 1) Instruction manual Figure 1 It can be seen that the graphene-modified lithium iron phosphate contains graphene (wrinkled in the figure); combined with Table 1, it can be obtained that in the graphene-modified lithium iron phosphate provided in the embodiment of the present application, the selected ferrous cysteine ​​can also dope the lithium iron phosphate with S and C elements, combined with graphene modification, not only the structural stability of the lithium iron phosphate is improved, but also the conductivity of the graphene-modified lithium iron phosphate is improved. Graphene-modified lithium iron phosphate has excellent conductivity, cycle stability and rate performance.

[0200] 2) The preparation method of the graphene-modified lithium iron phosphate of the embodiment of the present application is to subject the reaction solution containing lithium source, iron source, phosphoric acid, phosphoric acid-grafted graphene and lithium iron-grafted graphene to hydrothermal reaction. Phosphoric acid-grafted graphene and lithium iron-grafted graphene are not only used as raw materials of graphene, but also as "seeds" for hydrothermal reaction to generate graphene-modified lithium iron phosphate, which can promote the reaction of lithium source, iron source and phosphoric acid to generate lithium iron phosphate, thereby improving the reaction rate and yield; graphene-modified lithium iron phosphate improves the structural stability and conductivity of lithium iron phosphate. The graphene-modified lithium iron phosphate prepared by this preparation method has excellent conductivity, cycle stability and rate performance.

[0201] 3) In the preparation method of graphene-modified lithium iron phosphate of the embodiment of the present application, the radius of the lithium ion is smaller than the radius of the ferrous ion. The lithium ion is first grafted onto the second graphene oxide, so that the lithium ion can be fully grafted, and then the ferrous cysteine ​​is grafted onto the lithium-grafted graphene oxide, thereby avoiding the situation where the lithium ion grafting is difficult or impossible due to the presence of ferrous cysteine ​​with a large radius. In addition, when ferrous cysteine ​​is grafted onto the lithium-grafted graphene oxide, during the hydrothermal reaction, ferrous cysteine ​​can not only release ferrous ions slowly and control the reaction rate, but also the reducibility of cysteine ​​can prevent ferrous ions from being oxidized to ferric ions, thereby reducing the occurrence of side reactions and improving the yield. In addition, the S element and C element contained in the ferrous cysteine ​​are doped in the lithium iron phosphate after roasting, which improves the lithium ion diffusion rate, thereby improving the cycle performance and charge and discharge efficiency of the battery. Furthermore, the low decomposition temperature of cysteine ​​allows the hydrothermal reaction product to be calcined at a relatively low temperature (500°C~600°C) to obtain graphene-modified lithium iron phosphate.

[0202] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing graphene-modified lithium iron phosphate, characterized in that: The steps include: The reaction solution is subjected to a hydrothermal reaction and then calcined to obtain graphene-modified lithium iron phosphate; the reaction solution contains a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene, and lithium iron-grafted graphene; The mass ratio of the phosphoric acid grafted graphene, the lithium iron grafted graphene and the phosphoric acid is 0.1-0.2:0.7-0.9:1; The preparation of the lithium iron grafted graphene comprises the following steps: The first solution is subjected to a first ball milling, and then washed and dried to obtain lithium-grafted graphene oxide; the first solution contains graphene oxide and a lithium source; The second solution is subjected to a second ball milling, and then washed and dried to obtain the lithium iron grafted graphene; the second solution contains the lithium grafted graphene oxide and ferrous cysteine; The preparation of the phosphoric acid grafted graphene comprises the following steps: The graphene oxide and phosphoric acid are mixed and ball-milled, then washed to neutrality and dried to obtain the phosphoric acid-grafted graphene.

2. The method for preparing graphene-modified lithium iron phosphate according to claim 1, characterized in that: In the first ball milling step, the ball-to-material ratio is 1:2-5, the ball milling time is 5h-15h, and the ball milling speed is 100rpm-300rpm; In the second ball milling step, the ball-to-material ratio is 1:3-6, the ball milling time is 4h-10h, and the ball milling speed is 100rpm-300rpm.

3. The method for preparing graphene-modified lithium iron phosphate according to claim 1, characterized in that: In the step of preparing the phosphoric acid grafted graphene, the ball-to-material ratio of ball milling is 1:3-8, the ball milling time is 3h-12h, the ball milling speed is 200rpm-400rpm, and the drying temperature is 100°C-130°C.

4. The method for preparing graphene-modified lithium iron phosphate according to any one of claims 1 to 3, characterized in that: The calcination temperature is 500°C to 700°C.

5. A graphene-modified lithium iron phosphate prepared by the method for preparing graphene-modified lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The invention comprises lithium iron phosphate and graphene doped in the lithium iron phosphate.

6. The graphene-modified lithium iron phosphate according to claim 5, characterized in that: The powder resistivity of the graphene-modified lithium iron phosphate is 5Ω·cm~12Ω·cm.

7. A battery, characterized in that: The positive electrode active material of the battery comprises the graphene-modified lithium iron phosphate according to any one of claims 5 to 6.

8. Use of the battery as claimed in claim 7 in household appliances, electric vehicles, electric vehicles, electric trains, ships, satellites or energy storage systems.

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