A lithium-ion battery

Abstract

The present invention belongs to the technical field of lithium-ion batteries, and relates to a lithium-ion battery, in particular to a lithium-ion battery having excellent high-temperature performance and fast charge cycle performance. The lithium-ion battery includes: a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte; the positive electrode sheet includes a phosphate compound as a positive electrode active material; the non-aqueous electrolyte includes a first additive, a second additive, a lithium salt, and a solvent; the first additive includes at least one of Compound 1 to Compound 5; the second additive includes fluoroethylene carbonate, and the solvent includes the compound shown in Structural Formula 1; the lithium-ion battery satisfies the following conditions: 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, 10 ≤ e ≤ 35. The lithium-ion battery provided in the present invention can avoid the deterioration of the high-temperature performance of the battery by carboxylic acid esters, and has a higher ionic conductivity of the electrolyte.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, relates to a lithium-ion battery, and particularly relates to a lithium-ion battery with excellent high-temperature performance and fast charge and discharge cycle performance. Background Art

[0002] At present, the new energy vehicle industry is booming, and the market has a high demand for power lithium-ion batteries. Two common types of cathode materials for commercial power lithium-ion batteries are ternary nickel cobalt manganese lithium oxide materials and phosphate materials represented by lithium iron phosphate. The advantages of phosphate materials compared to ternary materials are long cycle life, low cost, and high safety performance. However, their disadvantages are low energy density and poor rate performance. If the rate performance of lithium iron phosphate cathode batteries can be improved to meet consumers' demand for fast charging of electric vehicles, the market competitiveness of lithium iron phosphate cathode batteries can be enhanced. The commonly used method in the industry is to use low-viscosity carboxylic esters as electrolyte solvents to reduce the transport resistance of lithium ions during high-rate charging and reduce the polarization internal resistance of the battery to improve the rate performance of lithium iron phosphate cathode batteries. However, the defect of carboxylic ester solvents is that they are prone to side reactions with the anode in a highly lithiated state at high temperatures, resulting in performance degradation at high temperatures. Therefore, how to balance the high-temperature performance and fast charge and discharge cycle performance of lithium iron phosphate cathode batteries is a research direction that has received much attention. Summary of the Invention

[0003] Based on this, the purpose of the present invention is to provide a lithium-ion battery to solve the problems of performance degradation at high temperatures and poor fast charge and discharge cycle performance of lithium iron phosphate cathode batteries in the prior art.

[0004] To achieve the above object, the present invention adopts the following technical solutions.

[0005] The present invention provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte;

[0006] The positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes a phosphate compound LiFe 1-x-y Mn x M y PO4, where 0 ≤ x ≤ 0.8, 0 ≤ y ≤ 0.05, and M includes any one or more of Co, B, Ga, F, W, Zr, Mg, Na, Pb, K, Al, Cr, Ba, Ca, Ni, Sr, Ti, Zn, Si, V, Mo, or Nb;

[0007] The non-aqueous electrolyte includes a first additive, a second additive, a lithium salt, and a solvent; the first additive includes at least one of the following compounds:

[0008] Compound 1

[0009] Compound 2

[0010] Compound 3

[0011] Compound 4

[0012] Compound 5;

[0013] The second additive includes vinylene carbonate fluoride;

[0014] The solvent includes the compound shown in Structural Formula 1;

[0015] ;

[0016] Structural Formula 1

[0017] Wherein, R1 is an alkyl group with a carbon atom number ≤ 2, R2 is an alkyl group with a carbon atom number ≤ 3, the hydrogen atoms in R1 can be partially substituted by fluorine atoms, and the total number of carbon atoms in R1 and R2 ≤ 4;

[0018] The lithium-ion battery satisfies the following conditions:

[0019] 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, 10 ≤ e ≤ 35;

[0020] Wherein, a is the mass percentage content of the first additive in the non-aqueous electrolyte, with the unit of %;

[0021] b is the mass percentage content of the second additive in the non-aqueous electrolyte, with the unit of %;

[0022] c is the mass percentage content of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, with the unit of %;

[0023] d is the specific surface area of the positive electrode material layer, with the unit of m 2 / g;

[0024] e is the porosity of the positive electrode material layer, with the unit of %.

[0025] In the field of power lithium-ion batteries, phosphate-based positive electrodes represented by lithium iron phosphate have the advantages of longer cycle life, better safety, and higher cost performance compared with nickel-cobalt-manganese ternary positive electrodes. However, the rate performance of phosphate-based positive electrodes is poor, mainly because of their poor conductivity. If the rate performance of lithium iron phosphate positive electrode batteries can be improved, the gap with ternary positive electrodes can be compensated to a certain extent, making them more competitive in the market.

[0026] The lithium-ion battery provided in the present invention uses at least one compound among Compound 1 to Compound 5 as the first additive, fluoroethylene carbonate as the second additive, and the compound shown in Structural Formula 1 as the solvent. And through extensive research by the inventor, it is found that when the mass percentage content a% of the first additive in the electrolyte, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer and the porosity e% of the positive electrode material layer satisfy the relational expression 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, 10 ≤ e ≤ 35, a lithium-ion battery with excellent high-temperature storage performance and fast-charge cycle performance can be obtained. Presumably, the reason is that, on the one hand, from the perspective of the non-aqueous electrolyte, one of the main factors restricting the rapid charging of the battery is the ionic conductivity of the electrolyte. The short-chain carboxylic ester solvent has a low viscosity, can reduce the resistance of lithium-ion movement, and can significantly improve the ionic conductivity of the electrolyte. However, the defect of the short-chain carboxylic ester solvent is poor electrochemical stability, and it is easy to react with the negative electrode in a highly lithiated state at high temperatures, resulting in deterioration of high-temperature storage and high-temperature cycle performance. By using the first additive and the second additive in combination, an organic matter protective film can be formed on the surface of the negative electrode, which can prevent the negative electrode from reacting with the carboxylic ester, avoid the deterioration of the high-temperature performance of the battery by the carboxylic ester, and has a low interfacial internal resistance, which is beneficial to further improving the fast-charge cycle performance of the battery. On the other hand, by restricting the specific surface area and porosity of the positive electrode material layer within a certain range, the lithium-ion transmission and charge transfer efficiency at the electrode-electrolyte interface are improved, thereby improving the fast-charge cycle performance of the battery. Generally speaking, in the present invention, by restricting the content of the first additive, the content of the second additive, the content of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area of the positive electrode material layer, and the porosity of the positive electrode material layer within a certain range, a synergistic effect is achieved among its various parameters, thereby obtaining a lithium-ion battery with excellent high-temperature storage performance and fast-charge cycle performance. Preferably, the lithium-ion battery satisfies: 0.5 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 10.

[0027] In a non-aqueous electrolyte, when at least one compound among Compounds 1 to 5 is used as a first additive in combination with fluoroethylene carbonate (FEC) as a second additive, a reduction reaction will occur on the surface of the negative electrode during the battery formation process. Among them, the first additive can generate a dense and thin organic matter protective film with a certain flexibility through a ring-opening polymerization reaction, and FEC introduces inorganic salt substances with high plasma conduction efficiency and strong thermal stability such as LiF into the protective film. This negative electrode protective film can prevent side reactions between the negative electrode and carboxylic esters, thereby avoiding the deterioration of the high-temperature performance of the battery by carboxylic esters. When the mass percentage content a% of the first additive in the non-aqueous electrolyte is too small, that is, a% < 0.01%, a dense interfacial film that completely covers the surface of the negative electrode cannot be formed, and it cannot play a role in protecting the negative electrode; when the mass percentage content a% of the first additive in the non-aqueous electrolyte is too high, that is, a% > 3%, the formed protective film layer is thicker, the interfacial impedance of the battery cell increases synchronously, which is not conducive to the rate performance of the battery, and more Joule heat will be generated, and the deterioration of high-temperature storage is too serious. Specifically, the mass percentage content a% of the first additive in the non-aqueous electrolyte is 0.01%, 0.03%, 0.05%, 0.07%, 0.09%, 1%, 1.3%, 1.5%, 1.7%, 1.9%, 2.0%, 2.3%, 2.5%, 2.7%, 3% or the range composed of any two of these values. Preferably, the mass percentage content a% of the first additive in the non-aqueous electrolyte is 0.05% - 1%.

[0028] When the mass percentage content b% of the second additive is too small, that is, b% < 0.05%, a dense interfacial film that completely covers the surface of the negative electrode cannot be formed, and the protective effect is insufficient; when the mass percentage content b% of the second additive is too high, that is, b% > 1%, the interfacial film will be too thick, resulting in an increase in the internal resistance of the battery and deterioration of the cycle performance. Specifically, the mass percentage content b% of the second additive in the total non-aqueous electrolyte is 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or the range composed of any two of these values. Preferably, the mass percentage content b% of the second additive in the non-aqueous electrolyte is 0.1% - 1%.

[0029] In a non-aqueous electrolyte, the short-chain carboxylic acid ester shown in Structural Formula 1, as a solvent, has a relatively low viscosity, can reduce the resistance to the movement of lithium ions, can significantly improve the ionic conductivity of the electrolyte, and can improve the fast charge cycle performance of the battery. When the mass percentage content c% of the solvent is too small, that is, c% < 30%, it cannot achieve an obvious viscosity reduction effect; if the mass percentage content c% of the solvent is too high, that is, c% > 50%, the deterioration during high-temperature storage is too serious. Specifically, the mass percentage content c% of the solvent is 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50% or the range composed of any two of these values. Preferably, the mass percentage content c% of the solvent in the non-aqueous electrolyte is 30% - 45%.

[0030] From the perspective of the electrode sheet, the main factor restricting the fast charging of the battery is the limited transport of lithium ions across the phase boundary at the electrode-electrolyte interface. By restricting the specific surface area of the positive electrode material layer within a suitable range, the contact area between the electrolyte and the electrode sheet can be increased. When the specific surface area d m 2 / g of the positive electrode material layer is too small, that is, d m 2 / g < 6 m 2 / g, it is not conducive to the fast charging performance of the battery; when the specific surface area d m 2 / g of the positive electrode material layer is too large, that is, d m 2 / g > 12 m 2 / g, it will cause excessive consumption of additives and lithium salts during the battery formation process to form a negative electrode protective film, resulting in an increase in the irreversible capacity during the first charge and deteriorating the first charge-discharge efficiency of the battery. Specifically, the specific surface area d m 2 / g of the positive electrode material layer is 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g or the range composed of any two of these values. Preferably, the specific surface area d m 2 / g of the positive electrode material layer is 7 m 2 / g - 10 m 2 / g.

[0031] Specifically, in some embodiments of the present invention, the following method is used to test the specific surface area of the positive electrode material layer: (1) After disassembling the battery, take out the positive electrode sheet, soak and clean the positive electrode sheet with EMC (ethyl methyl carbonate), and vacuum dry for 24 h. Scrape the positive electrode material layer from the positive electrode current collector into a powder, then add a small amount of EMC to the powder and disperse it using an ultrasonic device, and then dry the powder and put it into a sample tube; (2) Put the sample tube into a degassing station, heat and vacuum degas the sample to remove the surface adsorbed gas, and test the mass. Use a specific surface area analyzer to perform adsorption and desorption tests on the sample, and calculate the specific surface area of the positive electrode material layer from the measured adsorption isotherm.

[0032] The porosity of the positive electrode material layer is related to the adsorption and wetting of the electrode sheet to the electrolyte. Limiting the porosity of the positive electrode material layer within a suitable range is beneficial to the adsorption and wetting of the electrode sheet to the electrolyte, and improves the liquid retention capacity of the battery cell. When the porosity e% of the positive electrode material layer is too small, that is, e% < 10%, it is not conducive to the fast charging performance of the battery; when the porosity e% of the positive electrode material layer is too large, that is, e% > 35%, it will cause a decrease in the active material loading and a decrease in the electrode sheet surface density, deteriorating the energy density. Specifically, the porosity e% of the positive electrode material layer is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35% or the range composed of any two of these values. Preferably, the porosity e% of the positive electrode material layer is 10% - 30%. More preferably, the porosity e% of the positive electrode material layer is 15% - 25%.

[0033] Specifically, in some embodiments of the present invention, the following method is used to test the porosity of the positive electrode material layer: (1) After disassembling the battery, take out the positive electrode sheet, soak and clean the positive electrode sheet with EMC, and vacuum dry for 24 h. Cut the electrode sheet sample into slender strips of a certain size, measure the apparent volume of the positive electrode material layer, the apparent volume = the thickness of the positive electrode material layer × the length of the electrode sheet × the width of the electrode sheet, and then put it into a sample tube; (2) Put the sample tube into a degassing station, heat and vacuum degas the sample to remove the surface adsorbed gas, and test the mass. Use a mercury intrusion method tester to measure the pore volume of the sample, and the porosity (%) = pore volume / apparent volume × 100%.

[0034] Specifically, in some embodiments of the present invention, the compound shown in the structural formula 1 includes at least one of the following compounds 1-1 to compounds 1-6:

[0035] Compound 1-1

[0036] Compound 1-2

[0037] Compound 1-3

[0038] Compound 1-4

[0039] Compound 1-5

[0040] Compound 1-6.

[0041] Preferably, the compound represented by Structural Formula 1 includes at least one of Compound 1-1, Compound 1-2, or Compound 1-5, that is, at least one of methyl acetate, ethyl acetate, or ethyl propionate. More preferably, the compound represented by Structural Formula 1 includes at least ethyl acetate. Compared with other carboxylic ester compounds, ethyl acetate not only has a lower viscosity, can reduce the resistance of lithium ion movement, helps to improve the ionic conductivity of the electrolyte, but also is beneficial to stabilizing the cathode structure. Therefore, while improving the fast charging performance of the lithium ion battery, it can also improve its high temperature performance, thereby improving the overall performance of the electrochemical device.

[0042] Specifically, in some embodiments of the present invention, the cathode active material includes: LiFePO4, LiFe 0.6 Mn 0.4 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.2 Mn 0.8 PO4, LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.55 Zr 0.05 PO4, LiFe 0.4 Mn 0.55 Mg 0.05 PO4, LiFe 0.4 Mn 0.55 Na 0.05 PO4, LiFe 0.4 Mn 0.55 Zn 0.05 PO4, LiFe 0.4 Mn 0.55 Al 0.05 PO4, LiFe 0.4 Mn 0.55 Co 0.05 PO4, LiFe 0.4 Mn 0.55 B 0.05 PO4, LiFe0.4 Mn 0.55 Ga 0.05 PO4, LiFe 0.4 Mn 0.55 F 0.05 PO4, LiFe 0.4 Mn 0.55 W 0.05 PO4, LiFe 0.4 Mn 0.55 Pb 0.05 PO4, LiFe 0.4 Mn 0.55 K 0.05 PO4, LiFe 0.4 Mn 0.55 Cr 0.05 PO4, LiFe 0.4 Mn 0.55 Ba 0.05 PO4, LiFe 0.4 Mn 0.55 Ca 0.05 PO4, LiFe 0.4 Mn 0.55 Ni 0.05 PO4, LiFe 0.4 Mn 0.55 Sr 0.05 PO4, LiFe 0.4 Mn 0.55 Ti 0.05 PO4, LiFe 0.4 Mn 0.55 Si 0.05 PO4, LiFe 0.4 Mn 0.55 V 0.05 PO4, LiFe 0.4 Mn 0.55 Mo 0.05 PO4 or LiFe 0.4 Mn 0.55 Nb 0.05 Any one or more of PO4.

[0043] Specifically, in some embodiments of the present invention, the positive electrode sheet further includes a positive electrode current collector, the positive electrode material layer is located on both sides of the positive electrode current collector, and the positive electrode current collector includes a metal material that can conduct electrons. Preferably, the positive electrode current collector includes one or more of aluminum, nickel, tin, copper, and stainless steel.

[0044] 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 thermoplastic resins such as polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene, copolymers of tetrafluoroethylene - hexafluoropropylene, copolymers of tetrafluoroethylene - perfluoroalkyl vinyl ether, copolymers of ethylene - tetrafluoroethylene, copolymers of vinylidene fluoride - tetrafluoroethylene, copolymers of vinylidene fluoride - trifluoroethylene, copolymers of vinylidene fluoride - trichloroethylene, copolymers of vinylidene fluoride - fluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene, thermoplastic polyimide, polyethylene, and polypropylene; acrylic resins; and styrene - butadiene rubber. The positive electrode conductive agent includes one or more of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, or reduced graphene oxide.

[0045] Specifically, in some embodiments of the present invention, the non - aqueous electrolyte further includes an auxiliary additive, and the auxiliary additive includes at least one of cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, phosphate compounds, and borate compounds. Adding the auxiliary additive to the electrolyte can participate in the film - forming process during formation together with the first additive and the second additive, further improving the stability of the protective film, and thus further improving the high - temperature storage performance and fast - charge cycle performance of the battery.

[0046] In some preferred embodiments, the cyclic sulfate compounds include at least one of 4 - methyl vinyl sulfate, vinyl sulfate, and propylene sulfate.

[0047] In some preferred embodiments, the sultone compounds include at least one of 1,3 - propane sultone, 1,4 - butane sultone, and allyl - 1,3 - sultone.

[0048] In some preferred embodiments, the cyclic carbonate compounds include at least one of vinylene carbonate, ethylene vinylene carbonate, methylene vinylene carbonate, fluoroethylene carbonate, trifluoromethyl vinylene carbonate, difluoroethylene carbonate, and the compound shown in Structural Formula 2 below:

[0049] ;

[0050] Structural Formula 2

[0051] In the shown Structural Formula 2, R 21 、R 22 、R 23 、R 24 、R 25 、R 26Each is independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group.

[0052] In some preferred embodiments, the compound represented by Structural Formula 2 includes at least one of the compounds represented by Compounds 2-1 to 2-6 below:

[0053] Compound 2-1

[0054] Compound 2-2

[0055] Compound 2-3

[0056] Compound 2-4

[0057] Compound 2-5

[0058] Compound 2-6.

[0059] In some preferred embodiments, the phosphate compound includes at least one of the compounds represented by the following Structural Formula 3:

[0060] ;

[0061] Structural Formula 3

[0062] In Structural Formula 3, R 31 , R 32 , R 33 are each independently selected from a C1-C5 saturated hydrocarbon group, an unsaturated hydrocarbon group, a halogenated hydrocarbon group, -Si(C m H 2m+1 )3, and m is a natural number from 1 to 3; more preferably, the compound represented by Structural Formula 3 includes at least one of tris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate, tri(propargyl) 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, diallyl hexafluoroisopropyl phosphate, tri(allyl) 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, diallyl hexafluoroisopropyl phosphate.

[0063] In some preferred embodiments, the borate compound includes at least one of tris(trimethylsilyl) borate and tris(triethylsilyl) borate.

[0064] Specifically, in some embodiments of the present invention, based on the total mass of the non-aqueous electrolyte being 100%, the content of the auxiliary additive is 0.01% to 10%. Preferably, the content is 0.1% to 5%; more preferably, the content is 0.1% to 2%. Specifically, the content of any 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 a range composed of any two of these values.

[0065] Specifically, in some embodiments of the present invention, the lithium salt includes at least one of LiPF6, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, LiPO2F2, LiBF4, LiBOB, LiSbF6, LiAsF6, LiCF3SO3, LiDFOB, LiDFOP, LiN(SO2CF3)2, LiC(SO2CF3)3, LiN(SO2C2F5)2, LiCl, LiBr, LiI, LiClO4, LiB 10 Cl 10 , LiAlCl4, lithium chloroborane, lithium tetraphenylborate.

[0066] Specifically, in some embodiments of the present invention, the negative electrode sheet includes a negative electrode material layer containing a negative electrode active material, and the negative electrode active material is any one or several of a silicon-based material and a carbon material. The silicon-based material is selected from one or more of a silicon material, a silicon oxide material, a silicon-carbon material, and a silicon alloy material; preferably, the silicon material is a nanosilicon material; preferably, the silicon oxide material is a SiO x material, where 0 ≤ x < 2; preferably, the silicon-carbon material is: a silicon-based material containing silicon and a carbon material, and / or a silicon-based material containing SiO y and a carbon material, where 0 ≤ y < 2; preferably, the silicon alloy material is a Mg2Si alloy material and / or an 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 an artificial graphite material.

[0067] 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, which will not be elaborated here. The negative electrode binder and the negative electrode conductive agent can be the same as the positive electrode binder and the positive electrode conductive agent respectively, which will not be elaborated here.

[0068] Specifically, in some embodiments of the present invention, a separator is further included in the lithium-ion battery, and the separator is located between the positive electrode and the negative electrode.

[0069] The separator is a conventional separator, and is selected from one or more of a ceramic separator, a polymer separator, a non-woven fabric, and an inorganic-organic composite separator. For example, a single-layer polypropylene (PP) separator, a single-layer polyethylene (PE) separator, a double-layer PP / PE separator, a double-layer PP / PP separator, or a triple-layer PP / PE / PP separator.

[0070] In the lithium-ion battery of the present invention, any one of Compounds 1 to 5 is used as a first additive, fluoroethylene carbonate is used as a second additive, and a short-chain carboxylic acid ester compound represented by Structural Formula 1 is used as a solvent. The mass percentage content a% of the first additive, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound represented by Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer and the porosity e% of the positive electrode material layer satisfy the relationship of 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, and 10 ≤ e ≤ 35. A layer of organic matter protective film can be formed on the surface of the negative electrode, avoiding the deterioration of the high-temperature performance of the battery by the carboxylic acid ester, and at the same time, significantly improving the ionic conductivity of the electrolyte, thereby obtaining a lithium-ion battery with better high-temperature storage performance and fast charge and discharge cycle performance. Detailed implementation manners

[0071] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0072] Example 1

[0073] The preparation method of the lithium-ion battery in this embodiment includes the following steps:

[0074] (1) Preparation of non-aqueous electrolyte:

[0075] Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC) and ethyl acetate (EA), then add lithium hexafluorophosphate (LiPF6) until the molar concentration reaches 1.1 mol / L, and then add Compound 1 as the first additive, vinylene carbonate (VC), and fluoroethylene carbonate (FEC). Based on the total weight of the non-aqueous electrolyte being 100%, the content of the additive satisfying Structural Formula 1 is 0.05%, the content of VC is 3%, the content of FEC is 0.7%, and the content of ethyl acetate (Compound 1-2) is 30%.

[0076] (2) Preparation of the positive electrode sheet:

[0077] Mix the positive electrode active material LiFePO4, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 96:2:2, and then disperse them in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. Coat the slurry evenly on both sides of an aluminum foil current collector, and after drying, calendaring, vacuum drying, and welding an aluminum lead wire with an ultrasonic welder, a positive electrode sheet is obtained. The specific surface area of the positive electrode material layer is 8 m 2 / g, and the porosity of the positive electrode material layer is 23%.

[0078] (3) Preparation of the negative electrode sheet:

[0079] Mix the negative electrode active material artificial graphite, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a mass ratio of 95:1:1.5:2.5. Then disperse them in deionized water to obtain a negative electrode slurry. Coat the slurry on both sides of a copper foil, and after drying, calendaring, vacuum drying, and welding a nickel lead wire with an ultrasonic welder, a negative electrode sheet is obtained.

[0080] (4) Preparation of the battery cell:

[0081] Place a three-layer separator with a thickness of 20 μm between the positive electrode plate and the negative electrode plate, then wind the sandwich structure composed of the positive electrode plate, negative electrode plate, and separator, and after flattening the wound body, put it into an aluminum foil packaging bag and vacuum bake it at 85°C for 48 h to obtain a battery cell to be injected with electrolyte.

[0082] (5) Injection of electrolyte and formation of the battery cell:

[0083] In a glove box with the moisture and oxygen content controlled below 10 ppm, inject the above-prepared non-aqueous electrolyte into the battery cell, perform vacuum packaging, and let it stand at 45°C for 48 h. Then carry out the conventional formation for the first charge according to the following steps: constant current charge at 0.05C for 2 hours, constant current charge at 0.1C for 1 hour, and constant current charge at 0.2C for 1 hour.

[0084] (6) Capacity grading of the battery cell:

[0085] The battery cell was left standing at 45 °C for 48 h, then vacuum sealed for the second time to remove the gas generated during formation, and then charged at a constant current of 0.2C to 3.65V, followed by constant voltage charging until the current dropped to 0.05C. After standing for 5 min, it was discharged at a constant current of 0.2C to 2.5V to obtain a lithium iron phosphate cathode / graphite anode lithium-ion battery.

[0086] Examples 2 to 33 and Comparative Examples 1 to 17

[0087] This example and the comparative examples are used to compare and illustrate the lithium-ion battery disclosed in the present invention, including most of the operation steps in Example 1 above. The differences are as follows: the type of the positive electrode active material, the composition of the non-aqueous electrolyte and the content of each component, the specific surface area and porosity of the positive electrode material, as shown in Tables 1 to 5.

[0088] The following performance tests were carried out on the lithium-ion batteries prepared in each example and comparative example:

[0089] (1) High-temperature storage performance test

[0090] At 25 °C, the battery was charged at a constant current and voltage of 1C to 3.65V (cut-off current 0.05C), and discharged at a constant current of 1C to 2.5V, and the discharge capacity of the battery was recorded. Then the battery was charged at a constant current and voltage of 1C to 3.65V (cut-off current 0.05C) and placed in an oven at 60 °C. After 30 days, it was taken out. After the battery cooled down, at 25 °C, it was discharged at a constant current of 1C to 2.5V, and the discharge capacity and volume of the battery were recorded.

[0091] High-temperature storage capacity retention rate (%) = Discharge capacity after storage at 60 °C for 30 days / Discharge capacity before storage × 100%

[0092] (2) Normal-temperature fast-charging cycle life test

[0093] At 25 °C, the battery was charged at a constant current of 4C until the battery reached 80% SOC state, then charged at a constant current and voltage of 1C to 3.65V (cut-off current 0.05C), and then discharged at a constant current of 1C to 2.5V. This process was continuously used for cyclic charge and discharge tests, and the discharge capacity of each cycle was recorded.

[0094] Cycle capacity retention rate (%) = Discharge capacity of the current cycle / Discharge capacity of the first cycle × 100%

[0095] Record the capacity retention rate after 800 cycles.

[0096] (3) High-temperature fast-charging cycle life test

[0097] At 45 °C, the battery is charged at a constant current of 4C until the battery reaches 80% SOC state, then charged at a constant current and constant voltage of 1C until 3.65V (cut-off current 0.05C), and then discharged at a constant current of 1C until 2.5V. Use this process to continuously perform cyclic charge and discharge tests, and record the discharge capacity of each cycle.

[0098] Cyclic capacity retention rate (%) = Current cycle discharge capacity / First cycle discharge capacity × 100%

[0099] Record the capacity retention rate after 800 cycles.

[0100] Test results

[0101] As shown in Table 1 are the various parameters of the lithium-ion batteries prepared in Examples 1 to 20 and Comparative Examples 1 to 14; the differences between Examples 2 to 20 and Comparative Examples 1 to 14 and Example 1 are the relevant parameters in Table 1.

[0102] Table 1

[0103] Group Content of the first additive a / % Content of the second additive b / % Content of the solvent c / % <![CDATA[Specific surface area of the positive electrode material layer / d / m 2 / g]]> Porosity of the positive electrode material layer e / % 0.01×(c + e + 5d) / (a + b) Capacity retention rate after 30 days of high - temperature storage at 60℃ / % Capacity retention rate after 800 cycles of 4C / 1C at 25℃ / % Capacity retention rate after 800 cycles of 4C / 1C at 45℃ / % Example 1 0.05 0.7 30 8 23 1.42 82.5 93.4 88.6 Example 2 0.08 0.6 35 8 15 1.32 81.2 91.3 83.2 Example 3 0.1 0.4 37 7 20 1.84 81.6 93.1 84.4 Example 4 0.3 0.1 40 7 10 2.13 82.3 91.9 83.7 Example 5 0.7 0.8 43 9 27 0.77 82.9 94.3 90.2 Example 6 1 1 45 10 30 0.63 83.7 93.8 90.6 Example 7 1 1 30 10 20 0.50 84.5 91.6 89.4 Example 8 0.01 0.3 32 7 13 2.58 75.3 90.4 80.5 Example 9 0.06 0.05 33 9 17 8.64 73.8 91.1 79.7 Example 10 0.07 0.5 47 10 29 2.21 74.6 91.8 83.9 Example 11 0.09 0.7 38 11 27 1.52 76.8 91.4 83.6 Example 12 0.3 0.9 44 7 33 0.93 81.8 89.5 82.4 Example 13 0.03 0.08 36 10 24 10.00 72.3 86.6 80.3 Example 14 0.6 1.5 50 12 26 0.65 81.5 85.9 83.4 Example 15 0.8 0.06 36 6 31 1.13 76.1 89.5 85.6 Example 16 0.9 0.07 48 12 35 1.47 73.2 90.7 84.1 Example 17 3 0.05 49 7 34 0.39 85.1 86.4 81.5 Example 18 0.02 0.06 46 12 10 14.50 71.2 84.2 76.5 Example 19 0.01 0.05 35 10 35 20.00 72.5 85.6 77.8 Example 20 1 1 30 6 10 0.35 84.8 88.5 78.9 Comparative Example 1 0 0.1 30 7 10 7.50 65.7 80.6 70.3 Comparative Example 2 0.05 0 30 7 10 15.00 66.5 81.7 72.4 Comparative Example 3 0.008 0.1 30 7 10 6.94 66.9 80.8 70.8 Comparative Example 4 3.2 0.5 40 10 30 0.32 78.2 83.6 74.9 Comparative Example 5 0.5 0.03 30 7 10 1.42 73.3 84.8 73.8 Comparative Example 6 0.8 1.2 35 8 20 0.48 76.6 85.1 74.6 Comparative Example 7 0.6 0.5 28 8 25 0.85 79.8 83.2 75.6 Comparative Example 8 0.5 0.4 52 10 25 1.41 71.6 86.3 71.6 Comparative Example 9 2 0.7 45 5 25 0.35 80.8 83.5 75.3 Comparative Example 10 0.01 0.7 45 13 25 1.90 71.4 86.3 72.5 Comparative Example 11 0.09 0.05 45 9 4 6.71 70.3 83.9 75.6 Comparative Example 12 0.08 0.08 45 9 36 7.88 68.5 84.5 74.2 Comparative Example 13 0.01 0.05 30 12 35 20.83 66.8 81.2 70.8 Comparative Example 14 3 1 30 6 35 0.24 78.6 79.5 72.3

[0104] From the test results of Examples 1 to 20 and Comparative Examples 1 to 14, it can be seen that for the lithium-ion battery provided in the present invention, at least one compound among Compounds 1 to 5 is used as the first additive, fluoroethylene carbonate is used as the second additive, and the compound shown in Structural Formula 1 is used as the solvent, and it is specified that in the non-aqueous electrolyte, the mass percentage content a% of the first additive in the electrolyte, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer and the porosity e% of the positive electrode material layer satisfy the relational expression 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, 10 ≤ e ≤ 35, a lithium-ion battery with excellent high-temperature storage performance and fast charge and discharge cycle performance can be obtained.

[0105] From the test results of Examples 1 to 20 and Comparative Examples 1 to 14, it can be seen that when in the non-aqueous electrolyte of the lithium-ion battery, the mass percentage content a% of the first additive in the electrolyte, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2When any one of the parameters of / g and the porosity e% of the positive electrode material layer does not meet the range or the value of 0.01×(c + e + 5d) / (a + b) is too large or too small, it is impossible to ensure that an organic matter protective film can be formed on the surface of the negative electrode, resulting in side reactions between the negative electrode and the carboxylic acid ester, leading to the deterioration of the high-temperature performance of the battery by the carboxylic acid ester. In addition, it is also impossible to improve the lithium-ion transport and charge transfer efficiency at the electrode-electrolyte interface and the fast charge and discharge cycle performance of the battery.

[0106] When the mass percentage content a% of the first additive in the electrolyte, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, and the specific surface area d m 2 / g of the positive electrode material layer and the porosity e% of the positive electrode material layer further satisfy 0.5 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 10, 0.05 ≤ a ≤ 1, 0.1 ≤ b ≤ 1, 30 ≤ c ≤ 45, 7 ≤ d ≤ 10, 10 ≤ e ≤ 30, a dense and thin organic matter protective film with a certain flexibility can be better formed during the battery formation process, better preventing the carboxylic acid ester from deteriorating the high-temperature performance of the battery. Within this range, the positive electrode material layer also obtains the contact area with the electrolyte as much as possible, which is beneficial to the adsorption and infiltration of the electrolyte by the electrode sheet, improves the liquid retention capacity of the battery cell, ensures that at a large current density during high-rate charging, the lithium-ion transport and charge transfer efficiency at the electrode-electrolyte interface are improved, which is beneficial to further improving the fast charge and discharge cycle performance of the battery, thereby obtaining a lithium-ion battery that takes into account relatively good high-temperature storage performance and fast charge and discharge cycle performance.

[0107] Table 2 shows the various parameters of the lithium-ion batteries prepared in Example 1 and Examples 21 to 24; the differences between Examples 21 to 24 and Example 1 lie in the relevant parameters in Table 2.

[0108] Table 2

[0109]

[0110] From the measurement results in Table 2, it can be seen that when the mass percentage content a% of the first additive, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer, the porosity e% of the positive electrode material layer, and the value of the relationship 0.01×(c + e + 5d) / (a + b) meet the relevant requirements, adding Compounds 1 to 5 as the first additive can optimize the high-temperature storage performance and fast charge and discharge cycle performance of the lithium-ion battery, indicating that the battery system of the present invention has universality for different first additives.

[0111] Table 3 shows the various parameters of the lithium-ion batteries prepared in Example 1, Examples 25 to 29, and Comparative Examples 15 to 17; the differences between Examples 25 to 29 and Comparative Examples 15 to 17 and Example 1 lie in the relevant parameters in Table 3.

[0112] Table 3

[0113]

[0114] As can be seen from Table 3, when the mass percentage content a% of the first additive, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer, the porosity e% of the positive electrode material layer, and the value of the relational expression 0.01×(c + e + 5d) / (a + b) meet the relevant high requirements, adding different types of solvents shown in Structural Formula 1 can optimize the high-temperature storage performance and fast charge cycle performance of the lithium-ion battery, indicating that the battery system of the present invention has universality for different solvents. From the test results of Example 1, Examples 25 to 29, and Comparative Examples 15 to 17, it can be seen that when using a long-chain carboxylic acid ester compared with the second additive, the viscosity is relatively large, and the improvement effect on the fast charge cycle performance of the battery is not good.

[0115] Table 4 shows the various parameters of the lithium-ion batteries prepared in Example 1 and Examples 30 to 34; the differences between Examples 30 to 34 and Example 1 lie in the relevant parameters in Table 4.

[0116] Table 4

[0117]

[0118] As can be seen from Table 4, when the mass percentage content a% of the first additive, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer, the porosity e% of the positive electrode material layer, and the value of the relational expression 0.01×(c + e + 5d) / (a + b) meet the relevant requirements, using different types of positive electrode active materials can optimize the high-temperature storage performance and fast charge cycle performance of the lithium-ion battery, indicating that the battery system of the present invention has universality for different positive electrode active materials.

[0119] Table 5 shows the various parameters of the lithium-ion batteries prepared in Example 1 and Examples 35 to 37; the differences between Examples 35 to 37 and Example 1 lie in the relevant parameters in Table 5.

[0120] Table 5

[0121]

[0122] Wherein: VC is vinylene carbonate, DTD is ethylene sulfate, MMDS is methylene methanedisulfonate, and TMSP is tris(trimethylsilyl) phosphate.

[0123] As can be seen from the measurement results in Table 5, when the mass percentage content a% of the first additive, the mass percentage content b% of the second additive in the electrolyte, the mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, the specific surface area d m 2 / g of the positive electrode material layer and the porosity e% of the positive electrode material layer and the value of the relational expression 0.01×(c + e + 5d) / (a + b) meet the relevant high requirements, adding different auxiliary additives can optimize the high-temperature storage performance and fast charge cycle performance of the lithium-ion battery, indicating that the battery system of the present invention has universality for different positive electrode active materials.

[0124] The above embodiments only represent several implementation manners of the present invention, and the description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims

1. A lithium-ion battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet and a non-aqueous electrolyte; The positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes a phosphate compound LiFe 1-x-y Mn x M y PO4, where 0 ≤ x ≤ 0.8, 0 ≤ y ≤ 0.05, and M includes any one or more of Co, B, Ga, F, W, Zr, Mg, Na, Pb, K, Al, Cr, Ba, Ca, Ni, Sr, Ti, Zn, Si, V, Mo, or Nb; The non-aqueous electrolyte includes a first additive, a second additive, a lithium salt and a solvent; the first additive includes at least one of the following compounds: Compound 1 Compound 2 Compound 3 Compound 4 Compound 5; The second additive includes fluoroethylene carbonate; The solvent includes the compound shown in Structural Formula 1; ； Structural Formula 1 Wherein, R1 is an alkyl group or a fluoroalkyl group with a carbon atom number ≤ 2, R2 is an alkyl group with a carbon atom number ≤ 3, and the total number of carbon atoms in R1 and R2 ≤ 4; The lithium ion battery satisfies the following conditions: 0.3 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 20, 0.01 ≤ a ≤ 3, 0.05 ≤ b ≤ 1.5, 30 ≤ c ≤ 50, 6 ≤ d ≤ 12, 10 ≤ e ≤ 35; Wherein, a is the mass percentage content of the first additive in the non-aqueous electrolyte, with the unit of %; b is the mass percentage content of the second additive in the non-aqueous electrolyte, with the unit of %; c is the mass percentage content of the compound shown in Structural Formula 1 in the non-aqueous electrolyte, with the unit of %; d is the specific surface area of the positive electrode material layer, with the unit of m 2 / g; e is the porosity of the positive electrode material layer, with the unit of %.

2. The lithium-ion battery according to claim 1, characterized in that, The lithium ion battery satisfies: 0.5 ≤ 0.01×(c + e + 5d) / (a + b) ≤ 10.

3. The lithium-ion battery according to claim 1, wherein, The mass percentage content a% of the first additive in the non-aqueous electrolyte is 0.05% - 1%.

4. The lithium-ion battery according to claim 1, wherein The mass percentage content b% of the second additive in the non-aqueous electrolyte is 0.1% - 1%.

5. The lithium-ion battery according to claim 1, wherein, The mass percentage content c% of the compound shown in Structural Formula 1 in the non-aqueous electrolyte is 30% - 45%.

6. The lithium ion battery according to claim 1, characterized in that, The specific surface area d of the positive electrode material layer is 7 m 2 / g to 10 m 2 / g.

7. The lithium ion battery according to claim 1, wherein The porosity e% of the positive electrode material layer is 10% - 30%.

8. The lithium-ion battery according to claim 1, characterized in that, The compound shown in Structural Formula 1 includes at least one of the following Compound 1-1 to Compound 1-6: Compound 1-1 Compound 1-2 Compounds 1-3 Compounds 1-4 Compounds 1-5 Compounds 1-6.

9. The lithium-ion battery according to claim 1, wherein, The non-aqueous electrolyte further includes an auxiliary additive, and the auxiliary additive includes at least one of cyclic sulfate compounds, sulfonic acid lactone compounds, cyclic carbonate compounds, phosphate compounds and borate compounds; and / or, Based on the total mass of the non-aqueous electrolyte being 100%, the content of the auxiliary additive is 0.01% - 10%.

10. The lithium-ion battery according to claim 9, wherein The cyclic sulfate compounds include at least one of 4-methyl vinyl sulfate, vinyl sulfate, and propylene sulfate; and / or, The sulfonic acid lactone compounds include at least one of 1,3-propane sultone, 1,4-butane sultone, and allyl-1,3-sultone; and / or, The cyclic carbonate compounds include at least one of vinylene carbonate, ethylene vinylene carbonate, methylene vinylene carbonate, fluoroethylene carbonate, trifluoromethyl carbonate, difluoroethylene carbonate and the compound shown in the following Structural Formula 2: ； Structural Formula 2 In the shown structural formula 2, R 21 , R 22 , R 23 , R 24 , R 25 , R 26 are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group; and / or, The phosphate compounds include at least one of the compounds shown in the following Structural Formula 3: ； Structural Formula 3 In the said structural formula 3, R 31 , R 32 , R 33 are each independently selected from saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si(C m H 2m+1 )3, and m is a natural number from 1 to 3; and / or, The borate compounds include at least one of tris(trimethylsilyl) borate and tris(triethylsilyl) borate.

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