Lithium-ion battery and electrochemical device comprising the same

Abstract

The application relates to a lithium ion battery and an electrochemical device containing the same, the lithium ion battery comprising an electrolyte and a negative electrode sheet, the electrolyte comprising vinylene carbonate and fluoroethylene carbonate; the negative electrode sheet comprising a negative electrode active material, and the OI value of the negative electrode active material being a; the weight percentage content of the vinylene carbonate being b% and the weight percentage content of the fluoroethylene carbonate being c% based on the weight of the electrolyte; a, b and c satisfying: 0.3 <= a / (b+c) <= 6, 0.02 <= b+c <= 10, 0.1 < b / c < 3. When the OI value of the negative electrode active material, the weight percentage content of the vinylene carbonate and the weight percentage content of the fluoroethylene carbonate satisfy the above relationship, the destruction of the SEI film caused by the volume expansion of the negative electrode active material can be reduced, a SEI film with toughness is formed on the negative electrode surface, the internal resistance of the lithium ion battery is significantly reduced, and the high-temperature storage performance and the cycle performance of the lithium ion battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical devices, and in particular to a lithium ion battery and an electrochemical device comprising the same. BACKGROUND

[0002] Secondary batteries (e.g., lithium ion secondary batteries) have been widely used in the field of portable electronic devices such as mobile phones, notebook computers, and video cameras, and their use is expanding from small portable electronic devices to large electric vehicles and renewable energy storage fields, because they have high energy density, high operating voltage, long cycle life, no memory effect, and are environmentally friendly.

[0003] Currently, graphite-based negative electrode materials are the mainstream negative electrode materials for lithium ion secondary batteries, which are mainly divided into artificial graphite and natural graphite. Natural graphite has high capacity and high compaction density, and is relatively inexpensive. However, due to the different particle sizes and many surface defects, the compatibility with electrolyte is poor, there are many side reactions, and volume expansion occurs during the cycle process, which increases the internal stress of the battery and affects the service life and safety performance of the battery. SUMMARY

[0004] In view of the problems in the prior art, the present application provides a lithium ion battery and an electrochemical device comprising the same, to improve the cycle performance and high-temperature storage performance of the electrochemical device.

[0005] The first aspect of the present application provides a lithium ion battery, which comprises an electrolyte and a negative electrode sheet, the electrolyte comprises vinylene carbonate and fluoroethylene carbonate, the negative electrode sheet comprises a negative electrode active material, the OI value of the negative electrode active material is a; the weight percentage content of the vinylene carbonate is b% and the weight percentage content of the fluoroethylene carbonate is c% based on the weight of the electrolyte; the a, b and c satisfy: 0.3≤a / (b+c)≤6, 0.02≤b+c≤10, 0.1

[0006] In some embodiments, 0.4≤a / (b+c)≤3.

[0007] In some embodiments, 0.2≤b+c≤5.

[0008] In some embodiments, 0.5≤b / c≤2.5.

[0009] In some embodiments, the OI value of the negative active material is a, and a ranges from 2≤a≤15. In some embodiments, 3≤a≤7.

[0010] In some embodiments, the weight percentage of the vinylene carbonate in the electrolyte is b%, based on the weight of the electrolyte, and b ranges from 0.01≤b≤10. In some embodiments, 0.1≤b≤5.

[0011] In some embodiments, the weight percentage of the fluoroethylene carbonate in the electrolyte is c%, based on the weight of the electrolyte, and c ranges from 0.01≤c≤10. In some embodiments, 0.1≤c≤5.

[0012] In some embodiments, the negative active material comprises natural graphite. The OI value of the negative active material refers to the ratio of the peak area of the 004 characteristic diffraction peak in the X-ray diffraction pattern of the negative active material to the peak area of the 110 characteristic diffraction peak in the X-ray diffraction pattern of the negative active material.

[0013] In some embodiments, the liquid retention coefficient of the lithium ion battery is e, and b, c and e satisfy: 0.01≤e×(b+c)≤50.

[0014] In some embodiments, the liquid retention coefficient of the lithium ion battery is e, and e satisfies: 1≤e≤7.

[0015] In some embodiments, the specific surface area of the negative active material is f m 2 / g, and f ranges from 1≤f≤3.

[0016] In some embodiments, the electrolyte comprises a compound B represented by formula (II):

[0017]

[0018] wherein:

[0019] R6, R7, R8, R9and R 10 are each independently selected from the group consisting of hydrogen, a cyano group, a halogen atom, a C 1-10 alkyl group, a C 2-10 alkenyl group, a C 2-10 alkynyl group, a C 3-6 heterocyclic group, a C 5-6 aryl group, a C 5-6 heteroaryl group, -R 11 -(C=O)-R 12 -, -R 11 -(C=O)-O-R 12 -, -R 11 -(C=O)-NH-R 12R6, R7, R8, R9and R 10 any two of which adjacent to each other are linked to form a cyclic structure of 1-10 ; wherein R 11 and R 12 are each independently selected from a single bond, C 1-10 alkyl, C 2-10 alkenyl, C 2-10 alkynyl, C 3-6 heterocyclyl, C 5-6 aryl, or C 5-6 heteroaryl; the C 1-10 alkyl, C 2-10 alkenyl, C 2-10 alkynyl, C 3-6 heterocyclyl, C 5-6 aryl, C 5-6 heteroaryl, or C 1-10 of the cyclic structure is optionally substituted with one or more amino, cyano, halogen atom, C 1-3 alkyl, C 3-6 heterocyclyl, C 5-6 heteroaryl.

[0020] In some embodiments, the compound B represented by formula (II) comprises at least one of pyridine, 2-methylpyridine, 2-vinylpyridine, 2-ethynylpyridine, 2-fluoropyridine, 2-cyanopyridine, 3-vinylpyridine, 3-fluoropyridine, 2,6-difluoropyridine, pentafluoropyridine, 2,2'-bipyridine, terpyridine, 1,8-naphthyridine, 5,6,7,8-tetrahydroquinoline

[0021] In some embodiments, the compound B represented by formula (II) comprises at least one of pyridine, 2-vinylpyridine, 2-fluoropyridine.

[0022] In some embodiments, the weight percentage of the compound represented by formula (II) is k%, based on the weight of the electrolyte, k satisfying: 0≤k<3.

[0023] In some embodiments, the Dv50 of the negative electrode active material is g pm, g satisfying: 10≤g≤25.

[0024] In some embodiments, the compaction density of the negative electrode active material is h g / cm 3 , h satisfying: 1≤h≤1.8.

[0025] The second aspect of the present application provides an electrochemical device comprising the lithium ion battery provided by the first aspect of the present application.

[0026] The application can reduce the problems of cycle and storage performance deterioration caused by the destruction of SEI film due to the volume expansion of the negative active material, and can improve the expansion and gas production problems, and improve the cycle performance and high-temperature storage performance of the lithium ion battery, by setting the OI value of the negative active material of the lithium ion battery, the weight percentage content of vinylene carbonate in the electrolyte, and the weight percentage content of fluoroethylene carbonate in the electrolyte to satisfy a certain relationship. DETAILED DESCRIPTION

[0027] To make the objects, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in conjunction with embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The embodiments described herein are illustrative in nature and serve to provide a basic understanding of the present application. The embodiments of the present application should not be interpreted as a limitation of the present application.

[0028] The first aspect of the present application provides a lithium ion battery, which comprises an electrolyte and a negative electrode sheet, the electrolyte comprising vinylene carbonate and fluoroethylene carbonate, and the negative electrode sheet comprising a negative active material, the OI value of the negative active material being a; the weight percentage content of vinylene carbonate being b% and the weight percentage content of fluoroethylene carbonate being c% based on the weight of the electrolyte; and the a, b and c satisfying: 0.3≤a / (b+c)≤6, 0.02≤b+c≤10, 0.1<b / c<3.

[0029] In the present application, when the OI value of the negative active material is small, the compaction density is in a suitable range, the direction selectivity of natural graphite during lithium intercalation is small, the lithium intercalation expansion can be dispersed in all directions, the cycle expansion of the electrode sheet and the battery is reduced, and since the SEI film formed by vinylene carbonate and fluoroethylene carbonate has a large impedance, the additives should be added in an appropriate amount under the premise of ensuring performance; when the OI value of the negative active material is large, the cycle expansion of the electrode sheet and the battery is more serious, and the increase in the number of material end faces leads to an increase in surface side reactions and gas production deterioration, at which time more vinylene carbonate and fluoroethylene carbonate are needed to protect the negative active material and improve the expansion and gas production problems. In the present application, vinylene carbonate mainly generates PEO type polymer SEI film, and fluoroethylene carbonate mainly generates LiF and other SEI films containing inorganic components, and the contents of the two should satisfy the above relationship to form a SEI film rich in organic and inorganic components on the surface of the negative electrode. The film has a certain toughness and elasticity, and can reduce the problems of cycle performance and storage performance caused by the destruction of the SEI film due to the volume expansion of the natural graphite.

[0030] In some embodiments, the lithium ion battery according to the present application comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, etc., but is not limited thereto.

[0031] Electrolyte:

[0032] In some embodiments, 0.4≤a / (b+c)≤3. When the content of vinylene carbonate and fluoroethylene carbonate and the OI value of the negative electrode material are within this range, the lithium ion battery has more optimal cycle performance and high-temperature storage performance.

[0033] In some embodiments, 0.2≤b+c≤5 and / or 0.5≤b / c≤2.5. When the content of vinylene carbonate and fluoroethylene carbonate satisfies the above, an SEI film rich in organic and inorganic components can be formed on the surface of the negative electrode, the film has certain toughness and elasticity, and the destruction of the SEI film caused by the volume expansion of natural graphite can be reduced, so that the lithium ion battery has more optimal cycle performance and high-temperature storage performance.

[0034] In some embodiments, the weight percentage content of vinylene carbonate is b% based on the weight of the electrolyte, and b is in the range of 0.01≤b≤10, which can further improve the ion transmission of the electrolyte and improve the battery performance. In some embodiments, 0.1≤b≤5.

[0035] In some embodiments, the weight percentage content of fluoroethylene carbonate is c% based on the weight of the electrolyte, and c is in the range of 0.01≤c≤10, which can improve the toughness of the negative electrode SEI film. In some embodiments, 0.1≤c≤5.

[0036] In some embodiments, the liquid retention coefficient of the lithium ion battery is e, and b, c and e satisfy 0.01≤e×(b+c)≤50. The appropriate electrolyte retention amount can further improve the ion transmission in the electrolyte and improve the cycle performance of the battery. In some embodiments, the liquid retention coefficient e of the lithium ion battery satisfies 1≤e≤7.

[0037] In some embodiments, the electrolyte comprises a compound B represented by formula (II):

[0038]

[0039] wherein:

[0040] R6, R7, R8, R9and R 10 are each independently selected from hydrogen, cyano, a halogen atom, C 1-10 alkyl, C 2-10 alkenyl, C 2-10 alkynyl, C 3-6 heterocyclyl, C 5-6 aryl, C 5-6 heteroaryl, -R 11 -(C=O)-R 12 -, -R 11-(C=O)-OR 12 -、-R 11 -(C=O)-NH-R 12 - or R6, R7, R8, R9 and R 10 Any two adjacent elements in the middle are connected to form C. 1-10 A ring-shaped structure; wherein, R 11 and R 12 Each is independently selected from single bonds, C 1-10 Alkyl, C 2-10 alkenyl, C 2-10 alkynyl group, C 3-6 Heterocyclic group, C 5-6 Aryl or C 5-6 heteroaryl; the C 1-10 Alkyl, C 2-10 alkenyl, C 2-10 alkynyl group, C 3-6 Heterocyclic group, C 5-6 Aryl, C 5-6 heteroaryl or C 1-10 The cyclic structure is optionally separated by one or more amino, cyano, halogen atoms, C atoms 1-3 Alkyl, C 3-6 Heterocyclic group, C 5-6 It is replaced by heteroaryl compounds.

[0041] In some embodiments, compound B represented by formula (II) includes at least one of the following: pyridine, 2-methylpyridine, 2-vinylpyridine, 2-ethynylpyridine, 2-fluoropyridine, 2-cyanopyridine, 3-vinylpyridine, 3-fluoropyridine, 2,6-difluoropyridine, pentafluoropyridine, 2,2'-bipyridine, terpyridine, 1,8-naphthidine, 5,6,7,8-tetrahydroquinoline.

[0042] In some embodiments, compound B represented by formula (II) includes at least one of the following: pyridine, 2-vinylpyridine, and 2-fluoropyridine. Compound B represented by formula (II) can suppress the consumption of solvents and film-forming additives during cycling and storage, thereby further improving cycling performance.

[0043] In some embodiments, the weight percentage of the compound represented by formula (II) is k%, based on the weight of the electrolyte, where k satisfies: 0 ≤ k < 3. This allows the lithium-ion battery to have better cycle performance.

[0044] In some embodiments, the electrolyte includes lithium salts and organic solvents.

[0045] In some embodiments, the lithium salt comprises at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt is selected from at least one of the following compounds: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB).

[0046] In some embodiments, the organic solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0047] In some embodiments, the electrolyte can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, or an additive capable of improving certain properties of the battery, such as an additive capable of improving overcharge performance of the battery, an additive capable of improving high-temperature performance of the battery, an additive capable of improving low-temperature performance of the battery, and the like.

[0048] Negative electrode:

[0049] In some embodiments, the negative electrode includes a negative electrode tab including a negative electrode current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer including a negative electrode active material. The negative electrode active material includes graphite, the graphite including at least one of natural graphite or artificial graphite.

[0050] In some embodiments, the negative electrode active material is preferably natural graphite.

[0051] In some embodiments, the negative electrode active material has an OI value of a: 2≤a≤15. When the OI value of the negative electrode material is within this range, the cross-sectional proportion of the graphite material can be reduced, side reactions can be reduced, and the cycle performance and high-temperature storage performance of the lithium ion battery can be further improved. In some embodiments, 3≤a≤7.

[0052] In some embodiments, the specific surface area of the negative active material is f m2 / g, f satisfying 1≤f≤3. 2 The specific surface area of the negative active material in this range can ensure contact with the electrolyte and less side reaction with the electrolyte, further improving the cycle performance of the battery.

[0053] In some embodiments, the Dv50 of the negative active material is g μm, g satisfying 10≤g≤25.

[0054] In some embodiments, the tap density of the negative active material is h g / cm3, h satisfying 1≤h≤1.8. 3

[0055] In some embodiments, the negative active material can further include one or more of meso-carbon microbeads (MCMB for short), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithiated TiO2-Li4Ti5O12, Li-Al alloy, but is not limited thereto. 12

[0056] In some embodiments, the negative active material layer can include a binder and, optionally, a conductive material. The binder improves the binding between the negative active material particles and the binding between the negative active material and the current collector.

[0057] In some embodiments, non-limiting examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, epoxy resin, nylon, and the like.

[0058] In some embodiments, non-limiting examples of the conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, etc.), metal-based materials (e.g., metal powder, metal fibers, etc., such as copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0059] In some embodiments, the negative current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or a polymer substrate coated with a conductive metal.

[0060] In some embodiments, the negative active material is natural graphite, the conductive agent is Super P, and the binder is polyvinylidene fluoride (PVDF).

[0061] Positive electrode:​​

[0062] In some embodiments, the positive electrode includes a positive electrode tab including a positive electrode current collector and a positive electrode active material layer disposed on the current collector. The positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions. The positive electrode active material can include a composite oxide containing lithium and at least one element selected from cobalt, manganese, and nickel. The specific kind of positive electrode active material is not particularly limited and can be selected as needed. The positive electrode active material is optionally at least one of lithium cobaltate LiCoO2(LCO), lithium nickel manganese cobalt ternary material (NCM), lithium iron phosphate, lithium manganate. They can be used alone 1 kind, or 2 kinds and more in any combination.

[0063] In some embodiments, the positive electrode active material can have a coating on the surface. The compound used for the coating can be amorphous or crystalline, and the coating element contained in the coating can include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating can be applied by any method, as long as the method does not adversely affect the performance of the positive electrode active material. For example, the method can include any coating method well known to one of ordinary skill in the art, such as spraying, dipping, etc.

[0064] In some embodiments, the positive electrode active material layer further includes a binder, and optionally further includes a conductive material. The binder improves the binding between the positive electrode active material particles to each other, and also improves the binding of the positive electrode active material to the current collector.

[0065] In some embodiments, non-limiting examples of the binder include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, nylon, etc.

[0066] In some embodiments, non-limiting examples of the conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0067] In some embodiments, the positive electrode current collector can be an aluminum foil (Al), but is not limited thereto.

[0068] In some embodiments, the positive active material is LiFePO4, the conductive agent is Super P, and the binder is polyvinylidene fluoride (PVDF).

[0069] Separator:

[0070] In some embodiments, the electrochemical device is provided with a separator between the positive electrode and the negative electrode to prevent short circuit. The material and shape of the separator used in the electrochemical device are not particularly limited and can be any of the techniques disclosed in the prior art.

[0071] In some embodiments, the separator can include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be used. A surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic material. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from one or a combination of several of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from one or a combination of several of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride and poly(vinylidene fluoride-hexafluoropropylene).

[0072] In some embodiments, the lithium ion battery according to the present application is made by stacking the positive and negative electrode sheets described above.

[0073] In some embodiments, the lithium ion battery according to the present application can include an outer package, which can be a hard shell such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package such as a pouch-type soft package. The material of the soft package can be plastic such as one or several of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0074] In some embodiments, the present application also provides a battery module. The battery module comprises the above-mentioned lithium ion battery. The battery module adopts the above-mentioned lithium ion battery, thus at least has the same advantages as the lithium ion battery. The number of lithium ion batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0075] In some embodiments, the present application also provides a battery pack comprising the above-mentioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0076] The second aspect of the present application provides an electrochemical device comprising the lithium ion battery of the first aspect of the present application. In some embodiments, the electrochemical device includes, but is not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, a power storage system, etc. In order to meet the high power and high energy density requirements of the lithium ion battery for the electrochemical device, a battery pack or a battery module can be used. In other embodiments, the electrochemical device can be a mobile phone, a tablet computer, a notebook computer, etc. The electrochemical device usually requires thin and light, and a lithium ion battery can be used as a power source.

[0077] The present application will be further described in conjunction with the following examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application.

[0078] Preparation of lithium ion battery

[0079] (1) Preparation of positive electrode:

[0080] The positive electrode active material LiFePO4, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a solvent N-methyl pyrrolidone (NMP) according to a weight ratio of about 96:2:2, and stirred uniformly to obtain a slurry. The slurry is coated on a positive electrode current collector aluminum foil, dried, cold-pressed to obtain a positive electrode active material layer, and then subjected to piece cutting, slitting, and welding of tabs to obtain a positive electrode.

[0081] (2) Preparation of negative electrode:

[0082] Examples 1 to 44 and Comparative Examples 1 to 5:

[0083] The natural graphite with different OI values, the conductive agent Super P, and the thickening agent sodium carboxymethyl cellulose are fully stirred and mixed in a deionized water solvent according to a weight ratio of 95:2:3 to form a uniform negative electrode slurry. The slurry is coated on a negative electrode current collector copper foil, dried, cold-pressed to obtain a negative electrode active material layer, and then subjected to piece cutting, slitting, and welding of tabs to obtain a negative electrode.

[0084] (3) Preparation of electrolyte solution:

[0085] Examples 1-30 and Comparative Examples 1-3: In an argon glove box with water content <10 ppm, a certain amount of organic solvent ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed uniformly, and a fully dried lithium salt LiPF6 was dissolved in the above non-aqueous solvent to obtain an electrolyte solution. The weight percentage of the lithium salt LiPF6 was 12.5%, the weight percentage of ethylene carbonate (EC) was 26%, and the rest was DEC (the content of each substance in the electrolyte solution was calculated based on the weight of the electrolyte solution). Vinylene carbonate and fluoroethylene carbonate were added to the electrolyte solution in the amounts shown in Table 1.

[0086] Examples 31-44 and Comparative Examples 4-3: In an argon glove box with water content <10 ppm, a certain amount of organic solvent ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed uniformly, and a fully dried lithium salt LiPF6 was dissolved in the above non-aqueous solvent to obtain an electrolyte solution. The weight percentage of the lithium salt LiPF6 was 12.5%, the weight percentage of EC was 26%, the weight percentage of VC was 2.5%, the weight percentage of FEC was 1%, and the rest was DEC. Compound B was added to the electrolyte solution in the amounts shown in Table 2.

[0087] (4) Preparation of separator film:

[0088] A polyethylene (PE) porous film was used as the separator film, and the porosity was 35%.

[0089] (5) Preparation of lithium ion battery:

[0090] The positive electrode, the separator film, and the negative electrode were stacked in order, with the separator film between the positive electrode and the negative electrode to serve as a separator. Then, the bare cell was obtained by winding. The bare cell was placed in an outer packaging foil aluminum plastic film, and the above prepared electrolyte solution was injected into the dried bare cell. After vacuum packaging, standing, formation, shaping, capacity testing, and other processes, the preparation of the lithium ion battery was completed.

[0091] Test method:

[0092] 1. Lithium ion battery cycle test:

[0093] Room temperature cycling performance test: the lithium ion battery products of the above examples and comparative examples were placed in a 25°C constant temperature box for 30 minutes to allow the lithium ion battery to reach a constant temperature. The lithium ion battery that reached a constant temperature was charged at 1C constant current to 3.6V, then charged at 3.6V constant voltage until the current was less than or equal to 0.05C, then discharged at 1C constant current to 2.8V, which was one charge-discharge cycle, and the thickness of the cell was tested at the same time. The capacity at the first discharge was taken as 100%, and the charge-discharge cycle was repeated 1000 times, the test was stopped, and the corresponding discharge capacity was recorded. The capacity retention rate was calculated according to the following formula:

[0094] Capacity retention rate = (discharge capacity after cycling / first discharge capacity) x 100%.

[0095] Cycling expansion rate test of negative electrode sheet: the thickness of the negative electrode after cold pressing was recorded as H0, the thickness of the negative electrode sheet after 1000 cycles was recorded as H1, and the thickness change rate of the negative electrode sheet was calculated according to the following formula:

[0096] Negative electrode sheet thickness change rate = (H1 / H0-1) x 100%

[0097] 2. High temperature storage test of lithium ion secondary battery:

[0098] Full charge storage: the lithium ion battery products of the above examples and comparative examples were placed in a 25°C constant temperature box for 5 minutes, charged at 1C constant current to 3.6V, then charged at constant voltage until the current was less than or equal to 0.05C, then placed for 5 minutes, discharged at 1C, constant current to 2.8V, then charged at 1C constant current to 3.6V, and charged at constant voltage until the current was less than or equal to 0.05C. Then the lithium ion secondary battery in full charge state was placed in a 60°C oven for 90 days. After 90 days of storage, the lithium ion battery was taken out and its thickness change was observed and recorded. The thickness increase rate was calculated according to the following formula:

[0099] Thickness increase rate = (thickness after high temperature storage-thickness before high temperature storage) / thickness before high temperature storage x 100%.

[0100] 3. Direct current impedance test (DCR test) of lithium ion secondary battery:

[0101] The lithium ion battery products of the above examples and comparative examples were placed in a 25°C constant temperature box for 5 minutes, charged at 1C constant current to 3.6V, then charged at constant voltage until the current was less than or equal to 0.05C, and then placed for 30 minutes. Discharge at 0.1C current for 10 seconds (take one point every 0.1 second and record the corresponding voltage value U1), discharge at 1C current for 360 seconds (take one point every 0.1 second and record the corresponding voltage value U2). Repeat the charging and discharging steps 5 times. "1C" is the current value that completely discharges the battery capacity in 1 hour.

[0102] The direct current resistance (DCR) is calculated according to the following formula: R = (U1-U2) / (1C-0.1C). The DCR obtained is the concentration polarization resistance of the present application, which is the value at 50% SOC (state of charge), i.e. the 50% SOC DCR in the examples, in milliohms.

[0103] 4. Test of OI value of negative electrode material:

[0104] The OI value is tested by an X-ray diffractometer (e.g. Bruker D8 Discover X-ray diffractometer), which can refer to JIS K 0131-1996. The specific method is as follows: the negative electrode sheet is directly placed in the X-ray diffractometer, and the peak area C 004 and the peak area C 110 of the 110 crystal face diffraction peak of the negative electrode active material in the negative electrode sheet are obtained by X-ray analysis, and the OI value of the negative electrode sheet = C 004 / C 110 .

[0105] 5. Test of BET value of negative electrode material:

[0106] The specific surface area BET of the negative electrode can be tested by a method known in the art. For example, it can refer to GB / T 19587-2017, and is tested by a nitrogen adsorption specific surface area analysis test method and calculated by a BET (Brunauer Emment Teller) method, wherein the nitrogen adsorption specific surface area analysis test can be tested by a Tri-Star 3020 specific surface area pore size analyzer of the Micromeritics company in the United States. First, the battery in the examples and comparative examples is disassembled, the disassembled negative electrode sheet is soaked in a dispersant so that the negative electrode material is dispersed in the dispersant (ethanol), then the obtained material is dried in a vacuum drying box after ultrasonic treatment for 30 minutes, and finally the specific surface area of the negative electrode material is measured by a specific surface area tester.

[0107] 6. Test of Dv50 value of negative electrode material:

[0108] In the present application, the Dv50 value of the negative electrode active material can refer to the standard GB / T 19077.1-2016, and is measured by a laser particle size analyzer (e.g. Malvern Master Size 300). Wherein, the Dv50 represents the particle size corresponding to the cumulative volume distribution percentage of the negative electrode active material reaching 50%.

[0109] The lithium ion battery prepared according to the above test method is tested, and the test results are shown in Tables 1 and 2 [Table 1]

[0110]

[0111] By comparing examples 1-30 with comparative examples 1-3 in Table 1, when the OI value of the negative electrode material is small, the compaction density of the negative electrode sheet is in a suitable range, the direction selectivity of the natural graphite during lithium intercalation is small, the lithium intercalation expansion can be dispersed in all directions, the cycle expansion of the electrode sheet and the battery is reduced, and since the SEI film formed by vinylene carbonate and fluoroethylene carbonate has large impedance, the additives should be added in an appropriate amount under the premise of ensuring performance; when the OI value of the negative electrode material is large, the cycle expansion of the electrode sheet and the battery is serious, and the increase in the number of material end faces leads to an increase in surface side reactions and deterioration of gas production, at this time, more vinylene carbonate and fluoroethylene carbonate are needed to protect the negative electrode material and improve the expansion and gas production problems, so when 0.3≤a / (b+c)≤6, the lithium ion battery has better cycle performance and low impedance; in addition, vinylene carbonate mainly forms PEO type polymer SEI film, and fluoroethylene carbonate mainly forms LiF and other SEI films containing inorganic components, and only when the content of the two satisfies 0.01≤b+c≤10, 0.5≤b / c≤2.5, can an SEI film rich in organic and inorganic components be formed on the surface of the negative electrode, the film has certain toughness and elasticity, and can reduce the cycle and storage performance deterioration caused by the destruction of the SEI film due to the volume expansion of the natural graphite and a series of problems; and an appropriate amount of electrolyte can improve the cycle performance of the battery.

[0112] [Table 2]

[0113]

[0114]

[0115] When the BET of the negative electrode material is too large, the surface side reaction of the material increases, leading to deterioration of the DCR and gas generation, and in addition, the increase in the water content of the material caused by the large BET further leads to an increase in the HF. When the BET of the negative electrode material is too small, the lithium ion transfer speed is slow, affecting the cycle performance. The addition of the nitrogen-containing heterocyclic compound B represented by formula (II) to the electrolyte can significantly improve the high-temperature storage performance of the battery, which is mainly due to the fact that the nitrogen-containing heterocyclic compound can form a stable SEI film on the surface of the negative electrode, inhibiting the consumption of the solvent and other film-forming additives during the cycle and storage processes; in addition, the nitrogen-containing cationic part in the nitrogen-containing heterocyclic compound B represented by formula (II) not only can promote the dissociation of the lithium salt, but also can complex with Lewis bases such as ethylene carbonate (EC) and the like, improving the oxidation stability of the EC electrolyte. However, when the content of the nitrogen-containing heterocyclic compound is too high, the storage will not be further improved, which will lead to the formation of a SEI film that is too thick, increasing the impedance, and in turn increasing the DCR value, so the nitrogen-containing heterocyclic compound B represented by formula (II) can be appropriately added to make it play the optimal performance.

[0116] Although the illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments cannot be interpreted as a limitation of the present application, and changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the present application.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises an electrolyte and a negative electrode sheet, the electrolyte comprises an electrolyte additive, the electrolyte additive comprises vinylene carbonate and fluoroethylene carbonate; the negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises natural graphite, and The OI value of the negative electrode active material is a; the weight percentage content of the vinylene carbonate is b% and the weight percentage content of the fluoroethylene carbonate is c% based on the weight of the electrolyte; the a, b and c satisfy: 0.7≤a / (b+c)≤1.8, 1.1≤b+c≤8.9, 0.7≤b / c≤3, The liquid retention coefficient of the lithium ion battery is e, 2≤e≤5, The b, c and e satisfy: 5.1≤e×(b+c)≤26.

7.

2. The lithium-ion battery of claim 1, wherein, The lithium ion battery satisfies at least one of the following conditions: (2) 1.7≤b+c≤5.6; (3) 0.8≤b / c≤2.

5.

3. The lithium-ion battery of claim 1, wherein, The range of the a is: 3≤a≤15.

4. The lithium-ion battery of claim 3, wherein, The range of the a is: 3≤a≤7.

5. The lithium-ion battery of claim 1, wherein, The range of the b is: 0.1≤b≤5.

6.

6. The lithium-ion battery of claim 5, wherein, The range of the b is: 0.1≤b≤3.

7.

7. The lithium-ion battery of claim 1, wherein, The range of the c is: 0.3≤c≤6.

5.

8. The lithium-ion battery of claim 7, wherein, The range of the c is: 0.7≤c≤3.

2.

9. The lithium-ion battery of claim 1, wherein, The specific surface area of the negative active material is f m2 / g, f ranging from 1 to 3. 2 / g, f ranging from 1 to 3.

10. The lithium-ion battery of claim 1, wherein, The electrolyte comprises a compound B, the compound B comprises at least one of the following: pyridine, 2-vinylpyridine or 2-fluoropyridine.

11. The lithium-ion battery of claim 10, wherein, The weight percentage content of the compound B is k% based on the weight of the electrolyte, the range of the k is: 0.005≤k≤1.

12. An electrochemical device, characterized by, The electrochemical device comprises the lithium ion battery of any one of claims 1-11.

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