Composite coated high-stability lithium-rich lithium ferrite and preparation method thereof

Abstract

The invention provides composite coated high-stability lithium-rich lithium ferrite and a preparation method thereof, and belongs to the field of lithium ion battery positive electrode lithium supplements.The preparation method of the composite coated high-stability lithium-rich lithium ferrite comprises the steps that an iron source and a lithium source are mixed, and a first mixture is obtained; performing first calcining treatment on the first mixture to obtain a calcined product; mixing and dispersing the calcined product and a metal compound to obtain a second mixture; carrying out second calcining treatment on the second mixture to obtain composite coated high-stability lithium-rich lithium ferrite; wherein the ratio of the amount of substance of the lithium element in the lithium source to the amount of substance of the iron element in the iron source is greater than 5. According to the method, excessive lithium sources are introduced, so that a large amount of residual lithium on the surface of Li5FeO4 forms a residual alkali layer, the residual alkali layer is converted into a compact and complete metal lithium oxide coating layer under the action of the metal compound and the second calcination treatment, and the air stability of Li5FeO4 is effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of lithium-ion battery positive electrode lithium supplementing agents, and in particular to a composite-coated high-stability lithium-rich ferrite and a preparation method thereof. Background Art

[0002] Pre-lithiation is a technology to improve the performance of lithium-ion batteries. It aims to solve the problem of active lithium loss during the first charge and discharge process of the battery, and can increase the total capacity and energy density of the battery. Positive electrode pre-lithiation has become an industrially promising pre-lithiation method due to its advantages such as high safety, low price, easy synthesis, and high rechargeability.

[0003] Positive electrode pre-lithiation technology can compensate for the irreversible reaction of the negative electrode while being compatible with existing electrodes by adding positive electrode lithium supplements to the positive electrode materials. Among them, the performance of the positive electrode lithium supplement has a great influence on the effect of pre-lithiation and the performance of the battery. The material selection and preparation process of the positive electrode lithium supplement are the current research focuses.

[0004] Among the existing positive electrode lithium supplements, Li5FeO4 has a high theoretical specific capacity, is compatible with existing positive electrode materials, and has a relatively low price, and has good industrial prospects. However, Li5FeO4 has poor air stability and is easily affected by water and carbon dioxide in the air, which limits the industrial application of Li5FeO4. In order to improve the stability of Li5FeO4, it is usually necessary to coat the outside of Li5FeO4 with an inert layer. However, the existing method of coating the inert layer is generally to prepare the inert layer material separately first, and then use the inert layer material to coat it. Although this method can achieve a certain coating effect, it is difficult to obtain a dense and complete coating layer, and the stability still needs to be further improved; and this method requires an additional preparation process of the inert layer material, and the overall preparation process is relatively complicated. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present application provides a composite-coated high-stability lithium-rich lithium iron oxide and a preparation method thereof, aiming to solve the technical problems that Li5FeO4 has poor stability and it is difficult to easily form a dense and complete coating layer on the surface of Li5FeO4.

[0006] In a first aspect, the present application provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide, comprising the following steps:

[0007] mixing an iron source and a lithium source to obtain a first mixture;

[0008] performing a first calcination treatment on the first mixture to obtain a calcined product;

[0009] mixing and dispersing the calcined product and a metal compound to obtain a second mixture;

[0010] Performing a second calcination treatment on the second mixture to obtain a composite-coated high-stability lithium-rich ferrite;

[0011] Wherein, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is greater than 5.

[0012] In the technical solution of the embodiment of the present application, by controlling the molar ratio of lithium element in the lithium source to iron element in the iron source, an excess lithium source is deliberately introduced to form a uniform residual alkali layer on the surface of Li5FeO4 generated after the first calcination treatment; thereafter, a metal compound is introduced to be uniformly dispersed on the surface of the calcined product, and the residual alkali layer can be converted into a dense and complete coating layer through a second calcination treatment, thereby simply and efficiently improving the air stability of Li5FeO4.

[0013] In some embodiments, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is (5.0:1) to (6.0:1).

[0014] In this embodiment, by specifically controlling the molar ratio of lithium element in the lithium source to iron element in the iron source, the excess degree of the lithium source can be effectively controlled, and then the residual alkali layer on the surface of Li5FeO4 can be regulated to form a residual alkali layer uniformly coating the surface of Li5FeO4, thereby avoiding the problem that the residual alkali amount is too small to completely coat Li5FeO4 or the residual alkali layer is too thick to affect the performance of the lithium-rich lithium iron oxide material.

[0015] In some embodiments, the iron source includes at least one of iron oxide, ferroferric oxide, iron hydroxide, iron oxalate, iron nitrate, iron sulfate, and iron fluoride; the lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium nitrate, and lithium sulfate.

[0016] In this embodiment, the iron source and lithium source used are widely available, which is conducive to industrial large-scale production.

[0017] In some embodiments, the lithium source includes lithium hydroxide and lithium oxide, wherein the molar ratio of lithium hydroxide to lithium oxide is (1:1.5) to (1:2.5).

[0018] In this embodiment, by using lithium hydroxide and lithium oxide as lithium sources and controlling the molar ratio of the two, lithium hydroxide can be used to make the reaction more complete, while avoiding the occurrence of wall sticking, corrosion and the like caused by too high a proportion of lithium hydroxide.

[0019] In some embodiments, the first calcination treatment comprises: calcining at 550-650° C. for 32-40 hours.

[0020] In this embodiment, by controlling the calcination temperature and calcination time of the first calcination treatment process, it is possible to ensure that the iron source and the lithium source fully react to obtain Li5FeO4 with good crystallinity.

[0021] In some embodiments, the metal compound is a compound of a metal other than lithium, including at least one of a metal oxide and a soluble metal salt.

[0022] In this embodiment, the sources of metal compounds are wide, which can meet the needs of large-scale industrial production. In actual application, they can be selected according to needs, and have good industrial practicability.

[0023] In some embodiments, the metal oxide includes at least one of aluminum oxide, niobium oxide, and titanium oxide, and the soluble metal salt includes at least one of ammonium niobium oxalate and aluminum nitrate.

[0024] In this embodiment, the selected metal oxides and soluble metal salts can form coating layers such as LiAlO2, LiNbO3, Li2Ti3O7, etc. on the surface of Li5FeO4 through a second calcination treatment, thereby effectively improving the air stability of Li5FeO4.

[0025] In some embodiments, the mass ratio of the metal compound to the calcined product is (1-5):100.

[0026] In this embodiment, by controlling the mass ratio between the metal compound and the calcined product within an appropriate range, it is possible to ensure that the introduced metal compound is sufficient to react with the residual alkali layer on the surface of the calcined product to form a dense and complete coating layer, while also avoiding the influence of excessive metal compounds on the performance of the final lithium-rich lithium titanate.

[0027] In some embodiments, the second calcination treatment comprises: calcining at 620-680° C. for 2.5-3.5 hours.

[0028] In this embodiment, by controlling the calcination temperature and calcination time during the second calcination treatment, it is beneficial to promote the full reaction between the metal compound and the residual alkali layer on the surface of the calcined product to form a dense and complete metal lithium oxide protective layer.

[0029] In the second aspect, an embodiment of the present application provides a composite-coated high-stability lithium-rich iron ore, which is prepared by the preparation method of the composite-coated high-stability lithium-rich iron ore provided in the first aspect, and includes Li5FeO4 as an inner core and a metal lithium oxide coating layer coated outside the inner core.

[0030] In the technical solution of the embodiment of the present application, the metal lithium oxide coating layer can effectively coat the Li5FeO4 serving as the inner core, making it have higher air stability.

[0031] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0033] Figure 1 This is a flow chart for preparing the composite-coated high-stability lithium-rich iron oxide in the embodiment of the present application;

[0034] Figure 2 This is a schematic diagram of the structural changes on the surface of lithium ferrite in the embodiment of the present application;

[0035] Figure 3 This is a SEM image of the calcined product prepared in Example 1 of the present application;

[0036] Figure 4 This is a SEM image of the composite-coated high-stability lithium-rich ferrite prepared in Example 1 of the present application;

[0037] Figure 5 This is the XRD diagram of the composite-coated high-stability lithium-rich ferrite prepared in Example 1 of the present application;

[0038] Figure 6 This is the XRD diagram of the composite-coated high-stability lithium-rich ferrite prepared in Example 5 of the present application;

[0039] Figure 7 This is the XRD diagram of the composite-coated high-stability lithium-rich iron oxide prepared in Example 6 of the present application. DETAILED DESCRIPTION

[0040] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0042] 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 "multiple" is more than two, unless otherwise clearly and specifically defined.

[0043] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0044] Since Li5FeO4 has poor air stability and is easily affected by water and carbon dioxide in the air, it is usually necessary to coat the outside of Li5FeO4 with an inert layer. However, the existing coating method requires the preparation of the inert layer material separately and then the coating. Not only is the preparation process complicated, but it is also difficult to obtain a dense and complete coating layer, resulting in the air stability of Li5FeO4 still needing to be further improved.

[0045] In order to solve the technical problems that Li5FeO4 has poor stability and it is difficult to easily form a dense and complete coating layer on the surface of Li5FeO4, the present application provides a composite-coated high-stability lithium-rich ferrate and a preparation method thereof, which overcomes the technical prejudice of strictly limiting the amount of lithium source to reduce residual alkali. By introducing an excess lithium source when synthesizing Li5FeO4, a large amount of residual lithium on the surface of Li5FeO4 is deliberately formed into a residual alkali layer, and a metal compound is introduced so that the metal compound converts the residual alkali layer into a dense and complete metal lithium oxide coating layer under the action of a second calcination treatment, thereby effectively improving the air stability of Li5FeO4.

[0046] Please refer to Figure 1 In the first aspect, the present application provides a method for preparing a composite-coated high-stability lithium-rich ferrite, comprising the following steps:

[0047] mixing an iron source and a lithium source to obtain a first mixture;

[0048] performing a first calcination treatment on the first mixture to obtain a calcined product;

[0049] mixing and dispersing the calcined product and the metal compound to obtain a second mixture;

[0050] Performing a second calcination treatment on the second mixture to obtain composite-coated high-stability lithium-rich ferrite;

[0051] Wherein, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is greater than 5.

[0052] Through the above method, when the first mixture containing an iron source and a lithium source is subjected to the first calcination treatment, the iron source and the lithium source can react to generate Li5FeO4. Since the molar ratio of the lithium element in the lithium source to the iron element in the iron source is greater than 5, the unreacted lithium source will be evenly distributed on the surface of Li5FeO4 based on the uniformity of the solid-phase diffusion process during the calcination process, thereby forming a uniformly coated residual alkali layer on the surface of Li5FeO4. Compared with the coating layer prepared separately, the residual alkali layer is naturally formed by excess lithium source during the synthesis of Li5FeO4, omitting the step of preparing the coating layer separately, and can be completely and uniformly coated on the surface of Li5FeO4. On this basis, a metal compound is introduced and mixed with the calcined product, and the metal compound is dispersed on the surface of the calcined product. The metal compound is reacted with the residual alkali layer on the surface of Li5FeO4 through a second calcination treatment, and the residual alkali layer can be converted into a dense and complete metal lithium oxide coating layer. The schematic diagram of its structural change is shown as follows. Figure 2 As shown, the air stability of Li5FeO4 can be effectively improved.

[0053] Furthermore, in some embodiments, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is (5.0:1) to (6.0:1).

[0054] In the technical solution of the embodiment of the present application, by controlling the molar ratio of the lithium element in the lithium source to the iron element in the iron source to (5.0:1) to (6.0:1), the excess of the lithium source can be prevented from being too high. If the excess of the lithium source is too high, the residual alkali layer formed will be too thick, and the metal compound introduced subsequently may be difficult to react completely with the residual alkali layer. Even after the complete reaction, the thickness of the metal lithium oxide coating layer formed will be too thick, affecting the particle size, capacity and conductivity of the finally obtained lithium-rich lithium iron oxide. It can be understood that in the application, the molar ratio of the lithium element in the lithium source to the iron element in the iron source can be 5.1:1, 5.2:1, 5.3:1, 5.4:1, 5.5:1, 5.6:1, 5.7:1, 5.8:1, 5.9:1, 6.0:1 or any value in the range of (5.0:1) to (6.0:1), excluding the endpoint value when the molar ratio of the lithium element in the lithium source to the iron element in the iron source is 5.0:1.

[0055] In some embodiments of the present application, the molar ratio of the lithium element in the lithium source to the iron element in the iron source can be further preferably (5.1:1) to (5.5:1), and more preferably 5.5:1. Under this condition, the excess degree of the lithium source is more appropriate, which can ensure that the residual alkali content is sufficient to evenly and completely coat the Li5FeO4, and can avoid excessive thickness of the residual alkali layer, thereby ensuring that the lithium-rich lithium iron oxide has a higher capacity and conductivity while improving its air stability.

[0056] Further, in some embodiments, the iron source includes at least one of iron oxide, ferroferric oxide, iron hydroxide, iron oxalate, iron nitrate, iron sulfate, and iron fluoride; the lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium nitrate, and lithium sulfate.

[0057] In the technical solution of the embodiment of the present application, the iron source and lithium source used are widely available, which is conducive to industrial large-scale production.

[0058] Furthermore, in some embodiments, the lithium source includes lithium hydroxide and lithium oxide, wherein the molar ratio of lithium hydroxide to lithium oxide is (1:1.5) to (1:2.5).

[0059] In the technical solution of the embodiment of the present application, lithium hydroxide can become liquid at above 462°C, making the reaction more complete, but if the proportion of lithium hydroxide is too high, it will cause wall sticking, corrosion and other phenomena. By mixing lithium hydroxide and lithium oxide according to the molar ratio of (1:1.5) to (1:2.5), it is possible to promote the reaction to proceed fully while avoiding wall sticking, corrosion and other phenomena, and obtain the calcined product safely and efficiently. Specifically, the molar ratio of lithium hydroxide and lithium oxide can be 1:1.5, 1:2.0, 1:2.5 or any value in the range of (1:1.5) to (1:2.5).

[0060] Furthermore, in some embodiments, the first calcination treatment comprises: calcining at 550-650° C. for 32-40 hours.

[0061] In the technical solution of the embodiment of the present application, by controlling the calcination temperature of the first calcination treatment process at 550-650°C and calcining at this temperature for 32-40 hours, it is possible to promote the full reaction of the iron source and the lithium source to obtain Li5FeO4 with good crystallinity, and to cause the excess lithium source to form a uniform residual alkali layer on the surface of Li5FeO4.

[0062] Furthermore, in some embodiments, the metal compound is a compound of a metal other than lithium, including at least one of a metal oxide and a soluble metal salt.

[0063] In the technical solution of the embodiment of the present application, the sources of metal compounds are wide, which can meet the needs of large-scale industrial production. In actual application, they can be selected according to needs, and have good industrial practicability.

[0064] Furthermore, in some embodiments, the metal oxide includes at least one of aluminum oxide, niobium oxide, and titanium oxide, and the soluble metal salt includes at least one of ammonium niobium oxalate and aluminum nitrate.

[0065] In the technical solution of the embodiment of the present application, the selected metal oxide and soluble metal salt can be dispersed in a solvent, then uniformly attached to the surface of the calcined product, and then subjected to a second calcination treatment to form a corresponding metal lithium oxide coating layer. Specifically, aluminum oxide or aluminum nitrate can form a LiAlO2 coating layer on the surface of Li5FeO4, niobium oxide or ammonium niobium oxalate can form a LiNbO3 coating layer on the surface of Li5FeO4, and titanium oxide can form a Li2Ti3O7 coating layer on the surface of Li5FeO4, all of which can effectively improve the air stability of Li5FeO4. Among them, ammonium niobium oxalate is more preferably used for coating. Compared with other metal compounds, the coating formed by ammonium niobium oxalate is denser and more conducive to improving the stability of Li5FeO4.

[0066] Furthermore, in some embodiments, the mass ratio of the metal compound to the calcined product is (1-5):100.

[0067] In the technical solution of the embodiment of the present application, by controlling the mass ratio between the metal compound and the calcined product within (1-5):100, it is possible to ensure that the metal compound is evenly covered on the surface of the calcined product, so that the residual alkali layer on the surface of the calcined product is fully reacted to form a dense and complete coating layer, while avoiding the influence of the excessive metal lithium oxide layer caused by the excessive amount of metal compound on the performance of lithium-rich lithium titanate, so that the final lithium-rich lithium iron oxide has a higher capacity and better stability. Specifically, the mass ratio of the metal compound to the calcined product can be 1:100, 2:100, 3:100, 4:100, 5:100 or any value within the range of (1-5):100.

[0068] Furthermore, in some embodiments, the second calcination treatment comprises: calcining at 620-680° C. for 2.5-3.5 h.

[0069] In the technical solution of the embodiment of the present application, by controlling the calcination temperature of the second calcination treatment process at 620-680°C and calcining at this temperature for 2.5-3.5 hours, it is beneficial to promote the full reaction of the metal compound with the residual alkali layer on the surface of the calcined product to form a dense and complete metal lithium oxide protective layer.

[0070] In the second aspect, an embodiment of the present application provides a composite-coated high-stability lithium-rich iron ore, which is prepared by the preparation method of the composite-coated high-stability lithium-rich iron ore provided in the first aspect, and includes Li5FeO4 as an inner core and a metal lithium oxide coating layer coated outside the inner core.

[0071] In the technical solution of the embodiment of the present application, the surface of the prepared lithium-rich lithium iron oxide is coated with a dense and complete metal lithium oxide coating layer, which can effectively isolate the inner core Li5FeO4 from the air, making it have high air stability.

[0072] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used without specifying the manufacturer are all conventional products that can be obtained commercially.

[0073] 1. Preparation method

[0074] Example 1

[0075] This embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide, comprising the following steps:

[0076] S1. Use nano-Fe2O3 with a D90 of 50nm as an iron source and a mixture of Li2O and LiOH as a lithium source, weigh them and place them in a mixer, so that the molar ratio of LiOH and Li2O is 1:2, and the molar ratio of the lithium element in the lithium source to the iron element in the iron source is 5.5:1, stir for 30 minutes until they are evenly mixed, and obtain a first mixture.

[0077] S2. The first mixture is sent to a sintering furnace for a first calcination treatment, and high-purity nitrogen is introduced in advance (the amount of nitrogen introduced is 5m 3 / h), the oxygen content is less than 1ppm, the humidity is less than 5%, and then the sintering furnace is heated to 600°C at a heating rate of 5°C / min and kept at this temperature for 36h to obtain a calcined product. The calcined product is Li5FeO4 with a residual alkali layer on the surface.

[0078] S3. The calcined product is subjected to air flow milling treatment to maintain the dew point below -30°C; ammonium niobium oxalate is weighed so that the mass ratio of ammonium niobium oxalate to the calcined product is 3:100, the weighed aluminum nitrate is dispersed in ethanol, and then the calcined product after the air flow milling treatment is added, ball milling is performed, and stirring is performed until the anhydrous ethanol is completely evaporated to obtain a second mixture.

[0079] S4. Place the second mixture in a tube furnace, raise the temperature to 650°C, and keep the temperature for 3 hours to obtain LiNbO3-coated Li5FeO4.

[0080] Example 2

[0081] This embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the molar ratio of the lithium element in the lithium source to the iron element in the iron source in step S1 is changed to 5.1:1. The remaining steps are basically the same as in Example 1 and will not be repeated here.

[0082] Example 3

[0083] The present embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the molar ratio of the lithium element in the lithium source to the iron element in the iron source in step S1 is changed to 5.01:1. The remaining steps are basically the same as those in Example 1 and will not be repeated here.

[0084] Example 4

[0085] This embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the molar ratio of the lithium element in the lithium source to the iron element in the iron source in step S1 is changed to 6:1. The remaining steps are basically the same as in Example 1 and will not be repeated here.

[0086] Example 5

[0087] This embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the metal compound in step S3 is changed from ammonium niobium oxalate to titanium oxide, and the remaining steps are basically the same as Example 1 and will not be repeated here.

[0088] Example 6

[0089] This embodiment provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the metal compound in step S3 is changed from ammonium niobium oxalate to aluminum nitrate, and the remaining steps are basically the same as Example 1 and will not be repeated here.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that step S4 is omitted, that is, the second calcination treatment is not performed, and the remaining steps are basically the same as Example 1 and will not be repeated here.

[0092] Comparative Example 2

[0093] This comparative example provides a method for preparing a composite-coated high-stability lithium-rich lithium iron oxide. Compared with Example 1, the only difference is that the molar ratio of the lithium element in the lithium source to the iron element in the iron source in step S1 is changed to 5.1:1, and step S4 is omitted. The remaining steps are basically the same as in Example 1 and will not be repeated here.

[0094] 2. Test Method

[0095] 1. SEM test

[0096] The calcined product and the composite coated high stability lithium-rich ferrite obtained in Example 1 were tested using a scanning electron microscope. The results are as follows: Figure 3-4 shown.

[0097] 2. XRD test

[0098] The composite coated high stability lithium iron oxide prepared in Examples 1, 5 and 6 was tested by X-ray diffractometer. The results are as follows: Figure 5-7 shown.

[0099] 3. Capacity test

[0100] The electrochemical performance test was carried out using a button-type semi-battery: the composite-coated high-stability lithium-rich ferrite prepared in Examples 1-6 and Comparative Examples 1-2 were used as positive electrode materials, respectively, the positive electrode material, conductive carbon black and binder PVDF (polyvinylidene fluoride) were mixed into a slurry at a ratio of 8:1:1, and evenly coated on an aluminum foil to form a positive electrode sheet, a metal lithium sheet was used as a negative electrode sheet, 1 mol / L LiPF6 was used as an electrolyte, and the solvent was EC:DMC:EMC=1:1:1 (volume ratio). In a vacuum glove box, the battery case, positive and negative electrode sheets, diaphragm (PE double-layer ceramic diaphragm), spring sheet, and gasket were assembled into a button battery, and the battery capacity was tested under inert conditions and in air atmosphere, respectively, and the capacity test was performed again after being placed in air atmosphere for 2 hours. The results are shown in Table 1.

[0101] III. Analysis of test results of various embodiments and comparative examples

[0102] Depend on Figure 3-4 It can be seen that the preparation method provided in Example 1 can first form a uniform residual alkali layer on the surface of Li5FeO4, and then transform it into a dense and complete coating layer by using the effects of the metal compound and the second calcination treatment, thereby isolating it from the air.

[0103] Depend on Figure 5-7 It can be seen that Examples 1, 5, and 6 can successfully coat the surface of Li5FeO4 with a LiNbO3 coating layer, a Li2Ti3O7 coating layer, and a LiAlO2 coating layer, respectively, by introducing different types of metal compounds, so as to improve the air stability of Li5FeO4.

[0104] The capacity data of the composite-coated high-stability lithium-rich ferrite prepared in Examples 1-6 and Comparative Examples 1-2 under different test conditions are shown in Table 1.

[0105] Table 1 Capacity data corresponding to the lithium-rich iron oxide prepared in Examples 1-6 and Comparative Examples 1-2

[0106]

[0107]

[0108] It can be seen from Table 1 that based on the methods provided in the embodiments of the present application, while adding an excess of lithium source, introducing a metal compound and performing a second calcination treatment is beneficial to improving the air stability of Li5FeO4, so that it can still maintain a high capacity after being placed in the air for a long time.

[0109] Specifically, by comparing the data of Examples 1 to 2 and Comparative Examples 1 to 2, it can be seen that if the second calcination treatment is not performed, a dense and stable coating layer cannot be formed on the surface of Li5FeO4, resulting in it being extremely unstable in the air, and the capacity will decay rapidly, affecting practical applications. By comparing the data of Examples 1 to 4, it can be seen that the excess degree of the lithium source has an important influence on the air stability of the finally obtained lithium-rich iron oxide. When the excess degree of the lithium source is low, the residual alkali layer formed is thinner, and the coating layer formed thereby is relatively loose, and the stability of the obtained lithium-rich iron oxide is relatively low; as the excess degree of the lithium source increases, a dense and complete coating layer is gradually formed, so that the obtained lithium-rich iron oxide has a higher air stability; but too much lithium source will cause the coating layer to be too thick, affecting the electrochemical properties of the lithium-rich iron oxide, resulting in a relatively low initial capacity value. By comparing the data of Example 1 and Examples 5 to 6, it can be seen that although different types of metal compounds can improve the air stability of Li5FeO4 by forming a coating layer, ammonium niobium oxalate can form a denser coating layer than other metal compounds, and the effect of improving the air stability is more significant.

[0110] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing a composite-coated high-stability lithium-rich ferrite, characterized in that: The following steps are involved: mixing an iron source and a lithium source to obtain a first mixture; performing a first calcination treatment on the first mixture to obtain a calcined product; mixing and dispersing the calcined product and a metal compound to obtain a second mixture; Performing a second calcination treatment on the second mixture to obtain a composite-coated high-stability lithium-rich ferrite; Wherein, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is greater than 5.

2. The method for preparing the composite-coated high-stability lithium-rich ferrite according to claim 1, characterized in that: The molar ratio of the lithium element in the lithium source to the iron element in the iron source is (5.0:1) to (6.0:1).

3. The method for preparing the composite-coated high-stability lithium-rich ferrite according to claim 1, characterized in that: The iron source includes at least one of iron oxide, ferroferric oxide, iron hydroxide, iron oxalate, iron nitrate, iron sulfate, and iron fluoride; the lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium nitrate, and lithium sulfate.

4. The method for preparing the composite-coated high-stability lithium-rich ferrite according to claim 3, characterized in that: The lithium source includes lithium hydroxide and lithium oxide, wherein the amount ratio of lithium hydroxide to lithium oxide is (1:1.5) to (1:2.5).

5. The method for preparing the composite coated high stability lithium iron oxide according to claim 1, characterized in that: The first calcination treatment method includes: calcining at 550-650° C. for 32-40 hours.

6. The method for preparing the composite-coated high-stability lithium-rich ferrite according to claim 1, characterized in that: The metal compound is a compound of a metal other than lithium, including at least one of a metal oxide and a soluble metal salt.

7. The method for preparing the composite-coated high-stability lithium-rich ferrite according to claim 6, characterized in that: The metal oxide includes at least one of aluminum oxide, niobium oxide, and titanium oxide, and the soluble metal salt includes at least one of ammonium niobium oxalate and aluminum nitrate.

8. The method for preparing the composite coated high stability lithium iron oxide according to claim 1, characterized in that: The mass ratio of the metal compound to the calcined product is (1-5):

100.

9. The method for preparing the composite coated high stability lithium iron oxide according to claim 1, characterized in that: The second calcination treatment method includes: calcining at 620-680° C. for 2.5-3.5 hours.

10. A composite coated high stability lithium iron oxide, characterized in that: The lithium ion battery is prepared by the preparation method described in any one of claims 1 to 9, comprising Li5FeO4 as a core and a metal lithium oxide coating layer coated outside the core.

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