A negative electrode sheet for lithium battery and its preparation method and application

Abstract

The present invention discloses a negative electrode sheet for a lithium battery, a preparation method thereof, and an application thereof. The preparation method of the negative electrode sheet comprises the following steps: S1, adding silicon micron particles, carbon nanotubes, conductive carbon, and a dispersant to an organic solvent, stirring, and drying to obtain a mixed powder; S2, carbonizing, ball-milling, and sieving the mixed powder to obtain a silicon composite material with a particle size of 2-8 μm; S3, mixing the silicon composite material with graphite, conductive carbon, and polyacrylic acid emulsion to form a slurry, applying the slurry to a negative electrode current collector, and pressing to prepare the negative electrode sheet; the silicon-based composite material is secondary particles, including silicon micron particles, carbon nanotubes, and conductive carbon, wherein the carbon nanotubes and the conductive carbon are network-coated on the outer layer of the silicon micron particles, and the silicon secondary particles are assembled into a battery, thereby obtaining a battery with high capacity utilization, good conductivity, high coulombic efficiency, small volume expansion, and high cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon-based composite materials, and specifically relates to a negative electrode sheet for a lithium battery, a preparation method and application thereof, and particularly relates to the preparation of a silicon composite material and a battery containing the same, and especially relates to a silicon / carbon nanotube / conductive carbon composite material and the preparation and application of a battery containing the same. Background Art

[0002] Silicon is the negative electrode material with the highest known specific capacity, which can reach 4200mAh / g.

[0003] At the same time, compared to graphite negative electrode materials, silicon has a higher lithium intercalation and deintercalation potential, which can effectively prevent lithium precipitation during high-rate charge and discharge, thereby improving battery safety. Although silicon has a greater energy density advantage, it produces a large volume effect (up to 300%) during the process of alloying with lithium, leading to the collapse of the electrode structure and the peeling of active materials, causing the electrode material to lose electrical contact and causing rapid capacity decay. In addition, silicon has poor conductivity, which seriously hinders the practical application of pure phase silicon as a negative electrode material for lithium-ion batteries.

[0004] Currently available silicon-carbon and silicon-oxygen materials are expensive to produce. While they increase energy density, the cost per watt-hour of the battery cell also increases accordingly. Nano-silicon materials have poor cycling performance and cannot meet the performance requirements of power batteries. While nano-silicon that has undergone coating modification or porous treatment can improve cycling performance, its cost is further increased, making it unable to meet the lower cost requirements of power batteries. Micro-silicon is low-cost, but its cycling performance is poor, failing to meet the requirements of power batteries. Developing a low-cost silicon material with high energy density and cycling performance that meets the requirements of power batteries is urgent. Summary of the Invention

[0005] In light of this, the present invention aims to provide a negative electrode sheet for lithium batteries, its preparation method, and its application. The micronized silicon / carbon nanotube / conductive carbon composite material provided by the present invention not only maximizes silicon's high gram capacity but also improves its conductivity. When this silicon composite material (micronized silicon / carbon nanotube / conductive carbon composite) is assembled into a battery and cycled within a specific voltage range (0.2-0.6V), the resulting silicon-containing battery exhibits high capacity utilization, excellent conductivity, high coulombic efficiency, minimal volume expansion, and high cycling stability.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] <First Aspect>

[0008] A method for preparing a negative electrode sheet for a lithium battery, the method comprising the following steps:

[0009] S1. Add silicon microparticles, carbon nanotubes, conductive carbon, and a dispersant into an organic solvent, stir, and dry to obtain a mixed powder;

[0010] S2. The mixed powder is carbonized, ball-milled, and sieved to obtain a silicon composite material with a particle size of 2-8 μm (the silicon composite material is a secondary particle);

[0011] S3. Mix the silicon composite material with graphite, conductive carbon and polyacrylic acid emulsion to form a slurry, apply it on the negative electrode collector, and prepare a negative electrode sheet after pressing.

[0012] The silicon composite material is a secondary particle comprising silicon micron particles, carbon nanotubes, and conductive carbon. The carbon nanotubes and conductive carbon form a network-like coating around the outer layer of silicon micron particles. If the particle size is less than 2 μm, the particle specific surface area is too large, consuming excessive electrolyte during cycling and leading to accelerated decay in later cycles. If the particle size is greater than 8 μm, the volume change caused by particle expansion is too large, potentially damaging the negative electrode sheet structure. More preferably, the average particle size is 3 to 6 μm.

[0013] In step S2, the carbonization temperature is 300-700°C, and the carbonization time is 3-7 hours. Temperatures above 800°C cause the conductive carbon to aggregate and grow at high temperatures, resulting in reduced coating uniformity and poor conductivity of the silicon composite. Temperatures below 300°C prevent the carbon nanotubes and conductive carbon from being fully and tightly coated on the silicon micron particles, leading to poor structural stability of the silicon composite and, after repeated cycles, the secondary particles of the silicon composite may break and pulverize.

[0014] In step S3, the slurry is applied to the negative electrode current collector, and then rolled and vacuum dried to prepare a negative electrode sheet; the negative electrode current collector includes copper foil, nickel foil, or copper-nickel alloy; the rolling density is 1.45g / cc to 1.75g / cc; the vacuum drying temperature range is 80℃ to 150℃, and the drying time is 10h to 48h.

[0015] In step S1, the weight ratio of the silicon micron particles, carbon nanotubes, conductive carbon, dispersant, and organic solvent is (1-5): (0.01-0.05): (0.1-0.5): (0.1-1):100.

[0016] In step S3, the effective mass ratio of the silicon composite material, graphite, conductive carbon, and polyacrylic acid emulsion (PAA) ranges from 80%:0%:10%:10% to 3%:77%:10%:10%. That is, the mass ratio of the silicon composite material, graphite, conductive carbon, and polyacrylic acid emulsion in the slurry ranges from 8:0:1:1 to 0.3:7.7:1:1.

[0017] The carbon nanotubes are single-walled carbon nanotubes or multi-walled carbon nanotubes. The single-walled carbon nanotubes have an average diameter of 0.5-5 nm and a length of 10 nm-200 μm. The multi-walled carbon nanotubes have an average diameter of 5-20 nm and a length of 10 nm-200 μm. In addition to maintaining the structural stability of the silicon composite secondary particles, the carbon nanotubes provide an ion conduction path, eliminating the conventional step of carbon coating silicon particles to improve conductivity.

[0018] The average particle size of the silicon micron particles is 1-20 μm, more preferably 3-6 μm.

[0019] The negative electrode sheet is made into a lithium battery, and the charging and discharging process of the lithium battery limits the charging cut-off voltage range of the negative electrode to the lithium potential to 0.5-0.8V; the charging and discharging process of the lithium battery limits the discharge cut-off voltage range of the negative electrode to the lithium potential to 0.08-0.25V. When the negative electrode cycle process is too low to the lithium potential, the silicon is too deeply embedded with lithium to generate Li 15 The Si4 phase increases internal stress in the particles, leading to particle breakage and weakened electron-ion contact, which in turn deteriorates the cycle life. When the negative electrode cycle is subjected to a high lithium potential, excessive silicon delithiation causes drastic structural changes in the silicon composite particles, leading to particle breakage and thus deteriorating the cycle life.

[0020] Preferably, the charging voltage is cut off at 0.6V, and the discharging voltage is cut off at 0.2V.

[0021] As an embodiment of the present invention, in step S3, the surface density of the coating on the copper foil is 80g / m 2 ~200g / m 2 .

[0022] As an embodiment of the present invention, in step S1, the dispersant is polyethylene glycol with a weight average molecular weight ranging from 800 to 5000.

[0023] As an embodiment of the present invention, in step S1, the organic solvent is one or more of diphenylamine, toluene, carbon tetrachloride, N-methylpyrrolidone, acetone, and diethanol.

[0024] The negative electrode sheet prepared by the above-mentioned preparation method also falls within the protection scope of the present invention.

[0025] A lithium battery containing the negative electrode sheet also falls within the protection scope of the present invention.

[0026] Within the scope of use of the present invention, the first-efficiency gram capacity and cycle performance of the micron silicon composite material are not inferior to those of silicon oxide materials, and the cost is less than one-fifth, which greatly solves the cost problem.

[0027] The present invention obtains 2-8 micron silicon composite secondary particles by dispersing and drying single crystal silicon or polycrystalline silicon micron particles, carbon nanotubes and conductive carbon, carbonizing at high temperature, ball milling and sieving fine powder, and the specific surface area is 1-2m 2 / g. Silicon composite material is mixed with graphite, conductive carbon and polyacrylic acid emulsion to form a negative electrode sheet. Conventionally, the voltage application range of the negative electrode sheet is 0.05-1.5V. In this voltage range, micron silicon cyclic expansion is severe and easy to break, resulting in rapid cycle decay. The present invention proposes to limit the charging and discharging process of micron silicon to the negative electrode potential of lithium to a charging cutoff voltage range of 0.5-0.8V; the discharge cutoff voltage range is 0.08-0.25V, avoiding the formation of Li2+ by silicon intercalation. 15 The Si4 alloy phase causes rapid silicon decay, avoiding the dramatic volume expansion caused by the insertion and extraction of lithium from silicon, thereby achieving excellent cycle performance.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1) The present invention utilizes silicon micron particles, carbon nanotubes, and conductive carbon to disperse, dry, carbonize at high temperature, and ball mill to obtain a micron silicon / carbon nanotube / conductive carbon composite, which is then made into a negative electrode sheet, fully utilizing the high gram capacity of silicon and improving conductivity.

[0030] 2) By limiting the practical application lithium potential range of the micron silicon / carbon nanotube / conductive carbon composite, the volume expansion change of silicon particles during lithium insertion and removal is restricted, maintaining the structural stability of the composite during repeated lithium insertion and removal during long-term cycling;

[0031] 3) Batteries made from the composite materials prepared by the present invention have good cycle stability, small volume expansion, and excellent rate performance, and can be used in the automotive field;

[0032] 4) The process of the present invention is simple, and the cost of raw materials using silicon microparticles is reduced to one-fifth of that using silicon nanoparticles. The processing technology is simple and suitable for mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0034] Figure 1 Schematic diagram of a top view of the silicon composite material surface; carbon nanotubes and conductive carbon are evenly coated on the surface of silicon micron particles.

[0035] Figure 2 This is the morphology of the silicon composite material under a scanning electron microscope. DETAILED DESCRIPTION

[0036] The present invention will be described in detail below with reference to the examples. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make several adjustments and improvements without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0037] Example 1

[0038] (1) Preparation of silicon composite materials

[0039] 3 g of silicon micron particles (silicon micron particles with a particle size of 1-20 μm), 0.03 g of carbon nanotubes, 0.3 g of conductive carbon, and 0.2 g of polyethylene glycol with a molecular weight of 900 are sequentially added to 100 g of an organic solvent, acetone, and uniformly stirred. The mixture is spray-dried to obtain a mixed powder, which is carbonized at 300° C. for 6 h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The ball-milled composite is sieved to obtain a silicon composite material with a particle size of 2-8 μm, which is a secondary particle.

[0040] Schematic diagram of the top view of the silicon composite material surface Figure 1 As shown, carbon nanotubes and conductive carbon are evenly coated on the surface of silicon micron particles; Figure 2 This is the morphology of the silicon composite material under a scanning electron microscope.

[0041] (2) Silicon composite material is prepared into negative electrode sheet

[0042] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 8:0:1:1 and prepared into a slurry, which was applied on a copper foil with a thickness of 10μm at a surface density of 100g / m2, rolled to a rolling density of 1.6g / cc, and then dried at 120°C in vacuum for 24h to form a negative electrode sheet.

[0043] (3) Lithium battery assembly

[0044] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0045] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.7 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0046] Example 2

[0047] (1) Preparation of silicon composite materials

[0048] 4g of silicon micron particles (1-20μm in diameter), 0.05g of carbon nanotubes, 0.4g of conductive carbon, and 0.3g of polyethylene glycol (molecular weight 900) were sequentially added to 100g of acetone, an organic solvent, and stirred uniformly. The mixture was spray-dried to obtain a mixed powder, which was then carbonized at 400°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is the secondary particle.

[0049] (2) Silicon composite material is prepared into negative electrode sheet

[0050] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 8:0:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0051] (3) Lithium battery assembly

[0052] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0053] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.8 V, the discharge voltage cutoff at 0.25 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0054] Example 3

[0055] (1) Preparation of silicon composite materials

[0056] 2g of silicon micron particles (silicon micron particles with a particle size of 1-20μm), 0.02g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol with a molecular weight of 900 were added sequentially to 100g of the organic solvent acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixture was then carbonized at 600°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is the secondary particle.

[0057] (2) Silicon composite material is prepared into negative electrode sheet

[0058] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 8:0:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0059] (3) Lithium battery assembly

[0060] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0061] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.7 V, the discharge voltage cutoff at 0.1 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0062] Example 4

[0063] (1) Preparation of silicon composite materials

[0064] 5g of silicon microparticles (1-20μm in diameter), 0.05g of carbon nanotubes, 0.5g of conductive carbon, and 0.6g of polyethylene glycol (900 molecular weight) were added sequentially to 100g of acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixture was carbonized at 700°C for 6h and ball-milled to obtain a micronized silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain a silicon composite material with a particle size of 2-8μm, which served as secondary particles.

[0065] (2) Silicon composite material is prepared into negative electrode sheet

[0066] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 4:4:1:1 and prepared into a slurry, which was applied on a copper foil with a thickness of 10μm at a surface density of 100g / m2, rolled to make a rolling density of 1.6g / cc, and then dried at 120℃ in vacuum for 24h to prepare a negative electrode sheet.

[0067] (3) Lithium battery assembly

[0068] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0069] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.5 V, the discharge voltage cutoff at 0.08 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0070] Example 5

[0071] (1) Preparation of silicon composite materials

[0072] 3g of silicon micron particles (silicon micron particles with a particle size of 1-20μm), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol with a molecular weight of 900 were added sequentially to 100g of the organic solvent acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixed powder was then carbonized at 500°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting ball-milled composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0073] (2) Silicon composite material is prepared into negative electrode sheet

[0074] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry, which was applied on a copper foil with a thickness of 10μm at a surface density of 100g / m2, rolled to a rolling density of 1.6g / cc, and then dried at 120°C in vacuum for 24h to form a negative electrode sheet.

[0075] (3) Lithium battery assembly

[0076] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0077] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0078] Comparative Example 1

[0079] (1) Preparation of silicon composite materials

[0080] 3g of silicon micron particles (silicon micron particles with a particle size of 1-20μm), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol with a molecular weight of 900 were added sequentially to 100g of the organic solvent acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixed powder was then carbonized at 500°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting ball-milled composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0081] (2) Silicon composite material is prepared into negative electrode sheet

[0082] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0083] (3) Lithium battery assembly

[0084] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0085] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.05 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0086] Comparative Example 2

[0087] (1) Preparation of silicon composite materials

[0088] 3g of silicon micron particles (1-20μm in diameter), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol (molecular weight 900) were added sequentially to 100g of acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixture was carbonized at 500°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm.

[0089] (2) Preparation of secondary particles of silicon composite material into negative electrode sheets

[0090] The silicon composite secondary particles, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0091] (3) Lithium battery assembly

[0092] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0093] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 1.5 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0094] Comparative Example 3

[0095] (1) Preparation of silicon composite materials

[0096] 3g of silicon micron particles (1-20μm in size) and 0.2g of polyethylene glycol (molecular weight 900) were added sequentially to 100g of acetone, an organic solvent. The mixture was stirred uniformly and spray-dried to obtain a mixed powder. The mixture was carbonized at 500°C for 6h and ball-milled to obtain a micron silicon composite material. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm.

[0097] (2) Preparation of secondary particles of silicon composite material into negative electrode sheets

[0098] Silicon composite secondary particles, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry, which was applied on a copper foil with a thickness of 10μm at a surface density of 100g / m2, rolled to a rolling density of 1.6g / cc, and then dried at 120°C in vacuum for 24h to form a negative electrode sheet.

[0099] (3) Lithium battery assembly

[0100] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0101] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0102] Comparative Example 4

[0103] (1) Preparation of silicon composite materials

[0104] 3g of silicon micron particles (average particle size of 25μm), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol (molecular weight 900) were sequentially added to 100g of acetone, an organic solvent, and stirred uniformly. The mixture was spray-dried to obtain a mixed powder, which was then carbonized at 500°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0105] (2) Silicon composite material is prepared into negative electrode sheet

[0106] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0107] (3) Lithium battery assembly

[0108] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0109] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0110] Comparative Example 5

[0111] (1) Preparation of silicon composite materials

[0112] 3g of silicon micron particles (1-20μm in size), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol (molecular weight 900) were sequentially added to 100g of acetone, an organic solvent, and stirred uniformly. The mixture was spray-dried to obtain a mixed powder, which was then carbonized at 800°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0113] (2) Silicon composite material is prepared into negative electrode sheet

[0114] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry, which was applied on a copper foil with a thickness of 10μm at a surface density of 100g / m2, rolled to a rolling density of 1.6g / cc, and then dried at 120°C in vacuum for 24h to form a negative electrode sheet.

[0115] (3) Lithium battery assembly

[0116] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0117] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0118] Comparative Example 6

[0119] (1) Preparation of silicon composite materials

[0120] 3g of silicon micron particles (silicon micron particles with a particle size of 1-20μm), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol with a molecular weight of 900 were added sequentially to 100g of the organic solvent acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixed powder was then carbonized at 200°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The ball-milled composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0121] (2) Silicon composite material is prepared into negative electrode sheet

[0122] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 2:6:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0123] (3) Lithium battery assembly

[0124] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0125] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0126] Comparative Example 7

[0127] (1) Preparation of silicon composite materials

[0128] 3g of silicon micron particles (silicon micron particles with a particle size of 1-20μm), 0.03g of carbon nanotubes, 0.3g of conductive carbon, and 0.2g of polyethylene glycol with a molecular weight of 900 were added sequentially to 100g of the organic solvent acetone, stirred uniformly, and spray-dried to obtain a mixed powder. The mixed powder was then carbonized at 500°C for 6h and ball-milled to obtain a micron silicon / carbon nanotube / conductive carbon composite. The resulting ball-milled composite was sieved to obtain secondary silicon composite particles with a particle size of 2-8μm. This silicon composite material is a secondary particle.

[0129] (2) Silicon composite material is prepared into negative electrode sheet

[0130] Silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) were mixed in an effective mass ratio of 0.1:7.9:1:1 and prepared into a slurry at a rate of 100 g / m 2 It was coated on a copper foil with a thickness of 10 μm with an area density of 1.6 g / cc, rolled to make its rolling density 1.6 g / cc, and then dried at 120°C in vacuum for 24 hours to prepare a negative electrode sheet.

[0131] (3) Lithium battery assembly

[0132] The counter electrode is a 99.9% pure lithium metal sheet, and the electrolyte is a 1.1 mol lithium hexafluorophosphate (LiPF6) solution of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 3:5:2). The negative electrode sheet, lithium metal sheet, battery case, and separator obtained in step (2) are dried and then the electrolyte is added in an argon-protected glove box to assemble a button-type lithium-ion battery.

[0133] The battery was subjected to a charge-discharge cycle test, with the charge voltage cutoff at 0.6 V, the discharge voltage cutoff at 0.2 V, and the charge-discharge cycle rate at 0.1 C. The initial lithium insertion and delithiation capacity and the lithium delithiation capacity after 50 cycles are shown in Table 1.

[0134] Table 1 The initial lithium removal and insertion capacity of the battery prepared with the negative electrode material, the lithium removal capacity after 50 cycles and the electrode thickness test data

[0135]

[0136] Compared with Example 5, in Comparative Example 1, the battery discharge cut-off voltage is lowered, the battery cycle retention rate is lowered, and the expansion rate is sharply increased.

[0137] Compared with Example 5, in Comparative Example 2, the battery charging cut-off voltage is increased, the battery cycle retention rate is reduced, and the expansion rate is sharply increased.

[0138] In Comparative Example 3 compared with Example 5, carbon nanotubes and conductive carbon were not added when preparing the silicon composite material, and the battery cycle retention rate decreased and the expansion rate increased sharply.

[0139] Compared with Example 5, in Comparative Example 4, the size of the micron silicon is too large when preparing the silicon composite material, resulting in an excessively large particle size of the secondary particles of the silicon composite material, which leads to poor conductivity and expansion performance of the silicon composite material, resulting in a decrease in the battery cycle retention rate and a sharp increase in the expansion rate.

[0140] Compared with Example 5, in Comparative Example 5, the carbonization temperature during the preparation of the silicon composite material is 800°C, which results in a larger size of the conductive carbon in the secondary particles of the silicon composite material, resulting in poor uniformity of the conductive carbon on the surface of the secondary particles, poor conductivity of the silicon composite material, reduced battery cycle retention rate, and increased expansion rate.

[0141] Compared with Example 5, in Comparative Example 6, the carbonization temperature during the preparation of the silicon composite material is 200°C, resulting in the conductive carbon and carbon nanotubes in the secondary particles of the silicon composite material not tightly wrapping the silicon micron particles, resulting in poor structural stability of the silicon composite material, resulting in the silicon particle material breaking and pulverizing after multiple cycles, resulting in a decrease in the battery cycle retention rate and a sharp increase in the expansion rate.

[0142] In Comparative Example 7, compared with Example 5, the silicon composite material, graphite, conductive carbon and 0.6% polyacrylic acid emulsion (PAA) are arranged in a ratio of 0.1:7.9:1:1 by effective mass. The proportion of the silicon composite material is too small compared to the graphite, resulting in too low a negative electrode delithiation capacity and too little energy improvement.

[0143] As can be seen from Table 1, the silicon composite materials prepared in Examples 1-3 of the present invention exhibit higher coulombic efficiency, lower volume expansion and better cycle performance.

[0144] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A lithium battery control method, characterized in that: The control method limits the charging cut-off voltage range of the negative electrode to the lithium potential to 0.5-0.8V during the charging and discharging process of the lithium battery; and limits the discharging cut-off voltage range of the negative electrode to the lithium potential to 0.08-0.25V during the charging and discharging process of the lithium battery; The steps for preparing the negative electrode sheet of the lithium battery are as follows: S1. Add silicon microparticles, carbon nanotubes, conductive carbon, and a dispersant into an organic solvent, stir, and dry to obtain a mixed powder; S2, the mixed powder is carbonized, ball-milled, and sieved to obtain a silicon composite material with a particle size of 2-8 μm; S3, mixing the silicon composite material with graphite, conductive carbon and polyacrylic acid emulsion to form a slurry, applying the slurry on the negative electrode current collector, and pressing the slurry to prepare a negative electrode sheet; In step S2, the carbonization temperature is 300-700°C and the carbonization time is 3-7 hours; In step S3, in the slurry, the mass ratio of the silicon composite material, graphite, conductive carbon and polyacrylic acid emulsion is 8:0:1:1 to 0.3:7.7:1:1; In step S1, the weight ratio of the silicon micron particles, carbon nanotubes, conductive carbon, dispersant, and organic solvent is (1-5): (0.01-0.05): (0.1-0.5): (0.1-1): 100; and the average particle size of the silicon micron particles is 1-20 μm.

2. The lithium battery control method according to claim 1, characterized in that: In step S3, the slurry is applied to the negative electrode current collector, and then rolled and vacuum dried to prepare a negative electrode sheet; the negative electrode current collector includes copper foil, nickel foil, or copper-nickel alloy; the rolling density is 1.45g / cc to 1.75g / cc; the vacuum drying temperature range is 80℃ to 150℃, and the drying time is 10h to 48h.

3. The lithium battery control method according to claim 1, wherein: The carbon nanotubes are single-walled carbon nanotubes or multi-walled carbon nanotubes; the single-walled carbon nanotubes have an average diameter of 0.5-5 nm and a length of 10 nm-200 μm; the multi-walled carbon nanotubes have an average diameter of 5-20 nm and a length of 10 nm-200 μm.

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