Modified lithium iron phosphate and its preparation method and application

Abstract

The present application relates to the technical field of positive electrode active materials, and specifically to a modified lithium iron phosphate and its preparation method and application. The preparation method comprises the following steps: ball milling a ball milling solution in supercritical water and then calcining to obtain modified lithium iron phosphate; the ball milling solution contains a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide. In the preparation method, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide not only serve as raw materials for graphene, but also promote the directional reaction of lithium source, iron source and phosphoric acid to generate lithium iron phosphate during ball milling, thereby increasing the reaction rate and the yield of lithium iron phosphate; supercritical water has excellent solubility and good mass transfer characteristics, which promotes the ball milling solution to fully disperse and dissolve in supercritical water to form a precursor, and the modified lithium iron phosphate obtained by calcination has excellent conductivity, cycle stability and rate performance.

Description

Technical Field

[0001] The present application relates to the technical field of positive electrode active materials, and in particular to a modified lithium iron phosphate and a preparation method and 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] The existing preparation methods of lithium iron phosphate mainly include solid phase method and liquid phase method. However, the synthesis reaction of lithium iron phosphate is a complex multiphase reaction involving phosphate, iron salt (or oxide), lithium salt, carbon precursor and reducing gas phase; therefore, the conductivity, cycle stability and rate performance of the lithium iron phosphate obtained are poor, and the quality of the finished products obtained from different production batches varies greatly. Summary of the invention

[0004] The present application provides a modified lithium iron phosphate and a preparation method and application thereof. The preparation method at least solves the technical problem that the conductivity, cycle stability and rate performance of the lithium iron phosphate are all poor.

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

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

[0007] In supercritical water, the ball milling liquid is ball milled and then calcined to obtain modified lithium iron phosphate;

[0008] The ball milling solution comprises a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide, and lithium-iron-grafted graphene oxide;

[0009] The preparation of lithium iron grafted graphene oxide includes the following steps:

[0010] After subjecting the first solution to a first microwave ultrasound, the solution is filtered, washed and dried to obtain lithium-grafted graphene oxide; the first solution contains first graphene oxide and lithium chloride;

[0011] The second solution is subjected to a second microwave ultrasound, filtered, washed and dried to obtain the lithium iron grafted graphene oxide; the second solution contains the lithium grafted graphene oxide and the phenylalanine ferrous complex.

[0012] In some possible implementations, the mass ratio of the phosphoric acid grafted graphene oxide, the lithium iron grafted graphene oxide and the phosphoric acid is (0.1-0.2):(0.7-0.9):1.

[0013] In some possible implementations, the step of preparing the second solution includes:

[0014] Under an argon atmosphere and a stirring condition of 100 rpm to 200 rpm, phenylalanine is added to the ferrous chloride aqueous solution to obtain a phenylalanine ferrous complex solution, wherein the pH value of the ferrous chloride aqueous solution is 3 to 5;

[0015] Under stirring conditions at a rotation speed of 80 rpm to 150 rpm, lithium-grafted graphene oxide is added to the phenylalanine ferrous complex solution to obtain a second solution.

[0016] In some possible implementations, the mass ratio of the ferrous chloride, the phenylalanine, the lithium chloride, and the first graphene oxide is (2.9-5.8):(20-48):(2.5-5):1.

[0017] In some possible implementations, in the ball milling, the ball-to-material ratio is 1:(3-6), the ball milling time is 8h-20h, and the ball milling speed is 200rpm-400rpm.

[0018] In some possible implementations, the calcination temperature is 620°C to 700°C.

[0019] In some possible implementations, preparing the phosphoric acid grafted graphene oxide comprises the following steps:

[0020] The graphene oxide and phosphoric acid are mixed and subjected to a third microwave ultrasound, and then washed and dried to obtain the phosphoric acid-grafted graphene oxide.

[0021] In some possible implementations, the microwave power of the first microwave ultrasound is 150W~300W, the ultrasound frequency is 60kHz~80kHz; and the microwave ultrasound time is 30min~1h.

[0022] In some possible implementations, the microwave power of the second microwave ultrasound is 150W~300W, the ultrasound frequency is 60kHz~80kHz; and the microwave ultrasound time is 30min~1h.

[0023] In some possible implementations, the microwave power of the third microwave ultrasound is 150W~300W, the ultrasound frequency is 60kHz~80kHz; and the microwave ultrasound time is 30min~1h.

[0024] In a second aspect, the present application provides a modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate of the present application.

[0025] In a third aspect, the present application provides an application of the modified lithium iron phosphate provided in the present application in the field of lithium battery preparation.

[0026] A modified lithium iron phosphate and a preparation method thereof provided in the present application have at least the following beneficial technical effects compared with the prior art:

[0027] (1) The preparation method of the modified lithium iron phosphate of the present application comprises the following steps: ball milling a ball milling liquid containing a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide in supercritical water. The phosphoric acid-grafted graphene oxide and the lithium iron-grafted graphene oxide not only serve as raw materials for graphene, but also serve as "seeds" during ball milling to promote the directional reaction of the lithium source, the iron source and phosphoric acid to generate lithium iron phosphate, thereby improving the reaction rate and yield. Supercritical water has excellent solubility and good mass transfer characteristics, which promotes the ball milling liquid to be fully dispersed and dissolved in supercritical water and react to form a precursor. The precursor is then calcined to obtain a modified lithium iron phosphate having excellent conductivity, cycle stability and rate performance.

[0028] (2) In the preparation method of the modified lithium iron phosphate of the present application, during ball milling, the graphite-grafted phenylalanine ferrous complex can slowly release ferrous ions and control the reaction rate. In addition, the phenylalanine ferrous complex contains N and C elements. After calcination, the N and C elements are in-situ doped in the molecular gaps of the lithium iron phosphate, which can improve the lithium ion diffusion rate, thereby improving the battery's cycle performance and charge and discharge efficiency.

[0029] (3) The preparation method of the modified lithium iron phosphate of the present application, ball milling of the ball milling liquid in supercritical water, can generate lithium iron phosphate and graphene doping in one step. The preparation steps are simple, the cost is low, the preparation conditions are mild, and it is easy to implement.

[0030] (4) The modified lithium iron phosphate of the present application uses graphene to coat lithium iron phosphate and is doped with S and C elements, thereby improving the conductivity, cycle stability and rate performance of the lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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.

[0032] Figure 1 This is a SEM image of the modified lithium iron phosphate in Example 1 of the present application;

[0033] Figure 2This is the EDS element mapping general diagram of the modified lithium iron phosphate in Example 1 of the present application;

[0034] Figure 3 This is the EDS element mapping diagram of phosphorus, oxygen, iron and carbon in the modified lithium iron phosphate in Example 1 of the present application;

[0035] Figure 4 This is the EDS elemental analysis diagram of the modified lithium iron phosphate in Example 1 of the present application.

[0036] 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

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] Unless otherwise defined, all professional terms used below have the same meaning 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 this application.

[0045] 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.

[0046] 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.

[0047] The present invention provides a method for preparing modified lithium iron phosphate, comprising the following steps:

[0048] S1. In supercritical water, the ball milling liquid is ball milled and then calcined to obtain modified lithium iron phosphate.

[0049] The ball milling solution contains a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide, and lithium-iron-grafted graphene oxide;

[0050] The preparation of lithium iron grafted graphene oxide includes the following steps:

[0051] After subjecting the first solution to a first microwave ultrasound, the solution is filtered, washed and dried to obtain lithium-grafted graphene oxide; the first solution contains first graphene oxide and lithium chloride;

[0052] The second solution is subjected to a second microwave ultrasound, filtered, washed and dried to obtain lithium iron grafted graphene oxide; the second solution contains lithium grafted graphene oxide and phenylalanine ferrous complex.

[0053] The preparation method of the modified lithium iron phosphate of the present application is to ball-mill a ball-milling liquid containing a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide in supercritical water. Phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide are not only used as raw materials for graphene, but also as "seeds" during ball milling, which promote the lithium source, iron source and phosphoric acid to react in a directional manner to generate lithium iron phosphate, thereby increasing the reaction rate and the yield of lithium iron phosphate; supercritical water has excellent solubility and good mass transfer characteristics, which promotes the ball-milling liquid to be fully dispersed and dissolved in supercritical water, and react to form a precursor, and then calcine, and the obtained modified lithium iron phosphate 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.

[0054] In some embodiments, in the above step S1, the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid is (0.1~0.2):(0.7~0.9):1, which can be a typical mass ratio of 0.10:0.8:1, 0.2:0.7:1, 0.2:0.8:1, 0.15:0.85:1, 0.1:0.9:1, etc., or a range between any two mass ratios.

[0055] In some embodiments, in the above step S1, the iron source includes at least one of ferrous sulfate, ferrous chloride, and ferrous oxalate.

[0056] In some embodiments, in the above step S1, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium acetate.

[0057] In some embodiments, in the above step S1, the step of preparing the first solution includes: adding the first graphene oxide to the lithium chloride solution under a stirring condition of a rotation speed of 80 rpm to 150 rpm to obtain the first solution.

[0058] In some specific embodiments, the mass fraction of lithium chloride in the lithium chloride solution is 30% to 45%.

[0059] In some embodiments, in the above step S1, the microwave power of the first microwave ultrasound is 150W~300W, the ultrasound frequency is 60kHz~80kHz; the microwave ultrasound time is 30min~1h. In some embodiments, in the above step S1, the temperature of the first microwave ultrasound is 50℃~100℃, and in the exemplary example, it can be a typical temperature such as 50℃, 80℃, 100℃ or a range between any two temperatures. In this case, the grafting reaction rate can be accelerated without causing side reactions due to overheating.

[0060] In some embodiments, in the above step S1, after the first microwave ultrasound, pure water is used for suction filtration and washing, and the number of suction filtration and washing is 3 to 5 times. In this case, the use of pure water can remove excess reaction impurities and prevent impurities from affecting the subsequent reaction.

[0061] In some embodiments, in the above step S1, when preparing lithium-grafted graphene oxide, the drying temperature is 100° C. to 130° C., and in the exemplary embodiment, it can be a typical 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-grafted graphene.

[0062] In some embodiments, in the above step S1, the step of preparing the second solution includes:

[0063] Under an argon atmosphere and a stirring condition of 100 rpm to 200 rpm, phenylalanine is added to the ferrous chloride aqueous solution to obtain a phenylalanine ferrous complex solution, wherein the pH value of the ferrous chloride aqueous solution is 3 to 5;

[0064] Under the stirring condition of a rotation speed of 80 rpm to 150 rpm, lithium-grafted graphene oxide is added to the phenylalanine ferrous complex solution to obtain a second solution.

[0065] In some specific embodiments, the mass fraction of ferrous chloride in the ferrous chloride aqueous solution is 20% to 35%. At this concentration, the concentration of ferrous chloride is relatively high and is easy to form a complex with phenylalanine.

[0066] In some embodiments, the mass ratio of ferrous chloride, phenylalanine, lithium chloride and the first graphene oxide is (2.9-5.8): (20-48): (2.5-5): 1. In the exemplary embodiment, it can be a typical mass ratio such as 2.9:20:2.5:1, 4:40:3.45:1, 5.8:48:5:1 or a range between any two mass ratios. In this case, the excess phenylalanine can fully complex with ferrous chloride to form a complex, and the lithium chloride can be fully grafted with the first graphene oxide to increase the grafting rate. It should be noted that the mass ratio of ferrous chloride, phenylalanine, lithium chloride and the first graphene oxide is the original mass ratio, and their loss cannot be accurately detected during the preparation of the first solution and the second solution.

[0067] In some embodiments, in the above step S1, the microwave power of the second microwave ultrasound is 150W~300W, the ultrasound frequency is 60kHz~80kHz; the microwave ultrasound time is 30min~1h. In some embodiments, in the above step S1, the temperature of the second microwave ultrasound is 50℃~100℃, and in the exemplary example, it can be a typical temperature such as 50℃, 80℃, 100℃ or a range between any two temperatures. In this case, the grafting reaction rate can be accelerated without causing side reactions due to overheating.

[0068] In some embodiments, in the above step S1, after the second microwave ultrasound, pure water is used for suction filtration and washing, and the number of suction filtration and washing is 3 to 5 times. In this case, the use of pure water can remove excess reaction impurities and prevent impurities from affecting the subsequent reaction.

[0069] In some embodiments, in the above step S1, when drying after the second microwave ultrasound and suction filtration washing, the drying temperature is 100°C to 130°C. In the exemplary embodiment, it can be a typical 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 iron grafted graphene oxide.

[0070] In some embodiments, in the above step S1, preparing phosphoric acid grafted graphene oxide comprises the following steps:

[0071] S1.1. The second graphene oxide is mixed with phosphoric acid and subjected to a third microwave ultrasound treatment, and then washed and dried to obtain phosphoric acid-grafted graphene oxide.

[0072] In the above step of preparing phosphoric acid grafted graphene oxide, the second graphene oxide is mixed with phosphoric acid and ball-milled to make the phosphate group easy to graft with the second graphene oxide; washing is performed to remove excess phosphoric acid, and the phosphoric acid grafted graphene oxide is obtained after drying. It should be noted that the amount of phosphoric acid used here is not included in the amount of phosphoric acid used in the ball-milling solution.

[0073] In some embodiments, in the above step S1.1, the mass ratio of the second graphene oxide to phosphoric acid is (0.03-0.08):1. In the exemplary embodiment, it can be a typical mass ratio such as 0.03:1, 0.054:1, 0.073:1, 0.08:1, or a range between any two mass ratios.

[0074] In some embodiments, in the above step S1.1, the mass concentration of phosphoric acid is 75% to 85%.

[0075] In some embodiments, in the above step S1.1, the microwave power of the third microwave ultrasound is 150W-300W, the ultrasound frequency is 60kHz-80kHz, and the microwave ultrasound time is 30min-1h. In this case, microwave ultrasound accelerates the graphene oxide to fully contact and graft with phosphoric acid to obtain phosphoric acid-grafted graphene oxide.

[0076] In some embodiments, in the above step S1.1, the temperature of the third microwave ultrasound is 30°C to 70°C. In the exemplary embodiment, it can be a typical temperature such as 30°C, 50°C, 70°C, or a range between any two temperatures. In this temperature range, the grafting reaction rate can be accelerated without causing side reactions due to overheating.

[0077] In some embodiments, in the above step S1.1, in the washing step, pure water is used for suction filtration and washing until the filtrate is neutral. In this case, the use of pure water for suction filtration and washing can remove excess impurities such as phosphoric acid to prevent the impurities from affecting the subsequent reaction.

[0078] 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 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 oxide. It should be noted that the drying time can be determined according to the actual production volume and is not particularly limited in the embodiments of the present application.

[0079] In some embodiments, the total mass of the first graphene oxide and the second graphene oxide is the mass of graphene in the modified lithium iron phosphate.

[0080] In some embodiments, in the above step S1, the step of preparing the ball milling solution comprises:

[0081] According to the set raw material ratio, under the stirring condition of a rotation speed of 100 rpm to 200 rpm, a lithium source, an iron source, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide are added into phosphoric acid to obtain a ball milling solution.

[0082] In some embodiments, in the above step S1, during ball milling, the ball-to-material ratio (volume ratio) is 1:(3-6), the ball milling time is 8h-20h, and the ball milling speed is 200rpm-400rpm. In this case, the yield of the lithium iron phosphate precursor is improved in synergy with supercritical water and the particle size of the lithium iron phosphate particles is refined.

[0083] In some specific embodiments, the grinding balls are at least one of zirconium oxide grinding balls, stainless steel grinding balls, and agate.

[0084] In some embodiments, the grinding balls include large-diameter grinding balls, medium-diameter grinding balls, and small-diameter grinding balls, and the number ratio is 2: (10-12): (20-25). In this case, the large-diameter grinding balls, the medium-diameter grinding balls, and the small-diameter grinding balls fully cooperate to promote the generation and refinement of lithium iron phosphate particles.

[0085] In some embodiments, the diameter of the large-diameter grinding ball is 80 mm to 100 mm, the diameter of the medium-diameter grinding ball is 50 mm to 70 mm, and the diameter of the small-diameter grinding ball is 20 mm to 25 mm.

[0086] In some embodiments, in the above step S1, the calcination temperature is 620°C to 700°C. In the exemplary embodiment, it can be a typical temperature such as 620°C, 680°C, 700°C, or a range between any two temperatures. It should be noted that the calcination time is related to the amount of the reaction product and can be adjusted conventionally according to the common knowledge in the art, and is not particularly limited in the embodiments of the present application.

[0087] In some embodiments, a method for preparing modified lithium iron phosphate is provided, the steps are as follows:

[0088] S11. Preparation of LiFe-grafted Graphene Oxide

[0089] S111. According to the mass ratio of ferrous chloride, phenylalanine, lithium carbonate and first graphene oxide of (2.9-5.8):(20-48):(2.5-5):1, prepare a ferrous chloride solution with a mass fraction of 20%-35% and a lithium chloride solution with a mass fraction of 30%-45%.

[0090] S112. Under a stirring condition of a rotation speed of 80 rpm to 150 rpm, add the first graphene oxide to the lithium chloride solution to obtain a first solution.

[0091] S113. After subjecting the first solution to a first microwave ultrasound for 30 minutes to 1 hour, the solution is filtered and washed with pure water for 3 to 5 times, and dried at 100°C to 130°C to obtain lithium-grafted graphene oxide; wherein the temperature of the first microwave ultrasound is 50°C to 100°C, the microwave power is 150W to 300W, and the ultrasonic frequency is 60kHz to 80kHz.

[0092] S114. The pH value of the aqueous solution of ferrous chloride was adjusted to 3 to 5 with hydrochloric acid; phenylalanine was added to the aqueous solution of ferrous chloride under an argon atmosphere and a stirring speed of 100 rpm to 200 rpm to obtain a phenylalanine ferrous complex solution;

[0093] Under the stirring condition of a rotation speed of 80 rpm to 150 rpm, the lithium-grafted graphene oxide prepared in step S113 is added to the phenylalanine ferrous complex solution to obtain a second solution.

[0094] S115. After subjecting the second solution to a second microwave ultrasound for 30 minutes to 1 hour, the solution is filtered and washed with pure water for 3 to 5 times, and dried at 100°C to 130°C to obtain lithium iron grafted graphene oxide; wherein the temperature of the second microwave ultrasound is 50°C to 100°C, the microwave power is 150W to 300W, and the ultrasonic frequency is 60kHz to 80kHz.

[0095] S21. Preparation of Phosphoric Acid Grafted Graphene Oxide

[0096] S211. The mass ratio of the second graphene oxide and phosphoric acid is (0.03~0.08):1. After the second graphene oxide and phosphoric acid are mixed and subjected to the third microwave ultrasound, they are filtered and washed with pure water until the filtrate is neutral; wherein, the temperature of the third microwave ultrasound is 30℃~70℃, the microwave power is 150W~300W, and the ultrasonic frequency is 60kHz~80kHz; and the microwave ultrasound time is 30min~1h.

[0097] S212. Dry the filter residue at 100° C. to 130° C. until the water is completely removed to obtain phosphoric acid grafted graphene oxide.

[0098] S31. Preparation of ball milling solution

[0099] S311. Prepare ball milling liquid raw materials according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid of 0.2:0.7:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid of 1:1:1.

[0100] S312. According to the set raw material ratio, under the stirring condition of a rotation speed of 100rpm~200rpm, phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball milling medium to obtain a ball milling liquid.

[0101] S41.Ball milling

[0102] S411. Add stainless steel grinding balls to the ball milling liquid at a ratio of 1: (3~6) between balls and materials; among the grinding balls, the ratio of the number of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2: (10~12): (20~25), the diameter of the large-diameter grinding balls is 80mm~100mm, the diameter of the medium-diameter grinding balls is 50mm~70mm, and the diameter of the small-diameter grinding balls is 20mm~25mm.

[0103] S412. In supercritical water, ball mill the ball milling liquid (including stainless steel grinding balls) at a ball milling rate of 200 rpm to 400 rpm for 8 h to 20 h.

[0104] S51. After ball milling, the mixture is calcined at 620°C to 700°C to obtain modified lithium iron phosphate.

[0105] A second aspect of an embodiment of the present application provides a modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate provided in an embodiment of the present application.

[0106] The modified lithium iron phosphate of the embodiment of the present application includes lithium iron phosphate and graphene, N element and C element doped in the lithium iron phosphate, thereby improving the conductivity, cycle stability and rate performance of the lithium iron phosphate.

[0107] A third aspect of the embodiments of the present application provides an application of the modified lithium iron phosphate provided in the embodiments of the present application in the field of lithium battery preparation.

[0108] The following describes the invention in conjunction with specific embodiments.

[0109] Example 1

[0110] Example 1 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0111] (1) Preparation of lithium iron grafted graphene oxide:

[0112] ① According to the mass ratio of ferrous chloride, phenylalanine, lithium carbonate and first graphene oxide of 4:40:3.45:1, prepare a 30% by mass fraction ferrous chloride solution and a 35% by mass fraction lithium chloride solution.

[0113] ② Under the stirring condition of a rotation speed of 100 rpm, add the first graphene oxide to the lithium chloride solution to obtain a first solution.

[0114] ③ After subjecting the first solution to the first microwave ultrasound for 40 minutes, the solution was filtered and washed four times with pure water, and dried at 120° C. to obtain lithium-grafted graphene oxide; wherein the temperature of the first microwave ultrasound was 80° C., the microwave power was 200 W, and the ultrasonic frequency was 70 kHz.

[0115] ④ Adjusting the pH value of the aqueous solution of ferrous chloride to 3-5 with hydrochloric acid; adding phenylalanine to the aqueous solution of ferrous chloride under an argon atmosphere and a stirring condition of 180 rpm to obtain a phenylalanine ferrous complex solution;

[0116] Under stirring conditions at a rotation speed of 100 rpm, the lithium-grafted graphene oxide obtained in step ③ was added to the phenylalanine ferrous complex solution to obtain a second solution.

[0117] ⑤ After subjecting the second solution to a second microwave ultrasound for 1 hour, the solution was filtered and washed 4 times with pure water, and dried at 120° C. to obtain lithium iron grafted graphene oxide; wherein the temperature of the second microwave ultrasound was 60° C., the microwave power was 180 W, and the ultrasonic frequency was 80 kHz.

[0118] (2) Preparation of phosphoric acid grafted graphene oxide

[0119] ① The mass ratio of the second graphene oxide and phosphoric acid is 0.054:1. After the second graphene oxide and phosphoric acid are mixed and subjected to the third microwave ultrasound, they are filtered and washed with pure water until the filtrate is neutral; wherein, the temperature of the third microwave ultrasound is 40° C., the microwave power is 150 W, and the ultrasonic frequency is 60 kHz; and the microwave ultrasound time is 1 hour.

[0120] ② Dry the filter residue at 110° C. until the water is completely removed to obtain phosphoric acid-grafted graphene oxide.

[0121] (3) Preparation of ball milling solution:

[0122] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.2:0.7:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0123] ② According to the set raw material ratio, under the stirring condition of a rotation speed of 150 rpm, phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball milling medium to obtain a ball milling solution.

[0124] (4) Ball milling:

[0125] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:5; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:11:22, the diameter of the large-diameter grinding balls is 90 mm, the diameter of the medium-diameter grinding balls is 60 mm, and the diameter of the small-diameter grinding balls is 20 mm.

[0126] ② In supercritical water, ball mill the ball milling liquid (containing stainless steel grinding balls) at a ball milling rate of 400 rpm for 8 hours.

[0127] (5) After ball milling, the mixture was calcined at 650°C for 12 h to obtain modified lithium iron phosphate.

[0128] Example 2

[0129] Example 2 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0130] (3) Prepare the ball milling solution:

[0131] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.1:0.8:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0132] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0133] (4) Ball milling:

[0134] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:5; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:11:22, the diameter of the large-diameter grinding balls is 90 mm, the diameter of the medium-diameter grinding balls is 60 mm, and the diameter of the small-diameter grinding balls is 20 mm.

[0135] ② In supercritical water, ball mill the ball milling liquid (including stainless steel grinding balls) at a ball milling rate of 300 rpm for 15 h.

[0136] Example 3

[0137] Example 3 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0138] (3) Prepare the ball milling solution:

[0139] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.1:0.9:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0140] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0141] (4) Ball milling:

[0142] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:6; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:12:25, the diameter of the large-diameter grinding balls is 100 mm, the diameter of the medium-diameter grinding balls is 70 mm, and the diameter of the small-diameter grinding balls is 25 mm.

[0143] ② In supercritical water, ball mill the ball milling liquid (including stainless steel grinding balls) at a ball milling rate of 200 rpm for 20 hours.

[0144] Example 4

[0145] Example 4 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0146] (3) Prepare the ball milling solution:

[0147] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.1:0.9:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0148] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0149] (4) Ball milling:

[0150] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:6; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:10:20, the diameter of the large-diameter grinding balls is 80 mm, the diameter of the medium-diameter grinding balls is 50 mm, and the diameter of the small-diameter grinding balls is 20 mm.

[0151] ② In supercritical water, ball mill the ball milling liquid (including stainless steel grinding balls) at a ball milling rate of 400 rpm for 18 h.

[0152] Example 5

[0153] Example 5 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0154] (3) Prepare the ball milling solution:

[0155] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.2:0.9:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0156] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0157] (4) Ball milling:

[0158] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:3; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:10:20, the diameter of the large-diameter grinding balls is 80 mm, the diameter of the medium-diameter grinding balls is 50 mm, and the diameter of the small-diameter grinding balls is 20 mm.

[0159] ② In supercritical water, ball mill the ball milling liquid (including stainless steel grinding balls) at a ball milling rate of 400 rpm for 18 h.

[0160] Example 6

[0161] Example 6 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0162] (3) Prepare the ball milling solution:

[0163] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.15:0.85:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0164] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0165] (4) Ball milling:

[0166] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:4; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:12:25, the diameter of the large-diameter grinding balls is 100 mm, the diameter of the medium-diameter grinding balls is 70 mm, and the diameter of the small-diameter grinding balls is 25 mm.

[0167] ② In supercritical water, ball mill the ball milling liquid (containing stainless steel grinding balls) at a ball milling rate of 400 rpm for 20 hours.

[0168] Example 7

[0169] Example 7 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0170] (3) Prepare the ball milling solution:

[0171] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid grafted graphene oxide, lithium iron grafted graphene oxide and phosphoric acid being 0.2:0.8:1, and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0172] ② Phosphoric acid-grafted graphene oxide, lithium iron-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0173] (4) Ball milling:

[0174] ① Add stainless steel grinding balls to the ball milling liquid at a ball-to-material ratio of 1:3; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:12:25, the diameter of the large-diameter grinding balls is 80 mm, the diameter of the medium-diameter grinding balls is 70 mm, and the diameter of the small-diameter grinding balls is 25 mm.

[0175] ② In supercritical water, ball mill the ball milling liquid (containing stainless steel grinding balls) at a ball milling rate of 400 rpm for 20 hours.

[0176] Example 8

[0177] Example 8 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0178] (1) Preparation of lithium iron grafted graphene oxide:

[0179] ① According to the mass ratio of ferrous chloride, phenylalanine, lithium carbonate and first graphene oxide of 2.9:20:2.5:1, prepare a 30% by mass fraction ferrous chloride solution and a 35% by mass fraction lithium chloride solution.

[0180] ② Under the stirring condition of a rotation speed of 150 rpm, add the first graphene oxide to the lithium chloride solution to obtain a first solution.

[0181] ③ After subjecting the first solution to the first microwave ultrasound for 30 minutes, the solution was filtered and washed 5 times with pure water, and dried at 120° C. to obtain lithium-grafted graphene oxide; wherein the temperature of the first microwave ultrasound was 100° C., the microwave power was 300 W, and the ultrasonic frequency was 80 kHz.

[0182] ④ Adjust the pH value of the aqueous solution of ferrous chloride to 3-5 with hydrochloric acid; add phenylalanine to the aqueous solution of ferrous chloride under an argon atmosphere and a stirring condition of 200 rpm to obtain a phenylalanine ferrous complex solution;

[0183] Under stirring conditions at a rotation speed of 150 rpm, the lithium-grafted graphene oxide obtained in step ③ was added to the phenylalanine ferrous complex solution to obtain a second solution.

[0184] ⑤ After subjecting the second solution to a second microwave ultrasound for 1 hour, the solution was filtered and washed three times with pure water, and dried at 130° C. to obtain lithium iron grafted graphene oxide; wherein the temperature of the second microwave ultrasound was 80° C., the microwave power was 150 W, and the ultrasonic frequency was 70 kHz.

[0185] Example 9

[0186] Example 9 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0187] (1) Preparation of lithium iron grafted graphene oxide:

[0188] ① According to the mass ratio of ferrous chloride, phenylalanine, lithium carbonate and the first graphene oxide of 5.8:48:5:1, prepare a 30% by mass fraction ferrous chloride solution and a 35% by mass fraction lithium chloride solution.

[0189] ② Under the stirring condition of a rotation speed of 80 rpm, add the first graphene oxide to the lithium chloride solution to obtain a first solution.

[0190] ③ After subjecting the first solution to the first microwave ultrasound for 1 hour, the solution was filtered and washed 5 times with pure water, and dried at 120° C. to obtain lithium-grafted graphene oxide; wherein the temperature of the first microwave ultrasound was 80° C., the microwave power was 300 W, and the ultrasonic frequency was 80 kHz.

[0191] ④ Adjust the pH value of the aqueous solution of ferrous chloride to 3-5 with hydrochloric acid; add phenylalanine to the aqueous solution of ferrous chloride under an argon atmosphere and a stirring condition of 100 rpm to obtain a phenylalanine ferrous complex solution;

[0192] Under stirring conditions at a rotation speed of 80 rpm, the lithium-grafted graphene oxide obtained in step ③ was added to the phenylalanine ferrous complex solution to obtain a second solution.

[0193] ⑤ After subjecting the second solution to a second microwave ultrasound for 1 hour, the solution was filtered and washed 5 times with pure water, and dried at 100° C. to obtain lithium iron grafted graphene oxide; wherein the temperature of the second microwave ultrasound was 100° C., the microwave power was 300 W, and the ultrasonic frequency was 80 kHz.

[0194] Comparative Example 1

[0195] Comparative Example 1 provides a method for preparing lithium iron phosphate, the steps are as follows:

[0196] (1) lithium acetate, ferrous oxalate and phosphoric acid are obtained as reaction raw materials with a molar ratio of lithium acetate, ferrous oxalate 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;

[0197] (2) Mix lithium acetate, ferrous oxalate, phosphoric acid and water to form a ball milling solution.

[0198] (3) Stainless steel grinding balls are added to the ball milling liquid at a ratio of 1:5; among the grinding balls, the ratio of large-diameter grinding balls, medium-diameter grinding balls and small-diameter grinding balls is 2:11:22, the diameter of the large-diameter grinding balls is 90 mm, the diameter of the medium-diameter grinding balls is 60 mm, and the diameter of the small-diameter grinding balls is 20 mm.

[0199] (4) Mill the milling solution (containing stainless steel grinding balls) in supercritical water at 400 rpm for 8 h.

[0200] (5) After ball milling, the mixture was calcined at 650°C for 12 h to obtain lithium iron phosphate.

[0201] Comparative Example 2

[0202] Comparative Example 2 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0203] The process does not contain step (2), i.e., phosphoric acid grafted graphene oxide is not prepared.

[0204] (3) Prepare the ball milling solution:

[0205] ① The ball milling liquid raw materials were prepared according to the mass ratio of lithium iron grafted graphene oxide and phosphoric acid being 0.8:1 and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0206] ② Lithium iron grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball milling medium to obtain a ball milling solution.

[0207] Comparative Example 3

[0208] Comparative Example 3 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0209] The process does not contain step (1), i.e., lithium iron grafted graphene oxide is not prepared.

[0210] (3) Prepare the ball milling solution:

[0211] ① The ball milling liquid raw materials were prepared according to the mass ratio of phosphoric acid-grafted graphene oxide and phosphoric acid being 0.2:1 and the molar ratio of lithium acetate, ferrous oxalate and phosphoric acid being 1:1:1.

[0212] ② Phosphoric acid-grafted graphene oxide, lithium acetate, ferrous oxalate and phosphoric acid are mixed, and water is added as a ball-milling medium to obtain a ball-milling solution.

[0213] Comparative Example 4

[0214] Comparative Example 4 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0215] (4) Ball milling:

[0216] ① Add stainless steel grinding balls into the ball milling liquid at a ratio of 1:5; the grinding balls are small-diameter grinding balls with a diameter of 20 mm.

[0217] ② Ball mill the ball milling liquid (containing stainless steel grinding balls) at a ball milling rate of 400 rpm for 8 hours.

[0218] In order to verify the advancement of the modified lithium iron phosphate and its preparation method provided in the examples of the present application, taking Example 1 as an example, a scanning electron microscope was used to scan the modified lithium iron phosphate prepared in Example 1, and the corresponding Figure 1 The SEM image shown in the figure and EDS element analysis showed the following Figures 2~4 The elemental analysis diagram shown in Figure 2 This is the EDS element mapping diagram of modified lithium iron phosphate. Figure 3 This is the EDS element mapping diagram of phosphorus, oxygen, iron and carbon in modified lithium iron phosphate. Figure 4 This is the EDS elemental analysis diagram of modified lithium iron phosphate; Figure 4 The content of each element is shown in Table 1 below.

[0219]

[0220] Then the modified lithium iron phosphate or lithium iron phosphate provided in the embodiment and the comparative example is made into a button-type battery and then tested.

[0221] 1. The assembly steps of button batteries are as follows:

[0222] (1) Positive electrode sheet: Lithium iron phosphate / modified lithium iron phosphate, conductive carbon black and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, and N-methylpyrrolidone (NMP) was used as a solvent to prepare a slurry. The slurry was evenly coated on an aluminum foil, dried, compacted, and vacuum dried at 120°C for 12 h to obtain a positive electrode sheet. The mass of the active material in the positive electrode sheet was 12 mg.

[0223] (2) Negative electrode: The negative electrode uses a metal lithium sheet.

[0224] (3) The diaphragm was a polypropylene porous membrane, and the electrolyte was 1 mol / L NaPF6 / EC+DEC+DMC (EC:DEC:DMC=1:1:1 by volume).

[0225] EC: Ethylene Carbonate;

[0226] DEC: Diethyl Carbonate;

[0227] DMC: Dimethyl Carbonate.

[0228] 2. Test: Under 0.1C, the voltage range is 2.0V-4.3V to test the charge and discharge specific capacity. The calculation method of 0.1C efficiency is: 0.1C efficiency (%) = 0.1C discharge specific capacity (mAh / g) / 0.1C charge specific capacity (mAh / g) × 100%. The results are shown in Table 2 below.

[0229]

[0230] From Table 1~2 above and the instruction manual Figures 1 to 4 At least the following conclusions can be drawn:

[0231] 1) Instruction manual Figures 1 to 4 It can be seen from Neutralization Tables 1~2 that in the modified lithium iron phosphate provided in the embodiment of the present application, since the phenylalanine ferrous complex contains N and C elements, the obtained modified lithium iron phosphate must be doped with N and C elements. However, due to equipment limitations, only the EDS image of the C element can be obtained, and the N element cannot be photographed.

[0232] The doped N and C elements combined with the doped graphene not only improve the structural stability of lithium iron phosphate, but also improve the conductivity of lithium iron phosphate. The modified lithium iron phosphate has excellent conductivity, cycle stability and rate performance.

[0233] 2) The preparation method of the modified lithium iron phosphate in the embodiment of the present application is to react a ball-milling solution containing a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide and lithium iron-grafted graphene oxide in supercritical water. The phosphoric acid-grafted graphene oxide and the lithium iron-grafted graphene oxide not only serve as raw materials for graphene, but also serve as "seeds" during ball milling to promote the directional reaction of the lithium source, the iron source and phosphoric acid to generate lithium iron phosphate, thereby improving the reaction rate and yield. Supercritical water has excellent solubility and good mass transfer characteristics, which promotes the ball-milling solution to be fully dispersed and dissolved in supercritical water, and react to form a precursor. The precursor is then calcined to obtain a modified lithium iron phosphate with excellent conductivity, cycle stability and rate performance.

[0234] 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 modified lithium iron phosphate, characterized in that: The steps include: In supercritical water, the ball milling liquid is ball milled and then calcined to obtain modified lithium iron phosphate; The ball milling solution comprises a lithium source, an iron source, phosphoric acid, phosphoric acid-grafted graphene oxide, and lithium-iron-grafted graphene oxide; The preparation of lithium iron grafted graphene oxide includes the following steps: After subjecting the first solution to a first microwave ultrasound, the solution is filtered, washed and dried to obtain lithium-grafted graphene oxide; the first solution contains first graphene oxide and lithium chloride; The second solution is subjected to a second microwave ultrasound, filtered, washed and dried to obtain the lithium iron grafted graphene oxide; the second solution contains the lithium grafted graphene oxide and the phenylalanine ferrous complex.

2. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The mass ratio of the phosphoric acid grafted graphene oxide, the lithium iron grafted graphene oxide and the phosphoric acid is 0.1-0.2:0.7-0.9:

1.

3. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The steps of preparing the second solution include: Under an argon atmosphere and a stirring condition of 100 rpm to 200 rpm, phenylalanine is added to the ferrous chloride aqueous solution to obtain a phenylalanine ferrous complex solution, wherein the pH value of the ferrous chloride aqueous solution is 3 to 5; Under stirring conditions of a rotation speed of 80 rpm to 150 rpm, lithium-grafted graphene oxide is added to the phenylalanine ferrous complex solution to obtain a second solution.

4. The method for preparing modified lithium iron phosphate according to claim 3, characterized in that: The mass ratio of the ferrous chloride, the phenylalanine, the lithium chloride and the first graphene oxide is 2.9-5.8:20-48:2.5-5:

1.

5. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: In the ball milling, the ball-to-material ratio is 1:3-6, the ball milling time is 8h-20h, and the ball milling speed is 200rpm-400rpm.

6. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The calcination temperature is 620°C to 700°C.

7. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The preparation of the phosphoric acid grafted graphene oxide comprises the following steps: The graphene oxide and phosphoric acid are mixed and subjected to a third microwave ultrasound, and then washed and dried to obtain the phosphoric acid-grafted graphene oxide.

8. The method for preparing modified lithium iron phosphate according to claim 7, characterized in that: In the first microwave ultrasound, the second microwave ultrasound and the third microwave ultrasound, the microwave power, the ultrasonic frequency and the ultrasonic time can be independently: the microwave power is 150W~300W, the ultrasonic frequency is 60kHz~80kHz; the microwave ultrasound time is 30min~1h.

9. A modified lithium iron phosphate prepared by the method for preparing the modified lithium iron phosphate according to any one of claims 1 to 8.

10. Use of the modified lithium iron phosphate as claimed in claim 9 in the field of lithium battery preparation.

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