Positive electrode material and preparation method thereof, pole piece and secondary battery

Abstract

The invention provides a positive electrode material and a preparation method thereof, a pole piece and a secondary battery. The positive electrode material comprises porous carbon and lithium manganese iron phosphate powder embedded in the porous carbon, wherein the average particle size of the lithium manganese iron phosphate powder is 100 nm to 500 nm; through the confinement effect of the porous carbon on the lithium manganese iron phosphate powder, the size and the morphology of the lithium manganese iron phosphate powder are more uniform; moreover, the contact interface resistance of the nanoscale lithium manganese iron phosphate powder and the porous carbon is relatively low, so that uniform dispersion of the lithium manganese iron phosphate powder in the porous carbon is facilitated, and effective transportation of lithium ions among pores of the porous carbon can be realized; therefore, the technical problem of poor charging and discharging performance of the battery caused by poor conductivity of the lithium manganese iron phosphate is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a cathode material, a preparation method thereof, a pole piece and a secondary battery. Background Art

[0002] Currently, lithium iron phosphate (LiFePO4, abbreviated as LFP) is gradually becoming the main cathode material for electric vehicles or energy storage batteries due to its high safety. However, for LFP batteries, the development of their energy density has reached a limit, resulting in a very limited space for performance improvement.

[0003] In related technologies, due to the similar crystal structure of lithium manganese iron phosphate to that of LFP, it has characteristics such as stable chemical properties and excellent safety performance. In addition, the doped manganese element in lithium manganese iron phosphate can increase the charging voltage of the material, which helps to improve the theoretical energy density of lithium manganese iron phosphate batteries, thereby extending the cruising range. Therefore, lithium manganese iron phosphate is gradually replacing LFP and being applied in cathode materials.

[0004] However, due to the limitations of the structure of lithium manganese iron phosphate itself, it is unable to form a one-dimensional continuous channel, which restricts the transmission of lithium ions and leads to poor charge and discharge performance of the battery. Summary of the Invention

[0005] Embodiments of the present invention provide a cathode material, a preparation method thereof, a pole piece and a secondary battery, which can improve the technical problem of poor charge and discharge performance of the battery caused by the limitations of the structure of lithium manganese iron phosphate itself.

[0006] In a first aspect, embodiments of the present invention provide a cathode material, which includes porous carbon and lithium manganese iron phosphate powder embedded in the porous carbon;

[0007] Wherein, the average particle size of the lithium manganese iron phosphate powder is 100 nm to 500 nm, and the lithium manganese iron phosphate powder is prepared by in-situ reaction in the pores of the porous carbon.

[0008] In one embodiment, the average pore diameter of the porous carbon is at least 100 nm to 500 nm.

[0009] In one embodiment, the porosity of the porous carbon ranges from 80% to 90%; and / or

[0010] The porosity of the cathode material ranges from 20% to 30%.

[0011] In one embodiment, the average particle size of the cathode material is 5 μm to 10 μm

[0012] In one embodiment, the molecular formula of the lithium manganese iron phosphate powder is LiMnx Fe 1-x PO4; wherein, the value range of x is from 0.4 to 0.6.

[0013] In one embodiment, the mass percentage of the lithium iron manganese phosphate in the cathode material ranges from 90% to 95%.

[0014] In a second aspect, an embodiment of the present invention provides a method for preparing a cathode material, and the preparation method includes the following steps:

[0015] Provide porous carbon;

[0016] Heat and pressurize an iron salt to obtain an iron salt in a gaseous form, and the gaseous iron salt is embedded into the pores of the porous carbon, and after cooling, an iron salt / porous carbon is obtained;

[0017] Synthesize lithium iron manganese phosphate in the iron salt / porous carbon to obtain lithium iron manganese phosphate / porous carbon, that is, the cathode material.

[0018] In one embodiment, the providing of the porous carbon includes the following steps:

[0019] Dissolve a carbon source and a foaming agent in a first solvent, and stir and mix at room temperature to obtain a mixed solution;

[0020] Freeze-dry the mixed solution to obtain a precursor powder;

[0021] Then calcine the precursor powder in an inert atmosphere to obtain porous carbon.

[0022] In one embodiment, the carbon source includes at least one of glucose, sucrose, and fructose; and / or

[0023] The foaming agent includes one or a combination of two of sodium bicarbonate and ammonium chloride, and the first solvent includes water;

[0024] Preferably, the mass percentage content of the glucose is from 10% to 15%, the mass percentage content of the sodium bicarbonate is from 0.5% to 1.5%, the mass percentage content of the ammonium chloride is from 0.5% to 1.5%, and the mass percentage content of the water is from 82% to 89%; and / or

[0025] The temperature of the calcination is from 550°C to 750°C; and / or

[0026] The time of the calcination is from 12 h to 36 h.

[0027] In one embodiment, the heating and pressurizing treatment of the iron salt includes:

[0028] The iron salt and the porous carbon are subjected to pressure heating at a pressure of 1.2 Mpa to 1.5 Mpa and a temperature of 550 °C to 750 °C; and / or

[0029] The cooling includes cooling in an inert atmosphere;

[0030] Preferably, the inert atmosphere includes one or a combination of a nitrogen atmosphere, an argon atmosphere, a neon atmosphere, and a helium atmosphere.

[0031] In one embodiment, the iron salt includes at least one of iron chloride and iron acetate;

[0032] Preferably, the mass ratio of the iron chloride to the porous carbon ranges from 20 to 25.

[0033] In one embodiment, the synthesis of lithium iron manganese phosphate in the iron salt / porous carbon includes:

[0034] The iron salt / porous carbon, phosphoric acid, manganese salt, and reducing agent are dissolved in a second solvent to obtain a first blend;

[0035] A lithium salt is added to the first blend and stirred and mixed to obtain a second blend;

[0036] The second blend is subjected to microwave hydrothermal treatment, filtered, washed, and dried to obtain lithium iron manganese phosphate / porous carbon.

[0037] In one embodiment, the manganese salt includes manganese nitrate; and / or

[0038] The lithium salt includes at least one of lithium hydroxide and lithium carbonate; and / or

[0039] The iron salt / porous carbon, phosphorus salt, and manganese salt are configured according to a molar ratio of Mn:Fe:P of (0.8 to 1):(1 to 1.2):1; and / or

[0040] The conditions for the stirring and mixing include stirring at a temperature of 60 °C to 80 °C for 10 min to 50 min; and / or

[0041] The temperature of the microwave hydrothermal treatment is 150 °C to 200 °C, and the time of the microwave hydrothermal treatment is 5 min to 15 min.

[0042] In a third aspect, an embodiment of the present invention provides an electrode sheet, and the electrode sheet includes the positive electrode material as described above or the positive electrode material prepared by the method for preparing the positive electrode material as described above.

[0043] In a fourth aspect, an embodiment of the present invention provides a secondary battery, and the secondary battery includes the electrode sheet as described above.

[0044] The beneficial effects of the embodiments of the present invention:

[0045] In an embodiment of the present invention, the positive electrode material includes porous carbon and lithium iron phosphate manganese powder embedded in the porous carbon. Due to the confinement effect of the porous carbon on the lithium iron phosphate manganese powder, the size and morphology of the lithium iron phosphate manganese powder are more uniform. Moreover, the interfacial resistance between the nanoscale lithium iron phosphate manganese powder and the porous carbon is low, which helps the uniform dispersion of the lithium iron phosphate manganese powder in the porous carbon, and can realize the effective transportation of lithium ions between the pores of the porous carbon, thereby improving the technical problem of poor charge and discharge performance of the battery caused by the poor conductivity of lithium iron phosphate manganese itself. Description of the Drawings

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0047] Figure 1 is a scanning electron microscope image of the positive electrode material provided in Embodiment 1 of the present invention;

[0048] Figure 2 is a flowchart of the preparation method of the positive electrode material provided in the embodiment of the present invention. Detailed Embodiments

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention. In the present invention, unless otherwise stated, the orientation words such as "upper" and "lower" usually refer to the upper and lower in the actual use or working state of the device, specifically the drawing direction in the drawings; and "inner" and "outer" refer to the outline of the device.

[0050] Lithium iron phosphate batteries (LiFePO4, hereinafter simply referred to as LFP) and ternary lithium batteries (Ni-Co-Mn, hereinafter simply referred to as NCM) are two main battery types in the current market.

[0051] Lithium iron phosphate manganese (LiMn x Fe 1-xLiFePO4, hereinafter abbreviated as LMFP) is a cathode material for a new type of lithium phosphate battery. Doping manganese element (Mn) into LFP helps to increase the charging voltage of the cathode material. The charging voltage of LFP is about 3.4V, and the charging voltage of LMFP can reach 4.1V, which increases the theoretical energy density of the LMFP battery by 15% to 20%, thus significantly expanding the driving range of the vehicle.

[0052] In addition, the safety performance of LMFP is better than that of NCM, and the energy density of LMFP is higher than that of LFP. Moreover, LMFP has a low dependence on rare metals and can be produced in the same line as LFP, with obvious cost advantages.

[0053] Therefore, lithium manganese iron phosphate is gradually replacing LFP and being used in cathode materials.

[0054] However, due to the limitations of the structure of lithium manganese iron phosphate itself, it does not have a continuous FeO6 octahedron and MnO6 octahedron network structure, but is connected by PO4 tetrahedrons. Therefore, the structure of lithium manganese iron phosphate limits the transport of lithium ions, resulting in poor conductivity of lithium manganese iron phosphate, and further poor charge and discharge performance of the battery at high rates.

[0055] Currently, carbon materials are usually mixed with LMFP to coat the surface of LMFP with carbon materials, thereby improving the conductivity of LMFP. However, this coating is usually amorphous and the conductivity is significantly lower than that of the carbon material itself.

[0056] In view of this, the embodiments of the present application provide a cathode material, a preparation method thereof, a pole piece and a secondary battery, aiming to improve the problem of poor charge and discharge performance of the battery due to the poor conductivity of LMFP itself.

[0057] In a first aspect, the present application provides a cathode material, which may include porous carbon and lithium manganese iron phosphate powder embedded in the porous carbon;

[0058] Among them, the average particle size of the lithium manganese iron phosphate powder is 100nm to 500nm. The morphology of the lithium manganese iron phosphate powder can be tested and the particle size can be measured by using a scanning electron microscope (SEM) and a transmission electron microscope (TEM).

[0059] Through the above technical solution, the cathode material includes porous carbon and lithium iron phosphate manganese powder. The lithium iron phosphate manganese powder is embedded inside the porous carbon. Through the spatial confinement effect of the porous carbon, the size and morphology of the lithium iron phosphate manganese powder become more uniform. Moreover, the nano-scale lithium iron phosphate manganese powder is embedded inside the porous carbon, which helps to reduce the interfacial resistance when the lithium iron phosphate manganese powder contacts the porous carbon, improve the dispersion effect of the lithium iron phosphate manganese powder in the porous carbon, and enable the effective transport of lithium ions between the pores of the porous carbon, solving the problem of poor charge and discharge performance of the battery caused by the poor conductivity of the lithium iron phosphate manganese powder.

[0060] It should be noted that porous carbon is a carbon material with a highly developed pore structure. Porous carbon has a large number of pores, and these pores can be micropores (pore diameter less than 2 nm), mesopores (pore diameter between 2 nm and 50 nm), or macropores (pore diameter greater than 50 nm).

[0061] In some embodiments of the present application, the average pore diameter of the porous carbon is at least 100 nm to 500 nm, and the lithium iron phosphate manganese powder is prepared by in-situ reaction in the pores of the porous carbon. Further, the average pore diameter of the porous carbon is 150 nm to 400 nm. Exemplarily, the average pore diameter of the porous carbon is 150 nm, 170 nm, 200 nm, 220 nm, 250 nm, 270 nm, 300 nm, 330 nm, 350 nm, 380 nm, 400 nm, and any value between two adjacent values above. An optical microscope (Olympus GX51) or an electron microscope (JSM-7600F) can be used to magnify to an appropriate multiple, measure more than 10 pore diameters, and calculate the average pore diameter.

[0062] Through the above technical solution, since the average particle size of the lithium iron phosphate manganese powder to be synthesized is 100 nm to 500 nm, the average pore diameter of the porous carbon is 100 nm to 500 nm so that the lithium iron phosphate manganese powder can enter the porous carbon.

[0063] In some embodiments of the present application, the porosity of the porous carbon can range from 80% to 90%. Exemplarily, the porosity of the porous carbon can be 80%, 82%, 84%, 86%, 88%, 90%, and any value between two adjacent values above. The porosity of the porous carbon can be tested according to GB / T 19587-2017 Determination of Specific Surface Area of Solid Materials by Gas Adsorption BET Method.

[0064] Through the above technical solution, the porosity of the porous carbon is 80% to 90%, which helps to increase the specific surface area of the porous carbon. A larger specific surface area can provide more active sites, thereby improving the adsorption performance of the porous carbon, and further helping to adsorb more lithium iron phosphate manganese powder to embed; moreover, a higher porosity and a larger specific surface area can provide more charge transport channels, providing a continuous electron migration channel enables better contact between the lithium iron phosphate manganese powder and the porous carbon, reducing the self-aggregation effect, and thus helping to improve the battery performance.

[0065] It should be noted that during the preparation of the positive electrode material, if the porosity of the porous carbon is too large, it may be fragmented due to unstable structure; if the porosity of the porous carbon is too small, it is impossible to prepare the positive electrode material by good composite with the lithium iron phosphate manganese powder.

[0066] In some embodiments of the present application, the porosity of the positive electrode material can be in the range of 20% to 30%. Exemplarily, the porosity of the positive electrode material can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30% and any value between the above two adjacent values. The porosity of the positive electrode material can be tested in accordance with GB / T 19587-2017 "Determination of Specific Surface Area of Solid Substances by Gas Adsorption BET Method".

[0067] Through the above technical solution, the porosity of the porous carbon is 80% to 90%. Since a part of the lithium iron phosphate manganese powder will block some of the pore structures on the porous carbon, the porosity of the porous carbon loaded with the lithium iron phosphate manganese powder is reduced. The porosity of the positive electrode material in the embodiments of the present application is limited to 20% to 30%, that is, an appropriate proportion of the lithium iron phosphate manganese powder is added to the porous carbon, thereby improving the energy density of the battery.

[0068] In some embodiments of the present application, the average particle size of the positive electrode material can be 5 μm to 10 μm. Further, the average particle size of the positive electrode material can be 5.25 μm to 8.85 μm. Exemplarily, the average particle size of the positive electrode material can be 5.25 μm, 5.40 μm, 5.55 μm, 5.70 μm, 5.85 μm, 6.00 μm, 6.15 μm, 6.30 μm, 6.45 μm, 6.60 μm, 6.75 μm, 6.90 μm, 7.05 μm, 7.20 μm, 7.35 μm, 7.50 μm, 7.65 μm, 7.80 μm, 7.95 μm, 8.10 μm, 8.25 μm, 8.40 μm, 8.55 μm, 8.70 μm, 8.85 μm and any value between the above two adjacent values.

[0069] It should be noted that since the average pore diameter of the porous carbon is 100 nm to 500 nm, the average particle size of the porous carbon itself needs to be set to the micron level, that is, the average particle size of the positive electrode material is at the micron level.

[0070] Through the above technical solution, the average particle size of the positive electrode material in the embodiment of the present application is limited to 5 μm to 10 μm, which is beneficial to the embedding of nano-level lithium iron phosphate manganese powder into the porous carbon, so that the lithium iron phosphate manganese powder is confined in the porous carbon, and by utilizing the pore structure of the porous carbon, it is beneficial to the effective transmission of lithium ions therein, thereby contributing to improving the conductivity of the positive electrode material and thus improving the charge and discharge performance of the battery.

[0071] In some embodiments of the present application, the molecular formula of the lithium iron phosphate manganese powder is LiMn x Fe 1-x PO4, where the value range of x is 0.4 to 0.6. Further, the value range of x can be 0.45 to 0.55. Exemplarily, the value of x can be 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55 and any value between two adjacent values above.

[0072] Through the above technical solution, x of the lithium iron phosphate manganese powder in the embodiment of the present application is 0.4 to 0.6. By partially replacing Fe with Mn and with a moderate replacement ratio, it is beneficial to improving the electron transport performance, thereby enhancing the charge and discharge performance and cycle stability of the battery.

[0073] In some embodiments of the present application, the mass percentage of lithium iron phosphate manganese in the positive electrode material can have a value range of 90% to 95%. Exemplarily, the mass percentage of lithium iron phosphate manganese in the positive electrode material can be 90%, 91%, 92%, 93%, 94%, 95% and any value between two adjacent values above.

[0074] Through the above technical solution, the proportion of lithium iron phosphate manganese in the positive electrode material is 90% to 95%, indicating that the content of lithium iron phosphate manganese is relatively high, which is beneficial to enhancing the energy density and electrochemical performance of the battery.

[0075] In a second aspect, the embodiment of the present application provides a preparation method of a positive electrode material, and the preparation method includes the following steps:

[0076] S100. Prepare porous carbon;

[0077] S200. Heat and pressurize an iron salt to obtain an iron salt in a gaseous form, and the iron salt in the gaseous form is embedded into the porous carbon, and after cooling, an iron salt / porous carbon is obtained;

[0078] S300. Synthesize lithium iron manganese phosphate in iron salt / porous carbon to obtain lithium iron manganese phosphate / porous carbon, i.e., the cathode material.

[0079] An embodiment of the present application provides a method for preparing a cathode material. First, porous carbon is prepared, and the iron salt is heated and pressurized to obtain the iron salt in gaseous form. The gaseous iron salt is embedded into the porous carbon, and the iron salt is fixed in the porous carbon as the base point for synthesizing lithium iron manganese phosphate through cooling. Then, lithium iron manganese phosphate is continuously synthesized to obtain the cathode material. The cathode material includes porous carbon and lithium iron manganese phosphate powder embedded in the porous carbon, realizing the confinement of lithium iron manganese phosphate powder in the porous carbon, making the size and morphology of the lithium iron manganese phosphate powder more uniform. Moreover, the nano-scale lithium iron manganese phosphate powder is embedded inside the porous carbon, which helps to reduce the interfacial resistance when the lithium iron manganese phosphate powder contacts the porous carbon, improve the dispersion effect of the lithium iron manganese phosphate powder in the porous carbon, and contribute to the effective transport of lithium ions between the pores of the porous carbon, thereby improving the charge and discharge performance of the battery.

[0080] In some embodiments of the present application, in step S100, the preparation of porous carbon includes the following steps:

[0081] S110. Dissolve the carbon source and the foaming agent in the first solvent, and stir and mix at room temperature to obtain a mixed solution.

[0082] S120. Freeze-dry the mixed solution to obtain a precursor powder.

[0083] S130. Then calcine the precursor powder in an inert atmosphere to obtain porous carbon.

[0084] In some embodiments of the present application, in step S110, the carbon source may include at least one of glucose, sucrose, and fructose. Exemplarily, the carbon source may include glucose.

[0085] It should be noted that the foaming agent needs to have two properties. On the one hand, it should be soluble in the first solvent, and on the other hand, it should generate bubbles during the calcination process to promote the formation of the pore structure of the porous carbon.

[0086] In some embodiments of the present application, in step S110, the foaming agent may be a mixture of ammonium chloride and sodium bicarbonate, and the first solvent may be water.

[0087] It can be understood that when ammonium chloride dissolves in water, it absorbs heat and cools down, reducing the solubility of sodium bicarbonate in water, causing sodium bicarbonate to precipitate in water, so that gas is generated during the calcination process in step S130. Moreover, ammonium chloride itself can also decompose to generate gas during the calcination process in step S130, which is beneficial to the formation of the pore structure of the porous carbon.

[0088] It should be noted that the solubility of sodium bicarbonate and ammonium chloride is different. Sodium bicarbonate preferentially deposits to form foaming nuclei, and then ammonium chloride deposits and crystallizes on the surface of sodium bicarbonate. Since sodium bicarbonate is uniformly distributed in the aqueous solution of glucose, during the calcination process, sodium bicarbonate decomposes to generate gas. At the same time, ammonium chloride also decomposes to generate gas, which is beneficial to promoting the formation of the pore structure of porous carbon.

[0089] In addition, in step S110, after ammonium chloride is dissolved in water, the aqueous solution is weakly acidic. After adding sodium bicarbonate, sodium bicarbonate dissolves in water to make the aqueous solution alkaline, which can neutralize the acidic aqueous solution after ammonium chloride is dissolved in water.

[0090] In some embodiments of the present application, the mass percentage content of glucose can be 10% to 15%, the mass percentage content of sodium bicarbonate can be 0.5% to 1.5%, the mass percentage content of ammonium chloride can be 0.5% to 1.5%, and the mass percentage content of water can be 82% to 89%.

[0091] In some embodiments of the present application, the mass percentage content of glucose can be 12% to 14%, the mass percentage content of sodium bicarbonate can be 0.8% to 1.2%, the mass percentage content of ammonium chloride can be 0.8% to 1.2%, and the mass percentage content of water can be 84% to 87%.

[0092] In some embodiments of the present application, in step S110, room temperature refers to a temperature range of 25 ± 2 °C, that is, a temperature interval of 23 °C to 27 °C.

[0093] In some embodiments of the present application, in step S110, stirring means generating relative movement of two or more substances in a container through a stirrer or mechanical device to achieve the purposes of mixing, homogenization, dispersion, and dissolution.

[0094] In some embodiments of the present application, in step S120, the temperature of freeze-drying can be -50 °C to -70 °C. Further, the temperature of freeze-drying can be -55 °C to -65 °C. Exemplarily, the temperature of freeze-drying can be -55 °C, -56 °C, -57 °C, -58 °C, -59 °C, -60 °C, -61 °C, -62 °C, -63 °C, -64 °C, -65 °C and any value between two adjacent values above.

[0095] In some embodiments of the present application, in step S120, the precursor powder is sponge-like.

[0096] In some embodiments of the present application, in step S130, the inert atmosphere can include one or a combination of nitrogen atmosphere, argon atmosphere, neon atmosphere, and helium atmosphere.

[0097] In some embodiments of the present application, the calcination temperature can be from 550°C to 750°C. Further, the calcination temperature can be from 600°C to 700°C. Exemplarily, the calcination temperature can be 600°C, 605°C, 610°C, 615°C, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C, 650°C, 655°C, 660°C, 670°C, 680°C, 685°C, 690°C, 695°C, 700°C and any value between any two adjacent of the above values.

[0098] In some embodiments of the present application, the calcination time can be from 12 h to 36 h. Further, the calcination time can be from 18 h to 30 h. Exemplarily, the calcination time can be 18 h, 19 h, 20 h, 21 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, 28 h, 29 h, 30 h and any value between any two adjacent of the above values.

[0099] In some embodiments of the present application, in step S200, the heating and pressurizing treatment of the iron salt may include:

[0100] Pressurized heating of the iron salt and the porous carbon is carried out at a pressure of 1.2 Mpa to 1.5 Mpa and a temperature of 550°C to 750°C. Further, the pressure can also be from 1.25 Mpa to 1.45 Mpa. Exemplarily, the pressure can be 1.25 Mpa, 1.30 Mpa, 1.35 Mpa, 1.40 Mpa, 1.45 Mpa, 1.50 Mpa and any value between any two adjacent of the above values. Further, the temperature can be from 575°C to 675°C. Exemplarily, the temperature can be 575°C, 600°C, 625°C, 650°C, 675°C and any value between any two adjacent of the above values.

[0101] In some embodiments of the present application, in step S200, the cooling may include cooling in an inert atmosphere. Further, the inert atmosphere includes one or a combination of a nitrogen atmosphere, an argon atmosphere, a neon atmosphere, and a helium atmosphere. Further, the cooling temperature can be from -5°C to 20°C. Exemplarily, the cooling temperature can be -5°C, 0°C, 5°C, 10°C, 15°C, 20°C and any value between any two adjacent of the above values.

[0102] In some embodiments of the present application, in step S200, the iron salt can be at least one of ferric chloride and ferric acetate.

[0103] It should be noted that the ferric chloride used in the embodiments of the present application is anhydrous ferric chloride. Since ferric chloride has water absorption and can absorb moisture in the air and deliquesce, it needs to be dried before use.

[0104] In some embodiments of the present application, the mass ratio of iron chloride to porous carbon can range from 20 to 25. Exemplarily, the mass ratio of iron chloride to porous carbon can be 20, 21, 22, 23, 24, 25, and any value between two adjacent values above.

[0105] In some embodiments of the present application, synthesizing lithium iron manganese phosphate in iron salt / porous carbon includes:

[0106] S310. Dissolve the iron salt / porous carbon, phosphoric acid, manganese salt, and reducing agent in a second solvent to obtain a first blend;

[0107] S320. Add a lithium salt to the first blend, stir and mix to obtain a second blend;

[0108] S330. Perform microwave hydrothermal treatment on the second blend, filter, wash, and dry to obtain lithium iron manganese phosphate / porous carbon.

[0109] In some embodiments of the present application, in step S310, the iron salt / porous carbon, phosphorus salt, and manganese salt are configured according to the molar ratio of Mn:Fe:P of (1 - 1.6):1:1.

[0110] In some embodiments of the present application, in step S310, the manganese salt may include manganese nitrate.

[0111] In some embodiments of the present application, in step S320, the lithium salt may include at least one of lithium hydroxide and lithium carbonate.

[0112] In some embodiments of the present application, in step S320, after adding the lithium salt, adjust the pH to 6 to 6.5. Exemplarily, after adding the lithium salt, adjust the pH to 6, 6.1, 6.2, 6.3, 6.4, 6.5, and any value between two adjacent values above.

[0113] In some embodiments of the present application, in step S320, the conditions for stirring and mixing can be stirring for 10 min to 50 min at a temperature of 60°C to 80°C. Further, the conditions for stirring and mixing can be stirring for 15 min to 45 min at a temperature of 65°C to 75°C. Exemplarily, the conditions for stirring and mixing can be stirring for 45 min at a temperature of 65°C; or the conditions for stirring and mixing can be stirring for 35 min at a temperature of 68°C, can be stirring for 30 min at a temperature of 70°C; or the conditions for stirring and mixing can be stirring for 23 min at a temperature of 72°C or the conditions for stirring and mixing can be stirring for 15 min at a temperature of 75°C.

[0114] In some embodiments of the present application, the temperature of microwave hydrothermal treatment can be from 150°C to 200°C, and the time of microwave hydrothermal treatment can be from 5 min to 15 min. Exemplarily, the temperature of microwave hydrothermal treatment can be 150°C and the time can be 15 min; or the temperature of microwave hydrothermal treatment can be 170°C and the time can be 10 min; or the temperature of microwave hydrothermal treatment can be 200°C and the time can be 5 min.

[0115] In a third aspect, embodiments of the present application provide a pole piece, including the positive electrode material as described in the first aspect above or the positive electrode material prepared by the method for preparing the positive electrode material as described in the second aspect above.

[0116] Since the pole piece provided by the embodiments of the present application includes the above positive electrode material, the advantages possessed by the positive electrode material are also possessed by the pole piece, which is beneficial to improving the charge and discharge performance of the subsequent battery.

[0117] In a fourth aspect, embodiments of the present application provide a secondary battery, including the pole piece as described in the third aspect above.

[0118] Since the secondary battery provided by the embodiments of the present application includes the above pole piece, the advantages possessed by the pole piece are also possessed by the secondary battery, which is beneficial to improving the charge and discharge performance of the subsequent battery.

[0119] The present application will be specifically described below through specific embodiments. The following embodiments are only partial embodiments of the present application and do not limit the present application. The raw materials used in the following embodiments are all commercially available products unless otherwise specified.

[0120] Example 1

[0121] A battery is prepared by the following method:

[0122] I. Preparation of the positive electrode pole piece

[0123] S100. Preparation of porous carbon:

[0124] S110. Add 1 wt% ammonium chloride, 1 wt% sodium bicarbonate, and 12 wt% glucose to 86 wt% deionized water, and stir and mix at room temperature for 6 h to obtain a mixed solution;

[0125] S120. Freeze-dry the mixed solution at -50°C to obtain a precursor powder;

[0126] S130. Then calcine the precursor in an argon atmosphere for 24 h at a calcination temperature of 600°C to obtain porous carbon.

[0127] S200. Preparation of iron chloride / porous carbon:

[0128] S210, mixing ferric chloride and porous carbon in a mass ratio of 20:1, placing the mixture in a stainless steel reactor, and heating the mixture to 600° C. under a pressure of 1.35 MPa, so that the ferric chloride sublimates into gaseous ferric chloride, and the gaseous ferric chloride is embedded in the porous carbon to obtain an intermediate;

[0129] S220. Cool the intermediate under an inert atmosphere to obtain ferric chloride / porous carbon.

[0130] S300, preparing positive electrode materials:

[0131] S310, dissolving phosphoric acid, manganese nitrate, ferric chloride / porous carbon, and citric acid monohydrate in deionized water at a molar ratio of Mn:Fe:P of 1:1:1 to obtain a first mixed solution, and adding citric acid monohydrate at a mass ratio of 1% of the ferric chloride / porous carbon;

[0132] S320, adding 0.5 mol / L lithium hydroxide aqueous solution to the first mixed solution at a Li:P molar ratio of 3:1, adjusting the pH to 6.2 with aqueous ammonia, and stirring at 75° C. for 30 min to obtain a second mixed solution;

[0133] S330, subjecting the second mixed liquid to microwave hydrothermal treatment at 180° C., filtering, washing, and then drying at 95° C. for 24 h to obtain lithium manganese iron phosphate / porous carbon, i.e., the positive electrode material.

[0134] S400, the positive electrode material is mixed with carbon nanotubes CNT, polyvinylidene fluoride PVDF and polyvinyl pyrrolidone PVP in a mass ratio of 97.9:0.4:1.5:0.2 to prepare a slurry, and after stirring, the slurry is evenly coated on the aluminum foil current collector with a coating thickness of 175 mm, and then dried and cold-pressed to form positive electrode sheets.

[0135] 2. Preparation of negative electrode sheet

[0136] Graphite, conductive carbon black SP, sodium carboxymethyl cellulose CMC, and binder SBR were stirred in a mass ratio of 96.9:0.4:1.4:1.3 to prepare a slurry, which was evenly applied to a copper foil current collector, dried and cold-pressed, and then cut into negative electrode sheets.

[0137] 3. Assemble the battery

[0138] The positive electrode sheet, the negative electrode sheet and the separator are packaged, injected with liquid, formed and the capacity is divided to obtain a battery.

[0139] Example 2

[0140] A battery is different from Example 1 in that in step S130, the calcination temperature is 550°C.

[0141] Example 3

[0142] A battery, different from that of Example 1 in that in step S130, the calcination temperature is 750 °C.

[0143] Example 4

[0144] A battery, different from that of Example 1 in that in step S130, the calcination temperature is 500 °C.

[0145] Example 5

[0146] A battery, different from that of Example 1 in that in step S130, the calcination temperature is 800 °C.

[0147] Example 6

[0148] A battery, different from that of Example 1 in that in step S210, the mass ratio of ferric chloride to porous carbon is 25:1, and the remaining steps are the same as those of Example 1.

[0149] Example 7

[0150] A battery, different from that of Example 1 in that in step S210, the mass ratio of ferric chloride to porous carbon is 15:1, and the remaining steps are the same as those of Example 1.

[0151] Example 8

[0152] A battery, different from that of Example 1 in that in step S210, the temperature is raised to 400 °C under a pressure of 1.35 Mpa.

[0153] Example 9

[0154] A battery, different from that of Example 1 in that in step S310, phosphoric acid, manganese nitrate, and ferric chloride / porous carbon are configured according to the molar ratio of Mn:Fe:P of 0.8:1.2:1.

[0155] Example 10

[0156] A battery, different from that of Example 1 in that in step S330, the temperature of microwave hydrothermal treatment is 150 °C.

[0157] Example 11

[0158] A battery, different from that of Example 1 in that in step S330, the temperature of microwave hydrothermal treatment is 200 °C.

[0159] Example 12

[0160] A battery, different from that of Example 1 in that in step S330, the hydrothermal method is used instead of the microwave hydrothermal method, and the heating temperature is 180 °C.

[0161] Comparative Example 1

[0162] A battery, which is different from that in Example 1 in that the preparation method of the positive electrode material is different. The positive electrode material of this comparative example is prepared by the following method:

[0163] Mix LMFP and sucrose in a mass ratio of 9:1, and ball-mill in absolute ethanol to form a precursor;

[0164] Pre-sinter the precursor in an argon atmosphere, then add sucrose for secondary coating, and then sinter at 500 °C to obtain a carbon-coated LMFP material;

[0165] Stir the above carbon-coated LMFP material, carbon nanotubes CNT, polyvinylidene fluoride PVDF and polyvinylpyrrolidone PVP according to a mass ratio of 97.9:0.4:1.5:0.2 to prepare a slurry. After stirring, evenly coat it into an aluminum foil current collector, with a coating thickness of 175 mm, dry it, cold press it and cut it into a positive electrode plate.

[0166] Detection method:

[0167] 1. Porosity: Test the porosity of the porous carbon and lithium iron phosphate / mesoporous carbon in the example according to GB / T 19587-2017 "Determination of Specific Surface Area of Solid Materials by Gas Adsorption BET Method".

[0168] 2. Average pore diameter: Use an optical microscope (Olympus GX51) or an electron microscope (JSM-7600F), magnify to an appropriate multiple, measure more than 10 pore diameters, and calculate the average pore diameter.

[0169] 3. Particle size: Observe the morphology of the powder in the example by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and measure the particle size.

[0170] 4. Electrochemical test:

[0171] Use a Neware CT-4008-5V6A-164 test cabinet to perform 1C / 1C charge-discharge cycle tests at 25 °C or 45 °C respectively. The charging cut-off voltage is 4.2V, the cut-off current is 0.05C, the discharge cut-off voltage is 2.5V, and the capacity retention rates of the examples and comparative examples are tested after 500 charge-discharge cycles;

[0172] Use a Neware CT-4008-5V6A-164 test cabinet to fully charge at a temperature of 25 °C, with a cut-off voltage of 4.2V and a cut-off current of 0.05C; discharge at 0.33C at a temperature of -20 °C, with a cut-off voltage of 2.0V, and the capacity retention rates of the examples and comparative examples are tested after 500 charge-discharge cycles.

[0173] The detection results of the examples and comparative examples are shown in Table 1-2:

[0174] Table 1

[0175]

[0176]

[0177] Table 2

[0178]

[0179] Refer to Figure 1 For the cathode material of Example 1, it includes porous carbon and lithium iron phosphate manganese powder embedded in the porous carbon. Combining Tables 1 - 2, it can be known that the porosity of the porous carbon is 86%, the average pore diameter on the porous carbon is 300 nm, and the average particle size of the lithium iron phosphate manganese powder is 200 nm. Through the confinement effect of the porous carbon on the lithium iron phosphate manganese powder, the size and morphology of the lithium iron phosphate manganese powder are more uniform. Moreover, the interfacial resistance between the nanoscale lithium iron phosphate manganese powder and the porous carbon is relatively low, which helps the uniform dispersion of the lithium iron phosphate manganese powder in the porous carbon, and can realize the effective transport of lithium ions between the pores of the porous carbon, thereby improving the charge and discharge performance of the battery.

[0180] Combining Examples 2 - 3 with Example 1, in Examples 2 - 3, the calcination temperature in step S130 is changed. Combining the detection results in Table 1, it can be known that when the calcination temperature is 550 °C or 750 °C, the porosity of the prepared porous carbon is between 80% and 90%. The calcination temperature has little effect on the average pore diameter of the porous carbon. The capacity retention rate of the prepared battery at 25 °C / 45 °C and the discharge capacity retention rate at -20 °C both remain at a relatively high level, and the electrochemical performance is excellent.

[0181] Combining Comparative Example 1 with Example 1, it can be known that in Comparative Example 1, the cathode material is prepared by the traditional carbon coating method. Compared with the cathode material of Example 1, the electrochemical performance of the prepared battery is significantly reduced.

[0182] Comparing Examples 4 - 5 with Example 1, in Example 4, the calcination temperature was changed to 500°C. From the test results in Table 1, it can be seen that when the calcination temperature is 500°C, the porosity of the porous carbon decreases significantly. This is because when calcined at this temperature, fewer pore structures are formed on the porous carbon, affecting the porosity of the porous carbon. Combining the test results in Table 2, the decrease in porosity also affects the electrochemical performance of the battery, resulting in a decrease in the capacity retention rate of the battery. In Example 5, the calcination temperature was changed to 800°C. From Table 1, it can be seen that when the calcination temperature is 800°C, the porosity of the porous carbon will decrease. This may be because the calcination temperature is too high, and some pore structures are fragmented, thereby affecting the porosity of the subsequent target product LMFP / porous carbon. Combining the test results in Table 2, it can be seen that when the porosity of LMFP / porous carbon is low, the capacity retention rate of the battery at 25°C / 45°C for 500 cycles and the discharge capacity at -20°C will both be affected.

[0183] Comparing Examples 6 - 7 with Example 1, in Example 6, the mass ratio of ferric chloride to porous carbon was changed to 25:1, increasing the amount of ferric chloride added. This is beneficial for more gasified ferric chloride to enter the porous carbon, increasing the amount of ferric chloride attached inside the porous carbon, and thus generating more lithium iron manganese phosphate in the porous carbon. From Table 2, it can be seen that increasing the mass of ferric chloride helps to improve the capacity retention rate of the battery at 25°C / 45°C and the discharge capacity at -20°C. In Example 7, the mass ratio of ferric chloride to porous carbon was changed to 15:1, reducing the amount of ferric chloride added. This may lead to insufficient distribution of ferric chloride inside the porous carbon, and thus less of the target product lithium iron manganese phosphate / porous carbon is prepared. From Table 2, it can be seen that reducing the amount of ferric chloride added will affect the capacity retention rate of the battery at 25°C / 45°C and the discharge capacity at -20°C.

[0184] Comparing Example 8 with Example 1, in Example 8, the heating temperature in step S210 was changed to 400°C. Ferric chloride will also gasify to form gaseous ferric chloride at 400°C, but the generation rate may be slow. Under the condition of the same reaction time as in Example 1, the air pressure in the reactor is low, and the amount of ferric chloride entering the porous carbon becomes less, reducing the mass of lithium iron manganese phosphate in the porous carbon, thereby affecting the electrochemical performance of the battery.

[0185] Comparing Example 9 with Example 1, in Example 9, the molar ratio of Mn:Fe:P was changed. Compared with Example 1, the content of Mn in Example 9 decreased, which is beneficial for increasing the discharge capacity of the material in the battery, resulting in a decrease in the discharge capacity retention rate of the battery.

[0186] Comparing Example 10 - 11 with Example 1, in Example 10, the microwave hydrothermal temperature in step S330 is changed to 150 °C, and in Example 11, the microwave hydrothermal temperature in step S330 is changed to 200 °C. The suitable temperature for microwave hydrothermal treatment is between 150 °C and 200 °C, which results in a higher content of lithium iron manganese phosphate in the cathode material, thereby improving the electrochemical performance of the battery.

[0187] Comparing Example 12 with Example 1, Example 12 changes the heating method for synthesizing lithium iron manganese phosphate / porous carbon. As can be seen from Table 2, using microwave hydrothermal treatment in Example 1 is beneficial to increasing the supersaturation of the solution, which is conducive to increasing the content of lithium iron manganese phosphate in the cathode material, thereby improving the electrochemical performance of the battery.

[0188] The above has introduced the embodiments of the present invention in detail. Specific examples are used herein to illustrate the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention. At the same time, for those skilled in the art, based on the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A cathode material, characterized in that, the cathode material includes porous carbon and lithium iron phosphate manganese powder embedded in the porous carbon; wherein, the average particle size of the lithium iron phosphate manganese powder is 100 nm to 500 nm.

2. The cathode material according to claim 1, characterized in that, the average pore diameter of the porous carbon is 100 nm to 500 nm, and the lithium iron phosphate manganese powder is prepared by in-situ reaction in the pores of the porous carbon.

3. The cathode material according to claim 1, characterized in that, The porosity of the porous carbon ranges from 80% to 90%; and / or The porosity of the cathode material ranges from 20 to 30%.

4. The cathode material according to claim 3, characterized in that, the average particle size of the cathode material is 5 μm to 10 μm.

5. The cathode material according to claim 1, characterized in that, The molecular formula of the lithium iron manganese phosphate powder is LiMn x Fe 1-x PO4; wherein, the value range of x is from 0.4 to 0.

6.

6. The cathode material according to claim 1, characterized in that, the mass percentage of the lithium iron phosphate manganese in the cathode material ranges from 90% to 95%.

7. A method for preparing the cathode material according to any one of claims 1-6, characterized in that, the preparation method includes the following steps: providing porous carbon; heating and pressurizing an iron salt to obtain an iron salt in gaseous form, and the gaseous iron salt enters the pores of the porous carbon and is cooled to obtain iron salt / porous carbon; synthesizing lithium iron phosphate manganese in the iron salt / porous carbon to obtain lithium iron phosphate manganese / porous carbon, that is, the cathode material.

8. The method for preparing the cathode material according to claim 7, characterized in that, the providing of the porous carbon includes the following steps: dissolving a carbon source and a foaming agent in a first solvent, stirring and mixing at room temperature to obtain a mixed solution; freeze-drying the mixed solution to obtain a precursor powder; then calcining the precursor powder in an inert atmosphere to obtain porous carbon.

9. The method for preparing the cathode material according to claim 8, characterized in that, the carbon source includes at least one of glucose, sucrose, and fructose; and / or the foaming agent includes one or a combination of two of sodium bicarbonate and ammonium chloride, and the first solvent includes water; preferably, the mass percentage content of the glucose is 10% to 15%, the mass percentage content of the sodium bicarbonate is 0.5% to 1.5%, the mass percentage content of the ammonium chloride is 0.5% to 1.5%, and the mass percentage content of the water is 82% to 89%; and / or the temperature of the calcination is 550 °C to 750 °C; and / or the time of the calcination is 12 h to 36 h.

10. The method for preparing the cathode material according to claim 7, characterized in that, the heating and pressurizing treatment of the iron salt includes: heating and pressurizing the iron salt and the porous carbon at a pressure of 1.2 Mpa to 1.5 Mpa and a temperature of 550 °C to 750 °C; and / or the cooling includes cooling in an inert atmosphere; preferably, the inert atmosphere includes a combination of one or more of a nitrogen atmosphere, an argon atmosphere, a neon atmosphere, and a helium atmosphere.

11. The method for preparing the cathode material according to claim 10, characterized in that, the iron salt includes at least one of ferric chloride and iron acetate; Preferably, the mass ratio of the iron chloride to the porous carbon ranges from 20 to 25.

12. The method for preparing the positive electrode material according to claim 7, wherein The synthesis of lithium iron manganese phosphate in the iron salt / porous carbon includes: Dissolving the iron salt / porous carbon, phosphoric acid, manganese salt and reducing agent in a second solvent to obtain a first blend; Adding a lithium salt to the first blend, stirring and mixing to obtain a second blend; Subjecting the second blend to microwave hydrothermal treatment, filtering, washing and drying to obtain lithium iron manganese phosphate / porous carbon.

13. The method for preparing a cathode material according to claim 12, wherein the manganese salt includes manganese nitrate; and / or the lithium salt includes at least one of lithium hydroxide and lithium carbonate; and / or the iron salt / porous carbon, phosphoric acid and manganese salt are configured according to a molar ratio of Mn:Fe:P of (0.8-1):(1-1.2):1; and / or the conditions for the stirring and mixing include stirring at a temperature of 60°C to 80°C for 10 min to 50 min; and / or the temperature of the microwave hydrothermal treatment is 150°C to 200°C, and the time of the microwave hydrothermal treatment is 5 min to 15 min.

14. A pole piece, characterized in that, The electrode sheet includes the cathode material according to any one of claims 1 to 6 or the cathode material prepared by the method for preparing a cathode material according to any one of claims 7 to 13.

15. A secondary battery, characterized in that, The secondary battery includes the electrode sheet according to claim 14.

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