Evaluation method for cycling stability of lithium iron phosphate positive electrode material

Abstract

The invention discloses a method for evaluating the cycling stability of a lithium iron phosphate positive electrode material, and relates to the technical field of lithium iron phosphate positive electrode material evaluation, and the method comprises the following specific processes: S1, pretreatment operation of the lithium iron phosphate positive electrode material and battery assembly operation; s2, carrying out a multi-dimensional multi-working-condition cyclic charging and discharging dynamic test by virtue of a battery test system with ultrahigh precision and a powerful program control function; s3, carrying out performance analysis operation based on the obtained multi-dimensional multi-working-condition cyclic charging and discharging dynamic test data, wherein the performance analysis operation comprises capacity attenuation analysis, voltage characteristic analysis and internal resistance growth analysis; and S4, carrying out evaluation operation of comprehensive stability quantification based on the analyzed data information, and carrying out grade evaluation on the lithium iron phosphate positive electrode material after comprehensive evaluation. According to the evaluation method, preprocessing is carried out, multi-dimensional and multi-working-condition cyclic charging and discharging dynamic testing is carried out, finally evaluation of comprehensive stability quantification is carried out based on performance analysis, grades are evaluated, and evaluation of the cycling stability of the positive electrode material is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate positive electrode material evaluation, and in particular to a method for evaluating the cycle stability of a lithium iron phosphate positive electrode material. Background Art

[0002] At present, traditional evaluation methods often focus on a single performance dimension, such as simply paying attention to capacity attenuation, or the evaluation process is too complicated, with strict requirements on equipment and operating environment, time-consuming and labor-intensive, and difficult to reproduce. It is impossible to fully capture the cycle characteristics of lithium iron phosphate materials under various working conditions and complex environments, and it is difficult to meet the urgent needs of scientific research innovation and rapid industrial development for efficient material screening and accurate evaluation, resulting in low efficiency in identifying high-quality lithium iron phosphate materials, which restricts the process of battery performance improvement and cost optimization; Based on the above reasons, the present invention proposes a method for evaluating the cycle stability of lithium iron phosphate positive electrode materials, which can quantitatively evaluate lithium iron phosphate positive electrode materials from multiple dimensions. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a method for evaluating the cycle stability of a lithium iron phosphate positive electrode material, which solves the problems raised in the above-mentioned background technology.

[0004] To achieve the above objectives, the present invention is implemented by the following technical solutions: A method for evaluating the cycle stability of lithium iron phosphate positive electrode materials, comprising the following specific processes: S1: pretreatment operation of lithium iron phosphate cathode material and battery assembly operation; S2: Relying on a battery testing system with ultra-high precision and powerful program control functions, multi-dimensional and multi-condition cycle charge and discharge dynamic testing is carried out; S3: Based on the obtained multi-dimensional and multi-operating condition cycle charge and discharge dynamic test data, performance analysis operations are performed, including capacity decay analysis, voltage characteristic analysis and internal resistance growth analysis; S4: Conduct comprehensive stability quantitative evaluation based on the analyzed data information, and grade the lithium iron phosphate positive electrode material after the comprehensive evaluation.

[0005] Optionally, the specific process of the pretreatment operation of the lithium iron phosphate positive electrode material is as follows: S11: accurately weighing a certain amount of lithium iron phosphate positive electrode material, grinding the lithium iron phosphate positive electrode material at high speed, and performing screening operation after grinding to obtain lithium iron phosphate positive electrode material raw materials with a specified mesh size; S12: adding a conductive agent and a binder in a certain ratio to the obtained lithium iron phosphate positive electrode material raw material, and performing mixing and stirring operations to prepare a uniform, stable slurry with suitable viscosity; S13: Use precision coating equipment to evenly and evenly coat the slurry on the surface of the pre-cleaned and dried aluminum foil current collector, and strictly control the coating thickness; S14: After coating is completed, drying and rolling operations are performed, and the electrode sheet is cut to obtain a lithium iron phosphate positive electrode sheet that can match the specified specifications for battery assembly, completing the pretreatment operation.

[0006] Optionally, the conductive agent is graphene, the binder is polyvinylidene fluoride PVDF, and the ratio of lithium iron phosphate: conductive agent: binder = 88:8:4. Under the premise of taking energy density into consideration, the comprehensive performance and stability of the battery can be improved by means of the corresponding conductive and bonding effects of the conductive agent and the binder.

[0007] Optionally, the multi-dimensional and multi-operating condition cyclic charge-discharge dynamic test operation process is: S21: Carry out precision activation operation on the assembled battery, set the temperature to a constant 25°C, and steadily promote 8-12 charge and discharge cycles at an ultra-low rate of 0.05C-0.15C and a voltage range of 2.2-4.0V; S22: After activation, multiple groups of differentiated test conditions are programmed, covering various combinations of charge and discharge rates and temperature ranges, and 500-800 charge and discharge cycles are continuously carried out under each group of conditions; S23: At every 50 cycles, an electrochemical impedance spectroscopy (EIS) test is performed using an electrochemical workstation to obtain the battery internal resistance and impedance spectrum characteristic parameter information; S24: Full dynamic monitoring, real-time recording of the charge and discharge capacity, average charge and discharge voltage, and charge and discharge time data during each cycle.

[0008] Optionally, the calculation formula for the capacity decay analysis is: CR n =(C n / C1)×100%, where C n is the discharge capacity of the nth cycle, and C1 is the discharge capacity of the first cycle.

[0009] Optionally, the calculation formula for the voltage characteristic analysis is: ΔV=V x -V n , where V x is the average charging voltage, V n is the average discharge voltage.

[0010] Optionally, the calculation formula for the internal resistance growth analysis is: IR n =(R n -R1) / R1×100%, where R n is the internal resistance of the nth cycle, and R1 is the internal resistance of the first cycle.

[0011] Optionally, the quantitative evaluation of comprehensive stability based on the analyzed data information adopts a weighted summation method to calculate a quantitative score of comprehensive stability; The specific calculation formula is: A=0.45×CR+0.3×VS+0.25×IR; Among them, CR, VS, and IR are the capacity retention rate, voltage stability score, and internal resistance change rate score after normalization, respectively. The levels of the lithium iron phosphate positive electrode material include cutting-edge grade, excellent grade, general grade, passing grade, and poor grade. The cutting-edge grade is scored at 95-100 points, the excellent grade is scored at 85-94, the general grade is scored at 75-84, the passing grade is scored at 65-74, and the poor grade is scored below 64 points.

[0012] The present invention provides a method for evaluating the cycle stability of a lithium iron phosphate positive electrode material, which has the following beneficial effects: The evaluation method of the cycle stability of lithium iron phosphate cathode materials starts from the pretreatment of lithium iron phosphate cathode materials to the dynamic test of cycle charge and discharge under multi-dimensional and multi-operating conditions, and finally conducts a comprehensive stability quantitative evaluation based on performance analysis and assesses the grade, thus achieving a comprehensive evaluation of the cycle stability of lithium iron phosphate cathode materials. Through precise pretreatment of positive electrode materials, including high-speed grinding, screening, mixing, precision coating, drying and rolling, a stable positive electrode sheet is prepared. Then, a high-performance battery testing system is used to carry out a number of tests including activation, differential chemical condition testing, electrochemical impedance spectroscopy testing, etc., to obtain key parameter data such as capacity, voltage, and internal resistance. A comprehensive stability assessment is carried out by calculating indicators such as capacity attenuation, voltage change, and internal resistance growth. A weighted summation method is used to obtain a comprehensive score, thereby dividing the positive electrode materials into five grades: cutting-edge, excellent, general, qualified, and poor. This provides a scientific basis and effective guidance for practical applications and realizes the quantitative evaluation of lithium iron phosphate positive electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A flow chart of the evaluation method of the invention; Figure 2 This is a flow chart of the pretreatment operation of the lithium iron phosphate positive electrode material in the invention; Figure 3 This is a flow chart of the multi-dimensional and multi-operating condition cyclic charge and discharge dynamic test operation in the invention. DETAILED DESCRIPTION

[0014] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0015] like Figure 1 As shown, a method for evaluating the cycle stability of a lithium iron phosphate positive electrode material comprises the following steps: S1: pretreatment operation of lithium iron phosphate cathode material and battery assembly operation; like Figure 2 As shown, the pretreatment operation of the lithium iron phosphate positive electrode material includes the following steps: S11: Accurately weigh a certain amount of lithium iron phosphate positive electrode material, and grind the lithium iron phosphate positive electrode material at high speed. The grinding of the lithium iron phosphate positive electrode material is completed by a planetary ball mill, and the speed of the planetary ball mill is controlled to be 400-550r / min for 3-5h. After the grinding is completed, a screening operation is performed to obtain a lithium iron phosphate positive electrode material raw material with a specified mesh size; S12: Add a certain ratio of conductive agent and binder to the obtained raw material of lithium iron phosphate positive electrode material, and perform mixing and stirring operations. The mixing of the slurry is achieved by using a vacuum stirring device, and the vacuum degree of the vacuum stirring device is guaranteed to be between -0.08MPa and -0.1MPa, the stirring rate is 520-850r / min, and the duration is 7-10h to prepare a uniform, stable slurry with suitable viscosity; Furthermore, the conductive agent is graphene, the binder is polyvinylidene fluoride PVDF, and the ratio of lithium iron phosphate: conductive agent: binder is 88:8:4. Under the premise of taking into account the energy density, the comprehensive performance and stability of the battery can be improved by means of the corresponding conductive and bonding effects of the conductive agent and the binder. S13: Use precision coating equipment to evenly and evenly coat the slurry on the surface of the pre-cleaned and dried aluminum foil current collector, and strictly control the coating thickness; S14: After coating, drying and rolling operations are performed. The coating of the pole piece is dried by gradient drying at 60°C, 80°C, and 100°C for 3-5 hours respectively, which effectively removes the solvent. After rolling the pole piece, the compaction density of the pole piece is 2.2-2.6g / cm 2 , and cut the electrode sheet to obtain a lithium iron phosphate positive electrode sheet of a specified specification that can match the battery assembly, completing the pretreatment operation; S2: Relying on a battery testing system with ultra-high precision and powerful program control functions, multi-dimensional and multi-condition cycle charge and discharge dynamic testing is carried out; like Figure 3 As shown, the multi-dimensional and multi-operating condition cyclic charge and discharge dynamic test operation includes the following steps: S21: Carry out precision activation operation on the assembled battery, set the temperature to a constant 25°C, and steadily promote 8-12 charge and discharge cycles at an ultra-low rate of 0.05C-0.15C and a voltage range of 2.2-4.0V; S22: After activation, multiple groups of differentiated test conditions are programmed, covering various combinations of charge and discharge rates and temperature ranges. The charge and discharge rates include 0.2C, 0.5C, 1C, 2C, 3C and 5C, and the temperature ranges include -40℃, -20℃, 0℃, 25℃, 50℃ and 70℃. Each group of conditions is continuously charged and discharged for 500-800 cycles. S23: At every 50 cycles, an electrochemical impedance spectroscopy (EIS) test is performed using an electrochemical workstation to obtain the battery internal resistance and impedance spectrum characteristic parameter information; S24: Full dynamic monitoring, real-time recording of charge and discharge capacity, average charge and discharge voltage, and charge and discharge time data during each cycle; S3: Based on the obtained multi-dimensional and multi-operating condition cycle charge and discharge dynamic test data, performance analysis operations are performed, including capacity decay analysis, voltage characteristic analysis and internal resistance growth analysis; The calculation formula for capacity fade analysis is: CR n =(C n / C1)×100%, where C n is the discharge capacity of the nth cycle, C1 is the discharge capacity of the first cycle; The calculation formula for voltage characteristic analysis is: ΔV=V x -V n , where V x is the average charging voltage, V n is the average discharge voltage; The calculation formula for internal resistance growth analysis is: IR n =(R n -R1) / R1×100%, where R n is the internal resistance of the nth cycle, and R1 is the internal resistance of the first cycle; S4: Conduct comprehensive stability quantitative evaluation based on the analyzed data information, and grade the lithium iron phosphate cathode material after the comprehensive evaluation; The specific calculation formula is: A=0.45×CR+0.3×VS+0.25×IR; in, CR, VS, and IR are the normalized capacity retention rate, voltage stability score, and internal resistance change rate score, respectively. The grades of lithium iron phosphate cathode materials include advanced, excellent, general, qualified, and poor grades. The advanced grade is scored at 95-100 points, the excellent grade is scored at 85-94, the general grade is scored at 75-84, the qualified grade is scored at 65-74, and the poor grade is scored below 64 points, thus achieving intuitive, accurate, and multi-dimensional quantitative evaluation of the cycle stability of lithium iron phosphate cathode materials. The weight parameters of the capacity retention rate, the voltage stability score, and the internal resistance change rate can be assigned values ​​according to the evaluation requirements of the lithium iron phosphate positive electrode material. In this embodiment, the weight parameters are assigned values ​​of 0.45, 0.3, and 0.25.

[0016] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for evaluating the cycle stability of a lithium iron phosphate positive electrode material, characterized in that: The specific process includes the following: S1: pretreatment operation of lithium iron phosphate cathode material and battery assembly operation; S2: Relying on a battery testing system with ultra-high precision and powerful program control functions, multi-dimensional and multi-condition cycle charge and discharge dynamic testing is carried out; S3: Based on the obtained multi-dimensional and multi-operating condition cycle charge and discharge dynamic test data, performance analysis operations are performed, including capacity decay analysis, voltage characteristic analysis and internal resistance growth analysis; S4: Conduct comprehensive stability quantitative evaluation based on the analyzed data information, and conduct grade assessment for the lithium iron phosphate positive electrode material after the comprehensive evaluation.

2. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The specific process of the pretreatment operation of the lithium iron phosphate positive electrode material is as follows: S11: accurately weighing a certain amount of lithium iron phosphate positive electrode material, grinding the lithium iron phosphate positive electrode material at high speed, and performing screening operation after grinding to obtain lithium iron phosphate positive electrode material raw materials with a specified mesh size; S12: adding a conductive agent and a binder in a certain ratio to the obtained lithium iron phosphate positive electrode material raw material, and performing mixing and stirring operations to prepare a uniform, stable slurry with suitable viscosity; S13: Use precision coating equipment to evenly and evenly coat the slurry on the surface of the pre-cleaned and dried aluminum foil current collector, and strictly control the coating thickness; S14: After coating is completed, drying and rolling operations are performed, and the electrode sheet is cut to obtain a lithium iron phosphate positive electrode sheet that can match the specified specifications for battery assembly, completing the pretreatment operation.

3. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 2, characterized in that: The conductive agent is graphene, the binder is polyvinylidene fluoride (PVDF), and the ratio of lithium iron phosphate:conductive agent:binder is 88:8:

4. Under the premise of taking energy density into consideration, the comprehensive performance and stability of the battery can be improved by means of the corresponding conductive and bonding effects of the conductive agent and the binder.

4. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The multi-dimensional and multi-operating condition cyclic charge and discharge dynamic test operation process is as follows: S21: Carry out precision activation operation on the assembled battery, set the temperature to a constant 25°C, and steadily promote 8-12 charge and discharge cycles at an ultra-low rate of 0.05C-0.15C and a voltage range of 2.2-4.0V; S22: After activation, multiple groups of differentiated test conditions are programmed, covering various combinations of charge and discharge rates and temperature ranges, and 500-800 charge and discharge cycles are continuously carried out under each group of conditions; S23: At every 50 cycles, an electrochemical impedance spectroscopy (EIS) test is performed using an electrochemical workstation to obtain the battery internal resistance and impedance spectrum characteristic parameter information; S24: Full dynamic monitoring, real-time recording of the charge and discharge capacity, average charge and discharge voltage, and charge and discharge time data during each cycle.

5. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The calculation formula for the capacity decay analysis is: CR n =(C n / C1)×100%, where C n is the discharge capacity of the nth cycle, and C1 is the discharge capacity of the first cycle.

6. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The calculation formula for the voltage characteristic analysis is: ΔV=V x -V n , where V x is the average charging voltage, V n is the average discharge voltage.

7. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The calculation formula for the internal resistance growth analysis is: IR n =(R n -R1) / R1×100%, where R n is the internal resistance of the nth cycle, and R1 is the internal resistance of the first cycle.

8. The method for evaluating the cycle stability of a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The quantitative evaluation of comprehensive stability based on the analyzed data information adopts a weighted summation method to calculate the quantitative score of comprehensive stability; The specific calculation formula is: A=0.45×CR+0.3×VS+0.25×IR; Among them, CR, VS, and IR are the capacity retention rate, voltage stability score, and internal resistance change rate score after normalization, respectively. The levels of the lithium iron phosphate positive electrode material include cutting-edge grade, excellent grade, general grade, passing grade, and poor grade. The cutting-edge grade is scored at 95-100 points, the excellent grade is scored at 85-94, the general grade is scored at 75-84, the passing grade is scored at 65-74, and the poor grade is scored below 64 points.

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