Positive electrode sheet, secondary battery, and device

Abstract

The present invention provides a positive electrode sheet. The positive electrode sheet comprises a current collector and a material layer provided on the current collector; the material layer comprises a first active material and a second active material; the first active material comprises at least one of lithium nickel manganese cobalt oxide, lithium cobalt oxide, lithium manganese oxide, and lithium nickel manganese oxide; and the second active material comprises lithium iron phosphate and / or lithium iron manganese phosphate, and comprises a carbon coating layer. The positive electrode sheet satisfies the following relational expression: 0.5≤10,000×f×(g / h)×(j / k)≤150, wherein f represents a mass ratio of the second active material to the first active material; g represents a mass ratio of the carbon coating layer to the second active material; h represents the specific surface area of the first active material; j represents a Dv50 value of the first active material; and k represents a Dv50 value of the second active material. The positive electrode sheet has a good conductive network, thereby improving the low-temperature rate discharge capability of a lithium-ion battery.

Description

Positive electrode sheet, secondary battery and device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 20, 2023, with application number 202311221829.X and application name “Positive Electrode Plate, Secondary Battery and Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present disclosure relates to the technical field of energy storage materials, and in particular to a positive electrode sheet, a secondary battery including the electrode, and a device. Background Art

[0003] Lithium-ion batteries, with their outstanding advantages such as high energy density, low self-discharge, and lack of memory effect, have become the primary solution for mobile energy and are widely used in mobile phones, tablets, electric vehicles, drones, and other fields. With the expansion of lithium-ion battery applications, it is not only required that lithium-ion batteries have excellent performance at room temperature, but also that they perform in low-temperature environments. As the temperature decreases, the impedance of lithium-ion batteries will increase significantly, resulting in a decrease in discharge rate performance. This phenomenon is even more serious at low SOC states (for example, less than 20%), forcing electric vehicles to limit power output earlier, exacerbating users' anxiety about range.

[0004] Summary of the Invention

[0005] In response to the problems existing in the prior art, the present disclosure provides a positive electrode plate, which includes a current collector and a material layer arranged on the current collector; the material layer includes a first active material and a second active material, the first active material includes at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide and lithium nickel manganese oxide, the second active material includes lithium iron phosphate and / or lithium manganese iron phosphate, and the second active material includes a carbon coating layer.

[0006] In the present disclosure, the positive electrode sheet satisfies the following relationship:

[0007] 0.5≤10000×f×(g / h)×(j / k)≤150; wherein f is the mass ratio of the second active material to the first active material; g is the mass ratio of the carbon coating layer to the second active material; h is the specific surface area of ​​the first active material, in m 2 / g; j is the Dv50 value of the first active material, in μm; k is the Dv50 value of the second active material, in μm.

[0008] In one aspect of the present disclosure, the positive electrode sheet satisfies the following relationship:

[0009] 1.5≤10000×f×(g / h)×(j / k)≤78.

[0010] In one aspect of the present disclosure, preferably, the positive electrode sheet satisfies the following relationship:

[0011] 5≤10000×f×(g / h)×(j / k)≤25.

[0012] In one aspect of the present disclosure, the value of f is selected from 0.01 to 0.055.

[0013] In one aspect of the present disclosure, the value of g is selected from 0.005 to 0.05.

[0014] In one aspect of the present disclosure, preferably, the value of g is selected from 0.08 to 0.025.

[0015] In one aspect of the present disclosure, the value of h is selected from 0.5-2.

[0016] In one aspect of the present disclosure, preferably, the value of h is selected from 0.7 to 1.5.

[0017] In one aspect of the present disclosure, the value of j is selected from 1-8.

[0018] In one aspect of the present disclosure, preferably, the value of j is selected from 2-6.

[0019] In one aspect of the present disclosure, the value of k is selected from 0.2-3.

[0020] In one aspect of the present disclosure, preferably, the value of k is selected from 0.4 to 1.8.

[0021] In one aspect of the present disclosure, the value of g / h is selected from 0.005 to 0.055.

[0022] In one aspect of the present disclosure, the value of 10000×f×(g / h) is selected from 0.5 to 25.

[0023] In one aspect of the present disclosure, preferably, the value of 10000×f×(g / h) is selected from 2 to 15.

[0024] In one aspect of the present disclosure, the value of j / k is selected from 1-15.

[0025] In one aspect of the present disclosure, preferably, the value of j / k is selected from 2-9.

[0026] In one aspect of the present disclosure, the particle size distribution coefficient S of the first active material is selected from 0.5 to 2.5.

[0027] In one aspect of the present disclosure, the particle size distribution coefficient S of the second active material is selected from 0.5-12.

[0028] In one aspect of the present disclosure, the single particle mechanical strength of the first active material is selected from 20 MPa to 400 MPa.

[0029] In one aspect of the present disclosure, the ratio of the compacted density to the true density of the first active material is selected from 0.58 to 0.8.

[0030] In one aspect of the present disclosure, the ratio of the compacted density to the true density of the second active material is selected from 0.5 to 0.8.

[0031] The present disclosure also provides a method for preparing the aforementioned positive electrode sheet, the method comprising:

[0032] S1: mixing the first active material and the second active material, adding a solvent, kneading and stirring until uniform, to obtain a slurry;

[0033] S2: coating the slurry onto a current collector and drying the slurry to obtain the positive electrode sheet.

[0034] In one aspect of the present disclosure, the material layer further comprises a binder and a conductive agent.

[0035] In one aspect of the present disclosure, preferably, the single particle mechanical strength of the first active material is selected from 60 MPa to 250 MPa.

[0036] In one aspect of the present disclosure, the method for preparing the aforementioned positive electrode sheet includes the following:

[0037] S1: The first active material, the second active material, the binder, and the conductive agent are placed in a planetary vacuum disperser and dry-mixed. Then, N-methylpyrrolidone is added as a solvent, and the mixture is kneaded and stirred to obtain a slurry.

[0038] S2: coating the slurry onto an aluminum foil current collector and drying the slurry to obtain the positive electrode sheet.

[0039] The present disclosure also provides a secondary battery, which includes the aforementioned positive electrode sheet.

[0040] The present disclosure also provides a device comprising the aforementioned secondary battery.

[0041] The present disclosure has at least the following beneficial effects:

[0042] The present application controls the relationship between the mass ratio f of the second active material and the first active material, the mass ratio g of the carbon coating and the second active material, the specific surface area h of the first active material, the Dv50 value j of the first active material, and the Dv50 value k of the second active material, thereby controlling 0.5≤10000×f×(g / h)×(j / k)≤150, thereby ensuring that in a system containing the first active material and the second active material, the first active material and the second active material have good geometric configuration matching, and the carbon coating on the surface of the second active material can reduce the impedance between the active material particles. Especially under low temperature and low SOC conditions, the active material that meets this relationship has a good conductive network, improves the low temperature rate discharge capability of the lithium-ion battery, and has a longer cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG1 shows a SEM image of the positive electrode sheet in Example 1. DETAILED DESCRIPTION

[0044] To make the purpose, technical solutions, and advantages of the present disclosure more clear, the technical solutions of the present disclosure will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all the embodiments. The embodiments described herein are illustrative and are used to provide a basic understanding of the present disclosure. The embodiments of the present disclosure should not be interpreted as limiting the present disclosure.

[0045] the term:

[0046] For the sake of clarity, only some numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0047] As used herein, the terms "comprises," "comprising," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0048] In this document, it should be understood that the terms "upper", "lower", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0049] In the description herein, unless otherwise specified, “above” and “below” include the number itself.

[0050] Unless otherwise specified, the terms used in this disclosure have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this disclosure 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 disclosure).

[0051] The term "about" is used to describe and illustrate small variations. When used in conjunction with an event or situation, the term may refer to examples in which the event or situation occurs precisely as well as examples in which the event or situation occurs very approximately. For example, when used in conjunction with a numerical value, the term may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. In addition, amounts, ratios, and other numerical values ​​are sometimes presented herein in a range format. Such range formats are for convenience and brevity and should be flexibly understood to include not only the numerical values ​​explicitly designated as range limits, but also all individual numerical values ​​or subranges encompassed within the range, as if each numerical value and subrange were explicitly designated.

[0052] 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.

[0053] The terms "Dv10", "Dv50" and "Dv90" are the particle sizes corresponding to the cumulative particle size distribution percentages of 10%, 50% and 90%, respectively. In this article, the unit is μm; the particle size distribution coefficient S is calculated by (Dv90-Dv10) / Dv50.

[0054] The term "single particle mechanical strength" refers to the compressive strength obtained when an ultramicrodynamic hardness tester is used to press the indenter into a selected particle of the sample under a certain force until the particle is crushed.

[0055] The “specific surface area” referred to in this application is measured by the BET method.

[0056] The present disclosure will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present disclosure and are not used to limit the scope of the present disclosure.

[0057] Positive electrode:

[0058] In some embodiments of the present disclosure, the positive electrode plate includes:

[0059] a current collector; and

[0060] A material layer is provided on the current collector, wherein the material layer includes a first active material and a second active material,

[0061] The first active material includes at least one of lithium nickel manganese cobalt oxide, lithium cobalt oxide, lithium manganese oxide and lithium nickel manganese oxide,

[0062] The second active material includes lithium iron phosphate and / or lithium manganese iron phosphate, and the second active material includes a carbon coating layer,

[0063] The positive electrode sheet satisfies the following relationship: 0.5≤10000×f×(g / h)×(j / k)≤150;

[0064] in,

[0065] f is the mass ratio of the second active material to the first active material;

[0066] g is the mass ratio of the carbon coating layer to the second active material;

[0067] h is the specific surface area of ​​the first active material, in m 2 / g;

[0068] j is the Dv50 value of the first active material, in μm;

[0069] k is the Dv50 value of the second active material, in μm.

[0070] In some embodiments of the present disclosure, in a positive electrode sheet comprising both a first active material and a second active material, when the relationship 0.5≤10000×f×(g / h)×(j / k)≤150 is satisfied, the first active material and the second active material have good geometric configuration matching, the carbon coating layer on the surface can reduce the impedance between the particles, and a good conductive network is formed between the particles, thereby improving the low-temperature rate discharge capability of the battery and having a higher cycle life; when the relationship 5≤10000×f×(g / h)×(j / k)≤25 is satisfied, the first active material and the second active material have good geometric configuration matching, have a higher low-temperature rate discharge capability, and can further improve the cycle life of the battery.

[0071] In some embodiments of the present disclosure, the positive electrode current collector may be selected from aluminum foil, but is not limited thereto.

[0072] In some embodiments of the present disclosure, the mass ratio f of the second active material to the first active material is selected from 0.01 to 0.055. In the present disclosure, when the primary component of the positive electrode sheet is the first active material, the second active material having a carbon coating fills the voids in the first active material, thereby better constructing a conductive network and reducing inter-particle resistance.

[0073] In some embodiments of the present disclosure, the value of g / h is selected from 0.005 to 0.055.

[0074] In the present disclosure, the value of f×(g / h) represents the degree of matching between the second active material and the first active material. When the value is within a certain range, the second active material can be well filled and dispersed in the voids of the first active material, and the carbon coating layer on the surface of the second active material is in sufficient contact with the surface of the first active material, thereby constructing a good conductive network and achieving the effect of reducing impedance.

[0075] In some embodiments of the present disclosure, the value of j / k is selected from 1 to 15.

[0076] In the present disclosure, the value of j / k is in a suitable range, indicating that the first active material and the second active material have suitable particle size matching, thereby ensuring good geometric configuration matching, facilitating filling matching of the two active materials, and further reducing impedance.

[0077] In some embodiments of the present disclosure, the particle size distribution coefficient S of the first active material is selected from 0.5 to 2.5.

[0078] In some embodiments of the present disclosure, the particle size distribution coefficient S of the second active material is selected from 0.5 to 12.

[0079] In some embodiments of the present disclosure, the single particle mechanical strength of the first active material is selected from 20 MPa to 400 MPa; preferably, the single particle mechanical strength of the first active material is selected from 60 MPa to 250 MPa.

[0080] In the present disclosure, the particle strength of the suitable first active material can well absorb the stress release during the cycle, inhibit or delay the occurrence of cracks, cracks or pulverization of the first active material, reduce the exposure of fresh inner surfaces, thereby reducing new side reactions, which is beneficial to reducing the excessive increase in impedance during the cycle, improving the cycle life, and increasing the power output throughout the life cycle.

[0081] In some embodiments of the present disclosure, a ratio of the compacted density to the true density of the first active material is selected from 0.58 to 0.8.

[0082] In some embodiments of the present disclosure, a ratio of the compacted density to the true density of the second active material is selected from 0.5 to 0.8.

[0083] In some embodiments of the present disclosure, g is selected from 0.005 to 0.05; preferably, g is selected from 0.08 to 0.025.

[0084] In some embodiments of the present disclosure, the specific surface area h is selected from 0.5 to 2; preferably, h is selected from 0.7 to 1.5.

[0085] In some embodiments of the present disclosure, j is selected from 1 to 8; preferably, j is selected from 2 to 6.

[0086] In some embodiments of the present disclosure, k is selected from 0.2 to 3; preferably, k is selected from 0.4 to 1.8.

[0087] In the present disclosure, the ratio of the powder compaction density and true density of the first active material and the second active material represents the density inside the particles. Materials with this value within an appropriate range can reduce the degree of damage to their internal structure during pole piece rolling, and can also absorb more stress during the cycle process, thereby ensuring the cycle life under high-pressure density design.

[0088] In some embodiments of the present disclosure, the positive electrode active material layer further includes a binder and, optionally, a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector.

[0089] In some embodiments of the present disclosure, non-limiting examples of binders include 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, nylon, etc., but are not limited thereto.

[0090] In some embodiments of the present disclosure, non-limiting examples of conductive materials include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0091] Negative electrode:

[0092] In some embodiments of the present disclosure, the negative electrode sheet includes a current collector and a negative electrode material layer disposed on the current collector.

[0093] In the present disclosure, the specific types of negative electrode materials are not subject to specific restrictions and can be selected according to needs. Specifically, the negative electrode material is selected from natural graphite, artificial graphite, mesophase microcarbon beads (abbreviated as MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 , Li-Al alloy. Non-limiting examples of carbon materials include crystalline carbon, amorphous carbon, and mixtures thereof. Crystalline carbon can be amorphous or flake-shaped, platelet-shaped, spherical, or fibrous natural graphite or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, etc.

[0094] In some embodiments of the present disclosure, the negative electrode material layer may include a binder; the binder improves the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector.

[0095] In some embodiments of the present disclosure, non-limiting examples of binders include polyvinyl alcohol, carboxymethyl cellulose, 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, nylon, and the like.

[0096] In some embodiments of the present disclosure, the negative electrode material layer includes a conductive material so that the electrode has conductivity. The conductive material may include any conductive material as long as it does not cause chemical changes. Non-limiting examples of conductive materials include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., such as copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives) and mixtures thereof.

[0097] In some embodiments of the present disclosure, the negative electrode current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0098] Diaphragm:

[0099] In some embodiments of the present disclosure, the lithium battery and electrochemical device of the present disclosure are provided with a separator between the positive electrode and the negative electrode to prevent short circuit. The material and shape of the separator used in the electrochemical device of the present disclosure are not particularly limited and can be any technology disclosed in the prior art.

[0100] In some embodiments of the present disclosure, the separator includes a polymer or an inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0101] In some embodiments of the present disclosure, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film or composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide.

[0102] Electrolyte:

[0103] The lithium battery and electrochemical device disclosed herein also include an electrolyte.

[0104] In some embodiments of the present disclosure, the electrolyte includes a lithium salt and a solvent.

[0105] In some embodiments of the present disclosure, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments of the present disclosure, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB).

[0106] In some embodiments of the present disclosure, the solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0107] In some embodiments of the present disclosure, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0108] Secondary batteries:

[0109] In some embodiments of the present disclosure, the secondary battery involved in the present disclosure includes the above-mentioned positive electrode sheet, negative electrode sheet, separator, electrolyte, etc., but is not limited thereto.

[0110] In some embodiments of the present disclosure, the secondary battery involved in the present disclosure is manufactured by stacking the above-mentioned positive and negative electrode sheets.

[0111] In some embodiments of the present disclosure, the secondary battery involved in the present disclosure may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0112] In some embodiments of the present disclosure, a battery module is provided. The battery module includes the aforementioned secondary battery. The battery module may include multiple secondary batteries, the specific number of which may be adjusted based on the application and capacity of the battery module.

[0113] In some embodiments of the present disclosure, the present disclosure further provides a battery pack comprising the above-mentioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0114] Device:

[0115] The present disclosure also provides a device comprising at least one of the aforementioned secondary battery, battery module, or battery pack.

[0116] In some embodiments of the present disclosure, the device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. To meet the device's requirements for high power and high energy density of lithium batteries, a battery pack or battery module may be used.

[0117] In other embodiments of the present disclosure, the device may be a mobile phone, a tablet computer, a laptop computer, etc. The device is generally required to be lightweight and thin, and may use a lithium battery as a power source.

[0118] The present disclosure is further described below with reference to the following examples. It should be understood that these examples are only used to illustrate the present disclosure and are not intended to limit the scope of the present disclosure.

[0119] Example:

[0120] Electrochemical test method:

[0121] Rate discharge performance test at -20°C: At room temperature (25°C), charge the battery to the rated voltage at 0.33C, then switch to constant voltage charging with a cut-off current of 0.05C. At this time, the capacity is C0.

[0122] The fully charged battery was placed in a constant temperature cabinet at low temperature (-20°C) for 12 hours to allow the battery cell to reach the ambient temperature. It was then discharged to 2.0V at the required rate (0.1C, 0.33C, 1C, 2C). The capacity obtained from the test is C1. C1 / C0 is the capacity retention rate at the corresponding rate at -20°C, that is, the discharge performance at the corresponding rate at -20°C.

[0123] Cycle life test: At a constant temperature of 45°C, in the voltage range of 2.5V to 4.2V, charge at 2C constant current to 3.8V, then switch to 1C constant current and constant voltage charging to 4.2V, with a cutoff current of 0.05C and a discharge at 1C for cycle life test. The test cutoff condition is that the capacity decays to 80% of the initial capacity.

[0124] Example 1:

[0125] Example 1 includes the following steps:

[0126] (1) Preparation of positive electrode sheet:

[0127] The first active material, the second active material, the binder and the conductive agent were weighed in a mass ratio of 94.5:3.0:1.5:1.0, and all the solid substances were placed in a planetary vacuum disperser for dry mixing for 20 minutes; then, 35% of the mass of the above solid organic solvent NMP was added, and after stirring for 0.5 hours, an appropriate amount of NMP was added, and high-speed dispersion was performed for 4 hours to obtain a positive electrode slurry.

[0128] The positive electrode slurry is coated on an aluminum foil current collector and dried in an oven to obtain a positive electrode sheet containing the first active material and the second active material. The SEM image of the positive electrode sheet of Example 1 is shown in Figure 1. It can be seen that the small-particle size of the second active material fills the gaps in the first active material.

[0129] (2) Preparation of negative electrode sheet:

[0130] Artificial graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are thoroughly mixed in a suitable amount of deionized water at a weight ratio of 95.0:1.5:1.5:2 to form a uniform negative electrode slurry. This slurry is coated on the negative electrode current collector copper foil, dried, and cold-pressed to form the negative electrode active material layer. The negative electrode sheet is then cut, slit, and welded to the tabs.

[0131] (3) Preparation of lithium batteries:

[0132] The prepared positive electrode, negative electrode and separator (composed of a 9μm PE polyethylene film coated with 2μm boehmite ceramic coating on both sides) are stacked in order, so that the separator is placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging foil aluminum-plastic film, and the electrolyte (50% EMC + 30% EC + 20% DEC + 1.1M LiPF6 + 1% LiPO2F2 + 1% VC (vinyl carbonate) + 1% FEC) is injected into the dried bare battery cell. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, the preparation of the lithium-ion battery is completed.

[0133] Example 2-11:

[0134] The preparation steps of Examples 2-11 are the same as those of Example 1, except that the mass ratio f of the second active material to the first active material, the mass ratio of the carbon coating layer to the second active material, the specific surface area h of the first active material, the Dv50 j of the first active material, and the Dv50 k of the second active material are different. The specific values ​​are shown in Tables 1 and 2.

[0135] Comparative Example 1:

[0136] Comparative Example 1 comprises the following steps:

[0137] (1) Preparation of positive electrode sheet:

[0138] The first active material, binder and conductive agent were weighed in a mass ratio of 97.5:1.5:1.0, and all the solid substances were placed in a planetary vacuum disperser for dry mixing for 20 minutes; then, 35% of the mass of the above solid organic solvent NMP was added, and after stirring for 0.5 hours, an appropriate amount of NMP was added and high-speed dispersion was performed for 4 hours to obtain a positive electrode slurry.

[0139] The positive electrode slurry is coated on an aluminum foil current collector and dried in an oven to obtain a positive electrode sheet containing the first active material.

[0140] (2) Preparation of negative electrode sheet:

[0141] Artificial graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) are thoroughly mixed in a suitable amount of deionized water at a weight ratio of 95.0:1.5:1.5:2 to form a uniform negative electrode slurry. This slurry is coated on the negative electrode current collector copper foil, dried, and cold-pressed to form the negative electrode active material layer. The negative electrode sheet is then cut, slit, and welded to the tabs.

[0142] (3) Preparation of lithium batteries:

[0143] The prepared positive electrode sheet, negative electrode sheet and separator (composed of a 9μm PE polyethylene base film coated with 2μm boehmite ceramic coating on both sides) are stacked in order, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging foil aluminum-plastic film, and the electrolyte (50% EMC + 30% EC + 20% DEC + 1.1M LiPF6 + 1% LiPO2F2 + 1% VC (vinyl carbonate) + 1% FEC) is injected into the dried bare battery cell. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, the preparation of the lithium-ion battery is completed.

[0144] Comparative Examples 2-5:

[0145] Comparative Examples 2-5 have the same preparation steps as Example 1, except that the mass ratio f of the second active material to the first active material, the mass ratio g of the carbon coating layer to the second active material, the specific surface area h of the first active material, the Dv50 j of the first active material, and the Dv50 k value of the second active material are different. The specific values ​​are shown in Tables 1 and 2.

[0146] Table 1 discloses the contents of the first active material and the second active material in Examples 1-11 and Comparative Examples 1-5, wherein the contents of the first active material and the second active material are based on the total mass of the positive electrode sheet material layer.

[0147] [Table 1]

[0148] Table 2 discloses the mass ratio f of the second active material to the first active material, the mass ratio g of the carbon coating layer to the second active material, the specific surface area h of the first active material, the Dv50 j of the first active material and the Dv50 k of the second active material, as well as the values ​​of g / h, j / k, 10000×f×(g / h) and 10000×f×(g / h)×(j / k) for Examples 1-11 and Comparative Examples 1-5.

[0149] [Table 2]

[0150] Electrochemical test results:

[0151] Examples 1-11 and Comparative Examples 1-5 were subjected to electrochemical tests, and the results are shown in Table 3 below:

[0152] [Table 3]

[0153] As can be seen from Table 1, by comparing Examples 1-11 with Comparative Examples 1-5, it can be seen that the materials satisfying the relationship 0.5≤10000×f×(g / h)×(j / k)≤150 have a good conductive network under low temperature and low SOC conditions, thereby improving the low temperature rate discharge capability of the battery; by comparing Examples 1-3 with Examples 4-11, it can be seen that the materials satisfying the relationship 5≤10000×f×(g / h)×(j / k)≤25 further improve the cycle life of the battery; by comparing Example 4 with Examples 5-7, it can be seen that the materials satisfying the relationship 2≤10000 ×f×(g / h)≤15, compared with the materials not meeting the above conditions, the carbon coating layer on the surface of the second active material can be in more sufficient contact with the surface of the first active material, thereby constructing a better conductive network, so that the low-temperature rate discharge capability of the battery is further improved; through the comparison of Example 8 and Examples 9-11, it can be seen that the materials satisfying the relationship 2≤j / k≤9, compared with the materials not meeting the above conditions, the first active material and the second active material have better geometric configuration matching, thereby further reducing the impedance, so that the low-temperature rate discharge capability of the battery is further improved.

[0154] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limiting the present disclosure, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present disclosure.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet comprises: a current collector; and A material layer is disposed on the current collector, the material layer comprising a first active material and a second active material, The first active material includes at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide and lithium nickel manganese oxide, The second active material includes lithium iron phosphate and / or lithium iron manganese phosphate, and the second active material includes a carbon coating layer, The positive electrode sheet satisfies the following relationship: 0.5≤10000×f×(g / h)×(j / k)≤150; in, f is the mass ratio of the second active material to the first active material; g is the mass ratio of the carbon coating layer to the second active material; h is the specific surface area of ​​the first active material, in m 2 / g; j is the Dv50 value of the first active material, in μm; k is the Dv50 value of the second active material, in μm.

2. The positive electrode sheet according to claim 1, wherein: 1.5≤10000×f×(g / h)×(j / k)≤78.

3. The positive electrode sheet according to claim 1, wherein: 5≤10000×f×(g / h)×(j / k)≤25.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein: The positive electrode sheet satisfies at least one of the following conditions: (1) the value of f is selected from 0.01 to 0.055; (2) the value of g is selected from 0.005 to 0.05; (3) the value of h is selected from 0.5 to 2; (4) the value of j is selected from 1 to 8; (5) The value of k is selected from 0.2 to 3.

5. The positive electrode sheet according to claim 4, wherein: The positive electrode sheet satisfies at least one of the following conditions: (1) The value of g is selected from 0.08 to 0.025; (2) the value of h is selected from 0.7 to 1.5; (3) the value of j is selected from 2 to 6; (4) The value of k is selected from 0.4 to 1.

8.

6. The positive electrode sheet according to any one of claims 1 to 3, wherein: The positive electrode sheet satisfies at least one of the following conditions: (1) The value of g / h is selected from 0.005 to 0.055; (2) the value of 10000×f×(g / h) is selected from 0.5 to 25; (3) The value of j / k is selected from 1 to 15.

7. The positive electrode sheet according to claim 6, wherein: The positive electrode sheet satisfies at least one of the following conditions: (1) The value of g / h is selected from 0.01 to 0.02; (2) the value of 10000×f×(g / h) is selected from 2 to 15; (3) The value of j / k is selected from 2 to 9.

8. The positive electrode sheet according to claims 1 to 3, wherein: The positive electrode sheet satisfies at least one of the following conditions: (1) the particle size distribution coefficient S of the first active material is selected from 0.5 to 2.5; (2) the particle size distribution coefficient S of the second active material is selected from 0.5 to 12; (3) the single particle mechanical strength of the first active material is selected from 20 MPa to 400 MPa; (4) the ratio of the compacted density to the true density of the first active material is selected from 0.58 to 0.8; (5) The ratio of the compacted density to the true density of the second active material is selected from 0.5 to 0.

8.

9. A secondary battery, characterized in that: The secondary battery comprises the positive electrode sheet according to any one of claims 1 to 8.

10. A device, wherein: The device includes the secondary battery according to claim 9.

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