Lithium ion battery

Abstract

The invention discloses a lithium ion battery. The lithium ion battery comprises a positive electrode, a negative electrode, a diaphragm and an electrolyte, a positive electrode active material in the positive electrode comprises a positive electrode core and a solid electrolyte coated on a part of the surface of the positive electrode core, and a conductive agent is coated on an area, which is not coated by the solid electrolyte, of the positive electrode core; the positive electrode core comprises at least one of nickel cobalt manganese layered oxide, nickel cobalt aluminum layered oxide or lithium nickel manganese oxide; the electrolyte comprises a solvent, a lithium salt and a film-forming additive, the solvent comprises a low-viscosity solvent, the viscosity mu of the low-viscosity solvent at 25 DEG C is smaller than or equal to 0.6 cp, and the mass ratio of the film-forming additive in the electrolyte is larger than or equal to 3%; 0 < (mu / x) * (1-m) < 2 > < = 0.85. According to the lithium ion battery disclosed by the invention, the high energy density and the fast charging performance of the battery cell are considered on the whole, and the lithium ion battery also has good high-temperature circulation and storage performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, in particular to a lithium-ion battery. Background Art

[0002] Lithium-ion secondary batteries have become the primary energy storage device for new energy vehicles due to their high energy density, long cycle life, and high environmental friendliness. Based on user demands for vehicle range and rapid energy replenishment, the development of fast-charging lithium-ion batteries with higher energy density has become a hot topic in the new energy industry. To balance high energy density with fast charging speeds, the industry has generally applied high-ionic conductivity electrolytes to lithium iron phosphate cells, developed and applied dimethyl carbonate, and even developed solvents such as small molecule carboxylates, ethers, and nitriles with lower viscosity to improve the conductivity of lithium-ion electrolytes. These improvements have successively increased the ionic conductivity of the electrolyte from "9mS / cm" to "10-11mS / cm" and even higher levels of "13-20mS / cm", significantly improving the fast-charging rate of lithium iron phosphate batteries under high energy density designs. However, all of this is achieved based on the low operating voltage of lithium iron phosphate batteries (usually the upper limit of charging voltage is 3.65V, and the voltage after depolarization at 100% SOC usually does not exceed 3.5V). For cathode materials with higher operating voltage and energy density, such as nickel-cobalt-manganese layered oxide materials and spinel-phase lithium nickel manganese oxide, their application in such higher energy density batteries will be affected by the oxidation resistance of small molecule low-viscosity solvents. At the same time, due to the limitations of the ionic conductivity of existing electrolytes, the fast charging speed of batteries assembled with such cathode materials is difficult to achieve the same significant improvement as that of lithium iron phosphate.

[0003] Therefore, providing a battery that can adapt to positive electrode materials such as nickel-cobalt-manganese layered oxide and lithium nickel-manganese oxide, while achieving high energy density and improving the fast charging performance of the battery, is a technical problem that needs to be solved urgently. Summary of the Invention

[0004] In view of the above technical problems existing in the prior art, the present invention aims to provide a lithium-ion battery.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode active material in the positive electrode comprises a positive electrode core and a solid electrolyte coated on a portion of the surface of the positive electrode core, and the area of ​​the positive electrode core not coated by the solid electrolyte is coated with a conductive agent;

[0007] The positive electrode core comprises at least one of nickel-cobalt-manganese layered oxide, nickel-cobalt-aluminum layered oxide or lithium nickel-manganese oxide;

[0008] The electrolyte comprises a solvent, a lithium salt and a film-forming additive, wherein the solvent comprises a low-viscosity solvent, and the viscosity of the low-viscosity solvent at 25° C. is μ≤0.6 cp;

[0009] 0<(μ / x)×(1-m) 2 ≤0.85;

[0010] Wherein, m is the coverage rate of the solid electrolyte, in %, coverage rate = coverage area / total surface area of ​​the positive electrode core × 100%; the volume proportion of the low-viscosity solvent in the solvent is x, in %.

[0011] In the present invention, the viscosity μ of the low-viscosity solvent at 25° C. is ≤ 0.6 cp, for example, it may be 0.6 cp, 0.55 cp, 0.5 cp, 0.45 cp, 0.4 cp, 0.35 cp, 0.3 cp, 0.25 cp or 0.2 cp.

[0012] In the present invention, when the types of low-viscosity solvents are two or more types of solvents with viscosities μ≤0.6 cp, the viscosity in the formula is the weighted average of the material viscosities of the above low-viscosity solvents.

[0013] In the lithium-ion battery of the present invention, the positive electrode core includes at least one of nickel-cobalt-manganese layered oxide, nickel-cobalt-aluminum layered oxide or lithium nickel-manganese oxide, thereby improving the energy density of the material. By limiting the viscosity of the low-viscosity solvent at 25°C to ≤0.6cp, the ionic conductivity of the electrolyte can be significantly improved (for example, the ionic conductivity value at 25°C is ≥9.0mS / cm). By partially coating the particles of the positive electrode core with a solid electrolyte, the contact between the low-viscosity solvent and the active surface of the positive electrode can be reduced, thereby reducing the oxidative decomposition of the electrolyte on the positive electrode surface. The solid electrolyte layer has ion-conducting properties and does not affect the lithium ion conduction on the surface of the positive electrode material. At the same time, the loss of electron transmission points caused by the solid electrolyte coating can be compensated by a conductive agent. The present invention limits the formula 0<(μ / x)×(1-m) 2 ≤0.85, which can improve the conductivity of the electrolyte, increase the liquid-phase lithium ion transmission rate between the positive and negative electrodes, reduce the electrolyte concentration polarization, increase the power and charging speed, and effectively avoid the oxidative decomposition of the electrolyte at the positive electrode, thereby achieving an overall balance between high energy density and fast charging performance of the battery cell, and can also improve high-temperature cycle and storage performance.

[0014] In one embodiment, the upper operating voltage limit of the positive electrode active material is ≥4.15V, and the energy density of the positive electrode active material is ≥650wh / kg. The upper operating voltage limit here refers to the potential difference of the positive electrode relative to a lithium metal reference electrode after charging to 100% SOC, which is ≥4.15V. The energy density here refers to the product of the average voltage platform of the material and the material's specific capacity, and does not refer to the actual energy density of the lithium-ion battery.

[0015] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0016] Preferably, the mass proportion of the film-forming additive in the electrolyte is ≥3%, for example, it can be 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6% or 7%.

[0017] Preferably, the solid electrolyte has lithium ion conduction function but does not have electron transport function, and the lithium ion conductivity of the solid electrolyte is ≥10 -2 S / cm, the electronic conductivity of the solid electrolyte is ≤10 -5 S / cm.

[0018] Preferably, the solid electrolyte comprises lithium aluminum titanium phosphate (LATP, chemical formula is Li 1.3 Al 0.3 Ti 1.7 (PO4)3), lithium lanthanum zirconium oxide (LLZO, chemical formula Li7La3Zr2O 12 ), lithium phosphorus oxynitride (LiPON, chemical formula is Li x PO y N z , where x, y, and z satisfy valence balance), Li3PS4, Li7P3S 11 、Li 10 GeP2S 12 , at least one of Li2SiP2S8, Li6PS5Cl and Li3BO3.

[0019] Preferably, the coating method of the solid electrolyte on the surface of the positive electrode core is in-situ coating or non-in-situ coating, and the particle size D50 of the solid electrolyte core is D50 c The particle size D50 of the non-in-situ coated solid electrolyte is D50 s The particle size D50 of the in-situ coated solid electrolyte is D50 p , D50 c / D50 s ≥20, for example, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5 or 25, etc.; 1.01≤D50 p / D50 c ≤1.1, D50 p / D50 cFor example, it can be 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09 or 1.1, etc.

[0020] Preferably, the conductive agent comprises at least one of multi-walled carbon nanotubes, single-walled carbon nanotubes, and graphene, preferably multi-walled carbon nanotubes and / or single-walled carbon nanotubes, and more preferably single-walled carbon nanotubes. These materials can form an efficient electron transport network on the surface of the positive electrode core not covered by the solid electrolyte.

[0021] In one embodiment, a method for preparing a positive electrode active material includes the following steps: mixing a solid electrolyte, B2O3, and a conductive agent in a mass ratio of (10-50):(1-5):(1-10) to obtain a coating material; mixing the coating material with the positive electrode core; and sintering under protective gas conditions to obtain the positive electrode active material. In this solution, the solid electrolyte is selected in a range of "10-50", for example, 10, 15, 20, 25, 30, 35, 40, 45, or 50; the B2O3 is selected in a range of "1-5", for example, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5; and the conductive agent is selected in a range of "1-5", for example, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5.

[0022] Preferably, the sintering temperature may be, for example, 450°C to 550°C, for example, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C or 550°C.

[0023] Preferably, the sintering time is 4 h to 6 h, for example, 4 h, 4.2 h, 4.3 h, 4.5 h, 4.7 h, 5 h, 5.2 h, 5.4 h, 5.6 h, 5.8 h or 6 h.

[0024] Preferably, the mass ratio of the coating material to the positive electrode core is 1:(20-30), for example, it can be 1:20, 1:21, 1:22, 1:23, 1:24, 1:25, 1:26, 1:28 or 1:30.

[0025] Preferably, the viscosity μ of the low-viscosity solvent at 25° C. is ≤ 0.5 cp. Under this condition, the conductivity of the electrolyte can be further improved. For example, the ionic conductivity of the electrolyte at 25° C. is ≥ 10 mS / cm.

[0026] Preferably, x is 10 to 90, for example, 10, 12, 15, 18, 20, 25, 30, 35, 38, 40, 42, 44, 46, 48, 50, 55, 60, 65, 70, 75, 80, 85 or 90, etc.

[0027] Preferably, m is 50 to 90, for example, 50, 55, 60, 65, 70, 75, 80, 85 or 90.

[0028] Preferably, 2≤(μ / x)×C≤29, where C is the capacity of the active material loaded on one side of the positive electrode per unit area, expressed in mAh / cm 2 Under this condition, the battery cell can obtain better dynamic performance.

[0029] In one embodiment, C has different values ​​depending on the type of positive electrode core. For example, for lithium nickel manganese oxide, C can be 115 to 130. 0.8 Co 0.1 Mn 0.1 O2, C can take values ​​from 195 to 210.

[0030] Preferably, the film-forming additive includes at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, bisethylene carbonate, vinyl sulfate, propylene sulfate, 1,3-propanesulfonate, 1,4-butanesulfonate, vinyl bissulfate, dimethyl methanesulfonate, vinyl bissulfate, lithium difluorophosphate, lithium nitrate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorooxalatophosphate, lithium tetrafluorooxalatophosphate, lithium tetrafluoroborate, lithium difluorosulfonyl imide, lithium fluorosulfonate, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite and tetravinylsilane.

[0031] Preferably, the mass proportion of the film-forming additive in the electrolyte is 3% to 10%, for example, it can be 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5% or 10%, etc.

[0032] Preferably, when the viscosity μ of the low-viscosity solvent at 25° C. is ≤ 0.5 cp, the mass proportion of the film-forming additive in the electrolyte is 5-10%.

[0033] Preferably, the low viscosity solvent comprises at least one of carbonates, carboxylates, nitriles, ethers, fluorocarbons, fluorocarboxylates, fluoronitriles and fluoroethers.

[0034] For example, the fluoronitrile may be fluoroacetonitrile. The fluorocarboxylic acid ester may be methyl fluoroacetate.

[0035] Preferably, the low-viscosity solvent comprises at least one of dimethyl carbonate, ethyl acetate, ethyl propionate, methyl acetate, methyl formate, methyl propionate, acetonitrile, propionitrile, butyronitrile and ethylene glycol dimethyl ether;

[0036] Preferably, the solvent further includes a cyclic carbonate, and the volume proportion of the cyclic carbonate in the solvent is 10 to 40% (for example, it can be 10%, 12%, 15%, 17%, 18%, 20%, 23%, 26%, 30%, 33%, 35%, 37% or 40%, etc.); the cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate and fluoroethylene carbonate, wherein, when the solvent includes fluoroethylene carbonate, the mass proportion of fluoroethylene carbonate in the electrolyte is greater than 5%.

[0037] In the present invention, if the mass proportion of fluoroethylene carbonate in the electrolyte is less than or equal to 5%, the substance is regarded as an additive rather than a solvent.

[0038] Preferably, the solvent further comprises other solvents, and the other solvents include at least one of diethyl carbonate, ethyl methyl carbonate, propyl propionate and diethylene glycol dimethyl ether.

[0039] Preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorooxalatoborate, lithium bis(oxalatoborate) and lithium tetrafluoroborate, but is not limited to the lithium salts listed above. Other lithium salts commonly used in the art that can dissociate into lithium ions in the electrolyte are also applicable to the present invention.

[0040] Preferably, the mass proportion of the lithium salt in the electrolyte is 10-23%, for example, 10%, 12%, 13%, 15%, 16%, 18%, 20%, 21%, 22% or 23%.

[0041] Preferably, the negative electrode active material in the negative electrode has the following characteristics: a specific capacity ≥ 300 mAh / g, and a voltage plateau ≥ 0 mV and ≤ 0.5 V relative to a lithium reference electrode. Negative electrode active materials meeting these characteristics include, but are not limited to, at least one of graphite, hard carbon, soft carbon, silicon-carbon material, silicon oxide, tin alloy, or graphite fluoride.

[0042] In the present invention, the separator has the following characteristics: it has electronic insulation, a porous structure, and can pass the electrolyte and transfer lithium ions between the positive and negative electrodes.

[0043] The present invention does not specifically limit the type of diaphragm, which can be an organic polymer membrane or an inorganic fiber (such as glass fiber, alumina fiber); its structure can be a homogeneous single-layer porous structure or a non-homogeneous single-layer porous structure, or a non-homogeneous multi-layer porous structure.

[0044] In one embodiment, the separator may be polyethylene, polypropylene, or polysaccharide, among others.

[0045] Preferably, the lithium-ion battery further includes a packaging structure. For example, the packaging structure may be made of an aluminum shell, a steel shell, or a plastic shell, and may be shaped like a square, a cylinder, a polygonal prism, or any special-shaped three-dimensional geometric structure.

[0046] In one embodiment, the packaging structure is an aluminum-plastic soft package packaging structure.

[0047] The present invention does not limit the specific structures of the positive and negative electrodes. For example, the positive electrode includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, a conductive agent, and a binder. For another example, the negative electrode includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a conductive agent, and a binder.

[0048] The present invention does not specifically limit the preparation method of the positive electrode active material. It can be that a solid electrolyte is first coated on part of the surface of the positive electrode core, and then a conductive agent is coated; it can also be that a solid electrolyte and a conductive agent are coated on the surface of the positive electrode core at the same time; it can also be that a conductive agent is first coated on part of the surface of the positive electrode core, and then a solid electrolyte is coated. Those skilled in the art can make the selection according to their needs.

[0049] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0050] Compared with the existing technology, the present invention has the following beneficial effects:

[0051] (1) In the lithium-ion battery of the present invention, the positive electrode core includes at least one of nickel-cobalt-manganese layered oxide, nickel-cobalt-aluminum layered oxide or lithium nickel-manganese oxide, thereby improving the energy density of the material. By limiting the viscosity of the low-viscosity solvent to ≤0.6cp at 25°C, the ionic conductivity of the electrolyte can be significantly improved. By partially coating the particles of the positive electrode core with a solid electrolyte, the contact between the low-viscosity solvent and the active surface of the positive electrode can be reduced, thereby reducing the oxidative decomposition of the electrolyte on the positive electrode surface. The solid electrolyte layer has ion-conducting properties and does not affect the lithium ion conduction on the surface of the positive electrode material. At the same time, the loss of electron transmission points caused by the solid electrolyte coating can be compensated by a conductive agent. The present invention limits the formula 0<(μ / x)×(1-m) 2 ≤0.85, which can achieve a balance between high energy density and fast charging performance of the battery cell as a whole, and can also improve high-temperature cycle and storage performance.

[0052] (2) The lithium-ion battery of the present invention was tested, and its 0-80% SOC limit charging time at 25°C was less than 24.7 min, the charging DCR at 25°C was less than 22.26 mΩ, the discharging DCR at 25°C was less than 21.15 mΩ, the cycle retention rate after 1000 cycles at 25°C was more than 90.06%, the cycle retention rate after 1000 cycles at 45°C was more than 84.75%, and the capacity recovery rate after 30 days of storage at 60°C was more than 90.71%. DETAILED DESCRIPTION

[0053] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0054] In order to compare the performance differences between the examples and the comparative examples, the nickel-cobalt-manganese ternary layered oxide LiNi 0.9 Co 0.05 Mn 0.05 O2 serves as the positive electrode core to form the positive electrode active material, and graphite serves as the negative electrode active material. The example positive electrode material exhibits a specific capacity of 207 mAh / g and a plateau voltage of 3.71 V at a battery operating voltage limit of 4.2 V (equating to a positive electrode material energy density of 768 Wh / kg). The example negative electrode graphite exhibits a specific capacity of 352 mAh / g. It should be noted that the aforementioned selections of positive electrode core and negative electrode active materials are provided for illustrative purposes only and are not intended to limit the present invention.

[0055] In the present invention, ethylene carbonate is abbreviated as EC, propylene carbonate is abbreviated as PC, ethyl methyl carbonate is abbreviated as EMC, dimethyl carbonate is abbreviated as DMC, ethyl acetate is abbreviated as EA, methyl propionate is abbreviated as MA, acetonitrile is abbreviated as ACN, fluoroethylene carbonate is abbreviated as FEC, vinylene carbonate is abbreviated as VC, lithium bis(oxalatoborate) is abbreviated as LiBOB, and 1,3-propane sultone is abbreviated as PS.

[0056] Preparation Example 1

[0057] Provided is a positive electrode active material, comprising a positive electrode core (chemical formula LiNi 0.9 Co 0.05 Mn 0.05 O2) and a solid electrolyte (LATP) coated on a portion of the surface of the positive electrode core, and the area of ​​the positive electrode core not coated by the solid electrolyte is coated with a conductive agent (single-walled carbon nanotube).

[0058] The preparation method of the positive electrode active material includes:

[0059] LATP (particle size D50 is 200nm) and B2O3 and carbon nanotube (tube length> 2μm) powders are fully mixed in a mass ratio of 35:2:5 to obtain a coating material. The coating material is mixed with LiNi 0.9 Co 0.05 Mn 0.05 O2 is mixed in a mass ratio of 1:25, sintered at 475°C for 5 hours in an inert gas atmosphere, and then naturally cooled to room temperature to obtain the desired positive electrode active material.

[0060] The coverage rate m of the solid electrolyte in the positive electrode active material is shown in Table 1.

[0061] The coverage rate of the coated solid electrolyte is confirmed by the following method: taking the pure LATP solid electrolyte material as the benchmark, the Al element ratio is α through EDS element testing; the surface Al element ratio of the coated positive electrode material is β through EDS testing; then the coverage rate m = β / α.

[0062] Example 1

[0063] This embodiment provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode active material in the positive electrode is the positive electrode active material of Preparation Example 1. The preparation method of the lithium-ion battery comprises the following steps:

[0064] Positive electrode preparation:

[0065] LiNi 0.9 Co 0.05 Mn 0.05 O2, conductive carbon black, and PTFE were mixed uniformly in NMP at a mass ratio of 96:1:1:2 to obtain a positive electrode slurry. The positive electrode slurry was coated on a 10 μm aluminum foil and dried. The single-sided coating density of the positive electrode sheet was 18 mg / cm 2 After double-sided coating, the positive electrode sheet is rolled to 115μm, die-cut to a size of 30mm×50mm, and fully dried for use.

[0066] Negative electrode preparation:

[0067] Graphite, conductive carbon black, carboxymethyl cellulose, and styrene-butadiene rubber were mixed uniformly in deionized water at a mass ratio of 96:1:1.3:1.7 to obtain a negative electrode slurry. The negative electrode slurry was coated on both sides of a 5 μm copper foil and dried. The single-sided coating density of the negative electrode sheet was 11.43 mg / cm 2 The double-sided coated negative electrode sheet was rolled to a compaction density of 1.6 g / cm 3 , then die-cut into 33mm×54mm pole pieces and fully dry them for use.

[0068] The diaphragm is a PE diaphragm with a thickness of 12 μm.

[0069] The electrolyte includes a solvent, a lithium salt, and a film-forming additive. The solvent includes a low-viscosity solvent. The type and content of each substance, as well as the viscosity μ of the low-viscosity solvent at 25° C., are shown in Table 1.

[0070] The positive electrode sheet, negative electrode sheet, and separator are stacked in the order of negative electrode / separator / positive electrode to form 18 layers of positive electrode and 19 layers of negative electrode to assemble into a wound core. After welding the tabs, they are encapsulated with aluminum-plastic film to assemble into a soft-pack battery cell with a capacity of 2.0Ah. The electrolyte is injected for sealing (air bags are reserved), and after being fully soaked at 25°C for 24 hours, the battery cell is formed. After formation, the air bags are removed and sealed. After that, the battery cell can be used for testing.

[0071] The above-mentioned formation process is as follows: the battery cell is charged with a constant current of 0.1A for 5 hours, then placed in a 45°C incubator for aging for 12 hours, and the battery cell air bag is cut off to exhaust after aging. After exhausting, the battery cell is charged with a current of 0.67A to a full charge of 4.2V, left to stand for 10 minutes, and then discharged with a current of 0.67A to 3.0V, and then left to stand for 10 minutes. Finally, it is charged with a current of 0.67A for 1 hour to complete the formation process.

[0072] Examples 2 to 12

[0073] The differences from Example 1 are shown in Table 1.

[0074] Table 1

[0075]

[0076]

[0077] Note: The volume percentage of each component in the solvent is based on the total volume of the electrolyte; other components in the electrolyte are expressed as mass percentage, based on the total mass of the electrolyte.

[0078] The viscosities of various components in the solvent at 25°C are shown in Table 2.

[0079] Table 2

[0080] solvent EC PC EMC DMC EA MA ACN Viscosity / cp 1.9 2.51 0.65 0.58 0.45 0.36 0.34

[0081] In the embodiment of the present invention, the low-viscosity solvents involved are DMC, EA, MA and ACN. According to the viscosity μ of the low-viscosity solvent at 25°C, the volume proportion x of the low-viscosity solvent in the solvent, and the coverage m of the solid electrolyte, the (μ / x)×(1-m) of each embodiment and comparative example is calculated. 2 The results are shown in Table 3. Based on μ, x and the capacity C of the active material loaded per unit area on one side of the positive electrode sheet, (μ / x)×C of each embodiment and comparative example is calculated. The results are shown in Table 3.

[0082] Table 3

[0083]

[0084]

[0085] Performance testing:

[0086] ①25℃ DC internal resistance test:

[0087] The DC internal resistance of 3C-30S pulse charging and discharging at 50% SOC was tested respectively;

[0088] ②25℃ three-electrode charging capacity test:

[0089] The battery cell is charged at a constant current rate of 3C starting from 0% SOC, while monitoring the lithium reference electrode potential until the negative electrode potential drops to 2mV relative to the lithium reference electrode potential, and then gradually reducing the charging current to maintain the negative electrode potential relative to the lithium reference electrode potential at around 2mV until the battery cell is fully charged (100% SOC) to 4.2V, and the battery cell limit charging time is obtained (the time it takes for the battery cell to charge from 0% SOC to 80% SOC, that is, the fastest charging rate that the battery cell can achieve).

[0090] ③25℃ / 45℃ cycle 1000-cycle test:

[0091] The cell cycle voltage range is 3.0-4.2V, and the charge and discharge regime is: gradient charge and 1C discharge cycle. The gradient charge is: charging at a 2C rate in the 0-80% SOC range, a 1.5C rate at 80-85% SOC, a 1C rate at 85-90% SOC, a 0.75C rate at 90-95%, and a 0.5C rate at 95-100% SOC.

[0092] ④60℃ storage test:

[0093] The battery was charged and discharged at a rate of 0.33C for three weeks to constant capacity, with the final discharge capacity taken as the cell's baseline capacity, C1. The cell was then fully charged to 100% SOC, stored in a 60°C incubator for 30 days, and then discharged at a rate of 0.33C to an empty cell voltage of 3.0V. The discharge capacity was measured as C2. The cell was then charged and discharged at a rate of 0.33C for another three weeks, with the final discharge capacity taken as C3. The capacity recovery rate can then be calculated as: Capacity recovery rate = C3 / C1 × 100%.

[0094] See Table 4 for the results.

[0095] Table 4

[0096]

[0097]

[0098] In summary, in the lithium-ion battery of the present invention, the positive electrode core includes at least one of nickel-cobalt-manganese layered oxide, nickel-cobalt-aluminum layered oxide or lithium nickel-manganese oxide, so as to improve the energy density of the material. By limiting the viscosity of the low-viscosity solvent at 25°C to ≤0.6cp, the ionic conductivity of the electrolyte can be significantly improved (for example, the ionic conductivity value at 25°C is ≥9.0mS / cm). By partially coating the particles of the positive electrode core with a solid electrolyte, the contact between the low-viscosity solvent and the active surface of the positive electrode can be reduced, thereby reducing the oxidative decomposition of the electrolyte on the positive electrode surface. The solid electrolyte layer has ion-conducting properties and does not affect the lithium ion conduction on the surface of the positive electrode material. At the same time, the loss of electron transmission points caused by the solid electrolyte coating can be compensated by a conductive agent. The present invention limits the formula 0<(μ / x)×(1-m) 2 ≤0.85, which can improve the conductivity of the electrolyte, increase the liquid-phase lithium ion transmission rate between the positive and negative electrodes, reduce the electrolyte concentration polarization, increase the power and charging speed, and effectively avoid the oxidative decomposition of the electrolyte at the positive electrode, thereby achieving an overall balance between high energy density and fast charging performance of the battery cell, and can also improve high-temperature cycle and storage performance.

[0099] By comparing Comparative Example 1 and Comparative Example 3 with Example 1, it can be seen that when a low-viscosity solvent is not used, regardless of whether the surface of the positive electrode core is coated with a solid electrolyte, compared with Example 1, the following are presented: the fast charging performance is reduced, the charge and discharge DC internal resistance is increased, the cycle performance at 25°C and 45°C is deteriorated, and the storage performance at 60°C is deteriorated.

[0100] Comparative Example 2 does not coat the surface of the positive electrode core with a solid electrolyte, and does not satisfy the formula 0 < (μ / x) × (1-m) 2 ≤0.85, compared with Example 2, the cycle performance at 25°C and 45°C deteriorates, and the storage performance at 60°C deteriorates.

[0101] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A lithium-ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The positive electrode active material in the positive electrode includes a positive electrode core and a solid electrolyte coated on a portion of the surface of the positive electrode core, the area of ​​the positive electrode core not coated by the solid electrolyte is coated with a conductive agent, and the positive electrode core includes at least one of nickel-cobalt-manganese layered oxide, nickel-cobalt-aluminum layered oxide, or lithium nickel manganese oxide; The electrolyte comprises a solvent, a lithium salt and a film-forming additive, wherein the solvent comprises a low-viscosity solvent, and the viscosity of the low-viscosity solvent at 25° C. is μ≤0.6 cp; 0<(μ / x)×(1-m) 2 ≤0.85; Wherein, m is the coverage rate of the solid electrolyte, in %, coverage rate = coverage area / total surface area of ​​the positive electrode core × 100%; the volume proportion of the low-viscosity solvent in the solvent is x, in %.

2. The lithium-ion battery according to claim 1, wherein The mass percentage of the film-forming additive in the electrolyte is ≥3%; Preferably, the solid electrolyte has lithium ion conduction function but does not have electron transport function, and the lithium ion conductivity of the solid electrolyte is ≥10 -2 S / cm, the electronic conductivity of the solid electrolyte is ≤10 -5 S / cm; Preferably, the solid electrolyte includes lithium aluminum titanium phosphate LATP, lithium lanthanum zirconium oxide LLZO, lithium phosphorus oxynitride LiPON, Li3PS4, Li7P3S 11 、Li 10 GeP2S 12 , at least one of Li2SiP2S8, Li6PS5Cl and Li3BO3.

3. The lithium-ion battery according to claim 1 or 2, characterized in that The coating method of the solid electrolyte on the surface of the positive electrode core is in-situ coating or non-in-situ coating, and the particle size D50 of the solid electrolyte core is D50 c The particle size D50 of the non-in-situ coated solid electrolyte is D50 s The particle size D50 of the in-situ coated solid electrolyte is D50 p , D50 c / D50 s ≥20, 1.01≤D50 p / D50 c ≤1.

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4. The lithium-ion battery according to any one of claims 1 to 3, characterized in that: The conductive agent includes at least one of multi-walled carbon nanotubes, single-walled carbon nanotubes and graphene, preferably multi-walled carbon nanotubes and / or single-walled carbon nanotubes, and more preferably single-walled carbon nanotubes.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that: The viscosity of the low viscosity solvent at 25° C. is μ≤0.5 cp; Preferably, x is 10 to 90; Preferably, m is 50-90.

6. The lithium-ion battery according to any one of claims 1 to 5, characterized in that: 2≤(μ / x)×C≤29, where C is the capacity of the active material loaded on one side of the positive electrode per unit area, in mAh / cm 2 .

7. The lithium-ion battery according to any one of claims 1 to 6, characterized in that: The film-forming additive includes at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, bisethylene carbonate, vinyl sulfate, propylene sulfate, 1,3-propanesulfonate, 1,4-butanesulfonate, vinyl bissulfate, dimethyl methanesulfonate, vinyl bissulfate, lithium difluorophosphate, lithium nitrate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorooxalatophosphate, lithium tetrafluorooxalatophosphate, lithium tetrafluoroborate, lithium difluorosulfonylimide, lithium fluorosulfonate, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite and tetravinylsilane; Preferably, the mass percentage of the film-forming additive in the electrolyte is 3 to 10%; Preferably, when the viscosity μ of the low-viscosity solvent at 25° C. is ≤ 0.5 cp, the mass proportion of the film-forming additive in the electrolyte is 5-10%.

8. The lithium-ion battery according to any one of claims 1 to 7, characterized in that: The low viscosity solvent comprises at least one of carbonates, carboxylates, nitriles, ethers, fluorocarbons, fluorocarboxylates, fluoronitriles and fluoroethers; Preferably, the low-viscosity solvent comprises at least one of dimethyl carbonate, ethyl acetate, ethyl propionate, methyl acetate, methyl formate, methyl propionate, acetonitrile, propionitrile, butyronitrile and ethylene glycol dimethyl ether; Preferably, the solvent further includes a cyclic carbonate, and the volume proportion of the cyclic carbonate in the solvent is 10 to 40%; the cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate and fluoroethylene carbonate, wherein when the solvent includes fluoroethylene carbonate, the mass proportion of fluoroethylene carbonate in the electrolyte is greater than 5%.

9. The lithium-ion battery according to any one of claims 1 to 8, characterized in that: The solvent further includes other solvents, and the other solvents include at least one of diethyl carbonate, ethyl methyl carbonate, propyl propionate and diethylene glycol dimethyl ether; Preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorooxalatoborate, lithium bis(oxalatoborate) and lithium tetrafluoroborate; Preferably, the lithium salt accounts for 10 to 23% by mass in the electrolyte.

10. The lithium-ion battery according to any one of claims 1 to 9, characterized in that: The negative electrode active material in the negative electrode has the following characteristics: specific capacity ≥300 mAh / g, and a voltage platform ≥0 mV and ≤0.5 V relative to a lithium reference electrode.

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