Modified positive electrode material and preparation method and application thereof

Abstract

The invention discloses a modified positive electrode material and a preparation method and application thereof, and belongs to the technical field of new energy. The modified positive electrode material provided by the invention comprises an inner core and a coating arranged on the surface of the inner core, the inner core comprises a positive active material; the coating comprises graphite and molybdenum disulfide; the mass ratio of the graphite to the molybdenum disulfide is 1: (0.5-1.5); the mass ratio of the inner core to the coating is (4-7): 1. According to the modified positive electrode material, the compaction density of the positive electrode plate can be remarkably improved on the basis of ensuring the mechanical performance, the cycle performance and other electrochemical performance of the positive electrode plate, and finally the energy density of the lithium secondary battery comprising the modified positive electrode material is improved. The invention also provides a preparation method and application of the modified positive electrode material.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a modified positive electrode material and a preparation method and application thereof. Background Art

[0002] As the lithium-ion battery application market has higher and higher requirements for improving its energy density, the battery electrode design must consider the use of thinner aluminum foil, higher coating weight, and higher coating main material content, and ultimately need to achieve a high compaction density of the cathode sheet (positive electrode sheet). The design of the three highs and one thin pole sheet needs to take into account the high requirements of electrical performance. This puts forward higher requirements for the preparation of cathode sheets with higher compaction density. On the premise of ensuring that the energy density meets the product requirements, it is also necessary to take into account the technical difficulties of whether the performance can be improved. The most important thing is that the preparation of a higher compaction positive electrode sheet based on a high coating surface density and thin aluminum foil will greatly deteriorate its mechanical properties due to the reduction of its foil thickness. At the same time, the increase in the coating surface density of the positive electrode slurry will greatly increase the difficulty of the rolling process. In severe cases, it will cause the pole sheet to break during the rolling process. In addition, high compaction will also cause the rupture of the positive electrode active material particles, which will eventually seriously affect the electrochemical performance of lithium secondary batteries.

[0003] Therefore, it is necessary to provide a positive electrode material that matches the above-mentioned three highs and one thin electrode design and takes into account the electrochemical properties of lithium secondary batteries. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a modified positive electrode material that can significantly improve the compaction density of the positive electrode sheet while ensuring the mechanical properties (unbroken and opaque) and electrochemical properties of the positive electrode sheet such as cycling, and ultimately improve the energy density of the lithium secondary battery including the modified positive electrode material.

[0005] The present invention also provides a method for preparing the modified positive electrode material.

[0006] The present invention also provides application of the modified positive electrode material.

[0007] According to an embodiment of the first aspect of the present invention, there is provided a modified positive electrode material, the modified positive electrode material comprising a core and a coating disposed on a surface of the core;

[0008] The core includes a positive electrode active material; the coating includes graphite and molybdenum disulfide;

[0009] The mass ratio of the graphite to molybdenum disulfide is 1:0.5-1.5; the mass ratio of the core to the coating is 4-7:1.

[0010] The modified positive electrode material according to the embodiment of the present invention has at least the following beneficial effects:

[0011] The present invention uses graphite (C) and molybdenum disulfide (MoS2) to coat the surface of the positive electrode active material, which can significantly improve the roughness of the surface of the positive electrode active material and prepare a positive electrode sheet with a higher compaction density. Specifically:

[0012] Surface coating is one of the effective means of modifying positive electrode active materials. The coatings graphite (C) and molybdenum disulfide (MoS2) used in the present invention have similar layered hexagonal structures, with small interlayer resistance during sliding. They are common solid lubricants and are widely used to improve the characteristics of metal-to-metal friction. In the modified positive electrode material provided by the present invention, the roughness of the surface of the positive electrode active material (such as lithium cobalt oxide) is improved by arranging the above-mentioned coating on the surface of the positive electrode active material. In the process of preparing the positive electrode sheet (such as rolling), the mutual friction between the positive electrode active materials and between the positive electrode active materials and the current collector foil during high-pressure rolling is alleviated, the breakage of the positive electrode active material particles is reduced, and the damage to the current collector aluminum foil is reduced. The compaction can be improved to a certain extent without causing the electrode sheet to break or transmittance, and the particle integrity of the positive electrode active material can also be maintained.

[0013] In addition, graphite (C) and molybdenum disulfide (MoS2) have both good high ionic and electronic conductivity. The surface coating can change the physicochemical properties of the material surface to a certain extent while improving its lithium ion and electronic conductivity, thereby significantly improving its electrochemical performance at high rates.

[0014] Furthermore, the amount and ratio of the coating determine the uniformity of the coating layer formed by the coating, as well as the uniformity of the coating itself, and affect the effect of the coating function. Within the scope provided by the present invention, the electrochemical performance and safety performance of the obtained modified positive electrode material can be further improved.

[0015] According to some embodiments of the present invention, the mass ratio of graphite to molybdenum disulfide is 1:0.6-1.4, for example, about 1:0.6, 1:0.8, 1:1, 1:1.2 or about 1:1.4.

[0016] According to some embodiments of the present invention, the Dv90 particle size of the graphite is 1 to 1.2 μm. For example, it may be about 1.05 μm, 1.1 μm or about 1.15 μm. Unless otherwise specified, the particle size in the present invention is the Dv90 particle size.

[0017] According to some embodiments of the present invention, the Dv90 particle size of the molybdenum disulfide is 0.5-1 μm, for example, about 0.7 μm, 0.8 μm or about 0.9 μm.

[0018] According to some embodiments of the present invention, the positive electrode active material includes at least one of layered LiMO2 and a polyanion material; in LiMO2, M includes at least one of Ni, Co and Mn.

[0019] According to some embodiments of the present invention, the positive electrode active material includes at least one of lithium cobalt oxide and NCM, wherein the ratio of nickel, cobalt and manganese in NCM can be any ratio, for example, one of 1:1:1, 5:2:3, 6:2:2, 8:1:1 and 9:0.5:0.5.

[0020] According to some embodiments of the present invention, the positive electrode active material is lithium cobalt oxide. The compaction density of the lithium cobalt oxide under a pressure of 6 to 8 tons is 4.5 to 5 g / cm 3 .

[0021] According to some embodiments of the present invention, the mass ratio of the core to the coating is 4.5 to 6.5: 1. For example, it can be about 4.5: 1, 5: 1, 5.5: 1, 6: 1 or about 6.5: 1.

[0022] According to an embodiment of the second aspect of the present invention, a method for preparing the modified positive electrode material is provided, the method comprising the following steps:

[0023] S1. dispersing the graphite and molybdenum disulfide in an organic solvent to obtain a coating dispersion;

[0024] S2. mixing the coating dispersion and the positive electrode active material, and removing the organic solvent;

[0025] S3. Calcination of the solid product obtained in step S2.

[0026] The preparation method provided by the embodiment of the present invention has at least the following beneficial effects:

[0027] The invention adopts simple wet coating and calcination, and the preparation method is simple. The prepared modified positive electrode material has a higher compaction density, and at the same time avoids the rupture of the positive electrode active material, and avoids the problems of the positive electrode sheet being broken and light-transmitting.

[0028] According to some embodiments of the present invention, in step S1, the organic solvent includes at least one of methanol, ethanol and isopropanol.

[0029] According to some embodiments of the present invention, in step S1, the purity of the organic solvent is ≥ 99%, for example, about 99.9%.

[0030] According to some embodiments of the present invention, in step S1, the dispersion method includes stirring. The duration of the dispersion stirring is 7 to 9 hours. In actual production, the dispersion method and duration are not strictly limited, as long as sufficient mass transfer and uniform dispersion are achieved; for example, the actual stirring dispersion is 8 to 8.5 hours.

[0031] According to some embodiments of the present invention, in step S1, the solid content of the coating dispersion is 40-77%.

[0032] According to some embodiments of the present invention, in step S1, the solid content of the coating dispersion is 50-60%, for example, specifically about 55%.

[0033] According to some embodiments of the present invention, in step S2, the mixing method includes stirring, and the stirring method includes at least one of double planetary stirring and ball milling stirring.

[0034] According to some embodiments of the present invention, in step S2, the mixing temperature is 15-40°C, for example, about 25°C.

[0035] According to some embodiments of the present invention, in step S2, the mixed environment humidity is 20-30%, for example, specifically about 25%.

[0036] According to some embodiments of the present invention, in step S2, the mixing time is 6 to 8 hours, for example, 7 to 7.5 hours.

[0037] According to some embodiments of the present invention, in step S2, the method for removing the organic solvent includes at least one of drying, freeze drying and reduced pressure distillation. When the method for removing the organic solvent includes drying, the drying temperature is 70 to 80° C.; for example, it can be about 75° C. The drying time is 40 to 48 hours; for example, it can be about 45 hours.

[0038] According to some embodiments of the present invention, in step S3, the calcination temperature is 200-450°C, for example, 350-400°C.

[0039] According to some embodiments of the present invention, in step S3, the calcination time is 5 to 7 hours, for example, about 6 hours.

[0040] According to some embodiments of the present invention, in step S3, the calcination is performed in a protective atmosphere, which includes at least one of nitrogen and argon.

[0041] According to an embodiment of the third aspect of the present invention, a lithium secondary battery is provided, wherein the lithium secondary battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode collector and a positive electrode coating disposed on the surface of the positive electrode collector; the raw material for preparing the positive electrode coating comprises the modified positive electrode material.

[0042] Since the lithium secondary battery adopts all the technical solutions of the modified positive electrode material of the above embodiment, it has at least all the beneficial effects brought by the technical solutions of the above embodiment. Specifically: under high compaction conditions, the technical problems of light transmission, band breaking and crushing of positive electrode active material particles of the positive electrode sheet can still be avoided.

[0043] According to some embodiments of the present invention, the positive electrode current collector is an aluminum foil.

[0044] According to some embodiments of the present invention, the thickness of the positive electrode current collector is ≤ 8 μm. For example, it can be about 4 μm, 5 μm, 6 μm or 7 μm. When the thickness is above, the energy density of the obtained lithium secondary battery can be maximized, and the mechanical properties of the positive electrode sheet can be guaranteed without breaking.

[0045] According to some embodiments of the present invention, the raw materials for preparing the positive electrode coating further include a conductive agent and a binder. The conductive agent includes at least one of carbon black, graphene and carbon nanotubes. The binder includes PVDF.

[0046] The mass ratio of the modified positive electrode material to the conductive agent is 30 to 40:1, for example, specifically about 35:1.

[0047] The mass ratio of the modified positive electrode material to the binder is 35 to 45:1, for example, specifically about 40:1.

[0048] According to some embodiments of the present invention, the lithium secondary battery includes a negative electrode sheet.

[0049] According to some embodiments of the present invention, the negative electrode sheet is a lithium metal sheet.

[0050] According to some embodiments of the present invention, the negative electrode includes a negative electrode current collector and a negative electrode coating disposed on a surface of the negative electrode current collector.

[0051] According to some embodiments of the present invention, the negative electrode current collector comprises a copper foil having a thickness of 5 to 8 μm, for example, about 6 μm.

[0052] According to some embodiments of the present invention, the raw materials for preparing the negative electrode coating include a negative electrode active material, a negative electrode binder and a thickener. The negative electrode active material includes at least one of graphite and a silicon-based material. The negative electrode binder includes at least one of SBR. The thickener includes CMC.

[0053] The mass ratio of the negative electrode active material to the thickener is 80 to 85:1, for example, specifically about 82:1.

[0054] The mass ratio of the negative electrode active material to the negative electrode binder is 50 to 55:1, for example, specifically about 54:1.

[0055] According to some embodiments of the present invention, the lithium secondary battery includes at least one of a button battery, a soft pack battery, a square shell battery and a cylindrical battery.

[0056] According to some embodiments of the present invention, the lithium secondary battery includes at least one of a symmetrical battery, a half cell, and a full cell.

[0057] According to an embodiment of the fourth aspect of the present invention, there is provided an application of the lithium secondary battery in the power field, energy storage field, and 3C small household appliances field.

[0058] Since the application adopts all the technical solutions of the lithium secondary battery of the above embodiment, it at least has all the beneficial effects brought by the technical solutions of the above embodiment.

[0059] If there is no special explanation, the actual meaning of “about” in the present invention is that the error is allowed to be within the range of ±2%, for example, about 100 is actually 100±2%×100.

[0060] Unless otherwise specified, “between” in the present invention includes the number itself, for example, “between 2 and 3” includes the endpoint values ​​2 and 3.

[0061] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0063] Figure 1 This is an appearance diagram (after rolling) of a positive electrode sheet obtained by using the modified positive electrode material obtained in Example 3 as a raw material in the application example of the present invention;

[0064] Figure 2 This is a surface view (after rolling) of a positive electrode sheet obtained by using pure lithium cobalt oxide as the positive electrode active material (control example) in an application example of the present invention. DETAILED DESCRIPTION

[0065] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0066] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0067] Example 1

[0068] In this example, a modified positive electrode material was prepared, and the specific steps were as follows:

[0069] S1. Graphite (C) with a Dv90 particle size of 1 μm and molybdenum disulfide (MoS2) with a Dv90 particle size of 0.5 μm are dispersed in methanol with a purity greater than 99.88% in a mass ratio of 1:1, and a stirrer is used to stir for 8.5 hours to obtain a coating dispersion with a solid content of 55%;

[0070] S2. According to the mass ratio of solid and lithium cobalt oxide in the coating dispersion of 1:6, commercial lithium cobalt oxide (LiCoO2, abbreviated as LCO, density 4.82g / cm 3 , molecular weight 97.87 g / mol, Dv50 16 um) powder and the coating dispersion obtained in step S1 are mixed evenly.

[0071] During the mixing process, the ambient temperature was 25°C and the humidity was 28%; the mixing method was stirring, and the duration was 7.5 h;

[0072] The obtained mixture was dried at 75° C. for 40 h to obtain a coating precursor.

[0073] S3. The solid product obtained in step S2 is calcined at 400° C. for 6 h in an argon atmosphere to obtain a modified positive electrode material. During the calcination process, the ambient temperature is 25° C. and the ambient humidity is 28%.

[0074] Example 2

[0075] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0076] The graphite in Example 1 was replaced with graphite having a particle size of 1.2 μm;

[0077] The molybdenum sulfide in Example 1 was replaced with molybdenum sulfide having a particle size of 1 μm.

[0078] Example 3

[0079] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0080] The graphite in Example 1 was replaced with graphite having a particle size of 1.1 μm;

[0081] The molybdenum sulfide in Example 1 was replaced with molybdenum sulfide having a particle size of 0.8 μm.

[0082] Example 4

[0083] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0084] In step S1, the mass ratio of graphite to molybdenum disulfide is 1:0.5.

[0085] Example 5

[0086] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0087] In step S1, the mass ratio of graphite to molybdenum disulfide is 1:1.5.

[0088] Example 6

[0089] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0090] In step S2, the mass ratio of the solid to the lithium cobalt oxide in the coating dispersion is 1:4.

[0091] Example 7

[0092] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0093] In step S2, the mass ratio of the solid to the lithium cobalt oxide in the coating dispersion is 1:5.

[0094] Example 8

[0095] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0096] In step S2, the mass ratio of the solid to the lithium cobalt oxide in the coating dispersion is 1:7.

[0097] Comparative Example 1

[0098] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0099] Without step S3, a mixture of lithium cobalt oxide, molybdenum disulfide and graphite is formed, and no stable connection is formed between the coating layer and the core.

[0100] Comparative Example 2

[0101] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0102] Molybdenum disulfide is not included in step S1.

[0103] Comparative Example 3

[0104] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0105] Graphite is not included in step S1.

[0106] Comparative Example 4

[0107] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0108] In step S1, the mass ratio of graphite to molybdenum disulfide is 1:0.2.

[0109] Comparative Example 5

[0110] This example prepares a modified positive electrode material, which differs from Example 1 in that:

[0111] In step S1, the mass ratio of graphite to molybdenum disulfide is 1:1.8.

[0112] Comparative Example 6 This example prepares a modified positive electrode material, which is different from Example 1 in that in step S2, the mass ratio of the solid to the lithium cobalt oxide in the coating dispersion is 1:3.

[0113] Comparative Example 7 This example prepares a modified positive electrode material, which is different from Example 1 in that in step S2, the mass ratio of the solid to the lithium cobalt oxide in the coating dispersion is 1:8.

[0114] Some parameters of Examples 1 to 8 and Comparative Examples 1 to 7 are summarized in Table 1.

[0115] Table 1 Partial parameters of Examples 1 to 12 and Comparative Examples 1 to 3

[0116]

[0117]

[0118] Application Examples

[0119] This example provides a lithium secondary battery, and the specific preparation method includes:

[0120] D1. Add 4kg of positive electrode active material (from the examples, comparative examples or lithium cobalt oxide used in the examples), 0.1kg of PVDF, and 0.11kg of conductive carbon black into a stirring tank, and then add 1.8kg of NMP as a solvent, and stir and disperse them in a stirrer to obtain a positive electrode slurry. Use a coating machine to evenly coat the positive electrode slurry on the current collector (8μm aluminum foil) with a modified layer, dry it, and roll it to obtain a positive electrode sheet;

[0121] 4.9 kg of artificial graphite, 0.06 kg of sodium carboxymethyl cellulose (CMC), 0.09 kg of styrene-butadiene rubber (SBR) and water were added to a stirring tank, and after being fully stirred and dispersed by a stirrer, a negative electrode slurry with a solid content of 55% was obtained. The negative electrode slurry was evenly coated on a 6 μm copper foil and dried to obtain a negative electrode sheet.

[0122] D2. After the positive and negative electrode sheets are cut, the tabs are welded, and the glue is applied, they are combined with a 6μm separator to form a core, which is then packaged, baked, injected, formed, and molded to form a lithium secondary battery.

[0123] Test Case

[0124] In this example, the positive electrode sheet obtained from the corresponding use case was first subjected to a roller test. The test method is: a force of 160T (a force of 100-180T can be used for testing as needed in actual production) was used for roller pressing; the whole sheet after roller pressing was taken, a 20×10cm strip of sheet was cut from the center of the sheet, and then the sheet was pre-folded, and finally the pre-folded sheet was placed on the laboratory table, and a 25kg pressing wheel was used to continuously roll the sheet along the horizontal direction of the sheet for 3 times, and the roller was gently pushed to move freely, and the light transmittance level was observed. The test temperature was 25±2℃, among which the A-level light transmittance was 0-3 discontinuous light transmittance points; the B-level light transmittance was 4-6 discontinuous light transmittance points; the C-level light transmittance was >6 light transmittance points; and the D-level light transmittance was continuous points. The sheet was broken and gradually deteriorated; the positive sheet after roller pressing was taken for scanning electron microscopy test to observe the damage of the positive active material particles.

[0125] The above test results are shown in Table 2. Figures 1-2 shown.

[0126] Table 2 Rolling performance of positive electrode sheets obtained from implementation, comparative example and control example

[0127]

[0128]

[0129] The control example is a positive electrode sheet made by using the lithium cobalt oxide used in Example 1 as the positive electrode active material. For explanations of other tests on the control example, please refer to here.

[0130] The results show that the modified positive electrode material prepared by the present invention can improve the compaction density window of the positive electrode sheet, improve its flexibility, significantly reduce the probability of light transmission during rolling, the breakage rate of positive electrode active material particles, and improve the yield of the product. However, the use of simple mixing (Comparative Example 1) or no molybdenum disulfide-graphite synergistic effect (Comparative Examples 2-3) is very effective in reducing the probability of light transmission (compared with the control example).

[0131] By comparing the examples and comparative examples 4 to 7, it can be seen that if the ratio of graphite to molybdenum disulfide, or the ratio of the coating to lithium cobalt oxide is within the optimal range provided by the present invention, the flexibility can be further improved and the transmittance and breakage rate during the rolling process can be reduced.

[0132] This example also tests the safety of the lithium secondary battery obtained in the application example. The specific test method is: charge to 4.25V at 3.0C constant current and constant voltage, cutoff rate 2C, stand for 5 minutes, where 1C = 179mA / g is set. Put the battery cell into the oven (hanging test), heat it with convection or circulating hot air at a starting temperature of 25±3℃, and raise the oven temperature to 132±2℃ at a rate of 5℃ / min, and stop after 60 minutes. Test pass conditions: no fire, no explosion. The surface temperature is the average of the highest temperature of the cells that passed the test. The test results are shown in Table 3.

[0133] Table 3 Safety test results of embodiments, comparative examples and control examples

[0134]

[0135]

[0136] The safety test results show that the modified positive electrode material (the positive electrode active material is lithium cobalt oxide) provided by the present invention can effectively improve the thermal shock safety performance, while the direct mixing scheme has no obvious advantages compared with the control example. The reason is that with simple mixing, agglomeration and adhesion may occur between the coatings, poor dispersion, and poor adhesion to the inner core, and the lubricating effect cannot be fully exerted.

[0137] The safety test results also show that in the modified positive electrode material provided by the present invention, there is a significant synergistic effect between graphite and molybdenum disulfide in the coating, and the combination of the two can significantly improve safety performance.

[0138] The safety test results also show that with the increase of the ratio of graphite and molybdenum disulfide, and with the increase of the ratio of coating and lithium cobalt oxide, the safety shows a trend of first increasing and then decreasing. Due to the limitations of the embodiments, the above ratios can be further optimized in actual production to further improve safety performance.

[0139] This example also tests the electrochemical performance of the lithium secondary battery obtained in the application example, specifically: charging to 4.25V at 3.0C constant current and constant voltage, with a cut-off rate of 2.0C, charging to 4.35V at 2.0C constant current and constant voltage, with a cut-off rate of 1.5C, charging to 4.50V at 1.5C constant current and constant voltage, with a cut-off rate of 1.2C, charging to 4.55V at 1.2C constant current and constant voltage, with a cut-off rate of 0.05C, leaving for 5 minutes, discharging to 3.0V at 0.7C constant current, repeating the steps 600 times, and calculating the capacity retention rate. The test results are shown in Table 4.

[0140] Table 4 Electrochemical performance results of the embodiments, comparative examples and control examples

[0141]

[0142]

[0143] The results show that graphite has excellent electrical conductivity and can efficiently transfer electrons. The covalently assembled superstructure of molybdenum disulfide layers has unique advantages in the application of lithium-ion batteries. The constructed interlayer covalent bonds provide channels for the vertical transmission of electrons between layers, thereby improving the overall electron transmission efficiency of the material. At the same time, too much graphite will increase the amplitude of its volume change, aggravate structural deformation and looseness, thereby shortening the life of the battery, and too much molybdenum disulfide will lead to excessive costs. Due to the improvement of electronic conductivity and ionic conductivity by the graphite-molybdenum sulfide coating, as well as the inhibition of electrolyte side reactions, the cycle performance of lithium secondary batteries can be significantly improved. In Comparative Example 1, although graphite and molybdenum sulfide also have the effect of improving conductivity, they do not significantly help the cycle performance, but will reduce the cyclability to a certain extent. Further specific:

[0144] Comparative Examples 1 to 3 show that within the range provided by the present invention, as the particle size of graphite and molybdenum disulfide increases, the cycle performance of the modified positive electrode material decreases. The reason is that the smaller the particle size of the coating, the larger the contact area between the positive electrode and the electrolyte, the higher the reaction rate, and the better the cycle performance. However, too small a particle size can easily lead to agglomeration of the coating. Therefore, the above rule only applies to the preferred range provided by the present invention.

[0145] By comparing Example 1, Examples 4 to 5 and Comparative Examples 4 to 5, it can be seen that within the range provided by the present invention, as the mass ratio of graphite to molybdenum disulfide decreases, the cycle performance of the modified positive electrode material shows a trend of first increasing and then decreasing; an optimal solution appears near 1:1. The reason is that the synergistic effect between graphite and molybdenum disulfide, too much or too little graphite, will affect the full play of the synergistic effect.

[0146] By comparing Example 1, Examples 6 to 8, and Comparative Examples 6 to 7, it can be seen that within the scope provided by the present invention, as the amount of coating increases, the cycle performance of the modified positive electrode material obtained shows a trend of first rising and then falling. The reason is that if the coating amount is too small, the lubrication effect cannot be effectively achieved, and the positive electrode sheet after rolling is prone to defects, or the modified positive electrode material particles in the positive electrode sheet are prone to breakage; therefore, the cycle performance is poor; if the coating is too thick, the migration path required by lithium ions during charging and discharging is relatively long, and the wettability of the electrolyte to the modified positive electrode material is poor; therefore, the cycle performance is also poor.

[0147] In summary, the modified positive electrode material prepared by the present invention can improve the flexibility of the positive electrode sheet, avoid the problem of broken band and light transmission under high compaction conditions, and can also improve the safety performance and cycle performance of lithium secondary batteries including the modified positive electrode material to a certain extent. The obtained lithium secondary battery is expected to be widely used in the power field, energy storage field, and 3C small household appliances field.

[0148] The above is a detailed description of the embodiments of the present invention, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A modified positive electrode material, It is characterized in that The modified positive electrode material comprises a core and a coating disposed on the surface of the core; The core includes a positive electrode active material; the coating includes graphite and molybdenum disulfide; The mass ratio of the graphite to molybdenum disulfide is 1:0.5-1.5; the mass ratio of the core to the coating is 4-7:

1.

2. The modified positive electrode material according to claim 1, It is characterized in that The Dv90 particle size of the graphite is 1 to 1.2 μm.

3. The modified positive electrode material according to claim 1, It is characterized in that The Dv90 particle size of the molybdenum disulfide is 0.5-1 μm.

4. The modified positive electrode material according to claim 1, It is characterized in that The positive electrode active material includes layered LiMO 2 and at least one of a polyanionic material; LiMO 2 In the embodiment, M includes at least one of Ni, Co and Mn; or, the positive electrode active material is lithium cobalt oxide.

5. A method for preparing the modified positive electrode material according to any one of claims 1 to 4, It is characterized in that The preparation method comprises the following steps: S1. dispersing the graphite and molybdenum disulfide in an organic solvent to obtain a coating dispersion; S2. mixing the coating dispersion and the positive electrode active material, and removing the organic solvent; S3. Calcination of the solid product obtained in step S2.

6. The preparation method according to claim 5, It is characterized in that In step S1, the organic solvent includes at least one of methanol, ethanol and isopropanol; preferably, in step S2, the mixing method includes stirring; preferably, in step S2, the mixing time is 6 to 8 hours; preferably, in step S2, the method for removing the organic solvent includes at least one of drying, freeze-drying and reduced pressure distillation.

7. The preparation method according to claim 5, It is characterized in that In step S3, the calcination temperature is 200-450°C; and / or, in step S3, the calcination time is 5-7h.

8. A lithium secondary battery, It is characterized in that The lithium secondary battery comprises a positive electrode sheet, which comprises a positive electrode collector and a positive electrode coating disposed on the surface of the positive electrode collector; the raw material for preparing the positive electrode coating comprises the modified positive electrode material as claimed in any one of claims 1 to 4.

9. The lithium secondary battery according to claim 8, It is characterized in that The positive electrode current collector is made of aluminum foil, and / or the thickness of the current collector is ≤8 μm.

10. Application of the lithium secondary battery according to claim 8 or 9 in the fields of power, energy storage and 3C small household appliances.

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