Method for detecting high-temperature cyclicity of lithium ion battery cell

Abstract

The invention relates to the technical field of lithium battery testing, and discloses a lithium ion cell high-temperature cyclicity detection method, which comprises the following steps: testing the equivalent difference D of additive VC in electrolyte before and after cell capacity grading, and calculating the size of X according to the formula X = D / (A * R), where A is the specific surface area of a negative electrode active material, the unit is m < 2 > / g, R is the defect degree of the negative electrode material, and R = ID / IG, iD is the strength at the position of 1580 cm <-1 > in a Raman spectrum corresponding to the negative electrode active material, and IG is the strength at the position of 1360 cm <-1 > in the Raman spectrum corresponding to the negative electrode active material; if the X value is within the range of 0.06-0.16, the lithium ion battery cell has good high-temperature cyclicity; and the positive electrode active material of the lithium ion battery cell is lithium iron phosphate. According to the method provided by the invention, the high-temperature cyclicity of the produced battery can be quickly and conveniently judged, and guidance can be provided for designing an expected long-life energy storage battery product.

Description

Technical Field

[0001] The present invention relates to the technical field of battery testing, and in particular to a method for detecting high temperature cyclability of lithium-ion batteries. Background Art

[0002] Lithium-ion batteries have an indispensable position in today's energy storage and conversion field due to their significant advantages such as high energy density, long cycle life, low self-discharge rate and no memory effect. From portable electronic devices to electric vehicles to large-scale energy storage systems, lithium-ion batteries are the key to achieving efficient and convenient energy utilization. Their necessity is self-evident and they are an important force in promoting the development of modern science and technology and energy transformation. In particular, lithium iron phosphate (LiFePO4) energy storage batteries stand out in many application scenarios due to their high safety, good thermal stability and environmental protection characteristics. However, in the face of the market's increasingly high requirements for energy storage solutions, especially the pursuit of cycle life has reached an unprecedented height, it is also increasingly testing the selection and matching of battery raw materials. The quality, physical and chemical properties of battery raw materials and their synergy have a decisive influence on the cycle performance of the battery. From the selection of positive electrode materials to the adjustment of electrolyte formulas, every detail directly affects the performance of the final product. Therefore, it is particularly necessary to formulate a set of scientific and rigorous material selection standards and material matching guidelines.

[0003] In view of this, the present invention is proposed. Summary of the invention

[0004] The purpose of the present invention is to provide a method for detecting high temperature cycle performance of lithium ion batteries.

[0005] The present invention is achieved in that: In a first aspect, the present invention provides a method for detecting high temperature cyclability of a lithium ion battery cell, comprising: The difference D in the VC equivalent of the additive in the electrolyte before and after the test cell capacity division is calculated in wt%. The size of X is calculated according to the formula X=D / (A*R), where A is the specific surface area of ​​the negative electrode active material in m 2 / g, R is the defect degree of negative electrode material, R=I D / I G , I D The Raman spectrum of the negative electrode active material corresponds to 1580 cm -1 The intensity at I G The Raman spectrum of the negative electrode active material corresponds to 1360 cm -1 The strength of the place; If the X value is in the range of 0.06 to 0.16, it indicates that the lithium-ion battery has good high-temperature cycling performance; The positive electrode active material of the lithium-ion battery is lithium iron phosphate, and the additive is selected from one or more of unsaturated double bond or triple bond negative electrode film-forming additives, lithium salt additives, silicon sulfur additives and aromatic ring-containing additives; The VC equivalent refers to the amount when the content of all additives is converted into vinylene carbonate according to the coefficient; The coefficient refers to the ratio of the consumption of the additive other than vinylene carbonate to the consumption when it is equivalent to the vinylene carbonate after volume separation.

[0006] In an optional embodiment, the method of testing the difference in the equivalent of additives in the electrolyte before and after the battery cell capacity division includes: Sampling and testing the fresh electrolyte to obtain the equivalent of the additive in the electrolyte before volume separation; After capacity separation, the battery cell is discharged and the electrolyte in the battery cell is extracted to detect the equivalent of the additives therein.

[0007] In an optional embodiment, the content of each component in the electrolyte is detected by combining high performance liquid chromatography with gas chromatography-mass spectrometry.

[0008] In an optional embodiment, the specific surface area of ​​the negative electrode material is detected by a gas adsorption method.

[0009] In an optional embodiment, the mass proportion of the positive electrode active material in the active layer of the positive electrode sheet is 96.8% to 97.3%.

[0010] In an optional embodiment, the negative electrode active material accounts for 96.8% to 97.3% by mass in the active layer of the negative electrode sheet.

[0011] In an alternative embodiment, the method for preparing the negative electrode active material comprises: The carbonaceous raw material and the dopant are uniformly mixed to obtain a mixture, the mixture is subjected to high-temperature carbonization, and then subjected to high-temperature graphitization to obtain a negative electrode active material.

[0012] In an optional embodiment, at least one of the following features (1) to (4) is included: (1) The carbonaceous raw material is selected from at least one of needle coke and petroleum coke; (2) the dopant is at least one selected from a boron compound and a nitrogen compound; (3) The double-sided surface density of the negative electrode sheet is 100~300 g / m 2 ; (4) The double-sided density of the positive electrode is 110~390 g / m 2 .

[0013] In an optional embodiment, the solvent of the electrolyte is selected from at least two of a carbonate solvent and a carboxylate solvent; Optionally, the carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; Optionally, the carboxylate solvent is selected from at least one of ethyl acetate, ethyl propionate, propyl propionate, methyl acetate and methyl propionate.

[0014] In an optional embodiment, the lithium salt in the electrolyte is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium trifluoromethanesulfonate.

[0015] The present invention has the following beneficial effects: The present invention provides a high-temperature cycle detection method for lithium-ion batteries. By exploring the relationship between the specific surface area of ​​the negative electrode material, the degree of defects in the negative electrode material, and the difference in the additive equivalent before and after the battery cell is divided, the cycle performance of the battery cell at high temperature can be judged. Compared with the existing method of obtaining the performance of the battery cell only after a long period of circulation, the detection method provided by the present invention is highly convenient and the results are obtained quickly. In addition, the method also establishes the matching criteria of the electrolyte and the negative electrode material of the energy storage long-life battery system, accurately controls the matching ratio and compatibility between the materials, so as to design a long-life energy storage battery product that meets expectations. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 This is the Raman spectrum of the defect level of the negative electrode material. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0019] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0020] The following is a detailed description of the high temperature cycle performance testing method for a lithium-ion battery cell provided by an embodiment of the present invention.

[0021] The embodiment of the present invention provides a method for detecting high temperature cycle performance of a lithium-ion battery cell, comprising: The difference D in the VC equivalent of the additive in the electrolyte before and after the test cell capacity division is calculated in wt%. The size of X is calculated according to the formula X=D / (A*R), where A is the specific surface area of ​​the negative electrode active material in m 2 / g, R is the defect degree of negative electrode material, R=I D / I G , I D The Raman spectrum of the negative electrode active material corresponds to 1580 cm -1 The intensity at I G The Raman spectrum of the negative electrode active material corresponds to 1360 cm -1 The strength of the place; If the X value is in the range of 0.06 to 0.16, it indicates that the lithium-ion battery has good high-temperature cycling performance; The positive electrode active material of the lithium ion battery is lithium iron phosphate, and the additive is selected from one or more of unsaturated double bond or triple bond negative electrode film-forming additives, lithium salt additives, silicon sulfur additives and aromatic ring-containing additives; The VC equivalent refers to the amount when the content of all additives is converted into vinylene carbonate according to the coefficient; The coefficient refers to the ratio of the consumption of the additive other than vinylene carbonate to the consumption when it is equivalent to the vinylene carbonate after volume separation.

[0022] The present invention provides a high temperature cycle detection method for lithium ion batteries. By exploring the relationship between the specific surface area of ​​the negative electrode material, the degree of defect of the negative electrode material and the difference in the equivalent of the additive before and after the battery cell capacity division, the quality of the cycle performance of the battery cell at high temperature can be judged. When the specific surface area and degree of defect of the negative electrode material are reduced, its ion transmission efficiency and electronic conduction path are blocked, the internal resistance of the battery increases, the charging and discharging efficiency and power performance of the battery deteriorate, and the battery capacity decreases; increasing the specific surface area and degree of defect of the negative electrode material can improve its dynamics, but it will destroy the structural stability of the material, expose more active sites, increase the contact area between the electrolyte and the active substance, accelerate the side reaction, consume more active materials and electrolytes, and ultimately lead to capacity decay. The detection method formulated in this application, when designing and optimizing the negative electrode material, combines its consumption of electrolyte additives, and stipulates a balance point, so that the battery cell maintains high dynamics while the consumption of electrolyte additives is also within a reasonable range, thereby determining the battery cell that meets the requirements of the detection method of this application, and the battery cell cycle life is better. Compared with the existing method of obtaining the performance of the battery cell only after a long cycle, the detection method provided by the present invention is highly convenient and the results are obtained quickly. In addition, the significance of the present invention lies in the comprehensive control of the synthesis of the negative electrode raw materials of the lithium iron phosphate battery and the design of the electrolyte formula, from the defect degree and specific surface area of ​​the negative electrode raw materials, combined with the total equivalent consumption of the electrolyte additives after the battery capacity is divided, to establish the matching criteria of the electrolyte and the negative electrode material of the energy storage long-life battery system, accurately control the matching ratio and compatibility between the materials, so as to design a long-life energy storage battery product that meets expectations.

[0023] The specific plan is as follows: S1. Selection and preparation of negative electrode active materials: High-purity carbonaceous raw materials (such as needle coke, petroleum coke, etc.) and specific additives (such as a small amount of boron compounds or nitrogen compounds to improve the material structure and performance) are mixed evenly in precise proportions to obtain a preliminary molding material; the preliminary molding material is subjected to a high-temperature carbonization treatment under the protection of an inert gas, followed by a high-temperature graphitization treatment, and then pulverized and graded after cooling to obtain a graphite negative electrode material.

[0024] Optionally, the inert gas protection during the carbonization and graphitization process is argon.

[0025] S2. Preparation of negative electrode sheet The negative electrode active material, conductive agent acetylene black, thickener (such as CMC), and binder (such as PVDF) are stirred and mixed evenly, and then mixed evenly with a solvent to form a slurry, which is coated on the current collector, and then dried, pressed and cut to obtain a negative electrode sheet.

[0026] Optionally, the ingredients are formulated so that the negative electrode active material accounts for 96.8% to 97.3% of the negative electrode active layer.

[0027] Optionally, the double-sided surface density of the negative electrode sheet is 100~300g / m 2 .

[0028] For example: The negative electrode active material: conductive agent: thickener: binder are added to a vacuum mixer in a mass ratio of 97:1:1:1 and mixed evenly. Then, an appropriate amount of solvent (deionized water) is added and stirred until the slurry is uniform. The prepared slurry is evenly coated on both sides of the negative electrode current collector copper foil, dried at room temperature, and transferred to an oven for drying. After drying, the negative electrode sheet semi-finished product is obtained, and finally the negative electrode sheet to be assembled is obtained through cold pressing and slitting processes.

[0029] S3. Preparation of positive electrode The positive electrode active material (lithium iron phosphate), conductive agent and binder are evenly mixed and then evenly mixed with an appropriate amount of solvent to obtain a positive electrode slurry, and then the positive electrode slurry is coated on the current collector, and then dried, pressed and cut to obtain a positive electrode sheet.

[0030] Optionally, the mass proportion of the positive electrode active material is 96.8%~97.3%, the mass proportion of the conductive agent is 1.9%~2.2%, and the mass proportion of the binder is 0.8%~1.0%.

[0031] Optionally, the double-sided surface density of the positive electrode sheet is 110~390 g / m².

[0032] S4. Preparation of electrolyte The electrolyte solution includes an organic solvent, an electrolyte salt dissolved in the organic solvent, various additives, and the like.

[0033] Optionally, the organic solvent comprises two or more of conventional carbonate solvents or carboxylate solvents. Specific examples of carbonate solvents include ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate (EMC). Specific examples of carboxylate solvents include ethyl acetate, ethyl propionate, propyl propionate, methyl acetate, and methyl propionate.

[0034] Preferably, the mass percentage of cyclic ester to linear ester is ensured to be 3:7. Specifically, such as EC / EMC=3 / 7.

[0035] Optionally, the electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium trifluoromethanesulfonate.

[0036] Specifically, for example, the electrolyte lithium salt is LiPF6, and its content is 0.8~1.3 mol / L.

[0037] Optionally, the additives include one or more of unsaturated double bond or triple bond negative electrode film-forming additives, lithium salt additives, silicon sulfur additives, and aromatic ring-containing additives, and the total equivalent weight of the additives is preferably 3.50%.

[0038] Specifically, negative electrode film-forming additives include vinylene carbonate (VC) and fluoroethylene carbonate (FEC); lithium salt additives include lithium difluorooxalatoborate (LiODFB); silicon-sulfur additives include methyl methyl disulfonate (MMDS) and tris(trimethylsilyl) phosphate (TMSP); aromatic ring-containing additives include benzene sulfone (BS) and biphenyl (BP).

[0039] S5. Battery Assembly Assemble the negative electrode sheet, the positive electrode sheet and the separator (PP / PE / ceramic, etc.) to obtain a battery cell; inject the electrolyte into the battery cell, and obtain the battery cell after aging (25°C, 48h), formation (45°C), aging (25°C, 48h), and capacity separation. The assembly method of the battery cell is not limited, and it can be the assembly method of the existing lithium battery cell. The separator is located between the positive electrode sheet and the negative electrode sheet, and the battery cell can be prepared by lamination or winding.

[0040] Specifically, the formation method is, for example: ① Leave it for 5 minutes; ② Charge at constant current for 120 minutes, current 0.05C, protection voltage 3.8V; ③ Leave it for 5 minutes, charge at constant current for 120 minutes, current 0.3C, protection voltage 3.8V; ④ End.

[0041] S6. Detection method (1) Test method for specific surface area of ​​negative electrode active materials: The specific surface area of ​​the negative electrode material is tested using the gas adsorption method (BET specific surface area test method).

[0042] First, the negative electrode material sample is dried and activated using a BET surface area tester, and heated at 300°C for 4 hours in a vacuum environment to remove impurities and moisture on the sample surface; then, an adsorbed gas (such as nitrogen) is introduced into the system, the gas pressure is adjusted, and the temperature (usually liquid nitrogen temperature) is controlled. The instrument calculates the adsorption isotherm of the sample by measuring the adsorption amount under different pressures; finally, the BET formula is used to fit the data, calculate the specific surface area of ​​the negative electrode material, and output the test results through the software, including parameters such as specific surface area value and pore size distribution.

[0043] (2) Test method for the degree of defects in negative electrode active materials The defect degree is tested using Raman spectroscopy.

[0044] First, the sample is prepared into a form suitable for testing (such as powder or flakes), then placed in a sample cell, and the sample is irradiated by the instrument's laser (usually a laser of a specific wavelength, such as 532 nm or 785 nm) to stimulate molecular vibrations and generate Raman scattering signals. During the test, the spectrometer collects Raman signals and converts them into spectral data. The disorder of the sample can be evaluated by analyzing the position, intensity, and width of the peaks in the spectrum. Subsequently, the spectrum is subjected to background subtraction, peak fitting, and integration using supporting data processing software (such as Origin or Matlab) to extract characteristic parameters related to disorder (such as peak area, peak intensity ratio, or peak displacement change).

[0045] Take I D / I G As the defect degree of negative electrode material R, I D The Raman spectrum of the negative electrode active material corresponds to 1580cm -1 The intensity at I G The Raman spectrum of the negative electrode active material corresponds to 1360 cm -1 The intensity at the negative electrode material is shown in the Raman spectrum. Figure 1 shown.

[0046] (3) Test method for equivalent amount of additives in electrolyte The precise analysis was performed using a method combining high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS).

[0047] For organic solvents and additives, a high performance liquid chromatograph is used to separate the components by using the difference in the distribution coefficients between the stationary phase and the mobile phase. At the same time, an ultraviolet detector or a diode array detector is used to record the absorption signal of each component, obtain its retention time and peak area data, and obtain the absolute value content of the organic solvents and additives; For lithium salts and other volatile components in the electrolyte, gas chromatography-mass spectrometry technology is used. The sample is first diluted appropriately before injection, and a preliminary separation is achieved through a gas chromatography column. Then, it enters a mass spectrometer for qualitative and quantitative analysis to obtain the absolute value content of lithium salts and other volatile components. Finally, the measured results are converted according to the coefficient of each additive to VC, and the sum of the conversion results is the total equivalent of the additive.

[0048] The specific method of determining the coefficient is, for example: The same battery cell (positive, negative electrodes and separator are exactly the same) was injected with electrolytes with different additives. The solvent of the electrolyte was EC / EMC=3 / 7, the lithium salt was 1.0 M LiPF6, the additives were 1.5% VC (baseline group), and 1.5% VC + 0.5% of some other additives (control group).

[0049] If the measured VC consumption of the baseline group is 0.5%, the VC consumption of the control group is 0.2%, and the consumption of this other additive is 0.1%, then the coefficient of this other additive is the difference in VC consumption between the baseline group and the control group / this other additive in the control group, that is, (0.5%-0.2%) / 0.1%=3; then the VC equivalent of this other additive in the control group is 3×0.5%=1.5%, the total equivalent is 1.5%+1.5%=3%, and the consumption VC equivalent of this additive is 3×0.1%=0.3%.

[0050] Fresh electrolyte can be directly sampled and tested; the electrolyte after the battery is divided into capacity is first treated with an empty battery (to discharge the electrolyte in the pores of the electrode and improve the accuracy of the test results), and then discharged to 2.5V with a constant current of less than 0.5C. Then, 5~10ml of free electrolyte is extracted with a syringe in the glove box, transferred to a centrifuge tube for collection and storage, and then sent for sampling and testing.

[0051] The following examples only list some of the negative electrode active materials suitable for the method provided by the present invention. It should be noted that the currently disclosed negative electrode active materials that can be used in lithium-ion batteries are suitable for the preparation method provided by the present invention. The method provided by the present invention has no clear requirements for the type of negative electrode active material.

[0052] First embodiment (1) Prepare experimental samples Experimental Group 1 The preparation method of the negative electrode active material in this experimental group is: The needle coke and additives (such as urea) are mixed uniformly in a mass ratio of 95:5, and then high-temperature carbonization and graphitization are performed under the protection of inert gas. The high-temperature carbonization temperature is controlled at 800°C for 2 hours, and the high-temperature graphitization temperature is 2500°C, the heating rate is 5°C / min, and the holding time is 2 hours to obtain the negative electrode active material.

[0053] The components of the active layer in the negative electrode sheet are the negative electrode active material, conductive agent (acetylene black), thickener (CMC) and binder (PVDF) in a mass ratio of 97:1:1:1. The double-sided surface density is 160 g / m 2 .

[0054] The active layer of the positive electrode sheet is composed of positive electrode active material, conductive agent (acetylene black) and binder (PVDF) in a mass ratio of 97:2:1. The double-sided surface density is 300 g / m 2 .

[0055] The diaphragm is PP film.

[0056] The composition of the electrolyte is solvent EC / EMC=3 / 7, 1M LiPF6, additives unsaturated negative electrode film-forming additives (VC / FEC) / silicon sulfur additives (MMDS) / lithium salt additives (LiODFB) = 5 / 1 / 1, with an equivalent of 3.5%.

[0057] Experimental Group 2 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature is 2200℃, the heating rate is 5℃ / min, and the holding time is 2 hours. The additive equivalent in the electrolyte is 4.00%.

[0058] Experimental Group 3 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 2500°C, the heating rate was 2°C / min, and the holding time was 2 hours. The additive equivalent in the electrolyte was 3.20%.

[0059] Experimental Group 4 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 2800°C, the heating rate was 2°C / min, and the holding time was 2 hours. The additive equivalent in the electrolyte was 3.00%.

[0060] Experimental Group 5 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 2800°C, the heating rate was 2°C / min, and the holding time was 4 hours. The additive equivalent in the electrolyte was 2.80%.

[0061] Experimental Group 6 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 2800°C, the heating rate was 2°C / min, and the holding time was 6 hours. The additive equivalent in the electrolyte was 2.50%.

[0062] Experimental Group 7 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 3000°C, the heating rate was 2°C / min, and the holding time was 6 hours. The additive equivalent in the electrolyte was 2.30%.

[0063] Experimental Group 8 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 2000°C, the heating rate was 5°C / min, and the holding time was 2 hours. The additive equivalent in the electrolyte was 4.30%.

[0064] Experimental Group 9 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 1800°C, the heating rate was 10°C / min, and the holding time was 2 hours. The additive equivalent in the electrolyte was 4.50%.

[0065] Experimental Group 10 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 3000°C, the heating rate was 2°C / min, and the holding time was 10 hours. The additive equivalent in the electrolyte was 1.80%.

[0066] Experimental Group 11 The only difference between this experimental group and experimental group 1 is: The high temperature graphitization temperature was controlled at 3000°C, the heating rate was 2°C / min, and the holding time was 8 hours. The additive equivalent in the electrolyte was 2.00%.

[0067] (2) Experimental sample testing The parameter designs of experimental groups 1 to 11 are recorded in Table 1; The performance parameters of the batteries prepared in experimental groups 1 to 11 were tested according to the test method described in S6 above and recorded in Table 2; The batteries of experimental groups 1 to 11 were cycled tested in the following manner: charging at 0.5C constant current and constant voltage at 25°C, discharging at 0.5C constant current for three cycles to constant capacity; then heating to 45°C and leaving for 30 minutes, charging at 1.0C constant current and constant voltage to 3.65V, cutting off the current at 0.05C, leaving for 30 minutes, discharging at 1.0C constant current to 2.5V, and stopping the test after 4000 cycles. The test results are recorded in Table 2.

[0068] Table 1 Design parameters of lithium-ion batteries in each experimental group

[0069] Table 2 Specific parameters and performance data of lithium-ion batteries

[0070] It can be seen from Table 2 that when the X value is in the range of 0.06~0.16, the battery (45℃ 1C / 1C) can reach at least 2200 cycles @80% SOC.

[0071] Second embodiment The negative electrode raw material type selected in this embodiment is different from that in the first embodiment. The negative electrode active material is natural graphite. The preparation method is as follows: flake graphite and catalyst (iron salt) are mixed evenly in a mass ratio of 95:5, and then low-temperature pretreatment, medium-temperature conversion and high-temperature graphitization are carried out in sequence in a protective gas (argon) atmosphere. The low-temperature pretreatment temperature is set to 300°C for 1 hour; the medium-temperature conversion temperature reaches 1500°C, the heating rate is 3°C / min, and the temperature is kept for 3 hours; the high-temperature graphitization temperature, heating rate and holding time are controlled to obtain natural graphite materials with different specific surface areas and defect levels. The specific parameters and performance data of lithium-ion batteries are shown in Tables 3 and 4 below.

[0072] Table 3 Design parameters of lithium-ion batteries in each experimental group

[0073] Table 4 Specific parameters and performance data of lithium-ion batteries

[0074] Third embodiment The electrolyte components selected in this embodiment are different from those in the first embodiment. The solvent is EC / EMC / EA=3 / 5 / 2, 1M LiPF6, and the additives include unsaturated negative electrode film-forming additives (VC / FEC / VEC) / sulfur-containing additives (DTD) / lithium salt additives (LiFSI) = 8 / 1 / 1. The specific parameters and performance data of the lithium-ion battery are shown in Tables 5 and 6 below.

[0075] Table 5 Design parameters of lithium-ion batteries in each experimental group

[0076] Table 6 Specific parameters and performance data of lithium-ion batteries

[0077] In summary, the present invention provides a high-temperature cyclability detection method for lithium-ion batteries. By exploring the relationship between the specific surface area of ​​the negative electrode material, the degree of defects in the negative electrode material, and the difference in the additive equivalent before and after the battery cell is divided, the cycling performance of the battery cell at high temperature can be judged. Compared with the existing method of obtaining the performance of the battery cell only after a long period of circulation, the detection method provided by the present invention is highly convenient and the results are obtained quickly. The detection method provided by the present invention can be used as a quality inspection method for battery cells, and the method can provide direction for the formulation design of negative electrode materials and electrolytes.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for detecting high temperature cyclability of a lithium-ion battery cell, characterized in that: include: The difference D in the VC equivalent of the additive in the electrolyte before and after the test cell capacity division is calculated in wt%. The size of X is calculated according to the formula X=D / (A*R), where A is the specific surface area of ​​the negative electrode active material in m 2 / g, R is the defect degree of negative electrode material, R=I D / I G , I D The Raman spectrum of the negative electrode active material corresponds to 1580 cm -1 The intensity at I G The Raman spectrum of the negative electrode active material corresponds to 1360 cm -1 The strength of the place; If the X value is in the range of 0.06 to 0.16, it indicates that the lithium-ion battery has good high-temperature cycling performance; The positive electrode active material of the lithium ion battery is lithium iron phosphate, and the additive is selected from one or more of unsaturated double bond or triple bond negative electrode film-forming additives, lithium salt additives, silicon sulfur additives and aromatic ring-containing additives; The VC equivalent refers to the amount when the content of all additives is converted into vinylene carbonate according to the coefficient; The coefficient refers to the ratio of the consumption of the additive other than vinylene carbonate to the consumption when it is equal to the vinylene carbonate after volume separation.

2. The method according to claim 1, characterized in that The methods for testing the difference in the equivalent of additives in the electrolyte before and after the cell capacity division include: Sampling and testing the fresh electrolyte to obtain the equivalent of the additive in the electrolyte before volume separation; After capacity separation, the battery cell is discharged and the electrolyte in the battery cell is extracted to detect the equivalent of the additives therein.

3. The method according to claim 1, characterized in that The content of each component in the electrolyte was detected by combining high performance liquid chromatography with gas chromatography-mass spectrometry.

4. The method according to claim 1, characterized in that: The specific surface area of ​​the negative electrode material was detected by gas adsorption method.

5. The method according to claim 1, characterized in that The mass proportion of the positive electrode active material in the active layer of the positive electrode sheet is 96.8% to 97.3%.

6. The method according to claim 1, characterized in that The negative electrode active material accounts for 96.8% to 97.3% by mass in the active layer of the negative electrode sheet.

7. The method according to claim 1, characterized in that The method for preparing the negative electrode active material comprises: The carbonaceous raw material and the dopant are uniformly mixed to obtain a mixture, the mixture is subjected to high-temperature carbonization, and then subjected to high-temperature graphitization to obtain the negative electrode active material.

8. The method according to claim 7, characterized in that Includes at least one of the following features (1) to (4): (1) The carbonaceous raw material is selected from at least one of needle coke and petroleum coke; (2) The dopant is selected from at least one of a boron compound and a nitrogen compound; (3) The double-sided surface density of the negative electrode of the battery cell is 100~300 g / m 2 ; (4) The double-sided surface density of the positive electrode of the battery cell is 110~390 g / m 2 .

9. The method according to claim 1, characterized in that: The solvent of the electrolyte is selected from at least two of a carbonate solvent and a carboxylate solvent; Optionally, the carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; Optionally, the carboxylate solvent is selected from at least one of ethyl acetate, ethyl propionate, propyl propionate, methyl acetate and methyl propionate.

10. The method according to claim 1, characterized in that The lithium salt in the electrolyte is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium trifluoromethanesulfonate.

Images