Method for measuring internal resistance of all-vanadium redox flow battery under different charge and discharge states

Abstract

The application belongs to the technical field of analysis and test of all-vanadium redox flow battery, and discloses a method for measuring internal resistance of all-vanadium redox flow battery under different charging and discharging states. The all-vanadium redox flow battery system is subjected to two cycles of constant current charging and discharging by a charging and discharging instrument, and then the internal resistance of the battery in the charging process is measured, and the internal resistance of the battery in the discharging process is measured. (4) The application provides a method for measuring internal resistance, which is simple to operate, accurate in measurement data and has no negative effect on the all-vanadium redox flow battery. According to the resistance values of ohmic polarization internal resistance, electrochemical polarization internal resistance and concentration difference polarization internal resistance, improvements can be made in terms of battery materials, battery structure and the like, so as to indicate the direction for the next research and development work.

Description

Technical Field

[0001] This invention belongs to the field of analysis and testing technology of vanadium redox flow batteries, specifically relating to a method for measuring the ohmic internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance of vanadium redox flow batteries under different charge and discharge states. Background Technology

[0002] Energy storage technology can solve the problems caused by the intermittency and volatility of natural energy sources. Vanadium redox flow batteries have advantages such as high efficiency, fast response speed, intrinsic safety, ultra-long cycle life, independently designable capacity and power, and no geographical limitations. They are suitable for smoothing the supply side of wind and solar power generation, as well as for demand side power management, and are particularly suitable for large-scale energy storage power stations.

[0003] Internal resistance is an important technical indicator for measuring battery performance; batteries with lower internal resistance have stronger high-current discharge capabilities. Therefore, research on battery internal resistance has always been a crucial task in the battery industry. The internal resistance of vanadium redox flow batteries mainly consists of ohmic internal resistance, electrochemical internal resistance, and concentration polarization internal resistance.

[0004] Currently, the HPPC method is mainly used to test resistance in the field of lithium batteries. However, the electrolyte of vanadium redox flow batteries is a flowing liquid, characterized by a large concentration polarization between the battery flow inlet and outlet. The HPPC measurement method ignores the concentration polarization internal resistance, making it difficult to apply to vanadium redox flow batteries.

[0005] Chinese patent CN113805086A, entitled "A Rapid Estimation Method for the Internal Resistance of a Lithium-ion Battery," measures the ohmic internal resistance and polarization internal resistance of lithium batteries. The estimated ohmic internal resistance includes the electrochemical polarization internal resistance; however, the measurement of polarization internal resistance is time-consuming and subject to self-discharge effects, leading to inaccurate measurements. Furthermore, this method cannot determine the specific values ​​of the ohmic internal resistance, electrochemical polarization internal resistance, and concentration polarization internal resistance of lithium batteries. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a method for measuring the internal resistance of a vanadium redox flow battery under different charge and discharge states. By using an AC internal resistance meter and a charge / discharge meter to monitor the open-circuit voltage in real time, the method enables the analysis and measurement of the ohmic polarization internal resistance, electrochemical polarization internal resistance, and concentration polarization internal resistance of the vanadium redox flow battery.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution: a method for measuring the internal resistance of a vanadium redox flow battery under different charge and discharge states (process as follows). Figure 2 The process includes the following steps:

[0008] 1. Perform two constant-current charge-discharge cycles on the vanadium redox flow battery system using a charge-discharge apparatus;

[0009] 2. Measuring battery internal resistance during charging: When measuring the internal resistance during the charging process in the low SOC range (0%–90%), directly charge to the corresponding SOC value using constant current, then let it rest for 5–15 minutes. During the resting period, the battery open-circuit voltage is recorded in real time by the instrument at a data recording frequency of 0.1–2 seconds per point. Simultaneously, the ohmic internal resistance of the battery is measured using the instrument during the resting period. After the resting period, constant current discharge is continued. When measuring the internal resistance during the charging process in the high SOC range (90%–100%), after the constant current charging ends, continue constant voltage charging to the test SOC value, then let it rest for 5–15 minutes. During the resting period, the voltage value and ohmic internal resistance are also recorded. After the resting period, constant current discharge is continued to complete the test.

[0010] 3. Measuring battery internal resistance during discharge: First, charge with constant current, then continue charging with constant voltage until the battery SOC = 100%, then discharge with constant current to the SOC value to be measured, and let it rest for 5 to 15 minutes. During the resting process, record and measure the data using a charge / discharge meter and an AC internal resistance meter. After the resting period, continue discharging with constant current to complete the test.

[0011] Furthermore, the all-vanadium redox flow battery system described in step 1 ( Figure 1 It consists of a main battery, a SOC battery, a positive electrode storage tank, a negative electrode storage tank, pipelines, a magnetic pump, and a charge / discharge device. The main battery includes a positive terminal and a negative terminal. The positive terminal of the main battery is connected to the positive electrode storage tank and a magnetic pump in a closed loop through pipelines. The negative terminal of the main battery is connected to the negative electrode storage tank and another magnetic pump in a closed loop through pipelines. The positive and negative terminals of the main battery are respectively connected to the SOC battery and the charge / discharge device through pipelines. The SOC battery is connected to a voltage detection device.

[0012] The advantages of this invention compared to the prior art are:

[0013] (1) The method for testing the internal resistance of a vanadium redox flow battery of the present invention utilizes the open-circuit voltage change curve and data of the battery system to calculate the total internal resistance, ohmic polarization internal resistance, total electrochemical polarization internal resistance, and concentration polarization internal resistance of the battery system. Since the ohmic polarization internal resistance of the vanadium redox flow battery can be directly measured using an AC internal resistance meter, the present invention can accurately measure the ohmic internal resistance, electrochemical polarization internal resistance, and concentration polarization internal resistance of the vanadium redox flow battery.

[0014] (2) In the development of vanadium redox flow batteries, structural designs are made for electrode frames, electrodes, flow channels, etc., to improve the overall performance of the battery. However, it is far from sufficient to evaluate the impact of different structures on battery performance solely based on battery efficiency; a more detailed analysis of voltage efficiency is also required. This invention can accurately measure the ohmic polarization resistance, electrochemical polarization resistance, and concentration polarization resistance of different vanadium redox flow batteries, providing a basis for battery voltage efficiency analysis and battery performance.

[0015] (3) Based on the resistance values ​​of ohmic polarization internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance, improvements can be made to battery materials and battery structure, pointing the way for the next step of research and development;

[0016] (4) The present invention provides an internal resistance measurement method that is simple to operate, provides accurate measurement data, and has no negative impact on vanadium redox flow batteries. Attached Figure Description

[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0018] Figure 1 This is a schematic diagram of the structure of the all-vanadium redox flow battery system of the present invention;

[0019] Figure 2 This is a flowchart of the internal resistance test of the all-vanadium redox flow battery of the present invention;

[0020] Figure 3 This is a schematic diagram of the open-circuit voltage change curve of the battery system during the post-charging and storage process of this invention.

[0021] Figure 4 This is a graph showing the variation of internal resistances of the vanadium redox flow battery system of the present invention with the state of charge (SOC).

[0022] Figure 5 This is a schematic diagram of the open-circuit voltage change curve of the battery system during the post-discharge storage process of the present invention;

[0023] Figure 6 This is a schematic diagram of the internal resistance of the vanadium redox flow battery system of the present invention as a function of discharge SOC.

[0024] In the diagram: 1. Main battery; 2. Charge / discharge device; 3. SOC battery; 4. Positive electrode reservoir; 5. Negative electrode reservoir; 6. Magnetic pump; 7. Piping; 8. Voltage detection device; 11. Positive electrode of main battery; 12. Negative electrode of main battery. Detailed Implementation

[0025] The present invention is described in detail below through specific embodiments, but this does not limit the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be obtained commercially.

[0026] Example 1

[0027] The vanadium redox flow battery system used in the experiment is a 5kW stack composed of 18 individual cells. A sulfuric acid electrolyte system is used, with a total electrolyte volume of 320L, and 160L each for the positive and negative electrodes. A state-of-the-art (SOC) battery is connected in series in the electrolyte circuit. The open-circuit voltage of the SOC battery can be monitored in real time, and the SOC of the battery system can be read in real time by measuring the open-circuit voltage of the SOC battery. The battery system uses carbon felt electrodes with dimensions of 570mm × 220mm, and a constant-current charge / discharge current density of 150mA / cm². 2 The battery system operates within a temperature range of 36±1℃.

[0028] Method for measuring internal resistance during charging: Charge to the corresponding SOC value using constant current or constant current and constant voltage, let stand for 10 minutes, and record the battery open circuit voltage in real time using a charge / discharge meter during the stand period. The data recording frequency is 0.5s / point. During the stand period, measure the ohmic polarization internal resistance of the battery using an AC internal resistance meter. After the stand period, continue to discharge to 1V using constant current.

[0029] According to the method for measuring internal resistance during charging, the curve of open-circuit voltage change of the battery system during the resting period after charging is as follows: Figure 3 As shown, point A is the open-circuit voltage of the battery system when it is charged to the SOC value, point B is the position 0.5s during the resting process, which is the starting point of the slow decline after the instantaneous drop in open-circuit voltage, and point D is the starting point of the battery system open-circuit voltage entering the stable stage.

[0030] Table 1 shows the open-circuit voltage records during the resting process after charging to SOC values ​​of 10%, 30%, 50%, 70%, and 90%. Based on Table 1, the internal resistance values ​​of the vanadium redox flow battery at different SOC values ​​can be calculated, and the results are shown in Table 2. The curves of internal resistance versus charging SOC are shown below. Figure 4 As shown.

[0031] The method for calculating internal resistance is (U A -U B ) / I=R ohm +R act , (U B -U D ) / I=R com R ohm +R act It is the sum of the ohmic polarization resistance and the electrochemical polarization resistance, R con It is the concentration polarization internal resistance. Furthermore, R... ohm It can be directly measured using an AC internal resistance meter.

[0032] Table 1. Open-circuit voltage record during the post-charging resting process.

[0033]

[0034]

[0035] Table 2. Internal resistance results during charging at different SOCs

[0036]

[0037] Example 2

[0038] The vanadium redox flow battery system used in the experiment consists of a 5kW stack of 18 individual cells. A sulfuric acid electrolyte system is used, with a total electrolyte volume of 320L, and 160L each for the positive and negative electrodes. A state-of-the-art (SOC) battery is connected in series in the electrolyte circuit. The open-circuit voltage of the SOC battery can be monitored in real time, and the SOC of the battery system can be read in real time by measuring the open-circuit voltage of the SOC battery. The battery system uses carbon felt electrodes with dimensions of 570mm × 220mm, and a constant-current charge / discharge current density of 150mA / cm². 2 The battery system operates within a temperature range of 36±1℃. Following the method for measuring internal resistance during discharge, the open-circuit voltage change curve of the battery system during storage after discharge is shown below. Figure 5 As shown, point A is the open-circuit voltage of the battery system when it is discharged to the SOC value, point B is the position 0.5s in the resting process, which is the starting point of the slow rise of the open-circuit voltage after the instantaneous rise, and point D is the starting point of the battery system open-circuit voltage entering the stable stage.

[0039] Table 3 shows the open-circuit voltage records during the resting process after discharging to SOC values ​​of 10%, 30%, 50%, 70%, and 90%. Based on Table 3, the internal resistance values ​​of the vanadium redox flow battery at different SOC values ​​can be calculated, and the results are shown in Table 4. The curves showing the change of the battery's internal resistance with discharge SOC are as follows: Figure 6 As shown.

[0040] Table 3. Open-circuit voltage record during the post-discharge resting process.

[0041]

[0042] Table 4. Internal resistance results of the discharge process at different SOCs

[0043]

[0044] according to Figure 4 and Figure 6The results showed that the ohmic polarization resistance was the highest among the internal resistances of the vanadium redox flow battery, while the electrochemical polarization resistance was the lowest. The ohmic polarization resistance remained essentially constant with state of charge (SOC) during charging and discharging. This is because the ohmic polarization resistance is mainly affected by the experimental temperature, and the vanadium redox flow battery maintained an experimental temperature of approximately 36°C throughout operation. Therefore, the essentially unchanged ohmic polarization resistance demonstrates the reliability of the testing method. At the end of charging and discharging, the increased difficulty of charge transfer and electrochemical reactions at the positive and negative electrodes led to a significant increase in the electrochemical polarization resistance.

[0045] The measurement results of the examples are consistent with common sense. The results show that the present invention can effectively and accurately measure the ohmic polarization resistance, electrochemical polarization resistance, and concentration polarization resistance of vanadium redox flow batteries under different SOC states, which is of great significance for the evaluation of stack performance.

[0046] The embodiments described above are merely preferred embodiments of the present invention, and not all feasible embodiments of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for measuring the internal resistance of a vanadium redox flow battery under different charge and discharge states, characterized in that, Includes the following steps: S1. Perform two constant-current charge-discharge cycles on the vanadium redox flow battery system using a charge-discharge device; S2. Measuring battery internal resistance during charging: When measuring the internal resistance during charging in the low SOC range (0%~90%), directly charge to the corresponding SOC value using constant current, then let it rest for 5~15 minutes. During the resting period, the battery open-circuit voltage is recorded in real time by the instrument at a data recording frequency of 0.1-2 s / point. Simultaneously, the ohmic internal resistance of the battery is measured using the instrument during the resting period. After the resting period, constant current discharge is continued. When measuring the internal resistance during charging in the high SOC range (90%~100%), after the constant current charging ends, continue constant voltage charging to the test SOC value, then let it rest for 5~15 minutes. During the resting period, the voltage value and ohmic internal resistance are also recorded. After the resting period, constant current discharge is continued to complete the test. S3. Measure the battery internal resistance during the discharge process: First, charge with constant current, then continue charging with constant voltage until the battery SOC=100%, then discharge with constant current to the SOC value to be measured, and let it rest for 5~15 minutes. During the resting process, record and measure the data using a charge / discharge meter and an AC internal resistance meter. After the resting period, continue discharging with constant current to complete the test. The method for calculating internal resistance is (U A -U B ) / I=R ohm +R act , (U B -U D ) / I=R com R ohm +R act It is the sum of the ohmic polarization resistance and the electrochemical polarization resistance, R con It is the concentration polarization internal resistance; R ohm U is directly measured using an AC internal resistance meter. A It is the open-circuit voltage of the battery system when charged to the SOC value, U B This is the 0.5 s position in the resting process, the starting point where the open-circuit voltage begins to slowly decrease after the instantaneous drop. U D The point is the starting point for the battery system's open-circuit voltage to enter a stable phase.

2. The method for measuring the internal resistance of a vanadium redox flow battery under different charge and discharge states according to claim 1, characterized in that, The vanadium redox flow battery system described in step 1 consists of a main battery (1), a SOC battery (3), a positive electrode storage tank (4), a negative electrode storage tank (5), a pipeline (7), a magnetic pump (6), and a charge / discharge device (2). The main battery (1) includes a main battery positive electrode (11) and a main battery negative electrode (12). The main battery positive electrode (11) is connected in a closed loop to the positive electrode storage tank (4) and a magnetic pump (6) through the pipeline (7). The main battery negative electrode (12) is connected in a closed loop to the negative electrode storage tank (5) and another magnetic pump (6) through the pipeline (7). The main battery positive electrode (11) and the main battery negative electrode (12) are respectively connected to the SOC battery (3) and the charge / discharge device (2) through the pipeline (7). The SOC battery (3) is connected to a voltage detection device (8).

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