Lithium-rich lithium iron oxide material, and preparation method therefor and use thereof

Abstract

The present application belongs to the field of positive electrode materials, and provides a lithium-rich lithium iron oxide material, and a preparation method therefor and the use thereof. The lithium-rich lithium iron oxide material comprises a particle of a core-shell structure, wherein the particle of a core-shell structure comprises a core, a first coating layer that coats the core, and a second coating layer that coats the first coating layer; the core is Li5FeO4, the first coating layer is a carbon layer, and the second coating layer is a mixed layer comprising polyoxyethylene and a lithium salt; and the mass ratio of the first coating layer to the core is (2:100) to (10:100), and the mass ratio of the second coating layer to the core is (3:100) to (13:100). The present application is conducive to improving the air stability of the lithium iron oxide material, and can also enhance the overall ionic conductivity of an electrode.

Description

Lithium-rich lithium iron phosphate material, preparation method and application thereof

[0001] This application is based on the Chinese application CN application number 202410460708.9, application date 2024 / 04 / 16, and claims priority thereto, the disclosure of which is incorporated herein in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium ion battery cathode materials, in particular to a lithium-rich lithium iron phosphate material, a preparation method and application thereof. BACKGROUND

[0003] With the rapid development of domestic new energy field, lithium ion batteries have received widespread attention, and the endurance problem of electric vehicles has always been the key to further entering the market. The existing high-capacity anodes such as hard carbon anodes and silicon anodes have very high capacity, but they will form a large amount of solid electrolyte interface film (SEI film), which will cause the irreversible consumption of a large amount of Li + , and thus seriously affect the overall capacity of the lithium ion battery. The pre-lithiation method compensates for the consumption of Li + , which is the most effective solution to the low initial efficiency of high-capacity anodes.

[0004] To further promote the use of new generation high-capacity anodes in future lithium ion batteries, many pre-lithiation schemes have been developed to enable the battery to match high-capacity, low-initial-efficiency anodes and compensate for the loss of active lithium during the first cycle. The pre-lithiation strategy mainly includes positive electrode pre-lithiation and negative electrode pre-lithiation. Among them, the negative electrode pre-lithiation has been widely concerned in the early stage, and in recent years, the positive electrode pre-lithiation strategy has received more and more attention due to its high safety, low price, easy synthesis, and high chargeability.

[0005] Among the lithium supplementing agents used in positive electrode pre-lithiation, Li5FeO4 has a theoretical specific capacity of 864 mAh g -1 , which is theoretically sufficient to greatly improve the capacity decay caused by the combination of existing positive electrodes and new negative electrodes such as hard carbon and silicon oxide. Li5FeO4 can be well matched with existing positive electrode materials and is compatible with existing binder systems, and can be directly added to the positive electrode material. In addition, the price of Li5FeO4 is relatively low compared to other lithium supplementing agents, so it has good application prospects as a lithium-rich additive for positive electrodes. However, Li5FeO4 is very sensitive to water and CO2 in the air and has poor stability in the air, which seriously affects its industrialization process.

[0006] SUMMARY

[0007] In view of the technical problems existing in the background art, the present application provides a lithium-rich lithium-iron oxide material and a preparation method and application thereof, aiming to improve the stability of the lithium-rich lithium-iron oxide material in air.

[0008] In a first aspect, the embodiments of the present application provide a lithium-rich lithium-iron oxide material, comprising core-shell structure particles, the core-shell structure particles comprising an inner core, a first coating layer coated outside the inner core, and a second coating layer coated outside the first coating layer; wherein the inner core is Li5FeO4, the first coating layer is a carbon layer, and the second coating layer is a mixed layer comprising polyethylene oxide and lithium salt; wherein the mass ratio of the first coating layer to the inner core is (2:100) to (10:100), and the mass ratio of the second coating layer to the inner core is (3:100) to (13:100).

[0009] In the present application, the lithium-rich lithium-iron oxide material is provided with the first coating layer and the second coating layer outside the inner core containing Li5FeO4 in sequence, wherein the first coating layer is a carbon layer, and the second coating layer is a mixed layer comprising polyethylene oxide (PEO) and lithium salt. By sequentially arranging the first coating layer and the second coating layer outside the inner core, on the one hand, through double-layer coating, the Li5FeO4 in the inner core can be protected from the influence of moisture and carbon dioxide in the air, thereby improving the air stability of the lithium-rich lithium-iron oxide material. On the other hand, as the outermost layer, the second coating layer is a mixed layer comprising polyethylene oxide and lithium salt. The polyethylene oxide can increase the electrical conductivity of the material, improve the strength and uniformity of the coating layer, and the lithium salt can further improve the electrical conductivity of the coating layer, thereby effectively increasing the ionic conductivity of the lithium-rich lithium-iron oxide material.

[0010] In some embodiments, the mass ratio of the polyethylene oxide to the inner core is (0.5:100) to (5:100), and the mass ratio of the lithium salt to the inner core is (2:100) to (8:100).

[0011] In this embodiment, under the condition that the amounts of other components are the same, if the amount of polyethylene oxide is too high, the lithium-rich lithium-iron oxide material particles will be mutually bonded. If the amount of polyethylene oxide is too low, complete and continuous coating cannot be achieved. If the amount of lithium salt is too high, the content of polyethylene oxide in the mixed layer will decrease, the viscosity between the carbon layer and the mixed layer will decrease, and the strength of the mixed layer will also decrease. If the amount of lithium salt is too low, the electrical conductivity of the lithium-rich lithium-iron oxide material will decrease.

[0012] In some embodiments, the mass ratio of the polyethylene oxide to the inner core is (0.5:100) to (2:100), and the mass ratio of the lithium salt to the inner core is (2:100) to (4:100).

[0013] In this embodiment, by adjusting the amount of polyethylene oxide and lithium salt in the second coating layer, when the mass ratio of the polyethylene oxide to the inner core is (0.5:100)~(2:100), and the mass ratio of the lithium salt to the inner core is (2:100)~(4:100), the conductivity, uniformity and continuity of the coating can be considered, and mutual adhesion between the lithium-rich lithium iron phosphate material particles can be avoided as much as possible.

[0014] In some embodiments, the mass ratio of the carbon layer to the inner core is (5:100)~(7:100).

[0015] In this embodiment, by setting the mass ratio of the carbon layer to the inner core to (5:100)~(7:100), on the one hand, the carbon layer can realize continuous and uniform coating of the inner core, and on the other hand, the proportion of the carbon layer is not too high, which can avoid affecting other properties of the prepared lithium-rich lithium iron phosphate material, so that the prepared lithium-rich lithium iron phosphate material has higher capacity and stability under the premise of maintaining uniform coating of the inner core.

[0016] In some embodiments, the lithium salt is selected from at least one of lithium bisfluorosulfonylimide, lithium hexafluorophosphate and lithium fluoride.

[0017] In this embodiment, by selecting at least one of lithium bisfluorosulfonylimide, lithium hexafluorophosphate and lithium fluoride, the conductivity of the mixed layer can be effectively improved.

[0018] In some embodiments, the molecular weight of the polyethylene oxide is 50,000 Da~150,000 Da.

[0019] In this embodiment, high molecular weight polyethylene oxide is easy to form a uniform and continuous polymer network structure, providing better ion conduction performance. However, if the molecular weight of the polyethylene oxide is too high, it is difficult to achieve uniform coating on the surface of the carbon layer. Therefore, polyethylene oxide with a molecular weight of 50,000 Da~150,000 Da is selected.

[0020] In some embodiments, the D90 particle size of the inner core is 3 μm~5.5 μm.

[0021] In this embodiment, by controlling the particle size of the inner core, the particle size of the finally prepared lithium-rich lithium iron phosphate material can also be controlled within a smaller range. Small particle size of the lithium-rich lithium iron phosphate material is beneficial to uniform mixing with other components in the positive active material, and further beneficial to the relatively uniform distribution of the lithium-rich lithium iron phosphate material on the current collector.

[0022] In some embodiments, the average thickness of the first coating layer is 5 nm~10 nm, and the average thickness of the second coating layer is 5 nm~10 nm.

[0023] In this embodiment, the uniform and continuous coating is achieved while the thickness of the coating layer is reduced, which is beneficial to ensure the capacity of the lithium-rich lithium-iron-oxide material, improve the conductivity, and keep the lithium-rich lithium-iron-oxide material with a small particle size.

[0024] In a second aspect, the embodiments of the present application provide a preparation method of the lithium-rich lithium-iron-oxide material as described above, comprising:

[0025] providing a core and a coating liquid;

[0026] forming a first coating layer on the surface of the core to obtain an intermediate product;

[0027] mixing the intermediate product with the coating liquid to form a second coating layer on the surface of the first coating layer to obtain the lithium-rich lithium-iron-oxide material;

[0028] In the embodiments, the coating liquid comprises a solvent, polyethylene oxide and lithium salt.

[0029] In this embodiment, the lithium-rich lithium-iron-oxide material can be obtained by performing two-step coating on the core containing Li5FeO4, the preparation method is simple, and the lithium-rich lithium-iron-oxide material with a small particle size of 3 μm to 5.5 μm can be obtained. The lithium-rich lithium-iron-oxide material prepared has good air stability and ion conductivity.

[0030] In some embodiments, the step of forming the first coating layer on the surface of the core comprises: mixing the core with a carbon source and a grinding aid and performing ball milling, and then performing carbonization treatment to obtain the intermediate product.

[0031] In this embodiment, the ball milling method is beneficial to the uniform mixing of the core and the carbon source. Before the carbonization treatment, the grinding aid needs to be removed, and the first coating layer (carbon layer) can be formed on the surface of the core by the step of carbonization treatment, i.e., the intermediate product is obtained.

[0032] In some embodiments, the carbon source is selected from at least one of polypropylene, glucose and high-temperature coal pitch.

[0033] In this embodiment, polypropylene, glucose and high-temperature coal pitch are all widely available and inexpensive, which is beneficial to reduce the cost. In particular, high-temperature coal pitch has high stability, and the effect of using it as a carbon source to improve the stability of the lithium-rich lithium-iron-oxide material is more significant.

[0034] In some embodiments, the temperature of the carbonization treatment is 500°C to 700°C, and the time is 1.5 h to 3 h.

[0035] In this embodiment, if the carbonization temperature is too low, the carbonization cannot be sufficient, and if the carbonization temperature is too high, the structure of Li5FeO4 in the core will be affected. Therefore, in this embodiment, the carbonization temperature is selected to be 500-700°C, and the time is selected to be 1.5-3h, so that a uniform carbon layer can be formed without affecting the structure of the core.

[0036] In some embodiments, the method further comprises preparing the core before forming the first coating layer on the surface of the core to obtain an intermediate product, and the preparation method comprises:

[0037] The first mixture of the iron source and the lithium source is placed in an inert atmosphere and calcined at 500-700°C for 24-48h.

[0038] In this embodiment, the mixture of the iron source and the lithium source is placed in an inert atmosphere and calcined at 500-700°C for 24-48h, which is beneficial to the complete reaction of the iron source and the lithium source. If the calcination temperature is too low or the time is too short, the product will have poor crystallinity, and if the sintering temperature is too high or the time is too long, the structure will be damaged, which will affect the lithium supplement effect of the prepared lithium-rich lithium-iron-oxide material.

[0039] In some embodiments, the iron source is Fe2O3.

[0040] In this embodiment, compared with other iron sources such as iron hydroxide, Fe2O3 is more beneficial to obtaining a small-particle-size Li5FeO4 core.

[0041] In some embodiments, the D90 particle size of the iron source is less than or equal to 400nm.

[0042] In this embodiment, the particle size of the iron source will affect the particle size of the core, and further affect the particle size of the finally prepared lithium-rich lithium-iron-oxide material. If the particle size of the iron source is too large, the particle size of the finally prepared lithium-rich lithium-iron-oxide material will also be large, which is not conducive to the uniform distribution of the material on the current collector. Preferably, the D90 particle size of the iron source is 200-400nm, which is beneficial to obtaining a small-particle-size Li5FeO4 core.

[0043] In some embodiments, the lithium source is selected from at least one of lithium oxide, lithium hydroxide and lithium carbonate.

[0044] In this embodiment, lithium oxide, lithium hydroxide and lithium carbonate have a wide source, which is beneficial to mass production and improves the industrial applicability of the method.

[0045] In some embodiments, the lithium source is a second mixture of lithium oxide and lithium hydroxide.

[0046] In this embodiment, considering that the decomposition temperature of lithium carbonate is too high and causes serious corrosion of the container, and that lithium hydroxide alone as a lithium source is prone to caking and lithium oxide is relatively expensive, a second mixture of lithium hydroxide and lithium oxide is selected as the lithium source to reduce the cost and avoid caking.

[0047] In some embodiments, the lithium source is a second mixture of lithium oxide and lithium hydroxide in a molar ratio of (1.5-2.5):1.

[0048] In this embodiment, by setting the lithium source as a second mixture of lithium oxide and lithium hydroxide in a molar ratio of (1.5-2.5):1, on the one hand, caking can be avoided, and on the other hand, the cost can be reduced.

[0049] In some embodiments, the molar ratio of Fe to Li in the first mixture is (1:5.0)-(1:5.2).

[0050] In this embodiment, the introduction of a slight excess of lithium is beneficial to increasing the content of lithium in the lithium supplement, thereby facilitating the improvement of the lithium supplement effect.

[0051] In some embodiments, after mixing the intermediate product with the coating liquid, the solvent is removed and grinding and sieving are performed to form a second coating layer on the surface of the first coating layer, thereby obtaining the lithium-rich lithium iron phosphate material.

[0052] In this embodiment, the solvent can be acetonitrile or a solvent similar in polarity to acetonitrile. The presence of the solvent is beneficial to the uniform and continuous coating of polyethylene oxide and the lithium source on the surface of the intermediate product. However, it should be noted that only considering the properties of polyethylene oxide itself, although high molecular weight polyethylene oxide is easy to form a uniform and continuous polymer network structure, a too high molecular weight of polyethylene oxide will make it difficult to dissolve in solvents such as acetonitrile, thereby affecting the continuity and uniformity of the coating.

[0053] In a third aspect, the embodiments of the present application provide a positive electrode tab, which comprises a current collector and an active material layer arranged on at least one side of the current collector, and the active material layer comprises the lithium-rich lithium iron phosphate material of any one of the preceding embodiments or the lithium-rich lithium iron phosphate material prepared by the preparation method of any one of the preceding embodiments.

[0054] In this embodiment, the active material layer of the positive electrode tab comprises the lithium-rich lithium iron phosphate material. Since the stability of the lithium-rich lithium iron phosphate material in air is significantly improved, the lithium supplement effect of the lithium-rich lithium iron phosphate material can be exerted, thereby enabling the positive electrode tab containing the lithium-rich lithium iron phosphate material to be applied to a battery, which can ensure the exertion of the overall capacity of the battery.

[0055] In some embodiments, the mass fraction of the lithium-rich lithium iron phosphate material in the active material layer is 2wt%-8wt%, thereby helping to compensate for the Li consumed by the formation of the SEI film. +.

[0056] In a fourth aspect, the embodiments of the present application provide a secondary battery, comprising the positive electrode sheet as described above.

[0057] In this embodiment, the secondary battery contains the positive electrode sheet as described above, thus maintaining a high capacity.

[0058] In a fifth aspect, the embodiments of the present application provide an electric device, comprising the secondary battery as described above.

[0059] In this embodiment, the electric device contains the secondary battery as described above, thus having the advantage of long endurance time.

[0060] The above description is only a summary of the technical solutions of the present application. In order to enable the technical means of the present application to be more clearly understood, and to be implemented according to the content of the description, and in order to enable the above and other purposes, characteristics and advantages of the present application to be more apparent and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0061] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creating any creative labor on the basis of these drawings.

[0062] Fig. 1 is a structural schematic diagram of a lithium-rich lithium iron oxide material in the embodiments of the present application;

[0063] Fig. 2 is a flow schematic diagram of a preparation method of the lithium-rich lithium iron oxide material in the embodiments of the present application;

[0064] Fig. 3 is a flow schematic diagram of a preparation process of the lithium-rich lithium iron oxide material in embodiment 1 of the present application;

[0065] Fig. 4 is an SEM test diagram of the lithium-rich lithium iron oxide material prepared in embodiment 1 of the present application;

[0066] Fig. 5 is a partial enlarged view of the SEM test diagram of the lithium-rich lithium iron oxide material shown in Fig. 4;

[0067] Fig. 6 is specific capacity data of the lithium-rich lithium iron oxide material prepared in embodiments 1 to 3 of the present application;

[0068] Fig. 7 is an XRD test diagram of Li5FeO4 before and after coating in embodiment 1 of the present application.

[0069] Among them, the above drawings include the following reference signs: 1, Li5FeO4; 2, carbon layer; 3, mixed layer comprising polyethylene oxide and lithium salt. DETAILED DESCRIPTION

[0070] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0071] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0072] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0073] In this paper, the phrase "embodiment" means that the specific features, structures or properties described in conjunction with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment that is not mutually exclusive with other embodiments. The skilled person in the art explicitly and implicitly understands that the embodiments described herein can be combined with other embodiments.

[0074] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship between the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are in an "or" relationship.

[0075] In the description of the embodiments of the present application, the term "a plurality of" refers to two or more (including two), and similarly, "a plurality of groups" refers to two or more groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0076] In the description of the embodiments of the present application, the orientations or positional relationships indicated by the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the embodiments of the present application.

[0077] Li5FeO4 has a high theoretical specific capacity, is well matched with existing positive electrode materials, is well compatible with existing bonding agents, and is relatively low in price, and therefore has good application prospects. However, Li5FeO4 is very sensitive to water and CO2 in the air, and has poor stability in the air, which seriously affects its industrialization process. In order to solve the technical problem of poor stability of the lithium-rich lithium iron oxide material in the air, the present application provides a lithium-rich lithium iron oxide material, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device, wherein by arranging a carbon layer and a mixed layer composed of polyethylene oxide and a lithium salt outside the core containing Li5FeO4, the technical effect of improving the stability of the lithium-rich lithium iron oxide material in the air can be achieved, and the electrical performance of the positive electrode sheet, the secondary battery and the electric device is also improved.

[0078] The electric device provided by the embodiments of the present application can be, but is not limited to, a mobile phone, a tablet computer, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric automobile, a ship, a spacecraft, etc. The electric toy can include fixed or mobile electric toys, such as game consoles, electric automobile toys, electric ship toys and electric aircraft toys, etc. The spacecraft can include airplanes, rockets, space shuttles and spacecraft, etc.

[0079] Please refer to FIG. 1, in the first aspect, the present application provides a lithium-rich lithium iron oxide material, including core-shell structure particles, the core-shell structure particles include a core, a first coating layer coated outside the core and a second coating layer coated outside the first coating layer; wherein the core is Li5FeO4, the first coating layer is a carbon layer, and the second coating layer is a mixed layer including polyethylene oxide and a lithium salt; wherein the mass ratio of the first coating layer to the core is (2:100)~(10:100), and the mass ratio of the second coating layer to the core is (3:100)~(13:100).

[0080] The lithium-rich lithium-iron material of the present application has a first coating layer and a second coating layer arranged outside the core containing Li5FeO4in turn, wherein the first coating layer is a carbon layer, and the second coating layer is a mixed layer containing polyethylene oxide and lithium salt. By arranging the first coating layer and the second coating layer outside the core in turn, on the one hand, through double-layer coating, the Li5FeO4in the core can be protected from the influence of moisture and carbon dioxide in the air, and the air stability of the lithium-rich lithium-iron material can be improved. On the other hand, the second coating layer as the outermost layer is a solid electrolyte layer, which can increase the electronic conductivity and ionic conductivity of the whole lithium-rich lithium-iron material.

[0081] The mass ratio of the first coating layer to the core is (2:100)~(10:100), and the mass ratio of the second coating layer to the core is (3:100)~(13:100). For the first coating layer, if the proportion of the carbon layer is too large, the overall capacity of the prepared lithium-rich lithium-iron material will be reduced due to the low specific capacity of the carbon layer. If the proportion of the carbon layer is too small, complete and continuous coating cannot be achieved, and the core containing Li5FeO4cannot be effectively isolated from the air. For the second coating layer, if the proportion of the mixed layer is too large, the particles of the prepared lithium-rich lithium-iron material will be bonded to each other, thereby increasing the particle size of the lithium-rich lithium-iron material. If the proportion of the mixed layer is too small, complete and continuous coating cannot be achieved, and the core containing Li5FeO4cannot be effectively isolated from the air, and the electronic conductivity and ionic conductivity of the whole lithium-rich lithium-iron material will also be affected. Therefore, the proportion of the second coating layer is selected to be as small as possible and to form a complete and continuous coating layer.

[0082] Specifically, the mass ratio of the first coating layer to the core can be 2:100, 4:100, 6:100, 8:100, 10:100, or any value between (2:100)~(10:100); preferably, the mass ratio of the first coating layer to the core is (5:100)~(7:100); the mass ratio of the second coating layer to the core can be 3:100, 5:100, 7:100, 9:100, 11:100, 13:100, or any value between (3:100)~(13:100).

[0083] In some embodiments, the mass ratio of polyethylene oxide to the core is (0.5:100)~(5:100), and the mass ratio of lithium salt to the core is (2:100)~(8:100); preferably, the mass ratio of polyethylene oxide to the core is (0.5:100)~(2:100), and the mass ratio of lithium salt to the core is (2:100)~(4:100).

[0084] The polyethylene oxide, on one hand, increases the conductivity of the solid electrolyte material, and on the other hand, has certain adhesion, which can improve the strength and uniformity of the coating layer; the lithium salt can further improve the conductivity of the coating layer. Under the condition that the amounts of other components are the same, if the amount of polyethylene oxide is too high, the lithium-rich lithium-iron oxide material particles will be bonded to each other, and if the amount of polyethylene oxide is too low, complete and continuous coating cannot be achieved; if the amount of lithium salt is too high, the content of polyethylene oxide in the mixed layer is too low, the viscosity between the carbon layer and the mixed layer is reduced, and the strength of the mixed layer is also reduced, and if the amount of lithium salt is too low, the conductivity of the lithium-rich lithium-iron oxide material will be reduced.

[0085] Specifically, the mass ratio of polyethylene oxide to the inner core can be 0.5:100, 1:100, 2:100, 1.4:100, 3:100, 4:100, 5:100, or any value between (0.5:100) and (5:100); the mass ratio of lithium salt to the inner core can be 2:100, 3:100, 4:100, 5:100, 6:100, 7:100, 8:100, or any value between (2:100) and (8:100).

[0086] In some embodiments, the lithium salt is selected from at least one of lithium bisfluorosulfonylimide, lithium hexafluorophosphate, and lithium fluoride, and specifically can be one of lithium bisfluorosulfonylimide, lithium hexafluorophosphate, and lithium fluoride, or a mixture of two of lithium bisfluorosulfonylimide and lithium hexafluorophosphate, lithium hexafluorophosphate and lithium fluoride, or lithium bisfluorosulfonylimide, lithium hexafluorophosphate, and lithium fluoride.

[0087] In some embodiments, the molecular weight of the polyethylene oxide is 50,000 Da to 150,000 Da, and specifically can be 50,000 Da, 70,000 Da, 90,000 Da, 110,000 Da, 130,000 Da, 150,000 Da, or any value between 50,000 Da and 150,000 Da, which is conducive to forming a uniform and continuous polymer network structure and providing better ion conduction performance.

[0088] In some embodiments, the D90 particle size of the inner core is 3 μm to 5.5 μm, and specifically can be 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, or any value between 3 μm and 5.5 μm.

[0089] In some embodiments, the average thickness of the first coating layer is 5 nm to 10 nm, and specifically can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any value between 5 nm and 10 nm.

[0090] In some embodiments, the average thickness of the second coating layer is 5 nm to 10 nm, specifically 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm or any value between 5 nm and 10 nm.

[0091] In a second aspect, an embodiment of the present application provides a method for preparing any of the aforementioned lithium-rich lithium ferrite materials, as shown in FIG2 , comprising:

[0092] S1, providing the core and coating liquid;

[0093] S2, forming a first coating layer on the surface of the core to obtain an intermediate product;

[0094] S3, mixing the intermediate product with a coating liquid to form a second coating layer on the surface of the first coating layer to obtain a lithium-rich lithium ferrite material;

[0095] The coating solution includes a solvent, polyethylene oxide and lithium salt.

[0096] The preparation method of the present application only requires two-step coating of the Li5FeO4-containing core to obtain a lithium-rich lithium ferrite material. The preparation method is relatively simple and can obtain a lithium-rich lithium ferrite material with a small particle size of D90 of 3μm to 5.5μm.

[0097] Specifically, in step S1, the method for preparing the inner core includes:

[0098] The first mixture of the iron source and the lithium source is placed under an inert atmosphere and calcined at 500°C to 700°C for 24 to 48 hours, wherein the calcination temperature can be 500°C, 550°C, 600°C, 650°C, 700°C or any value between 500°C and 700°C, and the calcination time can be 24h, 28h, 32h, 36h, 40h, 44h, 48h or any value between 24h and 48h.

[0099] In some embodiments, a mixer can be used to uniformly mix the iron source and the lithium source. The mixing time can be adjusted according to the capacity and rotation speed of the mixer to ensure uniform mixing of the iron source and the lithium source as much as possible.

[0100] In some embodiments, during the calcination step, the first mixture may be fed into a sintering furnace for calcination, and an inert gas may be introduced in advance, with the amount of inert gas introduced being 5 m 3 / h~10m 3 / h, so that the oxygen content in the sintering furnace is less than 1ppm and the humidity is less than 5% to prevent carbon dioxide and water from reacting with the first mixture. Thereafter, the sintering furnace is heated for calcination at a heating rate of 3°C / min to 7°C / min.

[0101] In some embodiments, the iron source is Fe2O3, and the D90 particle size of the iron source is less than or equal to 400 nm, specifically, can be 400 nm, 350 nm, 300 nm, 250 nm, 200 nm, 150 nm, or any value less than or equal to 400 nm. Preferably, the D90 particle size of the iron source is 200 nm to 400 nm.

[0102] In some embodiments, the lithium source is selected from at least one of lithium oxide, lithium hydroxide, and lithium carbonate. Preferably, the lithium source is a second mixture of lithium oxide and lithium hydroxide. Specifically, the lithium source is a mixture of lithium oxide and lithium hydroxide in a molar ratio of (1.5-2.5):1, for example, the molar ratio of lithium oxide and lithium hydroxide can be 1.5:1, 1.7:1, 1.9:1, 2.1:1, 2.3:1, 2.5:1, or any value between 1.5-2.5:1.

[0103] In some embodiments, the molar ratio of Fe and Li in the second mixture of the iron source and the lithium source is 1:(5.0-5.2), specifically, can be 1:5.0, 1:5.05, 1:5.1, 1:5.15, 1:5.2, or any value between 1:(5.0-5.2).

[0104] It should be noted that the Li5FeO4 obtained in this embodiment needs to be crushed for use, specifically, the crushing can be performed by air flow crushing, for example, the sintered Li5FeO4 is subjected to air flow crushing, the dew point is maintained below -30°C, and pure phase Li5FeO4 with a particle size range of 3-5.5 μm is obtained.

[0105] Specifically, in step S2, the step of forming a first coating layer on the surface of the core includes: mixing the core with a carbon source and a grinding aid and ball milling, and then performing carbonization treatment to obtain an intermediate product.

[0106] In this embodiment, the core and the carbon source are uniformly mixed by ball milling, the rotation speed during ball milling can be set to 300-600 rpm, the time is 1-5 h, and the ball-to-material ratio is (8-12):1. Before the carbonization treatment, the grinding aid needs to be removed, the grinding aid is an alcohol compound, for example, ethanol, etc., wherein the grinding aid can be removed by drying to volatilize the grinding aid.

[0107] In some embodiments, the carbon source is selected from at least one of polypropylene, glucose, and high-temperature coal pitch. For example, the carbon source can be polypropylene, glucose, high-temperature coal pitch, a mixture of polypropylene and glucose, a mixture of glucose and high-temperature coal pitch, and a mixture of polypropylene, glucose, and high-temperature coal pitch, etc.

[0108] It should be noted that coal tar is a multiphase system, and its basic components are aliphatic hydrocarbons, cycloalkanes, polycyclic, condensed-ring and heterocyclic aromatic hydrocarbons. Because it is in a glassy phase at room temperature and softens and then melts when heated, it is classified according to the softening point into low-temperature coal tar (CTPD) ≤75°C, medium-temperature coal tar (CTPZ) 75.95°C and high-temperature coal tar (CTPG) ≥95°C.

[0109] In some embodiments, the temperature of the carbonization treatment is 500°C to 700°C, specifically 500°C, 550°C, 600°C, 650°C, 700°C or any value between 500°C and 700°C, and the time is 1.5h to 3h, specifically 1.5h, 2h, 2.5h, 3h or any value between 1.5h and 3h.

[0110] In this application, the carbonization step is to place the carbon source coated Li5FeO4 precursor in a sintering furnace for high temperature calcination. In order to reduce the oxidation of the carbon source, an inert gas needs to be introduced in advance. The amount of inert gas introduced can be 3m 3 / h~7m 3 / h, so that the oxygen content in the sintering furnace is less than 1ppm and the humidity is less than 5% to prevent oxygen and water from reacting with the carbon source and affecting the carbonization process; thereafter, the sintering furnace is heated to 500℃~700℃ at a heating rate of 3℃ / min~7℃ / min, and kept warm for 1.5h~3h to obtain an intermediate product, and the intermediate product is subjected to air flow pulverization to maintain the dew point below -30℃ to obtain an intermediate product with a particle size of 3μm~5.5μm.

[0111] Specifically, in step S3, the lithium iron oxide-rich material is obtained by mixing the intermediate product with the coating liquid, removing the solvent, grinding and sieving, wherein the solvent can be acetonitrile or a solvent with a polarity similar to acetonitrile, and the amount of solvent used needs to allow the polyethylene oxide to be completely dissolved. If there is too much solvent, it can be removed by drying later, which has little effect on the overall performance of the material, but will prolong the time for solvent removal and increase energy consumption, affecting production efficiency and cost. If there is too little solvent, the polyethylene oxide cannot be completely dissolved, which will affect the uniformity and continuity of the coating layer.

[0112] Further, in the coating liquid, the mass ratio of polyethylene oxide and lithium salt is (20-50):100, and specifically can be 20:100, 30:100, 40:100, 50:100, or any value between 20-50:100, preferably (30-40):100. By adjusting the ratio of polyethylene oxide and lithium salt, when the mass ratio of polyethylene oxide to the core is (0.5:100)-(2:100), the mass ratio of lithium salt to the core is (2:100)-(4:100), and the mass ratio of polyethylene oxide and lithium salt is (20-50):100, the conductivity, uniformity, and continuity of the coating can be further considered, and mutual adhesion between lithium-rich lithium-iron phosphate material particles is avoided.

[0113] Further, in the solvent removal process, the removal speed can be increased by increasing the temperature, but the temperature should not be too high to avoid damaging the structure of the lithium-rich lithium-iron phosphate material. In addition, after the solvent is removed, the lithium-rich lithium-iron phosphate material can be crushed. The crushing method can specifically use air flow crushing to reduce the particle size.

[0114] In a third aspect, the embodiments of the present application provide a positive electrode sheet, which comprises a positive electrode current collector and an active material layer arranged on at least one side of the positive electrode current collector. The active material layer comprises the lithium-rich lithium-iron phosphate material of any one of the preceding embodiments or the lithium-rich lithium-iron phosphate material prepared by the preparation method of any one of the preceding embodiments.

[0115] In some embodiments, the mass fraction of the lithium-rich lithium-iron phosphate material in the active material layer is 2wt%-8wt%, and specifically can be 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, or any value between 2wt%-8wt%. The specific amount can be determined according to the lithium supplement efficiency.

[0116] In a fifth aspect, the embodiments of the present application provide an electric device, such as a vehicle, comprising the secondary battery of the above embodiments.

[0117] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market.

[0118] I. Preparation method

[0119] Embodiment 1

[0120] The embodiment provides a preparation method of a lithium-rich lithium-iron phosphate material, which comprises the following steps:

[0121] (1) Take nano Fe2O3 with D90 of 300 nm, Li2O and LiOH, keep Fe: Li = 1:5.1 (this ratio refers to the molar ratio of Fe element to Li element, the same below), LiOH: Li2O = 1:2 (this ratio refers to the molar ratio of LiOH to Li2O, the same below) in a mixer, stir for 30 min until they are mixed uniformly, to obtain mixed raw materials;

[0122] (2) Put the mixed raw materials into a sintering furnace for calcination, pre-ventilate high-purity nitrogen, ventilate 5 m 3 / h, so that the oxygen content is less than 1 ppm and the humidity is less than 5%, then raise the sintering furnace to 600℃ at a temperature rising rate of 5℃ / min, keep for 36 h, to obtain pure phase Li5FeO4;

[0123] (3) Airflow crush the sintered Li5FeO4, keep the dew point below -30℃, to obtain pure phase Li5FeO4 with small particle size;

[0124] (4) Take 200 g of crushed Li5FeO4, 20 g of high-temperature coal pitch and appropriate amount of anhydrous ethanol, mix them uniformly, put them into a ball mill, ball mill according to the ball-to-material ratio of 10:1 at 450 rpm for 3 h, coat the high-temperature coal pitch on the Li5FeO4, to obtain high-temperature coal pitch coated Li5FeO4 precursor;

[0125] (5) Put the high-temperature coal pitch coated Li5FeO4 precursor into a sintering furnace for high-temperature calcination, pre-ventilate high-purity nitrogen, ventilate 5 m 3 / h, so that the oxygen content is less than 1 ppm and the humidity is less than 5%, then raise the sintering furnace to 550℃ at a temperature rising rate of 5℃ / min, keep for 2 h, airflow crush the product, keep the dew point below -30℃, to obtain Li5FeO4@C, wherein the mass ratio of the carbon layer to the core Li5FeO4 is 6:100;

[0126] (6) Put polyethylene oxide (molecular weight of 100,000 Da) and LIFSI (lithium bisfluorosulfonylimide) (according to the mass ratio of polyethylene oxide monomer to LIFSI = 40:100 (the same below)) into anhydrous acetonitrile, stir for 3 h until they are completely dissolved, to obtain coating liquid, put Li5FeO4@C (the mass ratio of polyethylene oxide to Li5FeO4 is 1:100) into the coating liquid, stir until the acetonitrile is completely volatilized, airflow crush the product, to obtain lithium-rich lithium iron phosphate material (wherein the mass ratio of polyethylene oxide to Li5FeO4 is 1:100, the mass ratio of LIFSI to Li5FeO4 is 2.5:100, and the mass ratio of the second coating layer to Li5FeO4 is 3.5:100), SEM and XRD are shown in FIG. 4, FIG. 5 and FIG. 7.

[0127] Example 2

[0128] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is only different from the example 1 in that the carbon layer content is reduced to a mass ratio of the carbon layer to the inner core Li5FeO4 of 2:100.

[0129] Example 3

[0130] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is only different from the example 1 in that the carbon layer content is increased to a mass ratio of the carbon layer to the inner core Li5FeO4 of 8:100.

[0131] Example 4

[0132] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is only different from the example 1 in that the carbon layer content is increased to a mass ratio of the carbon layer to the inner core Li5FeO4 of 10:100.

[0133] Example 5

[0134] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which comprises the following steps:

[0135] (1) Take nano Fe2O3 with D90 of 300 nm, Li2O and LiOH, keep Fe:Li = 1:5.1, LiOH:Li2O = 1:2 in the lithium source, and place them in a mixer to stir for 30 min until they are uniformly mixed to obtain mixed raw materials;

[0136] (2) Put the mixed raw materials into a sintering furnace for calcination, pre-vent in high-purity nitrogen, and vent in an amount of 5 m 3 / h to make the oxygen content less than 1 ppm and the humidity less than 5%, and then raise the sintering furnace to 600℃ at a temperature rising rate of 5℃ / min, and keep the temperature for 36 h to obtain pure phase Li5FeO4;

[0137] (3) Airflow crush the sintered Li5FeO4, keep the dew point below -30℃ to obtain small particle size pure phase Li5FeO4;

[0138] (4) Take 200 g of the crushed Li5FeO4, 20 g of polypropylene and an appropriate amount of anhydrous ethanol, mix them uniformly, place them in a ball mill, and ball mill at 450 rpm for 3 h according to a ball-to-material ratio of 10:1 to coat the Li5FeO4 with polypropylene to obtain a polypropylene-coated Li5FeO4 precursor;

[0139] (5) Put the polypropylene-coated Li5FeO4 precursor into a sintering furnace for high-temperature calcination, pre-vent in high-purity nitrogen, and vent in an amount of 5 m 3 / h, so that the oxygen content is less than 1 ppm, the humidity is less than 5%, then the sintering furnace is raised to 400℃ at a temperature raising rate of 5℃ / min, and the product is air-pulverized, and the dew point is kept below -30℃, to obtain Li5FeO4@C, wherein the mass ratio of the carbon layer to the core Li5FeO4 is 6:100;

[0140] (6) Polyethylene oxide (molecular weight 100,000 Da) and LIFSI (polyethylene oxide monomer: LIFSI = 40%) are placed in anhydrous acetonitrile, stirred for 3h to completely dissolve, and Li5FeO4@C is placed therein, stirred and volatilized until the acetonitrile is completely volatilized, and the product is air-pulverized to obtain a lithium-rich lithium-iron material (wherein the mass ratio of polyethylene oxide to Li5FeO4 is 1:100, the mass ratio of LIFSI to Li5FeO4 is 2.5:100, and the mass ratio of the second coating layer to Li5FeO4 is 3.5:100).

[0141] Example 6

[0142] This embodiment provides a preparation method of a lithium-rich lithium-iron material, which is different from example 5 only in that the carbon layer content is reduced to a mass ratio of the carbon layer to the core Li5FeO4 of 2:100.

[0143] Example 7

[0144] This embodiment provides a preparation method of a lithium-rich lithium-iron material, which is different from example 5 only in that the carbon layer content is increased to a mass ratio of the carbon layer to the core Li5FeO4 of 8:100.

[0145] Example 8

[0146] This embodiment provides a preparation method of a lithium-rich lithium-iron material, which is different from example 5 only in that the carbon layer content is increased to a mass ratio of the carbon layer to the core Li5FeO4 of 10:100.

[0147] Example 9

[0148] This embodiment provides a preparation method of a lithium-rich lithium-iron material, which comprises the following steps:

[0149] (1) Nanometer Fe2O3 with D90 of 300nm, Li2O and LiOH are weighed, and Fe:Li is kept at 1:5.1, and LiOH:Li2O in the lithium source is 1:2, which is placed in a mixer and stirred for 30min until it is uniformly mixed to obtain a mixed raw material;

[0150] (2) The mixed raw material is sent into a sintering furnace for calcination, high-purity nitrogen is introduced in advance, the amount of introduction is 5m 3 / h, so that the oxygen content is less than 1 ppm, and the humidity is less than 5%, and then the sintering furnace is raised to 600 DEG C at a temperature increasing rate of 5 DEG C / min, and is kept for 36 h, to obtain pure phase Li5FeO4;

[0151] (3) The sintered Li5FeO4 is subjected to airflow crushing, and the dew point is kept below -30 DEG C, to obtain pure phase Li5FeO4 with small particle size;

[0152] (4) 200 g of the crushed Li5FeO4, 60 g of glucose and a proper amount of anhydrous ethanol are uniformly mixed, and are placed in a ball mill, and are ball milled at 450 rpm for 3 h according to a ball-to-material ratio of 10:1, so that the glucose is coated on the Li5FeO4, to obtain a glucose-coated Li5FeO4 precursor;

[0153] (5) The glucose-coated Li5FeO4 precursor is placed in a sintering furnace for high-temperature calcination, high-purity nitrogen is introduced in advance, the amount of the introduced high-purity nitrogen is 5 m 3 / h, so that the oxygen content is less than 1 ppm, and the humidity is less than 5%, and then the sintering furnace is raised to 400 DEG C at a temperature increasing rate of 5 DEG C / min, and is kept for 2 h, and the product is subjected to airflow crushing, and the dew point is kept below -30 DEG C, to obtain Li5FeO4@C, wherein the mass ratio of the carbon layer to the core Li5FeO4 is 6:100;

[0154] (6) A proper amount of polyethylene oxide (with a molecular weight of 100,000 Da) and LIFSI (according to a ratio of polyethylene oxide monomer:LIFSI = 40:100) are placed in anhydrous acetonitrile, and are stirred for 3 h until they are completely dissolved, and the Li5FeO4@C is placed therein, and is stirred until the acetonitrile is completely volatilized, and the product is subjected to airflow crushing, to obtain a lithium-rich lithium-iron-oxide material (the mass ratio of the polyethylene oxide to the Li5FeO4 is 1%, the mass ratio of the LIFSI to the Li5FeO4 is 2.5:100, and the mass ratio of the second coating layer to the Li5FeO4 is 3.5:100).

[0155] Example 10

[0156] The embodiment provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the embodiment 9 only in that the content of the carbon layer is reduced to a mass ratio of the carbon layer to the core Li5FeO4 of 2:100.

[0157] Example 11

[0158] The embodiment provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the embodiment 9 only in that the content of the carbon layer is increased to a mass ratio of the carbon layer to the core Li5FeO4 of 8:100.

[0159] Example 12

[0160] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 9 is that the content of the carbon layer is increased to 10:100 of the mass ratio of the carbon layer to the core Li5FeO4.

[0161] Embodiment 13

[0162] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that the mass ratio of the polyethylene oxide to the Li5FeO4 is 0.5:100, the mass ratio of the LIFSI to the Li5FeO4 is 2.5:100, and the mass ratio of the second coating layer to the Li5FeO4 is 3:100.

[0163] Embodiment 14

[0164] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that the mass ratio of the polyethylene oxide to the Li5FeO4 is 1:100, the mass ratio of the LIFSI to the Li5FeO4 is 2:100, and the mass ratio of the second coating layer to the Li5FeO4 is 3:100.

[0165] Embodiment 15

[0166] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that the mass ratio of the polyethylene oxide to the Li5FeO4 is 2:100, the mass ratio of the LIFSI to the Li5FeO4 is 6:100, and the mass ratio of the second coating layer to the Li5FeO4 is 8:100.

[0167] Embodiment 16

[0168] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that the mass ratio of the polyethylene oxide to the Li5FeO4 is 5:100, the mass ratio of the LIFSI to the Li5FeO4 is 8:100, and the mass ratio of the second coating layer to the Li5FeO4 is 13:100.

[0169] Embodiment 17

[0170] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in the step (5), the Li5FeO4 precursor coated by the high-temperature coal pitch is placed in a sintering furnace for high-temperature calcination, the sintering furnace is increased to 500 DEG C at a temperature increasing rate of 5 DEG C / min, the product is airflow pulverized, the dew point is kept below-30 DEG C, the Li5FeO4@C is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0171] Embodiment 18

[0172] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (5), the Li5FeO4 precursor coated with high-temperature coal pitch is placed in a sintering furnace to perform high-temperature calcination, the sintering furnace is raised to 700 DEG C at a temperature raising rate of 5 DEG C / min, the product is air-flow pulverized, the dew point is kept below -30 DEG C, Li5FeO4@C is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0173] Embodiment 19

[0174] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (5), the Li5FeO4 precursor coated with high-temperature coal pitch is placed in a sintering furnace to perform high-temperature calcination, the sintering furnace is raised to 450 DEG C at a temperature raising rate of 5 DEG C / min, the product is air-flow pulverized, the dew point is kept below -30 DEG C, Li5FeO4@C is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0175] Embodiment 20

[0176] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (1), Fe:Li is kept as 1:5.2, the mixed raw material is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0177] Embodiment 21

[0178] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (1), Fe:Li is kept as 1:5.0, the mixed raw material is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0179] Embodiment 22

[0180] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (1), Fe:Li is kept as 1:5.3, the mixed raw material is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0181] Embodiment 23

[0182] The embodiment provides a preparation method of a lithium-rich lithium-iron material, and the difference from the embodiment 1 is that in step (1), Fe:Li is kept as 1:4.9, the mixed raw material is obtained, and finally the lithium-rich lithium-iron material is obtained.

[0183] Embodiment 24

[0184] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that in the step (6), the polyethylene oxide monomer: LIFSI = 30: 100, and finally a lithium-rich lithium-iron-oxide material is obtained.

[0185] Example 25

[0186] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that in the step (6), the polyethylene oxide monomer: LIFSI = 50: 100, and finally a lithium-rich lithium-iron-oxide material is obtained.

[0187] Example 26

[0188] The present example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that in the step (6), the polyethylene oxide monomer: LIFSI = 20: 100, and finally a lithium-rich lithium-iron-oxide material is obtained.

[0189] Comparative Example 1

[0190] The present comparative example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that the step (6) is not included.

[0191] Comparative Example 2

[0192] The present comparative example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that the steps (4) and (5) are not included.

[0193] Comparative Example 3

[0194] The present comparative example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that the Fe2O3 in the step (1) is replaced by iron hydroxide.

[0195] Comparative Example 4

[0196] The present comparative example provides a preparation method of a lithium-rich lithium-iron-oxide material, which is different from the example 1 only in that the polyethylene oxide in the step (1) is replaced by lithium titanium aluminum phosphate.

[0197] II. Test Method

[0198] Particle size test: the particle size of the lithium-rich lithium-iron-oxide material prepared in the examples and comparative examples is tested by using a laser particle size instrument, and the results are shown in Table 1.

[0199] Capacity test:

[0200] Electrochemical performance test was carried out by using button type half-electrode: the lithium-rich lithium-iron-oxide material prepared in the examples and comparative examples or the pure phase Li5FeO4 obtained in Example 2 was used as a positive electrode material, the above positive electrode material, conductive carbon black and binder PVDF (polyvinylidene fluoride) were mixed in a ratio of 8:1:1 to form a slurry, which was uniformly coated on an aluminum foil to prepare a positive electrode sheet (electrode mixing, coating and other preparation processes were completed in air), a lithium metal sheet was used as a negative electrode sheet, an electrolyte was 1 mol / L LiPF6, a solvent was EC:DMC:EMC = 1:1:1 (volume ratio), and a battery shell, positive and negative electrode sheets, a separator (PE double-layer ceramic separator), a spring and a gasket were assembled into a button cell in a vacuum glove box. Electrochemical performance test was carried out, and capacity test was carried out at 3.3-4.3 V, and the results are shown in Table 1 and FIG. 6.

[0201] Table 1

[0202] III. Analysis of test results of each example and comparative example

[0203] According to Table 1 and FIG. 6, the small-particle lithium-rich lithium-iron-oxide material with relatively optimal capacity can be obtained in Examples 1-26, and the capacity of Example 1 is the highest. As can be seen from the comparison of Examples 1-4, Examples 5-8 and Examples 9-12, the capacity decreases when the content of the carbon layer is too high or too low. As can be seen from the comparison of Example 1 and Examples 13-16, the capacity decreases when the content of the second coating layer is too high or too low. As can be seen from the comparison of Example 1 and Examples 13-14, the capacity decreases when the ratio of polyethylene oxide and lithium salt in the second coating layer is unreasonable. As can be seen from the comparison of Example 1, 5 and 9, although the use of conventional carbon source can improve the capacity of the battery, high-temperature coal pitch has a more obvious effect on the capacity improvement.

[0204] As can be seen from the comparison of Example 1, Comparative Example 1 and Comparative Example 2 according to Table 1 and FIG. 6, the capacity is improved much higher when the carbon layer and the second coating layer containing polyethylene oxide and lithium salt are simultaneously coated than when the carbon layer or the second coating layer containing polyethylene oxide and lithium salt is coated alone. As can be seen from the comparison of Example 1 and Comparative Example 3, the selection of iron oxide as the carbon source is beneficial to obtaining small-particle lithium-rich lithium-iron-oxide material, and if iron hydroxide is selected, the particle size of the lithium-rich lithium-iron-oxide material increases and the capacity significantly decreases. Common air-stable solid electrolytes include lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP) and lithium lanthanum zirconium oxide (LLZO). As can be seen from the comparison of Example 1 and Comparative Example 4, the application of the conventional solid electrolyte in the present application can significantly improve the capacity of the battery.

[0205] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A lithium-rich lithium-iron-phosphate material, characterized in that, The core-shell structure particle comprises an inner core, a first coating layer coated outside the inner core, and a second coating layer coated outside the first coating layer. The inner core is Li5FeO4, the first coating layer is a carbon layer, and the second coating layer is a mixed layer comprising polyethylene oxide and a lithium salt. The mass ratio of the first coating layer to the inner core is (2:100) to (10:100), and the mass ratio of the second coating layer to the inner core is (3:100) to (13:100).

2. The lithium-rich lithium-iron-phosphate material of claim 1, wherein, The mass ratio of the polyethylene oxide to the inner core is (0.5:100) to (5:100), and the mass ratio of the lithium salt to the inner core is (2:100) to (8:100). Optionally, the mass ratio of the polyethylene oxide to the inner core is (0.5:100) to (2:100), and the mass ratio of the lithium salt to the inner core is (2:100) to (4:100). And / or, the mass ratio of the carbon layer to the inner core is (2:100) to (8:100). And / or, the mass ratio of the carbon layer to the inner core is (5:100) to (7:100). And / or, the lithium salt is selected from at least one of lithium bisfluorosulfonylimide, lithium hexafluorophosphate, and lithium fluoride. And / or, the molecular weight of the polyethylene oxide is 50,000 Da to 150,000 Da.

3. The lithium-rich lithium-iron-phosphate material according to claim 1 or 2, characterized in that The D90 particle size of the inner core is 3 μm to 5.5 μm. And / or, the average thickness of the first coating layer is 5 nm to 10 nm. And / or, the average thickness of the second coating layer is 5 nm to 10 nm.

4. A method of producing the lithium-rich lithium-iron-phosphate material according to any one of claims 1 to 3, characterized in that The method comprises: providing an inner core and a coating liquid; forming a first coating layer on the surface of the inner core to obtain an intermediate product; mixing the intermediate product with the coating liquid to form a second coating layer on the surface of the first coating layer, thereby obtaining the lithium-rich lithium iron phosphate material; The coating liquid comprises a solvent, polyethylene oxide, and a lithium salt.

5. The preparation method according to claim 4, characterized in that The step of forming the first coating layer on the surface of the inner core comprises: mixing the inner core with a carbon source and a grinding aid and performing ball milling, and then performing carbonization treatment to obtain the intermediate product. Optionally, the carbon source is selected from at least one of polypropylene, glucose, and high-temperature coal tar. Optionally, the carbonization treatment is performed at a temperature of 500°C to 700°C for 1.5 h to 3 h.

6. The production method according to claim 4 or 5, characterized by, Before forming the first coating layer on the surface of the inner core to obtain the intermediate product, the method further comprises preparing the inner core, and the preparation method comprises: placing a first mixture of an iron source and a lithium source in an inert atmosphere and calcining at 500°C to 700°C for 24 h to 48 h; Optionally, the iron source is Fe2O3. Optionally, the D90 particle size of the iron source is less than or equal to 400 nm. And / or, the D90 particle size of the iron source is 200 nm to 400 nm. Optionally, the lithium source is selected from at least one of lithium oxide, lithium hydroxide, and lithium carbonate. Optionally, the lithium source is a second mixture of lithium oxide and lithium hydroxide. Optionally, the lithium source is the second mixture with a molar ratio of lithium oxide to lithium hydroxide of (1.5:1) to (2.5:1). Optionally, the molar ratio of Fe and Li in the first mixture is (1:5.0)-(1:5.2).

7. The production method according to any one of claims 4 to 6, characterized by, After mixing the intermediate product with a coating liquid, removing the solvent, and performing grinding and sieving, a second coating layer is formed on the surface of the first coating layer, thereby obtaining the lithium-rich lithium-iron-phosphate material.

8. The production method according to any one of claims 4 to 7, characterized by, In the coating liquid, the mass ratio of the polyethylene oxide and the lithium salt is (20-50):100, preferably (30-40):

100.

9. A positive electrode sheet characterized by comprising: The secondary battery comprises the positive electrode plate according to claim 9. The electric device comprises the secondary battery according to claim 10.

10. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode plate according to claim 9.

11. An electrical device, characterized by The electric device comprises the secondary battery according to claim 10.

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