Positive electrode materials and their preparation methods, positive electrode sheets and secondary batteries

Abstract

This application provides a cathode material, its preparation method, a cathode electrode sheet, and a secondary battery. The cathode material comprises lithium manganese iron phosphate nanorods, wherein the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30 ≤ d ≤ 40, and the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1. This lithium manganese iron phosphate material possesses a unique morphology, which significantly enhances its electronic and ion-conducting properties, resulting in a cathode material with high specific capacity and superior low-temperature performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to cathode materials and their preparation methods, cathode plates, and secondary batteries. Background Technology

[0002] Currently, lithium iron phosphate (LFP) cathode materials are widely used due to their abundant raw material sources and high safety performance. However, the relatively low energy density of LFP materials restricts their further development in the new energy market. Lithium manganese iron phosphate (LFP), which has the same olivine structure as LFP, has a higher voltage platform and higher energy density, but its specific capacity is low and its low-temperature performance is weak. The industry often addresses these issues through carbon coating, ion doping, and reducing material size, but the improvement effects are limited. Summary of the Invention

[0003] In view of this, this application provides a cathode material. This cathode material has a special morphology, which significantly improves the electronic and ion conduction properties of lithium manganese iron phosphate, thereby giving the cathode material a high specific capacity and superior low-temperature performance.

[0004] The first aspect of this application provides a cathode material comprising lithium manganese iron phosphate nanorods, wherein the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30≤d≤40, and the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1.

[0005] The aforementioned cathode material is lithium manganese iron phosphate nanorods. This special morphology forces the active lithium ions inside the material to diffuse along its short side. Furthermore, the radial dimension of the lithium manganese iron phosphate nanorods is controlled within the range of 30nm-40nm, thereby controlling the insertion and extraction path of active lithium ions to be shorter. This is beneficial for fully utilizing the specific capacity of the material and significantly improves the low-temperature performance and long-cycle stability of the material.

[0006] A second aspect of this application provides a preparation method, comprising the following steps:

[0007] (1) Mix the iron source, phosphoric acid, manganese source, lithium source and dispersant, and refine the particle size to obtain the first mixture;

[0008] (2) The first mixture is aged at 50℃-90℃ and then dried to obtain the second mixture;

[0009] (3) The second mixture is sintered to obtain a positive electrode material; wherein the positive electrode material includes lithium manganese iron phosphate nanorods, the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30≤d≤40, and the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1.

[0010] A third aspect of this application provides a positive electrode sheet, including the positive electrode material provided in the first aspect of this application. Because it incorporates the positive electrode material provided in this application, the positive electrode sheet can be used to provide a secondary battery with high specific capacity and good long-cycle performance under both room temperature and low-temperature conditions.

[0011] This application provides a fourth aspect of a secondary battery, including the positive electrode provided in the third aspect of this application. This secondary battery has a high specific capacity, high energy density, and excellent room temperature cycling performance and low temperature performance. Attached Figure Description

[0012] Figure 1 This is a scanning electron microscope (SEM) image of the positive electrode material prepared in Example 2 of this application;

[0013] Figure 2 This is a SEM image of the cathode material prepared in Comparative Example 1 of this application. Detailed Implementation

[0014] Lithium manganese iron phosphate (LFP) crystals possess a hexagonal close-packed structure, in which FeO6 octahedra, MnO6 octahedra, and PO4 tetrahedra are cross-connected. This structure offers good structural stability, resulting in high safety for LFP materials. However, this structure leads to a lack of a continuous FeO6-MnO6 shared-edge octahedral network within the material. Consequently, lithium ions diffuse primarily through one-dimensional conduction channels, severely restricting ion diffusion and reducing ionic conductivity, which in turn significantly impacts the material's low-temperature performance. Furthermore, during repeated charge-discharge cycles, the valence change of manganese is prone to the Jahn-Teller effect, leading to manganese dissolution and subsequent capacity decay, thus shortening the battery's cycle life.

[0015] This application provides a cathode material including lithium manganese iron phosphate nanorods, wherein the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30≤d≤40, and the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1.

[0016] The aforementioned cathode material is a nanorod structure, in which the

[101] facets of the lithium manganese iron phosphate crystals are mostly oriented radially along the nanorod. Radial orientation means that the

[101] facets of the lithium manganese iron phosphate can be completely parallel to the radial direction of the nanorod, or the

[101] direction can form an acute angle with the radial direction of the nanorod. Thus, lithium ions can diffuse radially (along the short side) within the lithium manganese iron phosphate nanorod. Furthermore, the short side dimension of the lithium manganese iron phosphate nanorod provided in this application is relatively short, approximately 30nm-40nm, which can significantly shorten the insertion / extraction path of active lithium ions in the lithium iron phosphate material, thereby facilitating the full release of the material's specific capacity. Simultaneously, the accelerated diffusion rate of active lithium ions can also significantly improve the problem of significant battery performance degradation caused by the slow diffusion rate of lithium ions under low-temperature conditions, improving the low-temperature performance of the battery. Understandably, an accelerated diffusion rate of active lithium ions can also improve the rate performance of the material. Furthermore, since the aspect ratio of the lithium manganese iron phosphate nanorods in this application is controlled within the range of (5-10):1, under this morphological condition, it is more conducive to controlling the proportion of one-dimensional ion channels along the short side inside the lithium manganese iron phosphate, making it easier to induce active lithium ions to diffuse along the short side, and fully reducing the proportion of lithium ions diffused along the long side; it is also conducive to the long cycle performance of the material, thereby ensuring that the electrochemical performance of the material is good.

[0017] For example, in this application, the aspect ratio of the aforementioned lithium iron manganese phosphate nanorods can be 5, 5.5, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, etc. For example, the radial dimension d of the aforementioned lithium iron manganese phosphate nanorods can be 30 nm, 30.1 nm, 30.5 nm, 31 nm, 32 nm, 33 nm, 34 nm, 35 nm, 36 nm, 37 nm, 38 nm, 39 nm, 39.5 nm, 39.9 nm, 40 nm, etc. If the radial dimension d of the rod-shaped nanomaterial is greater than 40 nm, Li + Excessive diffusion paths can lead to a decline in the electrochemical performance of the material and affect its specific capacity. If the radial dimension (d) of the rod-shaped nanomaterial is less than 30 nm, nanorod aggregation is likely, impacting battery performance and causing more irreversible loss of active ions during cycling, thus shortening the battery's cycle life. If the aspect ratio is too small (less than 5), the specific surface area is too small, hindering the insertion and extraction of active lithium ions and reducing the rate performance, especially at low temperatures. Conversely, a large aspect ratio (greater than 10) results in an excessively large specific surface area, increasing side reactions and negatively impacting the material's electrochemical performance.

[0018] In this application, particle size measurement software can be used to process the SEM images of the material, determine the radial dimension and length of the material, and calculate the aspect ratio.

[0019] In this application, the aforementioned cathode material exhibits a charging specific capacity of 159 mAh / g and a discharging specific capacity of 160.2 mAh / g at room temperature and 0.1C charge / discharge. After 500 charge / discharge cycles at 1C at room temperature, the capacity retention rate reaches 97.5%. When discharged at -20℃ / 1C, the aforementioned cathode material achieves a discharge efficiency (discharge capacity / charge capacity) as high as 62%.

[0020] In some embodiments of this application, the diffraction peak intensity of the (101) crystal plane of the cathode material is 58%-75% of that of the (131) crystal plane. At this time, the radial orientation of the lithium iron manganese phosphate crystal rod material along its

[101] direction is relatively high, which can ensure better rate performance of the cathode material. Exemplarily, the diffraction peak intensity of the (101) crystal plane of the cathode material can be 58%, 59%, 60%, 62%, 64%, 66%, 68%, 70%, 72%, 73%, 74%, or 75% of that of the (131) crystal plane.

[0021] In some embodiments of this application, the length of the lithium manganese iron phosphate nanorods is L nm, where 200 ≤ L ≤ 300. In other words, the length of the lithium manganese iron phosphate nanorods is in the range of 200 nm to 300 nm. Exemplarily, the length of the lithium manganese iron phosphate nanorods can be 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, etc. Controlling the length of the material within the above range also helps to reduce the risk of agglomeration between primary particles during preparation, storage, and application, thereby ensuring that the electrochemical performance of the material can be fully realized.

[0022] In some embodiments of this application, the specific surface area of ​​the positive electrode material is S m 2 / g, 18≤S≤22. In other words, the specific surface area of ​​the cathode material is within 18m². 2 / g-22m 2 Within the range of / g. For example, the specific surface area of ​​the cathode material can be 18m². 2 / g, 18.5m 2 / g、19m 2 / g、20m 2 / g, 20.5m 2 / g、21m 2 / g, 21.5m 2 / g、22m 2 / g etc. Controlling the specific surface area of ​​the material within the above range is more conducive to ensuring a high rate of lithium ion insertion / extraction and also ensures a small number of active sites on the surface of the cathode material, which helps to reduce the occurrence of side reactions on the cathode material surface, especially the side reactions when the cathode material comes into contact with the electrolyte phase; at the same time, it can also more fully reduce the risk of manganese ion dissolution, thereby ensuring better electrochemical performance of the material.

[0023] In some embodiments of this application, the lithium manganese iron phosphate nanorods satisfy the following condition: 110 ≤ (L / d) × S ≤ 180, where L and d are in the same unit, nm. Exemplarily, the value of (L / d) × S can be 110, 115, 120, 130, 140, 150, 160, 170, 180, etc. Although specific surface area and aspect ratio are closely related, quantitatively describing their aspect ratio and specific surface area and further controlling them within an optimal range is beneficial for better regulating the electrochemical performance of the material.

[0024] In some embodiments of this application, the surface of the lithium manganese iron phosphate nanorods is further coated with a conductive carbon layer. In some specific embodiments, the thickness of the conductive carbon layer can be 2 nm-4 nm. Coating with a conductive carbon layer is beneficial for further improving the electrochemical performance of the material.

[0025] This application also provides a method for preparing a cathode material, which can be used to prepare the aforementioned cathode material, including the following steps:

[0026] (1) Mix the iron source, phosphoric acid, manganese source, lithium source and dispersant, and refine the particle size to obtain the first mixture;

[0027] (2) The first mixture is aged at 50℃-90℃ and then dried to obtain the second mixture;

[0028] (3) The second mixture is sintered to obtain a positive electrode material; wherein the positive electrode material includes lithium manganese iron phosphate nanorods, the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30≤d≤40, and the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1.

[0029] In the above preparation method, the aging treatment in step (2) is the key to controlling the morphology of the material. The raw material can achieve a smaller particle size through particle size refinement treatment, laying the foundation for the formation of cathode material particles with a nanorod morphology. In this application, during the above aging treatment, under specific temperature conditions, the material can be induced to grow in a specific direction, thereby promoting the formation of nanorod structures, and these nanorod structures can form cathode materials with specific morphological characteristics during subsequent sintering. For example, the aging temperature can be 50℃, 55℃, 57℃, 58℃, 59℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc.

[0030] In this application, the aforementioned lithium source may be selected from materials well known to those skilled in the art. Specifically, the aforementioned lithium source may include, but is not limited to, one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium hydroxide, and lithium nitrate.

[0031] In this application, the aforementioned iron source may be selected from materials well known to those skilled in the art. Specifically, the aforementioned iron source may be one or more of anhydrous ferric phosphate, ferric phosphate dihydrate, ferric oxide, ferric chloride, and ferric nitrate.

[0032] In this application, the aforementioned phosphorus source may be selected from materials well known to those skilled in the art. Specifically, the aforementioned phosphorus source may be any one or more of phosphoric acid, iron phosphate, iron phosphate dihydrate, lithium phosphate, lithium dihydrogen phosphate, lithium monohydrogen phosphate, ammonium dihydrogen phosphate, and diamine hydrogen phosphate.

[0033] In this application, the aforementioned manganese source may be selected from materials well known to those skilled in the art. Specifically, the aforementioned manganese source may be one or more of manganese carbonate, manganese oxide, manganese sulfate, and manganese nitrate.

[0034] In this application, the dispersant is any one of deionized water, methanol, ethanol, or N-methylpyrrolidone.

[0035] In some embodiments of this application, the aging reaction in step (2) is carried out under normal pressure. This allows for a more successful preparation of lithium manganese iron phosphate with a nanorod morphology.

[0036] In some embodiments of this application, the aging time in step (2) is 0.5h-12h. Exemplarily, the aging time can be 0.5h, 1h, 1.5h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, etc. Controlling the aging time within the above range ensures that the material is fully oriented in a specific direction, guarantees that the final material structure meets expectations, and also helps improve the uniformity of the material.

[0037] In some embodiments of this application, the drying process in step (2) includes, but is not limited to, spray drying.

[0038] In some embodiments of this application, step (2) further includes adding a carbon source to the raw material during the drying process. The carbon source can be selected from materials well-known to those skilled in the art. Specifically, the carbon source includes, but is not limited to, one or more of sucrose, water-soluble phenolic resin, glucose, polyethylene glycol, hydroxymethyl cellulose, polyacrylamide, starch, polyvinyl alcohol, conductive carbon nanotubes, and graphene. In some specific embodiments, based on the mass of the added phosphorus, iron, and manganese sources converted to ferromanganese phosphate, the amount of carbon source added is 12 wt.%-20 wt.%.

[0039] In some embodiments of this application, step (1) involves sequential coarse grinding and fine grinding. The coarse grinding process uses milling balls with a diameter of 0.5 mm to 0.6 mm, such as zirconium balls. The fine grinding process uses milling balls with a diameter of 0.05 mm to 0.2 mm, such as zirconium balls. This facilitates grinding the material to obtain the desired structure, thereby enabling the successful fabrication of the cathode material provided in the embodiments of this application.

[0040] In some specific embodiments, the linear velocities of the coarse grinding and the fine grinding are each independently within the range of 8 m / s to 15 m / s. For example, the linear velocities of the coarse grinding and the fine grinding can each be independently 8 m / s, 9 m / s, 10 m / s, 11 m / s, 12 m / s, 13 m / s, 14 m / s, 15 m / s, etc. In some specific embodiments, the times of the coarse grinding and the fine grinding are each independently within the range of 0.5 h to 2 h. For example, the times of the coarse grinding and the fine grinding can each be independently 0.5 h, 1 h, 1.5 h, 2 h, etc.

[0041] The aforementioned particle size refinement process allows for more complete lattice deformation of the raw materials, making subsequent high-temperature chemical reactions easier to occur. More importantly, this process helps control the particle size of the raw materials within a more suitable range, thus facilitating orientation during subsequent aging treatment.

[0042] In this application, both the coarse grinding and fine grinding are preferably wet grinding methods. In some embodiments, after the particle size refinement treatment, the first mixture obtained is a slurry, and the viscosity of the slurry is in the range of 70 cP-200 cP. For example, the viscosity of the first mixture, i.e., the slurry, can be 70 cP, 80 cP, 90 cP, 100 cP, 110 cP, 120 cP, 130 cP, 140 cP, 150 cP, 160 cP, 170 cP, 180 cP, 190 cP, 200 cP, etc. The viscosity of the slurry within the above range is actually an apparent phenomenon that the particle size and morphology of each raw material particle are within a certain range. At this time, each raw material has a more suitable particle size and higher uniformity, and the viscosity of the slurry within the above range is more conducive to the orientation of the material along a specific direction during the aging process, thereby enabling the production of lithium manganese iron phosphate nanorods more smoothly, and the particle size distribution of the produced material is narrow.

[0043] In this application, step (3) further includes sequential coarse crushing, sieving, and air crushing after sintering. In some specific embodiments of this application, the coarse crushing includes, but is not limited to, grinding or roller crushing. The above post-treatment is to disperse the secondary particles formed by the agglomeration of primary nanorods in the material, thereby ensuring good dispersibility and high particle size uniformity of the final material.

[0044] This application also provides a positive electrode sheet, including the positive electrode material provided in this application embodiment. Because this electrode sheet incorporates the positive electrode material of this application, it can be used to provide a secondary battery with high specific capacity, good long-cycle performance, high safety performance, and good low-temperature performance.

[0045] In some embodiments of this application, the positive electrode material of this application can be formed on a positive electrode current collector (such as aluminum foil), and then rolled and slit to obtain a positive electrode sheet. Specifically, a slurry containing the positive electrode material can be coated on the positive electrode current collector, and then dried, rolled, and slit to obtain a positive electrode sheet.

[0046] This application also provides a secondary battery, specifically a lithium-ion secondary battery. This secondary battery can be a liquid battery using a liquid electrolyte, or a semi-solid or solid battery using a semi-solid electrolyte or a solid electrolyte. In some embodiments, the secondary battery may include the aforementioned positive electrode, negative electrode, and a separator and electrolyte disposed between the positive and negative electrodes. In other embodiments, the secondary battery may include the aforementioned positive electrode, negative electrode, and a semi-solid or solid electrolyte disposed between the positive and negative electrodes. Furthermore, when using a semi-solid or solid electrolyte, the positive and negative electrodes may also contain semi-solid or solid electrolyte materials.

[0047] In this application, the aforementioned lithium battery can be assembled using the following method:

[0048] S01, the above-mentioned positive electrode, separator and negative electrode are stacked or wound in sequence to form a battery cell;

[0049] S02 involves encapsulating the battery cell in a casing and injecting electrolyte. The battery can then undergo electrochemical performance testing after formation.

[0050] Alternatively, the above-mentioned lithium battery can be assembled using the following method:

[0051] S01, aligning and placing a positive electrode sheet with a solid or semi-solid electrolyte layer and a negative electrode sheet to form a battery cell; wherein, the solid or semi-solid electrolyte layer is close to the negative electrode sheet;

[0052] S02 involves encapsulating the battery cell to obtain a solid-state or semi-solid-state battery. This battery can then undergo electrochemical performance testing after formation.

[0053] In some embodiments of this application, the negative electrode sheet can be obtained by coating a positive electrode slurry containing a negative electrode active electrode material, a conductive agent, and a binder onto a positive electrode current collector (such as copper foil), followed by drying and pressing. The positive electrode active material is a material well-known to those skilled in the art. The conductive agent is a material well-known to those skilled in the art, specifically at least one of carbon black, conductive graphite, carbon fiber, carbon nanotubes, graphene, and mixed conductive slurry. The binder is a material well-known to those skilled in the art, specifically at least one of polyvinylidene fluoride, polyamide resin, polyacrylonitrile, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0054] In some embodiments of this application, the aforementioned electrolyte comprises an organic solvent, a lithium salt, and an additive; the organic solvent, lithium salt, and additive can all be materials well known to those skilled in the art. Exemplarily, the organic solvent can be at least one selected from ethylene carbonate, ethyl methyl carbonate, ethylene glycol, fluoroethylene carbonate, vinylene carbonate, tetraethylene glycol dimethyl ether, ethylene glycol dimethyl ether, dimethyl ether, and 1,3-dioxolane; the lithium salt can be at least one selected from lithium hexafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, and lithium trifluorosulfonylimide; and the additive can be at least one selected from fluoroethylene carbonate, vinylene carbonate, and lithium nitrate.

[0055] In some embodiments of this application, the diaphragm is made of a material well known to those skilled in the art. For example, the diaphragm may be a polyethylene membrane, a polypropylene membrane, a polyethylene / polypropylene bilayer membrane, a polyethylene / polypropylene / polypropylene trilayer membrane, etc.

[0056] In some embodiments of this application, the solid or semi-solid electrolyte is a material well known to those skilled in the art.

[0057] The technical solution of this application is further described below with reference to several embodiments.

[0058] The technical solution of this application will be further described in detail below with reference to several specific embodiments.

[0059] Example 1

[0060] (1) Accurately weigh the corresponding masses of lithium carbonate, ferrous oxalate, manganese carbonate, and phosphoric acid according to the molar ratio of lithium, iron, manganese, and phosphoric acid of 1:0.35:0.65:1. Then add a quantitative amount of dispersant (specifically deionized water), stir evenly, and then add it to a ball mill through a diaphragm pump for particle size refinement. The ball milling speed is 2000 rpm / min, and the ball milling time is 80 min to obtain the first mixture. The viscosity of the first mixture (slurry) is 64 cp.

[0061] (2) The first mixture was placed into an aging container and aged at 60°C for 1 hour. Then, 18% glucose was added and stirred evenly before spray drying to obtain the second mixture (dry powder).

[0062] (3) The second mixture is calcined at 700°C for 10 hours, and then coarsely ground, sieved and air-crushed to obtain the finished cathode material.

[0063] Example 2

[0064] The only difference from Example 1 is that in step (2), the aging conditions are: aging at 60°C for 2 hours.

[0065] Example 3

[0066] (1) Accurately weigh the corresponding masses of lithium carbonate, ferrous oxalate, manganese carbonate, and phosphoric acid according to the molar ratio of lithium, iron, manganese, and phosphoric acid of 1:0.35:0.65:1. Then add a quantitative amount of dispersant (specifically deionized water), stir evenly, and then add it to a coarse grinding device through a diaphragm pump for coarse grinding. The zirconium balls used in the coarse grinding have a particle size of 0.5-0.6 mm. After coarse grinding for 30 min at a linear velocity of 9 m / s, the slurry is fed into a fine grinding mill and then ground for 80 min at a linear velocity of 10 m / s. The zirconium balls used in the fine grinding have a particle size of 0.1 mm to obtain the first mixture. The viscosity of the first mixture (slurry) is 156 cp.

[0067] (2) The first mixture was placed into an aging container and aged at 60°C for 1 hour. Then, 18% glucose was added and stirred evenly before spray drying to obtain the second mixture (dry powder).

[0068] (3) The second mixture is calcined at 700°C for 10 hours, and then coarsely ground, sieved and air-crushed to obtain the finished cathode material.

[0069] Example 4

[0070] The only difference from Example 3 is that in step (2), the aging conditions are: aging at 90°C for 1 hour.

[0071] Example 5

[0072] The only difference from Example 3 is that in step (2), the aging conditions are: aging at 50°C for 1 hour.

[0073] Example 6

[0074] The only difference from Example 3 is that in step (2), the aging conditions are: aging at 60°C for 1.5 hours.

[0075] Example 7

[0076] The only difference from Example 3 is that in step (2), the fine grinding conditions are: grinding at a linear velocity of 10 m / s for 2 hours. The viscosity of the first mixture (slurry) obtained is 184 cp.

[0077] Example 8

[0078] The only difference from Example 3 is that in step (2), the fine grinding conditions are: grinding at a linear velocity of 13 m / s for 2 hours. The viscosity of the first mixture (slurry) obtained is 196 cp.

[0079] Example 9

[0080] The only difference from Example 3 is that in step (2), the fine grinding conditions are: grinding at a linear velocity of 7.5 m / s for 2 hours. The viscosity of the first mixture (slurry) obtained is 65 cp.

[0081] Example 10

[0082] The only difference from Example 3 is that in step (1), the lithium source is replaced by lithium hydroxide instead of lithium carbonate, and the dispersant is replaced by methanol.

[0083] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0084] Comparative Example 1

[0085] The only difference from Example 1 is that no aging process is performed in step (2).

[0086] Comparative Example 2

[0087] The only difference from Example 1 is that in step (2), the aging temperature is 40°C.

[0088] Comparative Example 3

[0089] The only difference from Example 1 is that in step (2), the aging temperature is 130°C.

[0090] Performance testing

[0091] (1) Morphology characterization test: SEM tests were performed on the cathode materials prepared in the above embodiments and comparative examples to observe their morphology and measure their particle size data. The SEM image of the cathode material prepared in Example 2 is shown below. Figure 1 As shown, the SEM image of the cathode material prepared in Comparative Example 1 is as follows. Figure 2 As shown in Table 1, the particle size data of each cathode material are summarized in Table 1.

[0092] (2) Specific surface area determination: The specific surface area of ​​the material was tested using Bestar instruments based on the nitrogen adsorption-desorption principle.

[0093] (3) X-ray diffraction (XRD) test: XRD test was performed on the cathode materials of each embodiment and comparative example.

[0094] (4) Electrochemical performance testing:

[0095] Prepare secondary batteries using the positive electrode materials obtained in the above embodiments and comparative examples:

[0096] ① Preparation of the positive electrode sheet: The positive electrode materials, binder-PVDF5130, and conductive agent-super P prepared in each example and comparative example are dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 100:2:3. After thorough stirring, a positive electrode slurry is obtained. The above positive electrode slurry is coated onto the positive electrode current collector-aluminum foil, and after drying, rolling, and slitting, a positive electrode sheet is obtained.

[0097] ② Preparation of negative electrode sheet: The negative electrode active material graphite, binder SBR / CMC and conductive agent are added to the solvent-water at a mass percentage of 100:5.2:1 and stirred thoroughly to obtain a negative electrode slurry; a certain amount of negative electrode slurry is coated on the surface of the negative electrode current collector-copper foil, and after drying, rolling and cutting, a negative electrode sheet is obtained.

[0098] ③ Preparation of secondary batteries: Multiple negative electrode sheets, separators, and positive electrode sheets are alternately stacked to prepare batteries through a stacking method. The positive and negative electrode sheets are arranged alternately, and adjacent positive and negative electrode sheets are separated by a separator, resulting in a dry cell. The dry cell is placed in an aluminum-plastic film outer packaging, electrolyte is injected, and then it is vacuum-sealed. After being placed at 60°C for 48 hours, a pressure layer is applied at 60°C, followed by secondary encapsulation, venting, and capacity testing to obtain a stacked soft-pack full battery with a capacity of 2.2 Ah. The batteries in each example are designated S1-S8, and the batteries in each comparative example are designated DS1-DS3. The room-temperature electrochemical performance data of each battery are summarized in Table 2.

[0099] ④ The above batteries were subjected to room temperature performance testing: At 25±2℃, they were charged at a constant current density of 0.1C to a cutoff voltage of 4.3V, and then charged at a constant voltage of 4.3V to a cutoff current of 0.02C; then discharged at a constant current density of 0.1C to 2.0V. After 500 cycles, the room temperature cycle retention rate of the batteries was measured.

[0100] ⑤ Low-temperature performance testing was conducted on the above batteries: At room temperature (25±2℃), the batteries were charged at a constant current of 0.1C to a cutoff voltage of 3.8V, then charged at a constant voltage of 3.8V to a cutoff current of 0.1C, and then discharged at a constant current of 0.1C to 2.5V; this process was repeated once more, followed by constant current charging at 0.5C to 3.8V, then constant voltage charging at 3.8V to a cutoff current of 0.1C, and finally the batteries were transferred to a -20℃ freezer and discharged at a constant current rate of 1C to 2.0V; the ratio of the discharge capacity at -20℃ to the charge capacity at 0.5C at room temperature is the capacity retention rate at -20℃. The low-temperature electrochemical performance data of each battery are summarized in Table 2.

[0101] Table 1. Summary of structural characterization data of the cathode materials prepared in each embodiment and comparative example.

[0102]

[0103]

[0104] Table 2. Summary of the electrochemical performance of batteries prepared in each embodiment and comparative example.

[0105]

[0106] As can be seen from the data in Tables 1 and 2, the electrochemical performance of the cathode material provided in this application is significantly better than that of the comparative cathode material, fully demonstrating the superiority of the cathode material and its preparation method. Furthermore, the electrochemical performance of the cathode material is even better when all parameters during the preparation process are within the preferred range of this application.

[0107] The above description is an exemplary embodiment of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A cathode material, characterized in that, The cathode material includes lithium manganese iron phosphate nanorods, the radial dimension of which is d nm, 30≤d≤40, and the aspect ratio of which is in the range of (5-10):1; the diffraction peak intensity of the (101) crystal plane of the cathode material is 58%-75% of that of the (131) crystal plane.

2. The cathode material according to claim 1, characterized in that, The length of the lithium manganese iron phosphate nanorods is L nm, where 200 ≤ L ≤ 300.

3. The cathode material according to claim 1, characterized in that, The specific surface area of ​​the cathode material is S m 2 / g, 18≤S≤22.

4. The cathode material according to claim 3, characterized in that, The lithium manganese iron phosphate nanorods satisfy the following condition: 110 ≤ (L / d) × S ≤ 180, where L and d are in the same unit.

5. The positive electrode material according to claim 1, characterized in that, The surface of the lithium manganese iron phosphate nanorods also has a coating layer.

6. A method for preparing a positive electrode material, characterized in that, Includes the following steps: (1) Mix the iron source, phosphoric acid, manganese source, lithium source and dispersant, and refine the particle size to obtain the first mixture; (2) The first mixture is aged at 50 ℃-90 ℃ and then dried to obtain the second mixture; (3) The second mixture is sintered to obtain a cathode material; wherein the cathode material includes lithium manganese iron phosphate nanorods, the radial dimension of the lithium manganese iron phosphate nanorods is d nm, 30≤d≤40, the aspect ratio of the lithium manganese iron phosphate nanorods is in the range of (5-10):1; the diffraction peak intensity of the (101) crystal plane of the cathode material is 58%-75% of that of the (131) crystal plane.

7. The preparation method according to claim 6, characterized in that, The aging time is 0.5 h to 12 h.

8. The preparation method according to claim 6, characterized in that, The particle size refinement process includes sequential coarse grinding and fine grinding. The coarse grinding process uses milling balls with a diameter of 0.5 mm to 0.6 mm, and the fine grinding process uses milling balls with a diameter of 0.05 mm to 0.2 mm.

9. The preparation method according to claim 8, characterized in that, The linear speeds of the coarse grinding and the fine grinding are each independently within the range of 8 m / s to 15 m / s; the times of the coarse grinding and the fine grinding are each independently within the range of 0.5 h to 2 h.

10. The preparation method according to claim 6, characterized in that, The viscosity of the first mixture is 70 cP-200 cP.

11. A positive electrode plate, characterized in that, The positive electrode sheet includes the positive electrode material as described in any one of claims 1-5.

12. A secondary battery, characterized in that, The secondary battery includes the positive electrode as described in claim 11.

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