Composite lithium iron phosphate material, and preparation method therefor and use thereof

Abstract

The present disclosure belongs to the technical field of batteries, and provides a composite lithium iron phosphate material, and a preparation method therefor and the use thereof. The composite lithium iron phosphate material comprises a lithium iron phosphate core and a shell, wherein the shell comprises an MXene material such as Ti3C2, and a semiconductor block polymer such as P3HT-PE. The MXene material having metal properties and the semiconductor block polymer are introduced and mixed, and jointly coat lithium iron phosphate, thereby forming a Schottky junction at a position where the MXene material is in close contact with the semiconductor block polymer, such that the directional movement of electrons can be promoted and the electron transfer rate can be increased; and the block polymer has certain tensile properties, and thus can not only tightly coat the lithium iron phosphate, but also does not easily fall off during long-time charging and discharging.

Description

A Composite Lithium Iron Phosphate Material, Its Preparation Method and Use Technical Field This disclosure belongs to the technical field of batteries and relates to a composite lithium iron phosphate material, its preparation method and use. Background Art In the field of new-generation energy storage battery technologies, lithium-ion batteries are widely used in the market due to their excellent performance such as high capacity, high voltage, high cycle performance, and high energy density. In lithium-ion batteries, the cathode material is one of the important factors determining battery performance and cost. Among many cathode materials, lithium iron phosphate (LiFePO4) occupies a large market share in power batteries due to its low raw material cost and excellent structural stability and cycle performance. However, the stable olivine crystal structure of LiFePO4 itself restricts its performance. Its lithium-ion diffusion rate is low and it can only diffuse in one-dimensional

[0010] channels, and it only has a low conductivity of 10 -9 ~10 -10 S / cm, etc. These problems greatly limit the future commercial development of lithium iron phosphate batteries. Therefore, some improvement methods are usually needed to solve the above problems, and carbon coating is a commonly used means. For example, CN117263161A discloses a preparation method of a uniformly carbon-coated lithium iron phosphate cathode material. After grinding iron phosphate and a lithium source, a water-soluble carbon source solution is added and mixed, followed by spray drying and then calcination to obtain uniformly carbon-coated lithium iron phosphate, effectively improving the electrochemical performance of the material and ensuring that carbon can still uniformly coat the lithium iron phosphate cathode material while reducing carbon source loss. CN112928268A discloses a carbon-coated lithium iron phosphate composite material and its preparation method. It performs high-temperature heat treatment on a lithium iron phosphate precursor in an inert atmosphere with an organic carbon source placed in the upper wind, completing reduction and carbon coating in one step. The obtained carbon-coated lithium iron phosphate composite material can maintain the electrochemical performance of lithium-ion batteries, such as capacity density, rate performance, etc. It can be seen that carbon, as a conductive layer, coats the cathode material. On the one hand, it can improve the conductivity of lithium iron phosphate and electron transfer between particles. On the other hand, it can effectively inhibit the agglomeration and overgrowth of LiFePO4 during the synthesis process, and at the same time avoid the conversion of Fe 2+ to Fe 3+ . However, this method also has its limitations. Due to its low mass density, carbon coating often reduces the overall tap density of the material, and after long-term charge and discharge, the carbon coating layer is prone to peeling off due to the lack of adhesion. Therefore, there is a need to find a new lithium iron phosphate composite material that can improve the electron transfer rate and reduce the peeling off of the coating layer, further effectively improving the rate performance and cycle performance of the battery. Summary of the Invention The following is an overview of the subject matter described in detail in this document. This overview is not intended to limit the scope of protection of the claims. In view of the problems existing in the prior art, the purpose of the present disclosure is to provide a composite lithium iron phosphate material, its preparation method and use. The composite lithium iron phosphate material includes a lithium iron phosphate core and a shell, and the shell includes an MXene material and a semiconductor block polymer. By introducing the mixture of the MXene material with metallic properties and the semiconductor block polymer to coat the lithium iron phosphate together, a Schottky junction is formed at the place where the MXene material and the semiconductor block polymer are in close contact, which can promote the directional movement of electrons and accelerate the electron transfer rate. The block polymer has certain stretching properties, which can not only tightly coat the lithium iron phosphate, but also is not easy to fall off during long-term charge and discharge. To achieve this purpose, the present disclosure adopts the following technical solutions: In the first aspect, the present disclosure provides a composite lithium iron phosphate material, which includes a core and a shell. The core includes lithium iron phosphate, and the shell includes an MXene material and a semiconductor block polymer. The present disclosure provides a composite cathode material that solves the problems of low electron transport rate of lithium iron phosphate and prevents the coating layer from falling off. By introducing an MXene material with metallic properties, such as Ti3C2, and a semiconductor block polymer, such as P3HT-PE, the two are mixed to coat the lithium iron phosphate together. A Schottky junction is formed at the place where the MXene material and the semiconductor block polymer are in close contact, which can promote the directional movement of electrons and accelerate the electron transfer rate. The block copolymer has certain stretching properties, which can not only tightly coat the lithium iron phosphate, but also is not easy to fall off during long-term charge and discharge. The following are optional technical solutions of the present disclosure, but not limitations to the technical solutions provided by the present disclosure. Through the following technical solutions, the technical objectives and beneficial effects of the present disclosure can be better achieved and realized. As an optional technical solution of the present disclosure, the shell is a coating layer composed of a mixture of an MXene material and a semiconductor block polymer. The present disclosure preferably makes the shell a single-layer shell in a mixed state. Compared with the double-layer shell formed by the MXene material and the semiconductor block polymer forming layers respectively, when they are mixed in a single layer, the two are more evenly mixed and the interface is larger, and the formed Schottky junction is more stable. In one embodiment, the MXene material includes Ti3C2 and / or Ti2C. Among the MXene materials, Ti3C2 has relatively excellent metallic conductivity, followed by Ti2C. Therefore, the present disclosure further preferably uses Ti3C2 as the MXene material. In one embodiment, the semiconductor block polymer comprises P3HT-PE. In one embodiment, in the P3HT-PE, the molar fraction of PE is 15% to 45%, such as 15%, 18%, 21%, 24%, 27%, 30%, 33%, 36%, 39%, 42% or 45%, etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the mass of the semiconductor block polymer accounts for 1% to 10% of the mass of the Ti3C2, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. P3HT (3-hexylthiophene polymer) is a semiconductor polymer with general stretchability. However, it can be synthesized with polyethylene (PE) to form a block copolymer P3HT-PE, and its tensile properties are enhanced. At the same time, the semiconductor block copolymer has general conductivity and mainly plays the role of manufacturing a PN junction in the present disclosure. An excessive proportion will affect the conductivity of the coating layer. In one embodiment, the thickness of the outer shell is 1 to 15 nm, such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm or 15 nm, etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the particle size of the inner core is 80 to 800 nm, such as 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm or 800 nm, etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In a second aspect, the present disclosure provides a method for preparing the composite lithium iron phosphate material described in the first aspect, comprising: Mixing lithium iron phosphate, Mxene material and semiconductor block polymer by heating, and then freeze-drying to obtain the composite lithium iron phosphate material. As an optional technical solution of the present disclosure, the heating and mixing is carried out in a liquid phase system. In one embodiment, the solvent of the liquid phase system includes water. ​In one embodiment, the temperature of the heating and mixing is 100 to 150 °C, such as 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 130 °C, 140 °C, 145 °C or 150 °C, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the mixing method of the heating and mixing includes ultrasonic waves; In one embodiment, the time of the freeze-drying is 6 to 10 h, such as 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h, 9.5 h or 10 h, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the freeze-drying is carried out under vacuum. As an alternative technical solution of the present disclosure, the method for preparing the lithium iron phosphate includes: Mixing a lithium source, a phosphorus source, an iron source and a carbon source, and successively performing primary ball milling, spray drying, secondary ball milling, pre-sintering and sintering to obtain a lithium iron phosphate material. In one embodiment, the molar ratio of the lithium source, the phosphorus source and the iron source is (1 to 1.05):1:1, such as 1:1:1, 1.01:1:1, 1.02:1:1, 1.03:1:1, 1.04:1:1 or 1.05:1:1, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the dosage of the carbon source is 1% to 5% of the total mass of the lithium source, the phosphorus source and the iron source, such as 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. The dosage of the carbon source described in the present disclosure is less, and it only needs to play a role in inhibiting the oxidation of divalent iron ions into trivalent iron ions. In one embodiment, the temperature of the pre-sintering is 200 to 350 °C, such as 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C, 340 °C or 350 °C, etc., and the time is 2 to 4 h, such as 2 h, 2.2 h, 2.4 h, 2.6 h, 2.8 h, 3 h, 3.2 h, 3.4 h, 3.6 h, 3.8 h or 4 h, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable. In one embodiment, the sintering temperature is 700 - 950 °C, such as 700 °C, 720 °C, 740 °C, 760 °C, 780 °C, 800 °C, 820 °C, 850 °C, 870 °C, 890 °C, 910 °C, 930 °C or 950 °C, etc., and the time is 4 - 12 h, such as 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable. As an alternative technical solution of the present disclosure, the method for preparing the MXene material includes: mixing a precursor with hydrofluoric acid, performing an etching reaction, washing and drying to obtain the MXene material. In one embodiment, the precursor includes Ti3AlC2. In one embodiment, the mass concentration of the hydrofluoric acid is 30% - 40%, such as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39% or 40%, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable. In one embodiment, the mass ratio of the precursor to the hydrofluoric acid is 1:(10 - 15), such as 1:10, 1:10.5, 1:11, 1:11.5, 1:12, 1:12.5, 1:13, 1:13.5, 1:14, 1:14.5 or 1:15, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable. In one embodiment, the temperature of the etching reaction is 30 - 50 °C, such as 30 °C, 32 °C, 34 °C, 36 °C, 38 °C, 40 °C, 42 °C, 44 °C, 46 °C, 48 °C or 50 °C, etc., and the time is 20 - 30 h, such as 20 h, 21 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, 28 h, 29 h or 30 h, etc., but is not limited to the listed values, and other unlisted values within the above value range are equally applicable. As an alternative technical solution of the present disclosure, the method for preparing the composite lithium iron phosphate material includes: Using deionized water as a medium, putting a lithium source, a phosphorus source, and an iron source into a ball mill according to a molar ratio of (1 - 1.05):1:1 for primary ball milling for 2 - 3 h. After being fully mixed evenly, adding a carbon source accounting for 1% - 5% of the total mass of the lithium source, the phosphorus source, and the iron source to the slurry, and continuing ball milling and mixing for 3 - 5 h until uniform; performing spray drying treatment on the ball-milled mixed slurry to obtain a powder, heating the powder to 200 - 350 °C in a protective atmosphere for pre-sintering, and holding for 2 - 4 h; taking out after the pre-sintering is completed for secondary ball milling for 1 - 2 h, and then heating to 700 - 950 °C in a protective atmosphere for sintering for 4 - 12 h to obtain lithium iron phosphate LiFePO4; The precursor Ti3AlC2 powder is dispersed in an HF solution with a mass fraction of 30% - 40%. The mass ratio of Ti3AlC2 to the HF solution is controlled to be 1:(10 - 15), and it is stirred and etched at 30 - 50 °C for 20 - 30 h. After centrifuging the suspension, it is repeatedly rinsed with deionized water until the pH of the solution is neutral. The powder obtained by centrifugation is dried overnight in a vacuum oven at 60 - 80 °C to obtain Ti3C2; The Ti3C2 powder and the block polymer P3HT-PE are added together into a beaker containing deionized water. The mass of P3HT-PE is controlled to be 1% - 10% of the mass of Ti3C2, and it is ultrasonically treated for 15 - 40 min under an argon protection atmosphere to obtain a uniformly dispersed dispersion A; The prepared lithium iron phosphate is placed in dispersion A, heated to 100 - 150 °C and ultrasonically treated continuously. After being mixed evenly, it is centrifuged and then dried in a vacuum freeze-drying oven for 6 - 10 h. After drying, the composite lithium iron phosphate material with Ti3C2 / P3HT-PE co-mixed and coated on LiFePO4 is obtained. In a third aspect, the present disclosure provides a positive electrode sheet, and the positive electrode sheet contains the composite lithium iron phosphate material described in the first aspect. In a fourth aspect, the present disclosure provides a battery, and the battery contains the positive electrode sheet described in the third aspect. In a fifth aspect, the present disclosure provides an electrical device, and the electrical device contains the battery described in the fourth aspect. Compared with the prior art solutions, the present disclosure has at least the following beneficial effects: In the present disclosure, by using Ti3C2 and a semiconductor block polymer, such as P3HT-PE, to co-coat on lithium iron phosphate instead of carbon coating. Among them, Ti3C2, as a graphene-like structure, has metallic properties and excellent conductivity, which can effectively improve the conductivity of the lithium iron phosphate material. The close contact between the semiconductor block polymer and Ti3C2 can form a Schottky junction, and an internal built-in electric field can be constructed in the lithium iron phosphate composite material to promote the directional movement of electrons and improve the electron transfer rate. At the same time, the stretchability of the semiconductor block polymer is improved, so that during long-term charge and discharge of lithium iron phosphate, the coating layer will not fall off due to repeated expansion and contraction of the crystal lattice, improving the cycle performance of lithium iron phosphate. Compared with the pure semiconductor polymer P3HT, both the field-effect mobility and stretchability of the P3HT-PE copolymer are further improved. The synergistic effect of the improvement of the field-effect mobility and the formation of the Schottky junction electric field can further accelerate the electron transfer rate and improve the rate performance of lithium iron phosphate. Other aspects can be understood after reading and understanding the drawings and the detailed description. Description of the Drawings The accompanying drawings are used to provide a further understanding of the technical solutions herein and form a part of the specification, which, together with the embodiments of the present application, are used to explain the technical solutions herein and do not constitute a limitation to the technical solutions herein. Figure 1 is a scanning electron micrograph of the lithium iron phosphate composite material prepared in Example 1 of the present disclosure. Specific Embodiments The technical solutions of the present disclosure will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only to help understand the present disclosure and should not be regarded as specific limitations to the present disclosure. Example 1 This example provides a composite lithium iron phosphate material, and the preparation method of the composite lithium iron phosphate material includes: (1) Using deionized water as a medium, take a certain mass of lithium carbonate, iron phosphate and phosphoric acid, and put them into a ball mill according to the molar ratio of lithium: iron: phosphorus = 1.03:1:1. Control the solid-liquid ratio to be 3:1 (unit: g:mL), and perform primary ball milling for 2.5 h. After thorough mixing, add a carbon source accounting for 2% of the total mass of the lithium source, phosphorus source and iron source to the slurry, and continue ball milling and mixing for 4 h until uniform. The mixed slurry after ball milling is subjected to spray drying treatment to obtain precursor powder, and the powder is heated to 300 °C in a protective atmosphere and pre-burned for 3 h. After the pre-burning is completed, the mixed deionized water is taken out, the solid-liquid ratio is controlled to be 2:1 (unit: g:mL), and secondary ball milling is carried out for 1.5 h in a wet state, and then heated to 850 °C in a protective atmosphere and sintered for 8 h to obtain the lithium iron phosphate LiFePO4 cathode material; (2) Take the precursor Ti3AlC2 powder and disperse it in an HF solution with a mass fraction of 40%. Control the mass ratio of Ti3AlC2 to the HF solution to be 1:12, and stir and etch at 40 °C for 25 h. After centrifuging the suspension, repeatedly rinse it with deionized water until the pH of the solution is neutral; dry the powder obtained by centrifugation in a vacuum oven at 70 °C overnight to obtain Ti3C2; (3) Add the Ti3C2 powder and the block copolymer P3HT-PE (the molar fraction of PE is 30%) in proportion together into a beaker containing deionized water, control P3HT-PE to be 5 wt% of the mass of Ti3C2, and ultrasonically treat for 25 min in an argon protective atmosphere to obtain a uniformly dispersed dispersion A; (4) Place the prepared lithium iron phosphate in dispersion liquid A, and control the ratio of the amount of lithium iron phosphate to the total mass of Ti3C2 powder and block copolymer P3HT-PE to be 10:1. Heat up to 130 °C and continue ultrasonic treatment. After mixing evenly, centrifuge and then place it in a vacuum freeze-drying oven for 8 h. After drying, the composite lithium iron phosphate material with Ti3C2 / P3HT-PE coated on LiFePO4 is obtained. Example 2 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the amount of P3HT-PE is adjusted from 5% of the mass of Ti3C2 to 0.5%. Except for the above, other conditions are exactly the same as those in Example 1. Example 3 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the amount of P3HT-PE is adjusted from 5% of the mass of Ti3C2 to 1%. Except for the above, other conditions are exactly the same as those in Example 1. Example 4 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the amount of P3HT-PE is adjusted from 5% of the mass of Ti3C2 to 9%. Except for the above, other conditions are exactly the same as those in Example 1. Example 5 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the amount of P3HT-PE is adjusted from 5% of the mass of Ti3C2 to 10%. Except for the above, other conditions are exactly the same as those in Example 1. Example 6 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the amount of P3HT-PE is adjusted from 5% of the mass of Ti3C2 to 11%. Except for the above, other conditions are exactly the same as those in Example 1. Example 7 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the molar fraction of PE is adjusted from 30% to 12%. Except for the above, other conditions are exactly the same as those in Example 1. Example 8 This example provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the molar fraction of PE is adjusted from 30% to 15%. Except for the above, other conditions are exactly the same as those in Example 1. Example 9 This embodiment provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the molar fraction of PE is adjusted from 30% to 45%. Except for the above, other conditions are exactly the same as those in Embodiment 1. Embodiment 10 This embodiment provides a composite lithium iron phosphate material. In step (3) of the preparation method of the composite lithium iron phosphate material, the molar fraction of PE is adjusted from 30% to 48%. Except for the above, other conditions are exactly the same as those in Embodiment 1. Embodiment 11 This embodiment provides a composite lithium iron phosphate material. In step (4) of the preparation method of the composite lithium iron phosphate material, the heating temperature is adjusted from 130 °C to 70 °C. Except for the above, other conditions are exactly the same as those in Embodiment 1. Embodiment 12 This embodiment provides a composite lithium iron phosphate material. In step (4) of the preparation method of the composite lithium iron phosphate material, the heating temperature is adjusted from 130 °C to 100 °C. Except for the above, other conditions are exactly the same as those in Embodiment 1. Embodiment 13 This embodiment provides a composite lithium iron phosphate material. In step (4) of the preparation method of the composite lithium iron phosphate material, the heating temperature is adjusted from 130 °C to 150 °C. Except for the above, other conditions are exactly the same as those in Embodiment 1. Embodiment 14 This embodiment provides a composite lithium iron phosphate material. In step (4) of the preparation method of the composite lithium iron phosphate material, the heating temperature is adjusted from 130 °C to 180 °C. Except for the above, other conditions are exactly the same as those in Embodiment 1. Comparative Example 1 In this comparative example, the lithium iron phosphate LiFePO4 cathode material obtained in step (1) of Embodiment 1 is used for subsequent tests. Comparative Example 2 This comparative example provides a composite lithium iron phosphate material. In the preparation method of the composite lithium iron phosphate material, Ti3C2 is not used, that is, step (2) is not carried out. In step (3), an equal amount of P3HT-PE is used to replace Ti3C2. Except for the above, other conditions are exactly the same as those in Embodiment 1. Comparative Example 3 This comparative example provides a composite lithium iron phosphate material. In the preparation method of the composite lithium iron phosphate material, P3HT-PE is not used. In step (3), an equal amount of Ti3C2 is used to replace P3HT-PE. Except for the above, other conditions are exactly the same as those in Embodiment 1. Comparative Example 4 This comparative example provides a composite lithium iron phosphate material. The preparation method of the composite lithium iron phosphate material uses an equal amount of pure P3HT to replace P3HT-PE in step (3). Except for this, other conditions are exactly the same as those in Example 1. Comparative Example 5 This comparative example provides a composite lithium iron phosphate material. The composite lithium iron phosphate material includes a lithium iron phosphate core and a carbon coating layer. The preparation method of the composite lithium iron phosphate material uses the lithium iron phosphate LiFePO4 cathode material obtained in step (1) of Example 1, and continues to add 8% of glucose by the mass of lithium iron phosphate and mix evenly according to the common carbon source coating amount, and sinter at 750 °C under a protective atmosphere for carbon coating, and control the thickness of the carbon coating layer to be the same as the thickness of the outer shell of the present disclosure. Figure 1 is the SEM image of the composite cathode material of the lithium ion battery prepared in Example 1. It can be seen that the lithium iron phosphate particles are mainly spherical, and the diameter is mainly concentrated between 100 and 550 nm, and there is an obvious coating effect on the surface. The composite lithium iron phosphate materials obtained in the examples and comparative examples were made into coin cells for testing the electrochemical performance of lithium ion batteries. The specific steps were as follows: The composite lithium iron phosphate material was used as the active substance of the cathode material, and was uniformly mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride in a ratio of 90:5:5 in N-methylpyrrolidone, and then coated on an aluminum foil and placed in a vacuum drying oven for drying. After drying, the battery was assembled in an argon glove box, and a 12 mm positive electrode sheet was pressed by a tablet press. The negative electrode was a lithium metal sheet, the electrolyte was 1 M LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as the separator. The electrochemical performance was tested, and the test voltage range was 2.4 to 4.6 V, and the rate test was 1C = 170 mA / g. The results are shown in Table 1 below. Table 1 As can be seen from Table 1: The discharge capacities of different embodiments at different rates (0.1C, 1C, 3C, 5C) are as follows. It can be seen from the table that the discharge capacities of Embodiment 1 at different rates are 159.3 mAh / g, 148.1 mAh / g, 135.2 mAh / g, and 117.8 mAh / g respectively, which is the best in terms of discharge capacity performance among all embodiments. This is due to the introduction of Ti3C2 and the semiconductor block polymer P3HT-PE for coating during the preparation of the cathode material. Ti3C2 and the semiconductor P3HT-PE form a Schottky junction, which can build an internal electric field in the lithium iron phosphate composite material, promote the directional movement of electrons, and improve the electron transfer rate. At the same time, the P3HT-PE block copolymer has the semiconductor properties of P3HT and the good stretchability of PE, and can adapt to long-term charge and discharge without the coating layer falling off. Its cycle performance is also excellent compared with other embodiments and comparative examples, and the cycle retention rate is 97.4% after 300 cycles of 1C discharge. By comparing Embodiment 1 with Embodiments 2-6 and adjusting the mass ratio of P3HT-PE to Ti3C2, it is found that when the mass ratio of P3HT-PE to Ti3C2 is too low, the battery discharge capacity also shows a decreasing phenomenon at 0.1C and other rates. This may be because too little P3HT-PE cannot form enough Schottky junctions with Ti3C2 to build an electric field, resulting in a low electron transfer rate. When the dosage ratio of P3HT-PE is too high or exceeds the limit value, the cycle retention rate of Embodiments 4-6 is above 96.8% after 300 cycles, and the conductivity is low. This may be because too much P3HT-PE affects the proportion of the conductive substance Ti3C2, resulting in a low conductivity. By comparing Embodiment 1 with Embodiments 7-10 and adjusting the dosage of PE in Ti3C2 and P3HT-PE, it is found that when the dosage of PE is too little, the cycle performance of the battery is low. This may be because too little PE dosage fails to make the block copolymer have good stretchability; as the dosage ratio of PE increases, the cycle performance becomes higher and higher, but too much PE dosage will hinder the electron transport rate, thereby affecting the rate performance. By comparing Embodiment 1 with Embodiments 11-14 and adjusting the ultrasonic temperature of the composite of Ti3C2, P3HT-PE and lithium iron phosphate, it is found that when the ultrasonic temperature is too low or too high, it will affect its rate discharge performance. Too low ultrasonic temperature may lead to insufficient composite and loose coating, and too high ultrasonic temperature may affect the properties of the block copolymer, such as molecular chain breakage, thereby affecting the rate performance and cycle performance. For example, molecular chain breakage will affect the rate performance and cycle performance. Compared with Comparative Examples 1-5, in Example 1, without using Ti3C2 and semiconductor block polymer-coated lithium iron phosphate, lithium iron phosphate was prepared in a normal manner. Its rate performance, cycling performance, and conductivity were all lower than those of this example, which proves the advantages of the present disclosure. In Comparative Example 2, P3HT-PE was not used and Ti3C2 was used entirely for coating. In Comparative Example 3, Ti3C2 was not used and P3HT-PE was used entirely for coating. In Comparative Example 4, P3HT was used instead of P3HT-PE for coating. The lithium iron phosphate prepared by the above methods had slightly lower performance compared with the example, but better performance compared with Comparative Example 1, which proves that the synergistic effect of Ti3C2 and P3HT-PE can effectively improve the electrical properties of lithium iron phosphate. As can be seen from the above, in the present disclosure, by introducing a mixture of MXene material with metallic properties and semiconductor block polymer to coat lithium iron phosphate together, a Schottky junction is formed at the place where the MXene material and the semiconductor block polymer are in close contact, which can promote the directional movement of electrons and accelerate the electron transfer rate, effectively improving the rate performance of lithium iron phosphate; the block polymer has certain stretching properties, which can not only tightly coat lithium iron phosphate, but also is not easy to fall off during long-term charge and discharge, effectively improving the cycling performance of lithium iron phosphate.

Claims

1. A composite lithium iron phosphate material, comprising a core and a shell, wherein the core comprises lithium iron phosphate, and the shell comprises MXene material and a semiconductor block polymer.

2. The composite lithium iron phosphate material according to claim 1, wherein, The shell is a coating layer formed by mixing MXene material and a semiconductor block polymer; Optionally, the MXene material comprises Ti3C2 and / or Ti2C; Optionally, the semiconductor block polymer comprises P3HT-PE.

3. The composite lithium iron phosphate material according to claim 2, wherein, In the P3HT-PE, the molar fraction of PE is 15% to 45%.

4. The composite lithium iron phosphate material according to claim 2 or 3, wherein The mass of the semiconductor block polymer accounts for 1% to 10% of the mass of Ti3C2.

5. The composite lithium iron phosphate material according to any one of claims 1-4, wherein, The thickness of the shell is 1 to 15 nm.

6. The composite lithium iron phosphate material according to any one of claims 1-5, wherein, The particle size of the core is 80 to 800 nm.

7. A preparation method of the composite lithium iron phosphate material according to any one of claims 1-6, comprising: Heating and mixing lithium iron phosphate, MXene material and a semiconductor block polymer, and then freeze-drying to obtain the composite lithium iron phosphate material.

8. The preparation method of the composite lithium iron phosphate material according to claim 7, wherein, The heating and mixing is carried out in a liquid phase system; Optionally, the temperature of the heating and mixing is 100 to 150 °C; Optionally, the mixing method of the heating and mixing includes ultrasonic.

9. The preparation method of the composite lithium iron phosphate material according to claim 7 or 8, wherein, The freeze-drying is carried out under vacuum.

10. The preparation method of the composite lithium iron phosphate material according to any one of claims 7-9, wherein, The method for preparing the lithium iron phosphate comprises: Mixing a lithium source, a phosphorus source, an iron source and a carbon source, and successively carrying out primary ball milling, spray drying, secondary ball milling, pre-sintering and sintering to obtain a lithium iron phosphate material; Optionally, the molar ratio of the lithium source, the phosphorus source and the iron source is (1 to 1.05):1:1; Optionally, the dosage of the carbon source is 1% to 5% of the total mass of the lithium source, the phosphorus source and the iron source; Optionally, the temperature of the pre-sintering is 200 to 350 °C, and the time is 2 to 4 h; Optionally, the temperature of the sintering is 700 to 950 °C, and the time is 4 to 12 h.

11. The preparation method of the composite lithium iron phosphate material according to any one of claims 7-10, wherein, The method for preparing the MXene material comprises: Mixing a precursor with hydrofluoric acid, carrying out an etching reaction, washing and drying to obtain the MXene material; Optionally, the precursor comprises Ti3AlC2; Optionally, the mass concentration of the hydrofluoric acid is 30% to 40%; Optionally, the mass ratio of the precursor to the hydrofluoric acid is 1:(10 to 15); Optionally, the temperature of the etching reaction is 30 to 50 °C, and the time is 20 to 30 h.

12. The preparation method of the composite lithium iron phosphate material according to any one of claims 7-11, wherein, The preparation method comprises: Using deionized water as a medium, putting the lithium source, the phosphorus source and the iron source into a ball mill according to a molar ratio of (1 to 1.05):1:1 for primary ball milling, mixing for 2 to 3 h, and adding a carbon source accounting for 1% to 5% of the total mass of the lithium source, the phosphorus source and the iron source to the slurry after being fully mixed evenly, and continuing to ball mill and mix for 3 to 5 h until uniform; the mixed slurry after ball milling is subjected to spray drying treatment to obtain powder, and the powder is heated to 200 to 350 °C in a protective atmosphere for pre-sintering, and kept warm for 2 to 4 h; after the pre-sintering is completed, take it out for secondary ball milling for 1 to 2 h, and then heat it to 700 to 950 °C in a protective atmosphere for sintering for 4 to 12 h to obtain lithium iron phosphate LiFePO4; The precursor Ti3AlC2 powder is dispersed in an HF solution with a mass fraction of 30% - 40%, the mass ratio of Ti3AlC2 to the HF solution is controlled to be 1:(10 - 15), and it is stirred and etched at 30 - 50 °C for 20 - 30 h. After centrifuging the suspension, it is repeatedly rinsed with deionized water until the pH of the solution is neutral; the powder obtained by centrifugation is dried overnight in a vacuum oven at 60 - 80 °C to obtain Ti3C2; The Ti3C2 powder and the block polymer P3HT-PE are added together into a beaker containing deionized water, and the mass of P3HT-PE is controlled to be 1% - 10% of the mass of Ti3C2. It is ultrasonicated for 15 - 40 min under an argon protection atmosphere to obtain a uniformly dispersed dispersion A; the prepared lithium iron phosphate is placed in the dispersion A, heated to 100 - 150 °C and ultrasonicated continuously. After being mixed evenly, it is centrifuged and then dried in a vacuum freeze-drying oven for 6 - 10 h. After drying, the composite lithium iron phosphate material with Ti3C2 / P3HT-PE co-mixed and coated on LiFePO4 is obtained.

13. A positive electrode plate containing the composite lithium iron phosphate material according to any one of claims 1 - 6.

14. A battery containing the positive electrode plate according to claim 13.

15. An electrical device containing the battery according to claim 14.

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