Lithium-ion batteries and their applications

Abstract

To provide a lithium-ion battery that can improve electrolyte stability, improve the cycle performance of the lithium-ion battery, and improve the performance of the lithium-ion battery at high temperatures.SOLUTION: The lithium-ion battery has a double-sided surface density of 35 mg / cm2 or more. The lithium-ion battery includes a positive electrode sheet 10 containing a positive electrode active material containing lithium iron manganese phosphate, a negative electrode sheet 20, a separator 30 disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte 40 filled between the positive electrode sheet, the negative electrode sheet, and the separator. The electrolyte includes a non-aqueous solvent. The non-aqueous solvent includes ethylene carbonate. The mass ratio of ethylene carbonate in the non-aqueous solvent is 10 wt.% to 35 wt.%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to lithium ion batteries and their applications. [Background technology]

[0002] Currently, the rapid development of lithium-ion batteries in electric vehicles and large-scale energy storage has significantly expanded the market size of lithium-ion batteries. In particular, lithium iron phosphate batteries have become the mainstream in the development of lithium-ion batteries due to their low cost and long cycle life. However, because the specific capacity of current lithium iron phosphate materials is close to the theoretical value, the energy density of lithium iron phosphate batteries is approaching its limit.

[0003] Lithium manganese iron phosphate is obtained by substituting manganese for part of the iron in lithium iron phosphate, which increases the energy density of lithium manganese iron phosphate without increasing the cost of the cathode active material. However, lithium manganese iron phosphate has low electrical conductivity. In lithium-ion batteries, this leads to poor dynamic performance and poor cycle performance. During cycling of lithium manganese iron phosphate batteries, the dissolution of manganese ions destroys the solid electrolyte interphase (SEI), causing increased consumption of active lithium in the battery and increased resistance of the anode SEI film, thereby affecting the battery's cycle life. Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention provides a lithium ion battery and its application, which can improve the stability of the electrolyte, enhance the cycle performance of the lithium ion battery, and improve the performance of the lithium ion battery at high temperatures. [Means for solving the problem]

[0005] In order to solve the above technical problems, the present invention provides a lithium ion battery comprising at least the following components:

[0006] Double-sided surface density is 35mg / cm 2 A positive electrode sheet comprising the above and a positive electrode active material including lithium iron manganese phosphate;

[0007] A negative electrode sheet;

[0008] a separator disposed between the positive electrode sheet and the negative electrode sheet;

[0009] An electrolytic solution filled between a positive electrode sheet, a negative electrode sheet, and a separator, the electrolytic solution containing a non-aqueous solvent, the non-aqueous solvent containing ethylene carbonate, and the mass ratio of the ethylene carbonate in the non-aqueous solvent being 10% by weight to 35% by weight.

[0010] In one embodiment of the present invention, the molecular formula of the lithium manganese iron phosphate is Li x Mn y Fe 1-y PO4, where 0.95≦x≦1.05 and 0.4≦y≦0.7.

[0011] In one embodiment of the present invention, the mass proportion of ethylene carbonate in the non-aqueous solvent is 15% by weight to 30% by weight.

[0012] In one embodiment of the present invention, the electrolyte further includes a positive electrode film-forming additive, the positive electrode film-forming additive including 1,3-propane sultone, and the content of the positive electrode film-forming additive in the electrolyte is 0.01% by weight to 5% by weight.

[0013] In one embodiment of the present invention, the electrolyte further includes an anode film-forming additive, the anode film-forming additive includes vinylene carbonate, and the content of the anode film-forming additive in the electrolyte is 0.1 wt % to 10 wt %.

[0014] In one embodiment of the present invention, the non-aqueous solvent further comprises any one or a combination of at least two of dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, or diethyl carbonate.

[0015] In one embodiment of the present invention, the non-aqueous solvent further comprises ethyl methyl carbonate and dimethyl carbonate, and the mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is (0.5-4.5):(2.75-4.75):(2.75-4.75).

[0016] In one embodiment of the present invention, the content of the non-aqueous solvent in the electrolytic solution is 60% by weight to 85% by weight.

[0017] In one embodiment of the present invention, the electrolyte further comprises a lithium salt, and the lithium salt comprises any one or a combination of at least two of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2, or CF3SO3Li.

[0018] In one embodiment of the present invention, the concentration of the lithium salt in the electrolyte solution is 0.1 mol / L to 2 mol / L.

[0019] The present invention further provides an electrochemical device comprising the lithium ion battery described above. [Effects of the Invention]

[0020] In summary, the present invention provides a lithium-ion battery and its applications that can improve the conductivity of the electrolyte, ensure the oxidative stability of the electrolyte, enhance the kinetics of the lithium-ion battery, and improve its cycling performance. A dense CEI film can be formed on the surface of the positive electrode to mitigate the oxidative decomposition of the electrolyte. A dense SEI film can be formed on the negative electrode. The SEI film is continuously repaired during cycling, thereby reducing damage to the SEI film on the negative electrode caused by manganese ion dissolution and effectively improving the cycle life of lithium manganese iron phosphate batteries. Controlling the type and content of the solvent, positive electrode additive, and negative electrode additive improves the stability of the electrolyte, improving the cycling performance of the lithium-ion battery and its performance at high temperatures. [Brief explanation of the drawings]

[0021] In order to more clearly describe the embodiments of the present disclosure or the technical solutions of the prior art, the drawings necessary for describing the embodiments or related art are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present disclosure. Those skilled in the art can obtain other drawings based on these drawings without creative efforts. [Figure 1] 1 is a schematic diagram of a lithium ion battery according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, the implementation of the present invention will be described with specific examples. Those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. The present disclosure can also be implemented or applied through other different specific implementations. Various details of the specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure.

[0023] It is understood that this disclosure may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0024] The technical solutions of the present disclosure are described in further detail below with reference to some embodiments and drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present disclosure. All other embodiments that can be obtained by those skilled in the art based on the embodiments of the present disclosure without any creative work fall within the scope of protection of the present disclosure.

[0025] Please refer to FIG. 1. The present invention provides a lithium ion battery including a positive electrode sheet 10, a negative electrode sheet 20, a separator 30, and an electrolyte 40. The separator 30 is located between the positive electrode sheet 10 and the negative electrode sheet 20. The electrolyte 40 is filled between the positive electrode sheet 10, the negative electrode sheet 20, and the separator 30. The present invention does not limit the type or shape of the lithium ion battery. In one embodiment of the present invention, the lithium ion battery is a primary battery or a secondary battery. The secondary battery is, for example, a pouch-type battery, a prismatic battery, or a cylindrical battery. In this embodiment, a pouch-type secondary battery will be described as an example.

[0026] See FIG. 1. In one embodiment of the present invention, the positive electrode sheet 10 includes a positive electrode current collector and a positive electrode active material layer coated on one side of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a binder, a conductive agent, etc. The positive electrode current collector is made of, for example, nickel, titanium, aluminum, silver Examples of the positive electrode current collector include a foil made of, for example, stainless steel or carbon, which has been subjected to a surface treatment. The positive electrode current collector may be in the form of a foil, or one or a combination of a film, mesh, micropores, foam, nonwoven fabric, etc. The thickness of the positive electrode current collector is, for example, 8 μm to 15 μm. In this embodiment, the positive electrode current collector is, for example, an aluminum foil.

[0027] In one embodiment of the present invention, the positive electrode active material is selected from, for example, lithium manganese iron phosphate, the molecular formula of which is Li x Mn y Fe 1-y PO4, where 0.95≦x≦1.05 and 0.4≦y≦0.7. The binder may be selected from one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR). The conductive agent may be selected from one or more of conductive carbon black (SuperP, SP), acetylene black, carbon nanotubes, graphene, etc.

[0028] See Figure 1. In one embodiment of the present invention, the positive electrode active material is, for example, LiMn 0.6 Fe 0.4 The positive electrode active material is PO4, the binder is selected from polyvinylidene fluoride, and the conductive agent is selected from acetylene black. After mixing the positive electrode active material, acetylene black, and polyvinylidene fluoride in a weight ratio of, for example, 95:3:2, an organic solvent is added and the mixture is stirred until the system becomes homogeneous to obtain a positive electrode slurry. The organic solvent is selected from, for example, N-methylpyrrolidone (NMP). The positive electrode slurry is uniformly applied to aluminum foil and dried, and then the dried aluminum foil is processed by cold pressing or the like to obtain a positive electrode sheet 10. In this embodiment, the surface density on both sides of the positive electrode sheet is 35 mg / cm 2 That's all.

[0029] See FIG. 1. In one embodiment of the present invention, the negative electrode sheet 20 includes, for example, a negative electrode current collector and a negative electrode active material layer applied to at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a binder, a conductive agent, a thickener, etc. The negative electrode current collector is, for example, any one selected from a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, a stainless steel current collector, etc. The thickness of the negative electrode current collector is, for example, 8 μm to 15 μm.

[0030] See FIG. 1. In one embodiment of the present invention, the negative electrode active material is selected from one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide compounds, silicon carbide compounds, and lithium titanate. The binder is selected from one or more of polyvinylidene fluoride, polyamide, polypropylene, polyacrylate, polyvinyl ether, polymethyl methacrylate, polyhexafluoropropylene, and styrene-butadiene rubber. The conductive agent is selected from one or more of conductive carbon black, acetylene black, carbon nanotubes, graphite, and the like. In one embodiment of the present invention, the negative electrode current collector is selected from, for example, copper foil. The negative electrode active material is selected from, for example, graphite. The conductive agent is selected from, for example, acetylene black. The binder is selected from, for example, styrene-butadiene rubber. The thickener is selected from, for example, sodium carboxymethyl cellulose. In one embodiment of the present invention, graphite, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of, for example, 96:2:1:1, and deionized water was added and the mixture was thoroughly stirred to obtain a negative electrode slurry. The negative electrode slurry was uniformly applied to a copper foil, and then subjected to processes such as drying and cold pressing to obtain a negative electrode sheet 20.

[0031] See Figure 1. In one embodiment of the present invention, the separator 30 is, for example, a polyethylene (PE) film, a polypropylene (PP) film, a glass fiber film, a polyethylene film, or a composite film, and the thickness of the separator is, for example, 9 μm to 15 μm. In this embodiment, a polyethylene film with a thickness of 8 μm to 10 μm is selected as the base film of the separator, and a nano-alumina coating with a thickness of 2 μm to 4 μm is coated on the base film to obtain the separator 30.

[0032] See Fig. 1. In one embodiment of the present invention, the electrolyte solution 40 contains at least a non-aqueous solvent, a lithium salt, and an additive. 2 In the above cases, the non-aqueous solvent contains ethylene carbonate (EC), which has a high dielectric constant, to improve the conductivity of the electrolyte. The mass ratio of ethylene carbonate in the non-aqueous solvent is 10% to 35% by weight, or 15% to 30% by weight. If the EC content is too low, the high conductivity of the electrolyte 40 cannot be ensured, and the dynamic performance of the high-density lithium manganese iron phosphate battery cannot be improved. If the EC content is too high, the oxidation stability of the electrolyte 40 decreases, causing irreversible oxidative decomposition reactions in the positive electrode and reducing the cycle characteristics of the battery. Therefore, the conductivity of the electrolyte can be improved by adding and controlling the content of ethylene carbonate in the electrolyte. At the same time, the amount of ethylene carbonate added can be controlled to prevent a decrease in the oxidation stability of the electrolyte.

[0033] See FIG. 1. In one embodiment of the present invention, the non-aqueous solvent further includes, for example, any one or a combination of at least two of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and diethyl carbonate (DEC). The non-aqueous solvent includes, for example, ethyl methyl carbonate and dimethyl carbonate, and the mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is (0.5-4.5):(2.75-4.75):(2.75-4.75). In one embodiment of the present invention, the mass content of the non-aqueous solvent in the electrolyte solution 40 is, for example, 60% to 85%. When preparing the electrolyte solution, when the nitrogen content in the glove box is 99.999%, the actual oxygen content in the glove box is 0.1 ppm or less, and the water content is 0.1 ppm or less, battery-grade ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are used. At a given mass ratio After uniformly mixing, the thoroughly dried lithium salt is added to the non-aqueous solvent, and additives are added to prepare a non-aqueous electrolyte solution for a lithium ion battery.

[0034] See FIG. 1. In one embodiment of the present invention, the additive further includes a cathode film-forming additive and an anode film-forming additive. The cathode film-forming additive includes 1,3-propanesultone (PS), and the content of the cathode film-forming additive in the electrolyte 40 is, for example, 0.01 wt % to 5 wt %, e.g., 0.1 wt % to 2 wt %. The cathode film-forming additive forms a dense cathode-electrolyte interphase (CEI) on the surface of the positive electrode, thereby reducing oxidative decomposition of the electrolyte 40. If the mass content of the cathode film-forming additive is too low, a dense CEI film cannot be formed on the positive electrode to prevent oxidative decomposition of the electrolyte 40. If the mass content of the cathode film-forming additive is too high, an excessively thick CEI film is formed, increasing the interfacial resistance of the positive electrode and reducing the dynamic performance and cycle life of the battery. Therefore, the mass content of the positive electrode film-forming additive is controlled to ensure the stability of the positive electrode, reduce the decomposition of the electrolyte, and improve the cycle characteristics of the lithium-ion battery.

[0035] See FIG. 1. In one embodiment of the present invention, the negative electrode film-forming additive includes vinylene carbonate (VC), and the content of the negative electrode film-forming additive in the electrolyte 40 is 0.1 wt % to 10 wt %, for example, 0.5 wt % to 3 wt %, to form a high-quality SEI film on the negative electrode. If the mass content of VC is too low, a dense SEI film will not form and the SEI film will not be able to be repaired in a timely and continuous manner during cycling. Under these circumstances, manganese ions dissolved from the positive electrode will rapidly deteriorate the negative electrode interface, causing lithium loss and destruction of the dynamic performance of the negative electrode, resulting in a rapid decrease in the cycle life of the battery. If the mass percentage of VC is too high, the viscosity of the electrolyte will increase rapidly, the conductivity of the electrolyte 40 will decrease, and the dynamic performance of the lithium-ion battery will deteriorate. Therefore, by controlling the VC content, VC can form a dense SEI film on the anode, and the SEI film can be continuously repaired during cycling, thereby reducing the damage to the SEI film of the anode caused by the dissolution of manganese ions. The combined use of EC, PS, and VC can improve the dynamic performance of lithium-ion batteries and effectively extend the cycle life of lithium manganese iron phosphate batteries.

[0036] See FIG. 1. In one embodiment of the present invention, the electrolyte solution 40 further includes a lithium salt. The lithium salt is selected from, for example, any one or a combination of at least two of lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium bisfluorosulfonimide (LiFSI), lithium bistrifluoromethanesulfonimide (LiTFSI), lithium bisoxaborate (LiBOB), lithium difluorobisoxalophosphate (LiODFP), lithium difluorooxaloborate (LiODFB), lithium difluorophosphate (LiPOF), and lithium triflate (LiCFSO). In one embodiment of the present invention, the concentration of the lithium salt in the electrolyte solution 40 is 0.1 mol / L to 2 mol / L.

[0037] See FIG. 1. In one embodiment of the present invention, the positive electrode sheet 10, separator 30, and negative electrode sheet 20 are stacked in this order, with the separator 30 positioned between the positive electrode sheet 10 and the negative electrode sheet 20, to separate them. These are then stacked to obtain a bare battery cell. The bare battery cell is placed in an aluminum plastic film and baked at 80°C to remove moisture, and then an electrolyte 40 is injected and sealed. The sealed bare battery cell is then left to stand, hot and cold pressed, molded, clamped, and volume-divided to obtain a completed soft-pack lithium-ion battery.

[0038] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Appropriate modifications are possible within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0039] Example 1

[0040] Preparation of positive electrode sheet: LiMn as positive electrode active material 0.6 Fe 0.4 PO4, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder were selected and mixed in a mass ratio of 95:3:2. After uniformly mixing the positive electrode active material, binder, and conductive agent, a solvent selected from N-methylpyrrolidone was added and stirred until the mixture became uniform and transparent to obtain a positive electrode slurry. The positive electrode slurry was uniformly applied to aluminum foil, dried, and then cold-pressed to obtain a positive electrode sheet. The surface density of the positive electrode sheet on both sides was 38 mg / cm. 2 It was.

[0041] Preparation of negative electrode sheet: Graphite was used as the negative electrode active material, acetylene black as the conductive agent, styrene butadiene rubber as the binder, and sodium carboxymethyl cellulose as the thickener. These materials were mixed in a mass ratio of 96:2:1, and deionized water was added and thoroughly stirred to obtain negative electrode slurry. The negative electrode slurry was uniformly applied to copper foil, dried, and then cold-pressed to obtain a negative electrode sheet.

[0042] Preparation of electrolyte: The nitrogen content in the glove box was 99.999%, the actual oxygen content in the glove box was 0.1 ppm, and the water content was 0.1 ppm. The non-aqueous solvent was composed of ethylene carbonate, ethyl methyl carbonate, and carbonic acid in a mass ratio of 2:4:4. The total mass of the electrolyte was taken as 100%, and lithium salt and additives were added so that the concentration of lithium hexafluorophosphate was 1 mol / L, the mass percentage of PS was 1 wt%, and the mass percentage of VC was 2 wt%, to obtain the electrolyte.

[0043] Selection of separator: 9 μm polyethylene was selected as the base film, and a 3 μm thick nano-alumina coating was coated on the base film to obtain a separator.

[0044] Battery fabrication: A positive electrode sheet, a separator, and a negative electrode sheet were stacked in this order with the separator interposed between the positive electrode sheet and the negative electrode sheet, and these were stacked to obtain a bare battery cell. The bare battery cell was wrapped in aluminum plastic film, baked at 80°C to remove moisture, and then injected with 40 ml of electrolyte and sealed. The sealed bare battery cell was then left to stand, hot and cold pressed, molded, clamped, and volume-divided to obtain a lithium-ion battery.

[0045] Example 2

[0046] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 0.5:4.75:4.75, and the other steps were the same as in Example 1.

[0047] Example 3

[0048] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 1:4.5:4.5, and the other steps were the same as in Example 1.

[0049] Example 4

[0050] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 1.5:4.25:4.25, and the other steps were the same as in Example 1.

[0051] Example 5

[0052] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 3:3.5:3.5, and the other steps were the same as in Example 1.

[0053] Example 6

[0054] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 3.5:3.25:3.25, and the other steps were the same as in Example 1.

[0055] Example 7

[0056] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 4.5:2.75:2.75, and the other steps were the same as in Example 1.

[0057] Example 8

[0058] The mass percentage of PS in the electrolyte was set to 0.005% by weight, and the other steps were the same as in Example 1.

[0059] Example 9

[0060] The mass percentage of PS in the electrolyte was set to 0.01% by weight, and the other steps were the same as in Example 1.

[0061] Example 10

[0062] The mass percentage of PS in the electrolyte was set to 0.1 wt %, and the other steps were the same as in Example 1.

[0063] Example 11

[0064] The mass percentage of PS in the electrolyte was 3 wt %, the concentration of LiFSI was 0.6 mol / L, and the other steps were the same as in Example 1.

[0065] Example 12

[0066] The mass percentage of PS in the electrolyte was set to 5 wt %, and the other steps were the same as in Example 1.

[0067] Example 13

[0068] The mass percentage of PS in the electrolyte was set to 8 wt %, and the other steps were the same as in Example 1.

[0069] Example 14

[0070] The mass percentage of VC in the electrolyte was 0.05% by weight, and the other steps were the same as in Example 1.

[0071] Example 15

[0072] The mass percentage of VC in the electrolyte was 0.1 wt %, and the other steps were the same as in Example 1.

[0073] Example 16

[0074] The mass percentage of VC in the electrolyte was set to 0.5 wt %, and the other steps were the same as in Example 1.

[0075] Example 17

[0076] The mass percentage of VC in the electrolytic solution was set to 3 wt %, and the other steps were the same as in Example 1.

[0077] Example 18

[0078] The mass percentage of VC in the electrolytic solution was 10 wt %, and the other steps were the same as in Example 1.

[0079] Example 19

[0080] The mass percentage of VC in the electrolyte was 15 wt %, and the other steps were the same as in Example 1.

[0081] Comparative Example 1

[0082] The mass ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the electrolytic solution was 0:5:5, and the other operations were the same as in Example 1.

[0083] Comparative Example 2

[0084] The procedure was the same as in Example 1, except that no PS was added to the electrolyte.

[0085] Comparative Example 3

[0086] The other operations were the same as in Example 1, except that VC was not added to the electrolyte.

[0087] In the present invention, performance tests such as tests of capacity retention rate in room temperature cycles and high temperature cycles were carried out on the lithium ion batteries obtained in Examples 1 to 19 and Comparative Examples 1 to 3. The results are shown in Table 1.

[0088] In one embodiment of the present invention, the capacity retention test during room temperature cycling was performed as follows. The lithium-ion battery was charged at a constant current of 1 C to 4.2 V at 25°C, and then charged at a constant voltage of 4.2 V until the current fell below 0.05 C. After leaving the battery for 10 minutes, the lithium-ion battery was discharged at a constant current of 1 C to 2.5 V. The discharge capacity of the lithium-ion battery under these conditions was tested, and the result was taken as the first-cycle discharge capacity. The above process was repeated multiple times on the battery, and the capacity retention of the battery after 400 cycles was calculated. The relative capacity retention after cycling was calculated according to the following formula: Capacity retention rate (%) = (discharge capacity equivalent to 400 cycles / discharge capacity at the first cycle) × 100%

[0089] In one embodiment of the present invention, the capacity retention rate during high-temperature cycling was tested as follows. A manganese iron lithium phosphate battery was charged to 4.2 V at a constant current of 1 C at 45°C, and then charged at a constant voltage of 4.2 V until the current dropped below 0.05 C. After leaving the battery for 10 minutes, the manganese iron lithium phosphate battery was discharged to 2.5 V at a constant current of 1 C. The discharge capacity of the manganese iron lithium phosphate battery under this condition was tested, and the result was recorded as the first-cycle discharge capacity. The above process was repeated multiple times on the battery, and the capacity retention rate of the battery after 400 cycles was calculated. The relative capacity retention rate after cycling was calculated according to the following formula: Capacity retention rate (%) = (discharge capacity equivalent to 400 cycles / discharge capacity at the first cycle) × 100% The performance test results of the lithium ion batteries of Examples 1 to 19 and Comparative Examples 1 to 3 are shown below.

[0090] [Table 1]

[0091] See Table 1. Examples 1 to 19 and Comparative Example 1 show that adding ethylene carbonate to the electrolyte and controlling the amount of ethylene carbonate simultaneously improves the retention rate of lithium-ion batteries after room-temperature cycling and after high-temperature cycling. That is, adding ethylene carbonate increases the conductivity of the electrolyte, improving the cycle performance of high-density lithium-ion batteries. Furthermore, controlling the ethylene carbonate content prevents the electrolyte from becoming unstable due to excessive ethylene carbonate content. Comparing Examples 1 to 19 and Comparative Example 2 shows that adding the positive electrode additive PS forms a dense CEI film on the positive electrode, improving the power performance and cycle performance of lithium-ion batteries. Comparing Examples 1 to 19 and Comparative Example 3 shows that adding the negative electrode additive VC forms a dense SEI film on the negative electrode, continuously repairs the SEI film during cycling, and reduces damage to the negative electrode SEI film due to manganese ion dissolution. This effectively improves the cycle life of lithium manganese iron phosphate batteries.

[0092] See Table 1. Comparative Examples 1 to 7 show that as the ethylene carbonate content increases, the capacity retention at room temperature and high temperature first increases and then decreases. When the ethylene carbonate content is 30 wt % of the nonaqueous solvent, the capacity retention is highest, and as the dimethyl carbonate content increases, the dynamic performance of the lithium ion battery improves. However, if the dimethyl carbonate content is too high, the oxidation stability of the electrolyte deteriorates, causing a rapid capacity loss. Therefore, to ensure the performance of the lithium ion battery, it is necessary to control the amount of ethylene carbonate.

[0093] See Table 1. Comparing Example 1 with Examples 8 to 13, we found that as the content of the positive electrode additive PS increased, the post-cycle capacity retention first increased and then decreased, and the capacity retention after room temperature cycling and the capacity retention after high temperature cycling changed synchronously. When the PS content was 3 wt%, the overall performance of the lithium-ion battery was optimized, indicating that increasing the PS content formed a dense CEI film on the positive electrode, preventing oxidative decomposition of the electrolyte. However, if the PS content was too high, the CEI film became too thick, increasing the interfacial resistance of the positive electrode and reducing the dynamic performance and cycle life of the lithium-ion battery.

[0094] See Table 1. Comparing Example 1 with Examples 14-19, we find that as the content of the anode additive VC increases, the post-cycling capacity retention rate first increases and then decreases. When the VC content is between 0.5 wt% and 3 wt%, the overall performance of the lithium-ion battery reaches an optimal level. This indicates that as the VC content increases, a dense SEI film is formed on the anode, and the SEI film is continuously repaired during cycling. This reduces damage to the SEI film of the anode caused by manganese ion dissolution and effectively improves the cycle life of the lithium manganese iron phosphate battery. At the same time, it prevents problems such as SEI damage and excessive impedance that occur when the VC content is too high or too low. That is, adding ethylene carbonate, PS, and VC to the electrolyte and controlling their contents can improve the performance of the lithium manganese iron phosphate battery and improve its cycle characteristics. [Industrial Applicability]

[0095] In summary, the present invention provides a lithium-ion battery and its applications. By selecting the components of the nonaqueous solvent, the conductivity of the electrolyte can be improved, ensuring the oxidative stability of the electrolyte, thereby improving the dynamic performance and cycle characteristics of the lithium-ion battery. By controlling the positive electrode additive and its content, a dense CEI film can be formed on the surface of the positive electrode, reducing the oxidative decomposition of the electrolyte. By controlling the negative electrode additive and its content, a dense SEI film can be formed on the negative electrode, which can be continuously repaired during cycling, thereby reducing damage to the negative electrode SEI film caused by manganese dissolution and effectively improving the cycle life of lithium manganese iron phosphate batteries. By controlling the type and content of the solvent, positive electrode additive, and negative electrode additive, the stability of the electrolyte can be improved, improving the cycle performance of the lithium-ion battery and its performance at high temperatures.

[0096] The above description is merely a preferred embodiment of the present disclosure and merely describes the technical principles used. Those skilled in the art should understand that the scope of disclosure contained in this disclosure is not limited to the technical solution formed by a specific combination of the above technical features, but should also encompass other technical solutions formed by any combination of the above technical features or their equivalent features, such as (but not limited to) technical solutions formed by replacing the above features with technical features having similar functions disclosed in the present application, within the scope of the present invention.

[0097] Except for the technical features described herein, the remaining technical features are known to those skilled in the art, and in order to highlight the innovative features of the present disclosure, the remaining technical features will not be described in detail here. [Explanation of symbols]

[0098] 10: Positive electrode sheet 20: Negative electrode sheet 30: Separator 40: Electrolyte

Claims

1. Both surfaces have a surface density of 35 mg / cm ２ A positive electrode sheet comprising the above and a positive electrode active material including lithium iron manganese phosphate; A negative electrode sheet; a separator disposed between the positive electrode sheet and the negative electrode sheet; The battery includes at least an electrolyte solution filled between the positive electrode sheet, the negative electrode sheet, and the separator, The electrolyte solution includes a non-aqueous solvent, 1,3-propane sultone, vinylene carbonate, and LiPF6; the content of the 1,3-propane sultone in the electrolytic solution is 0.01% by weight to 5% by weight, the content of the vinylene carbonate in the electrolytic solution is 0.1% by weight to 10% by weight, the non-aqueous solvent contains ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the mass ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is (0.5-4.5):(2.75-4.75):(2.75-4.75); The lithium ion battery, wherein the mass proportion of the ethylene carbonate in the non-aqueous solvent is 15% by weight to 30% by weight.

2. The molecular formula of the lithium manganese iron phosphate is Li ｘ Mn ｙ Fe １－ｙ P.O. ４ 2. The lithium ion battery of claim 1 , wherein 0.95≦x≦1.05 and 0.4≦y≦0.

7.

3. The lithium ion battery according to claim 1 , wherein the non-aqueous solvent further contains any one or a combination of at least two of propylene carbonate and diethyl carbonate.

4. 2. The lithium ion battery according to claim 1, wherein the content of the non-aqueous solvent in the electrolyte solution is 60% by weight to 85% by weight.

5. The LiPF in the electrolyte ６ The lithium ion battery according to claim 1, wherein the concentration of the lithium ion battery is 0.1 mol / L to 2 mol / L.

6. An electrochemical device comprising the lithium ion battery according to any one of claims 1 to 5.

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