A lithium-ion battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery, which comprises a positive electrode sheet, a negative electrode sheet and a nonaqueous electrolyte; the positive electrode sheet comprises a positive electrode material layer containing a positive electrode active material, and the positive electrode active material comprises lithium cobaltate containing at least one metal element of Ni or Mn; the nonaqueous electrolyte comprises a lithium salt, an organic solvent and an additive; the additive comprises at least one of the following compounds as a first additive and a nitrile compound as a second additive; the lithium ion battery satisfies the conditions of 3<=b+c<=8, 0.03<=b / c<=0.6, 2<=a / c<=1500, 10<=a<=5000, 0.1<=b<=2 and 2.5<=c<=6.5. The application can greatly reduce the dissolution of positive electrode Co 4+ , form a stable interface film on the positive and negative electrode surfaces and improve the stability of the positive electrode material; meanwhile, the synergistic use of the first additive and the second additive can complex the dissolved cobalt ions of the positive electrode, further improve the stability of the whole battery and greatly reduce the impedance growth in the high-temperature storage and cycle process.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically to a lithium-ion battery that suppresses impedance growth. Background Technology

[0002] Compared with traditional batteries (such as nickel-cadmium and nickel-metal hydride batteries), lithium-ion batteries have significant advantages such as high energy density, fast charging capability, and high operating voltage, which has led to their increasingly widespread use in modern consumer electronics devices and electric vehicles.

[0003] With the continuous upgrading of consumer electronics products, thinness and durability place increasingly stringent performance requirements on lithium-ion batteries. To meet these growing performance demands, the current strategy primarily involves increasing the cutoff voltage to improve battery energy density. However, increasing the cutoff voltage can lead to instability in the cathode material structure, resulting in reduced battery life and impacting the practical application of high-voltage lithium cobalt oxide. Currently, methods to optimize the electrochemical performance of lithium cobalt oxide cathode materials can be achieved through material modification, such as surface coating or bulk doping, to improve battery performance under high-voltage conditions. However, improvements in a single condition are often insufficient. Typically, a battery structure includes a cathode, anode, separator, and electrolyte. As the "blood" of the battery, the electrolyte acts as a carrier for ion transport, conducting lithium ions between the cathode and anode and providing an environment for free insertion and extraction. It significantly influences the energy density, specific capacity, operating temperature range, cycle life, and safety performance of lithium-ion batteries. Adding suitable electrolyte additives to the electrolyte can stabilize the cathode material and further improve the battery's performance in various aspects. Therefore, it is necessary to consider both the cathode material and the electrolyte comprehensively in order to obtain optimal battery performance. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a lithium-ion battery with excellent impedance growth suppression effect.

[0005] The present invention adopts the following technical solution:

[0006] A lithium-ion battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte;

[0007] The positive electrode sheet includes a positive electrode material layer containing a positive electrode active material, wherein the positive electrode active material includes lithium cobalt oxide containing at least one metal element selected from Ni or Mn;

[0008] The non-aqueous electrolyte includes lithium salts, organic solvents, and additives;

[0009] The additive includes a first additive and a second additive, wherein the first additive includes at least one of the following compounds:

[0010]

[0011] The second additive includes nitrile compounds;

[0012] The lithium-ion battery meets the following conditions:

[0013] 3≤b+c≤8, 0.03≤b / c≤0.6, 2≤a / c≤1500, 10≤a≤5000, 0.1≤b≤2, 2.5≤c≤6.5;

[0014] Where a is the total content of Ni and Mn elements in the cathode material layer, in ppm;

[0015] b% represents the mass percentage of the first additive in the non-aqueous electrolyte, expressed as %.

[0016] c% represents the mass percentage of the second additive in the non-aqueous electrolyte, expressed as %.

[0017] The lithium-ion battery of the present invention, starting from improving the stability of the lithium cobalt oxide positive electrode active material, contains at least one metal element selected from Ni or Mn; starting from the compatibility between the non-aqueous electrolyte and the positive and negative electrode materials, at least one of the following compounds is used as the first additive and a nitrile compound as the second additive in the non-aqueous electrolyte; the inventors have found through extensive research that when the relationship between the total content of Ni and Mn elements a in the positive electrode material layer, the mass percentage content b of the first additive in the non-aqueous electrolyte, and the mass percentage content c of the second additive in the non-aqueous electrolyte satisfies 3≤b+c≤8, 0.03≤b / c≤0.6, 2≤a / c≤1500, 10≤a≤5000, 0.1≤b≤2, and 2.5≤c≤6.5, the stability of the battery positive electrode active material can be improved, and the impedance growth during high-temperature cycling and storage can be suppressed. It is speculated that by doping or coating lithium cobalt oxide with Ni or Mn elements, the LCO structure can be controlled at the atomic scale, which is beneficial to enhance the structural stability under high voltage, greatly reduce the dissolution of cobalt ions in the cathode, and improve electrochemical performance. At the same time, by controlling the relationship between the total content of Ni and Mn elements in the cathode material layer and the first additive, the first additive can be ensured to form a uniform film on the cathode surface, further reducing the increase in impedance during cycling. The synergistic effect of the first and second additives can generate a stable interface film at the interfaces of the positive and negative electrodes and the electrolyte, reducing the impedance growth during battery operation.

[0018] Ni or Mn elements can be added to lithium cobalt oxide cathode active materials through doping. Ni or Mn can embed into the lithium cobalt oxide lattice, replacing some of the cobalt positions to form a stable doped state and suppress the dissolution of Co ions. Ni or Mn elements can also be coated onto the outer surface of lithium cobalt oxide as Ni or Mn oxides, preventing direct contact between the cathode active material and the non-aqueous electrolyte and further suppressing Co ion dissolution. Using at least one metal element from Ni or Mn as a doping or coating element, compared to elements such as Mg, Al, Zr, W, F, B, Cr, Mo, and rare earth elements, within a certain range, Ni can improve the material activity of lithium cobalt oxide cathodes and reduce cation mixing, while Mn can delay the phase transition of lithium cobalt oxide cathode materials under high voltage, thereby improving the electrochemical performance of lithium cobalt oxide cathode materials and maintaining good structural stability under high voltage.

[0019] The positive electrode active material contains Ni or Mn. Ni or Mn elements may dissolve into the electrolyte during battery storage or cycling, thus degrading battery performance. By adding a second additive and controlling the total content of Ni and Mn elements to the content of the second additive within the range of 2 ≤ a / c ≤ 1500, the second additive can complex small amounts of dissolved nickel and manganese ions, preventing the introduced metal elements from negatively impacting battery performance. Preferably, the total content of Ni and Mn elements to the content of the second additive is between 12.5 ≤ a / c ≤ 750.

[0020] In the non-aqueous electrolyte of this invention, the first additive and the second additive are used simultaneously. When their content ratio satisfies 3≤b+c≤8 and 0.03≤b / c≤0.6, a synergistic effect can be achieved, generating an interfacial film with good conductivity, suitable mechanical strength, and strong stability at both the positive and negative electrodes and the electrolyte interface. The presence of this excellent interfacial film can suppress side reactions generated by the electrolyte at the positive and negative electrodes. Furthermore, the presence of the two additives can also complex the transition metal ions dissolved from the positive electrode, thereby protecting the stability of the positive electrode active material structure and reducing impedance growth during battery operation. When the total content of the first and second additives is less than 3%, it will affect the quality of the positive and negative electrode interfacial film and will not effectively complex the transition metal ions dissolved from the positive electrode, resulting in limited protection for the positive and negative electrodes and difficulty in significantly improving the stability of the material. When the total content of the first and second additives is greater than 8%, a thicker interfacial film is formed at the positive and negative electrodes, leading to increased interfacial impedance and thus affecting battery performance. When the content ratio of the first additive and the second additive is less than 0.03 or greater than 0.6, they cannot exert a synergistic effect, resulting in excessive battery impedance and thus deterioration of battery performance. Preferably, the content relationship between the first additive and the second additive satisfies 3.5≤b+c≤7 and 0.05≤b / c≤0.5.

[0021] Specifically, in some embodiments of the present invention, by coating or doping the lithium cobalt oxide cathode active material with at least one metal element, Ni or Mn, at a content of 10–5000 ppm, the nickel and manganese elements of the coating or doping element can occupy Li, Co, and O lattice sites in the LCO lattice, thereby controlling the structure of LCO at the atomic scale. This is beneficial for enhancing structural stability under high voltage, including suppressing irreversible phase transitions and reducing strain / stress, mitigating redox reactions to stabilize the bulk phase and surface structure of the particles, and increasing the interlayer spacing to enhance Li… + Diffusion significantly reduces the positive electrode Co 4+ The dissolution of Ni and / or Mn elements improves electrochemical performance. If the amount of Ni and / or Mn elements introduced is too low, it will not improve the stability of the lithium cobalt oxide cathode; if the amount of Ni and / or Mn elements introduced is too high, it will destroy the layered structure of lithium cobalt oxide and promote the occurrence of microcracks and surface side reactions. More specifically, the total content 'a' of Ni and Mn elements in the cathode material layer is 10 ppm, 30 ppm, 50 ppm, 100 ppm, 200 ppm, 500 ppm, 600 ppm, 800 ppm, 1000 ppm, 1200 ppm, 1500 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2400 ppm, 2500 ppm, 2800 ppm, 3000 ppm, 3200 ppm, 3500 ppm, 3800 ppm, 4000 ppm, 4200 ppm, 4500 ppm, 4800 ppm, 5000 ppm, or any combination of these values; preferably, the total content of Ni and Mn elements in the cathode material layer is 100–3000 ppm; more preferably, the total content of Ni and Mn elements in the cathode material layer is 200–1000 ppm.

[0022] Specifically, in some embodiments of the present invention, at least one of compounds 1-5 is used as the first additive, which can simultaneously form films on the positive and negative electrodes, providing excellent protection for the positive and negative electrode materials, thereby improving the stability of the positive and negative electrode materials, suppressing high-temperature storage and cycle impedance growth of the battery, and also having a good inhibitory effect on gas generation during high-temperature storage of the battery. More specifically, the mass percentage b% of the first additive is 0.1%, 0.2%, 0.3%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2%, or any combination of these values; preferably, the mass percentage b% of the first additive is 0.3% to 1.5%.

[0023] Specifically, in some embodiments of the present invention, nitrile compounds are used as the second additive. These nitrile compounds contain cyano or cyanoxy groups, which can form a stable interfacial film on the surface of the cathode material. This inhibits the decomposition of the electrolyte and the dissolution of cobalt ions, maintains the structural integrity of the lithium cobalt oxide cathode active material, thereby suppressing the increase in interfacial impedance and improving the overall electrochemical performance of the battery. More specifically, the mass percentage c% of the second additive is 2.5%, 2.7%, 2.8%, 3%, 3.2%, 3.5%, 3.6%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.3%, 6.5%, or any combination of these values; preferably, the mass percentage c% of the second additive in the non-aqueous electrolyte is 4-6%.

[0024] Specifically, in some embodiments of the present invention, the nitrile compounds include at least one selected from succinic acid, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, nonadionitrile, and sebaconitol.

[0025] Specifically, in some embodiments of the present invention, increasing the porosity of the positive electrode material layer can increase the contact area between the positive electrode active material and the electrolyte, which is beneficial for lithium-ion transport and thus improves battery performance. However, if the porosity is too high, the contact resistance will also increase, and the internal connectivity may deteriorate during battery cycling, increasing the length of electron and ion transport paths, thereby increasing the internal resistance and cycling resistance of the battery. Therefore, the present invention controls the porosity ε% of the positive electrode material layer to be between 10% and 40%, which helps to improve the ion conduction rate, slow down the continuous growth of the interface film unevenness, and thus slow down the increase in resistance during cycling. More specifically, the porosity of the positive electrode material layer is 10%, 12%, 13%, 15%, 18%, 20%, 21%, 24%, 25%, 27%, 28%, 30%, 32%, 35%, 37%, 40%, or any combination of these values; preferably, the porosity ε of the positive electrode material layer is 13% to 30%.

[0026] Specifically, in some embodiments of the present invention, the content of the first additive and the porosity of the positive electrode material layer are limited. By controlling the content of the first additive (b%) and the porosity (ε%) of the positive electrode material layer to be between 0.01 ≤ b / ε ≤ 0.2, it can be ensured that the first additive forms a uniform film on the positive electrode surface, thereby reducing the increase in impedance during cycling. Preferably, the content of the first additive (b%) and the porosity (ε%) of the positive electrode material layer are between 0.01 ≤ b / ε ≤ 0.15.

[0027] Specifically, in some embodiments of the present invention, the non-aqueous electrolyte further includes a third additive, which includes at least one selected from fluorobenzene, 1,3,5-trifluorobenzene, 1,4-difluorobenzene, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The mass percentage (d%) of the third additive in the non-aqueous electrolyte is 1-10%. The molecular structure of the third additive contains fluorine atoms, which have strong electronegativity and small atomic radii. This makes the fluorine-containing third additive exhibit strong polarity and low surface tension, which helps to reduce the surface tension of the electrolyte at the positive and negative electrode interfaces, allowing the electrolyte to better wet the electrode surface and reduce interfacial impedance. More specifically, the mass percentage (d%) of the third additive is 1%, 2%, 3%, 5%, 7%, 8%, 9%, 10%, or any combination of these values; preferably, the mass percentage (d%) of the third additive is 1-5%.

[0028] Specifically, in some embodiments of the present invention, the mass percentages of the first additive (b), the second additive (c), and the third additive (d) satisfy the condition 0.1 ≤ d / (b+c) ≤ 2.5. The fluorine-containing third additive helps improve the wettability of the electrolyte to the positive and negative electrodes at high energy densities. The introduction of the first and second additives increases the electrolyte viscosity and reduces its fluidity, which is detrimental to improving the initial impedance of the battery at high energy densities. By using the third additive in combination and limiting the content relationship among the three additives, it helps to reduce the initial impedance of the electrolyte, increase the electrolyte retention, and thus improve the electrochemical performance of the battery. Preferably, the mass percentages of the first additive (b), the second additive (c), and the third additive (d) satisfy the condition 0.2 ≤ d / (b+c) ≤ 1.5.

[0029] Specifically, in some embodiments of the present invention, the organic solvent includes at least one of cyclic carbonate solvents, linear carbonate solvents, carboxylic acid ester solvents, and ether solvents.

[0030] In some preferred embodiments, the cyclic carbonate solvent includes at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, and butene carbonate.

[0031] In some preferred embodiments, the linear carbonate solvent includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

[0032] In some preferred embodiments, the carboxylic acid ester solvent includes at least one of ethyl acetate, ethyl propionate, propyl propionate, ethyl difluoroacetate, methyl acetate, methyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate.

[0033] In some preferred embodiments, the ether solvent includes at least one of ethylene glycol dimethyl ether, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0034] Specifically, in some embodiments of the present invention, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium bis(trifluoromethanesulfonyl)imide, and lithium difluorosulfonylimide.

[0035] Specifically, in some embodiments of the present invention, the additive further includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, phosphate compounds, and borate compounds.

[0036] In some preferred embodiments, the cyclic sulfate compound includes at least one of 4-methyl vinyl sulfate, vinyl sulfate, and propylene sulfate.

[0037] In some preferred embodiments, the sulfonyl lactone compound includes at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, and propenyl-1,3-sulfonyl lactone.

[0038] In some preferred embodiments, the cyclic carbonate compound includes at least one of vinylene carbonate, ethylene ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, and a compound represented by structural formula 1 below:

[0039]

[0040] In the structural formula 1 shown, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, or C1-C5 group;

[0041] In some preferred embodiments, the compound of structural formula 1 includes at least one of the compounds shown in compounds 1-1 to 1-6 below:

[0042]

[0043] In some preferred embodiments, the phosphate ester compound includes at least one of tris(trimethylsilane) phosphate and a compound represented by structural formula 2 below:

[0044]

[0045] In structural formula 2, R 31 R 32 R 33 Each is independently selected from saturated hydrocarbon groups, unsaturated hydrocarbon groups, and halogenated hydrocarbon groups of C1-C5; more preferably, the compound represented by structural formula 2 includes at least one of triphenyl phosphate, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, triargylpropyl phosphate, diargylpropyl methyl phosphate, diargylpropyl propyl phosphate, diargylpropyl-2,2,2-trifluoroethyl phosphate, diargylpropyl-3,3,3-trifluoropropyl phosphate, diargylpropyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, and diallyl hexafluoroisopropyl phosphate.

[0046] In some preferred embodiments, the borate ester compound is selected from tris(trimethylsilane)borate esters.

[0047] In some embodiments, the content of the auxiliary additive is 0.01% to 10% based on 100% of the total mass of the non-aqueous electrolyte. Preferably, the content is 0.1% to 5%; more preferably, the content is 0.1% to 2%. Specifically, the content of any one optional substance in the auxiliary additive can be 0.01%, 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, or any combination of these values.

[0048] Specifically, in some embodiments of the present invention, the positive electrode sheet further includes a positive electrode binder and a positive electrode conductive agent. The positive electrode binder includes one or more of the following: polyvinylidene fluoride (PVDF), copolymers of PVDF, polytetrafluoroethylene (PTFE), copolymers of PVDF-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ethers, copolymers of ethylene-tetrafluoroethylene, copolymers of PVDF-tetrafluoroethylene, copolymers of PVDF-trifluoroethylene, copolymers of PVDF-trichloroethylene, copolymers of PVDF-fluorinated vinylidene, copolymers of PVDF-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, polyethylene, and polypropylene; acrylic resins; and styrene-butadiene rubber. The positive electrode conductive agent includes one or more of the following: conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, or reduced graphene oxide.

[0049] Specifically, in some embodiments of the present invention, the positive electrode further includes a positive electrode current collector, and the positive electrode material layer is disposed on the surface of the positive electrode current collector. The positive electrode current collector includes a metallic material capable of conducting electrons; preferably, the positive electrode current collector includes one or more of aluminum, nickel, tin, copper, and stainless steel.

[0050] Specifically, in some embodiments of the present invention, the preparation method of the positive electrode sheet is as follows: the positive electrode active material, conductive agent and binder are mixed in a mass ratio, and then they are dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry is uniformly coated on both sides of an aluminum foil, and after drying, rolling and vacuum drying, aluminum leads are welded on using an ultrasonic welding machine to obtain the positive electrode plate.

[0051] Porosity of the cathode material layer

[0052] Where, ρ coat ω represents the bulk density of the coating, ρ represents the mass percentage of the coating components, and AM, CA, and B represent the positive electrode active material, conductive agent, and binder, respectively.

[0053] Specifically, in some embodiments of the present invention, the negative electrode sheet includes a negative electrode material layer containing a negative electrode active material, wherein the negative electrode active material is any one or more of silicon-based materials and carbon materials. The silicon-based material is selected from one or more of silicon materials, silicon oxide materials, silicon-carbon materials, and silicon alloy materials; preferably, the silicon material is nano-silicon material; preferably, the silicon oxide material is SiOx material, wherein 0 ≤ x < 2; preferably, the silicon-carbon material is: a silicon-based material containing silicon and carbon materials, and / or a silicon-based material containing SiOy and carbon materials, wherein 0 ≤ y < 2; preferably, the silicon alloy material is Mg2Si alloy material and / or Fe2Si alloy material. The carbon material is selected from one or more of artificial graphite, natural graphite, composite graphite, graphene, and hard carbon; preferably, the carbon material is artificial graphite material.

[0054] Specifically, in some embodiments of the present invention, the negative electrode material layer further includes a negative electrode binder, a negative electrode conductive agent, and a negative electrode current collector. The material of the negative electrode current collector can be the same as that of the positive electrode current collector, and will not be described again here. The negative electrode binder and the negative electrode conductive agent can be the same as those of the positive electrode binder and the positive electrode conductive agent, respectively, and will not be described again here.

[0055] Specifically, in some embodiments of the present invention, the lithium-ion battery further includes a separator located between the positive electrode and the negative electrode.

[0056] The diaphragm can be a conventional diaphragm, such as a ceramic diaphragm, a polymer diaphragm, a non-woven fabric, or an inorganic-organic composite diaphragm, including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP diaphragms.

[0057] The lithium-ion battery of the present invention uses lithium cobalt oxide as the positive electrode active material and contains at least one metal element selected from Ni or Mn. Simultaneously, it uses at least one of the following compounds as a first additive and a nitrile compound as a second additive. The total content a of Ni and Mn elements in the positive electrode material layer, the mass percentage b of the first additive in the non-aqueous electrolyte, and the mass percentage c of the second additive in the non-aqueous electrolyte are defined such that the relationship satisfies 3≤b+c≤8, 0.03≤b / c≤0.6, 2≤a / c≤1500, 10≤a≤5000, 0.1≤b≤2, and 2.5≤c≤6.5. This significantly reduces the Co content of the positive electrode. 4+ The dissolution of the positive and negative electrodes forms a stable interfacial film on the positive and negative electrode surfaces, further improving the stability of the positive electrode material. At the same time, the synergistic use of the first and second additives can also complex the cobalt ions dissolved from the positive electrode, further improving the overall stability of the battery and greatly reducing the impedance growth during high-temperature storage and cycling. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0059] Example 1

[0060] The method for preparing the lithium-ion battery in this embodiment includes the following steps:

[0061] 1) Preparation of the positive electrode sheet:

[0062] Lithium cobalt oxide (LiCoO2), a Ni-doped positive electrode active material, conductive carbon black Super-P, and polyvinylidene fluoride (PVDF) binder were mixed uniformly at a mass ratio of 93:4:3. These mixtures were then dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry was uniformly coated on both sides of an aluminum foil, and after drying, calendering, and vacuum drying, aluminum leads were welded on using an ultrasonic welder to obtain the positive electrode sheet. The porosity of the positive electrode material layer shown is 27%, and the Ni doping content in the positive electrode material layer is shown in Table 1.

[0063] 2) Preparation of the negative electrode sheet:

[0064] Artificial graphite, conductive carbon black Super-P, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 94:1:2.5:2.5 and then dispersed in deionized water to obtain a negative electrode slurry. The slurry was coated on both sides of a copper foil, dried, rolled, and vacuum dried, and then nickel leads were soldered on using an ultrasonic welder to obtain the negative electrode sheet.

[0065] 3) Preparation of electrolyte:

[0066] Ethylene carbonate (EC), diethyl carbonate (DEC), and propyl propionate (PP) were mixed in a mass ratio of EC:DEC:PP = 1:1:1. Then, lithium hexafluorophosphate (LiPF6) was added to a molar concentration of 1 mol / L. Finally, 0.5% of the first additive compound 1 and 3% of the second additive succinate were added.

[0067] 4) Cell fabrication:

[0068] A three-layer separator with a thickness of 20 μm is placed between the positive and negative plates. Then, the sandwich structure composed of the positive plate, negative plate and separator is wound up, and the wound body is flattened and placed in an aluminum foil packaging bag. It is then vacuum baked at 75°C for 48 hours to obtain the cell to be injected with electrolyte.

[0069] 5) Electrolyte injection and formation of battery cells

[0070] In a glove box with the dew point controlled below -40°C, the electrolyte prepared above was injected into the battery cell, vacuum sealed, and left to stand for 24 hours. Then, the first charge was performed according to the following steps: constant current charging at 0.05C for 180 minutes, constant current charging at 0.2C to 3.95V, vacuum sealing for the second time, and then further constant current charging at 0.2C to 4.5V. After being left to stand at room temperature for 24 hours, constant current discharging at 0.2C to 3.0V was performed.

[0071] Examples 2-57 and Comparative Examples 1-22

[0072] This embodiment and comparative example are used to illustrate the lithium-ion battery disclosed in this invention. They include most of the operating steps in the above embodiment 1, but differ in that: the composition and content of additives in the non-aqueous electrolyte, the types and content of doped elements in the positive electrode material layer, and the porosity of the positive electrode sheet are shown in Tables 1-6, respectively.

[0073] The lithium-ion batteries prepared in each embodiment and comparative example were subjected to performance testing according to the following methods:

[0074] 1. High-temperature storage performance test

[0075] After formation, the battery is charged at room temperature with a constant current of 1C to the cutoff voltage, then charged with constant current and constant voltage until the current drops to 0.05C, then discharged with a constant current of 1C to 3.0V, then fully charged and stored in an environment of 60℃ for 30 days, and then discharged with 1C to 3V.

[0076] Battery impedance growth rate (%) after high temperature storage = [(internal resistance after 30 days of storage - internal resistance before storage) / internal resistance before storage] × 100%.

[0077] Battery capacity retention rate after high-temperature storage (%) = [(Battery capacity after 30 days of storage - Capacity before storage) / Capacity before storage] × 100%.

[0078] 2. High-temperature cycling performance test

[0079] The battery was placed in a high-temperature oven at a constant temperature of 45°C and charged at a constant current of 0.5C to the cutoff voltage. Then it was charged at a constant voltage until the current dropped to 0.02C. After resting for 5 minutes, it was discharged at a constant current of 1C to 3.0V. This was the first cycle. 500 charge / discharge cycles were performed under the above conditions. The impedance growth rate and cycle capacity retention rate of the battery after 500 cycles at 45°C were calculated.

[0080] Battery impedance growth rate after cycling (%) = [(internal resistance after corresponding number of cycles - internal resistance after the first cycle) / internal resistance after the first cycle] × 100%.

[0081] Cyclic capacity retention rate (%) = [(Capacity after corresponding number of cycles - Capacity after the first cycle) / Capacity after the first cycle] × 100%.

[0082] Test Results

[0083] Table 1 shows the test results of Examples 1-22 and Comparative Examples 1-18; the differences between Examples 2-22 and Comparative Examples 1-18 and Example 1 are the relevant parameters in Table 1.

[0084] Table 1

[0085]

[0086]

[0087]

[0088] As can be seen from the test results of Examples 1-22 and Comparative Examples 1-18, the lithium-ion battery of the present invention uses lithium cobalt oxide as the positive electrode active material. By doping with at least one metal element from Ni or Mn, and using at least one of the following compounds and nitrile compounds as additives in the non-aqueous electrolyte, and further limiting the relationship between the total content a of Ni and Mn elements in the positive electrode material layer, the mass percentage content b of the first additive in the non-aqueous electrolyte, and the mass percentage content c of the second additive in the non-aqueous electrolyte to satisfy 3≤b+c≤8, 0.03≤b / c≤0.6, 2≤a / c≤1500, 10≤a≤5000, 0.1≤b≤2, and 2.5≤c≤6.5, the stability of the positive electrode active material of the battery can be improved, and the impedance growth during high-temperature cycling and storage can be suppressed.

[0089] As can be seen from the test results of Example 1 and Comparative Examples 1-18, when any one of the parameters—the total content of Ni and Mn elements in the cathode material layer (a), the mass percentage of the first additive in the non-aqueous electrolyte (b), and the mass percentage of the second additive in the non-aqueous electrolyte (c)—is outside the specified range, or when any one of the relationships b+c, b / c, or a / c is too large or too small, it is impossible to guarantee that the lithium-ion battery has good high-temperature performance and low impedance growth. This indicates that the total content of Ni and Mn elements in the cathode material layer (a), the mass percentage of the first additive in the non-aqueous electrolyte (b), and the mass percentage of the second additive in the non-aqueous electrolyte (c)—are strongly correlated in improving the high-temperature performance of lithium-ion batteries and suppressing impedance growth.

[0090] Table 2 shows the test results of Examples 7 and Examples 23-32; the difference between Examples 23-32 and Example 7 lies in the relevant parameters in Table 2.

[0091] Table 2

[0092]

[0093]

[0094] As can be seen from the test results in Table 2, when the total content of Ni and Mn elements a in the cathode material layer, the mass percentage content of the first additive in the non-aqueous electrolyte b, and the mass percentage content of the second additive in the non-aqueous electrolyte c meet the relevant requirements, and further when the porosity ε of the cathode material layer and the relationship between the content of the first additive and the porosity b / ε satisfy 10≤ε≤40 and 0.01≤b / ε≤0.2, the high-temperature performance of the lithium-ion battery can be further improved and the impedance growth can be suppressed.

[0095] Table 3 shows the test results for Examples 7 and 33-46; the difference between Examples 33-46 and Example 7 lies in the relevant parameters in Table 3.

[0096] Table 3

[0097]

[0098]

[0099] As can be seen from the test results in Table 3, when the total content of Ni and Mn elements a in the positive electrode material layer, the mass percentage content of the first additive in the non-aqueous electrolyte b, and the mass percentage content of the second additive in the non-aqueous electrolyte c meet the relevant requirements, further adding a third additive, while the mass percentage content of the third additive satisfies 1≤d≤10 and 0.1≤d / (b+c)≤2.5, helps to improve the initial impedance of the battery under high energy density and further optimize the high-temperature performance and impedance performance of the battery.

[0100] Table 4 shows the test results of Examples 7 and 47-54; the difference between Examples 47-54 and Example 7 lies in the relevant parameters in Table 4.

[0101] Table 4

[0102]

[0103] As can be seen from the test results in Table 4, when the total content of Ni and Mn elements a in the positive electrode material layer, the mass percentage content of the first additive in the non-aqueous electrolyte b, and the mass percentage content of the second additive in the non-aqueous electrolyte c meet the relevant requirements, adding different types of first additives and different types of second additives can optimize the high-temperature performance and impedance performance of lithium-ion batteries. This indicates that the battery system of the present invention has universality for different types of first additives and different types of second additives.

[0104] Table 5 shows the test results of Examples 32 and 55-57; the difference between Examples 55-57 and Example 32 lies in the relevant parameters in Table 5.

[0105] Table 5

[0106]

[0107] As can be seen from the test results in Table 5, when the total content of Ni and Mn elements in the positive electrode material layer (a), the mass percentage of the first additive in the non-aqueous electrolyte (b), the mass percentage of the second additive in the non-aqueous electrolyte (c), and the mass percentage of the third additive in the non-aqueous electrolyte (d) meet the relevant requirements, the addition of different types of third additives can optimize the high-temperature performance and impedance performance of lithium-ion batteries, indicating that the battery system of the present invention has universality for different types of third additives.

[0108] Table 6 shows the test results of Example 1 and Comparative Examples 19-22; the difference between Comparative Examples 19-22 and Example 1 lies in the relevant parameters in Table 6.

[0109] Table 6

[0110]

[0111]

[0112] As can be seen from the test results in Table 6, by doping the cathode material layer with at least one metal element, either Ni or Mn, the present invention helps to improve the activity and electrochemical performance of the lithium cobalt oxide cathode material compared with other doping elements, resulting in lithium-ion batteries with better high-temperature performance and resistance growth capability.

[0113] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A lithium-ion battery, characterized in that, Includes positive electrode, negative electrode, and non-aqueous electrolyte; The positive electrode sheet includes a positive electrode material layer containing a positive electrode active material, wherein the positive electrode active material includes lithium cobalt oxide containing at least one metal element selected from Ni or Mn; The non-aqueous electrolyte includes lithium salts, organic solvents, and additives; The additive includes a first additive and a second additive, wherein the first additive includes at least one of the following compounds: The second additive includes nitrile compounds; The lithium-ion battery meets the following conditions: 3≤b+c≤8, 0.03≤b / c≤0.6, 2≤a / c≤1500, 10≤a≤5000, 0.1≤b≤2, 2.5≤c≤6.5; Where a is the total content of Ni and Mn elements in the cathode material layer, in ppm; b% represents the mass percentage of the first additive in the non-aqueous electrolyte, expressed as %. c% represents the mass percentage of the second additive in the non-aqueous electrolyte, expressed as %.

2. The lithium-ion battery according to claim 1, characterized in that, The lithium-ion battery satisfies the following conditions: 3.5≤b+c≤7, 0.05≤b / c≤0.5, 12.5≤a / c≤750.

3. The lithium-ion battery according to claim 1, characterized in that, The total content (a) of Ni and Mn elements in the positive electrode material layer is 100 ppm to 3000 ppm; and / or, The mass percentage (b%) of the first additive in the non-aqueous electrolyte is 0.3% to 1.5%; and / or, The mass percentage (c%) of the second additive in the non-aqueous electrolyte is 4% to 6%.

4. The lithium-ion battery according to claim 1, characterized in that, The nitrile compounds include at least one of butadionitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, nonadionitrile, and sebaconitol.

5. The lithium-ion battery according to claim 1, characterized in that, The porosity ε% of the positive electrode material layer satisfies 10% to 40%.

6. The lithium-ion battery according to claim 5, characterized in that, The porosity ε% of the positive electrode material layer and the mass percentage b% of the first additive in the non-aqueous electrolyte satisfy the condition 0.01≤b / ε≤0.

2.

7. The lithium-ion battery according to claim 1, characterized in that, The non-aqueous electrolyte also includes a third additive, which includes at least one of fluorobenzene, 1,3,5-trifluorobenzene, 1,4-difluorobenzene, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

8. The lithium-ion battery according to claim 7, characterized in that, The mass percentage (d%) of the third additive in the non-aqueous electrolyte is 1% to 10%.

9. The lithium-ion battery according to claim 8, characterized in that, The mass percentage content b of the first additive, the mass percentage content c of the second additive, and the mass percentage content d of the third additive satisfy the condition 0.1 ≤ d / (b+c) ≤ 2.

5.

10. The lithium-ion battery according to claim 9, characterized in that, The mass percentage content b of the first additive, the mass percentage content c of the second additive, and the mass percentage content d of the third additive satisfy the condition 0.2 ≤ d / (b+c) ≤ 1.5.

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