A method for simultaneously measuring the open circuit potential curve and entropy coefficient curve of lithium ion battery electrode material

Abstract

A method for simultaneously measuring the open circuit potential curve and entropy coefficient curve of lithium ion battery electrode materials belongs to the technical field of lithium ion batteries. Specific steps: disassemble the battery to be simulated, assemble it into a button-type half-battery, first perform several cycles of charge and discharge cycle activation at room temperature, and then charge or discharge (positive electrode charging, negative electrode discharging) to full charge or full discharge, and then charge and discharge a fixed capacity (positive electrode discharge, negative electrode charging) with the same current at room temperature, and then stand at different temperatures, and then put it back to room temperature to charge and discharge a fixed capacity with the same current, repeat the above operation until the specified voltage upper or lower limit is reached, and record the voltage and voltage difference at different temperatures. Then the open circuit potential curve is fitted by combining the tangent function and the exponential function, and the entropy coefficient curve is fitted by polynomial fitting, so as to obtain the open circuit potential curve and the entropy coefficient curve.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for simultaneously measuring an open circuit potential curve and an entropy coefficient curve of a lithium ion battery electrode material. Background Art

[0002] In today's society, environmental pollution and energy crisis are becoming increasingly serious. The development and utilization of renewable energy has become a research hotspot for scientists from all over the world, such as solar energy, tidal energy and geothermal energy. However, these renewable energy sources cannot be used directly in social production. Chemical energy storage devices are needed as a medium. The rise and development of new energy storage technologies and chemical energy storage devices provide a strong guarantee for the practical application of new energy technologies. Among the many types of chemical energy storage devices, lithium-ion batteries have the advantages of high voltage platform, high energy density, high power density, low self-discharge rate, long cycle life, no memory effect and low environmental pollution. They have been widely used in electronics, automobiles, communications, aerospace and other fields.

[0003] The life prediction, status diagnosis and thermal management of lithium-ion batteries are the current research hotspots of scholars from all over the world, and most of these studies are based on battery simulation models. The electrochemical model of lithium-ion batteries can not only simulate the external characteristics of the battery such as voltage and current, but also simulate some microscopic physical quantities inside the battery that are difficult to measure in practice. Therefore, it is widely used in battery aging failure analysis and life prediction.

[0004] The electrode potential curve of a battery refers to the potential change curve of the electrode material caused by the change in the dissolution and ion adsorption degree of the electrode surface when the electrode contacts with electrolytes of different concentrations. The battery terminal voltage is equal to the battery open circuit voltage minus the battery polarization overpotential, so the acquisition of the electrode potential curve is particularly important for the simulation calculation of the electrochemical model. The entropy coefficient of the electrode is an important parameter for determining reversible heat and accurate thermal models. Therefore, determining the electrode entropy coefficient curve and accurately describing the reversible heat are of great significance to battery thermal modeling. However, most of the open circuit potential curves and entropy coefficient curves of electrode materials today are the results in references. In specific use, there may be individual differences in electrode materials produced by different manufacturers. Therefore, it is necessary to accurately measure the open circuit potential curve and entropy coefficient curve of the battery electrode material. Summary of the invention

[0005] The purpose of the present invention is to solve the problem of poor battery simulation accuracy caused by inaccurate open circuit potential curve and entropy coefficient curve of electrode materials, and to provide a method for simultaneously measuring the open circuit potential curve and entropy coefficient curve of lithium ion battery electrode materials.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for simultaneously measuring an open circuit potential curve and an entropy coefficient curve of a lithium ion battery electrode material, the method specifically comprising: disassembling a battery to be simulated, assembling it into a button-type half-cell, first performing 2-4 cycles of charge and discharge cycle activation at room temperature, then charging or discharging (positive electrode charging, negative electrode discharging) to full charge or full discharge, then charging and discharging a fixed capacity (positive electrode discharging, negative electrode charging) at the same current at room temperature, then standing at different temperatures in turn, after standing, returning to room temperature to charge and discharge a fixed capacity at the same current, repeating the above operation until a specified voltage upper limit or lower limit is reached, recording the voltage and voltage difference at different temperatures, and then fitting the open circuit potential curve and the entropy coefficient curve.

[0008] Furthermore, the battery to be simulated is a 18650 battery, a soft-pack battery, a square battery, etc.

[0009] Furthermore, the room temperature is 25 degrees Celsius, and the control temperature error is ±1 degree Celsius.

[0010] Furthermore, the half-cell is charged or discharged at 25 degrees Celsius.

[0011] Furthermore, the half-cell is activated by charge-discharge cycle, the number of charge-discharge cycles is 2-4, and the charge-discharge current is 0.2-0.5C.

[0012] Furthermore, the half-battery is charged and discharged (positive electrode charging, negative electrode discharging) until fully charged or fully discharged, the positive electrode is charged with constant current and constant voltage, the upper voltage limit needs to be greater than the upper voltage limit of the simulated full battery, the charging current is 0.5-1C, the cut-off current is 0.02C, the lower voltage limit is 0.001V, and the discharge current is 0.02-0.05C.

[0013] Furthermore, the half-cell is charged and discharged with a fixed capacity (positive electrode discharge, negative electrode charge), the positive electrode is discharged at a SOC of 10% each time, the negative electrode is charged at a SOC of 10% each time, and the charge and discharge current is 0.5-1C.

[0014] Furthermore, the battery is left standing at different temperatures, from high to low according to the simulated temperature as needed, and the standing time needs to be greater than 5 hours.

[0015] Furthermore, the battery is repeatedly charged and discharged at a fixed capacity (positive electrode discharge, negative electrode charge) at room temperature and left to stand at different temperatures until the positive electrode discharge voltage is less than the lower limit of the simulated full battery and the negative electrode charge voltage reaches 1.5-2V. The operation is stopped and the voltage and voltage difference at different temperatures are recorded.

[0016] Furthermore, for the voltage and voltage difference obtained at different temperatures of the battery, the open circuit potential curve is fitted by combining a tangent function with an exponential function, and the entropy coefficient curve is fitted by polynomial fitting, thereby obtaining an open circuit potential curve and an entropy coefficient curve.

[0017] The beneficial effects of the present invention compared to the prior art are:

[0018] (1) The present invention can measure the open circuit potential curve and entropy coefficient curve of the positive and negative electrode materials of the battery when the battery materials are unknown and only the upper and lower limits of battery charge and discharge are known;

[0019] (2) The method used in the present invention is simple and convenient, does not require too much equipment and operation, and can solve the entropy coefficient while measuring the open circuit potential curve, thereby providing reliable information for battery simulation and thermal management;

[0020] (3) The open circuit potential curve and entropy coefficient curve of the lithium ion battery electrode material obtained by the present invention are applied to the electrochemical model to improve the accuracy of battery simulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is an open circuit potential curve of the lithium ion battery positive electrode material measured by the present invention.

[0022] Figure 2 This is an open circuit potential curve of the negative electrode material of a lithium-ion battery measured by the present invention.

[0023] Figure 3 This is a curve diagram of the entropy coefficient of the lithium-ion battery positive electrode material measured by the present invention.

[0024] Figure 4 This is a curve diagram of the entropy coefficient of the negative electrode material of a lithium-ion battery measured by the present invention.

[0025] Figure 5 It is a comparison chart of the time-voltage simulation data and the measured value of the constant current discharge of the lithium ion battery at 25°C of the present invention.

[0026] Figure 6 It is a comparison chart of the time-voltage simulation data and the measured value of the constant current discharge of the lithium ion battery at 0°C of the present invention.

[0027] Figure 7 This is a comparison chart of the time-voltage simulation data and the measured value of the constant current discharge of the lithium ion battery at -20°C of the present invention.

[0028] Figure 8 This is a comparison chart of the time-voltage simulation data and the measured value of the constant current discharge of the lithium ion battery at -40°C of the present invention.

[0029] Fig. 9It is a comparison chart of the temperature simulation data and the measured value of the constant current discharge of the lithium ion battery at 25°C of the present invention.

[0030] Fig.10 It is a comparison chart of the temperature simulation data and the measured value of the constant current discharge of the lithium ion battery at 0°C of the present invention.

[0031] Fig.11 This is a comparison chart of the temperature simulation data and the measured values ​​of the constant current discharge of the lithium ion battery at -20°C of the present invention.

[0032] Fig.12 This is a comparison chart of the temperature simulation data and the measured values ​​of the constant current discharge of the lithium ion battery at -40°C of the present invention. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0034] The present invention applies the measured open circuit curve and entropy coefficient curve of the battery material to the electrochemical model. The model patent application number used is: CN201510337596.9, and the name is: Invention patent of method for obtaining electrochemical and thermal coupling model of lithium-ion battery. After actual use and experimental verification, it is known that the open circuit curve and entropy coefficient curve measured by the present invention can greatly improve the accuracy of low-temperature simulation.

[0035] Embodiment 1:

[0036] A 18650 lithium-ion battery with an upper charging limit of 4.4V, a lower discharging limit of 3.0V, a rated capacity of 2400mAh, a positive electrode of the battery being lithium cobalt oxide, and a negative electrode being graphite.

[0037] Step 1: First disassemble the battery, and assemble the positive and negative pole pieces into button-type half-cells. The negative poles of the half-cells all use lithium sheets, and five button-type half-cells are assembled for each material.

[0038] Step 2: Place the button half-cell at 25 degrees Celsius for 8 hours and start activation. The positive electrode is charged and discharged with a constant current, with an upper limit of 4.5V, a lower limit of 3.0V, and a charge and discharge current of 0.5C. The negative electrode is charged and discharged with a constant current, with an upper limit of 1.5V, a lower limit of 0.001V, and a charge and discharge current of 0.5C.

[0039] Step 3: Let the half battery stand for 3 hours after the cycle, and charge the positive electrode with constant current and constant voltage, with the upper voltage limit of 4.5V, the charging current of 0.5C, and the cut-off current of 0.02C. The negative electrode is discharged with constant current, with the lower voltage limit of 0.001V and the discharge current of 0.05C.

[0040] Step 4: Place the fully charged or fully discharged half-battery at 25 degrees Celsius for 5 hours, then discharge the positive electrode at a constant current of 0.1C for 1 hour, and charge the negative electrode at a constant current of 0.1C for 1 hour.

[0041] Step 5: Place the positive and negative electrodes at 25, 0, -20, and -40 degrees Celsius for 5 hours respectively, and record the voltage at different temperatures.

[0042] Step 6: Repeat the room temperature charge and discharge (positive electrode discharge, negative electrode charge) of the half-cell, and let it stand at different temperatures until the positive electrode discharge voltage is less than 3.0V and the negative electrode charge voltage reaches 1.5V, then stop the operation and record the open circuit voltage at different discharge depths and the voltage difference at different temperatures.

[0043] Step 7: Fit the open circuit voltage of the half-cell at different discharge depths measured by the tangent function and the exponential function (see the attached figure for the results). Figure 1-2 ).

[0044] Step 8: Use a polynomial to fit the voltage difference at different temperatures measured by the half-cell. (The results are shown in the attached Figure 3-4 ).

[0045] Step 9: Apply the fitted positive and negative electrode open circuit potential curves and entropy coefficient curves to the electrochemical model. (Simulation results are shown in the attached Figure 5-12 )

Claims

1. A method for simultaneously measuring an open circuit potential curve and an entropy coefficient curve of a lithium ion battery electrode material, characterized in that: The method specifically comprises: disassembling a battery to be simulated, assembling it into a button-type half-battery, performing 2-4 cycles of charge-discharge cycle activation at room temperature, then charging or discharging it to full charge or full discharge, then charging and discharging a fixed capacity at the same current at room temperature, then standing at different temperatures in turn, and after standing, returning it to room temperature to charge and discharge a fixed capacity at the same current, repeating the above operation until a specified voltage upper limit or lower limit is reached, recording the voltage and voltage difference at different temperatures, and then fitting an open circuit potential curve and an entropy coefficient curve; The half-cell is activated by charge-discharge cycle, the number of charge-discharge cycles is 2-4, and the charge-discharge current is 0.2-0.5C; The half-battery is charged and discharged until fully charged or fully discharged, the positive electrode is charged with constant current and constant voltage, the upper voltage limit needs to be greater than the upper voltage limit of the simulated full battery, the charging current is 0.5-1C, the cut-off current is 0.02C, the lower voltage limit is 0.001V, and the discharge current is 0.02-0.05C; The half-cell is charged and discharged at a fixed capacity, the positive electrode is discharged at a SOC of 10% each time, the negative electrode is charged at a SOC of 10% each time, and the charge and discharge current is 0.5-1C; The battery is placed at different temperatures, and the temperature is simulated from high to low, and the rest time needs to be greater than 5 hours; The battery is repeatedly charged and discharged at a fixed capacity at room temperature and left to stand at different temperatures until the positive electrode discharge voltage is less than the lower limit of the simulated full battery and the negative electrode charge voltage reaches 1.5-2V. The operation is stopped and the voltage and voltage difference at different temperatures are recorded.

2. The method for simultaneously measuring the open circuit potential curve and the entropy coefficient curve of a lithium ion battery electrode material according to claim 1, characterized in that: The batteries to be simulated are 18650 batteries, soft-pack batteries, and square batteries.

3. The method for simultaneously measuring the open circuit potential curve and the entropy coefficient curve of a lithium ion battery electrode material according to claim 1, characterized in that: The room temperature is 25 degrees Celsius, and the control temperature error is ±1 degree Celsius.

4. The method for simultaneously measuring an open circuit potential curve and an entropy coefficient curve of a lithium ion battery electrode material according to claim 1, characterized in that: When the half-cell is charged or discharged, it is carried out at 25 degrees Celsius.

5. The method for simultaneously measuring the open circuit potential curve and the entropy coefficient curve of a lithium ion battery electrode material according to claim 1, characterized in that: For the voltage and voltage difference obtained at different temperatures of the battery, the open circuit potential curve is fitted by combining a tangent function with an exponential function, and the entropy coefficient curve is fitted by polynomial fitting, thereby obtaining an open circuit potential curve and an entropy coefficient curve.

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