Lithium ion battery with long cycle life and good charge-discharge performance and preparation method thereof

Abstract

The invention provides a lithium ion battery with long cycle life and good charge-discharge performance and a preparation method thereof, and belongs to the technical field of lithium ion batteries. The preparation method specifically comprises the following steps: respectively adding potassium phosphate to the surfaces of a positive electrode material, a negative electrode material, an electrolyte or a diaphragm of the lithium ion battery; the potassium phosphate is at least one of potassium phosphate and dipotassium phosphate. After potassium phosphate is added into the lithium ion battery, the cycle performance, the low-temperature discharge efficiency and the storage stability of the battery are all improved, the preparation process is simple and flexible, and industrial production of the lithium ion battery is convenient to realize.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a lithium ion battery with high cycle life and good charge and discharge performance and a preparation method thereof. Background Art

[0002] Lithium-ion batteries, with their high energy density and low self-discharge rates, are widely used in electronics, electric vehicles, energy storage systems, and other fields. Despite these advantages, lithium-ion batteries also have the following disadvantages: they are sensitive to ambient temperature; extreme temperatures, whether too cold or too hot, can affect battery performance; and their capacity gradually decreases with increasing charge and discharge cycles, resulting in poor cycle performance.

[0003] In response to the shortcomings of the above-mentioned lithium-ion batteries, researchers have targeted the various components of lithium-ion batteries (electrode active materials, electrolytes, and separators) and carried out a series of improvement measures. Improvements to positive electrode materials generally include surface coating, doping modification, and nano-processing. For example, Chinese patent CN102437337A discloses a lithium-ion battery positive electrode material LiCoPO4 / Al2O3, which is composed of olivine-type LiCoPO4 material and aluminum salt. A layer of metal oxide film is coated on the surface of the lithium-ion battery positive electrode active material using surface coating technology to prepare a lithium-ion battery positive electrode composite material with high voltage and high cycle stability. Chinese patent CN102509804B discloses a method for preparing a lithium-ion battery Li3V2(PO4)3 / C composite material. β-cyclodextrin is introduced into the lithium vanadium phosphate system as a chelating agent and carbon source, which can achieve molecular-level mixing of the raw materials, and then a carbon-coated monoclinic lithium vanadium phosphate material with small and uniform particle size and excellent electrochemical performance can be synthesized at a lower sintering temperature and a shorter sintering time. Improvements to negative electrode materials generally include secondary granulation, carbon coating, and preparation of composite materials. For example, Chinese patent CN103682295B discloses a negative electrode material for lithium-ion batteries, including lithium titanate Li4Ti5O 12and transition metal sulfides, wherein the transition metal sulfides are one or more of NiS, FeS2, FeS, TiS2, MoS, and Co9S8. This lithium-ion battery negative electrode material has high capacity, excellent cycle stability, and durability. Improvements to electrolytes generally include developing solid-state electrolytes and adding additives to the electrolyte. For example, Chinese patent CN108199080A discloses an electrolyte for a lithium-ion battery comprising the following components: an electrolyte, a lithium salt, a polymer, and a ceramic lithium-ion solid electrolyte. This electrolyte can improve the safety of lithium secondary batteries while improving the compatibility and interfacial contact between the electrolyte and the electrodes. Improvements to the diaphragm include optimizing thickness, preparing ceramic coatings, and functional improvements. For example, Chinese patent CN114696040B discloses a lithium-ion battery diaphragm, comprising a substrate and at least one layer of ceramic coating and at least one layer of organic polymer electrolyte coating coated on the surface of the substrate. The lithium-ion battery diaphragm uses an alkali metal cationic polymer coating as an organic coating, and the ceramic coating is an inorganic coating. The diaphragm can prevent the gas generated by the battery reaction from passing through the diaphragm quickly, while lithium ions can be transmitted normally, thereby improving the cycle life and safety performance of the battery.

[0004] Although the above-mentioned improvement methods for lithium-ion batteries can improve their electrochemical performance to a certain extent, the improvement methods have certain complexities and limitations and are not flexible enough in practical applications. In addition, when improving the electrochemical performance of lithium-ion batteries on one hand, the electrochemical performance on the other hand is sacrificed. For example, when carbon coating is used to improve the negative electrode material, although carbon coating can improve the stability of the material, it may also hinder the insertion and extraction of lithium ions, thereby affecting the charge and discharge efficiency of the battery. Summary of the Invention

[0005] The purpose of the present invention is to provide a lithium ion battery with a long cycle life and good charge and discharge performance and a preparation method thereof, so as to solve the problems that the improvement method of the above lithium ion battery is not flexible and the electrochemical performance is not improved enough.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The first aspect of the present invention provides a lithium-ion battery with a long cycle life and good charge and discharge performance. The lithium-ion battery is prepared by adding potassium phosphate salt to the positive electrode material, negative electrode material, electrolyte or diaphragm surface of the lithium-ion battery.

[0008] Preferably, the potassium phosphate salt is at least one of potassium phosphate and dipotassium hydrogen phosphate.

[0009] More preferably, the potassium phosphate salt is potassium phosphate, as potassium dihydrogen phosphate will produce acidic substances at high temperatures, thereby corroding the electrode.

[0010] Preferably, the particle size of the potassium phosphate salt is less than 50 μm.

[0011] Preferably, potassium phosphate is added to the positive electrode material, the negative electrode material or the electrolyte, and the potassium phosphate is coated on the surface of the separator.

[0012] Preferably, the amount of potassium phosphate added to the positive electrode material, negative electrode material or electrolyte is 0.005-5 wt.%. 0.005-5 wt.% specifically refers to the mass fraction of potassium phosphate in the positive electrode material, negative electrode material or electrolyte.

[0013] The present invention specifically limits the amount of potassium phosphate added to the electrode active materials (positive electrode materials and negative electrode materials) and the electrolyte (0.005 to 5 wt.%). If the amount of potassium phosphate added is too small, the metal impurities inside the battery cannot be removed. If the amount of potassium phosphate added is too high, it will also have an adverse effect on battery performance, such as increasing the internal resistance of the battery, reducing the battery capacity, or causing irreversible damage to the electrode material.

[0014] Preferably, the coating amount of potassium phosphate on the diaphragm is 0.005 to 50 g / m 2 .

[0015] A second aspect of the present invention provides a method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to a positive electrode material, and the method specifically comprises the following steps:

[0016] (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material and potassium phosphate are added and stirred continuously to prepare a slurry 1, which is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet;

[0017] (2) Mixing and stirring the conductive agent, binder, and organic solvent, then adding the negative electrode material and continuing to stir to prepare slurry 2, coating the slurry 2 on copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet;

[0018] (3) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

[0019] A third aspect of the present invention provides a method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to an electrolyte, and the method specifically comprises the following steps:

[0020] (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 3. The slurry 3 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet;

[0021] (2) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a negative electrode material is added and stirred continuously to prepare a slurry 4, which is coated on a copper foil, dried, cut into strips, and spot-welded to obtain a negative electrode sheet;

[0022] (3) adding potassium phosphate to the electrolyte and stirring evenly to obtain a mixed electrolyte;

[0023] (4) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, which is then placed in a shell and injected with a mixed electrolyte, sealed and formed to form a lithium-ion battery.

[0024] A fourth aspect of the present invention provides a method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is coated on the surface of a diaphragm, specifically comprising the following steps:

[0025] (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 5. The slurry 5 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet;

[0026] (2) Mixing and stirring a conductive agent, a binder, and an organic solvent, then adding a negative electrode material and continuing to stir to prepare a slurry 6, coating the slurry 6 on a copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet;

[0027] (3) dispersing potassium phosphate and fluorine-based materials in a solvent to obtain a mixture, and coating the mixture on the surface of the diaphragm to obtain a modified diaphragm;

[0028] (4) The positive electrode sheet, the negative electrode sheet and the modified separator are wound to obtain an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

[0029] A fifth aspect of the present invention provides a method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to a negative electrode material, and the method specifically comprises the following steps:

[0030] (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 7. The slurry 7 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet;

[0031] (2) Mixing and stirring a conductive agent, a binder, and an organic solvent, then adding a negative electrode material and potassium phosphate and continuing to stir to prepare a slurry 8, coating the slurry 8 on a copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet;

[0032] (3) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

[0033] Preferably, the positive electrode material is lithium iron phosphate LiFePO4.

[0034] Preferably, the D50 of the positive electrode material is 2 μm.

[0035] Preferably, the negative electrode material is graphite material.

[0036] Preferably, the D50 of the negative electrode material is 20 μm.

[0037] Preferably, the conductive agent is conductive carbon black.

[0038] Preferably, the binder is at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber emulsion (SBR).

[0039] Preferably, the organic solvent is at least one of ethylene carbonate, dimethyl carbonate and diethyl carbonate.

[0040] Preferably, the electrolyte includes at least one of ethylene carbonate and lithium hexafluorophosphate.

[0041] Preferably, the electrolyte is 1.2M LiPF6.

[0042] Preferably, the separator is PE / PP.

[0043] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0044] 1. The addition of potassium phosphate of the present invention can effectively react with metal impurities inside the battery, help purify the electrode surface, reduce self-discharge, and reduce safety hazards.

[0045] 2. The addition of potassium phosphate salt of the present invention helps to form a more stable solid electrolyte interface (SEI) film, reduces the occurrence of side reactions, prolongs the battery life, and improves the working efficiency and charge and discharge performance of the battery.

[0046] 3. The method of the present invention has good compatibility and is applicable to various types of lithium-ion batteries. The specific type, particle size and dosage of potassium phosphate salt can be adjusted according to specific needs, showing high flexibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Graph showing the change in discharge efficiency of the lithium-ion batteries of the embodiment and comparative example at different temperatures;

[0048] Figure 2 FIG1 is a graph showing the change in expansion rate of the lithium-ion batteries of the embodiment and the comparative example at 85° C.;

[0049] Figure 3This is a test chart of the capacity recovery performance of lithium-ion batteries after temperature shock;

[0050] Figure 4 The temperature shock capacity recovery performance of the lithium ion batteries of the embodiment and the comparative example;

[0051] Figure 5 The graphs are the cycle life curves of the lithium-ion batteries of the embodiment and the comparative example. DETAILED DESCRIPTION

[0052] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0053] Polyvinylidene fluoride (CAS: 24937-79-9) was purchased from Hubei Wonder Chemical Co., Ltd.

[0054] Example 1

[0055] A method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to a positive electrode material, specifically comprising the following steps:

[0056] (1) 2g of polyvinylidene fluoride (PVDF) was mixed with 100g of N-methylpyrrolidone (NMP), stirred at 1000rpm in a 55°C water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, 3g of superconducting carbon black was added to the PVDF / NMP solution and dispersed at 2000rpm for 40 minutes to obtain a uniform conductive colloid. 95g of LiFePO4 with a D50 of 2μm and dipotassium hydrogen phosphate with a D50 of ≤1μm (accounting for 0.5% of the mass of LiFePO4) were added in batches and stirred at 1500rpm for 3 hours. The viscosity of the slurry was controlled at 4500-6500mPa·s and the solid content was 50% to prepare slurry 1. The slurry 1 was coated on an aluminum foil with a thickness of 12μm. The coating amount of the aluminum foil was 0.012g / cm 2 , after drying, slitting and spot welding, the positive electrode sheet is obtained;

[0057] (2) Mix 4g of sodium carboxymethyl cellulose (CMC) with 132g of deionized water, stir at 600rpm for 2 hours at 45℃ to form a transparent colloid. After standing and degassing for 30 minutes, add 2g of superconducting carbon black to the CMC solution and disperse it at 2000rpm for 30 minutes to obtain a uniform black conductive colloid. Add 94g of graphite with a D50 of 15-20μm in batches and continue stirring at 1500rpm for 2 hours to ensure that the slurry has no agglomerated particles and the viscosity is controlled at 3000-5000mPa·s. Slowly add 3g of styrene-butadiene rubber emulsion (SBR), switch to 500rpm and stir at low speed for 15 minutes, add deionized water to adjust the solid content to 45%, and make slurry 2. Apply slurry 2 on copper foil with a thickness of 8μm. The coating amount of copper foil is 0.0051g / cm 2 , after drying, slitting and spot welding, the negative electrode sheet is obtained;

[0058] (3) The positive electrode sheet, the negative electrode sheet and the PE separator are wound to form an electrode assembly, which is then placed into a shell and injected with 1.2M LiPF6 electrolyte, sealed and formed to obtain a lithium-ion battery.

[0059] Example 2

[0060] A method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to an electrolyte, specifically comprising the following steps:

[0061] (1) Mix 2g of polyvinylidene fluoride (PVDF) with 100g of N-methylpyrrolidone (NMP), stir at 1000rpm in a 55℃ water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, add 3g of superconducting carbon black to the PVDF / NMP solution and disperse it at 2000rpm for 40 minutes to obtain a uniform conductive colloid. Add 95g of LiFePO4 with a D50 of 2μm in batches and continue stirring at 1500rpm for 3 hours. Control the slurry viscosity at 4500-6500mPa·s and the solid content at 50% to make slurry three. Apply slurry three on aluminum foil with a thickness of 12μm. The coating amount of aluminum foil is 0.012g / cm 2 , after drying, slitting and spot welding, the positive electrode sheet is obtained;

[0062] (2) Mix 4g of sodium carboxymethyl cellulose (CMC) with 132g of deionized water, stir at 600rpm for 2 hours at 45℃ to form a transparent colloid. After standing and degassing for 30 minutes, add 2g of superconducting carbon black to the CMC solution and disperse it at 2000rpm for 30 minutes to obtain a uniform black conductive colloid. Add 94g of D50 15-20μm graphite in batches and continue stirring at 1500rpm for 2 hours to ensure that the slurry has no agglomerated particles and the viscosity is controlled at 3000-5000mPa·s. Slowly add 3g of styrene-butadiene rubber emulsion (SBR), switch to 500rpm and stir at low speed for 15 minutes, add deionized water to adjust the solid content to 45%, and make slurry four. Slurry four is coated on copper foil with a thickness of 8μm, and the copper foil coating amount is 0.0051g / cm 2 , after drying, slitting and spot welding, the negative electrode sheet is obtained;

[0063] (3) Potassium hydrogen phosphate (K2HPO4, purity ≥99.9%) and fluoroethylene carbonate (FEC) were mixed in a mass ratio of 1:5, and zirconium oxide grinding balls (ball-to-material mass ratio 5:1) were added. The mixture was ball-milled in a jar at 300 rpm for 4 hours to obtain a nanoscale suspension with a D50 of ≤200 nm. The suspension was vacuum-dried at 60°C for 2 hours to remove the residual solvent (FEC) and obtain dry nano-K2HPO4 powder (moisture content ≤50 ppm).

[0064] Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of EC / DMC / EMC (volume ratio 3:4:3) to prepare a 1.2M LiPF6 electrolyte. After dehydration with molecular sieves, the water content was ≤10ppm. The pretreated nano-K2HPO4 powder was added to the basic LiPF6 electrolyte at a mass ratio of 0.8%, followed by 1% FEC (volume ratio) and 0.5% LiODFB (mass ratio). In an argon glove box (dew point ≤-50°C), a magnetic stirrer (50°C, 500rpm) was used to stir continuously for 2 hours until the solution was clear and transparent with no visible particles or precipitation. The mixed solution was transferred to a high-shear emulsifier and dispersed at 10,000rpm for 15 minutes to ensure uniform distribution of K2HPO4 and LiODFB. Finally, the electrolyte was filter-filtered through a 0.22μm polytetrafluoroethylene (PTFE) membrane to remove undissolved particles and impurities to obtain a mixed electrolyte.

[0065] (4) The positive electrode sheet, the negative electrode sheet and the PE separator are wound to form an electrode assembly, which is then placed in a shell and injected with a mixed electrolyte, sealed and formed to produce a lithium-ion battery.

[0066] Example 3

[0067] A method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance comprises coating potassium phosphate on the surface of a diaphragm, and specifically comprising the following steps:

[0068] (1) Mix 2g of polyvinylidene fluoride (PVDF) with 100g of N-methylpyrrolidone (NMP), stir at 1000rpm in a 55℃ water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, add 3g of superconducting carbon black to the PVDF / NMP solution and disperse it at 2000rpm for 40 minutes to obtain a uniform conductive colloid. Add 95g of LiFePO4 with a D50 of 2μm in batches and continue stirring at 1500rpm for 3 hours. Control the slurry viscosity at 4500-6500mPa·s and the solid content at 50% to prepare slurry 5. Apply slurry 5 on aluminum foil with a thickness of 12μm. The coating amount of aluminum foil is 0.012g / cm 2 , after drying, slitting and spot welding, the positive electrode sheet is obtained;

[0069] (2) Mix 4g of sodium carboxymethyl cellulose (CMC) with 132g of deionized water, stir at 600rpm for 1-2 hours at 45℃ to form a transparent colloid. After standing and degassing for 30 minutes, add 2g of superconducting carbon black to the CMC solution and disperse it at 2000rpm for 30 minutes to obtain a uniform black conductive colloid. Add 94g of D50 (15-20μm) graphite in batches and continue stirring at 1500rpm for 2 hours to ensure that the slurry has no agglomerated particles and the viscosity is controlled at 3000-5000mPa·s. Slowly add 3g of styrene-butadiene rubber emulsion (SBR), switch to 500rpm and stir at low speed for 15 minutes, add deionized water to adjust the solid content to 45%, and make slurry 6. Slurry 6 is coated on a copper foil with a thickness of 8μm, and the copper foil coating amount is 0.0051g / cm 2 After drying, stripping and spot welding, the negative electrode sheet is obtained.

[0070] (3) Potassium hydrogen phosphate (purity ≥ 99.9%) was mixed with deionized water / ethanol (1:1 mass ratio), and zirconium oxide grinding balls (ball-to-material ratio 5:1) were added. The mixture was ball-milled at 400 rpm for 6 hours to obtain a nanosuspension with a D50 of ≤ 200 nm. The suspension was then vacuum-dried at 60°C for 2 hours to remove the solvent residue. A polyethylene (PE) membrane (thickness 16 μm, porosity 40%) was corona treated (surface energy ≥ 38 dyne / cm) and cleaned with argon plasma (power 50 W, flow rate 10 sccm) to improve the coating adhesion.

[0071] 8g of polyvinylidene fluoride was mixed with 96g of N-methylpyrrolidone (NMP) and stirred at 1000rpm in a 60℃ water bath for 2 hours to form a transparent viscous solution. 10g of nano-K2HPO4 and 5g of aluminum oxide (Al2O3) were then added in sequence. The mixture was dispersed at 2500rpm for 2 hours. The slurry viscosity was controlled at 2500±300mPa·s and the solid content was 23%. The slurry was evenly coated on one side of the PE diaphragm using a micro-gravure coater. The wet film thickness was 25μm and the coating speed was 5m / min. The thickness deviation was ensured to be ≤±2μm. The 90% solvent was first dried using an 80℃ hot air circulation system for 30 minutes. The mixture was then vacuum-dried at 120℃ for 2 hours. Finally, the coating layer was compacted to 6μm with a porosity of 35% by roller pressing at 8MPa pressure to obtain the modified diaphragm.

[0072] (4) The positive electrode sheet, the negative electrode sheet and the modified separator are wound to obtain an electrode assembly, which is then placed into a shell and injected with 1.2M LiPF6 electrolyte, sealed and formed to obtain a lithium-ion battery.

[0073] Example 4

[0074] A method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance, wherein potassium phosphate is added to a negative electrode material, specifically comprising the following steps:

[0075] (1) Mix 2g of polyvinylidene fluoride (PVDF) with 100g of N-methylpyrrolidone (NMP), stir at 1000rpm in a 55℃ water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, add 3g of superconducting carbon black to the PVDF / NMP solution and disperse it at 2000rpm for 40 minutes to obtain a uniform conductive colloid. Add 95g of LiFePO4 with a D50 of 2μm in batches and continue stirring at 1500rpm for 3 hours. Control the slurry viscosity at 4500-6500mPa·s and the solid content at 50% to prepare slurry 7. Apply slurry 7 on aluminum foil with a thickness of 12μm. The coating amount of aluminum foil is 0.012g / cm 2 , after drying, slitting and spot welding, the positive electrode sheet is obtained;

[0076] (2) Potassium hydrogen phosphate (purity ≥99.9%) was mixed with ethanol in a mass ratio of 1:5, and zirconium oxide grinding balls (ball-to-material ratio 5:1) were added. The mixture was ball-milled at 400 rpm for 6 hours to obtain a nanoparticle suspension with a D50 of ≤200 nm. The suspension was then vacuum-dried at 60°C for 2 hours to remove the residual solvent and obtain dry nano-K2HPO4 powder.

[0077] Plasma cleaning (power 50W, argon flow rate 10sccm) was performed on 8μm smooth copper foil, and the surface roughness R a ≤0.2μm, enhance slurry adhesion.

[0078] 1g of sodium carboxymethyl cellulose (CMC) was mixed with 50g of deionized water and stirred at 500rpm for 1 hour at 25°C to form a transparent colloid. 2g of superconducting carbon black (SP) was then added to the CMC solution and dispersed at 2000rpm for 30 minutes to form a uniform conductive colloid. 94g of graphite with a D50 of 15-20μm and 3g of nano-K2HPO4 were then added in batches. The mixture was stirred at 1500rpm for 2 hours. The slurry viscosity was controlled at 3000-5000mPa·s and the solid content was 45%. Finally, 4g of styrene-butadiene rubber latex (SBR) was added and stirred at 500rpm for 15 minutes to form slurry 8. Slurry 8 was coated on 8μm copper foil at a coating weight of 0.0051g / cm 2 , after drying, slitting and spot welding, the negative electrode sheet is obtained;

[0079] (3) The positive electrode sheet, the negative electrode sheet and the PE separator are wound to form an electrode assembly, which is then placed into a shell and injected with 1.2M LiPF6 electrolyte, sealed and formed to obtain a lithium-ion battery.

[0080] Comparative Example 1

[0081] The difference between this comparative example and Example 1 is that potassium phosphate is not added to the positive electrode sheet. The specific preparation method of the positive electrode material in this comparative example is:

[0082] Mix 2g of polyvinylidene fluoride (PVDF) with 100g of N-methylpyrrolidone (NMP), stir at 1000rpm in a 55℃ water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, add 3g of superconducting carbon black to the PVDF / NMP solution and disperse it at 2000rpm for 40 minutes to obtain a uniform conductive colloid. Add 95g of LiFePO4 with a D50 of 2μm in batches and continue stirring at 1500rpm for 3 hours. Control the slurry viscosity at 4500-6500mPa·s and the solid content at 50% to make slurry 1. Apply slurry 1 on aluminum foil with a thickness of 12μm, and the coating amount of aluminum foil is 0.012g / cm 2 After drying, slitting and spot welding, the positive electrode sheet is obtained. The other steps are the same as those in Example 1.

[0083] Comparative Example 2

[0084] The difference between this comparative example and Example 1 is that the potassium phosphate added to the positive electrode sheet is replaced with disodium hydrogen phosphate. The specific preparation method of the positive electrode material in this comparative example is:

[0085] 2g of polyvinylidene fluoride (PVDF) was mixed with 100g of N-methylpyrrolidone (NMP) and stirred at 1000rpm in a 55°C water bath for 2 hours to form a transparent viscous solution. After standing and degassing for 30 minutes, 3g of superconducting carbon black was added to the PVDF / NMP solution and dispersed at 2000rpm for 40 minutes to obtain a uniform conductive colloid. 95g of LiFePO4 with a D50 of 2μm and disodium hydrogen phosphate with a D50 ≤ 1μm (accounting for 0.5% of the mass of LiFePO4) were added in batches and stirred at 1500rpm for 3 hours. The slurry viscosity was controlled at 4500-6500mPa·s and the solid content was 50% to prepare slurry 9. Slurry 9 was coated on aluminum foil with a thickness of 12μm. The coating amount of aluminum foil was 0.012g / cm 2 After drying, slitting and spot welding, the positive electrode sheet is obtained. The other steps are the same as those in Example 1.

[0086] Test example

[0087] The electrochemical properties of the lithium-ion batteries of the embodiments and comparative examples were tested.

[0088] (1) The discharge efficiency of the lithium-ion batteries of the embodiment and the comparative example at different temperatures was tested. The test process is as follows:

[0089] At 20±5℃, the battery is fully charged at a current of 0.5C, and then discharged at an ambient temperature of 20±5℃ at a current of 0.5C to obtain a capacity C0;

[0090] The battery was fully charged at 0.5C at 20±5°C, and then discharged at 0.5C at different ambient temperatures (-30°C, -20°C, 0°C, 20°C, 60°C, 80°C) to obtain a capacity of C1; discharge efficiency = C0 / C1×100%.

[0091] The test results are shown in Table 1 and Figure 1 , from Table 1 and Figure 1 It can be seen that in Examples 1 to 3, potassium phosphate is added to the positive electrode material, electrolyte, and separator, respectively, and the lithium ion batteries prepared therefrom can maintain a high discharge efficiency both under low temperature conditions and under high temperature conditions. In Comparative Example 1, potassium phosphate is not added to the lithium ion battery, resulting in a poor discharge efficiency of the prepared lithium ion battery at low temperatures. This indicates that the method of adding potassium phosphate to a lithium ion battery in the present invention can significantly improve the discharge efficiency of the lithium ion battery at low temperatures.

[0092] Table 1 Discharge efficiency of lithium-ion batteries of Examples and Comparative Examples at different temperatures

[0093] Temperature / ℃ -30 -20 0 20 60 80 Example 1 75.3％ 85.2％ 93.7％ 100.0％ 97.4％ 89.6％ Example 2 69.4％ 78.1％ 88.6％ 100.0％ 92.8％ 82.7％ Example 3 71.8％ 81.5％ 90.9％ 100.0％ 95.1％ 85.3％ Example 4 46.8％ 58.2％ 74.9％ 100.0％ 76.5％ 64.2％ Comparative Example 1 57.6％ 66.3％ 80.2％ 100.0％ 85.2％ 72.9％ Comparative Example 2 52.1％ 61.8％ 77.4％ 100.0％ 80.7％ 68.4％

[0094] (2) The lithium ion batteries of the embodiment and the comparative example were subjected to expansion rate tests. The specific test process was as follows:

[0095] After the battery is fully charged at 20±5°C with a current of 0.5C, the battery thickness T1 is tested, and then stored at an ambient temperature of 85±2°C for three days, and the thickness T2 is tested again. The expansion rate = (T2-T1) / T1×100%.

[0096] The test results are shown in Table 2 and Figure 2 , from Table 2 and Figure 2 It can be seen that in Examples 1 to 3, potassium phosphate is added to the positive electrode material, electrolyte, and separator, respectively, and the expansion rate of the lithium ion batteries prepared therefrom at high temperature does not change substantially over time. In Comparative Example 1, potassium phosphate is not added to the lithium ion battery, and the expansion rate of the lithium ion battery changes significantly, indicating that the addition of potassium phosphate to the lithium ion battery in the present invention can significantly improve the stability of the battery at high temperature.

[0097] Table 2 Changes in expansion rate of lithium-ion batteries of Examples and Comparative Examples at 85°C

[0098] Day 1 the next day Day 3 Example 1 1.8％ 1.3％ 1.4％ Example 2 2.1％ 1.9％ 2.0％ Example 3 1.7％ 1.4％ 1.6％ Example 4 5.8％ 8.3％ 10.2％ Comparative Example 1 4.8％ 6.3％ 8.7％ Comparative Example 2 5.2％ 7.1％ 9.5％

[0099] (3) The capacity recovery performance of the lithium-ion batteries of the embodiment and the comparative example after temperature shock was tested. The specific test process is as follows:

[0100] In the temperature shock chamber, the test is carried out according to GB / T 2423.22-2012, and the shelf time is T soak =20min, maximum temperature T max =+80℃, minimum temperature T min = -30℃, 300 cycles, 1 cycle is 40min, total duration ≥ 200h, see attached for details Figure 3 .

[0101] The test results are shown in Table 3 and Figure 4 , from Table 3 and Figure 4 It can be seen that in Examples 1 to 3, potassium phosphate is added to the positive electrode material, electrolyte, and separator, respectively. The battery capacity of the prepared lithium ion batteries does not change substantially after high and low temperature shocks, and has good temperature stability. However, in Comparative Example 1, potassium phosphate is not added to the positive electrode material. After the second and third temperature shocks, the battery capacity of the lithium ion battery changes significantly, and the stability is poor.

[0102] Table 3 Temperature shock capacity recovery performance of lithium ion batteries of Examples and Comparative Examples

[0103] first Second time The third time Example 1 96.7％ 95.2％ 93.8％ Example 2 94.4％ 91.8％ 90.3％ Example 3 95.1％ 93.1％ 92.8％ Example 4 80.3％ 77.5％ 74.2％ Comparative Example 1 88.3％ 85.4％ 82.7％ Comparative Example 2 85.9％ 82.1％ 79.6％

[0104] (4) The lithium-ion batteries of the embodiment and the comparative example were subjected to a cycle performance test. The test process was as follows: charge and discharge 300 times at 0.5C. After 300 cycles, the remaining capacity was not less than 80% of the initial capacity. After the test, the internal resistance increase rate was ≤30%, indicating that the cycle performance of the lithium-ion battery was good.

[0105] The test results are shown in Table 4, Table 5, Table 6 and Figure 5 , from Table 4 and Figure 5 It can be seen that in Examples 1 to 3, potassium phosphate is added to the positive electrode material, electrolyte, and separator respectively, and the internal resistance proliferation of the lithium ion batteries prepared therefrom is small, and the lithium ion batteries have good cycle stability. However, in Comparative Example 1, potassium phosphate is not added to the positive electrode material, and the internal resistance proliferation of the lithium ion battery is higher than that of the embodiment.

[0106] Table 4 Internal resistance test results of lithium-ion batteries of Examples and Comparative Examples

[0107] Before internal resistance test After internal resistance test Value Added Example 1 43.2 51.8 19.9％ Example 2 45.6 56.7 24.3％ Example 3 44.1 54.3 23.1％ Example 4 48.9 70.5 44.2％ Comparative Example 1 46.3 63.9 38.0％ Comparative Example 2 47.5 67.2 41.5％

[0108] Table 5 Cycle life test results of lithium ion batteries of the embodiment

[0109]

[0110]

[0111] Table 6 Cycle life test results of lithium ion batteries of comparative example

[0112]

[0113] (5) The charge retention capability of the lithium-ion batteries of the embodiment and the comparative example was tested. The test process was as follows: the battery cells were first charged and discharged at a standard rate, and the initial capacity was recorded. After 28 days of storage at a standard rate, the cells were discharged at 0.2C to 3.0V, and the remaining capacity was recorded. The battery cells were then charged and discharged at a standard rate and the discharge capacity (recoverable capacity) was recorded. If the remaining capacity was ≥ 90% of the initial capacity and the recoverable capacity was ≥ 91% of the initial capacity, the lithium-ion battery had good charge retention capability.

[0114] The test results are shown in Table 7. It can be seen from Table 7 that in Examples 1 to 3, potassium phosphate is added to the positive electrode material, electrolyte, and separator, respectively, and the residual capacity ratio and recovery capacity ratio of the lithium ion batteries prepared therefrom are both above 99%. In Comparative Example 1, potassium phosphate is not added to the positive electrode material, and the residual capacity ratio and recovery capacity ratio of the lithium ion battery are both 97%. Compared with the lithium ion batteries of the embodiments, the residual capacity ratio and recovery capacity ratio of the lithium ion battery of Comparative Example 1 are reduced.

[0115] Table 7 Test results of charge retention capacity of lithium ion batteries of Examples and Comparative Examples

[0116]

[0117] In summary, after 300 cycles of the lithium-ion battery in the embodiment, potassium phosphate is added, and the capacity retention rate of the battery is higher than that of the lithium-ion battery without potassium phosphate, and the capacity retention rate is about 2% higher on average. At the same time, after the lithium-ion battery in the embodiment is added with potassium phosphate, the increase in the internal resistance of the battery after the cycle is also lower than that of the lithium-ion battery without potassium phosphate, and is about 6% lower on average. In addition, after the lithium-ion battery in the embodiment is added with potassium phosphate, the low-temperature discharge efficiency of the battery is higher than that of the lithium-ion battery without potassium phosphate, and the low-temperature discharge efficiency is about 8% higher on average. In addition, after 28 days of storage of the lithium-ion battery in the embodiment, the remaining capacity ratio and the recovery capacity ratio are both higher than those of the lithium-ion battery without potassium phosphate, and the capacity ratio is about 2% higher.

[0118] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A lithium-ion battery with a long cycle life and good charge and discharge performance, characterized in that: Lithium-ion batteries are made by adding potassium phosphate salts to the positive electrode material, negative electrode material, electrolyte or diaphragm surface of the lithium-ion battery.

2. A lithium-ion battery with a long cycle life and good charge and discharge performance according to claim 1, characterized in that: The potassium phosphate salt is at least one of potassium phosphate and dipotassium hydrogen phosphate.

3. A lithium-ion battery with a long cycle life and good charge and discharge performance according to claim 1, characterized in that: The particle size of the potassium phosphate salt is less than 50 μm.

4. A lithium-ion battery with a long cycle life and good charge and discharge performance according to claim 1, characterized in that: Potassium phosphate is added to the positive electrode material, the negative electrode material or the electrolyte, and the potassium phosphate is coated on the surface of the diaphragm.

5. A lithium-ion battery with a long cycle life and good charge and discharge performance according to claim 1, characterized in that: The amount of potassium phosphate added to the positive electrode material, the negative electrode material or the electrolyte is 0.005 to 5 wt.%.

6. A lithium-ion battery with a long cycle life and good charge and discharge performance according to claim 1, characterized in that: The coating amount of potassium phosphate on the diaphragm is 0.005~50g / m 2 .

7. The method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance according to any one of claims 1 to 6, characterized in that: Adding potassium phosphate salt to the positive electrode material specifically includes the following steps: (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material and potassium phosphate are added and stirred continuously to prepare a slurry 1, which is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet; (2) Mixing and stirring the conductive agent, binder, and organic solvent, then adding the negative electrode material and continuing to stir to prepare slurry 2, coating the slurry 2 on copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet; (3) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

8. The method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance according to any one of claims 1 to 6, characterized in that: Adding potassium phosphate to the electrolyte specifically comprises the following steps: (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 3. The slurry 3 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet; (2) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a negative electrode material is added and stirred continuously to prepare a slurry 4, which is coated on a copper foil, dried, cut into strips, and spot-welded to obtain a negative electrode sheet; (3) adding potassium phosphate to the electrolyte and stirring evenly to obtain a mixed electrolyte; (4) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, which is then placed in a shell and injected with a mixed electrolyte, sealed and formed to form a lithium-ion battery.

9. The method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance according to any one of claims 1 to 6, characterized in that: The potassium phosphate salt is coated on the surface of the diaphragm, specifically comprising the following steps: (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 5. The slurry 5 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet; (2) Mixing and stirring a conductive agent, a binder, and an organic solvent, then adding a negative electrode material and continuing to stir to prepare a slurry 6, coating the slurry 6 on a copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet; (3) dispersing potassium phosphate and fluorine-based materials in a solvent to obtain a mixture, and coating the mixture on the surface of the diaphragm to obtain a modified diaphragm; (4) The positive electrode sheet, the negative electrode sheet and the modified separator are wound to obtain an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

10. The method for preparing a lithium-ion battery with a long cycle life and good charge and discharge performance according to any one of claims 1 to 6, characterized in that: Adding potassium phosphate salt to the negative electrode material specifically includes the following steps: (1) A conductive agent, a binder, and an organic solvent are mixed and stirred, and then a positive electrode material is added and stirred continuously to prepare a slurry 7. The slurry 7 is coated on an aluminum foil, dried, slit, and spot-welded to obtain a positive electrode sheet; (2) Mixing and stirring a conductive agent, a binder, and an organic solvent, then adding a negative electrode material and potassium phosphate and continuing to stir to prepare a slurry 8, coating the slurry 8 on a copper foil, drying, slitting, and spot welding to obtain a negative electrode sheet; (3) The positive electrode sheet, the negative electrode sheet and the separator are wound to form an electrode assembly, and the electrode assembly is placed in a shell, and after injecting electrolyte, sealing and forming, a lithium-ion battery is obtained.

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