A modified lithium manganese iron phosphate cathode material, its preparation method and battery

Abstract

This invention belongs to the field of cathode material technology, and discloses a modified lithium manganese iron phosphate cathode material, its preparation method, and a battery. The preparation method includes the following steps: mixing lithium manganese iron phosphate, iron pyrophosphate, lithium source, and carbon source in a certain proportion to obtain a mixture; calcining the mixture under an inert atmosphere to obtain intermediate B; and depositing a carbon layer on the surface of intermediate B using vapor deposition technology to obtain a composite-coated modified lithium manganese iron phosphate cathode material, thereby improving the performance of existing lithium manganese iron phosphate cathode materials. 2+ It has disadvantages such as high dissolution and poor high-temperature cycling performance.

Description

Technical Field

[0001] This invention belongs to the field of cathode material technology, specifically relating to a modified lithium manganese iron phosphate cathode material, its preparation method, and a battery thereof. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Since Goodenough et al. first proposed LiMPO4 (M = Mn, Fe, Co, and Ni) lithium-ion battery cathode materials in 1997, lithium manganese phosphate (LMP) has attracted much attention as one of these materials due to its potential high energy density and stable performance. However, it faces a series of challenges in practical applications.

[0004] First, although LMP operates at 4.1V and has a higher energy density than LiFePO4 (LFP), its lower conductivity affects the performance of LiFePO4. + The transport and conduction of electrons by Mn limit its electrochemical performance. Simultaneously, due to the Jameuver-Taylor effect of Mn, Mn... 2+ It is easily dissolved from LMP, which further affects the cycle performance of the battery.

[0005] To address these issues, researchers have attempted to improve the electrochemical performance of LMPs through methods such as reducing particle size, rapid ion / electron conductor recombination, external conductive carbon coatings, and internal cation substitution. For example, LiMn... 1-x Fe x A solid solution system of PO4 (lithium manganese iron phosphate) was proposed, combining the advantages of LFP and LMP to improve electronic conductivity and suppress the Jameer-Taylor effect. However, despite some success, this system remains limited in improving conductivity, lithium diffusion, and high-temperature manganese dissolution. To further improve the conductivity of solid solution materials, carbon coating and nanostructuring have been widely used. However, these methods may lead to Li... + Problems include blocked diffusion channels, reduced volumetric energy density, and deteriorated processing performance.

[0006] In summary, although lithium manganese iron phosphate has the potential for high energy density and stability, its low conductivity and Mn content make it unsuitable for high-energy-density applications. 2+ Due to issues such as dissolution, its performance in practical applications is still limited. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a modified lithium manganese iron phosphate cathode material, its preparation method, and a battery, thereby improving upon existing lithium manganese iron phosphate cathode materials (Mn). 2+ It has disadvantages such as high dissolution and poor high-temperature cycling performance.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0009] In a first aspect, the present invention provides a modified lithium manganese iron phosphate cathode material, comprising a nano-lithium manganese iron phosphate matrix, a composite coating layer of lithium pyrophosphate and amorphous carbon coated on the surface of the nano-lithium manganese iron phosphate matrix, and a carbon layer coated on the surface of the composite coating layer.

[0010] In some embodiments, the thickness of the composite coating layer is 1-50 nm.

[0011] Preferably, the thickness of the composite coating layer is 10-40 nm.

[0012] Preferably, the modified lithium manganese iron phosphate cathode material contains 1.1-2% carbon by mass.

[0013] In some embodiments, the modified lithium manganese iron phosphate cathode material has a primary particle size of 200 nm-300 nm, an average particle size D50 of 0.4 μm-2.0 μm, and a specific surface area of ​​20.0 ± 6.0 m². 2 / g.

[0014] The primary grain size refers to the grain size of a single fine grain, also known as the original grain size.

[0015] Secondly, the present invention provides a method for preparing the modified lithium manganese iron phosphate cathode material, comprising the following steps:

[0016] Lithium manganese iron phosphate, iron pyrophosphate, lithium source and carbon source are mixed evenly in proportion to obtain a mixture.

[0017] The mixture was calcined under an inert atmosphere to obtain intermediate B;

[0018] A carbon layer was deposited on the surface of intermediate B using vapor deposition technology to prepare a composite-coated modified lithium manganese iron phosphate cathode material.

[0019] Vapor deposition technology is a technique that uses gaseous substances to chemically react or physically deposit on a solid surface to form a solid film or coating.

[0020] This invention involves the in-situ synthesis of lithium pyrophosphate on the surface of lithium manganese iron phosphate using a carbothermic reduction method to suppress manganese leaching and deposit a carbon layer on the surface of intermediate B, which is used to improve the electrical conductivity of the composite material.

[0021] In the process of solving the technical problem of manganese ion dissolution, the inventors tried to coat the surface of lithium manganese iron phosphate with a carbon layer, then immerse the carbon-coated lithium manganese iron phosphate in a mixture of iron phosphate, phosphoric acid and lithium carbonate, and then spray dry it. The prepared powder was then heated and calcined. However, it was found that the loading of lithium pyrophosphate on the surface of lithium manganese iron phosphate was too low, and the effect on inhibiting manganese ion dissolution was too poor.

[0022] When lithium manganese iron phosphate, iron pyrophosphate, lithium source, and carbon source are mixed in a solid phase and calcined in an inert atmosphere, the carbon formed by the carbon source has reducing properties and can reduce the trivalent iron in iron pyrophosphate to divalent iron, thus forming lithium manganese iron phosphate pyrophosphate in situ on the surface of lithium manganese iron phosphate. This method can effectively increase the loading of lithium ferrous pyrophosphate on the surface of lithium manganese iron phosphate, thereby effectively improving the inhibition of manganese ion dissolution from lithium manganese iron phosphate.

[0023] Furthermore, lithium iron pyrophosphate has poor electrical conductivity. The carbon produced during the calcination of the carbon source partially dops into the lithium iron pyrophosphate, forming a lithium iron pyrophosphate-carbon composite coating layer. This effectively improves the conductivity of the lithium iron pyrophosphate layer, thus making a positive contribution to improving the conductivity of the cathode material. Moreover, the inventors discovered that doping lithium iron pyrophosphate with a small amount of carbon promotes the inhibition of manganese dissolution.

[0024] The inventors also attempted to directly mix lithium manganese iron phosphate and lithium ferrous pyrophosphate and then perform solid-state sintering. However, they found that this method failed to achieve close contact between the lithium ferrous pyrophosphate-loaded lithium manganese iron phosphate and the lithium manganese iron phosphate, and the unavoidable gaps caused Li... + Diffusion is hindered, and the more layers of lithium iron pyrophosphate coated, the better the diffusion of Li. + The more severe the diffusion hindrance, the worse it is for improving the electrochemical performance of lithium-ion batteries.

[0025] Therefore, in this invention, a carbothermic reduction method is used to generate lithium iron pyrophosphate in situ on the surface of lithium manganese iron phosphate, and the particle size D of the iron pyrophosphate is controlled. 50 ≤30nm allows the composite coating of lithium iron pyrophosphate and carbon to be tightly coated on the surface of lithium manganese iron phosphate, effectively solving the problem of Li + The problem of blocked diffusion channels.

[0026] In some embodiments, lithium manganese iron phosphate, iron pyrophosphate, and lithium salt are all nanoscale.

[0027] Preferably, the D50 of ferric pyrophosphate is ≤30nm. Using ferric pyrophosphate with a smaller particle size allows for better coating of the substrate surface.

[0028] Preferably, the surface of the nano-sized lithium manganese iron phosphate is coated with a carbon layer. The chemical formula of lithium manganese iron phosphate can be LiMn. x Fe 1-xPO4, where 0 < X ​​< 1.

[0029] In some embodiments, the mass ratio of iron pyrophosphate to lithium manganese iron phosphate is 2000-10000 ppm. Here, ppm represents the proportion of iron pyrophosphate added, i.e., 2000-10000 kg of iron pyrophosphate is added to 1 million kg of lithium manganese iron phosphate.

[0030] In some embodiments, the molar ratio of lithium in the lithium source to iron in iron pyrophosphate is 1.03 to 1.06:1.

[0031] Preferably, the lithium source is selected from one or a combination of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium nitrate.

[0032] In some embodiments, the carbon source is selected from one or a combination of glucose, sucrose, fructose, citric acid, and PEG200. The carbon source refers to a substance used to provide carbon.

[0033] PEG200, or polyethylene glycol 200, is a colorless and transparent liquid with an average molecular weight of 200.

[0034] Preferably, the mass ratio of the residual carbon content of the carbon source to iron pyrophosphate is 1-3.5:100. In this invention, the residual carbon content refers to the mass of coke remaining after the carbon source undergoes high-temperature pyrolysis under air-isolated conditions.

[0035] In some embodiments, the calcination temperature in an inert atmosphere is 680-720°C, and the calcination time is 8-12 hours.

[0036] Thirdly, the present invention provides a battery in which the positive electrode is made of the aforementioned positive electrode material.

[0037] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0038] The main innovation of this invention is the coating modification of lithium manganese iron phosphate materials. Therefore, commercially available titanium-containing lithium manganese iron phosphate is used as the coating matrix, and the outer layer is a lithium pyrophosphate / amorphous carbon composite coating layer. Based on this material structure, the high-temperature electrochemical performance of lithium manganese iron phosphate cathode materials can be effectively improved.

[0039] This invention employs a composite coating modification of lithium manganese iron phosphate (LFP) material using lithium pyrophosphate / amorphous carbon. Lithium pyrophosphate possesses excellent resistance to acid and alkali corrosion, thermal conductivity, and a high melting point. By forming a composite coating layer on LFP using lithium pyrophosphate / amorphous carbon, the direct contact area between the LFP substrate and the electrolyte is reduced, preventing direct contact corrosion between the LFP electrode surface and the electrolyte. This reduces side reactions in the electrolyte, effectively inhibits the dissolution of manganese from the coated cathode material, and improves the material's high-temperature cycling stability. The outermost carbon layer also effectively addresses the issue of low electrical conductivity.

[0040] In this invention, lithium iron pyrophosphate is generated directly on the surface of the substrate using an in-situ synthesis method, resulting in a seamless connection between the lithium iron phosphate and the substrate. This method changes the traditional approach of re-coating the substrate with a carbon coating layer, thus improving the problem of hindered lithium ion migration. Attached Figure Description

[0041] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0042] Figure 1 This is a SEM image of the surface morphology of the sample prepared in Example 1 of this invention;

[0043] Figure 2 This is the XRD diffraction pattern of the sample in Embodiment 1 of the present invention. Detailed Implementation

[0044] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0045] The present invention will be further described below with reference to embodiments and comparative examples.

[0046] The chemical reagents (AR grade) used in the following examples were all purchased from Sinopharm Chemical Reagents and can be used directly without further processing.

[0047] Example 1

[0048] The preparation method of modified lithium manganese iron phosphate cathode material includes the following steps:

[0049] 1) Weigh an appropriate amount of commercially available, uniformly dispersed nano-sized lithium manganese iron phosphate cathode substrate A, whose chemical formula can be LiMn. 0.6 Fe 0.4 PO4;

[0050] 2) Weigh an appropriate amount of nano-sized (D50≤30nm) ferric pyrophosphate, with a mass ratio of ferric pyrophosphate to matrix A of 2000ppm;

[0051] 3) Weigh an appropriate amount of nano-sized lithium carbonate, with the amount of lithium carbonate weighed according to the ratio of the molar amount of lithium to the molar amount of iron in ferric pyrophosphate being 1.06.

[0052] 4) Weigh an appropriate amount of glucose and measure it according to the theoretical residual carbon content. The mass ratio of carbon to iron pyrophosphate is 1:100.

[0053] 5) The raw materials weighed in steps 1), 2), 3), and 4) are added into the mixing equipment in the order of matrix A, ferric pyrophosphate, lithium carbonate, and glucose, and mixed evenly.

[0054] 6) Calcine the mixed material from step 5) at 680°C for 12 hours under inert gas protection to obtain intermediate B;

[0055] 7) Using vapor deposition technology, the intermediate B obtained in step 6 was carbon-coated. The carbon content of the finished product was 1.4% by vapor deposition coating, and finally, the lithium manganese iron phosphate cathode material LiMn with composite coating modification was obtained. 0.6 Fe 0.4 PO4@lithium iron pyrophosphate / C. Figure 1 The image shown is a SEM image of the finished product. Figure 2 The XRD diffraction pattern of the material was compared with that of the standard sample card, and no obvious impurities were observed.

[0056] Example 2

[0057] The preparation method of modified lithium manganese iron phosphate cathode material includes the following steps:

[0058] 1) Weigh an appropriate amount of commercially available, uniformly dispersed nano-sized lithium manganese iron phosphate cathode substrate A, whose chemical formula can be LiMn. 0.6 Fe 0.4 PO4;

[0059] 2) Weigh an appropriate amount of nano-sized (D50≤30nm) ferric pyrophosphate, with a mass ratio of ferric pyrophosphate to matrix A of 6000ppm;

[0060] 3) Weigh an appropriate amount of nano-sized lithium oxalate. The amount of lithium salt should be weighed according to the ratio of the molar amount of lithium in lithium hydroxide to the molar amount of iron in iron pyrophosphate of 1.05.

[0061] 4) Weigh an appropriate amount of sucrose and measure it according to the theoretical residual carbon content. The mass ratio of carbon to iron pyrophosphate is 1.25%.

[0062] 5) The raw materials weighed in steps 1), 2), 3), and 4) are added into the mixing equipment in the order of matrix A, ferric pyrophosphate, lithium oxalate, and sucrose, and mixed evenly.

[0063] 6) Calcine the mixed material from step 5) at 700°C for 10 hours under inert gas protection to obtain intermediate B;

[0064] 7) Using vapor deposition technology, the intermediate B obtained in step 6) is carbon-coated. The carbon content of the finished product is 1.1% by vapor deposition coating, and finally, a lithium manganese iron phosphate cathode material LiMn with composite coating modification is obtained. 0.6 Fe 0.4 PO4@lithium iron pyrophosphate / C.

[0065] Example 3

[0066] The preparation method of modified lithium manganese iron phosphate cathode material includes the following steps:

[0067] 1) Weigh an appropriate amount of commercially available, uniformly dispersed nano-sized lithium manganese iron phosphate cathode substrate A, whose chemical formula can be LiMn. 0.6 Fe 0.4 PO4;

[0068] 2) Weigh an appropriate amount of nano-sized (D50≤30nm) ferric pyrophosphate, with a mass ratio of ferric pyrophosphate to matrix A of 10000ppm;

[0069] 3) Weigh an appropriate amount of nano-sized lithium acetate, the amount of which should be weighed according to the ratio of the molar amount of lithium in lithium hydroxide to the molar amount of iron in iron pyrophosphate of 1.06.

[0070] 4) Weigh an appropriate amount of citric acid and measure it according to the theoretical residual carbon content. The mass ratio of carbon to iron pyrophosphate is 1.5:100.

[0071] 5) The raw materials weighed in steps 1), 2), 3), and 4) are added into the mixing equipment in the order of matrix A, iron pyrophosphate, lithium salt, and carbon source, and mixed evenly.

[0072] 6) Calcine the mixed material from step 5) at 720°C for 8 hours under inert gas protection to obtain intermediate B;

[0073] 7) Using vapor deposition technology, the intermediate B obtained in step 6) is carbon-coated. The carbon content of the finished product is 2% by vapor deposition coating, and finally, the lithium manganese iron phosphate cathode material LiMn with composite coating modification is obtained. 0.6 Fe 0.4 PO4@lithium iron pyrophosphate / C.

[0074] Comparative Example 1

[0075] The preparation method of lithium manganese iron phosphate cathode material includes the following steps:

[0076] 1) Weigh the matrix A according to step 1) of Example 1;

[0077] 2) The substrate A, which is symmetrically selected according to the processing conditions in step 6) of Example 1, is processed.

[0078] The difference between this comparative example and Example 1 is that steps 2), 3), 4), 5), and 7 of Example 1 are omitted.

[0079] Comparative Example 2

[0080] The preparation method of lithium manganese iron phosphate cathode material includes the following steps:

[0081] 1) Weigh the matrix A according to step 1) of Example 1;

[0082] 2) Weigh the carbon source as described in step 4) of Example 1;

[0083] 3) Perform the mixing as described in step 5) of Example 1;

[0084] 4) Process the mixture from step 3 according to the processing conditions in step 6) of Example 1.

[0085] The difference between this comparative example and Example 1 is that steps 2), 3), and 7 of Example 1 are omitted.

[0086] Comparative Example 3

[0087] The difference between this comparative example and Example 1 is that the iron pyrophosphate and lithium carbonate raw materials in steps 2) and 3) of Example 1 are replaced with lithium iron pyrophosphate, and step 4) is omitted; everything else is the same as in Example 1. That is, the lithium manganese iron phosphate matrix and lithium iron pyrophosphate (D... 50 After being mixed evenly (≤30nm), the lithium iron pyrophosphate is calcined in an inert atmosphere to directly coat the lithium iron manganese phosphate matrix.

[0088] Comparative Example 4

[0089] The difference between this comparative example and Example 1 is that: in step 1), a carbon-coated nanoscale lithium manganese iron phosphate cathode substrate is used, the mass percentage of carbon is 1.4%, and glucose in step 4) is omitted; otherwise, it is the same as Example 1.

[0090] The composite cathode materials obtained in Examples 1-3 and Comparative Examples 1-4 were used as cathode materials to fabricate coin cells for electrochemical performance testing. The fabrication method is as follows:

[0091] a. The lithium iron manganese phosphate materials prepared in the examples and comparative examples were stirred in a ratio of positive electrode material powder: conductive agent (SP): adhesive (PVDF) = 90:5:5 to form a uniformly dispersed positive electrode slurry. The slurry was then coated, punched, and vacuum dried. Lithium metal sheets were used as the negative electrode material for the battery, and a polypropylene film with micropores was used as the battery separator. Ethylene carbonate (EC) / dimethyl carbonate (DMC) with a solvent volume ratio of 1:1 and 1 mol / L LiPF6 were used as the electrolyte. The batteries were assembled into 2032 button batteries in a glove box filled with dry high-purity argon gas and left to stand for 8 hours.

[0092] b. After the button batteries have been left to stand, charge and discharge them at an ambient temperature of 25°C, at a voltage of 2.5-4.5V, and at a current rate of 0.1C. Perform electrochemical performance tests on Examples 1-4 and Comparative Example 1. Calculate the initial discharge efficiency, i.e.: Initial efficiency = Initial discharge specific capacity / Initial charge specific capacity * 100%.

[0093] c. At 45°C, charge / discharge at 2.5-4.5V with 1C, and perform cycle performance tests on Examples 1-4 and Comparative Example 1. Calculate the capacity retention rate after 100 cycles using the following formula: Capacity retention rate = Specific capacity at 100th discharge / Specific capacity at first discharge * 100%.

[0094] Please refer to Table 1 for specific test data.

[0095] Table 1

[0096]

[0097] As shown in Table 1 above, compared with Comparative Example 1, the first-stage efficiency and discharge specific capacity of the composite modified cathode materials prepared in each embodiment of the present invention were not significantly affected, but the cycle performance was significantly improved. Table 1 shows that Examples 1, 2, and 3 all maintained a capacity retention rate of over 97% after 100 cycles at 45°C, while Comparative Example 1 only maintained a capacity retention rate of 85.93% after 100 cycles at 45°C. Although Comparative Example 2 achieved a capacity retention rate of 90.15% after 100 cycles at 45°C due to the addition of a carbon coating, it was still inferior to the comparative examples. The examples exhibited good cycle performance, indicating that the manganese leaching phenomenon was significantly improved.

[0098] In this invention, an LMFP composite cathode material was prepared by a solid-state method using a composite coating modification of lithium iron pyrophosphate and amorphous carbon. The process is simple, the conditions are controllable, the crystallinity is high, there are few impurities in the crystals, and the product has a relatively uniform particle size. Furthermore, it is cost-effective, environmentally friendly, and easily commercially scalable.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A modified lithium iron phosphate cathode material, characterized in that: It includes a nano-lithium manganese iron phosphate matrix, a composite coating layer of lithium iron pyrophosphate and amorphous carbon coated on the surface of the nano-lithium manganese iron phosphate matrix, and a carbon layer coated on the surface of the composite coating layer.

2. The modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The thickness of the composite coating layer is 1-50 nm.

3. The modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In the modified lithium manganese iron phosphate cathode material, the mass percentage of carbon is 1.1-2%.

4. The modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The modified lithium manganese iron phosphate cathode material has a primary particle size of 200nm-300nm, an average particle size D50 of 0.4μm-2.0μm, and a specific surface area of ​​20.0±6.0m². 2 / g.

5. The method for preparing the modified lithium manganese iron phosphate cathode material according to any one of claims 1-4, characterized in that: Includes the following steps: Lithium manganese iron phosphate, iron pyrophosphate, lithium source and carbon source are mixed evenly in proportion to obtain a mixture. The lithium source is selected from one or a combination of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate and lithium nitrate. The carbon source is selected from one or a combination of glucose, sucrose, fructose, citric acid and PEG200. The mixture was calcined under an inert atmosphere to obtain intermediate B; A carbon layer was deposited on the surface of intermediate B using vapor deposition technology to prepare a composite-coated modified lithium manganese iron phosphate cathode material.

6. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that: Lithium manganese iron phosphate, iron pyrophosphate, and lithium salts are all nanoscale; the D50 of iron pyrophosphate is ≤30nm.

7. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that: The mass ratio of iron pyrophosphate to lithium manganese iron phosphate is 2000-10000 ppm.

8. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that: The molar ratio of lithium in the lithium source to iron in iron pyrophosphate is 1.03~1.06:

1.

9. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that: The residual carbon content of the carbon source is in a mass ratio of 1-1.5:100 to iron pyrophosphate.

10. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that: The calcination temperature in an inert atmosphere is 680-720℃, and the calcination time is 8-12h.

11. A battery, characterized in that: Its positive electrode is made from any of the positive electrode materials described in claims 1-4.

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