Negative pole piece, secondary battery and device

Abstract

The invention provides a negative electrode piece, a secondary battery and a device, the negative electrode piece comprises a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-based material, and the silicon-based material is arranged on the surface of the negative electrode current collector. The negative pole piece meets the following relational expression: I / S is greater than or equal to 0.6 and less than or equal to 1.63; wherein I is the structural strength of the silicon-based material, and the structural strength refers to the ratio of the volume average particle size Dv50 of the silicon-based material after powder pressing under the pressure of 20 KN to the volume average particle size Dv50 before powder pressing; s is the full-filling expansion rate of the silicon-based material. According to the secondary battery, the cycle performance of the battery is effectively improved by adjusting the relationship between the full-charge expansion rate and the structural strength of the negative electrode active material.

Description

Technical Field

[0001] The present application relates to a negative electrode plate, and in particular to a negative electrode plate, a secondary battery and a device including the secondary battery, belonging to the field of batteries. Background Art

[0002] As the requirements for battery life of electric vehicles increase, power batteries will gradually use silicon negative electrode materials. However, the volume expansion effect of silicon materials is large, which causes the performance of power batteries to continue to decline. Therefore, constructing silicon-based materials with lower lithium insertion expansion rate will be a beneficial means to effectively improve the performance of power batteries.

[0003] Therefore, there is an urgent need to develop a novel secondary battery including silicon-based materials. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present application provides a negative electrode plate, a secondary battery and a device. The secondary battery of the present application effectively improves the cycle performance of the secondary battery by adjusting the relationship between the full charge expansion rate and the structural strength of the negative electrode active material.

[0005] The first aspect of the present application provides a negative electrode plate, which includes a negative electrode collector and a negative electrode active material layer arranged on the surface of the negative electrode collector, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material. The negative electrode plate satisfies the following relationship: 1.63≥I / S≥0.6; wherein I is the structural strength of the silicon-based material, wherein the structural strength refers to the ratio of the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN to the volume average particle size Dv50 before powder pressing; S is the full charge expansion rate of the silicon-based material.

[0006] A second aspect of the present application provides a secondary battery, which includes the negative electrode sheet and the positive electrode sheet according to the first aspect.

[0007] A third aspect of the present application provides a device comprising the secondary battery according to the second aspect.

[0008] The present application improves the structural strength of silicon-based materials while reducing the lithium-insertion volume expansion rate of silicon-based materials, thereby effectively improving the cycle performance of secondary battery silicon-containing negative electrodes. DETAILED DESCRIPTION

[0009] For simplicity, this application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, and any upper limit can be combined with any other upper limit to form an undefined range. In addition, each separately disclosed point or single value can itself be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an undefined range.

[0010] Unless otherwise specified, the terms used in this application have the commonly known meanings generally understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0011] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0012] As used herein, the term "structural strength" refers to the ratio of the volume average particle size Dv50 of the silicon-based material after powder compaction under a pressure of 2t (20KN) to the volume average particle size Dv50 before powder compaction.

[0013] As used herein, the term "full charge expansion ratio of silicon-based materials" refers to the full charge expansion ratio of a negative electrode sheet using silicon as the active material.

[0014] As used herein, the term "Dv50" refers to the particle size corresponding to when the cumulative particle size volume distribution percentage reaches 50%, wherein the particle size Dv50 of the silicon-based material can be measured by methods known in the art, such as using a laser particle size analyzer.

[0015] As used herein, the term "full charge expansion rate of silicon-carbon material" refers to (Ts1-Ts0) / Ts0×100%, wherein Ts0 is the thickness of the pure silicon negative electrode plate after cold pressing rebound (30μm~80μm), and Ts1 is the thickness of the pure silicon negative electrode plate after full charge.

[0016] Determination of the full charge expansion rate of the negative electrode: The thickness of the negative electrode after cold pressing and rebounding is recorded as T0, and the cold pressed negative electrode, positive electrode, separator and electrolyte are made into a full battery. At 25°C, the full battery after capacity conversion is charged and discharged with a voltage range of 2.5-4.25V. First, it is charged to 4.25V at a constant current of 0.33C, then charged to a current of 0.05C at a constant voltage. After standing for 10 minutes, it is discharged to 2.5V at a constant current of 0.33C. The charge and discharge cycle is repeated for 3 weeks. Then the full battery is charged to 100% charge state, the full battery is disassembled and the thickness of the negative electrode is tested, which is recorded as T1. The full charge expansion rate of the negative electrode is: (T1-T0) / T0×100%.

[0017] Determination of full filling expansion rate of silicon carbon material:

[0018] The thickness of the pure silicon negative electrode after cold pressing and rebounding is recorded as Ts0, and the cold pressed negative electrode is made into a button battery with a lithium sheet, a separator, and an electrolyte. At 25°C, the button battery is charged and discharged with a voltage range of 0.005-2V. First, it is discharged to 0.005V at a constant current of 0.1C, and then it is discharged to 0.005V at a constant current of 0.01C. After standing for 10 minutes, it is then discharged to 0.005V at a constant current of 0.01C. After standing for 30 minutes, it is charged to 2.0V at a constant current of 0.1C. The charge and discharge cycle is repeated for 3 weeks. Then the button battery is discharged to 0.005V at a current of 0.1C, the button battery is disassembled and the thickness of the negative electrode is tested, which is recorded as Ts1. The full charge expansion rate of the negative electrode is: (Ts1-Ts0) / Ts0×100%.

[0019] 1. Negative electrode

[0020] The negative electrode sheet provided by the present application includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, the negative electrode active material includes a silicon-based material, and the negative electrode sheet satisfies the following relationship: 1.63≥I / S≥0.6; wherein, I is the structural strength of the silicon-based material, wherein the structural strength refers to the ratio of the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN to the volume average particle size Dv50 before powder pressing; S is the full charge expansion rate of the silicon-based material. The higher the structural strength of the silicon-based material, the more it can resist cracking and crushing caused by expansion changes, and improve the cycle performance; the huge volume expansion of the silicon-based material during the lithiation process may cause the pulverization of the material, aggravate the side reaction between the material and the electrolyte, cause structural loss and active lithium loss, and form a thicker SEI film. Therefore, the smaller the expansion rate of the silicon-based material, the easier it is to exert the performance of the battery cell. The present application effectively improves the cycle performance of the battery by adjusting the relationship between the full charge expansion rate and the structural strength of the negative electrode active material.

[0021] In some embodiments, 1.58 ≥ I / S ≥ 0.65; if I / S is too small, it indicates that silicon undergoes a large volume expansion during the charge and discharge process, or the structural strength of the material is insufficient to resist the expansion change, further causing the silicon particles to pulverize, resulting in deterioration of the cycle performance. If I / S is too large, there may be deterioration of the battery cell dynamics. In some embodiments, I / S is 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.58 or any range therebetween.

[0022] In some embodiments, 1.0≥I≥0.68; silicon will expand and shrink in volume during the charge and discharge process, and this structural change will cause silicon to crack and pulverize. If I is too small, it will not be conducive to resisting the cracks caused by the expansion change, which will lead to deterioration of the cycle performance. In some embodiments, I is 0.68, 0.71, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0 or any range therebetween; in some embodiments, 0.98≥I≥0.66.

[0023] In some embodiments, 57% ≤ S ≤ 125%; if S is too large, the huge volume expansion of silicon particles during the lithiation process may cause the particles to pulverize, aggravate the side reaction between the material and the electrolyte, lead to structural loss and active lithium loss, and form a thicker SEI. Therefore, it is necessary to reduce the bulk expansion of the silicon material. In some embodiments, S is 57%, 60%, 70%, 80%, 90%, 100%, 110%, 120%, 125% or any range therebetween; in some embodiments, 60% ≤ S ≤ 122%.

[0024] In some embodiments, the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN satisfies 5μm≤Dv50≤10μm; the volume average particle size Dv50 after powder pressing is too large, indicating that the silicon-carbon material has high compressive strength and is not easy to crush, which is conducive to the improvement of cycle performance; the volume average particle size Dv50 after powder pressing is too small, which will increase the active specific surface area and cause more irreversible lithium loss, indicating that the silicon-carbon material has a lower compressive resistance. In some embodiments, the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN is 5μm, 6μm, 6.5μm, 7.0μm, 7.5μm, 8.0μm, 8.5μm, 9μm, 10μm or any range therebetween; in some embodiments, the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN satisfies 6μm≤Dv50≤9.5μm.

[0025] In some embodiments, the volume average particle size Dv50 of the silicon-based material before powder pressing under a pressure of 20KN satisfies 6μm≤Dv50≤11μm; if the volume average particle size Dv50 before powder pressing is too large, the lithium ion migration channels will be reduced between large particles, making it more difficult for lithium ions to be embedded and removed, resulting in poor kinetics; if the volume average particle size Dv50 before powder pressing is too small, its active effective surface area will increase, increasing the side reaction with the electrolyte. In some embodiments, the volume average particle size Dv50 of the silicon-based material before powder pressing under a pressure of 20KN is 6μm, 7μm, 7.5μm, 8.0μm, 8.5μm, 9μm, 10μm, 11μm or any range therebetween; in some embodiments, the volume average particle size Dv50 of the silicon-based material before powder pressing under a pressure of 20KN satisfies 7μm≤Dv50≤10μm.

[0026] In some embodiments, the negative electrode active material further comprises graphite. In some embodiments, the silicon-based material comprises at least one of silicon, silicon alloys, silicon-carbon compounds, silicon-oxygen compounds, and a composite of elemental silicon and graphite.

[0027] According to the embodiment of the present application, the preparation of the silicon-based material includes the following steps: firstly, carbonizing and activating the hard carbon material to obtain carbon particles with a porous structure, then introducing silane gas into the pores of the carbon particles with a porous structure, and precipitating the gas into silicon nanoparticles dispersed in the pores of the porous carbon through high-temperature pyrolysis to obtain the silicon-based material. Of course, the preparation method of the silicon-based material is not limited to this, but can also be prepared by other methods well known in the art.

[0028] In some embodiments, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a nickel-cobalt-based ternary material and a phosphate-based material.

[0029] In some embodiments, the nickel-cobalt ternary material includes LiNi m Co n At least one of A(1-mn)O2 materials, A is selected from at least one of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver and niobium, 0.5≤m≤1, 0≤n≤0.5, m+n≤1.

[0030] In some embodiments, m is 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two of these values. In some embodiments, n is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or a range consisting of any two of these values.

[0031] In some embodiments, the nickel-cobalt-based ternary material includes at least one of NCA, NCM111, NCM523, NCM622, NCM811, Ni90, Ni92, and Ni95.

[0032] In some embodiments, the phosphate-based material includes LiMn k B (1-k) PO4, wherein 0≤k≤1, and the B element is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium and lead. In some embodiments, k is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 or a range consisting of any two of these values. In some embodiments, the phosphate-based material includes lithium iron phosphate, LiMn 0.6 Fe 0.4 PO4 and LiMn 0.8 Fe 0.2 At least one of PO4.

[0033] In some embodiments, the positive electrode active material includes at least one of lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese cobalt magnesium oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate.

[0034] In some embodiments, the positive electrode active material layer further includes a binder, and optionally a conductive material. The binder improves the bonding of the positive electrode active material particles to each other and also improves the bonding of the positive electrode active material to the current collector.

[0035] In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0036] In some embodiments, the conductive material includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0037] In some embodiments, the positive electrode further includes a positive electrode current collector, which may be a metal foil or a composite current collector. For example, aluminum foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0038] In some embodiments, the negative electrode includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material, or a mixture of a silicon-based material and at least one material selected from a carbon-based material, a tin-based material, a phosphorus-based material, and metallic lithium.

[0039] In some embodiments, the negative electrode active material layer further comprises a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc.

[0040] In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0041] In some embodiments, the negative electrode further includes a negative electrode current collector, and the negative electrode current collector includes: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0042] In some embodiments, a separator is provided between the positive electrode and the negative electrode to prevent short circuit. The material and shape of the separator that can be used in the embodiments of the present application are not particularly limited, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed by a material that is stable to the electrolyte of the present application. In some embodiments, the separator can be selected from polyethylene film, polypropylene film, polyvinylidene fluoride film and their multilayer composite films.

[0043] For example, the isolation film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0044] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0045] The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene.

[0046] The polymer layer contains polymers, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride and poly(vinylidene fluoride-hexafluoropropylene).

[0047] When the secondary battery is a lithium-ion secondary battery, a lithium salt solution dissolved in an organic solvent is usually used as a non-aqueous electrolyte. The lithium salt may be an inorganic lithium salt (e.g., LiClO4, LiPF6, LiBF4, LiAsF6, LiSbF6), or an organic lithium salt (e.g., LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiN(CF3SO2)2, LiC(CF3SO2)3, LiCnF2n+1SO3 (n≥2)). The organic solvents used in the non-aqueous electrolyte include cyclic carbonates (such as ethylene carbonate, propylene carbonate, butylene carbonate or vinylene carbonate), chain carbonates (such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate), (chain esters, such as methyl propionate), cyclic esters (such as γ-butyrolactone), dimethoxyethane, chain ethers (such as diethyl ether, diethylene glycol dimethyl ether or triethylene glycol dimethyl ether), cyclic ethers (such as tetrahydrofuran or 2-methyltetrahydrofuran), acetonitrile, propionitrile or a combination thereof.

[0048] Lithium-ion secondary batteries are composed of negative electrode plates, positive electrode plates, separators and electrolytes. The positive and negative electrodes are immersed in the electrolyte. Lithium ions move between the positive and negative electrodes with the electrolyte as the medium to realize the charging and discharging of the battery. In order to avoid short circuits between the positive and negative electrodes through the electrolyte, a separator is required to separate the positive and negative electrodes. The shape of lithium-ion secondary batteries can be, for example, a cylindrical (square cylindrical or cylindrical) with an aluminum shell or a steel shell as the outer shell, or a soft-pack battery with an aluminum-plastic film as the outer shell.

[0049] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

[0050] In some embodiments, the secondary battery may include an outer package, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft shell, and the material of the soft shell may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0051] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other shape.

[0052] In some embodiments, the present application further provides a battery module. The battery module includes the above-mentioned secondary battery. The battery module of the present application uses the above-mentioned secondary battery, and therefore has at least the same advantages as the secondary battery. The number of secondary batteries contained in the battery module of the present application can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0053] In some embodiments, the present application further provides a battery pack, which includes the above-mentioned battery module. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0054] 2. Device

[0055] The present application also provides a device, which includes at least one of the above-mentioned secondary battery, battery module and battery pack.

[0056] In some embodiments, the device includes, but is not limited to: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. In order to meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0057] In other embodiments, the device may be a mobile phone, a tablet computer, a notebook computer, etc. The device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0058] Examples and Comparative Examples

[0059] In order to make the purpose, technical scheme and beneficial technical effect of the present application clearer, the present application is further described in detail below in conjunction with the examples. However, it should be understood that the examples of the present application are only for explaining the present application, not for limiting the present application, and the examples of the present application are not limited to the examples given in the specification. The specific experimental conditions or operating conditions not specified in the examples are made under conventional conditions, or are made under the conditions recommended by the material supplier.

[0060] The silicon-carbon materials and artificial graphite in the following examples and comparative examples are all commercially available.

[0061] Example 1

[0062] Preparation of positive electrode:

[0063] The positive electrode active material LiNi 0.9 Co 0.03 Mn 0.07 O2, conductive agent Super P / multi-walled carbon nanotubes and binder PVDF are dispersed in a proper amount of NMP in a mass ratio of 97.5:0.9 / 0.5:1.1 to form a uniform positive electrode slurry; the positive electrode slurry is coated on aluminum foil, and after drying, rolling and other processes, a positive electrode sheet is obtained.

[0064] Preparation of negative electrode sheet:

[0065] The silicon-carbon material and artificial graphite of the present application are premixed in a certain mass ratio (mass ratio of 1:9) to obtain a composite negative electrode active material, and the composite negative electrode active material, conductive agent carbon black / carbon nanotubes, thickener CMC, binder SBR and PAA are homogenized in a mass ratio of 96:0.9 / 0.1:0.6:1.2:1.2 to obtain a uniformly dispersed slurry, and the above slurry is coated on a 6μm or 8μm copper foil, dried at 100°C, and then roller pressed to obtain a negative electrode sheet.

[0066] Preparation of electrolyte:

[0067] LiPF6 was dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate (volume ratio 1:1:1), and 10 wt% of fluoroethylene carbonate was added as a film-forming additive to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0068] Release film: Polyethylene release film.

[0069] Outer packaging: aluminum plastic film.

[0070] Preparation of lithium-ion batteries

[0071] The positive electrode sheet, separator and negative electrode sheet are stacked in order, so that the separator is between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, and the prepared electrolyte is injected into the dried bare cell. After vacuum packaging, standing, forming, shaping and other processes, a lithium-ion secondary battery is obtained.

[0072] Examples 2-10 and Comparative Examples 1-3 are achieved by adjusting the structural strength and full-fill expansion rate of the silicon-carbon material on the basis of Example 1. Specific adjustment measures and detailed data are shown in Table 1.

[0073] Test Method

[0074] 1. Determination of structural strength of silicon-carbon materials

[0075] 3g of silicon-carbon powder was pressed with a weight of 2t for 30s on a powder compactor. The pressed silicon-carbon powder was taken out and the particle size distribution test was performed. The particle size D50 corresponding to the cumulative particle size volume distribution percentage reaching 50% was recorded. Structural strength I = Dv50 of silicon-carbon material after 2t pressure treatment (recorded as Dv50-2t) / Dv50 of silicon-carbon material before treatment (recorded as Dv50-initial)

[0076] 2. Determination of full filling expansion rate of silicon-carbon materials

[0077] Preparation of pure silicon negative electrode: Silicon-based material (negative electrode active material), conductive agent carbon black and PAA are homogenized in a mass ratio of 80:10:10 to obtain a uniformly dispersed slurry, which is then coated on a 6μm or 8μm copper foil, dried at 100°C, and then rolled to obtain a pure silicon negative electrode for later use. The single-sided surface density of the obtained pure silicon negative electrode is 4mg / cm 2 , compacted density is 1.0g / cm 3 .

[0078] The thickness of the pure silicon negative electrode plate after cold pressing and rebounding is recorded as Ts0, and the cold pressed negative electrode plate, lithium plate, isolation film, and electrolyte are made into a button battery. At 25°C, the button battery is charged and discharged with a voltage range of 0.005-2V. First, it is discharged to 0.005V at a constant current of 0.1C, and then it is discharged to 0.005V at a constant current of 0.01C. After standing for 10 minutes, it is then discharged to 0.005V at a constant current of 0.01C. After standing for 30 minutes, it is charged to 2.0V at a constant current of 0.1C. The charge and discharge cycle is repeated for 3 weeks. Then the button battery is discharged to 0.005V at a current of 0.1C, the button battery is disassembled and the thickness of the negative electrode plate is tested, which is recorded as Ts1. The full charge expansion rate of the negative electrode plate is: (Ts1-Ts0) / Ts0×100%.

[0079] 3. Determination of the expansion rate of the negative electrode when fully charged

[0080] The thickness of the negative electrode after cold pressing and rebounding is recorded as T0, and the cold pressed negative electrode, positive electrode, separator and electrolyte are made into a full battery. At 25°C, the full battery after capacity conversion is charged and discharged with a voltage range of 2.5-4.25V. First, it is charged to 4.25V at 0.33C constant current, then charged to 0.05C at constant voltage, and then discharged to 2.5V at 0.33C constant current after standing for 10 minutes; the charge and discharge cycle is repeated for 3 weeks. Then the full battery is charged to 100% charge state, the full battery is disassembled and the thickness of the negative electrode is tested, which is recorded as T1. The full charge expansion rate of the negative electrode is: (T1-T0) / T0×100%.

[0081] 4. Determination of cycle performance

[0082] At 25°C, first charge at 1C constant current to 4.25V, then charge at constant voltage to a current of 0.05C. After standing for 10 minutes, discharge at 1C constant current to 2.5V. Perform a cycle test in this full charge and discharge form until the discharge capacity of the lithium-ion battery decays to 80% of the initial capacity, and record the number of cycles at this time.

[0083] The test results are shown in Table 1.

[0084] Table 1

[0085]

[0086] As can be seen from Table 1, the full charge expansion rate of the negative electrode sheet of the present application is less than 33%, and after being assembled into a secondary battery with a ternary positive electrode, more than 1000 cycles can be achieved. Therefore, the lithium-ion battery of the present application exhibits excellent cycle stability.

[0087] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A negative electrode plate, comprising a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, wherein the negative electrode active material comprises a silicon-based material, and wherein the negative electrode plate satisfies the following relationship: 1.63≥I / S≥0.6; wherein, I is the structural strength of the silicon-based material, wherein the structural strength refers to the ratio of the volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN to the volume average particle size Dv50 before powder pressing; S is the full charge expansion ratio of the silicon-based material.

2. The negative electrode sheet according to claim 1, characterized in that: 1.58≥I / S≥0.

65.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: 1.0≥I≥0.68; and / or 57%≤S≤125%.

4. The negative electrode sheet according to claim 3, characterized in that: 0.98≥I≥0.66; and / or 60%≤S≤122%.

5. The negative electrode sheet according to claim 1 or 2, characterized in that: The volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN satisfies 5μm≤Dv50≤10μm, and / or The volume average particle size Dv50 of the silicon-based material before powder pressing under a pressure of 20KN satisfies 6μm≤Dv50≤11μm.

6. The negative electrode sheet according to claim 5, characterized in that: The volume average particle size Dv50 of the silicon-based material after powder pressing under a pressure of 20KN satisfies 6μm≤Dv50≤9.5μm, and / or The volume average particle size Dv50 of the silicon-based material before powder pressing under a pressure of 20KN satisfies 7μm≤Dv50≤10μm.

7. The negative electrode sheet according to claim 1 or 2, characterized in that: The negative electrode active material further comprises graphite, and / or The silicon-based material includes at least one of silicon, silicon alloys, silicon-carbon compounds, silicon-oxygen compounds, and a composite of elemental silicon and graphite.

8. A secondary battery comprising the negative electrode sheet and the positive electrode sheet according to claim 1 or 2.

9. A device comprising the secondary battery according to claim 8.

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