Cathode material for secondary battery and method for preparing the same

Abstract

The application relates to the technical field of secondary batteries, and discloses a cathode material for a secondary battery, which comprises silicon nanoparticles, a high-molecular polymer and a carbon material; the high-molecular polymer is subjected to heat treatment to form amorphous carbon material, the amorphous carbon material is coated on the surface of the silicon nanoparticles to obtain carbon-coated silicon nanoparticles; the carbon-coated silicon nanoparticles are distributed on the surface and inside of the carbon material to form a composite material, and irregular protrusions are formed on the surface of the composite material; the height of the protrusions is not more than 400 nm of the surface of the carbon material, and the volume of the open pores formed on the surface of the composite material by the protrusions is not more than 32% of the volume of the shape of the composite material. Part of the silicon nanoparticles is distributed inside the composite material, and part of the silicon nanoparticles is distributed on the surface of the composite material; by limiting the height of the protrusions on the surface of the composite material and limiting the volume of the open pores on the surface of the composite material formed by the protrusions, the structure of the composite material can be made more stable, and the life degradation in the process of repeated charging and discharging cycles can be slowed down.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a cathode material for secondary batteries and its preparation method. Background Technology

[0002] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. During charging, an applied voltage forces the current to move in the opposite direction, causing ions within the lithium battery to move in opposite directions, resulting in an electrochemical reaction that converts electrical energy into chemical energy. During discharge, reduction and oxidation reactions occur at the positive and negative electrodes, respectively, making it equivalent to a primary cell. During charging, oxidation and reduction reactions occur, making it equivalent to an electrolytic cell.

[0003] The active material for the negative electrode in existing lithium secondary batteries is mostly graphite. However, graphite has low capacity and low energy density. Silicon can be selected as a high-yield material to replace graphite negative electrodes. However, due to the change in material volume during charging and discharging, the lifespan of the negative electrode material deteriorates severely. At the same time, due to the low conductivity of silicon, its initial efficiency is low.

[0004] To address the shortcomings of silicon as a cathode material, research has focused on encapsulating silicon within nanoscale carbon, developing silicon-carbon composites with internal void structures. Existing silicon-carbon composites utilize conductive carbon materials, maintaining a bond between the silicon nanoparticles and the carbon material. This ensures the carbon material completely encapsulates the silicon nanoparticles, resisting silicon nanoparticle formation and structural volume changes. While this bonding method minimizes contact between the silicon nanoparticle surface and the electrolyte to maintain durability, in reality, during repeated charge-discharge cycles, the volume of the silicon particles within the composite structure changes, making it difficult to maintain the composite structure. These volume changes lead to carbon detachment and the formation of tiny gaps on the silicon particle surface. Furthermore, carbon detachment reduces the composite's conductivity, and the tiny gaps on the silicon nanoparticle surface cause electrolyte depletion, ultimately resulting in increased battery resistance and reduced battery life. Summary of the Invention

[0005] In view of this, the present invention provides a cathode material for secondary batteries to solve the problem that the composite structure of existing silicon-carbon composites is difficult to maintain, the volume is prone to change, and ultimately leads to a reduction in the electrochemical performance and lifespan of the battery.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] On the one hand, the present invention provides a cathode material for secondary batteries, comprising silicon nanoparticles, polymers, and carbon materials;

[0008] The polymer is heat-treated to form an amorphous carbon material, and the amorphous carbon material is coated on the surface of the silicon nanoparticles to obtain carbon-coated silicon nanoparticles.

[0009] The carbon-coated silicon nanoparticles are distributed on the surface and inside of the carbon material to form a composite material, and irregular protrusions are formed on the surface of the composite material; the height of the protrusions does not exceed 400 nm on the surface of the carbon material, and the volume of the openings formed by the protrusions on the surface of the composite material does not exceed 32% of the outer volume of the composite material.

[0010] Preferably, in the cathode material for secondary batteries described above, the volume of the opening on the surface of the composite material is the difference between the external volume of the composite material and the actual volume of the composite material.

[0011] Preferably, in the cathode material for secondary batteries described above, the amount of silicon nanoparticles distributed on the surface of the carbon material is less than or equal to 15% of the total amount of silicon nanoparticles.

[0012] Preferably, in the cathode material for the secondary battery described above, the mass ratio of the silicon nanoparticles, the polymer, and the carbon material is 1:1:(0.3-1).

[0013] Preferably, in the cathode material for the secondary battery described above, the polymer is a polymer with terminal functional groups of -OH and -NH2.

[0014] Preferably, in the cathode material for the secondary battery described above, the polymer includes polydopamine, hydroxy-PEG-amine, alanine, glycine, methionine, and lysine.

[0015] Preferably, in the cathode material for the above-mentioned secondary battery, the carbon material includes any one or a mixture of several of graphite, graphene, graphene oxide, reduced graphene, reduced graphene oxide, and carbon nanotubes.

[0016] On the other hand, the present invention also provides a method for preparing the above-mentioned cathode material for secondary batteries, comprising the following steps:

[0017] (1) Mix and dissolve silicon nanoparticles, polymer and solvent, add acidic buffer solution, stir to obtain solution A;

[0018] (2) Dissolve the carbon material in water to obtain solution B;

[0019] (3) Mix solution A and solution B under heating conditions, and then spray dry and heat treat to obtain the cathode material for secondary batteries.

[0020] Preferably, in the above-mentioned method for preparing cathode material for secondary batteries, the solvent in step (1) is water and / or an organic solvent, and the mass ratio of the total mass of the silicon nanopowder and the polymer to the mass of the solvent is 2:(1-1.5).

[0021] Preferably, in the above-mentioned method for preparing cathode material for secondary batteries, the heating temperature in step (3) is 85-98°C and the heating time is 10-15h;

[0022] And / or the inlet air temperature of the spray dryer is 180-250℃, the powder mass fraction is greater than 95%, and the feeding speed is 20-30 r / min;

[0023] And / or the heat treatment is to raise the temperature from room temperature to 700-1100℃ at a rate of 3-6℃ / min, and hold it at 700-1100℃ for 2-5 hours.

[0024] The present invention also provides a cathode material for secondary batteries, which is prepared by the above method.

[0025] The present invention also provides a battery comprising a cathode material for a secondary battery prepared by the above method.

[0026] This invention provides a cathode material for secondary batteries and its preparation method. Compared with the prior art, its advantages are as follows:

[0027] The silicon nanoparticles of the present invention are distributed in part inside the composite material and in part on the surface of the composite material. By limiting the height of the protrusions on the surface of the composite material and limiting the volume of the openings formed by the protrusions on the surface of the composite material, the structure of the composite material can be made more stable, and the life degradation during repeated charge and discharge cycles can be slowed down. At the same time, in terms of cathode material design, thin film can be realized, which exhibits performance that is very beneficial to fast charging and is suitable for mass production.

[0028] The polymer of the present invention has -OH and -NH2 at the ends of its molecular structure. The -OH group reacts with the -OH group on the surface of silicon nanoparticles to form hydrogen bonds, and the -NH2 group reacts with the -OH group of carbon material to form amide bonds, thus forming a precursor of silicon-carbon composite material with strong chemical bonds.

[0029] The cathode material of the present invention includes a polymer coating. The polymer is heat-treated to form an amorphous carbon material, which is then combined with silicon nanoparticles and carbon material to obtain the cathode material. The resulting composite structure is spherical. The present invention utilizes a polymer with -OH and -NH2 end groups to enhance the bonding between silicon nanoparticles and carbon material, so that silicon nanoparticles with amorphous carbon material distributed on the surface are irregularly distributed on the surface of the composite material.

[0030] The polymer derivative of the present invention is irregularly coated with amorphous carbon material on the surface of silicon nanoparticles. Furthermore, by limiting the proportion of reactants and the reaction conditions in the preparation method of the present invention, the amount of silicon nanoparticles on the surface of the composite structure can be kept to no more than 15% of the total silicon nanoparticles, so that the shape of the composite structure is well maintained, thereby enhancing the electrochemical performance and service life of the cathode material. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0032] Figure 1 Electron micrographs of the composite structure of cathode materials for secondary batteries, wherein (a) is the composite structure of the present invention and (b) is the composite structure of Comparative Example 1;

[0033] Figure 2 This is a schematic diagram of the detection sites of the composite structure of the cathode material for secondary batteries of the present invention;

[0034] Figure 3 This is a conceptual diagram of the composite structure of the cathode material for secondary batteries according to the present invention;

[0035] Figure 4 This is a schematic diagram of the volume of the cathode material for the secondary battery of the present invention;

[0036] Figure 5 This is a schematic diagram of the combined structure of the polymer, silicon nanoparticles, and carbon materials of the present invention.

[0037] Figure 6 This is a comparison chart showing the cycle life of the silicon nanoparticle content on the surface of the cathode material used in the secondary battery of this invention.

[0038] In the diagram,

[0039] 1 represents carbon-coated silicon nanoparticles, and 2 represents carbon materials. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] This invention provides a cathode material for secondary batteries, comprising silicon nanoparticles, polymers, and carbon materials;

[0042] The polymer is heat-treated to form an amorphous carbon material, and the amorphous carbon material is coated on the surface of the silicon nanoparticles to obtain carbon-coated silicon nanoparticles.

[0043] The carbon-coated silicon nanoparticles are distributed on the surface and inside of the carbon material to form a composite material, and irregular protrusions are formed on the surface of the composite material; the height of the protrusions does not exceed 400 nm on the surface of the carbon material, and the volume of the openings formed by the protrusions on the surface of the composite material does not exceed 32% of the outer volume of the composite material.

[0044] It should be noted that, as Figure 1 As shown in (a), this is a morphology diagram of the composite structure of the cathode material prepared in this invention. It is clearly visible that silicon nanoparticles are irregularly distributed on the surface of the composite structure. Figure 3 As shown, 1 is a carbon-coated silicon nanoparticle and 2 is a carbon material. In this embodiment of the invention, a portion of the carbon-coated silicon nanoparticles is irregularly distributed on the surface of the carbon material, and another portion is distributed inside the carbon material.

[0045] In some embodiments of the present invention, the volume of the opening on the surface of the composite material is the difference between the external volume of the composite material and the actual volume of the composite material.

[0046] like Figure 4 As shown, the external volume of the composite material is B. The external volume of the composite material includes the volume of the composite material body, the volume of the protrusions on the surface of the composite material, and the volume of the openings on the surface of the composite material. The external volume of the composite material is obtained as follows: the average particle size is obtained by analyzing the particle size of the final synthesized composite material, and then the external volume of the composite material is calculated using the radius of the average particle size.

[0047] like Figure 4 As shown, the actual volume of the composite material is A, which includes the volume of the composite material body and the volume of the protrusions on the composite material surface. The actual volume of the composite material is obtained as follows: the actual volume of the particles is calculated by measuring the true density of the composite material. Therefore, the volume of the openings on the surface of the composite material formed by the protrusions is M = (B - A), and M ≤ 32%B.

[0048] In some embodiments of the present invention, the amount of silicon nanoparticles distributed on the surface of the carbon material is less than or equal to 15% of the total amount of silicon nanoparticles.

[0049] In some embodiments of the present invention, the mass ratio of the silicon nanoparticles, the polymer, and the carbon material is 1:1:(0.3-1).

[0050] In some embodiments of the present invention, the carbon material includes any one or a mixture of several of graphite, graphene, graphene oxide, reduced graphene, reduced graphene oxide, and carbon nanotubes.

[0051] In some embodiments of the present invention, the polymer is a polymer with terminal functional groups of -OH and -NH2, and the polymer includes, but is not limited to, polydopamine, hydroxy-PEG-amine, alanine, glycine, methionine, and lysine; wherein hydroxy-PEG-amine is also known as aminodiethylene glycol monomethyl ether or amino polyethylene glycol.

[0052] In some embodiments of the present invention, the particle size of the silicon nanoparticles is D. 50 =50-110nm.

[0053] The following example uses polydopamine as the polymer and graphene oxide as the carbon material for illustration:

[0054] like Figure 5 The diagram shows a composite structure of polymer, silicon nanoparticles, and carbon materials. The -OH groups on the surface of the silicon nanoparticles form hydrogen bonds with the -OH groups of the polymer, while the -NH2 groups at the other end of the polymer bond with the carboxyl groups on the surface of the carbon material to form amide bonds, ultimately resulting in the composite structure shown. Figure 5 The silicon composite bonding structure shown.

[0055] The present invention also provides a method for preparing the above-mentioned cathode material for secondary batteries, comprising the following steps:

[0056] (1) Mix and dissolve silicon nanoparticles, polymer and solvent, add acidic buffer solution, stir to obtain solution A;

[0057] (2) Dissolve the carbon material in water to obtain solution B;

[0058] (3) Mix solution A and solution B under heating conditions, and then spray dry and heat treat to obtain the cathode material for secondary batteries.

[0059] In some embodiments of the present invention, the solvent in step (1) is water and / or an organic solvent, and the mass ratio of the total mass of the silicon nanoparticles and the polymer to the mass of the solvent is 2:(1-1.5); more preferably, the solvent is a mixed solution of distilled water and ethanol, and the mass ratio of the total mass of the silicon nanoparticles and the polymer to the mass of the solvent is 2:1.

[0060] In some embodiments of the present invention, the acidic buffer solution in step (1) is Tris HCl.

[0061] In some embodiments of the present invention, the mixing and dissolving time in step (1) is 10-15 hours, preferably 12 hours.

[0062] In some embodiments of the present invention, the dissolution in step (2) is ultrasonic dispersion, and the ultrasonic dispersion power is 800-1500W, the time is 1.5-3h, preferably 2h.

[0063] In some embodiments of the present invention, the heating temperature in step (3) is 85-98°C and the heating time is 10-15h; preferably, the heating temperature is 90°C and the heating time is 12h.

[0064] In some embodiments of the present invention, the inlet air temperature of the spray drying in step (3) is 180-250°C, the powder mass fraction is greater than 95%, and the feeding speed is 20-30 r / min. It should be noted that the feeding speed here is the pump speed. Preferably, the inlet air temperature of the spray drying is 200°C and the feeding speed is 25 r / min.

[0065] In some embodiments of the present invention, the number of spray cleaners in the spray drying process described in step (3) is at least one, and more preferably three.

[0066] In some embodiments of the present invention, the heat treatment is performed by heating from room temperature to 700-1100°C at a rate of 3-6°C / min, and holding at 700-1100°C for 2-5 hours; preferably, the heat treatment is performed by heating from room temperature to 900°C at a rate of 5°C / min, and holding at 900°C for 3 hours; more preferably, the heat treatment process is carried out in an H2 / Ar atmosphere.

[0067] Understandably, during the heat treatment process, the polymer carbonizes and is induced to become amorphous carbon material, which ultimately combines with silicon nanoparticles and carbon materials to form silicon-carbon complexes. In this process, silicon nanoparticles not only exist within the composite material but also irregularly on its surface. Experimental studies have shown that if the amount of silicon nanoparticles on the composite surface exceeds 15%, the volume change of silicon nanoparticles during repeated charge-discharge cycles will be exacerbated, making it difficult to maintain the composite structure and leading to a sharp deterioration in battery life. However, if the content is adjusted to below 15% of the silicon nanoparticles on the composite surface, there is little impact on the structural integrity of the composite material and the degradation of battery performance.

[0068] Example 1

[0069] A method for preparing a cathode material for a secondary battery includes the following steps:

[0070] (1) Mix 4g of silicon nanopowder, 4g of polydopamine, 3L of distilled water, 1L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 12 hours to obtain solution A;

[0071] (2) Add 2.66g of graphene oxide to water and disperse it ultrasonically for 2h to obtain solution B;

[0072] (3) Mix solution A and solution B in a heated reactor and react at 90°C for 12 hours to obtain a reaction solution;

[0073] (4) The reaction solution is spray-dried to obtain powder. The inlet air temperature of the spray dryer is 200℃ and the feed rate is 25r / min.

[0074] (5) Under H2 / Ar atmosphere, the powder material is heat-treated at 900℃ for 3h with a heating rate of 5℃ / min to obtain the cathode material for secondary batteries.

[0075] Elemental analysis was performed on the surface of the cathode material composite for secondary batteries prepared in this embodiment. Detection sites are shown in [reference needed]. Figure 2 The test results are shown in Table 1:

[0076] Table 1. Elemental analysis results of the composite structure surface in Example 1

[0077] Element types Point 1 (wt%) Point 2 (wt%) C 87.22 86.51 O 3.25 2.79 Si 9.53 10.7 total 100.00 100.00

[0078] Example 2

[0079] A method for preparing a cathode material for a secondary battery includes the following steps:

[0080] (1) Mix 4g of silicon nanoparticles, 4g of alanine, 3L of distilled water, 2L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 10 hours to obtain solution A;

[0081] (2) Add 1.2g of graphene to water and disperse it ultrasonically for 1.5h to obtain solution B;

[0082] (3) Mix solution A and solution B in a heated reactor and react at 85°C for 15 hours to obtain a reaction solution;

[0083] (4) The reaction solution is spray-dried to obtain powder. The inlet air temperature of the spray dryer is 180℃ and the feed rate is 20r / min.

[0084] (5) Under H2 / Ar atmosphere, the powder material is heat-treated at 700℃ for 5h with a heating rate of 3℃ / min to obtain the cathode material for secondary batteries.

[0085] Testing revealed that the average silicon nanoparticle content on the surface of the cathode material composite for secondary batteries prepared in this embodiment was 7.85%.

[0086] Example 3

[0087] A method for preparing a cathode material for a secondary battery includes the following steps:

[0088] (1) Mix 4g of silicon nanoparticles, 4g of hydroxy-PEG-amine, 2L of distilled water, 4L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 15 hours to obtain solution A;

[0089] (2) Add 4g of graphite to water and disperse it ultrasonically for 3h to obtain solution B;

[0090] (3) Mix solution A and solution B in a heated reactor and react at 98°C for 10 h to obtain a reaction solution;

[0091] (4) The reaction liquid is spray-dried to obtain powder. The inlet air temperature of the spray dryer is 250℃ and the feed rate is 30r / min.

[0092] (5) Under H2 / Ar atmosphere, the powder material is heat-treated at 1100℃ for 2h with a heating rate of 6℃ / min to obtain the cathode material for secondary batteries.

[0093] Testing revealed that the average silicon nanoparticle content on the surface of the cathode material composite for secondary batteries prepared in this embodiment was 10.93%.

[0094] Comparative Example 1

[0095] This comparative example demonstrates a conventional method for preparing cathode materials for secondary batteries, including the following steps:

[0096] (1) Add silicon nanoparticles to H2SO4 and H2O2 solutions and mix and stir for 12 hours.

[0097] (2) After stirring, wash with water and filter the water;

[0098] (3) Add the filtered silicon nanoparticles to the silane binder solution, stir evenly, wash with water, filter, and dry.

[0099] (4) The dried material is mixed and dissolved with carbon material, and ultrasonically dispersed for 2 hours to obtain a reaction solution;

[0100] (5) The reaction solution is spray-dried to obtain powder. The inlet air temperature of the spray dryer is 200℃ and the feed rate is 25r / min.

[0101] (6) Under H2 / Ar atmosphere, the powder material is heat-treated at 900℃ for 3h with a heating rate of 5℃ / min to obtain the cathode material for secondary batteries.

[0102] like Figure 1 (b) shows the morphology of the cathode material composite structure prepared in this comparative example. It can be clearly seen that the surface of the silicon nanoparticles and the carbon material in the composite structure are bonded together, and the carbon material is completely wrapped around the outside of the silicon nanoparticles. The surface of the composite structure is flat and smooth, and the contact area with the electrolyte is small.

[0103] Comparative Example 2

[0104] Comparative Example 2 is basically the same as Example 1, except that:

[0105] (1) Mix 4g of silicon nanopowder, 2g of polydopamine, 3L of distilled water, 1L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 12 hours to obtain solution A.

[0106] Comparative Example 3

[0107] Comparative Example 3 is basically the same as Example 1, except that:

[0108] (1) Mix 4g of silicon nanopowder, 3g of polydopamine, 3L of distilled water, 1L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 12 hours to obtain solution A.

[0109] Comparative Example 4

[0110] Comparative Example 4 is basically the same as Example 1, except that:

[0111] (1) Mix 4g of silicon nanopowder, 5g of polydopamine, 3L of distilled water, 1L of ethanol, and 1mol / L Tris HCl (0.04L) and stir for 12 hours to obtain solution A.

[0112] Comparative Example 5

[0113] Comparative Example 5 is basically the same as Example 1, except that:

[0114] (3) Mix solution A and solution B in a heated reactor and react at 80°C for 18 hours to obtain a reaction solution.

[0115] Comparative Example 6

[0116] Comparative Example 6 is basically the same as Example 1, except that:

[0117] (3) Mix solution A and solution B in a heated reactor and react at 100°C for 10 hours to obtain a reaction solution.

[0118] Comparative Example 7

[0119] Comparative Example 7 is basically the same as Example 1, except that:

[0120] (3) Mix solution A and solution B in a heated reactor and react at 75°C for 20 h to obtain a reaction solution.

[0121] Comparative Example 8

[0122] Comparative Example 8 is basically the same as Example 1, except that:

[0123] (3) Mix solution A and solution B in a heated reactor and react at 90°C for 8 hours to obtain a reaction solution.

[0124] The content of silicon nanoparticles on the surface of the cathode material composites for secondary batteries prepared in Comparative Examples 2-8 was detected, and the results are shown in Table 2.

[0125] Table 2 shows the detection results of silicon nanoparticle content on the surface of the composite structures in Comparative Examples 2-8.

[0126] Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 7 Comparative Example 8 content 20.48％ 18.59％ 16.64％ 16.08％ 18.23％ 17.46％ 15.57％

[0127] As shown in Table 2, after the heat treatment process, the content of silicon nanoparticles on the surface of the final synthesized composite structure will vary depending on the ratio between reactants, mixing reaction time, reaction temperature and other conditions. In order to keep the content of silicon nanoparticles coated with amorphous carbon material on the surface of silicon-carbon composite material below 15%, the limiting of reactant conditions in the preparation process is crucial.

[0128] Next, composite electrodes were fabricated using cathode materials with silicon contents of 10%, 15%, 18%, and 20% on the surface of the composite structure, and were successively labeled No.01, No.02, No.03, and No.04. Their charge-discharge cycle life was then measured.

[0129] Fabrication of the composite electrode: The above-mentioned cathode materials are mixed with binder and conductive carbon material in water at a weight ratio of 7:2:1 to prepare a slurry, which is then distributed on the electrode substrate. After drying and rolling, a 0.8 mg / cm³ electrode is produced. 2 Silicon composite electrode.

[0130] Composite electrode lifespan evaluation conditions: Under an ambient temperature of 25°C, the composite electrode was charged with a current of 300 mA and discharged with a current of 1500 mA for a total of 100 charge-discharge cycles. The test results are shown in [reference needed]. Figure 6 Clearly, when the silicon content on the surface of the composite structure is ≤15%, the cycle life is relatively high.

[0131] In addition, under the condition that the surface protrusion height of the composite material does not exceed 400 nm of the carbon material surface, composite electrodes were fabricated using cathode materials with surface opening volume relative to the overall volume of the composite material being 10%, 20%, 25%, 32%, 35%, and 45%, respectively, and their charge-discharge cycle life was measured. The preparation method and life evaluation conditions of the composite electrodes were the same as above, and the test results are shown in Table 3.

[0132] Table 3 Test Results

[0133] Percentage of surface openings 10％ 20％ 26％ 32％ 35％ 45％ Capacity retention after 100 cycles 82.0％ 83.4％ 88.5％ 86.8％ 75.1％ 66.9％

[0134] Clearly, when the volume of the openings on the surface of the composite material exceeds 32% of the total volume of the composite material, the cycle life is significantly reduced.

[0135] Meanwhile, while ensuring that the volume of the openings on the composite material surface is between 25% and 30% of the overall volume of the composite material, cathode materials with protrusion heights of 200 nm, 400 nm, 500 nm, and 600 nm were selected to fabricate composite electrodes, and their charge-discharge cycle life was measured. The preparation method and life assessment conditions for the composite electrodes were the same as above, and the test results are shown in Table 4.

[0136] Table 4 Test Results

[0137] Surface protrusion height of composite materials 200nm 400nm 500nm 600nm Capacity retention after 100 cycles 84.6％ 85.3％ 76.8％ 62.3％

[0138] As shown in Table 4, the cycle life is significantly higher when the protrusion height on the composite material surface is ≤400nm. Therefore, the protrusion height and volume formed by carbon-coated silicon nanoparticles on the composite material surface have a significant impact on the charge-discharge cycle life of the battery. Under the conditions of this invention, the capacity retention rate after 100 cycles can be effectively guaranteed to be above 80%.

[0139] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. The solutions disclosed in the embodiments are described simply because they correspond to the methods disclosed in the embodiments; relevant parts can be found in the method section.

[0140] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A cathode material for secondary batteries, characterized in that, Including silicon nanoparticles, polymers, and carbon materials; The polymer is heat-treated to form an amorphous carbon material, and the amorphous carbon material is coated on the surface of the silicon nanoparticles to obtain carbon-coated silicon nanoparticles. The carbon-coated silicon nanoparticles are distributed on the surface and inside of the carbon material to form a composite material, and irregular protrusions are formed on the surface of the composite material; the height of the protrusions on the surface of the composite material is 200-400 nm, and the volume ratio of the openings formed by the protrusions on the surface of the composite material is 10-32%. The volume of the opening on the surface of the composite material is the difference between the external volume of the composite material and the actual volume of the composite material; The amount of silicon nanoparticles distributed on the surface of the carbon material is less than or equal to 15% of the total amount of silicon nanoparticles; The mass ratio of the silicon nanoparticles, the polymer, and the carbon material is 1:1:(0.3-1); The polymer is a polymer with terminal functional groups of -OH and -NH2; The polymers include polydopamine, hydroxy-PEG-amine, alanine, glycine, methionine, and lysine.

2. A method for preparing the cathode material for a secondary battery according to claim 1, characterized in that, Includes the following steps: (1) Mix and dissolve silicon nanoparticles, polymer and solvent, add acidic buffer solution, stir to obtain solution A; (2) Dissolve the carbon material in water to obtain solution B; (3) Mix solution A and solution B under heating conditions, and then spray dry and heat treat to obtain the cathode material for secondary batteries.

3. The method for preparing the cathode material for secondary batteries according to claim 2, characterized in that, The solvent in step (1) is water and / or an organic solvent, and the mass ratio of the total mass of the silicon nanoparticles and the polymer to the mass of the solvent is 2:(1-1.5).

4. The method for preparing the cathode material for secondary batteries according to claim 2, characterized in that, The heating temperature in step (3) is 85-98℃, and the heating time is 10-15h; And / or the inlet air temperature of the spray dryer is 180-250℃, the powder mass fraction is greater than 95%, and the feeding speed is 20-30 r / min; And / or the heat treatment is to raise the temperature from room temperature to 700-1100℃ at a rate of 3-6℃ / min, and hold at 700-1100℃ for 2-5 hours.

5. A battery, characterized in that, The cathode material includes the cathode material for secondary batteries as described in claim 1 and the cathode material for secondary batteries prepared by the preparation method of the cathode material for secondary batteries as described in any one of claims 2-4.

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