A lithium-ion battery

Abstract

The present application relates to the field of battery manufacturing technology, and in particular, to a lithium-ion battery. The lithium-ion battery includes: a diaphragm, a positive electrode sheet, a negative electrode sheet, and an electrolyte. Among them, the coating of the positive electrode sheet includes a positive electrode material; the positive electrode material includes a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material includes at least one of lithium iron phosphate and lithium manganese oxide. The coating of the positive electrode sheet also includes a negative electrode film-forming additive; and / or, the electrolyte includes a negative electrode film-forming additive; the negative electrode film-forming additive includes at least one of elemental sulfur and metal sulfide. The electrolyte includes a film-forming solvent. The lithium-ion battery provided in the present application can effectively improve the cycle performance of lithium-ion batteries that use lithium iron phosphate or lithium manganese oxide as positive electrode active materials.

Description

Technical Field

[0001] The present application relates to the field of battery manufacturing technology, and in particular to a lithium-ion battery. Background Art

[0002] Compared with other rechargeable battery systems, lithium-ion secondary batteries have the advantages of high operating voltage, light weight, small size, no memory effect, low self-discharge rate, long cycle life and high energy density. They are currently widely used in mobile terminal products such as mobile phones, laptops or tablets, as well as electric vehicles and other fields.

[0003] During the first cycle of charging a lithium-ion battery, the electrolyte is reduced on the negative electrode surface to form the negative electrode SEI film. This SEI film, with its ion-conducting and electron-insulating properties, is crucial to the proper functioning of lithium-ion batteries. The composition, integrity, stability, density, thickness, and conductivity of the SEI film on the negative electrode surface of a lithium-ion battery directly influence the transport of lithium ions at the electrolyte-negative electrode interface, and thus the cycling performance of the lithium-ion battery.

[0004] Lithium iron phosphate and lithium manganese oxide are preferred cathode active materials in lithium-ion batteries due to their high capacity, low cost, and excellent safety. However, lithium-ion batteries using these materials often suffer from the inability to form a stable SEI film on the negative electrode surface, leading to a bottleneck in improving the cycling performance of these batteries. Summary of the Invention

[0005] The purpose of the present application is to provide a lithium-ion battery, which aims to improve the technical problem of bottlenecks in improving the cycle performance of existing lithium-ion batteries using lithium iron phosphate or lithium manganese oxide as positive electrode active materials.

[0006] The present application provides a lithium-ion battery, comprising: a diaphragm, a positive electrode sheet, a negative electrode sheet and an electrolyte.

[0007] The coating of the positive electrode sheet includes a positive electrode material; the positive electrode material includes a positive electrode active material, a conductive agent and a binder, and the positive electrode active material includes at least one of lithium iron phosphate and lithium manganese oxide.

[0008] The coating of the positive electrode plate further includes a negative electrode film-forming additive; and / or the electrolyte includes a negative electrode film-forming additive; the negative electrode film-forming additive includes at least one of elemental sulfur and metal sulfide.

[0009] The electrolyte includes a membrane-forming solvent, and the structural formula of the membrane-forming solvent is as follows:

[0010]

[0011] R1, R2, R3 and R4 are each independently a hydrogen atom, a halogen atom, an alkyl group or a hydrocarbon group.

[0012] Alternatively, the structural formula of the membrane-forming solvent is as follows:

[0013]

[0014] R5 and R6 are each independently a hydrogen atom, a halogen atom, an alkyl group or a hydrocarbon group.

[0015] When lithium iron phosphate and / or lithium manganese oxide are used as the positive electrode active material to prepare a lithium ion battery, the present application adds a negative electrode film-forming additive including elemental sulfur and at least one of metal sulfides to the coating and / or electrolyte of the positive electrode sheet. The additive can migrate to the negative electrode surface during the first cycle of charging of the lithium ion battery (i.e., during the electrochemical reaction process inside the lithium ion battery) and participate in the formation of the SEI film on the negative electrode surface, which is beneficial for the SEI film on the negative electrode surface to contain high-valent sulfur components with a valence of +4 and / or +6 and low-valent sulfur components with a valence of -2 to -1 / 4.

[0016] High-valent sulfur components can quickly transfer lithium ions and promote the transmission of lithium ions at the interface. The negative electrode film-forming additive is reduced to polysulfide at the negative electrode, and the polysulfide reacts with the film-forming solvent to generate a low-valent sulfur component with a PEO structure. The PEO structure has good toughness and can effectively alleviate the volume deformation of the negative electrode material during the charge and discharge process. The presence of high-valent sulfur components and low-valent sulfur components is conducive to the formation of a stable structure of the SEI film on the negative electrode surface, reducing charge transfer impedance and polarization, improving the stability of the battery cycle, and thus improving the rate performance of the lithium-ion battery.

[0017] In some embodiments of the present application, the metal sulfide includes at least one of lithium sulfide, iron sulfide, iron disulfide, titanium disulfide, titanium trisulfide, zinc sulfide, tin disulfide, molybdenum disulfide, tungsten disulfide, cobalt disulfide, and nickel sulfide.

[0018] In some embodiments of the present application, when the coating layer of the positive electrode sheet includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-10):100.

[0019] When the amount of negative electrode film-forming additive added meets the above conditions, it is beneficial to further reduce the charge transfer impedance and increase the cycle capacity, thereby improving the cycle performance of the battery.

[0020] Optionally, when the coating layer of the positive electrode sheet includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-3):100.

[0021] In some embodiments of the present application, when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.01-10 g / L.

[0022] When the amount of negative electrode film-forming additive added meets the above conditions, it is beneficial to further reduce the charge transfer impedance and increase the cycle capacity, thereby improving the cycle performance of the battery.

[0023] Optionally, when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.05-1 g / L.

[0024] In some embodiments of the present application, when the coating of the positive electrode plate includes a negative electrode film-forming additive, the preparation method of the positive electrode plate includes: applying a mixed slurry containing the negative electrode film-forming additive, the positive electrode active material, the conductive agent and the binder to the positive electrode foil, and then heat treating it at 80-160°C for 4-12 hours.

[0025] The above heat treatment conditions are beneficial to improving the uniformity of the mixed slurry on the surface of the positive electrode foil, which in turn is beneficial to further improving the cycle performance of the battery.

[0026] In some embodiments of the present application, the coating of the negative electrode plate includes a negative electrode active material, and the negative electrode active material includes at least one of a carbon-based material, a silicon-based material, an alloy, and lithium.

[0027] Selecting the above substances as the negative electrode active material is conducive to forming a stable SEI film on the negative electrode surface.

[0028] In some embodiments of the present application, the membrane-forming solvent includes at least one of ethylene carbonate, fluoroethylene carbonate, bisfluoroethylene carbonate, vinylene carbonate, and butylene carbonate.

[0029] The film-forming solvent includes the above-mentioned substances, which can react with the polysulfide formed by the reduction of the negative electrode film-forming additive at the negative electrode to generate a low-valent sulfur component with a PEO structure, further improving the stability of the battery cycle, and thereby achieving the improvement of the rate performance of the lithium-ion battery.

[0030] In some embodiments of the present application, the membrane-forming solvent accounts for 1-100% by mass of the total solvent in the electrolyte.

[0031] The film-forming solvent accounts for 1-100% of the total solvent mass in the electrolyte, which can effectively improve the stability of the battery cycle.

[0032] Optionally, the membrane-forming solvent accounts for 5-40% by mass of the total solvent in the electrolyte.

[0033] In some embodiments of the present application, the particle size D of the negative electrode film-forming additive is 50 0.01-20μm.

[0034] Particle size D of negative electrode film-forming additive 50 Within the above range, it is beneficial to further improve the stability of battery cycling.

[0035] Optionally, the particle size D of the negative electrode film-forming additive 50 0.05-5μm.

[0036] In some embodiments of the present application, after the lithium-ion battery is formed, the surface electrolyte interface film of the negative electrode plate contains a high-valent sulfur component and a low-valent sulfur component; in the surface electrolyte interface film of the negative electrode plate, the valence state of the high-valent sulfur component is +4 and / or +6, and the S2p spectrum peak is 168-172 eV; in the surface electrolyte interface film of the negative electrode plate, the valence state of the low-valent sulfur component is -2 to -1 / 4, and the S2p spectrum peak is 160-166 eV.

[0037] Optionally, in the surface electrolyte interface film of the negative electrode sheet, the ratio of the S2p spectrum peak area of ​​the high-valent sulfur component to the S2p spectrum peak area of ​​the low-valent sulfur component is (1-4):1;

[0038] Optionally, after the lithium-ion battery is formed, the surface electrolyte interface film of the positive electrode plate contains a high-valent sulfur component; in the surface electrolyte interface film of the positive electrode plate, the valence state of the high-valent sulfur component is +4 and / or +6, and the S2p spectrum peak is 168-172eV. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0040] Figure 1 The S2p spectrum of the negative electrode of the lithium-ion battery after formation prepared in Example 1 is shown in the X-ray photoelectron spectroscopy test.

[0041] Figure 2 The S2p spectrum of the positive electrode of the lithium-ion battery prepared in Example 1 after formation is shown in the X-ray photoelectron spectroscopy test.

[0042] Figure 3 The first week charging test curves of the lithium ion batteries prepared in Example 1 and Comparative Example 1 are shown.

[0043] Figure 4 The 100-cycle performance graph of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 is shown.

[0044] Figure 5 The battery rate diagrams of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 are shown.

[0045] Figure 6 The battery impedance diagrams of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 are shown. DETAILED DESCRIPTION

[0046] To make the purpose, technical solutions and advantages of the examples of the present application clearer, the technical solutions in the examples of the present application will be described clearly and completely below. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0047] Lithium-ion batteries that use lithium iron phosphate or lithium manganese oxide as positive electrode active materials generally have the problem that a stable SEI film cannot be formed on the negative electrode surface, resulting in a bottleneck in improving the cycle performance of lithium-ion batteries that use lithium iron phosphate or lithium manganese oxide as positive electrode active materials.

[0048] To this end, the present application provides a lithium-ion battery, comprising: a diaphragm, a positive electrode sheet, a negative electrode sheet, and an electrolyte.

[0049] The coating of the positive electrode sheet includes a positive electrode material; the positive electrode material includes a positive electrode active material, a conductive agent and a binder, and the positive electrode active material includes at least one of lithium iron phosphate and lithium manganese oxide.

[0050] The coating of the positive electrode plate further includes a negative electrode film-forming additive; and / or the electrolyte includes a negative electrode film-forming additive; the negative electrode film-forming additive includes at least one of elemental sulfur and metal sulfide.

[0051] The electrolyte includes a membrane-forming solvent, and the structural formula of the membrane-forming solvent is as follows:

[0052]

[0053] R1, R2, R3 and R4 are each independently a hydrogen atom, a halogen atom, an alkyl group or a hydrocarbon group.

[0054] Alternatively, the structural formula of the membrane-forming solvent is as follows:

[0055]

[0056] R5 and R6 are each independently a hydrogen atom, a halogen atom, an alkyl group or a hydrocarbon group.

[0057] It is understood that in the present application, the positive electrode active material may include both lithium iron phosphate and lithium manganese oxide, or may include only lithium iron phosphate, or may include only lithium manganese oxide. The negative electrode film-forming additive may be selected from both elemental sulfur and metal sulfide, or may include only elemental sulfur, or may include only metal sulfide. The coating of the positive electrode plate and the electrolyte may contain the negative electrode film-forming additive defined in the present application at the same time, or only the coating of the positive electrode plate may contain the negative electrode film-forming additive defined in the present application, or only the electrolyte may contain the negative electrode film-forming additive defined in the present application.

[0058] In the present application, the film-forming additive includes at least one of elemental sulfur and metal sulfide, and the role of the film-forming additive is to effectively improve the stability of the negative electrode SEI film. Specifically, when lithium iron phosphate and / or lithium manganate are used as the positive electrode active material to prepare a lithium-ion battery, the present application adds a negative electrode film-forming additive including at least one of elemental sulfur and metal sulfide to the coating and / or electrolyte of the positive electrode sheet. During the first cycle of charging of the lithium-ion battery (i.e., during the electrochemical reaction process within the lithium-ion battery), the film-forming additive can migrate to the negative electrode surface and participate in the formation of the negative electrode surface SEI film, which is beneficial for the negative electrode surface SEI film to contain both high-valent sulfur components with a valence of +4 and / or +6 and low-valent sulfur components with a valence of -2 to -1 / 4.

[0059] High-valent sulfur components can rapidly transfer lithium ions, promoting their transport at the interface. The negative electrode film-forming additive is reduced to polysulfide at the negative electrode, which reacts with the film-forming solvent defined in this application to generate a low-valent sulfur component with a PEO structure. The PEO structure exhibits excellent toughness, effectively mitigating the volumetric deformation of the negative electrode material during charge and discharge, enabling reaction platforms to be observed at 1-1.3V and 1.6-2V during the first-week charge curve of the lithium-ion battery.

[0060] The presence of high-valent sulfur components and low-valent sulfur components is conducive to the formation of a stable structure SEI film on the surface of the negative electrode, reducing the charge transfer impedance and polarization, improving the stability of the battery cycle, and thus improving the rate performance of the lithium-ion battery.

[0061] As an example, when metal sulfide is selected as the negative electrode film-forming additive, the metal sulfide can be selected from at least one of lithium sulfide, iron sulfide, iron disulfide, titanium disulfide, titanium trisulfide, zinc sulfide, tin disulfide, molybdenum disulfide, tungsten disulfide, cobalt disulfide and nickel sulfide.

[0062] It should be noted that the metal sulfides are not limited to the above-mentioned substances.

[0063] In the present application, when the negative electrode film-forming additive is selected from elemental sulfur, it is beneficial to further improve the cycle performance of the battery compared to when the negative electrode film-forming additive is selected from metal sulfide.

[0064] In this application, the particle size D of the negative electrode film-forming additive is 50 0.01-20 μm. Particle size D of negative electrode film-forming additive 50 Within the above range, it is beneficial to further improve the stability of battery cycling.

[0065] As an example, the particle size D of the negative electrode film-forming additive is 50 It can be 0.01μm, 0.05μm, 2μm, 5μm, 10μm or 20μm, etc.

[0066] Furthermore, the particle size D of the negative electrode film-forming additive 50 The particle size D of the negative electrode film-forming additive is 0.05-5μm. 50 Within the above particle size range, the stability of battery cycling is better.

[0067] In the present application, when the coating of the positive electrode sheet includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-10): 100. The addition amount of the negative electrode film-forming additive meeting the above conditions is conducive to further reducing the charge transfer impedance and increasing the cycle capacity, thereby improving the cycle performance of the battery.

[0068] As an example, when the coating of the positive electrode sheet includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material can be 0.1:100, 1:100, 2:100, 5:100 or 10:100, etc.

[0069] Furthermore, when the coating of the positive electrode plate includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-3):100; under the above ratio conditions, it is beneficial to further improve the cycle performance of the battery.

[0070] Furthermore, when the coating of the positive electrode plate includes a negative electrode film-forming additive, the preparation method of the positive electrode plate includes: applying a mixed slurry containing the negative electrode film-forming additive, the positive electrode active material, the conductive agent and the binder to the positive electrode foil, and then heat treating it at 80-160°C for 4-12h.

[0071] The above heat treatment conditions are beneficial to improving the uniformity of the mixed slurry on the surface of the positive electrode foil, which in turn is beneficial to further improving the cycle performance of the battery.

[0072] As an example, the temperature of the heat treatment can be 80°C, 100°C, 120°C, 140°C or 160°C, etc.; the time of the heat treatment can be 4h, 5h, 8h, 10h or 12h, etc.

[0073] It should be noted that the present application does not limit the order of adding the negative electrode film-forming additive, the positive electrode active material, the conductive agent and the binder into the mixed slurry.

[0074] In other feasible embodiments, when the coating of the positive electrode sheet includes a negative electrode film-forming additive, the positive electrode sheet can also be prepared using a vapor phase method or a liquid phase method, wherein the negative electrode film-forming additive is dispersed in a liquid or gas, and then uniformly deposited on the surface of the positive electrode sheet. As an example, the positive electrode sheet can also be prepared by dissolving sublimated sulfur powder in carbon disulfide, applying the carbon disulfide containing elemental sulfur to the surface of the positive electrode sheet, and then drying.

[0075] In this application, when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.01-10g / L. The addition amount of the negative electrode film-forming additive meeting the above conditions is conducive to further reducing the charge transfer impedance and increasing the cycle capacity, thereby improving the cycle performance of the battery.

[0076] As an example, when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte can be 0.01 g / L, 0.05 g / L, 0.5 g / L, 1.0 g / L, 2.0 g / L, 5.0 g / L or 10 g / L, etc.

[0077] Furthermore, when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.05-1 g / L.

[0078] As described above, the membrane-forming solvent structure contains a cyclic carbonate, and R1, R2, R3, and R4 in the membrane-forming solvent structure are each independently a hydrogen atom, a halogen atom, an alkyl group, or a hydrocarbon group. The alkyl group or hydrocarbon group may be a substituted alkyl group or a substituted hydrocarbon group, such as a haloalkyl group or a halohydrocarbon group; the alkyl group may also be a methyl group, an ethyl group, or a butyl group; and the hydrocarbon group may be a vinyl group, a propenyl group, or a butenyl group.

[0079] As an example, the membrane-forming solvent can be selected from at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), vinylene carbonate (VC), and butylene carbonate (BC). The membrane-forming solvent includes the above substances and can react with the polysulfide formed by the reduction of the negative electrode film-forming additive at the negative electrode to generate a low-valent sulfur component with a PEO structure, further improving the stability of the battery cycle and thereby improving the rate performance of the lithium-ion battery.

[0080] It should be noted that the membrane-forming solvent is not limited to the above substances, as long as it can satisfy the structural formula of the solvent defined in this application.

[0081] In the present application, the membrane-forming solvent accounts for 1-100% of the total solvent mass in the electrolyte, which can effectively improve the stability of the battery cycle.

[0082] As an example, the mass fraction of the membrane-forming solvent in the total solvent in the electrolyte can be 1%, 5%, 10%, 30%, 50% or 100%, etc.

[0083] Furthermore, the film-forming solvent accounts for 5-40% of the total solvent in the electrolyte, which is beneficial to further improve the stability of the battery cycle.

[0084] In the present application, when the positive electrode active material is selected from lithium iron phosphate, it is beneficial to further improve the cycle performance of the battery compared to when the positive electrode active material is selected from lithium manganese oxide.

[0085] In the present application, the coating of the negative electrode sheet includes a negative electrode active material, which includes at least one of a carbon-based material, a silicon-based material, an alloy, and lithium. The selection of the above-mentioned negative electrode active material facilitates the formation of a stable SEI film on the negative electrode surface.

[0086] As an example, the negative electrode active material may include graphite, silicon carbon 450 or silicon, etc.

[0087] Furthermore, when the negative electrode active material is selected from graphite, it is beneficial to further improve the cycle performance of the battery.

[0088] In the present application, after the lithium-ion battery is formed, the surface electrolyte interface film (SEI film) of the negative electrode sheet contains high-valent sulfur components and low-valent sulfur components; in the surface electrolyte interface film (SEI film) of the negative electrode sheet, the valence state of the high-valent sulfur component is +4 and / or +6, and the S2p spectrum peak is 168-172eV; in the surface electrolyte interface film (SEI film) of the negative electrode sheet, the valence state of the low-valent sulfur component is -2 to -1 / 4, and the S2p spectrum peak is 160-166eV.

[0089] Furthermore, in the surface electrolyte interface film (SEI film) of the negative electrode sheet, the ratio of the S2p spectrum peak area of ​​the high-valent sulfur component to the S2p spectrum peak area of ​​the low-valent sulfur component is (1-4):1, which is beneficial to further improve the cycle performance of the battery.

[0090] In the present application, after the lithium-ion battery is formed, the surface electrolyte interface film (CEI film) of the positive electrode sheet contains a high-valent sulfur component; in the surface electrolyte interface film (CEI film) of the positive electrode sheet, the valence state of the high-valent sulfur component is +4 and / or +6, and the S2p spectrum peak is 168-172eV.

[0091] The characteristics and performance of the lithium-ion battery provided by the present application are further described in detail below in conjunction with the embodiments.

[0092] Example 1

[0093] This embodiment provides a lithium ion battery, which is manufactured by the following method:

[0094] (1) Mix 10g of LiFePO4 and 0.1g of elemental sulfur powder, then add 0.56g of conductive carbon black (SP) and 0.56g of polyvinylidene fluoride (PVDF), and stir evenly to obtain a positive electrode mixed slurry. The positive electrode mixed slurry is evenly coated on Al foil, dried, and rolled to obtain a positive electrode sheet.

[0095] (2) 9.4 g of graphite, 0.3 g of conductive carbon black (SP), 0.15 g of sodium carboxymethyl cellulose (CMC), 0.15 g of styrene-butadiene rubber (SBR), and 25 mL of deionized water were stirred to obtain a negative electrode mixed slurry. The negative electrode mixed slurry was evenly coated on a Cu foil, dried, and rolled to obtain a negative electrode sheet.

[0096] (3) Place the diaphragm between the positive electrode obtained in step (1) and the negative electrode obtained in step (2), inject 120uL of electrolyte, place a gasket and a spring, and place it in a press to seal it to form a lithium-ion battery.

[0097] The electrolyte is 1.0M LiPF6 in EC / DMC / EMC=1:1:1, v / v / v.

[0098] Example 2

[0099] This embodiment provides a lithium ion battery, which is manufactured by the following method:

[0100] (1) 10 g of LiFePO4, 0.56 g of conductive carbon black (SP), and 0.56 g of polyvinylidene fluoride (PVDF) were mixed and uniformly prepared to obtain a positive electrode mixed slurry. The positive electrode mixed slurry was uniformly coated on Al foil, dried, and rolled to obtain a positive electrode sheet.

[0101] (2) 9.4 g of graphite, 0.3 g of conductive carbon black (SP), 0.15 g of sodium carboxymethyl cellulose (CMC), 0.15 g of styrene-butadiene rubber (SBR), and 25 mL of deionized water were stirred to obtain a negative electrode mixed slurry. The negative electrode mixed slurry was evenly coated on a Cu foil, dried, and rolled to obtain a negative electrode sheet.

[0102] (3) Place the diaphragm between the positive electrode obtained in step (1) and the negative electrode obtained in step (2), inject 120uL of the final electrolyte, place the gasket and spring, place it in a press to seal, and form a lithium-ion battery.

[0103] The electrolyte prepared in advance is 1.0M LiPF6 in EC / DMC / EMC=1:1:1, v / v / v, elemental sulfur powder is added to the electrolyte and stirred until completely dissolved, and the electrolyte is used as the final electrolyte; the final concentration of elemental sulfur in the final electrolyte is 0.5 mg / mL.

[0104] Example 3

[0105] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 is that the mass of elemental sulfur in step (1) is different. In this embodiment, the mass of elemental sulfur is 0.01 g.

[0106] Example 4

[0107] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 is that the mass of elemental sulfur in step (1) is different. In this embodiment, the mass of elemental sulfur is 0.5 g.

[0108] Example 5

[0109] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 is that the graphite in step (2) is replaced by silicon carbon 450.

[0110] Example 6

[0111] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that the graphite in step (2) is replaced by silicon.

[0112] Example 7

[0113] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that LiFePO4 in step (1) is replaced by LiMnO2.

[0114] Example 8

[0115] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that the elemental sulfur powder in step (1) is replaced by lithium sulfide.

[0116] Example 9

[0117] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that the elemental sulfur powder in step (1) is replaced by titanium disulfide.

[0118] Example 10

[0119] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that the elemental sulfur powder in step (1) is replaced by molybdenum disulfide.

[0120] Example 11

[0121] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 is that ethylene carbonate (EC) in step (3) is replaced by bisfluoroethylene carbonate (DFEC).

[0122] Example 12

[0123] This embodiment provides a lithium-ion battery. The difference between this embodiment and Example 1 is that ethylene carbonate (EC) in step (3) is replaced by vinylene carbonate (VC).

[0124] Example 13

[0125] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 lies in the difference in the electrolyte in step (3). In this embodiment, the electrolyte is 1.0M LiPF6 in EC.

[0126] Example 14

[0127] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 1 lies in the difference in the electrolyte in step (3). In this embodiment, the electrolyte is 1.0M LiPF6 in DMC / EMC=1:1, v / v+5wt%EC.

[0128] Example 15

[0129] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 2 is that the concentration of elemental sulfur in the electrolyte in step (3) is different. In this embodiment, the concentration of elemental sulfur in the electrolyte is 0.01 mg / mL.

[0130] Example 16

[0131] This embodiment provides a lithium-ion battery. The difference between this embodiment and embodiment 2 is that the concentration of elemental sulfur in the electrolyte in step (3) is different. In this embodiment, the concentration of elemental sulfur in the electrolyte is 2.0 mg / mL.

[0132] Comparative Example 1

[0133] This comparative example provides a lithium-ion battery. The difference between this comparative example and Example 1 lies in the difference in step (1). Step (1) of this comparative example is as follows:

[0134] 10g of LiFePO4, 0.56g of conductive carbon black (SP), and 0.56g of polyvinylidene fluoride (PVDF) were mixed and uniformly prepared to obtain a positive electrode mixed slurry. The positive electrode mixed slurry was evenly coated on Al foil, dried, and roll-pressed to obtain a positive electrode sheet.

[0135] Comparative Example 2

[0136] This comparative example provides a lithium-ion battery. The difference between this comparative example and Example 1 lies in the difference in the electrolyte in step (3). The electrolyte in this comparative example is 1.0M LiPF6 in DMC / EMC=1:1, v / v.

[0137] Experimental Example 1

[0138] After the lithium ion battery prepared in Example 1 was formed, the X-ray photoelectron spectroscopy (S2p) spectra of the negative electrode and the positive electrode were tested. The experimental results are as follows: Figure 1 and Figure 2 shown.

[0139] from Figure 1 It can be seen that the negative electrode X-ray photoelectron spectrum S2p spectrum of the lithium ion battery prepared in Example 1 after formation has a characteristic peak corresponding to the high-valent sulfur component at 168-172eV, and a characteristic peak corresponding to the low-valent sulfur component at 160-166eV; this indicates that after the lithium ion battery prepared in Example 1 is formed, elemental sulfur (the negative electrode film-forming additive) reacts to simultaneously generate high-valent sulfur components with a valence of +4 and / or +6 and low-valent sulfur components with a valence of -2 to -1 / 4 in the SEI film on the negative electrode surface. The generation of high-valent sulfur is due to the oxidation reaction of sulfur at the positive electrode, which generates a component similar to R-OSO2OLi and diffuses to the negative electrode, enabling rapid Li ion transport and reducing interfacial impedance; the presence of low-valent sulfur components is presumably because elemental sulfur (the negative electrode film-forming additive) is reduced to polysulfide at the negative electrode, which reacts with ethylene carbonate (EC) to generate an SEI film with a PEO structure.

[0140] from Figure 2 It can be seen that the positive electrode X-ray photoelectron spectrum S2p spectrum of the lithium ion battery prepared in Example 1 after formation has a characteristic peak corresponding to the high-valent sulfur component at 168-172 eV; this indicates that after the lithium ion battery prepared in Example 1 is formed, elemental sulfur (negative electrode film-forming additive) reacts to generate high-valent sulfur components with a valence of +4 and / or +6 in the CEI film on the positive electrode surface.

[0141] Experimental Example 2

[0142] The lithium-ion batteries prepared in Example 1 and Comparative Example 1 were subjected to the battery first-week charging test, 100-week cycle performance test, battery rate test, and battery impedance test, respectively. The experimental results are shown in Table 1. Figure 3 、 Figure 4 、 Figure 5 and Figure 6 shown.

[0143] Figure 3 Part a is the horizontal axis battery capacity (Capacity) in the range of 0-180mAh·g -1Cycle performance graph within the interval, Figure 3 Part b in the middle is the horizontal axis battery capacity (Capacity) in the range of 0-1.5mAh·g -1 Graph of cycle performance within interval.

[0144] from Figure 3 It can be seen that the first-week charging curve of the lithium-ion battery prepared in Example 1 is compared with the first-week charging curve of the lithium-ion battery prepared in Comparative Example 1. Reaction platforms can be observed at 1-1.3V and 1.6-2.0V.

[0145] from Figure 4 It can be seen that the 100-cycle performance of the lithium-ion battery prepared in Example 1 is significantly better than the 100-cycle performance of the lithium-ion battery prepared in Comparative Example 1.

[0146] from Figure 5 It can be seen that the rate performance of the lithium ion battery prepared in Example 1 is significantly better than the rate performance of the lithium ion battery prepared in Comparative Example 1.

[0147] from Figure 6 It can be seen that the charge transfer impedance of the lithium ion battery prepared in Example 1 is significantly lower than the charge transfer impedance of the lithium ion battery prepared in Comparative Example 1.

[0148] from Figures 3 to 6 The analysis results show that adding elemental sulfur (i.e., the negative electrode film-forming additive defined in this application) to the coating of the positive electrode of the lithium-ion battery can effectively improve the stability of the SEI film formed on the negative electrode surface, reduce the charge transfer impedance and polarization, and improve the stability of the battery cycle, thereby improving the rate performance of the lithium-ion battery.

[0149] Experimental Example 3

[0150] The lithium-ion batteries provided in Examples 1-17 and Comparative Examples 1-2 were subjected to impedance tests after 10 cycles and battery capacity tests after 100 cycles, respectively. The experimental results are shown in Table 1.

[0151] Table 1

[0152]

[0153] As can be seen from Table 1, the impedance of the lithium ion batteries provided in Examples 1-16 after 10 cycles is significantly lower than that of the lithium ion batteries provided in Comparative Examples 1-2 after 10 cycles, and the battery capacity of the lithium ion batteries provided in Examples 1-16 after 100 cycles is significantly higher than that of the lithium ion batteries provided in Comparative Examples 1-2 after 10 cycles, indicating that the addition of the negative electrode film-forming additive defined in this application to the coating of the positive electrode sheet or the electrolyte and the addition of the film-forming solvent defined in this application to the electrolyte can effectively improve the cycle performance of the lithium ion battery.

[0154] From the comparison between Examples 3-14 and Example 1, it can be seen that when the coating of the positive electrode sheet includes a negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material, the selection of the negative electrode active material, the selection of the positive electrode active material, the selection of the negative electrode film-forming additive, the selection of the film-forming solvent, and the mass fraction of the film-forming solvent in the electrolyte can further affect the cycle performance of the lithium-ion battery.

[0155] From the comparison between Examples 15-16 and Example 2, it can be seen that when the electrolyte includes a negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte can further affect the cycle performance of the lithium-ion battery.

[0156] In summary, the lithium-ion battery provided in the present application can effectively improve the cycle performance of lithium-ion batteries using lithium iron phosphate or lithium manganese oxide as the positive electrode active material.

[0157] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A lithium-ion battery, characterized in that: include: Diaphragm, positive electrode sheet, negative electrode sheet and electrolyte; Wherein, the coating of the positive electrode sheet comprises a positive electrode material; the positive electrode material comprises a positive electrode active material, a conductive agent and a binder, and the positive electrode active material comprises lithium iron phosphate; The coating of the positive electrode plate further comprises a negative electrode film-forming additive; and / or the electrolyte comprises a negative electrode film-forming additive; the negative electrode film-forming additive comprises elemental sulfur; The electrolyte includes a membrane-forming solvent, and the structural formula of the membrane-forming solvent is as follows: R1, R2, R3 and R4 are each independently a hydrogen atom, a halogen atom, a halogenated alkyl group or a hydrocarbon group; Alternatively, the structural formula of the membrane-forming solvent is as follows: R5 and R6 are each independently a hydrogen atom, a halogen atom, a halogenated alkyl group or a hydrocarbon group; When the coating of the positive electrode sheet includes the negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-10):100; When the electrolyte includes the negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.01-10 g / L; After the lithium-ion battery is formed, the surface electrolyte interface film of the negative electrode plate contains a high-valent sulfur component and a low-valent sulfur component; in the surface electrolyte interface film of the negative electrode plate, the high-valent sulfur component has a valence of +4 and / or +6, and an S2p spectrum peak of 168-172 eV; in the surface electrolyte interface film of the negative electrode plate, the low-valent sulfur component has a valence of -2 to -1 / 4, and an S2p spectrum peak of 160-166 eV; In the surface electrolyte interface film of the negative electrode plate, the ratio of the S2p spectrum peak area of ​​the high-valent sulfur component to the S2p spectrum peak area of ​​the low-valent sulfur component is (1-4):1; After the lithium-ion battery is formed, the surface electrolyte interface film of the positive electrode plate contains a high-valent sulfur component; in the surface electrolyte interface film of the positive electrode plate, the valence state of the high-valent sulfur component is +4 and / or +6, and the S2p spectrum peak is 168-172 eV.

2. The lithium-ion battery according to claim 1, wherein When the coating layer of the positive electrode sheet includes the negative electrode film-forming additive, the mass ratio of the negative electrode film-forming additive to the positive electrode active material is (0.1-3):

100.

3. The lithium-ion battery according to claim 1, wherein When the electrolyte includes the negative electrode film-forming additive, the mass concentration of the negative electrode film-forming additive in the electrolyte is 0.05-1 g / L.

4. The lithium-ion battery according to claim 1, wherein When the coating of the positive electrode plate includes the negative electrode film-forming additive, the preparation method of the positive electrode plate includes: applying a mixed slurry containing the negative electrode film-forming additive, the positive electrode active material, the conductive agent and the binder to the positive electrode foil, and then heat treating it at 80-160°C for 4-12h.

5. The lithium-ion battery according to claim 1, wherein The coating layer of the negative electrode plate includes a negative electrode active material, and the negative electrode active material includes at least one of a carbon-based material, a silicon-based material, an alloy, and lithium.

6. The lithium-ion battery according to claim 1, wherein The membrane-forming solvent includes at least one of ethylene carbonate, fluoroethylene carbonate, bisfluoroethylene carbonate, vinylene carbonate and butylene carbonate.

7. The lithium-ion battery according to claim 1, wherein The membrane-forming solvent accounts for 1% to 100% of the total solvent mass in the electrolyte.

8. The lithium-ion battery according to claim 7, characterized in that The membrane-forming solvent accounts for 5%-40% of the total solvent mass in the electrolyte.

9. The lithium-ion battery according to claim 1, wherein The particle size D of the negative electrode film-forming additive 50 0.01-20μm.

10. The lithium-ion battery according to claim 9, characterized in that The particle size D of the negative electrode film-forming additive 50 0.05-5μm.

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