A method for measuring the thermal entropy coefficient of lithium-ion batteries

Abstract

The present invention discloses a method for measuring the temperature entropy coefficient of a lithium-ion battery, comprising: obtaining, under a constant temperature environment, the real-time open-circuit voltage, the real-time discharge capacity at a constant temperature, and the real-time open-circuit voltage that varies with the state of charge of the lithium-ion battery under test; obtaining, under an adiabatic environment, the real-time voltage, the real-time adiabatic discharge capacity, the accumulated discharge time, the real-time temperature, the real-time voltage that varies with the state of charge, and the adiabatic temperature rise rate that varies with the state of charge of the lithium-ion battery under test; and determining the real-time temperature entropy coefficient based on the real-time open-circuit voltage that varies with the state of charge, the real-time voltage that varies with the state of charge, and the adiabatic temperature rise rate that varies with the state of charge. The measurement method of the present invention can calculate the temperature entropy coefficient of the lithium-ion battery that varies continuously over the entire state of charge range, and is a simple and efficient measurement method.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery thermal safety, and more particularly to a method for measuring the thermal entropy coefficient of a lithium-ion battery. Background Art

[0002] The widespread use of lithium-ion batteries has greatly facilitated people's lives. The popularity of electric vehicles, in particular, has not only brought a more comfortable travel experience but also significantly reduced energy consumption. However, safety issues with electric vehicles have become increasingly prominent, seriously hindering the development of lithium-ion batteries. Electric vehicle safety issues are primarily due to battery safety concerns, which in turn stem largely from heat generation during operation. Therefore, heat generation in lithium-ion batteries has become a key issue for electric vehicle safety.

[0003] Establishing a heat generation model during the charging and discharging process of lithium-ion batteries can provide a reference for the thermal management system of lithium-ion batteries. The heat generated by lithium-ion batteries is divided into two parts: reversible heat and irreversible heat. Irreversible heat can be obtained by measuring the internal resistance of the battery, and the calculation of reversible heat requires obtaining the temperature entropy coefficient of the battery under study. The temperature entropy coefficient of traditional lithium-ion batteries is obtained by the open circuit voltage method. The specific method is to obtain the slope of the open circuit voltage of the battery at a certain state of charge with temperature through testing, that is, the temperature entropy coefficient of the measured battery at this state of charge. This method can only obtain the temperature entropy coefficient of the battery at a specific state of charge, but cannot obtain the temperature entropy coefficient value of the battery that changes continuously over the entire state of charge range. In addition, this method requires a lot of testing time, which greatly affects the research efficiency. Summary of the Invention

[0004] The embodiment of the present invention provides a method for measuring the thermal entropy coefficient of a lithium-ion battery, so as to solve the problems raised in the above background technology.

[0005] An embodiment of the present invention provides a method for measuring the thermal entropy coefficient of a lithium-ion battery, comprising:

[0006] In a constant temperature environment, obtain the real-time open circuit voltage and constant temperature real-time discharge capacity of the lithium-ion battery under test;

[0007] According to the constant temperature real-time discharge capacity, the constant temperature real-time state of charge is obtained; and the real-time open circuit voltage and the constant temperature real-time state of charge are matched to obtain the real-time open circuit voltage that changes with the state of charge;

[0008] In an adiabatic environment, obtain the real-time voltage, adiabatic real-time discharge capacity, cumulative discharge time, and real-time temperature of the lithium-ion battery under test;

[0009] Derivative the real-time temperature with respect to the accumulated discharge time to obtain the adiabatic temperature rise rate;

[0010] According to the adiabatic real-time discharge capacity, the constant temperature real-time state of charge is obtained; the real-time voltage is matched with the constant temperature real-time state of charge to obtain the real-time voltage that changes with the state of charge; and the adiabatic temperature rise rate is matched with the constant temperature real-time state of charge to obtain the adiabatic temperature rise rate that changes with the state of charge;

[0011] The real-time temperature entropy coefficient is determined according to the real-time open circuit voltage that changes with the state of charge, the real-time voltage that changes with the state of charge, and the adiabatic temperature rise rate that changes with the state of charge.

[0012] Furthermore, obtaining the real-time open circuit voltage and constant temperature real-time discharge capacity of the lithium-ion battery under test specifically includes:

[0013] Place the battery under test in a constant temperature environment, set an initial temperature value T0, and maintain it for a long enough time to allow the lithium-ion battery under test to reach thermal equilibrium with the constant temperature environment;

[0014] After the lithium-ion battery under test is activated by charge and discharge cycles under the T0 temperature environment, it is charged to a fully charged state and left to stand for a sufficient time to allow the lithium-ion battery under test to reach a balanced state;

[0015] The fully charged lithium-ion battery under test is discharged at a constant current rate of 1 / 25C at temperature T0. The discharge is terminated when the voltage of the lithium-ion battery under test reaches the discharge cut-off voltage. The real-time open-circuit voltage at discharge time i and the constant temperature real-time discharge capacity are recorded.

[0016] Furthermore, the constant temperature real-time charging state is:

[0017]

[0018] Among them, Q0 is the actual total discharge capacity of the entire discharge process at constant temperature; Q 0,i is the actual total discharge capacity at constant temperature discharge to the i-th moment.

[0019] Furthermore, the real-time open circuit voltage that changes with the state of charge for:

[0020]

[0021] Among them, U OCV，i is the real-time open circuit voltage.

[0022] Furthermore, the real-time voltage, adiabatic real-time discharge capacity, cumulative discharge time, and real-time temperature of the lithium-ion battery under test are obtained, specifically including:

[0023] Place the lithium-ion battery under test in an adiabatic environment and record the real-time temperature of the lithium-ion battery under test;

[0024] Adjust the initial temperature of the lithium-ion battery under test to be consistent with the adiabatic environment temperature;

[0025] The lithium-ion battery under test is discharged at a constant current from a fully charged state. The discharge is terminated when the voltage of the lithium-ion battery under test reaches the discharge cut-off voltage. The real-time voltage, adiabatic real-time discharge capacity, cumulative discharge time, and real-time temperature of the lithium-ion battery under test at discharge time i are recorded.

[0026] Furthermore, the adiabatic real-time state of charge is:

[0027]

[0028] Among them, Q1 is the actual total discharge capacity of the entire discharge process under adiabatic environment; Q 1,i is the actual total discharge capacity at the time of discharge to the i-th moment under adiabatic environment.

[0029] Furthermore, the real-time voltage that changes with the state of charge for:

[0030]

[0031] Among them, U 1，i is the real-time voltage.

[0032] Furthermore, the adiabatic temperature rise rate that varies with the state of charge for:

[0033]

[0034] Among them, dTdt i is the adiabatic temperature rise rate.

[0035] Furthermore, the real-time temperature entropy coefficient is:

[0036]

[0037] Among them, T 1，i-1 is the temperature of the lithium-ion battery under test at time i-1; I1 is the discharge current under adiabatic environment; M is the mass of the lithium-ion battery under test; Cp is the specific heat capacity of the lithium-ion battery under test; is the real-time open circuit voltage that changes with the state of charge; is the real-time voltage that changes with the state of charge; is the adiabatic temperature rise rate that varies with the state of charge.

[0038] The embodiment of the present invention provides a method for measuring the thermal entropy coefficient of a lithium-ion battery. Compared with the prior art, the method has the following advantages:

[0039] By recording the temperature rise rate of a lithium-ion battery in an adiabatic environment and combining it with the open-circuit voltage at a constant temperature and the discharge voltage under adiabatic conditions, the present invention can calculate the continuously changing thermal entropy coefficient of the lithium-ion battery across its entire state of charge range. This method provides a simple and efficient method for measuring the thermal entropy coefficient of lithium-ion batteries, saving testing time. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A schematic flow chart of a method for measuring the thermal entropy coefficient of a lithium-ion battery provided by an embodiment of the present invention;

[0041] Figure 2 A schematic diagram of the voltage and temperature rise rate of a lithium-ion battery under test as the state of charge (SOC) changes, provided by an embodiment of the present invention;

[0042] Figure 3 The thermal entropy coefficient result of the 18650 cylindrical battery obtained in Example 1 provided in the embodiments of the present invention. DETAILED DESCRIPTION

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0044] See also Figure 1 The embodiment of the present invention provides a method for measuring the thermal entropy coefficient of a lithium-ion battery, the method comprising:

[0045] Step S1, in a constant temperature environment, obtaining the real-time open circuit voltage and constant temperature real-time discharge capacity of the lithium-ion battery under test.

[0046] Step S2, obtaining a constant temperature real-time state of charge according to the constant temperature real-time discharge capacity; and making the real-time open circuit voltage correspond to the constant temperature real-time state of charge to obtain a real-time open circuit voltage that changes with the state of charge.

[0047] Step S3, in an adiabatic environment, obtaining the real-time voltage, adiabatic real-time discharge capacity, accumulated discharge time, and real-time temperature of the lithium-ion battery under test.

[0048] Step S4: Derivative the real-time temperature with respect to the accumulated discharge time to obtain the adiabatic temperature rise rate.

[0049] Step S5, obtaining the constant temperature real-time state of charge based on the adiabatic real-time discharge capacity; and making the real-time voltage correspond to the constant temperature real-time state of charge to obtain the real-time voltage that changes with the state of charge, and making the adiabatic temperature rise rate correspond to the constant temperature real-time state of charge to obtain the adiabatic temperature rise rate that changes with the state of charge.

[0050] Step S6, determining the real-time temperature entropy coefficient according to the real-time open circuit voltage that changes with the state of charge, the real-time voltage that changes with the state of charge, and the adiabatic temperature rise rate that changes with the state of charge.

[0051] The specific description of the above steps S1 to S6 is as follows:

[0052] For step S1:

[0053] Obtain a lithium-ion battery to be tested, connect the battery to a charge and discharge tester and a voltage acquisition device respectively; place the battery to be tested in a constant temperature environment, set an initial temperature value T0, and maintain it for a long enough time so that the lithium-ion battery to be tested and the constant temperature environment reach thermal equilibrium; after the battery to be tested is activated by charge and discharge cycles at the temperature T0, it is charged to a fully charged state and left to stand for a long enough time so that the battery reaches a balanced state; the fully charged lithium-ion battery to be tested is subjected to a charge and discharge cycle at the temperature T0 at a rate of 1 / 25C (the corresponding discharge current is Q is the rated capacity of the lithium-ion battery under test) and constant current discharge is performed. When the battery voltage reaches the discharge cut-off voltage, the discharge is terminated and the voltage U at the i-th moment of discharge is recorded in real time. OCV，i and discharge capacity Q 0，i .

[0054] It should be noted that the lithium-ion battery tested in the present invention is a 18650 cylindrical battery, and its positive electrode material is LiNi 0。8 Co 0。15 Al 0。05 O2 (NCA), the negative electrode material is graphite. In the present invention, the battery charge and discharge tester is Neware, and the voltage acquisition equipment is Neware. The constant temperature environment should be able to ensure that it remains unchanged at a specific temperature. Preferably, the present invention adopts a constant temperature box to provide a constant temperature environment. The temperature of the constant temperature box can be adjusted. The constant temperature box adopts the Galaxy high and low temperature test box. In the present invention, the initial temperature T0 is the specified operating temperature of the lithium-ion battery. Preferably, the initial temperature T0 is 25°C. The present invention adopts a 1 / 25C rate for discharge, but is not limited to this, and other suitable discharge currents can also be adopted. The discharge cut-off voltage is the specified discharge cut-off voltage of the lithium-ion battery being tested. The battery and the environment reach thermal equilibrium when the temperature difference between the battery and the environment is not higher than 0.1°C.

[0055] For step S2:

[0056] Calculate the real-time state of charge of the lithium-ion battery during its discharge process Among them, Q0 is the actual total discharge capacity of the entire discharge process, Q 0，i is the actual discharge capacity at the time i.

[0057] During the discharge process of the lithium-ion battery under test, the real-time open circuit voltage U OCV，i Corresponding to the real-time state of charge, that is Get the open circuit voltage of the lithium-ion battery under test as the state of charge SOC changes

[0058] For step S3:

[0059] The lithium-ion battery under test is placed in an adiabatic environment, a temperature sensor is arranged on its surface and connected to a temperature acquisition device to record the temperature of the lithium-ion battery under test in real time; the initial temperature of the lithium-ion battery under test is adjusted to be consistent with the temperature of the adiabatic environment and both are T0; the lithium-ion battery under test is discharged from a fully charged state at a low current I1 at a constant current, and the discharge is terminated when the battery voltage reaches the discharge cut-off voltage, and the voltage U of the lithium-ion battery under test at the i-th moment of discharge is recorded in real time. 1，i , discharge capacity Q 1，i , cumulative discharge time t i and temperature T 1，i .

[0060] It should be noted that, in the present invention, an adiabatic environment means that there is no heat exchange between the battery and the environment, that is, all heat generated by the battery is absorbed by the battery itself, which should ensure that the temperature difference between the environment and the lithium-ion battery being tested is no more than 0.05°C. Preferably, the adiabatic environment is provided by an accelerating rate calorimeter (ARC). The present invention does not specifically limit the types of temperature sensors and temperature acquisition devices. Preferably, the temperature sensor is an ARC, and the temperature acquisition device is an ARC.

[0061] For step S4:

[0062] The real-time adiabatic temperature rise rate dTdt of the lithium-ion battery during discharge is obtained by taking the derivative of the real-time temperature T1 of the lithium-ion battery being discharged under an adiabatic environment with respect to the discharge time t. i .

[0063] For step S5:

[0064] Calculate the real-time state of charge of the lithium-ion battery during its discharge process Among them, Q1 is the actual total discharge capacity of the entire discharge process, Q 1，i is the actual discharge capacity at the time i.

[0065] During the discharge process of the lithium-ion battery under test, the real-time voltage U1，i With real-time SOC i Corresponding, that is The real-time adiabatic temperature rise rate dTdt of the lithium-ion battery during discharge i With real-time SOC i Corresponding, that is Get the voltage of the lithium-ion battery under test as the state of charge SOC changes and temperature rise rate

[0066] For step S6:

[0067] The temperature entropy coefficient of the lithium-ion battery under test is calculated by the following formula:

[0068]

[0069] Among them, T 1，i-1 is the temperature of the lithium-ion battery under test at time i-1; I1 is the discharge current under adiabatic environment; M is the mass of the lithium-ion battery under test; Cp is the specific heat capacity of the lithium-ion battery under test; is the real-time open circuit voltage that changes with the state of charge; is the real-time voltage that changes with the state of charge; is the adiabatic temperature rise rate that varies with the state of charge.

[0070] It should be noted that the mass M and specific heat capacity Cp of the lithium-ion battery under test are provided by the lithium-ion battery supplier, but this is not limited to this. It should be clear that those skilled in the art can also directly obtain the mass M and specific heat capacity Cp through testing, which will not be repeated here.

[0071] Example 1

[0072] A method for measuring the thermal entropy coefficient of a lithium-ion battery comprises the following steps:

[0073] 1. Provide a lithium-ion battery to be tested. Preferably, the lithium-ion battery is a 18650 cylindrical battery with a positive electrode material of LiNi 0。8 Co 0。15 Al 0。05 O2(NCA), the negative electrode material is graphite.

[0074] 2. Connect the lithium-ion battery to be tested to the battery charge and discharge tester Neware.

[0075] 3. Place the lithium-ion battery under test in a Galaxy high and low temperature test chamber, set the initial temperature T0 to 25°C, and let it stand for 12 hours.

[0076] 4. The lithium-ion battery under test was charged and discharged three times at a current of 0.6 A at a temperature of T0, and then charged to 4.2 V at a constant current of 0.6 A, and allowed to stand for no less than 2 hours.

[0077] 5. The lithium-ion battery under test is discharged at a constant current rate of 1 / 25C (corresponding to a discharge current I0 of 0.12A) at a temperature of T0. The discharge is terminated when the battery voltage reaches the discharge cut-off voltage of 2.5V. The voltage U at the i-th moment of discharge is recorded in real time. OCV，i and discharge capacity Q 0，i .

[0078] 6. Calculate the real-time value of the state of charge during the discharge process of the lithium-ion battery being tested Among them, Q0 is the actual total discharge capacity of the entire discharge process, Q 0，i is the actual discharge capacity at the time i.

[0079] 7. During the discharge process of the lithium-ion battery under test, the real-time open circuit voltage U OCV，i Corresponding to the real-time state of charge, that is Get the open circuit voltage of the lithium-ion battery under test as the SOC changes

[0080] 8. Place the lithium-ion battery under test in an ARC, arrange a thermocouple on its surface, and record the temperature of the lithium-ion battery under test in real time through the ARC.

[0081] 9. Adjust the initial temperature of the lithium-ion battery under test and the ARC cavity temperature to 25° C., and the difference between the ARC cavity temperature and the temperature of the lithium-ion battery under test is not higher than 0.05° C.;

[0082] 10. The lithium-ion battery under test is discharged from a fully charged state at a constant current of I1 = 0.6A. The discharge is terminated when the battery voltage reaches the discharge cut-off voltage of 2.5V. The voltage U of the lithium-ion battery under test at the time i of discharge is recorded in real time. 1，i , discharge capacity Q 1，i , cumulative discharge time t i and temperature T 1，i .

[0083] 11. Calculate the real-time value of the state of charge during the discharge process of the lithium-ion battery being tested Among them, Q1 is the actual total discharge capacity of the entire discharge process, Q 1，i is the actual discharge capacity at the time i.

[0084] 12. Derivative of the real-time temperature T1 of the lithium-ion battery being tested when discharged in an adiabatic environment with respect to the discharge time t to obtain the real-time adiabatic temperature rise rate dTdt of the lithium-ion battery during discharge i.

[0085] 13. During the discharge process of the lithium-ion battery under test, the real-time voltage U 1，i With real-time SOC i Corresponding, that is The real-time adiabatic temperature rise rate dTdt of the lithium-ion battery during discharge i With real-time SOC i Corresponding, that is Get the voltage of the lithium-ion battery under test as the state of charge SOC changes and temperature rise rate like Figure 2 shown.

[0086] 14. Obtain from the battery manufacturer the mass M of the lithium-ion battery being tested, which is 65.0 g, and the specific heat capacity Cp, which is 0.9654 J / (g·K). -1 .

[0087] 15. Calculate the temperature entropy coefficient of the lithium-ion battery under test using the following formula. The result is as follows: Figure 3 As shown:

[0088]

[0089] Among them, T 1，i-1 is the temperature of the lithium-ion battery under test at time i-1; I1 is the discharge current under adiabatic environment; M is the mass of the lithium-ion battery under test; Cp is the specific heat capacity of the lithium-ion battery under test; is the real-time open circuit voltage that changes with the state of charge; is the real-time voltage that changes with the state of charge; is the adiabatic temperature rise rate that varies with the state of charge.

[0090] The above disclosure is only a few specific embodiments of the present invention. Those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A method for measuring the thermal entropy coefficient of a lithium-ion battery, characterized in that: include: In a constant temperature environment, obtain the real-time open circuit voltage and constant temperature real-time discharge capacity of the lithium-ion battery under test; According to the constant temperature real-time discharge capacity, the constant temperature real-time charge state is obtained; The real-time open circuit voltage is matched with the real-time state of charge at constant temperature to obtain the real-time open circuit voltage that changes with the state of charge. In an adiabatic environment, obtain the real-time voltage, adiabatic real-time discharge capacity, cumulative discharge time, and real-time temperature of the lithium-ion battery under test; Derivative the real-time temperature with respect to the accumulated discharge time to obtain the adiabatic temperature rise rate; According to the adiabatic real-time discharge capacity, the constant temperature real-time state of charge is obtained; And the real-time voltage and constant temperature real-time charge state are matched to obtain the real-time voltage that changes with the charge state And correspond the adiabatic temperature rise rate to the constant temperature real-time state of charge to obtain the adiabatic temperature rise rate that changes with the state of charge According to the real-time open circuit voltage that changes with the state of charge Real-time voltage changes with state of charge Adiabatic temperature rise rate with state of charge Determine the real-time temperature entropy coefficient; The real-time temperature entropy coefficient is: Among them, T 1，i-1 is the temperature of the lithium-ion battery under test at time i-1; I1 is the discharge current under adiabatic conditions; M is the mass of the lithium-ion battery under test; Cp is the specific heat capacity of the lithium-ion battery under test; is the real-time open circuit voltage that changes with the state of charge; is the real-time voltage that changes with the state of charge; is the adiabatic temperature rise rate that varies with the state of charge.

2. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 1, wherein: The obtaining of the real-time open circuit voltage and constant temperature real-time discharge capacity of the lithium-ion battery under test specifically includes: Place the battery under test in a constant temperature environment, set an initial temperature value T0, and maintain it for a long enough time to allow the lithium-ion battery under test to reach thermal equilibrium with the constant temperature environment; After the lithium-ion battery under test is activated by charge and discharge cycles under the T0 temperature environment, it is charged to a fully charged state and left to stand for a sufficient time to allow the lithium-ion battery under test to reach a balanced state; The fully charged lithium-ion battery under test is discharged at a constant current rate of 1 / 25C at temperature T0. The discharge is terminated when the voltage of the lithium-ion battery under test reaches the discharge cut-off voltage. The real-time open-circuit voltage at discharge time i and the constant temperature real-time discharge capacity are recorded.

3. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 2, wherein: The constant temperature real-time state of charge is: Among them, Q0 is the actual total discharge capacity of the entire discharge process at constant temperature; Q 0,i is the actual total discharge capacity at constant temperature discharge to the i-th moment.

4. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 3, wherein: The real-time open circuit voltage that changes with the state of charge for: in, is the real-time open circuit voltage.

5. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 1, wherein: The obtaining of the real-time voltage, real-time adiabatic discharge capacity, accumulated discharge time, and real-time temperature of the lithium-ion battery under test specifically includes: Place the lithium-ion battery under test in an adiabatic environment and record the real-time temperature of the lithium-ion battery under test; Adjust the initial temperature of the lithium-ion battery under test to be consistent with the adiabatic environment temperature; The lithium-ion battery under test is discharged at a constant current from a fully charged state. The discharge is terminated when the voltage of the lithium-ion battery under test reaches the discharge cut-off voltage. The real-time voltage, adiabatic real-time discharge capacity, cumulative discharge time, and real-time temperature of the lithium-ion battery under test at discharge time i are recorded.

6. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 5, wherein: The adiabatic real-time state of charge is: Among them, Q1 is the actual total discharge capacity of the entire discharge process under adiabatic environment; Q 1,i is the actual total discharge capacity at the time of discharge to the i-th moment under adiabatic environment.

7. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 6, wherein: The real-time voltage that changes with the state of charge for: Among them, U 1，i is the real-time voltage.

8. The method for measuring the thermal entropy coefficient of a lithium-ion battery according to claim 6, wherein: The adiabatic temperature rise rate that varies with the state of charge for: Among them, dTdt i is the adiabatic temperature rise rate.

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