Fast-charging negative electrode sheet and use thereof in lithium battery

Abstract

A fast-charging negative electrode sheet and the use thereof in a lithium battery. The fast-charging negative electrode sheet comprises a negative electrode current collector and a negative electrode composite active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode composite active material layer comprises a negative electrode bottom-layer active material layer close to the negative electrode current collector and a negative electrode surface-layer active material layer away from the negative electrode current collector. The particle size D50 of a bottom-layer negative electrode active material in the negative electrode bottom-layer active material layer is less than the particle size D50 of a surface-layer negative electrode active material in the negative electrode surface-layer active material layer; and silicon in the bottom-layer negative electrode active material accounts for 60-80% of the total silicon mass, and silicon in the surface-layer negative electrode active material accounts for 20-40% of the total silicon mass.

Description

A fast-charging negative electrode sheet and application thereof in lithium batteries

[0001] The present application claims priority to the Chinese patent application No. 2024110987653 filed on August 9, 2024 with the China Patent Office, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a fast-charging negative electrode sheet and application thereof in lithium batteries. BACKGROUND

[0003] In the development process of new energy vehicles, in order to alleviate the problem of long charging time, developing fast-charging technology has become one of the main directions of the industry. Under this background, using high power to charge the vehicle power battery has become a trend in market application. Studies have shown that the charging time can be further reduced by high-rate charging, so the research of high-rate and high-performance power batteries has become a key element to promote the development of the industry.

[0004] The charging rate of the power battery used in the prior art is mainly 1C, and the fast-charging system is mainly concentrated in the graphite system. Some power batteries also use silicon negative electrode systems. Silicon negative electrode lithium batteries have high energy density. TECHNICAL PROBLEM

[0005] When the graphite negative electrode is directly applied in fast-charging lithium ion batteries, the energy density of the lithium battery is low and the rate performance is slightly poor. In the silicon-based material in the silicon negative electrode, there is a severe expansion and contraction effect in the charging and discharging process, which seriously even causes a series of failure conditions such as breaking of active material particles, peeling of active coating, and destruction of conductive network, which is not conducive to the fast-charging performance of lithium batteries and seriously affects the fast-charging cycle life of lithium batteries. TECHNICAL SOLUTION

[0006] In a first aspect, the present application provides a fast-charging negative electrode sheet, which adopts the following technical solution:

[0007] A fast-charging negative electrode sheet, comprising a negative electrode current collector and a negative electrode composite active material layer arranged on at least one surface of the negative electrode current collector; the negative electrode composite active material layer comprises a negative electrode bottom active material layer close to the negative electrode current collector and a negative electrode surface active material layer away from the negative electrode current collector;

[0008] The particle size D50 of the bottom layer negative active material in the negative electrode bottom active material layer is smaller than the particle size D50 of the surface layer negative active material in the negative electrode surface active material layer;

[0009] The silicon in the bottom layer negative electrode active material accounts for 60-80% of the total silicon mass, and the silicon in the surface layer negative electrode active material accounts for 20-40% of the total silicon mass.

[0010] In a second aspect, the application provides a lithium battery, which adopts the following technical scheme:

[0011] A lithium battery, comprising a positive electrode sheet, an electrolyte and the fast-charging negative electrode sheet as described above. Advantages

[0012] The application prepares a silicon-containing negative electrode sheet with a double-layer active material layer. First, on one hand, in the active material layer, the negative electrode bottom layer active material layer close to the current collector is more difficult to be infiltrated by the electrolyte than the negative electrode surface layer active material layer close to the electrolyte, which will lead to the situation that the ion transmission in the negative electrode bottom layer active material layer is difficult. By making the particle size D50 of the bottom layer negative electrode active material smaller than the particle size D50 of the surface layer negative electrode active material, the surface layer negative electrode active material has a larger particle size, which can form a negative electrode surface layer active material layer with a higher porosity, which is not only beneficial to the infiltration of the electrolyte, but also has a higher liquid retention performance, thereby helping the electrolyte to infiltrate the negative electrode bottom layer active material layer. On the other hand, since the particle size of the bottom layer negative electrode active material is smaller, it has a higher surface area to volume ratio, which also reduces the diffusion path of lithium ions in the solid phase to a certain extent. Therefore, the above two aspects work together to significantly improve the rate performance of the fast-charging negative electrode sheet.

[0013] Second, when the particle size of the bottom layer negative electrode active material is small, although more SEI films can be produced, which helps to buffer the expansion of the negative electrode composite active material layer during the charging and discharging process, but it will also increase the resistance of the negative electrode composite active material layer to a certain extent. The larger particle size of the surface layer negative electrode active material can to a certain extent alleviate the increase in resistance performance caused by the SEI film, which helps to improve the fast-charging cycle performance of the lithium battery. Moreover, since the particle size D50 of the bottom layer negative electrode active material is smaller than the D50 of the surface layer negative electrode active material, the degree of expansion and deformation of the bottom layer negative electrode active material during the fast-charging and discharging process is smaller than that of the surface layer negative electrode active material, which helps to improve the adhesion stability between the negative electrode bottom layer active material layer and the negative electrode current collector, and avoids the situation that the bottom layer negative electrode active material is deformed too much, causing the negative electrode active material particles to be crushed, or even the negative electrode composite active material layer to be peeled off from the negative electrode current collector, which helps to improve the stability of the negative electrode sheet during the fast-charging cycle.

[0014] Thirdly, the application adjusts the silicon content in the negative electrode bottom layer active material layer and the negative electrode surface layer active material layer, the silicon content in the negative electrode bottom layer active material layer is high, and the negative electrode surface layer active material layer and the negative electrode current collector serve as a buffer layer, which can buffer the silicon expansion in the bottom layer active material layer, thereby relieving the expansion of the silicon negative electrode, and helping to significantly improve the cycle performance of the lithium battery. Embodiments of the application

[0015] In some embodiments, the bottom layer negative electrode active material comprises first silicon-based active particles and first graphite particles; the particle size of the first silicon-based active particles satisfies 3pm < D50 < 6pm, and the particle size of the first graphite particles satisfies 8pm < D50 < 12pm.

[0016] In some embodiments, the particle size of the first silicon-based active material satisfies 1pm < D10 < 3pm and 6pm < D90 < 9pm; and the particle size of the first graphite particles satisfies 4pm < D10 < 6pm and 14pm < D90 < 17pm.

[0017] In some embodiments, the mass ratio of the first silicon-based active particles to the first graphite particles is 60-75: 15-20.

[0018] The application matches the particle size and mass ratio of the silicon-based active material and graphite in the negative electrode bottom layer active material layer, so that the negative electrode bottom layer active material layer can form a pore structure conducive to lithium ion transmission, which not only improves the rate performance of the lithium battery, but also promotes the formation of the SEI film in the negative electrode composite active material layer, greatly relieves the volume expansion of the silicon negative electrode in the fast charging and discharging process, and helps to improve the cycle performance of the fast charging lithium battery.

[0019] In some embodiments, the surface layer negative electrode active material comprises second silicon-based active particles and second graphite particles, the particle size of the second silicon-based active particles satisfies 9pm < D50 < 14pm, and the particle size of the second graphite particles satisfies 16pm < D50 < 19pm.

[0020] In some embodiments, the particle size of the second silicon-based active material satisfies 5pm < D10 < 8pm and 16pm < D90 < 21pm; and the particle size of the second graphite particles satisfies 10pm < D10 < 13pm and 21pm < D90 < 24pm.

[0021] In some embodiments, the mass ratio of the second silicon-based active particles to the second graphite particles is 15-20: 60-75.

[0022] The application can form a higher pore structure in the surface layer active material layer of the negative electrode by matching the particle size and mass ratio of the silicon-based active material and graphite in the surface layer active material layer of the negative electrode, which helps to improve the infiltration of the electrolyte into the composite active material layer of the negative electrode, and to a certain extent, increase the infiltration of the electrolyte into the bottom layer active material layer of the negative electrode close to the negative electrode current collector. In addition, it can greatly alleviate the increase in resistance caused by the increase in SEI, thereby helping to improve the rate performance of the lithium battery.

[0023] In the application, the particle size of the surface layer negative active material and the bottom layer negative active material is controlled within the above range, which reduces the expansion degree of the bottom layer active material layer of the negative electrode, reduces the difference in expansion degree between the surface layer active material layer and the bottom layer active material layer of the negative electrode, and avoids the peeling between the negative electrode current collector, the bottom layer active material layer of the negative electrode, and the surface layer active material layer of the negative electrode due to different stress effects, which helps to improve the structural stability of the negative electrode sheet and further improve the fast charging stability of the fast charging lithium battery.

[0024] In some embodiments, the thickness of the bottom layer negative active material layer is 30-130 pm, and / or the thickness of the surface layer negative active material layer is 80-190 pm.

[0025] In some embodiments, the fast charging negative electrode sheet is prepared by the following steps:

[0026] S1, the first silicon-based active material, the first graphite particle, the first conductive agent, and the first binder are mixed in a mass ratio of (60-75) :(15-25):(1-3):(8-15) to obtain a first slurry; the second silicon-based active material, the second graphite particle, the second conductive agent, and the second binder are mixed in a mass ratio of (15-25) :(60-75):(1-3):(8-15) to obtain a second slurry;

[0027] S2, the first slurry and the second slurry are simultaneously layered and coated on the surface of the negative electrode current collector, wherein the first slurry is in direct contact with the negative electrode current collector, to obtain a single-sided coated negative electrode sheet;

[0028] S3, the single-sided coated negative electrode sheet is dried, and then the other side is coated and dried by repeating step S2, and then roll pressing, cutting, and punching are sequentially performed to obtain the fast charging negative electrode sheet.

[0029] In some embodiments, the first conductive agent and the second conductive agent are independently selected from at least one of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0030] In some embodiments, the first binder and the second binder are independently selected from at least one of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and styrene butadiene rubber (SBR).

[0031] In some embodiments, the positive electrode sheet comprises a positive electrode current collector and a positive electrode composite active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode composite active material layer comprises a positive electrode bottom layer active material layer close to the positive electrode current collector and a positive electrode surface layer active material layer away from the positive electrode current collector.

[0032] The particle size D50 of the bottom layer positive electrode active material in the positive electrode bottom layer active material layer is greater than the particle size D50 of the surface layer positive electrode active material in the positive electrode surface layer active material layer.

[0033] In some embodiments, the particle size of the bottom layer positive electrode active material satisfies 14 μm < D50 < 16 μm; and the particle size of the surface layer positive electrode active material satisfies 9 μm < D50 < 11 μm.

[0034] In some embodiments, the negative electrode active material comprises at least one of a lithium iron manganese phosphate material and a ternary material; and in some embodiments, the positive electrode active material is a ternary material.

[0035] In some embodiments, the particle size of the bottom layer positive electrode active material satisfies 10 μm < D10 < 12 μm and 18 μm < D90 < 22 μm.

[0036] In some embodiments, the particle size of the surface layer positive electrode active material satisfies 4 μm < D10 < 7 μm and 13 μm < D90 < 15 μm.

[0037] By controlling the particle size of the bottom layer positive electrode active material and the particle size D50 of the surface layer positive electrode active material, the packing tortuosity of the large particles of the positive electrode active material is small, which is conducive to forming a path for lithium ion transmission in the positive electrode bottom layer active material layer, and is conducive to relieving the dramatic change in lithium ion concentration, while the specific surface area of the small particles of the positive electrode active material is large, which is helpful for the rapid embedding of lithium ions, so as to cooperate with the negative electrode sheet, so that the lithium ions can be rapidly embedded and extracted on the positive electrode side in the charging and discharging process, avoiding lithium deposition on the positive electrode side or the negative electrode side; by selecting the positive electrode sheet and the negative electrode sheet that satisfy the above conditions, the fast charging stability of the lithium battery can be significantly improved.

[0038] In some embodiments, the thickness of the positive electrode bottom layer active material layer is 50-150 μm, and / or the thickness of the positive electrode surface layer active material layer is 40-200 μm.

[0039] By controlling the bottom positive active material layer, the surface positive active material layer, the bottom negative active material layer, and the surface negative active material layer, the matching effect of the positive and negative active material layers of the positive and negative electrode sheet is more excellent, so that a fast-charging lithium battery with high rate capability and excellent cycle performance is prepared.

[0040] In some embodiments, the positive electrode sheet is prepared by the following steps:

[0041] A1, the bottom layer positive active material, the third conductive agent, the third binder are mixed in a mass ratio of (90-95):(1-5):(1-5) to obtain a third slurry; the surface layer positive active material, the fourth conductive agent, the fourth binder are mixed in a mass ratio of (90-95):(2-5):(3-7) to obtain a fourth slurry;

[0042] A2, the third slurry and the fourth slurry are simultaneously layered and coated on the surface of the positive current collector, wherein the third slurry is in direct contact with the negative current collector, to obtain a single-sided coated positive electrode sheet;

[0043] A3, the single-sided coated positive electrode sheet is dried, and then the other side is coated and dried by repeating step A2, and then roll pressing, slitting, and punching are sequentially performed to obtain the positive electrode sheet.

[0044] In some embodiments, the third conductive agent and the fourth conductive agent are independently selected from at least one of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0045] In some embodiments, the third binder and the fourth binder are independently selected from at least one of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), and styrene butadiene rubber (SBR).

[0046] Example 1

[0047] 1. Preparation of a negative electrode sheet

[0048] S1, the first silicon-based active material (particle size meets D10=2 μm, D50=5 μm, D90=7 μm), the first graphite particle (particle size meets D10=5 μm, D50=10 μm, D90=15 μm), the first conductive agent, and the first binder are mixed in a mass ratio of 60:20:2:10 to obtain a first slurry; the second silicon-based active material (particle size meets D10=6 μm, D50=11 μm, D90=18 μm), the second graphite particle (particle size meets D10=11 μm, D50=17 μm, D90=22 μm), the second conductive agent, and the second binder are mixed in a mass ratio of 20:68:2:10 to obtain a second slurry;

[0049] S2, using a double-layer coating machine, the first slurry is introduced into the lower die, the second slurry is introduced into the upper die, and the first slurry and the second slurry are simultaneously layered coated on the surface of the negative current collector, wherein the first slurry is in direct contact with the negative current collector, to obtain a single-sided coated negative electrode sheet;

[0050] S3, after drying the single-sided coated negative electrode sheet, repeating step S2 to coat the other side and drying, and then sequentially performing rolling, slitting, and punching processing, to obtain a fast-charging negative electrode sheet.

[0051] In the negative electrode composite active material layer on either side of the fast-charging negative electrode sheet, the thickness of the bottom layer of negative electrode active material is 80 μm, and the thickness of the surface layer of negative electrode active material is 130 μm.

[0052] 2. Preparation of positive electrode sheet

[0053] In some embodiments, the positive electrode sheet is prepared by the following steps:

[0054] A1, mixing the bottom layer of positive electrode active material NCM811 (particle size satisfying D10 = 11 μm, D50 = 15 μm, D90 = 20 μm), the third conductive agent, and the third binder according to a mass ratio of 92:3:5 to obtain a third slurry; mixing the surface layer of positive electrode active material (particle size satisfying D10 = 5 μm, D50 = 10 μm, D90 = 14 μm), the fourth conductive agent, and the fourth binder according to a mass ratio of 92:3:5 to obtain a fourth slurry;

[0055] A2, simultaneously layering coating the third slurry and the fourth slurry on the surface of the positive current collector, wherein the third slurry is in direct contact with the negative current collector, to obtain a single-sided coated positive electrode sheet;

[0056] A3, after drying the single-sided coated positive electrode sheet, repeating step A2 to coat the other side and drying, and then sequentially performing rolling, slitting, and punching processing, to obtain a positive electrode sheet.

[0057] In the positive electrode composite active material layer on either side of the positive electrode sheet, the thickness of the bottom layer of positive electrode active material is 100 μm, and the thickness of the surface layer of positive electrode active material is 120 μm.

[0058] 3. Assembly of lithium battery

[0059] The above positive electrode sheet, negative electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (wherein the volume ratio of EC, DMC, and EMC is 1:1:1) electrolyte, PE+aluminum oxide separator, and shell are assembled into a soft package battery using a conventional production process.

[0060] Example 2

[0061] S1, a first silicon-based active material (particle size meets D10=1 μm, D50=3 μm, D90=6 μm), first graphite particles (particle size meets D10=4 μm, D50=8 μm, D90=14 μm), a first conductive agent, and a first binder are mixed in a mass ratio of 75:25:3:15 to obtain a first slurry; a second silicon-based active material (particle size meets D10=5 μm, D50=9 μm, D90=16 μm), second graphite particles (particle size meets D10=10 μm, D50=16 μm, D90=21 μm), a second conductive agent, and a second binder are mixed in a mass ratio of 25:75:3:15 to obtain a second slurry;

[0062] S2, using a double-layer coating machine, the first slurry is introduced into the lower die, and the second slurry is introduced into the upper die, and the first slurry and the second slurry are simultaneously layered coated on the surface of the negative current collector, wherein the first slurry is in direct contact with the negative current collector, to obtain a single-sided coated electrode piece;

[0063] S3, after drying the single-sided coated electrode piece, the other side is coated and dried by repeating step S2, and then roll pressing, slitting, and punching processing are sequentially performed to obtain a fast-charging negative electrode piece.

[0064] In the negative electrode composite active material layer on either side of the fast-charging negative electrode piece, the thickness of the bottom layer of negative electrode active material is 40 μm, and the thickness of the surface layer of negative electrode active material is 80 μm.

[0065] 2. Preparation of positive electrode piece

[0066] In some embodiments, the positive electrode piece is prepared by the following steps:

[0067] A1, a bottom layer of positive electrode active material NCM811 (particle size meets D10=10 μm, D50=14 μm, D90=18 μm), a third conductive agent, and a third binder are mixed in a mass ratio of 90:5:5 to obtain a third slurry; a surface layer of positive electrode active material (particle size meets D10=4 μm, D50=9 μm, D90=13 μm), a fourth conductive agent, and a fourth binder are mixed in a mass ratio of 90:3:7 to obtain a fourth slurry;

[0068] A2, the third slurry and the fourth slurry are simultaneously layered coated on the surface of the positive current collector, wherein the third slurry is in direct contact with the negative current collector, to obtain a single-sided coated positive electrode piece;

[0069] A3, after drying the single-sided coated positive electrode piece, the other side is coated and dried by repeating step A2, and then roll pressing, slitting, and punching processing are sequentially performed to obtain a positive electrode piece.

[0070] The thickness of the bottom layer of the positive active material layer is 50 μm, and the thickness of the surface layer of the positive active material layer is 40 μm.

[0071] 3. Lithium battery assembly

[0072] The soft package battery is assembled by using the above positive electrode sheet, negative electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (the volume ratio of EC, DMC and EMC is 1:1:1) electrolyte, PE+aluminum oxide separator and a conventional production process.

[0073] Example 3

[0074] S1, a first silicon-based active material (particle size meets D10=3 μm, D50=6 μm, D90=9 μm), first graphite particles (particle size meets D10=6 μm, D50=12 μm, D90=17 μm), a first conductive agent and a first binder are mixed in a mass ratio of 60:15:1:8 to obtain a first slurry; a second silicon-based active material (particle size meets D10=8 μm, D50=14 μm, D90=21 μm), second graphite particles (particle size meets D10=13 μm, D50=19 μm, D90=24 μm), a second conductive agent and a second binder are mixed in a mass ratio of 15:60:1:8 to obtain a second slurry;

[0075] S2, using a double-layer coating machine, the first slurry is introduced into the lower die, and the second slurry is introduced into the upper die, and the first slurry and the second slurry are simultaneously layered coated on the surface of the negative current collector, wherein the first slurry is in direct contact with the negative current collector, to obtain a single-sided coated electrode sheet;

[0076] S3, after drying the single-sided coated electrode sheet, repeat step S2 to coat the other side and dry, and then sequentially perform rolling, cutting and punching processing to obtain a fast-charging negative electrode sheet.

[0077] The thickness of the bottom layer of the negative active material layer is 130 μm, and the thickness of the surface layer of the negative active material layer is 190 μm.

[0078] 2. Preparation of positive electrode sheet

[0079] In some embodiments, the positive electrode sheet is prepared by the following steps:

[0080] A1, the bottom layer positive electrode active material NCM811 (particle size meets D10=12 μm, D50=16 μm, D90=22 μm), the third conductive agent, the third binder are mixed according to the mass ratio of 95:1:4 to obtain the third slurry; the surface layer positive electrode active material (particle size meets D10=7 μm, D50=11 μm, D90=15 μm), the fourth conductive agent, the fourth binder are mixed according to the mass ratio of 95:2:3 to obtain the fourth slurry;

[0081] A2, the third slurry and the fourth slurry are simultaneously layered and coated on the surface of the positive electrode current collector, wherein the third slurry is in direct contact with the negative electrode current collector, to obtain a single-sided coated positive electrode sheet;

[0082] A3, the single-sided coated positive electrode sheet is dried, and the other side is coated and dried again by repeating step A2, and then roll pressing, slitting and punching processing are sequentially performed to obtain a positive electrode sheet.

[0083] The thickness of the bottom layer positive electrode active material layer in the negative electrode composite active material layer on either side of the positive electrode sheet is 150 μm, and the thickness of the surface layer positive electrode active material layer is 200 μm.

[0084] 3. Lithium battery assembly

[0085] The above positive electrode sheet, negative electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (wherein the volume ratio of EC, DMC and EMC is 1:1:1) electrolyte, PE+aluminum oxide separator and shell are assembled into a soft package battery by using a conventional production process.

[0086] Example 4

[0087] The difference between this embodiment and example 1 is that the particle size of the first silicon-based active material meets D10=5 μm, D50=10 μm, D90=15 μm; the particle size of the first graphite particle meets D10=2 μm, D50=5 μm, D90=7 μm; the other steps and parameter settings are consistent with example 1.

[0088] Example 5

[0089] The difference between this embodiment and example 1 is that the mass ratio of the first silicon-based active particle to the first graphite particle is 55:33; the other steps and parameter settings are consistent with example 1.

[0090] Example 6

[0091] The embodiment differs from example 1 in that the particle size of the second silicon-based active material satisfies D10 = 11 μm, D50 = 17 μm, and D90 = 22 μm, and the particle size of the second graphite particle satisfies D10 = 6 μm, D50 = 11 μm, and D90 = 18 μm; other steps and parameter settings are consistent with example 1.

[0092] Example 7

[0093] The embodiment differs from example 1 in that the mass ratio of the second silicon-based active particle to the second graphite particle is 12:76; other steps and parameter settings are consistent with example 1.

[0094] Example 8

[0095] The embodiment differs from example 1 in that the mass ratio of the second silicon-based active particle to the second graphite particle is 25:55; other steps and parameter settings are consistent with example 1.

[0096] Example 9

[0097] The embodiment differs from example 1 in that the thickness of the bottom layer of negative electrode active material is 10 μm, and the thickness of the surface layer of negative electrode active material is 200 μm; other steps and parameter settings are consistent with example 1.

[0098] Example 10

[0099] The embodiment differs from example 1 in that the thickness of the bottom layer of negative electrode active material is 150 μm, and the thickness of the surface layer of negative electrode active material is 60 μm; other steps and parameter settings are consistent with example 1.

[0100] Example 11

[0101] The embodiment differs from example 1 in that the particle size of the bottom layer of positive electrode active material satisfies D10 = 5 μm, D50 = 10 μm, and D90 = 14 μm; the particle size of the surface layer of positive electrode active material satisfies D10 = 11 μm, D50 = 15 μm, and D90 = 20 μm; other steps and parameter settings are consistent with example 1.

[0102] Example 12

[0103] The embodiment differs from example 1 in that the particle size of the bottom layer of positive electrode active material satisfies D50 = 18 μm; the particle size of the surface layer of positive electrode active material satisfies D50 = 12 μm; other steps and parameter settings are consistent with example 1.

[0104] Example 13

[0105] The difference between this example and Example 1 is that the positive electrode sheet comprises a positive electrode current collector and a single-layer positive electrode active material layer disposed on the surface of the positive electrode current collector, the thickness of the single-layer positive electrode active material layer is 220 μm, the particle size of the positive electrode active material in the single-layer positive electrode active material layer satisfies D10 = 11 μm, D50 = 15 μm, and D90 = 20 μm; the other steps and parameter settings are consistent with those of Example 1.

[0106] Example 14

[0107] The difference between this example and Example 1 is that the positive electrode sheet comprises a positive electrode current collector and a single-layer positive electrode active material layer disposed on the surface of the positive electrode current collector, the thickness of the single-layer positive electrode active material layer is 220 μm, the particle size of the positive electrode active material in the single-layer positive electrode active material layer satisfies D10 = 5 μm, D50 = 10 μm, and D90 = 14 μm; the other steps and parameter settings are consistent with those of Example 1.

[0108] Example 15

[0109] The difference between this example and Example 1 is that the thickness of the bottom positive electrode active material layer is 200 μm, and / or the thickness of the surface positive electrode active material layer is 20 μm; the other steps and parameter settings are consistent with those of Example 1.

[0110] Comparative Example 1

[0111] The difference between this example and Example 1 is that the negative electrode sheet comprises a negative electrode current collector and a single-layer negative electrode active material layer disposed on the surface of the negative electrode current collector, the thickness of the single-layer negative electrode active material layer is 210 μm; the single-layer negative electrode active material layer comprises a silicon-based negative electrode material and graphite particles, and the mass ratio of the silicon-based negative electrode material to the graphite particles is 68:20; the other steps and parameter settings are consistent with those of Example 1.

[0112] Comparative Example 2

[0113] The difference between this example and Example 1 is that the first silicon-based active material (particle size satisfies 5 μm < D10 < 8 μm, 9 μm < D50 < 14 μm, and 16 μm < D90 < 21 μm), the first graphite particle (particle size satisfies 10 μm < D10 < 13 μm, 16 μm < D50 < 19 μm, and 21 μm < D90 < 24 μm); the second silicon-based active material (particle size satisfies 1 μm < D10 < 3 μm, 3 μm < D50 < 6 μm, 6 μm < D90 < 9 μm, and silicon content), the second graphite particle (particle size satisfies 4 μm < D10 < 6 μm, 8 μm < D50 < 12 μm, and 14 μm < D90 < 17 μm); the other steps and parameter settings are consistent with those of Example 1.

[0114] Comparative Example 3

[0115] The difference between this embodiment and Example 1 is that the silicon in the bottom layer negative active material accounts for 20% of the total silicon mass, and the silicon in the surface layer negative active material accounts for 80% of the total silicon mass; other steps and parameter settings are consistent with those of Example 1.

[0116] Test method

[0117] I. Rate performance test

[0118] The lithium batteries of the above examples and comparative examples were subjected to rate performance test, and the specific test method was as follows:

[0119] 6C rate charging: 1C constant current discharge to 2.5V at 25℃, standing for 10min, 6C constant current constant voltage charging to 4.2V, 0.05C cutoff, recording the lithium battery constant current charging capacity, constant current constant voltage total charging capacity and the highest temperature in the fast charging process, constant current charging ratio = constant current charging capacity / constant current constant voltage total charging capacity x 100%.

[0120] 6C rate discharge: 1C constant current discharge to 2.5V at 25℃, standing for 10min, 1C constant current constant voltage charging to 4.2V, 0.05C cutoff, standing for 10min, 1C constant current discharge to 2.5V, standing for 10min, 1C constant current constant voltage charging to 4.2V, 0.05C cutoff, standing for 10min, 6C constant current discharge to 2.5V, recording 1C and 6C discharge capacity, 6C constant current discharge retention rate = 6C discharge capacity / 1C discharge capacity x 100%.

[0121] II. Cycle performance test

[0122] The lithium batteries of the above examples and comparative examples were subjected to cycle performance test, and the specific test method was as follows: 2C constant current constant voltage charging to 4.2V at 25℃, 0.05C cutoff, standing for 10min, 2C constant current discharge to 2.5V, standing for 10min, cycling for 500 times, recording the capacity retention rate of the lithium battery in 2C / 2C cycle for 500 times.

[0123] III. Energy density test

[0124] The lithium batteries of the above examples and comparative examples were subjected to energy density test, and the specific test method was as follows: weighing the weight of the test battery, recorded as m; placing the battery in a fixture and applying a force of 3000N, the single battery was charged to 4.3V at a constant current of 0.33C, standing for 30min, discharged to 2.5V at a constant current of 0.33C, standing for 30min, and continuously cycled for 3 times; calculating the discharge capacity (in Ah) and energy E (average value of three cycles), and the discharge energy density = E / m (in Wh / kg).

[0125] Table 1

[0126]

[0127] In combination with Examples 1-3, Comparative Examples 1-3 and Table 1, it can be seen that by arranging the negative electrode bottom layer active material layer and the negative electrode surface layer active material layer on the surface of the negative electrode current collector and keeping the particle size D50 of the bottom layer negative electrode active material smaller than the particle size D50 of the surface layer negative electrode active material, not only is it conducive to adjusting the porosity and pore structure in the negative electrode composite active material layer, conducive to the infiltration of the electrolyte to the negative electrode bottom layer active material layer close to the current collector, and conducive to the embedding and de-embedding of lithium ions, but also helps to improve the rate performance of the lithium battery. In addition, the arrangement of the negative electrode bottom layer active material layer and the negative electrode surface layer active material layer can greatly alleviate the expansion of the silicon-containing negative electrode during fast charging and discharging, and help to improve the fast charging cycle performance of the lithium battery.

[0128] In combination with Examples 1, 4, 6 and Table 1, it can be seen that by controlling the particle size of the first silicon-based active material and the particle size of the first graphite particles, and the particle size of the second silicon-based active material and the particle size of the second graphite particles, it is helpful to form a pore structure that facilitates the transmission of lithium ions, and to form a space that can buffer the expansion of the silicon negative electrode; therefore, the rate performance and fast charging cycle performance of the lithium battery in Example 1 are superior to those of the lithium battery in Example 4; the rate performance and fast charging cycle performance of the lithium battery in Example 1 are superior to those of the lithium battery in Example 7.

[0129] In combination with Examples 1, 5, 7-8 and Table 1, it can be seen that when the mass ratio of the first silicon-based active particles to the first graphite particles is too small, the mass ratio of the second silicon-based active particles to the second graphite particles is too small, or the mass ratio of the second silicon-based active particles to the second graphite particles is too large, it is not conducive to forming a pore structure that facilitates the transmission of lithium ions in the negative electrode active material layer, at this time, the embedding and de-embedding process of lithium ions in the negative electrode active material layer cannot achieve a relatively dynamic balance, and lithium deposition may occur at the negative electrode, affecting the cycle performance, discharge capacity retention rate and high temperature performance of the lithium battery.

[0130] In combination with Examples 1, 9-10 and Table 1, it can be seen that when the thickness of the negative electrode bottom layer active material layer is too small and the thickness of the negative electrode surface layer active material layer is too thick, or when the thickness of the negative electrode bottom layer active material layer is too large and the thickness of the negative electrode surface layer active material layer is too small, the rate performance and fast charging cycle performance of the lithium battery decrease.

[0131] In combination with Example 1, Examples 11-15 and Table 1, it can be seen that by setting a suitable positive electrode sheet, it is beneficial to form a path for lithium ion transmission in the positive electrode bottom active material layer, which is beneficial to alleviate the dramatic change of lithium ion concentration, cooperates with the negative electrode sheet, realizes the embedding and extraction of lithium ion on the positive electrode side in the charging and discharging process, avoids the lithium deposition on the positive electrode side, and significantly improves the rate performance and cycle performance of the lithium battery.

Claims

1. A fast-charging negative electrode sheet, comprising a negative electrode current collector and a negative electrode composite active material layer disposed on at least one surface of the negative electrode current collector; the negative electrode composite active material layer comprises a negative electrode bottom layer active material layer close to the negative electrode current collector and a negative electrode surface layer active material layer away from the negative electrode current collector; a particle size D50 of a bottom layer negative electrode active material in the negative electrode bottom layer active material layer is less than a particle size D50 of a surface layer negative electrode active material in the negative electrode surface layer active material layer; silicon in the bottom layer negative electrode active material accounts for 60%-80% of a total silicon mass, and silicon in the surface layer negative electrode active material accounts for 20%-40% of the total silicon mass.

2. The fast-charge negative electrode sheet of claim 1, wherein: the bottom layer negative electrode active material comprises first silicon-based active particles and first graphite particles; a particle size of the first silicon-based active particles satisfies 3 μm < D50 < 6 μm, and a particle size of the first graphite particles satisfies 8 μm < D50 < 12 μm; 3. The fast-charge negative electrode sheet of claim 2, wherein: a mass ratio of the first silicon-based active particles to the first graphite particles is 60-75:15-20.

4. The fast-charge negative electrode sheet of claim 1, wherein: the surface layer negative electrode active material comprises second silicon-based active particles and second graphite particles; a particle size of the second silicon-based active particles satisfies 9 μm < D50 < 14 μm, and a particle size of the second graphite particles satisfies 16 μm < D50 < 19 μm.

5. The fast-charge negative electrode sheet of claim 4, wherein: a mass ratio of the second silicon-based active particles to the second graphite particles is 15-20:60-75.

6. The fast-charge negative electrode sheet according to any one of claims 1 to 5, wherein: a thickness of the bottom layer negative electrode active material layer is 30-130 μm, and / or a thickness of the surface layer negative electrode active material layer is 80-190 μm. 7.A lithium battery, comprising a positive electrode sheet, an electrolyte and the fast-charging negative electrode sheet according to any one of claims 1-6.

8. The lithium battery of claim 7, wherein: the positive electrode sheet comprises a positive electrode current collector and a positive electrode composite active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode composite active material layer comprises a positive electrode bottom layer active material layer close to the positive electrode current collector and a positive electrode surface layer active material layer away from the positive electrode current collector; a particle size D50 of a bottom layer positive electrode active material in the positive electrode bottom layer active material layer is greater than a particle size D50 of a surface layer positive electrode active material in the positive electrode surface layer active material layer.

9. The lithium battery of claim 8, wherein: a particle size of the bottom layer positive electrode active material satisfies 14 μm < D50 < 16 μm; and a particle size of the surface layer positive electrode active material satisfies 9 μm < D50 < 11 μm.

10. The lithium battery according to any one of claims 8-9, wherein: a thickness of the bottom layer positive electrode active material layer is 50-150 μm, and / or a thickness of the surface layer positive electrode active material layer is 40-200 μm.

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