WO2024066070A1 - 一种二次电池 - Google Patents

一种二次电池 Download PDF

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WO2024066070A1
WO2024066070A1 PCT/CN2022/139466 CN2022139466W WO2024066070A1 WO 2024066070 A1 WO2024066070 A1 WO 2024066070A1 CN 2022139466 W CN2022139466 W CN 2022139466W WO 2024066070 A1 WO2024066070 A1 WO 2024066070A1
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positive electrode
active material
electrode active
secondary battery
carbon
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PCT/CN2022/139466
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English (en)
French (fr)
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张科
陈巍
褚春波
张耀
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欣旺达动力科技股份有限公司
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Publication of WO2024066070A1 publication Critical patent/WO2024066070A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0471—Processes of manufacture in general involving thermal treatment, e.g. firing, sintering, backing particulate active material, thermal decomposition, pyrolysis
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application belongs to the field of secondary batteries and relates to a secondary battery.
  • Lithium iron phosphate has become a widely used positive electrode active material due to its wide range of materials, low cost and no environmental pollution.
  • lithium iron phosphate positive electrode active materials also have the problems of low electronic conductivity and low lithium ion diffusion rate, resulting in low room temperature discharge capacity and low cycle performance.
  • the purpose of the present application is to provide a secondary battery.
  • the present application provides a secondary battery, comprising a negative electrode plate, an electrolyte, a separator and a positive electrode plate, characterized in that the positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium iron phosphate;
  • the DSC curve of the positive electrode active material has a first exothermic peak and a second exothermic peak at 300°C to 600°C;
  • the area of the first exothermic peak of the DSC curve of the positive electrode active material is H1, and the area of the second exothermic peak is H2, satisfying 0.45 ⁇ H1/(H1+H2) ⁇ 0.75.
  • the first exothermic peak of the positive electrode active material is in the range of 340°C to 390°C, and the second exothermic peak is in the range of 391°C to 550°C.
  • the DSC test conditions of the positive electrode active material are air atmosphere and a heating rate of 10° C./min.
  • the surface of the lithium iron phosphate contains carbon material, and based on the mass of the positive electrode active material, the mass percentage of the carbon material is 0.5% to 3%.
  • the carbon content needs to be maintained within a certain range.
  • the carbon content exceeds this range, it will affect the lithium iron phosphate, such as the gram capacity of the lithium iron phosphate positive electrode active material. Therefore, when the total carbon content in the positive electrode active material is controlled to be 0.5% to 3%, the secondary battery has better overall performance.
  • the particle size Dv50 of the positive electrode active material is 0.4 ⁇ m to 2 ⁇ m.
  • the positive electrode active material further comprises an M element, and the M element comprises at least one of Ti, V or Zr;
  • the content of the M element is 300 ppm to 5000 ppm based on the weight of the positive electrode active material.
  • the powder compaction density of the positive electrode active material under a pressure of 30 KN is greater than 2.3 g/cm 3 .
  • the electrolyte comprises an unsaturated cyclic carbonate additive
  • the unsaturated cyclic carbonate additive includes vinylene carbonate and/or vinyl cyclic carbonate.
  • the content of the unsaturated cyclic carbonate is A% based on the mass of the electrolyte, satisfying 0.04 ⁇ A ⁇ H2/(H1+H2) ⁇ 2.4.
  • the present application also provides a method for preparing the positive electrode active material in the above-mentioned secondary battery, comprising the following steps:
  • the sintered powder is crushed by air jet mill, and then sieved and iron removed by electric current to obtain the positive electrode active material.
  • step A) in order to catalyze the polymerization process of the organic carbon source and improve the integrity of the carbon material coating during high-temperature carbonization of the organic carbon source, an additive containing an M element is added, and the additive includes at least one of titanium dioxide, vanadium oxide and zirconium oxide.
  • the solid content of the slurry is 40% to 45%.
  • the particle size Dv50 of the sand-milled product is in the range of 0.1 ⁇ m to 0.4 ⁇ m.
  • the added mass percentage of the organic carbon source is 3% to 8%;
  • the organic carbon source includes at least one of glucose, sucrose, citric acid and chitosan;
  • the conditions for the polymerization of the organic carbon source are as follows:
  • the temperature of the slurry is 80°C to 85°C, and the stirring rate is 100r/min to 500r/min;
  • the organic carbon source is polymerized and then spray-dried to obtain carbon-coated lithium iron phosphate precursor powder.
  • step D) by appropriately controlling the carbonization time of the organic carbon source, the heating rate during the carbonization of the organic carbon source is reduced, thereby significantly increasing the content of amorphous coated carbon in the product and reducing the content of free carbon.
  • the sintering conditions are as follows 1) to 2):
  • step E the particle size Dv50 of the sintered powder is 0.4-2 ⁇ m.
  • the present application further provides a battery module, comprising a plurality of secondary batteries as described above.
  • FIG. 1 is an XRD pattern of the positive electrode active material in Example 1.
  • FIG. 2 is a DSC curve of the positive electrode active material in Example 1.
  • FIG. 3 is a TEM image of the positive electrode active material in Example 1.
  • a secondary battery provided in one embodiment of the present application includes a negative electrode plate, an electrolyte, a separator and a positive electrode plate, wherein the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode active material layer includes a positive electrode active material, and wherein the positive electrode active material includes lithium iron phosphate;
  • the DSC curve of the positive electrode active material has a first exothermic peak and a second exothermic peak at 300°C to 600°C;
  • the area of the first exothermic peak of the positive electrode active material is H1, and the area of the second exothermic peak is H2, satisfying 0.45 ⁇ H1/(H1+H2) ⁇ 0.75.
  • it can be 0.45, 0.48, 0.5, 0.51, 0.53, 0.55, 0.58, 0.6, 0.63, 0.65, 0.67, 0.69, 0.7, 0.71, 0.73, 0.74, 0.75 or a range consisting of any two of them.
  • H1/(H1+H2) is within this range, the integrity of the carbon material coating on the surface of the positive electrode active material particles can be within a suitable range, and at the same time, the content of free carbon material in the positive electrode active material can be reduced, thereby effectively ensuring the capacity of the positive electrode active material while allowing the battery to have better cycle performance.
  • the area of the first exothermic peak of the positive electrode active material is H1
  • the area of the second exothermic peak is H2, satisfying 0.5 ⁇ H1/(H1+H2) ⁇ 0.74.
  • H1/(H1+H2) is within the above range, the content of free carbon material is further reduced, so that the battery has better cycle performance and rate performance.
  • the area of the first exothermic peak of the positive electrode active material is H1
  • the area of the second exothermic peak is H2, satisfying 0.55 ⁇ H1/(H1+H2) ⁇ 0.72.
  • H1/(H1+H2) is within the above range, the carbon material coating on the surface of the positive electrode active material particles can be more complete, and the coating thickness is also within a suitable range, so that the battery has better comprehensiveness.
  • the positive electrode active material satisfies 0.25 ⁇ H2/(H1+H2) ⁇ 0.55.
  • it can be 0.25, 0.27, 0.29, 0.3, 0.31, 0.34, 0.36, 0.38, 0.4, 0.42, 0.45, 0.48, 0.5 or a range of any two thereof.
  • H2/(H1+H2) is within this range, the battery has better overall performance.
  • H2/(H1+H2) 0.31 ⁇ H2/(H1+H2) ⁇ 0.45.
  • H2/(H1+H2) is within the above range, the carbon material coating integrity and free carbon material content on the surface of the positive electrode active material particles are both within a suitable range, and the battery has better overall performance.
  • the DSC test conditions of the positive electrode active material are air atmosphere and a heating rate of 10°C/min; due to different heating rates, the position and area of the DSC curve peak may fluctuate. Analysis of the measurement results shows that the DSC curve of the positive electrode active material has a first exothermic peak at 340°C to 390°C and a second exothermic peak at 391°C to 550°C.
  • the surface of the lithium iron phosphate is provided with a carbon material, and based on the mass of the positive electrode active material, the mass percentage of the carbon material is 0.5% to 3%.
  • the mass percentage of the carbon material is 0.5% to 3%.
  • it can be 0.5%, 0.8%, 1%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, 2.0%, 2.3%, 2.5%, 2.8%, 3.0% or a range composed of any two of them.
  • the appropriate carbon content is conducive to the performance of the gram capacity of the lithium iron phosphate positive electrode active material.
  • the surface of the lithium iron phosphate contains carbon material, and based on the mass of the positive electrode active material, the mass percentage of the carbon material is 1% to 2.8%.
  • a carbon material is provided on the surface of the lithium iron phosphate, and based on the mass of the positive electrode active material, the mass percentage of the carbon material is 1.1% to 2.5%.
  • the carbon material includes amorphous carbon and/or conductive carbon black.
  • the particle size Dv50 of the positive electrode active material is controlled to be 0.4 ⁇ m to 2 ⁇ m.
  • it can be 0.4 ⁇ m, 0.7 ⁇ m, 0.9 ⁇ m, 1 ⁇ m, 1.2 ⁇ m, 1.4 ⁇ m, 1.5 ⁇ m, 1.7 ⁇ m, 2 ⁇ m, or a range consisting of any two of them.
  • the positive electrode powder obtained has a higher compaction
  • the positive electrode sheet has a higher uniformity, and can avoid the particle size being too small to produce more side reactions with the electrolyte and affect the improvement effect on the electrical performance. It can also avoid the particle size being too large to hinder the solid phase conduction of active ions inside the positive electrode active particles, thereby improving the electrical performance.
  • the positive electrode active material further comprises an M element, and the M element comprises at least one of Ti, V or Zr.
  • the M element can improve the integrity of the carbon material coating during high-temperature carbonization of the organic carbon source and improve the power performance of the battery.
  • the positive electrode active material further comprises an M element, and the M element comprises Ti.
  • the M element comprises Ti
  • the coating effect of the carbon material is better.
  • the content of the M element is 300ppm to 5000ppm.
  • the M element within this range can improve the overall performance of the battery.
  • the powder compaction density of the positive electrode active material under a pressure of 30 KN is greater than 2.3 g/cm 3 .
  • the powder compaction density of the positive electrode active material under a pressure of 30 KN is less than 2.7 g/cm 3 .
  • the compaction density of the positive electrode sheet is greater than or equal to 2.45 g/cm 3 .
  • the compaction density of the positive electrode sheet is less than or equal to 2.8 g/cm 3 .
  • the electrolyte comprises an unsaturated cyclic carbonate additive
  • the unsaturated cyclic carbonate additive includes vinylene carbonate (VC) and/or vinyl cyclic carbonate (VEC).
  • VC vinylene carbonate
  • VEC vinyl cyclic carbonate
  • the content of the unsaturated cyclic carbonate based on the mass of the electrolyte is A%, satisfying 0.04 ⁇ A ⁇ H2/(H1+H2) ⁇ 2.4.
  • it can be 0.04, 0.07, 0.09, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 1.2, 1.4, 1.5, 1.7, 1.9, 2.0, 2.4 or a range consisting of any two of them.
  • the unsaturated carbonate additive can react on the surface of the carbon material and form a protective film of suitable thickness on the surface of the positive electrode active material, thereby reducing the adverse effects of the electrolyte on the positive electrode active material and helping to improve the overall performance of the battery.
  • the content of the unsaturated cyclic carbonate is A%, satisfying 0.05 ⁇ A ⁇ 3%.
  • it can be 0.05, 0.07, 0.1, 0.3, 0.5, 0.7, 0.9, 1, 1.2, 1.5, 1.8, 2.0, 2.2, 2.5, 2.7, 3.0 or a range consisting of any two of them.
  • the electrolyte includes an electrolyte salt and an organic solvent.
  • the specific types of the electrolyte salt and the organic solvent are not subject to specific restrictions and can be selected according to actual needs.
  • the electrolyte may also include other additives.
  • the types of the additives are not particularly limited. They may be negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, additives that improve battery low temperature performance, etc. For example, additives containing sulfur-oxygen double bonds, silicon-containing additives, phosphorus-containing additives, etc.
  • the present application also provides a method for preparing the positive electrode active material in the above-mentioned secondary battery, comprising the following steps:
  • the sintered powder is crushed by air jet mill, and then sieved and iron removed by electric current to obtain the positive electrode active material.
  • the total carbon content in the carbon-coated lithium iron phosphate material prepared by the above method is controllable, the free carbon component is reduced, and the amorphous coated carbon component is increased, which is beneficial to improve the compaction density of the lithium iron phosphate material.
  • the above purpose is achieved specifically by regulating the type of raw materials and the proportion of use in the method, as well as controlling the conditions in each preparation step.
  • step A) in order to catalyze the polymerization process of the organic carbon source and improve the integrity of the carbon material, such as the amorphous carbon coating, during the high-temperature carbonization of the organic carbon source, an additive containing the M element is added.
  • the additive containing the M element includes at least one of titanium dioxide, vanadium oxide and zirconium oxide, and can also improve the electronic conductivity and ionic conductivity of the material in the lithium iron phosphate lattice, thereby improving the dynamic performance of the material, and as a positive active material for lithium-ion batteries, it exhibits excellent electrochemical properties.
  • the added mass percentage of the organic carbon source is 3% to 8%; the organic carbon source includes at least one of glucose, sucrose, citric acid and chitosan.
  • the sintering conditions are as follows 1)-2): 1) inert atmosphere; 2) programmed heating: heating from room temperature to 300°C-350°C at a heating rate of 5°C/min-10°C/min, and keeping at 300°C-350°C for 0.5h; then heating from 300°C-350°C to 350°C-420°C at 2°C/min-15°C/min, and keeping at 350°C-420°C for 1-8h; continuing to heat from 350°C-420°C to 650°C-730°C at a heating rate of 10°C/min, and keeping at 650°C-730°C for 8h-11h.
  • the room temperature is common knowledge in the art and may be 10°C to 30°C.
  • the specific type of the negative electrode active material is not specifically limited and can be selected according to actual needs.
  • the negative electrode active material can be selected from one or more of carbon materials, silicon-based materials, tin-based materials, and transition metal oxides.
  • the carbon material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, and mesophase carbon microspheres;
  • the graphite can be selected from one or two of artificial graphite and natural graphite;
  • the silicon-based material can be selected from one or more of elemental silicon, silicon oxides, and silicon alloys;
  • the tin-based material can be selected from one or more of elemental tin, tin oxide, and tin metal;
  • the transition metal oxide can be selected from lithium titanate.
  • the negative electrode active material includes at least one of a carbon material and a silicon-based material.
  • the separator is arranged between the positive electrode plate and the negative electrode plate to play a role of isolation.
  • the type of the separator is not specifically limited, and can be any separator material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride and their multilayer composite films, but not limited to these.
  • D. Transfer the precursor powder into the sintering furnace, and in a nitrogen atmosphere, heat it from room temperature to 300°C at a heating rate of 10°C/min, and keep it at 300°C for 0.5h; then heat it from 300°C to 380°C at 5°C/min and keep it at 4h; continue to sinter at a heating rate of 10°C/min from 380°C to 700°C for 10 hours, and then cool it naturally to obtain sintered powder;
  • E. The sintered material is crushed by air flow mill, and the crushed particle size Dv50 is controlled to 0.78 ⁇ m, and then sieved and iron removed by electric current to obtain carbon-coated lithium iron phosphate positive electrode active material.
  • Figure 1 is the XRD spectrum of the synthesized product, indicating that the synthesized lithium iron phosphate has no impurity phase.
  • the Dv50 of the positive electrode active material is 1.02 ⁇ m
  • Figure 2 is the DSC curve of the product (air atmosphere, rate 10°C/min), and the fitting analysis shows that the free carbon content in the product accounts for 52.83% of the total, and the coated carbon content accounts for 47.17% of the total.
  • Figure 3 is a TEM image of the synthesized product, from which it can be seen that the synthesized lithium iron phosphate material contains free carbon and coated carbon.
  • step A 0.1 wt % of titanium dioxide is mixed in as an ion doping additive.
  • step A 0.5 wt % of titanium dioxide is mixed in as an ion doping additive.
  • step C 3 wt % of glucose is added as an organic carbon source.
  • step C 8 wt % of glucose is added as an organic carbon source.
  • step C 1 wt % of glucose is added as an organic carbon source.
  • step C 10 wt % of glucose is added as an organic carbon source.
  • step C 5 wt % sucrose is added as an organic carbon source.
  • step C 5 wt % of citric acid is added as an organic carbon source.
  • step C 5 wt % chitosan is added as an organic carbon source.
  • step D the heating rate from 300° C. to 380° C. is 10° C./min.
  • step D the heating rate from 300° C. to 380° C. is 3° C./min.
  • step D the holding time at 380° C. is 6 h.
  • step D the holding time at 380° C. is 2 h.
  • step D the temperature is increased from room temperature to 300° C. at a heating rate of 5° C./min.
  • step D the temperature is increased from room temperature to 300° C. at a heating rate of 8° C./min.
  • step D the temperature is increased from room temperature to 350°C at a heating rate of 10°C/min, and kept at 350°C for 0.5h; then the temperature is increased from 350°C to 380°C at a heating rate of 5°C/min and kept for 4h; and the temperature is continued from 380°C to 700°C at a heating rate of 10°C/min, and kept at 700°C for 10h.
  • step A 0.3 wt % of vanadium oxide is mixed in as an ion dopant.
  • step A 0.3 wt % of zirconium oxide is mixed in as an ion dopant.
  • step A no ion doping additive is mixed in.
  • step C after adding 5 wt % glucose organic carbon source to the sand-milled slurry, the spray drying process is directly carried out, and the organic carbon source is not fully polymerized.
  • Example 1 The difference from Example 1 is that the precursor powder is transferred into a sintering furnace and sintered directly at 700° C. at a heating rate of 10° C./min for 10 h in a nitrogen protective gas atmosphere.
  • the secondary batteries were prepared according to the following method:
  • Step 1 The carbon-coated lithium iron phosphate positive electrode active material prepared in the above embodiment of the present application, the binder PVDF, and the conductive agent SP are mixed in a mass ratio of 96:2:2, and the solvent NMP is added, and the mixture is stirred under the action of a vacuum mixer to obtain a positive electrode slurry having a solid content of 50% to 58% and a viscosity of 3500 to 5000 mPa ⁇ s; the positive electrode slurry is evenly coated on both sides of a 14 ⁇ m carbon-coated aluminum foil (including a double-sided carbon coating layer with a thickness of 1 ⁇ m each);
  • Step 2 Dry the coated electrode in an oven at 100-120°C;
  • Step 3 After cold pressing and cutting, the positive electrode sheet is obtained.
  • the negative electrode active material graphite, the thickener sodium carboxymethyl cellulose, the binder styrene butadiene rubber, and the conductive agent acetylene black are mixed in a mass ratio of 97:1:1:1, and deionized water is added to obtain the negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry is evenly coated on a copper foil with a thickness of 6 ⁇ m; the coated electrode sheet is transferred to a 120°C oven for drying, and then cold pressed and cut to obtain the negative electrode sheet.
  • the organic solvent is a mixed solution containing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), wherein the volume ratio of EC, EMC and DEC is 20:20:60.
  • EC ethylene carbonate
  • EMC ethyl methyl carbonate
  • DEC diethyl carbonate
  • the volume ratio of EC, EMC and DEC is 20:20:60.
  • argon atmosphere glove box with a water content of ⁇ 10ppm fully dried LiPF6 lithium salt is dissolved in the organic solvent and mixed evenly to obtain an electrolyte.
  • the concentration of the lithium salt is 1 mol/L.
  • a 12 ⁇ m polypropylene diaphragm was selected.
  • the positive electrode sheet, separator and negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to play an isolating role, and then wound into a square bare battery cell, loaded with aluminum-plastic film, and then baked at 80°C to remove water, and then injected with the corresponding non-aqueous electrolyte and sealed. After standing, hot and cold pressing, formation, clamping, capacity division and other processes, the finished battery is obtained.
  • the positive electrode active materials prepared in each embodiment and comparative example were subjected to DSC test under the conditions of air atmosphere, temperature range of room temperature to 550° C., heating rate of 10° C./min, and the test instrument was a NETZSCH thermal analyzer.
  • the carbon content of the positive electrode active materials prepared in each embodiment and comparative example was tested using a carbon-sulfur analyzer.
  • the test parameters are as follows:
  • Gas pressure oxygen inlet 0.3Mpa, power gas nitrogen inlet 0.3Mpa;
  • Sample mass Weigh 0.1g sample, cover with 1.5g ⁇ 1.8g tungsten flux, and burn at high temperature.
  • the positive electrode active materials prepared in each embodiment and comparative example were subjected to a powder compaction density test under a pressure of 30 KN, and the testing instrument was a powder compaction density tester.
  • Example 1 and Comparative Example 2 by slowly stirring and maintaining the temperature of the slurry at 80°C to 85°C during the precursor preparation process, the process of polymerization of the organic carbon source on the surface of the lithium iron phosphate particles is accelerated, and the carbon component of the lithium iron phosphate coating and the uniformity of the coating can be effectively adjusted, thereby improving the compaction density of the positive electrode active material and further improving the rate performance and cycle performance of the battery.
  • the optimization of the powder sintering process can increase the component of coated carbon in the lithium iron phosphate material.
  • a material pre-sintering process is added so that the organic carbon source is fully carbonized during the pre-sintering process to completely coat the surface of the lithium iron phosphate particles, so that the positive electrode active material satisfies 0.45 ⁇ H1/(H1+H2) ⁇ 0.75.
  • a faster heating rate and a shorter pre-sintering time will lead to insufficient carbonization of the organic carbon source on the surface of the lithium iron phosphate particles, thereby forming more free carbon.
  • a slower heating rate and a longer pre-sintering time have limited effects on improving the coated carbon component in the lithium iron phosphate material, but will increase the energy consumption in the material production process and reduce the production capacity of material synthesis.
  • the secondary batteries were prepared according to the preparation method of the secondary batteries in Example 4 of the present application.
  • the positive electrode active material used in the positive electrode plate was the same as that in Example 4 of the present application. The difference was that the composition of the electrolyte also contained an unsaturated cyclic carbonate additive (i.e., the unsaturated carbonate in Table 3), as shown in Table 3.
  • the present application controls the areas of the first exothermic peak and the second exothermic peak of the positive electrode active material containing lithium iron phosphate at 300°C to 600°C to satisfy 0.45 ⁇ H1/(H1+H2) ⁇ 0.75, 0.25 ⁇ H2/(H1+H2) ⁇ 0.55, so that the integrity of the carbon material coating on the surface of the positive electrode active material particles is within an appropriate range, and the content of free carbon material in the positive electrode active material is reduced, thereby effectively ensuring the capacity of the positive electrode active material while allowing the battery to have better cycle performance.

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Abstract

本申请公开了一种二次电池。本申请二次电池，包括负极极片、电解液、隔膜和正极极片，所述正极极片包含正极集流体和正极活性材料层，所述正极活性材料层包含正极活性材料，所述正极活性材料包含磷酸铁锂；所述正极活性材料的DSC曲线在300℃～600℃有第一放热峰和第二放热峰；所述正极活性材料的第一放热峰的面积记为H1，第二放热峰的面积记为H2，满足0.45≤H1/(H1+H2)≤0.75。本申请通过控制包含磷酸铁锂的正极活性材料中两个放热峰的面积的比值在特定的范围内，从而使含磷酸铁锂的正极活性材料中碳材料包覆的完整度高，以提高二次电池的性能。

Description

一种二次电池 技术领域
本申请属于二次电池领域，涉及一种二次电池。
背景技术
二次电池，例如锂离子电池，具有重量轻、能量密度高、无污染、无记忆效应、使用寿命长等突出特点，因而被广泛应用于手机、电脑、家用电器、电动工具等领域。磷酸铁锂由于来料广泛、成本低廉且无环境污染等特点，使其成为广泛使用的一种正极活性材料。但磷酸铁锂正极活性材料也存在电子电导率低和锂离子扩散速率低的问题，导致其较低的室温放电容量和较低的循环性能。
因此由必要提供一种正极极片，能够改善二次电池的室温放电容量和循环性能。
发明公开
本申请的目的是提供一种二次电池。
本申请提供了一种二次电池，包括负极极片、电解液、隔膜和正极极片，其特征在于，所述正极极片包含正极集流体和正极活性材料层，所述正极活性材料层包含正极活性材料，所述正极活性材料包含磷酸铁锂；
所述正极活性材料的DSC曲线在300℃～600℃有第一放热峰和第二放热峰；
所述正极活性材料的DSC曲线的第一放热峰的面积为H1，所述第二放热峰的面积为H2，满足0.45≤H1/(H1+H2)≤0.75。
在本申请的一些实施例中，所述正极活性材料的所述第一放热峰在340℃～390℃范围内，所述第二放热峰在391℃～550℃范围内。
在本申请的一些实施例中，所述正极活性材料的DSC测试条件为空气氛围，升温速率为10℃/min。
进一步地，所述磷酸铁锂表面含有碳材料，基于所述正极活性材料的质量，所述碳材料质量百分含量为0.5％～3％。
本申请中，碳含量需保持在一定的范围内，当碳含量超过该范围会影响磷酸铁锂，例如磷酸铁锂正极活性材料克容量的发挥，因此控制所述正极活性材料中总的碳含量为0.5％～3％时，二次电池具有较优的综合性能。
进一步地，所述正极活性材料的粒径Dv50为0.4μm～2μm。
上述的二次电池中，所述正极活性材料还包含M元素，所述M元素包含Ti、V或Zr中的至少一种；
基于所述正极活性材料的重量，所述M元素含量为300ppm～5000ppm。
上述的二次电池中，所述正极活性材料在30KN压力下的粉体压实密度大于2.3g/cm 3。
所述电解液包含不饱和环状碳酸酯添加剂；
所述不饱和环状碳酸酯添加剂包括碳酸亚乙烯酯和/或乙烯基环状碳酸酯。
所述基于所述电解液的质量，所述不饱和环状碳酸酯的含量为A％，满足0.04≤A×H2/(H1+H2)≤2.4。
本申请还提供了上述的二次电池中所述正极活性材料的制备方法，包括如下步骤：
A)按照摩尔比1:0.9～1.2取无水磷酸铁、碳酸锂，与水混合，然后球磨，得到球磨后的浆料；
B)将所述球磨后的浆料进行砂磨，得到砂磨产物；
C)向砂磨后的浆料中添加有机碳源搅拌，进行聚合，得到碳包覆磷酸铁锂前驱体粉末；
D)将所述碳包覆磷酸铁锂前驱体粉末烧结，得到烧结粉料；
E)将所述烧结粉料经过气流磨粉碎，然后经过过筛，电流除铁，即得到所述正极活性材料。
进一步地，步骤A)中，为了催化有机碳源聚合化过程，提高有机碳源高温碳化时碳材料包覆的完整度，还包括添加含M元素的添加剂，所述添加剂包括二氧化钛、氧化钒和氧化锆中的至少一种。
进一步地，所述浆料中固含量为40％～45％。
进一步地，步骤B)中，所述砂磨产物的粒径Dv50的范围为0.1μm～0.4μm。
进一步地，步骤C)中，所述有机碳源的添加质量百分含量为3％～8％；所述有机碳源包括葡萄糖、蔗糖、柠檬酸和壳聚糖中的至少一种；
所述有机碳源聚合的条件如下：
所述的浆料的温度为80℃～85℃，搅拌的速率为100r/min～500r/min；
所述有机碳源聚合过程后经喷雾干燥得到碳包覆磷酸铁锂前驱体粉末。
进一步地，步骤D)中，通过适当控制有机碳源碳化时间，降低有机碳源碳化过程中的升温速率，从而明显提高产物中无定形包覆碳的含量，降低游离碳含量，所述烧结的条件如下1)～2)：
1)惰性气氛；
2)程序升温：从室温以5℃/min～10℃/min的升温速率升温至300℃～350℃，300℃～350℃保温0.5h；然后从300℃～350℃以2℃/min～15℃/min升温至350℃～420℃，350℃～420℃保温1～8h；继续从350℃～420℃以升温速率为10℃/min升温至650℃～730℃，650℃～730℃保温8h～11h；
进一步地，步骤E)中，所述烧结粉料粉碎的粒径Dv50为0.4～2μm。
本申请进一步提供了一种电池模块，包括多个如上所述的二次电池。
附图说明
图1为实施例1中正极活性材料的XRD图谱。
图2为实施例1中正极活性材料的DSC曲线。
图3为实施例1中正极活性材料的TEM图。
实施发明的最佳方式
下述实施例中所使用的实验方法如无特殊说明，均为常规方法。
下述实施例中所用的材料、试剂等，如无特殊说明，均可从商业途径得到。
本申请一实施例提供的二次电池，包括负极极片、电解质、隔膜和正极极片，所述正极极片包含正极集流体和正极活性材料层，所述正极活性材料层包含正极活性材料，所述正极活性材料包含磷酸铁锂；
所述正极活性材料的DSC曲线在300℃～600℃有第一放热峰和第二放热峰；
所述正极活性材料的第一放热峰的面积为H1，所述第二放热峰的面积为H2，满足0.45≤H1/(H1+H2)≤0.75。例如可以为0.45、0.48、0.5、0.51、0.53、0.55、0.58、0.6、0.63、0.65、0.67、0.69、0.7、0.71、0.73、0.74、0.75或其中任意二者组成的范围。当H1/(H1+H2)在此范围内，能够使正极活性材料颗粒表面碳材料包覆的完整度在合适的范围内，同时能够降低正极活性材料中游离碳材料的含量，从而有效的保证正极活性材料容量的同时，使电池具有较优的循环性能。
根据本申请的一些实施例，所述正极活性材料的第一放热峰的面积为H1，所述第二放热峰的面积为H2，满足0.5≤H1/(H1+H2)≤0.74。当H1/(H1+H2)在上述范围内，游离碳材料的含量进一步降低，使电池具有更优异的循环性能和倍率性能。
根据本申请的一些实施例，所述正极活性材料的第一放热峰的面积为H1，所述第二放热峰的面积为H2，满足0.55≤H1/(H1+H2)≤0.72。当H1/(H1+H2)在上述内，能使正极活性材料颗粒表面碳材料包覆的更完整，且包覆的厚度也在合适的范围内，使电池具有更优的综合性。
根据本申请的一些实施例，所述正极活性材料满足0.25≤H2/(H1+H2)≤0.55。例如可以为0.25、0.27、0.29、0.3、0.31、0.34、0.36、0.38、0.4、0.42、0.45、0.48、0.5或其中任意二者组成的范围。当H2/(H1+H2)在此范围内，电池具有更优的综合性能。
根据本申请的一些实施例，0.29≤H2/(H1+H2)≤0.5。
根据本申请的一些实施例，0.31≤H2/(H1+H2)≤0.45。当H2/(H1+H2)在上述范围内，正极活性材料颗粒表面的碳材料包覆完整度和游离碳材料含量均在合适的范围内，电池具有更优的综合性能。
根据本申请的一些实施例，所述正极活性材料的DSC测试条件为空气氛围，升温速率为10℃/min；因为升温速率不同，可能导致DSC曲线峰的位置和面积出现波动，测定结果分析可知，所述正极活性材料的DSC曲线在340℃～390℃具有第一个放热峰，在391℃～550℃具有第二个放热峰。
根据本申请的一些实施例，所述磷酸铁锂表面设有碳材料，基于所述正极活性材料的质量，所述碳材料质量百分含量为0.5％～3％。例如可以为0.5％、0.8％、1％、1.1％、1.3％、1.5％、1.7％、1.9％、2.0％、2.3％、2.5％、2.8％、3.0％或其中任意二者组成的范围。合适的碳含量有利于磷酸铁锂正极活性材料克容量的发挥。
根据本申请的一些实施例，所述磷酸铁锂表面含有碳材料，基于所述正极活性材料的质量，所述碳材料质量百分含量为1％～2.8％。
根据本申请的一些实施例，所述磷酸铁锂表面设有碳材料，基于所述正极活性材料的质量，所述碳材料质量百分含量为1.1％～2.5％。
根据本申请的一些实施例，碳材料包含无定形碳和/或导电炭黑。
根据本申请的一些实施例，为了实现高的正极粉体压实密度兼顾高的比容量目的，控制所述正极活性材料的粒径Dv50为0.4μm～2μm。例如可以为0.4μm、0.7μm、0.9μm、1μm、1.2μm、1.4μm、1.5μm、1.7μm、2μm或其中任意二者组成的范围。
根据本申请的一些实施例，所述正极活性材料的粒径Dv50为0.7μm～1.5μm时得到的正极粉体压实较高，正极极片均一性更高，并可以避免粒径过小与电解液产生较多的副反应而影响对电性能的改善效果，还可以避免粒径太大阻碍活性离子在正极活性颗粒内部固相传导，从而对电性能进行改善。
根据本申请的一些实施例，所述正极活性材料还包含M元素，所述M元素包含Ti、V或Zr中的至少一种。M元素可以提高有机碳源高温碳化时碳材料包覆的完整度和提高电池的动力性能。
根据本申请的一些实施例，所述正极活性材料还包含M元素，所述M元素包含Ti。当M元素包含Ti时，碳材料的包覆效果更优。
根据本申请的一些实施例，基于所述正极活性材料的重量，所述M元素含量为300ppm～5000ppm。M元素在此范围内能够使电池的综合性能更优。
根据本申请的一些实施例，所述正极活性材料在30KN压力下的粉体压实密度大于2.3g/cm 3。
根据本申请的一些实施例，所述正极活性材料在30KN压力下的粉体压实密度小于2.7g/cm 3。
根据本申请的一些实施例，所述正极极片的压实密度大于或等于2.45g/cm 3。
根据本申请的一些实施例，所述正极极片的压实密度小于或等于2.8g/cm 3。
根据本申请的一些实施例，1％≤(所述正极极片的压实密度-所述正极活性材料在30KN压力下的粉体压实密度)/所述正极极片的压实密度≤6％。
根据本申请的一些实施例，所述电解液包含不饱和环状碳酸酯添加剂；
所述不饱和环状碳酸酯添加剂包括碳酸亚乙烯酯(VC)和/或乙烯基环状碳酸酯(VEC)。当正极活性材料颗粒表面包含碳材料，且满足0.45≤H1/(H1+H2)≤0.75的情况下，进一步加入不饱和环状碳酸酯添加剂，可以进一步改善电池的循环性能。
根据本申请的一些实施例，所述基于所述电解液的质量，所述不饱和环状碳酸酯的含量为A％，满足0.04≤A×H2/(H1+H2)≤2.4。例如可以为0.04、0.07、 0.09、0.1、0.3、0.5、0.7、0.9、1.0、1.2、1.4、1.5、1.7、1.9、2.0、2.4或其中任意二者组成的范围。
当A×H2/(H1+H2)在上述范围内，不饱和碳酸酯添加剂能够在碳材料表面发生反应，可以在正极活性材料表面形成合适厚度的保护膜，减少电解液对正极活性材料的不利影响，有助于提高电池的综合性能。
根据本申请的一些实施例，所述不饱和环状碳酸酯的含量为A％，满足0.05≤A≤3％。例如可以为0.05、0.07、0.1、0.3、0.5、0.7、0.9、1、1.2、1.5、1.8、2.0、2.2、2.5、2.7、3.0或其中任意二者组成的范围。
根据本申请的一些实施例，电解液包括电解质盐以及有机溶剂，电解质盐和有机溶剂地具体种类不受到具体的限制，可以根据实际需求进行选择。所述电解质还可包括其他添加剂，所述添加剂种类没有特别的限制，可以为负极成膜添加剂，也可为正极成膜添加剂，也可以为能够改善电池某些性能的添加剂，例如改善电池过充性能的添加剂、改善电池高温性能的添加剂、改善电池低温性能的添加剂等。例如可以为包含硫氧双键的添加剂、含硅添加剂、含磷添加剂等。
本申请还提供了上述的二次电池中所述正极活性材料的制备方法，包括如下步骤：
A)按照摩尔比1:0.9～1.2取无水磷酸铁、碳酸锂，与水混合，然后球磨，得到球磨后的浆料；
B)将所述球磨后的浆料进行砂磨，得到砂磨产物；
C)向砂磨后的浆料中添加有机碳源搅拌，进行聚合，得到碳包覆磷酸铁锂前驱体粉末；
D)将所述碳包覆磷酸铁锂前驱体粉末烧结，得到烧结粉料；
E)将所述烧结粉料经过气流磨粉碎，然后经过过筛，电流除铁，即得到所述正极活性材料。
在一些实施例中，通过上述方法，制备得到的碳包覆磷酸铁锂材料中总碳含量可控，游离碳组分降低，无定形包覆碳组分提升，从而有利于提高磷酸铁锂材料的压实密度，具体从方法中原料种类和使用比例进行调控，以及在制备各个步骤中对条件的控制来实现上述目的。
在一些实施例中，步骤A)中，为了催化有机碳源聚合化过程，提高有机碳源高温碳化时碳材料，例如无定形碳包覆的完整度，还包括添加含M元素的添加 剂，所述含M元素的添加剂包括二氧化钛、氧化钒和氧化锆中的至少一种，还能在磷酸铁锂晶格中提升材料的电子电导和离子电导，从而提升其材料的动力性能，作为锂离子电池正极活性材料，表现出优异的电化学性能。
在另一实施例中，为了调控高温碳化后产物中总碳含量，通过控制磷酸铁锂材料前躯体中有机碳源的种类和比例来实现，步骤C)中，所述有机碳源的添加质量百分含量为3％～8％；所述有机碳源包括葡萄糖、蔗糖、柠檬酸和壳聚糖中的至少一种。
在又一实施例中，通过适当延缓有机碳源碳化时间，降低有机碳源碳化过程中的升温速率，从而明显提高产物中无定形包覆碳的含量，降低游离碳含量，步骤D)中，所述烧结的条件如下1)-2)：1)惰性气氛；2)程序升温：从室温以5℃/min～10℃/min的升温速率升温至300℃～350℃，300℃～350℃保温0.5h；然后从300℃～350℃以2℃/min～15℃/min升温至350℃～420℃，350℃～420℃保温1～8h；继续从350℃～420℃以升温速率为10℃/min升温至650℃～730℃，650℃～730℃保温8h～11h。
本申请中，上述室温为本领域公知的常识，可为10℃～30℃。
在本申请的二次电池中，所述负极活性物质的具体种类不受具体限制，可根据实际需求进行选择。例如，所述负极活性物质可选自碳材料、硅基材料、锡基材料、过渡金属氧化物中的一种或几种。其中，所述碳材料可选自石墨、软碳、硬碳、碳纤维、中间相碳微球中的一种或几种；所述石墨可选自人造石墨、天然石墨中一种或两种；所述硅基材料可选自单质硅、硅氧化合物、硅合金中的一种或几种；所述锡基材料可选自单质锡、锡的氧化物、锡金属中的一种或几种；所述的过渡金属氧化物可选自钛酸锂。在一些实施例中，所述负极活性物质包含碳材料、硅基材料中的至少一种。
在本申请的二次电池中，所述隔膜设置在正极极片和负极极片之间，起到隔离的作用。其中，所述隔膜的种类并不受到具体的限制，可以是现有电池中使用的任何隔膜材料，如聚乙烯、聚丙烯、聚偏氟乙烯以及他们的多层复合膜，但不仅限于这些。
下面以锂离子电池为例并结合具体实施例，进一步阐述本申请。应理解，这些实施例仅用于说明本申请而不用于限制本申请的范围。
实施例1
正极活性材料的制备方法具体步骤：
A先按摩尔比为1:1.05的比例称取无水磷酸铁、碳酸锂，再混入0.3wt％二氧化钛作为离子掺杂添加剂，然后加入纯水，配成浆料，之后进行球磨；B将球磨后的浆料转入砂磨机进行砂磨，控制砂磨产物的粒径Dv50的范围为0.25μm；C向砂磨后的浆料中添加5wt％的葡萄糖作为有机碳源，保持浆料的温度为80℃～85℃，以100r/min的速度缓慢搅拌，待有葡萄糖充分聚合在磷酸铁锂前驱体表面，然后经过喷雾干燥，得到碳包覆磷酸铁锂前驱体粉末。D将前驱体粉末转移入烧结炉里，在氮气保护氛围下，从室温以10℃/min的升温速率升温至300℃，300℃保温0.5h；然后以5℃/min，从300℃升温至380℃保温4h；继续以升温速率为10℃/min从380℃至升温700℃烧结10个小时，然后自然冷却得到烧结粉料；E将烧结的物料经过气流磨粉碎，控制粉碎粒径Dv50为0.78μm，然后经过过筛，电流除铁，得到碳包覆的磷酸铁锂正极活性材料。图1为所合成产物的XRD图谱，表明合成磷酸铁锂没有杂质相。其中正极活性材料的Dv50为1.02μm；图2为产物的DSC曲线(空气氛围，速率10℃/min)，经拟合分析得产物中游离碳含量占总量的52.83％，包覆碳含量占总量的47.17％。图3为所合成产物的TEM图，从图中可以看出合成磷酸铁锂材料中含有游离碳和包覆碳。
实施例2
与实施例1不同之处在于：在步骤A中，混入0.1wt％二氧化钛作为离子掺杂添加剂。
实施例3
与实施例1不同之处在于：在步骤A中，混入0.5wt％二氧化钛作为离子掺杂添加剂。
实施例4
与实施例1不同之处在于：在步骤C中，添加3wt％的葡萄糖作为有机碳源。
实施例5
与实施例1不同之处在于：在步骤C中，添加8wt％的葡萄糖作为有机碳源。
实施例6
与实施例1不同之处在于：在步骤C中，添加1wt％的葡萄糖作为有机碳源。
实施例7
与实施例1不同之处在于：在步骤C中，添加10wt％的葡萄糖作为有机碳源。
实施例8
与实施例1不同之处在于：在步骤C中，添加5wt％的蔗糖作为有机碳源。
实施例9
与实施例1不同之处在于：在步骤C中，添加5wt％的柠檬酸作为有机碳源。
实施例10
与实施例1不同之处在于：在步骤C中，添加5wt％的壳聚糖作为有机碳源。
实施例11
与实施例1不同之处在于：在步骤D中，从300℃到380℃之间的升温速率为10℃/min。
实施例12
与实施例1不同之处在于：在步骤D中，从300℃到380℃之间的升温速率为3℃/min。
实施例13
与实施例1不同之处在于：在步骤D中，380℃保温时间为6h。
实施例14
与实施例1不同之处在于：在步骤D中，380℃保温时间为2h。
实施例15
与实施例1不同之处在于：在步骤D中，从室温以5℃/min的升温速率升温至300℃。
实施例16
与实施例1不同之处在于：在步骤D中，从室温以8℃/min的升温速率升温至300℃。
实施例17
与实施例1不同之处在于：在步骤D中，从室温以10℃/min的升温速率升温至350℃，350℃保温0.5h；然后以5℃/min，从350℃升温至380℃保温4h；继续从380℃以升温速率为10℃/min至升温700℃，700℃保温10h。
实施例18
与实施例1不同之处在于：在步骤A中，混入0.3wt％氧化钒作为离子掺杂剂。
实施例19
与实施例1不同之处在于：在步骤A中，混入0.3wt％氧化锆作为离子掺杂剂。
对比例1
与实施例1不同之处在于：在步骤A中，不混入离子掺杂添加剂。
对比例2
与实施例1不同之处在于：在步骤C中，砂磨后浆料加入5wt％葡萄糖有机碳源后，直接进行喷雾干燥过程，有机碳源没有充分聚合化过程。
对比例3
与实施例1不同之处在于：前躯体粉末转入烧结炉中，在氮气保护气氛围下，直接以10℃/min升温速率至700℃烧结10h。
实施例20二次电池的制备
二次电池均按照以下所述方法制备：
(1)正极极片的制备：
步骤1：将本申请上述实施例制备得到的碳包覆的磷酸铁锂正极活性材料、粘结结PVDF、导电剂SP按照质量比96:2:2进行混合，加入溶剂NMP，在真空搅拌机作用下搅拌获得固含为50％～58％，粘度为3500～5000mPa·s的正极浆料；将正极浆料双面均匀涂覆在14μm的碳涂覆铝箔(含双面碳涂覆层厚度各为1μm)上；
步骤2:将涂覆后的极片经过100～120℃烘箱干燥；
步骤3：经过冷压、分切得到正极极片。
(2)负极极片制备：
将负极活性材料石墨、增稠剂羧甲基纤维素钠、粘接剂丁苯橡胶、导电剂乙炔黑按照质量比97:1:1:1进行混合，加入去离子水，在真空搅拌机作用下获得负极浆料；将负极浆料均匀涂覆在厚度为6μm的铜箔上；将涂敷后的极片转移至120℃烘箱干燥，然后经过冷压、分切得到负极片。
(3)电解液制备：
有机溶剂为含有碳酸亚乙酯(EC)、碳酸甲乙酯(EMC)和碳酸二乙酯(DEC)的混合液，其中，EC、EMC和DEC的体积比为20:20:60。在含水量<10ppm的氩气气氛手套箱中，将充分干燥的LiPF 6锂盐溶解于有机溶剂中，混合均匀，获得电解液。其中，锂盐的浓度为1mol/L。
(4)隔膜的制备：
选用12μm的聚丙烯隔膜。
(5)电池的制备：
将正极极片、隔膜、负极极片按顺序叠好，使隔膜处于正、负极片之间起到隔离的作用，再卷绕成方形的裸电芯后，装入铝塑膜，然后在80℃下烘烤除水后，注入相应的非水电解液、封口，经静置、热冷压、化成、夹具、分容等工序后，得到成品电池。
测试方法：
(1)DSC测试：
对各个实施例和对比例所制备的正极活性材料进行DSC测试，测试条件为空气氛围，温度范围为室温～550℃，升温速率为10℃/min，测试的仪器为耐驰热分析仪。
(2)碳含量测试：
对各个实施例和对比例所制备的正极活性材料进行碳含量测试，测试的仪器为碳硫分析仪。测试参数如下：
检测设备：钢研纳克CS-2800；
气体压力：氧气进口0.3Mpa，动力气氮气进口0.3Mpa；
试样质量：称取0.1g样品，覆盖1.5g～1.8g钨助溶剂，高温灼烧。
(3)粉体压实测试：
对各个实施例和对比例所制备的正极活性材料进行30KN压力下的粉体压实密度测试，测试仪器为粉体压实密度测试仪。
(4)循环容量保持率：
1.调节保温箱的温度为25℃，静置2h；
2. 0.33C恒流充电至3.65V，随后恒压充电至截止电流0.05C；
3.静置5min；
4. 0.33C恒流放电至2.5V；
5.静置5min；
6. 1C恒流充电至3.65V，随后恒压充电至截止电流0.05C；
7.静置5min；
8. 1C恒流放电至2.5V；
9.静置5min；
10.重复6～9步，直至1000个循环，计算电池的容量保持率。
(5)倍率放电性能测试
1.静置10min；
2. 1C恒流充电至3.65V，随后进行恒压充电，截止电流为0.05C；
3.静置5min；
4. 0.33C恒流放电至2.5V，静置5min；
5. 1C恒流充电至3.65V，随后进行恒压充电，截止电流为0.05C；
6.静置5min；
7. 4C恒流放电至2.5V
8.静置10min。
(6)0.33C放电容量测试
1.调节保温箱的温度为25℃，静置2h；
2. 0.33C恒流充电至3.65V，随后恒压充电至截止电流0.05C；
3.静置5min；
4. 0.33C恒流放电至2.5V；
5.静置5min；
表1 实施例和对比例的物性数据
Figure PCTCN2022139466-appb-000001
Figure PCTCN2022139466-appb-000002
Figure PCTCN2022139466-appb-000003
表2 实施例和对比例的正极活性材料制备的二次电池的电化学性能数据
Figure PCTCN2022139466-appb-000004
Figure PCTCN2022139466-appb-000005
从表1的数据来看，当正极活性材料第一放热峰的面积记为H1，所述第二放热峰的面积记为H2，满足0.45≤H1/(H1+H2)≤0.75时，在保持磷酸铁锂表面总碳含量不变时，提高包覆碳组分，是可以显著提升正极活性材料的粉体压实密度，从而提高极片的压实密度。表2的数据可以看出，第一放热峰和第二放热峰的面积再合适的范围内，正极极片与电解液界面荷电转移位点数量处于较优范围，使磷酸铁锂材料的比容量得到较好的发挥。
从实施例1、2、3和对比例1来看，当引入M元素，且H1/(H1+H2)在上述范围内，使正极活性材料中包覆碳组分的提升，可以提高磷酸铁锂颗粒表面包覆的完整度，减少与电解液界面的副反应，从而显著提升长循环的容量保持。同时，高温碳化过程完整的包覆在颗粒表面，提高正极活性材料的粉体压实密度和极片的压实密度。离子掺杂量过多时，所起到的催化作用有限，而较多的非活性的掺杂离子会降低材料自身克容量的发挥。
从实施例1、4～10来看，不同含量及种类的有机碳源在高温碳化过程中形成的游离碳与包覆碳的组分明显存在差异，而且游离碳和包覆碳的DSC放热峰的位置明显存在差异，以影响磷酸铁锂材料中游离碳和包覆碳的含量，从而影响正极活性材料的粉体压实密度，进而影响正极极片的压实密度。当控制正极活性材料放热峰在上述位置，且0.45≤H1/(H1+H2)≤0.75时，可使磷酸铁锂材料中碳材料的包覆情况处于较优状态。
从实施例1和对比例2来看，在前驱体制备过程中通过缓慢搅拌以及保持浆料的温度为80℃～85℃，加速有机碳源聚合在磷酸铁锂颗粒表面的过程，可以有效调整磷酸铁锂包覆碳组分及包覆的均匀性，从而提升正极活性材料的压实密度，进一步提高电池的倍率性能和循环性能。
从实施例1、11～17和对比例3来看，粉体烧结工艺的优化可以提高磷酸铁锂材料中包覆碳的组分，具体而言就是增设材料预烧结工艺，使得有机碳源在预烧结过程中充分碳化，以完整的包覆磷酸铁锂颗粒表面，使正极活性材料满足0.45≤H1/(H1+H2)≤0.75。在磷酸铁锂材料预烧结过程中，较快的升温速率和较短 的预烧结时间，都会导致有机碳源在磷酸铁锂颗粒表面碳化的不充分，从而形成较多的游离碳。较慢的升温速率和较长的预烧结时间，对磷酸铁锂材料中包覆碳组分提升的效果有限，反而会增加材料生产过程中的能耗，降低材料合成的产能。
从实施例1、18和19来看，不同M元素对正极极片的压实有一定的影响，其中M为Ti元素，正极极片的压实密度最高，电池的综合性能更优。
实施例20～实施例29
二次电池的制备：按照本申请实施例4中二次电池的制备方法制备，正极极片采用的正极活性材料与本申请实施例4中相同，不同之处在于：电解液的组成中还包含不饱和环状碳酸酯添加剂(即表3中不饱和碳酸酯)，具体如表3中所示。
表3 不同电解液组成的二次电池及其性能测试结果
Figure PCTCN2022139466-appb-000006
从表3数据可以看出，当A×H2/(H1+H2)范围为0.04～2.4时，能够进一步提升电池的倍率性能。
以上所述实施例的各技术特征可以进行任意的组合，为使描述简洁，未对上述实施例中的各个技术特征所有可能的组合都进行描述，然而，只要这些技术特征的组合不存在矛盾，都应当认为是本说明书记载的范围。
以上所述实施例仅表达了本申请的几种实施方式，其描述较为具体和详细，但并不能因此而理解为对发明专利范围的限制。应当指出的是，对于本领域的普通技术人员来说，在不脱离本申请构思的前提下，还可以做出若干变形和改进，这些都属于本申请的保护范围。因此，本申请专利的保护范围应以所附权利要求为准。
工业应用
本申请通过控制包含磷酸铁锂的正极活性材料在300℃～600℃时第一放热峰和第二放热峰的面积满足0.45≤H1/(H1+H2)≤0.75，0.25≤H2/(H1+H2)≤0.55，从而使正极活性材料颗粒表面碳材料包覆的完整度在合适的范围内，降低正极活性材料中游离碳材料的含量，从而有效的保证正极活性材料容量的同时，使电池具有较优的循环性能。

Claims (14)

  1. 一种二次电池，其特征在于，包括负极极片、电解液、隔膜和正极极片，所述正极极片包含正极集流体和正极活性材料层，所述正极活性材料层包含正极活性材料，所述正极活性材料包含磷酸铁锂；
    所述正极活性材料的DSC曲线在300℃～600℃有第一放热峰和第二放热峰；
    所述正极活性材料的第一放热峰的面积记为H1，所述第二放热峰的面积记为H2，满足0.45≤H1/(H1+H2)≤0.75。
  2. 根据权利要求1所述的二次电池，其特征在于，所述正极活性材料的所述第一放热峰在340℃～390℃范围内，所述第二放热峰在391℃～550℃范围内。
  3. 根据权利要求1所述的二次电池，其特征在于，所述磷酸铁锂表面含有碳材料，基于所述正极活性材料的质量，所述碳材料质量百分含量为0.5％～3％。
  4. 根据权利要求1所述的二次电池，其特征在于，所述正极活性材料的粒径Dv50为0.4μm～2μm。
  5. 根据权利要求1所述的二次电池，其特征在于，所述正极活性材料还含M元素，所述M元素包含Ti、V或Zr中的至少一种；
    基于所述正极活性材料的重量，所述M元素含量为300ppm～5000ppm。
  6. 根据权利要求1所述的二次电池，其特征在于，所述正极活性材料在30KN压力下的粉体压实密度大于2.3 g/cm 3。
  7. 根据权利要求1所述的二次电池，其特征在于，所述电解液包含不饱和环状碳酸酯添加剂；
    所述不饱和环状碳酸酯添加剂包括碳酸亚乙烯酯和/或乙烯基环状碳酸酯。
  8. 根据权利要求7所述的二次电池，其特征在于，所述基于所述电解液的质量，所述不饱和环状碳酸酯的含量为A％，满足0.04≤A×H2/(H1+H2)≤2.4。
  9. 一种权利要求1-6中任一项所述的二次电池中所述正极活性材料的制备方法，包括如下步骤：
    A)按照摩尔比1:0.9～1.2取无水磷酸铁、碳酸锂，与水混合，然后球磨，得到球磨后的浆料；
    B)将所述球磨后的浆料进行砂磨，得到砂磨产物；
    C)向砂磨后的浆料中添加有机碳源搅拌，进行聚合，得到碳包覆磷酸铁锂前驱体粉末；
    D)将所述碳包覆磷酸铁锂前驱体粉末烧结，得到烧结粉料；
    E)将所述烧结粉料经过气流磨粉碎，然后经过过筛，电流除铁，即得到所述正极活性材料。
  10. 根据权利要求9所述的制备方法，其特征在于，步骤A)中，还包括添加含M元素的离子掺杂添加剂，所述离子掺杂添加剂包括二氧化钛、氧化钒或氧化锆中的至少一种。
  11. 根据权利要求9所述的制备方法，其特征在于，所述浆料中固含量为40％～45％。
  12. 根据权利要求9所述的制备方法，其特征在于，步骤C)中，所述有机碳源的添加质量百分含量为3％～8％；所述有机碳源包括葡萄糖、蔗糖、柠檬酸和壳聚糖中的至少一种。
  13. 根据权利要求9所述的制备方法，其特征在于，步骤D)中，所述烧结的条件如下1)～2)：
    1)惰性气氛；
    2)程序升温：从室温以5℃/min～10℃/min的升温速率升温至300℃～350℃，300℃～350℃保温0.5h；然后从300℃～350℃以2℃/min～15℃/min升温至350℃～420℃，350℃～420℃保温1～8h；继续从350℃～420℃以升温速率为10℃/min升温至650℃～730℃，650℃～730℃保温8 h～11h。
  14. 一种电池模块，其特征在于，包括多个如权利要求1-8中任一项所述的二次电池。
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